Growth Of Self-assembled Monolayers Research Articles

Integration of Self-Assembled Monolayers for Cobalt/Porous Low-k Interconnects

The integration of self-assembled monolayers (SAM) into cobalt (Co)/porous low-dielectric-constant (low-k) dielectric interconnects is studied in terms of electrical characteristics and reliability in this work. Experimental results indicated that SAM derived from 3-aminopropyltrimethoxysilane (APTMS) improved breakdown field, time-dependent dielectric breakdown, and adhesion for Co/porous low-k integrated interconnects. However, the improvement magnitude was not large as compared to SAM in the Cu/porous low-k integration. Therefore, the integration of SAM into Co/porous low-k interconnects has a positive effect; however, in order to further promote the efficiency of SAM for Co/porous low-k interconnects, the option of precursors for the growth of SAM is required.

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.

Compact Hole-Selective Self-Assembled Monolayers Enabled by Disassembling Micelles in Solution for Efficient Perovskite Solar Cells.

Self-assembled monolayers (SAMs) are widely employed as effective hole-selective layers (HSLs) in inverted perovskite solar cells (PSCs). However, most SAM molecules are amphiphilic in nature and tend to form micelles in the commonly used alcoholic processing solvents. This introduces an extra energetic barrier to disassemble the micelles during the binding of SAM molecules on the substrate surface, limiting the formation of a compact SAM. To alleviate this problem for achieving optimal SAM growth, a co-solvent strategy to disassemble the micelles of carbazole-based SAM molecules in the processing solution is developed. This effectively increases the critical micelle concentration to be above the processing concentration and enhances the reactivity of the phosphonic acid anchoring group to allow densely packed SAMs to be formed on indium tin oxide. Consequently, the PSCs derived from using MeO-2PACz, 2PACz, and CbzNaph SAM HSLs show universally improved performance, with the CbzNaph SAM-derived device achieving a champion efficiency of 24.98% and improved stability.

Refining the Substrate Surface Morphology for Achieving Efficient Inverted Perovskite Solar Cells

AbstractSignificant advancements in perovskite solar cells (PSCs) have been driven by the engineering of the interface between perovskite absorbers and charge transport layers. Inverted PSCs offer substantial potential with their high power conversion efficiency (PCE) and enhanced compatibility for tandem solar cell applications. Conventional hole transport materials like poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and poly(triaryl amine) (PTAA) not only constrain the PSC efficiency but also elevate their fabrication costs. In the case of improving inverted structured PSCs according to the aforementioned concerns, utilizing self‐assembled monolayers (SAMs) as hole‐transporting layers has played a crucial role. However, the growth of self‐assembled monolayers on the substrates still limits the performance and reproducibility of inverted structured PSCs. In this study, the authors delve into the growth model of SAMs on different surface morphologies. Moreover, it is found that the plasma treatment can effectively regulate the surface morphologies of substrates and achieve conformal growth of SAMs. This treatment improves the uniformity and suppresses non‐radiative recombination at the interface, which leads to a PCE of 24.5% (stabilized at 23.5%) for inverted structured PSCs.

Influence of Temperature-Guided SAM Growth on Wetting and Its Mass Transfer Models.

The formation and growth of self-assembled monolayers (SAMs) composed of amphiphiles have garnered significant attention due to their diverse technical applications. This article reports the findings of molecular dynamics simulations aimed at elucidating the intricate relationship between the wetting behavior of amphiphiles, specifically n-alkanols, and the growth of their SAMs on a mica surface under varying temperature conditions. The investigation quantifies the structural characteristics of the formed SAMs, including density profiles, in-plane radial distribution functions, order parameters, and end-to-end length distributions of n-alkanol molecules within the SAM. Thermodynamic properties, such as the second virial coefficient and excess entropy, are examined in relation to temperature and time. The growth of the SAM is assessed by analyzing characteristic time scales at different temperatures and in-plane diffusion of n-alkanol molecules and utilizing classical theories of mass transfer to quantify the growth rate as a function of temperature. These results are then correlated with changes in the contact angle and spreading coefficient of n-alkanol droplets on the mica surface over time, providing insights into the impact of SAM growth on the wetting behavior and the mass transfer model of such systems.

