Photoconductivity of Nanofilaments That are Self-Assembled from a

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Photoconductivity of Nanofilaments that are Self-Assembled from a Porphyrin with Long Alkyl-chain Substituents Anna P Schall, Patrizia Iavicoli, Zhengqing John Qi, Julien Menko, Ye Lu, Mathieu Linares, Julio C. de Paula, David B. Amabilino, A.T. Charlie Johnson, and Walter F. Smith J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07902 • Publication Date (Web): 30 Oct 2015 Downloaded from http://pubs.acs.org on November 2, 2015

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The Journal of Physical Chemistry

1 Photoconductivity of Nanofilaments that are Self-assembled from a Porphyrin with Long Alkyl-chain Substituents Anna P. Schall†, Patrizia Iavicoli‡, Zhengqing John Qi§, Julien Menko†, Ye Lu§, Mathieu Linares, Julio C. de Paula⊥, David B. Amabilino‡,◊, A. T. Johnson§, Walter F. Smith*,† †Haverford College Physics Dept, Haverford PA 19041 USA ‡Institut de Ciència de Materials de Barcelona (CSIC), Campus Universitari de Bellaterra, 08193 Cerdanyola del Vallès Catalonia, Spain §University of Pennsylvania, Dept. of Physics and Astronomy, Philadelphia PA 19104 USA Linköping University, Department of Physics, Chemistry and Biology (IFM) and Swedish eScience Research Centre (SeRC), SE-58 183 Linköping, Sweden ⊥Lewis and Clark College, Dept. of Chemistry, Portland OR 97219 USA Abstract Photoelectronically-active nanostructures that are self-assembled from organic molecules hold the promise of tailored functionality with simple and inexpensive production. Comparison of nanowires assembled from related compounds can give important insights into the details of selfassembly and the conduction mechanisms. We report the photoconductivity of nanofibers that are self-assembled from a porphyrin with long alkyl substituents. In contrast to previously studied porphyrin nanowires, the photoconductivity increases as atmospheric O2 is increased. This can be explained using the same model as used in the previous studies, by assuming a different lineup of the bands of the nanofilaments with the electrode Fermi level. However, this model does not explain our observation that, at O2 concentrations above 1%, the conduction increases with continued illumination; this may be due to photoactivation of shallow O2 adsorption sites. The overall conduction level is low even at high O2 concentration, because the alkyl substituents form an insulating sheath around the nanofibers. Such insulation could be valuable in applications where it would prevent cross-talk between signals carried in different nanofilaments. Schottky barriers at the interface between organic nanostructures and electrodes strongly affect conduction and photoconduction, and are strongly influenced by atmospheric gases such as O2. Introduction Substances forming the basis for photoelectronically active, self-assembling nanodevices should absorb radiation strongly at the excitation wavelength of choice, self-assemble readily into useful nanoscale structures in response to appropriate chemical or physical cues, and show strong electronic coupling between molecules once self-assembly is complete. Porphyrins display all these characteristics, and are especially useful for excitation in the visible range of the spectrum. (For reviews, see 1-4.) To develop a full range of properties that include different responses to plane- or circularly-polarized light, it may be desirable to use chiral compounds. Moreover, to learn how the structure and photoelectronic properties of self-assembled molecular nanostructures are controlled by the structure of the parent molecules, it is helpful to compare closely related molecules, and also to compare the behavior when the forces that drive aggregation are changed. For some applications, it may be desirable to use nanowires that are electrically insulated, so as to prevent cross-talk between signals carried in different nanowires that may be in physical contact with one another. Here we report measurements of the electronic properties of nanoflilaments selfassembled from a porphyrin molecule with a stereogenic center, and with long alkyl substituents

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2 that form an insulating sheath. As described below, the forces controlling self-assembly of these molecules are quite different from those for tetrakis(4-sulfonatophenyl)porphine (TPPS4), which is based on the same central tetraphenylporphyrin core. The photoelectronic properties of nanowires formed from TPPS4 have been studied more thoroughly than those of any other organic self-assembled nanowires. In a solution of high enough ionic strength, TPPS4 forms straight nanowires of uniform cross-section when the pH of the solution is lowered. When deposited onto substrates the nanowires are 3.8 nm high and approximately 25 nm wide, with lengths up to several microns.5 These nanowires exhibit photoconductivity and trainable photovoltaic activity.6 The photoconductivity grows slowly over periods of hours, and is dramatically reduced by even low concentrations of atmospheric oxygen, as shown by Riley et al.7 After illumination for an extended period, the nanowires exhibit “persistent photoconductivity”, i.e. the conductivity does not drop to zero immediately when illumination is removed, but instead decays slowly. Combining their observations of this behavior with measurements of the effects of gate voltage, Riley and co-authors proposed a model in which the conductivity is controlled by Schottky barriers at the nanorod/electrode interfaces. The height of these barriers decreases when light causes physisorbed oxygen to desorb, resulting in “persistent photoconductivity”. Within the model, this type of conductivity may be thought of as a dark conductivity which is suppressed by adsorbed oxygen. Ohtsuka and co-authors8 studied larger diameter TPPS4 nanorods (with heights ranging from 15 to 35 nm) formed by air-drying from a 67 µM solution. (This is considerably different from the deposition conditions used by Schwab and co-authors6, where the nanorods were deposited from a 5 µM solution by spin drying.) They found dark conductivity, which was quenched when atmospheric oxygen was introduced. Yeats and co-authors9 studied nanotapes formed from a related compound, meso-tri(4sulfonatophenyl)monophenylporphine (TPPS3). They found qualitatively similar behavior to that shown by Schwab et al. for TPPS4 nanowires6, though with a lower conductivity level, and a better “memory” of the slow growth of the photoconductivity. They attributed these differences to the lower charge in solution of the ions that form the TPPS3 nanotapes (net charge of –e) compared that of the ions forming the TPPS4 nanorods (net charge of -2e). They speculated that the lower charge led to less structural rigidity, which in turn allowed more structural defects. Guo and co-authors10 studied nanofibers formed from oxo-[5,10,15,20-tetra(4pyridyl)porphyrinato]titanium(IV) (TiOTPyP) via surfactant-assisted self-assembly (SAS). They found dark conductivity, which was dramatically increased by atmospheric H2O2 vapor, and significantly increased by NH3 and C2H5OH vapors. They did not discuss possible mechanisms for the conductivity or the vapor sensitivity. For a review of other results using SAS, e.g. photocatalysis and photoelectrochemical cells, see reference 11. The studies cited above used porphyrins that self-assemble through π-π stacking interactions and electrostatic interactions. In this work, we employ a tetra-mesoamidophenyl substituted porphyrin (see fig. 1) with long alkyl side chains. We refer to this molecule as CP1. This neutral molecule is dissolved in a non-polar solvent, so the electrostatic interactions which are so important for the self-assembly of TPPS3 and Figure 1. Structure of CP1.


