Kun-Tong Tsai

Functionalized arrays of Raman-enhancing nanoparticles for capture and culture-free analysis of bacteria in human blood

Summary form only given. Detecting bacteria in clinical samples without the time-consuming culture process is most desired for rapid diagnosis. Such a culture-free detection needs to capture and analyze bacteria from a body fluid usually containing complicate constituents. Here we show that vancomycin (Van) coating of a special substrate with arrays of Ag-nanoparticles, which can provide label-free analysis of bacteria via surface enhanced Raman spectroscopy (SERS), leads to 1000 folds increase in its capability to capture bacteria without introducing significant spectral interference. Bacteria spiked in human blood can be concentrated onto a microscopic Van-coated area while blood cells are excluded. Furthermore, A Van-coated substrate provides distinctly different SERS spectra of Van-susceptible and Van-resistant Enterococcus, indicating its potential use for drug-resistance test. Our results represent a critical step towards the creation of SERS-based multifunctional biochips for rapid culture/label-free detection and drug-resistant testing of microorganisms in clinical samples.

Periodic Si nanopillar arrays by anodic aluminum oxide template and catalytic etching for broadband and omnidirectional light harvesting

Large-area, periodic Si nanopillar arrays (NPAs) with the periodicity of 100 nm and the diameter of 60 nm were fabricated by metal-assisted chemical etching with anodic aluminum oxide as a patterning mask. The 100-nm-periodicity NPAs serve an antireflection function especially at the wavelengths of 200~400 nm, where the reflectance is decreased to be almost tenth of the value of the polished Si (from 62.9% to 7.9%). These NPAs show very low reflectance for broadband wavelengths and omnidirectional light incidence, attributed to the small periodicity and the stepped refractive index of NPA layers. The experimental results are confirmed by theoretical calculations. Raman scattering intensity was also found to be significantly increased with Si NPAs. The introduction of this industrial-scale self-assembly methodology for light

Imaging visible light using anisotropic metamaterial slab lens

It has been shown that an anisotropic metamaterial made of nanowire array can realize negative refraction of light even without a negative phase index of refraction. Such non-resonant bulk material can be fabricated by bottom-up electrochemical method. Using this material, we were able to achieve lensing action with micron-thick slab and demonstrate imaging of a slit object. The details of the focused light beam in 3-dimensional space have been mapped with near field scanning optical microscope (NSOM).

Design, fabrication and characterization of indefinite metamaterials of nanowires

Indefinite optical properties, which are typically characterized by hyperbolic dispersion relations, have not been observed in naturally occurring materials, but can be realized through a metamaterial approach. We present here the design, fabrication and characterization of nanowire metamaterials with indefinite permittivity, in which all-angle negative refraction of light is observed. The bottom-up fabrication technique, which applies electrochemical plating of nanowires in porous alumina template, is developed and demonstrated in achieving uniform hyperbolic optical properties at a large scale. We developed techniques to improve the uniformity and to reduce the defect density in the sample. The non-magnetic design and the off-resonance operation of the nanowire metamaterials significantly reduce the energy loss of electromagnetic waves and make the broad-band negative refraction of light possible.

Contact transport of focused ion beam-deposited Pt to Si nanowires: From measurement to understanding

The Si nanowires (NWs) were contacted by focused ion beam (FIB)-deposited Pt as the Ohmic contacts. Ultralow specific contact resistivity of 1.2 × 10−6 Ω-cm2 has been measured. Due to the focused ion beam-induced amorphization of Si NWs, contact behavior is explained by diffusion theory, allowing accurate estimation of electron concentration, electron mobility, effective barrier height, and ideality factor. This study can be the guidance to correct measurement and understanding of the contact transport, which is useful for NWs device design and fabrication.

