Ultrafast Charge Transfer Dynamics in Photoexcited CdTe Quantum

Jul 7, 2012 - ... Ioannis Ierides , Franco Cacialli , Christoph G. Salzmann , Matteo Palma ... Ruifeng Li , Lorenz Maximilian Schneider , Wolfram Heim...
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Ultrafast Charge Transfer Dynamics in Photoexcited CdTe Quantum Dot Decorated on Graphene Sreejith Kaniyankandy,* Sachin Rawalekar, and Hirendra N. Ghosh* Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai, 400085 India S Supporting Information *

ABSTRACT: We report synthesis and ultrafast charge transfer dynamics of photoexcited CdTe quantum dots (QDs) decorated on graphene. We have synthesized CdTe QD particles of 2.2 nm sizes with first exciton (1S3/2-1Se) band ∼450 nm and then decorated the QD particles on graphene which has been confirmed by HRTEM studies. The CdTe QD decorated graphene has been named as GCdTe. Steady state emission studies revealed that on the graphene surface CdTe emission gets quenched drastically which indicates the charge transfer from photoexcited CdTe to graphene. To unravel the charge transfer dynamics in ultrafast time scale we have carried out femtosecond transient absorption studies by exciting the CdTe QD particles and monitoring the transients in the visible to near-IR region. Transient absorption studies indicate that exciton recombination time (as monitored the exciton bleach) of pure CdTe QD takes place within 50 ps; however, on graphene the surface exciton recombination time was found to be much longer (>1 ns). Our studies clearly indicate that charge separation of G-CdTe composite materials drastically improves as compared to that CdTe QD.

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INTRODUCTION Tunable optoelectronic properties in semiconductor quantum dots (QDs) in a size quantization regime has enabled the utilization of these materials in a variety of devices.1−4 The size quantization effect has been studied extensively, and there are enormous potential benefits as shown by “proof of principle” devices.5−7 One of the wide ranging applications for QD is in the field of photovoltaics, where tunability of optoelectronic properties helps in covering different regions of the solar spectrum thereby enabling the extraction of solar energy.8,9 However several independent studies have shown that the realization of efficient photovoltaic devices out of QD materials depends on the ability to extract the photoexcited charge carriers (either electron or hole) from the QD before trapping or recombination of the charge carriers (see ref 4 and references therein). In most of the QD both the trapping and charge recombination events take place on an ultrafast time scale. To prevent fast trapping and charge recombination and extract the charge carriers, conducting polymers and carbon nanotubes (CNT) have been used.10,11 Alivisatos and co-workers10 have demonstrated a conducting polymer can be used in a QD solar cell which prevents fast charge recombination and eventually increases the efficiency of the devices. Kamat and co-workers11 have demonstrated that single wall carbon nanotube (SWCNT) architectures when employed as conducting scaffolds in a TiO2 semiconductor based photoelectrochemical cell can boost the photoconversion efficiency. Similar in structure to CNTs, graphene has a remarkable potential as an electron relay due to its high electron drift velocity.12 Graphene by virtue of its unique © 2012 American Chemical Society

linear electronic dispersion near the K-point of the Brillion zone enables the electron to travel along the sheet at quasirelativistic velocities and therefore can in principle act as an efficient relay material for the injected electrons. The higher drift velocities in graphene enable a fast separation of electrons from the site of injection. However for an efficient extraction of the charge carriers, an electronic contact needs to be achieved between the QDs and graphene. Studies on synthesis have shown that the growth of the QD over graphene during the synthesis procedure can allow a good electronic contact with the QD to be achieved.13 Wang et al.14 and Cao et al.15 have shown the growth of QDs of CdS and CdSe by colloidal methods, and their photophysical and chemical properties have shown tremendous potential for different applications. In addition to that, QDs−graphene assemblies also have shown potential application in photovoltaic measurements.13,16 It has been realized that graphene can extract a photoexcited electron in QD materials very efficiently which can efficiently compete with multiexciton annihilation. Cao et al.15 have shown electron extraction in a CdS−graphene composite on the picosecond time scale as studied from timeresolved emission spectroscopy. In view of these studies it is necessary to know how fast graphene can extract an electron from photoexcited QD materials. To address this issue, it is very important to carry out transient absorption studies on the femtosecond time scale with a suitable QD−graphene composite Received: April 18, 2012 Revised: June 6, 2012 Published: July 7, 2012 16271

