Synthesis,COSMO-RS analysis and optical properties of surface modified ZnS quantum dots using ionic liquids

Zinc sulfide (ZnS) quantum dots (QDs) were synthesized using the microwave assisted ionic liquid (MAIL) route. Three ionic liquids (ILs), namely, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF₄]), trihexyl(tetradecyl) phosphonium bis(trifluoromethanesulfonyl) amide ([P_6,6,6,14][TSFA]) and trihexyl(tetradecyl) phosphonium chloride ([P_6,6,6,14][Cl]) were used in this study. The size and structure of the QDs were characterized by high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) pattern, respectively. The synthesized QDs were of wurtzite crystalline structure with size less than 5 nm. The QDs were more uniformly distributed while using the phosponium based ILs as a reaction medium during synthesis. The optical properties were investigated by UV–vis absorption and photoluminescence (PL) emission spectroscopy. The optical properties of QDs showed the quantum confinement effect in their absorption and the effect of cation and anion structural moiety was observed on their bandedge emission. The QDs emission intensity was measured higher for [P_6,6,6,14][Cl] due to their better dispersion as well as high charge density of Cl anion. The capability of the ILs in stabilizing the QDs was interpreted by density functional theory (DFT) computations. The obtained results are in good agreement with the theoretical prediction.     

Study of InAs/GaAs quantum dots grown by MOVPE under the safer growth conditions

InAs quantum dots (QDs) have been formed on GaAs (001) substrate by metal-organic vapor phase epitaxy (MOVPE) under the safer growth conditions: using tertiarybutylarsine (TBA) to replace AsH₃ as the arsenic source and replacing hydrogen by pure nitrogen as the carrier gas. Effects of growth conditions on the QD formation have been investigated. It is observed that the wetting layer is stabilized with some material being transferred to form the QDs due to the strain relaxation process during the QD formation. Dot size dispersion becomes broader when the post-growth interruption is more than 20 s. Compared with normal one-step grown QDs, dot density increases greatly by 213% after employing two-step deposition for QD growth. This is explained by considering the indium-flux-dependent nucleation density at step 1 and kinetically self-limiting growth at step 2. The two photoluminescence (PL) emission peaks, 1.203 μm and 1.094 μm, from the two-step grown QDs are attributed to E1–HH1 and E1–LH1 transitions of the QDs, respectively. The measured results agree well with those received by an 8 k·p theoretical calculation. The narrow PL linewidth of ~50 nm shows high quality of the QDs. This paves the way to develop safer MOVPE process, using TBA/N₂ instead of AsH₃/H₂, to grow QDs for device application.     

Synthesis of stable dispersion of ZnO quantum dots in aqueous medium showing visible emission from bluish green to yellow

Aqueous dispersion of 4-8 nm size stable ZnO quantum dots (QDs) exhibiting luminescence in the visible region have been synthesized by a simple solution growth technique at room temperature. Silica has been used as capping agent to control the particle size as well as to achieve uniform dispersion of QDs in aqueous medium. X-ray diffractometer (XRD) analysis reveals formation phase pure ZnO particles having wurzite (hexagonal) structure. Atomic force microscope (AFM) images show that the particles are spherical in shape, having average crystalline sizes ∼4, 5.5 and 8 nm for samples prepared at pH values of 10, 12 and 14, respectively. From the optical absorption studies, the band gap energy of QDs is found to be blue shifted as compared to bulk ZnO (3.36 eV) due to the quantum confinement effect and is consistent with the band gap calculated by using effective-mass approximation model. The photoluminescence (PL) observed in these QDs has been attributed to the presence of defect centers.     

