Gold nanostructures on chemically reinforced PDMS microwell arrays

This paper describes a facile strategy for fabricating arrays of two- and three-dimensional gold nanostructures using PDMS-infiltrated polystyrene (PS) colloidal crystals. PDMS molding of colloidal crystal, gold vapor deposition, and subsequent calcination of PS produced gold thin layers over hexagonal PDMS microwell arrays with hemispherical air-voids of approximately 140 nm on glass substrates. Vapor deposition of perfluoroalkylsilane thin layers improved the thermal stability of the colloidal template over 100 °C, providing a route to preparation of hollow architectures with gold thin layers supported by PDMS nanostructures. Surface modification of the PDMS using poly(allylamine hydrochloride) induced two-dimensional colloidal crystals of PS and PMMA spheres through electrostatic interactions. Particle aggregation of 13 nm gold nanoparticles in the PDMS microwells demonstrated a surface plasmon resonance band red-shifted to 810 nm, in comparison with that on the flat surface at 720 nm.     

Large-area silica nanotubes with controllable geometry on silicon substrates

The synthesis of a highly uniform, large-scale nanoarrays consisting of silica nanotubes above embedded nanohole arrays in silicon substrates is demonstrated. In situ anodized aluminium oxide (AAO) thin film masks on Si substrates were employed, and the nanotubes were fabricated by Ar ion milling through the masks. The geometries of the nanoarrays, including pore diameter, interpore distance and the length of both nanopores and nanotubes could be controlled by the process parameters, which included that the outer pore diameter of silica tube was tuned from ∼80 nm to ∼135 nm while the inner tube diameter from ∼40 nm to ∼65 nm, the interpore distance of the nanotube arrays was from 100 nm to 180 nm and the length of silica tube changed from ∼90 nm to ∼250 nm. The presented nanostructure fabrication method has strong potential for application in intensity and frequency adjustable high luminescence efficiency optoelectronic devices.     

Probing the diffusion of vacuum ultraviolet (λ = 172 nm) induced oxidants by nanoparticles immobilization

Vacuum ultraviolet (VUV, λ = 172 nm) patterning of alkyl monolayer on silicon surface has been demonstrated with emphasis on the diffusion of VUV induced oxygen-derived active species, which are accountable for the pattern broadening. The VUV photons photo-dissociates the atmospheric oxygen and water molecules into the oxygen-derived active species (oxidants). These oxidants photo-oxidize the hexadecyl (HD) monolayer in VUV irradiated regions (Khatri et al., Langmuir. 24 (2008) 12077), as well as the little concentration of oxidants diffuses towards the masked areas. In this study, we performed VUV patterning at a vacuum pressure of 10 Pa to track the diffusion pathways for the oxidants with help of gold nanoparticles (AuNPs; ? = 10 nm) immobilization. At VUV irradiated sites AuNPs are found as uniformly distributed, but adjacent to the pattern boundary we observed quasi-linear arrays of AuNPs, which are determined by diffusion pathways of the oxidants. The diffusion of oxidants plays vital role in pattern broadening. The site selective anchoring of AuNPs demonstrates the utility of VUV photons for the construction of functional materials with microstructural architecture.     

Field emission from patterned SnO2 nanostructures

A simple and reliable method has been developed for synthesizing finely patterned tin dioxide (SnO₂) nanostructure arrays on silicon substrates. A patterned Au catalyst film was prepared on the silicon wafer by radio frequency (RF) magnetron sputtering and photolithographic patterning processes. The patterned SnO₂ nanostructures arrays, a unit area is of ∼500 μm × 200 μm, were synthesized via vapor phase transport method. The surface morphology and composition of the as-synthesized SnO₂ nanostructures were characterized by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The mechanism of formation of SnO₂ nanostructures was also discussed. The measurement of field emission (FE) revealed that the as-synthesized SnO₂ nanorods, nanowires and nanoparticles arrays have a lower turn-on field of 2.6, 3.2 and 3.9 V/μm, respectively, at the current density of 0.1 μA/cm². This approach must have a wide variety of applications such as fabrications of micro-optical components and micropatterned oxide thin films used in FE-based flat panel displays, sensor arrays and so on.     

