The all-inorganic perovskite nanocrystals are currently in the research spotlight owing to their physical stability and superior optical properties—these features make them interesting for optoelectronic and photovoltaic applications. Here, we report on the observation of highly efficient carrier multiplication in colloidal CsPbI3 nanocrystals prepared by a hot-injection method. The carrier multiplication process counteracts thermalization of hot carriers and as such provides the potential to increase the conversion efficiency of solar cells. We demonstrate that carrier multiplication commences at the threshold excitation energy near the energy conservation limit of twice the band gap, and has step-like characteristics with an extremely high quantum yield of up to 98%. Using ultrahigh temporal resolution, we show that carrier multiplication induces a longer build-up of the free carrier concentration, thus providing important insights into the physical mechanism responsible for this phenomenon. The evidence is obtained using three independent experimental approaches, and is conclusive.
Abstract2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum. Nevertheless, the effective oscillator strengths of these transitions have been scarcely reported, nor is there a consistent interpretation of the obtained values. Here, we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect (OSE). Intriguingly, we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength FStark that is 50 times smaller than expected based on the linear absorption coefficient. We propose that the pronounced exciton absorption line should be seen as the sum of multiple, low oscillator strength transitions, rather than a single high oscillator strength one, a feat we assign to strong exciton center-of-mass localization. Within the quantum mechanical description of excitons, this 50-fold difference between both oscillator strengths corresponds to the ratio between the coherence area of the exciton’s center of mass and the total area, which yields a coherence area of a mere 6.1 nm2. Since we find that the coherence area increases with reducing temperature, we conclude that thermal effects, related to lattice vibrations, contribute to exciton localization. In further support of this localization model, we show that FStark is independent of the nanoplatelet area, correctly predicts the radiative lifetime, and lines up for strongly confined quantum dot systems.
Formation and decay of core-orbital vacancies in the water molecule.Chemical Physics Letters, http://dx.doi.org/10.1016/j.cplett. 2012.11.094 Access to the published version may require subscription. N.B. When citing this work, cite the original published paper. and 110 eV. Nuclear motion in these states competes with Auger decay and substantially modifies the final state spectra. The double core-hole state from ionisation of both 1s electrons is found at 1171±1 eV and calculated at 1170.85 eV.
We present a demonstration of online monitoring of gas-phase photocatalytic reactions. A cotton cloth impregnated with commercial titanium dioxide nanoparticles is used as a photocatalytic filter to clean air contaminated with a model pollutant. A fan forces air through the filter while it is irradiated by UV diodes. The concentration of the air pollutant is measured online by an inexpensive, commercially available semiconductor air quality sensor. The structural parts of the reactor were 3D printed in polylactide bioplastic. We provide all schematics, 3D printed model parts, hardware, firmware, and computer code of the reactor and control units. The device can be used for interactive learning of both gas phase photocatalysis and gas sensing, as well as in student laboratory classes for measuring air pollutants and their photodegradation. The experimental setup can also form the basis for a project work for chemical engineering university students, and it can be employed as a building block for development of other gas phase chemical reaction demonstrations.
We report on the epitaxial growth and the optical properties of Tm,Yb-codoped ZnO (ZnO:Tm,Yb) thin films by sputtering-assisted metalorganic chemical vapor deposition. The characteristic light emission due to the intra-4f shell transitions of Tm 3þ and Yb 3þ ions in the ZnO host is observed at 4 K by using photoluminescence (PL) characterization. The time-resolved PL measurements on the Tm 3þ and Yb 3þ emission reveals that the carrier decay time of each ion can vary by changing the concentration of both Tm 3þ and Yb 3þ ions, which depends on a mixing ratio of Tm 2 O 3 and Yb 2 O 3 in a sputtering target. The energy transfer from Tm 3þ to Yb 3þ ions occurs in the sample with the high mixing ratio, and the reversed energy transfer process occurs with the lower mixing ratio. These results indicate that the concentration, and thus the carrier dynamics of Tm 3þ and Yb 3þ ions in the ZnO:Tm,Yb host, can be controlled by altering the mixing ratio of the sputter target and the frequency power.
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