Control of Electron Transfer from Lead-Salt Nanocrystals to TiO2

LETTER pubs.acs.org/NanoLett

Control of Electron Transfer from Lead-Salt Nanocrystals to TiO2 Byung-Ryool Hyun,*,† A. C. Bartnik,† Liangfeng Sun,† Tobias Hanrath,‡ and F. W. Wise† †

School of Applied and Engineering Physics and ‡School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14850, United States

bS Supporting Information ABSTRACT: The roles of solvent reorganization energy and electronic coupling strength on the transfer of photoexcited electrons from PbS nanocrystals to TiO2 nanoparticles are investigated. We find that the electron transfer depends only weakly on the solvent, in contrast to the strong dependence in the nanocrystal
molecule system. This is ascribed to the larger size of the acceptor in this system, and is accounted for by Marcus theory. The electronic coupling of the PbS and TiO2 is varied by changing the length, aliphatic and aromatic structure, and anchor groups of the linker molecules. Shorter linker molecules consistently lead to faster electron transfer. Surprisingly, linker molecules of the same length but distinct chemical structures yield similar electron transfer rates. In contrast, the electron transfer rate can vary dramatically with different anchor groups. KEYWORDS: electron transfer, Marcus theory, solvent reorganization energy, electronic coupling, spacer length, anchor groups In Marcus theory the ET rate can be expressed as2,3

S

emiconductor nanocrystals (NCs) are poised to play a leading role in the revolution of photovoltaic devices1 due to their tunable energy gaps, large extinction coefficients, and extended photostability. They offer great opportunities to develop low-cost and high-efficiency photovoltaic devices. The successful extraction of photogenerated charges from within the NCs to external electrodes is contingent upon a series of charge separation and transport processes. The desire to improve the efficiency of photodevices has spurred considerable interest in the nature of the charge-transfer process itself, with the expectation that better understanding of the interfacial charge transport will lead to its control and then to better devices. Thus, an understanding of electron transfer (ET) between NCs and acceptors is of both scientific and technological importance. Although there has recently been quite a bit of research aimed at charge transfer from NCs, there is no systematic theory to guide experiments. Marcus theory is well-established for modeling of molecular ET-transfer reactions.2
6 Anecdotal connections to Marcus theory have been made in studies of charge transfer from semiconductor NCs to charge acceptors, over limited ranges of the parameters of the theory.7
11 It would be very valuable if Marcus theory could be extended to model nanoscale semiconductor ET accurately. Tvrdy et al. recently reported a systematic study of the rate of charge transfer from CdSe NCs to metal oxides as a function of NC size, and modeled the results in the framework of Marcus theory.10 No linker molecules were employed in that work. Here we report a systematic study of the transfer of photoexcited electrons from PbS NCs to TiO2. The effects of the solvent reorganization energy and the electronic coupling on ET are investigated, within the conceptual framework of Marcus theory. This work focuses on the role of the linker molecules and the solvent, and thus complements the recent study of Tvrdy et al. r 2011 American Chemical Society

kET

1 2π ¼ ¼ τET p

Z

" # jHDA ðEÞj2 FðEÞ ðλ þ ΔG0 þ EÞ2 p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp
dE 4λkb T 4πλkb T
¥ ¥

ð1Þ where τET is the charge transfer time, HDA(E) is the electronic coupling between the initial and final states, λ is the reorganization energy, ΔG0 is the total Gibbs free energy change for the electron transfer reaction, F(E) is density of accepting states, kb is Boltzmann’s constant, and T is the temperature. At a given temperature, there are three parameters to control the ET dynamics: λ, ΔG0, and HDA. In molecular donor
bridge
acceptor systems, the effects of these three parameters on the ET dynamics have been studied extensively.12
15 We will focus on transfer of electrons from NC to wide-gap oxides, exemplified by TiO2. The free-energy change is the difference between the lowest electron level of the NC and the acceptor level, which we take to be the conduction band minimum in TiO2. The reorganization energy includes all structural changes of both the reactants and the solvent during the charge transfer. The effects of the solvent, considered as a dielectric continuum, enter into the rate through the reorganization energy. Thus, the dielectric properties of the solvent or host materials clearly impact the ET dynamics, and this is welldocumented in molecular systems.16,17 Hyun et al. also showed that the rate of ET from PbS NCs to molecules increases dramatically with the static dielectric constant of the solvents.9 The electronic coupling strength is related to the wave function Received: March 3, 2011 Revised: April 9, 2011 Published: April 20, 2011 2126

