Elemental 2D Materials Research Articles

Mixed-dimensional Te/CdS van der Waals heterojunction for self-powered broadband photodetector

The 2D layered crystals can physically integrate with other non-2D components through van der Waals (vdW) interaction, forming mixed-dimensional heterostructures. As a new elemental 2D material, tellurium (Te) has attracted intense recent interest for high room-temperature mobility, excellent air-stability, and the easiness of scalable synthesis. To date, the Te is still in its research infancy, and optoelectronics with low-power consumption are less reported. Motivated by this, we report the fabrication of a mixed-dimensional vdW photodiode using 2D Te and 1D CdS nanobelt in this study. The heterojunction exhibits excellent self-powered photosensing performance and a broad response spectrum up to short-wave infrared. Under 520 nm wavelength, a high responsivity of 98 mA W−1 is obtained at zero bias with an external quantum efficiency of 23%. Accordingly, the photo-to-dark current ratio and specific detectivity reach 9.2 × 103 and 1.9 × 1011 Jones due to the suppressed dark current. This study demonstrates the promising applications of Te/CdS vdW heterostructure in high-performance photodetectors. Besides, such a mixed-dimensional integration strategy paves a new way for device design, thus expanding the research scope for 2D Te-based optoelectronics.

(Invited) Two-Dimensional Pnictogens: Van Der Waals Growth, Stability, and Phase Transformation

Pnictogens (P, As, Sb, Bi) emerged as a new class of elemental 2D materials displaying a wide range of electronic and topological properties. With their ns2 np3 valence electronic configuration and sp3 hybridization, group VA elements are the only elemental group which forms vdW and quasi-vdW structures throughout the entire group, thus providing a rich allotropic system. Light group VA elements crystallize in the semiconducting orthorhombic layered phase (A17) and heavier group VA elements prefer the semi-metallic rhombohedral layered phase (A7). At nanoscale thicknesses, group VA materials undergo various electronic phase transitions. For instance, A17 P – black phosphorus – is a high mobility 2D semiconductor with a thickness-dependent direct band gap gradually varying between 0.3-2 eV as its thickness decreases from bulk to single layer. Similarly, A7 Sb is predicted to exhibit topological semimetal, topological insulator, quantum spin Hall and indirect band gap semiconductor phases at thicknesses between 1 and 22 layers.Despite their highly attractive scientific and technological potential, 2D materials remain scarcely implemented in scalable devices and technologies due to several challenges including among others the lack of large-area and uniform synthesis methods. In fact, many 2D materials exist only in the realm of theoretical predictions and most 2D materials can only be produced by mechanical or chemical exfoliation from bulk crystals, yielding 2D flakes with limited dimensions and poor thickness control. Developing epitaxial growth methods is therefore a crucial step to enable the integration of 2D materials in emerging technologies. With this perspective, this presentation will address the development of the molecular beam epitaxy (MBE) growth of group VA 2D materials on semiconducting and vdW substrates. Furthermore, thermal, ambient and phase stability of these emerging 2D materials is studied using in situ low-energy electron microscopy, transmission electron microscopy, x-ray photoelectron emission microscopy, and ab initio calculations. MBE growth of 2D-Sb and 2D-AsSb is demonstrated on Ge(111) and graphene. Real-time LEEM measurements of the growth dynamics combined with in situ STM allow to determine the nucleation and growth mechanisms on semiconductor and vdW substrates. Subtle behavior during growth or thermal annealing was revealed based on these in situ studies including the metastability and layered-to-layered phase transformation mechanisms of A17 2D-Sb, which was previously considered as an unstable Sb phase. These results provide a deeper understanding of vdW growth of group VA 2D materials and layered-to-layered phase transformation mechanisms.

Recent trends and emerging challenges in two‐dimensional materials for energy harvesting and storage applications

AbstractBy itself, the physics of two‐dimensional (2D) materials are often fascinating. All the atoms of these elemental 2D materials are exposed to the surface, making it relatively easier to tune the structure and physical properties of the materials using surface treatments. The discovery of graphene has sparked a flurry of research into graphene‐related 2D materials such as silicene, germanene (a hexagonal network of silicon and germanium), stanene, borophene, and a zoo of other 2D materials. In this article, the discussion is on the 2D materials, the synthesis methods of the 2D materials, and the energy applications of the 2D materials.

