Tailoring SAM-on-SAM Formation - American Chemical Society

Letter pubs.acs.org/JPCL

Tailoring SAM-on-SAM Formation Albano Cossaro,*,† Michele Puppin,†,§ Dean Cvetko,∥,† Gregor Kladnik,∥,† Alberto Verdini,† Marcello Coreno,⊥ Monica de Simone,† Luca Floreano,† and Alberto Morgante‡ †

CNR-IOM, Laboratorio TASC, Trieste, Italy Dipartimento di Fisica, Università di Trieste, Trieste, Italy § Fritz-Haber Institut der MPG, Berlin, Germany ∥ University of Ljubljana, Ljubljana, Slovenia ⊥ CNR-IMIP, Montelibretti (I), Rome, Italy ‡

S Supporting Information *

ABSTRACT: We present the formation, under ultrahigh vacuum conditions, of a three-dimensional organic architecture on a Au(111) surface based on the NH2−COOH interaction. The surface is first functionalized with a selfassembled monolayer (SAM) of an amine-terminated molecule (cysteamine, CA); then, a layer of benzoic acid (BA) is grown on top. We characterized this self-assembled structure by means of X-ray photoemission and absorption spectroscopy. The formation of a hydrogen bond between the two molecular species anchors the BA molecules to the CA. The structure is homogeneous in terms of its morphology and chemical properties. We also show that the structure (molecular orientation) of the BA SAM formed on the CA SAM is different from that of the BA SAM on the bare Au surface. The chemical recognition and molecular ordering nature of the BA−CA self-assembly makes it a promising candidate for the bottom-up parallel fabrication of hierarchically assembled nanodevices starting from functionalized building blocks. SECTION: Surfaces, Interfaces, Catalysis

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he market of organic-based electronic devices is expected to follow an exponential growth in future years.1 Its development relies on the possibility that these technologies may become competitive with the existent silicon-based one in terms of efficiency and life-cycle costs. Regardless of the application (solar cells, organic-based transistors, biosensors), it is believed that in general the electronic transport properties at the interface between the metal electrodes and the organic active part of a device represent one of the major bottlenecks to an effective employment of these devices.2,3 Both the energy level alignment and the morphology at the metal−organic junction determine the charge injection process between the organic film and the substrate.4 In recent years, the attempts to improve the charge transport and reduce the contact resistance at electrodes followed two main routes. Many studies have been devoted to the control of the molecular orientation at the metal surface and to the integration of the desired organo-metallic interface into a suitable device architecture.5 This route has been extensively explored for the case of π-conjugated molecules because of their tendency to maximize the overlap of the π* molecular orbitals with the electronic cloud of the metal substrate. The possibility to steer the molecular orientation by coupling to a metal surface can favor either a top contact6 or a bottom contact device7 for the same kind of molecule−electrode pair, depending on the desired architecture. At the same time, this © 2011 American Chemical Society

characteristic behavior of the π-conjugated molecules on metals is the origin of most of the topological defects formed at the electrodes (thus increasing the contact resistance), when the thickness of the organic film exceeds a few layers and its bulk crystalline structure is recovered. As an alternative for the case of films composed of small organic molecules, a promising and widely adopted route is to interpose a self-assembled monolayer (SAM) to bridge the metal electrode and the organic film. On one side, the SAM modifies the work function of the electrode; on the other side, the organic semiconductor is simply physisorbed and may preserve its bulk crystalline structure down to to a few monolayers. It has been shown with this method that both the energy levels alignment8−10 and the interface morphology11 can be tuned to achieve higher device performance. A step forward in this direction is the exploitation of the SAM for directly anchoring a second organic layer through a specific ligand with the aim of enhancing the charge transport to the electrode. This route has been followed in a few seminal studies, where quite complex supramolecular architectures on a SAM were obtained by deposition of a self-assembled second-layer from the vapor Received: October 24, 2011 Accepted: November 30, 2011 Published: November 30, 2011 3124

dx.doi.org/10.1021/jz201415k | J. Phys. Chem.Lett. 2011, 2, 3124−3129

The Journal of Physical Chemistry Letters

Letter

Figure 1. N1s and C1s photoemission spectra taken on the CA pristine monolayer (bottom), on the anchored system (center), and on the CA monolayer upon the desorption of the BA. In the inset, the integrated intensity of the C1s peak is reported as a function of the sample temperature, revealing that the BA molecules are desorbed at T ∼ 275 K.

