Scanning Tunneling Microscopy Examination of Rubrene Deposited

Jan 2, 2015 - Rubrene has received much attention for its potential in the fabrication of organic thin film transistors (OTFTs), and the interface bet...
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Scanning Tunneling Microscopy Examination of Rubrene Deposited on Au(111) in Aqueous Solution Shu Rong Luo, Shueh Lin Yau,* Prabakaran Kumaresan, Sureshraju Vegiraju, and Ming-Chou Chen* Department of Chemistry, National Central University, Jhongli, Taiwan 320 ABSTRACT: Rubrene has received much attention for its potential in the fabrication of organic thin film transistors (OTFTs), and the interface between rubrene and metal leads, such as those made of gold, is important in this context. In this study in situ scanning tunneling microscopy (STM) was used to examine rubrene molecules deposited on a Au(111) electrode from a 8 or 80 μM benzene dosing solution under potential control in 0.1 M perchloric acid. Immersion in either dosing solution for 10 min or longer yielded ordered structures featuring parallel stripes aligned in the ⟨121⟩ direction of the Au(111)(1×1) surface. Unusual surface features of ragged step edges and one-atom-deep pits were observed on the heavily dosed Au(111) sample, which suggests unexpected rubrene-induced restructuring of the gold substrate. Roughly 0.1 monolayer gold adatoms were codeposited with rubrene molecules on the Au(111) electrode. Molecular-resolution STM imaging suggests an upright adsorption configuration of rubrene molecules anchored with two phenyl groups on the Au(111) electrode. Ordered rubrene adlattices were seen between 0.1 and 1.0 V (vs reversible hydrogen electrode), but disappeared at E < 0 V, leading to aggregates of gold atoms. These results lend support to the proposed restructuring of the gold surface.

1. INTRODUCTION Pentacene, rubrene, perylene, and their derivatives are renowned for their potential uses in fabricating organic thin film transistors.1−4 In addition to the intrinsic characteristics of these molecules, their spatial arrangements on metal electrodes and in the subsequently formed thin films also affect the mobility of charge shuffling between the source and drain terminals of a transistor. With a complicated molecular structure, rubrene can be adsorbed in various manners, depending on the environment, and the resulting spatial arrangements can differ from those in a single crystal.5 This implies that the charge mobility characteristics in a thin film and in a single crystal will also be different.5 Scanning tunneling microscopy (STM) has been used extensively to study the spatial structures of organic molecules adsorbed on ordered surfaces, including rubrene adsorbed on Cu,6,7 Au,8−11 and Bi.12 All studies are performed in a vacuum, and the results reveal a mainly “face-on” orientation with twisted tetracene backbones facing the metal substrate. This adsorption is classified as physisorption, which cannot afford enough energy to unfold the tetracene backbone of rubrene into a flat orientation. Meanwhile, rubrene and pentacene adsorbed on Bi(001) on Au(111) can change their adsorption orientations from parallel to upright as the coverage increases. Chiral rubrene thin films one or multiple layers thick have also been reported.12,13 STM was used in this study to examine rubrene adsorption on a Au(111) electrode from a benzene dosing solution. Molecular-resolution STM imaging conducted in 0.1 M © 2015 American Chemical Society

perchloric acid revealed the rubrene molecules could be adsorbed in a way that is very different from that seen in a vacuum.5,8,13−16 In addition, the STM results show that rubrene deposition resulted in restructuring of the Au(111) substrate, which has not been reported in a vacuum.11

2. EXPERIMENTAL SECTION The Au(111) electrodes used for voltammetry and STM experiments were single crystal bead electrodes made by melting the end of a poly gold wire using a hydrogen torch. For all experiments, a Au(111) electrode was annealed by the hydrogen torch, followed by rapid quenching in Millipore triple-distilled water bubbled with a hydrogen stream. This is the standard method used to prepare well-defined gold electrodes, which produces the reconstructed Au(111) - (√3 × 22) structure.17−19 The Au(111) electrode was removed from the quenching tube, rinsed with acetone, and blown dry with a nitrogen stream before it was placed in a rubrene/benzene dosing solution. As indicated by the STM results presented below, this method effectively produced a smooth rubrene adlayer, whose spatial structures varied with the concentrations of rubrene, soaking time, dosing temperature, and so on. We found the experimental conditions needed to obtain ordered rubrene Received: September 16, 2014 Revised: December 18, 2014 Published: January 2, 2015 1376

