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Solution-phase processes such as colloidal synthesis and transformations have enabled the formation of nanocrystals with exquisite control over size, shape, and composition. Self-assembly, in solution or at phase boundaries, can arrange such nanocrystal building blocks into ordered superlattices and dynamically reconfigurable "smart" materials. Ultimately, continued improvements in our ability to direct nanocrystal matter depend on progress in understanding colloidal chemistry and self-assembly in solution. The traditional approach for investigating the underlying, inherently dynamic processes involves sampling at different stages combined with ex situ characterization, for example, using electron microscopy. In situ studies have been restricted to a few methods capable of measuring in bulk liquids, either in reciprocal space by diffraction or scattering or using spatially averaging (e.g., optical) measurements. These strategies face clear limitations in obtaining mechanistic information, and they are unable to address heterogeneous systems that may harbor rich sets of configurations with different local properties. The development of microfabricated cells that hermetically encapsulate bulk solutions between ultrathin (electron transparent) membranes has paved the way for studying processes in liquids in real time by electron microscopy at resolution down to the atomic scale. Electrons interact much more strongly with matter than other probes, for example, X-rays. In ordinary inorganic samples, the main effects are atom displacements and defect formation via knock-on and ionization damage. In liquid-cell electron microscopy, the interaction of the beam with both the suspended nanostructures and the solution creates more diverse effects, so the straightforward scenario of imaging unperturbed nanocrystal chemistry in solution is rarely realized.In this Account, we discuss applications of real-time electron microscopy to the analysis of nanocrystal synthesis, transformations, and self-assembly in solution. While in the simplest case the effects of the electron beam are negligible, the interaction with high-energy electrons often provides excitation or stimulus for solution-phase processes or opens up competing chemical pathways. Real-time observations of self-assembly demonstrate particularly clearly the power of in situ microscopy in identifying key nucleation and growth mechanisms and providing information about preferred structural motifs that can be analyzed to quantify the balance of forces and the role of entropy in stabilizing ordered assemblies. Modifications of the solution by the electron beam can provide stimuli for on-demand self-assembly, for example, via an acid spike due to water radiolysis that locally lowers the pH in the imaged area. While in this and other cases (e.g., colloidal synthesis), beam-induced radicals become part of the experimental design, in imaging redox reactions such as galvanic transformations of nanocrystal templates, radicals need to be managed and if possible eliminated by suitable scavengers. Finally, excitation by the imaging electron beam can transfer energy to individual nanocrystals in solution, thus driving nonthermal (e.g., plasmon-mediated) synthesis or other chemistry while following the reaction progress with high resolution. Overall, with validation by ex situ control experiments, the unique ability of observing processes in solution at the nanometer scale should make liquid-cell electron microscopy an integral part of the toolkit for designing novel inorganic nanocrystal architectures.

Two-dimensional group IV monochalcogenide semiconductors (SnX, GeX; X = S, Se) are of fundamental interest due to their anisotropic crystal structure and predicted unique characteristics such as very large exciton binding energies and multiferroic order possibly up to above room temperature. Whereas growth on reactive supports produces mostly standing flakes, deposition on van der Waals (vdW) substrates can yield basal-plane oriented layered crystals. But so far, this approach invariably resulted in flakes that are several atomic layers thick, and the synthesis of monolayers has remained elusive. Here, we use in situ microscopy during molecular beam epitaxy of SnS on graphite and graphene to establish the origin of this predominant multilayer growth. The enhanced reactivity of group IV chalcogenide layers causes adsorption of precursor molecules primarily on the initial SnS nuclei instead of the vdW support. On graphite, this unusual imbalance in the material supply is the primary cause for fast vertical ...

Electron-beam (e-beam) irradiation damage is often regarded as a severe limitation to atomic-scale study of two-dimensional (2D) materials using electron microscopy techniques. However, energy transferred from the e-beam can also provide a way to modify 2D materials via defect engineering when the interaction of the beam with the sample is precisely controlled. In this article, we discuss the atomic geometry, formation mechanism, and properties of several types of structural defects, ranging from zero-dimensional point defects to extended domains, induced by an e-beam in a few representative 2D materials, including graphene, hexagonal boron nitride, transition-metal dichalcogenides, and phosphorene. We show that atomic as well as line defects and even novel nanostructures can be created and manipulated in 2D materials by an e-beam in a controllable manner. Phase transitions can also be induced. The e-beam in a (scanning) transmission electron microscope not only resolves the intrinsic atomic structure of materials with defects, but also provides new opportunities to modify the structure with subnanometer precision.

Solution-phase self-assembly of nanocrystals into mesoscale structures is a promising strategy for constructing functional materials from nanoscale components. Liquid environments are key to self-assembly since they allow suspended nanocrystals to diffuse and interact freely, but they also complicate experiments. Real-time observations with single-particle resolution could have transformative impact on our understanding of nanocrystal self-assembly. Here we use real-time in situ imaging by liquid-cell electron microscopy to elucidate the nucleation and growth mechanism and properties of linear chains of octapod-shaped nanocrystals in their native solution environment. Statistical mechanics modelling based on these observations and using the measured chain-length distribution clarifies the relative importance of dipolar and entropic forces in the assembly process and gives direct access to the interparticle interaction. Our results suggest that monomer-resolved in situ imaging combined with modelling can provide unprecedented quantitative insight into the microscopic processes and interactions that govern nanocrystal self-assembly in solution., Understanding the structure and transformation of colloidal matter requires probing configurations from monomers to extended assemblies. Here, the authors use liquid-cell electron microscopy to elucidate the nucleation and growth properties of linear chains of branched nanocrystals in solution.

