Crystallization Of Nanoclusters Research Articles

In Situ Crystallization, Differential Growth, and Multicolor Emission of Silver Nanoclusters.

The real-time monitoring of the stepwise growth process of the molecular crystal reveals a conclusive understanding of the morphological evolution, which otherwise remains elusive during the conventional crystallization processes. Herein, we report the in situ crystallization of silver nanoclusters (AgNCs) with special emphasis on their differential growth and multicolor emissive properties. A subtle variation of the methanol (MeOH) proportion in the reaction mixture induces the differential growth of these AgNCs, and thereby, a dramatic modulation in their optical properties was observed. Additionally, by increasing the temperature of the reaction (from a low temperature ice bath to 25 °C), an uncontrolled formation of AgNCs along with metallic silver nanoparticles (AgNPs) was observed, which was primarily induced by accelerating the reaction kinetics. We hope that this investigation comprehensively uncovers the serious bottlenecks of the conventional crystallization processes by showcasing systematic monitoring of structural evolution to the higher-ordered crystalline state.

Rearrangement of the optical properties of a-SiOx: H films after crystallization of silicon nanoclusters

The results of investigations on crystallization of silicon nanoclusters in a-SiOx matrix have shown that, even at the very fast annealing using pulse photonic annealing (PPA) formation of rather large silicon crystallites (≥ 100 nm) occurs, as well as the arrays of Si nanocrystals with average sizes of ~10 nm and small sizes of ~1–2 nm happens. It is shown that with an increase in the concentration of nanoclusters in the initial film from 15 to 53%, the contribution of large crystals (larger than 100 nm) increases from 15% (at 15% ncl-Si) to 65% (at 53% ncl-Si), which says on an increase in the probability of coalescence of crystallites into substantially larger ones with an increase of ncl-Si content in the initial film, despite the high annealing speed.

Формирование нанокристаллов германия в пленках GeO[SiO-=SUB=-2-=/SUB=-] и GeO[SiO

Films of nonstoichiometric germanosilicate glasses GeO0.5[SiO2]0.5 and GeO0.5[SiO]0.5 were obtained by co-evaporation of GeO2 and SiO2 or GeO2 and SiO powders, respectively, and by deposition on cold Si (100) and fused quartz substrates in a high vacuum. Then the films were annealed at temperatures up to 900 oC. The presence and phase composition of germanium nanoclusters in the films was investigated using Raman spectroscopy technique. The transformation after annealing of the GeSixOy matrix surrounding the nanoclusters was investigated using Fourier transform infrared spectroscopy. Analysis of the Raman spectra showed that annealing at a temperature of 800 oC led to the formation of germanium nanocrystals (NC-Ge), but the content of the amorphous germanium phase remains up to half of the volume. Annealing at a temperature of 900 oC led to almost complete crystallization of amorphous nanoclusters in all films. The sizes of the NC-Ge depended on the annealing temperature, the composition of the films, and on the substrate; in this case, the formation of Ge-Si nanocrystals was not observed.

On the Formation of IR-Light-Emitting Ge Nanocrystals in Ge:SiO2 Films

The study is concerned with light-emitting Ge nanocrystals formed during the annealing of Gex[SiO2]1 –x films produced by the high-vacuum cosputtering of germanium and quartz targets onto substrates at a temperature of 100°C. In accordance with the conditions of growth, the Ge molar fraction was varied from 10 to 40%. By means of electron microscopy and Raman spectroscopy, amorphous Ge nanoclusters ~4–5 nm in dimensions are detected in as-deposited films with a Ge content higher than 20 mol %. To crystallize amorphous nanoclusters, annealing at temperatures of up to 650°C is used. The kinetics of the crystallization of Ge nanoclusters is studied, and it is established that up to ~1/3 of the amorphous phase is retained in the system, supposedly at the interfaces between nanocrystals and the surrounding amorphous SiO2 matrix. It is found that, upon annealing in normal atmosphere, germanium nanoclusters are partially or completely oxidized (at a Ge molar fraction of 30% and smaller). An intense infrared photoluminescence signal from quantum-confined Ge nanocrystals and a visible photoluminescence signal defined by defect complexes (oxygen vacancy + excess Ge atoms) are observed.

