In situ ion beam sputter deposition and X‐ray photoelectron spectroscopy (XPS) of multiple thin layers under computer control for combinatorial materials synthesis

Deposition of ultra‐thin layers under computer control is a frequent requirement in studies of novel sensors, materials screening, heterogeneous catalysis, the probing of band offsets near semiconductor junctions and many other applications. Often large‐area samples are produced by magnetron sputtering from multiple targets or by atomic layer deposition (ALD). Samples can then be transferred to an analytical chamber for checking by X‐ray photoelectron spectroscopy (XPS) or other surface‐sensitive spectroscopies. The ‘wafer‐scale’ nature of these tools is often greater than is required in combinatorial studies, where a few square centimetres or even millimetres of sample is sufficient for each composition to be tested. The large size leads to increased capital cost, problems of registration as samples are transferred between deposition and analysis, and often makes the use of precious metals as sputter targets prohibitively expensive. Instead we have modified a commercial sample block designed to perform angle‐resolved XPS in a commercial XPS instrument. This now allows ion‐beam sputter deposition from up to six different targets under complete computer control. Ion beam deposition is an attractive technology for depositing ultra‐thin layers of great purity under ultra‐high vacuum conditions, but is generally a very expensive technology. Our new sample block allows ion beam sputtering using the ion gun normally used for sputter depth‐profiling of samples, greatly reducing the cost and allowing deposition to be done (and checked by XPS) in situ in a single instrument. Precious metals are deposited cheaply and efficiently by ion‐beam sputtering from thin metal foils. Samples can then be removed, studied and exposed to reactants or surface treatments before being returned to the XPS to examine and quantify the effects. Copyright © 2016 The Authors Surface and Interface Analysis Published by John Wiley & Sons Ltd.     

Surface and Interface Chemistry in Colloidal Quantum Dots for Solar Applications Studied by X‐Ray Photoelectron Spectroscopy

Control of the surface and interface chemistry of colloidal quantum dots (CQDs) is critical to achieving a product with good air stability and high performing optoelectronic devices. Through various surface passivation treatments, vast improvements have been made in fields such as CQD photovoltaics; however devices have not currently reached commercial standards. We show how X‐ray photoelectron spectroscopy (XPS) can provide a better understanding of exactly how surface treatments act on CQD surfaces, and the effect of surface composition on air stability and device performance.. We illustrate this with PbS‐based CQDs, using XPS to measure oxidation processes, and to quantify the composition of the topmost surface layer after different surface treatments. We also demonstrate the use of synchrotron radiation‐excited depth‐profiling XPS, a powerful technique for determining the surface composition, chemistry and structure of CQDs. This review describes our recent progress in characterization of CQD surfaces using SR‐excited depth profiling XPS and other photoemission techniques.     

Cryo ultra-low-angle microtomy for XPS-depth profiling of organic coatings

In X-ray photoelectron spectroscopy (XPS) Ar⁺ ion sputtering is usually used for depth profiling. However, for such samples as organic coatings, this is not feasible because of degradation. Also, measurement of a depth profile on a conventionally prepared cross-section is not possible if, for example, sample thickness is below the smallest available measurement spot size of the XPS system. In our approach we used a rotary microtome to cut samples under a shallow tilting angle of 0.5° to obtain an extended cross-section suitable for XPS investigations. We also used liquid nitrogen cooling to ensure an exposed area of higher quality: topography measurements with a novel optical 3D microscope and by atomic force microscopy revealed the linearity of the inclined sections. With our cryo ultra-low-angle microtomy (cryo-ULAM) preparation technique we were able to determine, by XPS, elemental and chemical gradients within a 25 μm thick polyester-based organic coating deposited on steel. The gradients were related to, for example, depletion of the crosslinking agent in the sub-surface region. Complementary reflection electron energy-loss spectroscopy measurements performed on the cryo-ULAM sections also support the findings obtained from the XPS depth profiles.

