p-Carborane on Au(111) - American Chemical Society

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High-Band-Gap Polycrystalline Monolayers of a 12-Vertex p-Carborane on Au(111) Florian von Wrochem,* Frank Scholz, Deqing Gao, Heinz-Georg Nothofer, Akio Yasuda, and Jurina M. Wessels Materials Science Laboratory, Sony Deutschland GmbH, Hedelfinger Strasse 61, 70327 Stuttgart, Germany

Subhadeep Roy,† Xudong Chen,† and Josef Michl†,‡ †

Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States, and Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nam. 2, 16610 Prague, Czech Republic

‡

ABSTRACT Carborane cages are interesting materials for electronics because of their pseudoaromatic, wide-band-gap molecular structures. We have observed that p-carboranes tethered to gold by thiolate linkers form polycrystalline monolayers which are incommensurate with the Au(111) substrate. The incommensurability results from the strong interactions between adjacent p-carborane cages (1.6 eV/ molecule) and from the adlayer lattice spacing, which is defined by the cage diameter. The high ionization potential of the carborane structures results in a significant shift in the position of the highest occupied molecular orbital (HOMO) of p-carborane (4.3 eV below the Fermi level) compared to the HOMO of oligophenylene derivatives (2.1 eV below the Fermi level). These findings suggest the combination of p-carboranes with established aromatic π-systems as a new design strategy for molecular electronic devices. SECTION Molecular Structure, Quantum Chemistry, General Theory

cosahedral closo-carboranes1,2 have been widely studied because of their peculiar electronic and structural properties, which make them attractive for the fabrication of boron carbide semiconductors,3 solid-state neutron detectors,4 and molecular scaffolds.5 The large band gap typical of boron carbides6 provides a high resistance to oxidation, and the strong bonds within such structures result in high temperature stability.7 While a number of studies have investigated the electronic structure of carboranes physisorbed on metals, generally using closo-carboranes directly evaporated onto metal substrates,8,9 little is known about the formation of self-assembled carborane monolayers covalently linked to the substrate. The symmetry and the electronic structure of closo-carboranes might lead to surfaces with new and unexpected physical properties. Even though low rate constants for electron transfer across carborane ligands were observed,10 their spectral density could yield transport properties different from those of aromatic hydrocarbon derivatives, which are commonly used in molecular electronics.11-13 Furthermore, in consideration of their specific size and symmetry, closocarborane rods could form monolayers with novel twodimensional periodic array structures. Herein, we show that closo-1,12-dicarbadodecaboranes (abbreviated as p-carboranes), covalently attached to Au substrates through a thiolate anchor group and a flexible alkyl linker (Scheme 1), form polycrystalline monolayers with

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interesting electronic properties. The periodicity of the hexagonally close-packed monolayer structure is determined by the physical size of the carborane cage, as reflected in the adlayer lattice spacing.14 However, we find that the strong intermolecular interactions between carborane cages lead to the formation of incommensurate monolayers15 having a polycrystalline nature never observed before within such a small range in real space. Electronically, the large band gap and high ionization potential typical of carboranes16 translate into a significant energy gap between the molecular HOMO and the Fermi level of the metal. The resulting energy level alignment could be of great interest for applications in molecular electronics. To secure firm adhesion to the gold surface and to provide the adsorbed carborane cages with the freedom to assemble into an optimal structure not dictated by the gold lattice itself, we have chosen structure 5, in which the carborane cage is attached rigidly and linearly to a bicyclo[1.1.1]pentane cage followed by a flexible trimethylene chain. Scheme 1 shows the synthetic sequence from 1 to 5 that was used to convert commercial p-carborane into the acetylprotected rod 4 using procedures patterned after a previous Received Date: August 27, 2010 Accepted Date: November 19, 2010 Published on Web Date: December 02, 2010

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DOI: 10.1021/jz101215h |J. Phys. Chem. Lett. 2010, 1, 3471–3477

