arXiv subjects

Alistair Rowe

Publications and source records attributed to Alistair Rowe.

7 recordsLinked to original sources

arXiv2024

High Strain Engineering of a Suspended WSSe Monolayer Membrane by Indentation and Measured by Tip-enhanced Photoluminescence

Straintronics involves the manipulation and regulation of the electronic characteristics of 2D materials through the use of macro- and nano-scale strain engineering. In this study, we utilized an atomic force microscope (AFM) coupled with an optical system to perform indentation measurements and tip-enhanced photoluminescence (TEPL), allowing us to extract the local optical response of a suspended monolayer membrane of ternary WSSe at various levels of deformation, up to strains of 10%. The photoluminescence signal is modelled considering the deformation, stress distribution and strain dependence of the WSSe band structure. We observe an additional TEPL signal that exhibits significant variation under strain, with 64 meV per percent of elongation. This peak is linked to the highly strained 2D material lying right underneath the tip. We discuss the amplification of the signal and its relation to the excitonic funnelling effect in a more comprehensive model. We will also compare the diffusion caused by Auger recombination against the radiative excitonic decay. We use TEPL to examine and comprehend the local physics of 2D semi-conducting materials subjected to extreme mechanical strain. Chemical vapour deposition-fabricated 2D ternaries possess high strain resistance, comparable to the benchmark MoS2, and a high Young's modulus of 273 GPa.

cond-mat.mes-hall↗

arXiv2021

Imaging Seebeck drift of excitons and trions in MoSe2 monolayers

Hyperspectral imaging at cryogenic temperatures is used to investigate exciton and trion propagation in MoSe$_2$ monolayers encapsulated with hexagonal boron nitride (hBN). Under a tightly focused, continuous-wave laser excitation, the spatial distribution of neutral excitons and charged trions strongly differ at high excitation densities. Remarkably, in this regime the trion distribution develops a halo shape, similar to that previously observed in WS2 monolayers at room temperature and under pulsed excitation. In contrast, the exciton distribution only presents a moderate broadening without the appereance of a halo. Spatially and spectrally resolved luminescence spectra reveal the buildup of a significant temperature gradient at high excitation power, that is attributed to the energy relaxation of photoinduced hot carriers. We show, via a numerical resolution of the transport equations for excitons and trions, that the halo can be interpreted as thermal drift of trions due to a Seebeck term in the particle current. The model shows that the difference between trion and exciton profiles is simply understood in terms of the very different lifetimes of these two quasiparticles.

cond-mat.mtrl-sci↗

arXiv2021

Spin/Valley pumping of resident electrons in WSe2 and WS2 monolayers

Monolayers of transition metal dichalcogenides are ideal materials to control both spin and valley degrees of freedom either electrically or optically. Nevertheless, optical excitation mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here we demonstrate a very efficient spin/valley optical pumping of resident electrons in n-doped WSe2 and WS2 monolayers. We observe that, using a continuous wave laser and appropriate doping and excitation densities, negative trion doublet lines exhibit circular polarization of opposite sign and the photoluminescence intensity of the triplet trion is more than four times larger with circular excitation than with linear excitation. We interpret our results as a consequence of a large dynamic polarization of resident electrons using circular light.

