J. Phys. Chem. C 2007, 111, 7735-7740
7735
Scanning Tunneling Microscopy and Orbital-Mediated Tunneling Spectroscopy Study of 1,5-Di(octyloxy)anthracene Adsorbed on Highly Ordered Pyrolytic Graphite from Various Solvents and in Different Environments Matthew Pokrifchak,† Tracey Turner,‡ Ian Pilgrim,† Martin R. Johnston,‡ and K. W. Hipps*,† Department of Chemistry and Materials Science Program, Washington State UniVersity, Pullman, Washington 99164-4630, and School of Chemistry, Physics and Earth Sciences, Flinders UniVersity, Adelaide 5001, South Australia ReceiVed: January 19, 2007; In Final Form: March 14, 2007
Scanning tunneling microscopy (STM) and orbital mediated tunneling spectroscopy (OMTS) are reported for 1,5-di(octyloxy)anthracene (15DA) adsorbed on highly ordered pyrolytic graphite (HOPG). 15DA forms wellordered monolayers either at the interface between HOPG and 1-phenyloctane or at the HOPG-air or HOPGvacuum interface when spin doped from octane or dichloromethane. Octyl chain interdigitation and planar adsorption of the anthracene ring combine to produce structures which are stable independent of the adsorption method. The observed unit cell has a ) 1.76 ( 0.06 nm, b ) 1.07 ( 0.06 nm, and R ) 77 ( 3°. Molecular orbital calculations are combined with bias-dependent imaging and OMTS results to show that the highestoccupied molecular orbital (HOMO) dominates the tunneling process over the voltage range from -2.5 to +1.0 V. The HOMO of 15DA on HOPG is found to occur at -7.0 eV below the vacuum level, in good agreement with previous UPS studies and with our calculations for the free molecule. We also report the first synthesis of 15DA.
1. Introduction Scanning tunneling microscopy (STM) has proved a tremendous boon to the study of surfaces. While it competes with lowenergy electron diffraction (LEED) and transmission electron micropscopy (TEM) studies for analyzing surface structures in an ultrahigh vacuum (UHV), it is without a competitor in the realm of solid-solution or solid-atmosphere interface problems. In particular STM studies have allowed us to become very familiar with adsorbate structures on graphite (highly oriented pyrolytic graphite, or HOPG) surfaces. Through STM studies, it is now well known that certain organic1-12 and metalorganic13-19 molecules having alkane substituents will form wellordered monolayers at the solid/solution interface. The literature contains numerous images with submolecular resolution of wellordered 2-dimensional monolayer structures at the solventHOPG interface. More recently, the role played by the solvent is being explored by using various solvents to probe solutesurface interactions.20-27 Less common in the literature, are STM studies of cast films (monolayers formed from solution then dried) on graphite.28-30 One of the important scientific consequences of this explosion in molecular monolayer imaging has been the realization that the various substrate-adsorbate, adsorbate-adsorbate, and adsorbate-solvent interactions can be used for designing new materials on the nanoscale.31-34 Thus, the study of solutionphase deposition of self-organizing molecules grew in importance from a scientific curiosity to a potentially highly significant technology. As a potential technology, it is necessary to examine more than just the atomic structure. In particular, an important characteristic of these 2-D structures is their electronic energy levels. The locations of the highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO),
for example, have great importance for properties such as electrical conductivity, optical absorption, and electron-transfer kinetics. The location of these critical occupied and unoccupied states can be determined with the STM, through a technique called orbital-mediated tunneling spectroscopy, OMTS.35,36 However, to be accessible by OMTS, the electronic levels must lie within a few electron volts of the Fermi energy of the substrate. Thus, a complete study of a given self-assembled structure will include both the molecular structure of the adlayer and also the electronic structure near the HOMO and LUMO levels. Moreover, for practical applications, it is highly desirable to determine if the structure that exists at the solid-solution interface can be prepared at the solid-air or solid-vacuum interface. In this paper we report the STM and OMTS study of a new organic molecule, 15DA, having both an electroactive component, which provides an electronic state within the accessible region of OMTS, and alkane chains to assist in creating a wellordered adlayer on HOPG. We demonstrate that the interaction between the molecule and surface dominates the adlayer structure so that the same surface results both at the 1-phenyloctane solution-HOPG interface and at spin-doped (spin-cast) solvent free surfaces. Transporting the pure monolayer into UHV we are able to obtain extremely reproducible OMTS over a relatively wide spectral range ((2.5 V) about the Fermi level of graphite. 2. Experimental 2.1. Compound Synthesis. 15DA.
