Surface-enhanced emission from single semiconductor nanocrystals

The fluorescence behavior of single CdSe(ZnS) core-shell nanocrystal (NC) quantum dots is dramatically affected by electromagnetic interactions with a rough metal film. Observed changes include a fivefold increase in the observed fluorescence intensity of single NCs, a striking reduction in their fluorescence blinking behavior, complete conversion of the emission polarization to linear, and single NC exciton lifetimes that are >10(3) times faster. The enhanced excited state decay process for NCs coupled to rough metal substrates effectively competes with the Auger relaxation process, allowing us to observe both charged and neutral exciton emission from these NC quantum dots.     

The effect of relaxation and core–hole decay on the M5O3 X-ray fluorescence spectrum of La metal

In the M₅O₃ X-ray emission spectroscopy (XES) spectrum of La metal the spectral intensity ratio of the satellite to the diagram line is very large, as compared to that of the shakedown satellite to the main line in the M₅ X-ray photoelectron spectroscopy (XPS) spectrum. The large spectral intensity ratio is due to the X-ray fluorescence of the shakedown satellite via the relaxation of the M₅ main-line state to the shakedown satellite state on the time scale of M₅–hole decay. We attribute the large line width of the M₅ main-line state of La metal as compared to that of Ba metal, to the relaxation on the time scale of M₅–hole decay.     

3-Pentanone LIF at elevated temperatures and pressures: measurements and modeling

The objective of this study is to investigate 3-pentanone fluorescence experiments in a constant volume vessel at high temperature and high pressure to underline the influent parameters in conditions close to those encountered in internal combustion engines. To obtain quantitative analysis, measured fluorescence signals must be corrected by considering the influence of preponderant parameters such as temperature, pressure and gas composition. Quantitative dependences of fluorescence on thermodynamic parameters are measured and compared with the predictions of a photophysical model, which combines the effects of temperature, pressure, excitation wavelength on fluorescence quantum yield. The increase of 3-pentanone fluorescence with pressure is due to the vibrational relaxation of energy levels. The fluorescence decreases with increasing temperature, except at low temperature where the fluorescence increase is due to an activation of intersystem crossing between triplet toward singlet levels. The influences of thermodynamic parameters are based on an increase of the non-radiative decay rate with the vibrational energy level of excited electronic state and the important collisions to remove the excess vibrational energy. Experimental and calculated results show a satisfactory agreement. PACS 33.20; 33.50; 34.90     

Wavelength and coherence effects on the growth mechanism of silicon nanopillars and their use in the modification of spontaneous lifetime emission of BODIPY dye molecules

Silicon nanopillars are grown by an electrochemical anodization of p-type silicon wafers at low current densities in a hydrofluoric acid solution. CW, white light, and various UV pulsed lasers are employed as illumination sources in sample preparation to study wavelength and coherence effects on the growth mechanism of the nanopillars. Coherence is observed to be the foundation of regularity in obtaining conical shapes. The pillar size is found to be almost linearly proportional to the employed illumination wavelength during their growth. BODIPY dye molecules are chemically attached to these silicon nanopillars and the radiative decay rates are investigated by means of a time-resolved fluorescence experiment. The decay rate of the dye molecules embedded in the vicinity of various size pillar tips is significantly affected due to different apex angles of the conical nature. It is demonstrated that the pillar size and the separation between pillars can be adjusted if one uses a coherent light source with an appropriate wavelength during the course of fabrication process. Since change in the decay rate is due to tips of the pillars only, separation of a few micrometers between pillar tips allows one to directly monitor a dye, which is embedded to the tip of a single nanopillar, via a confocal microscopic method for the spontaneous lifetime measurements, without having needed to any extra efforts for an in situ imaging process. It is observed that as the pillar size gets smaller, the inhibition in the spontaneous lifetime of BODIPY is more pronounced. In addition, a more regular pillar structure yields nonvarying decay rates of the dye molecules throughout the silicon sample.     

Fluorescence Quenching of Auramine in Fluid Solutions: A Femtosecond Spectroscopy Study

The quenching of Auramine fluorescence in ethanol is studied by two ultrafast spectroscopy techniques. The gain band, probed by transient absorption spectroscopy, vanishes in a few picoseconds, while a transient absorption band rises and the ground-state repopulation is delayed. In up-conversion experiments, nonexponential wavelength-dependent fluorescence decays are observed. The average decay times increase with the wavelength and the reconstructed instantaneous spectrum exhibits a few hundred-wavenumber red shift and a broadening while its intensity drops. The previously proposed relaxation model, involving a barrierless internal twisting motion toward a transient dark state, is further examined. In particular, the extinction coefficients of the transient state are extracted from the differential absorption spectra. The band is found to lie in the same wavelength range as the dimethylaniline cation radical. This result is discussed as a possible support for an internal twisting process involving a charge shift.     

