Nanometer-sized Clusters Research Articles

Theory of the charged cluster formation in the low pressure synthesis of diamond: Part I. Charge-induced nucleation

Based on several experimental observations, Hwang et al. recently proposed “the charged cluster model” [J. Cryst. Growth, 162, 55–68 (1996)] to disentangle the “puzzling thermodynamic paradox” encountered in the gas-activated chemical vapor deposition (CVD) of diamond. Many unusual phenomena observed in the CVD diamond process can be successfully approached by the charged cluster model. However, there are a couple of important subjects still unsolved quantitatively. The first question is connected with the main driving force for this unusual nucleation in the gas phase. The second issue is related to the difference in the thermodynamic stability between graphite and diamond for a nanometer-sized cluster during the growth. In this study, we have theoretically examined the thermodynamic driving forces for the charge-induced nucleation, in general, and have applied this idea to the nucleation of the charged carbon-atom cluster. It was shown that the short-range ion-induced dipole interaction and the ion-solvation electrostatic effect (Born term) were mainly responsible for this unusual nucleation in the gas phase. The theoretical analysis presented in this article is quite generic and, thus, can be applied to any process that involves the charge-induced nucleation.

Nanostructuring of Highly Ordered C60 Films by Charge Transfer

Highly ordered surfaces of C60 films are formed on highly ordered pyrolytic graphite (see Figure for a scanning tunneling microscopy image). Electrochemical reduction of these films leads to the first reported electrochemical nanostructuring of fullerene films, where nanometer-sized clusters of potassium fullerides are formed in fullerene layers at the electrode surface. The resulting stable structures cannot be destroyed by chemical reactions.

Optical characterization of porous silicon embedded with CdSe nanoparticles

Arrays of a few nanometer-size clusters have been realized using a porous silicon (PS) matrix by filling its pores with CdSe. The photoluminescence (PL) peak from the embedded area of the PS samples with different luminescence stabilizes at 1.79 eV. This has been interpreted as due to emission from the CdSe clusters with an average size of about 3–5 nm. Likewise, the PL and Raman scattering spectra of the pure PS area of the samples have been compared with those obtained from the embedded areas. PL spectra were examined as a function of laser irradiation. Finally, to analyze the possibility of the formation of metal/porous semiconductor contacts, cross-section structures have been observed by scanning electron microscopy in the electron-beam-induced current mode.

Unsupported nanometer-sized copper clusters studied by electron diffraction and molecular dynamics

Unsupported nanometer-sized copper clusters studied by electron diffraction and molecular dynamics

Mn-doped nanometer-size ZnS clusters in chitosan film matrix prepared by ion-coordination reaction

A novel method, the so called ion-coordination reaction, was introduced to prepare nanometer-sized Mn-doped ZnS clusters stuck to a chitosan medium. A possible growth mechanism of the Mn-doped ZnS clusters in a chitosan film has been proposed. The X-ray diffraction showed that the ZnS clusters have a cubic structure like bulk crystals and the cluster sizes were estimated from the diffraction linewidth to be 2–4 nm based on Scherre's equation. The emission of Mn ion was observed through band-to-band excitation of ZnS. The excitation spectrum showed a large blue shift relative to that of ZnS bulk material. The effect of Zn ion concentration on the luminescence intensity reveals that the emission efficiency of Mn ion increases with the decrease of the ZnS cluster size.

Recent developments in the preparation and properties of nanometer-size spherical and platelet-shaped particles and composite particles

