Magnetic Oscillation of Optical Phonon in ABA- and ABC-Stacked Trilayer Graphene

We present a comparative measurement of the G-peak oscillations of phonon frequency, Raman intensity and linewidth in the Magneto-Raman scattering of optical E2g phonons in mechanically exfoliated ABA- and ABC-stacked trilayer graphene (TLG). Whereas in ABA-stacked TLG, we observe magnetophonon oscillations consistent with single-bilayer chiral band doublets, the features are flat for ABC-stacked TLG up to magnetic fields of 9 T. This suppression can be attributed to the enhancement of band chirality that compactifies the spectrum of Landau levels and modifies the magnetophonon resonance properties. The drastically different coupling behaviour between the electronic excitations and the E2g phonons in ABA- and ABC-stacked TLG reflects their different electronic band structures and the electronic Landau level transitions and thus can be another way to determine the stacking orders and to probe the stacking-order-dependent electronic structures. In addition, the sensitivity of the magneto-Raman scattering to the particular stacking order in few layers graphene highlights the important role of interlayer coupling in modifying the optical response properties in van der Waals layered materials.Comment: 25 pages, 6 figure

Facile Synthesis of a 3,4-Ethylene-Dioxythiophene (EDOT) Derivative for Ease of Bio-Functionalization of the Conducting Polymer PEDOT

In the pursuit of conducting polymer based bio-functional devices, a cost-effective and high yield synthesis method for a versatile monomer is desired. We report here a new synthesis strategy for a versatile monomer 2-methylene-2,3-dihydrothieno (3,4-b) (1,4) dioxine, or 3,4-ethylenedioxythiophene with a exomethylene side group (EDOT-EM). Compared to the previously reported synthesis route, the new strategy uses less steps, with faster reaction rate, and higher yield. The presence of EM group opens up endless possibility for derivatization via either hydro-alkoxy addition or thiol-ene click chemistry. EDOT-EM could be polymerized into stable and low impedance PEDOT-EM polymer using electro-polymerization method on different conducting substrates at both macro and micro scales. Facile post-functionalization of PEDOT-EM with molecules of varying size and functionality (from small molecules to DNAs and proteins) was achieved. The new synthetic route of EDOT-EM and the ease of post-functionalization of PEDOT-EM will greatly accelerate the use of conducting polymer in a broad range of organic electronics and bioelectronics applications

Facile Synthesis of a 3,4-Ethylene-Dioxythiophene (EDOT) Derivative for Ease of Bio-Functionalization of the Conducting Polymer PEDOT

In the pursuit of conducting polymer based bio-functional devices, a cost-effective and high yield synthesis method for a versatile monomer is desired. We report here a new synthesis strategy for a versatile monomer 2-methylene-2,3-dihydrothieno (3,4-b) (1,4) dioxine, or 3,4-ethylenedioxythiophene with a exomethylene side group (EDOT-EM). Compared to the previously reported synthesis route, the new strategy uses less steps, with faster reaction rate, and higher yield. The presence of EM group opens up endless possibility for derivatization via either hydro-alkoxy addition or thiol-ene click chemistry. EDOT-EM could be polymerized into stable and low impedance PEDOT-EM polymer using electro-polymerization method on different conducting substrates at both macro and micro scales. Facile post-functionalization of PEDOT-EM with molecules of varying size and functionality (from small molecules to DNAs and proteins) was achieved. The new synthetic route of EDOT-EM and the ease of post-functionalization of PEDOT-EM will greatly accelerate the use of conducting polymer in a broad range of organic electronics and bioelectronics applications

Physiological effects of combined NaCl and NaHCO3 stress on the seedlings of two maple species

