Cesium Ion Bombardment Research Articles

Energy and Temperature Dependence of the Secondary Ion Emission of GaN/SiC Clusters under Cesium-Ion Bombardment

Energy and Temperature Dependence of the Secondary Ion Emission of GaN/SiC Clusters under Cesium-Ion Bombardment

Growth and chemical modification of silicon nanostructures templated in molecule corrals: Parallels with the surface chemistry of single crystalline silicon

Molecule corrals having diameters of 30–50 nm were created on highly oriented pyrolytic graphite (HOPG) using cesium ion bombardment. The molecule corrals were used as templates to grow silicon nanostructures by physical vapor deposition (PVD). The nanostructures could be grown with control over geometry (rings and mesas), and size distribution. In addition, transmission electron microscopy (TEM) results suggest that the silicon nanostructures are most likely polycrystalline.The chemical modification of these silicon nanostructures with nitrobenzene was compared to that of clean and hydrogen-terminated single crystalline silicon. X-ray photoelectron spectroscopy (XPS) of the modified nanostructures showed peaks located at 398.9 eV, 400.4 eV, and 402.1 eV for the N 1s region, which are consistent with those observed on a Si(100) single crystal. The chemical modification was further characterized by the presence of nitrogen-containing peaks in TOF-SIMS spectra. We conclude that the reaction of nitrobenzene on silicon nanostructures provides evidence that the reactivity of the nanostructures is similar to that of hydrogen-terminated Si(111) and Si(100).

Molecular depth profiling of model biological films using low energy monoatomic ions

As time-of-flight secondary ion mass spectrometry (ToF-SIMS) is increasingly used to characterize biological samples, fundamental studies on model samples are needed to better understand the interactions between primary ions and biological matter, and to help interpreting the complex ToF-SIMS data. In this work, model biological films were prepared by thermal evaporation and were characterized by XPS and ToF-SIMS. Eight biomolecules were carefully chosen to cover important physical and chemical properties such as aromaticity and oxygen content. The model films allowed studying systematically the sputter rates, ion yields and amenability to molecular depth profiling when using low energy Xe+ and Cs+ etching ions. The results show that aliphatic molecules such as cysteine and aspartic acid can be easily depth profiled using low energy monoatomic primary ions. However, aromatic samples such as phenylalanine could only be depth profiled using reactive Cs+ ions. The systematic study also revealed that the presence of oxygen strongly reduces the deprotonated ion signal during cesium ion bombardment. Finally, model multilayer structures were depth profiled using 500eV Xe+ and Cs+ primary ions with depth resolutions lower than 10nm. Isotopic variations of the same biomolecule were used for the first time to create multilayers structures with constant ion yields and etching rates.

The fate of the (reactive) primary ion: Sputtering and desorption

Accurate modeling of the enhancement of the ionization probabilities of secondary ions (positive and negative) by the presence of reactive species requires a precise determination of their concentration at the outermost surface and thus a fundamental understanding of their incorporation and release processes. In the present paper we discuss these processes for oxygen, cesium and cluster ion bombardment with a special emphasis on their importance at (very) low bombardment energies. At these energies the sputtering processes becomes very small and deposition instead of erosion is predicted while for some species such as oxygen and cesium, an additional process, i.e. desorption, becomes more important thereby enabling a stationary state without deposition. The desorption process is a property of the primary ion and element specific and can be predicted based on the sputter yields and heat of sublimation. For those species/conditions where desorption is not operational (such as for C 60-beams), depth profiling at very low energies and with low sputter yields (<1) will not be possible and accumulation of the primary ion (thin film growth) is observed. The absence of deposition under oxygen, cesium and (SF 5) n -bombardment and the positive impact of oxygen flooding on C 60 bombardment can all be explained when the desorption mechanism is included in the balance equation.

Accuracy of calibrated depth by delta-doped reference materials in shallow depth profiling

The influence of the initial surface roughness and the structure (crystalline/amorphous) of delta-doped reference materials (fabricated with molecular beam epitaxy (MBE) and magnetron sputtering) for depth-scale calibration of secondary ion mass spectrometry (SIMS) depth profiles on the sputtering rate and shift length under low-energy ion bombardment was investigated. The influence of the properties of the reference materials on the sputtering rate under normal oxygen ion bombardment is negligible, and the depth scale can be calibrated accurately using the sputtering rate. However, the shift length under normal oxygen ion bombardment and the sputtering rate under oblique cesium ion bombardment depend on the properties of the reference materials.

