Solid-State Densification of Spun-Cast Self-Assembled Monolayers for Use in Ultra-Thin Hybrid Dielectrics

High-k organic-inorganic hybrid dielectric material for flexible thin-film transistors and printed logic circuits

A new photopolymerizable organic-inorganic (O-I) hybrid sol-gel material, AUP@SiOx-184, has been synthesized and utilized as a gate dielectric in flexible organic thin-film transistors (OTFTs). The previously reported three-arm alkoxy-functionalized silane amphiphilic polymer has yielded stable O-I hybrid materials comprising uniformly dispersed nanoparticles in the sol state. In this study, a photosensitizer was introduced, facilitating curing effects under ultraviolet light. Photo-crosslinking enhances the stability of hydroxyl radicals within inorganic nanoparticles, thereby minimizing device hysteresis. This approach also contributes to achieving a low leakage current and a high dielectric constant (high-k) while maintaining reduced thickness. Moreover, AUP@SiOx-184 films are amenable to patterning through UV photopolymerization and can be successfully produced using printing techniques. Compared to other materials, they exhibit outstanding flexibility and improved insulating capabilities. Additionally, OTFTs incorporating AUP@SiOx-184 layers demonstrate extremely stable driving features on flexible substrates. Selective printing and specific patterning play crucial roles in the fabrication of logic circuits. This synthesis strategy has resulted in integrated logic devices that have successfully demonstrated their functionality, highlighting its value for producing functional O-I hybrid materials. Utilizing AUP@SiOx-184 as a gate dielectric in OTFTs showcases its potential to advance electronic technologies that are both flexible and high-performing.

Self-Assembled Monolayers for Interfacial Engineering in Solution-Processed Thin-Film Electronic Devices: Design, Fabrication, and Applications

Interfacial engineering has long been a vital means of improving thin-film device performance, especially for organic electronics, perovskites, and hybrid devices. It greatly facilitates the fabrication and performance of solution-processed thin-film devices, including organic field effect transistors (OFETs), organic solar cells (OSCs), perovskite solar cells (PVSCs), and organic light-emitting diodes (OLEDs). However, due to the limitation of traditional interfacial materials, further progress of these thin-film devices is hampered particularly in terms of stability, flexibility, and sensitivity. The deadlock has gradually been broken through the development of self-assembled monolayers (SAMs), which possess distinct benefits in transparency, diversity, stability, sensitivity, selectivity, and surface passivation ability. In this review, we first showed the evolution of SAMs, elucidating their working mechanisms and structure-property relationships by assessing a wide range of SAM materials reported to date. A comprehensive comparison of various SAM growth, fabrication, and characterization methods was presented to help readers interested in applying SAM to their works. Moreover, the recent progress of the SAM design and applications in mainstream thin-film electronic devices, including OFETs, OSCs, PVSCs and OLEDs, was summarized. Finally, an outlook and prospects section summarizes the major challenges for the further development of SAMs used in thin-film devices.

Structural investigations of self-assembled monolayers for organic electronics: results from X-ray reflectivity

Self-assembled monolayers (SAMs) have been established as crucial interlayers and electronically active layers in organic electronic devices, such as organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), organic thin film transistors (OTFTs), and nonvolatile memories (NVMs). The use of self-assembling functionalized organic molecules is beneficial due to mainly three advantages compared with common thin film deposition approaches. (1) Molecular self-assembly occurs with surface selectivity, determined by the interaction between the functional anchor group of the organic molecules and the target surface. (2) The film thickness of the resulting layers is perfectly controllable on the angstrom scale, due to the self-terminating film formation to only a single molecular layer. And finally, (3) the wide variability in the chemical structure of such molecules enables different SAM functionalities for devices, ranging from electrical insulation to charge storage to charge transport. The SAM approach can be further expanded by employing several functionalized molecules to create mixed SAMs with consequently mixed properties. The function of SAMs in devices depends not only on the chemical structure of the molecules but also on their final arrangement and orientation on the surface. A reliable and nondestructive in-depth characterization of SAMs on nonconductive oxide surfaces is still challenging because of the very small thickness and the impracticality of methods such as scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In this Account, we illustrate how X-ray reflectivity (XRR) provides analytical access to major questions of SAM composition, morphology, and even formation by means of investigations of pure and mixed SAMs based on phosphonic acids (PAs) of various chain structures on flat alumina (AlOx) surfaces. XRR is an analytical method that provides access to spatially averaged structural depth profiles over a relatively large area of several square micrometers. The key outcome of XRR, the surface-normal electron density profile of the SAMs, leads to precise information on the SAM thickness with subangstrom resolution and allows for the determination of molecular tilt angles and packing densities. We have systematically increased the chemical complexity of PA molecules and the resulting SAMs, utilizing XRR to provide insight into the SAM structures. In SAMs composed of functionalized molecules or complex chain structures, the distribution of electron rich and electron poor signatures is detected and thus the molecular order within the SAM is determined. In mixed SAMs of two different molecular species, electron density profiles reveal the morphology and how the surface-normal structure changes if one component of the mixed SAM is altered. Furthermore, XRR was applied to investigate in situ the self-assembly of functionalized PA molecules from solution by tracking the monolayer growth over time. Even though the results provided by XRR on in-plane molecular arrangement are limited, it presents excellent information on the molecular scale along the surface normal and in addition allows for drawing conclusions on the intermolecular interactions within the SAM.

High-throughput analysis of molecular orientation on surfaces by NEXAFS imaging of curved sample arrays

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy provides detailed information about the orientation and alignment of thin films. NEXAFS is a synchrotron-based technique-the availability of beam-time per user is typically limited to no more than a few weeks per year. The limited availability is currently a true barrier for using NEXAFS in combinatorial studies of molecular alignment. We have recently demonstrated how large area full field NEXAFS imaging allows users to pursue combinatorial studies of surface chemistry. Now we report an extension of this approach which allows the acquisition of orientation information from a single NEXAFS image. An array with 80 elements (samples), containing eight series of different surface modifications, was mounted on a curved substrate allowing the collection of NEXAFS spectra with a range of orientations with respect to the X-ray beam. Images collected from this array show how hyperspectral NEXAFS data collected from curved surfaces can be used for high-throughput molecular orientation analysis.

Amphiphobic surfaces from functionalized TiO2 nanotube arrays

Perfluorinated monolayer-coated TiO₂ nanotube surfaces are repellent to a broad spectrum of liquids, and are not only of immediate interest in anti-fouling applications but also present a platform to explore wetting and imbibition phenomena in nanostructures.