Optical properties of nanocrystalline silver halides

A review of quantum confinement effects in nanocrystals of silver bromide (AgBr) and silver iodide (AgI) is presented. AgBr is an indirect gap semiconductor while AgI has a direct band-to-band lowest energy transition. An examination of the low-temperature optical properties of quantum confined AgBr grown using a variety of synthetic techniques will be made. The dynamics of some of the involved excitonic processes will be measured and discussed in reference to a possible breakdown in the momentum selection rules as the nanocrystals are made smaller. Other explanations for this behavior such as impurity exclusion and surface effects will also be considered, as will the dynamics associated with the trapping of excitons at intrinsic iodide impurities in AgBr. Absorption measurements on AgI nanocrystals will be discussed and compared with the exciton photophysics in AgBr. Both AgBr and AgI display an increasing blue shift of their luminescence, arising from the recombination of excitons, as the crystallite size decreases. The luminescence intensity arising from this process increases with decreasing size in AgBr but it disappears in small crystals of AgI. This leads to the conclusion that in the latter material nonradiative decay channels are opening up as the size decreases.     

六方相LaOF纳米体系中Sm3+/Eu3+的能量转移效应

为了探讨Sm3+/Eu3+共掺体系的荧光光谱特性和能量转移机理,采用水热-烧结法制备了Sm3+/Eu3+共掺LaOF纳米晶体颗粒,并用X射线衍射和透射电子显微镜对纳米晶体颗粒进行了表征.结果显示,所制备的纳米晶体颗粒呈六方相,均匀性和分散性良好,平均粒径为70 nm左右.通过442 nm连续光激发,在六方相LaOF:Sm3+/Eu3+纳米体系中实现了Sm3+离子向Eu3+离子的能量转移,观测到了因能量转移效应而产生的Eu3+离子5D0能级的荧光发射谱线.光谱学研究发现,能量转移源于Sm3+离子4G5/2能级向Eu3+离子5D0能级的弛豫过程,并且随着受主离子Eu3+浓度的增加,能量转移效率也随之提高.     

(NaZSM-5)-AgI主体-客体纳米复合材料的研究

用热扩散法成功地把AgI组装于NaZSM5的孔道中,对制备的样品(NaZSM5)AgI进行了化学分析、粉末XRD分析、化学吸附、红外光谱表征,研究了所制备的样品固体扩散漫反射吸收光谱及发光性质.化学分析表明,AgI已进入NaZSM5主体中.粉末XRD分析显示了组装AgI后NaZSM5沸石骨架依然存在,红外光谱表明了(NaZSM5)AgI样品骨架振动与NaZSM5的骨架振动有细微差别,这主要是由组装了AgI引起的.吸附研究说明了AgI已进入NaZSM5的孔道.固体扩散漫反射吸收光谱表明,主体NaZSM5对制备的主体客体样品光吸收几乎没有影响,(NaZSM5)AgI的吸收光谱受AgI尺寸大小的影响.发光研究表明,制备的样品能带隙很高,辐射过程很强,(NaZSM5)AgI样品系具有发光功能的复合材料.     

用巯基乙酸作稳定剂制备CdSe纳米晶的光学性质

以巯基乙酸为稳定剂制备了CdSe纳米晶，通过尺寸选择沉淀得到2nm到3nm之间不同尺寸的纳米晶，利用室温光吸收，光致发光（PL）和光致发光激发（PLE）谱来研究了CdSe纳米团簇的光学性质。紫外－可见吸收谱给了具有清晰激光特征的尖锐吸收边，这表明样品的尺寸分布很窄。光致发光研究表明，样品有两个发射带，一个具有较高能量位于吸收边，来自电子－空穴对从最低激发态能级弛豫后的辐射复合，另一个低能发射带归属于基质与纳米晶界面存在的俘获中心。PLE谱中有2个吸收带，分别是S－S和P－P跃迁。最后还给出了不同激发能量下的发光特性。     

Luminescence and photosensitization properties of ensembles of silicon nanocrystals in terms of an exciton migration model