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Femtosecond Visible Spectrometer. A femtosecond tunable visible spectrometer has been developed based on a multipass amplified femtosecond Ti:sapphire laser system supplied by Thales, France as described previously.19 The instrument response function (IRF) for 400 nm excitation was obtained by fitting the rise time of the bleach of sodium salt of mesotetrakis (4 sulfonatophenyl) porphyrin (TPPS) at 710 nm and was found to be 120 fs. The concentration of CdTe samples was 10−5 M as calculated from the extinction coefficient from Rogach et al.20 at the exciton peak position. The ⟨N⟩ = jσ value was maintained at 0.1 excitons per particle to avoid multi particle relaxation processes like Auger process. Characterization of CdTe and G-CdTe Composite Materials. UV−vis spectra were recorded on CHEMITO, SPECTRASCAN UV 2600 Double Beam UV−vis spectrophotometer in slow scan mode. Photoluminescence studies were conducted on a fluorescence spectrometer (Hitachi model F4010). High-resolution TEM (HRTEM) measurements were carried out using a JEOL-JEM-2010 UHR instrument operated at an acceleration voltage of 200 kV with a lattice image resolution of 0.14 nm. SAED (selected area electron diffraction) and EDAX (energy dispersive X-ray analysis) patterns were also collected by the TEM instrument. To take TEM images of the graphene and graphene−CdTe composite materials we have dissolved the samples in water. A drop of sample was added to the TEM grid (200 mesh size holey carbon coated Cu grid). The sample on the grid was heated in an oven for 30 min at 40 °C to remove the excess solvent.

material. In the present investigation, we have synthesized watersoluble CdTe decorated on graphene (G-CdTe) and carried out ultrafast transient absorption spectroscopy and time-resolved emission spectroscopy by exciting G-CdTe nanocomposites with laser pulse. For this purpose we have chosen chemically exfoliated graphene oxide (GO) prepared by modified Hummers method to synthesize GO and reduced it to graphene by previously reported procedures and used this as the template for growth of CdTe in solution medium. The CdTe QDs were synthesized by colloidal methods in aqueous solution. High resolution TEM measurements have clearly shown that CdTe QD indeed decorated on graphene. Ultrafast transient absorption studies clearly indicate efficient charge separation on G-CdTe composite materials.

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EXPERIMENTAL SECTION Synthesis of GO. GO was synthesized from graphite by the modified Hummers method.17 Concentrated H2SO4 (10 mL) was heated to 90 °C in a 50 mL beaker. Later K2S2O8 (1.7 g) and P2O5 (1.7 g) was added to the acid, and then the mixture was stirred vigorous until it dissolved. The mixture was then cooled to 80 °C and graphite powder (2 g) was added to the H2SO4 solution mixture. It is to be noted that on addition of graphite the mixture bubbled for at least 20 min. The temperature was maintained at 80 °C for 4.5 h and the mixture diluted with 2 L of DI water and left overnight. The mixture was filtered and the powder dried in the ambient atmosphere. In the second step, the above dried graphite (2 g) was added to a 250 mL flask to which 50 mL of H2SO4 was added at room temperature. The above solution was cooled to 0 °C followed by addition of 7 g of KMnO4 slowly while maintaining the temperature below 10 °C at all times. After complete addition of KMnO4, the temperature was increased to 10 °C. The mixture was stirred for 2 h. Excess water was added into the mixture at 0 °C (ice bath), and then H2O2 (30 wt % in water) was added until the effervescence ceased. The GO powder was washed with copious amounts of water and dried under ambient conditions. Reduction of GO. A total of 1g of as prepared GO was dispersed in 1 L of 1 M NaOH and sonicated for 1 h. This solution was cooled to 0 °C. To this solution was added with vigorous stirring 2 g of NaBH4. This above solution was maintained at 0 °C for 2 h and heated to 50 °C for 2 h.18 Synthesis of CdTe Decorated Graphene. The CdTe QDs were prepared by colloidal methods as reported earlier.19,20 Briefly, 10 mmol of CdCl2. 2.5H2O was added to the above solution of reduced graphene (1 g/L). To this was added mercaptopropionic acid (MPA) with a mole ratio of MPA:Cd = 2.4:1. The solution was then purged with nitrogen gas (N2). The tellurium precursor NaHTe was prepared by the reaction between NaBH4 and tellurium powder in N2 purged water at 0 °C for 8 h. Part of this solution was added to the Cd precursor solution with the ratio of Cd:Te = 1: 0.5. The above mixed solution was then refluxed for 4 h. The solution was then concentrated at reduced pressure by a Buchi Rotavapor to 1/3 of the original volume, and isopropyl alcohol was added to the concentrated solution to precipitate the graphene−QDs composites. The collected composites were redissolved in nanopure water and precipitated with isopropyl alcohol, and this process was repeated 3 times to remove the precursors and clean the composites. Synthesis of CdTe QDs for the comparison of dynamics have been carried out by the same procedure as described above without the addition of exfoliated graphene in a 1 M NaOH solution.

