Submonolayer Amounts Research Articles

Structural properties and spatial ordering in multilayered ZnMgTe/ZnSe type-II quantum dot structures

We report the structural properties and spatial ordering of multilayer ZnMgTe quantum dots (QDs) embedded in ZnSe, where sub-monolayer quantities of Mg were introduced periodically during growth in order to reduce the valence band offset of ZnTe QDs. The periodicity, period dispersion, individual layer thickness, and the composition of the multilayer structures were determined by comparing the experimental high resolution x-ray diffraction (HRXRD) spectra to simulated ones for the allowed (004) and quasi-forbidden (002) reflections in combination with transmission electron microscopy (TEM) results. Secondary ion mass spectroscopy (SIMS) profiles confirmed the incorporation of Mg inside the QD layers, and the HRXRD analysis revealed that there is approximately 32% Mg in the ZnMgTe QDs. The presence of Mg contributes to higher scattering intensity of the HRXRD, leading to the observation of higher order superlattice peaks in both the (004) and (002) reflections. The distribution of scattered intensity in the reciprocal space map (RSM) shows that the diffuse scattered intensity is elongated along the qx axis, indicating a vertical correlation of the dots, which is found to be less defined for the sample with larger periodicity. The diffuse scattered intensity is also found to be weakly correlated along the qz direction indicating a weak lateral correlation of the dots.

Comment on “HCl adsorption on ice at low temperature: a combined X-ray absorption, photoemission and infrared study” by P. Parent, J. Lasne, G. Marcotte and C. Laffon, Phys. Chem. Chem. Phys., 2011,13, 7142

On the basis of NEXAFS, photoemission and FTIR spectra of ice films with low doses of adsorbed HCl, the authors of the PCCP paper "HCl adsorption on ice at low temperature: a combined X-ray absorption, photoemission and infrared study", Phys. Chem. Chem. Phys., 2011, 13, 7142, have come to conclusions regarding the behavior of submonolayer amounts of HCl at 50 K that contradict published results of the authors of this Comment. Our purpose is to argue that the conclusion, attributed going forward to PLML (authors' initials), that nearly 100% of HCl ionizes for dosage levels near to 0.16 monolayer (ML) or 0.3 Langmuir (L) at 50 K is questionable. Rather, we reaffirm our conclusions of much lower levels of ionization for similar temperatures and HCl dosages based on reactive ion scattering (RIS) and low energy sputtering (LES) data for ice films and FTIR spectra of ice nanocrystals. A second current paper by Ayotte et al., J. Phys. Chem. A, 2011, 115, 6002, that largely parallels in method and results the RAIR spectroscopy of PLML, is also given special notice.

Towards a detailed in situ characterization of non-stationary electrocatalytic systems

A complementary combination of cyclic voltammetry, impedance spectroscopy and quartz crystal microbalance techniques was used to: (i) control the assembly of a model electrocatalytic system consisting of monolayer and sub-monolayer amounts of Ag and Pb on a Au electrode, (ii) evaluate the system performance for the reduction of NO(3)(-) and (iii) study the disassembly of the electrocatalytic system to explore any changes which occurred during the assembly and/or catalytic stages. Physical models of the electrochemical interface (described in terms of equivalent electric circuits) at all stages are found to be considerably different but consistent with each other. Deposition of the Ag atomic layer on Au is accompanied by spontaneous surface alloying and specific adsorption of anions. In the following, deposition of the Pb atomic layer triggers further alloying in the Ag(ad)/Au layer while perchlorate-ions leave the surface. Approximately 1/3 of the Pb atomic layer on Ag(ad)/Au was found to demonstrate the best activity towards nitrate reduction. The developed experimental approach shows promise for the in situ characterization and control of all the non-stationary stages which are usually of particular importance in electrocatalytic research.

