Formation of Highly Ordered and Orientated Gold Islands: Effect of

Article pubs.acs.org/Langmuir

Formation of Highly Ordered and Orientated Gold Islands: Effect of Immersion Time on the Molecular Adlayer Structure of Pentafluorobenzenethiols (PFBT) SAMs on Au(111) Waleed Azzam,*,† Asif Bashir,*,‡ P. Ulrich Biedermann,‡ and Michael Rohwerder‡ †

Tafila Technical University, P.O. Box 179, 66110 Tafila, Jordan Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, 40237 Düsseldorf, Germany

‡

ABSTRACT: Self-assembled monolayers (SAMs) of pentafluorobenzenethiol (PFBT) on Au(111) substrates, prepared with different immersion times (ITs) at room temperature, were studied using scanning tunneling microscopy (STM) and infrared reflection−absorption spectroscopy (IRRAS). In the present study, the focus was on several important points of interest in the field of SAMs. First, the gold islands formed upon adsorption of PFBT molecules on the gold surface were monitored at different ITs in terms of their size, density, and shape. After short ITs (5 to 30 min), small gold islands with rounded shape were formed. These gold islands were arranged in a rather regular fashion and found to be quite mobile under the influence of the STM-tip during the scanning. When the IT was increased to 16 h, the results revealed the formation of highly ordered and orientated gold islands with very unusual shapes with straight edges meeting at 60° or 120° running preferentially along the ⟨110̅ ⟩ substrate directions. The density of the gold islands was found to decrease with increasing IT until they almost disappeared from the SAMs prepared after 190 h of IT. On top of the gold islands, the PFBT molecules were found to adopt the closely packed (10√3 × 2) structure. Second, a number of structural defects such as disordered regions at the domain boundaries and dark row(s) of molecules within the ordered domains of the PFBT SAMs were observed at different ITs. The SAMs prepared after 190 h of IT were found to be free of these defects. Third, at low and moderate ITs, a variation in the PFBT molecular contrast was observed. This contrast variation was found to depend mainly on the tunneling parameters. Finally, our results revealed that the organization process of PFBT SAMs is IT-dependent. Consequently, a series of structural phases, namely, α, β, γ, δ, and ε were found. The α-, β-, γ-, and δ-phases were typically accompanied by the ε-phase that appeared on top of gold islands. With increasing IT, the α→β→ γ→δ→ε phase transitions took place. The resulting ε-phase, which covered the entire gold surface after 190 h of IT, yielded well-ordered self-assembled monolayers with large domains having a (10√3 × 2) superlattice structure.

1. INTRODUCTION Self-assembled monolayers (SAMs) formed by the spontaneous adsorption of organo sulfur molecules on Au(111) have been intensively studied in the recent years due to their high structural order,1−16 ease of preparation,8−13 and a number of practical applications in chemical and biosensors,17,18 corrosion inhibition,19 and nanopatterning.17,18,20−25 Recently, SAMs of aromatic thiols received great topical interest because of their attractive electrical and optical properties that may lead to applications in molecular electronics.2−9,11,12,25−28 Most candidate aromatic thiols for molecular electronic applications are fully conjugated aromatic molecules such as benzenethiol (BT), biphenylthiol (BPT), or terphenylthiol (TPT). The © 2012 American Chemical Society

electrical properties of SAM-based molecular devices are obviously influenced by changes in both chemical functionality29 and orientation30 of the adsorbed molecules. However, to improve device performance and attain reproducible properties, the ability to fabricate two-dimensional (2D) wellordered SAMs from fully conjugated aromatic thiols is of essential importance. The self-assembly of purely aromatic thiols frequently yields SAMs of fairly low structural quality, significantly lower than that of the well-known aliphatic Received: April 19, 2012 Revised: June 12, 2012 Published: June 12, 2012 10192

dx.doi.org/10.1021/la301601c | Langmuir 2012, 28, 10192−10208

Langmuir

Article

systems.5,8,13 The self-assembly of purely aromatic thiol SAMs is usually accompanied by the formation of gold islands.5,8,13 These islands have a height of 2.4 Å and are randomly distributed over the entire gold surface. The simplest purely aromatic thiol is the benzenethiol (BT). SAMs of BT have been extensively investigated on the Au(111) surface by means of various surface analysis techniques.31−38 Disordered phases with bright gold adatom islands31−34,38or small ordered domains exhibiting lateral dimensions of less than 15 nm have been observed.37 Recently, 2D-ordered BT-SAMs with long-range-ordered domains have been successfully fabricated via the displacement of preadsorbed cyclohexanethiol SAMs on Au(111) by BT molecules.38 In their study, the BT molecules were found to adopt the (√13 × 2√5)R 46° structure on Au(111). Generally, enhanced structural ordering of aromatic thiol SAMs can be achieved either by introducing a flexible alkyl spacer between the sulfur head groups and the aromatic backbone6,8,11,13,39 or by changing the anchor group.15,40,41 However, insertion of such an alkyl spacer will considerably influence the electron transfer42,43 at the molecular interface and, consequently, the performance of the molecular devices. Therefore, preparation of highly orientated and ordered SAMs of purely aromatic thiols is prerequisite for the development of organic electronic devices with enhanced performance. Molecular electronic devices such as diodes and field effect transistors can be fabricated by incorporating different functional groups into the adsorbate backbone.44−47 SAMs with strong electron-withdrawing groups such as fluorine functionalized SAMs are found to decrease the barrier for electron or hole injection into the organic semiconductor.47−49 Previously, SAMs generated from partially fluorinated aliphatic thiols were used to improve the hole injection.50 By taking into account the much higher conductance of purely aromatic thiol SAMs, one could expect that application of fluorinated aromatic films would be a better choice for the fabrication of molecular electronic devices. Therefore, many studies have recently focused on the characterization of fluorinated aromatic thiols to figure out the optimum conditions for the preparation of 2Dordered SAMs.1,51−57 For example, parafluorothiophenol SAMs have been used to modify silver anodes of a top-emitting polymer light-emitting diode (T-PLED) to enhance the hole injection thereby improving the performance of the T-PLED device.51 Moreover, electron transfer at the molecule−metal interface of SAMs of terphenylthiol and its partially fluorinated counterpart (BFF: p-thiophenyl-nonafluoro-biphenyl) on Au(111) was investigated by core-hole clock spectroscopy.57 The results showed that the fluorination of phenyl rings considerably improves the localization of the excited electron in the LUMO and reduces the interface electron-transfer rate. More recently, SAMs of pentafluoro-benzenethiol (PFBT) on Au(111) were studied using scanning tunneling microscopy.53,58 The structural order of PFBT SAMs was found to depend significantly on the preparation conditions such as the temperature of the PFBT solution. At room temperature, the PFBT SAMs were found to include several structural defects such as disordered phases, dislocation of rows, and molecular rows having different contrast. Increasing the temperature of the solution to 75 °C, well-defined and highly ordered SAMs of PFBT were formed with a (2 × 5√13)R30° superlattice structure. Complementary to this previous work carried out on the PFBT SAMs, we have studied the effect of the immersion time (IT) to find the optimum conditions for the preparation of

highly ordered and orientated PFBT SAMs. Apart from this, we present the structural defects and a contrast variation phenomenon, which has never been reported so far for the PFBT SAMs on Au(111). Indeed, our results will add novel information toward the rational design of PFBT SAMs, which will be very helpful for the progress of SAM-based electronic devices.

2. EXPERIMENTAL SECTION 2.1. Chemicals. PFBT (Sigma-Aldrich 97%), ethanol (SigmaAldrich ≥99.8%, (GC)), acetone (Sigma-Aldrich ≥99.8% (HPLC)), and chloroform (Sigma-Aldrich ≥99.8% (HPLC)) were used without further purification. 2.2. Substrates. For STM, a freshly cleaved sheet of mica was heated to 370 °C for about 48 h inside a deposition chamber (Leybold) to remove residual water contained between the mica sheets. Au (100 nm, 99.995%, Chempur) was then evaporated at a substrate temperature of 380 °C and a pressure of 10−7 mbar. Thickness and deposition rate (20 Å s−1) were monitored using a crystal oscillator (Leybold Inficon). After deposition, the substrates were cooled and the vacuum chamber was filled with purified nitrogen. The substrates were stored under argon and flame-annealed in a butane/oxygen flame immediately before the adsorption experiments were carried out. This procedure yielded Au substrates with large terraces (diameters exceeding several 100 nm) exhibiting a (111) surface orientation. For the infrared reflection−absorption spectroscopy (IRRAS), polycrystalline Au substrates were prepared by evaporating 5 nm of titanium (99.8%, Chempur), and subsequently 100 nm of gold (99.995%, Chempur) onto polished silicon wafers (Wacker silicone) in an evaporation chamber operating at a base pressure of about 10−7 mbar. These substrates were stored in a vacuum desiccator until the adsorption experiments were carried out. 2.3. Preparation of PFBT SAMs. PFBT monolayers on Au(111) substrates were prepared at room temperature by immersing the gold substrates into 0.1 mM ethanolic solutions of the PFBT for different immersions times (see the text). After removal from the solution, the substrates were carefully rinsed with pure ethanol and dried in a nitrogen stream. 2.4. Structural Investigations. All the STM measurements were carried out in air using a Agilent technologies 5500 scanning probe microscope. Tips were prepared mechanically by cutting a 0.25 mm Pt/Ir alloy wire (8:2, Chempur). The data were collected at room temperature in constant current mode using tunneling currents between 10 and 150 pA and a sample bias between 400 and 1500 mV (tip positive). To reduce the drift, the sample was attached to the scanner 5 h before acquiring the STM micrographs. 2.5. Spectroscopic Characterization. Fourier-transform infrared (FTIR) spectra were recorded with a BioRad FTS-3000 spectrometer in absorption mode.

