J. Phys. Chem. C 2010, 114, 12659–12666
12659
Mechanistic Origins of Hierarchical Order in Organic Monolayers Deposited From Liquid Crystal Solvents Nick E. Gislason, Calvin Murphy, and David L. Patrick* Department of Chemistry, Western Washington UniVersity, 516 High Street, Bellingham, Washington 98225 ReceiVed: April 5, 2010; ReVised Manuscript ReceiVed: June 3, 2010
The orientational and chiral properties of monolayer films of the fluorescent dye 3-(2-benzothiazolyl)-7octadecyloxy coumarin (BOC) on highly oriented pyrolytic graphite have been studied by scanning tunneling microscopy and polarized absorption spectroscopy. Films were deposited from two oriented liquid crystal solvents and one isotropic solvent using a sacrificial template method in which a solvent film applied in a first deposition step is displaced by BOC in a second deposition step. Fluid-phase orientational order parameters, monolayer order parameters, and short-range orientational and chiral autocorrelation were analyzed to determine the mechanisms by which liquid crystal solvents influence order in the deposited BOC films. By comparing BOC molecular packing characteristics and ordering statistics it is concluded that the substrate potential provides the dominant influence, with rotational and chiral symmetry among the six substrate-allowed configurations broken by much weaker liquid crystal forces. A combination of elastic and anchoring forces explains most of the observed long-range order, with templating effects making a small additional contribution at submicrometer length scales. The orientation of BOC molecules in solution appeared not to be a significant factor in affecting order in the adsorbed monolayer. Introduction The recent observation that orientational order in solutiondeposited organic thin films can be influenced using a thermotropic liquid crystal (LC) as the deposition solvent raises the possibility of preparing new types of films with more precisely controlled micro- and macrostructured architectures.1-5 Organic film growth using LC solvents, which has been termed “liquid crystal imprinting” (LCI), could prove useful for the preparation of materials such as organic semiconductor films, where controlled molecular order leads to improved electrical performance,6 nonlinear optical films, which require specific, lowsymmetry molecular arrangements,7-9 oriented films of small particles,10,11 and structured “template” films for heteroepitaxial crystal growth.12-14 In order to further develop LC solvents for such applications, the underlying mechanisms through which LC solvents influence order in thin films deposited from them need to be better understood. Broadly speaking, these mechanisms may be grouped into two categories. One set involves solution-phase interactions, where guest-host effects lead to preferred orientational order15,16 and in some cases may affect the conformational distribution of flexible solute molecules.17 This produces a flux of solute molecules to the growth surface biased toward a particular orientational and conformational ensemble, which has the potential to influence order in the resulting film. The second group of mechanisms occurs at the surface, where interactions with adsorbed LC molecules and interfacial forces resulting from anchoring and curvature elasticity can select specific film orientations and even chirality.1,2 So far however only a few studies have attempted to investigate these phenomena, and their respective roles and relative contributions for films deposited from LC solvents are not well understood. * To whom correspondence should be addressed. Email: Patrick@ chem.wwu.edu. Tel: (360) 650-3128. Fax: (360) 650-2826.
Here we report experiments comparing order in organic thin films deposited from a set of LC and isotropic solvents, examining the effects of solution-phase order and surface forces on molecular- and macroscopic-scale order in the resulting films. Three chemically similar solvents were investigated, differing in the type of solution phase order (smectic-A LC, nematic LC, and isotropic liquid phases) and in whether the solvent formed an ordered monolayer of its own. Monolayer films of the fluorescent dye 3-(2-benzothiazolyl)-7-octadecyloxy-coumarin (BOC) were deposited from each solvent using a “sacrificial template” method,5 which provided a way to partly distinguish between the respective influences of surface and solvent interactions on molecular ordering. When films were deposited from LC solvents ordered using a magnetic field, the orientational degeneracy among molecular surface enantioconfigurations was broken, leading to preferential orientation among adsorbed molecules not present when an isotropic solvent was used. Using scanning tunneling microscopy (STM) to quantitatively measure the magnitude and persistence length of orientational and chiral order, along with linear dichroism UV-vis absorption spectroscopy to determine solute orientation in bulk solution, we compare the nature of molecular order in the three solvent systems to identify which solvent characteristics play the greatest role. Experimental Section BOC films were deposited onto highly oriented pyrolytic graphite (HOPG) from three different solvents: 4′-octyl-4cyanobiphenyl (8CB), a room-temperature smectic-A LC; 4′pentyl-4-cyanobiphenyl (5CB), a room temperature nematic LC; and a mixture of 5CB with ∼11 wt % 1-phenyloctane, which is isotropic.18 8CB and 5CB were obtained from Frinton Laboratories and BOC from Sigma-Aldrich. All reagents were used without further purification. Films were deposited on ZYHgrade HOPG (Advanced Ceramics Inc., Cleveland, Ohio) that
10.1021/jp103052p 2010 American Chemical Society Published on Web 06/30/2010
12660
J. Phys. Chem. C, Vol. 114, No. 29, 2010
Gislason et al.
