Article pubs.acs.org/JPCC
Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods Yasser Hassan,†,§ Chi-Hung Chuang,‡ Yoichi Kobayashi,† Neil Coombs,† Sandeep Gorantla,∥,⊥ Gianluigi A. Botton,∥ Mitchell A. Winnik,† Clemens Burda,‡ and Gregory D. Scholes*,† †
Department of Chemistry, 80 Street George Street, University of Toronto, 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 § Chemistry Department, Faculty of Science, Zagazig University, 44511 Zagazig, Egypt ∥ Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4L8 Canada ‡
S Supporting Information *
ABSTRACT: Linker-free TiO2/CdSe hybrid nanorods (NRs) were prepared by growing CdSe QDs in the presence of TiO2 NR seeds using seeded−growth type colloidal injection approach. TEM studies revealed the abundant formation of anatase TiO2 NRs with different lengths. We found that the as-prepared TiO2 seeds determined the morphology of TiO2/CdSe NRs. The photophysics of these materials were studied by photoluminescence (PL) and femtosecond transient absorption spectroscopy. While we observed the efficient PL quenching in TiO2/CdSe NRs, the bleach dynamics of TiO2/CdSe NRs is similar to that of CdSe NRs. It suggests that while surface traps that arise from the lattice mismatch between CdSe and TiO2 are mainly observed in transient absorption measurements the ultrafast exciton dissociation over the experimental time resolution occurs in TiO2/CdSe NRs.
1. INTRODUCTION Conversion of solar energy to electrical energy can be initiated by injecting electrons into a metal oxide from a photoexcited sensitizer. Using inorganic semiconductor materials as light harvesting sensitizers has drawn a lot of attention in past years.1−15 Semiconductor materials that absorb visible light can serve as sensitizers since they are able to transfer electrons into large band gap materials. Examples include CdSe/TiO2 and CdSe/SnO2.16,17 In addition, changing the size of semiconductor nanocrystals, also known as colloidal quantum dots (QDs), leads to tuning of their band gap and thereby optical properties can be tailored to optimize solar light absorption over a wide range of visible-IR light.18,19 A challenge, however, is that QDs have surfaces functionalized with organic ligands. Similarly, TiO2 surfaces are hydroxylated. These insulating organic surfactants act as a barrier for charge transport between the QDs and TiO2. The electronic coupling between QDs, and consequently the charge carrier mobility, can be enhanced by removing the organic surfactants from the QD surface.1,20−22 Three techniques have been reported to assemble semiconductor quantum dots (QDs) into a mesoporous thin film of TiO2 (large band gap material). One technique is to assemble the QDs using bifunctional linkers to attach them to the oxide.16,23,24 In this configuration, it was found the chemical nature of the linker plays a decisive role in determining the © 2014 American Chemical Society
efficiency of electron injection into the matrix. Second is physisorption of QDs to the metal oxide substrate, as reported by Nozik and co-workers.25−27 The third technique circumvents the use of in situ growth colloidal quantum dots by employing chemical bath deposition28−30 or successive ionic layer adsorption and reaction (SILAR) to grow semiconductor QDs directly on the oxide architecture.31−34 Although these techniques guarantee high optical densities in the visible region, they have disadvantages. For instance, there are two main disadvantages affecting their solar cell efficiency. First, linker molecules used to bridge nanocomposites hinder the electron injection into TiO2 and reduce carrier mobility, as in the case of using bifunctional linkers.24,35 Second, there may be difficulties preparing stable anions such as Se2− and Ti2− and losing the control over the QD size and size distributions can be a problem in the case of applying chemical bath deposition or SILAR.36 Here, we investigate direct contact between the nanocrystalline absorber and the metal−oxide surface. We present a colloidal route for the preparation of linker-free TiO2/CdSe hybrid nanorods (NRs) were prepared by growing CdSe QDs Received: December 2, 2013 Revised: January 8, 2014 Published: January 22, 2014 3347