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Table 1. Dielectric Constants of Various Solvents solvents

static dielectric constant46 optical dielectric constant47,a

tetrachloroethylene

2.30

chloroform

4.77

2.28 2.09

chlorobenzene

5.54

2.32

dichloromethane

8.51

2.03

a

Figure 1. Schematic of a donor
linker molecule
acceptor system. In the present work, the donor is a PbS NC and the acceptor is a TiO2 nanoparticle with a diameter of 25 or 50 nm. The linker molecule consists of functional groups R1 and R2 and a spacer (S).

overlap between initial and final states of the electron, when it is located on the donor and acceptor, respectively. Spatial overlap of the wave functions occurs within the linker molecule. Therefore, the role of the molecular linker is crucial for the electronic coupling, and its role on the ET has been investigated in molecular systems. However, only a few works on the role of linker molecules between NC and charge acceptor have been reported18,19 and we still lack understanding of their influence. In general, a linker molecule consists of a functional group (R1) for binding with the donor, a spacer (S), and another function group (R2) for binding with the acceptor—schematically, R1
S
R2 as illustrated in Figure 1. The spacer can be changed by varying its length, or its molecular structure (saturated hydrocarbons versus aromatics, e.g.). The electronic coupling strength is expected to decay exponentially with increasing separation between donor and acceptor, HDA = H0DA exp[(
1/2)]βdDA, where dDA is the separation, H0DA is the electronic coupling with dDA = 0, and β is the attenuation rate.6,20 Therefore, the electron transfer rate varies exponentially with dDA through the electronic coupling strength if the solvent reorganization energy is small, i.e., in the nonadiabatic and weak-coupling regime. The predicted exponential dependence of the electron transfer rate with dDA has been reported in molecular systems.21
23 Such spacer (bridge)assisted ET processes have been studied in molecular and NC.24
28 In the NC
metal oxide composite system, Dibbel et al. reported that the rate of ET from CdS NCs to TiO2 nanoparticles was faster for shorter ligands, with a saturated hydrocarbon chain for the linker molecule, along with the dependence of the rate on the presence of an alkyl chain or phenyl ring.18,19 Covalently bonded molecules are thought to strongly hybridize with the NC wave function, which causes the energy levels to broaden and facilitates transfer of charge to or from the NC. Changing the atom attached to the NC surface can change the energy distribution of the photoexcited electron of the NC, thus altering the ET rate.29 Recent measurements of the conductance of single molecules show that the electronic coupling strength is modified by different anchor groups.30
33 Further, it was reported that different anchor groups modified the electron transfer rate from molecules to TiO2 and SnO2 surfaces,34,35 and this was attributed to the change of the electronic coupling strength through the spatial extension of the donor orbital.36 However, no such study of ET between NCs and charge acceptors has been reported. This knowledge gap provides strong motivation to investigate the influences of the solvent dielectric constant and the anchor group on the ET of NC
bridge
acceptor systems. As a model system, we choose PbS NCs coupled to TiO2 using bifunctional linker molecules. The PbS NC system is a good

Optical dielectric constants are obtained by squaring the refractive indices measured at 589 nm.

