Jiuxiang Dai

Publication date: April 2022

Source: Materials Today, Volume 54

Author(s): Cordelia Sealy

Nature Reviews Materials, Published online: 28 April 2022; doi:10.1038/s41578-022-00446-9

Alloying two-dimensional (2D) materials is an effective approach to alter their lattice parameters and electronic band structure for achieving a specified band gap. By using our home-made AFM strain-stage, we successfully conducted the in situ strain Kelvin probe force microscopy (KPFM) and Conductive atomic force microscope (C-AFM) measurements of monolayer 2D TMD alloys (Mo_0.4W_0.6Se₂) on flexible substrate. The CPD of the 2D alloy decreased with the increase of uniaxial tensile strain. The SBH showed the drops upon the applied strain loadings. Our detailed study unveils that evolution of SBH is mainly due to the increase of the 2D TMD alloy electron affinity under strain. This indicates that the electrical properties of 2D alloy could be well designed and achieved through the controllable adjustment of strain.

Abstract

Atomically thin two-dimensional (2D) alloys have attracted wide interests of study recently due to their potential in flexible electronic and optoelectronic applications. In particular, monolayer transition metal dichalcogenide (TMD) alloys have emerged as unique 2D semiconductors with tunable bandgaps, by means of alloying. However, response of surface electrical potential and barrier height to strain for 2D TMD alloys–electrode interface is rarely explored. Apparently, revealing such strain-dependent evolution of electrical properties is crucial for developing advanced 2D TMD based flexible electronics and optoelectronics. Here we performed in situ strain Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM) investigations of monolayer Mo_0.4W_0.6Se₂ on Au coated flexible substrate, where controlled uniaxial tensile strain is applied. Both contact potential difference (CPD) and Schottky barrier heights (SBH) of monolayer Mo_0.4W_0.6Se₂ show obvious decreases with the increase of strain, which is mainly due to the strain-induced increment of TMD electron affinity. Our in situ strain photoluminescence (PL) measurements also indicate the changes of electronic band structures under strain. We further exploit the substrate effects on CPD by study the monolayer alloy on the mostly used substrates of SiO₂/Si and indium tin oxide (ITO)/glass. Our findings could strengthen the foundation for the potential applications of 2D TMD and their alloys in the fields of strain sensors, flexible photodetectors, and other wearable electronic devices.

Magnetic phase control is the source of fascinating correlated phenomena and the frontier research direction in materials science and spintronic devices. Magnetic anisotropy control and a dramatically enhanced T _c up to 400 K are realized in van der Waals ferromagnet Fe₅GeTe₂ during a high-pressure loading–deloading process, shedding light on exploring the exchange interaction mechanism and realizing practical spintronic applications.

Abstract

The technological appeal of van der Waals ferromagnetic materials is the ability to control magnetism under external fields with desired thickness toward novel spintronic applications. For practically useful devices, ferromagnetism above room temperature or tunable magnetic anisotropy is highly demanded but remains challenging. To date, only a few layered materials exhibit unambiguous ferromagnetic ordering at room temperature via gating techniques or interface engineering. Here, it is demonstrated that the magnetic anisotropy control and dramatic modulation of Curie temperature (T _c) up to 400 K are realized in layered Fe₅GeTe₂ via the high-pressure diamond-anvil-cell technique. Magnetic phases manifesting with in-plane anisotropic, out-of-plane anisotropic and nearly isotropic magnetic states can be tuned in a controllable way, depicted by the phase diagram with a maximum T _c up to 360 K. Remarkably, the T _c can be gradually enhanced to above 400 K owing to the Fermi surface evolution during a pressure loading–deloading process. Such an observation sheds light on the understanding and control of emergent magnetic states in practical spintronic applications.

Periodic substrate modulation perturbs the electronic states of graphene, duplicates Dirac cones, and effectively couples the two valleys of the Dirac states in epitaxial monolayer graphene. The intervalley interaction is controlled by the substrate lattice constant. Moiré flat bands emerge at certain magic values of the substrate lattice constant.

