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J. Phys. Chem. C 2010, 114, 1460–1466
Ultrafast Charge Carrier Relaxation and Charge Transfer Dynamics of CdTe/CdS Core-Shell Quantum Dots as Studied by Femtosecond Transient Absorption Spectroscopy Sachin Rawalekar, Sreejith Kaniyankandy, Sandeep Verma, and Hirendra N. Ghosh* Radiation and Photo Chemistry DiVision, Bhabha Atomic Research Center, Mumbai-400 085, India ReceiVed: September 22, 2009; ReVised Manuscript ReceiVed: NoVember 23, 2009
We are reporting ultrafast charge carrier and charge transfer dynamics of the CdTe quantum dot (QD) and type II CdTe/CdS core-shell QD materials with different shell (CdS) thicknesses. Herein, we have synthesized CdTe and CdTe/CdS core-shell quantum dots by using 3-mercaptopropionic acid as a capping agent. Steady state absorption and emission studies confirmed successful synthesis of CdTe QD and CdTe/CdS core-shell QD materials. Time-resolved emission studies indicate a longer emission lifetime of the CdTe/CdS core-shell as compared to CdTe QD materials, where in both cases only CdTe gets excited. We have carried out femtosecond transient absorption studies of these QD and core-shell materials by exciting them with 400 nm laser light and monitoring the transients in the visible to near-IR region to study charge carrier and charge transfer dynamics in the ultrafast time scale. On laser excitation, electron-hole pairs are generated which are confirmed by induced absorption signal for the charge carriers in the visible and near-IR region and an immediate bleach at excitonic position for both QD and QD core-shell. The carrier relaxation was found to be slower and the carrier lifetime was found to be longer in the QD core-shell as compared to the QD indicating charge transfer from core to shell. Carrier quenching studies have been carried out for both CdTe and CdTe/CdS by using benzoquinone (BQ, electron quencher) and Pyridine (Py, hole quencher) to assign the different relaxation processes. Details about the relaxation of hot carriers and the quenching effect on the relaxation dynamic of the charge carriers have been discussed for both QD and core-shell nanostructures. 1. Introduction Semiconductor quantum dots (QD) are quasi zero dimensional materials that have gained a great deal of research interest in the past decade due to their exciting size- and shape-dependent properties.1,2 Much effort has been made to synthesize QD materials with band gaps that can be easily tuned by changing their size and shape. These materials are highly luminescent with narrow emission line widths, which have enormous potential applications, including use in solar cells,1–3 biolabeling,4 QD-based lasers,5–7 and so forth. Various pathways of carrier cooling in QD-like nonradiative auger recombinations, phononmediated relaxations, and relaxations to surface state are well documented in the literature.8–16 However, for better application of QDs, efficient charge separation has remained a challenge. Different approaches, like metal-semiconductor nanocomposite17–21 use of polymers,22–24 core-shell nanostructure,3–7,25–32 have been used for the separation of charge carriers. Semiconductor core-shell nanostructure is one of the best possible methods for charge separation, which can be synthesized by combining two or more semiconductors with a proper lattice mismatch between them. Recently, considerable progress has been made in the synthesis of type II hetero junction nanostructures that spatially separate photoexcited electrons and holes in different parts of a composite QD. These core-shell nanocrystals are made up of two semiconductor materials with a particular alignment of conduction and valence band edges at the interface, which creates a step-like radial potential favoring the localization of one of the carriers in the core of a QD and the other one in the shell. The resulting charge separation leads to indirect band gap emission. This spatial * To whom correspondence should be addressed. Tel: 00-91-22-25590300; Fax: 00-91-22-2550-5151; E-mail:
[email protected].
separation of charge carriers leads to several characteristic differences from the type-I QDs, where both the conduction band and valence band levels of core are lower in energy than the shell. In type II core-shell QDs, the interband emission is allowable with an energy gap that would be otherwise inaccessible with type-I structures, extending the color tuning capability. Also, it is thought that these fascinating properties would facilitate the use of QDs in several technological applications including photovoltaics, where spatial charge separation can increase the light conversion efficiency. Considerable research effort has been devoted to charge transfer dynamics of type II semiconductor QDs.3–7,25–32 Bawendi and co-workers3 reported time-resolved emission in CdSe/ZnTe and CdTe/CdSe, where they have shown that charge recombination time in CdSe/ZnTe can be increased from 9.6 to 57 ns. Chen and co-workers4 have reported spatial separation of charge carrier by changing size of core and shell in CdSe/ZnTe. Feldman and co-workers25 reported charge separation and tunneling behavior between CdTe and CdSe QD material with the formation of core-shell structures. Banin and co-workers26 reported photophysical properties of ZnSe QD within CdS nanorods and demonstrate the charge separation. However, to the best of our knowledge, reports are not available on ultrafast charge (electron/hole) transfer and carrier relaxation in CdTe/ CdS type II core-shell material, which is very important in view of applications in optoelectronic devices and quantum dot (QD) solar cells. The CdTe/CdS hetero junction is particularly attractive due to a highly efficient charge separation across the interface. In these CdTe/CdS nanostructure materials, the band alignment is such that the valence band of CdS lies well below the valence band of CdTe, which leads to localization of photoinduced holes in the CdTe core, and the energetically lower
10.1021/jp909118c 2010 American Chemical Society Published on Web 12/30/2009
CdTe/CdS Core-Shell Quantum Dots SCHEME 1: Conduction and Valence Band Levels of CdTe and CdS Bulk Semiconductora
a
Energy levels are shown with respect to vacuum.
