The Influence of Surface Trapping and Dark States on the Fluorescence Emission Efficiency and Lifetime of CdSe and CdSe/ZnS Quantum Dots

To investigate the influence of surface trapping and dark states on CdSe and CdSe/ZnS quantum dots (QDs), we studied the absorption, fluorescence intensity and lifetime by using one-and two-photon excitation, respectively. Experimental results show that both one- and two-photon fluorescence emission efficiencies of the QDs enhance greatly and the lifetime increase after capping CdSe with ZnS due to the effective surface passivation. The lifetime of one-photon fluorescence of CdSe and CdSe/ZnS QDs increase with increasing emission wavelength in a supralinear way, which is attributed to the energy transfer of dark excitons. On the contrary, the lifetime of two-photon fluorescence of bare and core-shell QDs decrease with increasing emission wavelength, and this indicates that the surface trapping is the dominant decay mechanism in this case.     

Fluorescence Enhancement of CdSe Q-Dots with Intense Femtosecond Laser Irradiation

Effects of intense femtosecond (fs) laser irradiation on the optical properties of cadmium selenide (CdSe) nanocrystals are studied. We present the changes in emission and absorption of laser (800 nm, 110 fs, Ti–Sapphire) irradiated CdSe nanocrystals dispersed in dimethylformamide (DMF). It is observed that the absorbance of CdSe nanocrystals capped with trioctylphosphine (TOP) increases with the number of laser pulses. The trap state luminescence intensity of these crystals degrades, whereas the band edge luminescence intensity shows an increase as a function of the fs laser irradiation. We also report strong two photon absorption and reduction in the trap state luminescence intensity after irradiation with the laser pulses.     

CdSe/CdS核/壳型纳米晶的光谱特性

以巯基乙酸为稳定剂制备了CdSe/CdS核／壳型纳米晶。用光吸收谱（Abs)、光致发光谱（PL）及光致发光激发谱（PLE）研究了CdS壳层对CdSe纳米晶电子结构，从而对其吸收和发光性能的影响。根据PL和PLE的结果以及带边激子精细结构的计算结果，我们用尺寸很小的纳米晶中所形成的基激缔合物解释了PL光谱与吸收边之间较大的Stokes位移。     

Independence of Maximum Single Molecule Fluorescence Count Rate on the Temporal and Spectral Laser Pulse Width in Two-Photon FCS

We investigate the fluorescence emission characteristics of standard dye tetramethylrhodamine (TMR) in two-photon fluorescence correlation spectroscopy for different temporal and spectral properties of the femtosecond excitation pulses. After determining the second-order dispersion of our setup, including the microscope objective, a pulse stretcher was employed to control the temporal width at the location of the specimen. As expected, the fluorescence per molecule and therefore the signal-to-noise ratio of an FCS-measurement can be improved at constant average excitation power by altering either the temporal or spectral width of the excitation pulses. We found however, that the maximum achievable molecular brightness is largely independent of the temporal and spectral width in the regime analyzed. This observation confirms the current working hypothesis for two-photon fluorescence correlation spectroscopy that bleaching and saturation, and thus, the inherent properties of the dye system, are the dominant effects limiting the quality of measurements. As a practical consequence, elaborate optimization of temporal and spectral laser pulse width, e.g. by introducing pulse stretchers in the beam path, is less critical than previously expected.     

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。     

Near-unity quantum yields of biexciton emission from CdSe/CdS nanocrystals measured using single-particle spectroscopy

Biexciton photoluminescence (PL) quantum yields (Q(2X)) of individual CdSe/CdS core-shell nanocrystal quantum dots with various shell thicknesses are derived from independent PL saturation and two-photon correlation measurements. We observe a near-unity Q(2X) for some nanocrystals with an ultrathick 19-monolayer shell. High Q(2X)'s are, however, not universal and vary widely among nominally identical nanocrystals indicating a significant dependence of Q(2X) upon subtle structural differences. Interestingly, our measurements indicate that high Q(2X)'s are not required to achieve complete suppression of PL intensity fluctuations in individual nanocrystals.     

Acetylene-Substituted Two-Photon Absorbing Molecules With Rigid Elongated Pi-Conjugation: Synthesis,Spectroscopic Properties and Two-Photon Fluorescence Cell Imaging Applications

Two asymmetrical molecules with substituted acetylene as central rigid elongated conjugation are reported as potential chromophores for two-photon microscopic imaging. These molecules consist of a typical D–π–A structure, have different donors (D), the same π-conjugated center (π) and the same acceptor (A). Structural characterization and spectroscopic properties, including single-photon (linear) absorption, quantum yields, single-photon fluorescence, and two-photon absorption spectra, were studied in solvents with different polarity. These acetylene-substituted molecules were found to have high two-photon absorption cross-sections (for example, 690 GM for molecule 1 in toluene), which were determined by a two-photon induced fluorescence method using a femtosecond Ti: sapphire laser as excitation source. Single- and two-photon cellular imaging experiments demonstrate that the substituted acetylene derivatives could be one kind of promising two-photon fluorescence probes for cellular imaging.     

