Bing-Rui Lu

High-resolution plasmonic structural colors from nanohole arrays with bottom metal disks.

We present transmissive plasmonic structural colors from subwavelength nanohole arrays with bottom metal disks for scaled-up manufacturing by nanoimprint lithography (NIL). Comprehensive theoretical and experimental studies are carried out to understand the specific extraordinary optical transmission behavior of the structures with such bottom metal disks. Distinctive colors covering the entire visible spectrum can be generated by changing the structural dimensions of hole arrays in Ag covered by the metal disks. The plasmonic energy hybridization theory is applied to explain the unstable color output with shallow holes so that a large processing window during NIL could be achieved for mass production. A high-resolution of 127,000 dots per inch is demonstrated with potential applications, including color filters and displays, high-resolution color printing, CMOS color imaging, and anti-counterfeiting.

Duplication of nanoimprint templates by a novel SU-8/SiO2/PMMA trilayer technique

In this work, a trilayer technique used in the nanoimprint lithography process to replicate the templates is developed. The SU8/SiO2/PMMA trilayer was used. The photosensitive epoxy (SU8 resist) which has a low glass transition temperature was used as the imprint layer. Polymethylmethacrylate (PMMA) was used as the transfer layer. A SiO2 layer is placed between the SU8 and PMMA to act as a protective layer due to its strong resistance to oxygen reactive ion etching. By optimizing imprint and etching processes, master templates with minimum feature size of 150 nm and period of 300 nm can be successfully duplicated.

Photon nanojet lens: Design, fabrication and characterization

In this paper, a novel nanolens with super resolution, based on the photon nanojet effect through dielectric nanostructures in visible wavelengths, is proposed. The nanolens is made from plastic SU-8, consisting of parallel semi-cylinders in an array. This paper focuses on the lens designed by numerical simulation with the finite-difference time domain method and nanofabrication of the lens by grayscale electron beam lithography combined with a casting/bonding/lift-off transfer process. Monte Carlo simulation for injected charge distribution and development modeling was applied to define the resultant 3D profile in PMMA as the template for the lens shape. After the casting/bonding/lift-off process, the fabricated nanolens in SU-8 has the desired lens shape, very close to that of PMMA, indicating that the pattern transfer process developed in this work can be reliably applied not only for the fabrication of the lens but also for other 3D nanopatterns in general. The light distribution through the lens near its surface was initially characterized by a scanning near-field optical microscope, showing a well defined focusing image of designed grating lines. Such focusing function supports the great prospects of developing a novel nanolithography based on the photon nanojet effect.

APPLICATIONS OF NANOIMPRINT LITHOGRAPHY FOR BIOCHEMICAL AND NANOPHOTONIC STRUCTURES USING SU-8

Nanoimprint lithography (NIL) technology has aroused great interests in both academia and industry due to its high resolution, low-cost, and high-volume nanopatterning capability. And as an expoxy resin-based negative amplified photoresist, SU-8 is an ideal candidate for NIL because of its low-glass-transition temperature, low-volume shrinkage coefficient, and good optical properties. In this reviewing paper, we highlight the major technical achievements in NIL on epoxy resin and its applications for bio- and nanophotonic structures. NIL was also applied for the duplication of imprint templates, originally fabricated by e-beam lithography (EBL) followed by reactive ion etch (RIE), using a SU-8/SiO2/PMMA tri-layer technique. And nanoimprint properties were systematically investigated for optimization. The developed nanoimprint process for different applications indicates promising industrial potentials in the next generation lithography resolution.

Fabrication of Nano-Strctures on PEDOT:PSS Film by Nanoimprint Lithography

It is very hard to fabricate nano-strctures on PEDOT:PSS film by conventional Nanoimprint Lithography for its non-thermoplastic property. Here we demonstrated a new nanoimprint process to pattern the PEDOT:PSS film at low temperature and low pressure by adding proper amount of Glycerol into PEDOT:PSS solution and pressing the Si-mold into Glycerol-PEDOT:PSS film under a pressure of 6.2Mpa for 45min at 80°C. We also compared our result to L. Tan and co-workers’. They found that positive replica was left on PEDOT:PSS film after pressing the Si-mold into Glycerol-PEDOT:PSS film under a pressure of 10Kpa for 5min at 80°C, but our work showed negative replica formed. Pressing time maybe is the critical reason to explain the different results. Holding the pressure longer gave the PEDOT:PSS enough time to flow into Si-mold and also gave Glycerol enough time to evaporate so that PEDOT:PSS became strong enough when separated the Si-mold from the PEDOT:PSS film. At last, Roman spectra was measured to confirm adding glycerol to PEDOT:PSS will not influence its molecular structure.

