Bridging dimensions: demultiplexing ultrahigh-density nanowire circuits

A demultiplexer is an electronic circuit designed to separate two or more combined signals. We report on a demultiplexer architecture for bridging from the submicrometer dimensions of lithographic patterning to the nanometer-scale dimensions that can be achieved through nanofabrication methods for the selective addressing of ultrahigh-density nanowire circuits. Order log2(N) large wires are required to address N nanowires, and the demultiplexer architecture is tolerant of low-precision manufacturing. This concept is experimentally demonstrated on submicrometer wires and on an array of 150 silicon nanowires patterned at nanowire widths of 13 nanometers and a pitch of 34 nanometers.     

Room-temperature ultraviolet nanowire nanolasers

Room-temperature ultraviolet lasing in semiconductor nanowire arrays has been demonstrated. The self-organized, <0001> oriented zinc oxide nanowires grown on sapphire substrates were synthesized with a simple vapor transport and condensation process. These wide band-gap semiconductor nanowires form natural laser cavities with diameters varying from 20 to 150 nanometers and lengths up to 10 micrometers. Under optical excitation, surface-emitting lasing action was observed at 385 nanometers, with an emission linewidth less than 0.3 nanometer. The chemical flexibility and the one-dimensionality of the nanowires make them ideal miniaturized laser light sources. These short-wavelength nanolasers could have myriad applications, including optical computing, information storage, and microanalysis.     

Directed assembly of one-dimensional nanostructures into functional networks

One-dimensional nanostructures, such as nanowires and nanotubes, represent the smallest dimension for efficient transport of electrons and excitons and thus are ideal building blocks for hierarchical assembly of functional nanoscale electronic and photonic structures. We report an approach for the hierarchical assembly of one-dimensional nanostructures into well-defined functional networks. We show that nanowires can be assembled into parallel arrays with control of the average separation and, by combining fluidic alignment with surface-patterning techniques, that it is also possible to control periodicity. In addition, complex crossed nanowire arrays can be prepared with layer-by-layer assembly with different flow directions for sequential steps. Transport studies show that the crossed nanowire arrays form electrically conducting networks, with individually addressable device function at each cross point.     

Nanowire array composites

Long, nanometer-size metallic wires can be synthesized by injection of the conducting melt into nanochannel insulating plates. Large-area arrays of parallel wires 200 nanometers in diameter and 50 micrometers long with a packing density of 5 x 10(8) per square centimeter have been fabricated in this way. When charged, the ends of the wires generate strong, short-range electric fields. The nanowire electric fields have been imaged at high spatial resolution with a scanning force microscope.     

Fe-Co-P非晶态与晶态纳米线的磁性研究

[目的]研究双过渡金属-类金属的三元合金纳米线各成分的改变对磁学性质的影响。[方法]在氧化铝模板中用电化学沉积法制备2种体系的Fe-Co-P三元合金纳米线。[结果]用扫描电子显微镜和透射电子显微镜观测氧化铝模板和纳米线的形貌。选区电子衍射和X射线衍射结果表明,(Fe1-xCox)0.88P0.12纳米线为非晶结构,而(Fe1-xCox)0.92P0.08纳米线近似为晶态结构。用振动样品磁强计和穆斯堡尔谱仪研究了非晶态体系和晶态体系在室温下的宏观和微观磁性。随Co含量x的变化,每个体系磁参量的变化趋势基本相似,而当x相同时两体系的磁学性质又有所不同。[结论]纳米线中非晶体系的形状各向异性比晶态更明显,更适宜用于垂直磁记录中。     

不同成分和结构的CoCu纳米线阵列的沉积规律

利用直流电化学沉积法通过调节沉积参数在多孔阳极氧化铝模板中沉积制备出了一系列不同成分和结构的磁性CoCu纳米线阵列.XRD和TEM的结果显示,沉积溶液的pH值对纳米线的成分比例和相结构的影响表现出了明显的规律性.笔者认为pH值对纳米线成分比例和相结构的控制主要源于它对溶液中Co^2＋沉积速率的影响.基于此沉积规律,沉积出了立方结构与六角结构共存的复相结构的CoCu纳米线.由于复相结构的CoCu纳米线中包含较多的相边界,因此它们具有远好于单相结构CoCu纳米线的磁性,其矫顽力和剩磁比均是单相结构纳米线的3倍.     

Direct-current nanogenerator driven by ultrasonic waves

We have developed a nanowire nanogenerator that is driven by an ultrasonic wave to produce continuous direct-current output. The nanogenerator was fabricated with vertically aligned zinc oxide nanowire arrays that were placed beneath a zigzag metal electrode with a small gap. The wave drives the electrode up and down to bend and/or vibrate the nanowires. A piezoelectric-semiconducting coupling process converts mechanical energy into electricity. The zigzag electrode acts as an array of parallel integrated metal tips that simultaneously and continuously create, collect, and output electricity from all of the nanowires. The approach presents an adaptable, mobile, and cost-effective technology for harvesting energy from the environment, and it offers a potential solution for powering nanodevices and nanosystems.     

