Babak Nikoobakht

Horizontal growth and in situ assembly of oriented zinc oxide nanowires

The positioning and directed assembly of semiconductor nanowires (NWs) is of considerable current interest for “bottom-up” approaches to the engineering of intricate structures from nanoscale building blocks. We report a horizontal growth mode for ZnO NWs on the (112¯0) sapphire surface in which NWs grow in the [11¯00]sap direction. This growth mode strictly depends on the size and spacing of the Au nanodroplet catalysts and competes with the vertical growth of the NWs. An approach is presented which promotes the horizontal growth, in situ alignment, and predictable positioning of ZnO NWs. This strategy allows for the large scale assembly of NWs, width control, and production of quantum wires.

Growth habits and defects in ZnO nanowires grown on GaN/sapphire substrates

Growth habits and defects in epitaxial ZnO nanowires grown from Au catalyst on (00.1) GaN/sapphire substrate using the vapor-liquid-solid (VLS) technique were studied using electron microscopy and x-ray diffraction. The results revealed presence of both horizontal (crawling-like) and vertical nanowires having similar orientation relationship to the substrate (00.1)ZnO‖(00.1)GaN, [11.0]ZnO‖[11.0]GaN. The crawling-like growth precedes the vertical growth, and the coalescence and overgrowth of the crawling nanowires produce a highly defective layer which separates the substrate and vertical nanorods. Transmission electron microscopy revealed a high density of planar defects in this interfacial layer. A significant density of stacking faults residing on the (0001) planes was also observed in the shorter vertical nanorods. The crawling nanowires are under residual compressive strain, whereas the vertical nanorods grow strain-free.

Formation of Planar Arrays of One-Dimensional p−n Heterojunctions Using Surface-Directed Growth of Nanowires and Nanowalls

We report a surface-directed vapor-liquid-solid process for planar growth of one-dimensional heterojunctions of zinc oxide on single crystal gallium nitride (GaN) that enables their hierarchical assembly to light emitting diodes. An individual heterojunction is about 10 μm in length and 80 nm in width and is formed by planar growth of an n-type ZnO nanowire or nanowall on p-type GaN surface using Au catalyst. Our results show that a ZnO nanocrystal at its nucleation site has six possible growth directions that can be engineered and controlled using an intentional blockade of the nanocrystal growth in certain directions owing to similarities in crystal structures of ZnO and GaN. The ZnO nanowalls are formed when nanowires during their planar growth slowly grow in direction normal to the substrate via a self-catalytic process. The crystal structure of these heterojunctions is examined from two different crystallographic perspectives using high resolution transmission electron microscopy. Results indicate abrupt and epitaxial formation of n-p heterojunctions, which are difficult to achieve in thin film growth of these heterojunctions. The collective light emission of micrometer- to millimeter-size arrays of the heterojunctions is demonstrated via a simple design that is scalable to literally any platform size. This technique allows in situ growth and combinations of II-VI and III-V semiconductors and offers their easier integration to photonic and lab-on-chip platforms with applications in energy generation and light detection.

Analysis of copper incorporation into zinc oxide nanowires.

ZnO nanowires (NWs) are grown on a bulk copper half-transmission electron microscopy grid by chemical vapor deposition in a high temperature tube furnace. Photoluminescence (PL) microscopy revealed band gap emission at 380 nm and a more intense visible emission around 520 nm due to defect states in these NWs. High-resolution transmission electron microscopy shows that the ZnO NWs are single crystalline with hexagonal structure. Auger electron spectroscopy (AES) and energy dispersive X-ray spectroscopy reveal that copper atoms are present along the length of the NW. AES also found that the surface of the NWs is oxygen rich. The surface concentration of zinc increases moving from the tip toward the base of the NW while the concentration of oxygen decreases. The copper in this system not only remains at the tip of the growing NW but also acts as a dopant along the length of the NW, leading to a decrease in the intensity of the band gap PL of these NWs.

Two-dimensional nanomembranes: can they outperform lower dimensional nanocrystals?

Inorganic nanomembranes, analogues to graphene, are expected to impact a wide range of device concepts including thin-film or flexible platforms. Size-dependent properties and high surface area-two key characteristics of zero- (0D) and one-dimensional (1D) nanocrystals-are still present in most nanomembranes, rendering their use more probable in practical applications. These advantages make nanomembranes strong contenders for outpacing 0D and 1D nanocrystals, which are often difficult to integrate into commercial device technologies. This Perspective highlights important progress made by Wang et al. (doi: 10.1021/nn2050906) in large-scale fabrication of free-standing nanomembranes by using a solution-based technique, as reported in this issue of ACS Nano. The simplicity of this new approach and the elimination of typical delamination processes used in top-down nanomembrane fabrications are among the strengths of this technique. Areas for improvement along with an overview of other related work are also discussed.

