Staff directory Roberta Farris



  • Unraveling Heat Transport and Dissipation in Suspended MoSe2 from Bulk to Monolayer

    Saleta Reig D., Varghese S., Farris R., Block A., Mehew J.D., Hellman O., Woźniak P., Sledzinska M., El Sachat A., Chávez-Ángel E., Valenzuela S.O., van Hulst N.F., Ordejón P., Zanolli Z., Sotomayor Torres C.M., Verstraete M.J., Tielrooij K.-J. Advanced Materials; 34 (10, 2108352) 2022. 10.1002/adma.202108352. IF: 30.849

    Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood, in particular how transport is affected by material thickness and the material's environment. This combined experimental–theoretical study establishes a unifying physical picture of the intrinsic lattice thermal conductivity of the representative TMD MoSe2. Thermal conductivity measurements using Raman thermometry on a large set of clean, crystalline, suspended crystals with systematically varied thickness are combined with ab initio simulations with phonons at finite temperature. The results show that phonon dispersions and lifetimes change strongly with thickness, yet the thinnest TMD films exhibit an in-plane thermal conductivity that is only marginally smaller than that of bulk crystals. This is the result of compensating phonon contributions, in particular heat-carrying modes around ≈0.1 THz in (sub)nanometer thin films, with a surprisingly long mean free path of several micrometers. This behavior arises directly from the layered nature of the material. Furthermore, out-of-plane heat dissipation to air molecules is remarkably efficient, in particular for the thinnest crystals, increasing the apparent thermal conductivity of monolayer MoSe2 by an order of magnitude. These results are crucial for the design of (flexible) TMD-based (opto-)electronic applications. © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH


  • Giant thermoelectric figure of merit in multivalley high-complexity-factor LaSO

    Farris R., Ricci F., Casu G., Dahliah D., Hautier G., Rignanese G.-M., Fiorentini V. Physical Review Materials; 5 (12, 125406) 2021. 10.1103/PhysRevMaterials.5.125406. IF: 3.989

    We report a giant thermoelectric figure of merit ZT (up to six at 1100 K) in n-doped lanthanum oxysulphate LaSO. Thermoelectric coefficients are computed from ab initio bands within Bloch-Boltzmann theory in an energy-, chemical potential-, and temperature-dependent relaxation time approximation. The lattice thermal conductivity is estimated from a model employing the ab initio phonon and Grüneisen-parameter spectrum. The main source of the large ZT is the significant power factor which correlates with a large band complexity factor. We also suggest a possible n-type dopant for the material based on ab initio calculations. © 2021 American Physical Society.