Submonolayer Levels Research Articles

Probing the validity of the diffuse mismatch model for phonons using atomistic simulations

Due to it's simplicity the diffuse mismatch model (DMM) remains a popular description of phonon transmission across solid-solid boundaries. However, it remains unclear in which situations the DMM should be expected to be a valid model of the underlying physics. Here, its validity is investigated mode-by-mode using a 3-dimensional extension of the frequency domain, perfectly matched layer (FD-PML) method, to study the interface between face-centered cubic solids with interdiffused atoms. While submonolayer levels of interdiffusion are found to increase the number of available modes for transmission, consistent qualitatively with the DMM, we do not find quantitative or qualitative convergence toward the DMM at higher levels of interdiffusion. In particular, contrary to the fundamental assumption of the DMM, modes are not found to lose memory of their initial polarization and wavevector. The transmission coefficients of randomly interdiffused and smoothly-graded interfaces are also compared. While smoothly graded interfaces show strong anti-reflection properties, selection rules still prohibit transmission of many modes, whereas interdiffused interfaces are not subject to such rules and achieve similar thermal interface conductance by transmitting with lower probability but using a wider range of modes.

Influence of Step-Edge vs Terrace Sites on Temperature-Dependent C2H2 Hydrogenation with Ag-Doped Pt Nanoparticles

Ag-doped Pt nanoparticles (NP) were tested for continuous-flow C2H2 hydrogenation reactivity and CO adsorption–desorption from 100 to 300 °C to investigate how submonolayer levels of an insoluble metal dopant can distribute on steps and terraces of a NP surface and modify reactivity. High Ag coverage (0.5 monolayer, ML) on Pt promoted selective hydrogenation of C2H2 at T < 200 °C; at higher temperatures, both low (0.06 ML) and high (0.5 ML) Ag coverages were seen to increase selective C2H4 formation. Ag incorporation onto Pt step and terrace sites, and resultant temperature-dependent desorption behavior, were probed using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO. At low coverage, Ag appeared to occupy Pt step sites; for higher coverage, Ag occupied both step and terrace sites, which lowered the CO desorption temperature. Highly correlated reactivity and DRIFTS data show the importance of step site blocking and desorption from Pt during selective C2H2 hydrogenation. Multiple reaction cycles from 100 to 300 to 100 °C demonstrate how immiscible metal dopants can preferentially modify nanoparticle surface compositions and promote active, selective, and stable catalysts over large temperature operating windows.

Partial Hydrogenation of C2H2 on Ag-Doped Pt Nanoparticles

Pt nanoparticles (NPs) with submonolayer Ag coverage were synthesized and catalytically tested to investigate the effects of surface Ag and NP shape on C2H2 hydrogenation activity and selectivity. Various NP shapes (cubes, cuboctahedra, and octahedra) in the 6–8 nm range were synthesized using colloidal methods with PVP (polyvinylpyrrolidone) as a capping ligand and Ag+ for structure direction. Solid-state 13C NMR of PVP–NP interactions, as well as electrochemical measurements of oxygen evolution reaction (OER) activity, revealed that submonolayer levels of Ag significantly modify both the physical and electronic structure of the Pt NP surface. Octahedral NP catalysts with 0.5 monolayers of Ag were found to be highly selective for C2H2-to-C2H4 hydrogenation (C2H4/C2H6 >8) at reaction rates comparable to total hydrogenation on pure Pt. Traditionally prepared PtAg catalysts synthesized via incipient wetness showed analogously high selectivities but lower site time yields compared to NPs. In contrast, total ...

Effects of sodium chloride on the properties of chlorophyll &lt;italic&gt;a&lt;/italic&gt; submonolayer adsorbed onto hydrophobic and hydrophilic surfaces using broadband spectroscopy with single-mode integrated optical waveguides

