Ultralong Ordered Nanowires from the Concerted Self-Assembly of

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Ultra-long ordered nanowires from the concerted selfassembly of discotic liquid crystal and solvent molecules Ji Hyun Park, Kyung Ho Kim, Yung Woo Park, Jan P. F. Lagerwall, and Giusy Scalia Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b01332 • Publication Date (Web): 16 Jun 2015 Downloaded from http://pubs.acs.org on June 19, 2015

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Ultra-long ordered nanowires from the concerted self-assembly of discotic liquid crystal and solvent molecules Ji Hyun Park1, Kyung Ho Kim2, Yung Woo Park2, Jan P. F. Lagerwall1, ,† Giusy Scalia1, †. * 1

Graduate School of Convergence Science & Technology, Seoul National University, Suwon

(Korea); 2Department of Physics and Astronomy, Seoul National University, Seoul (Korea) KEYWORDS: self-assembly, discotic liquid crystals, lyonematic phase, molecular nanowires, organic semiconductors, solvent, organic electronics, atomic force microscopy, Raman spectroscopy

The realization of long and aligned molecular wires is a great challenge and different approaches have been proposed. Interestingly, hexapentyloxytriphenylene (HAT5) discotic liquid crystal molecules, model system of molecules with flat and aromatic cores, can spontaneously form well aligned, micrometer long yet only tens of nanometers thick nanowires on solid surfaces. We have investigated the formation mechanism of these wires by using different solvents with selected characteristics like chemical structure, boiling point, vapor pressure and surface tension. When casting from toluene and benzene solutions, atomic force microscopy reveals that the discotics

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spontaneously form very long and thin wires, self-aligning along a common orientation. If instead dodecane or heptane are used, different and in general thicker structures are obtained. The chemical structure of the solvent appears to have a key role, coupling to the liquid crystal self-assembly by allowing solvent molecules to enter in the ordered structure if their design matches the core of HAT5 molecules, thereby guiding the assembly. However, also other aspects are relevant in the assembly, like the nature of the substrate or the rate of solvent evaporation, and these can favor or interfere with the self-assembly into long structures. The use of solvents with aromatic structure is advantageous not only because it affects the geometry of the assembly, promoting long wire formation, but it is also compatible with good quality of the intermolecular order, as suggested by a high anisotropy of the Raman spectra of the nanowires formed from these solvents. Finally, the electrical properties of ordered systems show a clearly higher electrical conductivity compared to the disorganized aggregates.

INTRODUCTION Organization of molecules is essential in condensed matter, helping to bring properties from small to macroscopic scale or creating new features. Striking examples of the importance of the atomic or molecular assembly can be recognized in crystals or in liquid crystals (LCs) whose macroscopic properties are strongly connected to the way the building blocks are arranged. In liquid crystals the constituents have anisotropic shapes that are typically rod- or disk- like. Disclike molecules are interesting because they can show columnar phases in which the molecules spontaneously pile up on each other by π−π interaction. In turn, the columns can be arranged on a two-dimensional lattice. The columnar structures constitute molecular wires and, due to the natural geometrical confinement of the region where transport occurs, the type of transport is


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almost 1-dimensional. The liquid crystal self-assembly ability is particularly interesting because of the different scales at which the organization is manifested, spanning from the nanoscale for the formation of 1-dimensional molecular wires, to the macroscale related to the organization of the wires, all aligned along a common direction and positionally ordered in the perpendicular plane. This yields attractive properties for various applications in organic electronics . 1

The general interest in organic systems for devices is due to their relatively easy processability and low cost for applications such as organic field effect transistors (OFETs) ,1,2 organic solar cells ,1,3 organic light emitting diodes (OLEDs)

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or gas sensors .5 One of the most important

issues in organic electronics to achieve high performance is controlling the supramolecular organization 6 but also the reduction of structural defects since they critically interfere with the charge transport quality. In case of columnar LC phases defects can be annealed by thermal treatments and the orientation of the columns gives the direction of charge flow. Not only is the control of the alignment direction an open issue but also the uniformity of the alignment over large areas is a challenge .7 Thus, many methods have been explored to control the alignment of columnar systems like Langmuir-Blodgett technique ,8 zone-casting process ,9 surface-assisted treatments ,10,11 structural confinement ,12-14 thermal annealing process

