Small Gap Semiconductor Research Articles

Ta4SBr11: A Cluster Mott Insulator with a Corrugated, Van der Waals Layered Structure.

The compound Ta4SBr11 was prepared by a comproportionation reaction of tantalum bromide with tantalum and elemental sulfur. The crystal structure, as refined by single-crystal X-ray diffraction, is composed of clusters with Ta4S cores, arranged in corrugated van der Waals layers. Individual layers appear to be displaced relative to each other along one direction. Successful crystal growth in a melt of CsBr yielded black platelets of Ta4SBr11, which were used to investigate the electrical properties of the compound. The electronic structure was studied by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and by density functional theory (DFT) band structure calculations, revealing this material to be a small-gap semiconductor. DFT results, in combination with magnetic susceptibility measurements, suggest that metallicity originating from the one unpaired Ta d electron per cluster is most likely suppressed by electronic correlations, forming a cluster Mott insulator.

Theoretical study on arsenene as anode material for magnesium ion battery in rail transit

The development and regulation of high-performance anode materials contribute to the rapid development of ion batteries in rail transit. In this paper, based on the first-principles calculation method of density functional theory, the properties of two-dimensional arsenene materials are regulated by torsional deformation, and the feasibility of torsionally deformed arsenene as an electrode material for magnesium ion batteries is studied. The results show that the arsenene monolayer remains stable after being adsorbed by a single Mg ion. The properties of arsenene are indirect bandgap semiconductors with a band gap of 1.60 eV. In the current research scope, deformation does not change the semiconductor properties of intrinsic arsenene. As the torsion angle increases, the band gap gradually decreases. The Mg-adsorbed arsenene system exhibits quasi-metallic properties with a band gap of 0.07 eV. The torsional deformation transforms the adsorption system into a small band gap semiconductor. The torsion deformation makes the diffusion energy barrier of magnesium ions on the arsenene monolayer only 0.09 eV, which ensures an excellent charge–discharge rate. The research results provide a theoretical basis for the design of magnesium ion batteries in rail transit.

Pressure-driven perfection: Advancing lead-free halide perovskites Rb2AgBiX6 (X = Br, Cl) for optoelectronic applications

This research employs first-principles simulations to systematically study the structural, elastic, electronic mechanical, and optical characteristics of lead free halide Rb2AgBiX6 (X = Br, Cl) perovskites under pressure. The computed structural parameters are in good agreement with previous experimental and theoretical results. The obtained elastic constants met the Born stability requirements, showing that our materials are mechanically stable at variable hydrostatic pressures, as supported by the computed negative formation energy values. The covalent bond exhibits metallic characteristics, and induced hydrostatic pressure leads to a decrease in bond lengths. Mechanical analysis demonstrates that the studied materials are ductile and mechanically stable, with enhanced ductility under pressure. The materials are small band gap (1.30 eV, 1.801 eV for Rb2AgBiX6 (X = Br, Cl, respectively) semiconductors at ambient pressure with superior optoelectronic performance. Under hydrostatic pressure, Rb2AgBiX6 (X = Br, Cl) experiences a reduction in its band gap (0.545 eV, 1.305 eV for Rb2AgBiX6 (X = Br, Cl, respectively), accompanied by improved physical characteristics. This suggests the potential for increased utilization of this material in optoelectronic devices and solar cells compared to ambient pressure conditions.

Structural and electronic properties of amorphous silicon and germanium monolayers and nanotubes: A DFT investigation

A recent breakthrough has been achieved by synthesizing monolayer amorphous carbon (MAC). Here, we used ab initio (DFT) molecular dynamics simulations to study silicon and germanium MAC analogs. Typical unit cells contain more than 600 atoms. We also considered their corresponding nanotube structures. The cohesion energy values for MASi and MAGe are approximately 3.0 eV/atom lower than the energy ordering of silicene and germanene, respectively. Their electronic behavior varies from metallic to small band gap semiconductors. Since silicene, germanene, and MAC have already been experimentally realized, the corresponding MAC-like versions we propose are within our present synthetic capabilities.

