J. Phys. Chem. C 2007, 111, 12741-12746
12741
Homoepitaxial Growth of r-Hexathiophene Marcello Campione,* Silvia Caprioli, Massimo Moret, and Adele Sassella CNISM and Department of Materials Science, UniVersita` degli Studi di Milano Bicocca, Via R. Cozzi 53, 20125 Milano, Italy ReceiVed: December 15, 2006; In Final Form: June 29, 2007
The homoepitaxial growth by organic molecular beam epitaxy of the organic semiconductor R-hexathiophene has been described by scaling theories and classical kinetic tools. Atomic force microscopy has been employed for experimentally extracting kinetic parameters of the growth, in a range of substrate temperatures between 298 and 393 K. Within this range, the critical nucleus size of two-dimensional homoepitaxial islands has been observed to change from one to three molecules, and nucleation has been found to be an activated process with an energy barrier of 0.73 ( 0.18 eV (17 ( 4 kcal/mol). This energy barrier is consistent with a growth mechanism in which substantial material re-evaporation occurs.
1. Introduction R-Hexathiophene (R6T, C24H14S6) is definitely one of the most studied organic semiconductors. Thanks to its stability, high crystallinity in the solid state, and high conjugation of π electrons, this material shows good performance as an active layer in optoelectronic devices.1 Moreover, the relatively high molecular weight and conjugation of R6T, which ensure strong intermolecular π-π interactions, allows the two-dimensional (2D) nucleation and the subsequent layer-by-layer growth of the thin film phase on common inert substrates by deposition techniques in vacuum.2-5 The 2D nucleation of a semiconductor on a foreign crystalline substrate is the basic mechanism of the recent development of inorganic epitaxial heterostructures with applications in microand optoelectronics. In principle, the possibility to control the material dose at the submonolayer level with a molecular beam epitaxy apparatus permits formation of sharp interfaces among different materials, which is a necessary condition for the growth of, for example, quantum wells and superlattices. The coupling of different materials for the fabrication of heterostructures remains, in any case, the final step of the application of epitaxial growth of semiconductors. The recent literature provides only few examples of controlled layer-by-layer growth of organic semiconductors through molecular beam techniques, such as pentacene deposited on cyclohexene-saturated Si(001).6 In most cases, the deposition of an organic material on a foreign substrate leads to the nucleation of three-dimensional domains; in this context, a detailed study of the growth dynamics has been performed for para-hexaphenyl deposited on GaAs(001)-2 × 4.7 On the other hand, some attempts have been made for growing highly ordered heterostructures of organic semiconductors, coupling, for example, R-hexathiophene and para-hexaphenyl deposited on highly oriented inorganic substrates8,9 but achieving poor control over film morphology. In any case, the study of epitaxy finds its natural starting point in the homoepitaxy. The relevance of homoepitaxial growth resides in the possibility to ensure the control of crystallinity, purity, and surface quality of a semiconductor.10 In order to reproduce the * To whom correspondence should marcello.campione@ mater.unimib.it.
be
addressed.
E-mail:
same situation encountered in the field of inorganic semiconductors, the homoepitaxy of organic materials requires the use of oriented and high-quality single crystals as substrate for the ultrahigh vacuum deposition of the same material. We describe, in this work, the kinetic aspects of the homoepitaxial growth of the organic semiconductor R6T. In recent publications,11 our group demonstrated the possibility of growing homoepitaxial layers of R-quaterthiophene (R4T, C16H10S4) starting from a single crystal of the same material grown by vapor-phase techniques. Nonetheless, the growth kinetics of this system is complicated to a great extent because of the occurrence of desorption processes both during and after the deposition, involving even the bare substrate surface.11,12 On the contrary, R6T is a more stable and easily vacuumprocessable organic molecular material; it is used here as a source material in an organic molecular beam epitaxy (OMBE) apparatus, and it is homoepitaxially grown on the (100) surface of a single crystal of the low-temperature