Low temperature synthesis of colloidal CdSe quantum dots

In this study, the CdSe nanocrystals were prepared in phenyl ether and octyl amine to investigate the variations of their size, bandgap energy, and photoluminescence with growth time and temperature. The sizes of the CdSe nanocrystals were measured using High Resolution Transmission Electron Microscopy (HRTEM), and found to be nearly monodisperse for relatively low growth temperature, 130 degrees C. Their optic properties were characterized by photoluminescence measurements, which showed that the colors of the nanocrystals could be controlled. The bandgap energies of the nanocrystals were calculated theoretically and found to be in accord with quantum confinement theory. This synthetic method requires only a cheap solvent and offers good reproducibility at a lower price.     

Synthesis of CdSe nanocrystals in a noncoordinating solvent: effect of reaction temperature on size and optical properties

Colloidal synthesis of high quality CdSe nanocrystals with controllable size and tunable properties have been one of the most important topics of research over the last decade, in view of its huge technological potentials. CdSe is one of the most studied nanocrystals of this category because of its photoluminescence tunability across the visible spectrum. We have synthesized CdSe nanocrystals using CdO precursor in a noncoordinating solvent and studied the effect of the reaction temperature on the size and optical properties of the nanocrystals. The size of the nanocrystals could be varied systematically in the range of 3.5 to 6.6 nm diameter with a remarkably narrow size distribution by controlling only the reaction temperature, without any need for a post-synthesis processing. The band gap and the corresponding band edge emission could be tuned across the entire visible range by tuning the size of the nanocrystals. The narrow width of the photoluminescence emissions of different colours (blue to red) make these nanocrystals a potential candidate for different optical and optoelectronic devices.     

Preparation and characterization of LaNiO3 nanocrystals

Ten to twenty-three nanometers of LaNiO₃ nanocrystals were prepared by glycine combustion method using nickel nitrate and lanthanum nitrate as raw materials, glycine as gelating agent and fuel. The preparation process was monitored by thermogravimetric analysis and differential thermal analysis (TG-DTA); the final products were characterized by X-ray diffraction (XRD), electron diffraction (ED), high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). The effect of thermal treatment temperature on crystal size of the nanocrystals was studied and the activation energy of LaNiO₃ nanocrystals formation during calcinations was calculated to be 41.9 kJ/mol. Moreover, the interplanar distances of d₁₁₀ and d₁₀₁ measured from the TEM images were 0.272 and 0.363 nm, respectively, coinciding with the theoretical values.     

Colloidal CdSe nanocrystals from tri-n-octylphosphine: Part II: control of growth rate for high quality and large-scale production by tuning Cd-to-Se stoichiometry

Colloidal CdSe nanocrystals were synthesized in reaction media consisting of tri-n-octylphosphine (TOP) without addition of other species; the single-step approach used cadmium oxide (CdO) and TOPSe as Cd and Se sources, respectively. The temporal evolution of the optical properties of the growing TOP-capped CdSe nanocrystals was monitored for a couple of hours, showing that there are two distinguishable stages of growth: an early stage (less than 5 minutes) and a later stage; the growth kinetics of the two stages is a function of the Cd-to-Se precursor molar ratios. A rational choice of 2-6Cd-to-1Se molar ratio was found, based on the temporal evolution of the photoluminescent (PL) efficiency (studied as PL intensity and sensitivity to the media of dispersion, and non-resonant Stokes shifts). For a 2Cd-to-1Se synthesis, the growth in size was slow in the early stages and became fast in the later stages; this fast-later-stage feature could be suppressed by going to a synthesis with a 4-6Cd-to-1Se mole ratio: the nanocrystals between 0.5-60 min growth time exhibit very much similar optical properties, with less than 19 nm redshift of bandgap absorption and emission occurring. Thus, the synthetic route developed here, with a rational 4-6Cd-to-1Se molar ratio, enables us to produce high-quality CdSe nanocrystals on a large-scale with a high degree of synthetic reproducibility. The insights gained facilitate a deeper understanding of the concept of what constitutes high-quality nano-crystals: high PL efficiency resulting from a low growth rate, which can be thoroughly and readily investigated by the red-shift rate of the band-gap peak positions; in addition, the insights gained help us to define a proper synthetic approach for large-scale production with high-quality product.     

