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Carbon nanotubes (CNTs) have been grown by catalytical chemical vapor deposition (CCVD) with a thin iron layer as the catalyst. High surface tension metal, tantalum (Ta), and low surface tension, SiO2, have been deposited as the supporting layers before depositing the catalysts. SEM, TEM, STEM and EELS have been used to examine the morphology, structure, and chemical profile of iron nanoparticles and CNTs. The results have shown that the catalyst nanoparticle morphologies were distinctly different on two supports. In particular, Fe nanoparticles on SiO2 were found to follow a Vollmer-Weber (VW) growth mode and a Stranski-Krastanov (SK) growth mode on Ta. It was also found that CNT growth varied significantly on two supports in terms of morphology, growth rate and growth mode. Dense CNTs were grown on Ta with fast growth rates (> 1µm/min) and vertical alignment for the iron thicknesses of 1.5-9 nm. In contrast, CNTs grown on SiO2 exhibited a slow growth rate (< 100 nm/min) with all deposited iron thicknesses, indicating a severe catalyst poisoning. The results suggested that the catalyst morphology in combination with the presence of an iron wetting layer contributed to the enhanced CCVD growth of CNTs on Ta.
We report on a novel approach to grow platinum (Pt) nanostructure using a self-assembled fluorocarbon structure as a template. A ring-type structure of fluorocarbon residues forms during the reactive ion etching (RIE) of SiO2 using trifluoromethane (CHF3) and oxygen as etching gases. Typical dimensions of the ring-type fluorocarbon structure are found to be ∼50 nm in diameter, ∼10 nm in wall thickness, and ∼50 nm in height in this study. Platinum nanostructure up to 100 nm in height and 50 nm in diameter are grown on the template using a low-cost thin film sputter coater for 3 minutes. The morphology and growth mechanism of fluorocarbon structure and platinum nanostructure are discussed. This work provides a simple approach to platinum nanostructure growth for a fuel-cell application.
The use of Au nanoparticles as catalysts for growth of Si nanowires poses fundamental reliability concerns for applications in Si semiconductor technology. In this work we show that the choice of catalysts can be broadened when the need for catalytic precursor dissociation is eliminated. However, the requirements for selective deposition in a gas phase transport -limited regime become stringent. When competing deposition of amorphous Si can bury the particles faster than the incubation time for VLS growth, no nanowire growth will be initiated. We show that the use of a catalyst such as In, already in a liquid form at the growth temperature, is effective. Therefore, the choice of VLS catalysts among the low melting point metals from the III, IV and V groups is suggested.
In deposition of Pb atoms on Cu(111) or Si(111) surface growth of tower-shaped nanostructures is observed. This nanotower growth is caused by structure height dependent energetics, resulting from the quantum size effects (QSE) due to the vertical electron confinement in nanotowers. In this report we present a “wedding cake” -type phenomenological model describing the time evolution of nanostructures. The model reproduces the typical morphologies of nanotowers. The results demonstrate that nanohuts are formed by downward mass currents, due to the downward diffusion of adatoms. Also the stable layers due to the quantum size effects (QSE) can be modelled with a suitable choice of model parameters. In the case of altering stabilities of layers, simultaneous bi-layer growth takes place. The results are in agreement with experimental observations.
Recent research has shown that biologically inspired approaches to materials synthesis and self-assembly, hold promise of unprecedented atomic level control of structure and interfaces. In particular, the use of organic molecules to control the production of inorganic technological materials has the potential for controlling grain structure to enhance material strength; controlling facet expression for enhanced catalytic activity; and controlling the shape of nanostructured materials to optimize optical, electrical and magnetic properties. In this work, we use organic molecules to modify silver crystal shapes towards understanding the metal-organic interactions that lead to nanoparticle shape control.
Using in situ electrochemical AFM (EC-AFM) as an in situ probe, we study the influence of a cationic surfactant cetyltrimethylamminobromide (CTAB) on Ag growth during electrochemical deposition on Ag(100). The results show that the organic surfactant leads to a unique crystal growth habit. With CTAB present in the growth solution, Ag islands grow in a truncated pyramidal shape. To understand the shape evolution of the Ag islands, we utilize electron backscatter diffraction (EBSD) in conjunction with microscopic ellipsometry to characterize the facet-specific binding of the organic molecules to large-grained polycrystalline Ag substrates.
