Triangular Etch Research Articles

Fluorite dissolution at acidic pH: In situ AFM and ex situ VSI experiments and Monte Carlo simulations

Dissolution of the fluorite (1 1 1) cleavage surface was investigated by means of in situ atomic force microscopy (AFM) and ex situ vertical scanning interferometry (VSI) experiments at pH range 1–3 in HCl solutions. Surface retreat was quantified at different pH values, yielding dissolution rates that were used to derive an empirical rate law for fluorite dissolution: Rate fluorite ( mol m - 2 s - 1 ) = 2.0 ± 0.4 × 10 - 6 · ( a H + ) 0.38 ± 0.07 where a H + is the proton activity. The influence of Δ G on fluorite dissolution rate at pH 2 was investigated by means of AFM and VSI surface measurements and flow-through experiments with powdered fluorite. The fluorite dissolution rate decreases non-linearly with increasing Gibbs energy (Δ G) and a dissolution plateau is obtained at Δ G ⩽ −7 kcal mol −1. This Δ G effect can be expressed with a rate law of the form Rate fluorite mol m - 2 s - 1 = k · a H + n 1 - exp - 4.7 × 10 - 2 · | Δ G | RT 1.41 ± 0.24 An alternative form based on a formulation making use of a Temkin number is also possible Rate fluorite mol m - 2 s - 1 = k · a H + n 1 - exp Δ G 8 RT Dissolution proceeds by formation of equilateral triangular etch pits with trigonal pyramidal morphology and emanation of stepwaves that are responsible for the surface retreat. The results of Monte Carlo simulations are consistent with this reaction mechanism.

Nucleation kinetics and growth of nonlinear optical bis (dimethyl sulfoxide) manganese mercury thiocyanate single crystals

AbstractNonlinear optical (NLO) material of bis (dimethyl sulfoxide) manganese mercury thiocyanate (MMTD) was synthesized by two step reaction method. The solubility and metastable zonewidth were experimentally determined in order to optimize the growth parameters. Bulk crystals of MMTD were grown by slow cooling and slow evaporation methods. The structure of the grown crystal was confirmed by X‐ray diffraction analysis. Presence of functional groups and the coordination of Lewis base ligand of dimethyl sulfoxide (DMSO) were confirmed by FT‐IR analysis. Optical transparency of the grown crystals was studied by UV‐Vis spectroscopy. Nonlinear optical property of the grown crystal was confirmed by Kurtz powder method. Etching studies reveal the formation of triangular hillock etch patterns, indicative of 2D nucleation mechanism. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Morphological and chemical optimization of ex situ NH4F (40%) conditioned Si(111)-(1×1):H

Synchrotron radiation photoelectron spectroscopy was employed to investigate the chemical state of Si(111) surfaces upon anisotropic etching in concentrated NH4F solution. Minute amounts of oxidized silicon were detected and attributed to the fast Si–H–OH formation at atomic steps. Combining in situ optical and scanning probe techniques, consecutive chemical treatments were developed to achieve optimized morphological and chemical surface properties. Native oxides and a stressed SiO2/Si layer are removed by a two-step NH4F treatment leading to a terraced surface without triangular etch pits; subsequently, silicon in the Si1+/2+/3+ valence states is dissolved by HF (50%) while the surface topography is preserved.

A simple method for high yield fabrication of sharp silicon tips

A simple, high yield, method for the fabrication of sharp silicon tips is described. A triangular etch mask design is used to ensure that the tip forms with a single point. An anisotropic wet etch gives rise to a tip that continues to “self-sharpen” after the etch mask is released. The tip geometry comprises three converging {113} planes towards the apex with {313} planes forming at the base. The apex of each tip typically has a radius of curvature of <5nm, which can be reduced to <2nm by a subsequent oxide sharpening process. Tips of this kind have been successfully integrated into the fabrication of atomic force microscopy probes.