Mineralization of semiconductor nanocrystals on peptide-coated bionanotubes and their pH-dependent morphology changes

While various mineralizing peptides have been applied to grow metal nanoparticles on bionanotube templates, the semiconductor nanoparticle growth on nanotubes has not extensively been explored yet. In this paper, various semiconductor nanocrystals were grown on the bionanotubes surfaces with controlled sizes. When three synthetic peptides, which recognize and selectively bind Ge, Ti, and Cu ions, respectively, were incorporated on template bionanotube surfaces, highly crystalline and monodisperse Ge, TiO2, and Cu2S nanocrystals were grown on the tube surfaces. The sizes of these nanocrystals could be tuned as a function of pH, and larger semiconductor nanocrystals were grown as the pH of growth solutions was increased. All of these nanocrystals from smaller sizes to larger sizes had the same crystallinity. This peptide-controlled nanocrystal growth technique will be very useful to prepare semiconductor nanowires as building blocks for future microelectronics, whose band gaps can be tuned by the sizes of coated semiconductor nanoparticles via their quantum confinement effect. The novelty of this approach in the electronic device fabrication is that the semiconductor nanocrystal size control can be achieved by controlling peptide configurations via pH change, and this control may tune electronic structures and band gaps of the resulting semiconductor nanowires.     

The influence of parameters of chemical synthesis on the optical properties of CdS nanocrystals

A systematic analysis is presented on the influence of parameters of synthesis on the formation and optical properties of CdS nanocrystals prepared using dimethyl formamide as solvent and stabilizing agent. The parameters considered are the molar ratios of cadmium to sulfur, initial concentration of Cd²⁺ and S²⁻ at a fixed molar ratio and atmospheric conditions. Optical absorption and photoluminescence measurements were carried out during the synthesis to monitor the changes in optical properties. The results clearly indicate that the nanocrystals size could be tuned by varying the molar ratio of the starting materials. Ambient conditions are found to play a vital role on the stability and optical properties of the nanocrystals whereas the concentration of the starting materials had little influence on the optical absorption and luminescence properties. These results are expected to help in developing a better understanding of the mechanisms of solution growth and size stabilization of semiconductor nanocrystals during chemical synthesis.     

Catalytic Activity of the Spinel Ferrite Nanocrystals on the Growth of Carbon Nanotubes

We prepared three ferrite nanocatalysts: (i) copper ferrite (CuFe₂O₄) (ii) ferrite where cobalt was substituted by nickel (Ni _x Co_1?x Fe₂O₄, with x=0, 0.2, 0.4, 0.6), and (iii) ferrite where nickel was substituted by zinc (Zn _y Ni_1?y Fe₂O₄ with y=1, 0.7, 0.5, 0.3), by the sol-gel method. The X-ray diffraction patterns show that the ferrite samples have been crystallized in the cubic spinel structural phase. We obtained the size of grains by field emission scanning electron microscopy images and their magnetic properties by vibrating sample magnetometer. Next, carbon nanotubes were grown on these nanocatalysts by the catalytic chemical vapor deposition method. We show that the catalytic activity of these nanocrystals on the carbon nanotube growth depend on cation distributions in the octahedral and tetrahedral sites, structural isotropy, and catalytic activity due to cations. Our study may have applications in finding a suitable candidate of doped ferrite nanocrystals as catalysts for carbon nanotube growth. More interestingly, the yield of fabrication of carbon nanotubes can be considered as an indirect tool to study catalytic activity of ferrites.     

Highly graphitized carbon nanosheets with embedded Ni nanocrystals as anode for Li-ion batteries

Nano Research - A C/Ni composite was prepared via thermal decomposition of a nickel oleate complex at 700 °C, yielding disperse Ni nanocrystals with an average size of 20 nm, encapsulated by...     

碳包Ni纳米晶的形成及其特性

用直流碳弧法制备出碳包Ni金属纳米晶.在石墨阳极复合棒中掺入不同组份的Ni，研究其对碳包Ni纳米晶，富勒烯等碳微团特性的影响结果表明：碳包Ni纳米晶含Ni和石墨，不含碳化物和氧化物；纳米品数量随Ni增加而增加．而纳米晶的粒径出现极值；Ni催化碳微团石墨化并使C60／70产率下降；碳包Ni金属纳米晶单位质量的饱和磁化强度随Ni的增加而增加     