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3 TPPS4 are absent. However, unlike the previously studied compounds, the self-assembly of CP1 is strongly affected by hydrogen bonding between the amide groups and by London disperson interactions between the long alkyl chains.12 The molecule has one stereogenic center, in the R configuration (near the lower left in the figure). It forms ordered rows when dissolved in 1heptanol, applied to a graphite substrate, and imaged in solution with scanning tunneling microscopy (STM).12 The STM images clearly show that each macrocycle lies flat on the graphite, allowing for relatively little direct interaction between the electronic states of the macrocycles. However, when CP1 is dissolved in methycyclohexane, the Soret band of the porphyrin shows a significant shoulder at 400 nm12, attributed to the formation of an aggregate, and is blueshifted relative to the 421 nm peak for the monomer, indicating strong exciton coupling and Haggregation. (See, for example, 13 and references therein.) Circular dichroism (CD) 12 measurements show that the aggregates have a counterclockwise helicity. At higher concentrations, CP1 forms a gel in methycyclohexane; the aggregate absorbance peak near 420 nm is enhanced, while the monomer peak decreases. SEM and AFM images of the dried gel reveal bundles of oriented nanofibers. , Methods Molecular Modeling. Calculations were performed with the TINKER package14 and the MM3 force field15. 16, in the absence of solvent. To reduce computational time, alkyl chains of 13 carbons were used, rather than the 18 carbon chains in the actual molecule. However, this should not cause any qualitative change in the results. Preparation of CP1 Aggregates. The CP1 was prepared as described by Iavicoli and coauthors.12 Solid CP1 was dissolved in methylcyclohexane to a concentration of 0.78 g L-1, using shaking and heating as necessary. This solution was pipetted into a quartz cuvette, slowly brought to a boil, allowed to cool, and boiled again. At this point, a substrate was immersed in the solution for one minute, and then quickly blown dry. The substrates used for deposition were p++ doped silicon with a 400 nm silicon oxide layer. Interdigitated AuPd electrodes with a spacing of 400 nm were fabricated on the substrates using electron beam lithography.6 Before deposition, all substrates were cleaned in a Bioforce Nanoscience UV/ozone to oxidize any organics, and then baked in air at 300 ˚C to remove any remaining contaminants. AFM Imaging. All samples from CP1 aggregation experiments were imaged using an AFM, either a Digital Instruments Multimode or Bioscope, in tapping mode, using either Probe Plus silicon probes or App Nano SPM probes, with resonant frequencies of 75 kHz. Photoconductivity Measurements. The light source for the photoconductivity measurements was an SLM Spectral Energy model LH450 UV/vis Xenon lamp, directed through an SLM Instruments monochromator, or a laser diode operating at 405 nm. The beam was directed and focused by a series of mirrors and lenses onto the substrate. Prior to photoconductivity measurements, the intensity of the light source at the sample’s position was measured with a ThorLabs S20MM power meter with an aperture radius of 50 µm. A Keithley 6517A electrometer was used to carry out the electrical measurements. This electrometer applied the bias voltage between, and also measured the current. A LabVIEW v6.1 program was for data acquisition. Experiments were carried out under ambient air, dry air, and dry N2 (obtained from the boiloff of liquid nitrogen). The relative oxygen concentration, i.e. the percent by volume of the



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4 atmosphere within the sample chamber, was measured with a Vernier O2-BTA O2 gas sensor. The humidity was measured using a Lufft C200 humidity meter. Results Molecular modeling in the absence of solvent shows that the lowest-energy aggregate of CP1 is a counterclockwise helix, as shown in fig. 2. This is more stable than a clockwise helix, in which the methyl group attached to the stereogenic center disturbs the formation of hydrogen bonds between adjacent porphyrin molecules.

Fig. 2 Results of molecular modeling, using a version of CP1 with alkyl chains that are shortened (13 carbons rather that 18) to reduce computational time.


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5 Samples of CP1 take on a variety of morphologies when deposited from solution onto oxidized silicon substrates and imaged with AFM. This variation occurs even though all samples are prepared using nominally identical procedures. We presume that the differences arise from slight variations in the blow-drying step and/or from variations in the time the solution was heated prior to deposition. For some samples, CP1 aggregates appear as long and straight strand-like aggregates with relatively few kinks (fig. 3a). However, for most samples, we observe a web-like network (fig. 3b), or a bi-layer web-like network (fig. 3c). Electrical measurements on both types of networks show qualitatively similar behaviors (see fig. 7). We used bilayer network samples for all electrical measurements reported here, except the gray trace in fig. 7, which was taken using the sample shown in fig. 3b (a single-layer network). Figure 4 shows images of the sample used for the data shown in

Figure 4: AFM image of the sample deposited on interdigitated electrodes that was used for the data in figures 6 and 7. Three fingers from the electrode pattern are visible in this image; the fingers are denoted by the blue bars on the right side of the image. (The height difference between the fingers and the bare substrate has been suppressed.) This set of electrodes had protrusions on top of the electrodes (the white blobs), but these do not affect conduction through the nanofilaments that span the areas of bare silicon between the fingers. The z-scale is 10 nm from black to white.

