Controlling In-Plane Isotropic and Anisotropic Orientation of Organic

Article pubs.acs.org/JPCC

Controlling In-Plane Isotropic and Anisotropic Orientation of Organic Semiconductor Molecules on Ionic Fluoride Dielectrics Tingming Jiang,†,‡ Konstantin Koshmak,§ Angelo Giglia,§ Alexander Banshchikov,∥ Nikolai S. Sokolov,∥ Franco Dinelli,⊥ Raffaella Capelli,§ and Luca Pasquali*,‡,§,# †

Institut des Sciences Moléculaires d’Orsay, ISMO, UMR 8214, Université Paris Sud, Bâtiment 351, 91405 Orsay, France Dipartimento di Ingegneria E. Ferrari, Università di Modena e Reggio Emilia, Via Vivarelli 10, 41125 Modena, Italy § CNR-IOM s.s. 14, Km. 163.5 in AREA Science Park, 34149 Basovizza, Trieste, Italy ∥ Solid State Physics Division, Ioffe Physical-Technical Institute of Russian Academy of Sciences, 26 Polytechnicheskaya str., 194021 St. Petersburg, Russia ⊥ CNR-INO Area della Ricerca di Pisa - S. Cataldo, via Moruzzi,1, I-56124 Pisa, Italy # Department of Physics, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa ‡

ABSTRACT: α-Sexithiophene (6T) ultrathin films have been grown on CaF2(111)/Si(111) planar surfaces and on CaF2(110)/Si(001) ridged surfaces by molecular beam epitaxy. The growth mode has been studied by means of atomic force microscopy (AFM), photoemission, and near edge X-ray absorption fine structure (NEXAFS). AFM reveals a substantial difference in the film morphology on the two substrates: on CaF2(111) large islands with flat terraces form with no in-plane preferential growth direction; on CaF2(110) narrow and elongated islands develop following the substrate corrugation. Photoemission and X-ray absorption at Ca L2,3 and F K edges indicate that the interaction with the substrate is negligible. Near-edge X-ray absorption (NEXAFS) flanked by DFT calculations of the angular-resolved absorption cross section of 6T at the carbon K-edge reveal that the molecules on both substrates have their long axis vertically oriented with respect to the substrate plane. In addition, in-plane anisotropy of the molecular orientation has been observed on CaF2(110), and it has been interpreted in terms of well aligned molecules in the elongated islands.

1. INTRODUCTION Organic/dielectric/semiconductor stacks are at the basis of molecular electronics. One principal application is in thin-filmtransistor (TFT) devices, including light emitting ones.1,2 Major problems in the growth of organic thin film devices are related to the difficulties to induce a molecular order and alignment that favor the intermolecular π-orbital overlap over the large areas that are necessary for efficient operation. Besides this, it is recognized that the first few molecular layers at the interface with the dielectric are crucial for transport. Consequently, besides the reduction of the impurity levels and traps, the reduction of disorder at the interface is mandatory. It is also crucial to control and to promote the correct alignment of the molecules over large areas to foster high charge mobility along specific directions. Oxides are commonly employed as dielectrics (e.g., SiO2, Al2O3). In their place, suitably designed insulating layers based on epitaxial ionic fluorides can provide remarkable advantages in terms of higher dielectric constant, high crystallinity and sharp atomic interfaces, to guide the growth of organic semiconductors as active materials in devices. Ionic fluorides have demonstrated to be very attractive as gate dielectrics in hybrid organic−inorganic devices.3,4 In spite of this, studies regarding the molecular organization at the © XXXX American Chemical Society

interface with these materials are largely missing. Moreover, due to their inertness, the wide bandgap that makes them transparent up to the FUV (far ultraviolet, below 10−11 eV) and flatness of their interfaces, ionic fluorides can be ideal for fundamental studies, minimizing the chemical interaction between the molecular film and the support. Here we focus our attention on ultrathin films of sexithiophene (6T), a well-studied model system with high carrier mobility.5,6 6T tends to grow on dielectrics with the long molecular axis almost perpendicular to the growth plane, favoring π−π molecular interactions.7−10 Ordering and anisotropy of the in-plane orientation of the molecules can be obtained through the choice of specific substrate crystal faces,11,12 by exploiting growth at step edges,13 or through artificial patterning of the dielectric surface.14−17 This type of study is typically performed either on bulk single-crystal dielectrics or on artificially patterned substrates. Within the ionic fluorides family, calcium fluoride grows epitaxially of Si, due to the good lattice matching.18 Depending on the choice of the substrate orientation and growth Received: December 23, 2016 Revised: February 13, 2017 Published: February 13, 2017 A

DOI: 10.1021/acs.jpcc.6b12926 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C temperature, the morphology of the dielectric film can be tuned to form either flat layers with sharp interfaces on Si(111)19−21 or nanostructured films formed by separated huts of fluoride, parallel elongated stripes, or close parallel ridges on Si(001).22−26 In particular, CaF1/CaF2 striped surfaces on stepped Si(111) have been proposed for the organized growth of organic molecules along specific directions.27 Here 6T is grown by molecular beam epitaxy on CaF2(110)/ Si(001) ridged surfaces and on CaF2(111)/Si(111) planar surfaces. Atomic force microscopy (AFM), photoemission (XPS), and near edge X-ray absorption (NEXAFS) at the carbon K-edge are used to investigate morphology, chemical reactions between the molecules and with the fluoride substrate, and to get information on the molecular order. A clear in-plane anisotropic orientation of 6T is detected on CaF2(110)/Si(001), promoted by the CaF2 ridges. Theoretical calculation of the molecular orbitals and X-ray absorption cross sections are used to flank the experiments and to guide the interpretation of the spectra.

called tapping mode, employing cantilevers with a nominal spring constant of around 5−15 N/m and a resonant frequency of a few hundreds of kilohertz (NT-MDT, NSG11). The color code of the images is the following: a brighter color corresponds to a higher region.

