NANO LETTERS
Submolecular Potential Profiling Across Organic Monolayers
2006 Vol. 6, No. 12 2848-2851
Neta Filip-Granit,†,‡ Milko E. van der Boom,† Roie Yerushalmi,§ Avigdor Scherz,‡ and Hagai Cohen*,| Department of Organic Chemistry, The Weizmann Institute of Science, RehoVot, 76100, Israel, Department of Plant Sciences, The Weizmann Institute of Science, RehoVot, 76100, Israel, Electrical Engineering and Computer Science, UniVersity of California, Berkeley, California, and Department of Chemical Research Support, The Weizmann Institute of Science, RehoVot, 76100, Israel Received August 30, 2006; Revised Manuscript Received October 5, 2006
ABSTRACT Potential profiles across molecular layers are constructed by means of noncontact electrically stimulated photoelectron spectroscopy, probing for the first time the molecule−substrate interface potential and resolving local screening effects across inner phenyl groups.
Self-assembled molecular layers1-2 present flexible building blocks and a promising template for soft matter electronics, molecular machinery, and biotechnology.3-11 Their response to electrical signals and field-induced conformations are of applicative and fundamental interest. Besides challenging technical difficulties characterizing this field, a principal limitation is encountered: the inability of existing electrical tools to selectively access interior sites and interfaces; redox centers, as well as the molecule-substrate bond. By using charging effects in X-ray photoelectron spectroscopy (XPS),12,13 and more recently chemically resolved electrical measurements (CREM),14-18 one can in fact read potentials out of selected subsurface regions. However, extending CREM to the submolecular scale is highly demanding, involving small potential differences and weak signals under limited stability of the organic system. A first successful CREM analysis of top CF3 groups has been reported recently.18 While being an important common issue, consistent potential profiles and even the electrical characteristics of molecule-substrate interfaces have remained unachievable. To better access the submolecular information, we have developed the CREM method toward particularly low-energy conditions of the electron flood gun (eFG). A floating sample bias is applied via a Keithley 487 picoampermeter, such that the effective kinetic energy of incoming eFG electrons can * Corresponding author. E-mail:
[email protected]. † Department of Organic Chemistry, The Weizmann Institute of Science. ‡ Department of Plant Sciences, The Weizmann Institute of Science. § Electrical Engineering and Computer Science, University of California, Berkeley. | Department of Chemical Research Support, The Weizmann Institute of Science. 10.1021/nl0620435 CCC: $33.50 Published on Web 11/02/2006
© 2006 American Chemical Society
Table 1. Layer Thickness Evaluation (Given in Å): Present XPS Derived Data (Based on Standard Electron Attenuation Considerations); Literature Data20,22,23 for Similar Layers Grown on Si Substrates; and “Chem3D” Free-Molecule Calculations (Assuming Vertical Orientation)a layer thickness present work
other works
calculated
XdCl
8.1
10b
9.8
XdI
11.2
6.5c
9.9
BChl on XdI
25
21d
27
sample
stoichiometry (XPS) X/Si ) 0.95 C/Si ) 7.6 X/Si e 0.98 C/Si ) 6.9 N/S ) 5.7 N/Si ) 0.2
a Representative XPS derived stoichiometry values are provided for each sample. Experimental errors are ∼15%. Further characterization information can be found in ref 20 and the Supporting Information. b Taken after Lin et al. (ref 22) and references therein. c Taken after Facchetti et al. (ref 23). d Taken after Filip-Granit et al. (ref 20).
be evaluated in situ, referring to the sample work function in a fully consistent manner.17 A flow of electrons toward the ground is defined here as a negatiVe current. Siloxanebased mono- and bimolecular layers were studied, covalently assembled on indium-tin-oxide (ITO) coated glass, using a conductive carbon tape as contact to the ITO coating. Sample cooling to ca 220 K was used for damage suppression.19 Preparation and characterization (see Table 1 and the Supporting Information) of the samples by complementary techniques is described elsewhere.20 Figure 1a-c presents XPS line shifts in an iodineterminated molecular layer (XdI) (see Figure 1d). As shown below, these molecules propose useful markers for CREM
Figure 1. Electrically induced XPS line shifts observed upon changing the input current from -5 nA (dashed lines) to -20 nA (solid lines): (a) I(3d5/2); (b) Si(2p); (c) In(3d5/2); (d) a schematic illustration of the monolayer system, where XdI or Cl. Note the particularly small shifts to be quantified under these conditions. The spectra in b and c are smoothed for visual clarity; line shifts, however, are extracted from the raw spectra.
