Dehui Wan

Eco-friendly plasmonic sensors: using the photothermal effect to prepare metal nanoparticle-containing test papers for highly sensitive colorimetric detection.

Convenient, rapid, and accurate detection of chemical and biomolecules would be a great benefit to medical, pharmaceutical, and environmental sciences. Many chemical and biosensors based on metal nanoparticles (NPs) have been developed. However, as a result of the inconvenience and complexity of most of the current preparation techniques, surface plasmon-based test papers are not as common as, for example, litmus paper, which finds daily use. In this paper, we propose a convenient and practical technique, based on the photothermal effect, to fabricate the plasmonic test paper. This technique is superior to other reported methods for its rapid fabrication time (a few seconds), large-area throughput, selectivity in the positioning of the NPs, and the capability of preparing NP arrays in high density on various paper substrates. In addition to their low cost, portability, flexibility, and biodegradability, plasmonic test paper can be burned after detecting contagious biomolecules, making them safe and eco-friendly.

Using Spectroscopic Ellipsometry to Characterize and Apply the Optical Constants of Hollow Gold Nanoparticles

In this paper, we report the optical constants (refractive index, extinction coefficient) of self-assembled hollow gold nanoparticle (HGN) monolayers determined through spectroscopic ellipsometry (SE). We prepared a series of HGNs exhibiting various morphologies and surface plasmon resonance (SPR) properties. The extinction coefficient (k) curves of the HGN monolayers exhibited strong SPR peaks located at wavelengths that followed similar trends to those of the SPR positions of the HGNs in solution. The refractive index (n) curves exhibited an abnormal dispersion that was due to the strong SPR extinction. The values of Deltan and kmax both correlated linearly with the particle number densities. From a comparison of the optical constant values of HGNs with those of solid Au nanoparticles (NPs), we used SE measurements to demonstrate a highly sensitive Si-based chemical sensor. HGNs display a slightly lower value of k at the SPR peak but a much higher sensitivity to changes in the surrounding medium than do solid Au NPs.

One-shot deep-UV pulsed-laser-induced photomodification of hollow metal nanoparticles for high-density data storage on flexible substrates.

In this paper, we report a new optical data storage method: photomodification of hollow gold nanoparticle (HGN) monolayers induced by one-shot deep-ultraviolet (DUV) KrF laser recording. As far as we are aware, this study is the first to apply HGNs in optical data storage and also the first to use a recording light source for the metal nanoparticles (NPs) that is not a surface plasmon resonance (SPR) wavelength. The short wavelength of the recording DUV laser improved the optical resolution dramatically. We prepared HGNs exhibiting two absorbance regions: an SPR peak in the near-infrared (NIR) region and an intrinsic material extinction in the DUV region. A single pulse from a KrF laser heated the HGNs and transformed them from hollow structures to smaller solid spheres. This change in morphology for the HGNs was accompanied by a significant blue shift of the SPR peak. Employing this approach, we demonstrated its patterning ability with a resolving power of a half-micrometer (using a phase mask) and developed a readout method (using a blue-ray laser microscope). Moreover, we prepared large-area, uniform patterns of monolayer HGNs on various substrates (glass slides, silicon wafers, flexible plates). If this spectral recording technique could be applied onto thin flexible tapes, the recorded data density would increase significantly relative to that of current rigid discs (e.g., compact discs).

White-Light-Induced Collective Heating of Gold Nanocomposite/Bombyx mori Silk Thin Films with Ultrahigh Broadband Absorbance

This paper describes a systematic investigation of the phenomenon of white-light-induced heating in silk fibroin films embedded with gold nanoparticles (Au NPs). The Au NPs functioned to develop an ultrahigh broadband absorber, allowing white light to be used as a source for photothermal generation. With an increase of the Au content in the composite films, the absorbance was enhanced significantly around the localized surface plasmon resonance (LSPR) wavelength, while non-LSPR wavelengths were also increased dramatically. The greater amount of absorbed light increased the rate of photoheating. The optimized composite film exhibited ultrahigh absorbances of approximately 95% over the spectral range from 350 to 750 nm, with moderate absorbances (>60%) at longer wavelengths (750-1000 nm). As a result, the composite film absorbed almost all of the incident light and, accordingly, converted this optical energy to local heat. Therefore, significant temperature increases (ca. 100 °C) were readily obtained when we irradiated the composite film under a light-emitting diode or halogen lamp. Moreover, such composite films displayed linear light-to-heat responses with respect to the light intensity, as well as great photothermal stability. A broadband absorptive film coated on a simple Al/Si Schottky diode displayed a linear, significant, stable photo-thermo-electronic effect in response to varying the light intensity.

Using One-Step, Dual-Side Nanoimprint Lithography to Fabricate Low-Cost, Highly Flexible Wave Plates Exhibiting Broadband Antireflection

In this study, we used dual-side nanoimprint lithography to prepare sub-wavelength periodical grating (SPG)―based wave plates on flexible substrates. Without employing any etching, we directly imprinted SPGs on both sides of a polycarbonate (PC) substrate through a one-step imprinting process. This dual-side nanoimprint lithography process dramatically decreases the typical imprinting pressure required in conventional resist-based nanoimprint lithography. With this low-cost formation of PC-based wave plates, any degree of phase retardation could be obtained merely by stacking a set of wave plates with designed depths and working wave-lengths. Our SPG-based wave plates exhibited the excellent broadband antireflection properties of sub-wavelength structures possessing refractive index gradients on both sides of the PC substrates. The high flexibility, low cost, and low surface reflection of these dual-side SPG-based wave plates make them attractive alternatives to commonly used birefringent crystal―based wave plates, suggesting that this phase retardation technique has promising potential for use in the development of next-generation flexible optoelectronic devices and systems.

Using self-assembled nanoparticles to fabricate and optimize sub-wavelength textured structures in solar cells

In this study, we demonstrated the textured structure on silicon surface by metal assisted etching method, using Au nanoparticles as catalysts in HF and H2O2 solution. The size and density of the nanoparticles could be tuned easily. The porous layers filled with cylinder- or cone-shaped were uniformly formed by immersing the gold deposited silicon wafers in a mixed solution containing HF and H2O2 under different etching conditions. The optimized textured structure was close-packed pyramids-like surface in subwavelength scale and showed the lowest reflectance less than 0.5% over whole visible and near IR wavelengths. The large reduction of reflectance was attributed from the gradient refractive index of the silicon surface with the depth along the light propagation.

Fabrication of gold-nanoparticle-infiltrated inverse opal structures with both photonic bandgap and surface plasmon resonance characteristics

This paper demonstrates a simple method to fabricate gold-nanoparticle-infiltrated inverse-opal structures using annealing and sintering process. The transmission spectra indicates that the inverse opal structure have both photonic band-gap and surface plasmon resonance characteristics.