Nanostructures of Small-Molecule Organic Crystals on Capillary Wave

Article pubs.acs.org/Langmuir

Nanostructures of Small-Molecule Organic Crystals on Capillary Wave Surfaces with Controllable Capillary Lengths Hyun Hwi Lee,† Kwangseok Ahn,‡ Doris Yangsoo Kim,‡ Chung-Jong Yu,*,† and Dong Ryeol Lee*,‡ †

Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 790-784, Korea Department of Physics, Soongsil University, Seoul 156-743, Korea

‡

ABSTRACT: We report on the nanostructures of organic small-molecule pentacene crystals that have been vapor-deposited onto the capillary wave surfaces of thin liquid films. The characteristic lateral length of the capillary wave surface or the capillary length can be controlled by changing the thickness of the liquid films and, thus, the van der Waals interaction with the substrate. We find that the morphology of the organic crystals gradually varies from fractals to compact islands as the liquid film thickness increases. The square of average distance between organic crystal grains was also found to be proportional to the liquid film thickness. We discuss the possibility that these effects are driven by capillary fluctuations at the air−liquid interface.

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temperature but with the pair bond energy.11 Because liquid surfaces have more activated diffusion or longer diffusion lengths than solid surfaces, larger aggregates or molecular crystals can easily be formed on liquid surfaces.7,8 However, the morphological transitions in nanostructures because of variation in the thickness of the underlying liquid films have not previously been reported. We also found that the grain size and the grain−grain distance of the pentacene nanostructures increase with the increasing film thickness and the power-law relationships between these quantities are deduced.

INTRODUCTION Capillary waves are height fluctuations that are thermally induced along liquid surfaces.1 Thermal capillary waves play significant roles in fluid phenomena, such as spinodal dewetting2 and droplet coalescence.3 They are also involved in the self-assembly and aggregation of nanoparticles at fluid interfaces.4−6 The collective hydrodynamic motions of the capillary waves arise according to the thermal energy, which is comparable to the interface energy, and result in the aggregation of nanoparticles. The vapor depositions of atoms and molecules onto liquid surfaces are usually similar. The aggregation of large, branched Ag7 and the growth of large molecular organic crystals8 have been previously reported. However, direct evidence of the effects of capillary waves on the two-dimensional (2D) formation of nanostructures at liquid surfaces has not been previously reported. Here, we report the novel finding that the nanostructures of small-molecule organic crystals grown on thin liquid films change distinctively as a function of liquid film thickness, and we also speculate on the underlying causes, in terms of capillary fluctuations of the liquid surface. The nanostructured samples were prepared by vapor-depositing small-molecule pentacene crystals onto nanometer-thick polydimethylsiloxane (PDMS) films of silicone oil coated onto SiOx substrates. An organic semiconducting pentacene has been extensively studied because of its organic thin-film transistor applications.9 Typically, when pentacene molecules are deposited onto a bare SiOx substrate, they exhibit peculiar fractal island structures.10 Here, we found that, as the film thickness increases, the fractal islands are gradually replaced by compact islands. This fractal−compact island transition has a strong resemblance to the morphology variation in the sub-monolayer solid-on-solid epitaxial growth model, where the diffusion process varies not with the © 2013 American Chemical Society

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EXPERIMENTAL SECTION

Nanometer-thick liquid films were obtained using a dipping method.12 For non-evaporating and well-characterized liquid films, PDMS was chosen in this study. A liquid solution was prepared from 50 to 400 μL of PDMS (Sigma-Aldrich) of viscosity 10 cSt (Mw ∼ 1250) dissolved in 25 mL of hexane (95% pure, Sigma-Aldrich). Silicon pieces were cleaned in a strong oxidizer, rinsed with pure water, and blow-dried with nitrogen gas. They were dipped in the PDMS solution and withdrawn at a speed of ∼1 cm/s. The film thickness was controlled by varying the PDMS concentration of the solution. The solvent was evaporated promptly, leaving a nanometer-thick film of PDMS. The liquid film samples were investigated with X-ray measurements to determine their structural properties. Another set of liquid films and a reference bare SiOx were used for pentacene deposition. The samples were placed side by side to reduce the deposition area and, thereby, to obtain the same pentacene thickness. Pentacene was vapor-deposited with an evaporation rate of 0.1 nm/min. The deposition was stopped at the nominal thickness of three monolayers. The thicknesses of the liquid films were determined with X-ray reflectivity measurements, as shown in Figure 1b. The resulting data Received: November 2, 2012 Revised: January 21, 2013 Published: January 25, 2013 2646

