Direct Four-Probe Measurement of Grain-Boundary Resistivity and Mobility in Millimeter-Sized Graphene

Controlled Synthesis and Accurate Doping of Wafer-Scale 2D Semiconducting Transition Metal Dichalcogenides

2D semiconducting transition metal dichalcogenide (TMDCs) possess atomically thin thickness, a dangling-bond-free surface, flexible band structure, and silicon-compatible feature, making them one of the most promising channels for constructing state-of-the-art field-effect transistors in the post-Moore's era. However, the existing 2D semiconducting TMDCs fall short of meeting the industry criteria for practical applications in electronics due to their small domain size and the lack of an effective approach to modulate intrinsic physical properties. Therefore, it is crucial to prepare and dope 2D semiconducting TMDCs single crystals with wafer size. In this review, the up-to-date progress regarding the wafer-scale growth of 2D semiconducting TMDC polycrystalline and single-crystal films is systematically summarized. The domain orientation control of 2D TMDCs and the seamless stitching of unidirectionally aligned 2D islands by means of substrate design are proposed. In addition, the accurate and uniform doping of 2D semiconducting TMDCs and the effect on electronic device performances are also discussed. Finally, the dominating challenges pertaining to the enhancement of the electronic device performances of TMDCs are emphasized, and further development directions are put forward. This review provides a systematic and in-depth summary of high-performance device applications of 2D semiconducting TMDCs.

Retrieving Grain Boundaries in 2D Materials

The coalescence of randomly distributed grains with different crystallographic orientations can result in pervasive grain boundaries (GBs) in 2D materials during their chemical synthesis. GBs not only are the inherent structural imperfection that causes influential impacts on structures and properties of 2D materials, but also have emerged as a platform for exploring unusual physics and functionalities stemming from dramatic changes in local atomic organization and even chemical makeup. Here, recent advances in studying the formation mechanism, atomic structures, and functional properties of GBs in a range of 2D materials are reviewed. By analyzing the growth mechanism and the competition between far-field strain and local chemical energies of dislocation cores, a complete understanding of the rich GB morphologies as well as their dependence on lattice misorientations and chemical compositions is presented. Mechanical, electronic, and chemical properties tied to GBs in different materials are then discussed, towards raising the concept of using GBs as a robust atomic-scale scaffold for realizing tailored functionalities, such as magnetism, luminescence, and catalysis. Finally, the future opportunities in retrieving GBs for making functional devices and the major challenges in the controlled formation of GB structures for designed applications are commented.

Air-stable van der Waals PtTe2 conductors with high current-carrying capacity and strong spin-orbit interaction

High-performance van der Waals (vdW) integrated electronics and spintronics require reliable current-carrying capacity. However, it is challenging to achieve high current density and air-stable performance using vdW metals owing to the fast electrical breakdown triggered by defects or oxidation. Here, we report that spin-orbit interacted synthetic PtTe₂ layers exhibit significant electrical reliability and robustness in ambient air. The 4-nm-thick PtTe₂ synthesized at a low temperature (∼400°C) shows intrinsic metallic transport behavior and a weak antilocalization effect attributed to the strong spin-orbit scattering. Remarkably, PtTe₂ sustains a high current density approaching ≈31.5 MA cm⁻², which is the highest value among electrical interconnect candidates under oxygen exposure. Electrical failure is caused by the Joule heating of PtTe₂ rather than defect-induced electromigration, which was achievable by the native TeO_x passivation. The high-quality growth of PtTe₂ and the investigation of its transport behaviors lay out essential foundations for the development of emerging vdW spin-orbitronics.

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The synthesized PtTe₂ had a self-passivated surface under exposure to air

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Magnetoconductance study proved the realization of a 2D confined quantum system

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PtTe₂ sustained a remarkably high current density (∼31.5 MA cm⁻²) under air atmosphere

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The native TeO_x passivation retarded the defect-induced electromigration of PtTe₂

Construction and physical properties of low-dimensional structures for nanoscale electronic devices

Over the past decades, construction of nanoscale electronic devices with novel functionalities based on low-dimensional structures, such as single molecules and two-dimensional (2D) materials, has been rapidly developed. To investigate their intrinsic properties for versatile functionalities of nanoscale electronic devices, it is crucial to precisely control the structures and understand the physical properties of low-dimensional structures at the single atomic level. In this review, we provide a comprehensive overview of the construction of nanoelectronic devices based on single molecules and 2D materials and the investigation of their physical properties. For single molecules, we focus on the construction of single-molecule devices, such as molecular motors and molecular switches, by precisely controlling their self-assembled structures on metal substrates and charge transport properties. For 2D materials, we emphasize their spin-related electrical transport properties for spintronic device applications and the role that interfaces among 2D semiconductors, contact electrodes, and dielectric substrates play in the electrical performance of electronic, optoelectronic, and memory devices. Finally, we discuss the future research direction in this field, where we can expect a scientific breakthrough.

