Observation of 2D Conduction in Ultrathin Germanium Arsenide Field-Effect Transistors

Memory effect and coexistence of negative and positive photoconductivity in black phosphorus field effect transistor for neuromorphic vision sensors

Black phosphorus (BP) field-effect transistors with ultrathin channels exhibit unipolar p-type electrical conduction over a wide range of temperatures and pressures. Herein, we study a device that exhibits mobility up to 100 cm2 V-1 s-1 and a memory window up to 1.3 μA. Exposure to a supercontinuum white light source reveals that negative photoconductivity (NPC) and positive photoconductivity (PPC) coexist in the same device. Such behavior is attributed to the chemisorbed O2 molecules, with a minor role of physisorbed H2O molecules. The coexistence of NPC and PPC can be exploited in neuromorphic vision sensors, requiring the human eye retina to process the optical signals through alerting and protection (NPC), adaptation (PPC), followed by imaging and processing. Our results open new avenues for the use of BP and other two-dimentional (2D) semiconducting materials in transistors, memories, and neuromorphic vision sensors for advanced applications in robotics, self-driving cars, etc.

Triboelectrification and Unique Frictional Characteristics of Germanium-Based Nanofilms

In this study, germanium arsenide (GeAs) is investigated as a promising nanomaterial for application in triboelectric nanogenerators and green energy harvesting. The mechanical and electrical properties of mechanically exfoliated GeAs on silica substrates are evaluated through friction force microscopy and Kelvin probe force microscopy, respectively. First, it is observed that the surface potential/work function of GeAs varied with thickness. Second, thickness-dependent friction on GeAs films is found. However, the variation of friction with GeAs thickness followed an inverse trend typically observed for most other 2D material systems: larger friction is measured on thicker GeAs films. The higher friction is attributed to the higher surface potential of thicker GeAs, resulting from the accumulation of electrons on the GeAs surface that also resulted in higher adhesion between GeAs surface and the tip. Finally, history-dependent friction is observed and resulted from a continual increase in the friction force as the surface is scanned and originated from the triboelectrification of the surface. The dynamic triboelectrification behavior of thick GeAs during the scanning process is further verified and visualized by a serial experiment, where the GeAs is tribo-electrified through scanning and gradually de-electrified/discharged upon ceasing the scan.

Tunable ohmic van der Waals-type contacts in monolayer C3N field-effect transistors

Monolayer (ML) C3N, a novel two-dimensional flat crystalline material with a suitable bandgap and excellent carrier mobility, is a prospective channel material candidate for next-generation field-effect transistors (FETs). The contact properties of ML C3N-metal interfaces based on FETs have been comprehensively investigated with metal electrodes (graphene, Ti2C(OH/F)2, Zr2C(OH/F)2, Au, Ni, Pd, and Pt) by employing ab initio electronic structure calculations and quantum transport simulations. The contact properties of ML C3N are isotropic along the armchair and zigzag directions except for the case of Au. ML C3N establishes vertical van der Waals-type ohmic contacts with all the calculated metals except for Zr2CF2. The ML C3N-graphene, -Zr2CF2, -Ti2CF2, -Pt, -Pd, and -Ni interfaces form p-type lateral ohmic contacts, while the ML C3N-Ti2C(OH)2 and -Zr2C(OH)2 interfaces form n-type lateral ohmic contacts. The ohmic contact polarity can be regulated by changing the functional groups of the 2D MXene electrodes. These results provide theoretical insights into the characteristics of ML C3N-metal interfaces, which are important for choosing suitable electrodes and the design of ML C3N devices.

Spin-orbit splitting and piezoelectric properties of Janus Ge2XY (X ≠ Y = P, As, Sb and Bi)

The coexistence of spin-orbit coupling and piezoelectricity in a single material may have potential application in multifunctional devices, including spintronics, nanorobotics and piezotronics. Spin-orbit coupling provides a new means to manipulate electron's spin without an additional external magnetic field, while piezoelectricity refers to the interplay between mechanical stresses and electric polarization. Using first-principles calculations, the structural, electronic, optical, spin, and piezoelectric properties of the Janus Ge₂XY (X ≠ Y = P, As, Sb, and Bi) monolayers were systematically investigated. All the Ge₂XY are energetically and dynamically stable in the α phase. At the GW level, Ge₂AsSb, Ge₂AsBi, and Ge₂SbBi have direct fundamental band gaps of 0.65, 0.64, and 0.91 eV. At the GW + BSE level, their optical gaps are 0.42, 0.45, and 0.63 eV, and the optical absorption coefficients can reach about 10^-5 cm^-1 in the infrared light region, which reveals that they have potential for application in infrared photodetectors. For Ge₂PBi, Ge₂AsBi, and Ge₂SbBi containing the heavy Bi element, the lowermost conduction band and uppermost valence band have large spin splitting along the M-K and K-Γ lines, and the bands near the Fermi level possess Rashba spin splitting at the Γ point. Ge₂PBi and Ge₂SbBi have both large in-plane piezoelectric coefficients d₁₁ (-0.75 and -3.18 pm V^-1) and out-of-plane piezoelectric coefficients d₃₁ (0.37 and 0.30 pm V^-1). Our findings are helpful to understand the mechanism of the spin-orbit physics and piezoelectricity of Janus Ge₂XY monolayers and guide experiments in exploring novel multifunctional materials.

