Fluorinated Self-assembled Monolayer Research Articles

LiF-Rich Solid Electrolyte Interphase Formation by Establishing Sacrificial Layer on the Separator.

The formation of a stable solid electrolyte interphase (SEI) layer is crucial for enhancing the safety and lifespan of Li metal batteries. Fundamentally, a homogeneous Li+ behavior by controlling the chemical reaction at the anode/electrolyte interface is the key to establishing a stable SEI layer. However, due to the highly reactive nature of Li metal anodes (LMAs), controlling the movement of Li+ at the anode/electrolyte interface remains challenging. Here, an advanced approach is proposed for coating a sacrificial layer called fluorinated self-assembled monolayer (FSL) on a boehmite-coated polyethylene (BPE) separator to form a stable SEI layer. By leveraging the strong affinity between the fluorine functional group and Li+, the rapid formation of a LiF-rich SEI layer in the cell production and early cycling stage is facilitated. This initial stable SEI formation promotes the subsequent homogeneous Li+ flux, thereby improving the LMA stability and yielding an enhanced battery lifespan. Further, the mechanism behind the stable SEI layer generation by controlling the Li+ dynamics through the FSL-treated BPE separator is comprehensively verified. Overall, this research offers significant contributions to the energy storage field by addressing challenges associated with LMAs, thus highlighting the importance of interfacial control in achieving a stable SEI layer.

Wrinkle-Free Graphene Films on Fluorinated Self-Assembled Monolayer-Modified Substrates for Enhancing the Electrical Performance of Transistors

Self-assembled monolayer (SAM)-functionalized substrates are widely used for tailoring the electronic properties (i.e., p- and n-type doping) of two-dimensional materials, which might suppress the charge scattering, impurities, and wrinkles that can severely decline electrical properties. The fluorinated self-assembled monolayer (FSAM) is also used for promoting electrical properties by eliminating the small number of water molecules/residues between the substrate and graphene during the typical wet transfer process. However, taking graphene as an example, most of the graphene/SAM or graphene/FSAM studies are focused on the enhancement of their electrical properties and lack investigation on their surface morphology after transfer and the correlation between them. Herein, a strategy is discovered in which FSAM-modified substrates help construct a wrinkle-free graphene film with the dry transfer process due to the low surface energy between the graphene and FSAM. Trichloro(1H,1H,2H,2H-perfluorooctyl)silane (FDTS) was selected as a precursor toward the formation of the monolayer and highly uniform FSAM using a facile dip-coating route, which is feasible on versatile substrates such as Si, SiO2/Si, and poly(ethylene terephthalate) (PET). The surface roughness of the FSAM-modified SiO2 substrate reduces from 2.98 to 0.13 nm. Therefore, the lower surface energy (from 60.80 to 8.12 mN/m) was found to enhance the carrier mobility of the transferred graphene by about 1.77 times, increasing from 894.6 to 1588 cm2/(V·s). Furthermore, a top-gated field-effect transistor based on graphene/FSAM was fabricated and characterized in which the FSAM decouples from the strong substrate interference and significantly improved the field-effect mobility by up to 17 times compared to that of graphene without substrate modification. Thus, this work provides a facile strategy to avoid wrinkle defects and suppress charge scattering through the decoupling from the substrate, which could be applied to other two-dimensional (2D) materials for improving their electrical performance on transistors and flexible electronics.

Lubricant-infused directly engraved nano-microstructures for mechanically durable endoscope lens with anti-biofouling and anti-fogging properties

While a clear operating field during endoscopy is essential for accurate diagnosis and effective surgery, fogging or biofouling of the lens can cause loss of visibility during these procedures. Conventional cleaning methods such as the use of an irrigation unit, anti-fogging surfactant, or particle-based porous coatings infused with lubricants have been used but proven insufficient to prevent loss of visibility. Herein, a mechanically robust anti-fogging and anti-biofouling endoscope lens was developed by forming a lubricant-infused directly engraved nano-/micro-structured surface (LIDENS) on the lens. This structure was directly engraved onto the lens via line-by-line ablation with a femtosecond laser. This directly engraved nano/microstructure provides LIDENS lenses with superior mechanical robustness compared to lenses with conventional particle-based coatings, enabling the maintenance of clear visibility throughout typical procedures. The LIDENS lens was chemically modified with a fluorinated self-assembled monolayer (F-SAM) followed by infusion of medical-grade perfluorocarbon lubricants. This provides the lens with high transparency (> 70%) along with superior and long-lasting repellency towards various liquids. This excellent liquid repellency was also shown to be maintained during blood dipping, spraying, and droplet condensation experiments. We believe that endoscopic lenses with the LIDENS offer excellent benefits to endoscopic surgery by securing clear visibility for stable operation.

