X-ray Photoelectron Spectromicroscopy Research Articles

Real-Time Investigation of Sulfur Vacancy Generation and Passivation in Monolayer Molybdenum Disulfide via in situ X-ray Photoelectron Spectromicroscopy.

Understanding the chemical and electronic properties of point defects in two-dimensional materials, as well as their generation and passivation, is essential for the development of functional systems, spanning from next-generation optoelectronic devices to advanced catalysis. Here, we use synchrotron-based X-ray photoelectron spectroscopy (XPS) with submicron spatial resolution to create sulfur vacancies (SVs) in monolayer MoS2 and monitor their chemical and electronic properties in situ during the defect creation process. X-ray irradiation leads to the emergence of a distinct Mo 3d spectral feature associated with undercoordinated Mo atoms. Real-time analysis of the evolution of this feature, along with the decrease of S content, reveals predominant monosulfur vacancy generation at low doses and preferential disulfur vacancy generation at high doses. Formation of these defects leads to a shift of the Fermi level toward the valence band (VB) edge, introduction of electronic states within the VB, and formation of lateral pn junctions. These findings are consistent with theoretical predictions that SVs serve as deep acceptors and are not responsible for the ubiquitous n-type conductivity of MoS2. In addition, we find that these defects are metastable upon short-term exposure to ambient air. By contrast, in situ oxygen exposure during XPS measurements enables passivation of SVs, resulting in partial elimination of undercoordinated Mo sites and reduction of SV-related states near the VB edge. Correlative Raman spectroscopy and photoluminescence measurements confirm our findings of localized SV generation and passivation, thereby demonstrating the connection between chemical, structural, and optoelectronic properties of SVs in MoS2.

On the Promotion of Catalytic Reactions by Surface Acoustic Waves.

Surface acoustic waves (SAW) allow to manipulate surfaces with potential applications in catalysis, sensor and nanotechnology. SAWs were shown to cause a strong increase in catalytic activity and selectivity in many oxidation and decomposition reactions on metallic and oxidic catalysts. However, the promotion mechanism has not been unambiguously identified. Using stroboscopic X‐ray photoelectron spectro‐microscopy, we were able to evidence a sub‐nanosecond work function change during propagation of 500 MHz SAWs on a 9 nm thick platinum film. We quantify the work function change to 455 μeV. Such a small variation rules out that electronic effects due to elastic deformation (strain) play a major role in the SAW‐induced promotion of catalysis. In a second set of experiments, SAW‐induced intermixing of a five monolayers thick Rh film on top of polycrystalline platinum was demonstrated to be due to enhanced thermal diffusion caused by an increase of the surface temperature by about 75 K when SAWs were excited. Reversible surface structural changes are suggested to be a major cause for catalytic promotion.

Nitrogen ion casting on vertically aligned carbon nanotubes: Tip and sidewall chemical modification

Nitrogen inclusion in vertically aligned carbon nanotubes (v-CNTs) was performed in situ and in ultra-high vacuum by nitrogen ion implantation and evaluated by X-ray photoelectron spectromicroscopy. The creation of defects induced by the ions drives the formation of different nitrogen species (pyridinic, pyrrolic, and graphitic) at the CNT surface. While nitrogen implantation in CNT sidewalls has results similar to implantation in graphene, where mainly nitrogen sp2 bonding configuration occurs, we observed a different behaviour at the CNT tips, where nitrogen incorporation is also more efficient. A large amount of pyrrolic nitrogen is observed at the CNT tips compared to the amount at the CNT sidewalls for the same ion implantation parameters. This indicates a different reactivity of the CNT tips where the presence of natural defects may be involved in different nitrogen bonding formations between carbon and nitrogen with respect to the CNT sidewalls.

Plasma Fluorination of Vertically Aligned Carbon Nanotubes

Functionalization of vertically aligned multiwalled carbon nanotube carpets was performed via exposure to CF4 or Ar:F2 RF plasmas. Rapid fluorination was observed via X-ray photoelectron spectroscopy (XPS) with surface fluorine concentration, bonding type, and patterning dependent on gas mixture and exposure time. Surface properties of the v-MWCNTs forests have been changed by the introduction of fluorine-containing groups, as demonstrated via surface wettability studies, while scanning electron microscopy shows that overall nanotube alignment and separation is conserved. Scanning X-ray photoelectron spectromicroscopy (SPEM) shows that the plasma treatment results in selective functionalization of the surface tips of the nanotubes. This opens the way to nanotube carpet structures with activated surfaces, which maintain the desirable conductive properties of the pristine nanotubes near to the substrate.

