Mechanical Exfoliation Research Articles

Effect of surface preparation on PtSe2 crystal surface morphology

Transition metal dichalcogenides, a new class of layered materials, have recently been deemed as an excellent material platform for the further development of microelectronics. Contrary to the general trend, which is geared toward layers, we focus our attention on basic research regarding bulk PtSe2. The justification for this approach is based on the fact that some research (e.g., on the impact of the doping process on the material’s properties) can be performed on the bulk crystal. We believe that the conclusions drawn from our approximation can be extrapolated to thin films and monolayers. In this paper, we present a morphological study of the influence of the surface preparation procedure on the PtSe2 substrate. We show that mechanical exfoliation is one possible way to achieve a clean PtSe2 surface. However, STM measurements revealed that this process is insufficient to achieve an atomically clean surface. Subsequent additional annealing under UHV conditions led to an improved surface morphology by reducing the number of mobile PtSe2 flakes as well as the density of small surface clusters. Finally, STM measurements show other interesting surface structures, such as cracks, bulges, and flakes with heights lower than the apparent height typical of a PtSe2 monolayer.

Programmable Photo‐Induced Doping in 2D Materials

AbstractOver the past few decades, certain features of the bulk and surface properties of nanoscale two‐dimensional (2D) layered materials have been exploited to improve the performance and efficiency of optoelectronics functional devices. These findings include the role of surface modifications, including dopant engineering with chemical modifications, structural control through physical modifications, and bandgap modification through electrostatic control, but to enable n‐ or p‐type behavior in the same nanoflakes without photoresist and environmental stability in traded devices is challenging. In this state‐of‐the‐art focused review article, the most up‐to‐date and promising strategies to develop nanodevices with photoresist‐free possibilities of controlling the conduction type in 2D nanostructures based on the mechanical exfoliation of ultrathin nanomaterials are attempted to summarize. This article demonstrates how carrier type band modulation in transition metal dichalcogenides is generated by the charge storage interface or defect engineering such as optical‐electronic excitation of donor‐like states (or point defects) in BN upon illumination to configure opposite polarities, n, or the p‐type field‐effect transistor from an intrinsic semiconductor. It is believed that this brief overview of recent research on nanomaterials for application in various types of functional electronic devices may contribute in the development of future optoelectronic devices.

One-Step Exfoliation Method for Plasmonic Activation of Large-Area 2D Crystals.

Advanced exfoliation techniques are crucial for exploring the intrinsic properties and applications of 2D materials. Though the recently discovered Au-enhanced exfoliation technique provides an effective strategy for the preparation of large-scale 2D crystals, the high cost of gold hinders this method from being widely adopted in industrial applications. In addition, direct Au contact could significantly quench photoluminescence (PL) emission in 2D semiconductors. It is therefore crucial to find alternative metals that can replace gold to achieve efficient exfoliation of 2D materials. Here, the authors present a one-step Ag-assisted method that can efficiently exfoliate many large-area 2D monolayers, where the yield ratio is comparable to Au-enhanced exfoliation method. Differing from Au film, however, the surface roughness of as-prepared Ag films on SiO2 /Si substrate is much higher, which facilitates the generation of surface plasmons resulting from the nanostructures formed on the rough Ag surface. More interestingly, the strong coupling between 2D semiconductor crystals (e.g., MoS2 , MoSe2 ) and Ag film leads to a unique PL enhancement that has not been observed in other mechanical exfoliation techniques, which can be mainly attributed to enhanced light-matter interaction as a result of extended propagation of surface plasmonic polariton (SPP). This work provides a lower-cost and universal Ag-assisted exfoliation method, while at the same time offering enhanced SPP-matter interactions.

