Probing Driving Force and Electron Accepting State Density

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Probing Driving Force and Electron Accepting State Density Dependent Interfacial Electron Transfer Dynamics: Suppressed Fluorescence Blinking of Single-Molecules on Indium Tin Oxide Semiconductor Vishal Govind Rao, Bharat Dhital, and H. Peter Lu J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b08807 • Publication Date (Web): 15 Oct 2015 Downloaded from http://pubs.acs.org on October 20, 2015

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The Journal of Physical Chemistry

Probing Driving Force and Electron Accepting State Density Dependent Interfacial Electron Transfer Dynamics: Suppressed Fluorescence Blinking of Single-Molecules on Indium Tin Oxide Semiconductor

Vishal Govind Rao, Bharat Dhital, and H. Peter Lu* Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403 *Corresponding Author: [email protected]; 419-372-1840

Abstract Photo-induced, interfacial electron transfer (ET) dynamics between m-ZnTCPP and Sn-doped In2O3 (ITO) film has been studied using single-molecule photon-stamping spectroscopy. The observed ET dynamics of single m-ZnTCPP adsorbed on ITO was compared with that of mZnTCPP adsorbed on TiO2 NPs with and without applied electric potential. Compared to mZnTCPP on TiO2 NP surface, m-ZnTCPP on ITO surface shows reduced lifetime as well as suppressed blinking and quasi-continuous distribution of fluorescence intensities, presumably due to higher electron density in ITO. The higher electron density leads to the occupancy of CB acceptor states/trap states, which supports a higher backward electron transfer (BET) rate that results in quasi-continuous distribution of fluorescence intensities. The dependence of BET rate on electron density and charge trapping is consistent with our previous observations of quasi-continuous distribution of fluorescence intensities of m-ZnTCPP on TiO2 NPs with applied negative potential across the dye-TiO2 interface. The quasi-continuous distribution of fluorescence intensities in both cases of m-ZnTCPP on ITO surface and m-ZnTCPP on TiO2 NPs with applied negative potential indicates that the electron density in the semiconductor plays a dominant role in dictating the changes in rates of charge transfer in our system, rather than the relative energetics between electron in semiconductor and the oxidized sensitizer. Keywords: Single molecule spectroscopy, interfacial electron transfer, photon-stamping spectroscopy, blinking, ITO, density of states.

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1. Introduction Interfacial electron transfer (ET) dynamics between molecules and metallic or semiconductor electrodes at nanoscale is the core of surface chemistry, catalysis, and solar energy conversion, including solar photovoltaic and solar fuel science and technology.1-24 Improving our understanding of the underlying processes and mechanisms that govern interfacial ET is of foremost importance for basic energy science and technology. Comprehensive introspection of interfacial ET dynamics and associated intermittency of excited-state chemical reactivity requires molecular-level understanding of all related and regulating parameters: for example, adsorbate/semiconductor interactions, Franck-Condon factor, distribution of crystal surfaces and trap states, vibronic coupling, structures and energetics of both the adsorbate and the substrate surfaces, solvation, and local environment thermal fluctuations.5,

10, 13, 16, 25-47

There are two major processes involved in the photoinduced

interfacial ET at a dye/semiconductor interface: forward electron transfer (FET) from an excited state of dye molecule to the conduction band or energetically accessible surface states of semiconductor, and backward electron transfer (BET) of the excess electron which recombines with the parent oxidized dye molecule.10, 12, 16, 25, 28, 29, 31, 48 Extensive studies on interfacial ET dynamics have shown that FET kinetics are rather insensitive to variations in experimental condition,34,

49-53

whereas, BET kinetics exhibit greater sensitivity to

experimental condition. Designing an efficient solar energy harvesting system entails controlling the rate of BET process in order to generate long-lived charge-separated states so that the excess electron can diffuse away to generate photovoltaic potential energy.54-57 Despite the greater significance of BET dynamics, it has not been studied in great details, while FET dynamics has been well characterized. BET dynamics have been studied on an ensemble level using transient absorption spectroscopy and fluorescence measurements, but the underlying trapping and de-trapping processes have not been directly probed.32,

58-63

Ensemble-average measurements are not necessarily specific to resolve heterogeneity in local environment involving inhomogeneous substrate-dye molecule interactions. Detailed understanding of the molecular mechanics of interfacial ET dynamics requires studying the system at a single molecular level. Typical interfacial ET dynamics in sensitizer-semiconductor nanoparticle systems often involve temporal fluctuations and nanoscale spatial heterogeneities, i.e., the dynamic disorders and static disorders, respectively12,

13

presenting a significant challenge for

ensemble-averaged experiments for a molecular level characterization. Ensemble-averaged 2 ACS Paragon Plus Environment

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measurements often encounter interferences from molecular aggregation, multiple electron injection to a single particle and multiple electron-oxidized dye recombinations on the surface of a single particle. Single-molecule spectroscopy12,

