Shell Quantum

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C: Physical Processes in Nanomaterials and Nanostructures

Quasi-Type II Carrier Distribution in CdSe/CdS Core/ Shell Quantum Dots with Type I Band Alignment Li Wang, Kouhei Nonaka, Tomoki Okuhata, Tetsuro Katayama, and Naoto Tamai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11684 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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The Journal of Physical Chemistry

Quasi-Type II Carrier Distribution in CdSe/CdS Core/Shell Quantum Dots with Type I Band Alignment L. Wang, K. Nonaka, T. Okuhata, T. Katayama, and N. Tamai* School of Science and Technology, Kwansei Gakuin University, Sanda 669-1337, Japan

Abstract Band alignments are essential for understanding the optical properties and carrier transfer of core/shell QDs. As CdSe/CdS core/shell QDs with increasing shell thickness represent red-shifted absorption and luminescence spectra, weakened oscillator strength of the lowest electronic transition, elongated luminescence lifetime, they are assigned to quasi-type II band alignment. However, femtosecond transient absorption spectroscopy with state selective excitation revealed a type I band alignment of the CdSe/CdS QDs with thin CdS shell, in which the excited electron is localized in CdSe core with core excitation while delocalized in the whole QDs with shell excitation, even though a quasi-type II carrier distribution was observed with steady-state spectroscopy. In the type I core/shell QDs, CdS shell acts as an energy barrier in surface electron and hole trapping processes. Especially, the time constant of hole trapping process of CdSe core (~10 ps) was elongated ten times owing to tunnel effect through the high energy barrier of CdS shell, which was estimated from the decay related with the biexcitonic induced spectral shift. The biexcitonic spectral shift induced by ~100 ps hole trapping process was also observed at 1S(e)-2S3/2(h) transition. Our results by transient absorption spectroscopy with state selective excitation are useful to clarify band alignment and carrier distribution of heteronanostructures, which could help to objectively extract charge carriers in 1

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photovoltaic applications.

Introduction Semiconductor nanostructures, with tunable optical and excitonic properties as variations of geometries, sizes, and compositions, are applied in optical and electronic devices.1,2 To pursue high efficiencies of light harvest and energy conversion for solar cells, narrow line width within wide spectral region for light emission diodes, and low threshold with high optical gain for microlasers, core/shell heterostructures with a near unit quantum yield (QY) from surface passivation or long lifetime carriers from a charge separated state become hot topics.3-5 All of the interesting properties and potential applications are based on distributions and transfers of electron and hole in the nanostructures, which can be disclosed by time-resolved spectroscopies such as transient absorption (TA) spectroscopy, fluorescence decay measurements, 2D time-resolved spectroscopy, etc.6-9 In TA measurements of CdSe quantum dots (QDs), photogenerated exciton relaxes to ground state or is trapped in surface states.10,11 Spectroscopically, negative signals from electron filling in the lowest excitonic state go through a ~100 fs build-up process from hole-assisted Auger cooling of hot electron (in a high energy excitation case), a minor decay of ~1 ps surface trapping process, and a major component of ~10 ns recombination with hole in valence band.12 Positive signals in visible region are due to biexcitonic induced spectral shifts rather than intraband-transition induced absorption.13 On ~100 fs time scale, the whole TA spectra show a shape like the second-derivative of steady-state absorption spectrum if high excitation energy laser was used, that is induced by a biexcitonic induced spectral shift from pump populated initial excitonic state and probe generated excitonic state.14 On ~10 ps time scale, a weak narrow peak on the red side of the band edge is another biexcitonic induced spectral shift related with a hole trapping process.13 In near-IR region, positive and weak signals are assigned to and trapped carrier induced absorption.15 TA from hole-filling signal in visible region is too weak to be distinguishable owing to closely spaced energy levels of valence band and thus small occupation numbers per energy level. With state 2


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selective excitation or electron scavengers, hot hole relaxation by ligand vibrations and phonon couplings are declared on 500 fs time scale.16-19 In heteronanostructures, such as core/shell QDs, however all these processes become complicated because of different types of energy band alignments and trapping sites on surface and interface induced by the components.20,21 Their corresponding energy band alignments can be determined by offsets of conduction and valence bands of the core and shell. Depending on carrier distributions, band alignment is divided into type I (both carriers localized in either core or shell), quasi-type II (one carrier localized in core or shell and the other delocalized in a whole particle), or type II (e localized in core and h in shell and vice versa).22 CdSe QDs, one of the most studied II-VI semiconductors, are good candidates as cores for epitaxial growth of CdS shell with a ~4% small lattice mismatch to prepare CdSe/CdS heteronanostructures.23-25 Referring to the vacuum level as potential energy zero, the conduction- and valence-band edge positions are -4.04 and -5.78 eV for bulk CdSe, -3.84 and -6.34 eV for bulk CdS.8,26 Type I was expected for the energy band diagram of CdSe/CdS core/shell nanostructures based on the bulk properties.27 In direct measurements of the energy band alignments of CdSe/CdS heterostructures by scanning tunneling spectroscopy, type I band alignment was obtained.28,29 However, quasi-type II and type II band structures were also reported for CdSe/CdS core/shell nanostructures using time-resolved spectroscopic methods.30-34 The evolutions of reduced overlap of carrier wavefunctions were explained with theoretical methods, such as effective mass approximation, multi-band k⋅⋅p method, and first-principles density functional theory, to band-edge shifts and piezoelectric confinement induced by strain effect from epitaxial growth and lattice mismatch (compression of core and tension of shell).30,35-37 Moreover, Auger recombination in quasi-type II CdSe/CdS core/shell QDs was suppressed by the growth of CdS shell owing to reduction of electron-hole overlap from the penetration of electron into the shell and localization of hole in the core.36,38 3



