Microscopic Insights into the Sputtering of Thin Organic Films on Ag{111} Induced by C60 and Ga Bombardment

Ar cluster bombardment of phenylalanine deposited on graphene-Effect of kinetic energy and projectile size

This study investigates the effects of kinetic energy and projectile size on particle emission from a phenylalanine overlayer adsorbed on a graphene bilayer during argon projectile bombardment. Projectiles of various sizes (1-1000 atoms) and kinetic energies (0.5-40 keV) are used to study their impact on the ejection efficiency, mass distribution, and kinetic and angular spectra of the emitted substances. Particle ejection mechanisms are discussed in scenarios with and without sample perforation. Additionally, this study investigated the potential use of graphene in secondary ion mass spectrometry or neutral secondary mass spectrometry analyses.

Chemical Imaging and Analysis of Single Nerve Cells by Secondary Ion Mass Spectrometry Imaging and Cellular Electrochemistry

A nerve cell is a unit of neuronal communication in the nervous system and is a heterogeneous molecular structure, which is highly mediated to accommodate cellular functions. Understanding the complex regulatory mechanisms of neural communication at the single cell level requires analytical techniques with high sensitivity, specificity, and spatial resolution. Challenging technologies for chemical imaging and analysis of nerve cells will be described in this review. Secondary ion mass spectrometry (SIMS) allows for non-targeted and targeted molecular imaging of nerve cells and synapses at subcellular resolution. Cellular electrochemistry is well-suited for quantifying the amount of reactive chemicals released from living nerve cells. These techniques will also be discussed regarding multimodal imaging approaches that have recently been shown to be advantageous for the understanding of structural and functional relationships in the nervous system. This review aims to provide an insight into the strengths, limitations, and potentials of these technologies for synaptic and neuronal analyses.

Mechanism of FIB-Induced Phase Transformation in Austenitic Steel

The transformation of unstable austenite to ferrite or α′ martensite as a result of exposure to Xe+ or Ga+ ions at room temperature was studied in a 304 stainless steel casting alloy. Controlled Xe+ and Ga+ ion beam exposures of the 304 were carried out at a variety of beam/sample geometries. It was found that both Ga+ and Xe+ ion irradiation resulted in the transformation of the austenite to either ferrite or α′ martensite. In this paper, we will refer to the transformation product as a BCC phase. The crystallographic orientation of the transformed area was controlled by the orientation of the austenite grain and was consistent with either the Nishiyama–Wasserman or the Kurdjumov–Sachs orientation relationships. On the basis of the Xe+ and Ga+ ion beam exposures, the transformation is not controlled by the chemical stabilization of the BCC phase by the ion species, but is a result of the disorder caused by the ion-induced recoil motion and subsequent return of the disordered region to a more energetically favorable phase.

Hypervelocity cluster ion impacts on free standing graphene: Experiment, theory, and applications

We present results from experiments and molecular dynamics (MD) simulations obtained with C₆₀ and Au₄₀₀ impacting on free-standing graphene, graphene oxide (GO), and graphene-supported molecular layers. The experiments were run on custom-built ToF reflectron mass spectrometers with C₆₀ and Au-LMIS sources with acceleration potentials generating 50 keV C₆₀ ²⁺ and 440-540 keV Au₄₀₀ ⁴⁺. Bombardment-detection was in the same mode as MD simulation, i.e., a sequence of individual projectile impacts with separate collection/identification of the ejecta from each impact in either the forward (transmission) or backward (reflection) direction. For C₆₀ impacts on single layer graphene, the secondary ion (SI) yields for C₂ and C₄ emitted in transmission are ∼0.1 (10%). Similar yields were observed for analyte-specific ions from submonolayer deposits of phenylalanine. MD simulations show that graphene acts as a trampoline, i.e., they can be ejected without destruction. Another topic investigated dealt with the chemical composition of free-standing GO. The elemental composition was found to be approximately COH₂. We have also studied the impact of Au₄₀₀ clusters on graphene. Again SI yields were high (e.g., 1.25 C^-/impact). 90-100 Au atoms evaporate off the exiting projectile which experiences an energy loss of ∼72 keV. The latter is a summation of energy spent on rupturing the graphene, ejecting carbon atoms and clusters and a dipole projectile/hole interaction. The charge distribution of the exiting projectiles is ∼50% neutrals and ∼25% either negatively or positively charged. We infer that free-standing graphene enables detection of attomole to zeptomole deposits of analyte via cluster-SI mass spectrometry.

