Nanofabrication and electrochemical characterization of self-assembled monolayers sandwiched between metal nanoparticles and electrode surfaces

et al.Nanoscience and nanotechnology have reached the syllabi of many upper-division undergraduate and master-level courses all over the world. There is therefore a growing need for practical exercises that illustrate the fabrication, characterization, properties, and applications of nanomaterials. Here we describe an advanced-level laboratory experiment in which students had the opportunity to fabricate surfaces modified by ordered monolayers and nanostructured materials. The surface modification was quantified by means of a quartz crystal microbalance, while the electrochemical properties of the nanoarchitectures were assessed using cyclic voltammetry experiments. Electron transfer across self-assembled monolayers mediated by gold nanoparticles was presented as a topic for discussion, and consideration of potential practical applications of the observed phenomena (catalytic and electrocatalytic processes, as well as development of optical, (opto)electronic, and photovoltaic devices with enhanced properties) was proposed as a further reading exercise.Financial support from the Department of Physical Chemistry and DGA/Fondos Feder is acknowledge as well as CTQ2012-33198 and CTQ2013-50187-EXP grants.Peer Reviewe

Fabricación del electrodo superior en dispositivos moleculares mediante la formación de enlaces covalentes σ C-Au

Resumen del póster presentado a la 6ª Jornada de Jóvenes Investigadores en Física y Química de Aragón celebrada en Zaragoza el 20 de noviembre de 2014.-- et al.Peer reviewe

Electrical characterization of single molecule and Langmuir-Blodgett monomolecular films of a pyridine-terminated oligo(phenylene-ethynylene) derivative

Monolayer Langmuir–Blodgett (LB) films of 1,4-bis(pyridin-4-ylethynyl)benzene (1) together with the “STM touch-to-contact” method have been used to study the nature of metal–monolayer–metal junctions in which the pyridyl group provides the contact at both molecule–surface interfaces. Surface pressure vs area per molecule isotherms and Brewster angle microscopy images indicate that 1 forms true monolayers at the air–water interface. LB films of 1 were fabricated by deposition of the Langmuir films onto solid supports resulting in monolayers with surface coverage of 0.98 × 10−9 mol·cm−2. The morphology of the LB films that incorporate compound 1 was studied using atomic force microscopy (AFM). AFM images indicate the formation of homogeneous, monomolecular films at a surface pressure of transference of 16 mN·m−1. The UV–vis spectra of the Langmuir and LB films reveal that 1 forms two dimensional J-aggregates. Scanning tunneling microscopy (STM), in particular the “STM touch-to-contact” method, was used to determine the electrical properties of LB films of 1. From these STM studies symmetrical I–V curves were obtained. A junction conductance of 5.17 × 10−5 G 0 results from the analysis of the pseudolinear (ohmic) region of the I–V curves. This value is higher than that of the conductance values of LB films of phenylene-ethynylene derivatives contacted by amines, thiols, carboxylate, trimethylsilylethynyl or acetylide groups. In addition, the single molecule I–V curve of 1 determined using the I(s) method is in good agreement with the I–V curve obtained for the LB film, and both curves fit well with the Simmons model. Together, these results not only indicate that the mechanism of transport through these metal–molecule–metal junctions is non-resonant tunneling, but that lateral interactions between molecules within the LB film do not strongly influence the molecule conductance. The results presented here complement earlier studies of single molecule conductance of 1 using STM-BJ methods, and support the growing evidence that the pyridyl group is an efficient and effective anchoring group in sandwiched metal–monolayer–metal junctions prepared under a number of different conditions.S. M., M. C. L, and P. C. are grateful for financial assistance from Ministerio de Economía y Competitividad from Spain in the framework of project CTQ2012-33198 and support from DGA and Fondos Feder for funding the Platon research group. S. M. also thanks his JIUZ-2013-CIE-05 grant. S. M and P. C. are grateful for the award of the CTQ2013-50187-EXP grant. P. J. L. thanks EPSRC for funding and also gratefully acknowledges support from the Australian Research Council (DP 140100855) and the award of the Future Fellowship (FT120100073). R. J. N. and S. J. H. thank EPSRC for funding (grant EP/H035184/1 and EP/K007785/1).Peer Reviewe

Electrical characterization of single molecule and Langmuir–Blodgett monomolecular films of a pyridine-terminated oligo(phenylene-ethynylene) derivative