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Information. After the reaction flask was pumped under vacuum for ∼1 h at 120 °C, the solution was heated to 320− 340 °C under argon to yield a colorless Cd complex solution. The solution was then cooled to 120 °C and held under vacuum for 2 h to remove water. A fixed amount (in mg) of well-washed TiO2 short (30−50 nm in length), used for sample 1, or long (80−100 nm in length) NRs, used for samples 2, 3 and 4, was dissolved in a mixture of 2 mL ODE and 2 mL TOP, degassed well by nitrogen. This solution was then added to the Cd complex solution. The mixture was held under vacuum for a further 1 h before the flask was allowed to return to 270 °C under nitrogen. At that point the Se precursor was added dropwise into the reaction at a rate of 0.4 mL/min. The Se (0.1 mmol/mL) solution, prepared by dissolving 0.039 g (0.5 mmol) Se powder in 2 mL TOP and 3 mL ODE. This Se precursor was degassed and stored under nitrogen before it was used. The reaction temperature was adjusted to 250 °C, and the reaction was stopped after 30 min by the removal of the heating mantle and the injection of anhydrous toluene. The nanorods were isolated and cleaned by a few precipitation and redissolution cycles using toluene or chloroform as solvent and methanol and isopropanol mixture as the non-solvent (1:3 solvent to non-solvent ratio). Precipitation was achieved by centrifugation for 20−30 min under 6000 rpm. It is worth mentioning here that these heterostructures are easily soluble in organic nonpolar solvent (toluene) and precipitated in organic polar solvent like methanol. Moreover, it is a stable colloidal solution for more than 1 year if it is firmly sealed. The growth of TiO2/CdSe NRs was monitored by taking aliquots from the reaction and measuring absorption and photoluminescence spectra. For comparison, CdSe nanorods were also synthesized following the similar procedures without adding any TiO2 seeds. Characterization. Size distributions and energy-dispersive X-ray spectra of the prepared TiO 2/CdSe NRs were determined by transmission electron microscopy (TEM) with a Libra 200EF (Zeiss) at 200 kV. UV−vis absorption and PL spectra were recorded using a Varian Cary 50 and Varian Eclipse fluorescence spectrometer, respectively. UV−vis characterization took place in 1 cm cuvette. High-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM) images and electron energy loss spectroscopy (EELS) data were recorded using a FEI Titan 300 keV aberration-corrected microscope (CCEM facility at McMaster University). Femtosecond transient absorption (TA) measurements were conducted using a Clark MXR 2001 fs laser system producing 780 nm, 150 fs pulses from a regenerative amplifier.42,43 The laser pulse train was split to generate a white light continuum (450−800 nm) probe pulse in a sapphire crystal and a tunable (460−1100 nm) pump pulse. The TiO2/CdSe samples were excited at 620 or 650 nm wavelength light generated by using an optical parametric amplifier OPA (TOPAS, Light conversion). All femtosecond laser experiments were carried out in a 2 mm path length quartz cuvette at room temperature. The instrument time resolution was determined to be ∼150 fs via a pump− probe cross-correlation analysis. The pump power was carefully controlled to avoid multi-exciton production in the NRs. The excitation power in TA (∼150 μJ/cm2 fluence per pulse) measurements was considered carefully to ensure that the average number of excitations per rod was less than one.
in the presence of TiO2 NR seeds using seeded−growth type colloidal injection approach.37 We report characterization results of the materials and their photophysics with a focus on characterizing the exciton dynamics and electron transfer (ET) efficiency.
