Long-Range Coherent Tunneling in Physisorbed Molecular

Article pubs.acs.org/JPCC

Long-Range Coherent Tunneling in Physisorbed Molecular Ensembles Leandro Merces,†,‡ Rafael Furlan de Oliveira,† Davi Henrique Starnini de Camargo,† and Carlos César Bof Bufon*,†,‡,§ †

Brazilian Nanotechnology National Laboratory (LNNano), Brazilian Center for Research in Energy and Materials (CNPEM), 13083-970 Campinas, São Paulo, Brazil ‡ Institute of Physics “Gleb Wataghin” (IFGW), University of Campinas (UNICAMP), 13083-859 Campinas, São Paulo, Brazil § Department of Physical Chemistry, Institute of Chemistry (IQ), University of Campinas (UNICAMP), 13084-862 Campinas, São Paulo, Brazil S Supporting Information *

ABSTRACT: The charge transport in molecular systems is governed by a series of carrier-molecule quantum interactions, which result in a broad set of chemical and physical phenomena. The precise control of such phenomena is one of the main challenges toward the development of novel device concepts. In molecular systems, direct tunneling across 1−10 nm barriers and activated hopping over longer distances have been described as the main charge transport mechanisms. The continuous transition from one mechanism to the other, by increasing the transport distance, has mainly been reported for molecular chains covalently bonded to the electrodes. In elementary molecular junctions, like those formed by physisorbed organic semiconductor thin films, such transition remains unclear. Here, we report the first experimental evidence for sequential, long-range coherent tunneling across physisorbed ensembles by investigating the charge transport in copper phthalocyanine layers (5−60 nm thick films). Like observed for chemisorbed molecules, our junction exhibits a gradual transition from coherent tunneling to activated transport in the 10−22 nm thickness range. The present work contributes to connect the quantum transport to diffusive-related phenomena in such an elementary organic system.

1. INTRODUCTION The ongoing development of novel electronic applications based on organic/inorganic structures has motivated the investigations of the charge transport and related electrical conduction processes in different materials.1−5 The observation of long-range charge transport in DNA molecules,6−8 thiophenes,2 ferritins,9 and polymer structures10 has prompted organic molecules as the building blocks for nanoelectronics.11−13 In addition, the possibility of controlling the response of organic devices closer to the quantum frontiers by adjusting external parameters, such as temperature and electromagnetic field, apart from the device geometrical features, has further increased the range of applications envisioned for nanostructured elements.4,14,15 By considering the possibility of creating novel degrees of freedom, exploiting the variety of existing molecular systems, several unique chemical and physical properties have been identified.2,10,16 Thus, the investigation of the conduction processes at the molecular level is of substantial importance in order to fully explore both the fundamental and the technological potentials of molecular systems. © XXXX American Chemical Society

Regarding organic structures, the evaluation of the charge transport mechanisms is commonly performed by bridging a pair of electrodes with the organic layer and acquiring a set of current−voltage (I−V) traces.13,14,17 At the nanoscale, this approach is not a straightforward assignment. Several attempts to accommodate molecules in small gaps between electrodes have been made, leading to the fabrication of junctions based on single-layered molecular ensembles.2,18−21 However, measurements of charge transport properties across monolayers of n-alkanethiols on silver and gold substrates, for example, are known to present an unsolved mixture of consistencies and inconsistencies.22 Consequently, efficient strategies to address the fundamental properties of molecular ensembles depend on advances in different fronts, which include novel device architectures,22−25,16 theoretical models,26−28 and computer simulations.29,30 As foremost outcomes, coherent tunneling and activated hopping have emerged as the mainstream concepts to Received: March 17, 2017 Revised: May 22, 2017 Published: May 25, 2017 A

