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Temperature-dependent orientation study of the initial growth of pentacene on amorphous SiO2 by molecular dynamics simulations
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Temperature-dependent orientation study of the initial growth of pentacene on amorphous SiO2 by molecular dynamics simulations Yuanqi Zeng, Bo Tao, Jiankui Chen, Zhouping Yin
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S0022-0248(15)00498-4 http://dx.doi.org/10.1016/j.jcrysgro.2015.07.033 CRYS22943
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Journal of Crystal Growth
Received date: 27 January 2015 Revised date: 23 July 2015 Accepted date: 30 July 2015 Cite this article as: Yuanqi Zeng, Bo Tao, Jiankui Chen, Zhouping Yin, Temperature-dependent orientation study of the initial growth of pentacene on amorphous SiO2 by molecular dynamics simulations, Journal of Crystal Growth, http://dx.doi.org/10.1016/j.jcrysgro.2015.07.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Temperature-dependent orientation study of the initial growth of pentacene on amorphous SiO2 by molecular dynamics simulations Yuanqi Zeng, Bo Tao1, Jiankui Chen1 and Zhouping Yin State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, P. R. China
Abstract Temperature-dependent molecular orientations in the initial growth processes of pentacene on amorphous SiO2 surface with different substrate temperatures have been investigated using molecular dynamics simulations. As the substrate temperature ranges from 270K to 600K, there exists a transition behavior for pentacene cluster from the normal-oriented, ordered configuration to the lateral-oriented, disordered one as measured by the decreased average orientation angle and order parameter, showing the significant effect of the substrate temperature on the molecular orientation. The transition behavior is related to the strength relationship between molecule-molecule interactions and molecule-substrate interactions. During the optimal temperature range between 300K and 350K, the pentacene molecules tend to form the normal-oriented, well-ordered cluster driven by the dominant molecule-molecule interactions, which is affected by the substrate temperature in a greater degree than the molecule-substrate interactions. When the temperature is lower than 300K, the ordering of pentacene cluster becomes a little worse. A higher substrate temperature results in the lateral orientation with the weakening of the molecule-molecule interactions. Then the further intensification of molecular thermal motion gradually makes the molecules separate from the cluster or the substrate surface, resulting in the appearance of the undesirable separated configuration.
Keywords: A1. Computer simulation; A1. Substrate temperature; A3. Physical vapor deposition processes; B1. Organic compounds (pentacene); B3. Field effect transistors 1. Introduction Recently, organic thin film transistors (OTFTs) have been actively investigated for the microelectronic devices due to
1
Corresponding author: [email protected], [email protected]. 1
their potential for low-cost, facile manufacturing [1-3]. In particular, pentacene (C22H14) is one of the most prominent conjugated organic materials used in OTFTs because of its remarkably high charge-carrier mobility, which is strongly affected by the molecular orientation of pentacene [2, 4-6]. The thin film of pentacene takes a normal orientation (the molecular long axis vertical to the substrate) or a lateral orientation (the molecular long axis parallel to the substrate). The former orientation with maximum π-π overlap is desired for pentacene OTFTs to facilitate an isotropic electric transport from source to drain parallel to the gate electrode, as depicted in Figure 1 [7-8]. In particular, several factors can influence the molecular orientation, such as molecular number [9-11], substrate material [2], and substrate temperature [12-13]. We have theoretically investigated the molecular orientation of pentacene on amorphous SiO2 (a-SiO2) substrate [14]. It is found that the initially grown lateral-oriented pentacene cluster will transfer to the normal-oriented one at a critical molecular number and the existence of the sylanol groups on the a-SiO2 surface will increase the critical molecular number. Hence, controlling and optimizing the molecular orientation is essential to achieve high device performance for OTFT devices. The molecular orientation of pentacene thin film is known to change with the substrate temperature during the deposition process [2, 15]. A considerable amount of work on the substrate temperature has been carried out recently. Dimitrakopoulos and co-workers investigated the structure order and field-effect mobility of pentacene thin film with different substrate temperatures using X-ray diffraction [16-17]. They demonstrated that a very well-ordered film is deposited when the substrate temperature is held at room temperature, higher substrate temperature induces the lower mobility resulting from the coexistence of two phases, and lower substrate temperature results in an amorphous film. Street et al. concluded that enhanced substrate temperature will lead to an increase of the crystal size of pentacene and a further increase (> 363K) leads to no growth of pentacene on the substrate [18]. Bouchoms et al. reported that the pentacene bulk phase sets in after a critical thickness of the thin film phase, which is strongly dependent on the substrate temperature, and the XRD results they obtained show that the thin film phase is a substrate induced phase and the critical film thickness will decrease with increasing substrate temperature [19]. Their experiments indicate that the order and orientation of pentacene film will change with the substrate temperature during the deposition process, so there should set a reasonable temperature range to help the formation of well-ordered film with desired orientation. However, this issue has not yet been fully elucidated. On the other hand, many computational calculations have demonstrated that molecular dynamics (MD) simulation is a well-known tool to investigate molecular behavior and it can provide valuable microscopic information to understand the mechanism of the growth of thin film [20]. Fichthorn and co-workers used MD simulations to study the melting of pentane, whose results of the temperature-dependent structures and the melting temperatures agree well with 2
experiment [21]. Besides, Yoneya et al. have found that the low-temperature polymorph of pentacene thin film is unstable, and tends to transform into the high-temperature polymorph by MD simulations [22]. Mavrantzas used MD simulations to study the dependence of configurational properties of regioregular poly-3-hexylthiophene (P3HT) on temperature. They found that when the temperature was lowered below approximately 300 K, the disordered state of P3HT was found to exhibit a transition from a pure amorphous phase to a semicrystalline one [23]. These results, both experimental and computational, state briefly that the substrate temperature plays such an important role in the formation of films. Given that, the objective of this work is to study how the substrate temperature influences the molecular orientation and determine an optimal temperature range for the growth by means of MD simulations. In fact, limited by the slab size and time of the simulations, there is indeed a gap between the simulated sparse cluster and the practical condensed thin-film. Even so, as a well-known important research approach, molecular simulation can provide valuable insight into the molecular-scale mechanism with little hindrance, which experiment cannot emulate. It makes that the molecular simulation is often used to explain experimentally observed phenomena and predict the outcome unexplored by experiment. Besides, it’s noticeable that the initial growth plays a decisive role in the subsequent growth, even determines the quality of the thin film. However, due to the limitations of space and time scales of experimental observation, it’s difficult to explore the initial growth in directly. Here, our aim is attempt to qualitatively analysis the influence of substrate temperature on the initial growth process from the microscopic view by proposing the molecular simulations. In order to investigate it, we study the molecular orientation and order of pentacene cluster on a-SiO2 substrate using MD simulations. A wide range of temperatures from 270 K to 600 K are set. Within the substrate temperature range, we find that there exists a transition for pentacene molecules from the normal-oriented, ordered phase to the lateral-oriented, disordered one during the deposition process and an optimal temperature range is obtained, which agrees well with the experimental study [24]. A comparison of the interactions between molecule-substrate and molecule-molecule is also made to elucidate the temperature-dependent orientation transition mechanism. Our results may enrich the information on the initial growth of pentacene thin film to fabricate well-ordered configuration with desired orientation. 