2. EXPERIMENTAL SECTION General Methods. All procedures were carried out using standard Schlenk line techniques under oxygen-free conditions and nitrogen flow. Materials. All chemicals were used as received without further purification. Titanium isopropoxide (TTIP) (99%), cadmium oxide (99.99%), selenium (100 mesh, 99.5%,), oleic acid (OA) (90%), oleylamine (OLA) (70%), 1-octadecene (90%), lauric acid (LA) (99%), trioctylphosphine oxide (TOPO) (99%) and trioctylphosphine (TOP) (90%) were purchased from Sigma-Aldrich and used without any purification. Phosphonic acids like tetradecyl phosphonic acid (TDPA), n-hexyl phosphonic acid (HPA) and octyl phosphonic acid (OPA) were purchased from PCI Synthesis Inc., and used without any purification. All the solvents such as chloroform, methanol, toluene and isopropanol are anhydrous. Synthesis of TiO2 Nanorod Seeds. The synthesis of TiO2 nanocrystals with nanorod shapes were accomplished by an aminolysis reaction of titanium isopropoxide by oleylamine (OLA) at high temperature using a modification of previous methods.38−41 Typically, TiO2 nanocrystals were synthesized by the following procedure: (A) TiO2 short rods (30−50 nm): lauric acid (LA) and oleic acid (OA), each 16 mmol in 5 mL 1ODE were dried and degassed at 110 °C in a 50 mL three-neck flask for 1 h to remove all the traces of water and oxygen. After that, TTIP (8 mmol) was injected into the solution and hold under vacuum for 30 min. The solution gradually turned from colorless to a pale yellow, indicating the formation of titanium carboxylate complexes.39 Then the system was exchanged to nitrogen flow, and the temperature was raised to 270 °C. The solution turned to green yellowish, then become lighter when reaching 270 °C. The aminolysis reaction was initiated by the rapid injection of previously degassed OLA (3 mL) under vigorous stirring and the reaction kept at that point for 1.5 h at 250 °C. A white precipitate (suspension) was formed 30 min after OLA injection. (B) TiO2 long rods (80−100 nm): in order to elongate the rods length, we increased the reaction time, after OLA injection, to be 3 h instead of 1.5 h. In this case we used oleic acid as source of carboxylate complexes39 without the lauric acid. Once the system cooled down to room temperature, excess anhydrous toluene was added to the TiO2 nanorods, which were then precipitated by adding anhydrous methanol, and TiO2 was separated by centrifugation. The sample was washed several times by repeated methanol precipitation and toluene redispersion (volume ratio of methanol:toluene = 3:1). The resulting white powder could be redispersed easily into nonpolar solvents such as toluene or chloroform for further characterization. Synthesis of TiO2/CdSe Nanohybrids. We prepared five batches of TiO2/CdSe heterostructures. Four samples were prepared by using the two previously mentioned short and long TiO2 NR seeds. The fifth sample is presented in the Supporting Information in which we used bundle-shaped TiO2. In a typical synthesis of TiO2/CdSe short rods heterostructure, TOPO, TDPA, HPA or OPA, and CdO were mixed in a 50 mL threenecked flask. Details are presented in Table S1 in Supporting 3348
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Figure 1. Transmission electron microscope images (annular-dark field imaging mode) of isolated (a) short TiO2 NRs (used as a seed for sample 1) and (b) long TiO2 NRs (used as a seed for sample 2, 3, and 4).
Figure 2. Annular dark-field STEM images of TiO2/CdSe NRs (a) sample 1, (b) sample 2, (c) sample 3, and (d) sample 4, respectively.
3. RESULTS AND DISCUSSION The TiO2/CdSe nanohybrids investigated in our work were prepared according to a colloidal route. TiO2/CdSe nanorods (NRs), incorporating CdSe NCs grown in the presence of TiO2 NRs were synthesized by the seeded-type colloidal injection growth technique used previously for CdTe/CdSe type II heterostructures.37 The synthesis of the TiO2/CdSe NRs in this work demonstrated good reproducibility. TiO2 seeds were first synthesized using a slightly modified procedure from the previous work,38−41 by an ester aminolysis nonhydrolytic synthetic approach to form anatase titania nanocrystals44 capped with lauric acid and/or oleic acid ligands. TiO2 NRs (30−100 nm) length capped with lauric and/or oleic acids were used as a seed for the preparation of TiO2/CdSe nanohybrids. By changing the synthetic recipe, different shapes of TiO2 NRs, either “short or long” isolated nanorods or bundled nanorods, were obtained. The morphologies of TiO2 seeds (“short” rods, 50 nm in length and “long” rods of 100 nm in length) and TiO2/CdSe NRs were characterized by TEM, shown in Figures 1 and 2, respectively. It was found that the time of the aminolysis reaction affected the structure of the TiO2 product. In the first experiment, the