Oligonucleotide–nanoparticle conjugates, also called programmable atom equivalents, carry promise as building blocks for self-assembled colloidal crystals, reconfigurable or stimuli responsive functional materials, as well as bio-inspired hierarchical architectures in wet environments. In situ studies of the DNA-mediated self-assembly of nanoparticles have so far been limited to reciprocal space techniques. Liquid-cell electron microscopy could enable imaging such systems with high resolution in their native environment but to realize this potential, radiation damage to the oligonucleotide linkages needs to be understood and conditions for damage-free electron microscopy identified. Here, we analyze in situ observations of DNA-linked two-dimensional nanoparticle arrays, along with control experiments for different oligonucleotide configurations, to identify the mechanisms of radiation damage for ordered superlattices of DNA–nanoparticle conjugates. In a biological context, the results point to new avenues for studying direct and indirect radiation effects for small ensembles of DNA in solution by tracking conjugated nanoparticles. By establishing low-dose conditions suitable for extended in situ imaging of programmable atom equivalents, our work paves the way for real-space observations of DNA-mediated self-assembly processes.

Layered semiconductors show promise as channel materials for field-effect transistors (FETs). Usually, such devices incorporate solid back or top gate dielectrics. Here, we explore deionized (DI) water as a solution top-gate for field-effect switching of layered semiconductors including SnS2, MoS2, and black phosphorus. The DI water gate is easily fabricated, can sustain rapid bias changes, and its efficient coupling to layered materials provides high on–off current ratios, near-ideal subthreshold swing, and enhanced short-channel behavior even for FETs with thick, bulk-like channels, where such control is difficult to realize with conventional back gating. Screening by the high-k solution gate eliminates hysteresis due to surface and interface trap states and substantially enhances the field-effect mobility. The onset of water electrolysis sets the ultimate limit to DI water gating at large negative gate bias. Measurements in this regime show promise for aqueous sensing, demonstrated here by the amperom...

Mechanical exfoliation has been a key enabler of the exploration of the properties of two-dimensional materials, such as graphene, by providing routine access to high-quality material. The original exfoliation method, which remained largely unchanged during the past decade, provides relatively small flakes with moderate yield. Here, we report a modified approach for exfoliating thin monolayer and few-layer flakes from layered crystals. Our method introduces two process steps that enhance and homogenize the adhesion force between the outermost sheet in contact with a substrate: Prior to exfoliation, ambient adsorbates are effectively removed from the substrate by oxygen plasma cleaning, and an additional heat treatment maximizes the uniform contact area at the interface between the source crystal and the substrate. For graphene exfoliation, these simple process steps increased the yield and the area of the transferred flakes by more than 50 times compared to the established exfoliation methods. Raman and AFM characterization shows that the graphene flakes are of similar high quality as those obtained in previous reports. Graphene field-effect devices were fabricated and measured with back-gating and solution top-gating, yielding mobilities of ∼4000 and 12,000 cm(2)/(V s), respectively, and thus demonstrating excellent electrical properties. Experiments with other layered crystals, e.g., a bismuth strontium calcium copper oxide (BSCCO) superconductor, show enhancements in exfoliation yield and flake area similar to those for graphene, suggesting that our modified exfoliation method provides an effective way for producing large area, high-quality flakes of a wide range of 2D materials.

Materials integration in heterostructures with novel properties different from those of the constituents has become one of the most powerful concepts of modern materials science. Two-dimensional (2D) crystals represent a new class of materials from which such engineered structures can be envisioned. Calculations have predicted emergent properties in 2D heterostructures with nanoscale feature sizes, but methods for their controlled fabrication have been lacking. Here, we use sequential graphene and boron nitride growth on Ru(0001) to show that lateral heteroepitaxy, the joining of 2D materials by preferential incorporation of different atomic species into exposed 1D edges during chemical vapor deposition on a metal substrate, can be used for the bottom-up synthesis of 2D heterostructures with characteristic dimensions on the nanoscale. Our results suggest that on a proper substrate, this method lends itself to building nanoheterostructures from a wide range of 2D materials.

Two-dimensional Au islands of different thicknesses grown on graphene/Ru(0001) were used to study the corrugation of the moire structure of graphene/Ru(0001) and discriminate between its mainly structural or electronic character. A comparison of the apparent corrugation measured by scanning tunneling microscopy (STM) for different Au thicknesses with results of elasticity theory equations applied to a gold film over a corrugated substrate shows that the corrugation observed for the graphene/Ru(0001) moire is of structural nature rather than electronic. STM showed a large value for the corrugation of the first Au monolayer on graphene/Ru(0001), 1.7 A; using density functional theory calculations, we explain this large corrugation of the Au monolayer as the result of a strong (weak) binding of the Au layer at the valley (hill) regions of the graphene/Ru(0001) moire structure and infer an actual corrugation of the graphene/Ru(0001) moire structure of ∼1.2 A from the measured corrugation of the Au monolayer.

Using in situ surface microscopy, we study the processes associated with the intercalation and reaction of oxygen at the graphene/Ru(0001) interface. The decoupling of the graphene from the metal d...