2. Simulation methods and details The a-SiO2 substrate is prepared as an area of 64.2 × 64.2 Å2 with a thickness of approximately 15.6 Å, and a vacuum of 40 Å is enlarged in the z-direction of the simulation box where the pentacene molecules can move. In practice, it is well-known that the a-SiO2 substrate obtained by thermal oxidation of Si will be covered by sylanol groups (Si-O-H). To 3
reflect the situation more realistically, all top surface Si atoms of the a-SiO2 substrate are terminated with -OH groups. As we describe above, there exists a gap between the simulated sparse clusters and the practical condensed thin film. With regard to this and in order to elucidate the influence of the substrate temperature on the order of pentacene thin film more credible, two independent model systems of pentacene on a-SiO2 are constructed for comparison. The first model contains 24 pentacene molecules on a-SiO2 and the other one with 30 pentacene molecules, corresponding to the coverage from 0.7 to 0.9 ML (monolayer, is given in units of completed monolayer of lying pentacene, 1 ML corresponds to 34 molecules). We expect that it can prove that the influence of the substrate temperature is no coincidence through comparatively analyzing the influence of the substrate temperature on these two independent systems. The pentacene molecules depositing on sylanol-saturated a-SiO2 substrate prefer to form normal orientation when the cluster size is larger than 22 [14], so the initial pentacene configurations of the two systems are constructed in the normal orientation, as shown in Figure 2. The time of each simulation is 5 ns to ensure that the pentacene molecules have sufficient time to achieve the equilibrium shape and the data production time is 200 ps. All of the MD simulations were performed using the DISCOVER code [25], and both compounds are described using the COMPASS force field [26]. Details in calculations method were described in our previous work [14]. 3. Results To gain insight into the influence mechanism, we calculated two different quantities to characterize the orientation and order of pentacene molecules: the average orientation angle ( θ ) and the order parameter ( OPϕ ). Besides, given that the interplay between molecule-substrate interaction ( Em − s ) and molecule-molecule interaction ( Em − m ) is the key parameter in the self-assembly process of the nucleation and thin film growth [3], these interaction energies are used to describe the simulation results as well, which are defined as:
OPϕ =
Em − s (Tsub ) =
1 3cos 2 ϕ − 1 2
(1)
1 ( Etotal − Esub − E pen ) N
(2)
N 1 ( E pen − ∑ Em ) N 1
(3)
Em − m (Tsub ) =
Here θ is the angle between the long axis of individual molecule and the substrate surface, ϕ is the angle between the long axes of every two molecules, and the brackets operator denote an ensemble average over the data production time. The average orientation angle characters the orientation of the pentacene cluster and the order parameter quantifies the tendency 4
of the pentacene molecules axes to be aligned in one direction to evaluate the orderliness of the pentacene cluster. For the normal orientation, the average orientation angle θ is close to 90° and for the well-ordered film, the order parameter is close to 1. Besides, Etotal is the total energy of the substrate and the N pentacene molecules, Esub is the energy of the substrate without the pentacene molecules, E pen is the energy of the N pentacene molecules without the substrate, Em is the energy of each single pentacene molecule, and Tsub denotes the substrate temperature. In the case of the initial configurations shown in Figure 2, after energy minimization using smart minimizer method, the average orientation angles of the initial configurations for the two systems are 22° ( N = 24) and 46° ( N = 30), and the order parameters are 0.84 and 0.86, respectively. To afford a first glance on the simulation processes, the selected snapshots at the end of simulations of the pentacene cluster on the a-SiO2 surface at various substrate temperatures are displayed in Figure 3 and Figure 4 for the systems containing 24 and 30 pentacene molecules, respectively. When the substrate temperature ranges from 270K to 350K, the pentacene molecules keep aggregating into clusters on the a-SiO2 surface with an approximate perpendicular configuration. With the increase of the substrate temperature, however, the pentacene molecules exhibit the tendency to lie down on the substrate surface. Especially, when the substrate temperatures increase to 480K and 500K for the systems respectively, the pentacene molecules lie down entirely on the a-SiO2 surface with their molecular long axes almost parallel to the substrate. Even more serious, the separation phenomena gradually appear after further raising the substrate temperatures up to around 510K for both the two systems. It is clear to see that the trend of configuration transition from the normal-oriented, ordered to lateral-oriented, disordered is shown, and as the temperature increases, the separation phenomena become more obvious. When the substrate temperatures come to 600K, the molecules are in a decentralized state totally, even evaporate from the substrate. This can be attributed to a drop in sticking coefficient at elevated substrate temperatures [27]. We show the transition more intuitively in Figure 5a with how the average orientation angles θ vary as a function of substrate temperature. The clusters with 24 molecules and 30 molecules are denoted by subscripts 24 and 30. Despite the noise in the data, it is easy to see that the entire processes can be divided into three stages for the two models: I) for 270K <
Tsub < 350K, θ 24 increases steadily from 25° to 35°, meanwhile θ 30 keeps around 55° and there is a peak for θ 30 (69°) when Tsub comes to 320K; II) for 350K < Tsub < 510K, θ begin to decrease by large scale with large fluctuations until θ reach their lowest points (≈7°) at 480K and 500K, respectively, corresponding to the formation of the lateral orientation; and III) for Tsub > 510K, θ recover slightly to about 15° with little fluctuations, implying the steady state of the lateral 5