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or electric/magnetic

fields or their combinations . 17,18 One approach for improving the performance of organic molecules has been to focus on the synthesis of molecular structures that would self-assemble with improved intramolecular order via, for example, hydrogen bonds .19 The preparation of organic films is often based on solution process since it is practical and easy but particular attention is required regarding the conditions used for sample preparation that can affect the final performance .2,20,21 Solvents are used as


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means for preparation of very thin films but they are also in general regarded of importance for the organization of several types of systems .22 Solvent effects can be quite complex, going beyond the direct interaction between solvent and solute molecules, for instance via soluteinduced effects on the solvent-solvent interaction .23 Solvent effects are expected to influence also the aggregation of discotic LC molecules in solutions before deposition on substrates

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or

the annealing of spin-coated thin films, in the latter case using solvent vapor to assist the molecular reorganization into needles or fibers .2,25 Interestingly, solvents can influence the aggregation of fullerenes and the dimensionality of the resulting aggregates is suggested to be related to that of the solvent molecules used for deposition .26 In this work we show that the effect of solvents is crucial, playing an active role in the hierarchical organization of discotics. This role has not been recognized so far and it goes beyond the traditional views of interaction between solvent and solute molecules. Solvents act in combination with the spontaneous self-organization of the discotic LC, influencing the final arrangement and order, actively entering into the molecular organization via a novel mechanism. Hexapentyloxytriphenylene (HAT5) discotic liquid crystal has triphenylene as rigid aromatic core and six peripheral flexible alkyl chains. It exhibits an ordered hexagonal columnar liquid crystal phase (see Figure 1) between 69°C and 122°C in bulk and in sandwich cells it aligns quite easily with the columns perpendicular to the substrates .27 Nanostructures of HAT5 were observed on substrates

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but its strong tendency of forming elongated structures is apparent in

the formation of very long wires that may spontaneously and uniformly align along a common direction ,29 although this is not always the case .30 In this article we analyze the factors influencing the formation of HAT5 nanostructures on substrates, focusing on the effect of different solvents on the final organization and explaining the mechanism of formation of the


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observed assemblies. The solvent plays an active role in the formation of the structures, actively participating in the discotic assembly by matching parts of the discotic molecules. In this way, using the same type of discotic liquid crystal molecule, different self-assembled geometries can be realized by simply choosing a solvent with a specific chemical structure. Therefore the geometry of the final assembly is the result of a concerted action between the liquid crystal selforganization and the intercalation of solvent molecules. In particular, the aromatic structure of toluene and benzene matches the core of HAT5, promoting the formation of columns that seem to have a good molecular overlap without the need of modifications of the chemical structure of the discotic molecules. Finally, the nature of the substrate has also an effect on the assembly as well as the rate of solvent evaporation, influencing the final assembly. Very thin films were analyzed by atomic force microscopy (AFM) revealing the morphology of the resulting structures and their dimensions.

Figure 1. Molecular assembly of HAT5 in the ordered hexagonal columnar liquid crystal phase


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We focus our attention on a specific structural organization, also the most attractive for applications, namely the ultra-long wires. These structures are not induced by thermal annealing but appear spontaneously after deposition from toluene, being slightly shorter from benzene. Therefore their organization can be quite different from the one of annealed HAT5 films, deserving investigations of the alignment and of the electrical properties. Polarized Raman spectroscopy shows that the alignment of the discotics is planar and it also confirms the good quality of the intracolumnar order. This can be inferred from the higher polarizability along the columnar axis compared to the perpendicular direction, and from the local order parameter, deduced by analyzing the peak intensities. Electrical conductivity measurements were performed on aligned, long wires and on disorganized, shorter aggregates. The first configuration clearly shows better performances with values of the conductivity three times higher than the disorganized assembly.

RESULTS AND DISCUSSIONS Nanostructures from different solvents Unlike films of standard organic materials that can be fairly uniform, HAT5 produces nanostructured films when deposited on substrates in thin films. The thickness of the films can be easily controlled by changing the concentration of the solution used for spin-coating .29 Here we have used a concentration, 6mg/ml, that produces very thin films since they present more defined and distinguishable structures. This allows us to better identify the geometry of the structures and their macroscopic organization in the films. Discotic molecules of HAT5 were processed with different organic solvents: toluene, benzene, chloroform, heptane, and dodecane. The solvents were chosen for their molecular structure, resembling parts of the HAT5 molecule,


