Ultrashort XUV pulse absorption spectroscopy of partially oxidized cobalt nanoparticles

Abstract

High-order harmonic generation (HHG) based transient extreme ultraviolet (XUV) absorption spectroscopy is an emerging technique to trace photoinduced charge carrier dynamics in condensed phase materials with femtosecond and even attosecond temporal resolution and elemental specificity. However, its application to nanoparticulate samples that are relevant, for example, for novel photocatalytic light harvesting concepts, has been limited. This is in part due to the challenge to produce residual-free samples on ultrathin, XUV-transparent substrates as well as a widespread understanding that sparsely distributed nanoparticles do not provide sufficient contrast for XUV absorption measurements. Here, we present static XUV absorption spectra of partially oxidized Co nanowire-structures with diameters of approximately 4.5 nm and lengths between 10 and 40 nm, recorded with an ultrashort pulse HHG light source. Nanoparticles are synthesized by the agglomeration of Co atoms inside superfluid helium droplets, followed bysurfacedepositionandoxidationinambient air.Themethodisuniquely suited for residual-free synthesis of transition metal nanowires and their deposition on ultrathin substrates. Analysis by high-resolution transmission electron microscopy reveals the formation of CoO nanowires with regions of unoxidized Co in their interior. The nanoparticle samples are investigated in an HHGdriven ultrafast XUV absorption setup. Despite the low surface coverage of only 23%, the recorded spectrum exhibits a distinct absorption feature at the Co M 2,3 (2p) edge near 60 eV with a peak height of about 40 mOD. The results support the feasibility of table-top ultrafast transient XUV absorption studies of photoinduced dynamics in transition metal oxide nanoparticles with sub-monolayer surface coverage.

Metal clusters synthesized in helium droplets: structure and dynamics from experiment and theory

“…Similar to the creation of ZnO particles, Co clusters were assembled in helium droplets, deposited, and controlled oxidized. 131 Energy dispersive X-ray spectroscopy confirmed the elemental composition, and evaluation of high resolution TEM images are in agreement with the lattice distances of the CoO (111) and (200) planes. Samples with about 25% surface coverage were then investigated in the XUV energy range between 50 and 70 eV generated by high harmonic generation (HHG) of the output of a 785 nm, 3.3 mJ, 3 kHz, 40 fs full-width-at-half-maximum (FWHM) Ti:sapphire laser system.…”

“…Samples with about 25% surface coverage were then investigated in the XUV energy range between 50 and 70 eV generated by high harmonic generation (HHG) of the output of a 785 nm, 3.3 mJ, 3 kHz, 40 fs full-width-at-half-maximum (FWHM) Ti:sapphire laser system. The work 131 demonstrated the feasibility of studying nanoparticle samples prepared in helium droplets with tabletop ultrafast laser sources. .…”

Femtosecond Extreme Ultraviolet Spectroscopy of an Iridium Photocatalyst Reveals Oxidation State and Ligand Field Specific Dynamics

“…1−10 The M 2,3 -edge, corresponding to 3p → 3d transitions, is sensitive to oxidation state, ligand field, and spin-state, providing direct insight into key changes at the metal center. 9 This technique has been used to examine excited state dynamics at various first row transition metal centers, such as nickel, 2,4,6,11 iron, 2,7,8,12−14 and cobalt, 4,5,15,16 for photocatalysts and other molecular systems.…”

“…Table-top extreme ultraviolet (XUV) spectroscopy in the 40–100 eV energy range has been shown to be a powerful technique for probing the ultrafast photophysics of first row transition metal coordination complexes. − The M 2,3 -edge, corresponding to 3p → 3d transitions, is sensitive to oxidation state, ligand field, and spin-state, providing direct insight into key changes at the metal center . This technique has been used to examine excited state dynamics at various first row transition metal centers, such as nickel, ,,, iron, ,,,− and cobalt, ,,, for photocatalysts and other molecular systems.…”

Femtosecond extreme ultraviolet spectroscopy of an iridium photocatalyst reveals oxidation state and ligand field specific dynamics

“…Table-top extreme ultraviolet (XUV) spectroscopy in the 40-100 eV energy range has been shown to be a powerful technique for probing the ultrafast photophysics of 1 st row transition metal coordination complexes. [1][2][3][4][5][6][7][8][9][10] The M 2,3 -edge, corresponding to 3p → 3d transitions, is sensitive to oxidation state, ligand field, and spin-state, providing direct insight into key changes at the metal center. 9 This technique has been used to examine excited state dynamics at various 1 st row transition metal centers, such as nickel, 2,4,6,11 iron, 2,7,8,[12][13][14] & cobalt, 4,5,15,16 for photocatalysts and other molecular systems.…”