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Nanoscale Conductance in Lead Phthalocyanine Thin Films: Influence of Molecular Packing and Humidity K. Priya Madhuri, Prabhleen Kaur, Md. Ehesan Ali, and Neena Susan John J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09240 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017

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The Journal of Physical Chemistry

Nanoscale Conductance in Lead Phthalocyanine Thin Films: Influence of Molecular Packing and Humidity K. Priya Madhuri a, Prabhleen Kaur b, Md. Ehesan Ali b and Neena S. John a* a

b

Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore - 560013, India

Institute of Nano Science & Technology, Habitat Center, Phase-10, Sector-64, Mohali 160062, India

KEYWORDS. Lead phthalocyanine, molecular packing, substrate effect, graphite, conductingatomic force microscopy

ABSTRACT. The influence of crystalline phase and disorder on vertical charge transport in nonplanar metallophthalocyanines is studied by depositing thin films of shuttlecock molecules, lead phthalocyanine (PbPc) and SnPc, on various substrates such as graphite, silicon and gold and investigating the structural and electrical characteristics of the films. The films represent three cases of crystalline packing such as monoclinic, triclinic and disordered phases, directed by the substrate‒molecule interactions and intermolecular interactions. Theoretical calculations are performed to understand the molecular orientations on the substrate and give insights in to the possible origin of ordered and disordered packing. Vertical charge transport in the films are

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studied by employing conducting-atomic force microscopy and compared to bulk electrical measurements. It is revealed that the monoclinic phase of PbPc on highly oriented pyrolytic graphite exhibit two order higher conductance than for silicon or gold and is related to the ordered stacking of the macrocycles through which the charge transport is facilitated. Local conductance mapping reveals inhomogeneity among conducting domains and is correlated with the grain structure. The charge transport is best fitted with tunneling mechanism for nanoscale and space charge limited current for bulk measurements. The presence of humidity is also shown to influence the nanoscale charge transport with enhanced conductance. At saturating humidity conditions, the conductance is found to decrease. This study emphasizes the implication of graphite or graphite-like platforms that promote the ordered stacking of delocalized molecular systems for enhanced vertical charge transport.

INTRODUCTION

Zeal for designing new molecular materials, attempts to tailor the device architectures, and to maneuver the molecular organization or assembly for increased output characteristics is a topic of significant interest. Organic semiconductors have revolutionized the present technology by opening up new avenues over their inorganic counterparts. Metal phthalocyanines (MPcs) are aromatic molecules capable of stacking through π-π supramolecular interactions whose properties can be correlated to their structure. Their low cost, excellent stability, flexibility, good electrical response and easy fabrication have ventured them as multifunctional materials. These sensitive materials promise applications in the field of molecular electronics and optoelectronics.1,2 It is known that MPcs exhibit polymorphism with different crystalline packing that influence their optical and electrical properties. The study of molecular crystallinity and


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orientation of these molecules on different substrates have sought particular relevance of late in the field of organic photovoltaic cells (OPVs), gas sensors, organic field effect transistors (OFETs) and electrocatalysis.3-6 In particular, the properties of nonplanar MPcs or shuttlecock molecules with their out of plane centering of the metal or oxygen atom such as lead (II) phthalocyanine (PbPc, Figure 1a), tin (II) phthalocyanine (SnPc), titanyl phthalocyanine (TiOPc) and vanadyl phthalocyanine (VOPc) are strongly influenced by molecular packing. Wang Y et al. studied the geometry and electronic nature of shuttlecock structures and correlated with the substrate influence.7 The structure of the molecule also enables the metal head to be directed towards or away from the surface and the transition between these two conformations could be achieved via external electric field as in scanning tunneling microscopy.8 The polymorphism in nonplanar phthalocyanines has been extensively studied.9-11 Formation of crystalline film is dependent upon the film deposition method, thickness, morphology, deposition temperature and also on the substrate. The triclinic and the monoclinic polymorphs of PbPc differ in their arrangement of molecular macrocycles and the molecule‒substrate surface interactions, by differing in tilting angle of the molecular plane with respect to stacking axis (Figures 1b and 1c). The DC conductivity along the vertical axis becomes high (10-4 S cm-1) when PbPc crystallizes in monoclinic form than triclinic form12 and is known as a molecular 1-D organic conductor.13 Hence, there have been efforts to correlate the molecular packing in these films deposited on various substrates in relation to specific applications. Vasseur et al. has reported that the near IR absorption and corresponding solar cell efficiency greatly depends on the molecular orientation and degree of disorder in TiOPc and PbPc systems.14,15 The nature of the substrate surface could also be altered by using certain planar MPcs or other organic/ inorganic molecules as templates to direct a desired crystallite orientation or introduce


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amorphicity.16-19 Interestingly, it has been shown that the orientation of nonplanar MPcs may be altered by an external stimulus such as temperature.11

Figure 1. (a) Side-view representation of a PbPc molecule; (b) and (c) molecular arrangements in triclinic and monoclinic forms along their b-axis. In addition to the different parameters that influence the conductivity of the MPc thin films as discussed previously, the presence of different analytes in the environment can also interact with these films and affect their optoelectronic properties in particular. Hence, MPc thin films have been demonstrated as conductometric sensors for the detection of harmful gases such as ammonia, nitrogen dioxide and volatile hydrocarbons.20,21 It has been reported that MPc films are non-conducting in vacuum and become doped and conducting when exposed to the atmosphere, especially on interacting with oxygen and moisture.22,23 It is of great significance to understand the influence of substrate, molecular packing and environment on the electrical properties of metallophthalocyanines in order to employ them for suitable applications. In the literature, there are many reports on the charge transport in metallophthalocyanine films and their variation with doping and thickness.24,25 However, majority of the studies are focused on bulk electrical properties and has not been monitored with substrate effects or humidity influence. Bulk electrical measurements give an averaged electrical


