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Growth morphologies of dihydro-tetraaza-acenes on c-plane sapphire
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Growth morphologies of dihydro-tetraaza-acenes on c-plane sapphire Aleksandar Matkovića, Aydan Çiçeka, Markus Kratzera, Benjamin Kaufmanna, Anthony Thomasb, ⁎ Zhongrui Chenb, Olivier Sirib, Conrad Beckerb, Christian Teichert ,a a b
Institute of Physics, Montanuniversität Leoben, Franz Josef Strasse 18, Leoben 8700, Austria Aix Marseille Univ., CNRS, CINaM UMR 7325, Campus de Luminy 13288 Marseille cedex 09, France
A R T I C LE I N FO
A B S T R A C T
Keywords: Oligoacene derivates Dihydro-tetraaza-acenes Organic thin films H-bonding DHTAP AFM
Dihydro-tetraaza-acenes are promising candidates for future applications in organic electronics, since these molecules form crystals through an interplay between H-bonding, dipolar, and van der Waals interactions. As a result, densely packed π − π structures – favorable for charge transport – are obtained, with exceptional stability under ambient conditions. This study investigates growth morphologies of dihydro-tetraaza-pentacene and dihydro-tetraaza-heptacene on vicinal c-plane sapphire. Submonolayers and thin films are grown using hot wall epitaxy, and the structures are investigated ex-situ by atomic force microscopy. Molecular arrangement, nucleation densities, sizes, shapes, and stability of the crystallites are analyzed as a function of the substrate temperature. The two molecular species were found to assume a different orientation of the molecules with respect to the substrate. An activation energy of (1.23 ± 0.12) eV was found for the nucleation of dihydrotetraaza-heptacene islands (composed of upright standing molecules), while (1.16 ± 0.25) eV was obtained for dihydro-tetraaza-pentacene needles (composed of lying molecules). The observed disparity in the temperature dependent nucleation densities of the two molecular species is attributed to the different thermalization pathways of the impinging molecules.
1. Introduction Since the discovery of conducting polymers [1], the field of organic semiconductors has been driven by many potential applications in consumer electronics. With emphasis on the synthesis of high performance materials and enhanced control over the morphology in the thin films, acenes and their derivates have been in the focus as the most promising candidates for future organic field-effect devices [2–8]. Pentacene stands as a paradigm for the application of acenes in organic electronics, due to its electronic properties [9,10] and high field-effect mobility [11]. A rather straight-forward way to enhance charge-transfer processes is to increase the length of the π-conjugated systems [4,12]. However, longer acenes are not stable and suffer from very low solubility as well as poor stability under ambient conditions, and show the tendency to dimerize [12–16]. Photooxidation with molecular oxygen and dimerization is the main degradation pathway for longer acenes [12,14,17]. Therefore, protecting groups are commonly employed to stabilize these molecules [12,14,18]. Adding functionalized groups to the back-bone of acenes has been a well established molecular design pathway for tuning packing, charge transport properties, and photophysical phenomena [5,8,15,19–24]. In particular, nitrogen containing heteroacenes open a vast field of ⁎
designed organic semiconductors with the possibilities to control intermolecular interactions – mainly through H-bonding – in favor of both stabilizing the thin film structures and obtaining the desired functionality [3,4,15,21,22,25–30]. By tuning the intermolecular interactions, densely packed systems with maximized π-overlap between neighboring molecules can be achieved [3,31–33]. Incorporation of functional groups commonly leads to H-bonding between neighboring molecules, which then governs intermolecular interactions and thus packing in the solid [15,24,27,30]. This is also the case in many natural systems, as natural origin based dyes and pigments which have H-bonded π-stacked organic molecules [34,35], with indigo as a representative [35,36]. The impact of H-bonding on the molecular packing can have a significant benefit on many of the technologically relevant parameters of the organic semiconductors. This was clearly demonstrated in the case of epindolidione that shows higher field-effect mobility, better on-off ratio, and much better long term air stability than its linear oligoacene analogue, tetracene [15]. The key role of H-bonding has also been demonstrated in bottom up approaches, as for organic nanostructure self-assembly [37]. 2D supramolecular clusters and chains on metal surfaces can create well organized, very complex, and intriguing patterns principally controlled by Hbonding [38–40].
corresponding author. E-mail address: [email protected] (C. Teichert).
https://doi.org/10.1016/j.susc.2018.03.009 Received 30 January 2018; Received in revised form 9 March 2018; Accepted 12 March 2018 0039-6028/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Matković, A., Surface Science (2018), https://doi.org/10.1016/j.susc.2018.03.009
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Although nitrogen containing oligoacene derivates are promising candidates for future organic electronics applications, little is known on the epitaxial growth processes of these molecules and the morphologies of monolayers and thin films [30]. These morphological features will surely have a striking impact on the device performance, through trap states at localized defects and grain boundaries. However, a study that investigates epitaxial growth morphologies of dihydro-tetraaza-acenes in thin films, on device relevant scales, and on technologically relevant dielectric substrates is greatly lacking. Here, we investigate the growth morphologies of dihydro-tetraazapentacene (DHTA5 – also known as DHTAP) [28–30] and dihydro-tetraaza-heptacene (DHTA7) [41] on sapphire, which is commonly used as a gate dielectric in field-effect devices, and as a substrate for thin film growth. Thin films – up to 5 monolayers (ML) – and sub-MLs (0.10.2 ML) are grown using hot wall epitaxy (HWE) [42], and the structures are investigated ex-situ by atomic force microscopy (AFM). Molecular arrangement, nucleation densities, sizes, and shapes of the crystallites are analyzed as a function of the substrate temperature (TD). Furthermore, ambient stability of the DHTA5 and DHTA7 crystallites is compared, indicating the relevance of not only H-bonding but also van der Waals interactions of the neighboring molecules on the stability.
Fig. 2. (a) and (b) presents the chemical structure of DHTA5 and DHTA7, also indicating the expected lengths of the molecules considering van der Waals radii of the outer hydrogen atoms. H-donor (N-H) and H-acceptor (N]C) sites are partially charged and respectively labeled with δ + and δ - signs. (c) and (d) show ball-and-stick models of DHTA5 and DHTA7 dimers confined in a plane. Dotted lines between the molecules indicate (N-H⋅⋅⋅N) intralayer hydrogen bonding.
2. Materials and methods
presented in Fig. 1(b), and a three-dimensional representation of a 250 × 250 nm2 area is shown in Fig. 1(c) with the high symmetry directions of sapphire indicated. Interestingly, the vicinal sapphire surface structure did not influence the growth of dihydro-tetraaza-acenes, and no correlations of crystallite morphologies with the supporting sapphire steps were observed. Vicinal sapphire was reported to introduce templating of pentacene, rubrene, and 3,4,9,10-perylenetetracarboxylicdianhydride [49–51]. However, in those cases much longer annealing time and at higher temperatures (e.g. 120 h at 1500 °C) resulted in multi-layer steps ranging from 2 nm to 4 nm, thus having a height comparable to the size of the molecules. This is not the case in the present study, where the step heights are about one order of magnitude smaller than the length of the molecules.
