Zero-gap Semiconductor Research Articles

Study of carbon monoxide adsorption on the α- and γ-graphyne nanosheets for the sensor applications

Recently, graphyne-based gas sensors have drawn a lot of interest. One kind of graphene with acetylene bonds connecting its hexagons is graphyne. In this study, the density functional theory (DFT) method was used to investigate the physical parameters of the surface adsorption of carbon monoxide on the [Formula: see text]- and [Formula: see text]-graphyne nanosheet, taking into account van der Waals (vdW) interactions through the use of the SIESTA computational code. CO molecule adsorbed and optimized in two vertical and horizontal states from the side of carbon and oxygen atoms, at different distances and sites relative to the graphyne sheet. After optimization and finding the adsorption energy, the best adsorption sites, equilibrium distance of the molecule from the surface, electronic structure, charge transfer rate, and bandgap changes were calculated. It was observed that the changes in the electronic structure after the adsorption of CO molecule are insignificant and the [Formula: see text]- and [Formula: see text]-graphyne nanosheets remained zero-gap and narrow-gap semiconductors, respectively. The adsorption of CO molecule on the [Formula: see text]- and [Formula: see text]-graphyne nanosheets is physisorption. The results show that the [Formula: see text]-graphyne nanosheet has a good potential for adsorbing and detecting CO molecules and sensor applications.

Family behavior and Dirac bands in armchair nanoribbons with 4–8 defect lines

Bottom-up synthesis from molecular precursors is a powerful route for the creation of novel synthetic carbon-based low-dimensional materials, such as planar carbon lattices. The wealth of conceivable precursor molecules introduces a significant number of degrees-of-freedom for the design of materials with defined physical properties. In this context, a priori knowledge of the electronic, vibrational and optical properties provided by modern ab initio simulation methods can act as a valuable guide for the design of novel synthetic carbon-based building blocks. Using density functional theory, we performed simulations of the electronic properties of armchair-edged graphene nanoribbons (AGNR) with a bisecting 4–8 ring defect line. We show that the electronic structures of the defective nanoribbons of increasing width can be classified into three distinct families of semiconductors, similar to the case of pristine AGNR. In contrast to the latter, we find that every third nanoribbon is a zero-gap semiconductor with Dirac-type crossing of linear bands at the Fermi energy. By employing tight-binding models including interactions up to third-nearest neighbors, we show that the family behavior, the formation of direct and indirect band gaps and of linear band crossings in the defective nanoribbons is rooted in the electronic properties of the individual nanoribbon halves on either side of the defect lines, and can be effectively through introduction of additional ‘interhalf’ coupling terms.

First-principles calculations to investigate electronic, magnetic, and optical properties at (110) and (111) surfaces of Ni adsorption on CdO

In this study, we utilised the GGA+U method via the WIEN2k software to investigate the structural, magnetic, and optoelectronic responses of two distinct surfaces, (111) and (110), to Ni adsorption on CdO at the Cd site. The analysis revealed that Ni/CdO (111) demonstrates properties akin to a semimetal or a zero-gap semiconductor across both spin orientations, while Ni/CdO (110) exhibits a metallic nature under similar conditions. These phenomena are predominantly ascribed to the orbital contributions from Ni-d, Cd-d, and O-p for the Ni/CdO (111), and Ni-d, Cd-d, and O-s for the Ni/CdO (110), respectively. A notable blue shift in the optical absorption spectra was observed for both materials. Utilising the Dielectric function, we calculated the dispersion of energy bands along with essential optical properties, including the absorption coefficient, energy loss function, and reflectivity, refractive index, and extinction coefficient. Magnetisation values for Ni/CdO (110) and Ni/CdO (111) were recorded at (−0.11794) µB and (2.17926) µB, respectively, influenced by the absorption spectrum's blue shift. Additionally, transport properties were evaluated using the BoltzTraP software over a temperature range of 0 to 800 K. Our findings suggest that Ni/CdO (110) and Ni/CdO (111) are significant for applications in spintronics and energy devices.