polymorph of R6T (known as R6T/LT13). The crystal structure of R6T/LT belongs to the monoclinic space group P21/n with unit cell parameters a ) 44.708 Å, b ) 7.851 Å, c ) 6.029 Å, β ) 90.76°, and four molecules per unit cell.13 The R6T/LT single crystals possess a layered herringbone structure developing in the (100) plane. The cleavage face of the crystal is parallel to the (100) face. Slices of d200 thickness enclose layers of close-packed molecules arranged in a herringbone motif, with their axes parallel to one another and tilted by 24.14° (axis connecting R and ω carbon atoms) to the layer plane (see sketched structures in Figure 1). Due to the extremely weak interlayer interactions, the flat (100) surface of the crystal has the lowest surface energy and the lowest attachment energy as well,14 and consequently, it is always the widest singular surface of the grown crystals. The growth kinetics of homoepitaxial R6T is experimentally analyzed here by a quantitative morphological study performed by atomic force microscopy (AFM) measurements on films deposited at different substrate temperatures. This analysis consists of the evaluation of the density (number of islands per unit area) of 2D aggregates and their size distribution as a function of the substrate temperature. It has permitted evaluation of the size of the smallest stable nucleus as a function of substrate temperature and the activation energy barrier for
10.1021/jp068616j CCC: $37.00 © 2007 American Chemical Society Published on Web 08/09/2007
12742 J. Phys. Chem. C, Vol. 111, No. 34, 2007
Campione et al. critical nucleus i and of the parameters Ei, Ea, and Ed, and its expression depends on the condensation regime (see Table 1): complete (DτaN . 1), initially incomplete (S < DτaN < 1), and extreme incomplete (DτaN , S). Following eq 1, an Arrhenius-like behavior of the island density as a function of substrate temperature, within temperature ranges in which the critical nucleus size is constant, is expected. The experimental results were also validated by a calculation based on atomatom potentials of the binding energy of the R6T molecule on the R6T/LT(100) surface and of the energy barrier for surface diffusion, enabling the identification of the regime of condensation. 2. Methods
Figure 1. The 5 × 5 µm2 AFM images of the surface of R6T submonolayer homoepitaxial films grown on R6T/LT(100) under the same conditions at (a) 298 K (25 °C), substrate coverage S ) 0.41 ( 0.02, (b) 333 K (60 °C), S ) 0.42 ( 0.02, (c) 363 K (90 °C), S ) 0.27 ( 0.02, and (d) 393 K (120 °C), S ) 0.69 ( 0.02. The orientation of the surface unit cell axes is reported in each image. The cross-sectional profile is taken along the white line in image a. A structural model (view along the b-axis) of a R6T/LT island (blue C atoms) nucleated on top of the (100) surface of the R6T/LT substrate (gray C atoms) is sketched between panels b and d.
nucleation. Indeed, the island density on a defect-free substrate surface depends on the competition between the deposition of molecules, their migration on the substrate surface, their aggregation, and their re-evaporation from the substrate surface. These events can be quantitatively characterized by the deposition rate F (cm-2 s-1), the diffusion coefficient D (cm2 s-1) ) D0 exp(-Ed/kBTs), where D0 is the diffusivity and Ed is the activation energy for surface diffusion of a single molecule, the re-evaporation time τa ) ν-1 exp(Ea/kBTs), where ν is the attempt frequency and Ea is the binding energy of a molecule on the substrate surface, and the binding energy of an aggregate of j molecules Ej. Rate equation models developed for atomic systems15 suggest the total island density N (at fixed substrate temperature Ts and coverage S) scales with F and D in accordance with the relation
N(Ts,S) ∝
( ) F D0
γ
exp(Eact /kBTs)
(1)
where γ is a coefficient depending on the aggregation mechanism of molecules and Eact is the activation energy for nucleation. The parameter Eact is a function of the size of the