2.1. Substrate preparation: vicinal c-plane sapphire As substrates, c-plane sapphire platelets were used (SurfaceNet GmbH). Substrates were single side polished, with a miss-cut angle of 0.2-0.3° to the (0001) plane. In order to minimize any surface contaminations that might act as nucleation centers for the molecules, substrates were chemically cleaned by sonication in acetone and dried by nitrogen flow [43], followed by annealing in air at 1000 °C for 1 h with heat-up and cool-down ramps of 5 °C/min. Annealing of c-plane sapphire is well known to result in a vicinal surface [44–47]. Furthermore, it was demonstrated that heat treatment of the alumina surface introduces a substantial decrease in the number of surface -OH groups [48]. The cleaning and annealing procedure was carried out for each substrate immediately before the deposition of the molecules. The substrates were checked by AFM prior to the growth for obtaining a clearly resolved vicinal surface without contaminations. One example of a vicinal (0001) sapphire surface is shown in Fig. 1(a). Well-defined steps with height of ∼ 0.2 nm and width of ∼ 50 nm were found all across the 5 × 5 mm2 substrates. Steps follow the [1010] direction. A cross-section of four steps indicated by a solid line in Fig. 1(a) is
2.2. Dihydro-tetraaza-acene molecules Fig. 2(a,b) presents the chemical structure of DHTA5 and DHTA7 molecules, respectively. The expected length of the molecules – considering van der Waals radii of the outer hydrogen atoms – is also indicated in Fig. 2(a,b). Molecules were synthesized following the previously established routes [30,41]. The dihydropyrazine unit provides two additional electrons – compared to the parenting linear oligoacenes – that are delocalized over the entire molecule [41]. As a result, HOMO levels are shifted lower with respect to the Fermi level (and compared to the corresponding acenes) which was correlated to better air stability of the molecules [15]. Furthermore, in the case of DHTA5, H-donor (NH) and H-acceptor (N] C) sites – partially charged and respectively labeled with δ + and δ - in Fig. 2 – were shown to form H-bonds between the neighboring molecules [30], and accordingly the same is expected for DHTA7. The presence of intralayer H-bonds strengthens the molecular interactions, orients neighboring molecules into a headto-tail arrangement, and enables π − π stacking of the subsequent molecular layers [30]. Fig. 2(c,d) presents ball-and-stick models of DHTA5 and DHTA7 dimers confined in a plane, also indicating (NH⋅⋅⋅N) intralayer hydrogen bonding.
Fig. 1. (a) 1 × 1 µm2 AFM topography image of a c-plane sapphire substrate after annealing at 1000 °C for 1h, z scale 0.3 nm. (b) Height cross-section showing four subsequent sapphire steps - indicated by a solid line in (a) - leveled to have (0001) as horizontal plane and averaging 20 scan lines. (c) 250 × 250 nm2 AFM topography image, also with (0001) plane of sapphire as horizontal plane.
2.3. Deposition and growth morphology investigation DHTA5 and DHTA7 molecules were deposited using a HWE setup [42], with a base pressure of ∼ 8 × 10 −7 mbar. In the case of 2
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sub-MLs (0.1-0.2 ML) of DHTA7, source and wall temperatures of the HWE system were set to 580 K and 600 K, resulting in a growth rate of (0.35 ± 0.05) ML per hour. For multi-layer films of DHTA7, the temperatures of both source and wall heaters were increased by 10 K, yielding roughly a trippled deposition rate (1.05 ± 0.15) ML per hour. The deposition temperature (substrate temperature during the growth – TD) was varied between 310 K and 440 K, in the case of DHTA7. In the case of DHTA5, the temperatures for the source and wall heaters were set to 490 K and 510 K, respectively. The deposition rate was (1.5 ± 0.3) ML per hour, and TD was varied between 340 K and 380 K. A lower deposition rate of DHTA5 – that would be comparable to the deposition conditions of the DHTA7 molecules – was not achieved, since then much larger sample-to-sample fluctuations of the rate were observed. However, no significant difference in the crystallite morphologies were found at the growth attempts with lower rate. The surface coverage was estimated by considering the total deposited volume per unit area via AFM and comparing that to the volume equivalent of one complete ML of upright standing molecules. Morphology of the grown films was investigated ex-situ using AFM in tapping mode under ambient conditions. Asylum Research MFP-3D and Digital Instruments Nanoscope III systems were used. Olympus AC160TS and NuNano Scout350 probes were applied, with typical force constants of 20–80 N/m and tip curvature radii of 5–10 nm. AFM topography images of the samples were processed using the open source software Gwyddion (version 2.49) [52]. For the statistical analysis of the growth, several independent spots were measured on each sample, and for each spot multiple magnification topography images were acquired, ranging from 15 × 15 µm2 to 1 × 1 µm2.
Fig. 3. AN example of the island height analysis for DHTA7 deposited at 400 K on c-plane sapphire. (a) 2 × 2 µm2 (z scale 10 nm) AFM topography image showing several DHTA7 islands with different height (1L - first layer, 2L - second layer, 3L - third layer of upright DHTA7 molecules). (b) Histogram of (a) with Gaussian fits corresponding to substrate and first three layers of DHTA7 islands. Peak-to-peak distance of the Gaussian fits is used to estimate the height of each island layer.
Fig. 4. (a) The height of the 1L of DHTA7 as a function of TD. (b) Mean height of both DHTA5 and DHTA7 islands, averaged over the entire range of TD.
3. Results and discussion structure. The height of the islands was found not to depend on the number of layers either (see Fig. 4(b)), which indicates that intralayer H-bonds “lock” the structure [30], and interlayer interactions cannot introduce additional tilt of the molecules. In Fig. 4(b), an apparent increase in height for the first layer – of about 0.35 nm – both for DHTA5 and DHTA7 islands, is most likely a measurement artifact introduced by the material contrast (e.g. electrostatic effect) that exists between the first layer and the substrate. The height of the DHTA7 islands was found to be (1.94 ± 0.20) nm, while the height of DHTA5 islands was found to be (1.31 ± 0.20) nm, estimated from the second and third layer. These values indicate almost upright orientation of the molecules, within about 15° of tolerance. The height of DHAT5 islands well matches the length values reported from scanning tunneling microscopy measurements of flat-lying DHTA5 molecules [30], as well as the literature reports for upright standing islands of pentacene [49].
In the case of small rod-like molecules, a proper orientation of the πplanes with respect to the substrate is essential for the device functionality [6,53], since charge transport occurs most efficiently in the πstacking direction. Thus, having the molecules “flat-lying” with respect to the substrate will enhance vertical charge transport. The other option would be to have the molecules “upright” with respect to the substrate, favoring charge transport parallel to the substrate plane. The crystallite morphology can be a very clear indicator – without structural probes – for the arrangement of the molecules with respect to the substrate plane. In case when molecules assume an “upright” orientation, islandlike crystallites are formed commonly favoring layer-by-layer growth [53]. These features are easily distinguished, since then terraced structures are formed with the step height corresponding to the height of the molecules. Flat-lying molecules commonly form 3D structures (Volmer–Weber growth), and in the case of rod-like molecules, where the preferred growth direction is along overlapping π-orbitals, these structures will be elongated in one direction, resulting in needle-like crystallites [53]. In the case of DHTA5 and DHTA7 on cplane sapphire, both island-like and needle-like crystallites were observed.
3.2. Temperature dependent growth morphologies of DHTA7 Growth morphologies of DHTA7 have been analyzed as a function of TD in a range from 310 K to 440 K. Below 310 K, the crystallites were found to be too small for a reliable analysis, while above 422 K a relatively small amount of material was found on the surface due to reevaporation of the molecules from the surface. By changing TD surface diffusion of the molecules is changed [54–57]. Since source and wall heaters of the HWE system were kept fixed during these experiments, a constant flux of the incoming molecules can be assumed. Thus, the deposition rate should also be constant, unless reevaporation due to high substrate temperature starts to play an important role. Within the TD range from 310 K to 420 K, the surface coverage was found to be (0.15 ± 0.04) ML, indicating negligible desorption of the molecules from the substrate surface. For the temperatures above 420 K, the surface coverage was decreasing with an increase of TD, due to desorption of the molecules from the substrate surface. Majority of the crystallites (over 90%) form the first layer islands of DHTA7, except for the sample grown at 311 K, where ∼ 60% of the volume forms first layer islands. For this reason the sample at the lowest TD has been
3.1. Height of the island-like crystallites Focusing only on the island-like crystallites, the height of the islands was analyzed for all the samples where these crystallites were found. An example of the island height analysis is shown in Fig. 3(a,b), where a height histogram (b) of the AFM topography image (a) is fitted by Gaussian peaks. The height of each layer is analyzed as a peak-to-peak distance, while the uncertainty of the measured height is taken as a half-width-at-full-maximum of the considered peaks. In all cases subML samples have been considered, thus always having the substrate peak present in the histograms. No clear dependence of the island height on TD was found, as is shown in Fig. 4(a) for the height of the first layer of DHTA7 islands as a function of TD. This indicates that – in the considered range of TD – island-like crystallites have the same 3
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Fig. 5. (a)-(f) 1 × 1 µm2 images of (0.15 ± 0.04) ML of DHTA7 grown on sapphire (0001) in the TD range from 333 K to 422 K, z scales 5 nm. (g) Number of island nucleation centers per µm2, shown in logarithmic scale as a function of inverse TD. Black line shows the least-square fit where the slope is activation energy for island growth EA, excluding the two lowest temperature points due to hot precursor states. (h) Island compactness (mean island area divided by the mean island circumference square) as a function of TD. Inset of (h) illustrates a rhombus with a 60° angle, and its corresponding value of α = 54 × 10 −3 .
