Defects In Self-assembled Monolayers Research Articles

Repairing Interfacial Defects in Self-Assembled Monolayers for High-Efficiency Perovskite Solar Cells and Organic Photovoltaics through the SAM@Pseudo-Planar Monolayer Strategy.

Lately, carbazole-based self-assembled monolayers (SAMs) are widely employed as effective hole-selective layers (HSLs) in inverted perovskite solar cells (PSCs). Nevertheless, these SAMs tend to aggregate in solvents due to their amphiphilic nature, hindering the formation of a monolayer on the ITO substrate and impeding effective passivation of deep defects in the perovskites. In this study, a series of new SAMs including DPA-B-PY, CBZ-B-PY, POZ-B-PY, POZ-PY, POZ-T-PY, and POZ-BT-PY are synthesized, which are employed as interfacial repairers and coated atop CNph SAM to form a robust CNph SAM@pseudo-planar monolayer as HSL in efficient inverted PSCs. The CNph SAM@pseudo-planar monolayer strategy enables a well-aligned interface with perovskites, synergistically promoting perovskite crystal growth, improving charge extraction/transport, and minimizing nonradiative interfacial recombination loss. As a result, the POZ-BT-PY-modified PSC realizes an impressively enhanced solar efficiency of up to 24.45% together with a fill factor of 82.63%. Furthermore, a wide bandgap PSC achieving over 19% efficiency. Upon treatment with the CNph SAM@pseudo-planar monolayer, also demonstrates a non-fullerene organic photovoltaics (OPVs) based on the PM6:BTP-eC9 blend, which achieves an efficiency of 17.07%. Importantly, these modified PSCs and OPVs all show remarkably improved stability under various testing conditions compared to their control counterparts.

Correcting Edge Defects in Self-Assembled Monolayers through Thermal Annealing.

Self-assembled monolayers (SAMs) are emerging as platform technology for a myriad of applications, yet they still possess varied spatial stability and predictability issues as their properties are heavily dependent on subtle structural features. Reducing entropy within such a system serves as one of many potential solutions to increase order and therefore coherence/precision in measured properties. Here we explore controlled thermal annealing to improve edge disorders in SAMs and significantly reduce data variance. Using both odd- and even-numbered n-alkanethiol SAMs on Au, we observe statistically significant difference in the contact angles between edge and center. Thermal annealing at 40°C significantly narrows differences between edges and centre of the SAM, albeit with significant reduction in the parity dependent odd-even effect. This study provides a pathway to improve SAMs consistency through minimal external perturbation as reflected by the minimization of odd-even effect as SAMs become increasingly ordered.

Impact of defects in self-assembled monolayer on humidity sensing by molecular functionalized transistors

For highly sensitive and selective sensors, numerical study of transistors functionalized with a defected self-assembled monolayer (SAM) was performed. Adsorption of water molecules on the defected SAM was treated by molecular dynamics (MD) calculations. Based on time dependent atomic positions and charges derived from the MD calculations, carrier concentration and threshold voltage of defected-SAM-functionalized transistors were derived from the Poisson equation. The defects of the SAM caused a penetration of water molecules. Due to a strong polarization of water molecules, the water molecules penetrating the defects were stacked in the same direction. The alignment of the water molecules in the defects generated a large electrical dipole, and the threshold voltage of the transistors were shifted.

Probing Charge Reversal of Zwitterion at Defects on Self-Assembled Monolayers by Scanning Electrochemical Microscopy

Defects on self-assembled monolayers (SAMs) could have detrimental impact on their functionalities; thus, to optimize performances, researchers seek to manipulate the defects of SAMs. In this work,...