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and for some selected properties listed in Table 1. The solutions were deposited on substrates and spin coated using similar conditions for all the solvents. The films deposited on SiO2/Si were investigated by AFM and the morphologies are reported in Figure 2. Table 1 Chemical structures and relevant properties of used solvents Toluene

Benzene

Chloroform

Heptane

Dodecane

Vapor pressure 29.3 hPa at 20°C

100.0 hPa

213.3 hPa

53.3 hPa

0.4 hPa

Boiling point at atmospheric 110.6°C pressure

80.1°C

61.2°C

99°C

216°C

Molecular structure

Surface tension(γ) at 28.40 mN/m 28.88 mN/m 27.50 mN/m 20°C

20.14 mN/m 25.35 mN/m


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Figure 2 AFM images of thin films prepared by solutions of 6.0 mg/ml of HAT5 in (a) toluene (b) benzene, (c) chloroform, (d) heptane and (e) dodecane, deposited on SiO2/Si substrates. The scale is identical in all images (the full width of each image is 10 microns) Films from toluene and benzene are typically formed by molecular wire structures that easily span over several micrometers (even larger domains are often found), whereas films prepared from chloroform and heptane have small grainy structures. In case of dodecane isolated big fiber structures, instead of fully covering layers, were formed on the substrate. Since molecules of HAT5 possess a large polycyclic aromatic core with delocalized π orbitals, π-π interaction plays a key role for the self-organization into columnar structures. This spontaneous piling up can explain the tendency to form elongated structures but the difference in the final molecular arrangements obtained from the different solutions has to be related to solvent characteristics.


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The longest molecular wire structures, tens of micrometer long, were formed by HAT5 molecules initially dissolved in toluene. The nanowires have 30 to 40 nm of thickness and roughly 200 nm of width, as visible in Figure 2a. The fact that the films are composed by wires is reflected in the roughness of the structure, as shown in the surface profiles reported in Figure S1 in the Supporting Information. The fibers formed from benzene, Figure 2b, are a little thinner and shorter than the ones from toluene but with similar film thickness and similar overall appearance. Films deposited from chloroform solution, as visible in Figure 2c, show an entirely different structure, with domains a few micrometer in size containing very short features without long range alignment. In solvents, discotic molecules can easily diffuse and eventually aggregate and the anisotropic aggregates can spontaneously order above a certain concentration as in standard lyotropic liquid crystal systems. The formation of a nematic lyotropic phase was suggested in Ref [29] as main mechanism behind the formation of the large scale alignment of the nanowires, motivated by the AFM texture of assemblies resembling a nematic arrangement. The organization of our wires would be compatible with the occurrence of a nematic phase during evaporation although we could not confirm this. Chloroform has higher evaporation rate than toluene and benzene due to the higher vapor pressure and lower boiling point. The fast evaporation rate might be responsible for the shorter and disordered assemblies since not enough time is given to the molecules for self-organizing into macroscopically ordered structures. However, the observed type of self-assembly cannot be only dependent on the evaporation rate but also other factors must play a relevant role. This is evident from the results obtained with HAT5 dissolved in heptane, having a vapor pressure between toluene and benzene. The resulting structure does not have long range order, as is clear from Figure 2d, showing a few micrometer-sized domains with no clear organization. The


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domain formation looks similar to the one observed in HAT5 from chloroform solution but the domain size is larger. Within the domains there is also a difference in the morphology of the structures. There is a higher point at the center of the domains from heptane solutions and from there rod shaped aggregated structures spread out like from a nucleation point. Below these rods there are plateau-like areas, having a much smaller roughness than the top surfaces. AFM profiles of these structures, as well as the ones realized with the other solvents, are reported in the Supporting Information. The differences in the surface morphology might suggest that the molecular organization is also different and that the alignment can be homeotropic, with the molecules of HAT5 flat on the substrate. This tendency to vertical columnar alignment would explain why the film is about 20nm thicker than the others. Dodecane has very similar chemical structure to heptane but longer chain length. Interestingly, very different results were obtained compared with heptane or the other solvents as evident in Figure 2e. HAT5 molecules from dodecane solution tend to aggregate into even millimeter long and relatively large isolated fibers. Dodecane has very low vapor pressure and more than 24 hours are needed for evaporation of the solvent. In other words, HAT5 molecules in dodecane have sufficient time for diffusing and self-organizing and this may explain the formation of very anisotropic wires. While the length of the structures can be related, to some extent, to the evaporation rate, the shape and the macroscopic organization of the molecular aggregates are related to other properties of the solvent. If we consider the molecular structure of the solvents, heptane and dodecane are simple linear alkane hydrocarbons which would interact more with the peripheral chains of HAT5 molecules resulting in the formation of thicker structures. In contrast toluene or benzene can interact with the core of HAT5 due to the aromatic rings present in their structure, overlapping via π−π stacking in the columnar formation. This supports the picture that


