Growth Of Nanocrystallites Research Articles

Efficient tunability of size and optical properties of reduced graphene oxide-ZnO composite nanocrystallites on solid substrates

Growth of ultra-thin luminescent reduced graphene oxide (rGO)-zinc oxide (ZnO) composite nanostructures was accomplished by Langmuir-Blodgett (LB) transfer of GO-Zn composite layers onto solid substrates and subsequent oxidation. Layer structure of LB transferred composites and oxidation procedure determine the size distribution, density, structural order, optical quality of grown ZnO and the extent of GO reduction. Prolonged oxidation (120 min) of composite monolayers produce ZnO nanocrystallites nearly deprived of defects and exhibiting substantially enhanced band edge emission. Prior anchoring of Zn2+ ions on GO layers and oxidation facilitate controlled growth of ZnO nanocrystallites and nanoclusters (20–100 nm), offer tunability of absorption edge (325–355 nm) and assists in amendment of surface defects/band edge emission (376–384 nm). Conductivity of precursor and oxidized composite layers was mapped by recording STM images as well as STS spectra and, the obtained differential conductivity spectra substantiate the changes in GO conductivity and provide range of bandgap values of grown ZnO nanocrystallites (3.1–3.4 eV). The multiple GO-Zn compositing procedures and optimum oxidation conditions introduced in present work facilitate development of rGO-ZnO composite nanostructures with high optical, crystalline quality and conductive characteristics, which pave path for fabrication of such nanostructures for optoelectronic device applications.

Studies on photocatalytic performance and supercapacitor applications of undoped and Cu-doped ZnO nanoparticles

The structural, morphology and optical properties were characterised using XRD, FTIR, SEM, TEM, and UV-DRS for the undoped and Cu-doped ZnO NPs with varying Cu doping molar concentrations from 0 wt% to 6 wt% through the co-precipitation method. The presence of a ZnO hexagonal wurtzite structure for all samples was confirmed by the structural study of the Cu-doped ZnO NPs, indicating the incorporation of Cu2+ ions into the ZnO lattice by substitution and average crystallite sizes of 25–22 nm. The surface morphology analysis of undoped and Cu-doped ZnO NPs indicated the growth of spherical nanocrystallites with fine particles. Optical studies of Cu-doped ZnO nanostructured particles revealed that increasing the Cu doping agent from 0 wt% to 6 wt% resulted in a blue shift of the absorption band with increasing band gap energy from 3.27 eV to 3.44 eV. The degradation efficiency of MB is 89.2% for Cu-doped ZnO photocatalyst, which performs much better than undoped ZnO under natural sunlight irradiation. The electrochemical analysis determined the specific capacitance of 10 to 100 mV/s scan rate for all the samples. Amongst the various concentrations, 6 wt% Cu-doped ZnO had the most excellent specific capacitance of 539.87 F/g at 10 mV/s. Compared with undoped ZnO NPs, Cu-doped into the ZnO matrix enhanced photocatalytic activities and electrochemical performance for wastewater treatment and supercapacitor applications.

Effect of TeO2 on Ag/Si interface contact of crystalline silicon solar cells

• The effects of TeO 2 on the sintering of c-Si solar cell are systematically studied. • TeO 2 affects the morphology of the grids on the sintering process of Ag pastes. • TeO 2 promotes Ag pastes sintering and improves the conductivity of Ag grids. • TeO 2 promotes the growth of the Ag nanocrystallites in Ag/Si contact interface. • An optimal conversion efficiency of solar cells was acquired at TeO 2 content ∼40%. Glass frits, as a critical ingredient, plays an important role between Ag electrode and crystalline silicon (c-Si) solar cells and determines sintering properties of the silver paste and contact quality with c-Si solar cells. Here glass frits with different contents of TeO 2 are prepared and used to investigate the sintering process and effects of contact quality of Ag pastes on c-Si solar cells. Microstructures and electrical properties of the Ag/Si contact interface and performances of the resultant c-Si solar cells are characterized systematically. The results show that TeO 2 promotes the growth of Ag nanocrystallites in the glass layer between Ag grids and c-Si wafers, thus reduces the contact resistance of the Ag grids and improves the contact quality of the Ag/Si interfaces. Moreover, TeO 2 improves the densification and conductive performances of the Ag grids. However, excessive TeO 2 has a negative effect on the conductivity of the Ag grid due to the increased thickness of the glass layer. Based on the competing factors played by TeO 2 , the conversion efficiency is the highest at an optimized TeO 2 content ∼40%.

