Persistent Conductivity in TPyP:TSPP Organic Nanorods Induced by

Jun 30, 2016 - Persistent conductivity is observed following Ar+ bombardment of meso-tetra(4-pyridyl)porphyrin:meso-tetra(4-sulfonatophenyl)porphyrin ...
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Persistent Conductivity in TPyP:TSPP Organic Nanorods Induced by Ion Bombardment Marshall van Zijll, Bryan Borders, Ursula Mazur, and K. W. Hipps* Department of Chemistry and Materials Science and Engineering Program, Washington State University, Pullman, Washington 99164-4630, United States S Supporting Information *

ABSTRACT: Persistent conductivity is observed following Ar+ bombardment of meso-tetra(4-pyridyl)porphyrin:mesotetra(4-sulfonatophenyl)porphyrin (TPyP:TSPP) nanorods. The lifetime of the persistent conductivity in ultrahigh vacuum (UHV) is exceptionally long at room temperature, between 106 and 107 s. Ion beam currents can be used to both increase and decrease the level of persistent conductivity in these nanorods. Initial Ar+ bombardment of a sample causes an increase in the persistent current. Subsequent bombardment with low-energy Ar+ can cause a rapid decrease in the persistent current. A model is presented which presumes that persistent conductivity is carried by metastable defects with rates of excitation and relaxation following the Arrhenius relationship. Energy conservation suggests that ion bombardment introduces a thermal gradient across the nanorod which quickly quenches when ion bombardment ceases. This quick quenching results in a population of metastable defects which decay very slowly at room temperature.

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INTRODUCTION Meso-tetra(4-pyridyl)porphyrin:meso-tetra(4-sulfonatophenyl)porphyrin (TPyP:TSPP) nanorods exhibit an increase in conductivity during Ar+ bombardment and exhibit persistence in conductivity following Ar+ bombardment in UHV. Persistent conductivity differs from nonpersistent conductivity in that the conductivity does not cease when ion bombardment stops but slowly decays over time. Persistence is also observed following photoillumination of these nanorods.1 Previous studies have developed a working understanding of structure−property relationships of binary porphyrin nanorods with a goal to design and build high performance, stable, and durable molecular devices.2−6 They have also probed the nature and mechanism of nanocrystal formation and allowed the development for a predictive model for size relative to growth conditions.7 More recent studies focusing on the persistent photoconductivity of TPyP:TSPP nanorods are currently being prepared for publication.1 To our knowledge, persistent ioninduced conductivity (PIC) has not yet been studied in organic materials, and there is one inorganic study of PIC from ZnO nanostructures.8 The phenomenon of persistence has been discussed most often as a result of photoillumination and is often referred to as persistent photoconductivity (PPC).9−15 The previous study of PIC using ZnO nanostructures predicted that PIC would occur in all materials that exhibit PPC, as the energy brought into the material by ions can activate metastable states in a manner similar to photons.8 The experiments presented here using TPyP:TSPP nanorods © XXXX American Chemical Society

support this prediction. The long lifetimes associated with the decay in persistent conductivity of these nanorods are particularly interesting. The data obtained using ion bombardment have given us insight into the formation and relaxation of the defects responsible for conductivity, and a model consistent with our observations has been developed. In this paper, ion bombardment is presented as an additional means to study the formation and relaxation of electronic states as well as a means of inducing and controlling the level of persistent conductivity in TPyP:TSPP nanorods.

