Free-standing graphene at atomic resolution

Research interest in graphene, a two-dimensional crystal consisting of a single atomic plane of carbon atoms, has been driven by its extraordinary properties, including charge carriers that mimic ultra-relativistic elementary particles. Moreover, graphene exhibits ballistic electron transport on the submicrometre scale, even at room temperature, which has allowed the demonstration of graphene-based field-effect transistors and the observation of a room-temperature quantum Hall effect. Here we confirm the presence of free-standing, single-layer graphene with directly interpretable atomic-resolution imaging combined with the spatially resolved study of both the pi --> pi* transition and the pi + sigma plasmon. We also present atomic-scale observations of the morphology of free-standing graphene and explore the role of microstructural peculiarities that affect the stability of the sheets. We also follow the evolution and interaction of point defects and suggest a mechanism by which they form ring defects.     

Size-tunable Au nanoparticles on MoS(2)(0001)

Ultra-fine Au nanoparticles (NPs) show great application potential in catalysis. Size-tunable Au NPs have been fabricated on MoS(2) covered with monolayer 3,4,5,10-perylene tetracarboxylic dianhydride (PTCDA), and the morphological evolution as a function of Au deposition amount was investigated using scanning tunneling microscopy (STM). The PTCDA molecules act as a surfactant to stabilize ultra-fine Au NPs. Molecular scale STM images show that on MoS(2) the Au NPs with PTCDA molecules on top can be formed with height and lateral size down to 1.3?nm and 3.5?nm, respectively. By controlling the deposition amount and annealing temperature, the size of Au NPs can be tuned. After annealing at 270?°C to remove PTCDA, Au NPs with a linear size ≤5?nm can be obtained on MoS(2)(0001), facilitating the characterization of their intrinsic physical and chemical properties using various analytical techniques. In addition, photoemission spectroscopy data reveal charge transfer from Au NPs to PTCDA, indicating that the NPs possess more reactive chemical properties than bulk Au.     

Direct observation of atomic scale graphitic layer growth

The demand for better understanding of the mechanism of soot formation is driven by the negative environmental and health impact brought about by the burning of fossil fuels. While soot particles accumulate most of their mass from surface reactions, the mechanism for surface growth has so far been characterized primarily by measurements of the kinetics. Here we provide atomic-scale scanning tunneling microscope images of carbon growth by chemistry similar to that of importance in soot formation. At a temperature of 625 K, exposure of the surface of highly ordered pyrolytic graphite to 1 Langmuir of acetylene leads to the formation of both graphitic and amorphous carbonaceous material at the edges of nanoscale pits. Given the similarity of the electronic structure at these graphite defect sites to that of soot material growing in flames at higher temperatures, the present studies shed light on the mechanism for soot growth. These experiments also suggest that healing of defect sites in graphene nanostructures, which are of considerable interest as novel electronic devices, should be possible at modest surface temperatures by exposure of defected graphene to unsaturated hydrocarbons.     

Metallic Contact between MoS2 and Ni via Au Nanoglue

A critical factor for electronics based on inorganic layered crystals stems from the electrical contact mode between the semiconducting crystals and the metal counterparts in the electric circuit. Here, a materials tailoring strategy via nanocomposite decoration is carried out to reach metallic contact between MoS₂ matrix and transition metal nanoparticles. Nickel nanoparticles (NiNPs) are successfully joined to the sides of a layered MoS₂ crystal through gold nanobuffers, forming semiconducting and magnetic NiNPs@MoS₂ complexes. The intrinsic semiconducting property of MoS₂ remains unchanged, and it can be lowered to only few layers. Chemical bonding of the Ni to the MoS₂ host is verified by synchrotron radiation based photoemission electron microscopy, and further proved by first‐principles calculations. Following the system's band alignment, new electron migration channels between metal and the semiconducting side contribute to the metallic contact mechanism, while semiconductor–metal heterojunctions enhance the photocatalytic ability.     

