Enhanced Charge Separation in an Epitaxial MetalâSemiconductor

Article pubs.acs.org/JPCC

Enhanced Charge Separation in an Epitaxial Metal−Semiconductor Nanohybrid Material Anchored with an Organic Molecule Jayanta Dana, Tushar Debnath, Partha Maity, and Hirendra N. Ghosh* Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India

Downloaded by CENTRAL MICHIGAN UNIV on September 13, 2015 | http://pubs.acs.org Publication Date (Web): September 11, 2015 | doi: 10.1021/acs.jpcc.5b06055

S Supporting Information *

ABSTRACT: Heterostructure (HSs) composed of a Au metal nanoparticle and a CdSe semiconductor quantum dot (QD) have been synthesized and characterized by high-resolution TEM (HR-TEM) steady-state absorption and luminescence spectroscopic measurements. Time-resolved emission and femtosecond transient absorption (TA) spectrosopic measurements have been employed to study ultrafast charge-transfer dynamics after sensitizing CdSe{Au} HS with the bromopyrogallol red (Br-PGR) molecule. Charge-transfer (CT) complex formation between CdSe{Au} HS and Br-PGR was confirmed by steady-state absorption spectrocopy, making {Au}CdSe/Br-PGR a tricomposite system. Electron transfer from photoexcited CdSe QD to the Au NP was confirmed by both steady state and time-resolved emission studies and also by ultrafast TA measurements. Charge separation in the CdSe{Au}/Br-PGR tricomposite system took place in multiple ways: electron transfer from the conduction band (CB) of CdSe QD to Au NP, hole transfer from the valence band of CdSe QD to Br-PGR, and electron injection from photoexcited BrPGR to the CB of CdSe QD and finally to the Fermi level of Au NP. The electron-transfer time from photoexcited CdSe to Au NP was found to be ∼270 fs; however, the hole-transfer time from photoexcited CdSe to Br-PGR was measured to be ∼500 fs. Spectroscopic investigation suggests that in the CdSe{Au}/Br-PGR tricomposite system, upon photoexcitation, all the photoexcited electrons are localized in Au NP and all the holes are localized in Br-PGR, confirming spatial charge separation in the tricomposite system. The grand charge separation process implies that the CdSe{Au}/Br-PGR tricomposite system can be emoloyed as an advanced material for quantum dot solar cells (QDSCs). states of metal and semiconductor takes place very effectively.20 As a result, the interdomain charge transfer can take place very efficiently from the semiconductor domain to the metal domain.21−24 In most of the nanohybrid systems the Fermi level of the metal lies below the CB of the semiconductor. As a result, electron transfer from photoexcited QD to metal NP is thermodynamically viable. Park and co-workers25 demonstrated ultrafast electron transfer from photoexcited CdSe to Pt metal in Pt−CdSe−Pt nanodumbbells. Lian and co-workers26 demonstrated ultrafast electron transfer from the photoexcited CdS rod to the Au tip and also plasmonic hot electron transfer from the Au tip to the CB of the CdS rod. Again, ultrafast charge separation and a long-lived charge-separated state were observed by Lian and co-workers16 in CdS−Pt nanorod heterostructures due to ultrafast electron transfer from CdS QD to Pt metal. Earlier, Mongin et al.18 demonstrated sub-20-fs electron transfer from the band-edge state (1σe state) of CdS NRs. In addition to that, electron transfer was also demonstrated in a different kind of semiconductor−metal heterostructure such as metal-tipped CdS and CdSe nanorods (NRs),21,27−29 Au-appended CdSe and PbS quantum dots

1. INTRODUCTION Research has been focused on the design and development of higher-efficiency quantum-dot-sensitized solar cells (QDSSCs) in the last decade due to the excellent optoelectronic properties of quantum dot sensitizers.1−7 Due to the strong confinement of charge carriers in QD materials there is significant enhancement of the electron−hole Coulomb interaction. This enhancement leads to efficient multiple exciton generation (MEG),8 a process where a single photon can generate more than one exciton (electron−hole pair). However, due to the strong confinement effect, Auger recombination takes place, which reduces the lifetime of the QD excitons on the order of tens of picoseconds.9 So it is utmost important to extract the multiple excitons (charge carriers) before they go for efficient Auger recombination to design and develop higher efficient QD solar cell. The nanohybrid material composed of metal and semiconductor is the subject of great current interest due to their potential applications in solar energy conversion, biosensing and photocatalysis.10−19 In nanohybrid material ultrafast electron transfer takes place from semiconductor/QD to metal as a result of Auger recombination is minimized. In semiconductor−metal nanohybrid material, metal NPs are directly grown on the semiconductor surface without any spacer, hence the electronic mixing between the electronic © XXXX American Chemical Society

