J. Phys. Chem. C 2008, 112, 14907–14912
14907
Scanning Tunneling Microscopy and Orbital-Mediated Tunneling Spectroscopy of N,N′-Dioctyl-1,8:4,5-naphthalenediimide Adsorbed on Highly Ordered Pyrolytic Graphite from Various Solvents and in Different Environments Leslie Kleiner-Shuhler, Rebekah Brittain, Martin R. Johnston,† and K. W. Hipps* Department of Chemistry and Materials Science Program, Washington State UniVersity, Pullman, Washington 99164-4630 ReceiVed: May 3, 2008; ReVised Manuscript ReceiVed: July 3, 2008
Scanning tunneling microscopy (STM) and orbital-mediated tunneling spectroscopy (OMTS) are reported for N,N′-dioctyl-1,8:4,5-naphthalenediimide (diimide) adsorbed on highly ordered pyrolytic graphite (HOPG). The diimide forms well ordered monolayers either at the interface between HOPG and several phenylalkanes, or at the HOPG-air or HOPG-vacuum interface when adsorbed from toluene. Planar adsorption of the diimide ring on HOPG is observed. Hydrogen bonding, O and N interaction with HOPG, and π-π interactions appear to be the primary drivers for determining the monolayer structure which is stable and independent of the adsorption method. This is an unusual example since most alkane-substituted systems studied to date rely on alkane chain interactions (with HOPG and interdigitation) to drive the adsorbate structure on graphite. The observed unit cell has a ) 2.0 ( 0.2 nm, b ) 1.95 ( 0.2 nm, R ) 67 ( 2°. The STM imaging is highly bias dependent and appears to be controlled (in the (2 V bias region) by an unoccupied orbital. Orbitalmediated tunneling spectra reveal a single strong electron affinity band near 3.5 eV below the vacuum level. 1. Introduction Scanning tunneling microscopy (STM) has proved a tremendous boon to the study of surfaces. While it competes with LEED studies for analyzing surface structures in 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 metal-organic13-19 molecules having alkane substituents will form well ordered monolayers at the solid/solution interface. The literature contains numerous images with submolecular resolution of well ordered twodimensional monolayer structures at the solvent-HOPG interface. More recently, the role played by the solvent is being explored by using various solvents to probe solute-surface interactions.20-27 Less common in the literature are STM studies of cast films (monolayers formed from solution then dried) on graphite.28-30 Only a few examples exist of STM studies of stable monolayers formed both at the solution-solid interface that also in UHV.31,32 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.33-36 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 HOMO and LUMO, for example, have * Corresponding author. E-mail:
[email protected]. † Current address: School of Chemistry, Physics and Earth Sciences, Flinders University, Adelaide 5001, South Australia.
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 STM, through a technique called orbitalmediated tunneling spectroscopy, OMTS.37,38 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 N,N′dioctyl-1,8:4,5-naphthalenediimide (hereafter called diimide). This complex molecule has a rich potential for interaction on and with the HOPG surface. It has an electroactive component which provides at least one electronic state within the accessible region of OMTS. It contains electronegative species (oxygen and nitrogen) that can both hydrogen bond and also interact through induced dipolar interactions with the substrate. π-π interactions between the large aromatic ring and the HOPG may also influence the surface packing structure. Finally, it has alkane chains normally thought to play a critical role in creating a well ordered adlayer on HOPG. Although diimides have seen extensive use in supramolecular chemistry, mainly as electron acceptor units,39 there is little previous work on the STM and electronic spectroscopy of adsorbed diimide but more on its analogues. Santos et al.40 showed that a dibutyl version of the diimide was readily reduced once excited to the triplet state. Rodrigues and co-workers observed that naphthalenediimide triplet states cause photooxidation of tryptophan.41 Ito and co-workers produced large domains of relatively thick diimide films by a cover melting technique.42 Fuller and Wasielewski43 reported the first reduction potential of the diimide in solution to be -0.53 V vs SCE, while Gosztola et al.44 report -0.48 V vs SCE. The first singlet
10.1021/jp8039127 CCC: $40.75 2008 American Chemical Society Published on Web 08/28/2008
14908 J. Phys. Chem. C, Vol. 112, No. 38, 2008
Kleiner-Shuhler et al.
Figure 1. Structure of the diimide.
