Growth and aggregation of Cu nanocrystals on ionic liquid surfaces

We report a catalyst-free growth of Cu nanocrystals on ionic liquid surfaces by thermal evaporation method at room temperature. After deposition of Cu on ionic liquid surfaces, ramified Cu aggregates form. It is found that the aggregates are composed of both granules and nanocrystals with triangular or hexagonal appearances. The sizes of the nanocrystals are in the range of tens to hundreds of nanometers and increase with the nominal deposition thickness. The growth mechanism of the Cu aggregates and nanocrystals is presented.     

Numerical simulations of sonochemical production and oriented aggregation of BaTiO3 nanocrystals

Numerical simulations of sonochemical production and oriented attachment of BaTiO₃ nanocrystals are performed in aqueous solution with pH 14. It is suggested that most significant effect of ultrasound is the dissolution of Ti-based gel in aqueous solution. It results in the dissolution-precipitation mechanism in the production of BaTiO₃ nanoparticles, while with mechanical stirring without ultrasound it is the in situ mechanism that BaTiO₃ is gradually formed on Ti-based gel. The oriented attachment of spherical BaTiO₃ nanocrystals occurs by van der Waals torque (Casimir torque). Large aggregates of nanocrystals do not attach with each other as the repulsive double layer interaction is stronger for larger aggregates. For smaller spherical nanocrystals, the alignment of the crystal axes is less accurate due to more significant rotational Brownian motion of the nanocrystals.     

Carbon Nanotubes,Nanocrystal Forms,and Complex Nanoparticle Aggregates in common fuel-gas combustion sources and the ambient air

Aggregated multiwall carbon nanotubes (with diameters ranging from ∼3 to 30nm) and related carbon nanocrystal forms ranging in size from 0.4 to 2 μm (average diameter) have been collected in the combustion streams for methane/air, natural gas/air, and propane gas/air flames using a thermal precipitator. Individual particle aggregates were collected on carbon/formvar-coated 3mm nickel grids and examined in a transmission electron microscope, utilizing bright-field imaging, selected-area electron diffraction analysis, and energy-dispersive X-ray spectrometry techniques. The natural gas and propane gas sources were domestic (kitchen) stoves, and similar particle aggregates collected in the outdoor air were correspondingly identified as carbon nanocrystal aggregates and sometimes more complex aggregates of silica nanocrystals intermixed with the carbon nanotubes and other carbon nanocrystals. Finally, and in light of the potential for methane-series gas burning as major sources of carbon nanocrystal aggregates in both the indoor and outdoor air, data for natural gas consumption and corresponding asthma deaths and incidence are examined with a degree of speculation regarding any significance in the correlations.     

Cobalt induced nanocrystals on Ge(001)

The deposition of several monolayers of cobalt on germanium (001) substrates results in the formation of two types of clusters: flat-topped and peaked nanocrystals. Scanning tunneling spectroscopy and helium ion microscopy measurements reveal that these nanocrystals contain cobalt. The shape evolution of the flat-topped and peaked nanocrystals as a function of their size is investigated with scanning tunneling microscopy. For small sizes the nanocrystals are compact. Beyond a critical size, however, the peaked nanocrystals exhibit an elongated shape, whilst the flat-topped nanocrystals remain compact. The shape transition of the peaked nanocrystals is driven by a competition between boundary and strain energies. For small sizes the boundary energy is the dominant term leading to a minimization of the peaked nanocrystal's perimeter, whereas at larger sizes the strain energy wins resulting in a maximization of the perimeter. On the top facet of the flat-topped nanocrystals one-dimensional structures are observed that are comprised of small square shaped units of about 1 nm². Time-resolved scanning tunneling microscopy measurements reveal that these square shaped units are dynamic at room temperature.     

Photophysical manifestations of interactions of quantum dots with ortho-phenanthroline molecules

The interaction of CdSe/ZnS nanocrystals (NCs) (solubilized by trioctylphosphine oxide (TOPO) molecules) with ortho-phenanthroline (OP) molecules has been investigated. It is shown that OP molecules can replace TOPO; in this case, bonding of OP molecules with the nanocrystal surface does not lead to the formation of NC aggregates. It is established that the difference in the evolution of the spectral-luminescence NC characteristics in a mixture with OP is independent of the NC size. A model of the interaction of NCs with OP molecules is proposed.     

