J. Phys. Chem. B 2005, 109, 19411-19415
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Effect of Irradiation Dose in Making an Insulator from a Self-Assembled Monolayer Yian Tai, Andrey Shaporenko, Michael Grunze,† and Michael Zharnikov* Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany ReceiVed: June 21, 2005; In Final Form: August 25, 2005
A combination of functionalization and irradiation-induced cross-linking allows fabrication of stable metal film on top of an aromatic self-assembled monolayer, [1,1′;4′,1′′-terphenyl]-4,4′′-dimethanethiol (TPDMT) on Au. Using X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and ion-scattering spectroscopy the optimal irradiation dose for producing a stable metal overlayer was estimated to be 40-45 mC/cm2. This dose is necessary for complete 2D-polymerization and closure of transient channels, which would otherwise allow metal penetration into the SAM. What is also important, the majority of the thiol tail groups, responsible for 2D growth and chemical adherence of the metal film, remains intact even at this high dose. The optimal dose corresponds to a crossover in the response of the TPDMT film to ionizing radiation: the irradiation-induced processes progress fast at lower doses and saturate at higher doses.
1. Introduction The fabrication of thin metallic films and nanostructures (e.g. nanowires) on self-assembled monolayers (SAMs) is an important scientific and technological goal, e.g. for the preparation of top electrodes in molecular electronic devices and for the fabrication of monomolecular insulator layers to provide an alternative to commonly used oxide dielectrics in future electronic and spintronic devices (see e.g. refs 1 and 2). However, the formation of stable metal film on the SAM surface (i.e. at the SAM-ambient interface) is a nontrivial task, since the metal adsorbates are not stable on the top of a SAM but penetrate into the monomolecular layers and diffuse to the SAM-substrate interface.3-5 Diffusion occurs via structural defects in the film and is enhanced by formation of additional transient channels via dynamic hopping of the SAM constituents across the substrate.6,7 This process can, however, be partly suppressed by improving the SAM quality and by the introduction of chemically reactive tail groups, which bind the adsorbate atoms at the SAM-ambient interface and provide nucleation centers for the growth of the metal overlayer. Following this strategy, partial or temporary stabilization of some metal adsorbates, such as Ti, Cr, Pd, and Al, on the SAM surface has been achieved using HOOC-, CO2CH3-, CH2OH-, thiophene-, pyridine-, and SH-terminated films.3,6-11 Recently, we suggested a new approach to form a stable metal layer on the top of a SAM, combining functionalization of the monomolecular film with its 2D-polymerization by electron irradiation. With nickel as a test metal adsorbate we succeeded to form a stable metal film on the surface of [1,1′;4′,1′′terphenyl]-4,4′′-dimethanethiol (TPDMT) SAM on Au12 and showed that the polymerized TPDMT film in the metal/SAM/ substrate sandwich has good insulating properties.12,13 Note that this particular SAM substrate was specially designed for metal evaporation experiments. It possesses a reactive functional * To whom correspondence should be addressed. E-mail:
[email protected]. † Second address: Institute for Molecular Biophysics, University of Maine, Orono, ME.
group, high packing density, high orientational order, and high resistance to ionizing radiation.14,15 The degree of 2D-polymerization is of major importance to stabilize the metal film on top of the TPDMT SAM. Here we analyze the properties and performance of the TPDMT films as a diffusion barrier as a function of irradiation dose. For this purpose, we applied X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and ion-scattering spectroscopy (ISS). In the following section our experimental procedure will be described. The results are presented in section 3, followed by a discussion and a summary in sections 4 and 5, respectively. 2. Experimental Section The synthesis of TPDMT is described elsewhere.14 The gold substrates were prepared by thermal evaporation of 100 nm of gold onto Si(100) wafers (Silicon Sense) primed with a 5 nm titanium adhesion layer. In some cases, 20 nm of Au and 9 nm of Ti were used, which resulted in a smother gold film. The gold films are polycrystalline, with a grain size of 20-50 nm. The grains predominantly exhibit an (111) orientation. The SAMs were prepared by immersion of the freshly prepared substrates into a 1 mM solution of TPDMT in THF at 55 °C for 24 h. After immersion, the samples were carefully rinsed with pure solvent and blown dry with argon. No evidence for impurities or oxidative degradation products was found. The irradiation of the TPDMT films was performed with lowenergy electrons (10 eV). The current density was ≈2.5 µA/ cm2. The dose was calibrated by a Faraday cup. Ni (Goodfellow, 99.999% purity) was evaporated from a commercial e-beam evaporator (Omicron) with a deposition rate of 0.2 nm/min; the rate was calibrated by a commercial quartz crystal microbalance sensor (Inficon). The Ni coverage of the SAM samples (θNi) was estimated by the multiplication of the calibrated deposition rate by the evaporation time and related to the Au(111) surface. The sticking coefficient of Ni was assumed to be 1 for both quartz sensor and SAM samples. The evaporation was performed at a base pressure better than 1.5 × 10-9 Torr; the substrates were kept at room temperature.