Calculating a local oscillating quasiparticle’s lifetime at screened defects in solids

We report on a novel approach to calculate quasiparticle lifetimes of localized initial states, which decay into the continuum of underlying quasi-free quasiparticle states in the vicinity of point defects and steps in solids. By using this interpretation of the inelastic damping of wavefunctions, the lifetime becomes a local property. In particular, we consider electrons, which are injected by a scanning tunneling microscope tip into the surface state of a noble metal surface. We investigate numerically the configuration of a single scatterer, a chain of scatterers, and a triangular quantum corral. As compared to an exponential increase of the damping from prior theories, we find an oscillating damping together with a linear background of the resulting measurement signal. The different configurations show increased lifetimes with increasing dimensionality as their scattering phase space is decreased.     

Observations of the Effect of Confined Space on Fluorescence and Diffusion Properties of Molecules in Single Conical Nanopore Channels

In this work, we investigated the fluorescence emission spectra and diffusion properties of dye molecules confined in different positions of conical nanopore channels using a laser scanning confocal fluorescence microscope. The results showed that a red shift of the emission spectra is observed from the tip section to the bottom section and the diffusion rate is slower in the channel than that in bulk solution, indicating a single conical nanopore channel can be used as a convenient tool for investigating the effect of confined space on the behaviors of molecules.     

Effects of paramagnetic cations on the nonexponential spin-lattice relaxation of rare spin nuclei in solids

Nonexponential spin-lattice relaxation is often observed for rare spin nuclei in the solid state. Deviation from single-component decay may be amplified by the coupling of rare spin nuclei to paramagnetic centers. Nonexponential spin-lattice relaxation was observed in derivatized silica gels resins. This phenomenon was localized and enhanced when paramagnetic transition metal cations were bound to surface functional groups. A stretched exponential analysis method was determined to be robust in fitting nonexponential relaxation curves for silica gels both with and without bound paramagnetic ions. Spin-lattice relaxation rates (T₁⁻¹) for functional group nuclei increased as a function of percent surface coverage with metal ion. The magnitude of the relaxation rate increase was dependent upon internuclear distances from the paramagnetic center. At low surface coverages, a semi-random distribution of paramagnetic centers increased the degree of stretching of spin-lattice relaxation decays, as measured by decreases in the calculated stretching parameter β. At higher surface coverages, calculated β values reached a limiting value, indicating that while the spin-diffusion mechanism in metal-ex-changed silica gels is restricted, it is not completely diminished.     

基于电荷转移过程的PAN-C60共聚物薄膜的时间分辨荧光研究

利用吸收光谱和皮秒时间分辨荧光研究PAN－C60星状共聚物的电荷转移过程。PAN－C60共聚物的吸收和荧光光谱结果显示共聚物中存在着电荷转移过程。时间分辨荧光结果表明PAN的荧光衰减遵循双指数衰减规律（一快过程160ps和一慢过程1500ps)，快衰减过程主要来源于聚合物中主链间相互作用产生的空间间接极化子对的影响，慢变过程主要来源于单重态激子的辐射跃迁弛豫。在共聚物中，C60分子的存在除导致PAN激发态寿命缩短外，还影响聚合物链间的相互作用，C60分子对PAN荧光猝 灭作用主要通过慢变过程影响的，而对PAN的空间极化子对的影响主要与其快衰减过程有关。     

基于谷胱甘肽配体的ZnSe量子点对pH响应的时间分辨荧光光谱特性

当前,有关量子点pH响应方面的研究主要集中在含Cd（镉）类量子点,且都是研究其稳态荧光光谱对pH值的响应。然而,Cd类量子点对生物体系具有一定的毒性,且稳态荧光光谱法由于受浓度等因素的影响具有一定的不稳定性,因此应用于生物体系中作为pH探针具有明显的缺点。基于以上分析,通过水相合成法,我们制备出了基于谷胱甘肽配体的水溶性ZnSe量子点,该量子点具有毒性小,生物兼容性好等特点,适合被应用于生物体系中。利用所制备的ZnSe量子点,采用时间相关单光子计数技术,结合紫外可见吸收光谱和稳态荧光光谱,对pH值在5~11不同环境下的ZnSe量子点荧光动力学进行了系统性的研究。ZnSe量子点荧光衰减具有两个寿命组分,拟合得到分别为4和24 ns。通过采集不同探测波长下ZnSe量子点荧光衰减曲线,发现其长寿命组分随探测波长的增加而增加,而短寿命组分基本不随探测波长的改变而改变,结合有关报道分析判断,短寿命和长寿命组分分别来源于核内非局域载流子复合和表面态局域载流子复合。实验发现,处于不同pH值的环境下的ZnSe量子点具有不同的荧光寿命,其荧光寿命与pH值的变化呈负相关。通过比较ZnSe量子点两种荧光寿命组分随pH值的变化关系,发现ZnSe量子点的荧光寿命对pH值的响应主要来源于长寿命组分即表面态寿命,且在不同pH值范围内响应的灵敏度不同,在6~8的pH值范围内响应最为显著,表现为长寿命组分随pH值的增加出现一个较大幅度的衰减。实验进一步发现,ZnSe量子点两个寿命组分的比值在不同pH值范围内具有较好的线性相关性,但在不同pH值范围内斜率不同,通过比较,最大值在pH值为6~8的范围内。另外,与金属钠离子相互作用实验及相关报道表明,金属离子对ZnSe量子点荧光寿命的影响较小。以上研究表明,ZnSe量子点在生物体系pH值检测中具有良好的应用前景。     