3. RESULTS In the present article, three central issues concerning the preparation of highly ordered SAMs of PFBT on Au(111) will be presented and discussed. First, the gold islands that are formed due to the adsorption of PFBT on Au(111) will be examined after different ITs. A wide range of IT was used to follow carefully the time evolution of the size, density, and shape of the gold islands. Also, the PFBT adlayer structures over these gold islands were precisely determined and compared to those present in the vicinities of the gold islands. Second, several types of structural defects were observed during the STM measurements in the PFBT SAMs on Au(111). The structural defects correspond to deviations from the regular long-range periodic order. They include disordered regions at the domain boundaries and the dark row(s) of molecules within the ordered domains of the SAMs. These defects were found to 10193


Langmuir

Article

Figure 1. (a−g) STM images of PFBT SAMs on Au(111) prepared with different immersion times (ITs, indicated below each image). The dotted curves in (c) and (f) show the trenches. The images were recorded at tunneling conditions: I = 20−50 pA and V = 900−1200 mV.

significantly depend on the preparation conditions of the SAMs. Furthermore, contrast variations were observed in the STM images of the PFBT SAMs studied. It was found that they depend on the STM imaging conditions. The contrast variations usually become visible at the atomic scale such as the variation in the apparent molecular topographic heights within the same row of the ordered domains. In most cases, the contrast variation was found to disappear when appropriate tunneling conditions were used. Also, the polarity of the bias voltage was found to have a significant influence on the appearance of the contrast variation. Compared to previously investigated systems of thiols and selenols on Au(111),5,6 the fluorinated-SAMs are strongly affected by the STM imaging conditions. In this work, very extreme tunneling parameters were required to give nondestructive imaging after consecutive

STM scans over the same area such as a current set point (I) and the bias voltage (V) in the range of 10−60 pA and 900− 1300 mV, respectively. Also, the polarity of the bias voltage was found to have a significant influence on the appearance of the contrast variation. Recently, similar extreme tunneling conditions were reported for imaging perfluoro-terphenyl-substituted alkanethiols on Au(111).1 Such conditions were not required for STM imaging of the n-alkanethiols or aromatic thiols on Au(111).4,10 Third, the large body of high-quality STM data collected in the present work enables us to exactly determine the PFBT adlayer structures at different ITs. Therefore, the third section in this article will focus on analyzing and discussing the different structures, which were formed at different ITs, and their evolution with time. 10194


Langmuir

Article

Figure 2. (a) (i−iv) STM images of PFBT SAMs on Au(111) formed after 16 h of ITs. The images show the disappearance of some gold islands with time. All the images were recorded at tunneling conditions: I = 20−40 pA and V = −1200 mV. (b) Schematic illustration showing the growth of the gold islands with ITs.

3.2. Effect of the Immersion Times (ITs) on the Growth of the Gold Islands. The STM data showed that the PFBT SAMs prepared at 5 and 30 min of IT exhibit an identical surface morphology. An STM image showing this SAM morphology is displayed in Figure 1a (i). Interestingly, no etch-pits were observed for the PFBT-SAMs, which instead exhibit small adatom islands. These elevated islands have a height corresponding to that of a single atomic step on the

Au(111) surface (2.4 Å) and typical diameters of 2−5 nm. An analogous morphology has also been observed for the thioland selenol-derived SAMs with a purely aromatic backbone on Au(111).5,8,13,59,60 The appearance of such gold islands instead of the etch-pits has been attributed to the differences in the surface strain of the gold substrate upon formation of SAMs of different anchoring groups.5,8,15 The formation of the islands is due to Au atoms on the surface, which become highly mobile 10195


Langmuir

Article

because of the strong binding to the thiols.59 As can be seen from the image, the small gold islands are occasionally arranged along straight lines having different directions (see the arrows). The STM image displayed in Figure 1a (ii) is for the same surface area as shown in (i), but after successive STM scanning at nondestructive conditions (I = 30 pA and V = 1200 mV) carried out over the area located almost at the center of the image. The multiscanned area is marked with the rectangle in the image (ii). Clearly, successive STM scanning, even at safe conditions, causes the removal of the small adatom gold islands. This result indicates that, in the initial stage of the SAM growth, the small gold adatom islands are unstable on the gold surface and they move under the STM-tip. The STM image in Figure 1 (a−iii) shows an area of the PFBT SAM prepared with 0.5 h of IT, after successive STM scans. The small gold islands completely disappeared and the resulting surface is fully smooth analogous to the pure gold surface. The driving force behind the disappearance of these small gold islands is not well understood. One possible explanation for this might be the small size of these gold islands, which makes them mobile over the gold terraces under the influence of the STM-tip to finally coalescence with the substrate step-edges. In (b), an STM image showing the surface morphology of PFBT SAMs for samples prepared with 4 h of IT is displayed. The size of the majority of the islands has increased at the expense of their density. Also, small gold adatom islands as those observed at 0.5 h IT were observed. The large gold islands have lateral dimensions of 7−25 nm with mostly round or oval shape. These gold islands are arranged along straight rows (see the arrows). Successive STM scanning over the same area, even at perturbing tunneling conditions (I = 0.5 nA, V = 500 mV), did not eliminate these large gold islands. On the other hand, the small gold islands were observed to coalesce with either the larger ones or with the nearest step-edges, as will be discussed later. Further increase of the IT to 32 h significantly increases the size of the gold islands, as can be seen in the STM images presented in Figure 1c,d. This increase in the size of the gold islands occurred also at the expense of their density. The lateral dimensions of the gold islands are 20−80 nm and 40−120 nm at 16 and 32 h of ITs, respectively. A further increase of the IT to 115 h causes a reduction in both the density and the size of the gold islands (see the STM images displayed in Figure 1e,f). At an IT of 115 h, only a few islands are present on the surface and the terraces are isolated from their neighbors by deep trenches. The depth of these trenches reaches several gold layers. In fact, the trenches appeared in SAMs prepared with ITs of 16 h and higher (see the image in part (c)). The trenches are marked in Figure 1c,f by dotted loops. Moreover, the SAMs prepared with ITs longer than 90 h revealed the formation of two distinct structures of gold terraces; facets with straight step edges along the ⟨11̅0⟩ directions and structures with rounded step-edges. When the PFBT SAMs were prepared with very long ITs, such as 190 h, major changes concerning the size and density of the gold islands were observed. The surface became almost free of the gold islands (see Figure 1g). This dramatic change in the surface morphology of PFBT SAMs has not been reported for the other aromatic thiols that form gold islands on Au(111) at room temperature. Similar surface morphologies have only been observed for annealed SAMs of aromatic thiols on Au(111).12,61 Another important observation worth noting is the pronounced change in the shape of the gold islands that

were formed after long ITs (16 h and higher). The islands exhibited rather unusual shapes with straight edges meeting at 60° or 120° instead of the usual rounded shape. Representative STM images showing this interesting change are displayed in Figure 1c−f. Also, the gold islands were found to grow directionally, not randomly, as was observed for other purely aromatic thiol/selenol systems on Au(111).5,8,15 It is obvious that the irregularly shaped islands have a preferred orientation on the surface adopting three different directions separated by 60° or 120° from each other (see Figure 1c−f). The steps defining the borders of these gold islands run preferentially along the faceting of the Au(111) substrate edges, i.e., along the ⟨11̅0⟩ substrate directions, where a dense packing of step edge atoms can be attained. The striking correlation of the preferred edge orientation with the order and orientation of the adsorbed molecules covering the islands will be discussed in the section on the ε-phase. When PFBT SAMs were prepared with ITs of 32 h and higher, rough surfaces with multilayer gold steps terminated by step edges with zigzag structure were formed (see Figure 1d− g). Such roughness in the PFBT-modified gold substrates prepared at high IT has not been observed before for SAMs prepared from shorter ITs or for the pure Au(111) substrate. This indicates that the multistep restructuring of the Au top layer(s) at RT is unique for the PFBT molecules. This process can only take place if PFBT SAMs are prepared with long IT. Also, we would like to mention that, after this radical change in the surface morphology, the step-edges did not change their orientation that is running along the ⟨11̅0⟩ directions. To look closer at this unusual behavior of the PFBTmodified gold substrates, a series of STM images were recorded for the same area of a PFBT SAM, which was prepared with IT of 16 h, and are shown in Figure 2a(i−iv). These images were sequentially recorded, at safe tunneling conditions, from (i) to (iv) with a time interval of 5 min between two consecutive images. The STM image in (i) consists of a terrace carrying small and large gold islands and terminated by a step-edge. The small gold islands marked by the circles were found to almost disappear after short time and to dissipate at the nearest stepedge, which is marked by the small square, and consequently, the latter was expanded. Moreover, the shape of the large island (see the arrow) changed with a noticeable increment in its size. Apparently, a number of small islands were integrated into the large islands. As a result, the reorganized and oriented islands appeared with very sharp boundaries. On the basis of these observations, we propose a model for the evolution of the two-dimensional (2-D) gold islands. A schematic illustration of this model is displayed in Figure 2b. At short IT, the adsorbate(s)−gold atom(s) species will be initially nucleated into small and highly mobile gold islands (seeds) such as those observed in Figure 1a (see Figure 2b−i). The source of the gold atoms is expected to be the step-edges of the gold substrate, because the gold atoms at these substrate defects have a lower coordination number compared to that present in the bulk of the terraces. The driving force behind the disintegration of gold atoms in the top Au layer is to introduce an energetic stabilization for the thiolate adlayer.62 With increasing IT, the neighboring seeds are aggregated to form large islands as those observed in Figure 1b. Further increasing the IT results in a pronounced increase in the 2-D growth of the large islands (see Figure 2b(ii) and (iii)). The large islands are expected to grow following the Ostwald ripening mechanism; larger islands grow at the expense of smaller 10196


