J. Phys. Chem. C 2007, 111, 10993-10997
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Growth Morphology of Perylene-3,4,9,10-tetracarboxylic Dianhydride (PTCDA) Thin Films: Influence of Intermolecular Interactions and Step-Edge Barriers S. Yim,† K.-il Kim,‡ and T. S. Jones*,§ Department of Chemistry, Kookmin UniVersity, Seoul 136-702, South Korea, Department of Chemistry, Imperial College London, London, SW7 2AZ, United Kingdom, and Department of Chemistry, UniVersity of Warwick, CoVentry, CV4 7AL, United Kingdom ReceiVed: February 23, 2007; In Final Form: May 4, 2007
The growth evolution of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) thin films on glass substrates has been studied using atomic force microscopy measurements and scaling analysis. The results are consistent with an upward mound growth mechanism, with the measured scaling exponents indicating anomalous growth behavior. Simulation of one-dimensional surface lateral diffusion models and calculation of the intermolecular interaction energies indicate that the observed growth mode arises from the preference for deposited molecules to attach at the minimum energy position within the same PTCDA layer rather than in a lower layer, based on the existence of high step-edge barriers. The results highlight the important role played by the microscopic molecular properties in determining the macroscopic thin film morphology.
1. Introduction Molecular semiconductors are attracting increasing attention for application in a wide range of (opto)electronic devices, most notably organic light emitting diodes, photovoltaics, and field effect transistors. Thin film morphology plays a crucial role in the performance of these devices, for example, in determining charge carrier mobility, but also in affecting the quality of interfaces formed either between different organic layers (an organic heterojunction) or at the organic-inorganic (electrode) heterointerface.1,2 However, in contrast to atomic-based inorganic semiconductor thin films, the growth mechanisms of molecular thin films remains poorly understood, and there is relatively little understanding of the way in which the unique properties of molecules influence the macroscopic growth behavior and resulting morphology.3 Several recent studies have focused on a more fundamental understanding of the growth behavior of molecular thin films formed by vapor deposition, in particular, through the application of scaling theory,4-7 a common method for studying the growth dynamics of a variety of inorganic and metallic thin film systems.8 The combination of atomic force microscopy (AFM) measurements and height difference correlation function (HDCF) analysis allows the evolving morphology to be described by simple scaling laws characterized by a variety of exponents, the magnitude of which provides important information on the thin film growth mode. For example, Durr et al.5 studied the growth of diindenoperylene (DIP) films and found that the exponents were largely consistent with those observed in many inorganic thin film material systems except for the relatively large value for the growth exponent, β, which suggested rapid surface roughening. This observation was explained in terms of spatial inhomogeneities due to the presence of different tilt domains and grain boundaries. The growth of pentacene thin * Corresponding author. E-mail:
[email protected]. Phone: +44(0)24-7652-8265. Fax: +44-(0)24-7652-4112. † Kookmin University. ‡ Imperial College London. § University of Warwick.
films on SiO2 substrates has also been studied using scaling analysis, with Zorba et al.6 suggesting that the interplay between two different growth mechanisms (diffusion and step-edge barrier) leads to fractal-mound growth. In a recent study, we reported results for the growth of free-base phthalocyanine (H2Pc) films.7 The scaling exponents indicated anomalous behavior as a consequence of the pronounced upward growth of crystalline H2Pc mounds during the initial stages of film formation. It is clear from these studies that the growth behavior of molecular thin films is very different to that commonly observed for atomic-based inorganic systems. In this paper, we show that the macroscopic growth behavior of another important molecular semiconductor, perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), can be rationalized by understanding the way intermolecular interactions influence thin film growth. AFM analysis again indicates significant deviation from conventional scaling laws. A simple onedimensional (1D) lateral diffusion model and the calculation of lattice potentials show that the pronounced upward growth mechanism is a consequence of a high-energy barrier at the step edges of existing PTCDA islands. 2. Experimental Details PTCDA films were grown in an ultrahigh vacuum organic molecular beam deposition (OMBD) chamber with a base pressure of ∼2 × 10-9 torr. Commercially available PTCDA (Fluka, 98%) powder was purified using temperature gradient vacuum sublimation. The purified material was then outgassed in the vacuum chamber for ∼20 h before growth and sublimed from a miniature effusion cell onto glass substrates held at room temperature. The cell temperature was ∼325 °C, which corresponds to a growth rate of ∼1 Å‚s-1 as determined by a quartz crystal microbalance positioned near the substrate. Ex situ surface morphology analysis at different film thicknesses (20300 nm) was performed using tapping mode atomic force microscopy (AFM).
10.1021/jp0715272 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/29/2007
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Figure 1. (a) Schematic of the PTCDA herringbone structure; the molecular planes are parallel to the glass substrate. AFM images for PTCDA thin films deposited on glass substrates with various thicknesses, D, of (b) 21 nm, (c) 99 nm, and (d) 284 nm.