Ge nanocrystals in magnetron sputtered SiO2

Structural and optical properties of Ge nanocrystals in SiO₂ films created by magnetron sputtering and heat treatment have been investigated. The formation of nanocrystals is found to be influenced by the temperature of heat treatment and the Ge concentration in the films. After heat treatment at 1000 °C nanocrystals are present throughout the film, with the exception of a region close to the surface that does not contain nanocrystals. This effect is assigned to oxidation of Ge in this part of the film. The size distribution of the nanocrystals is analyzed by transmission electron microscopy for a range of deposition and heat-treatment parameters. By analyzing the transmission electron microscopy images, it is possible to estimate the fraction of nanoclusters that are crystalline for a given set of growth parameters. This analysis shows that all the nanoclusters are created in the crystalline state. Raman spectroscopy is employed to probe the Ge–Ge bonds. In combination with transmission electron microscopy, this information can be used to distinguish between growth modes such as nucleation or Ostwald ripening. The photoluminescence spectra exhibit a strong broad line at 625 nm, the presence of which is demonstrated to correlate to the presence of Ge nanocrystals. PACS 61.46.+w; 78.55.-m; 78.67.-n     

Nanostructured cellular networks

Au nanocrystals spin-coated onto silicon from toluene form cellular networks. A quantitative statistical crystallography analysis shows that intercellular correlations drive the networks far from statistical equilibrium. Spin-coating from hexane does not produce cellular structure, yet a strong correlation is retained in the positions of nanocrystal aggregates. Mechanisms based on Marangoni convection alone cannot account for the variety of patterns observed, and we argue that spinodal decomposition plays an important role in foam formation.     

Effect of temperature on the growth rate of tin dioxide whiskers formed by physical vapor deposition

The effect of temperature on the vertical growth rate of functional tin dioxide nanostructures formed by physical vapor deposition from a gas phase is studied. A model is proposed to describe the formation of tin dioxide nanocrystals. This model adequately describes the experimentally detected effect of the deposition zone temperature on the nanocrystal growth rate in the temperature range under study. An analytical relation between the growth rate of a nanowhisker, its geometric parameters, the nanowhisker material, the substrate material, and the technological parameters of its formation is revealed. The crystal growth rate is found to increase with the deposition zone temperature, which can be caused by an activation character of nucleation processes and precursor mass transport along the substrate surface and the lateral nanocrystal surface.     

Electrical and memory properties of silicon nitride structures with embedded Si nanocrystals

In this work, the electrical and memory behaviour of metal-silicon nitride-silicon structures with an embedded nanocrystalline silicon layer, which either consists of separated silicon nanocrystals, or is a continuous nanocrystalline layer, are presented. The structures were prepared by low-pressure chemical vapour deposition (LPCVD). The effect of the duration of deposition and the structure of the nanocrystalline layer were studied. The writing/erasing behaviour was similar for all the structures, but the retention properties were much worse in the structure with a continuous nanocrystalline layer, than in the structures with separated Si nanocrystals. This indicates that Si nanocrystals play role in charge storage in the studied structures.     

Structural evolvement of ZnO nanoporous films fabricated using prolonged electrodeposition under pulsed voltages

ZnO nanoporous films consisting nanocrystals on ITO-coated glass substrates were prolongedly electrodeposited under pulse voltages in zinc nitrate solution at room temperature. Their structures were found to be determined by experimental parameters, especially, the voltage and the deposition time. On the basis of the structural characterization, the growth mechanism was proposed as a combined growth process: the competitive growth between lateral overgrowth and nanovoid formation. The structural evolvement is believed to be due to the decrease of surface potential with deposition time, leading to the formation of a quasi-two-layer structure. The revealed mechanism here could be helpful to understand the growth and corresponding properties of ZnO films as well as their nanostructures.     

Room-temperature growth of high-quality ZnO nanocrystals using a dressed-photon-assisted near-field process

Single-crystalline ZnO nanocrystals were fabricated by room-temperature photo-chemical vapor deposition (PCVD). We further enhanced the growth of high-quality single-crystalline ZnO nanocrystals using dressed photons and phonons (DPPs). This resulted in greater position control and the growth of high-quality ZnO nanocrystals. The ZnO nanocrystals produced with DPPs had excellent cathodoluminescence characteristics, indicating that the near-field PCVD process could be a promising technique for nanophotonic integrated circuit production.     