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response and do not give us a picture of the varying details at local nanoscale regime. Hence, characterization of these materials at the nanoscale is of prime importance in understanding the film morphology and the local heterogeneity in conductance in relation to the locally varying surface features. For this, conducting atomic force microscopy (C-AFM) is a reliable diagnostic tool in the field of nanoelectronics for probing the local conductive domains of the materials under study and charge transfer properties.26-29 The main objective of the work is to explore the structural arrangement of PbPc molecules on different substrates and understand the effect of molecular arrangement on charge transport in thin films. The influence of humidity on charge transport is also studied since it is an important parameter that can affect the device performance under ambient conditions. We have sought to map the conductance distribution at nanoscale for PbPc films deposited on various substrates such as highly oriented pyrolytic graphite (HOPG), silicon, and Au(111)/mica by C-AFM and the results are compared to bulk electrical measurements. The variation in the vertical conductance values of PbPc films on different substrates is correlated to molecular packing and a probable mechanism for current conduction is proposed. The current maps give insights into the conducting domains of the film. EXPERIMENTAL DETAILS Lead phthalocyanine, PbPc (TCI, dye content >95.0%) was chosen as a target material without any further purification. The material contained in a molybdenum boat was thermally evaporated onto different substrates such as glass, quartz, SiO2/Si, n-Si, Au(111)/mica, and HOPG. Glass, quartz, SiO2/Si and Au(111) were cleaned thoroughly in acetone and distilled water. HOPG was freshly cleaved and n-Si was cleaned by the standard RCA procedure before loading into the chamber. The PbPc material was deposited at a base pressure of 1×10-5 mbar and the substrate


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was held at 100°C. The rate of deposition was 1‒1.5 Ǻ/s, as monitored by quartz crystal microbalance. The UV-visible spectrum of the as deposited PbPc film on quartz substrate was acquired using Perkin-Elmer Lambda 20. The crystal structure of the films was investigated by X-ray diffractometer (XRD) (Rigaku Smartlab) equipped with parallel beam optics monochromatic and Cu kα radiation (40 kV, 30 mA) was incident at a grazing angle of 0.3°. Raman spectra for the films deposited on different substrates was acquired using Horiba XploRA PLUS spectrometer with 532 nm laser through a 50X objective. Field emission scanning electron microscopy (FESEM) images were obtained using a TESCAN MIRA3 LM. The topography of the thin films on various substrates were acquired using an atomic force microscope (AFM, Agilent 5500) operating in tapping mode equipped with a rectangular probe having a resonance frequency of 325 kHz and a force constant of 40 Nm-1 (MikroMasch, USA). The surface features of the films along with the current mapping were investigated using C-AFM mode. C-AFM was performed in contact mode using a silicon probe (MikroMasch, USA), with a conductive Cr/Au coating and has a diameter less than 35 nm, force constant, k = 0.18 Nm-1. Tapping mode and C-AFM imaging were performed under ambient conditions. For humidity dependent studies, C-AFM was carried out in an environmental chamber wherein a humidity sensor was placed. Different levels in humidity were achieved by bubbling nitrogen gas through water and passing into the chamber. The sample was also heated inside AFM chamber to oust any adsorbed water molecules (Lake Shore temperature controller). Keithley 4200 semiconductor characterization system was employed for bulk I-V characteristics of PbPc films deposited on HOPG, n-Si substrates and Au(111) in vertical configuration similar to C-AFM. The conducting substrates were taken as


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bottom electrodes and silver paint coated PDMS blocks were used to achieve a soft contact on the top of the films.30

RESULTS AND DISCUSSIONS PbPc films are thermally evaporated in vacuum with an intention to probe the preferred orientation of the film with respect to different substrates. The films are thermally stable and the molecular identity of the evaporated film is confirmed by performing UV-Vis absorption spectroscopy of the deposited film on a quartz substrate. Figure 2 shows the absorption spectrum of a thin PbPc film deposited on a quartz substrate. The spectrum exhibits B-band and a broad Qband that originate due to the transitions from the HOMO(π) - LUMO(π*) orbitals.31 The Bbands, also called the Soret bands are seen at 350 nm and 450 nm which are due to the transitions among π-π* orbitals, b2u, b1u and a2u - eg*. The Q-band arises from a1u - eg* as a doublet between 600 to 950 nm, usually depicting the presence of different polymorphs; blue and the red spectral shifts observed for H and J-aggregates, respectively.2,32 The peaks around 660 nm and 710 nm correspond to the monoclinic phase or H-aggregate while the intense near infrared (NIR) peak at λ ~ 900 nm is due to the triclinic structure or J-aggregate.3 H-aggregate, wherein PbPc molecules are aligned by π stacking facilitate hole transport while J-aggregate with its upside-down stacking is important for NIR absorption in solar cells.33


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Triclinic form

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Figure 2. UV-Vis absorption spectrum of a PbPc thin film deposited on a quartz substrate. The molecular packing of PbPc thin films deposited on various substrates is probed by XRD at a glancing angle of 0.3° (Figure 3a). XRD from PbPc powder shows sharp peaks due to triclinic phase at 6.96° and 12.15°.3,34 The film deposited on quartz substrate shows a broad peak around 7.55° and 16.54° corresponding to (100) and (130) of triclinic phase. A peak at 12.61° due to (320) plane of monoclinic phase is also seen. Hence, PbPc on quartz consists of a mixture of monoclinic and triclinic phases that coexist with some degree of disorder. This is also in accordance with the optical absorption spectra (Figure 2) which showed the presence of H- and J-aggregates. The films obtained on Au(111) also show a broad peak at 7.3° (monoclinic at 6.9° and triclinic at 7.47°) indicating contributions from monoclinic and triclinic phases.3,34 However, unlike in the case of quartz, we do not see a pronounced monoclinic peak at 12.61°, which suggest that the monoclinic nanocrystallites are scattered or poorly ordered, within triclinic crystallites. In the case of Au(111) substrate, the crystallites seem to be more disordered than in the case of glass. The film on glass appears majorly triclinic with lesser degree of disorder. n-Si and SiO2/Si also exhibit triclinic phase of PbPc films at 7.55° indicating a highly (100) oriented


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and a more ordered structure. These triclinic moieties are usually described to have macrocycles that alternate the concave and convex sides. Also, the stacking could be upside down and macrocycles overlap laterally while increasing the central atomic spacing (Figure 1b). Interestingly, on HOPG substrate, a pronounced peak at 12.61° is seen which indicates a highly oriented monoclinic structure consisting of π stacked PbPc molecules (Figure 1c). The observed phases on different substrates show the preferred orientation of PbPc film. We clearly observe a change in PbPc structure from triclinic on Si to monoclinic on HOPG arising from the difference in the molecular packing in response to their interaction with the substrate. HOPG surface with hexagonal sp2 carbon network can promote π stacking with a high probability for face on orientation thus giving rise to an arrangement which has minimum contact angle with the substrate as shown in figure 3b. For multilayer, the π stacking will be extended along the b-axis of PbPc molecules with stronger intermolecular interaction, parallel to the basal plane of graphite. PbPc molecules may interact with Si and Au through van der Waals or chemical forces and can facilitate a different packing.18,35 During multilayer film deposition, it is also likely for the PbPc layers to contain the two polymorphs on a given substrate influenced by deposition rates, film thickness and temperature and can also lead to poorly ordered and amorphous segments among ordered crystallites.3,11,14,36 According to Vasseur et al. the disorder becomes more evident in the top layers of a thicker film where intermolecular dispersion forces overcome the molecule‒substrate interactions.15


