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.
Increasing power densities and decreasing transistor dimensions are hallmarks of modern computer chips. Both trends are increasing the thermal management challenge within the chip and surrounding packaging, as well as accelerating research progress on high-conductivity materials. This article reviews recent materials advances with a focus on novel composite substrates and interface materials, including those composites leveraging the high conductivities of carbon nanotubes. Furthermore, attention is given to the special properties of one-dimensional structures that are likely to be of increasing importance in future applications.
Thermal management is widely recognized to be an important aspect of computer design, with device performance being significantly affected by temperature. In addition, device lifetime can be decreased drastically because of large thermal stresses that occur especially at interfaces. The ability of a structure to remove heat is best quantified by its thermal resistance, which is given by the temperature difference divided by input power. In microprocessor design, the allowable temperature drop between the transistor (where most of the heat is generated) and the ambient air is constant. As a result, the challenge for thermal management is to develop high-conductivity structures that can accommodate this fixed temperature drop with the increasing power densities that characterize new generations of microprocessors.
We review the recent progress in thermal characterization and sensor applications of one-dimensional nanostructures employing microelectromechanical system (MEMS) devices. It was found by thermal measurements that the thermal conductance of a single wall carbon nanotube (SWCNT) was very close to the ballistic thermal conductance of a 1-nm-diameter SWCNT without signatures of phonon−phonon Umklapp scattering, a high thermoelectric figure of merit can potentially be obtained in bismuth telluride (BixTe1-x) nanowires with an optimized atomic ratio of x, and the thermal conductivity of metal oxide nanobelts was suppressed by increased phonon-boundary scattering. We further suggest that dielectrophoresis and other directed-assembly methods can be used for the large-scale integration of nanowires with MEMS to obtain ultrasensitive, stable, and selective sensor systems.
We have observed experimentally that the thermal conductance of a 2.76-μm-long individual suspended single-wall carbon nanotube (SWCNT) was very close to the calculated ballistic thermal conductance of a 1-nm-diameter SWCNT without showing signatures of phonon−phonon Umklapp scattering for temperatures between 110 and 300 K. Although the observed thermopower of the SWCNT can be attributed to a linear diffusion contribution and a constant phonon drag effect, there could be an additional contact effect.
For an electrodepositedbismuth telluride (BixTe1−x)nanowire from one batch with x found to be about 0.46, the Seebeck coefficient (S) was measured to be 15%–60% larger than the bulk values at temperature 300K. For four other nanowires from a different batch with x≈0.54, S was much smaller than the bulk values. The electrical conductivity of the nanowires showed unusually weak temperature dependence and the values at 300K were close to the bulk values. Below 300K, phonon-boundary scattering dominated phonon-phonon Umklapp scattering in the nanowires, reducing the lattice thermal conductivity.
In this paper, we have designed and fabricated a microfluidic channel to focus biological cells using dielectrophoresis for cytometry applications. The device consists of an elliptic-like channel fabricated by isotropic etching of soda lime glass wafers and a subsequent wafer-bonding process. Microelectrodes are patterned on the circumference of the channel to generate ac fringing fields that result in negative dielectrophoretic forces directing cells from all directions to the center of the channel. An analysis using a thin shell model and experiments with microbeads and human leukemia HL60 cells indicate that biological cells can be focused using an ac voltage of an amplitude up to 15 Vp-p and a frequency below 100 kHz, respectively. This design eliminates the sheath flow and the fluid control system that makes conventional cytometers bulky, complicated, and difficult to operate, and offers the advantages of a portable module that could potentially be integrated with on-chip impedance or optical sensors into a micro total analysis system.
A three-dimensional electrothermal model has been developed to investigate the spatial resolution of the scanning thermoelectric microscopy (SThEM). We found that if the electrical resistivity of the sample changes abruptly, the SThEM will measure a voltage close to the local thermoelectric voltage where electrical resistivity is relatively low, rather than a simple weighted average of the thermoelectric voltage distribution based on the temperature profile. This is due to the presence of internal currents in the sample. The spatial resolution of the Seebeck profiling is limited by the finite value of the phonon mean free path of the sample and the tip size of the microscopy. With a tip size around 1 nm, the scanning thermoelectric microscopy can achieve a spatial resolution of the physical limit defined by the statistical nature of the charge carrier and phonon behavior in a very small region.
We have probed the local thermoelectric power of semiconductor nanostructures with the use of ultrahigh-vacuum scanning thermoelectric microscopy. When applied to a p-n junction, this method reveals that the thermoelectric power changes its sign abruptly within 2 nanometers across the junction. Because thermoelectric power correlates with electronic structure, we can profile with nanometer spatial resolution the thermoelectric power, band structures, and carrier concentrations of semiconductor junctions that constitute the building blocks of thermoelectric, electronic, and optoelectronic devices.