# Polarized tip-enhanced Raman spectroscopy at liquid He temperature in
ultrahigh vacuum using an off-axis parabolic mirror

L. Peis  Walther Meissner Institut, Bayerische Akademie der Wissenschaften,
85748 Garching, Germany  School of Natural Sciences, Technische Universität
München, 85748 Garching, Germany  IFW Dresden, Helmholtzstrasse 20, 01069
Dresden, Germany  G. He<EMAIL_ADDRESS>Present address: Physics
department, University College Cork, College Road, Cork T12 K8AF, Ireland
Walther Meissner Institut, Bayerische Akademie der Wissenschaften, 85748
Garching, Germany  D. Jost  Present address: Stanford Institute for Materials
and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill
Road, Menlo Park, CA 94025, USA  Walther Meissner Institut, Bayerische
Akademie der Wissenschaften, 85748 Garching, Germany  School of Natural
Sciences, Technische Universität München, 85748 Garching, Germany  G. Rager
Walther Meissner Institut, Bayerische Akademie der Wissenschaften, 85748
Garching, Germany  School of Natural Sciences, Technische Universität München,
85748 Garching, Germany  R. Hackl<EMAIL_ADDRESS>Walther Meissner Institut,
Bayerische Akademie der Wissenschaften, 85748 Garching, Germany  School of
Natural Sciences, Technische Universität München, 85748 Garching, Germany  IFW
Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany

###### Abstract

Tip-enhanced Raman spectroscopy (TERS) combines inelastic light scattering
well below the diffraction limit down to the nanometer range and scanning
probe microscopy and, possibly, spectroscopy. In this way, topographic and
spectroscopic as well as single- and two-particle information may
simultaneously be collected. While single molecules can now be studied
successfully, bulk solids are still not meaningfully accessible. It is the
purpose of the work presented here to outline approaches toward this
objective. We describe a home-built, liquid helium cooled, ultrahigh vacuum
tip-enhanced Raman spectroscopy system (LHe-UHV-TERS). The setup is based on a
scanning tunneling microscope and, as an innovation, an off-axis parabolic
mirror having a high numerical aperture of approximately $0.85$ and a large
working distance. The system is equipped with a fast load-lock chamber, a
chamber for the in situ preparation of tips, substrates, and samples, and a
TERS chamber. Base pressure and temperature in the TERS chamber were
approximately $3\times 10^{-11}$ mbar and 15 K, respectively. Polarization
dependent tip-enhanced Raman spectra of the vibration modes of carbon
nanotubes were successfully acquired at cryogenic temperature. Enhancement
factors in the range of $10^{7}$ were observed. The new features described
here including very low pressure and temperature and the external access to
the light polarizations, thus the selection rules, may pave the way towards
the investigation of bulk and surface materials.

##  I Introduction

Tip-enhanced Raman spectroscopy (TERS) integrates scanning probe microscopy
with Raman spectroscopy, consequently enabling the acquisition of nanoscale
inelastic photon scattering fingerprints of matter Pettinger  _et al._ (2002);
Lucas and Riedo (2012); Pozzi  _et al._ (2017); Schultz  _et al._ (2020). The
original concept was proposed by John Wessel in 1985, where a surface plasmon
resonance in a metallic tip having a typical apex radius in the range of a few
nm. The geometry yields a strong spatial confinement of the electric field at
the tip apex and enables the use of evanescent waves. This allows studies of a
nearby sample surface in volumes much smaller than given by the diffraction
limit Wessel (1985).

It took another 15 years before the first TERS experiments were reported
independently by four groups Stöckle  _et al._ (2000); Pettinger  _et al._
(2000); Hayazawa  _et al._ (2000); Anderson (2000) triggering the rapid
development of TERS techniques. On the one hand, by optimizing design, tips
and substrates, the TERS enhancement factor was improved significantly Zhang
_et al._ (2013); Wickramasinghe  _et al._ (2014); Chiang  _et al._ (2016);
Sheng  _et al._ (2018) towards the theoretical limit of approximately
$10^{12}\,$ Pettinger (2010). On the other hand, the spatial resolution
reached nanometer or even Å ranges through improvements in the stability of
the tip-sample configurations Zhang  _et al._ (2013, 2019). Until now, the
vast majority of TERS studies focused on molecules either in ambient
conditions or under ultrahigh vacuum (UHV) and cryogenic temperatures Pozzi
_et al._ (2017); Schultz  _et al._ (2020). By taking advantage of increased
enhancement factors due to the so-called "gap mode" between a tip and a metal
surface, e.g. single crystal Au or Ag, in addition to substantially reducing
the far-field using optimized optics such as on-axis parabolic mirrors and
radially polarized light Steidtner and Pettinger (2007); Sackrow  _et al._
(2008).

The introduction of TERS to the study of bulk and thin-film materials
potentially facilitates the simultaneous observation of single-particle
tunneling spectra and two-particle Raman spectra in the same field of view
with atomic resolution Liu  _et al._ (2019). However, several challenges need
to be fixed: First, due to the excitation of the complete illuminated bulk
volume, the contrast between near and far field is decreased by a factor of
approximately $10^{-9}$, as for example shown for Si Steidtner and Pettinger
(2007); Sheng  _et al._ (2017) and MoS2 Voronine  _et al._ (2017). Second, the
requirements of UHV and low temperature for investigating quantum states, such
as superconductivity, magnetism, and novel topological states, entail
increasingly complicated TERS setups. So far, only few TERS instruments have
been demonstrated to enable experiments in UHV conditions and at either liquid
nitrogen Sheng  _et al._ (2018) or liquid helium Klingsporn  _et al._ (2014)
temperatures. TERS facilities achieving sufficiently high enhancement factors
and providing access to the Raman selection rules as well as being compatible
with low temperatures and UHV are still at their infancy.

In this publication, we describe a home-built ultrahigh vacuum tip-enhanced
Raman scattering system having a liquid-He-cooled sample stage (LHe-UHV-TERS),
a scanning tunneling microscope (STM) and an off-axis parabolic mirror. In
carbon nanotubes, we observed Raman signals with gain factors $g\approx
10^{7}$ (tip enhancement factors normalized to the scattering volume) at 15 K
and 3$\times$10-11 mbar. In addition, we studied the access to the Raman
selection rules. To our knowledge, this is the first demonstration of a
parabolic-mirror-based TERS at liquid helium temperature and UHV conditions.
Given the high numerical aperture, the access to clean polarization
configurations, and the cryogenic compatibility of a parabolic mirror
Steidtner and Pettinger (2007); Chelwani  _et al._ (2017); Peña Román  _et
al._ (2022), our work paves the way to the application of TERS in bulk
materials.