We formulate a global equilibrium model to describe the growth of 1-d nanostructures in the VLS process by including also the chemical tension in addition to the physical tensions. The chemical tension derives from the Gibbs free energy release due to the growth of a crystal layer. The system global equilibrium is attained via the balance of the static physical tensions and the dynamic chemical tension, which allows the system to reach the minimum Gibbs free energy state. The model predicts, and provides conditions for the growth of nanowires of all sizes exceeding a lower thermodynamic limit. The model also predicts the conditions distinguishing the growth of nanaohillocks from nanowires.
We present numerical calculations of tunneling through ultra thin wurtzite Gallium Nitride cap layers on p-doped wurtzite silicon carbide . We demonstrate the predominance of tunneling of the split-off holes to the total carrier flux, with the contribution of the heavy and the light holes damped by the large potential barrier. We calculate the contributions of spontaneous and piezoelectric polarizations to the tunneling profile seen by the holes. Two orders of magnitude enhancement is seen in the transmission probabilities for a 10 angstroms thick Gallium Nitride cap layer for holes very close to the valence band edge, compared to a barrier without any gallium nitride cap. The contact resistances are also calculated for the Gallium Nitride tunneling caps and more than two orders of magnitude lowering is seen with the ultra-thin caps. Larger cap widths induce hole accumulation layers, but the advantages of hole accumulation are offset by the higher effective tunneling width. Our calculations are relevant to nanostructures and nanodevices involving heterojunctions between gallium nitride and silicon carbide and provide the basis for low contact resistances with as-deposited metals. While our calculations focus on the regime of very high barriers to the metal of the order of 1.5 - 2 electron volts, where the method of ultra-thin caps is most useful, similar conclusions also hold for lower barrier heights.
There are multiple solution-phase approaches to synthesizing nanowires, yet some aspects of these syntheses are not well understood. Solution-phase methods are attractive for the ease of scaling to large quantities, which is a necessary step for applications utilizing nanowires. However, insight into the surface chemistry and mechanism of growth of the nanowires is essential for increasing the yield of nanowires. An improved synthesis of silver nanowires is presented along with insight into the mechanism of growth and stabilization of the nanowires. The techniques from this modified synthesis could be extended to a number of other solution-phase procedures to increase the yield of nanostructures.
A microscopic theory of dielectrical properties of thin molecular films, i.e. quasi 2D systems bounded by two surfaces parallel to XY planes was formulated. Harmonic exciton states were calculated using the method of two-time, retarded, temperature dependent Green's functions. It has been shown that two types of excitations can occur: bulk and surface exciton states. Analysis of the optical properties of these crystalline systems for low exciton concentration shows that the permittivity strongly depends on boundary parameters and the thickness of the film. Conditions for the appearance of localized exciton states have been especially analyzed.
Advances in nanoparticle technology enable the production of new types of electronic devices, catalytic systems and complex functional surface coatings. For most of these applications, random deposition or self-assembled arrangement of the particles on surfaces are sufficient. However, an increasing number of potential applications such as single electron transistors and quantum computers require exact placement of single nanoparticles with sub-10 nm resolution and specific size. Till date, techniques that provide an exact online placement of countable and size-selected nanoparticles for functional devices have not been reported. For this purpose a cluster-jet system, based on a gas-phase nanoparticle synthesis source, connected to a focussing collimator system has been developed. The objective of this technique is to assemble countable single nanoparticles with spatial resolution of 10 nm or below onto a pre-structured substrate. In the first stage of this system, nanoparticles in the size regime between 3 and 10 nm are synthesized in a lowpressure microwave plasma reactor. This reactor has the unique advantage of generating particles with defined size distribution, structure, morphology and low degree of agglomeration due to coulomb repulsion during particle formation and growth. Separated single particles are extracted by means of a particle laden molecular beam. A mass filter consisting of a particle mass spectrometer (PMS) coupled to the reactor is used to select nanoparticles of a specific size, according to their mass, charge and kinetic energy. In order to achieve the designated lateral resolution, the particle laden beam will be collimated by electromagnetic lenses and focused onto a pierced AFM-tip. Operation of the focusing mechanism and tip preparation have been successfully performed separatly and are currently being adapted to the use in the cluster-jet system. After completion, this technique is intended to enable the assembly of nanoparticles in almost any desired two-dimensional structure onto a substrate.
