The reliable production of two-dimensional crystals is essential for the development of new technologies based on 2D materials. However, current synthesis methods suffer from a variety of drawbacks, including limitations in crystal size and stability. Here, we report the fabrication of large-area, high-quality 2D tellurium (tellurene) using a substrate-free solution process. Our approach can create crystals with a process-tunable thickness, from monolayer to tens of nanometres, and with lateral sizes of up to 100 µm. The chiral-chain van der Waals structure of tellurene gives rise to strong in-plane anisotropic properties and large thicknessdependent shifts in Raman vibrational modes, which is not observed in other 2D layered materials. We also fabricate tellurene field-effect transistors, which exhibit air-stable performance at room temperature for over two months, on/off ratios on the order of 10 6 and field-effect mobilities of around 700 cm 2 /Vs. Furthermore, by scaling down the channel length and integrating with high-k dielectrics, transistors with a significant on-state current density of 1 A/mm are demonstrated. MainThe continuing development of two-dimensional materials, be it the exploration of new science 1-3 or the implementation of new technologies 4-8 , requires reliable methods of synthesising 2D crystals. Whether current approaches can be scaled up though remains uncertain 9,10 and are restricted by factors such as growth substrates and conditions 11-13 , small crystal sizes 14 and the instability of the synthesized materials 11,15,16 .

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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.

Topological insulators (TI) have attracted extensive research effort due to their insulating bulk states but conducting surface states. However, investigation and understanding of thermal transport in topological insulators, particularly the effect of surface states, are lacking. In this work, we studied thickness-dependent in-plane thermal and electrical conductivity of BiTeSe TI thin films. A large enhancement in both thermal and electrical conductivity was observed for films with thicknesses below 20 nm, which is attributed to the surface states and bulk-insulating nature of these films. Moreover, a surface Lorenz number much larger than the Sommerfeld value was found. Systematic transport measurements indicated that the Fermi surface is located near the charge neutrality point (CNP) when the film thickness is below 20 nm. Possible reasons for the large Lorenz number include electrical and thermal current decoupling in the surface state Dirac fluid, and bipolar diffusion transport. A simple computational model indicates that the surface states and bipolar diffusion indeed can lead to enhanced electrical and thermal transport and a large Lorenz number.