Synthesis and catalytic application of alumina@SAPO-11 composite via the in situ assembly of silicoaluminophosphate nanoclusters at an alumina substrate

An alumina@SAPO-11 composite with a core–shell structure has been synthesized via the in situ assembly and crystallization of silicoaluminophosphate nanoclusters.

Формирование светоизлучающих в ИК-диапазоне нанокристаллов германия в плeнках Ge:SiO-=SUB=-2-=/SUB=-

AbstractThe study is concerned with light-emitting Ge nanocrystals formed during the annealing of Ge_ x [SiO_2]_1 –_ x films produced by the high-vacuum cosputtering of germanium and quartz targets onto substrates at a temperature of 100°C. In accordance with the conditions of growth, the Ge molar fraction was varied from 10 to 40%. By means of electron microscopy and Raman spectroscopy, amorphous Ge nanoclusters ~4–5 nm in dimensions are detected in as-deposited films with a Ge content higher than 20 mol %. To crystallize amorphous nanoclusters, annealing at temperatures of up to 650°C is used. The kinetics of the crystallization of Ge nanoclusters is studied, and it is established that up to ~1/3 of the amorphous phase is retained in the system, supposedly at the interfaces between nanocrystals and the surrounding amorphous SiO_2 matrix. It is found that, upon annealing in normal atmosphere, germanium nanoclusters are partially or completely oxidized (at a Ge molar fraction of 30% and smaller). An intense infrared photoluminescence signal from quantum-confined Ge nanocrystals and a visible photoluminescence signal defined by defect complexes (oxygen vacancy + excess Ge atoms) are observed.

On the effect of heating and cooling rates on the melting and crystallization of metal nanoclusters

The effect of heating and cooling rates on melting (Tm) and crystallization (Tc) temperatures of metal nanoclusters is investigated in terms of the isothermal molecular dynamics. We report on the results obtained for nickel nanoclusters, although analogous results were also obtained for gold and aluminum nanoclusters. It is found that Tm increases, while Tc decreases with increasing heating and cooling rates, both Tm and Tc tending to the same value for heating and cooling rates tending to zero. The results indicate that the hysteresis of melting and crystallization of nanoparticles must be completely due to nonequilibrium conditions of heating and cooling. The transition of Ni nanoclusters to the amorphous state begins at very high cooling rates exceeding 10 TK/s.

Ge Nanostructures Embedded in ZrO2 Dielectric Films for Nonvolatile Memory Applications