Surface stiffness modification by e-beam irradiation for stem cell growth controla)

This article reports a novel method to effectively modify the surface stiffness for the differentiation of stem cell growth. To achieve large range of surface hardness, focused electron beam is first employed to radiate hydrogen silsesquioxane (HSQ) film. With different degrees of curing caused by certain e-beam exposure, the HSQ demonstrates various Young’s modulus from 0.5 to 2 GPa, measured by an atomic force microscope. Fourier transform infrared spectra were used to investigate the origin of the stiffness change, which is due to the e-beam irradiation induced network formation inside HSQ. The novel technique possesses a number of advantages such as precision control of stiffness in a broad matrix with high spatial resolution. It also offers a good opportunity to define the geometry shape with a constant stiffness in nanometer scale.

Novel techniques for modifying microtube surfaces with various periodic structures ranging from nano to microscale

Two dimensional (2-D) structured membranes have been well developed and widely studied to find potential applications in broad realms like optics, mechanics, fluidics, and electronics. In this work, the authors have successfully combined the top–down patterning techniques with the roll-up process to convert various structured flat membranes into three dimensional (3-D) microtubes with textured tube-walls. These 3-D textured microtubes may exhibit novel properties different from the original 2-D films and, thus, can be applied in wider research disciplines such as modern material sciences, biology, electrochemistry, etc. Depending on the parameters of the periodic templates including nanoscale porous anodic alumina and microscale imprinted templates in this work, the authors can curve these textured films into 3-D microtubes with structures on the tube-walls by the rolled-up nanotechnique. The specially designed microtubes here have the potential of interesting optical, electrical, and mechanical character...

Design and fabrication of Bragg reflectors based on SU-8

We report the design and fabrication of two kinds of distributed Bragg reflectors (DBRs). One is a Bragg filter which introduces a periodic height modulation in part of a waveguide, and the other is a horizontal DBR with a deeply patterned grating structure in a waveguide at a freespace wavelength of 1.55 um (the propagation loss of quartz fiber used in optical communication is lowest around 1.55 um wavelength) based on a Si/SiO2/SU8/air structure. The transfer matrix theory was used to calculate the spectral response of the Bragg filter and its dependence on the height modulation and the grating length. In addition, a multilayer film reflection theory was applied to simulate reflection spectrum of the DBR and estimate the least number of pairs of DBR mirrors needed for almost 100% (¿ 99.6%) reflection. Based on the simulation results, we designed and successfully fabricated these two types of DBRs with Si/SiO2/SU8/air structures.

Silver surface-enhanced raman scattering substrates prepared by a nanofabrication process using Electron Beam Lithography and magnetron sputtering

The tunable silver nano-pillar arrays were fabricated by using Electron Beam Lithography and direct current magnetron sputtering technique successfully. The as-prepared substrates retain an excellent Raman-enhancement characteristic. The developed process provides a reproducible and reliable method to fabricate the Surface-enhanced Raman Scattering substrate particularly for the high sensitivity biological detections.

Fabrication of Nanostructured Hydrophobic Surfaces with Laser Interference Lithography

The fabrication of large area nanoscale periodic structures on material surfaces for hydrophobicity engineering has been difficult due to the complex processes. Here we propose a two-step fabrication method for periodic nanostructures by combining laser interference lithography (LIL) and reactive ion etching (RIE). Sub-micron periodic nanotip patterns are fabricated in the photoresist by LIL, and then transferred into the silicon substrate using RIE. By measuring the contact angle (CA) of a water droplet on the substrate surface, the wettability of the surface with nanotip structures of various periods is studied. Our experiments show that the nanotip structures fabricated by the combined LIL and RIE process deliver satisfactory hydrophobic tendencies when the periods fall into the submicron scale. When the period of the structure is small enough, the hydrophilicity of the surface can be altered into hydrophobicity. The hydrophobicity achieved by this method is reusable and sustainable with low cost and no composition alteration comparing to chemical methods. The process developed in this work provides potential applications in biosensingand digital fluidics.