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Metal-free molecular junctions on ITO via amino-silane binding—towards optoelectronic molecular junctions
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 455204 (http://iopscience.iop.org/0957-4484/24/45/455204) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 455204 (8pp)
doi:10.1088/0957-4484/24/45/455204
Metal-free molecular junctions on ITO via amino-silane binding—towards optoelectronic molecular junctions S Sergani1 , Y Furmansky2 and I Visoly-Fisher2,3 1
Department of Chemistry, Ben Gurion University of the Negev, Be’er Sheva 84105, Israel Ilse Katz Institute for Nanoscale Science and Technology, Ben Gurion University of the Negev, Be’er Sheva 84105, Israel 3 Department of Solar Energy and Environmental Physics, Swiss Institute for Dryland Environmental and Energy Research, Jacob Blaustein Institutes for Desert Research, Ben Gurion University of the Negev, Be’er Sheva 84105, Israel 2
E-mail: [email protected]
Received 31 January 2013, in final form 15 August 2013 Published 15 October 2013 Online at stacks.iop.org/Nano/24/455204 Abstract Light control over currents in molecular junctions is desirable as a non-contact input with high spectral and spatial resolution provided by the photonic input and the molecular electronics element, respectively. Expanding the study of molecular junctions to non-metallic transparent substrates, such as indium tin oxide (ITO), is vital for the observation of molecular optoelectronic effects. Non-metallic electrodes are expected to decrease the probability of quenching of molecular photo-excited states, light-induced plasmonic effects, or significant electrode expansion under visible light. We have developed micron-sized, metal free, optically addressable ITO molecular junctions with a conductive polymer serving as the counter-electrode. The electrical transport was shown to be dominated by the nature of the self-assembled monolayer (SAM). The use of amino-silane (APTMS) as the chemical binding scheme to ITO was found to be significant in determining the transport properties of the junctions. APTMS allows high junction yields and the formation of dense molecular layers preventing electrical short. However, polar amino-silane binding to the ITO significantly decreased the conductance compared to thiol-bound SAMs, and caused tilted geometry and disorder in the molecular layer. As the effect of the molecular structure on transport properties is clearly observed in our junctions, such metal-free junctions are suitable for characterizing the optoelectronic properties of molecular junctions. S Online supplementary data available from stacks.iop.org/Nano/24/455204/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
metallic electrodes are expected to increase the probability of quenching of molecular photo-excited states [3], light-induced plasmonic effects [4], or significant electrode expansion under visible light [5], masking the molecular optoelectronic properties. Expanding the study of molecular junctions to non-metallic transparent substrates, such as indium tin oxide (ITO), is vital for the observation of molecular optoelectronic effects. Thermal conductance switching in molecular junctions [6] can also be practically overridden by the thermal expansion of metallic electrodes. Metal-free junctions, where both electrodes are non-metallic, are highly desirable for such applications.
The study of electrical transport through self-assembled monolayers (SAMs) is essential for the miniaturization of molecular electronic and optoelectronic devices. Light control over currents in molecular junctions is desirable as a non-contact input with high spectral and spatial resolution provided by the photonic input and the molecular electronics element, respectively. Molecular junctions commonly utilize thiol assembly on Au, due to Au chemical inertness and the ease of ordered SAM fabrication [1, 2]. Optically addressed junctions require at least one transparent electrode, which is difficult to achieve with metallic electrodes. Furthermore, 0957-4484/13/455204+08$33.00
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The nature of the molecule–electrode contact plays a major role in the measured characteristics of the junction. Molecular junctions comprising SAMs on semiconductors such as Si [7–15] and GaAs [16–19] were studied. In semiconducting electrodes the interfacial energy level alignment with the molecular levels depends on the doping type and concentration and not only on the adsorption chemistry. In addition, a potential drop within the electrode is feasible, and the chemical binding is typically much more stable than the thiol–gold bond. The latter results in less ordered SAMs as adsorbate diffusion to improve ordering is inhibited [7]. ITO is a degenerate n-type semiconductor, with a large band-gap enabling its use as a transparent conductive electrode. It is commercially available as sputtered thin films with nm-scale roughness, and is commonly used in organic optoelectronic devices. Only very few publications describe transport studies of molecular monolayers or single molecules adsorbed to ITO. An optoelectronic study of single molecule junctions of porphyrin–fullerene dyads, bonded to ITO substrate and Au STM probes, has shown photoconductance, suggesting the formation of a long-lived charge separated state on the ITO surface [20]. Junctions of 1-octadecanethiol SAM on pre-patterned ITO, top contacted by evaporated Al electrodes, were also fabricated and studied [21]. Both these examples utilize metal as one of the electrodes. More commonly, SAMs on ITO are used for interface modification in organic electronic devices [22–30], where transport across the SAM is not directly characterized. To the best of our knowledge, the only demonstration of metal-free junctions was of single molecule junctions of dicarboxylic terminated n-alkanes bonded to ITO substrate and ITO coated AFM probes, which showed a similar decay constant as n-alkanes sandwiched between Au electrodes, but different contact conductance [31]. Connecting molecules electrically to an external circuitry is anything but trivial, as it requires reproducible fabrication of nano-gaps without short-circuits. Technological applications of such junctions require them to be reliable, stable and reproducible, properties that are absent from single molecular junctions, but were demonstrated for SAM based nano-sized junctions [32]. Towards that end, de Boer et al developed micron-sized molecular junctions with high yields, excellent stability and reproducibility, where the molecular SAM was embedded in holes in a lithographically patterned insulating matrix [33–36]. The bottom electrode was made of Au and a transparent, non-metallic, conducting polymer was used as the counter-electrode. Such junctions were used for photochromic conductance switching [37], however they may still suffer from metal-induced effects. We have further developed this approach to form entirely metal-free molecular junctions on ITO as the bottom electrode with a conductive polymer as the counter-electrode. Such junctions can be made semi-transparent and allow efficient demonstration of molecular optoelectronic properties. Herein we describe the manufacture and validation of arrays of micron-sized junctions consisting of SAMs chemically bound to ITO via amino-silane binding, with PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate))
Figure 1. Schematic representation of a finite-area molecular junction (cross-section view, not to scale).
as the counter-electrode. The transport properties of the junctions and the role of amino-silane binding are discussed. Silane-bound SAM formation on a hydroxylated surface is driven by the formation of polysiloxane connected to the surface typically via metal–O–Si bonds, and has been demonstrated with a large variety of substrates [38]. Silane binding is of particular interest to the Si-based electronics industry, where it is used in MEMs and Si-organic hybrid devices [39], and is also widely utilized for ITO surface and interface modification in optoelectronic devices [29, 40, 41]. However, transport via silane-bound SAMs is rarely studied, and only on Si/SiOx electrodes [7, 42, 39]. Amino-silane binding provides significant flexibility in binding carboxylic acid-modified molecules via amide bond formation. However, to the best of our knowledge, its transport properties have not been studied hitherto.