reaction took place for only 30 min after oleylamine (OLA) injection and resulted in TiO2 bundled structure (consisting of a mixture of TiO2 nanoparticles and nanorods), shown in Figure S1 (Supporting Information). Accordingly, we modified the recipe by increasing both the concentration of ligand and the time of the reaction. The time of the reaction in this experiment considered after injection of oleylamine to the oleic/lauric acid solution mixture with titanium tetraisopropoxide. In other experiments, the time of the reaction was extended to 1.5 and 3 h. This adaptation led to well-formed and isolated TiO2 short (30−50 nm) and long NRs (80−100 nm) with anatase lattice structure, Figure 1 and Figure S1 (Supporting Information). Morphology Characterization. The morphology of TiO2 NR seeds after preparation was characterized by electron microscopy and powder X-ray diffraction. Short TiO2 (30−50 nm) NRs and long TiO2 (80−100 nm) NRs are shown in Figure 1a,b, respectively. High-resolution TEM (HRTEM) of the isolated long TiO2 NRs sample showed a crystalline structure with 0.35 nm spacing consistent with the (101) plane of anatase TiO2, Figure S1d. 3349
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Figure 3. The STEM (left) and EELS spectrum (right) from TiO2/CdSe NRs sample 3 hybrid. The spectrum shows the titanium L edge and the oxygen K edge in TiO2 plus the M edge in the case of Cd of the CdSe part.
Figure 4. Region of analysis for a TiO2/CdSe hybrid rod: (a) STEM with regions identified as 1, 2, and 3 in the field of analysis. (b) EELS spectra demonstrating the presence of Ti and oxygen in region 1, Cd in region 2, and Ti and oxygen within region 3.
dimensions of ∼100 nm in length and ∼7 nm in diameter, estimated by examining more than 200 NRs. It is clear from these TEM images that the particles are significantly different only in their thickness from the TiO2 rods used to seed the reaction. To check the chemical composition of the original TiO2 NRs compared to the TiO2/CdSe composite samples several measurements were carried out such as PXRD, energydispersive X-ray (EDX), TEM, HRTEM, and scanning transmission electron microscopy−electron energy loss spectroscopy (STEM−EELS). EDX results, shown in Figure S3, indicate the traces of Cd, Se, Ti, and O elements for TiO2/ CdSe composite sample, respectively. However, EDX measurements of our hybrids did not exhibit a strong peak for TiO2. Therefore, STEM−EELS measurements were performed on one patch of these rods to check whether CdSe did deposit on the TiO2. In the STEM−EELS technique, the inelastic
TiO2 NRs structure and crystallinity were characterized using powder X-ray diffraction (PXRD). It was found that our aminolysis reaction produced anatase titanium dioxide lattice structures for the bundled and isolated NRs TiO2, see Supporting Information Figure S4. The bundled TiO2 sample (30 min reaction time) does not exhibit clear PXRD peaks, Figure S4b, while the modified samples shown in Figure 1 do exhibit anatase features. In the former case, this is indicative of a lack of crystallinity whereas the later, this indicates a perfect anatase titanium dioxide structure. It is possible that a longer reaction time at high temperature (270 °C) improves the crystallinity of TiO2 by annealing of the defects. We found that the nature of the as-prepared TiO2 seeds dramatically determined the morphology of TiO2/CdSe NRs. Figure 2a shows TiO2/CdSe sample 1, “short” NRs with 50 nm in length, similar to their TiO2 seeds. The isolated “long” TiO2/ CdSe NRs samples 2, 3, and 4 in Figure 2b−d have average 3350
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Figure 5. Annular-dark-field STEM image of (a) TiO2 NRs as prepared, (b) TiO2 NR mixture with Cd-precursor after heating at 270 °C and before adding the Se precursor directly, (c) TiO2/ CdSe sample 2 formed after adding the Se precursor to TiO2 shown in (b), and (d) CdSe sample prepared under the same conditions used for sample 2, but without adding TiO2 NR seeds.
