Meng Lee Leek - Academia.edu (original) (raw)

Papers by Meng Lee Leek

IOP Publishing eBooks, Nov 30, 2023

Nature communications, Jan 11, 2018

The original version of this Article omitted an affiliation of Cheng Hon Alfred Huan: 'Instit... more The original version of this Article omitted an affiliation of Cheng Hon Alfred Huan: 'Institute of Materials Research and Engineering (IMRE), A*STAR, 2 Fusionopolis Way, Innovis #08-03 138634, Singapore'.This has now been corrected in both the PDF and HTML versions of the article.

Nature communications, Nov 3, 2017

Halide perovskites exhibit unique slow hot-carrier cooling properties capable of unlocking disrup... more Halide perovskites exhibit unique slow hot-carrier cooling properties capable of unlocking disruptive perovskite photon-electron conversion technologies (e.g., high-efficiency hot-carrier photovoltaics, photo-catalysis, and photodetectors). Presently, the origins and mechanisms of this retardation remain highly contentious (e.g., large polarons, hot-phonon bottleneck, acoustical-optical phonon upconversion etc.). Here, we investigate the fluence-dependent hot-carrier dynamics in methylammonium lead triiodide using transient absorption spectroscopy, and correlate with theoretical modeling and first-principles calculations. At moderate carrier concentrations (around 1018 cm-3), carrier cooling is mediated by polar Fröhlich electron-phonon interactions through zone-center delayed longitudinal optical phonon emissions (i.e., with phonon lifetime τ LO around 0.6 ± 0.1 ps) induced by the hot-phonon bottleneck. The hot-phonon effect arises from the suppression of the Klemens relaxation pat...

Applied Surface Science

Ti1-xSnxO2 nanofilms : layer-by-layer deposition with extended Sn solubility and characterization.

Optics express, Jan 10, 2015

Despite the concerted efforts to directly probe the electron-electron (e-e) scattering mediated r... more Despite the concerted efforts to directly probe the electron-electron (e-e) scattering mediated relaxation process in graphene using transient absorption spectroscopy, the initial sub-10 fs photoexcited carrier relaxation dynamics has remained elusive. Herein, we utilize a simple z-scan approach to elucidate this process and discern its mechanisms in CVD grown single layer graphene using femtosecond laser pulses with temporal pulse widths far longer than the relaxation time. We report the first experimental observation of e-e scattering lifetime shortening with increasing fluence, which had been theoretically predicted. Analysis from two-body Coulombic scattering suggests that Auger processes are essential relaxation channels in single layer graphene. Importantly, our straightforward approach on the graphene model system is applicable to the family of emergent layered materials.

Physical Review B, 2011

We report the coherent potential approximation method of treating quantum thermal transport prope... more We report the coherent potential approximation method of treating quantum thermal transport properties of nanoscale systems with mass disorder. Instead of massive efforts required in brute-force calculations, configuration averaging of disordered systems can be efficiently handled in a self-consistent manner by setting up the phonon version of nonequilibrium vertex correction theory. The accuracy of the method is verified by comparing with the exact results and Monte Carlo experiments in one-dimensional atomic chains. Results obtained for disordered harmonic chains and carbon nanotubes provide evidence of anomalous thermal transport in such systems. We also observe crossover in the transport where phonon scattering by disorder becomes important. Our results show that disorder plays a role in thermal conductance reduction.

In this thesis we showed that Non-equilibrium Green's Function Perturbation Theory (NEGF) is ... more In this thesis we showed that Non-equilibrium Green's Function Perturbation Theory (NEGF) is really the overarching perturbative transport theory. This is shown in great detail by using NEGF as a starting point and developing in 3 directions to obtain the usual transport-related expressions. The 3 directions are: Landauer-like theory, kinetic theory and Green-Kubo linear response theory. This thesis is concerned with using NEGF to generalize the 2 directions of Landauer-like theory and the kinetic theory. Firstly, NEGF is used to derive phonon-phonon Hedin-like functional derivative equations which generates conserving self energy approximations for phonon-phonon interaction. Secondly, for the Landauer-like theory, using the perturbation expansion, we easily obtain anharmonic (or phonon-phonon interaction) corrections to the ballistic energy current and to the noise associated to the energy current. The lowest 3-phonon interaction, the lowest and the second lowest 4-phonon inter...

Acta Materialia

Abstract The interplay among spin, lattice, charge and orbit is of central importance for several... more Abstract The interplay among spin, lattice, charge and orbit is of central importance for several rich and fascinating properties of oxides, and is the subject of intense research at present. Here, we present an approach to manipulate this interplay by Sn doping to effectively apply uniaxial strain on the TiO2 lattice. The evolution of this interplay in pseudo-homoepitaxial Ti1-xSnxO2 films is measured using a combination of X-ray absorption near edge spectroscopy at the O K and Ti L3,2-edges. Supported by various theoretical calculations, we find that the multiplet-type electronic correlations, long-range bonding and hybridization in the system can be controlled by independently modifying uniaxial strain, thereby allowing us to establish the correlations among these effects, doping concentration, and strain. This significantly widens the phase space for experimental exploration of predictive models and leads to new possibilities for manipulation over materials’ functional properties. The methodology presented here can be applied in general to study the nature of the multiplet-type electronic correlations and bonding properties in octahedral-coordinated 3dN transition metal oxides.