In this work, the thermal and electrical transport in an individual silver nanowire is characterized down to 35 K for in-depth understanding of the strong structural defect induced electron scattering. The results indicate that, at room temperature, the electrical resistivity increases by around 4 folds from that of bulk silver. The Debye temperature (151 K) of the silver nanowire is found 36% lower than that (235 K) of bulk silver, confirming strong phonon softening. At room temperature, the thermal conductivity is reduced by 55% from that of bulk silver. This reduction becomes larger as the temperature goes down. To explain the opposite trends of thermal conductivity (κ) ~ temperature (T) of silver nanowire and bulk silver, a unified thermal resistivity ("Equation missing") is used to elucidate the electron scattering mechanism. A large residual Θ is observed for silver nanowire while that of the bulk silver is almost zero. The same "Equation missing"~Ttrend proposes that the silver nanowire and bulk silver share the similar phonon-electron scattering mechanism for thermal transport. Due to phonon-assisted electron energy transfer across grain boundaries, the Lorenz number of the silver nanowire is found much larger than that of bulk silver and decreases with decreasing temperature.

Nanoscale novel devices have raised the demand for nanoscale thermal characterization that is critical for evaluating the device performance and durability. Achieving nanoscale spatial resolution and high accuracy in temperature measurement is very challenging due to the limitation of measurement pathways. In this review, we discuss four methodologies currently developed in nanoscale surface imaging and temperature measurement. To overcome the restriction of the conventional methods, the scanning thermal microscopy technique is widely used. From the perspective of measuring target, the optical feature size method can be applied by using either Raman or fluorescence thermometry. The near-field optical method that measures nanoscale temperature by focusing the optical field to a nano-sized region provides a non-contact and nondestructive way for nanoscale thermal probing. Although the resistance thermometry based on nano-sized thermal sensors is possible for nanoscale thermal probing, significant effort is still needed to reduce the size of the current sensors by using advanced fabrication techniques. At the same time, the development of nanoscale imaging techniques, such as fluorescence imaging, provides a great potential solution to resolve the nanoscale thermal probing problem. Keywords: nanoscale; scanning thermal microscopy; feature size; near-field; Raman spectroscopy; near-field; resistance thermometry (Published: 12 March 2012) Citation: Nano Reviews 2012, 3: 11586 - DOI: 10.3402/nano.v3i0.11586

High-thermal-conductivity polymers are very sought after for applications in various thermal management systems. Although improving crystallinity is a common way for increasing the thermal conductivity (k) of polymers, it has very limited capacity when the crystallinity is already high. In this work, by heat-stretching a highly crystalline microfiber, a significant k enhancement is observed. More interestingly, it coincides with a reduction in crystallinity. The sample is a Spectra S-900 ultrahigh-molecular-weight polyethylene (UHMW-PE) microfiber of 92% crystallinity and high degree of orientation. The optimum stretching condition is 131.5 °C, with a strain rate of 0.0129 s^–1 to a low strain ratio (∼6.6) followed by air quenching. The k enhancement is from 21 to 51 W/(m·K), the highest value for UHMW-PE microfibers reported to date. X-ray diffraction study finds that the crystallinity reduces to 83% after stretching, whereas the crystallite size and crystallite orientation are not changed. Cryogenic thermal characterization shows a reduced level of phonon-defect scattering near 30 K. Polarization Raman spectroscopy finds enhanced alignment of amorphous chains, which could be the main reason for the k enhancement. A possible relocation of amorphous phase is also discussed and indirectly supported by a bending test.

The photothermal phenomenon involves material photon heating in the form of continuous waves, modulated waves, or pulses. The resulting temperature rise carries very rich information about the material's structures and thermophysical properties. This review is constructed to cover some of the extended ideas of the photothermal phenomenon for measuring a material's thermophysical properties and interface thermal conductance/resistance. For frequency-domain photothermal phenomena, the photoacoustic (PA) and photothermal radiation (PT) techniques provide great ways to measure coatings and suspended samples, which can also be measured using the laser flash and time-domain thermoreflectance (TDTR) techniques. Thermal probing based on electrical thermal sensing is successfully implemented in the transient photo-electro-thermal (TPET) and pulsed laser-assisted thermal relaxation (PLTR) techniques, which significantly extend the capability of laser flash technique to micro/nanoscale 1D structures. The energy transport state-resolved Raman (ET-Raman) is distinct from traditional techniques in its material-specific thermal sensing capability and integral way of ultrafast thermal sensing. Detailed physics discussions are provided for all these techniques, including their advantages and limitations. Still, large development rooms exist for the relatively new techniques, including TPET, PLTR, and ET-Raman. These include measurements under extreme situations, sensitivity improvement, and distinguishing conjugated physical processes.