Modification Of Self-assembled Monolayers Research Articles

Fluorinated silane self-assembled monolayer modification of V2O5 for enhanced hole injection and device efficiency

This study introduces a new approach to enhance hole injection in organic light-emitting diodes (OLEDs) by modifying the energy band alignment. This enhancement is achieved by incorporating gap states near the Fermi level through the application of a 1H,1H,2H,2H-perfluorooctyltriethoxysilane self-assembled monolayer (F8SAM) coating on V2O5. Compared to thermally evaporated V2O5, the F8SAM coating induces greater oxygen deficiency, reducing the V2O5 (V5+) content from 84.2 % to 76.9 % and increasing the VO2 (V4+) content from 15.7 % to 23.0 %. This modification causes both the highest occupied molecular orbital and gap state levels to move closer to the Fermi level, facilitated by increased oxygen vacancies, high carrier concentration, and delocalized free electrons. The alteration in the gap state provides a more accessible pathway for efficient hole injection, as confirmed in optoelectronic green light devices, which demonstrate a wavelength of 530 ± 5 nm. Significantly, devices incorporating a double-layer hole-injection layer exhibit a 5-fold increase in maximum luminance, a 1.6-fold increase in current efficiency, and a 1.7-fold increase in power efficiency compared to devices with an unmodified V2O5 film. The enhanced performance is attributed to the well-aligned F8SAM on V2O5, which increases the density of hole states and thereby improves device performance. This study demonstrates a practical method for modulating the cation valence state of V2O5, offering a pathway toward improved hole injection and, consequently, more efficient and functional OLED devices.

Performance improvement mechanisms of perovskite solar cells by modification of NiO hole-selective contacts with self-assembled-monolayers

This study investigates effects of the modification of NiOx hole-selective contacts (HSCs) with self-assembled monolayers (SAMs), [2-(9H-carbazol-9-yl) ethyl]phosphonic acid (2PACz), on the characteristics of perovskite solar cells, used for perovskite–silicon tandem cells. After the NiOx layers were fabricated by RF sputtering, the 2PACz modification was performed by the spin-coating of a 2PACz solution and subsequent annealing at 100 °C for 10 min. The 2PACz-modified NiOx layer improved the solar cells' open-circuit voltage and short-circuit current density. Photoelectron yield spectroscopy measurements imply that the 2PACz monolayers increase the work function of the NiOx HSCs, thereby enhancing field-effect passivation at the perovskite–HSC interface. Density functional theory calculations and electron spin resonance measurements indicate that an increase in the work function results from a vacuum level shift due to the electric dipole moment of 2PACz molecules and formation of space-charge region at the NiOx–2PACz interface. The performance improvement is explained as a result of enhanced field-effect passivation caused by the vacuum level shift and thus the increase in the work function. These findings, which clarify SAM modification effects of HSCs on solar-cells’ performance, can contribute to the development of highly efficient and reliable perovskite–silicon tandem solar cells.

A Self-Assembled Monolayer Modification to Enhance the Performance of PVA Polymer Electret-Based Organic Synaptic Phototransistor

A Self-Assembled Monolayer Modification to Enhance the Performance of PVA Polymer Electret-Based Organic Synaptic Phototransistor

Amperometric Detection of NH3 by Aromatic SAM-Modified Graphene

Ammonia (NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) is a toxic substance resulting in various acute and chronic effects on individuals. NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> detection, monitoring methods, and detection tools are desperately needed. In this work, we improved the ammonia sensing capabilities of graphene films deposited by chemical vapor deposition (CVD) by modifying aromatic self-assembled monolayer (SAM) molecules such as 5-[(3-methylphenyl) (phenyl) amino] isophthalic acid (MeIPA) and 5-(diphenyl)amino] isophthalic acid (PhIPA) on amperometric detection method. Morphological investigations of the films were carried out by optical and scanning electron microscopy (SEM). Surface potential was characterized with Kelvin Probe Force Microscopy (KPFM), and vibrational properties were characterized with Raman Spectroscopy. MeIPA modification increased NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> uptake by two times compared to unmodified graphene. The results indicated that the SAM modification enhanced NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> molecule adsorption and improved its periodic reversible and reproducible response using the amperometric detection system, indicating that SAM molecules might be a feasible probe for ammonia.

