Monolayer InSe Research Articles

Two-dimensional g-C3N4/InSe heterostructure as a novel visible-light photocatalyst for overall water splitting: a first-principles study

The enhanced visible-light harvesting, low recombination of electron–hole pairs and high carrier mobility are found in a novel g-C3N4/InSe hybrid two-dimensional (2D) heterostructure photocatalyst by using first-principles calculations for the first time. The photocatalytic mechanism of g-C3N4/InSe is comprehensively investigated. Our calculations show that 2D g-C3N4/InSe heterostructure has a direct band gap of 1.93 eV and a typical type-II band alignment with holes and electrons located in metal-free g-C3N4 monolayer and non-noble metal InSe nanosheet, respectively. A remarkable visible-light absorption can thus be expected. The electrons and holes located in InSe and g-C3N4 monolayers have a high mobility (104 and 102 cm2 V−1 s−1), which is beneficial for improving the catalytic efficiency. The charge density difference and type-II band structure indicate that the photo-generated electrons easily transfer from g-C3N4 monolayer to InSe nanosheet, and the holes are transferred from InSe to g-C3N4, reducing the electron–hole recombination. Compared with the well-known 2D g-C3N4/MoS2 hybrid photocatalyst composed of g-C3N4 nanosheet and MoS2 monolayer with a low electron mobility (<200 cm2 V−1 s−1) and fast electron–hole recombination due to its direct bandgap, g-C3N4/InSe heterostructure photocatalyst has a distinctive advantage in improving the photocatalytic hydrogen evolution performance due to the high carrier mobility and suppressing the recombination of photo-generated electrons and holes by the indirect band gap of InSe monolayer. These clearly prove that g-C3N4/InSe is an energetic photocatalyst for overall water splitting under visible-light irradiation.

Intriguing electronic properties of germanene/ indium selenide and antimonene/ indium selenide heterostructures

The structural and electronic properties of the germanene/indium selenide (Ge/InSe), antimonene/indium selenide (Sb/InSe) heterostructures and the effects of in-plane strains on their electronic structures are investigated by performing comprehensive DFT calculations. The results show that the hybridization of the interbedded Se- and Ge- (Sb-) p orbitals can increase the binding strength between the InSe and Ge (Sb) monolayers. It is noteworthy that the band gap of germanene is opened by realizing the Ge/InSe heterostructure. In the case of the Sb/InSe heterostructure, all the stacks have direct bandgaps varying from 126.2 meV to 563.7 meV. In addition, strain has significant influence on the band structure. In the C-stacking Ge/InSe, a suitable −2% compression and 4% stretch can trigger the semiconductor-metal transition. Remarkably, an indirect-direct bandgap and semiconductor-metal transition have been observed under sufficient strain in the elastic range in the C-stacking Ge/InSe. These theoretical results could provide useful guidelines for applications of the Ge/InSe and Sb/InSe heterostructures in many fields of nanoelectronics, such as field-effect devices, optoelectronic applications, and strain sensors.

Phonon-mediated superconductivity in electron-doped monolayer InSe: A first-principles investigation

In this study, we investigated the electron–phonon coupling and superconductivity properties of an electron-doped indium selenide (InSe) monolayer for the first time. Electron doping at 0.1 e/cell induced significant phonon softening of the acoustic ZA mode, a lower frequency E″ mode, and higher frequency A′1 mode, which were responsible for the electron–phonon coupling. Under greater doping at 0.2 e/cell, the electronic density of states at the Fermi level increased remarkably and the two Fermi sheets around Γ and M expanded. The transition temperature increased to 3.41 K according to the McMillan–Allen–Dynes formula, which is higher than that of 1.7 K in silicene with doping at 0.44 e/atom and 2.85 K in Na-intercalated MoS2 bilayers calculated using the same method. The phonon-mediated superconductivity predicted in this study combined with the topologically nontrivial characteristics reported previously suggest that few-layer InSe is a potential platform for achieving topologically superconductivity in two dimensions.

