Grazing-exit electron probe X-ray microanalysis (GE-EPMA): Fundamental and applications

Electron probe microanalysis (or Scanning electron microscope-energy dispersive X-ray spectrometry) has been studied under grazing-exit conditions. That is, characteristic X-rays are detected at a very small take-off (exit) angle; the technique is known as grazing-exit electron probe microanalysis (GE-EPMA). Fundamental aspects, instrumentation, and characteristics of grazing-exit electron probe X-ray microanalysis method are described here. Since the observation depth decreases as the exit angle decreases, theoretically to a few nanometers, surface analysis is possible in grazing-exit electron probe X-ray microanalysis. Of course, the size of the electron beam is also small—less than 1 μm, enabling localized surface analysis. In the case of total reflection X-ray spectrometry that allows surface analysis, the whole sample surface must be flat. However, the requirement for flatness is not as strict in grazing-exit electron probe X-ray microanalysis. Grazing-exit electron probe X-ray microanalysis measurements can easily be applied using a commercially available electron probe microanalysis (or Scanning electron microscope-energy dispersive X-ray spectrometry) instrument. To change and control the exit angle in grazing-exit electron probe X-ray microanalysis, the inclination of the sample stage or movement of the X-ray detector is all that is required. Theoretically, this study shows that grazing-exit electron probe X-ray microanalysis would be useful in improving the lateral resolution of the sample surface. In addition, the study demonstrates that grazing-exit electron probe X-ray microanalysis can be applied successfully for surface, thin-film, and particle analyses. As an optional method of electron probe microanalysis, grazing-exit electron probe X-ray microanalysis will be useful in expanding the research fields of normal electron probe microanalysis.     

Detection Limits of Grazing-Exit EPMA for Particle Analysis

 In conventional electron probe microanalysis (EPMA), the electron-induced X-rays are measured at large take-off angles of about 45°. In the grazing exit EPMA (GE-EPMA) method, they are measured at small angles (< 1°). X-rays emitted from deep positions can not be detected at grazing exit angles due to refraction effects at sample-vacuum interface; therefore, it is possible to measure X-rays emitted only from near the surface with a low background. GE-EPMA is especially suitable for the analysis of particles deposited on a flat sample carrier. The detection limits of GE-EPMA were investigated for artificial particles (Al₂O₃, Fe₂O₃ and PbO₂ , particle sizes: 1 ∼ 18 μm) deposited on flat sample carriers of Au thin films–Si wafers. The detection limits improved with decreasing exit angle. The detection limits for characteristic X-rays at an exit angle of approximately 1.1° were 2–4 times lower than at 45°. A minimum detection limit of ca. 0.1% was obtained for Al in small particles.     

Grazing-exit X-ray spectrometry for surface and thin-film analyses.

Electron-probe x-ray micro analysis (EPMA) and particle-induced x-ray emission analysis (PIXE) were performed under grazing-exit conditions. To control the exit angle (take-off angle), a new sample holder having a stepping motor was developed for grazing-exit EPMA. A carefully polished surface of a stainless-steal sample was measured. The surface of stainless steel is normally covered with a thin native oxide layer. The intensity ratio of Cr K(alpha) to Fe K(alpha) increases significantly at the grazing angle, becoming about 5-times larger at 0.2 degrees than at 40 degrees. This result indicates that grazing-exit EPMA is useful for surface analysis. In addition, a new PIXE equipment was developed for grazing-exit x-ray measurements. The sample is fixed and the x-ray detector is moved by applying a linear stage. Preliminary experimental results of grazing-exit PIXE are also shown.     

Enhancement of electron-induced X-ray intensity for single particles under grazing-exit conditions

Grazing-exit electron probe microanalysis (GE-EPMA) was performed for single Al₂O₃ and atmospheric particles, deposited on a flat Si substrate coated by gold, by using an aperture (1 mm in diameter) in front of an energy-dispersive X-ray detector. Silicon Kα X-rays from the Si substrate were strongly observed at an exit angle of ∼45°. However, they disappeared at grazing-exit angles about 0° and only the X-rays from particles were detected. Furthermore, Al Kα and O Kα intensities from single Al₂O₃ particle were enhanced approximately three- and sixfold at the grazing-exit angles (∼1°), respectively, in comparison with those at large angle (∼7°). The background intensities at the energy of Al Kα and O Kα almost monotonously decreased with decreasing exit angle. As a result, the intensity ratios of Al Kα and O Kα X-rays to the background intensities were enhanced five- and sixfold, respectively. This enhancement is considered to be caused by the interference effect of both directly detected X-rays and reflected X-rays on the flat substrate. The similar results are also obtained for Al Kα, Si Kα, K Kα and Ca Kα emitted from single atmospheric particle. The significance of the matrix effect in the particle is also pointed out.     