Optical properties and Landau quantisations in twisted bilayer graphene

ABSTRACT The optical properties and Landau quantisations in twisted bilayer graphene are investigated using a generalised tight-binding model that takes into account all interlayer and intralayer atomic interactions in the Moire superlattice. The system is a zero-gap semiconductor with double-degenerate Dirac-cone structures, and saddle-point energy dispersions appear at low energies for small twisting angles. The magnetic quantisation phenomena in this system are rich and unique, with Landau-level subgroups specified owing to specific Moire zone folding through modulation of the stacking angles. The Landau-level spectrum shows hybridised characteristics associated with those in monolayer, as well as AA and AB stackings. The detailed analysis explores the complex relations among the different sublattices on the same and different graphene layers.

Colossal negative magnetoresistance in field-induced Weyl semimetal of magnetic half-Heusler compound

The discovery of topological insulators and semimetals triggered enormous interest in exploring emergent electromagnetic responses in solids. Particular attention has been focused on ternary half-Heusler compounds, whose electronic structure bears analogy to the topological zinc-blende compounds while also including magnetic rare-earth ions coupled to conduction electrons. However, most of the research in this system has been in band-inverted zero-gap semiconductors such as GdPtBi, which still does not fully exhaust the large potential of this material class. Here, we report a less-studied member of half-Heusler compounds, HoAuSn, which we show is a trivial semimetal or narrow-gap semiconductor at zero magnetic field but undergoes a field-induced transition to a Weyl semimetal, with a negative magnetoresistance exceeding four orders of magnitude at low temperatures. The combined study of Shubnikov-de Haas oscillations and first-principles calculation suggests that the exchange field from Ho 4f moments reconstructs the band structure to induce Weyl points which play a key role in the strong suppression of large-angle carrier scattering. Our findings demonstrate the unique mechanism of colossal negative magnetoresistance and provide pathways towards realizing topological electronic states in a large class of magnetic half-Heusler compounds.

Theoretical prediction of two-dimensional II-V compounds

Graphene has attracted significant attention as a pioneer of two-dimensional zero gap semiconductors, but the development of new two-dimensional materials with a finite band gap has been actively pursued. In this study, the structural stability of double bilayers (DBs) of group II-V compounds (II: Be, Zn, and Cd; V: P, As, and Sb) has been systematically investigated using first-principles calculations based on density functional theory. The thermodynamic calculations have confirmed that BeP, BeAs, ZnP, and ZnAs can be produced through exothermic reactions from their constituent bulk systems. It has also been confirmed that all the compounds have phonon dynamical stabilities. Only CdP and CdAs have been found to have an AB-stacked DB structure with threefold symmetry, while the other compounds have ${\text{AB}}^{\ensuremath{'}}$-stacked DB structure with broken symmetry. The difference in atomic radii between group II and group V results in the so-called size effect, which determines the stacking pattern. The structural stability of II-V DB thin films is explained by analogy with the surface structural stability of compound semiconductors: The change in the atomic arrangement of the DB structure alters the electronegativity of the surface orbitals of the II-V thin film, which does not result in any unsaturated bonds, i.e., no metallic bands across the Fermi level appear. The various DB II-V compounds proposed in this study will join the ranks of atomic-level 2D semiconductor materials.

