Atom Chemistry Research Articles

Interfaces e organização da pesquisa no Brasil: da Química à Nanotecnologia

Nanotechnology can be viewed as a powerful tool, capable of shaping the chemistry of atoms and molecules, converting them into exciting nanosized and nanostructured materials, devices and machines. However, in pursuing this task, an exceptional ability is required to deal with complex inter- and multidisciplinary approaches, as imposed by the nanoscale. A new research organization framework, capable of promoting cooperative interactions in many complementary areas, including the industries, is demanded. In this sense, an interesting example are the nanotechnology networks and millenium institutes recently created in Brazil. The highlights and weakness of such cooperative research networks are discussed, in addition to relevant nanotechnology themes focusing on the special needs and resources from the developing nations.

Incorporation of Oxygen Donors in AlGaN

The chemistry of oxygen atoms at the surface of an AlGaN layer for heterostructures was investigated by scanning photoemission microscopy (SPEM) using synchrotron radiation. SPEM imaging and space-resolved photoemission spectroscopy showed that the oxygen atoms were preferentially incorporated into AlGaN rather than GaN due to the high reactivity of Al with oxygen. In situ annealing at 1000°C could lead to the outdiffusion of oxygen impurities from the bulk AlGaN, resulting in a significant increase in the intensity of Al-O bonds at the AlGaN surface. Therefore, it is suggested that the unintentional doping of oxygen impurities in AlGaN could yield a heavily doped n-type AlGaN layer, resulting in a drastic reduction in effective Schottky barrier heights of metal contacts on AlGaN/GaN heterostructures. © 2004 The Electrochemical Society. All rights reserved.

From hot atom chemistry to epithermal chemistry

AbstractThe rise and fall of hot atom chemistry (HAC) over the years from 1934 to 2004 is reviewed. Several applications are discussed, in particular to astrophysics and the interaction of energetic ions and atoms in space. Epithermal chemistry (ETC) is proposed to substitute the old name, since it better fits the energy range as well as the non-thermal and non-equilibrium character of the reactions. ETC also avoids the strong connexion of HAC to nuclear chemistry and stands for the opening of the field to physical chemistry and astrophysics.

Interfacial chemistry and structures of ultrathin Si oxynitride films

Ultrathin SiON films with different nitrogen profiles grown by the plasma-enhanced CVD method and the rapid thermal nitridation (RTN) of SiO 2 with an NO gas have been analyzed by high-resolution angle-resolved photoelectron spectroscopy using bright synchrotron radiation to investigate interfacial chemistry and in-depth distribution of nitrogen atoms based on the second nearest neighbor effect of N 1s chemical shift. It is found that the CVD-deposited SiON film has a three-layer structure consisting of homogeneously-distributed N atoms in the [Si-Si 3− x N x ] 3N chemical state, N atoms in the (SiSi 3− x O x ) 3N chemical state of about two monolayers, and the top SiO 2 layer. In contrast, N atoms in the NO-RTN SiON film exist at the SiON/Si interface as a double layer consisting of the [Si-Si 3− x N x ] 3N lower layer and the (SiSi 3− x O x ) 3N upper layer with the concentration of 3.9×10 14 and 1.7×10 14 cm −2, respectively, based on the N 1s chemical shift of about 0.6 eV.

Interaction of alkali atoms with water multilayers adsorbed on TiO 2(1 1 0): a study with MIES and UPS

