Carbon nanofibers (CNFs) were incorporated into nylon 11 to form nylon 11-carbon nanofiber nanocomposites via twin screw extrusion. Injection molding has been employed to fabricate specimens that possess enhanced mechanical strength and fire retardancy. The thermal conductivity of these polymer nanocomposites was measured using a guarded hot plate method. The measurement results show that the room temperature thermal conductivity increases with the CNF loading from 0.24±0.01 W/m K for pure Nylon 11 to 0.30±0.02 W/m K at 7.5 wt % CNF loading. The effective medium theory has been used to determine the interface thermal resistance between the CNFs and the matrix to be in the range of 2.5–5.0×10−6 m2 K/W from the measured thermal conductivity of the nanocomposite.
The thermal conductivity of individual bismuth nanowires was characterized using a suspended microdevice and correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. Compared to bulk bismuth in the same crystal direction perpendicular to the trigonal axis, the thermal conductivity of a single-crystal bismuth nanowire of 232 nm diameter was found to be three to six times smaller than bulk in the temperature range between 100 and 300 K, and those of polycrystalline bismuth nanowires of 74–255 nm diameter are reduced by factors of 18–78 over the same temperature range. The thermal conductivity suppression in the single-crystal nanowire can be explained by a transport model that considers diffuse phonon-surface scattering, partially diffuse surface scattering of electrons and holes, and scattering of phonons and charge carriers by ionized impurities such as oxygen and carbon of a concentration on the order of 1019 cm−3. The comparable thermal conductivity values measured for polycrystalline nanowires of different diameters suggests a grain boundary scattering mean free path for all heat carriers in the range of 15–40 nm, which is smaller than the nanowire diameters.
The temperature distributions in current-carrying carbon nanotubes have been measured with a scanning thermal microscope. The obtained temperature profiles reveal diffusive and dissipative electron transport in multiwalled nanotubes and in single-walled nanotubes when the voltage bias was higher than the 0.1–0.2 eV optical phonon energy. Over 90% of the Joule heat in a multiwalled nanotube was found to be conducted along the nanotube to the two metal contacts. In comparison, about 80% of the Joule heat was transferred directly across the nanotube-substrate interface for single-walled nanotubes. The average temperature rise in the nanotubes is determined to be in the range of 5–42 K per microwatt Joule heat dissipation in the nanotubes.
The thermoelectric properties and crystal structure of individual electrodepositedbismuth telluride nanowires (NWs) were characterized using a microfabricated measurement device and transmission electron microscopy. Annealing in hydrogen was used to obtain electrical contact between the NW and the supporting Pt electrodes. By fitting the measured Seebeck coefficient with a two-band model, the NW samples were determined to be highly n-type doped. Higher thermal conductivity and electrical conductivity were observed in a 52 nm diameter monocrystalline NW than a 55 nm diameter polycrystalline NW. The electron mobility of the monocrystalline NW was found to be about 19% lower than that of bulk crystal at a similar carrier concentration and about 2.5 times higher than that of the polycrystalline NW. The specularity parameter for electron scattering by the NW surface was determined to be about 0.7 and partially specular and partially diffuse, leading to a reduction in the electron mean-free path from 61 nm in the bulk to about 40 nm in the 52 nm NW. Because of the already short phonon mean-free path of about 3 nm in bulk bismuth telluride, diffuse phonon-surface scattering is expected to reduce the lattice thermal conductivity of the 52–55 nm diameter NWs by only about 20%, which is smaller than the uncertainty in the extracted lattice thermal conductivity based on the measured total thermal conductivity and calculated electron thermal conductivity. Although the lattice thermal conductivity of the polycrystalline NW is likely lower than the bulk values, the lower thermal conductivity observed in this polycrystalline sample is mainly caused by the lower electron concentration and mobility. For both samples, the thermoelectric figure of merit (ZT) increases with temperature and is about 0.1 at a temperature of 400 K. The low ZT compared to that of bulk crystals is mainly caused by a high doping level, suggesting the need for better control of the chemical composition in order to improve the ZT of the electrodeposited NWs. Moreover, bismuth telluride NWs with diameter less than 10 nm would be required for substantial suppression of the lattice thermal conductivity as well as experimental verification of theoretical predictions of power factor enhancement in quantum wires. Such stringent diameter requirement can be relaxed in other NW systems with longer bulk phonon mean-free path or smaller effective mass and thus longer electron wavelength than those in bulk bismuth telluride.