Design of electrocatalysts with reduced Pt content supported on mesoporous NiWO4 and NiWO4-graphene nanoplatelets composite for oxygen reduction and hydrogen oxidation in acidic medium

Herein, a new direct synthesis route leading to a mesoporous NiWO4 with crystalline framework and NiWO4 - graphene nanoplatelets (GNP) composite is reported. Ni and W assembled into a mesoporous tungstate type of symmetry by co-precipitation synthesis route and its composite with GNP were used as supports for electrocatalysts, with reduced Pt content (8 wt.%), in oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) in acidic medium. A comprehensive assessment of the modifications related to the crystalline and porous structures, morphological aspects as well as the surface chemistry aiming to explain the electrochemical properties was performed. It was found that the presence of GNP during the synthesis process leads, mainly, to the enhanced growth of NiWO4 nanocrystallites, as well as induces changes in the surface chemistry. The electrochemical results show that the introduction of GNPs into the NiWO4 composite support leads to a significant improvement in the activity of the Pt electrocatalyst in ORR and HOR compared to both initial NiWO4 and Pt/NiWO4 samples, as well as mechanical mixtures of these catalysts with carbon. Mass activity for hydrogen oxidation, determined in a mixed kinetic-diffusion controlled region, obtained on the 8 wt.% Pt/NiWO4-GNP catalyst was significantly higher compared to the commercial 20 wt.% Pt/C Quintech catalyst. Our comprehensive structural and surface chemistry assessments indicate this composite material as a viable electrocatalyst for PEMFCs using a broader type of fuels.

Photon excitation effect on formation of graphene nanocrystallites during carbon film growth process

In this article, we propose a method to deposit nanocrystallite embedded carbon films by electron cyclotron resonance plasma sputtering with photon irradiation cooperated with electron or Ar+ ion irradiation. We found photon irradiation can enhance the growth of graphene nanocrystallites during carbon film deposition. The energy transfer from the photon to the metastable carbon structure excites the growth of sp2 hybridized graphene nanocrystallites, and photon-excited electrons can be accelerated by the bias and further promote the graphene nanocrystallite growth. Photons are the second quantum medium we found that can be used to deposit nanocrystallite embedded carbon films, and their quantum properties with electric neutrality can help us to further understand the formation of the carbon nanocrystallite structure and may shed light on the quantum fabrication of desired materials.

Thermodynamic analysis of structural transformations induced by annealing of amorphous Si–C–N ceramics derived from polymer precursors

Abstract Thermodynamic modeling has been used to explain structural transformations induced by heat treatment of amorphous Si –C–N ceramics derived from polymers. Nanocrystalline silicon carbide and nanocrystalline silicon nitride identified in the ceramic microstructure have been regarded as metastable NASIC and NASIN phases in the Si–C–N system. The Gibbs energies G(NASIC) and G(NASIN) have been derived and used together with the previously modeled Gibbs energy of the amorphous am-SICN to compute metastable phase diagrams. Computational results allow explanation of the crystallization process of amorphous Si –C–N ceramics. According to this model, the temperature of invariant reaction between carbon and silicon nitride changes with the growth of nanocrystallites, which explains the dependence of the thermal stability on the ceramic microstructure.

