Bending and precipitate formation mechanisms in epitaxial Ge-core/GeSn-shell nanowires.

Core-shell Ge/GeSn nanowires provide a route to dislocation-free single crystal germanium-tin alloys with desirable light emission properties because the Ge core acts as an elastically compliant substrate during misfitting GeSn shell growth. However, the uniformity of tin incorporation during reduced pressure chemical vapor deposition may be limited by the kinetics of mass transfer to the shell during GeSn growth. The balance between Sn precursor flux and available surfaces for GeSn nucleation and growth determines whether defects are formed and their type. On the one hand, when the Sn precursor delivery is insufficient, local variations in Sn arrival rate at the nanowire surfaces during GeSn growth produce asymmetries in shell growth that induce wire bending. This inhomogeneous elastic dilatation due to the varying composition occurs via deposition of Sn-poor regions on some of the {112} sidewall facets of the nanowires. On the other hand, when the available nanowire surface area is insufficient to accommodate the arriving Sn precursor flux, Sn-rich precipitate formation results. Between these two extremes, there exists a regime of growth conditions and nanowire densities that permits defect-free GeSn shell growth.

Revealing multiple classes of stable quantum emitters in hexagonal boron nitride with correlated optical and electron microscopy.

Defects in hexagonal boron nitride (hBN) exhibit high-brightness, room-temperature quantum emission, but their large spectral variability and unknown local structure challenge their technological utility. Here, we directly correlate hBN quantum emission with local strain using a combination of photoluminescence (PL), cathodoluminescence (CL) and nanobeam electron diffraction. Across 40 emitters, we observe zero phonon lines (ZPLs) in PL and CL ranging from 540 to 720 nm. CL mapping reveals that multiple defects and distinct defect species located within an optically diffraction-limited region can each contribute to the observed PL spectra. Local strain maps indicate that strain is not required to activate the emitters and is not solely responsible for the observed ZPL spectral range. Instead, at least four distinct defect classes are responsible for the observed emission range, and all four classes are stable upon both optical and electron illumination. Our results provide a foundation for future atomic-scale optical characterization of colour centres.

Two-dimensional limit of crystalline order in perovskite membrane films.

Long-range order and phase transitions in two-dimensional (2D) systems-such as magnetism, superconductivity, and crystallinity-have been important research topics for decades. The issue of 2D crystalline order has reemerged recently, with the development of exfoliated atomic crystals. Understanding the dimensional limit of crystalline phases, with different types of bonding and synthetic techniques, is at the foundation of low-dimensional materials design. We study ultrathin membranes of SrTiO3, an archetypal perovskite oxide with isotropic (3D) bonding. Atomically controlled membranes are released after synthesis by dissolving an underlying epitaxial layer. Although all unreleased films are initially single-crystalline, the SrTiO3 membrane lattice collapses below a critical thickness (5 unit cells). This crossover from algebraic to exponential decay of the crystalline coherence length is analogous to the 2D topological Berezinskii-Kosterlitz-Thouless (BKT) transition. The transition is likely driven by chemical bond breaking at the 2D layer-3D bulk interface, defining an effective dimensional phase boundary for coherent crystalline lattices.

Core-Shell Germanium/Germanium-Tin Nanowires Exhibiting Room-Temperature Direct- and Indirect-Gap Photoluminescence.

Germanium-tin alloy nanowires hold promise as silicon-compatible optoelectronic elements with the potential to achieve a direct band gap transition required for efficient light emission. In contrast to Ge1-xSnx epitaxial thin films, free-standing nanowires deposited on misfitting germanium or silicon substrates can avoid compressive, elastic strains that inhibit formation of a direct gap. We demonstrate strong room temperature photoluminescence, consistent with band edge emission from both Ge core nanowires, elastically strained in tension, and the almost unstrained Ge1-xSnx shells grown around them. Low-temperature chemical vapor deposition of these core-shell structures was achieved using standard precursors, resulting in Sn incorporation that significantly exceeds the bulk solubility limit in germanium.

