Shuit-Tong Lee

Bio: Shuit-Tong Lee is an academic researcher from Soochow University (Suzhou). The author has contributed to research in topics: Silicon & Nanowire. The author has an hindex of 138, co-authored 1121 publications receiving 77112 citations. Previous affiliations of Shuit-Tong Lee include University of British Columbia & Hong Kong University of Science and Technology.

Etching behavior of silicon nanowires with HF and NH4F and surface characterization by attenuated total reflection Fourier transform infrared spectroscopy: similarities and differences between one-dimensional and two-dimensional silicon surfaces.

Abstract: A systematic study of the etching behavior of one-dimensional (1-D) Si nanowires (SiNWs) in various HF and NH4F etching solutions is reported. The concentration and pH dependences of the etching time (which is inverse to the "stability") of the SiNWs in these solutions were investigated. A V-shaped bimodal etching curve was observed for HF solutions with concentrations of 0.5-40%. Specifically, SiNWs exhibit high stability in both low (0.5%) and high (40%) concentrations of HF solution, with the lowest stability (i.e., fastest etching rate) occurring at 2% (1 M) HF solution. With NH4F, the time needed to totally etch away the SiNWs sample decreases with increasing concentration (from 1-40%). The opposite is true when the pH of the NH4F solution was maintained at 14. These surprising results were rationalized in terms of "passivation" of the SiNW surfaces by HF or related molecules via hydrogen bonding for Si-H-terminated surfaces in HF solutions (with low pH values) and by NH4(+) ions via ionic bonding for Si-O(-)-terminated surfaces in NH4F solutions (with high pH values), respectively. Furthermore, it was found that SiNWs are stable only in relatively narrow pH ranges in these solutions. When SiNWs are etched with HF, the stability range is pH = 1-2 where the surface moieties are Si-H(x) species (x = 1-3). When SiNWs are etched with NH4F, the stability range is pH = 12-14 where the surface moieties are mainly Si-(O-)x species (x = 1-3). These rationales were confirmed by attenuated total reflection Fourier transform infrared spectroscopy measurements, which showed that, while etching SiNWs with HF gave rise to Si-H(x) surface species, no Si-H(x) species were observed when SiNWs were etched with NH4F. The latter finding is at odds with the corresponding results reported for the two-dimensional (2-D) Si wafers where etching with either HF or NH4F produces Si-H(x) species on the surface. This difference suggests either that the etching mechanisms for NH4F versus HF are different for SiNWs or, more likely, that the Si-H(x) surface species produced in NH4F solutions are so unstable that they are hydrolyzed readily at pH > 4. The similarities and differences of the etching behaviors and the resulting surface speciations between the 1-D SiNWs and the 2-D Si wafers suggest that the nanoscale structures as well as the low dimensionality of SiNWs may have contributed to the rapid hydrolysis of the surface Si-H(x) species in NH4F solutions, especially at high pH values.

Efficient red electroluminescence from organic devices using dye-doped rare earth complexes

Abstract: Using rare earth complexes with electron-transporting properties as host materials, and 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as a dopant, bilayer doped electroluminescent (EL) devices with efficient red light emission were fabricated. When a europium complex was adopted, the EL spectrum consisted of emissions from DCJTB and Eu3+ ions. At optimal dopant concentration, an EL efficiency of 5.7 cd/A at 0.04 mA/cm2 was observed. Although the EL efficiency decreased with an increase in current density, it remained higher than 2.0 cd/A with brightness of 347.0 cd/m2 at 5.7 V bias. DCJTB and Eu3+ ions collected the energy of singlet and triplet excitons, respectively, and then gave rise to pure red color emission, suggesting a promising way by which to utilize both singlet and triplet excited states in EL devices.

PES of atomic and molecular bismuth

Abstract: HeI photoelectron spectra of Bi and Bi2 were recorded by studying bismuth vapor at 750°C. The spectrum of Bi was found consistent with a j–j coupling description of the ground state. However, relativistic effects seemed to have little influence upon the relative photoionization cross sections of the 3P0,1,2 lines. Satellite peaks 1D2 at 11.49 eV, and possibly also 3D1,2 at 17.12 eV and 3F2,3 at 17.47 eV, were observed. They were assigned to admixtures of the configurations [(6s1/2)26p1/2(6p3/2)2]3/2 and [6s26p6d2]3/2 into the main ground‐state configuration [(6s1/2)2(6p1/2)26p3/2]3/2. An autoionization process at the energy of HeIγ radiation (23.74 eV) was suggested by the large intensity of the HeIγ spectrum. The photoelectron spectrum of Bi2 showed three distinguishable bands at 7.53, 8.94, and 9.30 eV corresponding, respectively, to the ionic states 2Πu, 3/2 , 2Πu, 1/2 , and 2Σ+g due to ionization of a (πu6p) or (σg6p) electron. A probable fourth band was observed at 14.87 eV, and was tentatively assig...