Monolayer Langmuir–Blodgett (LB) films of 1,4-bis(pyridin-4-ylethynyl)benzene (1) together with the “STM touch-to-contact” method have been used to study the nature of metal–monolayer–metal junctions in which the pyridyl group provides the contact at both molecule–surface interfaces. Surface pressure vs area per molecule isotherms and Brewster angle microscopy images indicate that 1 forms true monolayers at the air–water interface. LB films of 1 were fabricated by deposition of the Langmuir films onto solid supports resulting in monolayers with surface coverage of 0.98 × 10-9 mol·cm-2. The morphology of the LB films that incorporate compound 1 was studied using atomic force microscopy (AFM). AFM images indicate the formation of homogeneous, monomolecular films at a surface pressure of transference of 16 mN·m-1. The UV–vis spectra of the Langmuir and LB films reveal that 1 forms two dimensional J-aggregates. Scanning tunneling microscopy (STM), in particular the “STM touch-to-contact” method, was used to determine the electrical properties of LB films of 1. From these STM studies symmetrical I–V curves were obtained. A junction conductance of 5.17 × 10-5 G0 results from the analysis of the pseudolinear (ohmic) region of the I–V curves. This value is higher than that of the conductance values of LB films of phenylene-ethynylene derivatives contacted by amines, thiols, carboxylate, trimethylsilylethynyl or acetylide groups. In addition, the single molecule I–V curve of 1 determined using the I(s) method is in good agreement with the I–V curve obtained for the LB film, and both curves fit well with the Simmons model. Together, these results not only indicate that the mechanism of transport through these metal–molecule–metal junctions is non-reso- nant tunneling, but that lateral interactions between molecules within the LB film do not strongly influence the molecule conduc- tance. The results presented here complement earlier studies of single molecule conductance of 1 using STM-BJ methods, and support the growing evidence that the pyridyl group is an efficient and effective anchoring group in sandwiched metal–monolayer–metal junctions prepared under a number of different conditions

Low variability of single-molecule conductance assisted by bulky metal-molecule contacts

A detailed study of the trimethylsilylethynyl moiety, –C[triple bond]CSiMe3 (TMSE) , as an anchoring group in metalmoleculemetal junctions, using a combination of experiment and density functional theory is presented. It is shown that the TMSE anchoring group provides improved control over the molecule-substrate arrangement within metalmoleculemetal junctions, with the steric bulk of the methyl groups limiting the number of highly transmissive binding sites at the electrode surface, resulting in a single sharp peak in the conductance histograms recorded by both the in situ break junction and I(s) STM techniques. As a consequence of the low accessibility of the TMSE group to surface binding configurations of measurable conductance, only about 10% of gold break junction formation cycles result in the clear formation of molecular junctions in the experimental histograms. The DFT-computed transmission characteristics of junctions formed from the TMSE-contacted oligo(phenylene)ethynylene (OPE)-based molecules described here are dominated by tunneling effects through the highest-occupied molecular orbitals (HOMOs). This gives rise to similar conductance characteristics in these TMSE-contacted systems as found in low conductance-type junctions based on comparably structured OPE-derivatives with amine-contacts that also conduct through HOMO-based channels.R. R. F. thanks the Consejería de Educación del Principado de Asturias for a Severo Ochoa grant (BP11-069). V. M. G.-S. thanks the Spanish Ministerio de Economía y Competitividad for a Ramón y Cajal fellowship (RYC-2010-06053). R. R. F., J. F. and V. M. G.-S. wish to acknowledge financial support from the Spanish grant FIS2012-34858 and the Marie Curie Network MOLESCO. P. C. and S. M. are grateful for financial assistance from the Ministerio de Economía y Competitividad of Spain in the framework of the project CTQ2012-33198 as well as the award of the CTQ2013-50187-EXP grant. H. M. O., P. C., and S. M. thank the support from DGA and Fondos FEDER for funding through the Platon research group. H. M. O. is also grateful for financial assistance from the Secretaría Nacional de Educación Superior, Ciencia, Tecnología e Innovaciín from Ministerio de Educación (Ecuador). S. M. thanks the Ministerio de Educación from Spain for financial support through the framework of the Campus de Excelencia Internacional, CEI Iberus. S. J. H., R. J. N., P. J. L. and S. M.-G. thank the EPSRC for funding (EPSRC grants EP/K007785/1, EP/H035184/1, EP/K007548/1, EP/H005595/1). P. J. L. holds an Australian Research Council Future Fellowship (FT120100073) and gratefully acknowledges funding for this work from the ARC (DP140100855).Peer Reviewe

Towards the design of effective multipodal contacts for use in the construction of Langmuir-Blodgett films and molecular junctions

