Article pubs.acs.org/Langmuir
Electrical and Physical Characterization of Bilayer Carboxylic AcidFunctionalized Molecular Layers Sujitra Pookpanratana,† Joseph W. F. Robertson,† Cherno Jaye,‡ Daniel A. Fischer,‡ Curt A. Richter,† and Christina A. Hacker*,† †
Semiconductor and Dimensional Metrology Division and ‡Ceramics Division, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland S Supporting Information *
ABSTRACT: We have used flip chip lamination (FCL) to form monolayer and bilayer molecular junctions of carboxylic acidcontaining molecules with Cu atom incorporation. Carboxylic acid-terminated monolayers are self-assembled onto ultrasmooth Au by using thiol chemistry and grafted onto n-type Si. Prior to junction formation, monolayers are physically characterized by using polarized infrared absorption spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy, confirming the molecular quality and functional group termination. FCL was used to form monolayer junctions onto H-terminated Si or bilayer junctions of carboxylic acid monolayers on Au and Si. From the electrical measurements, we find that the current through the junction is attenuated as the effective molecular length within the junction increases, indicating that molecules are electrically active within the junction. We find that the electronic transport through the bilayer junction saturates at very thick effective distances possibly because of another electron-transport mechanism that is not nonresonant tunneling as a result of trapped defects or sequential tunneling. In addition, bilayer junctions are fabricated with and without Cu atoms, and we find that the electron transport is not distinguishably different when Cu atoms are within the bilayer.
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INTRODUCTION Incorporating molecular-based materials and elements into electronic components has been of great interest because of their potential applications in memory-based devices,1 organic thin-film transistors,2 and spintronic devices.3 The incorporation of organic monolayers with silicon is advantageous from a technological manufacturing point of view because of the extensive infrastructure that would allow for rapid integration with Si-based CMOS technologies. Another advantage of using silicon is the ability to use molecules to engineer and alter the surface energetics of the silicon surface and thus alter the interfacial energy alignment. This is particularly interesting because many electronic processes are dominated by interfaces. Moreover, organic molecules bind to silicon via a strong, robust Si−C covalent bond of about 4 eV, which would provide for a stable molecule−inorganic interface. However, because of the strong Si−C bond, the assembly of molecules onto Si is not as ordered for self-assembled monolayers (SAM) of thiolcontaining molecules on Au or Ag. This article not subject to U.S. Copyright. Published 2013 by the American Chemical Society
The incorporation of different molecular components allows for added functionality of the Si surface. Tailoring the functionality of the molecular layer is easily achieved by selecting the appropriate headgroup of a molecule. Carboxylic acid-containing monolayers have been studied for many applications because of their wide applicability in catalysis and sensors and their ability to tether a wide range of biological molecules. The metal−carboxylate chemistry of monolayers has been studied4−10 because of immense potential applications in biological systems and nanotechnology. Chelating metal ions to the carboxyl group of the molecules can be used as building blocks to make multilayers5,7,11 or “molecular rulers” for photolithographic approaches to making nanostructures.9 The incorporation of metal atoms into the molecular layer can also influence some electronic behaviors within a molecule-based Received: October 25, 2012 Revised: January 7, 2013 Published: January 30, 2013 2083
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device, such as tunneling characteristics,12 magnetic properties,13 nonlinear properties due to changing redox states,14 memristive characteristics,15 and negative differential resistance.16 The incorporation of metal ions is also important to studying electronic transport in molecular electronics. Yu and co-workers observed enhanced inelastic tunneling of electrons when incorporating nickel ions between dithiol molecules within their molecular junction.12 There are limited electrochemical studies that suggest an enhancement in charge transfer due to the presence of adsorbed metal ions.17 The design of molecular electronic junctions with active functional groups and the incorporation of chelating metal atoms may lead to advantageous electrical and physical properties. Metal ion incorporation onto monolayers has also been viewed as a strategy to aid in the metallization process of organic monolayers. Forming a robust physical and electrical contact to a molecular layer has been a challenge for ensemblescale devices (ensemble scale is defined as being one molecule thick and involving more than 103 molecules).18 From a fabrication point of view, the process of forming the top electrode by traditional physical vapor deposition often damages the molecular layer, leading to inconsistent molecular electronic junctions.19−22 Some approaches that incorporate metal atoms have been investigated as means to “soften” the metallization process, leading to more reliable molecular junction formation.6,8,23,24 There are