Among 2D/layered semiconductors, group IV monochalcogenides such as SnS(e) and GeS(e) have attracted attention as phosphorene/black phosphorus analogues with anisotropic structures and predicted ...unusual properties. In contrast to SnS, for which bottom-up synthesis has been reported, few-layer GeS has been realized primarily via exfoliation from bulk crystals. Here, we report the synthesis of large (up to >20 μm), faceted single crystalline GeS flakes with anisotropic properties using a vapor transport process. In situ electron microscopy is used to identify the thermal stability and sublimation pathways, and demonstrates that the GeS flakes are self-encapsulated in a thin, sulfur-rich amorphous GeS x shell during growth. The shell provides exceptional chemical stability to the layered GeS core. In contrast to exfoliated GeS, which rapidly degrades during exposure to air, the synthesized GeS–GeS x core–shell structures show no signs of chemical attack and remain unchanged in air for extended time periods. Measurements of the optoelectronic properties by photoluminescence spectroscopy show a tunable bandgap due to out-of-plane quantum confinement in flakes with thickness below 100 nm. Cathodoluminescence (CL) spectroscopy with nanoscale excitation provides evidence for interfacial charge transfer due to a type II heterojunction between the crystalline core and amorphous shell. Measurements by locally excited CL yield a minority carrier (electron) diffusion length in the p-type GeS core l diff = 0.27 μm, on par with diffusion lengths in the highest-quality layered chalcogenide semiconductors.
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 moire 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 moire structures with small e.g., (3 x 3){sub G}, ({radical}6 x {radical}6)R2{sub G}, and (2 x 2)R4{sub G} and large unit cells e.g., ({radical}44 x {radical}44)R15{sub G}, ({radical}52 x {radical}52)R14{sub G}, and (8 x 8){sub 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).
Epitaxial graphene on ruthenium Sutter, Peter W; Flege, Jan-Ingo; Sutter, Eli A
Nature materials,
05/2008, Letnik:
7, Številka:
5
Journal Article
Recenzirano
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.
Tin sulfide (SnS) is a part of the group IV chalcogenides (SnX, GeX; X:S, Se), a family of anisotropic layered materials considered for thin-film photovoltaics, optoelectronics, and valleytronics and ...predicted narrow band gap multiferroic materials. Large ultrathin SnS flakes, suitable for a variety of applications, are challenging to synthesize because of the enhanced surface reactivity due to the open layer structure of SnS, which gives rise to a competition between lateral and vertical growth. Here, we investigate the effects of added sulfur on passivating the surface and modifying the balance between lateral expansion and thickening of SnS flakes in chemical vapor transport from a SnS precursor. We investigate the growth of SnS flakes and compare the results of synthesis from a pure SnS precursor with growth, in which a slight excess of sulfur is supplied in different ways. Our results demonstrate that small amounts of excess sulfur can profoundly affect the size and thickness distributions of SnS flakes. The largest and thinnest flakes are obtained if (i) traces of sulfur are added and (ii) the sulfur source consists of either small S x fragments released at high temperatures from the reactor walls or atomic S supplied by sublimation from SnS2. The likely mechanism for the observed growth modifications is a transient surface passivation of SnS flakes during growth, which reduces the reactivity of the top facet of the flakes, limits vertical growth, and thus gives rise to ensembles with increased lateral size and reduced thickness.
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.
The prototypical photocatalyst TiO2 exists in different polymorphs, the most common forms are the anatase- and rutile-crystal structures. Generally, anatase is more active than rutile, but no ...consensus exists to explain this difference. Here we demonstrate that it is the bulk transport of excitons to the surface that contributes to the difference. Utilizing high -quality epitaxial TiO2 films of the two polymorphs we evaluate the photocatalytic activity as a function of TiO2-film thickness. For anatase the activity increases for films up to ~5 nm thick, while rutile films reach their maximum activity for ~2.5 nm films already. This shows that charge carriers excited deeper in the bulk contribute to surface reactions in anatase than in rutile. Furthermore, we measure surface orientation dependent activity on rutile single crystals. The pronounced orientation-dependent activity can also be correlated to anisotropic bulk charge carrier mobility, suggesting general importance of bulk charge diffusion for explaining photocatalytic anisotropies.
Engineered heterostructures create new functionality by integrating dissimilar materials. Combining different 2D crystals naturally produces two distinct classes of heterostructures, vertical van der ...Waals (vdW) stacks or 2D sheets bonded laterally by covalent line interfaces. When joining thicker layered crystals, the arising structural and topological conflicts can result in more complex geometries. Phase separation during one‐pot synthesis of layered tin chalcogenides spontaneously creates core–shell structures in which large orthorhombic SnS crystals are enclosed in a wrap‐around shell of trigonal SnS2, forcing the coexistence of parallel vdW layering along with unconventional, orthogonally layered core–shell interfaces. Measurements of the optoelectronic properties establish anisotropic carrier separation near type II core–shell interfaces and extended long‐wavelength light harvesting via spatially indirect interfacial absorption, making multifunctional layered core–shell structures attractive for energy‐conversion applications.
Core–shell structures of monocrystalline layered cores wrapped in few‐layer shells are realized via one‐pot synthesis and spontaneous phase separation of van der Waals crystals. Abundant interfaces with different core and shell layer orientations create unique optoelectronic properties, notably anisotropic charge transfers and spatially indirect interfacial absorption, promising efficient light harvesting to photon energies significantly below the bandgaps of the constituent semiconductors.
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 cm2/(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.
The growth of large-area hexagonal boron nitride (h-BN) monolayers on catalytic metal substrates is a topic of scientific and technological interest. We have used real-time microscopy during the ...growth process to study h-BN chemical vapor deposition (CVD) from borazine on Ru(0001) single crystals and thin films. At low borazine pressures, individual h-BN domains nucleate sparsely, grow to macroscopic dimensions, and coalescence to form a closed monolayer film. A quantitative analysis shows borazine adsorption and dissociation predominantly on Ru, with the h-BN covered areas being at least 100 times less reactive. We establish strong effects of hydrogen added to the CVD precursor gas in controlling the in-plane expansion and morphology of the growing h-BN domains. High-temperature exposure of h-BN/Ru to pure hydrogen causes the controlled edge detachment of B and N and can be used as a clean etching process for h-BN on metals.
We use transmission electron microscopy observations to establish the parts of the phase diagram of nanometer sized Au−Ge alloy drops at the tips of Ge nanowires (NWs) that determine their ...temperature-dependent equilibrium composition and, hence, their exchange of semiconductor material with the NWs. We find that the phase diagram of the nanoscale drop deviates significantly from that of the bulk alloy, which explains discrepancies between actual growth results and predictions on the basis of the bulk-phase equilibria. Our findings provide the basis for tailoring vapor−liquid−solid growth to achieve complex one-dimensional materials geometries.
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