Humphrey J. Maris

Nanoscale thermal transport

Rapid progress in the synthesis and processing of materials with structure on nanometer length scales has created a demand for greater scientific understanding of thermal transport in nanoscale devices, individual nanostructures, and nanostructured materials. This review emphasizes developments in experiment, theory, and computation that have occurred in the past ten years and summarizes the present status of the field. Interfaces between materials become increasingly important on small length scales. The thermal conductance of many solid–solid interfaces have been studied experimentally but the range of observed interface properties is much smaller than predicted by simple theory. Classical molecular dynamics simulations are emerging as a powerful tool for calculations of thermal conductance and phonon scattering, and may provide for a lively interplay of experiment and theory in the near term. Fundamental issues remain concerning the correct definitions of temperature in nonequilibrium nanoscale systems. Modern Si microelectronics are now firmly in the nanoscale regime—experiments have demonstrated that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thereby aggravating problems of thermal management. Microelectronic devices are too large to yield to atomic-level simulation in the foreseeable future and, therefore, calculations of thermal transport must rely on solutions of the Boltzmann transport equation; microscopic phonon scattering rates needed for predictive models are, even for Si, poorly known. Low-dimensional nanostructures, such as carbon nanotubes, are predicted to have novel transport properties; the first quantitative experiments of the thermal conductivity of nanotubes have recently been achieved using microfabricated measurement systems. Nanoscale porosity decreases the permittivity of amorphous dielectrics but porosity also strongly decreases the thermal conductivity. The promise of improved thermoelectric materials and problems of thermal management of optoelectronic devices have stimulated extensive studies of semiconductor superlattices; agreement between experiment and theory is generally poor. Advances in measurement methods, e.g., the 3ω method, time-domain thermoreflectance, sources of coherent phonons, microfabricated test structures, and the scanning thermal microscope, are enabling new capabilities for nanoscale thermal metrology.

Nanoscale thermal transport. II. 2003–2012

A diverse spectrum of technology drivers such as improved thermal barriers, higher efficiency thermoelectric energy conversion, phase-change memory, heat-assisted magnetic recording, thermal management of nanoscale electronics, and nanoparticles for thermal medical therapies are motivating studies of the applied physics of thermal transport at the nanoscale. This review emphasizes developments in experiment, theory, and computation in the past ten years and summarizes the present status of the field. Interfaces become increasingly important on small length scales. Research during the past decade has extended studies of interfaces between simple metals and inorganic crystals to interfaces with molecular materials and liquids with systematic control of interface chemistry and physics. At separations on the order of ∼1 nm, the science of radiative transport through nanoscale gaps overlaps with thermal conduction by the coupling of electronic and vibrational excitations across weakly bonded or rough interface...

Picosecond optical studies of amorphous diamond and diamondlike carbon: Thermal conductivity and longitudinal sound velocity

A picosecond pump‐probe technique is used to measure the room‐temperature thermal conductivity κ and longitudinal sound velocity cl of amorphous diamond (a‐D) and diamondlike carbon (DLC) thin films. Both κ and cl were found to decrease with film hydrogen content. Depending on the film deposition technique, κ is in the range 5–10×10−2 W cm−1 K−1 for a‐D, and 3–10×10−3 W cm−1 K−1 for DLC. Values of cl were found to be in the range 14–18×105 cm s−1 for a‐D, and 6–9×105 cm s−1 for DLC.

Picosecond Ultrasonic measurements using an optical cavity

We have implemented a new means of measuring very high frequency ultrasound in nanostructured materials (known as Picosecond Ultrasonics) by using a high-Q optical resonator that enables significant enhancement and detailed characterization of ultrasound signals.

Improved apparatus for picosecond pump‐and‐probe optical measurements

We present an improved method for making pump‐and‐probe optical measurements in the picosecond to nanosecond time range. In this type of measurement, a pump light pulse is used to excite the sample and the resulting changes in the optical properties are investigated by means of a probe pulse that is time‐delayed relative to the pump pulse. In most measurements of this type, a mechanical stage is used to introduce the variable time delay of the probe pulse. As a result of imperfections in the stage motion, alignment problems, and divergence of the probe beam, it has been difficult to make accurate measurements when the time delay of the probe relative to the pump is in the range above a few hundred picoseconds. To overcome these difficulties, we have developed an apparatus that utilizes a single‐mode optical fiber. In order to demonstrate the performance of this system, we present results of experiments in which the flow of heat from a thin film into a substrate has been measured.

Phonon attenuation and velocity measurements in transparent materials by picosecond acoustic interferometry

A detailed analysis of the method of picosecond acoustic interferometry to study attenuation and velocity of longitudinal acoustic phonons in transparent materials in the Brillouin frequency range with picosecond laser pulses is presented. Experimental results for fused quartz from 90 to 300 K show good agreement with previous Brillouin scattering data. Measurements on a borosilicate glass (Corning 7059) and sapphire have also been made. This method makes these measurements possible under conditions where conventional approaches are not applicable.

Thermal conductivity of isotopically enriched Si

We have used an optical pump-and-probe technique to measure the temperature dependence of the thermal conductivity, κ(T), of isotopically pure Si. The sample was made from 99.7% 28Si by liquid phase epitaxy. Measurements were performed over the temperature range of 100–375 K. We found an increase in the thermal conductivity of isotopically pure Si, as compared to Si of natural isotopic abundance, throughout the entire temperature range. The results were theoretically reproduced by appropriately scaling the parameters used recently to fit the thermal conductivity of Ge samples with different isotopic compositions. A maximum in κ(T) of ∼4×104 W m−1 K−1 is predicted for 28Si at T≃33 K.

Enhancement of Heat Pulses in Crystals Due to Elastic Anisotropy

The generation of thermal acoustic waves by a heated metallic film on the surface of a dielectric crystal at low temperatures is discussed. It is shown that even if the waves produced correspond to a uniform distribution of directions of wave vector, there may be a large enhancement of energy flow in some crystallographic directions compared to the average. This effect arises because of elastic anisotropy. Explicit expressions are given for the enhancement expected in the principal directions of cubic crystals.

Picosecond interferometric technique for study of phonons in the brillouin frequency range

Abstract We describe a new technique - picosecond interferometry - for the study of phonons in the Brillouin frequency range in transparent materials. This method makes possible the measurement of longitudinal phonon velocity and attenuation under conditions such that the conventional approach is not applicable.

Superfluidity of Grain Boundaries and Supersolid Behavior

When two communicating vessels are filled to a different height with liquid, the two levels equilibrate because the liquid can flow. We have looked for such equilibration with solid 4He. For crystals with no grain boundaries, we see no flow of mass, whereas for crystals containing several grain boundaries, we detect a mass flow. Our results suggest that the transport of mass is due to the superfluidity of grain boundaries.