Figure 3: AFM images of CP1 aggregates. a: The CP1 aggregates sometimes form straight strands. Under nominally identical conditions, the aggregates usually instead form web-like structures, either in a single layer (b) or double layer (c). All images have a z-scale of 10 nm from black to white.



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6 figures 6 and 7, with the network deposited on top of the inter digitated electrodes. The height of the nanofilaments is 3.6 ± 0.5 nm in both the lower and upper layers of the bilayer network samples, and also in the single layer network samples. This is much less than the 6.9 nm diameter of the helical model shown in fig. 2 (after adjusting for the 5 extra carbons per alkyl chain that are not included in that model). This difference suggests that, during the process of deposition onto a substrate and drying, the alkyl chains pack into a dense layer surrounding the central porphyrin cores. In the dark under a dry nitrogen environment, the aggregates are electrically insulating (less than 10 fA current for a bias voltage of 1 V, corresponding to a conductivity of less than 3 × 10-8 S/m). However, when illuminated with wavelengths between 350 nm and 525 nm with a 1 V bias Figure 5: Current as a function of time in ambient air with voltage, the aggregates show 1 V bias applied as the wavelength of illumination is varied; photoconductivity, in both ambient air the wavelength (in nm) used is indicated above each peak. and under dry nitrogen environment, as The current measured jumps up quickly upon illumination, then grows slowly. Most of the current vanishes as soon as shown in figure 5. The current jumps up illumination is removed, though there is a small component immediately upon illumination, and that decays away slowly. decreases rapidly when the light is blocked. Additionally, under most conditions (see below) the current increases steadily if the sample is exposed to light for longer times. In the data of figure 5, the peak current achieved at each wavelength may depend on light exposure from earlier-measured wavelengths, because of the slow growth of photoconductivity during each period of illumination. Therefore, to investigate the dependence of photoconductivity on wavelength, and also to minimize possible damage to the sample from O2 that has been excited to the singlet state by interaction with the porphyrin17, we illuminated for only 15 seconds at each wavelength (instead of the roughly 100 seconds used for figure 5), waited 180 seconds between periods of illumination, and took the data twice, once starting at low wavelengths and moving to higher wavelengths, and once from

Figure 6. The current produced by the sample shown in fig. 3 at an average oxygen concentration of 10%. The illumination is turned on for 15 s intervals (corresponding to the current spikes), with the wavelength changed for each interval. The wavelength used for the first pulse is at 390 nm. It is increased by increments up to 525 nm, then decreased in the same increments back to 390 nm, as indicated by the wavelength labels above each peak and by the schematic color bar at the top. A small drift of the current baseline is evident; this was subtracted out for the data shown in fig. 7.


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7 starting from higher wavelengths and moving to lower wavelengths. Peak values for each wavelength obtained between these two runs agreed to within 6%. We averaged these two values to obtain the current at each wavelength. A typical set of raw data is shown in fig. 6. We converted this current to the conductivity σ using the usual definition LI σ= , 1) AV where I is the measured photocurrent (after subtracting out the small dark Figure 7: Photoconductivity (defined as conductivity divided current), and V is the bias voltage (1 V). by illumination intensity) vs. wavelength. The We take L to be the distance between photoconductivity is defined as conductivity divided by electrodes (400 nm), and we take A to intensity. Colored traces (left axis): bilayer network sample be the average cross-sectional area of sample under dry nitrogen, with dry O2 added. Gray trace (right axis): single-layer network sample under ambient air. Solid aggregates between the electrodes, as black curve: absorption spectrum of the gel, with arbitrary determined by AFM. We did not units.12 The bias voltage used was 1 V, and the intensities attempt to correct for tip convolution ranged from 290 to 400 W/m2 for the bilayer sample and from 2 effects when determining this cross- 90 to 120 W/m for the single-layer sample. sectional area, so the area we used was an overestimate, leading to an underestimate of the conductivity. The intensity of light reaching the sample in our apparatus varies somewhat as a function of wavelength. For the variable-wavelength experiments on the bilayer network sample, the intensity varied from a minimum of 290 W/m2 to a maximum of 400 W/m2. By dividing the conductivity by the intensity, we compensate for this variation (assuming the photocurrent is proportional to intensity). Therefore, we define the “photoconductivity” as the conductivity divided by the intensity. The results of these experiments are shown as a series of action spectra (photoconductivity vs. wavelength) in figure 7. This figure also shows the effects of adding O2 to the nitrogen atmosphere surrounding the bilayer network sample. We allowed at least one hour for equilibration between each change in O2 concentration. The general features of the action spectra are similar to those of the UV/Vis absorption spectrum of the gel12, which is shown as a solid black line in the figure. However, there is a significant red shift in the Soret band, from 404 nm for the absorption spectrum of the gel to 425 nm for the action spectra. As shown in figure 7, higher O2 levels in the atmosphere surrounding the sample result in higher photoconductivity. Data taken on a second sample show the same shape of action spectra, the same dependence on O2 concentration, and a photoconductivity level that matches to within a factor of two. (Because AFM images were used to estimate the cross-sectional area for the conductivity calculation with no correction for tip convolution effects, the actual cross-sectional area is certainly larger than the area estimated from AFM images. The apparent difference in photoconductivity between samples may be due to different degrees of AFM tip convolution.) These data also match trends shown by a third and a fourth sample. The data in figure 7 were



