Monolayer InSe Research Articles

Electronic and Optical Properties of BP, InSe Monolayer and BP/InSe Heterojunction with Promising Photoelectronic Performance.

Two-dimensional (2D) materials provide a new strategy for developing photodetectors at the nanoscale. The electronic and optical properties of black phosphorus (BP), indium selenide (InSe) monolayer and BP/InSe heterojunction were investigated via first-principles calculations. The geometric characteristic shows that the BP, InSe monolayer and BP/InSe heterojunction have high structural symmetry, and the band gap values are 1.592, 2.139, and 1.136 eV, respectively. The results of band offset, band decomposed charge and electrostatic potential imply that the heterojunction structure can effectively inhibit the recombination of electron–-hole pairs, which is beneficial for carrier mobility of photoelectric devices. Moreover, the optical properties, including refractive index, reflectivity, electron energy loss, extinction coefficient, absorption coefficient and photon optical conductivity, show excellent performance. These findings reveal the optimistic application potential for future photoelectric devices. The results of the present study provide new insight into challenges related to the peculiar behavior of the aforementioned materials with applications.

Strong Influence of the Anion Radius on the Passivation of Selenium Vacancy in Monolayer InSe: Insights from Time-Domain Ab Initio Analysis

Using nonadiabatic molecular dynamics combined with the time-domain density functional theory, we have explored the influence of selenide vacancy and passivation of anions with different radii on the nonradiative charge trapping and recombination in monolayer InSe. Our results reveal that electron–hole recombination for pristine InSe occurs within several nanoseconds due to the weak nonadiabatic (NA) coupling. Selenide vacancy generates three trap states within the band gap, enhances the NA coupling, and provides new channels for charge carrier relaxation, resulting in the significantly decreased charge carrier recombination time. Passivating the selenide vacancy with anions (O2–, S2–, and Te2–) can eliminate the trap states within the band gap and extend the charge carrier lifetimes because of the decreased NA coupling. Meanwhile, the passivation effect of anions is dramatically dependent on the type of anions, and S2– is more suitable for repairing selenide vacancy than O2– and Te2– because S2– and Se2– have much closer radii. This study provides an atomistic mechanism of the effects of selenide vacancy and anion passivation on the performance of InSe thin-film solar cells and suggests that the choice of anions with a suitable radius is valuable for prolonging the excited-state lifetime.

CO2 gas sensor based on Pt-, Ag-, Au- and Pd-doped InSe monolayer: a first-principles study

Through a first-principles study based on density functional theory, a physical model of CO2 gas molecule adsorption on an InSe monolayer was designed and built. The geometric structures of the InSe monolayer doped with different transition metal elements were optimized, and the spin-polarized energy band structure and magnetoelectric properties, such as the density of states, semimetallicity and magnetic moment of nanosheets with stable adsorbed phases, were calculated. The microscopic mechanism of these properties was analyzed by crystal field theory, and it was found that InSe monolayers are typical semiconductors, but transition metal–InSe monolayers can conduct electricity and are typical semi-metallic nanosheets. Second, the adsorption mechanism of CO2 gas molecule adsorption on InSe monolayers is studied. The calculation results show that when the O atom of the CO2 gas molecule is adsorbed on the surface of an InSe monolayer, the adsorption structure is relatively stable. The surface adsorption is mainly due to the transition of electrons on the InSe monolayer surface to the CO2 gas molecule. The doping of metal atoms gives CO2 gas molecules strong adsorption energy, promotes the magnetic properties of the adsorption system, and completes the transition from semiconductor to metal. The change of the work function of the adsorption system before and after doping indicates that this doping method can also enhance the sensitivity of the InSe monolayer to CO2 gas molecules.

First-principles calculation of influence of biaxial strain on the electronic structure and optical properties of ZnSe/InSe van der waals heterojunction

The electronic properties of the InSe monolayer supported on the surface of ZnSe monolayer were studied by the DFT study. In this paper, a first-principles approach was used to study the regulation of the electronic structure and optical properties of InSe/ZnSe heterojunctions by biaxial strain. The results show that the forbidden band width of the InSe/ZnSe heterojunction is 1.040 eV. Due to the built-in electric field generated in the heterojunction system, the recombination rate of photogenerated carriers is reduced, and the light absorption band edge of the heterojunction is extended to the visible light region. After straining the heterojunction, its forbidden band width is reduced. Compared with the unstrained system, the strain has a stronger hindering effect on photogenerated electron carriers, and its photocatalytic ability is greatly improved. The above results show that the construction of the InSe/ZnSe heterojunction and the biaxial strain can significantly control the electronic structure and optical properties of the heterojunction, making it suitable for applications in the fields of narrow band gap and infrared and visible light semiconductor devices, such as photocatalytic materials.

