Significant Increase in Electrical Transport of Conducting Polymers

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C: Physical Processes in Nanomaterials and Nanostructures

Significant Increase in Electrical Transport of Conducting Polymers Confined in Alumina Nanopores Sukanya Das, and Kavassery Sureswaran Narayan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01563 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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The Journal of Physical Chemistry

Significant Increase in Electrical Transport of Conducting

Polymers

Confined

in

Alumina

Nanopores Sukanya Das and Kavassery Sureswaran Narayan* Chemistry and Physics of Materials Unit and School of Advanced Materials, Jawaharlal Nehru Centre for Advanced Scientific Research, Bengaluru- 560064, Karnataka, India.

ABSTRACT

We

probe

the

electrical

transport

in

mixed

conducting

polymer

poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a function of confinement. The scaling of electronic conductance as a function of dimension is expected to be different from that of ionic conductance in a mixed conducting system. The role of geometry and dimensions of the confinement in the form of alumina nanopillars significantly increases the electrical conductivity of PEDOT:PSS measured both at single nanochannel and macroscopic level. The increase in the transverse conductivity along the axis of nanopillar is discussed in terms of a binary resistor-capacitor (RC) network. The combination of results from structural and ac transport measurements indicate rearrangement of interconnected microstructures under confinement below a characteristic length scale of ≈ 20 nm.

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INTRODUCTION Mixed conducting polymers form a certain class of materials in polymer electronics which supports both electronic and ionic charge transport. Oxidation of conjugated polymers leading to formation of radical ions accompanied by a local lattice distortion gives rise to a polaronic form of transport which have been extensively studied.1-6 It should be noted that the resultant electronic transport takes place in the background of fluctuating ionic environment. PEDOT:PSS is a mixed conducting system which is traditionally used as a model system with a wide range of applications. PEDOT:PSS consists of colloidal particles of PEDOT which is an intrinsically conducting polymer carrying positive charge and PSS which is a poly-electrolyte (ionic conducting) dopant that acts as a chargecompensating counter-ion to stabilize the p-doped conducting polymer.7-9 The electronic transport is due to quasi-electronic particles, namely polarons, bipolarons4-5 along the PEDOT backbone. Ionic conduction takes place through hydrogen bonding, ionic carrier being H+ and the small displacements of Na+ and SO3- ions. The interplay of electronic and ionic carriers in conducting polymers (CPs) has been utilized in applications like electrochromic,10 actuators,11 sensors,12 switchable surfaces,13 supercapacitors,14-15 batteries,16 interfacing electronic biomedical devices,1719

electronic plants20. The lateral electronic conductivities of PEDOT:PSS is reported to be as high

as ~ 4000 S/cm.21 The range of conductivity estimates has been obtained by different optimization processes such as, inducing morphological changes, minimizing excess dopant PSS and other methods which involve different pre and post-processing techniques and change in formulation content.21-25 It is expected that the introduction of a geometrical and dimensional constraint should affect the organization, structure and consequently the electrical transport.26 It is noted that CPs have been polymerized from their monomer in confined channels and the transport properties have been studied.26-36 However, there have been very few reports of CP-blends in nanocavities from the


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perspective of studying mixed conduction. The scaling of ionic transport and electronic transport in the mixed conducting system can be expected to reveal contrasting features, with the ionic conductivity decreasing below a characteristic length scale, while the electronic conductivity can exhibit a non-linear increase. These trends can be primarily triggered by the structural rearrangement of the polymer chains under confinement. There are various physical interpretations of transport processes and its correlation to the microstructure in PEDOT:PSS films.37-38 The underlying morphology of the dried PEDOT:PSS films is strongly dependent on the various processing routes.39 The high electrical conductivity is essentially the lateral conductivity of PEDOT:PSS blend films and highlights the anisotropy prevailing in thin films.40 In the transverse direction PEDOT rich PEDOT-PSS cores form pancake like complexes of diameter ~ 20 - 25 nm, height ~ 5 - 6 nm and are separated by thick barriers formed by the PSS lamella, thereby reducing the conductance.21 The various mechanisms for the charge transport of PEDOT:PSS thin films can be represented by 3D variable range hopping (VRH) between the PEDOT rich islands whereas the transverse transport in thin films can be represented by nearest-neighbour hopping (nn-H) process across the PSS barrier.21,41 We emphasize the possibility to control and improve this transverse conductivity of PEDOT:PSS by confining it in cylindrical nanochannels. Upon reducing dimensionality and entering into a mesoscopic transport regime, below a critical transport length-scale, the transverse conductivity is expected to decrease. However, if the molecular and macromolecular structure evolves under confinement so as to introduce electronic-order into the system, the transport property can be enhanced. Indeed, conformation of polymers and its dynamics are perturbed under confinement, especially when the confinement length scales are comparable to the size of a polymer chain (size being represented by the radius of gyration, Rg, end-