Two dimensional XAs (X = Si, Ge, Sn) monolayers as promising photocatalysts for water splitting hydrogen production with high carrier mobility

Two dimensional (2D) materials are emerging as promising candidates for photocatalysts owing to their fascinating properties and ultra-high specific areas. Herein, using first principle calculations, we systematically investigated a new kind of 2D XAs (X=Si, Ge, Sn). After verifying their stabilities, we found that SiAs and SnAs monolayers possess direct band gaps of 2.35 and 1.69eV, while GeAs has an indirect band gap of 2.08eV by HSE hybrid functional with the inclusion of spin-orbit coupling (SOC) effect. Meanwhile, their exceptionally high and directionally anisotropic carrier mobilities (up to 8.98×103cm2V−1s−1 for SiAs) can be advantageous for the fast migration and separation of the photogenerated electron-hole pairs. Most importantly, their band gaps and band edge positions at neutral environment meet the requirement of water splitting. Moreover, the study of XAs heterobilayers (correspondingly stacking with GaS, GaSe and InSe monolayers) shows that they have type-II band alignment, enhanced optical absorption and proper CBM positions with respect to the reduction potential of water, indicating that they may have stronger driving force for the hydrogen evolution reaction. Finally, we explored the adsorption and decomposition of water as well as the subsequent generation of hydrogen on the surfaces of 2D SiAs and SiAs/GaS, which unravels the micro-mechanism of the photocatalytic hydrogen production process. Our finding shows that XAs monolayers can provide appealing alternatives for diverse nanodevices and be potential candidates for metal-free photocatalysts.

Modulation of electronic properties of monolayer InSe through strain and external electric field

In this work, we consider systematically the influence of uniaxial strain and external electric field E on electronic properties of a monolayer InSe using ab initio approach based on density functional theory. Our calculations indicate that the monolayer InSe has a medium indirect energy bandgap of 1.38 eV at equilibrium. The calculated results also demonstrate that we can adjust the bandgap of the monolayer InSe by strain engineering or electric field. The bandgap of the monolayer InSe changes dramatically when the uniaxial strain is applied. Especially, under the compressed uniaxial strain, an indirect–direct bandgap transition has been observed at certain elongations. Within the electric field magnitude E range from 0 to 5 V/nm, the calculated results show that the negative electric field changes the bandgap of the monolayer InSe up to 23% while the positive electric field effect on its bandgap is negligible.

Synthesis of Large-Area InSe Monolayers by Chemical Vapor Deposition.

Recently, 2D materials of indium selenide (InSe) layers have attracted much attention from the scientific community due to their high mobility transport and fascinating physical properties. To date, reports on the synthesis of high-quality and scalable InSe atomic films are limited. Here, a synthesis of InSe atomic layers by vapor phase selenization of In2 O3 in a chemical vapor deposition (CVD) system, resulting in large-area monolayer flakes or thin films, is reported. The atomic films are continuous and uniform over a large area of 1 × 1 cm2 , comprising of primarily InSe monolayers. Spectroscopic and microscopic measurements reveal the highly crystalline nature of the synthesized InSe monolayers. The ion-gel-gated field-effect transistors based on CVD InSe monolayers exhibit n-type channel behaviors, where the field effect electron mobility values can be up to ≈30 cm2 V-1 s-1 along with an on/off current ratio, of >104 at room temperature. In addition, the graphene can serve as a protection layer to prevent the oxidation between InSe and the ambient environment. Meanwhile, the synthesized InSe films can be transferred to arbitrary substrates, enabling the possibility of reassembly of various 2D materials into vertically stacked heterostructures, prompting research efforts to probe its characteristics and applications.

Direct Z-scheme photocatalytic overall water splitting on 2D CdS/InSe heterostructures

Using first-principles calculations, we investigated the photocatalytic mechanism of two-dimensional van der Waals CdS/InSe heterostructures (HSs). Our results show that the buckling distortion of a CdS monolayer sheet in the CdS/InSe HSs induces a built-in polarized electric field, which significantly changes the band edge positions of the InSe layer. The band alignment indicated that the photogenerated electrons in the conduction band (CB) of the InSe monolayer have a high possibility of recombining with the holes in the valence band (VB) of the CdS sheet, resulting in the electrons in the CB of CdS participating in a reduction reaction and the holes in the VB of InSe taking part in an oxidation reaction. This direct Z-scheme photocatalytic system can lead to spatial separation of photogenerated carriers and excellent redox ability, thus enhancing the photocatalytic efficiency. Meanwhile, CdS/InSe HSs can significantly extend the range of light harvesting from visible light to infrared light. Similar results can also be found in the CdS/InSe@2 HSs constructed by InSe bilayer deposit on a CdS nanosheet. These results indicate that CdS/InSe HSs are promising photocatalysts for water splitting.