Figure 1. Liquid crystal imprinting using a sacrificial template. Monolayer films of BOC were deposited on HOPG using a two-step procedure. In the first step (top) a droplet of pure solvent (8CB, 5CB, or 5CB/1-phenyloctane) was placed on the substrate located in a magnetic field. Samples were then removed from the magnet and BOC was gently applied to the supernatant (middle), through which it diffused to the substrate, displacing the physisorbed solvent monolayer and forming a polycrystalline film (bottom). The amount and length-scale of molecular order in the resulting film depended on the solvent used.
had been washed in acetone, baked in an open-air tube furnace at 600 °C to remove residual organic impurities, and cleaved immediately prior to use. Films of BOC were deposited in two steps, illustrated in Figure 1. First, a droplet of neat solvent was placed on a graphite substrate located between the poles of a 1.2 T electromagnet. For the two LC solvents the temperature was then raised to 100 °C for 2 min, and the system was allowed to gradually cool to room temperature with the field applied. This resulted in a uniformly oriented supernatant, and in the case of 8CB, produced a uniformly ordered polycrystalline 8CB monolayer adsorbed to the HOPG surface with uniaxial long-range orientational order, and short-range chiral order.1,2 The 5CB/1phenyloctane solution was not heated to prevent evaporation of phenyloctane. In the second step, a solution of BOC was prepared in the respective solvent with a 1:1000 molar ratio (BOC: solvent), and approximately 20 µL of this solution was carefully spread on top of the fluid layer applied in the first step. BOC diffused through the supernatant to the substrate, displacing adsorbed
solvent molecules over a period of 2-48 h. No magnetic field was applied during the displacement process. STM imaging was performed at the solution-graphite interface under ambient conditions using a home-built instrument (RHK model STM1000 control electronics). Tips were mechanically cut from 0.010′′ diameter Pt/Rh (87/13) wire, and imaging was performed in constant height mode. For measurements probing long-range order, images were acquired at multiple locations separated by at least 1 mm. For studies of short-range order, sets of images were acquired over areas up to 1 µm2, on single graphite terraces. In addition to investigating the molecular order of crystallized BOC monolayers, the orientational order of solution-phase (dissolved) BOC in each of the three bulk solvents was also determined. These measurements were performed using polarized UV-vis absorption spectroscopy on 200 µm thick solutionfilled cells, constructed from glass slides coated on their inner surfaces with mechanically rubbed polyvinyl alcohol (PVA). Rubbed PVA induces uniform in-plane alignment in LC cells with the director parallel to the rubbing direction. Cells were
Hierarchical Order in Organic Monolayers
J. Phys. Chem. C, Vol. 114, No. 29, 2010 12661
Figure 2. STM images of (a) BOC (Itunn ) 0.89 nA, Vb ) -0.323 V), and (b) an 8CB template film (Itunn ) 1.33 nA, Vb ) -1.2 V) on graphite. Both images show right-handed domains. Model overlays illustrate the arrangement of molecules in one unit cell.
