Paul Fenter

Surface speciation of calcite observed in situ by high-resolution X-ray reflectivity

High-resolution, in situ X-ray reflectivity measurements were made of the calcite (104)–water interface in calcite-saturated aqueous solutions at pH values ranging from 6.8 to 12.1 and low PCO2. The X-ray reflectivity data, taken over a momentum transfer range of 6 A−1, indicate that the calcite surface does not vary significantly over this range of experimental conditions. From an analysis of the data at pH 8.3, the best-fit reflectivity model requires the presence of 1.0 ± 0.4 monolayer of a hydroxyl species (OH or OH2) at 2.50 ± 0.12 A above the surface Ca ions and involves rotations of the surface carbonate groups toward the (104) plane. The X-ray reflectivity data for pH 6.8 and 12.1 can be explained without invoking changes other than protonation reactions in the surface speciation of terrace areas. This is consistent with scanning force microscopy studies of calcite growth and dissolution near equilibrium, which show that attachment and detachment of Ca and CO3 ions occurs primarily at step-edge kink sites. These results demonstrate how high-resolution X-ray reflectivity can be used for direct, in situ measurement of mineral surface structure, to provide strong constraints on chemical speciation and reactivity at the mineral-fluid interface.

Structures of quartz (100)- and (101)-water interfaces determined by x-ray reflectivity and atomic force microscopy of natural growth surfaces

Abstract The structures of prismatic (100) and pyramidal (101) growth faces of natural quartz crystals, and their modification upon annealing at T ≤ 400°C were investigated ex situ by atomic force microscopy (AFM) and in water by high-resolution X-ray reflectivity. AFM images revealed the presence of ∼ 0.1 to 1 μm-wide flat terraces delimited by steps of one to several unit cells in height. These steps follow approximately directions given by the intersection of growth faces. Modeling of X-ray reflectivity data indicates that surface silica groups on flat terraces have only one free Si-O bond each (presumably hydroxylated), except for some having two free Si-O bonds observed on a single (100) surface. Vertical relaxation of atomic positions (

Nanoscale Perturbations of Room Temperature Ionic Liquid Structure at Charged and Uncharged Interfaces

The nanoscale interactions of room temperature ionic liquids (RTILs) at uncharged (graphene) and charged (muscovite mica) solid surfaces were evaluated with high resolution X-ray interface scattering and fully atomistic molecular dynamics simulations. At uncharged graphene surfaces, the imidazolium-based RTIL ([bmim(+)][Tf(2)N(-)]) exhibits a mixed cation/anion layering with a strong interfacial densification of the first RTIL layer. The first layer density observed via experiment is larger than that predicted by simulation and the apparent discrepancy can be understood with the inclusion of, dominantly, image charge and π-stacking interactions between the RTIL and the graphene sheet. In contrast, the RTIL structure adjacent to the charged mica surface exhibits an alternating cation-anion layering extending 3.5 nm into the bulk fluid. The associated charge density profile demonstrates a pronounced charge overscreening (i.e., excess first-layer counterions with respect to the adjacent surface charge), highlighting the critical role of charge-induced nanoscale correlations of the RTIL. These observations confirm key aspects of a predicted electric double layer structure from an analytical Landau-Ginzburg-type continuum theory incorporating ion correlation effects, and provide a new baseline for understanding the fundamental nanoscale response of RTILs at charged interfaces.

AN UNEXPECTED PACKING OF FLUORINATED N-ALKANE THIOLS ON AU(111) : A COMBINED ATOMIC FORCE MICROSCOPY AND X-RAY DIFFRACTION STUDY

Atomic force microscopy (AFM) and grazing incidence x‐ray diffraction (GIXD) have been used to study the structure of self‐assembled monolayers of CF3(CF2)n(CH2)2SH (n=11, 7, and 5) on the Au(111) surface. Surprisingly, although the nearest‐neighbor fluorinated alkane thiol distance is very close to the lattice constant of a commensurate p(2×2) structure, the close‐packed rows of molecules are rotated ∼30° with respect to the underlying gold lattice. That packing is incommensurate or at most only close to the high‐order commensurate c(7×7) structure. The relative orientation of the organic monolayer and the Au(111) substrate has been determined unambiguously both with GIXD, and by AFM, taking advantage of an earlier finding (Ref. ) that AFM tips can reversibly displace the thiol molecules under high loads. In addition, we demonstrate that the two techniques provide complementary information on the order and the domain structures of these monolayers.

Resolving orthoclase dissolution processes with atomic force microscopy and X-ray reflectivity

