Ultrafast and large third-order nonlinear optical properties of CdS nanocrystals in polymeric film

We report the ultrafast and large third-order nonlinear optical properties of CdS nanocrystals (NCs) embedded in a polymeric film. The CdS NCs of 2 nm radius are synthesized by an ion-exchange method and highly concentrated in the two layers near the surfaces of the polymeric film. The two-photon absorption coefficient and the optical Kerr coefficient are measured with laser pulses of 250 fs duration at 800 nm wavelength. The one-photon and two-photon figures of merit are determined to be 3.1 and 1.3, respectively, at irradiance of 2 GW/cm(2). The observed nonlinearities have a recovery time of approximately 1 ps. The two-photon-generated free carrier effects have also been observed and discussed. These results demonstrate that CdS NCs embedded in polymeric film are a promising candidate for optical switching applications.     

Generation and optical properties of monodisperse wurtzite-type ZnS microspheres

Monodisperse wurtzite-type ZnS microspheres have been prepared by using glutathione (GSH) as a sulfur source at low reaction temperatures ranging from 160 to 210 degrees C. The diameter of the ZnS microspheres can be tuned from approximately 254 to approximately 597 nm by changing the reaction parameters such as temperature, molar ratio of reactants (GSH/Zn2+), and reaction medium (ethylenediamine or ammonia). Our results demonstrate that monodentate amines (ammonia) play the same role as that of bidentate amines (ethylenediamine) in the formation of the wurtzite-type ZnS microspheres. The formation process of the monodisperse ZnS microspheres consists of a GSH-dominated nucleation process and an amine-dominated assembly process. The as-synthesized monodisperse ZnS microspheres readily self-assemble into ordered hexagonal patterns and thus have potential applications as colloidal crystalline materials. Blue fluorescence emission peaks at 415 and 466 nm in wavelength, attributed to deep-trap emission, are observed at room temperature.     

Cauliflower-like CdS microspheres composed of nanocrystals and their physicochemical properties

Cauliflower-like cadmium sulfide (CdS) microspheres composed of nanocrystals have been successfully synthesized by a hydrothermal method using poly(ethylene glycol) (PEG) as the template coordination agent and characterized by a variety of methods. Our experiments confirmed that the size of the CdS microspheres could be easily modified by controlling the chain length of PEG. Powder X-ray diffraction and Raman spectroscopy measurements revealed the cubic structure of the CdS microspheres; morphological studies performed by HR-SEM and HR-TEM methods showed the cauliflower-like structure of the synthesized CdS microspheres. Each microsphere was identified to be created by the self-assembly of CdS nanocrystals and is attributed to the oriented aggregation of the CdS nanocrystals around a polymer-Cd(2+) complex spherical framework structure. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray (EDX) analysis confirmed the stoichiometries of the CdS microspheres. Diffuse reflectance spectrum (DRS) measurements showed that increasing the PEG chain length increased the band gap value of the CdS microspheres slightly, from 1.99 to 2.06 eV. The cauliflower-like CdS microspheres could be applied to photocatalytic degradation studies.     

Synthesis and optical properties of CdS nanoribbons

Rapid production of single crystalline CdS nanoribbons with hexagonal wurtzite phase has been achieved by thermal evaporation of CdS powder on Si wafers. The flow rate of the carrier (Ar) gas along with the synthesis temperature plays an important role in defining the size and shape of the CdS nanoribbons. Scanning electron and transmission electron microscopic observations revealed the nanoribbons to have a flat end as well as side surfaces which will make it ideal for optoelectronic devices such as nanolasers and light emitting diodes based on individual nanoribbons. The nanoribbons have widths within 200-400 nm and lengths approximately a few hundred micrometers. Room-temperature photoluminescence measurements show green emission centered at approximately 525 nm which may be ascribed to the near band edge emission. The Raman spectra of the CdS nanoribbons show peaks around 304, 609, 915, and 1220 cm(-1) corresponding to the first-, second-, third-, and fourth-order longitudinal optical phonon modes, respectively.     

