Dynamic distribution of growth rates within the ensembles of colloidal II-VI and III-V semiconductor nanocrystals as a factor governing their photoluminescence efficiency

The distribution of properties within ensembles of colloidally grown II-VI and III-V semiconductor nanocrystals was studied. A drastic difference in the photoluminescence efficiencies of size-selected fractions was observed for both organometallically prepared CdSe and InAs colloids and for CdTe nanocrystals synthesized in aqueous medium, indicating a general character of the phenomenon observed. The difference in the photoluminescence efficiencies is attributed to different averaged surface disorder of the nanocrystals originating from the Ostwald ripening growth mechanism when larger particles in the ensemble grow at the expense of dissolving smaller particles. At any stage of growth, only a fraction of particles within the ensemble of growing colloidal nanocrystals has the most perfect surface and, thus, shows the most efficient photoluminescence. This is explained by a theoretical model describing the evolution of an ensemble of nanocrystals in a colloidal solution. In an ensemble of growing nanocrystals, the fraction of particles with the highest photoluminescence corresponds to the particle size having nearly zero average growth rate. The small average growth rate leads to the lowest possible degree of surface disorder at any given reaction conditions.     

Photoluminescence brightening via electrochemical trap passivation in ZnSe and Mn(2+)-doped ZnSe quantum dots

Spectroelectrochemical experiments on wide-gap semiconductor nanocrystals (ZnSe and Mn(2+)-doped ZnSe) have allowed the influence of trap electrochemistry on nanocrystal photoluminescence to be examined in the absence of semiconductor band filling. Large photoluminescence electrobrightening is observed in both materials upon application of a reducing potential and is reversed upon return to the equilibrium potential. Electrobrightening is correlated with the transfer of electrons into nanocrystal films, implicating reductive passivation of midgap surface electron traps. Analysis indicates that the electrobrightening magnitude is determined by competition between electron trapping and photoluminescence (ZnSe) or energy transfer (Mn(2+)-doped ZnSe) dynamics within the excitonic excited state, and that electron trapping is extremely fast (k(trap) ≈ 10(11) s(-1)). These results shed new light on the complex surface chemistries of semiconductor nanocrystals.     

Colloidal gallium indium oxide nanocrystals: a multifunctional light-emitting phosphor broadly tunable by alloy composition

We demonstrate compositionally tunable photoluminescence in complex transparent conducting oxide nanocrystals. Alloyed gallium indium oxide (GIO) nanocrystals with variable crystal structures are prepared by a colloidal method throughout the full composition range and studied by different structural and spectroscopic methods, including photoluminescence and X-ray absorption. The structures and sizes of the GIO nanocrystals can be simultaneously controlled, owing to the difference in the growth kinetics of In(2)O(3) and Ga(2)O(3) nanocrystals and the polymorphic nature of both materials. Using the synthesized nanocrystal series, we demonstrate the structural and compositional dependences of the photoluminescence of GIO nanocrystals. These dependences, induced by the interactions between specific defect sites acting as electron donors and acceptors, are used to achieve broad emission tunability in the visible spectral range at room temperature. The nature of the photoluminescence is identified as donor-acceptor pair recombination and changes with increasing indium content owing to the changes in the energy states of, and interactions between, donors and acceptors. Structural analysis of GIO nanocrystals by extended X-ray absorption fine structure spectroscopy reveals that In(3+) occupies only octahedral, rather than tetrahedral, sites in the spinel-type γ-Ga(2)O(3) nanocrystal host lattice, until reaching the substitutional incorporation limit of ca. 25%. The emission decay dynamics is also strongly influenced by the nanocrystal structure and composition. The oxygen vacancy defects, responsible for the observed photoluminescence properties, are also implicated in other functional properties, particularly conductivity, enabling the application of colloidal GIO nanocrystals as integrated optoelectronic materials.     

