Elemental Two-dimensional Material Research Articles

Surfactant-Free Ultrasonication-Assisted Synthesis of 2d Tellurium Based on Metastable 1T'-MoTe2.

Elemental 2D materials (E2DMs) have been attracting considerable attention owing to their chemical simplicity and excellent/exotic properties. However, the lack of robust chemical synthetic methods seriously limits their potential. Here, a surfactant-free liquid-phase synthesis of high-quality 2D tellurium is reported based on ultrasonication-assisted exfoliation of metastable 1T'-MoTe2. The as-grown 2D tellurium nanosheets exhibit excellent single crystallinity, ideal 2D morphology, surfactant-free surface, and negligible 1D by-products. Furthermore, a unique growth mechanism based on the atomic escape of Te atoms from metastable transition metal dichalcogenides and guided 2D growth in the liquid phase is proposed and verified. 2D tellurium-based field-effect transistors show ultrahigh hole mobility exceeding 1000 cm2V-1s-1 at room temperature attributing to the high crystallinity and surfactant-free surface, and exceptional chemical and operational stability using both solid-state dielectric and liquid-state electrical double layer. The facile ultrasonication-assisted synthesis of high-quality 2D tellurium paves the way for further exploration of E2DMs and expands the scope of liquid-phase exfoliation (LPE) methodology toward the controlled wet-chemical synthesis of functional nanomaterials.

Calculation and prediction of sliding energy barriers by first-principles combined with machine learning

Recently, many novel two-dimensional materials have been identified through data-driven computational methods, whose tribological properties are bound to be further investigated. Here, the first principles calculation and machine learning were combined to predict the maximum sliding potential barrier of elemental two-dimensional materials and the importance of different descriptors were discussed. Due to literature reports and high correlation coefficients, RF and Bagging algorithms are used to present the final prediction results. The prediction results show that R2 is about 0.8, and the root mean square error (RMSE) of ∼0.0027 eV and 0.0029 eV, respectively. Importantly, when considering the interfacial charge transfer as the eigenvalue, the R2 of both models increased (R2 can reach 0.89), and the RMSEs decreased tõ0.0020 eV and 0.0024 eV. This indicates that interfacial charge transfer has a significant impact on the prediction of the maximum sliding energy barrier. That is, adjusting the charge transfer at the interface can improve the tribological properties of the interface. In addition, the geometric structure, such as lattice parameter, and mechanical properties of the material are also important factors affecting its maximum sliding energy barrier. Compared with traditional first principles calculation, machine learning based on statistics may be a promising choice for predicting friction performance, and it is expected to find the influencing factors directly related to friction behavior.

Recent Advances in Surface Modifications of Elemental Two-Dimensional Materials: Structures, Properties, and Applications

The advent of graphene opens up the research into two-dimensional (2D) materials, which are considered revolutionary materials. Due to its unique geometric structure, graphene exhibits a series of exotic physical and chemical properties. In addition, single-element-based 2D materials (Xenes) have garnered tremendous interest. At present, 16 kinds of Xenes (silicene, borophene, germanene, phosphorene, tellurene, etc.) have been explored, mainly distributed in the third, fourth, fifth, and sixth main groups. The current methods to prepare monolayers or few-layer 2D materials include epitaxy growth, mechanical exfoliation, and liquid phase exfoliation. Although two Xenes (aluminene and indiene) have not been synthesized due to the limitations of synthetic methods and the stability of Xenes, other Xenes have been successfully created via elaborate artificial design and synthesis. Focusing on elemental 2D materials, this review mainly summarizes the recently reported work about tuning the electronic, optical, mechanical, and chemical properties of Xenes via surface modifications, achieved using controllable approaches (doping, adsorption, strain, intercalation, phase transition, etc.) to broaden their applications in various fields, including spintronics, electronics, optoelectronics, superconducting, photovoltaics, sensors, catalysis, and biomedicines. These advances in the surface modification of Xenes have laid a theoretical and experimental foundation for the development of 2D materials and their practical applications in diverse fields.

Electronic Structure Modification of Rectangular Phosphorene Quantum Dots Via Edge Passivation

Phosphorene is an elemental two-dimensional material that shows great promise as a metal-free photocatalyst owing to its high carrier mobility and direct band gap. Doping and edge functionalization...

Emerging elemental two-dimensional materials for energy applications

Elemental two-dimensional (2D) materials possess distinct properties and superior performances across a multitude of fundamental and practical research fields.

