Jiuxiang Dai

An all-van-der-Waals (vdW) barrier-free metal–semiconductor contact is realized by using vdW semimetals to replace conventional metals, which can avoid the pinning effect of the Fermi levels and surface dangling bonds of conventional metal electrodes. Such a strategy can effectively reduce the contact resistance and enable record-setting performance metrics of 2D semiconductor transistors.

Abstract

Ultrathin 2D semiconductor devices are considered to have beyond-silicon potential but are severely troubled by the high Schottky barriers of the metal–semiconductor contacts, especially for p-type semiconductors. Due to the severe Fermi-level pinning effect and the lack of conventional semimetals with high work functions, their Schottky hole barriers are hardly removed. Here, an all-van-der-Waals barrier-free hole contact between p-type tellurene semiconductor and layered 1T′-WS₂ semimetal is reported, which achieves a zero Schottky barrier height of 3 ± 9 meV and a high field-effect mobility of ≈1304 cm² V^–1 s^–1. The formation of such contacts can be attributed to the higher work function of ≈4.95 eV of the 1T′-WS₂ semimetal, which is in sharp contrast with low work function (4.1–4.7 eV) of conventional semimetals. The study defines an available strategy for eliminating the Schottky barrier of metal–semiconductor contacts, facilitating 2D-semiconductor-based electronics and optoelectronics to extend Moore's law.

Recent progress on MXene-based PVs is discussed. Synthesis procedures and properties of MXene are presented. The role of MXene as an additive, interfacial layer, and as a conductive electrode in PVs is comprehensively studied. Later, the influence of MXene on the stability of PVs is described. Finally, guidelines to overcome the challenges of MXene based materials for PVs are provided.

Abstract

The 2D transition metal carbides/nitrides (2D MXenes) are a versatile class of 2D materials for photovoltaic (PV) systems. The numerous advantages of MXenes, including their excellent metallic conductivity, high optical transmittance, solution processability, tunable work-function, and hydrophilicity, make them suitable for deployment in PV technology. This comprehensive review focuses on the synthesis methodologies and properties of MXenes and MXene-based materials for PV systems. Titanium carbide MXene (Ti₃C₂T _x ), a well-known member of the MXene family, has been studied in many PV applications. Herein, the effectiveness of Ti₃C₂T _x as an additive in different types of PV cells, and the synergetic impact of Ti₃C₂T _x as an interfacial material on the photovoltaic performance of PV cells, are systematically examined. Subsequently, the utilization of Ti₃C₂T _x as a transparent conductive electrode, and its influence on the stability of the PV cells, are discussed. This review also considers problems that emerged from previous studies, and provides guidelines for the further exploration of Ti₃C₂T _x and other members of the 2D MXene family in PV technology. This timely study is expected to provide comprehensive understanding of the current status of MXenes, and to set the direction for the future development in 2D material design and processing for PVs.

Abstract

The synthesis of high-quality ultrathin overlayers is critically dependent on the surface structure of substrates, especially involving the overlayer-substrate interaction. By using in situ surface measurements, we demonstrate that the overlayer-substrate interaction can be tuned by doping near-surface Ar nanobubbles. The interfacial coupling strength significantly decreases with near-surface Ar nanobubbles, accompanying by an “anisotropic to isotropic” growth transformation. On the substrate containing near-surface Ar, the growth front crosses entire surface atomic steps in both uphill and downhill directions with no difference, and thus, the morphology of the two-dimensional (2D) overlayer exhibits a round-shape. Especially, the round-shaped 2D overlayers coalesce seamlessly with a growth acceleration in the approaching direction, which is barely observed in the synthesis of 2D materials. This can be attributed to the immigration lifetime and diffusion rate of growth species, which depends on the overlayer-substrate interaction and the surface catalysis. Furthermore, the “round to hexagon” morphological transition is achieved by etching-regrowth, revealing the inherent growth kinetics under quasi-freestanding conditions. These findings provide a novel promising way to modulate the growth, coalescence, and etching dynamics of 2D materials on solid surfaces by adjusting the strength of overlayer-substrate interaction, which contributes to optimization of large-scale production of 2D material crystals.

Covalent functionalization of graphene leads to excellent sensitivity for ammonia. Quick response and recovery of the functionalized graphene layers are observed, and a sensitivity benchmarking with other graphene chemiresistors shows a superior sensitivity in the sub-ppm range.

Abstract

Gas sensors are essential in several fields and, in general, features such as high sensitivity, quick response, and fast recovery are required, along with low power consumption and low cost. Graphene is considered a promising material for gas sensing applications, its functionalization often being a requisite. In the present study, we developed competitive and promising gas sensors for ammonia detection. Interestingly, we present an easy and efficient method to functionalize graphene by using diazonium chemistry with different functional groups. Moreover, we prove the superior sensing capability of our covalently modified graphene layers. These experimental data have been consistently interpreted by theoretical calculations, which reveal a defect-driven sensor's response to ammonia. These results open the possibility of a comprehensive design and use of these graphene-based sensors in real applications.

