Thermoelectric Performance of Tetrahedrite (Cu12Sb4S13) Thin Films: The Influence of the Substrate and Interlayer.

In the present work, tetrahedrite Cu12Sb4S13 thin films were deposited on various substrates via aerosol-assisted chemical vapor deposition (AACVD) using diethyldithiocarbamate complexes as precursors. A buffer layer of Sb2O3 with a small lattice mismatch to Cu12Sb4S13 was applied to one of the glass substrates to improve the quality of the deposited thin film. The buffer layer increased the coverage of the Cu12Sb4S13 thin film, resulting in improved electrical transport properties. The growth of the Cu12Sb4S13 thin films on the other substrates, including ITO-coated glass, a SiO2-coated Si wafer, and mica, was also investigated. Compared to the films grown on the other substrates, the Cu12Sb4S13 thin film deposited on the SiO2-coated Si wafer showed a dense and compact microstructure and a larger grain size (qualities that are beneficial for carrier transport), yielding a champion power factor (PF) of ∼362 µW cm-1 K-2 at 625 K. The choice of substrate strongly influenced the composition, microstructure, and electrical transport properties of the deposited Cu12Sb4S13 thin film. At 460 K, the highest zT value that was obtained for the thin films was ∼0.18. This is comparable to values reported for Cu-Sb-S bulk materials at the same temperature. Cu12Sb4S13 thin films deposited using AACVD are promising for thermoelectric applications. To the best of our knowledge, the first full thermoelectric characterization of the Cu12Sb4S13 thin film is performed in this work.

Exceptional Thermoelectric Performance of Cu2(Zn,Fe,Cd)SnS4 Thin Films.

High-quality Cu2(Zn,Fe,Cd)SnS4 (CZFCTS) thin films based on the parent CZTS were prepared by aerosol-assisted chemical vapor deposition (AACVD). Substitution of Zn by Fe and Cd significantly improved the electrical transport properties, and monophasic CZFCTS thin films exhibited a maximum power factor (PF) of ∼0.22 µW cm-1 K-2 at 575 K. The quality and performance of the CZFCTS thin films were further improved by postdeposition annealing. CZFCTS thin films annealed for 24 h showed a significantly enhanced maximum PF of ∼2.4 µW cm-1 K-2 at 575 K. This is higher than all reported values for single-phase quaternary sulfide (Cu2BSnS4, B = Mn, Fe, Co, Ni) thin films and even exceeds the PF for most polycrystalline bulk materials of these sulfides. Density functional theory (DFT) calculations were performed to understand the impact of Cd and Fe substitution on the electronic properties of CZTS. It was predicted that CZFCTS would have a smaller band gap than CZTS and a higher density of states (DoS) near the Fermi level. The thermal conductivity and thermoelectric figure of merit (zT) of the CZFCTS thin films have been evaluated, yielding an estimated maximum zT range of 0.18-0.69 at 550 K. The simple processing route and improved thermoelectric performance make CZFCTS thin films extremely promising for thermoelectric energy generation.

Ultimate Charge Transport Regimes in Doping-Controlled Graphene Laminates: Phonon-Assisted Processes Revealed by the Linear Magnetoresistance.

Understanding and controlling the electrical properties of solution-processed 2D materials is key to further printed electronics progress. Here, we demonstrate that the thermolysis of the aromatic intercalants utilized in nanosheet exfoliation for graphene laminates allows for high intrinsic mobility and the simultaneous control of doping type (n- and p-) and concentration over a wide range. We establish that the intraflake mobility is high by observing a linear magnetoresistance of such solution-processed graphene laminates and using it to devolve the interflake tunneling and intralayer magnetotransport. Consequently, we determine the temperature dependencies of the inter- and intralayer characteristics. The intraflake transport appears to be dominated by electron-phonon scattering processes at temperatures T > 20 K, while the interflake transport is governed by phonon-assisted tunneling. In particular, we identify the efficiency of phonon-assisted tunneling as the main limiting factor for electrical conductivity in graphene laminates at room temperature. We also demonstrate a thermoelectric sensitivity of around 50 µV·K-1 in a solution-processed metal-free graphene-based thermocouple.

Tunnel junctions based on interfacial two dimensional ferroelectrics.

Van der Waals heterostructures have opened new opportunities to develop atomically thin (opto)electronic devices with a wide range of functionalities. The recent focus on manipulating the interlayer twist angle has led to the observation of out-of-plane room temperature ferroelectricity in twisted rhombohedral bilayers of transition metal dichalcogenides. Here we explore the switching behaviour of sliding ferroelectricity using scanning probe microscopy domain mapping and tunnelling transport measurements. We observe well-pronounced ambipolar switching behaviour in ferroelectric tunnelling junctions with composite ferroelectric/non-polar insulator barriers and support our experimental results with complementary theoretical modelling. Furthermore, we show that the switching behaviour is strongly influenced by the underlying domain structure, allowing the fabrication of diverse ferroelectric tunnelling junction devices with various functionalities. We show that to observe the polarisation reversal, at least one partial dislocation must be present in the device area. This behaviour is drastically different from that of conventional ferroelectric materials, and its understanding is an important milestone for the future development of optoelectronic devices based on sliding ferroelectricity.