Dissolution of Brucite on the (001) Surface at Neutral pH: in situ Atomic Force Microscopy Observations

AbstractThe dissolution of brucke, Mg(OH)2, on the (001) surface was investigated using in situ atomic force microscopy in solutions at near-neutral pH. Dissolution proceeded by the formation of crystallographically oriented triangular etch pits with monolayer step and expansion of the pits. The sides of the triangle are parallel to the [100], [110] and [010] directions of the brucite structure, and the orientation of lines from the center of the triangle to the three apices are along the [210], [1¯10]$[\bar 110]$ and [1¯2¯0]$[\bar 1\bar 20]$ directions. This orientation may produce pit edges where OH groups coordinate to two Mg2+. Although triangular etch pits with monolayer depth formed mostly at random on the (001) surface, concentric pits penetrating several layers were also observed. Etch pits with spiral steps were rarely observed. Coalescence of the pits resulted in stranded terraces that diminished in size rapidly and formed a rounded irregular form. The step-retreat velocity around the triangular pit is 0.015–0.04 nm/s at pH 5–8. The retreat velocity around the stranded terraces was about three times more rapid than that around the triangular etch pits.

Analysis of the etching behavior of ZnO ceramics

ZnO ceramics doped with a small amount of Bi 2O 3 and Sb 2O 3 were prepared and their orientation-dependent etching patterns were analyzed. Three distinctive patterns produced by the oxidizing etchant HCl were observed: hexagonal etch pits on + C plane, triangular etch pits on prismatic plane, and hillocks of hexagonal pyramid on − C plane. From these results, the three-dimensional shape of etch pit could be suggested. The character of inversion boundary was also determined from the etching patterns.

Observations on the geometries of etched fission and alpha-recoil tracks with reference to models of track revelation in minerals

The kinetic and atomistic theories of crystal growth and dissolution are used to interpret the shapes and orientations of fission-track, recoil-track and dislocation etch pits in tri-octahedral phlogopite and di-octahedral muscovite. An atomistic approach combined with symmetry considerations lead to the identification of the periodic bond chains that determine the etch pit morphologies and relative etch rates at a chemical level: O–Mg–O in phlogopite, O–Mg–O–Fe in biotite and O–Al–O in muscovite. Using first-order estimates of the bond strengths, it is possible to account for the relative track etch rates in these minerals. The reported, sometimes simultaneous, occurrence of triangular, polygonal and hexagonal etch pit contours in phlogopite, some of which violate the crystal symmetry, suggests that the cohesion of the phlogopite lattice is lost over a much larger radius than that of the track core around the trajectories of particles for which the energy loss exceeds a threshold value. This is interpreted as an indication of pronounced sublattice and anisotropic effects during track registration.

Observation of dislocation etch pits in a sapphire crystal grown by Cz method using environmental SEM

The dislocation etch pits in an as-grown [0 0 0 1] direction sapphire crystal (α-Al 2O 3) produced by Cz method were investigated using an environmental scanning electron microscope for the first time. The clear and stable contrast images of triangular, hexagonal, and quadrilateral etch pits were observed, which demonstrate the structural symmetry of sapphire crystal along [0 0 0 1] direction. The dislocation density we measured is 1–2×10 5/cm 2 and the formation of the etch pits is also discussed.

Microscopic and spectroscopic investigation of the calcite surface interacted with Hg(II) in aqueous solutions

AbstractThe interaction of the {104} cleavage surface of calcite with Hg(CH3COO)2 aqueous solutions with concentration of 5 mM Hg(II) (pH ≈3.5), was investigated using microscopic and spectroscopic techniques. In situ atomic force microscopy experiments showed that surface microtopography changes significantly as a result of the interaction, and that the initial rhombic etch pits induced by H2O dissolution are rapidly transformed to deeper etch pits exhibiting an unusual triangular shape. The growth of these etch pits is strongly anisotropic, moving faster along the [21] direction than along the [010] direction (with step-retreat velocities of ∼12 nm s –1 and ∼4 nm s–1, respectively). The modified etch pits are due to Hg(II) sorption in the surface, rather than due to the effect of the acetate anion. The sorption (adsorption and probably absorption also) of Hg(II), in the first minutes of the interaction, is shown by X-ray photoelectron spectroscopy. After ∼2 h, the triangular etch pits are interconnected to form larger hexagonal etch pits, while Hg(II)-bearing phases (confirmed later by SEM-EDS) grow onto the surface through a heterogeneous nucleation process. The crystal growth of orthorhombic (montroydite-type) hydrated Hg(II) oxide (HgO·nH2O) on the surface of calcite was confirmed by XRD patterns and FT-IR spectra from samples exposed for longer times to Hg(CH3COO)2 solution.

