Comprehensive Characterization of Hybrid Junctions Comprised of a

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J. Phys. Chem. C 2008, 112, 9474–9485

Comprehensive Characterization of Hybrid Junctions Comprised of a Porphyrin Monolayer Sandwiched Between a Coinage Metal Overlayer and a Si(100) Substrate Franklin Anariba,† Hugo Tiznado,† James R. Diers,† Izabela Schmidt,‡ Ana Z. Muresan,‡ Jonathan S. Lindsey,*,‡ Francisco Zaera,*,† and David F. Bocian*,† Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403, and Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204 ReceiVed: March 19, 2008; ReVised Manuscript ReceiVed: April 1, 2008

The promise of molecular electronics has stimulated a variety of approaches for making hybrid junctions wherein molecules are sandwiched between two metal contacts or a metal and a semiconductor contact. However, the fate of molecules subsequent to deposition of a top metal contact has generally not been well characterized. Toward this goal, the interaction of evaporated Cu, Ag, and Au films deposited in varying thicknesses (3, 5, and 8 nm) on a series of monolayer-coverage porphyrins covalently attached to Si(100) substrates was investigated. Each porphyrin contains a triallyl tripod attached to the porphyrin via a p-phenylene unit, which anchors the porphyrin to the Si(100) substrate via hydrosilylation of the allyl groups. All of the porphyrins are Zn chelates bearing meso p-tolyl substituents orthogonal (lateral) to the tripodal surface anchor, but contain different substituents in the meso position opposite (distal) to the surface anchor, p-tolyl, R,R,Rtrifluoro-p-tolyl, pentafluorophenyl, or p-cyanophenyl, to test the potential for the distal meso substituents to impart different characteristics to the metal/molecule/Si junction. The methods of interrogation used include ellipsometry, atomic force microscopy, FTIR spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, and current-voltage measurements. The studies indicate that all of the porphyrin monolayers are robust under the conditions of metal deposition, experiencing no noticeable degradation, and that none of the distal functional groups reacts with the deposited metal, except for the nitrile group of the p-cyanophenyl-substituted porphyrin, which reacts/complexes with Cu. Neither Cu nor Ag penetrates through the monolayer to an appreciable extent to form electrically conductive filaments, whereas Au does penetrate through the porphyrin monolayer and contacts the Si substrate. Collectively, the studies provide a comprehensive assessment of the characteristics of the metal (Cu, Ag, Au)/porphyrin monolayer/Si junctions. I. Introduction The development of molecular-based materials for electronics applications has been stimulated by the prospect that devices relying on the bulk properties of semiconductors will fail to retain their characteristic properties as sizes decrease to nanoscale dimensions.1–3 Over the past decade, our laboratory has been engaged in the development of a molecular-based information-storage medium that can be readily incorporated into existing semiconductor processing technologies.4 Our general approach uses a collection of redox-active porphyrinic molecules covalently attached to an electroactive semiconductor (e.g., Si(100), TiN) surface wherein information is stored in the discrete redox states of the molecules.4–9 The construction of a functional hybrid semiconductor/molecular device requires formation of a “top contact” (typically a metal) to complete the electrical circuit. Thus, the understanding of the electrical performance of a metal/molecule/semiconductor junction requires detailed physicochemical characterization of both metal/ molecule and semiconductor/molecule interfaces. * To whom correspondence should be addressed. (D.F.B.) E-mail: [email protected]. Phone: (951) 827-3660. (J.S.L.) E-mail: jlindsey@ ncsu.edu. Phone: (919) 515-6406. (F.Z.) E-mail: Francisco.Zaera@ ucr.edu. Phone: (951) 827-5498. † University of California. ‡ North Carolina State University.

A variety of metal deposition techniques have been used for the construction of top contacts, including direct vacuum evaporation,10–12 indirect vacuum evaporation,13,14 electrochemical deposition,15,16 and soft deposition methods such as nanotransfer printing.17,18 Of these methods, direct evaporation of a metal onto the molecular layer under ultrahigh vacuum conditions is particularly easy to implement and therefore particularly appealing for practical applications. The interaction of deposited metal overlayers with organic underlayers, typically selfassembled monolayers or Langmuir-Blodgett films, has been investigated via a variety of analytical techniques, including X-ray photoelectron spectroscopy (XPS),11,12,16,19–29 infrared spectroscopy,25–27,30–35 secondary ion mass spectrometry,24,25,27,36,37 atomic force microscopy (AFM),37 ion scattering spectroscopy,32 Raman spectroscopy,38 impedance studies,15 current-voltage measurements,18,39,40 and cyclic voltammetry.15,16,26 On the other hand, studies of metal/molecule/Si junctions have been somewhat more limited.41–43 The studies of metal/molecule interfaces have assessed factors such as (1) the robustness of the molecules in the buried interface after the deposition of the metal overlayer, (2) the interaction of the metal with any functional groups that might be present on the molecules, and (3) the degree of metal penetration into the molecular layer. Herein, we report the results of our investigation of the interaction of evaporated Cu, Ag, and Au deposited at varying thicknesses on a series of porphyrins in monolayers covalently

10.1021/jp802428y CCC: $40.75  2008 American Chemical Society Published on Web 06/04/2008

Comprehensive Characterization of Hybrid Junctions CHART 1

attached to Si(100) substrates. The methods of interrogation used include ellipsometry, AFM, FTIR spectroscopy, Raman spectroscopy, XPS, cyclic voltammetry, and current-voltage (J-V) measurements. It is noteworthy that FTIR spectroscopy, Raman spectroscopy, and XPS are employed on intact junctions where the spectroscopic probes interrogate the molecules through the metal overlayer. The ability to probe intact junctions composed of a metal overlayer, a molecular monolayer, and a silicon substrate is invaluable for assessing the utility of methods for device fabrication. The objectives of the present study are multifold and include (1) an assessment of the morphology of the deposited metal overlayers, (2) the determination of the extent of the metal coverage and penetration into the molecular layer, (3) an evaluation of any chemical reactions between the molecules and the deposited metals, and (4) the correlation of the above-mentioned characteristics of the junctions with their electrical properties. The porphyrins included in the present study are shown in Chart 1 and have the following structural attributes: (1) Each molecule contains a triallyl tripod attached to the porphyrin via a p-phenylene unit.44 The tripod anchors the porphyrin to the Si(100) substrate via all three legs (owing to hydrosilylation upon reaction with the hydrogen-passivated Si45,46). This anchor unit was chosen for the present study because we have previously shown that the tripodal group affords more densely packed porphyrin monolayers than can be achieved with mono(or bi-) podal anchors.8,47 (2) All of the porphyrins are Zn chelates that bear meso p-tolyl substituents orthogonal (lateral) to the tripodal surface anchor but differ in the meso substituent that is opposite (distal) to the surface anchor. These latter substituents include p-tolyl (ZnP-CH3), R,R,R-trifluoro-p-tolyl (ZnP-CF3), pentafluorophenyl (ZnP-F5), and p-cyanophenyl

