Band Gap Of Monolayer Research Articles

Optoelectronic and transport properties of layer-dependent two-dimensional perovskite Cs3Bi2I9

The emergence of two-dimensional (2D) perovskite provides an ideal platform for designing and fabricating microelectronic and optoelectronic devices. Here, we present detailed ab initio calculations to comprehensively explore the layer-dependent optoelectronic and transport properties of the newly synthesized 2D perovskite Cs3Bi2I9. The calculations reveal that the indirect band gap of monolayer, bilayer, and trilayer Cs3Bi2I9 decreases from 2.41 to 2.35 eV The charge carriers of few-layered perovskite Cs3Bi2I9 are dominated by electrons, and the highest electron and hole carrier mobilities are 140 and 62 cm²V⁻¹s⁻¹ along the b axis for bilayer Cs3Bi2I9. Exciton binding energies decrease from 2.48 to 1.19 eV with an increment of layer, and the calculated exciton level drops down into the valence band to generate potential exciton insulator. 2D Cs3Bi2I9 exhibits potential in the field of ultraviolet detection and photoluminescent devices due to large exciton energy and ultraviolet absorption.

Tailoring the electronic and optical properties of ReS2 monolayer using strain engineering

In the present work, impact of various mechanical strains on the optoelectronic properties of monolayer ReS2 (m-ReS2) are investigated by using density functional theory. The bandgap of monolayer is determined to be 1.44 eV and 1.31 eV when computed using PBE and PBE + SOC methods. The monolayer displays outstanding electronic and optical tunability under biaxial compressive and shear strains. Under strain variations of 0 %–8 %, the bandgap for biaxial compression varies from 1.44 eV (1.31 eV) to 0.54 eV (0 eV), whereas for shear xx-yy strains, it varies from 1.44 eV (1.31 eV) to 0.34 eV (0.28 eV) when calculated using PBE (PBE + SOC) methods. A semiconductor-to-metal transition is observed for higher values of biaxial compressive strain. A pronounced impact of strain on the optical characteristics is likewise observed. We noticed that the absorption edge of monolayer ReS2 shifts from 1.32 eV to 0.50 eV with a 0 %–8 % increase in biaxial compression, leading to an 11 % red shift in wavelength per 1 % strain change. Moreover, high optical absorption (5 × 105 cm−1), lying from infrared to UV region is observed. The present study points out that strain engineering can be an efficient tool for modifying both the electronic and optical properties of m-ReS2 and may open new avenues for using this material in future optoelectronic applications.

Point Defects in Hexagonal SiP Monolayer: A Systematic Investigation on the Electronic and Magnetic Properties

AbstractIn this work, point defects are proposed to modify the electronic and magnetic properties of SiP monolayer. Pristine monolayer is a non‐magnetic semiconductor 2D material with energy gap of 1.52(2.21) eV as predicted by PBE(HSE06) functional. Single Si vacancy (), Si+P divacancy (), and substituting one P atom by one Si atom () magnetize significantly SiP monolayer. Herein, total magnetic moments between 1.00 and 2.00 are obtained . In contrast, no magnetism is induced by single P vacancy (), substituting one Si atom by one P atom (), and exchanging pair Si‐P positions (). Moreover, significant magnetization of SiP monolayer is also achieved by doping with single Li and Cu atoms ( and ), as well as pair of Li and Cu atoms ( and ). Total magnetic moments between 0.84 and 1.42 are obtained. Interestingly, the half‐metallicity is observed in and systems. When substituting P atom, F, Cl, and Br impurities reduce significantly the SiP monolayer bandgap, preserving its non‐magnetic nature. In contrast, a total magnetic moment of 1.26 is obtained by doping with I atom. Results presented herein may introduce the defected and doped SiP systems as prospective 2D candidates for spintronic and optoelectronic applications.

First-Principles Prediction of High and Low Resistance States in Ta/h-BN/Ta Atomristor.