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solvent molecules actively participate in the formation of the self-assembled structures influencing the final geometry. In order to confirm the described mechanism, we have prepared two solutions of HAT5, one in toluene and the other in dodecane at a concentration of 20 mg/ml, let the solvent evaporate and then filled two sandwich cells of about 7 micrometer cell gap. The phase transition temperatures were then investigated (see section 2 of the SI). While the transition temperature from the isotropic to the liquid crystal phase of HAT5 dried from toluene was around 123°C, similar to pure HAT5, thus indicating a good integration of toluene that does not alter the LC order, the phase transition temperature of HAT5 dried from dodecane was drastically decreased, occurring between 90 and 100°C. The decrease of order is compatible with the coupling of dodecane with the chains of HAT5, associated to disorder, reflected into the decrease in the phase transition temperature between the disorderd (isotropic) phase and the ordered LC phase. It is worth to point out that no phase separation between HAT5 and solvent was visible. The observed behavior, that is toluene compatible with the LC order and dodecane with disorder, is in line with the coupling of the solvent molecular structures with different parts of the HAT5 molecules allowing the tuning of the final arrangements.


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Figure 3 In the upper row (a-d) the schemes show the wetting behavior of HAT5 solutions (6.0 mg/ml of HAT5 in toluene) on various substrates before spin-coating and in the lower row (e-h) the AFM images of the films after spin-coating and drying are shown: (a), (e) bare glass, (b), (f) UV/O3 treated glass, (c), (g) ITO coated glass and (d), (h) SiO2 substrate. Influence of substrates The formation of the nanostructures is strongly determined by the solvents as we have shown in the previous paragraph but we have also observed influences of the substrates on which the solutions are deposited. In section 3 of the SI we report the structures formed by HAT5 from chloroform, heptane and dodecane on ITO (Figure S7) to be compared with Figure 2 of the structures following deposition on silicon. In Figure 3 we show the structures formed after deposition from toluene on different substrates. In general, the final structures on both substrates show similarities except for heptane. All HAT5 solutions had total wetting on the ITO coated substrates as well as on SiO2/Si substrates. The films deposited from chloroform solutions show different types of short grain structures on both SiO2/Si (Figure 2c) and ITO substrates (Figure S7a) although the grain formation looks different. They have in common that they form short structures and they look isotropic at macroscopic level. Heptane solution shows somewhat


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different structures on SiO2/Si (Figure 2d) and ITO substrates (Figure S7b) with noticeably different surface coverage. HAT5 did not cover the ITO surfaces like it did on the SiO2/Si surface, producing instead random sized, almost flat, bulkier and elongated structures on ITO substrates. Rod-like aggregates are present on SiO2/Si substrates while flat, but with much larger area, rod-like structures were dominant on ITO substrates. The structures deposited from dodecane solution on SiO2/Si (Figure 2e) and on ITO substrates (Figure S7c) have very similar characteristics with macroscopically large and long fibers of varying dimensions. Now we focus our attention on toluene, the best solvent for producing ultra-long wires, and investigate the effect of substrates on the final morphologies. Solutions with the same concentration as in the previous cases, 6.0 mg/ml, of HAT5 in toluene were deposited on SiO2/Si, on ITO-coated glass, glass and ozone cleaned glass substrates. Due to the low concentration of HAT5, the initial wetting behavior is mainly determined by the toluene wettability on the different substrates. In general, all substrates show a very good, almost “total” wetting, indicating a spreading parameter Sp > 0

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even if noticeable differences can be still

observed in the final structures. Surface roughness affects the degree of wettability enhancing the surface characteristics ,31 and as the roughness increases, a hydrophilic surface thus becomes more hydrophilic. ITO coated glass has higher surface roughness than normal glass substrates so, even though a glass substrate is wetted quite well by the solution, with about 10°contact angle between the toluene solution and the glass substrate, the wetting improves on ITO coated glass with almost 0° of contact angle. To increase the wettability of HAT5-toluene solution on bare glass, the substrate went through UV/O3 treatment for a deep cleaning of the surface. As a result, the contact angle showed a very similar value to the one on ITO coated glass. In case of SiO2, toluene solution shows an intermediate behavior between the previous two cases. It is worth to


