 stratified structures themselves gives rise to an extremely flexible system that, moreover, allows the facile variation of a number of important material parameters as well as the preparation of samples on large scales. This flexibility was used to modify the coupling distance between arrays, showing that both the positions and relative intensities of the resonances observed can be tuned with a high degree of precision. Our work renders research in the field of â€œplasmonic moleculesâ€� mature to the extent that it could be incorporated into functional optical devices.
Processing dye-sensitized solar cells gains more and more importance as interest in industrial applications grows daily. For large-scale processing and optimizing manufacturing in terms of environmental acceptability as well as time and material saving, a detailed knowledge of certain process steps is crucial. In this paper we concentrate on the sensitizing step of production, i.e., the anchoring of the dye molecules onto the TiO2 semiconductor. A vacuum-tight attentuated total reflection infrared (ATR-IR) flow-through cell was developed, thus allowing measurements using a vacuum spectrometer to monitor infiltration of dye molecules into the porous TiO2 film in situ at high sensitivity. In particular, the influence of the anchor and backbone of perylene dye molecules as well as the influence of solvents on the adsorption process was investigated. The experiments clearly show that an anhydride group reacts much slower than an acid group. A significantly lower amount of anhydride dye can be adsorbed on the films. Ex situ transmission experiments furthermore indicate that the availability of OH groups on the TiO2 surface may limit dye adsorption. Also the backbone and base frame of the dye can influence the adsorption time drastically. Electrical cell characteristics correlate with the amount of adsorbed dye molecules determined by in situ ATR-IR measurements. The latter is also sensitive toward the diffusion of the dye through the porous layer. To gain a deeper understanding of the interplay between diffusion and adsorption, simulations were performed that allowed us to extract diffusion and adsorption constants. Again it was demonstrated that the anchoring group has a strong effect on the adsorption rate. The influence of the solvent was also studied, and it was found that both adsorption and desorption are affected by the solvent. Protic polar solvents are able to remove bound dye molecules, which is a possible pathway of cell degradation. Most importantly, the analysis shows the potential of this approach for the evaluation of molecules or additives concerning their characteristics important for cell processing.
Thiolate-protected gold nanoparticles and clusters combine size-dependent physical properties with the ability to introduce (bio)chemical functionality within their ligand shell. The engineering of the latter with molecular precision is an important prerequisite for future applications. A key question in this respect concerns the flexibility of the gold â€“ sulfur interface. Here we report the first study on racemization of an intrinsically chiral gold nanocluster, Au38(SCH2CH2Ph)24, which goes along with a drastic rearrangement of its surface involving place exchange of several thiolates. This racemization takes place at modest temperatures (40 â€“ 80 Â°C) without significant decomposition. The experimentally determined activation energy for the inversion reaction is ca 28 kcal/mol, which is surprisingly low considering the large rearrangement. The activation parameters furthermore indicate that the process occurs without complete Au-S bond breaking.
Chirality unveiled: Thiolate-protected Au40(SR)24 clusters were enantioenriched using an HPLC approach. CD spectra show strong mirror-image responses, indicating the intrinsic chirality of a cluster of unknown structure protected with achiral ligands.
Silver nanoclusters protected by 2-phenylethanethiol (1), 4-fluorothiophenol (2), and l-glutathione (3) ligands were successfully synthesized. The optical properties of the prepared silver nanoclusters were studied. The absorption signal of Ag@SCH2CH2Ph in toluene can be found at 469 nm, and Ag@SPhF in THF shows two absorption bands at 395 and 462 nm. Ag@SG in water absorbs at 478 nm. Mie theory in combination with the Drude model clearly indicates the peaks in the spectra originate from plasmonic transitions. In addition, the damping constant as well as the dielectric constant of the surrounding medium was determined. In addition, the CD spectra of silver nanoclusters protected by the three ligands (1â€“3) were also studied. As expected, only the clusters of type 3 gave rise to chiroptical activity across the visible and near-ultraviolet regions. The location and strength of the optical activity suggest an electronic structure of the metal that is highly sensitive to the chiral environment imposed by the glutathione ligand. The morphology and size of the prepared nanoclusters were analyzed by using transmission electron microscopy (TEM). TEM analysis showed that the particles of all three types of silver clusters were small than 5 nm, with an average size of around 2 nm. The analysis of the FTIR spectra elucidated the structural properties of the ligands binding to the nanoclusters. By comparing the IR absorption spectra of pure ligands with those of the protected silver nanoclusters, the disappearance of the Sâ€“H vibrational band (2535â€“2564 cmâ€“1) in the protected silver nanoclusters confirmed the anchoring of ligands to the cluster surface through the sulfur atom. By elemental analysis and thermogravimetric analysis, the Ag/S ratio and, hence, the number of ligands surrounding a Ag atom could be determined.