Depth-profiling X-ray photoelectron spectroscopy (XPS) analysis of interlayer diffusion in polyelectrolyte multilayers

Functional organic thin films often demand precise control over the nanometer-level structure. Interlayer diffusion of materials may destroy this precise structure; therefore, a better understanding of when interlayer diffusion occurs and how to control it is needed. X-ray photoelectron spectroscopy paired with C60(+) cluster ion sputtering enables high-resolution analysis of the atomic composition and chemical state of organic thin films with depth. Using this technique, we explore issues common to the polyelectrolyte multilayer field, such as the competition between hydrogen bonding and electrostatic interactions in multilayers, blocking interlayer diffusion of polymers, the exchange of film components with a surrounding solution, and the extent and kinetics of interlayer diffusion. The diffusion coefficient of chitosan (M = ∼100 kDa) in swollen hydrogen-bonded poly(ethylene oxide)/poly(acrylic acid) multilayer films was examined and determined to be 1.4*10(-12) cm(2)/s. Using the high-resolution data, we show that upon chitosan diffusion into the hydrogen-bonded region, poly(ethylene oxide) is displaced from the film. Under the conditions tested, a single layer of poly(allylamine hydrochloride) completely stops chitosan diffusion. We expect our results to enhance the understanding of how to control polyelectrolyte multilayer structure, what chemical compositional changes occur with diffusion, and under what conditions polymers in the film exchange with the solution.

Use of TOF-SIMS in vascular biology

Secondary ion mass spectrometry is a technique used for surface analysis. The recent introduction of metal cluster ionization sources opened the possibility to probe biological tissues, due to its reduced in-source fragmentation. Despite its limitations in mass range of the molecules analyzed, a combination of minimal sample processing, sensibility, and unsurpassed lateral resolution makes TOF-SIMS a good tool for molecular imaging in pathology.

Modelling the sputtering of Au surfaces using a multi time-scale technique

We present results from an atomistic computer simulation model of the sputtering of gold crystal surfaces under 500 eV ion bombardment by Au and Ar ions for doses up to 10 14  ions cm −2 . The multi time-scale technique uses molecular dynamics to calculate the fast ballistic collision processes in the early stages of the cascade, whereas an on-the fly kinetic Monte Carlo technique is used to model the relaxation and diffusion processes between successive ion impacts when the defect motion has begun to be dominated by rare events. The results indicate a large amount of crystalline recovery between impacts, some facetting of the crystal surfaces but no large sub-surface defect accumulation. Because of this recovery process, sputtering yields and energy distributions are in good agreement with those obtained assuming a perfect crystal surface and also with those experimentally measured.

Exploring the surface sensitivity of TOF-secondary ion mass spectrometry by measuring the implantation and sampling depths of Bi(n) and C60 ions in organic films

The surface sensitivity of Bi(n)(q+) (n = 1, 3, 5, q = 1, 2) and C(60)(q+) (q = 1, 2) primary ions in static time-of-flight secondary ion mass spectrometry (TOF-SIMS) experiments were investigated for molecular trehalose and polymeric tetraglyme organic films. Parameters related to surface sensitivity (impact crater depth, implantation depth, and molecular escape depths) were measured. Under static TOF-SIMS conditions (primary ion doses of 1 × 10(12) ions/cm(2)), the 25 keV Bi(1)(+) primary ions were the most surface sensitive with a molecular escape depth of 1.8 nm for protein films with tetraglyme overlayers, but they had the deepest implantation depth (~18 and 26 nm in trehalose and tetraglyme films, respectively). The 20 keV C(60)(+2) primary ions were the second most surface sensitive with a slightly larger molecular escape depth of 2.3 nm. The most important factor that determined the surface sensitivity of the primary ion was its impact crater depth or the amount of surface erosion. The most surface sensitive primary ions, Bi(1)(+) and C(60)(+2), created impact craters with depths of 0.3 and 1.0 nm, respectively, in tetraglyme films. In contrast, Bi(5)(+2) primary ions created impact craters with a depth of 1.8 nm in tetraglyme films and were the least surface sensitive with a molecular escape depth of 4.7 nm.

