Monolayer GeSe Research Articles

Effect of biaxial strain on the gas-sensing of monolayer GeSe

The adsorptions of various gas molecules (H<sub>2</sub>, H<sub>2</sub>O, CO, NH<sub>3</sub>, NO, and NO<sub>2</sub>) on monolayer GeSe versus the external biaxial strain in a range of –8% to 8% are investigated by first-principles calculations. The band structures, the equilibrium heights, the adsorption energy, and the amount of charge transfer are determined. The calculated results show that monolayer GeSe changes from indirect-to-direct and semiconducting-to-metallic under a certain biaxial strain. The adsorbed gas molecules hardly change the band gap of monolayer GeSe even under a biaxial strain in the whole range from –8% to 8%. The calculated adsorption energies under different strains reveal that the external biaxial strain has no significant effect on the adsorption stability of the gas molecules on monolayer GeSe, so it seems impossible to promote the desorption of the gas molecules by applying strain. It is found that NO<sub>2</sub> under the biaxial tensile strain of 8% tends to be bound with the monolayer GeSe by chemical bond which leads to being-difficult-to-desorb. Besides that case, the investigated gas molecules are physisorbed on the GeSe surface and have a certain probability of adsorption and desorption. The charge transfers of CO, NH<sub>3</sub>, NO and NO<sub>2</sub> adsorbed systems under the biaxial strain from –8% to 8% change somehow but are still non-negligible, while for H<sub>2</sub> and H<sub>2</sub>O, their charge transfers are too small to be detected by the monolayer-GeSe-based gas-sensor. Thus, due to the moderate adsorption energy and charge transfer, monolayer GeSe can be a promising candidate as a sensor for CO, NH<sub>3</sub> and NO under the biaxial strain from –8% to 8%, and for NO<sub>2</sub> in the range from –8% to 6%. It is worth noting that because of the appropriately lower adsorption energy and bigger charge transfer, a bigger biaxial compressive strain, ranging from –6% to –8%, can improve the response speed and sensibility to CO and NO of monolayer GeSe. Furthermore, the effect of the external biaxial strain on the adsorption stability and the charge transfer are discussed based on the two mechanisms of charge transfers, i.e. the traditional and the orbital mixing charge transfer theory. The charge transfer of NH<sub>3</sub> is governed by mixing the molecular HOMO with the orbital of GeSe, while for CO, NO and NO<sub>2</sub>, their charge transfers are most likely determined by different mechanisms under different external strains, which results in different influences on the charge transfer. The present study would be valuable for fully excavating the gas-sensing potential of the two-dimensional GeSe, and then providing sufficient theoretical basis for designing high performance gas sensors based on two-dimensional materials.

Strain-engineering the in-plane electrical anisotropy of GeSe monolayers.

As a representative in-plane anisotropic two-dimensional (2D) material, germanium monoselenide (GeSe) has attracted considerable attention recently due to its various in-plane anisotropic material properties originating from the low symmetry of a puckered honeycomb structure. Although there have been plenty of reports on the in-plane anisotropic vibrational, electrical and optical properties of GeSe, the strain effect on those appealing anisotropies is still under exploration. Here we report a systematic first-principles computational investigation of strain-engineering of the anisotropic electronic properties of GeSe monolayers. We found that the anisotropic ratio of the effective mass and mobility of charge carriers (electrons and holes) of GeSe along two principle axes can be controlled by using simple strain conditions. Notably, the preferred conducting direction of GeSe can be even rotated by 90° under an appropriate uniaxial strain (>5%). Such effective strain modulation of the electronic anisotropy of GeSe monolayers provides them abundant opportunities for future mechanical-electronic devices.

