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Aug 10, 2014 - Attachment of a Diruthenium Compound to Au and SiO2/Si Surfaces by “Click” Chemistry. Sujitra Pookpanratana*† ... *E-mail sujitra...
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Attachment of a Diruthenium Compound to Au and SiO2/Si Surfaces by “Click” Chemistry Sujitra Pookpanratana,*,† Iulia Savchenko,§ Sean N. Natoli,§ Steven P. Cummings,§ Lee J. Richter,‡ Joseph W. F. Robertson,† Curt A. Richter,† Tong Ren,§ and Christina A. Hacker*,† †

Semiconductor and Dimensional Metrology Division and ‡Materials Measurement Science Division, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20899-1070, United States § Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: Fabrication of electrodes with functionalized properties is of interest in many electronic applications with the surface impacting the electrical and electronic properties of devices. We report the formation of molecular monolayers containing a redox-active diruthenium(II,III) compound to gold and silicon surfaces via “click” chemistry. The use of Cucatalyzed azide−alkyne cycloaddition enables modular design of molecular surfaces and interfaces and allows for a variety of substrates to be functionalized. Attachment of the diruthenium compound is monitored by using infrared and photoelectron spectroscopies. The highest occupied molecular (or system) orbital of the “clicked-on” diruthenium is clearly seen in the photoemission measurements and is mainly attributed to the presence of the Ru atoms. The “click” attachment is robust and provides a route to investigate the evolution of the electronic structure and properties of novel molecules attached to a variety of electrodes. The ability to attach this redox-active Ru molecule onto SiO2 and Au surfaces is important for the development of functional molecular devices such as charge-based memory devices.

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INTRODUCTION The functionalization of surfaces is a key component of many emerging device applications,1 including chemical sensing,2 photocatalysis,3,4 and organic photovoltaics.5 The ability to tether a variety of molecules to different surfaces while preserving the electronic properties of both the substrate and the molecules enables many of these paradigms to realize unique functionality. In molecular and organic-based electronics, the goal has largely been to impart electronic function and applications based on the composition and properties of a unique molecule and integrate them into electronic devices. Monolayer-based research has been limited to studying solution-processable molecules with limited functional groups and reaction sites. Recent work has highlighted the importance that redox functional groups can have on the electronic properties. Ru2(2-anilinopyridinate)4−(CC−C6H4−C CH) (henceforth referred to as Ru2) has multiple reversible redox states attributed to the ruthenium atoms and has a calculated band gap of about 1.28 eV.6,7 Earlier studies of Ru2 molecular variations have observed enhanced conductivity through the redox molecular layer8 and demonstrated multiple redox states9 applicable to nonvolatile memory and energy level alignment between the redox molecular orbitals and the electrode Fermi level leading to enhanced resonant tunneling.10 Molecular electronic junctions with ferrocene were shown to rectify the current 90 times at negative bias.11−13 This rectifying © 2014 American Chemical Society

property has been reported to be affected by the molecular structure, specifically odd−even chain length giving rise to the slight differences in the van der Waals forces.13 Making high quality monolayers of complex molecules has been a significant limitation in understanding the electronic properties on molecular length scales. Adding functionality for facile thiolbased self-assembly to a large, complex molecule is a chemical synthetic challenge. The progress of Cu-catalyzed azide−alkyne cycloaddition (CuAAC) “click” chemistry has provided a powerful method for attaching a variety of molecules with a plethora of diverse compositions and has opened up the field of interface and surface engineering that has been previously limited by the constraints in organic synthesis. Click chemistry-based reactions on surfaces are attractive because they bring together relatively smaller molecules that can be reacted to form a single unit utilizing chemically selective compounds and resulting in a high yield.14 These reactions occur under mild conditions and are tolerant to different chemical environments. Click chemistry builds upon a molecular framework that is already in place; therefore, complex molecular layers can be built readily from the bottom-up approach, thereby solving many synthetic and Received: April 30, 2014 Revised: August 8, 2014 Published: August 10, 2014 10280

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Scheme 1. Drawing of Ru2 (Upper Left) and Idealized Azide Molecular Layer on Au or SiO2/Si (Center), and the Idealized Molecular Layer after “Click” Chemistry Attachment of Ru2 (Right)a

a

One of the resonances of the −N3 group is depicted before the reaction.