Influence of Substituent and Solvent on the Radiative Process of Singlet Excited States of Novel Cyclic Azacyanine Derivatives

The photophysical properties of novel cyclic azacyanine derivatives have been investigated in acetonitrile, N-butyronitrile, methanol, ethanol, DMF and water. Introduction of electron donating or accepting groups on the cyclic azacyanine has a direct impact on the spectroscopic and photophysical properties. Irrespective of the nature of the substitution, azacyanine shows a general solvent relaxation in accordance with Lippert-Mataga’s prediction; however, in protic solvent, specific interactions are encountered. Fluorescence lifetime decay suggests a relaxation in the nanosecond time scale with monoexponential decay in polar solvents and biexponential decay in non polar solvents. The fluorescence lifetime of azacyanines are found to be longer than popular cy3 dyes. An electron donating substituent increases the fluorescence lifetime and influences the radiative process, whereas an electron withdrawing group marginally increases the excited state lifetime but remarkably enhances the radiative process. The fluorescence quantum yield of substituted cyclic azacyanine in water is noted to be at least five fold higher than the popular cy3 dye.     

邻羟基绿色荧光蛋白发色团光物理机理的理论研究

Herein we have employed the MS-CASPT2//CASSCF method to study the S₁ excited-state intramolecular proton transfers (ESIPTs) of recently synthesized ortho-hydroxyl GFP core chromophores, i.e. OHIM, CHBDI, and MHBID, and their excited-state relaxation pathways. We have found that in OHIM and CHBDI, the ESIPT process is associated with small barriers of 3.4 and 4.2 kcal/mol; while, in MHBDI, it becomes essentially barrierless. Moreover, we have found two main S₁ excited-state radiationless channels. In the first one, the enol S₁ species decays to the S₀ state via the enol S₁/S₀ conical intersection after overcoming considerable barriers of 7.0 and 7.7 kcal/mol in OHIM and CHBDI (however, in MHBDI, it is nearly barrierless). In the second one, the keto S₁ species is first generated through the ESIPT event; then, it is de-excited into the S₀ state in the vicinity of the keto S₁/S₀ conical intersection. These energetically allowed excited-state decay channels rationalize experimentally observed ultralow fluorescence quantum yields. The insights gained from the present work may help to guide the design of new ortho-hydroxyl GFP core chromophores with improved fluorescence emission and brightness.     

Red-Edge Excitation Study of Nonexponential Fluorescence Decay of Indole in Solution and in a Protein

Proteins are known to be heterogeneous systems with a hierarchy of internal motions. However, those properties are often ignored when the complex fluorescence decay of tryptophan residues is compared to model studies with indole derivatives in solution. Here two simple models are presented, which illustrate different aspects of protein organization: (1) Trp zwitterion in buffer exemplifies ground-state heterogeneity and (2) indole in water/glycerol mixture exemplifies excited-state reconfiguration of solvate. Both systems are known to produce nonexponential fluorescence decay, attributed to the existence of multiple species (rotamers) or to the effects of slow dipolar relaxation, for (1) and (2), respectively. In the latter case a substantial dependence of decay on the excitation wavelength is expected. Indeed such dependence is observed for indole in water/glycerol mixture but not for Trp zwitterion in buffer. Therefore, excitational dependence can be used as a criterion to distinguish effects of multiple conformations in the ground state from effects of excited state reactions on tryptophan decays in proteins. The example of the bee venom peptide melittin indicates that both phenomena are important for interpretation of heterogeneity of decay, and therefore, caution should be exercised when assigning individual decay components to conformational subspecies in proteins.     