Modification of ITO with Composite Self-Assembled Monolayer Enables Hole Injection and Transport Layer-Free TADF-OLED with Preferable Performance

A complex multilayer device structure severely obstructs the commercial development of thermally activated delayed fluorescence material-based organic light-emitting diodes (TADF-OLEDs) due to high preparation costs. Herein, simplified TADF-OLED without both hole injection layer (HIL) and hole transport layer (HTL) was obtained by embellishing indium tin oxide (ITO) with a composite self-assembled monolayer modification strategy. Perfluorobenzyl phosphoric acid with perfluoroalkyl carboxylic acids formed a denser composite monolayer on the surface of ITO. This increases the work function of ITO, improves the quality of emitting layer film, and facilitates hole injection, which realized direct hole injection without the HIL and HTL. The resultant HIL/HTL-free TADF-OLED gains favorable luminescence performance including a current efficiency (CE) of 76.9 cd A–1 and an external quantum efficiency (EQE) of 22.0% for green TADF-OLED and a CE of 21.4 cd A–1 and EQE of 13.4% for blue TADF-OLED. This work provides an effective method in simplifying TADF-OLED structures for its further commercial application.

Formation of Bright White Plasma Electrolytic Oxidation Films with a Uniform Maze-Like Structure by Anodizing Aluminum in Ammonium Tetraborate Solutions

Bright white plasma electrolytic oxidation (PEO) films with uniform maze-like structures were obtained by anodizing Al in an ammonium tetraborate solution. High-purity Al plates were galvanostatically anodized in 0.3–2.4 M ammonium tetraborate solutions at 303–343 K and 10–100 Am−2. A PEO film consisting of an outer porous layer consisting of amorphous alumina and crystalline alumina with α-and γ-phases and an inner amorphous barrier alumina layer was obtained on the Al surface. An extremely uneven PEO film with various pore sizes and many cracks was formed in a 0.3 M ammonium tetraborate solution, whereas a relatively uniform porous PEO film with similar pore sizes was obtained in 0.9–2.4 M solutions. This difference in the PEO film morphology was due to the plasma generation behavior while anodizing. The lightness of the PEO film increased with increasing anodizing time and PEO film thickness; thus, a bright white PEO film measuring 87.5 in lightness (L*) was formed on the Al surface. The water wettability of the PEO film exhibited weak hydrophilicity. Moreover, a superhydrophobic PEO film with a contact angle of 154° was easily fabricated by self-assembled monolayer modification. Similar bright white PEO coatings were successfully fabricated on various industrial alloys.

Removal of Thiol-SAM on a Gold Surface for Re-Use of an Interdigitated Chain-Shaped Electrode.

The self-assembled monolayer (SAM) is the most common organic assembly utilized for the formation of the monolayers of alkane-thiolates on gold electrode, resulting in a wide range of applications for the modified SAM on gold in various research areas. This study examined the desorption of a SAM that was developed on the gold surface of an interdigitated chain-shaped electrode (the ICE, a unique electrode design, was fabricated by our group) with the goal of determining the most efficient strategy of SAM removal for the ICE to be re-used. A simple and proficient solution-based cleaning procedure was applied for the removal of a SAM on the gold surface of the ICE by using a sodium borohydride solution within short-term treatment, resulting in efficiency for the recovery of the originally electrochemical characteristic of ICE of 90.3%. The re-use of ICE after the removal process was confirmed by the successful re-deposition of a SAM onto the electrode surface, resulting in the high efficiency percentage of 90.1% for the reusability of ICE with the SAM modification. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used as tools to investigate the changes in the electrode interface at each stage of the SAM removal and the electrode recycling. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy were employed, being powerful spectrum techniques, for the characterization of the bonding structure and chemical state of the bare ICE and the modified ICE at each treatment step. Based on the comprehensive discussion of analytical chemistry from the obtained EIS and CV data in this study, we confirmed and proved the effectiveness of this promising method for the removal of a SAM from the ICE and the re-use of ICE in the field of material deposition, with the aims of saving money, improving experimental handling, and protecting the environment.

