George K.L. Wong

Bio: George K.L. Wong is an academic researcher from Hong Kong University of Science and Technology. The author has contributed to research in topics: Thin film & Molecular beam epitaxy. The author has an hindex of 29, co-authored 117 publications receiving 3896 citations. Previous affiliations of George K.L. Wong include Lawrence Berkeley National Laboratory & Northwestern University.

Crystal Engineering of Acentric Diamondoid Metal–Organic Coordination Networks

Abstract: Acentric three-dimensional coordination polymers bis(isonicotinato)zinc (1) and bis(4-pyridylacrylato)cadmium⋅H2 O (2) were synthesized under hydro(solvo)thermal conditions; they exhibit a threefold (see picture) and fivefold diamondoid structure, respectively. Both 1 and 2 are active for second harmonic generation and exhibit remarkable thermal stability.

Impurity effect on weak antilocalization in the topological insulator Bi 2 Te 3

Abstract: We study the weak antilocalization (WAL) effect in topological insulator Bi(2)Te(3) thin films at low temperatures. The two-dimensional WAL effect associated with surface carriers is revealed in the tilted magnetic field dependence of magnetoconductance. Our data demonstrate that the observed WAL is robust against deposition of nonmagnetic Au impurities on the surface of the thin films, but it is quenched by the deposition of magnetic Fe impurities which destroy the π Berry phase of the topological surface states. The magnetoconductance data of a 5 nm Bi(2)Te(3) film suggests that a crossover from symplectic to unitary classes is observed with the deposition of Fe impurities.

Growth of ZnO Thin Film by Laser MBE: Lasing of Exciton at Room Temperature.

Abstract: High quality ZnO thin film was grown by Laser MBE. A pure ceramic ZnO target was ablated by the KrF laser pulses (248 nm, 10 Hz, 1 J/cm2) in an ultra high vacuum to deposit ZnO film on sapphire (0001) substrate. The lateral grain size was about 50 nm for the sample with thickness of 55 nm. At room temperature, the peak of the exciton absorption and the photoluminescence have the same energy. Under high density excitation (355 nm, 35 ps, 10 Hz), an exciton–exciton collision process was observed as P2 and P lines where 2S exciton and ionized exciton remain. From the edge of the sample, a very rapid increase of the P line was observed with the increase of the excitation power. A fine structure that comes from the cavity mode was also observed. These facts suggest that the lasing of the exciton was observed at room temperature.

Room-temperature stimulated emission of excitons in ZnO/(Mg, Zn)O superlattices

Abstract: We report on the observation of stimulated emission in ZnO/MgxZn1−xO superlattices well above room temperature. Two kinds of superlattices grown by laser molecular-beam epitaxy showed clear systematics on the quantum subband levels in absorption and spontaneous emission spectra. Stimulated emission with excitonic origin could be observed at very low optical pumping levels. The threshold excitation intensity changed from 11 to 40 kW/cm2, and the emission energy could be tuned between 3.2 and 3.4 eV, depending on the well thickness and/or the Mg content in the barrier layers. The excitonic stimulated emission could be observed up to 373 K and the characteristic temperature was as high as 87 K.

Click polymerization : facile synthesis of functional poly(aroyltriazole)s by metal-free, regioselective 1,3-dipolar polycycloaddition

Abstract: Heating mixtures of bis(aroylacetylene)s (5/6/9) and diazides (7/8) in polar solvents such as DMF/toluene at a moderate temperature of 100 °C readily affects their 1,3-dipolar polycycloadditions, producing poly(aroyltriazole)s (PATAs; PI−PXII) with high molecular weights (Mw up to 26 700) and regioregularities (F1,4 up to ∼92%) in high yields (up to ∼98%). The metal-free click polymerizations propagate smoothly in an open atmosphere without protection from oxygen and moisture. Through model reaction study and semiempirical calculation, the polymerization mechanism is proposed and discussed. Molecular structures of the PATAs are characterized spectroscopically. All the polymers are soluble in common organic solvents and are thermally stable, losing little of their weights when heated to ∼380 °C. The PATAs containing triphenylamine units emit visible light and show unique solvatochromism. The PATAs are nonlinear optically active, exhibiting large two-photon absorption cross sections due to the intramolecula...

Topological insulators and superconductors

Abstract: Topological insulators are new states of quantum matter which cannot be adiabatically connected to conventional insulators and semiconductors. They are characterized by a full insulating gap in the bulk and gapless edge or surface states which are protected by time-reversal symmetry. These topological materials have been theoretically predicted and experimentally observed in a variety of systems, including HgTe quantum wells, BiSb alloys, and Bi2Te3 and Bi2Se3 crystals. Theoretical models, materials properties, and experimental results on two-dimensional and three-dimensional topological insulators are reviewed, and both the topological band theory and the topological field theory are discussed. Topological superconductors have a full pairing gap in the bulk and gapless surface states consisting of Majorana fermions. The theory of topological superconductors is reviewed, in close analogy to the theory of topological insulators.

A comprehensive review of zno materials and devices

Abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

Cu-catalyzed azide-alkyne cycloaddition.

Abstract: The Huisgen 1,3-dipolar cycloaddition reaction of organic azides and alkynes has gained considerable attention in recent years due to the introduction in 2001 of Cu(1) catalysis by Tornoe and Meldal, leading to a major improvement in both rate and regioselectivity of the reaction, as realized independently by the Meldal and the Sharpless laboratories. The great success of the Cu(1) catalyzed reaction is rooted in the fact that it is a virtually quantitative, very robust, insensitive, general, and orthogonal ligation reaction, suitable for even biomolecular ligation and in vivo tagging or as a polymerization reaction for synthesis of long linear polymers. The triazole formed is essentially chemically inert to reactive conditions, e.g. oxidation, reduction, and hydrolysis, and has an intermediate polarity with a dipolar moment of ∼5 D. The basis for the unique properties and rate enhancement for triazole formation under Cu(1) catalysis should be found in the high ∆G of the reaction in combination with the low character of polarity of the dipole of the noncatalyzed thermal reaction, which leads to a considerable activation barrier. In order to understand the reaction in detail, it therefore seems important to spend a moment to consider the structural and mechanistic aspects of the catalysis. The reaction is quite insensitive to reaction conditions as long as Cu(1) is present and may be performed in an aqueous or organic environment both in solution and on solid support.

Fundamentals of zinc oxide as a semiconductor

Abstract: In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

Metal-organic frameworks

Abstract: Metal-organic frameworks are a recently-identified class of porous polymeric material, consisting of metal ions linked together by organic bridging ligands, and are a new development on the interface between molecular coordination chemistry and materials science. A range of novel structures has been prepared which feature amongst the largest pores known for crystalline compounds, very high sorption capacities and complex sorption behaviour not seen in aluminosilicate zeolites. The development of synthetic approaches to these materials and investigations of their properties are reviewed.