Characteristic fragmentation of polysiloxane monolayer films by bombardment with monatomic and polyatomic primary ions in TOF-SIMS

This study reports the characteristic fragmentation patterns from two polysiloxane polymers that form ordered overlayer on silver substrates. Results are compared for the bombardment of various monatomic and polyatomic projectiles of Cs(+), C(60)(+) (10 keV), Bi(1)(+), and Bi(3)(+) (25 keV) in the high mass range time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra. Results are reported from sub-monolayer (solution cast) coverages of poly(dimethylsiloxane)s with the number average molecular weights (M(n)) of 2200 and 6140 Da, respectively, and Langmuir-Blodgett monolayers of poly(methylphenylsiloxane) with molecular weights (MW) from 600 and 1000 Da. For each film, Bi projectiles resulted in the emission of positive silver cluster ions from the substrate under the polymer overlayer and peaks corresponding to silver cluster ions with larger mass were observed by impact of polyatomic 25 keV Bi(3)(+) projectiles. In addition, depending on the change of energy of Bi (3) (+) , a different pattern of fragments was observed. With Cs(+) and C(60)(+) impact, however, the emission of silver cluster ions was not detected. In the case of C(60)(+) impact for PDMS-6140, peaks corresponding to silver-cationized intact oligomers were not observed. In this paper, these results are explained by the possible bombardment mechanism for each projectile, based on its mass, energy, and split trajectories of the component atoms under the polyatomic impact.

ToF-SIMS Analysis of Adsorbed Proteins: Principal Component Analysis of the Primary Ion Species Effect on the Protein Fragmentation Patterns

In time-of-flight secondary ion mass spectrometry (ToF-SIMS), the choice of primary ion used for analysis can influence the resulting mass spectrum. This is because different primary ion types can produce different fragmentation pathways. In this study, analysis of single-component protein monolayers were performed using monatomic, tri-atomic, and polyatomic primary ion sources. Eight primary ions (Cs(+), Au(+), Au(3) (+), Bi(+), Bi(3) (+), Bi(3) (++), C(60) (+)) were used to examine to the low mass (m/z < 200) fragmentation patterns from five different proteins (bovine serum albumin, bovine serum fibrinogen, bovine immunoglobulin G and chicken egg white lysozyme) adsorbed onto mica surfaces. Principal component analysis (PCA) processing of the ToF-SIMS data showed that variation in peak intensity caused by the primary ions was greater than differences in protein composition. The spectra generated by Cs(+), Au(+) and Bi(+) primary ions were similar, but the spectra generated by monatomic, tri-atomic and polyatomic primary ion ions varied significantly. C(60) primary ions increased fragmentation of the adsorbed proteins in the m/z < 200 region, resulting in more intense low m/z peaks. Thus, comparison of data obtained by one primary ion species with that obtained by another primary ion species should be done with caution. However, for the spectra generated using a given primary ion beam, discrimination between the spectra of different proteins followed similar trends. Therefore, a PCA model of proteins created with a given ion source should only be applied to datasets obtained using the same ion source. The type of information obtained from PCA depended on the peak set used. When only amino acid peaks were used, PCA was able to identify the relationship between proteins by their amino acid composition. When all peaks from m/z 12-200 were used, PCA separated proteins based on a ratio of C(4)H(8)N(+) to K(+) peak intensities. This ratio correlated with the thickness of the protein films and Bi(1) (+) primary ions produced the most surface sensitive spectra.

Lipid imaging with time-of-flight secondary ion mass spectrometry (ToF-SIMS)

Fundamental advances in secondary ion mass spectrometry (SIMS) now allow for the examination and characterization of lipids directly from biological materials. The successful application of SIMS-based imaging in the investigation of lipids directly from tissue and cells are demonstrated. Common complications and technical pitfalls are discussed. In this review, we examine the use of cluster ion sources and cryogenically compatible sample handling for improved ion yields and to expand the application potential of SIMS. Methodological improvements, including pre-treating the sample to improve ion yields and protocol development for 3-dimensional analyses (i.e. molecular depth profiling), are also included in this discussion. New high performance SIMS instruments showcasing the most advanced instrumental developments, including tandem MS capabilities and continuous ion beam compatibility, are described and the future direction for SIMS in lipid imaging is evaluated.