Surface modification of c-TiO2 via different phenyl boronic acid SAMs for improved performance of inverted organic solar cells

In this study, phenyl boronic acid (PBA) self-assembled monolayers (SAMs) with different terminal groups were introduced on c-TiO2, as interfacial modification layer in organic solar cells (OSCs) with inverted structure. The morphological, optical, electrical properties and the surface wettability of c-TiO2 layer were successfully tuned by SAMs modification. The all five SAMs introduced to device configuration improved the photovoltaic performances. The NO2-PBA SAM modified device exhibited a power conversion efficiency (PCE) of 3.4 % with a short‐circuit current density of 9.68 mA cm−2, an open‐circuit voltage of 0.59 V and fill factor of 48.46 %. More than 50 % enhancement on the PCE was achieved thanks to better ohmic contact and energy level alignment at the interface resulting improved charge collection. The surface properties can be changed by varying the terminal group of molecules, which can control the charge transport properties leading high shunt resistance and low series resistance. Our results showed that the PBA SAMs can be used as efficient interfacial modification layer for inverted OSCs.

Enhancing hole injection by processing ITO through MoO3 and self-assembled monolayer hybrid modification for solution-processed hole transport layer-free OLEDs

Compared with current vacuum-deposited multilayer organic light-emitting diodes (OLEDs), solution-processed simplified-structured OLEDs are more appealing by merits of low costs and promising potentials in large-area OLEDs. Nevertheless, it remains challenging due to intermixing between adjacent organic layers. Herein, indium tin oxide (ITO) surface was deposited with molybdenum trioxide (MoO3) and then modified with self-assembled monolayer (SAM) of pentafluorobenzyl phosphonic acid. The SAM modification process made the MoOx film rich in oxygen vacancies, which ensured effective hole injection from the hybrid modified ITO directly to the emitting layer. This strategy facilitates HTL-free OLEDs and then completely avoids interlayer intermixing. Using the hybrid modified ITO as anode, HTL-free solution-processed green phosphorescent OLED was prepared and achieved a maximum current efficiency of 63.24 cd A−1 and a maximum external quantum efficiency of 18.18%. Meanwhile, preferable operational stability than standard multilayer OLEDs was obtained. For red phosphorescent OLED, a maximum external quantum efficiency of 17.49% was achieved. This work proposed a new way of adjusting cation valence to improve hole injection, contributing to solution-processed OLED fabrication.

Enhanced performance of ultra-thin polyaniline supercapacitor via aniline blue-WS SAMs with rich nucleation site

We report the ultrathin supercapacitor’s superior performance via the vertically aligned one-dimensional polyaniline structure (1D PANI) growth on the fluorine-doped tin oxide (FTO) substrate. FTO electrodes were treated by the molecular-self assembly, and the effects of different self assembled monolayers (SAMs) on both the evolution of PANI and electrochemical performance were examined. We obtained 809.09 F g−1 specific capacitance from ∼120 nm thick PANI (at 20 mV s−1 scan rate of cyclic voltammetry) via aniline blue water soluble SAMs modification. The supercapacitor’s internal dynamics were clarified by a new equivalent circuit model developed from the Graham model. Through the new model, accurate information about double-layer capacitance, percolation capacitance, and bulk capacitance, which composes the electrode’s capacitive performance, could be obtained. This work provided novel knowledge to develop PANI deposition and hence to achieve greater capacitances.

Impact of Poly(dimethylsiloxane) Surface Modification with Conventional and Amino Acid-Conjugated Self-Assembled Monolayers on the Differentiation of Induced Pluripotent Stem Cells into Cardiomyocytes.

