Cu2ZnSnS4纳米晶的制备及其三阶非线性光学性能的研究

以乙酰丙酮铜、醋酸锌、二氯亚锡、油胺和硫粉为前驱体,采用one-pot法合成出了单分散的Cu2ZnSnS4(CZTS)纳米晶.所得样品采用X射线粉末衍射仪(XRD),能量色散谱仪(EDS),透射电子显微镜(TEM),高分辨透射电子显微镜(HRTEM),光电子能谱仪(XPS),紫外-可见光谱仪(UV-vis)和Z-扫描(Z-scan)技术对其结构组成、形貌、性能等进行了表征.结果表明:所获得的产物为四方相结构的六边形CZTS纳米颗粒,直径约为10 nm.计算出尺寸大小为10 nm,13 nm的纳米晶的三阶非线性光学折射率γ(-1.08×10-15,-9.08×10-17 m2·W-1),三阶非线性光学吸收系数β(6.5×10-9,3.69×10-11 m·W-1)以及三阶非线性光学极化率χ(3)(1.49×10-9,4.35×10-10 esu).并探讨了CZTS纳米晶可能的形成机理,及引起三阶光学非线性发生变化的原因。     

溶剂热法制备球状Cu2ZnSnS4纳米晶及其表征

通过简单的溶剂热法合成了锌黄锡矿结构的Cu2ZnSnS4(CZTS)纳米晶,使用L-半胱氨酸作硫源和络合剂,以金属氯化物作前驱体,在180°C下反应16h成功获得了CZTS微球.使用X射线衍射(XRD)仪,场发射扫描电子显微镜(FESEM)、能量色散谱(EDS)、高分辨透射电子显微镜(HRTEM)、多功能X射线光电子能谱仪(XPS)、紫外-可见(UV-Vis)分光光度计对产物的物相、结构、形貌及光学性能进行表征.结果表明:所得的产物为纯相锌黄锡矿结构的CZTS纳米颗粒,CZTS微球直径为400-800nm,并可观察到微球是由大量厚度约20nm的纳米片构成;将CZTS颗粒均匀分散在异丙醇中,测试后估算其禁带宽度约1.58eV,与薄膜太阳能电池所需的最佳禁带宽度相近.并对其形成机理进行了初步探讨.     

Compositionally tunable Cu2ZnSn(S(1-x)Se(x))4 nanocrystals: probing the effect of Se-inclusion in mixed chalcogenide thin films

Nanocrystals of multicomponent chalcogenides, such as Cu(2)ZnSnS(4) (CZTS), are potential building blocks for low-cost thin-film photovoltaics (PVs). CZTS PV devices with modest efficiencies have been realized through postdeposition annealing at high temperatures in Se vapor. However, little is known about the precise role of Se in the CZTS system. We report the direct solution-phase synthesis and characterization of Cu(2)ZnSn(S(1-x)Se(x))(4) nanocrystals (0 ≤ x ≤ 1) with the aim of probing the role of Se incorporation into CZTS. Our results indicate that increasing the amount of Se increases the lattice parameters, slightly decreases the band gap, and most importantly increases the electrical conductivity of the nanocrystals without a need for annealing.     

Cu2ZnSnS4纳米晶微球的制备及其表征

以乙二醇为溶剂,采用溶剂热法合成出由纳米晶组装而成的Cu2ZnSnS4(CZTS)微球。采用X射线衍射仪(XRD)、拉曼光谱仪(Raman)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、紫外-可见漫反射光谱(DRS)对所得微球的结构与成分、颗粒大小与形貌和光学性质进行了测试分析。研究结果表明:溶剂热法制得的CZTS粉体具有四方晶相结构,微球由纳米晶组装而成,对可见光有良好的吸收;随着反应时间的增加,颗粒尺寸逐渐增大,且对形貌有一定影响;此外,文中还对CZTS微球的形成机理做了推测。     

Quantum confinement in silver selenide semiconductor nanocrystals

We prepare Ag(2)Se nanocrystals with average diameters between 2.7 and 10.4 nm that exhibit narrow optical absorption features in the near to mid infrared. We demonstrate that these features are broadly tunable due to quantum confinement. They provide the longest wavelength absorption peaks (6.5 μm) yet reported for colloidal nanocrystals.     

Ensemble Effects in the Temperature-Dependent Photoluminescence of Silicon Nanocrystals

In this work the temperature-dependent photoluminescence of alkyl-capped silicon nanocrystals with mean diameters of between 3 and 9 nm has been investigated. The nanocrystals were characterized extensively by FTIR, TEM, powder XRD, and X-ray photoelectron spectroscopy prior to low-temperature and time-resolved photoluminescence spectroscopy experiments. The photoluminescence (PL) properties were evaluated in the temperature range of 41–300 K. We found that the well-known temperature-dependent blueshift of the PL maximum decreases with increasing nanocrystal diameter and eventually becomes a redshift for nanocrystal diameters larger than 6 nm. This implies that the observed shifts cannot be explained solely by band-gap widening, as is commonly assumed. We propose that the luminescence of drop-cast silicon nanocrystals is affected by particle ensemble effects, which can explain the otherwise surprising temperature dependence of the luminescence peak.     