The relaxation processes that occur in ensembles of coupled silicon nanocrystals are described by a quantitative model that takes into account the energy transfer between them during exciton migration. This model is used to study the formation of singlet oxygen during the photoexcitation of silicon nanocrystals in porous silicon layers under various external conditions. It is experimentally found that, upon fine milling of as-deposited porous silicon films, the photoluminescence decay time increases despite an increase in the concentration of point defects. The photosensitized activity of ensembles of silicon nanocrystals degrades monotonically during their photostimulated oxidation. These experimental results agree completely with the conclusions of the model and are explained by a decrease in the number of exciton migration ways between nanocrystals when the granule size of a porous silicon powder decreases and by an increase in the efficiency of nonradiative recombination during the photostimulated oxidation of the nanocrystals. Our data indicate that nanocrystalline silicon is a promising material for the methods of nontoxic photodynamic therapy of oncological diseases.     

Breaking the phonon bottleneck in semiconductor nanocrystals via multiphonon emission induced by intrinsic nonadiabatic interactions

We observe ultrafast 1P-to-1S intraband relaxation in PbSe and CdSe nanocrystals (NCs) that have distinct energy spectra. While ultrafast dynamics in CdSe NCs has typically been interpreted in terms of electron-hole energy transfer, this mechanism is not active in PbSe NCs because of sparse densities of states in the conduction and valence bands. Our observations of temperature activation and confinement-enhanced relaxation in PbSe NCs can be explained by efficient multiphonon emission triggered by nonadiabatic electron-phonon interactions and are indicative of large, size-dependent, intraband Huang-Rhys parameters.     

ZnO filled opal arrays: Photo and cathodoluminescence studies

The photo and cathodoluminescence of zinc oxide (ZnO) filled opal arrays, as well as ZnO nanocrystals prepared by a chemical deposition method were investigated. The photoluminescence (PL) from the arrays was studied using angular resolution. The PL spectra of the filled opal exhibited a dip corresponding to the array’s photonic band gap. ZnO nanocrystals embedded in the opal matrix demonstrated quenched excitonic emission while the opal matrix showed enhanced emission. This effect is explained by an energy transfer from ZnO nanocrystals to the opal matrix.     

Emission of 1.53 microm originating from the lattice site of Er3+ ions incorporated in TiO2 nanocrystals

Er(3+)-ion-doped anatase TiO(2) nanocrystals were prepared by simple wet chemical synthesis. Very sharp and intense near-infrared luminescence at approximately 1.53 microm was observed that originated from the lattice site of Er(3+) ions incorporated in TiO(2) nanocrystals. Based on the high-resolution emission and excitation spectra at 10 K, an efficient energy transfer from the TiO(2) host to Er(3+) ions was verified. The luminescence decay for the I13/24-->I15/24 transition at 1.53 microm showed nonexponential characteristics, indicating a nonradiative relaxation to the defects in the nanocrystals, and the intrinsic lifetime of the I13/24 state was determined to be 1.56 ms.     

Electrical and optical properties of colloidal semiconductor nanocrystals in aqueous environments

Microscale and larger semiconductor crystals have electronic and optical properties that depend on their bulk band structures. When these crystals are reduced into the nanoscale, they enter a new regime in which the electrical and optical properties are no longer influenced solely by their bulk band structures, but are influenced by the crystallite size and shape. In this paper, dimensional confinement and proximity phenomena are examined for colloidal semiconductor nanocrystals in several cases of practical importance. Specifically, we determine the effective binding potentials of selected quantum dots in aqueous environments in various colloidal semiconductor nanocrystals and correlate them with experimentally obtained absorption spectra. We also study fluorescence resonance energy transfer (FRET) between semiconductor crystals connected by short peptide chains as well as the shift in photoluminescence spectra of CdTe nanowires made from a chain of CdTe quantum dots.     

Luminescence kinetics characteristics of lead-containing aggregates dispersed in Rb0.95Cs0.05Cl solid state solution

The relaxation of the electron excitations in an insulating matrix such as the Rb_0.95Cs_0.05Cl solid solution containing CsPbCl₃ nanocrystals and single lead centers is studied. Modification of the spectral kinetic parameters of the CsPbCl₃ nanocrystals due to the quantum confinement is explored. Mechanism of the energy transfer from matrix to the nanocrystals is established. The luminescence of the Rb_0.95Cs_0.05C–Pb crystal is characterised by means of measurements with subnanosecond time resolution under synchrotron radiation excitation in the energy range of 3.7–20 eV at T=10 K.     