Dimer-T3reconstruction of the Sm/Si(100)(2×3) surface studied by high-resolution photoelectron spectroscopy and density functional theory calculations

Silicon dimer--containing reconstructions on Si(100) can be induced by submonolayer amounts of rare earth (RE) metals. The tilt of dimer bonds in such reconstructions can be controlled by the coverage and electronic properties of RE adsorbates. In this study, we have utilized improved high-resolution photoelectron spectroscopy with the synchrotron radiation and density functional theory (DFT) calculations to exploit the structural and electronic properties of the Sm/Si(100)(2 $\ifmmode\times\else\texttimes\fi{}$ 3) system. A careful analysis of photoelectron spectra, in combination with DFT calculations of surface core-level shifts for silicon atoms in energetically plausible structural models, has allowed us to establish the favorable atomic configuration of Sm/Si(100)(2 $\ifmmode\times\else\texttimes\fi{}$ 3) with a buckled Si dimer and to explain characteristic features of Si 2$p$ line shape in detail. It is shown that the dimer buckling leads to a significant core-level binding-energy splitting of the first-layer Si atoms, affecting the lower-binding-energy region of Si 2$p$ spectra drastically. An interpretation of the Si 2$p$ line shape for RE/Si(100)(2 $\ifmmode\times\else\texttimes\fi{}$ 3) that is based on combined initial state and complete screening data is suggested. The mechanism underlying the buckling and symmetrization of silicon dimers in RE/Si(100) reconstructions is discussed.

Inversion boundary induced grain growth in ZnO ceramics: from atomic-scale investigations to microstructural engineering

In semiconducting materials special boundaries play the key role in crystal growth. They introduce an abrupt structural and chemical anisotropy, which is readily reflected in an unusual microstructure evolution, whereas their local structure affects the physical properties of semiconducting materials. These effects, however, can be exploited to tailor the electronic and optical properties of the materials, as demonstrated in this review. The presented topic fits in the field of preparatory stage of phase transformations, manifested through evolution of chemically induced structural faults. In the noncentrosymmetric structure of ZnO, inversion boundaries (IBs) are the most common type of planar faults that can be triggered by the addition of specific spinel-forming dopants (Sb2O3, SnO2, TiO2). In addition to conventional HRTEM techniques several new methods were developed to resolve crystallography and atomic-scale chemistry of IBs. The absolute orientation of the polar c-axes on both sides of the IB was determined by a novel quantitative microdiffraction method, providing a reliable identification of crystal polarity in noncentrosymmetric crystals. To determine sub-monolayer quantities of dopants on the IB, we developed a special technique of analytical electron microscopy using concentric electron probe (CEP) in EDS or EELS mode, providing more accurate and precise results than any other available technique. Knowing the local crystal chemistry of IBs we were able to design experiments to identify their formation mechanism. IBs nucleate in the early stage of grain growth as a dopant-rich topotaxial 2D reaction product on Zn-terminated surfaces of ZnO grains. Soon after their nucleation, ZnO is epitaxially grown on the inherent 2D phase in an inverted orientation, which effectively starts to dictate anisotropic growth of the infected crystallite. In very short time the grains with IBs dominate the entire microstructure in ZnO ceramics via IB-induced exaggerated grain growth mechanism. This phenomenon was used to design physical properties of ZnO-based varistor ceramics, whereas the bottom-up approach demonstrated here provides the basic tool for microstructural engineering of functional materials in virtually any system that is prone to formation of special boundaries.