Degradation of organic light-emitting diodes under different environment at high drive conditions

Abstract The origin of dark spots, crucial for understanding the degradation mechanisms in organic light-emitting diodes (OLEDs), is typically attributed to the penetration of moisture, oxygen and other active atmospheric agents. Employing scanning X-ray photoelectron spectromicroscopy we have investigated the morphology and chemical composition of degraded micro-areas created in OLEDs under different environment. The same confined degradation events, involving decomposition of the ITO film and organic layer and oxidation and delamination of the Al cathode were observed even for devices grown in situ and operated in ultra-high vacuum at pressures lower than 10 −9 mbar. Our results provide unambiguously prove that ‘uncontrollable’ imperfections in the fabricated structures are the major cause for ignition of degradation events, whereas external causes related to the air ambient act as efficient promoters.

Further developments in quantitative X-ray photoelectron spectromicroscopy: preliminary results from the study of germanium corrosion

The moisture-induced corrosion of an evaporated germanium film has been observed and studied using quantitative X-ray photoelectron spectromicroscopy. The application of recently reported software for the processing of multi-spectral image datasets (particularly noise reduction) to this real-world problem revealed the formation of oxide islands above the continuous passive oxide layer. The software has been developed to map the thickness of these structures within the XPS sampling depth, with a spatial resolution of a few microns. This exemplifies a characterisation methodology of use in any situation where (non-uniform) surface modification results in chemically shifted components of the same core level (or Auger peak) between the original substrate and new overlayer. Copyright © 2005 John Wiley & Sons, Ltd.

Noise reduction in X-ray photoelectron spectromicroscopy by a singular value decomposition sorting procedure

In spite of the fact that most X-ray photoelectron spectroscopy (XPS) instruments are capable of acquiring multispectral data sets, few analysts choose to do so due to the time required to obtain an adequate signal to noise ratio. A significant reduction in the acquisition time can be achieved by the use of principal component analysis to reduce noise in the data set. Additionally a reduction in computational requirements of Principal Component Analysis (PCA) can be gained by prior application of a singular value decomposition sorting procedure. Data transformation prior to processing minimises the difficulties arising due to the non-uniform distribution of Poissonian noise through the spectra. The use of the procedure is illustrated to produce quantified elemental images from wide energy spectra, and chemical state maps requiring curve fitting to high energy resolution spectra. Finally we discuss the effect sample charging during analysis of insulators has on the images.

A study of physical properties and gas-surface interaction of vanadium oxide thin films

Thin films of vanadium oxide were prepared by means of r.f. reactive sputtering. Their physical properties were studied as a function of the oxygen content in the sputtering atmosphere. Atomic force microscopy (AFM) and X-ray photoelectron spectromicroscopy (XPSM) investigations have been used for the study of the morphology and the chemical composition. In addition electrical characterisation in controlled atmosphere has been also performed for the evaluation of gas sensing properties. In particular, films deposited with 15% O2 in the sputtering chamber were found to be more resistive and to exhibit the best sensitivity when exposed to different gases. This behaviour has been explained taking into account the presence of different crystallographic phases and the influence of the polycrystalline nature of the films.

X-ray photo-electron spectromicroscopy

We report on the design implementation performances and experience of usage of the X-ray microscope system MAXIMUM. The project that began in 1987 culminated in 1992, achieving the design goal of a spatial resolution of sub 0.1 μm at the same time as an energy resolution of better than 250 meV. The system is based on the use of Schwarzschild optics coated with multilayers. We discuss the design implementation and alignment techniques used in achieving the design goals. We also clarify some issues related to the ultimate resolution of scanning systems in the presence of signal noise.

A Spectromicroscopic Study of Corrosion in AI-Cu-Si Metallization

AbstractSoft X-ray photoelectron spectromicroscopy has been employed to examine the microstructural chemistry of corrosion on thin films of AL-i%Cu-1%Si alloy. Cu-rich precipitates are formed after the film is annealed at around 320ºC Localized corrosion takes place around these precipitates, and circular microstructures have been observed. Energy distribution curves (EDCs) taken at specific points are analyzed and photoelectron images were obtained from different kinetic energies corresponding to the peaks in EDC. It was found that after corrosion the Al2Cu particles are revealed and distributed at the center of the corrosion structure with oxide as matrix, while the oxides form a ring-like band outside.

Photoelectron holography for high-resolution spectromicroscopy

X-ray photoelectron spectromicroscopy seeks to couple high spatial resolution with a contrast mechanism derived from the photoelectron binding-energy chemical shift. The two major approaches to direct surface imaging with photoelectrons are based either on using a focused incident X-ray beam which is scanned over the sample, or on electron optical techniques which form a magnified image of the surface using the emitted electrons. Due to practical problems and optical aberrations, both approaches appear to have an ultimate spatial resolution limit in the range of 10 nm. An alternative approach to mapping the locations of atoms in specific chemical states is X-ray photoelectron holography, which has the possibility for producing maps of the positions of photoemitting atoms with atomic resolution. Advantages and limitations of this new technique for emission spectromicroscopy are discussed.