observation of the redox electronic properties appears to be related to the device structure used to acquire the data with the degree of coupling36 between the molecule and electrode the critical factor impacting whether redox peaks10 or rectification is observed. Here, we present studies on the evolution of electronic structure and properties of monolayers on metallic and semiconducting surfaces as the monolayers proceed from an aliphatic chain to incorporate a redox active species by utilizing click chemistry. We begin with the characterization of monolayers of aliphatic chains terminated in an azido group on gold and silicon oxide surfaces. The diruthenium(II,III) tetrakis(2-anilinopyridinate) molecules with alkyne functionality successfully attached onto Au and SiO2/Si surfaces by using a click chemistry attachment route (depicted in Scheme 1) as confirmed by infrared and X-ray spectroscopies. The electronic states of the aliphatic azide and “clicked on” Ru2 surfaces as investigated with ultraviolet photoelectron spectroscopy and preliminary electrical measurements suggest this is a robust route to access the Ru2 electrical properties. Our results demonstrate the ability to probe the unique electronic properties of Ru2 molecules that have been covalently attached to different surfaces and have implications for designing molecular, organic, or hybrid device structures for specific functionality.

solubility challenges that have previously limited research. The CuAAC reaction results in a covalent heterocyclic ring formed by a terminal azide (−N3) functionality of one molecule and a terminal alkyne functionality of the other molecule. By utilizing CuAAC click chemistry, engineering molecular surfaces are broken into smaller building blocks while applying the following constraints: (1) a substrate functionalized by a molecular layer with an azide (alkyne) termination and (2) the intended molecule of interest contains an alkyne (azide) terminal functionality. CuAAC click chemistry has the added advantage for molecular electronics in that the molecular layer can be directly customized, enabling a researcher to tailor the density, composition, and conformation with greater ease than previous self-assembly methods. This approach has been used to attach a variety of molecules onto diverse surfaces, for example Au,15,16 SnO2,4,17 Si,18 and diamond.19 Building on such successful approaches, we have pursued the clickchemistry attachment of an electrochemically active diruthenium compound to electrodes, specifically Au or SiO2/Si for molecular electronic applications. Organometallic molecules possess unique chemical and electronic properties that offer great potential when integrated into electronic devices with applications including medicinal purposes,20 energy storage/conversion devices,21 and chemical sensors.22 Ruthenium oxide nanoparticles23 and rutheniumbased molecular catalysts24 oxidize water into molecular oxygen. Ru compounds are also catalysts in chemical production such as in the epoxidation of alkenes.25 Hydrogenase enzymes found in nature contain structures that consist of dinuclear metals and synthetic attempts have been made to mimic their design as molecular catalysts for hydrogen synthesis.26 Recent interest has focused on the attachment of such molecular catalysts to semiconductor surfaces.3,27 The electronic properties of redox active monolayers are of interest because the redox potentials are close in energy to the electrode work function28 and can be designed to tune the molecular orbitals with respect to the Fermi level,29 thus enabling charge transport for applications ranging from molecular spin to molecular memory. The electronic properties of redox active monolayers have been studied by a variety of mechanisms ranging from scanning tunneling-based molecular reports of bistable conducting states30,31 to metal−molecule−metal junctions demonstrating rectification11,12 or Coulomb blockade behavior32 and applications in flash memory devices.33−35 The

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EXPERIMENT

Note: the identification of commercial equipment or vendors in this 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. Molecules. The diruthenium(II,III) compound Ru2(ap)4(CC− C6H4−4-CCH), where ap is 2-anilinopyridinate (shown in Scheme 1), was synthesized following literature procedures.6 11-Azidoundecyltrimethoxysilane (AUS) was purchased from Prochimia (Poland) and used as-received. 11-Azido-1-undecanethiol (AUT), copper(II) tetrafluoroborate hydrate (CuBF4), tris[(1-benzyl-1H-1,2,3-triazol-4yl)methyl]amine (TBTA), sodium L-ascorbate, dimethyl sulfoxide (DMSO), toluene, and chloroform were purchased from SigmaAldrich and used as received. Substrate Preparation. Si wafers and Au-covered Si wafers were purchased from commercial vendors. Au substrates were cleaned by isopropanol (IPA) and UV/O3 treatment (10 min), followed by a deionized (DI) water rinse. All molecular layers were prepared in a N2filled glovebox. AUT was self-assembled onto Au in a 1 mmol/L solution in ethanol overnight, rinsed in ethanol, and dried in N2. 10281

