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Metal-to-Semiconductor Transition and Electronic Dimensionality Reduction of Ca 2 N Electride under Pressure
Abstract not provided
Structural, Optical, and Electronic Properties of Two Quaternary Chalcogenide Semiconductors: Ag 2 SrSiS 4 and Ag 2 SrGeS 4
Quaternary chalcogenide materials have long been a source of semiconductors for optoelectronic applications. Recent studies on the I2-II-IV-X4 (I = Ag, Cu, Li; II = Ba, Sr, Eu, Pb; IV = Si, Ge, Sn; X = S, Se) materials have shown particular versatility and promise among these compounds. These semiconductors take advantage of a diverse bonding scheme and chemical differences among cations to target a degree of anti-site defect resistance. Within this set of compounds, the materials containing both Ag and Sr have not been experimentally studied and leave a gap in the full understanding of the family. Here, we have synthesized powders and single crystals of two Ag- and Sr-containing compounds, Ag 2 SrSiS 4 and Ag 2 SrGeS 4 , each found to form in the tetragonal I4¯2m structure of Ag 2 BaGeS 4 . During the synthesis targeting the title compounds, two additional materials, Ag 2 Sr 3 Si 2 S 8 and Ag 2 Sr 3 Ge 2 S 8 , have also been identified. Furthermore, these cubic compounds represent impurity phases during the synthesis of Ag 2 SrSiS 4 and Ag 2 SrGeS 4 . We show through hybrid density functional theory calculations that Ag 2 SrSiS 4 and Ag 2 SrGeS 4 have highly dispersive band edge states and indirect band gaps, experimentally measured as 2.08(1) and 1.73(2) eV, respectively. Second-harmonic generation measurements on Ag 2 SrSiS 4 and Ag 2 SrGeS 4 powders show frequency doubling capabilities in the near-infrared range.
Quaternary Chalcogenide Semiconductors with 2D Structures: Rb 2 ZnBi 2 Se 5 and Cs 6 Cd 2 Bi 8 Te 17
In this work, two new layered compounds Rb 2 ZnBi 2 Se 5 and Cs 6 Cd 2 Bi 8 Te 17 are described. Rb 2 ZnBi 2 Se 5 crystallizes in the orthorhombic space group Pnma, with lattice parameters of a = 15.6509(17) Å, b = 4.218(8) Å, and c = 18.653(3) Å. Cs 6 Cd 2 Bi 8 Te 17 crystallizes in the monoclinic C2/m space group, with a = 28.646(6) Å, b = 4.4634(9) Å, c = 21.164(4) Å, and β = 107.65(3)°. The two structures are different and composed of anionic layers which are formed by inter connecting of BiQ 6 octahedra (Q = Se or Te) and MQ 4 (M = Zn or Cd) tetrahedra. The space between the layers hosts alkali metal as counter cations. The rubidium atoms of Rb 2 ZnBi 2 Se 5 structure can be exchanged with other cations (Cd 2+ , Pb 2+ and Zn 2+ ) in aqueous solutions forming new phases. Rb 2 ZnBi 2 Se 5 is an n-type semiconductor and exhibits an indirect band gap energy of 1.0 eV. Rb 2 ZnBi 2 Se 5 is a congruently melting compound (mp ~644 °C). The thermal conductivity of this semiconductor is very low with 0.38 W· m -1 ·K -1 at 873 K. Density functional theory (DFT) calculations suggest that the low lattice thermal conductivity of Rb 2 ZnBi 2 Se 5 is attributed to heavy Bi atom induced slow phonon velocities and large Gruneisen parameters especially in the a and c directions. The thermoelectric properties of Rb 2 ZnBi 2 Se 5 were characterized with the highest ZT value of ~0.25 at 839 K.
Discovery of a Wurtzite-like Cu 2 FeSnSe 4 Semiconductor Nanocrystal Polymorph and Implications for Related CuFeSe 2 Materials
I 2 -II-IV-VI 4 and I-III-VI 2 semiconductor nanocrystals have found applications in photovoltaics and other optoelectronic technologies because of their low toxicity and efficient light absorption into the near-infrared. Herein, we report the discovery of a metastable wurtzite-like polymorph of Cu 2 FeSnSe 4 , a member of the I 2 -II-IV-VI 4 family of semi-conductors containing only Earth-abundant metals. Density functional theory calculations on this metastable poly-morph of Cu 2 FeSnSe 4 indicate that it may be a superior semiconductor for solar energy and optoelectronics applications compared to the thermodynamically preferred stannite polymorph, since the former displays a sharper dispersion of energy levels near the conduction band minimum that can enhance electron mobility and suppress hot electron cooling. The experimental optical band gap was measured by the inverse logarithmic derivative method to be direct, in agreement with theory, and in the range of 1.48-1.59 eV. Mechanistic studies reveal that this metastable phase de-rives from intermediate Cu 3 Se 2 nanocrystals that serve as a structural template for the final hexagonal wurtzite-like product. We compare the chemistry of wurtzite-like Cu 2 FeSnSe 4 to the related CuFeSe 2 material system. Our experimental and computational comparisons between Cu 2 FeSnSe 4 and CuFeSe 2 help explain both the crystal chemistry of CuFeSe 2 that prevents it from forming wurtzite-like polymorphs and the essential role of Sn in stabilizing the metastable structure of Cu 2 FeSnSe 4 . Furthermore, this work provides insight into the importance of elemental composition when designing syntheses for metastable materials.
