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Ultrafast Charge Transfer and Trapping Dynamics in a Colloidal Mixture of Similarly Charged CdTe Quantum Dots and Silver Nanoparticles Navendu Mondal and Anunay Samanta* School of Chemistry, University of Hyderabad, Hyderabad 500 046, India S Supporting Information *

ABSTRACT: The interaction between colloidal CdTe quantum dots (QDs) and silver nanoparticles (Ag NPs), both in their negatively charged state, is studied to determine the importance of surface charge and epitaxial coupling of the two components to achieve the potential activity of the metal−semiconductor nanocomposites. An unexpected strong interaction between the two similarly charged species is evident from dramatic quenching of the excitonic emission of the QDs by the Ag NPs. Direct evidence of electron transfer from the photoexcited QDs to Ag NPs is obtained from an accelerated bleach recovery of the first exciton band of the QDs and a faster carrier cooling in the presence of Ag NPs in transient absorption measurements. The evidence of charge carrier trapping is demonstrated by the observation of a new broad positive transient absorption in the visible−near-infrared region, which was absent in their isolated counterparts. The electron transfer and charge carrier trapping processes are further substantiated by the results of similar measurements on core/shell (CdTe/ZnS) QDs. As ultrafast charge carrier (especially the hole) trapping slows the charge recombination process, the present findings open up new possibilities of harnessing solar energy and improving the photocatalytic activity by employing these colloidal mixtures. spacer/linker molecule.1,15 Weak plasmon−exciton interaction enhances the absorption cross sections, radiative rate, and exciton−plasmon energy transfer.15,17−19 Strong coupling of the metal and semiconductor components in plexcitonic materials promotes efficient electron transfer from the photoexcited semiconductor to the noble metal nanoparticles (Au, Ag, and Pt), which serve as potential electron reservoirs.20 These nanohybrids show enhanced charge separation and photocatalytic activity due to the trapping of holes in the semiconductor and electrons in the metal center and consequent reduction in the charge recombination rate.10,20 Intense research on a wide range of plexcitonic materials has provided evidence of efficient hot electron transfer (in subpicosecond time scale), cold electron transfer, and sometimes new optical signatures (like photoinduced absorption) resulting from the interaction between the participant materials.1,12,20 In this context we note that a mixture of QDs and metal NPs is seldom investigated, perhaps believing that poor coupling between the two will not help achieve novel properties, which are characteristics of the plexcitonic materials. Lian and co-workers,21 who compared the exciton bleach dynamics of the QDs in a colloidal mixture of CdS nanorods

1. INTRODUCTION Metal−semiconductor nanocomposites (M-S) have attracted considerable attention in recent years because of their multifunctional properties, combining the component material properties; often, new properties arise from the interaction of the two components,1,2 which are found to be useful in optoelectronics, photovoltaics,3 solar fuel production,4 photocatalysis,5 and lasers.6 The properties of these composites can be tuned by controlling the composition of the material constituents, their structures, and/or mode of interactions between them, such as by designing epitaxially coupled metaltipped semiconductor nanocomposite (dumbbells, dot on rod)7 or weakly and nonepitaxially coupled metal core−semiconductor shell nanoheterostructures.8,9 Attempts have been made in recent years to decorate a plasmonic metal tip onto an excitonic semiconductor nanorod to develop superior solar energy harvesting plexcitonic nanomaterials like CdS/Pt,10,11 CdS/Au,1,12 CdSe/Pt,13 and CdSe/Au.14 Optical excitation of semiconductor nanomaterials results in formation of a bound electron−hole pair or exciton.15 The surface plasmon of a metal nanoparticle comprises collective coherent oscillation of free electrons in the conduction band.16 Interaction between exciton and plasmon depends on the separation of the two units in nanoscale dimensions. In the weak coupling regime, there exists a substantial potential barrier in the interface of the metal and semiconductor domains coupled by using some © 2015 American Chemical Society

