Radiative mechanism and surface modification of four visible deep level defect states in ZnO nanorods

Abstract

Visible luminescence from ZnO nanorods (NRs) is attracting large scientific interest for light emission and sensing applications. We study visible luminescent defects in ZnO NRs as a function of post growth thermal treatments, and find four distinct visible deep level defect states (VDLSs): blue (2.52 eV), green (2.23 eV), orange (2.03 eV), and red (1.92 eV). Photoluminescence (PL) studies reveal a distinct modification in the UV (3.25 eV) emission intensity and a shift in the visible spectra after annealing. Annealing at 600 °C in Ar (Ar600) and O2 (O600) causes a blue and red-shift in the visible emission band, respectively. All samples demonstrate orange emission from the core of the NR, with an additional surface related green, blue, and red emission in the As-Prep, Ar600, and O600 samples, respectively. From PL excitation (PLE) measurements we determine the onset energy for population of the various VDLSs, and relate it to the presence of an Urbach tail below the conduction band due to a presence of ionized Zni or Zni complexes. We measured an onset energy of 3.25 eV for the as prepared sample. The onset energy red-shifts in the annealed samples by about 0.05 to 0.1 eV indicating a change in the defect structure, which we relate to the shift in the visible emission. We then used X-ray photoemission spectroscopy (XPS), and elastic recoil detection analysis (ERDA) to understand changes in the surface structure, and H content, respectively. The results of the XPS and ERDA analysis explain how the chemical states are modified due to annealing. We summarize our results by correlating our VDLSs with specific intrinsic defect states to build a model for PL emission in ZnO NRs. These results are important for understanding how to control defect related visible emission for sensing and electroluminescence applications.

Shallow acceptor state in ZnO realized by ion irradiation and annealing route

“…However, both of these PL peaks are either isolated VZn related or OZn type, as envisaged theoretically and experimentally [31,45,46]. Broad PL peak ~ 2.8 eV can be a signature of transition from different ionization states of Zn interstitials to VB band or shallow acceptor levels [47,48]. In this work, enhanced 2.8 eV PL emission is correlated with lower sheet Shift of (002) XRD peak to higher angle (compared to ZnO-I) can be due to partial recovery of interstitial as well as vacancy defects [50].…”

The effect of growth temperature of seed layer on the structural and optical properties of ZnO nanorods

“…from 400 to 525 nm. This emission is basically governed by the defect states present in the nanorod structure as explained by several researchers [19][20][21][22]. In addition to this, an emission is also observed in the UV region at 390 nm, which is the basic signature of the near band edge recombination in ZnO structure.…”

Photoluminescence Study of the Influence of Additive Ammonium Hydroxide in Hydrothermally Grown ZnO Nanowires

“…From the Figure 2a, we can also observe a broad visible level emission that expands from green to orange color wavelength. The presence of a broad visible emission peak may be explained with the hypothesis of existence of multiple defects and/or defect complexes which are dominantly present at the surface of ZnO nanostructures [29,39]. However, in spite of a number of reports about the presence of visible emission in the ZnO's PL spectrum, there is no clear consensus in the literature on the peak positions in the visible region or on their origin.…”