Salt stress impacts growth and physiological processes in plants, and some plants exposed to salt stress will produce physiological mechanisms to adapt to the new environment. However, the effects of combined NaCl and NaHCO3 stress on the seedlings of Acer species are understudied. In this study, we designed an experiment to measure physiological characteristics by establishing a range of NaCl and NaHCO3 concentrations (0, 25, 50, 75, and 100 mmol L-1) to estimate the compound salt tolerance of Acer ginnala and Acer palmatum. When the concentrations of NaCl and NaHCO3 were 25 mmol L-1, the leaf water content, relative conductivity, malondialdehyde (MDA) content, proline content, soluble sugar content, and chlorophyll did not change (p > 0.05) in two maple seedlings. At concentrations greater than 50 mmol L-1, the relative conductivity and MDA content increased, proline and soluble sugars accumulated, and the potential activity of PS II (Fv/Fo), potential photochemical efficiency of PS II (Fv/Fm), PS II actual photochemical efficiency (Yield), and photosynthetic electron transfer efficiency (ETR) decreased (p < 0.05). The superoxide dismutase (SOD) and catalase (CAT) activities showed the same trend of first increasing and then decreasing (p < 0.05). The peroxidase (POD) activity increased only when concentrations of NaCl and NaHCO3 were 100 mmol L-1, while there was no statistical difference between the other treatments and the control. Therefore, the two maple seedlings adjusted their osmotic balance and alleviated oxidative stress by accumulating proline, soluble sugars and increasing CAT and SOD activities. Further analysis showed that both species are salt tolerant and the salt tolerance of Acer ginnala is better than that of Acer palmatum

Photocurrent study of two dimensional materials

With the discovery of graphene, two dimensional materials have drawn great interest among researchers thanks to their unique properties. Research in graphene optoelectronics is one of the fastest developing platforms to study light-matter interaction. Beyond the active research on new fundamental aspects of this and related systems, the optical and electronic properties of graphene can be employed in numerous applications, such as highly sensitive bolometers, photodetectors, plasmonic or photovoltaic devices, which leverage on photo-induced thermal and electronic effects. Due to the lack of non-trivial band gap in graphene, researchers have explored various two-dimensional materials, among which transition metal dichalcogenide monolayers have shown various interesting properties such as charge density waves, unusual strain and thermal energy dependence, tunable light emission by chemical control and fast photoresponse, which can be used in interdisciplinary device applications. To reveal the intrinsic optical and electronic behavior of the materials mentioned above, photocurrent study turns out to be a powerful tool. This thesis covers three of my major contributions: 1. Helicity dependent photocurrent in biased bilayer graphene In this part, we report the experimental determination of the PC response of bilayer graphene as a function of light intensity and state of polarization, as well as carrier density and polarity. The data shows qualitative features in common with the PC that is expected to arise from the photon-drag and the circular photogalvanic effects, as seen in monolayer graphene. In addition, we identify a non-negligible contribution to the PC of different nature, with anomalous dependence on light polarization. These results highlight the richness of bilayer graphene photoresponse, providing an opportunity to establish light helicity as a means to manipulate the photoconductive behaviour of future optoelectronic graphene devices. 2. Low temperature photoresponse of monolayer tungsten disulphide High photoresponse can be achieved in monolayers of transition metal dichalcogenides. However, the response times are inconveniently limited by defects. Here, we report low temperature photoresponse of monolayer WS2 prepared by exfoliation and chemical vapour deposition method. The exfoliated device exhibits n-type behaviour; while the CVD device exhibits intrinsic behaviour. In off state, the CVD device has a photoresponse ratio for laser on/off four times larger than that of the device based on exfoliated WS2. And the photoresponse decay-rise time is 0.1s (limited by our setup), compared to few seconds in the exfoliated device. These findings are discussed in terms of charge trapping and localization. 3. Dichroic spin-valley PC in monolayer molybdenum disulphide The aim of valleytronics is to exploit confinement of charge carriers in local valleys of the energy bands of semiconductors as an additional degree of freedom in optoelectronic devices. Thanks to strong direct excitonic transitions in spin-coupled K valleys, monolayer MoS2 is a rapidly emerging valleytronic material, with high valley polarization in photoluminescence. Here we elucidate the excitonic physics of this material by light helicity-dependent PC studies of phototransistors. We demonstrate that large PC dichroism (up to 60%) can also be achieved in high-quality MoS2 monolayers grown by CVD, due to the circular photogalvanic effect on resonant excitations. This opens up new opportunities for valleytronic applications in which selective control of spin-valley-coupled PCs can be used to implement polarization-sensitive light-detection schemes or integrated spintronic devices, as well as biochemical sensors operating at visible frequencies.​Doctor of Philosophy (SPMS

