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We present a bottom-up approach for the construction of "Smart" active defects in colloidal photonic crystals (CPCs). These structures incorporate polyelectrolyte multilayer (PEM) planar defects embedded in silica CPCs through a combination of evaporation induced self-assembly and microcontact transfer printing. We show how the enormous chemical diversity inherent to PEMs can be harnessed to create chemically active defect structures responsive to solvent vapor pressures, light, temperature as well as redox cycling. A sharp transmission state within the photonic stopband, induced by the PEM defect, can be precisely, reproducibly and in some cases reversibly tuned by these external stimuli.
These materials could find numerous applications as optically monitored chemical sensors, adjustable notch filters and CPC-based tunable laser sources.
Crystallization experiments have been carried out in microfluidic devices to screen for polymorphs by crystallization on a range of surfaces. These devices consist of PDMS (polydimethylsiloxane) patterned with microchannels and then bonded to self-assembled monolayers (SAMs) of organic molecules on gold substrates. Barbital was crystallized in microchannels over five different SAMs functionalized with polar and nonpolar organic groups. Growth of polymorphs was examined under thermodynamic conditions from solutions at room temperature and under kinetic conditions by rapid cooling. The results of these experiments and the influence of chemically modified surfaces in microchannels in controlling polymorphism are discussed.
We recently developed a novel floating-potential dielectrophoretic method to selectively position individual single-walled carbon nanotubes between two floating electrodes while the bundles of nanotubes and impurities were attracted into the region between two control electrodes. In this study, we investigated effects of several process parameters including electric field distribution, electric field frequency, and solution media in order to understand the physical mechanisms of this dielectrophoretic process and to improve its efficiency. Results showed that both the magnitude and the direction of electrical force applied onto the nanotubes can be tailored by changing these process parameters. It was found that a 1 wt% sodium dodecyl sulfate in deionized water is an efficient solution for separating bundles of nanotubes into individual nanotubes and aligning individual nanotubes with a clean surface between two electrical contacts in comparison to N,N-dimethylformamide, 1,2-dichloroethane, 1,2-dichlorobenzene, 1,1-dichloromethane, ethanol, and isopropanol solutions. The fabricated carbon nanotube devices exhibit electronic properties comparable to nanotube transistors and interconnects fabricated by other methods.
In the present work we calculate the electronic band structure of single-wall helical carbon nanotubes following an effective-mass approach. We include curvature effects and strain due to bending in the band structure. The curvature energy ΔE, and the change in the electronic energy ΔEs due to strain, depend upon the coil pitch and coil diameter of the tube. We find 0.003 ≤|ΔE|≤ 1.3 eV and 0 ≤ΔEs ≤ 4.0 eV for the single-wall helical carbon nanotubes considered here.
Electric transport properties of chemically modificated carbon nanotubes (CNTs) using Si-containing organic molecules were investigated by means of the field effect transistors (FETs) technique. From the results of FET measurements, it was shown that p-type semiconducting CNTs can be converted to n-type ones by exohedral silylation. It is suggested that the electron carrier are doped into CNTs from the additional silyl groups, that is, the electronic properties of CNTs can be controlled by chemically modifications of outer surface.
Nickel monosilicide (NiSi) nanowires (NWs) were fabricated by metal-induced growth at 575 °C. The solid-state reaction of Ni and Si provides linear grown NWs. The parallel grown NW forms a nanobridge (NB) across a trench, patterned with a simple optical lithography and metal lift-off method. The Ni pads gave a good Ohmic contact without affecting the I-V transport characteristics through a NB. The metallic NB, 2.73 µm in length and 50 nm in diameter, gave a low resistance of 148 . The self-assembled nanobridge can be applied to form nanocontacts at relatively low temperatures. The MIG NB is a promising 1 dimensional nanoscale building block to satisfy the need of ‘self and direct’ assembled ‘bottom-up’ fabrication concepts.
Infrared vibrational spectroscopy in an attenuated total reflection geometry has been employed to investigate the presence of organic and inorganic thin layers on Si-wafer surfaces. Three different processes were compared for surface contaminant removal; microwave plasma, UV-ozone, and a piranha solution cleaning. The CH vibrations at 2928 and 2856 cm-1 characteristic of organic contaminants were monitored before and after each cleaning procedure to determine how well it removed surface contaminants. We found that native oxide removal from the Si surface should only be carried out after a cleaning essay. We observed that surface oxide removal exposed a hydrophobic bare Si surface, attracting organic molecules present in solution or the ambient. A large increase of the CH vibrational signature was observed for a Si wafer after an HF dip. A combination of plasma cleaning followed by UV-Ozone treatment was found the most effective one for Si wafer cleaning. We were able to evaluate the effectiveness of the cleaning methods, hydrogen surface passivation and oxide removal/regrowth.