Abstract

Tuning interactions between Dirac states in graphene has attracted enormous interest because it can modify the electronic spectrum of the 2D material, enhance electron correlations, and give rise to novel condensed-matter phases such as superconductors, Mott insulators, Wigner crystals, and quantum anomalous Hall insulators. Previous works predominantly focus on the flat band dispersion of coupled Dirac states from different twisted graphene layers. In this work, a new route to realizing flat band physics in monolayer graphene under a periodic modulation from substrates is proposed. Graphene/SiC heterostructure is taken as a prototypical example and it is demonstrated experimentally that the substrate modulation leads to Dirac fermion cloning and, consequently, the proximity of the two Dirac cones of monolayer graphene in momentum space. Theoretical modeling captures the cloning mechanism of the Dirac states and indicates that moiré flat bands can emerge at certain magic lattice constants of the substrate, specifically when the period of modulation becomes nearly commensurate with the (3 × 3)R30o\[(\sqrt 3 \; \times \;\sqrt 3 )R{30^o}\] supercell of graphene. The results show that epitaxial single monolayer graphene on suitable substrates is a promising platform for exploring exotic many-body quantum phases arising from interactions between Dirac electrons. 

Here it is summarized comprehensively and systematically the recent progress made in developing SnSe and SnSe-based nanostructures beyond thermoelectricity. Specifically, the main preparation methods, properties, and applications are covered. On that basis, the challenges and potential perspectives in applying SnSe nanomaterials are highlighted.

Abstract

Layered SnSe is an emerging class of black phosphorus, which is non-toxic, eco-friendly, and chemically stable. Recently, SnSe nanostructures have triggered more research interest and enabled broad applications beyond demonstrating their great performances on thermoelectricity. However, there are also a great many significant studies of SnSe nanostructures beyond thermoelectricity. SnSe quantum dots, nanosheets, nanowires, and thin films with diverse morphologies have been synthesized using various chemical and physical preparation approaches. SnSe is a multi-phase semiconductor, and its nanostructures endow unique properties, including small electron effective mass, ultralow thermal conductivity, huge anisotropy, and the largest 2D piezoelectric coefficient ever predicted. The versatility of SnSe nanostructures can enable potential applications ranging from ultrafast photonics, logic devices, photodetectors, solar cells, photocatalysis, energy storage, and biology to more cutting-edge interdisciplinary subjects. In this review, the recent advances made in SnSe nanostructures are summarized, covering basics, synthesis, properties, and applications, just giving a passing comment on thermoelectricity. An in-depth perspective on the challenges and prospects of SnSe nanostructures toward broad and practical applications is also given.

The gas-phase alkali metal-assisted MOCVD (GAA-MOCVD) for 2D semiconductors with advanced nucleation control realizes the grain size enhancement and generation of MoS₂ continuous films with large-scale spatial homogeneity. In addition, this method enables the grain size enhancement to be maximized as a result of precise and controllable feeding of gas-phase precursors.

Abstract

Advances in large-area and high-quality 2D transition metal dichalcogenides (TMDCs) growth are essential for semiconductor applications. Here, the gas-phase alkali metal-assisted metal-organic chemical vapor deposition (GAA-MOCVD) of 2D TMDCs is reported. It is determined that sodium propionate (SP) is an ideal gas-phase alkali-metal additive for nucleation control in the MOCVD of 2D TMDCs. The grain size of MoS₂ in the GAA-MOCVD process is larger than that in the conventional MOCVD process. This method can be applied to the growth of various TMDCs (MoS₂, MoSe₂, WSe₂, and WSe₂) and the generation of large-scale continuous films. Furthermore, the growth behaviors of MoS₂ under different SP and oxygen injection time conditions are systematically investigated to determine the effects of SP and oxygen on nucleation control in the GAA-MOCVD process. It is found that the combination of SP and oxygen increases the grain size and nucleation suppression of MoS₂. Thus, the GAA-MOCVD with a precise and controllable supply of a gas-phase alkali metal and oxygen allows achievement of optimum growth conditions that maximizes the grain size of MoS₂. It is expected that GAA-MOCVD can provide a way for batch fabrication of large-scale atomically thin electronic devices based on 2D semiconductors.