filled with 1 × 10-4 M solutions of BOC in 8CB, 5CB or 5CB/ 1-phenyloctane and linear dichroism spectra were acquired with light polarized parallel and perpendicular to the director. An order parameter for the absorption dipole corresponding to the transition at λmax ) 400 nm was calculated using
Sfluid )
A| - A⊥ A| - 2A⊥
(1)
where A| and A⊥ are the UV-vis absorbance for incident light polarized parallel and perpendicular to the LC director, respectively.19 Results At the molecular scale, BOC films deposited from all three solvents were indistinguishable, with molecules crystallizing into parallel rows composed of unit cells containing four molecules (Figure 2a, unit cell dimensions a ) 1.8 ( 0.2 nm, b ) 5.1 ( 0.2 nm, R ) 70 ( 2°). This differed from the arrangement of BOC molecules on graphite deposited from toluene reported by Pan et al.,20 where the angle between the lamellar rows and the alkyl tails was either 81 ( 2° or 36 ( 2° depending on film preparation conditions, versus 61 ( 4° found in this work, and the area per molecule was significantly smaller than in films deposited from toluene. In addition to the polymorph shown in Figure 2a, a second, much less common molecular arrangement was also sometimes observed (not shown). Since it accounted for less than 5% of the surface area we focused on the more prevalent polymorph. Although BOC is not chiral in 3-dimensions, its 2D monolayer is, with molecules adsorbing in either right- or left-handed enantioconfigurations. Molecules segregated into homochiral domains which were 0.01-0.1 µm2 in size for all three solvents. Over a macroscopic area there were an equal number of leftand right-handed domains and thus films were racemic overall. However, as discussed below, over short length scales in some cases BOC showed strongly correlated chirality with large homochiral patches comprised of numerous smaller domains. The extent of this autocorrelation depended on the solvent used. In addition to BOC, one of the three solvents, 8CB, formed its own crystalline monolayer on graphite at room temperature, shown in Figure 2b (unit cell dimensions: a ) 2.4 ( 0.2 nm, b ) 4.0 ( 0.2 nm, R ) 65 ( 2°).21-23 As with BOC, molecules segregated into homochiral domains, although the domains were
typically much larger (1-10 µm2), with each domain completely covering an entire graphite terrace. The molecular-scale arrangement of 8CB was identical in films deposited with and without a magnetic field, which only influences domain orientation, as discussed below. Based on repeated unsuccessful attempts to image them with STM, the other two solvents employed in this study, 5CB and 5CB/1-phenyloctane, did not form crystalline monolayers at room temperature. We note that 8CB exhibits both smectic-A and nematic phases, but the since 8CB monolayer melts at the same temperature as the smectic-nematic transition24 (33 °C), performing experiments at the temperature where 8CB is nematic would likely provide little new information since the conditions (disordered monolayer and nematic supernatant) would be very similar to using 5CB at room temperature. Long-range orientational order in BOC films deposited from the two LC solvents was measured by making STM observations at multiple macroscopically spaced locations on several independently prepared samples and recording the chirality and inplane molecular orientation with respect to the axis of the magnetic field at each location. The results are collected in Figure 3, which presents plots showing the frequency of observation of different molecular orientations for both left- and right-handed domains. In Figure 3 orientation is reported as the angle between the axis of the originally applied magnetic field and the axis of highest symmetry in the observed BOC domain j , defined below). For comparison, similar data are also shown (D for several representative 8CB template monolayers (i.e., 8CB films imaged before spreading BOC onto the surface). Using these data, the in-plane orientational component of order can be quantified through calculation of a 2D scalar orientational order parameter, SML, defined as the largest eigenvector of the 2 × 2 matrix25
[
]
Sxx Sxy ) Syx Syy
[
〈2 sin(χiθi) sin(χiθi) - 1〉 〈2 cos(χiθi) sin(χiθi)〉 〈2 sin(χiθi) sin(χiθi) - 1〉 〈2 cos(χiθi) sin(χiθi)〉
]
(2) where χi ) (1 is the chirality of the ith domain (right-handed ) +1), - π/2 < θi < +π/2 is the orientation of molecular rows
12662
J. Phys. Chem. C, Vol. 114, No. 29, 2010
Gislason et al.
Figure 3. Molecular orientational distributions based on STM observations. (a) The 8CB template (216 data points), (b) BOC deposited from 8CB j and the magnetic field. (41 data points), and (c) BOC deposited from 5CB (114 data points). The abscissa represents the acute angle between D Solids lines in (b) and (c) are fits using the model described in the text.
TABLE 1: Solution and Monolayer Order Parametersa system
Sfluid
SML
β (deg.)
no. obsv.
8CB BOC in 8CB BOC in 5CB
0.55 0.47
0.81 ( 0.02 0.82 ( 0.05 0.21 ( 0.07
56 ( 1 81 ( 3 57 ( 7
216 41 114
a fluid S ) solution-phase orientational order parameter for BOC determined by polarizing UV-vis spectroscopy. SML ) molecular orientational order in the monolayer adsorbed to HOPG, determined by STM. β ) average row-field angle, determined by STM. no. obsv. ) number of STM images used to compute SML and β.