Wafer‐Scale, Highly Uniform Surface Functionalization from Vapor Phase and Applications to Organic Transistors

AbstractThe surface functionalization by self‐assembled monolayers (SAMs) favors well‐packed organic semiconductor growth and reduces interfacial traps, which assists in developing high‐performance organic thin‐film transistors (OTFTs). Herein, the conventional SAM growth from the vapor phase is ameliorated and systematically studied. With 1H,1H,2H,2H‐Perfluorodecyltrichlorosilane as an example, it is found that deposition temperature of no less than 120 °C and deposition pressure of up to 0.02 bar is preferred for SAM deposition without morphological defects. The optimized SAMs are ultrasmooth with a surface roughness of 0.09 nm and can be escalated to wafer scale. It is verified that the growth condition is universal for other trichlorosilane species. Finally, it is shown that the OTFTs with defect‐free SAMs can achieve an average mobility of 1.79 cm2 V−1 s−1 using dinaphtho(2,3‐b:2′,3′‐f)thieno(3,2‐b)thiophene as the active layer, which is 2.06 times to the devices with defective SAMs and paces up the large‐area and high‐performance organic electronics.

Co-assembled Monolayers as Hole-Selective Contact for High-Performance Inverted Perovskite Solar Cells with Optimized Recombination Loss and Long-Term Stability.

Self-assembled monolayers (SAMs) have been widely employed as an effective way to modify interfaces of electronic/optoelectronic devices. To achieve a good control of the growth and molecular functionality of SAMs, we develop a co-assembled monolayer (co-SAM) for obtaining efficient hole selection and suppressed recombination at the hole-selective interface in inverted perovskite solar cells (PSCs). By engineering the position of methoxy substituents, an aligned energy level and favorable dipole moment can be obtained in our newly synthesized SAM, ((2,7-dimethoxy-9H-carbazol-9-yl) methyl) phosphonic acid (DC-PA). An alkyl ammonium containing SAM is co-assembled to further optimize the surface functionalization and interaction with perovskite layer on top. A champion device with an excellent power conversion efficiency (PCE) of 23.59 % and improved device stability are achieved. This work demonstrates the advantage of using co-SAM in improving performance and stability of PSCs.

Structural models and barrier properties of amine-terminated trialkoxysilane monolayers incubated in nonpolar vs. polar protic solvents

The effect of solvent chemistry and polarity on molecular structure and barrier capacity of a resulting trialkoxysilane self-assembled monolayer (SAM) has not yet been well explored. Here, comparative studies of synthesizing (3-aminopropyl)trimethoxysilane (APTMS) SAMs in nonpolar and polar protic solvents, toluene and ethanol, respectively, are made under strictly controlled silanization protocols favoring the growth of orderly SAMs. Atomic force, transmission and scanning electron microscopies suggest that (a) silanization for the two solvents proceeds concurrently from molecule adsorption, admolecule orientation adjustment to the formation of a stably assembled monolayer and (b) both SAMs effectively adsorb 2-nm-sized catalytic particles for electroless Cu deposits to evolve identically in terms of surface coverage. However, bonding structural analysis by angle-resolved X-ray photoelectron spectroscopy suggests that SAM (toluene) has a much higher ordering and closer packing than SAM (ethanol), and is the only qualified barrier for Cu. The effect of solvent's chemistry and polarity on molecular structure and barrier properties of the two SAMs is elucidated by molecular incubation models determined by the relative weighting of bimolecular nucleophilic substitution (SN2) reaction theoretically and uncontrolled molecular oligomerization found experimentally from dynamic light scattering.

Effects of Chiral Molecule Modification on Surface Biosorption Behavior.

Antifouling materials have many important applications in biomedical devices and marine coating. Oligo(ethylene glycol) (OEG) or poly(ethylene glycol) (PEG) exhibit promising antifouling properties and are widely used in biomedical engineering. Chiral selection is an important phenomenon in biological processes. Because of the influence of steric hindrance, the modification of chiral molecules with different chirality at interfaces will affect the intermolecular interaction at the interfaces and lead to different structures of interfacial molecules. The difference of surface structures such as surface hydration structure would impact the adsorption of biomolecules on the surface, thus causing different varieties of cell adhesion and cell growth. In this study, the influence on surface hydration and surface cell adhesion of OEG self-assembled monolayers (SAMs) modified with cysteine showing different chirality are explored. The water structure at the interfaces of OEG/water in different conditions was probed with sum frequency generation vibrational spectroscopy (SFG-VS). The results show that the interfacial water structure can change significantly with either d-cysteine or l-cysteine modification on OEG. Water molecules are more ordered at the OEG/water interface under the d-cysteine modification on OEG SAMs, which improves the protein adsorption resistance of the surface. In contrast, l-cysteine modification would make the water less ordered at the OEG/protein solution interface and enhance the protein adsorption. Additionally, optical micrographs indicate that l-cysteine can significantly promote the OEG SAMs cell adhesion and growth, while d-cysteine exhibits an inhibitory effect, which is consistent with the results of SFG-VS experiments.