Received: September 4, 2015 Revised: December 14, 2015 Published: December 14, 2015 650

DOI: 10.1021/acs.jpcc.5b08630 J. Phys. Chem. C 2016, 120, 650−658

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The Journal of Physical Chemistry C

the reaction was carried out at 180 °C for several minutes to reach the desired size of the CdTe QDs by monitoring through emission. After completion of reaction, the unreacted starting materials were washed out from the QDs using methanol followed by repeated precipitation and centrifugation. The purified precipitate was dissolved in CHCl3. A 0.5 M methanolic solution of MPA-KOH (20 mol % excess KOH) was added dropwise to the CdTe-HDA solution in CHCl3 under stirring. Because the thiol has a greater affinity toward the QD surface, the QDs will flocculate out of the solution in the presence of MPA. The solution was centrifuged to separate out the precipitate, which was dissolved in water. 2.3. Synthesis of MPA-Capped CdTe/ZnS (3 Monolayer) Core/Shell QDs. The shell of a few monolayers of ZnS was grown over the core CdTe QDs through the successive ion layer adsorption and reaction (SILAR) method.24,25 To prevent self-nucleation of the shell material and also to protect from the Ostwald ripening of the core QDs, low temperature was maintained for the shell growth when compared with that used for the core synthesis. In brief, 8 mL of 7 μM CHCl3 solution of CdTe-HDA core QDs, 3 g of HDA, and 10 mL of ODE were taken in a double-necked RB flask and kept under vacuum at 80 °C for 1 h to remove CHCl3 and residual air. Zinc oleate precursor was prepared by mixing zinc oxide in required amount of oleic acid and 5 mL of ODE in another RB flask and heated at 240 °C under N2 atmosphere until the solution became clear. Then the solution was allowed to cool to 80 °C.26 Sulfur precursor was prepared by sonicating sulfur powder in 3 mL of TOP and 5 mL of ODE for 1 h. These precursors were injected into the core CdTe QDs under vigorous stirring at 160 °C following the SILAR method25 over 5−10 min to form CdTe/ZnS core/shell QDs. We choose the precursor ratio such that 3 monolayers (ML) can form around the CdTe core. The excess starting materials were washed out by methanol, and similarly MPA ligand was used to exchange the ligand to make it soluble in water. 2.4. Synthesis of Citrate-Capped NPs (Ag-Citrate). To synthesize smaller Ag NPs, we have used an earlier report of Kamat an co-workers.27 Briefly, 25 mL of 1 mM AgNO3 in water was taken in a RB flask and then 1 mL of 47 mM citrate solution and 1 mL of 52 mM citric acid were added with vigorous stirring. After 15 min of stirring, 200 μL of aqueous NaBH4 was added dropwise to the solution with vigorous stirring to reduce all the Ag(I) to Ag(0). Appearance of the plasmon peak at 392 nm in the absorption spectrum indicates the formation of Ag NP. The concentrations of Ag NPs in the solutions are estimated from the Mie power law employing the diameter of the particle estimated from plasmon band maxima wavelength.28 The similar-sized citrate-capped Ag NPs was also synthesized at pH > 10.5 using a standard reported method.29 2.5. Experimental Methodologies and Instrumentation. The ζ-potential of all the nanomaterials has been measured using a Malvern Zetasizer instrument. Transmission electron microscopy (TEM) measurements of Ag NPs and CdTe QDs were performed by preparing the sample on carbon-coated copper grids. A Tecnai G2 FE1 F12 transmission electron microscope at an accelerating voltage of 120 kV was used to estimate the size of the nanomaterials. The steady-state absorption and fluorescence spectrum were recorded by an ultraviole−visible (UV−vis) spectrophotometer (Cary100, Varian) and a spectrofluorimeter (FluoroLog-3, Horiba Jobin Yvon), respectively. Time-resolved emission decay profiles of the QDs were recorded by using a time-correlated single-

and Au nanoparticles with that of the plexcitonic material (CdS-Au), observed efficient electron transfer from QDs to Au NPs only in the plexcitonic material, thus revealing the importance of coupling between the two constituents. In this work, we study the interaction between both negatively charged QDs and metal NPs (Figure 1) in aqueous

