Showing papers by "Peidong Yang published in 2008"

Enhanced thermoelectric performance of rough silicon nanowires

Abstract: Approximately 90 per cent of the world's power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30-40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT > 1, since the parameters of ZT are generally interdependent. While nanostructured thermoelectric materials can increase ZT > 1 (refs 2-4), the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20-300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT = 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.

Enhanced Thermoelectric Performance in Rough Silicon Nanowires

Abstract: Approximately 90 per cent of the world’s power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30–40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT > 1, since the parameters of ZT are generally interdependent. While nanostructured thermoelectric materials can increase ZT > 1 (refs 2–4), the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20–300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT = 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.

Shape Control of Colloidal Metal Nanocrystals

Abstract: Colloidal metal nanoparticles are emerging as key materials for catalysis, plasmonics, sensing, and spectroscopy. Within these applications, control of nanoparticle shape lends increasing functionality and selectivity. Shape-controlled nanocrystals possess well-defined surfaces and morphologies because their nucleation and growth are controlled at the atomic level. An overall picture of shaped metal particles is presented, with a particular focus on solution-based syntheses for the noble metals. General strategies for synthetic control are discussed, emphasizing key factors that result in anisotropic, nonspherical growth such as crystallographically selective adsorbates and seeding processes.

Silicon nanowire radial p-n junction solar cells.

Abstract: We have demonstrated a low-temperature wafer-scale etching and thin film deposition method for fabricating silicon n−p core−shell nanowire solar cells. Our devices showed efficiencies up to nearly 0.5%, limited primarily by interfacial recombination and high series resistance. Surface passivation and contact optimization will be critical to improve device performance in the future.

Sub-two nanometer single crystal Au nanowires

Abstract: Ultrathin single crystal Au nanowires with diameter of approximately 1.6 nm and length of few micrometers were synthesized with high yield by simply mixing HAuCl 4 and oleylamine at room temperature. High resolution transmission electron microscopy studies revealed that all of these nanowires are single crystalline and grew along the [111] direction. The valency evolution of the gold species during the synthesis was studied by X-ray photoelectron spectroscopy, which showed a clear Au (3+) --> Au (+) --> Au stepwise reduction at different reaction stages. Small angle X-ray scattering and small-angle X-ray diffraction suggest mesostructure formation upon HAuCl 4 and oleylamine mixing. The slow in situ reduction of this mesostructure leads to the formation of ultrathin nanowires in solution. This novel nanowire growth mechanism relies on cooperative interaction, organization, and reaction between inorganic precursor salts and oleylamine.

Tunable plasmonic lattices of silver nanocrystals

Abstract: Silver nanocrystals are ideal building blocks for plasmonicmaterials that exhibit a wide range of unique and potentially usefuloptical phenomena. Individual nanocrystals display distinct opticalscattering spectra and can be assembled into hierarchical structures thatcouple strongly to external electromagnetic fields. This coupling, whichis mediated by surface plasmons, depends on their shape and arrangement.Here we demonstrate the bottom-up assembly of polyhedral silvernanocrystals into macroscopic two-dimensional superlattices using theLangmuir-Blodgett technique. Our ability to control interparticlespacing, density, and packing symmetry allows for tunability of theoptical response over the entire visible range. This assembly strategyoffers a new, practical approach to making novel plasmonic materials forapplication in spectroscopic sensors, sub-wavelength optics, andintegrated devices that utilize field enhancement effects.

Enhanced Thermoelectric Performance of Rough Silicon Nanowires.

Abstract: Approximately 90 per cent of the world's power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30-40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT > 1, since the parameters of ZT are generally interdependent. While nanostructured thermoelectric materials can increase ZT > 1 (refs 2-4), the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20-300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT = 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.

Langmuir-Blodgettry of nanocrystals and nanowires.

Abstract: Although nanocrystals and nanowires have proliferated new scientific avenues in the study of their physics and chemistries, the bottom-up assembly of these small-scale building blocks remains a formidable challenge for device fabrication and processing. An attractive nanoscale assembly strategy should be cheap, fast, defect tolerant, compatible with a variety of materials, and parallel in nature, ideally utilizing the self-assembly to generate the core of a device, such as a memory chip or optical display. Langmuir−Blodgett (LB) assembly is a good candidate for arranging vast numbers of nanostructures on solid surfaces. In the LB technique, uniaxial compression of a nanocrystal or nanowire monolayer floating on an aqueous subphase causes the nanostructures to assemble and pack over a large area. The ordered monolayer can then be transferred to a solid surface en masse and with fidelity. In this Account, we present the Langmuir−Blodgett technique as a low-cost method for the massively parallel, controlled ...