candidate for the development of efficient photovoltaic devices due to its broad overlap with the solar spectrum. Here we focus specifically on electron transfer. In all of the studied cases, hole transfer should be negligible, and in a NC-sensitized Gratzel cell, the hole would be scavenged by a redox couple. The three key parameters of Marcus theory—the free energy change, the reorganization energy, and the electronic coupling—can be tuned independently for electrons. The free energy change can be controlled through the size of the NC.7,8,10,37,38 Three studies will be described: dependence of ET on the solvent, which probes the reorganization energy; investigation of the effects of the length and nature of the spacer group on ET, via the electronic coupling; investigation of the effects of the anchor groups on ET, also to probe the electronic coupling. Electron transfer was monitored primarily through quenching of the NC fluorescence. Other phenomena, such as trapping of charge carriers, can lead to fluorescence quenching, so careful controls are needed. For each combination of solvent and linker molecule studied, the fluorescence of NCs terminated with the linker molecules but no TiO2 was measured to confirm that the linkers do not quench the fluorescence. In addition, the same system but with ZrO2 substituted for TiO2 was measured. ET from PbS NCs to ZrO2 is energetically unfavorable, and the observed absence of fluorescence quenching in this system is an important control (details are in the Supporting Information). An advantage of the PbS
TiO2 system is that the use of fluorescence quenching to monitor charge transfer has been verified by photocurrent.8 The use of TiO2 particles that are much larger than the NCs ensures that there will be no more than one charge acceptor per NC. Colloidal PbS NCs were synthesized by literature procedures39,40 that build on Murray’s approach.41 The diameter of the NCs was estimated to be 3.5 nm by comparing the first absorption peak of the NCs with a k 3 P model,42 which has been previously demonstrated to match experimental sizes.43 The electron affinity (EA) of these NCs is 3.7 eV.8,9 Colloidal TiO2 nanoparticles were synthesized according to the approach of Robel et al.8,44 Details of the syntheses are in the Supporting Information. The EA of the TiO2 nanoparticles in organic solvent is 3.9 eV, and the EA when dried is 4.45 eV.45 Thus, the values of ΔG0 are 0.18 eV for the PbS
TiO2 complex in organic solvent and 0.73 eV for the PbS NCs coupled to dried TiO2. ET was monitored with time-integrated fluorescence and time-resolved fluorescence using time-correlated single photon counting (TCSPC). The excitation wavelength (780 nm) was chosen such that only the PbS NCs were optically excited. Our previous transient absorption measurements of the PbS NC
TiO2 system8 showed that there were no sub-nanosecond decay components in the fluorescence decay. The time scale of the fluorescence decay measurements presented here is a few nanoseconds. Thus, we expect there to be no difference between the fluorescence quenching and transient absorption measurements. 2127

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Figure 4. Electron transfer time vs 1/εop
1/εst of solvent. The lines are theoretical fits discussed in the text, with values of the reorganization energy indicated in the legend.

Figure 2. (a) The absorption of 3.5 nm diameter PbS NCs and PbS NC
MPA
TiO2 composite in TCE. (b) Fluorescence spectra of 3.5 nm diameter PbS NCs in TCE (black) and PbS NC
MPA
TiO2 composites in TCE (green line), chloroform (red line), chlorobenzene (blue line), and dichloromethane (cyan line), respectively. Excitation wavelength is 780 nm.

TiO2. The absorption spectra of the composites dispersed in the other solvents (not shown) are almost identical. The NC fluorescence is quenched in the presence of the TiO2, as expected.8 As the NC
MPA
TiO2 composites are dispersed in different solvents such as tetrachloroethylene (TCE), chloroform, chlorobenzene, and dichloromethane, the degree of quenching changes slightly, with the greatest quenching occurring in chloroform. The fluorescence of the PbS NCs in TCE decays with a time constant of 1.7 μs (black line in Figure 3). The fluorescence decays of the composite in other solvents are faster. As expected from the time-integrated fluorescence spectra, the fluorescence decays of the composites in different solvents are almost the same. The electron-transfer times inferred from the fluorescence decays48 1 1 1 ¼ kET ¼
τET τNC þ TiO2 τNC

Figure 3. Transient fluorescence traces of PbS NCs in TCE (black) and PbS NC-MPA-TiO2 composites in TCE (green), chloroform (red), chlorobenzene (blue), and dichloromethane (cyan), respectively. For clarity, the transients have been vertically displaced.