The R6T thin films were grown by means of OMBE,16 starting from commercially available R6T microcrystalline powder in a Knudsen effusion cell at 563 K and keeping the substrate temperature Ts in the range of 298-393 K, with an uncertainty of ( 1 K. This temperature range has been chosen on the basis of the results obtained on R6T thin-film transistors, which exhibit best performances when deposition is carried out at 393 K.4 The residual pressure in the growth chamber was below 5 × 10-8 Pa during deposition. The film nominal thickness was monitored by means of a quartz crystal microbalance17 and set to a value ensuring a submonolayer film covering about 50% of the substrate surface; the deposition rate, as measured by the quartz crystal microbalance, was 0.2 nm/min. After film deposition at temperatures higher than room temperature, the sample was kept in ultrahigh vacuum and slowly cooled down spontaneously. By conduction through the metallic sample holder, the sample was observed to cool down from 393 to 333 K in about 80 min, reaching room temperature in 6-7 h. The substrates were R6T/LT13 single crystals grown by physical vapor transport18 and placed on a glass plate contacting the (100) face of the crystal. The orientation of the substrate surface unit cell was determined with a goniometer mounted in the sample holder of a polarizing optical microscope, exploiting the birefringence and dichroism of the crystals.19 The precision of the given directions was intended to be within (2°. AFM images were collected ex situ under ambient conditions in the intermitting contact (Tapping) mode with a Nanoscope IIIa MMAFM (Digital Instruments) using silicon cantilevers, after complete sample cooling at room temperature. For each image, we extracted the actual film thickness (measured film volume per unit area), the substrate coverage, and the size of each 2D homoepitaxial island present. The size distribution of the 2D homoepitaxial islands was extracted from groups of 200-500 islands for each film. The total density of islands on the substrate surface was estimated both by direct counting in a certain area and by dividing the average substrate coverage by the average island size. The two methods were verified to produce the same result; however, the latter was preferred since it is not effected by finite-size effects and the error bar associated with the island density can be calculated by taking into account the error propagation due to the uncertainty in the best estimate
TABLE 1: Expressions for the Activation Energy in Various Regimes of Condensationa regime complete initially incomplete extreme incomplete
2D atomic islands Eact ) (Ei + iEd)/(i + 2) Eact ) 1/2 (Ei + iEa) Eact ) Ei +(i + 1)Ea - Ed
2D R6T islands Eact ) [Ei - i(Er - Ed)]/(i + 2) Eact ) 1/2 [Ei + i(Ea - Er)] Eact ) Ei + i(Ea - Er) + Ea - Ed
Eact (eV) i)3
i)4
-0.10 0.72 2.1
-0.040 1.2 3.0
a The energy parameters calculated from atom-atom potentials are Ea ) 0.71, Ed ) 0.063, Er ) 0.61, E2 ) 0.41, E3 ) 1.13, E4 ) 1.95, and E5 ) 2.66 eV.
Homoepitaxial Growth of R-Hexathiophene
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12743
Figure 3. AFM-measured thickness of R6T submonolayer homoepitaxial films as a function of substrate temperature. The experimental error bars in the abscissa coordinates is well within the width of the diamond symbols.
Figure 2. A 20 × 20 µm2 AFM image of the R6T submonolayer homoepitaxial film deposited at 393 K (see Figure 1d) and a crosssectional profile collected along the white line.
of the substrate coverage and in the average island size (that is, the standard deviation of the island size distribution). Image scales chosen for the island size statistics fell typically in a range between 25 and 400 µm2, thus allowing the neglect of the effects of thermal drift. 3. Experimental Results The morphology of homoepitaxial R6T submonolayer films grown at different temperatures is displayed in Figure 1. In each panel, 1 ML-thick islands are observed, some of them having a second layer on top. Indeed, from the cross-sectional profile of Figure 1a, one observes that each island (as well as the second layer, when present) is 2.0 ( 0.8 nm thick. This thickness is close, within the experimental uncertainty, to the d200 spacing of the R6T/LT crystals (22.35 Å),13 which enclose single layers of R6T molecules tilted by 24.14° (axis connecting R and ω carbon atoms) to the layer plane (see sketched structure in Figure 1). Second-layer nucleation before first-layer completion is a phenomenon favored by diffusion energy barriers at island edges (Ehrlich-Schwoebel barriers),20 contrasting the migration of molecules from the island surface to the substrate surface. As shown by the sequence of panels, from a to d, by increasing the substrate temperature, a progressive reduction of the island density is registered. Looking at larger scales on