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EXPERIMENTAL PROCEDURE

Ion bombardment was performed using an OCI Model IPS3-D ion source controller and ion gun. Experiments were performed in an RHK UHV 3000 system with a base pressure of 1 × 10−10 Torr. Residual gas analysis verified the chamber was free of leaks and that the Ar line was uncontaminated. A Bruker Multimode 8 atomic force microscope (AFM) was used in ambient conditions in peak-force tapping mode to monitor the physical state of the rods before and after ion-bombardments. Bias voltages were applied using a Data Translation DT9812 data acquisition system (DAQ). The current was measured using a Keithley 427 current amplifier and recorded by the Received: May 9, 2016 Revised: June 21, 2016

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The dimensions of the nanorods, as measured in both AFM and scanning tunneling microscopy (STM), typically range from 50 to 150 nm in height, and 100−500 nm width, with lengths around 10 μm. New samples were typically annealed to 60 °C for 5−8 h to encourage water desorption. Ion bombardment was performed in a direction normal to the surface using Ar+ with energies ranging from 0.1 to 2 keV and Ar partial pressures ranging from 1.5 × 10−7 to 5 × 10−6 Torr. An electrochemically formed oxide layer on Ta (Ta2O5) was used as a sputter calibration standard to calculate the ion flux.16 At 500 eV and 1.5 × 10−6 Torr (our most common ion bombardment parameters) the Ar+ beam flux was measured to be 0.6 μA/cm2. Exposure times ranged from 100s to 1000s of seconds. With the exception of IV curves, a 2.0 V bias was applied across the sample during current measurement.

DT9812 DAQ. Data were analyzed and plotted using OriginPro (OriginLab, Northampton, MA). TPyP:TSPP nanorods were formed by binary self-assembly, and the procedure for their preparation is described in the Supporting Information for another work.2 Drops of nanorod solution were deposited onto an interdigitated electrode (IDE) and allowed to evaporate, leaving a deposition of randomly oriented nanorods. The IDE, purchased from the UC Santa Barbara Nanofabrication Facility, consists of a 20 nm thick layer of Au deposited onto a glass slide, with interdigitated rows separated by 500 nm gaps. The interdigitated Au rows are seen in Figure 1a as wide strips on the substrate. Nanorods which lay

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RESULTS AND DISCUSSION Surface Study Following Ion Bombardment. Samples were observed in AFM before and after different lengths of ion bombardment (Figure 1). No significant changes to the rods were observed in AFM until about 3000 s of ion bombardment. Degradation of some rods was slightly apparent, but these differences were generally small enough that they could be attributed to a change in the AFM tip. It was rare to find a rod that was completely removed from the surface. STM images of sputtered nanorods make it clear that some material is being sputtered from the nanorods, and these images are included in the Supporting Information. Conductivity Experiments Using Ion Bombardment. For samples where the nanorods have negligible initial current, bombardment causes an increase in current followed by a very slow decay after bombardment ends (Figure 2). The gradual decay of the current following ion bombardment is persistent conductivity.

Figure 1. AFM images of TPyP:TSPP nanorods deposited onto an IDE: (a) before and (b) after 300 s of ion bombardment with 1 keV and 5 × 10−6 Torr of Ar+. (c) and (d) are line profiles taken along the similar lines on (a) and (b), respectively.

across multiple rows carry the current when a bias is applied. Depending on the concentration of rods in the drop of solution, 100s−1000s of rods were deposited onto the interdigitated section, a good portion of which appeared to bridge the 500 nm gap between rows. Other samples were prepared by scraping away a section of gold on the IDE, leaving a 1 mm gap between the anode and cathode. Drops of nanorod solution were allowed to evaporate over this gap, forming an ∼1 μm thick layer of nanorods. For this sample, measurable conduction required current to travel from rod to rod. After bombardment, samples prepared in this manner displayed ohmic I−V curves, while samples prepared with a typical IDE showed evidence of Schottky barrier formation. Apart from this difference, the data obtained using either substrate displayed similar trends. Figures 2, 3, 4, and 6 show data collected on an unmodified IDE, while the data in Figure 5 were collected using a substrate with a 1 mm gap between electrodes. The Supporting Information contains additional data collected using both types of substrates.

Figure 2. Current measured during (t = 0 to 300 s), and following, ion bombardment.