in the ith domain measured with respect to a fixed laboratory frame of reference, and the angled brackets denote an average over all observations. The largest eigenvalue is
SML )
1 S + Syy + √(Sxx)2 + (Syy)2 + 2SxxSyy - 4SxySyx 2 xx (3)
(
)
SML ranges from 0 to 1 for perfect disorder and perfect order, respectively. It is similar to the usual nematic orientational order parameter 〈P2(cos(θ))〉 commonly measured for bulk LCs and other uniaxial systems, except that the calculation involves two, rather than three dimensions.26 The uncertainty in SML was estimated using the jackknife method: from n nominally independent observations of molecular row orientation and chirality, we remove the first one, leaving n - 1, and calculate S1ML. The first observation is then replaced and the second removed, from which a new order parameter S2ML is computed, and so on. After repeating n times the uncertainty is 2 1/2 - 〈SML estimated as σ ) [∑in) 1(SML i i 〉) ] . The order parameters determined in this way are listed in Table 1. In addition to the overall degree of orientational order in the films, it was also of interest to determine along which direction the molecular rows tended to align. This average direction, j 〉, depends on chirality and can be found which we denote 〈R from the corresponding eigenvector of (2) using
〈R¯〉 ) nj ( β β)
[
(
Syx π + tan-1 2 Sxx - SML
(4)
)]
where nj is the axis of the LC director (parallel to the magnetic field for the LCs used in this work) and β is the average angle between the molecular rows and nj. The sign of β is defined as
a clockwise (counterclockwise) rotation from right- (left-) handed domains. The axis of highest symmetry in BOC monolayers (i.e., the axis that tended to align parallel to nj, so j i + χiβ. j 〉|nj) is then D ji ) R that 〈D Values for β are listed in Table 1 and shown schematically in Figure 4. On the basis of these data and the associated order parameters, it is evident that long-range order in BOC monolayers was very different for 5CB and 8CB solvents. When 8CB was used as the solvent, BOC molecules tended to orient with the long axis of the coumarin-benzothiazolyl headgroup approximately parallel to the bulk LC director nj. On the other hand, when the solvent was 5CB, the amount of orientational order was much smaller and the headgroup did not orient parallel to nj. The orientational order of solution-phase BOC molecules was determined using polarized UV-vis spectroscopy in planar LCfilled cells. These data are presented in Figure 5, which shows UV-vis spectra acquired using unpolarized illumination, and light polarized parallel and perpendicular to the director. The spectra are similar to those reported for BOC in heptane and chloroform by Dutta et al., where λmax ) 390 and 396 nm, respectively.27,28 Order parameters calculated using eq 1 for the transition at 400 nm are given in Table 1 and indicated in Figure 5. Note that the difference in orientational order for BOC in 8CB and 5CB was much less in solution than in the monolayer adsorbed to graphite, although care should be taken in the interpretation of this difference which compares order parameters measured in 2- vs 3-dimensions. It is worth mentioning that the order parameter Sfluid measured using eq 1 provides much less information on the orientational distribution function for molecules in solution than can be determined from STM observations of adsorbed molecules, where the full distribution is measured directly (viz. Figure 3). The solution-phase order parameter Sfluid )〈P2(cos φ)〉, where P2 is just the first nontrivial term in a series expansion in Legendre polynomials describing the complete distribution function, φ is the angle between the transition dipole in a given molecule for the absorption feature29 at 400 nm and the LC director, and the brackets denote an average over many molecules.30 Thus in the case where 8CB was used as the solvent for example, the order parameter for solution-phase BOC may have been smaller than that for BOC monolayers either because the orientational distribution function in solution was more broadened (i.e., molecules were less uniformly oriented), or because the transition dipole in solution tended to orient at an average angle 〈φ〉 ) 0 with respect to the LC director. From the measurement of Sfluid alone it is not possible to distinguish between these alternatives.
Hierarchical Order in Organic Monolayers
J. Phys. Chem. C, Vol. 114, No. 29, 2010 12663
Figure 4. The most probable orientations of BOC molecules in films deposited from 8CB (left) and 5CB (right). The two models show the average j 〉 observed for right-handed domains if the bulk LC director nj were along the horizontal. row orientation 〈R
Figure 5. UV-vis absorbance spectra of BOC in solution measured with light polarized parallel (solid line) and perpendicular (dashed line) to the director, and unpolarized light (dotted line).