Molecular depth profiling by wedged crater beveling

Time-of-flight secondary ion mass spectrometry and atomic force microscopy are employed to characterize a wedge-shaped crater eroded by a 40-keV C(60)(+) cluster ion beam on an organic film of Irganox 1010 doped with Irganox 3114 delta layers. From an examination of the resulting surface, the information about depth resolution, topography, and erosion rate can be obtained as a function of crater depth for every depth in a single experiment. It is shown that when measurements are performed at liquid nitrogen temperature, a constant erosion rate and reduced bombardment induced surface roughness is observed. At room temperature, however, the erosion rate drops by ∼(1)/(3) during the removal of the 400 nm Irganox film and the roughness gradually increased to from 1 nm to ∼4 nm. From SIMS lateral images of the beveled crater and AFM topography results, depth resolution was further improved by employing glancing angles of incidence and lower primary ion beam energy. Sub-10 nm depth resolution was observed under the optimized conditions on a routine basis. In general, we show that the wedge-crater beveling is an important tool for elucidating the factors that are important for molecular depth profiling experiments.

ToF-SIMS Depth Profiling of Trehalose: The Effect of Analysis Beam Dose on the Quality of Depth Profiles

In static secondary ion mass spectrometry (SIMS) experiments, an analysis dose of 10(12) ions/cm(2) typically produces optimum results. However, the same dose used in dual beam depth profiling can significantly degrade the signal. This is because during each analysis cycle a high-energy beam is rastered across the same x-y location on the sample. If a sufficient amount of sample is not removed during each sputter cycle, the subsequent analysis cycle will sample a volume degraded by the previous analysis cycles. The dimensionless parameter R' is used to relate the amount of damage accumulated in the sample to the amount of analysis beam dose used relative to the etching beam. Depth profiles from trehalose films spin-cast onto silicon wafers acquired using Bi(1) (+) and Bi(3) (+) analysis beams were compared. As R' increased, the depth profile and the depth resolution (interface width) both degraded. At R' values below 0.04 for both Bi(1) (+) and Bi(3) (+), the shape of the profile as well as the depth resolution (9 nm) indicated that dual beam analysis can be superior to C(60) single beam depth profiling.

Biological cluster mass spectrometry

This article reviews the new physics and new applications of secondary ion mass spectrometry using cluster ion probes. These probes, particularly C(60), exhibit enhanced molecular desorption with improved sensitivity owing to the unique nature of the energy-deposition process. In addition, these projectiles are capable of eroding molecular solids while retaining the molecular specificity of mass spectrometry. When the beams are microfocused to a spot on the sample, bioimaging experiments in two and three dimensions are feasible. We describe emerging theoretical models that allow the energy-deposition process to be understood on an atomic and molecular basis. Moreover, experiments on model systems are described that allow protocols for imaging on biological materials to be implemented. Finally, we present recent applications of imaging to biological tissue and single cells to illustrate the future directions of this methodology.

Strong-field Photoionization of Sputtered Neutral Molecules for Molecular Depth Profiling

Molecular depth profiles of an organic thin film of guanine vapor deposited onto a Ag substrate are obtained using a 40 keV C(60) cluster ion beam in conjunction with time-of-flight secondary ion mass spectrometric (ToF-SIMS) detection. Strong-field, femtosecond photoionization of intact guanine molecules is used to probe the neutral component of the profile for direct comparison with the secondary ion component. The ability to simultaneously acquire secondary ions and photoionized neutral molecules reveals new fundamental information about the factors that influence the properties of the depth profile. Results show that there is an increased ionization probability for protonated molecular ions within the first 10 nm due to the generation of free protons within the sample. Moreover, there is a 50% increase in fragment ion signal relative to steady state values 25 nm before reaching the guanine/Ag interface as a result of interfacial chemical damage accumulation. An altered layer thickness of 20 nm is observed as a consequence of ion beam induced chemical mixing. In general, we show that the neutral component of a molecular depth profile using the strong-field photoionization technique can be used to elucidate the effects of variations in ionization probability on the yield of molecular ions as well as to aid in obtaining accurate information about depth dependent chemical composition that cannot be extracted from TOF-SIMS data alone.

Cluster secondary ion mass spectrometry of polymers and related materials

Cluster secondary ion mass spectrometry (cluster SIMS) has played a critical role in the characterization of polymeric materials over the last decade, allowing for the ability to obtain spatially resolved surface and in-depth molecular information from many polymer systems. With the advent of new molecular sources such as C(60)(+), Au(3)(+), SF(5)(+), and Bi(3)(+), there are considerable increases in secondary ion signal as compared to more conventional atomic beams (Ar(+), Cs(+), or Ga(+)). In addition, compositional depth profiling in organic and polymeric systems is now feasible, without the rapid signal decay that is typically observed under atomic bombardment. The premise behind the success of cluster SIMS is that compared to atomic beams, polyatomic beams tend to cause surface-localized damage with rapid sputter removal rates, resulting in a system at equilibrium, where the damage created is rapidly removed before it can accumulate. Though this may be partly true, there are actually much more complex chemistries occurring under polyatomic bombardment of organic and polymeric materials, which need to be considered and discussed to better understand and define the important parameters for successful depth profiling. The following presents a review of the current literature on polymer analysis using cluster beams. This review will focus on the surface and in-depth characterization of polymer samples with cluster sources, but will also discuss the characterization of other relevant organic materials, and basic polymer radiation chemistry.

