Twin-free uniform epitaxial GaAs nanowires grown by a two-temperature process

We demonstrate vertically aligned epitaxial GaAs nanowires of excellent crystallographic quality and optimal shape, grown by Au nanoparticle-catalyzed metalorganic chemical vapor deposition. This is achieved by a two-temperature growth procedure, consisting of a brief initial high-temperature growth step followed by prolonged growth at a lower temperature. The initial high-temperature step is essential for obtaining straight, vertically aligned epitaxial nanowires on the (111)B GaAs substrate. The lower temperature employed for subsequent growth imparts superior nanowire morphology and crystallographic quality by minimizing radial growth and eliminating twinning defects. Photoluminescence measurements confirm the excellent optical quality of these two-temperature grown nanowires. Two mechanisms are proposed to explain the success of this two-temperature growth process, one involving Au nanoparticle-GaAs interface conditions and the other involving melting-solidification temperature hysteresis of the Au-Ga nanoparticle alloy.     

Self‐Aligned Anisotropic Plasmonic Nanostructures

Great opportunities emerge not only in the generation of anisotropic plasmonic nanostructures but also in controlling their orientation relative to incident light. Herein, a stepwise seeded growth method is reported for the synthesis of rod‐shaped plasmon nanostructures which are vertically self‐aligned with respect to the surface of colloidal substrates. Anisotropic growth of metal nanostructure is achieved by depositing metal seeds onto the surface of colloidal substrates and then selectively passivating the seed surface to induce symmetry breaking in the subsequent seed‐mediated growth process. The versatility of this method is demonstrated by producing nanoparticle dimers and linear trimers of Au, Au–Ag, Au–Pd, and Au–Cu₂O. Further, this unique method enables the automatic vertical alignment of the resulting plasmonic nanostructures to the surface of the colloidal substrate, thereby making it possible to design magnetic/plasmonic nanocomposites that allow the dynamic tuning of the plasmon excitation by controlling their orientation using an external magnetic field. The controlled anisotropic growth of colloidal plasmonic nanostructures and their dynamic modulation of plasmon excitation further allow them to be conveniently fixed in a thin polymer film with a well‐controlled orientation to display polarization‐dependent patterns that may find important applications in information encryption.     

A metallization and bonding approach for high performance carbon nanotube thermal interface materials

A method has been developed to create vertically aligned carbon nanotube (VACNT) thermal interface materials that can be attached to a variety of metallized surfaces. VACNT films were grown on Si substrates using standard CVD processing followed by metallization using Ti/Au. The coated CNTs were then bonded to metallized substrates at 220?°C. By reducing the adhesion of the VACNTs to the growth substrate during synthesis, the CNTs can be completely transferred from the Si growth substrate and used as a die attachment material for electronic components. Thermal resistance measurements using a photoacoustic technique showed thermal resistances as low as 1.7 mm(2) K W(-1) for bonded VACNT films 25-30 μm in length and 10 mm(2) K W(-1) for CNTs up to 130 μm in length. Tensile testing demonstrated a die attachment strength of 40 N cm(-2) at room temperature. Overall, these metallized and bonded VACNT films demonstrate properties which are promising for next-generation thermal interface material applications.     

Charge influence and growth mechanism of ZnO nanorods

We demonstrate the influence of charges near the substrate surface on vertically aligned ZnO nanorod growth. ZnO nanorods were fabricated on n-type GaN with and without H+ treatments by catalyst-free metal-organic chemical vapor deposition. The ZnO nanorods grown on n-GaN films were vertically well-aligned and had a well-ordered wurtzite structure. However, the ZnO did not form into nanorods and the crystal quality was very degraded as they were deposited on the H+ treated n-GaN films. The charge influence was also observed in the ZnO nanorod growth on sapphire substrates. These results implied that the charges near the substrate surface dominantly affected on the crystalization and formation of ZnO nanorods.     

Charge influence and growth mechanism of ZnO nanorods

We demonstrate the influence of charges near the substrate surface on vertically aligned ZnO nanorod growth. ZnO nanorods were fabricated on n-type GaN with and without H+ treatments by catalyst-free metal-organic chemical vapor deposition. The ZnO nanorods grown on n-GaN films were vertically well-aligned and had a well-ordered wurtzite structure. However, the ZnO did not form into nanorods and the crystal quality was very degraded as they were deposited on the H+ treated n-GaN films. The charge influence was also observed in the ZnO nanorod growth on sapphire substrates. These results implied that the charges near the substrate surface dominantly affected on the crystallization and formation of ZnO nanorods.     