Distinct interfaces of poly (9,9-dioctylfluorene-co-benzothiadiazole) with cesium and calcium as observed by photoemission spectroscopy

Abstract: We report that the green-emitting polymer—poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), a copolymer of poly(9,9-dioctylfluorene) (F8), formed two distinct interfaces with calcium and cesium, respectively, as observed in photoemission measurements. Ultraviolet photoemission spectroscopy showed that Ca formed a stable interface with F8BT without significantly changing the electronic structure of F8BT, which is in contrast to the Cs/F8BT interface where bipolaron states occurred in the forbidden gap as a result of charge transfer processes. X-ray photoemission spectroscopy revealed that Ca covalently bonded with the sulfur atoms whereas Cs preferably interacted with the nitrogen in the F8BT. The results are useful to account for the undesirable device performance using CsF/Al as a cathode in the F8BT-based polymer light-emitting device. The exposure of Cs/F8BT interface to residual gases at a pressure of 2.0×10−9 mbar for 2 and 12 h, respectively, slightly and largely eliminated the gap states. Depos...

Synthesis and optical properties of Pb‐doped ZnO nanowires

Abstract: ZnO nanowires and related materials are promising structures for new electronic and optical applications and devices. In the present article featured as Editor's Choice [1], ZnPbO nanowires have been synthesized and studied by structural and photoluminescence investigations. The cover picture shows in the lower left part a typical transmission electron microscopy (TEM) image of a 7.26% Pb-doped ZnO nanowire with a diameter of about 70 nm and the corresponding selected area electron diffraction pattern. The upper right figure is a high-resolution TEM image of an individual nanowire, where the growth direction 〈001〉 is indicated by a white arrow, and a lattice spacing of about 0.26 nm is found. The corresponding author, Xiaohong Zhang, works on semiconducting materials and nanomaterials synthesis, functionalization of nanomaterials, photochemistry and photophysics, and device characterization. He is Associate Director of the Nano-Organic Photoelectronic Laboratory in the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences.

A roadmap for graphene

Abstract: Recent years have witnessed many breakthroughs in research on graphene (the first two-dimensional atomic crystal) as well as a significant advance in the mass production of this material. This one-atom-thick fabric of carbon uniquely combines extreme mechanical strength, exceptionally high electronic and thermal conductivities, impermeability to gases, as well as many other supreme properties, all of which make it highly attractive for numerous applications. Here we review recent progress in graphene research and in the development of production methods, and critically analyse the feasibility of various graphene applications.

High-performance lithium battery anodes using silicon nanowires

Abstract: There is great interest in developing rechargeable lithium batteries with higher energy capacity and longer cycle life for applications in portable electronic devices, electric vehicles and implantable medical devices. Silicon is an attractive anode material for lithium batteries because it has a low discharge potential and the highest known theoretical charge capacity (4,200 mAh g(-1); ref. 2). Although this is more than ten times higher than existing graphite anodes and much larger than various nitride and oxide materials, silicon anodes have limited applications because silicon's volume changes by 400% upon insertion and extraction of lithium which results in pulverization and capacity fading. Here, we show that silicon nanowire battery electrodes circumvent these issues as they can accommodate large strain without pulverization, provide good electronic contact and conduction, and display short lithium insertion distances. We achieved the theoretical charge capacity for silicon anodes and maintained a discharge capacity close to 75% of this maximum, with little fading during cycling.

Nanobelts of Semiconducting Oxides

Abstract: Ultralong beltlike (or ribbonlike) nanostructures (so-called nanobelts) were successfully synthesized for semiconducting oxides of zinc, tin, indium, cadmium, and gallium by simply evaporating the desired commercial metal oxide powders at high temperatures. The as-synthesized oxide nanobelts are pure, structurally uniform, and single crystalline, and most of them are free from defects and dislocations. They have a rectanglelike cross section with typical widths of 30 to 300 nanometers, width-to-thickness ratios of 5 to 10, and lengths of up to a few millimeters. The beltlike morphology appears to be a distinctive and common structural characteristic for the family of semiconducting oxides with cations of different valence states and materials of distinct crystallographic structures. The nanobelts could be an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and building functional devices along individual nanobelts.

Raman spectroscopy as a versatile tool for studying the properties of graphene

Abstract: Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp(2)-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene.

Aggregation-Induced Emission: Together We Shine, United We Soar!

Abstract: Ju Mei,†,‡,∥ Nelson L. C. Leung,†,‡,∥ Ryan T. K. Kwok,†,‡ Jacky W. Y. Lam,†,‡ and Ben Zhong Tang*,†,‡,§ †HKUST-Shenzhen Research Institute, Hi-Tech Park, Nanshan, Shenzhen 518057, China ‡Department of Chemistry, HKUST Jockey Club Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, State Key Laboratory of Molecular Neuroscience, Division of Life Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China