Contact behavior of focused ion beam deposited Pt on p-type Si nanowires

Pt contact on p-Si nanowires (NWs) using Ga-ion-induced deposition by a focused ion beam was formed with a specific contact resistance (rho(c)) of 1.54 x 10(-6) Omega cm(2). Ohmic behavior is caused by Ga-ion-induced amorphization of Si NWs underneath the Pt contact. A very low Schottky barrier height associated with interface states raised from Pt-amorphized Si junction and with an image force induced by the applied bias can be implemented to elucidate ultralow rho(c). The value of rho(c) lower than that of any known contact to Si NWs demonstrates a practical method for integrating NWs in devices and circuits.

Probing surface plasmons in individual Ag nanoparticles in the ultra-violet spectral regime.

Previous investigations of surface plasmons in Ag largely focused on their excitations in the visible spectral regime. Using scanning transmission electron microscopy with an electron beam of 0.2 nm in conjunction with electron energy-loss spectroscopy, we spectrally and spatially probe the surface plasmons in individual Ag nanoparticles (approximately 30 nm), grown on Si, in the ultra-violet spectral regime. The nanomaterials show respective sharp and broad surface-plasmon resonances at approximately 3.5 eV (approximately 355 nm) and approximately 7.0 eV (approximately 177 nm), and the correlated spectral calculations established their multipolar characteristics. The near-field distributions of the surface plasmons on the nanoparticles were also mapped out, revealing the predominant dipolar nature of the 3.5 eV excitation with obvious near-field enhancements at one end of the nano-object. The unveiled near-field enhancements have potential applications in plasmonics and molecular sensing.

Electrochemically replicated smooth aluminum foils for anodic alumina nanochannel arrays

A fast electrochemical replication technique has been developed to fabricate large-scale ultra-smooth aluminum foils by exploiting readily available large-scale smooth silicon wafers as the masters. Since the adhesion of aluminum on silicon depends on the time of surface pretreatment in water, it is possible to either detach the replicated aluminum from the silicon master without damaging the replicated aluminum and master or integrate the aluminum film to the silicon substrate. Replicated ultra-smooth aluminum foils are used for the growth of both self-organized and lithographically guided long-range ordered arrays of anodic alumina nanochannels without any polishing pretreatment.

Custom-designed arrays of anodic alumina nanochannels with individually tunable pore sizes

We demonstrate a process to selectively tune the pore size of an individual nanochannel in an array of high-aspect-ratio anodic aluminum oxide (AAO) nanochannels in which the pore sizes were originally uniform. This novel process enables us to fabricate arrays of AAO nanochannels of variable sizes arranged in any custom-designed geometry. The process is based on our ability to selectively close an individual nanochannel in an array by using focused ion beam (FIB) sputtering, which leads to redeposition of the sputtered material and closure of the nanochannel with a capping layer of a thickness depending on the energy of the FIB. When such a partially capped array is etched in acid, the capping layers are dissolved after different time delays due to their different thicknesses, which results in differences in the time required for the following pore-widening etching processes and therefore creates an array of nanochannels with variable pore sizes. The ability to fabricate such AAO templates with high-aspect-ratio nanochannels of tunable sizes arranged in a custom-designed geometry paves the way for the creation of nanophotonic and nanoelectronic devices.

Photoluminescence from quasi-dendritic ZnO nanostructures grown in anodic alumina nanochannels

Atomic layer deposition (ALD) has been used to grow zinc oxide (ZnO) into a template of anodic aluminum oxide with quasi-dendritic nanochannels to form quasi-dendritic nanostructures. The characteristic photoluminescence (PL) emission from the inner region of the quasi-dendritic ZnO nanostructure peaks at 397 nm while that from its outer region at 424 nm. In between the two regions, the PL peak shows monotonic shift. In other words, the different layers of the single quasi-dendritic ZnO nanostructure emit PL with graded wavelengths spontaneously. The red shift in the PL peak positions is likely to be caused by the change in local stoichiometry between Zn and O, which are resulted from the limited supply of materials through the quasi-dendritic nanochannels during the ALD. The process to fabricate such quasi-dendritic ZnO nanostructures with spontaneously graded emission could help expand applications of ZnO-based devices.