Low temperature and cost-effective growth of vertically aligned carbon nanofibers using spin-coated polymer-stabilized palladium nanocatalysts

Abstract

We describe a fast and cost-effective process for the growth of carbon nanofibers (CNFs) at a temperature compatible with complementary metal oxide semiconductor technology, using highly stable polymer–Pd nanohybrid colloidal solutions of palladium catalyst nanoparticles (NPs). Two polymer–Pd nanohybrids, namely poly(lauryl methacrylate)-block-poly((2-acetoacetoxy)ethyl methacrylate)/Pd (LauMA_x-b-AEMA_y/Pd) and polyvinylpyrrolidone/Pd were prepared in organic solvents and spin-coated onto silicon substrates. Subsequently, vertically aligned CNFs were grown on these NPs by plasma enhanced chemical vapor deposition at different temperatures. The electrical properties of the grown CNFs were evaluated using an electrochemical method, commonly used for the characterization of supercapacitors. The results show that the polymer–Pd nanohybrid solutions offer the optimum size range of palladium catalyst NPs enabling the growth of CNFs at temperatures as low as 350 °C. Furthermore, the CNFs grown at such a low temperature are vertically aligned similar to the CNFs grown at 550 °C. Finally the capacitive behavior of these CNFs was similar to that of the CNFs grown at high temperature assuring the same electrical properties thus enabling their usage in different applications such as on-chip capacitors, interconnects, thermal heat sink and energy storage solutions.     

Low temperature and cost-effective growth of vertically aligned carbon nanofibers using spin-coated polymer-stabilized palladium nanocatalysts

We describe a fast and cost-effective process for the growth of carbon nanofibers (CNFs) at a temperature compatible with complementary metal oxide semiconductor technology, using highly stable polymer–Pd nanohybrid colloidal solutions of palladium catalyst nanoparticles (NPs). Two polymer–Pd nanohybrids, namely poly(lauryl methacrylate)-block-poly((2-acetoacetoxy)ethyl methacrylate)/Pd (LauMA_x-b-AEMA_y/Pd) and polyvinylpyrrolidone/Pd were prepared in organic solvents and spin-coated onto silicon substrates. Subsequently, vertically aligned CNFs were grown on these NPs by plasma enhanced chemical vapor deposition at different temperatures. The electrical properties of the grown CNFs were evaluated using an electrochemical method, commonly used for the characterization of supercapacitors. The results show that the polymer–Pd nanohybrid solutions offer the optimum size range of palladium catalyst NPs enabling the growth of CNFs at temperatures as low as 350 °C. Furthermore, the CNFs grown at such a low temperature are vertically aligned similar to the CNFs grown at 550 °C. Finally the capacitive behavior of these CNFs was similar to that of the CNFs grown at high temperature assuring the same electrical properties thus enabling their usage in different applications such as on-chip capacitors, interconnects, thermal heat sink and energy storage solutions.     

Influence of seed layers on the vertical growth of ZnO nanowires

We synthesized vertically aligned ZnO nanowires on SiO₂ wafer <100> using the Au, ZnO and Au/ZnO seed layers through the physical vapor deposition process. The growth direction of ZnO nanowire was controlled by using the three different seed layers. From the XRD results, we observed the highest intensity of the (002) peak on the Au/ZnO seed layer among the three seed layers. The SEM images show that all of the ZnO nanowires have an average diameter of about 100 ~ 200 nm and a length of about 5 μm, and the nanowires grown on the Au/ZnO seed layer are oriented the most perpendicularly to the substrate surface. From the PL analysis, we observed that the intensity of broad emissions at 400-600 nm relating the green emission for the ZnO nanowires on the Au/ZnO seed layer was much weaker than that for the ZnO nanowires on the ZnO seed layer. The experiment results indicate that the selection of seed layers is important to grow nanowires vertically for the application of nanoscale devices.     

A substrate-integrated and scalable templated approach based on rusted steel for the fabrication of polypyrrole nanotube arrays

We report here a facile, generalizable, and entirely scalable approach for the fabrication of vertically aligned arrays of Fe(2)O(3)/polypyrrole core-shell nanostructures and polypyrrole nanotubes. Our "all electrochemical" approach is based on the fabrication of α-Fe(2)O(3) nanowire arrays by the simple heat treatment of commodity low carbon steel substrates, followed by electropolymerization of conformal polypyrrole sheaths around the nanowires. Subsequently, electrochemical etching of the nanowires yields large-area vertically aligned polypyrrole nanotube arrays on the steel substrate. The developed methodology is generalizable to functionalized pyrrole monomers and represents a significant practical advance of relevance to the technological implementation of conjugated polymer nanostructures in electrochromics, electrochemical energy storage, and sensing.     

