# Publications

#### 2017 Journal Articles

**9. Photo-excited hot carrier dynamics in hydrogenated amorphous silicon imaged by 4D electron microscopy**Bolin Liao, Ebrahim Najafi, Heng Li, Austin J. Minnich, and Ahmed H. Zewail

Nature Nano. 12, 871–876 Jul 3, 2017

Nature Nano. 12, 871–876 Jul 3, 2017

In this work, carrier dynamics in hydrogenated amorphous silicon is imaged in space and time using scanning ultrafast electron microscopy (SUEM). An unexpected regime of fast diffusion immediately after photoexcitation, together with spontaneous electron-hole separation and charge trapping induced by atomic disorder, is observed.

**8. Experimental metrology to obtain thermal phonon transmission coefficients at solid interfaces**Chengyun Hua*, Xiangwen Chen*, Navaneetha K. Ravichandran and Austin J. Minnich

Phys. Rev. B, 95(20) May 15, 2017

Phys. Rev. B, 95(20) May 15, 2017

This work reports the first measurement of the Fresnel transmission coefficients for thermal phonons at a metal-semiconductor interface using ab-initio phonon transport modeling and a thermal characterization technique.

**7. Elastic and thermal properties of free-standing Molybdenum Disulfide Membranes measured using Ultrafast Transient Grating Spectroscopy**

Taeyong Kim, Ding Ding, Jong-Hyuk Yim, Young-Dahl Jho, and Austin J. Minnich

APL Materials, 5(086105) Aug 21, 2017

Molybdenum disulfide (MoS_{2}) is of intense interest due to its unique electronic and thermoelectric properties. However, reports of its in-plane thermal conductivity vary due to the difficulty of in-plane thermal conductivity measurements on thin films. Here, we use time-resolved transient grating spectroscopy to simultaneously measure the in-plane elastic and thermal properties of free-standing MoS_{2} membranes at room temperature. Our measurements provide useful insights into the elastic and thermal properties of MoS_{2} and demonstrate the capability of transient grating spectroscopy to investigate the in-plane vibrational properties of van der Waals materials.

**6. Propagating elastic vibrations dominate thermal conduction in amorphous silicon **

Jaeyun Moon, Benoit Latour, Austin Minnich

*arXiv:1704.08360, submitted on April 26, 2017*

In a-Si, prior works concluded that non-propagating waves are the dominant contributors to heat transport, while propagating waves are restricted to frequencies less than a few THz and are scattered by anharmonicity. Here, we present a lattice and molecular dynamics analysis of vibrations in a-Si that supports a qualitatively different picture in which propagating elastic waves dominate the thermal conduction and are scattered by elastic fluctuations rather than anharmonicity. Our work suggests that most heat is carried by propagating elastic waves in a-Si and demonstrates a route to achieve extreme thermal properties in amorphous materials by manipulating elastic fluctuations.

**5. Thermal response of materials to extreme temperature gradients and the role of the spatial frequency**

Xiangwen Chen*, Chengyun Hua*, Hang Zhang, Navaneetha K. Ravichandran, and Austin J. Minnich

The thermal response of materials to extreme temperature gradients has recently become of intense interest both scientifically and for applications. Here, we probe the thermal response of crystals to large thermal gradients generated by optical heating of nanoline arrays. We demonstrate that the ballistic thermal resistance associated with nanoscale heat sources can be nearly eliminated by rationally engineering the spatial arrangement of individual heaters, a finding that could impact strategies for heat dissipation in electronics and other applications.

** 4. Semiconductor-based Multilayer Selective Solar Absorber for Unconcentrated Solar Thermal Energy Conversion**

Nate Thomas*, Zhen Chen, Shanhui Fan, and Austin J. Minnich

Scientific Reports 7, 5362 July 13, 2017

We report a semiconductor-based multilayer selective absorber that exploits the sharp drop in optical absorption at the band gap energy achieve a measured absorptance of 76% at solar wavelengths and a low emittance of 5% at thermal wavelengths. In field tests we obtain a peak temperature of 225°C. With straightforward optimization, our work will allow unconcentrated solar thermal systems to reach stagnation temperatures exceeding 300°C, thereby eliminating the need for solar concentrators for mid-temperature solar applications.

