Low-coverage State Research Articles

Hydroxide films on mica form charge-stabilized microphases that circumvent nucleation barriers.

Crystal nucleation is facilitated by transient, nanoscale fluctuations that are extraordinarily difficult to observe. Here, we use high-speed atomic force microscopy to directly observe the growth of an aluminum hydroxide film from an aqueous solution and characterize the dynamically fluctuating nanostructures that precede its formation. Nanoscale cluster distributions and fluctuation dynamics show many similarities to the predictions of classical nucleation theory, but the cluster energy landscape deviates from classical expectations. Kinetic Monte Carlo simulations show that these deviations can arise from electrostatic interactions between the clusters and the underlying substrate, which drive microphase separation to create a nanostructured surface phase. This phase can evolve seamlessly from a low-coverage state of fluctuating clusters into a high-coverage nanostructured network, allowing the film to grow without having to overcome classical nucleation barriers.

Filled dietary fluoride supplement prescriptions for Medicaid-enrolled children living in states with high and low water fluoridation coverage

BackgroundAlthough dietary fluoride (F) supplements (DFS) are recommended for children who use F-deficient drinking water, no studies have examined filled DFS prescriptions across multiple states to examine the dosage consistency with current recommendations or prescription length. MethodsThis sequential cross-sectional analysis used Medicaid claims data for children aged 0.5 through 16 years who in 2011 lived in the 6 states with the lowest and the highest fluoridation coverage (≤ 34% and ≥ 95% of the public water system population fluoridated, respectively). For 2011, the authors calculated the mean percentage of children with filled DFS prescriptions and the change since 2000 across states with high and low fluoridation coverage, the percentage of children with filled DFS prescriptions containing F dosage consistent with current recommendations, and filled DFS prescription length and cost across states. ResultsIn states with high fluoridation coverage, the mean percentage of children with a filled prescription was < 1% in both years; in states with low fluoridation coverage, this value increased from 0.9% to 10.3%, the highest increase (16.4 percentage points) since 2000 among children aged 0.5 through 2 years. The average prescription length was 72 days. Across states, the mean costs per child prescribed supplements and per enrollee were $17.60 and $1.05, respectively. Conclusions and Practical ImplicationsFilled prescriptions largely followed current recommendations but reached only a small percentage of children in low-coverage states. The short prescription length indicated limited exposure for caries prevention. Results from these states suggest more children could have longer exposure to the caries-preventive benefits of F at a similar cost with water fluoridation as with DFS.

Associations between HPV vaccination among women and their 11–14-year-old children

ABSTRACT The human papillomavirus (HPV) vaccine has been available in the United States for over a decade. We sought to examine the associations between self-reported receipt of HPV vaccination among women and their 11–14-year-old children in 27 low-coverage states. Among the 3,261 mothers we surveyed, 18% reported receiving ≥1 dose of HPV vaccine. A significantly higher proportion of vaccinated women reported vaccination of their children compared to unvaccinated women (83% vs. 56%, p < .001). In multivariable logistic regression, vaccinated women (vs. unvaccinated) had 3.58 (95% CI: 2.81–4.56) times the adjusted-odds of vaccinating their children (≥1 dose HPV vaccine). Among unvaccinated children, vaccinated mothers (vs. unvaccinated) had 3.32 (95% CI: 2.09–5.26) times the adjusted odds of high intention to vaccinate their children in the next 12 months. We did not observe associations between mothers’ vaccination confidence and their vaccination status. We conclude that mothers who received ≥1 dose of HPV vaccine may be more likely to initiate or highly intend to initiate the HPV vaccine series for their children. This may have important implications for meeting population goals for HPV vaccination coverage as an increasing proportion of mothers are likely to be vaccinated over time.