Synthesis and optical properties of thiol-stabilized PbS nanocrystals

Thiol-capped water-soluble PbS nanocrystals (NCs) stabilized with 1-thioglycerol, dithioglycerol, or a mixture of 1-thioglycerol/dithioglycerol (TGL/DTG) were prepared via one-stage synthesis at room temperature. We found that NCs stabilized with a TGL/DTG mixture show efficient and stable infrared photoluminescence centered in the second "biological window" (1050-1200 nm). Under optimized conditions, full width at half-maximum of the PL emission peak was from 70 to 100 nm. PbS NCs were stable to precipitation and aggregation for the time period from 2 to 3 months when stored in the dark under room temperature. Room-temperature photoluminescence quantum efficiency of NCs was from 7 to 10%. When NCs were stored at 37 degrees C, their PL emission red-shifted, consistent with the NC growth.     

Surface-functionalization-dependent optical properties of II-VI semiconductor nanocrystals

We report a study of the surface-functionalization-dependent optical properties of II-VI zinc-blende semiconductor nanocrystals on the basis of ligand-exchange chemistry, isomaterial core/shell growth, optical spectroscopy, transmission electron microscopy, and X-ray powder diffraction. Our results show that the transition energy and extinction coefficient of the 2S(h3/2)1S(e) excitonic band of these nanocrystals can be strongly modified by their surface ligands as well as ligand associated surface atomic arrangement. The oleylamine exchange of oleate-capped zinc-blende II-VI nanocrystals narrows the energy gap between their first and second excitonic absorption bands, and this narrowing effect is size-dependent. The oleylamine exchange results in the quenching, subsequent recovery, and even enhancing of the photoluminescence emission of these II-VI semiconductor nanocrystals. In addition, the results from our X-ray powder diffraction measurements and simulations completely rule out the possibility that oleate-capped zinc-blende CdSe nanocrystals can undergo zinc-blende-to-wurtzite crystal transformation upon ligand exchange with oleylamine. Moreover, our theoretical modeling results suggest that the surface-functionalization-dependent optical properties of these semiconductor nanocrystals can be caused by a thin type II isomaterial shell that is created by the negatively charged ligands (e.g., oleate and octadecyl phosphonate). Taking all these results together, we provide the unambiguous identification that II-VI semiconductor nanocrystals exhibit surface-functionalization-dependent excitonic absorption features.     

表面修饰CdS和(CdS)ZnS纳米晶的性能研究

在水相中合成了CdS纳米微粒,以ZnS对其进行表面修饰,得到具有核壳结构的(CdS)ZnS水溶性纳米晶。采用红外光谱、X射线衍射(XRD)、透射电镜(TEM)表征其粒度和形貌,紫外-可见吸收光谱(UV)、荧光光谱表征其光学特性。制得的CdS近似呈球形,直径为8nm;CdS纳米颗粒表面经ZnS修饰后,其荧光发射峰强度显著增强,表面态发射减弱。     

CdS:Mn nanocrystals passivated by ZnS: synthesis and luminescent properties

Synthesis and characterization of highly luminescent ZnS-passivated CdS:Mn (CdS:Mn/ZnS) core/shell structured nanocrystals are reported. Mn-doped CdS core nanocrystals are produced ranging from 1.5 to 2.3 nm in diameter with epitaxial ZnS shell of wider band gap via a reverse micelle process. UV irradiation-stimulated photo-oxidation of the ZnS shell results in formation of sulfate (ZnSO(4)) as determined by x-ray photoelectron spectroscopy, which increases the photoluminescence emission intensity and subsequent photostability. Luminescent relaxation lifetime data present two different decay components, consisting of slow decay emission from the Mn center and a fast decay emission from a defect-related center. The impact of the density of surface defect states upon the emission spectra is discussed.     

