X-ray Photoelectron Spectroscopic Analysis of Si Nanoclusters in SiO₂ Matrix

Abstract

We investigated silicon nanoclusters Si(nc) in a SiO 2 matrix prepared by the plasma-enhanced chemical vapor deposition technique, using X-ray photoelectron spectroscopy (XPS) with external voltage stimuli in both static and pulsed modes. This method enables us to induce an additional charging shift of 0.8 eV between the Si2p peaks of the oxide and the underlying silicon, both in static and time-resolved modes, for a silicon sample containing a 6 nm oxide layer. In the case of the sample containing silicon nanoclusters, both Si2p peaks of Si(nc) and host SiO 2 undergo a charging shift that is 1 order of magnitude larger (>15 eV), with no measurable difference between them (i.e., no differential charging between the silicon nanoclusters and the oxide matrix could be detected). By use of a measured Auger parameter, we estimate the relaxation energy of the Si(nc) in the SiO 2 matrix as -0.4 eV, which yields a -0.6 eV shift in the binding energy of the Si(nc) with respect to that of bulk Si in the opposite direction of the expected quantum size effect. This must be related to the residual differential charging between the silicon nanoclusters and the oxide host. Therefore, differential charging is still the biggest obstacle for extracting size-dependent binding energy shifts with XPS when one uses the oxide peak as the reference.

Characterization of Si Nanocrystals

“…They used the C 1s signal exists due to the carbon contamination on the surface of the films as reference. The same amount of decrease was also reported by Aykutlu et al using the final state Auger parameter, which is free from charging effects [58]. In this study, the shift in the Si binding energy of Si nanoclusters was attributed to the relaxation energy differences.…”

Application of surface chemical analysis tools for characterization of nanoparticles

“…Some nanoparticle properties and measurement results can be impacted by agglomeration or aggregation or interactions of nanoparticles with the supporting substrate and/or with other nanoparticles (proximity effects [1,13]). Such effects include: i) buildup of charge during XPS measurements of metal clusters supported on insulating substrate [85-86], ii) coupling of plasmon modes in metal nanoparticles within close proximity [87-88], iii) coupling of quantum states [89], iv) impact of particle spacing on electronic and magnetic properties of composite [79,90], and v) effect of “buffer layers” on optical properties of silicon nanocrystal superlattices [91]…”

Characterization challenges for nanomaterials

“…In a more recent work, Suzer's group has looked at interface charging in different types of systems involving nanoparticles. [59] More generally, as nanoparticles become closer together, their magnetic, electronic, and optical properties may be altered. The coupling of quantum states of small particles as they approach each other is the subject of fundamental study.…”