Shape Dependence of Pressure-Induced Phase Transition in CdS Semiconductor Nanocrystals
Understanding structural stability and phase transformation of nanoparticles under high pressure is of great scientific interest, as it is one of the crucial factors for design, synthesis, and application of materials. Even though high-pressure research on nanomaterials has been widely conducted, their shape-dependent phase transition behavior still remains unclear. Examples of phase transitions of CdS nanoparticles are very limited, despite the fact that it is one of the most studied wide band gap semiconductors. Here we have employed in-situ synchrotron wide-angle X-ray scattering (WAXS) and transmission electron microscopy (TEM) to investigate the high-pressure behaviors of CdS nanoparticles as a function of particle shapes. We observed that CdS nanoparticles transform from wurtzite to rocksalt phase at elevated pressure in comparison to their bulk counterpart. Phase transitions also vary with particle shape—rod-shaped particles show a partially reversible phase transition and the onset of the structural phase transition pressure decreases with decreasing surface-to-volume ratios, while spherical particles undergo irreversible phase transition with relatively low phase transition pressure. Additionally, TEM images of spherical particles exhibited sintering-induced morphology change after high-pressure compression. Calculations of the bulk modulus reveals that spheres are more compressible than rods in the wurtzite phase. These results indicate that the shape of the particle plays an important role in determining their high-pressure properties. Our study provides important insights into understanding of the phase-structure-property relationship, guiding future design and synthesis of nanoparticles for promising applications.
Cubic AgMnSbTe 3 Semiconductor with a High Thermoelectric Performance
The reaction of MnTe with AgSbTe 2 in an equimolar ratio (ATMS) provides a new semiconductor, AgMnSbTe 3 . AgMnSbTe 3 crystallizes in an average rock-salt NaCl structure with Ag, Mn, and Sb cations statistically occupying the Na sites. AgMnSbTe 3 is a p-type semiconductor with a narrow band gap of ~0.33 eV. A pair distribution function analysis indicates that local distortions are associated with the location of the Ag atoms in the lattice. Density functional theory calculations suggest a specific electronic band structure with multi-peak valence band maxima prone to energy convergence. In addition, Ag 2 Te nanograins precipitate at grain boundaries of AgMnSbTe 3 . Here, the energy offset of the valance band edge between AgMnSbTe 3 and Ag 2 Te is ~0.05 eV, which implies that Ag 2 Te precipitates exhibit a negligible effect on the hole transmission. As a result, ATMS exhibits a high power factor of ~12.2 μWcm -1 K -2 at 823 K, ultralow lattice thermal conductivity of ~0.34 Wm -1 K -1 (823 K), high peak ZT of ~1.46 at 823 K, and high average ZT of ~0.87 in the temperature range of 400–823 K.
Dependence of hot electron transfer on surface coverage and adsorbate species at semiconductor–molecule interfaces
Developing a molecular-level understanding of how a hot electron transfer process can be enhanced at semiconductor–molecule interfaces is central to advancing various future technologies.
Advancing semiconductor–electrocatalyst systems: application of surface transformation films and nanosphere lithography
We demonstrate that shadow nanosphere lithography (SNL) is an auspicious tool to systematically create three-dimensional electrocatalyst nanostructures on the semiconductor photoelectrode through controlling their morphology and optical properties.
Crystal structure and orientation of organic semiconductor thin films by microcrystal electron diffraction and grazing-incidence wide-angle X-ray scattering
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Quantum shells versus quantum dots: suppressing Auger recombination in colloidal semiconductors
Semiconductor quantum shells – an emerging nanomaterial class for optical gain media, photovoltaic, and high-energy radiation detection applications.