orientation. The corresponding results also appear in the order parameter distributions presented in Figure 5b: during the substrate temperatures ranging from 270K to 350K, OPϕ (24) keeps at around 0.6 after a steep rise at 300K and OPϕ (30) remains around 0.7 with the highest point (0.84) at 320K, indicating that the molecules form the well-ordered clusters with normal orientation for both the two systems. Nevertheless, beyond this range OPϕ fall to about 0.2 and 0.3 rapidly, suggesting that the molecules tend to become an amorphous configuration. Interestingly, when the substrate temperatures of the systems increase to 450K and 480K respectively, OPϕ begin to increase promptly. It can be attributed to the gradual formation of the lateral-oriented configurations with rough order. The tendency continues to 480K and 500K, where the lateral orientations with a good order are fully formed and OPϕ are distributed at about 0.85 and 0.67, respectively. After that, the amorphous configurations present once again with steep drops at 490K and 510K. Then OPϕ remain at 0.18 with little fluctuations in the next elevated temperatures and the corresponding structures are disordered and unsystematic. Based on the above simulations, it is clear that the initially normal-oriented, ordered cluster will gradually transfer to the lateral-oriented, disordered one with the increase of substrate temperature. The transition can be explained using the calculated results of interaction energies, as depicted in Figure 6, which can be divided into three stages as well. For both the two systems it is easy to see that in the first stage Em − s and Em − m remain relatively steady, and the former is far less than the latter, which contributes to the formation of normal-oriented ordered stable cluster. In the second stage Em − m begins to decrease by large scale and Em − s slowly increases gradually, resulting in the metastable of the normal orientation. Especially, when Tsub comes to 480K for the system containing 24 pentacene molecules, Em − s finally surpasses Em − m and for the system containing 30 pentacene molecules the critical temperature is 510K, in which case the pentacene molecules prefer to combine with the a-SiO2 substrate but not with themselves. After that Em − s is always larger than
Em − m and the tendency becomes more apparent at the elevated substrate temperature, suggesting that the lateral orientation is completely dominant. Even so, the molecules still evaporate from the weakly interacting substrate due to thermal desorption [3]. Throughout the whole rise process of the substrate temperature of the system containing 24 pentacene,
Em − s (600) is just 0.05 eV larger than Em − s (270) , while Em − m (600) is almost 0.5 eV less than Em − m (270) , where the difference of the latter is fully 10 times larger than that of the former. For the other system, Em − s (600) is 0.2 eV larger than
Em − s (270) , while and Em − m (600) is 0.52 eV less than Em − m (270) . From the comparison, it is easy to observe that Em − s are less affected by temperature changes, while the influence of temperature on Em − m is more significant. Eventually, this 6
transition of the strength relationship between the two interactions leads directly the transition of the molecular orientation. In addition to the molecular orientation transition, the separation phenomena during the simulation process are also worthy of our close attention. To characterize the separation phenomena in detail, we analyze the temperature dependence of the average molecule-molecule distance between the mass centers of the pentacene molecules ( d m − m ) and the potential energy per pentacene molecule ( E p ). In Figure 7a, it is obvious to see that both d m − m remain nearly unchanged below 400K, indicating the aggregation of the pentacene molecules. There exists a steep rise with large fluctuations when the substrate temperature exceeds 410K associated with the separation phenomena in this temperature range. Figure 7b shows the potential energy per molecule as a function of temperature. E p is the potential energy per pentacene molecule, not only including the intramolecular interaction energy, but also the intermolecular interaction energy. At temperature of 400K and below, E p increase steadily. When the temperatures rise up to 420K and 480K for these two systems respectively, the irregular fluctuations appear. All these phenomena indicate that the pentacene molecules are entering a phase evolution process, not only from ordered to disordered, but also from aggregation to separation. It’s well-known that the intermolecular interactions between molecules contribute to the aggregation, whereas the molecular thermal motion is the source of separation. Thus, in order to explain the process more clearly, we present the comparison between the kinetic energy ( Ek ) and the intermolecular interaction energy ( Ei ) per pentacene molecule, which are defined as: Ek =
3n kBT 2
(4) (5)
Ei = Em − m + Em − s
Here 3n is the number of degrees of freedom in the system with n atoms, k B is the Boltzmann constant and T is the thermodynamic temperature. Given that there are two kinds of separations: the in-plane (the molecule separates from the cluster, but still adsorbs on the substrate surface) and the out-of-plane (the molecule separate from the substrate surface), we resolve Ek into two components, Ek (XY) and Ek (Z) . Ei is the intermolecular interaction energy per pentacene molecule, not only including the interactions between pentacene molecules ( Em − m ), but also the interactions between pentacene molecule and the surface molecule ( Em − s ). In order to compare the numerical size between Ei and Ek , and present the comparison more intuitively, the absolute values of both Ei and Ek , namely | Ei | and | Ek |, are used to compare with each other. Considering that the calculate values of Em − m and Em − s are all negative, it is reasonable to mathematically convert
7
| Ei | as | Ei | = | Em − m + Em − s | = | Em − m | + | Em − s |, which would not change the numerical size of Ei . The calculated results are plotted vs. the substrate temperature in Figure 8. Obviously, upon increasing the substrate temperature, there exists a strength transition between Ek and Ei for both the in-plane separation and out-of-plane separation. When the substrate temperature is 360K and below, Ek (XY) is always less than Ei , indicating that the pentacene molecules prefer to aggregate together on the substrate surface. After this Ek (XY) is always larger than Ei , which implies that it is probable for the pentacene molecules to separate from the cluster. It turns out again that the appropriate growth temperature for pentacene should be less than 360K. However, it is worth noting that in the temperature range between 360K and 390K, the different between Ek (XY) and Ei is very small and d m − m still remains unchanged. These mean that during this range, the intermolecular interaction energy still plays the leading role. Combining with the above analysis, when the temperature exceeds 400K where the d m − m begins to increase, it can be found that the kinetic energy takes over the leading role, and the pentacene molecules gradually spread apart, resulting in the rough normal configuration with low degree order parameter. After the temperatures reach 480K and 500K for these two systems where Em − s finally surpasses Em − m , the lateral orientation becomes the dominant configuration. During this process the molecules still adsorb on the substrate surface all along, which can be attributed to that Ek (Z) is less than Ei in this temperature range. At temperature of 530K and above, Ek (Z) is larger than Ei . By this time, the pentacene molecules have enough energy to separate from the substrate surface, just as presented in the simulations. Considering that the melt temperature of bulk pentacene is around 540K [28], there is reason to consider that a phase transition may occurs during the out-of-plane separation, which makes the pentacene molecules float above the substrate surface dispersedly. With these above analysis in mind, we may reasonably arrive at the conclusion that during the low temperature stage the intermolecular interaction energies play the decisive role to keep the molecules aggregating together to form stable cluster, where the stronger molecule-molecule interactions result in the normal-oriented clusters and the dominated molecule-substrate interactions conduce to the formation of lateral-oriented clusters. With the increase of temperature, the kinetic energy takes over the leading role gradually and the further intensification of molecular thermal motion leads to the evolution process of pentacene clusters from ordered, aggregated to disordered, separated.