Multispectral optical imaging combined in situ with XPS or ToFSIMS and principal component analysis

All X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) instruments have optical cameras to image the specimen under analysis, and often to image the sample holder as it enters the system too. These cameras help the user find the appropriate points for analysis of specimens. However they seldom give as good images as stand‐alone bench optical microscopes, because of the limited geometry, source/analyser solid angle and ultra‐high‐vacuum (UHV) design compromises. This often means that the images displayed to the user necessarily have low contrast, low resolution and poor depth‐of‐field. To help identify the different regions of the samples present we have found it useful to perform multispectral imaging by illuminating the sample with narrow‐wavelength‐range light emitting diodes (LEDs). By taking an image under the illumination of these LEDs in turn, each at a successively longer wavelength, one can build up a set of registered images that contain more information than a simple Red–Green–Blue image under white‐light illumination. We show that this type of multispectral imaging is easy and inexpensive to fit to common XPS and ToF‐SIMS instruments, using LEDs that are widely available. In our system we typically use 14 LEDs including one emitting in the ultraviolet (so as to allow fluorescent imaging) and three in the near infra‐red. The design considerations of this system are discussed in detail, including the design of the drive and control electronics, and three practical examples are presented where this multispectral imaging was extremely useful. Copyright © 2016 The Authors Surface and Interface Analysis Published by John Wiley & Sons Ltd.     

Nondestructive compositional depth profiling using variable‐kinetic energy hard X‐ray photoelectron spectroscopy and maximum entropy regularization

We discuss the calculation of nondestructive compositional depth profiles from regularization of variable kinetic energy hard X‐ray photoelectron spectroscopy (VKE‐XPS) data, adapting techniques developed for angle‐resolved XPS. Simulated TiO₂/Si film structures are analyzed to demonstrate the applicability of regularization techniques to the VKE‐XPS data and to determine the optimum choice of regularization function and the number of data points. We find that using a maximum entropy‐like method, when the initial model/prior thickness is similar to the simulated film thickness, provides the best results for cases where prior knowledge of the sample exists. For the simple structures analyzed, we find that only five kinetic energy spectra are necessary to provide a good fit to the data, although in general, the number of spectra will depend on the sample structure and noisiness of the data. The maximum entropy‐like algorithm is then applied to two physical films of TiO₂ deposited on Si. Results suggest interfacial intermixing. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.     

A method for self-attenuation and sample-height correction for counting efficiency of HPGe using Marinelli beaker geometry

A procedure for self-attenuation and sample height correction in HPGe gamma spectrometry efficiency has been presented. An MCNP model of an HPGe detector was used to calculate the full energy peak efficiency (FEPE) for a group of different samples with different heights in Marinelli beaker geometry. A proper function has been fitted to the simulation results to obtain the correction function. The function has been used to calculate the FEPE of a spiked soil sample in different sample heights by considering the experimentally known FEPE of another standard solution source. A good agreement between the experiments and calculations have been shown.     

Using XPS to determine solute solubility in room temperature ionic liquids

X-Ray Photoelectron Spectroscopy (XPS) was used to quantify the amount of bromide ions present in two samples of [C(4)mpyrr]Br dissolved in the room temperature ionic liquid (RTIL) [C(4)mpyrr][N(Tf)2]. One sample was of a known concentration (0.436 Br atom%); the other was a saturated solution. The results obtained from quantitative XPS analysis indicated that the saturated sample had a concentration, or solubility, of 0.90 Br atom% (746 mM) at 298 K, which was then independently confirmed by potential-step chronoamperometry of the same solution.     

Towards nanometric resolution in multilayer depth profiling: a comparative study of RBS, SIMS, XPS and GDOES