Low-temperature electrical conductivity of ion-beam irradiated Bi–Sb films

Bismuth-antimony alloys are among the most studied topological insulators and also have very promising thermoelectric properties. In addition, in the amorphous state they exhibit superconductivity with critical temperatures in the range 6.0–6.4 K. In this work, we have prepared and studied different polycrystalline films of Bi100–xSbx (x = 0, 5, 10, 15), and we have induced, through ion beam irradiation, significant damage in their internal structure with the aim of amorphizing the material. Specifically, we have irradiated Bi ions in the 10–30 MeV range, exploiting the capabilities of a 5 MV ion beam accelerator of tandem type. We have characterized the Bi–Sb films before and after irradiation from a morphological and structural point of view and measured their electrical resistivity from room temperature to near 2 K, to evaluate the influence of the preparation method and degree of disorder. We have found that the studied Bi–Sb system always behaves as a small energy gap semiconductor that follows the empirical Meyer–Neldel rule, which correlates the conductivity prefactor with the exponential value of the energy gap.

Electronic and Molecular Adsorption Properties of Pt-Doped BC6N: An Ab-Initio Investigation.

In the last two decades, significant efforts have been particularly invested in two-dimensional (2D) hexagonal boron carbon nitride h-BxCyNz because of its unique physical and chemical characteristics. The presence of the carbon atoms lowers the large gap of its cousin structure, boron nitride (BN), making it more suitable for various applications. Here, we use density functional theory to study the structural, electronic, and magnetic properties of Pt-doped BC6N (Pt-BC6N, as well as its adsorption potential of small molecular gases (NO, NO2, CO2, NH3). We consider all distinct locations of the Pt atom in the supercell (B, N, and two C sites). Different adsorption locations are also considered for the pristine and Pt-doped systems. The formation energies of all Pt-doped structures are close to those of the pristine system, reflecting their stability. The pristine BC6N is semiconducting, so doping with Pt at the B and N sites gives a diluted magnetic semiconductor while doping at the C1 and C2 sites results in a smaller gap semiconductor. We find that all doped structures exhibit direct band gaps. The studied molecules are very weakly physisorbed on the pristine structure. Pt doping leads to much stronger interactions, where NO, NO2, and NH3 chemisorb on the doped systems, and CO2 physiorb, illustrating the doped systems' potential for gas purification applications. We also find that the adsorption changes the electronic and magnetic properties of the doped systems, inviting their consideration for spintronics and gas sensing.

Tuning the electronic structure of gold cluster-assembled materials by altering organophosphine ligands.

Superatomic clusters can be assembled to build bulk matter, where the individual characteristics are preserved. The main benefit of these materials over conventional bulk species is the capability to tailor their features by altering the physicochemical identities of individual clusters. Electronic properties of metal clusters can be modified by a protective shell of ligands that attach to the surface and make the whole nanoparticle soluble in organic or aqueous solvents. In the present work, we demonstrate that properly chosen ligands provide not only steric protection from aggregation but also tune the redox activity of metal clusters. We investigate the role of the ligands in electronic structure tunability and ligand-field splitting. Our first-principles calculations agree with the experiments, showing that phosphine-protected gold materials are small gap semiconductors. The obtained bandgaps strongly depend on the ligand used. Hence, using phosphine and organophosphine ligands should be feasible and promising while designing the novel superatom-based materials since the desired range of the bandgap might be achieved (by the proper choice of the ligand).

