Octadecanethiol Monolayer Research Articles

Fluxless Soldering of Copper Substrates Using Self-Assembled Monolayers for Preservation

Self-assembled monolayers (SAMs) of octadecanethiol have been used as a preservative to enable fluxless soldering of copper substrates. The SAMs were deposited onto copper surfaces that had previously been etched to remove any existing oxide layers and wetting balance testing under an inert atmosphere was used to measure their fluxless solderability as a function of storage time and environment. It was found that SAM coated samples could be stored in air at room temperature for a short period of time before they became unwettable as a result of oxidation, however this storage time could be greatly increased by keeping them at low temperature. X-ray photoelectron spectroscopy was used to monitor the level of oxidation within the materials, which was found to correlate with the fluxless solderability. The SAM is thought to present a barrier to the penetration of oxygen to the copper surface, but at the same time can be readily displaced by heat and molten solder

Template-Directed Adsorption of Block Copolymers on Alkanethiol-Patterned Gold Surfaces

Functionalized alkanethiols have been self-assembled on gold to modify the wetting properties of the surface and promote or hinder the adsorption of block copolymers containing both hydrophobic and hydrophilic blocks. X-ray photoelectron spectroscopy (XPS) studies of spin-coated polyethylene-block-poly(ethylene oxide) (PE-b-PEO) copolymers on 16-mercaptohexadecanoic acid (MHDA)-, octadecanethiol (ODT)-, and 1H,1H,2H,2H-perfluorodecanethiol (PFDT)-covered surfaces have been performed. In the case of an 80 wt % PEO block copolymer, spin-coating on a gold surface precovered with MHDA results in a polymer film thick enough to completely attenuate Au 4f photoelectrons; spin-coating on the more hydrophobic ODT and PFDT monolayers leads to significantly thinner polymer films and incomplete attenuation of the gold photoelectrons. The opposite results are observed when a 20 wt % PEO block copolymer is used. Angle-resolved XPS studies of the 80 wt % PEO block copolymer spin-coated onto an MHDA-covered surface indicate that the PE blocks of the polymer segregate to the near-surface region, oriented away from the hydrophilic carboxylic acid tails of the monolayers; the surface concentration of PE is further enhanced by annealing at 90 degrees C. Microcontact printing and dip-pen nanolithography have been used to pattern gold surfaces with MHDA, and the surfaces have been backfilled with ODT or PFDT, such that the unpatterned regions of the surface are covered with hydrophobic monolayers. In the case of backfilling with PFDT, spin-coating the 80 wt % PEO copolymer onto these patterned surfaces and subsequent annealing results in the block copolymer preferentially adsorbing on the MHDA-covered regions and forming well-defined patterns that mimic the MHDA pattern, as determined by scanning electron microscopy and atomic force microscopy. Significantly worse patterning, characterized by micron-sized polymer droplets, results when the surface is backfilled with ODT instead of PFDT. Using PFDT and MHDA, polymer features having widths as small as 500 nm have been formed. These studies demonstrate a novel method to pattern block copolymers with nanoscale resolution.

Molecular Self-Assembly at Bare Semiconductor Surfaces: Preparation and Characterization of Highly Organized Octadecanethiolate Monolayers on GaAs(001)

Through rigorous control of preparation conditions, organized monolayers with a highly reproducible structure can be formed by solution self-assembly of octadecanethiol on GaAs (001) at ambient temperature. A combination of characterization probes reveal a structure with conformationally ordered alkyl chains tilted on average at 14 +/- 1 degrees from the surface normal with a 43 +/- 5 degrees twist, a highly oleophobic and hydrophobic ambient surface, and direct S-GaAs attachment. Analysis of the tilt angle and film thickness data shows a significant mismatch of the average adsorbate molecule spacings with the spacings of an intrinsic GaAs(001) surface lattice. The monolayers are stable up to approximately 100 degrees C and exhibit an overall thermal stability which is lower than that of the same monolayers on Au[111] surfaces. A two-step solution assembly process is observed: rapid adsorption of molecules over the first several hours to form disordered structures with molecules lying close to the substrate surface, followed by a slow densification and asymptotic approach to final ordering. This process, while similar to the assembly of alkanethiols on Au[111], is nearly 2 orders of magnitude slower. Finally, despite differences in assembly rates and the thermal stability, exchange experiments with isotopically tagged molecules show that the octadecanethiol on GaAs(001) monolayers undergo exchange with solute thiol molecules at roughly the same rate as the corresponding exchanges of the same monolayers on Au[111].