10.1021/jp053340l CCC: $30.25 © 2005 American Chemical Society Published on Web 09/23/2005
19412 J. Phys. Chem. B, Vol. 109, No. 41, 2005 The effect of electron irradiation and Ni evaporation was monitored by XPS, NEXAFS spectroscopy, and ISS. All experiments were performed in-situ and at room temperature. The XPS measurements were carried out with a Mg KR X-ray source and a LHS 11 analyzer. The spectra were acquired in normal emission geometry with an energy resolution of ≈0.9 eV. The energy scale was referenced to the Au 4f7/2 peak at 84.0 eV.16 The NEXAFS experiments were performed at the HE-SGM beamline of the synchrotron radiation facility BESSY II in Berlin. The spectra acquisition was carried out at the carbon K-edge in the partial electron yield mode with a retarding voltage of -150 V. Linearly polarized synchrotron light with a polarization factor of ≈82% was used. The energy resolution was ≈0.40 eV. The incidence angle of the light was varied to monitor the orientational order within the films. This approach is based on the linear dichroism in X-ray absorption, i.e., the strong dependence of the cross section of the resonant photoexcitation process on the orientation of the electric field vector of the linearly polarized light with respect to the molecular orbital of interest.17 The raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. The energy scale was referenced to the pronounced π* resonance of highly oriented pyrolytic graphite at 285.38 eV.18 ISS measurements were performed with a focused ion gun (Leybold-Heraeus) and a LHS 11 analyzer. ISS is a suitable technique to gain direct information on the depth distribution of the metal adsorbate in the SAM and at its interfaces with ambient and the substrate. In ISS experiments, an ion beam is directed onto a surface, and the energy distribution of the scattered ions is measured (a typical cross section for the scattering is about (5-10) × 10-19 cm2/sr), providing a mass spectrum of the scattering partners.19 Simultaneously, the sample is slowly eroded, so that the depth profile of the elements can be acquired by sequential ISS spectra. Under the conditions of our experiments the primary ion beam (He+, MESSER 99.999%) was accelerated to 1 keV and operated at a current of ≈100 nA/cm2. The beam was focused to a spot with a diameter of ca. 500 µm by two focal lenses. The angle between the primary beam and sample surface was 35°, i.e.; the scattering angle of the He+ ions was 145°. During data acquisition, the beam was gated with a 70% aperture (to cut out the low intensity part of the beam spot) and rastered over a 2 × 2 mm2 area. The depth profiles of Ni and Au were measured by sequential acquisition of the ISS spectra with an energy window from 750 to 1000 eV during the erosion of the samples by the ISS beam; both the Ni and Au signals were recorded in a single scan. The erosion rate, which was separately calibrated, was about 0.30.5 Å/min. No ion-induced intermixing occurred.20 3. Results Generally, ionizing radiation results in a complex modification of monomolecular films, including damage of both SAMambient and SAM-substrate interfaces, desorption of hydrogen and hydrocarbons, and conformational and orientational disordering.21 In aromatic SAMs, these processes are accompanied by irradiation-induced cross-linking of the aromatic backbones, which occurs after the abstraction of hydrogen via the cleavage of C-H bonds.22 For the use of TPDMT/Au as a monomolecular insulator, the intact character of the SAM-ambient interface, low extent of irradiation-induced desorption, maintenance of orientational order, and a high extent of 2D polymerization are of importance. In our study, the SAM-ambient interface and
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Figure 1. Ratio of the thiol and thiolate groups (a) and normalized total C 1s intensity (b) in the course of electron irradiation of TPDMT/ Au. The thiol/thiolate ratio was estimated from the S 2p XPS spectra; the signal of thiol was corrected for the attenuation by the overlayer.