Etching behaviour of alpha-recoil tracks in natural dark mica studied via artificial ion tracks

Alpha-recoil tracks (ARTs) created by the α-decay of U, Th, and their daughter nuclei, are used by a new dating method to determine the formation age of dark mica bearing Quaternary and Neogene volcanic rocks and the cooling age of plutonic and metamorphic rocks [Chem. Geol. 166 (2000) 127, Science 155 (1967) 1103]. The age equation combines the volumetric density of ARTs with the U and Th contents. Etching latent ARTs (diameter 30–100 nm) in the mica mineral phlogopite by HF and measuring the areal density of triangular etch pits by optical and scanning force microscopy (SFM) leads to a linear growth of ART areal density versus etching time. The ART volume density is a function of the slope of the areal density and the etching rate ( v eff). Therefore, the determination of v eff is essential for the calculation of an age value. To determine the etching parameters such as etching efficiency and v eff, phlogopite samples were irradiated with 80 keV Au ions. Irradiated surfaces were etched with 4% HF at 23±2 °C during successive time intervals and after each interval studied with SFM. The etching rate v eff was determined by different techniques. To evaluate the threshold of etchability, the energy losses of the Au ions and α-recoil nuclei in phlogopite were calculated with the SRIM00 code. The etching efficiency of the Au ion tracks was then used to predict the corresponding etching efficiency of the natural radioactive nuclei.

Simulation of step patterns on natural diamond {111} surfaces

Almost all {111} surfaces of natural diamond crystals show trigons: triangular etch pits centred on dislocations. The step patterns that arise when two trigons meet allow us to propose an analytical expression for the velocity of steps during etching as a function of their orientation. This step velocity function has a deep global minimum for 〈110〉 steps on a surface inclined towards {110} and a local minimum for 〈110〉 steps on a surface inclined towards {100}. Continuum simulations, based on the kinematic wave theory, of the evolution of step patterns using the step velocity function are able to reproduce quite satisfactorily intricate step patterns formed by multiple interacting trigons. This means that the assumption, implicit in the simulation, that bulk and surface diffusion are not rate limiting during etching is valid. Occasionally, growth hillocks are observed on a {111} surface of a natural diamond. The step patterns on these surfaces show that the step velocity function governing growth does not have the local minimum for 〈110〉 steps on a surface inclined towards {100}. This confirms that the occasionally observed growth patterns and the usually observed etching patterns are formed under fundamentally different conditions.

Growth, morphology, and microindentation analysis of Bi 2Se 3, Bi 1.8In 0.2Se 3, and Bi 2Se 2.8Te 0.2 single crystals

Bismuth Selenide single crystals have been grown by the vertical normal freezing technique. Confirmation of the compound formation and the lattice parameters are calculated from the X-ray diffractogram. SEM studies show the parallel cleavage lines in Te-doped samples. Triangular etch pits are obtained on the (111) planes of Bi 2Se 3. Microindentation studies are carried out on the same plane to understand the mechanical behavior. Annealing and quenching effects of microhardness are evaluated.

The picture tells the story: Using surface morphology to probe chemical etching reactions

Highly anisotropic etchants, which only attack specific crystallographic planes of a material, are prized by industry, but the inherent defect selectivity of these etchants has stymied investigations into their chemical mechanism. Because of their chemical specificity, these etchants produce characteristic nanoscale morphological features, such as triangular etch pits and jagged atomic steps, which reflect the site-specific chemical reactions that control the etching. Using a combination of scanning tunnelling microscopy and kinetic Monte Carlo simulations, these site-specific rates can be directly quantified. By correlating these measurements with the structure of the etched sites, information on the reaction mechanism can be obtained. These techniques are illustrated with examples drawn from aqueous silicon etching reactions. For example, the production of atomically flat Si(111) surfaces by NH 4 F etching can be explained by the existence of a pentacoordinate transition state to OH - attack. Sites held in a rigid tetrahedral geometry, such as the Si(111) terrace site, are relatively resistant to attack, while defect sites, such as steps and kinks, are rapidly etched.