phase, 12,13 in solution 14−16 and solution plus vacuum deposition.17 In any case, the possible influence of solvents for the growth in solution or the intrinsic low efficiency of the assembling process for the growth in vapor prevented a proper control of the structure and morphology of the systems, as well as the achievement of a large coverage of the double SAM film. The homogeneity of the molecular configuration (orientation and linking) within the organic architecture is an important aspect to be considered, since the presence of defects in the film crystalline structure hampers the transport performance.18,19 In this view, the growth of the organic structures by evaporation under ultrahigh vacuum (UHV) conditions permits a high level of control on the ordering properties of the systems20 and therefore represents a valuable route for the development of the thin organic films technology. Moving from these considerations, we present here a novel approach to the formation of organic architectures, where growth under UHV and exploitation of the functional groups of the chosen molecules, lead to a high degree of control over the interaction between the SAM and the organic film. We have recently shown that the tendency of amino acids to form zwitterions drives the formation of ordered two-dimensional nanostructures on metal surfaces.21,22 Two adjacent molecules interact by forming a hydrogen bond between their carboxylic and amino groups. Here we exploit the same interaction to build a threedimensional architecture where the SAM-film contact is controlled at the molecular level. The amino-carboxylic affinity has been already exploited for the synthesis of hetero-organic architectures by Crooks and co-workers,12 but the intrinsic limitation of the growth techniques yielded a poor coverage of the final double-SAM film. We have developed a procedure where the Au(111) surface is first functionalized by the formation of an amino-terminated SAM of cysteamine (CA, HSCH2CH2NH2); then a layer of carboxylic molecules (benzoic acid, BA, C6H5COOH) is grown on top, whose formation is guided by the chemical affinity

between the NH2 and COOH groups. The whole growth procedure is performed under UHV conditions and monitored by spectroscopic techniques. The characterization of the CA SAM on the Au(111) surface has been reported elsewhere; 23 CA molecules form a compact layer and are oriented mainly in a standing-up configuration, with the amino-group pointing out of the surface. Upon the BA deposition on top, anchoring between the carboxylic molecules and the amino SAM terminations takes place, with the formation of a hydrogen bond. By comparing the system with the BA SAM on the bare Au(111) surface, we demonstrate how the morphology of the BA SAM is changed by the interposition of the CA SAM. In Figure 1, we show the N1s and C1s photoemission spectra taken on the CA monolayer (lower panels), on the BA/CA junction (central panels) and on the system after the BA thermal desorption (upper panels). The spectra of the CA monolayer have been discussed extensively elsewhere.23 The CA molecules in the SAM are in a standing up geometry, with the amino group pointing out of the surface. The film is very homogeneous and compact; the molecular coverage is 85% of the coverage in the (√3 × √3R30°) phase of a methyl thiol SAM on a Au(111) surface. The N1s peak presents a main component Nn at 399.1 eV, assigned to the neutral NH2 group. In the C1s signal, two main components can be distinguished, Cα and Cβ corresponding to the two different carbons of the CA molecules. In both nitrogen and carbon spectra, minor components are evidenced by the fit (Ndef and Cdef), which have been assigned to CA molecules in their gauche conformation, with the amine group pointing downward to the surface. The central panels report the spectra taken after the deposition of a layer of BA on the CA film at low temperatures (240 K < LT < 270 K, see the Experimental Methods section). An additional nitrogen component, Nzw, appears at 401.3 eV, whereas the Nn component has strongly decreased and the Ndef one mostly unchanged. 3125



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Figure 2. O1s photoemission and C1s NEXAFS spectra taken on the BA monolayer on the bare Au(111) surface (bottom), on the anchored system (center) and in gas phase (top). The gas phase BE axis has been translated for clarity. The polarization-dependent absorption spectra reveal a different adsorption geometry for the BA molecules on the bare and functionalized gold substrate.

