J. Phys. Chem. C 2010, 114, 20843–20851
20843
Comparison of the Energy-Level Alignment of Thiolate- and Carbodithiolate-Bound Self-Assembled Monolayers on Gold† Philip Schulz,‡,§ Christopher D. Zangmeister,*,‡ Yi-Lei Zhao,‡,# Paul R. Frail,⊥ Sangameshwar R. Saudari,⊥ Carlos A. Gonzalez,‡ Cherie R. Kagan,⊥ Matthias Wuttig,§ and Roger D. van Zee‡ Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States, Institute of Physics (IA), RWTH Aachen UniVersity, 52056 Aachen, Germany, and Electrical & Systems Engineering, Materials Science and Engineering, and Chemistry Departments, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104, United States ReceiVed: July 30, 2010; ReVised Manuscript ReceiVed: October 8, 2010
This paper presents an experimental study of a carbodithiolate-bound monolayer on gold and finds the electronic structure of this surface to be different from those of thiolate-bound monolayers in important ways. Specifically, self-assembled monolayers formed from benzyl mercaptan and dithiobenzoic acid (molecules identical to each other except for the linker group) chemisorbed on gold are compared. The electronic structure of these modified surfaces was investigated using photoemission spectroscopy and electronic structure calculations. Surfaces covered with monolayers formed from benzyl mercaptan were found to be comparable to other thiolate-bound adsorbates. In contrast, surfaces covered with monolayers formed from dithiobenzoic acid have a markedly different electronic structure; the work function of the such modified surface is lowered by 1.6 eV, as compared to that of a bare gold surface, and the highest occupied π-band lies 1.3-1.6 eV below the Fermi level of the gold surface. Dithiobenzoic acid was also used to modify the contacts of organic transistors and found to be effective in engineering the metal-semiconductor interface to achieve ambipolar transport. Introduction The idea of using molecules as the active elements in electronic circuits began with the now famous visionary paper of Aviram and Ratner.1 In that prescient work, Aviram and Ratner identified many of the physical and chemical conditions required to realize useful molecular electronics components; one of the most important of these was the alignment among the various energy levels in the molecule relative to each other and the Fermi level of the substrate material. Some 40 years later, the use of organic materials in electronic applications remains a topic of scientific and technological interest. Most anticipated applications of organic materials are in devices that also include metals or inorganic semiconductors, an approach that leverages existing manufacturing capabilities and, potentially, allows engineers to design devices that exploit the best characteristics of a wide range of materials. While publicity is often focused on applications where the molecular entity performs a complex function,2 in logic and memory applications for example, molecules can also be used for simpler purposes, to modify the surface potential for example,3,4 and have already been shown to be powerful tools for engineering the interfaces between organic and inorganic materials in a wide range of electronic and optoelectronic applications. Especially in the application of organic field effect transistors, the dominant role of tailoring the electronic structure and electrical properties at the electrode †
Part of the “Mark A. Ratner Festschrift”. * To whom correspondence should be addressed. National Institute of Standards and Technology. § RWTH Aachen University. ⊥ University of Pennsylvania. # Current Address: Shanghai Jiao Tong University, Shanghai, China. ‡
interface by utilizing molecular self-assembling monolayers has been demonstrated in numerous studies.5-14 Considerable accomplishment toward the goal of using organic molecules in electronic devices has been achieved, and leading consumer electronics manufacturers now produce goods using organic light-emitting diodes. Yet, despite this progress, barriers to widespread application remain; among these is controlling the electrostatic landscape at organic-inorganic and organic-metal interfaces, where a variety of chemical and electrostatic phenomena conspire to produce effects that idealized, limiting-case models cannot anticipate.15-17 Trends in the work function of the modified surface and energy-level alignment at metal-monolayer interfaces, that is, the mismatch between the Fermi level of the metal and the frontier π-bands of the monolayer, exemplify this complexity. In the case of thiolate-bound aryls on gold, for example, it has been found