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The υasym(N3) is centered at 2111 and 2096 cm−1 for AUT/Au and AUS/SiO2/Si, respectively. The υasym(C−H) and υsym(C− H) are 2927 and 2854 cm−1, respectively, for both azide molecular layers on Au and SiO2/Si. The peak positions of the C−H stretches are in agreement with the trend for molecular layers of shorter alkyl lengths which tend to have stretching vibrations consistent with a liquid-like state (i.e., more disordered).38 For AUS/SiO2/Si, the relative intensity of the azide stretch compared to the C−H stretches is different from AUT/Au. The asymmetric azide stretch intensity on Au is about 3 times larger than on SiO2/Si; this peak does not scale proportionally with the C−H intensities. Both the AUS and AUT are attached on one side of a substrate, thus comparing the relative absorbance features should be valid after correcting for the physical geometry differences of the IR measurement between Au surface reflection and Si transmission. In the ppolarized reflection measurement, only the εzz (where ε is the dielectric constant) component of the molecular layer will be evident in the spectra. While in the p-polarized transmission through Si, both the εzz and εxx components will contribute making any in-plane anisotropy also evident in the spectra. This difference was confirmed by modeling a spectrum from an identical monolayer on Au and Si substrates by using WVASE software to show the differences in spectral intensity due to the measurement geometries. The modeling results showed the N3 vibration is at least 3 times more intense than the ν(C−H)asym when reflecting off of Au. For an identical monolayer on Si, measured in a p-polarized transmission geometry, the N3 stretch and ν(C−H)asym stretch are nearly identical in intensity. Comparison of the experimental and modeled spectra intensity indicates the N3 in the AUT/Au sample has more z-component contribution than the AUS/SiO2/Si sample shown in Figure 1 (black spectra) with the spectra from both surfaces indicative of a monolayer with terminal azide functionality present and accessible for Ru2 attachment by click chemistry. After undergoing the click reaction, the molecular structure on the surfaces show notable changes as indicated by the IR results (Figure 1, blue spectrum for the Au surface, red spectrum for SiO2/Si surface). First, the integrated azide (υasym(N3)) peak intensity decreases, which is direct evidence that a significant number of the azide sites have reacted and indirect evidence for the attachment of Ru2. Second, three additional spectral features are clearly visible for the clicked Au surface that were not present before (blue spectrum in Figure 1) and are labeled (1) (1438 cm−1), (2) (1475 and 1490 cm−1), and (3) (1592 and 1602 cm−1). We attribute these spectral features to the Ru2 molecule. The IR spectrum obtained from Ru2 in KBr is shown in Figure 1 (in green, topmost spectrum and rescaled) with three features at the same energetic positions that are associated with various vibrations in the anilinopyridinate moiety. The observation of Ru2 spectral features after the click reaction on the Au surface provides direct evidence that the Ru2 molecules are attached to the Au surface. The peak intensities of the methylene stretches obtained from the AUT samples do not significantly change after the click reaction, indicating the original azide-containing monolayer remains robustly attached to the Au surface after the slightly exothermic click reaction. IR spectra indicate that the click reaction has succeeded in attaching Ru2 through the presence of new spectral signatures and the decrease in azide features on the reacted surfaces. Direct evidence of Ru2 attachment by click chemistry to both Au and SiO2/Si surfaces is further confirmed by XPS. The N 1s