1T’-[formula omitted] hybrid monolayer as a novel magnetic material: A first principles study

The advent of two-dimensional (2D) materials, such as transition metal dichalcogenides (2D-TMD), has led to an extensive amount of interest amongst scientists and engineers alike in search of major breakthroughs in the electronic, magnetic, and thermodynamic properties of 2D materials, generating interest in novel device applications. This paper discusses a theoretical study of the structural, electronic, magnetic, stability, phonon dispersion, and thermodynamic properties of the monoclinic RuWTe2 monolayer, a type of TMD in its 1T’ phase. The study uses computational modeling, employing the Density Functional Theory (DFT) and Density Functional Perturbation Theory (DFPT) formalisms, using GGA approximation and pseudopotentials to replace nucleus electrons in each atomic species. Just for bandgap estimation, it was used a single point energy of HSE06 hybrid functional which resulted in a small gap semiconductor (≈ 0.4eV). Even after adding the DFT+U correction to PBE functional, it presented magnetic properties with a trend toward ferrimagnetism. It presents structural and energetic stability through calculations of cohesive, formation energy, and phonon dispersion, since it does not present virtual phonon frequency. Through thermodynamics, the free energy (F) indicates that the synthesis process for 1T’-RuWTe2 would be spontaneous even at low temperatures. These results demonstrate that the monoclinic 1T’-RuWTe2 hybrid monolayer is a promising candidate suitable for applications in magnetic and thermal devices.

Stacking and spin order in a van der Waals Mott insulator 1T-TaS2

Quasi-two-dimensional charge density wave system of 1T-TaS2 has attracted recent interest due to topological excitations, emergent superconductivity, ultrafast synaptic functionality, and the possibility of a quantum spin liquid state. While electron correlation has been known to be essential in this system, the nature of its insulating phase is currently under debate. Here, we reinvestigate the origin of the insulating band structures of the 1T-TaS2 surface using density-functional theory calculations to consider the recently-raised issues such as interlayer coupling, surface effect, and interlayer spin ordering. We identify four distinct electronic states of the surface layer such as a 2D Mott phase, a strongly-coupled antiferromagnetic insulator, a weakly-coupled ferromagnetic insulator, and a small-gap semiconductor, depending on types of the surface termination and the interlayer spin configuration. These distinct surface electronic states explain the different sizes of spectroscopic band gaps observed in scanning tunneling microscopy, revealing the complexity of the interlayer charge and spin couplings in layered correlated materials.

Explore the feasibility of Janus 2H-VSeTe monolayer as anode material for Li ion battery

Two-dimensional (2D) materials as potential energy storage systems have received extensive attention due to their high energy density and rate performance. Here, the electronic properties and feasibility of Janus 2H-VSeTe monolayer as LIBs anode material are systematically investigated using the first-principles calculations. No imaginary frequency in the phonon spectrum and the absence of obvious deformation simulated by AIMD simulations at 300 K prove the dynamic and thermal stability of Janus 2H-VSeTe monolayer, respectively. The pure Janus 2H-VSeTe monolayer exhibits a small direct band gap semiconductor character. The Li atom adsorption can result in the increase of electronic states near Ef and a transform of metallic behavior, ensuring good conductivity for the Janus 2H-VSeTe monolayer. On the other hand, the adsorbed Li atoms are fully ionized to ensure the charge and discharge process by the Bader analysis. Through the climbing-image nudged elastic band method, two possible migration paths are considered on the Se and Te surfaces, respectively. The lowest potential barriers are 0.159 eV and 0.188 eV, respectively, which proves that there is a high ion migration rate during charge and discharge process. The OCV and multi-layer Li atoms adsorption suggest that the corresponding specific capacity is 416 mA h g−1 for the Janus 2H-VSeTe monolayer, which reveals that Janus 2H-VSeTe monolayer is one of the feasible candidates for LIBs anode materials.

