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J. Phys. Chem. C 2008, 112, 10842–10854
Physical and Electronic Structure Effects of Embedded Dipoles in Self-Assembled Monolayers: Characterization of Mid-Chain Ester Functionalized Alkanethiols on Au{111} Orlando M. Cabarcos,† Andrey Shaporenko,‡ Tobias Weidner,‡ Sundararajan Uppili,†,⊥ Linda S. Dake,§ Michael Zharnikov,*,‡ and David L. Allara*,† Department of Chemistry and the Materials Research Institute, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, Department of Physics, Utica College, Utica, New York 13502, and Angewandte Physikalische Chemie, UniVerista¨t Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany ReceiVed: February 24, 2008; ReVised Manuscript ReceiVed: April 26, 2008
Self-assembled monolayers (SAMs) on Au{111} prepared from the midchain ester functionalized thiols, HS(CH2)mCO2(CH2)n-1CH3 with m,n ) 10,5; 10,10; 15,5; and 5,10, along with deuterated analogs, were characterized by wetting, ellipsometry, near edge X-ray absorption fine structure spectroscopy, quantitative infrared spectroscopy, high resolution X-ray photoelectron spectroscopy (HRXPS), and density functional theory calculations. The SAMs can be viewed as layered structures in which the bottom -(CH2)- alkyl segment (between the ester group and the substrate) exhibits typical conformational ordering and orientation analogous to long chain alkanethiolate SAMs, while the upper segment (ambient side) is significantly more conformationally disordered. The presence of the ester moiety leads to the formation of a strong electric dipole layer with a component of 1.05((0.09) Debye normal to the surface. This dipole layer exhibits a strong electrostatic effect on the XPS spectra in which the C 1s photoelectron kinetic energies are consistently shifted by 0.85((0.03) eV between the top and bottom -(CH2)- alkyl segments, regardless of relative lengths. This shift correlates, within error, with the value of 0.81((0.06) eV predicted via classical electrostatics due to the presence of the ester dipole layer. Overall, these data show that SAMs assembled from molecules with appropriately selected internal groups can be used to prepare internally layered structures with highly controlled electrical characteristics and further demonstrate that simple XPS shifts in core level energies can be used to derive accurate molecular dipole values in structured thin films. 1. Introduction Self-assembled monolayers (SAMs) prepared from alkanethiols on gold substrates have been extensively studied over the past 20 years due to their ability to form well-organized, reproducible organic films.1 The physical and chemical properties of such films are easily and most obviously modified by substituting the methyl tail with different functional groups. Relatively little research, however, has focused on incorporating functional groups within the midinterior portions of the SAM molecules and the resulting effects on monolayer structure and electronic characteristics.2–19 In essence, the buried functional groups act as small chemical and physical perturbations within the monolayer, which when compared against the extensively studied alkanethiol monolayers, can yield insight into the role of intermolecular interactions in monolayer formation and order. Additionally, the extent of the perturbation can be precisely controlled by selectively choosing the functional moiety introduced and by the moiety’s position within the chain, presenting new and unique avenues of investigation into some of the fundamental aspects and design of self-assembled monolayers and vertically stratified layers in general. Previous studies have incorporated phenoxy sulfones,2,16 sulfides,11,16 sulfones,3,16 diacetylenes,4 and ethers.5 Studies on * Corresponding author. E-mail:
[email protected]; Michael.Zharnikov@ ur3.unl-heidelberg.de. † The Pennsylvania State University. § Utica College. ‡ Universta ¨ t Heidelberg. ⊥ Current address: Conoco Phillips Co., Bartlesville Technology Center, Bartlesville, OK 74004.