In this work, we experimentally investigated the effects of sodium chloride on the molar absorptivity and surface density of a submonolayer of chlorophyll a adsorbed onto hydrophilic and hydrophobic solid/liquid interfaces. Those investigations were made possible by a broadband spectroscopic platform based on single-mode, integrated optical waveguides, which allows for extremely sensitive spectroscopic detection of analytes immobilized at submonolayer levels. Chlorophyll a with a constant bulk concentration (1.4 μM) was dissolved in phosphate buffer solutions (7 mM) of neutral pH, but with different sodium chloride concentrations. For a buffer solution of 1 mM of sodium chloride, the measured surface density of chlorophyll a was 0.209 pmol/cm2 for a hydrophilic and 0.125 pmol/cm2 for a hydrophobic surface. For a phosphate buffer solution of 10 mM of sodium chloride, the measured surface density of chlorophyll a was 0.528 pmol/cm2 for a hydrophilic and 0.337 pmol/cm2 for a hydrophobic surface. Additionally, a hypsochromic shift of the Soret band was observed for the adsorbed pigment in correlation with an increase in buffer ionic strength. The adsorption of chlorophyll a onto different surfaces can play an important role to elucidate several processes found in nature and provide a rationale for bio-inspired new material technologies.

ChemInform Abstract: Underpotential Deposition of Copper and Its Influence in the Oxygen Reduction on Platinum.

Abstract The underpotential deposition (UPD) of Cu on Pt was studied in acid media using cyclic voltammetry and rotating ring-disk electrode techniques. Experimental data at sub-monolayer levels demonstrate that ad-atom formation occurs in different energy states and no rearrangements take place in the ad-layer. Rotating ring-disk studies of the oxygen reduction reaction on Cu UPD show a strong inhibition of the reaction only in the region of diffusion control. At first, the 4 e reaction to H 2 O is inhibited in favor of the 2 e reaction to H 2 O 2 but at more negative potentials even the formation of H 2 O 2 is inhibited. A mechanism based on the degree of surface-active sites blocked by Cu ad-atoms is proposed.

Functional Nanostructured Plasmonic Materials

Plasmonic crystals fabricated with precisely controlled arrays of subwavelength metal nanostructures provide a promising platform for sensing and imaging of surface binding events with micrometer spatial resolution over large areas. Soft nanoimprint lithography provides a robust, cost-effective method for producing highly uniform plasmonic crystals of this type with predictable optical properties. The tunable multimode plasmonic resonances of these crystals and their ability for integration into lab-on-a-chip microfluidic systems can both be harnessed to achieve exceptionally high analytical sensitivities down to submonolayer levels using even a common optical microscope, circumventing numerous technical limitations of more conventional surface plasmon resonance techniques. In this article, we highlight some recent advances in this field with an emphasis on the fabrication and characterization of these integrated devices and their demonstrated applications.

Investigations on the Q and CT Bands of Cytochrome c Submonolayer Adsorbed on an Alumina Surface Using Broadband Spectroscopy with Single-Mode Integrated Optical Waveguides

In this work, we report experimental results on the molar absorptivity of cytochrome c adsorbed at different submonolayer levels onto an aluminum oxide waveguide surface; our data show a clear dependence of the protein optical properties on its surface density. The measurements were performed using the broadband, single-mode, integrated optical waveguide spectroscopic technique, which is an extremely sensitive tool able to reach submonolayer levels of detection required for this type of studies. This investigation focuses on the molar absorptivity at the Q-band (centered at 525 nm) and, for the first time to our knowledge, the weak charge transfer (CT) band (centered at 695 nm) of surface-adsorbed cyt c. Polarized light in the spectral region from 450 to 775 nm was all-coupled into an alumina thin film, which functioned as a single-mode planar optical waveguide. The alumina thin-film waveguide used for this work had a thickness of 180 nm and was deposited on a glass substrate by the atomic layer deposition process. The protein submonolayer was formed on the alumina waveguide surface through electrostatic adsorption from an aqueous buffer solution at neutral pH. The optical properties of the surface-adsorbed cyt c were investigated for bulk protein concentrations ranging from 5 nM to 8200 nM in the aqueous buffer solution. For a protein surface density of 2.3 pmol/cm(2), the molar absorptivity measured at the charge transfer band was 335 M(-1) cm(-1), and for a surface density of 15 pmol/cm(2) was 720 M(-1) cm(-1), which is much closer to the value of cyt c dissolved in an aqueous neutral buffer (830 M(-1) cm(-1)). The modification of the protein molar absorptivity and its dependence on the surface density can most likely be attributed to conformational changes of the surface-adsorbed species.

