Tianzhun Wu

Carbon nanotube sponge-array tandem composites with extended energy absorption range.

Controlled assembly of carbon nanotubes (CNTs) represents an exciting research area and provides opportunities for fabrication of various three-dimensional macroscopic structures with unexpected properties. Given that CNTs are one-dimensional with large aspect-ratios, synthesis of vertically aligned CNT arrays has stimulated tremendous interest with a series of important progressions made about one decade ago. [ 1–5 ] In an aligned array, individual CNTs grow vertically from the substrate creating a forest-like morphology with millimeter length over large area. Alignment of CNTs results in an anisotropic structure in which the excellent mechanical and electronic properties along the nanotube axis can be fully exploited. [ 6–10 ] For example, under large strain compression CNTs buckle collectively and then unfold during releasing, forming a highly compressible yet elastic foam with fatigue resistance over many cycles. [ 6 , 7 ] Solvent-induced zipping and densifi cation of aligned single-walled nanotube fi lms creates solid wafers that could be used as electromechanical actuators or stretchable strain sensors, [ 11 ] as well as other geometry-tunable multi-walled nanotube columns. [ 12 ]

Water as a colorful ink: transparent, rewritable photonic coatings based on colloidal crystals embedded in chitosan hydrogel

The invention of paper has greatly contributed to the development of information storage and spreading. However, the vast production and consumption have also brought significant environment problems to our society. Although several kinds of rewritable papers have been reported, their practical application is rare. Herein, a new rewritable paper with water as a colorful ink is proposed and demonstrated successfully by using water as the trigger to tune the band-gap of photonic coatings on solid substrates. Various colors are achieved by adjusting the particle size of photonic coatings, or writing with aqueous solutions of certain pH. The photonic coatings can be erased and rewritten multiple times with no significant loss in color quality. Furthermore, the photonic coatings, which are transparent, enable fast and convenient visualization of the invisible photonic patterns with good tenability and reproducibility, offering potential applications for steganography, identification marking and anti-counterfeiting purposes. This transparent and rewritable paper can serve as an environmentally friendly information storage device to meet the sustainability of modern society.

High-Precision Selective Deposition of Catalyst for Facile Localized Growth of Single-Walled Carbon Nanotubes

In the liquid-based dip-coating, the hydrophilicity of a Si/SiO(2) substrate is found to be critical for the successful deposition of catalyst and hence the growth of single-walled carbon nanotubes (SWNTs). When the surface is functionalized by a self-assembled monolayer (SAM) and becomes hydrophobic, no catalyst remains and no SWNT grows. This concept can be utilized to localize the growth of SWNTs at designated regions where SAMs were selectively removed by, e.g., UV or electron beam. Patterned high-quality as-grown SWNTs with a potential line width of approximately 10 nm can be obtained.

High-performance zero-bias ultraviolet photodetector based on p-GaN/n-ZnO heterojunction

Lattice-match p-GaN and n-ZnO bilayers were heteroepitaxially grown on the c-sapphire substrate by metal organic chemical vapor deposition and molecular beam epitaxy technique, respectively. X-ray diffraction and photoluminescence investigations revealed the high crystal quality of the bilayer films. Subsequently, a p-GaN/n-ZnO heterojunction photodetector was fabricated. The p-n junction exhibited a clear rectifying I-V characteristic with a turn-on voltage of 3.7 V. At zero-bias voltage, the peak responsivity was 0.68 mA/W at 358 nm, which is one of the best performances reported for p-GaN/n-ZnO heterojunction detectors due to the excellent crystal quality of the bilayer films. These show that the high-performance p-GaN/n-ZnO heterojunction diode is potential for applications of portable UV detectors without driving power.

Formation behavior of BexZn1−xO alloys grown by plasma-assisted molecular beam epitaxy

We report the phase formation behavior of BexZn1−xO alloys grown by plasma-assisted molecular beam epitaxy. We find the alloy with low- and high-Be contents could be obtained by alloying BeO into ZnO films. X-ray diffraction measurements shows the c lattice constant value shrinks, and room temperature absorption shows the energy band-gap widens after Be incorporated. However, the alloy with intermediate Be composition are unstable and segregated into low- and high-Be contents BeZnO alloys. We demonstrate the phase segregation of BexZn1−xO alloys with intermediate Be composition resulted from large internal strain induced by large lattice mismatch between BeO and ZnO.