to-end distance, Ree and other characteristic length scales like Flory’s radius and Kuhn length42-46). Polymers confined below the characteristic length scales can undergo stretching and chainalignment47-52 which enhances the conduction. An important question that needs to be addressed is how the dynamics change for a two-phase system having electronic transport in an ionic background, under similar geometrical constraints. There can be multiple factors which contribute to electrical conductivity enhancement; PSS chain alignment resulting in improved doping of PEDOT, interchain and intrachain hopping, PEDOT-rich PEODT-PSS cores getting denser or increase in phase segregation. In this paper, we report the higher conductivity of PEDOT:PSS in nanochannel than in bulk and seek to understand the behavior of this system. Anodized Aluminum Oxide (AAO) are utilized as nanochannels of various pore diameter, namely 20 nm, 50 nm and 100 nm, all having an equal thickness of 200 nm. AAOs have been recently used in our laboratory for fabricating polymer based vertical transistors. Apart from a collective response from an ensemble of nanopillars (109 per cm2), vertical FET action of individual nanochannel transport has also been demonstrated.53 We use such AAO templates to study the correlated transport processes in the polymer blend. With this picture of conformational changes of polymers in confined channels, we aim to analyze the frequency response of PEDOT:PSS as a function of temperature and confinement. Applying the hopping model and mapping ac conduction to binary RC networks the conductivity is shown to increase by nearly two orders of magnitude in 20 nm channels compared to bulk PEDOT:PSS in the transverse direction. We observe that at high frequency and low temperature the high conductivity is electronic in nature and dominates over ionic conduction at the characteristic length scale of ≈ 20 nm. These studies highlight the role of geometry to control the micro and macrostructure and reflect enhanced electrical transport of polymer blends. Achieving significant high current values from 20 nm


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channels implies that there is a net gain in the overall current even after discounting the nonutilization of the alumina space between the channels (detailed calculation shown in section S2, Supporting Information). These attributes may be well suited for other applications in the sandwich geometry requiring higher vertical current density.

EXPERIMENTAL SECTION Materials. PH1000 grade of PEDOT:PSS, a blue aqueous dispersion containing 30 wt.% solid content was purchased from Ossila. PEDOT:PSS contained PEDOT to PSS ratio as 1:2.5 (highest conducting grade PEDOT:PSS available) with work function 4.8 - 5.0 eV. The 3D nanopatterned templates were obtained from Top Membranes Technologies. Sample Preparation. The sample substrates consisted of 1.5*2 cm2 ITO coated glass slides with a specific sheet resistance of 7 Ω/□ and were etched in a shape as shown in Figure S3, Supporting Information using concentrated HCl and Zn powder. These substrates were cleaned in ultrasonic baths of acetone and isopropyl alcohol for 10 min each and rinsed with de-ionized water. The substrates were made hydrophilic using RCA treatment that involved cleaning in ammonia: hydrogen peroxide: water (1:1:5 mixture) at 100 °C till all the bubbles ceased to emerge from the samples. Substrates were again rinsed in deionized water and dried under ultra-high pure nitrogen flow followed by plasma cleaning. The AAO membranes were then transferred to the clean and hydrophilic ITO glass slides using acetone bath to remove the underlying PMMA base of AAO. The membranes adhered to the ITO surface. The cleaned hydrophilic substrates were dip-coated in PEDOT:PSS dispersion for 6 hrs. The hydrophilicity of the surface and dip coating for long 5 ACS Paragon Plus Environment