Fs-pulsed laser deposition of PbTe and PbTe/Ag thermoelectric thin films

For the first time, thermoelectric thin films were fabricated by femtosecond pulsed laser deposition (fs-PLD) that represents a challenging technological solution for this application since it provides a correct film stoichiometry compared to the starting target, capability of native nanostructuring and a high deposition rate. In particular, this paper shows a preliminary work on PbTe and PbTe/Ag thin films deposited at different substrate temperatures by fs-PLD from a microcrystalline PbTe target. Structural, morphological and compositional characterizations of the deposited films were performed to demonstrate the formation of films composed by crystalline nanograins (about 35 nm size) and characterized by a correct stoichiometry. A remarkable deposition rate of 1.5 nm/s was evaluated. The electrical conductivity and the Seebeck coefficient (thermopower) were measured as a function of operating temperature to derive the thermoelectric power factor that was found to be less than a factor 2 with respect to the bulk materials. Finally, a discussion about the influence of compositional and structural properties of the deposited films on the related thermoelectric performances was presented.     

Lead telluride nanocrystalline thin films: Structure,optical characterization and a broadband dielectric spectroscopy study

Nanocrystalline PbTe thin films were deposited on a glass substrate by thermal evaporation technique with two thicknesses namely, 45 and 250 nm. The structural studies revealed that the films have nanocrystalline cubic structure and the particle size was found to be 11 and 20.7 nm, for low and high thicknesses respectively. The FE-SEM study shows that the surface grains increase for higher thickness film. This indicates that samples lying under the strong regime of confinement for PbTe thin films. The optical properties confirm the occurrence of confinement process as the optical band gap are 1.67 and 0.9 eV for 45 and 250 nm films, respectively. The dielectric results indicated that the conductivity increases by about two orders of magnitude with increasing the thickness from 45 to 250 nm. Moreover, the permittivity shows a higher dispersion step at lower frequencies in both samples due to the hopping conduction mechanism in addition to the interfacial polarization in such heterogeneous structures. Another small dispersion step is noticed in case of the lower thickness. It is attributed to the polarization of the accumulated charge carriers near the grain boundaries interfaces. No indication of any electrode phenomena in both samples is shown here.     

Separated AlxIn1−xN quantum dots grown by plasma-reactive co-sputtering

Separated Al_xIn_1−xN quantum dots (QDs) embedded in amorphous AlN films have been produced by radio-frequency co-sputtering technique on silicon (1 1 1) and quartz glass substrates. The mean size and density of Al_xIn_1−xN QDs can be conveniently monitored by deposition parameters. Transparent electron microscope, and X-ray diffraction were used to detect the structure of the Al_xIn_1−xN QDs system; field-emission scanning-electron microscope was adopted to measure the surface morphology and anticipate the size of the QDs; X-ray photoelectronic spectroscopy was used to measure the stoichiometric ratios of the QDs.     

Synthesis of ZnO quantum dots and their agglomeration mechanisms along with emission spectra based on ageing time and temperature

The ZnO quantum dots (QDs) were synthesized with improved chemical solution method. The size of the ZnO QDs is exceedingly uniform with a diameter of approximately 4.8 nm, which are homogeneously dispersed in ethanol. The optical absorption edge shifts from 370 nm of bulk material to 359 nm of QD materials due to the quantum size effect, while the photoluminescence peak shifts from 375 nm to 387 nm with the increase of the density of ZnO QDs. The stability of ZnO QDs was studied with different dispersion degrees at 0 ℃ and at room temperature of 25 ℃. The agglomeration mechanisms and their relationship with the emission spectra were uncovered for the first time. With the ageing of ZnO QDs, the agglomeration is aggravated and the surface defects increase, which leads to the defect emission.     

Fluorescence Property of ZnO Nanoparticles and the Interaction with Bromothymol Blue

We synthesized ZnO quantum dots (QDs) simply in alcoholic solution, and investigated the interaction between ZnO QDs and bromothymol blue. The structural, morphological, size and spectral properties of ZnO QDs were studied. It was found that ZnO QDs were spherical nanoparticles in the crystal structure, and the average diameter of ZnO QDs was about 4.8 nm. The excitation and emission peaks were located at 346 nm and 520 nm, respectively, which were obtained on a common fluorophotometer. The quantum yield of ZnO QDs was obtained by using quinine sulfate as a reference reagent. In addition, the fluorescence of ZnO QDs can be quenched by bromothymol blue, and the quenching mechanism was proposed in a dynamic quenching mode.     