sulfones and ethers originated specifically to examine the effect of the incorporated functional group on monolayer packing and order.5,16 Buried phenoxy sulfone and phenoxy sulfide groups were studied to introduce a noncentrosymmetric arrangement of dipoles in the SAM for possible use in second-order nonlinear optical applications.2 The diacetylene groups were used for studies of photopolymerization in the interest of creating patterned surfaces and photolithography.4 Most recently, various sulfur-derived functional moieties were investigated as a deliberately introduced “weak link” for use as a monomolecular lithographic resist.11,16 In most cases, any changes in conformational order caused by the presence of even the bulkiest internal moieties were mitigated by ensuring that the length of the alkane chain above the moiety was greater than eight methylene units. The majority of previous work has focused on buried amide groups, where the amide functionality can form intermolecular hydrogen-bonds adding to monolayer stability and order when there is at minimum a methylene spacer unit between the sulfur and the amide.6–10,12–15,17–19 The films were originally contrived as a model system for the study of through-peptide electron transfer,7 though the extra stability imparted to the films led to further studies examining the fundamental dependence of monolayer structure on molecular constitution.9,10 Ultimately, the inclusion of functional groups capable of hydrogen bonding provides another facet by which to control the assembly of thin films instead of solely relying on the van der Waals interactions that dominate films formed by alkanethiols. Manipulating and
10.1021/jp801618j CCC: $40.75 2008 American Chemical Society Published on Web 06/28/2008
Embedded Dipoles in Self-Assembled Monolayers exploiting these interactions may lead the way to ultrahigh resolution nanolithography and nanostructured materials.12,14,20 In order to understand in detail the effects of chemical interactions between buried groups in SAMs, it is first useful to characterize the effects of nonbonding groups where only dispersive and electrostatic interactions can occur. In particular, there is considerable interest in simple polar groups which cannot H-bond but exhibit large electric dipoles since the dipoles can have a significant impact not only on the process of selfassembly but on the interfacial charge distribution that may arise at the organic/metal interface. It is well-known that the adsorption of dipole layers can affect the work function of a substrate.21 This is especially true at organic/metal interfaces where, even with nonpolar molecules, dipole layers can arise via charge transfer or chemical interaction at the substrate-adsorbate interface and is of particular importance for applications such as organic electroluminescent devices,22,23 organic solar cells,24 organic field-effect transistors,25,26 and other organic devices and functional structures.21,27–30 The adsorption of polar molecules can lead to additional effects. For example, a combined experimental and theoretical study by Ray and co-workers31 demonstrates that shifts in the charge distribution of the polar terminal groups of organothiolate SAMs on Au can induce changes in the sample work function and in observed X-ray photoelectron (XPS) binding energies of the polar group constituent atoms. In another series of studies, Weiss and co-workers have shown that conductance switching can be induced in dipole containing SAM molecules via electrostatically induced reorientation of adsorbate molecules under application of a potential bias from a scanning tunneling microscope (STM) tip.17,19,32 It is clear from these and a variety of other studies that a good understanding of and the ability to manipulate the electrostatic charge distributions in organic film structures is critical. This situation is ideally suited to SAMs in particular, due to their self-organization and inherent high degree of order, which offer a way to systematically study and control the electrical behavior via precise manipulations of the molecular architecture and substrate attachment. With these effects in mind and given our longstanding interest in molecular charge transport devices, we have initiated a series of experiments on SAMs with different buried polar groups located at varying, precise positions in the molecular skeleton. In this first paper, we examine the effect of a buried ester functional group and its associated large dipole on the monolayer structure and electrical properties of SAMs prepared by chemisorption and assembly onto Au{111}. The SAMs consist of a series of alkanethiols with varying -(CH2)- segment lengths on each side of the ester moiety. The molecular structures were characterized by wetting, ellipsometry, near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and quantitative infrared reflection spectroscopy (IRS), aided by density functional theory (DFT) quantum chemical calculations. Because these molecules have conductances too low for STM probes and because tip-sample contact issues arise with quantitative conducting probe atomic force microscopy measurements, we applied traditional and synchrotron based X-ray photoemission measurements (XPS) to characterize important aspects of the electrical behavior. Our results show that the ester group effectively splits the monolayer into a set of two stacked submonolayers, with different structural and electrical characteristics. The lower portion of the alkyl chain, the Au substrate side, is able to pack in a well organized fashion with high conformational order and orientation analogous to unsubstituted alkanethiolate/Au SAMs. The portion near
J. Phys. Chem. C, Vol. 112, No. 29, 2008 10843 TABLE 1: Compounds Used and Abbreviations for Their SAMsa compound SH(CH2)15CH3 SH(CH2)15COOCH3 SH(CH2)15COO(CH2)4CH3 SH(CH2)10COO(CH2)4CH3 SH(CH2)10COO(CD2)4CD3 SH(CH2)10COO(CH2)9CH3 SH(CH2)10COO(CD2)9CD3 SH(CH2)5COO(CH2)9CH3
shortened name C16 C15C1 ester C15C5 ester C10C5 ester C10D5 ester C10C10 ester C10D10 ester C5C10 ester
a The incorporation of deuterium atoms on the chain is signified by using D in the abbreviated name instead of C.