Polymeric arrayed waveguide grating using imprint method incorporating a flexible PDMS stamp

A polymeric arrayed waveguide grating (AWG) has been proposed and demonstrated by employing the nanoimprint method. A flexible PDMS (polydimethylsiloxane) stamp with the device patterns engraved was developed from a rigid master mold made of quartz glass, featuring uniform precision contact with and easy separation from polymer films. The device was fabricated by replicating the pattern on the stamp in a core polymer layer, with no etching process involved. For our device, the number of the output channels is eight and the center wavelength of each output channel is positioned from 1543.7 nm to 1548.3 nm with the spacing of 0.8 nm. The achieved channel crosstalk was about 10 dB and the 3 dB bandwidth approximately 0.8 nm.     

Direct imprinting of ordered and dense TiO2 nanopore arrays by using a soft template for photovoltaic applications

Highly ordered and dense TiO₂ nanopore arrays are directly nanoimprinted on a transparent conductive glass substrate by using a polymethylmethacrylate/polydimethylsiloxane (PMMA/PDMS) composite soft template, which is replicated from an anodic aluminum oxide (AAO) replica mold. Results indicate that heat infiltration under vacuum conditions can ensure complete filling of PMMA into the AAO pores, and that free-standing PMMA nanorods with an aspect ratio more than 5 can be obtained by adjusting the AAO pore depth based on a freeze-drying technique. TiO₂ nanopore arrays with different diameters from 30 to 300 nm and inter-pore distances between 70 and 450 nm can be easily fabricated by using the corresponding templates with different sizes. Preliminary solar cells are also assembled with a heterojunction of conjugated polymer/TiO₂ nanopore arrays. Results indicate that the construction of poly-(3-hexylthiophene) (P3HT)/TiO₂ nanopore arrays can be more helpful in quenching the PL emission of P3HT than that of P3HT/flat TiO₂ film, and a maximum efficiency of about 0.32% can be obtained for a photovoltaic device with a TiO₂/[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM)/P3HT structure.     

A sensor for adenosine triphosphate fabricated by laser-induced forward transfer of luciferase onto a poly(dimethylsiloxane) microchip

Laser-induced forward transfer (LIFT) of the enzyme luciferase was explored as a potential technique to be used in the fabrication of a microchip adenosine triphosphate (ATP) sensor. Poly(dimethylsiloxane) (PDMS) was selected as the substrate for deposition of the luciferase. In comparison with other solid substrates, such as glass and polystyrene, it was found that the flexibility of PDMS made it a superior substrate for the immobilization of micro-spots of luciferase. LIFT of luciferase onto a PDMS substrate using a 355 nm laser was successfully carried out, while the bioactivity of the enzyme was maintained. Yellow luminescence ascribed to luciferase was observed from a transferred spot on the PDMS chip from the enzymatic reaction between luciferin and ATP. A microchip ATP sensor was also fabricated by attaching a small photodiode to the PDMS chip. On the basis of the fabricated microchip, the Michaelis-Menten relation between the luminescence intensity from the spot, and the ATP concentration was confirmed. The potential for fabricating biosensors using a combination of the LIFT technique with a PDMS substrate was shown to be very good.     

Facile route to high-density, ordered ZnO nanowire arrays and their photoluminescence properties

A simple template-directed wet chemistry route based on traditional thermal decomposition technique has been developed for the preparation of high-density, ordered ZnO nanowire arrays. The fabrication was performed at relative low temperature without involving complex procedures, sophisticated equipment and rigorous experiment conditions, thereby providing a straightforward and mild method to produce metal/metal oxide ordered nanostructures. The nanowire array system was evaluated by SEM, XRD, TEM and PL. A stable luminescence at 425 nm was present.     

