Conversion of chirality to twisting via sequential one-dimensional and two-dimensional growth of graphene spirals.

The properties of two-dimensional (2D) van der Waals materials can be tuned through nanostructuring or controlled layer stacking, where interlayer hybridization induces exotic electronic states and transport phenomena. Here we describe a viable approach and underlying mechanism for the assisted self-assembly of twisted layer graphene. The process, which can be implemented in standard chemical vapour deposition growth, is best described by analogy to origami and kirigami with paper. It involves the controlled induction of wrinkle formation in single-layer graphene with subsequent wrinkle folding, tearing and re-growth. Inherent to the process is the formation of intertwined graphene spirals and conversion of the chiral angle of 1D wrinkles into a 2D twist angle of a 3D superlattice. The approach can be extended to other foldable 2D materials and facilitates the production of miniaturized electronic components, including capacitors, resistors, inductors and superconductors.

Charge State of Au Atoms on an Oxidized Rutile TiO2(110) Surface by AFM/KPFM at 78 K.

The charge state of noble metal atoms on a semiconductor surface is an important factor in surface catalysis. In this study, Au atoms were deposited on the rutile TiO2(110) surface to characterize its charge properties using atomic force microscopy with Kelvin probe force microscopy at 78 K. Au single atoms, dimers, and trimers at different sites on the surface were investigated. Positively charged Au atoms were verified at oxygen sites, while negatively charged Au atoms were found near oxygen vacancy sites. Furthermore, the charge states of small Au nanoclusters were clarified. Understanding the charge states of Au atoms is significant for identifying their efficient catalytic effects in surface catalysis.

Band Gap Engineering of MoxW1-xS2 Alloy Monolayers with Wafer-Scale Uniformity.

Modulating the band gap of two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors is critical for their application in a wider spectral range. Alloying has been demonstrated as an effective method for regulating the band gap of 2D TMDC semiconductors. The fabrication of large-area 2D TMDC alloy films with centimeter-scale uniformity is fundamental to the application of integrated devices. Herein, we report a liquid-phase precursor one-step chemical vapor deposition (CVD) method for fabricating a MoxW1-xS2 alloy monolayer with a large size and an adjustable band gap. Good crystalline quality and high uniformity on a wafer scale enable the continuous adjustment of its band gap in the range of 1.8-2.0 eV. Density functional theory calculations provided a deep understanding of the Raman-active vibration modes of the MoxW1-xS2 alloy monolayer and the change in the conductivity of the alloy with photon energy. The synthesis of large-area MoxW1-xS2 alloy monolayers is a critical step toward the application of 2D layered semiconductors in practical optoelectronic devices.

Fe Single Atom Catalysts Promoting Polysulfide Redox Reduction in Quantum Dot Photovoltaics.

Developing cost-effective and highly efficient photocathodes toward polysulfide redox reduction is highly desirable for advanced quantum dot (QD) photovoltaics. Herein, we demonstrate nitrogen doped carbon (N-C) shell-supported iron single atom catalysts (Fe-SACs) capable of catalyzing polysulfide reduction in QD photovoltaics for the first time. Specifically, Fe-SACs with FeN4 active sites feature a power conversion efficiency of 13.7% for ZnCuInSe-QD photovoltaics (AM1.5G, 100 mW/cm2), which is the highest value for ZnCuInSe QD-based photovoltaics, outperforming those of Cu-SACs and N-C catalysts. Compared with N-C, Fe-SACs exhibit suitable energy level matching with polysulfide redox couples, revealed by the Kelvin probe force microscope, which accelerates the charge transferring at the interfaces of catalyst/polysulfide redox couple. Density functional theory calculations demonstrate that the outstanding catalytic activity of Fe-SACs originates from the preferable adsorption of S42- on the FeN4 active sites and the high activation degree of the S-S bonds in S42- initiated by the FeN4 active sites.

Vanadium-Doped Molybdenum Diselenide Atomic Layers with Room-Temperature Ferromagnetism.