However if it is assumed that BOC adopts similar mean orientations in both 5CB and 8CB solvents (i.e., 〈φECS〉 ) 〈φECS〉), as seems reasonable given the chemical and structural similarities of these two compounds, then it can be inferred that molecules in both solutions are about equally well ordered since the order parameters are similar. This suggests surface ordering in BOC monolayers is not determined by solution-phase ML ML fluid fluid , S8CB even though S 5CB ≈ S 8CB . ordering alone, since S5CB To further investigate this possibility, additional measurements were performed to study submicrometer-scale correlations in BOC orientational and chiral order, which were then compared to the corresponding correlation functions for the 8CB template. Note that these measurements on BOC and 8CB monolayers had to be made at different locations from one another because it was not possible using our microscope to find and image the same surface region after a sample was removed from the STM to add BOC. Although the coverage of left- and right-handed domains was about equal over an entire sample for all films studied, in films deposited from liquid crystal solvents neighboring domains showed a tendency to possess the same chirality as one another over distances up to at least one micrometer, particularly when 8CB was used as the solvent. This phenomenon can be seen in Figure 6, which shows representative images of an 8CB template and BOC monolayers deposited from 8CB, 5CB, and 5CB/1phenyloctane with domains labeled according to chirality. 8CB formed large, essentially defect-free homochiral domains whose size was usually limited only by the extent of the graphite terrace they occupied (Figure 6a). BOC domains were much smaller, with films containing many more defects than 8CB, and the degree and length-scale of interdomain chiral correlation varied considerably according to the solvent. When 8CB was used as the solvent (Figure 6b) BOC domains were almost uniformly
oriented and homochiral over distances up to at least 1 µm, sharing these characteristics with the 8CB template monolayer they displaced. In contrast, BOC films deposited from 5CB and 5CB/1-phenyloctane solvents (Figures 6c,d) possessed somewhat less (in the case of 5CB) or essentially no chiral correlation (in the case of 5CB/1-phenyloctane). To quantify these trends, sets of STM images were acquired on several independently prepared samples for each solvent combination. At each location thirty 200 × 200 nm images were taken in a regularly spaced grid pattern, with images separated by up to 800 nm. This distance represents the largest area that could be scanned using our instrument at a single approach of the tip to the surface. Images were then analyzed, and the chirality and x,y positions of dozens of domains were recorded. The results were used to compute the chiral correlation function χ(r) )〈χiχj〉r, where χi ) (1 is the chirality of the ith domain and r is the distance between domains i and j. The pairwise products were averaged over a small window of separations to produce the radial chiral correlation functions presented in Figure 7a. The chiral correlation function shows that, at separations r < ∼100 nm, χ(r) f ∼1 for BOC monolayers in all three solvents, as expected because this was approximately the size of individual homochiral domains. At very large separations (beyond the scan range of the STM), χ(r) must approach 0 because on a macroscopic scale all films were racemic. Most interesting are the results for intermediate distances, where χ(r) showed differing trends, ranging from near perfect correlation in BOC films deposited from 8CB, to essentially uncorrelated chirality using 5CB/1-phenyloctane. Along with the chiral correlation function a similar measure of short-range orientational correlation was also computed, shown in Figure 7b for BOC deposited from 8CB and 5CB.
12664
J. Phys. Chem. C, Vol. 114, No. 29, 2010
Figure 6. Autocorrelation in short-range orientational and chiral order of BOC monolayers depends on the solvent. Domains are labeled according to chirality (R/L), and for films deposited from LC solvents, j is also shown as an the orientation of the highest symmetry axis D arrow. (a) STM image of an 8CB “template” monolayer, which BOC eventually displaces from the graphite surface (Itunn ) 1.21 nA, Vb ) -1.2 V). (b) A BOC monolayer deposited from 8CB. Molecular orientation is uniform and homochiral (Itunn ) 0.79 nA, Vb ) -0.51 V). (c) A BOC monolayer deposited from 5CB. Although domains are of mixed chirality, the symmetry axes are nearly collinear throughout the imaged region (Itunn ) 0.84 nA, Vb ) -0.41 V). (d) A BOC monolayer deposited from 5CB/1-phenyloctane (Itunn ) 0.92 nA, Vb ) -0.52 V). The magnetic field was horizontal in all cases.
This was done using the function S(r) ) 〈cos 2ij〉, where ij j i and D j j for domains i and j, and the is the angle between D products were averaged over a small window of interdomain separations. BOC monolayers deposited from 8CB were more highly oriented over distances up to at least 600 nm than their ML ) limiting macroscopic order parameter reported above (S8CB 0.81), suggesting that an additional mechanism operating over short length scales contributes to increase orientational order. This did not happen to the same extent with BOC films deposited from 5CB, which instead approached their limiting macroscopic order parameter almost immediately (SML 5CB ) 0.21). Discussion BOC molecules at the surface experience several interactions influencing their orientation: (i) a substrate potential due to graphite; (ii) a surface tension anisotropy due to contact with the LC supernatant; and in the case of films deposited from 8CB, (iii) the possibility of a “templating” interaction associated with displacement of the crystalline 8CB monolayer. In addition to these interactions, BOC molecules may arrive at the surface already oriented to a certain extent as a result of guest-host effects in the solvent. Based on the above-reported observations, let us consider how each of these contributes to order in the final BOC monolayer. Substrate Potential. STM observations showed that only certain interdomain row-row angles occurred in all BOC monolayers, regardless of the solvent used: 0°, ( 60°, ∼51°, and ∼9°. The fact that only certain angles were possible demonstrates that BOC molecules were constrained to adsorb
