Tunable metallic-like transport in polypyrrole

Conjugated polymers (CPs), organic macromolecules with a linear backbone of alternating C–C and C=C bonds, possess unique semiconductive properties, providing new opportunities for organic electronics, photonics, information, and energy devices. Seeking the metallic or metallic-like, even superconducting properties beyond semiconductivity in CPs is always one of the ultimate goals in polymer science and condensed matter. Only two metallic and semi-metallic transport cases—aniline-derived polyaniline and thiophene-derived poly(3,4-ethylenedioxythiophene)—have been reported since the development of CPs for four decades. Controllable synthesis is a key challenge in discovering more cases. Here we report the metallic-like transport behavior of another CP, polypyrrole (PPy). We observe that the transport behavior of PPy changes from semiconductor to insulator-metal transition, and gradually realizes metallic-like performance when the crystalline degree increases. Using a generalized Einstein relation model, we rationalized the mechanism behind the observation. The metallic-like transport in PPy demonstrates electron strong correlation and phonon–electron interaction in soft condensation matter, and may find practical applications of CPs in electrics and spintronics.

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Introduction
Over the past 40 years, CPs have been studied intensively for the combination of machinability inherited from polymer chains and semiconductor performances owing to delocalized π-electrons [1][2][3][4][5][6][7].However, semiconductor properties are not the only physical nature of CPs [8][9][10].In fact, the original intention of the development of CPs is to find a covalent organic polymer which would be a 'metal' [11].The flexibility of polymer chains provides excellent processability, making them a replacement of metals, but the folding, twisting, or rotating of the polymer chains caused by flexibility leads to uncontrollable transport behavior [12,13].With the development of advanced manufacture technology, especially the nano-fabrication techniques, high electrical-conductivity above 1000 S cm −1 at room temperature has been discovered in various CPs such as polyaniline (PANI), PPy and poly(3,4-ethylenedioxythiophene) (PEDOT) [14][15][16].
To explain the semiconducting or conducting transport behavior exhibited in different polymers, various models have been carried out such as the 1D Luttinger liquid model [17], the environment Coulomb blockade model [18] and the nuclear tunneling mediated polaron hopping model [19].Generally, the thermally activated conductivity in most polymers should be attributed to the inhomogeneous disordered structures caused by the charge carriers percolating from conducting ordered regions to poorly conducting disordered regions [20].Besides the doping effect on geometry, conductivity as well as thermoelectric property in the ordered PEDOT crystal have been analyzed [21].Though these progresses promote a deeper understanding of the relation between structure and performance in CPs, it is still a challenge to realize the metallic or metallic-like transport behavior in CPs.
In fact, seeking metallic or metallic-like transport behavior in organic materials is of great importance to both applications and fundamental studies in condensed matter [22][23][24][25].Yet such behaviors are found in only a handful of organic materials, including microbial nanowire networks [24], zirconium-based mesoporous metal-organic framework [25] and conducting polymer systems (PANI and PEDOT) [22,23].It is widely accepted that the crystallization behavior of CPs associated with the chain alignment, degree of disorder, interchain interactions and conjugation length determines their transport behaviors [22,23].In our previous work, PPy with different degrees of crystallinity, even single crystal, was synthesized via a multi-step solvent-free polymerization method [26][27][28][29][30].We found that the long-range orientational order in CPs can be obtained by regulating the growth environment, rate, and size [31].Herein, on this basis, we report the first observation of metallic-like transport behavior in PPy.We find that the transport performance of PPy has a direct correlation with its crystallization behavior.The PPy presents common semiconductor characteristics with a low degree of order.With the crystallinity increasing, the PPy demonstrates a temperature-dependent transition from the semiconducting to metallic phase, and to metallic-like or superconducting-like transport.

Synthesis of the paper-based PPy film
About 0.2, 0.5 and 1 M FeCl 3 •6H 2 O were prepared as water solution by stirring for 10 min at room temperature.The A4 paper was immersed into the FeCl 3 mixture using as the substrate.Then, the immersed A4 paper was sealed in a closed chamber with pyrrole vapor.The PPy film gradually grew on the surface of the A4 paper and the paper-based PPy film was obtained after a suitable period of time (1-75 days) at the different concentrations of Fe 3+ (#1-#3 were growing for 3 days using 1 M FeCl 3 as oxidant, samples #4-#6 were growing for 2 days using 1 M FeCl 3 as oxidant, samples #7 and #8 were growing for 9.5 days using 0.5 M FeCl 3 as oxidant and sample #9 was growing for 75 days using 0.2 M FeCl 3 as oxidant at 4 • C, respectively).