Composition dependence of the specific heat of FeSi

Recently, a high-mobility surface conduction channel and in-gap states were identified in the correlated small-gap semiconductor FeSi using electrical transport measurements and high-resolution tunneling spectroscopy. The mobility of the charge carriers in the surface channel is quantitatively reminiscent of topological insulators, but displays a lack of sensitivity to the presence of ferromagnetic impurities as studied by means of a series of single crystals with slightly different starting compositions. Here, we report measurements of the specific heat of these crystals. At low temperatures, a shallow maximum is observed in the specific heat divided by temperature. This maximum is suppressed under magnetic field, characteristic of a Schottky anomaly associated with magnetic impurities. In comparison, the height of this maximum decreases with increasing initial iron content.

Synthesis, Crystal Structure, and Electrical and Magnetic Characterizations of the Monoclinic Compounds Ln3Mo4+xSi1–xO14 (Ln = La, Ce, Pr, and Nd; x = 0 and 0.35) Containing Mo3 Clusters and Mo2 Dimers

Powder samples of the new monoclinic compounds Ln3Mo4SiO14 (Ln = La, Ce, Pr, and Nd) and single crystals of Pr3Mo4.35Si0.65O14 were obtained by the solid-state reaction. The crystal structure of Pr3Mo4.35Si0.65O14 was determined by single-crystal X-ray diffraction. Pr3Mo4.35Si0.65O14 crystallizes in the monoclinic space group P21/n with unit-cell parameters a = 5.6361 (2) Å, b = 17.5814 (8) Å, c = 10.9883 (4) Å, and Z = 4. Full-matrix least-squares refinement on F2 using 7544 independent reflections for 203 refinable parameters results in R1 = 0.0359 and wR2 = 0.0831. The structure contains chains of Mo3O13 clusters and chains of edge-sharing MoO6 octahedra with alternately short (2.508 Å) and long (3.161 Å) Mo-Mo distances running parallel to the a axis that are separated by 8- or 10-coordinate Pr-O polyhedra. Magnetic susceptibility measurements on the Ln3Mo4SiO14 (Ln = La, Ce, Pr, and Nd) are in agreement with a trivalent state of the rare earths for the Ce, Pr, and Nd compounds, while that on the lanthanum compound confirms the presence of one unpaired electron per Mo3 as expected. Resistivity measurements on a single crystal show that Pr3Mo4.35Si0.65O14 is a small band gap semi-conductor.

Probing FeSi, a d-electron topological Kondo insulator candidate, with magnetic field, pressure, and microwaves

Recently, evidence for a conducting surface state (CSS) below 19 K was reported for the correlated d-electron small gap semiconductor FeSi. In the work reported herein, the CSS and the bulk phase of FeSi were probed via electrical resistivity ρ measurements as a function of temperature T, magnetic field B to 60 T, and pressure P to 7.6 GPa, and by means of a magnetic field-modulated microwave spectroscopy (MFMMS) technique. The properties of FeSi were also compared with those of the Kondo insulator SmB6 to address the question of whether FeSi is a d-electron analogue of an f-electron Kondo insulator and, in addition, a "topological Kondo insulator" (TKI). The overall behavior of the magnetoresistanceof FeSi at temperatures above and below the onset temperature TS = 19 K of the CSS is similar to that of SmB6. The two energy gaps, inferred from the ρ(T) data in the semiconducting regime, increase with pressure up to about 7 GPa, followed by a drop which coincides with a sharp suppression of TS. Several studies of ρ(T) under pressure on SmB6 reveal behavior similar to that of FeSi in which the two energy gaps vanish at a critical pressure near the pressure at which TS vanishes, although the energy gaps in SmB6 initially decrease with pressure, whereas in FeSi they increase with pressure. The MFMMS measurements showed a sharp feature at TS ≈ 19 K for FeSi, which could be due to ferromagnetic ordering of the CSS. However, no such feature was observed at TS ≈ 4.5 K for SmB6.

Dramatic Plasmon Response to the Charge-Density-Wave Gap Development in 1T-TiSe_{2}.