Figure 1. Schematic representation of negatively charged MPA-capped CdTe QDs and citrate-capped Ag NPs.

medium to explore the importance of surface charges and epitaxial coupling of the components to achieve properties that arise from strong interaction of the two components. We have chosen CdTe QDs in this work because it is known to inject electrons (into TiO2) faster than CdSe QDs,22 and we have not come across any literature on plexcitonic materials involving CdTe. Our choice of Ag NPs over Au NPs is guided by the fact that the plasmon band of the former is well separated from the band edge emission of the CdTe QDs, thus enabling selective excitation and study the exciton dynamics of the latter in the presence of Ag NPs. Dramatic fluorescence quenching of a QD by a metal NP having similar surface charges observed for the first time is shown to be due to transfer of both hot and cold electrons from CdTe QDs to Ag NPs. Evidence of the trapping of charge carriers is also presented.

2. EXPERIMENTAL SECTION 2.1. Materials. Cadmium acetate dihydrate [Cd(CH3COO)2·2H2O], zinc oxide, and sulfur powder were obtained from Loba Chemie and used as precursors for the synthesis of QDs. Tellurium powder (Te) (99.997%, Aldrich), trioctylphosphine (TOP) (90%, Aldrich), hexadecylamine (HDA) (90%, Aldrich), 3-mercaptopropionic acid (MPA) (99%, Aldrich), 1-octadecene (ODE) (90%, Aldrich), oleic acid (90%, Aldrich), silver nitrate (AgNO3) (99%, SigmaAldrich), sodium borohydride (98%, Aldrich), and trisodium citrate dehydrate (Sigma-Aldrich) were used as purchased. The solvents, methanol, and chloroform, purchased from Merck, were purified and dried following standard procedures before their use. All the spectral measurements were performed using Milli-Q water. 2.2. Synthesis of MPA-Capped CdTe QDs (CdTe-MPA). Water-soluble CdTe-MPA was synthesized by preparing CdTeHDA followed by ligand exchange with MPA. CdTe-HDA QDs in CHCl3 were prepared by following a reported procedure.23 Briefly, 5 g of HDA and 5 mL of TOP were mixed in a doubleneck round-bottom flask (RB) and heated at 80 °C in an argon atmosphere. In another reagent bottle, a clear solution was prepared by sonication of a mixture of 0.41 g of cadmium acetate, 0.16 g of Te powder, and 3 mL of TOP for more than 1 h. The clear solution was quickly injected into the RB flask, and 651


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Figure 2. (A) Absorption spectra of Ag NPs (dotted line) and CdTe QDs (dashed line), emission spectra of CdTe QDs (solid line) in aqueous medium, and (B) TEM images of Ag NPs and CdTe QDs.

Figure 3. (a) Photoluminescence spectra (λex = 375 nm) and at 570 nm excitation (Figure S1) of the CdTe QDs (0.45 μM) with increasing concentration of colloidal Ag NPs (0, 33, 65, 98, 130, 163, 195, and 228 pM) in aqueous medium. (b) The conduction band (CB) and valence band (VB) potentials of quantum confined ∼3.58 nm sized CdTe QDs and Fermi level (Ef,m) potential of Ag NPs.

and focused onto a rotating CaF2 crystal to generate a white light continuum for broad band probe pulses of 350−800 nm. A near collinear geometry of the pump and probe beams was maintained to achieve a greater overlap between the two to produce a TA spectrum with high signal-to-noise ratio. All the TA data were chirp-corrected for the group velocity dispersion of the white light continuum by using the ΔA signals of neat ethanol. The incident excitation beam was attenuated before the rotating sample cell (path length of 1 mm) by using the adjustable neutral density filters to ensure a linear pump power dependence of the signal. The pump pulse energy was varied, and the estimated number of excitons per QD within was 1 by using the relation ⟨N⟩ = Jpσ, where Jp is the pump fluence and σ is absorption cross section of CdTe QDs. The pump−probe cross correlation was found to be around 80−90 fs.