Superior nonlinear optical responses from the near-infrared to visible range in non-centrosymmetric stacking spiral MoTe₂ nanopyramids are realized, enabled by their broken inversion symmetry, weak interlayer coupling, and strong light–matter interaction from the edge-rich quasi-multilayer structure. Moreover, the second-order nonlinear susceptibility of the spiral MoTe₂ is estimated to be around 1–2 order(s) of magnitude larger than those of most reported transition metal dichalcogenides.

Abstract

Transition metal dichalcogenides (TMDs) are of great promise for various nonlinear optical (NLO) applications due to their unique electronic and optoelectronic properties, such as tunable optical bandgap, strong spin–orbit coupling, and exciton effects. However, the desired NLO performances of regular 2H-TMDs are usually restricted by their limited absorption at atomic thickness. With this regard, a structurally novel spiral MoTe₂ (s-MoTe₂) nanopyramids is reported with unique and superior NLO response, enabled by their broken inversion symmetry, weak interlayer coupling, exciton resonance, and strong light–matter interaction from the edge-rich 3R-like quasi-multilayer structure. The excellent NLO response over a wide spectral range from the near-infrared to visible region is demonstrated, where second- and third-order NLO responses have been simultaneously observed. Moreover, the second-order nonlinear susceptibility of s-MoTe₂ is estimated to be around 1–2 order(s) of magnitude larger than those of most reported TMDs. The demonstration of a superior NLO response in such s-MoTe₂ not only paves a new way for designing the best NLO TMD structures, but also greatly prompts their practical applications in micro–nano NLO devices on chips in future.

By studying the synthesis of BiO_xF_y crystals on the surface of graphene oxide (GO) under environmental conditions, it is found that the surface interaction of GO will affect the reaction process by changing the microenvironment.

Abstract

Due to its rich functional groups and large specific surface area, graphene oxide (GO) will inevitably interact with the surrounding medium in practical applications. Although it has been found that this surface interaction of GO affects the crystallization process, it is still unclear whether it affects chemical reactions and reaction kinetics. Here, by studying the synthesis of BiO_xF_y crystals on the surface of GO under environmental conditions, it is shown that the surface interaction of GO affects the reaction process by changing the microenvironment. When the source ratio of F to Bi is 1, the morphology and structure of BiO_xF_y crystals synthesized on the surface of GO are significantly different from those obtained in the solution without GO. Moreover, the morphology, structure, and even chemical composition of the crystals formed on the surface of GO will change with the change of the element ratio of F/Bi. And a new bismuth-containing crystal can be formed on the GO nanosheets when the F/Bi ratio is less than 0.6 in the reaction solution. This research not only explains the effect of GO surface dynamic interaction on crystal synthesis, but also provides new ideas for the design and synthesis of new 2D materials.

A strategy of polarization-driven-orientation selective growth is proposed and it is demonstrated that single-crystalline nitrides can, in principle, be achieved on any substrate without any crystal symmetry. This strategy can be extended to the growth of any emergent single-crystalline semiconductor films on any arbitrary freestanding substrates by choosing appropriate 2D materials with matched crystal structures.

Abstract

III-nitride semiconductor films are usually achieved by epitaxial growth on single-crystalline substrates (sapphire, silicon (Si), and silicon carbide). It is important to grow these films on non-epitaxial substrates of interest such as polycrystalline substrates for exploring novel applications in electronics and optoelectronics. However, single-crystalline III-nitride films with uniform orientation on non-epitaxial substrates have not yet been realized, due to the lack of crystallographic orientation of the substrates. Here, this work proposes a strategy of polarization-driven-orientation selective growth (OSG) and demonstrate that single-crystalline gallium nitride (GaN) can in principle be achieved on any substrates. Taking polycrystalline diamond and amorphous-silicon dioxide/Si substrates as typical examples, the OSG is demonstrated by utilizing a composed buffer layer consisting of graphene and polycrystalline physical vapor deposited (PVD) aluminium nitride (AlN). The polarization of the PVD AlN can effectively tune the strength of interfacial orbital coupling between AlN nuclei and graphene at different rotation angles, as confirmed by atom-scale first-principles calculations, and align the AlN nuclei to form a uniform orientation. This consequently leads to continuous single-crystalline GaN films. The ability to grow single-crystalline III-nitrides onto any desired substrates would create unprecedented opportunities for developing novel electronic and optoelectronic devices.