Application of surface chemical analysis tools for characterization of nanoparticles

The important role that surface chemical analysis methods can and should play in the characterization of nanoparticles is described. The types of information that can be obtained from analysis of nanoparticles using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary-ion mass spectrometry (TOF-SIMS), low-energy ion scattering (LEIS), and scanning-probe microscopy (SPM), including scanning tunneling microscopy (STM) and atomic force microscopy (AFM), are briefly summarized. Examples describing the characterization of engineered nanoparticles are provided. Specific analysis considerations and issues associated with using surface-analysis methods for the characterization of nanoparticles are discussed and summarized, with the impact that shape instability, environmentally induced changes, deliberate and accidental coating, etc., have on nanoparticle properties.

ToF-SIMS Depth Profiling of Organic Films: A Comparison between Single Beam and Dual-beam Analysis

In dual-beam depth profiling, a high energy analysis beam and a lower energy etching beam are operated in series. Although the fluence of the analysis beam is usually kept well below the static SIMS limit, complete removal of the damage induced by the high energy analysis beam while maintaining a good depth resolution is difficult. In this study a plasma polymerized tetraglyme film is used as the model organic system and the dimensionless parameter R, (analysis beam fluence)/(total ion fluence), is introduced to quantify the degree of sample damage induced as a function of the analysis beam fluence. It was observed for a constant C(60) (+) etching beam fluence, increasing the analysis fluence (and consequently increasing the R parameter) increased in the amount of damage accumulated in the sample. For Bi(n) (+) (n = 1 and 3) and C(60) (+) depth profiling, minimal damage accumulation was observed up to R = 0.03, with a best depth resolution of 8 nm. In general, an increase in the Bi(n) (+) analysis fluence above this value resulted in a decrease in the molecular signals of the steady state region of the depth profile and a degradation of the depth resolution at the polymer/substrate interface.

Fundamental studies of molecular depth profiling and 3D imaging using Langmuir-Blodgett films as a model

Molecular depth profiling and three-dimensional imaging using cluster projectiles and SIMS have become a prominent tool for organic and biological materials characterization. To further explore the fundamental features of cluster bombardment of organic materials, especially depth resolution and differential sputtering, we have developed a reproducible and robust model system consisting of Langmuir-Blodgett (LB) multilayer films. Molecular depth profiles were acquired, using a 40-keV C(60) (+) probe, with LB films chemically alternating between barium arachidate and barium dimyristoyl phosphatidate. The chemical structures were successfully resolved as a function of depth. The molecular ion signals were better preserved when the experiment was performed under cryogenic conditions than at room temperature. A novel method was used to convert the scale of fluence into depth which facilitated quantitative measurement of the interface width. Furthermore, the LB films were imaged as a function of depth. The reconstruction of the SIMS images correctly represented the original chemical structure of the film. It also provided useful information about interface mixing and edge effects during sputtering.

C60 secondary ion mass spectrometry with a hybrid-quadrupole orthogonal time-of-flight mass spectrometer

A hybrid quadrupole orthogonal time-of-flight mass spectrometer optimized for matrix-assisted laser desorption ionization (MALDI) and electrospray ionization has been equipped with a C 60 cluster ion source. This configuration is shown to exhibit a number of characteristics that improve the performance of traditional time-of-flight secondary ion mass spectrometry (TOF-SIMS) experiments for the analysis of complex organic materials and, potentially, for chemical imaging. Specifically, the primary ion beam is operated as a continuous rather than a pulsed beam, resulting in up to 4 orders of magnitude greater ion fluence on the target. The secondary ions are extracted at very low voltage into 8 mTorr of N 2 gas introduced for collisional focusing and cooling purposes. This extraction configuration is shown to yield secondary ions that rapidly lose memory of the mechanism of their birth, yielding tandem mass spectra that are identical for SIMS and MALDI. With implementation of ion trapping, the extraction efficiency is shown to be equivalent to that found in traditional TOF-SIMS machines. Examples are given, for a variety of substrates that illustrate mass resolution of 12,000-15,600 with a mass range for inorganic compounds to m/ z 40,000. Preliminary chemical mapping experiments show that with added sensitivity, imaging in the MS/MS mode of operation is straightforward. In general, the combination of MALDI and SIMS is shown to add capabilities to each technique, providing a robust platform for TOF-SIMS experiments that already exists in a large number of laboratories.