Two-dimensional (2D) materials have received significant attention for their potential use in next-generation electronics, particularly in nonvolatile memory and neuromorphic computing. This is due to their simple metal-insulator-metal (MIM) sandwiched structure, excellent switching performance, high-density capability, and low power consumption. In this work, using comprehensive material simulations and device modeling, the thinnest monolayer hexagonal boron nitride (h-BN) atomristor is studied by using a MIM configuration with Ta electrodes. Our first-principles calculations predicted both a high resistance state (HRS) and a low resistance state (LRS) in this device. We observed that the presence of van der Waals (vdW) gaps between the Ta electrodes and monolayer h-BN with a boron vacancy (VB) contributes to the HRS. The combination of metal electrode contact and the adsorption of Ta atoms onto a single VB defect (TaB) can alter the interface barrier between the electrode and dielectric layer, as well as create band gap states within the band gap of monolayer h-BN. These band gap states can shorten the effective tunneling path for electron transport from the left electrode to the right electrode, resulting in an increase in the current transmission coefficient of the LRS. This resistive switching mechanism in monolayer h-BN atomristors can serve as a theoretical reference for device design and optimization, making them promising for the development of atomristor technology with ultra-high integration density and ultra-low power consumption.

To study the effect of strain engineering on the band gap of BaI2 monolayer: a first principle approach

Inspired by the recent proposal of improving materials performance by strain engineering, first principles calculations are performed to investigate the consequences of compressive and tensile strain on the band gap of BaI2 monolayer. Pristine monolayer is discovered to be a large band gap material with a band gap of 4.06 eV. The dynamical stability of material has been confirmed by the phonon dispersion curve which includes only real phonon mode. Calculations show that the application of strain decrease the band gap of monolayer. The effect of strain has been observed at the 4%, 8%, 12% compressive and tensile strain. The band gap has been reduced from 4.06 eV to 3.52 eV and 3.63 eV under the application of 12 % compressive and tensile strain respectively. In our knowledge the effect of strain on BaI2 monolayer has been studied for the first time and it opens new avenue to modify the properties of monolayer.

The MoS2-Graphene-Sapphire Heterostructure: Influence of Substrate Properties on the MoS2 Band Structure

Van der Waals MoS2/graphene heterostructures are promising candidates for advanced electronics and optoelectronics beyond graphene. Herein, scanning probe methods and Raman spectroscopy were applied for analysis of the electronic and structural properties of monolayer (ML) and bilayer 2H-MoS2 deposited on single-layer graphene (SLG)-coated sapphire (S) substrates by means of an industrially scalable metal organic chemical vapor deposition process. The SLG/S substrate shows two regions with distinctly different morphology and varied interfacial coupling between SLG and S. ML MoS2 nanosheets grown on the almost free-standing graphene show no detectable interface coupling to the substrate, and a value of 2.23 eV for the MoS2 quasiparticle bandgap is determined. However, if the graphene is involved in hydrogen bonds to the hydroxylated sapphire surface, an increased MoS2/graphene interlayer coupling results, marked by a shift of the conduction band edge toward Fermi energy and a reduction of the ML MoS2 quasiparticle bandgap to 1.98 eV. The surface topography reveals a buckle structure of ML MoS2 in conformity with SLG that is used to determine the dependence of the ML MoS2 bandgap on the interfacial spacing of this heterostructure. In addition, an in-gap acceptor state about 0.9 eV above the valence band minimum of MoS2 has been observed on locally elevated positions on both SLG/S regions, which is attributed to local bending strain in the grown MoS2 nanosheets. These fundamental insights reveal the impact of the underlying substrate on the topography and the band alignment of the ML MoS2/SLG heterostructure and provide the possibility for engineering the quasiparticle bandgap of ML MoS2/SLG grown on controlled substrates that may impact the performance of electronic and optoelectronic devices therewith.

Calculation of excited states of monolayer TPPA-COF based on first principles.