Device performance limits and negative capacitance of monolayer GeSe and GeTe tunneling field effect transistors

Exploring the device performance limits is meaningful for guiding practical device fabrication. We propose archetype tunneling field effect transistors (TFETs) with negative capacitance (NC) and use the rigorous ab initio quantum transport simulation to explore the device performance limits of the TFETs based on monolayer (ML) GeSe and GeTe along with their NC counterparts. With the ferroelectric dielectric acting as a negative capacitance material, the device performances of both the ML GeSe and GeTe NCTFETs outperform their TFET counterparts, particularly for the on-state current (Ion). Ion of the optimal ML GeSe and GeTe TFETs fulfills the demands of the International Technology Roadmap for Semiconductors (ITRS 2015 version) for low power (LP) and high performance (HP) devices, at the “6/5” node range, while with the aid of 80 nm and 50 nm thickness of ferroelectric SrBi2Nb2O9, both their NC counterparts extend the fulfillments at the “4/3” node range.

Intrinsic carrier mobility of monolayer GeS and GeSe: First-principles calculation

Abstract Recently, novel two-dimensional (2D) semiconductors of GeS and GeSe have attracted attention due to their potential applications in future nanoelectronic devices. The carrier mobility is first calculated from deformation potential theory (DPT) based on longitudinal acoustic and optical phonon model. Three important parameters of effective mass, elastic modulus and deformation potential are analyzed. Then, the intrinsic mobility is calculated from EPC matrix element. If the polarization effect of semiconductor is considered in the calculation, the electron mobility of GeS (GeSe) is reduced to 4.07 × 10 1 cm 2 V − 1 s − 1 (1.48 × 10 2 cm 2 V − 1 s − 1 ) for x direction, and 1.94 × 10 1 cm 2 V − 1 s − 1 (7.55 × 10 1 cm 2 V − 1 s − 1 ) for y direction. The hole mobility is 4.77 × 10 1 cm 2 V − 1 s − 1 (2.47 × 10 2 cm 2 V − 1 s − 1 ) for x direction, and 1.52 × 10 1 cm 2 V − 1 s − 1 (1.16 × 10 2 cm 2 V − 1 s − 1 ) for y direction. It is found that longitudinal optical phonon scattering plays a decisive role in carrier mobility. In this study, we illustrate that DPT model from the longitudinal acoustic and optical phonon scattering will overestimate carrier mobility. Some factors that affect carrier mobility are revealed.

Highly Efficient Polarized GeS/MoSe2 van der Waals Heterostructure for Water Splitting from Ultraviolet to Near‐Infrared Light

A high‐efficiency photocatalyst is critical for water splitting by solar light. Herein, via first principles calculations, the 2D polarized GeS/MoSe2 van der Waals (vdW) heterostructure is proposed as an efficient water redox photocatalyst. The performance of GeS/MoSe2 heterostructure is better than isolated materials, as the properties of GeS monolayer and MoSe2 monolayer are complementary by forming vdW heterostructure. GeS/MoSe2 heterostructure possesses suitable bandgap, dipole‐induced internal electric field, and excellent solar absorption performance. The band alignments of GeS/MoSe2 heterostructure are suitable compared with the redox potential of water. It is feasible to tune the optoelectronic properties and enhance photocatalytic activity of GeS/MoSe2 heterostructure via strain engineering. Biaxial compressive strain range from −2% to −3% induces the direct bandgap characteristic in GeS/MoSe2 heterostructure. The results suggest that 2D polarized GeS/MoSe2 vdW heterostructure is a potential novel high‐efficiency photocatalyst for water splitting under a wide range of spectra from ultraviolet to near infrared.

Injection current in ferroelectric group-IV monochalcogenide monolayers

We study the injection current response tensor (also known as circular photogalvanic effect or ballistic current) in ferrolectric monolayer GeS, GeSe, SnS, and SnSe. We find that the injection current is perpendicular to the spontaneous in-plane polarization and could reach peak (bulk) values of the order of ${10}^{10}\phantom{\rule{4pt}{0ex}}\mathrm{A}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ in the visible spectrum. The magnitude of the injection current is the largest reported in the literature to date for a two dimensional material. To rationalize the large injection current, we correlate the injection current spectrum with the joint density of states, electric polarization, strain, etc. We find that various factors such as anisotropy, in-plane polarization and wave function delocalization are important in determining the injection current tensor in these materials. We also find that compression along the polar axis can increase the injection current (or change its sign), and hence strain can be an effective control knob for their nonlinear optical response. Conversely, the injection current can be a sensitive probe of the crystal structure.