Abstract By using self-assembly molecules as a template, nanometer-sized plate-like metal oxide and semiconductor particles can be obtained by confined growth inside the lamellar bilayers of microemulsion systems. It was found that a strong chemical affinity between the metal salt and the polar head group of amphiphilic molecules and the anisotropic structure of microemulsion systems play a premier role in the anisotropic growth. Nanometer-sized composite particles (nano-composites) with a core-shell structure have been prepared by arrested precipitation of metal or semiconductor clusters in reverse micelles, followed by hydrolysis and condensation of organometallic precursors in the microemulsion matrices. Temporally discrete nucleation and growth at elevated temperature (70 °C) give the resulting particles a narrow size distribution and defined crystallinity. Both the size of the core particles and the thickness of the coating layers can be varied by controlling processing parameters such as the ratio of water to surfactant and the ratio of water to organometallic precursors. By controlling the pH conditions and aging temperatures, a transparent gel composing the nanometer-sized inorganic clusters has been obtained. Optical properties of nanometer-sized composite particles are reviewed. For silver metal clusters and nano-composites, the shift of the absorption peak at the surface-plasmon resonance frequency due to the classical limited mean-free path of the conduction electrons or quantum size effects has been observed. The enhanced third-order non-linear susceptibility of the silver nano-composites results from the local-field enhancement and size effects, which has been experimentally demonstrated by the optical phase-conjugation technique.

Thermodynamics of water-cubic ice and other liquid-solid coexistence in nanometer-size particles

When contributions from the interfacial energy become significant and comparable to the bulk energy, liquid and crystalline phases can coexist at a temperature much lower than the usual melting point. A formalism for this coexistence is given, and thermodynamic conditions for the melting of nanometer-size cubic ice crystals are derived when both the ice and water are at an equilibrium vapor pressure. By using the approximate values of surface energy and the enthalpy and entropy of melting, it is shown that nanometer-size water droplets can coexist with cubic ice particles of about the same size at temperatures in the 150–180 K range. The unusually large decrease in the temperature of a liquid-solid phase equilibrium is expected to be a general phenomenon in the nanometer-size films, clusters, and particles of materials.

Nanometer-size atomic clusters in semiconductors—a new approach to tailoring material properties

In this paper the physical mechanisms and fundamental methods of obtaining nanometer-size atomic clusters in semiconductors are discussed, along with the possibility of controlling the properties of these clusters and those of the resulting cluster-containing materials. A number of electronic properties of semiconductors containing nanometer-sized clusters are considered, along with potential uses of these materials in electronics.

Short-wavelength photoluminescence of SiO2 layers implanted with high doses of Si+, Ge+, and Ar+ ions

The short-wavelength (400–700 nm) photoluminescence (PL) spectra of SiO2 layers implanted with Si+, Ge+, and Ar+ ions in the dose range 3.2×1016–1.2×1017 cm−2 are compared. After Ar+ implantation an extremely weak luminescence, which vanishes completely after annealing for 30 min at 400 °C or 20 ms at 1050 °C, was observed. After implantation of group-IV elements the luminescence intensities were 1 to 2 orders of magnitude higher, and the luminescence remained not only with annealings but it could also increase. The dose and heating dependences of the luminescence show that it is due to the formation of impurity clusters and this process is more likely to be of a percolation than a diffusion character. For both group-IV impurities an intense blue band and a weaker band in the orange part of the spectrum were observed immediately after implantation. The ratio of the excitation and emission energies of the blue luminescence is characteristic of oxygen vacancies in SiO2; its properties are determined by the direct interaction of group-IV atoms. On this basis it is believed that the centers of blue PL are chains of Si (or Ge) atoms embedded in SiO2. The orange luminescence remained after annealings only in the case of Si+ implantation. This is attributed directly to the nonphase precipitates of Si in the form of strongly developed nanometer-size clusters.

Melting point depression of Al clusters generated during the early stages of film growth: Nanocalorimetry measurements

This work investigates the thermodynamic properties of small structures of Al using an ultrasensitive thin-film differential scanning calorimeter. Al thin films were deposited onto a Si3N4 surface via thermal evaporation over a range of thicknesses from 6 to 50 Å. The Al films were discontinuous and formed nanometer-sized clusters. Calorimetry measurements demonstrated that the melting point of the clusters is lower than the value for bulk Al. We show that the melting point of the clusters is size dependent, decreasing by as much as 140 °C for 2 nm clusters. The results have relevance in several key areas for Al metallization in micro-electronics including the early stages of film growth and texture formation, the Al reflow process, and the dimensional stability of high aspect ratio Al lines.

