Speaker: Luo Xin
Affiliation: Centre for Advanced 2D Materials, NUS
Abstract Details: Phonons are important collective excitations that can affect carrier scattering and thermal conductivity. Here, we explore from first principles calculations the properties of phonons in 2D layered materials, and compare our results with experimental Raman spectra. Our calculations explain the physical origin of frequency shifts observed due to changing film thickness,[1-3] or due to adsorbates. We find that the formation of a surface in the 2D material leads to larger interatomic force constants at the surface, which results in experimentally observed anomalous blue-shifts of the E12g mode in MoS2, and the E12g and B12g modes in WSe2. The effect of a substrate on the phonon frequencies will also be discussed. Finally, we also explore the ultra-low frequency interlayer phonon modes that are peculiar to 2D materials, and show that these frequencies can be described well using a nearest neighbor force constant model. We find that the frequencies of the interlayer shear modes blue-shift for AB stacked materials, and red-shift for ABC stacked materials, as the number of layers increases. This prediction is observed in experiments and can be understood from an intuitive bond polarizability model. Our studies shed light on a fundamental understanding of phonons in 2D layered materials and guide experimentalists to use Raman spectroscopy as a sensitive probe of the film thickness, stacking order, as well as environmental effects in 2D materials.
 Nano Lett. 13, 1007 (2013)
 Nano Lett. 15, 3931 (2015)
 Phys. Rev. B 90, 245428 (2014)
 Adv. Electron. Mater. 1400037 (2015)
 Phys. Rev. B 88, 075320 (2013)
 Phys. Rev. B 88, 195313 (2013)
 Scientific reports 5, 14565 (2015)
 Adv. Mater. 27, 4502 (2015)
About the Speaker: Dr. Luo Xin graduated from Sun Yat-sen University where he obtained his bachelor degree in 2006 and doctoral degree in 2011, both in physics. During his PhD, Luo Xin used first principles calculations and non-equilibrium Green functionâ€™s to investigate the strain tunable electroresistance of ferroelectric tunneling junctions and the 2D electron gas between perovskite oxide interfaces. After his graduation, he worked as a research scientist in the Institute of High Performance Computing, A*STAR, Singapore. Since 2013, he moved to the Centre for Advanced 2D materials and Graphene research center in the National University of Singapore as a research fellow. In his research in Singapore Luo Xin collaborates closely with experimentalists using first principles approaches and simple models to make predictions and explain the vibrational and transport properties of 2D layer materials such as Bi2Te3, MoS2 and few layer black phosphorus.