Langmuir

Article

ones. At this stage, the shape of the large islands is changed from a rounded to an unusual shape with very sharp boundaries meeting at 60° or 120°. As will be discussed later, each oriented gold island typically carries only one directionally ordered domain. Also, the direction of an ordered domain on top of an island is typically the same as the direction of the gold island beneath the ordered domain. Since the islands are rotated from each other by an angle of 60° or 120°, we conclude that the adlayer of PFBT molecules over the islands is commensurate with the underlying Au substrate. From the STM images in Figure 1c−f, the gold islands were found to adopt a preferential direction on the surface. This implies that the PFBT SAMs have a preference to form a single domain structure over the entire gold terrace. As we will see later, the PFBT-SAM structure covering the islands is different from that present between the islands. For reasons that will be mentioned later, the structure that exists on top of the islands is expected to be energetically more stable. Therefore, the adsorbates in the vicinities of the gold islands have a strong driving force to coalesce with the ordered domains that are growing on the islands. Since the diffusing species are in the form of adsorbate−gold atom(s), they will attach with the edges of the island borders along the ⟨11̅0⟩ directions. Therefore, the size of the oriented islands increases along the ⟨11̅0⟩ directions. It is still not clear why the restructuring of the gold islands requires such a long time. Since there are principle factors determining a SAM structure such as intermolecular interactions, molecule−substrate bonding, and interactions of the SAM with the environment, one of these factors will be predominant and is expected to play an important role in forming the energetically most stable structure. In the case of PFBT SAMs, the predominant factor is expected to be the intermolecular interactions, which will be discussed later. As mentioned above, the large oriented islands are found to coexist together with the small gold islands, which have round shapes. A number of these small islands are expected to migrate into the nearest step-edges, where they finally result in restructuring the step edges. The new structure of the step edges is very close to the zigzag structure. An additional increase in the IT to 190 h resulted in the disappearance of the orientated islands. In our model, we suggest that the large and orientated gold islands did not disappear from the surface, but with increasing IT, their size becomes very large and covers almost the entire underlying gold terraces as shown in Figure 2(iv). The fraction of the Au top layer that is covered by the large gold islands in conjunction with the step edges, where the random processes of removing and relinking gold atoms have occurred, is behind the formation of the multilayer gold steps. 3.3. Structural Defects. Although the SAMs of PFBT on Au(111) do not show the well-known vacancy island defects (etch-pits) in the top layer of the gold surface, a number of other types of defects have been detected especially at high magnification. The density of these defects was found to be comparatively high for the SAMs prepared at ITs between 16 and 115 h. In Figure 3a,b, the STM images exhibit at least three different types of defects. The first kind of defect appears in the STM images as cracks within the ordered domains. In the STM images, the cracks are marked by the white dotted loops. These cracks appear in the STM images as dark areas with an apparent decrease in height of 1.2 to 1.7 Å smaller than the typical depth of the etch-pits (2.5 Å). Moreover, the cracks exhibit irregular shapes, which are fundamentally different from the usual shapes

Figure 3. (a,b) STM images of PFBT SAMs on Au(111) formed after 115 and 90 h of IT, respectively. The images show different types of defects; cracks (white dotted loops), rows of dark molecules (white solid rectangles), bright spots (blue solid circles), and domain boundaries (blue dotted loops). The images were acquired at tunneling conditions: I = 15 pA and V = 950 mV and I = 25 pA and V = −1100 mV, respectively.

of the etch-pits, i.e., the round and triangular shapes. Higherresolution images, such as the image displayed in Figure 8b, reveal that the cracks are free of PFBT molecules. The second type of defects appears as dark molecular rows in the ordered domains. This type of defect can be distinguished from the cracks easily because it appears at a lower apparent height (∼0.75 Å) and has a regular shape. For examples, see the white-solid rectangles in Figure 3a. At high magnification such as the image displayed in (b), it can be seen that the dark rows are fully populated by adsorbate molecules exhibiting the same ordered structure present in their vicinity. The origin of these dark rows with a lower topographical height could be attributed to a mismatch between the ideal structure favored by the PFBT adsorbates and the lattice constant of the underlying gold substrate. Such a mismatch has been observed for different aromatic thiols on Au(111).63 The third type of the structural defect appears in the STM images as bright spots, marked in the images by blue solid circles. These spots have a height of about 2.5 Å, which is consistent with the expected height of a Au(111) single atomic step. Therefore, these spots are attributed to PFBT-gold species, which are expected to diffuse over the surface and combine with either the large islands or with the nearest neighboring step-edges. These bright spots are also expected to play a major role in the restructuring process of the gold surface top layers to finally yield the formation of the multilayer steps surfaces such as those displayed in Figure 1e and g. The defects remaining in PFBT SAMs after long ITs are expected to have little effect on the properties of the SAM because their densities are low, compared with those observed in the other aromatic thiol and n-alkanethiol SAMs on Au(111).6,10 In particular, n-alkanethiol and aromatic thiol SAMs usually accommodate a high density of defects having a significant effect on the SAM structure such as the etch-pits and the presence of disordered regions across the domain boundaries. The presence of the etch pit defects in these SAMs has a huge impact on the domain sizes, e.g., rather small domain sizes (5−20 nm) were observed for BPn SAMs at RT.7,11 In the case of PFBT SAMs, the etch pit defects are absent and the regions separating different ordered domains are well-ordered (see the dotted loops in the STM images displayed in Figures 3b and 8c). On the basis of the foregoing, the PFBT SAMs prepared at high ITs (such as 190 h) yield a 10197


Langmuir

Article

Figure 4. STM images displayed in (a), (b), and (c) show the PFBT molecular resolution on Au(111). The images were acquired for samples prepared from 32−115 h of ITs. The tunneling conditions are indicated in each image.

3.4. Contrast Variation. As mentioned above, a contrast variation was observed in the STM images, which corresponds

long-range order with domains sizes exceeding 200 nm at RT (see Figure 9b). 10198


Langmuir

Article

Figure 5. (a−e) High-resolution STM images of PFBT SAMs on Au(111) formed after 10 h of IT. The images show the α-phase. The rectangular unit cell of the (17 × √3) structure of the α-phase is marked in images (d) and (e). The tunneling conditions of (a−c) are I = 20 pA and V = −1000 mV. The tunneling conditions of (d) and (e) are as follows: I = 18 pA and V = −900 mV, I = 23 pA and V = −1300 mV, I = 30 pA and V = 900 mV, and I = 30 pA and V = −1200 mV, respectively. (f) Cross-section profiles along lines A and B in (d).

molecular resolution throughout the whole STM image; in a part of each image, the molecular-resolution appears with a lower quality than the remainder of the image (see the dashed lines in the images). Such results were obtained for the PFBT SAMs prepared at ITs lower than 190 h. It is worth noting that the quality of the molecular resolution was considerably improved when the value of tunneling current was extremely reduced (i.e., with increasing the gap between the STM tip and the SAM). Also, the topographic height of the molecular rows was found to alternate; at I = 400 and 70 pA, at least three adjacent rows are imaged more brightly, whereas at I = 12 pA, two brighter rows separated by a slightly darker row were observed. This means that when the same surface area of PFBT SAM is scanned several times at different imaging conditions, the contrast of the same molecular row changes and several unit cell dimensions for the same SAM structure could be proposed. For this reason, we believe that the variation in the apparent topographic height of the PFBT molecular rows is not related to the PFBT monolayer structure but to the imaging conditions. So, it is not sufficient to rely on one STM image to determine the dimensions of the superlattice unit cell as it is usually carried out for the other thiol-based SAMs on Au(111). A third parameter that was examined is the effect of bias voltage on the PFBT-SAMs. During the measurements, the tunneling current was fixed at an extremely low value (15 pA) and the bias voltage was increased incrementally. Under these imaging conditions, the STM images, that are displayed in Figure 4c (i−iv), were acquired over the same area. When moderate positive sample biases (V = 800−1000 mV) were used, the significant improvement in the quality of the resolution of the STM images was observed. Eventually, under these conditions the STM images appeared to be quite stable and reproducible with no change in the topographic height of the adsorbates.

to a change in the apparent height within the PFBT SAMs. This variation was found to depend significantly on the tunneling conditions. Under specific tunneling conditions, the contrast variation appears in the STM images, and when the imaging parameters are adjusted to the appropriate values, it disappears. Therefore, the contrast variation is not due to a slight misorientation of the adsorbed molecules or shifted adsorption site. Occasionally, it turns out to be extremely difficult to find suitable imaging parameters for the current set point and the bias voltage, which severely hampers assigning the precise dimension of the unit cell within the PFBT SAMs. On the basis of these observations, we found that it is essential to focus on this aspect. Figure 4a (i−iv) shows STM images recorded for the same surface area of PFBT-modified gold. The images displayed in (i) and (ii) were acquired at the same tunneling parameters (i.e., I and V) but the direction of the scanning was different (see the arrows in the images). At first glance, it seems that the two images were recorded for different areas on the surface. For example, paired and separated molecular rows are present in (i) and (ii), respectively. Also, the bright patch present in the lower part of the image (ii) is absent in (i). The other parameter which is worth mentioning is the polarity of the STM bias voltage. When the sign of the bias voltage was changed, the observed structures appeared rather different (see (iii) and (iv)). Rows of dark molecules that are absent in (i) and (ii) appear in the images displayed in (iii) and (iv) (see the white arrows). Moreover, the bottom half of the images exhibit low-quality molecular resolution at the negative bias voltage. Therefore, changing the polarity of the bias voltage or the direction of the scanning significantly affects the topographical contrast of the PFBT adsorbates and the quality of the molecular-resolution STM images. In the STM images of the same surface area that are displayed in Figure 4b (i−viii), the value of the tunneling current (I) was gradually decreased as we go from (i) to (viii). Again, it was not possible to obtain the same quality of 10199


Langmuir

Article

Figure 6. (a,b) High-resolution STM images of PFBT SAMs on Au(111) formed after 16 h of IT. The images show the β-phase. The rectangular unit cell of the (13 × √3) structure of the β-phase is marked in image (b). The tunneling conditions of (a) and (b) are I = 20 pA and V = −1000 mV and I = 15 pA and V = −1000 mV, respectively. (c) Cross-section profiles along lines A and B in (b).

