Presence Of Self-trapped Excitons Research Articles

CO2 Pressure‐Induced Self‐Trapped Excitons in SrTiO3

With strong electron–phonon coupling, self‐trapped excitons (STEs) are typically formed in perovskite materials, and radiative recombination of STEs can produce broadband emission with large Stokes shifts. STEs are essential to further improve the optoelectronic properties of materials. Surprisingly, 2D system is the edge case, with low even no self‐trapping barriers, leading to effortless formation of STEs. In this work, 2D strontium titanate (SrTiO3) with defects is prepared using supercritical carbon dioxide (SC CO2) and its carrier transport and transition are studied. The appearance of wide photoinduced positive absorption signals in the femtosecond transient absorption spectra is direct evidence for the formation of STEs. The presence of STEs is further supported by the increased Stokes shift and full width at half maximum in the steady‐state photoluminescence spectra.

UV to NIR photon conversion in Nd-doped rutile and anatase titanium dioxide films for silicon solar cell application

Undoped and Nd-doped titanium dioxide anatase and rutile films have been grown by pulsed-laser deposition at 700°C under 0.1mbar O2. By selecting adequate substrates, TiO2 films doped with 1, 2 or 5at.% Nd were grown and constituted with polycrystalline rutile, highly oriented (200) rutile film, or oriented (004) anatase. An UV to NIR photon conversion is evidenced in the films. Indeed, intense and well-resolved emission lines from Nd3+ have been observed upon excitation above the TiO2 bandgap at room temperature. The sensitised emission of Nd3+ is found to be much efficient in rutile than in anatase structure. Low temperature photoluminescence measurements lead to fine resolved peaks corresponding to the Nd3+ 4f transitions with different spectral characteristic according to the host matrix used. Photoluminescence dependence temperature evidences that the light emission from Nd3+ in anatase-based films is probably influenced by the presence of self-trapped excitons or by orbital interaction. Mechanisms of sensitisation host to Nd3+ are proposed for both matrixes. Finally, the Nd dopant concentration and the microstructure of TiO2 rutile films are found to affect the photoluminescence emission intensity. Rutile film (200) oriented is the most adapted host matrix to sensitise 1at.% Nd3+ ions for an emission around 1064nm making such Nd-doped layers interesting for photon conversion by down shifting process.