Long range ordering in self-assembled Ni arrays on patterned Si

We have succeeded in aligning self-assembled structures by using a lithographically defined stripe. The 140 nm wide by 100 nm high SiO₂ strip is shown to guide the assembly of 500 nm latex spheres so that spheres are aligned along the strip and are in registration on either side of the strip. This method can be used to increase long-range ordering in magnetic storage systems without compromising the density. Inverse sphere Ni arrays were made by electrodeposition through the latex template. We also show that the hexagonal symmetry of the resulting inverse sphere Ni arrays can be simulated using the approach presented below.     

Carbon nanodot arrays grown as replicas of specially widened anodic aluminum oxide pore arrays

Via a specially widened anodic aluminum oxide (AAO) pore arrays, carbon nanodot arrays with uniform size and high density were obtained through filtered cathodic arc plasma (FCAP) technique. The AAO template was prepared in oxalic acid by multi-steps to get a specially enlarged opening which plays an important role in the deposition of nanodots. The morphology of the nanodots was studied by a field emission scanning electron microscopy (FESEM). The diameter of the as-prepared nanodot demonstrated here is about 100 nm at the bottom and less than 40 nm at the top, and the density was estimated to 10¹⁰ cm⁻². Field emission properties of the nanodot arrays were investigated and a low threshold field of 5.1 V/μm at 10 mA/cm² was obtained. In this paper, the carbon nanodot arrays grown as replicas of the specially widened AAO template may support a strategy to realize the fabrication of nanodot arrays with various materials.     

Effects of thermal annealing temperature and duration on hydrothermally grown ZnO nanorod arrays

In this study, the effects of thermal annealing temperature and duration on ZnO nanorod arrays fabricated by hydrothermal method were investigated. The annealed ZnO/Si(1 1 1) substrate was used for ZnO nanorod array growth. The effects of annealing treatment on the structural and optical properties were investigated by scanning electron microscopy, X-ray diffraction, and room-temperature photoluminescence measurements. With the annealing temperature of 750 °C and the annealing duration of 10 min, both the structural and optical properties of the ZnO nanorod arrays improved significantly, as indicated in the X-ray diffraction and photoluminescence measurement.     

Temperature influence on the anodic growth of self-aligned Titanium dioxide nanotube arrays

We report about the ambient conditions and electrolytes influence on the synthesis of self-organized titania nanotube arrays prepared by anodic oxidation. Arrays of randomly disordered Titanium dioxide nanotubes with pores diameter ranging between 60 and 100 nm, wall thickness of 25 up to 40 nm and around 300 nm nanotubes length, can be prepared under HF electrolyte and its mixtures with sulphuric acid at RT anodization.     

Position-controlled synthesis of single-walled carbon nanotubes on a transparent substrate by laser-induced chemical vapor deposition

The synthesis of single-walled carbon nanotubes (SWCNTs) on a transparent substrate with multiple-catalyst layer (Fe/Al/Cr: 0.5/15/500 nm) using laser-induced chemical vapor deposition is reported. Ethylene (C₂H₄) mixed with hydrogen (H₂) and a continuous wave Nd:YVO₄ laser (532 nm) were used as the precursor gas and the irradiation source, respectively. It was found that the density and quality of the SWCNT dots varied sensitively to laser irradiance and chamber pressure. From subsequent micro-Raman analyses at different excitation sources (488, 514, 633, and 785 nm), the diameters of the SWCNTs were estimated to be within the range of 0.8-2 nm and that the SWCNT dots were composed of both semiconducting and metallic SWCNTs. It is demonstrated that an array of SWCNT dots can be fabricated at precisely controlled positions of a transparent substrate at room temperature with no need of catalysis patterning.     

