EPR and photoluminescence spectroscopy studies on the defect structure of ZnO nanocrystals

Structural and optical properties of ZnO nanoparticles can be fine tuned by a novel variant of milling performed at cryogenic temperatures. In this study intrinsic defect centers such as oxygen and zinc vacancies are characterized using electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. Three different surface defects with different $g$ factors were identified by EPR for which the spectral intensities change upon decreasing the crystal size. EPR and PL intensities revealed a linear correlation giving detailed information about optical and electronic properties of ZnO. The core-shell model established from optical emission and EPR suggests distinguished electronic states in the band gap belonging to negatively charged Zn vacancies and positively charged oxygen vacancies. This model indicates a correlation between red emission and positively charged oxygen vacancies, which lead to a possible transition from a typical $n$-type to a $p$-type ZnO semiconductor.

Figure 1

XRD patterns of ZnO powders for various milling times for 2θ in the range of (a) 30–80° and (b) 31–38°, and (c) average crystallite sizes and lattice strains of ZnO particles as a function of milling time. Asterisks (*) in (b) indicate residual Kα${}_2$.[35]

Figure 2

(a) AFM image of ZnO nanocrystallite particles obtained by 10 min of freezer milling; (b) an expanded image of the red region shown in (a); (c) line-scan profile analysis taken along the trace shown in (b).

Figure 3

(a) X-band (9.33 GHz) EPR measurements at room temperature for 0–10 min freezer-milled ZnO samples; (b) ratios of core and surface defect centers deduced from X-band EPR intensity as a function of the milling time; and (c) comparison of X- and Q-band EPR signals on a common $g$-factor scale.

Figure 4

Room temperature X-band (9.47 GHz) EPR spectrum for the 10 min freezer-milled ZnO sample (black curve) and its overall spectral simulation (red/dark gray curve). The green (light gray), blue (medium gray), and violet (gray) curves show spectral contributions of D3, D2, and D1, respectively. The overall EPR spectrum (intensity $I_{\mathrm{total}}$) is built up from the contributions of D1, D2, and D3 as follows: $I_{\mathrm{total}}$ $=$ 1.0097, $I_{\mathrm{D}1}$ + 0.1401, $I_{\mathrm{D}2}$ + 0.0489 $I_{\mathrm{D}3}$.

Figure 5

(a) PL spectra recorded at room temperature, (b) integrated area of core and shell signals from PL spectra (for details, see text), and (c) linear relation of EPR spectral intensity and PL signal intensity (at 655 nm) for the surface defects (shell).

Figure 6

Schematic illustration of a ZnO nanocrystal, obtained from freezer milling. It consists of a negatively charged interior core and a positively charged outer shell. The EPR and PL spectra are given for the 1 min milled sample where we clearly observe core and shell defects simultaneously.

Figure 7

Sketch of the band gap as a function of the defect structure in core-shell structured nano-sized ZnO synthesized by the freezer milling method. It shows that the oxygen vacancies at the surface act as deep donors and the Zn vacancies in the core act as shallow acceptors.