Octadecylphosphonic Acid Self-assembled Monolayers Research Articles

Hydrogenated graphene systems: A novel growth and hydrogenation process

Octadecylphosphonic acid self-assembled monolayers were used as a combined carbon and hydrogen source to grow graphene films on sapphire substrates via hot filament chemical vapor deposition. The functionalized substrates were sealed with a thin Cu film and heated to 950°C under Ar flow. After synthesis, the Cu was etched away. The graphene samples then underwent a hydrogenation treatment in the same reactor setup, exposed to a CH4/H2 gas mixture at 820°C for 2 hours. The structure and properties of the graphene films before and after hydrogenation were characterized. Raman spectroscopy was employed to probe the defect-related bands and C-H bonding. X-ray diffraction provided insights into the crystalline structure and interlayer spacing. The ferromagnetic response was measured using a PPMS system across a range of temperatures and magnetic fields. XPS was used to assess the chemical composition and bonding. This multi-step process enabled a detailed evaluation of the novel synthesis protocol and its effects on the resulting hydrogenated graphene material.

Parameters controlling octadecyl phosphonic acid self-assembled monolayers on titanium dioxide for anti-fouling biomedical applications

Octadecylphosphonic acid (ODPA) self-assembled monolayers (SAMs) have demonstrated potential for deterring bacterial attachment to titanium, however the coating process is time consuming and uses toxic solvents. In this study, ODPA SAM quantity, quality, and structure were evaluated on titanium quartz crystal microbalance (QCM) sensors by varying solvent type (anisole or cyclopentyl methyl ether (CPME), environmentally friendly non-polar solvents); ODPA concentration (0.5 mM or 1 mM); and temperature (21 °C or 60 °C). Surfaces were characterised using QCM frequency and dissipation measurements and Sauerbrey mass calculations; X-ray photoelectron spectroscopy; water contact angle measurements; and temperature-programmed desorption mass spectrometry (TPD-MS). Anti-fouling ability was established against Staphylococcus aureus. Incubation in 0.5 mM ODPA in CPME at 21 °C rapidly formed uniform rigid ODPA SAMs as demonstrated by high Sauerbrey mass (≈285-290 ng/cm2), viscoelastic modelling, high atomic percentage surface phosphorus (1.1%) and high water contact angles (117.6 ± 2.5°), consistent across the entire sample surface. High temperatures or the use of anisole resulted in suspected multilayer formation, which reduced bacterial attachment. TPD-MS confirmed covalent bonding of ODPA SAMs on TiO2 at ≈ 110–120 °C and thermal stability below 300 °C. This study demonstrates the key parameters that control ODPA SAM formation on titanium and their future potential for biomedical applications.

Octadecylphosphonic Acid Self-Assembled Monolayers Obtained Using Rapid Dipping Treatments

Octadecylphosphonic Acid Self-Assembled Monolayers Obtained Using Rapid Dipping Treatments

Reaction Mechanism of Area-Selective Atomic Layer Deposition for Al2O3 Nanopatterns.

The reaction mechanism of area-selective atomic layer deposition (AS-ALD) of Al2O3 thin films using self-assembled monolayers (SAMs) was systematically investigated by theoretical and experimental studies. Trimethylaluminum (TMA) and H2O were used as the precursor and oxidant, respectively, with octadecylphosphonic acid (ODPA) as an SAM to block Al2O3 film formation. However, Al2O3 layers began to form on the ODPA SAMs after several cycles, despite reports that CH3-terminated SAMs cannot react with TMA. We showed that TMA does not react chemically with the SAM but is physically adsorbed, acting as a nucleation site for Al2O3 film growth. Moreover, the amount of physisorbed TMA was affected by the partial pressure. By controlling it, we developed a new AS-ALD Al2O3 process with high selectivity, which produces films of ∼60 nm thickness over 370 cycles. The successful deposition of Al2O3 thin film patterns using this process is a breakthrough technique in the field of nanotechnology.