Monodisperse Two-Dimensional Nanometer Size Clusters of Partially Fluorinated Long-Chain Acids

Two-dimensional molecular clusters of a few tens nanometer size were found in spread monolayers of a series of partially fluorinated long-chain acids. Atomic force microscopy images have revealed that the size of the clusters is sharply monodisperse. The size changes systematically with changing structure of the hydrophobic chain of the amphiphiles. The smallest cluster has a circular shape of 17 nm diameter. One cluster is composed of about 700 film molecules. These clusters gather to form macroscopic domains of millimeter size without compression. A formation mechanism of these molecular clusters is discussed. Clusters are formed during the spreading process due to the instability of the film materials at the spreading process. It was made clear that cluster formation during the spreading is rather general for amphiphiles under the conditions where condensed monolayers are formed.

STM Observations of Ferromagnetic Clusters

Co, Fe and Ni clusters of nanometer size, deposited on silicon and graphite (highly oriented pyrolytic graphite), were observed by a scanning tunneling microscope. Deposition as well as the scanning tunneling micro- scope measnrements were carried out in an ultrahigh vacuum system at room temperature. Detailed analysis of Co cluster height was done with the scan- ning tunneling microscope equipped with a ferromagnetic tip in the magnetic field up to 70 Oe. It is found that bigger clusters (few nanometers in height) exhibit a dependence of their apparent height on applied magnetic field. We propose that such behaviour originates from the ferromagnetic ordering of cluster and associate this effect to spin polarized tunneling. PACS numbers: 34.80.Nz, 36.40.Cg, 61.16.Ch

Softening of nanocrystalline metals at very small grain sizes

Nanocrystalline solids, in which the grain size is in the nanometre range, often have technologically interesting properties such as increased hardness and ductility. Nanocrystalline metals can be produced in several ways, among the most common of which are high-pressure compaction of nanometre-sized clusters and high-energy ball-milling1,2,3,4. The result is a polycrystalline metal with the grains randomly orientated. The hardness and yield stress ofthe material typically increase with decreasing grain size, a phenomenon known as the Hall–Petch effect5,6. Here we present computer simulations of the deformation of nanocrystalline copper, which show a softening with grain size (a reverse Hall–Petch effect3,7) for the smallest sizes. Most of the plastic deformation is due to a large number of small ‘sliding’ events of atomic planes at the grain boundaries, with only a minor part being caused by dislocation activity in the grains; the softening that we see at small grain sizes is therefore due to the larger fraction of atoms at grain boundaries. This softening will ultimately impose a limit on how strong nanocrystalline metals may become.

Correlation effects among nanometre-sized clusters in Cu-Co melt-spun alloys with giant magnetoresistance

Abstract Magnetic correlation effects in nearly superparamagnetic granular Cu100_xCox systems exhibiting giant magnetoresistance (GMR) are studied by analysing the flat-top parabolae obtained when the GMR is plotted as a function of reduced magnetization. The nature of GMR in granular metallic systems is discussed in detail, taking into account the typical time constants involved in the short-time dynamics of magnetic moments in a local magnetic field. An outline of a theory providing the first quantitative explanation of the effect of magnetic correlation on GMR is given. The role of Co content in the sizes and distances of Co particles and in their degree of magnetic correlation are discussed, examining, for studied composition each, the samples characterized by the best GMR values.

Size-Dependent Icosahedral-to-fcc Structure Change Confirmed in Unsupported Nanometer-Sized Copper Clusters

Unsupported nanometer-sized copper particles in a molecular beam of inert carrier gas have been studied using electron diffraction. A distinct separation in the size distributions of fcc and icosahedral particles is observed: Icosahedra occur at sizes well below a diameter limit of 3.8 nm predicted for copper by molecular dynamics simulations, while fcc particles occur close to this limit or above. Molecular dynamics simulations have also provided information about particle temperature via thermal expansion and atomic dynamics.