Figure 1:  Blueprint (a) and photograph (b) of the customized LHe-UHV-TERS.
The setup consists of the TERS chamber combined with cryogenic equipment (1),
a sample preparation chamber (2), enabling annealing and cleaning via a home-
made heater and ion sputtering (6), respectively, and a small load-lock
chamber (3) for short initial pumping times. The transfer is facilitated
utilizing a height-adjustable (4) transfer arm (5) and a wobble stick (10).
Some other key components are: (7) CCD, (8) spectrometer, (12) laser mount,
(13) pneumatic vibration isolation system, and (9, 11) two observation
cameras.

##  II General design

The objective was an experimental setup for polarization-resolved Raman
scattering in far- and near-field configuration working at helium temperature
and UHV conditions. For providing enough space for the scanning stage and the
in situ exchange of samples and silver tips we introduced an off-axis
parabolic mirror.

A top-view blueprint and a photograph of the system are shown in Fig. 1. The
whole setup is installed on an optical table supported by four vibration
isolators (Newport, S-$2000\,$ stabilizer). The system consists of three
chambers having different functions: (1) TERS chamber, (2) preparation, and
(3) load lock (see Fig. 1(a) for details). The load-lock chamber is the access
point for samples and tips. The subsequent preparation chamber equipped with
an ion sputtering gun and a homemade annealing stage (up to approximately 600
degrees) allows the $in$-$situ$ cleaning of samples and tips. Experiments are
carried out in the TERS chamber which integrates a multi-axis STM head
(Attocube), an off-axis parabolic mirror, and an UHV cryostat (Janis Research,
CNDT series). The three chambers are individually pumped and separated by UHV
valves.

Ultrahigh vacuum (UHV) is achieved by a serial pumping system. The TERS
chamber is pumped by a 300 liter turbo pump (HiPace 300, Pfeiffer Vacuum),
which itself is pre-pumped by a dry Scroll pump (HiScroll 12, Pfeiffer
Vacuum). If needed, an additional ion getter pump can be installed on a CF150
flange. Both the preparation chamber and load-lock chamber are equipped with
80 liter turbo pumps (HiPace 80, Pfeiffer Vacuum) with additional dry fore
pumps.

After baking the system at 150 degrees for three days base pressures are
obtained reaching $8\times 10^{-9}\,$mbar in the TERS chamber, $3\times
10^{-8}\,$mbar in the preparation chamber and $5\times 10^{-8}\,$mbar in the
load-lock chamber. After cooling down to He temperature, a final minimal
pressure of $1.4\times 10^{-11}\,$mbar could be achieved in the TERS chamber.

The UHV cryostat is filled with 7 L liquid nitrogen in the outer tank and 4 L
liquid helium in the inner tank. While the sample stage may be operated at
temperatures from 15 to 320 K, it was kept at approximately 20 K for most of
the experiments. To reduce the vibrations induced by the bubbling of liquid
nitrogen, the nitrogen tank was pumped down to around 10 mbar with an
additional scroll pump. At 10 mbar the nitrogen solidifies and the noise from
the bubbles disappears. The LN2 had to be pumped for the entire measurement
runs.

Figure 2:  Schematic view of the setup. The upper row shows a perspective view
of the STM part with the parabolic mirror on the left (encircled) and a side
view along the $y$-axis as indicated by the inset on the right. The central
row displays the top view along the $z$-axis (see axes on the lower left).
Left: TERS chamber, middle: preparation chamber, right: load lock chamber and
transfer arm. The magnetically coupled transfer arm connects all three
chambers and facilitates the loading and transfer of samples and tips. The
height of the arm can be adjusted via the $z$-axis manipulator. The samples
and tips are stored on a pre-cooled, rotatable carousel with eight positions
in the TERS chamber (purple). From there they are being picked up by the
wobble-stick and inserted into the STM head. Two valves (V1, V2) separate the
individual chambers. The windows W(2-6) enable visual control in addition to
W1 and W2 being used for sample illumination and Raman measurements,
respectively.

##  III Chambers and transfer system

Figure 2 shows a schematic view of the three relevant chambers, TERS (left),
preparation(middle), and load lock (right). A transfer arm and a wobble stick
have been attached to the load-lock and TERS chamber, respectively, enabling
the smooth $in$-$situ$ transfer between the individual chambers.

The sample and tip are mounted in flag-style holders (sample and tip holder as
indicated in Fig. 2), which can be grabbed by a commercial wobble stick. Then,
the sample or the tip holder is inserted into a cylindric holder (transfer
holder as shown in Fig. 2), with the latter being designed to be forked by the
transfer arm. Two transfer holders may then be put into the load-lock chamber
and inserted into the available two voids of the storage table. A third holder
may stay in the fork. So in total three holders may be pumped simultaneously.
The transfer arm has a range of 1.5 m along $x$-direction and can be fine
adjusted in $z$ direction by a shaft guidance (4), having a range of 25 mm. A
fork is installed at the end of the transfer arm, in order to pick up the
transfer holder and deliver it to the preparation or TERS chamber.

In the preparation chamber, a homemade annealing stage heated by tungsten
coils is mounted at the bottom. By applying a current (maximum 5.6 A) through
the coils, the entire transfer holder heats up and starts glowing as seen in
the inset of Fig. 2. The annealing temperature can reach approximately 600
degrees.

Additionally, an ion gun is installed in the preparation chamber having a tilt
angle of $45^{\circ}$ with respect to the sample plane. The ion gun is
differentially pumped and operated with $99.995\%$ pure Ar at a chamber
pressure of approximately $5\times 10^{-5}\,$mbar, controlled via a needle
valve. Atomically flat surfaces of gold and silver crystals were achieved
after several cycles of alternating ion sputtering and annealing. The ion
sputtering approach can also be used for a first cleaning procedure of the
tip.

Samples and tips are finally transferred into the TERS chamber. A storage
carousel with eight positions is mounted on the bottom of the liquid nitrogen
radiation shield and can be turned by the wobble stick. For tip and sample
transfer to the STM head, the carousel has to be locked to the radiation
shield by screws. From the arrested carousel, the wobble stick grabs the
sample/tip holder and inserts it into the sample stage or tip stage on the STM
head.

The storage carousel substantially improves the space and time efficiency and
also simplifies the design of the STM chamber compared to other designs Wu
_et al._ (2018).