Ligand effects on optical properties of CdSe nanocrystals

We report effects of various organic and inorganic ligands on optical properties of CdSe nanocrystals (NCs) by changes in their photoluminescence and absorbance spectra. Surface ligand loss occurring during dilution and purification of solutions of CdSe NCs leads to a decrease of photoluminescence intensity. The complex of trioctylphosphine with Se atoms on the surface of CdSe NCs is found responsible for the trap emission band that is red-shifted relative to the photoluminescence band edge.     

Hierarchical CdSe-gold hybrid nanocrystals: synthesis and optical properties

The synthesis of hybrid nanostructures with controlled size, shape, composition and morphology has attracted increasing attention due to the fundamental and applicable interest. Here, we demonstrate the synthesis and optical properties of hierarchical CdSe-Au hybrid nanostructures with zinc blende (ZB) CdSe nanocrystals (NCs). For 3.5 nm ZB CdSe NCs, one Au cluster was deposited on each CdSe NC. Nevertheless, several Au clusters were selectively deposited on the apexes of 5 nm and 8 nm ZB CdSe NCs, resulting from the different reactivity of crystal facets. Furthermore, hierarchical CdSe-Au nanostructures with complex morphology were organized with the isolated CdSe-Au hybrid NCs by the coalescence of Au domains on the CdSe-Au hybrid NCs. UV-Vis spectra revealed a red tail upon the deposition of Au clusters. The chemical joint of Au on CdSe NCs was further confirmed by fluorescence quenching. The optical limiting performance of CdSe-Au hybrid NCs dispersed in toluene was investigated at 532 nm using a Nd:YAG laser with the pulse width of 8 ns.     

Controllable synthesis of functionalized CdS nanocrystals and CdS/PMMA nanocomposite hybrids

We reported controllable synthesis of CdS nanocrystal-polymer transparent hybrids by using polymethylmethacrylate (PMMA) as a polymer matrix. In a typical run, the appropriate amounts of cadmium chloride (CdCl₂) and sodium sulfide (Na₂S) in the presence of 2-mercaptoethanol (ME) as the organic ligand are well dispersed in H₂O/DMF solution without any aggregation. From a combination of transmission electron microscopy (TEM) and a computing method of Brus’s model according UV-vis absorption spectra, the particle size of as-prepared hydroxyl-coated CdS nanocrystal was determined to be about 5 nm. Then, with the surface treatment with methacryloxypropyltrimethoxysilane (MPS), CdS-PMMA hybrids were obtained via free radical polymerization in situ. FT-IR characterization indicates the formation of robust bonding between CdS nanocrystals and the organic ligand and the formation of double-bond functional CdS nanocrystals. The TGA measurement displays CdS-PMMA hybrids possess better thermal stability compared with pure PMMA polymer. The fluorescence measurement shows that CdS nanocrystals and CdS-PMMA hybrids exhibit good optical properties. Also, the luminescent photographs taken under ultraviolet light prove the luminescence properties.     

Controlled growth and photocatalytic properties of CdS nanocrystals implanted in layered metal hydroxide matrixes

Layered double hydroxide Cd(1)(-)(x)()Al(x)()(OH)(2)(DS)(x)().3.0H(2)O (CdAlDS) and a related hydroxide salt compound Cd(2)(OH)(3)(DS).2.5H(2)O (CdDS), where DS stands for dodecyl sulfate sandwiched between two adjacent inorganic layers, have been synthesized and used as precursors for CdS nanoparticle growth. Through a gas/solid reaction, CdS nanocrystals implanted in the layer matrixes of the layered double hydroxides are grown, and the sizes of the nanocrystals vary in the range of 3-6 nm in diameter. The presence of trivalent Al cations in the layered double hydroxide can be taken advantage of to control the size of the CdS nanocrystals, and it also helps to prevent the formed nanocrystals from extraction from the solid matrixes. The nano-CdS implanted composite exhibits high photocatalytic activity for degradation of the nonbiodegradable rhodamine B under both UV and visible irradiations.     

