Surface Sensitive Techniques for Advanced Characterization of Luminescent Materials

In 1925, P. Auger first observed the so-called Auger electrons in a Wilson cloud chamber. He explained this occurrence as being due to a radiationless transition in atoms excited by a primary x-ray photon source. In 1953, Lander first pointed out that Auger electrons arising from solid samples can be detected in the energy distribution curve of secondary electrons from surfaces subjected to electron bombardment. Moreover, low-energy Auger electrons (∼1 keV kinetic energy) can escape from only the first several atomic layers of a surface since they are strongly absorbed by even a monolayer of atoms. Thus Auger electron spectroscopy (AES) possesses high surface sensitivity. This is one characteristic that makes AES very useful for the study of thin films. For such applications, an important development in AES occurred when Harris showed that the sensitivity of the detection of Auger electrons can be improved by differentiating the electron energy distribution curve with respect to the energy. Furthermore, Weber and Johnson demonstrated that, provided the Auger line profile does not change, the peak-to-peak height in the differentiated energy distribution curves is proportional to the Auger current in the peak. Therefore, in addition to its surface sensitivity, AES also can be used for quantitative studies of thin films.Like AES, x-ray photoelectron spectroscopy (XPS) is a surface-sensitive technique that uses the energy distribution of electrons ejected from a thin film for quantitative analysis. However, in many ways the information provided by AES and XPS is complementary.

Auger Electron Spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) have many aspects in common. Therefore, tests of aspects for one spectroscopy, applicable to the other, should be validated for both. Digital databases of elemental spectra for both AES and XPS have thus been measured using an electron spectrometer that has been fully calibrated for its intensity and energy axes. These databases of true spectra give absolute Auger electron yields and relative photoelectron yields. The AES data are measured for both 5 and 10 keV electron beam energies, whereas the XPS data are measured for both Al and Mg unmonochromated x rays at the magic angle. In addition to these we have measured reflected electron energy loss spectroscopy (REELS) data to provide a third database. The combination of these databases allows a refinement of the theories to obtain an overall convergence between theory and experiment. Improvements to the theory have been obtained by identifying three classes of parameter, those (i) for both AES and XPS, such as electron transport, the methodology of evaluation of peak areas, and the spectrometer response function, (ii) for AES only, such as the electron ionization cross section, backscattering, and specific electron backgrounds, and (iii) for XPS only, such as the photon-ionization cross section. Additionally, the use of REELS data for background subtraction seems significantly better than the use of any one single Tougaard universal cross section in the determination of the AES and XPS intensities in the databases. The formalism for quantitative analysis in AES and XPS, using relative sensitivity factors, has been revised to develop an accurate matrix-less formalism that is very simple for use by the analyst. This formalism, involving “average matrix sensitivity factors” rather than “pure element sensitivity factors,” has the same accuracy as the matrix formalism but its simplicity permits ready extension to systems beyond binary.

The depth profile of thin film layers on bulk substrate, avoiding the cross-sectioning of samples, is commonly performed by techniques such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectroscopy (SIMS). Techniques based on X-ray emission intensity measurements by energy dispersive spectroscopy (EDS), with conventional matrix or ZAF correction, are normally applied to cross-sectioned samples. This article compares XPS with surface X-ray intensity measurements by EDS, carried out with a more realistic X-ray generation and absorption model, known as the ϕ(ρ Z) model. The ϕ(ρ Z) approach has been adopted together with Monte Carlo simulation for the proper selection of SEM accelerating voltages, in conjunction with the analysis of SEM morphological images for thin film density correction. The method discussed hereafter and compared with the XPS technique, has advantages of higher lateral resolution, non-destructive elemental analyses, morphological visualization, low cost, and faster performance. This methodology has been followed to verify the layered structure of SnO2/metal-based gas sensors. X-ray intensities were measured using an EDS ultra-thin window detector. Two different porous layers, 25-nm thick of SnO2 and 10-nm thick of Cu, were detected, showing better agreement with their nominal thickness compared to results obtained using XPS measurements where porosity affects XPS data. If confirmed to be reliable and as effective as XPS depth profiling, this technique may be adopted for process quality control purposes.

Si/O/N-doped single-walled carbon nanotubes (SWNTs) are synthesized using aerosol-assisted chemical vapor deposition (AACVD). The samples are characterized by Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), energy-dispersive X-ray spectroscopy (EDS), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). HRTEM and Raman spectroscopy studies indicate that doping plays a crucial role in the generation of stable small diameter SWNTs. In order to elucidate the role of the heterodoping (Si/N, O/N, and Si/O) on the electronic properties and stability of SWNTs, density functional theory (DFT) computations on semiconductor (10,0), semimetallic (9,0), and metallic (5,5) SWNTs are performed. It is found that in the heterodoped SWNTs substitutional nitrogen makes the inclusion of Si and O atoms more energetically favorable within the carbon lattice. Heterodoping with Si/O/N may have an important impact in the chemistry of SWNTs as they could be used for anchoring polymer chains, clusters, molecules, etc., thus leading to advanced composites and sensors.

