Monolayer SnSe Research Articles

Defect-Free Nanowelding of Bilayer SnSe Nanoplates.

Nanowelding is a bottom-up technique to create custom-designed nanostructures and devices beyond the precision of lithographic methods. Here, a new technique is reported based on anisotropic lubricity at the van der Waals interface between monolayer and bilayer SnSe nanoplates and a graphene substrate to achieve precise control of the crystal orientation and the interface during the welding process. As-grown SnSe monolayer and bilayer nanoplates are commensurate with graphene's armchair direction but lack commensuration along graphene's zigzag direction, resulting in a reduced friction along that direction and a rail-like, 1D movement that permits joining nanoplates with high precision. This way, molecular beam epitaxially grown SnSe nanoplates of lateral sizes 30-100nm are manipulated by the tip of a scanning tunneling microscope at room temperature. In situ annealing is applied afterward to weld contacting nanoplates without atomic defects at the interface. This technique can be generalized to any van der Waals interfaces with anisotropic lubricity and is highly promising for the construction of complex quantum devices, such as field effect transistors, quantum interference devices, lateral tunneling junctions, and solid-state qubits.

Carrier mobility and broadband performance of two-dimensional Sb/SnSe van der Waals heterostructure: A first-principles study

Compared to single two-dimensional (2D) materials, stacking layered 2D materials with van der Waals (vdW) heterostructures offers novel opportunities to achieve desired exotic properties. Herein, 2D Sb/SnSe vdW heterostructure is constructed by vertically stacking the antimonene (Sb) monolayer on the tin selenide (SnSe) monolayer. We have conducted a theoretical study by using the first-principles calculations to comprehensively examine the electronic, optical, and mechanical properties. Phonon dispersion and ab initio molecular dynamics simulations have demonstrated that the Sb/SnSe vdW heterostructure possesses remarkable stability, ensuring its robustness up to 900 K. The Sb/SnSe vdW heterostructure is characterized as a semiconducting material with a direct band gap of 0.24 eV, calculated by the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional method. Compared to the pristine Sb and SnSe monolayers, the Sb/SnSe vdW heterostructure exhibits a lower work function value of 3.82 eV. Furthermore, the carrier mobility of the heterostructure demonstrates anisotropic characteristics with a notable improvement in hole-mobility (12.05 × 103 cm2V−1s−1) along the y-direction. The Sb/SnSe vdW heterostructure shows enhanced broadband absorption spectra, especially in the visible to near-infrared ranges. Our findings underscore the potential of the Sb/SnSe vdW heterostructure for future nano-electronic and optoelectronic technologies.

First-principles calculations to investigate effect of strain on magnetic and optical properties of Mn-adsorbed SnSe2 monolayer

Nowadays, two-dimensional materials are ideal for fabricating optoelectronic and spintronic devices. We have investigated the optical and magnetic properties of −6 % to 6 % strain on Mn-adsorbed monolayer SnSe2 films using a first-principles approach. The Mn-adsorbed monolayer SnSe2 is a magnetic semiconductor in the absence of strain effects. As the tensile strain increases, the band gap of the Mn-adsorbed monolayer SnSe2 decreases and the structure becomes unstable, and Mn adsorption changes to exothermic adsorption at −6 % strain. The magnetic moment of the system increases slightly with increasing tensile strain and decreases rapidly when the strain reaches 4%. As the compressive strain increases, the magnetic moment first decreases slightly and then decreases rapidly when the strain reaches −4 %. Calculations of the optical properties show that the static dielectric constants at −6 %, −4 %, −2 %, 0 %, 2 %, 4 % and 6 % strains are 15.23, 10.19, 8.67, 7.78, 6.38, 5.99 and 5.92, respectively, which increase with increasing compressive strain. It decreases with increase in tensile strain. The static dielectric function increases as the compressive strain increases and decreases as the tensile strain increases. In the visible light region, the reflectivity, refractive index, extinction coefficient and photoconductivity in the XX and ZZ directions increase with increasing tensile strain. Our study shows that the optoelectronic and magnetic properties of the SnSe2 adsorbed Mn system can be improved to some extent by applying strain, and this study is expected to provide theoretical guidance for the fabrication of SnSe2-based magnetic optoelectronic devices.