Investigation of thin surface films by microprobe analysis

The electron microprobe has been applied to study thin films on metallic substrates. The intensities of the characteristic X-rays emitted by thin films of various elements and thicknesses on sublayers of different materials were measured. Two different theoretical approaches (Bishop and Poole, as well as Yakowitz and Newbury) were applied to interpet the X-ray intensities and to determine the film thickness from the intensity measurements. The effect of backscattering from the substrate, resulting in an increase of the intensity of characteristic X-rays of the film, is being described on the basis of a theory given by Hutchins. The corresponding equation for the backscattering factor has been modified to take into account the transmission of the electrons through the film, depending on the mass thickness of the film and the electron energy. The results obtained from theory and experiment are in good agreement for the different experimental parameters applied, except for very thick layers of high atomic numbers measured at low energies where the absorption of electrons in the film plays a dominant role.     

Feasibility study of three-dimensional XRF spectrometry using μ-X-ray beams under grazing-exit conditions

Grazing-exit XRF (GE-XRF), where the X-ray fluorescence is measured at small take-off angles, is a method related to TXRF. It has been demonstrated that GE-XRF is useful for surface, thin-film and particle analyses. In GE-XRF, it is possible to use a μ-X-ray beam at a normal incidence. Thus, a laboratory GE-XRF instrument was developed in combination with a μ-XRF setup. A μ-X-ray beam was produced by the combination of a single capillary and a pinhole aperture. It was demonstrated that depth information could be obtained by using this setup and changing the exit angle. Therefore, this instrument enables measurement of surface-sensitive line scanning and elemental mapping under grazing-exit conditions. In principle, measuring the elemental X-ray mappings at different exit angles enables the reconstruction of three-dimensional elemental distributions. To confirm the feasibility of three-dimensional XRF, a type of Japanese lacquerware, ‘Tamamushi-nuri’, which has a layered structure near the surface, was measured.     

Über die berücksichtigung von streuanteilen bei der messung von röntgenfluoreszenzintensitäten

A theoretical treatment of the contributions of scattered X-rays to the intensity of fluorescent radiation is given. These contributions are caused as well by scattering of white X-rays as by scattering of characteristic X-rays. The amount is calculated for different elements and thin films. From these considerations follows an increase of the X-ray fluorescence intensity of approximately 1 per cent for bulk materials and up to 30 per cent for thin films.

Since for a quantitative analysis reference samples are needed, values of the concentration or mass per unit area are found, which are higher or lower as compared with the true value, depending on the amount of fluorescent X-rays caused by scattering.     

Studies of tetracene- and pentacene-based organic thin-film transistors fabricated by the neutral cluster beam deposition method

The neutral cluster beam deposition (NCBD) method has been applied to produce and characterize organic thin-film transistors (OTFTs) based upon tetracene and pentacene molecules as active layers. Organic thin films were prepared by the NCBD method on hexamethyldisilazane (HMDS)-untreated and -pretreated silicon dioxide (SiO2) substrates at room temperature. The surface morphology and structures for the tetracene and pentacene thin films were examined by atomic force microscopy (AFM) and X-ray diffraction (XRD). The measurements demonstrate that the weakly bound and highly directional neutral cluster beams are efficient in producing high-quality single-crystalline thin films with uniform, smooth surfaces and that SiO2 surface treatment with HMDS enhances the crystallinity of the pentacene thin-film phase. Tetracene- and pentacene-based OTFTs with the top-contact structure showed typical source-drain current modulation behavior with different gate voltages. Device parameters such as hole carrier mobility, current on/off ratio, threshold voltage, and subthreshold slope have been derived from the current-voltage characteristics together with the effects of surface treatment with HMDS. In particular, the high field-effect room-temperature mobilities for the HMDS-untreated OTFTs are found to be comparable to the most widely reported values for the respective untreated tetracene and pentacene thin-film transistors. The device performance strongly correlates with the surface morphology, and the structural properties of the organic thin films are discussed.     