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The TA measurements of CdSe/CdS core/shell QDs or dot-in-rod nanocrystals suggested their energy diagram represented to be a quasi-type II structure that electron delocalization of the whole QDs and hole localized in the core.33,34,39-42 In the case of CdSe/CdS core/shell QDs, a tens of picosecond slow hole relaxation was reported based on the different dynamics of 1S(e)-1S3/2 and 1S(e)-2S3/2.40 For CdSe/CdS dot/rod nanocrystals, TA measurements by shell excitation with larger pumping photon energies (590 nm 1S state of the core and excitation shorter than 490 nm) revealed a 650 fs hole transfer process from shell to core, which was identified from the build-up of core bleach signal.43 Moreover, with state selective excitation, different ratio between the bleach signals of the core and the shell were observed, which has not been discussed or roughly assigned to carrier trapping on the surface.39,42,43 Therefore, details of carrier relaxations and distributions are still not clear in CdSe/CdS core/shell QD systems. In this paper, we performed TA measurements for CdSe/CdS core/shell QDs with state selective excitation of CdSe core and CdS shell. Quasi-type II carrier distribution in type I energy band alignment of CdSe/CdS core/shell QDs was obtained by combination of steady-state and transient absorption spectra. With core excitation, carriers were mainly localized in CdSe core and slightly trapped on the surface of the QDs by tunneling through energy barriers of CdS shell. The hole trapping process induced biexcitonic band shifts were observed in the transitions to the lowest electronic state 1S (e) of conduction band and became distinguishable in spectral regions with weak bleach signals from state filling of 1S(e) transitions. With shell excitation for the QDs, electron kinetically transferred from CdS shell to CdSe core by a two-step process and partially localized in CdS shell without effective Auger-assisted transfer channel. Finally, electron distributed in CdSe core and CdS shell radiatively recombined to core localized hole and surface trapped hole, respectively. Our results could help to understand the carrier relaxation and distribution in CdSe/CdS core/shell QDs and provide useful guide for their applications in photoelectronic devices.

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EXPERIMENTAL SECTION Reagents Cadmium oxide (CdO, 99%), trioctylphosphine (TOP, 90%), sulfur (99.99%, powder), and 1-octadecene (ODE, 90%) were purchased from Kanto Chemical Co., Inc., Alfa Aesar, Kojundo Chemical Laboratory, and Tokyo Chemical Industry Co., Ltd., respectively. Selenium (Se, 99.99%), tetradecylphosphonic acid (TDPA, 97%), trioctylphosphine oxide (TOPO, 99%), octadecylamine (ODA, 99%), and oleic acid (OA, 90%) were purchased from Sigma-Aldrich. All chemicals were used as received.

Synthesis CdSe and CdSe/CdS core/shell nanoparticles were prepared following the reported procedures with slight modifications.44,45 CdO (0.89 mmol), TDPA (0.15 mmol), and TOPO (18.1 mmol) were mixed in a flask under vacuum at 120 °C for 20 min, back-filled with Argon gas. Next, the mixture was heated to 360°C until it became clear, and then the temperature was reduced to 270°C. A stock solution (1 mmol selenium dissolved in 5 mL TOP) was quickly injected into the mixture. After 7 min, the reaction was stopped by air flowing and removing the heater. The CdSe nanoparticles were re-dissolved in 12 mL hexane after washing with acetone three times. The size and extinction coefficient of the synthesized CdSe QDs with 544 nm optical band gap were estimated to be 2.9 ± 0.6 nm (Figure S1) and 1.0 × 105 cm-1·M-1, respectively.46 CdSe/CdS core/shell QDs were synthesized by successive-ion-layer adsorption and reaction method under Argon atmosphere. The 4 mL CdSe solution was mixed with 1.5 g ODA and 6 mL ODE by two-step heating (at 60°C for 30 min and 100°C for 5 min) under vacuum, and then the temperature was increased to 240°C. The cadmium precursor was prepared by dissolving 0.8 mmol CdO in 6.4 mmol OA and 18.0 mL ODE at 250°C, and then kept at 5



