A Green Approach to Reversibly Tuning the Optical Properties of Metal Oxides
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A Green Approach to Reversibly Tuning the Optical Properties of Metal Oxides Szetsen Lee* and Jr-Wei Peng Department of Chemistry and Center for Nano-technology, Chung Yuan Christian University, Jhongli, Taoyuan, 32023, Taiwan ABSTRACT Metal oxide (MO) films (ZnO and CuO) were synthesized by hydrothermal methods and treated with hydrogen and oxygen plasmas. From uv-visible transmittance spectra, we have found that the optical band gaps of MO films blue-shifted with hydrogen plasma treatment, but red-shifted with oxygen plasma treatment. By alternating the treatment sequence of hydrogen and oxygen plasmas, the MO optical band gap values can be reversibly tuned with the tunable ranges as wide as 80 and 550 meV for ZnO and CuO, respectively. The mechanism for reversible tuning of optical property is proposed based on the results of optical emission, X-ray diffraction, and scanning electron microscopy characterization. Compared to conventional metal ion doping and high temperature annealing methods, the use of low-temperature hydrogen and oxygen plasmas is more environmentally friendly. INTRODUCTION Metal oxides (MOs) have been the focuses of research and development in recent years. In particular, zinc oxide (ZnO) is a well-known n-type semiconducting material with wide and tunable direct band gap energy of 3.37 eV and high exciton binding energy (60 meV) [1]. Copper oxide is also an important p-type semiconductor. There are two crystalline forms of copper oxide: cuprous oxide (Cu2O) with a higher band gap (2.1 eV) and cupric oxide (CuO) with a lower band gap (1.2 eV) [2]. The unique optoelectronic properties of MOs have attracted a great deal interest in the applications such as transparent conductors [3], solar cells [4], and chemical sensors [5]. Device applications of MOs often require the band gaps to be tuned in specific ranges. The commonly used hydrothermal [6,7] and sol–gel synthesis methods [8] are effective processes to prepare various nanostructures of MOs. However, in order to achieve specific band gap values, doping process are often used. The conventional doping method for tuning the band gap of MOs often brings pollution issues to the environment and is energy- and time- consuming. Particularly, the environmentally hazardous rare-earth/transition metals are often used in doping processes [9]. Besides, doping is irreversible and has a low precision of tuning optical properties when repeated. MOs doping also reflects problems in energy consumption due to high temperature annealing processes required to improve crystal quality and reach desired band gap values [10]. In this study, instead of using conventional metal ion doping and annealing methods, we adopted room temperature plasma treatment as a post-synthesis approach to fine-tune the band gaps of two kinds of MOs (ZnO and CuO) at room temperature. The room temperature plasma method used in this work has the advantages of less time-consuming, environmentally friendly, with water as the product, repeatability, reversibility, and provides a wide and flex
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