Kin Ming Ho

Broadband locally resonant sonic shields

We demonstrate a class of sonic shield materials based on the principle of locally resonant (LR) microstructures. Each local resonator is found to vibrate almost like an independent unit, and two layers of such resonators can even be regarded as a sonic crystal. By combining several LR layers of different resonant frequencies, a broadband (200–500 Hz) sound shield, with an average transmission intensity 11 dB lower than that dictated by mass density law, has been achieved.

High-Quality ZnO Nanowire Arrays Directly Fabricated from Photoresists

We report a simple and effective method for fabricating and patterning high-quality ZnO nanowire arrays using carbonized photoresists to control the nucleation site, density, and growth direction of the nanowires. The ZnO nanowires fabricated using this method show excellent alignment, crystal quality, and optical properties that are independent of the substrates. The carbonized photoresists provide perfect nucleation sites for the growth of aligned ZnO nanowires and they also perfectly connect to the nanowires to form ideal electrodes that can be used in many applications of ZnO nanomaterials.

Optimizing nanosheet-based ZnO hierarchical structure through ultrasonic-assisted precipitation for remarkable photovoltaic enhancement in quasi-solid dye-sensitized solar cells

For ZnO hierarchical structures composed of interlaced nanosheets, it has been proved that they are more favorable for electron transportation in the photoanodes of ZnO-based dye-sensitized solar cells (DSCs). Here, we introduce ultrasonic-assisted precipitation for fabricating novel nanosheet-based ZnO hierarchical flowers (HFs) in aqueous solution. With the powerful ultrasound irradiation, these nanosheets on the HFs are not only interlaced and monocrystalline, but also axially oriented, porous and ultrathin. Furthermore, broad channels enclosed by adjacent nanosheets can deeply extend into the inner parts of the HFs. Structural improvements reveal that the specific area of the novel HFs as well as their performances on light-capturing and electron transport have been largely improved compared with those prepared through direct precipitation. Remarkably, when assembled into quasi-solid DSCs, ZnO HF photoanodes show a high conversion efficiency up to 6.19% (under AM 1.5, 100 mW cm−2 illumination), the highest record of quasi-solid ZnO-based DSCs up to now.

Vertically aligned ZnO/amorphous-Si core–shell heterostructured nanowire arrays

We report the synthesis of vertically aligned ZnO/a-Si core-shell nanowire arrays (ZnO nanowires coated with amorphous silicon) through chemical vapor deposition. The core-shell heterostructured nanowires possessed uniform morphology and the thickness of the amorphous silicon shells could be controlled easily by tuning the deposition duration and temperature. The core-shell heterostructured nanowires exhibited enhanced antireflection and absorption performance as well as tunable PL properties. Because the individual ZnO/a-Si nanowires showed p-type characteristics and the ZnO cores were n-type semiconductors, the core-shell nanowires formed p-n junctions naturally.

van der Waals Epitaxial Growth of Atomically Thin Bi2Se3 and Thickness-Dependent Topological Phase Transition

Two-dimensional (2D) atomic-layered heterostructures stacked by van der Waals interactions recently introduced new research fields, which revealed novel phenomena and provided promising applications for electronic, optical, and optoelectronic devices. In this study, we report the van der Waals epitaxial growth of high-quality atomically thin Bi2Se3 on single crystalline hexagonal boron nitride (h-BN) by chemical vapor deposition. Although the in-plane lattice mismatch between Bi2Se3 and h-BN is approximately 65%, our transmission electron microscopy analysis revealed that Bi2Se3 single crystals epitaxially grew on h-BN with two commensurate states; that is, the (1̅21̅0) plane of Bi2Se3 was preferably parallel to the (1̅100) or (1̅21̅0) plane of h-BN. In the case of the Bi2Se3 (2̅110) ∥ h-BN (11̅00) state, the Moiré pattern wavelength in the Bi2Se3/h-BN superlattice can reach 5.47 nm. These naturally formed thin crystals facilitated the direct assembly of h-BN/Bi2Se3/h-BN sandwiched heterostructures without introducing any impurity at the interfaces for electronic property characterization. Our quantum capacitance (QC) measurements showed a compelling phenomenon of thickness-dependent topological phase transition, which was attributed to the coupling effects of two surface states from Dirac Fermions at/or above six quintuple layers (QLs) to gapped Dirac Fermions below six QLs. Moreover, in ultrathin Bi2Se3 (e.g., 3 QLs), we observed the midgap states induced by intrinsic defects at cryogenic temperatures. Our results demonstrated that QC measurements based on h-BN/Bi2Se3/h-BN sandwiched structures provided rich information regarding the density of states of Bi2Se3, such as quantum well states and Landau quantization. Our approach in fabricating h-BN/Bi2Se3/h-BN sandwiched device structures through the combination of bottom-up growth and top-down dry transferring techniques can be extended to other two-dimensional layered heterostructures.