**3. The role of low energy phonons in nanocrystalline Si and SiGe: an ab-initio based study**

Lina Yang and Austin J. Minnich

Scientific Reports 7, 44254, March 14, 2017

Using a variance-reduced Monte Carlo method with the full phonon dispersion and intrinsic lifetimes from first-principles as input, we examine phonon transport in nanocrystalline SiGe alloys with ab-initio electronphonon scattering rates. Our calculations show that low energy phonons still transport substantial amounts of heat in these materials, and thus improvements in ZT are still possible by disrupting these modes.

**2. Daytime Radiative Cooling Using Infrared Black Bodies**

Jun-long Kou, Zoila Jurado, Zhen Chen, Shanhui Fan, and Austin J. Minnich

ACS Photonics, February 3, 2017

We demonstrate that a polymer-coated fused silica mirror, as a near-ideal blackbody in the mid-infrared and near-ideal reflector in the solar spectrum, achieves radiative cooling below ambient air temperature under direct sunlight by 8.2 degrees. Our work demonstrates that abundant materials and straight-forward fabrication can be used to achieve daytime radiative cooling, advancing applications such as dry cooling of thermal power plants.

**1. Intrinsic localized mode and low thermal conductivity of PbSe**

Nina Shulumba, Olle Hellman, and Austin J. Minnich

Physical Review B, 95, 014302, January 4, 2017

In this work, we use ab-initio calculations that explicitly account for strong anharmonicity to identify the origin of the low thermal conductivity in PbSe as an anomalously large anharmonic interaction, exceeding in strength that in PbTe, between the transverse optic and longitudinal acoustic branches.

#### 2016 Journal Articles

**8. Sub-amorphous thermal conductivity in amorphous heterogeneous nanocomposites**

Jaeyun Moon and Austin J. Minnich

RSC Advances, 105154-105160 , Oct 31, 2016

This work employs equilibrium molecular dynamics to show an amorphous SiGe nanocomposite possesses thermal conductivity substantially lower than those of the amorphous Si and Ge constituents.

**7. Active Thermal Extraction and Temperature Sensing of Near-field Thermal Radiation**Ding Ding, Taeyong Kim, and Austin J. Minnich

Scientific Reports 6, Article number: 32744, Sep 06, 2016

Scientific Reports 6, Article number: 32744, Sep 06, 2016

This works reports a theoretical framework for active extraction of near field radiation in the context of laser cooling of solids with the direct application of temperature sensing.

**6. Exploring the extremes of heat conduction in anisotropic materials.**

Austin J. Minnich

Nanoscale and Microscale Thermophysical Engineering, Vol. 20, No. 1, 1-21, March 30, 2016

Anisotropic solids possess thermal conductivities ranging from among the highest found in nature, as in the in-plane thermal conductivity of graphite, to the lowest, as in the cross-plane thermal conductivity of disordered layered crystals. In this review article, we discuss the recent advances in our understanding of thermal phonon transport in anisotropic solids obtained using new theoretical, computational, and experimental tools.

**5. Thermal Characterization of Substrate Options for High-Power THz Multipliers Over a Broad Temperature Range**Tero Kiuru, Goutam Chattopadhyay, Theodore J. Reck, Austin J. Minnich, Robert Lin, Erich Schlecht, Jose V. Siles, Choonsup Lee, and Imran Mehdi

IEEE Transactions on Terahertz Science and Technology, vol. 6, no. 2, pp. 328-335, March 2016

IEEE Transactions on Terahertz Science and Technology, vol. 6, no. 2, pp. 328-335, March 2016

This paper presents thermal characterization results for three high-power THz Schottky frequency multipliers in the temperature range of 20–380 K. At temperatures below 125 K, the GaAs membrane is on par, or even has lower thermal resistance than the membrane on diamond.

**4. Temperature Dependent Mean Free Path Spectra of Thermal Phonons Along the c-axis of Graphite
**Hang Zhang, Xiangwen Chen, Young-Dahl Jho, Austin J. Minnich

Nano Letters, 2016, 16 (3), pp 1643–1649

Nano Letters, 2016, 16 (3), pp 1643–1649

This work reports the first quantitative measurements of c-axis phonon MFP spectra in graphite at a variety of temperatures using time-domain thermoreflectance measurements of graphite flakes with variable thickness. Our results indicate that c-axis phonon MFPs have values of a few hundred nanometers at room temperature and a much narrower distribution than in isotropic crystals. At low temperatures, phonon scattering is dominated by grain boundaries separating crystalline regions of different rotational orientation.