Femtosecond to nanosecond relaxation time scales in electronically excited tetrakis(dimethylamino)ethylene: identification of the intermediates

In the TDMAE molecule (title molecule), the time evolution has been analyzed from the very initial excitation step down to a fluorescent state, over widely different time scales. Pump probe measurements have been performed at 3 different excitation wavelengths 400, 266 and 200 nm. The decay has been followed over the femtosecond and subnanosecond ranges with this method and the decay of the final charge transfer state has been detected by its fluorescence emission. This allows an overview of the complete decay mechanism. The initial relaxation pathway is interpreted in a similar way to ethylenic molecules, where the initial wavepacket is quickly trapped in a doubly excited state Z with charge transfer character. Then the Z state decays slowly (10-100 picoseconds) into the final state. In difference to monoalkenes the final stage of this evolution is a charge transfer state. The decay of transient Z state to the charge transfer state is a further assessment of the partial ionic character of the Z state. This type of molecule with low ionization potential can be viewed as a demonstrative example of the interrelation between the charge induced forces and the deformations in excited state reaction dynamics. Received 17 January 2001 and Received in final form 23 February 2001     

Time-resolved fluorescence of carbon nanotubes and its implication for radiative lifetimes

The temporal evolution of fluorescence from isolated single-wall carbon nanotubes (SWNTs) has been investigated using optical Kerr gating. The fluorescence emission is found to decay on a time scale of 10 ps. This fast relaxation arises from nonradiative processes, the existence of which explains the relatively low observed fluorescence efficiency in isolated SWNTs. From the measured decay rate and a determination of fluorescence quantum efficiency, we deduce a radiative lifetime of 110 ns.     

Multiphonon relaxation of rare earth ions in borate,phosphate, germanate and tellurite glasses

Non-radiative multiphonon relaxation rates were obtained for excited electronic states in borate, phosphate, germanate and tellurite glasses. The rates were calculated from the intensities of fluorescence in the visible range of the spectrum, the measured radiative transitions and the decay times of fluorescence. A functional dependence was found between the relaxation rates and the energy gaps of the rare earth ion. It was shown that by changing the glass host from the borate to tellurite matrix, an increase in visible fluorescence was achieved. This was especially notable in Er³⁺, where the increase of fluorescence from germanate to tellurite was by a factor of 15.     

l-Dityrosine: A time-resolved fluorescence investigation

We have investigated the time-resolved fluorescence properties ofll-dityrosine in aqueous solution. Typically, three exponential components were needed to fit the fluorescence pattern adequately, with pure decay terms for the low-intensity, high-energy state (_em = nm) but with a pronounced subnanosecond rise phase for the predominant red-edge fluorescence (_em > 380). Dual fluorescence behavior is indicative of an intramolecular precursorsuccessor pair, i.e., a consecutive intramolecular excited-state reaction. We suggest that this reaction is a torsional motion of the (deprotonated) monoanion along the biphenolic bond. Analysis of the fluorescence anisotropy decay of dityrosine yielded two rotational correlation times, the longer of which is associated with a negative preexponential term. The increase with time in the horizontally polarized component of the intensity decay is interpreted as the result of an electronic rearrangement in the excited state when the successor form of dityrosine is generated. Lifetime distributions of experimental data were probed by an unbiased exponential series method which uses a Tikhonov-type regularization function. The procedure revealed three well-separated groups of lifetimes, the short-lived ensemble forming a formally negative distribution. A photophysical model is introduced which interprets the biexponential decay of dityrosine in terms of overlapping emission signals from the precursor and the successor molecule.     

Dipolar Relaxation Times of Tryptophan and Tyrosine in Glycerol and in Proteins: A Direct Evaluation from Their Fluorescence Decays

The dipolar relaxation process induced around tryptophan, indole and tyrosine in viscous media, as well as in several single tryptophan-containing proteins (staphylococcal nuclease, ribonuclease T1, melittin and albumin), has been studied by dynamic fluorescence measurements. A new theoretical model has been developed, including the relaxation dynamics directly in the fluorescence decay function. The phase shift and demodulation data have been fitted with this new algorithm which allows to resolve the different relaxation times influencing the fluorophore excited state. These parameters are in a good agreement with those measured with the traditional time-resolved emission spectroscopy. The results indicate that indeed a correlation exists between the radiative rate change obtained with the new model and the temporal spectral shift reported in the literature. Finally, this new approach has also been extended to the case of superoxide dismutase and phosphofructokinase, allowing to measure the relaxation time even in proteins lacking a temporal spectral shift during the fluorphore's lifetime.     

Visible fluorescence induced by the metal semiconductor transition in composites of carbon nanotubes with noble metal nanoparticles

We show that single-walled carbon nanotube (SWNT) bundles emit visible fluorescence in the presence of noble metal nanoparticles and nanorods in the solid state. Conductivity measurements with metallic nanotubes, isolated from pristine SWNTs, show that they become semiconducting in the presence of the metal nanoparticles. Nanoparticle binding increases the defects in the nanotube structures which is evident in the Raman spectra. The metal-semiconductor transition removes the nonradiative decay channels of the excited states enabling visible fluorescence. Nanotube structures are imaged using this emission with resolution below the classical limits.