The eutectic alloy of BiInSn was ablated in water by UV pulsed radiation. Electron microscopy of the ablated material shows spherical particles that fall into three size regimes: those with diameters of ∼0.5 μm, crystalline and amorphous particles with dimensions of ∼30 nm, and amorphous particles that are approximately 1 nm across. The 30-nm amorphous particles are homogeneous, while there are two types of 30-nm crystalline particles, those that separate into three phases and those that are homogeneous. The existence of different characteristic sizes is explained by two mechanisms: phase explosion and Rayleigh instability of the ejected melt.

The Pt monolayer-shell Pd nanowires/nanorods-core (PtML/PdNW(PdNR)) electrocatalysts, supported on carbon nanoparticles are notable for their very high activity and stable performance towards the oxygen reduction reaction (ORR). In the present work Pd nanostructures are electrochemically deposited on oxidized carbon (COX) surface and the impact of the deposition parameters on the deposits morphology is studied. The electrochemical deposition of Pd nanowires is demonstrated for the first time, and a viable mechanism for the growth of Pd nanostructures is proposed. The one-dimensional growth of the Pd nanowires is ascribed to the hydrogen-underpotential-mediated layer-by-layer deposition of Pd. In addition, the surface structure of deposited nanowires is composed predominantly of {111}-oriented facets that facilitate the enhanced ORR activity and stability of the PtML/PdNW/COX electrocatalysts. The Pt mass and specific activities of these electrocatalysts, as well as their stable performance under potential cycling conditions, are significantly higher than those of the commercial Pt/C electrocatalysts, and highlight their great potential for resolving the remaining obstacles to commercializing PEMFCs.

Ru nanoclusters self-assemble over macroscopic sample areas during vapor deposition of Ru on monolayer graphene (MLG) on Ru(0001). The Ru nanoclusters form arrays with a mean lateral cluster diameter of ~ 20 A, cluster heights of 1 or 2 ML, and a size distribution that remains nearly constant with increasing coverage. Combined scanning tunneling microscopy and density functional theory (DFT) show that the clusters are templated by the MLG/Ru(0001) moire unit cell and identify the preferred binding site of the clusters as the low fcc region of the moire. Cross-sectional transmission electron microscopy (TEM) and high-resolution TEM contrast simulations experimentally demonstrate that the interaction of the Ru clusters with the underlying MLG/Ru(0001) leads to a local lifting of the graphene layer of the template. DFT calculations confirm this mechanism of interaction of the Ru clusters with the strongly coupled MLG/Ru(0001). Weakening of the graphene-support coupling via oxygen intercalation is shown to have a major effect on the assembly of Ru nanocluster arrays. With a preferred binding site lacking on decoupled graphene, the Ru nanoclusters grow significantly larger, and clusters with 1 to 4 ML height can coexist.

Monolayer graphene was grown on polycrystalline Ru thin films on patterned fused silica. The Ru films grow with columnar structure with strongly aligned grains exposing flat (0 0 0 1) surface facets within the 3D geometric patterns and on the adjacent planar silica surface. The monolayer graphene was found to completely and uniformly cover the Ru films on the complex engineered substrates. In addition, we demonstrate that the single atomic layer graphene protects the underlying metal surface against reaction with ambient gases of particular importance for applications such as concave focusing mirrors, non-planar microelectrode arrays, etc.

Monitoring the photocurrent generated as a laser scans across a graphene field-effect device subjected to low temperature and high magnetic fields enables the spatial distribution of Landau levels across a graphene sheet to be mapped. This in turn allows the relative contribution of bulk and edge states to the macroscopic electrical characteristics of these devices to be determined. Band bending and the associated spatially inhomogeneous population of Landau levels play a central role in the physics of the quantum Hall effect (QHE) by constraining the pathways for charge-carrier transport and scattering1. Recent progress in understanding such effects in low-dimensional carrier gases in conventional semiconductors has been achieved by real-space mapping using local probes2,3. Here, we use spatially resolved photocurrent measurements in the QHE regime to study the correlation between the distribution of Landau levels and the macroscopic transport characteristics in graphene. Spatial maps show that the net photocurrent is determined by hot carriers transported to the periphery of the graphene channel, where QHE edge states provide efficient pathways for their extraction to the contacts. The photocurrent is sensitive to the local filling factor, which allows us to reconstruct the local charge density in the entire conducting channel of a graphene device.

The electronic structure of epitaxial monolayer, bilayer, and trilayer graphene on Ru(0001) was determined by selected-area angle-resolved photoelectron spectroscopy (micro-ARPES). Micro-ARPES band maps provide evidence for a strong electronic coupling between monolayer graphene and the adjacent metal, which causes the complete disruption of the graphene pi-bands near the Fermi energy. However, the perturbation by the metal decreases rapidly with the addition of further graphene sheets, and already an epitaxial graphene bilayer on Ru recovers the characteristic Dirac cones of isolated monolayer graphene. A graphene trilayer on Ru behaves like free-standing bilayer graphene. Density-functional theory based calculations show that this decoupling is due to the efficient passivation of metal d-states by the interfacial graphene layer.