Ageing of mass-selected Cu/Au and Au/Cu core/shell clusters probed with atomic resolution

Bimetallic clusters can display new characteristics that cannot be obtained by varying either the size of pure metallic systems or the composition of bulk bimetals. We have demonstrated that both copper (Cu)-rich core/gold (Au)-rich shell and Au-rich/Cu-rich shell clusters can be prepared using the radio frequency (RF) magnetron-sputtering gas phase cluster beam synthesis method with subsequent mass selection. Here we present an aberration-corrected scanning transmission electron microscopy (STEM) study of the ageing of the deposited clusters. We find that the Au-rich core/Cu-rich shell structure is unstable against both storage in vacuum and exposure to air, losing the Cu-rich shell. However, the Cu-rich core/Au-rich shell structure is found to be much more stable, even when exposed to air.     

Direct observation of local atomic order in a metallic glass

The determination of the atomic configuration of metallic glasses is a long-standing problem in materials science and solid-state physics. So far, only average structural information derived from diffraction and spectroscopic methods has been obtained. Although various atomic models have been proposed in the past fifty years, a direct observation of the local atomic structure in disordered materials has not been achieved. Here we report local atomic configurations of a metallic glass investigated by nanobeam electron diffraction combined with ab initio molecular dynamics simulation. Distinct diffraction patterns from individual atomic clusters and their assemblies, which have been theoretically predicted as short- and medium-range order, can be experimentally observed. This study provides compelling evidence of the local atomic order in the disordered material and has important implications in understanding the atomic mechanisms of metallic-glass formation and properties.     

Direct observation of isolated ultrananodimensional diamond clusters using atomic force microscopy

Isolated ultrananodimensional diamond (UND) particles obtained by means of detonation synthesis have been studied using atomic force microscopy (AFM). The UND particles were deposited onto the surface of highly oriented pyrolytic graphite from a suspension based on organic compounds. The deposited UND particles were deaggregated using a two-stage treatment with ultrasound and high-dynamic-pressure pulses. The isolated UND particles were stabilized in suspension by a benzene additive. AFM images of individual UND particles have been obtained, and the phenomenon of their alignment along atomic steps on the substrate surface has been observed.     

Direct Imaging of Dopant and Impurity Distributions in 2D MoS2

Molybdenum disulfide (MoS₂) nanosheet is a two-dimensional (2D) material with high electron mobility and with high potential for applications in catalysis and electronics. MoS₂ nanosheets are synthesized using a one-pot wet-chemical synthesis route with and without Re doping. Atom probe tomography reveals that 3.8 at% Re is homogeneously distributed within the Re-doped sheets. Other impurities are also found integrated within the material: light elements including C, N, O, and Na, locally enriched up to 0.1 at%, as well as heavy elements such as V and W. Analysis of the nondoped sample reveals that the W and V likely originate from the Mo precursor. It is shown how wet-chemical synthesis results in an uncontrolled integration of species from the solution that can affect the material's activity. The results of this work are expected to contribute to an improved understanding of the relationships linking composition to properties of 2D transition-metal dichalcogenide materials.     

Microlandscaping of Au Nanoparticles on Few‐Layer MoS2 Films for Chemical Sensing

Surface modification or decoration of ultrathin MoS₂ films with chemical moieties is appealing since nanointerfacing can functionalize MoS₂ films with bonus potentials. In this work, a facile and effective method for microlandscaping of Au nanoparticles (NPs) on few‐layer MoS₂ films is developed. This approach first employs a focused laser beam to premodify the MoS₂ films to achieve active surface domains with unbound sulfur. When the activated surface is subsequently immersed in AuCl₃ solution, Au NPs are found to preferentially decorate onto the modified regions. As a result, Au NPs can be selectively and locally anchored onto designated regions on the MoS₂ surface. With a scanning laser beam, microlandscapes comprising of Au NPs decorated on laser‐defined micropatterns are constructed. By varying the laser power, reaction time and thickness of the MoS₂ films, the size and density of the NPs can be controlled. The resulting hybrid materials are demonstrated as efficient Raman active surfaces for the detection of aromatic molecules with high sensitivity.     

Direct observation of inhomogeneous solid electrolyte interphase on MnO anode with atomic force microscopy and spectroscopy

Solid electrolyte interphase (SEI) is an in situ formed thin coating on lithium ion battery (LIB) electrodes. The mechanical property of SEI largely defines the cycling performance and the safety of LIBs but has been rarely investigated. Here, we report quantitatively the Young's modulus of SEI films on MnO anodes. The inhomogeneity of SEI film in morphology, structure, and mechanical properties provides new insights to the evolution of SEI on electrodes. Furthermore, the quantitative methodology established in this study opens a new approach to direct investigation of SEI properties in various electrode materials systems.     