Parallel preparation of plan-view transmission electron microscopy specimens by vapor-phase etching with integrated etch stops.

Specimen preparation remains a practical challenge in transmission electron microscopy and frequently limits the quality of structural and chemical characterization data obtained. Prevailing methods for thinning of specimens to electron transparency are serial in nature, time consuming, and prone to producing artifacts and specimen failure. This work presents an alternative method for the preparation of plan-view specimens using isotropic vapor-phase etching with integrated etch stops. An ultrathin amorphous etch-stop layer simultaneously serves as an electron transparent support membrane whose thickness is defined by a controlled growth process such as atomic layer deposition with sub-nanometer precision. This approach eliminates the need for mechanical polishing or ion milling to achieve electron transparency, and reduces the occurrence of preparation induced artifacts. Furthermore, multiple specimens from a plurality of samples can be thinned in parallel due to high selectivity of the vapor-phase etching process. These features enable dramatic reductions in preparation time and cost without sacrificing specimen quality and provide advantages over wet etching techniques. Finally, we demonstrate a platform for high-throughput transmission electron microscopy of plan-view specimens by combining the parallel preparation capabilities of vapor-phase etching with wafer-scale micro- and nanofabrication.

Spontaneous, Defect-Free Kinking via Capillary Instability during Vapor-Liquid-Solid Nanowire Growth.

Kinking, a common anomaly in nanowire (NW) vapor-liquid-solid (VLS) growth, represents a sudden change of the wire's axial growth orientation. This study focuses on defect-free kinking during germanium NW VLS growth, after nucleation on a Ge (111) single crystal substrate, using Au-Ge catalyst liquid droplets of defined size. Statistical analysis of the fraction of kinked NWs reveals the dependence of kinking probability on the wire diameter and the growth temperature. The morphologies of kinked Ge NWs studied by electron microscopy show two distinct, defect-free, kinking modes, whose underlying mechanisms are explained with the help of 3D multiphase field simulations. Type I kinking, in which the growth axis changes from vertical [111] to ⟨110⟩, was observed in Ge NWs with a nominal diameter of ∼ 20 nm. This size coincides with a critical diameter at which a spontaneous transition from ⟨111⟩ to ⟨110⟩ growth occurs in the phase field simulations. Larger diameter NWs only exhibit Type II kinking, in which the growth axis changes from vertical [111] directly to an inclined ⟨111⟩ axis during the initial stages of wire growth. This is caused by an error in sidewall facet development, which produces a shrinkage in the area of the (111) growth facet with increasing NW length, causing an instability of the Au-Ge liquid droplet at the tip of the NW.

Ultrafast electronic and structural response of monolayer MoS2 under intense photoexcitation conditions.

We report on the dynamical response of single layer transition metal dichalcogenide MoS2 to intense above-bandgap photoexcitation using the nonlinear-optical second order susceptibility as a direct probe of the electronic and structural dynamics. Excitation conditions corresponding to the order of one electron-hole pair per unit cell generate unexpected increases in the second harmonic from monolayer films, occurring on few picosecond time-scales. These large amplitude changes recover on tens of picosecond time-scales and are reversible at megahertz repetition rates with no photoinduced change in lattice symmetry observed despite the extreme excitation conditions.

Thermal stability and surface passivation of Ge nanowires coated by epitaxial SiGe shells.

Epitaxial growth of a highly strained, coherent SiGe alloy shell around a Ge nanowire core is investigated as a method to achieve surface passivation and carrier confinement, important in realizing nanowire devices. The high photoluminescence intensity observed from the core-shell nanowires with spectral features similar to that of bulk Ge indicates effective surface passivation. Thermal stability of these core-shell heterostructures has been systematically investigated, with a method demonstrated to avoid misfit strain relaxation during postgrowth annealing.

Hexagonal close-packed structure of au nanocatalysts solidified after ge nanowire vapor-liquid-solid growth.