花状铜锌锡硫纳米颗粒的溶剂热法制备和结构表征

以金属氯化物为金属源,硫脲为硫源,聚乙二醇和乙二醇为混合溶剂,采用溶剂热法一步合成了花状的铜锌锡硫纳米颗粒.利用X射线衍射仪,扫描电子显微镜、能谱仪、透射电子显微镜、紫外-可见分光光度计分析了铜锌锡硫纳米颗粒的物相、结构、形貌及光学性能,并初步探讨了铜锌锡硫的生长机理.结果表明,所得到的铜锌锡硫纳米颗粒具有锌黄锡矿结构,直径在500～2 000nm范围内可调,其中花状的铜锌锡硫纳米颗粒由大量厚度约25nm的纳米片构成.所制备的铜锌锡硫纳米颗粒对可见光具有明显的吸收;利用外延法推算得到其禁带宽度约为1.5eV,与太阳能电池所需的最佳禁带宽度相近,显示其有望在新一代太阳能电池中得到应用和推广.     

Size- and shape-dependent transformation of nanosized titanate into analogous anatase titania nanostructures

A size- and shape-dependent morphological transformation was demonstrated during the hydrothermal soft chemical transformation, in neutral solution, of titanate nanostructures into their anatase titania counterparts. Specifically, lepidocrocite hydrogen titanate nanotubes with diameters of approximately 10 nm were transformed into anatase nanoparticles with an average size of 12 nm. Lepidocrocite hydrogen titanate nanowires with relatively small diameters (average diameter range of < or = 200 nm) were converted into single-crystalline anatase nanowires with relatively smooth surfaces. Larger diameter (>200 nm) titanate wires were transformed into analogous anatase submicron wire motifs, resembling clusters of adjoining anatase nanocrystals with perfectly parallel, oriented fringes. Our results indicate that as-synthesized TiO2 nanostructures possessed higher photocatalytic activity than the commercial titania precursors from whence they were derived.     

Nucleation and growth of CdSe nanocrystals in a binary ligand system

The competing effects of two ligands, oleic acid (OA) and bis-(2,2,4-trimethylpentyl) phosphinic acid (TMPPA), on the nucleation rate and growth of CdSe nanocrystals in octadecene are reported. It is found that TMPPA acts as a high boiling point "nonsolvent" or "nucleating agent". Addition of TMPPA leads to higher initial particle yields and smaller particle diameters. Conversely, oleic acid inhibits nucleation and results in a drastic increase in "early time ripening" (ETR), a phenomenon that causes a rapid reduction in the number of particles within the first minutes of reaction. By controlling the number of nuclei formed with TMPPA and tuning the rate of ETR with oleic acid, high yields of particles can be obtained with sizes between 3 and 7 nm. Furthermore, in the absence of OA, the preparation of very small nanocrystals with diameters approximately 2 nm is facilitated.     

Sub-40 nm Zeolite Suspensions via Disassembly of Three-Dimensionally Ordered Mesoporous-Imprinted Silicalite-1

Zeolite nanocrystals were prepared from three-dimensionally ordered mesoporous-imprinted (3DOm-i) silicalite-1 by a fragmentation method involving sonication and dissolution within a certain pH range. 3DOm-i silicalite-1 with spherical elements with diameters ranging from 10 to 40 nm and a wide range of crystal sizes (100-200 nm, 500-600 nm, and 1-2 μm) was used as the starting material. The highest yield (57%) of isolated nanocrystals was obtained for 3DOm-i silicalite-1 with a crystal size of 100-200 nm and a spherical element diameter of 40 nm. The smallest nanocrystals obtained, albeit in very low yields, had a 10 nm diameter. Preparation of stable silicalite-1 nanocrystal suspensions fragmented from 20 and 40 nm 3DOm-i silicalite-1 was demonstrated. Cryogenic transmission electron microscopy showed that the isolated zeolite nanocrystals can be used as seeds for the epitaxial growth of silicalite-1. An application of these findings was demonstrated: silicalite-1 nanocrystal suspensions were used to deposit seed layers on porous α-alumina disks, which were converted to continuous thin (300-400 nm) films by secondary growth that exhibited both high permeances and separation factors (3.5 × 10(-7) mol m(-2) s(-1) Pa(-1) and 94-120, respectively, at 150 °C) for p- and o-xylene.     

