Static Conductance of Nitrile-Substituted Oligophenylene and Oligo

Nov 20, 2013 - Applied Physical Chemistry, University of Heidelberg, 69120 Heidelberg, Germany ... University, University Park, Pennsylvania 16802, Un...
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Static Conductance of Nitrile-Substituted Oligophenylene and Oligo(phenylene ethynylene) Self-Assembled Monolayers Studied by the Mercury-Drop Method Christine Joy Querebillo,†,∥ Andreas Terfort,‡ David L. Allara,§ and Michael Zharnikov*,† †

Applied Physical Chemistry, University of Heidelberg, 69120 Heidelberg, Germany Institut für Anorganische und Analytische Chemie, Universität Frankfurt, Max-von-Laue-Straße 7, 60438 Frankfurt, Germany § Departments of Chemistry and Material Science, Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡

ABSTRACT: Static charge transport (CT) properties of nitrile-substituted oligophenylenes and oligo(phenylene ethynylene)s (NC-OPh and NC-OPE, respectively) assembled via the thiolate anchor on gold substrates were measured by the mercury drop junction technique. The derived attenuation factors (β), viz. 0.53 ± 0.1 and 0.30 ± 0.08 Å−1 for the NC-OPh and NCOPE monolayers, respectively, correlate well with the literature values for the analogous nonsubstituted systems, suggesting that the attachment of the nitrile group to the OPh or OPE backbone does not significantly affect their transport properties. This finding provides a basis for the use of the nitrile moiety as a resonantly addressable group and as specific charge injection site in the measurements of dynamic CT by resonant Auger electron spectroscopy. The comparison between the static and dynamic β values for the NC-OPh monolayers implies that the static value corresponds to nonresonant injection conditions. This suggests, potentially, that the static CT can be performed more efficiently by controlling the specific molecular orbitals into which charge carriers are injected.

1. INTRODUCTION Reliable information about the charge transport (CT) through individual molecular groups is a prerequisite for continued progress in technologically important fields such as organic electronics and organic photovoltaics as well as for the development of future molecular electronics.1−4 Among different moieties, molecular backbones, denoted also “molecular wires”, are of particular importance since they serve as useful model systems for more complex species. In addition, in the framework of molecular electronics, molecular wires provide charge conduction paths between a charge injection site or an active element and an electrode which is typically connected to the molecule via a special anchor group to improve the CT between the molecule and electrode. A direct attachment of a charge injection site or an active element to the substrate cannot be realized in most cases since the performance of such a moiety can be disturbed considerably or even completely by the direct interaction with the substrate. Transport properties of typical molecular wires, both individual and within self-assembled monolayers (SAMs) on a conductive substrate, have been analyzed theoretically1,3,5 and studied experimentally4,6 using a variety of experimental tools. The most popular techniques in this regard are the mercury drop method,7−12 its variation with gallium−indium as contact medium,13−15 break junctions,16−21 scanning-probe based approaches,22−27 and large area junctions.28 In spite of the basic problems, such as the quality of the electrical contact and © 2013 American Chemical Society

uncertainty in the amount of molecules participating in CT, these experiments have provided reliable data for the timeaveraged, static CT properties for a variety of molecular wires. In most cases, the overall resistance of a molecular junction, R, can be described by the equation, R = R0 exp(βl), where R0 is the molecular contact resistance determined by the efficiency of the molecule−substrate (electrode) bond, β is the exponential attenuation factor, and l is the length of the molecular backbone.1,10,22,24,27 This behavior differs distinctly from the Ohmic one and is characteristic of a tunneling or hopping CT mechanism typical of insulators.1 The value of β which is the major parameter describing the CT efficiency depends, as expected, on the identity of the molecular wire. The lowest values, corresponding to a better conductance, were observed for unsaturated hydrocarbon chains such as alkenes (β = 0.27 Å−1),5 oligo(phenylene ethynylenes) (OPE; β = 0.27 Å−1),5 and oligophenyls (OPh, β = 0.41−0.7 Å−1).1,7,22,25 Saturated hydrocarbon chains such as alkanes are characterized by noticeably higher β values (0.57−1.0 Å−1),10 corresponding to poor CT properties. The large scattering of the β values is related to the basic problems of the measurements and specificity of individual experimental setups as mentioned above. Received: September 19, 2013 Revised: November 15, 2013 Published: November 20, 2013 25556

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The static conductance of the NC-OPh and NC-OPE assemblies was measured by the mercury drop method which, as mentioned above, is a well-established procedure for this purpose. To verify the performance of the experimental setup and the reliability of the results, measurements on a series of nonsubstituted alkanethiolate (NSAT) SAMs on silver substrates were performed before the experiments on the target systems. In the following section, we provide a brief description of the experimental setup and the procedure. Afterward, the results and discussion are presented in section 3. Finally, conclusions for the study are given in section 4.