Preparation of Bulk Ingots of the Spin-Polarized Zero-Gap Semiconductor Ti2MnAl

Preparation of Bulk Ingots of the Spin-Polarized Zero-Gap Semiconductor Ti2MnAl

New room-temperature 2D hexagonal topological insulator OsC: First Principle Calculations

The structural, electronic and topological characteristics of the two-dimensional (2D) binary compound OsC are investigated using DFT simulations. Phonon dispersion and molecular dynamics calculations confirm a strong structural, dynamic and thermal stability of the osmium carbide monolayer in hexagonal planar geometry where C−Os hybridization is sp2. Electronic band structures attest that OsC is a zero-gap semiconductor with the valence band coming mainly from osmium d orbitals and some contributions from carbon p orbitals. By tuning on the spin orbit coupling (SOC), the gap ΔSOC of 141 meV, corrected to 220 meV using HSE approximations, opens in the vicinity of the Γ point with the occurrence of a band inversion between d32−Os and d52−Os revealing that our system is a nontrivial QSH insulator with topological invariant Z2=1 and topological edge states. Our study shows that the room-temperature 2D topological insulator OsC is a potential material for possible applications in nanoelectronic and spintronic devices.

Quenching the bandgap of two-dimensional semiconductors with a perpendicular electric field.

Perpendicular electric fields can tune the electronic band structure of atomically thin semiconductors. In bilayer graphene, which is an intrinsic zero-gap semiconductor, a perpendicular electric field opens a finite bandgap. So far, however, the same principle could not be applied to control the properties of a broader class of 2D materials because the required electric fields are beyond reach in current devices. To overcome this limitation, we design double ionic gated transistors that enable the application of large electric fields of up to 3 V nm-1. Using such devices, we continuously suppress the bandgap of few-layer semiconducting transition metal dichalcogenides (that is, bilayer to heptalayer WSe2) from 1.6 V to zero. Our results illustrate an excellent level of control of the band structure of 2D semiconductors.

Tailoring Spectrally Flat Infrared Photodetection with Thickness-Controlled Nanocrystalline Graphite.

Graphene, a zero-gap semiconductor, absorbs 2.3% of incident photons in a wide wavelength range as a free-standing monolayer, whereas 50% is expected for ∼90 layers. Adjusting the layer number allows the tailoring of the photoresponse; however, controlling the thickness of multilayer graphene remains challenging on the wafer scale. Nanocrystalline graphene or graphite (NCG) can instead be grown with controlled thickness. We have fabricated photodetectors from NCG that are spectrally flat in the near-infrared to short-wavelength infrared region by tailoring the layer thicknesses. Transfer matrix simulations were used to determine the NCG thickness for maximum light absorption in the NCG layer on a silicon substrate. The extrinsic and intrinsic photoresponse was determined from 1100 to 2100 nm using chromatic aberration-corrected photocurrent spectroscopy. Diffraction-limited hyperspectral photocurrent imaging shows that the biased photoresponse is unipolar and homogeneous across the device area, whereas the short-circuit photoresponse gives rise to positive and negative photocurrents at the electrodes. The intrinsic photoresponses are wavelength-independent, indicative of bolometric and electrothermal photodetection.

Peculiarities of magnetic oscillations in HgSe monocrystal with Co impurities of low concentration (<1 at%)

The results of experimental studies of magnetoresistance oscillations are described for a HgSe monocrystal with Co impurities of low concentration (<1 at%). It has been discovered that at such impurity concentrations (~1018 cm-3) there exists two types of magnetic oscillations: Shubnikov de Haas oscillations at low temperatures (T<10 K) and magnetophonon oscillations at higher temperatures (T>10 K). The first ones are caused by the interaction of electrons with magnetic fieled inside of a Landau sublevel and the latter --- by the interaction with longitudinal optical phonons. The differences of these oscillations are characterized. A hypothesis of a nature of magnetophonon oscillations in such structures is proposed. Keywords: 3d-impurities of low concentration, zero-gap semiconductors, magnetophonon oscillations, SdH oscillations, optical and acoustic phonons.

