Probing Spatially Dependent Photoinduced Charge Transfer

Oct 28, 2013 - Department of Chemistry, Emory University, Atlanta, Georgia 30322 ..... which upon excitation forms positive trion state that is not em...
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Probing Spatially Dependent Photoinduced Charge Transfer Dynamics to TiO2 Nanoparticles Using Single Quantum Dot Modified Atomic Force Microscopy Tips Zheng Liu, Haiming Zhu, Nianhui Song, and Tianquan Lian* Department of Chemistry, Emory University, Atlanta, Georgia 30322 United States S Supporting Information *

ABSTRACT: Using single CdSe/CdS quantum dot (QD) functionalized atomic force microscopy (AFM) tips, we demonstrate that the spatial dependence of photoinduced electron transfer dynamics from the single QD to TiO2 nanoparticles can be controlled and probed with high spatial (subdiffraction-limited) and temporal (limited by fluorescence microscopy) resolutions. This finding suggests the feasibility of using electron donor or acceptor modified AFM tips for simultaneous high resolution imaging of morphology and photoinduced charge transfer dynamics in nanomaterials. KEYWORDS: Electron transfer, AFM, quantum dot, TiO2, single molecule spectroscopy, supraresolution imaging based tip-enhanced fluorescence,22,23 Raman,24−27 and nonlinear spectroscopic methods have been reported,28 and their spatial resolution is often limited by the dimension of the tip, typically on the 10−50 nm scale.21,29 The subdiffraction-limited spatial resolution results from the localization of the enhanced field near the metal tip.23,30 Significant effort has also been devoted to chemical functionalization of atomic force microscopy (AFM) tips, which can improve the chemical specificity and/or spatial resolution of AFM31,32 and extend its applications from atomic-force-based morphological imaging to studying dynamical processes between two interacting partners. For example, AFM tips functionalized with metal nanoparticles have been used to examine the effect of metallic nanostructure on the absorption and fluorescence properties of chromophores (radiative and nonradiative decays), similar to ANSOM.23,30 Carbon nanotube-functionalized AFM tips enable the study of the spatial dependence of energy transfer between single carbon nanotubes and QDs.33 In this paper, we report a new CT microscopic method for studying photoinduced CT dynamics in nanoscale electron donor−acceptor materials. The donor−acceptor complexes of interest are formed by immobilizing one of them on the AFM tip. The three-dimensional (3D) spatial separation between the donor and acceptor can be controlled by scanning the donor relative to the acceptor or vice versa, and its effect on the photoinduced CT dynamics can be studied using an integrated AFM and confocal fluorescence microscope (Figure 1A). As shown in Figure 1B, AFM tips are modified with fluorescent electron donors (or acceptors), such as QDs, which are used to

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anostructured composite materials containing electron donating and accepting components, such as dyesensitized nanocrystalline oxides,1,2 conjugated polymer/C60 blends,3,4 and quantum dot (QD) sensitized oxides or QD solids,5−7 are widely used in novel solar cells. The dynamics of interfacial charge separation and recombination across the electron donor/acceptor interface plays essential roles in determining the energy conversion efficiencies of these devices.8−10 Ensemble-averaged time-resolved spectroscopic studies showed that these dynamics are often highly heterogeneous and intimately dependent on the nanoscale and molecular structures of the interfaces.8−14 In recent years, in an effort to provide insight into the structural/chemical origins of the complex charge separation properties, interfacial charge transfer (CT) dynamics at the single donor−acceptor level have also been examined by single molecule fluorescence spectroscopy.15−18 This technique can provide the sensitivity, spectral resolution, and time resolution needed to reveal the static and dynamic heterogeneities that are hidden in the ensemble-averaged measurements. However, it remains challenging to correlate the observed heterogeneity of interfacial CT dynamics to the material and interfacial structures based on fluorescence measurement alone. Therefore, further progresses in material design and improvement would require new methods that can provide spectral and dynamical information offered by fluorescence spectroscopy as well as simultaneous spatial/morphological resolution on the nanometer or smaller length scales. Previous efforts to combine the capability of high spectral and time resolutions of time-resolved fluorescence spectroscopy with the capability of high spatial resolution of scanning probe microscopy have led to the development of near-field scanning optical microscopy (NSOM) and apertureless nearfield scanning optical microscopy (ANSOM).19−21 ANSOM © 2013 American Chemical Society

Received: August 24, 2013 Revised: October 14, 2013 Published: October 28, 2013 5563