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9475 (ZnP-CN). The resulting porphyrins represent a set wherein the distal meso substituents might impart different characteristics to the metal/molecule interface. In this regard, we have previously compared the effects of Cu deposition onto ZnPCH3 and ZnP-CN monolayers and showed that the distal methyl group of the former molecule is inert with respect to Cu deposition, whereas the nitrile functionality of the latter molecule reacts/complexes with the deposited Cu.48 Thus, it was of interest to determine whether the nitrile functionality also reacts/ complexes with the deposited Ag or Au. The ZnP-CF3 and ZnP-F5 porphyrins were included in the present study with the thought that distal fluorination (a “teflon-like” coating) might stabilize the monolayer/metal interface. Collectively, the studies reported herein provide a detailed assessment of the effects of Cu, Ag, and Au deposition onto the porphyrin monolayers. II. Experimental Section A. General. All porphyrins were synthesized as described previously: ZnP-CH3,44 ZnP-CF3,49 ZnP-F5,50 ZnP-CN.50 The substrates for surface attachment were prepared from commercially available heavily B-doped (F ) 0.01-0.03 Ω cm) p-type Si(100) wafers. The solvents used in the preparation of the porphyrin monolayers, anhydrous benzonitrile and tetrahydrofuran (THF), were used as received. The propylene carbonate used for the electrochemical studies was dried on molecular sieves before use. The Bu4NPF6 supporting electrolyte was recrystallized three times from methanol and dried at 110 °C under vacuum. B. Surface and Monolayer Preparation. The porphyrins were covalently attached to Si(100) using a high-temperature (400 °C), 2 min “baking” procedure described in detail elsewhere.51 The monolayers for the cyclic voltammetry and J-V experiments were prepared by dispensing a 2 µL drop of a 1-2 mM porphyrin solution onto the surface of a Si(100) microelectrode (100 × 100 µm2, prepared as previously described)6 contained in a sparged vial sealed under argon. The monolayers for the ellipsometry, AFM, FTIR, and XPS experiments utilized much larger areas (∼1 cm2) and, as a result, required a larger drop size of porphyrin solution (∼25 µL). After deposition, the vial containing the Si substrate was heated on a hotplate set at 400 °C for 2 min and then removed and purged with argon while being allowed to cool to room temperature. Finally, the Si substrate was rinsed twice with THF, sonicated once for 1 min in THF, rinsed again, and dried with an argon stream for 15 min. C. Metal Deposition. The Cu, Ag, and Au metal overlayers were deposited using the CHA Industries electron-beam evaporator at the Nanoelectronics Research Facility at UCLA. The residual gas pressure in the evaporator was below 8 × 10-7 Torr. The metals were deposited at a rate of 0.01 nm/s. To avoid spikes in the deposition rate, the evaporator power was set well below the threshold power and the rate was manually adjusted to reach the threshold power required to start the metal evaporation process. For the electrochemical, AFM, FTIR, and XPS experiments, thin (3, 5, or 8 nm) metal overlayers were deposited. For J-V experiments, a thicker (30 nm) metal overlayer was deposited. In the case of the structures fabricated with Cu and Ag overlayers, an additional layer of Au (20 nm) was deposited on top of the Cu or Ag layer to mitigate formation of oxide and provide a better electrical contact. The thickness of all the metal overlayers was measured with a quartz crystal microbalance. D. Ellipsometry. Ellipsometric thickness measurements were performed using a Sopra GES 5 instrument equipped with a Xenon light source with an intensity maximum at 450 nm. The