Polarization-Resolved and Helicity-Resolved Raman Intensity of Monolayer and Bilayer β-InSe

The structural and vibrational properties along with the polarization-resolved and helicity-resolved Raman spectra of monolayer and bilayer β-InSe excited by two commonly used laser lines (532 and 633 nm) in experiments are investigated by first-principles calculations so that we can establish the correlation between layer stacks and optical features in two-dimensional InSe. The stability of both monolayer and bilayer InSe are examined and confirmed by phonon dispersion. The monolayer and bilayer InSe are found to exhibit different point group symmetries and thereby the Raman properties due to the symmetry breaking of bilayer structure caused by interlayer van der Waals interaction. All Raman-active modes for monolayer (bilayer) InSe are listed, and the polarization-angle-dependent Raman intensities are accordingly discussed separately in terms of the configuration of the incident light (parallel or perpendicular to certain planes). By calculating the Raman tensor of each Raman-active mode, we obtained polarization-angle-dependent Raman intensities and discussed the location of each maximum under different polarization configurations, as well as the nonpolarized Raman intensity. The helicity-resolved Raman intensity is also obtained. It is found that more Raman-active modes can be observed under helical excitation when the incident light wave vector is in the y-direction. Among all of the Raman-active modes, the Raman intensity of A′1(A1g) is the strongest. Our work might help material scientists to characterize monolayer and bilayer InSe in experiments.

Spin Hall effect in two-dimensional InSe: Interplay between Rashba and Dresselhaus spin-orbit couplings

Materials with inversion asymmetries can exhibit strong spin Hall effect (SHE) in the presence of Dresselhaus and Rashba spin-orbit coupling (D/R SOC) interactions. Ideally, in a two-dimensional crystal, inversion asymmetry could be modulated by stacking order and external perturbations. Here, using first-principles calculations, we systematically investigate the interplay between DSOC and RSOC and their influences on SHE in mono- and bilayer InSe. We show that in the presence of DSOC, the introduction of Rashba interaction through gate voltages in monolayer InSe increases Zeeman-like spin splitting around the Brillouin-zone center and contributes to the enhancement of spin Hall conductivity (SHC), which reaches a saturation point due to the RSOC-enforced spiral-spin texture. The SHC in the unperturbed centrosymmetric $\mathit{AB}$ stacked bilayer shows a peak associated with the Mexican-hat-like valence-band edge; however, in a wide energy range the SHC stays insignificant. In the $\mathit{AB}$\ensuremath{'} stacked bilayer with the intrinsic RSOC present, the value of SHC can be comparable to that of $\mathit{AB}$ stacked bilayer with an external electric field. Moreover, we show that the spin-momentum locking in the $\mathit{AB}$\ensuremath{'} stacked bilayer is switchable by a gate voltage. These findings provide a promising route for spintronic and magneto-optical applications by exploiting the rich physics of spin-orbit effects.

Emergence of superconductivity in an InSe monolayer: Roles of deposited metal and biaxial strain

Two-dimensional (2D) superconductors are important for both fundamental superconducting physics and potential nanoscale applications. Here, using first-principles calculations, we unravel that an Na-deposited InSe monolayer (NaIn2Se2) is dynamically stable and may constitute a novel 2D superconductor with a transition temperature (Tc) of 4.42 K due to the combined effect of strong Fermi nesting and strong electron-phonon coupling (EPC) from in-plane phonons. In contrast, Li- and Mg-deposited InSe monolayers are dynamically unstable. Furthermore, a 3% biaxial tensile strain increases the EPC strength of NaIn2Se2 and Tc is increased to 4.88 K. However, a 3% compressive strain slightly lowers Tc to 4.12 K. Our work demonstrates that Na-deposited InSe monolayer may be a novel 2D superconductor and that strain is important to maximize its Tc.