The controlled delivery of fluids is a key process in nature and in many areas of science and technology, where pipettes or related devices are used for dispensing well-defined fluid volumes. Existing pipettes are capable of delivering fluids with attolitre (10−18 l) accuracy at best1. Studies on phase transformations of nanoscale objects would benefit from the controlled dispensing and manipulation of much smaller droplets. In contrast to nanoparticle melting whose fundamental pathway was studied extensively2, experiments on crystallization, testing classical nucleation theory3, are hindered by the influence of support interfaces. Experiments on free-standing fluid drops are extremely challenging4. Here, we demonstrate the operation of a pipette, which, observed by transmission electron microscopy, delivers a metal-alloy melt with zeptolitre (10−21 l) resolution. We use this exquisite control to produce nearly free-standing Au72Ge28 drops suspended by an atomic-scale meniscus at the pipette tip, and to image their phase transformations with near-atomic resolution. Our observations of the liquid–solid transition challenge classical nucleation theory3 by providing experimental evidence for an intrinsic crystallization pathway of nanometre-sized fluid drops that avoids nucleation in the interior, but instead proceeds via liquid-state surface faceting as a precursor to surface-induced crystallization.

Selective epitaxy growth (SEG) was used to build SiGe optoelectronic devices and nanoscale structures for the future nanotechnology. The growth of strained SiGe on small areas offers some advantages for improvement of device performances. In particular, with relative large area light emitting diodes (LED), the emission efficiency of SiGe diodes can be increased up to 0.1% internal value at room temperature. Further improvements are expected for nanoscale devices. By SEG on mesas, Ge islands can be obtained ordered in lines along the mesas edges. Precise localization of Ge dots can be obtained by SEG in very small oxide windows and even only one island/window is formed. It was shown that nanostructures of size down to 5 nm can be grown by this method.

Narrow graphene nanoribbons (GNRs) constructed by atomically precise bottom-up synthesis from molecular precursors have attracted significant interest as promising materials for nanoelectronics. But there has been little awareness of the potential of GNRs to serve as nanoscale building blocks of novel materials. Here we show that the substitutional doping with nitrogen atoms can trigger the hierarchical self-assembly of GNRs into ordered metamaterials. We use GNRs doped with eight N atoms per unit cell and their undoped analogues, synthesized using both surface-assisted and solution approaches, to study this self-assembly on a support and in an unrestricted three-dimensional (3D) solution environment. On a surface, N-doping mediates the formation of hydrogen-bonded GNR sheets. In solution, sheets of side-by-side coordinated GNRs can in turn assemble via van der Waals and π-stacking interactions into 3D stacks, a process that ultimately produces macroscopic crystalline structures. The optoelectronic properties of these semiconducting GNR crystals are determined entirely by those of the individual nanoscale constituents, which are tunable by varying their width, edge orientation, termination, and so forth. The atomically precise bottom-up synthesis of bulk quantities of basic nanoribbon units and their subsequent self-assembly into crystalline structures suggests that the rapidly developing toolset of organic and polymer chemistry can be harnessed to realize families of novel carbon-based materials with engineered properties.

β-FeSi2 embedded in a Si matrix was prepared by ion beam synthesis (IBS). Two step implantation, with energies 60 and 20 keV, of two different doses of the iron ions, 5 × 1015 and 5 × 1016 cm−1, was performed. After the implantation, the samples were subjected to rapid thermal annealing (RTA) at 900 °C. The crystal structure of the resulting material was studied using cross-sectional transmission electron microscopy (XTEM), including high-resolution electron microscopy (HREM). The comparison of the XTEM images with the initial iron ions implantation profiles, simulated by SRIM (Stopping and Range of Ions in Matter) demonstrate that the process of IBS, followed by RTA, preserves the initial implantation profile, implying a negligible Fe atoms diffusion velocity in comparison with the one of the chemical reaction between Fe and Si. The XTEM images show that continuous β-FeSi2 layers are fabricated when there is a stoichiometric region in the initial implantation profile. Fe concentration lower than the stoichiometric one in the whole implantation range results in formation of β-FeSi2 nanocrystallites embedded in the Si matrix. The behavior of the absorption coefficient energy dependences, obtained from the optical transmittance and reflectance measurements, reflects the different crystal structures forming in the two types of samples.