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The Journal of Physical Chemistry C

electrodes made with a low-dosage solution. The rubrene adlayer reduced the charging current of the Au(111) electrode by one-half. For the bare Au(111), a broad peak is seen at 0.6 V, which is ascribed to a phase transition from the reconstructed (22 × √3) to the ideal (1 × 1) structure. This feature was no longer seen with the modified electrode, suggesting that deposition of rubrene lifted the reconstructed Au(111) surface of the electrode prepared by the typical annealing and quenching method. These results show that rubrene admolecules were stably adsorbed on the unreconstructed (1 × 1) phase between 0.05 and 1.0 V. This view is supported by the STM results, presented below. 3.2. STM Imaging of Rubrene Adsorbed on Au(111). 3.2.1. Surface Morphology. The topographic STM image shown in Figure 2a reveals the surface morphology of a

adlattices by trial-and-error. It was found that a long soaking time (>10 min) in 8 and 80 μM rubrene at 10 and 25 °C was needed to render a highly ordered rubrene adlayer. STM efforts were directed to examine samples dosed under two conditions, 8 μM, 10 min, and 10 °C and 80 μM, 30 min, and 25 °C, referred as the low and high dosages, respectively. They produced different rubrene structures, as described below. Voltammetry experiments were performed in an electrochemical cell equipped with a reversible hydrogen electrode (as the reference electrode) and Pt counter electrode. The potentiostate was a CH 625. The STM scanner was an Ahead (Veeco, Santa Barbara, CA) with a maximal scan size of 500 nm. It was calibrated against highly oriented pyrolytic graphite (HOPG). The tip was a tungsten tip etched by AC in 6 M KOH. After thorough rinsing with Millipore water and drying with acetone, it was insulated by applying an Apeazon wax coating. All STM images were acquired with the constantcurrent mode and unfiltered.

3. RESULTS 3.1. Electrochemistry. Figure 1 shows CVs recorded at 50 mV/s with a Au(111) electrode exposed to high dosage of

Figure 2. In situ topographic STM images recorded with Au(111) precoated with a monolayer of rubrene at 0.4 V in 0.1 M HClO4. Stripes could be aligned in three ⟨121⟩ directions of the Au(111) surface, yielding three rotational domains I, II, and III. Arrows in panels a and b indicate the close-packed atomic direction of the Au(111) electrode. These images were collected with a 100 mV bias voltage and 1 nA feedback current.

Au(111) sample made with high dosage rubrene. It shows atomically smooth terraces spanning more than 300 nm and monatomic steps (Δz = 0.25 nm). No herringbone structure (or paired lines) are observed, which implies that deposition of rubrene lifted the reconstructed Au(111) surface, as inferred from the CV results and reported above. In contrast, the results obtained in a vacuum show that rubrene molecules are adsorbed on the reconstructed (22 × √ 3) structure, even after annealing to 370 K.9 It is surprising to see no gold aggregates in Figure 2, because roughly 4% of the gold atoms should have been injected on the electrode when the (√3 × 22) phase was converted to the (1 × 1) phase. These “extra” gold atoms could either migrate to nearby step defects or join the rubrene adlayer. Figure 2b is a higher-resolution STM scan, revealing parallel lines aligned in the three ⟨121⟩ direction of the Au(111), which accounts for the fact that three domains were formed. Owing to anisotropic attractive interaction between rubrene admolecules along the ⟨121⟩ direction, an ordered domain could span more than 50 nm. Each ordered rubrene domain likely resulted from local nucleation-and-growth mechanisms. Ordered rubrene structures were observed between 0.1 and 1 V, but became disordered at E < 0.1 V, where hydrogen was adsorbed. The smooth rubrene adlayer seen in Figure 2 implies that a lowdosage produces only a rubrene monolayer. The STM results obtained with Au(111) and a high-dosage of rubrene are shown in Figure 3. There were smooth terraces that could span more than 200 nm wide, with varying step morphology. The steps could be sharp or rugged, depending mainly on their alignment, as seen in Figure 3, parts a and b. Step edges aligned in the ⟨110⟩ direction of Au(111) were