CoO-SnSe monolayer: A high potential candidate for SF6 characteristic decomposition gas adsorption and detection

Insulation performance testing of gas-insulated switchgear (GIS) and treatment of SF6 decomposition products under partial discharge are two crucial studies. In this paper, the adsorption behavior and sensor properties of five characteristic decomposition gases of SF6 (HF, H2S, SO2, SOF2, SO2F2) on intrinsic and CoO-doped SnSe monolayers are investigated in detail based on density-functional theory. The results showed that the adsorption behaviors of HF, H2S, SOF2 and SO2F2 on SnSe monolayers were all physisorption among which the best adsorption was SO2 with an adsorption energy of −0.618 eV, which is a weak chemisorption. After doping with CoO, the adsorption capacity of SnSe monolayer for five gases was significantly enhanced, in which the adsorption energies for SO2 and SOF2 reached −1.356 eV and −1.175 eV, respectively. In addition, the conductivity of the system is greatly improved, with the bandgap changing from 1.129 eV to 0 eV. The microscopic mechanism of the interaction of gas molecules with CoO-SnSe monolayers has been revealed by energy band structure (Eg), density of states (DOS), Milligan charge analysis (∆Q) and electrostatic potential. Finally, the sensitivity (S) and desorption time (τ) of the five adsorption systems were calculated to illustrate the macroscopic gas-sensitive properties of the system. This work will help to explore the application of CoO-SnSe monolayers in SF6 decomposition gas sensing detection and adsorption treatment.

Weak interatomic interactions induced low lattice thermal conductivity in 2D/2D PbSe/SnSe vdW heterostructure

Inspired by the excellent thermoelectric performance of two-dimensional (2D) PbSe and SnSe monolayers, the thermal and electronic transport properties of 2D/2D PbSe/SnSe heterostructure is theoretically evaluated using first-principles calculations and Boltzmann transport theory. An indirect bandgap of 1.06 eV for the PbSe/SnSe heterostructure is calculated by the combination of Heyd-Scuseria-Ernzerhof (HSE06) functional and spin-orbit coupling (SOC) effect. The mechanical, thermal and dynamic stabilities of the PbSe/SnSe heterostructure are further demonstrated by the elastic modulus, ab initio molecular dynamics (AIMD) simulations and phonon dispersion curves, respectively. The honeycomb-like wrinkled structure introduce the asymmetry at the interface of the PbSe/SnSe heterostructure, thereby leading to enhanced phonon boundary scattering. Concurrently, the antibonding states originated from the weak interatomic interactions within PbSe/SnSe heterostructure also lead to strong anharmonicity. This synergistic effect of pronounced scattering and heightened anharmonicity contributes to the low lattice thermal conductivity of PbSe/SnSe heterostructure. The thermoelectric properties of PbSe/SnSe heterostructure are further investigated along the x- and y-direction, which reveals that the p-type PbSe/SnSe heterostructure attains an optimal dimensionless figure-of-merit (ZT) of 2.6 at 900 K along the y-direction. Our present study not only offers the fundamental insights into the thermal and electronic transport properties of the PbSe/SnSe heterostructure, but also provides an effective and feasible strategy for the design of 2D layered heterostructures as promising thermoelectric materials.

