Photophysics of Point Defects in ZnO Nanoparticles

Abstract

including oxygen vacancies ( V O ), [ 6 ] and zinc vacancies ( V Zn ), [ 7 ] interstitials (Zn i , O i ), [ 8 ] and antisite defects (Zn O , O Zn ) [ 9 ] as well as chemical impurities such as Cu. [ 10 ] The need to characterize point defects in ZnO is further amplifi ed with the recent applications of ZnO nanoparticles as hosts of single emitters for quantum information processing [11][12][13] and their use in random lasing [ 14 ] and other advanced sensing technologies. [ 15 ] These applications require precise control over the defect engineering in ZnO nanoparticles. Consequently, correlative characterization of point defects will be valuable to explore the luminescence properties and affi liate them with their chemical and paramagnetic spin features.In this work we perform comprehensive studies on the formation and photophysical properties of point defects in ZnO nanoparticles. In particular, we employ correlative characterization techniques to assign the optical emission peaks and electron paramagnetic resonance (EPR) lines to specifi c defects in ZnO nanoparticles. Our results provide new insight into optical luminescence properties of ZnO nanoparticles and promote them as potential candidate for nanophotonic technologies. [ 16 ] Results and DiscussionAfter being annealed in an Ar or Zn vapor environment at 700-900 °C the as-received ZnO nanoparticles (diameter ≈ 20 nm) coalescence [ 17,18 ] and display faceted morphologies ( Figure 1 ). The nanoparticles increase in average size up to about 120 nm after annealing in inert gas (Ar) or oxygen, and up to 150 nm for annealing in Zn vapor at 900 °C [see Figures S1 and S2, Supporting Information for nanoparticle sizes obtained from scanning electron microscopy (SEM) image and X-ray diffraction (XRD) analysis]. Since the Bohr radius of ZnO is 2.34 nm, [ 19 ] the nanoparticles are large enough to avoid quantum size effects but still possess a suffi ciently large surface area to allow defect engineering.To investigate the occurrence of defects in ZnO nanoparticles, EPR spectroscopy was performed. Figure 2 a shows EPR spectra of the as-received, O 2 annealed, Zn vapor annealed ZnO nanoparticles and a bare Si(100) substrate for comparison. The as-received ZnO nanoparticles exhibit a strong signal at g = 1.96, which has been previously assigned to zinc vacancies (V Zn − ), [ 20 ] oxygen interstitials (O i − ), [ 20 ] zinc interstitials (Zn i + ) [ 21 ] and to electrons in weakly bound or conduction band (CB) Zinc oxide (ZnO) nanoparticles have recently been identifi ed as a promising candidate for advanced nanophotonics applications and quantum technologies. This work reports the formation of luminescent point defects and describes their photophysical properties. In particular, it is shown using correlative photoluminescence, cathodoluminescence, electron paramagnetic resonance (EPR), and X-ray absorption near-edge spectroscopy that green luminescence at 2.48 eV and an EPR line at g = 2.00 belong to a surface oxygen vacancy (V o,s + ) center, while a second green emi...

Effects of annealing on optical and structural properties of zinc oxide nanocrystals

“…Figure 6b shows the relative intensities of each DL sub‐bands to the total DL intensities. On the one hand, annealing in O results in a quenching of GL1 and enhancement of GL2, which is in agreement with previous studies, suggesting that GL1 and GL2 originate from V and V related centers, respectively . On the other hand, annealing in Zn‐vapor reduces the V and produces V, leading to abrupt quenching of GL1 and enhancement of GL2.…”

“…Corresponding increase of the ZnO NC sizes were also observed for case of annealing in Zn‐vapor and O‐rich atmospheres. Such increase in particle sizes after annealing has been reported by us () and other authors . Briefly, during the annealing process, the surface diffusion and migration of atoms occur between neighboring NCs.…”

Ultrasensitive ZnO Nanowire Photodetectors with a Polymer Electret Interlayer for Minimizing Dark Current

“…To clarify the function of PVN interlayer, the charge distribution and the corresponding energy band diagram in the ZnO NW photodetector are schematically illustrated in Figure . It has been recognized that ZnO NW is an n‐type semiconductor containing abundant defects especially in the surface region, mainly consisting of oxygen vacancies and interstitial Zn atoms . In addition, the adsorption of O 2 molecules on the surface will capture free electrons in the ZnO NW via [O 2 (gas) + e ‐ → O 2 ‐ (adsorbed)], forming an important type of surface states and generating an internal electric field along the NW radial direction .…”

Zinc Oxide Nanophotonics

“…Cu is a major trace impurity in Zn metal used as a precursor material in ZnO growth accounting for the large number of reports on the structured green luminescence band. Following the works of Vanheusden et al and Choi et al, the green emission was attributed to transitions from singly ionized oxygen vacancies (V + O ) [47,48]. The origin of the green emission is still open and controversial, but there is strong experimental evidence that it is located at the surface.…”

“…Different treatment conditions can strongly affect on the formation of color centers in ZnO. For example, plasma treatment or annealing in different atmospheres and temperature can change emission properties, such as Zn-rich or Orich conditions [48]. In ZnO nanostructures, high temperature annealing (> 700 ∘ C) created other defects that showed strong green emission, rather than red emission showing SPSs.…”