Cu 2O(1 1 0) formation on Co 3O 4(1 1 0) induced by copper impurity segregation

The surface crystal structure of the Co 3O 4(1 1 0) spinel was characterized by low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Well-defined LEED diffraction patterns showed an unreconstructed Co 3O 4(1 1 0) surface in Type A termination, and XPS and Auger indicated the surface to be stoichiometric with octahedral and tetrahedral cation sites occupied by 3+ and 2+ cations, respectively. The experimental lattice parameters of 8.22 Å ± 0.2 Å and 5.50 Å ± 0.2 Å in the 〈0 0 1〉 and 〈 1 ¯ 10 〉 directions, respectively, are in agreement with a bulk-terminated unit cell. The impurities: K, Ca, Na, and Cu segregated to the surface after prolonged heating to 630 K. K, Ca and Na could easily be removed by routine cleaning procedures and did not affect the Co 3O 4(1 1 0) structure or stoichiometry detectably in the submonolayer levels at which they were observed. However, the copper impurity resulted in the formation of a Cu 2O(1 1 0) overlayer, with the accompanying reduction of the spinel surface to a rocksalt metal monoxide-like surface. The copper oxide formed a distorted hexagonal overlayer incommensurate with that of the Co 3O 4(1 1 0) stoichiometric surface and with periodic spacings of 3.86 Å ± 0.2 Å in the 〈0 0 1〉 and 4.10 Å ± 0.2 Å in the 〈 1 ¯ 10 〉 directions in agreement with Cu 2O(1 1 0) bulk termination. The Co 3O 4(1 1 0) substrate could not be fully re-oxidized until all detectable copper had been removed from the surface.

Measurement of ordered associate of protonated tetraphenylporphine formed at toluene/aqueous H 2SO 4 interface by attenuated total internal reflection spectroscopy with polarized light

Attenuated total internal reflection (ATR) spectroscopy with an s- or p-polarized visible light was examined for some species of protonated 5,10,15,20-tetraphenylporphine (tpp) at toluene/aqueous H 2SO 4 (3–6 mol dm −3) interface. Tpp initially dissolved in the toluene phase was diprotonated at the interface to form monomeric H 2tpp 2+, the absorption peak of which was 438 nm. At the same time, a long H 2tpp 2+ oligomer was formed, the absorption peak of which was 448 nm. The two interfacial species were transient. Just after their disappearance, a rod-shaped H 2tpp 2+ associate was formed at the interface, the absorption peak of which was 417 and 478 nm. The former and latter wavelengths corresponded to H- and J-bands of the associate, respectively. Theoretical calculation of the strength of electric field of light at the interface allowed one to estimate the interfacial concentration of the three species with measured reflection absorbance ( A R). The monomeric H 2tpp 2+ and its oligomer were at sub-monolayer levels, whereas the associate was at a multilayer level. Reflection absorption anisotropy ( K R), which was calculated from A R with the s- and p-polarized lights, was adopted for the evaluation of out-of-plane orientation of the interfacial species for the first time. The K R value suggested that the rod-shaped associate lay at the interface.

Surface chemistry effects on the processing and superplastic properties of nanocrystalline oxide ceramics

The unusual bulk behavior of nanoparticle and nanograined systems often originates in surface chemistry effects. Three examples are used to illustrate this point. In the first, newly precipitated nanocrystalline titania is washed with ethanol, and the mixture of these two supposedly inert substances causes the titania to lose its anatase crystal structure and become amorphous. This phenomenon is attributed to a reverse hydrolysis reaction at the particle surface. In the second example, nanocrystalline ZrO 2-3mol%Y 2O 3 is observed to partially dissolve on exposure to pH-adjusted water, due to the formation of soluble hydroxides at the particle surface. A major consequence of the dissolution is the formation of large, hard, multiparticle agglomerates on subsequent drying. In the final example, ZrO 2-3mol%Y 2O 3 particles are intentionally surface-doped with submonolayer levels of Cu-containing ions from ammoniacal solutions. The ceramics fabricated from such powders exhibit superplastic strain rates 100 or so times faster than in comparable undoped systems, due to the dopant’s role in lowering of the activation energy for diffusion along grain boundaries.