become more compact as TD increases. It is also observed that at higher temperatures the islands tend to form rather regular (diamond like) structures. The most likely reason for this is that due to high edge diffusion islands terminate at low index planes, thus revealing the structure of the unit cell of the DHTA7. The angle obtained between the edges of the more regular islands was found to be (61 ± 12)°. An inset in Fig. 5(h) shows a scheme of a rhombus with a 60° angle, and its corresponding value of α = 54 × 10 −3, which well matches the values for α obtained for the DHTA7 islands at higher TD.
omitted from the analysis of the nucleation density. An example for DHTA7 crystallite evolution with increasing TD is presented in Fig. 5(a–f), showing 1 × 1 µm2 AFM topography images. At the temperatures above 400 K, the areas were chosen to show islands of DHTA7, therefore are not representative for the actual surface coverage. Fig. 5(g) shows – in logarithmic scale – the number of DHTA7 island nucleation centers per µm2 (N) as a function of inverse TD. For each data point, five AFM topography images were analyzed ranging in size from 2 × 2 µm2 to 15 × 15 µm2 depending on the island size and density. The slope of a linear least-square fit to the data shown in Fig. 5(g) yields an activation energy of EA = (1.23 ± 0.12) eV for the nucleation of DHTA7 islands on c-plane sapphire. This is obtained by considering that N∝ exp(EA/kBT) [54–57], where kB stands for the Boltzmann constant. As can be seen from Fig. 5(g), lnN versus 1/TD clearly deviates from a linear behavior which would be expected if the impinging molecules would immediately assume substrate’s temperature and randomly diffuse across the surface [58]. In such a case, the slope is determined by the activation energies involved in the process. However, it was frequently observed that lnN versus 1/TD tends to deviate from a straight line (see [58] and the references therein). This behavior was explained by the fact that impinging molecules that have the temperature defined by the evaporation source (or walls) cannot immediately equilibrate to the substrate’s temperature and thus remain in a hot precursor state upon adsorption [58,59]. In such a case, the molecules have more energy than defined by the substrate’s temperature and can diffuse further, resulting in larger islands and a smaller island density than what would be expected if fast equilibration upon adsorption is assumed. This effect is more pronounced for larger differences between the temperature of the impinging molecules and the substrate. For this reason the linear fit to lnN versus 1/TD was carried out only for the four points at highest TD. Further, α = < A > / < C > 2 quantifies the ramified shape of the islands [60,61], and gives insight into the diffusion of the molecules along the rim of the growing islands. Here, < C > is the mean island circumference. As such, higher values of α relate to more densely packed islands, indicating higher diffusion along the island edges. Fig. 5(h) shows α as a function of TD, illustrating that the islands
3.3. Terraced mound formations in multi-layer DHTA7 films Samples with sub-ML coverage – between 0.1 ML and 0.8 ML of DHTA7 – have the majority of the molecules in the first layer, with only ∼ 10% of the islands consisting of the terraced structures which contain more than one layer of the molecules. This is a good indication that at least in the first layer a step-edge barrier is rather small [60], and the molecules arriving on the already grown islands can easily diffuse across the island edge onto the substrate and are incorporated into the first layer. However, in the case of the multi-layer films terraced mounds have been observed, clearly demonstrating the presence of a step-edge barrier [60]. One such example is shown in Fig. 6(a), with a nominal coverage of 4.5 ML grown at TD = 405 K, and the deposition rate (1.05 ± 0.15) ML per hour. In the case of the sample that is presented in Fig. 6(a), the third layer of the molecules is almost completely closed, while the fourth layer is coalescing between neighboring mounds, covering ∼ 86% of the substrate surface. On average, mounds consisting of ten layers were found. A height cross-section indicated by a solid line in Fig. 6(a) is shown in Fig. 6(b), with clearly observable constant height of the successive layers. The thickness of the film was determined by measuring the step-edge height on the areas where the DHTA7 film was scratched off the substrate, as presented in Fig. 6(c). The average mound separation (center-to-center) was found to be (5.8 ± 2.1) µm. Similar values were obtained for a coverage of ∼ 1.2 ML and ∼ 4.5 ML, which indicates that nucleation of the second layer dictates the formation of the mounds. The average mound separation was found to be almost an order of magnitude larger than the 4
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Fig. 6. (a) 6 × 6 µm2 AFM topography image (z scale 20 nm) of a terraced mound of DHTA7 ( ∼ 4.5 ML coverage, TD = 405 K). (b) Height cross-section marked by the solid line in (a), also indicating the number of the layers (right scale). (c) Step height cross-section (DHTA7 left, Al2O3 right) used to estimate the total coverage of DHTA7. (c) Inset, AFM topography image of the scratched edge of the film (10 × 10 µm2, z scale 30 nm).
at least five AFM topography images were used ranging in size from 2 × 2 µm2 to 15 × 15 µm2 depending on the crystallite size and density. Considering that the molecules within the needles are flat-lying means that the π-system of DHTA5 is exposed to the substrate, maximizing film-substrate interactions. In such case, the molecules commonly find rotationally commensurate states on the substrates as energetically most favored adsorption sites [53,63], giving preferred growth directions of the needles that arise from the interplay between individual adsorption sites and bulk packing of the molecules within the crystallites [63,66]. However, in the case of DHTA5 on c-plane sapphire, there was no evidence of the preferred growth directions of the needles. This indicates that the molecules cannot find a commensurate state on the substrate, although they assume a flat-lying orientation. Furthermore, many of the observed needles do not grow in a straight line and either have several kinks separated by straight segments, or appear – to the extent of the AFM resolution – as continually curved. This morphology feature could be explained considering that the growing needle-like crystallites are not in registry with the substrate, and thus any structural defect can alter the growth direction, without being “corrected for” by the molecule-substrate interaction. To the extent of this study, the reason why DHTA7 forms predominantly islands and DHTA5 form needles remains so far not completely clear. One possibility would be that for a larger molecule it is less-favored to stay on the surface as a flat-lying once the critical nucleus is formed, while smaller molecules can still remain in the flat lying configuration even after the formation of a stable nucleus. Another possible scenario would be related to the different source temperatures required to sublimate smaller DHAT5 and larger DHTA7 molecules. Namely, the source temperature is increased by 90 K for DHTA7 growth. Therefore, partial decomposition of the molecules within the HWE cell cannot be neglected anymore. This could introduce carbon contamination on the substrate surface, which could result in the change from flat-lying to up-right standing molecules as observed previously [67–69]. Besides needles, island-like crystallites of DHTA5 were also observed (see Fig. 7(b-d)). The height of the islands was analyzed in Section 3.1, implying that the islands consist of upright standing molecules. The volume per unit area of both island-like and needle-like crystallites as a function of TD is shown in Fig. 7(h). Needle-like crystallites were found in the entire considered range of TD, also at both lower and higher deposition rates, and at various surface coverages. Interestingly, islands were observed in a narrower TD range (354366 K), and were always found to nucleate along needles. In the case of lower coverage (0.1-0.2 ML), only needles were observed in the entire TD range. This indicates heterogeneous nucleation of the islands at already existing needles. Furthermore, islands of DHTA5 were found not to be stable under ambient conditions (see Section 3.5), while no significant changes were observed for the needle-like crystallites in the case of prolonged air exposure. To avoid any influence of the island
average island separation in the case of the samples with sub-ML coverage, grown at the same TD and with the same deposition rate. This indicates that the diffusion coefficient for DHTA7 molecules on an upright layer of DHTA7 is much larger than that on sapphire. Further, islands become more compact in higher layers. The inset of Fig. 6(b) shows α as the function of the terrace number for the mound formation presented in Fig. 6(a). These results imply that the number of kink sites at the step circumference is reducing with the number of layers. Nonuniform circumference and the presence of kink sites have been shown to reduce the step-edge barrier in the case of metal homoepitaxy [61,62], which is also a likely scenario for the DHTA7 molecules.