Figure 3:  Schematic of the optical configuration for Raman scattering. The
parabolic mirror, sample, and tip are located in the center of the TERS
cryostat. The laser for excitation on the left is linearly polarized. Two
confocal lenses (L1 and L2) expand the beam to 6 mm. A pinhole (PH1) in their
joint focus filters the light spatially. After the mirror, the desired
polarization and power are set (P1 and $\lambda/2$). Before hitting the
optical flat the beam is again spatially filtered (L3, PH2, L4). The optical
flat is adjustable in a way that the beam can be aligned parallel and at the
required distance to the optical axis of the parabolic mirror. The paraboloid
focuses the light on the sample and collects the inelastically scattered, red-
shifted light. The parallel Raman light is deflected towards lens O1 by a
semitransparent mirror (SM1). The focal length of lens L5 is smaller by
approximately a factor of three in comparison that of O1 thus leading to a
reduction of the beam diameter to 12 mm. Before the Raman light enters
analyzer P2 the laser is cut off by an edge filter (F1). Another $\lambda/2$
retarder rotates the selected polarization into the direction optimal for the
spectrometer. Lens L6 focuses the light on the entrance slit. The Köhler
illumination system and parts of the observation optics are also illustrated.
Tip, sample and laser spot may be imaged by two independent systems (C1, C2).
System C1 looks into the paraboloid through the semitransparent mirror SM2 and
uses part the paraboloid as an objective lens. Due to the off-axis conditions
the aberrations are substantial, and the laser spot is elongated (upper left
inset). When the tip gets close to the sample surface the tip and its shadow
approach each other (lower left inset). System C2 provides a direct image of
the sample with adjustable magnification (upper right inset with laser, lower
right inset without laser).

##  IV Optical path

A schematic drawing of the optical setup is displayed in Fig. 3. A diode-
pumped solid-state laser emitting at $532\,$nm (Coherent sapphire SF $532$)
was utilized for all measurements. The mounting platform allows an easy
exchange of the green sapphire laser to a red laser emitting at $660\,$nm
(Laser Quantum, Ignis-FS $660$) if desired. The laser beam is spatially
filtered and expanded to a diameter of $6\,$mm by the first pinhole system
(L1, $f=10$ mm; PH1, $d=30$ $\mu$m; L2, $f=60$ mm), reducing the divergence of
the beam to $2\times 10^{-4}\,$rad. Subsequently, the laser power and the
polarization direction of the light are selected by the combination of a
$\lambda/2$ waveplate and a polarizer (P1). After the polarizer, the beam is
spatially filtered again by a pinhole system (L3, $f=50$ mm; PH2, $d=30$
$\mu$m; L4, $f=60$ mm) before being directed to an adjustable optical flat (B.
Halle Nachfl., Berlin). The latter is a fused silica cuboid, polished to a
flatness of $\lambda/20$ and coated with silver on the back surface. Thus the
first and the third beam have an intensity of approximately 4 % of the
incoming laser beam, and the second one has 90 % and serves for excitation of
the Raman spectra. The wedge between the front and back side of the optical
flat does not exceed $\lambda/20$ over the entire length of 40 mm, and the
three beams are parallel to within $6\times 10^{-7}$ rad. With the central
beam blocked, the weaker side beams enable the accurate alignment of the
parabolic mirror inside the cryostat, as described in Ref. Chen (2012).
Cooling-induced changes in the position of the parabolic mirror inside the
cryostat can be tackled either by varying the $z$ position of the optical flat
or of the cryostat thus maintaining the parallel alignment of the incoming
beam and parabola axis.

The last optical element in front of the sample is the parabolic mirror having
a solid angle of $\pi$ and a numerical aperture of approximately 0.85. Its
diameter and focal length are $30$ and $8\,$mm, respectively. This
configuration guarantees a small laser focus. As an innovation, the sample
surface is not perpendicular but parallel to the optical axis of the parabola.
In this way, the directly back-reflected laser light never enters the
collection optics. Both incoming and scattered photons are reflected from the
aluminum coating of the parabola and thus the respective polarizations have
undesired components. This contamination was shown experimentally and
theoretically to not exceed 15 %. Consequently, the selection rules are by and
large effective. From the focal point, inelastically scattered light is
emitted in all directions. The part propagating into the direction of the
paraboloid is being collected and re-directed in an ideal parallel fashion.
The diameter of 30 mm (corresponding to the diameter of the paraboloid) of the
parallel light is reduced to 12 mm by an objective lens (O1, Canon,
$f=85\,$mm) and an achromat (L5, $f=40$ mm). Contributions of the elastically
scattered light (Rayleigh line) are blocked by an edge filter (F1, Iridian
532nm LPF). The desired polarization state of the inelastically scattered
photons is selected by analyzer P2. A $\lambda/2$ wave plate rotates the
selected polarization into the direction of the highest sensitivity of the
spectrometer. Finally, the inelastically scattered light is focused (L6,
$f=50$ mm) onto the entrance slit of the spectrometer (ISOplane SCT320,
Princeton Instruments). The photons transmitted with the desired energy are
detected by a charge-coupled device (CCD, PyLoN-400BRX, Princeton
Instruments). The spectral range in the green per single exposure for the chip
width of 27 mm is approximately 600 cm-1.

Figure 4:  Schematic of the STM head. The drawing is based on that shown in
the upper left part of Fig. 2. The front structure has been removed for
demonstration purposes. All components (paraboloid, sample tower, tip tower)
are mounted on the same platform. All structures are made of titanium. The
platform is connected to the He bath by a thermal resistor facilitating the
temperature to be varied in the range of 10 to 300 K. Each tower has three
linear degrees of freedom. One of the two operation modes allows sample and
tip to be positioned in the focal point of the parabolic mirror. The other
operation mode facilitates fine scanning for the measurements. Via a feedback
loop, the piezo stages enable the acquisition of STM topographic images. On
the lower left, the last lens of the Köhler illumination system is displayed
along with a mirror that directs the light towards the sample.

The related STM setup is shown in Fig. 4. The paraboloid on the right hand
side is fixed. The sample may be moved by the sample tower along three
independent axes in either coarse mode in a range of $\pm 2.5$ mm or fine
scanning mode in a range of approximately $\pm 2~{}\mu$m. The tip is mounted
on an identical second tower (tip tower). First, the sample is moved into the
focus of the paraboloid using the two weak parallel laser beams for fine-
adjustment as mentioned above and described in detail in Ref. Chelwani  _et
al._ (2017). Then the tip is approached and moved into the focus.

To this end, the sample and tip region are observed by two independent camera
systems (C1, C2), enabling the controlled movement of sample and tip, the tip
approach, and the optical alignment. A large field of view is accessible
through window W3 by camera C2. The attached zoom lens (MVL700, ThorLabs)
allows the variation of the field of view between $6\times 10$ mm2 and
$36\times 60$ mm2. The sample region is illuminated by a Köhler system for
optimal contrast.