3. Results and Discussion 3.1. AFM Analysis. PTCDA belongs to the P21/c space group and adopts a herringbone structure where the molecules lie in the (102) plane. This plane is usually parallel to the substrate (Figure 1a) when films are deposited at room temperature on a number of substrates,9 and our own previous X-ray diffraction studies have shown this to be the case for films grown on glass substrates.10 Representative AFM images of PTCDA thin films of different thicknesses (D) are shown in Figure 1b-d. The images were all taken with the same scan area of 2.5 µm × 2.5 µm, and in each case, the morphology is characterized by spherical crystallites.10 The film morphology was analyzed using HDCF scaling analysis,8 where the roughness (R), growth (β), and dynamic (z) exponents can be determined from the mean square surface fluctuation, g(R);
g(R) ) 〈[h(x,y) - h(x′,y′)]2〉 R ) x(x - x′)2 + (y - y′)2
(1)
The average is taken over all pairs of points (x,y) and (x′,y′) separated laterally by the length, R. There are two distinct regimes which depend on the relative magnitudes of R and the correlation length, ξ; g(R) ∝ R2R for R , ξ, and g(R) ) 2σ2 for R . ξ, where σ is the mean-square surface roughness. The parameters σ and ξ are related to the film thickness, D, according to the power laws σ ∝ Dβ and ξ ∝ D1/z. It has been shown that in many cases R, β, and z are not independent and obey the conventional scaling law, 1/z = β/R.11-13 In recent years, several experiments for inorganic thin film growth have revealed that the scaling of the long (global) and short (local) length characteristic interface fluctuations can be substantially different14,15 and that modified growth models with different roughness exponents at long (R) and short (Rloc) length scales have been proposed.16-18 The local roughness exponent, Rloc, can be determined experimentally, and the short length
Figure 2. (a) Plot of averaged g(R,D) vs R for PTCDA films with various D (b ) 21 nm, 4 ) 50 nm, 9 ) 99 nm, X with vertical bar ) 213 nm, and [ ) 284 nm). Also shown are plots of (b) Rloc, (c) σ, and (d) ξ as a function of film thickness, D. The horizontal dashed line in b denotes the average value of Rloc.
scales follow the so-called anomalous scaling ansatz: g(R,D) ∝ R2Rloc ‚ D(R-Rloc)/z for R , ξ, and g(R,D) ) 2σ2 for R . ξ. Figure 2a shows a plot of g(R,D) as a function of R for different values of PTCDA film thickness (D), and this yields an average value of Rloc ) 0.72 ( 0.12 for the range of thicknesses studied (Figure 2b). The surface roughness values (σ), determined experimentally from AFM images, are plotted in Figure 2c as a function of D, and a β value of 0.54 ( 0.06 is obtained from the power law, σ ∝ Dβ. The correlation length, ξ, at each thickness was determined by fitting the HDCF to the following analytical function: g(R,D) ) 2σ2g˜ (R/ξ) where g˜ (x) ) 1 - exp(-x2Rloc).19,20 Figure 2d shows a plot of ξ as a function of D and yields 1/z ) 0.25 ( 0.10, which is significantly smaller than the value of β/Rloc () 0.75 ( 0.25). Although deviation from conventional scaling laws has recently been reported for the growth of H2Pc thin films and was attributed to a steepening of the film as a consequence of upward growth,7 the anomaly in the case of PTCDA is much more pronounced. 3.2. Modelling. Molecular semiconductors such as PTCDA are bonded to each other by relatively weak van der Waals (vdW) forces, which enable impinging molecules to diffuse laterally across the surface until they find an energetically stable position (as indicated by the dashed empty squares in Figure 3a). However, in order for the impinging molecule to jump down and occupy the position on the lower layer (dashed square position on the right), it must overcome an additional energy barrier which exists at the step-edge; the so-called Schwoebel barrier (∆Eb). Our experimental analysis for PTCDA films, and in particular the pronounced anomalous growth behavior, suggests step-edge barriers play an important role. To assess the importance of step-edge barriers in determining the growth morphology, we have used a simple (1 + 1) dimensional surface lateral diffusion model in which different barrier heights have been simulated. The results are presented in Figure 3b-d. For each simulation, 50 000 particles were
Perylene-3,4,9,10-tetracarboxylic Dianhydride
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Figure 3. (a) Schematic of (1 + 1)-D diffusion model. Surface growth patterns generated by the model with various PF values of (b) 0, (c) 2, and (d) 4. (e) Plots of scaling exponents, Rloc, β, β/Rloc, and 1/z, as a function of PF.