As part of on-going efforts to optimize the electrical performance and stability of molecular electronic components, anchor groups that bind molecules to electrode surfaces via multiple points of connection (multipodal contacts) have begun to attract attention. Here an oligo(arylene)ethynylene (OAE) derivative with ‘tripodal’ 2,6-bis((methylthio)methyl)pyridine anchoring groups at both molecular termini has been prepared and used to form well-ordered monolayer Langmuir films at the air–water interface. These films were transferred onto solid supports (surface pressure of transference 8 mN m−1) to give homogeneous, densely packed, monolayer Langmuir–Blodgett (LB) films, which efficiently block a gold electrode surface. Within the surface-supported LB film, the molecules are oriented with a tilt angle of approximately 30° to the surface normal and contacted through both the ‘buttressed’ methylthioether groups and the pyridine nitrogen atom, as determined by X-ray photoelectron spectroscopy (XPS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Molecular junctions formed by contact of a single molecule within the film by the “STM touch-to-contact” method give a most probable molecular conductance of 4.4 × 10−5G0. This value compares well with the single molecule conductance of 1,4-bis(pyridin-4-ylethynyl)benzene determined by a variety of methods (3.2–5.4 × 10−5G0), indicating that the addition of the buttressing groups does not perturb the favourable electrical characteristics of the pyridyl contacting group. Consistent with these conductance data, a transition voltage (Vtrans = 0.48 V) was observed for this ‘buttressed’, pyridine-contacted OAE derivative, indicating relatively good alignment of the metal electrode Fermi level and the frontier molecular orbitals.E. E. gratefully acknowledges the award of a DGA fellowship from Government of Aragon. P. C. is grateful for financial assistance from Ministerio de Economía y Competitividad from Spain and fondos FEDER in the framework of the project MAT2016-78257-R. S. M. and P. C. also acknowledge DGA/fondos FEDER (construyendo Europa desde Aragón) for funding the research group Platón (E31_17R). R. J. N. and D. C. M are grateful for financial assistance from the EPSRC (grants EP/M029522/1, EP/M005046/1 and EP/M014169/1). P. J. L. gratefully acknowledges support from the Australian Research Council (DP190100073 and DP190100074). S. B. thanks the University of Western Australia for the award of an International Postgraduate Research Scholarship. The authors also thank Dr G. Antorrena for technical support in XPS studies. H. M. O. is grateful for financial assistance from Escuela Politécnica Nacional in the framework of projects PII-DFIS-02-2018 and PIS-17-12.Peer reviewe

Influence of surface coverage on the formation of 4,4 '- bipyridinium (viologen) single molecular junctions

Single-molecule conductance experiments using the STM-based I(s) method and samples of N,N’-di(4-(trimethylsilylethynyl)benzyl)-4,4’-bipyridinium bis(tetrafluoroborate) ([1](BF4)2) prepared on gold substrates with low-surface coverage of [1](BF4)2 (Γ = 1.25·10⁻¹¹ mol·cm⁻²) give rise to molecular junctions with two distinct conductance values. From the associated break-off distances and comparison experiments with related compounds the higher conductance junctions are attributed to molecular contacts between the molecule and the electrodes via the N,N’-dibenzyl-4,4´-bipyridinium (viologen) moiety and one trimethylsilylethynyl (TMSE) group (G = (5.4 ± 0.95)×10⁻⁵ G0, break-off distance (1.56 ± 0.09) nm). The second, lower conductance junction (G = (0.84 ± 0.09)×10⁻⁵ G0) is consistent with an extended molecular conformation between the substrate and tip contacted through the two TMSE groups giving rise to a break-off distance (1.95 ± 0.12) nm that compares well with the Si...Si distance (2.0 nm) in the extended molecule. Langmuir monolayers of [1](BF4)2 formed at the air-water interface can be transferred onto a gold-on-glass substrate by the Langmuir-Blodgett (LB) technique to give well-ordered, compact films with surface coverage Γ = 2.0·10⁻¹⁰ mol·cm⁻². Single-molecule conductance experiments using the STM-based I(s) method reveal only the higher conductance junctions (G = (5.4 ± 0.95)×10⁻⁵ G0, break-off distance (1.56 ± 0.09) nm) due to the restricted range of molecular conformations in the tightly packed, well-ordered LB films.S.M. and P.C. are grateful for financial assistance from Ministerio de Economía y Competitividad from Spain and fondos FEDER in the framework of projects MAT2016-78257-R. S.M. and P.C. also acknowledge DGA/fondos FEDER (construyendo Europa desde Aragón) for funding the research group Platón (E-54). S.M. acknowledges funding from the University of Zaragoza (grant number JIUZ02016-CIE-04). R.J.N, S.J.G and D.C.M are grateful for financial assistance from the EPSRC (grant EP/M029522/1). P.J.L. and J.B.G.G. gratefully acknowledge support from the Australian Research Council (FT120100073; DP140100855).Peer reviewe

Robust large area molecular junctions based on transparent and flexible electrodes