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Figure 3. (a) X-ray diffraction patterns of ~50 nm PbPc thin films deposited on various substrates (* peaks from the mica substrate); (b) Illustration of PbPc molecule lying flat on a graphite surface. MicroRaman spectra were acquired from PbPc films on various substrates employing 532 nm laser to glean further information on crystallinity. Earlier studies on VOPc and TiOPc have shown that amorphous and crystalline phases may be distinguished by higher frequency peaks associated with macrocycle deformation modes (670‒900 cm-1), isoindole stretching modes (1350‒1500 cm-1) and pyrrole stretching modes (1500‒1550 cm-1).10,14 In figure 4a, Raman


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spectra from PbPc on substrates show higher intensities relative to the powder spectra possibly due to the highly oriented films, however, the peak positions are comparable. The peaks have been indexed and are given in table S1. The pyrrole stretch peak for PbPc on Au(111) substrate appears as a doublet with 1510 and 1530 cm-1 components and the higher frequency peak is stronger, indicating a higher degree of disorder among the molecules.10,14 In all other cases, the peak appears single at 1505 cm-1 (Figure 4b). The very high intensities of PbPc peaks observed in the case of Au(111) compared to other substrates are clearly due to surface enhanced Raman spectroscopy (SERS) effect by nanostructured Au grains. PbPc on HOPG also shows relatively higher intensities and indicates the presence of ordered monoclinic domains of PbPc. Molecular resonance due to the absorption of 532 nm used for Raman studies by monoclinic domains can also give rise to intense Raman peaks.37 Peak intensities of PbPc from n-Si, glass and SiO2 are comparable and no shoulders observed at higher Raman shifts and represent triclinic phase with very less degree of disorder.


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Figure 4. (a) MicroRaman spectra of PbPc films deposited on different substrates; (b) Expanded view of the region of interest. Figures 5a, 5b and 5c are the SEM images of PbPc films deposited on HOPG, Si and Au(111), respectively. The images show that the deposited films are quite continuous and uniform showing good coverage. Interconnected structures with not so distinct grain boundaries constitute the film surface on HOPG (Figure 5a) while elongated grains of 150‒175 nm in length and 60‒70 nm in width are seen on the surface of n-Si (Figure 5b). The film on Au(111) is


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significantly different with both spherical and short rod like structures that are not uniformly distributed (Figure 5c). The topography of these films has also been imaged in dynamic AFM mode. Figures 5d, 5e and 5f show the varying morphology of PbPc films deposited on HOPG, Si and Au(111), respectively and are in close agreement with SEM images. Figure 5d shows the grains on HOPG which are mostly uniform and are found to be tightly packed and well connected. The grains on Si substrate (Figure 5e) are also similar to the one obtained on HOPG but the slightly larger grains with clear grain boundaries are found to be scattered over the small sized grains that constitute the film which is also seen from SEM image (Figure 5b). Spherical grains of PbPc are seen on Au(111) under AFM (Figure 5f). The thickness analysis is performed by taking section profile across the film edge (Figures 5g and 5h) and the height is found to be in the range 45‒50 nm. The roughness of these films are estimated and it is found that the PbPc on HOPG and Si exhibit RMS roughness of about 5 and 4.7 nm while Au(111) gives a higher value of 10.1 nm. The lesser roughness of the film in the case of HOPG and Si substrates is attributed to the atomically smoother surfaces. In the case of Au(111) substrate, the higher roughness of deposited PbPc film is introduced by the inherent roughness of the Au grains which is found to be 2.68 nm. This may also be correlated to the degree of disorder in PbPc packing introduced by the substrates. XRD (Figure 3a) shows that PbPc molecules have a more ordered packing with monoclinic or triclinic crystallites on HOPG and n-Si, respectively, and a higher degree of disorder on Au(111).


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Figure 5. (a), (b), (c) FESEM images and (d), (e), (f) 2×2 µm2 AFM topography of PbPc films deposited on (a, d) HOPG, (b, e) n-Si and (c, f) Au(111) substrate; (g) AFM image of a PbPc thin film scanned across the edge; (h) Profile section across the PbPc film showing the thickness. For vertical charge transport studies by C-AFM, conducting substrates are required as they form the bottom electrode and hence, we have focused on PbPc films deposited on to HOPG, nSi and Au(111) for further investigations. The molecular stacking of PbPc greatly influences the electrical transport properties of the thin films and in our experiments HOPG, n-Si and Au(111)


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substrates represent different molecular packing, monoclinic, triclinic and mixture of phases with amorphous character. Figures 6a, 6b and 6c show the topography of the films on various substrates and the corresponding current images obtained at fixed sample biases are given in figure 6d, 6e and 6f. It is seen that the film deposited on the HOPG show highest electrical response while the films on n-Si and Au(111) show lower currents. At 1 V sample bias (Figure 6d), PbPc domains on HOPG show slight inhomogenity in current distribution; the average current being 0.125 nA and the highest current being 0.25 nA. At higher biases (>1 V), we observe that more number of domains contribute to higher currents and the current conduction across the grains become homogenous. In the case of PbPc on Si (Figure 6e), the average current observed for the conducting domains is 5 pA at 2 V, which is two orders lower than that observed for HOPG at 1 V. However, the difference between the observed average current and maximum current is only a few pA in this case and hence, the domains can be considered more or less homogenous. For PbPc on Au(111) (Figure 6f), the conducting domains are homogenous with an average current of 2.25 pA at 3 V sample bias, which is slightly lesser than that for Si in a similar scenario. The current domains of topmost or underlying layers of PbPc cannot be differentiated for Au(111) substrate. The current profiles (Figure 6g, h, i) given below the corresponding C-AFM images indicate the variations across the domains and their sizes.