that the energy-level alignment is dominated by interfacial charge rearrangement and asymmetries associated with the thiolate linker group.18-21 Metal-linker group interactions play a major role in determining the work function of a monolayer-covered surface, and changing the linker group also changes the energy-level alignment and the dominant carriertype through molecular electronic junctions.22,23 Experiments have shown that the substrate plays a lesser role in determining energy-level alignment.24,25 Functionalization, and especially functionalization at the end molecule that is not bonded to the surface [i.e., the end pointing away from surface (metal-molecule interface)], plays an important role in determining the work function of monolayer-covered surfaces but has a lesser effect on the energy-level alignment.3,15,26 Because the linker group plays such a central role in determining the electrostatic potential of a monolayer-covered
10.1021/jp107186m 2010 American Chemical Society Published on Web 11/10/2010
20844 J. Phys. Chem. C, Vol. 114, No. 48, 2010 surface, many different linking chemistries have been proposed and investigated.27-29 Recently, monolayers linked by carbodithiolates, for which the sulfur-to-sulfur distance is wellsuited for forming densely packed ordered monolayers on gold surfaces, have been investigated.30 Dithiocarbamates (R2N-C-S2), for example, have been shown to form robust monolayers and structures with novel properties.31,32 To further explore the carbodithiolate linking group and its properties, we have synthesized dithiobenzoic acid (DTBA), grown monolayers from DTBA, measured X-ray and ultraviolet photoemission spectra of these monolayers, and calculated the electronic structure of representative structures. This dataset was used to determine the energy-level alignment of the occupied states relative to the Fermi level of the substrate, evaluate the nature of these frontier states, and test the performance of organic field-effect transistors with contacts modified with this carbodithiolate. These measurements and calculations are compared to the analogous arylthiol, benzyl mercaptan (BM), which is well-known to form stable monolayers.33 Experimental and Theoretical Methods Materials. All reagents were purchased from commercial sources and used without purification. Reactions were carried out under inert conditions. All solvents were redistilled from the corresponding HPLC-grade solvents using sodium/benzophenone or CaH2 as the drying agent/indicator. The corresponding synthesis was adapted from previously published work.34-36 1H and 13C NMR were acquired in the University of Pennsylvania Chemistry Department NMR facility. Benzenesulfonyl Methyl. A mixture of benzyl bromide (5.0 g, 0.029 mol), sodium phenylsulfinate (7.20 g, 0.044 mol), and a catalytic amount of tetrapropyl ammonium bromide (1.55 g, 5.8 mmol), in CH3CN (40 mL), was refluxed for 12 h. Then, the solvent was removed under vacuum, and the residue was dissolved in CH2Cl2 (20 mL), washed with brine (20 mL), and dried (Na2SO4), and the solvent was removed via vacuum. The product was recrystallized from hot ethanol and collected by filtration. The crude product was obtained in near-quantitative yield as a white solid and was used without purification in the next step. 1 H (360 MHz, CDCl3); 7.61 (m), 7.44 (t, 7.74 Hz), 7.28 (m), 7.07 (d, 7.79 Hz), 4.31 (s). 13C (90 MHz, CDCl3); 133.89, 131.02, 129.07, 128.96, 128.85, 128.77, 63.124. Dithiobenzoic Acid 2-(Trimethylsilyl)ethyl Ester.35 Sulfur (0.274 g, 8.54 mmol) and sodium methoxide (0.45 g, 8.34 mmol) were charged into a 50 mL Schlenk flask and stirred at room temperature under nitrogen in 20 mL of THF for 30 min. Benzenesulfonyl methyl (0.97 g, 4.17 m mol) was directly added to the reaction flask and allowed to stir overnight. A color change occurred from a pale yellow to a dark red-brown suspension. (2-Bromoethyl)trimethyl silane (1.37 mL, 4.25 m mol) was added dropwise, resulting in an immediate color change toward a more true red and precipitation of a white solid. The reaction was stirred overnight, which was then quenched with water, and the solid was extracted with CH2Cl2. The organic layer was washed with saturated NaCl, filtered, and dried over Na2SO4, and the solvent was removed via vacuum. Organic residue was purified via a short silica gel column using hexanes as the eluent. A red oil was isolated in 60% yield. 1 H (360 MHz, CDCl3): 7.97 (dd, 7.2 Hz), 7.53 (t, 6.81 Hz), 7.37 (t, 6.93 Hz), 3.37 (dd, 5.07 Hz), 1.05 (dd, 5.18 Hz), 0.11 (s). 13C (90 MHz, CDCl3): 132.32, 128.48, 126.98, 34.06, 15.062, -1.51.
