On the mechanical and electronic properties of thiolated gold nanocrystals

Redox-controlled conductance of polyoxometalate molecular junctions

We demonstrate the reversible in situ photoreduction of molecular junctions of a phosphomolybdate [PMo₁₂O₄₀]^3- monolayer self-assembled on flat gold electrodes, connected by the tip of a conductive atomic force microscope. The conductance of the one electron reduced [PMo₁₂O₄₀]^4- molecular junction is increased by ∼10, and this open-shell state is stable in the junction in air at room temperature. The analysis of a large current-voltage dataset by unsupervised machine learning and clustering algorithms reveals that the electron transport in the pristine phosphomolybdate junctions leads to symmetric current-voltage curves, controlled by the lowest unoccupied molecular orbital (LUMO) at 0.6-0.7 eV above the Fermi energy with ∼25% of the junctions having a better electronic coupling to the electrodes than the main part of the dataset. This analysis also shows that a small fraction (∼18% of the dataset) of the molecules is already reduced. The UV light in situ photoreduced phosphomolybdate junctions systematically feature slightly asymmetric current-voltage behaviors, which is ascribed to the electron transport mediated by the single occupied molecular orbital (SOMO) nearly at resonance with the Fermi energy of the electrodes and by a closely located single unoccupied molecular orbital (SUMO) at ∼0.3 eV above the SOMO with a weak electronic coupling to the electrodes (∼50% of the dataset) or at ∼0.4 eV but with a better electrode coupling (∼50% of the dataset). These results shed light on the electronic properties of reversible switchable redox polyoxometalates, a key point for potential applications in nanoelectronic devices.

Area-Selective Atomic Layer Deposition of TiN Using Trimethoxy(octadecyl)silane as a Passivation Layer

Area-selective deposition (ASD) offers tremendous advantages when compared with conventional patterning processes, such as the possibility of achieving three-dimensional features in a bottom-up additive fashion. Recently, ASD is gaining more and more attention from IC manufacturers and equipment and material suppliers. Through combination of self-assembled monolayer (SAM) surface passivation of the nongrowth substrate area and atomic layer deposition (ALD) on the growth area, ASD selective to the growth area can be achieved. With the purpose of screening SAM precursors to provide optimal passivation performance on SiO₂, various siloxane precursors with different terminal groups and alkyl chains were investigated. Additionally, the surface dependence and growth inhibition of TiN ALD on -NH₂, -CF₃, and -CH₃ terminations is investigated. We demonstrated the methyl termination of the SAM precursor combined with a C18 alkyl chain plays an important role in broadening the ALD selectivity window by suppressing precursor adsorption. Owing to the high surface coverage, excellent thermal stability and longer carbon chain length, an optimized trimethoxy(octadecyl)silane (TMODS) film deposited from liquid phase was able to provide a selectivity higher than 0.99 up to 20 nm ALD film deposited on hydroxyl-terminated Si oxide. The approach followed in this work can allow extending the ASD process window, and it is relevant for a wide variety of applications.

Long-range electron transport in Prussian blue analog nanocrystals

We report electron transport measurements through nano-scale devices consisting of 1 to 3 Prussian blue analog (PBA) nanocrystals connected between two electrodes. We compare two types of cubic nanocrystals, CsCoIIIFeII (15 nm) and CsNiIICrIII (6 nm), deposited on highly oriented pyrolytic graphite and contacted by conducting-AFM. The measured currents show an exponential dependence with the length of the PBA nano-device (up to 45 nm), with low decay factors β, in the range 0.11-0.18 nm-1 and 0.25-0.34 nm-1 for the CsCoFe and the CsNiCr nanocrystals, respectively. From the theoretical analysis of the current-voltage curve for the nano-scale device made of a single nanoparticle, we deduce that the electron transport is mediated by the localized d bands at around 0.5 eV from the electrode Fermi energy in the two cases. By comparison with previously reported ab initio calculations, we tentatively identify the involved orbitals as the filled Fe(ii)-t2g d band (HOMO) for CsCoFe and the half-filled Ni(ii)-eg d band (SOMO) for CsNiCr. Conductance values measured for multi-nanoparticle nano-scale devices (2 and 3 nanocrystals between the electrodes) are consistent with a multi-step coherent tunneling in the off-resonance regime between adjacent PBAs, a simple model gives a strong coupling (around 0.1-0.25 eV) between the adjacent PBA nanocrystals, mediated by electrostatic interactions.