Patterning of wettability using the photocatalytic decomposition of hydrophobic self-assembled monolayer on the TiO2 pattern

We report a patterning process for an octadecylphosphonic acid self-assembled monolayer (ODP-SAM) using TiO2 patterns fabricated by a combined use of reactive sputtering and CF4 plasma etching. The ODP-SAM with the remaining outer region of TiO2 patterns can be used as a constant hydrophobic region. The photocatalytic decomposition of the ODP-SAM on TiO2 thin films can be applied to the formation of hydrophobic/hydrophilic wettability patterns. The photoresponsive wettability of TiO2 thin films with different surface morphologies was examined on films deposited under different sputtering conditions. The contact angle on the ODP-SAM/TiO2 surface was successfully changed from 134.8 to 0° by exposing the raw TiO2 surface after the photodecomposition of the ODP-SAM, whereas the contact angle on the ODP-SAM in the outer region of TiO2 patterns remained constant at 112.1°. The width of the wettability switching range of the raw surface of sputter-deposited TiO2 thin films can be controlled by changing the deposition conditions of the TiO2 thin films. Droplet generation solely through the bottom surface with wettability patterns defined by a combined use of an ODP-SAM and TiO2 patterns in a microchannel was also demonstrated.

Metal-oxide assisted surface treatment of polyimide gate insulators for high-performance organic thin-film transistors.

We developed a facile method for treating polyimide-based organic gate insulator (OGI) surfaces with self-assembled monolayers (SAMs) by introducing metal-oxide interlayers, called the metal-oxide assisted SAM treatment (MAST). To create sites for surface modification with SAM materials on polyimide-based OGI (KPI) surfaces, the metal-oxide interlayer, here amorphous alumina (α-Al2O3), was deposited on the KPI gate insulator using spin-coating via a rapid sol-gel reaction, providing an excellent template for the formation of a high-quality SAM with phosphonic acid anchor groups. The SAM of octadecylphosphonic acid (ODPA) was successfully treated by spin-coating onto the α-Al2O3-deposited KPI film. After the surface treatment by ODPA/α-Al2O3, the surface energy of the KPI thin film was remarkably decreased and the molecular compatibility of the film with an organic semiconductor (OSC), 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10), was increased. Ph-BTBT-C10 molecules were uniformly deposited on the treated gate insulator surface and grown with high crystallinity, as confirmed by atomic force microscopy (AFM) and X-ray diffraction (XRD) analysis. The mobility of Ph-BTBT-C10 thin-film transistors (TFTs) was approximately doubled, from 0.56 ± 0.05 cm2 V-1 s-1 to 1.26 ± 0.06 cm2 V-1 s-1, after the surface treatment. The surface treatment of α-Al2O3 and ODPA significantly decreased the threshold voltage from -21.2 V to -8.3 V by reducing the trap sites in the OGI and improving the interfacial properties with the OSC. We suggest that the MAST method for OGIs can be applied to various OGI materials lacking reactive sites using SAMs. It may provide a new platform for the surface treatment of OGIs, similar to that of conventional SiO2 gate insulators.

Control of positive and negative threshold voltage shifts using ultraviolet and ultraviolet-ozone irradiation

The precise control of the threshold voltage (Vth) required to turn a metal-oxide semiconductor field-effect transistor on/off at the designed voltage was important to ensure that the Vth was constant for all the transistors in the circuit. In this study, Vth was adjusted in both the negative and positive directions by manipulating the number of O vacancies on the surface of an oxide nanowire channel using ultraviolet (UV) and UV-ozone (UVO) irradiation. UV irradiation shifted the Vth in the negative direction by an amount that increased with increasing exposure time (−0.96 V at 1 min, −1.42 V at 2 min, and −1.70 V at 3 min), whereas UVO irradiation shifted the Vth in the positive direction by an amount that increased with increasing exposure time (+0.37 V at 3 min, +0.69 V at 5 min, and +1.07 V at 10 min). The shifted values of Vth were maintained by passivating the channel region of the oxide nanowire with an octadecylphosphonic acid self-assembled monolayer.

TiO2 patterns with wide photo-induced wettability change by a combination of reactive sputtering process and surface modification in a microfluidic channel

This paper reports the formation of TiO2 patterns with a wide range of photo-induced wettability switching from high hydrophobic to superhydrophilic states for on-chip liquid manipulation. TiO2 thin films with rough surface morphology were formed by a combination of optimised reactive sputtering and CF4 plasma etching. Octadecylphosphonic acid self-assembled monolayer (ODP-SAM) surface modification was applied to the surface-roughened TiO2 thin films in order to obtain a highly hydrophobic surface initially. Photocatalytic decomposition of ODP-SAM on the surface-roughened TiO2 by ultraviolet (UV) irradiation caused a wetting transition from the Cassie–Baxter state to the Wenzel state. Switching of the flow direction into branch channels was also demonstrated by utilising the photoresponsive wettability of the surface-modified TiO2 patterns on a fluidic chip.