Magneto-photoconductivity of three dimensional topological insulator bismuth telluride

Magnetic field dependence of the photocurrent in a 3D topological insulator is studied. Among the 3D topological insulators bismuth telluride has unique hexagonal warping and spin texture which has been studied by photoemission, scanning tunnelling microscopy and transport. Here, we report on low temperature magneto-photoconductivity, up to 7 T, of two metallic bismuth telluride topological insulator samples with 68 and 110 nm thicknesses excited by 2.33 eV photon energy along the magnetic field perpendicular to the sample plane. At 4 K, both samples exhibit negative magneto-photoconductance below 4 T, which is as a result of weak-antilocalization of Dirac fermions similar to the previous observations in electrical transport. However the thinner sample shows positive magneto-photoconductance above 4 T. This can be attributed to the coupling of surface states. On the other hand, the thicker sample shows no positive magneto-photoconductance up to 7 T since there is only one surface state at play. By fitting the magneto-photoconductivity data of the thicker sample to the localization formula, we obtain weak antilocalization behaviour at 4, 10, and 20 K, as expected; however, weak localization behaviour at 30 K, which is a sign of surface states masked by bulk states. Also, from the temperature dependence of phase coherence length bulk carrier-carrier interaction is identified separately from the surface states. Therefore, it is possible to distinguish surface states by magneto-photoconductivity at low temperature, even in metallic samples.MOE (Min. of Education, S’pore)Published versio

The proximity-enabled sulfur fluoride exchange reaction in the protein context.

The proximity-enabled sulfur(vi) fluoride exchange (SuFEx) reaction generates specific covalent linkages between proteins in cells and in vivo, which opens innovative avenues for studying elusive protein-protein interactions and developing potent covalent protein drugs. To exploit the power and expand the applications of covalent proteins, covalent linkage formation between proteins is the critical step, for which fundamental kinetic and essential properties remain unexplored. Herein, we systematically studied SuFEx kinetics in different proteins and conditions. In contrast to in small molecules, SuFEx in interacting proteins conformed with a two-step mechanism involving noncovalent binding, followed by covalent bond formation, exhibiting nonlinear rate dependence on protein concentration. The protein SuFEx rate consistently changed with protein binding affinity as well as chemical reactivity of the functional group and was impacted by target residue identity and solution pH. In addition, kinetic analyses of nanobody SR4 binding with SARS-CoV-2 spike protein revealed that viral target mutations did not abolish covalent binding but decreased the SuFEx rate with affinity decrease. Moreover, off-target cross-linking of a SuFEx-capable nanobody in human serum was not detected, and the SuFEx-generated protein linkage was stable at cellular acidic pHs, suggesting SuFEx suitability for in vivo usage. These results advanced our understanding of SuFEx reactivity and kinetics in proteins, which is invaluable for ongoing exploration of SuFEx-enabled covalent proteins for basic biological research and creative biotherapeutics

Carrier density and light helicity dependence of photocurrent in mono- and bilayer graphene

Helicity-dependent photocurrent in monolayer graphene has been the subject of intense debate, and was recently ascribed to photon drag and circular photogalvanic effects. Unlike inversion symmetric monolayer graphene with no band gap, the most stable case of two-layer graphene, AB-stacked bilayer graphene, has broken inversion symmetry and can have a band gap upon electrical gating. Here we report the experimental determination of the photocurrent response of mono- and bilayer graphene as a function of light polarization, as well as carrier density and polarity. The mono- and bilayer graphene data show qualitative features in common with the photocurrent contribution that is expected to arise from the photon drag effect. On the other hand, the photocurrent due to the circular photogalvanic effect in bilayer (monolayer) graphene has asymmetric (symmetric) dependence on carrier density, which is attributed to particle-hole asymmetry.MOE (Min. of Education, S’pore

Observation of excitonic fine structure in a 2D transition-metal dichalcogenide semiconductor

10.1021/nn5059908ACS Nano91647-65