Formation mechanisms of Ni2P nanocrystals using XANES and EXAFS spectroscopy

Ni₂P nanocrystals have been synthesized by a ligand stabilization method with adopting nickel acetylacetonate (Ni(acac)₂) as a nickel precursor and trioctylphosphine (TOP) as a phosphorous precursor in the presence of trioctylphosphine oxide (TOPO) as a coordinating solvent. In order to better understand the formation of Ni₂P nanocrystals, various synthetic routes were employed with varying the mixing order of TOP or TOPO with Ni(acac)₂ in the complexation and nucleation steps, and the phosphidation temperatures of Ni-TOP solution in TOPO. In all cases the formation of Ni₂P nanocrystals having an average size in range of 5-11 nm was achieved, but with slightly larger particle distribution of ca. 11 nm being observed for the case where the TOP was injected after a nucleation of Ni-TOPO was formed. In particular, XAFS techniques of EXAXS and XANES which are effective to characterize either amorphous or crystalline materials were applied to examine the structural and electrical properties of the precursor, intermediate and final materials. This led to elucidate the role and the effect of TOP and TOPO on the formation of Ni₂P nanocrystals. It is concluded that the formation of Ni₂P nanocrystals proceeds through three consecutive steps of complexation, nucleation and phosphidation. In phosphidation, TOP works as a reducing agent on the Ni precursor as well as a phosphorous source through the cleavage of P-C bonds.     

Selective Formation of Size-Controlled Silicon Nanocrystals by Photosynthesis in SiO Nanoparticle Thin Film

The SiO_x thin film with a thickness of about 1 mum was formed on a GaAs substrate by bar-coating with the organic solution of the SiO_x nanoparticles (~40 nm). The as-formed SiO_x thin film consists of the SiO_x nanoparticles; thus the thin film is macroscopically discontinuous and is referred to as a nanoparticle thin film. Although there were no silicon (Si) nanocrystals in the as-formed SiO_x nanoparticle thin film, Si nanocrystals were observed by Raman scattering measurement after the thin film was exposed to the laser beam. The growth of Si nanocrystals by laser irradiation is referred to as photosynthesis. The photosynthesis of Si nanocrystals is found to be a self-limiting process. After the average size reaches a certain value, further increase of irradiation time or laser power does not increase the average size. The photosynthesis is similar to the thermal synthesis of Si nanocrystals from SiO_x but much faster and low-temperature growth of Si nanocrystals from SiO _x. Furthermore, the laser irradiation makes nanoparticles larger by merging. This suggests a possibility of low-temperature formation of a Si-nanocrystal array embedded in a SiO₂ thin film. Such a structure has many potential device applications     

Formation of cubic zinc sulfide nanocrystals in paraffin liquid

A green and cheap route to the production of cubic zinc sulfide nanocrystals is presented. Nanocrystals were synthesized in sole solvent paraffin liquid with zinc oxide, elemental sulfur and oleic acid. Their photophysical properties were investigated by UV-vis absorption and photoluminescence spectra. The temporal evolution of UV-vis spectra showed that the reactions could be terminated after about 5 min growth with a main absorption peak at 267 nm. The typical photoluminescence peaks (excited by 260 nm) were observed at about 285 nm and 397 nm. Transmission electron microscopy and selected area electron diffraction in combination with X-ray powder diffraction indicated the presence of cubic ZnS nanocrystals in good crystallization with the average size of 5.0 nm in diameter.     

Formation of ge nanocrystals in SiO2 by electron beam evaporation

Ge nanocrystals were formed by electron beam evaporation on SiO2 covered Si substrates. The size and distribution of the nanocrystals were studied by atomic force microscopy, scanning electron microscopy and cross-sectional transmission electron microscopy. Dependencies of the nanocrystal size, of the nanocrystal surface coverage, and sheet resistance obtained by van der Pauw method of the Ge layer have been found on the evaporation time. The suggested growth mechanism for the formation of nanocrystals is the Volmer-Weber type. The sheet resistance exhibited a power dependence on the nanocrystal size.     

Grain Size Effects on the Mechanical Behavior of Open‐cell Nickel Foams

The dependence of the mechanical behavior of nickel foams upon their grain size was studied. First, the grain coarsening phenomenon which occurs during the processing of foams was analyzed. A metallurgical characterization of the grain growth during heat treatment was performed. The grain size effects on the mechanical properties was then studied, namely, via the Hall‐Petch law. The foam walls being very thin, roughly 10 μm in thickness, grain growth and mechanical behavior might be different compared with conventional materials. The present results obtained with foams were compared with literature data on bulk pure nickel and with nickel foils of 10 and 50 μm in thickness which are good candidates for the modeling of the cell walls. The EBSD technique allowed observing the absence of preferred crystallographic orientations for both foams and foils. A mechanical model in the spirit of that by Gibson and Ashby was finally presented incorporating the grain size effect on yield strength and hardening modulus. This model provided a good estimation of the experimental data.     