Figure 1. CVs recorded at 50 mV/s with Au(111) electrode modified by high dosage and low dosage rubrene (inset) in 0.1 M HClO4. A pair of redox features (A1/C1) is seen at −0.05 V before hydrogen evolved at −0.1 V, and their current densities increased consecutively with the number of potential cycles. The solid trace in the inset was recorded with bare Au(111).

rubrene. Only a double-layer charging current was seen with repetitive potential cycled at 50 mV/s between 0.05 and 1.0 V, which suggests that rubrene admolecules were stable against redox reaction under these conditions. A pair of surface redox features (A1/C1) is seen at −0.04 V before hydrogen evolved at −0.1 V. Their current densities increased continuously with potential cycles. This feature is ascribed to concurrent processes of rearrangement of the rubrene adlayer, and reductive adsorption of hydrogen at the gold electrode. As revealed by the STM results described below, the pristine rubrene monolayer was compact and ordered at the beginning, but gradually opened up with continuous potential cycles. In situ STM imaging provided a direct view of these events, as described below. In comparison, the featureless CV profile seen between 0.1 and 1.0 V hardly changed with potential cycles, suggesting that rubrene molecules were still adsorbed on the electrode. These results are typical for Au(111) modified by organic molecules, including coronene20,21 and pentacene.22 The inset in Figure 1 shows that CV results obtained with bare (solid trace) and rubrene-coated (dotted trace) Au(111) 1377

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two spots were often 0.04 nm higher than the terminal ones, which could result from dissimilar adsorption orientations of these aromatic rings. As seen in Figure 4b, four-spot chains formed a herringbone pattern with respect to their neighbors, as also seen with a rubrene single crystal.24 The unit cell of this adlayer is defined by two unit vectors, a and b, measured as 0.50 ± 0.02 and 3.00 ± 0.05 nm long and intersecting perpendicular to each other. These results indicate a unit cell of (√3 × 10)rect −2 rubrene (θ = 2/20 = 0.1), which resembles the a−b plane of rubrene single crystal,24 except they have different dimensions. The vectors a and b in a rubrene single crystal are 1.449 and 0.721 nm, respectively.24 Shown in Figure 5 are higher resolution STM images intended to reveal the structure of heavily dosed rubrene on

Figure 3. In situ STM images revealing the surface morphology (a and b) and ordered molecular structures (c) formed on a Au(111)−(1 × 1) electrode made with a high dosage solution. Steps aligned in the ⟨110⟩ direction were mostly long and straight (a), but were rugged, otherwise (b). The inset of panel c reveals a pit on the Au(111) electrode. The potential was set at 0.4 V in 0.1 M HClO4, while the bias voltage and tunneling current were 150 mV and 1 nA, respectively.

usually long and well-defined, as seen on the left of Figure 3a. By contrast, steps aligned in other ways were frequently short and seesawed, as seen in Figure 3b. Because soaking Au(111) in a benzene-only solution did not produce these kinds of rough step edges, a high dosage of rubrene was responsible for these surface features. The higher resolution STM image shown in Figure 3c reveals protruding linear segments (Δz = 0.08 nm) found sparsely on the electrode. Other higher resolution STM scans, presented below, show that these segments were sitting atop the rubrene adlayer. Minor features of pit ∼7 nm wide and 0.27 nm deep were observed on the rubrene modified gold electrode (inset of Figure 3c, 25 × 25 nm2), which resemble those seen with a thiol-modified Au(111) electrode.23 Pits were not seen on Au(111) dosed with rubrene in a vacuum.8−10 3.2.2. Spatial Structures of Adsorbed Rubrene. Higher resolution STM scans were used to reveal submolecular features of adsorbed rubrene deposited with a low-dosage solution. As seen in Figure 4a, the rubrene adlayer was found to

Figure 5. Molecular-resolution STM images obtained with Au(111) exposed to high- dosage rubrene. Panel a shows parallel stripes running in the ⟨121⟩ direction of the Au(111) substrate. Linear segments marked in panel b highlight the herringbone pattern formed by neighboring stripes. The rectangular unit cell is defined by vectors a and b. Panel c highlights randomly dispersed linear segments (indicated by arrows) ascribed to chains of gold adatoms. The bias voltages and feedback currents were −150 mV and 1 nA, respectively.