Liposome-coated quantum dots targeting the sentinel lymph node

Sentinel lymph node (SLN) mapping with near-infrared (NIR) quantum dot (QDs) have many advantages over traditional methods. However, as an inorganic nanomaterial, QDs have low biocompatibility and low affinity to the lymphatic system. Here, we encapsulated QDs into nanoscale liposomes and then used these liposome-coated QDs for SLN mapping. The results showed that the liposome-coated QDs exhibited core–shell characterization, and their fluorescence emission did not decrease but slightly increased after being continuously excited by a xenon lamp source (150 W) at 488 nm at 37 °C for 1 h. After storing at 4 °C for more than one and half years, the liposome-coated QDs were found to have retained their spherical structure containing a large amount of QDs. When liposome-coated QDs with average size of 55.43 nm were injected intradermally into the paw of a mouse, the SLN was strongly fluorescent within only a few seconds and visualized easily in real time. Moreover, the fluorescence of the QDs trapped in the SLN could be observed for at least 24 h. Compared with the SLN mapping of QDs absent of liposomes and liposome-coated QDs with a larger average size (100.3 and 153.6 nm), more QDs migrated into the SLN when the liposome-coated QDs with smaller average size (55.43 nm) were injected. This technique may make a great contribution to the improvement of the biocompatibility of QDs and the targeting delivery capacity of QDs into the SLN.     

Water‐Dispersible Silicon‐Quantum‐Dot‐Containing Micelles Self‐Assembled from an Amphiphilic Polymer

The dispersion of silicon quantum dots (Si QDs) in water has not been established as well as that in organic solvents. It is now demonstrated that the excellent dispersion of Si QDs in water with photoluminescence (PL) quantum yields (QYs) comparable to those for hydrophobic Si QDs can be realized by combining the processes of hydrosilylation and self‐assembly. Hydrogen‐passivated Si QDs are initially hydrosilylated with 1‐dodecence. The toluene solution of the resulting dodecyl‐passivated Si QDs is mixed with the water solution of the amphiphilic polymer of Pluronic F127 to form an emulsion. Dodecyl‐passivated Si QDs are encapsulated in the micelles self‐assembled from F127 in the emulsion. The size of the Si‐QD‐containing micelles may be tuned in the range from 10 to 100 nm. Although self‐assembly in the emulsion causes the PL QY of Si QDs to decrease, after a few days of storage in ambient conditions, Si QDs encapsulated in the water‐dispersible micelles exhibit recovered PL QYs of ≈24% at the PL wavelength of ≈680 nm. The intensity of the PL from Si QDs encapsulated in the water‐dispersible micelles is >90% of the original value after 60 min ultraviolet illumination, indicating excellent photostability.     

PbTe/CdTe量子点的光学增益

PbTe/CdTe量子点是一类新型异系低维结构材料,实验发现具有强的室温中红外光致发光现象.为研究这一材料体系的发光特性,建立了理论模型,计算了PbTe/CdTe量子点的光学跃迁和增益.模型基于k·p包络波函数方法并考虑了PbTe能带结构的各向异性.分析了量子点光学增益与量子点尺寸、注入载流子浓度的关系.结果表明,当注入载流子浓度在(0.3—3)×10¹⁸cm^-3范围时,尺寸为15—20nm的量子点可以产生 关键词： PbTe/CdTe量子点 光学增益 铅盐矿半导体     

Well-crystallized zinc oxide quantum dots with narrow size distribution

In this study, pulsed laser ablation, online annealing, and following size classification using a differential mobility analyzer (DMA) were employed to fabricate quantum dots (QDs) of zinc oxide (ZnO). The irregularly shaped ZnO particles were obtained at annealing temperature less than 873 K, which gradually transformed into spherical QDs with increasing the annealing temperature. Finally, ZnO QDs with narrow size distribution having spherical shapes were successfully obtained at temperatures above 1173 K under the DMA classification at a nominal size of 10 nm. TEM observation demonstrated that the ZnO QDs obtained by this process were well-crystallized single crystallites with a wurtzite structure. Further, ZnO QDs with average sizes in the range of 4.8–8.1 nm were successfully fabricated by reducing the specified sizes of DMA. These features of the fabricated ZnO QDs are favorable for investigation of intrinsic quantum size effect in ZnO.     