4. Discussion All of these results show the significant effect of substrate temperature on the orientation of pentacene molecules. It 8
can be clearly concluded that as the substrate temperature goes up, the temperature-dependent orientation presents a transition from the normal-oriented, ordered phase to the lateral-oriented, disordered one for pentacene molecules on the sylanol-saturated a-SiO2 substrate. Besides, the optimal substrate temperature range can be determined between 300K and 350K, where pentacene molecules tend to form normal-oriented, well-ordered cluster. When the temperature is lower than 300K, the average orientation angle and order parameter become worse. Above this range, the normal-oriented cluster becomes metastable and an amorphous configuration presents gradually, while the disordered lateral orientation dominates completely with the further increase of the substrate temperature. The other theoretical confirmations of our results can be found in the recently literature: Yun and co-workers investigated the molecular structure of pentacene grown on PEDOT:PSS using in situ ultraviolet photoemission spectroscopy (UPS) [29]. The results indicated that the pentacene layer grown at room temperature shows a highly crystalline thin film phase with typical terrace-like structure, but that grown at 373K consists of a weakly oriented thin film/bulk mixed phase with a disordered structure. The similar re-orientation phenomena are observed by Dimitrakopoulos as well [16-17]. Therefore, the observed temperature-dependent molecular reorientation is a common growth mode and these results bring us to the conclusion that the temperature-dependent transition is an issue that cannot be ignored and an optimal substrate temperature range is necessary for the achievement of a well-ordered desired molecular configuration. Based on the kinetic models for the initial stage of anisotropic crystal growth, Kubono et al. deliberated on the re-orientation phenomena of long-chain molecules at various substrate temperatures. They found that there is a threshold substrate temperature for the orientation change and above 290K only normal-oriented clusters are formed [30]. Our results are agreement with their calculations, what’s more, we further point out beyond 290K there being another threshold substrate temperature for the re-orientation phenomena. The optimal substrate temperature range we obtained for the pentacene on a-SiO2 substrate agrees with that found by Shimada [24], who used reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) to reveal the epitaxial growth of pentacene on hydrogen-terminated Si(111). They considered that the best substrate temperature range is 338K-353K, while above this range the sticking coefficient of pentacene will decrease. The significant desorption at elevated substrate temperatures will drive the film growth mode toward the island growth mode [31], which may induce the dispersion phenomena observed in our simulations. Given that pentacene molecules are bound together more weakly than the atoms in inorganic crystals, Northrup considered that typical growth temperature for pentacene should be less than 373K [32], which further confirms the reliability of our result. During the reasonable substrate temperature range, pentacene molecules will tend to form ordered thin-film with good performance. 9
Utilizing low-energy electron microscopy (LEEM) to investigate the film growth of pentacene on SiO2, Al-Mahboob found that for the substrate temperatures from room temperature to about 353K, pentacene was observed to grow in a layer-by-layer mode [3]. And there was little variation observed in the carrier field-effect mobility for pentacene OTFTs fabricated on SiO2 film at different temperatures from 298K to 353K [33], implying that the structure of pentacene is stable in this range. Besides, we consider that the dependence of orientation transition on substrate temperature can be attributed to the greater effect of elevated substrate temperature on the molecule-molecule interactions, the weakening of which induces the formation of lateral orientation. In other words, these conclusions also indicate that under the same rate of deposition, on the same substrate, two different oriented pentacene thin film can be grown by properly controlling the substrate temperature: a normal orientation when low substrate temperature and a lateral orientation when higher substrate temperature. The recent works can confirm the inference. By controlling the coverage of pentacene on Cu(119), Annese demonstrates that highly ordered pentacene films can be obtained in a flat orientation for low coverages and in a bulk-like herringbone upright orientation for higher coverages by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning tunneling microscopy (STM) [34]. Djuric et al. shown that under identical growth conditions, two very different oriented films of pentacene on Cu(110) Surfaces can be grown [35]. These dissimilar growth behaviors are induced by subtle differences in the monolayer structures, which can be controlled by preparation procedures. Given this, we believe that the results of this study may enrich the information on the control of pentacene film growth to fabricate ordered configuration with desired orientation, and further work is needed to elucidate the details.
5. Conclusions In summary, the present work provides insight into the influence of substrate temperature on molecular orientation and order for the initial growth of pentacene molecules on sylanol-saturated a-SiO2 substrate using MD simulations with substrate temperature ranging from 270K to 600K. The results indicate that there exists a transition from the normal-oriented, ordered configuration to the lateral-oriented, disordered one and the optimal temperature range between 300K and 350K is determined, where pentacene molecules tend to form normal-oriented, well-ordered cluster driven by the dominant molecule-molecule interactions. Beyond this range, the lateral orientation becomes the dominant configuration because of the transition of the strength relationship between the molecule-substrate interactions and the molecule-molecule interactions, which is affected by the substrate temperature in a greater degree than the former. Further raising the substrate temperature, the further intensification of molecular thermal motion makes the molecules have enough kinetic energy to 10
overcome the intermolecular interaction energies to separate from the cluster or the substrate surface, resulting in the appearance of the undesirable separated configuration.
Acknowledgments We sincerely thank Dr. Jing Liu, Professor of School of Energy and Power Engineering of Huazhong University of Science and Technology, for providing the DISCOVER code. This work is supported by the National Science Foundation of China under Grant 91023033 and 51421062, the National Fundamental Research Program of China under Grant 2011CB013003.
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Materials Studio and Discover are products of Accelrys Inc. (see www.Accelrys.com).
[26] H. Sun, COMPASS: an ab initio force-field optimized for condensed-phase applications-overview with details on alkane and benzene compounds. J. Phys. Chem. B 102 (1998) 7338-7364. [27] D. Käfer, L. Ruppel and G. Witte, Growth of pentacene on clean and modified gold surfaces. Phys. Rev. B 75 (2007) 085309. [28] J. E. Goose, K. Wong, P. Clancy and M. O. Thompson, Direct melt processing of pentacene at temperatures above 1000 °C by pulsed laser irradiation. Appl. Phys. Lett. 93 (2008) 183306. [29] D. J. Yun, J. Chung, C. Jung, H. S. Han, J. Lee, B. Anass, S. Lee, Y. Kyung and S. H. Park, Pentacene orientation on source/drain electrodes and its effect on charge carrier transport at pentacene/electrode interface, investigated using in situ ultraviolet photoemission spectroscopy and device characteristics. J. Electrochem. Soc. 160 (2013) H436-H442. [30] A. Kubono and R. Akiyama, Orientational mechanism for long-chain organic molecules during physical vapor deposition. Mol. Cryst. Liq. Cryst. 378 (2002) 167-183. [31] A. C. Mayer, R. Ruiz, H. Zhou, R. L. Headrick, A. Kazimirov and G. G. Malliaras, Growth dynamics of pentacene thin films: Real-time synchrotron x-ray scattering study. Phys. Rev. B 73 (2006) 205307. 12
[32] J. Northrup, M. Tiago and S. Louie, Surface energetics and growth of pentacene. Phys. Rev. B 66 (2002). [33] C. Kim, A. Facchetti and T. J. Marks, Polymer gate dielectric surface viscoelasticity modulates pentacene transistor performance. Science 318 (2007) 76-80. [34] E. Annese, I. Vobornik, G. Rossi and J. Fujii, Pentacene films on Cu(119). Langmuir 26 (2010) 19142-7. [35] T. Djuric, T. Ules, H. G. Flesch, H. Plank, Q. Shen, C. Teichert, R. Resel and M. G. Ramsey, Epitaxially grown films of standing and lying pentacene molecules on Cu(110) surfaces. Cryst. Growth Des. 11 (2011) 1015-1020.
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Figure captions
Figure 1. In pentacene OTFT, pentacene molecules should be oriented in a normal orientation to facilitate an isotropic electric transport from source to drain parallel to the gate electrode.
14
Figure 2. The initial configurations of the pentacene molecules on the sylanol-saturated a-SiO2 substrate after energy minimization using smart minimizer method.
15
Figure 3. Selected snapshots from simulations of 24 pentacene molecules deposited on the a-SiO2 substrate at various substrate temperatures: (a) Tsub =270 K; (b) Tsub =280 K; (c) Tsub =300 K; (d) Tsub =350 K; (e) Tsub =390 K; (f) Tsub =480 K; (g) Tsub =510 K; (h) Tsub =600 K;
16
Figure 4. Selected snapshots from simulations of 30 pentacene molecules deposited on the a-SiO2 substrate at various substrate temperatures: (a) Tsub =270 K; (b) Tsub =280 K; (c) Tsub =300 K; (d) Tsub =320 K; (e) Tsub =480 K; (f) Tsub =500 K; (g) Tsub =510 K; (h) Tsub =600 K;
17
Figure 5. The average orientation angle θ between the molecular long axis and the substrate surface (a), and the order parameter OPϕ as a function of the substrate temperature for pentacene molecules on the a-SiO2 substrate.