13, 28, 31, 64-75

allows studying the

interfacial ET dynamics one molecule at a time in a specific nanoscale local environment, specifically powerful in resolving spatiotemporal heterogeneity. At single-molecule level, dye molecule adsorbed on metallic or semiconductor NPs shows fluorescence intensity fluctuation with time, also known as blinking. In this study we have used zinc(II)-5,10,15,20tetra(3-carboxyphenyl) porphyrin, m-ZnTCPP as dye molecule. In a single-molecule photonstamping measurement, m-ZnTCPP molecules are adsorbed on ITO film and on TiO2 NPs; each photon detected is emitted from the S1 excited state of a dye molecule and the same molecule will not be able to emit another photon until the dye molecule returns back to the ground state, S0 (Figure 1). For each photoexcitation, if the excited state of the dye molecule involves an ultrafast FET process reaching a charge-separation state of an oxidized dye and an excess electron in ITO/TiO2, then the dye molecule will not emit a photon through an S1 to S0 radiative transition. The oxidized dye is mostly nonfluorescent, and the dye molecule can only emit another photon when a BET occurs reducing the oxidized dye back to a ground state, S0.55 Therefore, a close look of single-molecule fluorescence intensity trajectories can provide information about FET and BET dynamics. While the blinking behavior of single-molecule on metallic or semiconductor NPs is still not fully understood, it can be considered to be the combined effect of (i) temporal fluctuation or dynamic disorder of interfacial ET dynamics,12, 76, 77 intermittency of ET, and (ii) BET rate (vide infra), which depends on the time spent by an excess electron on metallic or semiconductor NPs, which further depends on the time scales of the electron trapping and detrapping rate fluctuations.51, 55, 78-84 Detailed analysis of the fluorescence blinking behavior is therefore a direct method of probing both FET and BET dynamics.12, 13, 31, 55, 76, 83 Therefore, a systematic variation of parameters controlling blinking rate as well as blinking behavior at single-molecule level can yield mechanistic information about both FET and BET dynamics. Different variables have already been reported which influence BET dynamics to a larger extent than FET, e.g. effect of applied electric potential,34, 49, 82, 85, 86 effect of doping,26, 50, 57, 87 effect of oxidation and reduction of metallic or semiconductor NPs,88, 89 use of different dye molecule,32, 90 passivation of metallic or semiconductor NPs56, 60, 63 etc. In this study, we have compared the interfacial electron ET dynamics between mZnTCPP/Sn-doped In2O3, ITO film to that of m-ZnTCPP adsorbed on TiO2 NPs, with and 3 ACS Paragon Plus Environment


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without applied electric potential.31,

91

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Transparent n-type conducting oxides like ITO are

heavily doped and are of particular interest as semiconductor materials due to their relatively high electron densities (˃ 1019 cm-3).92,

93

The higher electron density in ITO than TiO2 is

likely to affect blinking behavior due to the increased occupancy of trap states in ITO. This can provide information regarding the role of trap states on BET rate. Furthermore, the conduction band of both ITO as well as TiO2 is lower than the LUMO energy of m-ZnTCPP indicating energetically allowed photoinduced interfacial ET. The Fermi-level lies above conduction band edge of ITO,92-95 whereas the Fermi-level lies below the conduction band edge of TiO212, 30, 36, 52, 53, 82, 96, 97 (Figure 1), which causes difference in the driving force for interfacial ET in ITO vs TiO2. This difference in driving force is again likely to affect the ET rate as well as fluorescence blinking behavior of m-ZnTCPP. Nevertheless, our singlemolecule studies provide novel insights in determining (i) the role of trap states on BET rate, hence on blinking behavior and (ii) the nature of semiconductor energy states that participate in interfacial ET dynamics. Figure 1 schematically depicts energy levels and photoinduced processes involved in the m-ZnTCPP-ITO/TiO2 systems. Here, we note that the blinking and fluctuation of the single-molecule fluorescence intensity are not due to the rotation or translational motions of the single molecules, m-ZnTCPP.12, 76 The covalently anchored mZnTCPP molecules through multiple carboxylic groups do not have the flexibility for rotational or translation motions.31,

76, 98

Additionally, according to our earlier studies, the

fluctuation of fluorescence intensity is not due to the triplet state of m-ZnTCPP molecules,31, 76

because the triplet-state lifetime of ZnTCPP dye lies close to 1 ms or less.99 In our control

experiment we have observed much less blinking for m-ZnTCPP on cover glass comparing to that in the m-ZnTCPP-ITO/TiO2 systems,31 which rule out a significant contribution from the dark triplet state to the blinking. Had the triplet state of m-ZnTCPP molecules been responsible for blinking, we should have observed similar blinking for m-ZnTCPP on glass. Additionally, the long dark time (from sub seconds to seconds) for m-ZnTCPP31 on TiO2 certainly shows that the fluctuation of fluorescence is not due to the triplet state.