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Scheme 1. Synthesis of the p-Carboranes 1-5 and of the Oligophenylene Derivative 6a

a

The monolayer structure was investigated in real space by STM using a bias of VGap = 0.4 V and a tunneling current of It = 2 pA. The STM scans (Figure 1) revealed a dense monolayer of carborane 5 forming a hexagonally close-packed array with a lattice spacing of 7.02 ( 0.08 Å, corresponding to a molecular area of 0.427 nm2. Because the carborane cage is the bulkiest group in structure 5 (Scheme 1), its size defines the dimensions of the unit cell. However, the distance between adjacent boron cages does not match the Au(111) lattice; thus, if superstructures of 5 on Au(111) exist, they must have a higher-order periodicity to be in registry with the substrate. Using short carboranethiols, √ √ Weiss et al. recently reported a commensurate ( 19 19)R23.4° lattice structure.14 However, as shown in Figure 2, the present monolayers show a polycrystalline behavior, that is, we find adjacent hexagonal domains rotated toward each other at variable angles. On a single Au(111) terrace, relative rotation angles of 5, 11, 18, 19, and 24° are observed among domains (Figure 2). The manifold of rotation angles is inconsistent with a unique registry of the monolayer to the Au surface, excluding hypothetic commensurate structures that would be compatible with √ the observed nearest-neighbor (NN) distance (as the ( 43 √ √ √ 43)R7.6° and ( 52 52)R13.9° unit cells (Figure 1C and D), having NN distances of 7.16 and 6.93 Å, respectively). Furthermore, we found a relatively broad distribution of intermolecular distances, expressed in a high standard deviation (0.3 Å) from the average NN distance (7.02 Å). The average STM height modulation of ∼0.5 Å, derived from height profiles along parallel rows of molecule 5 (Figure 2B), provides further evidence of this lack of registry. Altogether, these findings indicate the existence of an overlayer structure that is incommensurate with the Au(111) lattice, as it was found in the past, for example, for fluorinated n-alkane thiols.15 They also suggest a variation in the conformation of the thiolatetrimethylene-bicyclo[1.1.1]pentane spacer, linking the carborane cage to the Au substrate. Thus, whereas most thiolate-based monolayers are commensurate to the substrate due to the corrugation of the sulfurAu surface potential, carborane monolayers lack such commensurability for a number of reasons. First, the maximum packing density is limited by the closest distance between neighboring carborane cages (7.02 Å), which is ∼3/2 of the lattice spacing of alkanethiol monolayers. Second, the footprint of the trimethylene chain supporting the carborane cage (molecular area of alkanethiols: 0.216 nm2) covers an area equivalent to 50% of the area of 5 (molecular area: 0.427 nm2), allowing a flexibility in the coordination of the head group to Au. For thiolate head groups, a closer packing is energetically favored due to substrate corrugation, typically √ √ driving a ( 3 3)R30° phase.21 This aggregation of the head groups could further be supported by the attractive van der Waals forces acting between the flexible trimethylene groups.22 In short, the registry mismatch of monolayer 5 with the Au(111) substrate is a result of the strong interaction between neighboring carborane cages on the one hand and of the conformational flexibility of the trimethylene chains on the other hand,23 thereby enabling these chains to act as a “matching layer” between the positions of the cages and the locations of the

Details are provided in the Supporting Information.

synthesis of cubane- and bicyclo[2.2.2]octane-based rods.17 The deprotection to the desired thiol 5 was then performed in situ with piperidine. The carborane monolayers were prepared in an argon environment by immersion of atomically flat Au(111) surfaces into 1 mM ethanolic solutions of 4 containing 100 mM piperidine for 20 h at room temperature. The quality of the resulting monolayers was verified by X-ray photoelectron spectroscopy (XPS) measurements.18 The sulfur 2p signal at a binding energy of 162.1 eV is characteristic of the chemisorption of thiolates on Au substrates.19 The width of this peak (fwhm = 0.66 eVat the given resolution of the spectrometer) is identical to the fwhm observed for alkanethiol monolayers on Au, indicating a defined chemical bonding of the head group to Au,20 that is, a uniform distribution of thiolate adsorption sites on the Au(111) substrate (Supporting Information). Elemental quantification based on sulfur 2p, boron 1s, and carbon 1s corelevel spectra yielded a monolayer composition in excellent agreement with the stoichiometry of 5. Further details about the synthesis of 4 and XPS/scanning tunneling microscopy (STM) methods are provided in the Supporting Information.