DOI: 10.1021/acs.jpcc.7b02528 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) Schematics of the device structure before (top) and after (bottom) the roll-up process. The controlled strain relaxation curls up the Aucovered nanomembrane into a cylindrical shape, forming the junction top electrode. (b) SEM image of the rolled-up based device. (c) Lateral view of the molecular transport junction. The conductive channel is formed by CuPc films with thickness (tCuPc) varying from 5 to 60 nm.

layers to the device structure, a rolled-up nanomembrane was used to form the top electrode in a vertical junction configuration.16,52 This approach allows us to employ the same device architecture to connect films with different thicknesses, namely, from 5 to 60 nm. The extension of the coherent transport was also evaluated in terms of temperature and electric field, revealing a smooth thickness-dependent transition from tunneling to thermal activated charge transport. Such a transition is considered an indication that ST rules the transport process in films with thickness ranging from 10 to 22 nm, at temperatures below 40 K. For thicker films, longer transport distances are reached, and signatures of ST are observed at high electric fields (∼106 V/cm). Overall, the investigation of the charge transport mechanisms as a function of the organic layer thickness, electric field, and temperature has no previous report for transport junctions based on physisorbed molecular ensembles. The present work contributes to connect the quantum transport to diffusive-related phenomena in such an elementary organic system.

explain short- (∼1−10 nm) and long-range (>10 nm) charge transport in molecular junctions.2,8,13,16 In addition, transitions involving field-assisted mechanisms, such as Fowler−Nordheim (FN) tunneling and Frenkel−Poole emission, have also been reported.16 In the nanometer range, direct tunneling (DT) currents are expected to scale exponentially with both the attenuation factor (β) and the barrier width. Within the Simmons model,31 β is defined as the decay of the current density vs tunneling distance. For aliphatic molecules, β is found around 8−9 nm−1,19,21,32−37 while for the aromatic ones β ≈ 2−3 nm−1.38−41 Smaller values of β (∼1 nm−1)42,43 have both fundamental44−47 and technological48,49 implications. For instance, they reflect the possibility of charge carrier tunneling across distances longer than the molecular length. Such a possibility is itself an attractive appeal to develop complex molecular circuits.13 Usually, β < 1 nm−1 has been observed in temperaturedependent charge transport, being commonly attributed to activated hopping.19,50 However, recent works have shown that a non-negligible contribution of coherent tunneling, with a small temperature dependence, may occur in addition to the dominant activated regime.2,8,29 Such a contribution is due to both the strong electronic coupling between π-electrons of molecular neighboring sites29 and the molecular orbital energetically close to the contact Fermi level.19 These events provide the delocalization of charge carriers inside the molecular bridge, leading to the sequential tunneling (ST).9,51 Consequently, a transition from DT to a long-range coherent ST as a function of the barrier width may occur before the activated hopping dominates.2,8,9 The extension of the coherent transport has already been determined for chemisorbed molecular junctions by evaluating their electrical characteristics as a function of barrier width (i.e., molecular length).2,8,50 However, to the best of our knowledge, the experimental observation of ST and the subsequent transition to activated hopping have not been verified for physisorbed molecular ensembles. Here, we report the first experimental evidence of a longrange (>10 nm) coherent process occurring in physisorbed molecular ensembles. In order to precisely connect the active

2. EXPERIMENTAL SECTION 2.1. Defining the Top Electrode for the Vertical Transport Junction. In order to overcome the challenges4,40 of connecting molecules without damaging the active layer,53 the vertical junctions were fabricated using a strategy based on rolled-up nanomembranes.16,52,54 In addition to vertical junctions, the roll-up technology has been applied in a variety of three-dimensional elements, such as capacitors55−57 and sensors,58,59 as well as in magnetoelectronics60,61 and superconducting mesoscopic junctions.62 Figure 1a illustrates the device structure before and after the formation of the rolled-up based junction. The selective removal of a sacrificial layer promotes the relaxation of the strained nanomembrane,63 as shown on the top of Figure 1a. The freestanding structure bends up, assuming a cylindrical shape with typical diameter of ∼8 μm. Figure 1b shows the scanning electron microscopy (SEM) image of the device. The rolled-up nanomembrane contacts the organic thin film from the top, providing a soft and self-adjusted electrode.16 This strategy guarantees a robust and damage-free contact to the organic film and can be used with different nanoscale materials.16,52,57 These characteristics make B


