Growth of Highly Oriented (VNbMoTaW)S Layers.

Compositional tunability, an indispensable parameter for modifying the properties of materials, can open up new applications for van der Waals (vdW) layered materials such as transition-metal dichalcogenides (TMDCs). To date, multielement alloy TMDC layers are obtained via exfoliation from bulk polycrystalline powders. Here, we demonstrate direct deposition of high-entropy alloy disulfide, (VNbMoTaW)S, layers with controllable thicknesses on free-standing graphene membranes and on bare and hBN-covered AlO(0001) substrates via ultra-high-vacuum reactive dc magnetron sputtering of the VNbMoTaW target in Kr and HS gas mixtures.

Borazine Promoted Growth of Highly Oriented Thin Films.

We report on a phenomenon, where thin films sputter-deposited on single-crystalline AlO(0001) substrates exposed to borazine─a precursor commonly used for the synthesis of hexagonal boron nitride layers─are more highly oriented than those grown on bare AlO(0001) under the same conditions. We observed this phenomenon in face-centered cubic Pd, body-centered cubic Mo, and trigonal TaC thin films grown on AlO(0001). Interestingly, intermittent exposure to borazine during the growth of TaC thin films on TaC yields better crystallinity than direct deposition of monolithic TaC.

Self-Organized Growth of 111-Oriented (VNbTaMoW)N Nanorods on MgO(001).

Refractory high-entropy alloy nitride, (VNbTaMoW)N, layers are grown on single-crystalline MgO(001) via ultrahigh vacuum direct current magnetron sputtering of a VNbTaMoW target in Kr/N gas mixtures at 1073 K. X-ray diffraction, scanning and transmission electron microscopy, and energy dispersive X-ray spectroscopy characterizations revealed the formation of B1-structured, 111-textured (VNbTaMoW)N with lattice parameter = 0.4249 nm.

Ultra-high vacuum dc magnetron sputter-deposition of 0001-textured trigonal α-TaC/AlO(0001) thin films.

We report on the effects of substrate temperature (1073 K ≤ ≤ 1373 K) and deposition time (= 3 ~ 30 min.) on the crystallinity of TaC/AlO(0001) thin films grown via ultra-high vacuum direct current magnetron sputtering of TaC target in 20 mTorr (2.7 Pa) pure Ar atmospheres.

Growth Kinetics of Two-Dimensional Hexagonal Boron Nitride Layers on Pd(111).

Using variable-temperature scanning tunneling microscopy (300-673 K) during chemical vapor deposition of two-dimensional hexagonal boron nitride (hBN) on Pd(111) from borazine precursor at pressures up to 10 mbar, we identify the mechanisms leading to carpetlike uphill or downhill growth across the Pd steps. Deposition at a higher rate and lower temperature promotes uphill growth via preferential attachment at the ascending and descending step-edges, whereas a lower deposition rate and higher temperature lead to downhill growth via nucleation and growth of islands on Pd terraces. We attribute this unusual growth behavior to differences in temperature-dependent rates of hBN deposition at the steps versus on the Pd terraces.

Variable-Temperature Scanning Tunneling Microscopy Studies of Graphene Growth Using Benzene on Pd(111).

Using a combination of ultrahigh-vacuum variable-temperature scanning tunneling microscopy, Raman spectroscopy, and scanning electron microscopy, we investigated the growth of graphene using benzene on Pd(111) at temperatures up to 1100 K. Benzene adsorbs readily on Pd(111) at room temperature and forms an ordered superstructure upon annealing at 473 K. Exposure to benzene at 673 K enhances Pd step motion and yields primarily amorphous carbon upon cooling to room temperature.

Kinetics of Au-Ga Droplet Mediated Decomposition of GaAs Nanowires.