The chemistry of alkali atoms (Li, Na, K, Cs) embedded in a multilayer aqueous environment was studied with metastable impact electron spectroscopy (MIES) and ultraviolet photoemission spectroscopy (UPS) (HeI and II) under ultra high vacuum (UHV) conditions. The water multilayers were grown at 130 K on a rutile, 1×1 reconstructed, TiO 2(1 1 0) single crystal. The behavior of the multilayer system was investigated as a function of the temperature (130–500 K). Due to the relatively large escape depth of the emitted electrons UPS provides spectroscopic information about several layers in contrast to MIES which is only sensitive to the outermost layer. This allows us to discriminate between species adsorbed at the water multilayer and species, which are embedded in the solvent or at the substrate–solvent interface. Furthermore, MIES is, in contrast to UPS, very sensitive to the outermost s-states of adsorbed alkali atoms, which are considered to be responsible for the high reactivity of these metals. The present study gives insight into the complicated chemistry of alkali atoms added to an aqueous multilayer system. The chosen combination of MIES and UPS allows us to distinguish clearly between various phases depending on the amount of offered alkali atoms. For low alkali concentrations the alkali atoms penetrate the water surface whereby they dissociate some water. With increasing exposure more and more water molecules become dissociated, whereby the outermost water layer remains intact. Finally, the chemistry between water and alkali atoms takes place at the outermost surface too, which is manifested by the formation of OH-groups at the surface. With further increasing alkali concentration the atoms start to adsorb as neutral atoms; whereby the spectrum of the alkali species is then strongly influenced by the underlying solvent system. For very high exposure the observed spectra are not influenced by the underlying aqueous system anymore; the formation of an alkali metal film is observed. Under annealing, the entire multilayer system desorbs around 200 K; a titania surface covered by hydroxyl groups and alkali ions is remaining. This adlayer desorbs under further annealing above room temperature (around 400 K, but dependent on the particular alkali atom) resulting in a titania surface.

The chemistry of alkali atoms on solid water: a study with MIES and UPS

The chemistry of Na atoms in a multilayer aqueous environment was studied with metastable impact electron spectroscopy (MIES) and ultraviolet photoemission spectroscopy (UPS) under UHV conditions. Films of solid water were grown at 130 K on a rutile, 1×1 reconstructed, TiO 2(1 1 0) single crystal, and exposed to Na. The Na–water interaction was studied between 10 and 130 K. At small exposures, Na-induced water dissociation takes place whereby the Na gets ionized. Initially the outermost water layer remains intact. The number of OH species seen at the surface increases with exposure; finally, no molecular water, but only OH species can be detected with MIES and UPS. At sufficiently large exposures Na atoms are detected at the surface. We find that efficient water dissociation and Na solvation take place only above about 100 K. Under annealing, the mixed film is destroyed around 200 K; the titania surface is then covered by Na + and hydroxyl groups which desorb around 400 K.

Gas-phase chemistry of atmospheric Cl atoms: a PLP-RF kinetic study with a series of ketones

The pulsed laser photolysis-resonance fluorescence technique (PLP-RF) has been used to determine the absolute rate coefficients for the gas-phase reactions of Cl atoms with a series of ketones at room temperature (298 K). The rate coefficients obtained (in units of cm 3 molecule −1 s −1) are: 2-butanone (3.30±0.20)×10 −11, 2,3-butadione (4.92±0.44)×10 −13, 2-pentanone (4.57±0.28)×10 −11, 3-pentanone (4.50±0.32)×10 −11, 2-hexanone (6.54±0.58)×10 −11, 3-hexanone (6.69±0.62)×10 −11 and cyclohexanone (6.75±0.52)×10 −11. The measured values were independent of pressure over the range 20–200 Torr. These rate coefficients were compared with previous studies carried out by different techniques, when data were available. Results and atmospheric implications are discussed and compared with the reactivity with OH radicals.

Synthesis of nano-sized metal oxide powders and their application in separation technology

