Effects of intermolecular interaction on inelastic electron tunneling spectra

Single-Molecule Classification of Aspartic Acid and Leucine by Molecular Recognition through Hydrogen Bonding and Time-Series Analysis

Amino acid detection/identification methods are important for understanding biological systems. In this study, we developed the single-molecule measurement for investigated quantum tunneling enhancement by chemical modification and machine learning based time series analysis for develop accurate amino acid discrimination. We performed single-molecule measurement of L-aspartic Acid (Asp) and L-leucine (Leu) with mercaptoacetic acid (MAA) chemical modified nano-gap. The measured current was investigated by machine learning based time series analysis method for accurate amino acid discrimination. Compared to measurements using bare nano-gap, it is found that MAA modification improves the difference in the conductance-time profiles between Asp and Leu through the hydrogen bonding facilitated tunneling phenomena. It is also found that this method enables determination of relative concentration. even in the mixture of Asp and Leu. It improves selective analysis for amino acids, and therefore would be applicable in medicine, diagnosis, and single-molecule peptide sequencing.

Paving the way to single-molecule chemistry through molecular electronics

Since our understanding of single-molecule junctions, in which single molecules are connected between nanoelectrodes, has deepened, we have paved the way to single-molecule chemistry. Herein, we review fundamental properties, including the number of molecules connected to the electrode, their structure and type, the bonding force between the single molecule and electrode and the thermopower and quantum interference in single-molecule junctions. Additionally, we review the application of single-molecule junctions to biomolecules. Finally, we explore single-molecule chemical reaction analysis, which is one direction of single-molecule junction research.

Electrical characterization of benzenedithiolate molecular electronic devices with graphene electrodes on rigid and flexible substrates

We investigated the electrical characteristics of molecular electronic devices consisting of benzenedithiolate self-assembled monolayers and a graphene electrode. We used the multilayer graphene electrode as a protective interlayer to prevent filamentary path formation during the evaporation of the top electrode in the vertical metal-molecule-metal junction structure. The devices were fabricated both on a rigid SiO2/Si substrate and on a flexible poly(ethylene terephthalate) substrate. Using these devices, we investigated the basic charge transport characteristics of benzenedithiolate molecular junctions in length- and temperature-dependent analyses. Additionally, the reliability of the electrical characteristics of the flexible benzenedithiolate molecular devices was investigated under various mechanical bending conditions, such as different bending radii, repeated bending cycles, and a retention test under bending. We also observed the inelastic electron tunneling spectra of our fabricated graphene-electrode molecular devices. Based on the results, we verified that benzenedithiolate molecules participate in charge transport, serving as an active tunneling barrier in solid-state graphene-electrode molecular junctions.

The inelastic electron tunneling spectroscopy of curved finite-sized graphene nanoribbon based molecular devices

The inelastic electron scattering properties of the molecular devices of curved finite-sized graphene nanoribbon (GNR) slices have been studied by combining the density functional theory and Green's function method.

Inelastic tunneling spectroscopy of gold-thiol and gold-thiolate interfaces in molecular junctions: the role of hydrogen

It is widely believed that when a molecule with thiol (S-H) end groups bridges a pair of gold electrodes, the S atoms bond to the gold and the thiol H atoms detach from the molecule. However, little is known regarding the details of this process, its time scale, and whether molecules with and without thiol hydrogen atoms can coexist in molecular junctions. Here, we explore theoretically how inelastic tunneling spectroscopy (IETS) can shed light on these issues. We present calculations of the geometries, low bias conductances, and IETS of propanedithiol and propanedithiolate molecular junctions with gold electrodes. We show that IETS can distinguish between junctions with molecules having no, one, or two thiol hydrogen atoms. We find that in most cases, the single-molecule junctions in the IETS experiment of Hihath et al. [Nano Lett. 8, 1673 (2008)] had no thiol H atoms, but that a molecule with a single thiol H atom may have bridged their junction occasionally. We also consider the evolution of the IETS spectrum as a gold STM tip approaches the intact S-H group at the end of a molecule bound at its other end to a second electrode. We predict the frequency of a vibrational mode of the thiol H atom to increase by a factor ~2 as the gap between the tip and molecule narrows. Therefore, IETS should be able to track the approach of the tip towards the thiol group of the molecule and detect the detachment of the thiol H atom from the molecule when it occurs.

Inelastic electron tunneling spectroscopy of gold-benzenedithiol-gold junctions: accurate determination of molecular conformation

The gold-benzenedithiol-gold junction is the classic prototype of molecular electronics. However, even with the similar experimental setup, it has been difficult to reproduce the measured results because of the lack of basic information about the molecular confirmation inside the junction. We have performed systematic first principles study on the inelastic electron tunneling spectroscopy of this classic junction. By comparing the calculated spectra with four different experimental results, the most possible conformations of the molecule under different experimental conditions have been successfully determined. The relationship between the contact configuration and the resulted spectra is revealed. It demonstrates again that one should always combine the theoretical and experimental inelastic electron tunneling spectra to determine the molecular conformation in a junction. Our simulations have also suggested that in terms of the reproducibility and stability, the electromigrated nanogap technique is much better than the mechanically controllable break junction technique.

Inelastic electron tunneling spectroscopy of single-molecule junctions using a mechanically controllable break junction

We report the use of electrical measurements to identify simultaneously the number and type of organic molecules within metal-molecule-metal junctions. Our strategy combines analyses of single-molecule conductance and inelastic electron tunneling spectra, exploiting a nanofabricated mechanically controllable break junction. We found that the peak linewidth of the inelastic electron tunneling spectrum decreased as the modulation voltage and temperature decreased, and that the selection rule for inelastic electron tunneling spectroscopy agrees with that for Raman spectroscopy. Furthermore, the differential conductance curve of the single-molecule junction suggests that it has asymmetrical electrode-molecule coupling.

Molecular modeling of inelastic electron transport in molecular junctions

A quantum chemical approach for the modeling of inelastic electron tunneling spectroscopy of molecular junctions based on scattering theory is presented. Within a harmonic approximation, the proposed method allows us to calculate the electron-vibration coupling strength analytically, which makes it applicable to many different systems. The calculated inelastic electron transport spectra are often in very good agreement with their experimental counterparts, allowing the revelation of detailed information about molecular conformations inside the junction, molecule-metal contact structures, and intermolecular interaction that is largely inaccessible experimentally.