Article pubs.acs.org/JPCC
Evidence for Enhanced Electron Transfer by Multiple Contacts between Self-Assembled Organic Monolayers and Semiconductor Nanoparticles Nirit Kantor-Uriel,†,§ Partha Roy,†,§ Sergio Saris,† Vankayala Kiran,† David H. Waldeck,‡ and Ron Naaman*,† †
Department of Chemical Physics, The Weizmann Institute of Science, Rehovot 76100, Israel Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 United States
‡
S Supporting Information *
ABSTRACT: This study presents results on the charge transfer between CdSe nanoparticles (NPs) and a gold substrate, when the NPs are attached to the gold via self-assembled monolayers of alkanedithiols (DT) of various lengths. The study examines the dependence of the photoinduced charge transfer on the DT length. Two methods were applied for measuring the charge transfer yield, surface photovoltage (SPV) and temperature dependent photoluminescence. The results demonstrate a net transfer of electrons from the NPs to the gold, under constant illumination. Interestingly, the data reveal that the monolayer composed of 10 carbon methylene chains displays an unusually efficient electron transfer, which is attributed to a high local ligand density resulting in multiple links between the NPs and the substrate.
S
elf-assembled monolayers (SAMs) of organic molecules adsorbed on metal substrates have been an important architecture for investigating fundamental features of electron transfer and have proved useful in organic electronics and studies of molecular electronics.1−3 For example, the electron transfer rate through self-assembled monolayers of alkanethiols has been investigated intensively and among others the effect of the molecular length was considered.4,5 In addition to these studies of molecular electron transfer, SAMs that contain semiconductor nanoparticles (NPs) have been studied and found application in photovoltaic devices.6−8 Charge transfer between semiconductor NPs and the underlying surface, upon which the SAM is adsorbed, can be a critical factor in determining the overall solar cell efficiency.9 This study presents new results on the charge transfer between CdSe NPs and a gold substrate, when the NPs are attached to the gold via alkanedithiols (DT) of various lengths. This work examines the effect of the length of the DT and observes an anomalously large charge transfer rate for decanedithiol, which can be explained by an increase in the number of DT linkages to the NP. The transfer yield is studied by combining surface photovoltage (SPV) measurements and temperature dependent fluorescence measurements for different SAM assemblies. The findings show that electrons are transferred from the NPs to the gold and that the length dependence does not follow the commonly observed monotonic trend, presumably because of nanoscale organization within the SAM. The configuration of the systems studied is presented schematically in Figure 1. The samples consist of 6.2−7.7 nm © XXXX American Chemical Society
Figure 1. Schematic illustration of the CdSe NP/SAM/Au assembly motif. Typical NPs coverage in the experiments is 1.5 ± 1 × 1011 particles/cm2. Note that the SAM is represented by the zigzag blue lines and has a density of about 1014 chains/cm2.
diameter CdSe NPs that are adsorbed on the exposed surface of an alkanedithiol SAM which coats the Au substrate. The alkanedithiols that were used as linkers are 1,6-hexanedithiol (C 6 DT), 1,8-octanedithiol (C 8 DT), 1,10-decanedithiol (C10DT), and 1,16-hexadecanedithiol (C16DT). Upon photoexcitation of the NPs the excited electrons and holes can either Received: March 11, 2015 Revised: May 20, 2015
A
DOI: 10.1021/acs.jpcc.5b02367 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. (A) The CPD measuring system is illustrated. The light (red beam) passes through a gold wire mesh (1) and strikes the sample (2, yellow rectangle) that is located on a translation stage (for details, see the Supporting Information). The sample region is magnified in (B) and (C). (B) Direction of the surface dipole when electron is transferred from the substrate to the nanoparticle causing an increase in the work function. (C) Direction of the surface dipole when electron is transferred from the nanoparticle to the substrate causing a decrease in the work function.
recombine and emit photons (i.e., photoluminescence (PL)) or recombine by a nonradiative process (PL-quenching). The quenching of the PL can occur by several processes: internal conversion to the ground state of the NP, energy transfer, or charge transfer to the substrate. Depending on the conditions, like temperature, DT length and so forth, these competitive mechanisms can act simultaneously or one of them may dominate the fluorescence quenching. Note that while photovoltage measurements are directly sensitive to net charge transfer between the substrate and the adsorbed NPs, the PL reports only indirectly on the charge transfer.