Amorphous In-Ga-Zn-O thin-film transistors fabricated by microcontact printing

The authors present a facile, low-cost methodology to fabricate high-performance In-Ga-Zn-O (IGZO) bottom contact, bottom gate thin-film transistors (TFTs) by soft lithography. The IGZO channel and indium tin oxide (ITO) source and drain were patterned using microcontact printing of an octadecylphosphonic acid self-assembled monolayer (SAM). A polymer stamp was used for the pattern transfer of the SAMs, which were then used as a chemical protection layer during wet etching. Excellent pattern transfer was obtained with good resolution and sharp step profiles. X-ray photoelectron spectroscopy indicated that the microcontact printed SAMs can be effectively removed from the ITO source/drain surfaces, allowing a high-quality interface to the IGZO channel for good device performance. Scanning electron microscopy cross-sections of the devices indicate a smooth and defect-free transition regions between the source/drain and semiconductor regions. The fabricated TFTs have negligible gate-leakage currents, high average electron mobilities of 10.2 cm2/Vs, and excellent on-off ratios of 2.1 × 108. These results may provide new methodologies for low-cost and large-area integration of IGZO-TFTs for a range of applications including flexible and transparent displays.

Self-Correcting Process for High Quality Patterning by Atomic Layer Deposition.

Nanoscale patterning of materials is widely used in a variety of device applications. Area selective atomic layer deposition (ALD) has shown promise for deposition of patterned structures with subnanometer thickness control. However, the current process is limited in its ability to achieve good selectivity for thicker films formed at higher number of ALD cycles. In this report, we demonstrate a strategy for achieving selective film deposition via a self-correcting process on patterned Cu/SiO2 substrates. We employ the intrinsically selective adsorption of octadecylphosphonic acid self-assembled monolayers on Cu over SiO2 surfaces to selectively create a resist layer only on Cu. ALD is then performed on the patterns to deposit a dielectric film. A mild etchant is subsequently used to selectively remove any residual dielectric film deposited on the Cu surface while leaving the dielectric film on SiO2 unaffected. The selectivity achieved after this treatment, measured by compositional analysis, is found to be 10 times greater than for conventional area selective ALD.

Fabrication of controllable and stable In2O3 nanowire transistors using an octadecylphosphonic acid self-assembled monolayer

The controllability and stability of nanowire transistor characteristics are essential for the development of low-noise and fast-switching nano-electronic devices. In this study, the positive shift of threshold voltage and the improvement of interface quality on In2O3 nanowire transistors were simultaneously achieved by using octadecylphosphonic acid (OD-PA) self-assembly. Following the chemical bond of OD-PA molecules on the surface of In2O3 nanowires, the threshold voltage was positively shifted to 2.95 V, and the noise amplitude decreased to approximately 87.5%. The results suggest that an OD-PA self-assembled monolayer can be used to manipulate and stabilize the transistor characteristics of nanowire-based memory and display devices that require high-sensitivity, low-noise, and fast-response.

Improving area-selective molecular layer deposition by selective SAM removal.

Area selective molecular layer deposition (MLD) is a promising technique for achieving micro- or nanoscale patterned organic structures. However, this technique still faces challenges in attaining high selectivity, especially at large MLD cycle numbers. Here, we illustrate a new strategy for achieving high quality patterns in selective film deposition on patterned Cu/Si substrates. We employed the intrinsically selective adsorption of an octadecylphosphonic acid self-assembled monolayer (SAM) on Cu over Si surfaces to selectively create a resist layer only on Cu. MLD was then performed on the patterns to deposit organic films predominantly on the Si surface, with only small amounts growing on the Cu regions. A negative potential bias was subsequently applied to the pattern to selectively desorb the layer of SAMs electrochemically from the Cu surface while preserving the MLD films on Si. Selectivity could be enhanced up to 30-fold after this treatment.

Facile passivation of solution-processed InZnO thin-film transistors by octadecylphosphonic acid self-assembled monolayers at room temperature

A simple passivation method is developed to overcome the water susceptibility of solution-processed InZnO thin-film transistors (TFTs) by utilizing octadecylphosphonic acid (ODPA) self-assembled monolayers (SAMs). The unpassivated InZnO TFTs exhibit large hysteresis in their electrical characteristics due to the adsorbed water at the semiconductor surface. Formation of a SAM of ODPA on the top of InZnO removes water molecules weakly absorbed at the back channel and prevents water diffusion from the surroundings. Therefore, the passivated devices exhibit significantly reduced hysteretic characteristics.