Nano-sized inorganic oxides that find application in various fields (such as, in catalysis, sensors, pigments, membranes etc.) are also reported to be excellent reagents in various separation technologies through selective adsorption. Depending on the chemistry of the constituent atoms, they are known to exhibit remarkable adsorption properties due to their enhanced surface area and large interface volume. The nano-sized inorganic oxides can be produced by various reported physical and chemical methods. In general, the chemical methods offer the advantages of easy preparation of nearly any composition, maintaining compositional homogeneity and high purity. However, the chemical routes generally involve complex techniques and an improved level of skill is required to realize the benefits. We have been successful in standardizing the preparation of a variety of nano-sized inorganic oxides through thermolysis of a polymeric based aqueous precursor solution of the desired inorganic ions. The developed method is less cumbersome, more versatile, more cost effective and hence economically more viable than the existing ones for large-scale production of nano-sized inorganic oxides. In this paper we present the preparation of nano-sized powders of Fe 3 O 4 , Al 2 O 3 and ZrO 2 . The obtained nano-sized oxide powders were sonicated in water and incorporated in the matrix of activated charcoal through adsorption. The oxide incorporated charcoal materials have been used as the adsorbing bed for the removal of trace amounts of fluoride/arsenite and arsenate ions from industrial wastewater and aldehyde from the perfume grade alcohols. The oxides have been prepared through thermolysis of precursor solutions that contained the respective cations homogeneously dispersed in the polymeric matrix of sucrose and polyvinyl alcohol. Calcination of the fluffy, carbonaceous precursor powder at 200-500°/2 h resulted in the desired inorganic oxide phase with particle diameters ranging between 20 and 38 nm and BET surface area ranging between 120 and 200 m 2 /g. The charcoal embedded fine powders of the inorganic oxides have been able to remove fluoride/ arsenite and arsenate ions from industrial wastewater upto 0.01-0.02 ppm levels. Many of the perfume grade alcohols are produced by reduction of aldehyde by M.P.V. reduction using aluminum alkoxid or catalytic reduction using hydrogen. However, a ppm amount of residual aldehydes are perpetually retained in the alcohol even through the most efficient reduction by the mentioned process and changes the flavour of the perfume formulations. Here we report an efficient separation of cinnamaldehyde from cinnamyl alcohol using nano-sized Fe 3 O 4 powders. For this, the adsorbing bed has been further modified by impregnating it with 3-aminopropyltriethoxysilane in ethanol medium. The amino groups impregnated in the matrix readily undergoes reaction with the adsorbed aldehydes and results in the formation of Schiff bases thereby reducing the concentration of residual aldehyde contained in the alcohol to levels <0.2 ppm. The prepared adsorbent has been also tested for the removal of heptaldehyde from heptanol and octaldehyde from octanol. The major advantage of the process is that the adsorbent and the adsorbed aldehyde can be easily separated by applying an external magnetic field, thus avoiding the filtration process.

A Model of New VUV NLO Materials Based on Borate: A Novel Noncentrosymmetric Borophosphate Compound Be3BPO7

ABSTRACTA favorable borate anionic group can be selected as the basic structural unit for VUV NLO materials, in terms of the properties of known inorganic NLO crystals and the features of their anionic groups, based on anionic group theory from SHG coefficients, UV absorption edge and moderate birefringence. Considering the coordination chemistry of atoms in some inorganic compounds, and the relationship between a noncentrosymmetric structural arrangement and the composition of compounds, a suitable theoretical model that aids the search for new VUV NLO materials has been proposed. The model compounds, formulated as MxA1,2(BO3)1,2Oy, may be prospective candidates for new VUV NLO materials considering their SHG coefficients, UV absorption edge, and moderate birefringence. The model suggests that investigation for a new VUV NLO material would be profitably conducted by focusing on boroberylate, boroaluminate, and borophosphate, on the basis of anionic group theory. For example, a novel noncentrosymmetric beryllium borophosphate compound, Be3BPO7 was synthesized by solidstate reaction. Microcrystalline Be3BPO7 has a hexagonal system and contains the BO3 anionic group as its basic structural unit, which should play an important role for SHG coefficients. In fact, Be3BPO7 powder was found to have a significant SHG effect.

Non-methane hydrocarbons in the Arctic boundary layer

C 2–C 7 non-methane hydrocarbons were measured in clean maritime boundary layer air at latitudes between 53°N and 81°N. Measurements were made as part of the Arctic ice and environmental variability cruise aboard RRS James Clark Ross using a high sensitivity automated gas chromatography system. The data were collected during summer 1999, a period of continuous Arctic sunlight. Hydrocarbons of anthropogenic origin were observed to decrease in concentration with increasing latitude, a combination of dispersion and extensive atmospheric chemical processing. At high latitudes, low boundary layer conditions were common and species of exclusively anthropogenic origin reached highly stable although non-zero values (e.g. acetylene 27.8±2.4 pptV). A number of hydrocarbons believed to be of oceanic origin showed wide variability in these regions of atmospheric boundary layer stability (average ethene=37.2±20.9 pptV), highlighting inhomogeneity in the ocean to atmosphere flux rates. Whilst substantial increases in biological activity and productivity in the marginal ice zone have been previously reported, no evidence for increased biogenic hydrocarbon emissions were observed during this cruise. Removal mechanisms for atmospheric hydrocarbons were generally dominated by OH chemistry. Under clean maritime conditions the observed iso-/ n pentane and butane ratios were in good agreement with published sea-fluxes. Using the relative ratios of iso-/ n-butane and iso-butane/propane, there was evidence of significant chlorine atom chemistry at these high latitudes.