Highly polarized photoluminescence and photodetection from single indium phosphide nanowires

We have characterized the fundamental photoluminescence (PL) properties of individual, isolated indium phosphide (InP) nanowires to define their potential for optoelectronics. Polarization-sensitive measurements reveal a striking anisotropy in the PL intensity recorded parallel and perpendicular to the long axis of a nanowire. The order-of-magnitude polarization anisotropy was quantitatively explained in terms of the large dielectric contrast between these free-standing nanowires and surrounding environment, as opposed to quantum confinement effects. This intrinsic anisotropy was used to create polarization-sensitive nanoscale photodetectors that may prove useful in integrated photonic circuits, optical switches and interconnects, near-field imaging, and high-resolution detectors.     

Nanowire crossbar arrays as address decoders for integrated nanosystems

The development of strategies for addressing arrays of nanoscale devices is central to the implementation of integrated nanosystems such as biological sensor arrays and nanocomputers. We report a general approach for addressing based on molecular-level modification of crossed semiconductor nanowire field-effect transistor (cNW-FET) arrays, where selective chemical modification of cross points in the arrays enables NW inputs to turn specific FET array elements on and off. The chemically modified cNW-FET arrays function as decoder circuits, exhibit gain, and allow multiplexing and demultiplexing of information. These results provide a step toward the realization of addressable integrated nanosystems in which signals are restored at the nanoscale.     

On-wire lithography

We report a high-throughput procedure for lithographically processing one-dimensional nanowires. This procedure, termed on-wire lithography, combines advances in template-directed synthesis of nanowires with electrochemical deposition and wet-chemical etching and allows routine fabrication of face-to-face disk arrays and gap structures in the range of five to several hundred nanometers. We studied the transport properties of 13-nanometer gaps with and without nanoscopic amounts of conducting polymers deposited within by dip-pen nanolithography.     

A Silicon Single-Electron Transistor Memory Operating at Room Temperature

A single-electron memory, in which a bit of information is stored by one electron, is demonstrated at room temperature. The memory is a floating gate metal-oxide-semiconductor transistor in silicon with a channel width ( approximately 10 nanometers) smaller than the Debye screening length of a single electron and a nanoscale polysilicon dot ( approximately 7 nanometers by 7 nanometers) as the floating gate embedded between the channel and the control gate. Storing one electron on the floating gate screens the entire channel from the potential on the control gate and leads to (i) a discrete shift in the threshold voltage, (ii) a staircase relation between the charging voltage and the shift, and (iii) a self-limiting charging process. The structure and fabrication of the memory should be compatible with future ultralarge-scale integrated circuits.     

Encoding electronic properties by synthesis of axial modulation-doped silicon nanowires

We describe the successful synthesis of modulation-doped silicon nanowires by achieving pure axial elongation without radial overcoating during the growth process. Scanning gate microscopy shows that the key properties of the modulated structures-including the number, size, and period of the differentially doped regions-are defined in a controllable manner during synthesis, and moreover, that feature sizes to less than 50 nanometers are possible. Electronic devices fabricated with designed modulation-doped nanowire structures demonstrate their potential for lithography-independent address decoders and tunable, coupled quantum dots in which changes in electronic properties are encoded by synthesis rather than created by conventional lithography-based techniques.     

Small-diameter silicon nanowire surfaces

Small-diameter (1 to 7 nanometers) silicon nanowires (SiNWs) were prepared, and their surfaces were removed of oxide and terminated with hydrogen by a hydrofluoric acid dip. Scanning tunneling microscopy (STM) of these SiNWs, performed both in air and in ultrahigh vacuum, revealed atomically resolved images that can be interpreted as hydrogen-terminated Si (111)-(1 x 1) and Si (001)-(1 x 1) surfaces corresponding to SiH3 on Si (111) and SiH2 on Si (001), respectively. These hydrogen-terminated SiNW surfaces seem to be more oxidation-resistant than regular silicon wafer surfaces, because atomically resolved STM images of SiNWs were obtained in air after several days' exposure to the ambient environment. Scanning tunneling spectroscopy measurements were performed on the oxide-removed SiNWs and were used to evaluate the electronic energy gaps. The energy gaps were found to increase with decreasing SiNW diameter from 1.1 electron volts for 7 nanometers to 3.5 electron volts for 1.3 nanometers, in agreement with previous theoretical predictions.     

