ARTICLE pubs.acs.org/Langmuir
Tuning the Molecular Order of C60 Functionalized Phosphonic Acid Monolayers Armin Rumpel,†,§,|| Michael Novak,‡,|| Johannes Walter,† Bj€orn Braunschweig,†,|| Marcus Halik,‡,|| and Wolfgang Peukert*,†,§,|| †
Institute of Particle Technology (LFG), University of Erlangen-Nuremberg, Cauerstrasse 4, 91058 Erlangen, Germany Institute of Polymer Materials, University of Erlangen-Nuremberg, Martensstrasse 7, 91058 Erlangen, Germany § Erlangen Graduate School in Advanced Optical Technologies (SAOT), University of Erlangen-Nuremberg, Paul-Gordon-Strasse 6, 91052 Erlangen, Germany Cluster of Excellence Engineering of Advanced Materials (EAM), University of Erlangen-Nuremberg, N€agelsbachstrasse 49b, 91052 Erlangen, Germany
)
‡
bS Supporting Information ABSTRACT: Mixed self-assembled monolayers (SAM) of alkyl phosphonic acids and C60 functionalized octadecyl phosphonic acids (C60C18PA) are deposited on alumina substrates from solution and are shown to form wellordered structures with an insulating layer of alkyl chains and a semiconducting layer that comprises mainly C60. Such an ordered structure is a necessity for the application of SAMs in organic transistors but is difficult to obtain since C60C18PA without additional support do self-assemble in dense packaging but not in a well-ordered fashion. To avoid disordering of the SAM and to gain a better control of the interfacial properties we have investigated the stabilizing effects of fluorinated dodecyl phosphonic acids (FC12PA) on the C60C18PA monolayer. Vibrational sum-frequency (SFG) spectroscopy, ellipsometry, X-ray photoelectron spectroscopy, and electrical measurements were applied to study the mixed monolayers. Here, we make use of the differently labeled PA to determine surface coverages and molecular properties of the two species independently. Adsorption of FC12PA gives rise to vibrational bands at 1344 cm1 and 1376 cm1 in SFG spectra, while a pronounced vibrational band centered at 1465 cm1 is attributable to C60 vibrations. The coexistence of the bands is indicative for the presence of a mixed monolayer that is composed of both molecular species. Furthermore, a pronounced maximum in SFG intensity of the C60 band is observed for SAMs, which are deposited from solutions with ∼75% C60C18PA and ∼25% FC12PA. The intensity maximum originates from successful stabilization of C60 modified C60C18PA by FC12PA and a significantly improved molecular order. Conclusions from SFG spectra are corroborated by electric measurements that show best performance at these concentrations. Our results provide new information on the morphology and composition of C60 modified SAMs and establish a route to fabricate well-defined layers for molecular scale organic electronics.
’ INTRODUCTION Self-assembled monolayers (SAMs) of phosphonic acids (PAs) can be deposited on a large variety of oxides where surface functionalization is highly desirable to tune materials properties of the oxide support at low costs.17 For that reason, possible applications of phosphonic acids range from protective coatings that can enhance the corrosion resistance of less noble metals and alloys, to functional layers in organic electronics.8,9 In fact, phosphonic acid SAMs as auxiliary or functional layers in electronic devices have been proven to reduce the leakage currents of Al/AlOx(3.8 nm)/Au capacitors by 3 orders of magnitude.10 This remarkable improvement of the imperfect insulating oxide support was achieved with only a single self-assembled monolayer of an alkylphosphonic acid.10 More complex devices such as organic thin-film transistors require a more sophisticated approach where an additional semiconducting layer is deposited on top of the SAM.1013 This r 2011 American Chemical Society
layer can be either fabricated in a second step subsequent to the SAM deposition (by thermal evaporation) or even more efficiently in a single self-assembly process where specifically functionalized molecules are directly integrated into the monolayer.1417 The latter approach has already been demonstrated to be a promising route to self-assembled monolayer field-effect transistors (SAMFETs) based on the self-assembly of C60 functionalized alkylphosphonic acids on Al/AlOx (C60C18PA, Figure 1b).17 Under favorable conditions direct chemisorption of C60 modified with suitable organic groups has been shown to result in stable and homogeneous layers on gold.18,19 However, well-ordered structures are expected to be particularly difficult to obtain for C60C18PA since the relatively bulky C60 functional groups Received: October 5, 2011 Revised: November 2, 2011 Published: November 02, 2011 15016
dx.doi.org/10.1021/la203916h | Langmuir 2011, 27, 15016–15023
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ARTICLE
average Ææ of the Raman polarizability αk and the dynamic dipole moment μk. Therefore, χ(2) vanishes in the bulk of centrosymmetric materials such as cubic crystals, amorphous oxides, liquids, and gases where the orientational average is zero. Interfaces, however, break the prevailing bulk symmetry of these materials and give rise to additional nonzero χ(2) components. Thus, SFG is a powerful technique for studies of interfaces, their molecular composition, and structure.21,22 SFG has been applied extensively to study the structure of SAMs on metal and oxide substrates.2328 However, all applications of SFG at mixed SAMs on solid substrates were limited to the CH/OH region which impedes spectral discrimination of the two species.2934 This issue is resolved by using fluorinated alkyl chains which facilitate independent determination of surface coverage and molecular ordering for the two species.