Recently, we have complemented the above, time-averaged, static results by the measurement of CT dynamics for a variety of molecular wires including alkanes,29−31 OPh,32 OPE,32 and hybrid OPh−alkane moieties.32 In the respective experiments these wires were assembled in SAM fashion on a conductive substrate (gold) via the thiolate anchor. At the other end of the molecular chain, a nitrile tail group was attached. This group can be resonantly excited by narrow-band synchrotron radiation, resulting in the excitation of an electron above the Fermi level of the substrate. CT of this electron through the molecular framework to the substrate (bottom electrode) was monitored by resonant Auger electron spectroscopy within the core hole clock (CHC) approach. Using this method, we succeeded in (i) measuring the characteristic CT times for a variety of the molecules mentioned above, (ii) demonstrating that the CT rate behaves similar to the static conductance, increasing in an exponential fashion with increasing chain length, and (iii) calculating the corresponding β values for alkyl and OPh chains.29−32 In addition, we demonstrated that the efficiency and rate of CT through molecular backbones can be controlled by resonant injection of the charge carriers into specific molecular orbitals (MOs).31,32 In particular, for the OPh backbone, we calculated the β values for CT starting from MOs conjugated and nonconjugated with the MOs of the backbone. The corresponding values were estimated at 0.29 and 0.55 Å−1, respectively.32 The above results could only be obtained due to the use of the nitrile group serving as a trigger and a starting point for the CT. However, the attachment of this group to the molecular backbone might affect its transport characteristics to some extent, making the results for the nitrile-substituted molecules not fully representative of the respective molecular wires. In this context, we present here the results for the static conductance of nitrile-substituted OPh and OPE wires assembled over the thiolate anchor on the gold substrates. The respective molecules along with their abbreviations are shown in Figure 1, with NC-OPE1 being the first member of both NC-OPh and NC-OPE series; it could therefore also be named nitrilesubstituted phenylthiol, NC-PT.

2. METHODS Preparation of SAMs. Solvents, chemicals, and precursors for the preparation of the NSAT SAMs (ethanol, toluene, methylene chloride, hexadecane, 1-decanethiol, and 1-hexadecanethiol, 99%; 1-dodecanethiol and 1-tetradecanethiol, ≥98%) were obtained from Sigma-Aldrich. The NC-OPh and NC-OPE molecules were custom synthesized; the procedures are described elsewhere.33,34 The substrates were purchased from Georg Albert PVD, Silz, Germany. They were prepared by thermal evaporation of 100 nm of gold or silver (both 99.99% purity) onto polished single crystal silicon (100) wafers (Silicon Sense) primed with a 5 nm titanium adhesion layer. The evaporated films were polycrystalline, with a grain size of 20−50 nm as observed by atomic force microscopy and scanning tunneling microscopy. The grains predominantly possess a (111) orientation. The substrates were delivered in Ar filled containers which were opened only a short time before the SAM preparation. The NSAT SAMs were formed by immersion of freshly prepared silver substrates into a 0.1 mmol solution of 1decanethiol (C10), 1-dodecanethiol (C12), 1-tetradecanethiol (C14), and 1-hexadecanethiol (C16) in ethanol for 24−48 h at room temperature. The NC-OPh and NC-OPE SAMs were prepared by immersion of freshly prepared gold substrates into a 1 mmol solution of the respective precursors in toluene or methylene chloride.35 All solutions were saturated with argon before use, and the containers were backfilled with argon and sealed with Parafilm after immersion of the substrates. After assembly, the SAM samples were carefully rinsed with the respective solvent and blown dry with argon. The samples were then rinsed thoroughly with ethanol and dried with a stream of argon gas. The quality of the Cn SAMs was verified by laboratory X-ray photoelectron spectroscopy. The quality of the NC-OPh and NC-OPE SAMs was previously verified by a variety of synchrotron-based spectroscopic techniques,32,33,35,36 and some of these methods were applied in the present work as well to establish that high quality films were used. Overall, for all the NC-OPh and NC-OPE SAMs we used, the molecules were well organized, with the bonding to the substrate mediated by the thiol group and the nitrile tail group exclusively located at the SAM−ambient interface.32,33,35,36 Static Conductivity Measurements. A home-built mercury-drop setup was adapted from the works of Rampi et al.10,37 and von Wrochem et al.12,38 A schematic diagram of the setup is presented in Figure 2. Briefly, a gastight Hamilton syringe extrudes a mercury drop (∼400 μm diameter). A tungsten wire is made to protrude through the Teflon tip of the plunger to enable contact between the metallic core of the plunger and the mercury drop. The metallic plunger is