9476 J. Phys. Chem. C, Vol. 112, No. 25, 2008 incident angle was fixed at 75°, and the wavelength was changed from 300 to 800 nm in increments of 10 nm. The ellipsometric parameters were fitted using the Levenberg-Marquardt regression method. The film thickness was calculated using a model constructed by the Sopra Instruments group for porphyrins on Si.9 E. AFM. The AFM images were obtained in air using a Dimension 3100 instrument and a Nanoscope IV controller housed in a class-100 facility. To minimize environmental noise, the experiments were run under a metal hood and the samples were locked into placed by suction. The AFM tips used were rotated tapping-mode etched silicon probes (RTESP) with resonant frequencies of ∼250 KHz (Veeco Probes). All images were acquired with a scan rate of either 0.5 or 1.0 Hz and were flattened with a first-order polynomial before analysis. F. FTIR Spectroscopy. The IR spectra of the porphyrins in either solid or monolayer form were collected at room temperature with a spectral resolution of 4 cm-1 using a Bruker Tensor 27 spectrometer with a N2-cooled MCT detector. The spectra of the porphyrin solids were obtained in KBr pellets (∼1-5 wt % porphyrin) using transmission mode and averaging over 256 scans. The spectra of the porphyrin monolayers, both before and after deposition of the metal overlayer, were obtained using a Harrick Scientific Ge total reflection accessory (GATR; 65° incident angle relative to the surface normal) by averaging 256 scans. The Si substrates were placed in contact with the flat surface of a semispherical Ge crystal that serves as the optical element; IR spectra were collected with p-polarized radiation. Prior to data acquisition on each sample, the Ge crystal was cleaned with neat 2-butanone and dried with a stream of N2; the GATR accessory was continuously purged with dry air during data acquisition. The spectra of porphyrin monolayers were referenced against a hydrogen-passivated Si(100) substrate with no porphyrin attached but having undergone the same “baking” treatment used to prepare the monolayers. The spectra of the porphyrin monolayers with metal overlayers were referenced against hydrogen-passivated Si(100) substrates with no porphyrin attached but having undergone the same “baking” treatment used to prepare the monolayers and subsequent metal deposition. After the background subtractions, a manual correction was applied to produce a flat baseline (if necessary).52 G. Raman Spectroscopy. The Raman spectra were collected from samples wherein the plane of the Si substrate was positioned parallel to the spectrometer entrance slit with the incident laser beam impinging on the sample at ∼45° with respect to the plane of the substrate. The Si substrates were spun at ∼50 rpm during data acquisition to prevent photodamage. The scattered light was collected with a 50 mm (f/1.4) Nikon AF camera lens and dispersed by a Spex 1877 0.6 m triple spectrograph. The filter stage of the spectrograph contained 600 groove/mm gratings blazed at 500 nm. The spectrograph stage contained an 1800 groove/mm grating blazed at 500 nm. The detection system was a Princeton Instruments liquid N2cooled (-120 °C), 1152 × 298 pixel CCD. The exciting lines at 514.5 and 647.1 nm were provided by the outputs of a Coherent Innova 400-15UV Ar ion laser and a Coherent Innova 200-K3 Kr ion laser, respectively. The incident laser power was 20-50 mW and was line-focused on the sample using a cylindrical lens. The spectral resolution was ∼2 cm-1. H. XPS. The XPS data were collected using a Leybold EA11-MCD system equipped with an Al-KR X-ray source (1486.6 eV) and a 100 mm concentric hemispherical electron energy analyzer. The analytical chamber was maintained at a base pressure of >3 × 10-8 Torr. The samples were introduced

Anariba et al. through an intermediate pumping stage by using a fast-transfer mechanical moving rod, a procedure that required a total time of approximately 5 min. Survey scans and high-resolution spectra were acquired by averaging 5 and 20 scans and using dwell times of 100 and 250 ms per point and scan, respectively. Survey spectra were obtained with a band-pass energy of 100.8 eV (spectral resolution of ∼1.5 eV), whereas high-resolution spectra were obtained with a band-pass of 31.5 eV (spectral resolution of ∼0.8 eV). All spectra were acquired using a takeoff angle of 20° from the surface normal. The spectra from the highresolution scans were quantitatively analyzed by fitting them to Gaussian/Lorentzian peaks after linear background subtraction using the XPSPEAK 4.1 software. Sputtering of the metal/ porphyrin monolayer/Si junctions was carried out with an Ar ion gun in a raster pattern at a pressure of 10-5 Torr, a beam voltage of 2 KV, and an emission current of 25 mA. The samples were sputtered in intervals of 20 s with an initial “cleaning” sputtering of 10 s. I. Cyclic Voltammetry. The electrochemical measurements were performed in a two-electrode configuration using p-type Si(100) working microelectrodes and a Ag counter/reference electrode. Propylene carbonate containing 1.0 M n-Bu4NPF6 was used as solvent/electrolyte solution. The cyclic voltammograms were obtained by contacting the electrolyte solution (contained in a pipet tip) either directly to the porphyrin monolayers or to the metal overlayer. The cyclic voltammograms were recorded at a scan rate of 100 V s-1 using a Gamry Instruments PC4FAS1 femtostat running PHE 200 framework and Echem Analyst software. The charge density (σ, µC cm-2) of the porphyrins in the monolayers prior to metal deposition was determined by integration of the total charge of both anodic waves, normalized by the geometrical area of the microelectrode. The effective surface coverage of the porphyrins in the monolayers (Γ, mol cm-2) was determined from the integrated charge of the voltammetric waves (normalized by a factor of 2 to account for the fact that each porphyrin can undergo two oxidations). J. J-V Measurements. The J-V characteristics were measured in a two-electrode configuration using the same instrumentation as that used for the electrochemical experiments. The p-type Si(100) microelectrode served as one of the electrical contacts; a Ag wire attached to the metal overlayer provided the second contact. III. Results The four types of porphyrin monolayers on Si(100) were interrogated with the full complement of analytical techniques to probe their physicochemical properties before and after metal deposition. Ellipsometry, cyclic voltammetry, AFM, and FTIR studies of the monolayers prior to metal deposition examined the thickness, charge density, surface coverage, morphology, molecular structure, and adsorption characteristics (binding motif and surface orientation) of the molecules/monolayers on the Si(100) substrate. The monolayers were again interrogated using cyclic voltammetry, AFM, and FTIR methods subsequent to deposition of the three different metals at varying thicknesses to determine how formation of the metal overlayer altered the above-noted physicochemical properties. The studies of the monolayers after metal deposition were complemented by additional Raman, XPS, and J-V studies. The Raman studies provided additional information concerning the structure of the molecules in the monolayers; the XPS studies investigated the extent of metal penetration into the monolayers; the J-V studies probed whether metal penetration spans the molecular layer,

Comprehensive Characterization of Hybrid Junctions TABLE 1: Thickness, Charge Density (σ), and Surface Coverage (Γ) of Porphyrin Monolayers on Si(100) molecule

thicknessa (nm)

ZnP-CH3 ZnP-CF3 ZnP-F5 ZnP-CN

1.5 ( 0.1 2.3 ( 0.3 2.3 ( 0.8 2.7 ( 0.4

σb (µC · cm-2) 30 ( 7 35 ( 8 37 ( 8 34 ( 12

Γc (10-10 mol · cm-2) 1.6 ( 0.3 1.8 ( 0.4 1.9 ( 0.4 1.8 ( 0.6

a Measurements on different samples and at different locations on a given sample; average of 5-6 measurements. b Calculated from the integrated area of the E0/+1 and E+1/+2 voltammetric waves and normalized by the geometrical area of the microelectrode (10-4 cm2); average of 7-12 measurements. c Calculated using σ and normalized by a factor of 2 to account for the fact that each porphyrin can undergo two oxidations.

