Article pubs.acs.org/JPCC
Exploring Ultrafast Electronic Processes of Quasi-Type II Nanocrystals by Two-Dimensional Electronic Spectroscopy Yoichi Kobayashi,† Chi-Hung Chuang,‡ Clemens Burda,‡ and Gregory D. Scholes*,† †
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada Center for Chemical Dynamics and Nanomaterials Research, Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States
‡
S Supporting Information *
ABSTRACT: Colloidal CdTe/CdSe heteronanostructures are model systems for quasitype II nanocrystals (NCs) and have been examined extensively. However, the complex spectra in these heteronanostructures often make it difficult to reveal details of their optical properties by conventional techniques such as transient absorption spectroscopy. In the present study, two-dimensional electronic spectroscopy (2DES) is used to study colloidal CdTe, CdTe/CdSe, and CdTe/ZnS NCs revealing the nature of absorption bands and ultrafast dynamics in a quasi-type II system. We observe the electronic coupling between the lowest two transitions, oscillations in the population time due to the longitudinal optical (LO) phonon mode, and the high-frequency impulsive Raman modes of the solvent. We observed an excited state absorption near at the band edge only in CdTe/ CdSe NCs and established that it is related to the quasi-type II features: the redistribution of excitons among the fine-structured states or the biexciton level shift at the ultrafast time scale.
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INTRODUCTION
these questions, the analysis of the CT state and the electron delocalization in quasi-type II NCs is highly important. The excitonic relaxation processes in semiconductor NCs have been extensively studied by pump−probe spectroscopy.1,36−41 Especially state-resolved pump−probe spectroscopic studies have contributed to reveal various processes of semiconductor NCs such as hot carrier cooling, biexcitonic interaction, charge carrier trapping to the surface, and exciton− phonon coupling.37−43 On the other hand, the CT processes in type II NCs and surface trappings have been examined by analyses of wide-range emission decays and temperature dependences of steady-state emission spectra.44−49 Jones et al. have quantitatively revealed the complicated exciton trapping and recombination dynamics of heteronanostructures by wide-range emission decay measurements.44−46 They revealed that carrier trapping dynamics are explained with classical Marcus theory. Recently, the Kambhampati group has demonstrated that the semiclassical Marcus−Jortner electron transfer approach can fully explain the temperature dependence of the broad emission from deep surface traps.47−49 This model includes the tunneling electron transfer processes between the ground level in the initial potential curve and the higher vibrational level in the final potential curve.50,51 The above results suggested that higher vibrational modes are important for the carrier trapping processes, which are consistent with recent theoretical reports with ab initio time-domain
Colloidal semiconductor nanocrystals (NCs) have been studied extensively because of their widely tunable optical properties,1,2 multiexciton generation,3,4 hot carrier transfer,5 applications in solar cells,6 spintronics,7 and gain materials for lasers.8 Recent synthetic techniques allow us to extend these properties by combining two (or more) semiconductor materials in one single domain and then resulting in heterostructures. By tuning components, sizes, and shapes, we can control the spatial distribution of electron and hole wave functions. Type II heteronanocrystals, which have staggered band alignment,9,10 spatially separate electron and hole wave functions yielding charge transfer states, and therefore, it is promising for photovoltaics11 and broadband nonlinear optics applications.12 There have been many studies focused on the optical properties and photoinduced dynamics in type II heterostructures.9,10,13−33 In the case of CdTe/CdSe core/shell heteronanostructures, the electron wave function becomes more delocalized in the CdSe as the shell thickness increases, and eventually the electron is localized in the CdSe shell, i.e., CdTe/CdSe NCs transition from type I to type II.16,17,24,34 Scholes et al. have revealed that the electron transfer in type II CdTe/CdSe core/shell nanorods occurs in the Marcus inverted region,35 which means that electron transfer rates decrease with increasing energy separation between donor and acceptor states. Questions of interest include: When do charge transfer (CT) features appear as a function of the CdSe shell development? How does the relaxation rate from exciton to CT states correlate with the CdSe shell thickness? To answer © 2014 American Chemical Society
Received: May 8, 2014 Revised: July 2, 2014 Published: July 3, 2014 16255
dx.doi.org/10.1021/jp504559s | J. Phys. Chem. C 2014, 118, 16255−16263
The Journal of Physical Chemistry C
Article
approaches.52−56 However, to reveal further insight into the charge transfer processes and resolve overlapped spectra such as those in quasi-type II systems, an alternative technique to pump−probe spectroscopy would be necessary. Recently, two-dimensional electronic spectroscopy (2DES), which is a relatively newly developed nonlinear spectroscopy, has been shown to be very useful to reveal coherence, “coupling” between states, and pathways of population relaxation in various systems.57−89 Compared to time-resolved one-dimensional (1D) spectroscopy, such as pump−probe spectroscopy, 2DES reveals coherent and relaxation processes as a 2D map, in which the detection axis at a particular pump− probe time delay is obtained by frequency-resolving a signal field (like normal pump−probe spectroscopy), and the axis resolving excitation frequencies are obtained by Fourier transformation with respect to very precise timing between a pair of pump fields. 2DES resolves complex spectra and cross peaks in a 2D map help to visualize state-to-state interactions such as energy transfer and charge transfer and also indicate “coupling” between states (i.e., transitions involve common electronic orbitals). In recent years, 2DES has been used to reveal the coherent energy transfer, charge transfer, and the electronic coupling in light-harvesting proteins,57,63,66,67,70,72,73,75−77,81,82 conjugated polymers,65,83 epitaxial quantum wells, 69,74,79,89 and colloidal quantum dots.59−61,78 In colloidal NC studies, 2DES has shown biexcitonic fine structures,90,91 exciton superposition among the 1S, 2S, and 1P state (1S(e)−1S3/2(h), 1S(e)−2S3/2(h), and 1P(e)−1P3/2(h), respectively),61,78 and shape dependence of the ultrafast dynamics in CdSe NCs.92 In addition, multidimensional electronic spectroscopy, which is similar to 2DES, has represented surface trapping processes in PbSe NCs.60,93 In the present work, we used 2DES to analyze the electron delocalization in quasi-type II CdTe/CdSe NCs. CdTe and CdTe/ZnS NCs as bare cores and type I NCs, respectively, were examined for comparison. We observed the electronic coupling between the 1S and 2S states for CdTe NCs and |X1⟩ and |X2⟩ (shown in Figure 1) for CdTe/CdSe and CdTe/ZnS NCs and did not observe a clear dependence on the shell formation of CdSe or ZnS. We found a curious negative amplitude signal (possibly a photoinduced absorption or pump-
induced band shift) in the vicinity of the band edge only in CdTe/CdSe NCs, which indicates that this negative signal is related to the quasi-type II features. We propose two possibilities for the negative signal: the redistribution of excitons among the fine-structured states or the biexciton level shift at the ultrafast time scale.
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EXPERIMENTAL SECTION Synthesis of CdTe NCs and CdTe/CdSe NCs. CdTe NCs and CdTe/CdSe NCs were synthesized following the previously reported method.17 We estimated CdTe core diameters from the first excitonic peak positions reported elsewhere94,95 and estimated shell thicknesses based on transmission electron microscopy and the previous reports.16,17 The diameter of CdTe NCs was 3.4 nm. For the CdTe/CdSe NCs, the core diameter and the shell thickness were 2.8 and