Typical images are shown in the two insets of Fig. 3 on the right. The upper
inset shows the gold-plated silicon sample with the laser spot (red circle;
the big bright area is a reflex from the window). The arm with the tip is
retracted and thus out of focus (lower left part of the inset). The lower
inset shows the sample with the tip approached. The silver tip looks golden
because of the sample surface. The reflex of the tip is nearly perpendicular
to the tip close to the upper edge of the sample.

The final approach of the tip to the sample is optically controlled by
observation system C1. The zoom objective (Sigma Optics) is set at infinity
and uses a semi-transparent mirror (SM2) for looking into the paraboloid which
is used as an objective lens. In this way microscopic observation of sample
surface, tip, and laser spot with $\mu$m-resolution becomes possible.
Resulting images are depicted in the left insets in Fig. 3. The tip and its
shadow can be seen clearly (lower left inset) allowing a coarse approach down
to approximately $1~{}\mu$m. The laser spot (upper left inset) is elongated
because of the geometrical aberrations induced by the off-axis conditions. Its
lateral size is close to $5~{}\mu$m. All dimensions may only be estimated
relative to the tip thickness.

Figure 5:  Raman and STM performance of the LHe-UHV-TERS system. (a)
Electrochemically etched Ag-tip with a tip apex radius of around $50\,$nm. (b)
STM image of a gold surface where the height is color coded. The black line
indicated the position of the line cut in panel (c). (c) The terraces are flat
to within the noise level of approximately 0.5 Å. The steps between the
terraces can be clearly resolved. (d) Raman spectra of a (001)-Si surface in
$ab$-polarization configuration. For the Si phonon at $520\,$cm-1 the count
rate in the peak is approximately 2500 cts/(s mW).

##  V Preparation of the tip

Since Ag tips show the strongest surface plasmon excitation for green
wavelengths while having negligible luminescence (as opposed to Au), we
switched to Ag. Yet, Ag is much more tedious to handle because of the
instability of the surface. We first prepared the tips using electrochemical
etching as described in Ref. Chelwani  _et al._ (2017). Figure 5(a) displays
the SEM image of a typical Ag-tip having a radius of less than 20 nm. One of
those tips was selected and tested before being transferred into the STM
stage. The preparation is among the crucial parts of the experiment. The tip
is conditioned in close vicinity of either a Au single crystal or pristine
thin film using field emission. For this purpose, we apply a voltage between
the substrate and tip of up to 75 V so that the current reaches 10 $\mu$A for
up to 30 minutes. The tip can be further improved by short voltage pulses.
After that, the tip quality and cleanliness can be checked by STM.

##  VI Performance of STM and Raman

The performance of STM is illustrated by the topographic image of a single
crystalline Au(111) film shown in Fig. 5(b). The atomic terraces and the steps
are readily resolved as shown in the line cut in Fig. 5(c). The height of a
mono-layer atomic step (235 pm Walen  _et al._ (2015)) is used to calibrate
our STM head. This measurement and those shown below were performed with a
tunneling current of 1 nA and a bias voltage of 500 mV. Further checks are
possible upon scanning the topography of SWNTs on the gold substrate.

Before combining STM and Raman spectroscopy we measured the intensity of the
Si phonon at 520 cm-1 in the TERS system and compared it with earlier results
obtained in a scanning spectrometer. The experiments were carried out on
$\\{001\\}$ surfaces. An $ab$ polarization configuration (for the definition
see Fig. 7) was chosen for measurements covering a range of 338 to 1028 cm-1.
The point density is given by the pixel size of the CCD. For the wavelength
selected, $20~{}\mu$m corresponds to approximately 0.5 cm-1. The laser power
absorbed by the sample and the dwell time for a single acquisition were set at
1 mW and 1 s, respectively. The resulting spectrum is plotted in Fig. 5(d).
The peak intensity reaches $\approx 2500~{}$cts/(s mW). The corresponding peak
intensity in the scanning spectrometer using a resolution of 5 cm-1 is 7000
cts/(s mW). With all optical elements and the different resolutions in the two
experiments properly taken into account, the sensitivity of the TERS setup is
at least a factor of two higher owing to the higher aperture of the parabola.

Figure 6:  Tip enhanced Raman signal of SWNTs at $20\,$K. (a) Close-up of the
general setup of the sample, tip, and laser. (b) Topographic image of the
SWNTs sample, illustrating several different nanotube bundles on the gold
surface. The tip was positioned on an isolated region of an individual SWNT
bundle (white dot). (c) Enhancement of several characteristic vibration modes
of SWNTs. (d) Raman spectra visualizing the distance dependence of the TERS
signal. From the normalized intensity of the G+ excitation, the exponential
decrease upon removing the tip can be observed, as depicted in panel (e).

##  VII Enhancement in single wall carbon nanotubes

Polarization-resolved Raman spectra can be measured with very high sensitivity
in the temperature range between 10 and 300 K under UHV conditions using a
parabolic mirror as shown in Fig. 5(d). Both Au and Ag tips were prepared
routinely and may be transferred into the measurement position. The STM has a
$z$ resolution of approximately 50 pm (0.5 Å) as shown in Fig. 5(c). These
features allow us to perform test measurements using single-wall carbon
nanotubes (SWNT) as samples. Although these experiments are not new (see refs.
Liao  _et al._ (2016); Chen, Hayazawa, and Kawata (2014); Peica, Thomsen, and
Maultzsch (2011)) they help us to pin down the performance of our system and
compare it to earlier experiments.

SWNTs are in fact much easier to deal with than surfaces of crystals Steidtner
and Pettinger (2007) since they intrinsically limit the scattering volume (in
a fashion similar to single molecules) and allow the direct visualization of
the tip enhancement in the Raman spectra. In addition, the samples are easy to
prepare and stable at ambient conditions.

SWNTs are suspended in H2O. A drop of the suspension is dripped on a gold film
in ambient air. The liquid is evaporated in a continuous flow of clean
nitrogen for two minutes. The successful deposition of the SWNTs is validated
using an atomic force microscope (AFM) under ambient conditions. Finally, the
gold film with the attached SWNTs is loaded into the chamber.

A silver tip and, as above, a green laser emitting at 532 nm are used. Figure
6(a) displays the configuration of our TERS measurements. An STM image of the
sample with the SWNTs is shown in Fig. 6(b). It is difficult to resolve
individual nanotubes within our resolution limit, but rather several bundles
with each containing several nanotubes Hartschuh  _et al._ (2003).

