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Shell Thickness Dependent Bi-Exciton Lifetime in Type I and Quasi-Type II CdSe@CdS Core/Shell Quantum Dots Degui Kong, Yanyan Jia, Yueping Ren, Zhaoxiong Xie, Kaifeng Wu, and Tianquan Lian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01234 • Publication Date (Web): 25 Mar 2018 Downloaded from http://pubs.acs.org on March 25, 2018
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Shell Thickness Dependent Bi-exciton Lifetime in Type I and Quasi-type II CdSe@CdS Core/Shell Quantum Dots Degui Kong1,2#, Yanyan Jia3#, Yueping Ren4, Zhaoxiong Xie3, Kaifeng Wu2*, and Tianquan Lian5* 1
College of Electronic Engineering, Heilongjiang University, Harbin 150080, China
2
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China 3
State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of
Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China 4
School of Environment and Civil Engineering, Jiangnan University, Wuxi 214122, China
5
Department of Chemistry, Emory University,1515 Dickey Drive, NE, Atlanta, Georgia 30322, United States
# These authors contribute equally to this work. *Correspondence:
[email protected],
[email protected] Abstract Suppression of Auger recombination in colloidal quantum dots (QDs) is important for their many applications, ranging from biological tagging, QD lasing, to solar energy conversion. Although it has been reported that the biexciton Auger recombination time of core/shell QDs can be significantly prolonged compared to core only QDs, a systematic investigation of their dependence on the shell thickness is lacking. In this work, using CdSe@CdS core/shell QDs as a model system, we investigated the shell thickness dependence of bi-exciton lifetimes in both type I and quasitype II QDs, prepared using large and small core sizes, respectively. We observe a strong increase of bi-exciton lifetime with the shell thickness and a larger saturation volume in quasi-type II CdSe@CdS QDs, compared to type I CdSe@CdS QDs. These trends can be attributed to the different thickness dependences of electron-hole wave function overlaps in these materials, which reflect their different extent of conduction band electron delocalization. Our findings provide further insight for rational design of core/shell QDs with suppressed Auger recombination rates.
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Introduction Compared with conventional light harvesting materials, colloidal semiconductor nanocrystals or Quantum Dots (QDs) possess many attractive properties,1 including size-tunable optical absorption,2 high photoluminescence (PL) quantum yields (QYs),3-5 excellent photo-stability,6 and flexible chemical processability.7 Another significant advantage of QDs over molecular chromophores is their capability to accommodate multiple excitons (electron-hole pairs) in a single QD.8-10 Multiple excitons can be generated either through multiple photon absorption or through multiple exciton generation (MEG) in which a high energy photon creates two or more excitons.811
Although the efficiency of MEG in QDs has been under debate,12-15 it has indeed been observed
in photo detector and solar cell deives,16-18 providing an exciting opportunity to exceed ShockleyQueisser limit in power conversion efficiency of single-junction solar cells.19-20 Even in the absence of MEG, accommodation of multiple excitons in a single QD is important for many applications, such as solar-to-fuel conversion which involves multiple electron transfer reactions,21-22 and QD lasing where multiple excitons are required to achieve population inversion.10, 23 A complication or challenge associated with utilization of multiple excitons is the ultrafast decay of multiple exciton states through Auger recombination (AR) where an electronhole pair nonradiatively recombines through excitation of a third particle (electron or hole).8, 10 In CdSe QDs for example, the lowest multiple exciton states, i.e. bi-exciton states, typically have AR lifetimes on the order of several to hundreds of ps,8 competing with carrier transfer or radiative recombination processes.24-28 In addition to the situations where multiple excitons are deliberately created for certain applications, QDs are often unintentionally charged,29-30 for which reason performances of QD based solar cells and LEDs can be compromised by AR.29-30 In addition, AR is generally considered to be responsible for single QD PL blinking,31-32 hindering their reliable applications in biological tagging and imaging. Therefore, suppressed AR in colloidal QDs is important for their various applications. Currently well-established colloidal synthesis allows sophisticated control over QD size, shape, and composition to suppress AR.33,34 In terms of size control, bi-exciton lifetime has been demonstrated to universally scale with QD volume.8, 35 However, using this strategy, the extent to which bi-exciton lifetime can be prolonged is limited because further increasing of volume diminishes quantum confinement effect. With similar quantum confinement energy, onedimensional quantum rods were shown to have longer bi-exciton lifetime compared with zero2
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dimensional QDs.36,37 But the fast exciton diffusion along rod axis would lead to weak dependence of bi-exciton lifetime on rod length.38 In contrast, core/shell QDs provide a versatile approach for AR suppression by tuning the coulomb interaction between electrons and holes through wave function engineering.39-41 Specifically, electron and hole wavefunctions are both mostly confined in the core in type I QDs3, 42 and are (partially) spatially separated in (quasi-)type II QDs.4-5, 24, 4344
The electron and hole wavefunction distributions in different types of core/shell QDs should
lead to significantly different dependence of bi-exciton lifetime on shell thickness. Based on a simple argument that bi-exciton lifetime inversely varies with electron-hole overlap,34 bi-exciton lifetime should increase with shell thickness in (quasi-) type II QDs because of gradually decreasing electron-hole overlap, which has indeed been observed in type-II CdTe@CdSe QDs,34 whereas it should show little dependence on shell thickness in type I QDs. To our best knowledge, rational control of shell thickness dependence of bi-exciton lifetime by band alignment (electronic structure) has not been demonstrated yet. In this study, we systematically investigate the shell thickness dependence of bi-exciton lifetime in both type I and quasi-type II CdSe@CdS QDs using pump-probe transient absorption (TA) spectroscopy. Due to the large valence band (VB) offset (>0.45 eV) and small conduction band (CB) offset (