Unveiling the temperature-dependent thermoelectric properties of the undoped and Na-doped monolayer SnSe allotropes: a comparative study

Motivated by the excellent thermoelectric (TE) performance of bulk SnSe, extensive attention has been drawn to the TE properties of the monolayer SnSe. To uncover the fundamental mechanism of manipulating the TE performance of the SnSe monolayer, we perform a systematic study on the TE properties of five monolayer SnSe allotropes such as α-, β-, γ-, δ-, and ε-SnSe based on the density functional theory and the non-equilibrium Green’s functions. By comparing the TE properties of the Na-doped SnSe allotropes with the undoped ones, the influences of the Na doping and the temperature on the TE properties are deeply investigated. It is shown that the figure of merit ZT will increase as the temperature increases, which is the same for almost all the Na-doped and undoped cases. The Na doping can enhance or suppress the ZT in different SnSe allotropes at different temperatures, implying the presence of the anomalous suppression of the ZT. The Na doping induced ZT suppression may be caused basically by the sharp decrease of the power factor and the weak decrease of the electronic thermal conductance, rather than by the decrease of the phononic thermal conductance. We hope this work will be able to enrich the understanding of the manipulation of TE properties by means of dimensions, structurization, doping, and temperature.

Transition from fractal-dendritic to compact islands for the 2D-ferroelectric SnSe on graphene/Ir(111)

Epitaxial growth is a versatile method to prepare two-dimensional van der Waals ferroelectrics like group IV monochalcogenides which have potential for novel electronic devices and sensors. We systematically study SnSe monolayer islands grown by molecular beam epitaxy, especially the effect of annealing temperature on shape and morphology of the edges. Characterization of the samples by scanning tunneling microscopy reveals that the shape of the islands changes from fractal-dendritic after deposition at room temperature to a compact rhombic shape through annealing, but ripening processes are absent up to the desorption temperature. A two-step growth process leads to large, epitaxially aligned rhombic islands bounded by well-defined 110 -edges (armchair-like), which we claim to be the equilibrium shape of the stoichiometric SnSe monolayer islands. The relaxation of the energetically favorable edges is detected in atomically resolved STM images. The experimental findings are supported by the results of our first-principles calculations, which provide insights into the energetics of the edges, their reconstructions, and yields the equilibrium shapes of the islands which are in good agreement with the experiment.

Long-term monitoring of SF6 decomposition gases and water molecules using TiO2-doped SnSe monolayer for generator circuit breakers

In generator circuit breakers, monitoring the decomposition of Sulfur Hexafluoride (SF6) gas is a primary method to determine insulation and fault conditions. The presence of SF6 combined with H2O impurities also significantly impacts the degradation of equipment insulation. However, there is a lack of research on the simultaneous monitoring of both SF6 decomposition and H2O. To improve the system’s rapid recoverability, recyclability, and long-term usability, a SnSe monolayer doped with TiO2 nanoparticles (SnSe–TiO2) has been proposed as an SF6 gas decomposer detection sensor with humidity detection capabilities. The SnSe–TiO2 monolayer significantly enhances the conductivity and increases its adsorption energy for H2O (73.2%), H2S (13.54%), HF (59.70%), SO2 (96.33%), and SOF2 (52.04%). Furthermore, this material can be utilized for long-term cyclic gas monitoring as the SnSe–TiO2 monolayer is physically adsorbed to these gases and can be rapidly desorbed. This study establishes a theoretical foundation for the future advancement of gas detection sensors for insulating gases in circuit breakers.

Anharmonicity and weak bonding-driven extraordinary thermoelectric performance in wrinkled SnSe monolayer with low lattice thermal conductivity