numerous alternative approaches to forming an electrical contact to the monolayer in ensemble junctions (without metal ion incorporation), and they include transfer printing,25,26 spin coating a conductive polymer,27 liquid metal by Hg28 or E-GaIn,29 and direct22,30 and indirect30 metal evaporation. These ensemble-scale junctions present unique fabrication challenges compared to those of transient single-molecule (or isolated molecule as formed by scanning probe contacts) devices. Ensemble-scale junctions are necessary for the development of more amenable metrology, enabling devices to be tested for longer durations and at different laboratories. They are key to providing a route for scale-up and integration with conventional Si-based electronic technologies. Having in mind techniques compatible with scale-up production, we have utilized flip chip lamination (FCL) to form molecular junctions between Si and Au26 and have used them on a variety of different molecules with varying functionality.31 This method uses an ultrasmooth Au surface to minimize defects (such as grain boundaries) and provides surface uniformity by taking advantage of the facile selfassembly of thiol molecules. The SAM/Au is formed on a flexible substrate,26,31 which allows it to be laminated to a Si electrode. We have extended FCL to include the making of molecular bilayer junctions. Bilayer formation adds more functionality by allowing the opportunity to functionalize, optimize, and characterize each molecular layer separately before combining them. Typically, “bottom-up” bilayers have been created with an initial monolayer sequentially exposed to different monolayer-forming solutions.17,27,32,41 This is detrimental because defects are propagated and exacerbated as the layer thickness increases. An alternative approach is to form the layer on each electrode, which has been used for junctions of Au flakes and a patterned substrate33 and Hg−molecule−Hg junctions.34,35 Although this approach minimizes the electrical shorting of the molecular junction, the physical and chemical structures are uncertain in the formed bilayer junction. In our work presented here, each monolayer is formed and optimized
on a separate substrate, and the quality and physical structure can be determined prior to forming the bilayer structure. This method also introduces the flexibility of incorporating dissimilar molecules such as using different head groups, varying lengths, or different backbones for a resulting bimolecular layer complexity. The bilayers could also serve as well-defined spatial buffers with individual atoms, quantum dots, or nanoparticles incorporated within the molecular layer, thus ensuring close proximity to the electrodes without the loss of the specialized function of the nanoscale components. Our motivation is to bring together high-quality monolayers incorporating chemical species capable of redox reactions to form high-quality molecular junctions with the stability needed for long-term technological applications. We investigated the test case of copper ions on carboxylic acid-terminated monolayers because these present a relatively well studied system where there is evidence of copper ions being in the +2/ +1 states.4−7,10 Here, we have created carboxylic acidterminated monolayers on Au and Si electrodes and then used FCL to create bilayer junctions consisting of a carboxylic acid-terminated monolayer chemically bonded to each electrode and copper ions in the middle (illustrated in Scheme 1). We extensively characterized the chemical, physical, and Scheme 1. Idealized Drawings of Carboxylic AcidTerminated Molecular Layers on Au and Si Surfaces (Left) and Bilayer Junction Formation (Right)
electronic properties of the molecular surfaces with infrared and X-ray spectroscopy and electrochemistry prior to FCL (or junction formation). After FCL, the monolayer and bilayer junctions were interrogated with p-polarized backside infrared spectroscopy (pb-RAIRS) and with electrical measurements that confirmed the presence of the molecular layer between the electrodes.
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EXPERIMENTAL METHODS
Substrate Preparation. Ultrasmooth Au (uS Au) substrates were prepared according to ref 31, and details are provided in the Supporting Information document. For device-scale measurements, the uS Au substrates consisted of an array of 150-μm-diameter circles. Evaporated Au was brought into contact with a poly(ethylene terephthalate) (PET) sheet with a nanoimprint lithography tool (Nanonex NX-2000), and the Au layer was transferred from the Si substrate to the PET substrate, which resulted in the uS Au substrates. For alkene monolayer formation, n-type Si(111) double-side-polished silicon substrates were used. Heavily doped (ρ = 0.001−0.005 Ω·cm) silicon substrates were used for samples involved in electrical measurements, and lightly doped (ρ = 10−15 Ω·cm) silicon substrates were used for samples involved in physical characterization measurements. Monolayer Formation. All chemicals including mercaptoundecanoic acid (C11), mercaptohexadecanoic acid (C16), copper perchlorate hydrate, and undecylenic acid (C′11) were purchased from Sigma-Aldrich and used as received. Details of the monolayer preparation are provided in the Supporting Information document. 2084
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Figure 1. XPS spectra of (a) C 1s and (b) O 1s lines from carboxylic acid-terminated monolayer on Au and Si. (c) Cu 2p3/2 spectrum of Cu−C11/ Au.