18
Figure 6. The interaction energies per molecule of the molecule-molecule (M-M) and molecule-substrate (M-S) as a function of the substrate temperature for pentacene molecules on the a-SiO2 substrate.
19
Figure 7. The average molecule-molecule distance d m − m (a), and the potential energy per pentacene molecule E p (b) as a function of the substrate temperature for pentacene molecules on the a-SiO2 substrate.
20
Figure 8. The kinetic energy ( Ek ) and the interaction energy ( Ei ) per pentacene molecule as a function of the substrate temperature for pentacene molecules on the a-SiO2 substrate. Ek (XY) denotes the component of Ek in XY plane and
Ek (Z) is the component of Ek in Z direction.
21
Highlights
Substrate temperature change induces orientation transition of pentacene on a-SiO2.
Lower substrate temperature results in normal-oriented, ordered clusters.
Higher substrate temperature results in lateral-oriented, disordered clusters.
The optimal substrate temperature range is determined between 300K and 350K.
Molecule-molecule interaction is more affected by the substrate temperature.
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Temperature-dependent orientation study of the initial growth of pentacene on amorphous SiO2 by molecular dynamics simulations Yuanqi Zeng, Bo Tao, Jiankui Chen, Zhouping Yin
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PII: DOI: Reference:
S0022-0248(15)00498-4 http://dx.doi.org/10.1016/j.jcrysgro.2015.07.033 CRYS22943
To appear in:
Journal of Crystal Growth
Received date: 27 January 2015 Revised date: 23 July 2015 Accepted date: 30 July 2015 Cite this article as: Yuanqi Zeng, Bo Tao, Jiankui Chen, Zhouping Yin, Temperature-dependent orientation study of the initial growth of pentacene on amorphous SiO2 by molecular dynamics simulations, Journal of Crystal Growth, http://dx.doi.org/10.1016/j.jcrysgro.2015.07.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Temperature-dependent orientation study of the initial growth of pentacene on amorphous SiO2 by molecular dynamics simulations Yuanqi Zeng, Bo Tao1, Jiankui Chen1 and Zhouping Yin State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, P. R. China
Abstract Temperature-dependent molecular orientations in the initial growth processes of pentacene on amorphous SiO2 surface with different substrate temperatures have been investigated using molecular dynamics simulations. As the substrate temperature ranges from 270K to 600K, there exists a transition behavior for pentacene cluster from the normal-oriented, ordered configuration to the lateral-oriented, disordered one as measured by the decreased average orientation angle and order parameter, showing the significant effect of the substrate temperature on the molecular orientation. The transition behavior is related to the strength relationship between molecule-molecule interactions and molecule-substrate interactions. During the optimal temperature range between 300K and 350K, the pentacene molecules tend to form the normal-oriented, well-ordered cluster driven by the dominant molecule-molecule interactions, which is affected by the substrate temperature in a greater degree than the molecule-substrate interactions. When the temperature is lower than 300K, the ordering of pentacene cluster becomes a little worse. A higher substrate temperature results in the lateral orientation with the weakening of the molecule-molecule interactions. Then the further intensification of molecular thermal motion gradually makes the molecules separate from the cluster or the substrate surface, resulting in the appearance of the undesirable separated configuration.
Keywords: A1. Computer simulation; A1. Substrate temperature; A3. Physical vapor deposition processes; B1. Organic compounds (pentacene); B3. Field effect transistors 1. Introduction Recently, organic thin film transistors (OTFTs) have been actively investigated for the microelectronic devices due to
1
Corresponding author: [email protected], [email protected]. 1
their potential for low-cost, facile manufacturing [1-3]. In particular, pentacene (C22H14) is one of the most prominent conjugated organic materials used in OTFTs because of its remarkably high charge-carrier mobility, which is strongly affected by the molecular orientation of pentacene [2, 4-6]. The thin film of pentacene takes a normal orientation (the molecular long axis vertical to the substrate) or a lateral orientation (the molecular long axis parallel to the substrate). The former orientation with maximum π-π overlap is desired for pentacene OTFTs to facilitate an isotropic electric transport from source to drain parallel to the gate electrode, as depicted in Figure 1 [7-8]. In particular, several factors can influence the molecular orientation, such as molecular number [9-11], substrate material [2], and substrate temperature [12-13]. We have theoretically investigated the molecular orientation of pentacene on amorphous SiO2 (a-SiO2) substrate [14]. It is found that the initially grown lateral-oriented pentacene cluster will transfer to the normal-oriented one at a critical molecular number and the existence of the sylanol groups on the a-SiO2 surface will increase the critical molecular number. Hence, controlling and optimizing the molecular orientation is essential to achieve high device performance for OTFT devices. The molecular orientation of pentacene thin film is known to change with the substrate temperature during the deposition process [2, 15]. A considerable amount of work on the substrate temperature has been carried out recently. Dimitrakopoulos and co-workers investigated the structure order and field-effect mobility of pentacene thin film with different substrate temperatures using X-ray diffraction [16-17]. They demonstrated that a very well-ordered film is deposited when the substrate temperature is held at room temperature, higher substrate temperature induces the lower mobility resulting from the coexistence of two phases, and lower substrate temperature results in an amorphous film. Street et al. concluded that enhanced substrate temperature will lead to an increase of the crystal size of pentacene and a further increase (> 363K) leads to no growth of pentacene on the substrate [18]. Bouchoms et al. reported that the pentacene bulk phase sets in after a critical thickness of the thin film phase, which is strongly dependent on the substrate temperature, and the XRD results they obtained show that the thin film phase is a substrate induced phase and the critical film thickness will decrease with increasing substrate temperature [19]. Their experiments indicate that the order and orientation of pentacene film will change with the substrate temperature during the deposition process, so there should set a reasonable temperature range to help the formation of well-ordered film with desired orientation. However, this issue has not yet been fully elucidated. On the other hand, many computational calculations have demonstrated that molecular dynamics (MD) simulation is a well-known tool to investigate molecular behavior and it can provide valuable microscopic information to understand the mechanism of the growth of thin film [20]. Fichthorn and co-workers used MD simulations to study the melting of pentane, whose results of the temperature-dependent structures and the melting temperatures agree well with 2