(Invited) Octadecylphosphonic Acid Self-Assembled Monolayers in Low Voltage Electrowetting-on-Dielectric Systems

Electrowetting-on-dielectric (EWOD) is a useful phenomenon in which the apparent contact angle of a conductive liquid upon a dielectric-coated electrode changes in response to an applied electric voltage. Large electrowetting effects can be achieved when the dielectric film has small thickness, large permittivity, and high breakdown field strength [1]. EWOD is the fundamental building block to numerous devices: displays, lens, and electronic paper [2]. The use of self-assembled monolayers (SAMs) has the potential to drastically reduce voltage requirements for contact angle change, when used in conjunction with a high breakdown field strength layer.We studied self-assembled monolayers on anodic aluminum oxide. The monolayer films are formed using various solvents, immersion times, and formation temperatures. Characterization used contact angles and AFM to assess the quality of the films. Contact angles were studied in both a static and a dynamic configuration to assess the SAMs ability to serve as a hydrophobic layer for EWOD systems.Static contact angles are measured optically through a Ramé-Hart goniometer system, allowing full electrowetting characterization, such as in Figure 1. The three regions shown are low voltage EWOD, which is modeled according to the Lippman – Young (LY) curve at low voltage: cos(θ) = cos(θ0) + cV2/2γ; where c is capacitance, V is applied voltage, γ is the gas-liquid interface tension, and θ0is the equilibrium angle. This is followed by a saturation region, giving a limiting angle below which an increase in electric field has no effect. Finally, at high field the monolayer desorbs causing a large drop in wetting angle.Dynamic contact angles describe three phase boundary motion giving “advancing” and “receding” contact angles, with their difference defining the contact angle hysteresis (CAH). The measurements are made using the Wihelmy plate method and a Krüss tensiometer; the technique consists in dipping the plate in the liquid electrolyte, measuring the force as a function of the immersion depth. CAH gives an insight into the homogeneity of the SAM films, since any discontinuity in the SAM may obstruct the drop movement; this is observed in the saturation region of Figure 1. Additionally, the raw force-depth data from the tensiometer system show how the contact angle changes across the sample and through repeated cycles, as shown in Figure 2. The deviation from a linear behavior during immersion and emersion (Figure 2) suggests the presence of inhomogeneities in the SAM; the lines in fact smooth out for SAMs formed at higher immersion time.

(Invited) Octadecylphosphonic Acid Self-Assembled Monolayers in Low Voltage Electrowetting-on-Dielectric Systems

Octadecylphosphonic acid reacts with oxidized aluminum surfaces to form a self-assembled monolayer film. The effectiveness of the monolayers are examined for use in an electrowetting-on-dielectric application using anodized aluminum as the substrate. We report growth of the monolayers by monitoring contact angle and SEM imaging. Electrowetting is characterized in static and dynamic configuration, giving advancing, receding, and contact hysteresis. The films greatly reduce the voltage requirement for drop movement, but the high hysteresis currently limits the effectiveness at low voltage operation.

Solution-Processable Zirconium Oxide Gate Dielectrics for Flexible Organic Field Effect Transistors Operated at Low Voltages

We investigate solution based fabrication of high-k ZrO2 thin films for low-voltage-operated organic field effect transistors (OFETs). An alternative UV curing method for the densification of Zr-based gel films, which allows for low-temperature processing, is compared to the conventional thermal annealing method. Elemental and microstructural analysis shows that UV-curing induces the decomposition of organic-metal bonds and causes the densification of the metal oxide film, just as the conventional thermal annealing of gel films does, resulting in a high-k dielectric layer from Zr-based solutions. Furthermore, we found that the low temperature associated with UV-curing prevents the interface layer from intermixing with the substrate. Fabricated ZrO2 films (5–6 nm in thickness) treated with an octadecylphosphonic acid self-assembled monolayer exhibit low leakage current density (below 10–6 to 10–7 A/cm2) at 3 V and high dielectric breakdown strength (V > 4 V). Using this dielectric layer, solution processab...