2. Methodology: junction fabrication and validation Finite sized molecular junctions, as schematically described in figure 1, were fabricated via a multi-step process. Analytical grade solvents without further purification, 18 M water, and 99.995% grade N2(g) were used throughout the process. 2.1. Fabrication of the hole array 2.5 cm2 SiO2 passivated float-glass slides coated with a 150 nm thick ITO layer (Delta Technologies, sheet resistance 15–25 /sq) were sonicated in toluene, acetone, and isopropanol, for 5 min each, N2(g) dried, and then treated in an ozone cleaner (Novascan Technologies, PSD-UV) for 30 min. The cleaned slides were spin-coated with a permanent epoxy negative photoresist (MicroChem SU-8 2000.5). Photolithography was applied according to the manufacturer’s procedure, leaving an array of cylindrical holes of diameters 35, 50, 75 and 100 µm, in a 400 nm thick film of the cross-linked photoresist. This SU-8 photoresist thickness was found to be the thinnest showing no current when bias was applied to junctions formed across it by Au contact evaporation and etching (see below). The true area of the exposed ITO at the bottom of the holes was characterized by optical microscopy (Olympus, BX51) and atomic force microscopy (AFM, Nanosurf Easyscan 2 and Agilent 5500). 2.2. SAM adsorption The samples were treated for 30 min in a UV-ozone cleaner, to remove residual contaminants from the exposed 2
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Figure 2. UV–vis absorption spectra of TCPP: (a) in ethanol 0.0037 mM solution (b) APTMS-TCPP SAMs on ITO, (c) TCPP SAMs on ITO, (d) APTMS-TCPP SAM in a molecular junction, sandwiched between ITO and PEDOT:PSS.
that the exposed ITO at the bottom of the holes is highly conductive, while the surrounding SU-8 is insulating. It was also shown that the adsorbed monolayer covers the exposed ITO, decreasing its conductivity by several orders of magnitudes (figure S1 in the supplementary information available at stacks.iop.org/Nano/24/455204/mmedia). Topography imaging of the bottom of the holes after adsorption shows increased roughness compared to clean ITO (figures S2, S3 available at stacks.iop.org/Nano/24/455204/mmedia), confirming the adsorption inside the hole and the formation of a largely disordered SAM. Carboxylic acid-modified porphyrins (meso-Tetra(4carboxyphenyl)porphyrin, TCPP, Strem Chemicals, 98%) were adsorbed to the APTMS using a similar procedure (0.1 mg TCPP in 0.4 ml DMSO solution as described above). Porphyrins are photo-active and are used to study visible light effects on transport in such junctions [46]. TCPP molecules were also directly adsorbed to the ITO surface via the carboxylic acid groups for comparison, by 12 h immersion in a 2 mM solution in ethanol. The adsorption was followed by UV–vis spectrometry, which showed the porphyrin’s typical absorption spectrum and verified the binding procedures (figure 2). The presented UV–vis spectra indicate the absorption of TCPP adsorbed on both sides of the ITO/glass substrate. The small broadening and red shift of the Soret band absorption peak, compared to the absorption spectrum in solution, indicated a highly disordered SAM with heterogeneous adsorption modes, with possible partial J-aggregation (figures 2(a) and (b)). Comparing the absorption spectra of TCPP SAMs with and without APTMS showed lower coverage with carboxylic acid binding, and a narrower Soret band peak indicating better ordered adsorption (figures 2(b) and (c)). The molecular adsorption density on APTMS was deduced from absorption
ITO at the bottom of the holes, and immersed in a fresh solution of 1% (v/v) aminopropyl trimethoxysilane (APTMS, Alfa Aesar, 97%) in ethanol with 2% (v/v) water for 3 h at 70 ◦ C, rinsed with methanol and N2 dried. The chemisorption of a silane monolayer to the exposed ITO proceeds via surface hydroxyls. Sulfosuccinimidyl-4-o-(4,4dimethoxytrityl) butyrate (sulfo-SDTB) was used to estimate the APTMS adsorption density via characterization of the surface density of amine groups according to a published procedure [43]. The amine density was found to be 1.3±0.9× 1018 m−2 . The samples were then immersed for 3 h in solutions of the following n-alkanoic acids (1.25 mM): 2.3 µl butyric acid (Acros Organics, 99+%), or 4 µl octanoic acid (ABCR, 98%), or 6.4 mg palmitic acid (Acros Organics, 98%), in 20 µl dimethylsulfoxide (DMSO). 20 µl of N-ethyldiisopropylamine (Alfa Aesar, 99%) and 16 mg 2-(1H-benzotriazole-1-yl)-1,1,3,3tetramethyluroniumhexafluorophosphate (Novabiochem) were added to facilitate the formation of amide bonds with the APTMS. The samples were then rigorously rinsed with DMSO, methanol, and N2 dried. The alkanoic acid adsorption was verified by measuring static water contact angles (3 µl, Kruss, FM40) on an area exposed from SU-8 during the photolithography at the sample periphery. The measured contact angles (averaged for three locations) were: APTMS only—35 ± 3◦ , APTMS/butyric acid—48 ± 1◦ , APTMS/octanoic acid—82 ± 1◦ , APTMS/palmitic acid— 105 ± 1◦ . The values and the direction of the change in contact angles are in general agreement with previously published values [44, 45]. The cleanness of the exposed ITO at the bottom of holes prior to adsorption, and the adsorption quality, were followed using current sensing AFM (Agilent 5500), showing 3
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measurements to be about 1018 m−2 . A density of 0.9 ± 0.3 × 1018 m−2 was deduced from chemical analysis of the amine groups which remain un-reacted after porphyrin adsorption using sulfo-SDTB as described above, in good agreement. Rigorous washing and the low adsorption density reduced the chances of molecular multi-layer formation. However, such low adsorption density also points to a largely disordered monolayer. 2.3. Top contact formation The samples were spin-coated with an aqueous dispersion of PEDOT:PSS (Heraeus, CLEVIOS FE) filtered through a 0.45 µm PVDF membrane (Millipore Ireland, Millex-HV). The dispersion was diluted with water 50% (v/v), with 0.1% (v/v) surfactant (DuPont, Zonyl FSO-100) to improve the wetting of the aqueous dispersion over the hydrophobic adsorbed monolayer. The samples were dried and stored under vacuum. The resulting film thickness was measured by AFM to be 100 nm. In the case of porphyrin SAMs, the PEDOT:PSS dispersion, which was originally acidic (pH ≈ 2.2), was titrated to a pH of 6.5–7.0 with 100 mg ml−1 NaOH(aq) solution, in order to prevent the protonation of the basic amine groups of the porphyrin. The transparency of the metal-free junction is exemplified by the TCPP absorption spectrum measured within the junction, sandwiched between the electrodes (figure 2(d)). It should be noted that PEDOT:PSS is not a metal-like electrode, and changes in the processing can yield differences in its measured resistance [47]. However, as demonstrated below, if the fabrication technology is consistent, the effect of the molecular structure is clearly observed, enabling comparison of transport properties of different functional molecules. 150 nm thick circular Au electrodes, with diameters slightly larger than the junction diameters, were vapour deposited through a shadow mask over the cylindrical pits containing the junctions (figure 3). The Au electrodes served not only as top contacts but also as etching masks. The PEDOT:PSS surrounding the Au electrodes, and electrically connecting the junctions, was etched for 1 min by 700 W oxygen plasma at 12 PSI, 4 SCFH (Plasmatic Systems, Plasma-Preen II-862).
Figure 3. CCD image of a conductive AFM probe in contact with a 100 µm diameter junction.
to tip penetration. For verification, similar transport results were obtained using Au-coated cantilevers without a probe as contacts. The results below present transport in 100 µm diameter junctions only. Artefacts related to the junction dimensions are discussed in the supplementary information (available at stacks.iop.org/Nano/24/455204/mmedia).
