Chih-Yi Liu

Ordered anodic alumina nanochannels on focused-ion-beam-prepatterned aluminum surfaces

A combined process of electropolishing, focused-ion-beam lithography, and controlled anodization is used to fabricate anodic alumina films with ordered nanochannels. The ion beam is used to create a hexagonally close-packed lattice of concaves on a polished aluminum surface and the concaves act as pinning points for guiding the growth of nanochannels in the following anodization step. By carefully matching the lattice constant (100 nm) with the anodization voltage, ordered nanochannels with aspect ratio of ∼100 are fabricated. The effects of the ion dose and its corresponding depth of the concaves on the ordering of the nanochannel array are investigated and a minimum depth of 3 nm is found to be necessary for effective guidance of the growth of ordered nanochannels.

High-speed focused-ion-beam patterning for guiding the growth of anodic alumina nanochannel arrays

Long-range ordered arrays of anodic alumina nanochannels are grown by anodizing an aluminum covered with a patterned layer of polymethylmethacrylate (PMMA) resist. The two-dimensional hexagonal-closed-packed pattern is created by focused ion beam (FIB) exposure of the PMMA and transferred onto the aluminum surface by phosphoric acid etching. The required exposure time per channel is only ∼20 μs, more than two orders of magnitude reduction in comparison with the previous method employing FIB direct sputtering of the aluminum surface.

Anomalously enhanced Raman scattering from longitudinal optical phonons on Ag-nanoparticle-covered GaN and ZnO

The authors report experimental studies of surface-enhanced Raman scattering (SERS) of wurtzite-type GaN and ZnO crystalline samples covered with Ag-nanoparticles. The longitudinal optical phonons consistently exhibit unusually intense Raman enhancement in comparison with other phonons. The anomaly is interpreted by a proposed model based on a resonant Raman scattering process assisted by metal-induced gap states at the Ag/GaN and Ag/ZnO interfaces. This study suggests that SERS of lattice vibrations in inorganic semiconductors is sensitive to their propagation nature, providing a progressive perspective view on electron-mediated enhanced Raman scattering.

Order–disorder transition of anodic alumina nanochannel arrays grown under the guidance of focused-ion-beam patterning

By anodizing an aluminum surface that has been patterned with different guiding lattices using a focused ion beam, the resulting anodic alumina nanochannel arrays exhibit different degree of order arrangement. Long-range order is achieved only when the guiding lattice is carefully matched to that of the self-organized hcp array formed locally in an unguided area. Lattice mismatch between the guiding and locally self-organized lattice leads to an order–disorder transition via the creation and annihilation of nanochannels. The driving force of the transition is attributed to the unbalance in the stress among the nanochannels.

Ordered arrays of Ag nanoparticles grown by constrained self-organization

In a postdeposition annealing process, the Ag deposited at low temperature on a Si(100) substrate aggregates to regions that have been exposed to adequate dose of energetic ions. By patterning an array of such aggregation centers on the substrate, using a 50keV focused Ga+ ion beam, the migration of Ag on the patterned area during the subsequent annealing is constrained and ordered arrays of Ag nanoparticles with uniform size distribution are formed spontaneously. The mechanism and the optimum conditions for constraining the self-organization of Ag on Si by focused-ion-beam patterning are discussed.

Long-range ordered nanoaperture array with uniform diameter and interpore spacing

In application of focused-ion-beam lithography and grazing Ar+ milling on the U-shape barrier layer of anodic alumina nanochannels, we fabricated a hexagonally symmetry aperture array with nominal diameter of 12±2nm and interspacing of 100±2nm. Besides long-range spatial ordering, the focused-ion-beam guided-grown process has also significantly improved uniformity of both the interpore spacing and the aperture size. This aperture array membrane can be applied to the fabrication of nanostructures, such as a lithographic contact mask for ordered quantum-dot array.

Tip-enhanced Raman spectroscopy of graphite irradiated by focused ion beam

Tip-enhanced Raman spectroscopy with a spatial resolution of 25 nm is conducted on a sample of graphite that has been exposed to a 50 keV focused Ga ion beam with a diameter of 20 nm. The G mode located at 1579 cm(-1) does not exhibit a measurable tip enhancement, while the D mode at 1364 cm(-1) is significantly enhanced. A method is proposed to calculate the enhancement factor of anisotropic materials due to the electromagnetic field using the measured signal enhancement induced by the tip.

Morphological evolution of porous nanostructures grown from a single isolated anodic alumina nanochannel.

Porous anodic aluminum oxide (AAO) membranes have been widely used as templates for growing nanomaterials because of their ordered nanochannel arrays with high aspect ratio and uniform pore diameter. However, the intrinsic growth behavior of an individual AAO nanochannel has never been carefully studied for the lack of a means to fabricate a single isolated anodic alumina nanochannel (SIAAN). In this study, we develop a lithographic method for fabricating a SIAAN, which grows into a porous hemispherical structure with its pores exhibiting fascinating morphological evolution during anodization. We also discover that the mechanical stress affects the growth rate and pore morphology of AAO porous structures. This study helps reveal the growth mechanism of arrayed AAO nanochannels grown on a flat aluminum surface and provides insights to help pave the way to altering the geometry of nanochannels on AAO templates for the fabrication of advanced nanocomposite materials.

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.