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We dispersed electrochemically etched Si into ultrabright ultrasmall nanoparticles, with brightness higher than fluorescein or rhodamine. The emission from single particles is readily detectable. Aggregates or films of the particles exhibit emission with highly nonlinear characteristics. We observe directed blue beams at ∼ 410 nm between faces of aggregates excited by femtosecond radiation at 780 nm; and at ∼ 610 nm from aggregates of red luminescent Si nanoparticles excited by radiation at 550-570 nm from a mercury lamp. Intense directed Gaussian beams, a pumping threshold, spectral line narrowing, and speckle patterns manifest the emission. The results are analyzed in terms of population inversion and stimulated emission in quantum confinement-induced Si-Si dimer phase, found only on ultrasmall Si nanoparticles. This microlasing constitutes an important step towards the realization of a laser on a chip.
Mesostructured transition metal (Ti, Zr, V, Al and Ce-Zr) oxide-based hybrid thin films, templated by poly(ethylene oxide)-based surfactants or block copolymers, have been prepared reproducibly, displaying 2D-hexagonal (p6m) or 2D-centred rectangular (c2m) structure. By carefully adjusting the variables involved it is possible to combine both high organisation and excellent optical quality. TiO2 and ZrO2-based materials show thermal stability up to 400-550°C. The elimination of the template can be conducted efficiently and gives rise to high surface area mesoporous films. For the other metal oxide hybrids the inorganic framework is much more fragile, and requires a precise sequence of post-treatments to be stabilised. In addition, original and homogeneous macrotextures shaped with coral-like, helical or macroporous sieves morphologies have been obtained following a nanotectonic approach based on the template-directed assembly by poly-γ-benzyl-L-glutamate (PBLG) of organically functionalised CeO2 crystalline nanoparticles.
In the near future it will be more and more important to produce real nanometer-sized structures for semiconductor devices (e.g., quantum dot lasers) but also for nano-biomechanical applications like the so-called total analysis system implemented on one chip.
We describe here a technique to create nanometer-sized structures in semiconductors and metals by the use of self-assembling diblock copolymers as nano-lithographic masks. Semiconductor quantum structures with very high aspect ratio of 1:10 were fabricated from III-V semiconductor heterostructures by anisotropic dry etching. In a first step, so-called diblock copolymer micelles were generated in a toluene solution. These micelles were loaded by a noblemetal salt. With a “Langmuir Blodgett” technique we can decorate complete wafers with a monolayer of highly ordered micelles, covering almost the complete surface. After treatment in a hydrogen plasma all of the organic components are removed and only crystalline metal clusters of ~12 nm size remain. This metal cluster mask can be used directly in a highly anisotropic chlorine dry etching process to etch cylinders in GaAs and its In and Al alloys. It is also possible to etch through a quantum well layer underneath the surface in order to produce quantum dots.
By evaporating metals and applying a wet chemical image reversal process, we can invert the etched structure and generate a gauzy gold film with nano-holes inside. It is thinkable to use this porous gold film as a nano-filter in upcoming nano-biotechnology applications.
We have developed four different fabrication processes based on self-organizing diblock copolymers that all produce densely-spaced, uniformly-sized nanometer-scale dot arrays over large wafer areas. We demonstrate the versatility of these techniques through examples of dot arrays formed of metallic, insulating, and polymeric materials. These fabrication processes vary in complexity, utility, and degree of optimization, and we discuss the relative merits of each. The ability to create uniform nanoscale features below lithographic resolution limits may enable key applications in fields such as magnetic recording and microelectronics.