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Using single QD modified AFM tips, the relative threedimensional position of the QD and TiO2 can be controlled and the morphology of the TiO2 can be imaged using AFM technology. Simultaneously, the spatial dependence of photoinduced ET dynamics from the QD to TiO2 can be measured by QD fluorescence lifetime. Combining the strong exponential distance dependence of the interfacial CT rate with the small dimension of the single QD, we demonstrated that charge transfer dynamics can be measured with simultaneously high temporal (limited by time-resolved fluorescence microscopy) and spatial (similar to AFM) resolutions. The integrated AFM/fluorescence microscope, shown in Figure 1A, consists of an AFM head (Asylum’s MFP-3D-BIO) mounted on an inverted microscope (Olympus IX70) that is equipped with a 100× 1.4 NA oil immersion objective (Olympus). Tunable near UV−visible excitation pulses from 350 to 540 nm are generated by frequency doubling (in a BBO crystal) of the output (∼100 fs, 700−1080 nm, 80 MHz repetition rate) of a mode-locked Ti/Sapphire laser (Tsunami oscillator pumped by a 10 W Millennia Pro, Spectra-Physics). As shown in Figure 1B, a single QD-modified AFM tip is scanned over the TiO2 to acquire AFM images, during which the XY positions are controlled by the sample stage and the Z by the AFM tip. Simultaneously or subsequently, the tip is illuminated by laser pulses at 400 nm and the resulting fluorescence is detected by an avalanche photodiode (APD). For every detected photon, the tip position and photon information (delay time, relative to the excitation pulse, and arrival time, relative to the start of the measurement) are recorded using AFM and a time-correlated single photon counting (TCSPC) board, respectively.7,22,34−38 From the recorded photons, we can construct delay time histograms to obtain excited state lifetime (Figure 1C) and bin the photons to obtain emission intensity. These quantities can be measured as a function arrival time to construct lifetime and intensity trajectories of the single QD. We can also construct images of fluorescence lifetimes and intensities and correlate them with the AFM topography/phase images (Figure 1D).39 Correlation of AFM and fluorescence features was achieved by aligning the AFM tip to the center of the laser focus spot.

Figure 1. Integrated AFM and confocal fluorescence microscope for controlling and imaging electron transfer dynamics using single QDmodified AFM tips. (A) Schematics of an integrated AFM and confocal fluorecence microscope. (B) Expanded view of the tip sample contact region, showing an AFM tip modified by a single QD. (C) Illustration of QD fluorescence decay measured at different tip− sample positions. (D) Schematic depiction of correlated fluorescence (intensity and lifetime) and AFM morphology images.

probe nonemissive electron accepting (or donating) materials that are deposited on glass coverslips. The emission of the QD on the modified tip is monitored and its quenching (reduction in intensity and lifetime) is used to probe the interfacial CT processes. Alternatively, AFM tips can be modified with nonemissive electron or hole acceptors, which can then be used to probe emissive nanomaterials (such as QDs or nanorods) prepared on coverslips. In this paper, we demonstrate this method by examining the spatial dependence of photoinduced electron transfer (ET) from single QDs to TiO2 nanoparticles, a model system for QD-based solar cells.

Figure 2. Preparation of single QD-modified AFM tips. (a) Procedure for trapping a single QD by a MPTES functionalized AFM tip. (bi) Fluorescence images, (ci) AFM images, and (di) AFM line scan (along the line connecting QDs A and B) of a 3.5 μm × 3.5 μm area before (i = 1) and after (i = 2) the attachment of QD B to the AFM tip. Inset in c1 and c2 shows an expanded view of QD B. 5564

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Figure 3. Fluorescence properties of a single QD-modified AFM tip. (a) Fluorescence intensity (black) and lifetime (red) trajectories of a single QDmodified AFM tip when the tip is in contact with (Z = 0 nm, yellow shaded region) and withdrawn from (Z = 3 μm, blue shaded region) the surface of a glass coverslip. (b) Fluorescence intensity (black) and lifetime (red) trajectories from a single QD modified AFM tip when the tip is at Z = 0 nm (in contact, yellow shaded region) and at Z = 50 nm (blue shaded region) from the surface of a TiO2 nanoparticle. (c) The corresponding on-state fluorescence lifetime histogram for the trajectory in (b), showing two distinct peaks for Z = 0 and 50 nm. (d) Schematic energy level alignment, photoinduced electron transfer, and back electron transfer at QD/TiO2 interface.

