Bi Nanocrystals Research Articles

Growth and melting of metallic nanoclusters in glass: A review of recent investigations

The mechanisms of nucleation and growth and the solid-to-liquid transition of metallic nanoclusters embedded in sodium borate glass were recently studied in situ via small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). SAXS results indicate that, under isothermal annealing conditions, the formation and growth of Bi or Ag nanoclusters embedded in sodium borate glass occurs through two successive stages after a short incubation period. The first stage is characterized by the nucleation and growth of spherical metal clusters promoted by the diffusion of Bi or Ag atoms through the initially supersaturated glass phase. The second stage is named the coarsening stage and occurs when the (Bi-or Ag-) doping level of the vitreous matrix is close to the equilibrium value. The experimental results demonstrated that, at advanced stages of the growth process, the time dependence of the average radius and density number of the clusters is in agreement with the classical Lifshitz-Slyozov-Wagner (LSW) theory. However, the radius distribution function is better described by a lognormal function than by the function derived from the theoretical LSW model. From the results of SAXS measurements at different temperatures, the activation energies for the diffusion of Ag and Bi through sodium borate glass were determined. In addition, via combination of the results of simultaneous WAXS and SAXS measurements at different temperatures, the crystallographic structure and the dependence of melting temperature T m on crystal radius R of Bi nanocrystals were established. The experimental results indicate that T m is a linear and decreasing function of nanocrystal reciprocal radius 1/R, in agreement with the Couchman and Jesser theoretical model. Finally, a weak contraction in the lattice parameters of Bi nanocrystals with respect to bulk crystals was established.

Melting and freezing of spherical bismuth nanoparticles confined in a homogeneous sodium borate glass

The melting temperature and the crystallization temperature of Bi nanoclusters confined in a sodium borate glass were experimentally determined as functions of the cluster radius. The results indicate that, on cooling, liquid Bi nanodroplets exhibit a strong undercooling effect for a wide range of radii. The difference between the melting temperature and the freezing temperature decreases for decreasing radius and vanishes for Bi nanoparticles with a critical radius $R=1.9\text{ }\text{nm}$. The magnitude of the variation in density across the melting and freezing transitions for Bi nanoparticles with $R=2\text{ }\text{nm}$ is 40% smaller than for bulk Bi. These experimental results support a basic core-shell model for the structure of Bi nanocrystals consisting of a central crystalline volume surrounded by a structurally disordered shell. The volume fraction of the crystalline core decreases for decreasing nanoparticle radius and vanishes for $R=1.9\text{ }\text{nm}$. Thus, on cooling, the liquid nanodroplets with $R<1.9\text{ }\text{nm}$ preserve, across the liquid-to-solid transformation, their homogeneous and disordered structure without crystalline core.

Solution−Liquid−Solid (SLS) Growth of Silicon Nanowires

Here we report the solution-liquid-solid (SLS) synthesis of silicon (Si) nanowires. Nanowires are grown by trisilane (Si3H8) decomposition in a high boiling solvent, octacosane (C28H58) or squalane (C30H62), in the presence of either Au or Bi nanocrystals. To our knowledge, this is the first report of a colloidal synthetic route carried out in a solvent at atmospheric pressure that provides crystalline Si nanowires in large quantities.

Magnetic-field-assisted solvothermal growth of single-crystalline bismuth nanowires

The magnetic-field-assisted approach has been used in the shape-controlled synthesis ofsingle Bi nanocrystals. By tuning the magnetic field strength in the solvothermal process,Bi nanowires with dimension of 40–200 nm and lengths up to tens of micrometers weresynthesized. Various techniques such as x-ray diffraction, scanning electron microscopy,transmission electron microscopy and Fourier transform infrared spectrometryhave been used to characterize the products obtained. The results show that themagnetic field plays a key role in the crystal growth of the Bi nanowires. Allnanowires were highly oriented single crystals with the growth direction along thec-axis.

Bismuth Nanocrystal-Seeded III-V Semiconductor Nanowire Synthesis

Bismuth (Bi) nanocrystals are used for solution−liquid−solid (SLS) synthesis of crystalline InAs, GaP, GaAs, and InP nanowires at temperatures between 300 and 340 °C in trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO), and trioctylamine (TOA). Bi nanocrystals are observed at the nanowire tips, confirming their role as crystallization seeds. The nanowires are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD).

Electrical properties of bismuth nanocrystal–glass composite

Composites of bismuth nanocrystals and glass matrix have been obtained from Bi 0.33Ge 0.67O 1.84 and Bi 0.57Si 0.43O 1.72 glasses as a result of thermal treatment in hydrogen atmosphere. We have shown that the temperature dependence of the conductivity of bismuth germanate glass containing Bi nanocrystals, reduced in hydrogen exhibits crossover from T −1/4 to T −1/2 dependence in the temperature range between 170 and 190 K. Characteristic parameters describing conductivity were determined and their values were discussed within the models describing conductivity in granular metals. Conductivity in Bi–GeO 2 system may be described within the model of variable-range hopping between localized centers including Coulomb gap. On the other hand, in Bi–SiO 2 system the ES model is not a good explanation of conductivity. We suggest that Zvyagin’s model of conduction by hopping via virtual states (no Coulomb gap effects) may quite well describe conductivity in this system with high concentration of grains.