A metal-insulator-semiconductor (MIS) device that consists of germanium (Ge) nanocrystals embedded in a novel tri-layer insulator structure is proposed for memory applications . The tri-layer structure comprises a thin (≈5nm) rapid thermal oxidation (RTO) silicon dioxide (SiO2) layer, a Ge+SiO2 middle layer (6 - 20 nm) deposited by RF co-sputtering technique and a RF-sputtered silicon dioxide capping layer. High-resolution transmission electron microscopy (HRTEM) results show that Ge nanocrystals of sizes ranging from 6 –20 nm were found after rapid thermal annealing of the trilayer structure at 1000°C for 300s. The electrical properties of these devices have been characterized using capacitance versus voltage (C-V) measurements. A significant hysteresis was observed in the C-V curves of these devices, indicating charge trapping in the composite insulator. Comparison with devices having similar tri-layer insulator structure, but with a pure sputtered oxide middle layer (i.e. minus the Ge nanocrystals), clearly indicated that the observed charge trapping is due to the presence of the Ge nanocrystals in the middle layer. The C-V measurements of devices without the capping SiO2 layer exhibited no significant hysteresis as compared to the embedded Ge nanocrystal tri-layer devices. The HRTEM micrographs showed that the presence of the capping oxide is critical in the formation of nanocrystals for this structure. By varying the thickness of the middle layer, it was found that the maximum nanocrystal size correlates well with the middle layer thickness. This indicates that the nanocrystals are well confined by the RTO oxide layer and the capping oxide layer. In addition, Ge nanocrystals formed using a thinner middle layer were found to be relatively uniform in size and distribution. This structure, therefore, offers a possibility of fabricating memory devices with controllable Ge nanocrystals size.
Two-dimensional arrays of self-organized Si nanowires were synthesized using the metal induced growth (MIG) method. In MIG processing, the thermally evaporated 25∼100 nm thick Ni films serve as prelayers for magnetron sputtered Si. When sputtering at 550°C, the Si crystallization occurs via the formation of nickel disilicide followed by subsequent epitaxial growth of Si crystals on nickel disilicide due to an extremely small lattice mismatch. Scanning electron microscopy study showed that the nanowires originated from the Si thin film and grew upwards in bundles. The diameter of the nanowires was 20∼50 nm. The length of the nanowires was typically 1 νm. Transmission electron microscopy and electron diffraction analysis revealed the single crystal structure of nanowires. Quantum-size effects in the produced wires were investigated by measuring the photoluminescence spectra at both low and room temperature. An intense room temperature PL peak centered around 690 nm with FWHM of 180 nm showed the promise of MIG-Si nanowires for red light-emitting diode applications. In addition, self-aligned silicide film on the bottom provides an ultimate back Ohmic contact, which significantly simplifies the fabrication of optoelectronic devices.
In the interfaces of multilayered Co/Au and Co/Pd films, there were some interesting changes with annealing. For profile fittings, which is a comparison between measured peaks of X-ray diffraction and calculated peaks of the extended 3-step model profile fittings, the mixed layers between Co layers and noble metal layers were decreased in Co/Au films while increased in Co/Pd films. These results are based on the difference that Co-Au is a eutectic system while Co-Pd is an isomorphous system. This time, as new research, we fabricated multiple-structure multilayered Co/Au films (MSM Co/Au films). Magnetic properties and structural analyses were carried out with a vibrating sample magnetometer (VSM), X-ray diffraction (XRD), and extended 3-step model profile fittings. MSM Co/Au films have two periodic thicknesses. MSM Co/Au films showed the same perpendicular magnetization as Co/Au and Co/Pd films, which is dependent on the Co layer thickness. Perpendicular magnetization showed the maximum value after 1 h-annealing at 473 K. It was well confirmed that the mixed layers decreased at this annealing condition with the extended 3-step model profile fittings.
