Hui Li

Few-Layer Nanoplates of Bi2Se3 and Bi2Te3 with Highly Tunable Chemical Potential

A topological insulator (TI) represents an unconventional quantum phase of matter with insulating bulk band gap and metallic surface states. Recent theoretical calculations and photoemission spectroscopy measurements show that group V-VI materials Bi(2)Se(3), Bi(2)Te(3), and Sb(2)Te(3) are TIs with a single Dirac cone on the surface. These materials have anisotropic, layered structures, in which five atomic layers are covalently bonded to form a quintuple layer, and quintuple layers interact weakly through van der Waals interaction to form the crystal. A few quintuple layers of these materials are predicted to exhibit interesting surface properties. Different from our previous nanoribbon study, here we report the synthesis and characterizations of ultrathin Bi(2)Te(3) and Bi(2)Se(3) nanoplates with thickness down to 3 nm (3 quintuple layers), via catalyst-free vapor-solid (VS) growth mechanism. Optical images reveal thickness-dependent color and contrast for nanoplates grown on oxidized silicon (300 nm SiO(2)/Si). As a new member of TI nanomaterials, ultrathin TI nanoplates have an extremely large surface-to-volume ratio and can be electrically gated more effectively than the bulk form, potentially enhancing surface state effects in transport measurements. Low-temperature transport measurements of a single nanoplate device, with a high-k dielectric top gate, show decrease in carrier concentration by several times and large tuning of chemical potential.

Formation of Bilayer Bernal Graphene: Layer-by-Layer Epitaxy via Chemical Vapor Deposition

We report the epitaxial formation of bilayer Bernal graphene on copper foil via chemical vapor deposition. The self-limit effect of graphene growth on copper is broken through the introduction of a second growth process. The coverage of bilayer regions with Bernal stacking can be as high as 67% before further optimization. Facilitated with the transfer process to silicon/silicon oxide substrates, dual-gated graphene transistors of the as-grown bilayer Bernal graphene were fabricated, showing typical tunable transfer characteristics under varying gate voltages. The high-yield layer-by-layer epitaxy scheme will not only make this material easily accessible but reveal the fundamental mechanism of graphene growth on copper.

Topological insulator nanostructures for near-infrared transparent flexible electrodes

Topological insulators are an intriguing class of materials with an insulating bulk state and gapless Dirac-type edge/surface states. Recent theoretical work predicts that few-layer topological insulators are promising candidates for broadband and high-performance optoelectronic devices due to their spin-momentum-locked massless Dirac edge/surface states, which are topologically protected against all time-reversal-invariant perturbations. Here, we present the first experimental demonstration of near-infrared transparent flexible electrodes based on few-layer topological-insulator Bi(2)Se(3) nanostructures epitaxially grown on mica substrates by means of van der Waals epitaxy. The large, continuous, Bi(2)Se(3)-nanosheet transparent electrodes have single Dirac cone surface states, and exhibit sheet resistances as low as ~330 Ω per square, with a transparency of more than 70% over a wide range of wavelengths. Furthermore, Bi(2)Se(3)-nanosheet transparent electrodes show high chemical and thermal stabilities as well as excellent mechanical durability, which may lead to novel optoelectronic devices with unique properties.

Epitaxial heterostructures of ultrathin topological insulator nanoplate and graphene.

The authors present a van der Waals epitaxy of high-quality ultrathin nanoplates of topological insulator Bi(2)Se(3) on a pristine graphene substrate using a simple vapor-phase deposition method. Sub-10-nm-thick nanoplates of layered Bi(2)Se(3) with defined orientations can be epitaxially grown on a few-layer pristine graphene substrate. We show the evolution of Raman spectra with the number of Bi(2)Se(3) layers on few-layer graphene. Bi(2)Se(3) nanoplates with a thickness of three quintuple-layers (3-QL) exhibit the strongest Raman intensity. Strain effects in the Bi(2)Se(3)/graphene nanoplate heterostructures is also studied by Raman spectroscopy. 1-QL and 2-QL Bi(2)Se(3) nanoplates experience tensile stress, consistent with compressive stress in single-layer and bilayer graphene substrates. Our results suggest an approach for the synthesis of epitaxial heterostructures that consist of an ultrathin topological insulator and graphene, which may be a new direction for electronic and spintronic applications.

Novel fluorescent pH sensors and a biological probe based on anthracene derivatives with aggregation-induced emission characteristics.

