In-situ X-ray reflectivity study of alkane films grown from the vapor phase

AFM/XPS Analysis of the Growth and Architecture of Oriented Molecular Monolayer by Spin Cast Process and Its Cross-Linking Induced by Hyperthermal Hydrogen

We used atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) to comprehensively study the growth and the cross-linking of dotriacontane (C32H66) nanofilms that were deposited on a silicon wafer by the spin-coating process. It was found that the molecular structure of the nanofilms changed with C32H66 concentration at the given spin speed, of which a monolayer of oriented C32H66 molecules, formed at lower deposition concentrations, was composed of a perpendicular orientation state with the molecular long-chain axis perpendicular to the substrate surface and a parallel orientation state, while the perpendicular state was essentially dominant when the nanofilm was formed at higher deposition concentrations. The shortening of the first perpendicular layer in AFM topography could be attributed to the mixing of both parallel and perpendicular lamellas in the first layer. XPS analysis indicated that the average thickness of the layer almost linearly increased with the C32H66 concentration. The monolayer of C32H66 film could be cross-linked by a hyperthermal hydrogen-induced cross-linking (HHIC) at a few eV via kinetic collision to cleave C-H bonds. The water contact angle measurement of extensive HHIC on C32H66 nanofilms disclosed that the static contact angle decreased with the treatment time (or fluence) and saturated after full cross-linking of the film.

Two-Step Freezing in Alkane Monolayers on Colloidal Silica Nanoparticles: From a Stretched-Liquid to an Interface-Frozen State

The crystallization behavior of an archetypical soft/hard hybrid nanocomposite, that is, an n-octadecane C18/SiO2-nanoparticle composite, was investigated by a combination of differential scanning calorimetry (DSC) and variable-temperature solid-state (13)C nuclear magnetic resonance (VT solid-state (13)C NMR) as a function of silica nanoparticles loading. Two latent heat peaks prior to bulk freezing, observed for composites with high silica loading, indicate that a sizable fraction of C18 molecules involve two phase transitions unknown from the bulk C18. Combined with the NMR measurements as well as experiments on alkanes and alkanols at planar amorphous silica surfaces reported in the literature, this phase behavior can be attributed to a transition toward a 2D liquid-like monolayer and subsequently a disorder-to-order transition upon cooling. The second transition results in the formation of a interface-frozen monolayer of alkane molecules with their molecular long axis parallel to the nanoparticles' surface normal. Upon heating, the inverse phase sequence was observed, however, with a sizable thermal hysteresis in accord with the characteristics of the first-order phase transition. A thermodynamic model considering a balance of interfacial bonding, chain stretching elasticity, and entropic effects quantitatively accounts for the observed behavior. Complementary synchrotron-based wide-angle X-ray diffraction (WAXD) experiments allow us to document the strong influence of this peculiar interfacial freezing behavior on the surrounding alkane melts and in particular the nucleation of a rotator phase absent in the bulk C18.

Hydraulic transport across hydrophilic and hydrophobic nanopores: Flow experiments with water and n-hexane

We experimentally explore pressure-driven flow of water and n-hexane across nanoporous silica (Vycor glass monoliths with 7- or 10-nm pore diameters, respectively) as a function of temperature and surface functionalization (native and silanized glass surfaces). Hydraulic flow rates are measured by applying hydrostatic pressures via inert gases (argon and helium, pressurized up to 70 bar) on the upstream side in a capacitor-based membrane permeability setup. For the native, hydrophilic silica walls, the measured hydraulic permeabilities can be quantitatively accounted for by bulk fluidity provided we assume a sticking boundary layer, i.e., a negative velocity slip length of molecular dimensions. The thickness of this boundary layer is discussed with regard to previous capillarity-driven flow experiments (spontaneous imbibition) and with regard to velocity slippage at the pore walls resulting from dissolved gas. Water flow across the silanized, hydrophobic nanopores is blocked up to a hydrostatic pressure of at least 70 bar. The absence of a sticking boundary layer quantitatively accounts for an enhanced n-hexane permeability in the hydrophobic compared to the hydrophilic nanopores.

Determination of multilayer thicknesses of GaAs/AlAs superlattice by grazing incidence X-ray reflectivity

The grazing incidence X-ray reflectivity is used to determine the multilayer thickness of GaAs/AlAs supperlattice. The measurement process includes the fitting model and the measurement conditions (different powers of 45 kV × 40 mA, 40 kV × 40 mA and 35 kV × 40 mA, different step sizes of 0.005°, 0.008° and 0.010°, and different times per step of 1 s, 2 s, 3 s). In order to obtain the valid measurement process, the combined standard deviation is used as the normal of the fitting results selection. As a result, the measurement condition of 0.008° step size and 2 s time per step with the power 40 kV × 40 mA is selectable with the operation stability of facilities and smaller error.