Self assembly of inorganic nanocrystals in 3D supra crystals: Intrinsic properties

Here we describe how arrangements of nanocrystals can self-organize in 3D arrays called supra crystals. The 3D arrays can fall into the familiar categories of face centered cubic (fcc), hexagonal compact packing (hcp) crystals, and body centered (bcc) crystals. Intrinsic collective properties of these 3D arrangements are different from the properties of individual nanoparticles and from particles in bulk.We demonstrate by two various processes and with two types of nanocrystals (silver and cobalt) that when nanocrystals are self ordered in 3D superlattices, they exhibit a coherent breathing mode vibration of the supra crystal, analogous to a breathing mode vibration of atoms in a nanocrystal.Comparison between the approaches to saturation of the magnetic curve for supra crystals and disordered aggregates produced from the same batch of nanocrystals is similar to that observed with films or nanoparticles either highly crystallized or amorphous.     

Metal nanocluster formation in silica films prepared by rf-sputtering: an experimental study

Composite silica films containing metal nanoclusters were prepared by the rf- sputtering technique, in which SiO₂ was co-deposited with gold+copper, gold+silver, or copper+silver. The formation of either pure or alloy clusters was studied by extended X-ray absorption fine structure spectroscopy and transmission electron microscopy. For all systems, the presence of alloy aggregates was evidenced. Moreover, small amounts of pure metal aggregates as well as dispersed or oxidized dopants were observed. 61.46.+w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals – 61.10.Ht X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc. – 81.05.Pj Glass-based composites, vitroceramics Received 29 June 2001     

Formation of gold nanoparticles and their aggregates in a liquid at magnetron deposition

The influence of magnetron deposition conditions on the size of Au nanoparticles and their aggregates obtained by condensation in a neutral liquid is studied experimentally. A model is suggested in which the nanoparticles and aggregates form in a thin subsurface layer, which becomes oversaturated by atoms and resulting nanoparticles when the liquid flows through a localized deposition zone. The process stops when the products leave this zone because of stirring. The size of nanoparticles and aggregates depends on the particle flux density and exposure time in the deposition zone. The final size of nanoparticles depends on the exposure time only slightly, while that of aggregates significantly depends on the exposure time. This allows one to prepare a concentrated solution of almost monodisperse nanoparticles with a low degree of aggregation by properly selecting deposition conditions and multiply passing the liquid through the deposition zone.     

Infrared spectra of 1,2-difluoroethane aggregates obtained by matrix trapping of effusive and supersonic molecular beams

The formation of molecular aggregates was investigated by matrix ir spectroscopy. Molecular beams of 1,2-difluoroethane (DFE), either from an effusive or from a supersonic source, were deposited with an excess of argon onto a cryocooled target. (DFE)_x aggregates were formed either in the seeded supersonic beam or upon deposition of a matrix with a low $\text{M}\text{A}$ ratio. In the former case the ir matrix spectra were found to depend on the carrier gas used and to be different from the spectra obtained by deposition of effusive beams. The experimental results suggest the use of supersonic molecular beams for the selective isolation of specific aggregates in cryogenic matrices.     

Silicon rich silicon oxide films deposited by radio frequency plasma enhanced chemical vapor deposition method: Optical and structural properties

Silicon rich silicon oxide films have been deposited by plasma enhanced chemical vapour deposition using a gas mixture of silane, carbon-di-oxide and hydrogen. Silicon nanocrystals formations in the as deposited silicon rich silicon oxide films have been detected by high resolution transmission electron microscopy, scanning electron microscopy, Raman scattering and X-ray diffraction studies. Structural changes under different deposition condition have been studied by Fourier transform infrared spectroscopy. The oxygen and hydrogen bonding configurations have been obtained from Fourier transform infrared spectroscopy. Room temperature photoluminescence spectra have been observed for the as deposited films. The structural properties together with photoluminescence spectra allowed us to gain insight about the Si nanocrystal formation.     