Crystalline materials for quantum computing: Semiconductor heterostructures and topological insulators exemplars
Abstract High-purity crystalline solid-state materials play an essential role in various technologies for quantum information processing, from qubits based on spins to topological states. New and improved crystalline materials emerge each year and continue to drive new results in experimental quantum science. This article summarizes the opportunities for a selected class of crystalline materials for qubit technologies based on spins and topological states and the challenges associated with their fabrication. We start by describing semiconductor heterostructures for spin qubits in gate-defined quantum dots and benchmark GaAs, Si, and Ge, the three platforms that demonstrated two-qubit logic. We then examine novel topologically nontrivial materials and structures that might be incorporated into superconducting devices to create topological qubits. We review topological insulator thin films and move onto topological crystalline materials, such as PbSnTe, and its integration with Josephson junctions. We discuss advances in novel and specialized fabrication and characterization techniques to enable these. We conclude by identifying the most promising directions where advances in these material systems will enable progress in qubit technology.
Electronic states of semiconductor-metal-semiconductor quantum-well structures
Quantum-size effects are calculated in thin layered semiconductor-metal-semiconductor structures using an ideal free-electron model for the metal layer. The results suggest new quantum-well structures having device applications. Structures with sufficiently high-quality interfaces should exhibit effects such as negative differential resistance due to tunneling between allowed states. Similarly, optical detection by intersubband absorption may be possible. Ultrathin metal layers are predicted to behave as high-density dopant sheets.
Theoretical and experimental studies of radiation-induced damage to semiconductor surfaces and the effects of this damage on semiconductor device performance Semiannual progress report no. 3, 1 Mar. - 31 Aug. 1965
Effects of radiation-induced damage on performance of semiconductor devices - surface recombination velocity model
Algae-based sorbents for removal of gallium from semiconductor manufacturing wastewater
Gallium is increasingly used in industry, and particularly as a component of new materials that have entered semiconductor manufacturing. The release of gallium in factory’s wastewater is potentially an environmental issue and a human health concern. Utilization of algae-based sorbents in the treatment of wastewater polluted by heavy metals has been regarded as a potentially efficient and environmentally-friendly technology. Removal of gallium from simulated wastewater using dead freshwater green algae, ( Chlorella sorokiniana ), as a sorbent was studied. Both batch and continuous flow operations were investigated. Mass balance-based process models were developed and validated for both batch and flow systems. These models were found to be valuable for both understanding the process steps as well as for analyzing the experimental data and determining the fundamental kinetics and thermodynamic parameters. Adsorption capacity was found to be 14.1 mg/g at pH of 2.3, increasing to 38.5 mg/g when the pH is raised to 2.8. The effect of pH is attributed to the surface charge interactions between biomass and gallium and indicates that gallium can be acid-washed and desorbed from the biomass. This property allows for capture followed by recovery of gallium from wastewater using algae based biosorbents.
Theoretical and experimental studies of radiation-induced damage to semiconductor surfaces and the effects of this damage on semiconductor device performance semiannual progress report no. 2, 1 sep. 1964 - 28 feb. 1965
Radiation induced damage to semiconductor surfaces and effect on performance
Photo‐/Thermal‐Responsive Field‐Effect Transistor upon Blending Polymeric Semiconductor with Hexaarylbiimidazole toward Photonically Programmable and Thermally Erasable Memory Device
Abstract It is shown that the semiconducting performance of field‐effect transistors (FETs) with PDPP4T (poly(diketopyrrolopyrrole‐quaterthiophene)) can be reversibly tuned by UV light irradiation and thermal heating after blending with the photochromic hexaarylbiimidazole compound ( p ‐NO 2 ‐HABI). A photo‐/thermal‐responsive FET with a blend thin film of PDPP4T and p ‐NO 2 ‐HABI is successfully fabricated. The transfer characteristics are altered significantly with current enhanced up to 10 6 ‐fold at V G = 0 V after UV light irradiation. However, further heating results in the recovery of the transfer curve. This approach can be extended to other semiconducting polymers such as P3HT (poly(3‐hexyl thiophene)), PBTTT (poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b] thiophene)) and PDPPDTT (poly(diketopyrrolopyrrole‐dithienothiophene)). It is hypothesized that TPIRs (2,4,5‐triphenylimidazolyl radicals) formed from p ‐NO 2 ‐HABI after UV light irradiation can interact with charge defects at the gate dielectric–semiconductor interface and those in the semiconducting layer to induce more hole carriers in the semiconducting channel. The application of the blend thin film of PDPP4T and p ‐NO 2 ‐HABI is further demonstrated to fabricate the photonically programmable and thermally erasable FET‐based nonvolatile memory devices that are advantageous in terms of i) high ON/OFF current ratio, ii) nondestructive reading at low electrical bias, and iii) reasonably highly stable ON‐state and OFF‐state.