For the nano piezoelectric performances, the conclusions are clear and similar to those obtained on film: with the conventional process, the piezoelectric response disappears (even for large islands (diameter = 800 nm) and even with a post annealing treatment the properties can't be restored. The piezoelectric activity of the nano structure is maintained with the amorphous process. So, the amorphous process is used to fabricate piezoelectric nanostructures. PZT islands of 50 nm in diameter were fabricated and the piezoelectric response was unchanged in comparison with un-etched PZT films. Identical results have been obtained on PMN-PT films. We try in future work to decrease the island diameter and maybe found a diameter where the piezoelectric activity disappears as predicted by some authors (when the dimension of the island = dimension of the domains). A limit exist with the FIB process (due to the anisotropy) and we will try to find this limit and maybe the electron beam lithography will be more adapted for very narrow PZT island (< 50 nm in diameter).

## **4. Acknowledgement**

The authors gratefully acknowledge all the persons who are engaged in this work: Prof. R. Desfeux, and Dr. J.F. Blach from UCCS. Prof. Wang, Prof. Dong and Dr. R.H. Liang from SICCAS – Shanghai.

## **5. References**

[1] C. Ayela, T. Aleva, D. Lagrange, D. Rémiens, C. Soyer, T. Ondarcuhu, A. Greve, and L. Nicu, IEEE Sensors Journal 8, 210 (2008).

**14**

 *C/ Serrano Madrid*

*1Mexico 2Spain*

**Ion Implantation for the Fabrication of**

*1Instituto de Física, Benemérita Universidad Autónoma de Puebla, Puebla*

*Instituto de Óptica, Consejo Superior de Investigaciones Científicas (IO-CSIC),*

Metal nanoparticles (NPs) have been the subject of intense research in recent years, largely due to their characteristic surface plasmon resonance (SPR), a phenomenon arising from the collective oscillation of their conduction electrons. The SPR characteristics of metal NPs have been exploited to utilize them in medical diagnostics (Allain & Vo-Dinh, 2002), immunoassay (Hirsch *et al.*, 2003a; Cui *et al.*, 2006), cancer treatments (Hirsch *et al.*, 2003b; Zhang, 2010), biological or chemical sensing (Sun & Xia, 2002; Alivisatos, 2004; Cao *et al.*, 2009), and surface enhanced Raman scattering (SERS) (Jackson & Halas, 2004). Large thirdorder optical susceptibility and picosecond optical response of metal NPs made them excellent candidates for non-linear optical devices (Mazzoldi *et al.*, 1996; White *et al.*, 1998; Pavesi *et al.*, 2000; Borsella *et al.*, 2001). Strong SPR-induced optical absorption of these nanostructures has also been utilized to enhance their photocatalytic activity (Tihay *et al.*, 2001) and to increase the efficiency of dye-sensitized solar cells (Catchpole & Polman, 2008;

Frequently these NPs are incorporated into dielectric or semiconducting matrices to fabricate dielectric-metal or semiconductor-metal nanocomposites with the aim of manipulating their functionalities, exploiting the characteristics of both the matrix and the metal NPs. Among the nanocomposites, those in which the SPR effect is prominent are called plasmonic nanocomposites (PNCs). Frequently these nanocomposites are fabricated by incorporating nanoparticles of noble metals like Au, Ag and Cu in insulating matrices like silica (a-SiO2), quartz (c-SiO2), and other metal oxides like zinc oxide (ZnO), alumina (Al2O3), and titania (TiO2). Though, in principle, several other metal particles can be used for the fabrication of PNCs this is seldom done, due to their high proneness to oxidation or interband transitions in the usable spectral range. Consequently, in this short review we will

Two characteristics, particularly important for practical use of plasmonic nanocomposites, are: the location of the SPR in the desired spectral position and a high optical extinction. For

**1. Introduction**

Atwater & Polman, 2010; Ding *et al.*, 2011).

mainly focus on these three important metals.

*Universidad Autónoma de Madrid (UAM), Cantoblanco, Madrid*

Umapada Pal1 and Ovidio Peña Rodríguez2

*2Centro de Microanálisis de Materiales (CMAM),*

**Plasmonic Nanocomposites: A Brief Review**

## **Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review**

Umapada Pal1 and Ovidio Peña Rodríguez2

*1Instituto de Física, Benemérita Universidad Autónoma de Puebla, Puebla 2Centro de Microanálisis de Materiales (CMAM), Universidad Autónoma de Madrid (UAM), Cantoblanco, Madrid Instituto de Óptica, Consejo Superior de Investigaciones Científicas (IO-CSIC), C/ Serrano Madrid 1Mexico 2Spain*

## **1. Introduction**

326 Ion Implantation

[2] J. Verd, A. Uranga, G. Abadal, J. Teva, F. Torres, F. Pérez-Murano, J. Fraxeda, J. Esteve,

[4] Y. Cho, K. Fujimoto, Y. Hiranaga, Y. Wagatsuma, A. Onoe, K. Terabe, and K. Kitamura,

[6] C. S. Ganpule, A. Stanshevsky, S. Aggarwal, J. Melngailis, E. Williams, R. Ramesh, V.

[7] R. H. Liang, D. Rémiens, D. Deresmes, C. Soyer, D. Troadec, X.L. Dong, L.H. Yang, R. Desfeux, A. Da Costa, and J.F. Blach, J. Appl. Phys. 105, 044101 (2009).

[9] M. Detalle, G. Wang, D. Rémiens, P. Ruterana, P. Roussel, and B. Dkhil, J. Cryst. Growth,

[10] R. Herdier, M. Detalle, D. Jenkins, C. Soyer, and D. Rémiens, Sensors & Actuators A

[11] S. Hong, J. Woo, H. Shin, J.U. Jeon, Y.E. Park, E. L. Colla, N. Setter, E. Kim, and K. No,

[12] A. L. Kholkin, V. V. Shvartsman, A .Yu. Emelyanov, R. Poyato, M. L. Calzada, L. Pardo,

[14] A. L. Kholkin, V. V. Shvartsman, D. A. Kiselev, I.K. Bdikin, Ferroelectrics 341, 3 (2006).

[16] R. Desfeux, C. Legrand, A. Da Costa, D. Chateigner, R. Bouregba, and G. Poullain, Surf.

[17] A. Ferri, A. Da Costa, R. Desfeux, M. Detalle, G.S. Wang, and D. Rémiens, Integr.

[20] X. Lou, X. Hu, M. Zhang, F. D. Morrison, S. A. T. Redfern, and J. F. Scott, J. Appl. Phys.

[22] J. F. Meng, R. S. Katiyar, G. T. Zou, and X. H. Wang, Phys. Status Solidi A 164, 851, 1997. [23] K. Nomura, Y. Takeda, M. Maeda, and N. Shibata, Jpn. J. Appl. Phys., Part 1 39, 5247,

[25] R. H. Liang, D. Rémiens, C. Soyer, N. Sama, X. L. Dong, and G. S. Wang, Microelectron.

[26] A. Stanishevsky, B. Nagaraj, J. Melngailis, R. Ramesh, L. Khriachtchev, and E.

[18] J. F. Ziegel, The stoppingand Range of Ion in Solid (Pergamon, New York, 1985). [19] J. F. Meng, R. S. Katiyar, and G. T. Zou, Phys. Status Solidi A 164, 851, 1997.

[21] J. Frantti, V. Lantto, S. Nishio, and M. Kakihana, Phys. Rev. B 59, 12, 1999.

[24] C. Soyer, E. Cattan, and D. Rémiens, J. Appl. Phys. 92, 1048, 2002.

McDaniel, J. Appl. Phys. 92, 3275 (2002).

[13] S.V. Kalinin, A. Gruverman and D.A. Bonnel, Appl. Phys. Lett. 85, 795 (2004).

and N. Bardiol, Appl. Phys. Lett. 91, 013501 (2007).

Nanotechnology 14, 637 (2003).

J. Appl. Phys. 89, 1377 (2001).

Appl. Phys. Lett. 82, 2127 (2003).

305, 137 (2007).

148, 122 (2008).

Sci. 600, 219 (2006).

99, 044101, 2006.

Eng. 85, 670 \_2008\_.

2000.

Ferroelectr. 91, 80 (2007).

[8] K. Saito and M. Kaise, Jpn. J. Appl. Phys. 31, 3533 (1992).

[15] A. Gruverman and S.V. Kalinin, J. Mater. Sci. 41, 107 (2006).

[3] R. R. He, X. L. Feng, M. L. Roukes, and P. D. Yang, Nano Lett. 8, 1756 (2008).

[5] M. Alexe, C. Harnagea, D. Hesse, U. Gösele, Appl. Phys. Lett. 75, 1793 (1999).

Joshi, and Carlos Paz de Araujo, Appl. Phys. Lett. 75, 3874 (1999).

Metal nanoparticles (NPs) have been the subject of intense research in recent years, largely due to their characteristic surface plasmon resonance (SPR), a phenomenon arising from the collective oscillation of their conduction electrons. The SPR characteristics of metal NPs have been exploited to utilize them in medical diagnostics (Allain & Vo-Dinh, 2002), immunoassay (Hirsch *et al.*, 2003a; Cui *et al.*, 2006), cancer treatments (Hirsch *et al.*, 2003b; Zhang, 2010), biological or chemical sensing (Sun & Xia, 2002; Alivisatos, 2004; Cao *et al.*, 2009), and surface enhanced Raman scattering (SERS) (Jackson & Halas, 2004). Large thirdorder optical susceptibility and picosecond optical response of metal NPs made them excellent candidates for non-linear optical devices (Mazzoldi *et al.*, 1996; White *et al.*, 1998; Pavesi *et al.*, 2000; Borsella *et al.*, 2001). Strong SPR-induced optical absorption of these nanostructures has also been utilized to enhance their photocatalytic activity (Tihay *et al.*, 2001) and to increase the efficiency of dye-sensitized solar cells (Catchpole & Polman, 2008; Atwater & Polman, 2010; Ding *et al.*, 2011).

Frequently these NPs are incorporated into dielectric or semiconducting matrices to fabricate dielectric-metal or semiconductor-metal nanocomposites with the aim of manipulating their functionalities, exploiting the characteristics of both the matrix and the metal NPs. Among the nanocomposites, those in which the SPR effect is prominent are called plasmonic nanocomposites (PNCs). Frequently these nanocomposites are fabricated by incorporating nanoparticles of noble metals like Au, Ag and Cu in insulating matrices like silica (a-SiO2), quartz (c-SiO2), and other metal oxides like zinc oxide (ZnO), alumina (Al2O3), and titania (TiO2). Though, in principle, several other metal particles can be used for the fabrication of PNCs this is seldom done, due to their high proneness to oxidation or interband transitions in the usable spectral range. Consequently, in this short review we will mainly focus on these three important metals.

Two characteristics, particularly important for practical use of plasmonic nanocomposites, are: the location of the SPR in the desired spectral position and a high optical extinction. For

Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review 329

depth distribution located some micrometers underneath the surface and wide enough to be convenient for the production of optical waveguides. Ion implantation has been also successfully used to create more complex structures such as alloys (Mattei, 2002), sophisticated aggregates (Mattei *et al.*, 2003b; Pellegrini *et al.*, 2007a, 2007b), core-shell structures (Peña *et al.*, 2009) and nanorods (which are aligned, unlike those produced by

In this chapter, we intend to review the current status of the synthesis of plasmonic nanocomposites by means of ion-implantation. In the year 1984, Picraux (Picraux, 1984) has published a useful review on the status of ion-implantation in metals, improving the understanding on the kinetics and equilibria in metal alloy reactions, surface modification, and defect structures. Similarly, Buchal *et al.* (Buchal *et al.*, 1994) have reviewed the status of ion-implantation in optical materials in 1994, focusing mainly on the usefulness of the technique for producing linear and non-linear optical devices. However, apart from a good review on Cu implanted alumina by Stepanov and Khaibullin (Stepanov & Khaibullin, 2005) there exists no recent review summarizing the current advances made on the utilization of this technique for the fabrication and processing of plasmonic nanocomposites which have

> **Predicted (eV)**

**Au** SiO2 (1.48) 2.35 2.48-2.21 (Barreca *et al.*, 2004;

**Cu** SiO2 (1.48) 2.70 2.31-1.59 (De, 1997; Peña *et al.*,

**Au** Al2O3 (1.76) 2.24 2.38-1.79 (García-Serrano & Pal,

**Cu** Al2O3 (1.76) 2.20 2.10-2.00 (Ballesteros *et al.*, 1997;

**Ag** ZnO (2.00) 2.64 2.91-2.53 (Shan *et al.*, 2007; Zheng

**Ag** TiO2 (2.80) 2.09 2.58-2.10 (Okumu *et al.*, 2005;

**Cu** TiO2 (2.80) 1.82 2.15 (Sangpour *et al.*, 2010)

**Au** ZnO (2.00) 2.14 2.48-2.25 (Wang *et al.*, 2003;

**Ag** Al2O3 (1.76) 2.85 3.08-2.06 (Zeng *et al.*, 2004;

**Measured**

**(eV) References**

Kim *et al.*, 2008)

(Roiz *et al.*, 2004; Ma *et al.*, 2009; Duhan *et al.*, 2010)

2006; Tohidi *et al.*, 2010)

2003; Hazra *et al.*, 2004)

Manoilov, 2009)

Serna *et al.*, 2006)

Mishra *et al.*, 2008)

*et al.*, 2008)

(Ryasnyansky *et al.*, 2005; Kalidindi & Jagirdar, 2008)

(Subramanian *et al.*, 2004; Sonawane & Dongare, 2006)

Jiang *et al.*, 2009)

other methods) (Rodríguez-Iglesias *et al.*, 2010a; b).

emerging applications in diverse fields.