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Junction Formation. FCL molecular junctions were created by using the same nanolithography tool mentioned earlier. The thiolcontaining monolayer on uS Au was laminated against either a Hterminated Si substrate (thus forming a monolayer junction) or C′11/ Si (thus forming a bilayer junction). The temperature and pressure for successful FCL were optimized for each sample structure (e.g., monolayer or bilayer) in order to maximize the transfer of Au (initially on PET) to the Si substrates for electrical characterization. Lamination conditions for molecular junction formation are 1.4 MPa and 95 °C for monolayer junctions and 2.8 MPa and 105 °C for bilayer junctions. Physical Characterization. Structural characterization of the samples before junction formation was performed by transmission and reflection absorption infrared spectroscopy with a Thermo Nicolet 8700 instrument equipped with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector. Additional details of the infrared measurements are provided in the Supporting Information. To investigate the buried molecular junction formed by FCL, pb-RAIRS was utilized in the reflection setup. Details of the measurement can be found in refs 19 and 30 and in the Supporting Information. X-ray photoelectron spectroscopy (XPS) measurements were performed by using a Kratos Axis instrument with monochromatized Al Kα excitation. Additional details of the measurement and fit analysis are included in the Supporting Information. Near-edge X-ray absorption fine structure (NEXAFS) experiments were performed at the National Synchrotron Light Source at NIST beamline U7A at Brookhaven National Laboratory. Details of the measurement and fit analysis are included in the Supporting Information. Electrical Characterization. Two terminal current−voltage (I− V) measurements were acquired with an HP 4156A parameter analyzer. Bias was applied to the top Au contact (patterned circles, d = 150 μm), and current was collected on the bottom n-Si substrate (heavily doped, 1019 cm−3). The voltage was swept from −1 to +1 V with a step size of 0.01 V. About 40 to 60 I−V measurements were taken from each sample (each sample contained multiple junctions) that was fabricated from two device runs. The geometric mean of the I−V curve was determined for each sample.36 Electrochemical Characterization. Cyclic voltammetry and impedance measurements were performed using an Autolab PGSTAT30 potentiostat/galvanostat equipped with frequency response analyzers in a three-electrode cell setup. The cell was capped to minimize solution evaporation. The working electrode was a C11 (with and without Cu atoms) on the Au/Si substrate with an exposed area of 0.32 cm2. A platinum wire served as the counter electrode, and a Ag/AgCl reference electrode was used. The supporting electrolyte was 0.1 mol/L KCl buffered to pH 7.2.
RESULTS AND DISCUSSION Carboxylic Acid Surfaces Prior to Junction Formation. The chemical structures of the monolayers on uS Au and n-Si were extensively physically characterized before junction formation. Detailed C 1s, O 1s, and Cu 2p3/2 XPS spectra are shown in Figure 1. The C 1s spectra of C11 and C′11 are shown in Figure 1a (bottom two spectra) and are similar (as expected). There are two distinct features in the C 1s XPS spectra: the intense feature at 284.9 eV (deconvoluted into features a and b) and a high-binding-energy feature at 289.5 eV (feature c). The C 1s emission from features a and b arise from carbon atoms in the aliphatic chain.37 The high-binding-energy feature (c in Figure 1a) is representative of carbon atoms of the carboxyl group as a result of the electron-withdrawing effect and is consistent with values in the literature.10,38 The O 1s XPS spectra (Figure 1b) also confirm the presence of a carboxylic acid-terminated monolayer at the surface. The spectra of C11 and C′11 (bottom two in Figure 1b) indicate two distinct features and are deconvoluted into features i and ii. These two features arise from oxygen atoms in two chemical environments: (i) double bonded to C (OC) and (ii) protonated and bonded to carbon (C−O−H). The deconvoluted center values of i are 532.7 and 533.1 eV for C11 and C′11, respectively. The deconvoluted center values of feature ii are 534.0 and 534.6 eV for C11 and C′11, respectively. For both C11 and C′11, the peak intensity of the CO component is larger than that for the C−O−H component, which suggests fewer protons than one would expect. Our data are consistent with other works reporting O 1s XPS spectra of carboxylic acid-terminated monolayers.7,10 For both the C11 and C16 samples, S 2p XPS spectra were taken (Supporting Information) and showed that there was one chemical component, which is consistent with bound Au−S.39 No signs of sulfur oxidation were observed on the basis of the XPS measurement. The S 2p emission was used to estimate the density of thiol-containing molecules bound to the Au surface. The S 2p and Au 4f intensities were corrected for their respective photoionization cross sections, electron inelastic mean free paths, and spectrometer transmission. We find that the molecular coverage is about 3.5 ± 0.5 and 4.1 ± 0.5 molecules per nm−2 for C11 and C16 on Au/PET, respectively. The grafting density of C′11 molecules on Si was estimated by 2085