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Figure 6. (a), (b), (c) AFM topography of 1×1 µm2 scan area of PbPc film deposited on HOPG, n-Si and Au(111) substrates; (d), (e), (f) Corresponding current images obtained at a sample bias of 1 V for PbPc on HOPG, 2 V for n-Si and 3 V for Au(111); (g), (h), (i) Corresponding profile sections showing the current variation across the PbPc films. Figure 7a shows I vs. V plots on the conducting domains recorded in the vertical configuration using C-AFM. The schematic of the C-AFM setup is shown in the right inset of figure 7a in which the conductive tip is grounded and the sample is biased. The obtained C-AFM curves are


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nonlinear and rectified with higher currents beyond a threshold bias. The trend in substrate dependent conductance is similar to that observed for current maps. PbPc films on HOPG show higher electrical transport in vertical configuration with the maximum current value of about 3 nA at 5 V in the forward bias and on n-Si and Au(111) the observed values are 0.09 nA at 10 V and 0.17 nA at 5 V (left inset of Figure 7a). After a certain threshold bias, PbPc films on Au(111) exhibit higher currents than in the case of n-Si (Figure 7a and 7b), which is different from what is observed in the C-AFM image at lower biases. Figure 7b shows the I-V curves obtained from bulk electrical measurements of PbPc film. The top contact has been achieved using silver paint coated conductive PDMS films and the respective substrates serve as the bottom contact and the schematic is shown as a lower inset of figure 7b. The conductance trend similar to that of nanoscale I-V curves is observed. Current response from HOPG is around 2.5 µA at 5 V which is maximum when compared to the values of 0.38 µA at 10 V for Si and 0.25 µA at 5 V for Au(111) in the forward bias regime (upper inset of Figure 7b).

Figure 7. (a) I-V curves of PbPc film on different substrates obtained using C-AFM, Zoom-in view (left inset), Schematic of C-AFM setup (right inset); (b) I-V curves obtained from bulk electrical measurements and its schematic (lower inset), Zoom-in view (upper inset).


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The variations in electrical transport can be correlated to PbPc packing on various substrates inferred from our theoretical studies complemented by XRD and Raman spectra. PbPc molecule has been optimized on a single sheet of HOPG and Au(111) substrates using Perdew-BurkeErnzerhof (PBE) exchange-correlation (PBE) functional in density functional theory (DFT) (Figure 8) using Vienna Ab initio Simulation Package (VASP).38,39 The optimized structure of PbPc has a shuttlecock shape and Pb atom has been displaced on an average by 1.263 Å from the macromolecular ring. Hence two possible configurations, ‘face on’ and ‘face off’ are considered with respect to the substrates. The interface geometries are optimized by applying PBE+D2 to incorporate van der Waals interactions as proposed by Grimme in addition to the PBE.40,41 To calculate the molecular adsorption energies, single point calculations have also been performed using PBE as well as PBE+D3. It is evident that van der Waals interactions also play a role in molecular adsorption. The calculated adsorption energies indicate that on graphite/graphene sheet, PbPc exclusively deposits in a ‘face on’ configuration with Pb pointing upward and ‘face off’ configuration is thermodynamically unstable. However, on Au(111), PbPc can be deposited either ‘face on’ or ‘face off’ with almost equal probabilities. The calculated DFT energies for single molecule PbPc adsorption in different states on graphene and Au(111) substrates are given in tables S2a and S2b. Yin et al. have shown that the energetics required for the CuPc molecules to lie flat on HOPG is minimum compared to edge on.42 The substrate‒molecule interaction responsible for the ‘face on’ orientation directs the successive layers to be exactly on top of the preceding layer giving rise to highly ordered monoclinic PbPc on HOPG. However this is not the case for Au(111) substrate as i) both ‘face on’ and ‘face off’ have equal probabilities ii) PbPc in the case of ‘face on’ is highly mobile on Au(111) surface which is due to the surface dynamics of first


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two layers of Au substrate. These surface motions are not observed in the case of HOPG since graphene sheet has a very rigid structure and does not facilitate any prominent out of plane motions, which is responsible for the surface dynamics on Au(111). The charge density plots and density of states (DOS) for single PbPc interaction with the substrates are shown in figures S1a and b.

“face off”

PbPc/Au(111)

“face off”

PbPc/Gr

“face on”

“face on”

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PbPc/Gr

Figure 8. The PBE+D2 optimized geometries obtained by placing the PbPc molecule on HOPG and Au(111) substrates. Here, HOPG has been modeled with a single layer of graphene sheet.

The above calculations indicate a higher probability for the formation of highly ordered π stacked PbPc on HOPG, which facilitates enhanced charge transport in the vertical direction. In


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fact, the calculated DOS plots of stacked PbPc dimer (gas phase) show more energy states near the band gap (Figure S2). The heterogeneity among conducting domains seen in current image could be due to the different molecular orientations at nanoscale, particularly in multilayer films. About 84 ± 2% of the surface coverage can be considered to have highly ordered grains that show high conductance and there is no radical variation in conductance among these grains (Figures S3a, b). 2D GIXD of PbPc on various substrates such as CuI, MoO3 and organosilane modified ITO substrates by Vasseur et al. have shown that substrate‒molecule interactions direct the orientation of PbPc almost up to 20 nm and thereafter, molecule‒molecule interactions can cause orientational disorder.15 The triclinic packing in case of n-Si and less ordered phase in case of Au(111) has poor conducting channels in the vertical direction and hence the observed currents are lesser. Basova et al. have shown that in the case of substituted VOPcF16, a nonplanar Pc, the in-plane conductivity (lateral transport) is reduced by two orders of magnitude when the molecules change orientation from edge on to face on configuration.11 For Au(111), the vertical conductance is less at lower biases similar to the case of n-Si. However, beyond a threshold voltage, a percolation path is achieved among the disordered monoclinic and triclinic crystallites establishing a good charge transport than in the case of Si. This is evident from the C-AFM and bulk I-V characteristics that show enhanced current for PbPc on Au(111) than Si at higher bias voltages (Figures 7a and 7b). To verify the role of molecular stacking in charge transport we have achieved poorly ordered monoclinic PbPc on HOPG substrate by changing deposition conditions and indeed the current values are found to be lower (Figures S4a, b, c, d and e). It has been shown in the case of P3HT that 10 nm films on graphene and Si consists of ordered face on and edge on orientation, respectively, throughout the entire film structure and


20

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enhanced vertical charge transport is indeed observed for face on P3HT orientation on graphene compared to Si.30 50 nm P3HT film consists of disordered crystallites towards the top layers on graphene and vertical conductance is facilitated by charge hopping across amorphous fractions. The vertical charge transport observed in this study is analogous to the case of orientation dependent charge transport observed in VOPCF16 and P3HT films.11,30 The mechanism of charge transport was studied by analyzing nanoscale and bulk I-V curves using various reported charge transport treatments/formalisms. Vertical charge transport in P3HT films and ZnPc nanorods are reported to follow space charge limited injection current (SCLC) mechanism at higher biases.30,43 In vertical junctions of ultra-thin CuPc films of 10 nm thickness, a transition from thermally activated hopping to tunneling regime has been reported.25 In the present study, for 50 nm PbPc films, nanoscale I-V characteristics acquired by Au tip indicate charge transport by tunneling. At room temperatures, the thermal energy may not be enough for the charge carriers to overcome the interface barrier hence charge transport is dominated by tunneling mechanism. The generalized Simmons equation is given by: ln A plot of ln

!