Two-dimensional covalent organic frameworks (COFs) have always been a hot topic in condensed matter physics. Herein, the first 100 excited states of the TPPA-COF are calculated to investigate the optical absorption properties of the materials in the interval. The stable molecular structure of monolayer TPPA-COF is obtained by first-principles calculation, which can be regarded as a hexagon with an aperture size of 18.25 Å. By means of the band structure and density of state analysis, it is found that the monolayer band gap width of the TPPA-COF is 1.52 eV. All excited states of the TPPA-COF exhibit obvious pi → pi* (delocalized π to anti π) local excitation characteristics through analysing the spatial distribution of the electron-hole pairs of the 10 excited states with the highest oscillator strength among the first 100 excited states. In addition, the simulated UV-vis spectra show that the maximum absorption intensity of the TPPA-COF is about 357 684 mol-1 cm-1, indicating that the TPPA-COF is a potential light-absorbing material.

Measuring the Bandgap of Ambipolar 2D Semiconductors using Multilayer Graphene Contact

The bandgaps of monolayers and few layers in 2D semiconductors are usually measured by optical probing such as photoluminescence (PL). However, if their exfoliated thickness is as large as a few nanometers (multilayer over ≈5 L), PL measurements become less effective and inaccurate because the optical transition of a 2D semiconductor often changes from direct to indirect mode. Herein, a simple method to approximately estimate the bandgap of multilayer 2D van der Waals (vdW) semiconductors is introduced; that is utilizing a field‐effect transistor (FET) as a platform. Multilayer graphene (multi‐LG) contact for multilayer van der Waals channels in the FET is used, because multi‐LG contact would secure ambipolar behavior and somewhat enable Schottky barrier modulation in contact with vdW channels. As a result, the bandgaps of multilayer transition‐metal dichalcogenides (TMDs) and black phosphorus in unknown thicknesses are approximated through measuring the temperature‐dependent transfer curve characteristics. The bandgaps are confirmed with photoelectric responsivity measurements, which evidences the validity of the multi‐LG‐induced approximation.

A novel method for synthesis of γ-graphyne and their charge transfer properties

Graphyne represents a family of layered artificial carbon allotropes wherein both sp and sp2 hybridized carbon atoms coexist in the lattice with γ-graphyne being the most stable member of the group. Theoretical studies predict γ-graphyne has high carrier mobility (104–105 cm2V−1s−1), exceptional flexibility, low thermal conductance, and charge transport comparable to graphene with a monolayer bandgap of ∼2.23eV. At present experimental exploration of γ-graphyne is still impeded due to the lack of suitable synthesis techniques. We report a simple one-pot Sonogashira cross-linking strategy for the synthesis of pristine γ-graphyne under mild conditions with high yields. As synthesized product was of high purity and exhibited a bandgap of 2.3eV. We believe this synthetic strategy will be helpful in designing similar strategies for the experimental realization of other members of this family. Charge transfer is a fundamental process for understanding a material's properties and tailoring them for various electronic and optoelectronic applications. In this work, UV–visible absorption studies were carried out to understand the charge transfer behavior and defect density of γ-graphyne with donor/acceptor molecules wherein few-layered γ-graphyne showed a comparable/better charge transfer behavior with organic acceptor molecules (TCNQ) than few-layered graphene.

Monolayer Fullerene Networks as Photocatalysts for Overall Water Splitting.

Photocatalytic water splitting can produce hydrogen in an environmentally friendly way and provide alternative energy sources to reduce global carbon emissions. Recently, monolayer fullerene networks have been successfully synthesized [Hou et al. Nature2022, 606, 507], offering new material candidates for photocatalysis because of their large surface area with abundant active sites, feasibility to be combined with other 2D materials to form heterojunctions, and the C60 cages for potential hydrogen storage. However, efficient photocatalysts need a combination of a suitable band gap and appropriate positions of the band edges with sufficient driving force for water splitting. In this study, I employ semilocal density functional theory and hybrid functional calculations to investigate the electronic structures of monolayer fullerene networks. I find that only the weakly screened hybrid functional, combined with time-dependent Hartree-Fock calculations to include the exciton binding energy, can reproduce the experimentally obtained optical band gap of monolayer C60. All the phases of monolayer fullerene networks have suitable band gaps with high carrier mobility and appropriate band edges to thermodynamically drive overall water splitting. In addition, the optical properties of monolayer C60 are studied, and different phases of fullerene networks exhibit distinct absorption and recombination behavior, providing unique advantages either as an electron acceptor or as an electron donor in photocatalysis.