SnSe compound (Nature, 2014, 508, 373–377) has become the star in the field of thermoelectric (TE) materials owing to the environmental friendliness, abundant reserves, and excellent performance. In the present work, the electronic structures, thermal and electronic transport, as well as TE properties of a two-dimensional (2D) wrinkled SnSe monolayer are comprehensively investigated in combination with first-principles calculations and Boltzmann transport theory. The SnSe monolayer is an indirect semiconductor with a bandgap of 2.88 eV using Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional. The “multivalley” band structure of SnSe monolayer is not only beneficial for maximizing the power factor, but also leads to low lattice thermal conductivity (κl) (1.37/1.42 W/mK @ 300 K armchair-/zigzag-direction). The further crystal orbital Hamilton population (COHP) analysis shows that the low κl in SnSe monolayer is attributed to weak antibonding state below the Fermi level, which in turn weakens the chemical bonding and leads to the mutual exclusion of Sn and Se atoms. As a consequence, the softening of phonon modes and a significant reduction in the phonon group velocity are discovered for the SnSe monolayer, which is beneficial for strong anharmonicity and significant phonon scatterings. Additionally, the electronic transport properties of the 2D SnSe monolayer are evaluated by taking into account the electron-phonon interactions, and the optimal ZTs of 2.13 (armchair-direction) and 3.01 (zigzag-direction) are achieved for n-type and p-type SnSe monolayer at 900 K, respectively. Our present work can not only provide fundamental understanding of thermal and electronic transport properties of wrinkled SnSe monolayer, but also shed some light on the theoretical design of low dimensional SnSe-layered material in thermoelectric applications.

Ag2O doped SnSe: A highly discriminative sensor material for DNA sequencing

A DNA nucleobase sensor with high selectivity is of paramount importance for disease diagnosis, genetic analysis, and biological research. This study investigates the impact of incorporating Ag2O on the adsorption behavior of SnSe monolayer with four DNA nucleobases. Ag2O incorporation improves the conductivity of SnSe (87%) and increases the Eads of DNA nucleobases by 199% (A), 122% (C), 108% (G), and 181% (T). Analysis of ED and EDD reveals the distribution of electrons on nucleobases and SnSe-Ag2O monolayer and their activity trajectories during the adsorption process. Furthermore, desorption times were calculated, and at 698 K, all systems exhibited desorption times less than 7.1 h, indicating the potential of SnSe-Ag2O monolayer as a reusable material for nucleobase sensors. Additionally, the dielectric constant of solvents was found to be proportional to the adsorption strength and differentiation ability of the adsorbent towards different DNA nucleobases. Finally, comparisons with recent references validate the exceptional performance of the material used in this study. The remarkable nucleobase differentiation ability endows SnSe-Ag2O monolayer with immense potential in DNA sequencing technology.

Black phosphorene/SnSe van der Waals heterostructure as a promising anchoring anode material for metal-ion batteries

Black phosphorene (BP) is a glowing two-dimensional semiconducting layer material for cutting-edge microelectronics, with high carrier mobility and thickness-dependent band gap. Here, based on van der Waals (vdW)-corrected first-principles approaches, we investigated stacked BP/tin selenide (BP/SnSe) vdW heterostructure as an anode material for metal ion batteries, which exhibits a significant theoretical capacity, along with relatively durable binding strength compared to the constituent BP and SnSe monolayers. Our calculations demonstrated that the Li/Na adatom favors insertion into the interlayer region of BP/SnSe vdW heterostructure owing to synergistic interfacial effect, resulting in comparable diffusivity to the BP and SnSe monolayers. Subsequently, the theoretical specific capacities for Li/Na are found to be as high as 956.30 mAhg−1 and 828.79 mAhg−1, respectively, which could be attributed to the much higher storage capacity of Li/Na adatoms in the BP/SnSe vdW heterostructure. Moreover, the electronic structure calculations reveal that a large amount of charge transfer assists in semiconductor-to-metallic transition upon lithiation/sodiation, ensuring good electrical conductivity. These simulations verify that the BP/SnSe vdW heterostructure has immense potential for application in the design of metal-ion battery technologies.