Chemistry in grain aggregates: a source of complex molecules?

ABSTRA C T The aggregation of grains in dense protostellar clouds brings together materials such as silicates, carbons, polycyclic aromatic hydrocarbons and ices to form porous structures with high internal volume. Some physical and chemical properties of these aggregate grains are discussed in the context of the role that they may play in the formation of complex organic and organometallic compounds. One characteristic of such grains that is unique outside planetary systems is the availability of all elements and a number of their common compounds in a composite solid. In dark clouds, the chemistry inside aggregate grains will be driven by cosmic ray heating and sputtering. This occurs in an environment where the products of such reactions can be retained within the dust particle. Hot atom chemistry and secondary reactions are facilitated by the re-entrant nature of such aggregated structures, leading to the possible formation of complex organic compounds. In particular, the sputtering of Si, Mg and Fe from silicate dust is discussed, and it is shown that a variety of organometallic compounds could be expected in ices within aggregate grains. The optical depth for ultraviolet light within aggregates is large, so that materials inside such grains will be effectively shielded from ambient radiation. However, the incorporation of luminifors such as those grain components responsible for the extended red emission converts ultraviolet to visible and near-infrared radiation, and might moderate photochemistry within aggregates. It is suggested that the chemical environment within aggregates may be conducive to the formation and retention of complex molecules such as amino acids, peptides and a variety of organometallic compounds.

First‐Principles Simulations of Molecular Electronics

This paper gives an overview of recent work by the authors in first-principles, parameter-free calculations of electronic transport in molecules in the context of experimental measurements of current-voltage (I-V) characteristics of several molecules by Reed et al. The results show that the shape of I-V characteristics is determined by the electronic structure of the molecule in the presence of the external voltage, whereas the absolute magnitude of the current is determined by the chemistry of individual atoms at the contacts. A three-terminal device has been simulated, showing gain. Finally, recent data that show large negative differential resistance and a peak that shifts substantially as a function of temperature have been accounted for in terms of rotations of ligands attached to the main molecule, a phenomenon that is not present in semiconductor nanostructures.

SPECTRAL PROPERTIES OF CONFINED ATOMS

The changes in energy and photoionization spectra of atoms upon confinement by a spherical environment are explored theoretically. Two kinds of confinement are considered: an endohedral confinement, such as inside the bucky-ball C 60, and an impenetrable spherical confinement of adjustable radius. We demonstrate modifications in the energy spectrum and electron correlation effects in confined atoms, the appearance, nature and origin of "confinement" resonances in photoionization spectra of such atoms, as well as new regularities in the periodic table for "compressed" atoms. These findings are of importance for basic and applied physics and chemistry of atoms, molecules, surfaces, etc.

Impact of a major meteor storm on Earth's ionosphere: A modeling study

A comprehensive model of the effect of a major meteor storm on Earth's ionosphere is presented. The model includes meteor stream mass distributions based on visual magnitude observations, a differential ablation model of major meteoric metals, Fe and Mg, and state‐of‐the‐art modeling of the chemistry and transport of meteoric metal atoms and ions subsequent to deposition. Particular attention is paid to the possibility of direct ionic deposition of metallic species. The model is validated by calculating the effect of annual meteor showers on the background metal atom and ion abundances. A metallic ion density increase of up to 1 order of magnitude is observed, in agreement with in situ measurements during showers. The model is exercised for a hypothetical Leonid meteor storm of the magnitude reported in 1966. The model predicts the formation of a layer of metal ions in the ionospheric E region that reaches peak densities of around 1 × 105 cm−3, corresponding to a 2 order of magnitude increase of the quiescent nighttime E region density. Although sporadic E layers reaching or exceeding this density are relatively common, the effect is different in that it persists on the order of days and would be observed over nearly one‐half the globe. The model predictions are consistent with the available 1966 Leonid storm data. In particular, the observation of enhanced, predawn sporadic E activity points to efficient collisional ionization of meteoric metals, as assumed in the model.