Investigation of a Highly Sensitive Surface-Enhanced Raman Scattering Substrate Formed by a Three-Dimensional/Two-Dimensional Graphene/Germanium Heterostructure

Three-dimensional graphene (3D-graphene) is used in surface-enhanced Raman spectroscopy (SERS) because of its plasmonic nanoresonator structure and good ability to interact with light. However, a thin (3-5 nm) layer of amorphous carbon (AC) inevitably appears as a template layer between the 3D-graphene and object substrate when the 3D-graphene layer is synthesized, weakening the enhancement factor. Herein, two-dimensional graphene (2D-graphene) is employed as a template layer to directly synthesize 3D-graphene on a germanium (Ge) substrate via plasma-assisted chemical vapor deposition, bypassing the formation of an AC layer. The interaction and photoinduced charge transfer ability of the 3D-graphene/Ge heterojunction with incident light are improved. Moreover, the high density of electronic states close to the Fermi level of the heterojunction induces the adsorbed probe molecules to efficiently couple to the 3D-graphene-based SERS substrate. Our experimental results imply that the lowest concentrations of rhodamine 6G and rhodamine B that can be detected on the 3D/2D-graphene/Ge SERS substrate correspond to 10^-10 M; for methylene blue, it is 10^-8 M. The detection limits of the 3D/2D-graphene/Ge SERS substrate with respect to 3-hydroxytyramine hydrochloride and melamine (in milk) are both less than 1 ppm. This work may provide a viable and convenient alternative method for preparing 3D-graphene SERS substrates. It also constitutes a new approach to developing SERS substrates with remarkable performance levels.

Simultaneous Extraction of the Grain Size, Single-Crystalline Grain Sheet Resistance, and Grain Boundary Resistivity of Polycrystalline Monolayer Graphene

The electrical properties of polycrystalline graphene grown by chemical vapor deposition (CVD) are determined by grain-related parameters-average grain size, single-crystalline grain sheet resistance, and grain boundary (GB) resistivity. However, extracting these parameters still remains challenging because of the difficulty in observing graphene GBs and decoupling the grain sheet resistance and GB resistivity. In this work, we developed an electrical characterization method that can extract the average grain size, single-crystalline grain sheet resistance, and GB resistivity simultaneously. We observed that the material property, graphene sheet resistance, could depend on the device dimension and developed an analytical resistance model based on the cumulative distribution function of the gamma distribution, explaining the effect of the GB density and distribution in the graphene channel. We applied this model to CVD-grown monolayer graphene by characterizing transmission-line model patterns and simultaneously extracted the average grain size (~5.95 μm), single-crystalline grain sheet resistance (~321 Ω/sq), and GB resistivity (~18.16 kΩ-μm) of the CVD-graphene layer. The extracted values agreed well with those obtained from scanning electron microscopy images of ultraviolet/ozone-treated GBs and the electrical characterization of graphene devices with sub-micrometer channel lengths.

Nanoscale tip positioning with a multi-tip scanning tunneling microscope using topography images

Multi-tip scanning tunneling microscopy (STM) is a powerful method to perform charge transport measurements at the nanoscale. With four STM tips positioned on the surface of a sample, four-point resistance measurements can be performed in dedicated geometric configurations. Here, we present an alternative to the most often used scanning electron microscope imaging to infer the corresponding tip positions. After the initial coarse positioning is monitored by an optical microscope, STM scanning itself is used to determine the inter-tip distances. A large STM overview scan serves as a reference map. Recognition of the same topographic features in the reference map and in small scale images with the individual tips allows us to identify the tip positions with an accuracy of about 20 nm for a typical tip spacing of ∼1μm. In order to correct for effects such as the non-linearity of the deflection, creep, and hysteresis of the piezoelectric elements of the STM, a careful calibration has to be performed.