Identification of DNA bases using nanopores created in finite-size nanoribbons from graphene, phosphorene, and silicene

Graphene’s success for nanopore deoxyribonucleic acid (DNA) sequencing has shown that it is possible to explore other potential single-atom and few-atom thick layers of elemental 2D materials beyond graphene (e.g., phosphorene and silicene) and also that these materials can exhibit fascinating and technologically useful properties for DNA base detection that are superior to those of graphene. Using density functional theory (DFT), we study the interaction of DNA bases with nanopores created in finite-size nanoribbons from graphene, phosphorene, and silicene. Due to the small size of DNA bases, the bases interact with only a small section of the nanoribbon; hence, using a finite-size model is appropriate for capturing the interaction of bases and 2D membrane materials. Furthermore, by using a finite-size model, our system is approximated as a molecular system, which does not require a periodic DFT calculation. We observe that binding energies of DNA bases using nanopores from phosphorene and silicene are similar and generally smaller compared to those from graphene. This shows that minimal sticking of DNA bases to the pore is expected for phosphorene and silicene devices. Furthermore, nanopores from phosphorene and silicene show a characteristic change in the density of states for each base. The bandgaps of phosphorene and silicene are significantly altered due to interaction with DNA bases compared to those of graphene. Our findings show that phosphorene and silicene are promising alternatives to graphene for DNA base detection using advanced detection principles such as transverse tunneling current measurement.

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials.

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.

Structural, Topological, and Superconducting Properties of Two‐Dimensional Tellurium Allotropes from Ab Initio Predictions

AbstractThe discovery of tellurene has effectively extended the realm of two‐dimensional (2D) elemental materials to group‐VI, which demonstrates promising potential in next‐generation electronic and optoelectronic device applications. Beyond the three prevailing structural phases of tellurene, it is further predicted 31 2D tellurium allotropes combining the particle‐swarm optimization (PSO) searches with density functional theory (DFT) calculations. The corresponding dynamic and thermal stability are examined by both phonon spectrum calculations and ab initio molecular dynamics (AIMD) simulations. Those 2D tellurium allotropes can be well categorized in terms of their intrinsic number of atomic layers, and cohesive energy. It is found that the allotropes with tri‐atomic layers are energetically most favorable, and mainly demonstrate semiconducting feature at larger cohesive energies. Four of the bi‐atomic‐layer allotropes with larger cohesive energies exhibit topological insulating feature, and two with slightly smaller cohesive energies are intrinsic superconductors with critical temperature Tc (≈8 K) competing with their bulk counterpart at high pressure. Interestingly, both topological and superconducting properties can co‐exist in one of those bi‐atomic‐layer allotropes, which is unique in the elemental 2D materials ever reported. The present study enriches the 2D tellurium allotropes and provides a viable platform for exploring novel functionalities for future device applications.

Emerging beyond-graphene elemental 2D materials for energy and catalysis applications.

Elemental two-dimensional (2D) materials have emerged as promising candidates for energy and catalysis applications due to their unique physical, chemical, and electronic properties. These materials are advantageous in offering massive surface-to-volume ratios, favorable transport properties, intriguing physicochemical properties, and confinement effects resulting from the 2D ultrathin structure. In this review, we focus on the recent advances in emerging energy and catalysis applications based on beyond-graphene elemental 2D materials. First, we briefly introduce the general classification, structure, and properties of elemental 2D materials and the new advances in material preparation. We then discuss various applications in energy harvesting and storage, including solar cells, piezoelectric and triboelectric nanogenerators, thermoelectric devices, batteries, and supercapacitors. We further discuss the explorations of beyond-graphene elemental 2D materials for electrocatalysis, photocatalysis, and heterogeneous catalysis. Finally, the challenges and perspectives for the future development of elemental 2D materials in energy and catalysis are discussed.

Recent progress, challenges, and prospects in emerging group-VIA Xenes: synthesis, properties and novel applications.