3. THEORETICAL METHODS Spectroscopic characterization was flanked by DFT calculations of the molecular energy levels and optical transitions at the carbon K-edge of the free 6T molecule. Calculations were performed with the code StoBe.30 The 6T ground state properties and optimized geometry for the planar configuration were calculated using a gradient-corrected RPBE exchange/ correlation functional31,32 and triple-ζ basis set, including polarization functions.33 The transition-potential method34 was applied to simulate absorption spectra at the carbon K-edge. An IGLO-III basis on each excitation center was used, whereas effective core potentials (ECP) describing the core and the appropriate valence basis were applied for the remaining carbon atoms. ΔKohn−Sham adjustment35,36 was added to the spectra of nonequivalent atoms for energy scale alignment.

2. EXPERIMENTAL METHODS The experiments were performed in ultrahigh vacuum (UHV) at the BEAR beamline of the Elettra synchrotron.28,29 α-6T (Aldrich) was evaporated in situ with a Knudsen cell. Growth rate was 0.2 nm/min, with the substrate held at room temperature. Nominal thickness of the organic film was evaluated with a quartz microbalance. CaF2(111) (12 nm)/Si(111) and CaF2(110) (80 nm)/ Si(001) substrates were pregrown by molecular beam epitaxy.24 Si substrates were flash heated to 1100 °C before CaF2 evaporation to remove the native oxide. CaF2 films on Si(001) were grown with the substrates initially held at 780 °C, to promote the formation, at the initial stages of growth, of an interface-reacted CaF wetting layer, bonding with Si.22,24,25 After formation of the wetting layer, the substrate temperature was reduced to 440 °C, to favor groove uniformity on Si(001). On Si(111) the first four CaF2 layers were deposited at relatively low temperature of 250 °C, with further growth at 760 °C. This procedure promotes the formation of flat crystalline CaF2(111) films on Si(111). Substrates were degassed for a few hours in ultrahigh vacuum at the beamline before 6T growth. Surface substrate cleanliness was checked through photoemission and X-ray absorption. Photoemission was acquired in normal emission geometry with a hemispherical electron analyzer (66 mm mean radius) operated at constant pass energy. Overall energy resolution (analyzer and photon beam) was 0.1−0.3 eV (depending on spectral range). Near edge X-ray absorption was measured in total electron yield (TEY) mode, by recording the drain current while illuminating the sample with monochromatic light across the carbon and fluorine K-edges and Ca L2,3 edges. Resolution was 0.1 eV. The photon beam was linearly polarized (degree of linear polarization 84%). To get information on the orientation of 6T molecules, the incidence condition was varied to orient the electric field direction of the incoming beam along either specific in-plane or out-of-plane directions. In NEXAFS experiments the spectra were normalized to the signal acquired on a carbon-free Au reference sample to correct for the incoming flux fluctuations, due to the transmission of the optical elements of the beamline and possible beam instabilities. The AFM setup is composed of a commercial head (NTMDT, SMENA), with electronics and control software developed in-house. The AFM has been operated in the so-

4. RESULTS 4.1. Morphology. On Si(111) CaF2 grows in layer-by-layer mode, exposing the (111) face of CaF2 normal to the surface plane. The surface is fluorine terminated.37,38 The morphology of the bare CaF2(111)/Si(111) substrate is shown in Figure 1a. CaF2 forms wide flat terraces, extending over hundreds of nanometres, which follow the Si topography. Triangular shaped

Figure 1. AFM morphologies of pristine (a) CaF2/Si(111) and (b) CaF2/Si(001) surfaces, 6T layers with a nominal thickness of 4 nm deposited on (c) CaF2/Si(111) and (d) CaF2/Si(001). Vertical scales are 10 nm (a), 20 nm (b), 15 nm (c), and 20 nm (d). Inset of panel (b): RHEED pattern obtained for an electron beam parallel to the grooves. Inset of panel (d): schematic shape of 6T island with facets of type {011}, {001}, and (010} and the crystallographic axes of a 6T crystal (a pointing normal to the substrate plane and c parallel to the ridges). B


Article

The Journal of Physical Chemistry C

Figure 2. (a) Ca 2p levels of pristine surfaces, before 6T deposition; (b) S 2p levels and (c) C 1s levels of T6 for a 4 nm film; (d) comparison between the experimental valence band and shallow core level region before and after deposition. Theoretical calculations of the total and projected DOS of free 6T are also reported.

terraces are well visible, as expected for growth along the (111) direction. The growth of CaF2 on Si(001) is shown in Figure 1b. The surface presents a sequence of parallel ridges and grooves, extending over several microns and covering completely the Si substrate. The morphology is highly anisotropic, as discussed extensively in refs 23 and 24 with the ridges running parallel to the [110] direction of the Si substrate.24 The ridges form a roof-shaped morphology, with F-terminated CaF2 {111} facets inclined 55° with respect to the substrate normal. This is evidenced by the reflection high energy electron diffraction (RHEED) pattern shown in the inset of Figure 1b, obtained for an electron beam oriented parallel to the ridges. The ridges appear of uniform lateral dimensions of the order of 50 nm.

