Nanoscale physics

EPR study of luminescent SiO2:C nanocomposites prepared using sucrose solution
Savchenko D.V.1,2, Avramishin S.S.1, Kuz O.P.1, Vasin A.V.1,3, 
Kysil D.V.3, Rusavsky A.V.1,3, Nazarov A.N.1,3,  Kalabukhova E.N.3
1 National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, Prospect Peremohy, 37, Kyiv, 03056, Ukraine. 
E-mail: dariyasavchenko@gmail.com
2 Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21 Prague, Czech Republic.
3 V.E. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, Prospect Nauky 41, Kyiv, 03028, Ukraine.

The luminescent carbonized silica (SiO2:C) nanocomposites were studied by electron paramagnetic resonance (EPR) method. The samples were prepared by the infiltration of fumed silica precursor (A-300, S~295 m2/g, d~10-12 nm) by sucrose water solution as a carbon source. Different concentrations of sucrose solution (800 and 400 mmol/g) and annealing temperatures (300ºC and 400ºC) were used to study the effect of carbon incorporation. No EPR spectra were observed in the samples before thermal annealing. After samples annealing, the single EPR line of the Dysonian lineshape was detected. The temperature dependence of the EPR signal integral intensity was described by the sum of two contributions: Curie-Weiss (for localized spins) and Pauli (free electrons). From the analysis of the temperature dependence of g-factor the observed EPR signal was attributed to conduction electrons (g~2.0040(5)) coupled with localized spins with g~1.998 related to singly occupied orbital in ground-state carbon dots [1]. The temperature dependence of the EPR linewidth obeys variable-range hopping law. From the analysis of the EPR line asymmetry parameter, it was found that the sample with 800 mmol/g and Tann=300ºC possesses the highest conductivity that is in agreement with the obtained highest average barrier height for hopping motion in this sample. 
Thus, it was concluded that the intense photoluminescence in the investigated SiO2:C nanocomposites should be related to the presence of carbon dots.
The work was supported by MES of Ukraine (Project F2211), Projects No. CZ.02.1.01/0.0/0.0/16_013/0001406, LM2015088 and LO1409 (MEYS CR).

1.	Lin Z., Dou X., Li H.,et al Silicon-hybrid carbon dots strongly enhance the chemiluminescence of luminol // Microchim Acta.-2014. -181.-P. 805–811.