affording direct contact between the metal and the Si(100) substrate. Thus, the FTIR, Raman, and XPS methods were used to interrogate the molecules present in the intact junctions. In the sections below, the results of the different analytical studies of the monolayers are presented. The focus of the presentation is on the effects of metal deposition on the monolayers and the nature of the metal/monolayer/Si junction rather than on the physicochemical properties of the monolayers themselves; this latter topic has been examined in a number of previous publications.4–9,44,49 A. Monolayer Thickness, Charge Density, and Surface Coverage. The thicknesses of the porphyrin monolayers were determined via ellipsometry; they are summarized in Table 1. The average film thickness varied from ∼1.5 nm for the ZnPCH3 monolayer to ∼2.7 nm for the ZnP-CN monolayer; the ZnP-CF3 and ZnP-F5 monolayers displayed intermediate values, both ∼2.3 nm. However, any apparent differences in the thickness among the monolayers are likely insignificant (with the possible exception of ZnP-CH3), given the error bars on the measurement. Also, the average thickness for all of the monolayers is less than that expected for well-packed monolayers wherein the molecules stand straight up on the surface. This is consistent with previous studies that have shown that porphyrins bearing tripodal alkyl tethers on Si(100) are tilted ∼40° with respect to the surface normal.49 The average charge density (σ) and surface coverage (Γ) of the porphyrin monolayers, determined via cyclic voltammetry, are also reported in Table 1. The average charge density for all of the monolayers is the same within experimental error, ∼35 µC cm-2. This average charge density corresponds to an average surface coverage of ∼1.8 × 10-10 mol cm-2. The charge density and surface coverage values are typical of those exhibited by porphyrins bearing tripodal alkyl anchors on Si(100), and are indicative of a relatively well-packed monolayer wherein the molecular footprint is 90 Å2.44,49 B. Monolayer/Si Interface and Metal/Monolayer/Si Junction Morphology. The morphology of the porphyrin monolayers before and after metal deposition was studied by AFM. A series of tapping-mode images before and after deposition of 3 nm metal films are shown in Figure 1. Images of the bare hydrogenpassivated Si substrate before and after metal deposition are also included in the figure for reference (left panels). The rootmean-square (rms) roughness of the various surfaces, including data for 3 and 8 nm metal overlayers, is summarized in Table 2. The key features of the surface morphology are as follows. (1) The hydrogen-passivated Si substrate and the bare porphyrin monolayers on the Si surface are relatively smooth. The rms roughness of the Si substrate is ∼0.4 nm. The rms roughness values of the porphyrin monolayers are generally

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9477 comparable to one another, ZnP-CH3, ∼0.2 nm; ZnP-CF3, ∼0.5 nm; ZnP-F5, ∼0.3 nm; ZnP-CN, ∼0.4 nm, and similar to that of the Si substrate. (2) The roughness values of the metal/monolayer/Si junctions after deposition of the thinnest (3 nm) metal overlayers are similar to those of the bare monolayers on the surface. The roughness values observed for the thinnest metal/monolayer/Si junctions are somewhat less than those of the control metal/Si interfaces but increase again upon deposition of thicker (8 nm) metal overlayers. For example, the rms roughness of the 3 nm Ag/Si interface is ∼0.9 nm, whereas that of the 3 nm Ag/ZnPF5/Si junction is ∼0.6 nm, and that of the 8 nm junction ∼1.7 nm. This trend in surface roughness may be due to the fact that the deposited metal initially fills in gaps in the monolayer; once those gaps are filled, additional metal deposition goes into building a metal surface. (3) The roughness of all of the junctions varies as a function of the metal used with Ag > Au > Cu. This is illustrated in Figure 2, which shows tapping-mode images and line profiles for the 8 nm metal/ZnP-F5/Si junctions. The rms roughness of these junctions decreases in the following manner: Ag (1.7 nm) > Au (0.7 nm) > Cu (0.4 nm). (4) The Cu/ZnP-CN/junctions show clear evidence of cluster formation; this is not the case for the Cu interfaces with any of the other porphyrins. Clustering is consistent with our previous observation that Cu reacts/complexes with the distal nitrile group48 and has been previously associated with chemical reactions.20,53 None of the Au/monolayer/porphyrin junctions show evidence of clustering, whereas this feature is common to all of the Ag/monolayer/Si junctions. The clustering observed for the latter junctions cannot be attributed to reactivity of the porphyrin with Ag (vide infra) but is a general characteristic of slow vapor deposition of the Ag metal.54 C. Porphyrin Integrity, Reactivity, and Adsorption Characteristics in the Metal/Monolayer/Si Junctions. The integrity, reactivity, and adsorption characteristics of the porphyrins in the monolayers before and after metal deposition were examined using FTIR spectroscopy. The FTIR studies were complemented by a more limited series of Raman studies. We first describe the results of the former experiments and then turn to the latter. 1. IR Spectra. Representative midfrequency (650-1800 cm-1) IR spectra of ZnP-CH3 and ZnP-F5 in the solid form and in monolayers on Si(100) before and after deposition of varying thickness of Cu, Ag, or Au are shown in Figures 3 and 4, respectively. In the case of ZnP-CF3 and ZnP-CN, the midfrequency IR spectral characteristics and behavior as a function of metal type and deposition thickness (Supporting Information, Figures S1 and S2) are similar to those observed for ZnP-F5. We have previously reported and discussed the FTIR spectra of ZnP-CH3, ZnP-CF3, and ZnP-CN monolayers on Si(100);48,49 consequently, we will not elaborate on the characteristics of the bare monolayers here. The FTIR spectra of ZnP-F5 have not been previously reported; however, the general spectral features of this porphyrin are similar to those of the other porphyrins bearing the trialkyl tripodal surface anchor. We focus on the key features of the FTIR spectra that are important for understanding the effects of metal deposition on the monolayers, which include the following. (1) The IR spectra of all four porphyrins in the solid state and in monolayers exhibit two prominent spectral features that are common to all arylporphyrins and that are useful for evaluating the average tilt angle (R) with respect to the surface normal:6,8,49 the in-plane porphyrin pyrrole breathing mode, ν(pyr), at ∼999 cm-1,55 and the out-of-plane porphyrin β-pyrrole

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Figure 1. Tapping-mode AFM images of the various 3-nm-metal/monolayer/Si junctions. The leftmost set of images are those of the bare hydrogenpassivated Si(100) substrate before and after metal deposition. All images have a height scale of 5 nm and dimensions of 500 nm × 500 nm.