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60°C. The sulfur precursor solution was prepared by dissolving 0.4 mmol sulfur in 10 mL ODE at 200°C, and then kept at room temperature. The CdS shell was grown by alternating injection of the Cd and S precursors into the CdSe solution with 10 min interval. The synthesized CdSe/CdS core/shell QDs were purified by washing with acetone three times, and finally dispersed in chloroform. According to scanning transmission electron microscopy (STEM) measurements (Figure S1), the CdS shell thickness was estimated to 0.4 ± 0.1 and 0.6 ± 0.1 nm for the two CdSe/CdS core/shell QDs with 556 and 576 nm optical band gaps, respectively, which was in good agreement with exciton-wavelength shell-thickness sizing map.47,48 A monolayer is taken to be the average between metal or chalcogenide planes for (111) and (100) facets, which is 0.31 nm.49 The two core/shell QDs were labeled as CdSe/CdS1 and CdSe/CdS2 with the order of shell thickness. Based on the same concentration of parental CdSe core QDs and absorption spectra of CdSe/CdS core/shell QDs, the extinction coefficients of the lowest excitonic states of CdSe/CdS1 and CdSe/CdS2 core/shell QDs are estimated to be 5.4 × 104 and 8.5 × 103 cm-1·M-1, respectively.

Experimental Apparatuses The CdSe core and CdSe/CdS core/shell QDs were characterized by STEM (TECNAI 20, 200 keV, FEI). Steady-state absorption and fluorescence spectra were measured using a Hitachi U4100 spectrophotometer and a Fluorolog-3 spectrofluorometer (Jobinyvon-Spex), respectively. QYs of QDs were calculated relative to rhodamine B (QY = 31% in water under 505 nm excitation). A TA spectroscopy with 100 fs instrument response function (IRF) was performed using a conventional pump and probe method to monitoring the dynamics of the carrier populations. The pump beam was the second harmonic of an amplified Ti:sapphire laser system (Spitfire and Tsunami, Spectra-Physics, 800 nm, 60 fs, 1 kHz) or a selected output from an optical parametric amplification (TOPAS, Light Conversion). The probe beam was a white light continuum generated by focusing the fundamental laser into a 6


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quartz cell filled with D2O. The sample filled in a 2 mm cuvette was put on the overlap position of 500 Hz chopped pump beam and time-delayed probe beam. A small magnetic stirrer in the cuvette was used to avoid re-excitation of photoionized QDs.50 The probe beam through the sample was focused into an optical fiber and detected by a Princeton Spec-10 CCD after dispersed with a polychromator (Spectra Pro-275, Acton Research Co.). The transient absorption (the difference of the optical density ∆OD) at a certain time delay after the overlap time of the pump and probe pulses was finally obtained by comparing the intensities of the probe light passing a corresponding delay distance as the pump light on and off given by the chopper in open and close positions.

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RESULTS AND DISCUSSION:

Figure 1. Normalized steady-state absorption spectrum (thick solid line), its second derivative (thin solid line), and luminescence spectrum (dashed line) of CdSe core QDs (a), CdSe/CdS1 (b), and CdSe/CdS2 core/shell QDs (c). Schematic energy level diagram of CdSe/CdS2 core/shell QDs (d). Numbers 1−5 label the same transition species in the three QDs. Number 4* (2.7 eV) labels the transition to the lowest excitonic state of CdS shell in CdSe/CdS2 QDs determined by TA spectral analysis (Figures 2 and S5) and overlaps with number 4 position in (c).

The steady-state absorption spectra characterized with their second derivatives and luminescence spectra of the CdSe core and the two CdSe/CdS core/shell QDs are shown in Figure 1. In the absorption spectrum of CdSe core QDs (Figure 1a), four optical transitions were observed and distinguished to be 544, 508, 483, and 451 nm from its second derivative. Based on the previous experimental and theoretical results, the four bands are assigned to 1S(e)-1S3/2(h), 1S(e)-2S3/2(h), 1S(e)-1S1/2(h), and overlap of 1S(e)-2S1/2(h) and 1P(e)-1P3/2(h) transitions (labeled with number 1−4 in Figure 1).51,52 If without the second derivative, the transition of 1S(e)-1S1/2(h) was too weak to be distinguished from the steady-state absorption spectrum. In Figures 1b and 1c, these transition bands shift to 556/510/491/457 and 576/525/502/467 nm for CdSe/CdS1 and CdSe/CdS2 core/shell QDs, correspondingly. The lowest excitonic state was red shifted ~0.05 and ~0.13 eV, which was due to electron penetration from the CdSe 8