Electrografting of an oligophenylene ethynylene monolayer (OPEH) onto a Mylar®-supported PEDOT : PSS and graphene substrate prior to wet-transfer of a graphene top-electrode has been used to create flexible and transparent large-area Mylar–PEDOT : PSS|OPEH|Graphene molecular junctions. The electrical conductance of the Mylar–PEDOT : PSS|OPEH|Graphene structure was determined by conductive probe atomic force microscopy (c-AFM), sampling various regions across the surfaces of multiple devices. All registered I–V curves (sampling size ∼150 tests) give a sigmoidal response, consistent with through molecule conductance and ruling out the presence of direct electrode contacts or short-circuits. The demonstration of the combined use of Mylar-supported PEDOT : PSS and graphene as alternative electrode materials to conventional metal thin film electrodes in large-area molecular junctions opens avenues to enable flexible and transparent molecular (opto)electronic devices.P. C. and S. M. acknowledge PID2019-105881RB-I00 (MCIN/AEI/10.13039/501100011033), PID2022-14143300B-I00 and TED2021-131318B-I00 grant funded by the MCIN/AEI/10.13039/501100011033 and “European Union NextGenerationEU/PRTR”. These authors also acknowledge DGA/Fondos FEDER (Construyendo Europa desde Aragón) for funding the research group Platón (E31-23R). H. M. O. is grateful for financial assistance from Escuela Politécnica Nacional in the framework of project PIGR-19-04. G. R. acknowledges her contract under the programme Ayudas Ramon y Cajal, Reference RYC-2016-21412. P. J. L. gratefully acknowledges support from the Australian Research Council (DP220100790).Peer reviewe

Electrical characterization of single molecule and Langmuir–Blodgett monomolecular films of a pyridine-terminated oligo(phenylene-ethynylene) derivative

Monolayer Langmuir–Blodgett (LB) films of 1,4-bis(pyridin-4-ylethynyl)benzene (1) together with the “STM touch-to-contact” method have been used to study the nature of metal–monolayer–metal junctions in which the pyridyl group provides the contact at both molecule–surface interfaces. Surface pressure vs area per molecule isotherms and Brewster angle microscopy images indicate that 1 forms true monolayers at the air–water interface. LB films of 1 were fabricated by deposition of the Langmuir films onto solid supports resulting in monolayers with surface coverage of 0.98 × 10−9 mol·cm−2. The morphology of the LB films that incorporate compound 1 was studied using atomic force microscopy (AFM). AFM images indicate the formation of homogeneous, monomolecular films at a surface pressure of transference of 16 mN·m−1. The UV–vis spectra of the Langmuir and LB films reveal that 1 forms two dimensional J-aggregates. Scanning tunneling microscopy (STM), in particular the “STM touch-to-contact” method, was used to determine the electrical properties of LB films of 1. From these STM studies symmetrical I–V curves were obtained. A junction conductance of 5.17 × 10−5 G0 results from the analysis of the pseudolinear (ohmic) region of the I–V curves. This value is higher than that of the conductance values of LB films of phenylene-ethynylene derivatives contacted by amines, thiols, carboxylate, trimethylsilylethynyl or acetylide groups. In addition, the single molecule I–V curve of 1 determined using the I(s) method is in good agreement with the I–V curve obtained for the LB film, and both curves fit well with the Simmons model. Together, these results not only indicate that the mechanism of transport through these metal–molecule–metal junctions is non-resonant tunneling, but that lateral interactions between molecules within the LB film do not strongly influence the molecule conductance. The results presented here complement earlier studies of single molecule conductance of 1 using STM-BJ methods, and support the growing evidence that the pyridyl group is an efficient and effective anchoring group in sandwiched metal–monolayer–metal junctions prepared under a number of different conditions

Low variability of single-molecule conductance assisted by bulky metal–molecule contacts

A detailed study of the trimethylsilylethynyl moiety, –C[triple bond, length as m-dash]CSiMe3 (TMSE), as an anchoring group in metal|molecule|metal junctions, using a combination of experiment and density functional theory is presented. It is shown that the TMSE anchoring group provides improved control over the molecule–substrate arrangement within metal|molecule|metal junctions, with the steric bulk of the methyl groups limiting the number of highly transmissive binding sites at the electrode surface, resulting in a single sharp peak in the conductance histograms recorded by both the in situ break junction and I(s) STM techniques. As a consequence of the low accessibility of the TMSE group to surface binding configurations of measurable conductance, only about 10% of gold break junction formation cycles result in the clear formation of molecular junctions in the experimental histograms. The DFT-computed transmission characteristics of junctions formed from the TMSE-contacted oligo(phenylene)ethynylene (OPE)-based molecules described here are dominated by tunneling effects through the highest-occupied molecular orbitals (HOMOs). This gives rise to similar conductance characteristics in these TMSE-contacted systems as found in low conductance-type junctions based on comparably structured OPE-derivatives with amine-contacts that also conduct through HOMO-based channels