Au(111). As seen with the lightly dosed sample (Figure 4), rubrene molecules were arranged in stripes aligned in the ⟨121⟩ direction. However, a few differences in the rubrene adlayer compared with the results shown in Figure 4, should be noted. First, instead of single string, the structure seen in Figure 5a was clearly double-stringed. Moreover, the strings in Figure 5a were not equally spaced, and two neighboring stripes were separated alternatively by 1.20 ± 0.03 and 1.50 ± 0.05 nm. Compared with the uniform separation of 1.5 nm seen in Figure 4, this is a more compact adlayer. Because of the lack of a long-range ordering, the coverage of this adlayer is unknown. Second, as revealed by Figure 5c, linear segments protruding from the supporting rubrene stripes (Δz = 0.08 nm) were found at some local regions. They were found on molecular adsorbates, but were mobile in the course of STM imaging. Figure 5b is a further higher resolution STM scan intended to reveal the internal structure of rubrene admolecules. One stripe consisted of rows of bilobes separated by 0.8 nm, which formed a herringbone structure with its neighboring stripes, as indicated by the marked segments. This adlayer had a rectangular unit cell defined by unit vectors a and b measured 0.50 ± 0.02 and 2.70 ± 0.05 nm, which yielded a (√3 × 9)rect structure (θ = 2/18 = 0.11). This adlayer had an 11% higher coverage than seen with a low-dosage solution. Regardless of the dosage level, all stripe patterns exhibited irregular heights among spots. We ascribe these features to variations in the adsorption configuration of rubrene molecules. 3.2.3. Desorption of Rubrene Molecules at E < 0.1 V. The effect of potential on the structure of rubrene was examined first by lowering the potential from 0.4 to 0.1 V, and then to a

Figure 4. Molecular−resolution STM images (a and b) acquired with Au(111) of low-dosage rubrene at 0.4 V in 0.1 M HClO4. Stripes in the same dimension were evenly spaced at ∼1.5 nm apart. Linear segments marked in panel b denote rubrene molecules arranged in a herringbone pattern with respect to their neighbors in the next stripes. This structure is defined by unit vectors a and b intersecting perpendicularly. The imaging conditions were 100 mV in bias voltage and 1 nA in set point current.

organize itself in parallel, evenly spaced stripes aligned in the ⟨121⟩ direction. Two neighboring stripes were separated by 1.50 ± 0.05 nm (center-to-center). Figure 4b shows is a closeup STM scan revealing the internal molecular features in one of the stripes. We read this structure as slanted laminas of fourspot chains represented by the linear segments. The distance between spots at the two ends was 1.05 ± 0.05 nm long, which matches up with the distance between the carbon atoms at the terminals of the tetracene backbone of rubrene. The middle 1378

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The Journal of Physical Chemistry C negative voltage. The ordered rubrene adlayer hardly changed until 0.1 V, at which point the ordered rubrene structures residing on the upper end of step disappeared first, while those on terraces were unaffected (Figure 6a). Lowering the potential

Figure 7. In situ STM images obtained with Au(111) electrode treated with high-dosage rubrene at −0.1 (a and d), 0.4 V (b, c, e, and f) in 0.1 M HClO4. Panel a was recorded 7 min after the potential was held at −0.1 V. Panels b and c were recorded 11 and 13 min after the potential was switched from −0.1 to 0.4 V. A portion of panel a is highlighted in panel d, and panels e and f are portions of panel c. The tip potential was set at 0.4 V in this experiment, and the feedback current was 1 nA.

Figure 6. Real-time STM images acquired with Au(111) modified by a low dosage of rubrene at 0.1 (a), 0 (b−d), and 0.4 V (e, f) in 0.1 M HClO4. Local restructuring of the rubrene adlayer was first noted at the upper end of steps, highlighted in the lower inset of panel a. Ordered adlattices at terrace sites, as revealed in the upper inset of panel a, then disappeared. The step morphology also changed notably. Switching the potential back to 0.4 V partially restored the ordered molecular adlayer, as revealed by the 25 nm × 25 nm scan shown in the inset of panel f. The tip potential was fixed at 0.4 V in this experiment, and the feedback current was 1 nA.