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Figure 1: Schematic depiction of different energy levels and photoinduced processes involved in the m-ZnTCPP-ITO/TiO2 system: CB, conduction band; VB, valence band. The m-ZnTCPP binding on ITO and TiO2 surface as well as the Fermi-level position is demonstrated: In TiO2 the Fermi level lies below the trap states, which are assumed to follow an exponential distribution in energy below the conduction band;96, 97 In ITO the Fermi level lies above the conduction band edge. The higher electron density in ITO (which is obtained by substitutional doping with Sn) compared to TiO2, supports faster BET rate,89 charge recombination rate. 2. Experimental Section 2.1. Materials and Sample preparation Zn(II)-5,10,15,20-tetra (3-carboxyphenyl) porphyrin, m-ZnTCPP and dichloromethane were purchased from Frontier Scientific (research and development purpose only) and EMD chemicals (HPLC grade), respectively. Isopropyl alcohol (ACS reagent, ≥99.5%), ethanol (200 proof; anhydrous, ≥99.5%), and acetone (CHROMASOLV Plus, for HPLC; ≥99.5%) were purchased from Sigma Aldrich. All the reagents were used without further purification. Indium tin oxide (ITO)-coated transparent coverslips (SPI® Supplies; 18 mm × 18 mm; 8-12 ohms; thickness ∼ 170 µm) were thoroughly cleaned by sonication in deionized water, acetone, isopropyl alcohol and deionized water, each for 15 minutes, and dried in a jet of nitrogen gas before their use. Nanometer-size TiO2 particles were prepared by the hydrolysis 5 ACS Paragon Plus Environment


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of titanium isopropoxide (Aldrich, 99.999%) as precursor according to a literature protocol.100 The size range (diameter) of the TiO2 nanoparticles was found to be 5−15 nm as determined by atomic force microscopy (AFM). For the single-molecule experiments, we prepared the sample following our earlier reported procedures.13, 76 Briefly, 25 µL of 0.1 nM m-ZnTCPP in ethanol solution was first spin-coated on the transparent ITO-coated clean coverslip at 3000 rpm and covered by spin coating 50 µL PMMA (in CH2Cl2, 1 mg/mL) to form a thin film to protect the dye molecules from singlet O2 photobleaching. The sample of m-ZnTCPP on TiO2 NPs for interfacial ET study was prepared by first spin coating 25 µL of TiO2 NP solution on a clean cover glass (Fisher; 18 mm × 18 mm; thickness, 170 µm) at 3000 rpm followed by overlaying 25 µL of 0.1 nM mZnTCPP in ethanol and 50 µL of PMMA (in CH2Cl2, 1 mg/mL) by spin coating at 3000 rpm. 2.2. Single-molecule Fluorescence Spectroscopy and Imaging Single-molecule fluorescence spectroscopy and imaging were recorded by an Axiovert 135 inverted scanning confocal microscope equipped with a 100 × 1.3 NA oil immersion objective (Zeiss FLUAR) and a close-loop nanoscale-precision piezoelectric scanning stage (PI (Physik Instrumente) L.P.) to control the position of a sample. A picosecond pulsed laser (532 nm, Coherent Antares, YAG) was used to excite the sample with a repetition frequency of 76 MHz (excitation power at the sample was kept between 400 to 1200 nW). A beam splitter Z532rdc (Chroma) was used to reflect the excitation light into the objective (excitation spot size ~ 300 nm). The emission light passed through the emission filter HQ545lp (Chroma, for m-ZnTCPP, the emission wavelength typically ranges from 570 to 750 nm31) and collected by a single-photon counting avalanche photodiode (APD) detector (Perkin-Elmer SPCMAQR-14, pinhole size of APD is 170 µm). Photon-stamping data was recorded by a time-correlated single-photon counting (TCSPC) system (SPC-830, Becker & Hickl GmbH) in a FIFO (first-in first-out) mode (Figure 2A). We studied the interfacial ET dynamics of individual m-ZnTCPP molecule anchored to ITO/TiO2 NPs surface using single-molecule photon-stamping spectroscopic approach. This approach is capable of recording each detected photon with its chronic arrival time (absolute arrival time or real time) and the time delay between the photoexcitation and emission of the specific exited state of the individual molecule13, 76, 101 (Figure 2A-2C ).


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Figure 2: (A) Schematic representation of experimental setup for confocal fluorescence imaging and photon-stamping spectroscopy. (B) Schematic illustration of photon-stamping: for each detected photon two parameters were recorded: chronic arrival time or real time and delay time between the laser pulse excitation and the photon emission from the excited state. The distribution of the delay times gives a typical single-molecule fluorescence decay curve, and the histogram of chronic arrival times yields fluorescence intensity trajectories with a given time bin resolution. (C) Typical experimental photon-stamping trajectories for a particular time window (in blue) of single-molecule fluorescence trajectory and corresponding delay time distribution is shown in the red histogram. 3. Results and Discussion We recorded the fluorescence intensity trajectories of more than 100 molecules of m-ZnTCPP on Sn-doped In2O3 (ITO) film. Figures 3A-3F show three typical fluorescence intensity trajectories along with the fluorescence decays for three different single m-ZnTCPP molecules adsorbed on ITO film. The blinking behaviours of different m-ZnTCPP molecules adsorbed on ITO film are different, indicating static disorder of interfacial ET dynamics.12, 31, 76

Nevertheless, for majority of the molecules, the blinking is either dominated by bright

states indicating suppressed blinking behaviour (Figure 3B) or show a quasi-continuous 7 ACS Paragon Plus Environment


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(intensity trajectory with no distinct bright and dark states) distribution of fluorescence intensities (Figure 3A).