photon-counting (TCSPC) spectrometer (Horiba Jobin Yvon IBH). A PicoBrite diode laser (375 nm) was used as an excitation source with a repetition rate of 10 MHz and an MCP photomultiplier (Hamamatsu R3809U-50) as the detector. The instrument response function was found to be 55 ps for the 375 nm excitation source. Fluorescence decay curves were analyzed by a nonlinear least-squares iteration procedure using IBH DAS6 (Version 2.2) decay analysis software. The quality of the fit was assessed by the χ2 values and the distribution of residuals. Femtosecond laser setup for transient absorption (TA) measurements was described in our previous report.30 In brief, the setup consisted of a mode-locked Ti:sapphire oscillator (Mai Tai, Spectra Physics) which produced femtosecond pulses of nanojoule energy with a center wavelength of 800 nm. A part of this laser was injected into Ti:sapphire regenerative amplifier (Spitfire Ace, Spectra Physics) which was pumped by an output of 527 nm of a frequency-doubled Nd:YLF laser (Empower, Spectra Physics). The amplifier delivered 800 nm laser pulses (fwhm ∼100 fs) with pulse energy of ∼4.2 mJ at a repetition rate of 1 kHz. The laser output was split into two components: one was directed to an optical parametric amplifier (TOPASPrime, Spectra Physics) to generate the excitation pulses at 480 nm, and the second one was diverted through a 4 ns delay unit

3. RESULTS AND DISCUSSION 3.1. Steady-State and Time-Resolved Experiments. The ground-state absorption spectrum of mercaptopropionic acid (MPA)-capped CdTe QDs shows typical broad absorption with a sharp first exciton band maximum at ∼590 nm (Figure 2A).31 The average size of CdTe QDs estimated from the first exciton band maximum using Peng’s equation32 is found to be ∼3.58 nm. The citrate-capped Ag NPs show distinct surface 652


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The Journal of Physical Chemistry C plasmon band at ∼392 nm (Figure 2A), which according to the literature, corresponds to a diameter of ∼10 nm.33 The TEM images of the CdTe QDs and Ag NPs (Figure 2B) corroborate the calculated sizes from the spectral data. The interaction between metal and semiconductor NPs can be probed by monitoring the plasmon band of the former or the excitonic band of the latter. However, due to strong absorption of the CdTe QDs in the plasmon band region of Ag NPs, we concentrated on the exciton dynamics. The CdTe QDs exhibit a narrow band edge emission centered at ∼622 nm (Figure 2A). In the presence of minute quantities of Ag NPs, drastic quenching of fluorescence is observed (Figure 3), even though no significant change in the absorption spectra of the QDs (Figure S1) is noticed. Because drastic fluorescence quenching observed here is in contrast with earlier reports of no quenching between similar charged QDs by gold nanoparticles, but efficient quenching when they were oppositely charged,34,35 one may argue about the nature of surface charges of the two interacting particles. However, the measured ζ potentials (−28.9 ± 5.3 mV and −38.1 ± 4.4 mV for Ag NPs and CdTe QDs, respectively) confirm negative surface charges on both.36,37 Also, the measured ζ potential (−36.8 ± 7.7 mV) of the colloidal mixture of CdTe QDs and Ag NPs indicates the stability in the colloidal state and lack of electrostatic attractions between the two.38 An important point to note in this context is that at higher pH no such quenching was observed for CdTe QDs upon addition of similar sized Ag-citrate NPs with a highly negative surface charge (ζ potential ∼ −43.6 ± 7.1 mV at pH > 10.5). Metal nanoparticles are known to serve as both energy and electron acceptors and contribute to fluorescence quenching of molecular systems.34,39,40 Fluorescence quenching of molecular systems attached to the metal surfaces due to energy and electron-transfer processes have been demonstrated.41,42 However, energy transfer between the chosen pair in our case is ruled out already, and emission quenching of the CdTe QDs by Ag NPs depicted in Figure 3 can only be due to charge (electron/hole) transfer. The quenching effect of the Ag NPs is also evident from the emission decay profiles of the QDs (Figure S2). The radiative decay of the QDs originates from core state recombination and trap states (arising from surface defects) recombination processes.43,44 The three emission lifetime components of the QDs (monitored at 620 nm) in the absence of Ag NPs, 3.5 (17%), 0.3 (4%), and 20 (79%) ns (Table 1), are due to recombination of the intrinsic core and deep and shallow trap