Decays in persistent current following ion bombardment can be fit using a stretched exponential of the form9,10,12,17,18 ⎛ ⎛ t ⎞β⎞ I = I0 exp⎜ −⎜ ⎟ ⎟ ⎝ ⎝τ⎠ ⎠

(0 < β ≤ 1) (1)

where I0 is the maximum persistent current, τ is the average lifetime of the decaying states, and β is the dispersion parameter sometimes related to the distribution of states. Values for τ ranged from about 500 to 3000 s. Values for β were 0.12 ± 0.04. B

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for ΔI/t increased in magnitude with additional ion bombardment exposures. Figure 4c shows 22 normalized decay curves from this experiment. The decay curves overlap more precisely with more ion bombardment exposure. The consistency in the decay curves suggests consistency in the mechanisms underlying the persistent conductivity. Figure 5a shows an experiment including six ion bombardments performed on a system initially heavily defective (due to

Figures 3a and 3b show graphs of decay in persistent current following ion bombardment for two different samples, with

Figure 3. Graphs of decay in persistent current The thin line corresponds to the best fit stretched exponential. The fitting parameters are (a) β = 0.12, τ = 1000 s and (b) β = 0.106, τ = 2500 s.

best-fit curves shown in gray. Figure 3a was continuously recorded for 2.5 days. Figure 3b was not continuously recorded but includes data from current measurements taken at 21 and 42 days following ion bombardment; this sample remained undisturbed in UHV between measurements. After 42 days of decaying, the β and τ values for the best fit fell within the range of fits for the other experiments. For low-current experiments (≤1 μA), the current during ion bombardment could not be directly measured. This was due to nonpersistent components which added an offset of about 1 μA during ion bombardment. To measure the increase in persistence at low current, data were collected during repeated ion bombardment exposures performed on a single sample. Between each exposure, the persistent current was allowed to decay for some time. Figure 4a shows three decay graphs from one such experiment. The increasing magnitudes of the decay curves show that each additional ion bombardment exposure increases the persistent current. To determine the effect of a particular ion bombardment, best fit curves were determined for decay graphs taken before and after each exposure (shown as gray lines in Figure 4a). The best fit curves were extrapolated to overlap in time, and the difference in the extrapolated best fit curves at the time bombardment ceased is shown as ΔI in Figure 4a. This is a measure of the increase in persistent current caused by that particular ion bombardment exposure. The values of increase were normalized to show ΔI/t as a function of the total time of exposure to ion bombardment in Figure 4b. Successive values

Figure 5. Current measured during a sequence of ion bombardment exposures followed by decays in persistent current. The measurements during ion bombardment are shown in black, and the decaying persistence curves are gray. The parameters associated with each ion bombardment exposure are (a, c) 1 keV, 5 × 10−6 Torr, (e) 2 keV, 5 × 10−6 Torr, and (b, d, f) 0.2 keV, 1.5 × 10−7 Torr.

prior bombardment) and with alternating levels of bombardment energy. The ion bombardment exposures labeled a, c, and e all cause an increase in persistent current. The ion bombardment exposures labeled b, d, and f all cause the persistent current to decrease: the current falls below that predicted prior to the low dose exposure. The magnitude of the current during bombardment increases with an increase in either the ion energy or flux of the incident ions at the same energy. This is shown by the various conditions listed in Figure 5 that result in various levels of steady state current during ion bombardment. It is also shown in the Supporting Information. These steady states likely represent equilibrium between the formation and relaxation of conductive states. Figure 6 shows an extreme case of the phenomenon shown in Figure 5, with three ion bombardments shown in black and three decays shown in gray. The first ion bombardment (Figure 6a) reached a maximum persistent current of 20.5 μA. Next, a low energy/flux ion bombardment was performed for 2.5 h (Figure 6b). The current dropped to a value of 0.11 μA following this exposure. It would have taken 200 000 days for