We report that approximately 10% of the Au catalysts that crystallize at the tips of Ge nanowires following growth have the close-packed hexagonal crystal structure rather than the equilibrium face-centered-cubic structure. Transmission electron microscopy results using aberration-corrected imaging, and diffraction and compositional analyses, confirm the hexagonal phase in these 40-50 nm particles. Reports of hexagonal close packing in Au, even in nanoparticle form, are rare, and the observations suggest metastable pathways for the crystallization process. These results bring new considerations to the stabilization of the liquid eutectic alloy at low temperatures that allows for vapor-liquid-solid growth of high quality, epitaxial Ge nanowires below the eutectic temperature.

Facile synthesis, silanization, and biodistribution of biocompatible quantum dots.

A facile strategy for the synthesis of silica-coated quantum dots (QDs) for in vivo imaging is reported. All the QD synthesis and silanization steps are conducted in water and methanol under mild conditions without involving any organometallic precursors or high-temperature, oxygen-free environments. The as-prepared silica-coated QDs possess high quantum yields and are extremely stable in mouse serum. In addition, the silanization method developed here produces nanoparticles with small sizes that are difficult to achieve via conventional silanization methods. The silica coating helps to prevent the exposure of the QD surface to the biological milieu and therefore increases the biocompatibility of QDs for in vivo applications. Interestingly, the silica-coated QDs exhibit a different biodistribution pattern from that of commercially available Invitrogen QD605 (carboxylate) with a similar size and emission wavelength. The Invitrogen QD605 exhibits predominant liver (57.2% injected dose (ID) g(-1)) and spleen (46.1% ID g(-1)) uptakes 30 min after intravenous injection, whereas the silica-coated QDs exhibit much lower liver (16.2% ID g(-1)) and spleen (3.67% ID g(-1)) uptakes but higher kidney uptake (8.82% ID g(-1)), blood retention (15.0% ID g(-1)), and partial renal clearance. Overall, this straightforward synthetic strategy paves the way for routine and customized synthesis of silica-coated QDs for biological use.

Near-infrared light emitting luciferase via biomineralization.

A new strategy based on biomineralization is presented to rationally tune the emission wavelength of luciferase. In this study luciferase (Luc8) was used as a template to direct the synthesis of near-infrared (NIR) light emitting PbS quantum dots (QDs) at ambient conditions to form a Luc8-PbS nanocomplex. The as-synthesized PbS QDs exhibited photoluminescence in the range of 800-1050 nm, and the Luc8 enzyme remained active within the Luc8-PbS complex. Upon the addition of the substrate coelenterazine, the energy produced by Luc8 was nonradiatively transferred to PbS QDs via bioluminescence resonance energy transfer (BRET) and enabled the complex to emit NIR light. This is the first study to form dual functional bioinorganic hybrid nanostructures via active enzyme-templated synthesis of inorganic nanomaterials.

Gold-catalyzed vapor-liquid-solid germanium-nanowire nucleation on porous silicon.

Nanoporous Si(111) substrates are used to study the effects of Au catalyst coarsening on the nucleation of vapor-liquid-solid-synthesized epitaxial Ge nanowires (NWs) at temperatures less than 400 degrees C. Porous Si substrates, with greater effective interparticle separations for Au surface diffusion than nonporous Si, inhibit catalyst coarsening and agglomeration prior to NW nucleation. This greatly reduces the variation in wire diameter and length and increases the yield compared to nucleation on identically prepared nonporous Si substrates.

Inhibiting strain-induced surface roughening: dislocation-free Ge/Si and Ge/SiGe core-shell nanowires.

Elastic strain is a critical factor in engineering the electronic behavior of core-shell semiconductor nanowires and provides the driving force for undesirable surface roughening and defect formation. We demonstrate two independent strategies, chlorine surface passivation and growth of nanowires with low-energy sidewall facets, to avoid strain-induced surface roughening that promotes dislocation nucleation in group IV core-shell nanowires. Metastably strained, dislocation-free, core-shell nanowires are obtained, and axial strains are measured and compared to elasticity model predictions.