State Medicaid Coverage, ESRD Incidence, and Access to Care

The proportion of low-income nonelderly adults covered by Medicaid varies widely by state. We sought to determine whether broader state Medicaid coverage, defined as the proportion of each state's low-income nonelderly adult population covered by Medicaid, associates with lower state-level incidence of ESRD and greater access to care. The main outcomes were incidence of ESRD and five indicators of access to care. We identified 408,535 adults aged 20-64 years, who developed ESRD between January 1, 2001, and December 31, 2008. Medicaid coverage among low-income nonelderly adults ranged from 12.2% to 66.0% (median 32.5%). For each additional 10% of the low-income nonelderly population covered by Medicaid, there was a 1.8% (95% confidence interval, 1.0% to 2.6%) decrease in ESRD incidence. Among nonelderly adults with ESRD, gaps in access to care between those with private insurance and those with Medicaid were narrower in states with broader coverage. For a 50-year-old white woman, the access gap to the kidney transplant waiting list between Medicaid and private insurance decreased by 7.7 percentage points in high (>45%) versus low (<25%) Medicaid coverage states. Similarly, the access gap to transplantation decreased by 4.0 percentage points and the access gap to peritoneal dialysis decreased by 3.8 percentage points in high Medicaid coverage states. In conclusion, states with broader Medicaid coverage had a lower incidence of ESRD and smaller insurance-related access gaps.

Crystallographic dependence of 4-dimethylaminopyridine electrosorption on gold

Using differential capacity we have compared the electrosorption of dimethylaminopyridine (DMAP) on Au(poly), Au(100), and Au(111) electrode surfaces. Thermodynamic analyses of charge density data have provided quantitative information concerning the Gibbs excess of the films and the bonding motif between the adsorbate and the Au(111) surface. The results indicate that the orientation of DMAP is highly dependent on electrode potential, electrolyte pH, and surface crystallography. At pH equal to the pKa of the conjugate acid, the dominant adsorbed species is the free base, DMAP, but its adsorption state depends on the electrical state of the Au(111) surface. Negative polarizations favor a low coverage state with individual DMAP molecules coordinated through both the aromatic ring and the exocyclic tertiary amine. Positive polarizations induce a phase transition that provides a high density monolayer of DMAP coordinated to the Au(111) surface through the lone pair of the endocyclic nitrogen. The phase transition at high pH is unique to Au(111) as Au(100) and Au(poly) has been shown to only exhibit the high density state of adsorption. In slightly acidic electrolytes the adsorbed species on Au(111) and Au(100) is the protonated ion, DMAPH+, and no phase transitions were observed. The data provide the basis from which a rational strategy for synthesizing anisotropic gold nanocrystals can be developed.

Dynamic Response of Silver Monolayers Adsorbed on Gold(100) upon an Oscillatory Variation of the Chemical Potential: a Monte Carlo Simulation Study

On the basis that the underpotential electrochemical deposition of Ag atoms on the Au(100) surface exhibits sharp first-order phase transitions at well-defined values of the (coexistence) chemical potential (μcoex), we performed extensive simulations aimed at investigating the hysteretic dynamic behavior of the system close to coexistence upon the application of a periodic signal of the form μ(t) = μcoex + μo sin(2Πt/τ), where μo and τ are the amplitude and the period of the sweep, respectively. For relatively short periods and small enough amplitudes, the system becomes trapped either at low or high Ag coverage states, as shown by recording hysteresis loops. This scenario is identified as dynamically ordered states (DOS) such that the relaxation time (τrelax) of the corresponding metastable state obeys τrelax > τ. On the other hand, by properly increasing μo and/or τ, one finds that the Ag coverage gently follows the external drive (here τrelax < τ) and the system is said to enter into dynamically disord...

Time-Resolved Sum-Frequency Generation Spectroscopy of Cyclohexane Adsorbed on Ni(111) under Ultrashort NIR Laser Pulse Irradiation

The adsorption of cyclohexane on Ni(111) was studied by infrared-visible sum-frequency generation (SFG) spectroscopy with and without near-infrared (NIR) pump pulse irradiation. Two adsorption states of cyclohexane were found in the monolayer region, a low-coverage state showing SFG peaks at 2740, 2815, and 2865 cm(-1), and a high-coverage state showing peaks at 2740, 2815, and 2905 cm(-1). Both states coexisted on the saturated Ni(111) surface. The broad peak at 2740 cm(-1) was due to the softened CH stretching mode of the axial CH groups of cyclohexane that point toward the Ni(111) surface. The peaks at 2815 and 2865 (or 2905) cm(-1) were due to the symmetric and asymmetric stretching modes of CH(2) groups, respectively, that were free from the surface. Irradiation with NIR pulses caused a temporary jump in temperature at the Ni(111) surface and enhanced the intensity of the 2905 cm(-1) peak, but weakened the other peaks. This indicates that the temperature jump excited the cyclohexane molecules from the low-coverage state to the high-coverage state. The dynamics of the structural change observed in the adsorbed cyclohexane on NIR irradiation is discussed.