Effect of different pseudopotentials on the phonon frequencies, dielectric constant, and Born effective charge of SnSe and SnSe2 nanostructures: A density functional perturbation theory study

The choice of the pseudopotentials (PPs) is a crucial step towards an accurate atomistic calculation of physical properties of 2D materials. In this study, based on Density-Functional Perturbation Theory (DFPT), we investigated the dependency of the phonon properties such as phonon frequencies, dielectric constant, Born effective charge, and vibrational modes of SnSe and SnSe2 monolayers to the different types of PPs. On the basis of Generalized Gradient Approximation (GGA) using PBE functional, we computed eight phonon calculations for both lattices in combinations with four different PPs including Normconversing (NC) FHI/HGH, Ultrasoft (USPP), and Standard Solid-State (SSSP), while other parameters like cut-off energy, k-points mesh, and convergence threshold were kept fixed. However, the phonon properties of acoustic and optical phonons were better predicted by USPP and SSSP for phonon dispersion of SnSe and SnSe2, respectively. Regarding the dielectric constant and Bohr effective charge calculations, our results showed that SnSe and SnSe2 structures possess the dielectric tensor of 8.9 and 6.6 along the in-plane direction by using USPP calculations. Our outcomes also revealed less sensitivity of SnSe2 polarization to the type of PPs by applying the electric field. This study recommends USPP and SSSP to further phonon analysis of other semiconductors.

SnSe Monolayer-Based heavy metal sensors with high Sensitivity, Selectivity, and Reusability: Insights from first principle calculation

Heavy metal pollution has a negative impact on both human health and the environment. The monitoring of heavy metal is therefore very important from a practical standpoint. In this study, the adsorption energy, charge transfer, band, densities of states, and sensitivity of SnSe monolayers toward heavy metals (As, Cd, Cr, Hg, Ni, Pb) were investigated using density functional theory combined with the non-equilibrium Green's function approach. The calculations show that SnSe monolayers have excellent sensitivities exceeding the limit of quantitation (LOQ) towards As, Cd, Hg, and Pb, which are as high as 384467%, 1462%, 1791%, and 26160%, respectively. Moreover, analyzing response peaks at various bias voltages can identify the composition of heavy metals from diverse sources, providing valuable insights for selective heavy metal monitoring. SnSe monolayers exhibit wide bias voltage windows that enhance sensor sensitivity by reaching the detection threshold, while also simplifying sensor encapsulation, resulting in higher reliability. The SnSe monolayer exhibits rapid recovery times for As, Hg, and Cd at room temperature, while for Pb, heating to 498 K is required for rapid recovery. These findings show that SnSe monolayers have a strong potential for constructing extremely sensitive and selective heavy metal sensors that are also reusable, implying that SnSe monolayers might serve as a possible online sensor for environmental monitoring.

High Carrier Mobility and Controllable Electronic Property of the h-BN/SnSe2 Heterostructure.

Building two-dimensional (2D) vertical van der Waals heterostructures (vdWHs) is one of the effective methods to regulate the properties of single 2D materials. In this paper, we stack the hexagonal boron nitride (h-BN) monolayer (ML) on the SnSe2 ML to construct the stable h-BN/SnSe2 vdWH, of which the crystal and electronic structures, together with the optical properties, are also analyzed by the first-principles calculations. The results show that the h-BN/SnSe2 vdWH belongs to a type-I heterostructure with an indirect bandgap of 1.33 eV, in which the valence band maximum and conduction band minimum are both determined by the component SnSe2 ML. Interestingly, the h-BN/SnSe2 vdWH under the tensile strain or electric field undergoes the transitions both from type-I to type-II heterostructure and from the indirect to direct bandgap semiconductor. In addition, the carrier mobility of the h-BN/SnSe2 heterostructure has a significant enhancement relative to that of the SnSe2 ML, up to 104 cm2 V-1 s-1. Meanwhile, the h-BN/SnSe2 heterostructure presents the superb optical absorption and unique type-II hyperbolic property. Our findings will broaden the potential applications of SnSe2 ML and provide theoretical guidance for the related experimental studies.

A Comparative Study of Electronic, Optical, and Thermoelectric Properties of Zn-Doped Bulk and Monolayer SnSe Using Ab Initio Calculations.

