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Abstract
Treatment of ZnO films in a supercritical fluid (SCF) has been reported to improve the performance of devices in which the treated ZnO films are incorporated; however, the mechanism of this improvement remains unclear. In this paper, we study the transformation of the surface morphologies and emission properties of ZnO films before and after SCF treatment, establishing the relationship between the treated and untreated structures and thereby enabling tuning of the catalytic or opto-electronic performance of ZnO films or ZnO-film-based devices. Both undoped and N-doped ZnO nanostructures generated by SCF treatment of films are investigated using techniques to characterize their surface morphology (scanning electron microscopy (SEM) and atomic force microscopy (AFM)) as well as room-temperature photoluminescence (RT-PL) spectroscopy. The water-mixed supercritical CO2 (W-SCCO2) technology was found to form nanostructures in ZnO films through a self-catalyzed process enabled by the Zn-rich conditions in the ZnO films. The W-SCCO2 was also found to promote the inhibition of defect luminescence by introducing -OH groups onto the films. Two models are proposed to explain the effects of the treatment with W-SCCO2. This work demonstrates that the W-SCCO2 technology can be used as an effective tool for the nanodesign and property enhancement of functional metal oxides.
Highlights
IntroductionA wide band-gap of 3.37 eV and a high exciton binding energy of 60meV make ZnO a promising semiconductor material in a variety of opto-electronic applications, such as laser diodes (LDs), light-emitting diodes (LEDs), and UV detectors, because of the successful fabrication of highquality ZnO crystalline films.[1,2,3] Recently, numerous studies have focused on ZnO nanostructures.[4,5,6,7,8,9,10,11,12,13,14,15,16,17] Compared to ZnO crystalline films, ZnO nanostructures possess various structures and emission properties, enabling their further application in many other fields.[8,13,14,16] Among the preparation methods used to synthesize and grow ZnO crystalline films and ZnO nanostructures, a hydrothermal process and chemical vapor deposition (CVD) are commonly chosen for both crystalline films and nanostructures, and catalysts have been introduced into the ZnO-nanostructure2158-3226/2018/8(5)/055310/10055310-2 Li et al.AIP Advances 8, 055310 (2018)synthesis process.[18]
We find that the luminescence of ZnO films with broad defect-related emission can be substantially improved by water-mixed supercritical CO2 (W-SCCO2) treatment
The surface morphologies of sample ZB transformed into various nanostructures after the W-SCCO2 treatment, resembling mixed clumps of grass (Figure 2(c)) and honeycomb-like nonsymmetrical
Summary
A wide band-gap of 3.37 eV and a high exciton binding energy of 60meV make ZnO a promising semiconductor material in a variety of opto-electronic applications, such as laser diodes (LDs), light-emitting diodes (LEDs), and UV detectors, because of the successful fabrication of highquality ZnO crystalline films.[1,2,3] Recently, numerous studies have focused on ZnO nanostructures.[4,5,6,7,8,9,10,11,12,13,14,15,16,17] Compared to ZnO crystalline films, ZnO nanostructures possess various structures and emission properties, enabling their further application in many other fields.[8,13,14,16] Among the preparation methods used to synthesize and grow ZnO crystalline films and ZnO nanostructures, a hydrothermal process and chemical vapor deposition (CVD) are commonly chosen for both crystalline films and nanostructures, and catalysts have been introduced into the ZnO-nanostructure2158-3226/2018/8(5)/055310/10055310-2 Li et al.AIP Advances 8, 055310 (2018)synthesis process.[18]. Researchers have reported that ZnO nanostructures can be fabricated on glass substrates by annealing ZnO films at 500◦C for 1 h under an argon atmosphere in the absence of a catalyst.[19]
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