Microstructural engineering of ZnO-based varistor ceramics

In ceramic materials, special boundaries play the key role in crystal growth. They introduce an abrupt structural and chemical anisotropy, which is readily reflected in an unusual microstructure evolution, whereas their local structure affects the physical properties of polycrystalline materials. These effects, however, can be exploited to tailor the electronic and optical properties of the materials, as demonstrated in this review. The presented topic is related to a preparatory stage of phase transformations, manifested through the evolution of chemically induced structural faults. In non-centrosymmetric structure of ZnO, inversion boundaries (IBs) are the most common type of planar faults that is triggered by the addition of the specific IB-forming dopants (Sb2O3, SnO2, TiO2). In addition to conventional TEM techniques, new methods were developed to resolve crystallography and atomic-scale chemistry of IBs. The absolute orientation of the polar c-axes on both sides of an IB was determined by micro-diffraction, providing the most reliable identification of crystal polarity in non-centrosymmetric crystals. To determine sub-monolayer quantities of dopants on the IB, we developed a special technique of analytical electron microscopy using concentric electron probe (CEP) in EDS or EELS mode, providing more accurate and precise results than any other technique. Knowing the local crystal chemistry of IBs, we were able to design experiments to identify their formation mechanism. IBs nucleate in the early stage of grain growth as a dopant-rich topotaxial 2D reaction product on Zn-terminated surfaces of ZnO grains. Soon after their nucleation, ZnO is epitaxially grown on the inherent 2D phase in an inverted orientation, which effectively starts to dictate anisotropic growth of the infected crystallite. In very short time, the grains with IBs dominate the entire microstructure via IB-induced exaggerated grain growth mechanism. This phenomenon was used to design physical properties of ZnO-based varistor ceramics, whereas the bottom-up approach demonstrated here provides the basic tool for microstructural engineering of functional materials in virtually any system that is prone to the formation of special boundaries.

Effects of Varying MBE Growth Conditions on Layered Zn-Se-Te Structures

We report the effects of varying growth parameters on the photoluminescence (PL) and time-resolved photoluminescence (TRPL) emission from layered Zn-Se:Te structures with submonolayer quantities of ZnTe grown by a combination of migration-enhanced epitaxy (MEE) and molecular beam epitaxy (MBE). Dependences on Te effusion cell temperature (T Te source) and “wait time” (duration for which all shutters are closed, t all-off) are investigated to show that these parameters contribute towards changing the size and/or composition of the quantum dots (QDs). Optical studies showed that increasing T Te source resulted in the formation of larger QDs. Increasing t all-off from 0 s to 3 s contributed towards the formation of larger QDs, while further increasing t all-off to 9 s led to desorption of Te adatoms which resulted in smaller QDs. These parameters are easy to control, and thus understanding their effects should help us to gain better control towards obtaining high-quality QDs.

The structure formed by the deposition of a sub-monolayer quantity of platinum onto Cu(110) investigated using medium energy ion scattering

The structure of 0.35 monolayers of platinum deposited onto Cu(110) has been investigated using medium energy ion scattering. Quantitative analysis of the data has been performed using the VEGAS routine. It was found that platinum atoms mostly occupy the second layer with a first interlayer distance of d 12 = 123 ± 4 pm and a separation of first and third layers of d 13= 142 − 10 + 4 pm. These represent a contraction of 4% and an expansion of 11% respectively from the ideal termination of the Cu(110) surface. There is clear evidence of the presence of some platinum in the third layer.

Probing the Interaction of Hydrogen Chloride with Low-Temperature Water Ice Surfaces Using Thermal and Electron-Stimulated Desorption

The interaction and autoionization of HCl on low-temperature (80-140 K) water ice surfaces has been studied using low-energy (5-250 eV) electron-stimulated desorption (ESD) and temperature programmed desorption (TPD). There is a reduction of H(+) and H(2)(+) and a concomitant increase in H(+)(H(2)O)(n=1-7) ESD yields due to the presence of submonolayer quantities of HCl. These changes are consistent with HCl induced reduction of dangling bonds required for H(+) and H(2)(+) ESD and increased hole localization necessary for H(+)(H(2)O)(n=1-7) ESD. For low coverages, this can involve nonactivated autoionization of HCl, even at temperatures as low as 80 K; well below those typical of polar stratospheric cloud particles. The uptake and autoionization of HCl is supported by TPD studies which show that for HCl doses ≤0.5 ± 0.2 ML (ML = monolayer) at 110 K, desorption of HCl begins at 115 K and peaks at 180 K. The former is associated with adsorption of a small amount of molecular HCl and is strongly dependent on the annealing history of the ice. The latter peak at 180 K is commensurate with desorption of HCl via recombinative desorption of solvated separated ion pairs. The activation energy for second-order desorption of HCl initially in the ionized state is 43 ± 2 kJ/mol. This is close to the zero-order activation energy for ice desorption.