Two-dimensional ternary chalcogenides FeX2Y4 (X=Ga, In; Y=S, Se, Te): Promising materials for sustainable energy

Two-dimensional layered materials, particularly ternary chalcogenides, are promising for technological applications, as they exhibit tunable electronic, optical, and magnetic properties. Here we present detailed first-principles calculations of electronic, optical, and thermal transport properties of ${\text{Fe}X}_{2}{Y}_{4}$ $(X=\mathrm{Ga}, \mathrm{In}; Y=\mathrm{S}, \mathrm{Se}, \mathrm{Te})$ ternary layered chalcogenides, relevant for sustainable energy applications. We show that the monolayers, similar to bulk, are dynamically and thermally stable, as demonstrated by phonon dispersion and molecular dynamics simulations. The exfoliation energy of monolayers is comparable to graphene and transition-metal dichalcogenides, suggesting possible synthesis of monolayers by mechanical exfoliation. In the hexagonal crystal structure, these materials are nonmagnetic semiconductors with varied band-gap values, that are interesting for photocatalysis, photovoltaics, and thermoelectric applications. ${\mathrm{FeGa}}_{2}{\mathrm{S}}_{4}$ and ${\mathrm{FeIn}}_{2}{\mathrm{S}}_{4}$ monolayers exhibit suitable band gaps and band-edge positions for photocatalytic water splitting and ${\mathrm{CO}}_{2}$ reduction. ${\mathrm{FeGa}}_{2}{\mathrm{Se}}_{4}$ and ${\mathrm{FeIn}}_{2}{\mathrm{Se}}_{4}$ monolayers are promising photocatalysts for hydrogen evolution reaction. The calculated optical absorption spectra indicate that ${\mathrm{FeIn}}_{2}{\mathrm{Te}}_{4}$, ${\mathrm{FeIn}}_{2}{\mathrm{Se}}_{4}$, and ${\mathrm{FeGa}}_{2}{\mathrm{Se}}_{4}$ are promising photovoltaic absorbers with high spectroscopic limited maximum efficiencies of $\ensuremath{\sim}$ 30%, 27%, and 24.5%, respectively, comparable to existing high-performance thin-film absorber materials, such as CdTe $(\ensuremath{\sim}31.5%)$ and ${\mathrm{CuInSe}}_{2}$ $(\ensuremath{\sim}28%)$. The narrow band gaps and relatively low room-temperature lattice thermal conductivity of ${\mathrm{FeGa}}_{2}{\mathrm{Te}}_{4}$ (11.00 W/mK) and ${\mathrm{FeIn}}_{2}{\mathrm{Te}}_{4}$ (8.84 W/mK) monolayers suggest potential applications in thermoelectrics. We thereby propose ${\text{Fe}X}_{2}{Y}_{4}$ ternary layered chalcogenides as promising materials for sustainable energy applications due to their suitable electronic, optical, and thermal properties.

Thickness-dependent spin bistable transitions in single-crystalline molecular 2D material

The advent of two-dimensional (2D) crystals has led to numerous scientific breakthroughs. Conventional 2D systems have in-plane covalent bonds and a weak out-of-plane van-der-Waals bond. Here we report a new type of 2D material composed of discrete magnetic molecules, where anisotropic van-der-Waals interactions bond the molecules into a 2D packing. Through mechanical exfoliation, we can obtain single-crystalline molecular monolayers, which can be readily integrated into other 2D systems. Optical spectroscopy suggests the few-layered molecules preserve the temperature-induced spin-crossover switching observed in the bulk form but show a drastic increase in thermal hysteresis unique to these thin 2D molecule assemblies. The trapping of spin bistability with decreasing layer number can arise from domain wall dynamics in reduced dimensions. Our results establish molecular solids with strong anisotropy of intermolecular interactions as precursors to a new class of 2D materials, affording possibilities to control molecular functionalities through substrate and interlayer interactions.