Highly luminescent, stable, and water-soluble CdSe/CdS core-shell dendron nanocrystals with carboxylate anchoring groups

A dendron ligand with two carboxylate anchoring groups at its focal point and eight hydroxyl groups as its terminal groups was found to efficiently convert as-synthesized CdSe/CdS core-shell nanocrystals in toluene to water-soluble dendron-ligand stabilized nanocrystals (dendron nanocrystals). The resulting dendron nanocrystals retained 60% of the photoluminescence value of the original CdSe/CdS core-shell nanocrystals in toluene and were significantly brighter than the similar dendron nanocrystals with thiolate (deprotonated thiol group) as the anchoring group which retained just 10% of the photoluminescence value of the original CdSe/CdS core-shell nanocrystals in toluene. The carboxylate-based dendron nanocrystals survived UV irradiation in air for at least 13 days, about 9 times better than the thiolate-based dendron nanocrystals (35 h) and similar to that of the thiolate-based dendron-box stabilized CdSe/CdS core-shell nanocrystals (box nanocrystals). Upon UV irradiation, the dendron nanocrystals became even 2 times brighter than the original CdSe/CdS core-shell nanocrystals in toluene, and the UV-brightened PL can retain the brightness for at least several months. These stable and bright dendron nanocrystals were soluble in various aqueous media, including all common biological buffer solutions tested, for at least 1.5 years. In addition to their superior performance, the synthetic chemistry of carboxylate dendron ligands and the corresponding dendron nanocrystals is relatively simple and with high yield.     

Dispersion of ZnO Nanocrytals in Co-solvent and Its Application in Photovoltaic Material

To overcome the aggregation of nanocrystals in a blend of inorganic material with conjugated polymers to prepare photovoltaic material, we used a co-solvent blend of CHCl3 with MeOH at a certain volume fraction to disperse inorganic nanocrystals. The results show that when the volume fraction of MeOH is 50%, ZnO nanocrystals with an average diameter of 30 nm disperse well in the co-solvent solution. Its application in photovoltaic material was investigated in this work, and the photoluminescence(PL) spectra show that when ZnO was 50%(volume fraction) in solution and 25%(volume fraction) in film, the fluorescence quenching reached the maximum values 83.34% and 64.4%, respectively, indicating that electron could transfer from conjugated polymer to electron-acceptor ZnO effectively.     

Optical properties of single semiconductor nanocrystals

We present an overview of the current progress in the understanding of the (steady state) optical properties of individual II-VI semiconductor nanocrystals. We begin with a presentation of the conceptual development of the theory required to model the electronic structure of these systems. This is followed by an overview of the current experimental results obtained from the spectroscopy of individual semiconductor nanocrystals, and in particular, we focus on the study of photoluminescence intermittency (blinking) and spectral diffusion. Where possible, we link the experimental observations to the predictions of current theories. We conclude that the surface of small semiconductor crystals plays an important role in determining their optical properties.     

Role of surface coating in ZrO2/Eu3+ nanocrystals

The sol-emulsion-gel method is used for the preparation of Eu3+ ion-doped and coated ZrO2 nanocrystals. Here, we report the role of surface coating, dopant concentration, and temperature of heating in the modification of their crystal structure and photoluminescence properties. It is found that the volume fraction of the tetragonal phase increases from 28.08 to 91.56% because of surface coating. This is a significant modification of the crystal phase in ZrO2 nanocrystals due to surface coating by Eu2O3. It is found that the photoluminescence properties are sensitive to the crystal structure, which is again controlled by surface coating, concentration, and heating temperature. It is found that the decay time (tau) of Eu-doped ZrO2 nanocrystals increases with increasing the concentration of dopant and with increasing the temperature of heating because of changes in their crystal phase. The emission intensity of the peak at 611-617 nm (5D0 --> 7F2) of the Eu3+ ion-activated ZrO2 nanocrystals (doped and coated) is also found to be sensitive to the nanoenvironment. The average decay times are 770 and 488 mus for 1100 degrees C-heated 1.0 mol % Eu2O3-doped and coated ZrO2 nanocrystals, respectively. Our analysis suggests that the site symmetry of the ions plays the most important role in the modifications of the radiative and nonradiative relaxation mechanisms as a result of the overall photoluminescence properties.     

用于微生物体标记的CdTe半导体纳米晶超声波水相合成

利用超声波法制备高质量CdTe半导体纳米晶体, 采用该方法不仅降低了实验成本, 简化了实验流程, 而且还可制备出量子产率达到50%的CdTe纳米晶体, 具有较好的光谱学性质, 可用于莱姆病伯氏螺旋体(微生物)的荧光标记, 有利于提高对该病的检测效率.     