■
RESULTS AND DISCUSSION SPV Results. Surface photovoltage measurements were used to quantify the amount of photoinduced charge transfer in these assemblies. Contact potential difference (CPD) measurements provide the work function (Φ) of the sample by measuring the electric potential between the sample and a reference electrode.10 When the sample is illuminated and charge transfer occurs, a SPV is created; that is, a work function shift occurs. Based on the SPV measurements, it is possible to determine the direction and the extent of the electron transfer (for a given illumination intensity) between the NPs and the substrate. If an electron is transferred from the NP layer to the substrate, the surface’s work function decreases; whereas the work function increases if electrons are transferred in the opposite direction (see Figure 2). Thus, the sign of the SPV signal provides the direction of electron transfer and its magnitude is proportional to the change in the work function resulting from the light induced dipole moment that arises from charge transfer between the NPs and the substrate. Figure 3 and Table 1 present the SPV results for the CdSe NPs that are linked to the gold substrate by DT molecules of different lengths. The measurements were performed under constant illumination; hence, the values reported here are at steady state. Because the light induced dipole moment, which is measured by the SPV, is proportional to the molecular length, the amount of charge transferred, ΔQ, can be calculated by dividing the SPV signal by the length of the alkanedithiol molecules. Hence ΔQ = ΔV
Figure 3. Bar graph showing the average number of electrons transferred from the NPs to the gold substrate per the illuminated area of 0.8 mm2 in steady state, for the four different dithiol assemblies. See text for details.
Table 1. SPV Data for the Four Different NP/SAM/Au Dithiol Assembliesa
n
SPV (±0.5 mV)
d (Å)
Δq (C) (per illuminated area)
6 8 10 16
−15.0 −23.5 −62.0 −18.5
9.3 11.7 14.2 21.65
1.1 1.4 3.1 6.1
× × × ×
10−10 10−10 10−10 10−11
no. of electrons (per illuminated area)
avg no. of electrons (per nanoparticle)
× × × ×
0.6 0.7 1.5 0.3
7.2 9.0 2.0 3.8
108 108 109 108
a
The molecular length is d, the illuminated area is 0.8 mm2, and the average number of NPs per this area is 10 ± 2 × 108 particles.
The length of the alkanedithiol molecules was calculated by ChemDraw software and was verified by atomic force microscopy (AFM) measurements (see the Supporting Information). Three different sets of samples were investigated for each linker, and the results presented are the signals averaged over three different sample sets. The SPV results show that, for all molecules, upon photoexcitation of the NPs, the work function is reduced, so that the net flow of electrons is from the NPs to the metal substrate. In the approximation of a parallel plate capacitor, the SPV is proportional to the charge displaced divided by the distance (d, length of alkanethiol SAM) and times the area of the layer. To compare the amount of charge displaced for the
εA d
when ΔV is the SPV signal, A is the illuminated area, d is the molecular length, and ε is the dielectric constant. B
DOI: 10.1021/acs.jpcc.5b02367 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 4. PM-IRRAS spectra are shown in the C−H stretching region for each of the dithiol SAMs (clockwise from the top left, they are C6DT, C8DT, C10DT, and C16DT).