and a template for further chemistry. Two sets of spectra are presented, recorded under negative input eFG currents of 5 and 20 nA, respectively. The recorded line shifts are associated with changes in local potential (∆V). Note that the ∆V values are much smaller than the actual line widths. Practically, XPS line shifts can be determined (numerically) at high accuracy, 10 meV and even better, far beyond the spectral resolution of the instrument, in a procedure applied to the raw (unsmoothed) data.14,15 The accuracy of this procedure degrades with noisy signals, thus imposing high demands on data collection times and hence on system stability. Figure 2a summarizes the line-shift data of the XdI monolayer, induced via changes in the eFG flux at a fixed incoming kinetic energy, 0.5 eV. The element-specific I-V plots (I, C, and Si) can all be attributed to well-defined molecular sites, thus providing a fine mesh for profiling the induced potential changes across the monolayer. The indium curve (also Sn and O, not shown) represents the substrate, integrating over all back contact contributions. Potential scale zeroes refer to the initial data sets (recorded at low eFG current). We carefully follow each electrical step by a reversibility test (filled data points), lowering the eFG current back to its initial conditions. Chronologically, the experiment goes from “low” eFG current to “medium” conditions, reduced back to the “low” conditions (first reversibility test), raised to a “high” flux level, and finally lowered back to the initial “low” level. The repetitive test scans, combined with in situ XPS data, propose a detailed real-time follow up of surface modifications. The discussion below is restricted to relatively stable situations, such as the first step in Figure 2a, for which the test points get very close ((15 mV) to the origin. Initially, this layer undergoes irreversible changes, where partial desorption of iodine is detected with practically no additional Nano Lett., Vol. 6, No. 12, 2006
structural modifications (see the Supporting Information). System stabilization on a “diluted halogen” state is expressed by both the XPS and CREM characteristics, retained along time scales well above those required for the CREM analysis. At elevated eFG currents (see the -60 nA step in Figure 2a), the iodine desorption accelerates slightly and small additional drifts toward negative potential values are observed, mainly in the carbon signal. A unique result is presented in Figure 2a, resolving the molecule-substrate interface potential. Interestingly, a significant portion of the potential developed across the entire layer appears on the Si sites, a feature that cannot be resolved by standard methods. Figure 2a shows that the Si to In (Figure 1d) potential drop increases up to ca 40-50 mV, in other systems (ferrocene-based monolayers, not shown) even up to 150 mV. Saturation, or even breakdown features, has been observed above these values in all of our experiments. The magnitude of these fields, ∼50 mV across ∼3 Å, is comparable with typical breakdown fields of 0.1-1 V/nm. Similar potential drops across this interface evolve under positive currents as well. A qualitatively similar behavior is obtained by studying a similar monolayer, with XdCl and eFG electrons applied at a kinetic energy of 0.2 eV. The early stage of the experiment (before stabilization) is shown in Figure 2b, demonstrating the electrostatic changes, where the test points shift away from the origin. When this system stabilizes, full reversibility is exhibited, as demonstrated by the tests before and after the last step in Figure 2b (that of -40 nA). This last part of the experiment (a step up and down in the eFG current) is replotted in the inset of Figure 2b, with earlier drifts subtracted. Clearly, the interface potential drop in this case, ∼10 mV, is smaller than that in Figure 2a. Potential profiles are provided in Figure 3a, based on two fully reversible steps in the above experiments: the first in 2849
Figure 2a and that of the inset of Figure 2b. The z axis is constructed by means of the CREM chemical addresses, assuming vertical orientation of the molecules, an assumption supported by our in situ XPS data (layer thickness of ∼10 Å, see Table 1) and by complementary works.20,22,23 On the basis of free-molecule minimum-energy configuration calculations and neglecting field-induced structural distortions, an approximated submolecular ruler is thus provided. The center of the phenyl ring is taken as the mean position of the carbons. ∆V values are corrected for residual electrostatic drifts (given by the reversibility tests). Note that the two profiles, Figure 3a, correspond to different eFG conditions and that the XdI system exhibits generally stronger potential variations, as compared to the Cl-terminated layers. The profiles in Figure 3a manifest pronounced nonlinearity. In the XdCl system, the potential gradient between Cl and C is >3 times larger than that between C and Si. The corresponding gradient between I and C is also very large. Hence, the effect of molecular polarizability24 is resolved here for the phenyl ring, which screens the potential evolving
Figure 3. Submolecular potential profiles (a) across two monolayer systems: circles, XdCl; squares, XdI; (b) across a bilayer system, under two different input currents: circles, low; squares, high. The XdI curve from part a is replotted in part b (triangles). The inset of b illustrates the bilayer system. Vertical axes correspond to changes in potential (other than absolute potentials) under increasing the input eFG current: in part a -4.2 to -40 nA and -3.5 to -21 nA for XdCl and XdI, respectively, and in part b -3 to -21 nA and -4.4 to -60 nA for the low and high conditions, respectively. Atomic z-axis positions correspond to nonperturbed isolated molecules. The carbon in part a is considered at the center of the ring. The bilayer carbon in part b, which is broadly distributed and hence useful as a rough reference point only, is represented near the center of the double-layer film.
