Electrical Contacts in Monolayer Arsenene Devices

Recent advances in two-dimensional perovskite materials for light-emitting diodes

Light-emitting diodes (LEDs) are an indispensable part of our daily life. After being studied for a few decades, this field still has some room for improvement. In this regard, perovskite materials may take the leading role. In recent years, LEDs have become a most explored topic, owing to their various applications in photodetectors, solar cells, lasers, and so on. Noticeably, they exhibit significant characteristics in developing LEDs. The luminous efficiency of LEDs can be significantly enhanced by the combination of a poor illumination LED with low-dimensional perovskite. In 2014, the first perovskite-based LED was illuminated at room temperature. Furthermore, two-dimensional (2D) perovskites have enriched this field because of their optical and electronic properties and comparatively high stability in ambient conditions. Recent and relevant advancements in LEDs using low-dimensional perovskites including zero-dimensional to three-dimensional materials is reported. The major focus of this article is based on the 2D perovskites and their heterostructures (i.e., a combination of 2D perovskites with transition metal dichalcogenides, graphene, and hexagonal boron nitride). In comparison to 2D perovskites, heterostructures exhibit more potential for application in LEDs. State-of-the-art perovskite-based LEDs, current challenges, and prospects are also discussed.

The Contact Properties of Monolayer and Multilayer MoS2-Metal van der Waals Interfaces

The contact resistance formed between MoS2 and metal electrodes plays a key role in MoS2-based electronic devices. The Schottky barrier height (SBH) is a crucial parameter for determining the contact resistance. However, the SBH is difficult to modulate because of the strong Fermi-level pinning (FLP) at MoS2-metal interfaces. Here, we investigate the FLP effect and the contact types of monolayer and multilayer MoS2-metal van der Waals (vdW) interfaces using density functional theory (DFT) calculations based on Perdew-Burke-Ernzerhof (PBE) level. It has been demonstrated that, compared with monolayer MoS2-metal close interfaces, the FLP effect can be significantly reduced in monolayer MoS2-metal vdW interfaces. Furthermore, as the layer number of MoS2 increases from 1L to 4L, the FLP effect is first weakened and then increased, which can be attributed to the charge redistribution at the MoS2-metal and MoS2-MoS2 interfaces. In addition, the p-type Schottky contact can be achieved in 1L-4L MoS2-Pt, 3L MoS2-Au, and 2L-3L MoS2-Pd vdW interfaces, which is useful for realizing complementary metal oxide semiconductor (CMOS) logic circuits. These findings indicated that the FLP and contact types can be effectively modulated at MoS2-metal vdW interfaces by selecting the layer number of MoS2.

Ohmic contacts of the two-dimensional Ca2N/MoS2 donor-acceptor heterostructure

In the current stage, conventional silicon-based devices are suffering from the scaling limits and the Fermi level pinning effect. Therefore, looking for low-resistance metal contacts for semiconductors has become one of the most important topics, and two-dimensional (2D) metal/semiconductor contacts turn out to be highly interesting. Alternatively, the Schottky barrier and the tunneling barrier impede their practical applications. In this work, we propose a new strategy for reducing the contact potential barrier by constructing a donor-acceptor heterostructure, that is, Ca₂N/MoS₂ with Ca₂N being a 2D electrene material with a significantly small work function and a rather high carrier concentration. The quasi-bond interaction of the heterostructure avoids the formation of a Fermi level pinning effect and gives rise to high tunneling probability. An excellent n-type Ohmic contact form between Ca₂N and MoS₂ monolayers, with a 100% tunneling probability and a perfect linear I-V curve, and large lateral band bending also demonstrates the good performance of the contact. We verify a fascinating phenomenon that Ca₂N can trigger the phase transition of MoS₂ from 2H to 1T'. In addition, we also identify that Ohmic contacts can be formed between Ca₂N and other 2D transition metal dichalcogenides (TMDCs), including WS₂, MoSe₂, WSe₂, and MoTe₂.