islands, we believe the existence of a second overlayer structure on top of the gold islands coexists alongside the α-phase. Concerning the α-phase, the ordered domains are oriented with an angle of 60° or 120° to each other. This indicates that the formation of PFBT SAMs was clearly influenced by the threefold symmetry of the Au(111) lattice. The length of the ordered domains is fairly small with values ranging from 15 to 40 nm. Also, the molecular stripes do not appear clearly in the STM image displayed in (a) due to the presence of the gold islands. It was difficult to find a large area free of these elevated islands. Moreover, the α-phase suffers from the lack of the systematic distances between the stripes (see the images displayed in Figure 5b,c). Accordingly, determination of the unit cell dimensions of the α-phase was not an easy task. Therefore, the most accurate unit cell dimensions was determined from STM images that exhibit regularity in the topographic height of the molecules along the stripes, such as those displayed in Figure 5d,e. The cross-sectional profiles in Figure 5f along lines A and B corresponding to the unit cell in Figure 5d,e show the periodicities of the adsorbed molecules on the PFBT SAMs. The adlayer structure of the α-phase exhibits the lattice constants of a rectangular unit cell were a = 50 ± 1.5 Å = 17ah, b = 5.0 ± 0.6 Å = √3ah, and α = 90 ± 1°, where ah = 2.89 Å denotes the interatomic distance of the Au(111) lattice. The structure was assigned as a (17 × √3) lattice. The unit cell contains eight molecules corresponding to an area of 30.6 Å2 per molecule. β-Phase. In Figure 6a,b, STM images were taken for PFBT SAMs that are prepared with 16 h of IT. As in the case of αphase, the ordered domains of the new phase, which henceforth will be denoted as β, are consisting of uniform molecular clusters in a stripe configuration. The striped domains of βphase have the same orientation, parallel to the ⟨112̅⟩ direction of the Au(111) lattice. Again, the ordered domains of the βphase are coexisting on the surface with the gold islands. The length of the ordered domains was found to be in the range 15−50 nm. The unit cell of the β-phase is marked by the rectangular box in the STM image (b). The lattice constants extracted from image (b) (see the cross-sectional profiles in Figure 6c) are as follows: a = 37.5 ± 1 Å = 13ah, b = 5 ± 0.2 Å = √3ah, and α = 90 ± 1°. The unit cell contains eight molecules; therefore, packing structure was assigned as a (13 × √3) structure, corresponding to an area of 23.4 Å2 per molecule. γ-Phase. Further increasing the IT to 32 h, the adsorbed monolayer forms a new structural phase with different molecular arrangement from the previously observed α- and β-phases. Before further discussion on the structure of this new

In Figure 4c (i) and (iv), the polarity of the bias voltage was changed as displayed in the images. When negative values were used, the resolution was significantly improved. Depending on the above findings, we can conclude that obtaining reproducible STM data for the PFBT SAMs on Au(111) is not straightforward as expected. Such contrast variation has never been previously observed for the benzenethiol (BT) SAMs, the analogues to PFBT system. Since the only difference between the BT and PFBT molecular structures is the fluorine atoms in PFBT, they could be the main cause for the emergence of the contrast variation. In a nutshell, to achieve the highest and reproducible STM resolution for PFBT SAMs, a very low tunneling current (∼15 pA) and a high negative biased voltage (∼ −1000 mV) should be used. 3.5. Structural Analysis of PFBT SAMs at Different ITs. 3.5.1. Results. 3.5.1.1. STM. In this section, the periodic adsorbate structure of the PFBT SAMs observed at different ITs will be separately analyzed and discussed. As mentioned above, it was very difficult to determine precisely the dimensions of the superlattice unit cells formed by PFBT on Au(111) at different ITs. In samples prepared at short ITs (i.e., below 4 h), acquiring STM data with high-molecular resolution showing the exact molecular adlayer structure was not straightforward and turned out to be extremely difficult. This might be due to the small elevated gold islands that cover the largest fraction of the thiol-modified gold surface (see Figure 1a,b). Even when these small gold islands were removed by a repeated STM scanning (see Figure 1c), the multiscanned surface did not show adsorbed thiol molecules. α-Phase. Obtaining high-resolution STM images was only possible for SAMs prepared with ITs higher than 6 h. The SAMs prepared with ITs between 6 to 10 h yielded an identical SAM structure, and the data (recorded at nondestructive tunneling conditions) are summarized in Figure 5. In (a), the STM image shows the presence of stripe-like arrays coexisting with the gold islands. The stripes are aligned along the ⟨112̅⟩ substrate directions. This phase will, in the following, be referred to as phase α. At this stage, some gold islands started to adopt the unusual shapes with straight edges such as the islands labeled A and B in the images displayed in (a) and (b). Due to the small size of these islands, it was not possible to determine the adlayer structure on top of the islands. However, these islands were found to be aligned along the ⟨110̅ ⟩ substrate directions. As we will see later, the molecular rows on top take the same direction as the oriented island beneath them. Since the direction of the stripe arrays on the islands is different from that found in the surrounding areas of the 10200


Langmuir

Article

Figure 7. (a−d) High-resolution STM images of PFBT SAMs on Au(111) formed after 32 h of IT. The images in (c) and (d) show the γ-phase. The rectangular unit cell of the (10 × √3) structure of the γ-phase is marked in (d). The tunneling parameters of the images are I = 12−40 pA and V = 850−1200 mV. (e) Cross-section profiles along lines A and B in (d).

Figure 8. (a−f) High-resolution STM images of PFBT SAMs on Au(111) formed after 115 h of IT. The images (b−e) show the δ-phase. The inset in (b) shows the molecular resolution on top of the gold islands. The rectangular unit cell of the (5√3 × 2) structure of the δ-phase is marked in (d) and (e). The tunneling conditions of the images are I = 15−50 pA and V = 900−1100 mV. (f) Cross-section profiles along lines A and B in (e).

phase, denoted as the γ-phase, we would like to mention that the gold islands are now perfectly straight and are aligned along the ⟨11̅0⟩ direction (see Figure 7a). The results of the γ-phase are summarized in Figure 7a−e. Similar to the α- and β-phases, the stripes of the molecular rows are running along the ⟨112̅⟩ substrate directions. This indicates that the gold islands spread over the surface are taking different directions from that of the molecular rows between these islands. Unfortunately, the quality of the molecular resolution STM images on top of the islands is not sufficient to determine the overlayer structure and to tell whether this structure is the same as that existing in the surrounding areas of the islands. According to the γ-phase that is imaged in areas surrounding the gold islands, a variable

periodicity between the brighter molecular rows was observed. The analysis gave separation distances of 2.9, 3.7, and 5 nm (see the image presented in (b)). Among these values, the separation distance of 2.9 nm was the predominant one. The values of 5 and 3.7 nm are equal to those of the α- and βphases, respectively. The possibility that the α- and β-phases coexist with the γ-phase is likely, because their stripe domains have the same directions. The stripe separation with a periodicity of about 2.9 nm will be considered in the analysis of the γ-phase. As demonstrated by the cross-sectional profiles shown in part (e) of Figure 7, the distance between the protrusions along the rows amount to b = 5 ± 0.2 Å. Thus, the ad-structure of PFBT for the γ-phase exhibits a rectangular (10 10201


Langmuir

Article

Figure 9. (a−e) High-resolution STM images of PFBT SAMs on Au(111) formed after 190 h of IT. The images in (b−e) show the ε-phase. The rectangular unit cell of the (10√3 × 2) structure of the ε-phase is marked in (e). The tunneling conditions of the images are I = 10−80 pA and V = 800−1400 V. (f) Cross-section profiles along lines A and B in (e).

the unit cell is 90 ± 2°. Therefore, the structure of the PFBT SAMs with δ-phase can be assigned as the (5√3 × 2) overlayer structure. The rectangular unit cell of the δ-phase was found to accommodate five molecules, and therefore, the average areal density for the adsorbed PFBT molecule was calculated to be 28.8 Å2/molecule. ε-Phase. Several attempts have been carried out to figure out preparation and imaging conditions such as the IT and the tunneling parameters, respectively, in order to prepare highly ordered and defect-free PFBT SAMs. Our goal was achieved by increasing the IT to 190 h and using tunneling conditions of I = 10−80 pA and V = 900−1400 V. The resulting SAMs gave an extraordinary order with domain sizes exceeding 200 nm, which appear to be free of defects. In Figure 9a−e, the STM data obtained for PFBT SAMs prepared with 190 h of immersion time is summarized. In these images, the gold islands have almost disappeared from the surface (see Figures 1g and 9a). The images displayed in Figure 9b−d were intentionally chosen with such a high number of islands to provide a clear evidence that the orientation of the islands is in full accordance with that of the ordered domains present in the surrounding areas of the islands. In rare cases, triangular or round gold islands were also observed. A close examination of the molecular resolution on top of these islands shows the presence of two or more ordered domains of different directions (see Figure 9d). A detailed analysis of the PFBT adlayer structure on the gold islands (see Figure 9c,d) and comparison to the structure present in the areas between the islands clearly indicates that the two structures are exactly identical in nature. This indicates that after 190 h of IT only a single phase, denoted as ε-phase, is present in the PFBT SAMs. For PFBT SAMs prepared with lower ITs, the ε-phase was usually coexisting with one of the α-, β-, γ-, and δ-phases. Again, the ordered domains of this novel phase on and between the gold islands are composed of rows,

× √3) unit cell. The unit cell of the γ-phase is shown in Figure 7d. The unit cell contains six molecules yielding an area per molecule of 24.0 Å2. δ-Phase. After careful analysis, the PFBT SAMs prepared from 90 to 115 h exhibited a new structural phase, labeled δ, which is different from the previous α-, β-, and γ-phases. STM images showing the δ-phase are displayed in Figure 8a−e. In large-scale STM images as presented in (a), the surface looks very smooth with sharp step-edges having a zigzag structure. Also, only a small number of the oriented gold islands are present on the surface with directions that are collinear with the step-edges, i.e., along the ⟨110̅ ⟩ direction. In contrast to the α-, β-, and γ-phases, the stripes of the molecular rows are running along the ⟨11̅0⟩ substrate directions. Moreover, the long-range order in the PFBT overlayer was substantially improved so that the molecular domain sizes exceed 100 nm as evident by the arrow marked in (a). In Figure 8b, the inset shows the PFBT molecular resolution on top of the gold islands. It is obvious that each island carries a unidirectional ordered domain. Also, both the carrier (the gold islands) and the ordered domains on top of the islands are oriented in the same direction. Such a correlation of the orientation between the gold islands and the molecular rows covering these islands has never been reported for any system on Au(111). At molecular-scale resolution, the δ-phase is distinguished from its predecessors by the fact that it is free of the contrast variation of the adsorbed molecules (see the STM images in Figure 8d,e). The crosssectional profiles in Figure 8f taken along lines A and B labeled in the STM image (e), corresponding to the rectangular unit cell marked in Figure 8d,e indicate a periodic structure of PFBT SAMs in the δ-phase. The lattice constants extracted from the high-resolution images are as follows: a = 5.5 ± 0.5 Å = 2ah, b = 25 ± 1.5 Å = 5√3ah, and the angle between the two vectors of 10202


