Replication of mold for UV-nanoimprint lithography using AAO membrane

A simple and highly effective method to the replication of soft mold based on the anodic aluminum oxide (AAO) membrane was developed. The soft mold with nanopillar arrays was composed of the toluene diluted PDMS layer supported by the soft PDMS. A water contact angle as high as 114° was achieved. The hexagonally well-order arrays of holes of nanometer dimensions, ∼100 nm pore diameter and 125 nm center-to-center pore, could be gained over large areas by UV-nanoimprint lithography (UV-NIL) with the replicated soft PDMS mold. It is expected that the developed soft mold would find applications in light emitting diodes devices.     

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.     

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.     

Preparation of visible-light responsive PF-codoped TiO2 nanotubes

Fabrication of PF-codoped TiO₂ nanotubes was carried out using a one-step electrochemical anodization process by tailoring the composition of the electrolyte with the aim of PF-codoping to extend the optical absorption of TiO₂ to the visible-light region. The as-prepared PF-codoped TiO₂ nanotubes were characterized by SEM, XPS, and UV-vis diffuse reflectance absorption spectra (DRS). The results showed that the tube diameter of the nanotubes was approximately 100 nm and the tube length was approximately 510 nm. The phosphorus and fluorine were successfully doped into TiO₂ nanotubes, as evidenced by XPS. Moreover, the PF-codoped samples displayed remarkably strong visible-light response.     

Self-cleaning characteristics on a thin-film surface with nanotube arrays of anodic titanium oxide

Self-cleaning of a surface of nanotube arrays of anodic titanium oxide (ATO) is demonstrated. The ATO was prepared in fluoride ion containing sulfate electrolytes with a structure of 0.4 μm length, 100 nm pores diameter, 120 nm interpore distance, 25 nm pore wall thickness, a 8×10⁹ pores cm⁻² pore density, and 68.2% porosity. Prepared as thin films either directly from a Ti foil or on a glass substrate, these arrays have the property that water drops spread quickly over the surface of the films without irradiation. In contrast, a flat anatase TiO₂ film requires irradiation with UV light for several minutes before the contact angle decreases to zero. The observed self-cleaning behavior of the ATO thin films is due to the capillary effect of the nanochannel structure and the superhydrophilic property of the anatase TiO₂ surface inside the tube.     

Nonlinear optical properties of periodic gold nanoparticle arrays

Periodic Au nanoparticle arrays were fabricated on silica substrates using nanosphere lithography. The identical single-layer masks were prepared by self-assembly of polystyrene nanospheres with radius R = 350 nm. The structural characterization of nanosphere masks and periodic particle arrays was investigated by atomic force microscopy. The nonlinear optical properties of the Au nanoparticle arrays were determined using a single beam z-scan method at a wavelength of 532 nm with laser duration of 55 ps. The results show that periodic Au nanoparticle arrays exhibit a fast third-order nonlinear optical response with the nonlinear refractive index and nonlinear absorption coefficient being n₂ = 6.09 × 10⁻⁶ cm²/kW and β = −1.87 × 10⁻⁶ m/W, respectively.     

Efficient field emission from coiled carbon nano/microfiber on copper substrate by dc-PECVD

Crystalline coiled carbon nano/micro fibers in thin film form have been synthesized via direct current plasma enhanced chemical vapor deposition (PECVD) on copper substrates with acetylene as a carbon precursor at 10 mbar pressure and 750 °C substrate temperature. The as-prepared samples were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). XRD pattern as well as selected area electron diffraction (SAED) pattern showed that the samples were crystalline in nature. SEM and HRTEM studies showed that as synthesized coiled carbon fibers are having average diameter ∼100 nm and are several micrometers in length. The as-prepared samples showed moderately good electron field emission properties with a turn-on field as low as 1.96 V/μm for an inter-electrode distance 220 μm. The variation of field emission properties with inter-electrode distance has been studied in detail. The field emission properties of the coiled carbon fibrous thin films are compared with that of crystalline multiwalled carbon nanotubes and other carbon nanostructures.     

Carbon nanotube synthesis on oxidized porous silicon

Carbon nanotubes were grown on thermally oxidized porous silicon by catalytic chemical vapor deposition from the mixture of ferrocene and xylene precursor. The growth rate of carbon nanotubes showed dependence on the oxidation extent of porous silicon. On pristine porous silicon surfaces, only poor nanotube growth was observed, whilst samples oxidized in air at 200, 400, 600 and 800 °C prior to the deposition process proved to be suitable substrates for carbon nanotube synthesis. Networks of carbon tubes with diameter of ∼40 and ∼10 nm observed on the surfaces of samples were investigated by electron microscopy and by energy dispersive X-ray analysis.     

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).     

Self filling of Ni nanoparticles in amorphous AlN nanotubes and its field emission property

Unique aluminum nitride amorphous nanotubes filled with Ni nanoparticles have been successfully synthesized through the reaction of NH₃ over Ni-Al thin film at 1000 °C, which is similar to the extended vapor-liquid-solid technique. The X-ray diffraction and high-resolution transmission electron microscopic results are in good agreement with the amorphous nature of AlN nanotubes and crystallinity of Ni nanoparticles. The AlN nanotubes were having average diameter 35 nm and length ∼4 μm, whereas the Ni nanoparticles were having 5 nm in diameter. The unique structure showed excellent field emission property and high electrical conductivity ∼0.43 kmho/m at room temperature. The mechanism of good field emission property has been explained in detail.     

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.     

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.     