Double-side polished (DSP) p-type Si (111) (ρ = 10−15 Ω·cm) substrates were rinsed with acetone and isopropanol and then placed in the UV/O3 treatment (25 min). The oxide thickness on these substrates is about 3 nm, as measured from spectroscopic ellipsometry. AUS was grafted onto SiO2/Si by heating a 5 mmol/L solution in toluene at 90−100 °C for 2 h, then sonicated in a fresh toluene solution for 5 min, and dried by N2 gas. Note: only one side of the SiO2/Si substrate was fully exposed to the AUS. Click Chemistry Attachment. The Ru2 was attached to azideterminated Au and SiO2/Si surfaces in a DMSO:H2O (3:1) solution that contained: 2 mmol/L of Ru2, 2 mmol/L CuBF4, 2 mmol/L TBTA, and 15 mmol/L Na ascorbate. The samples were immersed in the “click” solution for 3 h, and the substrates were rinsed with H2O, sonicated in CH3Cl and IPA for 1 min each, and dried with N2. The Ru2 attached to AUT/Au and AUS/SiO2/Si substrates is idealized in the right side of Scheme 1. Physical Characterization. Molecular films were characterized by Fourier-transform infrared (FTIR) spectroscopy and photoelectron spectroscopies. Molecular films on Au and Si were characterized by using either p-polarized grazing angle (80°) reflection absorption IR (RAIRS) or Brewster angle transmission IR, respectively. A mercury cadmium telluride (MCT) and a deuterated triglycine sulfate (DTGS) IR detector were used for reflection and transmission measurements, respectively. Chemical analysis was done by using X-ray photoelectron spectroscopy (XPS; monochromatized Al Kα excitation) and electronic surface structure by UV-excited photoelectron spectroscopy (UPS; He I excitation). In the UPS measurements, a −5 V bias was applied to the sample and the Fermi edge of a clean Au surface was used for energy referencing. Electrical Measurements. Electrical measurements of Ru2 clicked onto Au and Si surfaces were performed by using a custom-built eutectic gallium indium (E-GaIn) setup. The E-GaIn is dispensed from a syringe and contacts the molecular layer where electrical bias is applied, and the Au substrate is grounded. The area of the junction is defined by the E-GaIn tip and is estimated by using an optical camera with a calibration reference. Typical E-GaIn tips are estimated to have an area of about 9500 μm2 (assuming a circular contact).

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RESULTS AND DISCUSSION Prior to the attachment of the Ru2 molecule, the azideterminated monolayer was formed onto SiO2/Si and Au surfaces as required for CuAAC-mediated attachment. The azide-terminated surfaces are confirmed by p-polarized FTIR measurements (in Figure 1) which show the presence of the asymmetric N 3 stretch 37 and the methylene stretches representative of the molecular backbone (see black spectra).

Figure 1. IR spectra of pre-“click” surfaces containing azide-terminated AUT and AUS monolayers (in black) and after “click” Ru2 surfaces on Au and SiO2/Si substrates. Ru2 in a KBr pellet is shown as reference (topmost, in green). 10282

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Figure 2. XPS spectra of the (a, b) N 1s and (c, d) C 1s and Ru 3d5/2 photoemission lines of surfaces before (in black) and after (in blue on Au, in red on SiO2/Si) click attachment of Ru2.

between the Si substrate and the molecular layer. As in the IR measurements, the presence of the azide on both SiO2/Si and Au surfaces is confirmed by the XPS data in Figure 2. The AUT/Au and AUS/SiO2/Si surfaces undergo the click chemistry reaction with Ru2, and the attachment of Ru2 is directly confirmed by the presence of Ru atoms. The Ru 3d5/2 photoemission peak (Figure 2c in red and Figure 2d in blue) is clearly seen; the Ru 3d3/2 XPS line coincides with the main C 1s feature and therefore is not discernible. Note: the Ru 3p XPS spectral lines are also detected. Since the Ru 3d5/2 energy level is very close to the C 1s level (and measured in the same energy window), we can accurately determine the amount of Ru2 that has clicked onto these surfaces because the transmission function of the spectrometer is assumed to be constant in this energy range. The ratio of Ru to C atoms is estimated by their integrated intensities and have been corrected for their respective photoionization cross sections.45 The C 1s and Ru 3d XPS spectra were decomposed into Voigt line shapes, and the peak area of Ru 3d is compared to that of C 1s. The effective C 1s area was corrected by subtracting the unresolved Ru 3d3/2 contribution (2/3 Ru 3d5/2 area, as determined by the spin orbital peak ratio). The “clicked” surfaces have a C:Ru ratio of about 60:1 (± 10%) and 68:1 (± 10%) on clicked Au and SiO2/Si surfaces, respectively. The Ru2 molecule (formula: C54H42N8Ru2) has a C:Ru of 27:1. For the click attachment of Ru2, the azide molecular layer has to be taken into account which will contribute 11 carbons per molecule and so the