* To whom correspondence should be addressed. E-mail:
[email protected]. † Washington State University. ‡ Flinders University.
10.1021/jp0704968 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/05/2007
7736 J. Phys. Chem. C, Vol. 111, No. 21, 2007 Purified anthrarufin (515 mg, 2 mmol) was suspended in DMF (dry, 35 mL) along with K2CO3 (515 mg, 5.2 mmol) and n-octyl bromide (2.5 mL, 13 mmol) added. NaI (815 mg, 5.4 mmol) was then added, and the resulting suspension stirred for eight days. The material was filtered and evaporated to dryness and purified using column chromatography (silica, hexane (20%) in DCM). Product was recrystallized from DCM/MeOH, 0.31 g (31%), mp ) 91-93 °C. 1H NMR (CDCl , 300 MHz); δ 7.88, (2H, d of d, J ) 1 Hz, 3 8.4 Hz), 7.64 (2H, t, J ) 8.4 Hz), 7.24 (2H, d of d, J ) 1, 8.4 Hz), 4.14 (4H, t, J ) 6.6 Hz), 1.94 (4H, quin, J ) 6.6 Hz), 1.56-1.29 (20H, m), 0.88 (6H, t, J ) 6.6 Hz). 13C NMR: 182.45, 159.28, 137.43, 134.70, 120.98, 119.39, 117.70, 68.95, 31.76, 29.25, 29.14, 29.04, 25.87, 22.59, 14.04. 1,5-Di(octyloxy)-9,10-dihydro-9,10-anthracenediol.
Method adapted from Norvez et al.37 The octyl anthraquinone derivative (162 mg, 0.35 mmol) was dissolved in THF (dry, 23 mL) and MeOH (dry, 23 mL) under an N2 atmosphere. NaBH4 (854 mg, 23 mmol) was added portion wise over 1.5 h with stirring. The solution was stirred for an additional 3 h followed by pouring onto ice water. The precipitated material was removed by filtration and dried in vacuo to yield a white pearly solid as a mixture of isomers, 123 mg (75%). It was used without further purification. 1H NMR (CDCl , 300 MHz); δ 10.38 (2H, s), 8.24 (2H, s), 3 7.96 (2H, d, J ) 8.4 Hz), 7.57 (2H, d, J ) 8.4 Hz), 7.29 (2H, t, J ) 8.4 Hz), 7.21 (2H, t, J ) 8.4 Hz), 6.75 (2H, d, J ) 8.4 Hz), 6.64 (2H, d, J ) 8.4 Hz), 4.28 (4H, t, J ) 6.6 Hz), 4.17 (4H, t, J ) 6.6 Hz), 1.99 (quin, 8H, J ) 6.6 Hz), 1.57-1.31 (40H, m), 0.89 (12H, t, J ) 6.6 Hz). 1,5-Di(octyloxy)anthracene.
Method adapted from Norvez et al.37 The dihydroxy derivative (123 mg, 0.26 mmol) was suspended in acetic acid (2.5 mL), and phenyl hydrazine (0.5 mL) was added. The solution was heated to 90 °C for 20 min followed by 120 °C for 20 min. The solution was cooled and filtered with the solids being washed well with MeOH and dried in vacuo. Yield 84 mg, 73%, mp)119-20 °C. 1H NMR (CDCl , 300 MHz); δ 8.78 (2H, s), 7.62 (2H, d, J 3 ) 8.4 Hz), 7.33 (2H, t, J ) 8.4 Hz), 6.72 (2H, d, J ) 8.4 Hz), 4.19 (4H, t, J ) 6.6 Hz), 2.00 (quin, 4H, J ) 6.6 Hz), 1.661.33 (20H, m), 0.91 (6H, t, J ) 6.6 Hz). 13C NMR; 154.78, 132.18, 125.43, 124.98, 120.18, 120.45, 102.56, 68.19, 31.86, 29.44, 29.29, 26.34, 22.69, 14.12. HR-MS calcd. C30H42O2Na 457.3077. Found 457.3081. 2.2 Materials. In addition to the 15DA, Figure 1, several commercial materials were used as supplied. 1-Phenyloctane was purchased from Avacado Research, dichloromethane was purchased from Baker, and octane was purchased from Sigma. All were 99+% purity. Pt0.8Ir0.2 wire (0.25 mm diameter) was
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Figure 1. Ball and stick model of 15DA based on the 6-311++ HF optimized ground state geometry.