Hybridization-Gap Formation and Superconductivity in the Pressure-Induced Semimetallic Phase of the Excitonic Insulator Ta2NiSe5

The excitonic insulator Ta$_2$NiSe$_5$ experiences a first-order structural transition under pressure from rippled to flat layer-structure at Ps = 3 GPa, which drives the system from an almost zero-gap semiconductor to a semimetal. The pressure-induced semimetal, with lowering temperature, experiences a transition to another semimetal with a partial-gap of 0.1-0.2 eV, accompanied with a monoclinic distortion analogous to that occurs at the excitonic transition below Ps. We argue that the partial-gap originates primarily from a symmetry-allowed hybridization of Ta-conduction and Ni-valence bands due to the lattice distortion, indicative of the importance of electron-lattice coupling. The transition is suppressed with increasing pressure to Pc = 8 GPa. Superconductivity with a maximum Tsc = 1.2 K emerges around Pc, likely mediated by strongly electron-coupled soft phonons. The electron-lattice coupling is as important ingredient as the excitonic instability in Ta2NiSe5.

Designing All Graphdiyne Materials as Graphene Derivatives: Topologically Driven Modulation of Electronic Properties

Designing new 2D systems with tunable properties is an important subject for science and technology. Starting from graphene, we developed an algorithm to systematically generate 2D carbon crystals belonging to the family of graphdiynes (GDYs) and having different structures and sp/sp2 carbon ratios. We analyze how structural and topological effects can tune the relative stability and the electronic behavior, to propose a rationale for the development of new systems with tailored properties. A total of 26 structures have been generated, including the already known polymorphs such as α-, β-, and γ-GDY. Periodic density functional theory calculations have been employed to optimize the 2D crystal structures and to compute the total energy, the band structure, and the density of states. Relative energies with respect to graphene have been found to increase when the values of the carbon sp/sp2 ratio increase, following however different trends based on the peculiar topologies present in the crystals. These topologies also influence the band structure, giving rise to semiconductors with a finite band gap, zero-gap semiconductors displaying Dirac cones, or metallic systems. The different trends allow identifying some topological effects as possible guidelines in the design of new 2D carbon materials beyond graphene.

Calculation of the Resonance States of Coulomb Acceptors in Zero-Gap Semiconductors

Calculation of the Resonance States of Coulomb Acceptors in Zero-Gap Semiconductors

Critical charge fluctuations and emergent coherence in a strongly correlated excitonic insulator

Excitonic insulator is a coherent electronic phase that results from the formation of a macroscopic population of bound particle-hole pairs—excitons. With only a few candidate materials known, the collective excitonic behavior is challenging to observe, being obscured by crystalline lattice effects. Here we use polarization-resolved Raman spectroscopy to reveal the quadrupolar excitonic mode in the candidate zero-gap semiconductor Ta2NiSe5 disentangling it from the lattice phonons. The excitonic mode pronouncedly softens close to the phase transition, showing its electronic character, while its coupling to noncritical lattice modes is shown to enhance the transition temperature. On cooling, we observe the gradual emergence of coherent superpositions of band states at the correlated insulator gap edge, with strong departures from mean-field theory predictions. Our results demonstrate the realization of a strongly correlated excitonic state in an equilibrium bulk material.

Magnetotransport signatures of chiral magnetic anomaly in the half-Heusler phase ScPtBi

Study of magnetotransport properties of ScPtBi revealed simultaneously: a negative contribution to the longitudinal magnetoresistance, the planar Hall effect, and distinct angular narrowing of the longitudinal magnetoresistance { three hallmarks of chiral magnetic anomaly (pumping of axial charge between Weyl nodes), a distinct property of topological semimetals. Electronic structure calculations show that structural defects, such as antisites and vacancies, bring substantial density of states at the Fermi level of ScPtBi, indicating that it is a semimetal, not a zero-gap semiconductor, as predicted earlier. This is in accord with electrical resistivity in ScPtBi, showing no characteristics of semiconductor. Moreover, below 0.7K we observed an onset of a superconducting transition, with the resistivity disappearing completely below 0.23 K.