Effect of Heating on the Microstructure of NiAl + CrB2 Coatings Deposited by Mechanical Embedding in a Ball Mill

AbstractThe thickness of NiAl + CrB2 coatings, produced by the mechanical embedding of powders, is limited due to the increasing brittleness of processed materials with milling time. Only the NiAl grain growth and resultant softening of the coating matrix could overcome this problem. Therefore, the effect of heating up to 750°C on the microstructure of NiAl + CrB2 coatings deposited in a ball mill rotating at 350 rpm was investigated through in situ TEM observations. The performed observations proved that defect annihilation starts at ~400°C in large intermetallic grains, which are first attached to the substrate. The growth in NiAl nanocrystallites forming most of the coating is activated only above ~600°C. The average crystallite size was measured to be 5, 14, and 19 nm at RT, 650°C, and 750°C, respectively. The first stage of nano-crystallite growth is relatively fast and connected with the reconstruction of crystallite boundaries using up the amorphous material accumulated in between them. The second stage is slower and involves the expansion of larger crystallites at the expense of smaller ones. The performed experiment proved that heating up to 750°C allows the microstructure recovery and grain coarsening of coatings to be activated.

Confining CsPbX3 perovskites in a hierarchically porous MOF as efficient and stable phosphors for white LED

Confining metal-halide perovskites within MOF hosts not only protects them from exterior environment stimulation and isolates them from each other without aggregation, but also enables a variety of new features and applications. In this work, a hierarchically porous MOF host was firstly explored for in-situ confined growth of CsPbX3 perovskite nanocrystallites (PNCs). Compared with the previously used microporous MOF matrixes, this hierarchically porous MOF not only enables the impregnation of perovskite precursors in one step, but also allows more facile diffusion of the reactants, thus giving rise to high loading capacities and uniform distribution of CsPbX3 perovskites. The MOF matrix serves as both template to guide the confined growth of monodispersed CsPbX3 PNCs and encapsulation to enhance the stability of CsPbX3 PNCs. The obtained CsPbX3@MOF nanocomposites show bright luminescence and good resistance to ambient moisture, UV light irradiation, and anion-exchange. By integrating the green-emitting CsPbBr2I@MOF and red-emitting CsPbI3@MOF nanocomposites with a commercial blue InGaN LED chip, we fabricate a primary WLED showing a NTSC value of 125% and Rec. 2020 of 93%, which highlights the potential in backlight display.

Effects of Seed-Layer N2O Plasma Treatment on ZnO Nanorod Based Ultraviolet Photodetectors: Experimental Investigation with Two Different Device Structures.

The crystalline quality of ZnO NR (nanorod) as a sensing material for visible blind ultraviolet PDs (photodetectors) critically depends on the SL (seed layer) material of properties, which is a key to high-quality nanocrystallite growth, more so than the synthesis method. In this study, we fabricated two different device structures of a gateless AlGaN/GaN HEMT (high electron mobility transistor) and a photoconductive PD structure with an IDE (interdigitated electrode) pattern implemented on a PET (polyethylene terephthalate) flexible substrate, and investigated the impact on device performance through the SL N2O plasma treatment. In case of HEMT-based PD, the highest current on-off ratio (~7) and spectral responsivity R (~1.5 × 105 A/W) were obtained from the treatment for 6 min, whereas the IDE pattern-based PD showed the best performance (on-off ratio = ~44, R = ~69 A/W) from the treatment for 3 min and above, during which a significant etch damage on PET substrates was produced. This improvement in device performance was due to the enhancement in NR crystalline quality as revealed by our X-ray diffraction, photoluminescence, and microanalysis.

Influence of composition and growth conditions on TiN nanocrystallite formation in TiN-metal films