Depth resolution during C60+ profiling of multilayer molecular films

Time-of-flight secondary ion mass spectrometry is utilized to characterize the response of Langmuir-Blodgett (LB) multilayers under the bombardment by buckminsterfullerene primary ions. The LB multilayers are formed by barium arachidate and barium dimyristoyl phosphatidate on a Si substrate. The unique sputtering properties of the C60 ion beam result in successful molecular depth profiling of both the single component and multilayers of alternating chemical composition. At cryogenic (liquid nitrogen) temperatures, the high mass signals of both molecules remain stable under sputtering, while at room temperature, they gradually decrease with primary ion dose. The low temperature also leads to a higher average sputter yield of molecules. Depth resolution varies from 20 to 50 nm and can be reduced further by lowering the primary ion energy or by using glancing angles of incidence of the primary ion beam.

Energy deposition during molecular depth profiling experiments with cluster ion beams

The role of the location of energy deposition during cluster ion bombardment on the quality of molecular depth profiling was examined by varying the incident angle geometry. Cholesterol films approximately 300 nm in thickness deposited onto silicon substrates were eroded using 40-keV C60(+) at incident angles ranging from 5 degrees to 73 degrees with respect to the surface normal. The erosion process was evaluated by determining at each incident angle the total sputtering yield of cholesterol molecules, the damage cross section of the cholesterol molecules, the altered layer thickness within the solid, the sputter yield decay in the quasi-steady-state sputter regime, and the interface width between the cholesterol film and the silicon substrate. The results show that the total sputtering yield is largest relative to the product of the damage cross section and the altered layer thickness at 73 degrees incidence, suggesting that the amount of chemical damage accumulated is least when glancing incident geometries are used. Moreover, the signal decay in the quasi-steady-state sputter regime is observed to be smallest at off-normal and glancing incident geometries. To elucidate the signal decay at near-normal incidence, an extension to an erosion model is introduced in which a fluence-dependent decay in sputter yield is incorporated for the quasi-steady-state regime. Last, interface width calculations indicate that at glancing incidence the damaged depth within the solid is smallest. Collectively, the measurements suggest that decreased chemical damage is not necessarily dependent upon an increased sputter yield or a decreased damage cross section but instead dependent upon depositing the incident energy nearer the solid surface resulting in a smaller altered layer thickness. Hence, glancing incident angles are best suited for maintaining chemical information during molecular depth profiling using 40-keV C60(+).

Computational view of surface based organic mass spectrometry

Surface based mass spectrometric approaches fill an important niche in the mass analysis portfolio of tools. The particular niche depends on both the underlying physics and chemistry of molecule ejection as well as experimental characteristics. In this article, we use molecular dynamics computer simulations to elucidate the fundamental processes giving rise to ejection of organic molecules in atomic and cluster secondary ion mass spectrometry (SIMS), massive cluster impact (MCI) mass spectrometry, and matrix-assisted laser desorption ionization (MALDI) mass spectrometry. This review is aimed at graduate students and experimental researchers.

Chemically alternating Langmuir-Blodgett thin films as a model for molecular depth profiling by mass spectrometry

Langmuir-Blodgett multilayers of alternating barium arachidate and barium dimyristoyl phosphatidate are characterized by secondary ion mass spectrometry employing a 40 keV buckminsterfullerene (C60) ion source. These films exhibit well-defined structures with minimal chemical mixing between layers, making them an intriguing platform to study fundamental issues associated with molecular depth profiling. The experiments were performed using three different substrates of 306 nm, 177 nm, and 90 nm in thickness, each containing six subunits with alternating chemistry. The molecular subunits are successfully resolved for the 306 nm and 177 nm films by cluster ion depth profiling at cryogenic temperatures. In the depth profile, very little degradation was found for the molecular ion signal of the underneath layers compared with that of the top layer, indicating that the formation of chemical damage is removed as rapidly as it is formed. The resolving power decreases as the thickness of the alternating subunits decrease, allowing a depth resolution of 20 to 25 nm to be achieved. The results show the potential of LB films as an experimental model system for studying fundamental features of molecular depth profiling.