Single-crystal germanium layers grown on silicon by nanowire seeding.

Three-dimensional integration and the combination of different material systems are central themes of electronics research. Recently, as-grown vertical one-dimensional structures have been integrated into high-density three-dimensional circuits. However, little attention has been paid to the unique structural properties of germanium nanowires obtained by epitaxial and heteroepitaxial growth on Ge(111) and Si(111) substrates, despite the fact that the integration of germanium on silicon is attractive for device applications. Here, we demonstrate the lateral growth of single crystal germanium islands tens of micrometres in diameter by seeding from germanium nanowires grown on a silicon substrate. Vertically aligned high-aspect-ratio nanowires can transfer the orientation and perfection of the substrate crystal to overlying layers a micrometre or more above the substrate surface. This technique can be repeated to build multiple active device layers, a key requirement for the fabrication of densely interconnected three-dimensional integrated circuits.

Synthesis and strain relaxation of Ge-core/Si-shell nanowire arrays.

Analogous to planar heteroepitaxy, misfit dislocation formation and stress-driven surface roughening can relax coherency strains in misfitting core-shell nanowires. The effects of coaxial dimensions on strain relaxation in aligned arrays of Ge-core/Si-shell nanowires are analyzed quantitatively by transmission electron microscopy and synchrotron X-ray diffraction. Relating these results to reported continuum elasticity models for coaxial nanowire heterostructures provides valuable insights into the observed interplay of roughening and dislocation-mediated strain relaxation.

Oxide-encapsulated vertical germanium nanowire structures and their DC transport properties.

We demonstrate the p-type doping of Ge nanowires (NWs) and p-n junction arrays in a scalable vertically aligned structure with all processing performed below 400 °C. These structures are advantageous for the large scale production of parallel arrays of devices for nanoelectronics and sensing applications. Efficient methods for the oxide encapsulation, chemical mechanical polishing and cleaning of vertical Ge NWs embedded in silicon dioxide are reported. Approaches for avoiding the selective oxidation and dissolution of Ge NWs in aqueous solutions during chemical mechanical polishing and cleaning of oxide-encapsulated Ge NWs are emphasized. NWs were doped through the epitaxial deposition of a B-doped shell and transport measurements indicate doping concentrations on the order of 10(19) cm(-3).

Metastability of Au-Ge liquid nanocatalysts: Ge vapor-liquid-solid nanowire growth far below the bulk eutectic temperature.

The vapor-liquid-solid mechanism of nanowire (NW) growth requires the presence of a liquid at one end of the wire; however, Au-catalyzed Ge nanowire growth by chemical vapor deposition can occur at approximately 100 degrees C below the bulk Au-Ge eutectic. In this paper, we investigate deep sub-eutectic stability of liquid Au-Ge catalysts on Ge NWs quantitatively, both theoretically and experimentally. We construct a binary Au-Ge phase diagram that is valid at the nanoscale and show that equilibrium arguments, based on capillarity, are inconsistent with stabilization of Au-Ge liquid at deep sub-eutectic temperatures, similar to those used in Ge NW growth. Hot-stage electron microscopy and X-ray diffraction are used to test the predictions of nanoscale phase equilibria. In addition to Ge supersaturation of the Au-Ge liquid droplet, which has recently been invoked as an explanation for deep sub-eutectic Ge NW growth, we find evidence of a substantial kinetic barrier to Au solidification during cooling below the nanoscale Au-Ge eutectic temperature.

Germanium nanowire epitaxy: shape and orientation control.

Epitaxial growth of nanowires along the 111 directions was obtained on Ge(111), Ge(110), Ge(001), and heteroepitaxial Ge on Si(001) substrates at temperatures of 350 degrees C or less by gold-nanoparticle-catalyzed chemical vapor deposition. On Ge(111), the growth was mostly vertical. In addition to 111 growth, 110 growth was observed on Ge(001) and Ge(110) substrates. Tapering was avoided by the use of the two-temperature growth procedure, reported earlier by Greytak et al.