Planar and van der Waals heterostructures for vertical tunnelling single electron transistors

Coulomb blockade and Coulomb staircases in CoBi nanoislands on SrTiO3(001)

We successfully fabricated two-dimensional metallic CoBi nanoislands on SrTiO3(001) substrate by molecular beam epitaxy, and systematically investigated their electronic structures by scanning tunneling microscopy and spectroscopyin situat 4.2 K. Coulomb blockade and Coulomb staircases with discrete and well-separated levels are observed for the individual nanoisland, which is attributed to single-electron tunneling via two tunnel junction barriers. They are in excellent agreement with the simulations based on orthodox theory. Furthermore, we demonstrated that the Coulomb blockade becomes weaker with increasing temperature and almost disappears at ∼22 K in our variable temperature experiment, and its full-width at half-maximum of dI/dVpeaks with temperature is ∼6 mV. Our results provide a new platform for designing single-electron transistors that have potential applications in future microelectronics.

2D Materials in the Display Industry: Status and Prospects

With advances in flexible electronics, innovative foldable, rollable, and stretchable displays have been developed to maintain their performance under various deformations. These flexible devices can develop more innovative designs than conventional devices due to their light weight, high space efficiency, and practical convenience. However, developing flexible devices requires material innovation because the devices must be flexible and exhibit desirable electrical insulating/semiconducting/metallic properties. Recently, emerging 2D materials such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have attracted considerable research attention because of their outstanding electrical, optical, and mechanical properties, which are ideal for flexible electronics. The recent progress and challenges of 2D material growth and display applications are reviewed and perspectives for exploring 2D materials for display applications are discussed.

2D Layered Materials for Fast-Charging Lithium-Ion Battery Anodes

The development of electric vehicles has received worldwide attention in the background of reducing carbon emissions, wherein lithium-ion batteries (LIBs) become the primary energy supply systems. However, commercial graphite-based anodes in LIBs currently confront significant difficulty in enduring ultrahigh power input due to the slow Li+ transport rate and the low intercalation potential. This will, in turn, cause dramatic capacity decay and lithium plating. The 2D layered materials (2DLMs) recently emerge as new fast-charging anodes and hold huge promise for resolving the problems owing to the synergistic effect of a lower Li+ diffusion barrier, a proper Li+ intercalation potential, and a higher theoretical specific capacity with using them. In this review, the background and fundamentals of fast-charging for LIBs are first introduced. Then the research progress recently made for 2DLMs used for fast-charging anodes are elaborated and discussed. Some emerging research directions in this field with a short outlook on future studies are further discussed.

NBOH Site-Activated Graphene Quantum Dots for Boosting Electrochemical Hydrogen Peroxide Production

Carbon materials are considered promising 2/4 e^- oxygen reduction reaction (ORR) electrocatalysts for synthesizing H₂ O₂ /H₂ O via regulating heteroatom dopants and functionalization. Here, various doped and functionalized graphene quantum dots (GQDs) are designed to reveal the crucial active sites of carbon materials for ORR to produce H₂ O₂ . Density functional theory (DFT) calculations predict that the edge structure involving edge N, B dopant pairs and further OH functionalization to the B (NBOH) is an active center for 2e^- ORR. To verify the above predication, GQDs with an enriched density of NBOH (NBO-GQDs) are designed and synthesized by the hydrothermal reaction of NH₂ edge-functionalized GQDs with H₃ BO₃ forming six-member heterocycle containing the NBOH structure. When dispersed on conductive carbon substrates, the NBO-GQDs show H₂ O₂ selectivity of over 90% at 0.7 -0.8 V versus reversible hydrogen electrode in the alkaline solution in a rotating ring-disk electrode setup. The selectivity retains 90% of the initial value after 12 h stability test. In a flow cell setup, the H₂ O₂ production rate is up to 709 mmol g_catalyst ^-1  h^-1 , superior to most reported carbon- and metal-based electrocatalysts. This work provides molecular insight into the design and formulation of highly efficient carbon-based catalysts for sustainable H₂ O₂ production.