bifunction linker for strong binding to the AFM tip and the thiol moiety for the subsequent QD attachment.39,45 Figure 2a depicts the procedure of attaching a single QD to a MPTMS-functionalized AFM tip. First, well-dispersed single QDs were deposited on glass coverslips and their surface coverage was verified by both AFM and fluorescence images.15,16 A raster-scanned fluorescence image in a 10 μm × 10 μm area collected by scanning the AFM sample stage in the XY plane with AFM tip not engaged to the sample shows well separated single QDs (Supporting Information Figure S2a). An expanded view of a small area (3.5 μm × 3.5 μm) containing QDs A and B (Figure 2b1) shows bright and dark lines caused by the blinking of QDs, indicating the presence of single QDs in each spot. This is confirmed by an AFM image of the same area (Figure 2c1) acquired in intermittent contact (or AC) mode and with the laser blocked to avoid photobleaching of QDs. These QDs can be clearly seen in the inset of Figure 2c1 and in a cross section line scan of the AFM image along the line connecting particle A and B (Figure 2d1). To attach QD B to the AFM tip, the functionalized tip was brought to contact with the particle in contact mode for over 1 min to establish chemical bonding between the MPTMS ligand and QD (step 1 in Figure 2a). After retrieving the tip from the sample (step 2 in Figure 2a), an AFM image of the same area was acquired in AC mode with the QD modified AFM tip, which clearly shows the absence of QD B from the glass coverslip (Figure 2c2,d2). This is also confirmed by the fluorescence image of the same area acquired after removing the tip far away from the sample (Figure 2b2 and Supporting Information Figure S2b). Multiple trials were often needed to obtain a successful QD attachment. The height profiles of QD A in Figure 2d1,d2 show full width at half-maximum of 25 and 23 nm, respectively. Because these features are much larger than the size of the QD, they reflect the lateral resolution of the AFM, which is limited by the size of the AFM tip and is not affected by QD attachment. From the fluctuation of the baseline height, a lower limit of height

The contrast mechanism of the AFM image acquired with QD-modified AFM tips remains unchanged compared to regular AFM and the spatial resolution reflects the convolution of the modified tip and sample dimensions. The contrast in fluorescence images comes from ET quenching. Because the ET rate decays exponentially with distance, that is, proportional to e−βZ, where β is estimated to be 10 nm−1 for a free electron tunneling through a 4 eV square potential barrier, it decreases by a factor of 10 when Z increases from 0 to 0.3 nm. Therefore, the spatial resolution of ET-induced emission quenching image is controlled by the size of the QD and the best resolution can be achieved with a single QD on the AFM tip. The temporal resolution of the technique is limited by the response of the APD detector, which is ∼500 ps in our experiment. Similar methodologies of correlated fluorescence intensity and AFM imaging using functionalized AFM tips have been reported previously and have been used to measure energy transfer dynamics.7,22,34−37,40,41 In this work, we extend these methods to probing electron transfer dynamics to TiO2 nanoparticles using single QD modified AFM tips. CdSe/CdS QDs with a CdSe core (∼1.2 nm in radius) and six monolayers of CdS (2.2 nm in thickness) were synthesized according to published procedures.42 These QDs have a lowest energy absorption peak at 575 nm and an emission band centered at 590 nm (see Figure S1 of the Supporting Information). Because of a quasi-type II band alignment between the core and shell, the lowest energy electron level in the conduction band is delocalized throughout the QD and the lowest energy valence band hole level is confined in the CdSe core.42 To reduce the quenching of QD emission by the n-Si AFM tips (MIKROMASCH HQ:NSC35/Al BS),43 we coated these tips with a thin (∼10 nm) layer of SiO2.44 The SiO2coated AFM tips were functionalized by (3-mercaptopropyl)trimethoxysilane (MPTMS, Aldrich, Figure 2a) according to a published procedure39,44 with the silane group of this 5565

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Figure 4. Spatial dependence of electron transfer dynamics from a single QD (attached on a AFM tip) to a TiO2 nanoparticle. (a) The 3D AFM image of a Degussa P25 TiO2 nanoparticle acquired with a single QD-modified AFM tip operating in ac mode. (b) Cross section line scan of AFM height (black solid line) and average QD on-state emission lifetimes at selected tip−sample contact positions (red circles) along the blue dashed line indicated in panel (a). (ci) Typical fluorescence intensity (black) and lifetime (red) trajectories, (di) histograms of intensity distribution, and (ei) histograms of on-state lifetime distribution of a QD-modified AFM tip at the left side (i = I), on top (i = II) and at right side (i = III) of the TiO2 nanoparticle. The tip is in contact with the glass coverslip in region I and III and with the TiO2 nanoparticle in region II. Blue dashed lines in cI-III indicates the fluorescence intensity threshold separating the on and off states.