Coexisting charge density wave and ferromagnetic instabilities in monolayer InSe

Recently fabricated InSe monolayers exhibit remarkable characteristics that indicate the potential of this material to host a number of many-body phenomena. In this work, we systematically describe collective electronic effects in hole-doped InSe monolayers using advanced many-body techniques. To this end, we derive a realistic electronic-structure model from first principles that takes into account the most important characteristics of this material, including a flat band with prominent van Hove singularities in the electronic spectrum, strong electron–phonon coupling, and weakly screened long-ranged Coulomb interactions. We calculate the temperature-dependent phase diagram as a function of band filling and observe that this system is in a regime with coexisting charge density wave and ferromagnetic instabilities that are driven by strong electronic Coulomb correlations. This regime can be achieved at realistic doping levels and high enough temperatures, and can be verified experimentally. We find that the electron–phonon interaction does not play a crucial role in these effects, effectively suppressing the local Coulomb interaction without changing the qualitative physical picture.

Stability and Bandgap Engineering of In1-xGaxSe Monolayer.

Bandgap engineering of semiconductor materials represents a crucial step for their employment in optoelectronics and photonics. It offers the opportunity to tailor their electronic and optical properties, increasing the degree of freedom in designing new devices and widening the range of their possible applications. Here, we report the bandgap engineering of a layered InSe monolayer, a superior electronic and optical material, by substituting In atoms with Ga atoms. We developed a theoretical understanding of stability and electronic properties in its whole compositional range () through first-principles density functional theory calculations, the cluster expansion method, and kinetic Monte Carlo simulations. Our findings highlight the possibility of modulating the InGaSe bandgap by ≈0.41 eV and reveal that this compound is an excellent candidate to be employed in many optoelectronic and photonic devices.

Tuning the band gap of the InSe monolayer by substitutional doping

Indium Selenide (InSe) has attracted widespread attention due to its excellent optoelectronic properties. Band-gap engineering provides opportunities for the application of InSe materials in the field of optoelectronics. Herein, the substitution of dopant Te atoms for Se atoms in the InSe monolayers was investigated. It was found that substitutional doping can provide an effective way to adjust the electronic structure and improve the performance of InSe monolayer semiconductor. Accordingly, increasing the concentration of the Te dopant brings a transition from the indirect to the direct band gap. Specifically, the transition occurs when the Te concentration is around 33%. With the gradual increase of the Te concentration, the band gap increased initially and then decreased. The band gap reached a maximum of 1.64 eV at a Te concentration of 33.3% and then gradually decreased to 1.08 eV beyond the maximum dopant concentration. These results showcase the controllable and changeable band gap of the InSe semiconductor and open up a new way for the potential application of InSe as a competent candidate in optoelectronic devices.

Point Defects in Two-Dimensional Indium Selenide as Tunable Single-Photon Sources.

In the past few years remarkable interest has been kindled by the development of nonclassical light sources and, in particular, of single-photon emitters (SPE), which represent fundamental building blocks for optical quantum technology. In this Letter, we analyze the stability and electronic properties of an InSe monolayer with point defects with the aim of demonstrating its applicability as an SPE. The presence of deep defect states within the InSe band gap is verified when considering substitutional defects with atoms belonging to group IV, V, and VI. In particular, the optical properties of Ge as substitution impurity of Se predicted by solving the Bethe–Salpeter equation on top of the GW corrected electronic states show that transitions between the valence band maximum and the defect state are responsible for the absorption and spontaneous emission processes, so that the latter results in a strongly peaked spectrum in the near-infrared. These properties, together with a high localization of the involved electronic states, appear encouraging in the quest for novel SPE materials.

Influence of Ce, Nd, Eu and Tm Dopants on the Properties of InSe Monolayer: A First-Principles Study.

Doping of foreign atoms may substantially alter the properties of the host materials, in particular low-dimension materials, leading to many potential functional applications. Here, we perform density functional theory calculations of two-dimensional InSe materials with substitutional doping of lanthanide atoms (Ce, Nd, Eu, Tm) and investigate systematically their structural, magnetic, electronic and optical properties. The calculated formation energy shows that the substitutional doping of these lanthanide atoms is feasible in the InSe monolayer, and such doping is more favorable under Se-rich than In-rich conditions. As for the structure, doping of lanthanide atoms induces visible outward movement of the lanthanide atom and its surrounding Se atoms. The calculated total magnetic moments are 0.973, 2.948, 7.528 and 1.945 μB for the Ce-, Nd-, Eu-, and Tm-doped systems, respectively, which are mainly derived from lanthanide atoms. Further band structure calculations reveal that the Ce-doped InSe monolayer has n-type conductivity, while the Nd-doped InSe monolayer has p-type conductivity. The Eu- and Tm-doped systems are found to be diluted magnetic semiconductors. The calculated optical response of absorption in the four doping cases shows redshift to lower energy within the infrared range compared with the host InSe monolayer. These findings suggest that doping of lanthanide atoms may open up a new way of manipulating functionalities of InSe materials for low-dimension optoelectronics and spintronics applications.