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Figure 1. Organic pentacene nanocrystals vapor-deposited on the capillary wave surfaces of a thin liquid PDMS film. (a) Schematic drawing. (b) Xray reflectivity curves from the liquid PDMS films. The solid lines were calculated using Parratt’s formula.13 The SiOx surface (no liquid) shows a very thin water interfacial layer. (c) AFM images of the pentacene crystals deposited onto the liquid PDMS films with various film thicknesses d = 2.9, 5.1, 8.6, and 19.8 nm. The pentacene crystals deposited onto the solid surface substrate without a liquid film are also shown as a reference. The image size is 5 × 5 μm2. (d) Plot of perimeter versus area for the islands. From the fitted slopes of the log−log plots, the fractal dimensions are found to be 1.75 and 1.15 for the bare substrate and the liquid film with a thickness of 8.6 nm, respectively.

Figure 2. Grain size and distribution of the pentacene nanocrystals. (a) PSDs obtained from the AFM data. The one-dimensional (1D) PSD data (symbols) extracted from the 2D PSD distributions in the inset were fitted to the total structure factor (lines), which was calculated with the polydisperse cylinder model and the 2D paracrystal model. (b) Schematic drawing of the gyration radius Rg and the average distance between islands Davg. (c) Plot of Rg and Davg as a function of the liquid film thickness d. curves were fitted with Parratt’s formula.13 The capillary lengths of the liquid surfaces were determined using transverse diffuse scattering measurements. The crystalline structures of the pentacene molecules deposited on the liquid films were investigated using conventional diffraction and two-dimensional grazing incidence X-ray diffraction (2D GIXRD). All X-ray measurements were carried out at the 5A beamline of the Pohang Accelerator Laboratory (PAL) in Korea, at an X-ray energy of 11.6 keV (a wavelength of 1.0688 Å). The surface morphologies of the pentacene crystals were examined using atomic force microscopy (AFM, Digital Instrument Multimode) operated in the tapping mode.

whereas the nanostructures deposited on the liquid films vary toward a compact island morphology as the thicknesses of the liquid films are increased. Second, in the case of the liquid film samples, the size of the pentacene grains and the grain−grain distances increase as the thickness of the liquid film d is increased. The fractal−compact transition in the morphologies in Figure 1c was quantitatively confirmed by calculating the fractal dimensions using the perimeter (L)−area (A) relationship of L ∝ DfADf/2, where Df is the fractal dimension of the island perimeter.14 To obtain the perimeters and areas of the islands, we first digitized the 20 × 20 μm2 AFM images into 1 and 0, where 1 is high and 0 is low with respect to the height of three monolayers, which is the nominal thickness. Figure 1d depicts log−log plots of L versus A obtained from the digitized islands. Using the L−A relationship and the slopes of the log−log plot, the fractal dimensions Df were estimated to be 1.75 (±0.07) for

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RESULTS AND DISCUSSION Figure 1c is a summary of the images from the AFM measurements on the pentacene crystals deposited on the liquid surfaces with various liquid film thicknesses. The AFM images have two important characteristics. First, the pentacene structure deposited on SiOx has a fractal island morphology, 2647

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the SiOx surface and 1.15 (±0.05) for the liquid film with d = 8.6 nm, respectively. While the former agrees well with Df = 1.6−1.7 for diffusion-limited aggregation (DLA) during pentacene sub-monolayer growth,10 the latter is close to Df = 1.0 for circles representing compact islands. For the liquid film samples with d = 5.1 and 19.8 nm, the fitted slopes of the log− log plots are close to that of d = 8.6 nm. On the other hand, both slopes shown in Figure 1d are observed for the sample with d = 2.9 nm. To quantify the effects of varying the liquid film thickness on the grain size and distribution of the pentacene crystals, we obtained the power spectral densities (PSDs) of the pentacene island distributions from the 20 × 20 μm2 AFM images. We first extracted the 2D PSDs from the digitized images that were obtained for the fractal dimension and then obtained the averaged PSD by circularly averaging the 2D PSDs, as shown in Figure 2a. These averaged PSDs were fitted by the total structure factor of the island distribution Stot(q), which can be expressed as Stot(q) ∝ ⟨|F(q,r)|2⟩rSis(q), where F(q) is the island form factor and Sis(q) is the structure factor of the island distribution.15 For polydisperse disks as a model of pentacene crystal islands, F(q,r) = πr2J1(qr)/(qr), where r is the disk radius and ⟨ ⟩r represents the averaging over the disk radii assuming a log-normal size distribution function n(r;r0,w) = exp[−ln(r/ 2 r0)2/2w2]/(2π)1/2 we0.5w with two parameters: r0 for radius at a maximum population and w for its broadness. From these two fitting parameters, we calculated the average radius of gyration Rg = (∑0.5r2n(r)(πr2)2/∑n(r)(πr2)2)1/2, which corresponds to the average size of pentacene crystal islands.16 The structure factor in the 2D paracrystal model is given as15 Sis(q) =