Zirconia and hafnia dielectrics are promising high-k materials to replace SiO2 gate in CMOS devices. However, in ultrathin film approach, the structure and the properties of these materials depend strongly on the deposition conditions and the post deposition treatment. It was predicted from the first principles that dielectric constant of these materials can be improved via silicon and/or germanium doping1. Recently it was demonstrated that silicon/germanium doping allows the tetragonal phase to be stabilized2,3. Besides, the formation of Si- and/or Ge-nanoclusters was observed. Nowadays, germanium has attracted a wide interest because of its improved hole mobility in comparison to silicon and the 0.6-eV-bandgap making it attractive for electronic applications. However, Ge oxides show an unstable behavior whereas for success application a good passivation of the Ge surface is mandatory. In this regard, high-k dielectrics are a promising candidate for Ge surface passivation. In this work, we will present the results obtained for Ge-doped ZrO2 dielectrics fabricated by RF magnetron sputtering in terms of the effect of deposition conditions on GeZrOx solid solution formation as well as the effect of annealing treatment on its phase separation and modification of Ge and ZrO2phases’ properties. The Ge-doped ZrO2 composite films as well as Ge-ZrO2/ZrO2 multilayer structures were investigated. The samples were fabricated by confocal RF magnetron sputtering of pure Ge and ZrO2targets in Ar plasma. The structural, optical and electrical properties of the samples were studied by means of spectroscopic ellipsometry, C-V (I-V), Raman scattering, FTIR and TEM methods versus deposition conditions and annealing treatment. It was observed that Ge-doped ZrO2 materials demonstrate phase separation upon thermal treatment. At the same time the “start point” of this phase separation depends significantly on Ge content in Ge-doped-ZrO2 materials and/or Ge-doped sublayer thickness in multilayer structures. Although the phase separation begins at about 600-700°C, the higher Ge content and/or thicker Ge-doped sublayer result in the lowering of the annealing temperature stimulated phase separation and an appearance and crystallization of Ge nanoclusters. Along with this the formation of tetragonal ZrO2 occurs that offers an achievement of higher dielectric constant. In addition, this leads to an orientation relation between the crystallized ZrO2and Ge nanosclustes. We will discussed in details the mechanism of phase separation that is the first step towards an in-situ passivation of the Ge surface. The obtained results demonstrate that the RF magnetron sputtering is a promising technique for the production of different structures and devices fully based on high-k dielectrics. This work is supported by DAAD grant (A/14/02380). D. Fischer and A. Kersch, Appl. Phys. Lett. 92, 012908 (2008)E. Talbot, M. Roussel, C. Genevois, P. Pareige, L. Khomenkova, X. Portier, F. Gourbilleau, J. Appl. Phys. 111, 103519 (2012).L.Khomenkova, X. Portier, M. Carrada, C. Bonafos, B. S. Sahu, A. Slaoui, F. Gourbilleau, ECS Trans. 45, 331 (2012)

Crystallization of silicon nanoclusters with inert gas temperature control

We analyze the fundamental process of crystallization of silicon nanoclusters by means of molecular dynamics simulations, complemented by magnetron-sputter inert gas condensation, which was used to synthesize polycrystalline silicon nanoclusters with good size control. We utilize two well-established Si interatomic potentials: the Stillinger-Weber and the Tersoff III. Both the simulations and experiments show that upon cooling down by an Ar gas thermal bath, initially liquid, free-standing Si nanocluster can grow multiple crystal nuclei, which drive their transition into polycrystalline solid nanoclusters. The simulations allow detailed analysis of the mechanism, and show that the crystallization temperature is size-dependent and that the probability of crystalline phase nucleation depends on the highest temperature the cluster reaches during the initial condensation and the cooling rate after it.

Atomically precise metal nanoclusters: stable sizes and optical properties.

Controlling nanoparticles with atomic precision has long been a major dream of nanochemists. Breakthroughs have been made in the case of gold nanoparticles, at least for nanoparticles smaller than ∼3 nm in diameter. Such ultrasmall gold nanoparticles indeed exhibit fundamentally different properties from those of the plasmonic counterparts owing to the quantum size effects as well as the extremely high surface-to-volume ratio. These unique nanoparticles are often called nanoclusters to distinguish them from conventional plasmonic nanoparticles. Intense work carried out in the last few years has generated a library of stable sizes (or stable stoichiometries) of atomically precise gold nanoclusters, which are opening up new exciting opportunities for both fundamental research and technological applications. In this review, we have summarized the recent progress in the research of thiolate (SR)-protected gold nanoclusters with a focus on the reported stable sizes and their optical absorption spectra. The crystallization of nanoclusters still remains challenging; nevertheless, a few more structures have been achieved since the earlier successes in Au102(SR)44, Au25(SR)18 and Au38(SR)24 nanoclusters, and the newly reported structures include Au20(SR)16, Au24(SR)20, Au28(SR)20, Au30S(SR)18, and Au36(SR)24. Phosphine-protected gold and thiolate-protected silver nanoclusters are also briefly discussed in this review. The reported gold nanocluster sizes serve as the basis for investigating their size dependent properties as well as the development of applications in catalysis, sensing, biological labelling, optics, etc. Future efforts will continue to address what stable sizes are existent, and more importantly, what factors determine their stability. Structural determination and theoretical simulations will help to gain deep insight into the structure-property relationships.