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point out that the contact angle measurements with the used method are not very accurate for small angles hence the values that we have reported should be considered as guides for giving a trend in the wetting behavior. The schemes of the wetting behavior on different substrates are reported in the upper row of Figure 3. In the final structures, shown in the lower row of Figure 3, there is still a general tendency of formation of long fibers due to the beneficial action of toluene but there are small differences in thickness and in the morphology as visible in Figure 3e-h for the four different substrates. In the first image, 3e), the AFM image shows the morphology of the film deposited on bare glass with some occurrence of non-fiber like, more homogenous texture. The thickness of the film is also larger than for the other structures being ~200nm. The films deposited on the UV/ ozone cleaned glass and ITO coated glass showed very similar wire formations with very long and fairly well aligned fibers, as visible in Figure 3f) and 3g), with similar film thickness, about 30 nm of height. The organization and the appearance of the wires when the solution was deposited on SiO2/Si substrate is very similar to the previous cases, as visible in Figure 3h), but the film thickness is slightly larger than in the samples of Figures 3f) and 3g), with a thickness around 30 - 40 nm. In summary, even if all solvents seem to wet the substrates quite well and in a similar way, the morphology of the structures appears affected by the details of the substrate, revealing a complex interplay between solvent and HAT5 molecules on the one hand and substrates on the other. Also, the effect of substrates appears to be influenced by the type of solvent used for the HAT5 deposition.


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Raman spectroscopy investigations on wires from toluene solutions Since the type of molecular assembly of HAT5 is strongly dependent on the solvent used in the solution but also on other factors entering during the preparation, the resulting structural organization is not necessarily the same as the one produced by solely HAT5, thus it is relevant to investigate the alignment of the molecules in the nanowires. For this, we used polarized Raman spectroscopy to analyze more visible fibers, prepared from toluene solutions. The change in intensity of Raman peaks observed with polarization of the probing light aligned along different measuring directions can be used for deriving information on the orientation of molecules as well as on the order parameter of uniaxial LCs .32 Polarized Raman spectroscopy has been used for assessing the alignment also of a triphenylene discotic liquid crystal .33 Here we make some simple considerations to deduce the alignment of the molecules and discriminate if they lie flat (homeotropic alignment) or edge-on (planar). The direction of the long axis of thicker fibers was used as reference for the direction of the input light polarization. In this way two polarizations were used, one parallel to the fiber direction (black spectrum in Figure 4 a)-c) and the other perpendicular to it (grey spectrum, Figure 4a)-c). In Figure 4d) we report the scheme of the used polarizations of light with respect to the wire orientation. We consider now the following peaks: 725 cm-1, 1622 cm-1 and 2904 cm-1. For the correlation of the peaks to molecular vibrations we look at the proximity of the peak positions to the values reported in the group frequency correlation table of Brandmueller and Moser reported in reference .34 The first peak can be associated to deformations of C-H around the aromatic rings while the second peak, 1622 cm-1, to ring vibrations. The third value is obtained by fitting a single Lorentzian function to what looks like more as a band, formed by several peaks, as visible in Figure 4c. Since the spectrum is noisy we carried out the fitting using only one peak function


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to have an indicative value for locating the spectral peak position. We associate this band to the C-H stretch of the aliphatic chains of the HAT5 molecules. We now point out that the intensities of all the peaks are higher when a polarization parallel to the fiber axis is used for exciting the molecular vibrations. If the molecules would align flat on the substrate, thus in homeotropic alignment, considering the three-fold rotational symmetry of the HAT5 molecules, the aliphatic chain vibrations should have similar response for the two perpendicular polarizations that would be both in the molecular core plane and generate similar effects on the induced polarization in the structure. The strong difference in peak intensities, more than 2 times higher when light is polarized parallel to the fiber axis than when it is perpendicular to it, suggests an asymmetric response to the field and thus indicates a vertical position of the molecules, edge-on to the substrate, in contrast to the behavior of a homeotropic sample shown in the SI. Similar considerations should hold for the aromatic vibrations and for the C-H around the aromatic rings although with different values for the ratio of intensities. Since larger fibers have morphology similar to the smaller ones on the nanometer scale 29 we can infer that the edge-on alignment is also present in the thinner fibers. Since the molecules overlap to form columns we expect that the higher polarizability is along the columnar axis. The larger electronic oscillations induced along the fiber axis couple more efficiently to the modes described previously if the molecular overlap is good. In this picture HAT5 molecules are not only aligned edge-on but they have their symmetry axis parallel to the orientation of the molecular wires observed by AFM. Below 900 cm-1, a peak at about 878 cm-1 can be clearly distinguished and it can be associated to CH out-of-plane vibrations probably from HAT5. However, on its left, a peak at about 864 cm-1 and one at 617 cm-1 could correspond to ring vibrations of toluene. Several small peaks are