Schulz et al. Self-Assembled Monolayer Formation. Monolayers were assembled on gold substrates (200 nm Au over a 10 nm Ti adhesion layer on Si) from a 10 mmol solution of dithiobenzoic acid dissolved in distilled THF and cooled to 0 °C under N2.35 A 1.0 M TBAF (0.1 mL, 10 mmol) solution in THF was added dropwise over 1 min, resulting in an instant darkening in color of the solution. The solution was stirred for an additional 9 min (total 10 min) and quickly transferred to a rotovap to remove the solvent to dryness. The reaction vessel was transferred to an inert environment glovebox and reconstituted in CHCl3 (for samples used in spectroscopic measurements) or toluene (for devices) via a syringe through a 0.2 µm filter into a vial containing either gold-coated substrates or organic field-effect transistor (OFET) devices with gold electrodes. Samples for spectroscopic measurements were allowed to react overnight and then removed from solution and rinsed with the corresponding organic solvent and dried with a stream of N2. DTBA modification of the source and drain electrodes for OFETs was also carried out using the following alternate method that gave higher device yields as the TBAF was observed to partially etch device Au electrodes which were fabricated without an underlying Ti adhesion layer. A 10 mmol solution of DTBA dissolved in distilled THF was refluxed over 2 equiv. of K2CO3 for 2-3 h. The reaction mixture was cooled to room temperature, and the solvent was removed by vacuum. The reaction vessel was transferred to an inert atmosphere and reconstituted in distilled toluene and filtered through a 0.2 µm filter, and then, device substrates were submerged for 12-16 h. Device Fabrication. Full details for fabricating pentacene bottom-contact/bottom-gate (BCBG) FETs are given elsewhere;37,38 the following is a brief description. Kapton films served as the substrate for all of the devices. Prior to device fabrication, the Kapton films were cleaned for 5 min each in an ultrasonic bath of ethanol and deionized water. The gate and source-drain electrodes were patterned by photolithography, and 20 nm of Au was deposited by thermal evaporation for each of the gate and the source and drain electrodes. A 420-500 nm parylene-C film was pyrolitically deposited in a PDS 2010, SCS coating chamber, intermediate to the steps of gate and source-drain electrode deposition. DTBA modification of the source and drain electrodes followed the self-assembly procedures described above. The semiconducting pentacene channel was deposited by spin coating and thermal conversion of its n-sulfinylacetamidopentacene precursor following literature procedures.39,40 Measurement Techniques. Reflection-absorption infrared spectra of the monolayer-covered surfaces were measured with a Nicolet 6700 Fourier-transform spectrometer equipped with a Harrick GATR. X-ray photoemission spectra (XPS) were taken at a Kratos Axis Ultra DLD equipped with a monochromatized Al X-ray source providing Al KR radiation with a line width of ∼0.3 meV. All X-ray spectra were normalized to the Au 4f level. The ultraviolet photoemission spectra (UPS) were recorded in the same chamber using a He I radiation source. In the UPS, 0 eV binding energy refers to the Fermi level, which was discernible in all spectra. Spectra of sputter-cleaned gold surfaces and decanethiolate-covered gold were taken as references in the photoemission experiments. OFETs were measured in a Karl Suss PM5 probe station mounted in a nitrogen-filled glovebox. Electrical measurements of current-voltage were collected using an Agilent 4156C semiconductor parameter analyzer, and measurements of capacitance were obtained by using a HP 4276A LCZ analyzer. Computational Methods. The electronic structures of DTBA and BM were calculated by standard density functional theory
Energy-Level Alignment of Self-Assembled Monolayers methods. The structures of the isolated, hydrogen-terminated molecules were optimized in order to determine the geometry and orbital energies of the isolated molecule. These calculations were performed using the B3LYP hybrid functional with the 6-311+g(d,p) basis set. The adsorption and density of states of the BM and DTBA on Au(111) were also studied using density functional theory calculations, using the generalized gradient approximation of the PBE functional and periodic slab models with three layers of gold atoms and a (3 × 2) overlayer structure of the adsorbed molecules on the fcc(111) face of the gold surface. The gold atoms were