Nature Communications, Published online: 22 April 2022; doi:10.1038/s41467-022-29929-7

Abstract

The edges of two-dimensional (2D) materials can exhibit special structure and thus distinctive properties differing from the interior, such as the modified photoluminescence emission, the improved electrocatalytic activity, and the enhanced nonlinear optical response. In this work, we report the observation of abrupt enhancement of Raman scattered light at edges of layered MoS₂, which is closely related to the polarization of the incident light. More importantly, the intensity of out-of-plane A_1g mode is enhanced much more obviously than the in-plane E _2g ¹ mode. The unique optical effect at edges is also reflected on the alteration of Raman selection rule. The forbidden Raman modes are observed at edge region, and the polarization of Raman scattered light can be modulated by the edge structure, which differs obviously from that of the interior. We attribute these particular performances to the modulation of the intensity and distribution of the electric field for the incident and Raman scattered light at the edge. This work provides a systematic research on the intensity and polarization modulation of the Raman scattering at edges, which would be helpful for understanding the distinctive optical properties of the edge structure.

A kind of Z-scheme carbon dot-based photothermal agent consisting of 2D ultrathin nonmetallic B_xC/C Janus quantum sheets is reported here to solve the limitation of fluorescence on photothermal conversion. The heterogeneous growth of Z-scheme B_xC/C enables the laser-driven quick injection of hot electrons from C into B_xC, realizing ultra-high photothermal conversion of 60.0% in the NIR-II biowindow for tumor elimination.

Abstract

Carbon dots (CDs) are one of the most popular photothermal agents (PTAs) as a noninvasive strategy for tumor treatment. However, because of the inherent dominant fluorescent emission, the CDs-based PTAs hardly achieve a single photothermal conversion, which causes low photothermal conversion efficiency and poor photothermal performance. In this regard, finding a new CDs-based material system to greatly restrain its fluorescence to enhance its photothermal conversion efficiency is highly required, however, it is still a grand challenge. Herein, a kind of Z-scheme CDs-based PTAs consisting of 2D ultrathin nonmetallic B_xC/C Janus quantum sheets (B_xC/C JQSs) is reported to greatly enhance the photothermal conversion efficiency. It is demonstrated that the heterogeneous growth of Z-scheme B_xC/C JQSs enables the NIR-driven quick injection of hot electrons from C into the conjugated B_xC, realizing a single conversion of light to heat, and resulting in a high photothermal conversion of 60.0% in NIR-II. Furthermore, these new Z-scheme B_xC/C-polyethylene glycol JQSs display outstanding biocompatibility and show effective tumor elimination outcome both in vitro and in vivo through the synergistic photothermal-immunotherapy in the NIR-II biowindow with undetectable harm to normal tissues.

A neuromorphic optoelectronic floating-gate transistor based on multilayer graphene/h-BN/MoS₂ vdW heterostructure exhibits programmable synaptic plasticity due to the unique light-induced carrier tunneling through vdW heterostructure. Ultra-low energy consumption for the electrical response to light stimulation is also realized under a low V _ds at program state, demonstrating its great potential in building efficient artificial neural networks based on vdW heterostructures.

Abstract

Van der Waals (vdW) heterostructures provide a unique opportunity to develop various electronic and optoelectronic devices with specific functions by designing novel device structures, especially for bioinspired neuromorphic optoelectronic devices, which require the integration of nonvolatile memory and excellent optical responses. Here, we demonstrate a programmable optoelectronic synaptic floating-gate transistor based on multilayer graphene/h-BN/MoS₂ vdW heterostructures, where both plasticity emulation and modulation were successfully realized in a single device. The dynamic tunneling process of photogenerated carriers through the as-fabricated vdW heterostructures contributed to a large memory ratio (10⁵) between program and erase states. Our device can work as a functional or silent synapse by applying a program/erase voltage spike as a modulatory signal to determine the response to light stimulation, leading to a programmable operation in optoelectronic synaptic transistors. Moreover, an ultra-low energy consumption per light spike event (~2.5 fJ) was obtained in the program state owing to a suppressed noise current by program operation in our floating-gate transistor. This study proposes a feasible strategy to improve the functions of optoelectronic synaptic devices with ultra-low energy consumption based on vdW heterostructures designed for highly efficient artificial neural networks.