Effect of Concentration, Chain Length, Hydrophobicity, and an External Electric Field on the Growth of Mixed Alkanethiol Self-Assembled Monolayers: A Molecular Dynamics Study.

Growing functionalized self-assembled monolayers (SAMs) with fewer defects and lower cost is the focus of ongoing investigations. In the present study, molecular dynamics simulations were performed to investigate the process of SAM formation on a gold substrate from mixed alkanethiolates in ethanol solution. Using the mixed-SAM system of 11-mercaptoundecanoic acid (MUA) with either 1-decanethiol (C9CH3) or 6-mercaptohexanol (C6OH) in a 3:7 ratio as the standard SAM model, we systematically investigated the effects of the concentration, chain length, functional group, and an external electric field on SAM growth. The results showed that the initial growth rate and surface coverage of the SAM are dependent on the ligand concentration. At a certain high concentration (about 1.2-1.5 times the minimum concentration), the final surface coverage is optimal. Reducing the chain length and increasing the proportion of hydrophobic diluting molecules are effective ways to improve the surface coverage, but the compositional ligands have to be changed, which may not be desirable for the functional requirements of SAMs. Furthermore, by investigating the behavior of the alkanethiolates and ethanol solvent under an applied external field, we find that a strong electric field with a proper field direction can facilitate the generation of defect-free monolayers. These findings will contribute to the understanding of mixed-SAM formation and provide insight into experimental design for efficient and effective SAM formation.

Thermodynamic Phase-like Transition Effect of Molecular Self-assembly.

The technique of self-assembled monolayers (SAMs) is frequently applied for grafting functional groups or area-selective deposition of thin films on a material surface. The formation and quality of SAMs are fundamentally determined by thermodynamic data, which are difficult to measure with available experimental methods. This work quantitatively extracted thermodynamic parameters including ΔH°, ΔG°, and ΔS° during the SAMs construction process with an ultrasensitive resonant microcantilever as molecule-surface interactions real-time recording tool. By correlating the thermodynamic parameters with self-assembling temperatures, a new thermodynamic phase-like transition effect of molecular self-assembly has been first revealed. The sharp transition of the thermodynamic parameters defines the critical condition for SAMs formation. The thermodynamic data further provide optimized reaction conditions for constructing high-quality SAMs. The explored quantitative thermodynamic analysis method not only plays as criterion for SAM growth but also helps to fundamentally elucidate physicochemical mechanism of spontaneous self-assembly.

Heat‐Assisted Photoacidic Oxidation Method for Tailoring the Surface Chemistry of Polymer Dielectrics for Low‐Power Organic Soft Electronics

AbstractThe use in low‐power soft electronics of the appropriate insulating polymer materials with a high dielectric constant (k) is considered a practical alternative to that of inorganic dielectric materials, which are brittle and have high processing temperatures. However, the polar surfaces of typical high‐k polymer insulators are problematic. Further, it is a huge challenge to control their surface properties without damage because of their soft and chemically fragile nature. Here, a heat‐assisted photoacidic oxidation method that can be used to effectively oxidize the outermost surfaces of high‐k rubbery polymer films without degradation is presented. The oxidized surfaces prepared with the developed method contain large numbers of hydroxyl groups that enable the subsequent growth of dense and ordered self‐assembled monolayers (SAMs) consisting of organosilanes. The whole process modifies the surface characteristics of polymer dielectrics effectively. The mechanisms of the oxidation of polymer surfaces and the subsequent SAM growth process are investigated. The resulting surface‐tailored rubbery dielectrics exhibit superior electrical characteristics when used in organic transistors. These results demonstrate that this method can be used to realize practical soft organic electronics based on high‐k polymer dielectrics.