Figure 6. (a) Solution phase UV−vis absorption spectra of TiO2 (red) and TiO2/CdSe NRs (blue). (b) PL spectra of CdSe QDs prepared alone and compared to the mixture of this CdSe with titanium isopropoxide, and a previously prepared TiO2 NRs. (c,d) UV−vis absorption (blue) and PL (red) spectra of TiO2/CdSe NRs of samples 1 and 2, respectively.
As a reference, we first measured the EELS spectra for the TiO2 NR seeds alone. These data are displayed in Figure S5 (Supporting Information). In a second step, the EELS spectrum was acquired for the TiO2/CdSe hybrid NRs sample 2 (shown in Figures 3 and 4). EELS spectra of the TiO2 NR seeds (Figure S5), shows significant Ti and O signals, in particular, the titanium L edge and the oxygen K edge in TiO2. EELS spectra for the TiO2/ CdSe hybrid sample 2 shows (Figure 3b) three edges including Ti−L2,3 edges (at ∼461 eV), oxygen−K edge (at ∼530 eV)
scattering of the incident beam of electrons on the sample (which in the present study is one nanorod or a collection of nanorods) is measured to provide chemical and spectroscopic (valence, coordination) information.45,46 To collect EELS spectrum image data of the area of interest, we used a reference areaa spot on the grid with no rodsfor tracking spatial drift occurring during acquisition. The EELS spectrum in Figure 3b was integrated over the rectangle marked “spectrum image” in the STEM image, red box shown in Figures 3a and 4a. 3351
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overlapping with the Cd−M4,5 edge at ∼437 eV, which indicates the coexistence of both TiO2 and CdSe in the sample. Figure 4 shows the EELS spectra recorded from one TiO2/ CdSe hybrid rod for the purpose of determining the chemical distribution at different points along the nanorod. Accordingly, EELS spectra were extracted at three points around the selected nanorod for the measurement; on the periphery of the rod and its core. Interestingly, it is pertinent to note that we could detect significant titanium signal at the edges of most of the NRs in sample 2 (shown in Figure 4a with spectra extracted from region 1 and 3) that we investigated. However, in the core of each nanorod, the Cd peak is predominant. We studied many single nanorods at different locations to test the reproducibility of the results. In order to investigate the role of TiO2 in the hybrid formation, we ran a control experiment in which we compared the STEM images of TiO2 NR seeds used in sample 2 before and after adding them to the Cd precursor. It was found that the TiO2 NRs maintained their shape even after heating at 270 °C with Cd precursor (Figure 5a,b. Similarly, for comparison, we prepared CdSe QDs following the same recipe used for preparing the TiO2/CdSe nanocomposite sample 2, but without adding TiO2 NR seeds to the reaction flask. In this experiment, we added Se precursor drop-wise similar to the preparation of TiO2/CdSe nanocomposite experiment. It was noticed from this CdSe sample TEM image that CdSe are in rice shape (short rods) (Figure 5d) and do not form elongated nanorods that resemble TiO2/CdSe sample 2, shown in figure 5c. This control experiment for CdSe formation in the absence of TiO2 but under the same conditions as heterostructure formation indicates that the presence of TiO2 NRs is responsible for the TiO2/CdSe NRs elongation (100 nm) in the case of sample 2. In sum, we have presented strong evidence that we have made a hybrid material wherein CdSe is precipitated on TiO2 NRs with irregular coverage. First, we did not find any sign of TiO2 in the EELS experiment in which the predominant spectral features arose from CdSe (Figure 4). However, we did find through TEM that the thin nanorods of TiO2 ended up much thicker after adding CdSe to them (Figure 5a−c), highly suggestive that CdSe is in fact growing along the TiO2 nanorods. This interpretation is further supported by the control experiment where we show that in the absence of TiO2 NR seeds, the CdSe nanorods did not grow long (rice shape) (Figure 5d) compared to the CdSe NRs formed in the presence of TiO2 NR seeds (Figure 5c). Optical Properties. We studied the optical properties of our hybrid NRs using UV−vis spectroscopy and PL measurements. Figure 6 displays the absorption and emission spectra of TiO2/CdSe nanohybrids. Since TiO2 has a wide band gap of ∼3.2 eV, the visible absorption from 500 to 700 nm (shown in Figure 6a; blue) are assigned to CdSe exciton transitions. Figure 6a shows the absorption of a dilute dispersion of TiO2/ CdSe hybrid NRs, in chloroform (the same behavior was found in toluene), and the absorption peak of TiO2 NR seeds capped with the organic surfactants, oleic acid ,and oleylamine, dissolved in toluene. The short TiO2/CdSe NRs (sample 1) have photoluminescence quantum yields on the order of ∼1% (Figure 6c), while the other four samples (samples 2−5) have no detectable emission (Figure 6d). It is likely that sample 1 contains a heterogeneous distribution of nanoparticles, where a small fraction of nanoparticles are CdSe that have not nucleated on a TiO2 NR.