An increasing amount of effort is currently being directed towards the development of new functionalized nanostructured materials (i.e., multilayers and nanocomposites). Using an appropriate combination of composition and microstructure, it is possible to optimize and tailor the final properties of the material to its final application. The analytical characterization of these new complex nanostructures requires high-resolution analytical techniques that are able to provide information about surface and depth composition at the nanometric level. In this work, we comparatively review the state of the art in four different depth-profiling characterization techniques: Rutherford backscattering spectroscopy (RBS), secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS) and glow discharge optical emission spectroscopy (GDOES). In addition, we predict future trends in these techniques regarding improvements in their depth resolutions. Subnanometric resolution can now be achieved in RBS using magnetic spectrometry systems. In SIMS, the use of rotating sample holders and oxygen flooding during analysis as well as the optimization of floating low-energy ion guns to lower the impact energy of the primary ions improves the depth resolution of the technique. Angle-resolved XPS provides a very powerful and nondestructive technique for obtaining depth profiling and chemical information within the range of a few monolayers. Finally, the application of mathematical tools (deconvolution algorithms and a depth-profiling model), pulsed sources and surface plasma cleaning procedures is expected to greatly improve GDOES depth resolution.     

Charging/discharging of Au (core)/silica (shell) nanoparticles as revealed by XPS

By recording XPS spectra while applying external voltage stress to the sample rod, we can control the extent of charging developed on core-shell-type gold nanoparticles deposited on a copper substrate, in both steady-state and time-resolved fashions. The charging manifests itself as a shift in the measured binding energy of the corresponding XPS peak. Whereas the bare gold nanoparticles exhibit no measurable binding energy shift in the Au 4f peaks, both the Au 4f and the Si 2p peaks exhibit significant and highly correlated (in time and magnitude) shifts in the case of gold (core)/silica (shell) nanoparticles. Using the shift in the Au 4f peaks, the capacitance of the 15-nm gold (core)/6-nm silica (shell) nanoparticle/nanocapacitor is estimated as 60 aF. It is further estimated that, in the fully charged situation, only 1 in 1000 silicon dioxide units in the shell carries a positive charge during our XPS analysis. Our simple method of controlling the charging, by application of an external voltage stress during XPS analysis, enables us to detect, locate, and quantify the charges developed on surface structures in a completely noncontact fashion.     

Quantitative portable gamma-spectroscopy sample analysis for non-standard sample geometries

Utilizing a portable spectroscopy system, a quantitative method for analysis of samples containing a mixture of fission and activation products in nonstandard geometries was developed. This method was not developed to replace other methods such as Monte Carlo or Discrete Ordinates but rather to offer an alternative rapid solution. The method can be used with various sample and shielding configurations where analysis on a laboratory based gamma-spectroscopy system is impractical. The portalle gamma-spectroscopy method involves calibration of the detector and modeling of the sample and shielding to identify and quantify the radionuclides present in the sample. The method utilizes the intrinsic efficiency of the detector and the unattenuated gamma fluence rate at the detector surface per unit activity from the sample to calculate the nuclide activity and Minimum Detectable Activity (MDA). For a complex geometry, a computer code written for shielding applications (MICROSHIELD) is utilized to determine the unattenuated gamma fluence rate per unit activity at the detector surface. Lastly, the method is only applicable to nuclides which emit gamma-rays and cannot be used for pure beta or alpha emitters. In addition, if sample self absorption and shielding is significant, the attenuation will result in high MDA's for nuclides which solely emit low energy gamma-rays. The following presents the analysis technique and presents verification results using actual experimental data, rather than comparisons to other approximations such as Monte Carlo techniques, to demonstrate the accuracy of the method given a known geometry and source term.     

How To Make Ohmic Contacts to Organic Semiconductors

The process of charge injection plays an important role in organic semiconductor devices. We review various experimental techniques that allow injection to be separated from other competing processes, and quantify the injection efficiency of a contact. We discuss the dependence of the injection efficiency on parameters such as the energy barrier at the interface, the carrier mobility of the organic semiconductor, its carrier density (doping level), the presence of mobile ions, and the sample geometry. Based on these findings, we outline guidelines for forming ohmic contacts and present examples of contact engineering in organic semiconductor devices.     