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present below 1000 cm-1 but none very intense, making it difficult to spot toluene. Also these peak intensities show a strong anisotropy by changing input polarization thus following the behavior of the overall HAT5 peak spectra. However, more systematic investigations are necessary to corroborate and analyze the presence of toluene within the structure. These studies will be performed in future work. In reference 35 the intensity of the Raman peaks was used for evaluating the liquid crystal order parameter using the formula S = (D-1)/(D+2) with D the ratio of the Raman peak intensities for parallel and perpendicular polarization, respectively, considered equivalent to the dichroic ratio of dyes in liquid crystals. Taking the fitted values of the intensities of the peaks associated to the vibration of C-H around the aromatic rings for parallel and perpendicular polarization, which is thus the most suitable for giving information of the order within the core of the columns, we obtain a value of S = 0.7, indicating a relatively high degree of order within the columns. For the C-H stretch of the aliphatic chains the value of S becomes drastically lower, S= 0.3, as expected for the disordered chains.


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Figure 4. Polarized Raman spectra of HAT5 film from toluene. Two polarizations for the


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incident light have been used, with polarization directions taken with respect to the long axis of a more visible fiber: parallel to the fiber axis (black spectrum) and perpendicular to it (grey spectrum). The images a) – c) highlight three different regions of the spectra with some peaks used for determining the alignment of HAT5 and the order within the columns. Panel d) shows a scheme of light polarization directions with respect to the column orientation.

Electrical properties In order to evaluate the effect of the alignment on the electrical properties, we have compared the conductivity of structures formed by aligned, straight nanowires, with that of disordered ensembles of short aggregates. Following our procedure for producing tailored nanostructures, we have used a solution of HAT5 in toluene for realizing the aligned long nanowires and one in chloroform for achieving disorganized, isotropic, structures. The two nanostructured films are shown in the AFM images of Figure 5. In Figure 5a) we report the AFM image of the straight nanowires from toluene solution, exhibiting a long-range orientational order that well bridges the electrodes, while in b) there is the isotropic structure achieved from chloroform solution. The electrical properties were measured for both samples and the results are reported in Figures 5c) and 5d). HAT5 has a very low intrinsic carrier concentration, in part due to the fairly large band gap and one of the strategies to face this issue is doping with iodine .36 This forms a charge transfer complex with the discotic molecules. The resulting current flow was monitored as function of time during the iodine doping process, shown in Figure 5c) for both aligned and isotropic structures of HAT5. As the level of doping increases, the charge density in the system also becomes higher and current starts to flow through the nanowires. When a saturation level of


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doping is reached, indicated by a stable level of current, the I-V characteristics of the samples were measured with a 4-probe method. Both samples showed a linear behavior as reported in Figure 5d. Since the samples have different thicknesses, h, it is worth to compare the current actually flowing in the two films per unit of height. The difference in magnitude, related to the structural film properties, is apparent in Figure 5e where the current per unit height (current / h) has been plotted against time. Analogously, in order to obtain a picture of the behavior closer to the structural properties, thus not affected by the difference in film thickness, the density of current against electric field is reported in figure 5f. Here the slope is directly connected to the conductivity σ of the structures, according to the local form of Ohm’s law, J=σE. The thicknesses of the films used for the calculations as well as the obtained values of resistance, resistivity and conductivity are summarized in table 2. The estimated resistance of the aligned sample was 418 kΩ while the isotropic one had 291 kΩ. The isotropic film is 5 times thicker than the aligned sample, being 100nm and 20 nm thick, respectively. Considering the film thicknesses, the electrode distance, L, of 2 micrometers and length, D, of 100 micrometers, we derived the conductivity that was 2.39 ⋅10-2 S/cm for the aligned sample and 0.7⋅10-2 S/cm for the isotropic one. This means that the well-aligned, long fiber nanowires show clearly better performance allowing a more efficient charge transport than the short and disorganized structures. We point out that the isotropic structure is densely packed, hence the charges can still travel from aggregate to aggregate but this process is less efficient. This is since the molecular wires from toluene offer straight and presumably defect-free paths for the charges, in contrast to the large amount of grain boundaries present in the isotropic structure.