Cesium ion sputtering with oxygen flooding: Experimental SIMS study of work function change

We investigated the work function (WF) change of a silicon surface being under cesium ion bombardment and simultaneous oxygen flooding with various oxygen pressures at the sample surface. It was found that WF of Cs + ion sputtered Si decreases under oxygen flooding. This decrease provides an essential grow of secondary ion yields of some negative ions, sputtered from Si. At the same time Si − ion yield decreases approximately in two times. In the paper we have discussed possible explanations of our experimental data: we considered a surface composition change, formation of surface dipoles and work function change caused by oxygen adsorption, and their relationships between each other.

Bombardment-induced ripple topography on GaAs and InP

Bombardment-induced ripples have possible application in nano-technology as nano-wires. This paper reviews some of the published experimental data of ripple formation on GaAs and InP resulting from noble gas, oxygen, nitrogen and cesium ion bombardment. The dependences of the ripples and their corresponding wavelengths on substrate temperature, areic dose and (to a lesser extent) angle of incidence were considered. The experimental results were tested against the predictions of the Bradley–Harper theory [J. Vac. Sci. Technol. A 6 (1988) 2390]. Surprisingly good agreement was obtained between the experimental data and this theory. This is probably due to the amorphisation of the substrates under ion bombardment and, thereby, minimising the effect of crystallographic-orientated self-diffusion on metals, and of energy barriers (Ehrlich–Schwoebel barriers) to interlayer surface diffusion. From the data of MacLaren et al. [J. Vac. Sci. Technol. A 10 (1992) 469], the activation energy for surface self-diffusion of GaAs in the temperature range 60–100 °C was determined as 0.26 eV.

Structural investigation of cyclic peptidolipids from Bacillus subtilis by high-energy tandem mass spectrometry.

The natural products belonging to the surfactin family are cycloheptapeptides bearing a long beta-hydroxy-fatty acyl chain at the N-terminal position. The structure of these compounds, often isolated as complex mixtures, can be elucidated by high-energy tandem mass spectrometry (MS/MS). The protonated molecules generated by cesium ion bombardment (LSIMS) undergo charge-proximate fragmentations leading to the b- and y-type ion series useful for the sequence determination. The sodium-cationised molecules show a radically different behaviour towards high-energy collisional activation. Besides the well-known charge-remote dissociation products of the alkyl side chain, complete series of d- and w-type fragments allow easy distinction between leucine and isoleucine. The complementary MS/MS data obtained from the protonated and cationised molecules prove to be of great interest for the structural characterisation of this type of compounds.

Charge-remote molecular hydrogen removal in protonated and alkali-cationized long-chain fatty acid esters upon cesium ion bombardment

The mass spectral behavior of cationized saturated fatty acid derivatives has been studied in order to gain insight into the loss of molecular hydrogen during cesium ion bombardment. It is shown that molecular hydrogen and hydrogen radical loss occurs for protonated and alkali (Li +/Na +)-cationized fatty acid methyl esters as well as for protonated acylcarnitines and that molecular hydrogen loss is dependent upon the acyl chain length. Investigation of ion structures by collision-induced dissociation tandem mass spectrometry indicates that in the case of alkali-cationization a hydrogen radical is lost from all positions of the saturated acyl chain, whereas in the case of protonated molecules a hydrogen radical is predominantly eliminated from the protonated ester group. For the primary reaction of the dehydrogenation chemistry, an ion-beam-induced excitation of a cationized molecule is proposed, yielding a diradical species that gives rise to an intermolecular reaction with a neutral analyte molecule. Subsequent cationization of the neutral [M–H 2] species formed in this primary reaction leads to the formation of the satellite ion at m/ z values 2 u lower compared to the cationized molecule.

ChemInform Abstract: Structures of Gas Phase Oligomeric Gold‐Oxygen‐Hydrogen Negative Ions Formed by Cesium Ion Bombardment of Vapor‐Deposited Gold Surfaces.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Structural characterization of a new class of penem beta-lactam antibiotics by triple quadrupole tandem mass spectrometry.

A series of new penem beta-lactam antibiotics, having at position 2 of the thiazoline ring an amido-substituted amino acid moiety spaced by a methylene group, was analyzed by collisionally activated dissociation (CAD) tandem mass spectrometry on a triple-quadrupole mass spectrometer, after ionization by cesium ion bombardment. This approach allowed complete structural characterization without any matrix interferences. All the compounds investigated underwent beta-lactam ring cleavage through a typical retro-Diels-Alder reaction, as well as cleavage of the 2-side chain at two different points, with charge retention on both sides of the molecule. Subsequent fragmentation reactions essentially involved combinations of the above processes.