Oxidative etching of cleaved synthetic diamond {1 1 1} surfaces

In this study, three commonly used methods for oxidative etching of diamond {1 1 1} faces are compared: gas phase etching using `dry' oxygen, gas phase etching using an oxygen/water vapour mixture and liquid etching in molten potassium nitrate. The synthetic diamond surfaces are prepared by cleavage. The morphology of the surfaces is studied using atomic force microscopy and the kinetics of the reactions is determined by measuring the decrease in thickness of the diamond. The atomic arrangement of the {1 1 1} surfaces etched in oxygen/water is studied using surface X-ray diffraction. Upon dry oxygen etching, the {1 1 1} faces are roughened and become morphologically unstable. This observation conflicts with standard theory, which predicts {1 1 1} to be a stable F-face that should etch via a layer mechanism. A possible explanation for this is chemical roughening. The addition of water vapour to the oxygen has a dramatic effect on the etching mechanism of the {1 1 1} faces. Now etching proceeds via a layer mechanism involving monoatomic steps. Shallow etch pits are formed, of which the slope increases for increasing etching temperature. Surface X-ray diffraction experiments show that the surface is –OH terminated. For potassium nitrate etching, the {1 1 1} face etches also via a layer mechanism and triangular etch pits with rounded corners are formed, having point or flat bottoms. This etching technique appears to be the best method to reveal different types of defects ending on diamond {1 1 1} surfaces.

Atomic force microscopy study of the CaF 2(111) surface: from cleavage via island to evaporation topographies

The development of (111) cleavage faces of CaF 2 upon thermal treatment is studied by atomic force microscopy (AFM). Vacuum-cleaved surfaces show the atomic periodicity of the uppermost fluorine layer. Cleavage steps have heights of about 0.32 nm, corresponding to the spacing of neighboring F −–Ca 2+–F − triple layers, or integer multiples thereof. The steps exhibit ‘lightning’-shaped arrangements with tips pointing in the direction of cleavage. Steps run essentially parallel to the 〈110〉, 〈211〉, and 〈321〉 directions. At about 820 K the original cleavage topography transforms into a topography with trigonal and hexagonal islands. The islands develop from the cleavage tips by a melt-off process mediated by step and/or surface diffusion. During further heat treatment at even higher temperatures the surface flattens and the formation of triangular islands with one type of 〈110〉 step is observed. The atomic configuration for this type of step is discussed. Strongly vaporized surfaces studied by an electron microscopic shadowing technique exhibit hexagonal evaporation pits. The exposure to stray electrons from the heater filament at a temperature of 820 K results in a strong surface erosion with deep triangular etch pits with edges parallel to 〈110〉. The formation of these etch pits is probably caused by a combined action of electron-stimulated desorption of fluorine, vaporization of metallic calcium clusters and a partial healing of the surface via step and/or surface diffusion.

Delineation of defect structures in flux-grown GdFeO 3 crystals by etching

Etch patterns on (1̄1̄0) and (001) faces of as-obtained GdFeO 3 crystals are illustrated and described. Rectangular etch pits oriented with their longer sides in 〈100〉 direction and shorter sides in 〈010〉 direction are exhibited by (001) faces, whereas almost circular etch pits are exhibited by (1̄1̄0) faces of as-obtained crystals. Hexagonal etch pits and triangular etch pits produced as a result of etching in boiling HNO 3 on (001) and (1̄1̄0) faces are respectively illustrated. Evidence in support of twinning in some crystals is offered. The etch pattern on as-grown crystals are attributed to some fault in their growth conditions. Edge nucleated growth pyramids on a (1̄1̄0) face and conical hillocks on (001) faces exhibiting pits at the centres of their initiation are illustrated. The conical hillocks are suggested to be as a result of preferential growth at the point of emergence of screw dislocations.