1T-TiSe_{2} is one of the most studied charge density wave (CDW) systems, not only because of its peculiar properties related to the CDW transition, but also due to its status as a promising candidate of exciton insulator signaled by the proposed plasmon softening at the CDW wave vector. Using high-resolution electron energy loss spectroscopy, we report a systematic study of the temperature-dependent plasmon behaviors of 1T-TiSe_{2}. We unambiguously resolve the plasmon from phonon modes, revealing the existence of Landau damping to the plasmon at finite momentums, which does not support the plasmon softening picture for exciton condensation. Moreover, we discover that the plasmon lifetime at zero momentum responds dramatically to the band gap evolution associated with the CDW transition. The interband transitions near the Fermi energy in the normal phase are demonstrated to serve as a strong damping channel of plasmons, while such a channel in the CDW phase is suppressed due to the CDW gap opening, which results in the dramatic tunability of the plasmon in semimetals or small-gap semiconductors.

Conductivity of Two-Dimensional Small Gap Semiconductors and Topological Insulators in Strong Coulomb Disorder

We are honored to dedicate this article to Emmanuel Rashba on the occasion of his 95 birthday. In the ideal disorder-free situation, a two-dimensional band gap insulator has an activation energy for conductivity equal to half the band gap, $\Delta$. But transport experiments usually exhibit a much smaller activation energy at low temperature, and the relation between this activation energy and $\Delta$ is unclear. Here we consider the temperature-dependent conductivity of a two-dimensional narrow gap semiconductor on a substrate containing Coulomb impurities, mostly focusing on the case when amplitude of the random potential $\Gamma \gg \Delta$. We show that the conductivity generically exhibits three regimes and only the highest temperature regime exhibits an activation energy that reflects the band gap. At lower temperatures, the conduction proceeds through nearest-neighbor or variable-range hopping between electron and hole puddles created by the disorder. We show that the activation energy and characteristic temperature associated with these processes steeply collapse near a critical impurity concentration. Larger concentrations lead to an exponentially small activation energy and exponentially long localization length, which in mesoscopic samples can appear as a disorder-induced insulator-to-metal transition. We arrive at a similar disorder driven steep insulator-metal transition in thin films of three-dimensional topological insulators with very large dielectric constant, where due to confinement of electric field internal Coulomb impurities create larger disorder potential. Away from neutrality point this unconventional insulator-to-metal transition is augmented by conventional metal-insulator transition at small impurity concentrations, so that we arrive at disorder-driven re-entrant metal-insulator-metal transition.

Non-Dyson Algebraic Diagrammatic Construction Theory for Charged Excitations in Solids.

We present the first implementation and applications of non-Dyson algebraic diagrammatic construction theory for charged excitations in three-dimensional periodic solids (EA/IP-ADC). The EA/IP-ADC approach has a computational cost similar to the ground-state Møller-Plesset perturbation theory, enabling efficient calculations of a variety of crystalline excited-state properties (e.g., band structure, band gap, density of states) sampled in the Brillouin zone. We use EA/IP-ADC to compute the quasiparticle band structures and band gaps of several materials (from large-gap atomic and ionic solids to small-gap semiconductors) and analyze the errors of EA/IP-ADC approximations up to the third order in perturbation theory. Our work also reports the first-ever calculations of ground-state properties (equation-of-state and lattice constants) of three-dimensional crystalline systems using a periodic implementation of third-order Møller-Plesset perturbation theory (MP3).

Atomic-scale 3D imaging of individual dopant atoms in an oxide semiconductor

The physical properties of semiconductors are controlled by chemical doping. In oxide semiconductors, small variations in the density of dopant atoms can completely change the local electric and magnetic responses caused by their strongly correlated electrons. In lightly doped systems, however, such variations are difficult to determine as quantitative 3D imaging of individual dopant atoms is a major challenge. We apply atom probe tomography to resolve the atomic sites that donors occupy in the small band gap semiconductor Er(Mn,Ti)O3 with a nominal Ti concentration of 0.04 at. %, map their 3D lattice positions, and quantify spatial variations. Our work enables atomic-level 3D studies of structure-property relations in lightly doped complex oxides, which is crucial to understand and control emergent dopant-driven quantum phenomena.