Semiconductor nanowires (NWs) have attracted significant interest recently due to quantum size effects and novel properties that they will bring to a wide range of electronic and optoelectronic devices, chemical and biological sensors, and photovoltaics. Among the broad range of materials from which NWs can be synthesized, group IV semiconductor (Si, Ge) NWs and NW heterostructures are distinct as they promise materials compatibility and facile integration with conventional Si-based circuits. This promise has stimulated extensive efforts in developing reliable synthesis routes that yield bulk quantities of high-quality group IV NW material, including laser ablation, chemical vapor deposition (CVD) from different precursors and catalysts, chemical vapor transport reactions, and low-temperature solution-phase synthesis. Ge offers a number of properties that are superior to those of Si in device applications, for example, higher carrier mobility and larger exciton radius, and hence stronger quantum confinement in a NW, and the prospect of lower processing temperatures and thus easier integration with conventional devices. However, in contrast to Si, which forms a stable oxide with an electronically benign Si/SiO2 interface, Ge forms oxides with poor chemical stability and unfavorable electrical properties. After exposure to air, Ge NWs invariably exhibit surface oxides with thicknesses of about 2–4 nm, a significant fraction of the NW diameter. Methods to inhibit or control surface oxidation, achieve chemical stability under ambient conditions, and thus prevent uncontrolled changes of the electronic properties due to oxidation, will be an important part of any future Ge NW-based device technology. First efforts to address this fundamental issue have been initiated only recently. Layered materials, such as graphite, MoS2, or BN, in which two-dimensional sheets with complete internal bonding (i.e., no dangling bonds) are joined by van der Waals forces, are chemically inert and could be used to prevent oxidation if assembled as a protective shell around individual Ge NWs. In addition to providing an oxidation barrier, the encapsulation of the semiconductor NWs in a graphitic carbon shell would have the benefit of an already comprehensive understanding of the chemical functionalization of C surfaces, which may open up new avenues for sensing applications based on Ge/C core/shell NWs. In the presence of sufficient C, nanoparticles of some metals (Au, Pb, Sn, Fe, Ni, Co, Cu, Ti) are readily encapsulated in graphitic C shells, as metal surfaces appear to catalyze and template the assembly of ordered C sheets. Surface chemistry and reactivity of most semiconductors, on the other hand, preclude the growth of ordered C sheets, both due to the absence of catalytic effects found for metals and because of the formation of stable carbide compounds. Here, we use in situ transmission electron microscopy (TEM) experiments to demonstrate the controlled encapsulation of Ge wires in C shells at moderate temperatures. Traces of Au at the NW surface, originating from the Au/Ge catalyst particle used for growing high-aspect-ratio wires, are shown to be critical to the assembly of fragments of graphene C sheets, which then transform into well-defined multilayer shells that completely encapsulate the Ge NWs. Examination of the encapsulated wires after prolonged exposure to ambient conditions shows that the C shells remain intact and unchanged, and provide an efficient oxidation barrier for the Ge wires. Our variable-temperature in situ experiments are carried out in the temperature range between room temperature and 400 °C on C-supported Ge NWs synthesized prior to the in situ studies using CVD. Figure 1 shows TEM images of the structure and morphology of Ge NWs, obtained at room temperature prior to our in situ experiment; these images are representative of the as-grown starting material in our study. The inset of Figure 1a shows an overview of a group of wires dispersed on the C film of the TEM grid. Our method of NW dispersal creates Ge wire segments severed from the Si substrate, comprising the wire shaft as well as the tip with the residual Au/Ge catalyst particle. The slightly tapered NWs have a typical diameter of 10–50 nm at the tip, 100–200 nm at the base, and are several micrometers long. The wires have monocrystalline cores and well-defined faceted surfaces, and all exhibit a 1–4 nm thick oxide shell, as would be expected after transfer through ambient air. The TEM images of Figure 1a–c show the detailed structure close to the NW tip for one of the wires from the inset. For all NWs the Au/Ge nanoparticle at the tip shows darker contrast than the Ge wire. In high-resolution TEM images (Fig. 1b and c) the (111) lattice planes of the Au/Ge alloy with separation (0.219 nm) close to the lattice spacing of Au are clearly resolved. The NW axis is aligned with the [111] direction. The high-resolution images show lattice fringes with separation of 0.327 nm perpendicular to the C O M M U N IC A IO N S

Selective epitaxial growth was used to fabricate narrow Si/Ge/Si pillar nanostructures in small holes in ultrathin oxide (UTO) on Si(100). The self-assembled holes with diameters of 5-30 nm were obtained by in situ partial removal of the UTO at high temperature. The UTO formation and the annealing process were optimized for a high density of holes. The SiGe nanopillars were grown with sizes determined by the initial hole diameter in the UTO. Crystalline Ge dots embedded in oxide were formed by oxidation of the pillar nanostructures. High-resolution transmission electron microscopy (HRTEM) was used to study the pillar nanostructures and the dot shapes before and after oxidation. Capacitors obtained with the oxidized samples showed a hysteresis in their C-V curves attributed to charge retention in the Ge dots embedded in the oxide.

Ion-beam synthesized (IBS) samples, prepared with a low dose of iron ions and subjected to rapid thermal annealing (RTA) were studied. The samples were characterized with cross-sectional transmission electron microscopy (XTEM), including high-resolution electron microscopy (HREM) and far infrared transmittance (FIRT) spectroscopy. The formation of ?-FeSi2 nanocrystallites, with various shapes and sizes, in the Si matrix was revealed. The optical constants dispersions of the samples were obtained from the reflectance (R) and transmittance (T) spectra, taken between 0.38 and 1.2?eV. From these dispersions, the energy band diagram of the interface ?-FeSi2/Si was determined, and compared to those reported by other authors.

In-situ liquid cell transmission and scanning transmission electron microscopy (TEM/STEM) experiments are important, as they provide direct insight into processes in liquids, such as solution growth of nanoparticles, among others. In liquid cell TEM/STEM redox reaction experiments, the hydrated electrons e(-)aq created by the electron beam are responsible for the reduction of metal-ion complexes. Here we investigate the rate equation of redox reactions involving reduction by e(-)aq generated by the electron beam during in situ liquid TEM/STEM. Specifically we consider the growth of Pd on Au seeds in aqueous solutions containing Pd-chloro complexes. From the quantification of the rate of Pd deposition at different electron beam currents and as a function of distance from a stationary, nanometer-sized exciting beam, we determine that the reaction is first order with respect to the concentration of hydrated electrons, [e(-)aq]. By comparing Pd- and Au-deposition, we further demonstrate that measurements of the local deposition rate on nanoparticles in the solution via real-time imaging can be used to measure not only [e(-)aq] but also the rate of reduction of a metal-ion complex to zerovalent metal atoms in solution.