Gas Sensing of Monolayer GeSe: A First-Principles Study

The adsorption of various gas molecules (H2, H2O, CO, NH3, NO and NO[Formula: see text] on monolayer GeSe were investigated by first-principles calculations. The most stable configurations, the adsorption energies, and the amounts of charge transfer were determined. Owing to the appropriate adsorption energies and the non-negligible charge transfers, monolayer GeSe could be a promising candidate as a sensor for NH3, CO, NO and NO2. According to the band structures of the H2O, CO, NH3, NO and NO2 adsorbed systems, the reductions of the bandgaps are caused by the orbital hybridizations between the gas molecules and the underlying GeSe. The partial densities of states reveal the degrees of these orbital hybridizations. The mechanisms of charge transfer are discussed in the light of both traditional and orbital mixing charge transfer theories. The charge transfer of the paramagnetic molecules NO and NO2 could be governed by both charge transfer mechanisms, while for the other gas molecules H2, H2O, CO and NH3, it was most likely determined by the mixing of the HOMO or LUMO with the GeSe orbitals.

Strain-tunable electronic and optical properties of monolayer GeSe: Promising for photocatalytic water splitting applications

In this work, we investigate the electronic and optical properties of monolayer GeSe and the possibility of enhancement the photocatalytic activities for the water splitting of monolayer GeSe through strain engineering using first-principles calculations. Our calculations indicate that monolayer GeSe is a semiconductor with a moderate indirect gap of 1.13 eV at equilibrium and we can control its band gap by biaxial strain. In the presence of biaxial strain εb, the semiconductor-metal phase transition happens at large compressive strain of -10% and the indirect-direct gap transition occurs at εb=4%. The optical spectrum of monolayer GeSe are highly anisotropic and biaxial strain can increase the absorption coefficient of monolayer GeSe up to about 6×105 cm−1. Our calculations demonstrate that monolayer GeSe possesses photocatalytic properties for water splitting at εb=5% and we can enhance its photocatalytic activity by strain.

Computational design of alkali metals decorated 2D GeSe for hydrogen storage: a first principle study

Based on first principles calculations, we computational design the alkali metal (AM) decorated GeSe monolayer for hydrogen storage. The adsorption mechanism of H2 molecules on AM decorated GeSe monolayer is explored. Our study proves that the clustering behavior of AMs on GeSe monolayer is suppressed. It is found that the bonding interaction between H2 molecule and pristine GeSe monolayer can be enhanced by the decoration of AM adatoms. We predict a gravimetric density of 5.53 wt% for hydrogen storage achieved by the adsorption of H2 molecules on two sides of Li decorated GeSe monolayer. Our simulation also predicts that the process of hydrogen adsorption on Li decorated 2D GeSe is reversible.

Optoelectronic properties of new direct bandgap polymorphs of single-layered Germanium sulfide

The single-layered Germanium sulfide exhibits interesting optoelectronic properties however the indirect bandgap hinders its practical applications for nanoscale optoelectronic and photovoltaic devices. Herein, we report two direct bandgap polymorphs of single-layered GeS (δ-GeS and ε-GeS) and explore their optoelectronic properties using the density functional theory based computational approaches. The indirect bandgap of the intrinsic GeS monolayer (α-GeS) calculated in present study amounts to 0.76 eV, whereas the direct bandgap energies for δ- and ε-derivatives of GeS have been recorded as 1.93 and 2.10 eV respectively. The optical spectra of these monolayers demonstrated a high degree of anisotropy and significantly different optical absorption, reflection, and refraction coefficients were seen in the x - and y -directions. They exhibited different plasmons energies along x - and y -directions which reveal the GeS monolayers as potential polarizers of electromagnetic radiations. Moreover, these monolayers demonstrated exceptionally large optical absorption spanning over a wide range of electromagnetic spectrum. The absorption coefficients were recorded typically larger along y -axis than x -axis. The designed direct bandgap monolayers are believed to overcome the major hurdles of single-layered GeS for nanoscale optoelectronic applications.

