Layered molybdenum disulfide (MoS2) has attracted great attention owing to its unique properties. However, synthesizing large area thin film with high crystal quality and uniformity remains a challenge. The present study explores large scale MoS2 growth methods, i.e., two-step method of sputtering-chemical vapor deposition and direct sputtering method, and applies them to fabricate field effect transistors and supercapacitors, respectively. The thickness modulated MoS2 films by two-step method exhibited high field effect mobility [∼12.24 cm2/(V s)] and current on/off ratio (∼106). The direct sputtering of MoS2 demonstrated excellent electrochemical performance with a high capacitance (∼30 mF/cm2) and cyclic stability upto 5000 cycles. Our growth methods reported here for the large scale MoS2 with high uniformity can trigger the development of several important technologies in two-dimensional materials.

Mechanical properties are of fundamental importance in materials science and engineering, and have been playing a great role in various materials applications in the human history. Measurements of mechanical properties of 2-dimensional (2D) materials, however, are particularly challenging. Although various types of 2D materials have been intensively explored in recent years, the investigation of their mechanical properties lags much behind that of other properties, leading to lots of open questions and challenges in this research field. In this review, we first introduce the nanoindentation technique with atomic force microscopy to measure the elastic properties of graphene and 2D transition metal dichalcogenides. Then we review the effect of defects on mechanical properties of 2D materials, including studies on naturally defective chemical-vapor-deposited and intentionally defective 2D materials. Lastly, we introduce a nano-electromechanical device, resonators, built on the basis of the excellent mechanical properties of 2D materials.

2D materials play a special role in the race to make smaller and smaller devices. Their unique and strong in-plane bonding makes them impervious to diffusion into other layers and provides excellent thickness control. Their van der Waal's bonding with other monolayers or substrates allows for heterostructures unattainable by any other technique. This is reflected by the abundant popularity of research into graphene and other 2D materials. In this review article, we will describe the out-of-plane properties of graphene and functionalized graphene. We will use three specific examples to illustrate how these out-of-plane properties can be used in spintronic devices, in section “Graphene as a Tunnel Barrier” we will describe a magnetic tunnel junction (MTJ) based on graphene. Section “Graphene Based MTJs” will describe the spin injecting properties of a graphene tunnel barrier on silicon. Section “Graphene in Semiconductor Spintronic Devices” describes how you can use functionalized graphene to make a homoepitaxial graphene device. The second part of this article reviews monolayer transition-metal dichalcogenides (TMDs). First, we will show how TMDs are grown and specifically how we can grow large-area TMDs by chemical vapor deposition. Secondly, we will describe the optical properties of several TMDs and compare the results from several authors. Finally, we choose a chemical sensor as a specific example to show how TMDs can be used in a device.

Density functional theory computations were performed to investigate the adsorption and diffusion properties of lithium (Li) on tin disulfides nanosheets and its derived nanoribbons (NRs), in comparison with SnS2 bulk in 1T phase. The Li adsorption energies and migration barriers are comparable in SnS2 bulk and bilayer, and Li adsorbed at the octahedral sites has the highest binding energy in both SnS2 bulk and bilayer. Reducing the dimension of SnS2 to monolayer significantly lowers the Li diffusion barrier while keeping a considerable binding energy, and lithium favors the hollow sites which corresponding to the octahedral sites in bulk phase. Due to the edge effect, SnS2NRs gain an enhanced Li binding strength, increased Li mobility, and improved Li capacity. Thus, SnS2 NRs are a promising candidate for anode materials of Li-ion batteries with a high power density and fast charge/discharge rates.

The [(BiSe)1+δ]1(VSe2)1 heterostructure was characterized structurally and electrically to determine the effects of interlayer interaction on the charge density wave (CDW) found in VSe2 and compared to previously reported [(SnSe)1.15]1(VSe2)1. Out-of-plane x-ray diffraction scans contain reflections that can be indexed as 00l reflections of a BiSe–VSe2 supercell. Structure refinement indicates that the VSe2 layer is very similar structurally to that found in [(SnSe)1.15]1(VSe2)1. Scanning transmission electron microscopy images show a turbostratically disordered layer structure and the formation of anti-phase boundaries in the BiSe bilayer. The [(BiSe)1+δ]1(VSe2)1 heterostructure is metallic with a negative Hall coefficient, in contrast to the positive Hall coefficient found for [(SnSe)1.15]1(VSe2)1. The CDW found [(SnSe)1.15]1(VSe2)1 is not present in [(BiSe)1+δ]1(VSe2)1. This work illustrates the importance of inter constituent interactions in determining the transport properties of single layer films.

Here we report on the photocurrent response of two-dimensional (2D) heterostructures of sputtered MoS2 on boron nitride (BN) deposited on (001)-oriented Si substrates. The steady state photocurrent (I ph) measurements used a continuous laser of λ = 658 nm (E = 1.88 eV) over a broad range of laser intensities, P (∼1 μW < P < 10 μW), and indicate that I ph obtained from MoS2 layers with the 80 nm BN under layer was ∼4 times higher than that obtained from MoS2 layers with the 30 nm BN under layer. We also found super linear dependence of I ph on P (I ph ∝ P γ, with γ > 1) in both the samples. The responsivities obtained over the range of laser intensity studied were in the order of mA/W (∼12 and ∼2.7 mA/W with 80 nm BN and 30 nm BN under layers, respectively). These investigations provide crucial insight into the optical activity of MoS2 on BN, which could be useful for developing a variety of optoelectronic applications with MoS2 or other 2D transition metal dichalcogenide heterostructures.