the ambient interface, however, exhibits significant conformational disorder in comparison. Most importantly, we find that the presence of the ester dipole layer induces significant shifts in the XPS photoelectron kinetic energies of the C 1s signals between the two submonolayers, even though the C atoms have identical chemical environments. These shifts further can be quantitatively understood in terms of the electrostatic potential difference across the ester dipole layer as determined from the electric dipole vector, which is independently deduced from the combination of the IR data and DFT calculations. This correlation thus shows that XPS measurements can be utilized to provide a new diagnostic for these types of SAM structures. Overall, we conclude that alkanethiolate SAMs with buried functional groups interacting only via simple van der Waals and dipole forces can be designed with predictably different structures in the top and bottom portions of the chains and can be used to create tailored buried dipole layers with specific electrostatic properties that can be characterized by simple photoemission measurements. 2. Experimental Methods 2.1. Sample Preparation. The starting thiols were synthesized using standard methods. Details are given in the Supporting Information. A list of the molecules and abbreviations for the SAMs are given in Table 1. Monolayers were prepared at room temperature using standard methods, briefly described here. A 10 nm Cr adhesion layer was thermally evaporated onto the native oxide of 2 in. silicon wafers immediately followed by a 200 nm layer of Au (background chamber pressures were kept at 18MΩ, NANOpure Diamond water system, Barnstead International, Boston, MA) and hexadecane (Sigma-
10844 J. Phys. Chem. C, Vol. 112, No. 29, 2008 Aldrich, 99+% anhydrous) contact angles were measured under ambient conditions using the sessile drop technique.38 A homebuilt apparatus with a computer interfaced CCD camera was used to capture images of the drop on the surface and analyzed digitally (ImageJ 1.37V, National Institute of Health, USA).39 A flat-tipped micrometer syringe (GS-1200, Gilmont Instruments, Barrington, IL) was used to place an ∼20 µL drop on the sample surface. The advancing contact angle was measured quasistatically as the drop was dispensed onto the surface and expanded. The receding contact angle was measured by partially retracting the drop back into the syringe. The syringe needle was kept inside the center of the drop during the measurements. Angles were measured at three different regions for each sample surface and averaged over, at minimum, three samples for each monolayer. 2.3. Infrared Reflection Spectroscopy (IRS). Infrared spectra were taken using an in-house customized Fourier transform infrared spectrometer (BioRad FTS-7000, Digilab, Randolph, MA) set up in an external configuration for grazing incidence reflection with a liquid nitrogen cooled MCT detector.39 All external optics were kept in an enclosure and continuously purged with dry N2 to remove water. Spectra were averaged over 800 scans using p-polarized light at an 86° incidence angle and at a resolution of 2 cm-1. The intensities displayed in each spectra are a function of the sample reflectivity, R, ratioed to the reflectivity of a reference sample, R0, and reported as absorbance units, -log(R/R0). The reference samples used were monolayers of C16H33SH on Au or C16D33SH on Au, depending on the spectral range of interest. 2.4. Infrared Spectral Simulations. The IRS spectra were used to deduce the ensemble averaged orientation of the adsorbed molecules using previously discussed methods.40 The C10C5 ester was picked as a representative model for all of the ester compounds. Measured quantities of the compound were dispersed in KBr powder and pressed into pellets of known concentration and path length to obtain the isotropic complex optical function spectra of the bulk solid. Specific vibrational modes were then assigned to each component peak in the bulk optical function spectra. In order to simulate the spectra of the ordered and uniformly oriented thin films, each vibrational mode must be assigned an associated transition dipole moment vector direction. These transition moment vectors were determined via quantum chemical calculations which are described in further detail below. This data was then used to calculate an anisotropic optical function tensor spectrum for an average molecule at a specific tilt and twist angle via a 3-layered structural model (air/ SAM/Au substrate) and then compared to the experimental SAM spectrum. The tilt and twist angles were then iteratively varied until an optimal fit between simulation and experiment was found. 2.5. Near Edge X-Ray Absorption Fine Structure (NEXAFS) Spectroscopy. Measurements were done at the HE-SGM beamline of the BESSY II synchrotron storage ring (Berlin, Germany). The experiments were performed at base pressures