Received: June 24, 2015 Revised: September 2, 2015

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DOI: 10.1021/acs.jpcc.5b06055 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C (QDs),30,31 and a Au@PbS core@shell nanohybrid material.32 Demortiere et al.29 demonstrated both electron and hole transfer processes at gold−sulfur sites in CdSe/CdS/Au heterostructures depending on the experimental condition while synthesizing the heterostructure. In all of the above systems charge-transfer dynamics were monitored by using different spectroscopic techniques such as single-molecule spectroscopy, time-resolved emission, and TA studies. Ultrafast dissociation of excitons through electron and hole transfer processes in different QD/molecule systems and molecular adsorbate/semiconductor (TiO2) systems have been extensively reported in the literature.33−46 Recently we have reported the dissociation of excitons through the extraction of both hot holes and thermalized holes by xanthenes,41 triphenyl methane,42−45 and catechol46 derivatives. Hot hole and thermalized hole extraction times were found to be 250 fs46 and 500 fs−1 ps,41−45 respectively. So far all the reports available in the literature are for the dissociation of excitons either by electron transfer or by hole extraction. However, no report is available in the literature where both electrons and holes are extracted from the photoexcited QD materials at the same time. To demonstrate exciton dissociation simultaneously by both electron and hole extraction, in this current investigation we have synthesized CdSe{Au} heterostructure (HS) materials and consequently sensitize CdSe{Au} HS by the bromo-pyrogallol red (Br-PGR) molecule. Subsequently, the charge-transfer reaction was monitored in the CdSe{Au}/Br-PGR tricomposite system with the help of time-resolved luminescence and ultrafast TA techniques. Both the time-resolved absorption and emission techniques suggest that in CdSe{Au} HS photoexcited electrons are transferred from the semiconductor domain to the metal domain. As the Fermi level of Au NP lies below the CB of CdSe QD, so electron transfer is thermodynamically viable. A steady-state optical absorption study suggests that Br-PGR molecules form a strong chargetransfer (CT) complex. As the LUMO and HOMO of the BrPGR molecule lie above the conduction band and valence band of CdSe QD,43 so electron injection from the photoexcited state to the CB of CdSe QD and hole transfer from the VB of photoexcited CdSe QD are thermodynamically viable. Both electron and hole transfer and charge recombination dynamics in the CdSe{Au}/Br-PGR tricomposite system have been unraveled by using both time-resolved luminescence and ultrafast TA spectroscopy techniques.

clear solution was obtained. Once the solution became clear the reaction temperature was increased up to 280 °C. To prepare the selenium precursor solution, 0.32 g of selenium powder (4 mmol) was mixed with 2.2 mL of TOP (5 mmol) in 9 mL of octadecene and sonicated for complete dissolution. After that, the TOP Se solution was injected in a single shot with a syringe into the reaction mixture at 280 °C. A rapid color change from yellow to orange to red occurred in the first 10−20 s after injection. The reaction mixture was kept at 280 °C for different times to get the desired size of CdSe. After the desired size of CdSe was obtained (by measuring the exciton position), the reaction was quickly quenched. The reaction mixture was left to cool. Finally the synthesized CdSe was precipipated by methanol three times, and then dissolved in chloroform for experimental purposes. c. Synthesis of the CdSe{Au} Heterostructure. For the synthesis of the Au-CdSe heterostructure, freshly prepared CdSe QD was used. The growth of Au NP on the CdSe QD surface was carried out after following the synthesis method reported earlier by Mokari et al.48 with some modifications. The gold precursor was prepared by the mixing of 15 mg of AuCl3 (0.05 mmol), 50 mg of DDAB (0.1 mmol), and 90 mg (0.46 mmol) of DDA in 4 mL of toluene. The color of the solution became dark orange to light yellow. Then 20 mg of synthesized CdSe was dissolved in 4 mL of toluene under an Ar atmosphere. The gold precursor was added dropwise to the CdSe solution for a time period of 4 min under an Ar atmosphere at room temperature. Gradually the solution color changed to dark brown. DDA stabilizes the QD, and DDAB acts as a surfactant for Au NP. Finally, the nanohybrid materials were precipitated in methanol. Then the precipitated nanohybrid materials were dissolved in chloroform for characterization and further use. d. High-Resolution Transmission Electron Microscopy Measurements. We have already explained our highresolution TEM (HRTEM) measurements in our previous reports.45 JEOL-JEM-2010 UHR instruments with 0.14 nm lattice image resolution was used at a 200 kV acceleration voltage. e. Time-Resolved Emission Spectrometry. Timeresolved fluorescence measurements were reported in detail in our previous reports.45 The working principle of this IBH (U.K.) instrument is time-correlated single photon counting (TCSPC). The 405 and 445 nm laser sources were used for excitation. f. Femtosecond Transient Absorption Spectroscopy. The details of the transient absorption spectroscopy measurement were demonstrated in our previous reports.43 A multipass Ti:sapphire laser ampliphire system (CDP, Moscow, 800 nm, < 100 fs, 1.2 mJ/pulse, and 1 kHz repetition rate) and an Excipro pump−probe spectrometer (CDP, Moscow) have been used.