3. Results and discussion To confirm the efficient penetration of PEDOT:PSS into the cylindrical pits produced in the insulating SU-8 matrix, transport measurements were performed in control junctions without SAM (direct contact between the ITO and the PEDOT:PSS electrodes). The measured conductance was 48 ± 4 S cm−2 . Scaling of the conductance with the junction area indicate that the measured conductance is indeed the junction’s conductance rather than that of the surrounding SU-8 film, and that the junctions were indeed isolated from each other (figure S4(b) available at stacks. iop.org/Nano/24/455204/mmedia). The resistance of a single junction of 100 µm diameter was about 265 (figure 4(a)), significantly larger than that of ITO or PEDOT:PSS films of similar area (resistivity of 3 × 10−4 and 7 × 10−3 cm, respectively), hence it is dominated by the ITO/PEDOT:PSS contact resistance. Kuila et al found that the contact resistance due to ITO contact was about 125 in junctions of a slightly larger area [21], indicating that the ITO contact is responsible for the measured resistance in our control junctions. Junctions with various SAMs were characterized to verify that the SAM controls the junction transport properties. The PEDOT:PSS/ITO junction conductance was larger by more than two orders of magnitude compared to similar junctions with adsorbed APTMS-based SAMs, and one order of magnitude larger than TCPP-based SAM (figure 4), confirming the SAM’s control over transport in the molecular junctions. The SAM’s resistance was significantly larger than the contact resistance, as expected. The SAM current–voltage characteristics were typical of the tunnelling transport mechanism (figures 4(b) and (c)), as expected [48]. Chemical attachment of the molecules to the ITO surface via amino-silane binding was found to significantly affect
2.4. Transport measurements Transport measurements were performed utilizing a current sensing AFM (Nanosurf, easyScan 2) as a micro-probe station (figure 3), using a conductive probe to contact the top Au electrode, while the ITO bottom electrode was grounded. The samples were dried at 80 ◦ C under vacuum for 60 min before transport measurements to prevent the adverse effect of humidity on the PEDOT:PSS conductivity. The measurements were conducted under dry nitrogen. All studies were performed on 12–18 similar junctions to maintain statistical significance, and were repeated on two identical sample plates. The tip did not penetrate the junctions due to the use of soft cantilevers (0.02–0.8 N m−1 ), and no shorts were noted in the current–voltage characteristics due 4
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Figure 4. Current–voltage characteristics of different 100 µm diameter junctions: (a) without SAM, (b) with different APTMS-alkanoic acid SAMs. Inset: representative characteristics at a larger bias range (−1 to +1 V), (c) with TCPP SAM, and (d) with APTMS-TCPP SAM. The curves in (a) and (b) are averaged while those in (c) and (d) are representative. The error bars in (a) and (b) represent the standard deviation upon averaging of at least 12 junctions.
that the higher resistance by two orders of magnitude results from the APTMS binding scheme. Surprisingly, we could not find published resistance values of porphyrin-based SAMs. We can therefore compare our result to the resistance of a single 5-(4-carboxyphenyl)-20-(4-pyridyl)-10,15-bis(2,4,6trimethylphenyl)porphyrin measured using the STM-based break junction method in a ITO/porphyrin/Au junction, which was 2.5 × 1010 [20]. However, a difference of several orders of magnitude in the transport properties between the same molecule measured as a single molecule or within a SAM is well documented and is attributed to changes in the molecular environment [49, 50], hence the comparison here is questionable. SAMs of APTMS only were shown to have lower conductance than SAMs of longer molecular chains attached to it (figure 4(b)), possibly due to partial protonation of unbound amine end groups [51]. Such protonation is expected in our junctions, where the basic amine groups are in contact with the acidic PEDOT:PSS dispersion (pH 2.2). The amide groups, formed by amidization of the carboxylic acid with the amine groups, are less basic, therefore they do not protonate upon contact with the PEDOT:PSS suspension. The polar amide bonds, as well as protonated unbound amines, are probably responsible for the increased resistance observed in our junctions. Binding via APTMS also induced a large molecular tilt and disorder in the attached layer, as expected from its deposition on the nm-rough ITO surface, in accordance with previous works on silane-bound SAMs on disordered surfaces [38], and with the expected reduced order in SAMs on semiconductor surfaces [7]. The SAM disorder was also demonstrated by the absorption spectra of adsorbed APTMSTCPP (figure 2(b)). Thus, APTMS binding is postulated to
the junction transport. Its advantages include reproducibility and excellent surface coverage preventing electrical shorts, as expressed in the yield of junctions which was evaluated from transport measurements as ∼80% for APTMS-based SAMs. This high yield is only slightly smaller than that observed by de Boer et al [35] on flat Au substrates (95%), probably due to worse homogeneity of the ITO and its surface roughness (figure S2 available at stacks.iop.org/Nano/24/ 455204/mmedia). The yield was lower when carboxylic acids were used for molecular binding (∼70%), due to the lower coverage with carboxylic acid binding, as shown above. We postulate that APTMS-SAM is dense due to its intermolecular binding, preventing shorts even if the molecular layer bound to it is of lower density. However, APTMS binding resulted in increased junction resistance compared to molecules of similar length but different molecule/electrode binding schemes. The properties of single molecules within the SAM junctions were calculated, taking into account the adsorption density of 1018 m−2 , and compared to published values. The resistance in a ITO/APTMS-palmitic acid (19 carbons)/PEDOT:PSS junction was found to be 5 × 1015 /molecule, compared to 2 × 1013 /molecule in ITO/octadecanethiol (18 carbons)/Al junctions [21]. The current through a single molecule in Au/decanedithiol (10 carbons)/PEDOT:PSS at 0.2 V was estimated to be 10 fA [35], while that in ITO/APTMSoctanoic acid (11 carbons)/PEDOT:PSS was 0.15 fA. Hence, APTMS binding increases the resistance by two orders of magnitude compared to thiol binding. The resistance of a single porphyrin molecule in our ITO/APTMS-TCPP SAM/PEDOT:PSS junctions was found to be 1.3 × 1016 , and that of a single porphyrin in our ITO/TCPP/PEDOT:PSS (no APTMS) junctions was 1.3 × 1014 , further confirming 5
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Figure 5. Current at 100 mV for 100 µm diameter junctions versus alkyl chain length. The error bars represent the standard deviation upon averaging of at least 12 junctions.
Figure 7. Current–voltage characteristics of junctions with APTMS-TCPP SAM, in the dark and under 532 nm laser illumination.
be responsible for the observed unexpectedly small tunnelling decay constant. Analysing the current dependence on the alkyl chain length at a given bias shows decay in current with increasing chain length (l), as expected from Simmon’s model suggesting that I ∼ exp(−βl). The tunnelling decay constant ˚ −1 (figure 5), significantly smaller than β is 0.125 ± 0.019 A decay constants found for thiol–alkyl monolayers adsorbed ˚ −1 at 1 V) [35]. This difference can be on Au (0.6–0.7 A explained by the large tilt angle of the alkyl chain with respect to the substrate plane [44], which will effectively reduce the monolayer thickness. A 48◦ –70◦ tilt was calculated for the alkyl chain bound to APTMS (figure 6), due to the amide ˚ −1 requires a tilt bond. A tunnelling decay constant of 0.65 A angle of 79◦ , in agreement with our modelling, taking into account the differences expected in a SAM with respect to single molecule modelling. Other possible explanations for a layer thickness smaller than expected are sub-monolayer coverage by alkyls due to incomplete binding to APTMS and alkyl chain folding [33]. A disordered and largely tilted SAM can cause intermolecular (rather than intra-molecular) electron transport, further limiting transport by a hopping mechanism, which implies weak length dependence of the transport, hence a small decay constant value. A weak dependence on molecular length may also be related to large contact resistance dominating the transport properties.
High contact resistance results from dipoles associated with APTMS binding to the ITO, as discussed above. Summarizing the various effects induced by molecular binding via APTMS, we find that APTMS allows high junction yields and the formation of dense molecular layers preventing short, but it also induces significant junction resistance and SAM disorder. Carboxylic acid binding provides better conductance but lower junction yield, probably due to worse surface coverage. Other binding schemes, such as via phosphonic acids, are currently being studied for molecular junctions on ITO. The optoelectronic capabilities of the junctions formed on ITO are demonstrated in figure 7, which shows photoconductance when junctions containing the photoactive APTMS-bound TCPP SAM are illuminated at a wavelength corresponding to the first Q-band absorption of the porphyrin (figure 2). Hence, molecular photoexcitation results in a change in the junction’s transport properties. The photoconductance mechanism and other optoelectronic properties of these junctions will be discussed elsewhere [46]. The junction’s optoelectronic capabilities are also demonstrated in the absorption spectrum of the TCPP SAM measured through the transparent electrodes in the completed junctions (figure 2(d)).