We present results on patterning microstructures using laser-guidance deposition of nanoparticles from particle-in-solvent suspensions. A laser beam axially confines and propels the particles inside a hollow optical fiber towards a substrate. Confining is provided by the gradient forces arising from light refraction or electrical forces on polarizable particles. The driving force results from the momentum conservation of photons scattered on particles. Polystyrene particles (100 and 400 nm in diameter) and gold particles (from 8 to 50 nm) with different surface organic functionality serve as a constructive material for fabrication of microstrips. In the experiments, the laser power varies from 0.1 to 1.6 W. The microstrips produced under different deposition conditions are studied using optical microscopy and atomic force microscopy. It was found that deposited polystyrene and gold particles form nanoclusters consisting of at least several particles. If deposited at an appropriate rate, such nanoclusters form multilayer microstrips of high particle density. The typical width of the microstrips ranges from less than 10 microns to 100 microns. This technique allows us to fabricate parallel arrays made of colloidal particles with different surface functionality, which seems to be an especially attractive approach for developing novel chemical and biological microsensors.
Zinc Oxide (ZnO) thin films have been deposited on silicon (100), quartz glass, sapphire (0001), and glass substrates by pulsed laser deposition over a range of process conditions such as versus ambient oxygen gas pressure and substrate temperature. Photoluminescence measurements have been carried out to characterize ZnO thin films for optical device applications. We have also studied the influence of the process conditions on crystal structure and morphology of the grown films. Ultraviolet luminescence near the band gap at 3.25 eV was obtained from the optical pumped grown films. And ZnO thin films deposited on sapphire single crystal substrates exhibited strong stimulated emission around 3.12 eV with increasing pumping energy. ZnO thin films deposited at ambient oxygen gas pressure of 5 mTorr and a substrate temperature of 550 °C were predominantly c-axis oriented regardless of substrate material. The crystallite size of 22.7-49.8 nm was estimated using Scherrer's formula. AFM images of films grown on sapphire substrates display hexagonally shaped grains at various sizes.
Mesostructured silica/diblock films with a 3D arrangement of spherical domains (bcc) were prepared through evaporation-induced self-assembly (EISA) using polystyrene-blockpoly( ethylene oxide) diblock copolymers as structure-directing agents and TEOS (Si(OCH2CH3)4) and/or MTES (Si(OCH2CH3)3CH3) as silica precursors. A detailed small angle x-ray scattering (SAXS) analysis of the calcined mesoporous films showed that, in contrast to recently reported studies, no additional microporosity due to the PEO was observed, indicating that the PEO block formed a layer at the interface between the PS domain and the silica matrix and thus contributed to the mesopore volume. These mesostructured porous silica films are believed to be the first in respect of isolated spheres with a 3D array distributed in a silica matrix without additional microporosity and with MTES as silica precursor. Such closed-cell mesostructured porous materials with high porosity and controllable hydrophobicity can be excellent candidates for low dielectric (K) insulator materials.
We demonstrate fabrication of periodic arrays of nanometre scale square helices, with potential applications in three-dimensional photonic bandgap (PBG) materials. Processing is performed using a thin film deposition method known as Glancing Angle Deposition (GLAD). Through advanced substrate motion, this technique allows for controlled growth of square helices in a variety of inorganic materials. Organization of the helices into periodic twodimensional geometries is achieved by prepatterning the substrate surface using electron beam lithography. The regular turns of the helices yield periodicity in the third dimension, perpendicular to the substrate. We present studies of tetragonal and trigonal arrays of silicon helices, with lattice constants as low as 300 nm. By deliberately adding or leaving out seeds in the substrate pattern, we have succeeded in engineering line defects. Our periodic nanoscale structure closely matches an ideal photonic band gap architecture, as recently proposed by Toader and John. While our fabrication technique is simpler than most suggested PBG schemes, it is highly versatile. A wide range of materials can be used for GLAD, manipulation of lattice constant and helix pitch ensures optical tunability, and the GLAD films are robust to micromachining.