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Figure 5 AFM images of spin-coated films of HAT5 on SiO2 substrates equipped with Ptelectrodes with 2µm distance: (a) planarly aligned molecular wires from toluene, (b) non-aligned


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molecular structures (isotropic) from chloroform. The corresponding electronic characteristics of the thin films: (c) current flow depending on the iodine exposure time, (d) I-V curve of the iodine doped samples. In (d) the current per unit of height is plotted against time and (e) the plot of the current density versus electric field. Table 2. Electrical characteristics of the aligned and isotropic samples. The resistance R is directly deduced from the linear fit of the current- voltage (I-V) measurements, the corresponding film thicknesses are indicated with h, D is the electrode length and L the distance between two electrodes.

Film type Aligned Isotropic

Thickness, h (nm) 20 100

Resistance, R (KΩ) 418 291

Resistivity, ρ=R (D*h)/L (Ωcm) 41.8 145.6

Conductivity, σ =1/ ρ (S/cm) 2.390 ⋅ 10-2 0.687⋅ 10-2

EXPERIMENTAL HAT5 was purchased from Synthon Chemicals (ST01612) and used as received or after purification. HAT5 was purified by column chromatography (silica gel; toluene, Rf = 0.58) and further purification by recrystallization from ethanol to give pure HAT5 (1.0g, 70%). However, in the morphology of the assemblies, no remarkable differences were noted between the unpurified and purified samples, indicating that the original commercial sample indeed had very high purity. Different organic solvents were used for preparing solutions of HAT5: toluene, benzene, chloroform, heptane, and dodecane, all from Sigma-Aldrich. The solvents were chosen for their molecular structures as well as for their characteristics such as boiling point and surface tension, as indicated in Table 1. All initial solutions were prepared with a concentration of 6 mg/ml. The deposition of the solution was performed by spin-coating with a speed of 1000 rpm


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for 30s with a Laurell Technologies Lite single Wafer Spin Processor WS-650S-6NPP. Films were spin-coated onto 10 x 10 mm2 square substrates: glass with 150 nm-thick indium tin oxide (ITO) coating, bare glass, UV/O3 treated glass and SiO2/Si substrates. The spin coated films were carefully transferred to the fume hood and kept for at least 2 hr at room temperature to obtain solid films. Longer waiting times provided very similar results with the exception of dodecane that needed more than a day for the evaporation. Glass, ITO-coated and silicon substrates were cleaned before deposition of HAT5 solutions. The substrates were cleaned in 4 steps using bath sonication, Power sonic406 from Hwashin instrument Co., Ltd, with a different cleaning agent at each stage. We started with a 2% Micro 90 concentrated cleaning solution in water, followed by acetone, isopropyl alcohol and finally DIwater. Each sonication step lasted for 15 minutes and then, at the end, the substrates were dried by compressed air. Some bare glass substrates were treated with a UV ozone treatment for deep cleaning using an Ozone Cure System, Minuta technology, for 30min. Structural analyses of the deposited structures were performed by atomic force microscopy (AFM) with a Dimension Edge atomic microscope system of Bruker/Veeco under ambient conditions in tapping mode, using commercial silicon cantilevers, named RTESP, with a nominal spring constant of 40N/m, resonance frequency of 300kHz and a tip radius of 8 nm. Polarized Raman spectroscopy was performed with a microRaman system Jobin-Yvon labram setup, at 633 nm wavelength and, for the spectrum in the SI, a Dongwoo optron system combined with an Olympus BX53 microscope, in the backscattering geometry, using two perpendicular directions for the incoming light. The spectra were analyzed by fitting the peaks with Lorentzian functions in order to infer the alignment of the discotic molecules as well as the columnar order.