From this starting point, we move the tip to one of the bundles as indicated
by the white spot in Fig. 6(b), and measure Raman spectra (see Fig. 6(c)). In
the far field with the tip retracted, one can identify the radial breathing
mode (RBM) at 170 cm-1, and the G- and G+ modes at $\Omega\approx 1569.7$ and
$1593.8$ cm-1, respectively. Once the tip is approached to the sample surface,
all three modes are enhanced substantially by factors in the range between 4
and 6 whereas the background changes only negligibly. This stability of the
background is another criterion for the quality of the tip. If the Ag tips are
not appropriately prepared the background always exhibits a substantial
increase upon tip approach. Further cleaning along the lines described above
removes this additional luminescence.

For a quantitative analysis of the Raman intensity, the phonon peaks were
fitted with Lorentzians and a flat background, giving the ratio of the peak
intensities, defined here as the enhancement factor $EF$ :

$EF=\frac{I_{\text{approached}}}{I_{\text{retracted}}}.$ (1)

Note that this is the actual intensity enhancement of the investigated Raman
processes. For the determination of the gain $g$ induced by the tip the
scattering volumina relevant for the near and far field scattering have to be
taken into account Steidtner and Pettinger (2007). The gain $g$ is given by:

$g=c*\left(\frac{I_{\text{total}}-I_{\text{far}}}{I_{\text{far}}}\right)*\frac{\text{A}_{\text{far}}}{\text{A}_{\text{near}}},$
(2)

where $c$ is the coverage of SWNTs and $A_{\text{far}}/A_{\text{near}}$ is the
ratio of the contributing areas $A_{i}$ with the tip retracted (far) and
approached (near). We can use the ratio of areas instead of the complete
illuminated volumes since the contributing SWNTs are quasi-2D in our
measurements. The coverage ratio may be estimated from the STM topographic
image as described in more detail in Appendix A. The resulting values for $EF$
and $g$ are listed in Table 1. The value for $g$ of order $10^{7}$ found here
for the G+-band of SWNTs is significantly larger than those described in the
literature. Most publications report $g$-factors in the range of $10^{4}$ to
$10^{6}$ Shiotari, Kumagai, and Wolf (2014); Vannier  _et al._ (2006); Kato
_et al._ (2020); Pettinger  _et al._ (2012). Recently, gains of $10^{8}$ were
observed by Liao et. al Liao  _et al._ (2016). We conclude that the LHe-UHV-
TERS-system described here meets the benchmarks.

Table 1: Intensity enhancement $EF$ and TERS gain $g$ of SWNT excitations. $EF$ and $g$ are calculated by applying Eqs. 1 and 2 to the data presented in Figs. 6 (c) and 7 for both $ab$ and $ba$ polarization. Excitation |  | $ab$ | $ba$
---|---|---|---
RBM | $EF$ | $4.88$ | $2.53$
| $g$ | $1.94\times 10^{7}$ | $7.65\times 10^{6}$
G--band | $EF$ | $5.58$ | $2.53$
| $g$ | $2.29\times 10^{7}$ | $7.65\times 10^{6}$
G+-band | $EF$ | 5.74 | $2.68$
| $g$ | $2.37\times 10^{7}$ | $8.40\times 10^{6}$

Next, the Raman spectra are studied as a function of tip/sample distance.
Starting from the tunneling mode we retract the tip slowly to $100\,$nm with a
step size of $5\,$nm by reducing the $dc$ voltage applied on the $z$-axis
scanner of the tip tower. At each step, a Raman spectrum was taken and is
displayed in Fig. 6(d). For close distances of the tip to the sample surface,
the intensities of the G+ and G- modes are increased clearly. The distance-
dependent intensity of the statistically more robust G+ mode is plotted in
Fig. 6(e), which can be fitted using an exponentially decaying function. We
find the distance for a decrease to 1/e at 32 nm, close to the values reported
in Kharintsev  _et al._ (2007).

Figure 7:  Polarization-dependent enhancement of the Raman signal. (a) and
(d): Definition of the polarization configurations. Vertical and horizontal
polarizations parallel to the laboratory $z$ and $y$ axes are labeled $a$
($a$-pol) and $b$ ($b$-pol), respectively. The incoming polarization ${\bf
e}_{i}$ of the photons on the sample surface depends on both the polarization
of the incoming light ($a$ or $b$) and the angle of the beam with respect to
the tip axis $\theta$. The enhancements $EF$ for $ab$ and $ba$ configuration
are depicted in the upper row and, respectively, lower row for the radial
breathing modes [(b) and (e)] and the G modes [(c) and (f)]. For better
visualization, the background signals were normalized for the two
configurations. All modes are enhanced in approximately the same way when the
polarization axis is aligned along the tip. This dichotomy may allow one to
distinguish contributions from far-field and near-field, as well as $c$-axis
contributions in bulk and thin film materials in future studies.

##  VIII Polarization dependence

The Raman selection rules allow the identification of excitations. In
addition, far- and near-field responses have to be separated. Thus, access to
the light polarizations is crucially important. However, the direction of the
tip limits the flexibility since the enhancement is much larger for light
polarizations along the tip axis. Nevertheless, for the potential use of the
selection rules and the distinction of far and near-field intensities we study
the influence of the light polarizations on the intensities of the vibrations
of SWNTs.

First, we align the incoming laser beam in a way that the polarizations can be
selectively adjusted to be either parallel or perpendicular to the tip. Since
the aluminum coating of the paraboloid generally adds elliptical components to
the originally linearly polarized photons special care is necessary to
optimize the light path. For best results, the light comes in next to the tip
[small azimuthal angle $\varphi$ measured from the plane defined by the tip
axis and the optical axis and large polar angle $\theta$ with respect to the
tip axis as shown in Fig. 7 (a) and (d)] since then the plane of incidence, by
and large, includes the tip and parabola axes facilitating the distinction
between parallel and perpendicular incident polarizations on the parabola’s
surface.

Raman measurements were then performed in $ab$ and $ba$ configurations as
defined in panels (a) and (d) and the caption of Fig. 7. The resulting spectra
for the retracted and tunneling mode are presented in Fig. 7 (b), (c), (e),
and (f). For better visualizing the enhancement factors, the $ab$ and $ba$
spectra were divided by factors of $89$ and $228$, respectively. Then, the
background of all spectra at low energies is close to 0.5 cts/(s mW) and close
to unity for high energies.