deposited randomly on a substrate of horizontal columns, L ) 500 (each figure shows 200 columns for clarity). The shading alternates after the deposition of each set of 5000 particles so that the increase in roughness can be easily recognized. The deposited particles are allowed to diffuse along the surface up to a finite distance (maximum of 10 columns in these simulations) before stopping, when they find an energetically stable position. It is convenient in this model to introduce a preference factor, PF ) n, which can be defined when the deposited particle is in a position where diffusion to a step edge in the same layer is n columns greater than diffusion down to a step edge in the lower layer, with equal probability of diffusion occurring in the two directions. In the case of PF ) 0, for example, the impinging particle at position ii in Figure 3a has equal
probability of moving left and right since both involve travelling 5 columns to find a stable position (dashed empty squares). A particle deposited at position i moves left, and particles deposited at position iii or iv will move right. In the case of PF ) 2, an impinging particle at position iii has an equal probability of moving in both directions since from this position diffusion in the same layer (six columns to the left stable position) is two columns greater than diffusion down to the lower layer (four columns to the right stable position). In this case, a particle deposited at position i or ii moves left, and a particle deposited at position iv will move right. Consequently, an increase in the PF values in the simulations leads to the preferential sticking of the deposited particle toward the stable position in the same layer, which is the same result as an increase in ∆Eb. Figure 3b
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Figure 4. Plan view of the molecular arrangement of the PTCDA film. Also shown are the intermolecular interaction energies calculated for the diffusion of a deposited molecule along the x and y directions.
Figure 5. Two PTCDA molecules shown with (a) a layered configuration, which maximizes the intermolecular interactions, and (b) a tilt configuration with a tilt angle, τ.
shows the (1 + 1) dimensional surface growth for PF ) 0, that is, ∆Eb ) 0, while Figure 3c,d represents the surface morphologies for PF ) 2 and 4, respectively (∆Eb * 0). As PF increases, the increase in the mound number density and the enhanced tendency for upward growth can be clearly observed. It should be noted that, although the lateral size of an individual mound depends on the ease of lateral diffusion and deposition rate, the tendency for anomalous behavior with increasing PF is essentially independent of the maximum distance the impinging particle is allowed to travel. The Rloc, β, β/Rloc, and 1/z values for these models are plotted as a function of PF in Figure 3e. The most interesting aspect is the fact that the 1/z value decreases significantly as PF increases before finally converging at zero for PF g 3. These results indicate that increases in ∆Eb lead to more pronounced upward growth of the films and hence an increase in the steepness of the crystallites, which consequently causes significant deviation from the conventional scaling law, 1/z = β/Rloc.
Potential energy calculations were performed in order to calculate the ∆Eb values for an impinging PTCDA molecule. Molecular structural parameters were obtained from density functional theory (DFT) methods, and the intermolecular interaction energies were calculated using a Lennard-Jones 96 vdW energy for all possible, nonbonded atom pairs. The molecular mechanics (MM) force field parameters used for the calculations have been described elsewhere.21 The results are presented in Figure 4. The energies of the impinging PTCDA molecule (black) were calculated as it diffuses laterally, along both the x and y directions, yielding ∆Eb values of 0.95 and 0.71 eV, respectively. These high step-edge energy barriers mean that the PTCDA molecules prefer to diffuse laterally within the same layer rather than move down to a lower layer, providing a clear explanation for the upward mound growth mechanism and anomalous scaling behavior. The ∆Eb values can be rationalized by the decrease in intermolecular interactions. PTCDA adopts a layered
Perylene-3,4,9,10-tetracarboxylic Dianhydride configuration with its molecular planes oriented parallel to the substrate, which maximizes the intermolecular interactions in a direction normal to the surface (Figure 5a). This configuration breaks at the step edges when the molecule moves down to the lower layer, leading to a significant decrease in the original intermolecular interactions and to large energy barriers. Figure 5b shows the energetically most stable molecular configuration obtained from the calculations at the step edge, which corresponds to a tilt angle of τ ) 25°. 4. Conclusions We have shown that the growth of PTCDA thin films is consistent with an upward mound growth mechanism. During diffusion, the high potential energy barriers at the step-edges of pre-existing PTCDA islands lead to anomalous scaling behavior and can be rationalized in terms of the intermolecular interactions and subsequent packing behavior within the film. This study highlights the importance of understanding the microscopic molecular properties in order to rationalize the macroscopic growth behavior and thin film morphology. References and Notes (1) Yang, F.; Shtein, M.; Forrest, S. R. Nat. Mater. 2005, 4, 37. (2) Mu, H.; Shen, H.; Klotzkin, D. Solid-State Electron. 2004, 48, 2085. (3) Schreiber, F. Phys. Status Solidi A 2004, 201, 1037. (4) Biscarini, F.; Samorı´, P.; Greco, O.; Zamboni, R. Phys. ReV. Lett. 1997, 78, 2389. (5) Du¨rr, A. C.; Schreiber, F.; Ritley, K. A.; Kruppa, V.; Krug, J.; Dosch, H.; Struth, B. Phys. ReV. Lett. 2003, 90, 016104.
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