Diluted two-dimensional magnetic semiconductors with high Curie temperature are highly sought after because of their potential applications in spintronics. Development of new techniques for preparation of high quality diluted magnetic semiconductors is critical for their applications. In this study, vanadium-doped molybdenum selenide, a new diluted magnetic semiconductor, was synthesized by a single-step chemical vapor deposition method. The merit of this method is that the molybdenum and vanadium precursors can be supplied to the growth substrate uniformly. Photoluminescence measurements reveal that the band gap of MoSe2 decreases after doping, which can be attributed to the formation of impurity energy band caused by p-type doping at the valence band maximum. Thus, the V-doped MoSe2 still maintains the semiconducting characteristics. Vibrating sample magnetometer studies clearly show the ferromagnetism of V-doped MoSe2 at room temperature. DFT calculations illustrates the joint contribution of V dopants and nearby atoms to the magnetic moments. This study provides future prospects for the multifunctional application of two-dimensional materials.

Direct growth of hBN/Graphene heterostructure via surface deposition and segregation for independent thickness regulation.

Hexagonal boron nitride/graphene (hBN/G) vertical heterostructures have attracted extensive attention, owing to the unusual physical properties for basic research and electronic device applications. Here we report a facile deposition-segregation technique to synthesize hBN/G heterostructures on recyclable platinum (Pt) foil via low pressure chemical vapor deposition. The growth mechanism of the vertical hBN/G is demonstrated to be the surface deposition of hBN on top of the graphene segregated from the Pt foil with pre-dissolved carbon. The thickness of hBN and graphene can be controlled separately from sub-monolayer to multilayer through the fine control of the growth parameters. Further investigations by Raman, scanning Kelvin probe microscopy and transmission electron microscope show that the hBN/G inclines to form a heterostructure with strong interlayer coupling and with interlayer twist angle smaller than 1.5°. This deposition-segregation approach paves a new pathway for large-scale production of hBN/G heterostructures and could be applied to synthesize of other van der Waals heterostructures.

An all two-dimensional vertical heterostructure graphene/CuInP2S6/MoS2for negative capacitance field effect transistor.

As scaling down the size of metal oxide semiconductor field-effect transistors (FETs), power dissipation has become a major challenge. Lowering down the sub-threshold swing (SS) is known as an effective technique to decrease the operating voltage of FETs and hence lower down the power consumption. However, the Boltzmann distribution of electrons (so-called 'Boltzmann tyranny') implements a physical limit to the SS value. Use of negative capacitance (NC) effect has enabled a new path to achieve a low SS below the Boltzmann limit (60 mV dec-1at room temperature). In this work, we have demonstrated a NC-FET from an all two-dimensional (2D) metal ferroelectric semiconductor (MFS) vertical heterostructure: Graphene/CuInP2S6/MoS2. The negative capacitance from the ferroelectric CuInP2S6has enabled the breaking of the 'Boltzmann tyranny'. The heterostructure based device has shown steep slopes switching below 60 mV dec-1(lowest to < 10 mV dec-1) over 3 orders of source-drain current, which provides an avenue for all 2D material based steep slope FETs.

Size-dependent strain-engineered nanostructures in MoS2monolayer investigated by atomic force microscopy.

The strain has been employed for controlled modification of electronical and mechanical properties of two-dimensional (2D) materials. However, the thermal strain-engineered behaviors of the CVD-grown MoS2have not been systematically explored. Here, we investigated the strain-induced structure and properties of CVD-grown triangular MoS2flakes by several advanced atomic force microscopy. Two different kinds of flakes with sharp-corner or vein-like nanostructures are experimentally discovered due to the size-dependent strain behaviors. The critical size of these two kinds of flakes can be roughly estimated at âˆ¼17µm. Within the small flakes, the sharp-corner regions show specific strain-modified properties due to the suffering of large tensile strain. While in the large MoS2flakes, the complicated vein-like nanoripple structures were formed due to the interface slipping process under the larger tensile strain. Our work not only demonstrates the size-specific strain behaviors of MoS2flakes but also sheds light on the artificial design and preparation of strain-engineered nanostructures for the devices based on the 2D materials.

