Letter pubs.acs.org/Langmuir
Highly Ordered Molecular Films on Au(111): The N‑Heteroacene Approach Tony Lelaidier, Thomas Leoni,* Pandurangan Arumugam, Alain Ranguis, Conrad Becker, and Olivier Siri* Aix Marseille Université, CNRS, CINaM UMR 7325, 13288 Marseille, France S Supporting Information *
ABSTRACT: This study presents an innovative synthesis of dihydrotetraazapentacene (DHTAP) and the scanning tunneling microscopy (STM) investigation of the initial stages of its growth on Au(111). We were able to demonstrate that, up to the fourth monolayer, the DHTAP films show a high structural order and growths in perfect epitaxy. This behavior can be unequivocally attributed to the stabilizing effect of intralayer hydrogen bonding interactions.
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INTRODUCTION Organic semiconductors offer promising applications in various technologies such as solar energy conversion,1 organic fieldeffect transistors (OFETs),2 or chemical/biological sensors3 essentially because of their mechanical flexibility and their compatibility with low-cost solution-based fabrication methods. For organic electronics, one of the major limiting factors is the low electronic mobility reached in the devices.4 Previous works have clearly indicated that thin organic semiconductor films maximize their electrical conductivity when π-orbitals are accessible for charge injection and hopping between neighboring molecules by orbital overlap. This unique characteristic is partially lost in amorphous or polycrystalline films, in which molecular disorder effectively reduces the mobility of the charge carriers. Improving carrier mobility in organic films is thus intimately related to molecular orientation and long-range order inside the films.5 Pentacene, although being one of the most studied organic semiconductors because of its high intrinsic mobility, failed to form large single-phase domain multilayer films when deposited on noble metal surfaces. For instance, on Au(111), pentacene films exhibit a large number of differently ordered structures in the monolayer regime, which ultimately leads to the growth of polycrystalline and partially nonepitaxial thin films.6−8 On the Cu(110) surface, a complex multiphase behavior comprising five different phases is observed.9 On Cu(111), two different phases are observed at monolayer coverage, and the second layer adopts the crystallized bulk structure with a tilted orientation of the molecule.10 Other examples of the coexistence of different monolayer structures and tilted molecular arrangements in the second layer have been published for the (111)11,12 and (110)13 surfaces of silver. Additionally, pentacene presents long-term stability issues due to its photooxidation and low thermal stability.14 These ascertainments have motived the community to search for isostructural derivatives of pentacene, and particular interest has been devoted to nitrogen-containing © 2014 American Chemical Society
oligoacenes derivatives because of their high stability under ambient conditions.15,16 More specifically, interest in dihydrotetraazapentacene (DHTAP) derivatives of type 1 has recently re-emerged owing to the presence of two different types of nitrogen atoms, which offer a number of opportunities to manipulate and control the electronic properties, the stability, and the supramolecular arrangement in the solid state.17−19 First measurements18 of the charge carrier mobilty of amorphous tetraazapentacene-based field effect transistors have shown a charge carrier mobility up to 0.02 cm2 V−1 s−1, which is in fact lower than the mobility of amorphous films of pentacene, for which values of 0.62 to 1.5 cm2 V−1 s−1 have been reported (see Table 1 in ref 4). The growth of wellordered azapentacene films seems thus to be an important challenge. Chart 1. 5,14-Dihydro-5,7,12,14-tetraazapentacene
Actually, DHTAP appears to be a very promising candidate for the growth of well-ordered molecular layers because of the presence of two H-donor (N−H) and two H-acceptor (NC) sites that could lead to a well-organized arrangement in the solid-state based on intermolecular H-bonding interactions.20−25 Here, we report a comprehensive scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) study of the initial stages of DHTAP film growth. We have been able to identify the different adlayer Received: January 20, 2014 Revised: April 11, 2014 Published: May 6, 2014 5700
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Letter
phases from the first up to the beginning of the fourth monolayer. In the course of this study, the chemistry of DHTAP was also reinvestigated, and we disclose a simple, versatile and solvent-free methodology for the synthesis of symmetrical DHTAP, which allows for the introduction of sensitive functions such as amide, previously unattainable. As far as we know, this study is the first one that describes the organization of an azaacene derivative on a metallic substrate at the molecular scale.