Graphene quantum dots: A comprehensive overview

Because of their prospective applications and exceptional features, graphene quantum dots (GQDs) have gotten a lot of recognition as a new class of fluorescent carbon materials. One of the carbon family’s newest superstars is the GQD. Due to its exceptional optoelectrical qualities, it has sparked a lot of curiosity since its debut in 2008. Two of the most important traits are a band gap that is not zero, biocompatibility, and highly changeable characteristics. GQDs have several important characteristics. GQDs have shown potential in a variety of fields, for instance, catalysis, sensing, energy devices, drug delivery, bioimaging, photothermal, and photodynamic therapy. Because this area constantly evolves, it is vital to recognize emerging GQD concerns in the current breakthroughs, primarily since some specific uses and developments in the case of GQDs synthesis have not been thoroughly investigated through previous studies. The current results in the properties, synthesis, as well as benefits of GQDs are discussed in this review study. As per the findings of this research, the GQD’s future investigation is boundless, mainly if the approaching investigation focuses on purifying simplicity and environmentally friendly synthesis, as well as boosting photoluminescence quantum output and manufacturing output of GQDs.

Hybrid G/BN@2H-MoS2 Nanomaterial Composites: Structural, Electronic and Molecular Adsorption Properties

Hybrid structures often possess superior properties to those of their component materials. This arises from changes in the structural or physical properties of the new materials. Here, we investigate the structural, electronic, and gas-adsorption properties of hybrid structures made from graphene/hexagonal boron nitride and 2H-molybdenum disulfide (G/BN@MoS₂) monolayers. We consider hybrid systems in which the G/BN patch is at the Mo plane (model I) and the S plane (model II). We find that the implanted hexagon of G or BN in MoS₂ alters its electronic properties: G@MoS₂ (I,II) are metallic, while BN@MoS₂ (I) is an n-type conducting and BN@MoS₂ (II) is semiconducting. We study the molecular adsorption of some diatomic gases (H₂, OH, N₂, NO, CO), triatomic gases (CO₂, NO₂, H₂S, SO₂), and polyatomic gases (COOH, CH₄, and NH₃) on our hybrid structures while considering multiple initial adsorption sites. Our results suggest that the hybrid systems may be suitable materials for some applications: G@MOS₂ (I) for oxygen reduction reactions, BN@MoS₂ (I,II) for NH₃-based hydrogen production, and G@MoS₂ (I) and BN@MoS₂ (I,II) for filtration of No, Co, SO₂, H₂S, and NO₂.

Patch formation on diblock copolymer micelles confined in templates for inducing patch orientation and cyclic colloidal molecules

HYPOTHESIS

Chemically or physically distinct patches can be induced on the micelles of amphiphilic block copolymers, which facilitate directional binding for the creation of hierarchical structures. Hence, control over the direction of patches on the micelles is a crucial factor to attain the directionality on the interactions between the micelles, particularly for generating colloidal molecules mimicking the symmetry of molecular structures. We hypothesized that direction and combination of the patches could be controlled by physical confinement of the micelles.

EXPERIMENTS

We first confined spherical micelles of diblock copolymers in topographic templates fabricated from nanopatterns of block copolymers by adjusting the coating conditions. Then, patch formation was conducted on the confined micelles by exposing them with a core-favorable solvent. Microscopic techniques of SEM, TEM, and AFM were employed to investigate directions of patches and structures of combined micelles in the template.

FINDINGS

The orientation of the patches on the micelles was guided by the physical confinement of the micelles in linear trenches. In addition, by confining the micelles in a circular hole, we obtained a specific polygon arrangement of the micelles depending on the number of micelles in the hole, which enabled the formation of cyclic colloidal molecules consisting of micelles.

White-emitting film of diblock copolymer micelles with perovskite nanocrystals

Perovskite nanocrystals are synthesized in diblock copolymer micelles to improve their processability and stability. The copolymer micelle approach allows fluorescence from a stretchable or flexible substrate by coating processes, and stable emission in water by protecting the nanocrystals in the micelles. Fluorescent films in three primary colors of blue, green, and red are also produced with the assistance of anion exchange reactions for perovskite nanocrystals in the micelles. Then, by stacking films in three primary colors, we are able to produce a white-emitting film of copolymer micelles containing only perovskite nanocrystals without the support of other kinds of emissive materials.

A white-emitting film is produced by stacking films in three primary colors of diblock copolymer micelles containing perovskite nanocrystals without the support of other kinds of emissive materials.

Nanomaterials for Quantum Information Science and Engineering

Quantum information science and engineering (QISE)-which entails the use of quantum mechanical states for information processing, communications, and sensing-and the area of nanoscience and nanotechnology have dominated condensed matter physics and materials science research in the 21st century. Solid-state devices for QISE have, to this point, predominantly been designed with bulk materials as their constituents. This review considers how nanomaterials (i.e., materials with intrinsic quantum confinement) may offer inherent advantages over conventional materials for QISE. The materials challenges for specific types of qubits, along with how emerging nanomaterials may overcome these challenges, are identified. Challenges for and progress toward nanomaterials-based quantum devices are condidered. The overall aim of the review is to help close the gap between the nanotechnology and quantum information communities and inspire research that will lead to next-generation quantum devices for scalable and practical quantum applications.