Theoretical Prediction and Thermal Transport Properties of Novel Monolayer TlPt2Se3

AbstractThe theoretical prediction, electronic properties, and thermal transport properties of novel monolayer TlPt2Se3 are investigated using the first‐principles calculations and semi‐classical Boltzmann transport theory. The calculated phonon band structure and exfoliation energy confirm that monolayer TlPt2Se3 is a stable material and can be exfoliated from its bulk counterpart. The exfoliation energy of the monolayer turns out to be 37 meV Å−2, comparable with the exfoliation energy of monolayer PdSe2. The HSE06 indirect bandgap of monolayer (bulk) TlPt2Se3 amounts to 1.18 eV (0.63 eV). The relaxation time is calculated considering three types of scattering mechanisms. The monolayer outperforms the bulk counterpart in the Seebeck coefficient and power factor for both p‐type and n‐type dopings. Monolayer TlPt2Se3 shows a high p‐type Seebeck coefficient of 211 µV K−1 compared to the n‐type Seebeck coefficient of 103 µV K−1 at maximum considered temperature (600 K) and a carrier concentration (1020 cm−3). The calculated lattice thermal conductivity of monolayer TlPt2Se3 is 1.92 W m−1 K−1 at 600 K which is lower than the monolayer PtSe2 and MoSe2. The p‐type figure of merit of 0.64 (at 600 K) affirms that the monolayer TlPt2Se3 is an excellent thermoelectric material.

Band gap engineering of 2H-MX2 (M = Mo; X = S, Se, Te) monolayers using strain effect

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted wide attention among researchers due to their extraordinary properties similar to graphene. In the present work, we have studied the electronic structure of 2H-MX2 (M = Mo; X = S, Se, Te) monolayer using density functional theory implanted in SIESTA Code. The calculated band structure shows the direct bandgap at the K point for all the MX2 monolayers and have values 1.56 eV, 1.38 eV, 0.98 eV for MoS2, MoSe2, MoTe2, respectively. We have carried out the systematic uniaxial tensile mechanical strain effect on the bandgap of monolayers in the range of −10% to + 10%. This systematic uniaxial tensile mechanical stain decreases the energy gap for all monolayers as we increase the strain from 0% to 10%. The strain variation from 0 to −10% yields varying bandgap, which firstly increases and then decreases with strain. Hence, we find that the application of strain could modulate the properties which forms the broken gap heterostructures within the 2H class.

Van der Waals heterostructure Pt2HgSe3/CrI3 for topological valleytronics

We identify a valley-polarized Chern insulator in a van der Waals heterostructure, monolayer ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$/monolayer ${\mathrm{CrI}}_{3}$, for potential applications with interplay between electric, magnetic, optical, and mechanical effects. The interlayer proximity magnetic coupling nearly closes the band gap of monolayer ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$, and the strong intralayer spin-orbit coupling further lifts the valley degeneracy by over 100 meV, leading to positive and negative band gaps at opposite valleys. In the valley with negative gap, the interfacial Rashba spin-orbit coupling opens a topological band gap of 17.8 meV, which is enlarged to 30.8 meV by adding a hexagonal boron nitride ($h$-BN) layer. We find large orbital magnetization in the ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ layer that is much larger than spin, which can induce a measurable optical Kerr effect. The valley polarization and Chern number are coupled to the magnetic order of the nearest-neighbor ${\mathrm{CrI}}_{3}$ layer, which is switchable by electric, magnetic, and mechanical means in experiments. The presence of $h$-BN protects the topological phase, allowing the construction of superlattices with valley, spin, and layer degrees of freedom.

Strain-tunable electronic structure of two-dimensional monolayer SiP

The 2D monolayer [Formula: see text]-SiP has a honeycomb lattice and an intrinsic indirect band gap. Herein, the density functional theory calculations are performed to modulate the electronic structure of 2D monolayer [Formula: see text]-SiP by applying strains. The band gap of monolayer [Formula: see text]-SiP is monotonously reduced by the strains. More interestingly, a direct band gap is more likely to be achieved by applying strains along the armchair direction than along the zigzag direction. Finally, 2D monolayer [Formula: see text]-SiP can possess a tunable direct band gap of 1.57–0.73 eV (HSE06) and considerable visible light absorption index, by applying compression strains of −6–−10% along the armchair direction. The work provides a route of modulating the electronic and optical properties of monolayer [Formula: see text]-SiP, which extends its application range for various fields such as electronic devices and solar energy conversion.

