Quantitative Chemical Mapping Research Articles

X-ray absorption spectroscopy of polyureas and polyurethanes and their use in characterizing chemical gradients in thin-walled polyurea capsules

The carbon 1s near edge X-ray absorption spectra of several aromatic and aliphatic polyureas and polyurethanes are reported and their spectral features are assigned. An example is given of the application of these spectra as references for quantitative chemical mapping by scanning transmission X-ray microscopy (STXM) which is being used for analytical feedback in the development of controlled release polyurea capsules with chemical structuring across the capsule wall.

Nanoscale Quantitative Chemical Mapping of Conjugated Polymer Blends

Utilizing scanning transmission X-ray microscopy (STXM), we present a quantitative chemical analysis of the bulk composition of spin-coated blends of poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) and poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) used in efficient polymer light emitting diodes. With a lateral resolution of 50 nm, we reveal nanoscale subsurface chemical structure and show that in particular while blend composition remains constant within the TFB-rich domain, in the F8BT-rich phase there is an enrichment of F8BT at the domain interface.

Quantitative chemical mapping of sodium acrylate- and N-vinylpyrrolidone-enhanced alginate microcapsules

Alginate microcapsules enclosing recombinant cells secreting therapeutic products have been used successfully to treat several murine models of human diseases. The mechanical and chemical properties of these alginate capsules can be improved by the addition and in situ photopolymerization of sodium acrylate and N-vinylpyrrolidone in the alginate capsule. The purpose of this modification was to form additional covalent cross-links. In this work we have used scanning transmission X-ray microscopy (STXM) to probe the nature and location of the chemical modifications in the modified capsules by comparison with unmodified capsules. Analysis of X-ray image sequences and selected area spectra has been used to map the calcium gradient in capsules, to identify the presence of polyacrylate throughout the capsules and the localization of poly-N-vinylpyrrolidone in the outer regions of the alginate capsules. The differences in the spatial distributions of these species have led to better understanding of the chemical modifications that provide a mechanically more stable capsule structure.

Quantitative Chemical Mapping of Nanostructured “Onionlike” Poly(methyl methacrylate)/Polystyrene Composite Particles by Soft X-ray Microscopy

In earlier work, we have produced micrometer-sized, monodisperse, poly(methyl methacrylate)/polystyrene (PMMA/PS) composite particles having a multilayered structure. A challenge to developing applications for these multilayered particles is the quantitative, spatially resolved chemical analysis of their onionlike layered structure, where some layers are thinner than 100 nm. Scanning transmission X-ray microscopy (STXM) has a spatial resolution of better than 50 nm and sufficient chemical contrast to both observe and quantitate the composition of individual layers without staining. The results of quantitative chemical analysis of the particles and standard P(S-co-MMA) random copolymers by STXM are shown to be in good agreement with 1H NMR measurements of their average composition. Results from chemical quantification of the individual layers of the onionlike particles by STXM are reported. Analyses of samples prepared with and without embedding in epoxy are compared. Our analysis takes into account the finite spatial resolution of the STXM. The extent of intermixing between layers was estimated by comparing the measured results to those generated from an assumed instrumental response function and trial compositional structures. This analysis suggests that the individual layers are relatively pure, although small amounts of intermixing (<15%) are also consistent with the results. Possible origins of intermixed layers are indicated and ways to achieve more pure layers are suggested.

Methods and Examples of Quantitative Chemical Mapping using Synchrotron Soft X-ray Spectromicroscopy

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Towards practical soft X-ray spectromicroscopy of biomaterials

Scanning transmission X-ray microscopy (STXM) is being developed as a new tool to study the surface chemical morphology and biointeractions of candidate biomaterials with emphasis on blood compatible polymers. STXM is a synchrotron based technique which provides quantitative chemical mapping at a spatial resolution of 50 nm. Chemical speciation is provided by the near edge X-ray absorption spectral (NEXAFS) signal. We show that STXM can detect proteins on soft X-ray transparent polymer thin films with monolayer sensitivity. Of great significance is the fact that measurements can be made in situ, i.e. in the presence of an overlayer of the protein solution. The strengths, limitations and future potential of STXM for studies of biomaterials are discussed.