purchased from California fine wire, and HOPG (grade 2) was purchased from SPI Supplies. 2.3 Ambient and Solution STM. Solutions of 15DA in 1-phenyloctane, dichloromethane, and octane were made at the 1 mg/mL level. Several drops of the 1-phenyloctane solution were placed on the HOPG surface, and STM imaging was performed at the solution-solid interface. In the case of the dichloromethane and octane solutions, a few drops were placed on the graphite surface and then the substrate was spun at several thousand rpm for 1-2 min and then further dried under argon. These dried samples were then imaged in air. The spin-doped samples had regions easily identified as monolayers both by the defects present and by the ready appearance of the underlying HOPG structure at reduced tip-substrate distances. They also had regions of multilayer coverage that were much harder to interpret and are not considered in this report. The STM used is a stand alone unit sold originally by Digital Instruments (now Veeco) using a Nanoscope E controller and version 4 of the software. Images were all collected in constant current mode. Both etched (in NaCl) and cut PtIr tips were used in this study. Any manipulation of the data beyond simple plane fitting and low pass filtering will be indicated in the figure caption. Scanners (two were used, a D and an A) were calibrated as follows. The 10-µm D scanner was calibrated using a 1-µm waffle grid. The 0.6-µm A scanner was calibrated using the atomic structure of clean HOPG. Attempts at performing OMTS in solution or air were only partially successful, in that several reproducible scans would be interrupted by wildly noisy scans. For this reason we will only report those spectra acquired in UHV. However, the low noise scans of the dried surfaces were comparable to the UHV results. 2.3 UHV STM and OMTS. A spin-doped sample of 15DA on HOPG was transferred into UHV for this study. The vacuum chamber used routinely operates at a pressure below 1 × 10-10 Torr. The STM was purchased from RHK and is controlled by RHK electronics and software. All images were collected in constant current mode. Calibration was performed using the known lattice spacing of clean graphite. Because our primary goal was to obtain high-quality spectra, efforts were employed to ensure the cleanliness of the tips, often at the cost of their sharpness. Electrochemically etched PtIr tips were transferred into an attached vacuum chamber and argon ion sputtered (1 kV, >10 µA beam current, 30-120 s) and then directly transferred into UHV. Current-voltage (IV) curves were acquired using the RHK software with the feedback loop turned off (fixed height) during IV scans. Generally 10-16 curves would be acquired and coadded. Over 50 such curves were taken from many points on the surface using several different tips, and all were essentially the same. Thus, these sputter-cleaned tips provided excellent reproducibility. Images taken with these
STM and OMTS of 15DA on HOPG
J. Phys. Chem. C, Vol. 111, No. 21, 2007 7737
Figure 3. Constant current STM images of a monolayer region at the air-solid interface of 15DA spin doped on HOPG from solution having: (A) octane solvent, set point current ) 50 pA, and sample bias ) -700 mV; (B) dichloromethane solvent, set point ) 50 pA, and sample bias ) -700 mV. The gray scale for both images is 1 nm.
Figure 2. Constant current STM image at the air-solid interface of a monolayer region of 15DA spin doped from a dichloromethane solution. The setpoint current was 50 pA, and the sample bias was -700 mV.
tips clearly showed the same lattice structure as observed in air and solution, but the resolution was inferior to those images. Care was taken to choose only those regions with a well-defined monolayer. Areas with possible multilayer structure were avoided. 2.4. Calculation Methods. The program Gaussian 0338 was used to perform geometry minimization, and to calculate the first ionization energy, electron affinity, and wavefunctions for the free molecule, 15DA. Calculations were carried out using the 6-311++(d,p) basis at the Hartree Fock level. Gaussview was used to generate the MO images and structures shown. The space filling models shown are based on the optimized geometry and CPK radii. 3. Results and Discussion 3.1. Surface Structure. The spin-doped samples generally showed well-defined coverage over the entire graphite surface. In some areas a complex pattern was observed that we interpreted as a multilayer. In this study these areas were avoided. Figure 2 shows a typical large area scan of a 15DA monolayer produced by spin doping from octane. Even with relatively dull tips, the long parallel rows separated by about 1.7 nm were easily observed. The monolayer grew in welldefined grains with clear grain boundaries, as shown in Figure 2. Grain boundaries generally also occurred at step edges, as can be seen on the right edge of Figure 2. The step in Figure 2 is about 1.1 nm high and is consistent with a three atomic layer step (3 × 3.36 nm). Typical corrugation observed with bias near -1 V was about 1 Å, but this was very bias dependent as will be discussed later. The monolayer areas of these spin-doped films were generally somewhat defective, with missing molecule sites present in most of the images we acquired. The individual anthracene rings making up the rows can be clearly seen in Figure 3A, obtained from a different sample spin doped from an octane solution. As will be demonstrated later in this paper the image is dominated by tunneling through the HOMO of 15DA so the alkane chains are almost invisible at this voltage. One can also see that the rings are sitting at an angle of about 35° with respect to the row direction. This is a feature seen in all the images acquired in all the different environments studied. Figure 3B is a higher resolution constant
Figure 4. Constant current STM image of 15DA at the solutionsolid interface of HOPG and 15DA in 1-phenyloctane. Inset into the Figure are CPK models of 15DA over the ) 1.76 nm × 1.07 nm lattice having an internal angle of 77°. The image has been filtered by removing high-frequency components in a circular region in frequency space. The set point was 200 pA and the sample bias was +700 mV.
