Cover Picture: Liquid Crystalline Peptide Nanowires (Adv. Mater. 22/2007)

Liquid‐crystalline peptide nanowires consisting of aromatic dipeptides are easily prepared by a sonication in a volatile organic solvent, report Hyotcherl Ihee, Sang Ouk Kim, and co‐workers on p. 3924. The colloidal nematic liquid crystalline phase of the rigid nanowires allows for a macroscopically ordered morphology of the nanowires under an external electric field. The cover image presents a broad field view of nanowires oriented by an electric field between two electrodes.     

Fabrication of a nickel nanowire mesh electrode suspended on polymer substrate

We report on an efficient strategy for the fabrication of an ultra-long suspended nanowire mesh suitable for nanodevice architectures on a polymer surface. First, nickel nanowires are synthesized directly on a template substrate by magnetron sputtering. Laser interference lithography followed by deep reactive ion etching is used to create the nanograted template substrate constituted of one-dimensional line pattern arrays of 240 nm in periodicity. Ordered alignment of ultra-long nanowires (～180 nm in diameter) with high fidelity to the template pattern is observed by scanning electron microscopy. The transfer of the pre-defined parallel nanowire array from the template surface to a target polymer substrate for electrical characterization of the system is demonstrated. The electrical behaviour of the nanowire mesh, suspended between two electrodes, was found to be linear, stable, and reproducible. This result suggests that this nanofabrication process will open an efficient way to the design and construction of novel nanodevices.     

Novel fabrication method of diverse one-dimensional Pt/ZnO hybrid nanostructures and its sensor application

A novel low-temperature, solution-phase method for the facile fabrication of a variety of one-dimensional (1D) metal/metal oxide hybrid nanostructures has been developed. This method is based on the wet chemical synthesis of metal oxide nanowires, followed by the surface coating of metal nanoparticles on metal oxide nanowire templates via reduction of metal ions along with controlled etching of metal oxide nanowires at the core, all in a low-temperature liquid environment. As a proof-of-concept, we applied this method to the fabrication of various 1D Pt/ZnO hybrid nanostructures including Pt nanoparticle-coated ZnO nanowires/nanotubes and Pt nanotubes on silicon and polymer substrates. The diverse morphology tuning is attributed to the control of pH in the solution with different metal precursor concentrations and amounts of reducing agent. The change of morphology, crystalline structure, and composition of various 1D Pt/ZnO hybrid nanostructures was observed by SEM, TEM (HRTEM), XRD and ICP-AES, respectively. Further, we have demonstrated a highly sensitive strain sensor (gauge factor = 15) with a Pt nanotube film fabricated by the developed method on a flexible polymer substrate.     

Biological assembly of nanocircuit prototypes from protein-modified CdTe nanowires

CdTe nanowires made by self-organization of CdTe nanoparticles in aqueous media were separately conjugated with complementary biological connectors, such as antigen-antibody and biotin-streptavidin. Transmission electron microscopy images and Forster resonance energy transfer measurements in nanowire superstructures with different diameters indicate that biological affinity of the attached proteins results in the formation of crossbar and end-to-side connections between the nanowires. A prototype of a logical circuit made from a triangular arrangement of the nanowires spontaneously assembled on a Si substrate was examined by conducting atomic force microscopy. While diode-like behavior was observed in the sides of the triangle, the nanowire junction points were found to be nonconductive. It was attributed to high tunneling barrier created by protein molecules wedged between the nanowires. Suggestions are made how to reduce it or use the insulating gap between the nanowires as a framework for single-electron devices.     

Wafer-scale assembly of highly ordered semiconductor nanowire arrays by contact printing

Controlled and uniform assembly of "bottom-up" nanowire (NW) materials with high scalability presents one of the significant bottleneck challenges facing the integration of nanowires for electronic applications. Here, we demonstrate wafer-scale assembly of highly ordered, dense, and regular arrays of NWs with high uniformity and reproducibility through a simple contact printing process. The assembled NW pitch is shown to be readily modulated through the surface chemical treatment of the receiver substrate, with the highest density approaching approximately 8 NW/mum, approximately 95% directional alignment, and wafer-scale uniformity. Such fine control in the assembly is attained by applying a lubricant during the contact printing process which significantly minimizes the NW-NW mechanical interactions, therefore enabling well-controlled transfer of nanowires through surface chemical binding interactions. Furthermore, we demonstrate that our printing approach enables large-scale integration of NW arrays for various device structures on both rigid silicon and flexible plastic substrates, with a controlled semiconductor channel width ranging from a single NW ( approximately 10 nm) up to approximately 250 microm, consisting of a parallel array of over 1250 NWs and delivering over 1 mA of ON current.     

