王晓勇南京大学物理学院固体微结构物理国家重点实验室

半导体量子点的多激子产生与能量转移 王晓勇南京大学物理学院固体微结构物理国家重点实验室 2012年12月29日基金委“单量子态探测及其相互作用”年度交流会议

内容 • 研究内容：半导体纳米材料的单光子发射与相干光学特性 • 单光子发射（共振能量转移方法） • 1、半导体纳米材料的多激子产生 • 2、半导体纳米材料的多激子存储 • 3、半导体纳米材料的双光子发射 • 相干光学特性 • 1、大尺寸单胶体量子点的光学特性研究 • 2、光学微腔中单胶体量子点的共振激发 • 总结

多激子产生（MulitpleExcitonGeneration-MEG） 多激子产生（MultipleExcitonGeneration More than one exciton generated upon absorption of a high energy photon (hν>2Eg) Auger 复合 Energy of multiple excitons transferred to lower-order excitons (tens to hundreds of picosecond) Schaller and Klimov, PRL 2004 False MEG signal • The existence of MEG effect has to be detected by ultrafast pump-probe or transient fluorescence measurements. • Charged excitons formed under high-energy photon excitation lead to a false MEG signal. New methods are needed to confirm the MEG effect!

Forster共振能量转移 (ForsterResonantEnergyTransfer - FRET） SE: single exciton BE: biexciton CB: conduction band VB: valence band Rad.: radiative decay When the same number of high-energy (hν>2Eg) photons is absorbed, the generation of biexcitons from the CM effect will cause enhanced dye fluorescence.

Quantum Dots-Organic Dyes FRET Couples Donor: CdSe nanocrystals (NCs) Qdot 525, 605, 655, 705 from Invitrogen Acceptor: Organic dyes molecules Superfluor 680 and 750 dyes same as the Alexa Fluor series from Invitrogen • The dyes cannot be directly excited at the wavelength range of 266-510 nm. • This FRET material system was used to study the MEG effect of NCs and the single photon fluorescence quantum yield of organic dyes.

Biexciton-Enhanced Dye Fluorescence a, PL spectra of the Qdot 525 NC-dye mixed sample excited at 266 nm (red, ~1.97Eg) and 488 nm (black, ~1.08Eg). Inset: PL spectra of the Qdot 655 NC-dye mixed sample excited at 266 nm (red, ~2.46Eg) and 488 nm (black, ~1.34Eg). b, Two sets of the NC and dye PL spectra decomposed by Gaussian fits from the two PL spectra shown in the inset of a. C/(A+B) is the fractional number of biexciton transferred to the dyes for each single exciton generated in the donor NCs.

Biexciton Generation Efficiency (BGE) BGE: the fractional biexciton number divided by τ1/(τ1 + τ2), where τ1 = ~1.7 ns and τ2 = ~320 ps are the biexciton Auger and FRET lifetimes, respectively. An average BGE of ~17.1% obtained after a statistically large number of measurements. TCSPC Transient absorption “Carrier multiplicationin semiconductor nanocrystals detected by energy transfer to organic dyemolecules”, JunXiao,YingWang,ZhengHua,XiaoyongWang*,ChunfengZhang,andMinXiao*,(*corresponding authors) Nature Communications 3, 1170(2012).

半导体纳米材料的超长时间多激子存储 胶体量子点与染料的能量转移体系不适合多激子存储：染料的~1ns荧光寿命与光漂白 • CdSe量子点中存在表面缺陷发光（量子点的能量转移）； • 该缺陷发光在Plasmonic作用下得到极大增强； • 实验中观察到单、双、三激子从量子点到缺陷态的转移； • 多激子的存储时间从几百ps延长到几百ns。缺陷发光量子点发光 CdSe量子点及其缺陷发光随激发功率的变化情况 “Extended storage of multiple excitons in the trap states of semiconductor nanocrystals”, QinfengXuet al.,manuscript to be submitted. （与浙大吴惠帧课题组合作）

Single NC-Dyes FRET couples single NC NC filter Dye filter dye film Single NCs Dye Film

利用FRET实现双光子对发射 • 首次实现单量子点和染料分子的相关光子对发射 • Anti-bunching测量； • Time-Tagged Time-Resolved (TTTR)测量； • 单光子吸收荧光量子产率测量。正在进行工作：

内容 • 研究内容：半导体纳米材料的单光子发射与相干光学特性 • 单光子发射 • 1、利用共振能量转移方法研究半导体纳米材料的多激子产生 • 2、利用共振能量转移方法实现半导体纳米材料的多激子存储 • 3、利用共振能量转移方法实现半导体纳米材料的双光子发射 • 4、利用共振能量转移方法实现半导体纳米材料的双光子发射 • 相干光学特性 • 1、超大尺寸单胶体量子点的光学特性研究 • 2、平面光学微腔中单胶体量子点的共振激发 • 总结

单个胶体量子点的相干光学特性测量 QuantumInterference，RabiOscillations,MollowFluorescence,etc. PL spectral image of single InGaAs/GaAs QDs Position 916 nm 921 nm 926 nm Wavelength (nm) 单胶体量子点相干光学测量的不利因素：SpectralDiffusionand Fluorescence Blinking

超大尺寸CdSe/CdS胶体量子点 超大尺寸CdSe/CdS量子点TEM Auger过程得到抑制原理图 • FluorescenceBlinking过程得到极大抑制； • SpectralDiffusion过程几乎消失； • 观察到几乎100%的双激子荧光量子产率。 Mahler et al., Nature Mater. 2008 Hermier et al., PRL 2009 Htoon et al., PRL 2011 Htoon et al., Nature 2011 Malko et al., Nano Letters 2012 Galland et al., Nature Commun. 2012 与LosAlamos的HanHtoon课题组以及东南大学张家雨课题组合作进行单个CdSe/CdS量子点的相干光学特性研究

单量子点的共振激发：平面微腔与光纤光学 共振激发示意图与设备图 A、微腔共振荧光测量设备的设计、订购与调试已经完成； B、用于共振激发的自组织InGaAs量子点平面光学微腔已经制备，中科院半导体所马文全研究员； C、用于共振激发的胶体CdSe量子点平面光学微腔已基本实现，东南大学张家雨教授； D、正在自主开发基于TaperedFiber的共振光学激发手段。

总结 • “Carrier nanocrystals detected by energy transfer to organic dye molecules”, • Nature Communications 3, 1170 (2012). • “Indirect optical transitions in hybrid spheres with alternating layers of titania and graphene oxide nanosheets”, Optics Express 20, 28801 (2012). • “Photoluminescence blinking and bleaching of single semiconductor nanocrystals”, manuscript to be submitted. • “Single photon quantum yield of dye fluorescence”, manuscript in preparation. • “Unified photoluminescence blinking mechanism for single semiconductor quantum dots”, manuscript in preparation. • “Missing evidence for the quantum confinement effect in single graphene nanoparticles”, manuscript to be submitted. • “Ultrafast fluorescence of semiconducting single-walled carbon nanotubes utilizing the exciton-exciton annihilation effect’’, manuscript to be submitted. • “Extended storage of multiple excitons in the trap states of semiconductor nanocrystals”,manuscript to be submitted.

王晓勇 南京大学物理学院 固体微结构物理国家重点实验室