Single Nanoporous Gold Nanowire Sensors

Chemisorption from the gas or liquid phase can result in a measurable resistance change in a metallic material when at least one dimension is smaller than the mean free path for electrons. Here we report on the fabrication of single nanoporous gold nanowires and demonstrate that adsorption of an alkanethiol can be monitored in real time. Single nanowire devices were fabricated by in situ etching of Au0.18Ag0.82 alloy nanowires in dilute nitric acid. The evolution of the porous structure was characterized by monitoring the resistance change and comparing to cross-sectional images. The feature size of about 10 nm is less than the mean free path for electrons in bulk gold, and hence the resistance is dominated by surface scattering. Adsorption of a monolayer of octadecanethiol onto the nanoporous gold nanowire results in a resistance change of about 3%. The sensitivity factor of 1.0x10(-16) cm2 is comparable to values reported for adsorption at ultrathin films.

Friction of Mixed and Single-Component Aromatic Monolayers in Contacts of Different Adhesive Strength

Friction force microscopy has been used to study single-component and mixed self-assembled monolayers of aminothiophenol and thiophenol on gold. The friction forces and transition pressures of mixed monolayers were intermediate to the ones of single-component monolayers, and varied systematically with composition. The strength of the adhesion was altered by working in dry N2 gas or in ethanol. In all systems studied, low adhesion (in ethanol) resulted in a linear dependence of the friction on load already at low loads, whereas high adhesion (in dry N2) gave an apparent area-dependence. However, for a given monolayer composition, similar transition pressures were observed in dry N2 and in ethanol, suggesting that the overall monolayer structure was not strongly altered by the presence of ethanol. Similar observations were made for very close-packed monolayers of octadecanethiol.

Langmuir–Blodgett films of octadecanethiol – properties and potential applications

Octadecanethiol (ODT) is known to form self-assembled monolayer on noble metal surfaces which has potential technological applications. Langmuir–Blodgett (LB) technique is another useful method of obtaining highly ordered assembly of molecules. It is of interest to find whether ODT molecules can also form a stable Langmuir monolayer which facilitates the preparation of LB films. In literature, it has been reported that ODT molecules form an unstable Langmuir monolayer. We have studied the stability of the monolayer of the ODT molecules at air–water interface using surface manometry and microscopy techniques. We find the monolayer to be stable on ultrapure water of resistivity greater than 18 MΩ cm. However, the behavior changes in the presence of even small amount of additives like NaOH or CdCl 2 in the subphase. Our AFM studies on the LB films of ODT deposited from ion-free ultrapure water showed streak-like bilayer domains. The LB films of ODT deposited from CdCl 2 containing aqueous subphase yield dendritic domains of the complexed unit grown over ODT monolayer. These nanostructures on surfaces may have potential applications in molecular electronics.

Preparation of Alkanethiol Monolayers on Mild Steel Surfaces Studied with Sum Frequency Generation and Electrochemistry

An n-alkanethiol, octadecanethiol (ODT), monolayer was successfully prepared onto an oxide-free mild steel (MS) surface under cathodic polarization in a 0.1 M LiCl/CH(3)OH solution containing 1 mM ODT. Cyclic voltammetry (CV) and electrochemical impedance (EIS) and sum frequency generation (SFG) spectroscopy were applied to study and characterize the adsorption of ODT at a MS surface. In 0.1 M LiCl/CH(3)OH solution containing 1 mM ODT, CV of the MS electrode shows a dramatic decrease in charging current and a positive shift in oxidation potential when compared to a solution without ODT. The interfacial capacitance was obtained as 2.52 microF/cm(2) from the impedance data. An average chain tilt angle of 48 degrees for the ODT molecules was deduced from the comparison of the interfacial capacitances of the ODT/MS and ODT/Au monolayers. X-ray photoelectron spectroscopy confirmed the formation of the ODT monolayer on mild steel. The ppp SFG spectrum of the ODT-modified MS features three strong methyl vibrational modes at 2877, 2943, and 2967 cm(-1), indicating the formation of the oriented and densely packed ODT monolayer. However, the appearance of the two weak CH(2) groups' vibrational modes at 2850 and 2914 cm(-1) implies the presence of defects in the ODT monolayer. ODT/Au films were prepared to compare with the ODT/MS films. Orientation analysis of the air/solid interface suggests that the methyl group of ODT/Au films has a tilt angle of 30 degrees , while the methyl group of ODT/MS films has a tilt angle of 23 degrees . Water was found to have an impact on the shape of the SFG spectra of ODT/MS. This suggests that the solution penetrated through the defects to reach the MS surface.