enough duration ensured easy filling of the polymer into the nanochannels via capillary forces. Exposing the inlets of the porous membrane to an aqueous environment for long hours removed any possibility of trapped air inside nanochannels. First, trapped air gets highly compressed when the pressure inside the bubble becomes sufficiently high, the trapped air gets dissolved with polymer dispersion resulting in the disappearance of the air bubble.54-55 The as-coated PEDOT:PSS samples were then annealed at 140 °C for 6 hrs. under atmospheric air and successively again for 1 hr. in a pure nitrogen environment. Methods. Electrical transport through single nanochannel was measured using Conducting Atomic Force Microscopy (CAFM). For imaging, JPK Nanowizard 3 with Au coated cantilever tips (Thickness: 2 μm/ Length: 450 μm/ Width: 50 μm, Resonance Frequency: 13 kHz/ Force constant: 0.2 N/m) was used in contact mode. The current amplifier was chosen so that the electrical noise could be suppressed on ~ fA. For macroscopic transport through an ensemble of nanopillars, the out-ofplane electrical conductivity was measured using Keithley 4200 SCS parameter analyzer. ITO served as the bottom electrode and 40 nm of gold evaporated on top of PEDOT:PSS containing nanochannels as the top electrode. Temperature-dependent frequency response was done using Linkam LTS420E-PB4 Temperature Controlled Stage.

RESULTS AND DISCUSSIONS (i)

DC conduction of an ensemble of PEDOT:PSS nanopillars Dc conductivity, σdc of PEDOT:PSS enclosed in AAO nanochannels is measured using Au as the top electrode and ITO as the bottom electrode. Apart from ensuring complete filling of the pores,


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the excess PEDOT:PSS on the non-porous region of the alumina template is minimized to ensure that the macroscopic σ represented the transverse transport arising from a parallel 109 pores distributed over 1 cm2 (details shown in section S6, Supporting Information). The I-V characteristics obtained is linear (0 V – 1 V range) and reversible across all samples of different dimensions. The geometrical information, i.e. pore diameter, channel length and density are utilized to estimate the corresponding σdc from the measurements shown in section S1, Supporting Information. The general trend of increase in σ with an increasing degree of confinement is observed (Figure 1a). σdc of PEDOT:PSS in 20 nm nanochannels is found to be two orders of magnitude higher than that of bulk PEDOT:PSS. The precipitous rise in σ clearly occurs below 100 nm range (AAOs having different aspect ratio are discussed in section S7, Supporting Information). This trend from measurements over a large area is then confirmed and studied at single nanopore level. (ii)

Single nanochannel electrical transport The PEDOT:PSS filled nanopores (without the top Au electrode) are locally probed using conductive atomic force microscopy (CAFM). Current is measured by applying a fixed bias voltage along the selected nanochannel between conducting cantilever tip and ITO. It is possible to measure the current magnitude across single pores using the narrow conducting tip (< pore diameter). The single-pore measurements (Figure 1b) of PEDOT:PSS reveal a trend similar to that obtained from macroscopic measurements. The direct observation of I20 nm > I50 nm > I100 nm > Ibulk for a constant voltage using the same CAFM tip clearly confirms the confinement effect. σ20 nm

is two orders of magnitude higher than σbulk. It should be stated that σnanopore of each pore has

been estimated by noting the pore dimensions and carried over randomly selected pores. The



relatively minor discrepancy in σ estimates from the two methods reveals the uniformity of PEDOT:PSS across the 109 parallel-identical channels. The organization of the polymer blend within the nanopore can be partially captured by examining the topography and current maps. The schematic in Figure 1e shows the different length of channels traced by cantilever tip, arrived from the constant-volume filling of the nanochannels. A smooth variation of the surface introduced by the pore-filling and drying process can be used for thickness dependent studies. These radial maps carried on partially filled pores are particularly informative and ensures a negligible presence of conductance contribution from the non-porous region. The linearity of current versus reciprocal of the transport-length in 20 nm and 50 nm nanochannels (Figure 2d,e,f) is indicative of the chain-ordering emerging within the nanopore. The sub-linear response in the case of 100 nm channel can then be indicative of chainslackening which emerges at these free-volume scales, with a gradual radial variation. (iii)