**Matrix material (refractive index)**

**Ag** SiO2 (1.48) 3.12 3.92-3.02

**Cu** ZnO (2.00) 2.09 2.14-2.06

**Au** TiO2 (2.80) 1.80 2.34-1.83

Table 1. SPR positions in common plasmonic nanocomposites.

**Incorporated species**

the very small metal NPs (*i.e.* those much smaller than the optical wavelength) the extinction is dominated by absorption and the absorption coefficient () can be expressed as (Arnold & Borders, 1977):

$$\approx \text{(cm}^{-1}\text{)} = \frac{18\pi \text{Qn}\_{\text{d}}^{3}}{\lambda} \frac{\varepsilon\_{2}}{\left(\varepsilon\_{1} + 2\text{n}\_{\text{d}}^{2}\right)^{2} + \varepsilon\_{2}^{2}} \tag{1}$$

where *Q* is the volume fraction occupied by the metallic nanoparticles, *nd* is the refractive index of the host matrix, *1* and *2* are the real and imaginary parts of the dielectric constant of the bulk metal, respectively, and is the optical wavelength. The value of becomes maximum at the surface plasmon frequency (*SPR*), when

$$
\mathfrak{e}\_1 \left( \omega\_{SPR} \right) + 2 \mathfrak{n}\_{\mathbf{d}}^2 = 0. \tag{2}
$$

As we see, the SPR frequency of metal nanoparticles depends on both the optical constant of bulk metal and the refractive index of the host matrix. Estimating the wavelength variation of *1* from the optical constants given by Johnson and Christy (Johnson & Christy, 1972) for Au, Ag, and Cu, and refractive indices of common oxide matrices we can calculate the position of the SPR band for different nanocomposites, as presented in Table 1. Though the position of SRP in different nanocomposites depends also on the size and size distribution of the metal NPs, for which we must introduce size correction terms (Hövel *et al.*, 1993) in equation (2), we can see that the tabulated SPR energies are in good agreement with the reported values for the nanocomposites fabricated by different methods (Ballesteros *et al.*, 1997; De, 1997; Pal *et al.*, 2000, 2001; Vazquez-Cuchillo *et al.*, 2001; Wang *et al.*, 2003; García-Serrano & Pal, 2003; Barreca *et al.*, 2004; Subramanian *et al.*, 2004; Zeng *et al.*, 2004; Hazra *et al.*, 2004; Okumu *et al.*, 2005; Ryasnyansky *et al.*, 2005; Serna *et al.*, 2006; Sonawane & Dongare, 2006; Mishra *et al.*, 2008; Zheng *et al.*, 2008; Kalidindi & Jagirdar, 2008; Kim *et al.*, 2008; Manoilov, 2009; Jiang *et al.*, 2009; Ma *et al.*, 2009; Sangpour *et al.*, 2010; Tohidi *et al.*, 2010; Duhan *et al.*, 2010). As the absorption coefficient depends on the volume fraction occupied by metallic NPs in the composites (eq. 1), the intensity of the SPR band increases on increasing the amount of incorporated metal NPs.

Several methods have been utilized to fabricate plasmonic nanocomposites like cosputtering, ion-implantation, sol-gel, laser ablation, etc. (De, 1997; Pal *et al.*, 2000; García-Serrano & Pal, 2003; Barreca *et al.*, 2004; Kim *et al.*, 2008; Manoilov, 2009). However, irrespective of the fabrication method, practical application of these materials strongly depends on parameters like the size and shape of the incorporated metal NPs, volume fraction occupied by them, homogeneity of NP distribution, and their composition. In this respect, each of the utilized methods has certain constrains. Although ion-implantation is not as versatile as other techniques used for the fabrication of plasmonic nanocomposites, it still has several attractive advantages. For example, ion implantation followed by an additional thermal annealing has proven to be a very useful method to obtain large volume fractions of NPs embedded in solid matrices. Additionally, those NPs are located in a well defined depth below the surface, chosen mainly by the ion energy (Oliver *et al.*, 2002). Some other advantages of ion implantation include controllability of depth profile and concentration, high purity, and the possibility to overcome low solubility restrictions. Particularly, deep ion implantation using energies of the order of MeV produces an ion

the very small metal NPs (*i.e.* those much smaller than the optical wavelength) the extinction is dominated by absorption and the absorption coefficient () can be expressed as (Arnold &

�

where *Q* is the volume fraction occupied by the metallic nanoparticles, *nd* is the refractive

As we see, the SPR frequency of metal nanoparticles depends on both the optical constant of bulk metal and the refractive index of the host matrix. Estimating the wavelength variation

Several methods have been utilized to fabricate plasmonic nanocomposites like cosputtering, ion-implantation, sol-gel, laser ablation, etc. (De, 1997; Pal *et al.*, 2000; García-Serrano & Pal, 2003; Barreca *et al.*, 2004; Kim *et al.*, 2008; Manoilov, 2009). However, irrespective of the fabrication method, practical application of these materials strongly depends on parameters like the size and shape of the incorporated metal NPs, volume fraction occupied by them, homogeneity of NP distribution, and their composition. In this respect, each of the utilized methods has certain constrains. Although ion-implantation is not as versatile as other techniques used for the fabrication of plasmonic nanocomposites, it still has several attractive advantages. For example, ion implantation followed by an additional thermal annealing has proven to be a very useful method to obtain large volume fractions of NPs embedded in solid matrices. Additionally, those NPs are located in a well defined depth below the surface, chosen mainly by the ion energy (Oliver *et al.*, 2002). Some other advantages of ion implantation include controllability of depth profile and concentration, high purity, and the possibility to overcome low solubility restrictions. Particularly, deep ion implantation using energies of the order of MeV produces an ion

*1* from the optical constants given by Johnson and Christy (Johnson & Christy, 1972) for Au, Ag, and Cu, and refractive indices of common oxide matrices we can calculate the position of the SPR band for different nanocomposites, as presented in Table 1. Though the position of SRP in different nanocomposites depends also on the size and size distribution of the metal NPs, for which we must introduce size correction terms (Hövel *et al.*, 1993) in equation (2), we can see that the tabulated SPR energies are in good agreement with the reported values for the nanocomposites fabricated by different methods (Ballesteros *et al.*, 1997; De, 1997; Pal *et al.*, 2000, 2001; Vazquez-Cuchillo *et al.*, 2001; Wang *et al.*, 2003; García-Serrano & Pal, 2003; Barreca *et al.*, 2004; Subramanian *et al.*, 2004; Zeng *et al.*, 2004; Hazra *et al.*, 2004; Okumu *et al.*, 2005; Ryasnyansky *et al.*, 2005; Serna *et al.*, 2006; Sonawane & Dongare, 2006; Mishra *et al.*, 2008; Zheng *et al.*, 2008; Kalidindi & Jagirdar, 2008; Kim *et al.*, 2008; Manoilov, 2009; Jiang *et al.*, 2009; Ma *et al.*, 2009; Sangpour *et al.*, 2010; Tohidi *et al.*, 2010; Duhan *et al.*, 2010). As the absorption coefficient depends on the volume fraction occupied by metallic NPs in the composites (eq. 1), the intensity of the SPR band increases

������� + 2n�

ε�

*2* are the real and imaginary parts of the dielectric constant

� = 0. (2)

is the optical wavelength. The value of

, (1)

becomes

�ε� + 2n� �� � + ε� �

*SPR*), when

λ

∝ �cm��� = 18πQn�

*1* and

on increasing the amount of incorporated metal NPs.

Borders, 1977):

of

index of the host matrix,

of the bulk metal, respectively, and

maximum at the surface plasmon frequency (

depth distribution located some micrometers underneath the surface and wide enough to be convenient for the production of optical waveguides. Ion implantation has been also successfully used to create more complex structures such as alloys (Mattei, 2002), sophisticated aggregates (Mattei *et al.*, 2003b; Pellegrini *et al.*, 2007a, 2007b), core-shell structures (Peña *et al.*, 2009) and nanorods (which are aligned, unlike those produced by other methods) (Rodríguez-Iglesias *et al.*, 2010a; b).

In this chapter, we intend to review the current status of the synthesis of plasmonic nanocomposites by means of ion-implantation. In the year 1984, Picraux (Picraux, 1984) has published a useful review on the status of ion-implantation in metals, improving the understanding on the kinetics and equilibria in metal alloy reactions, surface modification, and defect structures. Similarly, Buchal *et al.* (Buchal *et al.*, 1994) have reviewed the status of ion-implantation in optical materials in 1994, focusing mainly on the usefulness of the technique for producing linear and non-linear optical devices. However, apart from a good review on Cu implanted alumina by Stepanov and Khaibullin (Stepanov & Khaibullin, 2005) there exists no recent review summarizing the current advances made on the utilization of this technique for the fabrication and processing of plasmonic nanocomposites which have emerging applications in diverse fields.

Table 1. SPR positions in common plasmonic nanocomposites.

Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review 331

When high energy ions penetrate into a solid, they gradually lose energy due to non-elastic collisions with the free electrons of the solid and elastic collisions with the nuclei until they become immobilized. As the second process is an elastic one, most of the energy loss of the incident ion bean occurs very close to the surface of the solid, due to the first process. The penetration depth of the ions in the solid target depends on the ion energy, nature of ionic species, and the nature and composition of the target. Typically, the penetration depth or the *range* of the ions in a solid target varies in between a few nanometers to a few micrometers, and hence all the changes occurring during the implantation process remain near the surface

Fig. 1. Schematic presentation of the ion implantation process in a solid and the

Ion beam

Typical ion doses utilized for fabricating plasmonic nanocomposites vary in between 1.0x1016 cm-2 and 2.0x1017 cm-2. While a greater dose frequently induces the formation of undesired nanovoids or nanocavities in the target matrix (Kooi *et al.*, 2000) or inside metal precipitates (particles) (Ren *et al.*, 2006) due to aggregation of vacancies driven by the heat produced during the high-dose ion irradiation, lower doses frequently incorporate metallic species in ionic state inside the dielectric matrices (Haug *et al.*, 2009). While higher ion energy enhances the penetration depth of the implanted species, a higher ion dose shifts the maximum of the distribution of the implanted species in the target towards lower depths, due to the diffusion to the heavily-damaged region produced just before the ions are stopped (Figure 2). Though the penetration depth of ionic species remains within a few micrometers beneath the surface of the implanted solid, it can extend to a larger depth on

In the case of an amorphous solid target, the implanted ions get distributed randomly

� � = ��

where N(*x*) is the concentration of ions at a distance *x* from the surface; N0 is the concentration of ions at *x* = Rp, is the total implanted dose and Rp is the effective range of

�2π ∆R�

�x� �� �R� � x�� 2∆R�

Depth

Ion concentration

� �� (3)

following a Gaussian profile inside the matrix, which can be expressed as:

2∆R�

N�x� = N��x� �� �R� � x��

of the target material (Figure 1).

implantation profile.

increasing the temperature of the solid target.

## **2. Implantation of metal ions in dielectrics**

Since long ago, ion implantation has been used to modify the optical properties of materials. Dielectrics such as silica glass (SiO2), alumina (Al2O3), and quartz have been used traditionally for this purpose; but recently several other metal oxides like ZnO and TiO2 have also been utilized. Historically, ion implantation has been used to change the optical properties of dielectric target materials like reflectivity, absorption, refractive index, luminescence, and electro-optic coefficients (Mazzoldi, 1987; Townsend, 1987). Those modifications are not only associated to the implanted species, but also to a number of other phenomena occurring during the process like radiation damage caused by the ion beam, and formation of colloidal precipitates of the implanted species with diameters in between 2 and 20 nm (Hughes, 1983). As the mechanism of formation of radiation damages and their elimination processes have been widely discussed in the literature (Agulló-López *et al.*, 1988; Pal *et al.*, 2000; Bianconi *et al.*, 2009; Rivera *et al.*, 2010), we will concentrate our discussion on the latter mechanism only.