Growth of size-tunable periodic Ni silicide nanodot arrays on silicon substrates

The fabrications of size-tunable periodic arrays of nickel metal and silicide nanodots on (0 0 1)Si substrates using polystyrene (PS) nanosphere lithography (NSL) and heat treatments have been investigated. The growth of epitaxial NiSi₂ was found to be more favorable for the Ni metal nanodot arrays. The effect becomes more pronounced with a decrease in the size of the Ni nanodots. The sizes of the epitaxial NiSi₂ nanodots were tuned from 38 to 110 nm by varying the diameter of the PS spheres and heat treatment conditions. These epitaxial NiSi₂ nanodots formed on (0 0 1)Si were found to be heavily faceted and the faceted structures were more prone to form at higher temperatures. Based on TEM, HRTEM and SAED analysis, the faceted NiSi₂ nanodots were identified to be inverse pyramids in shape. Compared with the NiSi₂ nanodot arrays formed using single-layer PS sphere masks, the epitaxial NiSi₂ nanodot arrays formed from the double-layer PS sphere templates exhibit larger interparticle spacings and smaller particle sizes. Since the nanoparticle sizes, shapes and interparticle spacings can be adjusted by tuning the diameter of the PS spheres, stacking conditions, and heat treatment conditions, the PS NSL technique promises to be an effective patterning method for growth of other nanostructures.     

Structure and homoepitaxial growth of GaAs(6 3 1)

We have studied the surface atomic structure of GaAs(6 3 1), and the GaAs growth by molecular beam epitaxy (MBE) on this plane. After the oxide desorption process at 585 °C reflection high-energy electron diffraction (RHEED) showed along the [−1 2 0] direction a 2× surface reconstruction for GaAs(6 3 1)A, and a 1× pattern was observed for GaAs(6 3 1)B. By annealing the substrates for 60 min, we observed that on the A surface appeared small hilly-like features, while on GaAs(6 3 1)B surface pits were formed. For GaAs(6 3 1)A, 500 nm-thick GaAs layers were grown at 585 °C. The atomic force microscopy (AFM) images at the end of growth showed the self-formation of nanoscale structures with a pyramidal shape enlarged along the [5−9−3] direction. Transversal views of the bulk-truncated GaAs(6 3 1) surface model showed arrays of atomic grooves along this direction, which could influence the formation of the pyramidal structures.     

Characterization of MeWO4 (Me = Ba, Sr and Ca) nanocrystallines prepared by sonochemical method

Metal tungstates (MeWO₄, Me = Ba, Sr and Ca) were successfully prepared using the corresponding Me(NO₃)₂·2H₂O and Na₂WO₄·2H₂O in ethylene glycol by the 5 h sonochemical process. The tungstate phases with scheelite structure were detected with X-ray diffraction (XRD) and selected area electron diffraction (SAED). Their calculated lattice parameters are in accord with those of the JCPDS cards. Transmission electron microscopy (TEM) revealed the presence of nanoparticles composing the products. Their average sizes are 42.0 ± 10.4, 18.5 ± 5.1 and 13.1 ± 3.3 nm for Me = Ba, Sr and Ca, respectively. Interplanar spaces of the crystals were also characterized with high-resolution TEM (HRTEM). Their crystallographic planes are aligned in systematic array. Six different vibration wavenumbers were detected using Raman spectrometer and are specified as ν₁(A_g), ν₃(B_g), ν₃(E_g), ν₄(B_g), ν₂(A_g) and free rotation. Fourier transform infrared (FTIR) spectra provided the evidence of scheelite structure with W-O anti-symmetric stretching vibration of [WO₄]²⁻ tetrahedrons at 786-883 cm⁻¹. Photoluminescence emission of the products was detected over the range of 384-416 nm.     