2D Arsenene and Arsenic Materials: Fundamental Properties, Preparation, and Applications

As emerging 2D materials, arsenene and arsenic materials have attracted rising interest in the past few years. The diverse crystalline phases, exotic electrical characteristics, and widespread applications of 2D arsenene and arsenic bring them great research value and utilization potential. Herein, the recent progress of 2D arsenene and arsenic is reviewed in terms of fundamental properties, preparation, and applications. The fundamental properties of 2D arsenene and arsenic, including the crystal phases, environmental stability, and electrical structure, from theoretical to experimental reports are first summarized. Then, the experimental processes for preparing 2D arsenene and arsenic, along with their respective advantages and disadvantages, are introduced including epitaxial growth, mechanical exfoliation, and liquid-phase exfoliation. Moreover, applications of 2D arsenene and arsenic are discussed, suggesting a wide range of applications of 2D arsenene and arsenic in field-effect transistors, sensors, catalysts, biological applications, and so on. Finally, some perspectives about the challenges and opportunities of promising 2D arsenene and arsenic are provided. This review provides a helpful guidance and stimulates more focus on future explorations and developments of 2D arsenene and arsenic.

Chemistry, Functionalization, and Applications of Recent Monoelemental Two-Dimensional Materials and Their Heterostructures

The past decades have witnessed a rapid expansion in investigations of two-dimensional (2D) monoelemental materials (Xenes), which are promising materials in various fields, including applications in optoelectronic devices, biomedicine, catalysis, and energy storage. Apart from graphene and phosphorene, recently emerging 2D Xenes, specifically graphdiyne, borophene, arsenene, antimonene, bismuthene, and tellurene, have attracted considerable interest due to their unique optical, electrical, and catalytic properties, endowing them a broader range of intriguing applications. In this review, the structures and properties of these emerging Xenes are summarized based on theoretical and experimental results. The synthetic approaches for their fabrication, mainly bottom-up and top-down, are presented. Surface modification strategies are also shown. The wide applications of these emerging Xenes in nonlinear optical devices, optoelectronics, catalysis, biomedicine, and energy application are further discussed. Finally, this review concludes with an assessment of the current status, a description of existing scientific and application challenges, and a discussion of possible directions to advance this fertile field.

Schottky barrier heights in two-dimensional field-effect transistors: from theory to experiment

Over the past decade, two-dimensional semiconductors (2DSCs) have aroused wide interest due to their extraordinary electronic, magnetic, optical, mechanical, and thermal properties, which hold potential in electronic, optoelectronic, thermoelectric applications, and so forth. The field-effect transistor (FET), a semiconductor gated with at least three terminals, is pervasively exploited as the device geometry for these applications. For lack of effective and stable substitutional doping techniques, direct metal contact is often used in 2DSC FETs to inject carriers. A Schottky barrier (SB) generally exists in the metal-2DSC junction, which significantly affects and even dominates the performance of most 2DSC FETs. Therefore, low SB or Ohmic contact is highly preferred for approaching the intrinsic characteristics of the 2DSC channel. In this review, we systematically introduce the recent progress made in theoretical prediction of the SB height (SBH) in the 2DSC FETs and the efforts made both in theory and experiments to achieve low SB contacts. From the comparison between the theoretical and experimentally observed SBHs, the emerging first-principles quantum transport simulation turns out to be the most powerful theoretical tool to calculate the SBH of a 2DSC FET. Finally, we conclude this review from the viewpoints of state-of-the-art electrode designs for 2DSC FETs.

Anisotropic interfacial properties of monolayer C2N field effect transistors

Monolayer C2N is promising for next-generation electronic and optoelectronic applications due to its appropriate band gap and high carrier efficiency. However, relative studies have been held back due to the lack of high-quality electrode contacts. Here, we comprehensively study the electronic and transport properties of monolayer C2N with a series of electrode materials (Al, Ti, Ni, Cu, Ag, Pt, V2C, Cr2C and graphene) by using the nonequilibrium Green's function (NEGF) method combined with density functional theory (DFT). The monolayer C2N forms Ohmic contacts with the Ti/Cu/Ag electrode material in both armchair and zigzag directions, whereas Ohmic contact is only formed in the zigzag direction of the C2N-Al field effect transistor. However, the C2N-Ni, -Pt, -V2C, -Mo2C, -graphene contact systems form n-type Schottky contacts in either the armchair or zigzag direction owing to the relatively strong Fermi level pinning (the pinning factor S = 0.32 in the armchair direction and S = 0.26 in the zigzag direction). By insertion of BN or graphene between the C2N and Pt electrode in the armchair direction of contact systems, the Fermi level pinning can be effectively weakened due to the suppression of metal-induced gap states. Conspicuously, an Ohmic contact is realized in the C2N field effect transistors with the BN-Pt electrode, suggesting a possible approach to fabricating high-performance devices. Our study is conducive to selecting appropriate electrode materials for C2N-based field effect transistors.

