Article pubs.acs.org/Langmuir
Sensitivity to Molecular Order of the Electrical Conductivity in Oligothiophene Monolayer Films Florent Martin,†,‡ Bas L. M. Hendriksen,† Allard J. Katan,† Yabing Qi,†,‡ Clayton Mauldin,†,§ Jean M. J. Fréchet,†,§ and Miquel Salmeron*,†,‡ †
Materials Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States Materials Science and Engineering Department, University of California, Berkeley, Berkeley, California 94720, United States § Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States ‡
ABSTRACT: Using conducting probe atomic force microscopy (CAFM), we show that electrical conductivity in oligothiophene molecular films deposited on SiO2/Si wafers is extremely sensitive to degree of crystalline order in the film. By locally distorting the molecular order in the films through the controlled application of pressure with the AFM tip, the lateral charge transport was reduced by factors varying from 2 to 10, even when no changes in the height of the film could be observed.
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INTRODUCTION Electrical transport in polymers and in small molecules has been extensively studied because of potential applications in solar energy conversion, optical displays, and electrical switches.1 Polymers such as poly[2,5-bis(3-alkylthiophen-2yl)thieno(3,2-b)thiophene] (PBTTT) and poly(3-hexylthiophene) (P3HT) are among the materials considered for applications in organic electronics and are known to form heterogeneous films.2 Structural factors such as amorphous regions,3,4 grain boundaries,5,6 and local defects within crystalline domains7 have been shown to directly correlate with the electrical properties of these organic materials and more specifically with carrier mobility and threshold voltage in field effect transistors.8 Establishing this correlation by direct characterization of the defect structures responsible for it is therefore an important goal of molecular electronics science. Xray and electron diffraction provide information on the average size of crystalline domains, lattice structure, and intermolecular spacing.9 These diffraction techniques however sample areas many micrometers in size, which makes defect characterization on the nanoscale very difficult. Scanning probe microscopies, on the other hand, can be used to study properties on the nanometer scale, especially in monolayer or few-layer films.8,10 Atomic force microscopy (AFM) provides a direct view of the morphology and can be used to resolve the crystalline structure, size, and orientation of the crystalline domains and grain boundaries.11 Electrical characterization techniques such as conducting probe AFM (CAFM) and scanning Kelvin force probe microscopy provide local conductivity and surface potential information.10−15,16,17 In previous work using CAFM, we reported a decrease of the tip−sample current when high loads were applied to 4-mercaptophenylanthrylacetylene (MPAA) and trans-stilbene monolayers deposited on gold. A decrease in the overlap of π-orbitals of the aromatic © 2012 American Chemical Society
units in neighboring molecules as a result of pressure-induced disorder was proposed as the reason for the observation. However, a direct correlation between molecular order and conductivity could not be established.18,19 In this study we use lattice resolution imaging and CAFM to investigate the structure and transport properties of crystalline monolayers of 4-(5-decyl-2,2′;5′,2″;5″,2‴;5‴,2⁗-pentathiophen-5-yl)butyric acid, for brevity referred as D5TBA (Figure 1c). This molecule has a semiconducting core of five thiophene rings connected to each other as in P3HT. We recently demonstrated the efficient lateral electrical conductivity of D5TBA monolayers due to intermolecular hole hopping and its anisotropic dependence on lattice direction.20 The lateral conductivity studies in D5TBA were possible thanks to the high electrical resistance of the SiO2 native oxide film in the substrate, which efficiently prevented short-circuiting the molecules, effectively separating conduction channels through and across the molecules between tip and the Si−p+ substrate. Here we show how the lateral conductivity can be substantially reduced by locally distorting the crystallinity of the film through the application of loads above a certain threshold with the AFM tip. High-resolution lattice resolved images show that the decrease in conductivity directly correlates with a decrease in molecular ordering of the monolayer.
