**Xinwei Wang**

Dr. Xinwei Wang is an Anson Marston Distinguished Professor and Wilkinson Professor in Interdisciplinary Engineering at Iowa State University (http://web.me.iastate.edu/wang). He obtained his Ph.D. from the School of Mechanical Engineering, Purdue University in 2001 and his M.S. (1996) and B.S. (1994) from the University of Science and Technology of China. Over the past 20 years, he has led his laboratory to develop new techniques for the characterization of energy transport at the micro/nanoscale, including the TET, ET-Raman, TD-Raman, and FR-Raman techniques. His lab reported the first study that distinguished optical and acoustic phonon temperatures under intense photon excitation and determined their energy coupling factor. His work on conjugated phonon and hot carrier transport represents the first accomplishment in distinguishing these two physical processes and quantifying their transport diffusivities. The thermal reffusivity theory developed in his lab provides a novel way to characterize a material's structure domain size, similar to that measured via X-ray diffraction, but it has unique applications for nanomaterials. He received the inaugural Viskanta Fellow Award of Purdue University in recognition of his pioneering and independent work in thermal sciences. He has been the recipient of the Mid-career Award for Research (2014) and Award for Outstanding Achievement in Research (2018) from ISU. He is a Fellow of ASME and Associate Fellow of AIAA. He serves as the associate editor/editor/senior editor of five international journals and is on the editorial board of six international journals.

#### **Preface to "Energy Transport at the Micro/Nanoscale"**

With the rapid development of micro/nanoscale materials in the last two decades, there is now substantial demand for improvements in their thermophysical properties, especially thermal conductivity, thermal diffusivity and interface thermal conductance, as they are strongly affected by micro/nanoscale structures. The measurement of these properties is a significant task and requires new scientific and engineering developments. This Special Issue targets the state-of-art research in this area and is anticipated to provide timely reviews and research advancements which will benefit researchers and engineers in materials science, engineering, physics, etc.

> **Xinwei Wang** *Editor*

## *Editorial* **Perspectives on Energy Transport at the Micro/Nanoscale**

**Xinwei Wang**

> Department of Mechanical Engineering, Iowa State University, Ames, IA 50011, USA; xwang3@iastate.edu; Tel.: +1-515-294-8023

Over the last two decades, with the fast development of micro/nanomaterials, including micro/nanoscale and micro/nanostructured materials, significant attention has been attracted to study the energy transport in them [1–3]. This energy transport can be sustained by electrons and phonons, whose transport is strongly affected by micro/nanoscale structure scattering. Numerous computer models based on first-principle, molecular dynamics, and lattice Boltzmann transport have been reported [4–6]. Also tremendous efforts have been reported on development of new technologies to characterize the thermophysical properties at the micro/nanoscale [7,8]. Examples of such properties include thermal conductivity, thermal diffusivity, and specific heat [9–11].

Thermophysical characterization at the micro/nanoscale is extremely challenging due to the very small size of the sample under study. It is a non-trivial job to apply a very well defined heat flux along the sample and characterize the temperature drop over it. To overcome this challenge, new transient techniques have been developed by applying transient Joule or photon heating, and track the material's thermal response using electrical or optical methods. Examples include the transient electro-thermal (TET), transient photo-electro-thermal (TPET), pulsed laser-assisted thermal relaxation (PLTR), time domain differential Raman (TD-Raman), frequency-resolved Raman (FR-Raman), and energy transport state-resolved Raman (ET-Raman) techniques [7,8,12–16]. These techniques provide quick and high-level measurement of thermal diffusivity/conductivity of 1D and 2D materials down to atomic-level thickness [17].

This special issue includes some recent work on micro/nanoscale energy transport, including research progress reports and reviews. These reviews are in grea<sup>t</sup> technical detail since the authors have tremendous experience and expertise in the topics under review. In the paper by Lin et al. [18], the TET technique was reviewed thoroughly, especially about the differential TET concept. The TET technique measures 1D material's voltage/resistance change under step-current heating and can be used to measure thermal diffusivity, conductivity, and specific heat. The differential concept is especially useful in measuring materials of nm-thickness which is too thin to suspend between two electrodes. The energy transport in 2D materials features very unique characteristics due to the extreme phonon confinement and scattering. The review by Kalantari and Zhang [19] provides excellent coverage, discussions, and perspectives on computer modeling and experimental characterization. The works by Dai and Wang [20] and Liu et al. [21] are grea<sup>t</sup> additions to the review by Mohammad and Xian, with focus on detailed experimental development. The work by Zhou et al. [22] provides sound technical details on the photothermal technique for measuring the thermal conductivity and interface thermal resistance of coatings. This work covers the fundamental physical principles, mathematical solutions for data processing, and typical examples of measurement.

Of the research progress reports covered by this Special Issue, the work by Xu et al. provides inspiring measurement technique and knowledge about the dynamic thermal diffusivity evolution during fast heating of corn leaves and ultra-high-molecular-weight polyethylene (UHMW-PE) micro-fibers [23]. This indeed significantly extends the capability of the TET technique and provides unprecedented knowledge about thermal diffusivity evolution during fast heating. The work by Liu et al. presents pioneering efforts in studying

**Citation:** Wang, X. Perspectives on Energy Transport at the Micro/Nanoscale. *Nanomaterials* **2023**, *13*, 1746. https://doi.org/10.3390/ nano13111746

Received: 21 May 2023 Accepted: 24 May 2023 Published: 26 May 2023

**Copyright:** © 2023 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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the thermal conductivity variation of giant-scale graphene depending on temperature [24]. This work is very challenging since the thermal expansion mismatch between graphene and the poly(methyl methacrylate) (PMMA) substrate could easily break the graphene layer when temperature goes down. The experimental work by Wang et al. on the bolometric response of MoS2 nanoflowers and multi-walled carbon nanotube composite [25], and work on the effect of current annealing on thermal conductivity of carbon nanotubes by Lin et al. [26] present welcome efforts on studying the structural and temperature effects on energy transport. Nanoscale energy transport has tremendous applications in materials synthesis. The work by Deng et al. [27] reports the microstructure and superior corrosion resistance of NiTi-based intermetallic coatings. Such coating synthesis uses laser melting deposition, which is a strong energy transport-controlled process. The work by Nunes et al. reported detailed study of electrochemical behavior related to charge transport in doublelayer capacitors and pseudocapacitors [28]. An understanding of the physics of such transport phenomenon is critical to the design and optimization of capacitor performance.

Energy transport at the micro/nanoscale is still a very active research area, and current research is very diverse, including study of detailed phonon dynamics, structure, and behaviors, material design to either enhance or suppress energy transport, and new technology development to overcome challenges in characterizing special materials [29,30].

**Funding:** This research was funded by the US National Science Foundation (CBET1930866 and CMMI2032464).

**Conflicts of Interest:** The author declares no conflict of interest.
