Ultra-low-angle microtomy and static secondary ion mass spectrometry for molecular depth profiling of UV-curable acrylate multilayers at the nanoscale

Development of sustainable materials requires methods capable of probing the molecular composition of samples not only at the surface but also in depth. Static secondary ion mass spectrometry (S-SIMS) characterises the distribution of organic and inorganic compounds at the surface. Ultra-low-angle microtomy (ULAM) has been studied as an alternative or complementing method to the molecular depth profiling with, e.g. C₆₀⁺ projectiles. Acrylate-based multilayers relevant to industrial inkjet printing have been sectioned at a cutting angle below 1°. In this way, analysis of the section over a distance of 1 μm allows a depth range in the order of a few nm in the original sample to be achieved. Adequate procedures to optimise the ULAM step and minimise or control the cutting artefacts have been developed. The combination of ULAM with S-SIMS has allowed a depth resolution of 10 nm to be obtained for components at a distance of 35 μm from the surface.     

Icosahedral structure of large charged argon clusters

Measurements of the mass distribution of large argon clusters formed about positive ions in a free jet expansion are reported. The results support an icosahedrally derived shell model of cluster structure through completion of the fourth shell (N = 309 atoms), but significant differences are found near the completion of the fifth shell (N = 561).     

Backscattering effect in quantitative AES sputter depth profiling of multilayers

AES sputter depth profiles of multilayers with constituents of very different backscattering factors show characteristic distortions in the shape of the intensity–depth profiles. These distortions are quantified by introducing an extension of the local effective backscattering factor concept developed in an earlier paper in the mixing‐roughness‐information depth (MRI) model for profile quantification. The extension is based on a linear superposition of two newly defined parameters, the effective backscattering factors for each interface that are diminished with distance from the respective interface by another characteristic parameter, the mean effective backscattering decay length. As shown for a Ni/C multilayer structure of six alternating layers of Ni (38 nm) and C (25 nm) on a Si substrate, AES intensity depth profiles calculated with the presented modification of the MRI model, yield an excellent agreement with the measured profile after some adjustment of the initial mean effective backscattering decay lengths and, sometimes, after a slight change of the backscattering factors given by the Ichimura–Shimizu relations. The backscattering effect is studied as a function of the single layer thickness. A critical layer thickness can be determined, below which the backscattering influence becomes negligible for typical AES depth profiling results. Copyright © 2007 John Wiley & Sons, Ltd.     

Attempts for molecular depth profiling directly on a rat brain tissue section using fullerene and bismuth cluster ion beams

The capabilities of time of flight secondary ion mass spectrometry (TOF-SIMS) have been recently greatly improved with the arrival in this field of polyatomic ion sources. This technique is now able to map at the micron scale intact organic molecules in a range of a thousand Daltons or more, at the surface of tissue samples. Nevertheless, this remains a surface analysis technique, and three-dimensional information on the molecular composition of the sample could not be obtained due to the damage undergone by the organic molecules during their irradiation. The situation changed slightly with the low damage and low penetration depth of the C₆₀ fullerene ion beams. Recent promising studies have shown the possibility of organic molecular depth profiling using this kind of beams onto model samples. This possibility has been tried out directly onto a rat brain tissue section, which is the most commonly used biological tissue model in TOF-SIMS imaging method developments. The tissue surface has been sputtered with a 10 keV energy fullerene ion beam, and surface analyses were done with a 25 keV Bi₃⁺ ion beam at regular time intervals. The total depth which was analysed was more than two microns, with total primary ion doses of more than 10¹⁶ ions cm⁻². Although not in contradiction with results previously published but with much lower doses, it is found that the molecular damage remains too large, thus making molecular imaging very difficult. In addition, most of the lipids, which are usually the main observable molecules in TOF-SIMS, are concentrated close to the sample surface in the first hundreds of nanometers.     

Dynamics of the barium-molecule system within large argon clusters

The effects of adding molecules on the LIF at 540 nm of a barium atom at the surface of an argon cluster (average size 420) has been investigated. We showed that molecules like ethanol,n-hexane and O₂ from stable complexes with ground state barium. In the case of molecules like N₂, CH₄ and SF₆, the collisional quenching of solvated Ba(¹ P) is observed. The large quenching rates obtained are interpreted by a surface mobility of the collisional partners. Moreover, we showed that this collisional quenching leads to the ejection of free Ba(³ P ₁).     

Three-dimensional depth profiling of molecular structures

Molecular time of flight secondary ion mass spectrometry (ToF-SIMS) imaging and cluster ion beam erosion are combined to perform a three-dimensional chemical analysis of molecular films. The resulting dataset allows a number of artifacts inherent in sputter depth profiling to be assessed. These artifacts arise from lateral inhomogeneities of either the erosion rate or the sample itself. Using a test structure based on a trehalose film deposited on Si, we demonstrate that the “local” depth resolution may approach values which are close to the physical limit introduced by the information depth of the (static) ToF-SIMS method itself.     