Abstract

Recently, group-IVB semiconducting transition metal dichalcogenides (TMDs) of ZrS₂ have attracted significant research interest due to its layered nature, moderate band gap, and extraordinary physical properties. Most device applications require a deposition of high quality large-area uniform ZrS₂ single crystalline films, which has not yet been achieved. In this work, for the first time, we demonstrate the epitaxial growth of high quality large-area uniform ZrS₂ films on c-plane sapphire substrates by chemical vapor deposition. An atomically sharp interface is observed due to the supercell matching between ZrS₂ and sapphire, and their epitaxial relationship is found to be ZrS₂ (0001)[101̄0]∥Al₂O₃ (0001)[112̄0]. The epitaxial ZrS₂ film exhibits n-type semiconductor behavior with a room temperature mobility of 2.4 cm²·V⁻¹·s⁻¹, and the optical phonon is the dominant scattering mechanism at room temperature or above. Furthermore, the optoelectronic applications of ZrS₂ films are demonstrated by fabricating photodetector devices. The ZrS₂ photodetectors exhibit the excellent comprehensive performance, such as a light on/off ratio of 10⁶ and a specific detectivity of 2.6 × 10¹² Jones, which are the highest values compared with the photodetectors based on other group-IVB two-dimensional TMDs.

With metal chalcogenide layers covalently anchored by long-range ordered organic functional motifs, organic metal chalcogenides are a type of newly emerging 2D materials, which are exquisitely desired but impossible to realize by traditional methods for the synthesis of 2D materials. This Minireview gives a perspective on the structural design of bulk precursors, exfoliation methods, applications, and discusses research directions based on recently published work.

Abstract

The modification of inorganic two-dimensional (2D) materials with organic functional motifs is in high demand for the optimization of their properties, but it is still a daunting challenge. Organic metal chalcogenides (OMCs) are a type of newly emerging 2D materials, with metal chalcogenide layers covalently anchored by long-range ordered organic functional motifs, these materials are extremely desirable but impossible to realize by traditional methods. Both the inorganic layer and organic functional motifs of OMCs are highly designable and thus provide this type of 2D materials with enormous variety in terms of their structure and properties. This Minireview aims to review the latest developments in OMCs and their bulk precursors. Firstly, the structure types of the bulk precursors for OMCs are introduced. Second, the synthesis and applications of OMC 2D materials in photoelectricity, catalysis, sensors, and energy transfer are explored. Finally, the challenges and perspectives for future research on OMCs are discussed.

N=9 nitrogen-doped armchair graphene nanoribbons (GNRs) (N-9-AGNRs) with multiple substitutional sites were synthesized. Scanning tunneling spectroscopy (STS) analysis and density functional theory (DFT) calculations reveal that the doped sites of N atoms play a significant role in modulating the electronic structures of N-9-AGNRs (semiconducting for (C−C), metallic for (C−N) and (N−N)).

Abstract

Doped graphene nanoribbons (GNRs) with heteroatoms are a principal strategy to fine-tune the electronic structures of GNRs for future device applications. Here, we successfully synthesized the N=9 nitrogen-doped armchair GNR on the Au(111) surface. Due to the flexibility of precursor molecules, three different covalent bonds (C−C, C−N, N−N) are formed in the GNR backbone. Scanning tunneling spectroscopy analysis together with band structure calculations reveals that the band gap of the N-9-AGNRs (C−C) will be enlarged compared to pristine 9-AGNRs, and the C−N bond and N−N bond at the isolated site of N-9-AGNR (C−C) will introduce new defect states near the Fermi level. DFT calculations reveal that the electronic structure of N-9-AGNR (C−C) shows semiconductor character, while N-9-AGNR (C−N) and N-9-AGNR (N−N) display metallic character. Our results provide a promising route for creating more complex molecular heterostructures with tunable band gaps, which may be useful for future molecular electronics and memory device applications.