Langmuir

Article

whose orientations reflect the Au(111) threefold symmetry (see Figure 9c). On some surface areas, the ε-phase was found to adopt a preferred unidirectional orientation (see the STM images in (b) and (d)). As in the case of the δ-phase, the ordered domains of the ε-phase are running along the ⟨110̅ ⟩ substrate directions. Another typical feature that is noticed for this structure is the appearance of bright, densely packed equidistance pair rows of molecules which alternate consistently in the STM contrast. Also, the boundary regions between the ordered domains of ε-phase are clear and well-ordered. This high degree of structural order in the domain boundary regions within the organic adlayer seems to be unique to the PFBT SAMs so that it was not found in the commonly studied systems of n-alkanethiol and aromatic thiol SAMs on Au(111) at room temperature. For a detailed analysis of the structure of the ε-phase, we turn to the high-resolution STM image presented in Figure 9e together with the corresponding cross sections in Figure 9f. The measured lattice constants, for the ε-phase structure, are a = 50 ± 1 Å = 10√3ah, b = 5.7 ± 0.2 Å = 2ah, and α = 90 ± 1°. These findings could be associated with the (10√3 × 2) lattice. The unit cell consists of 10 molecules. Therefore, the area per PFBT molecule is about 28.8 Å2. Although the STM analysis concerning the dimensions of the unit cells of the α- and εphases yielded almost an identical lattice dimensions, we rule out the possibility that the two phases are only a single phase due to the following two reasons: first, in the case of the εphase, the orientation of the alternating molecular rows is along the ⟨11̅0⟩ instead of the ⟨112̅⟩ directions observed for the αphase. Finally, the two unit cells in the α- and ε-phases accommodate different numbers of PFBT molecules; ten and eight PFBT molecules were found in the (10√3 × 2) and (17 × √3) structures of the ε- and α-phases, respectively. 3.5.1.2. FTIR. Figure 10 shows the low-frequency regions of IRRAS spectra collected for PFBT SAMs formed at different ITs and the IR spectrum of a corresponding bulk sample on Au(111). Several peak positions in the spectra of the PFBT SAMs are slightly different from those of the bulk. The peak assignments were carried out by comparing the PFBT SAM spectra to theoretical calculations and by consulting the data in the literature65 (note: the PFBT molecule was fully optimized and vibratonal frequencies (not scaled) were calculated at the B3LYP/cc-pVT level using the Gaussian 03 program64). The results are summarized in Table 1. In the bulk spectrum, vibrational modes at 867, 925, 1024, 1090, 1496, and 1514 cm−1 are observed. The spectra of the SAMs are dominated by three vibrations located at 854, 1097, and 1517 cm−1. The peaks at 1097 and 1517 cm−1 are assigned to combined benzene ring C−C stretching and C−F stretching modes that have transition dipole moments (TDM) oriented in-plane, parallel to the molecular S1−F4 axis. The peak located at 854 cm−1 is assigned to the C−S stretching mode with the TDM parallel to this bond. To make the comparison between the spectra simple, the bands at 1517 cm−1 were normalized. According to the infrared surface selection rule, only vibrations with transition dipole moments oriented perpendicular to the metal substrate are observed. Therefore, peak intensities are directly related to the component of each transition moment that is perpendicular to the metal surface. The peaks at 854, 1097, and 1517 cm−1 observed for the SAMs prepared with different immersion times are assigned to vibrational modes with a TDM parallel to the long axis of the molecule through the sulfur and para fluorine (1,4-axis) as

Figure 10. (a) Low-frequency regions of IR spectra for PFBT bulk and SAMs prepared at different IT. (b) Calculated vibrational modes at 871, 1101, 1520, and 1531 cm−1, respectively. Atom labels are indicated in the left structure.

Table 1. Positions in cm−1 and Assignment of IR Modes in PFBT Surface Layers on Au(111) bulk

SAM

calc.

band assignment

867 925 1024 1090 1496 1514

854 n.r n.r 1097 n.r 1517

871 931 1036 1101 1520 1531

C−S stretch, parallel 1,4 axis C−S−H bend C−S−H bend ν (C−F) and ν (C−C), // 1,4-axis ν (C−F) and ν (C−C), // 3,5-axis ν (C−F) and ν (C−C), // 1,4-axis

can be seen from Table 1 and Figure 10b. Therefore, the molecules must be in a fairly upright orientation with the 1,4axis forming an acute angle with the surface normal. The vibrational mode located at 1496 cm−1 in the bulk spectrum is carrying a TDM in the molecular plane (ip), perpendicular to the 1,4-axis. This peak is not present in the SAMs spectra. We attribute this to an orientation of the transition dipole moment parallel to the surface. The fact that the unobserved peak near 1496 cm−1 is larger than the 1514 cm−1 peak in the bulk spectrum limits the deviation of its TDM direction from coplanarity with the surface. A peak larger than 10% of the 1517 cm−1 peak should be visible in the IRAS spectra. Hence, the TDM of the unobserved peak must be parallel to the 10203


Langmuir

Article

surface within ±6°. No significant changes in the spectra were observed in the SAMs prepared with different ITs. 3.5.2. Structural Models for the Different Phases. On the basis of the STM data, models are proposed for the series of the structures that formed at different ITs, namely, the α-, β-, γ-, δ-, and ε-phase in Figure 11. In these structural models, it was

angles are in good agreement with the fairly upright orientation of the molecules derived from the IR spectra. 3.5.3. Discussion. Our STM results showed that immersion of the pure gold substrates into diluted solutions of PFBT results in the formation of different structural phases. Before we start discussing these phases, it is essential to first summarize the most important results obtained for the gold islands, such as their growth morphologhy, and to identify the PFBT overlayer structure on these gold islands. Initially (at IT of 4 h), the islands appeared in a small size with rounded shape. Highresolution STM data could not resolve the individual PFBT molecules on these islands. Increasing the IT to 16 h, the gold islands become larger and are oriented along the ⟨11̅0⟩ directions of the substrate. Also, their shape gradually changed to become closer to unusual shapes with straight edges meeting at 60° or 120°. Again, it was not possible to identify the structure of the molecular layer on these islands. Upon further increase of the IT to 32 h, the orientation of the gold islands along the ⟨11̅0⟩ substrate directions becomes perfect, and STM imaging of the PFBT adlayer on these islands with molecular resolution became possible and was routinely achieved. Nevertheless, determination of the unit cell size on the gold islands was not possible due to the narrow width of the gold islands. Finally, as demonstrated in Figure 1g and Figure 9a, the elongated and oriented gold islands have almost disappeared from the surface at an IT of 190 h. A close inspection to the high-resolution STM images in Figure 9c,d obtained for areas on the top of the remaining gold islands yields the same (10√3 × 2) structure as that observed for the ε-phase. On the smaller gold islands present after shorter immersion times, the observed direction of the molecular rows along ⟨110⟩ is consistent with that of the δ- and ε-phases. Therefore, we may generalize that the gold islands formed at different ITs form δor ε-phases or a mixture of both. After identification of the structure on top of the gold islands, we turn to discuss in more detail the structural changes and the phase transitions in the PFBT films formed at different ITs. At low ITs (6−10 h), the low coverage α-phase with an area per molecule of 30.60 Å2 was initially assembled on the gold surface along with the ε-phase that is expected to be present on top of the round-shaped gold islands. The area occupied by a single PFBT molecule in the α-phase is large compared to that found for n-alkanethiols or aromatic thiols SAMs on Au(111) under the same preparation conditions.6−8,13,66,67 A rough estimate based on the van der Waals dimensions of the phenyl backbone yields a tilt angle of 46° with respect to the surface normal. This tilt angle value indicates that the PFBT is highly tilted away from the surface normal but not lying down on the surface. The gradual increase of the IT to 16, 32, and 115 h resulted in the formation of the β-, γ-, and δ-phases, respectively. Each of these phases was also accompanied by the δ- or ε-phase, i.e., by the gold islands. The molecular area occupied by a single

Figure 11. Proposed structural models for the different phases observed for PFBT SAMs at different ITs.

suggested that the sulfur atoms occupy different adsorption sites of the Au(111) lattice. Moreover, we propose a herringbone-like arrangement of the phenyl backbones in the different unit cells (not shown).5−7 The lattice dimensions, number of molecules in the unit cell, and area per molecule are summarized in Table 2. Taking into account the van der Waals dimensions of the molecule (a cross-sectional area of 21.1 Å28 for the phenyl ring) and the area per molecule in the α, β, γ, δ, and ε structures, the phenyl axes should be tilted away from the surface normal. The tilt angles are estimated by evaluating arccos(21.1/Amolecule(phase)) = 46°, 26°, 29°, 43°, and 43°, respectively, with respect to the surface normal. These tilt Table 2. Structural Data of the PFBT SAM Phases phase

lattice

α β γ δ ε

(17 × √3) (13 × √3) (10 × √3) (5√3 × 2) (10√3 × 2)