current image taken from a dry sample spin doped with a dichloromethane solution. This sample also shows the welldefined 1.7-nm row spacing dominated by tunneling through the anthracene rings. In all the samples studied, the unit cell parameters were a ) 1.76 ( 0.06 nm, b ) 1.07 ( 0.06 nm, and R ) 77 ( 3°. Note also the presence of defects in the images taken from monolayer regions of spin-doped samples (Figures 2 and 3). Images acquired at the 1-phenyloctane solution-HOPG interface were some of the best obtained. Figure 4 shows the very well ordered and nearly defect free surface seen by STM. Also shown in Figure 4 is our interpretation of the actual molecular structure giving rise to the STM image. The CPK models are based on gas-phase energy optimized structures of 15DA and are placed exactly on the proposed a ) 1.76 nm, b ) 1.07 nm, and R ) 77° lattice. As expected from solutionsolid interface studies on similar compounds,2,9 the interlacing of the alkane chains is an important component to the stability of this surface layer. For the first time, in this study, we find that interaction with the solvent is playing no significant role in determining the lattice structure. The same surface layer results if the film is formed from evaporated dichloromethane,
7738 J. Phys. Chem. C, Vol. 111, No. 21, 2007
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Figure 5. HOMO and LUMO of 15DA shown with the correlation average constant current image of 15DA at the 1-phenyloctane solution-HOPG interface. The set point current was 450 pA and the sample bias was +600 mV bias.
evaporated octane, or is in equilibrium with 1-phenylloctane. Our explanation involves several suppositions. First, we think that the interdigitation of the alkane chains is so tight in these layers that there is essentially no possibility for solvent molecules to be incorporated. Second, the π-π interactions between the anthracene ring and the graphite surface may be stronger than that between the solvents used here and the anthracene ring. However, despite these similarities in surface structure, there are some differences in the voltage dependence of the images associated with the presence of solvent during imaging. As noted previously, images taken at the solution solid interface had significantly fewer defects, primarily fewer vacancies, and no evidence of second layer molecules. Since the solution acts as both a source and sink for 15DA, this is not surprising. A second difference arises from the voltage dependence of the STM image intensity. This will be discussed in some detail in a later section. 3.2. Electronic Structure, Voltage-Dependent Imaging, and OMTS. We have performed SCF calculations on gas-phase 15DA using Gaussian03 in order to determine both its equilibrium nuclear structure and its electronic structure. Figure 5 presents the HOMO and LUMO obtained from these calculations. Also shown in Figure 5 is the correlation averaged structure of 15DA in 1-phenyloctane taken from a 50 × 50 nm2 image different than the one shown in Figure 4. This highly resolved image is essentially the same as can be seen in highly resolved images from spin-doped layers such as Figure 3B. It seems clear from Figure 5 that the HOMO is playing a dominant role in determining the STM image in the voltage range more negative than 800 mV. This interpretation is the opposite made by Hansen et al.2 in interpreting the images of some related anthracene derivatives but agree with the assignment of Flynn’s group9 in the case of sulfur-containing analogs. This assignment therefore requires greater attention, especially since there are a few irregularities that must be considered. If the tunneling current is strongly mediated by the HOMO, one would expect the apparent molecular height to be largest at negative voltage.35,36 On spin-doped samples this is clearly the case with the maximum corrugation in surface structure observed at -1.2 V. As shown in Figure 6, there is a dramatic drop in both total tunneling current and in apparent corrugation as the sample bias is varied from -1 to +0.5 V. This is exactly what one would expect for HOMO-mediated tunneling. A complication is that one sees very high corrugation, even at +0.7 V (Figure 4), in the case of tunneling through the
Figure 6. Constant current STM image and cross section at the airsolid interface of 15DA spin doped on HOPG from octane solution. The image was acquired by changing the bias voltage during the scan while holding the set point current at 200 pA. During the first and last thirds of the scan, the sample voltage was fixed at -1 V. During the middle third of the scan the voltage was held at +0.5 V.
