Octadecylthiol Research Articles

The nature of self-assembled octadecylphosphonic acid (ODPA) layers on copper substrates

HypothesisThe self-assembly of amphiphilic molecules onto solid substrates can result both in the formation of monolayers and multilayers. However, on oxidized and non-oxidized copper (Cu), only monolayer formation was reported for phosphonic acids possessing one phosphate head group. Here, the adsorption of octadecylphosphonic acid (ODPA) on Cu substrates through a self-assembly process was investigated with the initial hypothesis of monolayer formation. ExperimentsThe relative amount of ODPA adsorbed on a Cu substrate was determined by infrared reflection/absorption spectroscopy (IRRAS) and by atomic force microscopy (AFM) investigations before and after ODPA deposition. X-ray photoelectron spectroscopy (XPS) with sputtering was used to characterize the nature of the layers. FindingsThe results show that the thickness of the ODPA layer increased with deposition time, and after 1 h a multilayer film with a thickness of some tens of nm was formed. The film was robust and required long-time sonication for removal. The origin of the film robustness was attributed to the release of Cu ions, resulting in the formation of Cu-ODPA complexes with Cu ions in the form of Cu(I). Preadsorbing a monolayer of octadecylthiol (ODT) onto the Cu resulted in no ODPA adsorption, since the release of Cu(I) ions was abolished.

A facile method to mussel-inspired superhydrophobic thiol-textiles@polydopamine for oil/water separation

Due to the poor mechanical stability, the practical applications of superhydrophobic materials are limited. Inspired by the excellent adhesion of polydopamine (PDA), a kind of superhydrophobic coating is successfully fabricated on the textile. Herein, under weak acidic conditions (pH = 5.0), sodium periodate is selected as oxidant and the concentration of dopamine is controlled at 8 mg/mL, which leads to the fast and homogeneous deposition of PDA nanoaggregates on the pristine textile. Subsequently, the nanoaggregates are modified by octadecyl thiol. The PDA nanoaggregates work as a stable bond between the pristine textile and hydrophobic groups introduced from thiol, which contributes to the mechanical stability of the superhydrophobic textile. In addition, the superhydrophobic textiles display resistance to acetone, UV irradiation, and hot water. Significantly, the textiles can be used for oil/water separation with flux around 4500 Lm−2 h−1 and the separation efficiency more than 97%. These advantages provide the superhydrophobic textiles with multiple applications.

TCTAP A-101 Comparison of Hydrophilic Polyethylene Glycol-coating Stent and Hydrophobic Octadecyl Thiol-coating Stent Implantation in Porcine Coronary Restenosis Model

Drug-eluting stent had the concerns of stent thrombosis related to inflammation, polymer, and drug. We developed the novel polyethylene glycol (PEG)-coating stent with hydrophilic characteristic and octadecyl thiol (ODT)-coating stent with hydrophobic characteristic (Fig) to reduce inflammation and