long axis in Å and direction 50.0 37.5 29 25.0 50.0

± ± ± ± ±

1.5 1.0 1.0 1.5 1.0

⟨1 1̅ 0⟩ ⟨1 1̅ 0⟩ ⟨1 1̅ 0⟩ ⟨112̅ ⟩ ⟨112̅ ⟩

short axis in Å and direction 5.0 5.0 5.0 5.5 5.7

± ± ± ± ±

0.6 0.2 0.2 0.5 0.2

domain direction

number of moleules

area per molecule in Å2

tilt

⟨112̅ ⟩ ⟨112̅ ⟩ ⟨112̅ ⟩ ⟨1 1̅ 0⟩ ⟨1 1̅ 0⟩

8 8 6 5 10

30.6 23.4 24.0 28.8 28.8

46° 26° 29° 43° 43°

⟨112̅ ⟩ ⟨112̅ ⟩ ⟨112̅ ⟩ ⟨1 1̅ 0⟩ ⟨1 1̅ 0⟩ 10204


Langmuir

Article

PFBT molecule in the β- and γ-phases is ∼24 Å2, which is compatible with a tilt angle of ∼30° from the surface normal. This value is comparable to that obtained for the densely packed phases of n-alkanethiols or aromatic thiols SAMs on Au(111).6−8,13,66 Increasing the IT to 115 h results in a continuous increase of the area occupied by the ε-phase (∼28.8 Å2 per molecule) at the expense of the accompanying phase and yields a complete transition at 190 h of IT. At this point, the structural quality of the ε-phase is remarkably high. In this context, the long-range ordered structure of PFBT SAMs exceeds all other organothiol SAMs prepared at RT in terms of the size of the domains, which extend over 200 nm, and the absence of defects within these ultrathin organic layers. The formed α-, β-, γ-, δ-, and ε-phases were found to have common features: first, the appearances of densely packed rows of PFBT molecules with different STM contrast. In the case of the δ- and ε-phases, however, the orientation of the rows is along the ⟨110̅ ⟩ directions rather than the ⟨112⟩̅ directions observed for the α-, β-, and γ-phases. Rows with ⟨11̅0⟩ direction were frequently reported for the structures of BPn and TPn SAMs.4,6,7,61,68 The change in the direction of the molecular rows is expected to result from a major rearrangement of the adsorbed PFBT molecules and/or at the SAM− substrate interface during the transition between phases that are subjected to change in the orientation of their molecular rows, i.e., γ→δ phase transition. Another distinguishing feature of the β- and γ-phases is that they have a molecular packing density that is ∼20% higher than that in α- and ε-phases, respectively. Following this quick presentation for the main features of the phases that formed at different ITs, we turn now to discuss the phase transitions that took place with increasing ITs. The phase transition from a lower density phase to a higher one, i.e., α→β, closely resembles the behavior reported previously for nalkanethiolate adlayers, where more tilted molecules that lie flat on the surface were unstable when immersed in solutions and higher-density phases in which the molecules were approximately up-right oriented were obtained after longer immersion times.66,69,70 Regarding the β- and γ-phases, they have almost the same value of the packing density that is higher than that of the α-, δ-, and ε-phases, but they exhibit slight differences in their unit cell dimensions. The fact that the packing densities of the β- and γ-phases are high, with tilt angles of 26° and 29°, respectively, indicates that the Au−S−C angle will achieve a value that is not close to the most favorable angle for an isolated thiol adsorbed on Au(111). Therefore, these phases are expected to be less stable and hence less populated on the surface. Accordingly, the fraction of the ε-phase on the surface was found to increase with IT at the expense of the β and γ accompanying phases. Moreover, the structural quality of the resulting ε-phase was remarkably improved during the β→γ and γ→δ phase transitions. The γ→δ→ε phase transitions lead to a phase with a lower density, very large domains of a uniform structure, and defectfree structure similar to what has been previously observed for the phase transitions in the annealed BP4 and BP6 systems.6,61 On the basis of the high quality of the ε-structure, the γ→δ→ ε transitions are expected to be thermodynamically motivated by lowering the energy related to stress, via reducing the defects, and restructuring of the Au−S interface. The latter was evidenced in PFBT SAMs (at IT of 190 h) by the formation of rough surfaces, which exhibit high density of multilayer gold steps terminated by step-edges with zigzag structure. The high quality of the ε-phase SAM contrasts to a large extent with that

found in the previously studied systems of purely aromatic thiols at RT7,13,33,60,71,72 where the resulting SAMs were found to exhibit a misfit between the molecular lattice and the substrate, and accordingly, stress was released in the form of defects.60 Also, these defects were observed to have a remarkable negative influence on the structural quality of the purely aromatic thiol-dervied SAMs. As a result, the purely aromatic thiol monolayers exhibited a lack of structural quality with very high density of defects. Whereas such quality of SAMs was not observed in the final SAMs of PFBT films prepared from IT of 190 h in which highly ordered SAMs were formed and the density of the defects is very low to a limit that they are almost absent. The key reasons for this improvement in the quality of the PFBT SAMs could be the restructuring process of the Au−S interface, which forces the PFBT system to be more tolerant to the lattice mismatch between molecular lattice and the substrate. Another possible reason is the presence of fluorine moiety in the periphery of the phenyl ring, which delays the equilibrium process to attain the long-range ordering. Due to this unexpected behavior of PFBT SAMs, a number of questions could be raised about the actual driving force that is responsible for the phase transitions and about the requirement of a long time for the appearance of the ε-phase. Before answering these questions, we would like to sum up what has been obtained for systems similar to the PFBT SAMs. In general, the surface morphology of thiol-modified gold was found to be influenced by the nature and dynamic behavior of thiolate-Au complexes.33,73 For example, the stronger binding energy of sulfur to Au for arenethiols, compared with alkanethiols, can lead to less mobility of arenethiolate-Au complexes in the SAMs. Therefore, upon adsorption of aromatic thiols, such as benzenethiol (BT) and its derivatives, Au adatoms slowly diffuse out of the terraces, and Au adatom islands are observed as opposed to the depressions observed for n-alkanethiol SAMs. Therefore, the formation of the stable εphase after long IT is a sign of need of the PFBT adsorbates for long ordering time in order to compensate the low mobility of the molecules on the surface. A slow diffusion rate for PFBT and BT systems on gold surface was also reported in previous studies.55,56 This kinetic feature for PFBT and BT molecules was attributed to their inadequate dipole character. In some cases, the need for long-term rearrangement in a film formation was also reported for alkanethiols.74 It was found that the selfassembly at very cathodic electrode potentials leads to very slow chemisorption kinetics; therefore, a very long time is required for the diffusion of thiolate-Au complexes on the surface. This led finally to the formation of very large wellordered domains.74 An additional important reason for the phase transformations (α→β→γ→δ→ε) and the long time required for the PFBT molecular rearrangement could be the intermolecular C− F···F−C nonbonding interactions. There are a number of reports in the literature of such interactions that have been characterized both experimentally and theoretically.57,75−79 Derivatives of 1,8-difluoronaphthalene have been studied computationally and shown that the C−F···F−C interactions occur with an internuclear separation of 2.3−2.8 Å, and imparts up to 14 kcal/mol of local stability.80 A recent example of selfassembly of copper complexes directed by fluorine−fluorine interactions has been reported, demonstrating that fluorine− fluorine contacts can be used to modulate supramolecular structures.75 Therefore, such interaction is expected to be present between the neighboring PFBT molecules within the 10205


Langmuir

Article

following the ⟨110̅ ⟩ directions of the gold substrate with increasing the IT to 16 h. Further increasing the IT to 190 h results in the disappearance of most gold islands, and the PFBT molecular domains over the remaining gold islands exhibited the same structural order as that present in their vicinities, i.e., the (10√3 × 2) structure. Several types of structural defects were detected within the PFBT SAMs prepared from ITs below 190 h such as cracks and rows of dark molecules. Also, a contrast variation of the adsorbed molecules was observed, which mainly depends on the imaging conditions. The appearance of molecular rows with apparently different topographical heights seems to be particular for PFBT SAMs and, to our knowledge, has never been observed in any other thiol-based SAMs on Au(111). The structural defects and the contrast variation were absent in SAMs prepared from 190 h of ITs. The immersion time-dependent experiments revealed a multistep formation process of PFBT SAMs on the Au(111) surface. During this process, different structures came up due to the variation of the surface coverage and ordering time. Through the continuous transformation and reorganization, PFBT SAMs prepared with 190 h of IT finally result in closepacked, highly ordered, stable, and almost defect-free ε-phase on the Au(111) surface forming a (10√3 × 2) structure.

SAMs. Therefore, long IT was required in order to reach the most stable ε phase in which the van der Waals interactions are maximized. In a future study, we will investigate if this phase transition entails the PFBT SAM to be incubated in PFBT solution or can take place in air for samples prepared after short ITs. Recently, SAMs of PFBT were investigated on Au(111).53 Preparation of PFBT SAMs from short and moderate immersion times (2 and 24 h) at room temperature did not give ordered structures. The resulting SAMs were completely filled by different types of defects. The formation of wellordered SAMs was reported to only be possible when the SAMs were prepared from hot solutions at 75 °C after 2 h of IT. We expect that the phase transformations, which have been observed in our work, were thermally induced via the deposition of PFBT SAMs from hot solutions; preparation from hot solutions will enhance the diffusion rate of the PFBT adsorbates and the most stable ε-phase is formed faster than if the SAM deposition is carried out at RT. To conclude this point, high-temperature adsorption of PFBT molecules on Au(111) reduces the long time required for the structural transitions, which were observed in SAMs prepared at RT, to finally yield the most energetically stable phase very quickly. For PFBT SAMs deposited on Au(111) from hot solution, Kang et al. proposed the (2 × 5√13)R30° molecular packing structure.53 The reported rectangular unit cell was found to contain nine PFBT molecules. Therefore, the molecular area of PFBT is 33.3 Å2. Compared with the different structures proposed in this work, the ε-phase with the (10√3 × 2) structure is only the closest to the (2 × 5√13)R30° structure with respect to the dimensions of the unit cells. The lengths of the two vectors forming the superlattice unit cells of the (10√3 × 2) and the (2 × 5√13)R30° structures are almost identical, but the number of the PFBT molecules present in the two proposed unit cells was different; eight PFBT molecules were proposed in the (10√3 × 2) unit cell. In their study, it was not possible to visualize the individual PFBT molecules in some regions of the (2 × 5√13)R30° unit cell. In this work, the lack of clarity has been avoided by obtaining high-quality STM images from which the number of PFBT molecules present in the (10√3 × 2) was easily determined. Therefore, the proposed (10√3 × 2) structure with eight PFBT molecules is assumed to be most likely. What makes us believe that the two (10√3 × 2) and (2 × 5√13)R30°structures are the same is the similarity in their surface morphologies. Also, both structures are present in SAMs that are almost free of gold islands. What distinguishes our method of preparing PFBT SAMs (IT of 190 h) is the absence of the defects that are observed in the SAMs prepared from hot solution of 75 °C for IT of 2 h such as bright elevated features and missing rows of molecules. The bright features were attributed to PFBT molecules having unstable adsorption configuration resulting from the dynamic motion of the molecules. Therefore, our simple method of preparing PFBT SAMs from long IT (190 h) at RT results in the formation of well-ordered and almost defect-free SAMs.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS W. Azzam gratefully acknowledges the short-term research fellowship supported by the DFG and Tafila Technical University (TTU) for its partial support.