Figure 1. Schematic drawing of the SAM precursors along with their acronyms. The entire series will be abbreviated as NC-OPh and NCOPE. NC-OPE1 serves as the first member of both series. 25557

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bond (∼2.4 Å)35 and the van der Waals radius of the terminal H atom (∼1 Å)37 were added to give the total length.

3. RESULTS AND DISCUSSION Test and Reference Measurements on Cn/Ag. The performance of the mercury drop setup was verified by measuring the time-averaged static CT for a series of NSAT SAMs on silver, which have a reported β value of 0.6−0.9 Å−1 based on the results of the mercury drop measurements and the data by other techniques.8,10,37 The current density−bias voltage plots and the natural logarithm of the current density at a bias voltage of 0.5 V as a function of the molecular length are presented for Cn/Ag in Figures 3a and 3b, respectively. The

Figure 2. (a) Schematic diagram of the mercury-drop junction setup. (b) An image of the mercury drop in contact with the SAM functionalized substrate. (c) Schematic diagram of the junction.

connected to Keithley 2635A source meter. Before the measurement, the mercury drop was immersed in 10 mM hexadecanethiol solution in hexadecane for around 15 min. The SAMs-passivated mercury drop was then made to touch gently the SAM sample. The junction was assembled under 1 mM hexadecanethiol solution in hexadecane to increase its stability. A metal clip in contact with the sample was connected to the electrometer to close the electrical circuit. A CMOS camera with a Macro lens (The Imaging Source DMK22AUC03 1/3 in. Micron with MR 8/O) provided a sideway view of the junction which allowed to measure the diameter of the contact area. Using this technique, an accuracy of around 14% was generally achieved for the contact area (assuming a circular contact).12 The whole setup was placed onto a vibration isolation table and put inside a home-built Faraday cage to reduce vibrations and electrical noise, respectively. For each system, 3−5 different samples were used with 5 different positions for each sample. For each position, 4−6 I−V curves were obtained (on the average, at least 80 I−V curves were analyzed for each system). Only data from the series of curves converging to reproducible values of J(V) were included. This was done by selecting a representative curve (or getting the average) from each convergent set. Curves with initially higher impedances (indicative of a bad contact) or right before breakdown (amalgamation of gold or silver and mercury) were disregarded (adapted from ref 12). Data points were collected using a voltage ramp with a bias interval of 50 mV and an interval of at least 5 s between individual steps (adapted from ref 37). The logarithmic mean (or mean of the logarithms of the included curves) for each compound was calculated and plotted. The distribution of the current values at a particular bias voltage for all meaningful curves was used to estimate the error bars as an average deviation from the mean value. The respective error bars of the current values were taken into consideration at the calculation of the β factors. Estimation of the Molecular Chain Length. The lengths of the molecular chains used to compute were estimated using the ACD 3D-ChemSketch software. Molecules were drawn in their most extended conformation. 3D structure optimization was performed, and the distance along the molecular backbone from the sulfur atom to the terminal H atom was estimated. To this value, the length of the Ag−S bond (∼2.3 Å)37 or Au−S