Recent Advances in the Applications of Elemental Two-dimensional Materials and Their Derivatives Serving as Transport Layers in Solar Cells

Recent Advances in the Applications of Elemental Two-dimensional Materials and Their Derivatives Serving as Transport Layers in Solar Cells

Enhancement of the quantum capacitances of group-14 elemental two-dimensional materials by Ti-doping: A first principles study

Group-14 elemental two-dimensional materials are a key material for future supercapacitors because of their various advantages compared to activated carbon. However, the lack of density of states (DOS) near the Fermi level (EF) of these materials is one of the limiting factors for the performance of supercapacitors, especially for electric double-layer capacitors (EDLCs). In this study, Ti-doping was investigated as a strategy for providing a large amount of DOS near the EF. By using density functional theory calculations, it was confirmed that the d band of the Ti atom provides additional DOS to the materials. The calculated quantum capacitances and the surface charge densities of the doped systems were enhanced overall in both their gravimetric and specific aspects thanks to the increased DOS near the EF. In addition, it was revealed that Ti-doped silicene has superior characteristics at the low voltage range compared to other materials. These findings may provide practical guidelines for improving the performance of EDLCs.

Fabrication and Imaging of Monolayer Phosphorene with Preferred Edge Configurations via Graphene-Assisted Layer-by-Layer Thinning.

Phosphorene, a monolayer of black phosphorus (BP), is an elemental two-dimensional material with interesting physical properties, such as high charge carrier mobility and exotic anisotropic in-plane properties. To fundamentally understand these various physical properties, it is critically important to conduct an atomic-scale structural investigation of phosphorene, particularly regarding various defects and preferred edge configurations. However, it has been challenging to investigate mono- and few-layer phosphorene because of technical difficulties arising in the preparation of a high-quality sample and damages induced during the characterization process. Here, we successfully fabricate high-quality monolayer phosphorene using a controlled thinning process with transmission electron microscopy and subsequently perform atomic-resolution imaging. Graphene protection suppresses the e-beam-induced damage to multilayer BP and one-side graphene protection facilitates the layer-by-layer thinning of the samples, rendering high-quality monolayer and bilayer regions. We also observe the formation of atomic-scale crystalline edges predominantly aligned along the zigzag and (101) terminations, which is originated from edge kinetics under e-beam-induced sputtering process. Our study demonstrates a new method to image and precisely manipulate the thickness and edge configurations of air-sensitive two-dimensional materials.

Epitaxial growth of elemental 2D materials: What can we learn from the periodic table?

• We reviewed recent advances in epitaxial growth of elemental two-dimensional materials on coinage metals. • Surface alloys play key roles in both the layer-by-layer and segregation growth of these novel 2D materials. • The periodic table and phase diagram offer us valuable information for selecting suitable substrates. • Surface alloys with stable structures can be used for the heterogeneous growth of either monoelemental or binary 2D materials. Recent achievements in the fabrication of two-dimensional (2D) materials by molecular beam epitaxy have demonstrated that surface alloys are critical intermediates for either layer by layer or segregation growth. The resulting films show a homogenous growth mode that strongly depends on the lattice and properties of surface alloys. Learning from the periodic table, we address the importance of surface alloys in the heterogeneous growth of 2D materials. We demonstrate basic rules in the formation of surface alloys that are correlated with their periodic properties. We highlight the possibility of using 2D topological insulators as the substrates to accomplish quasifreestanding monolayers. Finally, we assess the current status and challenges of binary 2D materials and discuss prospects of their fabrication via surface alloy processes.

Infrared ultrafast spectroscopy of solution-grown thin film tellurium

Several materials studied intensively in their bulk forms several decades ago have re-emerged in recent years in their thin film and monolayer manifestations. Tellurium, like black phosphorus, is one such elemental two-dimensional material with promising semiconducting properties for electronic and optoelectronic applications. To study fundamental carrier properties such as hot carrier relaxation and recombination, we performed ultrafast femtosecond pump-probe spectroscopy on thin flakes of solution-grown tellurium. To access the low band gap of tellurium, we used infrared, near-band-gap and below-band-gap probes to monitor the relaxation processes. Sweeping the probing wavelengths across the band gap helps to shed light on the anisotropic band structure of the material. We find that relaxation in flakes of 60--160 nm thickness is on the order of 100 s of picoseconds. Thinner flakes (10--20 nm), on the other hand, exhibit fast relaxation times of sub-20 ps. Radiative recombination is identified as the relaxation mechanism in thick flakes, whereas midgap trap states arising from surface defects and impurities are responsible for the fast relaxation in thin flakes. A diffusion-recombination model accounting for the surface defect and radiative recombinations explains the experimental data well.