Frictional and morphological properties of AuMoS2 films sputtered from a compact target

AuMoS₂ films 0.02–1.2 μm thick were sputtered from a target compacted from 5 wt.% Au plus 95 wt.% MoS₂ to investigate the frictional and morphological film growth characteristics. The gold dispersion effects in MoS₂ films are of interest to increase the densification and strengthening of the film structure. Three microstructural growth stages were identified on the nano-micro-macrostructural level. During sliding both sputtered AuMoS₂ and sputtered MoS₂ films have a tendency to break within the columnar region. The remaining or effective film, about 0.2 μm thick, performs the lubrication. The AuMoS₂ films displayed a lower friction coefficient with a high degree of frictional stability and less wear debris generation compared with pure MoS₂ films. The more favorable frictional characteristics of the AuMoS₂ films are attributed to the effective film thickness and the high density packed columnar zone which has a reduced effect on the fragmentation of the tapered crystallites during fracture.     

Large-area vapor-phase growth and characterization of MoS(2) atomic layers on a SiO(2) substrate

Atomic-layered MoS(2) is synthesized directly on SiO(2) substrates by a scalable chemical vapor deposition method. The large-scale synthesis of an atomic-layered semiconductor directly on a dielectric layer paves the way for many facile device fabrication possibilities, expanding the important family of useful mono- or few-layer materials that possess exceptional properties, such as graphene and hexagonal boron nitride (h-BN).     

Bandgap broadening at grain boundaries in single-layer MoS2

Nano Research - Two-dimensional semiconducting transition-metal dichalcogenides have attracted considerable interest owing to their unique physical properties and future device applications. In...     

Formation of epitaxial NiSi2 nanowires on Si(100) surface by atomic force microscope nanolithography

Ni nanowries were fabricated by atomic force microscope nanolithography, evaporation, lift-off and annealing processes. Epitaxial NiSi2 nanowires on a Si(100) surface along Si(110) and (100) directions were formed by the rapid thermal annealing treatment of the Ni nanowires at 400 degrees C. The silicide nanowires along the Si(110) direction had coherent type-A Si(111) and Si(100) interfaces, while those along the Si(100) direction had a type-A Si(110) interface. Silicide nanowires were agglomerated when the Ni nanowires were annealed at high temperature (> or = 500 degrees C). The mechanism of formation of a faceted nanowire was discussed based on the minimization of the total surface energy.     

Direct observation of X-ray induced atomic motion using scanning tunneling microscope combined with synchrotron radiation

X-ray induced atomic motion on a Ge(111)-c(2 x 8) clean surface at room temperature was directly observed with atomic resolution using a synchrotron radiation (SR)-based scanning tunneling microscope (STM) system under ultra high vacuum condition. The atomic motion was visualized as a tracking image by developing a method to merge the STM images before and after X-ray irradiation. Using the tracking image, the atomic mobility was found to be strongly affected by defects on the surface, but was not dependent on the incident X-ray energy, although it was clearly dependent on the photon density. The atomic motion can be attributed to surface diffusion, which might not be due to core-excitation accompanied with electronic transition, but a thermal effect by X-ray irradiation. The crystal surface structure was possible to break even at a lower photon density than the conventionally known barrier. These results can alert X-ray studies in the near future about sample damage during measurements, while suggesting the possibility of new applications. Also the obtained results show a new availability of the in-situ SR-STM system.     

Direct self-assembly of CTAB-capped Au nanotriangles

Densely packed and ordered “suprastructures” are new types of nanomaterials exhibiting broad applications. The direct self-assembly of cetyltrimethylammonium bromide (CTAB)-capped gold nanotriangles to form “suprastructures” was systematically investigated by varying the temperature and tilt angle of the silicon wafer used in the assembly process. Under optimal conditions, nanotriangles form into regular patterns, maintain their integrity, and form edge-to-edge, point-to-point, and face-to-face connections to form ordered “suprastructures” within an area of hundreds of square microns, achieving a high level of regularity. The formation of the “suprastructures” under optimal conditions could be mainly attributed to the complex balance between multiple temperature-dependent factors, including the atom diffusion rate, solvent evaporation rate, self-assembly rate, and the time for which the nanoparticle stays in the wet medium.

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