The use of inductively coupled plasma mass spectrometry to provide an absolute measurement of surface coverage, and comparison with the quartz crystal microbalance

It is often difficult to obtain an absolute measure of the quantity of an ultrathin layer of an element deposited onto a dissimilar solid surface, even though this is an important parameter for many surface studies. Auger and X-ray photoelectron spectroscopies provide only a relative measure of the quantity of a deposit, and nuclear or ion-scattering spectroscopies are limited in the adsorbate/substrate pairs that can be measured and often require expensive accelerators that are not readily available. Inductively-coupled-plasma mass spectroscopy (ICPMS) has been used as an ex-situ method to determine the absolute coverage at sub-monolayer levels in semiconductor process control. However, it does not seem to have been used to date for determining surface coverage of adsorbates for other surface-science applications, even though it can do this routinely with an accuracy of few percent. This paper uses ICPMS to determine the coverage of a sub-monolayer of antimony on gold, and shows that the results are consistent with high-precision measurements taken with a quartz crystal microbalance on the same sample.

Controlled nitrogen incorporation at SiSiO 2 interfaces and in thin gate dielectrics by remote-plasma-assisted oxidation and deposition processes

Low-temperature, 300°C, plasma processes are described for controlled incorporation of N at SiSiO 2 interfaces, and in thin gate dielectrics. N incorporation at mono- and sub-monolayer levels have been achieved at SiSiO 2 interfaces by plasma-assisted oxidation in N 2O and N 2 O O 2 mixtures. N incorporation in stacked dielectrics with Si oxynitride or nitride layers has been demonstrated by remote plasma enhanced chemical-vapor deposition (PECVD).

Laser-induced decomposition of dimethyl cadmium on a quartz substrate

The photochemistry of adsorbed dimethyl cadmium at submonolayer levels on a fused quartz surface has been investigated at 193 and 248 nm using a rare-gas fluoride excimer laser. The desorbed gaseous products, which include CH 2, CH 3, CH 4, C 2H 4, C 2H 5, C 2H 6, Cd, CH 3Cd and (CH 3) 2Cd, have been detected by time-of-flight mass spectrometry with either electron impact or resonance-enhanced multiphoton ionization. The translational energies of these desorption products could be characterized in terms of Maxwell-Boltzmann temperatures, T MB ‘ s , which depend strongly on sample dosage (surface coverage), laser fluence and photon energy. The T MB ‘ s of the C 1 and C 2 hydrocarbon species were found to be much lower than those of Cd, CH 3Cd and (CH 3) 2Cd, suggesting that the electronic excitation/relaxation mechanism may be involved in the desorption of Cd and Cd-containing species. The rotational temperature of the CH 3 radical, determined by analysis of REMPI spectra, was found to be much colder than its translational temperature. A realistic mechanism is proposed for the UV-photochemistry of adsorbed (CH 3) 2Cd.

Microscopy of adsorbates by surface second-harmonic generation

A form of optical second-harmonic microscopy has been developed that spatially resolves concentration gradients of surface adsorbates. Surface concentration profiles at submonolayer levels are illuminated with a pulsed laser, and the reflected second-harmonic light is imaged into a photodiode array. Micrometer-scale resolution is obtainable while the surface damage and the spurious chemistry induced by more conventional electron- or ion-based techniques are avoided. We have used this microscopy to monitor the surface diffusion of Sb on Ge(111).

Irreversibility in formation of nominal monolayer and sub-monolayer levels of surface oxide attained at Fe at low temperatures

At Pt and other noble metals, the transition between sub-monolayer, through monolayer, to multilayer surface-oxide formation and growth can be followed quantitatively, especially by means of linear-sweep voltammetry. At base metals, on the other hand, multilayer oxide film formation proceeds rapidly in aqueous solutions at ordinary temperatures. By performing surface oxidation experiments at Fe at low temperatures, down to 180 K, in a methanoi + water mixture, it is shown that nominal sub-monolayer and monolayer levels of surface oxidation at this metal can be realized experimentally. Characterization of the stages of electrochemical formation and reduction of such films is described, and the behaviour is shown to be different from that of Pt at low temperatures, where the presence of an initial, reversibly formed oxide film can be demonstrated at sub-monolayer coverages by OH. By means of experiments conducted at a rotated Fe disc electrode, it is shown that the oxide film formation process at room and much lower temperatures does not significantly involve a dissolution/precipitation type of mechanism.