Diverse Properties of Carbon-Substituted Silicenes

The theoretical framework, which is built from the first-principles results, is successfully developed for investigating emergent two-dimensional materials, as it is clearly illustrated by carbon substitution in silicene. By the delicate VASP calculations and analyses, the multi-orbital hybridizations are thoroughly identified from the optimal honeycomb lattices, the atom-dominated energy spectra, the spatial charge density distributions, and the atom and orbital-decomposed van Hove singularities, being very sensitive to the concentration and arrangements of guest atoms. All the binary two-dimensional silicon-carbon compounds belong to the finite- or zero-gap semiconductors, corresponding to the thoroughly/strongly/slightly modified Dirac-cone structures near the Fermi level. Additionally, there are frequent π and σ band crossings, but less anti-crossing behaviors. Apparently, our results indicate the well-defined π and σ bondings.

Ab Initio Study of Structural, Electronic, and Elastic Properties of Graphene

Since its discovery in 2004, graphene has attracted the intention of several researchers in the world because of its fascinating electronic and mechanical properties. Various theoretical and experimental works have been devoted to this material. In this paper, we used a full-potential linearized augmented plane-wave (FP-LAPW) method to investigate the structural, electronic, and mechanical properties of graphene in hexagonal structure within local density and generalized gradient approximations (LDA and GGA). Our results are found in good agreement with other theoretical and experimental contributions. Using a modified Becke–Johnsone GGA approximation, we have also confirmed that graphene is a zero-gap semiconductor with the presence of a Dirac cone. In our contribution, we have also calculated the elastic constants, the Young’s modulus and Poisson’s ratio of graphene that are found in good agreement with the results published in the literature.

Various challenges in realizing spin-gapless semiconductivity in Ti2CoSi

Spin-gapless semiconductors are recently discovered class of materials that behave as an insulator for one spin channel and as a zero-gap semiconductor for the opposite spin. Here, we show from first-principle calculations that one such material Ti2CoSi predicted to exhibit spin-gapless semiconductivity has an energetically close non-spin-polarized phase. In particular, we show that the regular Heusler phase of this material is non-magnetic, while the inverted Heusler phase is nearly spin-gapless semiconducting, with a very small energy difference of ≈0.1 eV per 16-atom cell, in favor of the regular Heusler structure. Moreover, we also show that a 100% spin polarization in inverted Heusler phase is detrimentally affected by the emergence of surface states in thin-film geometry. These results need to be taken into account for realistic implementations of this and similar materials in nano-device applications, which rely on highly spin-polarized current in thin-film geometry.

Band Structure and Physical Properties of α-STF2I3: Dirac Electrons in Disordered Conduction Sheets

The compound being investigated is an organic charge-transfer complex of the unsymmetrical donor STF with I3 [STF = bis(ethylenedithio)diselenadithiafulvalene], which is isostructural with α-ET2I3 and α-BETS2I3 [ET = bis(ethylenedithio)tetrathiafulvalene, BETS = bis(ethylenedithio)tetraselenafulvalene]. According to recent studies, the calculated band structure should represent a zero-gap semiconductor at 1 bar that is similar to α-ET2I3 under high pressure (>15 kbar). Such materials have attracted extensive interest because the electrons at the Fermi level can be massless Dirac fermions (MDFs), with relativistic behaviors like those seen in graphene. In fact, α-STF2I3 exhibited nearly temperature-independent resistivity, ρ, (~100–300 K), a phenomenon that is widely observed in zero-gap semiconductors. The non-Arrhenius-type increase in ρ (<~100 K) was consistent with the characteristics of interacting MDFs. The paramagnetic susceptibility, χ, (2–300 K)—as well as the reflectivity, R and optical conductivity, σ, (25–300 K; 400–25,000 cm−1)—were also almost temperature independent. Furthermore, σ was practically independent of wavenumber at ~6000–15,000 cm−1. There was no structural transition based on X-ray studies (90–300 K). Considering all the electrical, magnetic, optical and structural properties of α-STF2I3 at 1 bar, it was concluded that the salt possesses a band structure characterized with Dirac cones, which was consistent with the calculation.