# Energy and Charge Transport in 2D Atomic Layer Materials: Raman-Based Characterization
## 1 Introduction  
Owing to their extraordinary electrical, optical, and mechanical properties, two-dimensional (2D) materials, for instance, graphene, black phosphorus, hexagonal boron nitride, transition metal dichalcogenides (TMDs), and MXenes, can be used in many different areas, for instance, optoelectronic devices, biosensing, energy storage, etc. [1; 2; 3; 4; 5; 6; 7]. Furthermore, the thermal transport, interface thermal transport, electronic structures, physical structures, and hot carrier transport properties of these 2D materials are of great importance in fundamental research, as well as in engineering applications [8]. From the perspective of fundamental research, it is of great importance to explore the physics behind thermal dissipation and thermal management at the micro/nanoscale. For application, with the rapid increase in power density in modern electronics, the heat accumulation becomes a bottleneck for further miniaturization. As the heat accumulation in electronic and optoelectronic devices raises the operating temperature, the device performance and lifetime can be influenced. Thus, it is in high demand to improve the heat conductance and reduce the interface thermal resistance [8; 9].  
Over the last few years, many simulation-based methods have been reported to characterize the thermal transport in 2D materials, such as molecular dynamics simulation, non-equilibrium Green function method, the Boltzmann transport equation, and the first-principles-based multi-temperature model [10; 11; 12; 13]. Many experimental methods, such as time-domain thermoreflectance (TDTR), the microbridge method, the 3\(\omega\) method, the laser flash technique, and Raman spectroscopy, are employed to explore the thermal properties of 2D materials [14; 15; 16; 17]. For TDTR, it requires complicated setups and careful operation. The accuracy of the microbridge method is affected by thethermal contact resistance between the sample and contact, and by the difficulty in evaluating the tiny heat flow sustained by very thin samples. The 3\(\omega\) method is vulnerable to the harmonic noises in the current source. The laser flash technique will become extremely difficult to use for measuring very thin samples (a few \(\upmu\)m) and for measurement at cryogenic temperatures. Raman spectroscopy, which carries signature information about materials regardless of their distance and size, provides a unique way of looking into the energy transport, hot carrier diffusion, and physical structure of 2D materials. As a noncontact optical method, Raman-based thermometry is able to realize precise (material specific) and specific thermal properties characterization of 2D materials with sub-micron size by focusing the excitation laser to a very small spot.  
Currently, many novel and new Raman-based techniques have been developed to meet different requirements of 2D materials measurement. For instance, different energy transport states are constructed to study the thermal conductivity, hot carrier diffusion, and interface thermal resistance of suspended or supported 2D materials. In the following sections, a comprehensive critical review of various Raman-based techniques developed for energy and charge transport in 2D materials is presented to give a clear picture of the progress in this field. In addition, potential research perspectives in the field of 2D materials using these Raman-based techniques are also discussed.