Kinetics of Molecular Diffusion and Self-Assembly: Glycine on Cu{110}

Nanofabrication and the growth of self-assembled monolayers (SAM) of organic molecules are increasingly important in various industries, including microelectronics and health care. Glycine adsorbed on Cu{110} provides a good model with a rich phenomenological space to explore and understand the self-assembly of more complex amino acids. Our focus is on (a) the dynamics exhibited by glycine molecules already adsorbed on Cu{110} when diffusing on the metal surface and (b) the chemical kinetics of how these molecules form clusters, networks, and islands. The stochastic discrete event algorithm we employ can be viewed as a multiscale approach, based on density functional energies and transition barriers. The method extends from the femtosecond time-scale of molecular rotations to the nano- and microsecond range of molecular self-assembly. Hydrogen-bonds and van der Waals forces play a crucial role in pattern formation. Investigations of chemical kinetics show that enantiopure, homochiral islands are an interm...

Molecular dynamics simulations of phosphonic acid-aluminum oxide self-organization and their evolution into ordered monolayers.

We outline an unprejudiced molecular dynamics simulation approach to study the mechanisms of self-organization encompassing the evolution of surfactant-surface interactions to the growth of self-assembled monolayers (SAMs). Therein, the time-length scale problem is tackled by combining an efficient docking-type procedure for implementing surfactant-by-surfactant association with detailed molecular simulations to explore structural relaxation. For this, nanosecond-scale molecular dynamics simulations unravel ordering processes during the gradual assembly of the monolayer. Along this line, different packing motifs of octadecyl phosphonic acid (ODPA) on the (0001) surface of α-alumina and implications for the final density and ordering of the resulting monolayers are elucidated. Moreover, the role of the solvent is discriminated by comparing SAM formation in 2-propanol, hexane and in a vacuum.

Plasma Tuning Effect on Silanol-Density of Silicon Substrate for Optimal Vapor-Phase Growth of Self-Assembled Monolayers

Plasma Tuning Effect on Silanol-Density of Silicon Substrate for Optimal Vapor-Phase Growth of Self-Assembled Monolayers

Synthesis of Noble Metal Nanoparticles By Atomic Layer Deposition Utilizing Self-Assembled Monolayers Templates

Self-assembled monolayers (SAMs) are widely used as surface modification structures for thin films synthesis and designed nanostructures fabrication1. Alkyltrichlorosilanes is a group of SAMs widely applied in the field of area-selective atomic layer deposition (AS-ALD). The closely packed octadecyltrichlorosilane (ODTS) SAMs could act as monolayer resist to prohibit ALD2, while the pinholes on unsaturated ODTS SAMs could act as nucleation sites to initiate ALD growth. Thus the distribution and structure of pinholes, including its size and density, are important factors for utilizing SAMs template to fabricate nanoparticles via ALD. Many studies characterize SAMs and pinholes by atomic force microscopy (AFM), scanning electron microscopy (SEM), scanning tunneling microscope (STM), etc3. Most of these methods could provide the microscopic information on the pinhole structures, yet the information from these methods is quite localized and hard to obtain statistical average and full-scale information about the pinhole size and distribution density. Here we report utilizing electrochemical impedance spectroscopy (EIS) to study the size distribution and loading of initial nanoparticles grown in those pinhole sites. EIS is a powerful and surface sensitive characterization technique to obtain the average and global characters of the detected surfaces4,5. In this study, the properties of unsaturated ODTS SAMs are characterized with EIS methods. The relationship between SAMs growth time and the pinholes properties has been established. With the SAMs immersing time increasing, the sizes and density distribution are both reduced. For SAMs sample immersed over long time, at least 24 hours, negligible pinholes are observed. Furthermore, Pd ALD process has been carried on those SAMs modified substrates. The distribution of Pd nanoparticles corresponds well with the pinholes. The results show that the nanoparticles grown on SAMs modified substrates depend on the pinholes properties. With SAMs modified substrate, ALD grown noble metal nanoparticles with uniform size and controlled loading density can be achieved.