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8 taken in order of increasing O2 concentration. Fig. 7 also shows data taken on a single-layer network sample (gray trace) under ambient conditions. The main shape of the curve is the same as for the bilayer sample under dry conditions. However, the photoconductivity is about 8 times higher than for the bilayer sample under a dry 15% O2 / 85% N2 atmosphere. The difference may be due to the relative humidity in the ambient air. The photoconductivity drops to zero for wavelengths above 475 nm, in contrast to the bilayer sample; this is discussed further below. Illumination under ambient atmosphere causes a permanent decrease in the photocurrent, as shown in figure 8. With each successive experiment, the magnitudes of the current measured through the sample diminished by as much as a factor of 7. Note that, in this experiment, we used illumination periods of approximately 100 seconds per wavelength. The 15 s periods used during the 15% O2 run in figure 7 did not cause a measurable decrease in current during the course of the run, since the currents measured in the second half of the run (moving from long wavelengths to short wavelengths) were the same to within 6%. The difference may be due to the shorter illumination period, or the lower relative humidity, in comparison with the data taken in ambient air. The damage caused by illumination in ambient air is presumably responsible for the lack of photoconductivity at wavelengths above 475 nm for the single-layer sample shown in fig. 7. (The data were taken in order of increasing wavelength.) Fig. 9 shows data taken on another sample, using a fixed wavelength of 405 nm and an intensity of 970 kW/m2, with a relative humidity of 1.3%. Prior to each data point, the oxygen concentration was held constant for at least four hours. For

Figure 8: Successive action spectra of CP1 aggregates taken in ambient air within a few days of each other. The first trial is shown in black with square symbols, the second in red with round symbols, and the third in blue with triangular symbols. The graph shows that the magnitudes of photoconductivity measured diminished with each successive trial.

Figure 9. Photoconductivity vs. relative O2 concentration. For each data point, the sample was illuminated at 405 nm and 970 W/m2 for 160 seconds, held in the dark for 260 seconds, and illuminated again for 160 seconds; the current shown is the average of that measured for the two periods of illumination. Points were taken in the order indicated by the arrows. There is significant hysteresis; as the O2 concentration is reduced, the sample partially “remembers” the earlier concentration.


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9 each data point, the sample was illuminated for 160 seconds, held in the dark for 260 seconds, and illuminated again for 160 seconds; the current shown is the average of that measured for the two periods of illumination. As expected from the results of fig. 7, the photoconductivity increases with the percentage of environmental oxygen. The experiment shows that there is significant hysteresis in this process: as the O2 level is reduced, the sample “remembers” the previous high oxygen level, so that the photoconductivity measured at each oxygen level is higher than those measured when the O2 level was being increased. For O2 levels of 1.3% and above, the current increased throughout the 160 s illumination period, as shown by the top four traces in fig. 10. However, for O2 levels of about 0.1%, the current decreased during the illumination period, as shown by the bottom four traces. Table 1 summarizes the average slope of the current during the illumination period. At the lowest O2 concentrations, current Figure 10. Current during illumination for 0.1% and 2% atmospheric oxygen levels Illumination was turned on at declines during illumination. At all other 30 s, and turned off at 190 s.. At an oxygen concentration O2 levels, current grows during of 0.1% the current falls during illumination. At 2% illumination. The rate of growth increases oxygen, the current grows during illumination. with increasing atmospheric O2 concentration. The rate of growth (or decline) is always greater in magnitude during the first illumination because current memory is exhibited during the second illumination. Table 1. Rate of growth of photocurrent during both the first and second illumination, and the average over the two illuminations, for the experiments summarized in Figure 9. Trial Number % O2 by volume Slope of 1st Illumination (fA/s) Slope of 2nd Illumination (fA/s) Average Slope (fA/s)

1 2 3 4 5 6 7 8 9 0.10% 1.90% 5.26% 10.25% 15.23% 10.14% 5.18% 2.22% 0.11% -1.10

0.49

1.21

1.17

1.44

1.42

1.14

0.85

-1.56

-0.41 -0.75

0.21 0.35

0.34 0.77

0.45 0.81

0.53 0.99

0.46 0.94

0.30 0.72

0.09 0.47

-0.73 -1.15



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10

Fig. 11 shows the response of this sample to a single pulse of O2. The current increases quickly when the O2 flow is turned on, but decreases more slowly when the flow is turned off. Note that the final value of current is lower than the initial value. The photoconductivity for the bilayer sample used for figures 9-11 much higher than for the bilayer sample used for fig. 7. Part of the difference is due to the longer illumination times: 15 s for fig. 8 vs. 160 s for fig. 9. Part may also be due to the fact that the sample used for fig. 7 was stored in a dessicant-filled jar for 25 days before electrical measurements were started, while the sample used for figures 9-11 was stored for only one day before measurements were started.

Figure 11. The photoconductivity (blue, left axis) through a CP1 nanofilament network as oxygen (red, right axis) is first introduced and then removed from the sample environment. Photoconductivity immediately responds to growing oxygen levels by growing. As oxygen is purged from the environment, however, the photoconductivity’s rate of decline is not as responsive as its previous rate of growth. However, the photoconductivity eventually falls below the original value.

