Trapping Layers Prevent Dopant Segregation and Enable Remote Doping of Templated Self-Assembled InGaAs Nanowires.

Selective area epitaxy is a promising approach to define nanowire networks for topological quantum computing. However, it is challenging to concurrently engineer nanowire morphology, for carrier confinement, and precision doping, to tune carrier density. We report a strategy to promote Si dopant incorporation and suppress dopant diffusion in remote doped InGaAs nanowires templated by GaAs nanomembrane networks. Growth of a dilute AlGaAs layer following doping of the GaAs nanomembrane induces incorporation of Si that otherwise segregates to the growth surface, enabling precise control of the spacing between the Si donors and the undoped InGaAs channel; a simple model captures the influence of Al on the Si incorporation rate. Finite element modeling confirms that a high electron density is produced in the channel.

Semiconductor-Ferromagnetic Insulator-Superconductor Nanowires: Stray Field and Exchange Field.

Nanowires can serve as flexible substrates for hybrid epitaxial growth on selected facets, allowing for the design of heterostructures with complex material combinations and geometries. In this work we report on hybrid epitaxy of freestanding vapor-liquid-solid grown and in-plane selective area grown semiconductor-ferromagnetic insulator-superconductor (InAs/EuS/Al) nanowire heterostructures. We study the crystal growth and complex epitaxial matching of wurtzite and zinc-blende InAs/rock-salt EuS interfaces as well as rock-salt EuS/face-centered cubic Al interfaces. Because of the magnetic anisotropy originating from the nanowire shape, the magnetic structure of the EuS phase is easily tuned into single magnetic domains. This effect efficiently ejects the stray field lines along the nanowires. With tunnel spectroscopy measurements of the density of states, we show that the material has a hard induced superconducting gap, and magnetic hysteretic evolution which indicates that the magnetic exchange fields are not negligible. These hybrid nanowires fulfill key material requirements for serving as a platform for spin-based quantum applications, such as scalable topological quantum computing.

Self-Seeded Axio-Radial InAs-InAs1-xPx Nanowire Heterostructures beyond "Common" VLS Growth.

Semiconductors are essential for modern electronic and optoelectronic devices. To further advance the functionality of such devices, the ability to fabricate increasingly complex semiconductor nanostructures is of utmost importance. Nanowires offer excellent opportunities for new device concepts; heterostructures have been grown in either the radial or axial direction of the core nanowire but never along both directions at the same time. This is a consequence of the common use of a foreign metal seed particle with fixed size for nanowire heterostructure growth. In this work, we present for the first time a growth method to control heterostructure growth in both the axial and the radial directions simultaneously while maintaining an untapered self-seeded growth. This is demonstrated for the InAs/InAs1-xPx material system. We show how the dimensions and composition of such axio-radial nanowire heterostructures can be designed including the formation of a "pseudo-superlattice" consisting of five separate InAs1-xPx segments with varying length. The growth of axio-radial nanowire heterostructures offers an exciting platform for novel nanowire structures applicable for fundamental studies as well as nanowire devices. The growth concept for axio-radial nanowire heterostructures is expected to be fully compatible with Si substrates.

Understanding Self-Catalyzed Epitaxial Growth of III-V Nanowires toward Controlled Synthesis.

The self-catalyzed growth of III-V nanowires has drawn plenty of attention due to the potential of integration in current Si-based technologies. The homoparticle-assisted vapor-liquid-solid growth mechanism has been demonstrated for self-catalyzed III-V nanowire growth. However, the understandings of the preferred growth sites of these nanowires are still limited, which obstructs the controlled synthesis and the applications of self-catalyzed nanowire arrays. Here, we experimentally demonstrated that thermally created pits could serve as the preferred sites for self-catalyzed InAs nanowire growth. On that basis, we performed a pregrowth annealing strategy to promote the nanowire density by enhancing the pits formation on the substrate surface and enable the nanowire growth on the substrate that was not capable to facilitate the growth. The discovery of the preferred self-catalyzed nanowire growth sites and the pregrowth annealing strategy have shown great potentials for controlled self-catalyzed III-V nanowire array growth with preferred locations and density.