Figure 6. Three-dimensional models of APTMS amides with (a) butyric acid, (b) octanoic acid, and (c) palmitic acid. The models were produced with Chem3D Pro 12.0 software utilizing the MM2 force field model. The alkanoic chain lengths in trans-configuration deduced ˚ octanoic acid—8.9 A, ˚ palmitic acid—19.1 A. ˚ from this modelling were: butyric acid—3.8 A, 6
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4. Summary and conclusions
[4] Arielly R, Ofarim A, Noy G and Selzer Y 2011 Accurate determination of plasmonic fields in molecular junctions by current rectification at optical frequencies Nano Lett. 11 2968–72 [5] Keller A, Atabek O, Ratner M and Mujica V 2002 Laser-assisted conductance of molecular wires J. Phys. B: At. Mol. Opt. Phys. 35 4981–8 [6] Chien C-C, Velizhanin K A, Dubi Y and Zwolak M 2013 Tunable thermal switching via DNA-based nano-devices Nanotechnology 24 095704 [7] Vilan A, Yaffe O, Biller A, Salomon A, Kahn A and Cahen D 2010 Molecules on Si: electronics with chemistry Adv. Mater. 22 140–59 [8] Yakuphanoglu F, Okur S and Oezgener H 2009 Modification of metal/semiconductor junctions by self-assembled monolayer organic films Microelectron. Eng. 86 2358–63 [9] Scott A, Janes D B, Risko C and Ratner M A 2007 Fabrication and characterization of metal–molecule–silicon devices Appl. Phys. Lett. 91 033508 [10] Pakoulev A V and Burtman V 2010 Study of the transport mechanism in molecular self-assembling devices Appl. Phys. A 98 717–34 [11] Liu Y and Yu H 2003 Molecular passivation of mercury–silicon (p-type) diode junctions: alkylation, oxidation, and alkylsilation J. Phys. Chem. B 107 7803–11 [12] Liu Y and Yu H 2003 Alkyl monolayer passivated metal–semiconductor diodes: 2: comparison with native silicon oxide ChemPhysChem 4 335–42 [13] Lenfant S, Guerin D, Van F T, Chevrot C, Palacin S, Bourgoin J P, Bouloussa O, Rondelez F and Vuillaume D 2006 Electron transport through rectifying self-assembled monolayer diodes on silicon: Fermi-level pinning at the molecule–metal interface J. Phys. Chem. B 110 13947–58 [14] Hiremath R K, Rabinal M K, Mulimani B G and Khazi I M 2008 Molecularly controlled metal–semiconductor junctions on silicon surface: a dipole effect Langmuir 24 11300–6 [15] Aswal D K, Petit C, Salace G, Guerin D, Lenfant S, Yakhmi J V and Vuillaume D 2006 Role of interfaces on the direct tunneling and the inelastic tunneling behaviors through metal/alkylsilane/ silicon junctions Phys. Status Solidi a 203 1464–9 [16] Vilan A, Shanzer A and Cahen D 2000 Molecular control over Au/GaAs diodes Nature 404 166–8 [17] Shpaisman H, Salomon E, Nesher G, Vilan A, Cohen H, Kahn A and Cahen D 2009 Electrical transport and photoemission experiments of alkylphosphonate monolayers on GaAs J. Phys. Chem. C 113 3313–21 [18] Nesher G, Shpaisman H and Cahen D 2007 Effect of chemical bond type on electron transport in GaAs-chemical bond-alkyl/Hg junctions J. Am. Chem. Soc. 129 734–5 [19] Nesher G, Vilan A, Cohen H, Cahen D, Amy F, Chan C, Hwang J and Kahn A 2006 Energy level and band alignment for GaAs-alkylthiol monolayer-Hg junctions from electrical transport and photoemission experiments J. Phys. Chem. B 110 14363–71 [20] Battacharyya S, Kibel A, Kodis G, Liddell P A, Gervaldo M, Gust D and Lindsay S 2011 Optical modulation of molecular conductance Nano Lett. 11 2709–14 [21] Kolipaka S, Aithal R K and Kuila D 2006 Fabrication and characterization of an indium tin oxide–octadecanethiol– aluminum junction for molecular electronics Appl. Phys. Lett. 88 233104 [22] Furmansky Y, Sasson H, Liddell P, Gust D, Ashkenasy N and Visoly-Fisher I 2012 Porphyrins as ITO photosensitizers: substituents control photo-induced electron transfer direction J. Mater. Chem. 22 20334–41
Metal free, micron-sized SAM-based junctions were realized on a transparent conductive oxide (ITO), with PEDOT:PSS as the counter-electrodes, with high fabrication yields. Importantly, the electrical transport was shown to be dominated by the nature of the SAM in these metal-free junctions. The use of amino-silane (APTMS) as the chemical binding scheme to ITO was found to be significant in determining the transport properties of the junctions. APTMS allows high junction yields and the formation of dense molecular layers preventing electrical short. However, the conductance per single molecule within the junction was found to be two orders of magnitude smaller than in corresponding thiol-bound molecular junctions due to the polar silane binding to the ITO and an amide bond between the SAM and the APTMS binder. The transport properties of the APTMS-only SAM were also measured, and protonation of the amine end groups was found to decrease the junction conductance compared to longer APTMS-SAMs. Since the amino-silane binding scheme is widely used in molecular adsorption to metal oxide surfaces, its electronic implications can be relevant also to fields other than molecular electronics, employing organic/metal oxide interfaces. Examples include organic photovoltaics, organic light emitting diodes, solar fuels etc. As the effect of the molecular structure on transport properties is clearly observed in our junctions, if the fabrication technology is consistent, we conclude that such metal-free junctions are suitable for characterizing the effect of external stimuli, which otherwise might be masked by the response of metallic electrodes. Such stimuli include the effect of photon excitation on the transport properties of photo-active molecules, i.e. molecular optoelectronic properties, as well as thermoelectric effects in molecular junctions. A preliminary demonstration of a change in the junction’s transport properties as a result of SAM photo-excitation is presented using APTMS-bound porphyrin SAM.
Acknowledgments The authors are grateful to Dr Rafi Shikler (BGU) for use of lab equipment and for helpful ideas, and to Professor Leeor Kronik (Weizmann Inst. of Science) for fruitful discussions.