Quantum Hall Effect across Graphene Grain Boundary

Charge carrier scattering at grain boundaries (GBs) in a chemical vapor deposition (CVD) graphene reduces the carrier mobility and degrades the performance of the graphene device, which is expected to affect the quantum Hall effect (QHE). This study investigated the influence of individual GBs on the QH state at different stitching angles of the GB in a monolayer CVD graphene. The measured voltage probes of the equipotential line in the QH state showed that the longitudinal resistance (R_xx) was affected by the scattering of the GB only in the low carrier concentration region, and the standard QHE of a monolayer graphene was observed regardless of the stitching angle of the GB. In addition, a controlled device with an added metal bar placed in the middle of the Hall bar configuration was introduced. Despite the fact that the equipotential lines in the controlled device were broken by the additional metal bar, only the R_xx was affected by nonzero resistance, whereas the Hall resistance (R_xy) revealed the well-quantized plateaus in the QH state. Thus, our study clarifies the effect of individual GBs on the QH states of graphenes.

Development of a multi-functional multi-probe atomic force microscope system with optical beam deflection method

We developed a multi-probe atomic force microscope (MP-AFM) system with up to four probes and realized various functions such as topography mapping, probing electrical property, and local temperature measurement. Each probe mounted on the corresponding probe scanner was controlled independently, and the system employed the optical beam deflection method to measure the deflection of each cantilever. A high-performance MP-AFM system with a compact optical design and rigid actuators was finally established. We demonstrated AFM high-resolution imaging in air and performed four-probe imaging in parallel and multi-functional characterization with the MP-AFM system.

A time-shared switching scheme designed for multi-probe scanning tunneling microscope

We report the design of a time-shared switching scheme, aiming to realize the manipulation and working modes (imaging mode and transport measurement mode) switching between multiple scanning tunneling microscope (STM) probes one by one with a shared STM control system (STM CS) and an electrical transport characterization system. This scheme comprises three types of switch units, switchable preamplifiers (SWPAs), high voltage amplifiers, and a main control unit. Together with the home-made software kit providing the graphical user interface, this scheme achieves a seamless switching process between different STM probes. Compared with the conventional scheme using multiple independent STM CSs, this scheme possesses more compatibility, flexibility, and expansibility for lower cost. The overall architecture and technique issues are discussed in detail. The performances of the system are demonstrated, including the millimeter scale moving range and atomic scale resolution of a single STM probe, safely approached multiple STM probes beyond the resolution of the optical microscope (1.1 µm), qualified STM imaging, and accurate electrical transport characterization. The combinational technique of imaging and transport characterization is also shown, which is supported by SWPA switches with ultra-high open circuit resistance (909 TΩ). These successful experiments prove the effectiveness and the usefulness of the scheme. In addition, the scheme can be easily upgraded with more different functions and numbers of probe arrays, thus opening a new way to build an extremely integrated and high throughput characterization platform.

Growth and Grain Boundaries in 2D Materials

Grain boundaries (GBs) are a kind of lattice imperfection widely existing in two-dimensional materials, playing a critical role in materials' properties and device performance. Related key issues in this area have drawn much attention and are still under intense investigation. These issues include the characterization of GBs at different length scales, the dynamic formation of GBs during the synthesis, the manipulation of the configuration and density of GBs for specific material functionality, and the understanding of structure-property relationships and device applications. This review will provide a general introduction of progress in this field. Several techniques for characterizing GBs, such as direct imaging by high-resolution transmission electron microscopy, visualization techniques of GBs by optical microscopy, plasmon propagation, or second harmonic generation, are presented. To understand the dynamic formation process of GBs during the growth, a general geometric approach and theoretical consideration are reviewed. Moreover, strategies controlling the density of GBs for GB-free materials or materials with tunable GB patterns are summarized, and the effects of GBs on materials' properties are discussed. Finally, challenges and outlook are provided.

Wrinkle-induced highly conductive channels in graphene on SiO2/Si substrates

A graphene wrinkle is a quasi-one-dimensional structure and can alter the intrinsic physical and chemical activity, modify the band structure and introduce transport anisotropy in graphene thin films. However, the quasi-one-dimensional electrical transport contribution of wrinkles to the whole graphene films compared to that of the two-dimensional flat graphene nearby has still been elusive. Here, we report measurements of relatively high conductivity in micrometer-wide graphene wrinkles on SiO2/Si substrates using an ultrahigh vacuum (UHV) four-probe scanning tunneling microscope. Combining the experimental results with resistor network simulations, the wrinkle conductivity at the charge neutrality point shows a much higher conductivity up to ∼33.6 times compared to that of the flat monolayer region. The high conductivity can be attributed not only to the wrinkled multilayer structure but also to the large strain gradients located mainly in the boundary area. This method can also be extended to evaluate the electrical-transport properties of wrinkled structures in other two-dimensional materials.