To aid the subsequent discussion of the transient absorption bleach features, we first discuss assignment of the bands observed in the CdSe absorption spectrum. Since CdSe covers TiO2 NRs and the mean rod radius is a few times smaller than the CdSe exciton Bohr radius, the CdSe exciton experiences strong−quantum confinement mainly in the direction of the thickness of the shell diameter.47 In this case, a onedimensional (1D) or two-dimensional (2D) exciton model is suitable to explain the electronic structure. Experimentally, the striking difference between 1D or 2D CdSe nanostructures is the oscillator strength of the second absorption peak (X1 in our transient absorption studies). While the second absorption of CdSe NRs (1S(e) −2S(h)) is weaker than the lowest absorption, the second absorption of CdSe nanopletelets (light hole) is comparable to the lowest absorption peak.48 When we see the bleaches of transient absorption spectra (shown later), the bleach of the second peak (X1) is much smaller than the lowest peak (X0). On the basis of this discussion, we use the 1D exciton model to explain the electronic structures of CdSe shells.49 The lowest peak and a shoulder at higher energy (∼540 nm) of the TiO2/CdSe absorption spectra are therefore assigned to the 1Σe−1Σ3/2 and the superposition of 1Σe−1Σ 1/2 and 1Πe−1Π3/2, respectively.50 Two control experiments were performed in order to examine the effect of TiO2 on the PL quenching in the TiO2/CdSe nanohybrids. CdSe QDs with high photoluminescence quantum yield (∼ 30%) were first prepared, and then mixed separately in two different vials with: (1) titanium tetraisopropoxide as a Ti-ion source and (2) TiO2 NRs (the same material used as a seed for the TiO2/CdSe nanohybrids) added at room temperature. These mixtures were stirred at room temperature for a few days before PL measurements were carried out. It was found that neither Ti-ions nor the TiO2 NRs affect the PL of the CdSe (Figure 6b), meaning no quenching occurs if the CdSe is not bound to the TiO2 surface. These results clearly show that CdSe PL is quenched only when TiO2 NRs are used as a seed for CdSe growth. The PL quenching in TiO2/CdSe NRs is rather due to the exciton dissociation by electron transfer than due to the surface defects at the interface of the CdSe and the TiO2 because the bleach decay due to the surface trapping does not explain the efficient PL quenching (discuss later).16,23,51−70 The charge separation process, in our system, is probably proceeding via tunneling transfer of electrons from the CdSe to the TiO2 NCs according to the type II alignment in a close-packed TiO2/ CdSe heterostructure.23,58,62,64,67,71 Kamat and his co-workers have reported that the TiO2 colloids are able to interact with excited CdSe23,58,62,64,67 and CdS particles.72 As the visible light excites the CdSe QDs nanocrystallites, electrons are excited to the conduction band (CB) with holes left in the valence band (VB) forming the excitons, then the excitons diffuse to the interface between the two materials of the heterostructure followed by the charge separation into the two different materials.53 Analysis of Pump−Probe Spectra. To examine the charge carrier relaxation pathways in TiO2/CdSe NRs following photoexcitation, we measured the exciton dynamics in TiO2/ CdSe NRs by femtosecond (fs) transient absorption (TA) spectroscopy.73 We tuned the pump wavelength to 650 nm to resonantly excite the CdSe exciton state, by an optical parametric amplifier (OPA) for the case of TiO2/CdSe NRs sample 1. The pump wavelength was set to 585 nm for sample 5, see the Supporting Information, while we tuned the pump 3352
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Figure 7. (a,b) Transient absorption spectra of TiO2/CdSe NRs for samples 2 and 1, respectively, recorded by a white light probe pulse after femtosecond excitation at 620 and 650 nm, respectively. (c) Transient absorption spectra of CdSe NRs recorded by a white light probe pulse after femtosecond excitation at 620 nm. (d,e,f) Decay of the bleach of TiO2/CdSe NRs (samples 2 and 1) and CdSe NRs assemblies in toluene following pulsed excitation.