In fact, the formation of colloidal precipitate of metals like Au and Ag has been utilized for hundreds of years to design beautiful colored glasses (Bamford, 1977). While the color of the implanted dielectrics depends mostly on the implanted species (metals) and to some extent the size of the formed colloidal particles, their intensity/contrast depends on the volume fraction occupied by the colloidal precipitates in the matrix. So, apart from the nature of implanted metal, a control over the implanted dose (amount of implanted species incorporated into the dielectric matrix) is essential to control the optical properties of the substrates (dielectrics). In contrast to the conventional melt growth process, where only about 1% of impurities (guest atoms) can be incorporated into a matrix, there is no such restriction in ion implantation, as the process occurs without thermodynamic restrictions.

For the ion implantation purpose, the ion beams of selected metals doses in the range of 1016-1018 cm-2 have been utilized to expose solid target surfaces as big as a few cm2. In fact, the process has the ability to fabricate technological grade composite wafers with uniform distribution of metal NPs at precise depths below the target surface. The range of metal NP distribution, their occupied volume fraction, size (diameter), and composition can also be controlled by suitable adjustment of implantation parameters. In the following section, we will discuss how these parameters can be controlled and their effects on the characteristics of the final product (nanocomposites).

## **2.1 Controlling parameters in ion implantation process**

Essentially any specie can be introduced into a solid matrix by means of ion implantation, thereby changing its physical properties. For this purpose, ions of the desired element are produced in an *ionization chamber*; then they are electrostatically accelerated to the desired energy and, finally, allowed to impinge on a solid target. As typically each ion is a single atom, the amount of material incorporated or implanted into the target can be precisely calculated by integrating the ion current over the exposure time. The amount incorporated is called *dose* or *fluence*. Generally the ion currents used for implantation are small, in the range of nano- to microamperes. Ion beam energies may vary from a few keV up to hundreds of MeV, depending on the requirement, though energies in the range of a few MeV are commonly used to produce PNCs. Greater ion energy causes higher structural damage to the target. However, due to broader depth distribution, the change in chemical composition at a certain point in the target would be small.

Since long ago, ion implantation has been used to modify the optical properties of materials. Dielectrics such as silica glass (SiO2), alumina (Al2O3), and quartz have been used traditionally for this purpose; but recently several other metal oxides like ZnO and TiO2 have also been utilized. Historically, ion implantation has been used to change the optical properties of dielectric target materials like reflectivity, absorption, refractive index, luminescence, and electro-optic coefficients (Mazzoldi, 1987; Townsend, 1987). Those modifications are not only associated to the implanted species, but also to a number of other phenomena occurring during the process like radiation damage caused by the ion beam, and formation of colloidal precipitates of the implanted species with diameters in between 2 and 20 nm (Hughes, 1983). As the mechanism of formation of radiation damages and their elimination processes have been widely discussed in the literature (Agulló-López *et al.*, 1988; Pal *et al.*, 2000; Bianconi *et al.*, 2009; Rivera *et al.*, 2010), we will concentrate our discussion on

In fact, the formation of colloidal precipitate of metals like Au and Ag has been utilized for hundreds of years to design beautiful colored glasses (Bamford, 1977). While the color of the implanted dielectrics depends mostly on the implanted species (metals) and to some extent the size of the formed colloidal particles, their intensity/contrast depends on the volume fraction occupied by the colloidal precipitates in the matrix. So, apart from the nature of implanted metal, a control over the implanted dose (amount of implanted species incorporated into the dielectric matrix) is essential to control the optical properties of the substrates (dielectrics). In contrast to the conventional melt growth process, where only about 1% of impurities (guest atoms) can be incorporated into a matrix, there is no such restriction in ion implantation, as the process occurs without thermodynamic restrictions. For the ion implantation purpose, the ion beams of selected metals doses in the range of 1016-1018 cm-2 have been utilized to expose solid target surfaces as big as a few cm2. In fact, the process has the ability to fabricate technological grade composite wafers with uniform distribution of metal NPs at precise depths below the target surface. The range of metal NP distribution, their occupied volume fraction, size (diameter), and composition can also be controlled by suitable adjustment of implantation parameters. In the following section, we will discuss how these parameters can be controlled and their effects on the characteristics

Essentially any specie can be introduced into a solid matrix by means of ion implantation, thereby changing its physical properties. For this purpose, ions of the desired element are produced in an *ionization chamber*; then they are electrostatically accelerated to the desired energy and, finally, allowed to impinge on a solid target. As typically each ion is a single atom, the amount of material incorporated or implanted into the target can be precisely calculated by integrating the ion current over the exposure time. The amount incorporated is called *dose* or *fluence*. Generally the ion currents used for implantation are small, in the range of nano- to microamperes. Ion beam energies may vary from a few keV up to hundreds of MeV, depending on the requirement, though energies in the range of a few MeV are commonly used to produce PNCs. Greater ion energy causes higher structural damage to the target. However, due to broader depth distribution, the change in chemical

**2. Implantation of metal ions in dielectrics**

the latter mechanism only.

of the final product (nanocomposites).

**2.1 Controlling parameters in ion implantation process**

composition at a certain point in the target would be small.

When high energy ions penetrate into a solid, they gradually lose energy due to non-elastic collisions with the free electrons of the solid and elastic collisions with the nuclei until they become immobilized. As the second process is an elastic one, most of the energy loss of the incident ion bean occurs very close to the surface of the solid, due to the first process. The penetration depth of the ions in the solid target depends on the ion energy, nature of ionic species, and the nature and composition of the target. Typically, the penetration depth or the *range* of the ions in a solid target varies in between a few nanometers to a few micrometers, and hence all the changes occurring during the implantation process remain near the surface of the target material (Figure 1).

Fig. 1. Schematic presentation of the ion implantation process in a solid and the implantation profile.

Typical ion doses utilized for fabricating plasmonic nanocomposites vary in between 1.0x1016 cm-2 and 2.0x1017 cm-2. While a greater dose frequently induces the formation of undesired nanovoids or nanocavities in the target matrix (Kooi *et al.*, 2000) or inside metal precipitates (particles) (Ren *et al.*, 2006) due to aggregation of vacancies driven by the heat produced during the high-dose ion irradiation, lower doses frequently incorporate metallic species in ionic state inside the dielectric matrices (Haug *et al.*, 2009). While higher ion energy enhances the penetration depth of the implanted species, a higher ion dose shifts the maximum of the distribution of the implanted species in the target towards lower depths, due to the diffusion to the heavily-damaged region produced just before the ions are stopped (Figure 2). Though the penetration depth of ionic species remains within a few micrometers beneath the surface of the implanted solid, it can extend to a larger depth on increasing the temperature of the solid target.

In the case of an amorphous solid target, the implanted ions get distributed randomly following a Gaussian profile inside the matrix, which can be expressed as:

$$\mathbf{N(x)} = \mathbf{N\_0} \exp\left[-\frac{\left(\mathbf{R\_p} - \mathbf{x}\right)^2}{2\Delta\mathbf{R\_p}^2}\right] = \frac{\phi\_0}{\sqrt{2\pi}\,\Delta\mathbf{R\_p}} \exp\left[-\frac{\left(\mathbf{R\_p} - \mathbf{x}\right)^2}{2\Delta\mathbf{R\_p}^2}\right],\tag{3}$$

where N(*x*) is the concentration of ions at a distance *x* from the surface; N0 is the concentration of ions at *x* = Rp, is the total implanted dose and Rp is the effective range of

Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review 333

(b)

Ion energy ( keV )

Fig. 3. a) Schematic illustration of the range of penetration; b) variation of implantation range Rp and standard deviation Rp with ion energy for 2 MeV Au ions implanted in silica; c) distributions of implanted ions and defects. All simulations were performed using

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Rp

 R p

Depth of implantation ( m )

10-3 10-2 10-1 100 101 102 100

Depth of implantation ( m )

R p

> R p

> > 0

1

2

Au conc. ( x104 ions )

3

4

5

**3. Controlled growth of noble metal nanoparticles through ion-implantation**  The noble metals like Au, Ag and Cu have been incorporated into dielectric matrices to produce several plasmonic nanocomposites with varied combinations of metals and dielectrics. The main objective of such works was to utilize the intense local field, strong linear optical absorption or SPR in applications such as optical sensing, biological and chemical sensing, and fabrication of all-optical devices. The electronic transitions responsible for plasmon absorption in metal NPs are also responsible for the generation of optical nonlinearities in the same spectral range, i.e. at the SPR position. To exploit the advantage of this SPR absorption in nanocomposites, it is desirable to have a high concentration of metal NPs (i.e., high filling factor) in the matrix (Flytzanis *et al.*, 1991). Therefore, as has been mentioned earlier, apart from the adequate energy of implantation, a high dose of implanted ions is required. By the terms "high metal ion dose" 0, we mean a dose exceeding the solubility limit of the metal atoms in the target (dielectric in the present case), which generally vary in between 1015 - 1017 cm-2, depending on the nature of ion species and target materials. The characteristics of the incorporated metal NPs also depend

SRIM 2008 (Ziegler, 1985, 2008).

Vacancies / nm

R p

(c)

Rc

Ions

(a)

penetration by an ion, dependent of real range Rc, where Rc, the real range of penetration, i.e. distance covered by the ion in the implanted solid, and Rp is the standard deviation of the Rp value. The location of Rp depends predominantly on the energy of ions and their atomic mass. The relative dispersion of implanted specie, i.e. Rp/Rp depends both on the atomic mass of the implanted species and the atomic mass of the solid target.

Fig. 2. a) Variation of profile and depth of implantation of Cu+ ions into Al2O3 with the variation of ion energy, calculated using SRIM 2008 (Ziegler, 1985, 2008); b) Implantation depth versus dose, calculated using dynamic computer code DYNA of Cu+ ions of 40 keV in Al2O3. Adapted with permission from AIP Conf. Proc. 680 (2003) 601-604 (Stepanov & Popok, 2003).

On the other hand, for the case of a crystalline solid target, the range of penetration and the implant distribution strongly depend on the orientation of the structure with respect to the direction of beam incidence. Generally the penetration depth or the implantation range is about one order higher when the ion beam is projected along a given crystallographic direction of the target like [111], [100], etc. (Burakowski & Wierzchon, 1998). As can be seen from Figure 3, the defect distribution profile remains at a shallower level with respect to the implant distribution.

penetration by an ion, dependent of real range Rc, where Rc, the real range of penetration, i.e. distance covered by the ion in the implanted solid, and Rp is the standard deviation of the Rp value. The location of Rp depends predominantly on the energy of ions and their atomic mass. The relative dispersion of implanted specie, i.e. Rp/Rp depends both on the

0 40 80 120 160 200

Dose ( x1015 cm-2

 1 - 0.1 2 - 0.3 3 - 0.6 4 - 0.9 5 - 1.2 6 - 1.5 7 - 1.8 8 - 2.1 9 - 2.4 10 - 2.7 )

Depth ( nm )

200 keV

atomic mass of the implanted species and the atomic mass of the solid target.

150 keV

100 keV

50 keV

Fig. 2. a) Variation of profile and depth of implantation of Cu+ ions into Al2O3 with the variation of ion energy, calculated using SRIM 2008 (Ziegler, 1985, 2008); b) Implantation depth versus dose, calculated using dynamic computer code DYNA of Cu+ ions of 40 keV in Al2O3. Adapted with permission from AIP Conf. Proc. 680 (2003) 601-604 (Stepanov &

1

0 10 20 30 40 50

Depth ( nm )

On the other hand, for the case of a crystalline solid target, the range of penetration and the implant distribution strongly depend on the orientation of the structure with respect to the direction of beam incidence. Generally the penetration depth or the implantation range is about one order higher when the ion beam is projected along a given crystallographic direction of the target like [111], [100], etc. (Burakowski & Wierzchon, 1998). As can be seen from Figure 3, the defect distribution profile remains at a shallower level with respect to the

Popok, 2003).

0

10

(b)

Atomic fraction ( 0.1 at% )

1

2

Cu concentration ( x105 ions )

3

4

5

(a)

implant distribution.

Fig. 3. a) Schematic illustration of the range of penetration; b) variation of implantation range Rp and standard deviation Rp with ion energy for 2 MeV Au ions implanted in silica; c) distributions of implanted ions and defects. All simulations were performed using SRIM 2008 (Ziegler, 1985, 2008).