High-throughput fabrication of organic nanowire devices with preferential internal alignment and improved performance

We demonstrate that arrays of nanowires of conjugated polymers can be easily produced by a simple embossing protocol, compatible with very large scale integration technology. The embossing process is shown to have the supplementary virtue to increase the internal degree of order of the nanowires, significantly enhancing their performance. This is applied to the fabrication of nanowire-based devices consisting of a liquid crystalline light-emitting polymer, of a liquid crystalline semiconducting polymer, and of an amorphous conducting polymer, illustrating the versatility and wide applicability of the method.     

图形化硅纳米线阵列的制备

本文主要研究了在常态(常温、常压等)条件下,利用金属催化化学腐蚀方法在硅片表面上大面积制备排列整齐、取向一致的硅纳米线阵列.同时,出于对后续制作硅纳米线传感器考虑,利用微电子标准加工工艺,以氮化硅做掩膜,通过选择合适的实验参数,在硅片表面选择性生长纳米线阵列,得到图形化的硅纳米线阵列.     

Capillary-driven assembly of ZnO nanowire arrays into micropatterns

The vertically aligned ZnO nanowire arrays prepared by vapor transport process can be assembled into complex micropatterns under capillary force. The deflection of the flexible ceramic nanowire is closely related to the liquid tension coefficient, mechanical and structural properties of the ZnO nanowires. The bended nanowires are adhesive together because the solid adhesion energy is sufficient to withstand the restoring elastic force of the deformed nanowires. The size of the bundling pattern can be controlled by varying the aspect ratio of the nanowire. The deflection of the ZnO nanostructure composed of a nanowire and a base is multifarious.     

交替刻蚀制备有序锯齿形硅纳米线阵列及其光学性能研究

采用金属催化化学刻蚀法(MCCE),以金属Ag为催化剂,在HF与H2O2体系中通过交替刻蚀在P(111)硅衬底上制备出锯齿形硅纳米线阵列.利用扫描电子显微镜对硅纳米线的形貌进行了表征,研究了HF浓度与H2O2浓度对纳米线刹蚀方向的调控作用.选取不同的HF与H2O2浓度配比,分别对硅基底各向同性刻蚀与各向异性刻蚀进行调控,使得刻蚀方向对溶液浓度的变化能够快速响应.在溶液Ⅰ([HF]=2.3 mol/L,[H2O2]=0.4 mol/L)与溶液Ⅱ([HF]=9.2 mol/L,[H2O2]=0.04 mol/L)中交替刻蚀,制备出刻蚀方向高度可控的大规模锯齿形硅纳米线.利用紫外-可见分光光度计对锯齿形硅纳米线的减反射性能进行研究,结果表明,其表现出优异的减反特性,最低反射率为5.9％.纳米线形貌的高度可控性使其在微电子器件领域也具有巨大的应用前景.     

Controlling the morphology of side chain liquid crystalline block copolymer thin films through variations in liquid crystalline content

In this paper, we describe methods for manipulating the morphology of side-chain liquid crystalline block copolymers through variations in the liquid crystalline content. By systematically controlling the covalent attachment of side chain liquid crystals to a block copolymer (BCP) backbone, the morphology of both the liquid crystalline (LC) mesophase and the phase-segregated BCP microstructures can be precisely manipulated. Increases in LC functionalization lead to stronger preferences for the anchoring of the LC mesophase relative to the substrate and the intermaterial dividing surface. By manipulating the strength of these interactions, the arrangement and ordering of the ultrathin film block copolymer nanostructures can be controlled, yielding a range of morphologies that includes perpendicular and parallel cylinders, as well as both perpendicular and parallel lamellae. Additionally, we demonstrate the utilization of selective etching to create a nanoporous liquid crystalline polymer thin film. The unique control over the orientation and order of the self-assembled morphologies with respect to the substrate will allow for the custom design of thin films for specific nanopatterning applications without manipulation of the surface chemistry or the application of external fields.     