Thermal transport properties in monolayer GeS

Utilizing the first-principle techniques combined with the Boltzmann transport equation, we investigate the thermal transport properties of monolayer GeS. The calculated lattice thermal conductivity κL is fairly low, e.g., 11.76 (12.53) Wm−1K−1 at 300 K, along the x (y) axis, which is much lower than the values for phosphorene, monolayer MoS2, graphene, etc. Our analysis reveals that the low lattice thermal conductivity is resulted from the strong three-phonon scattering process and the low phonon group velocity. Due to the weak anisotropy of GeS, we study the contribution of each phonon branch to the heat transport along x and y axis, and find that the longitudinal acoustic (LA) mode contributes most of the total κL along x axis, while the transverse acoustic (TA) branch has the largest contribution to the total κL along y axis. Additionally, the size dependence of thermal conductivity with respect to the nanoribbon width is also studied for the design of nanometer devices.

Recent Advances in Electrochemical Sensors for Detecting Toxic Gases: NO₂, SO₂ and H₂S.

Toxic gases, such as NOx, SOx, H2S and other S-containing gases, cause numerous harmful effects on human health even at very low gas concentrations. Reliable detection of various gases in low concentration is mandatory in the fields such as industrial plants, environmental monitoring, air quality assurance, automotive technologies and so on. In this paper, the recent advances in electrochemical sensors for toxic gas detections were reviewed and summarized with a focus on NO2, SO2 and H2S gas sensors. The recent progress of the detection of each of these toxic gases was categorized by the highly explored sensing materials over the past few decades. The important sensing performance parameters like sensitivity/response, response and recovery times at certain gas concentration and operating temperature for different sensor materials and structures have been summarized and tabulated to provide a thorough performance comparison. A novel metric, sensitivity per ppm/response time ratio has been calculated for each sensor in order to compare the overall sensing performance on the same reference. It is found that hybrid materials-based sensors exhibit the highest average ratio for NO2 gas sensing, whereas GaN and metal-oxide based sensors possess the highest ratio for SO2 and H2S gas sensing, respectively. Recently, significant research efforts have been made exploring new sensor materials, such as graphene and its derivatives, transition metal dichalcogenides (TMDs), GaN, metal-metal oxide nanostructures, solid electrolytes and organic materials to detect the above-mentioned toxic gases. In addition, the contemporary progress in SO2 gas sensors based on zeolite and paper and H2S gas sensors based on colorimetric and metal-organic framework (MOF) structures have also been reviewed. Finally, this work reviewed the recent first principle studies on the interaction between gas molecules and novel promising materials like arsenene, borophene, blue phosphorene, GeSe monolayer and germanene. The goal is to understand the surface interaction mechanism.

GeSe/BP van der Waals Heterostructures as Promising Anode Materials for Potassium-Ion Batteries

Potassium-ion batteries have attracted attention because of their abundant resources and similar electrochemistry to Li-ion batteries (LIBs). In the present work, GeSe/black phosphorus (BP) heterostructures as promising anode materials for K-ion batteries (KIBs) have been systematically investigated by first-principles calculations. The results reveal that GeSe/BP exhibits a semiconductor-to-metal transition after incorporating K atoms, indicating enhanced conductivity compared to monolayer GeSe. The energy barrier for K atom diffusion on GeSe/BP surface is relatively lower than that on monolayer GeSe. In addition, the GeSe/BP heterostructure can accommodate up to five layers of K atoms with negative adsorption energy, which greatly improves the storage capacity. Hence, the GeSe/BP heterostructure has great potential for application in advanced electrode materials in KIBs.