Photoinduced Degradation of the Optical Properties of Colloidal Ag<Subscript>2</Subscript>S and CdS Quantum Dots Passivated by Thioglycolic Acid

The results of studying degradation of the optical properties of colloidal Ag₂S and CdS quantum dots (QDs) 2.6–3.2 nm in size passivated by thioglycolic acid (TGA) are presented. The photoluminescence intensity of colloidal Ag₂S QDs has been found to decrease under laser irradiation at a wavelength of 445 nm, beginning with the effective power of 10 mW. The observed effect is interpreted as a photochemical reaction of formation of new nonradiative-recombination channels in Ag₂S QDs upon excitation. It is established for colloidal CdS QDs passivated by TGA that a decrease in the optical density in the entire absorption spectrum and the luminescence intensity is accompanied by precipitation of the colloidal particles in a cell and related to photodegradation of the passivating shell.     

Synthesis and photoluminescence properties of aqueous CdWO4 quantum dots with WO6 luminescence center

Aqueous CdWO₄ QDs were synthesized by the reaction of CdCl₂ and Na₂WO₄ in the presence of mercaptoacetic acid (TGA) as capping reagent. The crystal morphology, particle size and its distribution of as-prepared products were characterized by transmission electron microscopy (TEM, SAED) atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), and photon correlation spectroscopy (PCS), respectively. Qualitative assays for functional groups on the QDs’ surface were measured by fourier transform infrared spectroscopy (FTIR). Photoluminescence properties of QDs were studied by photoluminescence spectroscopy (PL). The results showed that the single QD with diameter of about 8 ± 2 nm was single-crystal. The particle size distribution of QDs was normal. Infrared absorption bands of carboxylic group on the surface of CdWO₄ QDs were observed around 1610-1550 cm⁻¹ (nonsymmetrical vibration of -COO⁻) and 1400 cm⁻¹ (symmetric vibration of C-O). With reaction-time going, PL peak position shifted from 498 to 549 nm and intensity of PL increased first and then decreased. PL peak position of QDs was blue-shift compared with 570 nm WO₆⁶⁻ luminescence center of bulk CdWO₄.     

RGDS-conjugated CdSeTe/CdS quantum dots as near-infrared fluorescent probe: preparation,characterization and bioapplication

In the experiments, high-quality, water-soluble and near-infrared (NIR)-emitting CdSeTe and CdSeTe/CdS quantum dots (QDs) were successfully prepared. The average size of CdSeTe?CdS QDs was 7.68 nm and CdSeTe QDs was 4.33 nm. Arginine-glycine-aspartic-serine acid (RGDS) peptides were linked to CdSeTe/CdS QDs by N-(3-(dimethylamino)propyl)-N′-ehtylcarbodiimide hydrochloride (EDC) and N′-hydroxysuccinimide (NHS). The prepared RGDS-tagged NIR CdSeTe/CdS QDs (denoted as RGDS-CdSeTe/CdS) had an average diameter of 24.83 nm and were used for cancer cell immunofluorescence imaging. The characteristics of RGDS-conjugated CdSeTe/CdS such as morphology, structure, spectra, stability, cytotoxicity, and near-infrared microscopic imaging were investigated in detail. HepG2 cells were incubated with the novel fluorescent probe (RGDS-CdSeTe/CdS), which realized immunofluorescence targeting and imaging. The results reported here open up new perspectives for integrin-targeted near-infrared imaging and may aid in tumor detection including imaging-guided surgery.     