8 2∗ 1 = − + ln 3ℎ 8 ℎ ∗

against '( (Figure 9a) shows a logarithmic variation in the low bias regime

indicating direct tunneling for all the cases of three substrates. A transition to linear dependence is seen in the high bias regime and the transition point varies for different substrates. The linear dependence in the high bias regime as shown in the inset of figure 9a for Au(111) and Si substrates is akin to field-emission tunneling or Fowler-Nordheim (F-N) tunneling. For HOPG substrate, signatures of resonant tunneling are seen after a bias of 0.25 V. The transition point gives us the idea of interface barrier existing across the organic semiconductor‒electrode


21


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Page 22 of 37

interface and it is least in case of HOPG (0.39 V) followed by Au(111) (2.13 V) and then n-Si (6.88 V) (Figure 9a). Several theoretical calculations and simulations in stacked oligoacenes and single molecules have shown that the cofacial alignment of phenyl rings with delocalized π electrons promote strong electron coupling and higher transfer integrals facilitating charge transport, which is analogous to that of stacked PbPcs on HOPG.24,44-46 The bulk I-V measurements follow ohmic behavior at low voltages with current values varying approximately linearly with voltage. While at higher biases, charge injection induces (space charge limited current) SCLC mechanism. The curves fit best with semi-empirical Mott-Gurney equation which is given as follows: )* (/ ln = 0.89γ + ln/α ξ ξ 01 * The log-log plot of I-V (Figure 9b) follow SCLC mechanism at higher applied biases where I~Vn. On extracting the data from SCLC regime and fitting a linear plot of ln 2

345

! 6 against 2 4 6

(7

, intercept of the line gives ln89 : : 0;, where V is the applied voltage, L

is the thickness of the film, J is the current density, α is the prefactor (α=8.2), ε is the relative permittivity of the MPc film (ε ~10), εo is the permittivity of the free space (8.85419×10-12 Fm-1), and µ is the mobility of the charge carriers. 27,39 Mobility values obtained for PbPc on HOPG, nSi and Au(111) was 1.45 × 10-6, 1.00 × 10-12 and 1.21 × 10-7 cm2V-1s-1, respectively.


22

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(a)

Au(111)

Vt = 2.13 V

-4

n-Si

Vt = 6.88 V -4

-6

Au(111)

-6 -8

ln(I/V2)

ln I/V2

HOPG

Vt = 0.39 V

-2

-8

-10 -7.2

0.2

-10

0.3

0.4

0.5

n-Si

-8.4

-9.6 -1.84

0.10

0.11

0.12

0.13

0.14

HOPG

-2.76 -3.68

(b)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

-1

-12

V

-2

0

2

4

-36

(b) -39 0.1

3 2 ln (JL /V )

1

6 V-1

0.01

8

HOPG

10

12

HOPG

Au (111)

-42

Au(111)

-45 -48

I ∼Vn

-51

I (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


n-Si

n-Si

-54 500

600

700

800

(V/L)

900 1000 1100

1/2

0.001

I ∼V 1E-4 1E-5 0.1

1

V (V) Figure 9. (a) Plot of ln

!

against '( for nanoscale I-V curves showing transition from direct

tunneling to F-N tunneling for HOPG, n-Si and Au(111) and inset is the linear fit for the F-N tunneling regime; (b) Log-log plot of the I-V characteristics obtained from bulk measurements at PbPc/HOPG, n-Si and Au(111) interface following SCLC mechanism and inset is the J-V curve extracted from the SCLC region of I-V curve. The differences in charge transport mechanisms operating for C-AFM and bulk electrical contacts could be arising due to the difference in the size of electrodes for individual


23


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Page 24 of 37

measurements. The semi-empirical Mott-Gurney equation for SCLC mechanism given by Ginger et al. accounts for the electric field dependent charge mobility developed due to the sharp CAFM probe.27 However, we could not validate this approach to fit our C-AFM data. The space charge limited region can be fit only at very high biases which do not seem appropriate (Figures S5a, b and c). The C-AFM curves are best fitted with the tunneling mechanism. A tunneling mechanism has been reported for pentacene film of channel length 200 nm by B. K. Sarkar et al. although below 200 K.47 Here, carbon nanotubes are used as electrodes for planar device measurements. In our case, we believe that tunneling is preferred under C-AFM conditions where a sharp conductive tip can produce high electric field than bulk electrodes. We have observed a similar trend in the vertical charge transport for another nonplanar MPc system, Sn(II)Pc. The UV-Vis spectra of SnPc show both H and J aggregates with a greater fraction of J aggregates (Figure S6). XRD shows a highly oriented monoclinic SnPc film on HOPG and triclinic on Si and mixture of phases on Au(111) similar to PbPc (Figure S7). Accordingly, higher conductance is observed for film on HOPG followed by films on Au(111) and Si (Figure S8). However, the absolute currents measured are very low due to the higher electronic band gap of SnPc (2.6 eV‒3.4 eV have been reported). The current images obtained by C-AFM reveal the trend for different substrates similar to that obtained for PbPc (Figure S8). This further confirms the influence of molecular packing and orientation on charge transport in nonplanar MPcs.


24

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nA pA

9 0.009

40

nA pA

(b)

0.01

(a)

0.1

90 0.09

Current (pA)

20

10

0.007

7

70 0.07

70 0.07

0.006

0.06

0.06

0.005

5

0.05

50

0.05

0.004

0.04

0.04

0.003

3

0.03

30

0.03

0.002

0.02

0.02

0.001

1

0.01

10

0.01

00

00

00

(e)

0.6 0.8 Distance (µm)

Current (pA)

80

1.0

30

10

80

0.12

60

60% RH-Start 10% RH

40

0.10

60% RH-Back 80% RH

0.08

0.0

0.2

0.4

Distance

(f)

0.6 (µm)

0.8

10% RH-after100°C

1.0

I (nA)

0.4

50

(g)

0

0.2

0.1

90 0.09 0.08

20

0 0.0

(c)

0.08

100

30

nA pA

0.008

120

(d) Current (pA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.06

60

0.04

40

0.02

20

0.00 0.0 0 0.0

0.2

0.4

0.6

0.8

0.5

1.0

1.0

1.5

2.0

V (V)

Distance (µm)