#### **3. Controlled growth of noble metal nanoparticles through ion-implantation**

The noble metals like Au, Ag and Cu have been incorporated into dielectric matrices to produce several plasmonic nanocomposites with varied combinations of metals and dielectrics. The main objective of such works was to utilize the intense local field, strong linear optical absorption or SPR in applications such as optical sensing, biological and chemical sensing, and fabrication of all-optical devices. The electronic transitions responsible for plasmon absorption in metal NPs are also responsible for the generation of optical nonlinearities in the same spectral range, i.e. at the SPR position. To exploit the advantage of this SPR absorption in nanocomposites, it is desirable to have a high concentration of metal NPs (i.e., high filling factor) in the matrix (Flytzanis *et al.*, 1991). Therefore, as has been mentioned earlier, apart from the adequate energy of implantation, a high dose of implanted ions is required. By the terms "high metal ion dose" 0, we mean a dose exceeding the solubility limit of the metal atoms in the target (dielectric in the present case), which generally vary in between 1015 - 1017 cm-2, depending on the nature of ion species and target materials. The characteristics of the incorporated metal NPs also depend

Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review 335

At a certain temperature, when the solute concentrations are above their solubility limits in a matrix, there occurs spontaneous condensation of solute. Within a homogeneous matrix and in conditions which may rule out coalescence, this condensation usually leads to the formation of spherical clusters. The theory of growth of spherical clusters has been developed and used by many authors (Lifshitz & Slezov, 1959; Aaron *et al.*, 1970). According to this theory, at relatively low temperatures (in comparison with the melting temperature of bulk materials), formed clusters predominantly go through a stage of nucleation and growth. The exact analytical solution of the growth process of a spherical cluster is very difficult to obtain. For this reason, various approaches have been used to represent the real solution to a greater or lesser extent. In particular, we want to solve the field equation:

2 *C D C*

where *D* is the volumetric coefficient of diffusion (assumed to be independent of position) and *C = C(r,t)* is the field of concentration in the matrix around the precipitate. Also, the

*dR C CC D*

*CP* is the concentration of the precipitate (constant), *CI* is the solute concentration in the matrix, at the precipitate-matrix interface and *R* is the value of *r* on that interface. For the condition (*CP* – *CM*)>>(*CI* – *CM*), the *R(t)* is a slowly varying function of time and a way to simplify the field equation (4) is by using the invariant-field approximation in time (Laplace approximation), which is *C t* 0 , thus being the Laplace equation 2 *C* 0 . Now, for the

*p I*

, where 1

Note that *k* is positive during the dissolution and negative during the growth.

**3.2 Fabrication of nanocomposites containing elemental noble metal clusters**

where *CM* is the concentration of solute in the matrix far away from the precipitate. The problem is almost identical for the dissolution of clusters. In this case *R*(*t* = 0) = *R0* > 0, we

2 2

Controlling the plasmonic behavior of noble metal clusters (nanoparticles), i.e. their SPR position and intensity in nanocomposites is of immense importance for their practical applications. Manipulation of the implantation variables (energy and dose) coupled with a post-implantation annealing allows the fabrication of metal nanoclusters with the desired SPR characteristics. Since the first report on ion implantation fabrication of such nanocomposites

steady state of the cluster, we can solve the equation (4) to obtain:

 1 2 *R Dt*

obtain the expression:

independent equation of flow balance must be satisfied:

*t*

*dt r*

*r R*

2 , 2 *I M*

*C C k k*

*P I*

0 *R R kDt*. (7)

, (6)

*C C*

(4)

. (5)

**3.1 Formation of nanocomposites**

on the utilized ion dose. While there is a limiting value of implanted dose for each combination of metal ions and target material to obtain well dispersed metal NPs with controlled nucleation and growth (i.e. with definite geometrical shape and size), doses higher than this limiting value cause aggregation or coalescence of the formed NPs, strongly affecting their optical properties (Figure 4). For example, for 25 keV Ag+ ion implantation into LiNbO3, the limiting dose was found to be 0 ~5.0x1015 cm-2 (Deying *et al.*, 1994), and ~1016 cm-2 for 30 keV Ag+ ions in solid epoxy resin (Stepanov *et al.*, 1995). In fact, the implantation range and concentration distribution of the metal NPs strongly depend on the implantation dose (Figure 2b).

Fabrication of plasmonic nanocomposites by ion implantation dates back to 1973, when Davenas and co-workers (Davenas *et al.*, 1973) pioneered this method to fabricate metal NPs of silver, calcium, etc. inside crystalline LiF and MgO matrices. Later, the method was extended for the fabrication of other metal NPs in several other matrices like sapphire (Al2O3), silica (SiO2) and even polymers. For practical applications, plasmonic nanocomposites containing metal NPs of desired sizes, high filling fraction, and controlled composition are the basic requirements. While the ion implantation technique can introduce virtually any element in any matrix without solubility restrictions, the typical cluster size distributions in ion implanted nanocomposites are not narrow. On the other hand, progress has been made to fabricate metal-dielectric nanocomposites with controlled composition of metal NPs like elemental metallic clusters, bimetallic alloy clusters and core-shell type bimetallic nanoclusters. Depending on the nature of the metallic element and the matrix, adequate adjustments of implantation conditions (e.g. implantation energy, dose) and post implantation treatments (thermal annealing, laser annealing, etc.) in adequate atmosphere (e.g. oxidizing atmosphere like air or oxygen, inert atmosphere like argon and reducing atmosphere like hydrogen or a mixture of hydrogen and argon) are necessary to fabricate each type of the above mentioned architectures. In the following sections we will discuss on the fabrication of each of these nanoarchitectures, their plasmonic behaviors, and application potentials.

Fig. 4. Basic physical processes (left to right) involved in the formation of metal nanoparticles from an implant at different ion doses.

#### **3.1 Formation of nanocomposites**

334 Ion Implantation

on the utilized ion dose. While there is a limiting value of implanted dose for each combination of metal ions and target material to obtain well dispersed metal NPs with controlled nucleation and growth (i.e. with definite geometrical shape and size), doses higher than this limiting value cause aggregation or coalescence of the formed NPs, strongly affecting their optical properties (Figure 4). For example, for 25 keV Ag+ ion implantation into LiNbO3, the limiting dose was found to be 0 ~5.0x1015 cm-2 (Deying *et al.*, 1994), and ~1016 cm-2 for 30 keV Ag+ ions in solid epoxy resin (Stepanov *et al.*, 1995). In fact, the implantation range and concentration distribution of the metal NPs strongly depend on the

Fabrication of plasmonic nanocomposites by ion implantation dates back to 1973, when Davenas and co-workers (Davenas *et al.*, 1973) pioneered this method to fabricate metal NPs of silver, calcium, etc. inside crystalline LiF and MgO matrices. Later, the method was extended for the fabrication of other metal NPs in several other matrices like sapphire (Al2O3), silica (SiO2) and even polymers. For practical applications, plasmonic nanocomposites containing metal NPs of desired sizes, high filling fraction, and controlled composition are the basic requirements. While the ion implantation technique can introduce virtually any element in any matrix without solubility restrictions, the typical cluster size distributions in ion implanted nanocomposites are not narrow. On the other hand, progress has been made to fabricate metal-dielectric nanocomposites with controlled composition of metal NPs like elemental metallic clusters, bimetallic alloy clusters and core-shell type bimetallic nanoclusters. Depending on the nature of the metallic element and the matrix, adequate adjustments of implantation conditions (e.g. implantation energy, dose) and post implantation treatments (thermal annealing, laser annealing, etc.) in adequate atmosphere (e.g. oxidizing atmosphere like air or oxygen, inert atmosphere like argon and reducing atmosphere like hydrogen or a mixture of hydrogen and argon) are necessary to fabricate each type of the above mentioned architectures. In the following sections we will discuss on the fabrication of each of these nanoarchitectures, their plasmonic behaviors, and

Fig. 4. Basic physical processes (left to right) involved in the formation of metal

nanoparticles from an implant at different ion doses.

implantation dose (Figure 2b).

application potentials.

At a certain temperature, when the solute concentrations are above their solubility limits in a matrix, there occurs spontaneous condensation of solute. Within a homogeneous matrix and in conditions which may rule out coalescence, this condensation usually leads to the formation of spherical clusters. The theory of growth of spherical clusters has been developed and used by many authors (Lifshitz & Slezov, 1959; Aaron *et al.*, 1970). According to this theory, at relatively low temperatures (in comparison with the melting temperature of bulk materials), formed clusters predominantly go through a stage of nucleation and growth. The exact analytical solution of the growth process of a spherical cluster is very difficult to obtain. For this reason, various approaches have been used to represent the real solution to a greater or lesser extent. In particular, we want to solve the field equation:

$$D\nabla^2 \mathbf{C} = \frac{\partial \mathbf{C}}{\partial t} \tag{4}$$

where *D* is the volumetric coefficient of diffusion (assumed to be independent of position) and *C = C(r,t)* is the field of concentration in the matrix around the precipitate. Also, the independent equation of flow balance must be satisfied:

$$D\left(\mathbf{C}\_p - \mathbf{C}\_I\right) \frac{dR}{dt} = D \left. \frac{\partial \mathbf{C}}{\partial r} \right|\_{r=R}. \tag{5}$$

*CP* is the concentration of the precipitate (constant), *CI* is the solute concentration in the matrix, at the precipitate-matrix interface and *R* is the value of *r* on that interface. For the condition (*CP* – *CM*)>>(*CI* – *CM*), the *R(t)* is a slowly varying function of time and a way to simplify the field equation (4) is by using the invariant-field approximation in time (Laplace approximation), which is *C t* 0 , thus being the Laplace equation 2 *C* 0 . Now, for the steady state of the cluster, we can solve the equation (4) to obtain:

$$R = \mathcal{A} \left( D t \right)^{1/2} \text{, where } \mathcal{A} = \left( -k \right)^{\frac{1}{2}} \text{, } k = 2 \frac{\mathcal{C}\_I - \mathcal{C}\_M}{\mathcal{C}\_P - \mathcal{C}\_I} \text{,} \tag{6}$$

where *CM* is the concentration of solute in the matrix far away from the precipitate. The problem is almost identical for the dissolution of clusters. In this case *R*(*t* = 0) = *R0* > 0, we obtain the expression:

$$R^2 = R\_0^2 - kDt.\tag{7}$$

Note that *k* is positive during the dissolution and negative during the growth.

#### **3.2 Fabrication of nanocomposites containing elemental noble metal clusters**

Controlling the plasmonic behavior of noble metal clusters (nanoparticles), i.e. their SPR position and intensity in nanocomposites is of immense importance for their practical applications. Manipulation of the implantation variables (energy and dose) coupled with a post-implantation annealing allows the fabrication of metal nanoclusters with the desired SPR characteristics. Since the first report on ion implantation fabrication of such nanocomposites

Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review 337

**Cu** SiO2 2000 (1.0-2.2)x1017 in air, 600-1050 0C, 1h (Pal *et al.*, 2000) **Cu** Al2O3 110 1.0x1017 (Donnet *et al.*, 1991) **Cu** Al2O3 160 (0.2-1.0)x1017 in air, 673 0C, 1h (Ila *et al.*, 1997) **Cu** Al2O3 40 1.0x1017 (Stepanov *et al.*, 2001)

**Cu** TiO2 20 5.0x1015 - (Wang *et al.*, 2006) **Cu** TiO2 60 1.0x1017 - (Takeda *et al.*, 2005) **Cu** TiO2 50 (0.3-1.0)x1017 in air, 400 0C, XX (Tsuji *et al.*, 2002) **Cu** ZnO 160 (0.1&1.0)x1017 - (Kono *et al.*, 2003;

Table 2. List of noble metal ions used to incorporate in common oxide matrices with

by Davenas' group in 1973 (Davenas *et al.*, 1973), a vast effort has been devoted to their controlled fabrication using different combinations of metal particles and matrices. As can be seen from table 2, Au, Ag and Cu NPs have been incorporated into several dielectric matrices with different implantation conditions and the conditions of subsequent annealing.

Fig. 5. Cross-sectional TEM micrographs showing colloidal Ag precipitates formed in fused silica by the implantation of Ag (1.8 MeV, 6x1016 cm-2) ions at room temperature. Adapted

Implantation of these metal ions of moderate energy (typically up to 200 keV) and low doses (1016 cm-2) generally produces metallic nanoclusters of homogeneous sizes, while a high dose and a high temperature post implantation treatment produce nanoclusters of bigger sizes with higher dispersion. However, both the size and the size dispersion of the formed nanoclusters depend on the nature of implanted metal, their solubility and diffusion behavior in the embedding matrix. While Ag nanoclusters of relatively uniform sizes and narrow size distributions can be grown in silica matrix even for a high ion dose (Figure 5)

with permission from Ann. Rev. Mater. Sci. 24 (1994) 125-158 (Buchal *et al.*, 1994).