Physical bounds of metallic nanofingers obtained by mechano-chemical atomic force microscope nanolithography

To obtain metallic nanofingers applicable in surface acoustic wave (SAW) sensors, a mechano-chemical atomic force microscope (AFM) nanolithography on a metallic thin film (50 nm in thickness)/piezoelectric substrate covered by a spin-coated polymeric mask layer (50-60 nm in thickness) was implemented. The effective shape of cross-section of the before and after etching grooves have been determined by using the AFM tip deconvolution surface analysis, structure factor, and power spectral density analyses. The wet-etching process improved the shape and aspect ratio (height/width) of the grooves and also smoothed the surface within them. We have shown that the relaxed surface tension of the polymeric mask layer resulted in a down limitation in width and length of the lithographed nanofingers. The surface tension of the mask layer can be changed by altering the initial concentration of the polymer in the deposition process. As the surface tension reduced, the down limitation decreased. In fact, an extrapolation of the analyzed statistical data has indicated that by decreasing the surface tension from 39 to 10 nN/nm, the minimum obtainable width and length of the metallic nanofingers was changed from about 55 nm and 2 μm to 15 nm and 0.44 μm, respectively. Using the extrapolation’s results, we have shown that the future SAW sensors buildable by this nanolithography method possess a practical bound in their synchronous frequency (∼58 GHz), mass sensitivity (∼6125 MHz-mm²/ng), and the limit of mass resolution (∼4.88 × 10⁻¹⁰ ng/mm²).     

Structure of dioctadecyl l-glutamide-derived lipid self-assembled monolayers on Au(1 1 1) surface

An l-glutamic acid-derived lipid with a terminal thiol has been synthesized and its corresponding self-assembled structure on Au(1 1 1) surfaces described. The surface morphology of the lipid self-assembled monolayer (SAM) exhibits nano-order patterning, where the height of the monolayer (approximately 1.5 nm) could be interpreted as due to the monolayer structure estimated from molecular models. The molecular orientation in the monolayer is almost perpendicular to the Au(1 1 1) surface depending on the three-point hydrogen-bonding sites in the molecule.     

Aligned growth of ZnO nanowires by NAPLD and their optical characterizations

Not only vertically aligned ZnO nanowires but also horizontally aligned ZnO nanowires have been successfully grown on the annealed (0 0 0 1) c-cut and (1 1 2 0) a-cut sapphire substrates, respectively using catalyst-free nanoparticle-assisted pulsed-laser ablation deposition (NAPLD). The as-synthesized ZnO nanowires exhibit an ultraviolet emission at around 390 nm and the absent green emission under room temperature. The single ZnO nanowire was collected in the electrode gap by dielectrophoresis (DEP). Under the optical pumping, the single ZnO nanowire exhibited UV emission at around 390 nm with several sharp peaks whose energy spacings are almost constant, which greatly differs from the broad UV emission of the film with many nanowires, suggesting ZnO nanowires as candidates for laser media. The single ZnO nanowire showed polarized photoluminescence (PL). The as-synthesized ZnO nanowires could find many interesting applications in short-wavelength light-emitting diode (LED), laser diode and gas sensor.     

Fabrication of nanostructures on Si(1 0 0) and GaAs(1 0 0) by local anodic oxidation

Atomic force microscopes have become useful tools not only for observing surface morphology and nanostructure topography but also for fabrication of various nanostructures itself. In this paper, the application of AFM for fabrication of nanostructures by local anodic oxidation (LAO) of Si(1 0 0) and GaAs(1 0 0) surfaces is presented. A special attention is paid to finding relations between the size of oxide nanolines (height and half-width) and operational parameters as tip-sample voltage and tip writing speed. It was demonstrated that the formation of silicon oxide lines obeys the Cabrera-Mott theory, i.e. the height of the lines grow, linearly with tip-sample voltage and is inversely proportional to logarithm of tip writing speed. As for GaAs substrates, the oxide line height grows linearly with tip-sample voltage as well but LAO exhibits a certain deviation from this theory. It is shown that the selective chemical etching of Si or GaAs ultrathin films processed by LAO makes it possible to use these films as nanolithographic masks for further nanotechnologies, e.g. fabrication of metallic nanostructures by ion-beam bombardment. The ability to control LAO and tip motion can be utilized in fabrication of complex nanostructures finding their applications in nanoelectronic devices, nanophotonics and other high-tech areas.