Tuning the Activity of Pt(111) for Oxygen Electroreduction by Subsurface Alloying

To enable the development of low temperature fuel cells, significant improvements are required to the efficiency of the Pt electrocatalysts at the cathode, where oxygen reduction takes place. Herein, we study the effect of subsurface solute metals on the reactivity of Pt, using a Cu/Pt(111) near-surface alloy. Our investigations incorporate electrochemical measurements, ultrahigh vacuum experiments, and density functional theory. Changes to the OH binding energy, ΔE(OH), were monitored in situ and adjusted continuously through the subsurface Cu coverage. The incorporation of submonolayer quantities of Cu into Pt(111) resulted in an 8-fold improvement in oxygen reduction activity. The most optimal catalyst for oxygen reduction has an ΔE(OH) ≈ 0.1 eV weaker than that of pure Pt, validating earlier theoretical predictions.

Analysis of thin films and molecular orientation using cluster SIMS

AbstractA study of phenylalanine films of different thicknesses from submonolayer to 55 nm on Si wafers has been made using Bin+ and C60+ cluster primary ions in static SIMS. This shows that the effect of film thickness on ion yield is very similar for all primary ions, with an enhanced molecular yield at approximately 1 monolayer attributed to substrate backscattering. The static SIMS ion yields of phenylalanine at different thicknesses are, in principle, the equivalent of a static SIMS depth profile, without the complication of ion beam damage and roughness resulting from sputtering to the relevant thickness. Analyzing thin films of phenylalanine of different thicknesses allows an interpretation of molecular bonding to, and orientation on, the silicon substrate that is confirmed by XPS. The large crater size for cluster ions has interesting effects on the secondary ion intensities of both the overlayer and the substrate for monolayer and submonolayer quantities. This study expands the capability of SIMS for identification of the chemical structure of molecules at surfaces. © Crown copyright 2010.

ChemInform Abstract: Solar Energy Conversion at the p-InP/Vanadium3+/2+ Semiconductor/ Electrolyte Contact. A Study Based on Differential Capacitance and Current-Voltage Data.

Previously unachieved values are obtained for the open-circuit voltage (0.7 to 0.8 V) and the fill-factor (76%) at the illuminated p-InP/V[sup 3+/2+]-HCl semiconductor/electrolyte contact ( face) coated with a submonolayer amount of silver. Photoelectrochemical solar cell efficiency is 11%, despite significant optical reflection and electrolyte absorption losses. Differential capacitance measurements demonstrate a semiconductor/surface energy barrier height of up to [psi][sub B] = 1.2 eV, independent of the Ag treatment. This value is considerably larger than typically found at solid-state p-InP/Ag contacts. Barrier heights derived from Mott-Schottky data are compared to photovoltages; results indicate that interface recombination and a slow charge-transfer (bare p-InP) and thermionic emission from the metal (Ag-treated p-InP) are important factors limiting the cell efficiency. Flatband potential shifts owing to a slow charge-transfer under photocurrent flow are reduced by the Ag treatment. In a finite potential range of about 400 mV (0 to [minus]400 mV/SCE), both photovoltages and Mott-Schottky data reveal an unpinned Fermi level at bare and Ag-treated electrodes. The unpinned barrier follows the ideal Schottky barrier model for an absolute SCE potential of [minus]5.0 eV/SCE. Reference to related work at InP and GaAs is made.