Electrical molecular switch addressed by chemical stimuli

We demonstrate that the conductance switching of benzo-bis(imidazole) molecules upon protonation depends on the lateral functional groups. The protonated H-substituted molecule shows a higher conductance than the neutral one (Gpro > Gneu), while the opposite (Gneu > Gpro) is observed for a molecule laterally functionalized by amino-phenyl groups. These results are demonstrated at various scale lengths: self-assembled monolayers, tiny nanodot-molecule junctions and single molecules. From ab initio theoretical calculations, we conclude that for the H-substituted molecule, the result Gpro > Gneu is correctly explained by a reduction of the LUMO-HOMO gap, while for the amino-phenyl functionnalized molecule, the result Gneu > Gpro is consistent with a shift of the HOMO, which reduces the density of states at the Fermi energy.

Charge transport through redox active [H7P8W48O184]33- polyoxometalates self-assembled onto gold surfaces and gold nanodots

Polyoxometalates (POMs) are redox-active molecular oxides, which attract growing interest for their integration into nano-devices, such as high-density data storage non-volatile memories. In this work, we investigated the electrostatic deposition of the negatively charged [H7P8W48O184]33- POM onto positively charged 8-amino-1-octanethiol self-assembled monolayers (SAMs) preformed onto gold substrates or onto an array of gold nanodots. The ring-shaped [H7P8W48O184]33- POM was selected as an example of large POMs with high charge storage capacity. To avoid the formation of POM aggregates onto the substrates, which would introduce variability in the local electrical properties, special attention has to be paid to the preformed SAM seeding layer, which should itself be deprived of aggregates. Where necessary, rinsing steps were found to be crucial to eliminate these aggregates and to provide uniformly covered substrates for subsequent POM deposition and electrical characterizations. This especially holds for commercially available gold/glass substrates while these rinsing steps were not essential in the case of template stripped gold of very low roughness. Charge transport through the related molecular junctions and nanodot molecule junctions (NMJs) has been probed by conducting-AFM. We analyzed the current-voltage curves with different models: electron tunneling though the SAMs (Simmons model), transition voltage spectroscopy (TVS) method or molecular single energy level mediated transport (Landauer equation) and we discussed the energetics of the molecular junctions. We concluded to an energy level alignment of the alkyl spacer and POM lowest occupied molecular orbitals (LUMOs), probably due to dipolar effects.

Nanoscale Mapping of Molecular Vibrational Modes via Vibrational Noise Spectroscopy

We have developed a "vibrational noise spectroscopy (VNS)" method to identify and map vibrational modes of molecular wires on a solid substrate. In the method, electrical-noises generated in molecules on a conducting substrate were measured using a conducting atomic force microscopy (AFM) with a nanoresolution. We found that the bias voltage applied to the conducting AFM probe can stimulate specific vibrational modes of measured molecules, resulting in enhanced electrical noises. Thus, by analyzing noise-voltage spectra, we could identify various vibrational modes of the molecular wires on the substrates. Further, we could image the distribution of vibrational modes on molecule patterns on the substrates. In addition, we found that VNS imaging data could be further analyzed to quantitatively estimate the density of a specific vibrational mode in the layers of different molecular species. The VNS method allows one to measure molecular vibrational modes under ambient conditions with a nanoresolution, and thus it can be a powerful tool for nanoscale electronics and materials researches in general.

Mechanically and Electrically Robust Self-Assembled Monolayers for Large-Area Tunneling Junctions

This paper examines the relationship between mechanical deformation and the electronic properties of self-assembled monolayers (SAMs) of the oligothiophene 4-([2,2':5',2″:5″,2‴-quaterthiophen]-5-yl)butane-1-thiol (T4C4) in tunneling junctions using conductive probe atomic force microscopy (CP-AFM) and eutectic Ga-In (EGaIn). We compared shifts in conductivity, transition voltages of T4C4 with increasing AFM tip loading force to alkanethiolates. While these shifts result from an increasing tilt angle from penetration of the SAM by the AFM tip for the latter, we ascribe them to distortions of the π system present in T4C4, which is more mechanically robust than alkanethiolates of comparable length; SAMs comprising T4C4 shows about five times higher Young's modulus than alkanethiolates. Density functional theory calculations confirm that mechanical deformations shift the barrier height due to changes in the frontier orbitals caused by small rearrangements to the conformation of the quaterthiophene moiety. The mechanical robustness of T4C4 manifests as an increased tolerance to high bias in large-area EGaIn junctions suggesting that electrostatic pressure plays a significant role in the shorting of molecular junctions at high bias.