and C 1s XPS spectra obtained from the preclicked aliphatic azide-terminated surfaces are shown in Figure 2 (in black). The high binding energy N 1s feature at 404−405 eV is representative of the center positively charged N atom of the azide, and the broad feature at 400 eV is representative of the other two N atoms in the azide group.39 Quantitative analysis of the XPS spectra indicates the AUS and AUT molecular layers are present as a monolayer based on the substrate signal attenuation. The estimated atomic ratios of N:Si, N:Au, and S:Au are consistent with the AUT layer having a slightly higher packing density (see Supporting Information). The ratio of the positively charged N (unique to azide functionality) to the total amount of N atoms present (i.e., [N 1s at 404 eV]/[N 1s all]) on AUT/Au and AUS/SiO2/Si are 0.22 ± 0.02 and 0.07 ± 0.01, respectively. The finding that AUT has a higher density of azide sites than AUS is in qualitative agreement with the IR results. Neither AUT/Au nor AUS/SiO2/Si has the ideal 0.33 for [N 1s at 404 eV]/[N 1s all], which indicates that not all of the molecules are properly azide terminated. This is consistent with previous studies often reporting N ratios less than 0.3340−44 attributed to some azide decomposition due to their highly reactive nature. The alkyl backbone of the azideterminated molecular layer is also confirmed by the main C 1s peak in the XPS spectra (Figure 2c,d, in black). The peak positions representative of the molecular layer (C 1s and N 1s XPS lines) on SiO2/Si have larger binding energies likely due to modest amount of charging from the thin SiO2 layer that is 10283

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the AUT HOSO leading edge.) The HOSO position of AUT/ Au is in agreement with previous reports of alkane-based SAMs on Au,48,49 while the HOSO position of AUS/SiO2/Si is closer to the Fermi energy level than previously reported UPS measurements of other alkane-based molecular layers on Si.50 The modified work function of the azide molecular layer on Au and SiO2/Si is 3.9 ± 0.1 and 2.9 ± 0.1 eV, respectively. Prior to molecular attachment, the Au and SiO2/Si measured work functions of ex situ prepared surfaces are 5.0 ± 0.1 and 4.1 ± 0.1 eV, respectively. The surface work function of each prepared surface is also plotted in Figure 3b to easily illustrate the work function evolution. This indicates that the azidebearing molecular layer has induced a dipole (ΔΦMO for AUT/ Au) of about −1 eV on both Au and SiO2 surfaces. This is consistent with reports of other azide-bearing molecules which have a positive dipole51−53 and with the resonance structure depicted in Scheme 1. Upon undergoing the “click” reaction, the surface electronic structure of Ru2 shows significant changes as seen in Figure 3a (in blue for the Au surface, and red for Si). A new spectral feature is clearly present that is at 1.6 ± 0.05 and 1.9 ± 0.05 eV below the Fermi level (indicated with a line in Figure 3a) for the Au and SiO2/Si substrates, respectively. Since it is seen on both Ru2 clicked surfaces, we ascribe this feature to the HOSO of the Ru2. Based on earlier density functional theory-based calculations,9 the ruthenium atoms and pyridinates contribute to the HOSO intensity seen in the UPS spectrum. This assignment is in agreement with calculations from other groups that place the Ru 4d energy level of Ru-containing organic molecules at about 1.5 eV below the Fermi energy.54 After the Ru2 molecules are attached to substrates by click chemistry, we find that the unique electronic properties of the Ru2 are still accessible at the surface and the HOSO is favorably aligned close to the Fermi energy. In addition to having the HOSO level closer to the Fermi energy, the work function of the “click” surfaces has changed (Figure 3b). After the attachment by “click” chemistry, the work function of Ru2/AUT/Au and Ru2/AUS/SiO2 are 4.0 ± 0.05 and 3.7 ± 0.05 eV, respectively. The experimental findings related to the electronic energy levels and dipoles with respect to the as-prepared Au and SiO2/ Si surfaces and the surfaces before and after “click” attachment of Ru2 are summarized in a simplified energy diagram in Figure 4. If we consider the Ru2 molecular layer separate from the AUS or AUT layer (thus having an organic−organic bilayer), this suggests that there is a dipole at the organic−organic interface (ΔΦOO) between AUT and Ru2 of 0.13 eV. Notably, both AUT and AUS surfaces show an increase in work function after the click attachment of Ru2 and start to converge to a similar value, and the HOSO of the Ru2 vary by 0.3 eV (with respect to EF) in spite of the large difference in the surface work functions between the Au and SiO2/Si surfaces and AUT/Au and AUS/SiO2/Si surfaces. The ionization energy of the clicked on Ru2 can be estimated by adding the work function to the HOSO onset (with respect to EF), and the result determined from both Au and SiO2/Si substrates is 5.6 eV ± 0.1, confirming that the clicked Ru2 surface on both substrates are identical from a surface electronic perspective. The electronic molecular properties such as the ionization energy are expected to be independent of the substrate. We do note that the surface is not entirely representative of Ru2 by itself as the (former) azide molecular layer and residual components from the click chemistry reaction (i.e., copper catalyst) are detected in the photoelectron spectroscopy measurement. In the UPS experi-