Preparation of Functional Silane-Stabilized Gold Colloids in the (Sub)nanometer Size Range

A synthesis method is introduced for very small uniform gold particles (diameter less than 5 nm), based on the reduction of hydrogen tetrachloroaurate(III) in ethanol in the presence of (γ-mercaptopropyl)trimethoxysilane (MPS). The surface layer of MPS molecules gives the gold particles a high colloidal stability and allows in principle further reaction with any silane coupling agent. Decrease of the HAuCl4:MPS ratio allows a controlled reduction of gold particle size, resulting in remarkably uniform gold clusters of (sub)nanometer size, observed with high-angle-annular-dark-field scanning transmission electron microscopy. After attachment of (γ-aminopropyl)triethoxysilane (APS) to the MPS surface layer, other molecules may be covalently bound to the gold colloid via the amine group of APS. As an illustrative example we prepared in this manner gold particles labeled with a fluorescent dye. The chemical structure of the surface silanes was studied with Fourier transform infrared spectroscopy.

Crystalline structure and orientation of gold clusters grown in preformed nanometer-sized pits

Gold clusters were produced by condensing evaporated gold in nanometer-sized preformed pits on the surface of highly oriented pyrolytic graphite (HOPG). The height of the clusters was 6.7 ± 0.7 nm as measured with scanning tunneling microscopy in ultrahigh vacuum, the lateral width was 10.1 ± 1.9 nm as determined with transmission electron microscopy (TEM). Using TEM for electron diffraction, we obtained information on the crystalline structure of the clusters. The intensity of the observed diffraction rings shows the preferential orientation of the clusters with the (111) plane of the gold lattice parallel to the (0001) surface of HOPG. This was compared to the diffraction pattern of gold clusters produced in the gas phase by inert-gas evaporation and deposited on a flat HOPG surface at room temperature as complete units which showed no preferential orientation. The directional alignment in the surface plane as it is described in the literature for larger gold crystallites grown on a flat HOPG surface is not observed for the nanometer-sized clusters grown in pits.

Spontaneous alloying of gold clusters into nanometer-sized antimony clusters

Alloying behavior of gold into nm-sized amorphous antimony (a-Sb) clusters has been studied by transmission electron microscopy (TEM), employing gold clusters in contact with a-Sb clusters. In order to produce gold clusters on individual a-Sb clusters, a-Sb clusters on an amorphous carbon film were cooled down to 96 K, and gold was then condenced on the film. When gold clusters in contact with a-Sb clusters are gradually heated from 96 to 290 K, dissolution of gold into a-Sb clusters sets in around 200K and clusters of a-(Sb-Au) alloys are produced. With increasing annealing temperture, more gold is absorbed into individual a-Sb clusters, and when the gold concentration in a-(Sb-Au) clusters reaches to the stoichiometric composition of AuSb2, these amorphous clusters crystallize into AuSb2 clusters. The crystallization temperature decreases with decreasing size of initial a-Sb clusters.

Halo-like structures studied by atomic force microscopy

Nanometer-sized clusters of copper have been produced in a hollow cathode sputtering source and deposited on SiOx. Halo-like structures consisting of micrometer sized protrusions in the silicon oxide surface surrounded by thin rings of smaller particles are observed. The area in between seems to be depleted of particles. We propose that the halo-like structures are a result of electrostatic forces acting between the incoming charged clusters and charged regions on the surface. A simple computer simulation supports this suggestion.

Encapsulation of Gold Nanoclusters in Silica Materials via an Inverse Micelle/Sol−Gel Synthesis

Nanometer-sized gold particles were encapsulated in the micropores of xerogels and aerogels. The synthesis involves the sequential reduction of a gold salt followed by sol-gel processing in an inverse micelle solution. The inverse micelle solution solubilizes the metal salt and provides a microreactor for the nucleation, growth, and stabilization of the nanometer-sized clusters. Hydrolysis and condensation of an added siloxane precursor produces a wet gel embedding the particles. Characterization of the particle size and composition and the particle growth process was completed with transmission electron microscopy (TEM), electron diffraction, and UV-visible absorption spectrometry. Characterization of the gel surface areas was completed with N{sub 2} porosimetry. Material properties determined as a function of the gel precursor (TEOS vs a prehydrolyzed form of TEOS), the water to gel precursor reaction stoichiometry, and surfactant concentration are discussed in terms of the unique solution chemistry occurring in the microheterogeneous inverse micelle solutions. 73 refs., 7 figs., 1 tab.