Figure 10. (a), (b), (c) Current images (scan area of 1×1 µm2) of PbPc film deposited on HOPG obtained at 10% RH, 60% RH and 80% RH, respectively, with 1 V bias; (d), (e), (f) Corresponding profile sections showing the current across the PbPc films; (g) I-V curves obtained from PbPc film on HOPG at different atmospheric conditions. We have also studied the effect of ambient humid conditions on the electrical properties in PbPc films. The humidity in the AFM chamber is varied to very low (10% RH), medium (60% RH) and high values (80% RH) and current images are acquired under these conditions. PbPc films on HOPG substrate that have exhibited the highest electrical response under ambient conditions is subjected to different humidity conditions. Here, we have not explored the material


25


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Page 26 of 37

as a humidity sensor but to see how humidity affects the conductivity of PbPc film and hence the RH conditions are varied coarsely. Figures 10a, b and c show the current images obtained at different RH levels and the corresponding section profiles are given in figures 10d, e and f. Figure 10g gives the I-V characteristics obtained at various RH. Initially, current image is obtained at ambient conditions (60% RH), wherein an average current of 0.055‒0.065 nA is observed and the peak current observed is 0.18 nA. I-V characteristics obtained by C-AFM shown in figure 10g also indicate the higher conductance of PbPc films in ambient. When nitrogen gas is purged into the environmental chamber of AFM, RH came down to 10% and the domains exhibit less current in the C-AFM image (Figure 10a). Average value of current is less than 0.01 nA and a few grain boundaries with current values of 0.03 nA are seen. The section profile clearly indicates the variation across the conducting domains. I-V plot also shows lesser current (Figure 10g). I-V curves have also been acquired after heating the sample to 100°C in nitrogen atmosphere in order to ensure complete desorption of water and the plot shows a behavior similar to the case of nitrogen purging with slightly lesser peak current. When water vapor is allowed into the chamber to regain RH to 60% corresponding to ambient humidity, however, a sudden increase in conductance is not observed. The film was allowed to equilibrate under the conditions for a few hours to regain the conductance. Figure 10b shows the current image after equilibration indicating an increase in the number of domains with higher conductance. The conductance distribution across the domains is largely non-uniform with an average current of 0.045‒0.055 nA and peak current of 0.1 nA (Figure 10b). I-V plot (Figure 10g), show that the conductance has regained to the starting ambient conditions. When the humidity is further increased to 80%, the conductance of the domains is seen to decrease with an average current of 0.025‒0.035 nA (Figure 10c). Most of the domains exhibit this average


26

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value and only a few domains show peak current. I-V curve also shows a decreased current at very high RH (Figure 10g). The section profiles shown alongside the current images indicate the variation in conductance from low RH to high RH across the conducting domains. The increase in conductance of MPcs when exposed to humidity and or oxygen atmosphere has been reported by other researchers and has been explored for sensing applications.21,48-51 The effects of oxygen adsorption or p-type dopant in general, on the energy levels of doped CuPc molecule are well studied using photoemission studies.52 However, several arguments have been reported concerning the interaction of MPcs with water molecules such as a p-dopant or a lewis base.53 The observed conductance changes in PbPc films under various RH conditions may be attributed to the different levels of water adsorption or diffusion into the films. At 60% RH levels, when the moderate population of water molecules adsorb on the surface, the I-V characteristics indicates an increase in charge transport and the behavior is similar to the case of oxygen adsorption. At 10% RH, the purging of nitrogen aids in desorption of water molecules and hence, conductance decreases by tenfold.21 On the other hand, at very high humidity (80% RH), the chamber is saturated with water molecules, which can diffuse into the interstitial sites of the PbPc disrupting the molecular stacking. This may be the cause of lesser current values than what is seen at moderate conditions. CONCLUSIONS Molecular packing in thin films of Pb(II)phthalocyanine, a shuttlecock molecule, is greatly influenced by the substrate‒molecule interactions. On atomically flat graphite surface, PbPc films acquire monoclinic structure directed by π-π and van der Waals interaction between basal plane of HOPG and phthalocyanine macrocycles facilitating a ‘face on’ configuration of PbPc while on the Si surface, the interactions favor a triclinic structure. On Au(111) surface, equal


27


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probability for ‘face on’ and

Page 28 of 37

‘face off’ configurations and surface dynamics promote a

disordered structure with both types of PbPc nanocrystallites. The molecular orientation and the origin of ordered and disordered phases have been deduced from DFT calculations on single PbPc adsorbed on graphite and Au(111) sheets. The vertical charge transport in PbPc films vary with the molecular packing, the highest conductance observed for films on HOPG substrates with higher degree of π overlap among the phthalocyanine domains. On Si and Au(111) surfaces, the conductance is almost two orders less with disordered packing favoring better transport than triclinic. DOS calculations on gas phase, stacked PbPc also support the enhanced conductance for monoclinic packing. Nanoscale current mapping reveals inhomogeneity among the conducting domains of PbPc among the ordered and disordered fractions on various substrates. Nanoscale I-V characteristics are best fitted with tunneling mechanism and a lower interface barrier is identified for PbPc on graphite. Local charge transport is also affected by humidity for a given substrate and the higher conductance at ambient conditions than in inert atmosphere is attributed to the doping effect by water molecules. At saturated humidity levels, the conductance decreases and the conducting domains are more uniform.

ASSOCIATED CONTENT Supporting information is available free of charge on the ACS Publications website at DOI: Charge density interactions and DOS of PbPc on HOPG and Au(111) substrates, DOS for stacked PbPc dimer, Current image of PbPc on HOPG showing surface coverage, C-AFM and XRD of PbPc on HOPG deposited at room temperature, Semi-empirical Mott-Gurney fit for CAFM curves, UV-Vis spectra, XRD and C-AFM images along with I-V curves for SnPc thin


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film deposited on HOPG, Si and Au(111) substrates, Indexed Raman signals of PbPc thin film, DFT energies for PbPc molecule on HOPG and Au(111) substrate. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT The authors acknowledge financial assistance from DST FastTrack project no. SR/FT/CS170/2011.

