Study of optical and dielectric properties of annealed ZnO nanoparticles in the terahertz regime

Abstract

The frequency-dependent optical and dielectric properties of annealed ZnO nanoparticles in the range of 0.1 to 0.9 THz are studied by using terahertz time-domain spectroscopy (THz-TDS). The refractive index, power absorption and complex dielectric constants are obtained and the experimental results are well fit with a simple effective medium theory in conjunction with a pseudo-harmonic model. This study reveals that annealed ZnO nanoparticles exhibit the similar phonon response characteristics to the single ZnO crystal and other ZnO nanostructures, such as tetrapods and nanowires.In recent years, ZnO has attracted great research interest because of its unique properties and a variety of potential applications in electro-optic, acousto-optic, optoelectronic devices, chemical sensors and piezoelectrics [1][2][3] . ZnO can be a highly potential choice for ultraviolet light emitters and low voltage applications [4,5] , and it also could be used in integrated active optic devices and generation of high power terahertz pulses [6] . Some ZnO nanoparticles have shown exciting applications in the field of optoelectronics due to its unique capability of being integrated into nano-devices with very low power consumption [7] . Recent advancement in nanomaterial research has revealed that a new type of ZnOorganic coated nanoparticles has luminescence properties [8] .In this paper, we present a study of far-infrared optical and dielectric properties of annealed ZnO nanoparticles made from the luminescence coated ZnO nanopowders in the regime of 0.1-0.9 THz using the terahertz time-domain spectroscopy system (THz-TDS).The THz-TDS system is a photoconductive switch-based spectrometer with four parabolic mirrors arranged in an 8-F confocal geometry [9] . This confocal configuration enables excellent beam coupling between the transmitter and the receiver. A frequency-independent 3.5 mm-diameter terahertz beam waist is achieved for sample characterization. The THz-TDS system is optically excited using a Kerr-lens modelocked Ti:sapphire laser of 26 fs pulse width and 88 MHz repetition rate and has an useful bandwidth of 0.1-4.5 THz (3 mm-67 m) and a signal-to-noise ratio (S/N) greater than 1.0 10 4 :1.The annealed ZnO nanopowder is prepared in the following two steps. First, wet chemistry method called reverse micelle technique is used to make the luminescence coated ZnO nanopowders [10] . The zinc acetylacetonate hydrate powder (1.4122 gm, 5.36 mmole) is dissolved in 35 ml ethanol and put into a flask that is connected with a water condenser. 1 ml DI water and stabilizer polyethylene glycol biscarboxymethyl ether (0.85 gm, 3.3 mmole) is then mixed with the prepared murky solution. The murky solution is then heated up to boiling point under vigorous magnetic stirring. Another cooled solution is prepared using sodium hydroxide pellets (0.4 gm, 10 mmole) which is dissolved in 35 ml ethanol. The cooled solution with temperature around 0 o C is then added slowly using a transfer pipet into the boiling solution. After that, a white preci...

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