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core into the CdS shell and hence decrease of quantum confinement.53 Except these four transitions, a new band becomes distinguishable in the second derivatives of the CdSe/CdS core/shell QDs. The new band locates around 427 and 445 nm for CdSe/CdS1 and CdSe/CdS2, respectively. It is assigned to the 1S(e)-3S1/2(h) transition of CdSe core (labeled as number 5 in Figure 1), which was red-shifted into the detection window with coating of CdS shell. With increasing CdS shell thickness, optical densities in the blue region of the absorption spectra for CdSe/CdS core/shell QDs are much enhanced as compared with that for CdSe core QDs. Absorption spectra normalized at 1S excitonic states for all the CdSe core and CdSe/CdS core/shell QDs are shown in Figure S2. As compared with large sized CdSe core QDs (1S at 585 nm), the absorption in blue region of detection window for CdSe/CdS core/shell QDs was further enhanced and assigned to large transition energy of 1S excitonic state of CdS shell. As shown in energy band schematics of Figure 1d, 1S(e)-1S3/2(h) transition of CdS shell in CdSe/CdS2 core/shell QDs is located around 461 nm (labeled by number 4*) and overlapped with 1S(e)-2S1/2(h) and 1P(e)-1P3/2(h) transitions of its CdSe core, which will be discussed in the following results from transient absorption measurements.

The luminescence spectra shift to the red from 559, 571, to 590 nm, and the QYs increase from 15%, 25% to 29% for CdSe core, CdSe/CdS1, and CdSe/CdS2 core-shell QDs, respectively. Average luminescence lifetimes of the three QDs increased from 16.5 to 30.6 and 58.2 ns ns with increasing CdS shell thicknesses from 0 to 1 and 2 monolayers, respectively (Figure S3a). The moderately increased QYs and luminescence lifetimes are attributed to enhanced surface passivation of CdSe core by CdS shell and limited by the sulfur-rich surface and the decreased overlapping of electron and hole wavefunctions with increasing shell thickness.54

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Figure 2. TA spectra of CdSe core QDs (a, d), CdSe-CdS1 core-shell QDs (b, e), and CdSe-CdS2 core-shell QDs (c, f) with different excitation fluences of 2.6, 5.2, and 10.4 µJ/cm2 at their corresponding lowest excitonic states, respectively, and 2.6 µJ/cm2 at 400 nm for all samples.

The TA measurements were performed to monitor the population of the excitons in the CdSe core and CdSe/CdS core/shell QDs with the excitations at their lowest excitonic states and 400 nm. The excitation intensities were chosen as low as less than 0.5, by which multiple generation of exciton was avoided (excitation intensity dependence of exciton recombination shown in Figure S6). In the top panels of Figure 2 show the TA spectra with the excitations of their lowest excitonic states of the three QDs. In the case of CdSe core QDs (Figure 2a), three bleach bands were observed, corresponding to the three 1S transitions detected in steady-state absorption spectra (Figure 1a): 1S(e)-1S3/2(h), 1S(e)-2S3/2(h), and 1S(e)-2S1/2(h). Except the three bleach bands, a weak and narrow positive absorption on the red side of band edge was detected. Following the convention, the four positions were labeled as B1, B2, B4, and A1 in Figure 2.10,55 The signal corresponding to 1S(e)-1S1/2(h) transition was too weak to be distinguishable for CdSe core QDs while would be detected for CdSe/CdS core/shell QDs and labeled as B3 10


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in Figures 2b and 2c. Herein only the transition to 1S electronic state of CdSe QDs was excited. Based on the Pauli exclusion principle, the state filling of 1S excitonic state represented the bleaching of the three 1S optical transitions.7,56 The induced narrow absorption A1 was explained to biexciton-induced level shifting similar to the case of untreated CdSe QDs.7,13,57 With the excitation of the lowest excitonic state of CdSe/CdS core/shell QDs (Figures 2b and 2c), the TA spectral characters are quite similar to those of CdSe core QDs. The three bleach bands (B1, B2, and B4) and one positive absorption band A1 were also detected and sequentially shifted to the red with increasing the shell thickness. The bleach signals were assigned to the state filling of 1S excitonic state in the core/shell QDs. A rising of A1 was combined with a decay of B2 from comparisons of the TA spectra at 2 and 100 ps delay times (Figure S4). Detail dynamics will be shown later (A1 and B1-B2 lines in Figures 3b and 3c). Moreover, new bleach bands located at 428 and 450 nm were detected for CdSe/CdS1 and CdSe/CdS2 QDs, respectively, and labeled as B5. This band is not originated from electron transition to the conduction band of CdS shell, but corresponding to the 1S(e)-3S1/2(h) transition and induced by the state filling of 1S electronic state of CdSe core. With a low excitation intensity of 400 nm, an exciton is generated in high lying state, and the hot carriers relax to the lowest excitonic states of the QDs. The TA spectra are illustrated in the bottom panels of Figure 2. For CdSe core QDs in Figure 2d, a fast formation (spectrum at 0.12 ps) of the bleach signal around B4 band corresponds to electron population in high lying 1P state. B4 is contributed from both 1S(e)-2S1/2(h) and 1P(e)-1P3/2(h) transitions.52 A relatively slow formation (spectrum at 0.6 ps) of the bleach signal around B1 band was owing to electronic cooling between 1P and 1S electronic states. For CdSe/CdS core/shell QDs in Figures 2e and 2f, three differences were observed as compared with the spectra in the upper panels in Figures 2b and 2c. First, bleach signals in the blue edge of the detection window (~420 nm) were formed instantaneously, which were induced by state filling of the high lying states in the core/shell QDs. 11