Localized Pd Overgrowth on Cubic Pt Nanocrystals for Enhanced Electrocatalytic Oxidation of Formic Acid

Abstract: Binary Pt/Pd nanoparticles were synthesized by localized overgrowth of Pd on cubic Pt seeds for the investigation of electrocatalytic formic acid oxidation. The binary particles exhibited much less self-poisoning and a lower activation energy relative to Pt nanocubes, consistent with the single crystal study.

Thermal conductance of thin silicon nanowires.

Abstract: The thermal conductance of individual single crystalline silicon nanowires with diameters less than 30 nm has been measured from 20 to 100 K. The observed thermal conductance shows unusual linear temperature dependence at low temperatures, as opposed to the ${T}^{3}$ dependence predicted by the conventional phonon transport model. In contrast to previous models, the present study suggests that phonon-boundary scattering is highly frequency dependent, and ranges from nearly ballistic to completely diffusive, which can explain the unexpected linear temperature dependence.

Selective growth of metal and binary metal tips on CdS nanorods.

Abstract: Here, we demonstrate an approach for the selective growth of Pt, PtNi, and PtCo on CdS nanorods. The hybrid nanostructures prepared via an organometallic synthesis have promise for photocatalytic and magnetic applications.

Surface-enhanced Raman spectroscopy for trace arsenic detection in contaminated water.

Abstract: Low-level arsenic contamination of drinking water in Bangladesh, India, and parts of China presents an international public health crisis, with over 300000 deaths attributed to chronic poisoning in Bangladesh alone. In 1993, the World Health Organization set a provisional guideline of 10 ppb (0.01 mgL ) for maximum arsenic content in groundwater. However, exposure to arsenic at these concentrations still results in increased rates of skin, lung, urinary bladder, and kidney cancer. New technologies allowing reliable detection of arsenic below 10 ppb should instigate a stricter standard. Current technologies for laboratory analysis (e.g. inductively coupled plasma (ICP) MS, atomic fluorescence spectroscopy (AFS), HPLC-MS) allow detection at these levels, but they are neither readily available in developing countries nor capable of on-site field detection. The current state of field-compatible technologies has been reviewed, and there remains significant room for improvement. Even if current chemical field tests are improved to meet these standards, there are no examples of chemical indicators that can distinguish the oxidation state of the arsenic species. For exposure studies, this knowledge is necessary for toxicology, remediation, and monitoring of the effects within the local populations. By developing a highly active substrate for surface-enhanced Raman spectroscopy (SERS) that can be used in conjunction with portable Raman technology, many of these challenges can be surmounted. Since the discovery of SERS in the late 1970s there has been a continual push to maximize the Raman signal for molecules near nanostructured surfaces. SERS enhancement results from an intense local amplification of the electric field near a metal surface when collective oscillations of conduction electrons resonate in phase with the incident light. The size, shape, and proximity of nanostructures all affect the frequency and magnitude of the localized surface plasmons (LSPs), thus directly influencing the degree of Raman enhancement exhibited. LSPs have been directly observed using experimental techniques such as scanning near field and TEM-correlated dark field microscopy. These experiments, along with more conventional light-scattering techniques, demonstrate the dramatic effects that size and shape have on the LSPs. Recent studies on electromagnetic coupling between nanostructures that are nearly touching indicate that such collective effects can excite LSPs that lead to even higher electromagnetic enhancement. Although it is widely known that silver shows the strongest plasmonic response, gold is often used for sensing applications because of its chemical stability and compatibility with many laser excitation wavelengths. For our SERS sensor, we have introduced two key features that lead to better analytical capability under typical sensing conditions. First, dense arrays of silver nanocrystals are fabricated using Langmuir–Blodgett (LB) assembly. These close-packed monolayers exhibit broadband scattering profiles, making them compatible with many different excitation wavelengths. The second key feature is the surface passivation of the silver particles with adsorbed polymer. Surface-adsorbed poly(vinyl pyrrolidone) (PVP) serves dual purposes: it functions as the passivating ligand during nanocrystal synthesis, and it stabilizes the silver particles to oxidation while still facilitating interaction between silver and arsenate during sensing experiments. The PVP coating makes these silver nanostructures airand water-stable over much longer periods then other passivating ligands. The synthesis of the polyhedral silver nanoparticles proceeds by the polyol process, in which the metal-salt precursors and a polymer capping agent (PVP) are alternately added to a solution of pentanediol heated near reflux. In this way the pentanediol acts as both the solvent and the reductant for the reaction, while PVP imparts shape control as the particles grow. The final shape of the particles is dictated by the length of the reaction; the particles are progressively capped by more [111] faces (see Figure 1). This growth results in an increase of the particle size as the reaction progresses, starting with cubes which are 80–100 nm on an edge, then cuboctahedra with diameters of 150–200 nm, and finally octahedra with edge lengths of 250–300 nm. Typically these nanocrystals can be isolated as nearly monodisperse suspensions, and final purification is achieved by filtration through 0.45-mm Durapore filters. Homogeneity of [*] M. Mulvihill, K. Benjauthrit, Prof. J. Arnold, Prof. P. Yang Department of Chemistry University of California, Berkeley Berkeley, CA 94720 (USA) Fax: (+1)510-642-7301 E-mail: p_yang@berkeley.edu