(Invited) InGaN/GaN Nanowire LEDs and Lasers on Nonconventional Substrates

Conventional planar group-III nitride epitaxy for light-emitting diode (LED) is typically grown on sapphire, which results in high threading dislocation densities in the order of 106-108 cm-2 because of thermal- and lattice-mismatch. The challenge in realizing efficient InGaN QWs in the green gap and beyond stems from the spontaneous and piezoelectric polarization fields induced electron-hole wave functions separation as the indium composition increased to 25% and above.1 SiC and GaN bulk substrates, and nonpolar nitride materials have been used to address the lattice-mismatch, efficiency-droop, polarization field, and green gap issues, at the expense of high development and material costs.2 Nanowire LEDs on silicon has numerous advantages over planar epitaxial LEDs, such as a reduced polarization field, wavelength tunability by increasing indium composition and light extraction.3, 4 The blue, green, red and infrared LEDs grown using molecular beam epitaxy (MBE) demonstrated the potential of nanowires emitters for practical applications.5, 6 However, the large specific surface, and therefore a high density of surface states was found to limit the quantum efficiency and output power of nanowire devices.7 The phonon and carrier confinement in nanowires also led to junction heating and reduced heat dissipation. In this talk, we will present our investigations on nanowires LEDs and lasers emitting at green-to-infrared wavelength range. The approach in surface states passivation in InGaN/GaN quantum-disk-in-nanowires LEDs, as well as enhancing heat dissipation by growing nanowires-crystal on metal substrates will also be presented. Ammonium sulfide was used to passivate the nanowires in the laser fabrication process to reduce the p-contact resistance of GaN:Mg.5 We also developed octadecylthiol (ODT) passivation process for LEDs, which led to a ~4 times increase in photoluminescence peak intensity, and 50% increase in relative peak external-quantum-efficiency in passivated LEDs.8 The passivation changed the surface dynamic charge and recovered the band-edge emission. The faster-increasing trend in quantum efficiency is due to the reduced Shockley–Read–Hall (SRH) non-radiative recombination on the nanowires surface, which was confirmed using 4D scanning ultrafast electron microscopy technique, and the associated rate-equations simulations.9 Current high-power planar LEDs have to be transferred to a heat sink, such as Cu or CuMo, via laser liftoff or wafer bonding because of the high junction temperature during operation. It is more severe for nanowire LEDs on Si because of the small diameter, and the formation of SiN layer before nanowires growth. We developed the first high-power red InGaN/GaN Qdisks-in-nanowire LEDs on bulk polycrystalline Mo; the platform simultaneously implements buffer layer, reflector, n-metal contact, and heat-sink, thus greatly simplifies the fabrication process for high-power devices.10 The LEDs showed a low turn-on voltage of ∼2 V without droop, much lower junction temperature compared to LEDs on Si, and longer lifetime, thus is promising for high-power device operation. In conclusion, the InGaN/GaN nanowire edge-emitting lasers at green-to-infrared wavelength range on Si were demonstrated. We showed the first organic chemical passivation treatment on nitride nanowires, to increase external quantum efficiency and reduce SRH non-radiative recombination. We also reported the first nanowires device growth process on metal substrates. The fabricated device from the high-crystal-quality nanowires shows low turn-on voltage, high-power, droop-free, and stable operation.

Nanowires: Enhanced Optoelectronic Performance of a Passivated Nanowire-Based Device: Key Information from Real-Space Imaging Using 4D Electron Microscopy (Small 17/2016).

Selective mapping of surface charge carrier dynamics of InGaN nanowires before and after surface passivation with octadecylthiol (ODT) is reported by O. F. Mohammed and co-workers on page 2313, using scanning ultrafast electron microscopy. In a typical experiment, the 343 nm output of the laser beam is used to excite the microscope tip to generate pulsed electrons for probing, and the 515 nm output is used as a clocking excitation pulse to initiate dynamics. Time-resolved images demonstrate clearly that carrier recombination is significantly slowed after ODT treatment, which supports the efficient removal of surface trap states.

Enhanced Optoelectronic Performance of a Passivated Nanowire-Based Device: Key Information from Real-Space Imaging Using 4D Electron Microscopy.

Managing trap states and understanding their role in ultrafast charge-carrier dynamics, particularly at surface and interfaces, remains a major bottleneck preventing further advancements and commercial exploitation of nanowire (NW)-based devices. A key challenge is to selectively map such ultrafast dynamical processes on the surfaces of NWs, a capability so far out of reach of time-resolved laser techniques. Selective mapping of surface dynamics in real space and time can only be achieved by applying four-dimensional scanning ultrafast electron microscopy (4D S-UEM). Charge carrier dynamics are spatially and temporally visualized on the surface of InGaN NW arrays before and after surface passivation with octadecylthiol (ODT). The time-resolved secondary electron images clearly demonstrate that carrier recombination on the NW surface is significantly slowed down after ODT treatment. This observation is fully supported by enhancement of the performance of the light emitting device. Direct observation of surface dynamics provides a profound understanding of the photophysical mechanisms on materials' surfaces and enables the formulation of effective surface trap state management strategies for the next generation of high-performance NW-based optoelectronic devices.

An enhanced surface passivation effect in InGaN/GaN disk-in-nanowire light emitting diodes for mitigating Shockley-Read-Hall recombination.