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EXPERIMENTAL METHODS
The D5TBA oligomer was synthesized by Stille cross-coupling methods.21 The semiconducting part of the molecule is the pentathiophene unit (5T). The butyric acid group serves as a hydrophilic anchoring group while the hydrophobic decyl group Received: September 7, 2012 Revised: December 13, 2012 Published: December 14, 2012 1206
dx.doi.org/10.1021/la303609g | Langmuir 2013, 29, 1206−1210
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Article
(∼0.1 nm in height) occurred only for loads well above 50 nN, although conductivity changes were visible earlier, at loads above ∼30 nN. The pressure-induced conductivity changes are the central topic of this paper. Streaks of low current were sometimes observed on CAFM current images due to attachment and detachment of molecules to the tip. Accumulation of material in the tip resulted in a gradual increase of the contact resistance between the tip and the monolayer. This did not constitute a significant limitation to this study since our conclusions are derived from comparing the conductivity of neighboring regions of the monolayer captured in the same AFM scan, so that any change undergone by the tip affects equally these different regions (most comparisons can be made within one single AFM scan line). Also, we frequently acquired images with new tips so that the transfer film was absent or very thin. The load manipulation experiments on the monolayer were carried out with no applied bias so that no current was flowing.
Figure 1. (a) Topographic AFM image of a D5TBA monolayer film formed on a Langmuir−Blodgett trough and transferred to mica. (b) Corresponding lateral force (friction) AFM image, with dark indicating low friction. D5TBA forms islands composed of elongated single crystal domains emanating from a nucleation center at the interior. The domain boundaries are best seen in the friction images due to their higher friction. Similar films are formed on SiO2/Si wafers and on Si3N4 or C films on TEM grid sample holders. Insets: (left) lattice resolution lateral force AFM image revealing the crystalline structure of a crystalline domain; (right) corresponding fast Fourier transform and top view model of the herringbone structure, the green segments representing thiophene planes of D5TBA. (c) Structure of 4-(5-decyl2,2′;5′,2″;5″,2‴;5‴,2⁗-pentathiophen-5-yl)butyric acid (D5TBA).
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RESULTS AND DISCUSSION Figure 1a shows a contact mode topographic AFM image of a 2 × 2 μm2 area of a D5TBA film deposited on mica. The film height is 2.7 nm. A large island and its domain structure filling most of the image is visible as well as some filamentous structures nearby. Figure 1b shows the corresponding lateral force (friction). The friction was always lower over the film than over the bare SiO2/Si substrate due to the lubricating properties of the exposed decyl chain tails.27 The radial domains in the islands are single crystals, as shown by lattice resolved images and by TEM.28 They are separated by boundaries where the friction is higher. The left inset in Figure 1b shows a lattice-resolved friction image from a region inside one of the crystalline domains. The Fourier transform (FT) of the image is also shown. The image is slightly distorted due to drift of the sample with respect to the scanner during imaging. After correcting for drift (using the adjacent mica lattice as reference), the spacing in the three lattice directions was determined to be 4.8 ± 0.4 Å and the angle between them close to 60°. These results are in agreement with a two molecule per unit cell herringbone structure (inset in Figure 1b), the most common in oligothiophene molecular crystals.29 The three close-packed directions of the herringbone unit cell, [1,1], [−1,1], and [2,0], are resolved in the AFM images. The relative orientation of molecules within the unit cell was determined by TEM diffraction28 because AFM images only the alkane chain terminations. To study the effect of molecular defects and disorder on the electrical transport properties of the film, we applied increased loads to the film followed by imaging at the lower 15 nN imaging load. Figures 2a and 2c show topographic and current images at −0.5 V sample bias of the D5TBA film on SiO2/Si− p+. Similar to our previous study,20 we observed that the current was higher on the D5TBA islands than on the bare substrate and that the current level scaled with the size of molecular islands. This size dependence is due to the efficient lateral transport through the islands, which increases the effective electrical contact area.20,30−32 When the tip is in contact with the D5TBA islands, the charge carriers (holes in this case20) travel parallel to the surface in the crystalline molecular layer hopping from molecule to molecule before tunneling to and across the oxide, thereby largely increasing the number of tip−substrate conducting channels. Figures 2b and 2d show topography and current images after a square area of 2.25 × 2.25 μm near the center of the image (marked by the
(C10H21 alkane chain) improves solubility and promotes ordering in self-assembly. Monolayers of D5TBA were prepared by the Langmuir−Blodgett method.20 A volume of 1 mL of 0.1 mM D5TBA dissolved in chloroform was filtered and deposited on the surface of ultrapure water contained in a PTFE Langmuir−Blodgett trough. After evaporation of the chloroform the amphiphilic molecular layer floating on the water was compressed to surface pressures in the range of 0−10 mN/m. The submerged substrate was then removed vertically so that the molecular monolayer was transferred to its surface. The substrates were either degenerately doped p-type Si(001) wafers with a resistivity