Photovoltaic Effect Related to Methylammonium Cation Orientation and Carrier Transport Properties in High-Performance Perovskite Solar Cells

Solar cells based on organic-inorganic hybrid halide perovskites (OIHHPs) have been widely studied because of their increasing power conversion efficiency. Extensive research has been conducted in electrical and optical properties and device fabrication. However, in terms of material science, the photovoltaic effects of OIHHP are still not well understood. Here, we theoretically investigate the photovoltaic phenomena of MAPbI₃ (MA = CH₃NH₃⁺) under standard AM 1.5G sunlight illumination, considering the MA cation orientation, light incident angle, polarization, and photon energy, using Keldysh nonequilibrium Green's function formalism combined with density functional theory calculations. It is found that the short-circuit current density J_sc has a maximum value of 383.149 A/m² when the MA orientation is parallel to the transport direction, whereas it is negligible when the MA orientation is orthogonal to the transport direction. In addition, full consideration is also given to the direction of incidence of sunlight and its polarization state. Nevertheless, of all factors considered, MA orientation plays the decisive role, for J_sc still has a respectable value of 364.112 A/m² even for a 90° sunlight incident angle relative to the transport direction, so long as the MAs are aligned in the transport direction. The increase in the photocurrent is attributed to an increase in the transmission coefficient of low-energy holes, as well as improvement of the velocities and mobilities of electrons and holes in the MAPbI₃-based device with [001] MA orientation. The results suggest that during the designing of high-performance OIHHP-based solar cells and photoelectronic devices, the crystal orientation and MA cation orientation relative to the transport direction should be carefully considered as they directly affect carrier transport properties.

Laser-Directed Assembly of Nanorods of 2D Materials

Herein, the previously unrealized ability to grow nanorods and nanotubes of 2D materials using femtosecond laser irradiation is demonstrated. In as short as 20 min, nanorods of tungsten disulfide, molybdenum disulfide, graphene, and boron nitride are grown in solutions. The technique fragments nanoparticles of the 2D materials from bulk flakes and leverages molecular scale alignment by nonresonant intense laser pulses to direct their assembly into nanorods up to several micrometers in length. The laser treatment process is found to induce phase transformations in some of the materials, and also results in the modification of the nanorods with functional groups from the solvent atoms. Notably, the WS₂ nanoparticles, which are ablated from semiconducting 2H WS₂ crystallographic phase flakes, reassemble into nanorods consisting of the 1T metallic phase. Due to this transition, and the 1D nature of the fabricated nanorods, the WS₂ nanorods display substantial improvements in electrical conductivity and optical transparency when employed as transparent conductors.

Adlayer-Free Large-Area Single Crystal Graphene Grown on a Cu(111) Foil

To date, thousands of publications have reported chemical vapor deposition growth of "single layer" graphene, but none of them has described truly single layer graphene over large area because a fraction of the area has adlayers. It is found that the amount of subsurface carbon (leading to additional nuclei) in Cu foils directly correlates with the extent of adlayer growth. Annealing in hydrogen gas atmosphere depletes the subsurface carbon in the Cu foil. Adlayer-free single crystal and polycrystalline single layer graphene films are grown on Cu(111) and polycrystalline Cu foils containing no subsurface carbon, respectively. This single crystal graphene contains parallel, centimeter-long ≈100 nm wide "folds," separated by 20 to 50 µm, while folds (and wrinkles) are distributed quasi-randomly in the polycrystalline graphene film. High-performance field-effect transistors are readily fabricated in the large regions between adjacent parallel folds in the adlayer-free single crystal graphene film.

Effects of buried grain boundaries in multilayer MoS2

Two-dimensional transition metal dichalcogenides have been the focus of intense research for their potential application in novel electronic and optoelectronic devices. However, growth of large area two-dimensional transition metal dichalcogenides invariably leads to the formation of grain boundaries that can significantly degrade electrical transport by forming large electrostatic barriers. It is therefore critical to understand their effect on the electronic properties of two-dimensional semiconductors. Using MoS₂ as an example material, we are able to probe grain boundaries in top and buried layers using conductive atomic force microscopy. We find that the electrical radius of the grain boundary extends approximately 2 nm from the core into the pristine material. The presence of grain boundaries affects electrical conductivity not just within its own layer, but also in the surrounding layers. Therefore, electrical grain size is always smaller than the physical size, and decreases with increasing thickness of the MoS₂. These results signify that the number of layers in synthetically grown 2D materials must ideally be limited for device applications.