The Nzw binding energy (BE) is consistent with the presence of amino groups in their protonated state NH3+.24 From the fit it can be stated that, as a result of the BA deposition, most of the standing up CA molecules change their ionic state from neutral to ionic. We assign this conversion to the interaction between the amino and the carboxylic groups of the CA and BA molecules, respectively. The chemical affinity between the terminations of the two molecules governs the formation of the BA layer and determines the anchoring of the BA molecules on the surface. From the ratio between the protonated and the residual neutral components, it results that the efficiency of the anchoring process is about 80%. Steric constraints probably prevent additional BA molecules from anchoring on the CA SAM. This result is nevertheless remarkable; systems based on the same anchoring interactions, grown both in vapor and in liquid, showed an efficiency of the anchoring process lower than the 40%.12 The C1s spectrum confirms that the BA has been adsorbed on the surface. The benzene ring atoms signal is superimposed to the CA signal, whereas the carboxylic carbon peak is at higher binding energies (288.4 ± 0.2 eV). The BA− CA double SAM is stable below room temperature RT. Increasing the sample temperature causes BA to desorb at T ∼ 275 K. The inset in the right upper panel reports the C1s integrated intensity as a function of the sample temperature; its drop indicates the desorption of anchored molecules. The C1s and N1s spectra taken after annealing show that the CA SAM recovers its initial chemical state and composition and that the anchoring-desorption process is reversible. In order to investigate the chemical state and the adsorption geometry of the anchored BA molecules, we measured the O1s photoemission spectra and the C1s absorption near edge X-ray absorption fine structure (NEXAFS) spectra. As a reference we took the same scans on the BA in the gas phase and as deposited on the bare Au(111) surface at LT. The spectra are reported in Figure 2. The gas phase O1s XPS (top panel, left) was reported in a previous work,25 with a lower signal-to-noise ratio due to the use of a conventional X-ray source. The two

main components are assigned to the carbonyl (lower BE) and to the hydroxyl (higher BE) oxygen, respectively. In our measurement, a small component at 541.7 eV can be distinguished, which we attribute to a shakeup satellite,26 i.e., to the kinetic energy loss of some O1s photoelectrons that excite a highest HOMO−LUMO transition before escaping the surface (final state effect). The energy difference between the two main components is 2.1 eV. Also in the BA/Au system (bottom, left) the spectrum is characterized by two components of the same intensity. This indicates that, as for the isolated molecule, the two oxygen atoms are inequivalent, i.e., the carboxylic group is not deprotonated by the interaction with the Au substrate. The energy difference between the two components is 1.4 eV, considerably smaller than in the gas phase. This might be due to the interaction with the substrate or, most likely, to the formation of an ordered BA structure on the surface. Carboxylic acids are known to assemble in dimers or catamers in their crystalline form27 and on poorly reactive surfaces such as the Au(111),28 through the formation of a hydrogen bond between the carboxylic groups of adjacent molecules. The different core level shift between the two O1s components in the BA/Au SAM with respect to the isolated molecule is the fingerprint of the interaction among the BA molecules. The spectrum taken on the anchored system (central panel) indicates that the BA molecules are in a different chemical state with respect to the previous situations and provides evidence that proton transfer occurs when BA anchors to CA. The peak profile is very similar to the one measured on a monolayer of cysteine on Au(110), 29 where the main component was attributed to the equivalent oxygen atoms of deprotonated carboxylic group of zwitterionic molecules, whereas the two minor components at higher binding energies were assigned to residual molecules still having the functional group in its neutral state. The main component at 531.3 eV highlights that for most of the BA molecules the formation of COO−-NH3+ hydrogen bonds takes place with the amino groups of the underlying CA. 3126



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through a defined and reliable interaction. In this view, our approach opens to the appealing perspective to describe the electronic transport properties of the system as the sum of the transport properties of the single amino-carboxylic molecular junction, which can be in principle determined by means of scanning tunnel microscope-based spectroscopy. Our anchored system represents an archetype of complex hetero-organic structures where the transport properties are known at the molecular level.

The O1s shoulder at higher energies, which can be fitted with two components, reveals that part of the BA molecules are still in their protonated form. This is consistent with the presence of the gauche defects in the CA monolayer, which show up as amine anchor-inactive, thus preventing part of the benzoic molecules from finding anchoring sites on the SAM. These components have higher binding energies than in the BA/Au case; this is due to the screening by the metallic substrate, which is lower here because of the larger distance of the BA molecules from the surface, due to the presence of the interposed SAM. The C1s NEXAFS spectra are presented on the right panels in Figure 2. The assignments of the BA gas-phase spectrum features can be made according to ref 30. In particular, the structure at 285.0 eV is due to the transitions from carbons of the ring to the first two unoccupied orbitals (C 1s(ring)−π*1,2) whereas the peak at 288.2 eV is attributed to the C1s(COOH) −π*1 excitation. Both features can be conveniently analyzed for the surface systems as a function of the angle between the light polarization and the surface normal, in order to determine the adsorption geometry of the BA molecules. In the case of the BA/Au(111) system (bottom panel), the intensity vanishes when the polarization is parallel to the surface, indicating that the BA molecules adsorb completely flat in the formation of the catamers. The linear dichroism measured in the anchored system (central panel) is much less pronounced, providing evidence that the anchored molecules are tilted away from the surface. The region of the carboxylic excitation is also affected by the contribution of the CA molecules.31 For this reason, we limit our attention to the variation of intensity of the C 1s(ring)π*1,2 transitions in order to determine the orientation of the molecules. Considering the measurement geometry of our apparatus 32 and the 3-fold symmetry of the substrate, the angle γ(R) between the BA rings and the surface can be derived from ref 33 to be a function of R, the ratio between the intensities measured at the two opposite p and s polarization angles, as follows:

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EXPERIMENTAL METHODS

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ASSOCIATED CONTENT

The solid state measurements were performed at the ALOISA beamline34 of the Elettra Synchrotron. The Au(111) surface was prepared by cycles of Ar+ sputtering (1 keV) and annealing up to ∼750 K. The surface quality after the preparation was checked by reflection high-energy electron diffraction (RHEED); the absence of contaminants was verified by XPS. XPS spectra were taken at grazing incidence angle (4°), using a photon energy of 500 eV for C1s and N1s and 650 eV for O1s. The signal was detected by a hemispherical electron analyzer in normal emission geometry; the overall energy resolution was about 300 meV. The spectra are reported as a function of BE after a Shirely-type background subtraction; the BE energy scale has been calibrated with respect to the bulk spectral component of the Au 4 f7/2 peak at 84.0 eV BE.35 The NEXAFS C K-edge spectra were acquired in partial electron yield by a wide acceptance angle channeltron. The photon energy resolution was better than 100 meV. Further details on the measurement geometry can be found in ref 32. CA film was prepared as described in ref 23. BA molecules (purity >98%, Fluka) were put in a pyrex cell and pumped down to high vacuum. For the deposition, the evaporation cell was operated at ∼330 K, and the sample chamber was exposed to a CA vapor pressure of 3 × 10−8 mbar, through a leak valve. The BA monolayer was obtained as saturation coverage by keeping the sample temperature Ts in the range 240 K < Ts < 270 K. The formation of a BA thick film was observed at Ts < 220 K. The measurements were performed at low sample temperature (Ts ∼ 180 K). The irradiated area was continuously displaced after each spectrum to minimize effects related to beam-induced damage. Gas phase spectra have been acquired at the gas phase beamline, Elettra, Trieste.36 BA powder was inserted in the experimental chamber inside a small capsule with an effusive nozzle. The absorption spectra (NEXAFS) were acquired by measuring the total ion yield with a channeltron multiplier placed near the ionization region. The photon resolution was set to 60 meV at the C K-edge. The O1s core photoemission spectrum was taken at 650 eV photon energy. The total resolution was better than 300 meV. Both NEXAFS photon energy and XPS BE were calibrated to the 1s energy of CO2.37 Molecular images in the present paper were produced using the ArgusLab software.38

where α is the photon incident angle (α = 7°). We obtain here an adsorption angle of γ (R) = 47° ± 5°, which reveals how the adsorption geometry of the carboxylic molecules change as a result of the interaction with the SAM. The effects of the anchoring process have been investigated also by measuring the valence band of both the anchored and gas phase BA. The results are reported in the Supporting Information. In conclusion, our study reports a novel route to the formation of complex three-dimensional organic heterostructures. We have successfully formed a BA SAM on top of a CA SAM by chemically anchoring the carboxylic termination of BA to the protruding amine functional group of the CA SAM. The hetero-organic interface formation is reversible and repeatable upon temperature cycling of the system. The BA-CA linkage, based on the hydrogen bond between the functional groups of the molecules, provides for a molecular recognition and a highly directional organic coupling. The anchoring process induces a different growth orientation on the carboxylic film when compared with the growth on the bare metal surface. Each carboxylic molecule is linked to one molecule of the SAM

S Supporting Information *

The valence band of the anchored BA compared with the spectrum taken in the gas-phase is reported. The effect of the anchoring is visible as a binding energy shift of some of the features in the valence band. This information is available free of charge via the Internet at http://pubs.acs.org/ 3127



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AUTHOR INFORMATION

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REFERENCES

Letter

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Corresponding Author *E-mail: [email protected].

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Tailoring SAM-on-SAM Formation - American Chemical Society

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