On both substrates, 6T layers with a nominal coverage of 4 nm have been evaporated. On CaF2(111)/Si(111), 3D islands with wide flat terraces form (Figure 1c). Overall, the growth mode strongly resembles what observed on SiO2 heated substrates.7,9,14,15,39,40 The actual coverage can be estimated on the order of 1.5 monolayers (ML). The islands have an irregular shape, giving rise to dendritic domains and do not show any preferential in-plane growth direction. The height profile shows that each ML has the same height value corresponding to 2.5 nm. This is consistent with a growth mode where the molecules align vertically having the long molecular axis almost perpendicular to the substrate plane. It can be noticed that the 6T islands extend over more than one CaF2 terrace. The step height at terrace edges for 6T and C


Article


position in the thiophene rings.44−46 The low binding energy emission down to 1.5−2 eV is due to π molecular orbitals, delocalized along the C backbone, including the HOMO. The intense feature between 8 and 10 eV is also reproduced by the calculation and is due to 6T. Features ascribed to molecular states with prevailing σ character are visible in the 18−22 eV energy region. Overall, the valence band of the molecular film largely masks the substrate features, partially superimposing to them. The molecular valence states do not appear to be strongly modified by the adsorption process. S 2p levels of 6T are reported in Figure 2b. A single asymmetric structure is observed with maximum at 163.9 eV. The asymmetry is due to the 2p3/2−2p1/2 spin orbit splitting of 1.2 eV of S 2p, which is not well resolved due to spectral broadening. C 1s peaks in Figure 2c consist of a single broad peak centered at 284.5 eV, for both samples. These features and energy positions are compatible with bulk-like 6T.47,48 No sizable differences are observed between the two samples, suggesting that interactions of the molecules in the film are comparable in the two cases and that the substrate is inert with respect to molecular bonding. The quality of the pristine surfaces before deposition is also clearly evidenced by NEXAFS spectra at Ca L2,3 and F K edges, as shown in Figure 3a,b. For both substrates, the spectral fine structure reflects pure CaF2 bulk crystal features.25,49 The line shape does not change after 6T deposition, which is a strong indication that chemical reactions do not occur between the molecules and the substrate, at both F and Ca sites.

CaF2(111) is around 2.5 and 0.2 nm, respectively. The topographical image shows that the fluoride substrate morphology is already masked after the growth of a complete 6T monolayer. The situation is markedly different on CaF2(110)/Si(001) (Figure 1d). The 6T film mimics the substrate corrugations: 3D narrow, parallel and highly elongated islands develop on top of the CaF2 ridges. The lateral dimension appears to be limited by the ridges-and-grooves pattern of the substrate and does not exceed ∼50 nm. The height profiles cannot be interpreted as straightforward as in the case of CaF2(111)/Si(111); it is, however, possible to measure steps along the CaF2 ridges, those corresponding to multiples of 2.5 nm. The long edges of the 6T islands are sharp and parallel to the direction of the CaF2(110) ridges. Their short edges are also sharp and either can be perpendicular with respect to the ridge direction or form an angle of around 50° (±5°), as shown in higher detail in the inset of Figure 1d): the observed shape of the 6T islands is compatible with that of elongated 6T crystal grains, with the c and a crystal axes parallel to the ridges and normal to the substrate plane, respectively, and delimited by {010}, {011}, and {001} facets.41,42 4.2. Soft X-ray Spectroscopy. Photoemission spectra of pristine surfaces and after deposition of 6T have been taken in correspondence of the Ca 2p, Ca 3p, F 2p, C 1s, and S 2p levels and of the valence band, to inspect possible chemical reactions within the molecules and between the molecules and the substrate (Figure 2a−d). Spectra were acquired at different photon energy to minimize the electron mean free path for the outgoing electrons and thus enhance the surface sensitivity. The Ca 2p spectra (Figure 2a) of the CaF2(111)/Si(111) and CaF2(110)/Si(001) substrates and the shallow levels F 2s, Ca 3p, and the F 2p valence band (bottom curves in Figure 2d) taken before 6T deposition are in agreement with literature reports for bulk CaF2.19,20,24,37 Slight broadening of the photoemission peaks exceeding the energetic resolution can be ascribed to some level of sample charging, due to the dielectric film. No sizable traces of contamination were found on the surfaces after annealing in ultrahigh vacuum. Deposition of 4 nm of 6T, corresponding to about 1.5 ML according to the AFM data, results in a deep modification of the valence band and shallow core level region (top of Figure 2d). In addition, after deposition on both substrates, the Ca 2p levels are no more visible, indicating that 6T film covers completely the CaF2 surface. Interestingly, Ca 3p and F 2s levels can still be observed. This can be exploited to derive the effective organic film thickness, as obtained by photoemission. At the used photon energies, Ca 2p levels are measured at kinetic energies around 100 eV, whereas Ca 3p/F 2s levels at energies around 230 eV. Applying the TPP-2 M formula43 to calculate at the two energies the inelastic electron free path in the 6T film and considering a value of 3λ as the maximum sampling depth, a film thickness between 2 and 3 nm is obtained. This is compatible with the AFM results: large portions of the surface are covered only with a thickness of 1 ML and the molecules align vertically with the long molecular axis almost perpendicular to the substrate plane. Concerning the valence band, new features show up for both samples below 1.5−2 eV of binding energy. In Figure 2d the calculated total and projected densities of states (DOS) of the free 6T molecule are also shown. The shoulder at about 4−5 eV of binding energy in the experimental spectra is associated with π states mainly localized on S atoms and C atoms in β