Template synthesis of nanocomposite CdS with nonlinear optical properties

Russian Chemical Bulletin -     

Temperature dependent spectral properties of type-I and quasi type-II CdSe/CdS dot-in-rod nanocrystals

We investigated systematically the temperature dependence of the spectral properties such as the band gap, bandwidth and fluorescence intensity of CdSe/CdS dot-in-rod nanocrystals. These asymmetry nanoparticles were synthesized by seeded growth techniques with band alignment of the type-I and quasi type-II with initial core sizes of 3.3 and 2.3 nm, respectively. With increasing temperature the band gap decreases and bandwidth increases, largely due to exciton-phonon scattering. Anomalous variations of the band gap and bandwidth were observed at 200-240 K, and the variations are attributed to the anisotropic strain in the CdSe/CdS interface due to temperature dependent lattice mismatch. The integrated intensity of fluorescence shows two variation regimes. In the low temperature regime, the intensity remained roughly constant due to the temperature dependent carrier mobility and trapping by the defect states in the CdS shell. However, in the higher temperature regime, the intensity decreased quickly due to thermal/phonon assisted escape from the CdSe dot. The barrier depths are estimated to be about 557 and 285 meV for type-I and quasi type-II samples, respectively.     

Tuning of copper nanocrystals optical properties with their shapes

Copper nanocrystals are obtained by chemical reduction of copper ions in mixed reverse micelles. A large excess of reducing agent favors producing a new generation of shaped copper nanocrystals as nanodisks, elongated nanocrystals, and cubes. By using UV-Visible spectroscopy and numerical optical simulations we demonstrate that the optical properties are tuned by the relative proportions of spheres and nanodisks.     

CdS和CdSe薄膜的制备及其光电化学性质

本文采用简易的化学水浴沉积法和自牺牲模板法制备CdS、CdSe薄膜,对两种薄膜进行了XRD表征,比较了两种薄膜的紫外吸收光谱并研究了CdS、CdSe薄膜作为太阳能电池中的光阳极时所产生的光电流和光电压,对两种薄膜的电化学性能进行了比较.     

Shape-controlled synthesis of polyhedral CdS flowerlike architectures and their optical properties

Fabrication of polyhedral CdS flower-like architectures have been achieved on a large scale through a mixed solvothermal method. The obtained CdS are characterized by X-ray diffraction, field-emission scanning electron microscopy and transmission electron microscopy, and the results indicate that the CdS flower-like architectures with diameters of 1.5–2.0 µm are hexagonal wurtzite phase and are assembled by some pyramids with the bottom side length of about 440 nm, which have some crystallographic faces. A series of relevant experiments through altering experimental parameters, indicate that the temperature, starting materials and solvent play key roles for the shape evolution of CdS flower-like architectures. The studies of optical properties for polyhedral CdS flower-like architectures indicate that the UV-vis spectroscopy shows a blue-shift absorption peak at 500 nm compared to that of bulk CdS, the photoluminescence spectroscopy shows an emission peak at 640 nm and another strong emission peak at 695 nm, which are believed to be attributed to excitonic emission and deep levels.      

Solvothermal synthesis of well-dispersed NaMgF3 nanocrystals and their optical properties

Complex metal fluoride NaMgF(3) nanocrystals were successfully synthesized via a solvothermal method at a relatively low temperature with the presence of oleic acid, and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectra, photoluminescence (PL) excitation and emission spectra, respectively. In the synthetic process, oleic acid as a surfactant played a crucial role in confining the growth and solubility of the NaMgF(3) nanocrystals. The as-prepared NaMgF(3) nanocrystals have quasi-spherical shape with a narrow distribution. A possible formation mechanism of the nanocrystals was proposed based on the effect of oleic acid. The as-prepared NaMgF(3) nanocrystals are highly crystalline and well-dispersed in cyclohexane to form stable and clear colloidal solutions, which demonstrate a strong emission band centered at 400 nm in photoluminescence (PL) spectra compared with the cyclohexane solvent. The PL properties of the colloidal solutions of the as-prepared nanocrystals can be ascribed to the trap states of surface defects.