Dendrimer Templated Synthesis of One Nanometer Rh and Pt Particles Supported on Mesoporous Silica: Catalytic Activity for Ethylene and Pyrrole Hydrogenation

Abstract: Monodisperse rhodium (Rh) and platinum (Pt) nanoparticles as small as ∼1 nm were synthesized within a fourth generation polyaminoamide (PAMAM) dendrimer, a hyperbranched polymer, in aqueous solution and immobilized by depositing onto a high-surface-area SBA-15 mesoporous support. X-ray photoelectron spectroscopy indicated that the as-synthesized Rh and Pt nanoparticles were mostly oxidized. Catalytic activity of the SBA-15 supported Rh and Pt nanoparticles was studied with ethylene hydrogenation at 273 and 293 K in 10 torr of ethylene and 100 torr of H2 after reduction (76 torr of H2 mixed with 690 torr of He) at different temperatures. Catalysts were active without removing the dendrimer capping but reached their highest activity after hydrogen reduction at a moderate temperature (423 K). When treated at a higher temperature (473, 573, and 673 K) in hydrogen, catalytic activity decreased. By using the same treatment that led to maximum ethylene hydrogenation activity, catalytic activity was also evaluate...

Dynamic manipulation and separation of individual semiconducting and metallic nanowires

Abstract: The synthesis of nanowires has advanced in the past decade to the point where a vast range of insulating, semiconducting and metallic materials1 are available for use in integrated, heterogeneous optoelectronic devices at nanometre scales2. However, a persistent challenge has been the development of a general strategy for the manipulation of individual nanowires with arbitrary composition. Here we report that individual semiconducting and metallic nanowires with diameters below 20 nm are addressable with forces generated by optoelectronic tweezers3. Using 100,000 times less optical power density than optical tweezers, optoelectronic tweezers are capable of transporting individual nanowires with speeds four times greater than the maximum speeds achieved by optical tweezers. A real-time array of silver nanowires is formed using photopatterned virtual electrodes, demonstrating the potential for massively parallel assemblies. Furthermore, optoelectronic tweezers enable the separation of semiconducting and metallic nanowires, suggesting a broad range of applications for the separation and heterogeneous integration of one-dimensional nanoscale materials.

Self-transducing silicon nanowire electromechanical systems at room temperature.

Abstract: Electronic readout of the motions of genuinely nanoscale mechanical devices at room temperature imposes an important challenge for the integration and application of nanoelectromechanical systems (NEMS). Here, we report the first experiments on piezoresistively transduced very high frequency Si nanowire (SiNW) resonators with on-chip electronic actuation at room temperature. We have demonstrated that, for very thin (∼90 nm down to ∼30 nm) SiNWs, their time-varying strain can be exploited for self-transducing the devices’ resonant motions at frequencies as high as ∼100 MHz. The strain of wire elongation, which is only second-order in doubly clamped structures, enables efficient displacement transducer because of the enhanced piezoresistance effect in these SiNWs. This intrinsically integrated transducer is uniquely suited for a class of very thin wires and beams where metallization and multilayer complex patterning on devices become impractical. The 30 nm thin SiNW NEMS offer exceptional mass sensitivities in the subzeptogram range. This demonstration makes it promising to advance toward NEMS sensors based on ultrathin and even molecular-scale SiNWs, and their monolithic integration with microelectronics on the same chip.