**3. Active Thermal Extraction of Near-field Thermal Radiation**D. Ding, T. Kim, and A. J. Minnich

Physical Review B, 93, 081402(R), February 2, 2016

Physical Review B, 93, 081402(R), February 2, 2016

This work proposes and numerically demonstrates an active scheme to extract near-field surface modes to the far-field. This approach exploits the monochromatic nature of near-field thermal radiation to drive a transition in a laser gain medium, which, when coupled with external optical pumping, allows the resonant surface mode to be emitted into the far-field.

**2. The Role of Thermalizing and Non-thermalizing Walls in Phonon Heat Conduction along Thin Films
**Navaneetha K. Ravichandran, Austin J. Minnich

Physical Review B, 93, 035314, January 28, 2016

Physical Review B, 93, 035314, January 28, 2016

Here we examine thermal transport along thin films with both thermalizing and non-thermalizing walls by solving the spectral Boltzmann transport equation (BTE) for steady state and transient transport. Our results provide insights into the boundary scattering process of thermal phonons over a range of heating length scales that are useful for interpreting thermal measurements on nanostructures.

**1. Heat conduction in multifunctional nanotrusses studied using
Boltzmann transport equation
**Nicholas G. Dou, Austin J. Minnich

Applied Physics Letters, 108, 011902, January 4, 2016

Applied Physics Letters, 108, 011902, January 4, 2016

Nanotrusses consisting of hollow nanoscale beams architected into a periodic truss structure can potentially achieve low density, low thermal conductivity yet also high stiffness due to their lattice architecture and nanoscale features. In this work, we study heat conduction in the exact nanotruss geometry by solving the frequency-dependent Boltzmann transport equation using a variance-reduced Monte Carlo algorithm.

#### 2015 Journal Articles

**9. Crystalline Polymers with Exceptionally Low Thermal Conductivity Studied using Molecular Dynamics
**Andrew B. Robbins, Austin J. Minnich

Applied Physics Letters, 107, 201908, November 19, 2015

Applied Physics Letters, 107, 201908, November 19, 2015

In this paper, we use molecular dynamics simulations to study heat transport in polynorbornene, a polymer that can be synthesized in semi-crystalline form using solution processing. We demonstrate that even perfectly crystalline polynorbornene has an exceptionally low thermal conductivity near the amorphous limit due to extremely strong anharmonic scattering. This scattering is sufficiently strong to prevent the formation of propagating phonons, with heat being instead carried by non-propagating, delocalized vibrational modes known as diffusons.

**8. Thermal phonon boundary scattering in anisotropic thin films
**A. J. Minnich

Applied Physics Letters, Vol. 107, Issue 18, November 3, 2015

Applied Physics Letters, Vol. 107, Issue 18, November 3, 2015

In this paper, we derive a generalization of the Fuchs-Sondheimer equation for solids with arbitrary dispersion relations and examine its predictions for graphite. We find that the isotropic equation vastly over-estimates the boundary scattering that occurs in thin graphite films due to the highly anisotropic group velocity, and that graphite can maintain its high in-plane thermal conductivity even in thin films with thicknesses as small as ten nanometers.

**7. Multidimensional quasiballistic thermal transport in transient grating spectroscopy
**A. J. Minnich

Physical Review B, 92, 085203, August 13, 2015

Physical Review B, 92, 085203, August 13, 2015

This paper analyzes quasiballistic transport in a two-dimensional transient grating experiment, in which heat conduction can occur both in- and cross-plane. Using an analytic Green's function of the Boltzmann equation, we demonstrate a method by which phonon MFPs can be extracted from these measurements, thereby extending the MFP spectroscopy technique using transient grating to opaque bulk materials.

**6. Semi-analytical solution to the frequency-dependent Boltzmann transport
equation for cross-plane heat conduction in thin films
**C. Hua and A. J. Minnich

J. Appl. Phys. 117, 175306 (2015)

J. Appl. Phys. 117, 175306 (2015)

This work presents a semi-analytical series expansion method to solve the transient frequency-dependent Boltzmann transport equation in a finite domain, which enables us to obtain solutions for the cross-plane heat transport in thin films.