Effect of Thiolated Ligands in Au Nanowire Synthesis.

Thiolated ligands are seldom used as morphology-directing reagent in the synthesis of Au nanostructures due to their low selectivity toward the different facets. Recently, we developed a thiolated ligands-induced synthesis of nanowires where the selective Au deposition only occurs at the ligand-deficient Au-substrate interface. Herein, the structural effect of thiolated ligands in this active surface growth is systematically investigated. It is revealed that their ability of rendering surface is closely related to the molecular structure. Ligands with aromatic backbones are capable of inducing nanowire formation, whereas those with aliphatic backbones cannot, likely because the former can pack better at short time scale of the rapid growth. The substituents of the ligands are critical for the colloidal stability of the final structure. It is further demonstrated that aromatic and aliphatic ligands could be mixed to turn on the continual lateral growth, leading to nanowires with tapered ends. The ligand generality in this growth mode also allows the creation of superhydrophobic surface, with the nanowire forest providing the nanoscale surface roughness and the hydrophobic ligand offering the surface property. These applications of the thiolated ligands in the nanosynthesis open a new approach for controlled synthesis of Au-based nanostructures with various morphologies and properties.

Kinetics-Driven Crystal Facets Evolution at the Tip of Nanowires: A New Implementation of the Ostwald-Lussac Law.

Nanocrystal facets evolution is critical for designing nanomaterial morphology and controlling their properties. In this work, we report a unique high-energy crystal facets evolution phenomenon at the tips of wurtzite zinc oxide nanowires (NWs). As the zinc vapor supersaturation decreased at the NW deposition region, the NW tip facets evolved from the (0001) surface to the {101̅3} surface and subsequently to the {112̅2} surface and eventually back to the flat (0001) surface. A series of NW tip morphology was observed in accordance to the different combinations of exposed facets. Exposure of the high-energy facets was attributed to the fluctuation of the energy barriers for the formation of different crystal facets during the layer-by-layer growth of the NW tip. The energy barrier differences between these crystal facets were quantified from the surface area ratios as a function of supersaturation. On the basis of the experimental observation and kinetics analysis, we argue that at appropriate deposition conditions exposure of the crystal facets at NW growth front is not merely determined by the surface energy. Instead, the NW may choose to expose the facets with minimal formation energy barrier, which can be determined by the Ehrlich-Schwoebel barrier variation. This empirical law for the NW tip facet formation was in analogy to the Ostwald-Lussac law of phase transformation, which brings a new insight toward nanostructure design and controlled synthesis.

Exploiting Rayleigh Instability in Creating Parallel Au Nanowires with Exotic Arrangements.

New types of nanowire arrangements are explored via active surface growth, where the use of Au seeds at room temperature means that the seed shape has major impacts on the subsequent nanowire growth. When Au nanorods are used as seeds, the original stripe-shape contact line with the substrate (the active surface) splits into a series of circular dots as the result of Rayleigh instability, giving coplanar nanowire bundles. The influence of a solid system by Rayleigh instability is exceptional, permitted by the dynamic active surface. The splitting is driven by the tendency to minimize the surface of the newly emerged nanowire section, whereas Rayleigh instability is responsible for overcoming the kinetic barriers. As a result, the average distance between the nanowires is only a few nanometers, much smaller than conventional lithographic methods. Conical and tubular bundles of nanowires are formed at low seed density, where the excessive growth material available for each seed leads to expansion and splitting of the active surface under the influence of both the diffusion limited growth and Rayleigh instability. Further designs of nanowire-based Au architectures demonstrate the feasibility of combining the multiple control of the system for new synthetic advances.

Optical and Electrical Characteristics of Hybrid ZnO Nanowire/a-Si:H Solar Cells on Flexible Substrates under Mechanical Bending.