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Second, the B1 bands reached to their maximum amplitudes on ~ps time scale while only 0.12 ps with their 1S excitations of the QDs. The slow build-up of B1 is related with hot electron relaxation and carrier transfer process, which will be discussed below with Figure 4. Third, on late delay time, the spectral region around 460 nm kept relatively large amplitude, corresponding to state filling of 1S electronic state of CdS shell. For CdSe/CdS2 QDs, the bleach peak around B4 located at 467 nm with 400 nm excitation (Figure 2f), which was shifted from 478 nm with 570 nm excitation (Figure 2c). Hence, the bleach band around 467 nm consisted of the state filling signals from 1S electronic states of CdSe core and CdS shell. Then the 1S excitonic transition of CdS shell of CdSe/CdS2 QDs is determined to be ~461 nm, labeled as B4*, after extracting the contribution of 1S electronic state of CdSe core around 478 nm from the TA spectra with 467 nm bleach band (Figure S5). In Figure S3c, luminescence spectrum of CdSe/CdS2 QDs in shorter wavelength shows a peak around 495 nm, which might be due to radiative recombination between electron in the lowest excitonic state of CdS shell and trapped hole on CdS shell surface.21 The two bands (461 and 478 nm) are too close to be distinguished, which results into one band around 467 nm in steady-state absorption spectrum and its second derivative of CdSe/CdS2 QDs (Figure 1c). Hence, the energy band diagrams of the CdSe/CdS core/shell QDs in Figure 1d was supported by the combination of steady-state absorption and TA spectra. For CdSe/CdS2 core/shell QDs, the bleach amplitude ratio of B4* to B1 was about 1:1, while 1:2 in the TA spectra with 570 nm excitation. The increased portion around B4* with 400 nm excitation was assigned to the state filling of the lowest excitonic state of CdS shell. Hence, with 400 nm excitation, the contribution ratio from CdSe and CdS to the bleach signal around B4* was 1:1. As concerned the steady-state absorption spectrum of CdSe/CdS2 QDs, contribution ratio of CdS and CdSe was roughly estimated to be 2:1. Hence, with 400 nm excitation, electron reserved in the lowest excitonic state of CdS shell was about 30% of the excited CdSe/CdS2 QDs (electron in 10% excited QDs was trapped on the surface as shown in Figure S9 and Table S3). 12


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Figure 3. Normalized decay profiles of bleach and absorption bands for CdSe core (a), CdSe/CdS1 (b), and CdSe/CdS2 core/shell QDs (c) with low excitation intensities at their corresponding lowest excitonic states (average number of exciton per QD 〈N0〉 less than 0.5), respectively. The data in b and c were average results from different excitation fluences (2.6, 5.2, and 10.4 µJ/cm2) clearly observe the biexcitonic effect because the decay profiles were too noisy with such low excitation intensities. The biexcitonic effect affected the decay profiles in the whole spectral region as shown with overlap of rising portions of A1 and B3 with the differences of B1-B2 and |B1-B5| for CdSe/CdS core/shell QDs in (b) and (c).

Figure 3 shows the normalized decay profiles at typical band positions of the three QDs with excitations at the lowest excitonic states. For CdSe core QDs, with a low excitation intensity at 540 nm, the bleach bands (B1, B2, and B4) were formed instantaneously owing to direct excitation of 1S electron state, and followed with a ~1.4 ps (5%) surface trapping and a long lifetime radiative decay (Scheme 1). The positive absorption band A1 shows a ~9 ps rising process assigned to biexciton-induced level shift as shown in Scheme 1.13 With excitations of the lowest excitonic states of the CdSe/CdS core/shell QDs, similar decay processes at different wavelengths were expected because electron only transited to the lowest excitonic state. However, as shown in Figures 3b, 3c and Scheme 1, the similar decay profiles were only detected at B1 and B4 bands, which were formed instantaneously and decayed with two components similar to those of CdSe core QDs. One slow decay component is on several ns time scale as the radiative recombination lifetime from luminescence decay measurements (Figure S3). One fast decay component was estimated to be 1.6 (12%) and 4.0 (10%) ps for CdSe/CdS1 and CdSe/CdS2 core/shell QDs, respectively (Figure S7 and Table S2), and assigned to surface trapping. The elongated trapping time constant with increasing CdS shell thickness may indicate that trapping sites distributed on the surface of CdS 13