Molecular tilting and its impact on frictional properties of n-alkane self-assembled monolayers

Hydrophobic, methyl-terminated self-assembled monolayer (SAM) surfaces can be used to reduce friction. Among methyl-terminated SAMs, the frictional properties of alkanethiol SAMs and silane SAMs have been well-studied. In this research, we investigated friction of methyl-terminated n-hexatriacontane (C36) SAM and compared its friction properties with the alkanethiol and silane SAMs. Alkane SAM does not have an anchoring group. The alkane molecules stand on the surface by physical adsorption, which leads to a higher surface mobility of alkane molecules. We found that C36 SAM has a higher coefficient of friction than that of octadecyltrichlorosilane (OTS) silane. When an atomic force microscope (AFM) tip was swiped across the alkane SAM with a loading force, we found that the alkane SAM can withstand the tip loading pressure up to 0.48 GPa. Between 0.48 and 0.49Ga, the AFM tip partially penetrated the SAM. When the tip moved away, the deformed SAM healed and maintained the structural integrity. When the loading pressure was higher than 0.49 GPa, the alkane SAM was shaved into small pieces by the tip. In addition, we found that the molecular tilting of C36 molecules interacted with the tribological properties of the alkane SAM surface. On one hand, a higher loading force can push the rod-like alkane molecules to a higher tilting angle; on the other hand, a higher molecular tilting leads to a lower friction surface.

Pentacene thin-film transistors encapsulated by a thin alkane layer operated in an aqueous ionic environment

The encapsulation of pentacene thin-film transistors (TFTs) by a 50 nm thick layer of a long chain alkane (C₄₄H₉₀) enables the operation in an aqueous solution of 1 mM NaCl. It is the first time that pentacene TFTs operate successfully in a liquid environment. This opens new perspectives for biocompatible sensor devices based on pentacene TFTs.

Stability of the parallel layer during alkane spreading and the domain structures of the standing-up layer

The spreading of liquid alkanes over surfaces plays an important role in applications such as lubrication, painting, and printing. To make significant advances in these fields, it is essential to increase our understanding of the interactions between alkanes and surfaces. Long-chain alkanes form two typical adsorption structures on a surface--the parallel phase and the standing-up phase. The most thermodynamically stable structure is the parallel phase, in which the alkane molecules lie flat on the surface. If the temperature is slightly below the bulk melting point, then alkanes form a thermodynamically stable standing-up phase on top of an existing parallel layer. At lower temperatures, the standing-up phase becomes metastable. Using atomic force microscopy, we have found that the standing-up alkane layer consists of multiple domains, indicating that the standing-up layer forms through a multinucleation process during the liquid-solid transition on the surface. If, however, the temperature is above the melting point, then we have found that the standing-up layer shrinks to a droplet and leaves a residue on its original position. During the spreading of an alkane droplet, the parallel layer forms on the substrate surface surrounding the droplet by adsorption from the vapor, which precedes the arrival of the liquid. There has been uncertainty, however, as to whether the parallel layer moves with the liquid alkane or remains stationary during spreading. In this study, we used the residue left on the parallel layer as a landmark to monitor the movement of the parallel layer during the spreading of an alkane droplet. Using this landmark, we found that the parallel layer remained stationary on the substrate, indicating that the liquid alkane spreads on a stationary parallel layer surface. Therefore, this study reveals that the surface properties of the parallel layer--not the surface properties of the substrate--control the spreading and wetting of a liquid alkane.

Role of vapor-phase mass transport during the spreading of a long-chain alkane drop