The enhancement factors are two times higher in the $ab$ channel than in the
$ba$ channel as summarized in Table 1. This is consistent with simulation
results, where the polarization along the tip axis has the highest evanescent
field Novotny, Bian, and Xie (1997). This behavior possibly allows the
distinction of the far and near field by only changing the polarization
outside of the cryostat, at least in a material in which $ab$ and $ba$ map the
same Raman symmetries. In such a scenario, the tip is kept at its position,
while the processes (enhanced and not) can be studied and compared easily.
Thin film samples with fourfold crystal symmetry are candidates, e.g.,
monolayer FeSe having an enhanced superconducting transition temperature Wang
_et al._ (2012).

An alternative approach for comparing far and near field while maintaining the
position of the tip and the polarization along $a$ is the variation of the
polar angle of the incoming light beam $\theta$ with respect to the tip axis.
The adjustable height of the optical flat provides comfortable access to
$\theta$. Along with the variation of $\theta$ the field enhancement at the
tip apex and, consequently, the $c$-axis contribution to the spectra of bulk
samples changes.

##  IX Conclusion

To summarize, we have described and successfully tested a home-built system
for tip-enhanced Raman scattering which operates at temperatures down to 15 K
close to the boiling point of liquid helium and in ultra-high vacuum. The
cryo-pumped vacuum reached a minimal pressure of $3\times 10^{-11}$ mbar.
Silver tips were prepared in situ. The light is focused on the tip and
collected after scattering by an off-axis paraboloid.

For benchmarking purposes, we studied topographic images and Raman spectra of
single-wall nanotubes dispersed on a thin gold film. The axial and lateral
resolution of the scanning tunneling microscopy is a fraction of an atomic
layer (approximately 0.5 Å) and in the Å-range, respectively. We demonstrated
that the Raman signal of all molecular vibrations of the nanotubes increased
exponentially by factors of up to 5.7 upon approaching the Ag tip. We observe
that the intensities of all excitations depend also on the photon
polarizations. The effect of rotating the incoming polarization from parallel
to perpendicular to the tip axis is stronger by a factor of two than that of
the rotation of the outgoing polarization. This dichotomy may either result
from the polarization dependent ratios of the scattering volumes or the
selection rules or from the different antenna effects of the tip for incoming
and outgoing photons.

For getting better in situ access to the sample and the tip we have changed
the orientation of the sample surface from perpendicular to the optical axis
of the parabola Steidtner and Pettinger (2007) to parallel. Since we can use
only half of the paraboloid the solid angle is only $\pi$ rather than $2\pi$
although the mounting of the STM and of the sample reduces the free aperture
in the latter case. The orientation used here allows reasonable (though not
full) access to the light polarizations. The stability of the STM system was
not as high as desirable but sufficient for the current proof of principles.
Using this equipment we could clearly demonstrate tip enhancement for SWNTs at
a temperature much lower than in previous experiments.

The advantage of nanotubes (similar to the case of single molecules) is the
confinement of the scattering volume in the far field facilitating the
visualization of the enhancement. The big challenge of future work is the
application of the technique to bulk samples such as topological insulators
having metallic surface states and an insulating bulk.

##  X Appendix A: Estimation of the enhancement factor $g$

As opposed to the enhancement factor $EF$ the gain $g$ is not directly
accessible. For an estimate, one would need to know the coverage ratio of
SWNTs and the related area ratio $A_{\text{far}}/A_{\text{near}}$ for far and
near field. We start with the minimal laser spot diameter which can be
calculated as

$d=1.27\cdot\lambda\cdot M^{2}\cdot\frac{f}{D}\approx 900\,\text{nm}$ (3)

where $M^{2}$ is assumed to be $1$ for a perfect Gaussian spot shape, $f$ is 8
mm, the beam diameter $D$ is 6 mm, and $\lambda=532$ nm. While the theoretical
limit $d$ is very hard to achieve we estimate the spot size via the image
taken from camera C1, yielding a more realistic spot diameter of approximately
$d^{\prime}=5~{}\mu$m. The coverage $c$ of the gold film by SWNT may be
estimated on the basis of the STM topographic image in Fig. 6(b), revealing
approximately $c=0.8$. The near field signal confined to the vicinity of the
tip is characterized by an area of $\pi r^{2}$ with $r=1$ nm. Therefore, we
can write for the relevant area ratio:

$\frac{c\cdot\text{A}_{\text{far}}}{\text{A}_{\text{near}}}=\frac{c\cdot(d^{\prime})^{2}}{(2r)^{2}}=5\times
10^{6}.$ (4)

The resulting $g$-values are listed in Tab. 1.

## Author Contribution

L.P. and G.H contributed equally to the work. D.J. helped with the optical
alignment and the data analysis. G.R. developed the electrochemical etching of
the Au and Ag tips. R.H. conceived the experiment. L.P., G.H., and R.H. wrote
the paper.

## Data availability

The data that support the findings of this study are available from the
corresponding author upon reasonable request.

###### Acknowledgements.

This work was supported by the Deutsche Forschungsgemeinschaft (DFG) through
the coordinated program TRR80 (Projekt-ID 107745057) and project HA2071/12-1.
G. H. would like to thank the Alexander von Humboldt Foundation for a research
fellowship. We thank S. Geprägs and M. Müller for the fabrication of thin Au
and Ag films and R. Gross for his continuous interest in the work. Finally, we
gratefully acknowledge the technical support by attocube systems over the
entire period during which the setup has been assembled.

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# Blockchain-based certificate authentication system with enabling correction

Md. Mijanur Rahman  Dept. of Computer Science and Engineering
Southeast University
<EMAIL_ADDRESS>Saifur Rahman Shihab  Md Tanzinul Kabir Tonmoy
Dept. of Computer Science and Engineering
Southeast University
<EMAIL_ADDRESS>Dept. of Computer Science and Engineering
Southeast University
<EMAIL_ADDRESS>Riya Farhana  Dept. of Computer Science and
Engineering
Southeast University
<EMAIL_ADDRESS>
Abstract–Blockchain has proven to be an emerging technology in the digital
world, changing the way everyone thinks about data security and bringing
efficiency to several industries. It has already been applied to a wide range
of applications, from financial services and supply chain management to voting
systems and identity verification. An organization must verify its candidates
before selecting them. Choosing an unqualified candidate can ruin an
organization’s reputation. In this digital era, many key fraudulent schemes
are rampant in many companies and one of them is certificate fraud. It is
possible to validate a candidate’s qualifications using traditional methods,
but there are drawbacks such as security issues and time consumption. In this
paper, a blockchain-based academic certificate authentication system will be
used to ensure authenticity and make the assertion of the decentralized system
secure. However, the system will generate, authenticate and make corrections
on academic certificates. Ultimately, some blockchain-based authentication
systems already exist, they can’t correct any errors that occur during
generation. The proposed system will help in many ways, such as providing a
user-friendly university admission, and smooth job hiring process, etc. In
conclusion, our proposed system can permanently eradicate certificate
forgeries and create and promote trust in society.