Unique markers of chlorine atom chemistry in coastal urban areas: The reaction with 1,3‐butadiene in air at room temperature

Chlorine atoms are generated in the marine boundary layer, including coastal urban areas, through heterogeneous reactions of sea‐salt particles. However, there are no measurements of tropospheric chlorine atoms, and few measurements of their precursors due to the difficulty in specifically identifying and measuring parts per trillion levels of these highly reactive species. We report here studies which suggest an alternate approach for elucidating the role of chlorine atoms in the chemistry of coastal urban areas: the identification and measurement of unique chlorine‐containing products from the reactions of Cl with organics, in this case 1,3‐butadiene, in air. The reaction of chlorine atoms with 1,3‐butadiene (19–61 ppm), which is emitted by motor vehicles, was studied at 1 atm pressure in air at room temperature in both the absence and presence of NO (0–20 ppm). Cl2 (4–16 ppm) was photolyzed to generate chlorine atoms, and the products were identified and measured using gas chromatography‐mass spectrometry (GC‐MS) and long path (40 m) Fourier transform infrared spectroscopy (FTIR). We identified 4‐chlorocrotonaldehyde (CCA) as a unique chlorine‐containing product from the Cl atom reaction with 1,3‐butadiene in both the presence and absence of NO. The yield of CCA was (29±7)% (2σ) in the absence of NO and (65±20)% (2σ) in its presence. The yield of chloromethyl vinyl ketone (CMVK), another chlorine‐containing compound from this reaction, was measured to be (19±4)% (2σ) in the absence of NO; it was not observed in its presence. The reaction system was modeled using ACUCHEM to clarify the reaction paths and to allow extrapolation to ambient air conditions. These studies predict that CCA will be formed in coastal urban areas in the early morning hours at concentrations of ∼100 ppt, which should be readily detectable by GC‐MS and GC with electron capture detection (ECD). Some studies were also carried out on the isoprene reaction, and some potential unique products of its reaction with chlorine atoms are suggested as candidates for field investigations.

Molecular electronics by the numbers

The paper gives an overview of recent work by the authors on first-principles, parameter-free calculations of electronic transport in molecules in the context of experimental measurements of current–voltage ( I– V) characteristics of several molecules by Reed et al. The results show that the shape of I– V characteristics is determined by the electronic structure of the molecule in the presence of the external voltage whereas the absolute magnitude of the current is determined by the chemistry of individual atoms at the contacts. A three-terminal device has been modeled, showing gain. Finally, recent data that show large negative differential resistance and a peak that shifts substantially as a function of temperature have been accounted for.

Im Memoriam Alfred P. Wolf

Dr. Alfred P. Wolf, Al, died on 17 December 1998. A great scientist has left us, whose 47 years´ service in the field of radioisotope applications in organic chemistry, hot atom chemistry, and radiopharmaceutical chemistry had a tremendous impact on applied radiochemsitry and nuclear medicine.

Cl and Br atom concentrations during a surface boundary layer ozone depletion event in the Canadian High Arctic

C2‐C5 hydrocarbons were measured in situ at Alert, Nunavut, Canada from April 14 to May 10, 1998 at a minimum frequency of 12 samples per day. During the sampling period, an event occurred where the O3 mixing ratio fell from about 15 ppbv to the instrumental detection limit of less than 1 ppbv in a timespan of about 24 hours. Mixing ratios of alkanes and ethyne decreased concurrently with the O3 decrease. Ancillary data suggested that this was essentially due to chemical reactions only, most probably involving halogen atoms. The hydrocarbon concentrations from the onset of O3 decrease until its virtually complete depletion are used in a first order kinetics analysis to calculate the concentrations of Cl and Br atoms that are required to explain these observations. The Cl atom concentration is calculated to be 7.5 × 104 molec.cm−3. Such a concentration is near the upper limit of earlier estimates, but cannot explain the observed decay of ethyne. Assuming the deficit to be due to bromine atom chemistry, the estimated Br atom concentration is 1.4 × 107 molec.cm−3. The ethyne mixing ratio continued to decrease after virtual total O3 depletion suggesting that Br atoms persisted when O3 was depleted. Hence it is postulated that the depletion of total gaseous mercury (TGM) which terminates upon virtual complete O3 depletion is due to a reaction between gaseous mercury and the BrO molecule.

Hot Atom Chemistry: Its Classical Studies and Modern Variations

Hot atom chemistry in the past and at present is reviewed, and its future direction is considered. Though it has still important meanings in nuclear and radiochemistry studies, new ideas to improve the present situation are expected to emerge.

The Rise and Fall of Hot Atom Chemistry

Hot Atom Chemistry is presented as a field which has been unable to define its border and to solve its own problems.