We present a detailed study of the effects of dangling bond passivation and the comparison of different sulfide passivation processes on the properties of InGaN/GaN quantum-disk (Qdisk)-in-nanowire based light emitting diodes (NW-LEDs). Our results demonstrated the first organic sulfide passivation process for nitride nanowires (NWs). The results from Raman spectroscopy, photoluminescence (PL) measurements, and X-ray photoelectron spectroscopy (XPS) showed that octadecylthiol (ODT) effectively passivated the surface states, and altered the surface dynamic charge, and thereby recovered the band-edge emission. The effectiveness of the process with passivation duration was also studied. Moreover, we also compared the electro-optical performance of NW-LEDs emitting at green wavelength before and after ODT passivation. We have shown that the Shockley-Read-Hall (SRH) non-radiative recombination of NW-LEDs can be greatly reduced after passivation by ODT, which led to a much faster increasing trend of quantum efficiency and higher peak efficiency. Our results highlighted the possibility of employing this technique to further design and produce high performance NW-LEDs and NW-lasers.

Removal of Surface States and Recovery of Band-Edge Emission in InAs Nanowires through Surface Passivation

Surface states in semiconductor nanowires (NWs) are detrimental to the NW optical and electronic properties and to their light emission-based applications, due to the large surface-to-volume ratio of NWs and the congregation of defects states near surfaces. In this paper, we demonstrated an effective approach to eliminate surface states in InAs NWs of zinc-blende (ZB) and wurtzite (WZ) structures and a dramatic recovery of band edge emission through surface passivation with organic sulfide octadecylthiol (ODT). Microphotoluminescence (PL) measurements were carried out before and after passivation to study the dominant recombination mechanisms and surface state densities of the NWs. For WZ-NWs, we show that the passivation removed the surface states and recovered the band-edge emission, leading to a factor of ∼19 reduction of PL linewidth. For ZB-NWs, the deep surface states were removed and the PL peaks width became as narrow as ∼250 nm with some remaining emission of near band-edge surface states. The passivated NWs showed excellent stability in atmosphere, water, and heat environments. In particular, no observable changes occurred in the PL features from the passivated NWs exposed in air for more than five months.

Comparison of Molecular Monolayer Interface Treatments in Organic-inorganic Photovoltaic Devices

AbstractIn this study, we explore the effects of alkyl surface terminations on ZnO for inverted, planar ZnO/poly(3-hexylthiophene) (P3HT) solar cells using two different attachment chemistries. Octadecylthiol (ODT) and octadecyltriethoxysilane (OTES) molecules were used to create 18-carbon alkyl surface molecular layers on sol gel-derived ZnO surfaces. Molecular layer formation was confirmed and characterized using water contact angle measurements, infrared (IR) transmission measurements, and X-ray photoelectron spectroscopy (XPS). The performances of the ZnO/P3HT photovoltaic cells made from ODT- and OTES-functionalized ZnO were compared. The ODT-modified devices had higher efficiencies than OTES-modified devices, suggesting that differences in the attachment scheme affect the efficiency of charge transfer through the molecular layers at the treated ZnO surface.

Manipulation ordered and close-packed nanoparticle monolayers at air/liquid interface coupling Langmuir-Blodgett and self-assembly techniques

Langmuir monolayers of octadecylamine (ODA) and octadecylthiol (ODT) at the air/liquid interface were used as template layers to adsorb gold nanoparticles (AuNPs) dispersed in the subphase. A small quantity of ethanol was introduced into the aqueous subphase to regulate the stability of dispersed AuNPs and, thereby, enhance the adsorption of NPs to a template layer. The effects of monolayer density and ethanol concentration on the adsorption of AuNPs and the relevant behavior of mixed NP/ODA (or ODT) monolayers were studied using the pressure-area (π-A) isotherms and the observation of transmission electron microscopy (TEM). The results show that ODA monolayer has a much higher ability, than the ODT monolayer, in incorporatiing AuNPs, attributed to the electrostatic interaction between ODA and AuNPs. Furthermore, the ODA molecules also act as capping agents, modifying the adsorbed NPs, providing van der Waals interaction among NPs for self-organization into ordered domains. By using an ODA monolayer with appropriate molecular density, as well as the adjusting of the ethanol concentration, a close-packed particulate film of AuNPs can be prepared through the monolayer compression. The results also show that over loading of the ODA molecules increases the fractional ratio of free ODA molecules at the interface which obstruct the coalescence of particulate domains and a close-packed particulate monolayer cannot be approached. The presence of ethanol in the subphase not only enhances the particle adsorption to the interface but also triggers a more expanded ODA monolayer, which plays a key role in preparing a close-packed particulate monolayer.