Synthesis, Applications, and Prospects of Graphene Quantum Dots: A Comprehensive Review

Graphene quantum dot (GQD) is one of the youngest superstars of the carbon family. Since its emergence in 2008, GQD has attracted a great deal of attention due to its unique optoelectrical properties. Non-zero bandgap, the ability to accommodate functional groups and dopants, excellent dispersibility, highly tunable properties, and biocompatibility are among the most important characteristics of GQDs. To date, GQDs have displayed significant momentum in numerous fields such as energy devices, catalysis, sensing, photodynamic and photothermal therapy, drug delivery, and bioimaging. As this field is rapidly evolving, there is a strong need to identify the emerging challenges of GQDs in recent advances, mainly because some novel applications and numerous innovations on the ease of synthesis of GQDs are not systematically reviewed in earlier studies. This feature article provides a comparative and balanced discussion of recent advances in synthesis, properties, and applications of GQDs. Besides, current challenges and future prospects of these emerging carbon-based nanomaterials are also highlighted. The outlook provided in this review points out that the future of GQD research is boundless, particularly if upcoming studies focus on the ease of purification and eco-friendly synthesis along with improving the photoluminescence quantum yield and production yield of GQDs.

Direct growth of hexagonal boron nitride on non-metallic substrates and its heterostructures with graphene

Hexagonal boron nitride (h-BN) and its heterostructures with graphene are widely investigated van der Waals (vdW) quantum materials for electronics, photonics, sensing, and energy storage/transduction. However, their metal catalyst-based growth and transfer-based heterostructure assembly approaches present impediments to obtaining high-quality and wafer-scale quantum material. Here, we have presented our perspective on the synthetic strategies that involve direct nucleation of h-BN on various dielectric substrates and its heterostructures with graphene. Mechanistic understanding of direct growth of h-BN via bottom-up approaches such as (a) the chemical-interaction guided nucleation on silicon-based dielectrics, (b) surface nitridation and N+ sputtering of h-BN target on sapphire, and (c) epitaxial growth of h-BN on sapphire, among others, are reviewed. Several design methodologies are presented for the direct growth of vertical and lateral vdW heterostructures of h-BN and graphene. These complex 2D heterostructures exhibit various physical phenomena and could potentially have a range of practical applications.

Nanodots Derived from Layered Materials: Synthesis and Applications

Layered 2D materials, such as graphene, transition metal dichalcogenides, transition metal oxides, black phosphorus, graphitic carbon nitride, hexagonal boron nitride, and MXenes, have attracted intensive attention over the past decades owing to their unique properties and wide applications in electronics, catalysis, energy storage, biomedicine, etc. Further reducing the lateral size of layered 2D materials down to less than 10 nm allows for preparing a new class of nanostructures, namely, nanodots derived from layered materials. Nanodots derived from layered materials not only can exhibit the intriguing properties of nanodots due to the size confinement originating from the ultrasmall size, but also can inherit some unique properties of ultrathin layered 2D materials, making them promising candidates in a wide range of applications, especially in biomedicine and catalysis. Here, a comprehensive summary on the materials categories, advantages, synthesis methods, and potential applications of these nanodots derived from layered materials is provided. Finally, personal insights about the challenges and future directions in this promising research field are also given.

Ultrafast Electron Tunneling Devices-From Electric-Field Driven to Optical-Field Driven

The search for ever higher frequency information processing has become an area of intense research activity within the micro, nano, and optoelectronics communities. Compared to conventional semiconductor-based diffusive transport electron devices, electron tunneling devices provide significantly faster response times due to near-instantaneous tunneling that occurs at sub-femtosecond timescales. As a result, the enhanced performance of electron tunneling devices is demonstrated, time and again, to reimagine a wide variety of traditional electronic devices with a variety of new "lightwave electronics" emerging, each capable of reducing the electron transport channel transit time down to attosecond timescales. In response to unprecedented rapid progress within this field, here the current state-of-the-art in electron tunneling devices is reviewed, current challenges and opportunities are highlighted, and possible future research directions are identified.