Adsorption states of acetaldehyde and butane-2,3-dione on Ni(1 1 1)

The chemisorption of butane-2,3-dione (biacetyl) on Ni(1 1 1) was studied using X-ray photoelectron spectroscopy, reflection absorption infrared spectroscopy (RAIRS) and thermal desorption measurements. An adsorbed state with both carbonyl groups bonded to the surface is formed at 105 K for coverages below 0.3 of a monolayer. Free butane-2,3-dione exists in the pure trans-conformation at room temperature, hence the low-coverage state results from chemisorption induced trans → cis isomerization. Chemisorption in the 0.5–1.0 ML range occurs into an η 1-state with one oxygen atom attached to the surface. Data for acetaldehyde on Ni(1 1 1) are presented to aid with the interpretation of the butane-2,3-dione results. Both η 1- and η 2-adsorption states of acetaldehyde are observed. The low coverage state of butane-2,3-dione displays the same C(1s) and O(1s) binding energies as η 2-(C,O)-acetaldehyde, consistent with a strong rehybridization of both keto groups. This low coverage di- η 2-type state is identified as a surface enediolate. A third adsorption state, involving one η 2-bond, is formed in the 0.3–0.5 coverage range. The thermal decomposition chemistry of butane-2,3-dione on Ni(1 1 1) is highly coverage dependent. The enediolate state undergoes decarbonylation to yield CO and H 2 desorption peaks. At higher coverages, desorption of acetaldehyde and butane-2,3-dione is observed. An acetyl intermediate was detected by RAIRS measurements.

Coverage dependence of activation barriers: Nitrogen on Ru(0001)

Exposing a Ru(0001) surface to an atomic beam of N produces a series of different states of atomic N adsorbed on the surface. For low atom doses, well-known low coverage states are produced, but for higher atom doses several previously unknown higher coverage states are sequentially filled up to a N coverage of almost 1 ML, N/Ru. These states show well-defined temperature programmed desorption (TPD) peaks. Recent density functional calculations demonstrate that the N–Ru bond strength decreases significantly with N coverage, and in fact that high coverage N adsorbate states are not thermodynamically stable relative to associative desorption. The observed high coverage states must, therefore, be only metastable, with lifetimes determined by the height of the barrier between gas phase N2 and the adsorbed atomic state. Analysis of the TPD in combination with the theoretical adsorption energies allows us to estimate these coverage dependent barriers. Independent measurements of the coverage dependent barrier heights were also obtained via the technique of laser assisted associative desorption i.e., by measuring the translational energy distribution of desorbing N2 via time of flight techniques induced by a short laser induced temperature jump. A barrier increases of ⩾1 eV with N atom coverage was observed by both methods.

Observation of metastable atomic nitrogen adsorbed on Ru(0001)

Exposing a Ru(0001) surface to an atomic beam of N produces a series of different states of atomic N adsorbed on the surface. For low atom doses, well-known low coverage states are produced, but for higher atom doses, several previously unknown higher coverage states are sequentially filled. These states exhibit well defined temperature programmed desorption (TPD) peaks which shift to considerably lower temperatures with N coverage. The highest N coverage obtainable is almost 1 ML N/Ru. Recent density functional calculations demonstrate that the N–Ru energy decreases significantly with N coverage, and in fact predict that N adsorbate states are not thermodynamically stable relative to associative desorption at high coverage. The observed high coverage states must, therefore, be metastable with lifetimes determined by the height of the barrier between gas phase N2 and the adsorbed atomic states. Simple analysis of the TPD features in combination with the theoretical adsorption energies allows us to estimate these coverage dependent barrier heights. We find that the barrier heights increase significantly with coverage, and this is important in the metastability of the adsorbed states. A comparison of nitrogen adsorption on Ru(0001) with oxygen adsorption on Ru(0001) surface is stressed throughout.