Figure 3: Typical Single-molecule fluorescence emission trajectories of m-ZnTCPP on ITO surface (A, B, and C) and corresponding single-molecule fluorescence decays (D, E, and F). Unlike m-ZnTCPP on ITO surface (Figure 4A), m-ZnTCPP adsorbed on TiO2 NPs show fluorescence intensity trajectories with distinct bright and dark states (Figure 4B). Once again the blinking behaviours of different m-ZnTCPP molecules adsorbed on TiO2 NPs are different, indicating static disorder of interfacial ET dynamics.12,

31, 76

For majority of the

molecules the fluorescence intensity trajectories are dominated by dark states, which demonstrates strong bonding between m-ZnTCPP and TiO2 as described in our earlier publication.31 8 ACS Paragon Plus Environment

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Figure 4: Typical Single-molecule fluorescence emission trajectories and corresponding intensity histograms of m-ZnTCPP on (A) ITO surface, (B) and (C) TiO2 NPs without and with applied electric potential respectively. Considering different semiconductor surfaces as the only difference between two systems, mZnTCPP on ITO surface vs m-ZnTCPP on TiO2 NPs, the change in the blinking behavior can be attributed to the effect of ITO surface and TiO2 NPs. The observed behavior of m-ZnTCPP on ITO surface can be associated with (i) inefficient FET leading to quasi-continuous distribution of fluorescence intensities and/or (ii) fast BET rate (vide infra). Considering larger driving force for FET in case of ITO than TiO2 NPs89, 102, 103 we posit that the FET is not inefficient in case of ITO, which is further corroborated by observed lifetime of mZnTCPP on ITO surface (vide infra). Therefore, to understand the observed difference in the



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single-molecule blinking behaviour, we focus our attention on BET dynamics, the interfacial charge recombination dynamics. The uniqueness of single-molecule photon-stamping spectroscopy lies in the fact that it can resolve the heterogeneities in both FET and BET dynamics through careful analysis of the observed blinking behavior. In single-molecule photon-stamping spectroscopy for each detected photon, two parameters are recorded: chronic arrival time or real time and delay time between the laser pulse excitation and the photon emission from the excited state.13, 95, 104

Therefore, the photon-to-adjacent-photon pair time can be precisely recorded (Figure 2B).

Fluorescence intensity trajectories, the photon counting rate as a function of time, were constructed from photon-stamping time trajectories by binning the detected photons with few ms bin. Multiple ET events typically occur in each intensity time bin of a few milliseconds, depending on the ET efficiency during that bin. Accordingly, bright or dark states of the molecules are observed. For each photoexcitation, the excited state of the single-molecule either undergoes radiative emission to yield an emission photon that contributes to a bright state or undergoes a nonradiative ET process that contributes to the dark state. The occurrence of the dark and bright states is ultimately determined by the dominance of radiative or the nonradiative process in term of their rates, which further depends on the other ET rate regulating parameters, such as the electronic and vibrational coupling between the molecule and the substrate. Following an interfacial ET to ITO/TiO2, the injected electrons go through charge trapping, detrapping, and electron Brownian and non-Brownian diffusion before mostly recombining with parent oxidized dye, the geminate charge recombination.10, 12, 30, 49, 53, 83

ITO/TiO2.

Surface traps play an important role in dynamics of injected electron in

A fundamental understanding of the surface states, such as their nanoscale

distribution and electronic coupling, is crucial to the interfacial ET dynamics and the mechanisms involving of the photoelectrical semiconductor materials.10,

30, 49, 53, 84, 105-108

Previously, a near-field scanning microscopic imaging analysis of surface states on a TiO2 (110) surface was demonstrated and reported.105 We have shown that for a molecule adsorbed on the semiconductor surface, the surface state charge-transfer pathways are primarily affected by the density and energy distributions of the surface states.105 The BET rate depends on the time spent by an electron on ITO/TiO2 NPs, which further depends on the distribution of trap states within the NPs as well as the electron density in the ITO/TiO2 NPs. Consequently, the BET rate should greatly influence the blinking behavior of single-molecules anchored to ITO/TiO2 NPs. The BET process, i.e., the electron and oxidized 10 ACS Paragon Plus Environment

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dye recombination, has been reported to occur in the range of picoseconds (ps) to several milliseconds (ms) under various conditions.40, 48, 49, 52, 53, 59, 82, 89, 109-112 Therefore, considering the nanosecond lifetime of the excited state of m-ZnTCPP and picoseconds to milliseconds timescale of BET, we suggest that fluorescence intensity fluctuation dynamics significantly depends on the BET rate. If the BET timescale is in the order of ms-µs, the dye molecule will be expected to spend more time in a given charge state M/M+ (M = m-ZnTCPP), leading to slow switching between M and M+ within the bin time, resulting in distinct bright and dark states. On the other hand, if the BET is of the order of ns-ps then the dye molecule will be expected to spend less time in a given charge state M/M+ leading to fast switching between M and M+ within the bin time, resulting in a quasi-continuous distribution of fluorescence intensities.113 Therefore, the observed suppressed blinking (Figure 3B) as well as quasicontinuous distribution of single-molecule fluorescence intensity (Figure 3A is likely the result of faster BET in of m-ZnTCPP on ITO surface than in m-ZnTCPP on TiO2 NPs. The higher BET rate in ITO is well supported by earlier reports.89, 95 For example, Meyer et al.89 have shown significant difference in the time scale for BET which ranges from nanoseconds on ITO to microseconds and milliseconds on TiO2, SnO2 and ZnO. Additionally on the basis of a convoluted interfacial electron-oxidized dye recombination model, we showed that the time scale of BET for p-ZnTCPP on TiO2 NPs lies in the range of microseconds to milliseconds.55 The electron density difference between ITO and TiO2 presumably play a major role in the BET kinetics. The electron density for a 10% Sn-doped In2O3 (ITO) has been estimated to be ~ 2.2 × 10