Material characterizations
The structures of paper-based PPy films were characterized by x-ray diffraction (XRD) on a PANalytical diffractometer with Cu K α radiation (λ = 1.5406Å) at room temperature and atmospheric pressure.The Fourier transform infrared (FTIR) spectra of paper-based PPy films were collected by a FT-IR spectrometer (JASCO FT/IR-6800).Electrical resistivity was measured by the conventional dc four-probe method and the measurements were performed on a Quantum Design physical properties measurement system (PPMS-9).Specifically, the temperature of samples was set from 300 K to 10 K with a cooling rate of 10 K min −1 , and then the temperature was set to 2 K with a cooling rate of 1 K min −1 .The heating process was set from 2 K to 300 K with a heating rate of 6 K min −1 .As the temperature changed, the resistance change was recorded.

Conductive behavior of PPy film
The samples #1, #2 and #3 prepared at a higher oxidant concentration and longer polymerization time show a thermalactivated semiconducting conductivity as seen in figure 1(a).As the temperature increases from 5 K to 25 K, the relative resistances (R T /R 300K ) decrease sharply by two orders of magnitude, where the values of R 5K /R 300K for three samples are 1.03 × 10 4 , 1.31 × 10 4 , and 1.67 × 10 4 , while the values of R 25K /R 300K are 0.53 × 10 2 , 0.75 × 10 2 , and 0.93 × 10 2 , respectively.The XRD patterns shown in figure 1(b) indicate amorphous structures in all the three PPy samples.The broad peak located at 2θ = 22.70 • corresponds to the π-π interchain stacking along the [010] direction.According toBragg's law (d spacing = nλ/2sinθ, for the use of Cu K α , λ = 1.5406Å), the d spacing is 3.9 Å.The increase in the intensity of the 22.70 • peak of the XRD patterns from sample #1 to sample #3 implies the improvement of the face-to-face interchain stacking.We further used FTIR spectroscopy to analyze the structures of PPy (figure 1(c)).Considering the IR spectra of the three samples are similar, we take sample #2 as an example to discuss the characteristics of IR absorptions of the semiconducting PPy.By referring to the theoretical simulation of FTIR spectrum of nonplanar PPy containing six units (figure S1 available online at stacks.iop.org/MF/1/011001/mmedia), the absorptions at 1289, 1443 and 1535 cm −1 are attributed to C-H in-plane bending, C-C and C-N stretching vibrations, the absorptions at 1000, 1094 and 1147 cm −1 correspond to C-H in-plane bending and C-N stretching vibrations, 836, 878 and 964 cm −1 correspond to Py ring in-plane bending vibrations, while the absorption at 784 cm −1 is attributed to C-H out-plane bending vibrations.It should be noted that the C-H out-plane bending vibrations of the first three samples gradually weaken, which demonstrates the decreasing of structure disorder, consistent with the XRD result.With the weakening of the out-plane bending vibrations, the torsion angles between the phenyl ring and the plane of the backbone are gradually reduced, which means that the higher the planar chain conformation is, the higher crystallinity is.This indicates that the effective conjugation length has been expanded to extend the delocalization degree of the charge carrier, resulting in the improved transport performance shown in the inset of figure 1(a).
The samples #4, #5 and #6, prepared with the polymerization time of two days, show an insulator-metal transition as seen in figure 2  be the major reason for the insulator-metal transition.Meanwhile, the intensity of absorption at 784 cm −1 is gradually weakened, which means the face-to-face interchain stacking is gradually strengthened from sample #4 to sample #6, resulting in improved transport performance shown in the inset of figure 2(a).
For samples #7, #8 and #9, the conductivities fully represent metallic-like transport performance, namely the resistance decreases along with reduced temperature (figure 3(a)).The R T /R 300K rapidly reduces to 0.01 when T is ca.50 K for both samples #7 and #8, while T is ca.200 K for sample #9.It should be noted that it is difficult for us to explain the obvious resistance jump at 51.6 K and 56.1 K for sample #7 and sample #8, respectively, which is perhaps due to the undiscovered phase transition.It can be seen that the XRD peak at 22.70 • of sample #8 is sharper than sample #7 with the same growing condition (figure 3(b)), representing the increase of crystallinity.The changes in crystallinity cause resistance behavior to change.Specifically, the insulator-metal transition between 200 K and 300 K disappears in sample #8 sample.Samples #7∼#9 and the former #1∼#6 samples were grown in different conditions, so the IR characterizations are used to analyze the specific crystallinity of these samples.The detailed peak strength of C-H out-plane bending (784 cm −1 ), C-H in-plane bending (1000 cm −1 ), C-N stretching (1147 cm −1 ) and Py ring in-plane bending vibrations (1544 cm −1 ) is shown in table S1.The intensities of absorptions between 900 and 1200 cm −1 , which are attributed to C-H in-plane bending, C-N stretching, and Py ring in-plane bending vibrations, are stronger than samples #1∼#6, while the absorption around 784 cm −1 disappears completely, indicating a good planarity of PPy (figure 3(c) and table S1).Along with increased planarity, the electronic background scattering is suppressed, leading to decreased interchain transport resistance and intrachain hoping resistance.Meanwhile, the contribution of the interlayer π-π interaction to transport behavior plays a more critical role.Thus, the metallic-like conducting property should be ascribed to the excellent crystalline degree.