Differences in adsorption behavior of N3 dye on flat and nanoporous TiO2 surfaces

The adsorption of N3 dye on two types of TiO2 surfaces (rutile single crystals and anatase nanocrystalline films) was studied. We used highly sensitive absorption spectroscopy to acquire absorption spectra of sub-monolayer amounts of dye adsorbed on rutile TiO2 single crystals. We found that absorption spectra were not sensitive to dye amount in single crystals, whereas the peak shifted toward longer wavelength region with increasing dye amount in nanocrystalline films. We also found that much less N3 dye was adsorbed on the nanoporous surface than on the flat (single-crystal) surface. Possible reason for these experimental findings will be discussed.

Structure of ultra-thin Ti film on the Al(001) surface

The atomic structure of sub-monolayer amounts of Ti deposited on the Al(001) surface at room temperature has been investigated using low-energy electron diffraction (LEED) and low-energy ion scattering spectroscopy (LEIS). The Ti coverage was determined using Rutherford backscattering spectroscopy (RBS). Though a crisp LEED image is inherently difficult to obtain, the symmetry of the observed c(2 × 2) LEED images allows us to infer a structure which places Ti atoms in every other Al lattice site. Analysis of the LEIS azimuth- and polar-angle scan spectra has been done to determine the best structural model which supports the c(2 × 2) symmetry of the LEED image as well as LEIS experimental data. It was concluded that the best model consistent with the experimental data, puts Ti preferentially below the surface of the Al substrate at every other lattice site for sub-monolayer coverage of Ti on Al(001). As Ti coverage increases, the presence if Ti atoms in the surface layer also increases. Results of this study are relevant to research pertaining to the possible use of Ti as a catalyst in sodium alanate (NaAlH 4) in hydrogen storage applications.

Oxygen-Induced Thermal Faceting of Pd Nanosized Crystals

The oxygen-induced faceting of [111] and [100] oriented Pd nanosized crystals (“tips”) was studied by field ion microscopy (FIM). Annealing at temperatures of 500 K in the presence of submonolayer amounts of oxygen caused major reconstruction to occur. Regions of the {100} plane broke up into small {100}, {112}, and {012} facets. In addition, only {111} and significantly enlarged {011} facets occurred at the surface of the reconstructed Pd crystal. The faceting behavior is in accordance with recently calculated equilibrium shapes of Pd crystals in the presence of small amounts of adsorbed oxygen.

Erratum: Positronium chemistry of the Ps quenchers adsorbed on to the mesoporous resin

The positron annihilation lifetime studies were carried out on XAD-4 resin loaded with different Ps quenchers in sub-monolayer quantities. The o-Ps intensity and lifetime of the longest lived component, corresponding to positronium annihilation in mesopores, were seen to reduce with adsorption of β-naphthol. In the case of nitroaniline and nitrobenzene adsorption, only the intensity of this component reduced. The studies showed that the physical dimension of the quencher should also be large to manifest as positronium quencher when adsorbed on to the porous materials if the porous material has large surface fractals. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation, electronic structure, and stability of film nanophases of transition metals on silicon

This paper is devoted to the study of the morphology, growth, electronic structure, and stability of ultrathin (0.03–3 nm) Co and Fe films on the Si(111) and Si(100) surfaces using Auger-electron spectroscopy, electron-energy loss spectrometry, low-energy electron diffraction, and atomic-force microscopy. It is shown that layer-by-layer growth of the metal with the formation of the film nanophase and the segregation of a submonolayer amount of Si on the film’s nanophase surface occurs during the process of layer-by layer growth of Co and Fe on Si(111)-7 × 7 and Si(100)-2 × 1 at room temperature after the growth of two-dimensional metal phases (the surface phase, the monolayer, and two metal monolayers). After these stages, the formation and growth of the bulk’s metal phase with the dissolution of silicon segregated before occur. It is shown that the upper layers of Si adjoining the surface phase, the monolayer, and two Co and Fe monolayers have respectively three different densities of the electron plasma that are higher than the density of the electron plasma in the volume of the silicon substrate. The nonmonotonous character of the morphological and chemical stability of Fe films with quantum-size thicknesses on Si(100) is discovered. After annealing, the film is first smooth, then it is nonuniform across its thickness; afterwards it is again smooth and then nonuniform across its thickness. In this case, the metal phase, different Fe silicides, and the bulk’s metal phase form successively in Fe films on Si(100) after annealing.