maximum C:Ru is 32.5:1 (assuming every alkyl molecule is azide terminated and every azide site reacts). It is estimated that the Ru2 molecule is “clicked” onto one out of every five AUT molecules (on Au) and one out of every seven AUS molecules (on SiO2/Si) on the surface. This ratio of clicked Ru2 is likely to be an upper bound since the C 1s electrons from AUT and AUS are somewhat attenuated (i.e., electron contribution from below the surface). Keeping the steric hindrance of the bulky Ru2 molecule in mind, we do find this to be very good packing density from click chemistry. The success of the “click” attachment of Ru2 is also observed in the N 1s XPS spectra. On both surfaces, the integrated intensity of the N 1s XPS line increases by at least 45% and the width of the low binding energy peak broadens after click chemistry, and this increase is from the N atoms present in Ru2. The broadening provides evidence that there are additional nitrogen chemical states present. The N 1s was not quantitatively fitted as there could be (at most) eight curves implemented in the fit where two contributions from the Ru2, three from the reacted triazole ring, and three from any unreacted azide. These results are in agreement with our earlier finding that Ru2 clicks onto one for every 5−7 azide sites. The ratio of clicked on Ru2 to aliphatic molecular layer (AUS or AUT) is comparable on both Au and SiO2/Si surfaces. Based on the direct spectroscopic evidence, Ru2 is covalently attached via CuAAC click chemistry onto both Au and SiO2/Si surfaces. The electronic properties of the molecular layer immobilized on a substrate are important to investigate since the nature of the molecule−electrode coupling has been linked to the resulting electronic properties.46,47 Here, the surface electronic structure was investigated for the azide-terminated surfaces and the Ru2-terminated surfaces by probing the occupied states with surface-sensitive UPS. In Figure 3a (black spectra), the leading edges of the highest occupied molecular or system orbital (HOMO or HOSO) of AUT/Au (denoted with an asterisk in Figure 3a) and AUS/SiO2/Si are about 4.5 ± 0.05 and 3.3 ± 0.05 eV below the Fermi energy, respectively. (See the Supporting Information for a detailed discussion on identifying

Figure 3. UPS spectra of the (a) highest occupied molecular/system orbital (HOSO) region and (b) surface work function as a function before azide-molecular layer, pre- and postclicked Ru2 surfaces. EF denotes the Fermi energy level as measured from a clean Au surface. 10284

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Figure 4. Energy level diagram of the Ru2 molecular layer on (a) Au and (b) SiO2/Si derived from experimental results in Figure 3. In this schematic drawing, it is assumed that each layer does not strongly interact with another layer (see scenario 1 in the text). Shown are the work function, interface dipole (Δ), and distance between the HOMO/HOSO and the Fermi energy (EF). Evac is the vacuum energy level.

Figure 5. Energy level diagram of the Ru2 molecular layer on (a) Au and (b) SiO2/Si derived from experimental results in Figure 3 as explained in scenario 2 in the text. Here, the initial substrate is considered to be the inorganic layer (Au or SiO2/Si) with the azide molecular layer. Shown are the work function, interface dipole (Δ), and distance between the HOMO/HOSO and the Fermi energy (EF). Evac is the vacuum energy level.