基于分子间作用力的敏感效应与痕量检测生化传感器

Surface Gibbs free energy can be changed by specific interaction between the targeted molecules and the functionalized molecule-layers pre-modified on MEMS/NEMS sensing structures. The generated nanomachining surface-stress can be electrically read out by the sensing element integrated on the MEMS/NEMS device. To explore the surface-stress generating mechanism that is originated from SAM (self-assembled monolayer) growth, we individually study on the three parts of head-group, molecule chain and tail-group. Based on the analysis of individual contribution from each part to surface energy change, a plotting method is developed to quantitatively assess and evaluate the effect of the grown SAM on surface energy change, i.e., generated surface-stress. In order to apply the nanomachining sensing effect into rapid biochemical detection and recognition, for the first time, we separately investigate the sensing effect from vertical specific interaction and lateral interaction between adjacent molecules. Following conclusion is finally declared based on theoretical analysis and experiment: the sign and the magnitude of the surface-stress are mainly determined by the lateral effect, while the vertical effect can make the molecule layer into ordered status thereby helping to the lateral effect. Besides, among various lateral molecule interaction, hydrogen-bond effect is the most effective one to generate the most significant sensing signal. Based on the nanomachining sensing mechanism, micro/nano cantilever sensors have been recently developed for rapid detection of trace-level biochemical molecules, which translate the surface-stress into cantilever bending and read out electric sensing signal by integrated piezoresitors. By modifying double-layer sensing molecules on gold surface of the cantilever, ppb-level organophorous agent is detected. To improve long-term stability of the sensing layer for TNT detection, a double-layer grafting technique is developed to directly modify siloxane sensing-groups on silicon surface of cantilever, thereby realizing long-term reliable detection to ppt-level TNT explosive. Finally, a SU8 selectively coated cantilever is developed for biological detection in liquid, with which 1.5×10-11 mol/mL streptavidin is real-time detected in solution.

Self-Assembled Monolayers Get Their Final Finish via a Quasi-Langmuir-Blodgett Transfer.

The growth of self-assembled monolayers (SAMs) of octadecylphosphonic acid (ODPA) molecules on α-Al2O3(0001) and subsequent dewetting of the SAMs were studied with a combination of in situ sum-frequency generation (SFG) and molecular dynamics (MD) simulations. Although SAM growth after deposition times >8 h reduces to nearly negligible values, the resultant ODPA SAMs in solution are still not in a well-ordered state with the alkyl chains in all-trans configurations. In fact, in situ SFG spectroscopy revealed a comparatively high concentration of gauche defects of the SAM in the ODPA 2-propanol solution even after a growth time of 16 h. Here, results of the MD simulations strongly suggest that defects can be caused by ODPA molecules which are not attached to the substrate but are incorporated into the SAM layer with the polar headgroup oriented into the 2-propanol solvent. This inverted adsorption geometry of additional ODPA molecules blocks adsorption sites and thus stabilizes the SAM without improving ordering to an extent that all molecules are in the all-trans configuration. While persistent in solution, the observed defects can be healed out when the SAMs are transferred from the solvent to a gas phase. During this process, a quasi-Langmuir-Blodgett transfer of molecules takes place which drives the SAM into a higher conformational state and significantly improves its quality.

Formation of cyclodextrin monolayer through a host–guest interaction with tailor-made phenyltriethoxysilane self-assembled monolayer

Abstract Host–guest systems have been widely used for nanostructure construction; however, studying host–guest behavior by immobilizing the guest molecule on a substrate's surface faces great challenges. In this study, phenyltriethoxysilane (PTES) was selected as the ideal candidate to fabricate a guest molecule self-assembled monolayer (SAM). A remarkable PTES SAM growth mechanism, which includes two growth stages, was revealed. By controlling the PTES SAM formation in the first growth stage, the residual PTES ethoxyl groups will hydrolyze in a cyclodextrin (CD) aqueous solution, and the hydrolyzed PTES SAM, as a guest molecule, can provide suitable steric space for forming α- and β-CD inclusion complexes. The experiments demonstrated that the inclusion configurations for α- and β-CD SAMs are different, with the α-CD molecule tilting relative to the silica surface in order to include the PTES phenyl group; whereas the secondary surface of the β-CD remains parallel to the silica surface due to its large cavity. The results corroborated that this simple, yet effective, platform can be readily used to investigate host–guest systems, and it has huge potential for fabricating materials containing CD monolayers.

Diffusion of self-assembled monolayers of thiols on the gold surfaces covered with polydimethylsiloxane stamps

Thiols can diffuse and form self-assembled monolayers (SAMs) on the gold surfaces covered with polydimethylsiloxane (PDMS) stamps. For the first time, with cells as the indicator of how far alkanethiols had diffused to form SAM, we studied the growth dynamics of SAMs of HS(CH2)11(OCH2CH2)3OH (EG3) and HS(CH2)11(OCH2CH2)6OH (EG6) on the gold surfaces covered with PDMS stamps. The growth of SAMs is well described by one-dimensional diffusion from a line source of concentration, with surface diffusion coefficient of 193.4 ± 19.2 μm2/min (EG3) and 95.8 ± 18.9 μm2/min (EG6).