Observations on X‐ray enhanced sputter rates in argon cluster ion sputter depth profiling of polymers

Traditionally polymer depth profiling by X‐ray photoelectron spectroscopy (XPS) has been dominated by the damage introduced by the ion beam rather than the X‐rays. With the introduction of polyatomic and especially argon gas cluster ion‐beam (GCIB) sources for XPS instruments, this is no longer the case, and either source of damage may be important (or dominate) under particular conditions. Importantly, while ion‐beam damage is a near‐surface effect, X‐ray damage may extend micrometres into the bulk of the sample, so that the accumulation of X‐ray damage during long depth profiles may be very significant. We have observed craters of similar dimensions to the X‐ray spot well within the perimeter of sputter craters, indicating that X‐rays can assist GCIB sputtering very significantly. We have measured experimentally sputter craters in 13 different polymers. The results show that X‐ray exposure can introduce much more topography than might previously have been expected, through both thermal and direct X‐ray degradation. This can increase the depth of a crater by a remarkable factor, up to three in the case of poly‐L‐lactic acid and polychlorotrifluorothylene under reasonably normal XPS conditions. This may be a major source of the loss of depth resolution in sputter depth profiles of polymers. Copyright © 2012 John Wiley & Sons, Ltd.     

Argon cluster-ion sputter yield: Molecular dynamics simulations on silicon and equation for estimating total sputter yield

Argon Gas Cluster-Ion Beam sources have become widely-used on X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) instruments in recent years, but there is little reference data on sputter yields in the literature as yet. Total sputter yield reference data is needed in order to calibrate the depth scale, of XPS or SIMS depth profiles. We previously published a semi-empirical ‘Threshold’ equation for estimating cluster total sputter yield from the energy-per-atom of the cluster and the effective monatomic sputter threshold of the material. This has been shown to agree extremely well with sputter yield measurements on a range or organic and inorganic materials for clusters of around a thousand atoms. Here we use the molecular dynamics (MD) approach to explore a wider range of energy and cluster size than is easy to do experimentally to high precision. We performed MD simulations using the ‘Large-scale Atomic/Molecular Massively Parallel Simulator’ (LAMMPS) parallel MD code on high-performance computer (HPC) systems. We performed 1150 simulations of individual collisions with a silicon (100) surface as an archetypal inorganic substrate, for cluster sizes between 30 and 3000 argon atoms and energies in the range 5 to 40 eV per atom. This corresponds to the most important regime for experimental cluster depth-profiling in SIMS and XPS. Our MD results show a dependence on cluster size as well as energy-per-atom. Using the exponent previously suggested by Paruch et al., we modified the Threshold model equation published previously to take this into account. The modified Threshold equation fits all our MD results extremely well, building on its success in fitting experimental sputter yield measurements. This work is submitted to the volume dedicated to Dr. Martin P Seah, MBE, who was a great influence on the early career of one of the authors (PJC) and who himself made many valuable contributions to the literature on sputtering as it relates to surface and interface analysis.     

Preparation of I2@SWNTs: synthesis and spectroscopic characterization of I2-loaded SWNTs

X-ray photoelectron spectroscopy (XPS) along with inductively coupled plasma analysis (ICP-AE) and Raman spectroscopy have been used to define the location and to quantify the amount of iodine in HiPco SWNT samples loaded with molecular I(2) via sublimation (I(2)-SWNTs). The exterior-adsorbed I(2) can be removed (as I(-)) by reducing the sample of filled nanotubes with Na(0)/THF or by heating the I(2)-SWNTs to 300 degrees C (without reduction), leaving I(2) contained only within the interior of the SWNTs (I(2)@SWNTs) as proven by XPS. These I(2)@SWNTs contain approximately 25 wt % of I(2) and are stable without the loss of I(2) even after exposure to additional reduction with Na(0)/THF or upon heating to ca. 500 degrees C.     