Low temperature plasma for the preparation of crater walls for compositional depth profiling of thin inorganic multilayers

An indirect, compositional depth profiling of an inorganic multilayer system using a helium low temperature plasma (LTP) containing 0.2% (v/v) SF₆ was evaluated. A model multilayer system consisting of four 10 nm layers of silicon separated by four 50 nm layers of tungsten was plasma‐etched for (10, 20, 30) s at substrate temperatures of (50, 75, and 100) °C to obtain crater walls with exposed silicon layers that were then visualized using time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) to determine plasma‐etching conditions that produced optimum depth resolutions. At a substrate temperature of 100 °C and an etch time of 10 s, the FWHM of the second, third, and fourth Si layers were (6.4, 10.9, and 12.5) nm, respectively, while the 1/e decay lengths were (2.5, 3.7, and 3.9) nm, matching those obtained from a SIMS depth profile. Though artifacts remain that contribute to degraded depth resolutions, a few experimental parameters have been identified that could be used to reduce their contributions. Further studies are needed, but as long as the artifacts can be controlled, plasma etching was found to be an effective method for preparing samples for compositional depth profiling of both organic and inorganic films, which could pave the way for an indirect depth profile analysis of inorganic–organic hybrid structures that have recently evolved into innovative next‐generation materials. Copyright © 2016 John Wiley & Sons, Ltd.     

Quantum chemical calculations of the molecular structures of hybrid amino acid derivatives of fullerene C60

The stereoselectivity of the formation of hybrid amino acid derivatives of fullerene (AADF) C₆₀ was studied. The energies of the model addition reactions C₆₀ + n C₂H₆ ? Me _n -C₆₀-Me _n (1) and C₆₀ + n EtNC₄H₇Me ? Et _n -C₆₀-(NC₄H₇Me) _n (2) (n = 1–3) were calculated by the DFT method B3LYP/6-31G*. The most stable products of reaction (1) are hexamethylated fullerene derivatives in which five Me groups are arranged in the form of a regular pentagon. Among the AADF obtained by reaction (2), 1,4-disubstituted fullerene isomers are most stable. The molecular structures of such isomers were calculated for six biologically active hybrid AADF; the solvent contact areas of these molecules were evaluated.     

Optical circular dichroism of the fullerene amino acid derivatives

The dependence of the circular dichroism spectra of the amino acid derivatives of fullerene C₆₀ on the structure of the initial α-amino acid was studied.     

Molecular depth profiling of multilayer structures of organic semiconductor materials by secondary ion mass spectrometry with large argon cluster ion beams

In this study, we present molecular depth profiling of multilayer structures composed of organic semiconductor materials such as tris(8‐hydroxyquinoline)aluminum (Alq₃) and 4,4′‐bis[N‐(1‐naphthyl)‐N‐phenylamino]biphenyl (NPD). Molecular ions produced from Alq₃ and NPD were measured by linear‐type time‐of‐flight (TOF) mass spectrometry under 5.5 keV Ar₇₀₀ ion bombardment. The organic multilayer films were analyzed and etched with large Ar cluster ion beams, and the interfaces between the organic layers were clearly distinguished. The effect of temperature on the diffusion of these materials was also investigated by the depth profiling analysis with Ar cluster ion beams. The thermal diffusion behavior was found to depend on the specific materials, and the diffusion of Alq₃ molecules was observed to start at a lower temperature than that of NPD molecules. These results prove the great potential of large gas cluster ion beams for molecular depth profiling of organic multilayer samples. Copyright © 2009 John Wiley & Sons, Ltd.     

Quantitative depth profiling of argon in tungsten films by secondary ion mass spectrometry.

Depth profiling of Ar in Ar-implanted tungsten (W) films with an excellent detection limit was investigated by secondary ion mass spectrometry (SIMS). Depth profiles of Ar with the detection of Ar+ and ArCs+ secondary ions, which were produced by O2+ and Cs+ primary ions, respectively, were compared in view of the detection limit and the depth resolution. The detection limit of Ar monitoring Ar+ was limited by the carbon- and oxygen-containing molecular ion (C2O+) in the sample as well as in the SIMS instrument. It was observed that some of the Ar+ ions were produced in the vacuum above the sample surfaces, whereas the ionization of almost all C2O+ occurred at the samples. By using different energy spectra between Ar+ and C2O+, we showed that the energy-filtering technique is advantageous for suppressing C2O+ ion detection. It is also confirmed that the ArCs+ secondary ion is only slighting by the C2OCs+ mass-interference ion. A detection limit of 4 x 10(18) cm(-3) for monitoring Ar+ and 3 x 10(16) cm(-3) for monitoring ArCs+ was achieved under a primary-ion current density of 0.16 mA/cm2. The detection of ArCs+ ion rather than Ar+ was found to be superior in the detection limit and the depth resolution. We conclude that SIMS is useful for the determination of the Ar depth distribution in W films.     

