Dajun Yuan

Wafer-Scale High-Throughput Ordered Growth of Vertically Aligned ZnO Nanowire Arrays

This article presents an effective approach for patterned growth of vertically aligned ZnO nanowire (NW) arrays with high throughput and low cost at wafer scale without using cleanroom technology. Periodic hole patterns are generated using laser interference lithography on substrates coated with the photoresist SU-8. ZnO NWs are selectively grown through the holes via a low-temperature hydrothermal method without using a catalyst and with a superior control over orientation, location/density, and as-synthesized morphology. The development of textured ZnO seed layers for replacing single crystalline GaN and ZnO substrates extends the large-scale fabrication of vertically aligned ZnO NW arrays on substrates of other materials, such as polymers, Si, and glass. This combined approach demonstrates a novel method of manufacturing large-scale patterned one-dimensional nanostructures on various substrates for applications in energy harvesting, sensing, optoelectronics, and electronic devices.

Large-area microlens arrays fabricated on flexible polycarbonate sheets via single-step laser interference ablation

A single-step method for the fabrication of large-area microlens arrays on flexible polycarbonate sheets is described. On areas of approximately 1 cm2, 17 million to 120 million microlenses ranging in size from sub-micrometer to several micrometers are fabricated via deep-UV pulsed laser interference ablation. The uniformity and surface quality of fabricated microlens arrays are examined and confirmed through atomic force microscopy and scanning electron microscopy. Optical imaging performance of the microlenses, and their use in massively parallel, pulsed laser nanofabrication on silicon is demonstrated. The microlens arrays can be fabricated in a matter of seconds, suggesting the potential for fast and low-cost production on flexible plastic substrates.

Fabrication of Patterned Polymer Nanowire Arrays

A method for the large-scale fabrication of patterned organic nanowire (NW) arrays is demonstrated by the use of laser interference patterning (LIP) in conjunction with inductively coupled plasma (ICP) etching. The NW arrays can be fabricated after a short ICP etching of periodic patterns produced through LIP. Arrays of NWs have been fabricated in UV-absorbent polymers, such as PET (polyethylene terephthalate) and Dura film (76% polyethylene and 24% polycarbonate), through laser interference photon ablation and in UV transparent polymers such as PVA (polyvinyl acetate) and PP (polypropylene) through laser interference lithography of a thin layer of photoresist coated atop the polymer surface. The dependence of the structure and morphology of NWs as a function of initial pattern created by LIP and the laser energy dose in LIP is discussed. The absence of residual photoresist atop the NWs in UV-transparent polymers is confirmed through Raman spectroscopy.

Reshaping the tips of ZnO nanowires by pulsed laser irradiation

AbstractVertically aligned ZnO nanowires have been synthesized by a hydrothermal method. After being irradiated by a short laser pulse, the tips of the as-synthesized ZnO nanowires can be tailored into a spherical shape. Transmission electron microscopy revealed that the spherical tip is a single-crystalline piece connected to the body of the ZnO nanowire, and that the center of the sphere is hollow. The growth mechanism of the hollow ZnO nanospheres is proposed to involve laser-induced ZnO evaporation immediately followed by re-nucleation in a temperature gradient environment. The laser-irradiated ZnO nanowire array shows hydrophobic properties while the original ZnO nanowire array shows hydrophilicity. The as-grown ZnO nanowire arrays with hollow spherical tips can serve as templates to grow ZnO nanowire arrays with very fine tips, which may be a good candidate material for use in field emission and scanning probe microscopy.

Rapid prototyping of microstructured hydrogels via laser direct-write and laser interference photopolymerisation

We present recent results on fabrication of micrometre- and sub-micrometre-scale structures in polyethylene glycol diacrylate (PEG-DA), a biocompatible hydrogel, useful in biomedical applications. The hydrogel structures were fabricated using two laser-based techniques; laser direct-write photopolymerisation, and multiple-beam laser interference photopolymerisation using continuous wave, nanosecond pulsed and femtosecond pulsed lasers, with light wavelengths of 266 nm, 355 nm, 532 nm and 800 nm. Through these techniques, we demonstrated the ability to fabricate fine structures over large areas without the use of templates or masks. Such structures can be useful for a variety of applications including cell biological studies, tissue engineering and drug delivery.

Silicon oxide nanowires: facile and controlled large area fabrication of vertically oriented silicon oxide nanowires for photoluminescence and sensor applications.

