© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Yunlong Liu, Chunfeng Zhang,* Huichao Zhang, Rui Wang, Zheng Hua, Xiaoyong Wang, Jiayu Zhang,* and Min Xiao* Broadband Optical Non-linearity Induced by Charge- Transfer Excitons in Type-II CdSe/ZnTe Nanocrystals Y. Liu, Prof. C. Zhang, R. Wang, Z. Hua, Prof. X. Wang National Laboratory of Solid State Microstructures School of Physics School of Engineering and Applied Science Nanjing University, Nanjing 210093, China E-mail: cfzhang@nju.edu.cn H. Zhang, Prof. J. Zhang Advanced Photonic Center Southeast University Nanjing 210096, China E-mail: jyzhang@seu.edu.cn Prof. M. Xiao National Laboratory of Solid State Microstructures School of Physics, School of Engineering and Applied Science Nanjing University Nanjing 210093, China Department of Physics University of Arkansas Fayetteville, Arkansas 72701, USA E-mail: mxiao@uark.edu DOI: 10.1002/adma.201301559 Advances in material science have enabled the tailoring of the physical properties of colloidal core–shell nanocrystals (NCs) with band alignment of the constituent semiconductors, [1–11] providing a myriad of opportunities for devising optoelectronic devices, including light-emitting diodes, [12–17] lasers, [4] solar cells, [18–22] transistors, [23] and sensors. [24,25] Type-II NCs are for- matted with a staggered line-up between the energy bands of the core–shell components, where the lowest energy excitonic states are achieved with charges transferred through the inter- face. [8,9,26–28] This formation of excitonic charge transfer (CT) states is valuable for light amplification [4] and solar conver- sion [18–20] since carrier recombination slows down significantly in such a band configuration. The excitonic effect has been regarded as a major factor that governs the optical properties in semiconductor NCs. The spa- tially separated excitons in type-II NCs are very different from the strongly-confined excitons in type-I NCs. [6,29] Recently, a rapidly growing interest has been drawn by the emergent prop- erties resulting from the charge separation in type-II NCs. [8,9] Besides the subject of intensively-studied linear optical proper- ties, the excitonic effect is also a primary source for the non- linear optical (NLO) properties in semiconductor NCs. Efficient NLO responses are of great significance for applications such as in-optical switching, [30–32] up-converted lasers, [33–35] and two- photon microscopy. [36] Optical non-linearity in type-I NCs has been well studied in the past few years. [37–47] The third-order NLO susceptibility ( χ (3) ) resonant to the excitonic states can be orders of magnitude larger than the off-resonant value. [37] In type-II NCs, the CT excitonic transition is determined by the band alignment rather than the size control. The resonant optical non-linearity may cover an ultrabroad spectral range with photon energy below the bandgaps of the constituent sem- iconductors. Such a broad spectral coverage can be achieved without reducing the quantum size confinement which is not realizable by simply changing the sizes of type-I NCs. The broadband NLO response in type-II NCs is of importance tech- nically, but, however, remains poorly understood. In this work, we report the characterization of third-order optical non-linearity induced by CT excitonic transitions in type-II CdSe/ZnTe NCs. With the technique of femtosecond (fs) degenerate four-wave mixing (FWM), we perform a com- parative study to investigate the excitonic effect on the NLO response in NCs. The third-order NLO susceptibility χ (3) is observed with different spectral dispersions in type-I and type- II NCs. Resulting from the CT excitonic transitions, χ (3) in type-II NCs is significantly enhanced when the incident photon energy lies below the bandgaps of the constituent semiconduc- tors. The relationship between the optical non-linearity and the depopulation dynamics of the excitonic CT states is elucidated with time-resolved (TR) measurements on the optical Kerr effect (OKE) and differential transmission, indicating the state- filling effect as a primary origin of the resonant NLO response. This work suggests that the band alignment can be used as an efficient strategy to manipulate the optical non-linearity in sem- iconductor NCs for broadband NLO photonic devices. We synthesize CdSe/ZnTe core–shell NCs as a model system to study the NLO properties in type-II NCs. The sample con- sists of CdSe cores with an average radius of ≈1.7 nm and ZnTe shells with an average thickness of ≈1 nm. As characterized by transmission electron microscopy (TEM), the NCs show con- siderably uniform distribution in shape and size Figure 1a. The good crystal quality of the NCs is evidenced by the clear fea- ture of lattice structures in the TEM images. A type-I sample of CdSe/CdS NCs with same core size and similar shell thickness is employed for comparison. The sample of NCs is spun-cast onto glass substrates for the optical studies. These samples are synthesized by standard procedures, as reported elsewhere. [1,48] Details of the sample preparation are given in the Experimental Section. To identify the excitonic CT states, we compare the absorp- tion/emission spectra of the type-I and type-II NCs in solution in Figure 1b. The sharp peak at ≈615 nm in the absorption spectrum of the type-I NCs is associated with the lS excitonic Adv. Mater. 2013, DOI: 10.1002/adma.201301559