Quantum plasmon and Rashba-like spin splitting in self-assembled Co_xC₆₀composites with enhanced Co content (x> 15)

Abstract

Driving by interplay between plasmonic and magnetic effects in organic composite semiconductors is a challenging task with a huge potential for practical applications. Here, we present evidence of a quantum plasmon excited in the self-assembled Co C nanocomposite films with x > 15 (interval of the Co cluster coalescence) and analyse it using the optical absorption (OA) spectra. In the case of Co C film with x = 16 (LF sample), the quantum plasmon generated by the Co/CoO clusters is found as the 1.5 eV-centred OA peak. This finding is supported by the establishment of four specific C-related OA lines detected at the photon energies E > 2.5 eV. Increase of the Co content up to x = 29 (HF sample) leads to pronounced enhancement of OA intensity in the energy range of E > 2.5 eV and to plasmonic peak downshift of 0.2 eV with respect to the peak position in the LF spectrum. Four pairs of the OA peaks evaluated in the HF spectrum at E > 2.5 eV reflect splitting of the C-related lines, suggesting great change in the microscopic conditions with increasing x. Analysis of the film nanostructure and the plasmon-induced conditions allows us to propose a Rashba-like spin splitting effect that suggests valuable sources for spin polarization.

Quantum plasmon excitations in gold-fullerene mixture films

“…However, in lowenergy interval (ω<2.5 eV), new OA peak is clearly arising with the x increasing. The peak energy position and the reported TEM results claim to ascribe the peak origin to the QP excited in the Au x C 60 film upon interaction of light with the Au clusters [24,31]. The quantum origin of the localized plasmon is provided by very small size of the Au clusters formed in the mixture films (see figure 2) [23], similar to that we established recently for the Co x C 60 films [24].…”

“…The peak energy position and the reported TEM results claim to ascribe the peak origin to the QP excited in the Au x C 60 film upon interaction of light with the Au clusters [24,31]. The quantum origin of the localized plasmon is provided by very small size of the Au clusters formed in the mixture films (see figure 2) [23], similar to that we established recently for the Co x C 60 films [24]. In contrast to the QP-related OA peak, the C 60 -related peaks become broader and less-resolved at higher x suggesting the effects of the Au-C 60 electronic interactions and possible coupling of the QP excitations [1,3,4,22,32].…”

“…The parameters were analysed using a model consisting of the substrate, the film and surface roughness layers. The ellipsometric spectra were evaluated using the WVASE32 software package by the generalized multi-oscillator model [22,24].…”

“…The optical effects clearly observed in the OA spectra allow one to distinguish two intervals in the applied photon energy ω. Thus, the high-energy interval with ω>2.5 eV displays basically several pronounced peaks related with C 60 -related OA transitions (namely, the A, B, C and D peaks, see figure 3(c) and references [22,24]). However, in lowenergy interval (ω<2.5 eV), new OA peak is clearly arising with the x increasing.…”

“…Exploiting the ability of the TM and C 60 mixture to selfassembling, this approach allows one to produce controllably the 3D array of ultra-small metal NPs with nanometre and subnanometre inter-cluster spacing assuming size-confinement quantum effects [17,18,20,23]. Thus, our recent study of optical absorption (OA) in the self-assembled Co x C 60 nanocomposites revealed quantum plasmon (QP) resonance caused by interaction of visible light with very small Co clusters (few nm in size) [24]. In such small plasmonic NPs the size-confinement effect is important, and the quantum mechanical behaviour of the conductive electrons should be taking into account for correct understanding of the QPrelated optical effects [23].…”