不同极性溶液中MEH-PPV链内激子扩散的研究（英文）

本文采用飞秒时间分辨瞬态光栅技术研究了MEH-PPV在不同极性溶液中的内激子扩散行为.采用宽带连续白光同时探测瞬态吸收和瞬态光栅信息.系统地研究了MEH-PPV的振动失相行为以及单激子弛豫和粒子数弛豫动力学.结果表明,在极性较大的溶剂中,样品溶液的弛豫扩散过程会加快.     

CdTe量子点的室温激子自旋弛豫动力学

利用交叉偏振三阶非线性瞬态光栅技术,研究了室温下CdTe胶体量子点激子自旋弛豫动力学的尺寸效应.在抽运-探测光子能量与CdTe量子点的最低激子吸收(1Se—1Sh)跃迁相共振时,量子点激子自旋弛豫显示了时间常数为0.1—0.5 ps的单指数衰减行为.CdTe量子点激子自旋的快速弛豫源于亮暗激子精细结构态跃迁,即J=±1←→■2跃迁.激子自旋弛豫主要由空穴的自旋翻转过程决定.研究结果表明:CdTe量子点激子自旋弛豫速率与量子点尺寸的4次方成反比.     

瞬态自旋光栅系统的建设及其在自旋输运研究中的应用

简要介绍了瞬态光栅系统原理及光路的建设,包括瞬态光栅的产生与探测.采用了外差探测法（heterodyne detection）,大大提高了信噪比.利用瞬态自旋光栅系统,研究了(110)方向生长的本征GaAs/AlGaAs单量子阱中自旋输运特性,测得室温下电子自旋的扩散常数D_s=551 cm/s. 关键词： 瞬态自旋光栅 自旋扩散 自旋输运 自旋弛豫     

激子旋转弛豫对低掺杂卟啉侧链聚合物荧光衰变过程的影响

利用激子旋转扩散理论研究了一类低掺杂卟啉侧链聚合物中卟啉侧链基团的旋转对其发光动力学过程的影响.研究表明,卟啉侧链基团的旋转行为是导致激发态无辐射能量弛豫的重要途径.基团旋转越容易,能量弛豫速度越快,这可导致一个快速的荧光衰变动力学过程.在卟啉低掺杂浓度和聚合物分子链间距离较大的情况下,卟啉侧链基团的旋转成为影响荧光寿命和发光效率的主要因素.对实验测得的两种样品的荧光弛豫过程进行了拟合,理论结果与实验结果符合较好. 关键词： 激子旋转弛豫 瞬态荧光 卟啉侧链聚合物     

聚(9,9-二辛基芴)和聚(9,9-二辛基芴-共-苯并噻二唑)的光物理特性的理论和实验研究

通过比较分析聚(9,9-二辛基芴)(PFO)和聚(9,9-二辛基芴-共-苯并噻二唑)(F8BT),对半导体聚合物的光物理特性进行了系统研究. 量子化学计算显示,苯并噻二唑单元的引入促进了链内电荷转移(ICT),调节了聚合物的电子跃迁机制. 瞬态吸收测定表明,在单分散系统中的受激PFO衰减时主要表现为链内激子弛豫.在F8BT溶液中,ICT状态出现,并参与到激发态的弛豫过程中. 凝聚相中PFO和F8BT的弛豫过程加速和显示了在高激发强度下具有显著的激子湮灭行为. 在相同的激发强度下,F8BT的平均寿命长于PFO,有助于实现良好的电荷离域.     