irradiation-induced desorption were monitored by XPS, orientational order was measured by NEXAFS spectroscopy, and the extent of 2D polymerization was indirectly probed by ISS, which gives an information about the diffusion of Ni into the organic film. The TPDMT film possesses an extraordinary resistance to ionizing radiation. In Figure 1, the ratio of the thiol and thiolate groups (top panel) and normalized total C 1s intensity (bottom panel) for TPDMT/Au are presented as functions of the irradiation dosage. The thiol/thiolate ratio was estimated from the S 2p XPS spectra, which exhibit two S 2p3/2,1/2 doublets at 162.06 eV (S2p3/2) and 163.3 eV (S2p3/2) assigned to the thiolate headgroups bound to the metal surface23,24 and to the thiol tail groups,25 respectively. During electron irradiation, the intensity of the thiol-related S 2p doublet decreases due to the damage of the respective species at the SAM-ambient interface, whereas the intensity of the thiolate-related doublet remains almost constant due to cross-linking of densely packed aromatic backbones, keeping the SAM constituents in their anchor places.26 According to Figure 1a, a noticeable amount of the terminal thiol groups at the film-ambient interface survives electron exposure. In particular, about 70% and 45% (with respect to the initial amount) of the thiol groups remain intact after exposure to a dose of 20 and 45 mC/cm2, respectively (8 mC/cm2 suffice to destroy an alkanethiol SAM on Au completely).21 Thus, even after an extensive irradiation, the SAMambient interface of the TPDMT films does not lose its chemical identity, providing nucleation centers for the growth of the metal film. According to Figure 1a, saturation of the electron-induced change occurs at a dose of 40-45 mC/cm2. Also, the extent of irradiation-induced desorption is small, as evident from the constancy of the C 1s XPS signal in Figure 1b. The suppression of the desorption processes is presumably related to the stability of the phenyl ring framework and the cross-linking of densely packed aromatic backbones.22,26 Note that while the desorption of hydrocarbon fragments is hindered, there is an extended desorption of hydrogen, which is a prerequisite for 2D polymerization of an aromatic SAM.22 Upon irradiation treatment, the TPDMT backbones maintain their orientational order as demonstrated in Figure 2, where we show the difference of the NEXAFS spectra acquired at X-ray incidence angles of 90 and 20° for pristine and cross-linked TPDMT/Au as a function of θNi. The difference spectra are a fingerprint of the orientational order and molecular tilt in the
Effect of Irradiation Dose in Making an Insulator
Figure 2. Difference of the NEXAFS spectra acquired at X-ray incidence angles of 90 and 20° as a function of θNi for pristine (a) and cross-linked (b, c) TPDMT/Au. The irradiation doses were 20 mC/ cm2 (b) and 45 mC/cm2 (c).