Preparation Engineering of Two-Dimensional Heterostructures via Bottom-Up Growth for Device Applications

Two-dimensional heterostructures with tremendous electronic and optoelectronic properties hold great promise for nanodevice integrations and applications owing to the wide tunable characteristics. Toward this end, developing construction strategies in allusion to large-scale production of high-quality heterostructures is critical. The mainstream preparation routes are representatively classified into two categories of top-down and bottom-up approaches. Nonetheless, the relatively low reproductivity and the limitation for lateral heterostructure formations of top-down methods at the present stage inherently impeded their further developments. To surmount these obstacles, assembling heterostructures via miscellaneous bottom-up preparation protocols has emerged as a potential solution, attributed to the controllability and clean interface. Three typical approaches of chemical/physical vapor deposition, solution synthesis, and growth under ultrahigh vacuum conditions have shown promise due to the possibilities for preparing heterostructures with predesigned structures, clean interfaces, and the like. Therefore, bottom-up preparation engineering of heterostructures in two dimensions for further device applications is of vital importance. Moreover, heterostructure integrations by these methods have experienced a period of flourishing development in the past few years. In this review, the classical bottom-up growth routes, characterization methods, and latest progress of diverse heterostructures and further device applications are overviewed. Finally, the challenges and opportunities are discussed.

Photodiode behaviors of the AgSbS2nanocrystals in a Schottky structure

Cubic phase AgSbS₂nanocrystals (NCs) were synthesized by the hot-injection method, and they were inserted between the Al andp-Si to fabricate Al/AgSbS₂/p-Si photodiode by the thermal evaporation method. AgSbS₂NCs were characterized by XRD, SEM and TEM instruments to confirm the crystal phase, surface morphology as well as crystalline size. The XRD pattern revealed that the cubic crystalline structure of the AgSbS₂. The spherical shapes and well surface morphology were affirmed by SEM and TEM analysis. Al/AgSbS₂/p-Si photodiode was characterized byI-Vmeasurements depending on the light power intensity and byC-Vmeasurement for various frequencies.I-Vcharacteristics revealed that the Al/AgSbS₂/p-Si exhibited good photodiode behavior and a high rectifying ratio. Various diode and detector parameters were extracted fromI-Vmeasurements, and they were discussed in detail. TheC-Vcharacteristics highlighted that the Al/AgSbS₂/p-Si photodiode showed voltage and frequency dependent profile at the accumulation region. The fabricated Al/AgSbS₂/p-Si photodiode can be thought for optoelectronic applications.

Electrophoretic Transport of Single-Stranded DNA through a Two Dimensional Nanopore Patterned on an In-Plane Heterostructure

Recent advances in nanotechnology have facilitated fabrication of various solid state nanopores as a versatile alternative to biological nanopores; however, effective transport of a single-stranded DNA (ssDNA) molecule through solid state nanopores for sequencing has remained a challenge. In particular, the nonspecific interactions between the ssDNA and the engineered nanopore surface are known to impose difficulties on both transport and interrogation. Here, we show that a two-dimensional (2D) nanopore patterned on an in-plane heterostructure comprising both graphene and hexagonal boron nitride (hBN) can be utilized to transport the ssDNA electrophoretically. Energetically, a ssDNA molecule prefers to stay on the hBN domain than the graphene one since the former has a stronger van der Waals attraction with the ssDNA, as demonstrated in both classic molecular dynamics (MD) simulations and density functional theory (DFT) based calculations, which leads to the confinement of the ssDNA in the 2D nanopore. Therefore, this nanopore enables the manipulation of the conformation of a highly flexible ssDNA molecule on a flat 2D heterostructure surface, making it possible for sensing ssDNA bases using the high resolution atomic force microscopy (AFM) or scanning tunneling microscopy (STM) in the third dimension (perpendicular to the 2D surface).

Blue emission at atomically sharp 1D heterojunctions between graphene and h-BN

Atomically sharp heterojunctions in lateral two-dimensional heterostructures can provide the narrowest one-dimensional functionalities driven by unusual interfacial electronic states. For instance, the highly controlled growth of patchworks of graphene and hexagonal boron nitride (h-BN) would be a potential platform to explore unknown electronic, thermal, spin or optoelectronic property. However, to date, the possible emergence of physical properties and functionalities monitored by the interfaces between metallic graphene and insulating h-BN remains largely unexplored. Here, we demonstrate a blue emitting atomic-resolved heterojunction between graphene and h-BN. Such emission is tentatively attributed to localized energy states formed at the disordered boundaries of h-BN and graphene. The weak blue emission at the heterojunctions in simple in-plane heterostructures of h-BN and graphene can be enhanced by increasing the density of the interface in graphene quantum dots array embedded in the h-BN monolayer. This work suggests that the narrowest, atomically resolved heterojunctions of in-plane two-dimensional heterostructures provides a future playground for optoelectronics.