**Post-implantation**

**References**

(Ikeyama *et al.*, 2001)

Norton *et al.*, 2003; Stepanov *et al.*, 2004)

**annealing**

0C, 1h

**Dose (cm-2)**

**Cu** Al2O3 2100 1.0x1017 in vacuum, 497-997

different implantation conditions and post implantation treatments.

**Metal Matrix Ion energy**

**(keV)**

**Post-implantation**

**References**

2007)

1988)

*al.*, 2005)

1993)

*al.*, 2006)

(Oliver *et al.*, 2002)

(Pan *et al.*, 2007)

(Xu *et al.*, 2006)

Xiao *et al.*, 2007)

(Oliver *et al.*, 2002)

in air, 400 0C (Tsuji *et al.*, 2002)

(3.0-7.0)x1016 in air, 500 0C, 1h (Joseph *et al.*, 2007;

1100 0C, 1h

1100 0C, 1h

per step with an interval of 100 0C

0C, 1h

**Cu** SiO2 60 1.0x1017 - (Bandourko *et al.*, 2005)

(Mohapatra *et al.*,

(Oliver *et al.*, 2002)

**annealing**

1100 0C, 1h

0C

**Au** SiO2 60 2.0x1017 - (Takeda *et al.*, 2006)

**Au** Al2O3 400 6.8x1016 - (Ohkubo & Suzuki,

**Au** Al2O3 160 1.0x1017 - (Marques *et al.*, 2004) **Au** Al2O3 2000 (0.2-2.0)x1017 in air, 978-1323 0C, 1h (Ila *et al.*, 1997) **Au** Al2O3 1800 (0.1-1.0)x1018 1000 0C & 1200 0C, 1h (Dhara *et al.*, 2004) **Au** Al2O3 60 2.0x1017 - (Takeda *et al.*, 2006) **Au** TiO2 260 5.0x1016 - (Zhu *et al.*, 2006) **Au** TiO2 1800 8.5x1016 480 0C (Nakao *et al.*, 2000) **Au** c-TiO2 2000 6.0x1016 1002 0C (Wang, 2004; Wang *et*

**Ag** SiO2 10 5.0x1015 in Ar, 500-800 0C (Arai *et al.*, 2006)

**Ag** Al2O3 25-30 (0.2-2.0)x1017 (Steiner *et al.*, 1998) **Ag** Al2O3 1500 (0.2-2.0)x1017 in air, 773 0C, 1h (Ila *et al.*, 1997) **Ag** Al2O3 1800 1.2x1017 in air, 1100 0C, 2h (White *et al.*, 1992,

**Ag** ZnO 30, 60 (0.3-1.0)x1017 - (Zhou *et al.*, 2011) **Ag** ZnO 20, 40 (3.0-7.0)x1016 - (Xiao *et al.*, 2010) **Ag** ZnO 40 1.0x1016 - (Mendoza-Galván *et*

**Dose (cm-2)**

**Au** SiO2 3000 6.0x1015 in air, 550-750, 1050

**Au** SiO2 2000 (0.7-6.0)x1016 in air & (N2+H2), 300-

4.0x1016

1.0x1017

1.0x1017

**Cu** SiO2 2000 (0.7-6.0)x1016 in air & (N2+H2), 300-

**Cu** SiO2 2000 6.0x1016 in vacuum, 600-800

**Cu** SiO2 60 (0.5-2.0)x1017 200 to 700 0C for 1 h

**Ag** SiO2 2000 (0.7-6.0)x1016 in air & (N2+H2), 300-

**Metal Matrix Ion energy**

**(keV)**

**Au** c-ZnO 300 8.0x1013-

2400

**Ag** SiO2 200 1.0x1015-

**Ag** TiO2 65 3.0x1016-

**Ag** SiO2 35, 1700,

Table 2. List of noble metal ions used to incorporate in common oxide matrices with different implantation conditions and post implantation treatments.

by Davenas' group in 1973 (Davenas *et al.*, 1973), a vast effort has been devoted to their controlled fabrication using different combinations of metal particles and matrices. As can be seen from table 2, Au, Ag and Cu NPs have been incorporated into several dielectric matrices with different implantation conditions and the conditions of subsequent annealing.

Fig. 5. Cross-sectional TEM micrographs showing colloidal Ag precipitates formed in fused silica by the implantation of Ag (1.8 MeV, 6x1016 cm-2) ions at room temperature. Adapted with permission from Ann. Rev. Mater. Sci. 24 (1994) 125-158 (Buchal *et al.*, 1994).

Implantation of these metal ions of moderate energy (typically up to 200 keV) and low doses (1016 cm-2) generally produces metallic nanoclusters of homogeneous sizes, while a high dose and a high temperature post implantation treatment produce nanoclusters of bigger sizes with higher dispersion. However, both the size and the size dispersion of the formed nanoclusters depend on the nature of implanted metal, their solubility and diffusion behavior in the embedding matrix. While Ag nanoclusters of relatively uniform sizes and narrow size distributions can be grown in silica matrix even for a high ion dose (Figure 5)

Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review 339

While a very low ion dose produces metal nanoclusters of extremely small sizes (typically below 1.0 nm) that exhibit no SPR band in absorption spectra, on increasing the ion dose, the clusters grow and the intensity of the SPR extinction band increases (Figure 7). On the other hand, for a high energy of the implanted ions, the size of the formed metallic clusters can be big enough to reveal an SPR peak in the absorption spectra of the composites (Figure 8B). While a high energy implantation can place the metallic clusters at micrometer depth below the irradiated surface, a higher implantation dose can extend the cluster distribution towards the target surface (Figure 8A). Such a broadening of cluster distribution towards lower depths is due to the interaction of the incident ions with the already formed metallic clusters, which effectively reduces the projected range of the metal ions that impinge at a

Fig. 8. a) The depth profiles of Cu concentration in silica matrix prepared with 2.0 MeV Cu ions of 9.95x1016, 1.25x1017, and 2.20x1017 ions cm-2 doses without post implantation

**9.95x1016 ions cm-2**

**1.25x1017 ions cm-2**

**2.20x1017 ions cm-2**

treatment, estimated through RBS analysis. The continuous lines show the TRIM simulated results. Optical absorption spectra of the Cu implanted silica before (b) and after (c) thermal annealing at 1050 0C. Adapted with permission from J. Non-Cryst. Sol. 275 (2000) 65-71 (Pal

Overall, incorporation of monometallic NPs in dielectric matrix through ion implantation has had a good success so far. At the present time, researchers can fabricate specific noble metal clusters of desired sizes and size distributions at precise depths inside most common dielectric matrices by controlling the implantation dose, energy and the conditions of post implantation treatment. However, from the application point of view the success has been limited. Though the SPR position in a nanocomposite could be

later stage.

*et al.*, 2000).

without post-implantation thermal treatment, the same is not true for the Au and Cu nanoclusters in silica matrix (Figure 6). As can be seen from Figure 6, Au nanoparticles formed with similar implantation conditions and thermal treatments have a broad size distribution.

Fig. 6. Cross-sectional bright field TEM images of Au implanted (190 keV, 3x1016 cm-2) silica: a) before and b) after 3h of thermal annealing at 900 0C, in air. Reproduced with permission from Nucl. Instrum. Meth. B 191 (2002) 323-332 (Mattei, 2002).

Fig. 7. Optical absorption spectra of 1.8 MeV Ag implanted fused silica at room temperature prepared with different doses. Reproduced with permission from Ann. Rev. Mater. Sci. 24 (1994) 125-158 (Buchal *et al.*, 1994).

without post-implantation thermal treatment, the same is not true for the Au and Cu nanoclusters in silica matrix (Figure 6). As can be seen from Figure 6, Au nanoparticles formed with similar implantation conditions and thermal treatments have a broad size

Fig. 6. Cross-sectional bright field TEM images of Au implanted (190 keV, 3x1016 cm-2) silica: a) before and b) after 3h of thermal annealing at 900 0C, in air. Reproduced with permission

Fig. 7. Optical absorption spectra of 1.8 MeV Ag implanted fused silica at room temperature prepared with different doses. Reproduced with permission from Ann. Rev. Mater. Sci. 24

from Nucl. Instrum. Meth. B 191 (2002) 323-332 (Mattei, 2002).

(1994) 125-158 (Buchal *et al.*, 1994).

distribution.

While a very low ion dose produces metal nanoclusters of extremely small sizes (typically below 1.0 nm) that exhibit no SPR band in absorption spectra, on increasing the ion dose, the clusters grow and the intensity of the SPR extinction band increases (Figure 7). On the other hand, for a high energy of the implanted ions, the size of the formed metallic clusters can be big enough to reveal an SPR peak in the absorption spectra of the composites (Figure 8B). While a high energy implantation can place the metallic clusters at micrometer depth below the irradiated surface, a higher implantation dose can extend the cluster distribution towards the target surface (Figure 8A). Such a broadening of cluster distribution towards lower depths is due to the interaction of the incident ions with the already formed metallic clusters, which effectively reduces the projected range of the metal ions that impinge at a later stage.

Fig. 8. a) The depth profiles of Cu concentration in silica matrix prepared with 2.0 MeV Cu ions of 9.95x1016, 1.25x1017, and 2.20x1017 ions cm-2 doses without post implantation treatment, estimated through RBS analysis. The continuous lines show the TRIM simulated results. Optical absorption spectra of the Cu implanted silica before (b) and after (c) thermal annealing at 1050 0C. Adapted with permission from J. Non-Cryst. Sol. 275 (2000) 65-71 (Pal *et al.*, 2000).

Overall, incorporation of monometallic NPs in dielectric matrix through ion implantation has had a good success so far. At the present time, researchers can fabricate specific noble metal clusters of desired sizes and size distributions at precise depths inside most common dielectric matrices by controlling the implantation dose, energy and the conditions of post implantation treatment. However, from the application point of view the success has been limited. Though the SPR position in a nanocomposite could be

Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review 341

Fig. 9. Comparison between Mie-theory based calculated extinction spectra of 3 nm clusters of Au, Ag, and Au0.4Ag0.6 alloy in silica (9a) with the experimental absorption spectra of similar clusters prepared by ion implantation (9b). The notations "A" and "H" at the end of sample names indicate the samples were annealed in air and hydrogen atmosphere, respectively. Reproduced with permission from Nucl. Instrum. Meth. B 191 (2002) 323-

Using sequential implantation technique, Mazzoldi and collaborators were able to fabricate alloy nanoclusters of different compositions in fused silica glass at room temperature using 190 keV gold and 90 keV Cu ions (Gonella *et al.*, 1999). The alloy clusters of compositions close to their nominal values were formed in the silica matrix even without any postimplantation thermal treatment (Figure 10). However, Cu migrates towards the particle surface during thermal treatment and a structural reordering of the alloy occurs. The effect was more prominent when the implanted samples were annealed in air, and Cu migrated to the surface gets oxidized, causing a phase separation (Au core and Cu2O shell). A complete separation of Cu from the alloy cluster and formation of CuO shell around the Au core could be obtained after 5 h of thermal annealing in air (Battaglin *et al.*, 2001b). The process could be reversed by annealing the sample again in a reducing atmosphere. It must be noted

332 (Mattei, 2002).

modified by varying the nature of implanted metal ions (e.g. the typical SPR position of Au nanoclusters in SiO2 is at about 540 nm, of Ag at about 420 nm, and of Cu at about 560 nm) and its intensity could be raised by increasing the implantation dose, it is not possible to tune the position of the SPR peak for a wider spectral range using monometallic nanoclusters. The fabricated monometallic plasmonic nanocomposites are suitable for the fabrication of nonlinear optical switches, effective waveguides, optical sensors, and devices covering a limited spectral range, but the limitation of spectral tunability of SPR peak position makes them unsuitable for most biological applications, such as medical diagnosis, immunoassay, and therapeutic treatments.

Therefore, researchers felt the need of fabricating plasmonic nanocomposites containing bimetallic nanoclusters to vary the SPR position of the composites in between the SPR positions of individual monometallic metal clusters, which we will discuss in the following section.