film, since an oriented layer exhibits pronounced dichroism, i.e., angular dependence of the absorption resonance intensity.15,17 The positive sign of the π1* peak (at ≈285 eV) in the difference spectra of the pristine and irradiated TPDMT films in Figure 2 (upper curves) is characteristic of an upright orientation of the SAM constituents, which is preserved even at a high irradiation dose. The average tilt angle of the terphenyl moieties is about 19.3° in the pristine film14 and increases by ≈5 and ≈12° upon irradiation with doses of 20 and 40 mC/cm2, respectively.15 According to the literature3-5 and our own data12,15 the orientational order in a SAM is destroyed upon extended penetration of the metal atoms into the film. Therefore, the degree of the molecular orientation in the TPDMT films, given by the amplitude of the difference peaks in NEXAFS spectra, can be used as a fingerprint for the penetration of the Ni adsorbate and formation of a Ni film on top of the SAM. Figure 2 shows that the amplitude of the difference peaks decreases with Ni evaporation in the case of the pristine and moderately (20 mC/cm2) irradiated TPDMT SAMs, until it becomes negligibly small at θNi ) 5-10, which corresponds to a fully disordered film. In the case of an extensively (45 mC/cm2) irradiated TPDMT film (see Figure 2c), the amplitude of the difference peaks remains constant upon Ni evaporation, indicating that the orientational order is preserved. This means that the penetration of the Ni adsorbate into the SAM does not occur and a Ni layer on top of the SAM is formed. These conclusions are supported by the ISS data, which show the architecture of the Ni/TPDMT/Au sandwich, giving the idea
J. Phys. Chem. B, Vol. 109, No. 41, 2005 19413 about the degree of irradiation-induced cross-linking. The ISS depth profiles of Ni and Au for pristine, moderately irradiated (20 mC/cm2), and extensively irradiated (45 mC/cm2) TPDMT/ Au and θNi ) 10 are shown in Figure 3a-c, respectively. In the case of the pristine SAM, there is a pronounced Ni signal on the SAM surface but even a stronger Ni signal at the SAMAu interface, which can be easily identified by the appearing Au signal (the continuous growth of the Ni signal is related to the formation of a Ni-Au alloy).27 Thus, most of the Ni atoms penetrated into the SAM, with subsequent diffusion to the SAM-Au interface. A similar situation, although with a lower extent of the Ni penetration, occurred for the moderately irradiated TPDMT film (see Figure 3b)sthe Ni signal at the SAM surface is slightly higher than that for pristine TPDMT/ Au but still small compared to the signal appearing at the SAMAu interface. Thus, the extent of cross-linking provided by 20 mC/cm2 is obviously not sufficient to block the penetration of the metal adsorbate into the SAM. However, the situation changed completely when the irradiation dose was increased to 45 mC/cm2, which resulted in a higher extent of crosslinking. In fact, the ISS Ni depth profile in Figure 3c taken immediately after Ni evaporation exhibits a strong Ni signal at the top of the SAM and no signal within the SAM or at the SAM-Au interface. Thus, diffusion of Ni into the film appears to be blocked, and a Ni film is formed on the SAM surface. What is very important, the Ni overlayer is stable, i.e., the penetration of Ni into the SAM is effectively hindered also in the long term, as shown by the comparison between the ISS profiles of Ni taken 5 min and 24 h after evaporation (down and up triangles in Figure 3c, respectively). 4. Discussion The XPS, NEXAFS, and ISS data provide a fully consistent picture of the effect of irradiation dose in making TPDMT SAM nonpenetratable for a metal adsorbate. The results are schematically sketched in Figure 4. Without irradiation (see Figure 4a), only a small portion of Ni adsorbate stays on the SAM surface, whereas most of the metal penetrates into the film and diffuses to the SAM-ambient interface, destroying the orientational order in the SAM. A moderate irradiation of the TPDMT SAM results in only partial polymerization of this film (see Figure 4b), which is not sufficient to block the penetration of the metal atoms into the SAM, even though the amount of Ni at the SAM surface increases to some extent as compared to the pristine TPDMT/Au. Only if the irradiation dose is as high as 45 mC/ cm2, penetration of Ni is effectively blocked (see Figure 4c) and irradiation-modified TPDMT SAM becomes a molecularthick insulator film, separating the top and bottom electrodes given by the evaporated metal film and substrate, respectively (see ref 13 for electric properties).
Figure 3. Ni (triangles) and Au (circles) ISS depth profiles for pristine (a) and cross-linked (b, c) TPDMT/Au. The irradiation doses were 20 mC/cm2 (b) and 45 mC/cm2 (c). θNi was 10. The depth profiles of Ni were acquired 5 min (down triangles) and 24 h (up triangles) after Ni deposition. The SAM region is tentatively marked light gray.