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Figure 1. (A) STM constant-current image of monolayer 5 on Au(111) forming a hexagonal close-packed network (scan range: 5 5 nm2). The bright spots are probably the terminal carbon atoms of the p-carborane cages. The scans were acquired at It = 2 pA and Ug = 400 mV. (B) Fourier transform of the STM image with reciprocal points indicative of a hexagonal lattice. The slight distortion of the hexagonal symmetry in k-space results from drift perpendicular to the scan direction. (C, models of two hypothetical structures of √ √ √ D) Schematic √ monolayer 5. A ( 43 43)R7.6° overlayer structure is adopted in (C), and a ( 52 52)R13.9° structure is used in (D). The two vectors (in blue) define the unit cell of the structure. The black circles represent the Au(111) surface with a lattice constant of aAu = 2.884 Å.

minima of the sulfur-Au surface potential. Figure 2c shows a model of the proposed structure. The different conformers in this model were obtained by varying the trimethylene bond angles of the density functional theory (DFT) energy-minimized structure 5. The model is consistent with our findings, for example, rotation of domains, STM apparent height modulation, lattice constant fluctuations, and the fact that the peak width of the XPS sulfur 2p signal is close to that of the alkanethiol data. The high cohesive energy between carboranes was verified by force field and DFT calculations on a periodic array of molecules of 5 (Figure S3 and S6, Supporting Information). The model converged to a lattice spacing of 7.0 Å (7.1 Å for DFT), in very good agreement with the experimental data. Homomolecular interaction energies of 1.22 and 1.69 eV/ molecule were found for the equilibrated layer from force field and DFT calculations, respectively. As the DFT model includes a hybrid semiempirical solution for the correction of dispersion forces,24 we expect the energies gained from this method to be more reliable than those from Dreiding force fields. In any case, the cohesive energy is significantly higher than the corrugation energy of the substrate reported for alkanethiols (ΔEcorr ≈ 0.26 eV),21,25 providing evidence that the forces acting between carborane rods determine the structural characteristics of the monolayer independently of the connectivity to the surface given by the matching layer. For structure 5, more significance is attributed to intermolecular dispersion forces among p-carborane cages than to possible intramolecular interactions.1


In view of the potential application of carboranes as hole conductors, we investigated to what extent the high ionization potentials16 of carboranes are reflected in the electronic structure of monolayers of 5. In particular, we determined the relative position of the higher-lying orbitals of 5, which play a crucial role in charge transport, with the Fermi energy (EF) of the Au substrate. Ultraviolet photoelectron spectroscopy (UPS) was used to reveal the valence band structure of the monolayers26,27 by directly probing the intensity and position of occupied electronic states at the surface. A He UV lamp with a photon energy of 40.8 eV was used as a source. The small attenuation length of the emitted photoelectrons (∼5 Å)28 (Supporting Information) and the implied high surface sensitivity allows this method to detect the electronic structure of 5 and, in particular, of the carborane cage. Figure 3A shows the UPS spectrum of the monolayer, where subtraction of the Au background results in the valence band structure of 5. The center of the HOMO level was determined by Gaussian fitting to the resonances with the lowest binding energy. Note that because we are only interested in orbitals delocalized on the molecular backbone, we neglect the higher-lying antibonding σ*(S-Au) orbital localized on the thiolate.29,30 The steep increase in the photoemission intensity below a binding energy (BE) of ∼4 eV reflects the significant density of states (DOS) in the frontier valence band of 5. A HOMO position of 4.3 eV was determined by Gaussian fitting. To identify the main resonances in the valence band, we calculated the DOS using an atomic orbital DFT model