Figure 3. (a) Plot of current density as a function of the bias voltage between the mercury and silver electrodes for the Hg-C16//Cn-Ag junctions, at n = 10 (circles), 12 (squares), 14 (up triangles), and 16 (down triangles). (b) Semilog plot of current density (at 0.5 V) vs molecular length for the Hg-C16//Cn-Ag junctions (diamonds). The dashed line represents the best fit by a straight line.

current density values are only shown for the positive bias voltages (steps from 0.05 to 0.5 V); the negative voltages (steps from −0.05 to −0.5 V) result in the symmetric curves. As shown in Figure 3b, the derived values of the current density can be perfectly fitted by a straight line. The slope of this line gives a β value of 0.78 ± 0.16 Å−1 for the Cn SAMs on silver, which is quite reasonable and agrees well with the literature values because it is well in the middle of the reported range of 0.6−0.9 Å−1. This confirms the performance of the mercury drop setup used for the given experiments as well as the reliability of the results. Static Conductivity Measurements on NC-OPh/Au and NC-OPE/Au. Current density−bias voltage plots and the natural logarithm of the current density at a bias voltage of 0.5 V as a function of the molecular length for the NC-OPh series are presented in Figures 4a and 4b, respectively. The error bars are typical for this type of junction and are attributed to statistical error and the uncertainty in estimating the contact area between the electrodes.9,37 As expected, NC-BP0 has lower current density values than NC-OPE1 but higher than 25558

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Figure 4. (a) Plot of current density as a function of the bias voltage between the mercury and gold electrodes for the Hg-C16//NC-OPhAu junctions, at OPh = phenylthiol (PT = OPE1) (circles), BPT (squares), and TPT (up triangles). (b) Semilog plot of current density (at 0.5 V) vs molecular length for the Hg-C16//NC-OPh-Au junctions (diamonds). The dashed line represents the best fit by a straight line.

Figure 5. (a) Plot of current density as a function of the bias voltage between the mercury and gold electrodes for the Hg-C16//NC-OPEAu junctions, at OPE = OPE1 (circles), OPE2 (squares), and OPE3 (up triangles). (b) Semilog plot of current density (at 0.5 V) vs molecular length for the Hg-C16//NC-OPE-Au junctions (diamonds). The dashed line represents the best fit by a straight line.

NC-TP0 at all points from 0.05 to 0.5 V. At a bias of 0.5 V, the conductance of the NC-OPE1 SAM is higher than those of the NC-BP0 and NC-TP0 monolayers by factors of ∼8 and ∼90, respectively. As shown in Figure 4b, the respective values of the current density can be perfectly fit by a straight line. The slope of this line gives a β value of 0.53 ± 0.1 Å−1 Similar results were obtained for the NC-OPE series. The corresponding current density−bias voltage plots are presented in Figure 5a, while the natural logarithm of the current density at a bias voltage of 0.5 V is depicted in Figure 5b as a function of the molecular length. Similar to the NC-OPh case, the second member of the series (NC-OPE2/Au) has current density values lower than the first (NC-OPE1/Au) but higher than the third (NC-OPE3/Au) at all points from 0.05 to 0.5 V. At a bias of 0.5 V, the conductance of the NC-OPE1 SAM is higher than those of the NC-OPE2 and NC-OPE3 monolayers by factors of ∼8 and ∼61, respectively. Significantly, NC-OPE2 has current density values that are comparable to those for NCBP0, which is its counterpart in the NC-OPh series, despite the noticeable difference in length (∼17.5 Å−1 vs ∼14.8 Å−1). This behavior is even more pronounced for the NC-OPE3 SAM which has a higher conductance (by a factor of 1.5 at 0.5 V) compared to the NC-TP0 monolayer, even though NC-OPE3 is noticeably longer than NC-TP0 (24.5 Å−1 vs 19.1 Å−1). This behavior can only be explained by the better CT properties of the OPE backbone as compared to the OPh one, with the effect becoming more pronounced with increasing chain length. The latter is presumably related to the changes in the electronic structure, including the exact positions of the HOMO and LUMO states and the width of the HOMO−LUMO gap.27 Taking the current density values at a bias voltage of 0.5 eV for the NC-OPE SAMs and plotting them in the semilogarithmic fashion as a function of molecular length, we got the experimental points which could be perfectly fitted by a