Evidence for shallow implantation during the growth of bismuth nanocrystals by pulsed laser deposition

The implantation of bismuth during pulsed laser deposition (PLD) has been directly observed and investigated. Bi was deposited on amorphous aluminum oxide (Al2O3) and the laser energy density on the Bi target was varied by one order of magnitude (0.4 to 5 J cm−2). Cross-sectional transmission electron micrographs reveal that, for laser energy densities above 2 J cm−2, in addition to the formation of Bi nanocrystals, there is a dark and apparently continuous layer in the Al2O3 underneath them. From previous velocity measurements, the kinetic energy of the Bi species in the plume generated at laser energy densities above 2 J cm−2 has been estimated to be around 200 eV, which gives a calculated implantation range of 1.8 nm in Al2O3. This is in good agreement with the position of the Bi-rich layer.

Size-dependent melting depression and lattice contraction of Bi nanocrystals

Several models, free of any adjustable parameters, for the size-dependent melting temperature and the size-dependent lattice contraction of nanocrystals and related solid–liquid interface energy are introduced and compared with the newest experimental results of Bi nanocrystals. It is found that the models may interpret the experimental results well.

The Shallow Implantation of Bismuth During the Growth of Bismuth Nanocrystals in Al2O3 by Pulsed Laser Deposition

AbstractThe effect of the laser energy density used to deposit Bi onto amorphous aluminum oxide (a-Al2O3) on the growth of Bi nanocrystals has been investigated using transmission electron microscopy of cross section samples. The laser energy density on the Bi target was varied by one order of magnitude (0.4 to 5 J cm-2). Across the range of energy densities, in addition to the Bi nanocrystals nucleated on the a-Al2O3 surface, a dark and apparently continuous layer appears below the nanocrystals. Energy dispersive X-ray analysis on the layer have shown it is Bi rich. The separation from the Bi layer to the bottom of the nanocrystals on top is consistent with the implantation range of Bi species in a-Al2O3. As the laser energy density increases, the implantation range has been measured to increase. The early stages of the Bi growth have been analyzed in order to determine how the Bi implanted layer develops.

Setup for in situ WAXS/SAXS studies of the formation and growth of Bi nanodroplets and the melting of Bi nanocrystals using synchrotron radiation

The setup was designed for in situ studies of structural transformations in materials at high temperatures. It makes possible simultaneous recording of wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) spectra from room temperature up to 1000 °C. The setup was installed in the National Synchrotron Light Laboratory SAXS beamline. It was applied to in situ studies of several structural transformations such as the formation and growth of liquid Bi nanodroplets embedded in a B 2O 3–Na 2O glass and the melting of Bi nanocrystals dispersed in the same glass matrix.

Structure and melting of Bi nanocrystals embedded in aB2O3−Na2Oglass

A composite material consisting of spherical Bi nanoclusters (nanocrystals and/or liquid nanodroplets) embedded in a $28{\mathrm{Na}}_{2}\mathrm{O}\ensuremath{-}72{\mathrm{B}}_{2}{\mathrm{O}}_{3}$ glass was studied by the wide-angle x-ray scattering (WAXS) and small-angle x-ray scattering (SAXS) techniques over the temperature range in which the Bi crystal-liquid transition occurs. Because of the wide radius distribution of Bi clusters and due to the dependence of the melting temperature on crystal radius, the overall transition occurs over a wide range, from 365 up to 464 K. In this transition range, large Bi nanocrystals coexist with small liquid droplets. A weak contraction in a and c lattice parameters of rhombohedral Bi nanocrystals with respect to the bulk crystal was detected. As expected, the average radius of crystalline Bi clusters, deduced from WAXS data, increases for increasing temperatures over the whole solid-to-liquid transition range. The SAXS spectrum recorded at different temperatures within the transition range is essentially invariant, indicating that the radius distribution of Bi nanoclusters (nanocrystals and nanodroplets) is temperature independent. The volume distribution of Bi nanoclusters is a single-mode function with the radius ranging from about 15 up to 41 \AA{} with a maximum at 28 \AA{}. The integral of Bragg peaks of Bi nanocrystals decreases for increasing temperatures as a consequence of the progressive melting of nanocrystals of increasing size. By combining the results of WAXS and SAXS experiments, we determined the melting temperature of the nanocrystals as a function their radius suppressing unwanted size dispersion effects. Our results clearly indicate a linear dependence of the melting temperature on nanocrystal reciprocal radius, thus confirming previous theoretical predictions.

