Low voltage scanning transmission electron microscopy for two-dimensional materials

Aberration-corrected scanning transmission electron microscopy has been demonstrated as a versatile tool to probe the atomic defects and local electronic states in 2D materials. However, the residual aberration of the probe may give rise to considerable deviations from the perfect imaging, especially in polyphase transition metal dichalcogenides. Here, we clarify the misleading imaging artifact of opposite subdomains in monolayer MoS2 through both image simulations and experimental in-plane rotation series. This non-honeycomb imaging artifact turns out to be caused by both probe residual threefold astigmatism and local sample tilt or fluctuation. Our results suggest that the direct STEM imaging should be cautiously treated to achieve accurate phase assignment.

Towards controlled synthesis of 2D crystals by chemical vapor deposition (CVD)

This chapter describes the prospects of integration of Two-dimensional (2D) materials with photonic nanostructures to enable future active nanophotonic devices. It focuses on development of controlling light emission from transition metal dichalcogenides (TMD) by nanophotonic integration. The monolayer form demonstrates a range of novel optical effects, including strong excitonic effects, photo-luminescence , valley polarisation, single-photon emission and second-harmonic generation, which show great potential for fabricating novel “flat” active nanophotonic devices in the future. Integration with photonic nanostructures is a solution to such challenges, since photonic nanostructures such as plasmonic nanoparticles show great capability to manipulate light–matter interaction at the nanoscale. A greater understanding of the coupling between photonic nanostructures and 2D materials is required, thus potentially better controlling the properties of these materials. For example, non-linear coupling between 2D materials, especially second-order non-linearity, has attracted intense interest. Dielectric nanostructures, such as photonic crystal cavities, have also been proposed for enhancing the emission from TMD monolayers.

Over the last decade, two-dimensional (2D) materials have drawn much attention for the next-generation electron devices, optoelectronics and bioelectronics [1, 2]. Recently, non-volatile resistance switching (NVRS) behavior has been observed in various 2D materials, including graphene oxide, functionalized MoS2 and partially degraded black phosphorus, etc. [3-5], where the resistance can be modulated between a high-resistance state (HRS) and a low-resistance state (LRS) [6]. In addition, there are several reports about NVRS in multi-layer hexagonal boron-nitride (h-BN) [7]. Although 2D materials are widely regarded as promising candidates to overcome vertical scaling obstacle in NVRS based on conventional metal-insulator-metal (MIM) structure [3], few researchers believe that single-layer 2D material can produce NVRS [8] due to excessive leakage current. Sangwan et al. discovered that grain boundaries in single-layer MoS2 can produce NVRS based on planar (horizontal) structure [9]. However, the planar structure without 3D stacking ability has the limitation of low integration density. Recently, we reported the discovery that NVRS phenomenon is accessible in a variety of single-layer transition metal dichalcogenides (TMDs) in vertical MIM structure [10, 11]. Herein, we report another 2D insulating material, single-layer h-BN, also shows stable and desirable NVRS behavior. Compared with monolayer TMDs with three atomic layers, monolayer h-BN has only one atomic layer and ~0.33 nm in thickness, which is the thinnest active layer in non-volatile resistance memory. These devices can be labelled as “atomristor”, which means the memristor effect in atomically thin nanomaterials.[12] The h-BN atomristors have been studied using a crossbar structure (Au/h-BN/Au) [See Figure a]. Figure b shows a representative I-V curve of h-BN atomristor, demonstrating forming-free switching with large on/off ratio (up to >106) and low switching voltage (down to <1V). In addition, pulse operation proves its fast switching speed (<15 ns), which is comparable to the state-of-the-art in 2D memory. Ab-initio simulation reveals that the metal ion substitution in sulfur vacancies may be related to the switching from HRS to LRS, and area scaling test result is also consistent with this conductive-bridge-like mechanism. The results of this work indicate a potential universal resistive switching behavior in 2D insulating monolayers, which is applicable to memory technology, neuromorphic computing, RF switch and flexible electronics.