cond-mat.mes-hall↗

arXiv2018

Giant, anomalous piezo-impedance of silicon-on-insulator

A giant, anomalous piezo-response of fully-depleted silicon-on-insulator (FD-SOI) devices under mechanical stress is demonstrated using impedance spectroscopy. This piezo-response strongly depends on the measurement frequency, $ω$, and consists of both a piezoresistance (PZR) and piezocapacitance whose maximum values are $π_R = -1100 \times 10^{-11}$ Pa$^{-1}$ and $π_C = -900 \times 10^{-11}$ Pa$^{-1}$ respectively. These values should be compared with the usual bulk PZR in p-type silicon, $π_R= 70 \times 10^{-11}$ Pa$^{-1}$. The observations are well described using models of space charge limited electron and hole currents in the presence of fast electronic traps having stress-dependent capture ($ω_c$) and emission rates. Under steady-state conditions (i.e. when $ω\ll ω_c$) where the impedance spectroscopy measurements yield results that are directly comparable with previously published reports of PZR in depleted, silicon nano-objects, the overall piezo-response is just the usual, bulk silicon PZR. Anomalous PZR is observed only under non-steady-state conditions when $ω\approx ω_c$, with a symmetry suggesting that the electro-mechanically active fast traps are native Pb$_0$ interface defects. The observations suggest new functionalities for FD-SOI, and shed light on the debate over the PZR of carrier depleted nano-silicon.

cond-mat.mes-hall↗

arXiv2017

Polarizers, optical bridges and Sagnac interferometers for nanoradian polarization rotation measurements

The ability to measure nanoradian polarization rotations, $θ_F$, in the photon shot noise limit is investigated for partially crossed polarizers (PCP), a static Sagnac interferometer and an optical bridge, each of which can in principal be used in this limit with near equivalent figures-of-merit (FOM). In practice a bridge to PCP/Sagnac source noise rejection ratio of $1/4θ_F^2$ enables the bridge to operate in the photon shot noise limit even at high light intensities. The superior performance of the bridge is illustrated via the measurement of a 3 nrad rotation arising from an axial magnetic field of 0.9 nT applied to a terbium gallium garnet. While the Sagnac is functionally equivalent to the PCP in terms of the FOM, unlike the PCP it is able to discriminate between rotations with different time ($T$) and parity ($P$) symmetries. The Sagnac geometry implemented here is similar to that used elsewhere to detect non-reciprocal ($\overline{T}P$) rotations like those due to the Faraday effect. Using a Jones matrix approach, novel Sagnac geometries uniquely sensitive to non-reciprocal $\overline{TP}$ (e.g. magneto-electric or magneto-chiral) rotations, as well as to reciprocal rotations (e.g. due to linear birefringence, $TP$, or to chirality, $T\overline{P}$) are proposed.

physics.optics↗

arXiv2014

VLS-HVPE growth of ultra-long and defect-free GaAs nanowires investigated by ab initio simulation coupled to near-field microscopy

High aspect ratio, rod-like and single crystal phase GaAs nanowires (NWs) were grown by gold catalyst-assisted hydride vapor phase epitaxy (HVPE). High resolution transmission electron microscopy (HRTEM) and micro-Raman spectroscopy revealed polytypism-free zinc blende NWs over lengths of several tens of micrometers for diameters ranging between 50 and 150 nm. Micro-photoluminescence studies of individual NWs showed linewidths smaller than those reported elsewhere which is consistent with the crystalline quality of the NWs. HVPE makes use of chloride growth precursors of which high decomposition frequency, after adsorption onto the catalyst particle, favors a direct and rapid introduction of the Ga atoms from the vapor phase into the catalyst liquid droplet. This yields high axial growth rate (more than 100 micron/h) of NWs. The fast diffusion of the Ga atoms in the droplet towards the interface between the liquid and the solid nanowire was investigated by using density functional theory calculations. The diffusion coefficient of Ga atoms was estimated to be 3x10-9 m2/s, which matches the experimental observations.

cond-mat.mes-hall↗

arXiv2012

Imaging ambipolar diffusion of photocarriers in GaAs thin films

Images of the steady-state luminescence of passivated GaAs self-standing films under excitation by a tightly-focussed laser are analyzed as a function of light excitation power. While unipolar diffusion of photoelectrons is dominant at very low light excitation power, an increased power results in a decrease of the diffusion constant near the center of the image due to the onset of ambipolar diffusion. The results are in agreement with a numerical solution of the diffusion equations and with a physical analysis of the luminescence intensity at the centre of the image, which permits the determination of the ambipolar diffusion constant as a function of electron concentration.

cond-mat.mes-hall↗