Role Of Self-assembled Monolayers Research Articles

The Role of Self-Assembled Monolayers in the Surface Modification and Interfacial Contact of Copper Fillers in Electrically Conductive Adhesives.

Printing of electrical circuits and interconnects using isotropic conductive adhesives (ICAs) is of great interest due to their low-temperature processing and compatibility with substrates for applications in sensors, healthcare, and flexible devices. As a lower cost alternative to silver (Ag), copper (Cu)-filled ICAs are desirable but limited by the formation of high-resistivity Cu surface oxides. To overcome this limitation, self-assembled monolayers (SAMs) of octadecanethiol (ODT) have been demonstrated to reduce the oxidation of micrometer-scale Cu powder particles for use in ICAs. However, the deposition and function of the SAM require further investigation, as described in this paper. As part of this work, the stages of the SAM deposition process, which included etching with hydrochloric acid to remove pre-existing oxides, were studied using X-ray photoelectron spectroscopy (XPS), which showed low levels of subsequent Cu oxidation when ODT coated. The treated Cu powders were combined with one- or two-part epoxy resins to make Cu-ICAs, and the effect of the Cu surface condition and weight loading on electrical conductivity was examined. When thermally cured in an inert argon atmosphere, ICAs filled with Cu protected by ODT achieved electrical conductivity up to 20 × 105 S·m-1, comparable to Ag-ICAs, and were used to make a functional circuit. To understand the function of the SAM in these Cu-ICAs, scanning and transmission electron microscopy were used to examine the internal micro- and nano-structures along with the elemental distribution at the interfaces within sections taken from cured samples. Sulfur (S), indicative of the ODT, was still detected at the internal polymer-metal interface after curing, and particle-to-particle contacts were also examined. XPS also identified S on the surface of cured Cu-ICAs even after thermal treatment. Based on the observations, electrical contact and conduction mechanisms for these Cu-filled ICAs are proposed and discussed.

Organic passivation of Al0.5Ga0.5N epilayers using self-assembled monolayer of Zn(II) porphyrin for improved solar-blind photodetector performance

We report on the passivation of surface states of Al0.5Ga0.5N epilayers by employing self-assembled monolayers (SAMs) of organic molecules, which led to a significant improvement in the performance of Al0.5Ga0.5N based solar-blind photodetector. The formation of SAM of meso-(5-hydroxyphenyl)-10,15,20-tri(p-tolyl) porphyrin (ZnTPP(OH)) on the surface of Al0.5Ga0.5N was probed by contact angle measurement, x-ray photoelectron spectroscopy, and atomic force microscopy. The successful passivation of surface states was confirmed by Kelvin probe force microscopy as a significant decrease in the surface potential of Al0.5Ga0.5N by ∼280 mV was observed. The inference was supported by a four-fold increase in the photoluminescence intensity of the near-band edge emission peak upon passivation. As a result, the dark current of the as-fabricated solar-blind photodetector reduced by two orders of magnitude, without compromising with the magnitude of the photo current at 270 nm. The role of SAM was evident in improving the performance of the photodetector as the peak value of photo-to-dark current ratio enhanced by ∼36 times. The peak responsivity of the photodetector increased from 1.6 to 2.2 mA W−1 at 10 V. The significant reduction in the dark current and enhancement in the responsivity led to an improvement in the specific detectivity by ∼10 times. Additionally, the response speed of the photodetector was found to improve significantly from 4 to 0.5 s.

The role of self-assembled monolayers in electronic devices

Today, the self-assembled monolayer (SAM) approach for surface functionalization is regarded as highly versatile and compelling, especially in the immobilization of biomolecules and fabrication of novel supramolecular architectures.

Self-Assembled Monolayers at the Conjugated Polymer/Electrode Interface: Implications for Charge Transport and Band-Bending Behavior.

The role of self-assembled monolayers (SAMs), trichloro(1 H,1 H,2 H,2 H-perfluorooctyl) (FTS) and octadecyltrichlorosilane (OTS), deposited on indium tin oxide (ITO) substrates, on electronic properties of the poly(3-hexylthiophene) (P3HT)/SAM/ITO system is reported. SAMs, well known for modifying the surface energies of materials, are also known to modify the work functions (WFs) of semiconductors. Unsurprisingly, differences between the band-bending behaviors of P3HT/ITO, P3HT/OTS/ITO, and P3HT/FTS/ITO systems were observed because the SAMs modify the WF of ITO. However, the degrees of band bending occurring in these systems could not be attributed solely to the modified WFs of the substrate. This was apparent based on measurements of samples that included P3HT films prepared with different morphological structures. Changes in the morphological structure, due to different deposition methods and surface energies of the substrates, are necessarily connected to changes in the electronic structure, including changes in the electronic density of states (DOS), of P3HT. An association between (i) the WF differences between P3HT, ITO, and SAM/ITO substrates, (ii) the surface energies of the ITO and SAM/ITO substrates, which influence the morphology of the deposited P3HT layer, (iii) the DOS widths of P3HT, and (iv) the degree of band bending is suggested.