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calculating the thickness of the C′11 layer on Si by using the signal attenuation of the Si 2s photoemission line. The overlayer thickness is about 1.7 nm, and the density of C′11 molecules on a Si surface is estimated to be 3.2 molecules per nm−2, which is consistent with a monolayer of C′11 formation and not multilayer formation. Successful Cu incorporation onto carboxylic acid monolayers on Au was confirmed by XPS, and the Cu 2p3/2 spectrum is shown in Figure 1c. The Cu atoms present on the surface are mostly in the Cu(II) oxidation state, in agreement with the literature, as evident by their signature shakeup features,40 and the Cu(I) state is likely an artifact due to the exposure of X-ray photons. We observed that the Cu(I) feature increases in intensity while the Cu(II) features decreases in intensity with prolonged X-ray exposure. We also note that Cl (from the Cuperchlorate salt) was not found on the surface of Cu−C11/Au. Thus, we presume that Cu exists in the form of Cu2+ chelated to two neighboring −COO− species or oxidized as CuO. To ascertain whether the Cu atoms are at the top of the monolayer (i.e., carboxyl end of the molecules) or on the Au surface (i.e., displacing the S−Au bond), we turn to the S 2p XPS spectrum (Supporting Information). The S 2p XPS spectra indicate that the sulfur atoms are in a single chemical environment, namely, that they are bonded to Au. Thus, we can be assured that the Cu atoms are not displacing the monolayer at the Au interface (i.e., the Au−S bond remains intact). The density of Cu atoms on the C11 surface was estimated by the relative intensities of XPS lines, and we find that there is one copper atom for every one to two sulfur atoms at the surface. Upon copper incorporation onto the monolayer, the chemical environment of the carbon and oxygen atoms are affected as seen by XPS (top spectra, Figure 1a,b). The carbon atoms in the carboxyl group (feature c in Figure 1a) shift to lower binding energy upon Cu incorporation, and the remaining carbon atoms in the aliphatic chain (features a and b) remain at the same binding energy. Similarly, all features in the O 1s spectrum (top spectrum, Figure 1b) also shift to lower binding energy after Cu incorporation. The chemical shifts indicated by the C 1s and O 1s spectra indicate that the Cu atoms interact with the terminal carboxyl group of the C11 monolayer. The CO component increases while the C−O−H component decreases, further indicating that the Cu atoms interact with the oxygen atoms of the molecular layer. We can estimate the relative number of Cu atoms interacting with the carboxylic acid sites by the relative decrease in the C−O−H component in the O 1s spectrum. The C−O−H contribution to the O 1s signal is about 44 ± 2% for C11 and 27 ± 2% for Cu−C11. With direct evidence based on the chemical shifts of the carbon and oxygen atoms and indirectly the sulfur atoms, we are able to deduce that the Cu atoms are at the surface, that is, chelated to the carboxyl group. Structural characterization and chemical conformation by IR confirmed the presence of carboxylic acid-terminated monolayers on Au and Si. For C′11/Si (bottom spectrum, Figure 2), the C−H and CO stretches confirm successful monolayer formation on the n-Si surface in the transmission IR measurements. The features at 2924 and 2853 cm−1 are assigned to the C−H asymmetric (νasym(C−H)) and symmetric (νsym(C−H)) stretches, respectively. Their energy positions are indicative of disordered molecular monolayer formation but are in agreement with values reported for monolayers on Si.41 A weak feature approximately centered at 1725 cm−1 is assigned to the carbonyl (ν(CO)) stretch. We note that in some monolayer fabrication runs the CC stretch (in −CHCH2,
Figure 2. Absorption infrared spectra of carboxylic acid-terminated monolayers: undecylenic acid (C′11) on Si (in black), mercaptoundecanoic acid (C11) on Au (in blue), Cu-incorporated C11 (red), and mercaptohexadecanoic acid (C16) on Au. The methylene symmetric and asymmetric stretches and carbonyl stretch are labeled. Note that the C′11/Si spectrum (measured in transmission) has been magnified by a factor of 5 to compare to the monolayers on Au within the same figure.