Construction of strongly coupled few-layer FePSe3-CNT hybrids for high performance potassium-ion storage devices

Given the layered crystal structure and tunable composition, ternary metal phosphorus trichalcogenides (MPCh3) is an attractive anode material for potassium-ion batteries with higher surface activity and mobility than their binary analogues. However, how to improve the electronic conductivity, ion diffusion, and maintain high reversible potassiation/depotassiation processes are the main key issues for the development of MPCh3-based electrodes with high rate and long cycle life. For example, theoretically, FePSe3 has a layered crystal structure with high electronic conductivity and low diffusion barrier, yet their K+-based electrochemical performance not well demonstrated. Here, we report a mechanical exfoliation method to prepare few-layer FePSe3-carbon nanotube (f-FePSe3/CNT) hybrids for use as high-efficiency K+ storage anodes. Electrochemical performance, kinetic analysis, reaction mechanism analysis, and density functional theory calculations show that this strongly coupled 1D-2D hybrids promotes the reaction and diffusion of potassium ions, giving full play to the inherent advantages of materials with different dimensions. Therefore, the f-FePSe3/CNT PIB anodes exhibit high capacity (472.1 mA h g−1, 0.05 A/g), high rate (124.9 mA h g−1, 10 A/g), and cycling stability (>1000 cycles), which significantly exceeds the performance of its binary analogue, here FeSe. In addition, the full cells of potassium-ion battery and hybrid capacitor coupled with f-FePSe3/CNT anodes exhibit good cycling stability (500 cycles) and high energy/power density of 54.7 W h kg−1/5790.8 W kg−1, respectively, revealing its practical applications in a wide range of K+-based storage systems. We believe that this work will provide a universal strategy for effectively activating the K+ electrochemical performance on a wide range of layered materials.

Surfactant assisted exfoliation of high purity graphene in aqueous solution as a nanofluid using kitchen blender: Influence on dispersion, thermal conductivity and rheological properties

In this work, graphene is exfoliated from graphite flakes in distilled water solution containing Tween 80 as surfactant and by using the mechanical exfoliation method through a kitchen blender. The blending time is an important parameter for the size reduction and concentration of graphene exfoliation in a nanofluid. Graphene obtained using a kitchen blender is of high purity having no basal plane defects, as depicted from Raman spectroscopic investigation. The viscosity of graphene nanofluid increases with blending time up to 7 h and thereafter the viscosity decreases. All graphene nanofluid samples prepared using different blending time shows shear thinning behavior by fitting the power law of non-linear viscoelastic measurement data. The linear viscoelastic region measured was in the strain range of 0.01–0.8%. Further, the frequency sweep measurement using a rheometer has indicated the gel-like behavior of graphene nanofluid in a certain frequency range and depends on the concentration of graphene. The use of two-dimensional nanosheets-based nanofluid has attracted the attention of researchers for improving pool boiling properties. Herein, the pool boiling of graphene nanofluid at 7 h of blending shows the 77.4% enhancement in critical heat flux (CHF). This work has established mechanical exfoliation as a process to synthesize the graphene nanofluid and can be used for heat transfer applications.

From Materials to Devices: Graphene toward Practical Applications.

Graphene, as an emerging 2D material, has been playing an important role in flexible electronics since its discovery in 2004. The representative fabrication methods of graphene include mechanical exfoliation, liquid-phase exfoliation, chemical vapor deposition, redox reaction, etc. Based on its excellent mechanical, electrical, thermo-acoustical, optical, and other properties, graphene has made a great progress in the development of mechanical sensors, microphone, sound source, electrophysiological detection, solar cells, synaptic transistors, light-emitting devices, and so on. In different application fields, large-scale, low-cost, high-quality, and excellent performance are important factors that limit the industrialization development of graphene. Therefore, laser scribing technology, roll-to-roll technology is used to reduce the cost. High-quality graphene can be obtained through chemical vapor deposition processes. The performance can be improved through the design of structure of the devices, and the homogeneity and stability of devices can be achieved by mechanized machining means. In total, graphene devices show promising prospect for the practical fields of sports monitoring, health detection, voice recognition, energy, etc. There is a hot issue for industry to create and maintain the market competitiveness of graphene products through increasing its versatility and killer application fields.

A Capillary-Force-Assisted Transfer for Monolayer Transition-Metal-Dichalcogenide Crystals with High Utilization.