### **3.3 Fabrication of nanocomposites containing alloy- and core-shell type bimetallic clusters**

Sequential ion implantation, where the ions of a particular metal are first implanted at a certain depth of the matrix and then the second type of metal ions are implanted at similar depths, has been utilized frequently to fabricate bimetallic nanocomposites. Depending on the conditions of implantation, the nature of metallic species and dielectric matrix, the process can lead to the formation of i) separated monometallic nanoclusters of individual metals, ii) alloy clusters, and iii) core-shell type bimetallic clusters. Formation of each of these clusters depends on their thermodynamic stability, and hence on the process of post-implantation thermal treatments along with the annealing atmosphere. Although the composition of alloy type clusters can be varied easily by sequential ion implantation of two elements in a matrix by adjusting their energy and doses to have maximum overlap between their implant-profiles and desired local relative concentrations, post implantation thermal treatments can frequently lead to (promote) separation of a component specie from the other or oxidation, instead of alloying. Formation of alloy type clusters also depends on the miscibility of the component elements, their chemical interaction with the matrix, and the creation of radiation induced defects, which influences the nucleation and growth of the nanoclusters.

As the criterion of bulk miscibility for two elements is not very stringent in the case of nanocrystals due to their high specific surface area (surface area/volume ratio) and characteristics closer to that of a molecule than to those of a massive (bulk) system (Yasuda & Mori, 1994), alloy type nanoclusters of several metal pairs like Au-Cu, Au-Ag, Pd-Ag, and Pd-Cu, have been successfully fabricated (Gonella *et al.*, 1999; Battaglin *et al.*, 2000, 2001a) through sequential ion implantation. The SPR position in such noble metal alloy clusters remains in between those of pure elements, depending on the complex interplay between the modified free electrons and interband absorptions. As can be seen from Figure 9, the measured absorption spectra of pure Au, Ag and Au0.4Ag0.6 alloy clusters of about 3 nm size in silica matrix reported by Mattei and co-workers (Mattei, 2002) agree well with the simulated extinction spectra of the NPs of similar compositions reported by Kreibig & Vollmer (Kreibig & Vollmer, 1995).

modified by varying the nature of implanted metal ions (e.g. the typical SPR position of Au nanoclusters in SiO2 is at about 540 nm, of Ag at about 420 nm, and of Cu at about 560 nm) and its intensity could be raised by increasing the implantation dose, it is not possible to tune the position of the SPR peak for a wider spectral range using monometallic nanoclusters. The fabricated monometallic plasmonic nanocomposites are suitable for the fabrication of nonlinear optical switches, effective waveguides, optical sensors, and devices covering a limited spectral range, but the limitation of spectral tunability of SPR peak position makes them unsuitable for most biological applications, such as medical

Therefore, researchers felt the need of fabricating plasmonic nanocomposites containing bimetallic nanoclusters to vary the SPR position of the composites in between the SPR positions of individual monometallic metal clusters, which we will discuss in the

**3.3 Fabrication of nanocomposites containing alloy- and core-shell type bimetallic**

Sequential ion implantation, where the ions of a particular metal are first implanted at a certain depth of the matrix and then the second type of metal ions are implanted at similar depths, has been utilized frequently to fabricate bimetallic nanocomposites. Depending on the conditions of implantation, the nature of metallic species and dielectric matrix, the process can lead to the formation of i) separated monometallic nanoclusters of individual metals, ii) alloy clusters, and iii) core-shell type bimetallic clusters. Formation of each of these clusters depends on their thermodynamic stability, and hence on the process of post-implantation thermal treatments along with the annealing atmosphere. Although the composition of alloy type clusters can be varied easily by sequential ion implantation of two elements in a matrix by adjusting their energy and doses to have maximum overlap between their implant-profiles and desired local relative concentrations, post implantation thermal treatments can frequently lead to (promote) separation of a component specie from the other or oxidation, instead of alloying. Formation of alloy type clusters also depends on the miscibility of the component elements, their chemical interaction with the matrix, and the creation of radiation induced defects, which influences the nucleation and

As the criterion of bulk miscibility for two elements is not very stringent in the case of nanocrystals due to their high specific surface area (surface area/volume ratio) and characteristics closer to that of a molecule than to those of a massive (bulk) system (Yasuda & Mori, 1994), alloy type nanoclusters of several metal pairs like Au-Cu, Au-Ag, Pd-Ag, and Pd-Cu, have been successfully fabricated (Gonella *et al.*, 1999; Battaglin *et al.*, 2000, 2001a) through sequential ion implantation. The SPR position in such noble metal alloy clusters remains in between those of pure elements, depending on the complex interplay between the modified free electrons and interband absorptions. As can be seen from Figure 9, the measured absorption spectra of pure Au, Ag and Au0.4Ag0.6 alloy clusters of about 3 nm size in silica matrix reported by Mattei and co-workers (Mattei, 2002) agree well with the simulated extinction spectra of the NPs of similar compositions reported by Kreibig &

diagnosis, immunoassay, and therapeutic treatments.

following section.

growth of the nanoclusters.

Vollmer (Kreibig & Vollmer, 1995).

**clusters**

Fig. 9. Comparison between Mie-theory based calculated extinction spectra of 3 nm clusters of Au, Ag, and Au0.4Ag0.6 alloy in silica (9a) with the experimental absorption spectra of similar clusters prepared by ion implantation (9b). The notations "A" and "H" at the end of sample names indicate the samples were annealed in air and hydrogen atmosphere, respectively. Reproduced with permission from Nucl. Instrum. Meth. B 191 (2002) 323- 332 (Mattei, 2002).

Using sequential implantation technique, Mazzoldi and collaborators were able to fabricate alloy nanoclusters of different compositions in fused silica glass at room temperature using 190 keV gold and 90 keV Cu ions (Gonella *et al.*, 1999). The alloy clusters of compositions close to their nominal values were formed in the silica matrix even without any postimplantation thermal treatment (Figure 10). However, Cu migrates towards the particle surface during thermal treatment and a structural reordering of the alloy occurs. The effect was more prominent when the implanted samples were annealed in air, and Cu migrated to the surface gets oxidized, causing a phase separation (Au core and Cu2O shell). A complete separation of Cu from the alloy cluster and formation of CuO shell around the Au core could be obtained after 5 h of thermal annealing in air (Battaglin *et al.*, 2001b). The process could be reversed by annealing the sample again in a reducing atmosphere. It must be noted

Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review 343

segregation of the component metals controlled by the minimization of surface free energy, resulting a phase separation as has been demonstrated theoretically for Co-Au (Johannessen *et al.*, 2005), Au-Pt (Esparza *et al.*, 2005), Au-Pd (Liu *et al.*, 2005, 2006), Au-Cu (Ascencio *et al.*,

Fig. 11. Cross-sectional TEM images and size distribution of Co-Au nanoclusters as well as monometallic nanoclusters formed by sequential implantation. The high resolution TEM image of the core-shell cluster reveals a significant bending of the lattice fringes due to interfacial strain (lattice mismatch). Reproduced with permission from Appl. Phys. Lett. 89

Formation of Au-Ag core-shell nanoclusters in silica matrix through sequential implantation and subsequent thermal annealing (in air) has been demonstrated by Peña and

2006) and many other systems.

(2006) 153118 (Kluth *et al.*, 2006).

that for the thermal treatment of metal ion implanted composites we frequently use the value of the bulk melting point temperature, even though this quantity is substantially lower for a metallic cluster (either elemental or bimetallic), due to thermodynamic size effect (Buffat & Borel, 1976). Thermal annealing of a sequentially implanted sample helps to migrate one component elemental species towards the small clusters of the other element through diffusion, enhancing their size and improving their stoichiometry. In fact, the probability of formation of homogeneous alloy clusters in sequential implantation process strongly depends on the solubility of the second elemental specie in the nanoclusters of the first element. To increase the solubility and helping the structural rearrangement of metal ions, sometimes it is useful to perform the implantation of the second element at elevated target (substrate) temperatures.

Fig. 10. (a) Optical absorption spectra of Au3Cu3 (as-implanted), Au3Cu3H, and Au3Cu3A (annealed at 900 0C for 1h in H2-Ar and air, respectively) samples. The bright-field TEM planer views of the corresponding samples are presented as (b), (c), and (d), respectively, at right. Reproduced with permission from Appl. Phys. Lett. 75 (1999) 55-57 (Gonella *et al.*, 1999).

Utilizing the above mentioned strategy, Kluth and coworkers (Kluth *et al.*, 2006) could fabricate core-shell type Co-Au nanoclusters of about 5.3 nm diameter in SiO2 matrix. Utilization of elevated target temperature (400 0C in the present case) during second metal (Co in this work) implantation helped to avoid the post-implanted thermal annealing process for the formation of bimetallic clusters. However, for the formation of Au clusters of uniform size before Co implantation, they used a thermal annealing treatment of the Au implanted sample at 1000 0C for one hour, in air. We can see the formation of uniform nanoparticles with Co core and Au shell (Figure 11) in their samples. As has been mentioned earlier, the formation of core-shell type bimetallic clusters depends strongly on the

that for the thermal treatment of metal ion implanted composites we frequently use the value of the bulk melting point temperature, even though this quantity is substantially lower for a metallic cluster (either elemental or bimetallic), due to thermodynamic size effect (Buffat & Borel, 1976). Thermal annealing of a sequentially implanted sample helps to migrate one component elemental species towards the small clusters of the other element through diffusion, enhancing their size and improving their stoichiometry. In fact, the probability of formation of homogeneous alloy clusters in sequential implantation process strongly depends on the solubility of the second elemental specie in the nanoclusters of the first element. To increase the solubility and helping the structural rearrangement of metal ions, sometimes it is useful to perform the implantation of the second element at elevated

Fig. 10. (a) Optical absorption spectra of Au3Cu3 (as-implanted), Au3Cu3H, and Au3Cu3A (annealed at 900 0C for 1h in H2-Ar and air, respectively) samples. The bright-field TEM planer views of the corresponding samples are presented as (b), (c), and (d), respectively, at right. Reproduced with permission from Appl. Phys. Lett. 75 (1999) 55-57 (Gonella *et al.*, 1999).

Utilizing the above mentioned strategy, Kluth and coworkers (Kluth *et al.*, 2006) could fabricate core-shell type Co-Au nanoclusters of about 5.3 nm diameter in SiO2 matrix. Utilization of elevated target temperature (400 0C in the present case) during second metal (Co in this work) implantation helped to avoid the post-implanted thermal annealing process for the formation of bimetallic clusters. However, for the formation of Au clusters of uniform size before Co implantation, they used a thermal annealing treatment of the Au implanted sample at 1000 0C for one hour, in air. We can see the formation of uniform nanoparticles with Co core and Au shell (Figure 11) in their samples. As has been mentioned earlier, the formation of core-shell type bimetallic clusters depends strongly on the

target (substrate) temperatures.

segregation of the component metals controlled by the minimization of surface free energy, resulting a phase separation as has been demonstrated theoretically for Co-Au (Johannessen *et al.*, 2005), Au-Pt (Esparza *et al.*, 2005), Au-Pd (Liu *et al.*, 2005, 2006), Au-Cu (Ascencio *et al.*, 2006) and many other systems.

Fig. 11. Cross-sectional TEM images and size distribution of Co-Au nanoclusters as well as monometallic nanoclusters formed by sequential implantation. The high resolution TEM image of the core-shell cluster reveals a significant bending of the lattice fringes due to interfacial strain (lattice mismatch). Reproduced with permission from Appl. Phys. Lett. 89 (2006) 153118 (Kluth *et al.*, 2006).

Formation of Au-Ag core-shell nanoclusters in silica matrix through sequential implantation and subsequent thermal annealing (in air) has been demonstrated by Peña and

Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review 345

Fig. 13. Experimental optical extinction spectra for the samples implanted with 5x1016 ions cm-2 of Ag at 1.8 MeV and coimplanted with different fluences of Au at 3.5 MeV, before and after thermal annealing in air (a), and simulated (Mie theory) extinction spectra for some possible configurations in silica matrix (b). Au-Ag represents Au core-Ag shell, whereas Ag-Au means the opposite. Reproduced with permission from J. Phys. Chem. C 113 (2009) 2296-

Indeed, the ion implantation process can produce both, alloy and core-shell type bimetallic nanoclusters in silica and other dielectric matrices by adequate manipulation of implantation

2300 (Peña *et al.*, 2009).

coworkers (Peña *et al.*, 2009). Considering the miscibility and segregation behavior of the two metals, they first implanted 1.8 MeV Ag+ ions at a 5.0x1016 ions cm-2 dose (fluence) to place most of them at about 1.0 m depth below the target surface (Figure 12). Then, the Au+ ions of higher energy (3.7 MeV) were implanted at a relative higher dose (6.7x1016 ion cm-2) to place them at greater depth (about 1.1 m) keeping a good portion of the implantation profiles overlapped. However, it has been observed that during the implantation of Au, preimplanted Ag segregates towards the sample surface (distribution maximum moves to 0.87 m depth). On thermal annealing at 900 0C for 1 h in air, the co-implanted samples produced core-shell type bimetallic nanoclusters with Au core and Ag shell. The formation of both the monometallic nanoclusters (embedded in silica matrix) before thermal treatment and bimetallic nanoclusters (after thermal treatment) could be verified through the experimental optical absorption spectra in comparison with the Mie theory based simulated spectra of similar structures (Figure 13).