We describe an electrochemical Langmuir-Blodgett (LB) trough for the synthesis and characterization of regular two-dimensional networks on a mercury surface. Two strategies were applied to the preparation of molecular grids on mercury. One was based on the mounting of lanthanum(III) bis(meso-tetrapyridylporphyrinate) sandwich anions (TPyP)2La on mercury subphase and coupling it with hydroquinone derivatives as temporary, or “phantom”, couplers. These couplers provide a weak and reversible bridging of (TPyP)2La- connectors on mercury. As a result a highly ordered square grid is created prior to the introduction of the real sturdy couplers for the replacement of the “phantom” ones. In the other strategy advantage has been taken of the high affinity of divalent sulfur compounds for mercury ions. The trigonal connectors, 1,3,5-triscarboranylbenzenes with attached alkylthioether chains, are firmly adsorbed on mercury at positive potentials and presumably form a hexagonal grid due to their mutual coupling through mercury ions.
Stamp deformation often produces undesirable effects that limit the practice and precision of micro-contact printing. We have experimentally studied one of the most pervasive consequences of undesired deformation: roof collapse of low-aspect-ratio recesses. Stamp behavior under increasing loads can readily be assessed by mounting stamps on flat glass and viewing stamps from below through an inverted microscope. Dynamic as well as limiting equilibrium behavior can be determined. Features with aspect ratios varying by a factor of ten were examined. We find that roof collapse initiates with the formation of a contact between the roof of the stamp and the substrate. This is followed by rapid growth of the contact region, driven by attractive interfacial forces. Contact growth usually terminates leaving a noncontacting “moat” region. Experimental measurements of the critical stress for roof collapse are in very good agreement with theory.
2,5-di(phenylethynyl)-4‘-4“-dithiolate-1-nitrobenzene has been shown to exhibit negative differential resistance (NDR) and spontaneous switching when inserted into inert molecular monolayers between metal contacts. We have used conducting atomic force microscopy to measure the electronic properties of individual dithiolated molecules 2,5-di(phenylethynyl)-4“ 4‘“dithiolate-1-nitrobenzene and 2,5-di(phenylethynyl)-4“4‘“thioacetyl-benzene inserted into an alkanethiol monolayer and chemically bonded to gold nano-contacts to form a covalentlyconnected molecular circuit (bonded contacts). The data show qualitative agreement with previously published results for similar molecules deposited in a nanopore containing several hundred molecules, allowing us to make the important conclusion that the measured negative differential resistance (NDR) is native to the molecule and not an intermolecular phenomenon.
Scanning Probe Microscopes (SPM) have been used to change surfaces at nanometer scales. We report the deposition of user defined patterns in a controlled manner using an electropulsed SPM. The patterns were fabricated by applying -12 V electrical pulses in the 10 to 40 Hz range between a commercial CoCr conductive tip and a crystalline n-doped Si wafer. The tip damage during deposition is negligible as measurements on the same surface region before and after deposition show no detectable differences. Immediately after deposition the same tip is used for measuring the fabricated patterns. Applying one isolated electrical pulse results in a pixel with a typical size of the order of 30 nm. By combining the scanning ability of the SPM with the atmospheric deposition induced by electrical pulses on the tip, patterns can be fabricated. For example, by applying electrical pulses during a 25 nm x 800 nm tip scan in AFM tapping mode, at 40 Hz, lines with 65 nm width by 828 nm length were obtained (in good agreement with the expected dimensions of 55 nm x 830 nm derived from the pixel size and the scan range). The height of the deposited patterns is of the order of 2 to 3 nm, and was found to increase with the density of scan lines. The RMS roughness of the deposited material is shown to be strongly dependent on the electrical pulse frequency. The smoother pattern surface results from the 40 Hz pulse frequency. No deposition was observed at higher frequencies.
We developed a novel growth method of aligned carbon nanotubes. Aligned carbon nanotubes are grown on a metal catalyst on a glass substrate using biased Helicon plasma chemical vapor deposition (HPECVD) of CH4/H2 gases from 400 C to 500 C. The Helicon plasma source is one of the high-density plasma sources and is promising for low temperature carbon deposition. A Ni film was used as a catalyst to reduce the activation energy of the nanotubes' growth. The carbon nanotubes were deposited on the nickel catalysis layer selectively.