The nanostructure of Ti in TiN-Cu films grown by Pulsed Layer Deposition (PLD) is investigated using Extended X-Ray Absorption Fine Structure (EXAFS) spectroscopy at the Ti-K edge. The TiN-Cu films were grown on Si (100) crystal wafers using either Ti (99.95%) and Cu (99.99%) elemental targets or a TiCu intermetallic target (99.95%), under flowing N2 (99.999 %). The influence of varying either the Ti concentration or the N2 partial pressure (PN2) was explored in a series of TiN-Cu films, in an effort to distinguish how these parameters affect the degree of TiN nanocrystalline formation. The Ti-K-EXAFS analysis results proposes that the growth of stoichiometric TiN nanocrystallites is favoured in all studied films, however the extent of TiN formation is strongly modulated as a consequence of changing growth conditions and film composition. In particular, when growth proceeds from a TiCu intermetallic target under constant PN2, decreasing the Ti concentration promotes the formation of a (Ti, Cu) solid solution at the expense of the formation of TiN nanocrystallites. Thus, PLD growth using intermetallic targets yields TiN-Cu films with extended TiN nanocrystalline regions embedded in a rich-amorphous Cu phase, where the excess of Ti atoms are also incorporated. When Cu and Ti elemental targets are used (Ti:Cu = 50:50), Ti preferentially bonds to N2 upon increasing PN2. The extent of TiN nanocrystalline formation is enhanced, while bond formation between the unreacted Ti atoms and the amorphous Cu regions is also revealed. Consequently, it can be proposed that a proper control of growth conditions and film composition can permit growth of TiN-metal films with improved mechanical response.

Nanocrystalline silicon thin film growth and application for silicon heterojunction solar cells: a short review.

Doped nanocrystalline silicon (nc-Si:H) thin films offer improved carrier transport characteristics and reduced parasitic absorption compared to amorphous silicon (a-Si:H) films for silicon heterojunction (SHJ) solar cell application. In this article, we review the growth conditions of nc-Si:H thin films as the carrier-selective layers for SHJ solar cells. Surface and growth zone models are analysed at different stages of incubation, nucleation, and growth of the silicon nanocrystallites within the hydrogenated amorphous silicon matrix. The recent developments in the implementation of nc-Si:H films and oxygen-alloyed nc-SiOx:H films for SHJ cells are highlighted. Furthermore, hydrogen and carbon dioxide plasma treatments are emphasised as the critical process modification steps for augmenting the nc-Si:H films' optoelectronic properties to enhance the SHJ device performance with better carrier-selective interfaces.

Study of variable range hopping conduction mechanism in nanocrystalline carbon thin films deposited by modified anodic jet carbon arc technique: application to light-dependent resistors

In this study, we report the deposition of nanocrystalline carbon thin films by modified anodic jet carbon arc technique assisted with inert-helium and reactive-nitrogen gaseous environments. The modified anodic jet technique facilitates to generate a very high localized pressure near the arc spot for the growth of nanocrystallites of carbon at high arc temperature and high pressure. The deposited films were analyzed for the growth of nanocrystallites in amorphous carbon structure under high-resolution transmission electron microscopy (HRTEM). The HRTEM studies reveal the distribution of nanocrystallites in the amorphous carbon matrix. The temperature-dependent conduction behavior of the deposited films has also been analyzed under Mott’s variable range hopping conduction mechanism. Both sets of the film are found to follow three-dimensional variable range hopping conduction mechanisms for the transport of charge carriers. The deposited films are also analyzed for their applications to energy-saving light-dependent resistors under an illumination intensity of ~ 100 mW/cm2 of white light. The reasonably high value of detectivity values ~ 1.26 × 106 and 3.12 × 106 Jones and relatively lower values of trap depth 0.6826 and 0.6834 eV for the nanocrystalline films deposited in helium and nitrogen environments suggest trapped state-assisted significantly high power conversion efficiency. This supports the suitability of the deposited films for light-dependent resistor applications.

Anisotropic Cathodic Corrosion of Gold Electrodes in the Absence and Presence of Carbon Monoxide