The fabrication of Si-Ge oxide composites in a two-stage photobioreactor cultivation process was systematically optimized by increasing the amount of germanium assimilated into the diatom cells. In this optimization process of the synthesis of Si-Ge oxides that maintain the original morphology of the diatoms, high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) both equipped with an energy dispersive X-ray spectrometer were extensively used to characterize the evaluation of the chemical composition and structural properties of the processed diatoms.
In this paper, the concepts of nanotechnology-based fan blade have been introduced and CNT-reinforced hollow micro-balloon-based syntactic foams/composites and damping coatings have been applied so as to develop the next generation aerospace components. The focus in this paper is directed toward the development of the next generation of vibration damping systems, providing a road map to manufacturing technology and design solutions.
We have employed protein-protein interactions for controlled assembly of gold nanoparticles. Stoichiometric 1:1 protein:nanoparticle conjugates were prepared for proteins known to strongly interact with each other and these interactions were used to self-assemble nanoparticles. Mixing equivalent amounts of the antigen-nanoparticle and antibody-nanoparticle conjugates resulted in the formation of nanoparticle dimers with a yield of about 60%. Trimers (yield ∼30%) can be obtained by mixing 2:1 antigen-nanoparticle with 1:1 antibody-nanoparticle conjugates in a molar ratio of 1:2. The structures are destroyed at low pH when the antibody-antigen complex dissociates.
For field-induced electron emission, the two factors that enable a high emission current density at low applied voltages are (a) low work function of the emitter and (b) sharpness of the emitter tip. We have developed and applied a chemical vapor deposition method to synthesize single-crystalline LaB6 nanowires for applications as point electron emitters. The crystallographic orientation of the grown nanowires can be controlled by the catalysts used in synthesis and their typical diameter is ranged from below 20 nm to over 100 nm. The nanowires’ tip is either hemispherical or flat top with rectangular cross-section depending on the catalyst being utilized. The field emission properties have also been measured from the single nanowire emitters and the results are discussed for applications as point electron sources used in high performance electron optical instruments such as the transmission and scanning electron microscopes.
Optical emission spectra of a CH4/H2/Ar gas mixture plasma were observed during carbon nanotube (CNT) growth in RF plasma-enhanced chemical vapor deposition. CNTs with diameters of ∼10-30 nm and length of ∼6 μm were grown on double- and triple-layered films of catalyst/support materials (FexOy/TiO2 and Al2O3/FexOy/Al2O3) at the total gas pressures of 1-10 Torr with gas flow rates of CH4 = 27 sccm, H2 = 3 sccm, and Ar = 1 sccm. The number density of CNTs increased with the gas pressure, and Al2O3/FexOy/Al2O3 (each thickness of 1 nm) film yielded the thinnest CNTs with a high number density among the present catalysts. The spatial distributions of H atom relative density in the plasma were obtained by actinometry. The H relative density decreased with the pressure, and this suggests the suppression of CH3 radical generation in the plasma.
HgTe nanotubes have been prepared by spray deposition of solvothermally synthesized iodine doped HgTe nanoparticles on glass substrates at low temperature (200°C). Spray deposition was done without voltage and with an externally applied voltage (700 V) to the nozzle and it is found from TEM studies that the length of the nanotubes increases from ∼4µm in case of without voltage to ∼ 6µm in case of the applied voltage, with an average diameter of ∼ 45nm. The nanotubes are found to have cubic lattice structure having Hg:Te in stoichimetric ratio (52:48).
Ion implantation into silica followed by thermal annealing is an established growth method for Si and Ge nanocrystals. We demonstrate that growth of Group IV semiconductor nanocrystals can be suppressed by co-implantation of oxygen prior to annealing. For Si nanocrystals, at low Si/O dose ratios, oxygen co-implantation leads to a reduction of the average nanocrystal size and a blue-shift of the photoluminescence emission energy. For both Si and Ge nanocrystals, at larger Si/O or Ge/O dose ratios, the implanted specie is oxidized and nanocrystals do not form. This chemical deactivation was utilized to achieve patterned growth of Si and Ge nanocrystals. Si was implanted into a thin SiO2 film on a Si substrate followed by oxygen implantation through an electron beam lithographically defined stencil mask. Thermal annealing of the co-implanted structure yields two-dimensionally patterned growth of Si nanocrystals under the masked regions. We applied a previously developed process to obtain exposed nanocrystals by selective HF etching of the silica matrix to these patterned structures. Atomic force microscopy (AFM) of etched structures revealed that exposed nanocrystals are not laterally displaced from their original positions during the etching process. Therefore, this process provides a means of achieving patterned structures of exposed nanocrystals. The possibilities for scaling this chemical-based lithography process to smaller features and for extending it to 3-D patterning is discussed.