Thermoelectric properties of two-dimensional selenene and tellurene from group-VI elements.

Single-layered two-dimensional (2D) materials have become very attractive due to their novel electronic behavior after the discovery of graphene. Here, we report the electronic structure and thermoelectric properties of single-layered 2D selerium (selenene) and tellurium (tellurene) by using density functional theory calculation. Both selenene and tellurene show three-phonon limited thermal conductivity. The prominent anharmonic phonon scattering process for tellurene makes it show the lowest lattice thermal conductivity among 2D single elemental materials till now. Their special square unit cells give rise to a highly anisotropic electronic structure along the zigzag and the armchair direction. The large effective mass and Seebeck coefficient along the armchair direction suggest that the thermal performance is better than that of the zigzag direction. The effect of spin-orbit coupling increases the band gap and is found to be crucial for tellurene. These studies provide a way to tune the thermoelectric properties of selenene and tellurene in the future.

All-Optical Switching of Two Continuous Waves in Few Layer Bismuthene Based on Spatial Cross-Phase Modulation

Bismuthene, the last and heaviest group-VA elemental two-dimensional material, has received tremendous interests owing to its advantages in electronic-transport, semimetallic bonding, and intrinsic spin-orbit coupling. However, light–bismuthene interaction is relatively less investigated. Herein, sonochemical exfoliation approach had been employed to deliver a successful synthesis of few-layer bismuthene with an average thickness of ∼3 nm and a lateral size of ∼0.2 μm. The corresponding band structure from mono- to sextuple-layer had been therotically calculated and it was found that bismuthene possesses a thickness dependent energy gap from almost zero to 0.55 eV, suggesting that bismuthene may also find unique applications from terahertz, mid-infrared toward infrared regime. The nonlinear optical absorption and refraction parameters had been well characterized by laser Z-scan and spatial phase modulation measurement techniques, respectively. By taking advantage of its strong nonlinear refraction effect,...

Carrier thermoelectric transport model for black phosphorus field-effect transistors

As a new elemental two-dimensional material, layered black phosphorus (BP) has revealed its intriguing properties and is considered to be a potential thermoelectric material. However, the ambiguous carrier thermoelectric transport in layered BP limits its development. In this work, based on variable-range hopping transport and percolation theory, we have developed a unified thermoelectric-transport model to investigate the Seebeck effect of BP field-effect transistors (FETs). The proposed model can characterize the effect of temperature, disorder, and the carrier concentration on carrier thermoelectric transport. The simulation results show abnormal temperature-dependence with decreasing disorder degree in BP FETs. The proposed model will be helpful to understand the thermoelectric transport properties of layered BP and design desired thermoelectric devices.

Partial Oxidized Arsenene: Emerging Tunable Direct Bandgap Semiconductor.

Arsenene, as a member of the Group V elemental two-dimensional materials appears on the horizon, has shown great prospects. However, its indirect bandgap limits the applications in optoelectronics. In this theoretical work, we reported that partial oxidation can tune the indirect bandgap of arsenene into the direct one. Attributed to the enthalpy decreasing linear to the oxygen ratio, partial oxidized arsenene can be controllably produced by the progressive oxidation under low temperature. Importantly, by increasing the oxygen content from 1O/18As to 18O/18As, the oxidation can narrow the direct bandgap of oxidized arsenene from 1.29 to 0.02 eV. The bandgap of partial oxidized arsenene is proportional to the oxygen content. Consequently, the partial oxidized arsenene with tunable direct bandgap has great potentials in the high efficient infra light emitter and photo-voltaic devices.

