Photoluminescence transient study of surface defects in ZnO nanorods grown by chemical bath deposition

Abstract

Two deep level defects (2.25 and 2.03 eV) associated with oxygen vacancies (V o ) were identified in ZnO nanorods (NRs) grown by low cost chemical bath deposition. A transient behaviour in the photoluminescence (PL) intensity of the two V o states was found to be sensitive to the ambient environment and to NR postgrowth treatment. The largest transient was found in samples dried on a hot plate with a PL intensity decay time, in air only, of 23 and 80 s for the 2.25 and 2.03 eV peaks, respectively. Resistance measurements under UV exposure exhibited a transient behaviour in full agreement with the PL transient indicating a clear role of atmospheric O 2 on the surface defect states. A model for surface defect transient behaviour due to band bending with respect to the Fermi level is proposed. The results have implications for a variety of sensing and photovoltaic applications of ZnO NRs.PACS numbers: 68.43. Fg, 68.43.Tj, 73.20.Hb, 73.20.At Zinc oxide is a wide band gap (∼ 3.2→3.4 eV) ntype semiconductor with a large exciton binding energy (60 meV) and is a promising material for a range of applications 1 . However, studies on low cost ZnO nanostructures (NSs) and thin films are unclear as to the source of n-type conductivity and persistent photoconductivity (PPC), the UV sensing mechanism, and the defect landscape 2,3 . In particular, the correlation between the defect landscape and sensing (whether gas, pH, or UV) responsivity in low cost ZnO NRs is debated. In one model, the neutral oxygen vacancy (V o ) is an n-type donor state 3 , atmospheric O 2 absorbs at this site 2 , and a depletion region forms beneath the surface 4 . UV excitation creates electron-hole pairs, holes migrate to the depletion region and O 2 desorption occurs, thus reducing the depletion region and increasing the conductivity 4,5 . Therefore, the kinetics of O 2 desorption determine the response time of a UV sensor. In a second model, V o is reported to be a deep level state (DLS) The quality, diameter, and length of the NRs was measured using a Gemini field emission scanning electron microscopy (SEM) Carl Zeiss SUPRATM 25. CBD introduces water based absorbates 13-15 , we controlled this parameter by comparing samples as-prepared not dried (ND), dried at 100 o C for 20 min on a hot plate, and left to dry over two weeks. Additionally, we annealed a sample at 600 o C for 30 min in O 2 to understand the role of V o .Resistance measurements were performed under exposure to 364 nm UV light. The samples were biased to force a current of 1 nA between two probes 1 mm apart to extract the resistance. This value was used to avoid compliance in the samples and gave a good overall response under UV exposure. PL measurements were performed by pumping at 1.5 mW the 325 nm line of a He-Cd laser chopped through an acousto-optic modulator at a frequency of 55 Hz. The PL signal was analyzed by a single grating monochromator, detected with a Hamamatsu visible photomultiplier, and recorded with a lock-in am-

ZnO Microflowers Grown by Chemical Bath Deposition: A Low-Cost Approach for Massive Production of Functional Nanostructures

“…The UV response of ZnO MFs was tested and the results clearly confirm that ZnO MFs are promising low-cost nanostructures for the construction of UV sensors. The above results are in agreement with the value ranges reported in the literature [40][41][42][43]. The slow response to UV illumination is partially determined by the desorption kinetics of the atmospheric oxygen molecules induced by UV photons [40,41].…”

“…The above results are in agreement with the value ranges reported in the literature [40][41][42][43]. The slow response to UV illumination is partially determined by the desorption kinetics of the atmospheric oxygen molecules induced by UV photons [40,41]. At ambient conditions, oxygen molecules were absorbed in correspondence with oxygen vacancies at ZnO surface.…”

“…As described in the literature, the photo-detection mechanism in ZnO structures is essentially based on the absorption-desorption of atmospheric oxygen occurring at the surface that causes a change in the conductivity of ZnO. The width of ZnO exposed area and the nature of the native defects in ZnO (as zinc and oxygen vacancies) were proven to have an important role in defining the UV-sensing performance [9,16,40].…”

Solvothermal synthesis of ZnO nanoparticles at low temperatures as cathode buffer layers for polymer solar cells with an inverted device structure

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