We report on the effects of chirality and diameter on the electron transport properties in individual semiconducting, single wall carbon nanotubes. The Boltzmann transport equation is solved indirectly by the Ensemble Monte Carlo method and directly by Rode's iterative technique. Results show considerable effects of chirality and group on band structure and transport properties of tubes with small diameters. However the effects of chirality and group become negligible for tubes with large diameters. Diameter affects these properties more strongly than either chirality or group.
Motivated by the extensive research on carbon nanotubes (CNTs), boron and its related nano-structures have attracted increasing interests for potential applications in nanodevices and nanotechnologies due to their extraordinary properties. B-related nanostructures are successfully grown on various substrates in a CVD process. The boron nanowires have diameters around 50-200 nanometers and lengths up to a few microns. The gas chemistry is monitored by the in-situ mass-spectroscopy, which helps to identify reactive species in the process. Modified vapor-solid growths as well as VLS growth mechanisms are proposed for the growth of these nanostructures. The role of the catalysts in the synthesis is also discussed.
Micro- and nano-structures of ZnO have been grown on substrates from flowing carrier gases in a tube furnace. We have investigated how variations in the carrier gas composition, gas flow rate and the position of the substrate control the formation and the morphology of the nanostructures. The source material was pure zinc powder evaporated in the temperature range 500ºC to 650ºC in flowing Ar plus oxygen at atmospheric pressure. It was found that precise control of the gas composition, gas flow rate, and growth time was necessary for reliable deposition. It was also found that zinc powder must be washed to remove the surface oxide. Scanning electron microscopy (SEM) images of samples grown from a Zn powder source show forested needles approximately 100 nm in diameter by 1 micron long, and faceted rods from 500nm to 700nm thick. Photoluminescence measurements at 4 K show a dominant line at ~3.36eV with additional features at 3.32 and 3.37eV. The line widths are ~ 3.5meV, indicating good quality material. The usual green-band emission is also observed.
ZnO is known to produce a wide variety of nanostructures that have enormous scope for optoelectronic applications. Using an aqueous electrochemical deposition technique, we are able to tightly control a wide range of deposition parameters (Zn2+ concentration, temperature, potential, time) and hence the resulting deposit morphology. By simultaneously conducting synchrotron x-ray absorption spectroscopy (XAS) experiments during the deposition, we are able to directly monitor the growth rates of the nanostructures, as well as providing direct chemical speciation of the films. In situ experiments such as these are critical to understanding the nucleation and growth of such nanostructures.
Recent results from in situ XAS synchrotron experiments demonstrate the growth rates as a function of potential and Zn2+ concentration. These are compared with the electrochemical current density recorded during the deposition, and the final morphology revealed through ex situ high resolution electron microscopy. The results are indicative of two distinct growth regimes, and simultaneous changes in the morphology are observed.
These experiments are complemented by modelling the growth of the rods in the transport-limited case, using the Nernst-Planck equations in 2 dimensions, to yield the growth rate of the volume, length, and radius as a function of time.
The electronic structure for a strained silicon quantum well grown on a tilted SiGe substrate is calculated using an empirical tight-binding method. For a zero substrate tilt angle the two lowest minima of the conduction band define a non-zero valley splitting at the center of the Brillouin zone. A finite tilt angle for the substrate results in displacing the two lowest conduction band minima to finite k0 and -k0 in the Brillouin zone with equal energy. The vanishing of the valley splitting for quantum wells grown on tilted substrates is found to be a direct consequence of the periodicity of the steps at the interfaces between the quantum well and the buffer materials.