Synthesis and optical properties of In-doped GaN nanocrystals

Indium-doped GaN nanocrystals with 5% and 10% In have been prepared by a low temperature solvothermal method using hexamethyldisilazane as the nitriding reagent. The nanocrystals show Raman bands at lower frequencies compared to GaN. Photoluminescence spectra of the In-doped GaN nanocrystals exhibit an increase in the FWHM with the decrease in the PL band energy, the band energy itself decreasing with increase in the In content.     

Dynamics of optical response of solutions of pseudoisocyanine <Emphasis Type="Italic">J</Emphasis> aggregates upon pico- and subnanosecond excitation

The luminescence kinetics of solutions of pseudoisocyanine iodide (PIC) J aggregates is studied upon excitation by pico- and subnanosecond laser pulses into the main superintense long-wavelength absorption band. The dependences of the transmission and luminescence energy on the pump energy are measured. It is found that the shape of the luminescence kinetics depends on the recording and excitation spectral regions, which is obviously related to the energy transfer between coherently coupled fragments of J aggregates.     

Formation of silver particles and silver oxide plume nanocomposites upon pulsed-laser induced liquid-solid interface reaction

Nanodendrites consisting of silver and silver oxide are grown upon a pulsed-laser induced reaction at the interface between the solid target and silver nitrate solution. By using a high-resolution transmission electron microscope (HRTEM) equipped with an energy dispersive X-ray spectrometer (EDS) and selected area diffraction (SAD), the fabricated nanopatterns are identified to be a composite structure that consisting of silver nanoparticles and silver oxide nanoplumes with polycrystalline structure. In detail, these silver nanocrystals are trunks of the nanodendrite, and their size is in the range of 30 to 50 nm. The silver oxide nanoplumes are branches of the nanodendrite, and their width and length are in the ranges of 20 to 50 nm and 30 to 100 nm, respectively. Additionally, we suggested a vapor-liquid mechanism for the formation of the nanopatterns using a pulsed-laser induced liquid-solid interface reaction, in which both silver clusters in vapor and silver ions in solution are simultaneously involved.Received: 6 April 2004, Published online: 5 November 2004PACS: 81.15.Fg Laser deposition - 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 81.07.Bc Nanocrystalline materials     

Study of interfacial charge transfer in nanosemiconductor–molecule composites

Interaction of sol–gel synthesized Ce–Ag‐codoped ZnO (CSZO) nanocrystals with (E)‐1‐(naphthalen‐1‐yl)‐2‐styryl‐1H‐phenanthro[9,10‐d]imidazole has been analysed. The synthesized nanocrystals and their composites with naphthyl styryl phenanthrimidazole have been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X‐ray spectroscopy, X‐ray diffractometry, X‐ray photoelectron spectroscopy (XPS) lifetime and Fourier transform infrared spectroscopy and cyclic voltammetry. XPS shows doped silver and cerium in Ag⁰ and Ce⁴⁺ states, respectively_. SEM and TEM images of CSZO nanoparticles show that they appear to be 3D trapezoid and cocoon‐like shape. The selected area electron diffraction pattern supports the nanocrystalline character of the synthesized material. The percentages of doping of cerium and silver in CSZO are 0.54 (at.) and 0.34 (at.), respectively. From the energy levels of the materials used in the imidazole–CSZO composite, the dominant CT direction has been analysed. Theoretical investigation shows that the binding energy and energy gap of the imidazole composites are highly dependent on the nature of the silver oxide cluster and that charge transfer in the imidazole–Ag₄O₄ composite is faster than the same in other composites. Molecular docking technique has also been carried out to understand the imidazole–DNA interactions. Copyright © 2016 John Wiley & Sons, Ltd.     

Excited state dynamics in silver nanoparticles embedded in phosphate glass

Excited state dynamics in silver nanoparticles embedded in aluminophosphate glass was studied by ultrafast optical pump–probe technique. The absorption process of pump radiation and the electron–phonon relaxation on the 10^?13–10^?11 s scale were analyzed in the framework of two-temperature model. The time evolution of the light-induced transient diffraction grating shows an uncommon relaxation on the nanosecond time scale. This relaxation is assigned to phonon–phonon scattering process as well as to the energy transfer from photoexcited electronic states in glass matrix to silver nanoparticles.     