A Novel Rectilinear Negative Carbon Ion Beam Source for Large-Area Amorphous Diamond Like Carbon Coatings

AbstractA novel rectilinear negative carbon ion beam source for large-area coatings has been developed, based on SKION's Solid State Ion Beam Technology. The negative carbon ion beam is effectively produced by a primary cesium ion bombardment and the secondary negative carbon ion yield has been observed to be about 0.5. The ion source produces a negative carbon ion current density of 0.25 mA/cm2 at the extraction voltage of 4 kV. The ion beam energy can be independently controlled from 0 eV to 300 eV. Due to the rectilinear geometry for the production of ion beams, the scale-up of the ion beam in length direction can be easily obtained with no limit. Furthermore, the ion source uses no gas discharge to generate ion beams and does not use any hydrogen gas. The ion source can be operated in a high vacuum (&lt;10-7 Torr), and the cesium vapors are filtered and recirculated. The ion source produces ultra-hard (50 GPa), atomically smooth (&lt; 1 nm Ra), and hydrogen-free amorphous diamond-like-carbon (DLC) films over large areas.

Structures of Gas Phase Oligomeric Gold-Oxygen-Hydrogen Negative Ions Formed by Cesium Ion Bombardment of Vapor-Deposited Gold Surfaces

Structures of Gas Phase Oligomeric Gold-Oxygen-Hydrogen Negative Ions Formed by Cesium Ion Bombardment of Vapor-Deposited Gold Surfaces

Low-energy collision-induced dissociation of deprotonated dinucleotides: determination of the energetically favored dissociation pathways and the relative acidities of the nucleic acid bases

Fourier transform ion cyclotron resonance mass spectroscopy has been used to examine the collision-induced dissociation pathways of all 16 of the possible deprotonated dinucleotides. These quasimolecular ions were generated by cesium ion bombardment of a mixture of triethanolamine, ammonium hydroxide and the dinucleotide. Collisional activation using continuous off-resonance excitation permits observation of energetically-favorable dissociation pathways. Dissociation products were examined over the range of center of mass energies from 0 to the minimum energy required to bring about complete dissociation of the reactant ion which did not exceed 7.7eV for deprotonated parent ions and 8.8eV for fragment ions in any of the systems. Semiempirical calculations were performed using the PM3 method, a variant of the AM1 method, to obtain gas-phase model structures and energies of the deprotonated dinucleotides and their collision-induced dissociation fragments. The acidities of the nucleic acid bases and dimethyl phosphate were calculated using the AM1 method. The deprotonated quasimolecular ions dissociate to yield several characteristic products. The major products formed in all systems are the deprotonated 5′-terminus base, the ion resulting from loss of the neutral 5′-terminus base, or the metaphosphate anion, PO − 3. Insight into the relative stabilities of the fragment ions is gained by comparing the product distributions observed in each of the systems. The relative yields of products involving either the 3′- or 5′-end of the molecule suggest the 3′-terminus base is stabilized through hydrogen bonding interaction with the phosphate group. The relative strength of this stabilization follows the order guanine > thymine > cytosine > adenine. Additionally, the relative abundances of the deprotonated nucleic acid fragments suggest that the relative acidities of the nucleic acid bases follow the order adenine > thymine > guanine > cytosine. Only minor yields of sequence ions in which one of the phosphate diester linkages is cleaved are observed with these quasimolecular ions. Reaction mechanisms which account for the observed products are proposed.

Investigation of some β‐lactam antibiotics by means of 2–35 keV cesium ion bombardment

AbstractA number of commercial β‐lactam antibiotics are investigated by means of cesium ion (Cs+) bombardment in the kinetic energy range 2–35 keV. The relative intensities of the protonated molecules, as well as some of the major fragment ions, are monitored as a function of the Cs+ kinetic energy. The dependence of the intact protonated molecule:fragment ion ratios on the primary‐ion kinetic energy is evident in some of the spectra investigated. An attempt is made to interpret these results in terms of existing secondary‐ion mass spectrometry data which have been obtained in a different kinetic energy range.