Figure 4. (a) Segment of data showing three sequential decays in persistent current. The current during ion bombardment is out of range. Best fits curves are shown as thin lines, with parameters β = 0.12 and τ = 500 s. ΔI indicates the increase in persistent current caused by one ion bombardment exposure. (b) ΔI/t for many sequential ion bombardment exposures, converted to show the average increase per second vs total time of ion bombardment. (c) 22 normalized decays from repeated ion bombardment exposures. The darker decays follow more ion bombardment. C

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Figure 6. Current measured during a sequence of ion bombardments followed by decays in persistent current. The measurements during ion bombardment are shown in black, and the decaying persistence curves are gray. The parameters associated with each ion bombardment exposure are (a, c) 2 keV, 5 × 10−6 Torr and (b) 0.2 keV, 1.5 × 10−7 Torr. Compare to Figure 9.

the current to naturally decay to this value following the initial bombardment Finally, the sample was bombarded with a high energy/flux to show integrity of the sample. This experiment shows that the increase in conductivity induced by ion bombardment is not a permanent effect, as at least 99.4% of the current was controllably relaxed! The inset in Figure 6 shows the normalized decay curves; the decay curve following the decrease in persistence is associated with a decrease in the proportion of short-lived states. This was a repeatable trend. The decay following the final ion bombardment has a decay curve similar to the first ion bombardment. Model and Calculations. The energies we used for ion bombardment (0.1−2 keV) are small and are often used to sputter a surface clean. In comparison, the study of PIC on ZnO nanorods used energies ranging from 50 to 150 keV, but with much lower flux. They observed distinguishable effects due to the impacts of single ions.8 Qualitative analysis suggests that the formation of conductivity in the nanorods relies primarily on the average rate of incident energy rather than the specific energy or flux of ion bombardment. This is supported by the observation that changing either the flux or the energy of the ions has a similar effect on changing the steady state current during ion bombardment and similarly on changing the level of persistent conductivity. Calculations performed using the Stopping Range of Ions in Matter (SRIM) software predict a shallow penetration depth of ions into the nanorods of about 35 ± 10 Å. Defects formed within this thin layer do not necessarily account for the entirety of the measured current. It seems unlikely that the ions cause defects simply by collision with the atoms in the nanowires; this process would not explain the quick relaxation of defects by using lower energy bombardment. We postulate that ion impacts can be treated as phonon sources, with each impact resulting in a distribution of phonons. We further postulate that these phonons quickly equilibrate at a new local temperature. In the local heating produced by the ion beam at the surface of the nanorods, a temperature gradient is formed between the surface of the nanorods and the substrate. Considering energy conservation W dE in dT dT = er + Cv +k dt dt dt dz

Figure 7. Schematic describing the ion-induced conductivity in the TPyP:TSPP nanorods. (a) Before ion bombardment there are too few defects present to allow for hopping conductivity. (b) After ion bombardment, the defect concentration has increased, thereby decreasing the defect−defect distance which increases the hopping rate and the hopping mobility. (c) When ion bombardment stops, defects begin to relax, decreasing the defect concentration and thereby causing the hopping rate and hopping mobility to decrease.

where Ein is the incident energy due to ion bombardment, Wer is the work due to the formation and relaxation of metastable states, Cv is the specific heat of the nanorods, k is the thermal conductivity of the nanorods, and z is in the direction perpendicular to the substrate. Metastable defects associated with conductivity are suspected to be formed randomly within a spatial region at a given temperature. These metastable defects support current through a defect−defect hopping scheme. Let us assume that the rates of excitation and relaxation, Ke and Kr, for these metastable defects may be described by Arrhenius relationships: ⎛ E ⎞ Ke(T ) = ke exp⎜ − e ⎟ , ⎝ k bT ⎠

⎛ E ⎞ K r(T ) = k r exp⎜ − r ⎟ ⎝ k bT ⎠ (3)

where ke and kr are pre-exponential factors and Ee and Er are the energy barriers for exciting and relaxing metastable sites, and T is a function of time. The low proportion of excited states at