3.4. Temperature dependent growth morphologies of DHTA5 In a similar manner as for DHTA7, a temperature series of DHTA5 samples was examined, within a range of TD between 340 K and 380 K. Below 340 K the crystallites were too small for a reliable analysis, while above 372 K almost no material was deposited due to reevaporation. In the TD range between 354 K and 366 K, the samples had a surface coverage of (0.5 ± 0.1) ML. Interestingly, unlike DHTA7 which predominantly forms island-like crystallites, DHTA5 molecules mainly form needle-like crystallites, implying that the molecules assume a “flat-lying” orientation on the substrate. Similar needle-like crystallites were also observed in the case of DHTA7, but only at high coverage and higher growth rates than presented in Section 3.2. An example of the evolution of the DHTA5 crystallite morphology with an increase of TD is shown in Fig. 7(a–e). Frequently these needles are not straight, as usually observed for oligophenylenes [53,63,64], but kinked or constantly curved. They also vary in width and height. These peculiarities are addressed later. Fig. 7(f) shows in logarithmic scale the number of nucleation centers of DHTA5 needle-like crystallites per unit area, as a function of inverse TD. Unlike the case of DHTA7 islands (see Section 3.2), lnN versus 1/TD follows a straight line for DHTA5 needles. This does not indicate that the molecules impinging on the surface are meditatively equilibrating their potential energy with the substrate [58]. Since the needle-like crystallites have their growth front only at the needle apexes, the molecules will most likely reflect many times from the needle side walls before they are incorporated into the crystallite. As a result, impinging molecules will thermalize to the substrate’s temperature thus yielding a straight line in lnN versus 1/TD curve. In Fig. 7(f) the slope of a least-square fit to the experimental data gives EA = (1.16 ± 0.25) eV. These results would imply that DHTA5 molecules have rather large probability for the diffusion on sapphire surface, and that nucleation is governed by the attachment limited aggregation [65]. Further, Fig. 7(g) shows that both average length of the needles < l > and average volume of the needles < V > increases with TD, indicating a three-dimensional growth of the crystallites. For each data point used in the statistical analysis presented in Fig. 7(f-h), 5
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Fig. 7. (a-e) 5 × 5 µm2 (z scale 10 nm) AFM topography images of DHTA5 grown in the TD range from 343 K to 372 K. (f) Number of DHAT5 needle-like crystallite nucleation centers per µm2, shown in logarithmic scale as a function of inverse TD, where the slope of the linear fit gives activation energy (EA) for the aggregation of the molecules on the c-plane sapphire. (g) An average length of the needles (left), and average needle volume (right), as a function of TD. (h) Volume nm3 per µm2 of DHTA5 needles (solid line) and island (dashed line) as a function of TD.
Fig. 8(a–c), and even up to several months. However, DHTA5 islands were found to degrade rather fast once the sample had been exposed to ambient air. Within the first several hours, there are noticeable changes on the island’s surface (perturbations, holes, trenches) while after 3–4 days the height of the islands reduces from (1.65 ± 0.20) nm to (0.50 ± 0.40) nm. Fig. 8(d,e) presents the same sample area measured immediately after the growth, and after 4 days of air exposure. Height cross sections – shown in Fig. 8(f) – illustrate the change of island’s height. In addition, a video file that follows with AFM the degradation of DHTA5 island over the time span of four days is available in supplementary data (link). Prolonged exposure does not introduce any further change in the island’s morphology. Needle-like crystallites of DHTA5 were found to be unaffected by air exposure. In the case when DHTA5 samples were left in high vacuum (below 1 × 10 −6 mbar) for several days, the degradation of the islands started
degradation, all the images that were used to evaluate the volume per unit area of DHTA5 islands (Fig. 7(h)) were measured within the first two hours from the point of air exposure of the samples.
3.5. Stability of crystallites under ambient conditions Forming H-bonds between the neighboring molecules [30] is expected to provide better air stability of both individual molecules and crystallites [12,14,18], finally yielding stable electrical properties of the thin films [15]. This was indeed observed for all the crystallites that were analyzed in this study, with the exception of DHTA5 islands. Fig. 8(a–c) compares DHTA7 islands, grown at TD = 400 K, with DHTA5 islands and needles grown at TD = 360 K (Fig. 8(d–f)). No significant changes in crystallite morphologies of DHTA7 islands were detected in the case of air exposure, up to 45 days as shown in
Fig. 8. (a) and (b) 5 × 5 µm2 (z scale 7 nm) AFM topography images of DHTA7, as grown (day 1) and after 45 days of the sample being exposed to ambient air. Note that (a) and (b) do not show the exact same sample area. (c) AFM height cross sections (indicated by solid lines in (a) and (b)) showing negligible change of island height. (d) and (e) 5 × 5 µm2 (z scale 7 nm) AFM topography images of DHTA5, as grown (day 1) and after 4 days of the sample being exposed to ambient air. (f) AFM height cross sections of the same DHTA5 island (indicated by solid lines in (d) and (e)), showing a significant reduction in island height (indicated by the bold arrows). The horizontal arrow points to a DHTA5 needle in the middle of both cross sections.
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again from the point of air exposure of the samples. This does indicate that interaction with ambient air (most likely polar water molecules) introduces the observed morphological changes in the case of DHTA5 islands. An influence of exposure to moisture on the crystallite structure has been reported for para-phenylenes on muscovite mica [70] as well as a reorganization of the crystallites upon exposure to ambient conditions [67]. Understanding why only these crystallites degrade upon ambient exposure – and the exact degradation pathway for these crystallites – goes beyond the scope of this study. Compared to air stable DHTA7 islands, DHTA5 islands are expected to have mainly weaker vdW interactions between H-bonded molecular rows. As mentioned in the Section 2.1, annealing of the sapphire substrate is expected to reduce the amount of surface -OH groups. It is highly likely that exposure to the ambient conditions after the growth reintroduces formation of surface -OH groups, which could be a reason for the dewetting of DHTA5 islands. In addition, steps in the sapphire surface are expected to introduce stacking folds at the molecular pairs above and below the step edge. Being that DHTA5 is a shorter molecule than DHTA7, relatively greater area of the π-network will be exposed at these steps, which could be enough to destabilize the DHTA5 islands in ambient conditions, but not DHTA7. The studies on the influence of TiO2 atomic steps on the growth and stability of parahexaphenyl reported island-like crystallite degradation through the above mentioned mechanism [71,72].
[2]
[3] [4] [5]
[6]
[7] [8]
[9] [10]
[11]
[12] [13]
[14]
4. Summary [15]
In summary, we have investigated the growth morphologies of dipolar DHTA5 and DHTA7 molecules on vicinal (0001) sapphire surface. The longer – DHTA7 – molecules were found to form predominantly island-like crystallites consisting of up-right standing molecules, exhibiting quasi layer-by-layer (terraced mound) growth. The shorter – DHTA5 – molecules were found to mainly form 3D needle-like crystallites, favoring Volmer-Weber growth. Further, DHTA5 island-like crystallites were the only observed structure that was not stable upon ambient exposure. The morphology of both DHTA7 islands and DHTA5 needles was found to be stable in ambient air. Dihydro-tetraaza-acenes are promising candidates for future applications in organic electronics, since these molecules form crystals through an interplay between H-bonding, dipolar, and van der Waals interactions. As a result, densely packed π − π structures – favorable for charge transport – are obtained, with exceptional stability under ambient conditions. Morphological features of the films as orientation of the molecules with respect to the substrate plane, grain boundaries, and grain sizes can have a dominant impact on the device performance. Therefore, understanding the growth morphologies of monolayers and thin films is an important step towards realization of efficient devices based on these novel oligoacene derivates.
[16] [17]
[18] [19] [20]
[21] [22] [23] [24]
[25] [26] [27]
Acknowledgments
[28]
This work has been supported by Austrian Science Fund (FWF Der Wissenschaftsfonds) through project I 1788-N20 and by Agence Nationale de la Recherche (ANR) through grant ANR 14 CE34 0003 01. The authors would like to thank Andreas Egger (Montanuniversität Leoben, Austria) for the help in preparation of the sapphire substrates.
[29] [30]
[31]
Supplementary material [32]
Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.susc.2018.03.009.
