Electronic Properties Of Self-assembled Monolayers Research Articles

How far the chemistry of self-assembled monolayers on gold surfaces affects their work function?

Self-assembled monolayers composed of various long-chain aliphatic molecules and different tail functional groups have been synthesized on the Au(111) surface and characterized by Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy. Carboxy, amino, thio and methyl terminal groups have been considered in the design of self-assembled monolayers with different aliphatic chain lengths (from C6 to C16). Work function measurements by Kelvin probe force microscopy have been carried out under a controlled and room atmosphere. Remarkably, a reduction of the relative humidity from 40% to 3% has induced a work function shift of up to 0.3 eV. As expected, the changes of the chain length of the aliphatic moiety and of the tail group have a significant impact on the tuning of the measured work function (3.90 eV for dodecanethiol versus 4.57 eV for mercaptohexadecylamine). Surprisingly, the change of the net dipole moment of the tail group (sign and amplitude) does not dominate the work function variations. In contrast, the change of the chain length and the possibility of the tail group to form a complex hydrogen bond network between molecules lead to significant modulations of the work function. In order to interpret these original findings, density functional theory models of equivalent self-assembled monolayers adsorbed on the Au(111) surface have been developed at an unprecedented level of description with large supercells including simultaneously 27 co-adsorbed molecules and weak van der Waals interactions between them. Such large systems have allowed the theoretical modeling of complex hydrogen bond networks between molecules when possible (carboxy tail group). The comparison between computed and measured work functions shows a striking agreement, thus allowing the disentanglement of the previously mentioned competing effects. This consistency between experiment and theory will help in designing the electronic properties of self-assembled monolayers in the context of molecular electronics and organic transistors.

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.

Effect of molecular desorption on the electronic properties of self-assembled polarizable molecular monolayers

We investigated the interfacial electronic properties of self-assembled monolayers (SAM)-modified Au metal surface at elevated temperatures. We observed that the work functions of the Au metal surfaces modified with SAMs changed differently under elevated-temperature conditions based on the type of SAMs categorized by three different features based on chemical anchoring group, molecular backbone structure, and the direction of the dipole moment. The temperature-dependent work function of the SAM-modified Au metal could be explained in terms of the molecular binding energy and the thermal stability of the SAMs, which were investigated with thermal desorption spectroscopic measurements and were explained with molecular modeling. Our study will aid in understanding the electronic properties at the interface between SAMs and metals in organic electronic devices if an annealing treatment is applied.

Imaging Photoelectron Transmission through Self-Assembled Monolayers: The Work-Function of Alkanethiols Coated Gold

In this paper, we present a new approach for studying the electronic properties of self-assembled monolayers and their interaction with a conductive substrate, the low-energy photoelectron imaging spectroscopy (LEPIS). LEPIS relies on imaging of photoelectrons ejected from a conductive substrate and subsequently transmitted through organic monolayers. Using this method, we measure the relative work-function of alkanethiols of different length on gold substrate, and we are able to follow the changes occurring when the surface coverage is varied. We also computed the work-function of model alkanethiols using a plane-wave density functional theory approach, in order to demonstrate the correlation between changes in the work-function with the monolayer organization and density.

Probing a molecular electronic transition by two-colour sum-frequency generation spectroscopy

We demonstrate that a new emerging technique, two-colour sum-frequency generation (SFG) spectroscopy, can be used to probe the molecular electronic properties of self-assembled monolayers (SAMs). In the CH spectral range (2800–3200cm−1), we show that the sum-frequency generation signal of a porphyrin alkanethiol derivative adsorbed on Pt(111) reaches a maximum intensity at ∼435nm SFG wavelength. This wavelength corresponds to the porphyrin moiety specific π–π∗ molecular electronic transition which is called the Soret or B band. This resonant behaviour is not observed for 1-dodecanethiol SAMs, which are devoid of molecular electronic transition in the investigated visible spectral range.

Interactions between Thiol Molecular Linkers and the Au13 Nanoparticle

The structure and binding of thiol molecular linkers to gold surfaces and nanoparticles is central to the understanding of the electronic properties of self-assembled monolayers and, of relevance to recent studies, to nanoscale assemblages consisting of molecular wires and metal nanoparticles. The study of mono-molecular electron transport generally requires consideration of bonding with irregular metallic contacts or poorly defined surfaces such as break junctions, electromigration generated gaps, and scanning probe microscopy tips. These structures can locally bear a closer resemblance to atomic clusters, as compared to neat metallic surfaces. It has also emerged that the prediction and understanding of the electronic transport properties for molecular wires and nanoscale assemblies requires detailed knowledge of thiolate−gold cluster interactions. Recent debate has focused on the nature of the thiolate bonding to surfaces, and the effect of disordering and distortion in gold cluster structures on thiol...

Electronic properties of self-assembled monolayers on Au(111) studied by electrical force spectroscopy

Localized voltage dependent capacitive currents through different self-assembled monolayers on Au(111) surfaces were studied by a spectroscopic method in a modified atomic force microscope that detects surface potentials and $\ensuremath{\partial}C/\ensuremath{\partial}z$ data as functions of an applied modulated and biased voltage between tip and sample. On flame-annealed Au surfaces, the $\ensuremath{\partial}C/\ensuremath{\partial}z$ signal is completely voltage independent. The signal amplitudes on 2-amino-alkanethiol layers change with both ac and dc voltage. The $\ensuremath{\partial}C/\ensuremath{\partial}{z(U}_{\mathrm{dc}})$ data obtained represent the energy-dependent density of states of the investigated surface layer. Moreover, the charges of ligand stabilized clusters and the dependence of the clusters' electrical properties on their size were reproduced. The results are discussed in connection with schematic energy models and replacement circuits.