References [1] Akkerman H and de Boer B 2008 Electrical conduction through single molecules and self-assembled monolayers J. Phys.: Condens. Matter 20 013001 [2] Haick H and Cahen D 2008 Making contact: connecting molecules electrically to the macroscopic world Prog. Surf. Sci. 83 217–61 [3] He J et al 2005 Switching of a photochromic molecule on gold electrodes: single-molecule measurements Nanotechnology 16 695–702 7
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[37] Kronemeijer A J, Akkerman H B, Kudernac T, van Wees B J, Feringa B L, Blom P W M and de Boer B 2008 Reversible conductance switching in molecular devices Adv. Mater. 20 1467–73 [38] Ulman A 1996 Formation and structure of self-assembled monolayers Chem. Rev. 96 1533–54 [39] Richter C A, Hacker C A, Richter L J, Kirillov O A, Suehle J S and Vogel E M 2006 Interface characterization of molecular-monolayer/SiO2 based molecular junctions Solid-State Electron. 50 1088–96 [40] Cook R M, Pegg L-J, Kinnear S L, Hutter O S, Morris R J H and Hatton R A 2011 An electrode design rule for organic photovoltaics elucidated using molecular nanolayers Adv. Energy Mater. 1 440–7 [41] Heo S W, Lee E J, Seong K W and Moon D K 2013 Enhanced stability in polymer solar cells by controlling the electrode work function via modification of indium tin oxide Sol. Energy Mater. Sol. Cells 115 123–8 [42] Chauhan A K, Aswal D K, Koiry S P, Gupta S K, Yakhmi J V, Surgers C, Guerin D, Lenfant S and Vuillaume D 2008 Self-assembly of the 3-aminopropyltrimethoxysilane multilayers on Si and hysteretic current–voltage characteristics Appl. Phys. A 90 581–9 [43] Gaur R K and Gupta K C 1989 A spectrophotometric method for the estimation of amino groups on polymer supports Anal. Biochem. 180 253–8 [44] Flink S, Van Veggel F C J M and Reinhoudt D N 2001 Functionalization of self-assembled monolayers on glass and oxidized silicon wafers by surface reactions J. Phys. Org. Chem. 14 407–15 [45] Karsi N, Lang P, Chehimi M, Delamar M and Horowitz G 2006 Modification of indium tin oxide films by alkanethiol and fatty acid self-assembled monolayers: a comparative tudy Langmuir 22 3118–24 [46] Furmansky Y, Sergani S, Ashkenasy N and Visoly-Fisher I 2013 in preparation [47] Kronemeijer A J, Katsouras I, Huisman E H, van Hal P A, Geuns T C T, Blom P W M and de Leeuw D M 2011 Universal scaling of the charge transport in large-area molecular junctions Small 7 1593–8 [48] Tao N J 2006 Electron transport in molecular junctions Nature Nanotechnol. 1 173–81 [49] Selzer Y, Cai L, Cabassi M A, Yao Y, Tour J M, Mayer T S and Allara D L 2005 Effect of local environment on molecular conduction: isolated molecule versus self-assembled monolayer Nano Lett. 5 61–5 [50] Salomon A, Cahen D, Lindsay S, Tomfohr J, Engelkes V B and Frisbie C D 2003 Comparison of electronic transport measurements on organic molecules Adv. Mater. 15 1881–90 [51] Shaya O, Einati H, Fishelson N, Shacham-Diamand Y and Rosenwaks Y 2010 Transistor gating by polar molecular monolayers Appl. Phys. Lett. 97 053501
[23] Khodabakhsh S, Poplavskyy D, Heutz S, Nelson J, Bradley D D C, Murata H and Jones T S 2004 Using self-assembling dipole molecules to improve hole injection in conjugated polymers Adv. Funct. Mater. 14 1205–10 [24] Macaraig L, Sagaw T and Yoshikawa S 2011 Self-assembly monolayer molecules for the improvement of the anodic interface in bulk heterojunction solar cells Energy Procedia 9 283–91 [25] Lee J, Jung B, Lee J, Chu H Y, Do L and Shim H 2002 Modification of an ITO anode with a hole-transporting SAM for improved OLED device characteristics J. Mater. Chem. 12 3494–8 [26] Kim D H, Chung C M, Park J W and Oh S Y 2008 Effects of ITO surface modification using self-assembly molecules on the characteristics of OLEDs Ultramicroscopy 108 1233–6 [27] Yun D, Lee D, Jeon H and Rhee S 2007 Contact resistance between pentacene and indium-tin oxide (ITO) electrode with surface treatment Org. Electron. 8 690–4 [28] Tokudome Y, Fukushima T, Goto A and Kaji H 2011 Enhanced hole injection in organic light-emitting diodes by optimized synthesis of self-assembled monolayer Org. Electron. 12 1600–5 [29] Hatton R A, Day S R, Chesters M A and Willis M R 2001 Organic electroluminescent devices: enhanced carrier injection using an organosilane self assembled monolayer (SAM) derivatized ITO electrode Thin Solid Films 394 292–7 [30] Koide Y, Wang Q, Cui J, Benson D D and Marks T J 2000 Patterned luminescence of organic light-emitting diodes by hot microcontact printing (HmCP) of self-assembled monolayers J. Am. Chem. Soc. 122 11266–7 [31] Chen F, Huang Z and Tao N 2007 Forming single molecular junctions between indium tin oxide electrodes Appl. Phys. Lett. 91 162106 [32] Wang W, Lee T and Reed M A 2003 Mechanism of electron conduction in self-assembled alkanethiol monolayer devices Phys. Rev. B 68 035416 [33] Akkerman H B, Kronemeijer A J, Hal P A v, Leeuw D M d, Blom P W M and Boer B d 2008 Self-assembledmonolayer formation of long alkanedithiols in molecular junctions Small 4 100 [34] Akkerman H B, Kronemeijer A J, Harkema J, van Hal P A, Smits E C P, de Leeuw D M and Blom P W M 2010 Stability of large-area molecular junctions Org. Electron. 11 146–9 [35] Akkerman H B, Blom P W M, de Leeuw D M and de Boer B 2006 Towards molecular electronics with large-area molecular junctions Nature 441 69–72 [36] Akkerman H B, Naber R C G, Jongbloed B, Van Halt P A, Blom P W M, de Leeuw D M and de Boer B 2007 Electron tunneling through alkanedithiol self-assembled monolayers in large-area molecular junctions Proc. Natl Acad. Sci. 104 11161–6
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Metal-free molecular junctions on ITO via amino-silane binding—towards optoelectronic molecular junctions
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 455204 (http://iopscience.iop.org/0957-4484/24/45/455204) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 455204 (8pp)
doi:10.1088/0957-4484/24/45/455204
Metal-free molecular junctions on ITO via amino-silane binding—towards optoelectronic molecular junctions S Sergani1 , Y Furmansky2 and I Visoly-Fisher2,3 1
Department of Chemistry, Ben Gurion University of the Negev, Be’er Sheva 84105, Israel Ilse Katz Institute for Nanoscale Science and Technology, Ben Gurion University of the Negev, Be’er Sheva 84105, Israel 3 Department of Solar Energy and Environmental Physics, Swiss Institute for Dryland Environmental and Energy Research, Jacob Blaustein Institutes for Desert Research, Ben Gurion University of the Negev, Be’er Sheva 84105, Israel 2
E-mail: [email protected]
Received 31 January 2013, in final form 15 August 2013 Published 15 October 2013 Online at stacks.iop.org/Nano/24/455204 Abstract Light control over currents in molecular junctions is desirable as a non-contact input with high spectral and spatial resolution provided by the photonic input and the molecular electronics element, respectively. Expanding the study of molecular junctions to non-metallic transparent substrates, such as indium tin oxide (ITO), is vital for the observation of molecular optoelectronic effects. Non-metallic electrodes are expected to decrease the probability of quenching of molecular photo-excited states, light-induced plasmonic effects, or significant electrode expansion under visible light. We have developed micron-sized, metal free, optically addressable ITO molecular junctions with a conductive polymer serving as the counter-electrode. The electrical transport was shown to be dominated by the nature of the self-assembled monolayer (SAM). The use of amino-silane (APTMS) as the chemical binding scheme to ITO was found to be significant in determining the transport properties of the junctions. APTMS allows high junction yields and the formation of dense molecular layers preventing electrical short. However, polar amino-silane binding to the ITO significantly decreased the conductance compared to thiol-bound SAMs, and caused tilted geometry and disorder in the molecular layer. As the effect of the molecular structure on transport properties is clearly observed in our junctions, such metal-free junctions are suitable for characterizing the optoelectronic properties of molecular junctions. S Online supplementary data available from stacks.iop.org/Nano/24/455204/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
metallic electrodes are expected to increase the probability of quenching of molecular photo-excited states [3], light-induced plasmonic effects [4], or significant electrode expansion under visible light [5], masking the molecular optoelectronic properties. Expanding the study of molecular junctions to non-metallic transparent substrates, such as indium tin oxide (ITO), is vital for the observation of molecular optoelectronic effects. Thermal conductance switching in molecular junctions [6] can also be practically overridden by the thermal expansion of metallic electrodes. Metal-free junctions, where both electrodes are non-metallic, are highly desirable for such applications.