Using in situ liquid cell electron microscopy we investigate Pd growth in dilute aqueous Pd salt solutions containing Au nanoparticle seeds. Au-Pd core-shell nanostructures are formed via deposition of Pd(0), generated by the reduction of chloropalladate complexes by radicals, such as hydrated electrons (eaq(-)) induced by the electron beam in the solution. The size and shape of the Au seeds determine the morphology of the Pd shells, via preferential Pd incorporation in low-coordination sites and avoidance of extended facets. Analysis of the Pd incorporation on Au particles at different distances from a focused electron beam provides a quantitative picture of the growth process and shows that the growth is limited by the diffusion of eaq(-) in the solution.

(100)-oriented ultrathin silicon-on-insulator (SOI) with Si template thickness at or below 10nm, normally fully depleted due to carrier trapping in surface states, can be made conducting by terminating the surface with monolayer amounts of Ge. With this modification, which preserves the morphology of the surface, imaging with atomic resolution by scanning tunneling microscopy (STM) becomes possible. STM imaging is used to demonstrate surface smoothing by a thin (few monolayers) Si buffer at high temperature. Tunneling spectroscopy shows a high sheet resistance of the SOI sample, caused by a partial dewetting of the oxide supported Si template during thermal surface cleaning. High-resolution imaging and spectroscopy by STM will enable studies, at the atomic scale, of technologically relevant processes on ultrathin SOI.

The sequence of shape transitions in low mismatch, dilute coherent Si1−xGex (x

The multiscale mechanisms leading to the formation of faceted quantum dot (QD) islands during Si1−xGex/Si(100) heteroepitaxy were studied by scanning tunneling microscopy. The entire evolution from an initial morphological instability of the strained alloy to final {105} faceted pyramidal QD islands is continuous. Shallow mounds, generated by the alloy instability, are bounded by alternating (100) terraces and single-layer steps. Growth of the mounds in height without lateral expansion drives a continuous increase in mound slope and step density up to a critical angle of about 6°. Kinetic constraints then force the nucleation of small {105} protofacets, which expand and ultimately coalesce to form the {105} facets bounding pyramidal QD islands.

We have studied the formation of Ru nanocluster arrays on several monolayer graphene/Ru Moire structures with different relative orientations of the graphene and Ru lattices. Experiments and ab initio calculations clearly show that the presence of a graphene/Ru Moire does not guarantee the ordered adsorption of Ru nanoclusters. The simultaneous deposition of Ru onto coexisting Moires demonstrates that a structure with aligned graphene and Ru lattices templates the formation of arrays of small Ru clusters with narrow size spread and adsorption exclusively in a single site (the 'low fcc' site). The other Moires considered here gave rise to substantially larger clusters with broader size distribution and without detectable site selectivity. Calculations explain these findings via the density of states (DOS) at different sites of the graphene/Ru Moire. The ordered nucleation of many small clusters instead of incorporation of metal atoms into larger ones requires one Moire site with a large DOS at the Fermi level, so that the binding of metal adatoms to this site is stronger than to competing sites in the Moire and to existing metal clusters.

A eutectic AuGe target immersed in distilled water was ablated by pulsed ultraviolet laser light. The structure of the ablated material was investigated by high-resolution transmission electron microscopy (HRTEM). The images show formation of nanowire structures of AuGe up to 100 nm in length, with widths of 5–10 nm. These nanostructures have Ge content significantly lower than the target material. Electron diffraction demonstrates that they crystallize in the α-AuGe structure. For comparison, laser ablation of pure Au and pure Ge targets was also performed under the same conditions. HRTEM shows that Ge forms spherical nanoparticles with a characteristic size of ~30 nm. Au forms spherical nanoparticles with diameters of ~10 nm. Similar to AuGe, it also forms chainlike structures with substantially lower aspect ratio.

The following article appeared in Applied Physics Letters Volume 99, Issue 21, 21 November 2011, Article number 211907 and may be found at http://scitation.aip.org/content/aip/journal/apl/99/21/10.1063/1.3663866, We report a He and H2 diffraction study of graphene-terminated Ru(0001) thin films grown epitaxially on c-axis sapphire. Even for samples exposed for several weeks to ambient conditions, brief annealing in ultrahigh vacuum restored extraordinarily high specular reflectivities for He and H 2 beams (23 and 7 of the incident beam, respectively). The quality of the angular distributions recorded with both probes exceeds the one obtained from in-situ prepared graphene on Ru(0001) single crystals. Our results for graphene-terminated Ru thin films represent a significant step toward ambient tolerant, high-reflectivity curved surface mirrors for He-atom microscopy, Work performed in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which issupported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Work supported by the Ministerio de Educacion y Ciencia through projects CONSOLIDER-INGENIO 2010 on Molecular Nanoscience (CSD 2007-00010), FIS2010-18847, and byComunidad de Madrid through the program NANOBIOMAGNET (S2009/MAT-1726).

Epitaxial transition metal films have recently been introduced as substrates for the scalable synthesis of transferable graphene. Here, real-time microscopy is used to study graphene growth on epitaxial Ru films on sapphire. At high temperatures, high-quality graphene grows in macroscopic (>100 μm) domains to full surface coverage. Graphene nucleation and growth characteristics on thin (100 nm) Ru films are consistent with a pure surface chemical vapor deposition process, without detectable contributions from C segregation. Experiments on thicker (1 μm) films show a systematic suppression of the C uptake into the metal to levels substantially below those expected from bulk C solubility data, consistent with a strain-induced reduction of the C solubility due to gas bubbles acting as stressors in the epitaxial Ru films. The results identify two powerful approaches--i) limiting the template thickness and ii) tuning the interstitial C solubility via strain--for controlling graphene growth on metals with high C solubility, such as Ru, Pt, Rh, Co, and Ni.