Surface Dipoles Induce Uniform Orientation in Contacting Polar Liquids

Molecular interactions at the solid–liquid interface have a profound impact on the macroscale wetting properties of surfaces. Surface-specific sum frequency generation (SFG) spectroscopy, one of a limited number of techniques having the capability to probe such interfaces, generates a surface vibrational spectrum that is sensitive to molecular structure and is used to determine the orientation of molecules at the interface. This study utilized SFG spectroscopy to study the interactions between the polar liquid acetonitrile and selectively fluorinated self-assembled monolayer (FSAM) interfaces generated by the adsorption of CF3(CH2)17SH and CD3(CF2)6(CH2)10SH on gold; these SAMs exhibit strong oriented molecular dipoles at their chain termini. We used SFG spectroscopy to examine the C–N and C–H stretching regions, 2000–2300 and 2800–3050 cm–1, respectively, to probe the solid–liquid interface of the FSAMs in contact with acetonitrile. The appearance of positive- or negative-going peaks in the SFG spectrum of acetonitrile as a function of the variation in the terminal group of the FSAMs could be attributed, to a first-order approximation, to the orientation (CH3 up or down) of the acetonitrile molecules with respect to changes in the direction of the FSAM dipole. Coherent molecular spectroscopy was used as a direct probe of this effect by examining the differences in the SFG spectra to determine the relative orientation of acetonitrile with respect to the underlying SAM and, therefore, provided direct evidence for the dipole-influenced wettability behavior of these unique model surfaces.

Effective hole‐injection characteristics of organic light‐emitting diodes due to fluorinated self‐assembled monolayer embedded as a buffer layer

AbstractThe electrical and optical properties of organic light‐emitting diodes (OLEDs) are demonstrated by varying the length of the alkyl chain of a fluorinated self‐assembled monolayer (F‐nSAM). OLEDs containing F‐nSAMs that have a long alkyl chain length were found to exhibit excellent properties in terms of current density, luminance, turn‐on voltage, etc. The obtained current density at 6 V, which was the highest measurement voltage, was up to about 36 times higher for an OLED including an F‐12SAM thin film with the longest chain length than for an OLED including only an indium tin oxide (ITO) substrate. With regard to luminance characteristics depending on voltage, the luminance was about 13 times higher for the OLED including the F‐12SAM thin film than for the OLED including only the ITO substrate. Also, the turn‐on voltage of the OLED including the F‐12SAM thin film was decreased by approximately 1 V compared to that of the OLED including only the ITO substrate. Although F‐nSAMs with alkyl chains have insulating film properties, F‐nSAMs with long alkyl chains exhibited good electrical and optical properties because of an improvement in the hole‐injection barrier due to a large positive shift of the vacuum level and smooth carrier injection resulting from a high contact angle due to strong hydrophobic properties caused by the good alignment properties of F‐nSAMs resulting from strong van der Waals forces between the molecules due to the long alkyl chains. © 2019 Society of Chemical Industry

Alignment of liquid crystal molecules on self-assembled monolayer with fluorinated alkyl chain at different deposition time

We demonstrated a homeotropic liquid crystal (LC) alignment state on fluorinated self-assembled monolayers (FSAMs) deposited by gas-phase method as a function of deposition time. The gas-phase method to deposit FSAMs was simple process and cost-effective. Contact angle data and surface energy analyses of the FSAMs showed that film formation occurs, and showed a correlation with the homeotropic alignment of LC molecules on the FSAMs. Stable LC alignments on FSAMs were achieved at deposition time over 60 min. In addition, high transparency and thermal stability of FSAM further enhanced its possibility as a LC alignment layer.

Photoelectric Properties of OLEDs With and Without Fluorinated Self-Assembled Monolayer and by Varying Thickness of α-NPD

In this study, we investigated the photoelectric effect and optimization of an organic light-emitting diode (OLED) depending on the presence or absence of a fluorinated self-assembled monolayer (FLSAM) and by varying the thickness of N,N'-Di (1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (α-NPD) from 0 nm to 50 nm. The large distinction in electronegativity between the carbon and the fluorine replacing hydrogen in the alkyl chain of FLSAM generates a strong dipole moment to elevate the vacuum level, resulting in a change of the work function. This eliminates the injection barrier between the work function of the ITO modified by FLSAM and the highest occupied molecular orbital (HOMO) level of the hole-transport layer, thus leading to excellent driving voltage characteristics. Devices without FLSAM had a driving voltage more than twice that of devices using with FLSAM. The introduction of α-NPD as the hole-transport layer enhanced the electrical conductivity by facilitating the transport of holes. However, due to the inherent insulating film properties of α-NPD, the increase in its thickness resulted in a decrease in current density.