states, respectively.30,45,46 A decrease in lifetime with increase in [Ag NPs] points to dynamic nature of the quenching, which we attribute to photoinduced electron transfer from CdTe QDs to Ag NPs. A point to note here is that the long lifetime component (∼20 ns) and its contribution to the decay are not affected initially. However, above [Ag NPs] of 130 pM, the lifetime and amplitude of this component (due to shallow trap states) drop drastically with an increase in the contribution of the deep trap state (∼0.3 ns) and core state (∼3.5 ns). This observation can be attributed to the worsening of the surface quality of the QDs in the presence of Ag NPs based on an earlier report.43 These densely spaced shallow trap states serve as a nonradiative relaxation pathway for the photogenerated charge carriers and contribute to the decrease in emission quantum yield of the QDs.47 3.2. Ultrafast Transient Absorption Measurements. To understand the mechanism of observed exciton quenching, femtosecond transient absorption (TA) measurements have been carried out by exciting the solution at 480 nm and monitoring the transients in the vis−NIR region. The TA spectra of the system (Figure 4) are characterized by excitonic bleach centering around 590 nm, which closely corresponds to the first exciton band maximum of the CdTe QDs and an additional broad positive absorption beyond 625 nm in the presence of Ag NP. The time dependence of the 1S excitonic bleach signal monitored at 590 nm (Figure 5) is characterized by an ultrafast growth component and three decay components (see Table 2 for time constants and associated amplitudes). The growth of the bleach signal becomes faster (from 165 to ∼90 fs), and the bleach amplitude is lowered by ∼60% in the presence of Ag NPs (Figure 5). 3.2.1. Dynamics of the First Exciton Bleach. As 480 nm excitation populates the states well above the conduction band edge, the intraband relaxation of nonthermalized electrons to the conduction band edge 1S state (thus producing the cold electrons)48,49 is mainly expected to contribute to the growth of the bleach amplitude. Hence, the rise time of 165 fs corresponds to the thermalization (or intraband relaxation) time of the hot electrons. The bleach is more sensitive to the electron because of its lower effective mass compared to that of hole (for CdTe, mh/me = 4) and higher degeneracy of the valence band.50 In the presence of an electron acceptor (Ag NPs), as the hot electrons are decoupled from the QDs, the bleach formation is mainly governed by the cooling of hot holes. Hence, a faster rise of the bleach signal (90 fs) in the presence of electron acceptor reflects the cooling of the hot holes in the VB48,51,52 and provides the necessary evidence of ultrafast transfer of the hot electron from CdTe QDs to Ag NPs on a time scale faster than the temporal resolution of our femtosecond setup.12,53 A comparison of the amplitude of the 1S bleach signals (Figure 5) in the absence and presence of Ag NPs suggests that ∼60% of the hot electrons are transferred to Ag NP. The 8.5 and 50 ps components of the bleach recovery kinetics of the bare QDs (Figure 5) are attributed to shallow and deep electron trapping, respectively, and the 308 ps long component to recombination of the charge carriers.48,54 In the presence of Ag NP, a faster bleach recovery of the CdTe QD (3, 10, 113 ps) is a reflection of the transfer of electrons from the QDs to Ag NP; the bleach recovery dynamics under these conditions essentially represents the hole-trapping dynamics and subsequent recombination process, considering that a

