Disentangling the Evolution of Electrons and Holes in photoexcited ZnO nanoparticles

The evolution of charge carriers in photoexcited room temperature ZnO nanoparticles in solution is investigated using ultrafast ultraviolet photoluminescence spectroscopy, ultrafast Zn K-edge absorption spectroscopy and ab-initio molecular dynamics (MD) simulations. The photoluminescence is excited at 4.66 eV, well above the band edge, and shows that electron cooling in the conduction band and exciton formation occur in <500 fs, in excellent agreement with theoretical predictions. The X-ray absorption measurements, obtained upon excitation close to the band edge at 3.49 eV, are sensitive to the migration and trapping of holes. They reveal that the 2 ps transient largely reproduces the previously reported transient obtained at 100 ps time delay in synchrotron studies. In addition, the X-ray absorption signal is found to rise in ~1.4 ps, which we attribute to the diffusion of holes through the lattice prior to their trapping at singly-charged oxygen vacancies. Indeed, the MD simulations show that impulsive trapping of holes induces an ultrafast expansion of the cage of Zn atoms in <200 fs, followed by an oscillatory response at a frequency of ~100 cm-1, which corresponds to a phonon mode of the system involving the Zn sub-lattice

Testing elementary and secondary school students’ ability to perform historical perspective taking: the constructing of valid and reliable measure instruments

Historical reasoning competencies play an important role in history education. However, valid and reliable large-scale measurement instruments to assess these competencies are scarce. This study considers two instruments for measuring students’ ability to perform historical perspective taking (HPT) as a historical reasoning competency. The instruments have been tested for validity and reliability among 1,270 Dutch upper elementary and secondary school students, ranging in age from 10 to 17 years. One instrument offers effective validity and reliability and can map HPT performance among a large and heterogeneous student population. The results show that even upper elementary school students are capable of performing HPT. However, as students age, their ability to perform HPT increases. Differences regarding the ability to perform HPT were also found between educational levels. Pre-university students performed HPT more successfully compared to students at lower educational levels. The results of this study can be used to gain insight into the construct of HPT and into how historical reasoning competencies such as HPT can be measured. Furthermore, the results provide insight into how differences between students, such as age and educational levels, influence the performance of HPT