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Figure 3. Valence band structure of 5. (A) He II photoemission spectrum from a monolayer on Au (blue), referenced to the spectrum of a clean (sputtered) Au surface (black). The spectral contribution of 5 was obtained by subtracting the normalized Au spectrum from the total intensity. (B) DOS of molecule 5 from DFT calculations, projected on the carborane cage (red line), on the bicyclo[1.1.1]pentane cage (black line), and on the trimethylene group (dotted black line). The energy scale of the DOS is rigidly shifted (by 2.9 eV) to match the experimental data (the peaks in the DOS were aligned with the most prominent peak-shaped resonances at 9.7 and 14.4 eV BE; see vertical dotted lines). The binding energy is referred to the Fermi level of Au.

Figure 2. (A) STM image of monolayer 5 on Au(111) (scan range: 41 41 nm2). The marked areas define domains that are rotated relative to each other (It = 2 pA, Ug = 400 mV). The zoom image (scan range: 6 11 nm2) shows three adjacent domains marked by A, B and C. Domain A is rotated by 24 and 11° with respect to domains B and C, respectively. Rotation angles of 5, 11, 18, 19, and 24° are found for the total scan area. (B) Section from a STM height profile along parallel carborane rows, showing an average height modulation of ∼0.05 nm. (C) Model explaining the incommensurate structure of the monolayer, where the bicyclo[1.1.1]pentane/ trimethylene spacer acts as a “matching layer”. The height modulation (dashed lines) is in agreement with the experimental values obtained from STM (B).

On the basis of experimental gas-phase ionization potentials (IP)33 and electron affinities (EA),34 as well as on semiempirical calculations,8,35 a HOMO-LUMO gap of ∼11.0 eV was determined for the p-carborane cage. In conjunction with the UPS results discussed above, the Fermi level of Au is found to be closer to the HOMO than to the LUMO of 5 (EHOMO = 4.3 eV, ELUMO ≈ -6.7 eV), which implies that charge transport through Au-p-carborane-Au junctions would probably occur as a hole transport process for carboranes linked to the electrodes by thiolates. Of course, due to polarization effects, in close proximity to the metal substrate an energy shift of molecular levels relative to gas phase values is possible. However, using a dielectric continuum model for the estimation of HOMO shifts resulting from electrostatic screening,36 a change of only ∼0.1 eV compared to the gas phase is computed (first order approximation). Note that the HOMO is significantly closer to EF

(Supporting Information).31 A fairly good agreement was found between the simulated DOS and the experimental data (Figure 3B). The peak at ∼5 eV in the DOS is mainly attributed to pseudoaromatic orbitals on the p-carborane as it arises from boron and carbon p orbitals on the carborane cage and also partially on the bicyclo[1.1.1]pentane cage. Note that the ionization potential of the latter is known to be slightly lower than that of p-carborane.32 However, even though the bicyclo[1.1.1]pentane cage makes a significant contribution to the low-energy tail of the DOS (at ∼5.5 eV), the DOS within the extended range from 4 to 8 eV predominantly stems from delocalized molecular orbitals on p-carborane.