Intercalation of metal atoms is an established route for tuning the coupling of graphene to a substrate. The extension to reactive species such as oxygen would set the stage for a wide spectrum of interfacial chemistry. Here we demonstrate the controlled modification of a macroscopic graphene-metal interface by oxygen intercalation. The selective oxidation of a ruthenium surface beneath graphene lifts the strong metal-carbon coupling and restores the characteristic Dirac cones of isolated monolayer graphene. Our experiments establish the competition between low-temperature oxygen intercalation and graphene etching at higher temperatures and suggest that small molecules can populate the space between graphene and metals, with the adsorbate-metal interaction being modified significantly by the presence of graphene. These findings open up new avenues for the processing of graphene for device applications and for performing chemical reactions in the confined space between a metal surface and a graphene sheet.

In situ low-energy electron microscopy (LEEM) of graphene growth combined with measurements of the graphene structure and electronic band structure has been used to study graphene on Pt(111). Growth by carbon segregation produces macroscopic monolayer graphene domains extending continuously across Pt(111) substrate steps and bounded by strongly faceted edges. LEEM during cooling from the growth temperature shows the propagation of wrinkles in the graphene sheet, driven by thermal stress. The lattice mismatch between graphene and Pt(111) is accommodated by moir\'e structures with a large number of different rotational variants, without a clear preference for a particular interface geometry. Fast and slow growing graphene domains exhibit moir\'e structures with small [e.g., ${(3\ifmmode\times\else\texttimes\fi{}3)}_{\text{G}}$, $(\sqrt{6}\ifmmode\times\else\texttimes\fi{}\sqrt{6})\text{R}{2}_{\text{G}}$, and $(2\ifmmode\times\else\texttimes\fi{}2)\text{R}{4}_{\text{G}}$] and large unit cells [e.g., $(\sqrt{44}\ifmmode\times\else\texttimes\fi{}\sqrt{44})\text{R}{15}_{\text{G}}$, $(\sqrt{52}\ifmmode\times\else\texttimes\fi{}\sqrt{52})\text{R}{14}_{\text{G}}$, and ${(8\ifmmode\times\else\texttimes\fi{}8)}_{\text{G}}$], respectively. A weak substrate coupling, suggested by the growth and structural properties of monolayer graphene on Pt(111), is confirmed by maps of the band structure, which is close to that of isolated graphene aside from minimal hole doping due to charge transfer from the metal. Finally, the decoupled graphene monolayer on Pt(111) appears impenetrable to carbon diffusion, which self-limits the graphene growth at monolayer thickness. Thicker graphene domains, which can form at boundaries between monolayer domains, have been used to characterize the properties of few-layer graphene on Pt(111).

Graphene has been used to explore the fascinating electronic properties of ideal two-dimensional carbon, and shows great promise for quantum device architectures. The primary method for isolating graphene, micromechanical cleavage of graphite, is difficult to scale up for applications. Epitaxial growth is an attractive alternative, but achieving large graphene domains with uniform thickness remains a challenge, and substrate bonding may strongly affect the electronic properties of epitaxial graphene layers. Here, we show that epitaxy on Ru(0001) produces arrays of macroscopic single-crystalline graphene domains in a controlled, layer-by-layer fashion. Whereas the first graphene layer indeed interacts strongly with the metal substrate, the second layer is almost completely detached, shows weak electronic coupling to the metal, and hence retains the inherent electronic structure of graphene. Our findings demonstrate a route towards rational graphene synthesis on transition-metal templates for applications in electronics, sensing or catalysis.

The motion of Pt-Si liquid nanodroplets and their interaction with atomic steps on Si(100) has been studied using low-energy electron microscopy. Submicron droplets migrate across the surface in the direction of the lateral temperature gradient as demonstrated by overlain maps of migration velocity and sample temperature. The thermal migration motion is opposed by a drag force normal to substrate steps, which strongly deflects smaller droplets to move parallel to step edges but hardly affects larger ones. In addition, steps are dragged along by moving droplets before eventually depinning. The resulting curved steps can create ``roadways'' that collect droplets and guide their motion, resulting in a non-Ostwald coarsening process involving guided collision and coalescence.

The use of cleaved, [111]-oriented monocrystalline InAs probe tips enables state-specific imaging in constant-current filled-state scanning tunneling microscopy. On Si(111)-(7 x 7), the adatom or rest-atom dangling-bond states can thus be mapped selectively at different tip-sample bias. This state-selective imaging is made possible by energy gaps in the projected bulk band structure of the semiconductor probe. The lack of extended bulk states in these gaps gives rise to efficient energy filtering of the tunneling current, to which only sample states not aligned with a gap contribute significantly.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

We assess scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) for thickness measurements on few-layer hexagonal boron nitride (h-BN), the layered dielectric of choice for integration with graphene and other two-dimensional materials. Observations on h-BN islands with large, atomically flat terraces show that the secondary electron intensity in SEM reflects monolayer height changes in films up to least 10 atomic layers thickness. From a quantitative analysis of AES data, the energy-dependent electron escape depth in h-BN films is deduced. The results show that AES is suitable for absolute thickness measurements of few-layer h-BN of 1 to 6 layers.