The photoluminescence measurements indicate that the CdSe NP coverage on the C10DT layer is not anomalously high (see “PL Results” subsection). SAM Characterization. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was used to investigate the quality of the SAMs and their packing. The frequency of the asymmetric methylene stretch has been shown to be a sensitive indicator for the intermolecular interaction among the alkyl chains in the monolayer. A disordered liquidlike monolayer has bands at higher frequencies than a closedpacked, crystalline monolayer (2924 versus 2918 cm −1 respectively).12,13 Figure 4 shows IR spectra for each of the dithiol SAMs. For each alkyl chain length the spectra are consistent with a closely packed, highly ordered monolayer film. Furthermore, the optical density of the IR transitions imply that there is little difference in the average monolayer density for the different molecular lengths (Figure 4). The quality of the monolayers was further examined by electrochemical measurements. Reductive desorption of alkanethiolates (see eq 1) can be used as a method for determining the average surface coverage.14,15
different assemblies, Figure 3 shows a bar graph of number of electrons transferred under illumination from the NPs to the gold in the steady state. Table 1 presents the data with all the parameters used for calculating the amount of charge transferred. Whereas the amount of charge transferred is almost the same for the C6DT and C8DT SAMs, it is somewhat smaller for C16DT, and it is significantly larger for the C10DT SAM; in the latter case the amount of charge transferred is about twice that of the other samples. The length dependence of the amount of charge transferred is rarely discussed; as most electron transfer studies relate to the rates (current), and the amount of charge transferred is sensitive to the area. However, this value is an important parameter for the characterization of photovoltaic devices. The expression for the amount of charge transferred per unit area at steady state, Δq, was derived before,11 and it is proportional to the difference between the electron transfer rate, ke, and the hole transfer rate, kh, For the tunneling limit ki = Ai exp(−βil), where i refers to electrons (e) or holes (h), β is the inverse decay length (damping) factor, and l is the length. Because βe and βh need not be equal, the length dependence of the two rates can be different and cause a length dependence for the amount of charged transferred. However, the dependence is expected to be monotonic with length and therefore the results obtained for C10DT are surprising. The differences observed for Δq could arise from a difference in the density of NPs and/or from changes in the density of molecules in the SAM and their packing. The SAM films were characterized by a combination of infrared spectroscopy, cathodic stripping of the SAM, and thickness measurements.
Au−SR + e− ⇒ Au + RS−
(1)
It has been established that the longer the alkane chain, the more negative the reductive peak potential, presumably because of increasing van der Waals attraction among the alkyl chains.14 It has been found that the shift is approximately linear in the chain length. Figure 5 shows the reductive peak potential as a function of the alkyl chain length. The dependence is small (300 mV over the whole range) and linear, indicating that on average there is little difference in packing of the different C
DOI: 10.1021/acs.jpcc.5b02367 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
energy transfer and the charge transfer from the NPs to the surface are expected to change sensitively with the SAM thickness. Figure 6 shows data for the temperature dependent PL intensity from CdSe NPs adsorbed on alkanedithiol/Au SAMs of different thickness. The NPs were excited at 514 nm. The results show that, below 239 K, C10DT displays an anomalous behavior, as was observed with SPV measurements. In this case, the PL intensity is lower than that observed for the other SAM thicknesses. Note that this could be caused by a lower coverage of the CdSe NPs on the C10DT SAM or by a higher charge transfer rate of the CdSe NPs on the C10DT. Note that a lower coverage of CdSe NPs is in direct contrast with the higher coverage of CdSe NPs that would be needed to explain the anomaly in the SPV data, but that the presence of a higher electron transfer rate f rom the CdSe NPs to the Au electrode on the C10DT is consistent with both a higher SPV value and a lower photoluminescence intensity. Three primary relaxation mechanisms contribute to the quenching of the PL in these assemblies: internal conversion, energy transfer, and electron transfer. The internal conversion (electron−phonon coupling) within the NPs is strongly temperature dependent. This process is most efficient at the higher temperatures, and indeed, for temperatures of 239 K and above, the emission does not depend on the chain length. Namely, in this high temperature limit, the PL intensity is dominated by the quenching of the excited state of the NPs by phonons. By monitoring the PL intensity as a function of temperature and fitting the data to an Arrhenius plot, one can extract an activation energy for the fluorescence quenching (see Figure 7). This analysis gives an average activation energy of about 50 cm−1 (see Table 2) which does not vary significantly from one system to another. In contrast to the recombination quenching in the NPs, the energy transfer, via dipole−dipole interaction from the NPs to the gold substrate, should not display a significant temperature dependence. Thus, an upper limit on the energy transfer can be
Figure 5. Cathodic peak potential as a function of the chain length. Data are from cyclic voltammograms recorded at 100 mV/s in 0.5 mM KOH. Each data point represents the average of the peak potential for desorption of four electrodes. The potentials are reported vs Ag/ AgCl/saturated KCl.