In this study, we explore the effects of Zn doping on the electronic, optical, and thermoelectric properties of α-SnSe in bulk and monolayer forms, employing density functional theory calculations. By varying the doping concentrations, we aim to understand the characteristics of Zn-doped SnSe in both systems. Our analysis of the electronic band structure using (PBE), (SCAN), and (HSE06) functionals reveals that all doped systems exhibit semiconductor-like behavior, making them suitable for applications in optoelectronics and photovoltaics. Notably, the conduction bands in SnSe monolayers undergo changes depending on the Zn concentration. Furthermore, the optical analysis indicates a decrease in the dielectric constant when transitioning from bulk to monolayer forms, which is advantageous for capacitor production. Moreover, heavily doped SnSe monolayers hold promise for deep ultraviolet applications. Examining the thermoelectric transport properties, we observe that Zn doping enhances the electrical conductivity in bulk SnSe at temperatures below 500 K. However, the electronic thermal conductivity of monolayer samples is lower compared to bulk samples, and it decreases consistently with increasing Zn concentrations. Additionally, the Zn-doped 2D samples exhibit high Seebeck coefficients across most of the temperature ranges investigated.

Adsorption of SF6 gas and insulating oil decomposition gas by CoO-doped SnSe monolayer in various environments: A study of strong adsorption performance

Taking into account that Sulfur hexafluoride (SF6) gas leakage can result in environmental pollution, the insulating oil used in high-voltage equipment can decompose into various detectable dissolved gases, allowing for the monitoring of its operational status. In this study, we use a CoO-doped SnSe (SnSe-CoO) monolayer to achieve strong adsorption of SF6 gas, CO, and C2H2 gas, and highly sensitive monitoring of H2 and CH4. Thus, the SnSe-CoO monolayer can serve as an adsorbent for SF6 gas leakage and a material for monitoring insulating oil-dissolved gas. Furthermore, we also observed that the adsorption energy of each gas on the intrinsic material increases with the solvent dielectric constant, facilitating the control of gas adsorption. Our research provides a significant theoretical foundation for future adsorption and monitoring of insulating gases and insulating oil decomposition gases.

Strong Adsorption Capacity for SF₆-Leaking Gas With NiO-Doped and Ag₂O-Doped SnSe Monolayer

Sulfur hexafluoride (SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> ) gas is widely used in Gas Insulation Switchgear (GIS) systems of power systems due to its efficient arc extinguishing characteristics. However, long-term equipment operation or electrical faults may cause SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> insulating gas leakage, leading to environmental pollution. Although there are numerous 2-dimensional materials for detecting SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> decomposition, few studies on SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> leakage gas exits. This paper proposes an SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> adsorbent material based on a SnSe monolayer doped with NiO and Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O. Theoretical calculations demonstrate that the SnSe monolayer doped with NiO and Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O exhibits extremely strong adsorption performance for SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas, with adsorption energies of -2.824 eV and -2.812 eV, respectively, which is an 890.88% enhancement compared to pure SnSe monolayer. Furthermore, the adsorption energy of SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas in water solvent exceeds -3.5 eV, indicating that water can serve as an effective enhancer of SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas adsorption. This study provides a new theoretical basis and method for detecting and adsorbing SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> leaking gas.

Fe-doped SnSe monolayer: A promising 2D material for reusable SO2 gas sensor with high sensitivity

Recently, 2D materials have attracted great interest in the gas-sensing field, in which TMMC GeS, GeSe, SnS, and SnSe monolayers have been reported as promising SO2 sensing materials [Chem. Phys. Lett. 686 (2017) 83–87; Appl. Surf. Sci. 484 (2019) 33–38]. However, the transport properties, sensitivity, and recovery times of such structures related to the adsorption of SO2 molecule have not been published to date. In this work, we found that the recovery times of the pristine GeS, GeSe, SnS, and SnSe monolayers after SO2 adsorption are too short, indicating their low application possibility for SO2 sensing materials. Interestingly, the transition metals doping (Cr, Mn, or Fe) improves the recovery time of the SnSe monolayer in respect of SO2 molecules. Especially the calculated recovery time of the Fe-doped SnSe monolayer in respect of SO2 molecules at 373 K is 27 s, which is suitable for reusable gas sensors. Moreover, we first designed an SO2 gas sensor based on the Fe-doped SnSe monolayer with two Au electrodes. The NEGF formalism-based calculations show its high sensitivity to an SO2 molecule. Our results suggest that the Fe-doped SnSe monolayer is a promising 2D material for reusable SO2 gas sensors with high sensitivity.