A detailed study of poly(alkylthiophene) self-assembly and organization on single-walled carbon nanotubes is presented. We show that ordered polymer domains are formed when a conjugated polymer is blended with small amounts of carbon nanotubes. By correlating the lowest energy feature in the absorption spectra of the polymer with ordering, we demonstrate that the degree of ordering in the polymer is enhanced when it is blended with carbon nanotubes. Furthermore, we elucidated the conformation of the polymer chain when it is absorbed onto the nanotube surface and imaged the high degree of ordering in the polymer/carbon nanotube complex by microscopy.
A micro-reflectance spectroscopy was performed on a single island of photonic crystal array. An array of islands of photonic crystal with colloidal polystyrene beads was assembled on Si using ink-jet printing. The polystyrene colloids with three different sizes (190 nm, 210 nm, and 270 nm in diameter) were used and the polystyrene colloidal particles were self-assembled to form fcc lattices with three different lattice spacing. It was observed from the reflectance spectra that the position of the optical stop band shifts as the size of the colloidal particle changes. Effective medium approximations were used to model the dielectric properties of the colloid/air composite. The theoretically expected reflectance peak position agrees well with those of the experimentally observed peaks. The effect of finite size of the photonic crystal island on its reflectance spectroscopy was investigated by comparing the reflectance spectra collected from four different photonic crystal islands assembled from the same polystyrene colloidal particles, but with different lateral size for each island. It was found that the primary reflection peak was broadened and its intensity was reduced when the lateral size of the island was decreased.
Single walled carbon nanotubes have been filled with a variety of metal oxides and the structural and morphological characteristics of the metal_oxide@SWNT composites studied. Advanced techniques of software aberrations correction for transmission electron microscopy were used for characterisation. This research shows that, despite their higher reactivity compared to salts such as halides, oxides can be encapsulated within SWNTs with some compounds attaining remarkable filling yields.
The photoluminescence (PL) intensity of undecylenic acid surface functionalized planar Si (001) was investigated in the presence of colloidal Ag nanoparticles. The acid passivated Si surface has a weak PL at 1125 nm. Upon exposure to a Ag nanoparticle sol, the PL quenched exponentially with a characteristic decay time of ∼ 18 minutes. It is known that the metal mediated charge-transfer process provides a pathway for energy decay and leads to a quenching of luminescence in light emitting material. An in-situ study of the surface passivated Si revealed that the Ag nanoparticle was likely to have come into contact or was sited close enough to the semiconductor surface through adsorption to cause effective PL quenching.
Thin films synthesized by assembling clusters present interesting chemical and physical properties and a large specific surface, and are appealing for functional applications (e.g. sensing and catalysis). Also, clusters supported on surfaces are interesting both for nanocatalysis applications and for fundamental research. By means of pulsed laser deposition (PLD) in a background atmosphere we can induce cluster aggregation in the ablation plume and control the deposition kinetic energy of the clusters. These phenomena depend on the plume expansion dynamics and their influence on the properties of the deposited films has been investigated as a function of the background gas mass and pressure. The control of these parameters permits variation of the film surface morphology, from a compact structure with a very smooth surface, to a film with a controlled roughness at the nanoscale, to an open, low density meso- and nanostructure characterized by a high fraction of voids and by a large specific area. Thin films of WOx, TiOx, Pd/PdO, and Ag were deposited and characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM) and Raman spectroscopy. Post-deposition annealing permits control of the crystalline degree of the films, which in the case of tungsten and titanium oxide is found to depend on the original nanostructure, while a different degree of oxidation can be induced by controlling the amount of oxygen in the deposition chamber. In-situ scanning tunneling microscopy (STM) was employed to study the first stages of growth of W films on different substrates. This opens the possibility to tailor the material properties through the control of the building nano-units.
Nanostructured one-dimensional materials, such as nanowires, tubes and rods, are gaining increasing attention due to interesting properties and confinement effects, however controlled synthesis of these structures is still limited to a few methods. We present here the synthesis of SnO2 nanowires (Ø, 50 – 1000 nm) at moderate temperatures (550 – 900 °C) using a molecular source [Sn(OBut)4] with pre-existent Sn-O bonds. The growth occurs via a catalyst driven vapor-solid-solid mechanism. Size-selective synthesis of NWs in high areal density was achieved by choosing Au particles of appropriate size. HR-TEM analysis reveals the single crystalline behaviour of wires with a preferred growth direction . Use of SnO2 nanowires as potential optical switches for UV applications was demonstrated by the photo-response measurements. Determination of band gap values confirmed the blue-shift of the main photo-response peak with shrinking radial dimensions of the wires. Furthermore, deposition of vanadium oxide onto SnO2 led to a red-shift of the main conduction value of the nanowires.