Anisotropic characteristics and morphological control of silicon nanowires fabricated by metal-assisted chemical etching

Low-cost fabrication methods enabling the morphological control of silicon nanowires are of great importance in many device application fields. A top-down fabrication method, metal-assisted chemical etching, is proved to be a feasible solution. In this paper, some novel approaches based on metal-assisted chemical etching, alkaline solution etching, and electrochemical anodic etching are presented for fabricating micro- and nano-structures, which reveal the anisotropic characteristics of metal-assisted chemical etching in silicon. A new model is proposed to explain the motility behavior of Ag particles in metal-assisted chemical etching of silicon. It is shown that Ag particle forms a self-electrophoresis unit and migrates into Si substrate along [100] direction independently. Diameter and length control of silicon nanowires are achieved by varying Ag deposition and etching durations of metal-assisted chemical etching, respectively, which provide a facilitation to achieve high-aspect-ratio silicon nanowires at room temperature in a short period. These results show a potential simple method to microstructure silicon for devices application, such as solar cells and sensors.     

Optical trapping and integration of semiconductor nanowire assemblies in water

Semiconductor nanowires have received much attention owing to their potential use as building blocks of miniaturized electrical, nanofluidic and optical devices. Although chemical nanowire synthesis procedures have matured and now yield nanowires with specific compositions and growth directions, the use of these materials in scientific, biomedical and microelectronic applications is greatly restricted owing to a lack of methods to assemble nanowires into complex heterostructures with high spatial and angular precision. Here we show that an infrared single-beam optical trap can be used to individually trap, transfer and assemble high-aspect-ratio semiconductor nanowires into arbitrary structures in a fluid environment. Nanowires with diameters as small as 20 nm and aspect ratios of more than 100 can be trapped and transported in three dimensions, enabling the construction of nanowire architectures that may function as active photonic devices. Moreover, nanowire structures can now be assembled in physiological environments, offering new forms of chemical, mechanical and optical stimulation of living cells.     

Field emission properties of carbon nanotube arrays on the thickness-controlled flexible substrate by the pattern transfer process

A technigal with the polydimethylsiloxane (PDMS) solution infiltrated into the SiOx-coated CNTAs has been utilized to directly transfer the CNTAs away from the silicon substrate. The oxide coating layer was utilized to protect the morpholgy of as-grown patterned vertical aligmed carbon nanotube (CNTs) arrays. The high density plasma reactive ions etching (HDP-RIE) system was used to make the CNTs emerge from the surface of the flexible substrate and modify the crystallines of CNTs. After the protecting oxide was HDP-RIE-processed for 8 min, the emission current properties were enhanced to be 1.03 V/microm and 1.43 V/microm, respectively, for the turn-on field and the threshold field, as compared with 1.25 V/microm and 1.59 V/microm for the as-grown CNTs, accordingly. The Field Emission (FE) enhancement after dry etching could be attributed to the open-ended structures and better crystalline.     

Peptide‐Based Nanotubes and Their Applications in Bionanotechnology

In nature, biological nanomaterials are synthesized under ambient conditions in a natural microscopic‐sized laboratory, such as a cell. Biological molecules, such as peptides and proteins, undergo self‐assembly processes in vivo and in vitro, and these monomers are assembled into various nanometer‐scale structures at room temperature and atmospheric pressure. The self‐assembled peptide nanostructures can be further organized to form nanowires, nanotubes, and nanoparticles via their molecular‐recognition functions. The application of molecular self‐assemblies of synthetic peptides as nanometer‐scale building blocks in devices is robust, practical, and affordable due to their advantages of reproducibility, large‐scale production ability, monodispersity, and simpler experimental methods. It is also beneficial that smart functionalities can be added at desired positions in peptide nanotubes through well‐established chemical and peptide syntheses. These features of peptide‐based nanotubes are the driving force for investigating and developing peptide nanotube assemblies for biological and non‐biological applications.     

New silicon architectures by gold-assisted chemical etching

Silicon nanowires (SiNWs) were produced by nanosphere lithography and metal assisted chemical etching. The combination of these methods allows the morphology and organization control of Si NWs on a large area. From the investigation of major parameters affecting the etching such as doping type, doping concentration of the substrate, we demonstrate the formation of new Si architectures consisting of organized Si NW arrays formed on a micro/mesoporous silicon layer with different thickness. These investigations will allow us to better understand the mechanism of Si etching to enable a wide range of applications such as molecular sensing, and for thermoelectric and photovoltaic devices.     