Effect of Solvent on Electron Transfer. To investigate the effect of the dielectric constant of the solvent (which is a factor in the reorganization energy), colloidal PbS NCs were attached to TiO2 nanoparticles by 3-mercaptopropionic acid (MPA) in different organic solvents. The solvents we studied are listed in Table 1 along with their static and optical dielectric constants. The absorption spectra of 3.5 nm diameter PbS NCs and PbS NC
TiO2 composites in tetrachloroethylene (TCE) are shown in Figure 2a, and their fluorescence spectra in different solvents are in Figure 2b. The lowest absorption peak is broadened, and there is a red shift of ∼50 nm, when the NCs are coupled to the

are plotted versus 1/εop
1/εst of the solvent in Figure 4. The near-independence from the solvent contrasts with the strong solvent-dependent rate observed in ET from PbS NCs to molecules.9 To understand the experimental results, we consider the dependence of the reorganization energy on the solvent. In Marcus theory, the reorganization energy of the solvent is written as2,49 !Z 1 1 1 ðDt
Df Þ2 dv λsolvent ¼
ð2Þ 8π εop εst where εst and εop are the static and optical dielectric constants of the solvent, and Dt and Df are the inductions created in the medium by the distribution of charges in the reactants and products. In Marcus’s two-sphere donor
acceptor model for molecules,2,5,9,17,50 the above expression becomes !  e2 1 1 1 1 1
þ
ð3Þ dD dA LDA 4πε0 εop εst where dD, dA, and LDA are the diameters of the donor and acceptor, and the separation distance between the donor and the acceptor centers, respectively. It is assumed that the charge on either dielectric sphere is spread evenly over its surface, and both are embedded in a structureless dielectric medium. This model 2128

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Nano Letters has previously been used successfully to model ET reactions.5,9,17,50 The total reorganization energy λtotal = λr þ λs, where λs is the solvent reorganization energy calculated as above and λr is the unknown energy of the reactants. We left λr as a free parameter, assumed independent of solvent or NC size. We previously found that for a PbS NC donor with small molecule acceptor, the ET rate depends strongly on the choice of solvent,9 while in the current case the variation is negligible. These results can be understood qualitatively through eq 3. For charge transfer to a molecule, the reorganization energy is dominated by the small radius of the acceptor. Electron transfer to the molecular acceptor is accounted for empirically with only the static dielectric constant of the solvent, which seems consistent with the slow time scale of the transfer. In contrast, the large TiO2 acceptor size (roughly 50 nm18,44) in the present work leads to a negligible contribution to the reorganization energy, so the energy scale is dominated by discharging of the nanocrystal. The order-of-magnitude decrease in the solvent reorganization energy in the present case leads to a reduced sensitivity to it, as long as the reorganization energy of the reactants is sufficiently large. This is illustrated in Figure 4, which includes the values predicted by eqs 2 and 3. (Similar results are obtained with the modified model that includes only the static dielectric constant.) The diameters of the PbS NC and TiO2 nanoparticle and their separation distance are set to be 3.5, 50, and 27 nm, respectively. The static and optical dielectric constants are set to the bulk values of 17051 and 1752,53 for PbS NC and 3154 and 5.255 for TiO2 nanoparticle, respectively. Finally, the electronic coupling parameter |HDA|2 was adjusted to set the proper scale of the transfer times. The value inferred from the fit is |HDA| ∼ 10 μeV, which indicates that the ET between PbS NC and TiO2 nanoparticles is through a nonadiabatic reaction.4 Figure 4 shows fits with various values of λr in the 50
1000 meV range. Importantly, above λr =100 meV, the model predicts a roughly constant trend with solvent dielectric constant. As expected, this insensitivity is due to the relatively small λs ∼ 30
100 meV, which causes λr to dominate above that scale. However, if TiO2 nanoparticles of diameter ∼2 nm are used, the ET rate should depend more strongly on the solvent, as in our previous work.9 We conclude that the weak dependence on solvent dielectric constant is a fundamental consequence of the relatively large size of the TiO2 particles. Extension of the correlation between ET rate and solvent dielectric constant would be desirable but is limited by the solubilities of PbS NCs and TiO2 nanoparticles. In this work, solvents are chosen to meet two conditions simultaneously: (1) PbS NCs coated with oleic acid should be well-dispersed in the solvents, and their optical properties should not change, and (2) The MPA-capped TiO2 nanoparticles should be well-dissolved in the same solvents. Unfortunately, many polar solvents with high static dielectric constants such as acetonitrile, dimethylformamide, and dimethyl sulfoxide cannot be used due to the limited solubility of both the PbS NCs and MPA-capped TiO2 nanoparticles. Electronic Coupling. As a consequence of the large size of the acceptor, the remaining parameter to control the ET rate is the electronic coupling strength, which can be changed by the spacers and anchor groups of the linker molecules. As for the spacers of the linker molecules, aliphatic chains make the energy gap of the molecules much wider than those of PbS NCs and TiO2 nanoparticles, which helps avoid charge transfer to the

