Lifetime effects and satellites in the photoelectron spectrum of tungsten metal

Abstract

Tungsten (W) is an important and versatile transition metal and has a firm place at the heart of many technologies. A popular experimental technique for the characterization of tungsten and tungsten-based compounds is x-ray photoelectron spectroscopy (XPS), which enables the assessment of chemical states and electronic structure through the collection of core level and valence band spectra. However, in the case of tungsten metal, open questions remain regarding the origin, nature, and position of satellite features that are prominent in the photoelectron spectrum. These satellites are a fingerprint of the electronic structure of the material and have not been thoroughly investigated, at times leading to their misinterpretation. The present work combines high-resolution soft and hard x-ray photoelectron spectroscopy (SXPS and HAXPES) with reflected electron energy loss spectroscopy (REELS) and a multitiered ab initio theoretical approach, including density functional theory (DFT) and many-body perturbation theory (G0W0 and GW + C), to disentangle the complex set of experimentally observed satellite features attributed to the generation of plasmons and interband transitions. This combined experiment-theory strategy is able to uncover previously undocumented satellite features, improving our understanding of their direct relationship to tungsten's electronic structure. Furthermore, it lays the groundwork for future studies into tungsten-based mixed-metal systems and holds promise for the reassessment of the photoelectron spectra of other transition and post-transition metals, where similar questions regarding satellite features remain.

Evaluation of the Thermal Stability of TiW/Cu Heterojunctions Using a Combined SXPS and HAXPES Approach

“…Peak fit analysis of the W 4f core level is particularly challenging due to the close proximity of the W 4f core lines to the W 5p 3/2 core line. 49 In the present case another complication that needs addressing is the overlap of the Ti 3p core level located at 32.6 eV for titanium metal with the W 4f 5/2 core level peak. 50 In this system, the composition is heavily tungsten dominated and one would expect a considerably lower Ti 3p peak intensity relative to the W 4f peaks.…”

“…59,60 However, the binding energy position of the W 4f 7/2 and W 4f 5/2 are located on average at 30.9 eV and 33.0 eV, respectively, which are approximately 0.5 eV lower compared to pure metallic tungsten. 49 This negative shift in BE position away from the pure tungsten metal BE position can be attributed to an alloying effect in the presence of titanium. Similar shifts have also been observed for Ti-Ni, 61 Ti-Cu 62 and Ti-Pd 63 alloys when varying the alloying concentration and are a consequence of unique electronic environments that develop when two metals are alloyed together.…”

“…71,72 In contrast, the HAXPES measurements probe predominately the bulk TiW and closely resemble the VB of tungsten metal. 49,60 The SXP spectra show differences between the samples. The gradient of the region between 0-2 eV appears to fluctuate between the samples.…”

Role and Dynamics of Transition Metal Carbides in Methane Coupling

“…While no WO3 was found in the spent WC, reduction to metallic W(0) with the 4f 7/2 component at 32.1 eV was observed. For W(0) metal, a characteristic loss feature is reported, 25 here it can be observed as a broad peak at 38 eV. Overall, XPS measurements show a change in composition for the surface from a mixture of WO3 and WC in the pristine catalyst to WC and W(0) in the spent catalyst hinting towards carbon depletion on the surface of WC with concomitant reduction to metallic W. Intrigued by this finding, the reaction of 13 CH4 with these materials was investigated in order to elucidate the role of surface C in transition metal carbides and to examine whether there is incorporation of the carbide surface C in the final products.…”

Satellites in the Ti~1$s$ core level spectra of SrTiO$_3$ and TiO$_2$

“…Here, HAXPES brings an additional advantage over SXPS, providing access to deeper core states, which can be advantageous for spectral analysis due to absence of SOS, favourable intrinsic line widths, reduction of overlap with other spectral features including core and Auger lines, and higher photoionisation cross sections. [15][16][17][18][19][20][21][22] This has already been exploited in the case of 3d TMOs, by accessing their 1s core states using HAXPES, particularly for the late TMOs. [15,16] Due to the complexity of the spectra, theoretical approaches to aid their interpretation are crucial.…”