瞬态荧光方法研究溶剂诱导的三苯胺衍生物的对称性破缺电荷转移动力学

在缺乏特征红外振动的情况下追踪具有四极或八极对称性分子的激发态对称性破缺电荷转移通常是很困难的.本文以一种具有八极对称性的三苯胺衍生物为研究对象,利用飞秒时间分辨瞬态荧光光谱方法获得发光跃迁偶极矩的演化动力学,进而实时表征了其溶剂诱导对称性破缺电荷转移的动力学过程.当该分子处于弱极性甲苯溶液中时,在激发态弛豫过程中其发射偶极矩变化较小;当处于较强极性的四氢呋喃溶液中时,其发射偶极矩在数皮秒内快速减小.在对比单体偶极分子的荧光动力学后,推断八极分子的发光态在强极性溶剂中经历溶剂诱导的结构变化,由激子耦合的八极对称性降低至激发定域的偶极对称性;而在较弱极性的溶剂中,其八极对称性在溶剂化稳定中得以较大程度的保持.     

链构象对稀溶液中MEH-PPV的光致发光的影响

聚(2-甲氧基-5-(2’-乙基己氧基)-1,4-对苯乙炔)(MEH-PPV)在溶液中的链构象依赖于溶剂的性质,共轭聚合物的发光特性受链构象影响明显。在稀溶液中,不良溶剂含量的增加使MEH-PPV分子链更加紧缩卷曲,单个分子链内更多共轭链段发生聚集,光激发形成的链间激子增加。通过对MEH-PPV稀溶液的光谱分析,发现链间激子的形成过程依赖于激发光的能量(hv)。在激发光能量大于聚合物的最低激发能(E4)的情况下,大部分链内激子通过辐射复合发光,少部分链内激子沿分子链转移到聚集的共轭链段上形成链间激子。在hv＜Ea的情况下,单个分子链内聚集共轭链段吸收光子能量形成链问激子并复合发光。     

基于三维简并四波混频的PDT光敏剂HA瞬态光栅特性研究

竹红菌甲素(hypocrellin A,HA)具有丰富的激发态特性,有可能在新型光动力疗法(PDT)光敏剂、激光染料和新型光电器件方面有一定的应用前景。利用瞬态光栅技术研究了竹红菌甲素分别在不同极性和粘度溶剂中以及溶剂本身的瞬态光栅特性,把其瞬态光栅的超快过程归结为竹红菌甲素质子转移形成的过渡态TS*的衰减,得出过渡态TS*瞬态光栅的寿命为10.5 ps。竹红菌甲素分子的质子转移过渡态TS*瞬态光栅寿命不受溶剂极性和粘度的影响。     

神经网络法研究多溴代二苯胺热力学性质

通过时间分辨光谱技术详细地研究了ZnCuInS/ZnSe/ZnS量子点的激子弛豫动力学. 基于速率分布模型,波长依赖的发射动力学表明本征激子、界面缺陷态中的激子、给-受体对态中的激子都会参与量子点的发光过程,整个发光过程主要依赖于给-受体对态发射. 瞬态吸收数据表明激发后本征激子和界面缺陷物种可能会同时出现,在高激发光强下,光强依赖的俄歇复合过程也存在于量子点中     

偏置电场对聚对苯乙烯激发态弛豫特性的影响

为探讨洞悉电场对有机发光二极管电致荧光量子效率的影响,通过激发-探测超快光谱技术研究了激子在电场下的瞬态行为.与单重态激子相应的激发态在230 μJ/cm2激发强度下,显示了快慢两个弛豫过程. 快慢组分的权重因子及快组分弛豫时间常数是电场相关的, 在6.4×10⁵ V/cm的电场下,与无偏置电场相比,激子的快组分弛豫时间加速,快组分的权重因子由22%增加为72%,约50%的初始激子又通过电场而离解. 慢组分是电场无关的,其弛豫时间常数为890 ps. 实验结果还揭示了由激发光所产生的长程声学声子,其声速为17 /ps. 关键词： 聚对苯乙烯 超快光谱 激子 有机发光二极管     

Transient grating studies of phase and spin relaxations of excitons in GaAs single quantum wells

Exciton spin and phase relaxations at low temperatures in GaAs/AlGaAs single quantum wells were investigated by using transient grating technique. The technique allows us to obtain the exciton lifetime, spin relaxation, and phase relaxation in the same setup. In combination with a series of single quantum wells grown on the same substrate, the well width dependence of exciton spin relaxation was studied. The obtained spin relaxation results were analyzed with their phase relaxation times in a framework of MAS mechanism, and were in good agreement with the calculated results. Especially, the motional narrowing character of the exciton spin relaxation was well demonstrated.     