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Figure 2. (a) Detailed lateral view of the Au/CuPc/Au junction. The topography of the electrodes defines both the contact and the injection areas (dotted lines and red marks, respectively). The film thickness is used to calculate the applied electric field (E). (b) Fowler−Nordheim (FN) plot for high-E regimes at 18 K. (c) Junction areas as a function of tCuPc and the tube diameter compression. The dotted line was calculated using the Hertzian approach, considering the mechanical contact area between a rigid flat surface and a cylinder. The surface roughness was also considered. The experimental points were obtained from the analysis of the FN traces.

samples in which the electrical response is equivalent to the statistic average of at least five transport junctions, evaluated with the same tCuPc value.

the rolled-up based vertical junction a suitable platform to reliably characterize thin and ultrathin films, allowing stable electrical measurements as a function of thickness (t). 2.2. Materials Used to Form the Molecular Junctions. We have chosen copper phthalocyanine (CuPc) as the material of interest because of its stability and versatility in both the fields of molecular16 and organic electronics.64,65 The CuPc thin films were sublimated in a high vacuum chamber (10−6 Torr) at the growth rate of ∼0.2 nm/s. The device test patterns were kept at room temperature during the CuPc deposition. To connect the active layer, we have used Au-covered electrodes (Figure 1c) to ensure an efficient carrier injection.66 2.3. Electrical Measurements. We have evaluated the current density vs electric field (J−E) characteristics of the junctions by measuring the I−V traces as a function of the temperature (T) and the CuPc film thickness (tCuPc). Prior to the electrical measurements, the samples were stored in high vacuum (10−5 Torr) for 24 h. The I−V characteristics were acquired using a Keithley 4200 SCS semiconductor parameter analyzer connected to a LakeShore EMPX-HF cryogenic probe station (at 10−5 Torr). The V-bias was limited (equivalent to E ≤ 106 V/cm) in order to prevent the junction damage by heating and/or electrode metal interdiffusion.13 The bias configuration is illustrated in Figure 1a,c. The I−V characterization was performed for tCuPc ranging from 5 to 60 nm and T varying from 18 to 300 K. The tCuPc values were evaluated by atomic force microscopy (AFM). The detailed topographies of the electrodes and active layers are shown in the Supporting Information (SI). The results reported here are related to

3. RESULTS AND DISCUSSION 3.1. Determining the Mechanical Contact Area and the Effective Injection Area for the 5−60 nm Thick Junctions. Figure 2a illustrates the lateral view of the rolled-up based junction, considering the electrode roughness and the average CuPc film topographies. The roughness (rms) of the CuPc films is 2.0 ± 0.5 nm, while the values for finger and tube electrodes are 1.0 ± 0.2 and 3.0 ± 0.4 nm, respectively. Thus, we have considered the deviation of the film thickness as ΔtCuPc = ±3 nm. In cases where the sum of the maximum peak-topeak distance exceeds tCuPc ± ΔtCuPc, electrical short circuit between finger and tube electrodes is observed. Such a value is important to precisely control the electric field across the Au/ CuPc/Au junction. Furthermore, the determination of the electrode’s topography is of fundamental importance to define both the mechanical contact and the effective injection areas. The mechanical contact region is highlighted in Figure 2a by the dotted lines, while the circular marks illustrate the effective injection regions. As the injection areas are expected to be much smaller than the geometrical cross-sectional areas,13,16 the electric field (E) was calculated using an approximation of a parallel-plate capacitor spaced by a distance tCuPc. At high-E regimes (E > 8 × 105 V/cm) all the 5−60 nm thick junctions presented a strong E-dependence, which is characteristic of field emission.51,67 The C