Our ability to precisely manipulate size, shape and composition of nanoscale carriers is essential for controlling their in-vivo transport, bio-distribution and drug release mechanism. Shape-specific, “smart” nanoparticles that deliver drugs or imaging agents to target tissues primarily in response to disease-specific or physiological signals could significantly improve therapeutic care of complex diseases. Current methods in nanoparticle synthesis do not allow such simultaneous control over particle size, shape and environmentally-triggered drug release, especially at the sub 100 nm range. We report here a high-throughput nanofabrication technique using synthetic and biological macromers (peptides) to produce highly monodisperse, enzymatically-triggered nanoparticles of precise sizes and shapes. Particles as small as 50 nm were fabricated on silicon wafers and harvested directly into aqueous buffers using a biocompatible, one-step release technique. We further demonstrate successful encapsulation and precisely controlled enzyme-triggered release of antibodies and nucleic acids from these nanoparticles, thus providing a potential means for disease-controlled delivery of biomolecules.
Thermal transport in carbon nanotubes is explored using different laser powers to heat suspended single-walled carbon nanotubes∼5μm in length. The temperature change along the length of a nanotube is determined from the temperature-induced shifts in the G band Raman frequency. The spatial temperature profile reveals the ratio of the contact thermal resistance to the intrinsic thermal resistance of the nanotube. Moreover, the obtained temperature profiles allow differentiation between diffusive and ballisticphonontransport. Diffusive transport is observed in all nanotubesmeasured and the ratio of thermal contact resistance to intrinsic nanotube thermal resistance is found to range from 0.02 to 17.
The effect of surface roughness on phonon transport in a nanowire has often been described by treating the surface as flat with a specularity parameter (p) in the range between 0 and 1. A lower thermal conductivity limit is approached at p=0 for diffuse surface. It is demonstrated here by Monte Carlo simulation that sawtooth roughness on a nanowire can cause phononbackscattering and suppress the thermal conductivity below the diffuse surface limit. The backscattering effect can be accounted for only by a negative p if the detail of the surface roughness is ignored.
The objective of this U.S.–Japan joint seminar series is to provide a cross-disciplinary and international forum for discussing and identifying outstanding science and technology issues in the area of nanoscale thermophysics and energy conversion and to foster collaboration among researchers in these areas. The first of this seminar series, championed by the late Professors Chang-Lin Tien and Kunio Hijikata, was held in Kanazawa, Japan, in June of 1993. Subsequent meetings have been held every three years, alternating venues between the United States and Japan. The Sixth U.S.–Japan Joint Seminar on Nanoscale Transport Phenomena—Science and Engineering was held in Boston, Massachusetts, July 13–16, 2008, and was organized by Professors Gang Chen from MIT, Fushinobu Kazuyoshi from Tokyo Institute of Technology, Shigeo Maruyama from Tokyo University, and Pamela Norris from University of Virginia. Nearly 100 scientists participated in the seminar. (The agenda of the seminar is attached at the end at this report.) The seminar included keynote sessions and invited sessions, as well as a dedicated poster session of selected presentations from an open call for papers. All papers presented in the regular sessions, the invited sessions, were upon invitation by the organizers. Invited sessions used a mixed form of communication: each speaker gave a 5-minute summary of his work followed by a 30-minute poster session of just the papers summarized orally, and then these speakers came back to the podium, serving as panelists to answer questions regarding their papers and session themes. This format offered good opportunities for the presenters to discuss their work with the participants. Reports for each session were summarized by session chairs. Following is a brief summary of the sessions.