In this work, we investigate gold surface dissolution under cathodic potentials as a function of crystal orientation using cyclic voltammetry, scanning electron microscopy, and atomic force microscopy to determine to what extent cathodic corrosion is anisotropic in nature. Additional experiments were performed to gain insights into the effect of the presence of CO on cathodic corrosion. Carbon monoxide experiments were conducted to investigate the effect of more realistic reaction conditions on cathodic corrosion as they might be found in gold-driven CO2 electrolyzers, although the effects of the presence of CO2 (reactant) and carbonate salts (typically present in the reaction medium) were excluded from this study. It was found that cathodic corrosion of gold is strongly anisotropic, with stepped surfaces and the {110} plane being most susceptible to dissolution, whereas the {111} and {100} planes were much more resilient to dissolution. On these more stable surfaces, nanocrystallite growth was observed instead, which we hypothesize originates from redeposition of the gold that dissolves from the more corrosive, more open, and/or defective facets. Finally, the presence of carbon monoxide was found to slightly enhance the rate of surface change, as evidenced by increases in charge in cyclic voltammograms after corrosion and an earlier onset of crystallite growth on the {111} and {100} planes, which we ascribe to enhanced gold mobility due to the strong bond formed between CO and gold in alkaline media.

Growth of GaN on monolayer hexagonal boron nitride by chemical vapor deposition for ultraviolet photodetectors

Chemical vapor deposition (CVD) technology is a simple and flexible method used to prepare high-quality crystalline materials. Traditional CVD technology, based on pre-deposited thin catalyst metal, usually produces nanostructures instead of continuous films. In this work, a continuous GaN film on a monolayer boron nitride (h-BN) insertion layer is demonstrated using CVD technology. The experimental results and theoretical calculations indicate that abundant GaN nanocrystallites are firstly formed at the edges or grain boundaries of the monolayer h-BN by quasi-van der Waals epitaxy. Then, the vapor-solid mechanism will control further growth of the GaN nanocrystallites, causing them to merge into a continuous GaN film. Meanwhile, the CVD-grown GaN ultraviolet detector exhibits a relatively high responsivity with a value of 0.57 A W−1 at 2 V. In this paper, a simple low-cost CVD method is proposed for preparing continuous films on two-dimensional materials for electronic and optoelectronic devices.

On the adsorption kinetics and mechanism of enhanced photocatalytic activity of Fe3 O4 -SiO2 -TiO2 core-multishell nanoparticles against E. coli.

In the present study, a Fe3 O4 -TiO2 (FT) core-shell and a core-multishell structure of Fe3 O4 -SiO2 -TiO2 (FST) were synthesized, and their bactericidal capability was investigated on Escherichia coli (E. coli). Scanning electron microscopy (SEM), ultraviolet-visible spectroscopy (UV-vis), X-ray diffraction, Brunauer-Emmett-Teller, zeta potential, and fluorimetry were carried out to characterize properties of synthesized nanoparticles. An efficiency of 98% adsorption and harsh bacterial damage was observed when E. coli was put in contact with FST. Weaker adsorption of bacteria in contact with FT demonstrated that heterojunction has destructive effects on nanostructure. Further investigation proved that more OH were produced on the surface of FST, which is a sign of its longer lifetime. Moreover, results revealed that the presence of SiO2 in the structure caused enhanced coverage, surface area, and porosity in TiO2 outer layer, all of which have positive effects on adsorption. However, UV-vis showed smaller band gap for FT. It suggests that although photoactivity of FST is less influenced by light absorption, it possesses more e/h lifetime for generation of reactive oxygen species. Results point to the importance of SiO2 as an obstacle of heterojunction on both adsorption and photoactivity. It was also proposed that increasing band gap in FST can be attributed to the porosity of SiO2 that causes suppression of TiO2 nanocrystallite growth.

Liquid-mediated crystallization of amorphous GeSn under electron beam irradiation

Crystallization processes of amorphous germanium–tin (GeSn) under low-energy electron-beam irradiation were examined using transmission electron microscopy (TEM). Freestanding amorphous GeSn thin films were irradiated with a 100 keV electron beam at room temperature. The amorphous GeSn was athermally crystallized by electron-beam irradiation, when the electron flux exceeded the critical value. Heterogeneous structures consisting of nano- and micro-crystallites were formed after crystallization of amorphous GeSn with ∼24 at. % Sn in the as-sputtered amorphous state. In situ TEM observations of structural changes under electron-beam irradiation revealed that random nucleation and growth of nanocrystallites occur at the early stage of crystallization, followed by rapid formation of micro-grains surrounding the nanocrystals. It has been suggested that the growth of micro-grains progresses via supercooled liquid Sn at the amorphous/crystalline interface. The resultant GeSn grains with a size of a few micrometers contained ∼15 at. % Sn, much larger than the solubility limit of Sn in Ge (∼1 at. % Sn).