The study of electrical transport through self-assembled monolayers (SAMs) is essential for the miniaturization of molecular electronic and optoelectronic devices. Light control over currents in molecular junctions is desirable as a non-contact input with high spectral and spatial resolution provided by the photonic input and the molecular electronics element, respectively. Molecular junctions commonly utilize thiol assembly on Au, due to Au chemical inertness and the ease of ordered SAM fabrication [1, 2]. Optically addressed junctions require at least one transparent electrode, which is difficult to achieve with metallic electrodes. Furthermore, 0957-4484/13/455204+08$33.00
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c 2013 IOP Publishing Ltd Printed in the UK & the USA
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The nature of the molecule–electrode contact plays a major role in the measured characteristics of the junction. Molecular junctions comprising SAMs on semiconductors such as Si [7–15] and GaAs [16–19] were studied. In semiconducting electrodes the interfacial energy level alignment with the molecular levels depends on the doping type and concentration and not only on the adsorption chemistry. In addition, a potential drop within the electrode is feasible, and the chemical binding is typically much more stable than the thiol–gold bond. The latter results in less ordered SAMs as adsorbate diffusion to improve ordering is inhibited [7]. ITO is a degenerate n-type semiconductor, with a large band-gap enabling its use as a transparent conductive electrode. It is commercially available as sputtered thin films with nm-scale roughness, and is commonly used in organic optoelectronic devices. Only very few publications describe transport studies of molecular monolayers or single molecules adsorbed to ITO. An optoelectronic study of single molecule junctions of porphyrin–fullerene dyads, bonded to ITO substrate and Au STM probes, has shown photoconductance, suggesting the formation of a long-lived charge separated state on the ITO surface [20]. Junctions of 1-octadecanethiol SAM on pre-patterned ITO, top contacted by evaporated Al electrodes, were also fabricated and studied [21]. Both these examples utilize metal as one of the electrodes. More commonly, SAMs on ITO are used for interface modification in organic electronic devices [22–30], where transport across the SAM is not directly characterized. To the best of our knowledge, the only demonstration of metal-free junctions was of single molecule junctions of dicarboxylic terminated n-alkanes bonded to ITO substrate and ITO coated AFM probes, which showed a similar decay constant as n-alkanes sandwiched between Au electrodes, but different contact conductance [31]. Connecting molecules electrically to an external circuitry is anything but trivial, as it requires reproducible fabrication of nano-gaps without short-circuits. Technological applications of such junctions require them to be reliable, stable and reproducible, properties that are absent from single molecular junctions, but were demonstrated for SAM based nano-sized junctions [32]. Towards that end, de Boer et al developed micron-sized molecular junctions with high yields, excellent stability and reproducibility, where the molecular SAM was embedded in holes in a lithographically patterned insulating matrix [33–36]. The bottom electrode was made of Au and a transparent, non-metallic, conducting polymer was used as the counter-electrode. Such junctions were used for photochromic conductance switching [37], however they may still suffer from metal-induced effects. We have further developed this approach to form entirely metal-free molecular junctions on ITO as the bottom electrode with a conductive polymer as the counter-electrode. Such junctions can be made semi-transparent and allow efficient demonstration of molecular optoelectronic properties. Herein we describe the manufacture and validation of arrays of micron-sized junctions consisting of SAMs chemically bound to ITO via amino-silane binding, with PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate))
Figure 1. Schematic representation of a finite-area molecular junction (cross-section view, not to scale).
as the counter-electrode. The transport properties of the junctions and the role of amino-silane binding are discussed. Silane-bound SAM formation on a hydroxylated surface is driven by the formation of polysiloxane connected to the surface typically via metal–O–Si bonds, and has been demonstrated with a large variety of substrates [38]. Silane binding is of particular interest to the Si-based electronics industry, where it is used in MEMs and Si-organic hybrid devices [39], and is also widely utilized for ITO surface and interface modification in optoelectronic devices [29, 40, 41]. However, transport via silane-bound SAMs is rarely studied, and only on Si/SiOx electrodes [7, 42, 39]. Amino-silane binding provides significant flexibility in binding carboxylic acid-modified molecules via amide bond formation. However, to the best of our knowledge, its transport properties have not been studied hitherto.
2. Methodology: junction fabrication and validation Finite sized molecular junctions, as schematically described in figure 1, were fabricated via a multi-step process. Analytical grade solvents without further purification, 18 M water, and 99.995% grade N2(g) were used throughout the process. 2.1. Fabrication of the hole array 2.5 cm2 SiO2 passivated float-glass slides coated with a 150 nm thick ITO layer (Delta Technologies, sheet resistance 15–25 /sq) were sonicated in toluene, acetone, and isopropanol, for 5 min each, N2(g) dried, and then treated in an ozone cleaner (Novascan Technologies, PSD-UV) for 30 min. The cleaned slides were spin-coated with a permanent epoxy negative photoresist (MicroChem SU-8 2000.5). Photolithography was applied according to the manufacturer’s procedure, leaving an array of cylindrical holes of diameters 35, 50, 75 and 100 µm, in a 400 nm thick film of the cross-linked photoresist. This SU-8 photoresist thickness was found to be the thinnest showing no current when bias was applied to junctions formed across it by Au contact evaporation and etching (see below). The true area of the exposed ITO at the bottom of the holes was characterized by optical microscopy (Olympus, BX51) and atomic force microscopy (AFM, Nanosurf Easyscan 2 and Agilent 5500). 2.2. SAM adsorption The samples were treated for 30 min in a UV-ozone cleaner, to remove residual contaminants from the exposed 2
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Figure 2. UV–vis absorption spectra of TCPP: (a) in ethanol 0.0037 mM solution (b) APTMS-TCPP SAMs on ITO, (c) TCPP SAMs on ITO, (d) APTMS-TCPP SAM in a molecular junction, sandwiched between ITO and PEDOT:PSS.