Figure 2. STM constant current images of the adlayer formed at the solution-HOPG interface at room temperature. (A) diimide in phenylheptane, (B) diimide in phenyloctane, and (C) diimide in phenyldecane. Typical corrugation is 0.1 to 0.4 nm. A single unit cell is shown in B.
absorption band was reported at 375 nm or 3.36 eV. Seki has studied the diimide with phenyl groups replacing the octyl chains adsorbed on aluminum.45 From UPS data, he determined that the HOMO of the diimide ring lies deeper than 8.7 eV below the vacuum level. His calculations indicate that the LUMO lies deeper than 2.2 eV below the vacuum level. Ruiz-Oses has reported the STM of naphthalene tetracarboxylic dianhydride (NTCDA) (this is the parent imide having H replacing the octyl chains) and of the coadsorption of this with 1,4-bis(4,6-diamino1,3,5-triazin-2-yl)benzene on Au(111).46 Keeling and co-workers47 also studied NTCDA but adsorbed on the Ag/Si(111)3x3R30° surface. They found that H-bonding induced the formation of long rows of molecules. A larger conjugated analogue of the diimide, perylene3,4,9,10-tetracarboxylic diimide (PTCDI), has been studied on metal surfaces in UHV by STM and EELS. Nowakowski and Seidel48 used UHV STM to study a nonplanar analogue of PTCDI on silver. Guillermet and co-workers49 studied PTCDI on Pt(100) by STM and EELS. They report that the HOMOLUMO optical excitation occurs near 2.4 eV and that the surface band gap (determined by EELS) is 1.8 eV. Seidel and coworkers50 studied the methyl-disubstituted PTCDI Ag(110). In this study, we will demonstrate that the interaction between the diimide and HOPG surface dominates the adlayer structure so that the same monolayer structure results both at the solution-HOPG interface and at the solvent free surfaces. Further, the structures do not depend on the specific solvent. Transporting the pure monolayer into UHV we are able to obtain extremely reproducible OMTS over a relatively wide spectral range ((2.0 V) about the Fermi level of graphite. 2. Experimental Section 2.1. Materials. The diimide (Figure 1) has been previously reported51,52 and was prepared by known methods.53 Calculated/ theoretical element percentages were C: 73.18/73.44; H: 7.86/ 7.61; O: 12.50/13.04; N: 5.90/5.71. The structure of the diimide was confirmed by 1H NMR and 13C NMR (see Supporting Information). All solvents are commercially available and were used as supplied. The substrate used was highly ordered pyrolytic graphite (HOPG) grade 2 from SPI Supplies. The organic solvents used were purchased from Alfa Aesar (nhexylbenzene, n-heptylbenzene, and n-octylbenzene) and Aldrich (n-decylbenzene and toluene).
Solutions of the diimide in n-hexylbenzene, n-heptylbenzene, n-octylbenzene, and n-decylbenzene were made at concentrations of 5 × 10-4 M, 2 × 10-3 M, 4 × 10-4M, and 2 × 10-3 M, respectively. For all four solutions, the diimide was brought into solution by heating to approximately 55 °C. The diimide was readily soluble in toluene at the millimolar level, and 2 × 10-3 M diimide in toluene solutions were used. 2.2. Ambient and Solution STM. 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. Pt0.8Ir0.2 tips of 0.010 in. diameter were etched in 1 M NaCl solution and used in this study. Solutions of the diimide in the above indicated alkylbenzene solutions were used to wet a freshly cleaved HOPG surface. STM imaging was performed with the tip inserted through the solution layer. These samples had large regions of organized monolayers in which grain boundaries and defects were visible. The best images obtained from each solution were analyzed by the commercially available scanning probe image processor software (SPIP), version 4.6.0.54 Attempts at performing OMTS in solution or air were unsuccessful in that reproducible scans would be interrupted by wildly noisy scans. Often, the spectra would either invert or become symmetrical, indicating that tunneling was occurring from molecules on the tip. For these reasons we will only report those spectra acquired in UHV. Images were typically acquired at 15 to 25 pA and sample bias ranging from +1.2 to +1.8 V. Bias settings more negative than about 1 V and currents greater than about 100 pA resulted in loss of the image. 2.3. UHV STM and OMTS. The sample for UHV studies was made by evaporating a drop of 2 × 10-3 M solution of diimide in toluene on a HOPG surface. It was then dried under argon and inserted into the UHV-STM chamber for imaging and spectroscopy. While in UHV, the sample was heated up to 91 °C for 7 min to remove molecules adsorbed on the monolayer. The vacuum chamber used is routinely operated 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, great efforts were employed to ensure the cleanliness of the tips, often at the cost of their sharpness. Electrochemically etched W tips were transferred into an attached vacuum chamber
STM and OMTS of a Diimide on HOPG
J. Phys. Chem. C, Vol. 112, No. 38, 2008 14909
Figure 3. Constant current UHV STM image of diimide on HOPG. Diimide adsorbed from toluene and then annealed in UHV. Corrugation is 0.09 nm. 1.5 V bias and 20 pA setpoint. Typical corrugation is 0.12 nm.
and argon ion sputtered (1 kV,