"Top-down" Arsenene Production by Low-Potential Electrochemical Exfoliation

Arsenene, as an exotic representative of two-dimensional (2D) materials, has received great interest, yet the interest is mainly based on theoretical study. The reason for this is a restricted ability to operate the material from its synthesis to implementation. Beginning with the production, electrochemical exfoliation has been found as an extremely effective method for the preparation of 2D materials from bulk materials. Here, for the first time, we demonstrate the electrochemical exfoliation of bulk black arsenic in the anhydrous electrolyte medium. Spectro- and microscopic analyses evidence micrometer lateral size few-layer arsenene in a netlike porous shape formed of 2D flakes. We demonstrate that the surfactant-free exfoliation successfully resulted in a stable dispersion for which only washing with the corresponding solvent was sufficient. This electrochemistry route for the black arsenic exfoliation toward few-layer arsenene will broaden the materials' scope applications in new-generation devices.

Emerging Applications of Elemental 2D Materials

As elemental main group materials (i.e., silicon and germanium) have dominated the field of modern electronics, their monolayer 2D analogues have shown great promise for next-generation electronic materials as well as potential game-changing properties for optoelectronics, energy, and beyond. These atomically thin materials composed of single atomic variants of group III through group VI elements on the periodic table have already demonstrated exciting properties such as near-room-temperature topological insulation in bismuthene, extremely high electron mobilities in phosphorene and silicone, and substantial Li-ion storage capability in borophene. Isolation of these materials within the postgraphene era began with silicene in 2010 and quickly progressed to the experimental identification or theoretical prediction of 15 of the 18 main group elements existing as solids at standard pressure and temperatures. This review first focuses on the significance of defects/functionalization, discussion of different allotropes, and overarching structure-property relationships of 2D main group elemental materials. Then, a complete review of emerging applications in electronics, sensing, spintronics, plasmonics, photodetectors, ultrafast lasers, batteries, supercapacitors, and thermoelectrics is presented by application type, including detailed descriptions of how the material properties may be tailored toward each specific application.

Two-Dimensional Pnictogen for Field-Effect Transistors

Two-dimensional (2D) layered materials hold great promise for various future electronic and optoelectronic devices that traditional semiconductors cannot afford. 2D pnictogen, group-VA atomic sheet (including phosphorene, arsenene, antimonene, and bismuthene) is believed to be a competitive candidate for next-generation logic devices. This is due to their intriguing physical and chemical properties, such as tunable midrange bandgap and controllable stability. Since the first black phosphorus field-effect transistor (FET) demo in 2014, there has been abundant exciting research advancement on the fundamental properties, preparation methods, and related electronic applications of 2D pnictogen. Herein, we review the recent progress in both material and device aspects of 2D pnictogen FETs. This includes a brief survey on the crystal structure, electronic properties and synthesis, or growth experiments. With more device orientation, this review emphasizes experimental fabrication, performance enhancing approaches, and configuration engineering of 2D pnictogen FETs. At the end, this review outlines current challenges and prospects for 2D pnictogen FETs as a potential platform for novel nanoelectronics.

Phase stability of three-dimensional bulk and two-dimensional monolayer As1-x Sb x solid solutions from first principles

The mixing thermodynamics of both three-dimensional bulk and two-dimensional mono-layered alloys of As_1-x Sb _x as a function of alloy composition and temperature are explored using a first-principles cluster-expansion method, combined with canonical Monte-Carlo simulations. We observe that, for the bulk phase, As_1-x Sb _x alloy can exhibit not only chemical ordering of As and Sb atoms at x  =  0.5 to form an ordered compound of AsSb stable upon annealing up to [Formula: see text] K, but also a miscibility gap at 475 K [Formula: see text] T [Formula: see text] 550 K in which two disordered solid solutions of As_1-x Sb _x of different alloy compositions thermodynamically coexist. At T  >  550 K, a single-phase solid solution of bulk As_1-x Sb _x is predicted to be stable across the entire composition range. These results clearly explain the existing uncertainties in the alloying behavior of bulk As_1-x Sb _x alloy, as previously reported in the literature, and also found to be in qualitative and quantitative agreement with the experimental observations. Interestingly, the alloying behavior of As_1-x Sb _x is considerably altered, as the dimensionality of the material reduces from the three-dimensional bulk state to the two-dimensional mono-layered state-for example, a single-phase solid solution of monolayer As_1-x Sb _x is predicted to be stable over the whole composition range at T  >  250 K. This distinctly highlights an influence of the reduced dimensionality on the alloying behavior of As_1-x Sb _x .