Nature Communications, Published online: 25 April 2022; doi:10.1038/s41467-022-29817-0

Nature Electronics, Published online: 25 April 2022; doi:10.1038/s41928-022-00754-6

Nature Electronics, Published online: 25 April 2022; doi:10.1038/s41928-022-00753-7

Recent research progress on the use of liquid-phase exfoliation beyond layered van der Waals (vdW) crystals is summarized. Questions remain unanswered on how these 3D strongly bonded nonlayered crystals exfoliate into nanoplatelets, and on the insight of the exfoliation mechanism. The answer to the questions helps in revealing the future direction of this newly grown sub-family of 2D-class of materials.

Abstract

For nearly 15 years, researchers have been using liquid-phase exfoliation (LPE) to produce 2D nanosheets from layered crystals. This has yielded multiple 2D materials in a solution-processable form whose utility has been demonstrated in multiple applications. It was believed that the exfoliation of such materials is enabled by the very large bonding anisotropy of layered materials where the strength of intralayer chemical bonds is very much larger than that of interlayer van der Waals bonds. However, over the last five years, a number of papers have raised questions about our understanding of exfoliation by describing the LPE of nonlayered materials. These results are extremely surprising because, as no van der Waals gap is present to provide an easily cleaved direction, the exfoliation of such compounds requires the breaking of only chemical bonds. Here the progress in this unexpected new research area is examined. The structure and properties of nanoplatelets produced by LPE of nonlayered materials are reviewed. A number of unexplained trends are found, not least the preponderance of isotropic materials that have been exfoliated to give high-aspect-ratio nanoplatelets. Finally, the applications potential of this new class of 2D materials are considered.

2D layered materials, exhibiting exotic structural, electrical, and magnetic properties, provide a superior platform for implementing novel quantum devices—from tunneling diodes and transistors, to spin-FETs, valley-FETs, and qubits. The physics are highlighted and the opportunities and challenges of exploiting the unique quantum properties of 2D materials to enable revolutionary ultra-energy-efficient quantum devices are analyzed.

Abstract

As an approximation to the quantum state of solids, the band theory, developed nearly seven decades ago, fostered the advance of modern integrated solid-state electronics, one of the most successful technologies in the history of human civilization. Nonetheless, their rapidly growing energy consumption and accompanied environmental issues call for more energy-efficient electronics and optoelectronics, which necessitate the exploration of more advanced quantum mechanical effects, such as band-to-band tunneling, spin–orbit coupling, spin–valley locking, and quantum entanglement. The emerging 2D layered materials, featured by their exotic electrical, magnetic, optical, and structural properties, provide a revolutionary low-dimensional and manufacture-friendly platform (and many more opportunities) to implement these quantum-engineered devices, compared to the traditional electronic materials system. Here, the progress in quantum-engineered devices is reviewed and the opportunities/challenges of exploiting 2D materials are analyzed to highlight their unique quantum properties that enable novel energy-efficient devices, and useful insights to quantum device engineers and 2D-material scientists are provided.

Nature Chemistry, Published online: 25 April 2022; doi:10.1038/s41557-022-00924-1

Abstract

Atomically thin two-dimensional (2D) materials are promising candidates to develop flash memories with premium performances as compared to conventional bulk materials, because of their ultra-thin thickness and highly tunable electrical properties. So far, most of the reported 2D material based flash memories work in the uni-polar mode, which usually further integrate additional local gate to achieve bi-polar function. However, such approach is volatile, meaning that the gate bias has to be applied persistently to maintain the polarity change and thus increases the power consumption. Here, we report a bi-polar memory based on MoTe₂/h-BN/graphene semi-floating gate (SFG) heterostructure, which has non-volatile and dynamically tunable polarity. The SFG configuration has the channel layer of MoTe₂ and dielectric layer of h-BN half-stacked on the floating gate layer of graphene. The off-graphene half of the MoTe₂ channel can be tuned between n-type and p-type by simultaneously applying ultraviolet (UV) illumination and electrical field through the back gate, which maintains this polarity after the removal of both stimuli. As a result, the SFG memory can work in the non-volatile bi-polar mode, with a on/off ratio of ∼ 100 and switching speed of 1 ms. On the other hand, the on-graphene half of the MoTe₂ channel remains n-type under UV illumination and electrical bias, so that the MoTe₂ full floating gate memory maintains n-type, which implements the integration of both n- and p-type memories in a single 2D heterostructure. This capability provides great flexibility for memory devices adapting in various emerging applications.