2D materials have spectacular electrical and optical properties but their device fabrication and integration are arduous and expensive. Optical modification methods offer less detrimental processes that can often be done in one optical setup, sometimes even simultaneously with one another, saving time and money. Additionally, the extreme locality of lasers can be utilized in designable laser direct writing processes.

Abstract

2D materials are under extensive research due to their remarkable properties suitable for various optoelectronic, photonic, and biological applications, yet their conventional fabrication methods are typically harsh and cost-ineffective. Optical modification is demonstrated as an effective and scalable method for accurate and local in situ engineering and patterning of 2D materials in ambient conditions. This review focuses on the state of the art of optical modification of 2D materials and their applications. Perspectives for future developments in this field are also discussed, including novel laser tools, new optical modification strategies, and their emerging applications in quantum technologies and biotechnologies.

For the first time, the growth of nitride on sapphire by metal–organic chemical vapor deposition can be monitored layer-by-layer, starting at the substrate, to observe the inverse domain boudary formation and the evolution of the polarity inversion. This work offers a breakthrough in the understanding of the underlying mechanism that dictates the polarity of the nitride epilayer.

Abstract

The performance of nitride devices is strongly affected by their polarity. Understanding the polarity determination and evolution mechanism of polar wurtzite nitrides on nonpolar substrates is therefore critically important. This work confirms that the polarity of AlN on sapphire prepared by metal–organic chemical vapor deposition is not inherited from the nitrides/sapphire interface as widely accepted, instead, experiences a spontaneous polarity inversion during the growth. It is found that at the initial growth stage, the interface favors the nitrogen-polarity, rather than the widely accepted metal-polarity or randomly coexisting. However, the polarity subsequently converts into the metal-polar situation, at first locally then expanding into the whole area, driven by the anisotropy of surface energies, which results in universally existing inherent inverse grain boundaries. Furthermore, vertical two-dimensional electron accumulation originating from the lattice symmetry breaking at the inverse grain boundary is first revealed. This work identifies another cause of high-density defects in nitride epilayers, besides lattice mismatch induced dislocations. These findings also offer new insights into atomic structure and determination mechanism of polarity in nitrides, providing clues for its manipulation toward the novel hetero-polarity devices.

Nature Nanotechnology, Published online: 10 February 2022; doi:10.1038/s41565-022-01075-7

Highly oriented and long graphene folds are spontaneously formed in chemical-vapor-deposition (CVD)-grown single-crystal graphene film on a Cu(111) foil surface, and this is triggered by the formation of highly ordered bunched Cu steps under the graphene. Such folds are fractured into “back-and-forth” subfold patterns at bunched Cu step edge regions, with cracks propagating along zigzag or armchair directions.

Abstract

A single-crystal graphene film grown on a Cu(111) foil by chemical vapor deposition (CVD) has ribbon-like fold structures. These graphene folds are highly oriented and essentially parallel to each other. Cu surface steps underneath the graphene are along the <110> and <211> directions, leading to the formation of the arrays of folds. The folds in the single-layer graphene (SLG) are not continuous but break up into alternating patterns. A “joint” (an AB-stacked bilayer graphene) region connects two neighboring alternating regions, and the breaks are always along zigzag or armchair directions. Folds formed in bilayer or few-layer graphene are continuous with no breaks. Molecular dynamics simulations show that SLG suffers a significantly higher compressive stress compared to bilayer graphene when both are under the same compression, thus leading to the rupture of SLG in these fold regions. The fracture strength of a CVD-grown single-crystal SLG film is simulated to be about 70 GPa. This study greatly deepens the understanding of the mechanics of CVD-grown single-crystal graphene and such folds, and sheds light on the fabrication of various graphene origami/kirigami structures by substrate engineering. Such oriented folds can be used in a variety of further studies.

Due to the formation of the built-in electric field, the mixed dimensional 1D CdS nanowire/2D graphene van der Waals heterostructure shows strong electron–phonon coupling with the average intensity ratio of 2LO/1LO Raman mode is 1.2. Additionally, CdS nanowires are the typical polar semiconductor, the intensity of 2LO/1LO presents polarization dependence, and the maximum value can reach up to 8.95.