Strain-tunable p-type Ag doping in the native n-type InSe monolayer

As a new hopeful two-dimensional (2D) layered semiconductor material in nanoscaled electronic devices, 2D InSe nanosheet has been constructed successfully and identified to be a native n-type semiconductor. In this work, we provide theoretical studies that Ag substituting In atom can generate p-type carriers in the InSe monolayer, and its formation energy is lower under Se-rich condition, which shows that the p-type Ag-doped InSe system can be fabricated experimentally. More importantly, compressive strain makes the impurity states move toward the valence band edge, and thus p-type conducting characteristics are further improved. These findings show that for native n-type 2D InSe nanosheet, the properties of p-type carriers and band gap can be tuned, which will be useful to design 2D InSe material-based electronic devices in the future.

Ultrahigh Conductivity in Two-Dimensional InSe via Remote Doping at Room Temperature.

Conductivity of two-dimenstional (2D) materials, which largely determines the efficiency and reliability of nanodevices, is proportional to the product of carrier concentration and mobility. Conventional doping, such as ionic substitution or introduction of vacancies, increases carrier concentration and decreases carrier mobility due to the scattering or trapping of carriers. We propose a remote-doping strategy that enables the simultaneous enhancement of both parameters. Density functional theory calculations in 2D InSe reveal that adsorbing the molecule tetrathiafulvalene (TTF) and applying a 4% external tensile strain leads to an increase in the carrier concentration of the TTF-InSe system that is 13 orders of magnitude higher than that of the pristine counterpart, whereas the carrier mobility is enhanced by 35% compared with the InSe monolayer. As a consequence of the synergetic role of molecule doping and strain engineering, ultrahigh conductivity of 1.85 × 105 S/m is achieved in the TTF-InSe system at room temperature.

First principles study on the electronic properties and Schottky barrier of Graphene/InSe heterostructure

Graphene-based van der Waals heterostructures by stacking graphene on other two-dimensional materials have recently attracted much attention due to their extraordinary properties and greatly extend the applications of the parent materials. By means of the density functional theory from first-principles calculations, in this work, the electronic properties and Schottky contact of the Graphene/InSe heterostructure, together with the effect of strain, are investigated systematically. Our results show that in the graphene/InSe heterostructure, graphene is very weakly bound to the InSe monolayer. Furthermore, we find that due to the sublattice symmetry breaking, a tiny band gap of 5 meV is opened in the graphene/InSe heterostructure, making it suitable for applications in electronic and optoelectronic devices. Moreover, we also find that the n-type Schottky contact is formed in the graphene/InSe heterostructure with a very small Schottky barrier height of 0.05 eV. The Schottky barrier height as well as Schottky contact types in the graphene/InSe heterostructure could be controlled by vertical strain applied perpendicularly to the heterostructure. When the interlayer distance between graphene and the topmost InSe monolayer is smaller than 2.40 Å, one can observe a transformation of the Schottky contact of the graphene/InSe heterostructure. Our results may provide helpful information for designing novel high-performance graphene-based van der Waals heterostructures and explore their potential applications in future nanoelectronic and optoelectronic devices.

Many-Body Effect and Device Performance Limit of Monolayer InSe.

Due to a higher environmental stability than few-layer black phosphorus and a higher carrier mobility than few-layer dichalcogenides, two-dimensional (2D) semiconductor InSe has become quite a promising channel material for the next-generation field-effect transistors (FETs). Here, we provide the investigation of the many-body effect and transistor performance scaling of monolayer (ML) InSe based on ab initio GW-Bethe-Salpeter equation approaches and quantum transport simulations, respectively. The fundamental band gap of ML InSe is indirect and 2.60 eV. The optical band gap of ML InSe is 2.50 eV for the in-plane polarized light, with the corresponding exciton binding energy of 0.58 eV. The ML InSe metal oxide semiconductor FETs (MOSFETs) show excellent performances with reduced short-channel effects. The on-current, delay time, and dynamic power indicator of the optimized n- and p-type ML InSe MOSFETs can satisfy the high-performance and low-power requirements of the International Technology Roadmap for Semiconductors 2013 both down to 3-5 nm gate length in the ballistic limit. Therefore, a new avenue is opened to continue Moore's law down to 3 nm by utilizing 2D InSe.