Investigation of micron-sized fish-scale surface structures on tool steel surfaces using laser galvanometer scanning

The cathodoluminescence (CL) intensities of Y2SiO5:Ce3+, Gd2O2S:Tb3+ and SrAl2O4:Eu2+,Dy3+ phosphor thin films that were grown by pulsed laser deposition (PLD) were investigated for possible application in low voltage field emission displays (FEDs) and other infrastructure applications. Several process parameters (background gas, laser fluence, base pressure, substrate temperature, etc.) were changed during the deposition of the thin films. Atomic force microscopy (AFM) was used to determine the surface roughness and particle size of the different films. The layers consist of agglomerated nanoparticle structures. Samples with good light emission were selected for the electron degradation studies. Auger electron spectroscopy (AES) and CL spectroscopy were used to monitor changes in the surface chemical composition and luminous efficiency of the thin films. AES and CL spectroscopy were done with 2 keV energy electrons. Measurements were done at 1×10−6 Torr oxygen pressure. The formation of different oxide layers during electron bombardment was confirmed with X-ray photoelectron spectroscopy (XPS). New non-luminescent layers that formed during electron bombardment were responsible for the degradation in light intensity. The adventitious C was removed from the surface in all three cases as volatile gas species, which is consistent with the electron stimulated surface chemical reaction (ESSCR) model. For Y2SiO5:Ce3+ a luminescent SiO2 layer formed during the electron bombardment. Gd2O3 and SrO thin films formed on the surfaces of Gd2O2S:Tb3+ and SrAl2O4:Eu2+,Dy3+, respectively, due to ESSCRs.

Role of TiF3 catalyst in the tribological properties of biofuel soot-contaminated liquid paraffin

X‐ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) are two of the most powerful materials analysis techniques available today. First, the fundamental concepts underlying these methods are presented in simplified form. Then the most important problem areas of concern to workers in these fields are examined. These are: radiation damage, the technology of ion milling, and quantitative analysis. The complexity of these areas is illustrated from what is already known, and some problems that arise in areas that are still incompletely understood are discussed. Finally, the relative merits of XPS and AES are considered by evaluating the performance of each technique with respect to: (1) sensitivity, (2) speed, (3) spatial resolution, (4) depth resolution, (5) chemical bonding information, (6) sample damage, (7) quantitative analysis, and (8) electrostatic charging problems. Scores have been assigned for each of these 8 criteria for XPS and AES; at present, AES is found to have a significantly higher total score. These criteria have also been evaluated assuming that all the theoretical limits can be attained. In that limit, some problems are expected to remain for AES with respect to sensitivity, chemical bonding information, damage, and charging; however, with XPS, all of the performance criteria become superior to AES except for spatial resolution. The total performance score for XPS then becomes much higher than for AES.

10 - Surface Analysis Methods for Contaminant Identification

Microstructure and properties of low friction TiCC nanocomposite coatings deposited by magnetron sputtering

Synthesis, field emission properties and optical properties of ZnSe nanoflowers

Various commercially available surface treatments are being explored for use on stainless steel components in mass spectrometer inlet systems. Type A-286 stainless steel coupons, approximately 12.5 mm in diameter and 3 mm thick, were passivated with one of five different surface treatments; an untreated coupon served as a control. The surface and near-surface microstructure and chemistry of the coupons were investigated using sputter depth profiling using Auger electron spectroscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy (SEM). All the surface treatments studied appeared to change the surface morphology dramatically, as evidenced by lack of tool marks on the treated samples in SEM images. In terms of the passivation treatment, Vendors A-D appeared to have oxide layers that were very similar in thickness to each other (0.7–0.9 nm thick), as well as to the untreated samples (the untreated sample oxide layers appeared to be somewhat larger). Vendor E’s silicon coating appears to be on the order of 200 nm thick.

Cd 2+ uptake by calcite, solid-state diffusion, and the formation of solid-solution: Interface processes observed with near-surface sensitive techniques (XPS, LEED, and AES)

Results concerning the calibration and use of a new ultrahigh vacuum (UHV) surface preparation and analysis system are reported. This Sample Preparation Evaluation Analysis and Reaction (SPEAR) side chamber system replaces an older surface side chamber that was attached to a Hitachi UHV H-9000 microscope. The system combines the ability to prepare clean surfaces using sample heating, cooling, ion milling, or thin film growth with surface analytical tools such as Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM), along with atomic surface structure information available from high-resolution transmission electron microscopy (HREM). The chemical sensitivity of the XPS and AES are demonstrated in preliminary studies of catalytic and semiconductor samples. In addition, the surface preparation capabilities are also demonstrated for the Si(100) and Ge(100) surfaces, including the ability to acquire secondary electron images during milling. During operation, the entire system is capable of maintaining the UHV conditions necessary for surface studies.

Ag-dispersive chalcogenide media for readily activated electronic memristor