Discussion of conductive behavior of PPy film
To better understand the reason for the changes of conducting performance with the change of crystalline degree, we applied the generalized Einstein relation (GER) [32] to predict the temperature-dependent conductivity in different crystalline degrees.The GER model can unify the theoretical models from hopping to band-like and achieve good fitting for a series of organic semiconductors (OSCs) with various crystallinities.According to the Kubo-Greenwood integral, the overall conductivity for all the carriers can be expressed as: where the Fermi-Dirac distribution is applied: E F is the Fermi energy.σ ′ is the microscopic conductivity of electronic states which has been evaluated in Gaussian-like [33]: Here, σ 0 is the characteristic conductivity as materialspecific property.The characteristic parameter ∆D (⩽1) represents the degree of delocalization for the electronic states near the edge of the density of states (DOSs).∆E is the variance of the Gaussian distributed DOS (usually below 0.2 eV) and E 0 is the energy of the DOS center (E 0 is set as 0 eV by default).It should be noted that, ∆E characterizes the site energy disorder which can be affected by crystallinity of structure and intermolecular interactions, while ∆D describes the charge delocalization degree which can be affected by the similar factors as well as material defects or traps and lattice scattering.Namely, the higher crystallinity, the lower ∆E and the higher ∆D.
The resistivity (ρ) is the reciprocal of conductivity and the resistance is R = ρL/A.Assuming the length (L) and the crosssectional area (A) are independent on temperature, the relative resistance is proportional to the relative resistivity.Considering all the parameters can reflect the material properties, different values of parameters were set to calculate the relative resistivity in the relation of temperature, and discuss the relationship between the material structure and the conducting property.From the equations, it is derived that the conductivity is actually dependent on |E F |, ∆E and ∆D.When ∆D•∆E is unchanged, the conductivity also stays the same.Liu et al have applied the GER model to fit the experimental charge transport properties in more than 20 kinds of OSCs [32].The fitted parameters ∆E and ∆D were found to be approximately linearly related.Thus, by referring the fitted sets of ∆E and ∆D, assuming that ∆D = 1 − ∆E/0.15, the relation between ∆E and ∆D•∆E has been drawn in figure 4(a).It is easily found that ∆D•∆E will be raised when the system crystallinity becomes worse from the perfect crystal structure, or when the crystallinity becomes better from the poor amorphous structure.
By altering the values of |E F | and ∆D•∆E, the different characteristics of temperature-dependent conductivity occurs.When the Fermi energy (E F = 0) equals the DOS center energy (E 0 = 0), the conducting performance is metallic-like (figure 4(b)).However, when the E F deviates from the E 0 , the conductivity characteristics change dramatically.For instance, when ∆D•∆E is 0.0375 eV (∆E = 0.075 eV and ∆D = 0.5), |E F | is around 0.14-0.17eV, the insulator-metal transition appears (figure 4 is Gaussian distributed DOS.The carrier density is calculated in relation with E F as seen in figure S2.The carrier densities at all temperatures show a positive relationship with E F .The doping concentration increases as the PPy crystallinity becomes poor in experiment, while the carrier density increases as well.Thus, it can conclude that the transport property change from metallic-like to semiconducting is related to the increase of charge carrier density (corresponding to the gradually decrease of FeCl 3 concentration).
According to the experimental FTIR spectra, the samples (#1-#3) with semiconducting transport behavior possess poor crystalline structure, while the samples (#7-#9) with metallic-like characteristics have good crystalline structure.In theory, the systems in figure 4(b) are considered to possess good crystallinity (strong delocalization degree and low site energy disorder), while the systems in figure 4(d) present the bad crystalline (weak delocalization degree and high energy disorder).Therefore, in the metallic-like systems, it is easily found from figure 4(b) that the stronger delocalization degree and lower site energy disorder, the stronger the positive temperature-dependence of the relative resistivity.To the contrary, in semiconducting systems, the weaker the delocalization degree and the higher the site energy disorder, the lower thermal-activated temperature of the system.