On the applicability of radionuclides emitting low energy X-rays for in situ radiotracer adsorption studies

In this paper, methodology and applicability of the in situ radiotracer “thin gap” method are presented and discussed via some studies of Cr(VI) electroreduction on Pt electrode by measuring the low energy (E = 4.90 keV) X-rays of ⁵¹Cr used for labeling. Two types of scintillators, 2 mm thick β-plastic and 300 μm thick CaF₂(Eu), were tested for nuclear detection. The plastic scintillator cannot be utilized for selective measurement of the soft X-rays emitted by ⁵¹Cr, hence, it does not allow us the estimation of the surface excess of chromium containing species on Pt electrode. In contrast to this, CaF₂(Eu) is suitable for selective detection of low-energy X-rays; therefore, the radioelectrochemical cell equipped with the latter detector provides very promising measuring conditions for the extension of the radiotracer “thin gap” method towards application of the low-energy X-ray emitters.     

Grazing incidence small-angle X-ray scattering: an advanced scattering technique for the investigation of nanostructured polymer films

Hamburg workshop on the "application of synchrotron radiation in chemistry"With grazing incidence small-angle X-ray scattering (GISAXS) the limitations of conventional small-angle X-ray scattering with respect to extremely small sample volumes in the thin-film geometry are overcome. GISAXS turned out to be a powerful advanced scattering technique for the investigation of nanostructured polymer films. Similar to atomic force microscopy (AFM), a large interval of length between molecular and mesoscopic scales is detectable with a surface-sensitive scattering method. While with AFM only surface topographies are accessible, with GISAXS the buried structure is also probed. Because a larger surface area is probed, GISAXS also has a much larger statistical significance compared to AFM. Due to the high demand on collimation, GISAXS experiments are based on synchrotron radiation. Nanostructures parallel and perpendicular to the sample surface observable in thin poly(styrene- block-isoprene) diblock copolymer films are presented as an example of the possibilities of GISAXS.     

Impact of structure and morphology on charge transport in semiconducting oligomeric thin-film devices.

We investigated various thin-film morphologies of vacuum-deposited highly luminescent 2,5-di-n-octyloxy-1,4-bis[4'-(styryl)styryl]benzene (Ooct-OPV5) in a typical light-emitting-diode device structure. Important modifications in the thin-film morphology and structure were obtained by changing the substrate temperature in the range 23-90 degrees C. Structural analysis by X-ray and electron diffraction provided clear evidence for polymorphism in evaporated thin films of Ooct-OPV5. Concomitantly, the hole mobility in the corresponding devices was determined by transient electroluminescence measurements. We demonstrate that the substrate temperature T(sub) is a key parameter that controls the hole mobility of the devices. Increasing T(sub) between 23 and 84 degrees C results in a progressive increase of the zero-field hole mobility from 10(-6) to 10(-4) cm(2) V(-1) s(-1). The increase in hole mobility is correlated to the average grain size in the thin films. In addition, we give evidence for the existence of a peculiar growth mode in the bulk crystal structure of Ooct-OPV5, whereby the (a,b) and (b,c) planes can grow in a homoepitaxial manner.     

pH值对电沉积CuInSe2薄膜的组份、结构及光电性能的影响

本文采用一步恒电位沉积法在铟锡氧化物(ITO)基底上制备CuInSe2薄膜,研究了沉积过程中不同的离子浓度配比及pH值对CuInSe2膜结构性能的影响。利用扫描电子显微镜(SEM)、X射线能谱仪(EDS)及X射线衍射仪(XRD)研究了薄膜材料的结构性能,结果发现pH值对薄膜的化学成份、表面形貌、晶格结构都有显著影响,通过控制合适的浓度及酸度分别制备了高质量富铟与富铜薄膜。利用表面光电压(SPS)技术分别对富铟与富铜薄膜的光电分离特性进行了研究,结果发现富铟薄膜具有很强的光电响应;而富铜薄膜由于Cu-Se相的存在,在薄膜中形成了新的界面,电子-空穴对在其界面处因捕获而发生复合,从而导致其光电响应的强烈降低。所得到的结果为提高铜铟硒薄膜的光电效率提供了有价值的新思路。     

Effect of Pt loading on the photocatalytic reactivity of titanium oxide thin films prepared by ion engineering techniques