Nanovolume analysis with secondary ion mass spectrometry using massive projectiles

A variant of secondary ion mass spectrometry is presented where the surface is bombarded with individual gold nanoparticles each resolved in time and space with a corresponding event-by-event detection of the secondary ions (SIs). The projectile used, Au400(4+), with impact energy of 136 keV, generates high SI yields. Typically, there is co-emission of multiple SIs from a single impact, i.e., emission of SIs from molecules co-located within a nanovolume with dimensions in the 10-nm range. The ability to detect co-located molecules was tested on samples consisting of alternating nanometric layers of oppositely charged polyions, poly(diallyldimethylammonium chloride), poly(styrenesulfonate) (PSS), and clay nanoplatelets. To achieve signal statistics, the chemical analysis was carried out with a sequence of stochastic impacts making this method suitable for characterization of similar nanoparticles or spots dispersed on a surface. Attomole detection sensitivity was achieved for PSS. The homogeneity of assembled layers could be assessed with approximately 10-nm resolution.

Effect of cluster size in kiloelectronvolt cluster bombardment of solid benzene

Emission of benzene molecules by 5-keV cluster bombardment of a range of carbon projectiles from C6H6 to C180 is studied by a coarse-grained molecular dynamics (MD) technique. This approach permits calculations that are not feasible using more complicated potential energy functions, particularly as the interesting physics associated with the ion impact event approaches the mesoscale. These calculations show that the highest ejection yields are associated with clusters that deposit their incident energy 15-20 A below the surface. The highest yield for the projectiles is produced by the C20 and C60 projectiles. The results from the MD simulations are also compared favorably to an analytical model based on fluid dynamics to describe the energy deposition. The analytical model is then utilized to extend the range of the calculations to higher incident energies. The issue of the relative amount of chemical fragmentation and intact molecular desorption is also examined for the benzene crystal. These results show that damage accumulation at high-incident fluence should not be problematic and that it should be possible to perform molecular depth profiling via secondary ion mass spectrometry experiments. In general, the approach presented here illustrates the power of combining a simplified MD method with analytical strategies for describing a length scale that is difficult to achieve with traditional MD calculations.

Multivariate analysis strategies for processing ToF-SIMS images of biomaterials

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a hyperspectral imaging technique. Each pixel in a two-dimensional ToF-SIMS image (or each voxel in a three-dimensional (3-D) ToF-SIMS image) contains a full mass spectrum. Thus, multivariate analysis methods are being increasingly used to process biomaterial ToF-SIMS images so the maximum amount of information can be extracted from the images. This study examines the use of principal component analysis (PCA) and maximum autocorrelation factors (MAF) on four different ToF-SIMS images. These images were selected because they represent significant challenges for biomedical ToF-SIMS image processing (topographical features, low count rates, surface contaminants, etc.). With PCA four different types of scaling methods (auto, root mean, filter, and shift variance scaling) were used. The effect of two preprocessing methods (normalization and mean centering) was also examined for both PCA and MAF. The more computational intense MAF provided the best results for all the images investigated in this study, doing the best job of reducing the number of variables required to describe the image, enhancing image contrast and recovering key spectral features. MAF was particularly good at identifying subtle features that were often lost in PCA and impossible to visualize in single peak images. However, the combination of PCA with either root mean or shift variance scaling provided similar results to MAF. Thus, these combinations offer promising alternatives to MAF for working with large data sets encountered in 3-D imaging. Also, the new method of filter scaling is promising for processing low count rate images with salt and pepper noise. Normalization proved an important tool for deconvoluting chemical effects from topographic and/or matrix effects. Mean centering aided in reducing the dimensionality of the data, but in one case resulted in a loss of information.

Surface sensitivity in cluster-ion-induced sputtering

The ion beam-induced removal of thin water ice films condensed onto Ag and bombarded by energetic Au, Au2, Au3, and C60 projectiles is examined both experimentally and with molecular dynamics computer simulations. For water overlayers of thicknesses greater than 10 A, the yields of sputtered Ag+ secondary ions decay exponentially with increasing ice thickness, revealing characteristic decay lengths of 24, 20, 18, and 7.0 A, respectively. It is shown that these values manifest the characteristic depths of projectile energy loss, rather than escape depths of the sputtered Ag atoms through the water ice overlayer. Computer simulations show that the mechanism of ejection involves the sweeping away of overlayer water molecules, allowing for an unimpeded escape of ejected Ag atoms. The relevance of these data with respect to surface sensitivity in secondary ion mass spectrometry is discussed.