∼1630 cm−1) indicative of alkene functional groups on the surface was observed in the IR spectrum and is attributed to the upside-down attachment of C′11. The relative intensities of the carbonyl stretch and alkene stretch varied from sample to sample (Supporting Information), which is indicative of the limitation of chemical selectivity when grafting bifunctional molecules onto Si surfaces. For the carboxylic acid-terminated monolayer on Au (e.g., C11/Au), the RAIR spectra in Figure 2 indicate dense molecular packing with carboxylic acid termination on the surface. The νasym(C−H) and νsym(C−H) stretches at 2922 and 2850 cm−1 are consistent with other reported values for carboxylic acid-terminated thiols7,42 and indicate a densely packed molecular chain. The stretching mode of the carbonyl group in C11 and C16 is approximately centered at 1715 to 1725 cm−1, and because of its broadness and asymmetry, it is likely a product of a superposition of multiple CO stretches in slightly different chemical environments. A contribution of around 1718 cm−1 is indicative of an OC−OH chemical environment.6 On the basis of the energy position of νasym (∼2922 cm−1), the C11 and C16 monolayers on Au are more densely packed than the C′11 monolayer on Si, consistent with subtle differences observed in the XPS spectra. The methylene scissor deformation (∼1415 cm−1) was not detected in the spectra. After Cu incorporation, the vibrational modes associated with the methylene group do not change in terms of peak positions and relative intensities, but the stretch associated with the carbonyl group changes slightly. The spectral contribution at lower energy (1715 cm−1) increases with respect to the higher-energy (∼1740 cm−1) carbonyl stretch. The higher-energy (1740 cm−1) stretch is associated with the free (non-hydrogen-bonded) −COOH group (see ref 42 and references therein). Thus, upon Cu incorporation at the surface, the population of this group decreases, as expected, indicative of decreased protonated carbonyl sites. We are confident that the Cu atoms interact directly with the carboxyl group based on our XPS and IR findings. We find that our carboxylic acid monolayer on Au and Si is of good quality, and for C11/Au, the structural quality of the monolayer is unchanged upon Cu chelation. The effect of Cu atoms on the electronic structure of carboxylic acid-terminated monolayers was also investigated by 2086
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where spectra of each sample at different photon angles of incidence were fitted simultaneously. The fit results of the carboxyl π* feature for both samples are shown in Figure 3c and show that C16 and Cu−C16 follow a weak but opposite trend. This implies that the Cu incorporation has altered the mean orientation of the CO group. And at the same angle of incidence, the relative intensity of the π* contribution from Cu−C16 is less than that from C16. This also suggests that Cu incorporation has affected the unoccupied density of states that correspond to the carboxyl group. This finding is in agreement with our XPS results mentioned earlier and further supports the fact that, upon Cu incorporation, the Cu atoms are adsorbed (or chelated) at the surface of the monolayer (i.e., the carboxyl group). The Cu atoms neither interact between the molecules nor attack the S−Au bond of the monolayer. To assess the electrochemical properties of C11 and Cu− C11 monolayers, we have performed impedance spectroscopy measurements on both of these surfaces. In Figure 4, the
NEXAFS. In Figure 3a (top panel), the carbon K-edge NEXAFS spectra of C16 is shown at three different angles of
Figure 4. Impedance and phase shift shown as a function of frequency from electrochemical impedance measurements of C11 monolayers with and without Cu atoms. Measurements were made in 0.1 mol/L KCl at 0 V vs Ag/AgCl. Symbols represent data, and solid lines represent fit results from a Randles-like equivalent circuit; the arrows refer to the data axis reference. Figure 3. Angle-dependent C−K edge NEXAFS of C16 (a) before and (b) after Cu chelation. Resonances and Rydberg transitions are labeled. (c) Fit results of the carboxyl π* feature, with error bars representing the margin of confidence derived from the fitting algorithm.