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Figure 3. J−E curves for the Au/CuPc/Au rolled-up based junctions at (a) 300 K and (b) 18 K. Each tCuPc is indicated next to the respective curve (values in nanometers).

value E > 8 × 105 V/cm was experimentally determined as the electric fields where the best fittings for field emission were obtained (Table S1). Thermoactivated charge transport is not expected to occur in CuPc thin films at low T since activation energies of hundreds of meV are commonly observed for T > 40 K, as discussed hereafter.16 Furthermore, field-emission tunneling was recently reported for CuPc as the main charge transport mechanism operating at both low T and high E.16 In such a regime, the tunneling current I(E), described by Simmons,31 follows the equation I(E) = S1AE2e−S2 / E

contact area (dotted line). The error bars are related to both ΔtCuPc and the tube diameter deviation and represent the model reliability. It is important to notice that the surface roughness coupled to small channel lengths makes the injection areas smaller than the mechanical contact areas in nanoscaled junctions.13,22 Here, such an effect is pronounced for tCuPc < 15 nm, where the injection point contacts may be nonnegligible.22 In general, the correlation between areas can be done by crossing the information presented in Figures 2a and 2c: (i) the dotted lines represent the mechanical contact areas obtained via Hertzian approach, and (ii) the red circles (Figure 2a) and the data points (Figure 2c) are the effective injection areas obtained from the FN plots. From the AFM analyses (SI), we have obtained the correlation lengths for both CuPc thin films and tube electrode, namely, 70 ± 30 nm. This figure-ofmerit provides information on the extension of topography uniformity around a surface point. Such correlation lengths correspond to uniform regions of thousands of nm2, sufficiently larger than the values calculated for the injection areas ( 15 nm, the rectification is ∼1:8 at 300 K (Figure 3a) and increases to ∼1:16 at 18 K (Figure 3b). The distinct rates arise from the intrinsically different finger/CuPc and CuPc/tube interfaces.16 While vacuum-deposited CuPc molecules form the finger/ CuPc interface, a mechanical contact is formed onto the CuPc film after the roll-up process, which occurs in water. Therefore, at the CuPc/tube interface, the presence of impurities and/or defects is expected to be higher.70 Furthermore, the reduction of CuPc in water could also contribute to the charge carrier localization next to the top interface.71 Similar rectification rates, reaching values of 1:5, have been previously reported for equivalent junctions at 300 K.16

(1)

where A is the injection area, and S1 and S2 are parameters dependent on the tunnel barrier heights. The FN plot exhibited in Figure 2b makes eq 1 linear with the same generality level as ln(I/E2) vs 1/E (see the SI for details). The slope and intercept of the FN plot allow us to empirically obtain the injection areas that increase with tCuPc (Figure 2c). Since the device architecture is the same for all the transport junctions, the mechanical stress applied by the finger electrode toward the tube scales with tCuPc as well. Most models applied to understand mechanical contacts68 consider, under such conditions, deformations on both structures. Thus, one may expect that both the CuPc film and the tube electrode deform, increasing the electrical contact area. However, no substantial changes in tCuPc have been observed (Figure S1). Instead, the rolled-up nanomembrane leaves a footprint on the CuPc film surface, indicated by a partial roughness transfer from the tube to the film. Consequently, in the case presented here, the tube electrode supports the whole deformation and becomes responsible for the increment observed in the contact area as tCuPc increases. Such a behavior is well predicted by the Hertzian approach for the mechanical contact between a rigid flat surface and a cylindrical shell. There, the geometrical contact area is proportional to the square root of the diameter compression.69 As the involved surfaces are rough at the nanoscale, affecting the injection area, a compensation accounting for the electrode topographies was required. The geometrical description using the Hertzian model was applied disregarding any topographic contribution below ∼3 nm from the maximum electrode peak heights. Such improvement of the Hertzian model results in an effective contact area ∼0.03% of the geometrical contact one. Figure 2c exhibits the effective D


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Figure 4. Attenuation plots (J vs tCuPc) at (a) E ≈ 0.5 MV/cm and (b) E ≈ 1.0 MV/cm, obtained at 300 K (full squares) and at 18 K (open circles). Regions with distinct exponential decays (β) are indicated by the continuous and dotted lines, corresponding to exponential fittings. The horizontal error bars refer to the junction thickness uncertainty (∼3 nm). The vertical error bars have origin in both the dispersion of the current around the applied E and the propagation of the uncertainty related to the injection areas.