straight line. The slope of this line gives a β value of 0.30 ± 0.08 Å−1. Comparison to Literature−Static Conductivity Measurements. As mentioned above, the static β values for nonsubstituted OPh and OPE “wires” arranged in the SAM fashion are 0.4−0.7 and 0.27 Å−1, respectively.1,5,8,22,25,37 The OPh system has been measured more frequently than the OPE one for which, to our knowledge, only one β value (0.27 Å−1)1,5 has been reported. A large scattering of the β values for the OPh system, viz. 0.61,8,37 0.5−0.7,25 0.4−0.5,1 and 0.42 Å−1,22 is presumably related to both the general problems of the conductivity measurements in molecular junctions and specific problems of the individual techniques used for this purpose. Our values for the NC-OPh and NC-OPE series are 0.53 ± 0.1 and 0.30 ± 0.08 Å−1, respectively, which is in very good agreement with the above literature values for the analogous nonsubstituted systems. Moreover, the value of 0.53 ± 0.1 Å−1 for NC-OPh/Au is well in the middle of the 0.4−0.7 Å−1 range from the literature. Thus, the attachment of the nitrile moiety to the OPh and OPE backbones does not affect noticeably the CT properties of the respective SAMs.27 Note that CT is predominately mediated by the individual backbones, and the effects of molecular packing as well as the exact molecular arrangement are believed to be less important. It is known, however, at least for some of the OPh and OPE monolayers, that the attachment of the nitrile moiety to the molecular backbone does not significantly affect the molecular orientation in the SAMs.35 In some cases, this attachment results in an increase of the average tilt angle of the backbone,33 reflecting, presumably, slight disordering mediated by the strong dipole− dipole interactions between the terminal nitrile moieties at the SAM−ambient interface.39 An important parameter in this regard is the torsion of the individual rings, which, at least in the case of the OPh backbone (flexible conformation), can affect noticeably the molecular conductance.21,40 Obviously, 25559

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based on the good agreement between the β values for the NCOPE and OPE SAMs, the attachment of the nitrile moiety to the OPh backbone does not result in a significant change of the torsion angles. Comparison to Dynamic CT Measurements. The β values for the static CT can be compared to the analogous values for the dynamic CT. Regretfully, the latter values are only available for the NC-OPh series.32 The measurement of β for the NC-OPE monolayers was not possible because of the limitations of the CHC approach.32,35 For the NC-OPh SAMs, the dynamic β value depends on the character of the MO serving as the starting point for CT. If CT starts from the π1* orbital of the terminal nitrile group which is strongly conjugated with the π* system of the adjacent phenyl ring, a value of 0.29 Å−1 can be derived from the experimental data. In contrast, if CT occurs in a nonresonant way, starting from the nonconjugated π3* orbital of the nitrile moiety, a value of 0.55 Å−1 is determined.32 Comparing these dynamical values to the static one (0.53 ± 0.1 Å−1), it can be found that the static value correlates very well with the dynamic one for the case of nonresonant injection but is significantly higher than the value for the resonant injection. This is understandable considering that no special care was taken in the mercury drop experiments to guarantee a favorable injection of the charge carriers into the suitable MOs. On the other hand, the comparison of the static and dynamic values suggests that there probably is a potential to improve the CT parameters in the static case as well, by controlling the specific orbitals into which charge carriers are injected.

well, by controlling the specific orbitals into which charge carriers are injected. We hope that the above results and related discussion can stimulate theory work on the CT dynamics in molecular junctions and provide useful input for molecular electronics where improvement of performance of molecular wires can be certainly of importance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +496221-54 4921; Fax +49-6221-54 6199. Present Address ∥

Department of Chemistry, Ateneo de Manila University, Loyola Heights, Quezon City 1108, Philippines. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to M.-A. Rampi, F. von Wrochem, and F. Scholz for useful advice, M. Grunze for the support, and F. Staier and N. Meyerbröker for valuable discussions. This work has been supported by DFG (ZH 63/14-1 and ZH 63/14-2) and EMMA (a scholarship to C.J.Q.).