electrochemical responses of the two surfaces show similarities and are quantitatively analyzed with a Randles-type equivalent circuit47 and fit by a nonlinear least-squares method (shown in Figure 4 as solid lines). The estimated error of the interface capacitance is representative of the margin of confidence derived from the fitting algorithm. The equivalent circuit consists of a resistor (for the electrolyte solution) in series with a capacitor (for the interface between the monolayer/electrode and electrolyte); the capacitor is in parallel with a resistor (for charge transfer) and a diffusive constant phase element to account for defects within the monolayer. We find that the interface capacitance of the C11 layer is about 2.4 ± 0.1 μF cm−2, which is in agreement with previously published results.48 The Cu−C11 surface has a nearly identical interface capacitance, 2.2 ± 0.1 μF cm−2, to that of C11. Though the C11 and Cu−C11 surfaces are chemically different, the electrochemical capacitance at their surfaces is nearly identical. Bilayer Junction Formation. The junction formation of carboxylic acid-terminated monolayers on Au and Si was formed by FCL. For large-area (ca. 1 cm2) samples, the molecular layer within the junctions was physically characterized by pb-RAIRS (Figure 5). This technique allows us to probe the buried monolayer nondestructively after junction formation. The molecular monolayer and bilayer junctions
incidence. Resonance-labeled ii is assigned to the Rydberg states associated with the alkyl backbone of the monolayer.43 Feature iii is the C 1s electron transition into the π* orbital of the carboxyl group, and iv is the transition into the σ* orbital of the C−C bond.44,45 Note that the feature labeled i is an artifact assigned to a π* transition due to X-ray beam damage. The C16 monolayer has a carbon K-edge polarization dependence as seen by the intensity variation of π* and σ* with respect to the angle of incidence between the sample and X-ray photons. After Cu incorporation, the carbon polarization dependence differs from the carboxyl π* (feature iii) transition as seen in Figure 3b (bottom panel). For the Cu−C16 sample, there is an additional feature at 286.8 eV (near feature i) that could be assigned to the coordination of Cu atoms to the carboxyl group (CO → π*).46 The NEXAFS spectra were further analyzed by deconvoluting with an error function (modeled as the continuum step) and Gaussian line shapes as the absorption orbitals. A linear least-squares approach was implemented 2087
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Figure 5. Pb-RAIRS spectra of the fabricated molecular junctions of monolayers C′11, C11, and C16 and bilayers C11 + C′11 and C11-Cu + C′11. The methylene symmetric and asymmetric stretches are labeled. Figure 6. Average electrical transport measurements for device structures varying from C11 to C27 effective molecular length within the FCL junction. The average current was determined using the geometric average. The standard deviation of the measured I−V curves at a few biases is also shown.
formed by FCL clearly display the spectral feature associated with the methylene stretches of the molecular layer. The positions of νsym and νasym are 2851 and 2920 cm−1 for the monolayers initially assembled on Au (C11 and C16), which confirm that the molecular layer is intact after FCL. For the monolayer assembled on Si (C′11), the νsym and νasym features have broadened and grown asymmetric in spectral shape so it is difficult to identify the peak position of the vibrational transitions. In the bilayer molecular junctions of C11 + C′11 and C11 − Cu + C′11 (where + denotes the interface where lamination occurred), the peak intensities have decreased and the positions of their C−H stretches are shifted to higher frequencies of 2922 to 2926 cm−1 and 2852 to 2853 cm−1 for νsym and νasym, respectively, which suggests the introduction of disorder into the layer. There is evidence of substantial restructuring of the molecular layers when a second monolayer is formed on top of the Au−SAM−Cu layer,11 which could also give rise to changes in the peak intensity due to IR surface selection rules. The significant change in the magnitude of the observed spectra cannot be explained by reorientation alone. There are other possible physical explanations of the variation in the peak intensities of the bilayer molecular junctions. Peak variations could be due to the Au atoms penetrating the molecular layer, the presence of microscopic gaps within the molecular layer, and/or a void