The discovery of graphene (G) attracted considerable attention to the study of other novel two-dimensional materials (2DMs), which is identified as modern day "alchemy" since researchers are converting the majority of promising periodic table elements into 2DMs. Among the family of 2DMs, the newly invented monoelemental, atomically thin 2DMs of groups IIIA-VIA, called "Xenes" (where, X = IIIA-VIA group elements, and "ene" is the Latin word for nanosheets (NSs)), are a very active area of research for the fabrication of future nanodevices with high speed, low cost and elevated efficiency. Currently, any novel structure of 2DMs from the typical Xenes will probably be applicable in electronic technology. Analysis of their possible highly sensitive synthesis and characterization present opportunities for theoretically examining proposed 2D-Xenes with atomic precision in ideal circumstances, thus providing theoretical predictions for experimental support. Several theoretically predicted and experimentally synthesized 2D-Xene materials have been investigated for the group-VIA elements (tellurene (2D-Te), and selenene (2D-Se)), which are similar to topological insulators (TIs), thus potentially rendering them suitable materials for application in upcoming nanodevices. Although the investigation and device application of these materials are still in their infancy, theoretical studies and a few experiment-based investigations have proven that they are complementary to conventional (i.e., layered bulk-derived) 2DMs. This review focuses on the synthesis of novel group-VIA Xenes (2D-Te and 2D-Se) and summarizes the current development in understanding their basic properties, with the current advancement in signifying device applications. Lastly, the future research prospects, further advanced applications and associated shortcomings of the group-VIA Xenes are summarized and highlighted.

Elemental 2D Materials: Progress and Perspectives Toward Unconventional Structures

Elemental 2D materials have attracted a great deal of interest due to their excellent properties for various applications in catalysis, sensing, photonics, intracellular gene regulation, magnetism, superconductivity, and so on. It is broadly accepted that the physicochemical properties are highly associated with their atomic arrangements and coordination. Recent studies have demonstrated that unconventional structures of elemental 2D materials exhibit unexpected physicochemical properties and innovative applications beyond conventional structures. Those unconventional structures bring new life to elemental 2D materials and deserve special attention. However, there are considerable challenges to controllably prepare them due to their metastable characters. Herein, their synthetic strategies are focused on to shed light on the reasonable design of new structures with special atomic arrangements to open up tremendous opportunities for expanding their functionalities and potential applications. In addition, the correlations among the types of unconventional structures, the element types, and corresponding synthetic strategies are uncovered. Finally, based on the achievements in the representative studies, some prospects and potential opportunities for extending the scope of elemental 2D materials with unconventional structures and potentially distinctive applications are provided.

Monocrystalline Antimonene Nanosheets via Physical Vapor Deposition

AbstractAmong the family of elemental 2D materials, antimonene is predicted to have a desirable combination of bandgap tunability and exceptional physical properties. However, there is a lack of a facile synthesis technique to prepare high‐quality antimonene with large aspect ratios on standard SiO2 substrates, hindering wide scale exploration of this material. Here, a physical vapor deposition process to controllably achieve millimeter‐scale, β‐phase, monocrystalline antimonene nanosheets on a SiO2 dielectric substrate is reported. The temperature gradient across the deposition tube is exploited to realize either large‐area nanosheets or single antimonene crystals on‐demand. The composition and quality of the nanosheets and crystals is assessed using spectroscopy, diffraction, and microscopy techniques which suggest the formation of the β‐phase allotrope. The band structure of as‐synthesized nanosheets which matches the theoretically calculated values is experimentally extracted. Finally, contrary to earlier reports, the oxidation of antimonene under ambient conditions over a period of time indicating the need for further experimental studies of the material's stability is revealed. The reported controllable growth of antimonene nanosheets and single crystals on conventional dielectric substrates opens a spectrum of possible applications of this material in electronics and optoelectronics devices.

Mid‐Infrared Photodetector Based on Selenium‐Doped Black Phosphorus

Black phosphorus (BP), a new elemental 2D material, is a suitable candidate for broadband photodetection from the visible to the infrared (IR) range of the spectrum because of its tunable bandgap and high carrier mobility. Herein, the electrical and optical properties of pristine and Se‐doped BP have been investigated, using the density functional theory (DFT) method. The results show that the Se‐doped BP has an indirect bandgap of 0.4 eV which is smaller than that of the pristine monolayer BP. Moreover, the Se‐doped BP exhibits higher optical absorption compared with pristine BP in the low energy regions. Furthermore, the simulation results of designed (12 nm thick) field‐effect transistors, based on Se doped BP, show excellent optoelectronic properties in the mid‐IR photodetector applications. The responsivity of this device reaches up to about in a mid‐IR region of 4 μm with an excitation intensity of 0.0001 W cm−2 and bias voltage and drain voltage of 1 and 0.5 V, respectively. Finally, the results demonstrate that doping BP with Se improves device performance by increasing optical properties and enhances responsivity of the mid‐IR photodetector.