Role of Self-Assembled Monolayers on Improved Electrical Stability of Amorphous In-Ga-Zn-O Thin-Film Transistors

Self-assembled monolayers (SAMs) have been used to improve both the positive and negative bias-stress stability of amorphous indium gallium zinc oxide (IGZO) bottom gate thin film transistors (TFTs). N-hexylphosphonic acid (HPA) and fluorinated hexylphosphonic acid (FPA) SAMs adsorbed on IGZO back channel surfaces were shown to significantly reduce bias-stress turn-on voltage shifts compared to IGZO back channel surfaces with no SAMs. FPA was found to have a lower surface energy and lower packing density than HPA, as well as lower bias-stress turn-on voltage shifts. The improved stability of IGZO TFTs with SAMs can be primarily attributed to a reduction in molecular adsorption of contaminants on the IGZO back channel surface and minimal trapping states present with phosphonic acid binding to the IGZO surface.

Adsorption of Oxygenates on Alkanethiol-Functionalized Pd(111) Surfaces: Mechanistic Insights into the Role of Self-Assembled Monolayers on Catalysis

Recent work shows that coating a supported palladium catalyst with a self-assembled monolayer (SAM) of alkanethiols can dramatically improve selectivity in the hydrogenation of 1-epoxy-3-butene (EpB) to 1-epoxybutane. Here, we present the results of surface-level investigations of the adsorption of EpB and related molecules on SAM-coated Pd(111), with an aim of identifying mechanistic explanations for the observed catalytic behavior. Alkanethiol SAM-covered Pd(111) surfaces were prepared by conventional techniques and transferred to ultrahigh vacuum, where they were characterized using Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) of EpB and other probe molecules. Whereas previous studies have shown that EpB undergoes rapid decomposition via epoxide ring opening on uncoated Pd(111), TPD studies show that EpB does not undergo substantial ring opening on SAM-covered surfaces but rather desorbs intact at temperatures less than 300 K. Systematic comparisons of EpB desorption spectra to spectra for other C(4) oxygenates suggest that the SAM creates a kinetic barrier to epoxide ring-opening reactions that does not exist on the uncoated surface. The EpB desorption spectra as a function of exposure show behavior similar to the desorption of olefins from Pd(111), indicating that the binding of the olefin functionality, in contrast to that of the epoxide ring, is not significantly perturbed. EpB desorption spectra from surfaces with less well-ordered SAMs show the presence of weakly bound states not observed on well-ordered SAM surfaces. The lower activity observed on catalysts covered with less well-ordered SAMs is hypothesized to occur due to partial confinement of adsorbates into these weakly bound, less active states.

Molecular gated transistors: Role of self-assembled monolayers

In order to understand the biosensing mechanism of field-effect based biosensors and optimize their performance, the effect of each of its molecular building block must be understood. In this work the gating effect of self-assembled linker molecules on field-effect transistor was studied in detail. We have combined Kelvin probe force microscopy, current–voltage measurements, capacitance–voltage measurements, equivalent circuit modeling and device simulations in order to trace the mechanism of silicon-on-insulator biological field-effect transistors. The measurements were conducted on the widely used linker molecules (3-aminopropyl)-trimethoxysilane (APTMS) and 11-aminoundecyl-triethoxysilane (AUTES), which were self-assembled on ozone activated silicon oxide surface covering the transistor channel. In a dry environment, the work function of the modified silicon oxide decreased by more than 1.5 eV, and the transistor threshold voltage increased by about 30 V following the self-assembly. A detailed analysis indicates that these changes are due to negative induced charges on the top dielectric layer, and an effective dipole due to the polar monolayer. However, the self-assembly did not change the silicon flat-band voltage when in contact with an electrolyte. This is attributed to electrostatic screening by the electrolyte.

Preparation, characterization and mechanistic features of zirconia films on bare and functionalized gold surfaces

Electrochemical synthesis of microcrystalline ZrO2 at room temperature on bare and functionalized gold surfaces is reported to illustrate the biomimetic conditions of nucleation and growth. ZrO2 films prepared on self-assembled monolayers (SAMs) of pentane-1,5-dithiol (PDT) on polycrystalline gold surfaces reveal a high degree of orientation and a distinct cleavage face, in contrast to monoclinic films, obtained on bare gold surfaces at room temperature. Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), impedance measurement, current-transients, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to follow the monolayer formation, Zr-attachment on the SAM-modified surface and subsequent difference in nucleation conditions of ZrO2 formation. The role of SAMs in controlling the size and habit during crystallization of ZrO2 is discussed in terms of a proposed reaction mechanism.

Characteristics of redox systems on self-assembled monolayer-covered electrodes

Electrochemical studies on gold electrodes covered with self-assembled monolayers (SAM) of aminoethane thiol (AET), mercaptobenzothiazole (MBT) and octadecyl mercaptan (ODM) have been carried out using cyclic voltammetry. Study of the influence of these monolayers on the double layer capacity of the interfaces involving the Au/SAM electrodes and of the electron transfer kinetics of chosen redox reaction probes, viz., Fe(CN) 6 4−/3− , Fe2+/3+, hydroquinone/quinone(H2 Q/Q) and Cu underpotential deposition, offers a wealth of information that can throw light on the role of SAMs in allowing/moderating/blocking the electron transfer at such interfaces. These details are presented and discussed in the paper.