Layered materials are an actively pursued area of research for realizing highly scaled technologies involving both traditional device structures as well as new physics. Lately, non-equilibrium growth of 2D materials using molecular beam epitaxy (MBE) is gathering traction in the scientific community and here we aim to highlight one of its strengths, growth of abrupt heterostructures, and superlattices (SLs). In this work we present several of the firsts: first growth of MoTe2 by MBE, MoSe2 on Bi2Se3 SLs, transition metal dichalcogenide (TMD) SLs, and lateral junction between a quintuple atomic layer of Bi2Te3 and a triple atomic layer of MoTe2. Reflected high electron energy diffraction oscillations presented during the growth of TMD SLs strengthen our claim that ultrathin heterostructures with monolayer layer control is within reach.

In this article, we have explored the interface states that form between the channel of a monolayer MoS2 transistor and a high-κ gate dielectric. These interface states lead to large hysteresis in the drain current versus gate voltage characteristic or the so-called transfer characteristic of the transistor. By applying carefully designed pulses to the gate of the transistor, we show that it is possible to both understand the nature of the interface states and minimize the hysteresis, so that the transfer characteristic can be reliably used for subsequent extraction of material parameters such as mobility.

Layered transition metal dichalcogenides which are part of the two dimensional materials family are experiencing rapidly growing interest owing to their diverse physical and optoelectronic properties. Large area controllable synthesis of these materials is required for transition from lab scale research to practical applications. In this work, we present a single step chemical vapor deposition process for large area monolayer growth of molybdenum selenide (MoSe2). We also demonstrate controllable thermal conversion from molybdenum selenide to molybdenum sulfide.

Van der Waals (vdW) heterojunctions consisting of vertically-stacked individual or multiple layers of two-dimensional layered semiconductors, especially the transition metal dichalcogenides (TMDs), show novel optoelectronic functionalities due to the sensitivity of their electronic and optical properties to strong quantum confinement and interfacial interactions. Here, monolayers of n-type MoSe2 and p-type Mo1−x W x Se2 are grown by vapor transport methods, then transferred and stamped to form artificial vdW heterostructures with strong interlayer coupling as proven in photoluminescence and low-frequency Raman spectroscopy measurements. Remarkably, the heterojunctions exhibit an unprecedented photoconductivity effect that persists at room temperature for several days. This persistent photoconductivity is shown to be tunable by applying a gate bias that equilibrates the charge distribution. These measurements indicate that such ultrathin vdW heterojunctions can function as rewritable optoelectronic switches or memory elements under time-dependent photo-illumination, an effect which appears promising for new monolayer TMDs-based optoelectronic devices applications.

Transition metal dichalcogenides such as WS2 show exciting promise in electronic and optoelectronic applications. Significant variations in the transport, Raman, and photoluminescence (PL) can be found in the literature, yet it is rarely addressed why this is. In this report, Raman and PL of monolayered WS2 produced via different methods are studied and distinct features that indicate the degree of crystallinity of the material are observed. While the intensity of the LA(M) Raman mode is found to be a useful indicator to assess the crystallinity, PL is drastically more sensitive to the quality of the material than Raman spectroscopy. We also show that even exfoliated crystals, which are usually regarded as the most pristine material, can contain large amounts of defects that would not be apparent without Raman and PL measurements. These findings can be applied to the understanding of other two-dimensional heterostructured systems.

Graphene-covered copper surfaces have been exposed to borazine, (BH)3(NH)3, with the resulting surfaces characterized by low-energy electron microscopy. Although the intent of the experiment was to form hexagonal boron nitride (h-BN) on top of the graphene, such layers were not obtained. Rather, in isolated surface areas, h-BN is found to form μm-size islands that substitute for the graphene. Additionally, over nearly the entire surface, the properties of the layer that was originally graphene is observed to change in a manner that is consistent with the formation of a mixed h-BN/graphene alloy, i.e., h-BNC alloy. Furthermore, following the deposition of the borazine, a small fraction of the surface is found to consist of bare copper, indicating etching of the overlying graphene. The inability to form h-BN layers on top of graphene is discussed in terms of the catalytic behavior of the underlying copper surface and the decomposition of the borazine on top of the graphene.

A key issue in two-dimensional structures composed of atom-thick sheets of electronic materials is the dependence of the properties of the combined system on the features of its parts. Here, we introduce a simple framework for the study of the electronic structure of layered assemblies based on perturbation theory. Within this framework, we calculate the band structure of commensurate and twisted bilayers of graphene (Gr) and hexagonal boron nitride (h-BN), and of a Gr/h-BN heterostructure, which we compare with reference full-scale density functional theory calculations. This study presents a general methodology for computationally efficient calculations of two-dimensional materials and also demonstrates that for relatively large twist in the graphene bilayer, the perturbation of electronic states near the Fermi level is negligible.

A scalable approach for synthesis of ultra-thin (<10 nm) transition metal dichalcogenides (TMD) films on stretchable polymeric materials is presented. Specifically, magnetron sputtering from pure TMD targets, such as MoS2 and WS2, was used for growth of amorphous precursor films at room temperature on polydimethylsiloxane substrates. Stacks of different TMD films were grown upon each other and integrated with optically transparent insulating layers such as boron nitride. These precursor films were subsequently laser annealed to form high quality, few-layer crystalline TMDs. This combination of sputtering and laser annealing is commercially scalable and lends itself well to patterning. Analysis by Raman spectroscopy, scanning probe, optical, and transmission electron microscopy, and x-ray photoelectron spectroscopy confirm our assertions and illustrate annealing mechanisms. Electrical properties of simple devices built on flexible substrates are correlated to annealing processes. This new approach is a significant step toward commercial-scale stretchable 2D heterostructured nanoelectronic devices.