The isolation of graphene has sparked an unprecedented world wide research activity on the topic of two-dimensional (2D) materials. However, even though graphene has a high potential for various applications, its metallic character is a significant limitation for a large variety of electron devices, starting with the conventional field effect transistor. This limitation could be overcome in the near future by the use of layered van der Waals crystals such as black phosphorus (i.e., phosphorene, which is one to few layers of black phosphorus) and also by other semiconducting 2D materials such as transition metal dichalcogenides (TMDCs) of general formula MX2, where M is the metal and X the chalcogen (S, Se and Te). For instance, phosphorene exhibits a direct band gap varying between ~ 0.3 eV for bulk crystals to ~ 2 eV for a monolayer. In layered TMDC structures, each layer typically comprises 3 atomic planes (corresponding to a thickness below 8 Å), and consists of an hexagonally-organized atomic layer of metal atoms sandwiched between two planes of chalcogen atoms, also in hexagonal configurations. M–X bonds inside each layer are covalent; however, the sandwiching chalcogen layers are linked by weak van der Waals bonds, so that a bulk crystal is easy to cleave along the chalcogen planes. As with phosphorene, most semiconducting TMDC materials also see their band gap increasing as thickness decreases and they switch from indirect to direct band gap for a monolayer. To date, the most studied semiconductor TMDCs are MoS2 and WS2. Because they are atomically thin, 2D materials are particularly well suited for flexible electronics applications. After a brief description of the major features of phosphorene and TMDCs (crystal structures, relevant electronic properties as a function of thickness …), the talk will highlight recent results concerning field effect transistor characteristics. In particular, carrier mobility values as high as 5000 cm2/Vs have been measured in phosphorene films. Since local substitutional-type doping cannot be used in 2D materials, there is a general contacting problem (Schottky barrier formation), which is of paramount importance for device applications. In particular, two types of contacts can be formed: the top contact and the edge contact, which behave differently. Generally speaking, the semiconductor thickness is much smaller than the depletion length at the contact, which is an intriguing and novel situation. In the second part of the talk, various synthesis methods will be presented and discussed, particularly in view of their possible use in the field of large area electronics. For most devices fabricated so far, the 2D layers were obtained by exfoliation of bulk crystals. However, recent progress in synthesis include molecular beam epitaxy, controlled chemical vapour deposition and vapour phase transport. Those various approaches will be discussed, as well as the newly-introduced atomic layer deposition (ALD) process. As most 2D materials are grown on a separate substrate, the transfer problem will be highlighted, particularly concerning phosphorene which is highly hygroscopic, and sensitive to the presence of light. The presentation will be concluded by some perspectives concerning the industrialization of 2D materials.

Nanoscale variations of optical properties in transition metal dichalcogenide (TMD) monolayers can be explored with cathodoluminescence (CL) and electron energy loss spectroscopy (EELS) using electron microscopes. To increase the CL emission intensity from TMD monolayers, the MoSe2 flakes are encapsulated in hexagonal boron nitride (hBN), creating van der Waals (VdW) heterostructures. Until now, the studies have been exclusively focused on scanning transmission electron microscopy (STEM-CL) or scanning electron microscopy (SEM-CL), separately. Here, we present results, using both techniques on the same sample, thereby exploring a large acceleration voltage range. We correlate the CL measurements with STEM-EELS measurements acquired with different energy dispersions, to access both the low-loss region at ultra-high spectral resolution, and the core-loss region. This provides information about the weight of the various absorption phenomena including the direct TMD absorption, the hBN interband transitions, the hBN bulk plasmon, and the core losses of the atoms present in the heterostructure. The S(T)EM-CL measurements from the TMD monolayer only show emission from the A exciton. Combining the STEM-EELS and S(T)EM-CL measurements, we can reconstruct different decay pathways leading to the A exciton CL emission. The comparison with SEM-CL shows that this is also a good technique for TMD heterostructure characterization, where the reduced demands on sample preparation are appealing. To demonstrate the capabilities of SEM-CL imaging, we also measured on a SiO2/Si substrate, quintessential in the sample preparation of two-dimensional materials, which is electron-opaque and can only be measured in SEM-CL. The CL-emitting defects of SiO2 make this substrate challenging to use, but we demonstrate that this background can be suppressed by using lower electron energy.

Two dimensional (2D) materials have become a growing subject in the last 15 years mainly due to the isolation of graphene, which created a completely different class of material based on its unique, monolayer design. Since then, various stable materials of few atoms thick are showing emerging capabilities in optical electronics and photonics. Semiconducting monolayers of transition metal dichalcogenides (TMDs) such as MoS2, Mo1-xWxS2, and WS2 exhibit direct electronic band gaps; bulk crystals display indirect band gaps. Interestingly, these 2D materials show significant light interaction over a broad bandwidth ranging from infrared to ultraviolet wavelengths. The materials allow photodetection in this bandwidth without the need of cooling, thus creating new potential for uncooled detection. In this review, we discuss various 2D materials and their interaction with light for photodetection applications.