Macroscopic ac studies over an ensemble of PEDOT:PSS nanopillars AC measurements provide considerable insight into these systems with the observation of the systematic trend of response as a function of the confinement dimensions. The σ is measured by driving ac input signal of Vrms = 20 mV (Vdc = 0 V) over a frequency range from 10 kHz to 10 MHz. σ𝑎𝑐(𝜔) of PEDOT:PSS films25,56 generally exhibit universal power law, σ ~ ω𝑠 57-60 and is typically associated with disordered systems. At low 𝜔, the conductance asymptotically approaches a constant value and takes on a form of ∼ ωs (0 < s < 1) above a certain threshold frequency ωo. The total measured conductivity which is σ = σ𝑑𝑐 + σ𝑎𝑐(𝜔) can be expressed as σ = σ𝑑𝑐 + 𝐴ω𝑠 (where 0 < s < 1). We use this general expression to characterize the confined PEDOT:PSS system response. The features from results (Figure 3a,b) are the following: (i) σ() is relatively constant over the 10 kHz -100 kHz range and over varying T range, (ii) σ ~ ωs in the


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high frequency region (> 100 kHz) beyond certain threshold frequency ωo. The dc and ac conductivity (at low ω) magnitudes are comparable, i.e., σ20 nm ≈ 10-4 S/cm, σ50 nm ≈ 10-5 S/cm, σ100 nm

≈ 10-6 S/cm. Thus, the ionic contribution to conductivity does not appear to be largely

significant. The source of the increase in conductivity at high ω, arising from ionic or electronic processes, needs to be addressed. To investigate this, we analyze ac conduction further. The onset frequency, ωo is generally used as a parameter for probing correlation length scale, λ of network connectivity between nodes and junctions in the system. Higher ωo corresponds to shorter λ, i.e., ωo  1/λ. For  < o, the carrier transport is primarily across the conducting clusters, which are separated by the insulating PSS lamellae. The normalized σ() is studied using the procedure adopted in Ref.61 and the dependence of onset frequency with temperature and pore variation is analyzed. Normalization of each graph is done w.r.t its corresponding σdc values. It is evident from Figure 3c,d that σnorm (20 nm) at low T ~ 175 K is an order of magnitude higher than σdc, whereas for 100 nm channels it is only a factor of two. The o(T) is extracted from the results shown in Figure 3c,d and noting that σ(ωo) = 1.1 σdc , it is observed that o(T) increases with T (inset of Figure 3c,d) for all the nanopillars dimensions studied as expected. It is noted that o (20 nm) ~ 0.07 MHz < o (50 nm) ~ 0.2 MHz < o (100 nm)

~ 1.18 MHz channel at T ~ 175 K. At higher T ~ 400 K, o (50 nm, 100 nm) is very high

greater than 10 MHz and so within our operational frequency range of 10 kHz to 10 MHz, we couldn’t assign any value (Figure 3d inset). Based on these trends, it can be interpreted that the charge carriers in PEDOT-PSS confined in 20 nm channels can be transported across longer distances between two nodes since λ  1/ωo. This can be taken as a measure of ordering or alignment of PEDOT-PSS chains in smaller diameters which will enforce charge carriers to travel over larger distances to encounter subsequent nodes. This is supported by earlier findings of length 9 ACS Paragon Plus Environment


scales of 30 – 50 nm globular structures of PEDOT-PSS structures.62-63 Beyond this length scale, the globular grains are expected to collapse to form stretched or elongated structures. (iv)