Gislason et al. in registry with the graphite substrate, indicating a strong azimuthal orientational interaction. This led to six possible configurations: (3 distinguishable orientations) × (2 chiralities), and 62/2 ) 18 row-row angles, of which 5 are unique. Since the same interdomain angles and unit cell structures were observed in all three solvents, the substrate potential in combination with intermolecular interactions among adsorbed molecules must be the dominant influence determining domain orientation, with solvent-related forces being of secondary importance. The role of the latter is to break the degeneracy among the six substrate-allowed configurations, increasing the probability of forming certain ones and reducing the probability of others. Orientationally Biased Flux of BOC Reaching the Surface. Linear dichroism measurements of Sfluid in filled cells showed that solution-phase molecules were oriented in both LC solvents. Although some disruption of the fluid must have occurred when BOC was spread atop the supernatant in the second sample preparation step, this procedure was performed very carefully, in an attempt to leave the initial director alignment imparted by the magnetic field near the surface largely undisturbed. Consequently, BOC molecules diffusing through this layer could have approached the substrate already oriented to a certain degree, potentially favoring the nucleation and/or growth of monolayer domains having a similar orientation. However, the evidence shows this mechanism is apparently not very important, for two reasons. First, Sfluid was about the same for both LC solvents, but SML was very different. Thus orientational order in the solvent can at most account for only a small portion of orientational order in the monolayer. Second, STM studies involving monolayers of the structurally similar compounds 1-pyrenedodecanoic acid and 5-(octadecanoylamino) fluoroscein deposited from 5CB and 8CB showed no orientational order even though both compounds are almost certainly oriented in the LC,16 indicating that solution-phase order is insufficient to result in an oriented monolayer.31 Consequently, although the flux of BOC molecules arriving at the surface may have been biased toward a particular orientation, this appeared not to have had much influence on order in the adsorbed monolayer. We do note however that the fluid phase order parameter reflects dye orientation statistics in the bulk solvent, far from the interface. Near a solid surface order in the LC may differ, possibly substantially so, from its bulk state. To fully assess the role of solution-phase order one should perform measurements of dye orientation using techniques sensitive to the near surface region. Elastic and Anchoring Forces. After adsorption BOC molecules remain in contact with the interfacial LC supernatant, experiencing an anisotropic surface tension that results from elastic forces in the LC which favor uniform director alignment j i|nj. This and azimuthal anchoring forces which encourage D leads to an effective orientational potential which raises the total energy (monolayer + interfacial LC) in the region of a misaligned domain. Let us attempt to roughly estimate the magnitude of this effect. If W , Kd-1, where W is the azimuthal anchoring energy, K is an elastic constant for the LC and d ∼ A is the domain size, the director will remain nearly uniform (∇nj ∼ 0) in the region of the surface over a length scale much larger than the domain size. Under these conditions the director can be considered stiff relative to anchoring and the orientational energy can be approximated from anchoring alone by considering the energetic cost of a misaligned domain in contact with a homogeneously aligned LC. This approach neglects contribu-
Hierarchical Order in Organic Monolayers
J. Phys. Chem. C, Vol. 114, No. 29, 2010 12665
Figure 7. Short-range orientational and chiral autocorrelation in BOC monolayers.
tions from the free energy density of the interfacial LC but allows us to conveniently estimate the size of the effect (a more comprehensive treatment based on Landau-de Gennes theory with a surface coupling term could be developed to include the interfacial fluid32,33). With these simplifications the alignment energy Ut of a domain in contact with a uniformly oriented interfacial LC can be approximated using the Rapini-Papolar potential34
W j W Ui ) Ai (D , nj)2 ) Ai sin2 φi 2 i 2
(5)
where Ai is the area of the domain and φi is the angle between j i and nj. Given Ui, the probability P(φi) of observing a domain D with orientation φi can be computed if one assumes the observed molecular alignment statistics reflect an equilibrium distribution
P(φ) ) Q-1e-U(W,φ)/kT Q)
∫-ππ e-U(W,φ)/kT
(6a) (6b)