21

cm-3.92, 95 The increased electron density leads to shift of Fermi level to more

negative potential and filled trap states.10, 16, 30, 52, 53, 82 For 10% Sn-doped In2O3 the Fermi level of ITO is estimated to be ~ 100 meV higher than its conduction band edge.95 Due to the filled trap states as well as increase in number of electrons per nanoparticle, the photoinduced electrons spend less time within the semiconductor resulting in faster charge recombination.10,

11, 30, 35, 52, 53, 60, 82, 114

The effect of increased electron density on BET

dynamics is well supported by our experimental results of applied negative bias across dyeTiO2 interface.91 Figure 4C shows a quasi-continuous distribution of fluorescence intensities observed with applied potential of -0.60 V across dye-TiO2 interface. It has been reported that applied electric potential across dye-TiO2 interface modulate the Fermi level of TiO2 NP as well as the electron occupancy in TiO2 conduction band and energetically accessible surface traps states.33, 34, 49, 52, 82, 85, 86, 115-117 The applied negative bias across dye-TiO2 interface also 11 ACS Paragon Plus Environment


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increases the electron density inside the TiO2 NPs which supports faster BET rate.34, 49, 82, 85 Evidently, the similar BET dynamics at ITO and TiO2 surfaces with applied potential, at single molecule level, highlights the role of electron density and surface states density in governing charge transfer dynamics. The dependence of BET kinetics on electron density and surface states density has been revealed by observed effect of applied bias and doping using transient absorption spectroscopy.26, 34, 49, 50, 57, 82, 85-87 BET rate was found to vary over nine orders of magnitude, from milliseconds to picoseconds with changes in applied bias.34, 49, 82, 85 For TiO2 NPs, the BET rate was found to be in the rage of milliseconds to microseconds at positive or zero applied potential, whereas a dramatic increase in BET rate has been reported at negative potential.37,

49, 82

For TiO2 NPs, the BET rate was found to be in the rage of

nanoseconds to picoseconds at an applied bias of -0.6 V versus Ag/AgCl.37, 49, 82 Similarly, the BET rate increases depending on increases of the dopant and doping concentration.26, 50, 57, 87

In the simplest consideration, with increase in doping concentration, the electron density

in the materials increase and raises the Fermi level to a more negative potential, which results in increase in BET rate. In fact, the reduced blinking as well as quasi-continuous distribution of fluorescence intensity of (i) m-ZnTCPP on ITO surface (Figure 3) and (ii) m-ZnTCPP on TiO2 NPs with applied negative potential are well in agreement with the observed fluorescence intensity trajectories of QDs in affect to different doping level and applied bias.50, 57, 81, 95, 103, 115 To explain the huge variation of BET rate, Haque et al.49, 82 proposed a mechanism, which supports electron transport being controlled by the energetically distributed trap sites within the TiO2 NPs. According to Haque et al.,49,

82

BET kinetics

exhibits a nonlinear dependence on the TiO2 electron occupancy, with dramatic increase of BET rate with increasing occupancy. Interestingly, the quasi-continuous distribution of fluorescence intensities in both cases of m-ZnTCPP on ITO surface (Figure 3) and mZnTCPP on TiO2 NPs with applied negative potential, indicates similar range of BET rate in spite of difference in the relative energetics between excited state of m-ZnTCPP and ITO/TiO2 (Figure 5). The insensitivity of BET kinetics on the relative energetics between electron in TiO2 and the oxidized sensitizer corroborate with the mechanism of Haque et al., which suggests the electron occupancy, rather than the relative energetics between electron in TiO2 and the oxidized sensitizer, is the most important factor in determining the BET rate.49, 82