Large scale synthesis of highly pure single crystalline tellurium nanowires by thermal evaporation method

Single crystalline tellurium nanowires were successfully synthesized in large scale by a facile approach of vaporizing tellurium metal and condensing the vapor in an inert atmosphere onto a Si substrate. Tellurium was evaporated by heating at 300 degrees C at 1 torr and condensed on the Si substrate at 100-150 degrees C, in the downstream of argon (Ar) gas at a flow rate of 25 sccm for 30 min. The as-synthesized nanowires have diameters between 100-300 nm and lengths up to several micrometers. The single crystalline nanowires grew in a preferred [0001] direction. The obtained nanowires were highly pure as only tellurium metal was used in the vaporization process, and no other reagent, surfactant, or template were used for the growth. This low temperature and high-yield approach to the tellurium nanowires synthesis may facilitate its industrial production for various applications.     

Promoted growth of Bi single-crystalline nanowires by sidewall-induced compressive stress in on-film formation of nanowires

To increase the density of Bi nanowires grown by our unique on-film formation of nanowires (OFF-ON) method, we introduced a technique for enhancing compressive stress, which is the driving force for the nanowire growth. The compressive stress could be controlled by modifying the substrate structure. A combination of photolithography and a reactive ion etching technique was used to fabricate patterns on a thermally oxidized Si(100) substrate. It was found that the density of Bi nanowires grown from Bi films in 100 x 100 microm2-sized SiO2 patterns increases by a factor of seven over that from non-patterned substrates. Our results indicate that the density of Bi nanowires can be increased by enhanced compressive stress arising from a sidewall effect in the optimized pattern size and array.     

Template-grown NiFe/Cu/NiFe nanowires for spin transfer devices

We have developed a new reliable method combining template synthesis and nanolithography-based contacting technique to elaborate current perpendicular-to-plane giant magnetoresistance spin valve nanowires, which are very promising for the exploration of electrical spin transfer phenomena. The method allows the electrical connection of one single nanowire in a large assembly of wires embedded in anodic porous alumina supported on Si substrate with diameters and periodicities to be controllable to a large extent. Both magnetic excitations and switching phenomena driven by a spin-polarized current were clearly demonstrated in our electrodeposited NiFe/Cu/ NiFe trilayer nanowires. This novel approach promises to be of strong interest for subsequent fabrication of phase-locked arrays of spin transfer nano-oscillators with increased output power for microwave applications.     

Effect of molecular adsorption on the electrical conductance of single au nanowires fabricated by electron-beam lithography and focused ion beam etching

Metal nanowires are one of the potential candidates for nanostructured sensing elements used in future portable devices for chemical detection; however, the optimal methods for fabrication have yet to be fully explored. Two routes to nanowire fabrication, electron-beam lithography (EBL) and focused ion beam (FIB) etching, are studied, and their electrical and chemical sensing properties are compared. Although nanowires fabricated by both techniques exhibit ohmic conductance, I-V characterization indicates that nanowires fabricated by FIB etching exhibit abnormally high resistivity. In addition, the resistivity of nanowires fabricated by FIB etching shows very low sensitivity toward molecular adsorption, while those fabricated by EBL exhibit sensitive resistance change upon exposure to solution-phase adsorbates. The mean grain sizes of nanowires prepared by FIB etching are much smaller than those fabricated by EBL, so their resistance is dominated by grain-boundary scattering. As a result, these nanowires are much less sensitive to molecular adsorption, which mediates nanowire conduction through surface scattering. The much reduced mean grain sizes of these nanowires correlate with Ga ion damage caused during the ion milling process. Thus, even though the nanowires prepared by FIB etching can be smaller than their EBL counterparts, their reduced sensitivity to adsorption suggests that nanowires produced by EBL are preferred for chemical and biochemical sensing applications.     

Nanowire lithography on silicon

Nanowire lithography (NWL) uses nanowires (NWs), grown and assembled by chemical methods, as etch masks to transfer their one-dimensional morphology to an underlying substrate. Here, we show that SiO2 NWs are a simple and compatible system to implement NWL on crystalline silicon and fabricate a wide range of architectures and devices. Planar field-effect transistors made of a single SOI-NW channel exhibit a contact resistance below 20 kOmega and scale with the channel width. Further, we assess the electrical response of NW networks obtained using a mask of SiO2 NWs ink-jetted from solution. The resulting conformal network etched into the underlying wafer is monolithic, with single-crystalline bulk junctions; thus no difference in conductivity is seen between a direct NW bridge and a percolating network. We also extend the potential of NWL into the third dimension, by using a periodic undercutting that produces an array of vertically stacked NWs from a single NW mask.