that the exposed ITO at the bottom of the holes is highly conductive, while the surrounding SU-8 is insulating. It was also shown that the adsorbed monolayer covers the exposed ITO, decreasing its conductivity by several orders of magnitudes (figure S1 in the supplementary information available at stacks.iop.org/Nano/24/455204/mmedia). Topography imaging of the bottom of the holes after adsorption shows increased roughness compared to clean ITO (figures S2, S3 available at stacks.iop.org/Nano/24/455204/mmedia), confirming the adsorption inside the hole and the formation of a largely disordered SAM. Carboxylic acid-modified porphyrins (meso-Tetra(4carboxyphenyl)porphyrin, TCPP, Strem Chemicals, 98%) were adsorbed to the APTMS using a similar procedure (0.1 mg TCPP in 0.4 ml DMSO solution as described above). Porphyrins are photo-active and are used to study visible light effects on transport in such junctions [46]. TCPP molecules were also directly adsorbed to the ITO surface via the carboxylic acid groups for comparison, by 12 h immersion in a 2 mM solution in ethanol. The adsorption was followed by UV–vis spectrometry, which showed the porphyrin’s typical absorption spectrum and verified the binding procedures (figure 2). The presented UV–vis spectra indicate the absorption of TCPP adsorbed on both sides of the ITO/glass substrate. The small broadening and red shift of the Soret band absorption peak, compared to the absorption spectrum in solution, indicated a highly disordered SAM with heterogeneous adsorption modes, with possible partial J-aggregation (figures 2(a) and (b)). Comparing the absorption spectra of TCPP SAMs with and without APTMS showed lower coverage with carboxylic acid binding, and a narrower Soret band peak indicating better ordered adsorption (figures 2(b) and (c)). The molecular adsorption density on APTMS was deduced from absorption
ITO at the bottom of the holes, and immersed in a fresh solution of 1% (v/v) aminopropyl trimethoxysilane (APTMS, Alfa Aesar, 97%) in ethanol with 2% (v/v) water for 3 h at 70 ◦ C, rinsed with methanol and N2 dried. The chemisorption of a silane monolayer to the exposed ITO proceeds via surface hydroxyls. Sulfosuccinimidyl-4-o-(4,4dimethoxytrityl) butyrate (sulfo-SDTB) was used to estimate the APTMS adsorption density via characterization of the surface density of amine groups according to a published procedure [43]. The amine density was found to be 1.3±0.9× 1018 m−2 . The samples were then immersed for 3 h in solutions of the following n-alkanoic acids (1.25 mM): 2.3 µl butyric acid (Acros Organics, 99+%), or 4 µl octanoic acid (ABCR, 98%), or 6.4 mg palmitic acid (Acros Organics, 98%), in 20 µl dimethylsulfoxide (DMSO). 20 µl of N-ethyldiisopropylamine (Alfa Aesar, 99%) and 16 mg 2-(1H-benzotriazole-1-yl)-1,1,3,3tetramethyluroniumhexafluorophosphate (Novabiochem) were added to facilitate the formation of amide bonds with the APTMS. The samples were then rigorously rinsed with DMSO, methanol, and N2 dried. The alkanoic acid adsorption was verified by measuring static water contact angles (3 µl, Kruss, FM40) on an area exposed from SU-8 during the photolithography at the sample periphery. The measured contact angles (averaged for three locations) were: APTMS only—35 ± 3◦ , APTMS/butyric acid—48 ± 1◦ , APTMS/octanoic acid—82 ± 1◦ , APTMS/palmitic acid— 105 ± 1◦ . The values and the direction of the change in contact angles are in general agreement with previously published values [44, 45]. The cleanness of the exposed ITO at the bottom of holes prior to adsorption, and the adsorption quality, were followed using current sensing AFM (Agilent 5500), showing 3
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measurements to be about 1018 m−2 . A density of 0.9 ± 0.3 × 1018 m−2 was deduced from chemical analysis of the amine groups which remain un-reacted after porphyrin adsorption using sulfo-SDTB as described above, in good agreement. Rigorous washing and the low adsorption density reduced the chances of molecular multi-layer formation. However, such low adsorption density also points to a largely disordered monolayer. 2.3. Top contact formation The samples were spin-coated with an aqueous dispersion of PEDOT:PSS (Heraeus, CLEVIOS FE) filtered through a 0.45 µm PVDF membrane (Millipore Ireland, Millex-HV). The dispersion was diluted with water 50% (v/v), with 0.1% (v/v) surfactant (DuPont, Zonyl FSO-100) to improve the wetting of the aqueous dispersion over the hydrophobic adsorbed monolayer. The samples were dried and stored under vacuum. The resulting film thickness was measured by AFM to be 100 nm. In the case of porphyrin SAMs, the PEDOT:PSS dispersion, which was originally acidic (pH ≈ 2.2), was titrated to a pH of 6.5–7.0 with 100 mg ml−1 NaOH(aq) solution, in order to prevent the protonation of the basic amine groups of the porphyrin. The transparency of the metal-free junction is exemplified by the TCPP absorption spectrum measured within the junction, sandwiched between the electrodes (figure 2(d)). It should be noted that PEDOT:PSS is not a metal-like electrode, and changes in the processing can yield differences in its measured resistance [47]. However, as demonstrated below, if the fabrication technology is consistent, the effect of the molecular structure is clearly observed, enabling comparison of transport properties of different functional molecules. 150 nm thick circular Au electrodes, with diameters slightly larger than the junction diameters, were vapour deposited through a shadow mask over the cylindrical pits containing the junctions (figure 3). The Au electrodes served not only as top contacts but also as etching masks. The PEDOT:PSS surrounding the Au electrodes, and electrically connecting the junctions, was etched for 1 min by 700 W oxygen plasma at 12 PSI, 4 SCFH (Plasmatic Systems, Plasma-Preen II-862).
Figure 3. CCD image of a conductive AFM probe in contact with a 100 µm diameter junction.
to tip penetration. For verification, similar transport results were obtained using Au-coated cantilevers without a probe as contacts. The results below present transport in 100 µm diameter junctions only. Artefacts related to the junction dimensions are discussed in the supplementary information (available at stacks.iop.org/Nano/24/455204/mmedia).
3. Results and discussion To confirm the efficient penetration of PEDOT:PSS into the cylindrical pits produced in the insulating SU-8 matrix, transport measurements were performed in control junctions without SAM (direct contact between the ITO and the PEDOT:PSS electrodes). The measured conductance was 48 ± 4 S cm−2 . Scaling of the conductance with the junction area indicate that the measured conductance is indeed the junction’s conductance rather than that of the surrounding SU-8 film, and that the junctions were indeed isolated from each other (figure S4(b) available at stacks. iop.org/Nano/24/455204/mmedia). The resistance of a single junction of 100 µm diameter was about 265 (figure 4(a)), significantly larger than that of ITO or PEDOT:PSS films of similar area (resistivity of 3 × 10−4 and 7 × 10−3 cm, respectively), hence it is dominated by the ITO/PEDOT:PSS contact resistance. Kuila et al found that the contact resistance due to ITO contact was about 125 in junctions of a slightly larger area [21], indicating that the ITO contact is responsible for the measured resistance in our control junctions. Junctions with various SAMs were characterized to verify that the SAM controls the junction transport properties. The PEDOT:PSS/ITO junction conductance was larger by more than two orders of magnitude compared to similar junctions with adsorbed APTMS-based SAMs, and one order of magnitude larger than TCPP-based SAM (figure 4), confirming the SAM’s control over transport in the molecular junctions. The SAM’s resistance was significantly larger than the contact resistance, as expected. The SAM current–voltage characteristics were typical of the tunnelling transport mechanism (figures 4(b) and (c)), as expected [48]. Chemical attachment of the molecules to the ITO surface via amino-silane binding was found to significantly affect
2.4. Transport measurements Transport measurements were performed utilizing a current sensing AFM (Nanosurf, easyScan 2) as a micro-probe station (figure 3), using a conductive probe to contact the top Au electrode, while the ITO bottom electrode was grounded. The samples were dried at 80 ◦ C under vacuum for 60 min before transport measurements to prevent the adverse effect of humidity on the PEDOT:PSS conductivity. The measurements were conducted under dry nitrogen. All studies were performed on 12–18 similar junctions to maintain statistical significance, and were repeated on two identical sample plates. The tip did not penetrate the junctions due to the use of soft cantilevers (0.02–0.8 N m−1 ), and no shorts were noted in the current–voltage characteristics due 4
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Figure 4. Current–voltage characteristics of different 100 µm diameter junctions: (a) without SAM, (b) with different APTMS-alkanoic acid SAMs. Inset: representative characteristics at a larger bias range (−1 to +1 V), (c) with TCPP SAM, and (d) with APTMS-TCPP SAM. The curves in (a) and (b) are averaged while those in (c) and (d) are representative. The error bars in (a) and (b) represent the standard deviation upon averaging of at least 12 junctions.