(2) D

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The Journal of Physical Chemistry C room temperature (lack of initial conductivity) suggests that Ee > Er. In our model, ion bombardment will act to temporarily increase the temperature of the nanorods, thus increasing the proportion of excited states. As the temperature increases, the ratio Ke(T)/Kr(T) becomes larger (Ee > Er), resulting in a larger proportion of excited states. The first term in eq 2 can be written as Wer d θ (t ) = N (Ee − Er) dt dt

(4) Figure 8. Results from a numerical calculation of two 200 s ion bombardments performed with dEin/dt =1 × 1019 eV/s each followed by 600 s decays with no incident energy. The current (black); temperature (gray).

where N is the number of sites that are able to be either excited or relaxed and θ(t) is the fraction of sites that are excited. The first term in this equation is related to eq 3 as d θ (t ) = (1 − θ(t ))Ke(T ) − θ(t )K r(T ) dt

the surface of the nanorods increases. The increase in current is not significant until a temperature of about 380 K has been reached. The simulated increase in current qualitatively matches experimental observations during ion bombardment of a new sample (Figure 2). When ion bombardment ceases at 200 s in Figure 8, the temperature quickly drops below 380 K. The quick drop in temperature at the moment dEin/dt = 0 is facilitated by a nearby substrate which was measured to be near room temperature throughout these experiments. This quenching of temperature causes a number of sites to become “stuck” in an excited state and is the origin of the persistent conductivity. In Figure 8, the second bombardment results in additional increase of persistent conductivity. As already shown with Figures 5 and 6, low-energy ion bombardment caused quick relaxation of the persistent conductivity. Figure 9 shows results from a numerical

(5)

Because of the complicated dependence of these terms on the temperature, these are best solved by numerical calculations. A value for Er was estimated by determining the lifetimes, τ, associated with the decay curves, while assuming a singleexponential decay and also omitting the first 24 h of data in order to guarantee a constant-temperature segment. The lifetimes for these decays were between 106 and 107 s. Using the relationship Kr = 1/τ, eq 3 provides a value for Er of about 1.10 ± 0.3 eV. The high value of Er is the reason for the slow decay of persistence. An estimate for Ee will be provided shortly. The pre-exponential factors ke and kr are likely on the order of typical molecular vibrational frequencies, about 1013 Hz. The heat capacity and thermal conductivity were estimated to be 7.89 × 1025 eV/(m3 K) and 2 × 1018 eV/(m s K) using published estimates for organic materials.19,20 The incident energy was set to values between 5 × 1018 and 1 × 1020 eV/s, on par with the ion beam current. The electron mobility was estimated using the generalized Einstein equation:21,22 μe =

ea(t )2 vij(t ) k bT

,

a(t ) = (ρN θ(t ))−1/3

(6)

where a(t) is the average distance between defects and ρN is density of TPyP:TSPP dimers. The hopping frequency, vij(t), between neighboring sites i and j can be approximated with the Miller−Abrahams model:22,23 ⎛ a(t ) dE ⎞ − vij = v0 exp⎜ −1 ⎟ b k bT ⎠ ⎝

Figure 9. Results from a numerical calculation. The black curves are during simulated sputtering and the gray curves are when the sputtering is off. The calculation was performed with dEin/dt = 2 × 1019 eV/s during the first 100 s, dEin/dt = 5 × 1018 eV/s from 800 to 900 s, and dEin/dt = 0 eV/s otherwise.