Gated proton transport in aligned mesoporous silica films

Abstract: Modulated proton transport plays significant roles in biological processes such as ATP synthesis as well as in technologically important applications including, for example, hydrogen fuel cells. The state-of-the-art proton-exchange membrane is the sulphonated tetrafluoroethylene copolymer Nafion developed by DuPont in the late 1960s, with a high proton conductivity. However, actively switchable proton conduction, a functional mimic of the ion transport within a cell membrane, has yet to be realized. Herein, we report the electrostatic gating of proton transport within aligned mesoporous silica thin film. It is observed that surface-charge-mediated transport is dominant at low proton concentrations. We have further demonstrated that the proton conduction can be actively modulated by two-fourfold with a gate voltage as low as 1 V. Such artificial gatable ion transport media could have potential applications in nanofluidic chemical processors, biomolecular separation and electrochemical energy conversion.

Highly Selective Synthesis of Catalytically Active Monodisperse Rhodium Nanocubes

Abstract: Monodisperse sub-10 nm Rh nanocubes were synthesized with high selectivity (>85%) by a seedless polyol method. The {100} faces of the Rh NCs were effectively stabilized by chemically adsorbed Br− ions from trimethyl(tetradecyl)ammonium bromide (TTAB). This simple one-step polyol route can be readily applied to the preparation of Pt and Pd nanocubes. Moreover, the organic molecules of PVP and TTAB that encapsulated the Rh nanocubes did not prevent catalytic activity for pyrrole hydrogenation and CO oxidation.

Vertical nanowire array-based light emitting diodes

Abstract: Electroluminescence from a nanowire array-based light emitting diode is reported. The junction consists of a p-type GaN thin film grown by metal organic chemical vapor deposition (MOCVD) and a vertical n-type ZnO nanowire array grown epitaxially from the thin film through a simple low temperature solution method. The fabricated devices exhibit diode like current voltage behavior. Electroluminescence is visible to the human eye at a forward bias of 10 V and spectroscopy reveals that emission is dominated by acceptor to band transitions in the p-GaN thin film. It is suggested that the vertical nanowire architecture of the device leads to waveguided emission from the thin film through the nanowire array.

Self-organized silver nanoparticles for three-dimensional plasmonic crystals.

Abstract: Metal nanostructures that support surface plasmons are compelling as plasmonic circuit elements and as the building blocks for metamaterials. We demonstrate here the spontaneous self-assembly of shaped silver nanoparticles into three-dimensional plasmonic crystals that display a frequency-selective response in the visible wavelengths. Extensive long-range order mediated by exceptional colloid monodispersity gives rise to optical passbands that can be tuned by particle volume fraction. These metallic supercrystals present a new paradigm for the fabrication of plasmonic materials, delivering a functional, tunable, completely bottom-up optical element that can be constructed on a massively parallel scale without lithography.

Silver ion mediated shape control of platinum nanoparticles: Removal of silver by selective etching leads to increased catalytic activity

Abstract: A procedure has been developed for the selective etching of silver from platinum nanoparticles of well-defined shape, resulting in the formation of nearly elementally pure Pt cubes, cuboctahedra, or octahedra, with a largest vertex-to-vertex distance of ∼9.5 nm from Ag-modified Pt nanoparticles. The characterization of mesoporous silica-supported Pt nanoparticles by XRD, TEM, and N2 adsorption measurements demonstrated that the structure of the nanoparticles and the mesoporous support was conserved after etching in concentrated nitric acid. Both elemental analysis and ethylene hydrogenation indicated that etching of Ag is only effective when [HNO3] ≥ 7 M; below this concentration, the removal of Ag is limited to ∼10%. The activity for ethylene hydrogenation increased by four orders of magnitude after etching Pt octahedra containing the highest fraction of silver. High-resolution transmission electron microscopy of the unsupported particles after etching demonstrated that etching does not alter the surface...

Near-Monodisperse Ni-Cu Bimetallic Nanocrystals of Variable Composition: Controlled Synthesis and Catalytic Activity for H2 Generation

Abstract: Near-monodisperse Ni1−xCux (x = 0.2−0.8) bimetallic nanocrystals were synthesized by a one-pot thermolysis approach in oleylamine/1-octadecene, using metal acetylacetonates as precursors. The nanocrystals form large-area 2D superlattices, and display a catalytic synergistic effect in the hydrolysis of NaBH4 to generate H2 at x = 0.5 in a strongly basic medium. The Ni0.5Cu0.5 nanocrystals show the lowest activation energy, and also exhibit the highest H2 generation rate at 298 K.