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Figure 4. Effect of electron irradiation and Ni evaporation in the case of pristine (a) and cross-linked (b, c) TPDMT/Au. The irradiation doses were 20 mC/cm2 (b) and 45 mC/cm2 (c). See text for details.
Even though we have not continuously monitored the extent of irradiation-induced cross-linking in the TPDMT film during irradiation, we can reasonably assume that the behavior of this parameter mimics to some extent the dose dependence of the portion of the intact thiol tail groups at the SAM-ambient interface. According to literature data, different irradiationinduced processes in a SAM occur simultaneously and are strongly interrelated to each other.21 These processes mostly follow an exponential law,21 similar to the curve in Figure 1a. The respective dependence suggests that the irradiation-induced processes in the TPDMT SAM progress quite fast with the irradiation up to a dose of about 35-45 mC/cm2 and slow at higher doses. Following, the extent of irradiation-induced crosslinking after the treatment with 45 mC/cm2 should be more or less close to the saturation value, which corresponds to the fully polymerized graphite-like film. Moreover, the latter value can already be achieved close to the SAM-ambient interface (which is most important for the blocking of the penetration channels), since the cross-linking occurs preferentially in this region.15 Thus, the dose of 45 mC/cm2, necessary for the formation of the metal overlayer, seems to be quite reasonable. Probably, a slightly lower dose, such as e.g. 40 or 35 mC/cm2, will be still sufficient, but it is better to stay at the safe side and apply a dose which is somewhat higher than the threshold value. At the same time, an increase of the dose far beyond 45 mC/cm2 should not provide much impact except for an additional reduction in the amount of the thiol tail groups. Considering that the occurrence of these groups promotes the growth of 2D metal film and improves the adherence of this film to the SAM support,12 one should keep the amount of such groups as high as possible and choose a reasonably low irradiation dose, which will be still sufficient for an extensive cross-linking. According to the data presented in this article, it should be something around 40-45 mC/cm2. Note that this dose is very high; it corresponds to about 600 electrons hitting every TPDMT molecule. Such a high resistance to ionizing radiation is explained by the high packing density of the terphenyl moieties, which leads to a large delocalization and fast quenching of the initial electronic excitations.26 Theoretically, this dose can be lowered if a pure aromatic backbone (i.e. without the methylene linker) is used, but it is
not clear whether the respective SAMs will exhibit the same behavior upon metal evaporation as the TPDMT film. Introduction of the methylene linker results in a significant increase of the packing density and structural quality of the aromatic SAMs,28,29 which is important for the blocking of diffusion channels. 5. Conclusion Combination of functionalization and irradiation-induced cross-linking of the TPDMT SAM can be used to create a monomolecular insulator layer. The optimal radiation exposure is a balance between a sufficient degree of cross-linking to close both static and dynamic diffusion channels at the SAM-ambient interface and the limitation to keep the amount of the functional tailgroups at this interface as high as possible. For TPDMT/ Au, the optimal value is estimated to be 40-45 mC/cm2, which causes an extensive modification of the film. Irradiation-induced processes, which progress quite fast at lower doses, slow and saturate at higher exposures to the electron beam. It would be interesting to compare the results to these for pure aromatic SAMs, such as e.g. biphenyldithiol and terphenyldithiol or a “shorter” analogue of the TPDMT filmsa SAM of [1,1′-biphenyl]-4,4′-dimethanethiol. The use of these systems may results in a decrease of the necessary irradiation dose, provided that a lower structural quality of these films14,30,31 (as compared to the TPDMT SAM) will be not a problem for their transformation into a monomolecular insulator layer. Acknowledgment. We thank W. Eck (Universita¨t Heidelberg) for the synthesis of the TPDMT substance, Ch. Wo¨ll (Universita¨t Bochum) for providing us with experimental equipment at BESSY II, and the BESSY II staff for assistance at the synchrotron. This work has been supported by the DFG (Grant JA 883/4-2), BMBF (Grant 05KS4VHA/4), and the EU 6th Framework STREP Nanocues. References and Notes (1) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dehm, C.; Schu¨tz, M.; Maisch, S.; Effenberger, F.; Brunnbauer, M.; Stellacci, F. Nature 2004, 431, 963.
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