Discussion As noted above, the peak in the action spectrum of the nanofibers occurs at a significantly longer wavelength (about 425 nm) than the peak in the absorption spectrum of the gel (about 404 nm). There are several possible explanations. The peak in the action spectrum is close to the peak absorbance for the CP1 monomer (421 nm). This suggests the possibility of a small fraction of CP1 molecules that continue to exist in monomer form, perhaps physisorbed to the exterior of the nanofilaments. These monomers would be small in number compared to those that are fully incorporated into the nanofilaments, and so would have a small effect on the absorption spectrum. (There is a suggestion of a shoulder in the gel absorption spectrum close to the 421 nm absorption maximum of the monomers.) However, these monomers could still be important for conduction, perhaps serving the role of hopping sites. Another model is that the nanofilaments may be heterogeneous. There is a clear shoulder in the absorption spectrum of the gel at about 445 nm. Perhaps most of each nanofilament is composed of molecules in an arrangement that leads to absorption at 404 nm, while in other parts of the nanofilament the molecules are arranged differently, so that they absorb at 445 nm. If these two regions are electrically in series, then both would need to be photoexcited for conduction to occur. In this model, one must assume that the absorption peaks for the two regions are broad enough that both have non-zero absorption at 425 nm; changing the excitation wavelength away from this value results in a decrease in absorption in one region or the other, choking off the current. Finally, it is possible that the absorption maximum for the nanofilaments when deposited onto the silicon surface is different from that of the gel. It may well be that interaction with the substrate induces a rearrangement of molecules within the nanofilaments, changing the absorption maximum from 404 nm to 425 nm. It seems likely that some rearrangement, at least


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11 of the alkyl chains occurs, given the measured 3.6 nm height of the deposited nanofilaments. The photoconductivity measured for this system is approximately 2000 times lower than that for TPPS4 nanorods.6 Much of the difference may be due to the long hydrocarbon tails attached to the porphyrin molecules in this study. The combination of the computer model shown in fig. 2 and the AFM-measured height of the nanofilaments suggests that the porphyrin core is insulated by a layer formed by the hydrocarbon tails. This would prevent good electrical contact with the metal electrodes, limiting conduction, as discussed more quantitatively below. Another surprising result is the dependence of photoconductivity on the concentration of O2 in the atmosphere surrounding the sample. For the CP1 nanofilaments studied here, adding O2 increases photoconductivity, whereas Riley and co-authors showed that for TPPS4 nanorods, adding even small concentrations of O2 drastically decreases photoconductivity.7 However, the model presented by Riley and co-authors can explain an increase in photoconductivity with O2 if the band line-up is shifted. For Figure 12. Model for the effect of adsorbed O2 on photoconductivity. a: The filled states of the electrodes TPPS4, they assumed that the Fermi level are shown with dark shading, and the empty states with of the porphyrin nanorods was initially light shading. The nanofilaments are modeled as below that of the metal electrodes, so that semiconductors, with a filled valence band (shaded) and when the two were put into contact, an empty conduction band (hatched). We assume that the electrons moved from the electrodes into initial line up of Fermi levels results in electron transfer from the nanofilaments into the electrodes, resulting in the the nanorods, resulting in an upward bend Schottky barriers shown. An electron-hole pair can be of the bands. However, it may be that the created by photon absorption (step 1). The hole can then Fermi level for the CP1 nanofilaments is easily move to the left electrode, but the electron must be initially above that of the electrodes. This thermally promoted over the Schottky barrier (step 2) to might occur because of impurities serving reach the right electrode. b: When O2 adsorbs onto the nanofilaments, it forms charge transfer complexes with the as n-type dopants, or because the packing nanofilaments. The resulting empty energy levels attract in CP1 is H-type, which shifts the energy additional electrons into the nanofilaments, populating the levels higher with respect to the isolated energy levels (as shown), and unbending the bands. This porphyrin, while the energy levels of the J- reduces the height of the Schottky barrier, so that the aggregates formed by TPPS4 are shifted energy needed for step 2 is smaller, and the conductivity lower. If the Fermi level of the increases. nanofilaments is initially above that of the electrodes, then electrons will move from the nanofilaments into the electrodes, resulting in a downward bending of the bands, as shown in figure 12a. As for TPPS4, this results in Schottky barriers at the contacts, though with an opposite bend. For TPPS4, after an electron-hole pair is created by photoexcitation, the electron can easily move into the electrodes, while the hole must be thermally promoted over the Schottky barrier. In the model shown in figure 12 for CP1, it is instead the hole that can easily move into the electrodes, and the electron which must be promoted over the Schottky barrier.