Our findings, particularly the blinking behavior corroborate earlier reports of suppressed

blinking on ITO,95,

103

dopant concentration dependent blinking,50,

26, 115

dependent blinking

57, 81

and applied bias

all of which posit the considerable contribution of the electron

occupancy and density of states. 12 ACS Paragon Plus Environment

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Figure 5: Schematic illustration of the sub-bandgap or trap states, Fermi level, and forward ET free energy driving force (∆G) for ITO, TiO2 NPs, and TiO2 NPs with applied negative bias: M*/M+ indicate excited state energy of m-ZnTCPP. In TiO2 the Fermi level lies below the trap states, application of negative potential raises the TiO2 Fermi level, which leads to occupancy of CB electron acceptor states as well as decrease in ∆G value. In case of ITO the Fermi level lies above the conduction band edge and the forward ET free energy driving force (∆G) is also large compared to that of TiO2. Blue and green areas denote electron acceptor states unavailable for electron injection from m-ZnTCPP excited state. To further analyze the complex interfacial ET dynamics and energetics involving in both FET and BET, we have studied the effect of free energy driving force on FET from the excited states of dye molecules to the conduction band and energetically accessible surface states of ITO and TiO2 NPs, and hence on the lifetime of m-ZnTCPP. Figure 6 shows single-molecule fluorescence lifetime distributions of m-ZnTCPP on ITO surface and m-ZnTCPP on TiO2 NPs. The lifetime distribution of m-ZnTCPP on ITO surface, from 0.7 ns to 2.6 ns demonstrates the individuality of each molecule in a specific environment associated with the inhomogeneous surface state distribution of m-ZnTCPP on rough ITO surface. Similarly, the lifetime distribution of m-ZnTCPP on TiO2 NPs, from 1.4 ns to 4.8 ns indicates the individuality of each molecule in a specific environment associated with the inhomogeneous surface state distribution of m-ZnTCPP molecules on TiO2 NPs.



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Figure 6: Fluorescence lifetime distributions of m-ZnTCPP on ITO surface and on TiO2 NPscoated cover glass. The shift towards longer lifetime indicates lower efficiency of FET for m-ZnTCPP on TiO2 NPs than m-ZnTCPP on ITO surface, which can be assigned to smaller FET free energy driving force in case of m-ZnTCPP on TiO2 NPs than m-ZnTCPP on ITO surface (Figure 5). We note that the difference in refractive index of the two semiconductor materials ITO (~2.05) and TiO2 (~2.30) should also affect radiative rates of m-ZnTCPP molecule. Following Strickler-Berg equation118 higher index of refraction of the TiO2 surface than that of ITO should result in a higher radiative rate and shorter lifetime for m-ZnTCPP on a TiO2 surface. On the contrary we observed longer lifetime for m-ZnTCPP on TiO2 surface than that of m-ZnTCPP on ITO surface. Therefore, the lifetime difference between the m-ZnTCPP on ITO vs m-ZnTCPP on TiO2 is not dominated by the refractive index difference of the two semiconductor materials, and we attribute the changes in lifetime to nonradiative FET processes. Unlike charge recombination, where the BET rate is significantly influenced by the density of surface states and electron occupancy, the FET rate difference is mainly contributed by FET free energy driving force differences; although, assuming the other critical factors of electronic coupling, vibrational reorganization energy changes, and surface dye-substrate chemical interactions remain essentially constant. The insensitivity of FET rate on density of electron accepting states is reflected by higher efficiency of FET for m-ZnTCPP on ITO resulting in shorter lifetime. Had the density of electron accepting states been a 14 ACS Paragon Plus Environment

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deciding factor, the higher electron density within the ITO, which reduces the density of unoccupied states available for photoinduced electron injection (Figure 5) should have lower efficiency of FET. The insensitivity of FET rate on density of electron accepting states corroborate the observed insensitivity of FET rate to the applied electric potential effect and doping effect.34, 49, 81, 82, 115 Because the Fermi level in ITO at 10 % doping level is just 100 meV higher than its conduction band edge, only states near the band edge are filled.89,

95

Therefore, most electron accepting states of ITO conduction band remain unoccupied and the density of accepting states in ITO for FET from excited m-ZnTCPP molecule should not be significantly smaller than that of m-ZnTCPP on TiO2 NPs. We have also analyzed the correlations in temporal variations of fluorescence intensity and fluorescence decay time calculated for a 10-ms bin size. Figure 7A and Figure 7B show fluorescence lifetime-intensity distribution (FLID) of m-ZnTCPP on ITO and m-ZnTCPP on TiO2 NPs, respectively. In Figure 7, the distribution of intensity and lifetime are shown along the horizontal and vertical lines for both m-ZnTCPP on ITO and on TiO2 NPs, respectively. Figure 7A and 7B show fluorescence lifetime-intensity distributions of m-ZnTCPP on ITO and on TiO2 NPs deduced from the single-molecule fluorescence intensity trajectories shown in Figure 3B and 4B, respectively. The single-molecule FLID of m-ZnTCPP on ITO and on TiO2 NPs show wide distribution of intensity as well as lifetime. The broadness of intensity and lifetime distribution for a single-molecule indicates the fluctuations in the reactivity of interfacial ET dynamics, i.e., intermittent interfacial ET dynamics,12, 76, 119-121 and variation in BET rate or trapping and detrapping dynamics.30 The m-ZnTCPP adsorbed on TiO2 NPs shows a broad fluorescence intensity distribution as well as a broad lifetime distribution. However, unlike m-ZnTCPP adsorbed on TiO2 NPs, m-ZnTCPP on ITO surface shows a narrower fluorescence intensity distribution as well as a narrower lifetime distribution (Figure 7). Furthermore, Figure 7 shows that there are two different regions with distinct intensity for m-ZnTCPP adsorbed on TiO2 NPs, and only one region of intensity for m-ZnTCPP on ITO surface. This observation again suggests suppressed blinking activity of m-ZnTCPP on ITO surface.