A novel approach for the synthesis of advanced functional inorganic materials with atomic-scale control over the size of periodic features on the sub-30 nm scale is presented. The key innovative aspect of this technique is the direct, bottom-up formation of a two-dimensional periodic array of spatially separated nanostructures in a self-organized thin-film porous template. This thin-film template is fabricated via biologically inspired hierarchical self-assembly of organic surfactant molecules in the presence of inorganic charged silicate species. The removal of organic molecules from such an organic/inorganic hybrid system creates a periodic array of pore channels of ∼3-30 nm diameter inside the thin-film silica template. This porous template is employed as a shadow mask to directly grow various functional nanostructures inside the confined environment of the periodic pore arrays. In the present study, silicon nanostructures were grown inside the templates by both chemical and physical (sputtering) vapor deposition. The quantum size effect was clearly pronounced in the room temperature photoluminescence spectra of the samples prepared by sputtering from a Si target, which makes the approach highly promising for the fabrication of nanoscale optoelectronic devices.
The alignment, orientation and morphologies of multi-walled carbon nanotubes (MWNTs) can be tailored by controlling catalyst deposition on porous silicon substrates. During the growth of MWNTs, H2 promoted the growth of carbon nanotubes and prevented the formation of amorphous carbon particles. With the introduction of H2, the average diameter of MWNTs decreased from 130 nm to 15 nm, and the average growth rate of nanotubes increased from 50 nm/s to 145 nm/s. The use of CH4 as the carbon source resulted in single-walled carbon nanotubes (SWNTs) with an average diameter of 2 nm, and the use of C2H2 as the carbon source resulted in MWNTs with an average diameter of 15 nm.
The field emission properties of carbon nanotubes (CNTs) from various sources are investigated for the application of field emission displays. Comparisons are made between graphite with Ni metal as catalyst and polycyclic aromatic hydrocarbon as precursor by the arc discharge method. Cathode deposits are examined using scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) to determine microstructure. Carbon structure is studied using Raman spectroscopy. Electron field emission characteristics are measured with the diode method at 10-6 torr pressure. In this study, SEM micrographs of cathode deposits show dense random fiber-like carbon nanotubes. The HRTEM images clearly exhibit characteristic features of multiwalled carbon nanotubes. Microstructural investigation provides evidence that both the metal catalyst and the precursor can be used to synthesize carbon nanotubes. The Raman spectrum shows a stronger peak at about 1580 cm-1 indicating formation of a well-graphitized carbon nanotube. The degree of carbon nanotube graphitization is high and is in good agreement with the HRTEM result. From field emission measurements, the lowest onset field is about 1.0 V/μm and can be attributed to highly sharp tips and the high density of carbon nanotubes. Based on microstructure characterization and field emission measurements, the influence on field emission properties including turn on voltage and threshold voltage of carbon nanotubes synthesized from different sources is discussed.
Both thin films and nanowires of silver selenide were synthesized by electrodeposition from an aqueous acid electrolyte containing silver ion complexed with SCN- and selenium dioxide at room temperature. Orthorhombic Ag2Se films with Ag slightly in excess were obtained. After annealing in argon atmosphere, the films are highly (002) oriented. A positive transverse magnetoresistance of about 20–25% at T = 5 K, and 10–13% at T = 300 K, in fields of H=50 kOe were observed in the electrodeposited films. Furthermore, silver selenide nanowires were synthesized from the same aqueous system by electrodeposition in porous anodic alumina templates. X-ray diffraction (XRD), Transmission electron microscopy (TEM), and Energy dispersive absorption X-ray (EDAX) characterization results showed that the nanowires are highly crystalline with (002) growth direction after annealing. In addition, the atomic ratio of Ag/Se in the films and the nanowire samples can be controlled from about 3.0 to 2.1 by adjusting the concentration of AgI and SeIV source and the deposition potential.