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Figure 5. Transient fluorescent lifetime traces of PbS NC-sensitized TiO2 film with (a) mercaptocarboxylic acid with chain lengths and (b) mercaptocarboxylic acids with different spacers. The solid black trace in (a) is the fluorescent lifetime decay of the PbS NCs in TCE. The 1/e decay time is 1.7 μs. The 1/e decay times for MBA, MPA, MAA, MPCA, and MBZA are 12, 4.2, 4.2, 12, and 12 ns, respectively.

linker molecules. To investigate the effect of the length of the spacer, mercaptocarboxylic acids with varying alkyl chain lengths are used. The effect of the chemical structure of the spacer can be investigated by comparing mercaptocarboxylic acids with benzene rings, pyridine rings, and alkyl chains. Regarding the anchor groups, on the NC side, the thiol (SH
) group is most commonly used due to its rapid binding to the surface of NCs. In principle, amines, phosphonic acid (
PO3H2), and carboxylic acid (
COOH), e.g., could be used as anchor groups. However, identification of functional groups that bind rapidly through a nondestructive ligand exchange method is difficult. In the interest of controlled study, we decided to maintain the thiol group on the NC end of the linker. On the other hand, there are various choices for the anchor groups on the TiO2 side. It is well-known that molecules with anchor groups such as carboxylic acid, phosphonic acid, silane (
SiOx), or sulfonic acid (
SO3H) groups56
59 can be adsorbed on the TiO2 surface via chemical bonds, which results in a strong interaction compared to physical adsorption by van der Waals forces. Therefore, aliphatic thiol ligands with these four functional groups were used to investigate the effect of the anchor group on the TiO2 side. The absence of ET from PbS NCs to all of these linker molecules was confirmed in separate experiments (Supporting Information). PbS NCs were attached to mesoporous TiO2 films by the respective linker molecules. To focus on the role of the linker molecules, the films were dried completely and sealed under nitrogen environment in a glovebox to avoid the degradation of 2129

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Figure 6. Variation of fluorescence decay of PbS NCs coupled to TiO2 with different anchor groups binding to the TiO2. Aliphatic linker molecules have thiol (
SH) groups at one end to bind to the NC and different anchor groups at the other end to bind with TiO2: MPTMS (HS(CH2)3Si(OCH3)3, solid blue line), MBA (SH
(CH2)3
COOH, solid green line), MBPA (SH
(CH2)4
PO3H2, solid orange line), and S3MPS (SH
(CH2)3
SO3Na, solid red line). IR (solid cyan line) is the instrument response function. All linker molecules have a threecarbon chain except the MBPA. 1/e decay times are 77 ns (silane group), 12 ns (carboxylic acid), 5.8 ns (phosphonic acid), and 2.5 ns (sulfonic acid), respectively.