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Figure 7: Correlated variations of fluorescence intensity and fluorescence decay time (calculated for 10 ms bin size): (A) fluorescence lifetime-intensity distribution (FLID) of mZnTCPP on ITO for single-molecule fluorescence intensity trajectory shown in Figure 3 B. (B) fluorescence lifetime-intensity distribution (FLID) of m-ZnTCPP on TiO2 NPs for singlemolecule fluorescence intensity trajectory shown in Figure 4 B. In both the FLID images the dark states of single-molecule fluorescence intensity trajectories (shown in Figure 3 B and 4B) were not included because the fluorescence intensity level for dark states is almost equal to the background level. To compare the intermittency or blinking dynamics of m-ZnTCPP on ITO and on TiO2 NPs, we have performed statistical analysis on the stochastic durations of the dark state involving high ET reactivity compared to the bright state associated with low ET reactivity. Specifically, we have applied threshold method for the analysis of dark states to provide 16 ACS Paragon Plus Environment

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information of BET dynamics involving charge trapping and detrapping processes. The following steps are used to determine the distribution of dark-time durations and probability density distribution of the dark states for single-molecule fluorescence emission trajectories. (i) The threshold fluorescence intensity (photon count), ITh, separating the on and off states is determined using equation 1

= + 3 (1) where Iav is the average fluorescence intensity of the background and σ is its standard deviation. (ii) The distribution of dark-time durations, including occurrence of different dark times are determined using the threshold value. (iii) Finally the probability density distribution68, 78, 79, 122-124 of the dark states P(tdark) are generated by using equation 2

(t ) =

Occurrence (t) (2) ∆t

where Occurrence (t) is the occurrence of dark event having duration time of t, and ∆t is the average of the dark time intervals to the preceding and following events. Figure 8 shows the normalized probability densities of the dark-time durations of m-ZnTCPP on ITO and m-ZnTCPP on TiO2 NPs. The probability density built from different dark time events show linear dependence on tdark in log-log plots for majority of events, particularly at short dark time. This linear dependence of the single-molecule dark time probability density distributions shows typical power-law behavior, which can be mathematically described by equation 3

(t ) ∝ t ! (3) The observed power-law kinetics is consistent with our earlier results.31, 76 We have shown that the probability density distributions of the dark times for p-ZnTCPP on TiO2 NPs also follow power-law behaviour.76 However, the structural change of the dye molecule from pZnTCPP to m-ZnTCPP is always accompanied by the change in interfacial molecular interactions between the probe molecules and their fluctuating local environment leading to the difference in interfacial ET reactivity. Several existing models explain the power-law blinking behavior of single molecules on semiconductor surfaces.68, 78, 123-129 Following our earlier studied system and based on the data shown in Figure 8,76 we attribute that the observed power-law blinking and fluctuations of the fluorescence from m-ZnTCPP on ITO



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and on TiO2 NPs, are due to temporal fluctuations in the reactivity of interfacial ET dynamics i.e., the dynamic disorder,121 and variation in BET rate or trapping and detrapping dynamics.

Figure 8: Normalized probability densities, (t ) of the dark-time durations of mZnTCPP on ITO and m-ZnTCPP on TiO2 NPs. In log-log scale plots the solid lines are linear fits indicating power-law behavior. The power-law exponent, mdark, which is the slope of the linear fit, is also indicated in the figure. The power law exponent mdark is found to be 1.36 and 2.06 for m-ZnTCPP on TiO2 NPs and m-ZnTCPP on ITO, respectively. The lower mdark value of m-ZnTCPP on TiO2 NPs than that of m-ZnTCPP on ITO indicates a relatively higher probability for longer dark times in the case of m-ZnTCPP on TiO2 NPs. For m-ZnTCPP on ITO, no dark events with durations of longer than 2 seconds were observed, whereas dark events with durations up to 8 seconds were observed for m-ZnTCPP on TiO2 NPs. To explain the long dark-events observed for m-ZnTCPP on TiO2 NPs compared to that of mZnTCPP on ITO, we proposed two possibilities (i) long dark events may occur due to high FET activity for a long time55 in m-ZnTCPP on TiO2 NPs and (ii) lower BET rate, owing to the presence of a large number of trap states in m-ZnTCPP on TiO2 NPs, resulting from longer trapping and slower detrapping processes. From our earlier discussion, it is evident that FET efficiency is lower in case of m-ZnTCPP on TiO2 NPs than to m-ZnTCPP on ITO.