Magnetic Field-Effect on Photoluminescence of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) Nanoparticles in a Poly[vinyl butyral] Matrix

Abstract

In this work the magnetic field effects (MFE) on the photoluminescence of MEH-PPV film and nanoparticles were obtained using the MFE technique with continuous-wave photoexcitation. The analysis of the MFE dependence for the MEH-PPV film gave a HWHM (half-width at half maximum) ～ 65?mT and a total MFE value about 102.8% at 200?mT whereas the MFE dependence of the MEH-PPV nanoparticles showed significant broadening, with their HWHM ～ 280?mT and a total MFE value of 99% at 550?mT. By a simple theoretical model based on rate equations, the mechanism of the negative magnetic field effect was explained. The observed negative effect was the result of the dominant singlet exciton formation rate in the MEH-PPV nanoparticles. For explaining this negative effect the enhanced triplet-triplet annihilation due to confinement of triplet excitons in the nanoparticles and singlet molecular oxygen-triplet exciton annihilation were proposed.     

Anomalous energy transfer dynamics due to torsional relaxation in a conjugated polymer

In isolated conjugated polymers two explanations are in discussion for the redshift of the emission on a picosecond time scale-exciton energy transfer (EET) between conjugated segments along the chains and conformational changes of these segments themselves, i.e., torsional relaxation. In order to resolve this question we perform femtosecond time-resolved transient absorption measurements of the energy relaxation of poly[3-(2,5-dioctylphenyl)thiophene] in toluene solution. We show that torsional relaxation can be distinguished from EET by site-selectively exciting low-energy conjugated segments. We present a unified model that integrates EET and torsional dynamics. In particular, comparison to ultrafast depolarization measurements shows that torsional dynamics cannot be neglected when analyzing EET dynamics and furthermore reveals that the exciton extends itself by about 2 monomer units during torsional relaxation.     

Exciton relaxation in Ga1 − xInxAs/GaAs self-organized quantum dots

The exciton dynamics in Ga_1 − xIn_xAs/GaAs self-organized quantum dots grown on GaAs (111)B substrates are studied by the time-resolved photoluminescence (PL). We have found the intra-dot exciton relaxation by the reduction of the linewidth and peak energy and also by the energy-dependent PL rise time in the transient PL spectra. Compared with the energy relaxation in the reference quantum wells, we have confirmed that the exciton relaxation in three-dimensionally confined quantum dots is slower than in the quantum wells.     

Effects of excitation spectral width on decay profile of weakly confined excitons

We report the effect due to a simultaneous excitation of several exciton states on the radiative decay profiles on the basis of the nonlocal response of weakly confined excitons in GaAs thin films. In the case of excitation of single exciton state, the transient grating signal has two decay components. The fast decay component comes from nonlocal response, and the long-lived component is attributed to free exciton decay. With an increase of excitation spectral width, the nonlocal component becomes small in comparison with the long-lived component, and disappears under irradiation of a femtosecond-pulse laser with broader spectral width. The transient grating spectra clearly indicates the contribution of the weakly confined excitons to the signal, and the exciton line width hardly changes by excitation spectral width. From these results, we concluded that the change of decay profile is attributed not to the many-body effect but to the effect of simultaneous excitation of several exciton states.     