the same samples, one realizes that a 3D crystalline phase coexists with 2D islands. Figure 2 reports a 20 × 20 µm2 AFM image of the homoepitaxial film grown at 393 K, where 3D R6T needles (white segments) are observed to decorate the film surface. Their length reaches micrometers, and their thickness, as shown by the cross-sectional profile, is some tens of nanometers, whereas their width is on the order of the tip diameter (typically 20-50 nm). Some of them show multiple branching deriving from twinning of the original needle, as observed in R-quaterthiophene needle-like crystallites grown on potassium hydrogen phthalate.21,22 Finally, the presence of needles is the signature of a 3D mechanism of
nucleation, which is competitive with the 2D film growth. The absence of specific preferential azimuthal orientation of the needles is indicative of a marginal role played by the interface energy during their nucleation. For rigid rod-like molecules, the presence of needles is indicative of a contact plane with the substrate which contains the molecular axes,21,23 for example, the (010) plane of R6T/LT. Then, in contrast to 2D islands, molecules within needles lie flat on the substrate surface. Since the contact plane is different with respect to the plane exposed by the substrate, the nucleation of the R6T needles on the (100)R6T/LT face is favored when occurring in a 3D fashion. The film coverages S at the various substrate temperatures are reported in the caption of Figure 1, whereas the film thickness texp, as deduced by AFM from the measurement of the film volume, is reported in Figure 3 as a function of Ts after normalization to the output of the quartz crystal microbalance (nominal thickness tnom). The ratio texp/tnom is equivalent to the ratio between the amount of the material crystallized on the substrate and that deposited on the microbalance. A value inferior to unity of the ratio texp/tnom and its decreasing trend with temperature indicate that desorption occurs during growth and its effect increases with temperature. An overestimation of texp on the order of 15% is expected for the film deposited at 393 K due to the high density of needle-like crystallites, whose estimated volume is effected by tip shape convolution. The morphological details of the 2D islands formed on the R6T/LT(100) surface can be inferred from Figure 4. In Figure 4, the normalized island size distribution is shown; the density Na (µm-4) of islands of size a (µm2) divided by the total island density N (µm-2) is reported as a function of the island area a. By increasing the substrate temperature, the distribution changes from a narrow curve, centered at small sizes, to a progressively broader curve, with the center displaced toward larger sizes. Nonetheless, these strong discrepancies among the curves disappear when the distributions are scaled in accordance with scaling relations expected in the case of diffusion-mediated growth.24 According to the dynamic scaling assumption, the density Na scales with the average island size am as
Na(S) ) Sam-2f(u)
(2)
where u ) a/am and f(u) is a dimensionless S-independent scaling function. The particular shape of the scaling function is determined by the critical nucleus size i (in terms of number of molecules). The expression of f(u) has been obtained empirically on the basis of numerical simulations and the comparison of some experiments, resulting in
f(u) ) Ciui exp(-biiu1/bi)
(3)
12744 J. Phys. Chem. C, Vol. 111, No. 34, 2007
Campione et al.
Figure 5. Logarithmic density N of homoepitaxial 2D islands of R6T as a function of substrate temperature. The linear regression of data for temperatures between 333 and 393 K (dotted line) gives an activation energy of 0.73 ( 0.18 eV. The experimental error bars in the abscissa coordinates is well within the width of the diamond symbols.
in Figure 5. The linear behavior found for Ts g 333 K is indicative of a process activated by temperature involved in the formation of islands. The activation barrier calculated by the linear regression of the data is 0.73 ( 0.18 eV. 4. Kinetic Analysis and Discussion
Figure 4. Upper panel: normalized size distribution of 2D homoepitaxial islands of R6T at different substrate temperatures. Lower panels: the scaled island size distribution is reported for the films grown at the four substrate temperatures and compared with the empirical curves calculated for critical nuclei between 1 and 5 molecules.