PbS NCs through electron scavenging by oxygen60 (details in Supporting Information). The sealed PbS NC
TiO2 system did not show any change in the optical spectra and fluorescence decays during measurments. The use of the dried form of TiO2 increases its electron affinity to 4.45 eV, compared to 3.9 eV in organic solvents.45 Thus, the free energy difference between the lowest electron level of the PbS NCs and the conduction band of TiO2 is increased correspondingly and much faster ET can be expected. Spacer Length. The fluorescence decay of PbS NC
MPA
dried TiO2 (solid blue line) is displayed in Figure 5a. The 1/e time is 4.2 ns. Because the quenching is so rapid, the fluorescence decay is a good approximation to the ET process. The ET time is 2 orders of magnitude faster than that of the same system in organic solvent.8 Results of adding or removing one hydrocarbon are shown in Figure 5a. With 4-mercaptobutyric acid (MBA, SH
(CH2)3
COOH), the ET time is 12 ns, and for 2-mercaptoacetic acid (MAA, SH
(CH2)
COOH) it is 4.2 ns. The reduction from three hydrocarbons (MBA) to two hydrocarbons (MPA) leads to about 3 times faster ET. However, there is no further increase in the rate with the reduction to one hydrocarbon (MAA). This indicates that the electronic coupling strength was saturated with the two hydrocarbons of the mercaptocarboxylic acids. At this point, it is not clear if the ET rate of PbS NC
TiO2 linked with mercaptocarboxylic acids will decrease exponentially with hydrocarbon chain length, because experimental data with longer hydrocarbons are not completely consistent.18,61 This is not surprising, as the chain length is not necessarily proportional to the number of hydrocarbons owing to the high flexibility of the aliphatic chain.62 Thus, we cannot compare our experimental results to the theoretical expectation, HDA = H0DA exp[(
1/2)βdDA]. Structure of the Spacer Molecule. In the superexchange or through-bond model, the coupling between the donor and acceptor states is mediated by the orbitals of the atoms that lie between the donor and acceptor.63,64 An aromatic spacer with σ and π orbitals should increase the electronic coupling compared