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12 When atmospheric O2 adsorbs to the CP1 nanofilaments, we assume in this model that it creates charge transfer complexes. Due to the large oxidizing power of the O2, the net result is to attract more negative charge into the nanofilaments. This causes the bands to unbend partially, reducing the barrier over which the electrons must be promoted, and increasing the photoconductivity, as shown in fig. 12b. Therefore, for a band lineup in which the Fermi level of the nanostructure is initially above that of the electrodes, adding atmospheric O2 causes an increase in photoconductivity. Riley and co-authors observed a growth in conductivity when their TPPS4 samples were continuously illuminated, and attributed it to light-induced O2 desorption. Their measurements of this effect were carried out at O2 concentrations below 0.1%. For this O2 concentration, we observe a decrease in conductivity under continuous illumination. This is consistent with the model of photo-induced desorption of O2, since in this model for the CP1 nanofilaments, a decrease in adsorbed O2 causes a decrease in conductivity. However, for all higher O2 levels, we instead observe a growth of conductivity. It is difficult to explain this observation within the context of this model. It is conceivable that illumination activates relatively shallow O2 binding sites on the nanorods, leading to an increase in the level of adsorbed O2, and a corresponding increase in conductivity. Perhaps because of their shallowness, these sites are less important under the conditions of lowest O2 concentration. The results of fig. 11 suggest that the O2 associated from the pulse could be cleared from the nanofilaments within 2000 s. However, the results of fig. 8 show a memory of earlier O2 levels that persists for at least 4 hours. There are two possible explanations. First, illumination was applied continuously for fig. 10, whereas it was applied only for 160 s at a time for fig. 8. The continuous illumination would be expected to cause more damage to the sample, due to creation of singlet O2 by the interaction with the photoexcited porphyrin. Therefore, the decrease in current shown in fig. 11 is presumably partly due to the decreasing O2 levels, but also partly due to sample damage; this would explain why the final current level is lower than the initial level. A second possible explanation is that, when the sample is exposed to O2 for several hours (as in fig. 8), the O2 has time to diffuse to interior sites. When the O2 level is later reduced, the interior O2 takes a correspondingly long time to diffuse back out. During the relatively short O2 exposure of fig. 10, there would be less time for such diffusion to the interior. In the model described above, the resistance of the nanofilaments is dominated by the contacts, and the effects of O2 are due to the changes it creates in the Schottky barriers. The action of O2 cannot be due to a doping effect, since the mobile carriers thus created would give dark conductivity, which we do not observe. It seems unlikely that the effect of O2 could be due to changes it creates in the charge mobility of the nanofilaments, since the adsorbed O2 should create a more uneven electrostatic landscape, which would reduce mobility. Although the photoconductivity of this system is very significantly affected by atmospheric O2, it is much less sensitive than TPPS4 nanorods. As shown in fig. 9, for CP1 nanofilaments, changing the O2 concentration from 0.1% to 1.9% causes about a factor of two increase in photoconductivity, while Riley and co-authors7 found for TPPS4 nanorods that a change of O2 concentration from 0 to only about 0.2% causes a decrease in photoconductivity of at least 44%. We can understand this difference in O2 sensitivity with the Schottky barrier model. In this model, the current is limited by thermal promotion of electrons over the Schottky barrier (step 2 in fig. 12). We assume the activation rate for promotion over this barrier is k = Ce− Eb / k BT

,


2)

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13 Where C is the relevant attempt rate and Eb is the relevant barrier height. The photocurrent, and therefore the photoconductivity γ is proportional to this rate: 3) γ = ACe − Eb / k BT , where A is a constant. As suggested by the model, we make the simplest possible assumption for the dependence of the barrier height on O2 concentration for low values of the concentration: Eb = E0 − BPO2 , 4) where E0 is the barrier height in the absence of O2, B is a constant (positive for CP1 nanofilaments and negative for TPPS4 nanorods) , and PO2 is the percent concentration of O2. Plugging this into 3) gives 5) γ = ACe − E0 / k BT e BPO2 / k BT . The rate at which the photoconductivity changes when the O2 concentration is changed is then dγ B BPO2 / k BT e = ACe − E0 / kBT dPO2 k BT dγ B ⇒ =γ 6) dPO2 k BT This means that systems with high photoconductivity should tend to have high sensitivity to atmospheric O2, and explains qualitatively why the sensitivity of TPPS4 nanorods is higher than that of CP1 nanofilaments. We can rearrange 6) to isolate the constant B, which determines the sensitivity of the Schottky barrier height to atmospheric O2: d γ dPO2 B= k BT 7) γ Plugging in the numbers from the first two points in figure 9 (and using the average values of γ and PO2 for these two points), we find that, for CP1, B = 0.029 eV/% where the units are eV divided by percent O2. For TPPS4 from fig. 4 of ref. 7, we find B = 0.26 eV/% . The higher value of B for TPPS4 nanorods may be due to a higher density of binding sites for O2, to a higher net charge for the charge transfer complex formed by each adsorbed O2 , and/or to a stronger effect of that charge on the height of the Schottky barrier. The latter two effects could well be associated with the insulating effect of the alkyl chains in CP1. Fig. 9 suggests that, at an O2 level of 15%, the effect of O2 has nearly saturated. In the context of the above model, this could occur either because the O2 binding sites on the nanofilaments are nearly all populated, or because the Schottky barrier height has been reduced enough that some other factor is limiting conduction. We can estimate whether the insulating sheath created by the alkyl chains is creating this limit. We assume that electrons tunnel through this barrier into the central core of the nanofilament, where they can travel relatively easily between the central porphyrin macrocycles. We approximate this tunneling using a simplified Simpson model18, giving Ralkyl β d R= e , 8) A Where Ralkyl is the exponential pre-factor for tunneling through the alkyl layer, A is the crosssectional area available for tunneling, β is the tunneling decay constant, and d is the thickness of the alkyl layer. Using impedance spectroscopy, Sangeeth and co-authors18 were able to separate the effects of tunneling through an alkyl layer (in the form of a self-assembled monolayer) from