Ion-Beam-Assisted Nanocrystal Formation in Silicon Implanted with High Doses of Pb+ and Bi+ Ions

In this paper we discuss ion-beam-assisted nanocrystal nucleation in amorphized silicon (a-Si) layers produced by high-dose implantation of Pb+ and Bi+. (100)-oriented Si wafers were implanted at room temperature (RT) with 50 keV Pb+ and Bi+ ions at doses ranging from 5×1013 to 1×1018 cm-2 and a constant ion current density of 10 µA cm-2. The resulting structures were studied by conventional transmission electron microscopy (CTEM), high resolution transmission electron microscopy (HRTEM) and Rutherford backscattering spectroscopy (RBS) in combination with computer simulations. The dynamics of the ion-beam-induced crystallization of new phases and precipitates evolution in the implanted layer were studied as a function of implant dose. It is established that the front of the new phase crystallization (cubic Pb and hexagonal Bi nanocrystals) starts approximately at the peaks of the implanted species profiles; the crystallography of the nucleated nanocrystal is examined as a function of the dose.

Size effects investigated by Raman spectroscopy in Bi nanocrystals

Bismuth nanocrystals, embedded in amorphous germanium thin films and synthesized by pulsed-laser deposition, are investigated by Raman spectroscopy. Raman spectra are strongly dependent upon the size of the deposited nanocrystals. For the larger nanocrystals, two peaks corresponding to the optical phonons ${A}_{1g}$ and ${E}_{g}$ can be observed. As the size of the particles decreases, the ${E}_{g}$ mode shifts to higher frequencies while the ${A}_{1g}$ mode shifts to lower frequencies. Such behavior is a result of the curvature of the respective phonon-dispersion curves in Bi. A third size-dependent low-frequency mode has also been observed and attributed to acoustic phonons confined in Bi nanocrystals.

Crystallisation of thin Bi/Ge films: role of Bi crystal size

The kinetics of metal-induced-crystallisation of layered films containing amorphous Ge (a-Ge) in contact with Bi nanocrystals (NCs) have been studied by in situ transmission electron microscope annealing. The a-Ge crystallisation temperature depends strongly on NC size and shape, the former influencing the nucleation process through the metal surface area in contact with the semiconductor and the latter controlling the initial Ge crystal growth direction.

The role of size effects on the crystallization of amorphous Ge in contact with Bi nanocrystals

The kinetics of metal-induced crystallization of amorphous Ge in contact with Bi nanocrystals (NCs) have been studied by in situ transmission electron microscope annealing. Series of nanostructured films consisting of layers of Bi NCs in an amorphous Ge matrix have been grown by pulsed laser deposition. The a-Ge crystallization temperature depends strongly on both the size and shape of the NCs and the separation between the NCs in the film-normal direction. The size of the NCs controls the crystal nucleation process through the amount of metal surface in contact with the semiconductor, the shape of the NCs determines the initial Ge crystallization in the direction perpendicular to the film plane, and the separation between the NCs in the film-normal direction controls the overall pattern of the Ge crystal growth process.

Spectroscopic ellipsometry of composite thin films with embedded Bi nanocrystals

Spectroscopic ellipsometry together with an effective medium model is used to determine simultaneously the effective refractive index, thickness, and metal volume fraction of thin nanocomposite films. The films are formed by Bi nanocrystals embedded in amorphous matrices, either semiconducting (Ge) or dielectric (Al2O3). For the Bi:Ge films (metal in an absorbing host), the values obtained for both the real and imaginary parts of the refractive index vary continuously from that of Ge to that of Bi. The metal contents determined from the ellipsometry analysis are in excellent agreement with those obtained from direct measurements of the composition. For the Bi:Al2O3 films (metal in a nonabsorbing host), the extinction coefficient (k) exhibits a maximum around 360 nm which is related to the metal plasmon resonance frequency of Bi nanocrystals. The metal content determined from the ellipsometry analysis in this case is underestimated, probably due to interaction of the Bi crystals with the Al2O3 host.

Bi nanocrystals embedded in an amorphous Ge matrix grown by pulsed laser deposition

Bi nanoclusters embedded in an amorphous Ge matrix have been produced by alternate pulsed laser deposition in a vacuum. The resulting thin films have been analyzed by high resolution electron microscopy, and the images show that the Ge matrix is amorphous and that Bi nanocrystals are formed. The size of the nanoclusters was easily changed within the range 2 nm to 25 nm by varying the number of pulses on the Bi target. Both the dependence of the nanocrystal size on the number of laser pulses on the Bi target and the results of tilting experiments in the electron microscope show that the nanocrystals have an oblate ellipsoidal shape with a much shorter axis in the direction of the film normal. This particular shape is related to their growth mechanism after nucleation at the substrate.