A Tribute to Richard M. Crooks on the Occasion of His 65th Birthday

A Tribute to Richard M. Crooks on the Occasion of His 65th Birthday

Defects in Self-Assembled Monolayers on Nanoparticles Prompt Phospholipid Extraction and Bilayer-Curvature-Dependent Deformations

Metal nanoparticles (NPs) functionalized with self-assembled monolayers (SAMs) of long alkanethiol ligands are subject to defects in the SAM structure due to the interplay between alkyl chain packing and the free volume available in space per ligand. We find via dissipative particle dynamics (DPD) simulations that hydrophobic contact between protruding acyl chains of phospholipids from lipid vesicles and exposed alkyl chains in SAM defects prompts NP insertion and that defects become sites for phospholipid extraction. Experiments show that AuNPs coated with cationic (11-mercaptoundecyl)trimethylammonium bromide, analogous to the models used for the simulations, attach to and acquire lipids from planar supported lipid bilayers, while AuNPs coated with anionic 11-mercaptoundecanoic acid do not. Phospholipid extraction and the structure of the ligands inserted in bilayers collectively contribute to bilayer thinning at the site of NP insertion and bilayer-curvature-dependent deformability, revealing how these typical engineered SAM-coated NPs interface with lipid-bilayer systems with potential biological consequences.

Bottom-electrode induced defects in self-assembled monolayer (SAM)-based tunnel junctions affect only the SAM resistance, not the contact resistance or SAM capacitance.

In large area molecular junctions, defects are always present and can be caused by impurities and/or defects in the electrode materials and/or SAMs, but how they affect the electrical characteristics of junctions has rarely been studied. Usually, junctions are characterized by two-terminal current–voltage measurements where only the total current across the junction is measured, but with these methods one cannot distinguish how the individual components of the junctions are altered by the defects. Here we show that the roughness of the bottom-electrode is a crucial factor in determining the electrical properties of self-assembled monolayer (SAM)-based junctions. We used potentiodynamic impedance spectroscopy to reveal which components of the junctions are altered by defective bottom electrodes because this method allows for direct determination of all components that impede charge transport in the equivalent circuit of the junctions. We intentionally introduced defects via the roughness of the bottom electrode and found that these defects lower the SAM resistance but they do not alter the capacitance of the SAM or the contact resistance of the junction. In other words, defective junctions can be seen as “leaky capacitors” resulting in an underestimation of the SAM resistance of two orders of magnitude. These results help to improve the interpretation of data generated by SAM-based junctions and explain in part the observed large spread of reported tunneling rates for the same molecules measured across different platforms.

Ordering and defects in self-assembled monolayers on nanoporous gold

Self-assembled monolayers are commonly used to tailor nanoporous structures for applications, and they also provide a model system for determining the effects of nanoscale structure on self-assembly. We have investigated the ordering and defects in alkanethiol self-assembled monolayers on nanoporous gold, a high surface area mesoporous material. Infrared reflection absorption spectroscopy was used to characterize the effects of alkyl chain length and nanoporous gold pore size on molecular layer ordering. Cyclic voltammetry was used to characterize the monolayer density and ordering, with ferrocenylalkylthiolates used to quantify and characterize defect sites. We find that dense and well-ordered molecular layers form quickly with low defect levels. However, we do not observe differences in molecular layer ordering or defects with changes in pore size.

One-Nanometer Thin Monolayers Remove the Deleterious Effect of Substrate Defects in Molecular Tunnel Junctions.

Defects in self-assembled monolayer (SAMs) based junctions cause the largest deviation between predicted and measured values of the tunnelling current. We report the remarkable, seemingly counterintuitive finding that shorter, less-ordered SAMs provide, unlike taller crystalline-like SAMs, higher quality tunnelling barriers on defective substrates, which points to self-repair of liquid-like SAMs on defects. The molecular dynamics show that short-chain molecules can more easily rotate into low-density boundary regions and smoothen out defects than thick solid-like SAMs. Our findings point to an attractive means of removing their deleterious effects simply by using flexible molecules.

Identification of Domain Boundary Defects in Crystalline Self-Assembled Monolayers.

One of the major challenges to the fabrication of functionalized templates using self-assembled monolayers (SAMs) is the characterization of nanoscale defects, particularly SAM domain boundaries (DBs). In this study, an etchant was used to chemically amplify the DBs in a SAM by forming microscale pits in the underlying SiO2 layer. This approach revealed that the naturally occurring DBs acted as structural defects in the SAMs. The DB structures were characterized by systematically varying the octadecyltrichlorosilane (ODTS) monolayer domain size from the nanoscale to the microscale by varying the preparation temperature. These approaches showed that the SAM DBs, which were visualized as having round- and line-shaped nanoscale structures, provided potentially chemical and mechanical surface defect sites. Our principal findings open up exciting new possibilities for understanding the structural defects in SAMs on the molecular level and suggest an approach for optimizing the conditions used to generate defect-free SAM templates.