(7) 13

where v0 is the attempt frequency, estimated to be 10 Hz, b is the charge carrier localization radius estimated to be 1 × 10−9 m, and dE is the energy difference between sites i and j estimated to be 0. The value of the excitation barrier for defects, Ee = 1.30 ± 0.03 eV, was chosen to make the calculated persistent currents the same order of magnitude as experimental results. The incident energy was distributed throughout a volume 10 nm deep into the rod (this is the order of the penetration depth estimated using SRIM24). The sample was assumed to be a 1 μm thick deposition of rods with an area of 1 mm × 200 μm and in contact with a 2 mm thick glass substrate (this mimics some experimental samples). A 2 V bias was simulated across the length of 1 mm. These assumptions and the values of Cv and k were then used to solve eqs 2−7 self-consistently. Results of such a numerical calculation are shown in Figure 8. During the simulated ion bombardments, the temperature near

calculation which naturally reproduces ion induced relaxation. The sections labeled a and c were bombarded using dEin/dt = 2 × 1019 eV/s and section b using dEin/dt = 5 × 1018 eV/s. The bombardment performed with lower energy resulted in a decrease of current rather than an increase! The magnitude of the temperature during bombardment b is less than that reached during a or c, but this low increase in temperature allows the “stuck” excited sites to relax more quickly. Loss of defects due to the second exposure results in a lower persistent current. The inset in Figure 9 shows the normalized decays of persistent current. The trends of this calculation follow the same trends obtained experimentally and shown in Figure 6. The calculated decay curves could be fit using a stretched exponential function with low values for β (between 0.1 and 0.2) in good agreement with the experimental values of 0.12 ± E

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0.04. The values for β are often interpreted to describe the distribution of lifetimes within the decay.9 These numerical calculations provide an intuitive basis for these lifetimes which are determined by Arrhenius rate equations and the temperature. Quenching was not observed in calculations using values of Cv and k that produce a slowly dropping temperature. If the temperature was allowed to slowly relax, the current measurements showed no evidence of quenching, and the decay curves were well-fit using a single-exponential decay. These calculations fall short when considering long exposures to incident energy. The calculated currents are 1−2 orders of magnitude too high, but they quickly fall to realistic levels following exposure. This may be because the calculations do not take into account the loss of material through sputtering or structural damage done by ion bombardment. Additionally, the numerical calculation assumes perfect contacts to the nanorods at their cross section. In experiment the current must travel from the gold electrodes through the perimeter of the nanorods and also from nanorod to nanorod.

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (K.W.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the US National Science Foundation for their support in the form of grants CHE-1152951 and CHE1403989.