■

REFERENCES

(1) Chesneau, F.; Schuepbach, B.; Szelagowska-Kunstman, K.; Ballav, N.; Cyganik, P.; Terfort, A.; Zharnikov, M. Self-assembled Monolayers of Perfluoroterphenyl-Substituted Alkanethiols: Specific Characteristics and Odd−even Effects. Phys. Chem. Chem. Phys. 2010, 12, 12123−12137. (2) Al-Rawashdeh, N. A. F.; Azzam, W.; Woell, C. Fabrication and Preparation of Amino-Terminated SAMs by Chemical Reduction of Aromatic Nitro Groups. Z. Phys. Chem. 2008, 222, 965−978. (3) Arnold, R.; Azzam, W.; Terfort, A.; Woell, C. Preparation, Modification, and Crystallinity of Aliphatic and Aromatic Carboxylic Acid Terminated Self-Assembled Monolayers. Langmuir 2002, 10, 3980−3992. (4) Azzam, W. Temperature-Induced Phase Transition; Polymorphism in BP2 SAMs on Au(111). Cent. Eur. J. Chem. 2009, 7 (4), 884−899. (5) Azzam, W.; Novel, A Method for Elimination of The GoldIslands Formed in The Self-Assembled Monolayers of Benzeneselenol on Au(111) Surface. Applid Surface Science 2010, 256, 2299−2303. (6) Azzam, W.; Bashir, A.; Terfort, A.; Strunskus, T.; Woell, C. Combined STM and FTIR Characterization of Terphenylalkanethiol Monolayers on Au(111): Effect of Alkyl Chain Length and Deposition Temperature. Langmuir 2006, 22, 3647−3655. (7) Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; Woell, C. Pronounced Odd-Even Changes in the Molecular Arrangement and Packing Density of Biphenyl-Based Thiol SAMs: A combined STM and LEED Study. Langmuir 2003, 19, 8262−8270.

4. CONCLUSIONS In summary, the surface morphology, the defects, and molecular ordering of PFBT SAMs formed after different ITs at RT were studied using STM and FTIR techniques. At short ITs (0.5−4 h), small gold islands with almost round shape were formed. These islands become larger and highly oriented 10206


Langmuir

Article

(8) Azzam, W.; Fuxen, C.; Birkner, A.; Rong, H.-T.; Buck, M.; Woell, C. Coexistance of Different Structural Phases in Thioaromatic Monolayers on Au(111). Langmuir 2003, 19, 4958−4968. (9) Azzam, W.; Wehner, B. I.; Fischer, R. A.; Terfort, A.; Woell, C. Bonding and Orientation in SelfAssembled Monolayers of Oligophenyldithiols on Au Substrates. Langmuir 2002, 18 (21), 7766−7769. (10) Azzam, W.; Woell, C. In The Direction of Perfect Design of Self-Assembled Monolayers: Scanning Tunneling Microscopy Study of SAMs Made of Decanethiol on Au(111). JJC 2006, 2, 143−154. (11) Cyganik, P.; Buck, M.; Azzam, W.; Woell, C. Self-Assembled Monolayers of ω-Biphenylalkanethiols on Au(111): Influence of Spacer Chain on Molecular Packing. J. Phys. Chem. B 2004, 108, 4989−4996. (12) Cyganik, P.; Buck, M.; Strunskus, T.; Shaporenko, A.; Witte, G.; Zharnikov, M.; Woell, C. Influence of Molecular Structure on Phase Transitions: A Study of Self-Assembled Monolayers of 2-(Aryl)-ethane Thiols. J. Phys. Chem. C 2007, 111, 16909−16919. (13) Fuxen, C.; Azzam, W.; Arnold, R.; Terfort, A.; Witte, G.; Woell, C. Structural Characterization of Organothiolate Adlayers on Gold: The Case of Rigid, Aromatic Backbones. Langmuir 2001, 17, 3689− 3695. (14) Hipler, F.; Gil Girol, S.; Azzam, W.; Fischer, R. A.; Woell, C. Interaction of Thiadiazole Additives with Metal Surfaces: Reactions and Thin-film Formation on Gold As a Model Surface. Langmuir 2003, 19, 6072−6080. (15) Kaefer, D.; Bashir, A.; Witte, G. Interplay of Anchoring and Ordering in Aromatic Self-Assembled Monolayers. J. Phys. Chem. C 2007, 111 (28), 10546−10551. (16) Siemeling, U.; Bruhn, C.; Bretthauer, F.; Borg, M.; Traeger, F.; Vogel, F.; Azzam, W.; Badin, M.; Strunskus, T.; Wöll, C. Photoresponsive SAMs on Gold Fabricated From Azobenzene-Functionalised Asparagusic Acid Derivatives. Dalton Trans. 2009, 40, 8593−8604. (17) Flink, S.; van Veggel, F.; Reinhoudt, D. N. Sensor Functionalities in Self-Assembled Monolayers. Adv. Mater. 2000, 12, 1315−1328. (18) Crooks, R. M.; Ricco, A. J. New Organic Materials Suitable for Use in Chemical Sensor Arrays. Acc. Chem. Res. 1998, 31, 219−227. (19) Muglali, M. I.; Bashir, A.; Rohwerder, M. A study on Oxygen Reduction Inhibition at Pyridine-Terminated Self Assembled Monolayer Modified Au(111) Electrodes. Phys. Status Solidi A 2010, 207 (4), 793−800. (20) Joachim, C.; Gimzewski, J. K.; Aviram, A. Electronics Using Hybrid-Molecular and Mono-Molecular Devices. Nature 2000, 408, 541−548. (21) Kraemer, S.; Fuierer, R. R.; Gorman, C. B. Scanning Probe Lithography Using Self-Assembled Monolayers. Chem. Rev. 2003, 103 (11), 4367−4418. (22) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103−1170. (23) Noh, J.; Kato, H. S.; Kawai, M.; Hara, M. Surface Structure and Interface Dynamics of Alkanethiol Self-Assembled Monolayers on Au(111). J. Phys. Chem. B 2006, 110 (6), 2793−2797. (24) Rawlett, A. M.; Hopson, T. J.; Amlani, I.; Zhang, R.; Tresek, J.; Nagahara, L. A.; Tsui, R. K.; Goronkin, H. A molecular Electronics Toolbox. Nanotechnology 2003, 14, 377−384. (25) Tour, J. M. Molecular Electronics. Synthesis and Testing of Components. Acc. Chem. Res. 2000, 33, 791−804. (26) Sikes, H. D.; Smalley, J. F.; Dudek, S. P.; Cook, A. R.; Newton, M. D.; Chidsey, C. E.; Feldberg, S. W. Rapid Electron Tunneling Through Oligophenylenevinylene Bridges. Science 2001, 291, 1519− 1523. (27) Felgenhauer, T.; Rong, H. T.; Buck, M. Electrochemical and Exchange Studies of Self-Assembled Monolayers of Biphenyl Based Thiols on Gold. Electroanal. Chem. 2003, 550, 309−319. (28) Kaefer, D.; Bashir, A.; Dou, X.; Witte, G.; Muellen, K.; C., W. Evidence For Band-like Transport in Graphene-based Organic Monolayers. Adv. Mater. 2010, 22 (3), 384−388.

(29) Seminario, J. M.; Zacarias, A. G.; Tour, J. M. Molecular CurrentVoltage Characteristics. J. Phys. Chem. A 1999, 103, 7883−7887. (30) Donhauser, J. L.; Mantooth, B. A.; Kelly, K. F.; Bumn, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W.; Rawlett, J., A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Conductance Switching in Single Molecules Through Conformational Changes. Science 2001, 292, 2303−2307. (31) Tao, Y. T.; Wu, C. C.; Eu, J. Y.; Lin, W. L.; Wu, K. C.; Chen, C. H. The Structure Evolution of Aromatic-Derivatized Thiols Monolayers on Evaporated Gold. Langmuir 1997, 13, 4018−4023. (32) Noh, J.; Park, H.; Jeong, Y.; Kwon, S. Structure and Electrochemical Behavior of Aromatic Thiol Self-Assembled Monolayers on Au(111). Bull. Korean Chem. Soc. 2006, 27 (3), 403−406. (33) Yang, G.; Liu, G. Y. New Insights for Self-Assembled Monolayers of Organothiols on Au(111) Revealed by Scanning Tunneling Microscopy. J. Phys. Chem. B 2003, 107 (34), 8746−8759. (34) Dhirani, A. A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. Self Assembly of Conjugated Rigid Rods: A Molecular Resolution STM Study. J. Am. Chem. Soc. 1996, 118, 3319−3320. (35) Wan, L.-J.; Terashima, M.; Noda, H.; Osawa, M. Molecular Orientation and Ordered Structure of Benzenethiol Adsorbed on Gold(111). J. Phys. Chem. B 2000, 104 (15), 3563−3569. (36) Frey, S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Structure of Thioaromatic SelfAssembled Monolayers on Gold and Silver. Langmuir 2001, 17 (8), 2408−2415. (37) Jung, H. H.; Won, Y. D.; Shin, S.; Kim, K. Molecular Dynamics Simulation of Benzenethiolate and Benzylthiolate on Au(111). Langmuir 1999, 15, 1147−1154. (38) Kang, H.; Lee, H.; Kang, Y.; Hara, M.; Noh, J. Two-dimensional Ordering of Benzenethiol Self-assembled Monolayers Guided by Displacement of Cyclohexanethiols on Au(111). Chem. Commun. 2008, 41, 5197−5199. (39) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y.-J.; Heister, K.; Buck, M.; Grunze, M. The Effect of Sulfur−metal Bonding on the Structure of Self-assembled Monolayers. Phys. Chem. Chem. Phys. 2000, 2, 3359−3362. (40) Bashir, A.; Kaefer, D.; Mueller, J.; Woell, C.; Terfort, A.; Witte, G. Selenium as a Key Element for Highly Ordered Aromatic SelfAssembled Monolayers. Angew. Chem. 2008, 47 (28), 5250−5252. (41) Shaporenko, A.; Cyganik, P.; Buck, M.; Terfort, A.; Zharnikov, M. Self-Assembled Monolayers of Aromatic Selenolates on Noble Metal Substrates. J. Phys. Chem. B 2005, 109, 13630−13638. (42) Cyganik, P.; Szelagowska-Kunstman, K.; Terfort, A.; Zharnikov, M. Odd-Even Effect in Molecular Packing of Biphenyl-Substituted Alkaneselenolate Self-Assembled Monolayers on Au(111): Scanning Tunneling Microscopy Study. J. Phys. Chem. C 2008, 112, 15466− 15473. (43) Hamoudi, H.; Neppl, S.; Kao, P.; Schuepbach, B.; Feulner, P.; Terfort, A.; Allara, D.; Zharnikov, M. Orbital-Dependent Charge Transfer Dynamics in Conjugated Self-Assembled Monolayers. Phys. Rev. Lett. 2011, 107 (2), 0278011−0278014. (44) Pernstich, K. P.; Haas, S.; Oberfoff, D.; Goldmann, C.; Gundlach, D. J.; Batlogg, B.; Rashid, A. N.; Schitter, G. Threshold Voltage Shift in Organic Field Effect Transistors by Dipole Monolayers on the Gate Insulato. J. Appl. Phys. 2004, 96 (11), 6431−6439. (45) Bock, C.; Pham, D. V.; Kunze, U.; Kaefer, D.; Witte, G.; Terfort, A. Influence of Anthracene-2-thiol Treatment on The Device Parameters of Pentacene Bottom-contact Transistors. Appl. Phys. Lett. 2007, 91 (5), 052110−052113. (46) Saudari, S. R.; Frail, P. R.; Kagan, C. R. Ambipolar Transport in Solution-deposited Pentacene Transistors Enhanced by Molecular Engineering of Device Contacts. Appl. Phys. Lett. 2009, 95, 023301− 023303. (47) Campbell, I. H.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Controlling Charge Injection in Organic Electronic Devices Using Self-assembled Monolayers. Appl. Phys. Lett. 1997, 71 (24), 3528−3531. 10207


