Moreover, thermal quenching is found to be more severe for the high energy PL components which lead to an apparent red shift of the PL maximum position at high T. To get further insights into the mechanisms responsible for the observed thermal quenching, we have analyzed Arrhenius plots of the PL intensity at

different detection energies (E det) as shown in Figure  2a. The analysis was performed for constant detection JQ1 in vivo energies since (a) the temperature-induced shift of the bandgap energy is significantly suppressed in GaNP alloys [15], and (b) spectral positions of the excitons bound to various deep-level N-related centers do not one-to-one follow the temperature-induced shift of the bandgap energy. This approximation defines error bars of the deduced values as specified below. All experimental data (shown by the symbols in Figure  2) can be fitted bywhere I(T) is the temperature-dependent PL intensity, I(0) is its value at 4 K, E 1 and E 2 are the activation energies

for two different thermal quenching processes, and k is the Boltzman constant (the results of the fitting are shown by the solid lines in Figure  2a). The first activation process that occurs within the 30 to 100 K temperature range is characterized by the activation energy E 1 ranging between 40 (at E det = 2.17 eV) and 60 meV (at E det = 2.06 eV). The contribution of this process is most pronounced for high energy PL components that correspond to the radiative recombination at the N-related localized states with CT99021 price their energy levels close to the GaNP band

edge. The quenching of the high energy PL components is accompanied by a slight increase in the PL intensity at low E det. Therefore, this process can be attributed to the thermal ionization of the N-related localized states. Such ionization is expected to start from the N-states that are shallower in energy. The thermally activated excitons can then be recaptured by the deeper N states, consistent with our experimental observations. We note that the determined values of E 1 do not one-to-one correspond to the ‘apparent’ depth of the involved localized states deduced simply from the distance between E det and the bandgap energy of the GaNP. Celastrol This is, however, not surprising since such correspondence is only expected for the no-phonon excitonic transitions whereas recombination of excitons at strongly localized states (such as the monitored N states) is usually dominated by phonon-assisted transitions due to strong coupling with phonons. Figure 2 Arrhenius plots of the PL intensity measured at different detection energies from the GaP/GaNP NWs (a) and GaNP epilayer (b). (1) The second thermal quenching process is characterized by the activation energy E 2 of approximately 180 ± 20 meV, which is the same for all detection energies. This process becomes dominant at T > 100 K and leads to an overall quenching of the PL intensity irrespective of detection energies.