Comparative molecular dynamics study of melting and crystallization of Ni and Au nanoclusters

The melting and crystallization hysteresis for nanoclusters of two transition metals (nickel and gold) have been comparatively studied using the molecular dynamics method and the tight-binding potential. It is established that the onset and end temperatures of the corresponding first kind of phase transition (i.e., melting and crystallization temperatures) should be considered for Ni nanoclusters, whereas Au nanoclusters are melted and crystallized at certain temperatures. The size dependence of the melting temperature of Au nanoclusters determined using the computation results is compared with the molecular dynamics data from other researchers and the results of two independent laboratory experiments. It is shown that our molecular dynamics results are in much better agreement with the experimental data than those obtained by other researchers.

Crystallization of nickel nanoclusters by molecular dynamics

We investigated the melting properties of bulk nickel and the crystallization of nickel nanocrystals via molecular dynamics using a potential in the framework of the second moment approximation of tight-binding theory. The melting behavior was simulated with the hysteresis approach by subsequently heating and cooling gradually the system over a wide range of temperatures. The crystallization of nickel nanoclusters consisting of 55, 147 and 309 atoms was achieved after repeatedly annealing and quenching the corresponding quasicrystals several times to avoid being trapped in a local energy minimum. The time over which the global minimum was reached was found to increase with the cluster size.

Study of the thermodynamic characteristics of gold nanoclusters using a Gupta many-body potential

The Monte Carlo method has been employed to investigate the hysteresis of the process of melting and crystallization of gold nanoclusters using a many-body Gupta potential. The size dependence of the effective specific total surface energy of gold nanoclusters has been calculated and analyzed based on the Gibbs surface phase method.

Formation and crystallization of silicon nanoclusters in SiN x :H films using femtosecond pulsed laser annealings

SiN x :H films of different compositions grown on glass and silicon substrates using plasma-chemical vapor deposition at a temperature of 380°C have been subjected to pulsed laser annealings. The treatments are performed using titanium-sapphire laser radiation with a wavelength of 800 nm and a pulse duration of 30 fs. Structural changes in the films are studied using Raman spectroscopy. Amorphous silicon nanoclusters are detected in as-grown films with molar fractions of excess silicon of ∼1/5 and larger. Conditions required for pulsed crystallization of nanoclusters were determined. According to the Raman data, no silicon clusters were detected in as-grown films with a small amount of excess silicon (x > 1.25). Pulsed treatments resulted in the formation of silicon nanoclusters 1–2 nm in size in these films.

Simulation of the processes of structuring of copper nanoclusters in terms of the tight-binding potential

The processes of melting and crystallization of copper nanoclusters with a radius ranging from 0.69 to 3.05 nm have been investigated using the molecular dynamics simulation. The performed simulation has shown that the melting begins with the surface of the cluster. Another feature of this phase transition is that it occurs in a temperature range where the liquid and solid phases can coexist. However, it is found that, for small copper clusters, the melting and crystallization temperatures coincide with each other. Moreover, it is established that the parent face-centered cubic structure of these small clusters (N < 150 atoms) transforms into a structure with fivefold symmetry even at temperatures of the order of 150–170 K. The behavior of some thermodynamic characteristics of copper nanoclusters is investigated in the vicinity of the solid-liquid phase transition. Analysis of the data obtained has revealed a number of regularities that are in agreement with the results of analytical calculations. In particular, the melting and crystallization temperatures of copper nanoparticles are linear functions of N−1/3. However, the melting heat ΔHm and the melting entropy ΔSm vary in a more complex manner. It is noted that the formation of a cluster structure depends on the conditions used for cooling from the liquid phase. Slow cooling results predominantly in the formation of a face-centered cubic phase, whereas rapid cooling in the majority of cases leads to the formation of an icosahedral modification. Therefore, the simulation performed has demonstrated the possibility of controlling the formation of a structure of copper nanoclusters during crystallization.