Figure 3. (a) Ca L2,3 and (b) F K absorption edges after deposition of T6 on the two substrates. The comparison with the bare CaF2 spectra is also shown. No differences of line-shape were observed at the edges for the two pristine substrates. D


Article


Figure 4. NEXAFS spectra taken in s- and p-light scattering conditions on (a) 6T/CaF2(111)/Si(111) and (b) 6T/CaF2(110)/Si(001). In sscattering, two perpendicular in-plane directions of the electric field were measured. (c) and (d) simulation of the 6T X-ray absorption, assuming a molecular arrangement, as shown in the insets for the two different substrates. (e) Calculated absorption cross sections of 6T along three perpendicular molecular axes. (f) Model for 6T alignment on CaF2(110) ridges. See text for the meaning of labels.

CaF2(110) though, a sizable anisotropy is also observed in the s-polarization experimental curves, pointing to an additional inplane preferential orientation of the molecules. To aid in the interpretation of the spectral features and anisotropies, calculations of the absorption cross sections (dipole transitions) of the single 6T molecule are reported in Figure 4c−e. In the calculated curves, the electric field vector was aligned along specific molecular directions, emphasizing the spectral weight directionality of allowed dipole transitions with respect to the molecular axes.50−52 In particular, in Figure 4e the calculated absorption curves are shown for the electric filed vector oriented along the long molecular axis zm, the axis ym that is parallel to the ring planes and the xm axis that is perpendicular to the plane of the thiophene rings. The different spectral features have been labeled in Figure 4a,e and are in agreement with literature reports for 6T.53,54 In particular, the prominent structure A and feature C are associated with π* transitions, respectively, involving mainly carbons in the α position and in the β position. Structure B is related to σ*

The NEXAFS spectra taken at the carbon K edge are reported in Figure 4a,b for 6T/CaF2(111)/Si(111) and for 6T/ CaF2(110)/Si(001), respectively. Spectra were acquired at a grazing incidence of 20° in three different geometries: with the electric field parallel to the substrate plane (s-polarization) but at two orthogonal directions, that for the CaF2(110) surface corresponds to the direction of the ridges (s-pol||) and perpendicular to them (s-pol⊥), and with the electric field at an angle of 20° with the substrate normal (p-pol). This is illustrated in the insets of Figure 4b−d. While presenting a similar fine structure, the spectra also show clear angular effects that are related to molecular orientation. On CaF2(111) the two perpendicular s-polarization spectra do not present sizable differences. This indicates substantial in-plane isotropy for the average orientation of the molecules, consistent with the AFM observations. Instead, the marked differences between in-plane and out-of-plane (p-polarization) spectra indicate a preferential orientation of the molecules with respect to the substrate normal. This is also the case for the CaF2(110) substrate. For E


Article


crystallographic data of bulk 6T.41,42 Random in-plane orientation of the domains was assumed. As a consequence, considering a beam footprint much larger than each domain, an average in-plane isotropy is expected for the NEXAFS curves. In the simulation this is achieved by applying an in-plane average over varying in-plane orientations of the crystal grains.50,51 Clearly, NEXAFS calculated along the two perpendicular in-plane directions indicated in Figure 4c give equivalent results, which reflects the experimental curve behavior reported in Figure 4a. On the contrary, a marked difference is present when the electric field is oriented at 70° with respect to the substrate plane, representing the ppolarization scattering condition of Figure 4a. Resonances A and C, associated with π* transitions, are depressed with respect to the s-polarization case. The opposite occurs for features D−F. The behavior of feature B is less evident, due to partial superposition of π* and σ* type transitions. Concerning the CaF2(110) case, we observe an additional preferential organization of the molecular domains along the directions of the ridges. Consistent with AFM, we assume that the 6T islands are crystal grains with their c axis parallel to the ridges. The molecular arrangement is schematized in the inset of Figure 4d and in Figure 4f. This induces anisotropy in the inplane NEXAFS, which is reflected both in the simulations and in the experimental curves, with overall qualitative agreement of the angular behavior of the spectral features. In particular, feature A is more pronounced when the electric field is perpendicular to the grooves than when it is parallel to them. The opposite evolution is observed for features E and F. The particular orientation of the 6T domains with respect to the ridges-and-grooves structure can be tentatively explained considering the crystal structure of 6T. In the crystal the molecules have their long axes inclined toward the c axis and they form an angle of 23.5° with the a axis.41 For 6T islands with their c axis parallel to the ridges, the molecules growing on adjacent CaF2 {111} facets would be parallel to each other, maximizing their contact interaction through π states. This would enhance the stability of the system. Consequently, the long edges of adjacent 6T elongated grains would be facing each other. Overall, the principal factor that determines the alignment of the molecules on the ridged CaF2(110) seems to be the interaction between the organic molecules at the edges of the inclined facets of CaF2. This gives rise to a highly anisotropic in-plane growth, with preferential orientation of the molecules in the film. The model of the molecular arrangement of 6T on CaF2(110) is sketched in Figure 4f. With respect to the 6T bulk crystal, here the molecular arrangement appears to be slightly distorted, because perpendicular to the c axis the molecular grains grow on the inclined facets of CaF2. We attempted to simulate the NEXAFS spectra assuming a vertical alignment of the molecules with respect to the inclined facets (and not to the substrate plane), but this resulted in poor agreement with the experiment. Therefore, for the determination of the in-plane NEXAFS anisotropy, a simple arrangement of the molecules as for CaF2(111) on the alternate inclined {111} facets seems to be excluded. Good qualitative agreement between simulations and experiment is obtained only if 6T molecules are assumed to be vertically standing with respect to the substrate plane.