Two-dimensional SnSe/GeSe van der Waals heterostructure with strain-tunable electronic and optical properties

Two dimensional materials have a bright prospect for some strain-tunable applications due to their ultrathin and flexible characteristics. In this work, SnSe/GeSe van der Waals heterostructure is first constructed based on the GeSe and SnSe monolayers, and then their electrical and optical properties are investigated by first-principles calculations. Furthermore, the effects of strain on the electrical and optical properties of SnSe/GeSe are also studied. SnSe/GeSe is found to be a direct semiconductor with a band gap range from 0.19 eV to 0.83 eV, which can be flexiblely tuned by the in-plane strain. Under the uniaxial strains εa and εb, the tunable ranges of strain and direct bandgap are much smaller than those under the biaxial strain εu. Transformation between direct and indirect bandgap in SnSe/GeSe occurs under some applied strains. Being capable to absorb ultraviolet and far-ultraviolet light, SnSe/GeSe exhibits higher peak of absorption coefficient and the imagery part of dielectric function than those of GeSe and SnSe. Under small in-plane strains range from −0.06 to 0.06, anisotropy of SnSe/GeSe can be found in the optical parameters. The peaks of the absorption coefficient and dielectric function both increase with the decrease of strain. The light absorption capability of SnSe/GeSe becomes stronger with smaller in-plane strains, in which the biaxial strain is the most effective way to tune the optical properties. With these intriguing properties, SnSe/GeSe will be a promising two-dimensional material for applications in nanomechanics and optoelectronics.

Ultrafast Zero-Bias Surface Photocurrent in Germanium Selenide: Promise for Terahertz Devices and Photovoltaics.

Theory predicts that a large spontaneous electric polarization and concomitant inversion symmetry breaking in GeSe monolayers result in a strong shift current in response to their excitation in the visible range. Shift current is a coherent displacement of electron density on the order of a lattice constant upon above-bandgap photoexcitation. A second-order nonlinear effect, it is forbidden by the inversion symmetry in the bulk GeSe crystals. Here, we use terahertz (THz) emission spectroscopy to demonstrate that ultrafast photoexcitation with wavelengths straddling both edges of the visible spectrum, 400 and 800 nm, launches a shift current in the surface layer of a bulk GeSe crystal, where the inversion symmetry is broken. The direction of the surface shift current determined from the observed polarity of the emitted THz pulses depends only on the orientation of the sample and not on the linear polarization direction of the excitation. Strong absorption by the low-frequency infrared-active phonons in the bulk of GeSe limits the bandwidth and the amplitude of the emitted THz pulses. We predict that reducing GeSe thickness to a monolayer or a few layers will result in a highly efficient broadband THz emission. Experimental demonstration of THz emission by the surface shift current in bulk GeSe crystals puts this 2D material forward as a candidate for next-generation shift current photovoltaics, nonlinear photonic devices, and THz sources.

Enhanced photocatalytic activity for water splitting of blue-phase GeS and GeSe monolayers via biaxial straining.

The structural, electronic, dipole-induced internal electric field, optical and photocatalytic properties of monolayer GeS and GeSe under external biaxial strain were investigated by using first-principles calculations. The monolayer GeS and GeSe are indirect semiconductors with the band gaps of 3.265 eV and 2.993 eV, respectively. The band alignment of the monolayer GeS and GeSe manifests the photocatalytic activity for water splitting. Especially, it is effective to tune the properties including structures, band gaps, surface potential difference, dipole moment P, dipole-induced internal electric field, absorption and photocatalytic activity of the monolayer GeS and GeSe via biaxial strain. Our results suggest that monolayer GeS and GeSe possess photocatalytic properties for water splitting, and strain engineering, especially tensile strain, can enhance the photocatalytic activity under ultraviolet and visible light.

Sub-10 nm tunneling field-effect transistors based on monolayer group IV mono-chalcogenides.