Thermoelectric properties of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;&lt;i&gt;X&lt;/i&gt;

Two-dimensional (2D) materials with lower lattice thermal conductivities and high figures of merit are useful for applications in thermoelectric (TE) devices. In this work, the thermoelectric properties of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;S and Cu&lt;sub&gt;2&lt;/sub&gt;Se are systematically studied through first-principles and Boltzmann transport theory. The dynamic stability of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;S and Cu&lt;sub&gt;2&lt;/sub&gt;Se through elastic constants and phonon dispersions are verified. The results show that monolayer Cu&lt;sub&gt;2&lt;/sub&gt;S and Cu&lt;sub&gt;2&lt;/sub&gt;Se have small lattice constants, resulting in lower phonon vibration modes. Phonon transport calculations confirm that monolayer Cu&lt;sub&gt;2&lt;/sub&gt;Se has lower lattice thermal conductivity (1.93 W/(m·K)) than Cu&lt;sub&gt;2&lt;/sub&gt;S (3.25 W/(m·K)) at room temperature, which is due to its small Debye temperature and stronger anharmonicity. Moreover, the heavier atomic mass of Se atom effectively reduces the phonon frequency, resulting in an ultra narrow phonon band gap (0.08 THz) and a lower lattice thermal conductivity for monolayer Cu&lt;sub&gt;2&lt;/sub&gt;Se. The band degeneracy effect at the valence band maximum (VBM) of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;S and Cu&lt;sub&gt;2&lt;/sub&gt;Se significantly increase their carrier effective mass, resulting in higher Seebeck coefficients and lower conductivities under p-type doping. The electric transport calculation at room temperature shows that the conductivity of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;S (Cu&lt;sub&gt;2&lt;/sub&gt;Se) under n-type doping about 10&lt;sup&gt;11&lt;/sup&gt; cm&lt;sup&gt;–2&lt;/sup&gt; is 2.8×10&lt;sup&gt;4&lt;/sup&gt; S/m (4.5×10&lt;sup&gt;4&lt;/sup&gt; S/m), obviously superior to its conductivity about 2.6×10&lt;sup&gt;2&lt;/sup&gt; S/m (1.6×10&lt;sup&gt;3&lt;/sup&gt; S/m) under p-type doping. At the optimum doping concentration for monolayer Cu&lt;sub&gt;2&lt;/sub&gt;S (Cu&lt;sub&gt;2&lt;/sub&gt;Se), the n-type power factor is 16.5 mW/(m·K&lt;sup&gt;2&lt;/sup&gt;) (25.9 mW/(m·K&lt;sup&gt;2&lt;/sup&gt;)), which is far higher than p-type doping 1.1 mW/m·K&lt;sup&gt;2&lt;/sup&gt; (6.6 mW/(m·K&lt;sup&gt;2&lt;/sup&gt;)). Through the above results, the excellent figure of merit of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;S (Cu&lt;sub&gt;2&lt;/sub&gt;Se) under optimal n-type doping at 700 K can approach to 1.85 (2.82), which is higher than 0.38 (1.7) under optimal p-type doping. The excellent thermoelectric properties of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;S (Cu&lt;sub&gt;2&lt;/sub&gt;Se) are comparable to those of many promising thermoelectric materials reported recently. Especially, the figure of merit of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;Se is larger than that of the well-known high-efficient thermoelectric monolayer SnSe (2.32). Therefore, monolayer Cu&lt;sub&gt;2&lt;/sub&gt;S and Cu&lt;sub&gt;2&lt;/sub&gt;Se are potential thermoelectric materials with excellent performances and good application prospects. These results provide the theoretical basis for the follow-up experiments to explore the practical applications of 2D thermoelectric semiconductor materials and provide an in-depth insight into the effect of phonon thermal transport on improvement of TE transport properties.