Langmuir

Article

(48) Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J. The Interface Energetics of Self-Assembled Monolayers on Metals. Acc. Chem. Res. 2008, 41 (6), 721−729. (49) Demirkan, K.; Mathew, A.; Weiland, C.; Yao, Y.; Rawlett, A. M.; Tour, J. M.; Opila, R. L. Energy Level Alignment at Organic Semiconductor/Metal Interfaces: Effect of Polar Self-Assembled Monolayers at The Interface. J. Chem. Phys. 2008, 128 (7), 074705. (50) Campbell, I. H.; Rubin, S.; Zawodzinski, T. A.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Controlling Schottky Energy Barriers in Organic Electronic Devices Using Selfassembled Monolayers. Phys. Rev. B: Condens. Matter 1996, 54 (20), R14321−R14324. (51) Chong, L. W.; Lee, Y. L.; Wen, T. C.; Guo, T. F. Self-assembled Monolayer-Modified Ag Anode for Top-Emitting Polymer LightEmitting Diodes. Appl. Phys. Lett. 2006, 89 (23), 233513−233517. (52) Jiang, P.; Deng, K.; Fichou, D.; Xie, S.-S.; Nion, A.; Wang, C. STM Imaging ortho- and para-fluorothiophenol Self-assembled Monolayers on Au(111). Langmuir 2009, 25 (9), 5012−5017. (53) Kang, H.; Lee, N.-S.; Ito, E.; Hara, M.; Noh, J. Formation and Superlattice of Long-Range-Ordered Self-Assembled Monolayers of Pentafluorobenzenethiols on Au(111). Langmuir 2010, 26 (5), 2983− 2985. (54) Hong, J.-P.; Park, A.-Y.; Lee, S.; Kang, J.; Shin, N.; Yoon, D. Y. Tuning of Ag Work Functions By Self-Assembled Monolayers of Aromatic Thiols For an Efficient Hole Injection For Solution Processed Triisopropylsilylethynyl Pentacene Organic Thin Film Transistors. Appl. Phys. Lett. 2008, 92 (14), 143311−143314. (55) Wong, K.; Kwon, K. Y.; Rao, B. V.; Liu, A. W.; Bartels, L. Effect of Halo Substitution on the Geometry of Arenethiol Films on Cu(111). J. Am. Chem. Soc. 2004, 126, 7762. (56) Wong, K. L.; Lin, X.; Kwon, K. Y.; Pawin, G.; Rao, B. V.; Liu, A.; Bartels, L.; Stolbov, S.; Rahman, T. S. Halogen-Substituted Thiophenol Molecules on Cu(111). Langmuir 2004, 20 (25), 10928−10934. (57) Reichenbaecher, K.; Suess, H. I.; Hulliger, J. Fluorine in Crystal Engineering−the Little Atom that Could. Chem. Soc. Rev. 2005, 34, 22−30. (58) Chesneau, F.; Schuepbach, B.; Szelagowska-Kunstman, K.; Ballav, N.; Cyganik, P.; Terfort, A.; Zharnikov, M. Phys. Chem. Chem. Phys. 2010, 12, 12123. (59) Jin, Q.; Rodriguez, J. A.; Li, C. Z.; Darici, Y.; Tao, N. J. Selfassembly of Aromatic Thiols on Au(111). Surf. Sci. 1999, 425 (1), 101−111. (60) Kaefer, D.; Witte, G.; Cyganik, P.; Terfort, A.; Woell, C. A Comprehensive Study of Self-Assembled Monolayers of Anthracenethiol on Gold: Solvent Effects, Structure, and Stability. J. Am. Chem. Soc. 2006, 128 (5), 1723−1732. (61) Cyganik, P.; Buck, M.; Strunskus, T.; Shaporenko, A.; Wilto, J. D. E. T.; Zharnikov, M.; Woell, C. Competition As a Design Concept: Polymorphism in Self-assembled Monolayers of Biphenyl-based Thiols. J. Am. Chem. Soc. 2006, 128 (42), 13868−13878. (62) Nara, J.; Higai, S.; Morikawa, Y.; Ohno, T. Density Functional Theory Investigation of Benzenethiol Adsorption on Au(111). J. Chem. Phys. 2004, 120 (14), 6705−6711. (63) Cyganik, P.; Buck, M.; Wilton-Ely, J. D. E.; Woell, C. Stress in Self-Assembled Monolayers: ω-Biphenyl Alkane Thiols on Au(111). J. Phys. Chem. B 2005, 109, 10902−10908. (64) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, J., T.;; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Lyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,

J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Cliffordxxx, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2004. (65) McCoy, C. P.; Cowley, J. F.; Gorman, S. P.; Andrews, G. P.; Jones, D. S. Reduction of Staphylococcus Aureus and Pseudomonas Aeruginosa colonisation on PVC through covalent surface attachment of fluorinated thiols. J. Pharm. Pharmacol. 2010, 61 (9), 1163−1169. (66) Poirier, G. E. Characterization of Organosulfur Molecular Monolayers on Au(111) using Scanning Tunneling Microscopy. Chem. Rev. 1997, 97, 1117−1127. (67) Poirier, G. E.; Tarlov, M. J. The c(4 × 2) Superlattice of nAlkanethiol Monolayers Self-Assembled on Au(111). Langmuir 1994, 10 (9), 2853−2856. (68) Shaporenko, A.; Brunnbauer, M.; Terfort, A.; Grunze, M.; Zharnikov, M. Structural Forces in Self-Assembled Monolayers: Terphenyl-Substituted Alkanethiols on Noble Metal Substrates. J. Phys. Chem. B 2004, 108 (38), 14462−14469. (69) Camillone, N.; Eisenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Poirier, G. E.; Tarlov, M. J. New Monolayer Phases of nalkane Thiols Self-assembled on Au(111): Preparation, Surface Characterization, and Imaging. J. Chem. Phys. 1994, 101 (12), 11031−11037. (70) Toerker, M.; Staub, R.; Fritz, T.; Schmitz-Hubsch, T.; Sellam, F.; Leo, K. Annealed Decanethiol Monolayers on Au(111): Intermediate Phases Between Structures with High and Low Molecular Surface Density. Surf. Sci. 2000, 445 (1), 100−108. (71) Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Schreiber, F.; Ulman, A. Structure and Growth of 4-methyl-4′-mercaptobiphenyl Monolayers on Au(111): A Surface Diffraction Study. Surf. Sci. 2000, 458 (1−3), 34−52. (72) Yang, G.; Qian, Y.; Engtrakul, C.; Sita, L. R.; Liu, G.-Y. Arenethiols Form Ordered and Incommensurate Self-Assembled Monolayers on Au(111) Surfaces. J. Phys. Chem. B 2000, 104, 9059−9062. (73) Whelan, C. M.; Barnes, C. J.; Gregoire, C.; Pireaux, J. J. Influence of Step Sites on the Bonding of Benzenethiol on Au Surface. Surf. Sci. 2000, 454, 67−72. (74) Rohwerder, M.; de Weldige, K.; Stratmann, M. Potential Dependence of the Kinetics of Thiol Self-Organization on Au(111). J. Solid State Electrochem. 1998, 2 (2), 88−93. (75) Halper, S. R.; Cohen, S. M. Self-Assembly of Heteroleptic [Cu(dipyrrinato)(hfacac)] Complexes Directed by Fluorine−Fluorine Interactions. Inorg. Chem. 2005, 44 (12), 4139−4141. (76) Thakur, T. S.; Kirchner, M. T.; Blaser, D.; Boese, R.; Desiraju, G. C−H...F−C Hydrogen Bonding in 1,2,3,5-tetrafluorobenzene and Other Fluoroaromatic Compounds and the Crystal Structure of Alloxan Revisited. CrystEngComm 2010, 12, 2079−2085. (77) Berger, R.; Resnati, G.; Metrangolo, P.; Weber, E.; Hulliger, J. Organic Fluorine Compounds: a Great Opportunity for Enhanced Materials Properties. J. Chem. Soc. Rev. 2011, 40, 3496−3508. (78) Chopra, D.; Guru Row, T. N. Role of Organic Fluorine in Crystal Engineering. CrystEngComm 2011, 13, 2175−2186. (79) O’Hagan, D. Understanding Organofluorine Chemistry. An Introduction to the C−F Bond. Chem. Soc. Rev. 2008, 37, 308−319. (80) Matta, C. F.; Castillo, N.; Boyd, R. Characterization of a ClosedShell Fluorine−Fluorine Bonding Interaction in Aromatic Compounds on the Basis of the Electron Density. J. Phys. Chem. A 2005, 109 (16), 3669−3681.

10208


Formation of Highly Ordered and Orientated Gold Islands: Effect of

Recommend Documents