2. EXPERIMENTAL SECTION a. Materials. Br-PGR dye was purchased from SigmaAldrich and used as is. Selenium powder (Se, 99.99%, Aldrich), cadmium oxide (CdO, Aldrich), oleic acid (OA, Aldrich), 1octadecene (ODE, Aldrich), trioctylphosphine (TOP, Aldrich, 90%), gold chloride, dodecylamine (DDA), didodecylammonium bromide (DDAB), and anhydrous toluene (Aldrich) were used to prepare CdSe QDs and the Au/CdSe metal semiconductor nanohybrid. For precipitation, AR-grade methanol and chloroform were used. b. Synthesis of CdSe QDs. CdSe QDs were synthesized in noncoordinating solvent octadecene (ODE) after following a previously reported high-temperature reaction method.47 Briefly, in the beginning a cadmium oleate complex was formed by heating the mixture of 6.8 mL of OA (21.2 mmol), 1 g of CdO (8.0 mmol), and 20 mL of ODE in a round-bottomed three-necked flask at 240 °C under an Ar atmosphere until a

3. RESULTS AND DISCUSSION a. Characterization and Optical Properties of CdSe{Au} HS. To characterize the Au-CdSe nanohybrid structure, HRTEM measurements have been performed and are shown in Figure 1A. It is clearly seen from the HRTEM image that Au NP is nicely coupled on the surface of CdSe QD, where the size of Au NP can be determined to be 3−3.5 nm and the size of CdSe QD is found to be 4−4.5 nm. The TEM image clearly shows the homogeneous deposition of Au on the surface of CdSe QD. Separate lattice images of CdSe QD and Au NP are clearly seen in the TEM image. The optical absorption B


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Figure 2. Steady-state photoluminescence spectra of (a) a CdSe quantum dot and (b) CdSe{Au} HS. Inset: Time-resolved luminescence decay trace of (c) a CdSe quantum dot and (d) CdSe{Au} HS at 580 nm after exciting the samples at 406 nm. (L is the lamp profile of the 406 nm laser.)


Figure 1. (A) HR-TEM picture of CdSe{Au} HS material. (Inset: magnified image of Au/CdSe.) (B) Steady-state optical absorption spectra of (a) a CdSe quantum dot, (b) Au NP, and (c) CdSe{Au} HS in chloroform.

Scheme 1. Schemetic Diagram Showing Electron Transfer from the CdSe Semiconductor to the Metal Domain in the CdSe{Au} Nanohybrid Material

measurements have also been carried out to characterize the CdSe QD and CdSe{Au} heterostructures (HSs) in chloroform solution, which are shown in Figure 1B. At this point, we have also synthesized Au NPs with size of 3 to 4 nm and have compared the optical properties with those of CdSe{Au} HS, as also shown in Figure 1B. Figure 1a shows that the steady-state optical absorption spectra of CdSe QD consists of two excitonic absorption bands at 560 and 450 nm which can be ascribed to 1S(e)−1S3/2(h) and 1P(e)−1P3/2(h) transitions, respectively. Figure 1B(b) shows the optical absorption spectra of Au NPs with a particle size of 3 to 4 nm, which has optical absorption due to the plasmon band at 527 nm. Figure 1c shows optical absorption spectra of CdSe{Au} HS where Au NP is grown on the surface of CdSe QD. It can be seen clearly that the optical absorption band becomes broad as compared to both CdSe QD and Au NP. From the optical absorption studies it is clear that the electronic interaction between the plasmonic band of Au NP and the excitonic band of CdSe QD is quite strong. In an earlier investigation, Lian and co-workers26 demonstrated the broadening of an optical absorption band for the CdS-Au composite materials where Au NP is located in the tip of the CdS nanorod. Due to direct coupling between the electronic states of the plasmon band of Au NP and the excitonic band of the QD, there is a change in the optical absorption spectra of the heterostructure materials. Due to mixing, the refractive index of the interdomain area changes as a result of excited-state properties, and the charge carrier dynamics of both Au NP and CdSe QD are expected to be changed. To monitor the excited-state behavior of the heterostructure, steady-state and time-resolved luminescence studies have been carried out for both CdSe QD and CdSe{Au} HS. Figure 2a shows the photoluminescence spectra of CdSe QD with an excitonic emission peak at 580 nm (emission quantum yield(QY) ϕf = 0.34). The emission intensity of high QY CdSe is completely quenched (Figure 2b) in the case of CdSe{Au} HS. Scheme 1 suggests that the Fermi level of Au NP lies below the conduction band of CdSe QD. The emission quenching of CdSe luminescence in CdSe{Au} HS materials can be attributed to the electron transfer from the CB of CdSe QD to Au NP which is thermodynamically viable. The electron transfer reaction can be explained by the equations given below: CdSe{Au} + hν → CdSe(e− + h+){Au}