Adsorption and stability of self-assembled organophosphonic acid monolayers on plasma modified Zn–Mg–Al alloy surfaces

The adsorption and stability of adhesion promoting organophosphonic acid monolayers on plasma modified Zn–Mg–Al alloy surfaces were investigated by means of microscopic and spectroscopic techniques. A strip hollow cathode (SHC) was chosen for the plasma surface modification. The chemical composition of the plasma treated surfaces and the influence of reducing and oxidizing plasma modifications on the corrosion properties of Zn–Mg–Al alloy surfaces were analyzed by means of X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry, respectively. The adsorption and stability of phosphonic acid monolayers were comparatively studied on plasma modified and non-plasma modified Zn–Mg–Al alloy surfaces. Self-organization of monofunctional monolayers was confirmed by means of polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS) and XPS. Contact angle measurements were performed to prove the stability of the octadecylphosphonic acid (ODPA) monolayer on the native and different plasma treated surfaces in aqueous media. Plasma modification and ODPA adsorption resulted in a synergistic inhibition of the redox current densities of the alloy surface. The strongest inhibition was observed for an Ar/H2 plasma+O2 plasma treatment followed by octadecylphosphonic acid self-assembled monolayer adsorption.

Oxidizing octadecylphosphonic acid molecules without disrupting their self-assembled monolayers

In examination of oxidation of octadecylphosphonic acid (OPA) self-assembled monolayers (SAMs) prepared on the native oxide of a Si wafer upon exposure to UV/ozone (UVO) for up to 5 min, atomic force microscopy studies of the surface elucidated that there were no changes in morphology of the SAMs. Surprisingly, investigations on the same samples of OPA SAMs using time-of-flight secondary ion mass spectrometry revealed that the intensity of the deprotonated molecular ion fragment decreased exponentially to approximately one fourth of that of the control. We clarify that UVO-induced oxidation of alkyl chains of the OPA molecules in their SAMs precedes the disruption of the monolayer structure, which is the basis for controlling surface chemistry of SAMs with brief UVO exposure.

Interface Chemistry and Molecular Interactions of Phosphonic Acid Self-Assembled Monolayers on Oxyhydroxide-Covered Aluminum in Humid Environments

Barrier properties of self-assembled octadecylphosphonic acid (ODPA) monolayers on plasma-modified oxyhydroxide-covered aluminum surfaces were analyzed by means of in situ photoelastic modulated infrared reflection absorption spectroscopy (PM-IRRAS). The surface hydroxyl density prior to ODPA adsorption was increased by means of a low-temperature H(2)O-plasma treatment. Adsorption isotherms of H(2)O on ODPA self-assembled monolayer (SAM) modified surfaces in comparison to bare oxide covered aluminum surfaces showed that the ODPA SAM leads to a strongly reduced amount of adsorbed water based on the inability of water to form hydrogen bonds to the low-energy aliphatic surface. However, the ODPA SAM covered surfaces did not show a significant inhibition of the H(2)O/D(2)O isotope exchange reaction between the D(2)O gas phase and the hydroxyl groups of the aluminum oxyhydroxide film, as the interfacial layer between the ODPA SAM and the metal substrate, while the interfacial phosphonate group as well as the orientation of the SAM is not affected by the adsorption of water. It can be followed that the strong adhesion promoting and high corrosion resistances of organophosphonate monolayers on oxyhydroxide-covered aluminum is a result of the strong acid-base interaction of the phosphonate headgroup with the Al ions in the oxyhydroxide film, even in the presence of high interfacial water activity and the molecular interactions of the aliphatic chains. However, the barrier effect of such monolayers on the transport of water is negligible.

Revealing Different Bonding Modes of Self-Assembled Octadecylphosphonic Acid Monolayers on Oxides by Time-of-Flight Secondary Ion Mass Spectrometry: Silicon vs Aluminum

Condensed octadecylphosphonic acid (OPA) dimers, i.e., two OPA molecules combined with the loss of a water molecule, were detected by time-of-flight secondary ion mass spectrometry (TOF-SIMS) on OPA self-assembled monolayers (SAMs) that are only weakly bonded on the native oxide layer of a silicon wafer. In contrast, these condensed dimers were absent on OPA SAMs formed on the oxide layer of an aluminum film, where the OPA molecules are chemically bonded on the substrate through a P-O-Al linkage. These observations lead us to conclude that the OPA molecules in their SAMs have to be free from chemical bonding with the substrate in order for the primary ion beam to generate ion fragments of the condensed dimer. We demonstrate that the detection of condensed OPA dimers serves as an analytical criterion for TOF-SIMS to reveal the bonding mode of OPA molecules in their SAMs on different oxides.