Forming reliable and reproducible molecule−nanoelectrode contacts is one of the key issues for the implementation of nanoparticles as functional units into nanoscale devices. Utilizing heterometallic electrodes and Janus-type nanoparticles equipped with molecules allowing selective binding to a distinct electrode material represents a promising approach to achieve this goal. Here, the directed immobilization of individual Janus-type gold nanoparticles (AuNP) between heterometallic electrodes leading to the formation of asymmetric contacts in a highly controllable way is presented. The Janus-AuNP are stabilized by two types of ligands with different terminal groups on opposite hemispheres. The heterometallic nanoelectrode gaps are formed by electron beam lithography in combination with a self-alignment procedure and are adjusted to the size of the Janus-AuNP. Thus, by choosing adequate molecular end group/metal combinations, the immobilization direction of the Janus-AuNP is highly controllable. These results demonstrate the striking potential of this approach for the building-up of novel nanoscale organic/inorganic hybrid architectures.
Chemical templates for the patterned immobilization of gold nanoparticles were fabricated by soft UV nanoimprint lithography. The template structures were fabricated by means of the consecutively performed process steps of nanoimprint lithography, reactive ion etching, chemical functionalization with amino groups, and lift-off of imprint resist. These chemical templates were used for the defined assembly of 20 nm diameter citrate stabilized gold nanoparticles from aqueous solution. By reducing the ionic strength of the solution, one- and zero-dimensional particle assemblies were generated on sub-100-nm template structures. By this means, the pattern resolution predefined by the lithography process could be easily enhanced by dilution of the nanoparticle solution.
Integration of molecule-capped gold nanoparticles (AuNP) into nanoelectronic devices requires detailed knowledge about the AuNP-electrode interface. Here, we report the pH-dependent adsorption of amine or carboxylic acid-terminated gold nanoparticles on platinum or gold/palladium (30% Pd) alloy, respectively. We synthesized amine-terminated AuNP, applying a new solid phase supported approach, as well as AuNP exhibiting carboxylic acid as terminal groups. The pH-induced agglomeration of the synthesized AuNP was investigated by UV-vis, DLS, and ζ-potential measurements. Depending on the pH and the ionic strength of the AuNP solution a preferential adsorption on the different metals occurred. Thereby, we demonstrate that by choosing the appropriate functional group and adjusting the pH as well as the ionic strength a directed binding can be achieved, which is an essential prerequisite for applications of these particles in nanoelectronics. These findings will pave the way for a controlled designing of the interface between molecule-capped AuNP and metallic electrodes for applications in nanoelectronics.
One concept to build up hybrid electronic devices based on molecules or nanoparticles with rectifying properties is based on nanoscale objects that are immobilized between two electrodes composed of different metals forming asymmetric contacts. Following this concept, we introduce an optimized procedure to fabricate heterometallic nanoelectrodes with a separation of only 5 nm. Gold nanoparticles (AuNPs) with a diameter of 15 nm, stabilized with 4-mercaptophenylamine, were used to form electrode1molecule/AuNP/molecule-electrode2 devices comprising at most a small number of AuNPs. Immobilization was performed by dielectrophoretic trapping. The molecular properties of 4-mercaptophenylamine are reflected in transition voltage spectroscopy features of the device. Cyclic current−voltage measurements on 20 functional devices revealed distinct differences in conductivities based on minor differences in device geometry. Analysis of the electron transport characteristics discloses that under these experimental conditions an asymmetric contact configuration alone is not sufficient for building up a molecule-based rectifier.
Transition metal complexes are electrofunctional molecules due to their high conductivity and their intrinsic switching ability involving a metal-to-ligand charge transfer. Here, a method is presented to contact reliably a few to single redox-active Ru-terpyridine complexes in a CMOS compatible nanodevice and preserve their electrical functionality. Using hybrid materials from 14 nm gold nanoparticles (AuNP) and bis-{4′-[4-(mercaptophenyl)-2,2′:6′,2″-terpyridine]}-ruthenium(II) complexes a device size of 302 nm2 inclusive nanoelectrodes is achieved. Moreover, this method bears the opportunity for further downscaling. The Ru-complex AuNP devices show symmetric and asymmetric current versus voltage curves with a hysteretic characteristic in two well separated conductance ranges. By theoretical approximations based on the single-channel Landauer model, the charge transport through the formed double-barrier tunnel junction is thoroughly analyzed and its sensibility to the molecule/metal contact is revealed. It can be verified that tunneling transport through the HOMO is the main transport mechanism while decoherent hopping transport is present to a minor extent.