experiment [21]. Besides, Yoneya et al. have found that the low-temperature polymorph of pentacene thin film is unstable, and tends to transform into the high-temperature polymorph by MD simulations [22]. Mavrantzas used MD simulations to study the dependence of configurational properties of regioregular poly-3-hexylthiophene (P3HT) on temperature. They found that when the temperature was lowered below approximately 300 K, the disordered state of P3HT was found to exhibit a transition from a pure amorphous phase to a semicrystalline one [23]. These results, both experimental and computational, state briefly that the substrate temperature plays such an important role in the formation of films. Given that, the objective of this work is to study how the substrate temperature influences the molecular orientation and determine an optimal temperature range for the growth by means of MD simulations. In fact, limited by the slab size and time of the simulations, there is indeed a gap between the simulated sparse cluster and the practical condensed thin-film. Even so, as a well-known important research approach, molecular simulation can provide valuable insight into the molecular-scale mechanism with little hindrance, which experiment cannot emulate. It makes that the molecular simulation is often used to explain experimentally observed phenomena and predict the outcome unexplored by experiment. Besides, it’s noticeable that the initial growth plays a decisive role in the subsequent growth, even determines the quality of the thin film. However, due to the limitations of space and time scales of experimental observation, it’s difficult to explore the initial growth in directly. Here, our aim is attempt to qualitatively analysis the influence of substrate temperature on the initial growth process from the microscopic view by proposing the molecular simulations. In order to investigate it, we study the molecular orientation and order of pentacene cluster on a-SiO2 substrate using MD simulations. A wide range of temperatures from 270 K to 600 K are set. Within the substrate temperature range, we find that there exists a transition for pentacene molecules from the normal-oriented, ordered phase to the lateral-oriented, disordered one during the deposition process and an optimal temperature range is obtained, which agrees well with the experimental study [24]. A comparison of the interactions between molecule-substrate and molecule-molecule is also made to elucidate the temperature-dependent orientation transition mechanism. Our results may enrich the information on the initial growth of pentacene thin film to fabricate well-ordered configuration with desired orientation. 2. Simulation methods and details The a-SiO2 substrate is prepared as an area of 64.2 × 64.2 Å2 with a thickness of approximately 15.6 Å, and a vacuum of 40 Å is enlarged in the z-direction of the simulation box where the pentacene molecules can move. In practice, it is well-known that the a-SiO2 substrate obtained by thermal oxidation of Si will be covered by sylanol groups (Si-O-H). To 3
reflect the situation more realistically, all top surface Si atoms of the a-SiO2 substrate are terminated with -OH groups. As we describe above, there exists a gap between the simulated sparse clusters and the practical condensed thin film. With regard to this and in order to elucidate the influence of the substrate temperature on the order of pentacene thin film more credible, two independent model systems of pentacene on a-SiO2 are constructed for comparison. The first model contains 24 pentacene molecules on a-SiO2 and the other one with 30 pentacene molecules, corresponding to the coverage from 0.7 to 0.9 ML (monolayer, is given in units of completed monolayer of lying pentacene, 1 ML corresponds to 34 molecules). We expect that it can prove that the influence of the substrate temperature is no coincidence through comparatively analyzing the influence of the substrate temperature on these two independent systems. The pentacene molecules depositing on sylanol-saturated a-SiO2 substrate prefer to form normal orientation when the cluster size is larger than 22 [14], so the initial pentacene configurations of the two systems are constructed in the normal orientation, as shown in Figure 2. The time of each simulation is 5 ns to ensure that the pentacene molecules have sufficient time to achieve the equilibrium shape and the data production time is 200 ps. All of the MD simulations were performed using the DISCOVER code [25], and both compounds are described using the COMPASS force field [26]. Details in calculations method were described in our previous work [14]. 3. Results To gain insight into the influence mechanism, we calculated two different quantities to characterize the orientation and order of pentacene molecules: the average orientation angle ( θ ) and the order parameter ( OPϕ ). Besides, given that the interplay between molecule-substrate interaction ( Em − s ) and molecule-molecule interaction ( Em − m ) is the key parameter in the self-assembly process of the nucleation and thin film growth [3], these interaction energies are used to describe the simulation results as well, which are defined as:
OPϕ =
Em − s (Tsub ) =
1 3cos 2 ϕ − 1 2
(1)
1 ( Etotal − Esub − E pen ) N
(2)
N 1 ( E pen − ∑ Em ) N 1
(3)
Em − m (Tsub ) =
Here θ is the angle between the long axis of individual molecule and the substrate surface, ϕ is the angle between the long axes of every two molecules, and the brackets operator denote an ensemble average over the data production time. The average orientation angle characters the orientation of the pentacene cluster and the order parameter quantifies the tendency 4
of the pentacene molecules axes to be aligned in one direction to evaluate the orderliness of the pentacene cluster. For the normal orientation, the average orientation angle θ is close to 90° and for the well-ordered film, the order parameter is close to 1. Besides, Etotal is the total energy of the substrate and the N pentacene molecules, Esub is the energy of the substrate without the pentacene molecules, E pen is the energy of the N pentacene molecules without the substrate, Em is the energy of each single pentacene molecule, and Tsub denotes the substrate temperature. In the case of the initial configurations shown in Figure 2, after energy minimization using smart minimizer method, the average orientation angles of the initial configurations for the two systems are 22° ( N = 24) and 46° ( N = 30), and the order parameters are 0.84 and 0.86, respectively. To afford a first glance on the simulation processes, the selected snapshots at the end of simulations of the pentacene cluster on the a-SiO2 surface at various substrate temperatures are displayed in Figure 3 and Figure 4 for the systems containing 24 and 30 pentacene molecules, respectively. When the substrate temperature ranges from 270K to 350K, the pentacene molecules keep aggregating into clusters on the a-SiO2 surface with an approximate perpendicular configuration. With the increase of the substrate temperature, however, the pentacene molecules exhibit the tendency to lie down on the substrate surface. Especially, when the substrate temperatures increase to 480K and 500K for the systems respectively, the pentacene molecules lie down entirely on the a-SiO2 surface with their molecular long axes almost parallel to the substrate. Even more serious, the separation phenomena gradually appear after further raising the substrate temperatures up to around 510K for both the two systems. It is clear to see that the trend of configuration transition from the normal-oriented, ordered to lateral-oriented, disordered is shown, and as the temperature increases, the separation phenomena become more obvious. When the substrate temperatures come to 600K, the molecules are in a decentralized state totally, even evaporate from the substrate. This can be attributed to a drop in sticking coefficient at elevated substrate temperatures [27]. We show the transition more intuitively in Figure 5a with how the average orientation angles θ vary as a function of substrate temperature. The clusters with 24 molecules and 30 molecules are denoted by subscripts 24 and 30. Despite the noise in the data, it is easy to see that the entire processes can be divided into three stages for the two models: I) for 270K <
Tsub < 350K, θ 24 increases steadily from 25° to 35°, meanwhile θ 30 keeps around 55° and there is a peak for θ 30 (69°) when Tsub comes to 320K; II) for 350K < Tsub < 510K, θ begin to decrease by large scale with large fluctuations until θ reach their lowest points (≈7°) at 480K and 500K, respectively, corresponding to the formation of the lateral orientation; and III) for Tsub > 510K, θ recover slightly to about 15° with little fluctuations, implying the steady state of the lateral 5