Fabrication of a micro-optical lens using femtosecond laser 3D micromachining for two-photon imaging of bio-tissues

We report, for the first time to our knowledge, on the application of a micro-optical lens fabricated by three-dimensional (3D) femtosecond laser direct writing for two-photon fluorescence imaging of biological tissues. We show that the two-photon fluorescence images of a plant leaf tissue acquired with the micro-optical lens are comparable to that of a 5× objective lens. Our result represents an important step towards the application of micro-optical components fabricated by femtosecond laser micromachining in miniaturized nonlinear fluorescence microscopy applications, such as two-photon endoscopy.     

Single-molecule measurements of gold-quenched quantum dots

We report the study of the quenching of quantum dots (CdSe) by gold nanoparticles at the single-molecule level. Double-stranded DNA is used as a rigid spacer to tune the distance between the two nanoparticles. The width of the fluorescent intensity distribution, monitored at different interparticle distances, reflects both the nanoparticle heterogeneity and the fluorescence intermittency of the quantum dot. The fluorescence distribution emitted by single CdSe nanocrystals can easily be distinguished from the fluorescence of partially quenched CdSe. Our results show that the distance-dependence quenching is compatible with a F?rster-type process.     

Two-photon induced optical-power limiting and upconverted superradiance properties of a new organic dye HEASPI

A new organic dye trans-4-[p-(N-ethyl-N-hydroxyethylamino)styryl]-N-methylpyridinium iodide (abbreviated as HEASPI hereafter) with large two-photon absorption (TPA) cross section and excellent upconverted superradiance properties was synthesized in our group recently. The TPA cross section was measured to be σ₂′=7.0×10⁻⁴⁸ cm⁴ s/photon by using an open aperture Z-scan system. Linear absorption, single-photon induced fluorescence, two-photon induced fluorescence and two-photon pumped (TPP) upconverted superradiance properties were systematically studied. The highest net upconversion efficiency from the absorbed pump energy to the output upconverted superradiance energy is as high as 19.6% at the pump energy of 2.07 mJ from a mode-locked Nd : YAG picosecond laser. The dye solution also shows a clear optical power limiting effect.     

Size-dependent electron-hole exchange interaction in CdSe quantum dots

Il Nuovo Cimento D - We have investigated the fine structure of luminecence of CdSe nanocrystals observed with size-selective excitation. We show that the luminescence line closest to the laser...     

Size-dependent electron-hole exchange interaction in CdSe quantum dots

Il Nuovo Cimento D - We have investigated the fine structure of luminecence of CdSe nanocrystals observed with size-selective excitation. We show that the luminescence line closest to the laser...     

Upconversion luminescence of CdTe nanocrystals by use of near-infrared femtosecond laser excitation

We study the steady-state and time-resolved luminescent properties of CdTe nanocrystals by one- and two-photon excitation with a femtosecond laser. We observe that 1208 nm excitation causes a shift of the emission peak of about 20 nm to the infrared compared with 400 nm laser excitation. It is found that upconversion luminescence is composed of a photoinduced trapping and a band edge excitonic state and produces the observation of biexponential decay kinetics. We conclude that the redshift of the emission peak is caused by the relative change in luminescence intensity between excitonic and trapping states.     

Non-injection and one-pot approach to CdSe: Eu<Superscript>3+</Superscript> hybrid nanocrystals with tunable photoluminescence from green to red

Europium ion-doped CdSe hybrid nanocrystals (CdSe:Eu³⁺ NCs) as a class of new luminescent materials have drawn increasing attention in recent years owing to their remarkable optical properties. In this paper, we report a facile method to prepare CdSe:Eu³⁺ NCs using oleic acid (OA) as the capping agent. With this non-injection and one-pot synthesized approach, the formation and surface passivation of CdSe:Eu³⁺ NCs are performed simultaneously and result in intrinsic luminescence. The as-prepared CdSe:Eu³⁺ NCs are characterized by transmission electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy (EDX). Their optical properties are also studied by UV–vis and photoluminescence spectra. Moreover, the effects of feed ratios and reaction temperatures on the optical properties are further investigated. The results show that the luminescent spectra of CdSe:Eu³⁺ NCs are tunable from green (490 nm) to red (630 nm) and gradually redshift with the increase of the nanoparticle size from 2.5 to 4.4 nm. Upon decoration with 2-thenoyltrifluoroacetone (TTA), the luminescence of europium ion drastically increases and efficient energy transfer from CdSe host to the europium ion is proposed. In addition, an MTT and apoptosis assay show CdSe:Eu³⁺ NCs have low cellular toxicity and could be used as fluorescence imaging for human epithelial type 2 (Hep-2) cells. These properties make CdSe:Eu³⁺ NCs a potential candidate for biological labeling, immunoassays, and optical sensing.