Three functionalized 9,10-distyrylanthracene (DSA) derivatives, namely, 9,10-bis(4-hydroxystyryl)anthracene (2), 9,10-bis{4-[2-(diethylamino)ethoxy]styryl}anthracene (4), and 9,10-bis{4-[2-(N,N,N-triethylammonium)ethoxy]styryl}anthracene dibromide (5), were synthesized and their fluorescence properties were investigated. The three DSA derivatives possess a typical aggregation-induced emission (AIE) property (i.e., they are nonluminescent in dilute solutions but are efficiently fluorescent as induced by molecular aggregation). Different AIE properties were tuned through molecular structure control. Dye 2 is a phenol-moiety-containing compound, which shows aggregation at pH values smaller than 10, resulting in a high fluorescence intensity. Thus, dye 2 has a pK(a) of 9.94. 4 is an amine-containing compound that starts to aggregate at slightly basic conditions, resulting in a pK(a) of 6.90. Dye 5 is an ammonium-salt-containing compound. Because it is very soluble in water, this compound has no AIE phenomenon but can interact strongly with protein or DNA to amplify its emission. Therefore, 5 is a fluorescent turn on biological probe for protein and DNA detection and it is also selective, which works for native BSA and ct DNA but not their denatured forms. Therefore, we not only developed a few new compounds showing the AIE phenomena but also controlled the AIE through environmental stimulation and demonstrated that the new AIE molecules are suitable for pH and biomacromolecule sensing.

Controlled Synthesis of Topological Insulator Nanoplate Arrays on Mica

The orientation- and position-controlled synthesis of single-crystal topological insulator (Bi(2)Se(3) and Bi(2)Te(3)) nanoplate arrays on mica substrates was achieved using van der Waals epitaxy. Individual ultrathin nanoplates with the lateral dimension up to ~0.1 mm or uniform thickness down to 1-2 nm were produced. Single-Dirac-cone surface states of nanoplate aggregates were confirmed by angle-resolved photoemission spectroscopy measurements. The large-grain-size, single-crystal nanoplate arrays grown on mica can act as facile platforms for a combination of spectroscopy and in situ transport measurements, which may open up new avenues for studying exotic physical phenomena, surface chemical reactions, and modification in topological insulators.

Observation of glassy ferromagnetism in Al-doped 4H-SiC.

Glassy ferromagnetism is observed in diluted magnetic semiconductor Al-doped 4H-SiC. We propose a possible explanation for the origin of ferromagnetism order that is the coeffect of sp(2)/sp(3) configuration along with the structural defects. This result unambiguously demonstrates the existence of intrinsic ferromagnetism order in nonmagnetic sp systems.

Langmuir–Blodgett film and second harmonic generation of a new type of amphiphilic non-linear optical bis-chromophore complex dye

A bivalent zinc complex anion: bis(2-thione-1,3-dithiol-4,5-dimercapto)zinc, has been chosen to balance several non-linear optical (NLO) dye cations in the design and synthesis of a series of novel bis-chromophore complex dyes. The Langmuir film-forming properties of these new NLO complex dyes at the air/water interface, the stability and second-order non-linear optical properties of the monolayer and Langmuir–Blodgett multilayer films were investigated. Results show that both the film-forming properties and NLO coefficiencies were improved greatly when the zinc complex anion was incorporated into four types of NLO-active organic dyes.

Influence of substrate bias and post-deposition Cl treatment on CdTe film grown by RF magnetron sputtering for solar cells

CdTe thin films were grown by RF magnetron sputtering at deposition pressures of 1.5–4 Pa and substrate potentials of 8.8, 0, −8.2, −18.9, −28.3, −38.1, −48.0, and −98.7 V. All as-grown CdTe thin films had undergone CdCl2 treatment in dry air. The deposition pressure had a large influence on the crystalline quality, morphology, and grain size. At 1.5–3 Pa, CdTe thin films with good crystalline quality and column morphology were successfully obtained. The substrate potential also influenced the phase composition, grain size, morphology, microstress, and roughness of the obtained CdTe thin films. The phase composition, morphology, grain size, microstress, and crystalline quality changed significantly after CdCl2 annealing treatment. For CdTe grown at 3 Pa, the grain size reached a maximum when the Cl treatment time was 20 min; for CdTe thin films deposited at 2 Pa, grain size was the greatest when the Cl treatment time was 43 min. The maximum grain size corresponded to the best performance of the CdTe solar cell. CdTe thin films grown at different substrate potentials were subjected to Cl treatment at 400 °C for 43 min. The internal structure was observed using scanning electron microscopy of sample cross-sections sliced by a focused ion beam. After Cl treatment, voids were seen at the CdTe grain boundaries, and at the interfaces CdS/F:SnO2 and CdS/CdTe. The CdTe film grown at a substrate potential of −18.9 V produced the best solar cell, with device parameters of η = 12.78%, Voc = 779 mV, Jsc = 22.91 mA cm−2, and FF = 71.62%.

Investigation of hepatic fibrosis in rats with x-ray diffraction enhanced imaging

X-ray diffraction enhanced imaging (DEI) is a phase contrast technique that generates excellent contrast of biological soft tissues compared to conventional absorption radiography. We explore the application of DEI in the diagnosis of hepatic fibrosis. The produced refraction contrast images of fibrous rat liver samples show clearly abnormal liver architectures. Moreover, by comparing to histological pictures, different stages of fibrosis are discriminated, and the corresponding morphological features are analyzed. Besides, quantitative analyses of texture features are presented. The results reported herein show that DEI can be a potential noninvasive technique to diagnose and stage hepatic fibrosis.