Microstructured horizontal alumina pore arrays as growth templates for large area few and single nanowire devices

We demonstrate the fabrication of horizontally aligned and well‐defined nanopore structures by anodic oxidation of aluminum thin films and micro stripes on a Si substrate. We are able to control both, the pore diameters and interpore distances from 10 nm to 130 nm and from 30 nm to 275 nm, respectively. The anisotropy of the system induces some deviations in the pore configuration from the typical honeycomb structure. By decreasing the dimensions of the Al structures, the final pore diameter and interpore distance remains constant, enabling the transition from multiple to a few nanowire porous structures. Finally, we successfully filled the nanopores by pulsed electroplating, as demonstrated both by Scanning Electron Microscopy and by current–voltage measurements. Having full control over the size, the density, the position and the orientation of the porous structure, our approach is promising for many exciting applications, including nanoelectronics and sensing. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nonlinear optics of nontoxic nanomaterials

We studied the linear and nonlinear optical properties of halloysite nanotubes using Z-scan technique. Halloysite is alumina silicate clay rolled into 50 nm diameter hollow cylinders, where the silica layer is at the outer surface of the tube and alumina layer is in the inner surface. Optical absorption spectra show an absorption peak around 600 nm. Open aperture Z-scan measurements using 3 ns laser pulses at 532 nm reveal two-photon induced absorption. The closed aperture Z-scan indicates a positive nonlinear refractive index. As these nanotubes are nontoxic and biocompatible, they have advantages over conventional carbon nanotubes for biomedical applications.     

Fabrication of PECVD-grown fluorinated hydrocarbon nanoparticles and circular nanoring arrays using nanosphere lithography

Nanosphere lithography (NSL) masks were created by spin-coating of polystyrene particles onto silicon surfaces. Fluorinated hydrocarbon films were coated on the nanosphere lithography masks using plasma-enhanced chemical vapor deposition (PECVD) to obtain ordered arrays of fluorinated hydrocarbon. Atomic force microscope images show hexagonally ordered nanodots of dimension 225 ± 11 nm with a height of 23 ± 4 nm. Every hexagon encloses a circular ring of diameter 540 ± 24 nm having a height and width of 13.5 ± 0.6 nm and 203 ± 16 nm, respectively. FTIR analysis shows two distinct zones of atomic bonding of CH_x and CF_x in the plasma coated ordered fluorinated hydrocarbon films.     

Synthesis of lanthanum nickelate perovskite nanotubes by using a template-inorganic precursor

A new route to obtain metal oxide nanotubes is presented: an inorganic coordination complex precursor containing the metal ions and impregnated into alumina membrane templates yield hollow tubular nanostructures of LaNiO₃ by calcination at 600 °C as characterized by powder X-ray diffraction (XRD). Scanning electron microscopy (SEM) shows that the resulting nanotubes have 200 nm in diameter in good agreement with the template pore. Transmission electron microscopy (TEM) and dark field transmission electron microscopy (DF-TEM) show that the nanotubes with 10-20 nm walls and internal separations are composed of 3-5 nm crystals.     

Effect of seed layer on the growth of well-aligned ZnO nanowires

We demonstrate that vertical well-aligned crystalline ZnO nanowire arrays were grown on ZnO/glass substrates by a low-temperature solution method. Different thicknesses of ZnO seed layers on glass substrates were prepared by radio-frequency sputtering. In this work it was found that the morphology of ZnO nanowires strongly depends on the thickness of ZnO seed layers. The average diameter of nanowires is increased from 50 to 130 nm and the nanowire density is decreased from 110 to 60 μm⁻² while the seed layer thickness is varied from 20 to 1000 nm. The improved control of the morphology of ZnO nanowire arrays may lead to an enhanced carrier collection of hybrid polymer photovoltaic devices based on ZnO.     

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.     

Synthesis and electrochemical properties of Ni doped titanate nanotubes for lithium ion storage

Ni doped titanate nanotubes were synthesized by hydrothermal method using Ni doped rutile TiO₂ nanopowders as a starting material. The electrochemical properties were investigated by cyclic voltammmetric methods. The microstructure and morphology of the synthesized powders were characterized by XRD (X-ray diffraction), and HRTEM (high resolution transmission electron microscopy). Ni doped nanotubes were composed of H₂Ti₂O₅·H₂O with outer and inner diameter of ∼10 nm and 6 nm and showed a initial discharge capacity of 305 mAh/g with poor cycling performance. However, after firing, the Ni doped nanotubes revealed better cycling performance due to lower reaction with hydrate and smaller diameter of the tubes.     

Structural features of natural and acids modified chrysotile nanotubes

The structural features of the natural chrysotile have been studied by transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectrometry, thermogravimetric and low-temperature nitrogen adsorption techniques. The chrysotile fibers are present as nanotubes of cylindrical morphology of various forms (rectilinear cylinders, cylinders with cup-like ends, tube twins, cylinder-in-cylinder and cone-in-cone tubes) with the outer diameters of 15-30 nm and the inner ones of 2-6 nm. The surface areas of the raw and the acid leached chrysotile samples obtained by nitrogen adsorption are 15.3 and 63.6 m²/g with the average pore diameter 9.8 and 3.9 nm, respectively. The inner and the outer surfaces of the chrysotile nanotubes are evaluated by the geometric method as 16 and 80 m²/g. The thermogravimetric analysis reveals two main phases of mass loss associated with dehydration and dehydroxylation (with two overlaying steps) processes. The first phase is attributed to the dehydration reaction at low temperature range 293-450 K with activation energy in the range 22-32 kJ/mol. The second phase occurs between 798 and 985 K with activation energy 249-298 kJ/mol for the raw sample and 130-146 kJ/mole for the acid treated one.