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Graphical abstract Stable and luminescent CdSe:Eu³⁺ hybrid nanocrystals were synthesized, and the luminescence is tunable from green to red by the variation of reaction temperatures and feed ratios. Moreover, CdSe:Eu³⁺ NCs show low cellular toxicity and could be used as fluorescence probes for Hep-2 cells.

     

Two- and three-photon excitation of Gd in CaAl12O19

We have employed two-photon excitation to study the higher energy levels of Gd³⁺ ions in CaAl₁₂O₁₉ and we compare the results with those obtained using conventional UV excitation techniques. Under two-photon excitation, the luminescence intensity exhibits an unusual temporal behavior, a very long build-up followed by a decrease by orders of magnitude, ascribed to a recombination-assisted luminescence excitation mechanism assuming photo-ionization of Gd³⁺ ions and trapping of free electrons on deep traps.

We also find that the two-photon excitation spectra contain an additional broadening contribution which can be attributed to homogeneous broadening of excitation levels caused by excited state absorption into the conduction band. We believe that this may be a general phenomenon whenever participating photons produce ionization of impurity ions from metastable excited states. The phenomenon can manifest itself also in two-photon ionization spectral hole burning and in up-conversion processes (in the latter case, the homogeneous broadening can be caused by an intra-ion excited-state absorption).     

Frequency-dependent spontaneous emission rate from CdSe and CdTe nanocrystals: influence of dark states

We studied the rate of spontaneous emission from colloidal CdSe and CdTe nanocrystals at room temperature. The decay rate, obtained from luminescence decay curves, increases with the emission frequency in a supralinear way. This dependence is explained by the thermal occupation of dark exciton states at room temperature, giving rise to a strong attenuation of the rate of emission. The supralinear dependence is in agreement with the results of tight-binding calculations.     

CdSe纳米晶的制备条件对发光特性的影响

以巯基乙酸为稳定剂,在水相中制备了CdSe纳米晶水溶胶,并用X射线光电子能谱和透射电子显微镜对其进行了表征,证明了CdSe纳米晶的形成。研究了在不同反应时间所制备的CdSe纳米晶的XRD谱以及吸收光谱和发射光谱的变化情况,并且还研究了不同反应物配比情况下的发射光谱的变化,结果表明改变CdSe纳米晶的制备条件,将会对CdSe纳米晶的发光特性有显著的影响。     

Fluorescence upconversion properties of a class of improved pyridinium dyes induced by two-photon absorption

We report the fluorescence upconversion properties of a class of improved pyridinium toluene-p-sulfonates having donor–π–acceptor (D–π–A) structure under two-photon excitation at 1064 nm. The experimental results show that both the two-photon excited (TPE) fluorescence lifetime and the two-photon pumped (TPP) energy upconversion efficiency were increased with the enhancement of electron-donating capability of the donor in the molecule. It is also indicated that an overlong alkyl group tends to result in a weakened molecular conjugation, leading to a decreased two-photon absorption (TPA) cross section. By choosing the donor, we can obtain a longest fluorescence lifetime of 837 ps, a highest energy upconversion efficiency of 6.1%, and a maximum TPA cross-section of 8.74×10⁻⁴⁸ cm⁴ s/photon in these dyes.     

氨基酸稳定的CdSe纳米晶对溶菌酶进行荧光标记

利用氨基酸中惟一带有巯基的L-半胱氨酸盐酸盐(L-Cys)作为稳定剂在水相体系中合成CdSe纳米晶,通过表面包覆的L-Cys与生物试剂溶菌酶进行结合,实现CdSe纳米晶对溶菌酶的荧光标记.利用TEM、荧光光谱仪及荧光显微镜等多种手段对样品的结构、形貌及光学性能进行表征.试验结果表明,利用L-Cys为稳定剂合成的CdSe纳米晶具有较强的生物活性,L-Cys本身所带的氨基和羧基可以和多种生物试剂通过不同途径进行结合.对溶菌酶(Lys)标记前后,CdSe纳米晶的发射光谱的峰位发生微小红移,大约为2nm,半峰全宽基本保持不变,在40~50nm之间,荧光强度随所标记的Lys浓度的不同均发生明显增强.所有特性均表明实验所合成的CdSe纳米晶在生物标记方面具有很好的实用价值.     

Up-conversion luminescence polarization control in Er~(3+)-doped NaYF_4 nanocrystals

We propose a femtosecond laser polarization modulation scheme to control the up-conversion(UC) luminescence in Er~(3+)-doped NaYF_4 nanocrystals dispersed in the silicate glass. We show that the UC luminescence can be suppressed when the laser polarization is changed from linear through elliptical to circular, and the higher repetition rate will yield the lower control efficiency. We theoretically analyze the physical control mechanism of the UC luminescence polarization modulation by considering on- and near-resonant two-photon absorption, energy transfer up-conversion, and excited state absorption, and show that the polarization control mainly comes from the contribution of near-resonant two-photon absorption. Furthermore, we propose a method to improve the polarization control efficiency of UC luminescence in rare-earth ions by applying a two-color femtosecond laser field.