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Growth morphologies of dihydro-tetraaza-acenes on c-plane sapphire Aleksandar Matkovića, Aydan Çiçeka, Markus Kratzera, Benjamin Kaufmanna, Anthony Thomasb, ⁎ Zhongrui Chenb, Olivier Sirib, Conrad Beckerb, Christian Teichert ,a a b
Institute of Physics, Montanuniversität Leoben, Franz Josef Strasse 18, Leoben 8700, Austria Aix Marseille Univ., CNRS, CINaM UMR 7325, Campus de Luminy 13288 Marseille cedex 09, France
A R T I C LE I N FO
A B S T R A C T
Keywords: Oligoacene derivates Dihydro-tetraaza-acenes Organic thin films H-bonding DHTAP AFM
Dihydro-tetraaza-acenes are promising candidates for future applications in organic electronics, since these molecules form crystals through an interplay between H-bonding, dipolar, and van der Waals interactions. As a result, densely packed π − π structures – favorable for charge transport – are obtained, with exceptional stability under ambient conditions. This study investigates growth morphologies of dihydro-tetraaza-pentacene and dihydro-tetraaza-heptacene on vicinal c-plane sapphire. Submonolayers and thin films are grown using hot wall epitaxy, and the structures are investigated ex-situ by atomic force microscopy. Molecular arrangement, nucleation densities, sizes, shapes, and stability of the crystallites are analyzed as a function of the substrate temperature. The two molecular species were found to assume a different orientation of the molecules with respect to the substrate. An activation energy of (1.23 ± 0.12) eV was found for the nucleation of dihydrotetraaza-heptacene islands (composed of upright standing molecules), while (1.16 ± 0.25) eV was obtained for dihydro-tetraaza-pentacene needles (composed of lying molecules). The observed disparity in the temperature dependent nucleation densities of the two molecular species is attributed to the different thermalization pathways of the impinging molecules.
1. Introduction Since the discovery of conducting polymers [1], the field of organic semiconductors has been driven by many potential applications in consumer electronics. With emphasis on the synthesis of high performance materials and enhanced control over the morphology in the thin films, acenes and their derivates have been in the focus as the most promising candidates for future organic field-effect devices [2–8]. Pentacene stands as a paradigm for the application of acenes in organic electronics, due to its electronic properties [9,10] and high field-effect mobility [11]. A rather straight-forward way to enhance charge-transfer processes is to increase the length of the π-conjugated systems [4,12]. However, longer acenes are not stable and suffer from very low solubility as well as poor stability under ambient conditions, and show the tendency to dimerize [12–16]. Photooxidation with molecular oxygen and dimerization is the main degradation pathway for longer acenes [12,14,17]. Therefore, protecting groups are commonly employed to stabilize these molecules [12,14,18]. Adding functionalized groups to the back-bone of acenes has been a well established molecular design pathway for tuning packing, charge transport properties, and photophysical phenomena [5,8,15,19–24]. In particular, nitrogen containing heteroacenes open a vast field of ⁎
designed organic semiconductors with the possibilities to control intermolecular interactions – mainly through H-bonding – in favor of both stabilizing the thin film structures and obtaining the desired functionality [3,4,15,21,22,25–30]. By tuning the intermolecular interactions, densely packed systems with maximized π-overlap between neighboring molecules can be achieved [3,31–33]. Incorporation of functional groups commonly leads to H-bonding between neighboring molecules, which then governs intermolecular interactions and thus packing in the solid [15,24,27,30]. This is also the case in many natural systems, as natural origin based dyes and pigments which have H-bonded π-stacked organic molecules [34,35], with indigo as a representative [35,36]. The impact of H-bonding on the molecular packing can have a significant benefit on many of the technologically relevant parameters of the organic semiconductors. This was clearly demonstrated in the case of epindolidione that shows higher field-effect mobility, better on-off ratio, and much better long term air stability than its linear oligoacene analogue, tetracene [15]. The key role of H-bonding has also been demonstrated in bottom up approaches, as for organic nanostructure self-assembly [37]. 2D supramolecular clusters and chains on metal surfaces can create well organized, very complex, and intriguing patterns principally controlled by Hbonding [38–40].
corresponding author. E-mail address: [email protected] (C. Teichert).
https://doi.org/10.1016/j.susc.2018.03.009 Received 30 January 2018; Received in revised form 9 March 2018; Accepted 12 March 2018 0039-6028/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Matković, A., Surface Science (2018), https://doi.org/10.1016/j.susc.2018.03.009
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Although nitrogen containing oligoacene derivates are promising candidates for future organic electronics applications, little is known on the epitaxial growth processes of these molecules and the morphologies of monolayers and thin films [30]. These morphological features will surely have a striking impact on the device performance, through trap states at localized defects and grain boundaries. However, a study that investigates epitaxial growth morphologies of dihydro-tetraaza-acenes in thin films, on device relevant scales, and on technologically relevant dielectric substrates is greatly lacking. Here, we investigate the growth morphologies of dihydro-tetraazapentacene (DHTA5 – also known as DHTAP) [28–30] and dihydro-tetraaza-heptacene (DHTA7) [41] on sapphire, which is commonly used as a gate dielectric in field-effect devices, and as a substrate for thin film growth. Thin films – up to 5 monolayers (ML) – and sub-MLs (0.10.2 ML) are grown using hot wall epitaxy (HWE) [42], and the structures are investigated ex-situ by atomic force microscopy (AFM). Molecular arrangement, nucleation densities, sizes, and shapes of the crystallites are analyzed as a function of the substrate temperature (TD). Furthermore, ambient stability of the DHTA5 and DHTA7 crystallites is compared, indicating the relevance of not only H-bonding but also van der Waals interactions of the neighboring molecules on the stability.
Fig. 2. (a) and (b) presents the chemical structure of DHTA5 and DHTA7, also indicating the expected lengths of the molecules considering van der Waals radii of the outer hydrogen atoms. H-donor (N-H) and H-acceptor (N]C) sites are partially charged and respectively labeled with δ + and δ - signs. (c) and (d) show ball-and-stick models of DHTA5 and DHTA7 dimers confined in a plane. Dotted lines between the molecules indicate (N-H⋅⋅⋅N) intralayer hydrogen bonding.
2. Materials and methods
presented in Fig. 1(b), and a three-dimensional representation of a 250 × 250 nm2 area is shown in Fig. 1(c) with the high symmetry directions of sapphire indicated. Interestingly, the vicinal sapphire surface structure did not influence the growth of dihydro-tetraaza-acenes, and no correlations of crystallite morphologies with the supporting sapphire steps were observed. Vicinal sapphire was reported to introduce templating of pentacene, rubrene, and 3,4,9,10-perylenetetracarboxylicdianhydride [49–51]. However, in those cases much longer annealing time and at higher temperatures (e.g. 120 h at 1500 °C) resulted in multi-layer steps ranging from 2 nm to 4 nm, thus having a height comparable to the size of the molecules. This is not the case in the present study, where the step heights are about one order of magnitude smaller than the length of the molecules.
