文献讲解

the [Zn29O57]-56cluster model
the [Zn29O 57]56-cluster model that was used to calculate the electronic states in this study. Larger clusters such as [Zn47O87]80-gave slightly more accurate results but,because the difference was verysmall, [Zn29O57]56clusterswere mainly used to enable smooth and faster operation of the software.
0 a
I G [ I G 0 Ri ( IU 0 IU )i ]
i
1 1 CG exp( Ea , G ) KT
IU
IU 0 1 CU exp( Ea,U )
the variation in the calculated band-gap energy ( Eg) with the number of atoms in the cluster models.
Experimental
• Commercial ZnO powder (<5lm, 99.9%, Sigma Aldrich)was heat treated at 700℃ in four different atmospheres: an air atmosphere; pure nitrogen gas flow; mixed gas flow of 5% H2 and 95% N2 ; and a CO/CO2 atmosphere generated using activated carbon (Junsei Chemicals). • PL was measured using a Xe lamp and a monochromator system (Acton Research Corp.). To observe the thermal quenching behavior, the PL emission was measured in the temperature range 80–300 K.
2.This article main research contents:
In this study, the luminescence properties of ZnO were investigated experimentally and by first-principles methods. Thermal treatment in various atmospheres provided insight into the UV and green emission mechanisms. These mechanisms were examined in detail from the thermal quenching behavior of emission spectra. The structure of electronic energy states was calculated by a first-principlesmethod known as the discrete variational (DV) Xa molecular orbital method. Cluster models with and without an oxygen vacancy were analyzed. Based on the results, the mechanisms and interrelation of the photoluminescence (PL) emissions of ZnO are discussed.
Investigation of photoluminescence mechanisms of ZnO through experimental and fiirst-principles calculation methods
报告人：吴凤霞
contents：
• • • • • Abstract Introduction Experimental Results and discussion Conclusions
the measured UV intensities (symbols) and the fitting results (continuous lines)
• were calculated as13meV for the 370 nm peak and~45 meV for the peaksmat 375 I I and 383 nm, using 1 C exp( E ) kT These results agree wellwith those for the localization and donor-binding energies related to hydrogen interstitials, which are reported to be13.1 and 46.1 meV, respectively
As the cluster size increases, the calculated Eg approached the measured value. Eg for [Zn29O57]56-and [Zn47O87]80are 4.03 and 3.95 eV, respectively. Compared with other firstprinciples calculations , DV-Xa provides reasonably accurate results, suggesting that it is a reliable simulation tool for investigating the elec-tronic and optical properties of ZnO.
Introduction
1.Why ZnO can be used as a phosphor material
ZnO has been actively investigated for use in optical devices because it has a wide band gap ( ~3.37 eV) and a high exciton-binding energy ( ~ 60 meV) . Ultraviolet(UV) lasers and UV ight-emitting diodes have been fabricated from ZnO nanorods arrays and thin films.ZnO has also been investigated as a phosphor material for display devices, because it exhibits strong green emission.Although the exact mechanism remains a matter of debate many researchersconcur that the green emission originates from oxygen vacancies
Results and discussion room-temperature PL emission spectra of ZnO after heat treatment.
• These results appear to indicate that the enhancement mechanism of intensity is different for UV and green emission: UV emission seems to be related to hydrogen atoms (interstitials or substitutions for lattice atoms), although their identification in ZnO crystal is still controversial, whereas the green emission is enhanced by the formation of oxygen vacancies by reduction.
Abstract
The photoluminescence properties of ZnO were investigated experimentally and by a first principles method. Thermal treatment of ZnO in various atmospheres provides insight into the ultraviolet (UV) and green emission mechanisms. These mechanisms were investigated in detail from the thermal quenching behavior of the emission spectra of ZnO. The negative thermal quenching of the green emission could be fitted exactly with a model equation based on the assumption that UV emission loss by thermal quenching affects green emission. The electronic states of ZnO clusters with and without an oxygen vacancy were calculated by first-principles calculations usingbthe discrete variational (DV) Xa molecuBaidu Nhomakorabeaar orbital method. This method gave a reasonably accurate calculation of band-gap energy. The energy levels calculated by the DV-Xa method agree well with experimentally measured thermal quenching activation energy and the zero-phonon energy of green emission. The results indicate that oxygen vacancies function as luminescent centers for green emission.
The thermal quenching behavior
The UV peak positions are the same for both samples, although their intensities are greatly enhanced by reduction. This may indicate that defects (presumably interstitial hydrogen atoms) are natively present in ZnO powder and that their concentration increases rapidly with annealing in H2 /N2