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REFERENCES

(1) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; et al. Charge Transfer on the Nanoscale: Current Status. J. Phys. Chem. B 2003, 107, 6668−6697. (2) Tour, J. M. Molecular Electronics: Commercial Insights, Chemistry, Devices, Architecture and Programming; World Scientific: Singapore, 2003. (3) Karthäuser, S. Control of Molecule-Based Transport for Future Molecular Devices. J. Phys.: Condens. Matter 2011, 23, 013001. (4) Branchi, B.; Simeone, F. C.; Rampi, M. A. Active and Non-Active Large-Area Metal−Molecules−Metal Junctions; Top. Curr. Chem.; Springer-Verlag: Berlin, 2011. (5) Tomfohr, J.; Sankey, O. Complex Band Structure, Decay Lengths, and Fermi Level Alignment in Simple Molecular Electronic Systems. Phys. Rev. B 2002, 65, 245105. (6) Haick, H.; Cahen, D. Making Contact: Connecting Molecules Electronically to the Macroscopic World. Prog. Surf. Sci. 2008, 83, 217. (7) Rampi, M. A.; Schueller, O. J. A.; Whitesides, G. M. Alkanethiol Self-Assembled Monolayers as the Dielectric of Capacitors with Nanoscale Thickness. Appl. Phys. Lett. 1998, 72, 1781. (8) Rampi, M. A.; Whitesides, G. M. A Versatile Experimental Approach for Understanding Electron Transport Through Organic Materials. Chem. Phys. 2002, 281, 373−391. (9) Grave, C.; Risko, C.; Shaporenko, A.; Wang, Y.; Nuckolls, C.; Ratner, M. A.; Rampi, M. A.; Zharnikov, M. Charge Transport Through Oligoarylene Self-Assembled Monolayers: Interplay of Molecular Organization, Metal-Molecule Interactions, and Electronic Structure. Adv. Funct. Mater. 2007, 17, 3816−3828. (10) Weiss, E. A.; Chiechi, R. C.; Kaufman, G. K.; Kriebel, J. K.; Li, Z.; Duati, M.; Rampi, M. A.; Whitesides, G. M. Influence of Defects on the Electrical Characteristics of Mercury-Drop Junctions: SelfAssembled Monolayers of n-Alkanethiolates on Rough and Smooth Silver. J. Am. Chem. Soc. 2007, 129, 4336−4349. (11) Ferri, V.; Elbing, M.; Pace, G.; Dickey, M. D.; Zharnikov, M.; Samorì, P.; Mayor, M.; Rampi, M. A. Light-Powered Electrical Switch Based on Cargo Lifter Azobenzene SAMs. Angew. Chem., Int. Ed. 2008, 47, 3407−3409. (12) Von Wrochem, F.; Gao, D.; Scholz, F.; Nothofer, H.-G.; Nelles, G.; Wessels, J. M. Efficient Electronic Coupling and Improved Stability

4. CONCLUSIONS We have measured current as a function of bias voltage in metal−molecule−metal junctions containing NC-OPh and NC-OPE SAMs. The measurements were carried out by the mercury drop method using a custom-built setup, the performance of which was verified by experiments on a series of Cn SAMs on silver. In all cases, the mercury drop served as the top electrode and the metal substrate (Au in the case of the NC-OPh and NC-OPE SAMs) as the bottom one. In spite of the longer backbone (at the same number of the phenyl rings), NC-OPE SAMs were found to have a higher conductance than the analogous NC-OPh monolayers, which became even more significant with increasing number of rings in the molecular backbone. Also, the β factor, which is the major parameter describing CT in molecular wires, was found to be significantly lower in the NC-OPE case (∼0.30 Å−1) as compared to the NC-OPh system (∼0.53 Å−1), suggesting, as reported earlier, better CT properties of the OPE backbone. The derived β values for the static conductance agree well with the literature values for the analogous nonsubstituted systems, suggesting that the attachment of the nitrile moiety to the OPh or OPE backbone does not affect significantly their transport properties. This is an important finding, which provides a basis for the use of nitrile group as a specifically addressable group and as charge injection site in the measurements of dynamic CT by the CHC method. A comparison of the static and dynamic β values for the NCOPh monolayers suggests that the static value, measured by the mercury drop method and other techniques, corresponds to the case of the nonresonant injection of the charge carriers. A much lower dynamic β value for the case of CT starting from a conjugated molecular orbital implies that there probably is a potential to improve the CT parameters in the static case as 25560

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp409366e | J. Phys. Chem. C 2013, 117, 25556−25561