Underpotential deposition of copper and its influence in the oxygen reduction on platinum

The underpotential deposition (UPD) of Cu on Pt was studied in acid media using cyclic voltammetry and rotating ring-disk electrode techniques. Experimental data at sub-monolayer levels demonstrate that ad-atom formation occurs in different energy states and no rearrangements take place in the ad-layer. Rotating ring-disk studies of the oxygen reduction reaction on Cu UPD show a strong inhibition of the reaction only in the region of diffusion control. At first, the 4 e reaction to H 2O is inhibited in favor of the 2 e reaction to H 2O 2 but at more negative potentials even the formation of H 2O 2 is inhibited. A mechanism based on the degree of surface-active sites blocked by Cu ad-atoms is proposed.

The electrodeposition of rhenium species at the sub-monolayer level on a platinum electrode and its effect on formic acid electrooxidation

The electrodeposition of oxygenated rhenium species from ReO − 4 acid solutions on polycrystalline Pt electrodes has been investigated. Results suggest that ReO 2 at sub-monolayer levels is probably involved as adsorbed species. Electrocatalytic effects observed in HCOOH electrooxidation can be interpreted on the basis that ReO 2 may act as a third-body catalyst, preventing or eliminating the strongly bound intermediates that poison the electrode, as well as a bi-functional catalyst facilitating the existence of oxygen-containing species such as OH radicals formed during its oxidation to ReO − 4.

The influence of halide ions at submonolayer levels on the formation of oxide layer and electrodissolution of copper in neutral solutions

The formation of submonolayers of copper (I) halide preceding anodic oxide layer growth and copper electrodissolution has a remarkable influence on these two processes. Chloride electroadsorption on copper at the submonolayer level takes place as two successive stages at potentials 0.7 V lower than the reversible potential of the Cu/CuCl redox couple. Similar results are obtained for iodide, bromide and fluoride ions although the driving force for electroadsorption decreases in the order I − > Br − > Cl − > F −. The relative contributions of OH − and halide ion electroadsorption can be modified through the solution composition, applied potential and electroadsorption time. The increase surface coverage by the electroadsorbate diminishes the amount of the passive oxide layer and strongly increases the metal electrodissolution rate, particularly when the amount of passive oxide layer becomes smaller than that required to form a compact monolayer.

Adsorption of Model Compounds of Lubricant on Nascent Surfaces of Mild and Stainless Steels under Dynamic Conditions

Mild steel and stainless steel specimens were activated mechanically by cutting under high vacuum at room temperature, and sub-monolayer levels were adsorbed. It was found that the compounds could be divided into two groups according to their adsorption activity on the cut surface. Benzene, 1-hexene, methyl proprionate and diethyl disulfide exhibited a high adsorption activity, while the polar compounds propionic acid, propylamine, trimethyl phosphate and trimethyl phosphite exhibited comparatively low activity. The functional groups of the former compounds have a higher polarizability than that of the latter compounds. The results are discussed on the basis of the Pearson's HSAB (Hard and Soft Acids and Bases) principle. It was found that the nascent surface was so active that benzene decomposition and the reaction of diethyl disulfide with the surface occurred even at room temperature.

Temperature Dependence of InP and GaAs Etching in a Chlorine Plasma

and etching in chlorine plasmas at 0.3 Torr follows an Arrhenius dependence on substrate temperature. Apparent activation energies, , of and , respectively, were determined from both optical emission of product species, and step height or weight change measurements. For , equals the heat of vaporization of , and the absolute etch rate (7 μm/min at 250°C) is in reasonable agreement with the predicted vaporization rate of . Hence, volatilization of is most likely the rate‐controlling step for etching . Sputter Auger analysis shows that etching in a chlorine plasma leaves multiple layer coverage of on (removable by washing with deionized water), and submonolayer levels of chlorine on . Both surfaces are rich in the group III element. The etched surface morphologies of and are strongly dependent on temperature, exhibiting a rough‐to‐smooth texture transition above ∼250° and ∼120°C, respectively.