Disordered 3-D hybrid ZnO nanowire/a-Si:H thin-film radial-junction solar cells are directly fabricated onto flexible substrates. A 41% reduction in optical reflectivity resulted in a 15% increase in the current density when the substrate is mechanically bent concave-up toward the incoming light. The light scattering of the nanowire devices was enhanced by decreasing the spacing between the nanowire solar cell by bending the substrate.

Creating New VLS Silicon Nanowire Contact Geometries by Controlling Catalyst Migration.

The formation of self-assembled contacts between vapor-liquid-solid grown silicon nanowires and flat silicon surfaces was imaged in situ using electron microscopy. By measuring the structural evolution of the contact formation process, we demonstrate how different contact geometries are created by adjusting the balance between silicon deposition and Au migration. We show that electromigration provides an efficient way of controlling the contact. The results point to novel device geometries achieved by direct nanowire growth on devices.

Factors Influencing the Growth of Pt Nanowires via Chemical Self-Assembly and their Fuel Cell Performance.

This work reports a detailed investigation of the template-free synthesis of Pt nanowires via the chemical reduction of Pt salt precursors with formic-acid. The results indicate that both the oxidation state of Pt in the salt and the pH value of the aqueous solution comprising the platinum salt and formic acid are critical factors for the formation of Pt nanowires. Nanowires are obtained from platinum atoms in a +IV oxidation state, with ligating chloride anions (H2 PtCl6 and K2 PtCl6 ) or nonligating chloride anions (PtCl4 ). Increasing the pH of the aqueous Pt salt and HCOOH solution leads to a drastic reduction of the nanowires' length between pH 3 and 4.5. A mechanism involving formate as a reducing agent and formic acid as a structure directing agent explains these results. The Pt nanowires are stable up to 200 °C; therefore, these nanowires are suitable for use as catalysts in proton-exchange-membrane fuel cell. The optimized synthesis conditions are then selected for investigating the kinetics of the oxygen reduction reaction (ORR) of such nanowires in a fuel cell. The ORR mass activity of the Pt nanowires is 130 A g(-1) Pt at 0.9 V iR-free potential; significantly higher than that of two commercial Pt/C catalysts tested in the same conditions. The higher mass activity is explained based on a higher surface specific activity. Accelerated degradation tests indicate that Pt nanowires supported on carbon are as stable as Pt nanoparticles supported on carbon.

3D Shape Reconstruction of Ge Nanowires during Vapor-Liquid-Solid Growth under Modulating Electric Field.

Bottom-up growth offers precise control over the structure and geometry of semiconductor nanowires (NWs), enabling a wide range of possible shapes and seamless heterostructures for applications in nanophotonics and electronics. The most common vapor-liquid-solid (VLS) growth method features a complex interaction between the liquid metal catalyst droplet and the anisotropic structure of the crystalline NW, and the growth is mainly orchestrated by the triple-phase line (TPL). Despite the intrinsic mismatch between the droplet and the NW symmetries, its discussion has been largely avoided because of its complexity, which has led to the situation when multiple observed phenomena such as NW axial asymmetry or the oscillating truncation at the TPL still lack detailed explanation. The introduction of an electric field control of the droplet has opened even more questions, which cannot be answered without properly addressing three-dimensional (3D) structure and morphology of the NW and the droplet. This work describes the details of electric-field-controlled VLS growth of germanium (Ge) NWs using environmental transmission electron microscopy (ETEM). We perform TEM tomography of the droplet-NW system during an unperturbed growth, then track its evolution while modulating the bias potential. Using 3D finite element method (FEM) modeling and crystallographic considerations, we provide a detailed and consistent mechanism for VLS growth, which naturally explains the observed asymmetries and features of a growing NW based on its crystal structure. Our findings provide a solid framework for the fabrication of complex 3D semiconductor nanostructures with ultimate control over their morphology.

Thermodynamics Controlled Sharp Transformation from InP to GaP Nanowires via Introducing Trace Amount of Gallium.