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surface. Below 170 K, the diffusivity of the DHTAP molecules is too low, which prevents self-organization. Consequently, all STM images presented in this communication have been obtained after DHTAP deposition in the favorable substrate temperature range. Under these deposition conditions, the apparent height of the DHTAP molecules in STM is 0.14 nm, the apparent width is 0.65 nm, and the length is 1.43 nm (for details, see Supporting Information). As DHTAP 1 is similar in size to pentacene,29 these measurements suggest a flat lying adsorption of the molecules on the Au(111) surface. As one can see in Figure 1, the DHTAP molecules form a well-organized
EXPERIMENTAL SECTION
The STM experiments were performed under ultrahigh vacuum (low 10−11 mbar) conditions using a commercial low-temperature STM (Omicron). The LT-STM was used at nitrogen temperature (78 K both sample and tip) for all experiments presented here. The Au (111) single crystal was cleaned by several cycles of Ar+ sputtering and thermal annealing at about 700 K. DHTAP 1 was thermo-evaporated from a quartz crucible heated to 500 K and deposited on the Au(111) surface for substrate temperature in the range from 170 K to room temperature.
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RESULTS AND DISCUSSION Synthesis of (un)substituted DHTAPs is generally accomplished by the condensation of o-phenylenediamines and 2,5dihydroxybenzoquinone in the presence of different Bronsted acids,18,26 except for Miao et al.17 who described a solvent and acid-free preparation. However, all these syntheses require long reaction times (several hours) at high temperatures, which limits the ability to introduce sensitive groups. We have been able to considerably accelerate the synthesis by using benzoic acid as catalyst in a solvent-free reaction because this solid acid allows for higher temperature conditions (up to 300 °C) and therefore activation of the reaction, which is completed in only 2 min. Thus, the condensation of 2 and 3 in the presence of excess PhCO2H at 300 °C (heated with a heat gun) yields 1 that could be easily isolated by filtration with 80% yield (Scheme 1). Interestingly, this extremely short time
Figure 1. STM images (90 nm × 90 nm) of the two domains (R and L) of the monolayer phase of DHTAP on Au (111), It = 150 pA, Ubias = 1.2 V. Red arrows indicate orientation of the R domain and green arrow orientation of the L domain along the long molecular axis. Dark arrows indicate the Au(111) crystallographic directions. The inset shows a zoom of a second-layer island.
Scheme 1. Synthesis of DHTAPsa
a
structure at coverage of roughly 1.1 monolayers (ML). A closer inspection of this image reveals that a great number of domains are present, which are indicated by the red and the green arrows. These domains can be classified into righthanded (R) and left-handed (L) structure, which possess a particular orientation with respect to the [110̅ ] crystallographic direction of the surface, e.g., the densely packed rows of gold atoms. We have observed three different orientations for each domain, which are rotated by 60° with respect to each other. Thus, the orientations of the domains perfectly preserve the symmetry of the Au(111) substrate. The straight steps of the substrate present in the image in Figure 1, which run in the [11̅0] direction, allow us to unambiguously determine the rotation angle of the domains. It turns out that the R-domain is rotated by 32°, and the L-domain is rotated by 28° with respect to the [11̅0] direction of the Au(111) surface. This result is confirmed by the LEED pattern shown in Figure 2A. In this image, only the bright double spots can be unequivocally attributed to the DHTAP layer. A close analysis of the pattern using the information about the orientation and the lattice size of the crystal obtained for the clean Au(111) surface leads us to the simulation presented in Figure 2B. Here the red spots correspond to the DHTAP layer and clearly possess the rotation angles found in STM images that is 28° and 32° with respect to the close-packed direction of the
(i) PhCO2H, 300 °C, 2 min.