Figure 5. Arrhenius plots of J vs 1000/T at (a) E ≈ 0.5 MV/cm and (b) E ≈ 1.0 MV/cm for the 5−60 nm rolled-up based junctions. The dashed line separates the two distinct charge transport mechanisms: (1) coherent tunneling and (2) activated hopping.

3.4. Verifying the Occurrence of DT, ST, and Hopping Conduction in Physisorbed Molecular Ensembles. In Figure 4a, at 18 K, β = 0.7 ± 0.1 nm−1 was found for tCuPc < 22 nm and β = 0.10 ± 0.02 nm−1 for tCuPc > 22 nm. At 300 K, β = 0.11 ± 0.04 nm−1 was obtained for the whole t-range. Complementary attenuation curves for 18 K < T < 300 K are shown in the SI (Figure S4). For tCuPc < 10 nm, the weak activation16 associated with β = 0.7 nm−1 indicates that DT is the dominant charge transport mechanism. The T dependence of J smoothly increases with tCuPc toward ∼22 nm, suggesting a possible contribution of activated charges to the conduction.9 Nevertheless, β remains ∼0.7 nm−1, ensuring that coherent tunneling still dominates the charge transport. Supporting this statement, β ∼0.7 nm−1 in the 10−22 nm range agrees with the values reported for aromatic molecules, when a transition from coherent tunneling to activated charge transport occurs.19,50 The presence of an intermediate, coherent charge transport mechanism, which provides the continuous transition from DT to activated hopping in molecular ensembles, has already been reported for chemisorbed structures at similar t-ranges: DNA molecules for few nanometers;8 ferritins from 7 to 12 nm;9 and

In Figures 3a and 3b, the shape and intensities of the J−E plots are strongly affected by tCuPc. For tCuPc < 22 nm, J−E strongly depends on tCuPc. Such a dependence reduces as tCuPc increases over 22 nm (the traces overlap). Additionally, the analysis of the J−E characteristics at both 300 and 18 K reveals a stronger T-dependence for tCuPc > 22 nm in comparison to tCuPc < 10 nm. Then, as tCuPc increases, thermally activated charges dominate the transport. 3.3. Attenuation Factor (β) Is Mainly Dependent on tCuPc and T, Being Slightly Modified by E. In order to visualize the direct influence of tCuPc on the electrical response of our devices, Figure 4a exhibits the attenuation plots obtained at E = 0.5 ± 0.1 MV/cm from the J−E curves for T = 300 K and T = 18 K. The exponential decay observed in the J−t plot gives β, which is associated with the transport mechanism, the molecule type, and its conjugation length.2 Figure 4b exhibits the corresponding J−tCuPc characteristics obtained at E = 1.0 ± 0.1 MV/cm. As shown in Figures 4a and 4b by the straight lines (exponential fittings), distinguishable β-values are identified as tCuPc, T, and E change. E