**5. Length dependent thermal conductivity measurements yield phonon mean free path spectra in nanostructures
**H. Zhang, C. Hua, D. Ding and A. J. Minnich

Scientific Reports, 5, Article Number: 9121, March 13, 2015

Scientific Reports, 5, Article Number: 9121, March 13, 2015

This work develops a reconstruction method to obtain mean free path spectra of nanostructures from variable-length thermal conductivity measurements. Using this method, we investigate recently reported length-dependent thermal conductivity measurements on SiGe alloy nanowires and suspended graphene ribbons.

**4. Selective radiative heating of nanostructures using hyperbolic metamaterials
**D. Ding, and A. J. Minnich

Optics Express, Vol. 23, Issue 7, pp. A299-A308, 2015

Optics Express, Vol. 23, Issue 7, pp. A299-A308, 2015

This work demonstrates the concept of using hyperbolic metamaterials (HMM) to enable selective heating of a sub-wavelength plasmonic nanowire by controlling the angular mode number of a plasmonic resonance.

**3. The best nanoparticle size distribution for minimum thermal conductivity
**H. Zhang and A. J. Minnich

Scientific Reports, 5, Article Number: 8995, March 11, 2015

Scientific Reports, 5, Article Number: 8995, March 11, 2015

This work demonstrates the best size distribution of nanoparticles embedded in a crystalline solid, which can yield the lowest thermal conductivity. Counterintuitively, nanoparticles with several well-chosen discrete sizes can most effectively scatter the continuous broad thermal phonon spectrum.

**2. Phonon heat conduction in layered anisotropic crystals
**A. J. Minnich

Physical Review B, 91, 085206, February 17, 2015

Physical Review B, 91, 085206, February 17, 2015

The work extends an analytical solution of the Boltzmann transport equation to highly anisotropic solids and demonstrates a method to reconstruct the anisotropic mean free path spectrum of crystals with arbitrary dispersion relations without any prior knowledge of their harmonic or anharmonic properties using observations of quasiballistic heat conduction.

**1. Advances in the measurement and computation of thermal phonon transport properties
**A. J. Minnich

J. Phys. Cond. Matt., 27, 053202, January 21, 2015

J. Phys. Cond. Matt., 27, 053202, January 21, 2015

In this topical review, the author provides an overview of the methods in both computation and experiment, which enable an unprecedented microscopic view of thermal transport by phonons, as well as the insights they are providing and their impact on the science and engineering of heat conduction.

#### 2014 Journal Articles

**7. Analytical Green's function of the multidimensional frequency-dependent phonon Boltzmann equation
**C. Hua and A. J. Minnich

Physical Review B, 90,214306, December 31, 2014

Physical Review B, 90,214306, December 31, 2014

This paper presents an analytical Green's function for the frequency-dependent, multidimensional Boltzmann equation and provides simple closed-form solutions for a number of multidimensional problems for which the only prior solution methods have been computationally expensive numerical simulations.

**6. Phonon blackbody radiation limit for heat dissipation in electronics
**J. Schleeh, J. Mateos, I. Iniguez-de-la-Torre, N. Wadefalk, P. A. Nilsson, J. Grahn, and A. J. Minnich

Nature Materials, 14, 187-192, November 10, 2014

Nature Materials, 14, 187-192, November 10, 2014

This paper shows that the origin of the noise saturation in cryogenic amplifiers is due to ballistic thermal phonon transport near the transistor and demonstrates that the lowest noise that can be achieved in these transistors at cryogenic temperatures is largely governed by the heat dissipation in the device.

**5. Heat under the microscope
**I. Maasilta and A. J. Minnich

Physics Today (invited, peer-reviewed), August, 2014

Physics Today (invited, peer-reviewed), August, 2014

In this article, the authors review recent advances in computational and spectroscopic tools which offer new insights into the nature of thermal conduction at ever-finer length scale and ways to control it.