TABLE 2: Root-Mean-Square (RMS) Surface Roughness (nm) of the Si(100) Substrate and Porphyrin Monolayers on Si(100) Before and After Metal Deposition Cu

Ag

Au

molecule

monolayer

3 nm

8 nm

3 nm

8 nm

3 nm

8 nm

Si(100) ZnP-CH3 ZnP-CF3 ZnP-F5 ZnP-CN

0.4 0.2 0.5 0.3 0.4

0.7 0.2 0.3 0.3 0.7

a 0.4 0.7 0.4 1.1

0.9 0.7 0.6 0.6 0.8

a 1.1 0.9 1.7 1.3

0.4 0.3 0.2 0.4 0.3

a 0.5 0.6 0.7 0.6

a

Not measured.

hydrogen deformation, γ(CH), at ∼798 cm-1.56 The average tilt angles for ZnP-CH3 (R ∼ 42°), ZnP-CF3 (R ∼ 38°), Zn-F5 (R ∼ 38°), ZnP-CN (R ∼ 43°) monolayers are generally similar to one another.9,49 (2) The IR spectra of ZnP-CF3 and ZnP-F5 in the solid state and in monolayers exhibit spectral features that are characteristic of the presence of the C-F bonds. For the former molecule, the symmetric stretch of the CF3 group, νs(CF3), is observed at 1326 cm (Supporting Information, Figure S1).57 For the latter molecule, a series of bands that are characteristics of fluorinated aryl groups are observed at 943, 1494, and 1520 cm-1 (Figure 4).58 (3) The IR spectra of ZnP-CN in the solid state and in monolayers exhibit a spectral feature that is characteristic of the nitrile group, the stretch, ν(C≡N), at ∼2229 cm-1.59 This feature is shown in the expanded IR spectra (2200-2260 cm-1) region presented in Figure 5. Comparison of the IR spectra of the porphyrin monolayers before and after metal deposition reveals certain features that are common among the different monolayers and others that are monolayer and/or metal overlayer specific. Specifically, the ZnP-F5, ZnP-CF3, and ZnP-CN monolayers (Figure 4, and Supporting Information, Figures S1 and S2) appear to be relatively unperturbed by metal deposition, whereas the ZnPCH3 monolayers (Figure 3) are more sensitive to the presence of the metal overlayer. In the case of the junctions containing the ZnP-CF3, ZnP-F5, and ZnP-CN monolayers, the prominent ν(pyr) (999 cm-1) and γ(CH) (798 cm-1) vibrations of the porphyrin skeleton do not exhibit any significant frequency shifts, and no significant new spectral features signaling the presence of adventitious organic material (due to porphyrin

Figure 2. Tapping-mode AFM images (left panels) and line profiles (right panels) of the 8 nm metal/ZnP-F5/Si junctions. Note the difference in z-scales for the different images.

degradation) appear after metal deposition. Some differences are observed in the weaker spectral features before versus after metal deposition, but those may originate from background subtraction artifacts. The deposition of the metals on the ZnPCF3, ZnP-F5, and ZnP-CN monolayers also results in minimal changes in the surface orientation of the porphyrins (R varies by 5° or less). In the case of the junctions containing the ZnP-CF3 and ZnPF5 monolayers (Supporting Information, Figure S1 and Figure 4), the bands characteristic of the C-F bonds, (νs(CF3), 1326 cm-1 for the former and the triplet of bands, 943, 1494, and 1520 cm-1 for the latter), are not affected by metal deposition. Accordingly, neither the -CF3 nor -C6F5 groups react with any of the metals. In the case of the interfaces containing the ZnP-CN monolayers, ν(C≡N) (2229 cm-1) is not affected by

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Figure 3. FTIR spectra of ZnP-CH3 in the solid form, in monolayers on Si(100), and in metal/monolayer/Si junctions: Cu (left), Ag (middle), Au (right).

Figure 4. FTIR spectra of ZnP-F5 in the solid form, in monolayers on Si(100), and in metal/monolayer/Si junctions: Cu (left), Ag (middle), Au (right).


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Figure 5. FTIR spectra of ZnP-CN in the region of νs(C≡N) in the solid form, in monolayers on Si(100), and in metal/monolayer/Si junctions: Cu (left), Ag (middle), Au (right).

deposition of either Ag or Au, indicating that these metals do not react/complex with the nitrile group (Figure 5). The lack of reactivity of Ag and Au with the nitrile group can be contrasted with behavior observed upon deposition of Cu, which we have previously shown does react/complex with the nitrile;48 this is evidenced here by the absence of ν(C≡N) from the IR spectrum (Figure 5). As noted above, metal deposition on the ZnP-CH3 monolayers appears to induce larger perturbations on the molecular layer than for the other three types of porphyrins. In most cases, the ν(pyr) (999 cm-1) and γ(CH) (798 cm-1) modes characteristic of the porphyrin skeleton are clearly observable in the spectra, as are a number of weaker porphyrin modes. However, under certain conditions (e.g., deposition of 8 nm of Cu or 3 and 5 nm of Au) the IR spectra are extremely noisy and the porphyrinic vibrational signatures are difficult to detect. More sample-to-sample variation was also observed in the IR spectra of the junctions containing the ZnP-CH3 monolayers than for those containing the monolayers of the other three types of porphyrins. The deposition of metals on the ZnP-CH3 monolayers also appears to result in a larger change in surface orientation (R increases by 10-15°, as evidenced by the increased intensity of γ(CH) versus ν(pyr) bands) than for the other monolayers. It is important to note that increasing the tilt angle with respect to the surface normal reduces the intensity of the in-plane porphyrinic modes relative to the out-of-plane modes. Because most of the bands in the IR spectrum are in fact in-plane modes, the overall effect on the spectrum is an apparent loss of intensity of most of the IR bands (with the exception of γ(CH)). The generally lower IR intensity reduces the S/N ratio, which likely contributes to the poor quality of the spectra for many of the interfaces containing the ZnP-CH3 monolayers.