2D-Pnictogens: alloy-based anode battery materials with ultrahigh cycling stability

There is an increasing demand for efficient energy storage systems in our modern mobile society for a wide range of applications such as smart grids, portable electronic devices, and electric vehicles. The performance of advanced batteries in terms of energy density, power density, cyclability, and safety is mainly determined by the primary functional components, particularly by the electrode materials. Black phosphorus (BP) and the following elements in group V (pnictogens) including arsenic, antimony, and bismuth with layered structures have attracted tremendous attention to replace the graphite anode. This is due to their extremely high specific-capacities for lithium and sodium storage based on the alloying reaction mechanism; however, the same mechanism causes an irreversible volume expansion and thus low cycling stability. Since the discovery of single layer BP and its outstanding physical properties such as tunable band gap, strong in-plane anisotropy, and high carrier mobility, the battery community have intensively studied this material as well as the 2D structures of other pnictogens. In this review, first, the preparation and properties of 2D-pnictogens including crystal structure and chemical stability are briefly described. Second, the theoretical and experimental details of the intercalation and alloying mechanisms are discussed. Finally, the excellent performance of 2D-pnictogens for lithium ion and sodium ion batteries and their principal advantages compared to their parent 3D structures are presented.

n- and p-type ohmic contacts at monolayer gallium nitride-metal interfaces

Recently, two-dimensional (2D) gallium nitride (GaN) was experimentally fabricated, and has promising applications in next-generation electronic and optoelectronic devices. A direct contact with metals to inject the carrier is often required for potential 2D GaN devices. Herein, the first systematic study on the interface properties of monolayer (ML) planar and buckled GaN with different metal electrodes (Al, Ti, Ag, Au, Sc, and Pt) in a field-effect transistor framework is presented using first-principles energy band calculations and quantum transport simulations. Because of moderate Fermi level pinning (electron pinning factor S = 0.63), ML planar GaN and the Ag electrode form an n-type lateral Schottky contact, while ML planar GaN and Ti, Al, and Au electrodes form a p-type lateral Schottky contact. The ML buckled GaN, Ag, Al, Ti, and Sc electrodes form a p-type lateral Schottky contact as a result of Fermi level pinning with a hole pinning factor of S = 0.75. Notably, a highly desirable n-type/p-type lateral ohmic contact is formed at the lateral interface of the ML planar GaN and Sc/Pt electrodes, and a p-type lateral ohmic contact is formed at the lateral interface of the ML buckled GaN and Pt/Au electrodes. Therefore, a low resistance contact can be realized in ML planar and buckled GaN devices.

Tuning electronic and optical properties of arsenene/C3N van der Waals heterostructure by vertical strain and external electric field

Searching for new van der Waals (vdW) heterostructure with novel electronic and optical properties is of great interest and importance for the next generation of devices. By using first-principles calculations, we show that the electronic and optical properties of the arsenene/C₃N vdW heterostructure can be effectively modulated by applying vertical strain and external electric field. Our results suggest that this heterostructure has an intrinsic type-II band alignment with an indirect bandgap of 0.16 eV, facilitating the separation of photogenerated electron-hole pairs. The bandgap in the heterostructure can be tuned from 0-0.35 eV via the strain, experiencing an indirect-to-direct bandgap transition. Moreover, the bandgap of the heterostructure varies linearly with respect to a moderate external electric field, and the semiconductor-to-metal transition can be realized in the presence of a strong electric field. The calculated band alignment and the optical absorption reveal that the arsenene/C₃N heterostructure could present excellent light-harvesting performance. Our designed vdW heterostructure is expected to have great potential applications in nanoelectronic devices and photovoltaics.

Wannier Koopmans Method Calculations of 2D Material Band Gaps

A major drawback of the widely successful density functional theory is its underestimation of the material band gap. Various methods have been proposed to correct its band gap predictions. Wannier Koopmans method (WKM) is recently developed for this purpose to predict the band gap of extended 3D bulk systems. While the WKM has also been shown to be successful for isolated molecules, it is still a question whether it will work for 2D materials that are in between the 0D molecules and 3D bulk systems. We apply the WKM to 16 commonly known well studied 2D materials and find that the WKM predicted band gaps are on par with their GW calculated results.

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

We explore the contact properties of monolayer InSe transistors and obtain n-type Ohmic/p-type Schottky contacts.

Two dimensional allotropes of arsenene with a wide range of high and anisotropic carrier mobility

Considering the rapid development of experimental techniques for fabricating 2D materials in recent years, various monolayers are expected to be experimentally realized in the near future.