SAMs. Note that the error bar in the case of C10DT is smaller than the other layers, indicating that it was easy to make SAMs of C10DT with reproducible packing. In principle, all the characterization techniques applied indicate no significant change in the monolayers average packing. PL Results. In order to validate the SPV results, photoluminescence (PL) of the CdSe NP/SAM/Au assemblies was measured at different temperatures. As mentioned above, the PL intensity can be used as an indirect measure for charge transfer processes. The CdSe NPs are all obtained from the same batch, namely, all synthesized under identical conditions; and they are in a similar chemical environment, other than the SAM thickness, so that any changes in the photoluminescence yield should arise from the change in the distance between the NP and the Au electrode. Given the small thicknesses (much less than the emission wavelength of the NP) the radiative rate is not expected to change significantly over the 1.2 nm range16,17 of the different SAM assemblies. In contrast, the
Figure 6. (A) Plots of the area under the PL curve as a function of the alkyl chain length at different temperatures. (B) Plot of the total photoluminescence normalized to the result obtained at room temperature, for the different molecules and temperatures. AT is the area of the PL curve at temperature T and A298K is the area of the PL curve at room temperature. D
DOI: 10.1021/acs.jpcc.5b02367 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
sample is weaker than that from the C6DT and C8DT. This lower intensity is attributed to more efficient charge transfer in the C10DT samples. This anomaly is consistent with the larger SPV signal observed for the same sample. The results obtained by both experimental techniques support the case of a more efficient electron transfer from the NPs adsorbed to the substrate for C10DT than for the other alkanedithiols. Mixed SAMs. The efficient electron transfer observed both in the SPV and PL measurements indicates that the Fourier transform infrared (FTIR) and electrochemistry methods, used for characterization of the SAM films, fail to probe an important feature. SAMs of alkanethiols on Au surfaces form a closed packed hexagonal (√3 × √3)R30° structure,18 and the alkyl chains adopt an all-trans configuration with an average tilt angle of about 30° from the surface normal. This lattice presents distances of 0.50 nm between the sulfur heads of nearest neighbor molecules. The unit cell contains one molecule and its area is 0.22 nm2.19 On average, only one molecule can be attached to each NP. We propose that the efficient charge transfer in the case of C10DT monolayers results from nanoscale organization of these molecules which is not reflected in the conventional characterization methods that provide information on the “average packing”. This special organization may result from the C10DT being long enough to have strong van der Waals interaction (that scales with the length) but short enough not to contribute too much to the entropy. Hence, the free energy for the intermolecular interactions of C10DT is lower than that for all other alkyl chains. Indeed, it has already been reported that SAMs made of C10DT are at a transition point in terms of SAM packing as a function of the chain length.20,21 This may result in closed-packed domains (see Figure 8) of nanoparticles, but an overall average density that is not much different from that of the other SAMs. If the domains are small enough and the domain boundaries reduce the overall density, then it may be possible for two or more molecules of C10DT to bind to a single NP while for other SAMs each NP is bound by one thiol link. This multiple binding can open more tunneling pathways and enhance the electron transfer rate. This hypothesis was tested by preparing mixed monolayers of monothiols (MT) and dithiols in a concentration ratio of 9:1. Each sample contained mono- and dithiols of the same length (e.g., the C6DT was mixed with C6MT, etc.). In the case of monothiols, where the tail is a methyl group, NPs cannot be attached to them. By mixing mono- and dithiols of the same
Figure 7. Temperature dependent fluorescence intensity for the four monolayers studied.