Preparation of vertically aligned ZnO crystal rods in aqueous solution at external electric field

In this study, an external electric field was used to facilitate the growth of vertically aligned ZnO crystal rods on the surface of indium tin oxide (ITO) glass substrates in an aqueous solution. We used Zn(NO₃) and C₆H₁₂N₄ as precursor and reagent. We found that the external electric field generated by DC potential of 5 kV between two electrodes that were placed outside the bottle could facilitate the growth of homogeneous, high density and vertically aligned ZnO crystal rods. Position of the substrate during the growth of crystal was found to be important to obtain well aligned crystal. The crystals that were grown near the negative electrode had the best properties. Photoluminescence measurement at room temperature revealed sharp peaks at around 360 and 380 nm and a broad peak around 420 nm that indicated good properties of ZnO crystals grown with external electric field.     

Aligned nanowire growth using lithography-assisted bonding of a polycarbonate template for neural probe electrodes

This research presents a fabrication method of vertically aligned nanowires on substrates using lithography-assisted template bonding (LATB) towards developing highly efficient electrodes for biomedical applications at low cost. A polycarbonate template containing cylindrical nanopores is attached to a substrate and the nanopores are selectively opened with a modified lithography process. Vertically aligned nanowires are grown by electrochemical deposition through these open pores on polyimide film and silicon substrates. The process of opening the nanopores is optimized to yield uniform growth of nanowires. The morphological, crystalline, and electrochemical properties of the resulting vertically aligned nanowires are discussed using scanning electron microscopy (SEM), x-ray diffraction (XRD), and electrochemical analysis tools. The potential application of this simple and inexpensive fabrication technology is discussed in the development of neural probe electrodes.     

Controllable fabrication of carbon nanotubes on catalysts derived from PS-b-P2VP block copolymer template and in situ synthesis of carbon nanotubes/Au nanoparticles composite materials

Highly ordered iron oxide nanoparticles with controlled size and spacing over a large surface area were prepared with polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP) diblock copolymer template, the obtained nanoparticles could be used as catalysts for CNTs growth in a plasma-enhanced chemical vapor deposition (PECVD) system. This route offers the capability of controlling the density of CNTs on the substrate by altering the growing time, and aligned CNTs grew vertically onto the substrates with a pre-coating of aluminium oxide (Al₂O₃) layer. In addition, Au nanoparticles were successfully attached to the sidewall of deposited CNTs through in situ synthetic method.     

van der Waals epitaxy of InAs nanowires vertically aligned on single-layer graphene

Semiconductor nanowire arrays integrated vertically on graphene films offer significant advantages for many sophisticated device applications. We report on van der Waals (VDW) epitaxy of InAs nanowires vertically aligned on graphene substrates using metal-organic chemical vapor deposition. The strong correlation between the growth direction of InAs nanowires and surface roughness of graphene substrates was investigated using various graphene films with different numbers of stacked layers. Notably, vertically well-aligned InAs nanowire arrays were obtained easily on single-layer graphene substrates with sufficiently strong VDW attraction. This study presents a considerable advance toward the VDW heteroepitaxy of inorganic nanostructures on chemical vapor-deposited large-area graphenes. More importantly, this work demonstrates the thinnest epitaxial substrate material that yields vertical nanowire arrays by the VDW epitaxy method.     

Uniform and position-controlled InAs nanowires on 2" Si substrates for transistor applications

This study presents a novel approach for indirect integration of InAs nanowires on 2' Si substrates. We have investigated and developed epitaxial growth of InAs nanowires on 2' Si substrates via the introduction of a thin yet high-quality InAs epitaxial layer grown by metalorganic vapor phase epitaxy. We demonstrate well-aligned nanowire growth including precise position and diameter control across the full wafer using very thin epitaxial layers (<300 nm). Statistical analysis results performed on the grown nanowires across the 2' wafer size verifies our full control on the grown nanowire with 100% growth yield. From the crystallographic viewpoint, these InAs nanowires are predominantly of wurtzite structure. Furthermore, we show one possible device application of the aforementioned structure in vertical wrap-gated field-effect transistor geometry. The vertically aligned InAs nanowires are utilized as transistor channels and the InAs epitaxial layer is employed as the source contact. A high uniformity of the device characteristics for numerous transistors is further presented and RF characterization of these devices demonstrates an f(t) of 9.8 GHz.     

