9 research outputs found
Hot carrier optoelectronic phenomena in van der Waals heterostructures
No full textTwo-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) exhibit fascinating optical properties including strong light-matter coupling, prominent excitonic effects, and valley-selective circular dichroism. These properties render TMDs as a potential component for advanced optoelectronics devices. One of the most remarkable phenomena of 2D TMDs is the ultrafast charge transfer of photocarriers across van der Waals (vdW) interfaces, which potentially allows realization of hot carrier optoelectronic devices. Hot photocarrier and exciton dynamics in 2D TMDs depend on intrinsic and extrinsic factors such as many-body effects and interface electronic states. Thus, the prospects for exploiting the photophysical properties of 2D TMDs and their heterostructures for hot carrier optoelectronic devices remain largely unexplored. In this thesis, experimental studies on the photophysics and optoelectronic device properties of 2D TMD-based vdW heterostructures are presented. The first part of this thesis discusses an unconventional electro-optic light upconverter. It is shown that ultrafast interlayer transfer of photoexcited carriers can be electrically induced, allowing upconversion of relatively weak continuous wave laser light. The second part reports on efficient exciton-exciton annihilation (EEA) process in monolayer TMDs. We discover that encapsulation of TMD within hexagonal boron nitride (hBN) films strongly suppresses EEA. The last part reports on unusually robust photocurrent response in a metal-insulator-semiconductor (MIS) heterostructure. It is shown that generation of high energy free carriers via efficient EEA plays a fundamental role in the observed unusual photoresponse of the heterostructures.Open Acces
Electro‐Optic Upconversion in van der Waals Heterostructures via Nonequilibrium Photocarrier Tunneling
No full text
Adsorption of hyaluronic acid on solid supports: Role of pH and surface chemistry in thin film self-assembly
No full textOwing to its biocompatibility, resistance to biofouling, and desirable physicochemical and biological properties, hyaluronic acid (HA) has been widely used to modify the surface of various materials. The role of various physicochemical factors in HA adsorption remains, however, to be clarified. Herein, we employed quartz crystal microbalance with dissipation (QCM-D) in order to investigate HA adsorption at different pH conditions onto three substrates—silicon oxide, amine-terminated self-assembled monolayer (SAM) on gold, and carboxylic acid-terminated SAM on gold. The QCM-D experiments indicated specific pH conditions where either strong or weak HA adsorption occurs. The morphology of the adsorbed HA layers was investigated by atomic force microscopy (AFM), and we identified that strong HA adsorption produced a complete, homogenous and smooth HA layer, while weak HA adsorption resulted in rough and inhomogeneous HA layers. The observed specifics of the kinetics of HA adsorption, including a short initial linear phase and subsequent long non-linear phase, were described by using a mean-field kinetic model taking HA diffusion limitations and reconfiguration in the adsorbed state into account. The findings extend the physicochemical background of design strategies for improving the use of passive HA adsorption for surface modification applications.NRF (Natl Research Foundation, S’pore)NMRC (Natl Medical Research Council, S’pore
Nanoplasmonic ruler to measure lipid vesicle deformation
No full textA nanoplasmonic ruler method is presented in order to measure the deformation of adsorbed, nm-scale lipid vesicles on solid supports. It is demonstrated that single adsorbed vesicles undergo greater deformation on silicon oxide over titanium oxide, offering direct experimental evidence to support membrane tension-based theoretical models of supported lipid bilayer formation.NMRC (Natl Medical Research Council, S’pore)Published versio
Influence of pH and Surface Chemistry on Poly(l‑lysine) Adsorption onto Solid Supports Investigated by Quartz Crystal Microbalance with Dissipation Monitoring
No full textPoly(l-lysine) (PLL)
adsorption onto various materials
has been widely applied as a surface modification strategy and layer-by-layer
fabrication method. Considering the role of electrostatic charges,
a detailed understanding of the influence of solution pH on PLL adsorption
process is important for optimization of PLL coating protocols. Herein,
PLL adsorption onto different polar and hydrophilic substratessilica,
an amine-terminated self-assembled monolayer (SAM) on gold, and a
carboxyl-terminated SAM on goldacross a range of pH conditions
was investigated using the quartz crystal microbalance with dissipation.
The adsorption kinetics consisted of an initial rapid phase, followed
by a second phase where adsorption rate gradually decelerated. These
features were interpreted by applying a mean-field kinetic model implying
diffusion-limited adsorption in the first phase and reconfiguration
of adsorbed PLL molecules in the second phase. The adsorption kinetics
and uptake were found to be sensitive to the pH condition, surface
chemistry, and flow rate. The strongest PLL adsorption occurred at
pH 11 on all three surfaces while weak PLL adsorption generally occurred
under acidic conditions. The surface morphology and roughness of adsorbed
PLL layers were investigated using atomic force microscopy, and strong
PLL adsorption is found to produce a uniform and smooth adlayer while
weak adsorption formed a nonuniform and rough adlayer
Harnessing Exciton–Exciton Annihilation in Two-Dimensional Semiconductors
No full textStrong many-body
interactions in two-dimensional (2D) semiconductors
give rise to efficient exciton–exciton annihilation (EEA).
This process is expected to result in the generation of unbound high
energy carriers. Here, we report an unconventional photoresponse of
van der Waals heterostructure devices resulting from efficient EEA.
Our heterostructures, which consist of monolayer transition metal
dichalcogenide (TMD), hexagonal boron nitride (hBN), and few-layer
graphene, exhibit photocurrent when photoexcited carriers possess
sufficient energy to overcome the high energy barrier of hBN. Interestingly,
we find that the device exhibits moderate photocurrent quantum efficiency
even when the semiconducting TMD layer is excited at its ground exciton
resonance despite the high exciton binding energy and large transport
barrier. Using ab initio calculations, we show that EEA yields highly
energetic electrons and holes with unevenly distributed energies depending
on the scattering condition. Our findings highlight the dominant role
of EEA in determining the photoresponse of 2D semiconductor optoelectronic
devices
Plasmonic Nanohole Sensor for Capturing Single Virus‐Like Particles toward Virucidal Drug Evaluation
No full textA plasmonic nanohole sensor for virus-like particle capture and virucidal drug evaluation is reported. Using a materials-selective surface functionalization scheme, passive immobilization of virus-like particles only within the nanoholes is achieved. The findings demonstrate that a low surface coverage of particles only inside the functionalized nanoholes significantly improves nanoplasmonic sensing performance over conventional nanohole arrays.NRF (Natl Research Foundation, S’pore)NMRC (Natl Medical Research Council, S’pore