Conclusion
In conclusion, the conducting performance of PPy changes from semiconducting to insulator-metallic transition, to a metallic-like property and it could be ascribed to the increasing crystallinity as well as the decreasing carrier density.The appearance of metallic-like transport in PPy, which is one of the fundamental categories of the CPs, will allow further, in-depth study on the electron strong correlation and phonon-electron interaction in the condensed state physics of polymers.From an applications perspective, it will offer the possibility for plastic electronics and spintronics with the rapid progress both in the preparation and theory of highconductivity CPs.
Furthermore, some superconducting-like behaviors (figure S3) have been observed in some specific samples.By combining Little's prediction of high-T c organic superconductor [34] and the observed electron attraction mediated by Coulomb repulsion [35], it would be possible to find high-T c superconductivity in CPs.

Figure 1 .
Figure 1.The performance of the PPy samples with semiconducting properties.All these samples were synthesized using FeCl 3 (1 M) as an oxidant and paper as substrates with the growing time of three days.(a) The relationship between resistance and temperature from 5 K to 300 K. (b), (c) The XRD patterns and the FTIR spectra of the three PPy samples.

Figure 2 .
Figure 2. The characteristics of PPy samples with insulator-metal transition properties.The samples were synthesized for two days by using FeCl 3 (1 M) as an oxidant.(a) The temperature dependence of resistance of these PPy samples.(b), (c) The XRD patterns and the FTIR spectra of three PPy samples.
(a), where the transition temperatures (T trans ) are 7∼15 K and the R T /R 300 K at T trans are 160∼340.The XRD patterns shown in figure 2(b) exhibit two obvious peaks at 8.74 • and 22.70 • , which correspond to d spacing of 10.1 and 3.9 Å, respectively.The new peak at 8.74 • may relate to the diffraction of the lateral chain spacing along the [100] direction as traditionally conceived, but further detailed research is needed.Compared with the XRD patterns shown in figure 1(b), the intensity of the 22.70 • peak shown in figure 2(b) increases.The XRD results indicate the further improved crystallinity both in the face-to-face interchain stacking and lateral chain arrangement.The IR spectra in figure 2(c) are different from those in figure 1(c): the intensity of absorptions between 900 and 1200 cm −1 becomes stronger while the intensity of absorption around 784 cm −1 becomes weaker compared to #1∼#3 PPy samples.In other words, the C-H in-plane bending and the C-C and C-N stretching vibrations are strengthened while C-H out-plane bending vibrations weaken in #4∼#6 samples, indicating a better crystalline degree than the first three samples, which should

Figure 3 .
Figure 3.The performance of the PPy samples with metallic-like properties.The samples were fabricated using paper as substrates, FeCl 3 as an oxidant (0.5 M (sample #7 and sample #8), 0.2 M (sample #9)) with a different growing time (9.5 days (sample #7 and sample #8), 75 days (sample #9)).(a) The relationship between resistance and temperature from 5 K to 300 K. (b), (c) The XRD patterns and the FTIR spectra of the three PPy samples.

Figure 4 .
Figure 4. (a) The hypothetic relation between ∆E and ∆D (∆D = 1 − ∆E/0.15) and the relation between ∆E and ∆D•∆E.(b)-(d) The theoretical simulation results of temperature-dependent relative resistivity (ρ/ρ 300 ) under different parameter settings.The units of |E F |, ∆E and ∆D•∆E are eV, and ∆D has no unit.
(c)), and the corresponding relative resistivity at the transition temperature (T trans = 6 K) is 30-400.However, when |E F | raises up to 0.2 eV (figure 4(d)), the conductivity becomes almost insulating at low temperatures, close to the semiconducting transport property.E F mainly determines the overall carrier density and n = ´+∞ −∞ N(E)f(E)dE where N