positioned conduction band of CdS favored the localization of electrons in the CdS shell. As a result in the CdTe/CdS heterostructure, it is expected to have an efficient electron-hole (e-h) separation for a minimal shell thickness. The band alignment in CdTe/CdS can enhance other vital electronic properties which in turn can be useful for number of practical applications. In the present investigation, we have synthesized CdTe/CdS type II core-shell by using 3-mercaptopropionic acid as a capping agent by a previously reported method. Formation of core-shell material was confirmed by optical absorption and photoluminescence studies. In Scheme 1, we have shown the band alignment of bulk CdTe/CdS nanostructured materials, which show that this nanostructured pair forms type-II core-shell material. We have carried out steady state emission studies which show that on core-shell formation, the emission properties of these materials drastically changes. To unravel the charge transfer dynamics, we have carried out transient absorption studies in ultrafast time scale, detecting the transients in the visible to near-IR region by exciting CdTe in both the CdTe core and the CdTe/CdS core-shell material by exciting at 400 nm laser light. Electron and hole quenching experiments have been carried out with quantum dots and its core core-shell to separate out the electron and hole dynamics, which helps to assign different relaxation processes. 2. Experimental Section 2.1. Materials. Cadmium chloride (g99%), Te powder (g98), 1-mercaptopropionic acid (g99%), NaBH4, and benzoquinone were purchased from Aldrich and used as-received without further purification. Isopropyl alcohol was used as received from S.D. Fine Chemicals. Pyridine (Fluka) was used after distillation. 2.2. Synthesis. 2.2.1. Synthesis of NaHTe. NaHTe was synthesized by reducing Te powder with sodium borohydride at 0 °C for 6 h in nitrogen-purged nanopure water. The molar ratio between the Te powder and the sodium borohydride was maintained at 1:2. The clear solution of NaHTe obtained was further used as Te precursors. 2.2.2. Synthesis of CdTe. The water-soluble CdTe QD was prepared by a previously reported method.33 In brief, cadmium chloride, a cadmium precursor was dissolved in nitrogen purged nanopure water followed by addition of 1-mercaptopropionic acid (MPA) and pH was adjusted to 10 slowly by using 1 M NaOH. NaHTe solution was further added in the above solution with vigorous stirring at temperature 90 °C. A ratio between Cd, MPA, and Te was maintained at 1:2.5:0.5. Reaction was continued for six hours. The reaction mixture was allowed to cool naturally and was concentrated to 1/3rd of its original volume by using a rotary evaporator. Iso-propyl alcohol was added to this concentrated solution as a nonsolvent to precipitate the particle. The supernatent solution at this stage of precipitation was thrown out and the precipitate was redissolved in nanopure
J. Phys. Chem. C, Vol. 114, No. 3, 2010 1461 water. This cleaning procedure was repeated three times, and the pure CdTe QD sample was used as the core material for further steps. The QDs obtained by this method are normally spherical in shape, so the size of the particle can be calculated by using the equation given by Peng and co-workers.34 2.2.3. Synthesis of CdTe/CdS. The CdTe/CdS core-shell quantum dot was synthesized using a previously reported method.30 Briefly, the CdTe QD (core) sample (10-6M) was dissolved in nanopure water, followed by addition of MPA, and finally, the pH of the mixed solution was adjusted to 10. Thioacetamide (TAA) was further added as the sulfur source. The reaction temperature was maintained at 75 °C for 2 h and 90 °C for 4 h. After the reaction, the solution mixture was cleaned by the same method as discussed earlier for CdTe. This cleaned sample was dissolved in nanopure water and used for further study. The thickness of the CdS shell was varied by adjusting the concentration of the injected precursors of cadmium and sulfur into CdTe core solution. 2.3. Steady State Absorption and Emission Spectrometer. Steady-state absorption spectra were recorded on a ThermoElectron model Biomate spectrophotometer. Fluorescence spectra, which were corrected for the wavelength dependence of the instrument sensitivity, were recorded using Hitachi model 4010 spectrofluorimeter. 2.4. Time-Correlated Single Photon Counting. Timeresolved fluorescence measurements were carried out using a diode laser based spectrofluorometer from IBH (UK). The instrument works on the principle of time-correlated singlephoton counting (TCSPC).35 In the present work, a 453-nm LED was used as the excitation light sources and a TBX4 detection module (IBH) coupled with a special Hamamatsu PMT was used for fluorescence detection. 