The capillary-force-assisted transfer has shown application potential for constructing two-dimensional (2D) electronic and optoelectronic devices for the advantage of free of spin coating the organic compound and etching the substrate. Currently, the transfer mechanism remains obscure. The capillary adhesion mechanism and capillary invasion separation mechanism were proposed independently and rarely discussed in a comprehensive manner. What is more, the integrity and utilization remain to be improved. Here, we developed the capillary-force-assisted transfer method with high utilization and integrity. Uniformity of water transport was improved by introducing water from the sidewall of the small polydimethylsiloxane (PDMS) stamp driven by capillary force. The transfer integrity rate increased, and the location of the complete samples became predictable. The transfer utilization increased as the limited water transportation minimized the impact on the surrounding WS2. The monolayer triangle WS2 crystals from adjacent areas on the sapphire substrate were transferred one after another. Besides, local mechanical exfoliation of the continuous WS2 thin films was demonstrated, implying that the capillary adhesion is strong enough to break the strong in-plane covalent bond and overcome the van der Waals force between WS2 and sapphire substrate. Finally, the water transport model between two surfaces with different hydrophobicity combinations was derived on the basis of the Young-Laplace equation. The analysis of water transport between different interfaces reveals how capillary adhesion and capillary invasion work together to achieve capillary force transfer. This study highlights the potential of the capillary-force-assisted transfer as an efficient technique for fabricating van der Waals structures based on two-dimensional atomic crystals, especially periodic structures.

Wear and corrosion resistance of fluorocarbon/epoxy blended coatings nanofilled by mechanically peeled few-layer fluorinated graphene

In order to prepare novel fluorocarbon resin coatings with outstanding wear and corrosion resistance, this manuscript prepared few-layer fluorinated graphene (FG) nanosheets through a simple mechanical exfoliation method, and nanofilled them into the fluorocarbon resin/epoxy resin (FEVE/EP) blended matrix to prepare FG-FEVE/EP nanocomposite coating. The exfoliated FG was 4–7 layers and contained active O–H and C=C functional groups in addition to abundance of C–F bonds that were identical to those of FEVE. The as-prepared FG layers were freely stacked and possessed low orientation, and exhibited good dispersion and interfacial compatibility in the FEVE/EP matrix. The FG-FEVE/EP nanocomposite coating filled with 0.1 wt% FG (0.1% FG-FEVE/EP) achieved the hardness of 3H and adhesion strength of 7.7 MPa. Compared with pure FEVE/EP coating, the 0.1% FG-FEVE/EP reduced the friction coefficient by 10.6% and 30.7% and the wear rate by 55.1% and 56.1% under dry friction and simulated seawater friction conditions, respectively. In addition, the EIS testing and fitting data yielded that the impedance modulus of the 0.1% FG-FEVE/EP was two orders of magnitude higher than that of the FEVE/EP coating after 15 days of immersion in NaCl solution, and the coating resistance (Rc) still reached 1.6 × 109 Ω cm2, showing excellent corrosion resistance. The excellent wear and corrosion resistance of the FG-FEVE/EP was attributed to the fact that the FG prepared by mechanical peeling was uniformly dispersed and interfacially compatible in the FEVE/EP matrix, which contributed to the excellent interlayer lubrication, stress resistance and barrier corrosion protection of FG.

Electron Irradiation Effects on Single‐Layer MoS2 Obtained by Gold‐Assisted Exfoliation

Mechanical exfoliation assisted by gold is applied to obtain good quality large lateral size single‐layer MoS2. The effects of 2.5 MeV electron irradiation are investigated at room temperature on structural and electronic features by Raman and microluminescence spectroscopy. The exciton recombination emission in the direct bandgap of single‐layer MoS2 is affected during irradiation starting from the minimum explored dose of 1 kGy. At higher doses, Raman bands show no relevant modifications whereas the exciton emission is quenched, suggesting that irradiation‐induced point defects affect exciton dynamics.