orientation. The corresponding results also appear in the order parameter distributions presented in Figure 5b: during the substrate temperatures ranging from 270K to 350K, OPϕ (24) keeps at around 0.6 after a steep rise at 300K and OPϕ (30) remains around 0.7 with the highest point (0.84) at 320K, indicating that the molecules form the well-ordered clusters with normal orientation for both the two systems. Nevertheless, beyond this range OPϕ fall to about 0.2 and 0.3 rapidly, suggesting that the molecules tend to become an amorphous configuration. Interestingly, when the substrate temperatures of the systems increase to 450K and 480K respectively, OPϕ begin to increase promptly. It can be attributed to the gradual formation of the lateral-oriented configurations with rough order. The tendency continues to 480K and 500K, where the lateral orientations with a good order are fully formed and OPϕ are distributed at about 0.85 and 0.67, respectively. After that, the amorphous configurations present once again with steep drops at 490K and 510K. Then OPϕ remain at 0.18 with little fluctuations in the next elevated temperatures and the corresponding structures are disordered and unsystematic. Based on the above simulations, it is clear that the initially normal-oriented, ordered cluster will gradually transfer to the lateral-oriented, disordered one with the increase of substrate temperature. The transition can be explained using the calculated results of interaction energies, as depicted in Figure 6, which can be divided into three stages as well. For both the two systems it is easy to see that in the first stage Em − s and Em − m remain relatively steady, and the former is far less than the latter, which contributes to the formation of normal-oriented ordered stable cluster. In the second stage Em − m begins to decrease by large scale and Em − s slowly increases gradually, resulting in the metastable of the normal orientation. Especially, when Tsub comes to 480K for the system containing 24 pentacene molecules, Em − s finally surpasses Em − m and for the system containing 30 pentacene molecules the critical temperature is 510K, in which case the pentacene molecules prefer to combine with the a-SiO2 substrate but not with themselves. After that Em − s is always larger than
Em − m and the tendency becomes more apparent at the elevated substrate temperature, suggesting that the lateral orientation is completely dominant. Even so, the molecules still evaporate from the weakly interacting substrate due to thermal desorption [3]. Throughout the whole rise process of the substrate temperature of the system containing 24 pentacene,
Em − s (600) is just 0.05 eV larger than Em − s (270) , while Em − m (600) is almost 0.5 eV less than Em − m (270) , where the difference of the latter is fully 10 times larger than that of the former. For the other system, Em − s (600) is 0.2 eV larger than
Em − s (270) , while and Em − m (600) is 0.52 eV less than Em − m (270) . From the comparison, it is easy to observe that Em − s are less affected by temperature changes, while the influence of temperature on Em − m is more significant. Eventually, this 6
transition of the strength relationship between the two interactions leads directly the transition of the molecular orientation. In addition to the molecular orientation transition, the separation phenomena during the simulation process are also worthy of our close attention. To characterize the separation phenomena in detail, we analyze the temperature dependence of the average molecule-molecule distance between the mass centers of the pentacene molecules ( d m − m ) and the potential energy per pentacene molecule ( E p ). In Figure 7a, it is obvious to see that both d m − m remain nearly unchanged below 400K, indicating the aggregation of the pentacene molecules. There exists a steep rise with large fluctuations when the substrate temperature exceeds 410K associated with the separation phenomena in this temperature range. Figure 7b shows the potential energy per molecule as a function of temperature. E p is the potential energy per pentacene molecule, not only including the intramolecular interaction energy, but also the intermolecular interaction energy. At temperature of 400K and below, E p increase steadily. When the temperatures rise up to 420K and 480K for these two systems respectively, the irregular fluctuations appear. All these phenomena indicate that the pentacene molecules are entering a phase evolution process, not only from ordered to disordered, but also from aggregation to separation. It’s well-known that the intermolecular interactions between molecules contribute to the aggregation, whereas the molecular thermal motion is the source of separation. Thus, in order to explain the process more clearly, we present the comparison between the kinetic energy ( Ek ) and the intermolecular interaction energy ( Ei ) per pentacene molecule, which are defined as: Ek =
3n kBT 2
(4) (5)
Ei = Em − m + Em − s
Here 3n is the number of degrees of freedom in the system with n atoms, k B is the Boltzmann constant and T is the thermodynamic temperature. Given that there are two kinds of separations: the in-plane (the molecule separates from the cluster, but still adsorbs on the substrate surface) and the out-of-plane (the molecule separate from the substrate surface), we resolve Ek into two components, Ek (XY) and Ek (Z) . Ei is the intermolecular interaction energy per pentacene molecule, not only including the interactions between pentacene molecules ( Em − m ), but also the interactions between pentacene molecule and the surface molecule ( Em − s ). In order to compare the numerical size between Ei and Ek , and present the comparison more intuitively, the absolute values of both Ei and Ek , namely | Ei | and | Ek |, are used to compare with each other. Considering that the calculate values of Em − m and Em − s are all negative, it is reasonable to mathematically convert
7
| Ei | as | Ei | = | Em − m + Em − s | = | Em − m | + | Em − s |, which would not change the numerical size of Ei . The calculated results are plotted vs. the substrate temperature in Figure 8. Obviously, upon increasing the substrate temperature, there exists a strength transition between Ek and Ei for both the in-plane separation and out-of-plane separation. When the substrate temperature is 360K and below, Ek (XY) is always less than Ei , indicating that the pentacene molecules prefer to aggregate together on the substrate surface. After this Ek (XY) is always larger than Ei , which implies that it is probable for the pentacene molecules to separate from the cluster. It turns out again that the appropriate growth temperature for pentacene should be less than 360K. However, it is worth noting that in the temperature range between 360K and 390K, the different between Ek (XY) and Ei is very small and d m − m still remains unchanged. These mean that during this range, the intermolecular interaction energy still plays the leading role. Combining with the above analysis, when the temperature exceeds 400K where the d m − m begins to increase, it can be found that the kinetic energy takes over the leading role, and the pentacene molecules gradually spread apart, resulting in the rough normal configuration with low degree order parameter. After the temperatures reach 480K and 500K for these two systems where Em − s finally surpasses Em − m , the lateral orientation becomes the dominant configuration. During this process the molecules still adsorb on the substrate surface all along, which can be attributed to that Ek (Z) is less than Ei in this temperature range. At temperature of 530K and above, Ek (Z) is larger than Ei . By this time, the pentacene molecules have enough energy to separate from the substrate surface, just as presented in the simulations. Considering that the melt temperature of bulk pentacene is around 540K [28], there is reason to consider that a phase transition may occurs during the out-of-plane separation, which makes the pentacene molecules float above the substrate surface dispersedly. With these above analysis in mind, we may reasonably arrive at the conclusion that during the low temperature stage the intermolecular interaction energies play the decisive role to keep the molecules aggregating together to form stable cluster, where the stronger molecule-molecule interactions result in the normal-oriented clusters and the dominated molecule-substrate interactions conduce to the formation of lateral-oriented clusters. With the increase of temperature, the kinetic energy takes over the leading role gradually and the further intensification of molecular thermal motion leads to the evolution process of pentacene clusters from ordered, aggregated to disordered, separated.