Homeotropic alignment behavior of liquid crystal molecules on self-assembled monolayers with fluorinated alkyl chain

This study demonstrates the orientation characteristics of liquid crystals (LCs) on fluorinated self-assembled monolayers (FSAMs) deposited using the gas phase method. The authors confirmed that the FSAMs were properly deposited on indium-doped tin oxide glass substrates using x-ray photoelectron spectroscopy and atomic force microscopy. Compared with conventional polyimide (PI) layers, the authors confirmed that the FSAM is an alternative to the conventional PI layer. Regardless of the positive and negative LC characteristics, the LC molecules were vertically aligned on the FSAMs, with hydrophobic properties. In addition, an LC cell using an FSAM showed faster response times than that with a conventional PI layer when the electro-optical characteristics were examined. Therefore, these results indicate that FSAMs are suitable for LC applications as homeotropic LC alignment layers.

Performance enhancement of organic light-emitting diodes using fluorinated self-assembled monolayer

ABSTRACTIn order to enhance the performance of organic light-emitting diodes, fluorinated self-assembled monolayer (F-SAM) with long alkyl chain was introduced to the device between the anode and hole-transport layer. CF3-terminated F-SAM has a relatively high electric dipole moment, which may affect hole injection at the indium-tin-oxide (ITO) anode. Two kinds of F-SAM with different alkyl chain lengths (F3-SAM and F10-SAM) were formed on the ITO substrate through hydrolysis and dehydration process. Electric dipole moments of these F-SAMs were obtained from a simulation using Gauss View program. A device was made in a structure of ITO (170 nm) / F-SAM / TPD (40 nm) / Alq3 (60 nm) / Al (100 nm). Formation of the SAM was confirmed by contact-angle measurements. It was found that the contact angles for the F3-SAM and F10-SAM are about 84° and 112°, respectively. A device with properly treated self-assembled monolayer at the anode gives an improvement in turn-on voltage, luminance, and efficiency compared to those of the device without SAM. Turn-on voltage was lowered from 4.25 V to 3.50 V in the device with SAM compared to the one without SAM, which indicates a reduction of hole-injection barrier height due to the electric dipole moment of the F-SAM. An external quantum efficiency of the device with F3-SAM was increased by a factor of about 3.5 compared to the one without SAM.

Interface engineering to enhance charge injection and transport in solution-deposited organic transistors

Soluble small molecule organic semiconductors combine the high-performance of small molecule organic semiconductors with the versatile processability of polymeric materials, but the control of device performance and uniformity is challenged by the complex film microstructure formed in these materials, and its strong dependence on processing conditions. These films crystallize via a nucleation and growth mechanism that can be difficult to control. In this study we used highly fluorinated self-assembled monolayers (SAMs) to modify the surface of the source and drain contacts and improve the performance of organic thin-film transistors (OTFTs) through controlling film microstructure and lowering the contact resistance. We reached charge carrier mobilities as high as 5.7 cm2/V in 2,8-Difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT), one order of magnitude greater than what we obtained in devices on untreated substrates, and on par with the value reported for single crystal devices. Kelvin probe measurements distinguished an increase in the work function between 0.28 eV and 0.5 eV, depending on the molecular structure of the SAM. Selected area electron diffraction (SAED) confirmed the preferential “edge-on” molecular orientation of the semiconductor. We discuss the device performance in relation to the film morphology and contact resistance.

Influence of fluorinated self-assembled monolayer on wetting dynamics during evaporation of refrigerant–oil mixture on metal surface

Reducing wettability of a metal surface is a promising method for enhancing boiling heat transfer of refrigerant–oil mixture on the metal. As fluorinated self-assembled monolayer (F-SAM) coating is effective for wettability reduction, its influence on wetting dynamics including meniscus shape, contact angle, contact line velocity and rising liquid height during evaporation of refrigerant–oil mixture on metal surface were experimentally investigated. The refrigerant–oil mixture was prepared by R141b and NM56, the oil mass fraction ranged from 0 to 10 wt%, and the surface roughness ranged from 0.028 to 1.166 µm. The results show that during evaporation of refrigerant–oil mixture, the presence of F-SAM changes the evaporation mode to be constant contact line velocity followed by both constant contact angle and contact line velocity, while decreases the rising liquid height. The results suggest that larger surface roughness and higher oil mass fraction are preferred when using F-SAM to reduce surface wettability.