present between the molecular layer and the Au surface. However, we avoid putting too much significance on the changes in peak intensities because we are not confident in the MCT detector’s constant efficiency over that time period when the different samples were made and measured. The CO stretch at around 1700 cm−1 is not visible in any of the FCL junctions, but we have previously been able to detect the carbonyl stretch in C11 and C16 junctions that were formed under milder conditions of 0.8 MPa and 90 °C.49 Nonetheless, the pb-RAIRS data shows that a molecular layer is present after mono- and bilayer molecular junction formation by FCL and thus a metal/molecule/semiconductor structure is created. To demonstrate the utility of FCL to make molecular electronic junctions, we report the electrical response of the junctions and the corresponding Au−Si control in Figure 6. The electrical characterization of the molecular junctions was accomplished by applying the voltage to the top gold electrode (circular pads with diameter of 150 μm) and grounding the bottom silicon substrate (heavily n-type doped, 1019 cm−3). The geometric average and standard deviation of the current density, J, for each molecular junction are displayed in Figure 6,
and the data are derived from molecular junctions fabricated from two different runs (except for C16, which was from one run, and J is in agreement with our earlier results31). The fraction of molecule/Au pads that transfer to H−Si or C′11/Si is about 25 to 40%. Of the ones that mechanically transfer successfully, the ratio of the number of nonshorting junctions to the total number of junctions measured (i.e., yield) is 100%. For each sample, at least 20 different pads (or junctions) were measured and at least 60 I−V traces were obtained for each type of molecular junction. The variance of the I−V traces between the same samples from different runs is negligible when compared to the variation within the junctions fabricated from the same sample run. As seen in Figure 6, J is attenuated as the molecular length increases. For the C16 monolayer junctions, we observe that J at 0.5 V is the same order of magnitude as reported for other aliphatic large-area junctions.31,50 To a first approximation, charge transport through a saturated molecule is described by nonresonant tunneling where the magnitude of the current is dependent on the electrodes and molecular orbitals. The current decays exponentially with increased molecule length (I = I0e−βd), and the exponential term is often reported via a β parameter that is a structurally dependent tunneling attenuation factor. On the basis of the data shown in Figure 6, we find that the current through a junction with a C11 monolayer is about 7 times larger than a junction with C16 and about 5000 times larger than a junction with C11 + C′11 (C22) (at +0.3 V). The attenuation of the current can be estimated for a variety of β values51,52 to see how it compared to our results. We estimate that the current ratio through a C11 molecule to a C16 molecule is expected to be between 12 (for β = 0.5 per carbon) and 150 (for β = 1 per carbon) times greater whereas the ratio of C11 to C22 (C11 + C′11) is expected to be between 250 (for β = 0.5) to 60 000 (for β = 1) times greater. Thus, we can infer that our experimental β parameter lies closer to 0.5 than 1, the value expected for saturated aliphatics. To determine the experimental β parameter from our data, we adapted the method introduced by Engelkes and coworkers51 and compared it to other published results reviewed in ref 52. Using the geometric average I−V data of each sample, the data were linearly fit in the range of −0.3 to +0.3 V where 2088
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the Cu bilayer devices, the Cu atoms incorporated at the carboxylate interface can provide an intermediate electronic state. In all bilayer junction devices, it is likely that the carboxylate interface is poorly defined and defects are present at this interface. These defects can provide intermediate electronic states within the bilayer molecular junctions (even without Cu atoms). Thus, sufficient intermediate electronic states could be present to enable the sequential tunneling process in carboxylate bilayers with and without Cu atoms. Because the current density through our bilayer devices does not scale with β = 1 and the Cu atoms within the junction do not actively influence the electronic properties, it is clear that charge -transport mechanisms other than simple nonresonant tunneling must be considered.