Two-phase solvothermal synthesis of rare-earth doped NaYF4 upconversion fluorescent nanocrystals

The synthesis, characterization, and fluorescent spectra of rare-earth doped NaYF₄ upconversion nanocrystals are introduced in this paper. The nanocrystals were synthesized in the water-ethanol-oleic acid system via a two-phase solvothermal approach, by using rare-earth stearate as the precursor. The as-prepared nanocrystals were of hexagonal phase, strong UC fluorescent intensity, with an average size of about 25 nm, which have been characterized by TEM, SAED, powder XRD, and luminescence spectroscopy. The possible mechanism of this synthesis that the nucleation and growth of nanocrystals occurred at the solid-liquid interface was also discussed. The nanocrystals are hydrophilic, and expected to fulfil the demand for biological applications via further modification.     

Preparation and characterization of NiO nanoparticles through calcination of malate gel

NiO nanoparticles have been successfully prepared by calcining malate gel, using basic nickel carbonate (BNC) and malic acid as the raw materials and H₂O as the solvent. The reaction was studied by TG–DTA, XRD and FT-IR. The particle size and morphology of NiO nanoparticles was characterized by TEM. The results show that nickel malate could be formed by the reaction of BNC and malic acid; the gel was composed of nickel malate and residual malic acid; the precursor can translate into NiO nanoparticles completely at 400 °C under the air, and the as-prepared sample has a cubic crystal structure with about 14 nm of average diameter by the Scherrer formula. The NiO calcined at 400 °C for 1 h was of narrow particle size distribution, weak agglomeration and small particle size. The particle size of NiO increased with the increase in temperature. Compared with other methods, the developed method is simple and the raw materials were inexpensive, so that it has the potential for further scale-up application in the industry. Furthermore, the reason for the metallic nickel formation in the course of the preparation was explained.     

Structure Transition of Nanocrystalline Fe_2O_3

A structure transition of Fe2O3 nanocrystal was studied by using DTA and TG thermal analysis and X-ray diffraction method. We found that size increase of the nanocrystals is larger after the structure transition than that before the transition. It means that the structure transition is beneficial on growth of nanocrystals     

Monodispersed quinacridone nanocrystals prepared by a high-temperature and high-pressure liquid crystallization method

Monodispersed quinacridone nanocrystals were fabricated by a high-temperature and high-pressure liquid crystallization method, which proved to be an advanced technique for fabricating nanocrystals of pigment compound. The aqueous dispersion liquid of quinacridone nanocrystals was very stable. The nanocrystats had a spherical shape with an average size of 60 nm when water was used as the high-temperature and high-pressure liquid at 260 degrees C and cooling solvent. The crystal structure of the nanocrystals could be controlled by varying the experimental conditions.     

Characterization of CdTe nanocrystals during their synthesis in liquid paraffin: optical properties and particle growth

This article presents a systematic investigation of the optical properties and the growth process of CdTe nanocrystals during their hot-injection-based synthesis in liquid paraffin. Cadmium(II) stearate and tributylphosphine telluride are used as precursors. The as-obtained nanocrystals are characterized by transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS), X-ray powder diffraction (XRD), UV–vis absorbance, and fluorescence spectroscopy. The changes in optical absorbance and fluorescence during the nanocrystal synthesis are studied. The average nanocrystal size and the mean nanocrystal concentration are derived from the optical spectra and their changes during the synthesis are investigated. It is found that synthesis at lower temperature (150 °C) favors the continuous nucleation and leads to the formation of relatively smaller nanocrystals (~3 nm in size), whereas the nanocrystal concentration is relatively constant during synthesis at higher temperature (250 °C) thus leading to the formation of larger nanocrystals (~5 nm in size).     

直接沉淀法制备纳米ZnO

以ZnCl2 为原料 ,直接沉淀法制备ZnO纳米粒子 ;研究了制备过程中Zn2 + 浓度、焙烧温度等条件对ZnO纳米晶体粒径的影响 ,并对其机理进行了分析。实验结果表明 ,较小的反应浓度可以获得较小的晶体粒径 ;在其它反应条件相同的情况下 ,制备的纳米ZnO粒子 ,其晶粒尺寸随着焙烧温度的增加 ,晶粒逐渐增大 ,与体相ZnO粒子相比 ,纳米ZnO粒子在紫外区光吸收能力显著增强 ,为ZnO的应用开辟了更为广阔的前景。