transitions, with more pronounced spectral weight for carbons in the α position (involving C−S bonding) than in the β position. Contributions to structure C also originate from σ* C−H excitation. Features D−F are associated with σ* C−C resonances.

5. DISCUSSION Photoemission and X-ray absorption at Ca L2,3 and F K edges show that the chemical interaction between the molecules in the film and the fluoride surface is indeed absent. High resolution NEXAFS at Ca L2,3 and F K edges do not change with 6T film formation (except of an intensity reduction due to the overlayer). Correspondingly, photoemission from the molecular core levels in the ultrathin films show a bulk-like behavior. In this sense, CaF2 can be considered as an ideal substrate, where molecular interactions dominate the formation and organization of the organic film. For both CaF2(111) and CaF2(110) surfaces and at 1.5 ML coverage (as deduced from AFM data), the substrates appear completely covered by the 6T film that gives rise to extended islands. AFM and photoemission suggest that the molecules tend to be vertically aligned in the layers, with their long axis perpendicular to the substrate plane. This is analogous to other dielectrics, as SiO2. For CaF2(111) the growth mode is actually very close to what observed on SiO2, with the formation of islands with wide flat terraces and with irregular edges.7,9,14,15,39,40 The situation is markedly different for CaF2(110). The groove-and-ridges morphology guides the islands development. A similar behavior has been observed on patterned SiO2 substrates.14−17 However, at variance with respect to these cases, here patterning of CaF2(110) is naturally obtained by the choice of the preparation conditions, without the necessity of artificial fabrication of the grooved structure. Growth on stepped dielectrics,13,55,56 including CaF2,27 was also successfully exploited to induce in-plane growth anisotropy, but this requires vicinal surfaces, which strongly limits application possibilities. In the present case, the islands are elongated and extend several micrometers in their length. According to AFM (Figure 1d), these islands have definite edges, which are compatible with 6T well aligned grains, with the c axis of the 6T crystallites aligned with the direction of the ridges. Partial alignment of crystal grains was previously observed on artificially patterned SiO2.16,17 But in that case the alignment was not as uniform as on CaF2(110)/Si(001). Here the islands show sharp edges. Assuming crystal order in each island, the observed shapes are consistent with a uniform orientation of the c axis parallel to the ridges. Long edges would correspond to {010}-like facets, and short terminal edges to {011} or {001} facets. Further information on the molecular orientation can be obtained from angular resolved NEXAFS. Linear dichroism effects in NEXAFS curves in Figure 4a,b clearly show that the 6T molecules tend to adopt a vertical configuration on both CaF2(111) and CaF2(110). This interpretation is supported by the theoretical simulations that we performed with the electric field oriented along specific molecular directions, as shown in Figure 4c and 4d. The simulation curves reported in Figure 4c for 6T on CaF2(111) were obtained by supposing molecules organized in crystal grains, with b and c axes of 6T parallel to the substrate and a axis perpendicular to the surface. The geometrical arrangement of the molecules in the grains was taken from

6. CONCLUSIONS The growth of organic ultrathin films of 6T on CaF2 epitaxial layers strongly depends on the morphology of the substrate. F


Article

The Journal of Physical Chemistry C The interactions between the molecules in the film dominate the growth mode. The interaction between the molecules and the substrate is instead negligible. Although patterning or corrugations at the nanoscale of the substrate surface can strongly influence the 6T molecular organization over extended regions. This is evidenced by the striking difference in molecular organization on CaF 2 (111)/Si(111) and CaF2(110)/Si(001) substrates. In the first case only a preferential vertical alignment of the molecules is observed, with the formation of large islands with wide flat terraces and dendritic edges, in analogy with other poorly reactive dielectrics, whereas in the second case the molecules organize in well aligned domains that follow the natural ridges-andgrooves roof-like morphology of the substrate. On CaF2(110)/ Si(001) the vertical alignment of the long molecular axis is preserved. In addition, in-plane anisotropy is obtained over extended macroscopic areas. The c axis of the 6T elongated grains appears parallel to the ridges direction. We believe that this is due to the interaction of molecules growing at the edges of two adjacent inclined {111} CaF2 facets, which tend to maximize their contact interaction through π states assuming a parallel alignment. This creates a pattern of extended parallel nanowires of highly oriented molecules, which can be exploited to favor charge transport along the direction of the wires.