NBN-Doped Bis-Tetracene and Peri-Tetracene: Synthesis and Characterization.

Combining solution-based and surface-assisted synthesis, we demonstrate the first synthesis of NBN-doped bis-tetracene (NBN-BT) and peri-tetracene (NBN-PT). The chemical structures are clearly elucidated by high-resolution scanning tunneling microscopy (STM) in combination with noncontact atomic force microscopy (nc-AFM). Scanning tunneling spectroscopy (STS) characterizations reveal that NBN-BT and NBN-PT possess higher energy gaps than bis-tetracene and peri-tetracene. Interestingly, NBN-BT can undergo stepwise one-electron oxidation and convert into its corresponding radical cation and then to its dication. The energy gap of the NBN-BT dication is similar to that of bis-tetracene, indicating their isoelectronic relationship. Moreover, a similar energy gap between the NBN-PT dication and peri-tetracene can be predicted by DFT calculations. This work provides a novel synthesis along with characterizations of multi-NBN-doped zigzag-edged peri-acenes with tunable electronic properties.

Force-Activated Isomerization of a Single Molecule.

Understanding and controlling isomerization at the single molecular level should provide new insight into the molecular dynamics and design guidelines of functional devices. Scanning tunneling microscopy (STM) has been demonstrated to be a powerful tool to study isomerization of single molecules on a substrate, by either electric field or inelastic electron tunneling mechanisms. A similar molecular isomerization process can in principle be induced by mechanical force; however, relevant study has remained elusive. Here, we demonstrate that isomerization of a N,N-dimethylamino-dianthryl-benzene molecule on Ag(100) can be mechanically driven by the STM tip. The existence of an out-of-plane dimethylamino group in the molecule is found to play a pivotal role in the isomerization process by providing a steric hindrance effect for asymmetric interaction between the STM tip and the molecule. This underlying mechanism is further confirmed by performing molecular dynamics simulations, which show agreement with experimental results. Our work opens the opportunity to manipulate the molecular configuration on the basis of mechanical force.

Wafer-Scale Growth of Pristine and Doped Monolayer MoS2 Films for Electronic Device Applications.

Scalable production and controlled doping of large-area two-dimensional transition-metal dichalcogenide films are fundamental steps toward their applications in electronic devices. Although a variety of methods for preparation of wafer-scale transition-metal dichalcogenide films have been developed, it is still challenging to realize homogeneous doping of the large-area films to modulate their electronic properties. In this paper, we report a new chemical vapor deposition (CVD) method for preparation of wafer-scale pristine and doped monolayer MoS2 films on 2-inch sapphire wafers. The molybdenum precursors are supplied in a "face-to-face" manner from a silica gel plate to the sapphire wafer, which guarantees uniform nucleation and growth of monolayer MoS2. This method can be used to prepare substitutionally doped monolayer MoS2 films. By using ReCl3 as the dopant precursor, we have obtained continuous Re-doped monolayer MoS2 films on sapphire wafers. Elemental analysis confirms successful Re-doping of the MoS2 film. Spherical aberration-corrected scanning transmission electron microscopy characterization reveals that the Re atoms are incorporated at the substitutional Mo sites in the MoS2 lattice. The incorporation of Re atoms leads to n-type doping of MoS2 as evidenced by Kelvin probe force microscope studies. Electrical measurements reveal that the transport properties of the Re-doped monolayer MoS2 is dramatically enhanced as compared with the pristine MoS2. The CVD method developed in this study can be applied to the production of a variety of two-dimensional transition-metal dichalcogenide films suitable for applications in electronic devices.

On-Surface Synthesis of NBN-Doped Zigzag-Edged Graphene Nanoribbons.