TRANSPORT OF LIGHT IN TISSUE IN PHOTODYNAMIC THERAPY

Abstract The dose rate in photodynamic therapy is proportional to the energy fluence rate and the concentration of the photosensitizer. Calculations of the energy fluence rate have been performed in slab, cylindrical and spherical geometries with the discrete ordinates transport method and diffusion theory. The attentuation of the energy fluence rate is least in slab geometry and greatest in spherical geometry. Violet (405 nm) light is attenuated much more rapidly than red (630 nm) light. Small tissue dimensions or narrow beam irradiation further decrease the energy fluence rate with radius and depth. Anisotropic scattering increases the energy fluence rate at large depths, but decreases it near the source. Measurements of the absolute energy fluence rate vs depth in a mouse tumor model exhibit an order of magnitude attenuation through the skin and a 3 mm thick tumor. Calculations of the energy fluence rate of the DHE fluorescence have been carried out to guide measurement of the concentration. Violet light excitation is much more efficient than red light excitation.     

Use of monte carlo modeling to aid interpretation and quantification of the low energy-loss electron yield at low primary energies

Experimental low-loss electron (LLE) yields were measured as a function of loss energy for a range of elemental standards using a high-vacuum scanning electron microscope operating at 5 keV primary beam energy with losses from 0 to 1 keV. The resulting LLE yield curves were compared with Monte Carlo simulations of the LLE yield in the particular beam/sample/detector geometry employed in the experiment to investigate the possibility of modeling the LLE yield for a series of elements. Monte Carlo simulations were performed using both the Joy and Luo [Joy, D.C. & Luo, S., Scanning 11(4), 176988 (2005)] to assess the influence of the more recent stopping power data on the simulation results. Further simulations have been conducted to explore the influence of sample/detector geometry on the LLE signal in the case of layered samples consisting of a thin C overlayer on an elemental substrate. Experimental LLE data were collected from a range of elemental samples coated with a thin C overlayer, and comparisons with Monte Carlo simulations were used to establish the overlayer thickness.     

Monte Carlo generated spectra for QA/QC of automated NAA routine

A quality check for an automated system of analyzing large sets of neutron activated samples has been developed. Activated samples are counted with an HPGe detector, in conjunction with an automated sample changer and spectral analysis tools, controlled by the Canberra GENIE 2K and REXX software. After each sample is acquired and analyzed, a Microsoft Visual Basic program imports the results into a template Microsoft Excel file where the final concentrations, uncertainties, and detection limits are determined. Standard reference materials are included in each set of 40 samples as a standard quality assurance/quality control (QA/QC) test. A select group of sample spectra are also visually reviewed to check the peak fitting routines. A reference spectrum was generated in MCNP 4c2 using an F8, pulse-height, tally with a detector model of the actual detector used in counting. The detector model matches the detector resolution, energy calibration, and counting geometry. The generated spectrum also contained a radioisotope matrix that was similar to what was expected in the samples. This spectrum can then be put through the automated system and analyzed along with the other samples. The automated results are then compared to expected results for QA/QC assurance.     

Improved depth information from routine analysis of the inelastic background of XPS and HAXPES spectra using optimized two- and three-parameter cross-sections

Determination of the depth distribution of complex nanostructures by X-ray photoelectron spectroscopy (XPS) inelastic background analysis may be complicated if the sample materials have widely different inelastic scattering cross-sections. It was recently demonstrated that this may be solved by using a mixture of cross-sections. This permits retrieval of depth distributions of complex stacks and deeply buried layers with a typical 5% accuracy. This requires however that the cross-sections of the individual sample materials are known which is often not the case and this can complicate practical use for routine analysis. In this paper, we explore to what extent a suitable two- or three-parameter cross-section can be defined independent of prior knowledge of the cross-sections involved but simply defined by fitting the cross-section parameters to the spectrum being analyzed. This paper presents a theoretical study following our recent paper that explored how to make the best choice of inelastic mean free path and inelastic scattering cross-section for the inelastic background analysis with the Quases-Tougaard software. It was previously shown that a rough analysis of the inelastic background could give a good idea of the depth distribution. Here, we demonstrate with model spectra from buried layers created with Quases-Tougaard Generate software that a rather accurate analysis can be performed for very different cases with an average ~5% error. This analysis is easy to apply as it only needs the two- or three-parameter cross-sections generated with the Quases-Tougaard software. This study is aimed to improve routine analysis of the inelastic background of XPS and hard X-ray photoelectron spectroscopy (HAXPES) spectra.