2.5. Femtosecond Visible Spectrometer. The femtosecond tunable visible spectrometer has been developed based on a multipass amplified femtosecond Ti:sapphire laser system supplied by Thales, France. The pulses of 20 fs duration and 4 nJ energy per pulse at 800 nm obtained from a self-mode-locked Ti-Sapphire laser oscillator (Synergy 20, Femtolaser, Austria) were amplified in a regenerative and two-pass amplifier pumped by a 20 W DPSS laser (Jade) to generate 40 fs laser pulses of about 1.2 mJ energy at a repetition rate of 1 kHz. The 800 nm output pulse from the multipass amplifier is split into two parts to generate pump and probe pulses. In the present investigation, we have used frequency doubled 400 nm as excitation sources. To generate pump pulses at 400 nm one part of 800 nm with 200 µJ/pulse is frequency doubled in BBO crystals. To generate visible probe pulses, about 3 µJ of the 800 nm beam is focused onto a 1.5-mm thick sapphire window. The intensity of the 800 nm beam is adjusted by iris size and ND filters to obtain a stable white light continuum in the 400 nm to over 1000 nm region. The probe pulses are split into the signal and reference beams and are detected by two matched photodiodes with variable gain. We have kept the spot sizes of the pump beam and probe beam at the crossing point around 500 and 300 µm, respectively. The noise level of the white light is about ∼0.5% with occasional spikes due to oscillator fluctuation. We have noticed that most laser noise is low-frequency noise and can be eliminated by comparing the adjacent probe laser pulses (pump blocked vs unblocked using a mechanical chopper). The typical noise in the measured absorbance change is about 20 meV below the bright exciton state in wurtzite state. Recently, Scholes and co-workers41,42 studied timeresolved PL of CdSe/CdS/ZnS at low temperature and found different lifetime regimes because of the splitting of dark and bright exciton fine structure states. They have also observed the effect of carrier trapping on the average lifetime of CdSe/ CdS/ZnS nanocrystals, which indicates trapping dynamics of the particles are size and temperature dependent. However, at room temperature, carrier trapping can also influence the average lifetime in addition to radiative recombination lifetime. So, to assign various relaxation processes in QDs and their core-shell nanostructures, it is very important to carry out ultrafast transient absorption studies which will be useful to understand their detailed photophysics. 3.3. Transient Absorption Spectroscopy of CdTe QD. To understand charge carrier relaxation and charge recombination dynamics in CdTe QDs, we have carried out femtosecond transient absorption studies detecting the transients in visible and near-IR region after excitation at 400 nm laser light, and the transient spectrum has been shown in Figure 3. The transient absorption spectra show negative absorption in the 450-600 nm region with a bleach peak ∼540 nm and positive absorption band beyond 610 nm region. It is interesting to see that the bleach peak appears at 540 nm which exactly matched with 1S exciton peak appeared in the steady state absorption of CdTe QD (Figure 1). Emission peak for CdTe appears at 618 nm which has relatively lower quantum yield does not interfere with
J. Phys. Chem. C, Vol. 114, No. 3, 2010 1463 the bleach at 540 nm. So we have attributed that the bleach at 540 nm to be exclusively due to state filling of the 1S exciton. The bleach recovery kinetics at 540 nm can be attributed to carrier recombination dynamics between photogenerated electron and holes. It is well-known that the magnitude of the statefilling induced bleach is proportional to the sum of the occupation number of the quantized electron and hole states involved in the transition. As degeneracy of the valence band is high and the difference in the effective mass of the hole as compared to the electron is very high, the room temperature occupation probability of the lower electron state is much greater than that of the coupled hole state and hence the state filling induced bleach portion of the transient absorption is sensitive to the electron.8 The transient positive absorption beyond 620 nm region can be attributed to absorption of light by photogenerated charge carriers (both electrons and holes) in free or defect state trapped QDs. In addition, positive absorption can also arise due to nonlinear effects like two electron-hole pair interaction (biexciton effect) and trapped-carrier induced stark effect. To exclude these many-particle interactions, we have recorded the transient data at low pump intensity so that average number of electron-hole pair () is less than 1. This average number of electron-hole pairs can be calculated by using relation 400 ps (35%). On excitation of QDs, initially electrons are populated in higher excitonic levels and then gradually it populates the lower excitonic level after depopulating from upper levels. In the present investigation, the bleach position at 540 nm signify lowest excitonic position, so it will populate slowly after depopulation of higher exciton. Here, the 220 fs time constant can be attributed to state filling to 1S excitonic level. The bleach recovery time constants suggest the charge recombination time constants between photogenerated electrons and holes. We have also monitored decay kinetics of excited state absorption (ESA) of photo-excited CdTe at 900 nm (Figure 3B) which can be fitted multiexponentially 1 ps (42%), 10 ps (25%), and >200 ps (37%). It is interesting to see that the growth time of the signal can be fitted single exponentially with the pulsewidth limited time constant (