Quantitative Chemical Mapping with Low-Voltage EDS Spectrum Imaging and Multivariate Statistical Analysis (MSA)

Abstract One of the advantages of performing X-ray microanalysis at low (≤5 kV) operating voltages is that high spatial resolution (≤250 nm) chemical maps of the specimen can be acquired. Spectrum imaging, where a full spectrum is acquired for each pixel in a two-dimensional array, provides the most comprehensive method of characterization, as long as the sampling density (pixel size) is sufficiently smaller than the spatial resolution. Multivariate statistical analysis (MSA) methods are effective in reducing the large (typically ∼10 MByte) raw spectrum images to the modest (typically ∼100 kByte) data files that contain all of the statistically significant information of interest about the specimen. Preliminary analysis of a cross section of a computer chip from a major semiconductor company was previously performed using limited spectrum imaging capabilities available with the 4pi X-ray mapping module, which allowed for simultaneous acquisition of only 48 channels. MSA of images acquired with only a 960 eV portion of the spectrum containing the Al-K, Si-K and W-M lines showed that excellent contrast between the Si- and W-rich regions of the specimen could be achieved in spite of the strong overlap between Si-Kα and W-Mα (∼34 eV separation).

An approach to quantitative compositional profiling at near-atomic resolution using high-angle annular dark field imaging

The compositional profile of a GaAs Al 0.6Ga 0.4As interface is investigated through analysis of a high resolution high-angle annular dark field image. Image calculations are carried out using a multislice code that incorporates thermal diffuse scattering, and an algorithm is developed for quantitative matching between these images and the experimental image. The resulting compositional profile (with monolayer spatial resolution) is compared with an analogous quantitative chemical mapping experiment. The extension of this new technique to high resolution compositional mapping (in two dimensions) is briefly explored.

Quantitative surface chemical mapping with Auger and backscattered electron signals

The objective of surface analytical imaging methods is to map the quantitative distribution of the elements in the surface of a solid. Auger spectroscopy and imaging are often used for this purpose but the contrast in scanning Auger microscope (SAM) images of inhomogeneous, rough, surfaces show some features which are not due to the spatial variations of the surface chemical composition. Various schemes devised to reduce the effects on the image contrast of roughness and sub-surface inhomogeneity are reviewed in this paper. A model sample with a range of known surface inclinations and with known surface and sub-surface composition variations is used to demonstrate the effects of two general classes of correction schemes intended to reduce the contrast of such artefacts. The first class is based upon various arithmetic combinations of the number of electrons detected by a spectrometer set at the energy of an Auger peak and then at an energy (or a number of energies) on the spectral background above the peak. The extent to which these different combinations are successful in reducing topographical contrast and revealing otherwise hidden chemical contrast is summarised. It will be shown how a simple estimate of the Auger peak height leaves large topographical effects in the image contrast and that ratios of measurements at two energies provide a good first approximation for the removal of topographical artefacts. However, even those ratios which provide the best topographic compensation can overcorrect for variations in the sub-surface composition and its effects upon the Auger backscattering factor. In addition, they can introduce new problems for quantitative surface microanalysis. A second class of corrections involves the use of the signals from a set of four backscattered electron (BSE) detectors which collect BSE imaes at the same time as the energy-analysed images. The sum and difference signals from these detectors can be used, together with appropriate calibration, to compute three images in which the pixel intensities give the local surface inclinations and effective atomic numbers of the sub-surface material. These images can be used, in turn, to derive correction images for the Auger maps so as to yield a more accurate map of the surface chemical composition. The combination of the correlated Auger and BSE signals for each place on the surface requires the use of an instrument which collects these signals simultaneously from several detectors. Various methods of using such correlations are discussed. Surfaces with large variations in the contrast in the “raw” Auger images can be analysed with a precision of approximately ±5 at% after exploitation of such correlations.