Figure 3. (a) X-ray intensities measured along the surface normal direction. (b) Two-dimensional GIXRD patterns of the pentacene crystals deposited on the capillary wave surface of the liquid film with a thickness of 19.8 nm and on the bare surface of the silicon substrate.

thickness instead of the temperature. Thin liquid films have different intrinsic properties from the bulk liquid because of the confinement effect and the disjoining pressure.19 For example, surface tension and viscosity would affect the dynamics of adsorbed particles and would be relevant to our experimental results. Surface tension changes as a function of d because of the disjoining pressure. However, this change is noticeable only when the thickness is much thinner than our sample.20 Viscosity also changes with the thickness of the liquid film, but this change becomes significant only when the liquid is tightly confined between solid surfaces.15 Another example could be dependency upon the relaxation time. There have been reports on the significant change of the relaxation time from liquid in bulk status to confined status.21 The reason is known to be the structural changes of liquid; this situation does not apply to the air−liquid interface, unless the liquid molecules have a strong interaction with the substrate, such as grafting. The main observation in our experiment is the dependence of the crystal island size as a function of the film thickness. Therefore, as long as there is no explicit dependence of the relaxation time upon the film thickness, the dynamic behavior of capillary waves in relation to relaxation time seems to be irrelevant to our observation. Because the effects of these intrinsic properties are negligible, we turn to the capillary wave instead, which is characterized by the capillary length (ξ) and causes the interface height fluctuation (Δz), i.e., ⟨Δz⟩2 ∼ kT log ξ.22 For thin films, the disjoining pressure is significant, causing ξ ∼ d2 and ⟨Δz⟩2 ∼ kT log d.20 ⟨Δz⟩2 is also roughly proportional to a dynamical parameter, the self-diffusion constant of liquid molecules normal to the interface, whose value is about the same as that parallel to the interface.23 The pentacenes adsorbed onto the liquid surface are likely to have similar diffusion behavior, which effectively increases average distances between them as the thickness increases and, similarly, the probability of pentacenes detaching away from each other. This would be the condition of the fractal−compact transition in the “deposition, diffusion, and aggregation” (DDA) model.11,22,24 In this model, if the energy of a bond formed by a molecule with its nearest neighbors is large, the diffusing molecule attaches almost irreversibly to the island and forms a fractal. On

1 − A2 (q) 1 − 2A(q)cos(qDavg ) + A2(q)

where Davg is the average distance between islands, A(q) = exp(−q2σ2), and σ is the variation of Davg. The lines in Figure 2a represent the best fits using Stot(q), from which the average island size Rg and the island−island distance Davg were precisely determined. Figure 2c shows a plot of these parameters Rg and Davg as a function of the liquid film thickness d. It is noticeable that Rg is nearly the same as Davg/4. The plot in Figure 2c also reveals the following power-law relationship: Davg ∼ 4Rg ∝ d1/2. To check the crystalline properties of the pentacene crystals, we measured the (00L) peaks along the surface normal (Figure 3a) and the 2D GIXRD along the in-plane direction (Figure 3b). When we compare the (00L) peaks in Figure 3a, the pentacene crystals on the liquid films have smaller peak widths and, thus, longer crystalline lengths than those on the bare solid surface, which is due to the enhanced three-dimensional (3D) island structures on the liquid surfaces. From good crystallinity of the pentacene crystals on both the liquid film and the bare substrate, as shown in Figure 3, we found that the crystal structure of the pentacene on the liquid surfaces is not significantly affected by the liquid film thickness or the resulting grain size. We also note that this good crystallinity is quite different from that of the pentacene crystals deposited on liquid-like polymer thin films at T > Tg, the glass transition temperature, which are nearly amorphous and have a smaller grain size than those grown at T < Tg.17 The fractal−compact island transition in the morphology seems to depend upon the temperature, while all other growth parameters are fixed.18 However, we only varied liquid film 2648

