Argon plasma-enhanced UV light emission from ZnO submicrowires grown by hydrothermal method

Abstract

In this work, the optical, morphological, and structural improvements of vertically aligned, hydrothermally grown ZnO submicrowires (SMWs) treated with a 250 W, 250 V RF argon plasma (Ar) during different exposure times were investigated by scanning and transmission electrons microscopy, X-ray diffraction, and micro-Raman and photoluminescence (PL) spectroscopies. In two steps, the SMWs were synthesized in an aqueous medium at low temperatures. The plasma-treated samples showed significantly improved room-temperature PL compared to untreated samples. All the treated samples exhibited a substantial increase of the near band edge UV emission intensity and a decrease of the deep level emission in the visible. The samples treated for 4 minutes presented the best UV/vis intensity ratio of ∼ 1568 (an increase of ⁓174-fold with respect to the untreated sample). The analysis of the UV band in terms of the first and second phonon replica of the excitonic emission indicated that the Ar plasma treatment favors the multiple phonon-exciton coupling, with partial correlation with the observed overall increase of the UV/visible intensity ratio. From the PL measurements at low temperatures, the presence of excitons bound to H donors in the ZnO structure was inferred. The possible reasons for the Ar plasma-induced enhancement of the UV emission from the treated SMWs are discussed in terms of previous work, the observed morphology and changes expected to occur at the SMW surfaces due to Ar ion impacts from the plasma.