M. Y. Lai

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.

Random and ordered arrays of surface magic clusters

Surface magic clusters (SMCs) are clusters exhibiting enhanced stability at certain sizes on a particular surface. Through the formation of SMCs, it is possible to grow an ensemble of nanostructures on a surface with extremely small or essentially zero size dispersion. Such an ensemble of nanostructures with identical size and atomic structure is highly desirable for certain nanotechnologies that rely on the homogeneity in the physical and chemical properties of the constituent nanostructures. This review summarizes current experimental observations and understanding of SMCs and discusses the most recent progress in the formation of a two-dimensional lattice of SMCs, whose constituent clusters have not only identical size and structure but also the same local environment due to the translational symmetry of the system.

Stepwise self-assembly of C60 mediated by atomic scale moiré magnifiers

Self-assembly of atoms or molecules on a crystal surface is considered one of the most promising methods to create molecular devices. Here we report a stepwise self-assembly of C₆₀ molecules into islands with unusual shapes and preferred sizes on a gold-indium-covered Si(111) surface. Specifically, 19-mer islands prefer a non-compact boomerang shape, whereas hexagonal 37-mer islands exhibit extraordinarily enhanced stability and abundance. The stepwise self-assembly is mediated by the moiré interference between an island with its underlying lattice, which essentially maps out the adsorption-energy landscape of a C₆₀ on different positions of the surface with a lateral magnification factor and dictates the probability for the subsequent attachment of C₆₀ to an islands periphery. Our discovery suggests a new method for exploiting the moiré interference to dynamically assist the self-assembly of particles and provides an unexplored tactic of engineering atomic scale moiré magnifiers to facilitate the growth of monodispersed mesoscopic structures.

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.

Formation of surface magic clusters: a pathway to monodispersed nanostructures on surfaces

Surface magic clusters (SMC) are clusters exhibiting enhanced stability at certain sizes on a particular surface. Through the formation of SMC, it is possible to grow an ensemble of nanostructures on a particular surface with extremely narrow size dispersion. Such monodispersed nanostructures are highly desirable for the realization of some emerging nanotechnology. This review summarizes the recent experimental observations and current theoretical understanding of SMC and discusses the possibility of exploiting the formation of such unusual clusters as a pathway to the growth of monodispersed nanostructures on surfaces.

Fabrication of an ordered nanoparticle array with a nanoaperture membrane used as a contact-mask

In this paper, we provide a useful technology to fabricate a long-range ordered nanoparticle array with a feature size under 30 nm. By adjusting the incident angle of Ar + beam milling on a U-shaped barrier layer of anodic alumina oxide, we can create a long-range ordered nanoaperture array with samples prepared by a focused-ion-beam-guided process. Compared to the naturally self-organized alumina nanochannels, the FIB-guided process has increased long-range ordering and uniformity of aperture size, and the aperture size can be varied by changing the grazing angle. The nanoaperture membrane can be used as a contact-mask and its undercut structure has another advantage for nanolithography. This technique could be extensively applied to the manufacturing of advanced nanodevices in large areas and as a catalyst to fabricate one-dimensional nanosized materials. (Some figures in this article are in colour only in the electronic version)

Autocatalytic reaction in hydrolysis of difructose anhydride III.

Hydrolysis of several polysaccharides in neutral and weak acid environment has been shown to exhibit autocatalytic behavior. Because the pH value of the solution decreases during hydrolysis, it has been proposed that proton is the catalyst of the autocatalytic reaction. We monitored the hydrolysis of difructose anhydride III (DFA III) in both strong and weak acid environment using Raman spectroscopy and found that it is also an autocatalytic reaction. Its Raman signatures were analyzed with ab initio method. When the reaction product, fructose, is added in the beginning of the reaction, the speed of hydrolysis increases to a magnitude that cannot be explained by the rate enhancement due to a decrease in the pH value, indicating that proton alone is not an effective catalyst for the reaction. It is the combination of proton and a certain form of reaction product such as monosaccharide or its derivatives that catalyzes the hydrolysis of difructose anhydride III. Similar results are observed in the hydrolysis of cellobiose, suggesting the universality of this autocatalytic reaction. Our findings provide the first clue to a new autocatalytic pathway in the hydrolysis of polysaccharides.

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.

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.