kept in their bulk structure, though other geometric parameters were optimized at this level of theory. The convergence criteria have been set to 1.0 × 10-6 Hartree for the scf procedure and 2.0 × 10-5 Hartree/Bohr for the geometry optimization. The density of states and the orbital analysis were obtained using pseudoatomic basis orbitals and pseudopotentials. The Au7.0-s2p2d2f1, Au_PBE, S5.5-s2p2d1, S_PBE, H4.5-s2, H_PBE, and C4.5-s2p1, C_PBE basis sets and pseudopotentials utilized in the openMX program were employed with a 7 × 7 × 7 k-point grid. The electronic structure was analyzed by site-projected local densities of states for all atoms in the adsorbent and plotted against their distances to the first layer of gold atoms. In order to simulate the ultraviolet photoemission spectra, the projected density of states Np(E) of the metal-molecule system was calculated. The Kohn-Sham eigenstates, Ψj, were projected onto the atomic orbitals φi,n by using the tetrahedron broadening method for k-point integration.41 The total density of states, N(E), for a specific eigenstate Ψj is given by
N(E) )
∑ δ(E - εj)
J. Phys. Chem. C, Vol. 114, No. 48, 2010 20845 dicular to the surface. The monolayers were also evaluated using X-ray photoemission spectroscopy (XPS). The binding energy of the S 2p was used to determine the chemical state of the sulfur. Both BM and DTBA S 2p peak maxima were at 162.3 eV, consistent with molecule-metal covalent bonding. No higher-energy peaks, typical of oxidized sulfur, were observed for either monolayer. Molecular coverages were calculated from X-ray photoemission spectra by determining the S/Au ratio derived relative to the intensity of the S 2p and Au 4f peaks.43 The absolute coverages were estimated by comparing S/Au ratios of BM and DTBA to that of decanethiolate on gold, which forms well-ordered monolayers of known coverage (surface coverage ) 4.5 × 1014 molecules/cm2). Using this approach, the coverage of BM on gold was found to be 118 ( 9% (one standard deviation) that of decanethiolate, and that of DTBA was 110 ( 28% that of decanethiolate. These results are supported by our calculations that show the computed adsorption geometry to be nearly perpendicular to the surface and a Au-S distance of 0.278 nm for BM and a mean value of 0.291 nm for DTBA. The lowest-energy bonding configuration is on top of the gold atom sites (cf. Figure 1), as has been recently calculated for other thiol-based molecules on Au(111) surfaces. Standard DFT calculations reveal the fcc-bridge site to be most favorable for the adsorption of thiolate compounds on Au(111) surfaces. Since it is well-known from theory and experiment that thiolate molecules adsorb on the on-top site of a reconstructed gold surface, we chose the on-top position as the adsorption site44-46 (cf. Figure 1). For DTBA, the S-C-S angle among the atoms of the carbodithiolate group is calculated to be 123.9°, only a slight deviation from the calculated equilibrium angle of 121.3° in the hydrogen-capped, isolated molecule.
j
Ultraviolet Photoemission Spectra where εj is the respective Kohn-Sham eigenvalue, and the projected density of states on a specific atom n is calculated by
Nn(E) )
∑ ∑ 〈φi,nψj〉δ(E - εj) i
j
Eventually, the length-attenuated projected density of states, used for comparison to the ultraviolet photoemission spectra, is given by
Np(E) )
∑ µnNn(E) n
In this expression, variable attenuation of electrons from different parts of the monolayer (e.g., S-Au bands versus phenyl bands) is rudimentarily incorporated by multiplying the projected density of states of the single atoms by a factor µn that decreases as the distance away from the vacuum side of the monolayer increases. Finally, the length-attenuated projected density of states is convoluted with Gaussian functions with a full width at half-maximum of 0.3 eV, to simulate the peak broadening. Monolayer Structure. Previous reports have found complex trends in the self-assembly process of monolayers grown from small aromatic molecules,42 demonstrating that it is important to verify the structure of these monolayers. BM and DTBA monolayer formation was evaluated using grazing-incidence reflection-absorption infrared measurements, in which the most pronounced features (e.g., 1210 cm-1) correspond to C-C stretches, suggesting that the phenyl group is oriented perpen-
Ultraviolet photoemission spectra of monolayers of BM and DTBA on gold are shown in Figure 2. The most interesting and relevant features are those at low binding energy (
