Nd3+:Y3Al5O12 laser ceramics: Influence of the size of yttrium oxide particles on sintering

The influence of the size of Y₂O₃ powder particles on the structure formation and densification of Nd³⁺:Y₃Al₅O₁₂ laser ceramics has been studied. It is shown that the use of 50- and 100-nm yttrium oxide particles makes it possible to synthesize single-phase yttrium aluminum garnet at temperatures of 1200 and 1500°C, respectively, whereas in the case of 5000-nm yttrium oxide particles 2-h exposure at a temperature of 1500°C yields only 80 wt % of the Nd³⁺:Y₃Al₅O₁₂ phase. Bulk swelling of pressed samples during sintering of 2.94Y₂O₃-0.06Nd₂O₃-5Al₂O₃ powders with the size ratio of the initial particles R(Al₂O₃/Y₂O₃) ～ 5 is observed. The application of different-sized powders (R ～ 2.5) provides quantitative ratios between phases in the 3Y₂O₃-5Al₂O₃ system at which shrinkage in a temperature range of 20–1500°C is dominant. Laser ceramics 0–2 at % Nd³⁺:Y₃Al₅O₁₂ have been obtained by the solid-phase sintering of oxide powders (R ～ 2.5). The slope efficiency for 1 at % Nd³⁺:Y₃Al₅O₁₂ laser ceramics is found to be 33%.

     

Molecular self-assembly and passivation of GaAs (0 0 1) with alkanethiol monolayers: A view towards bio-functionalization

Properties of as prepared or nanoengineered III-V semiconductor surfaces provide attractive means for photonic detection of different adsorbants from surrounding gaseous or liquid environments. To be practical, this approach requires that the surface is made selectively sensitive (functionalized) to targeted species. In addition, such surface has also to stay stable over extended period of time to make it available for rapid testing. Numerous reports demonstrate attractive properties of GaAs for sensing applications. One of the most fundamental issues relevant to these applications concerns the ability to functionalize chemically, or biologically, the surface of GaAs. The most studied method of GaAs surface functionalization is based on formation of self-assembled monolayers (SAMs) of various n-alkanethiols, HS-(CH₂)_n-T (T = CH₃, COOH, NH₂, etc.). In spite of multi-year research concerning this issue, it has only been recently that a comprehensive picture of SAMs formation on GaAs and an understanding of the natural limitation of the SAM-GaAs interface in some bio-chemical sensing architectures has begun to emerge.     

Pure Cubic‐Phase Hybrid Iodobismuthates AgBi2I7 for Thin‐Film Photovoltaics

Bismuth‐based hybrid perovskites are candidates for lead‐free and air‐stable photovoltaics, but poor surface morphologies and a high band‐gap energy have previously limited these hybrid perovskites. A new materials processing strategy to produce enhanced bismuth‐based thin‐film photovoltaic absorbers by incorporation of monovalent silver cations into iodobismuthates is presented. Solution‐processed AgBi₂I₇ thin films are prepared by spin‐coating silver and bismuth precursors dissolved in n‐butylamine and annealing under an N₂ atmosphere. X‐ray diffraction analysis reveals the pure cubic structure (Fd3m) with lattice parameters of a=b=c=12.223 Å. The resultant AgBi₂I₇ thin films exhibit dense and pinhole‐free surface morphologies with grains ranging in size from 200–800 nm and a low band gap of 1.87 eV suitable for photovoltaic applications. Initial studies produce solar power conversion efficiencies of 1.22 % and excellent stability over at least 10 days under ambient conditions.     

Structure of thiol self-assembled monolayers commensurate with the GaAs (001) surface

Observed properties of thiol self-assembled monolayers (SAMs) on GaAs (001) surfaces can be explained by the presence of surface reconstructions, but their exact form is generally unknown. We propose a new approach to modeling the SAM-surface interface based on using alkanethiol dense packing structures as a starting point and adjusting the surface reconstruction to accommodate them. Obtained in such a way, model SAMs adsorb along the trenches in the [110] direction and exhibit a 19 degrees tilt and +/- 45 degrees twist angles, in agreement with available experimental data. The molecules of the SAM bind to both Ga and As, and cover only 50% of the available surface sites. The requirements for the SAM formation process to achieve the proposed structures are discussed.     

Gold adatom as a key structural component in self-assembled monolayers of organosulfur molecules on Au(1 1 1)

Chemisorption of organosulfur molecules, such as alkanethiols, arenethiols and disulfide compounds on gold surfaces and their subsequent self-organization is the archetypal process for molecular self-assembly on surfaces. Owing to their ease of preparation and high versatility, alkanethiol self-assembled monolayers (SAMs) have been widely studied for potential applications including surface functionalization, molecular motors, molecular electronics, and immobilization of biological molecules. Despite fundamental advances, the dissociative chemistry of the sulfur headgroup on gold leading to the formation of the sulfur–gold anchor bond has remained controversial. This review summarizes the recent progress in the understanding of the geometrical and electronic structure of the anchor bond. Particular attention is drawn to the involvement of gold adatoms at all stages of alkanethiol self-assembly, including the dissociation of the disulfide (S–S) and hydrogen-sulfide (S–H) bonds and subsequent formation of the self-assembled structure. Gold adatom chemistry is proposed here to be a unifying theme that explains various aspects of the alkanethiol self-assembly and reconciles experimental evidence provided by scanning probe microscopy and spectroscopic methods of surface science. While several features of alkanethiol self-assembly have yet to be revisited in light of the new adatom-based models, the successes of alkanethiol SAMs suggest that adatom-mediated surface chemistry may be a viable future approach for the construction of self-assembled monolayers involving molecules which do not contain sulfur.     

Structure, bonding nature, and binding energy of alkanethiolate on As-rich GaAs (001) surface: a density functional theory study

Chemisorption of alkanethiols on As-rich GaAs (001) surface under a low coverage condition was studied using first principles density functional calculations in a periodic supercell approach. The thiolate adsorption site, tilt angle and its direction are dictated by the high directionality of As dangling bond and sulfur 3p orbital participating in bonding and steric repulsion of the first three CH2 units from the surface. Small charge transfer between thiolate and surface, strong dependence of total energy on tilt angle, and a relatively short length of 2.28 A of the S-As bond indicate the highly covalent nature of the bonding. Calculated binding energy of 2.1 eV is consistent with the available experimental data.