Excitonic insulators and Gross-Neveu models

We introduce a generalized Gross-Neveu (GN) model to describe the excitonic instabilities in two different systems: a small overlap semi-metal (SM) and a small gap semi-conductor (SMC), both in two (2d) and three-dimensions (3d). We identify the excitonic order parameter (EOP) and obtain the effective potential within the Large $N$ limit approach where the GN model can be exactly solved. We obtain the excitonic insulator (EI) phase diagrams as a function of temperature, chemical potential, overlap between bands and gaps of the system. We show that the EI may undergo first- or second-order thermal transitions depending on the regime whereupon this phase is approached. We also investigate the expected thermodynamic signatures for the specific heat above the fine-tuned excitonic quantum critical point (EQCP), in both 2d and 3d, in the SMC regime. We show that the EQCP is a different kind of critical point since although the EOP vanishes at the EQCP, there is always a finite gap in the SMC regime. We find that for high temperatures, the specific heat might exhibit a scaling behavior in the form $C_V/T \propto T^{(d-z)/z}$, where $d$ is the dimension of the system and $z$ is the dynamical critical exponent. The very low temperature behavior has a dominant exponential thermally activated term due to the presence of a gap that does not vanish at the excitonic transition.

New Charge Transfer Cocrystals of F2TCNQ with Polycyclic Aromatic Hydrocarbons: Acceptor–Acceptor Interactions and Their Contribution to Supramolecular Arrangement and Charge Transfer

A series of new charge-transfer cocrystals of F2TCNQ with anthracene, tetracene, and chrysene was prepared and characterized. The donor and acceptor molecules are arranged in alternating D–A–D–A stacks. The linear acenes in combination with F2TCNQ form layered structures due to in-plane lateral donor–acceptor and acceptor–acceptor interactions via multiple C–H···N and C–H···F hydrogen bonds, which govern the crystal structure and significantly alter face-to-face π–π interactions. In the cocrystal of F2TCNQ with chrysene no interactions between acceptor molecules are observed, and the π–π interactions prevail. Thus, the donor–acceptor interplanar distance is the smallest in the chrysene complex despite its lower energy level of the HOMO and weaker donor ability, as determined through electrochemical oxidation potentials in this series of PAHs. The charge-transfer values estimated through empirical correlations and QTAIM analysis also do not manifest a direct dependence on the donor ability of PAHs. Thus, though face-to-face π–π interactions dictate the formation of cocrystals, the lateral noncovalent interactions are as important for the supramolecular arrangement and charge transfer. UV/vis spectroscopy and electronic structure quantum chemical calculations show that these cocrystals may be classified as small-gap semiconductors with energy gaps of 0.7–1.3 eV.

Crystal chemistry rationale and ab initio investigation of ultra-hard dense rhombohedral carbon and boron nitride

Rhombohedral dense forms of carbon, rh-C2 (or hexagonal h-C6), and boron nitride, rh-BN (or hexagonal h-B3N3), are derived from rhombohedral 3R graphite based on original crystal chemistry scheme backed with full cell geometry optimization to minimal energy ground state computations within the quantum density functional theory. Considering throughout hexagonal settings featuring extended lattices, the calculation of the hexagonal set of elastic constants provide results of large bulk moduli with B0 (rh-C2) = 438 GPa close to that of diamond, and B0 (rh-BN) = 369 GPa close to that of cubic BN. The hardness assessment in the framework of three contemporary models enables both phases to be considered as ultra-hard. From the electronic band structures calculated in the hexagonal Brillouin zones, 3R graphite is a small-gap semiconductor, oppositely to rh-C2 that is characterized by a large band gap of 6 eV, and rh-BN is a large band gap insulator with Egap = 5 eV.