Carbon-13 Labeled Polymers: An Alternative Tracer for Depth Profiling of Polymer Films and Multilayers Using Secondary Ion Mass Spectrometry

13C labeling is introduced as a tracer for depth profiling of polymer films and multilayers using secondary ion mass spectrometry (SIMS). Deuterium substitution has traditionally been used in depth profiling of polymers but can affect the phase behavior of the polymer constituents with reported changes in both bulk-phase behavior and surface and interfacial interactions. SIMS can provide contrast by examining various functional groups, chemical moieties, or isotopic labels. 13C-Labeled PS (13C-PS) and unlabeled PS (12C-PS) and PMMA were synthesized using atom-transfer radical polymerization and assembled in several model thin-film systems. Depth profiles were recorded using a Cameca IMS-6f magnetic sector mass spectrometer using both 6.0-keV impact energy Cs+ and 5.5-keV impact energy O2+ primary ion bombardment with detection of negative and positive secondary ions, respectively. Although complete separation of 12C1H from 13C is achieved using both primary ion species, 6.0-keV Cs+ clearly shows improved detection sensitivity and signal-to-noise ratio for detection of 12C, 12C1H, and 13C secondary ions. The use of Cs+ primary ion bombardment results in somewhat anomalous, nonmonotonic changes in the 12C, 12C1H, and 13C secondary ion yields through the PS/PMMA interface; however, it is shown that this behavior is not due to sample charging. Through normalization of the 13C secondary ion yield to the total C (12C + 13C) ion yield, the observed effects through the PS/PMMA interface can be greatly minimized, thereby significantly improving analysis of polymer films and multilayers using SIMS. Mass spectra of 13C-PS and 12C-PS were also analyzed using a PHI TRIFT I time-of-flight mass spectrometer, with 15-keV Ga+ primary ion bombardment and detection of positive secondary ions. The (12)C7(1)H7 ion fragment and its 13C-enriched analogues have significant secondary ion yields with negligible mass interferences, providing an early indication of the potential for future use of this technique for cluster probe depth profiling of high molecular weight 13C-labeled fragments.

Nanomaterials in mass spectrometry ionization and prospects for biological application

The rapid development of nanotechnology has revolutionized scientific developments in recent decades. Mass spectrometry (MS) measurements are no exception and have benefited greatly from integration of nanomaterials in every step of analysis. This brief review summarizes recent developments in the field with the focus on the use of nanomaterials as alternative media to facilitate analyte ionization in laser-desorption ionization-mass spectrometry (LDI-MS) and secondary ion mass spectrometry (SIMS). The biological applications of both techniques are also detailed. The use of nanomaterials in other aspects of MS analysis, for example in sample clean-up and indirect analyte quantification, is briefly discussed.

Organic surfaces excited by low-energy ions: atomic collisions, molecular desorption and buckminsterfullerenes

This article reviews the recent progress in the understanding of kiloelectronvolt particle interactions with organic solids, including atomic displacements in a light organic medium, vibrational excitation and desorption of fragments and entire molecules. This new insight is the result of a combination of theoretical and experimental approaches, essentially molecular dynamics (MD) simulations and secondary ion mass spectrometry (SIMS). Classical MD simulations provide us with a detailed microscopic view of the processes occurring in the bombarded target, from the collision cascade specifics to the scenarios of molecular emission. Time-of-flight SIMS measures the mass and energy distributions of sputtered ionized fragments and molecular species, a precious source of information concerning their formation, desorption, ionization and delayed unimolecular dissociation in the gas phase. The mechanisms of energy transfer and sputtering are compared for bulk molecular solids, organic overlayers on metal and large molecules embedded in a low-molecular weight matrix. These comparisons help understand some of the beneficial effects of metal substrates and matrices for the analysis of molecules by SIMS. In parallel, I briefly describe the distinct ionization channels of molecules sputtered from organic solids and overlayers. The specific processes induced by polyatomic projectile bombardment, especially fullerenes, are discussed on the basis of new measurements and calculations. Finally, the perspective addresses the state-of-the-art and potential developments in the fields of surface modification and analysis of organic materials by kiloelectronvolt ion beams.