In situ synthesis of horizontally aligned metal-boron alloy nanotubes on a silicon substrate with liquid crystal template

In this paper, an interesting strategy has been described for the direct synthesis of parallel aligned metal-boron alloy nanotubes on silicon substrates, involving the use of lyotropic liquid crystal of non-ionic-anionic mixed surfactants. In particular, super-long, up to the scale of millimetres, nanotubes with well-controlled inner and outer diameters can be obtained. The current lyotropic liquid crystal template method could be used as an effective strategy for the in situ synthesis of aligned one-dimensional nanostructures with the wet chemical method. The results further prove the rolling-up mechanism for the formation of noncrystal metal-boron alloy nanotubes with a layered lyotropic liquid crystal precursor.     

3D Orientational Control in Self‐Assembled Thin Films with Sub‐5 nm Features by Light

While self‐assembled molecular building blocks could lead to many next‐generation functional organic nanomaterials, control over the thin‐film morphologies to yield monolithic sub‐5 nm patterns with 3D orientational control at macroscopic length scales remains a grand challenge. A series of photoresponsive hybrid oligo(dimethylsiloxane) liquid crystals that form periodic cylindrical nanostructures with periodicities between 3.8 and 5.1 nm is studied. The liquid crystals can be aligned in‐plane by exposure to actinic linearly polarized light and out‐of‐plane by exposure to actinic unpolarized light. The photoalignment is most efficient when performed just under the clearing point of the liquid crystal, at which the cylindrical nanostructures are reoriented within minutes. These results allow the generation of highly ordered sub‐5 nm patterns in thin films at macroscopic length scales, with control over the orientation in a noncontact fashion.     

Aligned Growth of Millimeter‐Size Hexagonal Boron Nitride Single‐Crystal Domains on Epitaxial Nickel Thin Film

Atomically thin hexagonal boron nitride (h‐BN) is gaining significant attention for many applications such as a dielectric layer or substrate for graphene‐based devices. For these applications, synthesis of high‐quality and large‐area h‐BN layers with few defects is strongly desirable. In this work, the aligned growth of millimeter‐size single‐crystal h‐BN domains on epitaxial Ni (111)/sapphire substrates by ion beam sputtering deposition is demonstrated. Under the optimized growth conditions, single‐crystal h‐BN domains up to 0.6 mm in edge length are obtained, the largest reported to date. The formation of large‐size h‐BN domains results mainly from the reduced Ni‐grain boundaries and the improved crystallinity of Ni film. Furthermore, the h‐BN domains show well‐aligned orientation and excellent dielectric properties. In addition, the sapphire substrates can be repeatedly used with almost no limit. This work provides an effective approach for synthesizing large‐scale high‐quality h‐BN layers for electronic applications.     

van der Waals epitaxy of InAs nanowires vertically aligned on single-layer graphene

Semiconductor nanowire arrays integrated vertically on graphene films offer significant advantages for many sophisticated device applications. We report on van der Waals (VDW) epitaxy of InAs nanowires vertically aligned on graphene substrates using metal-organic chemical vapor deposition. The strong correlation between the growth direction of InAs nanowires and surface roughness of graphene substrates was investigated using various graphene films with different numbers of stacked layers. Notably, vertically well-aligned InAs nanowire arrays were obtained easily on single-layer graphene substrates with sufficiently strong VDW attraction. This study presents a considerable advance toward the VDW heteroepitaxy of inorganic nanostructures on chemical vapor-deposited large-area graphenes. More importantly, this work demonstrates the thinnest epitaxial substrate material that yields vertical nanowire arrays by the VDW epitaxy method.     

Enhancement of field emission and photoluminescence properties of graphene-SnO2 composite nanostructures

In this study, the SnO(2) nanostructures and graphene-SnO(2) (G-SnO(2)) composite nanostructures were prepared on n-Si (100) substrates by electrophoretic deposition and magnetron sputtering techniques. The field emission of SnO(2) nanostructures is improved largely by depositing graphene buffer layer, and the field emission of G-SnO(2) composite nanostructures can also further be improved by decreasing sputtering time of Sn nanoparticles to 5 min. The photoluminescence (PL) spectra of the SnO(2) nanostructures revealed multipeaks, which are consistent with previous reports except for a new peak at 422 nm. Intensity of six emission peaks increased after depositing graphene buffer layer. Our results indicated that graphene can also be used as buffer layer acting as interface modification to simultaneity improve the field emission and PL properties of SnO(2) nanostructures effectively.     