Particle-assisted III-V semiconductor nanowire growth and applications thereof have been studied extensively. However, the stability of nanowires in contact with the particle and the particle chemical composition as a function of temperature remain largely unknown. In this work, we use in situ transmission electron microscopy to investigate the interface between a Au-Ga particle and the top facet of an ⟨1̅1̅1̅⟩-oriented GaAs nanowire grown via the vapor-liquid-solid process.

Ultrahigh vacuum dc magnetron sputter-deposition of epitaxial Pd(111)/AlO(0001) thin films.

Pd(111) thin films, ∼245 nm thick, are deposited on AlO(0001) substrates at ≈0.5, where is the Pd melting point, by ultrahigh vacuum dc magnetron sputtering of Pd target in pure Ar discharges. Auger electron spectra and low-energy electron diffraction patterns acquired from the as-deposited samples reveal that the surfaces are compositionally pure 111-oriented Pd.

One-Step Synthesis of Tunable-Size Gold Nanoplates on Graphene Multilayers.

Au nanoplates (quasi-two-dimensional single crystals) are most commonly synthesized using a mixture of Au precursors via approaches involving multiple processing steps and the use of seed crystals. Here, we report the synthesis of truncated-hexagonal {111}-oriented micrometer-scale Au nanoplates on graphene multilayers using only potassium tetrabromoaurate (KAuBr) as the precursor. We demonstrate that the nanoplate sizes can be controllably varied from tens of nanometers up to a few micrometers by introducing desired concentrations of chloroauric acid (HAuCl) to KAuBr and their thicknesses from ∼13 to ∼46 nm with the synthesis time.

Control of Growth Front Evolution by Bi Additives during ZnAu Electrodeposition.

The performance of many electrochemical energy storage systems can be compromised by the formation of metal dendrites during charging. Additives in the electrolyte represent a useful strategy to mitigate dendrite formation, but understanding the mechanisms involved requires knowledge of the nanoscale effects of additives during electrochemical deposition. Here we quantify the effects of an inorganic additive on the morphology of an evolving electrochemical growth front, using liquid cell electron microscopy to provide the necessary spatial and temporal resolution.

Electrochemical electron beam lithography: Write, read, and erase metallic nanocrystals on demand.

We develop a solution-based nanoscale patterning technique for site-specific deposition and dissolution of metallic nanocrystals. Nanocrystals are grown at desired locations by electron beam-induced reduction of metal ions in solution, with the ions supplied by dissolution of a nearby electrode via an applied potential. The nanocrystals can be "erased" by choice of beam conditions and regrown repeatably.

Control of Electron Beam-Induced Au Nanocrystal Growth Kinetics through Solution Chemistry.

Measurements of solution-phase crystal growth provide mechanistic information that is helpful in designing and synthesizing nanostructures. Here, we examine the model system of individual Au nanocrystal formation within a defined liquid geometry during electron beam irradiation of gold chloride solution, where radiolytically formed hydrated electrons reduce Au ions to solid Au. By selecting conditions that favor the growth of well-faceted Au nanoprisms, we measure growth rates of individual crystals.

Shape-directional growth of Pt and Pd nanoparticles.

The design and synthesis of shape-directed nanoscale noble metal particles have attracted much attention due to their enhanced catalytic properties and the opportunities to study fundamental aspects of nanoscale systems. As such, numerous methods have been developed to synthesize crystals with tunable shapes, sizes, and facets by adding foreign species that promote or restrict growth on specific sites. Many hypotheses regarding how and why certain species direct growth have been put forward, however there has been no consensus on a unifying mechanism of nanocrystal growth.

Strategies to control morphology in hybrid group III-V/group IV heterostructure nanowires.

By combining in situ and ex situ transmission electron microscopy measurements, we examine the factors that control the morphology of "hybrid" nanowires that include group III-V and group IV materials. We focus on one materials pair, GaP/Si, for which we use a wide range of growth parameters. We show through video imaging that nanowire morphology depends on growth conditions, but that a general pattern emerges where either single kinks or inclined defects form some distance after the heterointerface.