Table 2. Activation Energies for Different Monolayer Chain Lengths, Estimated from Arrhenius-Type Fitting Shown in Figure 7 activation energy (cm−1) C6-DT C8-DT C10-DT C16-DT
40 31 37 34
± ± ± ±
7 11 7 17
found by considering the difference in the PL intensity between the CdSe adsorbed on the short vs long chains in the high temperature regime. In addition, the fluorescence intensity has been plotted as a function of d−3 or d−4 where d is the length of the organic molecules (plots not shown). If the energy transfer mechanism was dominant, we would expect one of these plots to show a linear dependence. However, this was not found. Therefore, we conclude that the energy transfer process is small, if compared to the electron−phonon interaction at high temperatures and the electron transfer process at low temperatures (see below). The third mechanism for the excited state quenching is electron transfer to the substrate, and it becomes more dominant at lower temperatures when the electron−phonon quenching mechanism (recombination in the NPs) becomes inefficient. At lower temperature, the PL from the C10DT
Figure 8. (A) Schematic illustration of the closed packed monolayers of CdSe NP/C10DT SAM/Au assemblies. (B) Schematic illustration of the mixed mono- and dithiols monolayers. E
DOI: 10.1021/acs.jpcc.5b02367 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
solution in methanol for 20 h, followed by rinsing with ethanol and drying under a N2 flow. Solvents were reagent grade or better and purchased from Merck, Baker, or Bio-Lab. Nanoparticle Adsorption. In order to form a monolayer from CdSe NPs, the SAM coated substrates were immersed for 3 h. in anhydrous toluene solution (99.8%, Aldrich) of coreonly CdSe (MK Nano) with a diameter of 6.2−7.7 nm. The samples were than sonicated twice (30 s each) in toluene to remove excess NPs and dried rapidly under N2 flow. Surface Photovoltage (SPV). The SPV of the surfaces was determined using a commercial Kelvin probe instrument (Delta Phi Besocke, Jülich, Germany) within a Faraday cage at atmospheric pressure of nitrogen. The reference probe consisted of a gold grid. The measurements were held in the dark and under 633 nm (red) excitation. Prior to illumination, the CPD signal was allowed to stabilize and was recorded as the dark CPD; after stabilizing, the illuminated CPD was recorded. The photovoltage data was calculated as the difference between the voltage measured with a dark substrate and that measured with an illuminated one:
length, the average number of molecules attached to each NP is reduced (see Figure 8B). From Figure 9, it is evident that the
Figure 9. Comparison between monolayers of dithiols and mixed monolayers of mono- and dithiols (9:1) at low temperature (77 K).
photovoltage = CPDilluminated − CPDdark
Photoluminescence (PL) Measurements. The PL measurements were performed using a LabRam HR800-PL spectrofluorimeter microscope (Horiba Jobin-Yivon) with an excitation source at 514 nm (argon-ion CW laser at ∼15 mW/ cm2). The incident light impinged on the surface at an angle of about 90° relative to the sample’s surface, and the PL spectra were collected using the microscope (with 5× high working distance lens). For cooling and for keeping the stability of the signal, we used a liquid nitrogen cooled sample holder (Linkam) and the samples were under constant flow of N2. The measurements were carried out with a fully open confocal aperture (1100 μm) and integration time of 15 s. Polarization Modulation-Infrared Reflection−Absorption Spectroscopy (PM-IRRAS). Infrared spectra were recorded in PM-IRRAS mode using a Nicolet 6700 FTIR instrument, at an 80° incidence angle, equipped with a PEM-90 photoelastic modulator (Hinds Instruments, Hillsboro, OR). Electrochemical Measurement. Cyclic voltammetry (CV) measurements were performed using a Bio-Logic potentiostat SAS (model SP-200) with EC Lab software (version 10.36), by employing a typical three-electrode electrochemical cell arrangement. The working electrode used was gold. The counter electrode was platinum, and the reference electrode was Ag/AgCl/saturated KCI. Data was recorded at 100 mV/s in 0.5 mM KOH.