adsorbed rubrene molecules. Holding the potential at 0.4 V for ∼10 min partially restored the ordered rubrene adlayer, and the protruding linear segments ascribed to gold adatoms (such as those shown in Figures 3c and 5c) were no longer present. These results are similar to those observed with thiol-modified Au(111) reacting with hydrogen in a vacuum, with which possible incorporation of gold adatoms in the thiol adlayer was investigated. In that study, desorption of thiol admolecules also yielded one-atom-thick gold islands as gold adatoms were released from desorption of thiol molecules.25

to 0 V eliminated nearly (∼95%) all the ordered rubrene adlayer in 3 min (Figure 6b−d). Meanwhile, the step edge seen in these images (marked by an arrow in Figure 6a) turned notably ragged. Since holding the potential at 0.1 V for 10 min did not yield the reconstructed (√3 × 22) structure, it seems that the rubrene molecules were not truly desorbed from the electrode, but simply became disordered. This is likely associated with the poor solubility of rubrene in aqueous solution, and hydrogen adsorption is probably responsible for the changes seen here. Parts e and f of Figure 6 were obtained when the potential was raised from 0 to 0.4 V. The ordered rubrene adlayer was partially restored after 10 min STM imaging, as indicated by the 25 × 25 nm2 scan shown in the inset of Figure 6f. Steps also became better defined as the potential was made to 0.4 V. Lowering the potential to −0.1 V yielded even more drastic changes at the rubrene/Au(111) interface. Expectedly, hydrogen adsorption became more important and H2 evolution was about to occur at the rubrene−modified Au(111) electrode, according to the CV results shown in Figure 1. As revealed by Figure 7a, acquired 7 min after the potential was set at −0.1 V, protruded islands popped up evenly on the electrode to displace the ordered rubrene adlayer. As seen in Figure 7d, these islands measured 2−7 nm wide and 0.26 nm high, and were stable against protracted STM imaging. Because the Au(111) electrode should have assumed the smooth (√3 × 22) reconstructed structure at −0.1 V in 0.1 M HClO4, it is surprising to see protruding gold islands. The potential was gradually raised from −0.1 to 0.4 V to see if the ordered rubrene adlayer could be restored. This act resulted in dendrites seen at 11 min scanning at 0.4 V (at the lower end of Figure 7b). These dendrites grew at the expense of the protruding islands, as seen in Figure 7c, obtained 2 min later. All dendrites seen here were 0.26 nm or one gold atom high (Figure 7e) and neighbored with stripes (Figure 7f) due to

4. DISCUSSION 4.1. Restructuring of Au(111) Electrode Surface by the Deposition of Rubrene. The STM results reveal a number of unusual features on the Au(111) electrode surface after being dosed with rubrene. First, distinct seesaw steps, unseen with bare or hydrocarbon-coated Au(111), were observed (Figure 2). Second, protruding (Δz = 0.08 nm) linear segments attributed to gold adatoms were identified in Figure 5c, which were not seen when the same experiment was performed in a vacuum. Third, pits one atom deep were produced (Figure 3c) and grew notably along with dosing time. Fourth, compared with the reconstructed Au(111) reported in a vacuum,8,9 deposition of rubrene lifted the reconstructed Au(111) surface to the (1 × 1) atomic structure. Fifth, lowering the potential to 0 V or negative values induced notable changes in step morphology (Figure 6) or generated clusters on terraces (Figure 7). These results imply that deposition of rubrene from a solution phase resulted in substantial restructuring of the Au(111) electrode surface. This presumed adsorbate−induced restructuring can occur with strongly adsorbed species, such as alkanethiol,25,26 but would be surprising for rubrene, which is not expected to interact with Au(111) strongly enough to induce surface restructuring. Nonetheless, the STM results obtained in this work strongly implicate restructuring of the Au(111) surface, possibly driven by the need to accommodate 1379

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The Journal of Physical Chemistry C adsorbing species. It seems that rubrene could be similar to methanelthiol and amine molecules27 in forcing restructuring of the Au(111) surface, such as by producing gold adatoms under the conditions employed in this study. Neither dosing with rubrene in a vacuum, nor deposition of aromatic molecules, such as coronene and pentacene, from solution, would induce restructuring of Au(111).20−22,28,29 The results of this study thus highlight the importance of experimental conditions in guiding the interfacial structure. The dosing time of 10−30 min used in this study was notably longer than those used previously (