**4. Importance of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon-germanium thermoelectrics
**C. Hua and A. J. Minnich

Special invited issue of Semiconductor Science and Technology, 29 (2014) 124004, Nov 15, 2014

Special invited issue of Semiconductor Science and Technology, 29 (2014) 124004, Nov 15, 2014

This work examines the impact of frequency-dependent grain boundary scattering in nanocrystalline silicon and silicon-germanium alloys in a realistic 3D geometry using frequency-dependent variance-reduced Monte Carlo simulations.

**3. Radial quasiballistic transport in time-domain thermoreflectance studied using Monte Carlo simulations
** D. Ding, X. Chen and A. J. Minnich

Applied Physics Letters 104, 143104, April 8, 2014

Applied Physics Letters 104, 143104, April 8, 2014

This paper presents a numerical study to identify the radial suppression function that allows the mean free path spectrum to be quantitatively reconstructed from variable pump-size measurements in time-domain thermoreflectance.

**2. Coherent and incoherent thermal transport in nanomeshes
**N. K. Ravichandran and A. J. Minnich

Physical Review B, 89, 205432, May 27, 2014

Physical Review B, 89, 205432, May 27, 2014

This paper demonstrates that thermal transport in nanosctructures that can be created with available lithographic techniques is dominated by incoherent boundary scattering at room temperature.

**1. Transport regimes in quasiballistic heat conduction
**C. Hua and A. J. Minnich

Physical Review B, 89,094302, March 14, 2014

Physical Review B, 89,094302, March 14, 2014

This work presents a new analytical solution of the frequency-dependent Boltzmann transport equation in transient grating decay, which leads to a more accurate measurement of phonon mean free path spectrum.

#### 2013 Journal Articles

**2. Thermal transport: Naturally glassy crystals
**A. J. Minnich

Nature Nanotechnology, Vol. 8 pp. 392-393, June 05, 2013

Nature Nanotechnology, Vol. 8 pp. 392-393, June 05, 2013

This News and Views article describes a recent work (See [Delaire et al]) that reports the origin of a glass-like thermal conductivity in a perfectly crystalline semiconductor.

**1. Direct Measurement of Room-Temperature Nondiffusive Thermal Transport Over Micron Distances in a Silicon Membrane
**J. A. Johnson, A. A. Maznev, J. Cuffe, John, J. K. Eliason, A. J. Minnich, T. Kehoe, C. M. Torres, G. Chen and K. A. Nelson.

Physical Review Letter Vol. 110, 025901, January 08, 2013

Physical Review Letter Vol. 110, 025901, January 08, 2013

This work presents a simple experiment demonstrating that room-temperature thermal transport in Si significantly deviates from the diffusion model already at micron distances.

#### 2012 Journal Articles

**2. Coherent Phonon Heat Conduction in Superlattices**

Maria N. Luckyanova, Jivtesh Garg, Keivan Esfarjani, Adam Jandl,
Mayank T. Bulsara, Aaron J. Schmidt, Austin J. Minnich, Shuo Chen, Mildred S. Dresselhaus, Zhifeng Ren, Eugene A. Fitzgerald, Gang Chen

Science, Vol. 338 no. 6109 pp. 936-939, November16, 2012

This paper describes the first observation of coherent heat conduction, in which thermal phonons travel as waves rather than particles, in a GaAs/AlAs superlattice.

**1. Determining Phonon Mean Free Paths from Observations of Quasiballistic Thermal Transport
**A. J. Minnich

*, 109, 205901 (2012)*

Physical Review Letters

Physical Review Letters

This work demonstrates how the phonon mean free path distribution may be reconstructed from quasiballistic thermal measurements by solving a common inverse problem.

#### 2011 Journal Articles

**3. Power Factor Enhancement by Modulation Doping in Bulk Nanocomposites**** **

Mona Zebarjadi, Giri Joshi, Gaohua Zhu, Bo Yu, Austin Minnich, Yucheng Lan, Xiaowei Wang, Mildred Dresselhaus, Zhifeng Ren, and Gang Chen

Nano Lett., 2011, 11 (6), pp 2225–2230

**2. Quasiballistic heat transfer studied using the frequency-dependent boltzmann transport equation**

A. J. Minnich, G. Chen, S. Mansoor, and B. S. Yilbas*
Physical Review B*, 84(23):235207, December 2011

This paper presents a computational study of one-dimensional quasiballistic in a metal/semiconductor structure.