Dissolution of the barite (001) surface by the chelating agent DTPA as studied with non-contact atomic force microscopy

DTPA (diethylenetriaminepentaacetic acid) is a chelating agent widely used for removal of barium sulfate (barite) scale in the petroleum industry. In this paper we report ex-situ investigations of barite dissolution in deionized water and in 0.18 M DTPA aqueous solutions. Non-contact atomic force microscopy (NC-AFM) was used to observe dissolution on the BaSO 4 (001) cleavage surface. Dissolution was carried out at room temperature in a 10 ml reactor. Each sample was first etched in solution and dried before examination by NC-AFM. Dissolution on the BaSO 4 (001) surface took place via development of etch pits. In deionized water, triangular etch pits were observed on the (001) terraces at room temperature. And, zigzag shaped etch pits were found at the edges of steps. In DTPA solutions, etch pits on the (001) terraces were observed and these became deeper and longer with increasing time. The geometry of these etch pits was trapezoidal, and/or trapezohedral. To explain this characteristic morphology caused by dissolution we suggest that the active sites of one DTPA molecule bind to two or three Ba 2+ cations exposed on the (001) surface.

Scanning Force Microscopy Study of Etch Pits Formed during Dissolution of a Barite (001) Surface in CDTA and EDTA Solutions

Dissolution of the barite (001) surface in aqueous solutions of 0.18 M CDTA (trans-1,2-cyclohexylenediaminetetraacetic acid) and 0.18 M EDTA (ethylenediaminetetraacetic acid) at pH 12 was investigated using ex situ scanning force microscopy. In both solutions, triangular and trapezoidal etch pits developed on the (001) surface and became deeper and longer with increasing dissolution time. The orientation of the etch pits in CDTA and EDTA solutions was elongated along the crystallographic b axis. Furthermore, dissolution of the (001) surface in a layer-by-layer fashion was observed. This resulted in the formation of “alternating” etch pits with heights of one half-unit cell (about 3.6 Å), with the orientations of any two consecutive etch pits pointing oppositely to each other. In CDTA, etch pits within the half-unit cell were frequently bounded along the 〈120〉 and 〈010〉 directions. However, in EDTA, etch pits within the half-unit cell were bounded along the 〈110〉 and 〈010〉 directions. The dissolution behavior of barite in these two solutions is different based on the observed differences in the etch pits geometries as an assay for specific interactions between the crystal surface and organic molecules. Thus, we suggest that CDTA molecules bind to one Ba2+ cation along the 〈120〉 and/or 〈010〉 directions and EDTA molecules bind along the 〈110〉 directions to two Ba2+ cations exposed on the (001) surface.

Surface morphology of quartz grains from tropical soils and its significance for assessing soil weathering

SummaryThe surface morphology of quartz grains can indicate the degree of weathering of soil material. We have compared two methods of assessing the relative weathering of soils on the basis of differences in the surface morphology of quartz sand grains in a catena of soils in Rwanda. One method is based on the presence or absence of surface features indicative of weathering or freshness, while the other uses the frequency and size of dissolution etch pits. A ranking of relative weathering could be obtained using the first technique for the slightly and somewhat weathered soils but not for the weathered soils. On the other hand, weathering trends and differences between the horizons studied were detected in weathered soils using the second method. The introduction of more specific definitions of the weathering classes used in the latter method leads to clear improvement of the inter‐observer reproducibility of the weathering classification.The surface features on the quartz grains suggest that the soil at the summit is less weathered than the other soils of the sequence. Quartz grains from the well‐drained soils on the slopes, which are subjected to more intense leaching and thus to stronger chemical weathering, have more triangular etch pits and chatter marks. In the imperfectly drained soils in the valley bottom quartz grains are less etched because dissolution is inhibited by the oversaturation in silica of the drainage waters.

Dissolution of Magnesium Oxide in Aqueous Acid: An Atomic Force Microscopy Study

The dissolution of the surfaces (100), (110), and (111) of MgO in aqueous hydrochloric acid is studied by in-situ AFM experiments in a flow cell with known hydrodynamics,which permits the modeling of the rate of proton transport to the solid surface. Comparison with directly measured rates of dissolution determined via monitoring the absolute height of the surface in real time shows that the dissolution of all three surfaces is a surface-controlled reaction. Examination of the surface morphology shows that the (100) plane dissolves via the growth of etch pits which are of circular or square shape depending on the acid concentration. In contrast, the (110) surface dissolves to form a corrugated surface of parallel ridges whose surfaces are predominantly composed of (100) planes. The (111) surface dissolves via triangular etch pits of a fixed orientation where (100) planes are partly re-expressed during the pit growth. These observation are in concordance with previous reports that the (100) plane is the most stable surface of MgO.