Near room-temperature synthesis of transfer-free graphene films.

Large-area graphene films are best synthesized via chemical vapour and/or solid deposition methods at elevated temperatures (~1,000 °C) on polycrystalline metal surfaces and later transferred onto other substrates for device applications. Here we report a new method for the synthesis of graphene films directly on SiO(2)/Si substrates, even plastics and glass at close to room temperature (25-160 °C). In contrast to other approaches, where graphene is deposited on top of a metal substrate, our method invokes diffusion of carbon through a diffusion couple made up of carbon-nickel/substrate to form graphene underneath the nickel film at the nickel-substrate interface.

Moiré superstructures of graphene on faceted nickel islands.

Using scanning tunneling microscopy and spectroscopy, in combination with density functional theory calculations, we investigated the morphology and electronic structure of monolayer graphene grown on the (111) and (110) facets of three-dimensional nickel islands on highly oriented pyrolytic graphite substrate. We observed graphene domains exhibiting hexagonal and striped moiré patterns with periodicities of 22 and 12 Å, respectively, on (111) and (110) facets of the Ni islands. Graphene domains are also observed to grow, as single crystals, across adjacent facets and over facet boundaries.

Synthesis and surface activity of single-crystalline Co3O4 (111) holey nanosheets.

Single crystalline, thermally stable, Co(3)O(4) (111) holey nano-sheets were prepared by an efficient, template-free, wet chemical synthetic approach. The high energy (111) surfaces formed can be used as highly active heterogeneous catalysts for methanol decomposition.

Three-dimensional morphology control during wet chemical synthesis of porous chromium oxide spheres.

Controlling the morphological evolution in nanostructures is essential for improving their functionality, for example, in catalysis. Here, we demonstrate, using chromium oxide as a model system, that morphologies of functional binary oxide particles can be tailored by an efficient template-free synthetic approach. We construct a morphological "phase diagram" for chromium oxide spheres that shows the evolution of size and surface roughness as a function of the precursor and urea concentrations.

Growth of semiconducting graphene on palladium.

We report in situ scanning tunneling microscopy studies of graphene growth on Pd(111) during ethylene deposition at temperatures between 723 and 1023 K. We observe the formation of monolayer graphene islands, 200-2000 A in size, bounded by Pd surface steps. Surprisingly, the topographic image contrast from graphene islands reverses with tunneling bias, suggesting a semiconducting behavior.

Kinetic control of self-catalyzed indium phosphide nanowires, nanocones, and nanopillars.

The morphological phase diagram is reported for InP nanostructures grown on InP (111)B as a function of temperature and V/III ratio. Indium droplets were used as the catalyst and were generated in situ in the metalorganic vapor-phase epitaxy reactor. Three distinct nanostructures were observed: wires, cones, and pillars.

Self-catalyzed epitaxial growth of vertical indium phosphide nanowires on silicon.

Vertical indium phosphide nanowires have been grown epitaxially on silicon (111) by metalorganic vapor-phase epitaxy. Liquid indium droplets were formed in situ and used to catalyze deposition. For growth at 350 degrees C, about 70% of the wires were vertical, while the remaining ones were distributed in the 3 other <111> directions.

The morphology of axial and branched nanowire heterostructures.

We present an extensive investigation of the epitaxial growth of Au-assisted axial heterostructure nanowires composed of group IV and III-V materials and derive a model to explain the overall morphology of such wires. By analogy with 2D epitaxial growth, this model relates the wire morphology (i.e.

Control of Si nanowire growth by oxygen.

Semiconductor nanowires formed using the vapor-liquid-solid mechanism are routinely grown in many laboratories, but a comprehensive understanding of the key factors affecting wire growth is still lacking. In this paper we show that, under conditions of low disilane pressure and higher temperature, long, untapered Si wires cannot be grown, using Au catalyst, without the presence of oxygen. Exposure to oxygen, even at low levels, reduces the diffusion of Au away from the catalyst droplets.