reaction allowed for the use of diaminobenzene precursor bearing amide functions (4)27 that could react similarly with 3 affording 5 in 60% yield. The current protocol appeared to be unsuccessful in the absence of benzoic acid. It is known that the morphology of organic thin films depends strongly on the substrate temperature, especially for pentacene, a molecule structurally very similar to DHTAP.28 In order to find the best growth conditions for an ordered DHTAP film on Au(111), we have first determined the ideal substrate temperature for the formation of a well ordered film after DHTAP deposition. We have found that the temperature window between 170 and 250 K leads to the formation of wellordered domains. Above 250 K, 3D molecular aggregates are instead observed, which are randomly distributed on the 5701
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which correspond to the double spots in the LEED pattern can be easily identified in the STM images shown in Figure 2C,D. The R-domain in Figure 2C has a unit cell angle θR = 72 ± 1° and the L domain on Figure 2D has a measured unit cell angle θL = 108 ± 1°. The experimentally measured unit cell distance between two neighboring DHTAP molecules (measured center to center) in the long molecular axis direction is 1.56 ± 0.08 nm, which corresponds to the value obtained by LEED. The second unit cell vector has a length of 0.61 ± 0.03 nm for both the R- and L-domain. However, as mentioned above, we were not able to find any spots in LEED corresponding to this length and direction. Consequently, we have closely inspected a large number of STM images in order to confirm these observations. It turned out that a relatively large angular uncertainty of ±5° has been found for the angle θ. This indicates a non-negligible degree of freedom of the molecules along the long molecular axis and suggests a nondirectional intermolecular lateral interaction between the molecular rows, which explains the absence of the corresponding LEED spots. The results presented so far have revealed the overall structure and the orientation of the DHTAP films, but the internal structure of the molecular rows has not been elucidated. The results suggest, however, that an interaction between rows stabilizes the DHTAP layers. We have thus undertaken to explore the detailed internal structure of the layers. Molecule 1, as opposed to pentacene, does not have a symmetry plane perpendicular to the long molecular axis. It should thus be possible to identify the molecular orientation from STM images. Unfortunately, STM images of the first ML have not revealed this asymmetry independently of the applied bias voltage, probably due to screening effects by the metallic substrate. We have thus turned our attention to the investigation of the second layer, in which the molecules should be better decoupled from the substrate. Figure 3A shows an STM image obtained for coverage of 1.6 ML coverage at a bias voltage of +1 V. The apparent height of the second layer is 0.125 nm, again indicating flat-lying molecules. It can
Figure 2. (A) LEED pattern of DHTAP at monolayer coverage (17.6 eV). (B) Schema of the LEED pattern with the Au (111) dense azimuth. (C) STM image of the R domain. White arrow indicates the Au (111) dense azimuth. It = 80 pA, Ubias = 1 V. (D) STM image of the L domain It = 80 pA, Ubias = 1 V.
Au(111) surface. This indicates that the long molecular axis is oriented by ±2° with respect to next nearest neighbors in the Au(111) surface. The LEED pattern also gives us access to the periodicity in this direction, which is 1.54 ± 0.02 nm (see Supporting Information), a value that is corroborated by the STM measurements. The remaining spots, showing a rotation of ±8° with respect to the [11̅0] direction, do not correspond to any structure that we have found in STM. Their origin thus remains unexplained for the time being. The two domains,
Figure 3. (A) STM image of the first and second layer of DHTAP on Au (111) acquired in forward scan direction. It = 30 pA, Ubias = 1 V. (B) STM image of the same area of panel A acquired in backward scan direction. It = 30 pA, Ubias = −1 V. (C) Superposition of panels A and B. (D) Corresponding ball-and-stick model. 5702
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Figure 4. (A) STM image of a DHTAP layer at 1.6 ML coverage. It = 50 pA, Ubias = 1.6 V, 150 nm × 150 nm. (B) STM image of a DHTAP layer at 2.6 ML. It = 40 pA, Ubias = 1.5 V, 20 nm × 20 nm.