Chemical Vapor Deposition of Quaternary 2D BiCuSeO p-Type Semiconductor with Intrinsic Degeneracy

2D BiCuSeO is an intrinsic p-type degenerate semiconductor due to its self-doping effect, which possesses great potential to fabricate high-performance 2D-2D tunnel field-effect transistors (TFETs). However, the controllable synthesis of multinary 2D materials by chemical vapor deposition (CVD) is still a challenge due to the restriction of thermodynamics. Here, the CVD synthesis of quaternary 2D BiCuSeO nanosheets is realized. As-grown BiCuSeO nanosheets with thickness down to ≈6.1 nm (≈7 layers) and domain size of ≈277 µm show excellent ambient stability. Intrinsic p-type degeneracy of BiCuSeO, capable of maintaining even in a few layers, is comprehensively unveiled. By varying the thicknesses and temperatures, the carrier concentration of BiCuSeO nanosheets can be adjusted in the range of 10¹⁹ to 10²¹ cm^-3 , and the Hall mobility of BiCuSeO is ≈191 cm² V^-1 s^-1 (at 2 K). Furthermore, taking advantage of the p-type degeneracy of BiCuSeO, a prototypical BiCuSeO/MoS₂ TFET is fabricated. The emergence of the negative differential resistance trend and multifunctional diodes by modulating the gate voltage and temperature reveal the great practical implementation potential of BiCuSeO nanosheets. These results pave way for the CVD synthesis of multinary 2D materials and rational design of high-performance tunnel devices.

Solid-source vapor growth and optoelectronic properties of arsenic-based layered group-IV monopnictides

We investigated the preparation of SiAs and GeAs films using solid-source vapor growth. The shapes of the SiAs and GeAs films were rectangular, reflecting the thermally stable monoclinic phase (C2/m)...

Screening in Graphene: Response to External Static Electric Field and an Image-Potential Problem

We present a detailed first-principles investigation of the response of a free-standing graphene sheet to an external perpendicular static electric field E. The charge density distribution in the vicinity of the graphene monolayer that is caused by E was determined using the pseudopotential density-functional theory approach. Different geometries were considered. The centroid of this extra density induced by an external electric field was determined as zim = 1.048 Å at vanishing E, and its dependence on E has been obtained. The thus determined zim was employed to construct the hybrid one-electron potential which generates a new set of energies for the image-potential states.

Structural, electrical and optical properties of bilayer SiX (X = N, P, As and Sb)

In this work, the structural, electrical, and optical properties of bilayer SiX (X= N, P, As, and Sb) are studied using density functional theory. Five different stacking orders are considered for every compound and their structural properties are presented. The band structure of these materials demonstrates that they are indirect semiconductors. The out-of-plane strain has been applied to tune the bandgap and its electrical properties. The bandgap increases with tensile strain, whereas, compressive strain leads to semiconductor-to-metal transition. The sensitivity of the bandgap to the pressure is investigated and bilayer SiSb demonstrates the highest bandgap sensitivity to the pressure. These structures exhibit Mexican hat-like valence band dispersion that can be approved by a singularity in the density of states (DOS). The Mexican-hat coefficient can be tuned by out-of-plane strain. Optical absorption of these compounds shows that the second and lower valence bands due to the high DOS display a higher contribution to optical transitions.

Electron Irradiation of Metal Contacts in Monolayer MoS2 Field-Effect Transistors

Metal contacts play a fundamental role in nanoscale devices. In this work, Schottky metal contacts in monolayer molybdenum disulfide (MoS2) field-effect transistors are investigated under electron beam irradiation. It is shown that the exposure of Ti/Au source/drain electrodes to an electron beam reduces the contact resistance and improves the transistor performance. The electron beam conditioning of contacts is permanent, while the irradiation of the channel can produce transient effects. It is demonstrated that irradiation lowers the Schottky barrier at the contacts because of thermally induced atom diffusion and interfacial reactions. The simulation of electron paths in the device reveals that most of the beam energy is absorbed in the metal contacts. The study demonstrates that electron beam irradiation can be effectively used for contact improvement through local annealing.