Mapping ac conduction to binary RC random percolation network Models based on electrical networks are informative and provides a useful physical representation for transport processes. Earlier reports on this line have modelled the ac transport to an equivalent circuit consisting of a binary RC random network, where each pair of nodes is linked by a randomly placed resistors and capacitors.58,60,64 The resistor currents represent the free carrier transport while the capacitor contributes to Maxwell’s displacement currents. For a binary RC network, in the low

 regime, the admittance across resistors R-1 > ωC; so, conduction takes place through the resistive network. At high  regime, ωC > R-1, conduction occurs through the capacitive path. The admittance across the capacitor, CP and conductance, GP are measured as a function of  to obtain a quantitative estimate of the number of parallel capacitors required to contribute to the conduction. (Series capacitance, Cs is not accounted for due to reasons mentioned in section S3, Supporting Information). Admittance across the capacitor, ωCP increases linearly as shown in Figure 4b,c. In fact, the capacitance decreases (Figure 4a) by only one order of magnitude (10-1 nF to 10-2 nF) over a change of four orders of  (103 Hz to 107 Hz), hence the product CP effectively increases. Also, CP (20 nm) > CP (100 nm). Equivalent capacitance increases in a parallel capacitor circuit, so it can be quantified that the effective number of CP’s present in 20 nm channels is greater than 100 nm channels. Also, in Figure 4b,c it is observed that: (i) ω ≥ 1 MHz, admittance across C’s and R’s are comparable, i.e., ωCP ~ GP for 20 nm, (ii) GP > ωCP by one order higher in magnitude for 100 nm channels. The frequency at which ωτ = 1 signifies the relaxation frequency, ω in Nyquist plot (Figure 5a). The relaxation time scales (~ µs) for 20 nm and 100 nm channel diameters versus 10 ACS Paragon Plus Environment

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temperature is plotted in Figure 5b. These results can provide a realistic sketch to a schematic of the RC circuit in different pore dimensions. A schematic of equivalence of RC network to the alignment of PEDOT-PSS polymer chains is shown in Figure 5c. The effective impedance decreases in 20 nm channels. Decreasing impedance marks for larger polaron transport. That is, if n = number of Cp’s in the 20 nm channel, R = parallel resistance in 100 nm channels, R’ = parallel resistance in 20 nm channels, Rs = series resistance and ZCp = 1/jωCP, impedance across parallel capacitor, the total impedance of PEDOT:PSS in 20 nm channels is given by:

Z20nm =

𝑍 𝑅′∗( 𝐶𝑃 ) 𝑛 𝑍𝐶𝑃 𝑅′+ 𝑛

+ RS

𝑅′∗𝑍

= 𝑛𝑅′+𝑍𝐶𝑃 + RS 𝐶𝑃

and Z100nm =

𝑅∗(𝑍𝐶𝑃 ) 𝑅+𝑍𝐶𝑃

+ RS

∴ Z20nm < Z100nm (since n > 1) At ωτ = 1, solving for the parallel capacitance values in RC network of Figure 5c (e.g. at T = 175 K) gives CP (100 nm) ≈ 34.2 pF and CP (20 nm) ≈ 102.8 pF with detailed calculation shown in section S4, Supporting Information. This leads to an estimate of n ≈ 3, which accounts for the three capacitors in 20 nm channel and one in 100 nm channel. The increasing capacitance for increasing confinement marks for the rearrangement of the PEDOT-PSS chain orientation and higher electronic conduction. This approach then provides a framework to describe the higher conductivity of narrow pore channels. The RC equivalence to the microscopic picture of ac conductivity is indicative of the number of PSS chains/ unit volume decrease for 20 nm channels which enhances large domain polaron transport, suppressing ionic conduction.



CONCLUSIONS The combination of macroscopic dc, ac measurements and CAFM results clearly point to a scenario that a structural realignment within the channel enhances transport of PEDOT:PSS, at confinement characteristic lengths of ≈ 20 nm. The description of ac conduction in terms of RC network provides an interpretation to the higher conductivity in 20 nm channels, based on the dominant polaron transport. These results will help further understanding of transport studies of mixed polymer system on increasing confinement below the characteristic length of the polymer blend system. A key implication of the results is the outcome that the absolute conductance of PEDOT:PSS in narrow alumina pores exceeds the transverse conductance of a plain film, in spite of the reduced effective area of transport. The conductance of 20 nm membranes at room temperature is nearly an order of magnitude higher than bulk film. It is expected that the σ can be further improved by narrower channels and functionalization of the pore walls. The magnitude of transverse σ to access the high magnitude of lateral σ of doped PEDOT systems may be possible, leading to its use in high current density devices.


