Facile SILAR Approach to Air-Stable Naked Silver and Gold Nanoparticles Supported by Alumina

A synthetically convenient and scalable SILAR (successive ion layer adsorption and reaction) method is used to make air-stable films of silver and gold nanoparticles supported on alumina scaffolds. This solution-based deposition technique yields particles devoid of insulating capping agents or ligands. The optical properties of the nanoparticle films were investigated using femtosecond transient absorption spectroscopy. A linear absorption arising from intraband excitation (775 nm laser pulse) is seen only for Au nanoparticles at low intensity. However, both Au and Ag particles exhibit plasmon resonance responses at high excitation intensity via two photon absorption of the 775 nm pump pulse. The difference in optical response to near-IR laser excitation is rationalized based on the known density of states for each metal. To demonstrate the potential applications of these films, alumina-supported Ag nanoparticles were utilized as substrates for surface enhanced Raman spectroscopy, resulting in a 65-fold enhancement in the Raman signal of the probe molecule rhodamine 6G. The exceptional stability and scalability of these SILAR films opens the door for further optical and photocatalytic studies and applications, particularly with ligand-free Ag nanoparticles that typically oxidize under ambient conditions. Additionally, isolating plasmonic and interband electronic excitations in stable AgNP under visible light irradiation could enable elucidation of the mechanisms that drive noble metal-assisted photocatalytic processes

Two Distinct Transitions in Cu_<i>x</i>InS₂ Quantum Dots. Bandgap versus Sub-Bandgap Excitations in Copper-Deficient Structures

Cu-deficient CuInS₂ quantum dots (QDs) synthesized by varying the [Cu]:[In] ratio allow modulation of optical properties as well as identification of the radiative emission pathways. Absorption and emission spectral features showed a strong dependence on the [Cu]:[In] ratio of Cu_<i>x</i>InS₂ QDs, indicating two independent optical transitions. These effects are pronounced in transient absorption spectra. The bleaching of band edge absorption and broad tail absorption bands in the subpicosecond–nanosecond time scale provide further evidence for the dual optical transitions. The recombination process as monitored by photoemission decay indicated the involvement of surface traps in addition to the bandgap and sub-bandgap transitions. Better understanding of the origin of the optical transitions and their influence on the photodynamics will enable utilization of ternary semiconductor quantum dots in display and photovoltaic devices

How Lead Halide Complex Chemistry Dictates the Composition of Mixed Halide Perovskites

Varying the halide ratio (e.g., Br^–:I^–) is a convenient approach to tune the bandgap of organic lead halide perovskites. The complexation between Pb²⁺ and halide ions is the primary step in dictating the overall composition, and optical properties of the annealed perovskite structure. The complexation between Pb²⁺ and Br^– is nearly 7 times greater than the complexation between Pb²⁺ and I^–, thus making Br^– a dominant binding species in mixed halide systems. Emission and transient absorption measurements show a strong dependence of excited state behavior on the composition of halide ions employed in the precursor solution. When excess halide (X = Br^– and I^–) are present in the precursor solution (0.3 M PbX₂ and 0.9 M CH₃NH₃X), the exclusive binding of Pb²⁺ with Br^– results in the formation of CH₃NH₃PbBr₃ perovskites as opposed to mixed halide perovskite

Boosting the Photovoltage of Dye-Sensitized Solar Cells with Thiolated Gold Nanoclusters

Glutathione-capped gold nanoclusters (Au_<i>x</i>-GSH NCs) are anchored along with a sensitizing squaraine dye on a TiO₂ surface to evaluate the cosensitizing role of Au_<i>x</i>-GSH NCs in dye-sensitized solar cells (DSSCs). Photoelectrochemical measurements show an increase in the photoconversion efficiency of DSSCs when both sensitizers are present. The observed photoelectrochemical improvements in cosensitized DSSCs are more than additive effects as evident from the increase in photovoltage (Δ<i>V</i> as high as 0.24 V) when Au_<i>x</i>-GSH NCs are present. Electron equilibration and accumulation within gold nanoclusters increase the quasi-Fermi level of TiO₂ closer to the conduction band and thus decrease the photovoltage penalty. A similar beneficial role of gold nanoclusters toward boosting the <i>V</i>_oc and enhancing the efficiency of Ru­(II) polypyridyl complex-sensitized solar cells is also discussed

Size-Dependent Photovoltaic Performance of CuInS₂ Quantum Dot-Sensitized Solar Cells

The optical and electronic properties of quantum dots (QDs), which are drastically affected by their size, have a major impact on their performance in devices such as solar cells. We now report the size-dependent solar cell performance for CuInS₂ QDs capped with 1-dodecanethiol. Pyramidal shaped CuInS₂ QDs with diameters between 2.9 and 5.3 nm have been synthesized and assembled on mesoscopic TiO₂ films by electrophoretic deposition. Time-resolved emission and transient absorption spectroscopy measurements have ascertained the role of internal and surface defects in determining the solar cell performance. An increase in power conversion efficiency (PCE) was observed with the increasing size of QDs, with maximum values of 2.14 and 2.51% for 3.9 and 4.3 nm size particles, respectively. The drop in PCE observed for larger QDs (5.3 nm) is attributed to decreased charge separation following bandgap excitation. Because the origin of photocurrent generation in CuInS₂ QDSC arises from the defect-dominated charge carriers, it offers the opportunity to further improve the efficiency by controlling these defect concentrations

Interplay between Size, Composition, and Phase Transition of Nanocrystalline Cr³⁺-Doped BaTiO₃ as a Path to Multiferroism in Perovskite-Type Oxides

Multiferroics, materials that exhibit coupling between spontaneous magnetic and electric dipole ordering, have significant potential for high-density memory storage and the design of complex multistate memory elements. In this work, we have demonstrated the solvent-controlled synthesis of Cr³⁺-doped BaTiO₃ nanocrystals and investigated the effects of size and doping concentration on their structure and phase transformation using X-ray diffraction and Raman spectroscopy. The magnetic properties of these nanocrystals were studied by magnetic susceptibility, magnetic circular dichroism (MCD), and X-ray magnetic circular dichroism (XMCD) measurements. We observed that a decrease in nanocrystal size and an increase in doping concentration favor the stabilization of the paraelectric cubic phase, although the ferroelectric tetragonal phase is partly retained even in ca. 7 nm nanocrystals having the doping concentration of ca. 5%. The chromium­(III) doping was determined to be a dominant factor for destabilization of the tetragonal phase. A combination of magnetic and magneto-optical measurements revealed that nanocrystalline films prepared from as-synthesized paramagnetic Cr³⁺-doped BaTiO₃ nanocrystals exhibit robust ferromagnetic ordering (up to ca. 2 μ_B/Cr³⁺), similarly to magnetically doped transparent conducting oxides. The observed ferromagnetism increases with decreasing constituent nanocrystal size because of an enhancement in the interfacial defect concentration with increasing surface-to-volume ratio. Element-specific XMCD spectra measured by scanning transmission X-ray microscopy (STXM) confirmed with high spatial resolution that magnetic ordering arises from Cr³⁺ dopant exchange interactions. The results of this work suggest an approach to the design and preparation of multiferroic perovskite materials that retain the ferroelectric phase and exhibit long-range magnetic ordering by using doped colloidal nanocrystals with optimized composition and size as functional building blocks