Quantitative chemical mapping: Spatial resolution

We show that the spatial resolution of quantitative chemical mapping is determined by the larger of two factors: the periodicity of the chemically sensitive reflections, or the size of the unit cell adopted for pattern recognition. Under appropriate chemically sensitive imaging conditions, nonlocal effects due to dynamical scattering and Fresnel fringing are insignificant, even when artifically large discontinuities in the sample projected potential are introduced.

Chemical characterization of (In,Ga)As/(Al,Ga)As strained interfaces grown by metalorganic chemical vapor deposition

We have used quantitative chemical mapping to determine the chemical abruptness of interfaces in In0.2Ga0.8As/Al0.2Ga0.8As strained multilayers grown by metalorganic chemical vapor deposition. We observe a large difference in the interfacial width, depending on the order in which successive layers are grown; the (Al0.2Ga0.8As on In0.2Ga0.8As) interface is twice as wide as the (In0.2Ga0.8As on Al0.2Ga0.8As).

Interaction of energetic ions with inhomogeneous solids.

We use quantitative chemical mapping and statistical analysis to explore the microscopic consequences of the passage of energetic ions through AlAs/GaAs multilayers. We thus quantitatively characterize and microscopically identify the damage cascades produced by individual 320-keV ${\mathrm{Ga}}^{+}$ ions. Our results reveal a strong drift of the damage in the applied electric field, indicating the effects to be charged. We exploit this effect to show that the defects produced by the passage of energetic ions may be microscopically steered in a solid.

Quantitative chemical mapping: Spatial resolution

Quantitative Chemical Mapping is an electron microscopic technique capable of revealing compositional variations in crystalline materials. It combines chemical lattice imaging which maps the sample composition, with vector pattern recognition, which quantifies the local information content of the image to measure the local sample composition. Here we briefly address the spatial resolution of this technique, assuming complete familiarity with its theoretical underpinnings.In chemical imaging, we are concerned with the way that a compositional inhomogeneity is imaged under conditions appropriate for chemical sensitivity, and how the pattern recognition algorithm extracts information from a chemical lattice image. The problem can be formulated as follows. Given a “chemical impulse” of a specific shape, such as a column of Al atoms imbedded in GaAs (approximating a δ-function), an abrupt interface (a θ-function), or a diffuse interface (e.g., with an error function profile), what is the shape of the impulse on the analyzed chemical image? Or, alternatively, what region of the sample contributes to the information content of an image unit cell? By reciprocity, these two formulations are equivalent.

Diffusion in multilayers, studied by quantitative chemical mapping at the atomic level

Using chemical lattice imaging in combination with vector pattern recognition, we study diffusion and interdiffusion at single interfaces in mulilayered semiconductors quantitatively with atomic plane resolution. 20 periods of 50Å C-doped GaAs/50Å undoped GaAs/50Å Al0.4Ga0.6As grown by Molecular Beam Epitaxy(MBE) at 600C was examined to study interdiffusiuori across single GaAs/AlGaAs interfaces as a function of temperature (650C to 750C), depth of interface beneath the surface, and doping . After annealing in bulk form, cross-sectional TEM samples were prepared chemically. Chemical lattice images in the &lt;100&gt; zone axis obtained at an accelerating voltage of 400KV with a JEOL 4000-EX high resolution transmission electron microscope were analyzed by a digital pattern recognition method to obtain composition profiles at each interface before and after annealing.Figure 1 is a chemical lattice image of a C:GaAs/AlGaAs interface. The compositional information in the sample is contained in the local patterns that make up such images. The analysis of such chemical images by the pattern recognition yields quantitative composition profiles across single interfaces. The composition profiles shown in Figure 2 refer to a single interface at a depth of 300Å, before and after annealing at 700C for one hr.