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Samples for electrical measurements were prepared by spin-coating the HAT5 solution from toluene and chloroform onto 6 mm x 6 mm SiO2/Si substrates with 2 μ m width platinum interdigitated electrodes, produced by photolithography. The distance between two adjacent electrodes is 2 micrometers and their length is 100 micrometers. A sketch of the electrode configurations is reported in the Supporting Information. The samples were placed in a glass tube connected to a high vacuum line, at a pressure lower than 10 /5 torr, and to another glass tube containing iodine. After closing the vacuum line, the valve to the chamber containing iodine was opened to let it sublimate onto the sample to dope the film. Electrical measurements were performed in vacuum with a Keithley 6517A for 2-probe measurements to monitor the current resulting from the doping while applying a voltage equal to 1 V. Once the level of doping gave a constant current then the I-V characteristics were measured using a standard 4-probe method with a Keithley 6221 current source and a Keithley 2182 nanovoltmeter.

CONCLUSIONS We have shown that the self-assembly process observed from solution is not only determined by intrinsic mechanisms as in pristine discotics but is also affected by the presence of solvent molecules during the evaporation process. Solvents do not just serve for dispersing the discotic liquid crystal molecules but they also affect the way molecules arrange during evaporation by actively entering into the assembly. HAT5 molecules form films composed by long fiber-like structures after spin-coating toluene solutions on substrates. We have studied very thin films, below 50 nm, obtaining clearer structures compared to thicker films that helped to discriminate important factors that contribute to the formation of the fibers. The type of solvent used for dispersing HAT5 strongly influences


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the aggregation of the discotic molecules. Toluene and benzene support the formation of very long fibers that align along a common direction while dodecane forms isolated, long and thicker fibers. Chloroform and heptane induce smaller anisotropic aggregates although for films from heptane solutions there are also more uniform features underlying the anisotropic structures. We argue that the molecular structure of the solvent plays a fundamental role in the formation of aggregates entering into play in the fiber formation. The aromatic ring matches the aromatic structure of HAT5 molecules, thus participating into the stacking of the molecules and promoting a columnar formation of wires. Instead dodecane has a structure similar to the chains of HAT5 molecules therefore interacting with its sides, increasing the effective chain size and thus promoting bulkier formations during the assembly. Finally, substrates also influence the final organization, even if the effect is smaller than that of the solvent type. The fibers show anisotropic response in the Raman spectrum indicating that the HAT5 molecules are aligned edge-on to the substrates, thus planarly. In addition, the final structure appears to have a high degree of orientational order by an estimate of the order parameter of the core. This suggests a good level of orbital overlaps, attractive for the wire formation and for the conductive properties of the wires. Electrical measurements on molecular wires from toluene showed that these structures are indeed clearly better performing than disorganized assemblies from chloroform, with a value of conductivity three times higher for the former than the latter. Our findings provide an attractive route for tailoring or changing the geometry of supramolecular assemblies not necessarily by changing the discotic molecule structure but by simply choosing a solvent with a desired molecular structure. We expect that our results can be of general interest, being applicable also to other type of molecules.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Present Addresses † University of Luxembourg, Faculty of Science, Technology and Communication, Physics and Material Science Research Unit, Luxembourg

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT We acknowledge financial support from the National Research Foundation (NRF) Korea, grant number 490-20130016 (2011-0014835), the European Commission for Solcanta, in the framework of the Marie Curie EIF program (GS), the Leading Foreign Research Institute Recruitment Program (2009-00514) of the NRF, Korea, (YWP). Youn Sang Kim is thanked for


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the access to his equipment and for discussions. Armin Shultz, Miro Haluska are thanked for the support with Raman measurements and Siegmar Roth for general support related to the EIF. Yo Shimizu is thanked for the purification process and Hi Gyu Moon for the drawings. Supporting Information Available: Atomic force microscopy profiles of the surface of structured films obtained from toluene, benzene, chloroform, heptane and dodecane; Raman spectra of bulk HAT5 in sandwich cell with homeotropic alignment; sketch of the electrodes used for the electrical measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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TOC Graphic

Synopsis


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The formation of very long and ordered nanowires from the disc-like liquid crystal hexapentyloxytriphenylene in toluene solution is a combination of the self-assembly behavior typical of liquid crystals and of the solvent structure. The solvent appears to have an active part in the assembly process leading, in case of toluene, to nanowire formation on substrates. Other factors can also have an impact on the final morphology such as substrate nature and solvent evaporation rate.


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Ultralong Ordered Nanowires from the Concerted Self-Assembly of

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