Harnessing Photovoltage: Effects of Film Thickness, TiO2 Nanoparticle Size, MgO and Surface Capping with DSCs.

High photovoltage dye-sensitized solar cells (DSCs) offer an exceptional opportunity to power electrocatalysts for the production of hydrogen from water and the reduction of CO2 to usable fuels with a relatively cost-effective, low-toxicity solar cell. Competitive recombination pathways such as electron transfer from TiO2 films to the redox shuttle or oxidized dye must be minimized to achieve the maximum possible photovoltage (Voc) from DSC devices. A high Voc of 882 mV was achieved with the iodide/triiodide redox shuttle and a ruthenium NCS-ligated dye, HD-2-mono, by utilizing a combined approach of (1) modulating the TiO2 surface area through film thickness and nanoparticle size selection, (2) addition of a MgO insulating layer, and (3) capping available TiO2 film surface sites post film sensitization with an F-SAM (fluorinated self-assembled monolayer) treatment. The exceptional Voc of 882 mV observed is the highest achieved for the popular NCS containing ruthenium sensitizers with >5% PCE and compares favorably to the 769 mV value observed under common device preparation conditions.

Hole-Injection Properties Characteristics of Fluorinated Self-Assembled Monolayer on Polymeric and Organic Light-Emitting Diodes

We examine the electro-optic effects of a fluorinated self-assembled monolayer (FSAM) on polymeric light-emitting diodes (PLEDs) and organic light-emitting diodes (OLEDs). Owing to the high electronegativity of fluorine atoms, which decreases the hole-injection barrier-height, the LEDs with FSAM as the hole-injection layer (HIL) have the highest current density. The current densities of the LEDs with FSAM, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and without HIL were estimated to be approximately 1, 0.24, and 0.06 A/cm2 respectively at 10 V. During the operational lifetime of the PLEDs, the driving voltage with FSAM is suppressed owing to the ohmic contact of the FSAM-modified ITO.

Surface Modification of TiO2 Photoanodes with Fluorinated Self-Assembled Monolayers for Highly Efficient Dye-Sensitized Solar Cells

Dye aggregation and electron recombination in TiO2 photoanodes are the two major phenomena lowering the energy conversion efficiency of dye-sensitized solar cells (DSCs). Herein, we introduce a novel surface modification strategy of TiO2 photoanodes by the fluorinated self-assembled monolayer (F-SAM) formation with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFTS), blocking the vacant sites of the TiO2 surface after dye adsorption. The F-SAM helps to efficiently lower the surface tension, resulting in efficient repelling ions, e.g., I3(-), in the electrolyte to decrease the electron recombination rate, and the role of F-SAM is characterized in detail by impedance spectroscopy using a diffusion-recombination model. In addition, the dye aggregates on the TiO2 surface are relaxed by the F-SAM with large conformational perturbation (i.e., helix structure) seemingly because of steric hindrance developed during the SAM formation. Such multifunctional effects suppress the electron recombination as well as the intermolecular interactions of dye aggregates without the loss of adsorbed dyes, enhancing both the photocurrent density (11.9 → 13.5 mA cm(-2)) and open-circuit voltage (0.67 → 0.72 V). Moreover, the combined surface modification with the F-SAM and the classical coadsorbent further improves the photovoltaic performance in DSCs.

Evaluation of Fluorinated Self‐Assembled Monolayer by Photoelectron and Near Edge X‐Ray Absorption Fine Structure Spectroscopy

SUMMARYThe electronic structure of fluorinated self‐assembled monolayers (F‐SAMs) with different chain lengths was investigated by photoelectron and near edge X‐ray absorption fine structure (NEXAFS) spectroscopy. From the measurements of the photoelectron spectra in the wide region and in the C 1s core‐level region, the chemical compositions and components of the F‐SAMs with different chain length were clarified. In the C K‐edge NEXAFS spectra of the F‐SAMs, the several spectral features were observed and the intensity of the features at 292 eV and 299 eV decreased with increasing incidence angle of the excitation photons. Based on the results of the photoelectron and NEXAFS spectral measurements, the electronic structure of four F‐SAMs with different chain length is discussed.