the present case, the presence of hydrogen bonds between molecules inverses the balance between the molecule−substrate and intermolecular interactions. This guides the assembly of molecules into a two-dimensional network that then controls and templates the growth of thicker layers. In order to validate this hypothesis, we have explored the structure of thicker DHTAP layers. Figure 4A illustrates the long-range order of the first and second DHTAP layer at 1.6 ML coverage. The STM image clearly demonstrates the high degree of order even in the second monolayer, which is reflected in the large domain sizes, which can exceed 100 nm × 100 nm. In Figure 4A, different domain orientations are noticeable as well as the herringbone structure of the Au (111). For the identical directions of the herringbone structure, different domain orientations are observed, confirming once again that orientation of DHTAP domain is only weakly influenced by molecule−substrate interactions. At a coverage of 2.6 ML, we could even identify the beginning of the formation of a fourth layer island (see Figure 4B). It is noteworthy that a perfect epitaxy can be again observed between the third and fourth layer. For thicker films, we could not obtain conclusive STM results, probably due to accumulation of molecules on the tip during scanning and because of the high tunneling barrier.
furthermore easily be seen, as in the inset of Figure 1, that the second layer is in perfect epitaxy with the first layer. Concurrently with the forward scan, we have imaged the layer with a sample bias of −1 V on the backward motion of the scan for each scan line. The resulting image is shown in Figure 3B. The two images thus correspond to exactly the same position on the sample. In Figure 3B, an important alteration of the molecular contrast can be seen as compared to Figure 3A, clearly indicating the above-mentioned asymmetry of the molecule. However, the identification of the individual DHTAP molecules becomes more difficult in this case. We have thus superimposed the two images, which resulted in the image displayed in Figure 3C. Here the individual DHTAP molecules can easily be identified in the second layer, and, moreover, the above-mentioned asymmetry is clearly visible. A bright lobe can be identified for each DHTAP molecule, which is either on the right-hand side or the left-hand side of the molecule. We can, therefore, confirm a head-to-tail arrangement of the DHTAP molecules in adjacent lines, which gives rise to N−H···N hydrogen bonding between adjacent molecules. Based on the unit cell parameters and orientations deduced above, we propose the model of the R-domain shown in Figure 3D. One should note that our measurements do not unequivocally allow assigning a specific adsorption site to the DHTAP molecules but rather show the orientation of the long molecular axis with respect to the substrate. From this model we can deduce the length of the N−H···N hydrogen bond, which corresponds to 3.3 Å. This bond length is well within the typical range for N−H···N hydrogen bonds proposed by Steiner et al.,30 which is 2.5 Å to 3.4 Å. Within this model, the ordered DHTAP layer domains are noncommensurate with the Au(111) substrate. As Au(111) is a nonreactive surface, the adsorption of lying down aromatic molecules is believed to be of van der Waal type. Thus, energetic variation between adsorption surface sites seems to be too small to result in well-defined molecular positions with respect to the Au(111) surface along the long molecular axis. If we compare our results to the extensive previous works on the pentacene/noble metal interface, which is one of the benchmark molecules for molecular electronics, we note that large closed packed domains of only one phase or epitaxial multilayer are never observed for these systems.6−12 One of the main reasons advanced for these observations is the domination of the molecule−substrate interactions on the growth process. In
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CONCLUSION
In conclusion, we described a fast and versatile synthesis of DHTAPs derivative that allowed introduction of sensitive functions, previously unknown. The presence of H-donor (N− H) and H-acceptor sites (NC) on the skeleton of the azapentacene led to the formation of well-ordered epitaxial DHTAP layers on Au(111), which have been confirmed up to the fourth monolayer. The internal structure of the film revealed that molecules are assembled with a head-to-tail arrangement in adjacent rows, thus stabilizing the structure by hydrogen bonding between molecules. Our observation clearly demonstrated that increasing intermolecular interaction via hydrogen bonding favors the growth of epitaxial large-range self-assembled domains of DHTAP. The important stabilization of the DHTAP layers by hydrogen bonding suggests that this mechanism will also be operative on dielectric surfaces such as alumina and silica, which are often used for the fabrication of thin film transistors. 5703
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ASSOCIATED CONTENT
S Supporting Information *
STM image of DHTAP with detailed height and width profiles. LEED pattern of the DHTAP/Au(111) showing diffraction spots of the Au (111) substrate. Details of the synthesis of 1. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]; Phone: +33(0)4 91 17 28 00; Fax: +33(0)4 91 41 89 16. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS C.B. gratefully acknowledges the financial support of the project by the BQR Grant NanoMolFab of Aix-Marseille Université.
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REFERENCES
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