The spreading of liquid alkanes over solid surfaces has important applications in painting, coatings, lubrication, and petroleum tertiary recovery. The role of the vapor-phase mass transport accompanying liquid spreading has not been well studied because it is difficult to separate the contributions from the liquid spreading and the vapor-phase transport that occurred at the same time. We used the engineered surface patterns to study the vapor-phase mass transport during liquid spreading. First, we fabricated several hydrophilic, carboxylic acid-terminated patterns (OTSpd) on a hydrophobic, methyl-terminated octadecyltrichlorosilane (OTS) surface. These OTSpd patterns did not connect to each other. Next, we let an alkane drop spread within one OTSpd pattern. The liquid alkane could not spread to other OTSpd patterns because OTS separated them; however, the alkane molecules in the vapor phase could migrate and adsorb on other OTSpd patterns. Therefore, the contributions from the liquid spreading and the vapor-phase transport were separated and could be investigated independently. We found that during the spreading of the liquid alkane, mass transport through the vapor phase cannot be ignored. Alkane molecules adsorbed on the OTSpd surface with their backbones parallel to the surface in the first few layers. Additional alkane molecules adsorbed on these parallel layers to form the seaweed-shaped layers in which the alkane molecules stood up. Our study showed that the parallel layers formed from the vapor-phase mass transport before the liquid alkane spread. Therefore, the liquid alkane does not spread over the more strongly binding OTSpd surface. It actually spreads over the parallel alkane layer, which formed from its own vapor.

Structure and growth of vapor-deposited n-dotriacontane films studied by X-ray reflectivity

We have used synchrotron X-ray reflectivity measurements to investigate the structure of n-dotriacontane (n-C(32)H(66) or C32) films deposited from the vapor phase onto a SiO(2)-coated Si(100) surface. Our primary motivation was to determine whether the structure and growth mode of these films differ from those deposited from solution on the same substrate. The vapor-deposited films had a thickness of approximately 50 A thick as monitored in situ by high-resolution ellipsometry and were stable in air. Similar to the case of solution-deposited C32 films, we find that film growth in vacuum begins with a nearly complete bilayer adjacent to the SiO(2) surface formed by C32 molecules aligned with their long axis parallel to the interface followed by one or more partial layers of perpendicular molecules. These molecular layers coexist with bulk particles at higher coverages. Furthermore, after thermally cycling our vapor-deposited samples at atmospheric pressure above the bulk C32 melting point, we find the structure of our films as a function of temperature to be consistent with a phase diagram inferred previously for similarly treated solution-deposited films. Our results resolve some of the discrepancies that Basu and Satija (Basu, S.; Satija, S. K. Langmuir 2007, 23, 8331) found between the structure of vapor-deposited and solution-deposited films of intermediate-length alkanes at room temperature.

Spontaneous imbibition dynamics of an n-alkane in nanopores: evidence of meniscus freezing and monolayer sticking

Capillary filling dynamics of liquid n-tetracosane (n-C24H50) in a network of cylindrical pores with 7 and 10 nm mean diameter in monolithic silica glass (Vycor) exhibit an abrupt temperature-slope change at Ts = 54 degrees C, approximately 4 degrees C above bulk and approximately 16 degrees C, 8 degrees C, respectively, above pore freezing. It can be traced to a sudden inversion of the surface tension's T slope, and thus to a decrease in surface entropy at the advancing pore menisci, characteristic of the formation of a single solid monolayer of rectified molecules, known as surface freezing from macroscopic, quiescent tetracosane melts. The imbibition speeds, that are the squared prefactors of the observed square-root-of-time Lucas-Washburn invasion kinetics, indicate a conserved bulk fluidity and capillarity of the nanopore-confined liquid, if we assume a flat lying, sticky hydrocarbon backbone monolayer at the silica walls.

The partially degraded hydrophilic silane pattern and its application in studying the structures of long chain alkane films

We developed a protocol to fabricate hydrophilic patterns over an octadecyltrichlorosilane (OTS) film surface with an atomic force microscope (AFM). Through a local probe oxidation under a 100% humidity environment, the OTS was converted into a hydrophilic, carboxylic acid-terminated surface (OTSpd). The OTSpd pattern grew with the voltage dwell time applied on the conducting AFM probe. Eighty nanometer to submillimeter sized OTSpd patterns could be fabricated with a single scanning probe. The OTSpd patterns were used to study the spreading of long chain alkanes. Hexatriacontane (C36H74) was dip-coated on an OTSpd pattern. Subsequently, an additional hydrophilic OTSpd region was fabricated surrounding the coated C36H74. The alkane spread over this newly created region when heated above its melting point. After cooling to room temperature, the shape and structures of the solidified alkane patterns were characterized. On the methyl-terminated, low-energy surface, the alkane molecules stood directly on the surface. In contrast, on the hydrophilic, high-energy surface, the alkane formed seaweed-shaped patterns after spreading. On the OTSpd surface, the alkane molecules initially adsorbed on the hydrophilic surface with their alkyl chains parallel to the surface. Additional alkane molecules stood vertically or tilted on top of the parallel layer, forming the seaweed-shaped layer. The seaweed patterns were previously thought to consist of only vertically standing alkane molecules. We found that three additional tilted phases existed in the seaweed-shaped structures.