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The Journal of Physical Chemistry C thiophenes from 8 to 22 nm.2 Here, we have identified the intermediate coherent regime as the ST,9,51 which dominates the charge transport for tCuPc in the 10−22 nm range, at T < 40 K. The transition from coherent transport to a thermal activated regime follows a change from a temperature-independent J(tCuPc)which decays as tCuPc increasesto a behavior where J(T) decays as T decreases.2,9 We observe such a specific transition at tCuPc ≈ 22 nm, when β changes from ∼0.7 to ∼0.1 nm−1 at 18 K (Figure 4a). As E increases (Figure 4b), β decreases to 0.3 ± 0.1 nm−1 for tCuPc < 22 nm and to zero for tCuPc > 22 nm. The observed transition could also be attributed to Joule heating, which is more prominent at high E-values.72 Consequently, the increasing E may enhance the contribution of hopping to the transport at 18 K, leading to the slight decrease of β. Such an effect also occurs at 300 K, where β decreases from 0.11 to 0.06 (±0.04) nm−1 as E increases from 0.5 to 1.0 MV/cm. Therefore, we conclude that the transition from coherent tunneling to activated transport mainly depends on tCuPc and T, being further assisted by E. As reported for 6.5 nm thick CuPc films connected by Au electrodes,16 the transition from coherent tunneling to activated hopping occurs at T ≈ 100 K for E around 105 V/cm. A similar transition was identified here for tCuPc < 22 nm when T increases from 18 to 300 K. In this case, the transition is evidenced by β changing from 0.7 nm−1 (DT, ST) to 0.1 nm−1 (hopping). In summary, two relevant charge transport transitions occur at low T: (1) from DT to ST (both coherent) at tCuPc = 10 ± 3 nm and (2) from ST to activated hopping at tCuPc = 22 ± 3 nm. Finally, we have validated our observations by evaluating the electrical response of another physisorbed small molecule, namely, dinaphtho[2,3-b:2′,3,-f]thieno[3,2-b]thiophene (DNTT), widely used in thin-film transistors. The preliminary results on the attenuation of DNTT reveal a transition from coherent to activated transport occurring for tDNTT = 20 ± 3 nm, at E ≈ 0.5 MV/cm and T = 18 K, similar to our findings for CuPc. The DNTT attenuation plot is shown in the SI (Figure S5). 3.5. Changes Observed in the Transition Temperature (Ttr) between Coherent and Activated Charge Transport Weakly Depend on E. Figures 5a and 5b exhibit the Arrhenius plots obtained for different tCuPc values at E = 0.5 MV/cm and E = 1.0 MV/cm, respectively. The dashed line indicates the transition temperature (Ttr) between two distinct transport regions: (1) coherent tunneling, where β = 0.3−0.7 nm−1, and (2) activated transport, where β = 0−0.1 nm−1. The coherent tunneling is intrinsically associated with T-independent charge transport characteristics.9 The carrier interaction time within the organic bridge directly scales with tCuPc, promoting decoherence of the charge transport. Thus, a thermoactivated response is observed (region 2). However, before the transport loses its coherence, long-range sequential tunneling takes place. Figure 5a confirms that coherent tunneling dominates across long distances (ca. 10−22 nm) for T < Ttr. A weak contribution of coherent transport is also observed for longer distances (>22 nm) around T ≈ 18 K, once J is weakly activated. By increasing E to 1.0 MV/cm (Figure 5b), the field-assisted mechanism starts contributing,16 and a change in the Ttr trace occurs. The activated transport still governs the conduction mechanism for tCuPc > 22 nm in region 2, while the coherent tunneling dominates for tCuPc < 22 nm for T < Ttr. Overall, the contribution of long-range coherent tunneling remains tending to zero as tCuPc increases above 22 nm.