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Figure 4. Valence band structure of monolayers 5 and 6 from He II (40.8 eV) photoelectron spectra. For 6, the photoelectron attenuation of the Au signal was ∼96%; thus, the background correction was neglected. The intensities of the two spectra were normalized to the relative molecular surface density, and accordingly, the intensity of 6 was scaled down by a factor of 2. The HOMOs of 5 and 6 were found at 4.3 and 2.1 eV, respectively. For monolayer 5, a weak shoulder at ∼3.9 eV is observed. The two bumps in the spectrum of 6 at 1 and -2 eVare not related to the electronic structure but rather to the first satellite of He II radiation (ΔE = 7.54 eV), doubling the 8.5 eV and 6 peaks, respectively. The energy scheme on the right (energy values in eV) was obtained from measured HOMO values and from reported IP and EA data.

than was reported by Balaz et al.9 for p-carboranes directly adsorbed to Au (7-8 eV below EF). A comparison with oligophenylene derivatives, which are frequently used in molecular electronics, is instructive when discussing the alignment of the HOMO and EF at the metalmolecule interface.37-40 Figure 4 illustrates the difference between p-carboranes and oligophenylenes based on the band structure of 5 and on that of [1,10 ;40 ,100 ]terphenyl-400 -ylmethanethiol (6; see Scheme 1), which was synthesized according to reported procedures.41 A distinctive feature of 5 and 6 is the high DOS (significantly higher than that for hydrocarbons) found for p-carborane 5 in the valence band, that is, over the whole spectral range from 4 to 8 eV. We assign this effect to the larger number of atoms within the icosahedral skeleton, whose interaction results in a splitting of levels over a broad energy range. The energy gap between the HOMO of 6 (derived from the 1e1g orbital of benzene)42 and the Fermi level of Au is only 2.1 eV, rather close to the gap recently reported for (oligo-phenylene-ethynylene)thiols on Au.43 This is a drastic difference in the HOMO position as for p-terphenyl, the gap HOMO-EF is about half as large as that for p-carborane. Using the resonances from UP spectra and combining them with reported values for gas-phase IPs and EAs of p-terphenyl (IP = 8.7 eV. EA = 0.27 eV)44 and of p-carborane, an energy scheme as depicted in Figure 4 is obtained. To further tune the energy level alignment of closocarboranes with the electrodes, either a modification of cage substituents or a variation in the length of the carborane-Au spacer could be envisioned. On the basis of these results, the large hole-injection barrier observed for p-carboranes on Au could be of considerable


interest, for example, to fabricate diodic molecular junctions with high rectification ratios, realized by combining carborane cages with aromatic π-systems in a single molecular junction. Furthermore, despite their similar band gap, carborane rods could behave quite differently than hydrocarbon chains when used as structural elements in electronic devices. Specifically, a combination of the enhanced transmission of electronic effects through carborane cages known from NMR studies (antipodal effect)45 with the high ionization potential typical for uncharged p-carboranes could result in transport properties much different than those of saturated alkane chains. In conclusion, we have identified a densely packed and polycrystalline 2d arrangement of closo-carborane units with unique electronic properties. The thiolate anchor group with the flexible trimethylene/bicyclo[1.1.1]pentane spacer allows both the linkage of p-carborane cages to gold surfaces and the formation of a monolayer structure that is incommensurate to Au(111). The high energy gap between the molecular HOMO and the Fermi level of Au is a characteristic feature of carboranes, and it could be tuned either by varying the linkage to the substrate or by changing the chemical substitution of the cage. The band structure of p-carborane monolayers significantly differs from that of aromatic hydrocarbons, making this class of compounds a promising expansion of available functional materials for applications in molecular electronics.

SUPPORTING INFORMATION AVAILABLE Experimental and computational details on sample preparation, XP and UP spectroscopy, STM characterization, and electronic structure and

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force field calculations. This material is available free of charge via the Internet at http://pubs.acs.org. (15)

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Tel.: þ49-711-5858838. Fax: þ49-711-5858-99838. E-mail: Florian.vonWrochem@ eu.sony.com.

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ACKNOWLEDGMENT The authors thank William Ford, Yvonne

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Joseph, and Nikolaus Knorr for their support and helpful discussions. We extend particular gratitude to Carsten Menke (Accelrys) for his support in molecular modeling (Dreiding force field calculations). Work in Boulder was supported in part by the U.S. National Science Foundation (CHE-0848477).

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p-Carborane on Au(111) - American Chemical Society

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