2.1. Substrate preparation: vicinal c-plane sapphire As substrates, c-plane sapphire platelets were used (SurfaceNet GmbH). Substrates were single side polished, with a miss-cut angle of 0.2-0.3° to the (0001) plane. In order to minimize any surface contaminations that might act as nucleation centers for the molecules, substrates were chemically cleaned by sonication in acetone and dried by nitrogen flow [43], followed by annealing in air at 1000 °C for 1 h with heat-up and cool-down ramps of 5 °C/min. Annealing of c-plane sapphire is well known to result in a vicinal surface [44–47]. Furthermore, it was demonstrated that heat treatment of the alumina surface introduces a substantial decrease in the number of surface -OH groups [48]. The cleaning and annealing procedure was carried out for each substrate immediately before the deposition of the molecules. The substrates were checked by AFM prior to the growth for obtaining a clearly resolved vicinal surface without contaminations. One example of a vicinal (0001) sapphire surface is shown in Fig. 1(a). Well-defined steps with height of ∼ 0.2 nm and width of ∼ 50 nm were found all across the 5 × 5 mm2 substrates. Steps follow the [1010] direction. A cross-section of four steps indicated by a solid line in Fig. 1(a) is
2.2. Dihydro-tetraaza-acene molecules Fig. 2(a,b) presents the chemical structure of DHTA5 and DHTA7 molecules, respectively. The expected length of the molecules – considering van der Waals radii of the outer hydrogen atoms – is also indicated in Fig. 2(a,b). Molecules were synthesized following the previously established routes [30,41]. The dihydropyrazine unit provides two additional electrons – compared to the parenting linear oligoacenes – that are delocalized over the entire molecule [41]. As a result, HOMO levels are shifted lower with respect to the Fermi level (and compared to the corresponding acenes) which was correlated to better air stability of the molecules [15]. Furthermore, in the case of DHTA5, H-donor (NH) and H-acceptor (N] C) sites – partially charged and respectively labeled with δ + and δ - in Fig. 2 – were shown to form H-bonds between the neighboring molecules [30], and accordingly the same is expected for DHTA7. The presence of intralayer H-bonds strengthens the molecular interactions, orients neighboring molecules into a headto-tail arrangement, and enables π − π stacking of the subsequent molecular layers [30]. Fig. 2(c,d) presents ball-and-stick models of DHTA5 and DHTA7 dimers confined in a plane, also indicating (NH⋅⋅⋅N) intralayer hydrogen bonding.
Fig. 1. (a) 1 × 1 µm2 AFM topography image of a c-plane sapphire substrate after annealing at 1000 °C for 1h, z scale 0.3 nm. (b) Height cross-section showing four subsequent sapphire steps - indicated by a solid line in (a) - leveled to have (0001) as horizontal plane and averaging 20 scan lines. (c) 250 × 250 nm2 AFM topography image, also with (0001) plane of sapphire as horizontal plane.
2.3. Deposition and growth morphology investigation DHTA5 and DHTA7 molecules were deposited using a HWE setup [42], with a base pressure of ∼ 8 × 10 −7 mbar. In the case of 2
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sub-MLs (0.1-0.2 ML) of DHTA7, source and wall temperatures of the HWE system were set to 580 K and 600 K, resulting in a growth rate of (0.35 ± 0.05) ML per hour. For multi-layer films of DHTA7, the temperatures of both source and wall heaters were increased by 10 K, yielding roughly a trippled deposition rate (1.05 ± 0.15) ML per hour. The deposition temperature (substrate temperature during the growth – TD) was varied between 310 K and 440 K, in the case of DHTA7. In the case of DHTA5, the temperatures for the source and wall heaters were set to 490 K and 510 K, respectively. The deposition rate was (1.5 ± 0.3) ML per hour, and TD was varied between 340 K and 380 K. A lower deposition rate of DHTA5 – that would be comparable to the deposition conditions of the DHTA7 molecules – was not achieved, since then much larger sample-to-sample fluctuations of the rate were observed. However, no significant difference in the crystallite morphologies were found at the growth attempts with lower rate. The surface coverage was estimated by considering the total deposited volume per unit area via AFM and comparing that to the volume equivalent of one complete ML of upright standing molecules. Morphology of the grown films was investigated ex-situ using AFM in tapping mode under ambient conditions. Asylum Research MFP-3D and Digital Instruments Nanoscope III systems were used. Olympus AC160TS and NuNano Scout350 probes were applied, with typical force constants of 20–80 N/m and tip curvature radii of 5–10 nm. AFM topography images of the samples were processed using the open source software Gwyddion (version 2.49) [52]. For the statistical analysis of the growth, several independent spots were measured on each sample, and for each spot multiple magnification topography images were acquired, ranging from 15 × 15 µm2 to 1 × 1 µm2.
Fig. 3. AN example of the island height analysis for DHTA7 deposited at 400 K on c-plane sapphire. (a) 2 × 2 µm2 (z scale 10 nm) AFM topography image showing several DHTA7 islands with different height (1L - first layer, 2L - second layer, 3L - third layer of upright DHTA7 molecules). (b) Histogram of (a) with Gaussian fits corresponding to substrate and first three layers of DHTA7 islands. Peak-to-peak distance of the Gaussian fits is used to estimate the height of each island layer.
Fig. 4. (a) The height of the 1L of DHTA7 as a function of TD. (b) Mean height of both DHTA5 and DHTA7 islands, averaged over the entire range of TD.
3. Results and discussion structure. The height of the islands was found not to depend on the number of layers either (see Fig. 4(b)), which indicates that intralayer H-bonds “lock” the structure [30], and interlayer interactions cannot introduce additional tilt of the molecules. In Fig. 4(b), an apparent increase in height for the first layer – of about 0.35 nm – both for DHTA5 and DHTA7 islands, is most likely a measurement artifact introduced by the material contrast (e.g. electrostatic effect) that exists between the first layer and the substrate. The height of the DHTA7 islands was found to be (1.94 ± 0.20) nm, while the height of DHTA5 islands was found to be (1.31 ± 0.20) nm, estimated from the second and third layer. These values indicate almost upright orientation of the molecules, within about 15° of tolerance. The height of DHAT5 islands well matches the length values reported from scanning tunneling microscopy measurements of flat-lying DHTA5 molecules [30], as well as the literature reports for upright standing islands of pentacene [49].
In the case of small rod-like molecules, a proper orientation of the πplanes with respect to the substrate is essential for the device functionality [6,53], since charge transport occurs most efficiently in the πstacking direction. Thus, having the molecules “flat-lying” with respect to the substrate will enhance vertical charge transport. The other option would be to have the molecules “upright” with respect to the substrate, favoring charge transport parallel to the substrate plane. The crystallite morphology can be a very clear indicator – without structural probes – for the arrangement of the molecules with respect to the substrate plane. In case when molecules assume an “upright” orientation, islandlike crystallites are formed commonly favoring layer-by-layer growth [53]. These features are easily distinguished, since then terraced structures are formed with the step height corresponding to the height of the molecules. Flat-lying molecules commonly form 3D structures (Volmer–Weber growth), and in the case of rod-like molecules, where the preferred growth direction is along overlapping π-orbitals, these structures will be elongated in one direction, resulting in needle-like crystallites [53]. In the case of DHTA5 and DHTA7 on cplane sapphire, both island-like and needle-like crystallites were observed.
3.2. Temperature dependent growth morphologies of DHTA7 Growth morphologies of DHTA7 have been analyzed as a function of TD in a range from 310 K to 440 K. Below 310 K, the crystallites were found to be too small for a reliable analysis, while above 422 K a relatively small amount of material was found on the surface due to reevaporation of the molecules from the surface. By changing TD surface diffusion of the molecules is changed [54–57]. Since source and wall heaters of the HWE system were kept fixed during these experiments, a constant flux of the incoming molecules can be assumed. Thus, the deposition rate should also be constant, unless reevaporation due to high substrate temperature starts to play an important role. Within the TD range from 310 K to 420 K, the surface coverage was found to be (0.15 ± 0.04) ML, indicating negligible desorption of the molecules from the substrate surface. For the temperatures above 420 K, the surface coverage was decreasing with an increase of TD, due to desorption of the molecules from the substrate surface. Majority of the crystallites (over 90%) form the first layer islands of DHTA7, except for the sample grown at 311 K, where ∼ 60% of the volume forms first layer islands. For this reason the sample at the lowest TD has been
3.1. Height of the island-like crystallites Focusing only on the island-like crystallites, the height of the islands was analyzed for all the samples where these crystallites were found. An example of the island height analysis is shown in Fig. 3(a,b), where a height histogram (b) of the AFM topography image (a) is fitted by Gaussian peaks. The height of each layer is analyzed as a peak-to-peak distance, while the uncertainty of the measured height is taken as a half-width-at-full-maximum of the considered peaks. In all cases subML samples have been considered, thus always having the substrate peak present in the histograms. No clear dependence of the island height on TD was found, as is shown in Fig. 4(a) for the height of the first layer of DHTA7 islands as a function of TD. This indicates that – in the considered range of TD – island-like crystallites have the same 3
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Fig. 5. (a)-(f) 1 × 1 µm2 images of (0.15 ± 0.04) ML of DHTA7 grown on sapphire (0001) in the TD range from 333 K to 422 K, z scales 5 nm. (g) Number of island nucleation centers per µm2, shown in logarithmic scale as a function of inverse TD. Black line shows the least-square fit where the slope is activation energy for island growth EA, excluding the two lowest temperature points due to hot precursor states. (h) Island compactness (mean island area divided by the mean island circumference square) as a function of TD. Inset of (h) illustrates a rhombus with a 60° angle, and its corresponding value of α = 54 × 10 −3 .