Image Contrast in Sum Frequency Generation Microscopy Based on Monolayer Order and Coverage

Sum frequency generation imaging microscopy (SFGIM) was used to study a binary system of self-assembled monolayers (SAMs) on evaporated gold. A two-step process that consists of microcontact printing (μCP) and the solution backfilling method is used to generate a sample surface with two distinct SAMs domains. The high contrast chemical identification and the homogeneity map of the SAMs for two different head groups are revealed by processing the three-dimensional images (xy-surface and infrared wavenumbers) obtained from SFGIM. For conformation order analysis, the amplitude ratios of the methylene and methyl symmetric resonance obtained by a nonlinear curve fit of the spatially and spectrally resolved SFG images are used to reconstruct spatial image maps. The conformation order map shows the distribution of gauche defects in SAMs with improved contrast across monodentate and bidentate alkanethiol monolayer regions. The results show that variation of the head group attached on the surface dominantly influences the conformation disorder and surface coverage of the molecules, while the terminal methyl group in both monodentate and bidentate SAMs shows a similar orientation angle.

Ab initio DFT study of 6-mercapto-hexane SAMs: effect of Au surface defects on the monolayer assembly

This paper reports a novel ab initio density functional theory (DFT) simulation approach to the study of surface defects in self-assembled monolayers (SAMs) on gold (0 0 1) surfaces. Surfaces defects can cause changes in gold atom spacing and the ligand footprint. DFT calculations of 6-mercapto-hexane on Au(0 0 1) surfaces have been performed as a function of ligand footprints of 18, 20, 22, 24 and 26 Å2, using a slab geometry and periodic boundary conditions with varying gold atom spacing. The simulation results indicate that as the ligand packing density increases, the position of ideal binding sites changes from fcc to bridge sites. The packing density of 24 Å2 corresponds to the highest binding energy and to a 28.85° cant angle. These results are in good agreement with the experiments from ligand-capped gold nanoparticles. The simulation results also prove that the SAM structure of short-chain alkanethiols on gold is dominated by surface interactions rather than ligand interactions. The electron density mapping study confirms this observation, where only the top two layers of gold atoms contributed charge transfer to sulphur atom and no charge transfer was observed from the alkane chain.

The fabrication of a carbon nanotube array using a catalyst-poisoning layer in the inversenano-sphere lithography method

A new method for the fabrication of periodic CNT arrays was developed in this study,which involves the use of the inverse nano-sphere lithography (INSL) process. Mo of ahoneycomb pattern, acting as a catalyst-poisoning layer, was produced by the nano-spherelithography (NSL) process; the Mo poisoned the catalyst and prevented CNT growth wheredeposited, and as a result, a periodic CNT pattern was obtained. Using this method, theuniformity of the CNT array can be improved by preventing the negative effect ofarrangement defects in self-assembled monolayers. The size and the period of the CNTarray can be adjusted by careful selection of the polystyrene (PS) sphere diameter. X-rayphotoelectron spectroscope (XPS) analysis revealed that the Co catalyst wasineffective on the areas of Mo deposition due to the diffusion of Co into the Mo layer.

Identification and Passivation of Defects in Self-Assembled Monolayers

We demonstrate imaging of nanoscale defects in self-assembled monolayers (SAMs). Atomic layer deposition of aluminum oxide (AlO(x)) onto hydrophobic SAMs is followed by imaging using scanning electron microscopy (SEM). The insulating AlO(x) selectively deposits onto the exposed substrate at defect sites and becomes charged during imaging, providing high contrast even for nanometer scale defects. The deposited AlO(x) also acts as a barrier for electron transfer, thereby simultaneously electrically passivating the defects in the SAM as it labels them.