Cardiomyocytes, differentiated from induced pluripotent stem cells (iPSCs), have the potential to produce patient- and disease-specific pharmacological and toxicological platforms, in addition to their cardiac cell therapy applications. However, the lack of both a robust and a simple procedure for scalable cell substrate production is one of the major limitations in this area. Mimicking the natural healthy myocardium extracellular matrix (ECM) properties by altering the cell substrate properties, such as stiffness and chemical/biochemical composition, can significantly affect cell substrate interfacial characteristics and potentially influence cellular behavior and differentiation of iPSCs to cardiomyocytes. Here, we propose a systematic and biomimetic approach, based on the preparation of poly(dimethylsiloxane) (PDMS) substrates having the similar stiffness as healthy heart tissue and a well-defined surface chemistry obtained by conventional [(3-aminopropyl)triethoxysilane (APTES) and octadecyltrimethoxysilane (OTS)] and amino acid (histidine and leucine)-conjugated self-assembled monolayers (SAMs). Among a wide range of different concentrations, the 50:1 prepolymer cross-linker ratio of PDMS allowed adaptation of the myocardium stiffness with a Young's modulus of 23.79 ± 0.61 kPa. Compared with conventional SAM modification, amino acid-conjugated SAMs greatly improved iPSC adhesion, viability, and cardiac marker expression by increasing surface biomimetic properties, whereas all SAMs enhanced cell behavior, with respect to native PDMS. Furthermore, leucine-conjugated SAM modification provided the best environment for cardiac differentiation of iPSCs. This optimized approach can be easily adapted for cardiac differentiation of iPSCs in vitro, rendering a very promising tool for microfluidics, drug screening, and organ-on-chip platforms.

Temperature dependence of resistivity increases induced by thiols adsorption in gold nanosheets

By utilizing stable thiol self-assembled monolayers (SAMs) on gold (Au) nanosheets, we investigated the temperature dependence of the amount of resistivity increases by SAM modification. increased with increasing temperature from 5 to 375 K in all the measured nanosheets for various thicknesses (9–20 nm). We consider from the DFT calculation that the temperature dependence originates from unoccupied states of sulfur atoms above the Fermi level of Au. The available unoccupied states increase at higher temperatures, which enhances scattering rates and thus increases the resistivity. Furthermore, we calculated the temperature dependence of using the Bloch–Grüneisen theory and Fuchs–Sondheimer (FS) model. Based on the results, we conclude that for ultra-thin metal films, the temperature-dependent specularity parameter which represents the fraction of electrons scattered specularly should be introduced into the conventional FS model to predict over a wide temperature range.

Effect of different terminal groups of phenyl boronic acid self-assembled monolayers on the photovoltaic performance of organic solar cells

Abstract Here, a new series of phenyl boronic acid derivative self-assembled monolayers (SAMs) with various functional groups terminated was presented as modification layer for the high performance P3HT:PCBM based organic solar cells (OSCs) with the configuration of ITO/SAM/PEDOT:PSS/P3HT:PCBM/LiF:Al. The photovoltaic device parameters of non-modified and modified ITO surfaces were compared. The electrical and morphologic characteristics of non-modified and modified ITO surfaces were determined by different techniques. It was obtained that the device performances were improved by using all boronic acid derivatives SAM molecules. The best performance of OSC was observed for NO2-PBA SAM modified device, which yielded a power conversion efficiency of 3.17%. Thanks to improved charge transport properties, serial resistance reduced, whereas shunt resistance improved after boronic acid SAMs modification. As a result, it could be said that the using of these SAMs as novel molecules for interfacial modification is an efficient way to improve the device performance of OSCs.