Figure 2. Chemically resolved I-V plots of (a) the XdI system at its “stable state” and with the incoming eFG electron energy set to 0.5 eV; (b) the XdCl system at eFG effective energy of 0.2 eV, starting before stabilization is realized. Filled data points stand for intermediate checks of reversibility, performed after each step by lowering back the eFG current: colored and gray filling represent the tests before and after the last step, respectively. The last step (“up and down” in the eFG current) of the experiment in b, when stabilization is accomplished, is replotted in the inset. Potential zeroes are determined arbitrarily (for each element) by the first data sets in each experiment, thus subjected to small inconsistency in their relative positions. Note the finite contribution of the back contact to the In potential. 2850
at the halogens effectively. The XdI sample shows an even more striking feature: its interface potential drop (Si to In) appears to be larger than the ∆V value of carbon. Such an effect implies rather high polarization where the top halogen affects the (remote) interface. It also indicates efficient charge injection into the iodine sites, to which the neighboring conjugated system responds by a strong repulsion of π electrons toward the Si atom. A similar effect has been indicated recently by a top CF3 moiety.18 It reflects the nonclassical (discrete) nature of the medium: The potential is probed at atomic cores, while its changes, ∆V, reflect variations in (the surrounding) charge distribution. Hence, under certain circumstances, the CREM potentials propose a direct probe of submolecular polarizability and an interesting tool for predicting electronegativity and chemical reactivity. Using the same approach, we now demonstrate the analysis of a bilayer system, Figure 3b. A layer of bacteriochlorophyll Nano Lett., Vol. 6, No. 12, 2006
(BChl) derivative, chemically attached to the XdI layer (see inset, Figure 3b), has been found to form a saturated, relatively compact second layer, showing particularly high stability under the CREM experiments. Understanding the charge distribution within BChl is motivated by optical and biological applications.25,26 Addresses for the electrostatic data are provided here by the nitrogen, the residual interlayer iodine, and the Si, considering a 40° tilt of the top BChl molecules.20 For comparison, the curve of the XdI single layer, Figure 3a, is replotted. Apparent similarity between the latter’s profile and that of the bilayer is demonstrated, for example, in their pronounced Si-substrate potential drops. Note that carbon atoms are distributed all over the bilayer structure, thus providing a rough reference only, with no fine sensitivity to the intramolecular details achieved in Figure 3a. Remarkably, the structurally complex bilayer, which is a challenging system for conventional electrical measurements, manifests in Figure 3b a well-behaving smooth and roughly linear profile across the organic medium, with appreciable response to the increase in eFG flux. In summary, potential profiling across organic monolayers has been demonstrated at submolecular resolution. By resolving inner sites and molecular interfaces, we propose a unique reference for the standard electrical characterizations and a direct access to the inherent complexity of electrical properties in molecular assemblies. Supporting Information Available: Preparation and characterization of the samples by complementary techniques. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgment. This research was supported by BMBF and the MJRG for Molecular Materials and Interface Design. M.E.vd.B. is the incumbent of the Dewey David Stone and Harry Levine career development chair. References (1) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674-676. (2) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991.
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