Table 1. Fluorescence Decay Parameters of the CdTe QDs at Different Concentrations of Ag NPs; Estimated ⟨τint⟩ and ⟨τamp⟩a Values [AgNP] (pM) 0 33 65 98 130 163 195 228

τ1 (a1) [ns]

τ2 (a2) [ns]

τ3 (a3) [ns]

⟨τint⟩ [ns]

⟨τamp⟩ [ns]

3.56 3.01 2.25 1.74 1.47 0.91 0.73 0.48

21.20 (0.79) 20.03 (0.79) 19.60 (0.79) 19.60 (0.79) 19.97 (0.74) 15.82 (0.66) 11.78 (0.56) 4.61 (0.30)

0.28 0.24 0.20 0.17 0.15 0.11 0.09 0.06

17.30 16.34 15.90 15.70 15.10 10.67 6.87 1.72

4.04 3.29 2.47 1.89 1.32 0.67 0.42 0.20

(0.17) (0.16) (0.15) (0.14) (0.17) (0.21) (0.26) (0.45)

(0.04) (0.05) (0.06) (0.07) (0.09) (0.13) (0.17) (0.25)

⟨τint⟩ = (a1τ12 + a2τ22 + a3τ32)/(a1τ1 + a2τ2 + a3τ3) and ⟨τamp⟩ = (a1τ1 + a2τ2 + a3τ3)/(a1 + a2 + a3). a

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Figure 4. Transient absorption spectra of CdTe QDs in the (a) absence and (b) presence of Ag NPs at different delay times (1, 2.5, 5, 10, 25, 50, and 250 ps) after 480 nm excitation. Excitation at 480 nm used to probe the exciton dynamics preferentially because Ag NPs are spectrally apart from the QDs at the excitation wavelength.

Figure 5. (A) Comparison of normalized growth kinetics at 590 nm of CdTe QDs only and a mixture of CdTe QDs and Ag NPs. Inset shows the extent of decrease in amplitude of the 1S bleach signal at 590 nm in the presence of Ag NPs. (B) Bleaching recovery profiles monitored at 590 nm of CdTe QDs in the (a) absence and (b) presence of Ag NPs. Inset shows the carrier cooling and bleach recovery dynamics in the early time region.

Table 2. Kinetic Parameters of Bleaching Recovery of CdTe QDs and a Mixture of CdTe QDs and Ag NPs and Photoinduced Absorption at 710 nm of a Mixture of CdTe QDs and Ag NPs kinetics

sample

τrise (a0)

τdecay1 (a1)

τdecay2 (a2)

τdecay3 (a3)

1S bleach recovery

only CdTe CdTe and Ag NPs CdTe and Ag NPs

164 ± 4 fs (100%) 90 ± 6 fs (100%) 196 ± 6 fs (100%)

8.5 ± 0.3 ps (45%) 3.0 ± 0.3 ps (23%) 3.2 ± 0.4 ps (70%)

50.4 ± 5.2 ps (34%) 10.2 ± 1.1 ps (60%) 120 ± 35 ps (30%)

308 ± 59 ps (21%) 113 ± 24 ps (17%) −

PA (710 nm)

photoinduced absorption (PA) in the 625−740 nm region,58 which is negligible for CdTe QDs alone and completely absent for Ag NPs (Figure S4). This PA, which develops within ∼200 fs and decays with lifetime of 3 ps (70%) and 120 ps (30%) (Table 2), can arise from photogenerated charge carriers in their free or trapped state and/or be due to nonlinear interactions like Stark-induced shift54 and exciton−exciton annihilation.59,60 That many-particle nonlinear interactions do not contribute to this broad PA is evident from the fact that the pump fluence used in our experiments is low enough (