where the coefficients Ci and bi are fixed by implicit hypergeometrical equations which ensure normalization and proper asymptotic behavior of f(u). The lower panel of Figure 4 reports the experimentally measured scaling functions f(u) as squares together with the curves calculated with eq 3 for i ) 1-5. This procedure permits one to point out a difference among the films grown at room temperature and the ones grown at 333 e Ts e 393 K. Indeed, at room temperature, the scaled distribution closely resembles the curve corresponding to a critical size equal to 1 molecule, whereas at the other growth temperatures, the experimental distributions collapse into a curve closely resembling the ones corresponding to i ) 3-4. At this stage, we limit our comparison between experimental and empirical curves only to the identification of possible similarities. Indeed, so far, these empirical curves have been verified for 2D islands only under the assumption of a regime of complete condensation,24 even in the case of organic materials.25 This change of size of the smallest stable island with substrate temperature is a phenomenon already observed, for example, in metal heteroepitaxy; in the initial stages of Cu epitaxy on Ni(100) at 320 K, the critical nucleus was observed to change from i ) 1 to 3.26 Finally, an Arrhenius plot of the logarithmic total island density N as a function of the reciprocal temperature is reported
On the basis of eq 1, the activation barrier for nucleation calculated for the homoepitaxial growth of R6T for Ts g 333 K is deduced from Figure 5 to be Eact ) 0.73 ( 0.18 eV. Analogous procedures have been adopted for calculating the activation energy for para-hexaphenyl deposited on GaAs(001),7 R-quaterthiophene deposited on silica,27 and 3,4,9,10-perylene tetracarboxylicdianhydride on Ag(111),28 for example, obtaining Eact ) 0.90 ( 0.40, 0.122 ( 0.011, and 0.6 ( 0.2 eV, respectively. It must be noted that for molecular systems, the aggregation of diffusing molecules into islands must involve a rotation of molecules from a flat laying to a standing up configuration. This process is associated with another energy barrier Er, which is not contemplated by the model of eq 1. However, it can be included as a subtractive term in the parameter Ei. Table 1 reports the expressions for the activation energy of nucleation modified consistently with this assumption. An atom-atom potential well-established for organic molecules29 can be exploited for calculating all of the energy terms necessary for evaluating the activation energy for nucleation in various growth regimes. These same atom-atom potentials have been previously used30 to model the surface potential of the R6T(100) surface in order to estimate the activation energy barriers to diffusion and nucleation and the energy of adsorption of the R6T molecules on R6T(100). We performed a similar analysis for the normal configuration according to Biscarini et al.,30 that is, an R6T molecule lying flat on the crystal surface but with explicit atoms instead of an all united atoms approach. Our simulation was based on a (100) slab comprising 216 R6T molecules in a single layer of ca. 22 Å thickness. The surface potential was calculated on an equally spaced grid of 0.1 Å along the b and c axes for different rotations about the normal to the surface (with 5° steps) and different distances between the probe molecule and the R6T(100) surface. Results of the surface potential simulation are reported in Table 1. With respect to the results reported in ref 30, a significant discrepancy is noted only for the value of the parameter Ed (0.063 versus 0.27 eV); this can be probably ascribed to the very simplified model
Homoepitaxial Growth of R-Hexathiophene used in ref 30 representing the molecule as a rigid chain made of six identical spherical units. The last column of Table 1 reports the activation energy calculated for critical nuclei of three and four R6T molecules, the size suggested by the analysis of the island size distribution reported in Figure 4 for Ts g 333 K. Note that for the regime of complete condensation, negative values are found, positive values of the activation energy being obtained only for i g 6. A close correspondence of our experimental value of the activation energy and that calculated is found for a growth regime of initially incomplete condensation with i ) 3. On the other hand, a regime of complete condensation can be ruled out a priori in the light of the results reported in Figure 3, and the calculated activation energy (see Table 1) supports this conclusion. On the basis of the results reported in Figure 4, clearly showing a critical nucleus composed of one molecule when Ts ) 298 K, if we assume a regime of complete condensation, the activation energy for nucleation would be negative (see Table 1). This indicates that re-evaporation is present even when the substrate is kept at room temperature. In the light of these results, re-evaporation is to be considered a dominant event during the homoepitaxial growth of R6T. In order to give a realistic picture of the competing events occurring during growth, under a regime of initially incomplete condensation, the mean diffusion length of molecules is comparable with respect to the interisland distance. Our experimental results provide other pieces of evidence in favor of this conclusion. The increment of substrate temperature influences the kinetics of island growth by enhancing the diffusion processes of molecules on the substrate surface and, simultaneously, the probability of re-evaporation. An important process is the diffusion of adsorbed molecules at the island edge along its perimeter. This process is favored by an increment of the thermal energy and is driven by the minimization of the excess energy at the island edges. For this reason, by increasing the substrate temperature, the island shape is expected to become more compact and assume a polygonal shape, reflecting the thermodynamically stable shape of the crystal. Actually, in our homoepitaxial system, islands grown at 393 K possess a more accentuated polygonal shape with respect to those grown at lower temperature, as shown by Figure 1. Nonetheless, the island perimeter is observed to get rougher. This behavior, looking anomalous at first sight, is again attributed to film desorption, which occurs preferably at island edges both during the growth and the post-growth stages (slow substrate cooling down under ultrahigh vacuum conditions) of the film preparation.12 The effect of post-growth material re-evaporation is believed to be limited to edge roughening of the islands, without substantial variations of their area and number. Indeed, as shown in Figure 4, when scaled in accordance with eq 2, the island size distributions in the temperature range of 333-393 K collapse into the same curve, indicating no dependence on temperature of the mechanism of formation of islands. As a general conclusion, we are confident that the complete condensation regime must be ruled out as a possible mechanism of growth for R6T homoepitaxial films. Rather, when a regime of initially incomplete condensation is assumed, the best accordance between kinetic and scaling theories is achieved. 5. Conclusions The homoepitaxial growth of organic R6T presents strong analogies with the homoepitaxial growth of traditional inorganic semiconductors. Under typical growth conditions employed for the OMBE of R6T, the supersaturation level is sufficient for
J. Phys. Chem. C, Vol. 111, No. 34, 2007 12745 growing stable 2D nuclei with sizes as small as 2 molecules when the substrate is kept at room temperature, increasing up to 4 molecules in a temperature range of 333-393 K. These characteristics are the basis for the controlled preparation of the semiconductor surface, which is, in general, a fundamental step for the development of a technology based on organic materials. Hence, from this point of view, the know-how of the inorganic semiconductor technology can be exploited for the development of an “all-organic” technology. Other characteristics of the R6T homoepitaxial films suggest it is less trivial to control the surface of organic semiconductors; in particular, material re-evaporation, both during the growth and post-growth stages, is found to be an important phenomenon which is responsible for a decrease in the fractal dimension of 2D islands when the substrate temperature is increased even slightly above room temperature. Simultaneously, the nucleation of a crystalline 3D phase incommensurate with the substrate surface is favored. Acknowledgment. Dino Aquilano is kindly acknowledged for the critical reading of the paper and for useful discussions. References and Notes (1) (a) Horowitz, G.; Garnier, F.; Yassar, A.; Hajlaoui, R.; Kouki, F. AdV. Mater. 