2. Raman Spectra. As noted above, a limited set of Raman experiments was also conducted on the metal/monolayer/Si junctions. The objective of these studies was not only to further probe the structure of the molecules in the junction, but also to determine whether deposition of the metal induces any surfaceenhanced Raman scattering (SERS). In this regard, Raman experiments (647.1 nm excitation) conducted on various junctions fabricated with Cu and Au overlayers failed to reveal any detectable Raman signal from the porphyrin. In contrast, Raman experiments (514.5 nm excitation) conducted on junctions fabricated with Ag overlayers exhibited a clear SERS effect. The SERS effect is illustrated by the data shown in Figure 6, which compares the Raman spectra of the ZnP-F5 monolayers before and after Ag deposition. The Raman spectrum of the ZnP-F5 monolayers prior to Ag deposition shows no detectable signal from the porphyrin. Upon deposition of 3 nm of Ag, Raman bands characteristic of the porphyrin appear. It is difficult to estimate the magnitude of the SERS effect because no Raman signal can be observed for the bare monolayer; however, it must be many orders of magnitude. Increasing the thickness of the Ag overlayer from 3 to 5 nm further amplifies (5-fold) the SERS effect. Interestingly, an additional increase of the thickness of the Ag layer from 5 to 8 nm diminishes the SERS effect somewhat, and further increases in the thickness of the metal overlayer result in further diminution of the SERS effect (not shown). Thus, there appears to be an optimum Ag overlayer thickness of about 5 nm for SERS in the unconventional inverted geometry (metal on molecules versus molecules on metal). D. Metal Penetration into the Monolayers in the Metal/ Monolayer/Si Junctions. XPS depth-profile experiments were performed on all of the metal/monolayer/Si junctions in attempts to probe the extent of metal penetration into the molecular layer.


Figure 6. Raman spectra of ZnP-F5 in monolayers on Si(100) and in Ag/monolayer/Si interfaces.

Figure 7 displays representative data for the metal/ZnP-F5/Si junctions. These data compare the evolution of the XPS signal intensities for the metal (Cu 2p3/2, Ag 3d5/2, or Au 4f7/2), the Zn ion in the porphyrin (Zn 2p3/2), and the silicon substrate (Si 2p) as a function of sputtering time. The signals are reported in arbitrary units but were scaled according to reported cross sections for the different XPS peaks.60 Inspection of the sputtering profiles for the Ag and Au cases reveals a monotonic decrease in the signal of the metal with time at the expense of that of the Si, as expected given that the metal layer is on top of the Si substrate. In the case of Cu, a plateau is first seen, before the expected decay that is accompanied by larger Si signals, indicating the presence of a thicker film. In fact, considerable oxygen is also detected in this case, as evidenced by a XPS peak centered at 530.1 eV.

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9481 This, together with the position of the Cu 2p3/2 XPS peak, which was observed at 932.2 eV, is indicative of the formation of Cu2O (at least in the topmost layers). The XPS signal for Zn first increases and then decreases with sputtering time in all three cases (Cu, Ag, Au), as expected given that the porphyrins are sandwiched between the metal and Si layers. Nevertheless, the transition is sharper and better defined in the case of Ag, where the depth profiles are consistent with the view that the Ag metal is primarily on top of the monolayer and therefore, preferentially removed by sputtering (in fact, the Ag signal does not start decaying until after about 20 s of sputtering). The changes in Zn signal are almost imperceptible with Cu, perhaps because of the considerable thickness of the Cu2O film deposited on top. The case of Au shows intermediate behavior, but significant Au is observed long after the Zn signal starts to decay, suggesting some degree of intermixing between the Au and porphyrin layers. Also apparent from these data is the difference in film thicknesses for the three different types of metals, which is reflected by the differences in time scale of the plots. The profile for Ag required the least amount of sputtering time, indicating a thin layer and well-defined boundaries among the metal, porphyrin, and Si layers. The profile for Au required approximately twice the amount of time to reach the silicon substrate, implying more intermixing among the layers. Finally, the Cu case required quite long sputtering times because of the particularly large thickness of the Cu2O film. E. Electrical Properties of the Metal/Monolayer/Si Junctions. The electrical properties of the metal/monolayer/Si junctions were investigated using both “wet” electrochemical (cyclic voltammetry) and “dry” electrical (current-voltage) techniques. These studies probed the extent of electrolyte penetration into the molecular layer and also provided additional insight into the effects of metal penetration into the molecular layer. We first present the results of the electrochemical studies and then turn to the current-voltage measurements. 1. Cyclic Voltammetry. Representative cyclic voltammograms (100 V s-1) of the metal/ZnP-F5/Si junctions are shown in Figure 8. For reference, the cyclic voltammograms of the

Figure 7. XPS elemental composition profiles for the 3 nm metal/ZnP-F5/Si junctions as a function of sputtering time. The three panels correspond to Cu (left), Ag (middle) and Au (right) films and display the XPS signals for the metal, the Zn atoms in the porphyrin, and the Si substrate. The signal for oxygen is also reported in the case of copper to highlight the formation of a Cu2O film.


Figure 8. Representative cyclic voltammograms (100 V s-1) of the metal/ZnP-F5/Si junctions: Cu (top), Ag (middle), Au (bottom). The cyclic voltammogram of the ZnP-F5 monolayer on Si(100) prior to metal deposition is shown in each panel for reference.