Abstract Direct measuremens of orthoclase (001) were performed using in situ atomic force microscopy (AFM) and synchrotron X-ray reflectivity to reveal the A-scale dissolution process as a function of pH and temperature. Distinct processes were observed, involving mainly terrace roughening at pH = 1.1 and step motion at pH = 12.9. A gel-like surface coating was observed to form at acidic pH under slow fluid flow-rate conditions. No coating was observed either at alkaline pH or at acidic pH under high fluid flow-rate conditions. The corresponding dissolution rates were measured directly at pH = 1.1 and 12.9 at ∼50°C using real-time X-ray reflectivity measurements, and reacted interface structures were derived from crystal truncation rod measurements after reaction at both acidic and alkaline pH. Our observations reveal, under these experimental conditions, that 1) orthoclase dissolution is controlled by at least two separate surface reactions having distinct reactive sites; 2) dissolution is stoichiometric at alkaline pH and only minimally nonstoichiometric (limited to one unit-cell depth) at acidic pH; previously identified nonstoichiometric layer thicknesses derived from macroscopic measurements are associated with the formation of the gel-like coatings; 3) dissolution rates measured at freshly cleaved (001) surfaces are comparable to those derived from steady-state powder dissolution rates for both alkaline and acidic pH; and 4) elevated transient dissolution rates are not observed for freshly cleaved surfaces but are obtained under alkaline conditions after reacting the orthoclase (001) surface at acidic pH. These observations clarify differences in orthoclase dissolution mechanisms as a function of pH, demonstrate the utility of AFM and X-ray scattering methods for measuring A-scale structures and face-specific dissolution rates on single crystals and place new constraints on the understanding of alkali feldspar weathering processes.

Thermally induced failure mechanisms of organic light emitting device structures probed by X-ray specular reflectivity

Abstract The thermal evolution of organic light emitting device structures and materials has been measured using X-ray specular reflectivity. Thermally induced failure of these structures is found to be due to the large thermal expansion of the hole transport layer (N,N′-diphenyl-N,N′-bis(3-methylphenyl)1,1′-biphenyl-4,4′diamine, or TPD) associated with its low glass transition temperature, suggesting a strain-driven failure mechanism (as opposed to TPD recrystallization). These results suggest the possibility of optimizing organic light emitting devices through direct in-situ measurements of their structure and stability.

Atomic-scale structure of the orthoclase (001)-water interface measured with high-resolution x-ray reflectivity.

In situ X-ray specular reflectivity and atomic force microscopy were used to determine the structure of the orthoclase (001) cleavage surface in contact with deionized water at 25°C. These are the first in situ measurements of the orthoclase–water interface structure performed to Angstrom-scale resolution. The orthoclase (001) cleavage surface has minimal roughness, and only one of two possible surface terminations is exposed. The X-ray data show that (1) the silica network at the orthoclase surface is terminated by an oxygen-containing species (e.g., O or OH) having a coverage of 1.9 ± 0.25 ML (the expected coverage is 2.0 ML, where 1 ML = 1 atom/55.76 A2), (2) the outermost layer of K+ ions have been removed with a derived coverage of 0.0 ± 0.08 ML (the bulk truncated K+ coverage is 1.0 ML), and (3) a complex relaxation profile affecting the near-surface structure propagates ∼26 A into the orthoclase with a maximum relaxation of ∼0.15 A near the surface. These data are inconsistent with K+ ion depletion below the topmost K+ layer. These results provide a new baseline for understanding the initial steps of the feldspar dissolution process, demonstrate the power of combining X-ray scattering techniques with scanning probe microscopies for understanding the intrinsic characteristics of complex mineral–water interface systems, and suggest a new approach for understanding feldspar dissolution mechanisms.

Structural defects in self‐assembled organic monolayers via combined atomic beam and x‐ray diffraction

We present the results of a combined He atom and x‐ray diffraction study of CH3(CH2)n−1SH monolayers self assembled on Au(111) surfaces. By combining these two complementary probes, we have characterized both the surface and the interior structure of the monolayers. In both cases, we find the same structure containing four molecules per unit mesh. However, we demonstrate that there are significant differences in both the diffraction linewidths and the dependence of the linewidth upon chain length for these two techniques.

Epitaxy and chainlength dependent strain in self-assembled monolayers

We use grazing incidence x-ray diffraction to systematically study the structure of an archetypal self-assembled monolayer as a function of the hydrocarbon chain length, n. The monolayers consists of n-alkyl thiol molecules, CH3(CH2)n−1SH (Cn, 10⩽n⩽30), self-assembled on single crystal Au(111) surfaces. At room temperature, the 2D structure is described by a C(4×2) unit mesh for all chain lengths. However, we demonstrate that there is a systematic dependence of the tilt structure (i.e., the tilt angle and tilt direction) of the hydrocarbon chains as a function of the chain length. Furthermore, we show that the monolayer structures are characterized by distinct “long’’ (n⩾16) and “short’’ (n⩽14) chain length regimes, as well as a smooth variation of the structural parameters within each regime. We associate these systematic structural changes with the conflicting requirements of epitaxy and molecular packing, and argue that the driving force is the changing intra-layer interaction strength (which is propor...

On the structure and evolution of the buried S/Au interface in self-assembled monolayers : x-ray standing wave results.

We describe a structural study of the S/Au interface for decanethiol monolayers (C10) on a Au(111) surface using the technique of X-ray standing waves ( XSWs). The XSW results for full-coverage monolayers are inconsistent with any model incorporating a single sulfur adsorption site, such as the widely assumed threefold hollow site on the Au(111) surface. Instead, the XSW results reveal two distinct sulfur head group sites, each with a distinct lateral and vertical location with respect to the underlying gold lattice. We discuss structural models that are consistent with these results. We have also studied the evolution of the structure versus coverage with XSW and X-ray photoelectron spectroscopy ( XPS) and have determined that the local S/Au interface structure of the ‘‘lying down’’ striped phase at low coverages (0.27 ML, 1 ML=4.62◊1014 molecules cm’2) is indistinguishable from that of the ‘‘standing up’’ c(4◊2) phase at saturation (1 ML). Some important implications concerning the structure and growth of these monolayers are discussed.