Juanyuan Hao

Biomimetic Antireflective Si Nanopillar Arrays

The Fresnel reflection of incident light comes from the large refractive index discontinuity at the interface of two media. The high reflective index of Si results in the reflection of up to 40% of the incident light, which severely limits the performance of Si based optical and optoelectronic devices, such as solar cells, displays, and light sensors. Thin film coatings with intermediate or gradient refractive indices are commonly utilized to suppress undesired Fresnel reflection. However, the stability problems induced by adhesiveness and thermal mismatch are often associated with such approaches. Since the corneas of nocturnal-moth eyes were found to have antireflective properties, surface-relief arrays with dimension smaller than the wavelength of incident light have been considered an alternative to thin-film coatings, which are more stable and durable than surface coatings since only one material is involved. In the last decade, increasing effort has been devoted to the fabrication of such surface antireflection structures. Many different structures, such as nanorods and nanopillars, gratings, porous structures, and nanotubes have been created in order to suppress surface reflection. The basic purpose of this technique is to introduce a refractive index gradient between air and a substrate material by creating a structured layer. Reflection can be substantially suppressed for a wide spectral bandwidth and over a large field of view. Due to the applications of Si in modern optical and optoelectronic industry, a variety of techniques for producing subwavelength ‘‘moth eye’’ structures on Si have been proposed, such as electron-beam lithography, laser interference lithography, and nanoimprint lithography. However, these

Tuning the Intensity of Metal‐Enhanced Fluorescence by Engineering Silver Nanoparticle Arrays

It is demonstrated that silver nanoparticle (SNP) arrays fabricated by combining nanoimprint lithography and electrochemical deposition methods can be used as substrates for metal-enhanced fluorescence, which is widely used in optics, sensitive detection, and bioimaging. The method presented here is simple and efficient at controlling the nanoparticle density and interparticle distance within one array. Furthermore, it is found that the fluorescence intensity can be tuned by engineering the feature size of the SNP arrays. This is due to the different coupling efficiency between the emission of the fluorophores and surface plasmon resonance band of the metallic nanostructures.

High‐Resolution Triple‐Color Patterns Based on the Liquid Behavior of Organic Molecules

Driven by anticipation of novel applications, enormous progress has been made in organic semiconductors since the 1950s. Historically, much of the initial work was centered on organic light-emitting diodes (OLEDs) [ 1 , 2 ] and photovoltaics (PVs) [ 3 , 4 ] due to their intensive applications. Owing to efforts from both academia and industry over the last two decades, the research interest has dramatically expanded to transistors, [ 5 ] sensors, [ 6 ] memories, [ 7 ] lasers, [ 8 ] etc. In organic microelectronics and optoelectronics, the ability to create scalable high-resolution patterns of semiconductor organic molecules still presents challenges for fabrication technology. As an example, a full-color microdisplay requires high-resolution red, green, and blue (RGB) OLEDs to be patterned in close proximity of micrometers. Current patterning techniques based on shadow-mask [ 9 ] and vapor-jet printing [ 10 ] suffer from low resolution, poor scalability, and complicated multistep processing. Previously, we demonstrated the template-directed growth technique as a promising method for patterning organic semiconductors with tunable physical properties. [ 11–13 ] Herein, we show the liquid behavior of appropriately selected dye molecules on prepatterned substrates. By further controlling the diffusion of molecules between designed patterns, we achieve heteropatterning of organic materials, that is, the thickness of organic molecule patterns differs in a controlled way over predefi ned areas. Further deposition of a second type of molecule on the heteropatterned structure produces high-resolution, ordered, triple-color organic patterns using only two dyes. In this study, we use N , N ′ -di[( N -(3,6-ditertbutylcarbazyl))n -decyl] quinacridone (DtCDQA), an orange light-emitting material with high quantum yield and multiple emission states in both liquid and solid solvents. [ 13 ]

Site-Selective Patterning of Organic Luminescent Molecules via Gas Phase Deposition

In this paper, we present a bottom-up approach to pattern organic luminescent molecules with a feature size down to sub-100 nm over wafer-sized areas. This method is based on the selective gas deposition of organic molecules on self-organized patterned structures, which consist of an organic monolayer with two different phases rather than different materials. The site selectivity is controllable by deposition rate and the pattern features. The reason for the site selectivity may be due to the nucleation and diffusion behaviors of the deposited organic molecules on different monolayer phases.

Creating bicolor patterns via selective photobleaching with a single dye species.

Bicolor fluorescent pattern in thin polymer film is fabricated via a photobleaching process. Dye molecules exhibit monomer emission when they are dispersed inside the polymer and aggregate emission when they are on the surface of the polymer. Thus, a mixed emission of monomer and aggregate can be obtained by evaporating a single dye species on the polymer film. Bicolor pattern in thin polymer film is readily formed by selective photobleaching. This process is particularly attractive for the fabrication of bicolor patterns on flat substrates using a single dye species, which is of potential applications in photonic/electronic devices.

Anisotropic growth of organic semiconductor based on mechanical contrast of pre-patterned monolayer

Site-selective anisotropic growth of perylene film is achieved by using a striped Langmuir–Blodgett (LB) monolayer as an alignment layer. The stripes significantly increase the grain size along the stripe direction due to increased diffusion length. By controlling the grain boundaries and the molecular alignment, a mobility anisotropic ratio of ∼10 for the current flow parallel and perpendicular to the stripes has been observed.

Fabrication of split-ring resonators by tilted nanoimprint lithography

An efficient fabrication technique for large area periodic metallic split-ring arrays has been demonstrated by the combination of tilted nanoimprint lithography and nanotransfer imprinting. The feature size of the split-rings can be adjusted by varying the key geometry parameters of the original imprinting mold. Due to the flexible nature of PDMS molds, these arrays can be patterned on curved surfaces. The molds for nanoimprint lithography and nanotransfer imprinting can be used multiple times without a loss of fidelity.

Selective deposition of organic molecules onto DPPC templates--a molecular dynamics study.

The site-selective deposition of organic molecules onto template structures to create ordered micro/nanoscale arrangements has drawn more and more attention because of the broad possibility, for example, application in organic electronic devices. Here we present a molecular dynamics study toward the selective deposition of organic molecules 3(5)-(9-anthryl) pyrazole (ANP), perylene and sexiphenyl (6P) onto template structures made of the phospholipid L-α-dipalmitoyl-phosphatidylcholine (DPPC) in alternating liquid expanded (LE) and liquid condensed (LC) states. The simulation results indicate, first of all, that the molecules immerge into both LE and LC phases instead of staying on top of them. Furthermore, the simulations replicate the empirically observed higher diffusion constants of the organic molecules on LE phase compared to LC phase of the underlying DPPC layer. Additionally, we propose a possible mechanism for the diffusion barrier between LE/LC phase needed to explain the experimental findings of the selective deposition. Altogether, this study supports the notions suggested by the experiments on the causes of the selective deposition while giving a deeper insight into the molecular processes involved.

Fabrication of multicolor patterns with a single dye species on a polymer surface.

Multicolored patterns can be fabricated by evaporating a single dye species on a prepatterned polymer substrate. The ratios of dye to polymer are different on protrusion and recess areas of the prepatterned surface, which can result in different aggregates and emissions. The polymer substrate was prepatterned using nanoimprint lithography (NIL) without any further process. This method may provide a facile route for fabricating large-area multicolored patterns.