Growth of high-quality III-V nanowires at a low cost for optoelectronic and electronic applications is a long-term pursuit of research. Still, controlled synthesis of III-V nanowires using chemical vapor deposition method is challenge and lack theory guidance. Here, we show the growth of InP and GaP nanowires in a large area with a high density using a vacuum chemical vapor deposition method. It is revealed that high growth temperature is required to avoid oxide formation and increase the crystal purity of InP nanowires. Introduction of a small amount of Ga into the reactor leads to the formation of GaP nanowires instead of ternary InGaP nanowires. Thermodynamic calculation within the calculation of phase diagrams (CALPHAD) approach is applied to explain this novel growth phenomenon. Composition and driving force calculations of the solidification process demonstrate that only 1 at.% of Ga in the catalyst is enough to tune the nanowire formation from InP to GaP, since GaP nucleation shows a much larger driving force. The combined thermodynamic studies together with III-V nanowire growth studies provide an excellent example to guide the nanowire growth.

Drastic Effect of Sequential Deposition Resulting from Flux Directionality on the Luminescence Efficiency of Nanowire Shells.

Core-shell nanowire heterostructures form the basis for many innovative devices. When compound nanowire shells are grown by directional deposition techniques, the azimuthal position of the sources for the different constituents in the growth reactor, substrate rotation, and nanowire self-shadowing inevitably lead to sequential deposition. Here, we uncover for In0.15Ga0.85As/GaAs shell quantum wells grown by molecular beam epitaxy a drastic impact of this sequentiality on the luminescence efficiency. The photoluminescence intensity of shell quantum wells grown with a flux sequence corresponding to migration enhanced epitaxy, that is, when As and the group-III metals essentially do not impinge at the same time, is more than 2 orders of magnitude higher than for shell quantum wells prepared with substantially overlapping fluxes. Transmission electron microscopy does not reveal any extended defects explaining this difference. Our analysis of photoluminescence transients shows that co-deposition has two detrimental microscopic effects. First, a higher density of electrically active point defects leads to internal electric fields reducing the electron-hole wave function overlap. Second, more point defects form that act as nonradiative recombination centers. Our study demonstrates that the source arrangement of the growth reactor, which is of mere technical relevance for planar structures, can have drastic consequences for the material properties of nanowire shells. We expect that this finding holds good also for other alloy nanowire shells.

In Situ Transmission Electron Microscopy Measurements of Ge Nanowire Synthesis with Liquid Metal Nanodroplets in Water.

The growth of Ge nanowires in water inside a liquid transmission electron microscope (TEM) holder has been demonstrated at room temperature. Each nanowire growth event was stimulated by the incident electron beam on otherwise unsupported liquid Ga or liquid In nanodroplets. A variety of conditions were explored, including liquid metal nanodroplet surface condition, liquid metal nanodroplet size and density, formal concentration of dissolved GeO2, and electron beam intensity. The cumulative observations from a series of videos recorded during growth events suggested the following points. First, the conditions necessary for initiating nanowire growth at uncontacted liquid metal nanodroplets in a liquid TEM cell indicate the process was governed by solvated electrons generated from secondary electrons scattered by the liquid metal nanodroplets. The attained current densities were comparable to those achieved in conventional electrochemical liquid-liquid-solid (ec-LLS) growths outside of a TEM. Second, the surface condition of the liquid metal nanodroplets was quite influential on whether nanowire growth occurred and surface diffusion of Ge adatoms contributed to the rate of crystallization. Third, the Ge nanowire growth rates were limited by the feed rate of Ge to the crystal growth front rather than the rate of crystallization at the liquid metal/solid Ge interface. Estimates of an electrochemical current for the reduction of dissolved GeO2 were nominally in line with currents used for Ge nanowire growth by ec-LLS outside of the TEM. Fourth, the Ge nanowire growths in the liquid TEM cell occurred far from thermodynamic equilibrium, with supersaturation values of 104 prior to nucleation. These collective points provide insight on how to further control and improve Ge nanowire morphology and crystallographic quality by the ec-LLS method.