Synthesis of vertical arrays of ultra long ZnO nanowires on noncrystalline substrates

Vertically aligned arrays of ultralong ZnO nanowires were synthesized on SiO₂ substrates with carbothermal vapor phase transport method with Au seeding layer. High density of vertically aligned ZnO nanowires with lengths from a few to ∼300 μm could be grown by controlling growth conditions. Supply of high concentration of Zn vapor and control of the ratio between Zn vapor and oxygen are found to have the most significant effects on the growth of long ZnO nanowires in the vapor-solid growth mechanism. The nanowires are of high crystalline quality as confirmed by various structural, compositional, and luminescent measurements. Luminescent and electrical properties of ZnO nanowires with different growth conditions were also investigated.     

Controlled growth of Si nanowire arrays for device integration

Silicon nanowires were synthesized, in a controlled manner, for their practical integration into devices. Gold colloids were used for nanowire synthesis by the vapor-liquid-solid growth mechanism. Using SiCl4 as the precursor gas in a chemical vapor deposition system, nanowire arrays were grown vertically aligned with respect to the substrate. By manipulating the colloid deposition on the substrate, highly controlled growth of aligned silicon nanowires was achieved. Nanowire arrays were synthesized with narrow size distributions dictated by the seeding colloids and with average diameters down to 39 nm. The density of wire growth was successfully varied from approximately 0.1-1.8 wires/microm2. Patterned deposition of the colloids led to confinement of the vertical nanowire growth to selected regions. In addition, Si nanowires were grown directly into microchannels to demonstrate the flexibility of the deposition technique. By controlling various aspects of nanowire growth, these methods will enable their efficient and economical incorporation into devices.     

Effect of ZnO seed layer on the catalytic growth of vertically aligned ZnO nanorod arrays

We have grown vertically aligned ZnO nanorods and multipods by a seeded layer assisted vapor–liquid–solid (VLS) growth process using a muffle furnace. The effect of seed layer, substrate temperature and substrate material has been studied systematically for the growth of high quality aligned nanorods. The structural analysis on the aligned nanorods shows c-axis oriented aligned growth by homoepitaxy. High crystallinity and highly aligned ZnO nanorods are obtained for growth temperature of 850–900 °C. Depending on the thickness of the ZnO seed layer and local temperature on the substrate, some region of a substrate show ZnO tetrapod, hexapods and multipods, in addition to the vertically aligned nanorods. Raman scattering studies on the aligned nanorods show distinct mode at ∼438 cm⁻¹, confirming the hexagonal wurtzite phase of the nanorods. Room temperature photoluminescence studies show strong near band edge emission at ∼378 nm for aligned nanorods, while the non-aligned nanorods show only defect-emission band at ∼500 nm. ZnO nanorods grown without the seed layer were found to be non-aligned and are of much inferior quality. Possible growth mechanism for the seeded layer grown aligned nanorods is discussed.     

Misfit dislocations in epitaxial films III. Theory

Observations on the growth of Pd on (001)Au and (111)Au substrates have shown that Pd deposits grow pseudomorphically up to a few monolayers thickness, whereupon misfit dislocations nucleate at the Pd surface and grow into the Pd-Au interface to relieve misfit strains. In this paper the critical thicknesses for coherency loss in Pd/(001)Au and Pd/(111)Au films are measured accurately using an X-ray microanalysis technique. The nucleation and growth of the observed misfit dislocation sources, described in previous papers, is examined in detail theoretically, and the results are compared with experiment. It is shown that misfit dislocation sources in Pd/(001)Au and Pd/(111)Au films nucleate at different deposit thicknesses in excellent quantitative agreement with experiment and are the most energetically favourable sources.     

Chemical vapour deposition of single walled carbon nanotubes freely suspended over nanotube supports

Single walled carbon nanotubes (SWNTs) suspended above the substrate can be fabricated simply and rapidly by chemical vapour deposition growth over pre-grown multi-walled carbon nanotubes (MWNTs). SWNTs are suspended either on a randomly organized carbon nanotube network on an unpatterned substrate, or between organized pillars made from vertically aligned nanotube forests on a patterned substrate. All nanotubes are produced during a single growth run using a two step growth technique. This approach enables the fabrication of laterally suspended SWNT networks which are well suited for optical applications.     

Fabrication of vertically aligned ultrafine ZnO nanorods using metal-organic vapor phase epitaxy with a two-temperature growth method

We report the fabrication of vertically aligned ultrafine ZnO nanorods using metal-organic vapor phase epitaxy and applying a two-temperature growth method. First, thick nanorods were grown vertically on the substrate at a lower temperature. Then, ultrafine ZnO nanorods with an average diameter of 17.7?nm were grown from the tips of the thick nanorods at a higher temperature. The direction of the ultrafine ZnO nanorods followed that of the preformed vertically aligned thick nanorods. Electron microscopy revealed that the ultrafine nanorods were single crystals and the growth direction was along the c axis. Excellent photoluminescence characteristics of the nanorods were confirmed.