CdSe(e− + h+){Au} → CdSe(h+){Au(e−)}

(2)

To reconfirm the electron-transfer process, time-resolved luminescence measurements have also been carried out. Figure 2c,d shows the luminescence decay trace at 580 nm after exciting the sample at 406 nm for CdSe QD and CdSe{Au} HS, respectively. The emission decay trace of CdSe QD can be fitted biexponentially with time constants of τ1 = 1.03 (±0.05) ns (30%) and τ2 = 16.13 (±0.5) ns (70%) with τavg = 11.66 (±0.5) ns. Interestingly, the emission kinetics for CdSe{Au} HS are found to decay much faster and can be fitted biexponentially with time constants of τ1 = 0.31 (±0.01) ns (86%) and τ2 = 3.72 (±0.15) ns (14%) with τavg = 0.72 (±0.05) ns. In our earlier report49 we have demonstrated (and also described by others50) that the shorter component of QD emission can be attributed to e/h trapping in sub-bandgap localized states of the QDs. On the other hand, the longer component can be assigned to the radiative recombination (orbital forbidden) of trapped charge carriers. In Au-CdSe the emission lifetime can be fitted biexponentially with time constants of τ1 = 0.31 ns (86%) and τ2 = 3.72 ns (14%). The faster decay components can be attributed to the nonradiative transfer of electrons from photoexcited CdSe to Au NP. The average lifetime of the CdSe{Au} HS system is more than 16 times shorter as compared to that of CdSe QD, which confirms the electron transfer process in the composite

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increases, which facilitates exciton−plasmon coupling in CdSe{Au} HS. Interestingly, the broad positive absorption band in the 580−700 nm region in the CdSe{Au} HS system is totally different from those of isolated photoexcited CdSe QD (Figure 3A) and Au NP (SI Figure 3A). This positive absorption band can be assigned to the appearance of the electron in the Au NP52,53 (in the Fermi level) from the CB of CdSe QD which is a thermodynamically viable process (Scheme 1). Earlier, Zamkov et al.51,52 also reported the appearance of a positive absorption band due to the transfer of electrons from the semiconductor domain (S) to the metal domain (M). To monitor the charge carrier and transfer dynamics, transient kinetics at different wavelengths for both CdSe QD and CdSe{Au} HS have been monitored and are shown in Figure 4. The transient data have also been fitted

materials as shown in eq 2. In all probability the observed reduction in lifetime arises due to electron transfer (ET) from CdSe QD to Au NP, and then the ET rate constant can be calculated from the following expression: 1 1 KET = − τCdSe{Au} τCdSe (3)


From the average lifetime values, 11.66 (±0.5) ns (CdSe) and 0.72 (±0.05) ns (CdSe{Au}), the electron-transfer rate constant was determined to be 1.39 × 109 s−1. b. Transient Absorption Study of CdSe{Au} HS. To confirm the electron transfer on the early time scale and to monitor the excited-state properties specifically for CdSe{Au} HS, we have carried out femtosecond TA spectroscopic measurements by exciting CdSe QD and CdSe{Au} HS with a 400 nm laser pulse. Figure 3A shows the TA spectra of CdSe

Figure 4. Bleach recovery kinetics of (a) a CdSe quantum dot and (b) CdSe{Au} HS at 560 nm and (c) transient absorption kinetics at 650 nm for CdSe{Au} HS in chloroform after 400 nm laser excitation. Inset: Bleach recovery kinetics for traces a and b after normalizing at 5 ps.