(with respect to EF) of the click surfaces with respect to the initial electrode work functions suggests the surfaces are likely in the Fermi level pinning regime. The presence of Fermi level pinning has been reported in a bilayer organic system55 and could be prevalent here rather than vacuum level alignment. To properly confirm the existence of Fermi level pinning, the Ru2 attachment by click chemistry should be studied on substrates of varying work functions,56 but this is beyond the scope of our work presented here. The interaction between the Ru2 and AUS or AUT layer is a covalent triazole ring that links the Ru2 molecule to the former azide molecular layer, and so there are multiple scenarios to consider for energy level alignment and to properly understand the electronic structure evolution in this system which have implications for strategies and limitations of interface engineering with molecular layers. The first scenario considers only weak (e.g., van der Waals) interaction between the organic layers where there are three discrete layers: inorganic substrate/ SAM/Ru2 as mentioned earlier (see Figure 4), and this scenario also reflects the measurement order of the samples. This scenario is not likely an accurate representation of the electronic structure present in our results because of the

ments, the inelastic mean free path of the photoelectron is (at most) about 0.1−0.3 nm, and thus the derived HOSO level and work function values are most heavily influenced by the Ru2 molecule that is at the surface. The electronic properties of the click chemistry interface presented here are more complicated than what is commonly reported and studied of organic−organic interfaces which consist of weak physical interactions at the interface (i.e., van der Waals interaction). In the field of organic electronics, controlling the work function of the substrate to tune the holeor electron-injection barrier is important, and so understanding the limits of modifying work functions for efficient charge injection is vital. In the vacuum level alignment regime (Schottky−Mott limit), the work function of the thin organic layer on an electrode varies linearly (slope = 1; S = d(Φorganic/substrate)/dΦsubstrate) with the unmodified electrode work function or the hole-injection barrier (HIB) varies linearly with the unmodified electrode work function (S = d(HIB)/ dΦsubstrate). Whereas in the Fermi level pinning regime, the work function of the organic layer on an electrode or the holeinjection barrier remains a constant value regardless of the substrate work function. The small shift in the HOSO distance 10285

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Figure 6. Energy level diagram of the Ru2 molecular layer on (a) Au and (b) SiO2/Si derived from experimental results in Figure 3 as explained in scenario 3 in the text. Here, only the final molecular layer (after click attachment) is considered with respect to the initial substrate. There is no distinction from the initial azide-containing molecular layer. Shown are the work function, interface dipole (Δ), and distance between the HOMO/ HOSO and the Fermi energy (EF). Evac is the vacuum energy level.

former azide molecule and Ru2 into the one organic layer. Studying the electronic structure evolution is important because of the increasing use and reliance of SAMs to modify substrate work functions in order to decrease the hole- or electroninjection barrier into an organic semiconductor (i.e., decrease the difference between the electrode Fermi level and the occupied or unoccupied frontier orbitals of the organic semiconductor). It is clear based on our results and others56,57 that SAMs can be used to engineer favorable energy level alignment between a substrate and a molecular solid, but there are also limitations to this strategy that must be considered. Stepwise modification and measurement of the electronic properties provide insight into the actual circumstances (interaction and bonding, initial work function values, etc.) that will lead to improved charge carrier injection. There is still debate within the organic electronics community about models describing the interface electronic structure of strongly interacting molecular interfaces, but it is clear that all of these scenarios should be considered to properly describe the electronic structure of these samples and samples that are similarly constructed by strong interaction or chemisorption. Preliminary electrical measurements of the Ru2 molecular layer obtained by using eutectic GaIn (E-GaIn) as a solid-state, soft, top electrical contact were performed, and data are shown in Figure 7. The current−voltage characteristics were measured for the AUS/Au surface (before “click” attachment) and Ru2 attached (after “click” attachment) using the same E-GaIn formed tip. The current is significantly attenuated (in blue, Figure 7) within the junctions containing Ru2 when compared to the AUT-only junctions (in black open circles, Figure 7). The attenuation of the current is not surprising since Ru2 contributes to the overall molecular length of the molecular layer and thus increases the tunneling distance for electrons from the E-GaIn tip to the Au substrate (see the right portion of Scheme 1 for the idealized sample depiction). For example, the current at −0.2 V is 0.34 and 0.008 μA for the AUT-only and Ru2-containing junctions, respectively. The bias applied in this measurement was limited to a range of −0.5 to 0.5 V due to a maximum current compliance to preserve the molecular layer (set to 10 mA). Additional experiments are currently ongoing to limit the device area (and thus lowering the measured