Molecular Orientation of Self-assembled Monolayer of Octadecanethiol on Platinum Surface Studied by Femtosecond Broad-bandwidth Sum Frequency Generation Spectroscopy

Abstract The molecular orientation in self-assembled monolayer (SAM) of octadecanethiol (ODT) on platinum surface prepared in ethanol solution was examined by broad-bandwidth sum frequency generation (BB-SFG) spectroscopy. The bands attributed to the CH3 symmetric and asymmetric stretching vibrational modes were observed in the SFG spectra. From the analysis of SFG intensity ratio of these two modes, the alkyl chain on platinum was found to be more perpendicular to the substrate than those on gold surface.

Oxidation protection of copper surfaces using self-assembled monolayers of octadecanethiol

Self-assembled monolayers (SAMs) of alkanethiols adsorbed onto clean surfaces of face centred cubic (fcc) metals have been studied extensively for their ability to control the chemical functionality of the surface and as a means of preventing the oxidation and corrosion of the substrate metal. However, in many cases it has been found that on reactive substrates such as copper, it is difficult to prepare SAMs without the incorporation of some oxygen into the structure. In this work, self-assembled monolayers of octadecanethiol (ODT) were formed on copper foil substrates using a series of etching treatments to remove the native oxide layer prior to deposition of the ODT coating from a modified solution. X-ray photoelectron spectroscopy was used to analyse the SAMs and showed that monolayers with no detectable oxygen content could be produced. The effect of exposing the samples to air at different temperatures was monitored to examine the rate of the oxidation process, which was found to vary strongly with temperature. Samples stored at room temperature were found to oxidise relatively quickly, while those kept in a refrigerator were slower. Storing samples in a freezer dramatically reduced the oxidation of the copper, such that samples kept for 10 weeks still did not show any clear evidence of oxygen incorporation.

An integrated study of dropwise condensation heat transfer on self-assembled organic surfaces through Fourier transform infra-red spectroscopy and ellipsometry

Self-assembled monolayers (SAMs) formed by adsorption of 1-octadecanethiol [CH 3(CH 2) 17SH] and 16-mercaptohexadecanoic acid [CO 2H(CH 2) 15SH] onto gold-coated-copper substrates were applied to investigate the enhancement of dropwise condensation (DWC) heat transfer of steam at atmosphere pressure. A durability test was also conducted. Although hydrophobic SAMs increase the heat transfer coefficient by nearly an order of magnitude from that of filmwise condensation, it was found that DWC using octadecanethiol SAM as a promoter is a dynamic process in that the heat transfer coefficient decreases with time over 2 h. These results were reconfirmed by an integrated study using Fourier transform infra-red spectroscopy (FT-IR) and Spectroscopic Ellipsometry. We found that the monolayer of octadecanethiol becomes less crystalline with time, causing the film thickness and heat transfer coefficient to decrease. There was also some indications that, as SAMs were partly removed (leaving patches of the bare metal), contaminant from steam spontaneous adsorbed onto these high energy sites, resulting in a slightly higher heat transfer coefficient at a later stage of condensation.

Tribological behavior of self-assembled double layer measured by a pin-on-plate method

We investigated the frictional properties and the durability of self-assembled double layers using pin-on-plate methods and X-ray photoelectron spectroscopy (XPS). We found that double layers made from octadecanethiol (ODT) and mercaptohexadecanoic acid (MHA) connected via Cu ions exhibited lower frictional coefficients (approximately 0.1–0.15) than that of ODT monolayer (∼0.2). XPS measurements revealed that ratios of C(1s)/Au(4f) after friction tests were lower than that before friction but was higher than that measured on ODT monolayer, suggesting that some of ODT molecules were desorbed from the surface, i.e. formation of loosely packing molecular layers after friction. These findings suggest that the loosely packed molecular layer contributed to decrease the friction.