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RESULTS AND DISCUSSION Figure 1 shows the TEM images of G-CdTe samples. The images clearly show decoration of CdTe QDs over graphene sheets. Our

Figure 1. TEM (high and low magnifications) and electron diffraction of G-CdTe samples.

TEM images clearly show that most of the QDs are isolated with some of the QDs coupled together. The size of CdTe QDs was found to be ∼2 nm. The HRTEM images show a d spacing of 0.24 nm as obtained from lattice fringes which matches well with the (220) plane for cubic CdTe (JCPDS database no. 150770).21 The selected area electron diffraction shows predom16272

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inantly two rings with a d-spacing of 0.24 nm and 0.194 nm which is close to the (220) and (311) planes of Cubic CdTe. Figure 2, inset, shows optical absorption spectra of CdTe (curve a) and graphene−CdTe (G-CdTe) (curve b). The clear

decay kinetics of CdTe in water and on the graphene surface. The emission kinetics of CdTe can be fitted multiexponentially with time constants of τ1 = 0.27 ns (10.5%), τ2 = 2.3 ns (28.2%), and τ3 = 14.6 ns (61.3%). The luminescence decay kinetics may be governed by both intrinsic and surface states. According to previous studies by Rogach et al.20 on CdTe, multiexponential dynamics arises from Te related surface traps. Since there is an energetic heterogeneity of traps states, it leads to a trap state energy distribution. Additionally the Te related trap position decreases in energy for large QDs. Therefore, these states for large QDs remain well within the valence band edge; however, on smaller QDs some part of the trap state distributions appears above the valence band leading to trapping of holes. It should also be mentioned that the thiol capping influences position of these traps. The crossover size of CdTe QD for appearance of multiexponential to monoexponential decay is 4−4.5 nm.20 In the present case the size of CdTe QDs is ∼2 nm; therefore, it can be safely argued that multiexponential decay arises from surface traps. On the graphene surface the emission kinetics of CdTe can be fitted with time constants τ1 = 0.19 ns (57%), τ2 = 0.75 ns (25%), and τ3 = 3.4 ns (18%). It is clearly seen from Figure 3 that emission kinetics of CdTe on the graphene surface decay much faster which is a clear indication of electron transfer from photoexcited CdTe. The faster lifetimes appear due to a carrier quenching due to the electron accepting nature of graphene. However from nanosecond time-resolved emission spectroscopy, it is difficult to comment on the electron transfer time from photoexcited CdTe to graphene. To understand the charge transfer dynamics on the early time scale, we have carried out femtosecond transient absorption spectroscopy exciting both CdTe QD and G-CdTe composites at a 400 nm laser pulse. Figure 4 shows the transient absorption spectra of CdTe and G-

Figure 2. Photoluminescence spectra of (c) CdTe and (d) G-CdTe after exciting the sample at 380 nm. Inset: optical absorption spectra of (a) CdTe and (b) G-CdTe.

discerning feature in the spectrum is a band at 440 nm for CdTe assigned to 1Se-1S3/2 exciton. The size of the particles has been determined to be 2.2 nm as determined from sizing curves as reported previously.20 This size obtained from the optical absorption spectrum matches well with the size obtained from TEM. A closer look at the absorption spectra in Figure 2d reveals a feature at ∼300 nm which matches well with the graphene π−π* absorption. We have carried out steady state photoluminescence studies for both CdTe and G-CdTe samples after exciting the samples at 380 nm. Figure 2c shows the emission spectra of CdTe QD with a single feature in the visible region with emission maxima at 500 nm which can be attributed to radiative recombination of the 1S3/2-1Se exciton. Figure 2d show the emission spectra of the G-CdTe composite. A similar peak position of CdTe and G-CdTe for both emission and excitation spectra clearly suggest that the sizes are very similar (Supporting Information). It is clearly seen that 90% of emission CdTe QD is quenched on the graphene surface as compared to that of bare CdTe. The quenching of emission could be due to transfer of electrons from photoexcited CdTe to graphene. Earlier Cao et al.15 reported emission quenching of CdS on the graphene surface by time-resolved emission studies. In the present investigation also we have carried out time-resolved emission of CdTe on the graphene surface and compared the emission decay trace with that of bare CdTe. Figure 3 shows the emission

Figure 4. Femtosecond transient absorption spectrum at different time delay for CdTe (a) and G-CdTe (b) after exciting the samples at 400 nm laser light.