Stretchable photodetectors based on 2D materials: materials synthesis, fabrications and applications

The layered two-dimensional (2D) materials, e.g., the well-known graphene, transition metal dichalcogenides, and topological insulators, have attracted great interest of researchers from fundamental research as well as industries owing to their intriguing properties in a number of aspects. In practical applications, the efficient modification of the features of 2D materials is compelling and essential to achieve desired functionalities. Ion implantation has been successfully applied to synthesize graphene. The most exotic advantage of ion beam technology is that it offers distinct options of energetic beams, which has recently shown the unique capability to modify and tailor the properties of versatile 2D materials. To name a few, the energetic ion beams could implement the surface morphology or layer-to-layer structural engineering of 2D materials. At the microcosmic level, the introduction of ion beam induced defects and intentional doping of specific ions are the basis of tailoring properties of 2D materials. By manipulating the parameters of ion beams (energies, species, fluences, incident angles, etc.), the modified 2D materials may possess novel properties, which are unprecedented in pristine ones. Promising applications based on these 2D materials with ion beam tailored features have been realized in a broad range of fields. In this work, we systematically review the latest research progress on the deployed techniques and property modifications of graphene and other 2D materials under the treatment of various ion beams. A few selected applications are presented to indicate the practical potentials of ion beam modified 2D materials in distinct areas. Perspectives on future developments are also provided by focusing on several promising topics.

The fascinating two-dimensional (2D) materials are being unconsciously applied in various fields from science to engineering, which is benefited from the glamorous physical and chemical properties of mechanics, optics, electronics, and magnetism. The representative 2D thermoelectric/piezoelectric materials can directly convert thermal/mechanical energy into electrical energy, which can resolve the energy issues and avoid further environmental deterioration. The thermoelectric or piezoelectric properties of various 2D materials, such as graphene, hexagonal boron nitride, black phosphorus, transition metal dichalcogenides (TMDs), arsenene, metal carbides and nitrides (MXenes), and so on, have been investigated in detail. Although tremendous progress has been achieved in the past few years, these properties still need to be improved for their practical application by designing new 2D materials, strain engineering, chemical functionalization, etc. In addition to this, in 2D materials, there are many other novel physical properties, such as magnetism, topology, valley, and so on. The combination of thermoelectricity/piezoelectricity with other unique properties may lead to novel device applications or scientific breakthroughs in new physics. Overall, the emergence of 2D thermoelectric and piezoelectric materials has expanded energy conversion research dramatically. By combing this new device concept with the novel 2D materials, original devices should have potential applications in energy harvesting.

Significant progress has been made in growing large-area transition metal dichalcogenide (TMD) monolayers, but most rely on stitching small flakes, which can affect electronic and optical performance. High-quality single-crystal monolayers are needed for device applications. Here, we report a wafer-bonder-assisted transfer (WBAT) method to produce uniform, crack-free, single-crystal TMD monolayers over large areas. This technique integrates gold-tape exfoliation with wafer-scale bonding, enabling flake sizes over 106 times larger than those from traditional Scotch tape exfoliation. Compared to hand-press transfer, WBAT yields higher-quality monolayers with fewer cracks, reduced strain, and over 2-fold improvement in photoluminescence uniformity, as confirmed by Raman and PL mapping. Fabricated field-effect transistor arrays show high mobility and on-off ratio. The WBAT method offers high yield, reproducibility, and compatibility with various 2D materials, heterostructures, and substrates, aligning with standard semiconductor processes. It provides a scalable solution for integrating high-quality TMD monolayers into next-generation electronic and optoelectronic devices.

Transition metal dichalcogenide (TMDC) monolayers exhibit great photocatalytic performances and tunabilities. By using first-principles simulations based on density functional theory, we demonstrate here that external electric bias can be employed to further improve the photocatalytic hydrogen production efficiencies of the six AB2 (A = Mo, W and B = S, Se, Te) TMDC monolayers since these are influenced by their piezoelectric response. In particular, when subjected to a proper amount of electrically induced tensile biaxial strain, most TMDC monolayers turn into potentially ideal photocatalysts toward the hydrogen evolution reaction (HER). The beneficial effects of introducing tensile biaxial strain on the TMDC monolayers are not limited to the reduction of the band gap and proper adjustment of the band edge positions; the concomitant hydrogen adsorption free energies are also modified in such a way that the HER reaction is noticeably favored.

Two-dimensional (2D) materials are crystals with one to a few layers of atoms and are being used in many fields such as optical modulator, photodetector, optical switch, and ultrafast lasers. Their exceptional optoelectronic and nonlinear optical properties make them as a suitable saturable absorber for laser cavities. This review focuses on the recent progress in ultrafast laser use 2D materials as a saturable absorber. 2D materials traditionally include graphene, topological insulators, transition metal dichalcogenides, as well as new materials such as black phosphorus, bismuthene, antimonene, and MXene. Material characteristics, fabrication techniques, and nonlinear properties are also introduced. Finally, future perspectives of ultrafast lasers based on 2D materials are also addressed.

A systematic review on 2D materials for volatile organic compound sensing

Environmental resilience with 2D materials: A futuristic perspective