Temperature-Programmed Desorption Investigation of the Adsorption and Reaction of Butene Isomers on Pt(111) and Ordered Pt−Sn Surface Alloys

The influence of alloyed Sn on the chemistry of C4 butene isomers, including 1-butene, cis-2-butene, and isobutene, chemisorbed on Pt(111) was investigated by temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Pt−Sn alloy chemistry was probed by investigation of two ordered surface alloys formed when Sn atoms were incorporated within the topmost layer on a Pt(111) substrate to form a (2 × 2) Sn/Pt(111) alloy with ϑSn = 0.25 and a (√3 × √3)R30° Sn/Pt(111) alloy with ϑSn = 0.33. Low-coverage states of chemisorbed 1-butene, cis-2-butene, and isobutene on Pt(111) have desorption activation energies of 17.5, 17, and 17 kcal/mol, respectively. These energies are reduced to 16, 15.5, and 15 kcal/mol on the (2 × 2) alloy and 13.5, 12, and 11 kcal/mol on the (√3 × √3)R30° alloy. Changing the surface Sn concentration from ϑSn = 0.25 to ϑSn = 0.33 causes a relatively larger decrease in the chemisorption bond strength of these alkenes, and we associate this with the importance of a pure Pt 3-fold site for strong alkene bonding. All three butenes undergo decomposition on Pt(111) during TPD which accounts for 50−60% of the chemisorbed monolayer. Alloying Sn into the surface causes a large reduction in the reactivity of the surface, and the fraction of the chemisorbed layer which decomposes is decreased to 3−7% on the (2 × 2) alloy, and no decomposition occurs on the (√3 × √3)R30° alloy. The strong reduction of decomposition on these two surface alloys may be due to the elimination of adjacent pure Pt 3-fold hollow sites. No large changes occur in the coverage of the chemisorbed monolayer of butenes in the presence of up to 33% of a monolayer of alloyed Sn, showing that the adsorption ensemble requirement for chemisorption of these alkenes on Pt(111) and the two Sn/Pt(111) alloys is at most a few Pt atoms. To the extent that alloying or direct Pt−Sn interactions occur in supported, bimetallic Pt−Sn catalysts, the chemistry reported here would lead to increased isobutene yields and decreased coking of the catalyst.

Local structure determination for low-coverage CO on Ni(111)

Scanned-energy-mode photoelectron diffraction has been used to determine the local adsorption structure of CO on Ni(111) at low temperature at a coverage of approximately 0.25 ML, corresponding to a nominal phase. CO molecules are found to occupy the same two (`hcp' and `fcc') inequivalent hollow sites as in the 0.5 ML phase, and while the results do indicate a preference for the hcp hollow site directly above a second-layer Ni atom in the low-coverage state, this preference is of marginal statistical significance. The similarity of this parameter in both the and phases can only be reconciled by assuming significant local disorder. It does, however, indicate that the local binding energies at the two hollow sites are very similar and that the presence of both sites in the phase is not simply a consequence of near-neighbour adsorbate - adsorbate repulsion.

Sensitivity of electron stimulated desorption to surface oxygen on Rh(111) and Pt(111)

Surface oxygen on Rh(111) and Pt(111) has been investigated using Auger electron spectroscopy, X-ray photoelectron spectroscopy and electron stimulated desorption (ESD). Pt(111) surfaces have been investigated with temperature programmed desorption as well. ESD kinetic energy distributions (KEDs) for positive and negative ions indicate the presence of multiple bonding states for oxygen on both surfaces. ESD KEDs from surfaces judged clean with AES and XPS show the persistence of one oxygen state after cleaning. This ESD peak remains after repeated high temperature anneals, suggesting that it arises from oxygen bonded to impurities or as near-surface metal oxides. This persistent signal makes ESD a much more sensitive probe for this low-coverage oxygen state on the Rh(111) and Pt(111) surfaces than common surface spectroscopies such as AES and XPS.

Comment on ‘Equilibrium structure of the InP(100) surface by Jian-Min Jin and Laurent J. Lewis’ [Surface Science 325 (1995) 251

We extend previous first-principles calculations of the equilibrium structure of the InP(100) surface to include atomic chemical potential contributions to the surface formation energies at various coverages. We confirm earlier conclusions that the surface will most likely assume a low indium coverage state (i.e., ≤ 25%).

