Photovoltage Oscillations in Encapsulated Graphene

We theoretically analyze the rise of photovoltage oscillations in hexagonal boron-nitride (h-BN) encapsulated monolayer graphene (h-BN/graphene/h-BN) when irradiated with terahertz radiation. We use an extension of the radiation-driven electron orbit model, successfully applied to study the oscillations obtained in irradiated magnetotransport of GaAs/AlGaAs heterostructures. The extension takes mainly into account that now the carriers are massive Dirac fermions. Our simulations reveal that the photovoltage in these graphene systems presents important oscillations similar to the ones of irradiated magnetoresistance in semiconductor platforms but in the terahertz range. We also obtain that these oscillations are clearly affected by the voltages applied to the sandwiched graphene: a vertical gate voltage between the two hBN layers and an external positive voltage applied to one of the sample sides. The former steers the carrier effective mass and the latter the photovoltage intensity and the oscillations amplitude. The frequency dependence of the photo-oscillations is also investigated.

Stoichiometric modulation on optical nonlinearity of 2D MoS x Se2−x alloys for photonic applications

The composition-engineered band structures of two-dimensional (2D) ternary transition-metal dichalcogenides (TMDCs) semiconductor alloys directly dominate their electronic and optical properties. Herein, in this paper, a detailed theoretical and experimental study on the composition-dependent nonlinear optical properties of 2D MoS x Se2−x alloys was carried out. The first-principles calculations were performed to investigate the compositionally modulated properties of monolayer 2D MoS x Se2−x (x = 0.25, 0.5, 1.0, 1.5, and 1.75) in terms of the carrier effective mass, carrier density and mobility, as well as band-gaps. Furthermore, high-quality few-layered MoS x Se2−x (x = 0.2, 0.5, 1.0, 1.5, and 1.8) nanosheets were fabricated by using liquid phase exfoliation method. The third-order nonlinear optical response was investigated by open-aperture Z-scan technique, revealing composition-dependent saturable absorption, and light modulation properties, which were correlated to the theoretical calculations and further confirmed by using MoS x Se2−x nanosheets as saturable absorbers (SAs) for all-solid-state pulsed lasers. In particular, a mode-locked solid-state laser with pulse width of 227 fs was realized with MoS0.2Se1.8 as SA, for the first time to our best knowledge. Our work not only provides a comprehensive understanding of the compositionally and defectively modulated nonlinear optical responses of ternary TMDCs alloys, but also paves a way for the development of 2D materials-based novel optoelectronic devices.

Restructured single parabolic band model for quick analysis in thermoelectricity

The single parabolic band (SPB) model has been widely used to preliminarily elucidate inherent transport behaviors of thermoelectric (TE) materials, such as their band structure and electronic thermal conductivity, etc. However, in the SPB calculation, it is necessary to determine some intermediate variables, such as Fermi level or the complex Fermi-Dirac integrals. In this work, we establish a direct carrier-concentration-dependent restructured SPB model, which eliminates Fermi-Dirac integrals and Fermi level calculation and emerges stronger visibility and usability in experiments. We have verified the reliability of such restructured model with 490 groups of experimental data from state-of-the-art TE materials and the relative error is less than 2%. Moreover, carrier effective mass, intrinsic carrier mobility and optimal carrier concentration of these materials are systematically investigated. We believe that our work can provide more convenience and accuracy for thermoelectric data analysis as well as instructive understanding on future optimization design.

High mobility transparent and conducting oxide films of La-doped SrSnO3

The synthesis and characterization of high mobility thin films of La-doped SrSnO 3 are reported. The mobility for the 7% La-doped sample is found to be 228 cm 2 V -1 s -1 . The observed high mobility is associated with the reduced carrier effective mass and scattering centers of various scattering mechanisms. The enhancement in mobility and the increase in carrier concentration after doping reduced the resistivities of the thin films by 5 orders of magnitude. X-ray absorption spectroscopy and X-ray photoelectron spectroscopy revealed that La-dopant and oxygen vacancies donate the electrons in the films. Films were highly transparent (> 90%) in the visible region. These materials have great potential to be used in the optoelectronic and heterostructure devices.