Keywords—Blockchain, Certificate Authentication, Modifiable System.

##  I INTRODUCTION

In our daily lives, certificates play an invaluable role [1]. It is a document
that is provided to a student when he/she graduates from a school, college or
university. Human capital is defined as the knowledge, skills, and abilities
gained via education [2]. A student’s credentials serve as a testament to his
or her diligent work over the years. Those graduate students need these
certificates to apply for higher education or to obtain employment in
respectable places. In both circumstances, they need to provide their
credentials for validation. To validate, institutes check the authenticity of
certificates provided by candidates. After validation, candidates proceed to
the next stage. To validate the authenticity of a certificate, the institute
uses a third-party system or direct contacts with the institute from which it
was generated which is often a time-consuming and complicated process.

There are currently 2 million fake degree certificates in circulation in the
United States and 300 unauthorized universities operating [3]. If proper
validation was not conducted, then any unworthy candidates who had resorted to
forgery of their qualifications would be hired. The reason behind certificate
forgery is that many unethical people want to get their dream job without
having the necessary qualifications, so they provide fake certificates in
order to deceive the hiring company. This makes the validation process much
more difficult and time-consuming, as the hiring company must now carefully
scrutinize the qualifications of each candidate to ensure their legitimacy. A
study shows that almost 10$\%$ of applications given by candidates are forged
[4]. Applicants tend to lie about their education and experience [5]. Every
year, academic certificate fraud costs employers approximately $\$$600 billion
[6].

“The phenomenon of fake academic degrees is a threat to any community. It is
an enormous threat, both for the present and the future” [7]. Nowadays there
are numerous cases of certificate forging in the news. In recent years,
certificate forgeries have increased due to advances in scanning and printing
technologies. As a result, the integrity of both the certificate holder and
the institute that issued the certificate is at risk. [8]. For 28 years, the
Massachusetts Institute of Technology’s dean of admissions was found to have
faked and misled the institute about her academic qualifications [9].

Certificate forging is also causing problems in the medical field. Many people
pretend to be a doctor by forging fake certificates. As a result, people from
all socioeconomic backgrounds will not receive equal treatment when they visit
different types of doctors [10]. A similar report was published in Bangladesh.
It was for using expired reagents and selling unapproved drugs. Seven
employees, including two fake doctors, were sentenced to indeterminate jail
terms and penalties for taking high charges from patients and providing fake
medical reports [11]. In Mexico, there are some aesthetic pseudo-clinics with
non-medical personnel performing surgery on patients which caused many
complications [12].

Blockchain is one of the most popular technologies that transform the way we
live at the moment. Blockchain is a decentralized database that contains
records called a block. Each block contains its timestamp, hash or address of
the previous block and the data of the block. Each block has a unique hash
that can be used to track back to its previous block. A hash is a function
that converts a set of data into a fixed-size data structure, called a hash
value. Blockchain technology has several applications, including
cryptocurrency, smart contracts, supply chain management, and more. Stuart
Haber and W. Scott Stornetta, two mathematicians who wanted to implement a
system where document timestamps could not be tampered with, proposed
blockchain technology in 1991 [13]. Since the information on the blockchain is
unchangeable, it gives assurance that no one can tamper with the information.
If any attempt is made, it can easily be identified because of the changes in
the hash value [13]. As the authors point out [14], traditional authentication
methods do not provide security, tamper-proofing, or authentication for
documents. For this reason, blockchain is best suited to our system.

Based on this study, we proposed a system to resolve a crucial pair of
problems in the world of education and job hiring using blockchain technology.
The system will verify whether the certificates are given by any legitimate
organization or academy. The system will permanently store all the credentials
securely, making the authentication process much easier and more convenient,
and eliminating the crime of certificate forging. The system will also have
correction functionality so that if any modifications need to be done, the
authority can easily accomplish them.

##  II LITERATURE REVIEW

Blockchain was introduced long before Bitcoin was introduced by Satoshi
Nakamoto[15], [16]. However, blockchain gained its popularity with Bitcoin.
Although bitcoin is sometimes referred to as a blockchain by some people, but
the fact is bitcoin was built using blockchain technology. In a short period
of time, blockchain became popular when people discovered its salient
benefits. It has become one of the top technologies worldwide. For this
reason, many studies and research were conducted and this proposed system
rewired some of them.

Authors of [17] have made some excellent points about blockchain architecture
and consensus algorithms. They have also examined the limitations of
blockchain and given some possible solutions. They also investigate various
consensus algorithms in different respects and discuss their differences. By
analyzing those differences, they can determine the most appropriate consensus
algorithm.

The author of [4] Rishabh Garg, has examined how blockchain technology can be
applied to solve problems within academic institutes and the employment
sector. According to the author, blockchain will be the panacea for this
issue. He proposed a framework that can issue new certificates as well as
validate them.

In Osman Ghazali and Omar S. Sale [18], emphasized the significance of
certificate verification and its influence in our society. In the paper,
traditional verification processes were briefly explained, as well as their
limitations. To eliminate those limitations, they proposed a blockchain-based
verification system. A system that is not only used for verification but also
for generating new certificates. They will generate digitally signed
certificates using the asymmetric key and timestamp. Students will receive a
copy, which they can verify in the system.

Jayesh G. Dongre and his colleagues proposed[19] to solve the problems of the
current system of certificate verification. In the paper, they talked about
the current verification process and the proliferation of certificate fraud.
Using blockchain, they have developed a platform for validating and generating
certificates. In their view, the use of blockchain for certificate
verification is beneficial to society. It will eliminate certificate fraud.

R.Suganthalakshmi[20] and her colleagues investigated an optimal method of
verifying academic credentials in 2022. The number of graduating students is
increasing every year, and it is becoming increasingly necessary to validate
their academic credentials. Without proper validation, any ineligible
candidate will be blessed with opportunities. Their solution is to create a
platform for all certificates a student may possess. Students upload all their
credentials to the system, and the system stores them on the blockchain. To
verify a credential, a person needs the student’s ID and password. They used
the PoW(Proof of work) consensus algorithm for validation, but the validation
method was not well elucidated.

In the papers mentioned, researchers are interested in building a system to
verify and generate academic credentials. Although their system design is
different from each other but they all agree on using blockchain for security.
Our proposed system can also generate and verify academic credentials, as well
as modify them. Making errors is a part of human nature. Our study found that
none of the papers offered editing options if any modifications were needed.
We provide an editing facility option to fix those errors.