Synthesis of Ag and AgI quantum dots in AOT-stabilized water-in-CO2 microemulsions

Silver and silver iodide nanocrystals have been synthesized in the water-in-CO(2) reverse microemulsions formed by the commonly used surfactant, sodium bis(2-ethylhexyl)sulfosuccinate (AOT), in the presence of 2,2,3,3,4,4,5,5-octafluoro-1-pentanol as cosurfactant. The nanometer-sized aqueous domains in the microemulsion cores not only act as nanoreactors, but the surfactant interfacial monolayer also helps the stabilization of the metal and semiconductor nanoparticles. The transmission electron microscopy results show that silver and silver iodide nanocrystals with average diameters of 6.0 nm (standard deviation, SD=1.3 nm) and 5.7 nm (SD=1.4 nm), respectively, were formed. The results indicate that the method can be utilized as a general and economically viable approach for the synthesis of metal and semiconductor quantum dots in environmentally benign supercritical carbon dioxide.     

Continuous production of Cu2ZnSnS4 nanocrystals in a flow reactor

A procedure for the continuous production of Cu(2)ZnSnS(4) (CZTS) nanoparticles with controlled composition is presented. CZTS nanoparticles were prepared through the reaction of the metals' amino complexes with elemental sulfur in a continuous-flow reactor at moderate temperatures (300-330 °C). High-resolution transmission electron microscopy and X-ray diffraction analysis showed the nanocrystals to have a crystallographic structure compatible with that of the kesterite. Chemical characterization of the materials showed the presence of the four elements in each individual nanocrystal. Composition control was achieved by adjusting the solution flow rate through the reactor and the proper choice of the nominal precursor concentration within the flowing solution. Single-particle analysis revealed a composition distribution within each sample, which was optimized at the highest synthesis temperatures used.     

Synthesis of tungsten carbide nanocrystals and their electrochemical properties

Tungsten carbide (WC) nanocrystals have been prepared by a solvothermal method with Mg as the reductant and WO₃ and anhydrous ethanol as the precursors. The effects of time and temperature on the synthesis of WC were investigated and a probable formation mechanism was discussed. The obtained WC nanocrystals were characterized by X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy and electrochemical methods. Hexagonal closepacked WC was successfully synthesized when the temperature was as low as 500°C. The content of carbon was more than that of W, indicating that the composition of the treated sample was C and WC only. The diameters of WC nanocrystals were ranged from 40 nm to 70 nm and the nanocrystals were dispersed on carbon films. The electrochemical measurements reveal that WC nanocrystals obviously promote Pt/C electrocatalytic ability for the oxygen reduction reaction. __________ Translated from Chinese Journal of Catalysis, 2008, 29(7) (in Chinese)     

Optical gap and excitation energies of small Ge nanocrystals

Using the density functional theory (DFT) with the hybrid nonlocal exchange correlation functional of Becke and Lee, Yang and Parr (B3LYP), we have calculated the optical gap and the oscillator strengths for several of the lowest, spin and symmetry allowed, electronic transitions of small Ge nanocrystals passivated by hydrogen. The largest nanoparticle has an approximate diameter of 2 nm. Our results show that the optical gap exhibits size dependence (due to quantum confinement) roughly similar to silicon nanoparticles. However, for this range of diameters, there is an indirect-to-direct transition in the spectra of Ge as the size of the nanocrystals decrease. The first allowed excitation (fundamental optical gap) of each germanium nanoparticle has relatively larger oscillator strengths compared to silicon. The diameter of the smallest Ge nanocrystal capable to emit in the visible region of the spectrum, is approximately 1.9 nm, compared to 2.2 nm for silicon nanocrystals.     

Understanding of the finite size effects on lattice vibrations and electronic transitions of nano alpha-Fe2O3

Alpha-Fe(2)O(3) nanocrystals with controlled diameters ranging from 10 to 63 nm were successfully prepared. The finite size effects in alpha-Fe(2)O(3) nanocrystals were probed by X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, UV-visible spectrum, and magnetization measurements. With a size reduction, alpha-Fe(2)O(3) nanocrystals showed a lattice expansion and an enlarged axial ratio of c/a that is in apparent contradiction to the previous conjecture of high lattice symmetry for alpha-Fe(2)O(3) nanocrystals at small sizes. The surface terminations of alpha-Fe(2)O(3) nanocrystals were found to be highly hydrated with a size dependence that surprisingly follows the surface hydration chemistry of anatase TiO2 nanocrystals reported recently by us. The lattice vibrations, electronic transitions, and magnetic properties of alpha-Fe(2)O(3) nanocrystals were significantly modified by surface hydration and lattice expansion. The finite size effects that occurred in alpha-Fe(2)O(3) nanocrystals at small sizes were first found to give a red shift in frequencies of perpendicular mode at 540 cm(-1), a blue shift in the electronic transition of double exciton process in visible region, and a significant decrease in the coercive force.     