Temperature-dependent structural relaxation of BeZnO alloys

The thermal stability of BeZnO was examined in this study. Structural relaxation and reconstruction of the Be0.4Zn0.6O alloy film started at 500 °C and came to a halt at 800 °C. Be atoms were found to be diffused out from the host position, and BeO-based second phase was formed. The bandgap of Be0.4Zn0.6O was reduced to the value of pure ZnO after annealing at 600 °C. Therefore, the thermodynamic solubility of BeO in ZnO is far below than that of MgO in ZnO. Finally, the long term stability of BeZnO at room temperature was verified after aged for one year.

Engineering superlyophobic surfaces on curable materials based on facile and inexpensive microfabrication

This paper proposes a facile, versatile, and low-cost approach for batch production of engineered superlyophobic surfaces (SLSs, simultaneously superhydrophobic and superoleophobic) on various curable materials. Based on the soft replication using poly(dimethylsiloxane) (PDMS) as the intermediate mold, T-shaped overhang microstructures on Si and dual-resist masters have been transferred to curable materials including poly(methyl methacrylate) (PMMA), PDMS and glass resin. The as-fabricated polymer SLS replicas exhibit high structure fidelity, comparable nonwettability and excellent reproducibility for both water and oil during the 10 × 10 replication and possess new features such as tunable transparency. The proposed microfabrication approach for SLSs decouples the material and process dependence, greatly dilutes the fabrication cost and enables high-performance SLSs on a wide range of materials, which may initialize the broad applications of engineered SLSs for various low-contamination, low-adhesion and self-cleaning requirements in academy, industry and daily life.

Solar-blind wurtzite MgZnO alloy films stabilized by Be doping

MgxZn1?xO alloy films were deposited on c-plane sapphire substrates by radio frequency plasma-assisted molecular beam epitaxy (rf-PMBE). The phase segregation occurred when x was larger than 33%. Be doping was found experimentally able to stabilize the high-Mg-content MgZnO alloy. By alloying 1?2% Be into MgZnO, the band gap of as-prepared quaternary alloys can be raised to the solar-blind range (4.5?eV). Calculated formation energy of the alloys based on first principle reveals that a small amount of Be incorporation can reduce the formation energy of high-Mg-content MgZnO alloys and results in a more stable system, which justifies our experimental observations.

Stabilization of p-type dopant nitrogen in BeZnO ternary alloy epitaxial thin films

We demonstrate the growth of BexZn1−xO alloy thin films on c-sapphire substrates by plasma-assisted molecular beam epitaxy. The formation of BexZn1−xO is very sensitive to the Be content in the alloy. Be atoms occupy the Zn lattice sites at low Be content, but move partly to the interstitial sites with increasing Be content. We also investigated the thermal stability of N, one of the most frequently used p-type dopants, in the BexZn1−xO films. Secondary ion mass spectrometry shows that Be element plays a crucial role in stabilizing N in the BexZn1−xO host, which is favourable in improving the solid solubility and the thermal stability of acceptor dopants in ZnO-based wide-gap semiconductors.

Shrunk to femtolitre: Tuning high-throughput monodisperse water-in-oil droplet arrays for ultra-small micro-reactors

We report a facile, low-cost, and high-yielding microfluidic technology for in situ generating and arraying water-in-oil droplets by shrinking them to the order of femtolitres (fLs) as scalable batch micro-reactors. Instead of generating ultra-small droplets by the direct atomization, which requires dedicate control and high energy input, we shrink droplets to stable smaller ones by utilizing the controlled water diffusion in oil. This “shrunk to fL” method is combined with a three-dimensional microwell design to create high-density addressable droplet arrays. As the result, scalable, high-throughput, and well-aligned W/O arrays with excellent long-term stability and predicable droplet sizes have been achieved.