HgCdTe/CdTe Multiple Quantum Wells: Growth, Stability, and Optical Properties

ABSTRACTWe have grown HgCdTe/CdTe multiple quantum wells by molecular beam epitaxy which show room temperature photoluminescence and sharp absorption steps at mid-infrared wavelengths. Quantitative chemical mapping, performed by transmission electron microscopy, indicates minimal interdiffusion during growth. Annealing experiments performed at higher temperatures show that the interdiffusion coefficient is a strong function of the depth of the interface below the surface. Absorption spectra have been accurately modeled with a square well/envelope function approach. The films have been used to passively mode lock color center lasers and produce pulses as short as 120 fsec near 2.7 μm.

Prospects for quantitative chemical mapping of surfaces using the ‘surface analysis by laser ionization’ (SALI) technique

Determination of the lateral distributions of constituents on surfaces is important in a large variety of applications. Although scanning Auger electron spectroscopy (AES) is commonly used, better sensitivity can often be achieved by scanning SIMS within practical time constraints. However, this depends strongly on both the species of interest and the local matrix of the sample. Large variations in secondary ion yields among species lead to severe difficulties in the quantitative interpretation of SIMS chemical maps. Surface Analysis by Laser Ionization (SALI) is a recently developed technique that uses non-resonant multiphoton ionization of neutral species emitted from the sample surface. In most situations, the neutrals constitute a much greater portion of the sputtered material than do the secondary ions, and so are much less sensitive to variations in secondary ion yields. The result is that SALI data can be quantified more easily than SIMS data, while achieving comparable detection sensitivities. In this paper, the prospects for quantitative chemical mapping of surfaces is evaluated for SALI using recent experimental results for achievable useful yields. The theoretical detection capabilities of SALI is compared to that for SIMS and AES as a function of spatial resolution. It is demonstrated that the capabilities of SALI compare very well with those of SIMS and AES in applications of surface chemical mapping at high spatial resolution.

Quantifying the Information Content of Lattice Images

Quantitative information may be extracted from local areas of images that consist of one or more types of unit cell. Fourier-space analysis, real-space intensity analysis, and real-space vector pattern recognition are discussed. The pattern recognition approach efficiently exploits the available information by representing the intensity distribution within each unit cell of the image as a multidimensional vector. Thus, the amount and the effect of noise present are determined, statistically significant features are identified, and quantitative comparisons are made with model images. In the case of chemical lattice images, the position of a vector can be directly related to the atomic composition of the unit cell it represents, allowing quantitative chemical mapping of materials at near-atomic sensitivity and resolution. More generally, the vector approach allows the efficient and quantitative extraction of information from images, which consist of mosaics of unit cells.

Nonlinear diffusion in multilayered semiconductor systems

Using quantitative chemical mapping, we study the low-temperature interdiffusion of HgCdTe/CdTe multilayers, sampling volumes 14 orders of magnitude smaller than previously needed for such measurements. Our results show interdiffusion to be strongly depth dependent and nonlinear, and allow the direct determination of thermodynamic parameters of interest at individual interfaces. The high spatial resolution of our technique establishes that the stability of a layer can depend sensitively on its depth from the surface.

Diffusion and Interdiffusion in Multilayered Semiconductor Systems

AbstractWe apply quantitative chemical mapping techniques to study thermal interdiffusion and ion-implantation induced intermixing at single heterointerfaces at the atomic level. Our results show thermal interdiffusion to be strongly depth dependent. This is related to the need for the presence of native point defects (interstitials and vacancies) to bring about interdiffusion. Since their initial concentration in the bulk is negligible, the point defects must be injected at the surface and transported to the interface for interdiffusion to occur. In the case of ion-implanted samples, we find the passage of a single energetic ion through a sample at 77 K causes significant intermixing, even when the sample receives no subsequent thermal treatment.