On the Problem of the Formation of Structural Modifications in Cu and Ni Nanoclusters by the Crystallization

The processes of melting and crystallization of Ni and Cu nanoclusters with a diameter to 3.7 nm have been studied by the molecular-dynamics method based on tight-binding potentials. We find that the nanoparticles melt via surface processes. The effect of the particle size on the stability of the structural modifications obtained has been investigated. The dependence of the process of the cluster-structure formation on the conditions of cooling from the liquid state has been noted. Upon slow crystallization, an fcc structure was mainly formed, whereas upon rapid cooling an icosahedral phase was formed in the majority of cases. Thus, the simulation performed indicates the possibility of some control of the structure formation of Cu and Ni clusters upon crystallization.

Modeling the crystallization of gold nanoclusters—the effect of the potential energyfunction

The crystallization dynamics of 5083 atom gold nanoclusters, which were quenched fromthe melt, were studied by molecular dynamics (MD) using the EAM ‘Glue’ and‘Force-matched’ potentials to compare and contrast how the crystallization dynamics isaffected by these potential energy functions. MD simulations from each potential showedthe formation of gold nanoclusters of icosahedral morphology during the quenching process,which is in good agreement with the experimental studies of gold nanoclustersformed under vacuum. The effect of the potential on the evolution of cluster(surface and interior) morphology during the crystallization process is discussed.

Formation of structural modifications during crystallization of Cu and Ni nanoclusters

The processes of melting and crystallization of Cu and Ni nanoclusters to 3.6 nm in diameter have been studied by the molecular-dynamics method. The effect of the particle size on the stability of structural modifications has been analyzed. The data obtained indicate a possibility of controlling to a certain extent the formation of the structure of Cu and Ni clusters during crystallization.

Silicon nanocluster crystallization in SiOx films studied by Raman scattering

Precipitation and crystallization of Si nanocrystals have been monitored by means of Raman spectroscopy. SiOx films with different compositions have been deposited by low-pressure chemical-vapor deposition technique onto silica substrates and treated to temperatures exceeding 800 °C. The evolution of the Raman signal with the thermal budget reveals that the silicon transition from amorphous to crystalline state shifts to higher temperatures as the Si content in the layers is lowered. A rather complete crystallization of the nanoparticles is achieved after annealing at 1250 °C for a Si excess lower than 20%, while for higher excesses the crystalline fraction reaches only 40%, suggesting the formation of a crystalline core surrounded by an amorphous shell. The Raman spectra have been analyzed by a phonon confinement model that takes into account stress effects. An increasing nanocrystal size, from 2.5 to 3.4 nm, has been estimated when the Si excess varies from 16 to 29 at. %. For small Si nanocrystals a strong hydrostatic stress has been observed, induced by a very abrupt transition with the surrounding SiO2. Its magnitude correlates with the increase in thermal budget required for the crystallization of the amorphous clusters. This study underlines the fundamental role of hydrostatic stress in retarding the crystallization of Si nanoclusters.

Formation of structural modifications in copper nanoclusters

The melting and crystallization of copper nanoclusters are investigated using the molecular dynamics simulation with tight-binding potentials. The formation of a cluster structure depends on the conditions used for cooling from the liquid phase. Slow cooling results predominantly in the formation of a face-centered cubic structure, whereas rapid cooling in the majority of cases leads to the formation of an icosahedral structure. Therefore, the simulation performed has demonstrated the possibility of controlling the formation of a structure of copper nanoclusters during crystallization.