Figure 1. (a-c) Molecules used in this study and (d) scheme of the investigated field-effect transistor (FET) with a mixed SAM that comprises insulating FC12PA and semiconducting C60C18PA.
are attached to a long and flexible alkyl chain and deposition may result in disordered layers. A possible approach to support the bulky C60 head groups (∼1 nm diameter) and to optimize the morphology of C60C18PA monolayers would be the incorporation of phosphonic acids with shorter alkyl chains and without C60 functional groups. The result a mixed monolayer of both species will be a necessary trade-off between interfacial order and surface coverage. Tailoring of semiconducting functionality requires a high concentration of C60, while an increased molecular order and insulating functionality should originate from elongated alkyl chains (Figure 1d). Characterization of these systems on a molecular level is highly challenging due to the complex nature of the interfaces. Therefore, a detailed knowledge on the molecular arrangement does not exist so far9 but is necessary to exploit the full potential of self-assembly for organic electronics. Using ellipsometry, X-ray photoelectron spectroscopy (XPS), vibrational sum-frequency generation (SFG), and additional electrical characterizations we present new information on the composition and the molecular order of mixed C60C18 and FC12PA monolayers and address the question if an optimum mixing ratio regarding their application in SAM-FETs exists. Theoretical Basis of Sum-Frequency Generation. SFG is a nonlinear optical technique where a frequency fixed visible (ωVIS) and a tunable infrared (ωIR) laser beam are combined at the interface of interest. The electric field of the two beams induces an electric polarization which is dependent on the second-order nonlinear susceptibility χ(2) of the sample.20 The intensity of the sum-frequency signal ωSF = ωIR + ωVIS is measured as a function of ωIR and given by 2 ð2Þ ð2Þ χk IðωVIS ÞIðωIR Þ with IðωSF Þ χNR þ k
∑
ð2Þ
χk ¼
Ak expðijk Þ ωk ωIR þ iΓk
ð1Þ
(2) (2) where χ(2) NR is the nonresonant and χk the resonant part of χ . Ak, ωk, Γk, and jk are the amplitude, frequency, bandwidth, and phase of the vibrational mode k. Ak NÆαk μkæ is proportional to the molecular number density N and to the orientational
’ EXPERIMENTAL SECTION Substrate and Monolayer Preparation. Molecules used for the self-assembly are depicted in Figure 1. Tetradecylphosphonic acid (C14PA) was obtained from PCI Synthesis and used as received. The synthesis of 1H,1H,2H,2H-perfluorododecylphosphonic acid (FC12PA) and C60C18PA has been reported elsewhere.35,36 Monolayers were prepared with the procedure that was previously described by Klauk et al.10 Briefly, an aluminum layer (30 nm) was thermally evaporated onto a silicon wafer and mildly oxidized in oxygen plasma. The root-mean-square roughness of the resulting AlOx layer was determined to 1.05 ( 0.15 nm by subsequent AFM measurements. Pure SAMs were formed by immersing as-prepared substrates into 0.02 mM solutions of FC12PA or C60C18PA in 2-propanol for 24 h. Several sample series of mixed SAMs where the substrates were cut from the same wafer were prepared from solutions containing mixtures with varying fractions of both molecules. The given concentrations throughout this paper refer always to the ratio of the molecules in the bulk solution. Electronic devices of pure and mixed SAMs were fabricated according to the previously described procedure17 where a bottom-gate and a topcontact setup is provided (Figure 1d). The bottom-gate of 30 nm aluminum and top electrodes of 30 nm gold were structured by shadow mask techniques, defining an active transistor channel of 150 μm in width and 10 μm in length. Sum-Frequency Generation. SFG measurements were performed with a home-built broadband SFG spectrometer. An amplified Ti:Sapphire laser system (MaiTai oscillator and Spitfire XP chirped pulse amplifier, Spectra Physics) with a repetition rate of 1 kHz and 800 nm wavelength generated laser pulses of 100 fs duration and 3.5 mJ per pulse. 2.6 mJ of the pulse energy were used to pump an optical parametric amplifier (TOPAS-C, Light Conversion) and a subsequent conversion stage. The latter was composed of a noncollinear difference frequency generator (NDFG) where signal and idler photons from the first amplification were mixed, and broadband IR pulses tunable from 2.6 to 11 μm were generated. The remainder of the pulse energy of the amplified Ti:Sapphire laser was spectrally filtered by an air spaced etalon (TecOptics) in order to narrow the spectral width of the 800 nm pulse to e6 cm1 which determined the spectral resolution of our broadband SFG spectrometer. The ethalon filtered pulses were highly time-asymmetric with a sharp rise time of ∼0.1 ps and a ∼1.2 ps long tail. As was previously reported by Lagutchev et al.37 such a time-asymmetric VIS pulse is ideally suited to suppress nonresonant χ(2) NR contributions effectively. IR and VIS beams were combined at the interface of interest at incidence angles of 60 and 50, respectively. The IR beam was focused with a f = +100 mm ZnSe lens to a 1/e beam diameter of ∼300 μm, while a f = +500 mm BK7 focusing lens for the VIS leads to a beam diameter of 500 μm. 15017
dx.doi.org/10.1021/la203916h |Langmuir 2011, 27, 15016–15023
Langmuir Adjustment of pulse energies for both the IR and the VIS beam was critical due to the small damage threshold of C60 modified molecules. Therefore, intense laser radiation that could cause radiation damages to the samples had to be avoided in order to keep the mixed SAMs with C60 intact. As a consequence, a reduction of IR and VIS pulse energies to 6 μJ at 7 μm (1430 cm1) and 12 μJ at 800 nm was necessary and resulted in stable SFG signals for more than 60 min (g3.6 3 106 laser shots) where the samples were constantly exposed to the laser beams. The generated SF signal was collimated, filtered, and spectrally dispersed with a spectrograph (Andor Shamrock 303i) onto a gated intensified CCD (Andor iStar). In order to reduce infrared adsorption due to gaseous H2O and CO2 in ambient air, the NDFG unit and the sample compartment were purged with dry (humidity