become more compact as TD increases. It is also observed that at higher temperatures the islands tend to form rather regular (diamond like) structures. The most likely reason for this is that due to high edge diffusion islands terminate at low index planes, thus revealing the structure of the unit cell of the DHTA7. The angle obtained between the edges of the more regular islands was found to be (61 ± 12)°. An inset in Fig. 5(h) shows a scheme of a rhombus with a 60° angle, and its corresponding value of α = 54 × 10 −3, which well matches the values for α obtained for the DHTA7 islands at higher TD.
omitted from the analysis of the nucleation density. An example for DHTA7 crystallite evolution with increasing TD is presented in Fig. 5(a–f), showing 1 × 1 µm2 AFM topography images. At the temperatures above 400 K, the areas were chosen to show islands of DHTA7, therefore are not representative for the actual surface coverage. Fig. 5(g) shows – in logarithmic scale – the number of DHTA7 island nucleation centers per µm2 (N) as a function of inverse TD. For each data point, five AFM topography images were analyzed ranging in size from 2 × 2 µm2 to 15 × 15 µm2 depending on the island size and density. The slope of a linear least-square fit to the data shown in Fig. 5(g) yields an activation energy of EA = (1.23 ± 0.12) eV for the nucleation of DHTA7 islands on c-plane sapphire. This is obtained by considering that N∝ exp(EA/kBT) [54–57], where kB stands for the Boltzmann constant. As can be seen from Fig. 5(g), lnN versus 1/TD clearly deviates from a linear behavior which would be expected if the impinging molecules would immediately assume substrate’s temperature and randomly diffuse across the surface [58]. In such a case, the slope is determined by the activation energies involved in the process. However, it was frequently observed that lnN versus 1/TD tends to deviate from a straight line (see [58] and the references therein). This behavior was explained by the fact that impinging molecules that have the temperature defined by the evaporation source (or walls) cannot immediately equilibrate to the substrate’s temperature and thus remain in a hot precursor state upon adsorption [58,59]. In such a case, the molecules have more energy than defined by the substrate’s temperature and can diffuse further, resulting in larger islands and a smaller island density than what would be expected if fast equilibration upon adsorption is assumed. This effect is more pronounced for larger differences between the temperature of the impinging molecules and the substrate. For this reason the linear fit to lnN versus 1/TD was carried out only for the four points at highest TD. Further, α = < A > / < C > 2 quantifies the ramified shape of the islands [60,61], and gives insight into the diffusion of the molecules along the rim of the growing islands. Here, < C > is the mean island circumference. As such, higher values of α relate to more densely packed islands, indicating higher diffusion along the island edges. Fig. 5(h) shows α as a function of TD, illustrating that the islands
3.3. Terraced mound formations in multi-layer DHTA7 films Samples with sub-ML coverage – between 0.1 ML and 0.8 ML of DHTA7 – have the majority of the molecules in the first layer, with only ∼ 10% of the islands consisting of the terraced structures which contain more than one layer of the molecules. This is a good indication that at least in the first layer a step-edge barrier is rather small [60], and the molecules arriving on the already grown islands can easily diffuse across the island edge onto the substrate and are incorporated into the first layer. However, in the case of the multi-layer films terraced mounds have been observed, clearly demonstrating the presence of a step-edge barrier [60]. One such example is shown in Fig. 6(a), with a nominal coverage of 4.5 ML grown at TD = 405 K, and the deposition rate (1.05 ± 0.15) ML per hour. In the case of the sample that is presented in Fig. 6(a), the third layer of the molecules is almost completely closed, while the fourth layer is coalescing between neighboring mounds, covering ∼ 86% of the substrate surface. On average, mounds consisting of ten layers were found. A height cross-section indicated by a solid line in Fig. 6(a) is shown in Fig. 6(b), with clearly observable constant height of the successive layers. The thickness of the film was determined by measuring the step-edge height on the areas where the DHTA7 film was scratched off the substrate, as presented in Fig. 6(c). The average mound separation (center-to-center) was found to be (5.8 ± 2.1) µm. Similar values were obtained for a coverage of ∼ 1.2 ML and ∼ 4.5 ML, which indicates that nucleation of the second layer dictates the formation of the mounds. The average mound separation was found to be almost an order of magnitude larger than the 4
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Fig. 6. (a) 6 × 6 µm2 AFM topography image (z scale 20 nm) of a terraced mound of DHTA7 ( ∼ 4.5 ML coverage, TD = 405 K). (b) Height cross-section marked by the solid line in (a), also indicating the number of the layers (right scale). (c) Step height cross-section (DHTA7 left, Al2O3 right) used to estimate the total coverage of DHTA7. (c) Inset, AFM topography image of the scratched edge of the film (10 × 10 µm2, z scale 30 nm).
at least five AFM topography images were used ranging in size from 2 × 2 µm2 to 15 × 15 µm2 depending on the crystallite size and density. Considering that the molecules within the needles are flat-lying means that the π-system of DHTA5 is exposed to the substrate, maximizing film-substrate interactions. In such case, the molecules commonly find rotationally commensurate states on the substrates as energetically most favored adsorption sites [53,63], giving preferred growth directions of the needles that arise from the interplay between individual adsorption sites and bulk packing of the molecules within the crystallites [63,66]. However, in the case of DHTA5 on c-plane sapphire, there was no evidence of the preferred growth directions of the needles. This indicates that the molecules cannot find a commensurate state on the substrate, although they assume a flat-lying orientation. Furthermore, many of the observed needles do not grow in a straight line and either have several kinks separated by straight segments, or appear – to the extent of the AFM resolution – as continually curved. This morphology feature could be explained considering that the growing needle-like crystallites are not in registry with the substrate, and thus any structural defect can alter the growth direction, without being “corrected for” by the molecule-substrate interaction. To the extent of this study, the reason why DHTA7 forms predominantly islands and DHTA5 form needles remains so far not completely clear. One possibility would be that for a larger molecule it is less-favored to stay on the surface as a flat-lying once the critical nucleus is formed, while smaller molecules can still remain in the flat lying configuration even after the formation of a stable nucleus. Another possible scenario would be related to the different source temperatures required to sublimate smaller DHAT5 and larger DHTA7 molecules. Namely, the source temperature is increased by 90 K for DHTA7 growth. Therefore, partial decomposition of the molecules within the HWE cell cannot be neglected anymore. This could introduce carbon contamination on the substrate surface, which could result in the change from flat-lying to up-right standing molecules as observed previously [67–69]. Besides needles, island-like crystallites of DHTA5 were also observed (see Fig. 7(b-d)). The height of the islands was analyzed in Section 3.1, implying that the islands consist of upright standing molecules. The volume per unit area of both island-like and needle-like crystallites as a function of TD is shown in Fig. 7(h). Needle-like crystallites were found in the entire considered range of TD, also at both lower and higher deposition rates, and at various surface coverages. Interestingly, islands were observed in a narrower TD range (354366 K), and were always found to nucleate along needles. In the case of lower coverage (0.1-0.2 ML), only needles were observed in the entire TD range. This indicates heterogeneous nucleation of the islands at already existing needles. Furthermore, islands of DHTA5 were found not to be stable under ambient conditions (see Section 3.5), while no significant changes were observed for the needle-like crystallites in the case of prolonged air exposure. To avoid any influence of the island
average island separation in the case of the samples with sub-ML coverage, grown at the same TD and with the same deposition rate. This indicates that the diffusion coefficient for DHTA7 molecules on an upright layer of DHTA7 is much larger than that on sapphire. Further, islands become more compact in higher layers. The inset of Fig. 6(b) shows α as the function of the terrace number for the mound formation presented in Fig. 6(a). These results imply that the number of kink sites at the step circumference is reducing with the number of layers. Nonuniform circumference and the presence of kink sites have been shown to reduce the step-edge barrier in the case of metal homoepitaxy [61,62], which is also a likely scenario for the DHTA7 molecules.