Optimization of porous silicon preparation technology for SERS applications

A series of porous silicon samples prepared at different etching parameters, namely etchant composition, etching time and current density, was investigated as substrates for surface-enhanced Raman scattering (SERS). Silver nanostructures were deposited on porous silicon by immersion plating method and Rhodamine 6G was used as analyte. The relation between the etching parameters, morphology of porous silicon surface and its SERS efficiency after silver deposition is examined. We show that a high HF content in the etchant allows the formation of a film with close-packed silver nanocrystals, which possess strong surface enhancement properties.     

Ferromagnetic glass on the base of aggregates of Ni amorphous nanogranules

We have studied theoretically (including computer simulations) magnetic properties of aggregates of ferromagnetic amorphous nanogranules in the presence of direct exchange between the neighboring granules and random anisotropy fields. We show that such a system can be considered as ferromagnetic glass. We demonstrate (basing on analytical considerations as well as on the results of numerical simulations) that the system is decomposed to clusters or domains with nearly collinear orientation of magnetization. The size of the domains depends on the ratio of the exchange interaction and random anisotropy. For quasi-2D structures we predict that the dipole-dipole interactions between the granules lead to a formation of magnetic vortices. Moreover, the computer simulations also reproduce the puzzling increase of the thermoremanent magnetization observed experimentally, which is expected to be a result of a temperature-dependent decrease in the anisotropy (or a temperature-dependent increase in the exchange). We also consider the structures with weak intergranular exchange and show that they are characterized by the presence of two critical temperatures.     

氧化锌纳米晶体的光谱分析

采用沉淀法并通过控制前驱体的煅烧温度来制备粒径不同的氧化锌(ZnO)纳米晶体,对粒子的透射电镜照片进行分析,结果表明,制备出的纳米粒子分散性好、形貌一致、粒径分布集中。样品的X射线衍射光谱分析表明,随着前驱体煅烧温度增加,晶体粒径增大、结晶度提高;样品的紫外-可见吸收光谱的峰位随粒径减小而发生蓝移,这一实验结果表明ZnO纳米晶体呈现出较明显的量子限域效应;红外吸收光谱测量结果表明,用沉淀法制备的ZnO纳米晶体的表面会吸附一小部分残余的离子,对红外吸收光谱中的ZnO特征振动峰随粒径减小发生宽化和红移的现象进行了理论分析;光致发光光谱测量结果表明,ZnO纳米晶体在紫外区(360 nm)存在一较弱的发光峰,而在可见区(468 nm)存在一较强的发光峰,与理论计算结果进行比较后,认为锌空位点缺陷是导致ZnO纳米晶体可见区发光的主要原因。     

Evolution of epitaxial titanium silicide nanocrystals as a function of growth method and annealing treatments

Titanium silicide grows on silicon in a form of discontinuous layers, which is the most serious obstacle to the formation of high-quality epilayers for VLSI applications. At the same time, nanometric dimensions of the epitaxial silicide islands attract interest as quantum nanostructures. However, for this purpose, nanocrystals in a self-assembled array have to be defect-free, and exhibit high shape and size uniformity. In this work titanium silicide was grown on Si(1 1 1) substrates by reactive deposition epitaxy and by solid-phase epitaxy. Since the reaction and phase-formation kinetics depend on the growth method, accordingly different lattice matching and facet energies may result in different morphological shapes of the nanocrystals. Nanocrystals from reaction in a solid-state could be characterized as highly non-uniform in both shape and size, and their evolution due to post-deposition anneals increased that non-uniformity even further. Relaxation of epitaxial mismatch strain by misfit dislocations could be inferred from a gradual reduction of the nanocrystal vertical aspect ratio and development of flat top facets out of the initially sharp conical crests, in accord with generalized Wulf-Kaishew theorem. On the other hand, the silicide nanocrystals formed by reactive deposition exhibited high uniformity and thermal stability. Significant strain relaxation, as could be judged by the nanocrystal flattening, took place only at temperatures in excess of 650 °C, followed by progressive nanocrystal coalescence. It thus could be inferred, that better titanium silicide nanocrystal arrays (in the sense of uniformity and stability) are more easily obtained by reactive deposition epitaxy than by solid-phase epitaxy. While terminal, stable C54-TiSi₂ phase, did eventually form in the epilayers in both methods, different evolution pathways were manifested by different respective morphologies and orientations even in this final state.