Recent Advances of Preparation and Application of Two-Dimension van der Waals Heterostructure

With paramount electrical, optical, catalytic, and other physical and chemical properties, van der Waals heterostructures (vdWHs) have captured increasing attention. vdWHs are two-dimension (2D) heterostructures formed via van der Waals (vdW) force, paving the way for fabricating, understanding, and applications of 2D materials. vdWHs materials of large lattice constant difference can be fabricated together, forming a series of unique 2D materials that cannot form heterostructures earlier. Additionally, vdWHs provide a new platform to study the interlayer interactions between materials, unraveling new physics in the system. Notably, vdWHs embody short-range bonds weaker than covalent and ionic bonds, almost only interactions between nearest particles are considered. Owing to a clear interface, vdW interaction between two different components, devices made by vdWHs can bring amazing physicochemical properties, such as unconventional superconductivity, super capacitance in intercalation 2D structure, etc. Recently, impressive progress has been achieved in the controlled preparation of vdWHs and various applications, which will be summarized in this review. The preparation methods comprise mechanical exfoliation, liquid phase stripping, physical vapor deposition, chemical vapor deposition, and metalorganic chemical vapor deposition. The applications sections will focus on photoelectric devices, logic devices, flexible devices, and piezotronics. Finally, some perspectives in the future on the controlled preparation of vdWHs with desired properties for advanced applications will be discussed.

Influence of the properties of different graphene-based nanomaterials dispersed in polycaprolactone membranes on astrocytic differentiation

Composites of polymer and graphene-based nanomaterials (GBNs) combine easy processing onto porous 3D membrane geometries due to the polymer and cellular differentiation stimuli due to GBNs fillers. Aiming to step forward to the clinical application of polymer/GBNs composites, this study performs a systematic and detailed comparative analysis of the influence of the properties of four different GBNs: (i) graphene oxide obtained from graphite chemically processes (GO); (ii) reduced graphene oxide (rGO); (iii) multilayered graphene produced by mechanical exfoliation method (Gmec); and (iv) low-oxidized graphene via anodic exfoliation (Ganodic); dispersed in polycaprolactone (PCL) porous membranes to induce astrocytic differentiation. PCL/GBN flat membranes were fabricated by phase inversion technique and broadly characterized in morphology and topography, chemical structure, hydrophilicity, protein adsorption, and electrical properties. Cellular assays with rat C6 glioma cells, as model for cell-specific astrocytes, were performed. Remarkably, low GBN loading (0.67 wt%) caused an important difference in the response of the C6 differentiation among PCL/GBN membranes. PCL/rGO and PCL/GO membranes presented the highest biomolecule markers for astrocyte differentiation. Our results pointed to the chemical structural defects in rGO and GO nanomaterials and the protein adsorption mechanisms as the most plausible cause conferring distinctive properties to PCL/GBN membranes for the promotion of astrocytic differentiation. Overall, our systematic comparative study provides generalizable conclusions and new evidences to discern the role of GBNs features for future research on 3D PCL/graphene composite hollow fiber membranes for in vitro neural models.

Effect of Annealing on the Optical Properties of WSe2 Monolayer Obtained by Gold-Assisted Mechanical Exfoliation

Effect of Annealing on the Optical Properties of WSe2 Monolayer Obtained by Gold-Assisted Mechanical Exfoliation

Molybdenum disulfide homogeneous junction diode fabrication and rectification characteristics

The chemical vapor transport method was used in this research to synthesize MoS2 bulk. Through mechanical exfoliation, we limited the thickness of MoS2 flakes from 1 to 3 μm. In order to fabricate a p–n homogeneous junction, we used oxygen plasma treatment to transform the MoS2 characteristics from n-type to p-type to fabricate a p–n homogenous junction and demonstrate the charge neutrality point shift from −80 to +102 V successfully using FET measurement. The MoS2 p–n homogeneous junction diode showed an excellent p-n characteristic curve during the measurements and performed great rectifying behavior with 1–10 Vpp in the half-wave rectification experiment. This work demonstrated that MoS2 flake had great potential for p-n diodes that feature significant p–n characteristics and rectifying behavior.