Mapping of defects in self-assembled monolayers by polymer decoration

A new method mapping the defects in self-assembled monolayers (SAMs) is described. The method is based on electrochemical polymerisation of nonconductive tyramine in defect sites of a monolayer and subsequent visualisation of the polymer structures by atomic force microscopy (AFM). SAMs of hexadecanthiol (HDT) on gold prepared by deposition from solution and microcontact printing were used as a model for this study. The method allows easy mapping of defects on monolayers and provides information about their shape, size, size distribution, defect density and spatial distribution. Comparative electrochemical characterisation of defects in SAMs before and after polymerisation shows that polymer growth occurs on the sites of uncovered gold. The approach should be applicable for the characterisation of defects in other types of ultra-thin organic films on conducting surfaces.

Electrochemical investigation of dynamic interfacial processes at 1-octadecanethiol-modified copper electrodes in halide-containing solutions

1-Octadecanethiol (C18SH) monolayers were self-assembled on the fresh and active copper surface pretreated by nitric acid etching method. The surface properties of the alkanethiol-modified copper electrode in halide-containing solutions were characterized systematically by using several electrochemical methods, including polarization curves, cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN). The results show that C18SH self-assembled monolayers (SAMs) onto copper provide a flexible method that can protect the underlying copper against corrosion. With the immersion time of SAMs-coated copper electrode in NaCl and HCl corrosive solutions increasing, a slow loss of corrosion protection ability of SAMs indicates dynamic processes occurring at the electrode/solution interface and in the monolayers, such as expansion of the defects and transport of corrosive ions through defects of SAMs. Electrochemical noise (EN) is employed to detect the alkanethiol-modified copper surfaces immersed in HCl solution. This observation suggests the pitting process associate with dynamic processes in the 1-octadecanethiol layer.

The Influence of Bromine Adsorption on Copper Electrodeposition on Polycrystalline Gold Electrodes Modified with Self-Assembled Monolayers

The influence of bromine adsorption on copper electrodeposition on a polycrystalline gold electrode modified with self-assembled monolayers (SAMs) has been investigated by chronoamperometry and cyclic voltammetry. It was found that the deposition potential of copper was shifted negatively due to the SAMs. The hydrogen bond interaction between ω-carboxyl thiols decreased the defect density of the SAMs and significantly retarded the deposition of copper. The presence of bromide anions also shifted the potential more negatively through adsorption into the defects of SAMs.

Using Two-Stage Chemical Amplification To Determine the Density of Defects in Self-Assembled Monolayers of Alkanethiolates on Gold

This paper describes a technique to measure the surface density of the defects in self-assembled monolayers (SAMs) of hexadecanethiolates on gold that lead to etching of the gold when the system is etched with aqueous ferricyanide solution. This technique uses two steps of amplification through chemical reaction to convert pinhole defects in SAMs into easily imaged, micron-scale pits in an underlying Si support. With this technique, it is possible to evaluate the density of defects in SAMs and in the structures prepared by using SAMs as resists under conditions that may be encountered in lithographic processing. At present, the lowest density of pits that we have measured for SAMs of hexadecanethiolates on 50 nm thick gold is ∼5 pits/mm2. This density is an upper limit for the density of defects that expose bare gold to the etching solution or that comprise regions of SAM sufficiently thin that the SAM is not able to block access of the etchant to the gold.

Using Two-Stage Chemical Amplification To Determine the Density of Defects in Self-Assembled Monolayers of Alkanethiolates on Gold

ADVERTISEMENT RETURN TO ISSUEPREVCorrectionUsing Two-Stage Chemical Amplification To Determine the Density of Defects in Self-Assembled Monolayers of Alkanethiolates on GoldXiao-Mei Zhao, James L. Wilbur, and George M. WhitesidesCite this: Langmuir 1996, 12, 22, 5504Publication Date (Web):October 30, 1996Publication History Published online30 October 1996Published inissue 1 January 1996https://doi.org/10.1021/la9608674Copyright © 1996 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views246Altmetric-Citations10LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (18 KB) Get e-AlertsSUBJECTS:Defects,Etching,Gold,Self-assembled monolayers Get e-Alerts