We describe a technique for the fabrication of dense and patterned arrays of aligned silicon oxide nanowires for applications in surface modification, optoelectronic, and electromechanical based devices. Conventional techniques for the fabrication of silicon oxide nanowires based on the vapor-liquid-solid (VLS) chemical vapor deposition (CVD) processes involve the use of high temperatures and catalysts. We demonstrate a technique that extends the use of a plasma thermal reactive ion etching for the fabrication of aligned silicon oxide nanowires with aspect ratios extending up to 20 and lengths exceeding 1 μm. The process incorporates phase separated PS-b-P4VP block copolymer loaded with an iron salt. The iron salt preferentially segregates into the P4VP layer and during an O2 etch is not removed but forms a hexagonally packed array on the silicon oxide substrate. Further etching with CHF3/O2 gas mixture over time can generate nanodots, to nanopillars, and then nanowires of silicon oxide. The photoluminescence property of the as-fabricated nanowire arrays as well as the parasitic ferromagnetic effect from the iron oxide-tipped section of the wires resulting in coalescence under an scanning electron microscope (SEM) are demonstrated. This technique is simpler compared to existing VLS fabrication approaches and can be used for the direct fabrication of patterned arrays of nanowires when a laser interference ablation step is incorporated into the fabrication procedure.

Patterning of periodic nano-cavities on PEDOT–PSS using nanosphere-assisted near-field optical enhancement and laser interference lithography

A simple approach for creating periodic nano-cavities and periodic stripes of nano-cavity arrays on poly (3,4-ethylene dioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) thin films using a combination of optical near-field enhancement through self-assembled silica nanospheres and laser interference lithography is presented. Monolayers of close-packed silica nanospheres (800, 600, and 430 nm in diameter) are self-assembled on 2 µm thick PEDOT-PSS electropolymerized films and are subsequently irradiated with 10 ns pulses of 355 nm wavelength laser light. Over areas spanning 2 cm(2), circular nano-cavities with central holes of size 50-200 nm and surrounding craters of size 100-400 nm are formed in the PEDOT-PSS films directly underneath the nanospheres due to strong enhancement (11-18 fold) of the incident light in the near-field, which is confirmed through Mie scattering theory. Predictions from theoretical simulations examining the combined effects of near-field enhancement and interference are in good agreement with the experimental results. The results illustrate the versatility of the described technique for creating nano-cavity arrays or nano-pores in PEDOT-PSS over large areas with designed periodicity and hole size.

Rapid fabrication of pentaerythritol triacrylate periodic structures on large areas by laser interference patterning with nanosecond pulses

We report on rapid fabrication of two-dimensional periodic structures in pentaerythritol triacrylate (PETIA) through laser interference patterning with 10 ns pulses from a frequency-tripled Nd:YAG laser emitting at 355 nm. Different periodic arrays including line-, cross-, honeycomb-, and dotlike structures were fabricated using two and three interfering laser beams. The composition of the photoinitiator was changed from 2% to 15% w/w to determine the threshold laser fluences necessary to photopolymerize the PETIA solution. The effects of the PETIA layer thickness and periodic geometries on the mechanical stability of the fabricated structures as well as self-organization processes are reported.

Direct periodic patterning of GaN-based light-emitting diodes by three-beam interference laser ablation

We report on the direct patterning of two-dimensional periodic structures in GaN-based light-emitting diodes (LEDs) through laser interference ablation for the fast and reliable fabrication of periodic micro- and nano-structures aimed at enhancing light output. Holes arranged in a two-dimensional hexagonal lattice array having an opening size of 500 nm, depth of 50 nm, and a periodicity of 1 μm were directly formed by three-beam laser interference without photolithography or electron-beam lithography processes. The laser-patterned LEDs exhibit an enhancement in light output power of 20% compared to conventional LEDs having a flat top surface without degradation of electrical and optical properties of the top p-GaN layer and the active region, respectively.

Hierarchical metallic and ceramic nanostructures from laser interference ablation and block copolymer phase separation

We report on the formation of hierarchical nanostructures of Au, PtOx, FexOy and PdOx using a hybrid technique by combining laser interference ablation (LIA) and block copolymer phase separation (BCPS). Different types of hierarchical arrays can be obtained including square, triangular, linear and circular arrays by varying the loading time of the block co-polymer with metallic salts, and the laser interference technique. The primary ordering of the as-generated nanoarrays (30-100 nm) is tunable by changing either the composition of the block copolymer spun from solution or by changing other parameters that affect the phase separation kinetics of the block copolymer, while the secondary ordering of the features can be tuned from 200 nm to 2 μm, by changing the angle of convergence of the laser beams on the patterned substrate. Such a robust method can be applied to the fabrication of other metallic and ceramic materials including Ag, Co, and Ni (O) and has potential use in the large scale fabrication of hierarchical arrays of catalysts that can be used to grow germanium, silicon nanowires using the vapour-liquid-solid growth (VLS) technique. The as-generated arrays can also find use in optical as well as sensor applications for biodetection and biosensing.