the inverse of the slope is the average resistance. Thus, our data follow a trend with a β parameter of 0.6 ± 0.1 per carbon atom. In ensemble-scale devices, there have been reports of experimental β parameters that are less than unity.28,53 In a further step, we determined the average resistance per molecule (Rmol) for each sample (by using the grafting density of molecules onto Au estimated by XPS). We find that the resistance per molecule (Rmol) is at least (6.5 ± 1) × 109 MΩ (as determined for C11). On the basis of data that Akkerman and de Boer have compiled,52 our large Rmol is consistent with large-area Hg measurements. In our junction fabrication method, we expect to form two chemisorbed contacts; however, our Rmol value more closely matches trends reported in ref 52 attributed to the presence of an extra tunneling barrier or a poorly conducting layer in the molecular junction. Alternatively, as recently pointed out by Whitesides et al., the extracted β value is very sensitive to how the data are mathematically treated,36 and we can obtain a significantly (and artificially) larger β (∼0.8 per carbon) and Rmol of at least 2.7 × 108 MΩ if we use the arithmetic mean. Two key trends stand out in the electrical data presented in Figure 6. First, as the effective molecular length of the junction increases, the current density reaches saturation, with J obtained from the C11 + C′11 bilayer, C11-Cu + C′11 bilayer, and C16 + C′11 bilayer equivalent within the standard deviation. Second, the current density is equivalent for the bilayer structures with and without the incorporated Cu. There could be several reasons that we observe these two trends in the current density. First, the current density is quite small and the monolayers behave as an effective dielectric. In this thick dielectric regime, any alternate resistance channels can dominate the charge transport. For example, in previous work Stein and co-workers observed that series resistance due to trapped solvent dominated in small junction areas, making J insensitive to the molecular length.54 A second possible cause for the observed J in the bilayer junctions could be that transport is dominated by a single barrier and the copper ions are decoupled from this transport pathway. Within the bilayer junction, the carboxylic acid head groups (with and without Cu atoms) likely provide an abrupt discontinuity for electron tunneling when compared to a single, long molecule (e.g., C22) within the same junction. Galperin and co-workers simulated I−V characteristics of Hg/S(CH2)nCH3 + H3C(CH2)mS/Au bilayer molecular junctions with potential drops at the molecule−contact interface (i.e., Hg/molecule and molecule/Au).55 Simulated current with the introduction of a second potential at the molecule−molecule interface was found to affect J substantially.55 The Cu ions may not be integral in the charge transport across the junction because of a lack of electronic coupling between the Cu atoms and the carboxylic acid head groups, in agreement with our electrochemical results. A third possible understanding of the observed current density trends would be a sequential electron-transport mechanism. Sequential tunneling requires the electrons to tunnel through two barriers via an intermediate allowable electronic state, and the electron transmission through each barrier is independent of the other.56,57 Each of the molecular layers attached to an electrode (e.g., C′11/Si and C11/Au) could be viewed as an independent potential barrier. For the same total tunneling length, it is expected that there is higher current (or conduction) through a double-barrier device (i.e., sequential tunneling) than through a single-barrier device. In
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CONCLUSIONS In summary, we have demonstrated the versatility of FCL in forming both monolayer and bilayer molecular electronic junctions. We utilized carboxylic acid-terminated monolayers preformed on two separate substrates prior to FCL. Because the junctions were formed from monolayers optimized on the electrodes prior to junction formation, these molecular electronic architectures are more highly ordered than complementary structures built solely from the bottom electrode. Electronic properties of different monolayers and bilayers display an exponential trend until very thick bilayer junctions are formed where the current saturates and are indicative of the presence of another transport mechanism aside from nonresonant tunneling. Incorporating redox-active copper atoms into the bilayer junction does not enhance or alter the charge transport through the molecular layer, possibly because of (1) there is a lack of electronic overlap between the Cu atoms and the carboxylic acid head groups and (2) the potential barrier inherent in the bilayer junction is larger or indistinguishable from what the Cu atoms introduce. With FCL, we are able to (1) fabricate large-area silicon−based solidstate molecular electronic junctions, (2) incorporate multiple molecular layers and redox centers to tailor functionality, (3) modify the device area and electrical properties, and (4) provide stability for long-term metrology and technological applications. We have expanded the utility of FCL, and it is paving the way for the future design and engineering of complex molecular junctions that may yield different molecules or materials for added device functionality and capabilities.
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ASSOCIATED CONTENT
S Supporting Information *
Additional XPS spectra and analyses, spectroscopic ellipsometry, and contact angle measurements of the samples mentioned. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.P. acknowledges support from the NRC-NIST ARRA program. We thank Drs. Lee Richter for fruitful discussions and Marlon Walker for assistance with fluorinated monolayer 2089
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formation. The devices and samples were fabricated in part at the NIST Center for Nanoscale Science and Technology. The identification of commercial equipment or vendors in the Experimental Methods section is not intended to imply recommendation or endorsement by NIST nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose.
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