Here, the authors explore the blue photoluminescence signal arising from the interface between graphene and h-BN arranged in in-plane heterostructures, and fabricate a blue light emitting device utilizing the heterojunction as the emitting layer.

Spatially controlled lateral heterostructures of graphene and transition metal dichalcogenides toward atomically thin and multi-functional electronics

Edge contacts between two-dimensional (2D) materials in the in-plane direction can achieve minimal contact area and low contact resistance, producing atomically thin devices with improved performance. Particularly, lateral heterojunctions of metallic graphene and semiconducting transition metal dichalcogenides (TMDs) exhibit small Schottky barrier heights due to graphene's low work-function. However, issues exist with the fabrication of highly transparent and flexible multi-functional devices utilizing lateral heterostructures (HSs) of graphene and TMDs via spatially controlled growth. This review demonstrates the growth and electronic applications of lateral HSs of graphene and TMDs, highlighting key technologies controlling the wafer-scale growth of continuous films for practical applications. It deepens the understanding of the spatially controlled growth of lateral HSs using chemical vapor deposition methods, and also contributes to the applications that depend on the scale-up of all-2D electronics with ultra-high electrical performance.

Inlaid ReS2 Quantum Dots in Monolayer MoS2

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) are prospective materials for quantum devices owing to their inherent 2D confinements. They also provide a platform to realize even lower-dimensional in-plane electron confinement, for example, 0D quantum dots, for exotic physical properties. However, fabrication of such laterally confined monolayer (1L) nanostructure in 1L crystals remains challenging. Here we report the realization of 1L ReS₂ quantum dots epitaxially inlaid in 1L MoS₂ by a two-step chemical vapor deposition method combining with plasma treatment. The lateral lattice mismatch between ReS₂ and MoS₂ leads to size-dependent crystal structure evolution and in-plane straining of the 1L ReS₂ quantum dots. Optical spectroscopies reveal the abnormal charge transfer between the 1L ReS₂ quantum dots and the MoS₂ matrix, resulting from electron trapping in the 1L ReS₂ quantum dots. This study may shed light on the development of in-plane quantum-confined devices in 2D materials for potential applications in quantum information.

Resonant tunnelling spectroscopy of van der Waals heterosystems

The review concerns the most interesting aspects of (mainly experimental) resonance tunnelling spectroscopy studies of a new type of heterosystems called van der Waals heterostructures. The possibility to compose such systems is a result of the recent discovery of two-dimensional crystals, a new class of materials derived from graphene. The role of the angular mismatch of the crystal lattices of conductive graphene electrodes in the tunnelling of charge carriers between them, as well as the closely related issues associated with fulfillment of the conservation laws during tunnelling transitions are considered. The experimental results on inelastic tunnelling in the graphene/h-BN/graphene heterosystems with strong angular mismatch are discussed. The experiments made it possible to determine the phonon density of states spectra of the constituent layers and to detect and describe tunnelling transitions involving localized states of structural defects in the h-BN barrier. We consider new results of studies on tunnelling and magnetotunnelling in van der Waals heterosystems that demonstrate the possibilities of practical application of resonant tunnelling effects in, e.g., microwave engineering, based on realization of electronic devices having I – V curves with negative differential conductance (NDC) regions at tunnelling through defect levels of the barrier layers in such systems. These studies revealed two new types of heterosystems characterized by the formation of NDC regions as a result of resonant tunnelling through the defect levels in the h-BN barrier and by defect-assisted generation of tunnelling current.

The bibliography includes 40 references.

Tuning the Electronic and Magnetic Properties of Graphene Flake Embedded in Boron Nitride Nanoribbons with Transverse Electric Fields: First-Principles Calculations

The electronic and magnetic properties of h-BN nanoribbions embedded with graphene nanoflakes (CBNNRs) are systematically studied by ab initio calculations. The CBNNRs with zigzag or armchair edges are all bipolar magnetic semiconductors (BMSs). The band gaps of zigzag CBNNRs (zCBNNRs) change linearly with the transverse electric field (E-field) for the first-order Stark effect, whereas for the armchair CBNNRs (aCBNNRs), the band gaps vary quadratically with the E-field for the second-order Stark effect. For zCBNNRs and aCBNNRs, they could transform from BMS to spin gapless semiconductor (SGS), metal, and half-metal (HM) under different transverse E-fields. The CBNNRs may transform into a semiconductor or HM, under the same E-fields, depending on the position of graphene flakes. The CBNNRs introduce local magnetic moment at carbon atoms, and the magnetic moment is determined by the size of the graphene flakes. These observations open the door to applications of CBNNRs in spintronic devices.