Fig. 12. Depth profiles of Au and Ag distribution for the silica samples sequentially implanted with Ag (1.8 MeV) and Au (3.5 MeV) ions, obtained from the fitting of the corresponding RBS spectra. Reproduced with permission from J. Phys. Chem. C 113 (2009) 2296-2300 (Peña *et al.*, 2009).

Though it was not possible to differentiate the core and shell regions from their HRTEM images, due to very similar lattice parameters of Au and Ag (4.078 Å for Au, JCPDS 4-0784; 4.077 Å for Ag, JCPDS 87-0720), the formation of core-shell structures could be demonstrated using high angle annular dark field (DAADF) images of the nanoclusters (Figure 14).

coworkers (Peña *et al.*, 2009). Considering the miscibility and segregation behavior of the two metals, they first implanted 1.8 MeV Ag+ ions at a 5.0x1016 ions cm-2 dose (fluence) to place most of them at about 1.0 m depth below the target surface (Figure 12). Then, the Au+ ions of higher energy (3.7 MeV) were implanted at a relative higher dose (6.7x1016 ion cm-2) to place them at greater depth (about 1.1 m) keeping a good portion of the implantation profiles overlapped. However, it has been observed that during the implantation of Au, preimplanted Ag segregates towards the sample surface (distribution maximum moves to 0.87 m depth). On thermal annealing at 900 0C for 1 h in air, the co-implanted samples produced core-shell type bimetallic nanoclusters with Au core and Ag shell. The formation of both the monometallic nanoclusters (embedded in silica matrix) before thermal treatment and bimetallic nanoclusters (after thermal treatment) could be verified through the experimental optical absorption spectra in comparison with the Mie theory based simulated

Fig. 12. Depth profiles of Au and Ag distribution for the silica samples sequentially implanted with Ag (1.8 MeV) and Au (3.5 MeV) ions, obtained from the fitting of the corresponding RBS spectra. Reproduced with permission from J. Phys. Chem. C 113 (2009)

Though it was not possible to differentiate the core and shell regions from their HRTEM images, due to very similar lattice parameters of Au and Ag (4.078 Å for Au, JCPDS 4-0784; 4.077 Å for Ag, JCPDS 87-0720), the formation of core-shell structures could be demonstrated using high angle annular dark field (DAADF) images of the nanoclusters

spectra of similar structures (Figure 13).

2296-2300 (Peña *et al.*, 2009).

(Figure 14).

Fig. 13. Experimental optical extinction spectra for the samples implanted with 5x1016 ions cm-2 of Ag at 1.8 MeV and coimplanted with different fluences of Au at 3.5 MeV, before and after thermal annealing in air (a), and simulated (Mie theory) extinction spectra for some possible configurations in silica matrix (b). Au-Ag represents Au core-Ag shell, whereas Ag-Au means the opposite. Reproduced with permission from J. Phys. Chem. C 113 (2009) 2296- 2300 (Peña *et al.*, 2009).

Indeed, the ion implantation process can produce both, alloy and core-shell type bimetallic nanoclusters in silica and other dielectric matrices by adequate manipulation of implantation

Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review 347

Utilizing the above mentioned concept, ion implantation technique has been utilized for the fabrication of several unusual nanostructures either inside dielectric matrices or in other dielectric-metal combinations with distinct plasmonic behaviors. Though all of these nanostructures do not fall into the class of commonly known nanocomposites (where nanoparticles of one material are embedded/dispersed into another matrix material), as they are in combination with at least one material apart from the metal in question, we can call them as *composite nanostructures*. For example, Mattei and co-workers (Mattei *et al.*, 2003a, 2005) could disintegrate alloy clusters like Au-Cu and Au-Ag embedded in silica matrix by high energy Ne, He and Kr ions to form Au-rich satellite clusters around the

While the optical absorption spectrum of the co-implanted (190 keV Au and 90 keV Cu, 3.0x1016 ions cm-2) AuCu sample after annealing in inert atmosphere revealed SPR peak of AuCu alloy particles (550 nm, curve (a) of the Figure 15), after irradiation of 190 keV Ne ions (1.0x1017 ions cm-2) the SPR peak shifts to 575 nm, which correspond to the SPR band of the main particle along with the satellite ones (aggregate). Similar phenomenon occurred for the AuAg nanoclusters embedded in silica matrix. It has been observed that in both cases, mainly the Au segregates out of the main bimetallic cluster to form Au-rich satellite clusters of smaller sizes. The size of the satellite clusters depends on the atomic mass of the irradiating ions. Higher atomic mass of the irradiating ions produces satellite clusters of bigger sizes (Figure 16). However, the phenomenon of disintegration of the preformed noble metal clusters in dielectric matrix occurs when the atomic mass of the irradiating ions is lower than the atomic mass of the embedded metal nanoclusters. The process was tentatively related to the nuclear component of the energy released to the system during

Fig. 16. Bright-field TEM cross-sectional micrographs of the sample AuAgA (annealed in air atmosphere at 800 0C, 1h) before (a) and after irradiation at room temperature with 380 keV Kr ions at a dose of 2.0x1016 ions cm-2 (b) (Mattei et al., 2003a). Adapted with permission

On the other hand, irradiation by high energy ions of higher atomic mass (with respect to the implanted metals in a dielectric matrix) generates very different phenomena. Generally,

from J. Non-Cryst. Sol. 322 (2003) 17-21 (Mattei *et al.*, 2003a).

initial bimetallic clusters (Figures 15 & 16).

irradiation with different ions.

parameters and proper post-implantation treatments. However, apart from producing monometallic and bimetallic nanoclusters of common types, ion implantation can produce several special metallic structures and aggregates, which we will discuss in the next section.

Fig. 14. Typical HAADF micrograph of the sample co-implanted with 5×1016 ions cm-2 of Ag (1.8 MeV) and 5×1016 ions cm-2 of Au (3.5 MeV) after thermal annealing at 900 °C (a) along with the intensity profiles for the selected particle (b) and (c). Typical HRTEM micrograph of a core-shell structure of about 12 nm diameter and the fast Fourier transforms (FFTs) of core (e) and shell (f) regions. Adapted with permission from J. Phys. Chem. C 113 (2009) 2296-2300 (Peña *et al.*, 2009).

## **3.4 Fabrication of nanocomposites containing unusual metallic nanostructures**

The anisotropic plastic deformation process of amorphous materials under MeV ion irradiation has been reported by Klaumünzer and Schumacher as early as 1983 (Klaumünzer & Schumacher, 1983). The process can be understood by means of a visco-elastic model, in which the thermal expansion of the cylindrically-shaped high temperature ion track region causes shear stresses that relax by viscous flow, with the viscous strains subsequently frozen in upon rapid cooling of the ion track region (Trinkaus & Ryazanov, 1995; van Dillen *et al.*, 2005). The deformation is induced by the electron energy loss, and thus occurs mostly with ions in the MeV energy range.

Fig. 15. Bright-field TEM cross-sectional micrographs of the AuCuH (annealed in H2 (4.0%)-N2 atmosphere at 900 0C, 1h) before (a), after a thermal annealing in air at 900 0C for 15 min (b) and after irradiation at room temperature with 190 keV Ne ions, at a dose of 1.0x1017 ions cm-2 (c). Adapted with permission from J. Non-Cryst. Sol. 322 (2003) 17-21 (Mattei *et al.*, 2003a).

parameters and proper post-implantation treatments. However, apart from producing monometallic and bimetallic nanoclusters of common types, ion implantation can produce several special metallic structures and aggregates, which we will discuss in the next section.

Fig. 14. Typical HAADF micrograph of the sample co-implanted with 5×1016 ions cm-2 of Ag (1.8 MeV) and 5×1016 ions cm-2 of Au (3.5 MeV) after thermal annealing at 900 °C (a) along with the intensity profiles for the selected particle (b) and (c). Typical HRTEM micrograph of a core-shell structure of about 12 nm diameter and the fast Fourier transforms (FFTs) of core (e) and shell (f) regions. Adapted with permission from J. Phys. Chem. C 113 (2009)

**3.4 Fabrication of nanocomposites containing unusual metallic nanostructures**

The anisotropic plastic deformation process of amorphous materials under MeV ion irradiation has been reported by Klaumünzer and Schumacher as early as 1983 (Klaumünzer & Schumacher, 1983). The process can be understood by means of a visco-elastic model, in which the thermal expansion of the cylindrically-shaped high temperature ion track region causes shear stresses that relax by viscous flow, with the viscous strains subsequently frozen in upon rapid cooling of the ion track region (Trinkaus & Ryazanov, 1995; van Dillen *et al.*, 2005). The deformation is induced by the electron energy loss, and thus occurs mostly with

Fig. 15. Bright-field TEM cross-sectional micrographs of the AuCuH (annealed in H2 (4.0%)-N2 atmosphere at 900 0C, 1h) before (a), after a thermal annealing in air at 900 0C for 15 min (b) and after irradiation at room temperature with 190 keV Ne ions, at a dose of 1.0x1017 ions cm-2 (c). Adapted with permission from J. Non-Cryst. Sol. 322 (2003) 17-21 (Mattei *et al.*, 2003a).

2296-2300 (Peña *et al.*, 2009).

ions in the MeV energy range.

Utilizing the above mentioned concept, ion implantation technique has been utilized for the fabrication of several unusual nanostructures either inside dielectric matrices or in other dielectric-metal combinations with distinct plasmonic behaviors. Though all of these nanostructures do not fall into the class of commonly known nanocomposites (where nanoparticles of one material are embedded/dispersed into another matrix material), as they are in combination with at least one material apart from the metal in question, we can call them as *composite nanostructures*. For example, Mattei and co-workers (Mattei *et al.*, 2003a, 2005) could disintegrate alloy clusters like Au-Cu and Au-Ag embedded in silica matrix by high energy Ne, He and Kr ions to form Au-rich satellite clusters around the initial bimetallic clusters (Figures 15 & 16).

While the optical absorption spectrum of the co-implanted (190 keV Au and 90 keV Cu, 3.0x1016 ions cm-2) AuCu sample after annealing in inert atmosphere revealed SPR peak of AuCu alloy particles (550 nm, curve (a) of the Figure 15), after irradiation of 190 keV Ne ions (1.0x1017 ions cm-2) the SPR peak shifts to 575 nm, which correspond to the SPR band of the main particle along with the satellite ones (aggregate). Similar phenomenon occurred for the AuAg nanoclusters embedded in silica matrix. It has been observed that in both cases, mainly the Au segregates out of the main bimetallic cluster to form Au-rich satellite clusters of smaller sizes. The size of the satellite clusters depends on the atomic mass of the irradiating ions. Higher atomic mass of the irradiating ions produces satellite clusters of bigger sizes (Figure 16). However, the phenomenon of disintegration of the preformed noble metal clusters in dielectric matrix occurs when the atomic mass of the irradiating ions is lower than the atomic mass of the embedded metal nanoclusters. The process was tentatively related to the nuclear component of the energy released to the system during irradiation with different ions.

Fig. 16. Bright-field TEM cross-sectional micrographs of the sample AuAgA (annealed in air atmosphere at 800 0C, 1h) before (a) and after irradiation at room temperature with 380 keV Kr ions at a dose of 2.0x1016 ions cm-2 (b) (Mattei et al., 2003a). Adapted with permission from J. Non-Cryst. Sol. 322 (2003) 17-21 (Mattei *et al.*, 2003a).

On the other hand, irradiation by high energy ions of higher atomic mass (with respect to the implanted metals in a dielectric matrix) generates very different phenomena. Generally,

Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review 349

Fig. 18. Experimental optical extinction spectra of elongated Au nanoclusters (a)-(c), measured for the light incident at (d) 90°, and (e) 29° and polarization angles varying between 0° (p-polarized) and 90° (s-polarized). Corresponding simulated spectra (Mie theory) are presented in (f), and (g), respectively. All the spectra were normalized to 1 at the 450 nm wavelength for better visualization. Adapted with permission from J. Phys. Chem. C

Fig. 19. Scanning electron microscopy images of (a) silica colloid (1.0 m diameter) before (left) and after (right) 16 MeV Au irradiation (1.1x1015 ions cm-2); (b) silica-Au core-shell colloid (dSiO2 = 438 nm, tAu = 46 nm); (c) same as (b), after 30 MeV Cu irradiation (5.0x1014 ions cm-2); (d) silica-Au-silica core-shell-shell colloid (dSiO2 = 300 nm, tAu = 25 nm, tSiO2 = 133 nm) after 6 MeV Au irradiation (6.5x1014 ions cm-2). Scale bars represent 500 nm. Reproduced with permission from Nucl. Instrum. Meth. B 242 (2006) 523-529 (Penninkhof *et al.*, 2006).