Self‐Aligned Anisotropic Plasmonic Nanostructures

Great opportunities emerge not only in the generation of anisotropic plasmonic nanostructures but also in controlling their orientation relative to incident light. Herein, a stepwise seeded growth method is reported for the synthesis of rod‐shaped plasmon nanostructures which are vertically self‐aligned with respect to the surface of colloidal substrates. Anisotropic growth of metal nanostructure is achieved by depositing metal seeds onto the surface of colloidal substrates and then selectively passivating the seed surface to induce symmetry breaking in the subsequent seed‐mediated growth process. The versatility of this method is demonstrated by producing nanoparticle dimers and linear trimers of Au, Au–Ag, Au–Pd, and Au–Cu₂O. Further, this unique method enables the automatic vertical alignment of the resulting plasmonic nanostructures to the surface of the colloidal substrate, thereby making it possible to design magnetic/plasmonic nanocomposites that allow the dynamic tuning of the plasmon excitation by controlling their orientation using an external magnetic field. The controlled anisotropic growth of colloidal plasmonic nanostructures and their dynamic modulation of plasmon excitation further allow them to be conveniently fixed in a thin polymer film with a well‐controlled orientation to display polarization‐dependent patterns that may find important applications in information encryption.     

Patterned Growth of Graphene over Epitaxial Catalyst

Rectangle‐ and triangle‐shaped microscale graphene films are grown on epitaxial Co films deposited on single‐crystal MgO substrates with (001) and (111) planes, respectively. A thin film of Co or Ni metal is epitaxially deposited on a MgO substrate by sputtering while heating the substrate. Thermal decomposition of polystyrene over this epitaxial metal film in vacuum gives rectangular or triangular pit structures whose orientation and shape are strongly dependent on the crystallographic orientation of the MgO substrate. Raman mapping measurements indicate preferential formation of few‐layer graphene films inside these pits. The rectangular graphene films are transferred onto a SiO₂/Si substrate while maintaining the original shape and field‐effect transistors are fabricated using the transferred films. These findings on the formation of rectangular/triangular graphene give new insights on the formation mechanism of graphene and can be applied for more advanced/controlled graphene growth.     

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.     

Patterning of Lyotropic Chromonic Liquid Crystals by Photoalignment with Photonic Metamasks

Controlling supramolecular self‐assembly in water‐based solutions is an important problem of interdisciplinary character that impacts the development of many functional materials and systems. Significant progress in aqueous self‐assembly and templating has been demonstrated by using lyotropic chromonic liquid crystals (LCLCs) as these materials show spontaneous orientational order caused by unidirectional stacking of plank‐like molecules into elongated aggregates. In this work, it is demonstrated that the alignment direction of chromonic assemblies can be patterned into complex spatially‐varying structures with very high micrometer‐scale precision. The approach uses photoalignment with light beams that exhibit a spatially‐varying direction of light polarization. The state of polarization is imprinted into a layer of photosensitive dye that is protected against dissolution into the LCLC by a liquid crystalline polymer layer. The demonstrated level of control over the spatial orientation of LCLC opens opportunities for engineering materials and devices for optical and biological applications.     

The role of nanoscale architecture in supramolecular templating of biomimetic hydroxyapatite mineralization

Understanding and mimicking the hierarchical structure of mineralized tissue is a challenge in the field of biomineralization and is important for the development of scaffolds to guide bone regeneration. Bone is a remarkable tissue with an organic matrix comprised of aligned collagen bundles embedded with nanometer-sized inorganic hydroxyapatite (HAP) crystals that exhibit orientation on the macroscale. Hybrid organic-inorganic structures mimic the composition of mineralized tissue for functional bone scaffolds, but the relationship between morphology of the organic matrix and orientation of mineral is poorly understood. Herein the mineralization of supramolecular peptide amphiphile templates, that are designed to vary in nanoscale morphology by altering the amino acid sequence, is reported. It is found that 1D cylindrical nanostructures direct the growth of oriented HAP crystals, while flatter nanostructures fail to guide the orientation found in biological systems. The geometric constraints associated with the morphology of the nanostructures may effectively control HAP nucleation and growth. Additionally, the mineralization of macroscopically aligned bundles of the nanoscale assemblies to create hierarchically ordered scaffolds is explored. Again, it is found that only aligned gel templates of cylindrical nanostructures lead to hierarchical control over hydroxyapatite orientation across multiple length scales as found in bone.     