Realizing Few-Layer Iodinene for High-Rate Sodium-Ion Batteries.

Elemental 2D materials with fascinating characteristics are regarded as an influential portion of the 2D family. Iodine is as a typical monoelemental molecular crystal and exhibits great prospects of applications. To realize 2D iodine, not only is it required to separate the weak interlayer van der Waals interactions, but also to reserve the weak intramolecular halogen bonds; thus, 2D iodine is still unexploited until now. Herein, atomically thin iodine nanosheets (termed "iodinene") with the thickness around 1.0nm and lateral sizes up to hundreds of nanometers are successfully fabricated by a liquid-phase exfoliation strategy. When used for the cathode of rechargeable sodium-ion batteries, the ultrathin iodinene exhibits superb rate properties with a high specific capacity of 109.5mA h g-1 at the high rate of 10 A g-1 owing to its unique 2D ultrathin architecture with remarkably enhanced pseudocapacitive behavior. First-principles calculations reveal that the diffusion of sodium ions in few-layered iodinene changes from the original horizontal direction in bulk to the vertical with a small energy barrier of 0.07eV because of the size effect. The successful preparation and intensive structural investigation of iodinene paves the way for the development of novel iodine-based science and technologies.

Emerging Group-VI Elemental 2D Materials: Preparations, Properties, and Device Applications.

Due to the ultrathin thickness and dangling-bond-free surface, 2D materials have been regarded as promising candidates for future nanoelectronics. In recent years, group-VI elemental 2D materials have been rediscovered and found superior in electrical properties (e.g., high carrier mobility, high photoconductivity, and thermoelectric response). The outstanding semiconducting properties of group-VI elemental 2D materials enable device applications including high-performance field-effect transistors and optoelectronic devices. The excellent environmental stability also facilitates fundamental studies and practical applications of group-VI elemental 2D materials. This Review first focuses on the crystal structures of group-VI elemental 2D materials. Afterward, preparation methods for nanostructures of group-VI materials are introduced with comprehensive studies. A brief Review of the electronic structures is then presented with an understanding of the electrical properties. This Review also contains the device applications of group-VI elemental 2D materials, emphasizing transistors, photodetectors, and other appealing applications. Finally, this Review provides an outlook for the development of group-VI elemental 2D materials, highlighting the challenges and opportunities in fundamental studies and technological applications.

Tellurene: An elemental 2D monolayer material beyond its bulk phases without van der Waals layered structures

Due to the quantum confinement effect, atomically thin two-dimensional (2D) monolayer materials possess distinct characteristics from their corresponding bulk materials, which have received wide attention from science and industry. Among all the 2D materials, elemental 2D materials with the simplest components are most striking. As an emerging group-VIA elemental 2D monolayer material, tellurene exhibits many exciting fundamental properties, such as chemical and mechanical stabilities, bandgap and high carrier mobilities compared to phosphorene, graphene and MoS2, respectively. Besides, in further exploration, it was found that tellurene or tellurene-based device presents excellent thermoelectric properties, piezoelectric properties, quantum Hall effects, and superb optical properties especially nonlinear optics characteristics, etc. The properties of tellurene can be modulated by virtue of strain, defects, edges, and heterojunction effects. In view of so many unique properties, it has drawn significant interest since tellurene was predicted and fabricated successfully in 2017. In this paper, we review the 2D tellurene allotropes, experimental preparation, excellent properties, performance modulation and future development.

High‐Performance Phosphorene‐Based Transistors Using a Novel Exfoliation‐Free Direct Crystallization on Silicon Substrates

AbstractPhosphorene is one of the most promising elemental 2D materials. So far, mechanical exfoliation is the main route toward the formation of phosphorene sheets. Although field‐effect transistors are realized on phosphorene, their arrangement and size are dictated by the randomness of the exfoliation and transfer methods. Here, the evolution of highly crystalline and large‐area phosphorene sheets with a laser‐assisted phase transition from red phosphorus directly on silicon substrates in desired locations is reported. The original red phosphorus can be patterned prior to laser‐assisted crystallization to achieve the desired shape, location, and layer thickness (either mono or few). A 1064 nm laser is employed as the source to impart energy onto the red phosphorus layer and to achieve allotrope transformation. A combination of argon–oxygen (Ar:O2) and helium–hydrogen (He:H2) gases is exploited in a sequential manner to lead to phosphorene sheets. Various techniques are used to examine the physical properties of phosphorene sheets. Field‐effect transistors are made at different phases of crystallization and a high field‐effect mobility of 1450 cm2 V−1 s−1 and on/off ratio of 103 are achieved. Moreover, the optoelectrical characteristics of the phosphorene‐based phototransistors under different illumination powers and back‐gate voltages are investigated, showing superior performance.