WS2 Nanotubes: Electrical Conduction and Field Emission Under Electron Irradiation and Mechanical Stress

This study reports the electrical transport and the field emission properties of individual multi-walled tungsten disulphide (WS₂ ) nanotubes (NTs) under electron beam irradiation and mechanical stress. Electron beam irradiation is used to reduce the nanotube-electrode contact resistance by one-order of magnitude. The field emission capability of single WS₂ NTs is investigated, and a field emission current density as high as 600 kA cm^-2 is attained with a turn-on field of ≈100 V μm^-1 and field-enhancement factor ≈50. Moreover, the electrical behavior of individual WS₂ NTs is studied under the application of longitudinal tensile stress. An exponential increase of the nanotube resistivity with tensile strain is demonstrated up to a recorded elongation of 12%, thereby making WS₂ NTs suitable for piezoresistive strain sensor applications.

Air Pressure, Gas Exposure and Electron Beam Irradiation of 2D Transition Metal Dichalcogenides

In this study, we investigate the electrical transport properties of back-gated field-effect transistors in which the channel is realized with two-dimensional transition metal dichalcogenide nanosheets, namely palladium diselenide (PdSe2) and molybdenum disulfide (MoS2). The effects of the environment (pressure, gas type, electron beam irradiation) on the electrical properties are the subject of an intense experimental study that evidences how PdSe2-based devices can be reversibly tuned from a predominantly n-type conduction (under high vacuum) to a p-type conduction (at atmospheric pressure) by simply modifying the pressure. Similarly, we report that, in MoS2-based devices, the transport properties are affected by pressure and gas type. In particular, the observed hysteresis in the transfer characteristics is explained in terms of gas absorption on the MoS2 surface due to the presence of a large number of defects. Moreover, we demonstrate the monotonic (increasing) dependence of the width of the hysteresis on decreasing the gas adsorption energy. We also report the effects of electron beam irradiation on the transport properties of two-dimensional field-effect transistors, showing that low fluences of the order of few e-/nm2 are sufficient to cause appreciable modifications to the transport characteristics. Finally, we profit from our experimental setup, realized inside a scanning electron microscope and equipped with piezo-driven nanoprobes, to perform a field emission characterization of PdSe2 and MoS2 nanosheets at cathode–anode separation distances as small as 200 nm.

Emerging 2D Materials and Their Van Der Waals Heterostructures

Two-dimensional (2D) materials and their van der Waals heterojunctions offer the opportunity to combine layers with different properties as the building blocks to engineer new functional materials for high-performance devices, sensors, and water-splitting photocatalysts. A tremendous amount of work has been done thus far to isolate or synthesize new 2D materials as well as to form new heterostructures and investigate their chemical and physical properties. This article collection covers state-of-the-art experimental, numerical, and theoretical research on 2D materials and on their van der Waals heterojunctions for applications in electronics, optoelectronics, and energy generation.

Two-dimensional silicon bismotide (SiBi) monolayer with a honeycomb-like lattice: first-principles study of tuning the electronic properties

Using density functional theory, we investigate a novel two-dimensional silicon bismotide (SiBi) that has a layered GaSe-like crystal structure. Ab initio molecular dynamic simulations and phonon dispersion calculations suggest its good thermal and dynamical stability. The SiBi monolayer is a semiconductor with a narrow indirect bandgap of 0.4 eV. Our results show that the indirect bandgap decreases as the number of layers increases, and when the number of layers is more than six layers, direct-to-indirect bandgap switching occurs. The SiBi bilayer is found to be very sensitive to an E-field. The bandgap monotonically decreases in response to uniaxial and biaxial compressive strain, and reaches 0.2 eV at 5%, while at 6%, the semiconductor becomes a metal. For both uniaxial and biaxial tensile strains, the material remains a semiconductor and indirect-to-direct bandgap transition occurs at a strain of 3%. Compared to a SiBi monolayer with a layer thickness of 4.89 Å, the bandgap decreases with either increasing or decreasing layer thickness, and at a thicknesses of 4.59 to 5.01 Å, the semiconductor-to-metal transition happens. In addition, under pressure, the semiconducting character of the SiBi bilayer with a 0.25 eV direct bandgap is preserved. Our results demonstrate that the SiBi nanosheet is a promising candidate for designing high-speed low-dissipation devices.

The modulation of the electronic properties of SiBi monolayer via external means, including layer thickness, electric field and mechanical strain are explored with DFT method.

The mechanical, electronic, optical and thermoelectric properties of two-dimensional honeycomb-like of XSb (X = Si, Ge, Sn) monolayers: a first-principles calculations

Herein, by using first-principles calculations, we demonstrate a two-dimensional (2D) of XSb (X = Si, Ge, and Sn) monolayers that have a honey-like crystal structure.