Impact of Oxygen Coadsorption on Intercalation of Cobalt under the h-BN Nanomesh

The process of penetration of cobalt atoms through the h-BN nanomesh on Rh(111) is investigated with both spectroscopic and microscopic techniques. It is discovered that oxygen coadsorption can drastically modify the physical properties and behavior of the deposited Co clusters upon postannealing. In the absence of oxygen, Co forms small nanoparticles in the pores (bonding parts) of the h-BN nanomesh, which start to agglomerate at elevated temperatures without any considerable intercalation. However, even a tiny amount of coadsorbed oxygen reduces cobalt agglomeration and greatly promotes its intercalation and trapping under h-BN. The oxygen exposure necessary for a complete intercalation of 1-2 monolayers of Co is very low, and the formation of oxidic species can be easily avoided. The nanomesh structure remains intact upon intercalating submonolayer amounts of Co, while further intercalation gradually distorts and finally destroys the periodic corrugation. Fortunately, this process is not accompanied by damaging the h-BN sheet itself, and the original structure can be restored by removing Co upon annealing at higher temperatures.

Conversion of cis- and trans-2-butene with Deuterium on a Pd/Fe 3O 4 model catalyst

Combining molecular beam (MB) techniques, reflection–absorption infrared spectroscopy (RAIRS), and temperature-programmed desorption (TPD), we have investigated the adsorption and kinetics of conversion of cis- and trans-2-butene with deuterium on a well-defined model supported Pd catalyst. Particular emphasis was placed on the quantitative comparison of the reaction rates of the competing cis-trans isomerization and hydrogenation pathways between both steric isomers. It was observed that the reaction temperature critically affects the cis-trans selectivity of 2-butene in both reaction paths. At low-temperatures (195–210 K), cis-2-butene shows higher activity toward both hydrogenation and isomerization reactions han the trans isomer. At temperatures above 250 K, on the other hand, very similar reaction rates were found for both isomers. Remarkably, deposition of sub-monolayer amounts of strongly dehydrogenated carbonaceous species on the Pd particles results in a significant decrease of the cis-trans isomerization rate for trans-2-butene, but does not affect the rate of this pathway for the cis isomer. Additionally, under high-temperature conditions (250–260 K), co-deposited carbon was shown to assist the catalytic and sustained hydrogenation of both cis- and trans-2-butene, both at similar rates.

Growth and Desorption Kinetics of Ultrathin Zn Layers on Pd(111)

A scanning tunneling microscopy (STM) study, combined with density functional theory (DFT) calculations and thermal desorption spectroscopy (TDS) data, on the growth and structure of Zn on a Pd(111) surface is presented. The STM results demonstrate that PdZn(111) surface alloy bilayer islands with a p(2 × 1) structure already form during the room temperature deposition of submonolayer amounts of Zn on Pd(111). DFT calculations predict that the PdZn bilayers are energetically more stable than single PdZn layers and establish their structural parameters. The TDS data show that Zn desorbs below 600 K in a multiple-peak desorption structure with fractional order desorption kinetics of the individual components, which is due to multilayer Zn desorption. The experimentally derived adsorption energies of the low temperature desorption peaks are reproduced in the DFT calculations. At temperatures above 750 K, Zn desorbs from the PdZn alloy, and the desorption kinetics is a mixture of a first-order and a diffusion-limited desorption process.