Comparative study of monolayers self-assembled from alkylisocyanides and alkanethiols on polycrystalline Pt substrates.

This paper compares the properties of self-assembled monolayers (SAMs) derived from octadecylisocyanide (ODI) and octadecanethiol (ODT) on polycrystalline Pt substrates. Both monolayers formed at a similar rate using 1.0 mM solutions in ethanol and achieved a thickness of 22-23 A after 24 h as determined by ellipsometry measurements. The advancing contact angles of ODI and ODT monolayers were found to be 113 and 117 degrees, respectively, suggesting a slight difference in structure between them. X-ray photoelectron spectroscopy revealed that SAMs of ODT were more stable than those of ODI, which was supported by experiments that probed desorption of these layers in prewarmed hexadecane. Cyclic voltammetry measurements indicated that both monolayer systems could diminish electron-transfer rates substantially, although ODT monolayers were more effective and robust than their ODI counterparts. The resistance of the SAMs to ion penetration differed in a similar way, and a microcontact-printed monolayer of ODT could protect the underlying Pt better in an HCl/Cl2-based etch process than the one formed from ODI.

Fabrication of Arrays of Gold Islands on Self-Assembled Monolayers Using Pulsed Laser Deposition through Nanosieves

Sandwich structures of gold-self-assembled monolayer-gold were prepared by deposition of gold on alkylthiolate self-assembled monolayers on polycrystalline gold, using pulsed laser deposition (PLD) through a nanosieve. The arrays of sandwiches, around 600 nm in diameter, approximately 10 nm high, and spaced 1.6 im apart, were analyzed using tapping mode atomic force microscopy. Electrochemical copper deposition experiments showed that of the islands deposited on octadecanethiolate monolayers about 15% were electrically insulated from the bottom gold electrode. This means that PLD is a suitable technique for the fabrication of metal-SAM-metal sandwich structures.

DPN-Generated Nanostructures Made of Gold, Silver, and Palladium

Dip-pen nanolithography (DPN) has been used to generate resist layers on Au, Ag, and Pd that when combined with wet-chemical etching can lead to nanostructures with deliberately designed shapes and sizes. Monolayers of mercaptohexadecanoic acid (MHA) or octadecanethiol (ODT), patterned by DPN, were explored as etch resists. They work comparably well on Au and Ag, but ODT is the superior material for Pd. MHA seems to attract the FeCl3 etchant and results in nonuniform etching of the underlying Pd substrate. Dots, lines, triangles, and circles, ranging in size from sub-100 to several hundred nanometers have been fabricated on Si/SiOx substrates. These results show how one can use DPN as an alternative to more complex and costly procedures such as electron beam lithography to generate nanostructures from inorganic materials.

Chemical and Thermal Stability of Alkanethiol and Sulfur Passivated InP(100)

InP(100) surfaces treated with Na2Sx9H20 and CnH(2n+1)SH are examined by contact angle measurement, X-ray photoelectron spectroscopy, and atomic force microscopy to determine the chemical and thermal behavior of these passivated surfaces. The surfaces coated by octadecanethiol (n = 18) self-assembled monolayers (SAMs) are found to be more stable toward oxidation than the S-passivated surface. The chemical stability of octadecanethiol SAMs in various environments is examined. The thiol monolayer is found to be stable in 0.1 M HCl but degrades in 0.1 M NaOH, boiling chloroform, and water. The behavior of these surfaces at elevated temperatures under a vacuum is also investigated. The octadecanethiol-coated InP(100) is stable up to 473 K, above which the films begin to degrade. Unlike other substrates on which the entire molecule including the sulfur headgroup desorbs together, on InP, the sulfur headgroup remains on the surface even after annealing to 673 K. These observations suggest that the desorption occurs by S-C bond cleavage as well as In-S bond cleavage. The sulfur of S-passivated InP is found to be more thermally stable than that of the octadecanethiol monolayer, perhaps due to their different bonding geometries and hence energies.

Microcontact printing with octadecanethiol

We investigate various alkanethiol self-assembled monolayer inks for microcontact printing patterns on gold substrates. Our research compares hexadecanethiol with an alkanethiol of higher molecular weight, octadecanethiol. Transport mechanisms of alkanethiols along the gold surface and through the ambient are highly dependant upon the molecular weight of the thiols used, and thus are of particular importance when patterning submicron features. Monolayers of octadecanethiol are well suited for patterning such features on gold. The quality of transferred patterns printed with octadecanethiol and hexadecanethiol are examined. The gold patterns fabricated in this work were analyzed via scanning electron microscopy.