**1. A thermal conductivity spectroscopy technique to measure phonon mean free paths**

A. J. Minnich, , J. A. Johnson, A. J. Schmidt, K. Esfarjani, M. S. Dresselhaus, K. A. Nelson, and G. Chen*Physical Review Letters*, 107:095901, Aug 2011. Chosen as Editor's suggestion

This paper introduces the first experimental technique that is able to measure thermal phonon mean free paths over a wide range of length scales and materials.

#### 2011 Conference Papers

**Quasi-ballistic heat transfer from metal nanostructures on sapphire**

Austin Minnich and Gang Chen

In *Proceedings of the 8th ASME/JSME Thermal Engineering Joint Conference*, page 44094, 2011

#### 2010 Journal Articles

**2. Theoretical studies on the thermoelectric figure of merit of nanograined bulk silicon**

Qing Hao, Gaohua Zhu, Giri Joshi, Xiaowei Wang, Austin Minnich, Zhifeng Ren, and Gang Chen *
Applied Physics Letters*, 97(6):063109, 2010

** 1. Thermal conductance and phonon transmissivity of metal-graphite interfaces**

Aaron J. Schmidt, Kimberlee C. Collins, Austin J. Minnich, and Gang Chen

*Journal of Applied Physics*, 107(10):104907, 2010

#### 2009 Journal Articles

**4. Modeling study of thermoelectric SiGe nanocomposites**

A. J. Minnich, H. Lee, X. W. Wang, G. Joshi, M. S. Dresselhaus, Z. F. Ren, G. Chen, and D. Vashaee*
Physical Review B*, 80(15):155327, October 2009

**3. Bulk nanostructured thermoelectric materials: Current research and future prospects***
*A. J. Minnich, M. S. Dresselhaus, Z. F. Ren, and G. Chen

*, 2(5):466 - 479, 2009 (Invited, peer-reviewed. Among the top 10 downloaded articles in EES for July, August, December 2009, January-May, July-December 2010, January 2011)*

Energy & Environmental Science

Energy & Environmental Science

**2. Enhancement of thermoelectric figure-of-merit by a bulk nanostructuring approach***
*Yucheng Lan, Austin Minnich, Gang Chen, and Zhifeng Ren

*, 20:357 - 376, 2009*

Advanced Functional Materials

Advanced Functional Materials

**1. Solubility study of Yb in n-type skutterudites YbxCo4Sb12 and their enhanced thermoelectric properties**

J. Yang, Q. Hao, H. Wang, Y. C. Lan, Q. Y. He, A. J. Minnich, D. Z. Wang, J. A. Harriman, V. M. Varki, M. S. Dresselhaus, G. Chen, and Z. F. Ren*
Physical Review B*, 80(11):115329, 2009

#### 2008 Journal Papers

**High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys***
*Bed Poudel, Qing Hao, Yi Ma, Yucheng Lan, Austin Minnich, Bo Yu, Xiao Yan, Dezhi Wang, Andrew Muto, Daryoosh Vashaee, Xiaoyuan Chen, Junming Liu, Mildred S. Dresselhaus, Gang Chen, and Zhifeng Ren

*, 320(5876):634-638, 2008*

Science

Science

#### 2008 Conference Papers

**2. Modeling grain boundary scattering in nanocomposites**

A. Minnich, D. Vashaee, and G. Chen

In *Proceedings of the 2008 ASME International Mechanical Engineering Congress & Exposition*, pages IMECE2008-67385, Boston, November 2008

**1. Modeling the thermoelectric properties of nanocomposites**

A. Minnich and G. Chen

In *Proceedings of the 3rd ASME Energy Nanotechnology International Conference*, Jacksonville, August 2008

#### 2007 Journal Articles

**Modified effective medium formulation for the thermal conductivity of nanocomposites***
*Austin Minnich and Gang Chen

*, 91(7):073105, 2007 (also in August 27, 2007 issue of Virtual Journal of Nanoscale Science & Technology)*

Applied Physics Letters

Applied Physics Letters

#### Book Chapters

**Measuring phonon mean free paths using thermal conductivity spectroscopy****
**Austin J. Minnich

*Annual Review of Heat Transfer*,

*Belsevere*2012.