4. Discussion All of these results show the significant effect of substrate temperature on the orientation of pentacene molecules. It 8
can be clearly concluded that as the substrate temperature goes up, the temperature-dependent orientation presents a transition from the normal-oriented, ordered phase to the lateral-oriented, disordered one for pentacene molecules on the sylanol-saturated a-SiO2 substrate. Besides, the optimal substrate temperature range can be determined between 300K and 350K, where pentacene molecules tend to form normal-oriented, well-ordered cluster. When the temperature is lower than 300K, the average orientation angle and order parameter become worse. Above this range, the normal-oriented cluster becomes metastable and an amorphous configuration presents gradually, while the disordered lateral orientation dominates completely with the further increase of the substrate temperature. The other theoretical confirmations of our results can be found in the recently literature: Yun and co-workers investigated the molecular structure of pentacene grown on PEDOT:PSS using in situ ultraviolet photoemission spectroscopy (UPS) [29]. The results indicated that the pentacene layer grown at room temperature shows a highly crystalline thin film phase with typical terrace-like structure, but that grown at 373K consists of a weakly oriented thin film/bulk mixed phase with a disordered structure. The similar re-orientation phenomena are observed by Dimitrakopoulos as well [16-17]. Therefore, the observed temperature-dependent molecular reorientation is a common growth mode and these results bring us to the conclusion that the temperature-dependent transition is an issue that cannot be ignored and an optimal substrate temperature range is necessary for the achievement of a well-ordered desired molecular configuration. Based on the kinetic models for the initial stage of anisotropic crystal growth, Kubono et al. deliberated on the re-orientation phenomena of long-chain molecules at various substrate temperatures. They found that there is a threshold substrate temperature for the orientation change and above 290K only normal-oriented clusters are formed [30]. Our results are agreement with their calculations, what’s more, we further point out beyond 290K there being another threshold substrate temperature for the re-orientation phenomena. The optimal substrate temperature range we obtained for the pentacene on a-SiO2 substrate agrees with that found by Shimada [24], who used reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) to reveal the epitaxial growth of pentacene on hydrogen-terminated Si(111). They considered that the best substrate temperature range is 338K-353K, while above this range the sticking coefficient of pentacene will decrease. The significant desorption at elevated substrate temperatures will drive the film growth mode toward the island growth mode [31], which may induce the dispersion phenomena observed in our simulations. Given that pentacene molecules are bound together more weakly than the atoms in inorganic crystals, Northrup considered that typical growth temperature for pentacene should be less than 373K [32], which further confirms the reliability of our result. During the reasonable substrate temperature range, pentacene molecules will tend to form ordered thin-film with good performance. 9
Utilizing low-energy electron microscopy (LEEM) to investigate the film growth of pentacene on SiO2, Al-Mahboob found that for the substrate temperatures from room temperature to about 353K, pentacene was observed to grow in a layer-by-layer mode [3]. And there was little variation observed in the carrier field-effect mobility for pentacene OTFTs fabricated on SiO2 film at different temperatures from 298K to 353K [33], implying that the structure of pentacene is stable in this range. Besides, we consider that the dependence of orientation transition on substrate temperature can be attributed to the greater effect of elevated substrate temperature on the molecule-molecule interactions, the weakening of which induces the formation of lateral orientation. In other words, these conclusions also indicate that under the same rate of deposition, on the same substrate, two different oriented pentacene thin film can be grown by properly controlling the substrate temperature: a normal orientation when low substrate temperature and a lateral orientation when higher substrate temperature. The recent works can confirm the inference. By controlling the coverage of pentacene on Cu(119), Annese demonstrates that highly ordered pentacene films can be obtained in a flat orientation for low coverages and in a bulk-like herringbone upright orientation for higher coverages by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning tunneling microscopy (STM) [34]. Djuric et al. shown that under identical growth conditions, two very different oriented films of pentacene on Cu(110) Surfaces can be grown [35]. These dissimilar growth behaviors are induced by subtle differences in the monolayer structures, which can be controlled by preparation procedures. Given this, we believe that the results of this study may enrich the information on the control of pentacene film growth to fabricate ordered configuration with desired orientation, and further work is needed to elucidate the details.
5. Conclusions In summary, the present work provides insight into the influence of substrate temperature on molecular orientation and order for the initial growth of pentacene molecules on sylanol-saturated a-SiO2 substrate using MD simulations with substrate temperature ranging from 270K to 600K. The results indicate that there exists a transition from the normal-oriented, ordered configuration to the lateral-oriented, disordered one and the optimal temperature range between 300K and 350K is determined, where pentacene molecules tend to form normal-oriented, well-ordered cluster driven by the dominant molecule-molecule interactions. Beyond this range, the lateral orientation becomes the dominant configuration because of the transition of the strength relationship between the molecule-substrate interactions and the molecule-molecule interactions, which is affected by the substrate temperature in a greater degree than the former. Further raising the substrate temperature, the further intensification of molecular thermal motion makes the molecules have enough kinetic energy to 10
overcome the intermolecular interaction energies to separate from the cluster or the substrate surface, resulting in the appearance of the undesirable separated configuration.
Acknowledgments We sincerely thank Dr. Jing Liu, Professor of School of Energy and Power Engineering of Huazhong University of Science and Technology, for providing the DISCOVER code. This work is supported by the National Science Foundation of China under Grant 91023033 and 51421062, the National Fundamental Research Program of China under Grant 2011CB013003.
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Figure captions
Figure 1. In pentacene OTFT, pentacene molecules should be oriented in a normal orientation to facilitate an isotropic electric transport from source to drain parallel to the gate electrode.
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Figure 2. The initial configurations of the pentacene molecules on the sylanol-saturated a-SiO2 substrate after energy minimization using smart minimizer method.
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Figure 3. Selected snapshots from simulations of 24 pentacene molecules deposited on the a-SiO2 substrate at various substrate temperatures: (a) Tsub =270 K; (b) Tsub =280 K; (c) Tsub =300 K; (d) Tsub =350 K; (e) Tsub =390 K; (f) Tsub =480 K; (g) Tsub =510 K; (h) Tsub =600 K;
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Figure 4. Selected snapshots from simulations of 30 pentacene molecules deposited on the a-SiO2 substrate at various substrate temperatures: (a) Tsub =270 K; (b) Tsub =280 K; (c) Tsub =300 K; (d) Tsub =320 K; (e) Tsub =480 K; (f) Tsub =500 K; (g) Tsub =510 K; (h) Tsub =600 K;
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Figure 5. The average orientation angle θ between the molecular long axis and the substrate surface (a), and the order parameter OPϕ as a function of the substrate temperature for pentacene molecules on the a-SiO2 substrate.
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Figure 6. The interaction energies per molecule of the molecule-molecule (M-M) and molecule-substrate (M-S) as a function of the substrate temperature for pentacene molecules on the a-SiO2 substrate.
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Figure 7. The average molecule-molecule distance d m − m (a), and the potential energy per pentacene molecule E p (b) as a function of the substrate temperature for pentacene molecules on the a-SiO2 substrate.
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Figure 8. The kinetic energy ( Ek ) and the interaction energy ( Ei ) per pentacene molecule as a function of the substrate temperature for pentacene molecules on the a-SiO2 substrate. Ek (XY) denotes the component of Ek in XY plane and
Ek (Z) is the component of Ek in Z direction.
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Highlights
Substrate temperature change induces orientation transition of pentacene on a-SiO2.
Lower substrate temperature results in normal-oriented, ordered clusters.
Higher substrate temperature results in lateral-oriented, disordered clusters.
The optimal substrate temperature range is determined between 300K and 350K.
Molecule-molecule interaction is more affected by the substrate temperature.
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