■

(5) Horowitz, G.; Garnier, F.; Yassar, A.; Hajlaoui, R.; Kouki, F. Field-Effect Transistor Made with a Sexithiophene Single Crystal. Adv. Mater. 1996, 8, 52−54. (6) Liu, S.; Mannsfeld, S. C. B.; Wang, W. M.; Sun, Y.-S.; Stoltenberg, R. M.; Bao, Z. Patterning of α-Sexithiophene Single Crystals with Precisely Controlled Sizes and Shapes. Chem. Mater. 2009, 21, 15−17. (7) Loi, M. A.; da Como, E.; Dinelli, F.; Murgia, M.; Zamboni, R.; Biscarini, F.; Muccini, M. Supramolecular Organization in Ultra-Thin Films of α-Sexithiophene on Silicon Dioxide. Nat. Mater. 2004, 4, 81− 85. (8) Lorch, C.; Banerjee, R.; Frank, C.; Dieterle, J.; Hinderhofer, A.; Gerlach, A.; Schreiber, F. Growth of Competing Crystal Phases of αSexithiophene Studied by Real-Time in Situ X-Ray Scattering. J. Phys. Chem. C 2015, 119, 819−825. (9) Muccini, M.; Murgia, M.; Biscarini, F. Morphology Controlled Energy Transfer in Conjugated Molecular Thin Films. Adv. Mater. 2001, 13, 355−358. (10) Blumstengel, S.; Koch, N.; Sadofev, S.; Schäfer, P.; Glowatzki, H.; Johnson, R. L.; Rabe, J. P.; Henneberger, F. Interface Formation and Electronic Structure of α-Sexithiophene on ZnO. Appl. Phys. Lett. 2008, 92, 193303. (11) Cerminara, M.; Tubino, R.; Meinardi, F.; Ivanco, J.; Netzer, F. P.; Ramsey, M. G. Optical Properties of Sexithiophene Films Grown on Ordered and Disordered TiO2(110) Surfaces. Thin Solid Films 2008, 516, 4247−4251. (12) Haber, T.; Ivanco, J.; Ramsey, M. G.; Resel, R. Epitaxial Growth of Sexithiophene on TiO2(110). J. Cryst. Growth 2008, 310, 101−109. (13) Ikeda, S.; Kiguchi, M.; Yoshida, Y.; Yase, K.; Mitsunaga, T.; Inaba, K.; Saiki, K. Epitaxial Growth and Domain Coalescence of Sexithiophene Induced by the Steps on Cleaved KBr(001). J. Cryst. Growth 2004, 265, 296−301. (14) Garcia, R.; Tello, M.; Moulin, J. F.; Biscarini, F. Size and Shape Controlled Growth of Molecular Nanostructures on Silicon Oxide Templates. Nano Lett. 2004, 4, 1115−1119. (15) Albonetti, C.; Barbalinardo, M.; Milita, S.; Cavallini, M.; Liscio, F.; Moulin, J.-F.; Biscarini, F. Selective Growth of α-Sexithiophene by Using Silicon Oxides Patterns. Int. J. Mol. Sci. 2011, 12, 5719−5735. (16) Ikeda, S.; Saiki, K.; Tsutsui, K.; Edura, T.; Wada, Y.; Miyazoe, H.; Terashima, K.; Inaba, K.; Mitsunaga, T.; Shimada, T. Graphoepitaxy of Sexithiophene on Thermally Oxidized Silicon Surface with Artificial Periodic Grooves. Appl. Phys. Lett. 2006, 88, 251905. (17) Ikeda, S.; Saiki, K.; Wada, Y.; Inaba, K.; Ito, Y.; Kikuchi, H.; Terashima, K.; Shimada, T. Graphoepitaxy of Sexithiophene and Orientation Control by Surface Treatment. J. Appl. Phys. 2008, 103, 084313. (18) Thin Films: Heteroepitaxial Systems; Liu, W. K., Santos, M. B., Eds.; Series on Directions in Condensed Matter Physics; World Scientific: Singapore, 1999; Vol. 15. (19) Olmstead, M. A.; Uhrberg, R. I. G.; Bringans, R. D.; Bachrach, R. Z. Photoemission Study of Bonding at the CaF2-on-Si(111) Interface. Phys. Rev. B: Condens. Matter Mater. Phys. 1987, 35, 7526− 7532. (20) Rieger, D.; Himpsel, F. J.; Karlsson, U. O.; McFeely, F. R.; Morar, J. F.; Yarmoff, J. A. Electronic Structure of the CaF2/Si(111) Interface. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 34, 7295− 7306. (21) Tromp, R. M.; Reuter, M. C. Structure of the Si(111)-CaF2 Interface. Phys. Rev. Lett. 1988, 61, 1756−1759. (22) Pasquali, L.; D’Addato, S.; Selvaggi, G.; Nannarone, S.; Sokolov, N. S.; Suturin, S. M.; Zogg, H. Formation of CaF2 Nanostructures on Si(001). Nanotechnology 2001, 12, 403−408. (23) Pasquali, L.; Suturin, S. M.; Kaveev, A. K.; Ulin, V. P.; Sokolov, N. S.; Doyle, B. P.; Nannarone, S. Interface Chemistry and Epitaxial Growth Modes of SrF2 on Si(001). Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 075403. (24) Pasquali, L.; Suturin, S. M.; Ulin, V. P.; Sokolov, N. S.; Selvaggi, G.; Giglia, A.; Mahne, N.; Pedio, M.; Nannarone, S. Calcium Fluoride on Si(001): Adsorption Mechanisms and Epitaxial Growth Modes. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 45448.