state. Second, bleach of the 1.98 eV feature lasts much longer than the intraband transition of electron and hole from higher states. For example, we still see the bleach even on a time scale of nanoseconds, which means the 1.98 eV feature is not the bleach of a higher energy electronic transition. At short times, such as sub-picoseconds, after photoexcitation, the peak should involve a bleach corresponding to the 1Πe−1Π1/2 transition, but after several picoseconds, the 1Πe−1Π1/2 transition relaxes to the lowest excited sigma-related transition. As a consequences, the reason we see the bleach at the X2 even on a nanosecond time scale is because the 1Σe−1Σ1/2 exists near the 1Πe−1Π1/2 transition and X0 and X2 populate the same electron state, most probably 1Σ(e). These results are in agreement with the report by Norris et al.50 They measured photoluminescence excitation line-narrowed spectra of CdSe nanocrystals and assigned the third peak, i.e., X2, to 1Se−1S1/2 and not 1Pe−1P1/2. Although, the notation, fine structures and polarity of nanocrystals and nanorods systems are different, the order of states is basically the same, even when shapes change. As we can see from the figures (Figure 7a−c and Figure S8a−c), the bleach peaks shift with time. This time-dependent shift means that the decay extracted at only one wavelength does not give correct time constants for recovery of the bleach. We fitted a TA spectrum at each time delay with three Gaussian functions and determined the time evolution of the amplitude, peak, and width of each Gaussian component. Dynamics of all
wavelength for samples (2, 3, and 4) to 620 nm. With the 150 fs white light continuum as the probe pulse, we obtained the TA spectra shown in Figure 7 and Figure S8. The pump power (less than 20 μW) was carefully controlled to avoid multiexciton production in the NCs. We also investigated the relaxation dynamics of CdSe-NRs sample as a reference to investigate the surface traps accumulated with CdSe, which is shown in Figures 5d and 7f. Figures 7a and 7b show the TA spectra of TiO2/CdSe NRs samples 2 and 1, respectively, while Figure 7c shows spectra of CdSe NRs prepared without TiO2 seeds. In the TA spectra of TiO2/CdSe sample 2 (Figure 7a), two bleaches are mainly observed at 1.82 and 2.2 eV. However, since the peak at 1.82 eV has a tail at higher energy there is an additional peak around 1.98 eV. We determined the spectral peaks by fitting the spectrum with three Gaussian functions. We observed similar spectral features for the samples 3 and 4, but the TA spectra of samples 1 and 5 do not have the intermediate peak. We refer to these peaks as X0, X1, and X2 from low to high energy as indicated in Figure 7a. We assign X0 and X1 to the bleach of 1Σe−1Σ 1/2 and 1Σe−1Σ3/2, respectively. Although some reports assign X2 to 1Πe−1Π1/2, we assign X2 to the bleach of 1Σe−1Σ1/2 for the following reasons.47,50 First, the X2 bleach appears in spite of the excitation energy being lower than X2, which indicates that the electron or hole state is the same as X0 or X2, i.e., they share a common ground or excited 3353
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Table 1. Time Constants for Transient Bleach Recovery Measured for CdSe NRs and TiO2/CdSe Hybrid NRs Using Femtosecond Pump−Probe Experiments X0 bleach τ1 CdSe NRs TiO2/CdSe TiO2/CdSe TiO2/CdSe TiO2/CdSe TiO2/CdSe
#1 #2 #3 #4 #5
τ2
11 ± 2 ps (25%) 800 ± 200 ps (33%) 190 ± 40 ps (23%) 1.2 ± 0.5 ns (23%) -480 ± 20 ps (54%) 11 ± 1 ps (23%) 170 ± 20 ps (51%) -270 ± 390 ps (14%) 49 ± 8 ps (34%) 390 ± 30 ps (66%)