Time-Resolved IR Studies of Monolayer Self-Assembly on a Gold Substrate Using Planar Array Infrared (PA-IR) Spectroscopy

Time-resolved and spatially resolved infrared spectra (line images) of self-assembled monolayers of octadecylthiol (ODT) on gold substrates were investigated by planar array infrared (PA-IR) spectroscopy, with a grazing incidence reflection attachment. It was observed that PA-IR spectra with a good signal-to-noise ratio that is comparable to those obtained from Fourier transform infrared spectroscopy could be acquired with a significantly shorter collection time. Focusing on the peak intensities and peak positions of the CH2 asymmetric stretch (approximately 2918 cm(-1)) and the CH3 symmetric stretch (approximately 2960 cm(-1)), respectively, the molecular orientation and organization of the monolayers were determined. The ODT formed a uniform monolayer on the surface of the gold whether it was deposited within 1-2 s or over a 40 h period. Disordered monolayers formed instantly, and with an increase in dipping time, the monolayer became more ordered, becoming highly ordered after dipping times of many ( > 24) hours.

Enhancement of photoluminescence from near-surface quantum dots by suppression of surface state density

We report the functionalization of semiconductor nanostructures near the surface with the self-assembled monolayer of octadecylthiol (ODT). In the first part of this study, photoluminescence properties of the near-surface quantum dots and quantum wells grown on GaAs [100] substrates were investigated as a function of the distance to the surface, and the luminescence signals were also systematically compared in terms of the confinement potential. In the second part, the ODT monolayer was deposited on the nanostructures based on indium arsenide quantum dots (InAs QDs). The monolayer deposition resulted in a significant enhancement in the photoluminescence from the QDs, which can be attributed to the effective suppression of the surface state densities by arsenide–sulfide coupling. It is noteworthy that the enhancement in the luminescence signal was more significant for QDs closer to the surface. Since the hydrophobic surface of the monolayer can be functionalized with polymer films and model cell membranes, this strategy is applicable for the design of local detectors in the very proximity of the surface.

Stable Surface Coating of Gallium Arsenide with Octadecylthiol Monolayers

In this paper, we describe the deposition of octadecylthiol (ODT) monolayers on highly doped n-GaAs electrode surfaces, which showed high stability both in air and in aqueous electrolytes. In the first part of this study, four different wet chemical etching procedures were investigated to optimize surface treatment before ODT deposition. The chemical composition of the surface was evaluated by X-ray photoelectron spectroscopy (XPS), demonstrating that the photochemical etching procedure (called “etch P” in this study) can generate a surface enriched with arsenides, which can serve as the binding sites for sulfides. In the second part of this study, the surface prepared by etch P was coated with an ODT monolayer. The monolayer showed high stability in air, as indicated by the constant ellipsometric thickness. In electrolytes, the stability of the monolayer-coated surface was verified by impedance spectroscopy under zero-current potential (Uj=0 = −360 mV) for more than 10 h; then the stability of the interface was monitored under different bias potentials. Electrochemical passivation of the GaAs surface has been demonstrated for the first time under physiological conditions (in aqueous electrolyte, near neutral pH), which allows for the application of GaAs electrodes to biological systems.

Electrochemical passivation of gallium arsenide surface with organic self-assembled monolayers in aqueous electrolytes

Self-assembled monolayers of octadecylthiol (ODT) were reconstituted on freshly etched gallium arsenide (n-GaAs) for the electrochemical stabilization against decomposition of the surfaces (passivation) in aqueous buffers. The surface composition was evaluated by x-ray photoelectron spectroscopy to optimize the surface treatment before ODT deposition. Electrochemical properties of the monolayers were monitored by cyclic voltammetry and impedance spectroscopy. The impedance spectrum of the photoetched n-GaAs after the deposition of the ODT monolayer was stable in an aqueous electrolyte at pH=7.5 for more than 24 h within the sensitivity of our experimental technique. The effective passivation of GaAs surfaces is an essential step towards biosensor applications.