Finite difference time domain simulation reveals plasmon coupling and local field enhancement at the gap between the gold nanoparticle (NP) tip of a ZnOnanowire (NW) waveguide and a gold-coated substrate or a gold NP probe. The region of field enhancement is about three times smaller than the 100 nm diameter of the gold NP tip, making the NW waveguide grown on a transparent microcantilever well-suited for near field imaging of single molecules immobilized on a gold substrate or gold NP-labeled cell membranes with superior spatial resolution and signal to noise ratio.
Measuring in-plane thermoelectric properties of submicron thin films has remained a challenging task. Here we report a method based on a suspended microdevice for four-probe measurements of the Seebeck coefficient, thermal conductivity,electrical conductivity, and thermoelectric figure of merit of patterned indium arsenide (InAs) nanofilms assembled on the microdevice. The contact thermal resistance and intrinsic thermal resistance of the 40nm thick InAs nanofilm sample were measured by using the nanofilm itself as a differential thermocouple to determine the temperature drops at the contacts. The microdevice was also used to measure a 190nm thick silicon dioxide (SiO2)film and the results were compared with those reported in the literature. A through-substrate hole under the suspended microdevice allows for transmission electron microscopy characterization of the nanofilm sample assembled on the device. This capability enables one to correlate the measured thermoelectric properties with the crystal structures of the nanofilm.
It has been suggested by theoretical calculation that indium antimonide (InSb)nanowires can possess improved thermoelectric properties compared to the corresponding bulk crystal. Here we fabricated a device using electron beam lithography to measure the thermopower and electrical conductivity of an individual InSbnanowire grown using a vapor-liquid-solid method. The comparison between the measurement results and transport simulations reveals that the nanowire was unintentionally degenerately doped with donors. Better control of the impurity doping concentration can improve the thermoelectric properties.
A comparison study has been conducted on the formation of catalyst nanoparticles on a high surface tension metal and low surface tension oxide for carbon nanotube(CNT)growth via catalytic chemical vapor deposition (CCVD). Silicon dioxide (SiO2) and tantalum have been deposited as supporting layers before deposition of a thin layer of iron catalyst. Ironnanoparticles were formed after thermal annealing. It was found that densities, size distributions, and morphologies of ironnanoparticles were distinctly different on the two supporting layers. In particular, ironnanoparticles revealed a Volmer-Weber growth mode on SiO2 and a Stranski-Krastanov mode on tantalum. CCVD growth of CNTs was conducted on iron∕tantalum and iron∕SiO2. CNTgrowth on SiO2 exhibited a tip growth mode with a slow growth rate of less than 100nm/min. In contrast, the growth on tantalum followed a base growth mode with a fast growth rate exceeding 1μm/min. For comparison, plasma enhanced CVD was also employed for CNTgrowth on SiO2 and showed a base growth mode with a growth rate greater than 2μm/min. The enhanced CNTgrowth rate on tantalum was attributed to the morphologies of ironnanoparticles in combination with the presence of an iron wetting layer. The CNTgrowth mode was affected by the adhesion between the catalyst and support as well as CVD process.
The Seebeck coefficient, electrical conductivity, and thermal conductivity of individual chromium disilicide nanowires were characterized using a suspended microdevice and correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. The obtained thermoelectric figure of merit of the nanowires was comparable to the bulk values. We show that combined Seebeck coefficient and electrical conductivity measurements provide an effective approach to probing the Fermi Level, carrier concentration and mobility in nanowires.
It was recently reported that misoriented layered WSe2 and (W)x(WSe2)yfilms possess extremely low cross-plane thermal conductivity. Here, we report that the in-plane thermal conductivity results for WSe2 and W4(WSe2)10films measured by using a suspended device are about 30 times higher than the cross-plane values because of the in-plane ordered and cross-plane disordered structures and about six times lower than that of compacted single-crystalWSe2 platelets. The additional W layers in the W4(WSe2)10films were found to greatly increase the in-plane electrical conductivity relative to the WSe2films, but reduce the in-plane lattice thermal conductivity assuming the Wiedemann-Franz law.