Structure of the human adult hemoglobin minor fraction A1b by electrospray and secondary ion mass spectrometry. Pyruvic acid as amino-terminal blocking group

Hemoglobin A1b is a minor hemoglobin component from human hemolysate (less than 0.5% of total hemoglobin) whose structure has never been established. It was purified and studied by mass spectrometry. Electrospray ionization of its abnormal beta-chain indicated a 70-Da mass increase. Separation of the trytic digest by reversed-phase liquid chromatography revealed an abnormal beta T1 peptide. Cesium ion bombardment ionization produced a protonated molecular ion at m/z 1022.516, showing an additional C3H2O2 residue to normal beta T1. The amino acid sequences of both abnormal and normal beta T1 peptides were found identical by comparison of their collision activation spectra. Time course hydrolysis of abnormal beta T1 indicated a rapid loss of the modifying group, leading to normal beta T1. At least, mild treatment with acidic methanol showed an additional methylated site, comparatively with normal beta T1. All these results are consistent with a ketimine-linked pyruvic acid at the amino end of the beta-chain of hemoglobin.

Catalytic oxidation of silicon by cesium ion bombardment

Results for room-temperature oxidation of silicon using cesium ion bombardment and low oxygen exposure are presented. Bombardment with cesium ions is shown to allow oxidation at O2 pressures orders of magnitude smaller than with noble gas ion bombardment. Oxide layers of up to 30 Å in thickness are grown with beam energies ranging from 20–2000 eV, O2 pressures from 10−9 to 10−6 Torr, and total O2 exposures of 100 to 104 L. Results are shown to be consistent with models indicating that initial oxidation of silicon is via dissociative chemisorption of O2, and that the low work function of the cesium- and oxygen-coated silicon plays the primary role in promoting the oxidation process.

Interface broadening in sputter depth profiling through alternating layers of isotopically purified silicon: I. Experimental results

Samples composed of alternating layers of 30Si and 28Si were sputter depth profiled at near normal incidence using a variety of primary ion beams (Ne +, Ar +, Xe +, N 2 +, O 2 + and Cs +) at energies between 2 and 10 keV. The profiles exhibit the well-known asymmetric interface broadening, i.e. a relatively sharp rise of the signal at the leading edge (described by a characteristic width δ) and a comparatively slow exponential fall-off at the trailing edge (defined by a decay length λ). Both δ and λ increase with increasing beam energy. For inert gas and cesium ion bombardment at a fixed energy, the width δ reveals the qualitatively expected decrease with increasing primary ion mass. By contrast, the decay length λ does not show any dependence on the projectile mass or on the matrix sputtering yield. These findings are discussed in terms of ion ranges, cascade mixing, and possible shapes of the stationary internal mixing profiles. The interface broadening observed under N 2 + and O 2 + bombardment is much (up to a factor of more than two) smaller than expected on the basis of the data for inert gas and cesium ion bombardment. This effect is attributed to beam induced nitride and oxide formation which results in a reduced depth of mixing in the Si subsystem. Comparison of the present data with previous results shows that the decay length for several impurities in Si is shorter than for Si in Si.

The effect of low glancing angle sputtering on depth resolution in secondary ion mass spectrometry

This paper describes the development of the depth resolution in secondary ion mass spectrometry using a low glancing angle sputtering method under cesium (Cs +) or oxygen (O + 2) primary ion bombardment. The primary ion glancing incident angle is less than 10° in this method. Using this method, the depth resolution increases with increase in the primary ion incident energy. W/C multilayer sample analysis shows 9 Å depth resolution under 8° 10 keV Cs + ion bombardment. The high resolution surface or interface profiling is achieved using the low glancing angle sputtering method in InP/InGaAs and Ga-implanted Si samples.

Work function of metallic surfaces bombarded with cesium ions

Reduction of work function due to the build-up of cesium coverage on beryllium, molybdenum, and tungsten caused by cesium ion bombardment was investigated for incident energies between 5 eV and 1 keV. Surface characterization was performed as a function of incident ion energy and dose using Auger spectra and work function measurements. A steady state surface coverage is achieved in all cases for a dose of less than 10 16 ions/cm 2. The coverage is strongly dependent on target mass. Beryllium has nearly monolayer coverage at all beam energies, while above 150 eV molybdenum has less than 50% coverage and tungsten has less than 20% coverage. The minimum steady state work function at 60% coverage can be achieved on molybdenum and tungsten at 100 and 45 eV, respectively. The effects of hydrogen coadsorption are also examined. The development of the surface coverage occurs due to the effects of implantation, reflection, and sputtering. A Monte Carlo calculation shows that a surface coverage of cesium on beryllium inhibits sputtering, which should be true for other light elements.