Generation of MoS<Subscript>2</Subscript> quantum dots by laser ablation of MoS<Subscript>2</Subscript> particles in suspension and their photocatalytic activity for H<Subscript>2</Subscript> generation

MoS₂ quantum dots (QDs) have been obtained in colloidal suspensions by 532 nm laser ablation (7 ns fwhp/pulse, 50 mJ/pulse) of commercial MoS₂ particles in acetonitrile. High-resolution transmission electron microscopy images show a lateral size distribution from 5 to 20 nm, but a more homogeneous particle size of 20 nm can be obtained by silica gel chromatography purification in acetonitrile. MoS₂ QDs obtained by laser ablation are constituted by 3–6 MoS₂ layers (1.8–4 nm thickness) and exhibit photoluminescence whose λ_PL varies from 430 to 530 nm depending on the excitation wavelength. As predicted by theory, the confinement effect and the larger periphery in MoS₂ QDs increasing the bandgap and having catalytically active edges are reflected in an enhancement of the photocatalytic activity for H₂ generation upon UV–Vis irradiation using CH₃OH as sacrificial electron donor due to the increase in the reduction potential of conduction band electrons and the electron transfer kinetics.     

Quantum confinement in amorphous silicon quantum dots embedded in silicon nitride

Amorphous silicon quantum dots (a-Si QDs) were grown in a silicon nitride film by plasma enhanced chemical vapor deposition. Transmission electron micrographs clearly demonstrated that a-Si QDs were formed in the silicon nitride. Photoluminescence and optical absorption energy measurement of a-Si QDs with various sizes revealed that tuning of the photoluminescence emission from 2.0 to 2.76 eV is possible by controlling the size of the a-Si QD. Analysis also showed that the photoluminescence peak energy E was related to the size of the a-Si QD, a (nm) by E(eV) = 1.56+2.40/a(2), which is a clear evidence for the quantum confinement effect in a-Si QDs.     

Grid-pattern aligned InAs self-assembled quantum dots grown on InP substrate

We present a method to control the nucleation sites of InAs self-assembled quantum dots (QDs). Tensile-strained material, such as GaAs used here, was grown on InP substrates before InAs deposition. This thin GaAs layer can provide a surface with grid-pattern trenches which have the same function as atomic-steps and are promising for the formation of QDs with controlled nucleation sites. Atom force microscopy (AFM) measurement was performed and the AFM images indicate that the InAs islands grown with our technique are grid-pattern aligned and have good homogeneity and low size fluctuation. In addition, another kind of three-dimensional structure with larger size would coexist with normal QDs if a 30nm thick GaAs layer was deposited. This revised version was published online in June 2006 with corrections to the Cover Date.     

Tailoring Cu2−xTe quantum-dot-decorated ZnO nanoparticles for potential solar cell applications

Cu_2−xTe QDs on ZnO nanoparticles were synthesized using a successive ionic layer absorption and reaction technique (SILAR) at room temperature. The as-synthesized QDs which were distributively deposited on ZnO nanoparticles surface were characterized by field emission scanning electron microscope (FE-SEM), X-ray diffraction and high-resolution transmittance microscope (HR-TEM). It revealed that the average diameter of the QDs was ∼2 nm. The synthesized Cu_2−xTe QDs were solely orthorhombic Cu_1.44Te phase. The growth mechanism was supposed that it based on ions deposition. The energy gap of as-synthesized Cu_2−xTe QDs was determined ∼1.1 eV and the smallest energy gap of 0.76 eV was obtained, equal to that of bulk material. Raman spectroscopy and FTIR were also used to study the Cu_2−xTe QDs on ZnO nanoparticles. These characteristics suggest a promising implication for a potential broadband sensitizer of QDSCs.     

Carrier gas effects on the SiGe quantum dots formation

SiGe quantum dots (QDs) grown by ultra-high vacuum chemical vapor deposition using H₂ and He carrier gases are investigated and compared. SiGe QDs using He carrier gas have smaller dot size with a better uniformity in terms of dot height and dot base as compared to the H₂ carrier gas. There is a higher Ge composition and less compressive strain in the SiGe QDs grown in He than in H₂ as measured by Raman spectroscopy. The Ge content is higher for He growth than H₂ growth due to hydrogen induced Si segregation and the lower interdiffusivity caused by the more strain relaxation in the He-grown SiGe dots. The photoluminescence also confirms more compressive strain for H₂ growth than He growth. Hydrogen passivation and Ge-H cluster formation play an important role in the QDs growth.