To better understand thermal transport at nanoscale point contacts such as the tip-sample contact of a scanning probe microscope and at the contact between a nanotube and a planar surface, we have used a nonequilibrium molecular dynamics (MD) method to calculate the temperature distribution and thermal resistance of a nanometer scale constriction formed between two planar silicon substrates of different temperatures. Surface reconstruction was observed at the two free siliconsurfaces and at the constriction. The radius of the heated zone in the cold substrate was found to approach a limit of about 20 times the average nearest-neighbor distance of borondoping atoms when the constriction radius (a) is reduced below the interdopant distance. The phonon mean free path at the constriction was found to be suppressed by diffuse phonon-surface scattering and phonon-impurity scattering. The MD thermal resistance is close to the ballistic resistance when a is larger than 1nm, suggesting that surface reconstruction does not reduce the phonon transmission coefficient significantly. When a is 0.5nm and comparable to the dominant phonon wavelength, however, the MD result is considerably lower than the calculated ballistic resistance because bulk phonon dispersion and bulk potential are no longer accurate. The MD thermal resistance of the constriction increases slightly with increasing doping concentration due to the increase in the diffusive resistance.
Selective growth of vertically aligned and highly dense carbon nanotubes was achieved by using thermal chemical vapor deposition via careful selection of a thin catalyst layer and an appropriate supporting layer. It was found that carbon nanotubegrowth was significantly enhanced when tantalum was used as the supporting layer on which a thin iron catalyst was deposited. Cross-sectional transmission electron microscopy revealed a Stranski-Krastanov mode of iron island growth on tantalum with relatively small contact angles controlled by the relative surface energies of the supporting layer, the catalyst, and their interface. The as-formed iron island morphology promoted vertical growth of carbon nanotubes.
We have measured the electrical conductance and thermopower of a single InSb nanowire in the temperature range from 5 to 340 K. Below temperature (T) 220 K, the conductance (G) shows a power-law dependence on T and the current (I)–voltage (V) curve follows a power-law dependence on V at large bias voltages. These features are the characteristics of one-dimensional Luttinger liquid (LL) transport. The thermopower (S) also shows linear temperature dependence for T below 220 K, in agreement with the theoretical prediction based on the LL model. Above 220 K, the power law and linear behaviours respectively in the G–T and S–T curves persist but with different slopes from those at low temperatures. The slope changes can be explained by a transition from a single-mode LL state to a multi-mode LL state.
We have measured the thermal resistance of a 152‐nm-diameter carbon nanofiber before and after a platinum layer was deposited on the contacts between the nanofiber and the measurement device. The contact resistance was reduced by the platinum coating for about 9–13% of the total thermal resistance of the nanofiber sample before the platinum coating. At a temperature of 300K, the axial thermal conductivity of the carbon nanofiber is about three times smaller than that of graphite fibers grown by pyrolysis of natural gas prior to high-temperature heat treatment, and increases with temperature in the temperature range between 150K and 310K. The phonon mean free path was found to be about 1.5nm and approximately temperature-independent. This feature and the absence of a peak in the thermal conductivity curve indicate that phonon-boundary and phonon-defect scattering dominate over phonon-phonon Umklapp scattering for the temperature range.
We have assembled tin dioxide nanobelts with low-power microheaters for detecting dimethyl methylphosphonate (DMMP), a nerve agent simulant. The electrical conductance of a heated nanobelt increased for 5% upon exposure to 78 parts per billion DMMP in air. The nanobelt conductance recovered fully quickly after the DMMP was shut off, suggesting that the single-crystalnanobelt was not subject to poisoning often observed in polycrystalline metal oxide sensors. While the sensitivity can be improved via doping nanobelts with catalytic additives, directed assembly or growth of nanobelts on microsystems will potentially allow for the large-scale fabrication of nanosensor arrays.