Quenching of surface-exciton emission from ZnO nanocombs by plasma immersion ion implantation

Abstract

Surface modification of ZnO nanocombs was performed through a Ti plasma immersion ion implantation (PIII) with low bias voltages ranging from 0to5kV to quench surface-originated exciton emission. The ion energy dependent surface modification on ZnO was investigated using transmission electron microscopy and temperature-dependent photoluminescence (PL). The surface exciton (SX) was clearly identified for the as-grown sample at 4.5K, and complete quenching was observed for sample treated with 5kV PIII due to surface state passivation. The SX related surface states were located within 5nm in depth from the surface corresponding to the implantation depth of 5kV PIII. Room-temperature PL enhancement of these surface-modified ZnO nanocombs was observed and discussed. The results show that PIII can become a viable technique for nanostructure surface passivation.

Microscopic origins of the surface exciton photoluminescence in ZnO nanostructures

“…3,2,6,10,15 However, a number of reports of low temperature PL from ZnO nanostructures with varying aspect ratios show no consistent correlation from one report to another [22][23][24] and within the same report 2,15 between the SX peak relative intensity and the nanostructure aspect ratio. Based on the entirety of these reported data we question whether the SX peak's relative intensity is solely or mainly determined only by the nanostructure surface-to-volume ratio.…”

“…Adsorption and/or chemisorption processes at semiconductor surfaces, and specifically in the case of ZnO. 18,19,20 Adsorbates 5,6,9,21 with particular attention on O and O 2 , but no conclusion has been reached. Also, the assignment of the large surface-to-volume ratio of nanostructures as the sole or main determinant of the SX peak relative intensity is questionable given the existence of a number of reports of low temperature PL from ZnO nanostructures with varying aspect ratios, which show no consistent correlation from one report to another between the SX peak relative intensity and the nanostructure aspect ratio (as well as variations with morphology variations seen within individual reports) see e.g.…”

“…This was observed in freshly cleaved ZnO crystals by Travnikov et al 8 and more recently only in emission from ZnO nanostructures with high surface-to-volume ratios. 2,3,5,6,9,10 These properties, and the parallels with similar emissions observed in a range of other materials, 4,[11][12][13][14] have led to its labelling as a bound exciton transition at a surface or near surface defect. Although the SX peak itself is visible only at temperatures below 25 K, nanostructures showing the SX peak also have rapidly decaying bandedge intensity with increasing temperature 7 and it appears that the surface conditions responsible for the SX peak lead also to temperature activated non-radiative recombination processes at higher temperatures.…”

“…However, an asymmetric peak, broader than the I lines (~ 5 meV), at ∼3.367 eV, denoted as I 2 by Meyer et al, 1 and as a surface-related bound exciton (SX) peak by various authors, can be seen also, mostly in nanostructured materials. [2][3][4][5][6][7] . This was observed in freshly cleaved ZnO crystals by Travnikov et al 8 and more recently only in emission from ZnO nanostructures with high surface-to-volume ratios.…”

Ultrasound Assisted Synthesis and Properties of ZnO:B Nanorods and Micro Flowers

“…Several studies have been made in this direction 2-6 and many methods have been tried by various investigators to suppress the defect emission and enhance UV emission. [7][8][9] In recent years, the spectrum of synthesis options for production of nanocrystalline materials such as; thin films, nanorods, nanowires and nanocomposites has become considerably broader. [10][11][12] Zhao et al 13 have used plasma treatment to enhance the band edge emission.…”