Electron-Induced Modification of Self-Assembled Monolayers of Aromatic Carboxylic Acids

The effects of low-energy electrons on aromatic self-assembled monolayers (SAMs) with carboxylic acid (CA) docking groups were studied with a focus on the dose range below 5 mC/cm2. The SAMs were prepared on an underpotentially deposited Ag bilayer and comprised nonsubstituted and CA-substituted monolayers with a rod-like biphenyl backbone and a monolayer of a Y-shaped, CA-substituted molecule, 1,3,5-benzenetribenzoic acid (H3BTB), formed either as a single-component film or as a binary one by mixing with adamantane-CA (Ad-CA). X-ray photoelectron and near-edge X-ray absorption fine structure spectra suggest high proneness of the CA groups to electron irradiation at both SAM/substrate and SAM/ambient interfaces. Cleavage of the carboxylate-substrate bond results in substantial molecular desorption at the initial stage of irradiation until electron-induced cross-linking gradually takes over. The CA groups at the outer SAM interface undergo substantial chemical changes, indicating that they participate in the cross-linking chemistry. The electron-induced processes are accompanied by molecular reorientation. Disordering for the SAMs formed by the rod-like molecules is contrasted by the H3BTB-based systems where changes also occur but some molecular order is preserved as explained by a proposed model invoking conformational changes. In SAMs of H3BTB mixed with Ad-CA, the latter shows higher proneness to irradiation-induced desorption than the former, as well as an influence on the cross-linking chemistry. The results of the present study suggest that CA-based SAMs on Ag offer additional options for cross-linking in SAMs and, as exemplarily demonstrated by the generation of Cu patterns on structured H3BTB templates, can be efficiently used for lithography and nanofabrication.

Nanoporous Thin Films Formed from Photocleavable Diblock Copolymers on Gold Substrates Modified with Thiolate Self-Assembled Monolayers.

Nanoporous thin films formed on electrodes are considered functional elements of electrochemical sensing systems, thus motivating methods for their development. We report a preparative strategy detailing the effects of surface modification of gold substrates with thiolate self-assembled monolayers (SAMs) on the properties of nanoporous thin films derived from polystyrene-block-poly(ethylene oxide) having a photocleavable o-nitrobenzyl ester junction (PS-hν-PEO). Two PS-hν-PEO having similar PEO volume fractions (≈0.2) but different molecular weights (10 and 23 kg/mol) were used to prepare films (30-100 nm thick) spin-cast on gold substrates unmodified and modified with cysteamine, thioctic acid, and 6-hydroxy-1-hexanethiol SAMs. Solvent vapor annealing followed by PEO removal led to the formation of nanopores with average diameters of 12 and 19 nm from the smaller and larger PS-hν-PEO, respectively. Cyclic voltammograms of 1,1'-ferrocenedimethanol showed that nanoporous films on cysteamine SAMs afforded nanopores reaching the underlying substrates at higher density than those on the other substrates. This result was attributed to balanced affinity of the cysteamine SAM surface with PS and PEO, which enhanced the vertical orientation of PEO microdomains. The generation of carboxyl groups associated with the photocleavage reaction was revealed by pH-dependent changes in the voltammogram of Fe(CN)63- that reflected electrostatic effects regulated by the protonation state of the carboxyl groups. The SAMs underneath the nanoporous films could be replaced by treatment with a thiol solution, as verified by voltammograms of l-ascorbic acid. These results suggest that thiolate SAM modification provides a simple means to control the interfacial orientation of PEO microdomains in thin PS-hν-PEO films.

Self-Assembled Monolayer Formation on a Dental Orthodontic Stainless Steel Wire Surface to Suppress Metal Ion Elution

Metal ion elution, including Cr and Ni from dental orthodontic stainless steel, accounts for some allergies. In this study, a self-assembled monolayer (SAM) on a wire surface is proposed for suppressing such elution. This method involves modifying the stainless steel surface using phosphonic acid containing a long alkyl chain. The uncoated and coated wires are immersed in different acidic solutions, and the supernatant is analyzed by inductively coupled plasma mass spectrometry after 1–4 weeks. The results reveal that Cr and Ni ion elution is significantly suppressed by SAM modification. These findings will help in minimizing potential allergens from dental orthodontics.

