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On May 18, the prestige jounal Science publishes the work entitled ‘3D charge and 2D phonon transports leading to high out-of-plane ZT in n-type SnSe crystals’, which is the fruit of a close collabation of Department of Physcs, SUSTech, and Beihang University (previously known as Beijing University of Aeronautics and Astronautics).Other contributors include Tsinghua University, University of Hong Kong and Shanghai Synchrotron Radiation Facility, CAS Shanghai Institute of Applied Physics. SUSTech Physics Professor He Jiaqing and Beihang University Professor Zhao Lidong are the corresponding authors of this paper.
Thermoelectric conversion technology is a clean energy technology that uses semiconductors to make the thermal energy and electeric energy directly interconvertable . The thermoelectric conversion efficiency of the key material is always the bottleneck for further development of this technology. In recent years, SnSe (tin selenide) single crystal is a star in the field of thermoelectric materials because of its environmental friendship, abundance, low price, and excellent performance.In 2016, the workco-authored by Prof. He Jiaqing in the journal “Science” shows thatadjusting the energy band structure enables the regulation of the electric conductivity and thermal voltage of p-type SnSe, thus leading to the breakthough of the thermoelectric performance in the low and moderatetemperature region. Through nearly two years of continuous in-depth effort in this field, breakthrough has been made again on the thermoelectric performance of n-type SnSe.
It was found that the two-dimensional interface of SnSe with a layered structure has a strong scattering effect on the phonons (Fig. 1 left), causing SnSe to have a very low thermal conductivity along the out-of-plane direction, approaching the minimum theoretical value at 773 K, which is ~ 0.18 W/mK. Searching for low thermal conductivity materials or lowering the thermal conductivity is an effective way to improve the ZT value of thermoelectricity in the field of thermoelectricity for a long time. On the basis of the low thermal conductivity between SnSe layers, high thermoelectric performance can be achieved if high electrical transpot property is realised in this direction. By simplifying theWiedemann-Franz and Pisarenko's relationship, the ZT value can be described as:
It is obvious that improving the electric transport property between layers requires optimizing both the carrier mobility (m) and effective mass (m). The SnSe material undergoes a phase transition from Pnma to Cmcm at 800K, in this work by using synchrotron radiation X-ray diffraction and in-situ TEM , it is found that the phase transites continuously from 600K. Through this continuous phase transition and adjusting the electron doping concentration, the material experiences a process of increasing the effective mass and reducing the mobility within the light-conducting band while reducing the effective mass and increasing the mobility within the heavy-conducting band. By makde use of this process, the product of the mobility and the effective mass (in Figure 1) is exactly optimized, so that SnSe maintains high electrical transport performance over the entire temperature range. By comparing electron- and hole-doped n-type and p-type SnSe materials, it was found that the p-orbital of Sn and Se after electron doping generates electron delocalization at the bottom of the conducting band (but not at the top of the valence band ). This phenomenon exists so that the charge density of the n-type SnSe is increased enough to fill the gaps between the layers and tunneling of the electrons between the layers is achieved (FIG. 1 right).
Figure 1. "Two-dimensional phonon/three-dimensional charge" transpots significantly improves the thermoelectric properties of n-type SnSe: interfacial scattering prevents the phonon transport thus leading to ultra-low thermal conductivity ; mobility and effective mass are optimized by continuous phase transformation; large charge density makes electrons (n-type) easier to transport than holes (p-type).
This phenomenon can be simply described as follows: the intrinsic layered structure of SnSe acts like a wall and can simultaneously block the transports of phonons and carriers (electrons and holes). However, after heavy-electron doping, the electronic delocalization at the bottom of the conduction band increases the charge density, and a tunnel for the transport of electrons is tailored through the wall l, as shown in Figure 2. In addition to the large charge density, changes in the band structure and crystal symmetry caused by the continuous phase change are the three main factors make SnSe along the out-of-plane direction exhibit excellent electrical transport performance when the temperature is higher than 700K,even better than the in–plane electric transport. This "two-dimensional phonon/three-dimensional charge" transport property greatly improves the thermoelectric properties of n-type SnSe.
Figure 2. Illustration of Two-dimensional phonon/three-dimensional chargetransports (a) Electrons at the bottom of the conduction band create delocalized hybridization, increasing the charge density, providing channels for electrons to travel between layers, and phonons and holes are blocked by the layer interface; (b) Aircraft (phonons) that are not constrained by orbits are blocked by mountains (layer interfaces), trains (electrons) can cross tunnels, and cars (holes) cannot cross tunnels due to mismatched orbits.
SUSTech’s Physics Department has made important contributions to this work. Assistant Professor Huang Li and Postdoctoral Researcher Wu Minghui provided theoretical calculations and explained the mechanism behind the improvement of the performance. The engineer He Dongsheng used in-situ transmission electron microscopy (TEM) to directly observe the structural changes in materials as temperature increased. Associate Professor Wang Kedong and Postdoctoral Researcher Wu Xuefeng measured the charge density distribution with a scanning tunneling microscope. The work was supported by the in-situ TEM of SUSTech Pico Center, Major Cultivating Project of Guangdong Natural Science Foundation, funding from the Shenzhen Peacock team, and leading talents of the Guangdong “Pearl River Talents Program.”