Monolayer Cu&lt;sub&gt;2&lt;/sub&gt;X (X=S, Se): excellent thermoelectric material with low lattice thermal conductivity

Two-dimensional (2D) materials with lower lattice thermal conductivity and high figure of merit are useful for applications in thermoelectric (TE) devices.In this work,the thermoelectric properties of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;X (X=S,Se) have been systematically researched through first-principles and Boltzmann transport theory.We have verified the dynamic stability of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;X (X=S,Se) through elastic constants and phonon dispersion.The results show that monolayer Cu&lt;sub&gt;2&lt;/sub&gt;X (X=S,Se) together with small lattice constants,resulting in lower phonon vibration modes.Phonon transport calculations confirm that monolayer Cu&lt;sub&gt;2&lt;/sub&gt;Se has lower lattice thermal conductivity (1.93W/mK) than Cu&lt;sub&gt;2&lt;/sub&gt;S (3.25W/mK) at room temperature,which is due to its small Debye temperature and stronger anharmonicity.Moreover,the heavier atomic mass of Se atoms effectively reduces the phonon frequency,resulting in a ultra narrow phonon band gap (0.08THz) and lower lattice thermal conductivity for monolayer Cu&lt;sub&gt;2&lt;/sub&gt;Se.The band degeneracy effect at the VBM of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;X (X=S,Se) significantly increases its carrier effective mass,resulting in higher Seebeck coefficient and lower conductivity under p-type doping.The electric transport calculation at room temperature shows that the conductivity of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;S (Cu&lt;sub&gt;2&lt;/sub&gt;Se) under n-type doping about 10&lt;sup&gt;11&lt;/sup&gt;cm&lt;sup&gt;-2&lt;/sup&gt; is 2.8×10&lt;sup&gt;4&lt;/sup&gt;S/m (4.5×10&lt;sup&gt;4&lt;/sup&gt;S/m),obviously superior to their conductivity about 2.6×10&lt;sup&gt;2&lt;/sup&gt;S/m (1.6×10&lt;sup&gt;3&lt;/sup&gt;S/m) under p-type doping.At the optimum doping concentration for monolayer Cu&lt;sub&gt;2&lt;/sub&gt;S (Cu&lt;sub&gt;2&lt;/sub&gt;Se),the n-type power factor is 16.5mW/mK&lt;sup&gt;2&lt;/sup&gt;(25.9mW/mK&lt;sup&gt;2&lt;/sup&gt;),which is far higher than p-type doping 1.1mW/mK&lt;sup&gt;2&lt;/sup&gt;(6.6mW/mK&lt;sup&gt;2&lt;/sup&gt;).Through the above results,the excellent figure of merit of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;S and Cu&lt;sub&gt;2&lt;/sub&gt;Se under optimal n-type doping at 700 K can approach 1.85 and 2.82,which are higher than 0.38 and 1.7 under optimal p-type doping.The excellent thermoelectric properties of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;S and Cu&lt;sub&gt;2&lt;/sub&gt;Se are comparable to those of many promising thermoelectric materials reported recently.Especially,the figure of merit of monolayer Cu&lt;sub&gt;2&lt;/sub&gt;Se is larger than the well-known high-efficient thermoelectric monolayer SnSe (2.32).Therefore,monolayer Cu&lt;sub&gt;2&lt;/sub&gt;X (X=S,Se) are potential thermoelectric material with excellent performance and good application prospects.Such results provide theoretical basis for the follow-up experiments exploring the practical applications of 2D thermoelectric semi-conductors materials and offer an in-depth insight into the effect of phonon thermal transport on improvement of TE transport properties.