Time-resolved luminescence quenching in thin films of perylene-tetracarboxylic-dianhydride

We present luminescence quenching experiments and determine the exciton diffusion length in polycrystalline thin films of PTCDA (perylene-3,4,9,10-tetracarboxylic-dianhydride). From an analysis of time-resolved experiments, we can distinguish between exciton transport during an ultra-fast initial relaxation phase and transport in the long-living emitting states. The temperature dependence of the exciton diffusion constant in the emitting states indicates thermally activated hopping.     

MEH-PPV/CdSe纳米复合器件的光电导特性的研究

以CdO和Se粉作为前驱物,在TOPO/TOP有机体系中制备了CdSe纳米晶,将其与聚合物MEH-PPV复合制备了复合光电导器件,研究了它的光电导特性,并将其与单层MEH-PPV光电导器件的特性进行了比较。结果发现纳米复合光电导器件的光电流响应光谱的2个峰的位置基本上与MEH-PPV和CdSe纳米晶的吸收峰的位置相对应,这说明CdSe纳米晶和聚合物MEH-PPV的吸收对光电流都有贡献,主要是由于CdSe纳米晶和MEH-PPV界面处的激子离化和电荷转移造成的。而且复合器件的光电流较单层有所增强,且MEH-PPV器件光谱的响应范围更宽。     

CdTe/CdS核壳结构量子点超快载流子动力学

在水相合成CdTe以及CdTe/CdS核壳结构量子点基础上, 利用基于抽运-探测技术的瞬态差分透射技术研究了CdTe量子点以及不同CdS壳层厚度的CdTe/CdS量子点的最低激子能态的超快激发与弛豫动力学. 研究表明:相比于CdTe,CdTe/CdS量子点的电子空穴由于空间分离,其所需的激发时间要长于电子空穴空间重叠态所需要的激发时间.随着壳层厚度的增加, 量子点表面的钝化有效地减少了表面态相关弛豫机理,并延长相对应的弛豫时间.     

DMSO溶剂中All-trans-astaxanthin激发态的飞秒时间分辨光谱

基于飞秒时间分辨瞬态吸收和多元瞬态光栅光谱技术对全反式Astaxanthin(AXT)在DMSO溶剂中的超快激发态弛豫动力学进行了观测.结果表明,光激发后AXT/DMSO体系直接发生S_0→S_2跃迁,基态漂白对应光谱范围为420~550nm.由S_2→S_1的内转换过程发生的时间常数为120~160fs.S_1态激发态吸收对应的光谱范围为550~740nm,基态漂白恢复过程对应的是S_1→S_0的内转换过程,其时间尺度为4.50~5.50ps.     

CdSeS合金结构量子点的多激子俄歇复合过程

多激子效应通常是指吸收单个光子产生多个激子的过程,该效应不仅可以为研究基于量子点的太阳能电池开拓新思路,还可以为提高太阳能电池的光电转换效率提供新方法.但是,超快多激子产生和复合机制尚不明确.这里以CdSeS合金结构量子点为研究对象,研究了其多激子生成和复合动力学.稳态吸收光谱显示, 510, 468和430 nm附近的稳态吸收峰,分别对应1S_(3/2)(h)-1S(e)(或1S), 2S_(3/2)(h)-1S(e)(或2S)和1P_P(3/2)(h)-1P(e)(或1P)激子的吸收带.通过飞秒时间分辨瞬态吸收光谱和纳秒时间分辨荧光光谱两种时间分辨光谱技术对CdSeS合金结构量子点的超快动力学进行了探究,结果显示, 1S激子的双激子复合时间大概是80 ps,这一时间比传统量子点的双激子复合时间(小于50 ps)延长了近一倍,结合最近发展的超快界面电荷分离技术,在激子湮灭之前将其利用起来,这一时间的延长将有很大的应用前景;其中,在2S和1P激子中除上述双激子复合外,还存在一个通过声子耦合路径的空穴弛豫过程,时间大概是5—6 ps.最后,利用纳秒时间分辨荧光光谱得到该样品体系单激子复合的时间约为200 ns.