1-phenyloctane solution. Unusual voltage dependences in images at the solution-HOPG interface have been associated with solvent effects,2,39 and it is likely that both solvent-induced surface work function changes and increased coupling between substrate and tip orbitals play a role. Another possibility involves weak but nonzero solvent mediated electronic state mixing between empty surface states of graphite and the 15DA HOMO. This type of mixing is similar (but reversed) to that first described in the case of Xenon on Ni where tailing of the weakly mixed (unoccupied) 6s orbital of the Xe and metal-occupied states produced localized occupied state density at the Xe atom.40 What role the solvent plays in enhancing this process is a mystery to us. To better define the electronic state energies of 15DA near the Fermi energy (EF) of graphite, the orbital mediated tunneling spectrum was measured in UHV using extremely clean Pt0.8Ir0.2 electrochemically etched tips. A typical result is shown in Figure 7. The intense band at -2.0 V dominates the spectrum between -2.5 and 2.5 V bias. Because it occurs at negative sample bias, it is clearly an occupied orbital. If we take EF of HOPG to be 5.0 V, this places the energy of the ionization associated with the -2 V bias band as 7.0 V below the vacuum level. Our best density functional theory (DFT) calculations [B3LYP/6-311++(d,p)] estimate the vertical ionization potential for the HOMO as 6.6 V below the vacuum level. The experimentally observed first ionization band of a 2-nm film of anthracene on gold occurs at 6.5 V41 and anthracene in the gas phase has its first ionization peak near 7.4 V (below the vacuum level). Thus, the observed strong resonance near 7 V
STM and OMTS of 15DA on HOPG
Figure 7. Current-voltage curve and OMTS of a monolayer region of 15DA spin doped from octane on HOPG under UHV conditions and T ) 295 K.
below the vacuum level can be confidently assigned to the first ionization of 15DA; or, more colloquially, the HOMO. On the basis of the OMTS results (Figure 7), the first affinity level (the LUMO) must be something less than 2.5 V below the vacuum level. The DFT calculations on the vertical excitation from the optimized ground state of the neutral molecule to the negative ion produces an unbound ion. We do not take this to mean that the first affinity level is necessarily unbound, only that it is at most weakly bound. Thus, the calculations are consistent with the OMTS data. 4. Conclusions We have successfully imaged self-assembled monolayers of an anthracene molecule appended with alkoxy chains on HOPG. The monolayer was found to consist of ordered rows of molecules with spacing of the rows determined by the alkyl chain length. This study is one of very few that compares the structure of an adlayer at the solvent-solid surface with that formed by spin doping at the air-solid or vacuum-solid interface. In the case of this particular molecule, and unlike other systems reported to date,21-27 the use of dichloromethane, octane, or 1-phenyloctane had no significant effect on the monolayer of 15DA produced on HOPG. The constant current STM image of monolayer regions of 15DA is dominated by HOMO tunneling, and the anthracene rings appear bright while the octane chains are nearly invisible. Bias voltage dependent images in air and in vacuum show great loss of corrugation for small positive bias voltage. However, images of 15DA in 1-phenyloctane can be obtained over a much wider voltage range. OMTS placed the first ionization level at 7.0 V below the vacuum level, and the first affinity level higher than 2.5 V below the vacuum level. DFT calculations indicate that the negative ion may be very weakly bound or even unbound. Further work is now continuing in our laboratories to investigate the influence of anthracene moiety derivitization on the structure of self-assembled monolayers. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant CHE0555696 and from an REU Grant DMR-0453554. This work was also funded by the Australian Research Council (MRJ). We thank Clarisa Carrizales for her assistance with the program Gaussian calculations. References and Notes (1) Giancarlo, L. C.; Fang, H.; Avila, L.; Fine, L. W.; Flynn, G. W. J. Chem. Ed. 2000, 77, 66-71.
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