Adsorption and co-adsorption of ethylene and carbon monoxide on silica-supported monodisperse Pt nanoparticles: volumetric adsorption and infrared spectroscopy studies.

Abstract: The adsorption of carbon monoxide and ethylene, and their sequential adsorption, was studied over a series of Pt/SBA-15 catalysts with monodisperse particle sizes ranging from 17 to 71 nm by diffuse-reflectance infrared spectroscopy and chemisorption Gas adsorption was dependent on the Pt particle size, temperature, and sequence of gas exposure Adsorption of CO at room temperature on Pt/SBA-15 gives rise to a spectroscopic feature assigned to the C−O stretch: ν(CO) = 2075 cm-1 (19 nm); 2079 cm-1 (29 nm); 2082 cm-1 (36 nm); and 2090 cm-1 (71 nm) The intensity of the signal decreased in a sigmoidal fashion with increasing temperature, thereby providing semiquantitative surface coverage information Adsorption of ethylene on Pt/SBA-15 gave rise to spectroscopic features at ∼1340, ∼1420, and ∼1500 cm-1 assigned to ethylidyne, di-σ-bonded ethylene, and π-bonded ethylene, respectively The ratio of these surface species is highly dependent on the Pt particle size At room temperature, Pt particles sta

Nanostructures having high performance thermoelectric properties

Abstract: The invention provides for a nanostructure, or an array of such nanostructures, each comprising a rough surface, and a doped or undoped semiconductor. The nanostructure is an one-dimensional (1-D) nanostructure, such a nanowire, or a two-dimensional (2-D) nanostructure. The nanostructure can be placed between two electrodes and used for thermoelectric power generation or thermoelectric cooling.

Synthesis of Lead Chalcogenide Alloy and Core–Shell Nanowires

Abstract: Control over the dimensions and shape of nanostructures represents one of the main challenges in modern materials science. Morphology control of a variety of materials can be achieved using vapor–liquid–solid or solution–liquid–solid techniques to obtain one-dimensional (1D) systems. The unique optical and electrical properties of 1D nanostructures make them one of most important building blocks for nanoscience and nanotechnology applications, and provide the opportunity for their integration in electronic, photonic, thermoelectric, and sensor-based devices. Size control has been traditionally important and necessary to tune the optical and electrical properties of nanomaterials by changing the band gap. This is particularly important in the strong confinement region, where one of the dimensions is smaller than the corresponding excitonic Bohr diameter. Semiconductor alloy and core–shell nanowire systems represent another interesting direction towards functional nanostructures with enhanced structural and property tunability. Herein, we focus on preparing novel 1D heterostructures of IV–VI semiconductor nanomaterials. Lead chalcogenides are known to be good materials for thermoelectrics due to their low thermoconductivity. Pseudobinary (e.g. PbSeTe) and pseudoternary alloys (e.g. PbSnSeTe) have even lower lattice thermal conductivities than the binary compounds due to disorder-induced phonon scattering processes. Lead chalcogenide materials are also good candidates for multiexciton-generation (MEG) solar cells. For example, previous reports showed quantum efficiencies as high as 300% and 700% for PbSe nanoparticles. Heterostructured alloy and core–shell nanomaterials have previously been shown for various materials, mainly II–VI semiconductor nanocrystals. For example, a quasi 1D system of CdSe–ZnS has been reported, other systems include PbSe–PbS core–shell and alloy spherical nanoparticles developed by Lifshitz and co-workers. In addition, Talapin et al. have demonstrated the growth of PbS and Au onto PbSe nanowires. The physical properties of these heterostructured nanosystems are of interest for various applications as shown by the electronic structure calculations carried out by different groups. Here we demonstrate the formation of lead chalcogenide heterostructure nanowires by a solution-phase synthesis at moderate temperatures (see the Experimental Section). Two types of heterostructures (alloy and core–shell) were prepared by changing the concentration and temperature of the reaction. We were able to control the composition of the alloy and the thickness of the shell by changing the growth parameters. Three different systems, PbSexS1 x alloys, and PbSe–PbS and PbSe–PbTe core–shell nanowires were prepared. Achieving these three targeted structures is nontrivial due to various competitive processes such as ripening and formation of pure PbS (PbTe) nanoparticles. The synthesis of PbSe nanowires is based on a previous report by Murray and co-workers. The same procedure was used to prepare the PbSe nanowires used here as templates for further growth to give the alloy and core–shell nanostructures. The diameter of the core nanowires could be controlled and varied from 4 nm up to 100 nm, with a length of a few tens of micrometers. The PbSe nanowires (Figure 1A) were used as templates to form PbSexS1 x alloy wires. Figure 1B shows PbSe0.4S0.6 alloy nanowires that were prepared by the slow addition of Pb and S precursors to a hot solution containing PbSe nanowires. (a detailed description of the synthesis can be found in the Experimental Section). The diameter of the alloy nanowires increased from 6 nm (pure PbSe nanowires) to ca. 10 nm, indicating the incorporation of additional material into the nanowires. Structural characterization of the alloy system was carried out using various methods as shown in Figure 1. Figure 1D shows a high-resolution transmission electron microscopy (HRTEM) image of the PbSe0.4S0.6 nanowires. The latticeresolved image indicates that the nanowires are growing along the h100i direction. X-ray diffraction (XRD) measurements of the alloy nanowires are shown in Figure 1C. The pattern can be indexed to a structure intermediate between the cubic PbSe and cubic PbS bulk phases, which strongly supports the formation of an alloyed structure. An energydispersive X-ray (EDX) spectrum (Figure 1E) taken on a small area of the alloy nanowire, shown in Figure 1D, indicates the presence of Se from the original PbSe nanowires, Pb from the original and added materials, and Cu from the TEM grid. However, due to overlap between the Pb and S peaks, electron energy loss spectroscopy (EELS) was necessary to detect the incorporation of S. The energy loss peak for S was observed at 165 eV (Figure 1F), providing clear evidence for the existence of S in the alloy nanowires. The EDX and EELS spectra were taken from the same area of the nanowire shown in Figure 1D. Tuning the alloy composition can be achieved by simply controlling the reaction conditions. For example, altering the S concentration will act to tune the alloy composition. The actual composition was determined by [*] Dr. T. Mokari, S. E. Habas, Prof. P. Yang Department of Chemistry, University of California Berkeley, CA 94720 (USA) Fax: (+1)510-642-7301 E-mail: p_yang@berkeley.edu