Nucleation and growth of nano-crystallites in a new multicomponent Fe-based BMG during the partial crystallization process

In this study, the non-isothermal kinetic of nucleation and growth of nano-crystallites in [(Fe0.9Ni0.1)77Mo5P9C7.5B1.5]100-0.1Cu0.1 (at %) BMG was systematically investigated. The crystallization process in the BMG occurred in four stages. The KAS, Augis–Bennett, and Gao–Wang methods were used to determine activation energies of all four crystallization stages. Also, to investigate the mechanism of nucleation and growth phenomena, which occurred during the crystallization process, other kinetic parameters including n, Kp, and m were determined using Augis–Bennett, Gao–Wang, and Matusita methods. Also, the JMAK method, as an accurate isokinetic method, was used to confirm the results obtained by other methods. The research findings revealed that the growth mechanisms of all crystallization stages were controlled by the interface. Moreover, the values of n and b parameters were increased in the fourth crystallization stage; indicating an increase in nucleation rate in this stage.

Dynamics at the crystal-melt interface in a supercooled chalcogenide liquid near the glass transition

Direct quantitative measurements of nanoscale dynamical processes associated with structural relaxation and crystallization near the glass transition are a major experimental challenge. These type of processes have been primarily treated as macroscopic phenomena within the framework of phenomenological models and bulk experiments. Here, we report x-ray photon correlation spectroscopy measurements of dynamics at the crystal-melt interface during the radiation induced formation of Se nano-crystallites in pure Se and in binary AsSe4 glass-forming liquids near their glass transition temperature. We observe a heterogeneous dynamical behaviour where the intensity correlation functions g2(q, t) exhibits either a compressed or a stretched exponential decay, depending on the size of the Se nano-crystallites. The corresponding relaxation timescale for the AsSe4 liquid increases as the temperature is raised, which can be attributed to changes in the chemical composition of the melt at the crystal-melt interface with the growth of the Se nano-crystallites.

Influence of the Preparation Method on Amorphous-Crystalline Transition in Fe84B16 Alloy

The amorphous-crystalline transition in Fe84B16 alloys prepared by melt spinning and high-energy ball milling was studied. Time-resolved X-ray diffraction showed that the kinetics of transition into a crystalline state depends on the method of preparing a metastable alloy. In amorphous Fe84B16 alloy prepared by melt spinning, crystallization proceeded within a time period of below 1 s and was accompanied by the formation of eutectic α-Fe–Fe3B. At temperatures above 600°C, metastable phase Fe3B was found to transform into Fe2B and α-Fe. In the high-energy ball milling produced alloy, structural changes were accomplished in 4–8 s and the transition into a state with a perfect crystalline structure was caused by the growth of nanocrystallites formed during processing.

Ultrasmooth nanocrystalline carbon film induced by low concentration doping: Carbide disorienting graphene nanocrystallite

The fabrication of nanocrystalline films is usually accompanied by undesired surface roughening, which limits their applications in tribology, optics and electronics. Here, we report a technique to reduce the average roughness of graphene nanocrystalline carbon film from 18.3 nm to 0.66 nm by ∼5 at.% Ti-doping. The surface of the Ti-doped graphene nanocrystalline carbon film stay ultrasmooth from few nanometer thick up to 1 μm thick. Based on the nanostructure characterization, the ultrasmooth mechanism is interpreted that Ti-doping leads to the formation of Ti–C bond, and changes the growth of graphene nanocrystallite from preferred vertical-aligned orientation to random orientation. The ultrasmooth mechanism is general, as shown by similar effects with low concentration doping of Si, Al, Cr, Zr and W. This approach paves a way for fabricating ultrasmooth nanocrystalline carbon films without a requirement of ion-impact-induced downhill current, and significantly improving the hardness of the graphene nanocrystalline carbon film.