chemical bonds that forms between the original azide-bearing molecular layer and the Ru2 are covalent in nature giving rise to through-bond charge transfer at the organic−organic interface. An alternative electronic structure considers the case where the monolayer strongly interacts with the substrate such that the “substrate” combines both the inorganic layer (Au or SiO2/ Si) and the original azide molecular layer together, and the changes of the electronic energy levels are observed after the inclusion of the Ru2 organic layer (illustrated as scenario 2 in Figure 5). In the second scenario, the small interface dipole (Δ = 0.1 eV) between the Ru2 layer and the azide−substrate suggests that there’s likely vacuum level alignment on the AUT/Au substrate. While in the case for attaching Ru2 onto the AUS/SiO2/Si substrate, there is a large interface dipole (Δ = 0.8 eV). The surface work function values of the starting substrates in scenario 2 are very dissimilar (see Figure 3b, “before click” values or Figure 5), and yet the HIB vary by only 0.3 eV on the two surfaces which suggests that the on the AUS/ SiO2/Si the surface is Fermi level pinned. The final scenario is consistent with all three layers strongly interacting, and the energy levels are depicted as an inorganic substrate with one organic layer that contains both the original azide-bearing layer and the Ru2 (depicted as scenario 3 in Figure 6). The third scenario is consistent with the presence of Fermi level pinning since the HOSO position with respect to EF are similar on both substrates (varying by 0.3 eV) even though the work function of the initial substrates vary by almost 1 eV. We find that scenarios 2 and 3 are most likely the valid picture of the electronic structure evolution in the clickchemistry fabricated molecular layers. A similar phenomenon is observed where there is a sharp transition between the vacuumlevel alignment regime to a Fermi-level pinning regime in numerous weakly interacting organic semiconductors deposited onto substrates of varying work functions56 and calculated for an organic semiconductor deposited onto different SAMs.57 The integer-charge transfer model is used to explain this phenomena in weakly interacting and physisorption-formed interfaces. In these click chemistry formed interfaces, the molecular layers are strongly interacting because of the use of chemical bonds to attach the azide layer to the substrate (Au−S or SiO2−O−Si linkages) and the triazole ring linking the 10286

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ACKNOWLEDGMENTS S.P. acknowledges support from the NIST NRC-ARRA postdoctoral associateship. Work at Purdue is supported by the National Science Foundation (CHE 1057621).

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Figure 7. Electrical properties of the molecular layer before (black open circles) and after (blue solid line) Ru2 click attachment. The “before” represents an AUT molecular layer on Au.

current) in order to probe the molecular layer within a larger bias window and to avoid breakdown of the molecular layer (i.e., electrical shorts) as well as integrating the Ru2 molecule into a memory device structure by click chemistry attachment. Thorough investigations of the electronic properties of the Ru2 molecule immobilized to an electrode and integrated into a memory device structure are currently ongoing, and these results will be presented in a future publication.

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CONCLUSION In summary, we have shown that a redox-rich molecule, Ru2, can be covalently attached to Au and Si substrates by utilizing “click” chemistry. This attachment is robust and allows for direct access to the electronic properties of Ru2 molecule as accessed by UPS. The click chemistry route for designing and creating molecular layers has significant impact for the field of molecular, organic, and hybrid electronics as a means for interface and surface engineering of inorganic components within these devices and provides a platform to study the electronic structure of strongly interacting molecular layers. This approach of surface engineering allows for a variety of complex molecules (such as Ru2) of varying electronic function and properties to be attached onto different substrates and allows for facile integration into existing device architectures. Our results have implications for preparing “smart” electrodes by modular fabrication, with the ability to attach complex, bulky molecules of specific chemical and electronic functionality to specific electrodes for a targeted device.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details; Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (S.P.). *E-mail [email protected] (C.A.H.). Notes

The authors declare no competing financial interest. 10287

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