Opening band gaps of low-dimensional materials at the meta-GGA level of density functional approximations

The quasiparticle band structure can be properly described by Hedin's GW approximation (GW), at a high computational cost. For band gaps, semilocal density functionals up to the generalized gradient approximation (GGA) level cannot compete with the accuracy of hybrid-based approximations or GW. Meta-GGA density functionals with a strong dependence on the kinetic energy density ingredient can potentially give wider band gaps compared with GGAs. The recent TASK meta-GGA density functional from Aschebrock and K\"ummel [Phys. Rev. Research 1, 033082 (2019)], is constructed with an enhanced nonlocality in the generalized Kohn-Sham scheme and therefore harbors great opportunities for band gap prediction. Although this approximation was found to yield excellent band gaps of bulk solids, this accuracy cannot be straightforwardly transferred to low-dimensional materials. The reduced screening of these materials results in larger band gaps compared with their bulk counterparts, as an additional barrier to overcome. In this paper we demonstrate how the alteration of this functional affects the band gaps of monolayers and nanoribbons and present accurate band gaps competing with the revised Heyd-Scuseria-Ernzerhof (HSE06) approximation. In order to achieve this goal, we have modified the TASK functional (a) by changing the tight upper bound for one- or two-electron systems (${h}_{X}^{0}$) from 1.174 to 1.29 and (b) by changing the limit of the interpolation function ${f}_{X}(\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\infty}$) of the TASK functional that interpolates the exchange enhancement factor ${F}_{X}(s,\ensuremath{\alpha})$ from $\ensuremath{\alpha}=0$ to 1. The resulting modified TASK (mTASK) was tested for various materials from three dimensions to two dimensions to one dimension (nanoribbons) and was compared with the results of the higher-level hybrid functional HSE06 or with the ${\mathrm{G}}_{0}{\mathrm{W}}_{0}$ approximation within many-body perturbation theory. We find that mTASK systematically improves the band gaps and band structures of two-dimensional (2D) and 1D systems, without significantly affecting the accuracy of the original TASK for the bulk 3D materials, when compared with the Perdew-Burke-Ernzerhof (PBE) GGA and the strongly constrained and appropriately normed (SCAN) meta-GGA. We further demonstrate the applicability of mTASK by assessing the band structures of transition metal dichalcogenide nanoribbons with respect to various bending curvatures.

Laser stimulation ofMoSe2/Au(111)in an STM junction: Photoinduced versus thermally induced current response

A laser beam is focused onto the junction of a scanning tunneling microscope (STM) to probe the local photocurrent response of highly ordered $\mathrm{Mo}{\mathrm{Se}}_{2}$ islands. The islands, grown by physical vapor deposition onto Au(111), are characterized by uniform triangular shapes, Mo-terminated edges, and the typical 1.8 V band gap of monolayer $\mathrm{Mo}{\mathrm{Se}}_{2}$. The laser-induced signals are detected with nanometer spatial resolution by a lock-in amplifier, both in the form of point spectra and surface maps. To distinguish real photocurrents from current contributions, arising from periodic tip expansions due to the dissipated laser power, blind experiments are performed in which the tip height is modulated without laser irradiation. In addition, the expected current responses are simulated as a function of sample bias for the two scenarios. Comparison between experimental and simulated data indicates that periodic tip expansions dominate the detected current modulations and photoinduced signals are of minor importance. Our work provides various indicators to safely distinguish photo- and thermally induced current components in a laser-driven STM experiment and discusses means to enhance the desired photoresponse with respect to parasitic thermal effects.

