Tellurium (Te) is an intrinsically p-type doped narrow bandgap semiconductor with excellent electrical conductivity and low thermal conductivity. Bulk trigonal Te has been theoretically predicted and experimentally demonstrated to be an outstanding thermoelectric material with high value of thermoelectric figure-of-merit ZT. In view of the recent progress in developing synthesis route of two-dimensional (2D) tellurium thin films as well as the growing trend of exploiting nanostructures as thermoelectric devices, here for the first time we report excellent thermoelectric performance of tellurium nanofilms, with room temperature power factor of 31.7 μW/cm•K 2 and ZT value of 0.63. To further enhance the efficiency of harvesting thermoelectric power in nanofilm devices, thermoelectrical current mapping was performed with a laser as a heating source, and we found high work function metals such as palladium can form rare accumulationtype metal-to-semiconductor contacts to 2D Te, which allows thermoelectrically generated carriers to be collected more efficiently. High-performance thermoelectric 2D Te devices have broad applications as energy harvesting devices or nanoscale Peltier coolers in microsystems.Thermoelectricity emerges as one of the most promising solutions to the energy crisis we are facing in 21 st century. It generates electricity by harvesting thermal energy from ambient or wasted heat, which is a sustainable and environmental-friendly route compared to consuming fossil fuels 1,2 . The efficiency of converting heat to electricity is evaluated by the key thermoelectrical figure of merit:= 2 , where is the Seebeck coefficient defined as = , and are electrical and thermal conductivity, V is the measured thermal voltage, and is the operating temperature. However the ZT value has not been significantly enhanced since 1960's 3 and so far the most state-of-the-art bulk materials can merely surpass 1 at room temperature 4,5 . This is because the parameters in defining ZT, Seebeck coefficient, electrical conductivity and thermal conductivity are usually correlated through the Wiedemann-Franz law 6-8 and by engineering one parameter, generally other parameters will compensate the change, which poses a dilemma for drastically improving ZT.For the past decades enormous efforts have been made to increase thermoelectric efficiency along two major pathways: either by developing new high-efficiency thermoelectric bulk materials, or by developing novel nano-structured thermoelectric materials 9 . From material perspective, the paradigm of an excellent thermoelectric material should be a heavily doped narrow-bandgap semiconductor with good conductivity meanwhile because of the existence of a finite bandgap, the separation of electrons and holes can avoid opposite contributions to the Seebeck coefficient.Also, heavy elements are preferred for thermoelectrical applications since they can enhance the ZT values by providing more effective phonon scattering centers and reducing the thermal conductivity 1,10 . Furthermore, vall...
Two-dimensional tellurium (2D-Te) has been recently synthesized and shown potential in electronics, optoelectronics, and thermoelectric applications, with the merits of high mobility, environmental stability, high thermoelectric power-factor, and simplicity of mass production. These 2D-Te films have unique atomic structures: the Te atoms form trigonal helical chains and are then stacked into hexagonal lattice by van der Waals force, which brings up distinctive transport behaviors. Here we report anisotropic thermal conductivity of suspended 2D-Te films measured by micro-Raman thermometry and the time-domain thermal reflectance (TDTR) method. The in-plane along-chain and cross-chain thermal conductivities are found to be around 2.5 and 1.7 W m −1 K −1 , respectively, for thicker films (>100 nm), and reduced to 1.6 and 0.64 W m −1 K −1 for the thinner films (<20 nm). The measured anisotropy is >1.3 for all the films studied. The cross-plane (also across-chain) thermal conductivity is found to be around 0.8 to 1.2 W m −1 K −1 for thicker films, slightly lower than that along the in-plane across-chain direction due to the stronger suppression by the thin film boundary. Theoretical modeling reveals that the anisotropy mainly originates from anisotropic phonon dispersion. The long mean-free-path phonons in Te are also shown to be strongly suppressed by boundary scattering. The large reduction of anisotropic thermal conductivity from the bulk makes it the best single-element thermoelectric material and enables potential thermoelectric generation or cooling devices at room temperature. Our results also provide critical information for thermal management of 2D-Te electronic devices.
Sn-based halide perovskites are promising for thermoelectric (TE) device applications because of their high electrical conductivity as well as the low thermal conductivity associated with their soft lattices. However, conventional three-dimensional Sn-based perovskites are not stable under typical TE device operating conditions. Here, we report a stable two-dimensional Sn-based perovskite for thermoelectric energy conversion by incorporating bulky conjugated ligands. We demonstrate a thin film with a large power factor of 5.42 ± 3.07 (average) and 7.07 (champion) μW m–1 K–2 at 343 K with an electrical conductivity of 5.07 S cm–1 and a Seebeck coefficient of 118.1 μV K–1. Importantly, these thin films show excellent operational stability (i.e., for over 100 h) at 313 K. This work suggests that the novel hybrid two-dimensional perovskites are a promising platform for thermoelectric energy conversion applications.
Radiation greatly exceeding blackbody between two objects separated by microscale distances has attracted great interest. However, challenges in reaching such a small separation between two plates have so far prevented studies below a separation distance of about 25 nm. Here, we report a study of radiation enhancement in the near-field regime of less than 10 nm between two parallel plates. We make use of bulk, rigid plates to approach small separation distances without the adverse snap-in effect, develop embedded temperature sensors to allow near-zero separation, and employ advanced sensing method to level the plates and approach and maintain small separations. Our findings agree with theoretical predictions between parallel surfaces with separations down to 7 nm where an 18000 times enhancement in radiation between two quartz plates is observed. Our method can also be used to explore heat transfer between other materials and can possibly be extended to smaller separation gaps.
A high resolution spatiotemporal ultrafast pump–probe system is developed to examine the interactions and transport phenomena between the electrical and the lattice thermal subsystems during ultrafast laser–matter interactions. This system incorporates an ultrafast pump–probe scheme with a stationary probe beam that interrogates the response to a spatial scanning pump beam, providing a full spatiotemporal mapping of a material’s response due to an ultrafast pump excitation. The material’s response, which is highly sensitive to its transport properties, is measured with a high spatial accuracy of up to ±10 nm and subpicosecond time resolution. Details of achieving this high spatial accuracy are described, and a study of the ultrafast transport processes in thin film gold is demonstrated. With the aid of transport and optical response models, the electrical thermal transport properties of gold and the electron–lattice coupling constant are simultaneously determined.
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