Electronic structure and magnetism in transition metal doped InSe monolayer: A GGA + U study

The electronic structures and magnetic properties of X-doped (X = Ni, Pd, Pt) InSe monolayers were investigated using first-principles calculations based on the GGA+U method. We show that the chemical doping is easier to take place at Se-rich than In-rich environment, and that Pt substitution of In is energetically most favorable among the three types of doping, followed by Ni and Pd. We further conduct calculation of ground state and band structures of Ni-, Pd- and Pt-doped InSe monolayers and find that the doped systems show diluted magnetic semiconducting nature. The magnetic moments of the X-doped systems are dominantly originated from the X dopants, and their extent of spin density expansion to the neighboring atoms is stronger in Pd- and Pt-doped cases than Ni-doped case. Further analysis of the impurity states near the Fermi energy reveals a strong spin-polarization feature in the three doped systems. Our results demonstrate that substitutional doping with Ni, Pd and Pt in InSe monolayer represents an effective manner to modulate electronic and magnetic properties of InSe for spintronic applications.

Interaction of the O atom with the InSe monolayer: A first-principles study

The interaction of the O impurity atom with the InSe monolayer has been investigated by using first-principles calculations. O atoms energetically highly tend to be located at the inner of the material between the In-In bond. The energy barrier (0.53 eV) for the O diffusion from the topmost atomic layer to the material inner is much smaller than that (1.12 eV) for the diffusion across the topmost atomic layer. These suggest that the O atom on the topmost atomic layer can rapidly diffuse into the material inner far below room temperature. However, the rapid diffusion of the O atom in the material inner may need an elevated temperature, due to its higher diffusion energy barrier of 0.99 eV. For the electronic structure, O impurity single atoms are most likely to only induce a slight change in the bandgap, and moderate changes in the band dispersion and the carrier effective masses of the valence and conduction bands. Further, a higher concentration of O impurities in the inner of InSe has been studied. Our studies are helpful for understanding of the oxidation, enviromental stability, and electronic properties of InSe two-dimensional semiconductors at the atomic level, due to the O impurity.

First-Principles Simulations of FETs Based on Two-Dimensional InSe

We explore the performance limits of monolayer InSe ${n}$ -type and ${p}$ -type FETs by means of first-principle simulations of carrier transport in nanoscale devices. We evaluate the impact on device performance of different device parameters, such as channel length, oxide thickness, and gate underlap. Finally, we assess the operation of a 32-bit CMOS ALU, based on InSe FETs with 10-nm channel length, for both high-performance and low-power applications, and find promising figures of merit with respect to CMOS and other beyond-CMOS proposals.

Adsorption of 3d transition-metal atom on InSe monolayer: A first-principles study

The adsorption of 3d transition metal (TM) atoms (from Sc to Zn) on the InSe monolayer is studied by using density functional theory. The most preferable adsorption site is on top of In atoms for Sc, Ti, and V atoms, while the other seven 3d TM atoms (from Cr to Zn) are found to prefer the hollow adsorption site of InSe monolayer. The sizable adsorption energies are obtained for these systems except Zn-InSe system. The electrons are transferred from the adsorbed TM atoms to the support, thus a n-doping effect is achieved in the InSe monolayer by the TM atoms adsorption. The interesting magnetic properties are exhibited in those TM-InSe systems for TM from Sc to Co, which can be understood based on the orbital alignments and occupations of adsorbed TM atoms. The spin configurations are listed for each system. Our results indicate that great potential exists in these systems for 2D InSe utilization in nanoelectronics and spintronics.

Graphene-like monolayer InSe–X: several promising half-metallic nanosheets in spintronics

Several half-metallic graphene-like nanosheets, namely halogen atom adsorbed InSe–X (X = F, Cl, Br and I) nanosheets, are predicted by first-principles calculations. Then, their structural, electric and magnetic properties are studied in detail. The calculated negative adsorption energies of these InSe–X nanosheets ensure that they attain stable adsorption structures, which suggests that they may be prepared experimentally. The pristine InSe monolayer is a typical semi-conductor, whereas it is interesting that the X ion (X = F, Cl, Br and I) adsorbed InSe–X nanosheets are electronically conductive. They can be promising and good candidates for applications of half-metallic 2D materials. The calculated magnetic moments of these nanosheets are close to 1.0 µB. In the InSe–F nanosheet, there are sp2 hybridized orbitals due to the crystal field effect, and its electroconductibility, half-metallicity and magnetic moments originate from the In and Se ions, not the F ion. However, in InSe–X (X = Cl, Br and I) nanosheets, there are sp3 hybridized orbitals, and their electroconductibility, half-metallicity and magnetic moments originate mainly from X ions, together partially with the In and Se ions.