Liquid-Phase Exfoliation of Bismuth Telluride Iodide (BiTeI): Structural and Optical Properties of Single-/Few-Layer Flakes.

Bismuth telluride halides (BiTeX) are Rashba-type crystals with several potential applications ranging from spintronics and nonlinear optics to energy. Their layered structures and low cleavage energies allow their production in a two-dimensional form, opening the path to miniaturized device concepts. The possibility to exfoliate bulk BiTeX crystals in the liquid represents a useful tool to formulate a large variety of functional inks for large-scale and cost-effective device manufacturing. Nevertheless, the exfoliation of BiTeI by means of mechanical and electrochemical exfoliation proved to be challenging. In this work, we report the first ultrasonication-assisted liquid-phase exfoliation (LPE) of BiTeI crystals. By screening solvents with different surface tension and Hildebrandt parameters, we maximize the exfoliation efficiency by minimizing the Gibbs free energy of the mixture solvent/BiTeI crystal. The most effective solvents for the BiTeI exfoliation have a surface tension close to 28 mN m–1 and a Hildebrandt parameter between 19 and 25 MPa0.5. The morphological, structural, and chemical properties of the LPE-produced single-/few-layer BiTeI flakes (average thickness of ∼3 nm) are evaluated through microscopic and optical characterizations, confirming their crystallinity. Second-harmonic generation measurements confirm the non-centrosymmetric structure of both bulk and exfoliated materials, revealing a large nonlinear optical response of BiTeI flakes due to the presence of strong quantum confinement effects and the absence of typical phase-matching requirements encountered in bulk nonlinear crystals. We estimated a second-order nonlinearity at 0.8 eV of |χ(2)| ∼ 1 nm V–1, which is 10 times larger than in bulk BiTeI crystals and is of the same order of magnitude as in other semiconducting monolayers (e.g., MoS2).

Turbostratic Boron-Carbon-Nitrogen and Boron Nitride by Flash Joule Heating.

Turbostratic layers in 2D materials have an interlayer misalignment. The lack of alignment expands the intrinsic interlayer distances and weakens the optical and electronic interactions between adjacent layers. This introduces properties distinct from those structures with well-aligned lattices and strong coupling interactions. However, direct and rapid synthesis of turbostratic materials remains a challenge owing to their thermodynamically metastable properties. Here, a flash Joule heating (FJH) method to achieve bulk synthesis of boron-carbon-nitrogen ternary compounds with turbostratic structures by a kinetically controlled ultrafast cooling process that takes place within milliseconds (103 to104 K s-1 ) is reported. Theoretical calculations support the existence of turbostratic structures and provide estimates of the energy barriers with respect to conversion into the corresponding well-aligned counterparts. When using non-carbon conductive additives, a direct synthesis of boron nitride is possible. The turbostratic nature facilitates mechanical exfoliation and more stable dispersions. Accordingly, the addition of flash products to a poly(vinyl alcohol) nanocomposite film coating a copper surface greatly improves the copper's resistance to corrosion in 0.5 m sulfuric acid or 3.5 wt% saline solution. FJH allows the use of bulk materials as reactants and provides a rapid approach to large quantities of the hitherto hard-to-access turbostratic materials.

Large-Scale Multilayer MoS2 Nanosheets Grown by Atomic Layer Deposition for Sensitive Photodetectors