Phonon spectra and allowed phonon states, as well as thermodynamic characteristics of nanowires of simple cubic crystalline structure, are analyzed using the method of two-time dependent Green's functions, adjusted to bounded crystalline structures. Poles of Green's functions, defining phonon spectra, can be found by solving of the secular equation. For different boundary parameters, this problem is presented graphically. The presence of boundaries as well as the change of boundary parameters leads to appearance of new properties of low dimensional structures (thin film and nanowire). The most important feature is that beside allowed energy zones (which are continuous as in the bulk structure), zones of forbidden states appear. Different values of boundary parameters lead to appearance of lower and upper energy gaps, or dispersion branches spreading out of bulk energy zone. The correlation with spectra of phonons in corresponding unbounded structures is maintained in the work. Determination of phonon spectra and allowed phonon energies has great importance for kinetic and thermodynamic properties of semiconductive nanostructures and devices. The temperature behavior of nanowire thermal capacitance is compared to that of bulk structures. It is shown that at extremely low temperature nanowire thermal capacitance is considerably lower than the thermal capacitance of bulk sample. It was discussed what are the consequences of this fact to the thermal, conducting and superconducting properties of materials.
Nanosphere lithography was used to form a nano-scaled SiO2 template on a GaAs substrate. Especially, a simple method for fabricating nanopatterns with multi feature sizes within one step has been invented. These nanopatterns, as a template, can be used to grow selectively ordered nanostructures arrays such as quantum dots, quantum bars with different feature sizes in one step. Two sized In0.25Ga0.75As nano-bar arrays have been successfully grown on a GaAs substrate by MOCVD.
ZnO films were grown on (0001) sapphire substrates by atomic layer deposition (ALD) using diethylzinc (DeZn) and nitrous oxide (N2O) in an inductively heated reactor operated at atmospheric pressure. Low-temperature (LT) ZnO buffer layers having various thicknesses were deposited at 400¢J followed by subsequent growth of ZnO films at 600¢J. Some of the ZnO films were then post-annealed at 1000¢J in the N2O flow. Under certain growth conditions, ZnO nanowires were formed on the post-annealed ZnO samples. Room temperature (RT) photoluminescence (PL) spectra of the ZnO nanowires show strong ultraviolet (UV) near band edge emissions at 3.27 eV with a typical full width at half-maximum ( FWHM ) of ~130 meV and quenched defect luminescence at 2.8 eV. 10 K PL spectra of the post-annealed ZnO all exhibit sharp excitonic emissions with the dominant emission being located at 3.36 eV having a FWHM of 4.6 meV.
Novel meso-/macroporous SiO2 monoliths have been reached by applying a nanotectonic pathway within a confined geometry, i.e. a non-static air-liquid foam pattering process. Final scaffolds are a very close transcription of the tailored periodic air-liquid foam template while highly ordered close-packed silica colloids are texturing the as-synthesized foam walls. The interconnected nanoparticles and associated void space between adjacent particles allow generating intrinsic mesopores, thereby defining hierarchically organized porous scaffolds. The good control over both the air-liquid foam's water volume fraction and the bubble size allow a rational tuning of the macropore shape (diameter, Plateau border's width). At the nano-scale, heterogeneous textural character is associated with abrupt variation in the film's topology certainly governed by the complex liquid flow present within the foam film. This flow induces a surfactant concentration gradient that causes a sort of marginal regeneration on the side of the film. According to these observations, the heterogeneous character of the film surface revealed by AFM can be interpreted like a direct expression of the liquid flow within the air-liquid foam's film.
We report on the extraction of carrier type, and mobility in semiconductor nanowires by adopting experimental nanowire field-effect transistor device data to a long channel MISFET device model. Numerous field-effect transistors were fabricated using n-InAs nanowires of a diameter of 50 nm as a channel. The I-V data of devices were analyzed at low to medium drain current in order to reduce the effect of extrinsic resistances. The gate capacitance is determined by an electro-static field simulation tool. The carrier mobility remains as the only parameter to fit experimental to modeled device data. The electron mobility in n-InAs nanowires is evaluated to µ = 13,000 cm2/Vs while for comparison n-ZnO nanowires exhibit a mobility of 800 cm2/Vs.