Electronic structure of fluorinated self-assembled monolayer investigated by photoelectron spectroscopy in the valence band region

The electronic structure of the fluorinated self-assembled monolayer (F-SAM) was investigated by using the photoelectron spectroscopy in the valence band region. The photon energy dependence of the photoelectron spectra in the valence band region of the F-SAM was measured from 50 to 420 eV and the orbital components of the photoelectron spectra were clarified. By measuring the photoelectron spectra in the valence band region of four types of F-SAM with different chain lengths, we obtained the evolution of the electronic structures in the F-SAM. In addition, the energy level of the F-SAM molecule obtained by ab initio molecular orbital calculation in a previous study was compared with the photoelectron spectrum in the valence band region. On the basis of the results of the photoelectron spectroscopy measurements in the valence band region, the electronic structure of the F-SAMs is discussed.

Fluoroalkylsilanes with Embedded Functional Groups as Building Blocks for Environmentally Safer Self-Assembled Monolayers.

The fabrication of silane-based fluorinated self-assembled monolayers (FSAMs) on indium tin oxide (ITO, a transparent electrode) was carried out making use of the following fluoroalkylsilanes (FAS): 2,2,3,3,4,4,5,5,6,6,6-undecafluoro-N-[3-(trimethoxysilyl)propyl]hexanamide (1; R(F) = C5F11) and 1,1,2,2,3,3,4,4,4-nonafluoro-N-[3-(trimethoxysilyl)propyl]butane-1-sulfonamide (2; R(F) = C4F9), containing an embedded amide and a sulfonamide group, respectively, between the short perfluoroalkyl chain (R(F) with C < 6) and the syloxanic moiety. The obtained FSAM-modified/ITO systems were characterized by X-ray photoelectron spectroscopy (XPS), contact angle (CA), surface energy measurements, and electrochemical impedance spectroscopy (EIS) and compared to ITO modified with a 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyltriethoxysilane (3; R(F) = C6F13), with the perfluoroalkyl group linked to the syloxanic moiety through a simple hydrocarbon chain. The results obtained show that the presence of the -NHCO- and -NHSO2- groups have a different mode of action and, with the former, despite the short perfluoroalkyl chain, the ITO-1 system presents a CA (Θ(water )= 113.5°) and surface energy (γl = 14.0 mJ m(-2)) typical of amphiphobic materials. These properties can be exploited in a variety of applications, such as self-cleaning, anti-fouling, and anti-fingerprint coatings, and in advanced microelectronic components.

Effects of Self-Assembled Monolayers with Fluorinated Alkyl Chain

The electrical characteristics of fluorinated self-assembled monolayers (FSAMs) in organic light-emitting diodes (OLEDs) were investigated according to the change in the lengths of their alkyl chains. As the number of fluorine atom increases, the device with long alkyl chain length allows the flow of a large amount of current because of the improved barrier height of the hole injection. However, the current efficiency under long alkyl chain length was decreased by the switched carrier balance. The device with the shortest FSAM alkyl chain length demonstrated an approximately 68% higher current efficiency than that with the longest alkyl chain length.

Evaluation of the fluorinated antisticking layer by using photoemission and NEXAFS spectroscopies

The electronic structures of four kinds of fluorinated self-assembled monolayers (F-SAMs) with different chain length, which were used for an antisticking layer, were investigated by the photoemission and the near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. From the photoemission spectra in the wide and in the C 1s core-level regions, chemical compositions and components of the F-SAMs with different chain length were evaluated. By using the curve fitting analysis of the photoemission spectra in C 1s core-level region, it was found that the CF3 site is located at the top of the surface in the C sites of the F-SAM. From the C K-edge NEXAFS spectra of the F-SAMs as a function of the incidence angle of the excitation photon, it was shown that the σ*(C–F) and σ*(C–C) orbitals in the F-SAMs are parallel and perpendicular to the surface, respectively. This indicates that the C–C chain in (CF2) n part of the F-SAMs is perpendicular to the surface. Based on these results, the electronic structures of the F-SAMs are discussed.

Electrical conduction behavior of organic light-emitting diodes using fluorinated self-assembled monolayer with molybdenum oxide-doped hole transporting layer.

The electrical conductivity behavior of a fluorinated self-assembled monolayer (FSAM) of a molybdenum oxide (MoOx)-doped α-naphthyl diamine derivative (α-NPD) in organic light-emitting diodes (OLEDs) was investigated. The current density of the MoOx-doped α-NPD/FSAM device was proportional to its voltage owing to smooth carrier injection through the FSAM and the high carrier density of its bulk. The temperature-dependent characteristics of this device were investigated. The current density-voltage characteristics at different temperatures were almost the same owing to its very low activation energy. The activation energy of the device was estimated to be 1.056 × 10(-2) [eV] and was very low due to the inelastic electron tunneling of FSAM molecules.