Ligand exchange reactions on size-selected Au38(2-PET)24 and Au40(2-PET)24 clusters (2-PET: 2-phenylethylthiol) with mono- and bidentate chiral thiols was performed. The reactions were monitored with MALDI mass spectrometry and the arising chiroptical properties were compared to the number of incorporated chiral ligands. Only a small fraction of chiral ligands is needed to induce significant optical activity to the clusters. The use of monodentate camphor-10-thiol (CamSH) leads to comparably fast exchange on both clusters. The arising optical activity is weak. In contrast, the use of bidentate 1,1â€™-binaphthyl-2,2â€™-dithiol (BINAS) is slow, but the optical activity measured is strong. Moreover, a non-linear behaviour between optical activity and number of chiral ligands is found in the BINAS case for both Au38 and Au40, which may indicate different exchange rates of enantiopure BINAS with the enantiomers of inherently chiral clusters. This is ascribed to effects arising from the bidentate nature of BINAS. This is the first study where chiroptical effects are directly correlated with the composition of the ligand shell.
N-Aryl,Â N-branched alkyl carbamates react withÂ in situÂ generated chiral Pd-NHC catalysts by coupling a Pd-Ar moiety with an aliphatic Câ€“H bond at high temperature to give enantioenriched 2-substituted and 2,3-disubstituted indolines. Prochiral precursors give single products with very high asymmetric induction. Chiral racemic precursors react in a regiodivergent reaction of a racemic mixture to yield enantioenriched indolines resulting from either methyl Câ€“H activation or asymmetric methylene Câ€“H activation. In favorable cases this can result in a complete separation of an enatiomeric mixture into two different highly enantioenriched indolines.
Chiral gold clusters stabilised by enantiopure thiolates were prepared, size-selected and characterised by Circular Dichroism and mass spectrometry. The product distribution is found to be ligand dependent. Au25 clusters protected with camphorthiol show clear resemblance of their chiroptical properties with their glutathionate analogue.
We investigate the optical properties of a true three-dimensional metamaterial that was fabricated using a self-assembly bottom-up technology. The metamaterial consists of closely packed spherical clusters being formed by a large number of non-touching gold nanoparticles. After presenting experimental results, we apply a generalized Mie theory to analyze its spectral response revealing that it is dominated by a magnetic dipole contribution. By using an effective medium theory we show that the fabricated metamaterial exhibits a dispersive effective permeability, i.e. artificial magnetism. Although this metamaterial is not yet left-handed it might serve as a starting point for achieving bulk metamaterials by using bottom-up approaches.
We theoretically analyze, fabricate, and characterize a three-dimensional plasmonic nanostructure that exhibits a strong and isotropic magnetic response in the visible spectral domain. Using two different bottom-up approaches that rely on self-organization and colloidal nanochemistry, we fabricate clusters consisting of dielectric core spheres, which are smaller than the wavelength of the incident radiation and are decorated by a large number of metallic nanospheres. Hence, despite having a complicated inner geometry, such a coreâ€“shell particle is sufficiently small to be perceived as an individual object in the far field. The optical properties of such complex plasmonic coreâ€“shell particles are discussed for two different core diameters.
Size Exclusion Chromatography (SEC) on a semi-preparative scale (10 mg and more) was used to size-select ultrasmall gold nanoclusters (< 2 nm) from polydisperse mixtures. In particular, the ubiquitous byproducts of the etching process towards Au38(SR)24 (SR: thiolate) clusters were separated and gained in high monodispersity (based on mass spectrometry). The isolated fractions were characterized by UV/Vis spectroscopy, MALDI mass spectrometry and electron microscopy. Most notably, the separation of Au38(SR)24 and Au40(SR)24 clusters is demonstrated.
Nanofluids have been proposed to improve the performance of microchannel heat sinks. In