Transition from freestanding SnO2 nanowires to laterally aligned nanowires with a simulation-based experimental design.

In this study, we used simulations as a guide for experiments in order to switch freestanding nanowire growth to a laterally aligned growth mode. By means of finite element simulations, we determined that a higher volumetric flow and a reduced process pressure will result in a preferred laterally aligned nanowire growth. Furthermore, increasing the volumetric flow leads to a higher species dilution. Based on our numerical results, we were able to successfully grow laterally aligned SnO2 nanowires out of gold film edges and gold nanoparticles on a-plane sapphire substrates. In our experiments a horizontal 2-zone tube furnace was used. The generation of Sn gas was achieved by a carbothermal reduction of SnO2 powder. However, we observed no elongation of the nanowire length with an increase of the process time. Nevertheless, an alternating gas exchange between an inert gas (Ar) and an oxygen-containing process atmosphere yielded an elongation of the laterally aligned nanowires, indicating that the nanowire growth takes place in a transient period of the gas exchange.

Impact of Rotational Twin Boundaries and Lattice Mismatch on III-V Nanowire Growth.

Pseudomorphic planar III-V transition layers greatly facilitate the epitaxial integration of vapor-liquid-solid grown III-V nanowires (NW) on Si(111) substrates. Heteroepitaxial (111) layer growth, however, is commonly accompanied by the formation of rotational twins. We find that rotational twin boundaries (RTBs), which intersect the surface of GaP/Si(111) heterosubstrates, generally cause horizontal NW growth and may even suppress NW growth entirely. Away from RTBs, the NW growth direction switches from horizontal to vertical in the case of homoepitaxial GaP NWs, whereas heteroepitaxial GaAs NWs continue growing horizontally. To understand this rich phenomenology, we develop a model based on classical nucleation theory. Independent of the occurrence of RTBs and specific transition layers, our model can generally explain the prevalent observation of horizontal III-V NW growth in lattice mismatched systems and the high crystal quality of horizontal nanowires.

Atomic-Scale Observation of Vapor-Solid Nanowire Growth via Oscillatory Mass Transport.

In situ atomic-scale transmission electron microscopy (TEM) can provide critical information regarding growth dynamics and kinetics of nanowires. A catalyst-aided nanowire growth mechanism has been well-demonstrated by this method. By contrast, the growth mechanism of nanowires without catalyst remains elusive because of a lack of crucial information on related growth dynamics at the atomic level. Herein, we present a real-time atomic-scale observation of the growth of tungsten oxide nanowires through an environmental TEM. Our results unambiguously demonstrate that the vapor-solid mechanism dominates the nanowire growth, and the oscillatory mass transport on the nanowire tip maintains the noncatalytic growth. Autocorrelation analysis indicates that adjacent nucleation events in the nanowire growth are independent of each other. These findings may improve the understanding of the vapor-solid growth mechanism of nanowires.

Centrifugal Shape Sorting of Faceted Gold Nanoparticles Using an Atomic Plane-Selective Surfactant.

Highly refined shape populations of gold nanoparticles (AuNPs) are important for emerging applications in catalysis, plasmonics, and nanomaterials growth. To date, research efforts have focused on achieving monodisperse shape by synthetic control or postsynthetic processing that relies on centrifugal sedimentation-based sorting schemes where differences in the particle mass and aspect ratios (e.g., rods and spheres) provide a driving force for separation. Here, we present a technique to reversibly modify the sedimentation coefficients of AuNPs possessing different shapes that would otherwise be virtually indistinguishable during centrifugal sedimentation due to their similar densities, masses, and aspect ratios by exploiting the preferential affinity of the surfactant cetyltrimethylammonium bromide (CTAB) for the Au(100) facet. The resulting tailored sedimentation coefficients enable AuNP shape sorting via density gradient centrifugation (DGC). DGC-refined populations of faceted AuNPs are shown to significantly enhance the growth rate of InAs nanowires when used as seed particles, emphasizing the importance of shape control for nanomaterials growth applications.