where k is Boltzmann’s constant, T is the temperature, and Q is the partition function. To determine the anchoring energy necessary to reproduce the experimentally observed order parameters, eqs 5 and 6 were fit to the distributions in Figure 3 (solid lines) treating all domains as having the same size (d ) 100 nm) and allowing W to vary. This gave W5CB ) 8 × 10-7 J m-2, and W8CB ) 6 × 10-6 J m-2. Although this treatment includes some simplifications, the shape of P(φ) matches the experimental distributions fairly well and the anchoring energies needed to reproduce the experimentally observed alignment statistics are consistent with typical values for W measured for many different LCs on a variety of organic surfaces.35 We also note that the assumption W , Kd-1 is found to be self-consistent even for 5CB, where K ∼ 10-11 N, and taking d ) 10-8 m. These results show that anchoring and elastic forces may be large enough to account for the observed degree of orientational order. Monolayer Templating. As noted above 8CB formed its own oriented crystalline monolayer on HOPG, whereas 5CB did not. Since BOC had to displace 8CB from the surface, the existence of this 8CB monolayer may have affected adsorption and domain nucleation, potentially altering the orientational or chiral distributions. For example, a recent study examining displacive
absorption of 8CB monolayers by n-tetracontane found evidence for a strong templating effect causing molecular orientation in the resulting alkane film to be influenced by the orientation of the 8CB monolayer it displaced.36 If templating affected molecular order in BOC films its influence should be most pronounced over short length scales, up to the size of individual 8CB domains (1-10 µm2). Over this size scale the 8CB monolayer was essentially uniformly oriented (Figure 6a), so templating influences should have the effect of inflating local order in BOC films. This is indeed observed in BOC/8CB samples, where SML ∼ 1 at short length scales, significantly larger than its limiting macroscopic value of 0.81. Furthermore, a similar effect was not observed in BOC/ 5CB films, which instead approached their limiting order parameter almost immediately (Figure 7b). BOC/8CB films also exhibited a high degree of chiral autocorrelation over short length scales (Figure 7a), even though at macroscopic scales films were racemic overall. Since the 8CB monolayer was also homochiral over this size scale, it may seem as if this difference should be attributed to a templating effect as well. However chiral autocorrelation in BOC/8CB films is more simply explained as a byproduct of orientational autocorj j if j i||D relation, since a pair of domains i and j can only have D χi ) χj. At macroscopic length scales, the correspondence between orientation and chirality breaks down because the substrate is polycrystalline, with random long-range azimuthal crystallographic orientation, but over short distances where the substrate has a single crystallographic orientation the correlation can be very strong. Note that unlike BOC/8CB films, orientation and chirality for BOC films deposited from 5CB need not be correlated over any length scale because β ) 57°, close to the azimuthal rotational periodicity of the substrate potential. Thus the combination of a 60° rotation and chirality inversion can j almost unchanged in BOC/5CB films, and so the leave D alignment axes of a pair of domains can be nearly collinear even if their chiralities differ (several examples of domain pairs with this relationship can be seen in Figure 6c). Finally, we note that the in-plane orientations of BOC and 8CB molecules were rather different, as were their respective mean row orientations (Table 1). This makes a direct replacement mechanism, with BOC molecules substituionally displacing 8CB unlikely. On the basis of these arguments, we conclude that monolayer templating made only a small additional contribution to short-range orientational order in films deposited from 8CB, which in turn led to an increase in chiral correlation between
12666
J. Phys. Chem. C, Vol. 114, No. 29, 2010
nearby domains. There is no evidence for any templating influence in films deposited from 5CB. Conclusions In summary, the orientational and chiral properties of BOC films deposited from two oriented LCs and one isotropic solvent using a sacrificial template method have been studied by STM and polarized UV-vis absorption spectroscopy in order to determine the mechanisms leading to short- and long-range order. BOC films were hierarchically ordered, with different types and degrees of order over different length scales. From the structural properties and ordering statistics of the resulting films it appears that the substrate potential provides the dominant influence, with rotational and chiral symmetry broken by much weaker LC forces. A combination of elastic and anchoring forces explains most of the observed long-range order, with templating effects making a small additional contribution at submicrometer length scales. The orientation of BOC molecules in solution appeared largely uncorrelated to order in the adsorbed monolayer. These findings provide a clearer picture of the mechanisms through which LC solvents can influence order in organic thin films and demonstrate their potential for controlling structural properties over a range of different length scales. Acknowledgment. This work was supported by the National Science Foundation under CHE-0518682 and DMR-0705908. N.G. thanks the Arnold and Mabel Beckman Foundation for fellowship support. References and Notes (1) Mougous, J. D.; Brackley, A. J.; Foland, K.; Baker, R. T.; Patrick, D. L. Phys. ReV. Lett. 2000, 84, 2742. (2) Berg, A.; Patrick, D. L. Angew. Chem. Int. Ed. 2005, 43, 1744. (3) Wilkinson, F. S.; Norwood, R. F.; McLellan, J. M.; Lawson, L. R.; Patrick, D. L. J. Am. Chem. Soc. 2006, 128, 16468. (4) Patrick, D. L.; Wilkinson, F. S.; Fegurgur, T. L. Proc. SPIE 2005, 5936, 59360A. (5) Hickman, S.; Hamilton, A.; Patrick, D. L. Surf. Sci. 2003, 537, 113. (6) Dimitrakopoulos, C. D.; Mascaro, D. J. IBM J. Res. DeVel. 2001, 45, 11. (7) Makinen, A. J.; Melnyk, A. R.; Schoemann, S.; Headrick, R. L.; Gao, Y. Phys ReV. B 1999, 60, 14683. (8) Ohring, M. The materials science of thin films; Academic Press: New York, 1992.
