Chi-Te Liang

Ultrahigh-Gain Photodetectors Based on Atomically Thin Graphene-MoS2 Heterostructures

Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic graphene is attractive for optoelectronics. Another two-dimensional semiconducting material molybdenum disulfide (MoS2) is also known as light- sensitive. Here we show that a large-area and continuous MoS2 monolayer is achievable using a CVD method and graphene is transferable onto MoS2. We demonstrate that a photodetector based on the graphene/MoS2 heterostructure is able to provide a high photogain greater than 108. Our experiments show that the electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently separated across the layers. Contradictory to the expectation based on the conventional built-in electric field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the presence of a perpendicular effective electric field caused by the combination of the built-in electric field, the applied electrostatic field, and charged impurities or adsorbates, resulting in a tuneable photoresponsivity.

Nitrogen-Doped Graphene Sheets Grown by Chemical Vapor Deposition: Synthesis and Influence of Nitrogen Impurities on Carrier Transport

A significant advance toward achieving practical applications of graphene as a two-dimensional material in nanoelectronics would be provided by successful synthesis of both n-type and p-type doped graphene. However, reliable doping and a thorough understanding of carrier transport in the presence of charged impurities governed by ionized donors or acceptors in the graphene lattice are still lacking. Here we report experimental realization of few-layer nitrogen-doped (N-doped) graphene sheets by chemical vapor deposition of organic molecule 1,3,5-triazine on Cu metal catalyst. When reducing the growth temperature, the atomic percentage of nitrogen doping is raised from 2.1% to 5.6%. With increasing doping concentration, N-doped graphene sheet exhibits a crossover from p-type to n-type behavior accompanied by a strong enhancement of electron-hole transport asymmetry, manifesting the influence of incorporated nitrogen impurities. In addition, by analyzing the data of X-ray photoelectron spectroscopy, Raman spectroscopy, and electrical measurements, we show that pyridinic and pyrrolic N impurities play an important role in determining the transport behavior of carriers in our N-doped graphene sheets.

Synthesis of graphene-ZnO-Au nanocomposites for efficient photocatalytic reduction of nitrobenzene.

A simple hydrothermal method of preparing highly photocatalytic graphene-ZnO-Au nanocomposites (G-ZnO-Au NCs) has been developed. Zinc acetate and graphene oxide are reduced by catechin to form graphene-zinc oxide nanospheres (G-ZnO NSs; average diameter of (45.3 ± 3.7) nm) in the presence of ethylenediamine (EDA) as a stabilizing agent and gold nanorods (Au NRs) at 300 °C for 2 h. Then Au NRs are deposited onto as-formed G-ZnO NSs to form G-ZnO-Au NCs. Upon ultraviolet light activation, G-ZnO-Au NCs (4 mg mL(-1)) in methanol generates electron-hole pairs. Methanol (hydroxyl group) assists in trapping holes, enabling photogenerated electrons to catalyze reduction of nitrobenzene (NB) to aniline with a yield of 97.8% during a reaction course of 140 min. The efficiency of G-ZnO-Au NCs is 3.5- and 4.5-fold higher than those provided by commercial TiO2 and ZnO NSs, respectively. Surface assisted laser desorption/ionization mass spectrometry has been for the first time applied to detect the intermediates (nitrosobenzene and phenylhydroxylamine) and major product (aniline) of NB through photoelectrocatalytic or photocatalytic reactions. The result reveals that the reduction of NB to aniline is through nitrosobenzene to phenylhydroxylamine in the photoelectrocatalytic reaction, while via nitrosobenzene directly in the photocatalytic reaction. G-ZnO-Au NC photocatalyst holds great potential in removal of organic pollutants like NB and in the production of aniline.

Intermixing-seeded growth for high-performance planar heterojunction perovskite solar cells assisted by precursor-capped nanoparticles

This work proposes a novel approach to modulate the nucleation and growth of perovskite crystals in planar perovskite (CH3NH3PbI3−xClx) solar cells by intermixing precursor-capped inorganic nanoparticles of PbS. A small amount of dispersed PbS nanoparticles which were covered with perovskite precursor molecules of methylammonium iodide (CH3NH3I, MAI) through the ligand-exchange treatment functioned as effective seed-like nucleation sites to promote the formation of perovskite lattice structures. Through this intermixing-seeded growth technique, substantial morphological improvements, such as increased crystal domains, enhanced coverage, and uniformity, were realized in the perovskite thin films, and the corresponding solar cell devices exhibited a promising power conversion efficiency of 17.4%, showing an enhancement of approximately 25% compared to that of the controlled pristine solar cell device. The substantially enhanced crystal orientation, particularly along the direction perpendicular to the substrate, was evident from the synchrotron-based grazing incidence wide-angle X-ray scattering data. This observation was consistent with the enhanced carrier diffusion lengths and excellent reproducibility of high fill factors of the planar heterojunction perovskite devices fabricated through the proposed technique.