Lateral black phosphorene P–N junctions formed via chemical doping for high performance near-infrared photodetector

Black phosphorene (BP), a newly discovered elemental two-dimensional material, is attractive for optoelectronic and photonic applications because of its unique in-plane anisotropy, thickness-dependent direct bandgap and high carrier mobility. Since its discovery, black phosphorene has become an appealing candidate well-suited for polarization-resolved near- and mid-infrared optoelectronics due to its relative narrow bandgap and asymmetric structure. Here, we employ benzyl viologen (BV) as an effective electron dopant to part of the area of a (p-type) few-layer BP flake and achieve an ambient stable, in-plane P–N junction. Chemical doping with BV molecules modulates the electron density and allows acquiring a large built-in potential in this in-plane BP P–N junction, which is crucial for achieving high responsivity photodetectors and high quantum efficiency solar cells. As a demonstrative example, by illuminating it with a near-infrared laser at 1.47µm, we observe a high responsivity up to ~180mA/W with a rise time of 15ms, and an external quantum efficiency of 0.75%. Our strategy for creating environmentally stable BP P–N junction paves the way to implementing high performance BP phototransistors and solar cells, which is also applicable to other 2D materials.

Substitutionally doped phosphorene: electronic properties and gas sensing

Phosphorene, a new elemental two-dimensional material, has attracted increasing attention owing to its intriguing electronic properties. In particular, pristine phospohorene, due to its ultrahigh surface–volume ratio and high chemical activity, has been shown to be promising for gas sensing (Abbas et al 2015 ACS Nano 9 5618). To further enhance its sensing ability, we perform first-principles calculations based on density functional theory to study substitutionally doped phosphorene with 17 different atoms, focusing on structures, energetics, electronic properties and gas sensing. Our calculations reveal that anionic X (X = O, C and S) dopants have a large binding energy and highly dispersive electronic states, signifying the formation of covalent X–P bonds and thus strong structural stability. Alkali atom (Li and Na) doping is found to donate most of the electrons in the outer s-orbital by forming ionic bonds with P, and the band gap decreases by pushing down the conduction band, suggesting that the optical and electronic properties of the doped phosphorene can be tailored. For doping with VIIIB-group (Fe, Co and Ni) elements, a strong affinity is predicted and the binding energy and charge transfer are correlated strongly with their electronegativity. By examining NO molecule adsorption, we find that these metal doped phosphorenes (MDPs) in general exhibit a significantly enhanced chemical activity compared with pristine phosphorene. Our study suggests that substitutionally doped phosphorene shows many intriguing electronic and optic properties different from pristine phosphorene and MDPs are promising in chemical applications involving molecular adsorption and desorption processes, such as materials growth, catalysis, gas sensing and storage.

Phosphorene under electron beam: from monolayer to one-dimensional chains.

Phosphorene, a single sheet of black phosphorus, is an elemental two-dimensional material with unique properties and potential applications in semiconductor technology. While few-layer flakes of the material have been characterized using transmission electron microscopy, very little is known about its response to electron irradiation, which may be particularly important in the context of top-down engineering of phosphorus nanostructures using a focused electron beam. Here, using first-principles simulations, we study the production of defects in a single phosphorene sheet under impacts of energetic electrons. By employing the McKinley-Feshbach formalism and accounting for the thermal motion of atoms, we assess the cross section for atom displacement as a function of electron energy. We further investigate the energetics and dynamics of point defects and the stability of ribbons and edges under an electron beam. Finally, we show that P atomic chains should be surprisingly stable, and their atomic structure is not linear giving rise to the absence of a gap in the electronic spectrum.

Black phosphorus gas sensors.

The utilization of black phosphorus and its monolayer (phosphorene) and few-layers in field-effect transistors has attracted a lot of attention to this elemental two-dimensional material. Various studies on optimization of black phosphorus field-effect transistors, PN junctions, photodetectors, and other applications have been demonstrated. Although chemical sensing based on black phosphorus devices was theoretically predicted, there is still no experimental verification of such an important study of this material. In this article, we report on chemical sensing of nitrogen dioxide (NO2) using field-effect transistors based on multilayer black phosphorus. Black phosphorus sensors exhibited increased conduction upon NO2 exposure and excellent sensitivity for detection of NO2 down to 5 ppb. Moreover, when the multilayer black phosphorus field-effect transistor was exposed to NO2 concentrations of 5, 10, 20, and 40 ppb, its relative conduction change followed the Langmuir isotherm for molecules adsorbed on a surface. Additionally, on the basis of an exponential conductance change, the rate constants for adsorption and desorption of NO2 on black phosphorus were extracted for different NO2 concentrations, and they were in the range of 130-840 s. These results shed light on important electronic and sensing characteristics of black phosphorus, which can be utilized in future studies and applications.