Pyridine on Ru(001): Thermal evolution

The thermal evolution of the various states of chemisorbed pyridine on Ru(001) has been studied in detail using vibrational spectroscopy and thermal desorption. At low coverages a parallel π -bonded pyridine state is formed, but at higher coverages the configuration changes to an inclined, mainly N-bonded state. This disparity of adsorption is reflected in the formation of different intermediate states and dissociation products upon heating as well as in the evolution and nature of the desorption products. It is demonstrated that the low coverage state maintains a parallel ring orientation as dehydrogenation occurs; no inclined ring can be found, and in this respect the decomposition is analogous to that of benzene o on the same surface. The onset of ring cleavage can be correlated with the appearance of adsorbed N atoms around 450 K. In contrast, the decomposition of inclined pyridine leads to the formation of α -pyridyl with a strongly inclined ring; the formation and decomposition of this and other species have been followed by vibrational spectroscopy. Contrary to the thermal evolution of low coverage layers the vibrational features of N ads are observed above 600 K for the saturated layer. This is interpreted as hindered decomposition of the (originally standing) pyridine species. As a consequence a HCN species is formed which desorbs around 660 K and which also shows up in the vibrational spectra by an increase of the v C-H loss intensity above 600 K.

Oxygen on an approximately W(110) single crystal: electron reflection, electron energy-loss spectroscopy, and work-function measurements

Adsorption of oxygen on W(110) at room temperature has been studied by means of electron reflection, high-resolution electron energy-loss spectroscopy, and work-function measurements. Data collected with these different techniques show that a change in the structure of the adlayer takes place at a coverage of about a 0.2 monolayer. In the low-coverage state, sharp low-energy electron diffraction fine structures in the reflected current, a slow variation in the work-function variation, and a single vibration loss at 68 meV are observed. At higher coverage, a damping of the fine-structure features, a normal work-function increase, and a shift of the 68 meV loss peak to 73 meV together with the growth of a 48 meV loss are observed. These observations seem to support models involving initial dissociation near surface defects and subsequent p(2 × 1) island growth).

Bonding and electronic structure of the GaAs(1 1 0)-Al interface

The electronic structure of the GaAs(110)-Al interface is studied by using the ordered metal layer and metal droplet models simulating the low and high coverage states. It is found that the character of the Fermi level pinning states depends on the level of metal coverage. Upon the formation of the metal cluster, the metal-semiconductor interactions weaken and become nondirectional.

Evidence for a thermally induced state conversion in low coverage states of hydrogen on platinum

Evidence for a thermally induced state conversion in low coverage states of hydrogen on platinum

Evidence for a thermally induced state conversion in low coverage states of hydrogen on platinum

Evidence for a thermally induced state conversion in low coverage states of hydrogen on platinum

Carbon interaction with nickel surfaces: Monolayer formation and structural stability

A first order phase transition occurs at Ni (111) surfaces under conditions of controlled chemical potential. For a dilute Ni–C alloy of fixed bulk composition an abrupt change occurs (as temperature is lowered) in the surface carbon coverage from a very low coverage state to a two dimensional crystalline C overlayer. In the present work the possible effects of substrate orientation on the occurrence and characteristics of the transition have been investigated. On surfaces vicinal to (111) and on the (311) and (110) planes abrupt segregation of a graphitic monolayer was observed. The observed coverage–temperature relationship was similar to that for (111) with the monolayer formation occurring at a temperature between 10% and 12% higher than the equilibrium precipitation temperature. Surface reconstruction accompanies the monolayer condensation and precipitation processes. At dilute carbon coverages and high temperatures the vicinal planes have arrays of monoatomic height steps; upon monolayer formation rearrangement of the surface steps occurs to produce facets of (111) and (110) for surfaces along the [11̄0] zone and of (111) and (311) for surfaces along the [011̄] zone. Qualitatively different behavior was observed with (100) and (210). Monolayer graphite condensation was also observed with both of the surfaces but the high temperature region was characterized by a gradually decreasing carbon coverage for which the Auger line shape was that of nickel carbide. Upon graphite precipitation at the solubility limit all surfaces become unstable with respect to formation of {111} facets.