Analysis of fragmentation data and molecular orbital calculations of small argon ion clusters

Distributions of argon clusters (Ar _n ⁺ wheren=1–27) obtained in a molecular beam/time-of-flight system were analyzed in order to assess the influence of fragmentation. The results from rudimentary pseudopotential molecular calculations were calculated to predict the most stable structures and stabilities for the smaller argon ion sized clusters (Ar _n ⁺ wheren=3−9).     

A simple erosion dynamics model of molecular sputter depth profiling

A simple model which describes the essential features commonly observed in a molecular sputter depth profile is presented. General predictions of the dependence of measured molecular ion signals on the primary ion fluence are derived for the specific case where a mass spectrometric technique such as SIMS or secondary neutral mass spectrometry (SNMS) is used to analyze the momentary surface. The results are compared with recent experimental data on molecular depth profiles obtained by cluster‐ion‐initiated SIMS of organic overlayers. Copyright © 2008 John Wiley & Sons, Ltd.     

Kinetics of phase changes in large molecular clusters

A method has been devised to study the kinetics of phase changes in an unfamiliar regime of extreme undercooling and rate of transformation. We show how electron diffraction can monitor the time evolution of phase changes in molecular clusters condensed from the vapor in supersonic flow. Transitions taking place in microseconds are readily followed. Examples include solid state transformations as well as the freezing of liquid clusters. Aspects of the experiment making it possible to observe familiar transitions under highly unusual conditions will be discussed along with some advantages of the new technique.     

Comparison between the classical theory predictions and molecular simulation results for heterogeneous nucleation of argon

We have performed Monte Carlo simulations of homogeneous and heterogeneous nucleations of Lennard-Jones argon clusters. The simulation results were interpreted using the major concept posing a difference between the homogeneous and heterogeneous classical nucleation theories-the contact parameter. Our results show that the multiplication concept of the classical heterogeneous nucleation theory describes the cluster-substrate interaction surprisingly well even for small molecular clusters. However, in the case of argon nucleating on a rigid monolayer of fcc(111) substrate at T=60 K, the argon-substrate atom interaction being approximately one-third as strong as the argon-argon interaction, the use of the classical theory concept results in an underestimation of the heterogeneous nucleation rate by two to three orders of magnitude even for large clusters. The main contribution to this discrepancy is induced by the failure of the classical theory of homogeneous nucleation to predict the energy involved in bringing one molecule from the vapor to the cluster for clusters containing less than approximately 15 molecules.     

Modification of a micellar system for amino acid separation by MEKC--application for amino acid profiling in formulations for parenteral use

The paper proposes a new method for amino acid determination which can be applied for amino acid profiling in solutions for parenteral nutrition. The MEKC method based on a mixed micellar system was developed for the separation of 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) derivatized amino acids. Background electrolyte was based on tris-borate buffer with high alkaline pH. Sodium dodecyl sulfate micelles were modified using 1,2-hexanediol as a co-surfactant. The effect of the modifier on amino acid migration was studied with respect to hydrophobicity of the analytes. The modifier appeared to be suitable to improve the separation of AQC-tagged amino acids without an adverse effect on buffer ionic strength or EOF velocity. The method was successfully validated and applied for amino acid profiling in medicinal preparations for parenteral nutrition. The results obtained were compared with a reference chromatographic method (amino acid analyser).     

Electronic absorption and resonance Raman spectra of large linear carbon clusters isolated in solid argon

Neutral and anionic carbon clusters have been generated via a laser-induced graphite-based plasma and deposited in a solid argon matrix. Anionic clusters were formed from neutral clusters by using crossed electron/carbon cluster beams. Thermal annealing (to 36 K) resulted in the aggregation of the smaller carbon species, leading to the formation of long chain neutral and anionic clusters. Spectroscopic measurements in the ultraviolet, visible, near-infrared and infrared regions revealed a series of bands attributable to a homologous set of odd-numbered C5-C29 neutral clusters and even-numbered C6(-)-C36- anionic clusters. Good agreement is found for the band positions of carbon chains containing odd C15-C21 neutrals and even C6(-)-C22- anions, with species previously identified by Maier and coworkers using mass selection or laser vaporization, followed by neon matrix isolation. Resonance Raman frequencies for the neutral C17, C21 and C23 species are shown to be consistent with the above attributions. Density functional theory calculations agree well with the observed bands. It is found that certain low frequency Raman stretching frequencies decrease in a predictable way with increasing chain length. Comparison of the 0(0)0 absorption transitions of the even C18(-)-C36- anionic clusters with the 'unidentified' infrared (UIR) interstellar emission bands suggests that the electronic emission from specific long chain carbon anions may contribute to the some of the UIR bands.