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14 the contact resistances at each end of the alkyl chain. They found Ralkyl = (1.9 ± 0.3 ) × 10 −8 Ωm 2 and β = 1.03 ± 0.04 nC−1 , where nC is the number of carbons in the alkyl chain. The latter translates to β = 8.2 ± 0.3nm −1 , assuming the standard geometry for carbon-carbon bonds in the alkyl chains. We assume that, in our bilayer network samples, the conduction is dominated by the bottom layer, which is in direct contact with the electrodes. For the sample used for figure 9. this layer has a coverage of 76% (as estimated from AFM images). Within the covered regions, we assume the nanofilaments are close-packed, with a center-to-center spacing of 4.1 nm, as suggested by the center-to-center spacing of the individual molecules when they deposit as a monolayer on graphite12. Since the nanofilaments are deposited with random orientations, we assume for simplicity that each one crosses the electrode fingers at an angle of 45o. We further assume that each nanofilament crosses the full width (580 nm) of the electrode fingers, giving one of the two dimensions needed to calculate the cross-sectional tunneling area A. We set the other dimension equal to the edge-to-edge span of the central porphyrin macrocycle, 1.08 nm. To approximate the tunneling distance, we assume that the disordered alkyl layer directly contacts the central macrocycle. Given the total thickness of 3.6 nm for the nanofilaments, and assuming the alkyl layer has the same thickness above and below the central core, this gives 3.6 nm − 1.08 nm d= = 1.26 nm . Plugging in these numbers gives a tunneling resistance for the 2 sample of R = 12 GΩ. However, this must be doubled because the electrons must tunnel from the electrode into the nanofilament on one end, and then out of the nanofilament into the electrode at the other end. Finally, the nanofilaments have no covalent connection to the electrodes. Engelkes and co-authors19, using a Landauer approach to the electron transfer process, measured that a noncovalent contact between a gold surface and a methyl group multiplies the tunneling resistance by a factor of between 150 and 450, the uncertainty arising from the estimate of the number of molecules in their tunnel junction. As an approximation, we use the geometric mean of these numbers. Multiplying this by the combined tunneling resistance from both ends yields an estimated resistance of 4.4 TΩ. The highest current we have measured (the peak shown in fig. 11) is 3.6 pA, with a bias voltage of 1 V, an O2 level of 10%, and illumination at 404 nm and 950 W/m2. Dividing the voltage by the current yields a resistance of 0.28 TΩ. We made several assumptions and approximations in estimating the theoretical resistance, including modeling what is likely a dense layer of alkyl chains by comparison with a well-ordered self-assembled monolayer. Furthermore, tunneling resistances are notoriously irreproducible from one situation to another, with orders of magnitude disparity between similar experiments. Therefore, it is not at all surprising that our theoretical estimate for the resistance is 15 times higher than our measured value. For example, if we assumed a tunneling distance of 0.92 nm rather than 1.26 nm, we would get an exact match. This could occur for example if the alkyl layer were uneven between the top and the bottom of the nanofilaments, with a thicker layer on the top side of the porphyrin cores and a thinner layer on the bottom side. However, again, the difference could come from several other sources. The calculation suggests the possibility that charge conduction through the porphyrin core of the nanofilaments may be relatively facile, since we can approximately account for all the


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15 observed resistance by the resistances of the contacts. Conclusions Nanofilaments self-assembled from the porphyrin CP1 display photoconductivity, but at a level 2000 times lower than nanorods self-assembled from TPPS4. In our model, this is due to a higher Schottky barrier (in the absence of atmospheric O2) and to the insulating effects of the long alkane chains (at higher levels of O2); computer modeling suggests that these lie on the outside of the nanofilaments. Approximate calculations of the resistance associated with the insulating layer created by the alkyl chains suggests that conduction through the porphyin core of the nanofilaments may be relatively facile. For applications, it would likely be necessary to “strip the ends” of the nanofilaments by removing the alkyl chains at the ends, thus permitting better electrical contact. With the alkyl layers intact, the photoconductivity displayed by this system is too low for practical applications, but comparison with related systems yields useful insights. The wavelength dependence of the photoconductivity qualitatively matches the absorption spectrum of the gel, but with a significant red shift of the peak. This may be due to a small change in structure of the nanofilament cores when the nanofilaments are deposited on substrates. The photoconductivity increases when environmental O2 is added; this is opposite the behavior of TPPS4 nanorods. Most of the behaviors observed are consistent with the model proposed by Riley and co-workers, in which the conductivity is dominated by Schottky barriers, whose height is determined by the amount of adsorbed O2. In this case, the Fermi level of the nanofilaments must initially be above that of the electrodes, leading to a band bending that is opposite that of TPPS4. The H-aggregation displayed by CP1 shifts the energy levels higher, and this may cause the difference in band line-up comparted to TPPS4, which displays J-aggregation. It would be interesting to examine other J- and H-aggregating porphyrin systems to determine if this pattern always holds. The sensitivity of the Schottky barrier height to atmospheric O2 can be determined via measurements of the photoconductivity as a function of O2 concentration. We find that CP1 nanofilaments have Schottky barrier heights that are about nine times less sensitive than those for TPPS4 nanorods; this difference may be due to the insulating effects of the alkyl chains on the CP1. We observe that conductivity decreases with duration of illumination for the lowest O2 levels, as predicted by the model, but surprisingly increases with duration of illumination for higher O2 levels. This effect may arise from photoactivation of shallow O2 adsorption sites on the nanomaterial, possibly associated with its unique structure. The model predicts that atmospheric H2 should produce the opposite effects from atmospheric O2, since O2 is generally electron withdrawing, while H2 is generally electron donating. The model suggests the central importance of Schottky barriers at the electrodes. This could be tested by varying the electrode material, especially by the use of organic electrodes such as reduced graphene oxide. 20,21 The sample shows a “memory” of previous exposure to O2, with elevated levels of photoconductivity for at least 4 hours after exposure, especially when the exposure was prolonged. This suggests that, in a prolonged exposure, O2 can diffuse to interior adsorption sites.