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where K0(x) is the modified Bessel function and r0 is the lowlength cutoff related to the molecule size. The parameter B and the capillary length ξ are two adjustable parameters for the fits. From the best fits represented by the lines in Figure 4, B was estimated to be 5.3 (±0.2) × 10−2 nm2 for the two thickest films. Using the relationship of B = kBT/(πγ), we obtained the surface tension of PDMS, γ = 24 (±1) mN/m, which is consistent with the reported value of 20 mN/m.30,31 This supports the liquid nature of our PDMS films. The inset of Figure 4b shows a log−log plot of ξ−1, which was determined from the best fits, versus the liquid film thickness d. The powerlaw relationship between ξ−1 and d, predicted by eq 1, is clearly confirmed by a linear fit, which results in a power of −2.0 (±0.1) and the effective Hamaker constant, Aeff, of 3.1 (±0.7) × 10−20 J. This is comparable to 6 (±2) × 10−20 J obtained for cyclohexane on Si in the work by Tidswell et al.25 Therefore, all of the physical values from the experiment are in the reasonable range.

the other hand, a lower bond energy increases the probability of a molecule detaching itself from the island or diffusing along the island perimeter, resulting in a compact shape. Because the bond energy does not vary in our experiment, the detaching or the diffusing probability of pentacene molecules is likely to be larger as a result of the capillary fluctuations. We then characterized the samples in terms of the capillary length, ξ, and checked the related physical parameters for the clarity of our experiments. ξ is related to the thickness of the liquid film according to

ξ(d) =

2πγ /Aeff d 2

(1)

where γ, Aeff, and d are the surface tension, the effective Hamaker constant, and the thickness of the liquid film, respectively.25,26 Figure 4 shows the results of the X-ray diffuse

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SUMMARY The organic pentacene crystals were grown on the capillary wave surfaces of the PDMS liquid films. Our AFM measurement reveals a transition from fractal to compact island structures as the liquid film thickness is increased. We also found that the structure factors of the pentacene crystals strongly depend upon the liquid film thickness. We have discussed the possibility that these effects are driven by capillary fluctuations controlled by the capillary length in the liquid film. These results provide new insights into solid-on-liquid growth with the controllable length scale on thermally fluctuating liquid surfaces. A detailed explanation awaits the development of a theoretical formulation in terms of capillary fluctuations or computer simulation of these effects.

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Figure 4. X-ray diffuse scattering from liquid PDMS films on silicon substrates with various film thicknesses. The diffuse scattering intensities along the in-plane component of the scattering vector were measured at fixed out-of-plane components of (a) qz = 2.05 nm−1 and (b) qz = 3.70 nm−1 . The arrows represent the positions of the inverse of the capillary length, ξ−1, obtained from the best fits (lines), which roughly correspond to the kinks in the curves. (Inset) Plot of ξ−1 versus the liquid film thickness, d. Error bars are smaller than the symbol size.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.-J.Y.); [email protected] (D.R.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science and Technology (MEST) (2009-0076010 and R15-2008-00601002-0).

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scattering measurements, which can be used to determine the capillary lengths of the liquid film samples with different thicknesses. To characterize the in-plane structures of the capillary wave surfaces, the diffuse scattering intensities were measured along the in-plane component of the scattering vector at fixed out-of-plane components qz = 2.05, 2.87 (not shown), and 3.70 nm−1. For a precise determination of the capillary length, the diffuse intensities at three qz values were simultaneously fitted by a quantitative model of the X-ray diffuse scattering from the capillary wave surfaces with a set of fitting parameters. The intensity of X-ray diffuse scattering from a non-ideal surface is proportional to the absolute square of the Fourier transform of exp[qz2C(R)], where C(R) = ⟨z(0)z(R)⟩ is the height−height correlation function.27 For the capillary wave surfaces, the correlation function is given as28,29

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