Odd-Integer Quantum Hall States and Giant Spin Susceptibility in p -Type Few-Layer WSe2

We fabricate high-mobility p-type few-layer WSe_{2} field-effect transistors and surprisingly observe a series of quantum Hall (QH) states following an unconventional sequence predominated by odd-integer states under a moderate strength magnetic field. By tilting the magnetic field, we discover Landau level crossing effects at ultralow coincident angles, revealing that the Zeeman energy is about 3 times as large as the cyclotron energy near the valence band top at the Γ valley. This result implies the significant roles played by the exchange interactions in p-type few-layer WSe_{2}, in which itinerant or QH ferromagnetism likely occurs. Evidently, the Γ valley of few-layer WSe_{2} offers a unique platform with unusually heavy hole carriers and a substantially enhanced g factor for exploring strongly correlated phenomena.

Luminescence enhancement of ZnO-core/a-SiN x :H-shell nanorod arrays

We report a remarkable improvement of photoluminescence from ZnO-core/a-SiN(x):H-shell nanorod arrays by modulating the bandgap of a-SiN(x):H shell. The a-SiN(x):H shell with a large bandgap can significantly enhance UV emission by more than 8 times compared with the uncoated ZnO nanorods. Moreover, it is found that the deep-level defect emission can be almost completely suppressed for all the core-shell nanostructures, which is independent of the bandgaps of a-SiN(x):H shells. Combining with the analysis of infrared absorption spectrum and luminescence characteristics of NH(x)-plasma treated ZnO nanorods, the improved photoluminescence is attributed to the decrease of nonradiative recombination probability and the reduction of surface band bending of ZnO cores due to the H and N passivation and the screening effect from the a-SiN(x):H shells. Our findings open up new possibilities for fabricating stable and efficient UV-only emitting devices.

Effective control of photoluminescence from ZnO nanowires by a-SiN x :H decoration

The a-SiNx:H with a large bandgap of 3.8 eV was utilized to decorate ZnO nanowires. The UV emission from the a-SiNx:H-decorated ZnO nanowires are greatly enhanced compared with the undecorated ZnO nanowire. The deep-level defect emission has been completely suppressed even though the sample was annealed at temperatures up to 400 °C. The incorporation of H and N is suggested to passivate the defect states at the nanowire surface and thus result in the flat-band effect near ZnO surface as well as reduction of the nonradiative recombination probability.

Imaging 0.4nm single-walled carbon nanotubes with atomic force microscopy

The discovery of the single-walled carbon nanotubes (SWCNTs) with a diameter of 0.4 nm has attracted extensive attentions. In this paper we report our attempt with two methods to directly observe these SWCNTs by AFM. The first one is to deposit the SWCNTs extracted from the zeolite matrix to a flat surface for AFM observation. While one-dimensional features have been observed, the SWCNT was suspected not to adhere well to the substrate. To overcome the difficulties of weak adhesion, we attempt to expose only part of the SWCNT from the zeolite channel by cutting the zeolite crystal at an angle. This alternative method, in which the SWCNT contained zeolite crystal is polished and etched by HCl, however, did not result in a smooth enough surface and thus no one-dimensional features can be observed. The difficulties in sample preparation and possible improvements are discussed.