AUTHOR INFORMATION

Corresponding Author

*L. Pasquali. E-mail: [email protected]. ORCID

Angelo Giglia: 0000-0002-1672-9029 Luca Pasquali: 0000-0003-0399-7240 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS T. Jiang thanks the Chinese Scholarship Council for his Ph.D. scholarship. This work was partly supported by Italian FAR 2015. The research leading to some of these results has received funding from the European Community’s Seventh Framework Program (FP7- IRSES-2009) grant agreement no. 247518 (ONDA). Parts of the experiments were carried out in proposal n. 20150163 at Elettra. S. Nannarone is acknowledged for useful discussions and suggestions.

■

REFERENCES

(1) Facchetti, A. Semiconductors for Organic Transistors. Mater. Today 2007, 10, 28−37. (2) Capelli, R.; Toffanin, S.; Generali, G.; Usta, H.; Facchetti, A.; Muccini, M. Organic Light-Emitting Transistors with an Efficiency That Outperforms the Equivalent Light-Emitting Diodes. Nat. Mater. 2010, 9, 496−503. (3) Yu, S.; Yi, M.; Ma, D. Copper Phthalocyanine Organic Thin-Film Transistors with Calcium Fluoride Gate Insulator. Semicond. Sci. Technol. 2006, 21, 1452−1454. (4) Rahimi, R.; Korakakis, D. Transport Mechanism in Ambipolar Pentacene Organic Thin Film Transistors with Lithium Fluoride Gate Dielectric. J. Appl. Phys. 2011, 110, 013702. G


Article

The Journal of Physical Chemistry C (25) Pasquali, L.; Suturin, S.; Sokolov, N.; Selvaggi, G.; D’Addato, S.; Nannarone, S. Electronic Properties of CaF2 Nanodimensional Islands on Si(001): An MDS and UPS Study. Nucl. Instrum. Methods Phys. Res., Sect. B 2002, 193, 474−479. (26) Sokolov, N. S.; Suturin, S. M.; Ulin, V. P.; Pasquali, L.; Selvaggi, G.; Nannarone, S. Initial Stages of MBE Growth and Formation of CaF2/Si(001) High-Temperature Interface. Appl. Surf. Sci. 2004, 234, 480−486. (27) Rauscher, H.; Jung, T.; Lin, J.-L.; Kirakosian, A.; Himpsel, F.; Rohr, U.; Müllen, K. One-Dimensional Confinement of Organic Molecules via Selective Adsorption on CaF1 versus CaF2. Chem. Phys. Lett. 1999, 303, 363−367. (28) Pasquali, L.; DeLuisa, A.; Nannarone, S. The UHV Experimental Chamber For Optical Measurements (Reflectivity and Absorption) and Angle Resolved Photoemission of the BEAR Beamline at ELETTRA. AIP Conf. Proc. 2003, 705, 1142−1145. (29) Nannarone, S.; Borgatti, F.; DeLuisa, A.; Doyle, B. P.; Gazzadi, G. C.; Giglia, A.; Finetti, P.; Mahne, N.; Pasquali, L.; Pedio, M.; et al. The BEAR Beamline at Elettra. AIP Conf. Proc. 2003, 705, 450−453. (30) Hermann, K.; Pettersson, L. G. M.; Casida, M. E.; Daul, C.; Goursot, A.; Koester, A.; Proynov, E.; St-Amant, A.; Salahub, D. R. Contributing authors Carravetta, V.; et al. StoBe-deMon Version 3.0; 2007. (31) Hammer, B.; Hansen, L. B.; Nørskov, J. K. Improved Adsorption Energetics within Density-Functional Theory Using Revised Perdew-Burke-Ernzerhof Functionals. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 7413−7421. (32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (33) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Optimization of Gaussian-Type Basis Sets for Local Spin Density Functional Calculations. Part I. Boron through Neon, Optimization Technique and Validation. Can. J. Chem. 1992, 70, 560−571. (34) Triguero, L.; Pettersson, L. G. M.; Ågren, H. Calculations of near-Edge X-Ray-Absorption Spectra of Gas-Phase and Chemisorbed Molecules by Means of Density-Functional and Transition-Potential Theory. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 8097− 8110. (35) Cavalleri, M.; Odelius, M.; Nordlund, D.; Nilsson, A.; Pettersson, L. G. M. Half or Full Core Hole in Density Functional Theory X-Ray Absorption Spectrum Calculations of Water? Phys. Chem. Chem. Phys. 2005, 7, 2854−2858. (36) Kolczewski, C.; Puttner, R.; Plashkevych, O.; Agren, H.; Staemmler, V.; Martins, M.; Snell, G.; Schlachter, A. S.; Sant’Anna, M.; Kaindl, G.; et al. Detailed Study of Pyridine at the C1s and N1s Ionization Thresholds: The Influence of the Vibrational Fine Structure. J. Chem. Phys. 2001, 115, 6426−6437. (37) Olmsead, M. A. Heteroepitaxy of Disparate Materials: From Chemisorption to Epitaxy in CaF2/Si(111). In Thin Films: Heteroepitaxial Systems; Liu, W. K., Santos, M. B., Eds.; Series on Directions in Condensed Matter Physics; World Scientific: Singapore, 1999; Vol. 15, pp 211−266. (38) Rotenberg, E.; Denlinger, J. D.; Leskovar, M.; Hessinger, U.; Olmstead, M. A. Layer-by-Layer Resolved Core-Level Shifts in CaF2 and SrF2 on Si(111): Theory and Experiment. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 11052−11069. (39) Ivanco, J.; Krenn, J. R.; Ramsey, M. G.; Netzer, F. P.; Haber, T.; Resel, R.; Haase, A.; Stadlober, B.; Jakopic, G. Sexithiophene Films on Clean and Oxidized Si(111) Surfaces: Growth and Electronic Structure. J. Appl. Phys. 2004, 96, 2716−2724. (40) Ohno, S.; Tanaka, H.; Tanaka, K.; Takahashi, K.; Tanaka, M. Sexithiophene Ultrathin Films on Passivated Si(001) Surfaces: Growth and Electronic Structure. Org. Electron. 2015, 25, 170−177. (41) Horowitz, G.; Bachet, B.; Yassar, A.; Lang, P.; Demanze, F.; Fave, J.-L.; Garnier, F. Growth and Characterization of Sexithiophene Single Crystals. Chem. Mater. 1995, 7, 1337−1341. (42) Porzio, W.; Destri, S.; Mascherpa, M.; Brückner, S. Structural Aspects of Oligothienyl Series from X-ray Powder Diffraction Data. Acta Polym. 1993, 44, 266−272.