Resonant electron transfer and luminescent enhancement in a toluene suspension of Si nanocrystals

Efficient resonant electron transfer from the surface bonding structure to the conduction band of quantum confined Si nanocrystals is observed by Si nanocrystals in a toluene suspension. Based on the electron transfer mechanism, the enhancement of photoluminescence originates from the band-to-band recombination in the p-type Si nanocrystals suspended in a toluene solution. The energy levels of the electrons in the Si nanocrystals chemisorbed with toluene molecules are calculated using the method of linear combination of atomic orbitals, and the characteristics of the obtained density of states is in good agreement with the observed photoluminescence properties.     

Preparation of CdSe/ZnSe Core-shell Nanocrystal in One-step Reaction

IntroductionSemiconductor nanocrystals show strong size-de-pendent properties when their size is similar to or smal-ler than the excition Bohr radius of the bulk materialsand quantum confinement occurs for the space-con-fined motion of the electrons and holes in the nano-re-gion of materials[1—5].Because of the excellent opticaland electronic properties,semiconductor nanocrystalsare currently being investigated as emitting materials forthin-film light-emitting devices(LED)[6,7],low-thresh-ol…     

Microwave-assisted aqueous synthesis: a rapid approach to prepare highly luminescent ZnSe(S) alloyed quantum dots

In this paper, we present a new procedure for the rapid synthesis of luminescent ZnSe nanocrystals in aqueous phase by microwave irradiation with controllable temperature. The effects of microwave irradiation and experimental conditions on the synthesis of nanocrystals were investigated systematically. It was found that there were significant effects of pH value of reaction solutions, molar ratio of precursors, and heating time of microwave irradiation on the optical properties of the ZnSe nanocrystals. A series of nanocrystals with different size was prepared in 1 h, and the photoluminescence quantum yield reached up to 17% at the optimal reaction condition. The results of HRTEM and XRD showed that the as-prepared nanocrystals had high crystallinity. The characterizations of EDS spectra and elemental analysis showed that the sulfur content of nanocrystals increased with the growth of nanocrystals. We speculated that the structure of nanocrystals was an alloy ZnSe(S) shell on the surface of the ZnSe particles core. Furthermore, we found that the oxygen from air in the reaction vessel played an important role in the decomposition of the thiol group under microwave irradiation.     

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.     

Room temperature synthesis of HgTe nanocrystals

We report a new method to synthesize monodisperse zinc blende HgTe nanocrystals at room temperature in noncoordinating solvent-octadecene. Thiol was needed to control the reaction at a suitable nucleation and growth speed. In the early stage of the reaction, HgTe nanocrystals formed aggregates, and then the aggregates were dispersed and individual dot-shaped nanocrystals were formed with stronger photoluminescence emitting. UV-vis, photoluminescence, and TEM have been used to study the properties of as-prepared HgTe nanocrystals.     

功能化ZnxCd1-xS纳米晶的制备及其光学性能的研究

以氯化锌、氯化镉和硫化钠为原料，采用巯基乙醇为表面修饰剂，H₂O/DMF为溶剂，制得未团聚且分散均匀的纳米晶溶液，同时得到表面富含羟基基团不同锌镉组成的复合Zn_xCd_1-xS纳米晶。利用FTIR、EDS、UV-Vis、XRD、荧光光谱(PL)考察了复合纳米晶结构和光学性能的关系，并利用紫外灯摄像技术对Zn_xCd_1-xS纳米晶的光致发光性能进行     

Synthesis and properties of nanosilicon stabilized by butyl and perfluorobutyl ligands

Nanocrystalline silicon stabilized by butyl and perfluorobutyl ligands that form improper surface states of silicon nanocrystals were synthesized. The presence of perfluorobutyl ligands on the surface of silicon nanocrystals was proved by IR spectroscopy. Nanocrystals with perfluorobutyl ligands form aggregates, which decreases the efficiency of photoluminescence. The nanocrystals with butyl ligands have smaller size but their photoluminescence can be clearly recorded.     