Theoretical investigations on the high light yield of the LuI3:Ce scintillator

The extremely high scintillation efficiency of lutetium iodide doped by cerium is explained as a result of at least three factors controlling the energy transfer from the host matrix to activator. We propose and theoretically validate the possibility of a new channel of energy transfer to excitons and directly to cerium, namely the Auger process when Lu 4f hole relaxes to the valence band hole with simultaneous creation of additional exciton or excitation of cerium. This process should be efficient in LuI₃, and inefficient in LuCl₃. To justify this channel, we perform calculations of density of states using a periodic plane-wave density functional approach. The second factor is the increase of the efficiency of valence hole capture by cerium in the row LuCl₃-LuBr₃-LuI₃. The third one is the increase of the efficiency of energy transfer from self-trapped excitons to cerium ions in the same row. The latter two factors are verified by cluster ab initio calculations. We estimate either the relaxation of these excitations and barriers for the diffusion of self-trapped holes (STH) and self-trapped exciton (STE). The performed estimations theoretically justify the high LuI₃:Ce³⁺ scintillator yield.     

Effects of Metallic Silver Particles on Resonance Energy Transfer Between Fluorophores Bound to DNA

We examined the effects of metallic silver island films on resonance energy transfer (RET) between a donor and acceptor bound to double helical DNA. The donor was 4',6-diamidino-2-phenylindole (DAPI) and the acceptor was propidium iodide (PI). Proximity of the labeled DNA to the silver particles resulted in a dramatic increase in RET as seen from the emission spectra and the donor decay times. Proximity to silver particles results in an increase of the Förster distance from 35 Å an to an apparent value of 166 Å. These results suggest a new type of DNA hybridization assays based on RET over distances much longer than the free-space Forster distance.     

紫细菌捕光天线LH2中的超快光物理研究

采用飞秒泵浦探测技术研究了紫细菌外周捕光天线LH2中的超快光动力学过程.从B800蓝侧的激发态动力学中观察到B800到B850的能量传递时间,实验结果与理论计算结果的差别说明激发B800时可能引起B850上激子带的直接激发,或存在由B800到B850上激子态的能量传递通道.在B800红侧激发的动力学过程中,漂白信号前端存在的一个快速光吸收信号主要来源于B850上激子带的直接激发.在天然RS601和突变体GM309的LH2中,800 nm激发时的动力学过程都表现为一个类似的光漂白过程,动力学曲线的衰减时间常量在天然LH2中明显快于突变体中,说明在GM309中B800到B850的激发能传递速率有所降低.而在845 nm激发下两个样品中的快过程类似,但慢过程在GM309中有所增快,激发态中的能量重新分布包括逆向的能量传递也受到类胡萝卜素微结构的影响.     

CuI晶体及其缺陷态电子结构的模拟

利用相对论密度泛函理论和嵌入分子团簇方法,模拟计算了具有闪锌矿结构的γ态CuI晶体及其缺陷态的电子结构。结果显示晶体的本征能级结构:价带顶主要由I5p和Cu3d轨道杂化组成,导带底由Cu4s轨道组成,禁带宽度为3.1eV,该结果与实验相符。在不同缺陷态的计算中,四面体间隙铜缺陷相对其他间隙缺陷更易于在晶体中形成,其中Cu3d→4s跃迁能量为3.2eV,推测与CuI晶体发光密切相关。     

Sensitized Fluorescence of Silver Nanoparticles in the Presence of Pyrene

The fluorescence of pyrene adsorbed onto the surface of the cetyltrimethylammonium-coated silver nanoparticles was studied. Pyrene molecules adsorbed on freshly prepared silver particles were found to be in close proximity to silver surface thus providing the possibility of energy transfer from excited pyrene to silver cores of the particles. In that case along with the expected fluorescence of pyrene we observed the fluorescence of the silver nanoparticles induced by the excited pyrene molecules. In due course the restructuring of the cetyltrimethylammonium layer resulted in moving of pyrene molecules away from silver surface and simultaneous disappearance of the silver nanoparticles fluorescence band. These data strongly support the recent hypothesis of fluorophore-plasmon coupled emission.