CC BY
9LD-I-22
Pulsed laser fabrication of Zn and ZnO nanoparticles meant for implementation in chemical sensors
M.D. Komissarov, N.B. Leonov, T.A. Vartanyan
ITMO University, Kronverkskypr. 49, 197101 St. Petersburg, Russia
email address: tavartanyan@itmo.ru
Zinc oxide as a wide bandgap semiconductor with large exciton bounding energy has found numerous applications. It is used as an effective UV light absorber, transparent electrode in thin film solar cells, and transducer material in chemical sensors [1]. The last application is based on its intrinsic nonstoichiometry that may be affected by the gas environment. While the thin film conductivity changes in response to the different gases exposure is most explored, changes of the photoluminescence properties are also promising. Oxygen vacancies are believed to be responsible for the bright green luminescence under UV irradiation. In the presence of ambient oxygen they are healed and the luminescence intensity drops. To function as an oxygen pressure transducer zinc oxide is to be prepared in a thin film form, preferably, as nanoparticles with large surface area.
Based on our experience in physical vapor deposition of granular metal films [2,3] we adopted the following procedure. First, the granular zinc films were produced via the pulsed laser fabrication method. The metallic zinc target of 99,99% purity was irradiated by the second harmonic of Nd:YAG laser. The laser pulses with 10 ns duration follow with the repetition rate of 10 Hz. The amount of the deposited material varies in dependence of the laser pulse energies and the number of pulses.
The atomic force microscopy images show the formation of granular films of different mean thicknesses. Absorbance spectra confirm the formation of an array of densely packed flat zinc nanoparticles with the plasmon resonance that shifts from 320 to 650 nm when the mean thickness of the film changes from 5 to 12 nm and then stabilizes. Simultaneously, the surface conductivity of the granular film rapidly drops by 6 orders of magnitude. This observation attests to the arriving at the percolation threshold at this rather low amount of deposited material.
The obtained granular zinc films were transformed into zinc oxide via heating in a muffle furnace at 800°C for 3 hours. This transformation is accompanied by the plasmon bands disappearance and emerging of the exciton band at 360 nm.
Photoluminescence of zinc oxide was observed in two bands with maxima at 390 and 510 nm. The short-wavelength band corresponds to the exciton luminescence while the long-wavelength band is due to the defects. The relative intensity of the two photoluminescence bands depends on the nature of the excitation source. When the photoluminescence was excited by the 10 ns pulses of the 3-rd harmonic of the Nd:YAG laser at 355 nm the short-wavelength band dominate, while at low intensity cw excitation at somewhat shorter wavelengths both bands have comparable intensities. The reason for this difference was found to be the saturation of the long-wavelength fluorescence known as "green" luminescence of zinc oxide band and associated with defects. This saturation happens already at the fluencies as small as 2 mJ/cm2. On the other hand, the short-wavelength fluorescence band associated with the exciton recombination remains unsaturated below at least 40 mJ/cm2
[1] Transparent Conductive Zinc Oxide. Basics and Applications in Thin Film Solar Cells. Ed.: K. Ellmer, A. Klein, B. Rech (SpringerVerlag Berlin Heidelberg) (2008)
[2] N.B. Leonov, I.A. Gladskikh, V.A. Polishchuk and T.A. Vartanyan, Evolution of the Optical Properties and Morphology of Thin Metal Films during Growth and Annealing, Optics and Spectroscopy, vol. 119, pp. 458-463, (2015).
[3] N.B. Leonov, V.A. Polishchuk and T.A. Vartanyan, An investigation of major factors affecting metal nanoparticle morphology in island films. In: Metal Nanoparticles: Properties, Synthesis and Applications, Ed.: Y. Saylor and V. Irby (Nova Science Publishers, Inc., New York) (2018)