Terpyridine derivatives reveal rich coordination chemistry and are frequently used to construct reliable metallo-supramolecular wires, which are promising candidates for optoelectronic or nanoelectronic devices. Here, we examine especially the terpyridine/electrode interface, which is a critical point in these organic/inorganic hybrid architectures and of utmost importance with respect to the device performance. We use the approach to assemble nanodevices by immobilization of single terpyridine-functionalized gold nanoparticles with a diameter of 13 nm in between nanoelectrodes with a separation of about 10 nm. Conductance measurements on the formed double-barrier tunnel junctions reveal several discrete conductance values in the range of 10–9–10–7 S. They can be attributed to distinct terpyridine/electrode contact geometries by comparison with conductance values estimated based on the Landauer formula. We could clearly deduce that the respective terpyridine/metal contact determines the length of the tunneling path through the molecule and thus the measured device conductance. Furthermore, the formation of a distinct terpyridine/electrode contact geometry correlates with the chemical pretreatment of the terpyridine ligand shell of the gold nanoparticles with an alkaline solution. By applying infrared reflection absorption spectroscopy, we found that only a chemical treatment with a concentrated ammonia solution results in effective deprotonation of the terpyridine anchor group. This enables the electrical contact to the middle pyridyl ring and thus a short tunneling path through the molecule corresponding to a high conductance value. These findings indicate a way to control the contact geometry at the terpyridine/metal interface, which is a prerequisite for reliable nanodevices based on this class of molecules.
The chiral half-sandwich complexes 3 and 4 are formed with high diastereoselectivities in the reaction of cyclopentadienides 2a,b, bearing tethered phosphaferrocene donor moieties with planar chirality, and [(PPh 3 ) 3 RuCl 2 ] in toluene at 90°C. The diastereoisomers of the Cp complex 3 are obtained in a 95:5 ratio, whereas for the Cp* derivative 4 only one isomer is detectable, the structure of which has been determined by X-ray diffraction. Substitution of the chloride ligand in 4 by other anionic (H -, I -) or neutral (H 2 , py) ligands proceeds stereospecifically in all cases. In contrast, conversion of the chlorides 3a,b (95:5) to the respective hydrides 9a,b proceeds with complete epimerization at Ru. In CHCl 3 the 1:1 mixture of hydrides 9a,b is reconverted to the chlorides to give a kinetically controlled 4:1 mixture of isomers 3a,b. Equilibration of this mixture in toluene at 90°C restores the original ratio of isomers of 95:5, which we therefore believe to reflect the thermodynamically controlled value. The cationic H 2 complex 7, generated via Cl abstraction from 4 in the presence of H 2 , was characterized to be a η 2 -dihydrogen complex by measuring the T 1 value of the coordinated H 2 ligand.
Self-assembled monolayers (SAMs) of ruthenium-based molecular wires on solid surfaces are of great interest for optoelectronic and nanoelectronic applications. Here, we present a novel reactive Ru precursor, which enabled us to grow SAMs of Ru complex wires on Au surfaces even at room temperature. Thus, the Ru complex wire growth can be performed easily by sequential reaction of the reactive Ru precursor with terpyridine ligands without the harsh reaction conditions needed otherwise. Subsequently, we monitored the stepwise growth using infrared reflection absorption spectroscopy (IRRAS) and surface-enhanced Raman spectroscopy (SERS). A comparison of IRRAS and SERS data with theoretical spectra, derived from density functional theory calculations, enabled us to verify the formation of each individual growth step. Furthermore, we used these data to determine the orientation of the Ru-based molecular wires with respect to the Au surface. Growth step-dependent layer thicknesses obtained from variable angle spectroscopic ellipsometry verify the spectroscopic results. Thus, we provide a room-temperature method to realize Ru complex wire growth based on a reactive Ru precursor.
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