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to a saturated hydrocarbon spacer with only σ orbitals.65 We tested this hypothesis by investigating PbS NC
TiO2 linked with mercaptocarboxylic acids with pyridine (6-mercaptopyridine-3-carboxylic acid (MPCA), solid orange line) and benzene (4-mercaptobezonic acid (MBZA), solid magenta line) rings. We found that the nature of the linker does not significantly influence ET rate (Figure 5b). The decay times with MPCA, MBZA, and MBA are all about 12 ns. The carbon
carbon bond length is 0.154 nm, and MBA has two carbon bonds, so its length is 0.308 nm. The length of the benzene or pyridine ring is also estimated to be ∼0.3 nm.28 Thus, similar spacer length yields similar electronic coupling strength regardless of the molecular structures. The slower ET times of PbS NC
MPCA/ MBZA
TiO2 compared to PbS NC
MPA
TiO2 are attributed to the longer length of the spacer. This result strongly indicates that the spacer length of the linker molecules plays a more important role than the molecular structure of the linkers in the ET. Anchor Groups. Next, we consider the effects of the anchor group, using aliphatic thiol ligands with different functional groups, such as 4-mercaptobutyric acid (MBA, carboxylic acid group), 4-mercaptobutylphosphonic acid (MBPA, phosphonic acid), 3-(mercaptopropyl)-trimethoxysilane (MPTMS, silane group), and sodium 3-mercaptopropane-1-sulfonate (S3MPS, sulfonic acid). Each linker molecule has a (CH2)3 spacer, with the exception of MBPA, which has (CH2)4. We were unable to obtain MPBA with a three-carbon chain. MBPA was synthesized according to literature procedures.66 Figure 6 displays the fluorescence decays measured with these anchor groups. It shows that the change of the anchor groups on the TiO2 side can dramatically influence the ET. The 1/e decay times with silane, carboxylic acid, and sulfonic acid groups are 77, 12, and 2.5 ns, respectively. The 1/e decay time for the phosphonic acid group with four hydrocarbons (MBPA) is 5.8 ns. Considering the results obtained with varying chain length of mercaptocarboxylic acids (discussed above), we would expect the ET time to be even faster than 5 ns with a three-carbon chain. The physical mechanism behind the observed strong dependence on the anchor group on the TiO2 end is not clear, and more work is needed to understand it thoroughly. A theoretical study of dye
TiO2 systems found that the electronic coupling is sensitive to the chemical nature of the anchor group and only weakly sensitive to the adsorption mode.67 The ET times of dyes with different anchor groups were calculated by estimating the width of the projected density of states (PDOS) of the dye
metal oxides, with broader width correponding to stronger coupling. ET times calculated this way agree reasonably with experimenal results.34,36,68 It would be interesting to do similar theoretical work for NC
metal oxides coupled with linker molecules having different anchor groups to investigate their effect on ET dynamics. Conclusions. In summary, we find that the rate of electron transfer between PbS NCs and TiO2 nanoparticles is weakly dependent on the solvent dielectric constant. This is attributed to the relatively large size of TiO2 nanoparticles and consistent with expectations from Marcus theory. The electron transfer rate decreases systematically with the length of the spacer molecule. The spacer length plays a much more important role on the ET rate than the chemical structure of the spacer. Modification of the anchor groups of the linker molecule leads to significant changes of the electron transfer rate. We attribute this to variation of the electronic coupling, although the precise mechanism is still under 2130

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Nano Letters investigation. However, the results presented here clearly show that electron transfer can be controlled by modifying the electronic coupling through the length and anchor groups of the linker molecules.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information on absorption and emission spectrum measurements, fluroescence lifetime measurements, synthesis, and PbS
NC linker
ZrO2 composite system. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank Professor Masa-aki Haga (Chuo University, Japan) for providing MBPA. This work was supported by the Cornell Center for Materials Research (CCMR) with funding from the Materials Research Science and Engineering Center program of the National Science Foundation (cooperative agreement DMR 0520404) and in part by Award No. KUSC1-018-02, made by King Abdullah University of Science and Technology (KAUST). ’ REFERENCES (1) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (2) Marcus, R. A. J. Chem. Phys. 1956, 24, 966. (3) Marcus, R. A. J. Chem. Phys. 1956, 24, 979. (4) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15, 155. (5) Marcus, R. A. J. Chem. Phys. 1965, 43, 679. (6) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta, Rev. Bioenerg. 1985, 811, 265. (7) Robel, I.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2007, 129, 4136. (8) Hyun, B.-R.; Zhong, Y.-W.; Bartnik, A. C.; Sun, L.; Abrun?a, H. D.; Wise, F. W.; Goodreau, J. D.; Matthews, J. R.; Leslie, T. M.; Borrelli, N. F. ACS Nano 2008, 2, 2206. (9) Hyun, B.-R.; Bartnik, A. C.; Lee, J.-K.; Imoto, H.; Sun, L.; Choi, J. J.; Chujo, Y.; Hanrath, T.; Ober, C. K.; Wise, F. W. Nano Lett. 2010, 10, 318. (10) Tvrdy, K.; Frantsuzov, P. A.; Kamat, P. V. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 29. (11) Pijpers, J. J. H.; Koole, R.; Evers, W. H.; Houtepen, A. J.; Boehme, S.; de Mello Doneg
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