Mechanism of giant enhancement of light emission from Au/CdSe nanocomposites

Based on the enhanced electron–hole recombination rate generated by surface plasmon (SP) waves of Au nanoparticles (NPs) and electrons transferred from CdSe quantum dots (QDs) to Au NPs, we propose a mechanism to elucidate the luminescent behavior in Au and CdSe nanocomposites. With our proposed model, the enhancement of the spectrally integrated PL intensity can be manipulated by up to a factor of ~33, the largest value ever reported. Our study can be used to clarify the ambiguity in controlling the light emission enhancement and quenching of semiconductor nanocrystals coupled with the SP waves of metal NPs. It should be very useful for the creation of highly efficient solid-state emitters.

Extrinsic Origin of Persistent Photoconductivity in Monolayer MoS2 Field Effect Transistors

Recent discoveries of the photoresponse of molybdenum disulfide (MoS2) have shown the considerable potential of these two-dimensional transition metal dichalcogenides for optoelectronic applications. Among the various types of photoresponses of MoS2, persistent photoconductivity (PPC) at different levels has been reported. However, a detailed study of the PPC effect and its mechanism in MoS2 is still not available, despite the importance of this effect on the photoresponse of the material. Here, we present a systematic study of the PPC effect in monolayer MoS2 and conclude that the effect can be attributed to random localized potential fluctuations in the devices. Notably, the potential fluctuations originate from extrinsic sources based on the substrate effect of the PPC. Moreover, we point out a correlation between the PPC effect in MoS2 and the percolation transport behavior of MoS2. We demonstrate a unique and efficient means of controlling the PPC effect in monolayer MoS2, which may offer novel functionalities for MoS2-based optoelectronic applications in the future.

Plant leaf-derived graphene quantum dots and applications for white LEDs

Graphene quantum dots (GQDs) have been prepared for the first time using raw plant leaf extracts of Neem (Azadirachta indica) and Fenugreek (Trigonella foenum-graecum) by a facile, hydrothermal method at 300 °C for 8 hours in water, without the need of any passivizing, reducing agents or organic solvents. High resolution transmission electron microscope studies showed that the average sizes of the GQDs from Neem (N-GQDs) and Fenugreek (F-GQDs) were 5 and 7 nm respectively. N-GQDs and F-GQDs exhibit high quantum yields of 41.2% and 38.9% respectively. Moreover, the GQDs were utilized to prepare a white light converting cap based on the red-green-blue (RGB) color mixing method.

Strong luminescence from strain relaxed InGaN/GaN nanotips for highly efficient light emitters

Semiconductor heterostructures represent the most important building block for current optoelectronic devices. One of the common features of semiconductor heterostructures is the existence of internal strain due to lattice mismatch. The internal strain can tilt the band alignment and significantly alter the physical properties of semiconductor heterostructures, such as reducing the internal quantum efficiency of a light emitter. Here, we provide a convenient route to release the internal strain by patterning semiconductor heterostructures into nanotip arrays. The fabrication of the nanotip arrays was achieved by self-masked dry etching technique, which is simple, low cost and compatible with current semiconductor technologies. By implementing our approach to InGaN/GaN multiple quantum wells, we demonstrate that the light emission can be enhanced by up to 10 times. Our approach renders an excellent opportunity to manipulate the internal strain, and is very useful to create highly efficient solid state emitters.

DETECTION OF COULOMB CHARGING AROUND AN ANTIDOT IN THE QUANTUM HALL REGIME

We have detected oscillations of the charge around a potential hill (antidot) in a two-dimensional electron gas as a function of a large magnetic field B. The field confines electrons around the antidot in closed orbits, the areas of which are quantised through the Aharonov-Bohm effect. Increasing B reduces each states area, pushing electrons closer to the centre, until enough charge builds up for an electron to tunnel out. This is a new form of the Coulomb blockade seen in electrostatically confined dots. Addition and excitation spectra in DC bias confirm the Coulomb blockade of tunnelling.

Experimental Evidence for Coulomb Charging Effects in an Open Quantum Dot at Zero Magnetic Field

We have measured the low-temperature transport properties of an open quantum dot formed in a clean one-dimensional channel. For the first time, at zero magnetic field, continuous and periodic oscillations superimposed upon ballistic conductance steps are observed when the conductance through the dot