The growth of two-dimensional (2D) materials is a topic of very high scientific and technological interest. While chemical vapour deposition on catalytic metals has become a well developed approach for the growth of graphene and hexagonal boron nitride (BN), very few alternative approaches for synthesis on non-reactive supports have been explored so far. Here we report the growth of BN on gold, using magnetron sputtering of B in N2/Ar atmosphere, a scalable method using only non-toxic reagents. Scanning tunnelling microscopy at low coverage shows primarily triangular monolayer BN islands exhibiting two 'magic' orientations on the Au(111) surface. Such rotational alignment of BN on Au(111) is surprising, given the expected weak binding and the high lattice mismatch (~14%) between BN and Au. Our observations are consistent with a strong coupling between the edges of BN flakes and the substrate, which leads to the selection of BN orientations that maximize the orbital overlap between edge atoms and Au surface atoms. Diverse flake morphologies resembling the shape of butterflies, six-apex stars and diamonds, implying alternating B- and N- terminated edges, are observed as well. Our results provide insight into the growth mechanisms of 2D materials on weakly interacting and chemically inert substrates, and provide the basis for integrating other 2D materials with atomically precise graphene nanostructures synthesized from molecular precursors on Au.

We introduce a method for isolating graphene grown on epitaxial Ru(0001)/α-Al2O3. The strong graphene/Ru(0001) coupling is weakened by electrochemically driven intercalation of hydrogen underpotentially deposited in aqueous KOH solution, which allows the penetration of water molecules at the graphene/Ru(0001) interface. Following these electrochemically driven processes, the graphene can be isolated by electrochemical hydrogen evolution and transferred to arbitrary supports. Raman and transport measurements demonstrate the high quality of the transferred graphene. Our results show that intercalation, typically carried out in vacuum, can be extended to solution environments for graphene processing under ambient conditions.

Integration of graphene with other materials by direct growth, i.e., not using mechanical transfer procedures, is investigated on the example of metal/graphene/dielectric heterostructures. Such structures may become useful in spintronics applications using graphene as a spin-filter. Here, we systematically discuss the optimization of synthesis procedures for every layer of the heterostructure and characterize the material by imaging and diffraction methods. 300 nm thick contiguous (111) Ni-films are grown by physical vapor deposition on YSZ(111) or Al2O3(0001) substrates. Subsequently, chemical vapor deposition growth of graphene in ultra-high vacuum (UHV) is compared to tube-furnace synthesis. Only under UHV conditions, monolayer graphene in registry with Ni(111) has been obtained. In the tube furnace, mono- and bilayer graphene is obtained at growth temperatures of ∼800 °C, while at 900 °C, non-uniform thick graphene multilayers are formed. Y2O3 films grown by reactive molecular beam epitaxy in UHV covers the graphene/Ni(111) surface uniformly. Annealing to 500 °C results in crystallization of the yttria with a (111) surface orientation.

Single crystalline Ru(0001) thin films epitaxially grown on sapphire (0001) substrates were used as sacrificial metal templates for the synthesis of high-quality graphene with uniform monolayer thickness and full surface coverage. Removal of the metal template by etching transferred monolayer graphene with good crystal quality onto the insulating sapphire support. Our findings demonstrate epitaxial Ru(0001) films on sapphire (0001) as a substrate for the scalable synthesis of high-quality graphene for applications.

Monolayer graphene has been grown on polycrystalline Ru thin films on SiO2/Si substrates. The Ru films have columnar structure with strongly aligned grains exposing flat (0001) surface facets. Adjacent grains show small relative tilts of their [0001] axes and variations in in-plane orientation. Graphene layers grown on this template cover the entire surface and have uniform monolayer thickness. Analysis of the graphene/Ru moire structure shows that monocrystalline graphene domains are coherent across a large number of substrate grains. Hence, the size of monolayer graphene domains is not limited by grain boundaries in the metal template.

The atomic structure of epitaxial single and bilayer graphene on Ru(0001) was studied by scanning tunneling microscopy (STM). High-resolution imaging of the surface of single layer graphene shows a moire with pronounced buckling and broken A/B carbon sublayer symmetry due to a strong interaction with the metal substrate. The top sheet of bilayer graphene is largely unperturbed by residual interactions with the substrate. Screened from the metal substrate, it shows the hallmarks of freestanding monolayer graphene: a honeycomb structure with equivalent carbon sublattices imaged in STM and a linear dispersion of π-bands near the Dirac point.

Using scanning tunneling microscopy and density functional theory, we characterize the population of low-coverage Ti atoms on Al(111) as a model surface system for transition metal doped alanate hydrogen storage compounds, such as NaAlH4. When deposited at room temperature, Ti is kinetically trapped in first-layer substitutional sites, avoids nearest-neighbor locations, and preferentially forms next-nearest-neighbor pairs, similar to a structure that has been predicted to dissociate H2 with no energy barrier. The results on this well-defined system suggest the presence of a large population of Ti-pair complexes that may catalyze the dissociative chemisorption of hydrogen in Ti-doped alanate storage materials.

Variable temperature transmission electron microscopy of individual 5 nm Au nanoparticles shows a striking increase in the particle size on raising the temperature from room temperature to 500 °C in the presence of carbon from amorphous carbon support. Using the assembly of ordered graphene shells on the surface of individual nanoparticles at elevated temperatures—and the high pressures induced by such shells—as an experimental tool to study the origins of this swelling, we find that the volume increase is associated with the uptake of carbon to concentrations exceeding the bulk solubility by more than four orders of magnitude. The formation of stable metal–carbon nanostructures that have no bulk equivalent may have important implications on the functional properties of metal nanoparticles.