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shell, but not on the interfaces between CdSe core and CdS shell. As shown in Figures 3b and 3c, the decay profiles at B2 show additional fast decay components on late decay time scale. The difference of B1 and B2, labeled as B1-B2, is overlapped with the long lifetime rising portions of A1 and B3,58 which is not induced by hot hole relaxation from 2S3/2(h) to1S3/2(h) because the lowest excitonic state was excited and no hot hole was generated. Hence, the additional fast component of B2 is assigned to the biexcitonic induced spectral shift as that for A1. The biexcitonic state was formed by binding pump generated and probe created 1S excitonic states, in which the pump generated exciton progressed in a hole trapping process. Hence, the spectral shift might exist in all transitions related with 1S excitonic state. In core/shell QDs, the additional fast component of B2 became distinguishable owing to the increased energy separation between the transitions of 1S(e)-1S3/2(h) and 1S(e)-2S3/2(h), which was increased from 176 to 202 and 210 meV of CdSe core and the two core/shell QDs. Similar to B2 band, the biexcitonic induced spectral shift was detected in the bands with weak bleach signals, such as B3 and B5. For B5 band, it represented a long lifetime decay component (as that of B2) in CdSe/CdS1 QDs while a slow rising component (as that of B3) in CdSe/CdS2 QDs owing to the different energy space between B5 and B4. Similar to B1-B2, differences between B1 and B5 (labeled as absolute value of B1-B5 in Figures 3b and 3c) overlapped with the rising portions of the normalized profiles of A1 and B3. On the initial time scale, the decay behaviors were also affected by phonon oscillation for these weak TA signal bands. The influence of phonon oscillation became obvious with high excitation intensities and was qualitatively discussed in Figure S8. Therefore, biexcitonic effect related with hole-trapping state induced spectral shift in the whole detection window and characterized by the additional decay components in the decay profiles corresponding to the lowest excitonic state of CdSe/CdS core/shell QDs.

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Figure 4. Normalized decay profiles of bleach and absorption bands for CdSe core (a), CdSe/CdS1 (b), and CdSe/CdS2 core/shell QDs (c) with low excitation intensities at 400 nm (average number of exciton per QD 〈N0〉 less than 0.5), respectively. Decay profiles for core/shell QDs were average results from different excitation intensities (1.3, 2.6, and 5.2 µJ/cm2), similar to Figure 3.

For CdSe core QDs with low excitation intensity of 400 nm, B1 represented a fast rising component, ~170 fs, that was assigned to Auger cooling process of hot electron with hole assistance, a ~1.8 ps (10%) surface trapping, and a long lifetime decay owing to the radiative recombination (Figure 4a and Table S3).28 B5 built up on the time scale of IRF, and its lifetime of fast decay component became ~800 fs (30%) owing to a mix of electron cooling between 1P and 1S states and surface trapping process. Induced absorption A1 was dominated by two biexcitonic processes: ~1 ps decay for exciton populated in initial high energy state and ~10 ps rising for surface trap state from hole trapping process. For CdSe/CdS core/shell QDs with low excitation intensity of 400 nm, one exciton was excited in CdSe core or CdS shell in one QD (Figure S2), and relaxed from high lying state or transferred from CdS shell to the lowest excitonic state. As shown in Figures 4b, 4c, and S9, the decay profiles of B1 represent two rising components ~190 fs (86%) and ~1.2 ps for CdSe/CdS1 QDs and ~270 fs (78%) and ~3.1 ps for CdSe/CdS2 QDs. The fast rising component of B1 in CdSe/CdS QDs represents the hot electron directly relaxed into the lowest excitonic state of CdSe core. This build-up time elongates from 170 fs of CdSe core QDs to 270 fs of CdSe/CdS2 core/shell QDs, which was due to the weak confinement of electrons delocalized in the whole QDs.40,59 The second rising component 3.1 ps of B1 for CdSe/CdS2 core/shell QDs could be assigned to electron transfer 15



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process from CdS shell to CdSe core. As mentioned in the above and discussed in Figure S5, the large amplitude of bleach signal around B4* in the TA spectra of CdSe/CdS2 QDs with 400 nm excitation represented the hot electron relaxed to the lowest excitonic state of CdS shell and localized in CdS shell. The electron relaxed into CdS shell should transfer to CdSe core because of the higher electronic energy level of CdS shell. Both electron cooling in bare CdSe QDs and electron transfer from QDs to electron acceptors are facilitated by Auger-assisted electron-hole coupling rather than electron-phonon coupling due to quantum confine effect and large density of states in valence band.8,60 The electron transfer from CdS shell to CdSe core also resorts to electron-hole coupling with hole excitation.61 The excited hole may locate at an energy level near valence band edge of CdS shell while corresponding to a high energy state in valence band of CdSe core, concerned the large band offset between valence bands of CdSe and CdS. Hence, the hot hole may directly transfer into CdSe core and relax to the lowest state of valence band of CdSe through phonon coupling.62 Otherwise, the hot hole may relax to the lowest state of valence band in CdS shell through coupling with vibration mode of surface ligand. For 2.0 nm radius CdSe QDs, a 0.38 eV/ps hole transition rate was detected through ligand coupling as dominant pathway.16 The hole distributed in CdS shell still has strong Coulomb interaction with the electron relaxed to the lowest excitonic state in CdS shell and can assist electron transfer from CdS shell to CdSe core (Figure S10). Therefore, the slow rising component 3.1 ps may be contributed from a three-step process: hot electron relaxation to the lowest excitonic state of CdS shell with hole excitation to high energy state, a small portion of excited hole relaxation to the lowest state of valence band in CdS shell, and electron transfer realized by coupling with the relaxed hole in CdS shell. After hole transferred from CdS shell into CdSe core within several ps, electron distributed in CdS shell in a part of all excited QDs can’t efficiently transfer to CdSe core owing to reduced overlap of electron and hole wavefunctions and hence lacking of Auger-assisted transfer process. Therefore, electron in 30% excited CdSe/CdS2 core/shell QDs was localized in CdS shell as mentioned above and gave large bleach amplitude around B4* in TA spectra 16