##  III METHODOLOGY

In this study, the general idea of the proposed system is depicted in Fig-1.
There will be two actors, the university (Admin), and other users
(Students/Employer). The university is authorized to generate new certificates
for students, make corrections if necessary, and authenticate the
certificates. On the other hand, general users are allowed only to confirm the
authenticity or view the certificates. The user in this category can’t make
any corrections or generate new certificates. We utilize two chains in our
system. One is for storing certificate data and the other is for keeping track
of corrections.

Figure 1: System Architecture

The certificate generation process is shown in Fig-2. In this system, everyone
can check the authenticity of the certificates but not everyone can generate
or add new certificates to the system. Only authorized persons will be
allowed. First, an authorized person needs to prove their identity by login
in. After proving their identity, they can add a new certificate to the
blockchain. To add a certificate, the user may need to fill up a form. In that
form, they have to provide the information of the candidate. After hitting
submission, the system will generate a new block and store the data on the
certificate chain. After that, it will return the block address to the
university and the student.

Figure 2: Generating new certificates.

PESUDO CODE 1: Function to generate Certificate

function generate Certificate(Arguments) public {
check if authorized to generate a Certificate then
store the certificate return the address of the certificate
}

The certificate correction process is depicted in Fig-3. To correct a
certificate, we need to provide the system with the address of the old block
and the updated data. After getting those two pieces of information our system
will generate two new blocks. The first block will contain corrected data and
it will be stored in the certificate chain. The second block will contain the
hash address of the old certificate and the hash address of the corrected
block. However, the modified block will be stored in the correction chain as a
reference point.

Fig-3: Certificate correction

PESUDO CODE 2: Function to make correction

function correct Certificate (Argument old_Certificate_addressother arguments)
public {
Check if authorized to perform the correction.
Then check if old_Certificate_address exists on the certificate chain.Store
new certificate on certificate chain.
Store old and new certificate addresses on the correction chain.
return the corrected certificate address.
}

The validation-checking process is shown in Fig-4. In our system, a
certificate can be checked in two ways. This can either be done by manual
typing or scanning the QR code. QR code will contain the hash address of the
block where the data is stored. Apparently, the system will look for the hash
address in the correction chain. Upon finding a match, the system will get the
corresponding new certificate hash from the block. Later, it will search for
the hash in the certificate chain and display the result. When the system is
not able to locate the hash address in the correction chain, it searches the
certificate chain and displays the result. If the system could not locate the
hash address in both chains, then it will show certificate does not exist.

Fig-4: Checking the authenticity

PESUDO CODE 3: Function to Authenticate

function authenticate (Argument address) public {
check if exists on the correction chain
then
get a new certificate address and check if
a new certificate address exists on the
certificate chain
then return the corrected certificate.
else return ”Not Exist”
else check if exists on the certificate chain then
return the certificate
else return ”Certificate does not exist”
}

SHA-256: SHA-256 is a part of SHA-2 (Secure Hash Algorithm 2). It is a popular
hashing algorithm. A cryptographic hash, known as a fingerprint, or signature,
is a nearly identical string of characters generated from a different piece of
input text. SHA-256 generates a 256-bit signature. SHA-2 is widely known for
its security (it hasn’t weakened as much as SHA-1) [21].

Smart Contract: Nick Szabo introduced this concept in 1994 and defined it as a
smart contract [22]. A smart contract is nothing but a computer procedure that
verifies, facilitates, or enforces renegotiation or efficiency. Smart
contracts enable the execution of credible exchanges without the involvement
of outsiders such as lawyers or notaries. On the blockchain, smart contracts
execute exactly as programmed, with no potential for censorship, shutdown,
forgery, or third-party interface [23]. Smart contracts operate by executing
basic ”if/when…then…” phrases encoded into blockchain code. When preset
conditions are met and validated, a network of computers executes the actions.
When the transaction is finished, the blockchain will be updated. This
indicates that the transaction is final and that only parties to whom
permission has been granted can view the outcome [24].

Solidity: Solidity is a high-level contract-oriented language for creating a
smart contract. It was inspired by C++, Python, and JavaScript and is intended
for use with the Ethereum Virtual Machine. Solidity is a programming language
that is similar to OOP (object-oriented programming) [12]. The significance of
Solidity programming for the Ethereum blockchain is based on the ability to
develop industry-grade blockchain applications. The Ethereum Network team
created it specifically for designing and creating smart contracts on
blockchain platforms [21].

##  IV RESULT AND DISCUSSION

In our proposed system, we adopted Remix IDE for writing and developing our
smart contract. For testing, we deployed the smart contract on Remix’s virtual
network. Our study estimated three primary functions: (1) Generate
Certificate, (2) Correct Certificate and (3) Authenticate Certificate. Upon
deploying the smart contract in the Remix virtual machine, it gives us
information about the transaction hash, gas fee, and more which is illustrated
in Fig-5.

Fig-5: Deploy on Remix IDE

After successfully testing the smart contract on Remix’s virtual network, we
deploy the code on the Goerlitestnet network. During the deployment phase, it
asks for confirmation. Our metamask wallet confirms it. After successfully
deploying the smart contract on the blockchain, we obtained the address and
the ABI (Application Binary Interface) of the smart contract from Remix IDE
and the medium of our front-end connection with the blockchain.

Fig-6: Deploy to Goerlitestnet

Fig-7: Web Application

Fig-7 shows the frontend web application of the proposed system.

##  V CONCLUSION

Because many candidates fabricate certificates, traditional validation methods
are facing new challenges in the digital age. Fabrication of certificates
occurs in order to gain admission to prestigious universities or to obtain a
satisfactory job. To overcome those challenges the validation process needs to
be more advanced and secure. Blockchain technology is ideal for tackling those
challenges. Due to the enormous characteristics of blockchain, such as
decentralization, census, and security, it has become one of the leading
technologies in recent years. Our proposed system is built on a blockchain
architecture. It will not only validate certificates but also has the ability
to generate brand-new certificates for students. Other blockchain-based
verification platforms only offer verification and the ability to generate new
certificates, but don’t allow the correction of certificates. However, during
the process of creating the certificate, a mistake can occur. If there is no
way to make up for those mistakes, then it is not an ideal platform. In this
proposed system if any mistakes were made, they can be fixed and by
eliminating certificate forgeries and creating a trustworthy environment, the
system will guarantee the integrity of certificates.

As the system develops, it can be expanded to include blockchain-based
professional degree management, implementation of a NID (National Identity
Card) verification system, and other blockchain-based systems.

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