Effects of octa decyl thiol (ODT) treatment on the gallium arsenide surface and interface state density

The ability of Octa Decyl Thiol (ODT) compound in passivating Gallium Arsenide (GaAs) surfaces is studied by comparing the XPS spectra on the ODT treated and untreated GaAs samples and by comparing the electrical characteristics of Schottky diodes and MIS capacitors fabricated on them. A detailed study on MIS capacitors and an evaluation of the interface state density (Dit) has clearly demonstrated that even though the ODT treatment reduces the interface state density in the upper half of the band gap more than an order of magnitude compared to untreated samples, it is not effective in reducing the midgap Dit values. From the XPS spectra taken on the ODT treated GaAs surfaces, this is attributed to an island-like ODT coating and to the inability of this sulphur compound treatment to remove the native oxide present on the GaAs surface.

Structural studies of sulfur-passivated GaAs (100) surfaces with LEED and AFM

We present the results of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and atomic force microscopy (AFM) analysis of sulfur-passivating layers on the GaAs (100) surface. The GaAs surfaces were passivated with both inorganic [(NH 4) 2S x ] and organic [octadecylthiol (ODT)] S-based compounds. We prepared the inorganic sulfur-passivated GaAs(100) surfaces with a wet chemical treatment using (NH 4) 2S solution. This was followed by thermal annealing of the treated sample in an ultra-high vacuum (UHV). After ex-situ and in-situ treatments, the surface resulted in a (2×1) LEED pattern. The LEED data ( I– E curves) were recorded and compared with dynamical LEED calculations for different structural models for the sulfur-passivated GaAs (100) surface. The results showed that the sulfur-passivated (2×1) surface structure is an arsenic–sulfur dimer on a Ga-terminated substrate. The ex-situ AFM results also revealed a (2×1) structure for the inorganic passivation and a very smooth surface for the organic ODT in the ethanol-treated sample.

Selective scanning tunneling microscope-induced light emission from self-assembled monolayer-covered Au surfaces

We have studied the scanning tunneling microscope-induced light emission from Au surfaces in air. The effect of adsorbates on the light emission was investigated. Atmospheric contaminants appear as nonemissive clusters in the luminescence images. Preliminary results suggest that UV/ozone exposure partially reverses the effect of atmospheric contamination. Self-assembled monolayer chemisorption was also studied. The presence of an octadecylthiol (ODT) monolayer on the Au surface decreases the light emission intensity by one order of magnitude. Selective inhibition of luminescence is demonstrated using a microcontact printed pattern of ODT.

Near-surface electronic structure in GaAs (100) modified with self-assembled monolayers of octadecylthiol

Passivation of the GaAs (100) surface by self-assembled monolayers of octadecylthiol (ODT) has been studied using inelastic light scattering as a probe of the near-surface electronic structure. Application of the ODT self-assembled monolayers reduces the width of the depletion region at the surface of GaAs resulting in a reduction of the surface band bending, and the electron scattering time is increased as well. The ODT passivated surfaces are more stable to environmental degradation, over time and under temperature stress, than inorganic sulfide treated surfaces which have been reported. Organic thiol passivation may provide an attractive alternative to inorganic sulfide protocols for reduction of surface recombination velocities in III–V devices.

GaAs Interfaces with Octadecyl Thiol Self-Assembled Monolayer: Structural and Electrical Properties

A passivation layer on chemically etched (100) GaAs surface was synthesized with octadecyl thiol, CH3(CH2)17SH, and its structural and electrical properties were investigated. The layer is a self-assembled monolayer (SAM) in all-trans-planar-zig-zag conformation and is the first conformationally ordered SAM on any semiconductor surface. The barrier height of GaAs Schottky diodes can be modified by the octadecyl thiol SAM passivation without sacrificing good diode characteristics. Octadecyl and other alkane thiol SAM's enable introduction of a nano-scale, controlled structure on a GaAs surface and prove useful in an assessing the GaAs surface electrical properties.

Structure of octadecyl thiol self-assembled on the silver(111) surface: an incommensurate monolayer

We describe the first experimental study of the molecular structure of the self-assembled monolayer of CH 3 (CH 2 ) 17 SH formed on the Ag(111) surface. Using both low-energy He diffraction and grazing incidence X-ray diffraction, we show that the monolayer formed on Ag is both incommensurate and rotated with respect to the Ag lattice, with a lattice spacing which is very similar to n-alkane chains in bulk hydrocarbon crystals.