Adhesion contact deformation in nanobridge tests

An accurate grasp of the mechanical properties, especially Young's moduli, of one dimensional nanomaterials plays a crucial role in the design and safe service of flexible electronic devices and implanted biomedical sensors. Nanobridge tests are widely used in the characterization of the mechanical properties of nanowires. In these tests, an atomic force microscope (AFM), functioning as a test machine, exerts a force to bend a nanowire suspended across a trench or a hole with the two ends fixed on a template or substrate. Adhesion contact deformation occurs inevitably during nanobridge testing between the AFM tip and the tested sample, thereby underestimating the Young's modulus of the tested nanowire and causing a pseudo-size effect in the determined Young's modulus. The present work systematically investigates the adhesion contact deformation in nanobridge tests and provides an analytical approach to evaluate the contact deformation and to determine the Young's modulus. To illustrate the developed methodology, AFM nanobridge tests were conducted on gold nanowires (180-340 nm wide, 3.6-5.1 μm long and 90 nm thick). The results indicate that when the contact deformation was taken into consideration, the average Young's modulus increased by 4.63%. Guidelines for minimizing the impact of contact deformation in practical experiments are presented. Furthermore, the results provide insight into the probable causes of the variation in experimentally obtained size-dependencies of Young's moduli of nanowires.

Estimation of π-π Electronic Couplings from Current Measurements

The π-π interactions between organic molecules are among the most important parameters for optimizing the transport and optical properties of organic transistors, light-emitting diodes, and (bio-) molecular devices. Despite substantial theoretical progress, direct experimental measurement of the π-π electronic coupling energy parameter t has remained an old challenge due to molecular structural variability and the large number of parameters that affect the charge transport. Here, we propose a study of π-π interactions from electrochemical and current measurements on a large array of ferrocene-thiolated gold nanocrystals. We confirm the theoretical prediction that t can be assessed from a statistical analysis of current histograms. The extracted value of t ≈35 meV is in the expected range based on our density functional theory analysis. Furthermore, the t distribution is not necessarily Gaussian and could be used as an ultrasensitive technique to assess intermolecular distance fluctuation at the subangström level. The present work establishes a direct bridge between quantum chemistry, electrochemistry, organic electronics, and mesoscopic physics, all of which were used to discuss results and perspectives in a quantitative manner.

Revealing tunnelling details by normalized differential conductance analysis of transport across molecular junctions

A new analysis tool reveals hidden fingerprints of tunnelling mechanisms and quantifies the bias response via a scaling bias parameter.

A 17 GHz molecular rectifier

Molecular electronics originally proposed that small molecules sandwiched between electrodes would accomplish electronic functions and enable ultimate scaling to be reached. However, so far, functional molecular devices have only been demonstrated at low frequency. Here, we demonstrate molecular diodes operating up to 17.8 GHz. Direct current and radio frequency (RF) properties were simultaneously measured on a large array of molecular junctions composed of gold nanocrystal electrodes, ferrocenyl undecanethiol molecules and the tip of an interferometric scanning microwave microscope. The present nanometre-scale molecular diodes offer a current density increase by several orders of magnitude compared with that of micrometre-scale molecular diodes, allowing RF operation. The measured S₁₁ parameters show a diode rectification ratio of 12 dB which is linked to the rectification behaviour of the direct current conductance. From the RF measurements, we extrapolate a cut-off frequency of 520 GHz. A comparison with the silicon RF-Schottky diodes, architecture suggests that the RF-molecular diodes are extremely attractive for scaling and high-frequency operation.

Molecular electronics holds promise to overcome scaling limits of conventional technologies, but is currently limited to low frequency operation. Here, Trasobares et al. show radio frequencies of up to 17.8 GHz in a molecular diode based on ferrocenyl undecanethiol self-assembled monolayers on gold nanodots.

Effects of functional groups on the tribological properties of hairy silica nanoparticles as an additive to polyalphaolefin

Hairy silica nanoparticles with different kinds of functional groups were prepared and dispersed into polyalphaolefin, the effects of the functional groups on the tribological properties of the hairy silica nanoparticles were investigated.

Important issues facing model-based approaches to tunneling transport in molecular junctions

Extensive studies on thin films indicated a generic cubic current-voltage I-V dependence as a salient feature of charge transport by tunneling. A quick glance at I-V data for molecular junctions suggests a qualitatively similar behavior. This would render model-based studies almost irrelevant, since, whatever the model, its parameters can always be adjusted to fit symmetric (asymmetric) I-V curves characterized by two (three) expansion coefficients. Here, we systematically examine popular models based on tunneling barriers or tight-binding pictures and demonstrate that, for a quantitative description at biases of interest (V slightly higher than the transition voltage Vt), cubic expansions do not suffice. A detailed collection of analytical formulae as well as their conditions of applicability is presented to facilitate experimentalist colleagues to process and interpret their experimental data obtained by measuring currents in molecular junctions. We discuss in detail the limits of applicability of the various models and emphasize that uncritically adjusting the model parameters to experiment may be unjustified because the values deduced in this way may fall in ranges rendering a specific model invalid or incompatible to ab initio estimates. We exemplify with the benchmark case of oligophenylene-based junctions, for which the results of ab initio quantum chemical calculations are also reported. As a specific issue, we address the impact of the spatial potential profile and show that it is not notable up to biases V ≳ Vt, unlike at higher biases, where it may be responsible for negative differential resistance effects.