Enhanced mobility for increasing on-current and switching ratio of vertical organic field-effect transistors by surface modification with phosphonic acid self-assembled monolayer

Abstract For typical organic transistors with a lateral source/drain contact geometry, self-assembled monolayer (SAM) has been extensively applied to modify the organic/dielectric interface state and to improve the in-plane charge transport. Here, we demonstrate that SAM modification can also be applied to improve the out-of-plane charge transport in vertical organic field-effect transistors (VOFETs), leading to a simultaneous increase in the on-current and ON/OFF ratio. We investigate a series of phosphonic acid (PA) SAMs with different alkyl chain length for modification of p-type dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) VOFETs and their corresponding lateral transistors for comparison. The perforated source electrode of VOFETs is fabricated using surfactant-assisted colloidal lithography, which provides excellent control of the perforations fill factor and allows the introduction of an insulating cap layer to eliminate the off-current. Overall, the VOFET modified with dodecyl-PA SAM shows the optimal performance, with a 6 times enhanced mobility (and on-current) and a larger ON/OFF ratio compared to the unmodified device. Similar performance enhancement is also observed in the lateral transistors with SAM modifications. From detailed analyses of DNTT film morphology, crystallinity, and molecular orientation, we demonstrate that SAM modification exerts a strong impact on film morphology not only near the interface but also in the bulk, which plays a major factor for enhancing the mobilities in two types of transistors. The highest mobility obtained in the dodecyl-PA SAM modified VOFET is primarily attributed to the formation of largest grains with a resultant decrease of boundary defect density, despite that the crystallinity and molecular packing are not much improved than the other devices.

Vapor-deposition of naphthalene diimide derivatives and interface control between aluminum cathode

Two types of naphthalenediimide derivatives, N,N′-bis(benzyl)naphthalenediimide (Bzl-NDI) and N,N′-bis(p-vinylbenzyl)naphthalenediimide (PVB-NDI), were vapor-deposited either on a bare aluminum or an aluminum modified with a self-assembled monolayer (SAM) of (3-mercaptopropyl)trimethoxysilane. The deposition on the SAM-modified surface was achieved under UV irradiation to enhance thiol-ene reaction of the SAM with vinyl groups of the deposited molecules. The films of Bzl-NDI gradually crystallized, while PVB-NDI formed stable amorphous films. Electron-only devices were prepared with combinations of Bzl- and PVB-NDIs with bare and the SAM-modified aluminum cathodes. PVB-NDI was less conductive than Bzl-NDI due to the amorphous nature of the film. The SAM modification on the cathode was effective to increase the electron current of the PVB-NDI device, whereas the SAM modification did not increase the current of the Bzl-NDI device. The implication is that the thiol-terminated SAM can bind with PVB-NDI that has the vinyl group, leading to an improvement of the organic/cathode interface.

Electrochemical Stability Enhancement of Electrochromic Tungsten Oxide by Self-Assembly of a Phosphonate Protection Layer.

The presented work demonstrates an innovative method to overcome electrolyte restrictions for electrodeposited tungsten oxide (WO3) electrochromic electrodes. By self-assembly of a phosphonic acid protection layer on top of the WO3 electrode, the cycle life of a WO3 electrode in aqueous electrolytes of potassium (KCl) and lithium chloride (LiCl) is dramatically enhanced. Based on the hydrophobic nature of the self-assembled monolayer (SAM), the modification allows for ion intercalation while it prevents etching of the electrode. The cycle life of a WO3 electrode in 1 M KCl increased from under 100 to over 1000 cycles between -0.6 and 0.6 V versus Ag/AgCl. Furthermore, the current-voltage cycling and simultaneous optical transparency measurements show that a WO3 electrode having a self-assembled monolayer of an n-dodecylphosphonic acid exhibits no degradation through detachment of the electrochromic material. Our results suggest that SAM modification of electrochromic oxides is a promising new route toward long lifetime electrochromic devices even in hostile electrolyte environments.

Modification of Self-Assembled Monolayers by Electron Irradiation: The Effect of Primary Energy (10–500 eV)

Well-defined alkanethiolate self-assembled monolayers (SAMs) represent a useful model system for a variety of thin organic films on solid supports, in particular in the context of their modificatio...