Materials science: Lilliputian light sticks

Abstract: Building two different fluorescing dyes into a composite organic nanocrystal makes a tunable light generator. At just the right dye proportions, a low-cost, highly efficient source of white light is the result.

Monolithic Multinozzle Emitters for Nanoelectrospray Mass Spectrometry

Abstract: Novel and significantly simplified procedures for fabrication of fully integrated nanoelectrospray emitters have been described. For nanofabricated monolithic multinozzle emitters (NM2 emitters), a bottom up approach using silicon nanowires on a silicon sliver is used. For microfabricated monolithic multinozzle emitters (M3 emitters), a top down approach using MEMS techniques on silicon wafers is used. The emitters have performance comparable to that of commercially-available silica capillary emitters for nanoelectrospray mass spectrometry.

Process for altering thermoelectric properties of a material

Abstract: A process for altering the thermoelectric properties of an electrically conductive material is provided. The process includes providing an electrically conducting material and a substrate. The electrically conducting material is brought into contact with the substrate. A thermal gradient can be applied to the electrically conducting material and a voltage applied to the substrate. In this manner, the electrical conductivity, the thermoelectric power and/or the thermal conductivity of the electrically conductive material can be altered and the figure of merit increased.

Inorganic nanotubes and electro-fluidic devices fabricated therefrom

Abstract: Nanofluidic devices incorporating inorganic nanotubes fluidly coupled to channels or nanopores for supplying a fluid containing chemical or bio-chemical species are described. In one aspect, two channels are fluidly interconnected with a nanotube. Electrodes on opposing sides of the nanotube establish electrical contact with the fluid therein. A bias current is passed between the electrodes through the fluid, and current changes are detected to ascertain the passage of select molecules, such as DNA, through the nanotube. In another aspect, a gate electrode is located proximal the nanotube between the two electrodes thus forming a nanofluidic transistor. The voltage applied to the gate controls the passage of ionic species through the nanotube selected as either or both ionic polarities. In either of these aspects the nanotube can be modified, or functionalized, to control the selectivity of detection or passage.