that the higher resistance by two orders of magnitude results from the APTMS binding scheme. Surprisingly, we could not find published resistance values of porphyrin-based SAMs. We can therefore compare our result to the resistance of a single 5-(4-carboxyphenyl)-20-(4-pyridyl)-10,15-bis(2,4,6trimethylphenyl)porphyrin measured using the STM-based break junction method in a ITO/porphyrin/Au junction, which was 2.5 × 1010 [20]. However, a difference of several orders of magnitude in the transport properties between the same molecule measured as a single molecule or within a SAM is well documented and is attributed to changes in the molecular environment [49, 50], hence the comparison here is questionable. SAMs of APTMS only were shown to have lower conductance than SAMs of longer molecular chains attached to it (figure 4(b)), possibly due to partial protonation of unbound amine end groups [51]. Such protonation is expected in our junctions, where the basic amine groups are in contact with the acidic PEDOT:PSS dispersion (pH 2.2). The amide groups, formed by amidization of the carboxylic acid with the amine groups, are less basic, therefore they do not protonate upon contact with the PEDOT:PSS suspension. The polar amide bonds, as well as protonated unbound amines, are probably responsible for the increased resistance observed in our junctions. Binding via APTMS also induced a large molecular tilt and disorder in the attached layer, as expected from its deposition on the nm-rough ITO surface, in accordance with previous works on silane-bound SAMs on disordered surfaces [38], and with the expected reduced order in SAMs on semiconductor surfaces [7]. The SAM disorder was also demonstrated by the absorption spectra of adsorbed APTMSTCPP (figure 2(b)). Thus, APTMS binding is postulated to
the junction transport. Its advantages include reproducibility and excellent surface coverage preventing electrical shorts, as expressed in the yield of junctions which was evaluated from transport measurements as ∼80% for APTMS-based SAMs. This high yield is only slightly smaller than that observed by de Boer et al [35] on flat Au substrates (95%), probably due to worse homogeneity of the ITO and its surface roughness (figure S2 available at stacks.iop.org/Nano/24/ 455204/mmedia). The yield was lower when carboxylic acids were used for molecular binding (∼70%), due to the lower coverage with carboxylic acid binding, as shown above. We postulate that APTMS-SAM is dense due to its intermolecular binding, preventing shorts even if the molecular layer bound to it is of lower density. However, APTMS binding resulted in increased junction resistance compared to molecules of similar length but different molecule/electrode binding schemes. The properties of single molecules within the SAM junctions were calculated, taking into account the adsorption density of 1018 m−2 , and compared to published values. The resistance in a ITO/APTMS-palmitic acid (19 carbons)/PEDOT:PSS junction was found to be 5 × 1015 /molecule, compared to 2 × 1013 /molecule in ITO/octadecanethiol (18 carbons)/Al junctions [21]. The current through a single molecule in Au/decanedithiol (10 carbons)/PEDOT:PSS at 0.2 V was estimated to be 10 fA [35], while that in ITO/APTMSoctanoic acid (11 carbons)/PEDOT:PSS was 0.15 fA. Hence, APTMS binding increases the resistance by two orders of magnitude compared to thiol binding. The resistance of a single porphyrin molecule in our ITO/APTMS-TCPP SAM/PEDOT:PSS junctions was found to be 1.3 × 1016 , and that of a single porphyrin in our ITO/TCPP/PEDOT:PSS (no APTMS) junctions was 1.3 × 1014 , further confirming 5
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Figure 5. Current at 100 mV for 100 µm diameter junctions versus alkyl chain length. The error bars represent the standard deviation upon averaging of at least 12 junctions.
Figure 7. Current–voltage characteristics of junctions with APTMS-TCPP SAM, in the dark and under 532 nm laser illumination.
be responsible for the observed unexpectedly small tunnelling decay constant. Analysing the current dependence on the alkyl chain length at a given bias shows decay in current with increasing chain length (l), as expected from Simmon’s model suggesting that I ∼ exp(−βl). The tunnelling decay constant ˚ −1 (figure 5), significantly smaller than β is 0.125 ± 0.019 A decay constants found for thiol–alkyl monolayers adsorbed ˚ −1 at 1 V) [35]. This difference can be on Au (0.6–0.7 A explained by the large tilt angle of the alkyl chain with respect to the substrate plane [44], which will effectively reduce the monolayer thickness. A 48◦ –70◦ tilt was calculated for the alkyl chain bound to APTMS (figure 6), due to the amide ˚ −1 requires a tilt bond. A tunnelling decay constant of 0.65 A angle of 79◦ , in agreement with our modelling, taking into account the differences expected in a SAM with respect to single molecule modelling. Other possible explanations for a layer thickness smaller than expected are sub-monolayer coverage by alkyls due to incomplete binding to APTMS and alkyl chain folding [33]. A disordered and largely tilted SAM can cause intermolecular (rather than intra-molecular) electron transport, further limiting transport by a hopping mechanism, which implies weak length dependence of the transport, hence a small decay constant value. A weak dependence on molecular length may also be related to large contact resistance dominating the transport properties.
High contact resistance results from dipoles associated with APTMS binding to the ITO, as discussed above. Summarizing the various effects induced by molecular binding via APTMS, we find that APTMS allows high junction yields and the formation of dense molecular layers preventing short, but it also induces significant junction resistance and SAM disorder. Carboxylic acid binding provides better conductance but lower junction yield, probably due to worse surface coverage. Other binding schemes, such as via phosphonic acids, are currently being studied for molecular junctions on ITO. The optoelectronic capabilities of the junctions formed on ITO are demonstrated in figure 7, which shows photoconductance when junctions containing the photoactive APTMS-bound TCPP SAM are illuminated at a wavelength corresponding to the first Q-band absorption of the porphyrin (figure 2). Hence, molecular photoexcitation results in a change in the junction’s transport properties. The photoconductance mechanism and other optoelectronic properties of these junctions will be discussed elsewhere [46]. The junction’s optoelectronic capabilities are also demonstrated in the absorption spectrum of the TCPP SAM measured through the transparent electrodes in the completed junctions (figure 2(d)).
Figure 6. Three-dimensional models of APTMS amides with (a) butyric acid, (b) octanoic acid, and (c) palmitic acid. The models were produced with Chem3D Pro 12.0 software utilizing the MM2 force field model. The alkanoic chain lengths in trans-configuration deduced ˚ octanoic acid—8.9 A, ˚ palmitic acid—19.1 A. ˚ from this modelling were: butyric acid—3.8 A, 6
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4. Summary and conclusions
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Metal free, micron-sized SAM-based junctions were realized on a transparent conductive oxide (ITO), with PEDOT:PSS as the counter-electrodes, with high fabrication yields. Importantly, the electrical transport was shown to be dominated by the nature of the SAM in these metal-free junctions. The use of amino-silane (APTMS) as the chemical binding scheme to ITO was found to be significant in determining the transport properties of the junctions. APTMS allows high junction yields and the formation of dense molecular layers preventing electrical short. However, the conductance per single molecule within the junction was found to be two orders of magnitude smaller than in corresponding thiol-bound molecular junctions due to the polar silane binding to the ITO and an amide bond between the SAM and the APTMS binder. The transport properties of the APTMS-only SAM were also measured, and protonation of the amine end groups was found to decrease the junction conductance compared to longer APTMS-SAMs. Since the amino-silane binding scheme is widely used in molecular adsorption to metal oxide surfaces, its electronic implications can be relevant also to fields other than molecular electronics, employing organic/metal oxide interfaces. Examples include organic photovoltaics, organic light emitting diodes, solar fuels etc. As the effect of the molecular structure on transport properties is clearly observed in our junctions, if the fabrication technology is consistent, we conclude that such metal-free junctions are suitable for characterizing the effect of external stimuli, which otherwise might be masked by the response of metallic electrodes. Such stimuli include the effect of photon excitation on the transport properties of photo-active molecules, i.e. molecular optoelectronic properties, as well as thermoelectric effects in molecular junctions. A preliminary demonstration of a change in the junction’s transport properties as a result of SAM photo-excitation is presented using APTMS-bound porphyrin SAM.
Acknowledgments The authors are grateful to Dr Rafi Shikler (BGU) for use of lab equipment and for helpful ideas, and to Professor Leeor Kronik (Weizmann Inst. of Science) for fruitful discussions.
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