Platinum-loaded titanium oxide thin-film photocatalysts were prepared by using an ionized cluster beam (ICB) deposition method and a RF magnetron sputtering (RF-MS) deposition method as dry processes. From the results of the photocatalytic oxidation of acetaldehyde with O₂ under UV light irradiation, small amounts of Pt loading (less than 10 nm film thickness) were found to dramatically enhance the photocatalytic reactivity. However, when TiO₂ thin films were loaded with relatively larger amounts of Pt (more than 30 nm as the film thickness), the photocatalytic reactivity became lower than for the pure TiO₂ thin films. Moreover, investigations of the ratio of Pt loaded onto the surface of the thin film catalysts by XPS measurements revealed that the small amounts of Pt loaded exist as very small clusters working to efficiently enhance the charge separation, whereas, large amounts of Pt covers the entire surface of the TiO₂ thin films, resulting in a decrease of the photocatalytic reactivity.     

Room-temperature reaction of laser-photolytically generated Te nanosols with silver

KrF laser photolysis of diphenyl ditelluride in 2-propanol yields a stable solution of tellurium nanosols, which reacts with immersed Ag sheets to yield thin silver telluride films. The nanosols were identified by UV–vis spectroscopy and the films were characterized by electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction analysis. It is revealed that the films are mostly amorphous and contain small contributions of cubic as well as monoclinic Ag₂Te structures. The procedure provides the first example of the fast formation of silver telluride thin films by reaction between the elements in inert solvent at room-temperature.     

二维多层PtRu/PtNd纳米薄膜的结构效应及电催化氧化活性

采用离子束多靶溅射技术控制膜层结构制备出二维多层PtRu/PtNd纳米合金薄膜作为微型直接甲醇燃料电池(DMFCs)阳极催化电极材料. 应用X射线光电子能谱(XPS)、原子力显微镜(AFM)、X射线衍射(XRD)、掠入射X射线衍射(GIXD)研究了薄膜表面的化学状态、形貌以及薄膜表层、次表层和体相的结构,并用CO-stripping伏安法、循环伏安法(CV)、线性扫描伏安法(LSV)、计时电流法(CA)等电化学方法测试薄膜催化剂的电化学活性比表面积及其对甲醇的电催化氧化. 结果表明, 多次交替沉积制备的PtRu/PtNd薄膜, 由于溅射产生的Pt+、Ru+和Pt+、Nd+之间的相互作用, 使薄膜表面的化学状态和膜层结构发生变化, 其衍射谱峰呈现异常宽化, Pt与Nd之间产生电子转移, 证实了PtRu/PtNd纳米合金薄膜是一种具有特殊膜层结构和电子结构的二维多层PtRu/PtNd纳米合金薄膜, 电化学活性比表面积高达115.00m2 ·g-1, 在酸性溶液中电催化氧化甲醇的活性显著提高.     

Analysis of elemental composition of Fe1-xNix and Si1-xGex alloy thin films by electron probe microanalysis and micro-focus X-ray fluorescence

The present study reports on results of analysis of the elemental composition of thin films by electron probe microanalysis with energy dispersive (ED-EPMA) X-ray spectrometry in conjunction with the dedicated thin-film analysis software package Stratagem and by X-ray fluorescence in its version with a micro-focus X-ray fluorescence (μ-XRF) source attached to a scanning electron microscope (SEM). Two thin-film systems have been analyzed: Fe_1-xNi_x on silicon wafer and Si_1-xGe_x on Al₂O₃ substrate, in both cases the layers being grown to a thickness of about 200 nm by ion beam sputter deposition. Samples of five different atomic fractions have been produced and analyzed for each thin-film system. Moreover, reference samples with certified elemental composition and thickness have been also available. This study is part of an interlaboratory comparison organized in the frame of standardization technical committee ISO/TC 201 “Surface chemical analysis.” Two laboratories have been analyzed by ED-EPMA (one laboratory standardless and one laboratory using both standardless and with standards variants) and one laboratory by μ-XRF (standardless and with standards). All the elemental compositions obtained with different methods are in very good agreement for the complete two sets of five samples each.     