CdTe nanocomposites at different time delays. The spectra are comprised of bleach below 500 nm for CdTe QD samples and below 550 nm for G-CdTe nanocomposites. We have observed a broad absorption feature in the red region of the spectra for both of the samples. We ascribe the bleach feature to the first exciton of the CdTe QD in both cases. This bleach could be attributed to state filling transitions as the position matches well with the excitonic absorption of the CdTe QDs. The bleach in the case of CdTe mostly arises from the electron due to much higher effective mass of the electron as compared to the hole and greater

Figure 3. Time-resolved emission kinetics of (a) CdTe and (b) G-CdTe after exciting the sample at 400 nm. 16273

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degeneracy of the valence band. Therefore the bleach dynamics that we monitor mostly pertains to electron dynamics.19,22,23 It is interesting to see that, although the transient spectra in both CdTe and G-CdTe look very similar, the intensity of excited state absorption (ESA) in comparison to its bleach is much higher in CdTe as compared to that in G-CdTe. The positive charge carriers can be attributed to trapped charge carriers in QDs.19 This observation indicates that trapping of charge carriers in the defect states is much higher in CdTe as compared to that in G-CdTe. This observation also clearly indicates that phtoexcited electrons in QD prefer to transfer to the graphene sheet in GCdTe rather than to be trapped in the defect states of QD. To understand the charge transfer dynamics in G-CdTe, we have compared bleach recovery kinetics for both CdTe QD and GCdTe composite materials monitoring at 460 nm (excitonic position) and shown in Figure 5. The bleach recovery kinetics

Figure 6. Kinetics decay traces at 460 nm for G-CdTe and G-CdTe-EA after excitation at 400 nm.

observed that the bleach signal reduces by 60% but the bleach kinetics does not change in the presence of BQ. The bleach recovery kinetics can be fitted multiexponentially with time constants τ1 = 6 ps (22.4%), τ2 = 50 ps (24.2%), and τ3 = >200 ps (51.6%) which is very similar to that without quencher. The reduction of the bleach signal in G-CdTe in the presence of BQ is due to the quenching of a photoexcited electron in the QD. Here there is a competition between electron injection to the graphene sheet and reaction with BQ. So from our observation, we can safely conclude that the slower components in G-CdTe composite both in the presence and absence of BQ can be attributed to an injected electron in graphene and a hole in QD. Furthermore the dynamics of the bleach also does not arise on excitation of pure graphene, where we have observed an insignificant positive signal for the current graphene concentration used in the present investigation. From the above experimental observation, we demonstrate the electron transfer reaction in the following schematic diagram (Scheme 1). In the literature, the reported energy level for

Figure 5. Bleach recovery kinetics at 460 nm for (a) CdTe and (b) GCdTe after 400 nm laser excitation.

primarily gives the information of recombination dynamics of photoexcited charge carriers. The bleach recovery kinetics can be fitted multiexponentially with time constants of τ1= 0.2 ps (49%), τ2= 3 ps (23%) and τ3= 17 ps (28%) for CdTe QD and τ1 = 6 ps (25.4%), τ2 = 50 ps (20.2%), and τ3 = >200 ps (54.4%) for GCdTe composite. The bleach kinetics at 460 nm clearly indicates that recombination dynamics is much faster in CdTe QD as compared to that of the G-CdTe composite. From the above observation it can be concluded that graphene plays a significant role in relaxation dynamics in the photoexcited G-CdTe composite. As discussed earlier, the bleach has a significant component arising from the electron in the case of CdTe. Therefore one can conclude that the delayed dynamics in the case of G-CdTe is clearly due to an electron transfer into graphene on photoexcitation of CdTe QDs. This observation clearly indicates that graphene can act as an electron relay which enables an efficient charge separation. To gain further insight into the charge transfer dynamics, we study the dynamics in the presence of an electron quencher. Electron quencher (benzoquinone, BQ in this case) leads to a fast separation of electrons from the conduction band. In our earlier investigation we observed very efficient electron quenching in CdTe QDs by BQ.19 The results of these have unambiguously verified the role of benzoquinone as an electron shuttler on the surface of CdSe and CdTe QD. BQ accepts the electron within the pulse width and the BQ anion situated on the surface donates the electron back to the valence band hole within a few picoseconds. We have also observed that even the trapped electrons are also got quenched in presence of BQ which eventually drastically reduces (more than 95%) the bleach signal, and bleach kinetics recovers very fast. Figure 6 shows the bleach recovery kinetics of G-CdTe at 460 nm in the presence and absence of BQ. Here we have