Thermoelectric and Transport Properties of Permingeatite Cu3SbSe4 Prepared Using Mechanical Alloying and Hot Pressing

Permingeatite (Cu3SbSe4) is a promising thermoelectric material because it has a narrow band gap, large carrier effective mass, and abundant and nontoxic components. Mechanical alloying (MA), which is a high-energy ball mill process, has various advantages, e.g., segregation/evaporation is not required and homogeneous powders can be prepared in a short time. In this study, the effects of MA and hot-pressing (HP) conditions on the synthesis of the Cu3SbSe4 phase and its thermoelectric properties were evaluated. The electrical conductivity decreased with increasing HP temperature, while the Seebeck coefficient increased. The power factor (PF) was 0.38–0.50 mW m−1 K−2 and the thermal conductivity was 0.76–0.78 W m−1 K−1 at 623 K. The dimensionless figure of merit, ZT, increased with increasing temperature, and a reliable and maximum ZT value of 0.39 was obtained at 623 K for Cu3SbSe4 prepared using MA at 350 rpm for 12 h and HP at 573 K for 2 h.

Rationally optimized carrier effective mass and carrier density lead to high average ZT value in n-type PbSe

Among these intricately coupled thermoelectric parameters, the carrier effective mass (m*) and carrier density (n) are two key parameters to determine the electrical transport properties. To enhance the broad-temperature thermoelectric...

Tin Oxynitride-based Ferroelectric Semiconductors for Solar Energy Conversion Applications

Lead-halide perovskites have emerged as a promising class of semiconductors; however they suffer from issues related to lead-toxicity and instability. We report results of a firstprinciples-based design of heavy-metal-based oxynitrides as alternatives to lead-halide perovskites. We have used density-functional-theory calculations to search a vast composition space of ABO2N and ABON2 compounds, where B is a p-block cation, and A is an alkaline, alkaliearth, rare-earth or transition metal cation, and identify 10 new ABO2N oxynitride semiconductors that we expect to be formable. Specifically, we discover a new family of ferroelectric semiconductors with A3+SnO2N stoichiometry (A = Y, Eu, La, In, and Sc) in the LuMnO3-type structure, which combine the strong bonding of metal oxides with the low carrier effective mass and small, tunable band gaps of the lead-halide perovskites. These tin oxynitrides have predicted direct band gaps ranging from 1.6 – 3.3 eV, and a sizeable electric polarization up to 17 μC/cm2 , which is predicted to be switchable by an external electric field through a non-polar phase. With their unique combination of polarization, low carrier effective mass and band gaps spanning the entire visible spectrum, we expect ASnO2N ferroelectric semiconductors will find useful applications as photovoltaics, photocatalysts, and for optoelectronics.

Tin Oxynitride-based Ferroelectric Semiconductors for Solar Energy Conversion Applications

Lead-halide perovskites have emerged as a promising class of semiconductors; however they suffer from issues related to lead-toxicity and instability. We report results of a firstprinciples-based design of heavy-metal-based oxynitrides as alternatives to lead-halide perovskites. We have used density-functional-theory calculations to search a vast composition space of ABO2N and ABON2 compounds, where B is a p-block cation, and A is an alkaline, alkaliearth, rare-earth or transition metal cation, and identify 10 new ABO2N oxynitride semiconductors that we expect to be formable. Specifically, we discover a new family of ferroelectric semiconductors with A3+SnO2N stoichiometry (A = Y, Eu, La, In, and Sc) in the LuMnO3-type structure, which combine the strong bonding of metal oxides with the low carrier effective mass and small, tunable band gaps of the lead-halide perovskites. These tin oxynitrides have predicted direct band gaps ranging from 1.6 – 3.3 eV, and a sizeable electric polarization up to 17 μC/cm2 , which is predicted to be switchable by an external electric field through a non-polar phase. With their unique combination of polarization, low carrier effective mass and band gaps spanning the entire visible spectrum, we expect ASnO2N ferroelectric semiconductors will find useful applications as photovoltaics, photocatalysts, and for optoelectronics.

Type-II tunable SiC/InSe heterostructures under an electric field and biaxial strain

A novel type II band alignment with lower carrier effective mass can be adjusted by an electric field and strain.

Descriptors for Electron and Hole Charge Carriers in Metal Oxides

<div> <div> <div> <p>Metal oxides can act as insulators, semiconductors or metals depending on their chemical composition and crystal structure. Metal oxide semiconductors, which support equilibrium populations of electron and hole charge carriers, have widespread applications including batteries, solar cells, and display technologies. It is often difficult to predict in advance whether these materials will exhibit localized or delocalized charge carriers upon oxidation or reduction. We combine data from first-principles calculations of the electronic structure and dielectric response of 214 metal oxides to predict the energetic driving force for carrier localization and transport. We assess descriptors based on the carrier effective mass, static polaron binding energy, and Frohlich electron–phonon coupling. Numerical analysis allows us to assign p and n type transport of a metal oxide to three classes: (i) band transport with high mobility; (ii) small polaron transport with low mobility; and (iii) intermediate behaviour. The results of this classification agree with observations regarding carrier dynamics and lifetimes and are used to predict 10 candidate p-type oxides. </p> </div> </div> </div>