3.4. Temperature dependent growth morphologies of DHTA5 In a similar manner as for DHTA7, a temperature series of DHTA5 samples was examined, within a range of TD between 340 K and 380 K. Below 340 K the crystallites were too small for a reliable analysis, while above 372 K almost no material was deposited due to reevaporation. In the TD range between 354 K and 366 K, the samples had a surface coverage of (0.5 ± 0.1) ML. Interestingly, unlike DHTA7 which predominantly forms island-like crystallites, DHTA5 molecules mainly form needle-like crystallites, implying that the molecules assume a “flat-lying” orientation on the substrate. Similar needle-like crystallites were also observed in the case of DHTA7, but only at high coverage and higher growth rates than presented in Section 3.2. An example of the evolution of the DHTA5 crystallite morphology with an increase of TD is shown in Fig. 7(a–e). Frequently these needles are not straight, as usually observed for oligophenylenes [53,63,64], but kinked or constantly curved. They also vary in width and height. These peculiarities are addressed later. Fig. 7(f) shows in logarithmic scale the number of nucleation centers of DHTA5 needle-like crystallites per unit area, as a function of inverse TD. Unlike the case of DHTA7 islands (see Section 3.2), lnN versus 1/TD follows a straight line for DHTA5 needles. This does not indicate that the molecules impinging on the surface are meditatively equilibrating their potential energy with the substrate [58]. Since the needle-like crystallites have their growth front only at the needle apexes, the molecules will most likely reflect many times from the needle side walls before they are incorporated into the crystallite. As a result, impinging molecules will thermalize to the substrate’s temperature thus yielding a straight line in lnN versus 1/TD curve. In Fig. 7(f) the slope of a least-square fit to the experimental data gives EA = (1.16 ± 0.25) eV. These results would imply that DHTA5 molecules have rather large probability for the diffusion on sapphire surface, and that nucleation is governed by the attachment limited aggregation [65]. Further, Fig. 7(g) shows that both average length of the needles < l > and average volume of the needles < V > increases with TD, indicating a three-dimensional growth of the crystallites. For each data point used in the statistical analysis presented in Fig. 7(f-h), 5
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Fig. 7. (a-e) 5 × 5 µm2 (z scale 10 nm) AFM topography images of DHTA5 grown in the TD range from 343 K to 372 K. (f) Number of DHAT5 needle-like crystallite nucleation centers per µm2, shown in logarithmic scale as a function of inverse TD, where the slope of the linear fit gives activation energy (EA) for the aggregation of the molecules on the c-plane sapphire. (g) An average length of the needles (left), and average needle volume (right), as a function of TD. (h) Volume nm3 per µm2 of DHTA5 needles (solid line) and island (dashed line) as a function of TD.
Fig. 8(a–c), and even up to several months. However, DHTA5 islands were found to degrade rather fast once the sample had been exposed to ambient air. Within the first several hours, there are noticeable changes on the island’s surface (perturbations, holes, trenches) while after 3–4 days the height of the islands reduces from (1.65 ± 0.20) nm to (0.50 ± 0.40) nm. Fig. 8(d,e) presents the same sample area measured immediately after the growth, and after 4 days of air exposure. Height cross sections – shown in Fig. 8(f) – illustrate the change of island’s height. In addition, a video file that follows with AFM the degradation of DHTA5 island over the time span of four days is available in supplementary data (link). Prolonged exposure does not introduce any further change in the island’s morphology. Needle-like crystallites of DHTA5 were found to be unaffected by air exposure. In the case when DHTA5 samples were left in high vacuum (below 1 × 10 −6 mbar) for several days, the degradation of the islands started
degradation, all the images that were used to evaluate the volume per unit area of DHTA5 islands (Fig. 7(h)) were measured within the first two hours from the point of air exposure of the samples.
3.5. Stability of crystallites under ambient conditions Forming H-bonds between the neighboring molecules [30] is expected to provide better air stability of both individual molecules and crystallites [12,14,18], finally yielding stable electrical properties of the thin films [15]. This was indeed observed for all the crystallites that were analyzed in this study, with the exception of DHTA5 islands. Fig. 8(a–c) compares DHTA7 islands, grown at TD = 400 K, with DHTA5 islands and needles grown at TD = 360 K (Fig. 8(d–f)). No significant changes in crystallite morphologies of DHTA7 islands were detected in the case of air exposure, up to 45 days as shown in
Fig. 8. (a) and (b) 5 × 5 µm2 (z scale 7 nm) AFM topography images of DHTA7, as grown (day 1) and after 45 days of the sample being exposed to ambient air. Note that (a) and (b) do not show the exact same sample area. (c) AFM height cross sections (indicated by solid lines in (a) and (b)) showing negligible change of island height. (d) and (e) 5 × 5 µm2 (z scale 7 nm) AFM topography images of DHTA5, as grown (day 1) and after 4 days of the sample being exposed to ambient air. (f) AFM height cross sections of the same DHTA5 island (indicated by solid lines in (d) and (e)), showing a significant reduction in island height (indicated by the bold arrows). The horizontal arrow points to a DHTA5 needle in the middle of both cross sections.
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again from the point of air exposure of the samples. This does indicate that interaction with ambient air (most likely polar water molecules) introduces the observed morphological changes in the case of DHTA5 islands. An influence of exposure to moisture on the crystallite structure has been reported for para-phenylenes on muscovite mica [70] as well as a reorganization of the crystallites upon exposure to ambient conditions [67]. Understanding why only these crystallites degrade upon ambient exposure – and the exact degradation pathway for these crystallites – goes beyond the scope of this study. Compared to air stable DHTA7 islands, DHTA5 islands are expected to have mainly weaker vdW interactions between H-bonded molecular rows. As mentioned in the Section 2.1, annealing of the sapphire substrate is expected to reduce the amount of surface -OH groups. It is highly likely that exposure to the ambient conditions after the growth reintroduces formation of surface -OH groups, which could be a reason for the dewetting of DHTA5 islands. In addition, steps in the sapphire surface are expected to introduce stacking folds at the molecular pairs above and below the step edge. Being that DHTA5 is a shorter molecule than DHTA7, relatively greater area of the π-network will be exposed at these steps, which could be enough to destabilize the DHTA5 islands in ambient conditions, but not DHTA7. The studies on the influence of TiO2 atomic steps on the growth and stability of parahexaphenyl reported island-like crystallite degradation through the above mentioned mechanism [71,72].
[2]
[3] [4] [5]
[6]
[7] [8]
[9] [10]
[11]
[12] [13]
[14]
4. Summary [15]
In summary, we have investigated the growth morphologies of dipolar DHTA5 and DHTA7 molecules on vicinal (0001) sapphire surface. The longer – DHTA7 – molecules were found to form predominantly island-like crystallites consisting of up-right standing molecules, exhibiting quasi layer-by-layer (terraced mound) growth. The shorter – DHTA5 – molecules were found to mainly form 3D needle-like crystallites, favoring Volmer-Weber growth. Further, DHTA5 island-like crystallites were the only observed structure that was not stable upon ambient exposure. The morphology of both DHTA7 islands and DHTA5 needles was found to be stable in ambient air. Dihydro-tetraaza-acenes are promising candidates for future applications in organic electronics, since these molecules form crystals through an interplay between H-bonding, dipolar, and van der Waals interactions. As a result, densely packed π − π structures – favorable for charge transport – are obtained, with exceptional stability under ambient conditions. Morphological features of the films as orientation of the molecules with respect to the substrate plane, grain boundaries, and grain sizes can have a dominant impact on the device performance. Therefore, understanding the growth morphologies of monolayers and thin films is an important step towards realization of efficient devices based on these novel oligoacene derivates.
[16] [17]
[18] [19] [20]
[21] [22] [23] [24]
[25] [26] [27]
Acknowledgments
[28]
This work has been supported by Austrian Science Fund (FWF Der Wissenschaftsfonds) through project I 1788-N20 and by Agence Nationale de la Recherche (ANR) through grant ANR 14 CE34 0003 01. The authors would like to thank Andreas Egger (Montanuniversität Leoben, Austria) for the help in preparation of the sapphire substrates.
[29] [30]
[31]
Supplementary material [32]
Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.susc.2018.03.009.
[33] [34]
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