3.6. Barrier Energies for the Activated Regime Indicate That Hopping Plays a Role, But High E Can Lead to Field-Emission Currents. The transition from coherent to activated charge transport mechanism strongly depends on tCuPc and T, while E slightly modifies the Ttr position, as shown in Figures 5a and 5b. The transition to activated hopping implies the observation of an exponential decay of J(T−1).73 The maximum decay observed for high Tvalues, in region (2) of the figures, allows the calculation of the activation energy (Ea) for the CuPc molecular ensembles. For E = 0.5 MV/cm, Ea was found to be 100 ± 20 meV and drops to 70 ± 20 meV as E increases to 1.0 MV/cm. The Ea values agree with those reported for thiophenes2 and ferritins9 but have been found to be smaller than that reported for hopping conduction in CuPc ultrathin films.16 We believe the reason for it stands for the possibility of Ea to be dependent on E, once field-assisted mechanisms were already observed around 18 K. Bof Bufon et al. have reported a transition from hopping to Frenkel−Poole emission74 in CuPc ultrathin films, in which Ea varies from 180 to 50 meV under E. Such a transition cannot be confirmed here because of the large Ea error bars. Our calculated Ea values are, in addition, much smaller than the energy difference between the contact Fermi level and the CuPc highest occupied molecular orbital (HOMO). Photoelectron spectroscopy measurements reveal energy differences larger than 600 meV.16 This indicates that the Ea values found are characteristic of hopping conduction into the molecular orbital, instead of a result of the thermal activation of charges from the electrodes into the first hopping energy level.19,75 Furthermore, Ea < 250 meV indicates that the injection limited current cannot fully explain the J−E characteristics,16,76 ensuring hopping as the main charge transport mechanism in the activated region. Hence, the low Ea values found indicate the conduction sites are shallow traps in the film. Such traps may arise from the unintentional doping of the organic semiconductor layer, localized defects in the film grain boundaries, and states created due to the electrochemical reduction of CuPc in water.16

4. CONCLUSIONS Here, we report the first experimental evidence of a long-range, sequential coherent tunneling occurring in physisorbed molecular ensembles, by varying the CuPc film thickness. Three distinct charge transport mechanisms were identified: direct coherent tunneling, sequential coherent tunneling, and activated hopping. The direct tunneling (DT) dominates the transport for ultrathin CuPc junctions (tCuPc < 10 nm) temperature-independent J−E curves below 100 K and a strong thickness dependence (β = 0.7 nm−1) are the main features. The sequential coherent tunneling (ST) dominates for junctions with barrier widths between 10 and 22 nm, and it is characterized by the weak J(E) dependence on temperature below 40 K and the strong thickness dependence (β = 0.7 nm−1). Finally, activated hopping takes place for films thicker than 22 nm, with a strong J(E) dependence on temperature above 40 K and a weak thickness dependence (β = 0.1 nm−1). It is worth mentioning that the observed sequential coherent tunneling is equivalent to the activationless mechanism observed by Yan et al.2 for oligothiophenes in the 8−22 nm range. Similar observations have been described by Kumar et al.9 and Vilan et al.51 as well. Regarding longer charge transport distances (larger than 22 nm), signatures of coherent transport are observed around 18 K for high electric fields (∼1.0 MV/ F


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The Journal of Physical Chemistry C cm), suggesting a possible field assistance on the coherent tunneling processes. Overall, the present work contributes to unify the fields of short-range tunneling and activated charge transport in the most elementary molecular system, composed of conjugated physisorbed molecular films.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02528. Description of the FN current plot, the calculated FN coefficients, the AFM characterization of the vertical junction components, discussion about the tube-footprint left on the CuPc film, CuPc attenuation plots for different temperatures, and the DNTT preliminary attenuation curve (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +55 (0) 19 35175098. ORCID

Leandro Merces: 0000-0002-6202-9824 Rafael Furlan de Oliveira: 0000-0001-8980-3587 Author Contributions

L.M. fabricated the vertical junctions, performed the measurements and data analysis, and wrote the manuscript. R.F.O. contributed to the data interpretation and the manuscript writing. D.H.S.C. contributed to the vertical junction’s fabrication. C.C.B.B. supervised the experiments, discussed the results, wrote the manuscript, and led the work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge CAPES, CNPq (Project 483550/ 2013-2), and FAPESP (Project 2014/25979-2) for the financial support. We thank Erico Teixeira-Neto (CNPEM-Brazil) for the acquisition of SEM images and Evandro M. Lanzoni and Christoph Deneke (CNPEM-Brazil) for the support with the AFM measurements.

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