Nanostructured biointerfaces

Colloidal lithography is used to create nanostructured interfaces suitable for studying and interacting with cellular biosystems. Large areas of patterned surface can be produced. We investigate the use of plasma etching for transfer of the pattern of individual colloidal particles into the substrates to create short-range ordered arrays of topographic and/or chemical nanostructures. Colloidal masks perform differently to traditional photoresist masks and local redeposition of material around the particle has significant impact on the resultant structures. The colloidal particles can be reshaped to allow the fabrication of flat-topped structures. Topographic and chemical nanostructures can be used to template the self assembly of macromolecules. The phase separation of thin films of PS-PMMA symmetric block copolymers above nanostructure sites is aligned at the surface nanostructure by topographic features. PLL-PEG assembly at alkanethiol-modified nanoscale chemical patterns of gold/silicon allows the production of nanoscale protein patterns. Patterns of ferritin 100 nm in diameter are demonstrated.     

Alignment of colloidal graphene quantum dots on polar surfaces

Controlling the orientation of nanostructures with anisotropic shapes is essential for taking advantage of their anisotropic electrical, optical, and transport properties in electro-optical devices. For large-area alignment of nanocrystals, so far orientations are mostly induced and controlled by external physical parameters, such as applied fields or changes in concentration. Herein we report on assemblies of colloidal graphene quantum dots, a new type of disk-shaped nanostructures, on polar surfaces and the control of their orientations. We show that the orientations of the graphene quantum dots can be determined, either in- or out-of-plane with the substrate, by chemical functionalization that introduces orientation-dependent interactions between the quantum dots and the surfaces.     

Quantum Confinement of Dirac Quasiparticles in Graphene Patterned with Sub-Nanometer Precision

Quantum confinement of graphene Dirac-like electrons in artificially crafted nanometer structures is a long sought goal that would provide a strategy to selectively tune the electronic properties of graphene, including bandgap opening or quantization of energy levels. However, creating confining structures with nanometer precision in shape, size, and location remains an experimental challenge, both for top-down and bottom-up approaches. Moreover, Klein tunneling, offering an escape route to graphene electrons, limits the efficiency of electrostatic confinement. Here, a scanning tunneling microscope (STM) is used to create graphene nanopatterns, with sub-nanometer precision, by the collective manipulation of a large number of H atoms. Individual graphene nanostructures are built at selected locations, with predetermined orientations and shapes, and with dimensions going all the way from 2 nm up to 1 µm. The method permits the patterns to be erased and rebuilt at will, and it can be implemented on different graphene substrates. STM experiments demonstrate that such graphene nanostructures confine very efficiently graphene Dirac quasiparticles, both in 0D and 1D structures. In graphene quantum dots, perfectly defined energy bandgaps up to 0.8 eV are found that scale as the inverse of the dot’s linear dimension, as expected for massless Dirac fermions.     

Hyperfine interactions in graphene and related carbon nanostructures

Hyperfine interactions, magnetic interactions between the spins of electrons and nuclei, in graphene and related carbon nanostructures are studied. By using a combination of accurate first principles calculations on graphene fragments and statistical analysis, I show that both isotropic and dipolar hyperfine interactions in sp2 carbon nanostructures can be accurately described in terms of the local electron spin distribution and atomic structure. A complete set of parameters describing the hyperfine interactions of 13C and other nuclear spins at substitution impurities and edge terminations is determined. These results permit the design of graphene-based nanostructures allowing for longer electron spin coherence times which are required by spintronics and quantum information processing applications. Practical recipes for minimizing hyperfine interactions in carbon nanostructures are given.     