presence of MT in the SAM eliminates the anomalous behavior of C10DT monolayers. Namely, once only one molecule is attached to the NPs, due to the low density of the dithiols, the anomaly in the behavior of C10DT disappeared. The importance of the actual binding of the molecules to the NPs indicates that charge transfer “through bond” is more efficient than charge transfer through “nonbonded contacts”, which is in keeping with conventional expectations.
■
CONCLUSIONS By combining SPV and temperature-dependent photoluminescence studies, it is possible to reveal the role that charge transfer plays in quenching the fluorescence from NPs attached via alkyl chain linkers to a gold substrate. While at room temperature internal recombination is the dominant excited NP relaxation mechanism, electron transfer still takes place, as is evident from the room-temperature SPV studies; and it becomes the dominant decay mechanism at lower temperatures. The present work demonstrates that C10DT SAMs display an anomalously efficient charge transfer, and it is hypothesized that it occurs because of a special nanoscale organization that allows for multiple dithiol contacts between the CdSe NP and the Au electrode. This special organization is difficult to detect because most SAM characterization methods provide properties averaged on a large (micrometer) scale. In addition, this study shows that through bond electron transfer is more efficient than through space electron transfer in these assemblies.
■
ASSOCIATED CONTENT
S Supporting Information *
■
Three-dimensional AFM topography images; relative signal from Cd atoms obtained by XPS; schematic band diagram of a parallel plate capacitor; CPD signal for gold coated with dithiols; and temperature-dependent PL intensity spectra are provided. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b02367.
EXPERIMENTAL SECTION Self-Assembled Monolayers. Monolayers of 1,6-hexanedithiol (C6DT,97% Alfa Aesar), 1,8-octanedithiol (C8DT,98% Alfa Aesar), 1,10-decanedithiol (C10DT,95% Alfa Aesar), and 1,16-hexadecanedithiol (C16DT,99% Aldrich) were formed on Au by adsorption from thiol-containing methanol solutions. Substrates were cleaned prior to SAM adsorption by dipping in acetone and ethanol for 10 min each, followed by UV/ozone oxidation (UVOCS) for 10−15 min. The samples were then soaked in warm ethanol for 20 min and dried with a flow of N2 gas. The samples were placed immediately in 1 mM dithiol
■
AUTHOR INFORMATION
Author Contributions §
N.K.-U. and P.R. contributed equally.
F
DOI: 10.1021/acs.jpcc.5b02367 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Notes
(19) Vericat, C.; Vela, M. E.; Salvarezza, R. C. Self-Assembled Monolayers of Alkanethiols on Au(111): Surface Structures, Defects and Dynamics. Phys. Chem. Chem. Phys. 2005, 7, 3258−3268. (20) Camillone, N.; Chidsey, C. E. D.; Liu, G.; Putvinski, T. M.; Scoles, G. Surface Structure and Thermal Motion of N-Alkane Thiols Self-Assembled on Au(111) Studied by Low Energy Helium Diffraction. J. Chem. Phys. 1991, 94, 8493. (21) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. Formation of Monolayer Films by the Spontaneous Assembly of Organic Thiols from Solution onto Gold. J. Am. Chem. Soc. 1989, 111, 321−335.
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We acknowledge the financial support through the US DOE (Award # ER46430) and the Israel Ministry of Science.