Gislason et al. (9) Silinsh, E. A.; Capek, V. Organic Molecular Crystals Interaction, Localization, and Transport Phenomena; AIP Press: New York, 1994. (10) Lynch, M. D.; Patrick, D. L. Chem. Mater. 2004, 16, 762. (11) Lynch, M. D.; Patrick, D. L. Nano Lett. 2002, 2, 1197. (12) Carter, P. W.; Ward, M. D. J. Am. Chem. Soc. 1993, 115, 11521. (13) Mitchell, C. A.; Yu, L.; Ward, M. D. J. Am. Chem. Soc. 2001, 123, 10830. (14) Oehzelt, M.; Koller, G.; Ivanco, J.; Berkebile, S.; Haber, T.; Resel, R.; Netzer, F. P.; Ramsey, M. G. AdV. Mater. 2006, 18, 2466. (15) Zannoni, C. In Polarized spectroscopy of ordered systems; Samori, B., Thulstrop, E. W., Eds.; Kluwer: Dordrecht, The Netherlands, 1987; pp 421-453. (16) Ivashchenko, A. V. Dichroic dyes for liquid crystal displays; CRC Press: Boca Raton, FL, 1994. (17) Weiss, R. G. Tetrahedron 1988, 44, 3413. (18) The transition temperatures for 8CB are Cr f Sm-A, 21.5 °C; Sm-A f N, 33.5 °C; N f I, 40.5 °C. The transition temperatures for 5CB are Cr f N, 24 °C; N f I, 35 °C. (19) Mair, W.; Saupe, A. Z. Naturforsch. 1961, 16a, 816. (20) Pan, L. L.; Zeng, Q.; Lu, J.; Wu, D.; Xu, S.; Tan, Z.; Wan, L.; Bai, C. Surf. Sci. 2004, 559, 70. (21) Foster, J. S.; Frommer, J. E. Nature 1988, 333, 542. (22) Smith, D. P. E.; Horber, J. K. H.; Binnig, G.; Nejoh, H. Nature 1990, 344, 641. (23) Smith, D. P. E. J. Vac. Sci. Technol. 1991, B9, 1119. (24) Rivera Hernandez, M.; Miles, M. J. Probe Microsc. 2000, 2, 45. (25) Scott, D. M.; Smith, N. A.; Valente, J. J.; Adams, R.; Bufkin, K.; Patrick, D. L. J. Phys. Chem. B 2010, 114, 1810. (26) In previous work we used a somewhat different method for calculating the order parameter based on the function s ) 〈cos 2(Rt- R j )〉, where the brackets denote an average over all observations, Ri is the orientation of molecule i, and R j ) 〈Ri〉. Both methods yield the same result, but Eq. 3 avoids certain complexities associated with defining how molecular orientation Ri is measured, which must be done in such a way that the order parameter is maximal. (27) Ray, K.; Dutta, A. K.; Misra, T. N. J. Lumin. 1997, 71, 123. (28) Dutta, A. K.; Ray, K.; Misra, T. N. Solid State Commun. 1995, 94, 53. (29) The transition moment associated with this absorption is approximately parallel to the long axis of the coumarin-benzothiazolyl head group, based on a semiempirical calculation of an isolated molecule using the ZINDO/S method with configuration interaction on the AM1 optimized geometry, performed with the software package Hyperchem. (30) De Gennes, P. G.; Prost, J. The Physics of Liquid Crystals, 2nd ed.; Clarendon Press: Oxford, U.K., 1993. (31) Fegurger, T.; Plank, K.; Patrick, D. L. unpublished results. (32) Schuddedboom, P. C.; Jerome, B. Phys. ReV. E 1997, 56, 4294. (33) Lacaze, E.; Michel, J.-P.; Goldmann, M.; Gailhanou, M.; de Boissieu, M.; Alba, M. Phys. ReV. E 2004, 69, 041705. (34) Rapini, A.; Papoular, M. J. J. Phys. (Paris), Colloq. 1969, 30, C4. (35) Sonin, A. A. The Surface Physics of Liquid Crystals; Gordon and Breach: Luxembourg, 1995. (36) Baker, R. T.; Mougous, J. D.; Brackley, A.; Patrick, D. L. Langmuir 1999, 15, 4884.
JP103052P