Thermal conductivities of naked and monolayer protected metal nanoparticle based nanofluids: Manifestation of anomalous enhancement and chemical effects

Thermal conductivities of two kinds of Au nanoparticles were measured in water and toluene media. The water soluble particles, 10–20 nm in mean diameter, made with citrate stabilization showed thermal conductivity enhancement of 5%–21% in the temperature range of 30–60 °C at a loading of 0.000 26 (by volume). The effect was 7%–14% for Au particles stabilized with a monolayer of octadecanethiol even for a loading of 0.011%. Comparatively lower thermal conductivity enhancement was observed for larger diameter Ag particles for significantly higher loading. Effective enhancement of 9%, even at vanishing concentrations, points to additional factors in the thermal conductivity mechanism in nanofluids. Results also point to important chemical factors such as the need for direct contact of the metal surface with the solvent medium to improve enhancement.

Effects of Surface Defects, Polycrystallinity, and Nanostructure of Self-Assembled Monolayers for Octadecanethiol Adsorbed onto Au on Wetting and Its Surface Energetic Interpretation

We report low-rate dynamic contact angle data of various liquids on self-assembled monolayers (SAMs) of octadecanethiol on annealed and nonannealed gold. It was found that interpretation of solid surface tensions using contact angle data on less well-prepared polycrystalline nonannealed gold surfaces can be misleading. Our findings were supported by reflectance infrared spectra and atomic force microscopy data that the surface nanostructure, defects, and polycrystallinity can be important factors for a systematic study of wettability on SAMs in terms of surface energetics. We found that the contact angle and adhesion patterns of various liquids on SAMs of octadecanethiol adsorbed onto annealed gold substrates are consistent with recent experimental data for the relatively thick polymer-coated surfaces. The variation of surface structure in terms of surface energetics can be estimated only when a fundamental understanding of contact angles and surface tensions is known. We estimated an increase in the soli...

Preparation, Characterization, and Heck Reaction of Multidentate Thiolate Films on Gold Surfaces

Multidentate thiolate films were prepared by solution phase deposition of dendritic oligothiols on gold surfaces. The dendritic molecules contain three (G0) and nine (G1) thiol groups at the periphery and a functional group (bromophenyl) at the corn. The films were characterized by contact-angle goniometry, ellipsometry, polarization-modulation infrared reflection−absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). Results of the XPS measurement indicate the presence of ∼20% of unbound S in the G0 films while ∼28% in the G1 films. Some of the unreacted thiol groups may be located at the upper part of the films and some of the bromophenyl groups at the lower part of the films. Three G0 molecules (containing three SH groups) cover about the same area on the substrate as one G1 molecule (containing nine SH groups), and both of the films exhibit similar thicknesses, indicating the flattening of the dendrons to maximize the bonding of SH groups to the gold surface. The G1 films exhibit higher stability than the G0 films, and both of them are much more stable than octadecanethiolate monolayers on gold in hot solvents, attributed to the formation of multiple S−Au bonds per dendron unit. The high stability of the films allows for the palladium-catalyzed Heck reaction on thiolate films. Treatment of both the G0 and G1 films with 4-fluorostyrene under Heck reaction conditions led to the disappearance of about 76 and 68% of the Br atoms among which about 60−70% were replaced by the 4-fluorostyryl groups, as shown by XPS. Prolonging the reaction time did not consume the remaining Br atoms, probably due to unfavorable orientation of the bromophenyl groups and/or the presence of adjacent thiol groups that may deactivate the catalyst in the film assembly.

Tribological Properties of Alkoxyl Monolayers on Oxide Terminated Silicon

The tribological properties of alkoxyl monolayers on oxide terminated silicon surfaces have been investigated using interfacial force microscopy. For a C18 alkoxyl monolayer, both adhesion and frictional properties are similar to those of a self-assembled monolayer of octadecanethiol on gold. Friction is shown to increase as the alkyl chain length of the molecules decreases. Analysis using contact mechanics models has been carried out to estimate reduced modulus, adhesion energy, and friction shear strength. These interfacial mechanical properties are correlated to molecular structures at the interface.