Scheme 1. Schematic Diagram of the Electron Transfer Reaction from Photoexcited CdTe QD to Graphene in GCdTe Composite Material

valence band (EVB) and conduction band (ECB) found to be −5.2 and −3.5 eV, respectively, with respect to vacuum.24 Now the expressions for the valence and conduction band shift with respect to diameter of the QDs used are given below 19.03 d1.13 16.38 + 0.92 d

bulk E VB = E VB − bulk ECB = E VB

The positions of the valence and conduction band can be determined as ∼−5.8 and −2.6 eV, respectively, in the present investigation. The Fermi level of graphene is reported to be −4.5 eV with respect to vacuum.25 In G-CdTe the band alignment is such that the Fermi level of graphene is much lower as compared to the conduction band of CdTe. As a result electron transfer 16274

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(17) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771−778. (18) Shin, H.-J.; Kim, Ki. K.; Benayad, A.; Yoon, S.-Mi.; Park, H. K.; Jung, I.-S.; Jin, M. H.; Jeong, H.-K.; Kim, J. M.; Choi, J.-Y.; Lee, Y. H. Adv. Func. Mater. 2009, 19, 1987−1992. (19) Kaniyankandy, S.; Rawalekar, S.; Verma, S.; Palit, D. K.; Ghosh, H. N. Phys. Chem. Chem. Phys. 2010, 12, 4210−4216. (20) Rogach, A. L.; Franzl, T.; Klar, T. A.; Feldmann, J.; Gaponik, N.; Lesnyak, V.; Shavel, A.; Eychmuller, A.; Rakovich, Y. P.; Donegan, J. F. J. Phys. Chem. C 2007, 111, 14628−14637. (21) Powder Diffract. File, JCPDS International Centre Diffract. Data, PA 19073−3273, U.S.A., 2001. (22) Klimov, V. I. J. Phys. Chem. B 2000, 104, 6112−6123. (23) Klimov, V. I.; McBranch, D. W.; Leatherdale, C. A.; Bawendi, M. G. Phys. Rev. B 1999, 60, 13740−13749. (24) Sapra, S.; Sarma, D. D. Phys. Rev. B 2004, 69 (125304), 1−7. (25) Lightcap, I. V.; Kosel, T. H.; Kamat, P. V. Nano Lett. 2010, 10, 577−583.

reaction from the conduction band of CdTe to graphene is thermodynamically feasible, which we have confirmed by timeresolved photoluminescence and ultrafast transient absorption studies. In conclusion we state that we have synthesized water-soluble G-CdTe nanocomposites by a colloidal method. UV−visible absorption revealed an absorption at 450 nm indicating a particle size of ∼2.2 nm for CdTe QD which is also confirmed by HRTEM studies. The interaction between graphene and CdTe was studied by luminescence spectroscopy which indicated a 90% quenching of CdTe luminescence in G-CdTe nanocomposites. Transient absorption studies by monitoring bleach recovery of the lowest exciton state revealed extended lifetime photoexcited charge carriers in G-CdTe which indicates higher charge separation. This was also further ascertained by electron quenching in transient absorption studies. Our studies clearly show the role of graphene in charge separation in photoexcited CdTe QD. The inferences from the present study highlight the role of graphene as an efficient electron relay material.

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ASSOCIATED CONTENT

S Supporting Information *

Photoluminescence and photoluminescence excitation spectra of CdTe and G-CdTe samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.N.G.); [email protected] (S.K.). Fax: (+) 91-22-25505331/25505151. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely thank Dr. D. K. Palit, Dr. S. K. Sarkar, and Dr. T. Mukherjee for their encouragement and support. REFERENCES

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