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Nevertheless, FET activity does not describe the higher probability of longer dark time of mZnTCPP on TiO2 NPs than that of m-ZnTCPP on ITO surface. Therefore, the BET rate or electron trapping and detrapping dynamics plays an important role in the observed intermittency or blinking dynamics. Overall the systematic variation of driving force and electron accepting state density (Figure 5) for interfacial ET in ITO vs TiO2 NPs with and without applied potential and corresponding effect on blinking rate as well as blinking behavior at single-molecule level yield mechanistic information about both FET and BET dynamics. In our working range, the Fermi levels of both the TiO2 NPs with applied negative potential91 as well as ITO at 10 % doping level reside close to their respective conduction band edge.60, 89, 91, 95, 130 This implies that only states near the band edge are filled. We believe the results reported here, combined with our previous studies,91, study,

91

105

particularly the single-molecule spectroelectrochemistry

advance our understanding of the nature of interfacial charge transfer dynamics and

energetics. Although our observations corroborate the findings of extensive literature on ensemble-averaged measurements, this study provides unique and fundamental understanding of the complex BET rate processes at single-molecule level. For example, our singlemolecule results overcome interferences from molecular aggregation, multiple electron injection to a single particle, energy transfer, hopping, and multiple electron-oxidized dye recombinations on the surface of a single particle. Additionally, the observed nanoseconds lifetimes of m-ZnTCPP on ITO and TiO2 NPs as well as the nanoseconds lifetime variation range with applied potential indicates the presence of a time component with nanosecond FET rates. Had the time scale of the FET rate been solely in the femtosecond to picosecond range, the excited state radiative emission efficiency would have been as low as 10-3 to 10-6, and the single molecules would essentially be undetectable in a typical single-molecule imaging measurement. We also note that most of the molecules, visible in single-molecule imaging, show nanoseconds lifetime components, and the fluctuations of FET rates at nanosecond time scale are significant. The highly inhomogeneous interfacial ET rate processes are typically reported in literature as non-exponential dynamics with ultrafast decay components and nanosecond decay components that present as a constant level off at ultrafast time scales. Although, the level-off amplitude is typically only a low percentage of the overall time decay amplitudes, the integrated photon counts can be more than 70% or even 90%, which cannot be ignored. There are two significantly different dynamics and mechanisms: (1) Statically-disordered inhomogeneous dynamics: Majority of molecules is 19 ACS Paragon Plus Environment


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involved in ultrafast interfacial ET dynamics and only a small portion of molecules which does not engage in ultrafast ET participates only in radiative decays of nanosecond time scale; and (2) Dynamically-disordered inhomogeneous dynamics, the intermittent rate fluctuation dynamics: Majority of the molecules exhibit broad rate fluctuations, from ultrafast ET rate processes to nanosecond radiative rate processes. Seemingly identical individual molecules have significant different interfacial electron transfer rates reflects static disorder; whereas, for each individual molecule, the interfacial electron transfer rate also fluctuates significantly from time to time, which constitutes the dynamic disorder of the reaction dynamics.12, 101, 119, 129, 131-143 Should the interfacial ET be controlled by a statically-disordered inhomogeneous dynamics, the ET dynamics would be dominated by the ultrafast dynamics, and the molecules with slower ET rate and small amplitude will not be in the majority. However, should the interfacial ET be controlled by intermittent rate fluctuation dynamics, most molecules will show broad ET rate fluctuations and the molecules with slower ET rate and small amplitude will be in the majority. Since the ensemble-averaged time resolved nonexponential decay dynamics are widely reported in the literature,40,

80, 144, 145

it is highly

significant to identify if the controlling mechanism is only the static disorder or both the static and dynamic disordered rate processes, i.e., the intermittent interfacial ET dynamics. Nevertheless, our single-molecule spectroscopic analysis suggests that the systems we have studied are mostly controlled by the intermittent dynamics. Certainly, the quantification of the inhomogeneous interfacial ET dynamics, namely, the BET rate process, and FET rate process at single-molecule level, requires more research on this topic. We are currently working on developing single-molecule ultrafast spectroscopy to resolve the interfacial ET at picoseconds

to

sub-picoseconds

time

scale.

In

the

future,

single-molecule

spectroelectrochemical studies as well as the correlated single-molecule fluorescence spectroscopy approach with various other techniques such as atomic force microscopy (AFM), scanning tunneling microscopy (STM), electrochemistry, and Raman spectroscopy will continue to reveal and specify underlying processes and factors governing interfacial ET dynamics. 4. Conclusion We have examined the role of driving force and density of accepting state on interfacial ET dynamics by comparative single-molecule photon-stamping studies of m-ZnTCPP adsorbed on ITO and m-ZnTCPP adsorbed on TiO2 NPs. The two different semiconductors (ITO and TiO2) present disparate adsorbing surfaces, characterized by large difference in electron 20 ACS Paragon Plus Environment

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densities and distinct driving forces. Without applied bias, we observed a blinking pattern associated with slower rate of BET for TiO2 and a quasi-continuous distribution of fluorescence intensities and reduced lifetime for ITO. With applied negative potential, mZnTCPP adsorbed on TiO2 NPs has an even larger difference in driving force compared to mZnTCPP adsorbed on ITO but similar electron densities. In both cases, a quasi-continuous fluorescence trajectory is observed. Our results suggest that the electron density in the semiconductor plays a significant role in dictating the changes in rates of charge transfer in the system of m-ZnTCPP adsorbed on ITO, rather than the relative energetics between electron in semiconductor and the oxidized sensitizer.

Acknowledgements This work is supported by the Office of Basic Energy Sciences within the Office of Science of the U.S. Department of Energy (DOE).



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Probing Driving Force and Electron Accepting State Density

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