Aqueous Synthesis of Methylammonium Lead Halide Perovskite Nanocrystals

Methylammonium lead halide perovskite nanocrystals offer attractive optoelectronic properties but suffer from fast degradation in the presence of water. In contradiction to this observation, we demonstrate the possibility of a direct aqueous synthesis of CH₃NH₃PbX₃ (X=Br or Cl/Br) nanocrystals through the reaction between the lead halide complex and methylamine when the pH is maintained in the range of 0–5. Under these synthetic conditions, the positively charged surface of the perovskite nanocrystals and the proper ionic balance help to prevent their decomposition in water. Additional surface capping with organic amine ligands further improves the photoluminescence quantum yield of the perovskite nanocrystals to values close to 40 %, ensures their stability under ambient conditions for several months, and their photoluminescence performance under continuous 0.1 W mm^?2 405 nm light irradiation for over 250 hours.     

Study of quasi-monodisperse In2O3 nanocrystals: synthesis and optical determination

In this communication, we report a successful synthesis of quasi-monodisperse In2O3 nanocrystals with high crystallinity in a high-temperature organic solution. The average size of nanocrystals can be tuned using a dynamic injection technique. TEM and XRD investigations indicate that each nanocrystal is a single crystal. The optical determination implies that the photoluminescence behavior of these In2O3 nanocrystals is different from that of the bulk, probably due to the combination of weak quantum-confinement-effects and the nature of high crystallinity in nanocrystals.     

Luminescence of Ce3+ in Y2SiO5 nanocrystals: Role of crystal structure and crystal size

Here, we report the role of crystal structure and crystal size on the photoluminescence properties of Ce3+ ions in Y2SiO5 nanocrystals. The emission at 430 nm (5d1 --> 4f1) and lifetime of the excited state of Ce3+ ion doped Y2SiO5 nanocrystals are found to be sensitive to the crystal structure, crystal size, and dopant concentration. It is found that the overall lifetime tau of 0.5 mol % Ce doped Y2SiO5 nanocrystals are 8.78 and 3.45 ns for 1000 and 1100 degrees C heat-treated samples with the same crystal structure (X1-Y2SiO5 phase), respectively. However, a significant increase in the overall lifetime (35.21 ns) is observed for the 1300 degrees C annealed 0.5 mol % Ce doped Y2SiO5 sample having a different crystal structure (X2-Y2SiO5 phase). We found that the decay kinetic is biexponential. It is explained that the fast component arises due to sequential hole-electron capture on the luminescent ions and the slow component arises from isolated ions. Our analysis suggests that modifications of radiative and nonraditive relaxation mechanisms are due to local symmetry structure of the host lattice and crystal size, respectively.     

Energy transfer dynamics of nanocrystal-polymer composites

Steady-state and time-resolved photoluminescence spectroscopy are used to examine the photoluminescent properties of nanocrystal-polymer composites consisting of colloidal PbS nanocrystals blended with poly(2-methoxy-5(2-ethylhexyloxy)-p-phenylene vinylene). Quenching of the emission from the conjugated polymer due to the PbS nanocrystals is observed along with band edge emission from the ligand capped PbS nanocrystals. A decrease in the photoluminescence lifetime of MEH-PPV is also observed in the thin film nanocrystal-polymer composite materials. Photoluminescence excitation spectroscopy of the PbS nanocrystal emission from the composite shows features attributed to MEH-PPV providing evidence of a F?rster transfer process.     

Ultrafast Microwave-hydrothermal Synthesis of CdTe Nanocrystal and Its Application to Fingerprint Detection

We have developed a microwave-assisted hydrothermal method,where both microwave and hydrothermal techniques are utilized to synthesize CdTe nanocrystals with high quality in less than 55 s.Among the known preparation methods,the monodisperse CdTe nanocrystals showing narrow emission spectra,widest scope of emission peak(from 500 nm to 822 nm) and high quantum yield(up to 80%) were synthesized by this route.The quantum yield of CdTe with near-infrared emission(800 nm) is up to 10%,which is better than that of InAs(0.5%―2%) synthesized in organic phase.Moreover,we detected the latent fingerprints on aluminum,plastic,glass and papers by prepared CdTe nanocrystals,and photoluminescence fingerprints images were taken with a digital camera.Then the images would transmitt into fingerprint detection system of computer to identify the criminals in suit.