contact periods. This demonstrates that the attached single QD is stable under these experimental conditions for at least 600 s. In the off state, the emission intensity is too weak to allow accurate determination of lifetimes and is estimated to be ∼5 nm) the tip position could not be stabilized by the feedback loop and the extent of its fluctuation was not clear. As shown in Figure 3a, the QD emission can be observed when the tip is in contact with the coverslip (Z = 0 nm) and the emission returns to the background level when the tip is retrieved from the coverslip (to a distance of ∼3 μm), confirming the presence of the QD on the tip. Furthermore, the fluorescence intensity and lifetime shows positively correlated fluctuations, switching between on states with high intensity and long lifetime to off states with intensity at the background level, consistent with previously reported blinking dynamics of single QDs.16,46−51 This abrupt switching from the on to the off states is often attributed to the formation of charged QDs in the off state in which the exciton lifetime is shortened by the presence of fast Auger recombination, although the exact nature of the charge and degree of charging remain unclear.52−59 As shown in Figure 3a, the QD on-state fluorescence lifetime remains the same during the three contact periods. This can be better seen in the histograms of lifetime distributions (Supporting Information Figure S3b) that show average lifetimes (±standard deviation) of 18.1 (±4.66), 17.4 (±4.3), and 19.7 (±5.25) ns in the three 5566

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to average ET rates of 2.2(±1.0) × 107 s−1 and 1.8(±0.7) × 107 s−1, respectively. These rates are consistent with the previously reported ET rates in similar systems.61 In the off state, the QD exciton decay is dominated by Auger recombination and interfacial ET to TiO2 is no longer a competitive pathway.61 It is important to point out that the observed change in exciton on-state lifetime (and the corresponding ET rate) as a function of QD-TiO2 distance is reproducible and reversible for multiple tip−sample contact and withdraw processes, indicating negligible photo- or ET-induced long-term damage of the QD. The long-term photostability of the QD can be attributed to the confinement of hole in the CdSe core away from the surface in these core/multiple shell QDs, which is also consistent with the finding of the previous study of single QD-TiO 2 complexes.61 At the QD-TiO2 interface, forward ET from the QD conduction band to TiO2 is followed by the back ET from the TiO2 to fill the hole in the QD valence band (Figure 3d), regenerating the system in the ground state and enabling repetitive measurement. The observed ET-induced quenching of QD emission suggests that single QD-modified AFM tips can be used to control and probe the spatial dependence of ET dynamics between single QDs and TiO2 nanoparticles. As shown by the AFM image acquired in ac mode (Figure 4a) with a single QDmodified AFM tip and a cross section line scan (Figure 4b), the TiO2 nanoparticle has a height of about 28 nm and an apparent width of about 60 nm. Assuming a roughly spherical shape of the TiO2 particle, the width reflects lateral resolution of the AFM image. Next, we measured the QD fluorescence intensity and lifetime trajectory at a few tip−sample contact points along the blue dashed line indicated in Figure 4a. In this measurement, the tip was brought to contact with sample (glass surface or TiO2) in contact mode at which a 40 s long fluorescence trajectory was collected. After withdrawing the tip and moving the sample stage laterally, the tip was brought to contact with the next position along the blue dashed line indicated in Figure 4a. To avoid physical damage of the QDmodified AFM tip, only selected number of points were measured: five points to the left of (region I), four points on top of (region II), and five points to the right (region III) of the TiO2 nanoparticle. As indicated in Figure 4b, the lateral position of these points are from −95 to −55 nm, from −11 to 11 nm, and from 70 to 110 nm to the center of the TiO2 nanoparticle in region I, II, and III, respectively. The measured fluorescence lifetime and intensity trajectories at the tip−sample contact points are shown in Supporting Information Figure S3. Representative trajectories for regions I, II, and III are shown in Figure 4cI−cIII, respectively, and the corresponding intensity and lifetime distribution histograms are shown in Figure 4dI−III and 4eI-III. Although in all regions, positively correlated fluorescence intensity and lifetime fluctuations are observed, the probability of dark states in region II are higher than regions I and III. To quantify this difference, for each trajectory we attribute all points with intensity within three standard deviations of the background level to off states and all points with higher intensities to on states. The intensity histograms (Figure 4d) for points in region I and III are similar, showing peaks of off and on states at 250 and 1100 counts per second. In comparison, in region II (on top of TiO2), the probability of on states is much smaller and the off state dominates. This trend can also be seen in the on and off time probability density distributions shown in Supporting Information Figure S5. It is clear that the

