Semiconductor nanowires represent an important class of nanostructure building block for photovoltaics as well as direct solar-to-fuel application because of their high surface area, tunable bandgap and efficient charge transport and collection. In this talk, I will highlight several recent examples in this lab using semiconductor nanowires and their heterostructures for the purpose of solar energy harvesting. In addition, we have also discovered that the thermoconductivity of the silicon nanowires can be significantly reduced due to phonon scattering, pointing to a very promising approach to design better thermoelectrical materials. It is important to note that the engines that generate most of the world's power typically operate at only 30–40 per cent efficiency, releasing roughly 15 terawatts of heat to the environment. If this “wasted heat” could be recycled, the impact globally would be enormous. Our silicon nanowire thermoelectric technology could have a significant impact in alternative energy generation.

Semiconductor nanowires for energy conversion.

Understanding energy dissipation and transport in nanoscale structures is of great importance for the design of energy-efficient circuits and energy-conversion systems. This is also a rich domain for fundamental discoveries at the intersection of electron, lattice (phonon), and optical (photon) interactions. This review presents recent progress in understanding and manipulation of energy dissipation and transport in nanoscale solid-state structures. First, the landscape of power usage from nanoscale transistors (∼10−8 W) to massive data centers (∼109 W) is surveyed. Then, focus is given to energy dissipation in nanoscale circuits, silicon transistors, carbon nanostructures, and semiconductor nanowires. Concepts of steady-state and transient thermal transport are also reviewed in the context of nanoscale devices with sub-nanosecond switching times. Finally, recent directions regarding energy transport are reviewed, including electrical and thermal conductivity of nanostructures, thermal rectification, and the role of ubiquitous material interfaces. 
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Energy dissipation and transport in nanoscale devices

Understanding energy dissipation and transport in nanoscale structures is of great importance for the design of energy-efficient circuits and energy-conversion systems. This is also a rich domain for fundamental discoveries at the intersection of electron, lattice (phonon), and optical (photon) interactions. This review presents recent progress in understanding and manipulation of energy dissipation and transport in nanoscale solid-state structures. First, the landscape of power usage from nanoscale transistors (~10^-8 W) to massive data centers (~10^9 W) is surveyed. Then, focus is given to energy dissipation in nanoscale circuits, silicon transistors, carbon nanostructures, and semiconductor nanowires. Concepts of steady-state and transient thermal transport are also reviewed in the context of nanoscale devices with sub-nanosecond switching times. Finally, recent directions regarding energy transport are reviewed, including electrical and thermal conductivity of nanostructures, thermal rectification, and the role of ubiquitous material interfaces.

/pdf/energy-dissipation-and-transport-in-nanoscale-devices-17sw2ubrhz.pdf

Energy Dissipation and Transport in Nanoscale Devices

Recently a significant figure-of-merit (ZT) improvement in the most-studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high-energy ball milling and a direct-current-induced hot-press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.

Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring Approach

A review on the enhancement of figure of merit from bulk to nano-thermoelectric materials

For an electrodeposited bismuth 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.

Thermoelectric properties of individual electrodeposited bismuth telluride nanowires

It has been suggested that CNTs and carbon nanofibers CNFs can be used as thermal interface materials to enhance contact thermal conductance for electronic packaging applications. Several groups have reported mixed experimental results from no improvements to large improvements in the thermal contact conductance due to the CNTs and CNFs 8‐12. These mixed results can be caused by the difference in surface coverage and perpendicular alignment of the CNTs or CNFs. Moreover, the results can be affected by two other factors. First, the CNTs and CNFs grown using different methods possess different defect densities and different intrinsic thermal conductivities. Secondly, the contact thermal resistance of the nanometer scale point and line contacts between a CNT or CNF and a planar surface can be high due to enhanced phonon-boundary scattering at the nanocontacts. We have used a microfabricated device to measure the thermal resistance of an individual CNF from a vertically aligned CNF film for applications as thermal interface materials. The measurement was conducted before and after a platinum Pt layer was deposited on the contacts between the CNF and the microdevice so as to investigate the thermal contact resistance between the CNF and a planar surface. The contact resistance was reduced by the platinum coating for about 9‐13% of the total thermal resistance of the nanofiber sample before the Pt coating. At temperature 300 K, the obtained axial thermal conductivity of the carbon nanofibers was about three times smaller than that of graphite fibers grown by pyrolysis of natural gas prior to high-temperature heat treatment.

Thermal Contact Resistance and Thermal Conductivity of a Carbon Nanofiber

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.

Thermal probing of energy dissipation in current-carrying carbon nanotubes

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.

Thermal characterization and sensor applications of one-dimensional nanostructures employing microelectromechanical systems

We have measured the thermoelectric properties of electrochemically deposited bismuth telluride (Bi/sub x/Te/sub 1-x/) nanowires with different atomic ratio or x. In this paper, we report the measurement method and the results for an individual nanowire from a batch with x found to be about 0.54. The Seebeck coefficient of the nanowire was found to be -30 /spl mu/V/K at temperature 300 K. The obtained electrical conductivity of the nanowire showed unusually weak temperature dependence, and the value at 300 K was only 5.6% lower than that of bulk Bi/sub 0.485/Te/sub 0.515/ crystal. The thermal conductivity of the nanowires was found to be 44% lower than that of bulk Bi/sub 0.485/Te/sub 0.515/ crystals.

Jianhua Zhou

Papers

Thermoelectric properties of individual electrodeposited bismuth telluride nanowires

Thermal Contact Resistance and Thermal Conductivity of a Carbon Nanofiber

Thermal probing of energy dissipation in current-carrying carbon nanotubes

Thermal characterization and sensor applications of one-dimensional nanostructures employing microelectromechanical systems

Measurement of thermoelectric properties of individual bismuth telluride nanowires