Ultrahigh-density nanowire arrays grown in self-assembled diblock copolymer templates

We show a simple, robust, chemical route to the fabrication of ultrahigh-density arrays of nanopores with high aspect ratios using the equilibrium self-assembled morphology of asymmetric diblock copolymers. The dimensions and lateral density of the array are determined by segmental interactions and the copolymer molecular weight. Through direct current electrodeposition, we fabricated vertical arrays of nanowires with densities in excess of 1.9 x 10(11) wires per square centimeter. We found markedly enhanced coercivities with ferromagnetic cobalt nanowires that point toward a route to ultrahigh-density storage media. The copolymer approach described is practical, parallel, compatible with current lithographic processes, and amenable to multilayered device fabrication.     

A laser ablation method for the synthesis of crystalline semiconductor nanowires

A method combining laser ablation cluster formation and vapor-liquid-solid (VLS) growth was developed for the synthesis of semiconductor nanowires. In this process, laser ablation was used to prepare nanometer-diameter catalyst clusters that define the size of wires produced by VLS growth. This approach was used to prepare bulk quantities of uniform single-crystal silicon and germanium nanowires with diameters of 6 to 20 and 3 to 9 nanometers, respectively, and lengths ranging from 1 to 30 micrometers. Studies carried out with different conditions and catalyst materials confirmed the central details of the growth mechanism and suggest that well-established phase diagrams can be used to predict rationally catalyst materials and growth conditions for the preparation of nanowires.     

Dislocation-driven nanowire growth and Eshelby twist

Hierarchical nanostructures of lead sulfide nanowires resembling pine trees were synthesized by chemical vapor deposition. Structural characterization revealed a screwlike dislocation in the nanowire trunks with helically rotating epitaxial branch nanowires. It is suggested that the screw component of an axial dislocation provides the self-perpetuating steps to enable one-dimensional crystal growth, in contrast to mechanisms that require metal catalysts. The rotating trunks and branches are the consequence of the Eshelby twist of screw dislocations with a dislocation Burgers vector along the 110 directions having an estimated magnitude of 6 +/- 2 angstroms for the screw component. The results confirm the Eshelby theory of dislocations, and the proposed nanowire growth mechanism could be general to many materials.     

Spontaneous organization of single CdTe nanoparticles into luminescent nanowires

Nanoparticles of CdTe were found to spontaneously reorganize into crystalline nanowires upon controlled removal of the protective shell of organic stabilizer. The intermediate step in the nanowire formation was found to be pearl-necklace aggregates. Strong dipole-dipole interaction is believed to be the driving force of nanoparticle self-organization. The linear aggregates subsequently recrystallized into nanowires whose diameter was determined by the diameter of the nanoparticles. The produced nanowires have high aspect ratio, uniformity, and optical activity. These findings demonstrate the collective behavior of nanoparticles as well as a convenient, simple technique for production of one-dimensional semiconductor colloids suitable for subsequent processing into quantum-confined superstructures, materials, and devices.     

Germanium nanowire growth below the eutectic temperature

Nanowires are conventionally assumed to grow via the vapor-liquid-solid process, in which material from the vapor is incorporated into the growing nanowire via a liquid catalyst, commonly a low-melting point eutectic alloy. However, nanowires have been observed to grow below the eutectic temperature, and the state of the catalyst remains controversial. Using in situ microscopy, we showed that, for the classic Ge/Au system, nanowire growth can occur below the eutectic temperature with either liquid or solid catalysts at the same temperature. We found, unexpectedly, that the catalyst state depends on the growth pressure and thermal history. We suggest that these phenomena may be due to kinetic enrichment of the eutectic alloy composition and expect these results to be relevant for other nanowire systems.     

Magnetic domain-wall racetrack memory

Recent developments in the controlled movement of domain walls in magnetic nanowires by short pulses of spin-polarized current give promise of a nonvolatile memory device with the high performance and reliability of conventional solid-state memory but at the low cost of conventional magnetic disk drive storage. The racetrack memory described in this review comprises an array of magnetic nanowires arranged horizontally or vertically on a silicon chip. Individual spintronic reading and writing nanodevices are used to modify or read a train of approximately 10 to 100 domain walls, which store a series of data bits in each nanowire. This racetrack memory is an example of the move toward innately three-dimensional microelectronic devices.     

Ordering of ruthenium cluster carbonyls in mesoporous silica

The anionic ruthenium cluster carbonylates [Ru6C(CO)16]2- or [H2Ru10(CO)25]2- interspersed with bis(triphenylphosphino)iminium counterions (PPN+) are incorporated from solution into the pores of MCM-41 mesoporous silica (3 nanometers in diameter), where they form tightly packed arrays. These arrays were shown by high-resolution transmission electron microscopy, Fourier transform optical diffraction, and computer simulations to be well ordered both along and perpendicular to the axis of the cylindrical pores. In their denuded state produced by gentle thermolysis, the cluster carbonylates yield nanoparticles of ruthenium that are less well ordered than their assimilated precursors but show good activity as hydrogenation catalysts for hexene and cyclooctene. In both their as-prepared and denuded states, these encapsulated clusters are likely to exhibit interesting electronic and other properties.