Local electronic properties of graphene on a BN substrate via scanning tunneling microscopy

The use of boron nitride (BN) as a substrate for graphene nanodevices has attracted much interest since the recent report that BN greatly improves the mobility of charge carriers in graphene compared to standard SiO(2) substrates. We have explored the local microscopic properties of graphene on a BN substrate using scanning tunneling microscopy. We find that BN substrates result in extraordinarily flat graphene layers that display microscopic Moire? patterns arising from the relative orientation of the graphene and BN lattices. Gate-dependent dI/dV spectra of graphene on BN exhibit spectroscopic features that are sharper than those obtained for graphene on SiO(2). We observe a significant reduction in local microscopic charge inhomogeneity for graphene on BN compared to graphene on SiO(2).     

The driving force for homeotropic alignment of a triphenylene derivative in a hexagonal columnar mesophase on single substrates

An interesting discotic triphenylene derivative, 2,7-biscarbethoxy-3,6,10,11-tetrapentyloxytriphenylene, can be homeotropically aligned on a large scale as a film on single substrates, i.e. the film with one side on the substrate and the other side freestanding. The alignment is achieved in a controlled condition close to a thermodynamic equilibrium state in a liquid crystal phase when interactions at the substrate surface are limited. Phase changing properties of this material are also a key factor in the alignment processes. The aligned structures are observed with an optical microscope. Within a single domain a hexagonal lattice of a d-spacing 16.3 Å is confirmed by X-ray diffraction measurement.     

Nanoscale strainability of graphene by laser shock-induced three-dimensional shaping

Graphene has many promising physical properties. It has been discovered that local strain in a graphene sheet can alter its conducting properties and transport gaps. It is of great importance to develop scalable strain engineering techniques to control the local strains in graphene and understand the limit of the strains. Here, we present a scalable manufacturing process to generate three-dimensional (3D) nanostructures and thus induce local strains in the graphene sheet. This process utilizes laser-induced shock pressure to generate 3D tunable straining in the graphene sheet. The size dependent straining limit of the graphene and the critical breaking pressure are both studied. It is found that the graphene film can be formed to a circular mold (～50 nm in diameter) with an aspect ratio of 0.25 and strain of 12%, and the critical breaking pressure is 1.77 GPa. These values were found to be decreasing with the increase of mold size. The local straining and breaking of graphene film are verified by Raman spectra. Large scale processing of the graphene sheet into nanoscale patterns is presented. The process could be scaled up to roll-to-roll process by changing laser beam size and scanning speed. The presented laser shock straining approach is a fast, tunable, and low-cost technique to realize strain engineering of graphene for its applications in nanoelectrical devices.     

Growth and process conditions of aligned and patternable films of iron(III) oxide nanowires by thermal oxidation of iron

A simple, catalyst-free growth method for vertically aligned, highly crystalline iron oxide (α-Fe(2)O(3)) wires and needles is reported. Wires are grown by the thermal oxidation of iron foils. Growth properties are studied as a function of temperature, growth time and oxygen partial pressure. The size, morphology and density of the nanostructures can be controlled by varying growth temperature and time. Oxygen partial pressure shows no effect on the morphology of resulting nanostructures, although the oxide thickness increases with oxygen partial pressure. Additionally, by using sputtered iron films, the possibility of growth and patterning on a range of different substrates is demonstrated. Growth conditions can be adapted to less tolerant substrates by using lower temperatures and longer growth time. The results provide some insight into the mechanism of growth.     

Carbon-assisted synthesis of nanowires and related nanostructures of MgO

MgO nanowires and related nanostructures have been prepared by carbon-assisted synthesis, starting from polycrystalline MgO or Mg without the use of metal catalysts. The study has been carried out with different sources of carbon, all of them yielding the nanostructures with some differences. It has been possible to obtain nanotrees and other interesting nanostructures by this method. It has also been possible to obtain aligned MgO nanowires by carbon-assisted synthesis over Au-coated Si substrates. A vapor-solid mechanism of one-dimensional growth seems to be operative in the reactions carried out in bulk, but a vapor-liquid-solid mechanism applies when Si substrates are used.     

Large physisorption strain in chemical vapor deposition of graphene on copper substrates

Graphene single layers grown by chemical vapor deposition on single crystal Cu substrates are subject to nonuniform physisorption strains that depend on the orientation of the Cu surface. The strains are revealed in Raman spectra and quantitatively interpreted by molecular dynamics (MD) simulations. An average compressive strain on the order of 0.5% is determined in graphene on Cu(111). In graphene on Cu (100), MD simulations interpret the observed highly nonuniform strains.