■
REFERENCES
(1) Fan, F.-R. F.; Yang, J.; Cai, L.; Price, D. W.; Dirk, S. M.; Kosynkin, D. V.; Yao, Y.; Rawlett, A. M.; Tour, J. M.; Bard, A. J. Charge Transport through Self-Assembled Monolayers of Compounds of Interest in Molecular Electronics. J. Am. Chem. Soc. 2002, 124, 5550−5560. (2) Nijhuis, C.; Reus, W. F.; Barber, J. R.; Dickey, M. D.; Whitesides, G. M. Charge Transport and Rectification in Arrays of SAM-Based Tunneling Junctions. Nano Lett. 2010, 10, 3611−3619. (3) Aswal, D. K.; Lenfant, S.; Guerin, D.; Yakhmi, J. V.; Vuillaume, D. Self Assembled Monolayers on Silicon for Molecular Electronics. Anal. Chim. Acta 2006, 568, 84−108. (4) Janek, R. P.; Fawcett, W. R.; Ulman, A. Impedance Spectroscopy of Self-Assembled Monolayers on Au(111): Sodium Ferrocyanide Charge Transfer at Modified Electrodes. Langmuir 1998, 14, 3011− 3018. (5) Liu, B.; Bard, A. J.; Mirkin, M. V.; Creager, S. E. Electron Transfer at Self-Assembled Monolayers Measured by Scanning Electrochemical Microscopy. J. Am. Chem. Soc. 2004, 126, 1485−1492. (6) Kamat, P. V. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737−18753. (7) Milliron, D. J.; Gur, I.; Alivisatos, A. P. Hybrid Organic− Nanocrystal Solar Cells. MRS Bull. 2005, 30, 41−44. (8) Wu, M. H.; Ueda, A.; Mu, R. Semiconductor Quantum Dot Based Nanocomposite Solar Cells. In Organic Photovoltaics: Mechanisms, Materials, and Devices; Sun, S.-S., Sariciftci, N. S., Eds.; CRC Press: Boca Raton, FL, 2005. (9) Gotesman, G.; Guliamov, R.; Naaman, R. Horizontal versus Vertical Charge and Energy Transfer in Hybrid Assemblies of Semiconductor Nanoparticles. Beilstein J. Nanotech. 2012, 3, 629−636. (10) Kronik, L.; Shapira, Y. Surface Photovoltage Spectroscopy of Semiconductor Structures: At the Crossroads of Physics, Chemistry and Electrical Engineering. Surf. Interface Anal. 2001, 31, 954−965. (11) Gotesman, G.; Waldeck, D. H.; Naaman, R. Self-Assembly of Nanoparticle Arrays on Semiconductor Substrate for Charge Transfer Cascade. J. Phys. Chem. A 2009, 113, 7213−7217. (12) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. Carbon-Hydrogen Stretching Modes and the Structure of N-Alkyl Chains. 1. Long, Disordered Chains. J. Phys. Chem. 1982, 86, 5145−5150. (13) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (14) Widrig, C. A.; Chung, C.; Porter, M. D. The Electrochemical Desorption of N-Alkanethiol Monolayers from Polycrystalline Au and Ag Electrodes. J. Electroanal. Chem. Interfacial Electrochem. 1991, 310, 335−359. (15) Walczak, M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Reductive Desorptionof Alkanethiolate Monolayers at Gold: A Measure of Surface Coverage. Langmuir 1991, 2687−2693. (16) Waldeck, D. H.; Alivisatos, A. P.; Harris, C. B. Nonradiative Damping of Molecular Excited States by Solid Surfaces. Surf. Sci. 1985, 158, 103−125. (17) Kuhn, H.; Foersterling, H.-D.; Waldeck, D. H. Principles of Physical Chemistry; Wiley: New York, 2009. (18) Schreiber, F.; Eberhardt, A.; Leung, T.; Schwartz, P.; Wetterer, S.; Lavrich, D.; Berman, L.; Fenter, P.; Eisenberger, P.; Scoles, G. Adsorption Mechanisms, Structures, and Growth Regimes of an Archetypal Self-Assembling System: Decanethiol on Au(111). Phys. Rev. B 1998, 57, 12476−12481. G
DOI: 10.1021/acs.jpcc.5b02367 J. Phys. Chem. C XXXX, XXX, XXX−XXX