A methodology for investigating new nonprecious metal catalysts for PEM fuel cells

This paper reports an approach to investigate metal-chalcogen materials as catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane (PEM) fuel cells. The methodology is illustrated with reference to Co-Se thin films prepared by magnetron sputtering onto a glassy-carbon substrate. Scanning Auger microscopy (SAM), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) have been used, in parallel with electrochemical activity and stability measurements, to assess how the electrochemical performance relates to chemical composition. It is shown that Co-Se thin films with varying Se are active for oxygen reduction, although the open circuit potential (OCP) is lower than for Pt. A kinetically controlled process is observed in the potential range 0.5-0.7 V (vs reversible hydrogen electrode) for the thin-film catalysts studied. An initial exposure of the thin-film samples to an acid environment served as a pretreatment, which modified surface composition prior to activity measurements with the rotating disk electrode (RDE) method. Based on the SAM characterization before and after electrochemical tests, all surfaces demonstrating activity are dominated by chalcogen. XRD shows that the thin films have nanocrystalline character that is based on a Co(1-x)Se phase. Parallel studies on Co-Se powder supported on XC72R carbon show comparable OCP, Tafel region, and structural phase as for the thin-film model catalysts. A comparison for ORR activity has also been made between this Co-Se powder and a commercial Pt catalyst.     

一种在室温合成具有宽带隙CdS的简单方法

采用简单的磁控溅射方法, 在室温合成了CdS多晶薄膜. 在溅射CdS多晶薄膜过程中, 分别在Ar 气中通入0%、0.88%、1.78%、2.58%和3.40% (体积分数, φ)的O₂, 得到不同O含量的CdS多晶薄膜. 通过X射线衍射仪、拉曼光谱仪、扫描电子显微镜、X射线光电子能谱仪、紫外-可见光谱仪对得到的CdS多晶薄膜进行表征.分析结果表明: O的掺入能得到结合更加致密, 晶粒尺寸更小的CdS多晶薄膜; 与溅射气体中没有O₂时制备的CdS多晶薄膜的光学带隙(2.48 eV)相比, 当溅射气体中O₂的含量为0.88%和1.78% (φ)时, 制备得到的CdS多晶薄膜具有更大的光学带隙, 分别为2.60和2.65 eV; 而当溅射气体中O₂的含量为2.58%和3.40% (φ)时, 得到的CdS光学带隙分别为2.50 和2.49 eV, 与没有掺杂O的CdS的光学带隙(2.48 eV)相当; 当溅射气体中O₂的含量为0.88% (φ)时, 制备的CdS多晶薄膜具有最好的结晶质量. 通过磁控溅射方法, 在溅射气体中O₂含量为0.88% (φ)条件下制备的CdS多晶薄膜表面沉积了CdTe 多晶薄膜并在CdCl₂气氛中进行了高温退火处理, 对退火前后的CdTe多晶薄膜进行了表征. 表征结果显示: CdS中掺入O能得到结合更紧密、退火后晶粒尺寸更大的CdTe多晶薄膜. 通过磁控溅射方法, 在CdS制备过程中于Ar 中掺入O₂, 在室温就能得到具有更大光学带隙的CdS多晶薄膜, 该方法是一种简单和有效的方法, 非常适用于大规模工业化生产.     

Influence of morphology and topography on potentiometric response of magnesium and calcium sensitive PEDOT films doped with adenosine triphosphate (ATP)

Poly(3,4-ethylenedioxythiophene) (PEDOT) films doped with adenosine triphosphate (ATP) are used to study the biologically relevant competitive magnesium and calcium ion-exchange at ATP membrane sites. It is shown, by atomic force microscopy (AFM) and scanning electron microscopy (SEM), that the surface topography and morphology of the PEDOT-ATP films determines the quality of their potentiometric response. More smooth and less rough films result in better potentiometric characteristics, particularly in a faster response. The topography/morphology of the PEDOT-ATP films is influenced by conditions during electrodeposition (electrochemical method of deposition, pH, concentration of electrolytes) and post-deposition soaking (including net-time of soaking), as evidenced by X-ray photoelectron spectroscopy (XPS) and energy dispersive analysis of X-rays (EDAX).     

Three-dimensional molecular packing of thin organic films of PTCDI-C8 determined by surface X-ray diffraction

We have determined the full molecular 3D packing of thin organic films of the archetypical organic n-type semiconductor N, N'-dioctyl-3,4:9,10-perylene tetracarboxylic diimide (PTCDI-C 8) by surface X-ray crystallography. We show that PTCDI-C 8 forms smooth layered films on Al 2O 3 (11-20) with an outstanding degree of molecular order. The thin-film structure is found to consist of a triclinic unit cell with the plane of the aromatic core tilted by 67 +/- 2 degrees with respect to the surface plane, which differs significantly from the bulk structure. The 3D crystallites extend with vertical coherent order across the entire film thickness.