Spin polarized quantum oscillations in monolayer InSe induced by magnetic field

We study the quantum capacitance and the Landau levels (LLs) in n-type monolayer InSe subjected to a vertical magnetic field. Starting from a single band k⋅p Hamiltonian, we analytically obtain the LLs by solving the Hamiltonian and find that the LLs have a uniform distribution on the LL index and magnetic field, i.e., the equal LL spacings. Further, the LLs are fully spin polarized under moderate magnetic fields originating from the spin–orbit coupling and Zeeman splitting. The magneto-capacitance peaks show one-to-one correspondence to the LLs, resulting in a perfect square-wave-shaped spin polarization. Our results is useful to detect the effective mass and spin–orbit coupling parameters of n-type monolayer InSe and design spintronics devices based on it.

Tuning the electronic properties of two dimensional InSe/In2Se3 heterostructure via ferroelectric polarization and strain

Two dimensional (2D) ferroelectric heterostructures with tunable electronic properties show many novel physical properties, which have gradually aroused great interest. Herein, based on first-principle calculations, we present a comprehensive research on the electronic properties of InSe/In2Se3 ferroelectric van der Waals heterostructures with and without strains. We have obtained the most stable stack configuration of the heterostructure. Under a certain strain, the pure electric field can control the change of semiconductor types and the transition between direct and indirect band gap. In addition, we found that the built-in electric field served by ferroelectric polarization makes the InSe/In2Se3 heterostructure have higher light absorption than monolayer InSe and In2Se3. Our results show that InSe/In2Se3 heterostructure may have great potential applications in electronic and optoelectronic devices or other fields.

InSe/Te van der Waals Heterostructure as a High-Efficiency Solar Cell from Computational Screening.

Designing the electronic structures of the van der Waals (vdW) heterostructures to obtain high-efficiency solar cells showed a fascinating prospect. In this work, we screened the potential of vdW heterostructures for solar cell application by combining the group III–VI MXA (M = Al, Ga, In and XA = S, Se, Te) and elementary group VI XB (XB = Se, Te) monolayers based on first-principle calculations. The results highlight that InSe/Te vdW heterostructure presents type-II electronic band structure feature with a band gap of 0.88 eV, where tellurene and InSe monolayer are as absorber and window layer, respectively. Interestingly, tellurene has a 1.14 eV direct band gap to produce the photoexcited electron easily. Furthermore, InSe/Te vdW heterostructure shows remarkably light absorption capacities and distinguished maximum power conversion efficiency (PCE) up to 13.39%. Our present study will inspire researchers to design vdW heterostructures for solar cell application in a purposeful way.

Strain-tunable electronic and optical properties of novel MoSSe/InSe van der Waals heterostructures

van der Waals heterostructures (vdWHs) have been widely studied due to their particular properties. In this work, the electronic structure and the optical properties of MoSSe/InSe vdWH are studied by first-principles calculations, and the changes of strain modulation properties are also discussed. Theoretical research shows that the MoSSe/InSe vdWHs have an indirect band gap and the band alignments are type-I and type-II correspond to two different stacking patterns, respectively. Compared with MoSSe and InSe monolayer, the optical absorption of the MoSSe/InSe vdWH in the visible and ultraviolet region is improved. In addition, the band gap, the band alignment, and the optical absorption of the MoSSe/InSe vdWH can be effectively modulated by biaxial strain. This work provides possibilities for the application of the MoSSe/InSe vdWH in optical devices, electronic devices and photocatalysis.