Soluble precursors for CuInSe2, CuIn(1-x)Ga(x)Se2, and Cu2ZnSn(S,Se)4 based on colloidal nanocrystals and molecular metal chalcogenide surface ligands

We report a new platform for design of soluble precursors for CuInSe(2) (CIS), Cu(In(1-x)Ga(x))Se(2) (CIGS), and Cu(2)ZnSn(S,Se)(4) (CZTS) phases for thin-film potovoltaics. To form these complex phases, we used colloidal nanocrystals (NCs) with metal chalcogenide complexes (MCCs) as surface ligands. The MCC ligands both provided colloidal stability and represented essential components of target phase. To obtain soluble precursors for CuInSe(2), we used Cu(2-x)Se NCs capped with In(2)Se(4)(2-) MCC surface ligands or CuInSe(2) NCs capped with {In(2)Cu(2)Se(4)S(3)}(3-) MCCs. A mixture of Cu(2-x)Se and ZnS NCs, both capped with Sn(2)S(6)(4-) or Sn(2)Se(6)(4-) ligands was used for solution deposition of CZTS films. Upon thermal annealing, the inorganic ligands reacted with NC cores forming well-crystallized pure ternary and quaternary phases. Solution-processed CIS and CZTS films featured large grain size and high phase purity, confirming the prospects of this approach for practical applications.     

Synthesis of organic monolayer-stabilized copper nanocrystals in supercritical water

When water is heated and pressurized above the critical point, it becomes a suitable solvent to employ organic capping ligands to control and stabilize the synthesis of nanocrystals. Without alkanethiol ligands, Cu(NO(3))(2) hydrolyzes to form polydisperse copper(II) oxide particles with diameters from 10 to 35 nm. However, in the presence of 1-hexanethiol, X-ray photoelectron spectroscopy, selected area electron diffraction, and transmission electron microscopy reveal the formation of copper nanocrystals approximately 7 nm in diameter. The use of a different precursor, Cu(CH(3)COO)(2), leads to particles with significantly different morphologies. A mechanism is proposed for sterically stabilized nanocrystal growth in supercritical water that describes competing pathways of hydrolysis to large oxidized copper particles versus ligand exchange and arrested growth by thiols to produce small monodisperse Cu nanoparticles.     

Hollow boron nitride (BN) nanocages and BN-nanocage-encapsulated nanocrystals

Hollow boron nitride (BN) nanocages (nanospheres, image on the left) and BN-nanocage-encapsulated GaN nanocrystals (right) have been synthesized by using a homemade B-N-O precursors. The as-prepared BN hollow nanocages have typically spherical morphologies with diameters ranging from 30 to 200 nm. The nanocages have crystalline structures. Peanutlike nanocages with double walls have also been observed; their internal space is divided into seperated compartments by the internal walls. The method is extended to sheathe nanocrystals with BN nanocages; BN-shell/GaN-core nanostructures have been successfully fabriacted. The method may be generally applicable to the fabrication BN-sheathed nanocrystals.     

Specific Features of the Preparation of AgI Nanocrystals in Reverse Micelles of Aerosol OT

It was shown that the interrelation between the mean size of AgI nanocrystals prepared in the water pools of reverse micelles as a result of reaction between KI and AgNO₃ and the pool diameter has a complex pattern and is determined mainly by the stability of micellar system. Microemulsions with small micelles (the pool diameter is up to 2 nm) are stable due to the presence of micelle oligomeric phase, regardless of the effect of water structurization in pools. In microemulsions with pool diameters from 2 to 6 nm, such an oligomeric phase is absent that leads to the avalanche-like growth of nanocrystals due to their aggregation in organic phase. As the diameter of micelle pools increases above 6 nm, the microemulsions are stabilized because of the limited dimensions of structured water layer and the deceleration of intermicellar exchange. When the excess amount of KI is added to microemulsion, the screening shell of ions is formed around nanocrystals, thus complicating the formation of icelike structure of water in micelle pools.     

α-ZnS纳米粒子的制备及其光致发光和拉曼特性

以湿化学法合成了直径约35 nm的蓬松的α-ZnS球形纳米粒子. XRD与TEM结果表明, 硫化锌球形纳米粒子由粒径约6 nm的二次晶粒组成. 光致发光光谱表明, 所得样品分别在约430 nm及360 nm处各有一个发射谱带, 前者源于ZnS的表面缺陷发射, 后者可被指认为六方相ZnS的近带边发射. 拉曼表征结果表明, 在较高激光功率的照射下, 没有观察到样品的光氧化现象, 表明所制备的硫化锌纳米粒子结构较为稳定.