ZnP-F5 monolayers prior to metal deposition are also included in the figure. Similar voltammograms and behavior as a function of metal type and deposition thickness were observed for the three other types of porphyrins (not shown). Comparison of the voltammograms of the Cu/ZnP-F5/Si and Ag/ZnP-F5/Si junctions as a function of metal overlayer thickness reveals similar trends. In particular, the amplitudes of the voltammetric waves are attenuated upon deposition of metal, and monotonically decrease as the thickness of the metal overlayer increases. For the thickest (8 nm) Ag overlayer, the voltammetric signature of the porphyrin is barely discernible. This trend is consistent with the view that the thinner metal overlayers contain defects that permit the electrolyte to penetrate into some fraction of the monolayer, thereby affording a redox response, albeit of diminished amplitude from that of the bare monolayer. On the other hand, for the thickest (8 nm, not shown) Cu overlayer, the electrochemical responses resembles that of bulk Cu, suggesting a well ordered film has formed that prevents the electrolyte from reaching the monolayer. Comparison of the voltammograms of the Au/ZnP-F5/Si junctions as a function of metal overlayer thickness reveals behavior that is different from that of the Cu/ZnP-F5/Si and Ag/ZnP-F5/Si junctions. In particular, deposition of Au of any thickness results in the complete loss of the voltammetric signature of the porphyrin, and is accompanied by a very large increase in background capacitance, to ∼20 µF cm-2, approximately that expected for bulk Au. In fact, Au/ZnP-F5/Si junctions with even thinner (1.5 nm) overlayers of Au exhibited the same electrochemical characteristics (not shown) as those observed for that fabricated with the thicker Au overlayers. The absence of the redox signature of the porphyrin for all of the

Anariba et al. Au/monolayer/Si junctions suggests that upon deposition of even minimal amounts of Au, the metal can penetrate into the monolayer, perhaps bridging to the Si substrate, and effectively short-circuiting the electrochemical cell. 2. J-V Measurements. Representative J-V curves (-2 to +1 V) for the metal/ZnP-F5/Si junctions are shown in Figure 9. Similar J-V curves were observed for the three other types of porphyrins in the junctions (not shown). The left set of panels show the J-V curves on a large current-density scale and include J-V curves of control metal/Si interfaces. The right set of panels show the J-V curves of the metal/ZnP-F5/Si junctions on a smaller current-density scale. Inspection of the J-V curves for the metal/Si control interfaces shows that these interfaces exhibit extremely high current densities, tens of A cm-2 at voltages of tenths of volts. This behavior is expected because of the direct contact of the metal with the Si substrate. Note, however, that the J-V curves for the metal/Si controls are not ohmic because of the semiconducting nature of the Si substrate. Comparison of the J-V characteristics of the metal/ZnP-F5/Si junctions with those of the metal/Si control samples reveals that the presence of the porphyrin monolayer dramatically alters the appearance of the J-V curves. In all cases, the current density is reduced by several orders of magnitude. Inspection of the J-V curves for the metal/monolayer/Si junctions on a smaller current-density scale reveals other notable features. These features are conveniently summarized as lowvoltage resistance (LVR) values for all of the porphyrin and control junctions in Table 3. The LVR values reveal the following: (1) the resistance of the control Cu/Si interface (∼7.7 kΩ) is considerably higher than that of either the Ag/Si (∼0.9 kΩ) or Au/Si (∼0.4 kΩ) interface. This higher resistance for the Cu/Si interface is likely due to the formation of Cu2O during metal deposition, as indicated by the XPS data. (2) The LVR values of the various metal/monolayer/Si junctions vary among the different types of porphyrins. However, no clear trends can be ascertained because the sample-to-sample variations for junctions fabricated using a given porphyrin are relatively large. (3) The Au junctions showed the lowest LVR values, most likely because of the formation of filaments interdigitated with the porphyrin monolayer and making contact with the silicon substrate. (4) The Cu/monolayer/Si and Ag/monolayer/Si junctions exhibit much higher LVR values than the Au/monolayer/ Si junctions, with the Ag/monolayer/Si junctions arguably exhibiting the highest LVR values. This observation is consistent with the results of both the XPS and cyclic voltammetric studies, which indicate that, contrary to the case with Au, neither Cu nor Ag substantially penetrate into the monolayer. (5) Although Au does penetrate into the monolayer, the LVR values of the Au/monolayer/Si junctions are still 1-2 orders of magnitude higher than that of the control Au/Si interface. This suggests that the Au filaments that form spanning the molecular layer and contacting the Si substrate are of relatively small diameter (higher resistance). IV. Discussion The fabrication of electronic devices typically requires hundreds of processing steps. Many of those steps involve conditions that are extremely harsh by the standards of molecular materials, including extended times at high temperatures and exposure to chemically and/or mechanically abrasive processes. To fulfill their promise for device applications and realistically gain acceptance as a next-generation technology, any molecular materials must be compatible with semiconductor-industry


J. Phys. Chem. C, Vol. 112, No. 25, 2008 9483

Figure 9. Representative J-V curves (+1 to -2.2 V) of the metal/ZnP-F5/Si junctions. The left set of panels show the J-V curves on a larger current-density scale and include J-V curves of control metal/Si interfaces. The right set of panels show the J-V curves of the metal/ZnP-F5/Si junctions on a smaller current-density scale.

TABLE 3: Low-Voltage Resistance of the Metal/Porphyrin Monolayer/Si(100) Junctionsa molecule

Cu resistance (KΩ)b

Ag resistance (KΩ)

Au Resistance (KΩ)

no molecule ZnP-CH3 ZnP-CF3 ZnP-F5 ZnP-CN

7.7 ( 0.4 151 ( 43 1100 ( 1200 8300 ( 9500 144 ( 49

0.9 ( 1.0 13500 ( 5,500 11400 ( 11,000 55600 ( 8000 5000 ( 6100

0.4 ( 0.05 3.4 ( 4.2 3.8 ( 5.1 21 ( 14 57 ( 21

a Average of five different samples per molecule and per metal; junction area, 10-4 cm2. b Values measured in the linear J-V range, -0.3 to +0.1 V.

