[1] Univ Pretoria, Dept Phys, ZA-0002 Pretoria, South Africa
[2] Univ Pretoria, ENGAGE, ZA-0002 Pretoria, South Africa
[3] Univ South Africa, Inst Nanotechnol & Water Sustainabil, Coll Sci Engn & Technol, Florida Sci Campus, ZA-1710 Johannesburg, South Africa
[4] Walter Sisulu Univ, Dept Chem & Phys Sci, Private Bag x1, Mthatha, South Africa
[5] Sefako Makgatho Hlth Sci Univ, Dept Phys, POB 94, ZA-0204 Medunsa, South Africa