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REFERENCES

(1) Borders, B.; Rosenkrantz, N.; van Zijll, M.; Eskelsen, J. R.; Hipps, K. W.; Mazur, U. Private communication. (2) Eskelsen, J. R.; Qi, Y.; Schneider-Pollack, S.; Schmitt, S.; Hipps, K. W.; Mazur, U. Correlating Elastic Properties and Molecular Organization of an Ionic Organic Nanostructure. Nanoscale 2014, 6, 316−327. (3) Shelnutt, J.; Medforth, C. J. Self-Assembled Porphyrin Nanostructures and their Potential Applications. In Organic Nanomaterials: Synthesis, Characterization, and Device Applications; Bottari, G., Torres, T., Eds.; Wiley: 2013; Chapter 5, pp 103−130. (4) Tian, Y.; Beavers, C.; Busani, T.; Martin, K.; Jacobsen, J.; Mercado, B.; Swartzentruber, B.; van Swol, F.; Medforth, C. J.; Shelnutt, J. Binary Ionic Porphyrin Nanosheets: Electronic and LightHarvesting Properties Regulated by Crystal Structure. Nanoscale 2012, 4, 1695−1700. (5) Friesen, B. A.; Wiggins, B.; McHale, J.; Mazur, U.; Hipps, K. W. Differing HOMO and LUMO Mediated Conduction in a Porphyrin Nanorod. J. Am. Chem. Soc. 2010, 132, 8554−8556. (6) Mazur, U.; Hipps, K. W.; Eskelsen, J.; Adinehnia, M. Functional Porphyrin Nanostructures for Molecular Electronics: Structural, Mechanical, and Electronic Properties of Self-Assembled Ionic Metal-Free Porphyrins. In Handbook of Porphyrin Science; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World Scientific: 2016; Vol. 40. (7) Adinehnia, M.; Mazur, U.; Hipps, K. W. Predicting the Size Distribution in Crystallization of TSPP: TMPyP Binary Porphyrin Nanostructures in a Batch Desupersaturation Experiment. Cryst. Growth Des. 2014, 14, 6599−6606. (8) Johannes, A.; Niepelt, R.; Gnauck, M.; Ronning, C. Persistent Ion Beam Induced Conductivity in Zinc Oxide Nanowires. Appl. Phys. Lett. 2011, 99, 252105. (9) Klafter, J.; Shlesinger, M. F. On the Relationship among Three Theories of Relaxation in Disordered Systems. Proc. Natl. Acad. Sci. U. S. A. 1986, 83, 848−851. (10) Lee, C. H.; Yu, G.; Heeger, A. J. Persistent Photoconductivity in Poly(p-phenylenevinylene): Spectral Response and Slow Relaxation. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 15543−15553. (11) Riley, C’; Muller, E.; Feldman, B.; Cross, C.; Van Aken, K.; Johnston, D.; Lu, Y.; Johnson, A.; de Paula, J.; Smith, W. F. Effects of O2, Xe, and Gating on the Photoconductivity and Persistent Photoconductivity of Porphyrin Nanorods. J. Phys. Chem. C 2010, 114, 19227−19233. (12) Santos, L.; Faria, R.; Del Cano, T.; de Saja, J.; Constantino, C.; Amorim, C.; Mergulhao, S. Observation of Persistent Photoconductivity in Vanadyl Phthalocyanine. J. Phys. D: Appl. Phys. 2008, 41, 125107−125114. (13) Hamed, A. J. Persistent Photoconductance in DopingModulated and Compensated α-Si:H. Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 44, 5585. (14) Frello, T.; Veje, E.; Leistiko, O. Observation of Time-Varying Photoconductivity and Persistent Photoconductivity in Porous Silicon. J. Appl. Phys. 1996, 79, 1027−1031. (15) Polyakov, A.; Smirnov, N.; Govorkov, A.; Belogorokhov, A.; Kozhukhova, E.; Markov, A.; Osinsky, A.; Dong, J.; Pearton, S. J. Persistent Photoconductivity in p-type ZnO(N) Grown by Molecular Beam Epitaxy. Appl. Phys. Lett. 2007, 90, 132103−132106.

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CONCLUSIONS The very long lifetimes associated with the decay in persistent conductivity and the ability to quickly relax persistent states by using low-energy Ar+ are unique observations associated with ion bombardment of TPyP:TSPP nanorods. The conductivity data obtained using ion bombardment has given us insight into how the observed currents can be related to the formation and relaxation of defects. A model has been developed which assumes the population of metastable conductive states is controlled by an Arrhenius barrier. Numerical calculations qualitatively show that ion bombardment causes surface heating of the porphyrin nanorods, which leads to an increase in the density of conductive states. These states have the highest density near the surface and do not extend uniformly through the entirety of the rod. The long lifetimes associated with the decay in persistent conductivity are explained by the phenomenon of quenching. The relaxation in persistent conductivity caused by low-energy ion bombardment is also reproduced. Calculations suggest the low-energy ion bombardment causes a local increase in temperature which allows previously excited states to quickly relax. This model qualitatively reproduces all the observed data at low and intermediate sputtering times. Our model attributes the current to electron hopping between metastable defect sites. It was determined that the barrier to conversion from ground state molecules to the structurally defective entity was about 1.3 eV and the barrier for return to the ground state was about 1.1 eV. Thus, the proposed structural defect only lies about 0.2 eV above the ground state and could easily be a simple rotation or bond angle change in the parent molecule(s).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04689. Sample preparation details, control experiments, ion bombardment effects, STM images of bombarded nanorods, additional conductivity results, and the C++ code used in the calculation (PDF) F

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