Atomic-Scale Structural Modification of 2D Materials

2D materials have attracted much attention since the discovery of graphene in 2004. Due to their unique electrical, optical, and magnetic properties, they have potential for various applications such as electronics and optoelectronics. Owing to thermal motion and lattice growth kinetics, different atomic-scale structures (ASSs) can originate from natural or intentional regulation of 2D material atomic configurations. The transformations of ASSs can result in the variation of the charge density, electronic density of state and lattice symmetry so that the property tuning of 2D materials can be achieved and the functional devices can be constructed. Here, several kinds of ASSs of 2D materials are introduced, including grain boundaries, atomic defects, edge structures, and stacking arrangements. The design strategies of these structures are also summarized, especially for atomic defects and edge structures. Moreover, toward multifunctional integration of applications, the modulation of electrical, optical, and magnetic properties based on atomic-scale structural modification are presented. Finally, challenges and outlooks are featured in the aspects of controllable structure design and accurate property tuning for 2D materials with ASSs. This work may promote research on the atomic-scale structural modification of 2D materials toward functional applications.

Selective detection of water pollutants using a differential aptamer-based graphene biosensor

Graphene field-effect transistor (GFET) sensors are an attractive analytical tool for the detection of water pollutants. Unfortunately, this application has been hindered by the sensitivity of such sensors to nonspecific disturbances caused by variations of environmental conditions. Incorporation of differential designs is a logical choice to address this issue, but this has been difficult for GFET sensors due to the impact of fabrication processes and material properties. This paper presents a differential GFET affinity sensor for the selective detection of water pollutants in the presence of nonspecific disturbances. This differential design allows for minimization of the effects of variations of environmental conditions on the measurement accuracy. In addition, to mitigate the impact of the fabrication process and material property variations, we introduce a compensation scheme for the individual sensing units of the sensor, so that such variations are accounted for in the compensation-based differential sensing method. We test the use of this differential sensor for the selective detection of the water pollutant 17β-estradiol in buffer and tap water. Consistent detection results can be obtained with and without interferences of pH variations, and in tap water where unknown interferences are present. These results demonstrate that the differential graphene affinity sensor is capable of effectively mitigating the effects of nonspecific interferences to enable selective water pollutant detection for water quality monitoring.

Invited Review Article: Multi-tip scanning tunneling microscopy: Experimental techniques and data analysis

In scanning tunneling microscopy, we witness in recent years a paradigm shift from "just imaging" to detailed spectroscopic measurements at the nanoscale and multi-tip scanning tunneling microscope (STM) is a technique following this trend. It is capable of performing nanoscale charge transport measurements like a "multimeter at the nanoscale." Distance-dependent four-point measurements, the acquisition of nanoscale potential maps at current carrying nanostructures and surfaces, as well as the acquisition of I - V curves of nanoelectronic devices are examples of the capabilities of the multi-tip STM technique. In this review, we focus on two aspects: How to perform the multi-tip STM measurements and how to analyze the acquired data in order to gain insight into nanoscale charge transport processes for a variety of samples. We further discuss specifics of the electronics for multi-tip STM and the properties of tips for multi-tip STM, and present methods for a tip approach to nanostructures on insulating substrates. We introduce methods on how to extract the conductivity/resistivity for mixed 2D/3D systems from four-point measurements, how to measure the conductivity of 2D sheets, and how to introduce scanning tunneling potentiometry measurements with a multi-tip setup. For the example of multi-tip measurements at freestanding vapor liquid solid grown nanowires, we discuss contact resistances as well as the influence of the presence of the probing tips on the four point measurements.

Anisotropic Strain Relaxation of Graphene by Corrugation on Copper Crystal Surfaces

Corrugation is a ubiquitous phenomenon for graphene grown on metal substrates by chemical vapor deposition, which greatly affects the electrical, mechanical, and chemical properties. Recent years have witnessed great progress in controlled growth of large graphene single crystals; however, the issue of surface roughness is far from being addressed. Here, the corrugation at the interface of copper (Cu) and graphene, including Cu step bunches (CuSB) and graphene wrinkles, are investigated and ascribed to the anisotropic strain relaxation. It is found that the corrugation is strongly dependent on Cu crystallographic orientations, specifically, the packed density and anisotropic atomic configuration. Dense Cu step bunches are prone to form on loose packed faces due to the instability of surface dynamics. On an anisotropic Cu crystal surface, Cu step bunches and graphene wrinkles are formed in two perpendicular directions to release the anisotropic interfacial stress, as revealed by morphology imaging and vibrational analysis. Cu(111) is a suitable crystal face for growth of ultraflat graphene with roughness as low as 0.20 nm. It is believed the findings will contribute to clarifying the interplay between graphene and Cu crystal faces, and reducing surface roughness of graphene by engineering the crystallographic orientation of Cu substrates.