InSe monolayer: synthesis, structure and ultra-high second-harmonic generation

III–IV layered materials such as indium selenide have excellent photoelectronic properties. However, synthesis of materials in such group, especially with a controlled thickness down to monolayer, still remains challenging. Herein, we demonstrate the successful synthesis of monolayer InSe by physical vapor deposition (PVD) method. The high quality of the sample was confirmed by complementary characterization techniques such as Raman spectroscopy, atomic force microscopy (AFM) and high resolution annular dark field scanning transmission electron microscopy (ADF-STEM). We found the co-existence of different stacking sequence (β- and γ-InSe) in the same flake with a sharp grain boundary in few-layered InSe. Edge reconstruction is also observed in monolayer InSe, which has a distinct atomic structure from the bulk lattice. Moreover, we discovered that the second-harmonic generation (SHG) signal from monolayer InSe shows large optical second-order susceptibility that is 1–2 orders of magnitude higher than MoS2, and even 3 times of the largest value reported in monolayer GaSe. These results make atom-thin InSe a promising candidate for optoelectronic and photosensitive device applications.

N-Type Ohmic contact and p-type Schottky contact of monolayer InSe transistors.

Owing to their few lateral dangling bonds and enhanced gate electrostatics, two-dimensional semiconductors have attracted much attention for the fabrication of channels in next-generation field-effect transistors (FETs). Herein, combining first-principle band structure calculations with more precise quantum transport simulations, we systematically explore the interface properties between monolayer (ML) indium selenide (InSe) and a sequence of common electrodes in an FET. The ML InSe band structure is damaged by Sc, Au, Cr, Pt, and Pd electrodes but identifiable in contact with Ag, Cu, In, graphene and ML O-terminated Cr2C electrodes. A lateral n-type Schottky contact is generated with Sc, Au, Cr, Pt, Pd, and ML graphene electrodes owing to Fermi level pinning originating from the metal-induced gap states, which feature a pinning factor of 0.32. Luckily, a highly desirable lateral n-type Ohmic contact is generated with the Ag, Cu, and In electrodes. The calculated contact polarity is in agreement with the available experimental results using Au, Cr, ML graphene, Ag, and In as electrodes. Remarkably, a lateral p-type Schottky contact is generated with ML O-terminated Cr2C despite the very high work function of ML InSe. Therefore, this study offers a deeper understanding of ML InSe device interfaces and instructions for the design of ML InSe transistors.

Unusual phonon behavior and ultra-low thermal conductance of monolayer InSe.

Monolayer indium selenide (InSe) possesses numerous fascinating properties, such as high electron mobility, quantum Hall effect and anomalous optical response. However, its phonon properties, thermal transport properties and the origin of its structural stability remain unexplored. Using first-principles calculations, we show that the atoms in InSe are highly polarized and such polarization causes strong long-range dipole-dipole interaction (DDI). For acoustic modes, DDI is essential for maintaining its structural stability. For optical modes, DDI causes a significant frequency shift of its out-of-phase vibrations. Surprisingly, we observed that there were two isolated frequency regimes, which were completely separated from other frequency regimes with large frequency gaps. Within each frequency regime, only a single phonon mode exists. We further reveal that InSe possesses the lowest thermal conductance among the known two-dimensional materials due to the low cut-off frequency, low phonon group velocities and the presence of large frequency gaps. These unique behaviors of monolayer InSe can enable the fabrication of novel devices, such as thermoelectric module, single-mode phonon channel and phononic laser.

Electric field analyses on monolayer semiconductors: the example of InSe.

External electric fields can be used to manipulate the electronic properties of two-dimensional (2D) materials. 2D InSe semiconductors possess high electron mobility and wide band gap tunability. Therefore, they have been proposed for use in ultrathin electronic devices. Here, using first-principles calculations, we study the charge polarization, structure, electronic structure, and gas adsorption of an InSe monolayer under vertical electric fields. We find that both the structural evolution and charge polarization rely on the directions of the electric fields. The hole effective mass at the valance band maximum can be decreased by fields that offer a possible route to increase mobility. In contrast, the fields have little impact on the effective mass of electrons at the conduction band minimum. Therefore, high electron mobility in InSe is retained under the fields. Besides, electric fields could alter the absorption intensity for gas molecules. Therefore, gas sensors could be an expected application. More importantly, this work systematically points out some key steps for setting up electric-field calculations in the popular VASP code, such as the cancellation of the symmetrisation of the charge density, avoiding electrons spilling out into the vacuum under high fields.