Controlling The Growth Direction of ZnO Nanowires on c-Plane Sapphire

Well oriented vertical ZnO nanowires (NWs) are grown on c -plane sapphire via a vapor- phase transport process using an Au thin film as a catalyst. This new finding is unexpected due to the fact that the lattice mismatch between the zinc oxide and the underlying substrate is 18%. X- ray diffraction (XRD) analysis shows that single-crystal, wurtzite NWs grow in the [0001] direction normal to the basal sapphire plane, which proves that a -plane sapphire is not essential for growth of vertical ZnO NWs, as has been previously stated.[1] We have found that by controlling the thickness of the Au-film and pre-growth annealing of the Au/sapphire substrate NWs can be grown either tilted or vertical. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) studies on Au films with thicknesses ranging from 1 to 10 nm show that in the absence of film annealing, NWs can be grown 32° tilted from the surface normal, whereas pre-annealed Au films result in growth of NWs in the surface normal direction. We attribute the formation of the normal and tilted growth directions to the surface concentration of O and Al ions on sapphire.

Vapor-Liquid-Solid Etch of Semiconductor Surface Channels by Running Gold Nanodroplets.

We show that Au nanoparticles spontaneously move across the (001) surface of InP, InAs, and GaP when heated in the presence of water vapor. As they move, the particles etch crystallographically aligned grooves into the surface. We show that this process is a negative analogue of the vapor-liquid-solid (VLS) growth of semiconductor nanowires: the semiconductor dissolves into the catalyst and reacts with water vapor at the catalyst surface to create volatile oxides, depleting the dissolved cations and anions and thus sustaining the dissolution process. This VLS etching process provides a new tool for directed assembly of structures with sublithographic dimensions, as small as a few nanometers in diameter. Au particles above 100 nm in size do not exhibit this process but remain stationary, with oxide accumulating around the particles.

Separation, Sizing, and Quantitation of Engineered Nanoparticles in an Organism Model Using Inductively Coupled Plasma Mass Spectrometry and Image Analysis.

For environmental studies assessing uptake of orally ingested engineered nanoparticles (ENPs), a key step in ensuring accurate quantification of ingested ENPs is efficient separation of the organism from ENPs that are either nonspecifically adsorbed to the organism and/or suspended in the dispersion following exposure. Here, we measure the uptake of 30 and 60 nm gold nanoparticles (AuNPs) by the nematode, Caenorhabditis elegans, using a sucrose density gradient centrifugation protocol to remove noningested AuNPs. Both conventional inductively coupled plasma mass spectrometry (ICP-MS) and single particle (sp)ICP-MS are utilized to measure the total mass and size distribution, respectively, of ingested AuNPs. Scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS) imaging confirmed that traditional nematode washing procedures were ineffective at removing excess suspended and/or adsorbed AuNPs after exposure. Water rinsing procedures had AuNP removal efficiencies ranging from 57 to 97% and 22 to 83%, while the sucrose density gradient procedure had removal efficiencies of 100 and 93 to 98%, respectively, for the 30 and 60 nm AuNP exposure conditions. Quantification of total Au uptake was performed following acidic digestion of nonexposed and Au-exposed nematodes, whereas an alkaline digestion procedure was optimized for the liberation of ingested AuNPs for spICP-MS characterization. Size distributions and particle number concentrations were determined for AuNPs ingested by nematodes with corresponding confirmation of nematode uptake via high-pressure freezing/freeze substitution resin preparation and large-area SEM imaging. Methods for the separation and in vivo quantification of ENPs in multicellular organisms will facilitate robust studies of ENP uptake, biotransformation, and hazard assessment in the environment.

Realization of vertically-aligned GaN n-p core-shell nanoscale structures using top-down fabrication

Vertically aligned core-shell p-n nanostructures are technologically significant due to their potential applications in a variety of devices such as light-emitting diodes, laser diodes, photodetectors, and solar cells. Such structures developed in III-Nitride material system are expected to increase device efficiency, partially due to mitigating detrimental effects of spontaneous electrical polarization. Despite significant progress in the synthesis of nitride core-sell nanostructures using both bottom-up and top-down paradigms, fabrication of large arrays of these nanostructures with controlled morphology, orientation, dopant incorporation, and site-specific nucleation is still a challenge. One of the most challenging aspects of making high quality GaN nanopillar arrays using top-down approach is to design an etching protocol for producing high-aspect ratio structures while inducing minimal damage to the sidewalls and preventing the etch-mask erosion.[1]

Structural modulation of nanowire interfaces grown over selectively disrupted single crystal surfaces

Recent breakthroughs in deterministic approaches to the fabrication of nanowire arrays have demonstrated the possibility of fabricating such networks using low-cost scalable methods. In this regard, we have developed a scalable growth platform for lateral fabrication of nanocrystals with high precision utilizing lattice match and symmetry. Using this planar architecture, a number of homo- and heterostructures have been demonstrated including ZnO nanowires grown over GaN. The latter combination produces horizontal, epitaxially formed crystals aligned in the plane of the substrate containing a very low number of intrinsic defects. We use such ordered structures as model systems in the interests of gauging the interfacial structural dynamics in relation to external stimuli. Nanosecond pulses of focused ion beams are used to slightly modify the substrate surface and selectively form lattice disorders in the path of nanowire growth to examine the nanocrystal, namely: its directionality and lattice defects. High resolution electron microscopies are used to reveal some interesting structural effects; for instance, a minimum threshold of surface defects that can divert nanowires. We also discuss data indicating formation of surface strains and show their mitigation during the growth process.