We report the first bottom-up synthesis of NBN-doped zigzag-edged GNRs (NBN-ZGNR1 and NBN-ZGNR2) through surface-assisted polymerization and cyclodehydrogenation based on two U-shaped molecular precursors with an NBN unit preinstalled at the zigzag edge. The resultant zigzag-edge topologies of GNRs are elucidated by high-resolution scanning tunneling microscopy (STM) in combination with noncontact atomic force microscopy (nc-AFM). Scanning tunneling spectroscopy (STS) measurements and density functional theory (DFT) calculations reveal that the electronic structures of NBN-ZGNR1 and NBN-ZGNR2 are significantly different from those of their corresponding pristine fully-carbon-based ZGNRs. Additionally, DFT calculations predict that the electronic structures of NBN-ZGNRs can be further tailored to be gapless and metallic through one-electron oxidation of each NBN unit into the corresponding radical cations. This work reported herein provides a feasible strategy for the synthesis of GNRs with stable zigzag edges yet tunable electronic properties.

Precise Organization of Metal and Metal Oxide Nanoclusters into Arbitrary Patterns on DNA Origami.

The development of facile techniques for precisely patterning complex metal and metal oxide nanostructures is essential for catalytic nanosystems and optical and electronic nanodevices. Herein, we report a general strategy for designing and fabricating metal and metal oxide nanoclusters (MMONs) with arbitrarily prescribed patterns on DNA origami templates. The valuable feature of our approach lies in the site-specific arrangement of thiol groups on DNA origami, which act as reaction centers, initiating in situ MMONs growth. This strategy can be generalized to the patterning of arbitrary geometries and various inorganic materials, which will aid the generation of complex and precisely arranged components for customized functional nanoarchitectures.

Interfacial water intercalation-induced metal-insulator transition in NbS2/BN heterostructure.

Interfacial engineering, such as molecule intercalation, can modify properties and optimize performance of van der Waals heterostructures and their devices. Here, we investigated the pristine and water molecule intercalated heterointerface of niobium disulphide (NbS2) on hexagonal boron nitride (h-BN) (NbS2/BN) using advanced atomic force microscopy (AFM), and observed the metal-insulator transition (MIT) of first layer (1L-) of NbS2 induced by water molecule intercalation. In pristine sample, interfacial charge transfers were confirmed by the direct detection of trapped static charges at the post-exposed h-BN surface, produced by mechanically peeling off the 1L-NbS2 from the substrate. The interfacial charge transfers facilitate the intercalation of water molecules at the heterointerface. The intercalated water layers make a MIT of 1L-NbS2, while the pristine metallic state of the following NbS2 layers remains preserved. This work is of great significance to help understand the interfacial properties of 2D metal/insulator heterostructures and can pave the way for further preparation of an ultrathin transistor.

Nanoscale charge transfer and diffusion at the MoS2/SiO2 interface by atomic force microscopy: contact injection versus triboelectrification.

Understanding the process of charge generation, transfer, and diffusion between two-dimensional (2D) materials and their supporting substrates is very important for potential applications of 2D materials. Compared with the systematic studies of triboelectric charging in a bulk sample, a fundamental understanding of the triboelectrification of the 2D material/insulator system is rather limited. Here, the charge transfer and diffusion of both the SiO2 surface and MoS2/SiO2 interface through contact electrification and frictional electrification are investigated systematically in situ by scanning Kelvin probe microscopy and dual-harmonic electrostatic force microscopy. Different from the simple static charge transfer between SiO2 and the PtSi alloy atomic force microscope (AFM) tip, the charge transfer between the tip and the MoS2/SiO2 system is complicated. Triboelectric charges, generated by contact or frictional electrification with the AFM tip, are trapped at the MoS2/SiO2 interface and act as floating gates. The local charge discharge processes can be obtained by monitoring the surface potential. The charge decay time (τ) of the MoS2/SiO2 interface is one (or two) orders of magnitude larger than the decay time τ of the SiO2 surface. This work facilitates an understanding of the triboelectric and de-electrification of the interface between 2D materials and substrates. In addition to the charge transfer and diffusion, we demonstrate the nanopatterns of surface and interfacial charges, which have great potential for the application of self-assembly of charged nanostructures.

Electrically configurable graphene field-effect transistors with a graded-potential gate.