1996, 8, 52. (b) Dinelli, F.; Murgia, M.; Levy, P.; Cavallini, M.; Biscarini, F.; de Leeuw, D. M. Phys. ReV. Lett. 2004, 92, 116802. (c) Gomes, H. L.; Stallinga, P.; Dinelli, F.; Murgia, M.; Biscarini, F.; de Leeuw, D. M.; Muck, T.; Geurts, J.; Molenkamp, L. W.; Wagner, V. Appl. Phys. Lett. 2004, 84, 3184. (d) Horowitz, G. AdV. Funt. Mater. 2003, 13, 53. (2) Biscarini, F. In Scanning Probe Microscopy of Polymers; Ratner, B. D., Tsukruk, V. V., Eds.; Americam Chemical Society: Washington, D. C., 1998; p 163. (3) Garcia, R.; Tello, M.; Moulin, J. F.; Biscarini, F. Nano Lett. 2004, 4, 1115. (4) Loi, M. A.; Da Como, E.; Dinelli, F.; Murgia, M.; Zamboni, R.; Biscarini, F.; Muccini, M. Nat. Mater. 2005, 4, 81. (5) Campione, M.; Sassella, A.; Moret, M.; Thierry, A.; Lotz, B. Thin Solid Films 2006, 500, 169. (6) Meyer zu Heringdorf, F.-J.; Reuter, M. C.; Tromp, R. M. Nature 2001, 412, 517. (7) Mu¨ller, B.; Kuhlmann, T.; Lischka, K.; Schwer, H.; Resel, R.; Leising, G. Surf. Sci. 1998, 418, 256. (8) Koller, G.; Berkebile, S.; Krenn, J. R.; Netzer, F. P.; Oehzelt, M.; Haber, T.; Resel, R.; Ramsey, M. G. Nano Lett. 2006, 6, 1207. (9) Oehzelt, M.; Koller, G.; Ivanco, J.; Berkebile, S.; Haber, T.; Resel, R.; Netzer, F. P.; Ramsey, M. G. AdV. Mater. 2006, 18, 2466. (10) (a) Herman, M. A.; Richter, W.; Sitter, H. Epitaxy; Springer: Berlin, Germany, 2004. (b) Tringides, M. C. Surface Diffusion: Atomistic and CollectiVe Processes; NATO Science Series B: Physics; Plenum: New York, 1997. (11) (a) Sassella, A.; Campione, M.; Papagni, A.; Goletti, C.; Bussetti, G.; Chiaradia, P.; Marcon, V.; Raos, G. Chem. Phys. 2006, 325, 193. (b) Sassella, A.; Borghesi, A.; Campione, M.; Tavazzi, S.; Goletti, C.; Bussetti, G.; Chiaradia, P. Appl. Phys. Lett. 2006, 89, 261905. (12) Campione, M.; Sassella, A.; Moret, M.; Marcon, V.; Raos, G. J. Phys. Chem. B 2005, 109, 7859. (13) Horowitz, G.; Bachet, B.; Yassar, A.; Lang, P.; Demanze, F.; Fave, J.-L.; Garnier, F. Chem. Mater. 1995, 7, 1337. (14) Hartmann, P.; Bennema, P. J. Cryst. Growth 1980, 49, 145. (15) Venables, J. A.; Spiller, G. D. T.; Handbu¨cken, M. Rep. Prog. Phys. 1984, 47, 399. (16) (a) Forrest, S. R. Chem. ReV. 1997, 97, 1793. (b) Tubino, R.; Borghesi, A.; Dalla Bella, L.; Destri, S.; Porzio, W.; Sassella, A. Opt. Mater. 1998, 9, 437. (17) Campione, M.; Cartotti, M.; Pinotti, E.; Sassella, A.; Borghesi, A. J. Vac. Sci. Technol., A 2004, 22, 482. (18) Kloc, Ch.; Simpkins, P. G.; Siegrist, T.; Laudise, R. A. J. Cryst. Growth 1997, 182, 416. (19) Vrijmoeth, J.; Stok, R. W.; Veldman, R.; Schoonveld, W. A.; Klapwijk, T. M. J. Appl. Phys. 1998, 83, 3816. (20) (a) Ehrlich, G.; Hudda, F. G. J. Chem. Phys. 1966, 44, 1039. (b) Schwoebel, R. L.; Shipsey, E. J. J. Appl. Phys. 1966, 37, 3682. (21) Campione, M.; Moret, M.; Sassella, A.; Trabattoni, S.; Resel, R.; Lengyel, O.; Marcon, V.; Raos, G. J. Am. Chem. Soc. 2006, 128, 13378. (22) Marcon, V.; Raos, G.; Campione, M.; Sassella, A. Cryst. Growth Des. 2006, 6, 1826.
12746 J. Phys. Chem. C, Vol. 111, No. 34, 2007 (23) (a) Koller, G.; Berkebile, S.; Krenn, J.; Tzvetkov, G.; Hlawacek, G.; Lengyel, O.; Netzer, F. P.; Teichert, C.; Resel, R.; Ramsey, M. G. AdV. Mater. 2004, 16, 2159. (b) Haber, T.; Oehzelt, M.; Andreev, A.; Thierry, A.; Sitter, H.; Smilgies, D.-M.; Schaffer, B.; Grogger, W.; Resel, R. J. Nanosci. Nanotechnol. 2006, 6, 698. (24) Amar, J. G.; Family, F. Phys. ReV. Lett. 1995, 74, 2066. (25) (a) Ruiz, R.; Nickel, B.; Koch, N.; Feldman, L. C.; Haglund, R. F.; Kahn, A.; Family, F.; Scoles, G. Phys. ReV. Lett. 2003, 91, 136102. (b) Stadlober, B.; Haas, U.; Maresch, H.; Haase, A. Phys. ReV. B 2006, 74, 165302. (c) Wu, Y.; Toccoli, T.; Koch, N.; Iacob, E.; Pallaoro, A.; Rudolf, P.; Iannotta, S. Phys. ReV. Lett. 2007, 98, 076601.
Campione et al. (26) Mu¨ller, B.; Nedelmann, L.; Fischer, B.; Brune, H.; Kern, K. Phys. ReV. B 1996, 54, 17858. (27) Berlanda, G.; Campione, M.; Sassella, A.; Moret, M.; Borghesi, A. Phys. ReV. B 2004, 69, 085409. (28) Krause, B.; Du¨rr, A. C.; Ritley, K.; Schreiber, F.; Dosch, H.; Smilgies, D. Phys. ReV. B 2002, 66, 235404. (29) Gavezzotti, A. Theoretical Aspects and Computer Modeling of the Molecular Solid State; John Wiley & Sons: Chichester, U.K., 1997; p 67. (30) Biscarini, F.; Zamboni, R.; Samorı`, P.; Ostoja, P.; Taliani, C. Phys. ReV. B 1995, 52, 14868.