Figure 3. TA spectrum of (A) a CdSe quantum dot and (B) CdSe{Au} HS in chloroform solvent after exciting the samples at 400 nm.

multiexponentially and are shown in Table 1. The bleach recovery kinetics at 560 nm for CdSe{Au} HS is much faster as compared to that for pure CdSe QD. The faster recovery kinetics in the excitonic bleach position of CdSe{Au} HS can be attributed to photoinduced electron transfer from the CB of CdSe QD to the Fermi level of Au. The bleach at 560 for CdSe QD can be fitted with biexponential growth with time constants of τg1 = < 100 (±15) fs (67%) and τg2 = 750 (±50) fs (33%) and multiexponential recovery with time constants of τ1 = 8.5 (±0.35) ps (28%), τ2 = 45 ps (±2) (12%), τ3 = 350 (±15) ps (15%), and τ4 = >1 (±0.05) ns (45%) (Table 1). The second growth time constant can be attributed to cooling dynamics of the photoexcited electron in CdSe QD.53 The multiexponential recovery with different time constants can be attributed to carrier recombination dynamics of the electron and hole at different trapped depths. However, the bleach at 560 nm for CdSe{Au} HS can be fitted with a single-exponential pulse width limited growth (1 (±0.05) ns (34%) (Table 1). The biexponential growth at 560 nm bleach becomes single-exponential in CdSe{Au} HS, clearly suggest the photoexcited electron in CdSe QD transfer to Au NP before cooling down to the lower excitonic states. It is also interesting that the majority of the bleach signal at 560 nm recovers very fast, which suggests the transfer of an electron from the semiconductor domain to the metal domain. The kinetics at 650 nm has also been monitored and fitted with biexponential growth with time constants of τg1 = 1(±0.05) ns (6%)

500 (±50) fs (51%)

Notes


electron in Au NP and the localization of a hole in Br-PGR (Scheme 2), which are spatially separated. To the best of our knowledge, for the first time in the literature we are reporting charge-transfer (both electron and hole transfer) dynamics in a metal/semiconductor/molecular adsorbate tricomposite system.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the DAE-SRC Outstanding Research Investigator Award (project/scheme no. DAE-SRC/ 2012/21/13-BRNS) awarded to H.N.G. Also, J.D. and T.D. acknowledge CSIR, and P.M. acknowledges DAE for providing a fellowship. We also thank Dr. D. K. Palit and Dr. B. N. Jagatap for their encouragement.

4. CONCLUSIONS The CdSe{Au} heterostructure (HS) has been successfully synthesized and characterized by steady-state absorption and emission spectroscopy and HRTEM measurements. The surface plasmon of Au NP was found to be suppressed in the presence of the CdSe QD due to strong exciton-plasmon coupling between Au NP and CdSe QD. A photoexcited electron in CdSe QD is found to be transferred to the Au NP as monitored by both time-resolved luminescence and ultrafast TA studies. Spectrosopic measurements have been performed to study charge-transfer dynamics after sensitizing CdSe{Au} HS with bromo-pyrogallol red (Br-PGR). The steady-state optical absorption studies have confirmed the CT complex formation between CdSe{Au} HS and Br-PGR. Charge separation in the CdSe{Au}/Br-PGR tricomposite system takes place via different routes, such as electron transfer from the CB of the CdSe QD to the Au NP, hole transfer from the VB of the CdSe QD to Br-PGR, and the injection of a electron from the LUMO of photoexcited Br-PGR to the CB of CdSe QD. Spectroscopic investigations suggest that upon photoexcitation the CdSe{Au}/Br-PGR tricomposite system cascading electron transfer and hole transfer take place where all of the electrons are localized in Au NP and all of the holes are localized in Br-PGR. The experimentally measured electronand hole-transfer times from photoexcited CdSe QD to Au NP and Br-PGR were ∼270 fs and ∼500 fs, respectively. The chrage recombination reaction between an electron in Au NP and a hole in Br-PGR becomes very slow due to spatial charge separation in the CdSe{Au}/Br-PGR tricomposite system. Our experimental investigations clearly suggest that the metal/ semiconductor/molecular adsorbate tricomposite system can be reality for multiple exciton dissociation and finally higher efficiency for photoconversion devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06055. HR-TEM image of CdSe, steady state and TA spectroscopic study of Au and after interaction with the BrPGR molecule, TA spectroscopy of the BrPGR molecule, steady state and TA absorption study of CdSe sensitized with the BrPGr molecule, and time-resolved luminescence study of CdSe and CdSe/BrPGR (PDF)

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