processing standards. In this regard, we have previously demonstrated that porphyrinic molecules in the form of covalently attached monolayers on Si(100) are extremely stable both thermally (400 °C) and electrically (>1010 redox cycles).4 The studies reported herein provide new insights into the characteristics of the porphyrinic materials under conditions suitable for fabricating top metal contacts. The studies also provide detailed information concerning important metal overlayer characteristics such as surface morphology, extent of metal reaction and penetration into the molecular layer, and electrical characteristics of the metal/monolayer/Si junctions. We discuss these issues in more detail below. A. Porphyrin Monolayer Characteristics. The studies reported herein indicate that the porphyrin monolayers on Si(100) are generally robust under conditions wherein all the coinage metals (Cu, Ag, or Au) are deposited via electron-beam evaporation. In particular, there is no evidence that metal deposition significantly compromises the integrity of the molecules or degrades the molecular layer, with the possible exception of the case of the ZnP-CH3 porphyrin. The studies

further suggest that the presence of distal functional groups such as the -CF3, -C6F5 and -C≡N in ZnP-CF3, ZnP-F5 and ZnPCN, respectively, offers advantages in metal overlayer formation compared with distal hydrocarbon functionalization such as the -CH3 group in ZnP-CH3, as evidenced by the minimal versus quite noticeable perturbations of the IR spectra of the former versus the latter porphyrins. The enhanced robustness of the monolayers of the former group of porphyrins cannot be attributed to any specific reactivity of the functional groups with the metal (except perhaps for the -C≡N group and Cu), because these groups are little perturbed by metal deposition. Most likely, the larger mass of the distal fluorinated or nitrile groups (relative to hydrogen) serves to better disperse the kinetic energy and/or thermal energy of the impinging metal atoms. In general, the maintenance of molecular integrity under metal deposition conditions suggests that metal contacts to porphyrinic materials might be established via relatively straightforward processing conditions. B. Metal Overlayer Characteristics. A relevant issue regarding the nature of the metal overlayer is the extent to which the deposited metal covers over and/or penetrates into the molecular layer. In terms of coverage, it can be argued that it is incomplete in the case of the thinnest (3 nm) layers investigated; this view is consistent with the fact that the thinnest metal/monolayer/Si junctions exhibit an electrochemical response in the ”wet” electrochemical experiments (except those fabricated with Au), suggesting exposure of the molecules to electrolyte. However, the metal coverage of the monolayer appears to be effectively complete upon reaching deposition thicknesses of 8 nm, as evidenced by the near absence of an electrochemical signature from these thicker metal/monolayer/ Si junctions. The case of the Au films is unique, because the

9484 J. Phys. Chem. C, Vol. 112, No. 25, 2008 electrochemical measurements provide no information regarding the extent of the coverage owing to the fact that all of the Au/ monolayer/Si junctions fail to exhibit an electrochemical response. This behavior cannot be realistically attributed to complete coverage of the monolayer by Au, because even junctions fabricated from very thin (1.5 nm) layers of Au do not exhibit any observable redox behavior. Instead, the collective body of experimental evidence (cyclic voltammetry, J-V measurements, XPS) is most consistent with Au penetrating through the monolayers to form a direct electrical connection to the Si substrate. In contrast, metal penetration of either the Ag or Cu layers to the Si substrate is minimal. This does not imply that Cu and Ag sit exclusively atop the molecular layer, but rather that filament formation is negligible.39 V. Conclusions The studies reported herein demonstrate that porphyrin monolayers covalently anchored to Si(100) via tripodal alkyl groups are robust to coinage-metal (Cu, Ag, Au) deposition; however, only the Cu and Ag films are not subject to significant metal penetration that results in continuous filaments between the top and bottom electrical contacts. These qualities of the porphyrin monolayers are important for potential device applications. In particular, the fact that even relatively gentle electron-beam deposition of Au onto the porphyrin monolayers results in electrical shorts implies that the fabrication of Au/ monolayer/Si junctions, at least by this method, will not afford functionally robust electronic devices. On the other hand, electron-beam deposition of Cu and Ag does appear to be a viable method of fabricating a robust electrical contact. However, the use of Cu requires ultraclean conditions, otherwise Cu oxides are formed, resulting in junctions with higher resistivities. Acknowledgment. This work was supported by the Center for Nanoscience Innovation for Defense and DMEA (H9400307-2-0708) and by ZettaCore, Inc. Supporting Information Available: Midfrequency FTIR spectra of ZnP-CF3 and ZnP-CN in solid form and in monolayers on Si(100) before and after metal deposition. This material is free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kwok, K. S.; Ellenbogen, J. C. Mater. Today 2002, 5, 28–37. (2) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378–4400. (3) Lindsay, S. M.; Ratner, M. A. AdV. Mater. 2007, 19, 23–31. (4) Liu, Z.; Yasseri, A. A.; Lindsey, J. S.; Bocian, D. F. Science 2003, 301, 1543–1545. (5) Roth, K. M.; Dontha, N.; Dabke, R. B.; Gryko, D. T.; Clausen, C.; Lindsey, J. S.; Bocian, D. F.; Kuhr, W. G. J. Vac. Sci. Technol., B 2000, 18, 2359–2364. (6) Roth, K. M.; Yasseri, A. A.; Liu, Z.; Dabke, R. B.; Malinovskii, V.; Schweikart, K.-H.; Yu, L.; Tiznado, H.; Zaera, F.; Lindsey, J. S.; Kuhr, W. G.; Bocian, D. F. J. Am. Chem. Soc. 2003, 125, 505–517. (7) (a) Yasseri, A. A.; Syomin, D.; Loewe, R. S.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Am. Chem. Soc. 2004, 126, 15603–15612. (b) Yasseri, A. A.; Syomin, D.; Loewe, R. S.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Am. Chem. Soc. 2005, 127, 9308. (8) Wei, L.; Syomin, D.; Loewe, R. S.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Phys. Chem. B 2005, 109, 6323–6330. (9) Jiao, J.; Anariba, F.; Tiznado, H.; Schmidt, I.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Am. Chem. Soc. 2006, 128, 6965–6974. (10) Pitts, J. R. Ph.D. Dissertation, University of Denver, Denver, CO, 1985. (11) Herdt, G. C.; Jung, D. R.; Czanderna, A. W. Prog. Surf. Sci. 1995, 50, 103–129. (12) Herdt, G. C.; Czanderna, A. W. J. Vac. Sci. Technol., A 1995, 13, 1275–1280.

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Comprehensive Characterization of Hybrid Junctions Comprised of a

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