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FIGURES

Figure 1. DC transport studies of PEDOT:PSS in AAO nanochannels: (a) dc conductivity of PEDOT:PSS in different nanochannels versus the nanochannel diameter over an ensemble of nanopillars for a constant bias of 1 V between ITO (bottom electrode) and Au (top electrode) at T = 300 K using parameter analyzer 4200 SCS, inset shows linear I-V characteristics of PEDOT:PSS in 20 nm channels. (b) CAFM studies show similar trend of dc conductivity of PEDOT:PSS versus the nanochannel diameter when measured with conducting cantilever tip at single nanochannels. Inset shows AFM image of surface morphology of polymer PEDOT:PSS filled nanochannels, where the high current IHIGH corresponds to conducting tip in contact with the polymer inside an AAO nanochannel and low current ILOW corresponds to conducting tip in contact with the non-



porous AAO region. (c,d) Surface and current profile of PEDOT:PSS in 50 nm channels, respectively. (e) Schematic of PEDOT:PSS filling the length of AAOs with 50 nm pore diameter (interpore distance of 100 nm) and thickness of 200 nm.


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Figure 2. Schematic of different length of channels traced by CAFM cantilever tip, arrived from the constant-volume filling of polymer blend PEDOT:PSS in the nanochannels: (a,b,c) Surface profile scanned by AFM tip in 20 nm, 50 nm, 100 nm channels, respectively. It is evident that the extent of polymer filling in nanochannels follows the order of 20 nm > 50 nm > 100 nm. (d,e,f) shows the current dependence, I(d-1) on reciprocal of channel length in a nanochannel (Number of points in each graph vary due to varying pore diameter and a fixed dimension of CAFM tip).



Figure 3. Temperature dependent frequency response of PEDOT:PSS in 20 nm and 100 nm AAO channels: AC conductivity of PEDOT:PSS, σac (a,b) and normalized conductivity, σnorm (c,d) variation with frequency, ω within interval of 10 kHz to 10 MHz of input sinusoidal voltage of Vrms = 20 mV and Vd.c = 0 V for 20 nm (a,c) and 100 nm (b,d) channel diameters for a range of temperature from 400 K to 168 K. Inset of (c,d) shows the onset frequency, ωo variation with temperature for the respective channel diameters.


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Figure 4. (a) Parallel capacitance variation with frequency, ω of input sinusoidal voltage of Vrms = 20 mV and Vdc = 0 V at T = 200 K. (b,c) shows conductance, GP (left y-axis) and admittance, ωCP across CP (right y-axis) versus frequency, ω of input signal over temperature range from T = 400 K to 168 K (only few temperature scales are shown in graph).



Figure 5. (a) Nyquist plot for 20 nm channel diameter over temperature range from T = 400 K to 168 K, (b) Relaxation timescale, τ (derived from graph (a) using ωτ = 1) variation with temperature for 20 nm and 100 nm channel diameters. (c) Schematic of PEDOT-PSS chain alignment (vertical cross-sectional view) in different nanoconfinement scales represented to an equivalent RC network in each nanochannel. The RC network is shown for τ at T = 175 K.


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ASSOCIATED CONTENT Supporting Information The following files are available free of charge in PDF. Calculation of dc conductivity over an ensemble of nanopillars, Quantification of non-utilization of alumina in different diameter nanochannels, Non-consideration of series capacitance in nanochannels, Quantification of parallel capacitors in each nanochannels from Nyquist plot, Different steps of sample preparation, Minimization of the excess PEDOT:PSS on the non-porous region of alumina to ensure that the macroscopic σ represented the transverse transport, AAOs with different aspect ratios. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID K. S. Narayan: 0000-0001-8550-6868 Author Contributions All authors contributed equally to this work. Notes The authors declare no competing financial interest.



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ACKNOWLEDGEMENT We thank JNCASR facilities and Department of Science and Technology (DST), Government of India for its funding. ABBREVIATIONS PEDOT:PSS, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate); CPs,

Conducting

Polymers; ITO, Indium Tin Oxide; AAO, Anodized Aluminum Oxide; RC, Resistor Capacitor; AFM, Atomic Force Microscopy; CAFM, Conductive Atomic Force Microscopy.

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