Tunable electronic and magnetic properties of MoSi2N4 monolayer via vacancy defects, atomic adsorption and atomic doping

The two dimensional MoSi2N4 (MSN) monolayer exhibiting rich physical and chemical properties was synthesized for the first time last year. We have used the spin-polarized density functional theory to study the effect of different types of point defects on the structural, electronic, and magnetic properties of the MSN monolayer. Adsorbed, substitutionally doped (at different lattice sites), and some kind of vacancies have been considered as point defects. The computational results show all defects studied decrease the MSN monolayer band gap. We found out the H-, O-, and P-doped MSN are n-type conductors. The arsenic-doped MSN, and MSN with vacancy defects have a magnetic moment. The MSN with a Si vacancy defect is a half-metallic which is favorable for spintronic applications, while the MSN with a single N vacancy or double vacancy (N + S) defects are metallic, i.e., beneficial as spin filters and chemical sensors.

New two-dimensional arsenene polymorph predicted by first-principles calculation: robust direct bandgap and enhanced optical adsorption

In this work, we predict a new polymorph of 2D monolayer arsenic. This structure, named δ-As, consists of a centrosymmetric monolayer, which is thermodynamically and kinetically stable. Distinctly different from the previously predicted monolayer arsenic with an indirect bandgap, the new allotrope exhibits a direct bandgap characteristic. Moreover, while keeping the direct bandgap unchanged, the bandgap of monolayer δ-As can be adjusted from 1.83 eV to 0 eV by applying zigzag-direction tensile strain, which is pronounced an advantage for solar cell and photodetector applications.

Thickness- and strain-tunable electronic structures of two-dimensional Bi2O2Se

Two-dimensional (2D) Bi2O2Se that exhibits high carrier mobility, moderate band gap, and good stability is a promising candidate of 2D semiconductors for next-generation computing technologies. However, the electronic properties of ultrathin Bi2O2Se films are poorly understood, especially the thickness and strain tunability of their electronic structures. In this work, we performed a systematic investigation of electronic structures of 2D Bi2O2Se using HSE06 method with high accuracy. Our results show that Bi2O2Se multi-layers display a weak quantum size effect. The origin of this unusual phenomenon is elucidated by the crystal orbital Hamilton population (COHP) analysis. Imposed with biaxial strain, band gaps of monolayer and bilayer can be modulated in a large energy range, e.g., direct band gap of monolayer from 3.08 eV to 1.17 eV which strides over the whole visible light region. Besides, the band gap of bilayer closes up at large tensile strain of ~8%, indicating a semiconductor-metal transition. Further analysis of valence band and conduction band edges shows that high tunability of band gap originates from the sensitivity of band edge with strain, especially conduction band minimum. With the strain from compressive to tensile, Bi-Se bond length gradually increase with the stretch of lattice, which reduces the Bi-Se bond strength and thus lowers the energies of valence band and conduction band edge states due to their antibonding character. Highly tunable electronic properties via changing thickness or adding strain impart Bi2O2Se more possibilities for its application in electronic and optoelectronic devices.

Opening the germanene monolayer band gap using halogen atoms: An efficient approach studied by first-principles calculations

Similar to graphene, the absence of an energy gap has restricted considerably the germanene monolayer practical applications. In this work, we propose an efficient method to create artificially the band gap of the germanene through the chemical functionalization and doping with halogen atoms. Our density functional theory (DFT)-based calculations indicate the semimetal nature of the pristine single layer with a Dirac cone generated at the K point. The applied methods notice structural distortion, however the good stability remains. Relatively large direct band gaps ranging from 0.416 to 1.596 eV can be obtained by the full- and Janus-decoration as confirmed by the HSE06 hybrid functional, where the Cl-Ge-Cl, Br-Ge-Br, F-Ge-Cl, F-Ge-Br, and Cl-Ge-Br exhibit values suitable for optoelectronic applications. In contrast the half-functionalization produces the metallic nature as induced by the large p electrons of the Ge2 atoms. The standard PBE functional shows that the F-, Cl-, Br-, and I-doping give place to the indirect band gap of 0.052, 0.107, 0.116, and 0.110 eV, respectively. Besides, the analysis of the chemical bonds suggests that the halogen atoms are ionically bonded to the host Ge atoms. We believe that results presented herein may pave a solid way to engineer the germanene monolayer electronic properties at the time of designing its applications in nano-devices.