Properties of graphene flakes obtained by treating graphite with ultrasound - V. O. Yukhymchuk; Ya. Valakh; O. M. Hreshchuk; Ye. O. Havrylyuk; I. B. Yanchuk; A. V. Yefanov; R. N. Arif; A. G. Rozhin; M. A. Skoryk (Ukrainian Journal of Physics, 2017)