X2 bleach τ3
τ1
28 ± 12 ns (42%) 8.0 ± 0.9 ns (54%) 4.3 ± 0.2 ns (46%) 2.1 ± 0.1 ns (27%) 3.2 ± 0.2 ns (86%) 3.9 ± 0.1 ps (28%)
36 16 18 11 14 17
± ± ± ± ± ±
9 3 5 2 1 1
ps ps ps ps ps ps
τ3
red shift (meV)
24 ± 10 ns (38%) 5.0 ± 0.2 ns (59%) 3.5 ± 0.6 ns (47%) 2.6 ± 0.3 ns (26%) 3.1 ± 0.1 ns (44%) 3.5 ± 0.1 ps (55%)
33 23 46 116 79 25
τ2 (35%) 700 ± 170 ps (27%) (14%) 310 ± 20 ps (27%) (24%) 350 ± 70 ps (31%) (32%) 170 ± 20 ps (42%) (23%) 270 ± 60 ps (33%) (13%) 280 ± 30 ps (31%)
Figure 8. Relaxation models of excited carriers observed at (a) X0 bleach and (b) X2 bleach for TiO2/CdSe NRs.
parameters obtained by fitting (amplitude, peak energy, and bandwidth) are shown in the Supporting Information (Figures S9−S14). In Figure 7f we display the amplitude dynamics of X0 and X2 bleaches of CdSe NRs, while Figure 7d,e displays the amplitude dynamics of X0 and X2 bleaches of TiO2/CdSe NRs samples 2 and 1, respectively. (Other samples are shown in the Supporting Information, Figure S8). It is obvious from Figure 7a−c that the bleaches shift with time in both CdSe NRs and TiO2/CdSe NRs. The red-shift of the bleach band of each sample is tabulated in Table 1. The significant red-shift bleaching in TA measurements have not been observed in spherical nanoparticles.74,75 However, a red-shift of the bleach feature has been reported in studies of anisotropic heterostructures.12,13,60 Trapping on the surface of CdSe layers weakens the quantum confinement and causes a dynamical shift of the lowest bleach.76 In this case, the analysis of the higher bleach, such as X2, helps us to characterize the dynamics. For instance, the bleach of the 1S(e)−1S3/2(h) transition of CdSe nanocrystals contains predominantly the state filling of the excited electron state for the main contribution and the decrease of the ground state population, which has been revealed by electrochemical and time-resolved spectroscopic studies.77−79 This is explained by the fewer density of states of conduction band states due to the lighter effective mass of electron (0.13m0 for electron and 0.45m0 for hole respectively;
m0 is the mass of free electron).77−79 On the other hand, the bleaches of 1S(e)−2S3/2(h) and 1S(e)−1S1/2(h) excited at lower energy than these bleaches contain only the effect of the state filling of the electron because the hole cannot be populated to the higher state than the excited energy.80 Since we excited our samples at X1 in this experiment, the X2 bleach contains electron dynamics, and the X0 bleach contains the electron and hole dynamics. We analyzed X0 and X2 dynamics to obtain more reliable information. It is worth mentioning that this behavior is valid only for a strong confinement regime, in which the electron and hole move independently. Analysis of X0 and X2 Decays. We observe slightly different dynamics between X0 and X2, which may be a result of hole relaxation process in the X0 dynamics (Figure 8). The decay of TiO2/CdSe 1, 3 and 5 are fitted with triexponential functions, but 2 and 4 can be fitted with biexponential functions. Each decay time constant for bleach recovery kinetics is tabulated in Table 1. The fast (τ1) and middle (τ2) decay time constants become about 2 times faster than those of CdSe NRs, while the long decay (τ3) time constants become more than 5 times faster than those of CdSe NRs. However, these time constants do not explain the huge decrease of the emission quantum yield (from 30% for CdSe NRs to