(43) Tanuma, S.; Powell, C. J.; Penn, D. R. Calculations of Electron Inelastic Mean Free Paths (IMFPS). IV. Evaluation of Calculated IMFPs and of the Predictive IMFP Formula TPP-2 for Electron Energies between 50 and 2000 eV. Surf. Interface Anal. 1993, 20, 77− 89. (44) Heiner, C. E.; Dreyer, J.; Hertel, I. V.; Koch, N.; Ritze, H.-H.; Widdra, W.; Winter, B. Anisotropy in Ordered Sexithiophene Thin Films Studied by Angle-Resolved Photoemission Using Combined Laser and Synchrotron Radiation. Appl. Phys. Lett. 2005, 87, 093501. (45) Lögdlund, M.; Dannetun, P.; Fredriksson, C.; Salaneck, W. R.; Brédas, J. L. Theoretical and Experimental Studies of the Interaction between Sodium and Oligothiophenes. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 53, 16327−16333. (46) Chandekar, A.; Whitten, J. E. Ultraviolet Photoemission and Electron Loss Spectroscopy of Oligothiophene Films. Synth. Met. 2005, 150, 259−264. (47) Schwieger, T.; Liu, X.; Peisert, H.; Adolphi, B.; Kiriy, N.; Knupfer, M. Electronic Properties of Interfaces between Different Sexithiophenes and Gold. J. Appl. Phys. 2005, 97, 123712. (48) Murr, J.; Ziegler, C. Interaction of Na with Sexithiophene Thin Films. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57, 7299−7304. (49) Gao, Y.; Tiedje, T.; Wong, P. C.; Mitchell, K. A. R. X-RayAbsorption near-Edge Structure at the Fluorine K Edge in CaF2 and BaF2. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 15578− 15583. (50) Capelli, R.; Mahne, N.; Koshmak, K.; Giglia, A.; Doyle, B. P.; Mukherjee, S.; Nannarone, S.; Pasquali, L. Quantitative Resonant Soft X-Ray Reflectivity of Ultrathin Anisotropic Organic Layers: Simulation and Experiment of PTCDA on Au. J. Chem. Phys. 2016, 145, 024201. (51) Pasquali, L.; Mukherjee, S.; Terzi, F.; Giglia, A.; Mahne, N.; Koshmak, K.; Esaulov, V.; Toccafondi, C.; Canepa, M.; Nannarone, S. Structural and Electronic Properties of Anisotropic Ultrathin Organic Films from Dichroic Resonant Soft X-Ray Reflectivity. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 045401. (52) Stohr, J. Nexafs Spectroscopy; Springer: Berlin, Heidelberg, 1996. (53) Okajima, T.; Narioka, S.; Tanimura, S.; Hamano, K.; Kurata, T.; Uehara, Y.; Araki, T.; Ishii, H.; Ouchi, Y.; Seki, K.; et al. Study of Alpha-Sexithienyl Thin Film by Polarized near Edge X-Ray Absorption Fine Structure. Japanese J. Appl. Phys. Part 1-Regular Pap. Short Notes Rev. Pap. 1996, 35, 2822−2825. (54) Okajima, T.; Narioka, S.; Tanimura, S.; Hamano, K.; Kurata, T.; Uehara, Y.; Araki, T.; Ishii, H.; Ouchi, Y.; Seki, K.; et al. NEXAFS Spectroscopic Studies of Molecular Orientation in α-Sexithienyl Evaporated Thin Films on Metal Films. J. Electron Spectrosc. Relat. Phenom. 1996, 78, 379−382. (55) Aldahhak, H.; Schmidt, W. G.; Rauls, E. Single PTCDA Molecules on Planar and Stepped KCl and NaCl(100) Surfaces. Surf. Sci. 2015, 641, 278−281. (56) Guo, Q.; Paulheim, A.; Sokolowski, M.; Aldahhak, H.; Rauls, E.; Schmidt, W. G. Adsorption of PTCDA on Terraces and at Steps Sites of the KCl(100) Surface. J. Phys. Chem. C 2014, 118, 29911−29918.

H


Controlling In-Plane Isotropic and Anisotropic Orientation of Organic

Recommend Documents