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with 400 nm excitation. The shell localized electron may bind with surface trapped hole, form exciton, and recombine with a 69 ns lifetime constant (Figure S3b).21 As shown in Figures 4b and 4c, biexcitonic induced spectral shift was also detected in the case of 400 nm excitation. On late delay time scale the decay profiles of B2 were faster than that of B1, and the difference of B1 and B2 were overlapped with the slow rising component of A1. Similar to the excitation case of the lowest excitonic state (Figure 3), here the difference of B1 and B2 is assigned to biexcitonic induced spectra shift rather than the cooling of hot hole. Decay profile of B4 was totally overlapped with that of B1 for CdSe/CdS1 QDs. For CdSe/CdS2 QDs, the dynamics of B4* was corresponding to 1S excitonic state of CdS shell in CdSe/CdS2 QDs. A 160 fs rising time constant of B4* was detected and assigned to electron cooling in CdS shell (Figure S11). The decay process of electron localized in CdS shell can’t be distinguished from that of B1 (electron distributed in CdSe core) owing to similar long lifetimes of the two excitonic states (Figure S3b). For B5 of both CdSe/CdS core/shell QDs (Figure S12 and Table S4), the rising portion represents a weak signal of the build-up of the lowest excitonic state of CdSe core and a large contribution of the transition from the high lying electronic state, and the decay processes include electron trapping, biexcitonic induced spectral shift, and radiative recombination. The contributions from each species were overlapped and difficult to give clear assignment on time scale of the decay profiles at B5. The decay profiles of A1 for the three QDs are shown in Figures 3 and 4 and normalized in Figure S13. The rising process of A1 showed no shell-thickness dependence while excitation wavelength dependence. With core (1S) excitation, the rising time constant was elongated from 9 ps for CdSe core QDs to ~110 ps for CdSe/CdS core/shell QDs, which was assigned hole surface-trapping process. The elongation was induced by hole tunneling of the barrier CdS shell. Hole tunneling rate k depends on the thickness l of barrier with k0e-βl (k0 is the rate at 0 Å separation, and β is the measured tunneling decay constant).32 In our results, the difference of the CdS shell thickness of the two CdSe/CdS core/shell QDs may be too small to distinguish the dependence of hole trapping rate. 17



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With an average barrier thickness 0.5 nm of the two core/shell QDs, β was roughly estimated to be 5.0 nm-1, which is in good agreement with the reported value 5.4 nm-1 for CdSe/CdS core/shell QDs for the hole tunneling process. With shell excitation, hole trapping with ~60 ps time constant was observed for both core/shell QDs, which may be induced by the sum of two trapping pathways from the highest states in the valence bands of CdSe core and CdS shell. Hence, ~100 ps hole trapping process in CdSe/CdS core/shell QDs was much elongated as compared with ~9 ps in CdSe core QDs owing to tunneling effect of CdS shell (Scheme 1). Both electron and hole were trapped on the surface of CdS shell rather than the interface between CdSe core and CdS shell. Hence, the CdSe core was well passivated by CdS shell and the interfacial defect was negligible. The small lattice mismatch 4% of CdSe and CdS and the uniform spherical structures of CdSe/CdS QDs in the present study may not induce large grain boundaries on the interfaces, and hence no defect-induced energy barrier between the conduction bands of CdSe core and CdS shell as reported in previous references.31,63,64 The large bleach signal B4* of CdSe/CdS2 QDs with 400 nm excitation (in Figures 2f and 4c), corresponding to the long lifetime of electron in CdS shell, were not originated from blocking electron transfer by the energy barrier of grain boundaries. The population of excited electron in CdS shell might be explained to lacking of efficient Auger-assisted electron transfer process and bounded with surface trapped hole.

Scheme 1 Carrier dynamics of CdSe core and CdSe/CdS2 core/shell QDs. Surface trapping processes of electron and hole were shown in (a) for CdSe core and (b) for CdSe/CdS2 core/shell QDs with their 1S excitations. With 400 nm excitation (c), electron excited in CdS shell may transfer into CdSe core or local in the shell. For CdSe/CdS core/shell QDs, biexcitonic induced spectral shift was observed for both excitation conditions.