Two-dimensional (2D) transition-metal dichalcogenides have promising optoelectronic applications. However, most reported MoS2-based photodetectors are based on MoS2 flakes, dozens of microns in size, prepared by mechanical exfoliation or chemical vapor deposition, which limits their practical applications. In this study, a wafer-scale, continuous, and uniform multilayer MoS2 nanosheet was grown on an insulating single-crystal sapphire substrate by atomic layer deposition (ALD), with MoCl5 and hexamethyldisilathiane as precursors, because of the smaller lattice mismatch between crystalline sapphire and hexagonally arranged MoS2 compared with polycrystalline substrates. Then, large-area MoS2 nanosheets with high uniformity and homogeneity were obtained on Al2O3/Si and SiO2/Si substrates via large-scale peeling and vacuum stacking transfer. Subsequently, MoS2 photodetectors were fabricated on both substrates with ALD-MoS2 as a photosensitive channel material and high-k Al2O3 or SiO2 as a dielectric layer, which resulted in gate control with relatively low and high voltages, respectively. A high photoresponsivity (R) of 577.4 A W–1 was achieved for the MoS2 photodetector fabricated on the Al2O3/Si substrate at an illumination power density (Pin) of 5.01 μW cm–2 (λ = 500 nm). In addition, the MoS2 photodetectors exhibited a light response over a wide wavelength range (400–700 nm). Moreover, a MoS2 photodetector array containing 4 × 3 photodetectors with a uniform photoelectric response was demonstrated. These results will facilitate future applications of 2D materials in large-scale, high-performance photoelectronic devices and circuits.

Two-dimensional V-shaped PdI2: Auxetic semiconductor with ultralow lattice thermal conductivity and ultrafast alkali ion mobility

Using first-principles calculations, we predicted a multifunctional two-dimensional (2D) semiconductor PdI2 from its van der Walls layered bulk. Monolayer or multilayer PdI2 obtained by mechanical exfoliation exhibit high stability as freestanding 2D materials. Originating from its special corner-shared quasi-plane crystal structure, an amazing in-plane negative Poisson's ratio in monolayer PdI2 has been confirmed. The layer dependent electronic and thermal properties have also been discussed in detail. Encouragingly, it has been found that strong phonon scattering which is induced by low symmetry will further reduce the lattice thermal conductivity in monolayer PdI2 at room temperature. Besides, the monolayer PdI2 is seen as an excellent platform for alkali ions migration with ultralow diffusion barriers, benefiting from its distinctive V-shaped crystal structure. Considering these interesting findings, we believe the 2D PdI2 will be a promising candidate for future's low dimensional electron devices.

(Invited) Synthesis and Application of One-Dimensional Van Der Waals Heterostrucures Based on Single-Walled Carbon Nanotubes

A typical one-dimensional (1D) van der Waals (vdW) heterostructure consists of single-walled carbon nanotubes (SWCNT), boron nitride nanotube (BNNT), and molybdenum disulfide nanotube (MoS2NT), can be grown coaxially by successive chemical vapor deposition (CVD) steps [1]. The coaxially nested structure based on SWCNTs can expand the board application possibilities of 1D vdW heterostructures [2]. Semiconductor SWCNT wrapped with BNNT can be regarded as the ideal building blocks of field-effect transistors (FET) [3]. We have demonstrated the radial semiconductor–insulator–semiconductor (S-I-S) tunneling heterojunction diode by using a micrometer long 1D vdW heterostructure SWCNT@BNNT@MoS2NT [4]. By comparing optical properties of films of BNNT@MoS2NT and SWCNT@BNNT@MoS2NT, we found strong photoluminescence (PL) from monolayer MoS2NT and quenching of PL by coupling to SWCNT through thin BNNT [5]. The remarkable population of free charges and inter-tube excitons are demonstrated by ultrafast optical spectroscopy [6]. The inter-tube excition is regarded as the inter-layer excition for 2D heterostructures. Precise control of the chemical vapor deposition (CVD) process of each layer is essential since the mechanical exfoliation & stacking technique for 2D counterpart is not possible for 1D vdW heterostructures. The surprisingly sharp edge of BNNT grown on SWCNT is the signature of preference of nitrogen terminated zig-zag edge of h-BN common to 2D counterpart [7]. The next challenge is the CVD growth of various transition metal dichalcogenides on BNNTs. In addition to MoS2 nanotubes [1], we will discuss the growth control of WS2 nanotubes and NbS2 nanotubes. A general strategy we can tune the CVD condition from 2D flake to 1D tube is proposed [8]. Part of this work was supported by JSPS KAKENHI Grant Number JP20H00220, and by JST, CREST Grant Number JPMJCR20B5, Japan.