Stable halogen 2D materials: the case of iodine and astatine

Two-dimensional (2D) materials have applications towards electronic devices, energy storages, and catalysis, et al. So far, most of elemental 2D materials are composed based on groups IIIA, IVA or VA. To expand the 2D material family, the orbital hybridization becomes a key factor to determine stability. Here we predict that sp2d3 hybridization of the outmost electrons in iodine and astatine can build up 2D triangle lattices, delta-iodiene and delta-astatiene, using first-principles calculations. Each atom is connected by σ bonds with nearest 6 atoms and the π bonds are thus introduced. The band gaps can approach zero because of interaction of unpaired single electron between each atom, if the identical bond length is reduced. By inducing compression strain, the Dirac points or topological nontrivial points can be created in the delta-iodiene and delta-astatiene. Our discovery paves a new way to construction of 2D materials.

Synthesis of Monolayer Blue Phosphorus Enabled by Silicon Intercalation.

The growth of entirely synthetic two-dimensional (2D) materials could further expand the library of naturally occurring layered solids and provide opportunities to design materials with finely tunable properties. Among them, the synthesis of elemental 2D materials is of particular interest as they represent the chemically simplest case and serve as a model system for exploring the on-surface synthesis mechanism. Here, a pure atomically thin blue phosphorus (BlueP) monolayer is synthesized via silicon intercalation of the BlueP-Au alloy on Au(111). The intercalation process is characterized at the atomic scale by low-temperature scanning probe microscopy and further corroborated by synchrotron radiation-based X-ray photoelectron spectroscopy measurements. The evolution of the band structures from the BlueP-Au alloy into Si-intercalated BlueP are clearly revealed by angle-resolved photoemission spectroscopy and further verified by density functional theory calculations.

Two-dimensional graphene-like Xenes as potential topological materials

Since the discovery of quantum spin Hall (QSH) effect in graphene, searching for two-dimensional (2D) QSH materials with larger bulk gap has been an active field in the past decade. As cousins of graphene, the elemental graphene-like 2D materials (Xenes, X refers to group-IV, group-V, or group-VI elements) have been particularly interested in search of the QSH state as they share a similar honeycomb lattice with graphene. For those Xenes with heavier elements, theory predicted an extraordinarily large bulk gap suitable for room-temperature applications that have been experimentally verified in recent epitaxially grown bismuthene and stanene samples. We briefly review the progress in theoretical and experimental investigations of 2D Xenes as possible QSH topological materials.

Design, characterization, and application of elemental 2D materials for electrochemical energy storage, sensing, and catalysis

Elemental 2D materials have emerged as promising candidates for electrochemical applications that require miniaturized devices and superior performance.

Staggering transport of edge states and symmetry analysis of electronic and optical properties of stanene.

As one of the most intriguing elemental 2D materials beyond graphene, stanene is a unique material possessing strong quantum spin Hall effect and is promising for spintronics applications. Since most of these exotic phenomena are associated with the edge states of stanene and their responses under external stimuli, here, we first investigate the electronic and transport behavior of the edge states of stanene. Through examining the acoustic phonon-limited scattering of transporting carriers, we reveal a staggering behavior in the effective mass and mobility varying with the width of stanene nanoribbons. Remarkably, an opposite oscillating trend of the quantum confinement effect with respect to the electrons and holes is found and this trend is in sharp contrast to graphene. Moreover, through group-theory analysis, we further analyze the symmetry-permitted light absorptions and predict a much smaller band gap at Γ compared with other IV-group 2D materials like graphene and silicene, allowing for a red-shift of optical π-π* absorption in stanene. The presence of the narrow flat bands along the M-K path in stanene suggests appreciable density of states of low-energy carriers and a strong light-matter interaction for low-energy photons, which are beneficial for its applications in low-frequency optoelectronics.