A device architecture for electrically configurable graphene field-effect transistor (GFET) using a graded-potential gate is present. The gating scheme enables a linearly varying electric field that modulates the electronic structure of graphene and causes a continuous shift of the Dirac points along the channel of GFET. This spatially varying electrostatic modulation produces a pseudobandgap observed as a suppressed conductance of graphene within a controllable energy range. By tuning the electrical gradient of the gate, a GFET device is reversibly transformed between ambipolar and n- and p-type unipolar characteristics. We further demonstrate an electrically programmable complementary inverter, showing the extensibility of the proposed architecture in constructing logic devices based on graphene and other Dirac materials. The electrical configurable GFET might be explored for novel functionalities in smart electronics.

Quasi-freestanding monolayer heterostructure of graphene and hexagonal boron nitride on Ir(111) with a zigzag boundary.

In-plane heterostructure of hexagonal boron nitride and graphene (h-BN-G) has become a focus of graphene research owing to its tunable bandgap and intriguing properties. We report herein the synthesis of a quasi-freestanding h-BN-G monolayer heterostructure on a weakly coupled Ir(111) substrate, where graphene and h-BN possess distinctly different heights and surface corrugations. An atomically sharp zigzag type boundary has been found to dominate the patching interface between graphene and h-BN, as evidenced by high-resolution Scanning tunneling microscopy investigation as well as density functional theory calculation. Scanning tunneling spectroscopy studies indicate that the graphene and h-BN tend to exhibit their own intrinsic electronic features near the patching boundary. The present work offers a deep insight into the h-BN-graphene boundary structures both geometrically and electronically together with the effect of adlayer-substrate coupling.

Toward single-layer uniform hexagonal boron nitride-graphene patchworks with zigzag linking edges.

The atomic layer of hybridized hexagonal boron nitride (h-BN) and graphene has attracted a great deal of attention after the pioneering work of P. M. Ajayan et al. on Cu foils because of their unusual electronic properties (Ci, L. J.; et al. Nat. Mater. 2010, 9, 430-435). However, many fundamental issues are still not clear, including the in-plane atomic continuity as well as the edge type at the boundary of hybridized h-BN and graphene domains. To clarify these issues, we have successfully grown a perfect single-layer h-BN-graphene (BNC) patchwork on a selected Rh(111) substrate, via a two-step patching growth approach. With the ideal sample, we convinced that at the in-plane linking interface, graphene and h-BN can be linked perfectly at an atomic scale. More importantly, we found that zigzag linking edges were preferably formed, as demonstrated by atomic-scale scanning tunneling microscopy images, which was also theoretically verified using density functional theory calculations. We believe the experimental and theoretical works are of particular importance to obtain a fundamental understanding of the BNC hybrid and to establish a deliberate structural control targeting high-performance electronic and spintronic devices.

Angle-Resolved Optical Imaging of Interlayer Rotations in Twisted Bilayer Graphene.

Twisted bilayer graphene (TBG) is a prototypical layered material whose properties are strongly correlated to interlayer coupling. The two stacked graphene layers with distinct orientations are investigated to generate peculiar optical and electronic phenomena. Thus, the rapid, reliable, and nondestructive twist angle identification technique is of essential importance. Here, we integrated the white light reflection spectra (WLRS), the Raman spectroscopy, and the transmission electron microscope (TEM) to propose a facile RGB optical imaging technique that identified the twist angle of the TBG in a large area intuitively with high efficiency. The RGB technique established a robust correlation between the interlayer rotation angle and the contrast difference in the RGB color channels of a standard optical image. The angle-resolved optical behavior is attributed to the absorption resonance matching with the separation of van Hove singularities in the density of states of the TBG. Our study thus developed a route to identify the rotation angle of stacked bilayer graphene by means of a straightforward optical method, which can be further applied in other stacked van der Waals layered materials.

Charge transfer and current fluctuations in single layer graphene transistors modified by self-assembled C60 adlayers.

Flicker noise in a "fullerene + graphene" hybrid transistor is investigated to reveal the electrical coupling between the graphene channel and C60 adsorbates. The charge trapping and detrapping events at the C60 /graphene interface induce current fluctuations in the devices. The evolution of noise characteristics at varying temperatures indicates the different contributions related to Coulomb scattering and charge exchange kinetics.