The development of air-stable channels with a high on-state current (Ion) is in high demand for the feasible application of TFETs. Monolayer group IV mono-chalcogenides (i.e., GeS, GeSe, SnS, and SnSe), as emerging air-stable atomic-thin semiconductors, are potential channels for sub-10 nm tunneling field-effect transistors due to their high carrier mobility and anisotropic electronic properties. Herein, we investigated the performances of sub-10 nm monolayer (ML) group IV mono-chalcogenide TFETs using ab initio quantum transport simulation. The ML GeSe TFET exhibited the best performance with regards to both high Ion and low leakage current (Ileak) among the four devices, followed by the ML SnSe TFET with a high Ion. The Ion of the optimal ML GeSe TFET with a physical gate length of Lg = 10 nm surpasses the International Technology Roadmap for Semiconductors (ITRS, 2013 Edition) requirements for high-performance (HP) and low-power (LP) devices, simultaneously, and that of the ML SnSe TFET with Lg = 10 nm surpasses the requirement of ITRS HP devices. In combination with our former works, we suggest an Eg of 0.77-1.19 eV and of 0.11-0.15m0 to search for competitive 2D channels with a high Ion for HP application in TFET devices with a planar homogeneous p-i-n architecture.

First-principles study of five isomers of two-dimensional GeSe under in-plane strain

Using first-principles calculations, we investigate the stability and electronic properties of five isomers of two-dimensional (2D) GeSe monolayer under in-plane strain. Our calculated results show that the five isomers of GeSe monolayer are all stable. It is found that the α-GeSe has a direct band gap, while each of the β-GeSe, γ-GeSe, δ-GeSe and ε-GeSe possesses an indirect band gap. By applying compressive or tensile uniaxial and biaxial strain to the five GeSe isomers, the indirect-to-direct transition in band gap is found. In the α-GeSe, the changes from indirect-to-direct and semiconducting-to-metallic are both found under an applied strain. In the 2D β-GeSe and γ-GeSe, an adjustable range of indirect band gap under strain is found. Moreover, a direct band gap in the δ-GeSe is found separately under the biaxial compression strain of <i>σ</i><i><sub>xy</sub></i> = –2% and <i>σ</i><i><sub>xy</sub></i> = –4%. By applying a tensile strain of 10% along the armchair direction in ε-GeSe, a transition from an indirect to direct band gap occurs. When the tensile strain is continuously increased to 20%, the band structure of ε-GeSe maintains direct character. This direct band gap can be tuned from 1.21 eV to 1.44 eV. When 10% tensile strain is applied along the biaxial direction, the transition in band gap from indirect-to-direct also occurs. Our results indicate that the direct band gap can be tuned from 0.61 eV to 1.19 eV when the tensile strain is increased from 10% to 19% in ε-GeSe.

An accurate and compact tight-binding model for GeS

In this work we calculate the bands structure for the GeS monolayer from a multi-orbital tight-binding model, where the effect of s-p mixing are considered in slater-Koster equations. The model parameters are adjusting and optimising compared with the band structure obtained by density-functional theory calculation based on the hybrid HSE06 functional. It reproduces the conduction and valence bands as well as the orbital compositions. The anisotropic effective masses and Fermi velocities are identified directly from the anisotropy bands structure.

Computational design of GeSe/graphene heterojunction based on density functional theory

Based on density functional theory, we computational designed the hetero junction composed by GeSe monolayer and graphene. The effects of interlayer coupling, strains and electric fields on the electronic structures of the designed GeSe/graphene (G/g) hetero structure are explored. We demonstrated that both the intrinsic electric properties of the GeSe monolayer and graphene are well preserved in G/g hetero structure. It is found that an energy gap of 0.17 eV in graphene is opened by decreasing the interlayer distance in G/g hetero structure. The height of Schottky barrier can be effectively tuned by the interlayer distance between GeSe monolayer and graphene. Moreover, we found that applying in-plane strains and the electric fields perpendicular to the G/g hetero structure can control the Schottky barriers at the G/g interface. Our results predict that the ultra-thin G/g hetero structure can be used as two-dimensional semiconductor-based optoelectronic devices.