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16 Prolonged exposure to O2 and illumination causes a permanent decrease of the photoconductivity. This is presumably due to the well-known mechanism in which a photoexcited porphyrin molecule interacts with an O2 molecule to create singlet O2, which then attacks the nanofilaments. This presents a challenge for any porphyrin-based nanodevice, though one that can be avoided, e.g. by operation in an O2-free environment. The experiments reported here do not attempt to probe any influence of the stereogenic center present in CP1 on the photoconductivity. In a future study of chiral porphyrins, it would be interesting to study the effects of circularly polarized illumination, as well as to compare the properties of nanofilaments formed from each of the two enantiomers. Acknowledgements This work was supported by National Science Foundation grants CHE-0616615 and BMAT- 1306170 (Haverford College), by the Nano/Bio Interface Center at the University of Pennsylvania through the National Science Foundation NSEC DMR08-32802 (University of Pennsylvania), by MINECO (Spain) Projects CTQ2010-16339 and MAT2013-47869-C4 (CSIC). Z.J.Q. acknowledges support from the IBM Ph.D. Fellowship and the NSF IGERT program (Grant DGE02-21664). M.L. acknowledges SeRC (Swedish e-Science Research Center) for funding and SNIC (Swedish National Infrastructure for Computing) for providing computer resources. Author information * Corresponding author: [email protected] ◊ Current address: School of Chemistry, University of Nottingham, University Park NG7 2RD England References 1- Balaban, T. S. Tailoring porphyrins and chlorins for self-assembly in biomimetic artificial antenna systems, Acc. Chem. Res. 2005, 38, 612-623. 2- Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H, . About supramolecular assemblies of pi-conjugated systems, Chem. Rev. 2005, 10, 1491–1546. 3- Elemans, J. A. A. W.; van Hameren, R., Nolte, R. J. M.; Rowan, A. E., Molecular materials by self-assembly of porphyrins, Pthalocyanines, and Perylenes, Adv. Mater. 2006, 18, 1251-1266. 4- Drain, C. M.; Varotto, A.; Radivojevic, I., Self-organized porphyrinic materials, Chem. Rev. 2009, 5, 1630-1658. 5- Schwab, A. D.; Smith, D. E.; Rich, C. S.; Young, E. R.; Smith, W. F.; de Paula, J. C., Porphyrin nanorods. Journal of Physical Chemistry B 2003, 107, 11339-11345. 6- Schwab, A. D.; Smith, D. E.; Bond-Watts, B.; Johnston, D. E.; Hone, J.; Johnson, A. T.; de Paula, J. C.; Smith, W. F., Photoconductivity of self-assembled porphyrin nanorods. Nano Lett. 2004, 4, 1261-1265. 7- Riley, C. K.; Muller, E. A.; Feldman, B. E.; Cross, C. M.; Van Aken, K. L.; Johnston, D. E.; Lu, Y.; Johnson, A. T.; de Paula, J. C.; Smith, W. F., Effects of O(2), Xe, and gating on the photoconductivity and persistent photoconductivity of porphyrin nanorods. Journal of Physical Chemistry C 2010, 114, 19227-19233.


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17 8- Otsuka, Y.; Naitoh, Y.; Matsumoto, T.; Mizutani, W.; Tabata, H.; Kawai, T., A simple fabrication method of nanogap electrodes for top-contacted geometry: application to porphyrin nanorods and a DNA network, Nanotechnology 2004, 15, 1639–1644. 9- Yeats, A. L.; Schwab, A. D.; Massare, M.; Johnston, D. E.; Johnson, A. T.; de Paula, J. C.; Smith, W. F., Photoconductivity of self-assembled nanotapes made from meso-tri(4sulfonatophenyl)monophenylporphine, J. Phys. Chem. C 2008, 112, 2170-2176. 10- Guo, P.; Zhao, G.; Chen, P.; Lei, B.; Jiang, L.; Zhang, H.; Hu, W.; Liu, M., Porphyrin nanoassemblies via surfactant-assisted assembly and single nanofiber nanoelectronic sensors for high-performance H2O2 vapor sensing, ACS Nano 2014, 8, 3402-3411. 11- Zhang, C.; Chen, P.; Dong, H.; Zhen, Y.; Liu, M.; Hu, W., Porphyrin supramolecular 1D structures via surfactant-assisted self-assembly, Adv. Mater. 2015, 27, 5379–5387. 12- Iavicoli, P.; Xu, H.; Feldborg,L. N.; Linares, M.; Paradinas, M.; Stafström, S.; Ocal,C.; NietoOrtega, B.; Casado, J.; López Navarrete, J. T.; et al., Tuning the supramolecular chirality of oneand two-dimensional aggregates with the number of stereogenic centers in the component porphyrins, J. Am. Chem. Soc. 2010, 132, 9350–9362. 13- Parkash, J.; Robblee, J. J.; Agnew, J.; Collings, P.; Gibbs, E.; Pasternack, R. F.; de Paula, J. C.. Depolarized resonance light scattering by porphyrin and chlorophyll a aggregates, Biophys. J. 1998, 74, 2089-2099. 14- http://dasher.wustl.edu/tinker/ 15- Allinger, N. H.; Yuh, Y. H.; Lii, J. H., Molecular mechanics, The MM3 force field for hydrocarbons. 1, J. Am. Chem. Soc. 1989, 111, 8551–8556. 16- Many of the results of these calculations were presented in reference 12, but these did not include the image presented here. 17- Wilkinson, F.; Helman, W. P., and Ross A. B., Quantum yields for the photosensitized formation of the lowest electronically excited singlet-state of molecular-oxygen in solution, J. Phys. Chem. Ref. Data, 1993, 22, 113-262, and references therein. 18- Sangeeth, C. S.; Wan, A.; Nijhuis, C. A., Equivalent circuits of a self-assembled monolayerbased tunnel junction determined by impedance spectroscopy, J. Am. Chem. Soc. 2014, 136, 11134-11144. 19- Englekes, V. B.; Beebe, J. M.; Frisbie, C. D., Length-dependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols: effect of metal work function and applied bias on tunneling efficiency and contact resistance, J. Am. Chem. Soc. 2004, 126, 14287-14296. 20- Gomez-Navarro, C.; Weitz, R.T.; Bittner, A.M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K., Electronic transport properties of individual chemically reduced graphene oxide sheets, Nano Lett. 2007, 7, 3499-3503. 21- Eda, G; Lin, Y.-Y.; Miller, S.; Chen, C.-W.; Su, W.-F.; Chhowalla, M., Transparent and conducting electrodes for organic electronics from reduced graphene oxide, Appl. Phys. Lett. 2008, 92, 233305.



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Photoconductivity of Nanofilaments That are Self-Assembled from a

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