114 (2010) 746-751 (Rodríguez-Iglesias *et al.*, 2010b).

when an embedded metallic nanocluster is irradiated by heavier ions of higher energy (in comparison to the energy of implantation of the metal clusters), the preformed nanocluster can grow in bigger size or get deformed. For the low dose/fluence of irradiation, the preformed metallic nanoclusters grow in size maintaining their initial shape. However, an increased dose of irradiation generates deformation of the nanoclusters. In fact, several researchers have reported this deformation process for the noble metal nanoclusters embedded in amorphous dielectric matrices. While D'Orléans and co-workers (D'Orléans *et al.*, 2003) could observe this effect for the preformed Co nanoclusters embedded in silica matrix upon irradiating them by 200 MeV 127I ions, Awazu and co-workers (Awazu *et al.*, 2009) could observe this effect for the pre-implanted gold and silver nanoclusters embedded in silica matrix (Figure 17).

Fig. 17. Cross-sectional TEM image of Au (left) and Ag (right) nanoparticles embedded in SiO2 before and after irradiation with 110 MeV Br10+ ions at different doses. The Ag nanoparticles were irradiated with 1.0x1014 ions cm-2 dose. Adapted with permission from Nucl. Instrum. Meth. B 267 (2009) 941-943 (Awazu *et al.*, 2009).

Ag and Au nanoclusters could also be deformed on irradiating them by high energy Si ions to form elongated or rod shaped aligned nanoclusters in silica matrix (Rodríguez-Iglesias *et al.*, 2010a; b). As we can see from Figure 18, the elongated nanoclusters are well aligned, and generate SPR absorption bands corresponding to longitudinal and transversal vibrational modes of prolate nanoclusters on oblique incidence. Indeed, the metallic clusters get deformed upon irradiation of high energy heavy ions along the direction of incident ions; i.e. they get elongated in this direction. On the contrary, the deformation or expansion of nonmetallic or dielectric clusters occurs in the perpendicular direction of the irradiating ion beams.

when an embedded metallic nanocluster is irradiated by heavier ions of higher energy (in comparison to the energy of implantation of the metal clusters), the preformed nanocluster can grow in bigger size or get deformed. For the low dose/fluence of irradiation, the preformed metallic nanoclusters grow in size maintaining their initial shape. However, an increased dose of irradiation generates deformation of the nanoclusters. In fact, several researchers have reported this deformation process for the noble metal nanoclusters embedded in amorphous dielectric matrices. While D'Orléans and co-workers (D'Orléans *et al.*, 2003) could observe this effect for the preformed Co nanoclusters embedded in silica matrix upon irradiating them by 200 MeV 127I ions, Awazu and co-workers (Awazu *et al.*, 2009) could observe this effect for the pre-implanted gold and silver nanoclusters embedded

Fig. 17. Cross-sectional TEM image of Au (left) and Ag (right) nanoparticles embedded in SiO2 before and after irradiation with 110 MeV Br10+ ions at different doses. The Ag nanoparticles were irradiated with 1.0x1014 ions cm-2 dose. Adapted with permission from

Ag and Au nanoclusters could also be deformed on irradiating them by high energy Si ions to form elongated or rod shaped aligned nanoclusters in silica matrix (Rodríguez-Iglesias *et al.*, 2010a; b). As we can see from Figure 18, the elongated nanoclusters are well aligned, and generate SPR absorption bands corresponding to longitudinal and transversal vibrational modes of prolate nanoclusters on oblique incidence. Indeed, the metallic clusters get deformed upon irradiation of high energy heavy ions along the direction of incident ions; i.e. they get elongated in this direction. On the contrary, the deformation or expansion of nonmetallic or

dielectric clusters occurs in the perpendicular direction of the irradiating ion beams.

Nucl. Instrum. Meth. B 267 (2009) 941-943 (Awazu *et al.*, 2009).

in silica matrix (Figure 17).

Fig. 18. Experimental optical extinction spectra of elongated Au nanoclusters (a)-(c), measured for the light incident at (d) 90°, and (e) 29° and polarization angles varying between 0° (p-polarized) and 90° (s-polarized). Corresponding simulated spectra (Mie theory) are presented in (f), and (g), respectively. All the spectra were normalized to 1 at the 450 nm wavelength for better visualization. Adapted with permission from J. Phys. Chem. C 114 (2010) 746-751 (Rodríguez-Iglesias *et al.*, 2010b).

Fig. 19. Scanning electron microscopy images of (a) silica colloid (1.0 m diameter) before (left) and after (right) 16 MeV Au irradiation (1.1x1015 ions cm-2); (b) silica-Au core-shell colloid (dSiO2 = 438 nm, tAu = 46 nm); (c) same as (b), after 30 MeV Cu irradiation (5.0x1014 ions cm-2); (d) silica-Au-silica core-shell-shell colloid (dSiO2 = 300 nm, tAu = 25 nm, tSiO2 = 133 nm) after 6 MeV Au irradiation (6.5x1014 ions cm-2). Scale bars represent 500 nm. Reproduced with permission from Nucl. Instrum. Meth. B 242 (2006) 523-529 (Penninkhof *et al.*, 2006).

Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review 351

plasmonic behaviors, which are highly attractive for several applications from optical sensing to biological treatments. Considering the current interest on plasmonic nanostructures and nanocomposites and the demonstrated potential of available commercial ion implanters, utilization of this technique for fabricating new exotic nanostructures in the

The authors extend their apology to the several other authors who have contributed significantly in this field, but were not cited due to limited space of this review. The work was supported by CONACyT, Mexico (Grant # 151767) and VIEP-BUAP (Grant # VIEP/EXC/2011). OPR thanks CONACyT, Mexico for extending a postdoctoral fellowship.

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near future is very much expected.

**5. Acknowledgments**

**6. References**

Utilizing this deformation behavior of metal and dielectric nanoparticles, Pennikhof and coworkers (Penninkhof *et al.*, 2006) could produce colloidal prolate core-shell type Au-SiO2 and reverse nanoparticles as big as 1 m in size (Figure 19) by irradiating high energy Au or Cu ions. As the ion irradiation induced deformation is more prominent for a dielectric like SiO2 than a metal like Au, the extent of deformation of the SiO2-core Au-shell particles is not as drastic as of Au-core SiO2-shell particles. In fact, the presence of a thin layer (shell) of metal imposes a mechanical constrain on the deforming silica core, reducing the net deformation compared to that of pure silica (Penninkhof *et al.*, 2006).

Optical characteristics of all the above plasmonic structures, which we defined as 'special structures' are quite different from that of conventional nanocomposites where the shape of incorporated/embedded noble metal nanoclusters remains in its most stable or spherical form. As the technological application of plasmon-based nanocomposites remains on the ability of manipulating their optical behavior, which is possible mainly by controlling the size, shape and other geometrical parameters, rather than just changing their composition. Ion implantation technique has demonstrated its great potential for their fabrication and manipulations.

## **4. Prospects of ion-implantation for the fabrication of plasmonic nanocomposites**

Significant advances have been made in the last few decades on the fabrication of noble metal plasmonic nanocomposites through ion implantation. By controlling the implantation parameters and post implantation annealing, noble metal nanocomposites with embedded elemental, alloy and core-shell type NPs could be produced with moderate control over the NP size, size distribution, composition and location in the matrix. The significant advantage of ion implantation technique is that the treated surface is an integral part of the sample and consequently does not suffer from possible adhesion problems associated with coatings. Therefore, the implanted structures remain as the integral parts of an optical device. The moderate heating associated with the process virtually eliminates any risks of distortion or oxidation effects. On the other hand, ion implantation produces no macroscopic change in the workpiece. From the point of view of applications, though several plasmonic nanocomposites with embedded noble metal clusters of tailored compositions and even of core-shell geometries could be fabricated extending their SPR peak position to a broader spectral range (in comparison with monometallic nanocomposites), tuning the relative thickness of the core and shell layer has not been sufficiently demonstrated. As has been shown recently (Peña-Rodríguez & Pal, 2010, 2011a; b; Peña-Rodríguez *et al.*, 2011a; b), application potential of plasmonic nanostructures depends on their enhanced SPR extinction, along with a precise control over its peak position. For this purpose, it is possible that in the near future we will need to fabricate multi-layer metallic nanoclusters with precise control over the thickness of each layer. Though the modern ion implanters are capable of introducing virtually any element in a solid matrix with a wide variation of energy and implantation dose, there has been no attempt to fabricate nanocomposites with trimetallic nanoclusters incorporated into dielectric matrix. On the other hand, fewer efforts have been devoted to control the shell-thickness in a core-shell type nanocluster or the fabrication of multilayer metallic nanoclusters. However, very recently this technique has been utilized for the fabrication of unusual composite nanostructures with distinct plasmonic behaviors, which are highly attractive for several applications from optical sensing to biological treatments. Considering the current interest on plasmonic nanostructures and nanocomposites and the demonstrated potential of available commercial ion implanters, utilization of this technique for fabricating new exotic nanostructures in the near future is very much expected.

## **5. Acknowledgments**

The authors extend their apology to the several other authors who have contributed significantly in this field, but were not cited due to limited space of this review. The work was supported by CONACyT, Mexico (Grant # 151767) and VIEP-BUAP (Grant # VIEP/EXC/2011). OPR thanks CONACyT, Mexico for extending a postdoctoral fellowship.

## **6. References**

350 Ion Implantation

Utilizing this deformation behavior of metal and dielectric nanoparticles, Pennikhof and coworkers (Penninkhof *et al.*, 2006) could produce colloidal prolate core-shell type Au-SiO2 and reverse nanoparticles as big as 1 m in size (Figure 19) by irradiating high energy Au or Cu ions. As the ion irradiation induced deformation is more prominent for a dielectric like SiO2 than a metal like Au, the extent of deformation of the SiO2-core Au-shell particles is not as drastic as of Au-core SiO2-shell particles. In fact, the presence of a thin layer (shell) of metal imposes a mechanical constrain on the deforming silica core, reducing the net

Optical characteristics of all the above plasmonic structures, which we defined as 'special structures' are quite different from that of conventional nanocomposites where the shape of incorporated/embedded noble metal nanoclusters remains in its most stable or spherical form. As the technological application of plasmon-based nanocomposites remains on the ability of manipulating their optical behavior, which is possible mainly by controlling the size, shape and other geometrical parameters, rather than just changing their composition. Ion implantation technique has demonstrated its great potential for their fabrication and

Significant advances have been made in the last few decades on the fabrication of noble metal plasmonic nanocomposites through ion implantation. By controlling the implantation parameters and post implantation annealing, noble metal nanocomposites with embedded elemental, alloy and core-shell type NPs could be produced with moderate control over the NP size, size distribution, composition and location in the matrix. The significant advantage of ion implantation technique is that the treated surface is an integral part of the sample and consequently does not suffer from possible adhesion problems associated with coatings. Therefore, the implanted structures remain as the integral parts of an optical device. The moderate heating associated with the process virtually eliminates any risks of distortion or oxidation effects. On the other hand, ion implantation produces no macroscopic change in the workpiece. From the point of view of applications, though several plasmonic nanocomposites with embedded noble metal clusters of tailored compositions and even of core-shell geometries could be fabricated extending their SPR peak position to a broader spectral range (in comparison with monometallic nanocomposites), tuning the relative thickness of the core and shell layer has not been sufficiently demonstrated. As has been shown recently (Peña-Rodríguez & Pal, 2010, 2011a; b; Peña-Rodríguez *et al.*, 2011a; b), application potential of plasmonic nanostructures depends on their enhanced SPR extinction, along with a precise control over its peak position. For this purpose, it is possible that in the near future we will need to fabricate multi-layer metallic nanoclusters with precise control over the thickness of each layer. Though the modern ion implanters are capable of introducing virtually any element in a solid matrix with a wide variation of energy and implantation dose, there has been no attempt to fabricate nanocomposites with trimetallic nanoclusters incorporated into dielectric matrix. On the other hand, fewer efforts have been devoted to control the shell-thickness in a core-shell type nanocluster or the fabrication of multilayer metallic nanoclusters. However, very recently this technique has been utilized for the fabrication of unusual composite nanostructures with distinct

deformation compared to that of pure silica (Penninkhof *et al.*, 2006).

**4. Prospects of ion-implantation for the fabrication of plasmonic**

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