probability density of long on-states decreases and the probability density of long off-states increases in Region II compared to region I and III. Furthermore, as shown in Figure 4dI−III the on-state lifetimes are similar in region I and III and are significantly shortened in region II. Similar changes of single QD fluorescence characteristics have been observed in previous comparisons of QDs deposited on glass and TiO2 (single crystals and nanoparticles).61 Previous studies of electron transfer from single QDs to various electron acceptors have shown similar ET-induced change of blinking statistics.15,16,18,61 Electron transfer from QDs to acceptors creates positively charged QDs, which upon excitation forms positive trion state that is not emissive, leading to the increase (decrease) of the long off-state (on-state) probability densities. Therefore, the reduced on-state lifetime, reduced probability of long on states and increased probability of long off states observed for contact points in region II can be attributed to ET from the QD to TiO2. The variation of QD on-state lifetime as a function of its lateral position relative to TiO2 is shown in Figure 4b. The average of on-state lifetime of QD is about 23.1 ± 4.2 ns and 23.5 ± 3.7 ns at region I and III, respectively, when they are in contact with the glass substrate at either side of the TiO2 particle. When the tip is in contact with the TiO2 nanoparticle, the average QD on state lifetime is shortened to 16.5 ± 2.2 ns. Using the average on-state lifetime of region I and III as the intrinsic lifetime of the QD, the average ET rates for the four points in region II is 1.8 (±0.9) × 107 s−1. No measurements were attempted at the edge of the TiO2 particle to avoid displacing the particle by the AFM tip. A sharp decrease of QD on-state lifetime of QD from region II to regions I and III, over a tip lateral displacement of ∼40 nm, suggests that the lateral resolution of the lifetime contrast is on the order of 40 nm or better, which is similar to the lateral resolution of the AFM height image and significantly smaller than the optical diffraction limit. It is important to note that the suboptical diffraction limited spatial resolution in this measurement is achieved with electron transfer induced fluorescence quenching, a novel imaging contrast mechanism that has yet to be reported in the literature. In ANSOM, ultrahigh special resolution is achieved through the strongly enhanced local electric fields near sharp metal tips;22−25 its spatial resolution is determined by the electric field decay length scale, which is controlled by the size of the tip.62−64 In our study, the charge transfer rate between the QD and substrate is controlled by the exponential decay of electron wave function outside the QD, which is expected to occur on subnanometer length scale.65 Thus the spatial resolution of this technique is determined by contact area between the single QD and sample, which is controlled by the QD size and can in principle be on the nanometer or smaller length scale. In the current study, we have used the contact mode to ensure a close contact of the QD with the sample. For fast imaging applications, it would be more desirable to operate the AFM in AC mode, in which the tip−sample distance (Z) oscillates around an average value of 10 −100 nm. Because at most times within the oscillation period, the tip−sample distance is more than 1 nm, where ET rate is slow and quenching is negligible, it would be necessary to sort the detected photons according to the tip−sample distance.7,22,34−37,40,41 The implementation of such scheme is currently ongoing. Another limitation of the current method is the stability of the attached QD. Better attachment methods 5567

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will be needed to allow more reproducible and stable attachment, which would facilitate more systematic studies of interesting materials with functionalized tips. In summary, we have developed a new method for controlling and probing the spatial dependence of photoinduced charge transfer dynamics in nanoscale electron donor− acceptor materials that are widely used in solar cells. The spatial control is achieved by functionalizing either the donor or acceptor on the AFM tip, whereas the charge transfer dynamics is probed by fluorescence spectroscopy in an integrated AFM/ confocal fluorescence microscopy setup. We show that single QD-modified AFM tips can be prepared by bifunctional molecular linkers and can be used for AFM and fluorescence imaging studies. We demonstrate that using single CdSe/CdS core/shell QD functionalized AFM tips, the spatial dependence of photoinduced electron transfer dynamics from the single QD to TiO2 nanoparticles can be controlled and probed with a suboptical-diffraction-limited spatial resolution and high temporal resolution. Our findings demonstrate a new general method for high-resolution imaging of the spatial dependence of ultrafast charge transfer dynamics in nanocomposite materials.

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ASSOCIATED CONTENT

S Supporting Information *

Absorption and emission spectra of QD, fluorescence images, fluorescence intensity, and lifetime trajectories. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Science Foundation (CHE-1212907 and CHE-1309817). We thank Professor U. Banin for helpful discussion on the experimental implementation of the correlated AFM and fluorescence imaging method.

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REFERENCES

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dx.doi.org/10.1021/nl403181k | Nano Lett. 2013, 13, 5563−5569