REFERENCES 1. Guillaud, G.; Simon, J.; Germain, J. P. Metallophthalocyanines: Gas sensors, resistors and field effect transistors. Coord. Chem. Rev. 1998, 178-180, 1433-1484. 2. Claessens, C. G.; Hahn, U.; Torres, T. Phthalocyanines: From Outstanding Electronic Properties to Emerging Applications. The Chemical Record 2008, 8, 75-97. 3. Vasseur, K.; Rand, B. P.; Cheyns, D.; Froyen, L.; Heremans, P. Structural Evolution of Evaporated Lead Phthalocyanine Thin Films for Near-Infrared Sensitive Solar Cells. Chem. Mater. 2011, 23, 886-895. 4. Alessio, P.; de Oliveira, R. F.; Aoki, P. H. B.; Pereira, J. D. A. S.; Braunger, M. L.; Furini, L. N.; Vieira, M.; Teixeira, S. R.; Job, A. E.; Saenz, et al. Molecular


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 37

Architecture and Electrical Properties in Evaporated Films of Cobalt Phthalocyanine. J. Nanosci. Nanotechnol. 2012, 12, 7010-7020. 5. Li, L.; Tang, Q.; Li, H.; Hu, W. Molecular Orientation and Interface Compatibility for High Performance Organic Thin Film Transistor Based on Vanadyl Phthalocyanine. J. Phys. Chem. B 2008, 112, 10405-10410. 6. Somashekarappa, M. P.; Sampath, S. Orientation dependent electrocatalysis using selfassembled molecular films. Chem. Commun. 2002, 12, 1262-1263. 7. Wang, Y.; Kroger, J.; Berndt, R.; Hofer, W. Structural and Electronic Properties of Ultrathin Tin-Phthalocyanine Films on Ag(111) at the Single-Molecule Level. Angew. Chem. Intl. Ed. 2009, 48, 1261-1265. 8. Toader, M.; Hietschold, M. Tuning the Energy Level Alignment at the SnPc/Ag(111) Interface Using an STM Tip. J. Phys. Chem. C. 2011, 115, 3099-3105. 9. Jennings, C. A.; Aroca, R.; Kovacs, G. J.; Hsaio, C. FT-Raman Spectroscopy of Thin Films of Titanyl Phthalocyanine and Vanadyl Phthalocyanine. J. Raman Spectrosc. 1996, 27, 867-872. 10. Coppede, N.; Toccoli, T.; Pallaoro, A.; Siviero, F.; Walzer K.; Castriota, M.; Cazzanelli, E.; Iannotta, S. Polymorphism and Phase Control in Titanyl Phthalocyanine Thin Films Grown by Supersonic Molecular Beam Deposition. J. Phys. Chem. A 2007, 111, 12550-12558. 11. Basova, T. V.; Kiselev, V. G.; Dubkov, I. S.; Latteyer, F.; Gromilov, S. A.; Peisert, H.; Chasse, T. Optical Spectroscopy and XRD Study of Molecular Orientation,


30

Page 31 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Polymorphism, and Phase Transitions in Fluorinated Vanadyl Phthalocyanine Thin Films. J. Phys. Chem. C 2013, 117, 7097-7106. 12. Papageorgiou, N.; Ferro, Y.; Salomon, E.; Allouche, A.; Layet, J. M. Geometry and electronic structure of lead phthalocyanine: Quantum calculations via densityfunctional theory and photoemission measurements. Phys. Rev. B 2003, 68, 235105. 13. Frauenhein, T.; Hamann, C.; Muller, M. Electric Field-Induced Disorder- Order Transition in Organic Polycrystalline Films of Quasi-One-Dimensional LeadPhthalocyanine. Phys. Stat. Sol. A 1984, 86, 735-747. 14. Vasseur, K.; Rand, B. P.; Cheyns, D.; Temst, K.; Froyen, L.; Heremans, P. Correlating the Polymorphism of Titanyl Phthalocyanine Thin Films with Solar Cell Performance. J. Phys. Chem. Lett. 2012, 3, 2395-2400. 15. Vasseur, K.; Broch, K.; Ayzner, A. L.; Rand, B. P.; Cheyns, D.; Frank, C.; Schreiber, F.; Tonet, M. F.; Froyen, L.; Heremans, P. Controlling the Texture and Crystallinity of Evaporated Lead Phthalocyanine Thin Films for Near-Infrared Sensitive Solar Cells. Appl. Mater. Interfaces 2013, 5, 8505-8515. 16. Rochford, L. A.; Ramadan, A. J.; Woodruff, D. P.; Heutz, S.; Jones, T. S. Ordered Growth of Vanadyl Phthalocyanine (VOPc) on an Iron Phthalocyanine (FePc) Monolayer. Phys. Chem. Chem. Phys. 2015, 17, 29747-29752. 17. Kim, T. -M.; Shim, H. -S.; Choi, M. -S.; Kim, H. J.; Kim, J. -J. Multilayer Epitaxial Growth of Lead Phthalocyanine and C(70) Using CuBr as a Templating Layer for


31


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Page 32 of 37

Enhancing the Efficiency of Organic Photovoltaic Cells. Appl. Mater. Interfaces 2014, 6, 4286-4291. 18. Wang, C.; Liu, X.; Wang, C.; Xu, X.; Li, Y.; Xie, F.; Gao, Y. Molecular Orientation of Copper Phthalocyanine Thin Films on Different Monolayers of Fullerene on SiO2 or Highly Oriented Pyrolytic Graphite. Appl. Phys. Lett. 2015, 106, 121603. 19. Roy, S. S.; Bindl, D. J.; Arnold, M. S. Templating Highly Crystalline Organic Semiconductors Using Atomic Membranes of Graphene at the Anode/Organic Interface. J. Phys. Chem. Lett. 2012, 3, 873-878. 20. Azim-Araghi, M. E.; Krier, A. The Influence of Ammonia, Chlorine and Nitrogen Dioxide on Chloro-Aluminium Phthalocyanine Thin Films. Appl. Surf. Sci. 1997, 119, 260-266. 21. Bohrer, F. I.; Sharoni, A.; Colesniuc, C.; Park, J.; Schuller, I. K.; Kummel, A. C.; Trogler, W. C. Gas Sensing Mechanism in Chemiresistive Cobalt and Metal-Free Phthalocyanine Thin Films. J. Am. Chem. Soc. 2007, 129, 5640-5646. 22. Kerp, H. R.; Westerduin, K. T.; van Veen, A. T.; van Faassen, E. E. Quantification and effects of molecular oxygen and water in zinc phthalocyanine layers. J. Mater. Res. 2001, 16, 503-511. 23. de Haan, A.; Debliquy, M.; Decroly, A. Influence of atmospheric pollutants on the conductance of phthalocyanine films. Sens. Actuators B 1999, 57, 69-74. 24. Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Bredas, J. -L. Charge Transport in Organic Semiconductors. Chem. Rev. 2007, 107, 926-952.