Abstract

Electron–phonon coupling plays a key role in affecting the properties of the semiconducting nanostructures, such as providing the possibility for obtaining higher superconducting transition temperatures. Here, using Raman, temperature-dependent and polarized Raman scattering measurements, ultra-strong electron–phonon coupling in 1D CdS nanowires and 2D graphene heterostructures is demonstrated. The intensity ratio of 2LO/1LO mode in CdS nanowires provides a spectroscopy-based method to quantify electron–phonon coupling, the strength of which is temperature and polarization dependent. The intensity ratio mode of 2LO/1LO in heterostructure reached up to 8.95 when the incident laser polarization is parallel to the c-axis of the nanowire. It is ≈2.37 times higher than in an individual nanowire. In addition, in situ and time-resolved photoluminescence spectra demonstrate the dynamics of the exciton recombinations, providing a comprehensive understanding of the enhancement of electron–phonon coupling in heterostructures. Via optical waveguiding characterization, the graphene layer is demonstrated to not only be an ultrafast carrier transfer channel but also a low Fermi level channel that induces the formation of the built in electrical field, elevating the electron–phonon coupling. Such new mixed dimensional heterostructures illustrate a straightforward approach to enhance the electron–phonon coupling, which may be applied to many integrated superconducting photonic and optoelectronic devices.

Van der Waals (vdW) epitaxy represents a powerful way for growing heterostructures made of stacked sequences of 2D crystals, potentially exhibiting new phenomena and peculiar properties. The Ge-Sb-Te (GST) alloys are phase-change materials, widely studied for their applications in memories and neuromorphic devices. In this study, the key parameters to predict the interaction between GST layered materials and substrate surface are identified in order to contribute to a generalized guideline for the design and mastering of vdW epitaxy of 2D materials.

Abstract

In this study, a generalized guideline is identified to predict the interaction between two-dimensional (2D) layered materials and substrate surfaces. Additionally, the van der Waals (vdW) heterostructures commensurability, the phase formation and the strain relaxation are identified during interface growth. To achieve such a general overview, the case of Ge-Sb-Te (GST) alloys on InAs(111) is studied. In this system, low-lattice mismatch conditions are fulfilled to avoid relaxation due to formation of misfit dislocations and allow to correctly identify vdW epitaxy. At the same time, the substrate can be efficiently prepared into self- and un-passivated surfaces to clarify the role of the surface interaction. Furthermore, the GST epilayer exhibits two different highly ordered 2D structures and a three-dimensional disordered structure, allowing to directly infer the nature of the epitaxy. This study opens the way for the design and mastering of vdW epitaxial growth of 2D heterostructures as well as hybrid 2D and non-layered materials.

Regardless of the type or the thickness of the channel, holes of electron-hole pairs generated by electron beam (e-beam) irradiation are trapped in SiO₂, resulting in negative shifts of transfer curves according to different bias applications. Understanding and interpretation of the e-beam effect are conducted in parallel with DC analysis, time-dependent current, and low-frequency noise.

Abstract

In this study, the radiation effects of electron beam (e-beam) on field-effect transistors (FETs) using transition-metal dichalcogenides (TMD) as a channel are carefully investigated. Electron-hole pairs (EHPs) in SiO₂ generated by e-beam irradiation induce additional traps, which change the surface potential of the TMD channel, resulting in strong negative shifts of transfer characteristics. These negative shifts, which remind one of n-doping effects, are highly affected not only by the condition of e-beam irradiation, but also by the gate bias condition during irradiating. As a result of the e-beam irradiation effect, band bending and contact resistance are affected, and the degree of formation of oxide traps and interface traps varies depending on the gate bias conditions. In the case of V _G > 0 V application during e-beam irradiation, the negative shifts in the transfer characteristics are fully recovered after ambient exposure. However, the interface traps increase significantly, resulting in variations of low-frequency (LF) noise and time-dependent current fluctuations.

Zero-degree grain boundaries, called domain boundaries (DB), exist in 2D materials that have a strong epitaxial relationship with their substrate. Here four distinct DBs are observed in a monolayer of Ti₂O₃ supported on Au(111). The strong epitaxial constraint from the substrate stabilizes unique DB structures that are not observed in van-der-Waals-bonded 2D materials such as graphene.

Abstract

Grain boundaries (GBs) are ubiquitous in solids. Their description is critical for understanding polycrystalline materials and explaining their mechanical and electrical properties. A GB in a 2D material can be described as a line defect and its atomic structures have been intensively studied in materials such as graphene. These GBs accommodate the relative rotation of two neighboring grains by incorporating periodic units consisting of nonhexagonal rings along the boundary. Zero-degree GBs, called domain boundaries (DBs), where there is only a lattice offset between two grains without any rotation, are rare in 2D van-der-Waals (vdW) bonded materials where the grains can easily move. However, this movement is not possible in 2D materials that have a strong epitaxial relationship with their substrate such as the M₂O₃ (2 × 2) honeycomb monolayers on noble metal (111) supports. Involving experimental and theoretical investigations, four main DBs are observed here in a monolayer of Ti₂O₃ supported on Au(111) and their atomic structures are solved. The DB formation energies explain why some DBs are more frequently observed than others. The strong epitaxial constraint from the Au(111) substrate stabilizes some unique Ti₂O₃ monolayer DB structures that are not observed in vdW-bonded 2D materials.