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CONCLUSION: In this paper, two different CdSe/CdS core/shell QDs with one and two monolayer CdS shells were prepared from 2.9 nm diameter CdSe core QDs. The lowest excitonic state was shifted to the red with increasing shell thickness due to electron penetration into CdS shell. Hence quasi-type II carrier distribution of CdSe/CdS core/shell QDs was detected in steady-state spectra and time-resolved luminescence dynamics. However, with the lower pump energy (550 and 579 nm) of core excitation, electron was only localized in CdSe core and didn’t distribute in CdS shell while it was delocalized in both core and shell with the higher pump energy (400 nm) of shell excitation. Hence, type I energy band alignment of the core/shell QDs was declared by transient absorption measurements with state selective excitation. Moreover, in the case of core excitation, the trapping processes of electron and hole on the surface of CdS shells were controlled by energy barriers CdS shell, which were determined from elongated trapping time with increasing the shell thickness. Related with hole trapping, biexcitonic induced spectral shift was observed in the whole spectral region and became obvious from the different decay profiles of 1S(e)-1S3/2(h) and 1S(e)-2S3/2(h) transitions. In the case of shell excitation, electron generated in CdS shell would relax into CdSe core or localize in CdS shell. The build-up of the lowest excitonic state of CdSe core was formed by electron relaxation from the higher lying states of CdSe core or CdS shell and electron transfer from the lowest excitonic state of CdS shell. Electron in part of all excited QDs was localized in CdS shell, which might be due to lacking of efficient Auger-type transfer pathway after hole localized into CdSe core. Furthermore, by state selective excitation, the overlapping spectral species of 1S(e)-2S1/2(h) in CdSe core and 1S(e)-1S3/2(h) in CdS shell can be distinguished. Our results proved that transient absorption measurements of heteronanostructures with state selective excitation of different compositions were useful to make clear the band alignment and carrier distribution, which could help to objectively extract charge carriers in photovoltaic applications.

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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Additional experimental details and figures (PDF). Additional text and 13 figures showing STEM images of CdSe core and CdSe/CdS core/shell QDs, normalized steady-state absorption spectra, time-resolved luminescence decay profiles and steady-state luminescence spectrum of CdS shell of CdSe/CdS2 QDs, original and normalized TA spectra, normalized decay profiles of the QDs with 1S and 400 nm excitations and their fitting results, excitation intensity dependence of exciton recombination of 1S and a discussion of hole transfer from CdS shell to CdSe core. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. NOTES The authors declare no competing financial interest. ACKNOWLEDGEMENT This work was supported by JSPS KAKENHI Grants Number JP26107005 in Scientific Research on Innovative Areas “Photosynergetics” and Number JP15H03773 in Scientific Research (B).

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Figure 1. Normalized steady-state absorption spectrum (thick solid line), its second derivative (thin solid line), and luminescence spectrum (dashed line) of CdSe core QDs (a), CdSe/CdS1 (b), and CdSe/CdS2 core/shell QDs (c). Schematic energy level diagram of CdSe/CdS2 core/shell QDs (d). Numbers 1−5 label the same transition species in the three QDs. Number 4* (2.7 eV) labels the transition to the lowest excitonic state of CdS shell in CdSe/CdS2 QDs determined by TA spectral analysis (Figures 2 and S5, Supporting Information) and overlaps with number 4 position in (c). 82x70mm (300 x 300 DPI)


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Figure 2. TA spectra of CdSe core QDs (a, d), CdSe-CdS1 core-shell QDs (b, e), and CdSe-CdS2 core-shell QDs (c, f) with low excitation intensities at their corresponding lowest excitonic states and 400 nm (average number of exciton per QD less than 0.5). 139x119mm (300 x 300 DPI)



Figure 3. Normalized decay profiles of bleach and absorption bands for CdSe core (a), CdSe/CdS1 (b), and CdSe/CdS2 core/shell QDs (c) with low excitation intensities at their corresponding lowest excitonic states (average number of exciton per QD áN0ñ less than 0.5), respectively. The data in b and c were average results from different excitation fluences (2.6, 5.2, and 10.4 µJ/cm2) clearly observe the biexcitonic effect because the decay profiles were too noisy with such low excitation intensities. The biexcitonic effect affected the decay profiles in the whole spectral region as shown with overlap of rising portions of A1 and B3 with the differences of B1-B2 and |B1-B5| for CdSe/CdS core/shell QDs in (b) and (c). 169x59mm (300 x 300 DPI)


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Figure 4. Normalized decay profiles of bleach and absorption bands for CdSe core (a), CdSe/CdS1 (b), and CdSe/CdS2 core/shell QDs (c) with low excitation intensities at 400 nm (average number of exciton per QD áN0ñ less than 0.5), respectively. Decay profiles for core/shell QDs were average results from different excitation intensities (1.3, 2.6, and 5.2 µJ/cm2), similar to Figure 3. 160x59mm (300 x 300 DPI)



Scheme 1 279x165mm (300 x 300 DPI)


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Shell Quantum

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