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Page 33 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25. Bufon, C. C. B.; Vervacke, C.; Thurmer, D. J.; Fronk, M.; Salvan, G.; Lindner, S.; Knupfer, M.; Zahn, D. R. T.; Schmidt, O. G. Determination of the Charge Transport Mechanisms in Ultrathin Copper Phthalocyanine Vertical Heterojunctions. J. Phys. Chem. C 2014, 118, 7272-7279. 26. Liu, Y.; He, J.; Kwon, O.; Zhu, D. -M. Probing Local Surface Conductance using Current Sensing Atomic Force Microscopy. Rev. Sci. Instrum. 2012, 83, 013701/1-5. 27. Reid, O. G.; Munechika, K.; Ginger, D. S. Space Charge Limited Current Measurements on Conjugated Polymer Films using Conductive Atomic Force Microscopy. Nano Lett. 2008, 8, 1602-1609. 28. Wood, D.; Hancox, I.; Jones, T. S.; Wilson, N. R. Quantitative Nanoscale Mapping with Temperature Dependence of the Mechanical and Electrical Properties of Poly(3hexylthiophene) by Conductive Atomic Force Microscopy. J. Phys. Chem. C 2015, 119, 11459-11467. 29. Madhuri, K. P.; Bramhaiah, K.; John, N. S. Nanoscale photocurrent distribution in ultra-thin films of zinc oxide nanoparticles and their hybrid with reduced graphene oxide. Mater. Res. Express. 2016, 3, 035004. 30. Skrypnychuk, V.; Boulanger, N.; Yu, V.; Hilke, M.; Mannsfeld, S. C. B.; Toney, M. F.; Barbero, D. R. Enhanced Vertical Charge Transport in a Semiconducting P3HT Thin Film on Single Layer Graphene. Adv. Funct. Mater. 2015, 25, 664-670.


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31. Rio, Y.; Rodriguez Morgade, M. S.; Torres, T. Modulating the Electronic Properties of Porphyrinoids: A Voyage from the Violet to the Infrared Regions of the Electromagnetic Spectrum. Org. Biomol. Chem. 2008, 6, 1877-1894. 32. Dexters, W.; Bourgeois, E.; Nesladek, M.; D’Haen, J.; Goodvaerts, E.; Haenen, K. Molecular Orientation of Lead Phthalocyanine on (100) Oriented Single Crystal Diamond Surfaces. Phys. Chem. Chem. Phys. 2015, 17, 9619-9623. 33. Hiramoto, M.; Kitada, K.; Iketaki, K.; Kaji, T. Near Infrared Light Driven Organic p-i n Solar Cells Incorporating Phthalocyanine J -Aggregate. App. Phys. Lett. 2011, 98, 023302. 34. Ottaviano, L.; Lozzi, L.; Phani, A. R.; Ciattoni, A.; Santucci, S.; Di Nardo, S. Thermally Induced Phase Transition in Crystalline Lead Phthalocyanine Films Investigated by XRD and Atomic Force Microscopy. App. Surf. Sci. 1998, 136, 81-86. 35. Niu, T.; Zhang, J.; Chen, W. Molecular Ordering and Dipole Alignment of Vanadyl Phthalocyanine Monolayer on Metals: The Effects of Interfacial Interactions. J. Phys. Chem. C. 2014, 118, 4151-4159. 36. Rochford, L. A.; Ramadan, A. J.; Heutz, S.; Jones, T. S. Selective nucleation of iron phthalocyanine crystals on micro-structured copper iodide. Phys. Chem. Chem. Phys. 2014, 16, 25404-25408. 37. Zhao, J.; McCreery, R. L. Polarized Raman Spectroscopy of Metallophthalocyanine Monolayers on Carbon Surfaces. Langmuir 1995, 11, 4033-4040.


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38. Kresse, G, and Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. 39. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 40. Bučko, T,; Hafner, J.; Lebègue, S.; Ángyán, J. G. Improved Description of the Structure of Molecular and Layered Cryatals: Ab Initio DFT Calculations with van der Waals Corrections. J. Phys. Chem. A 2010, 114, 11814-11824. 41. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2016, 132, 154104. 42. Yin, S.; Wang, C.; Xu, B.; Bai, C. Studies of CuPc Adsorption on Graphite Surface and Alkane Adlayer. J. Phys. Chem. B. 2002, 106, 9044-9047. 43. Lu, Z.; Zhan, C.; Yu, X.; He, W.; Jia, H.; Chen, L.; Tang, A.; Huang, J.; Yao, J. LargeScale, Ultra-Dense and Vertically Standing Zinc Phthalocyanine π-π Stacks as a HoleTransporting Layer on an ITO Electrode. J. Mater. Chem. 2012, 22, 23492-23496. 44. Rochefort, A.; Bedwani, S.; Lopez-Bezanilla, A. Evidence for π-Interactions in Stacked Polymers by STM Simulations. J. Phys. Chem. C 2011, 115, 18625-18633. 45. Rochefort, A.; Boyer, P.; Nacer, B. Resonant Tunneling Transport in Highly Organized Oligoacene Assemblies. Org. Electron. 2007, 8, 1-7. 46. Solomon, G. C.; Herrmann, C.; Vura-Weis, J.; Wasielewski, M. R.; Ratner, M. A. The Chameleonic Nature of Electron Transport through π-Stacked Systems. J. Am. Chem. Soc. 2010, 132, 7887-7889.


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47. Sarkar, B. K. and Khondaker, S. Thermionic Emission and Tunneling at Carbon Nanotube-Organic Semiconductor Interface. ACS Nano. 2012, 6, 4993-4999. 48. Chani, M. T. S.; Karimov, Kh. S.; Khalid, F. A.; Raza, K.; Farooq, M. U.; Zafar, Q. Humidity Sensors Based on Aluminum Phthalocyanine Chloride Thin Films. Physica E 2012, 45, 77-81. 49. Jafari, M. J.; Azim-Araghi, M. E.; Barhemat, S. Effect of Chemical Environments on Palladium Phthalocyanine Thin Film Sensors for Humidity Analysis. J. Mater. Sci. 2012, 47, 1992-1999. 50. Bialek, B.; Bragiel, P. Influence of Oxygen Adsorption on the Electronic Structure of Iron Phthalocyanine. Acta. Phys. Pol. A 1996, 89, 443-449. 51. Yasunaga, H.; Kojima, K.; Yohda, H. Effect of Oxygen on Electrical Properties of Lead Phthalocyanine. J. Phys. Soc. Jpn. 1974, 37, 1024-1030. 52. Ding, H.; Gao, Y. Photoemission Study of Oxygen and Au Modification of Doped Copper Phthalocyanine. J. Appl. Phys. 2007, 102, 043703. 53. Gomes, T. C.; de Oliveira, R. F.; Lopes, E. M.; dos Santos Klem, M.; da Silva Agostini, D. L.; Constantino, C. J. L.; Alves, N. Effects of Humidity on the Electrical Properties of Thermal Inkjet-Printed Films of Copper Tetrasulfonated Phthalocyanine (CuTsPc) onto Paper Substrates. J. Mater. Sci. 2015, 50, 2122-2129.


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Nanoscale Conductance in Lead Phthalocyanine ... - ACS Publications

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