Bandgap engineering of layered mono-chalcogenides via pressure

The layered mono-chalcogenide semiconductors MX (M = In, Ga; X = Se, S) have attracted considerable attention due to their high carrier mobility and tunable bandgap, which gives them potential applications in the development of new optoelectronic devices. We have systematically studied the effect of pressure on the band structure evolution and electronic properties of monolayer indium selenide (InSe) using first-principles calculations. The bandgap of monolayer InSe first increases and then decreases with increasing pressure. In addition, monolayer InSe undergoes an indirect to direct bandgap transition at 6.8 GPa. Increasing the pressure further to 10.4 GPa leads to recovery of the indirect bandgap. This indirect-direct bandgap transition is absent in other monolayer MX compounds. The continuous tuning of band structure of monolayer InSe gives them potential applications for pressure-response optoelectronic devices.

Theoretical insight into two-dimensional g-C6N6/InSe van der Waals Heterostructure: A promising visible-light photocatalyst

The monolayer InSe has drawn extensive attention since it was firstly fabricated by exfoliation and encapsulation in 2017. In the paper the g-C6N6/InSe heterostructure is designed to achieve high-performance photocatalytic application, and it is unveiled that the g-C6N6/InSe is a typical type-II heterstructure. There is significant charge transferred from the g-C6N6 to the InSe in the heterostructure, which is demonstrated by charge density difference and Bader charge analysis. Moreover, the built-in electric field exists between the InSe layer and the g-C6N6 layer, which is favorable for the effective separation of photogenerated electron-hole pairs. In the meanwhile the binding energy shows that the g-C6N6/InSe is a van der Waals heterostructure, and the band gap for that can be effectively tuned by interfacial strain. In addition, it is manifested that absorption coefficient for the g-C6N6/InSe heterostructure in the visible-light range is enhanced remarkably as compared with individual InSe layer, which implies that constructing heterostructure is an effective way to improve optical performance. Furthermore, elementary reactions are also discussed in detail to evaluate photocatalytic activity. These results pave a path to deepen the understanding of the InSe-based heterostructure, which is useful to design and synthesize new water-splitting photocatalysts applied in the visible-light range.

Electronic and transport properties of GaAs/InSe van der Waals heterostructure

Recently, the GaAs/InSe heterostructure was synthesized experimentally, and the spatial redistribution of charge and motion of energy-resolved photoelectron was imaged. To gain more insight into the detailed electronic structure and expand knowledge to transport behaviors and physical field tuning effects, we here construct the GaAs/InSe van der Waals heterostructure theoretically and perform investigations in depth, especially focusing on the comprehensive understanding and mechanism analysis of results, trying to create new predictions. The results show that such a heterostructure holds a typical type-II band alignment and its band gap is narrowed significantly as compared with its each component, and the indirect-direct band gap transition is realized, which holds great potential applications in developing excellent photovoltaic materials and new photo-electronic devices. Particularly, the carrier mobility of heterostructure is predicted to be enhanced dramatically to μ > 104 cm2 V−1 s−1, greatly overcoming the intrinsic weakness of InSe monolayer with a very low hole mobility, ~101 cm2 V−1 s−1. Meanwhile, we find that the electronic structure of the heterostructure is flexibly controllable. Under higher compressive strain and whole tensile strain, its band-gap size drops linearly. While by applying an external electric field, the band offset is increased significantly, and the type-II band alignment is well preserved and the band gap drops significantly under positive external electric field, These flexibly tunable properties are more favorable to design the photovoltaic materials and photo-electronic devices.

Charge density wave and superconducting phase in monolayer InSe

In this paper, the completed investigation of a possible superconducting phase in monolayer indium selenide is determined using first-principles calculations for both the hole and electron doping systems. The hole-doped dependence of the Fermi surface is exclusively fundamental for monolayer InSe. It leads to the extensive modification of the Fermi surface from six separated pockets to two pockets by increasing the hole densities. For low hole doping levels of the system, below the Lifshitz transition point, superconductive critical temperatures $T_c \sim 55-75$ K are obtained within anisotropic Eliashberg theory depending on varying amounts of the Coulomb potential from 0.2 to 0.1. However, for some hole doping above the Lifshitz transition point, the combination of the temperature dependence of the bare susceptibility and the strong electron-phonon interaction gives rise to a charge density wave that emerged at a temperature far above the corresponding $T_c$. Having included non-adiabatic effects, we could carefully analyze conditions for which either a superconductive or charge density wave phase occurs in the system. In addition, monolayer InSe becomes dynamically stable by including non-adiabatic effects for different carrier concentrations at room temperature.