Giant Exciton Mott Density in Anatase TiO2

Abstract

Elucidating the carrier density at which strongly bound excitons dissociate into a plasma of uncorrelated electron-hole pairs is a central topic in the many-body physics of semiconductors. However, there is a lack of information on the high-density response of excitons absorbing in the near-to-mid ultraviolet, due to the absence of suitable experimental probes in this elusive spectral range. Here, we present a unique combination of many-body perturbation theory and state-of-the-art ultrafast broadband ultraviolet spectroscopy to unveil the interplay between the ultraviolet-absorbing two-dimensional excitons of anatase TiO 2 and a sea of electron-hole pairs. We discover that the critical density for the exciton Mott transition in this material is the highest ever reported in semiconductors. These results deepen our knowledge of the exciton Mott transition and pave the route toward the investigation of the exciton phase diagram in a variety of wide-gap insulators.

Optoelectronic properties and ultrafast carrier dynamics of copper iodide thin films

“…3 . The persistence of such excitonic feature at carrier density above the Mott density is most likely due to the formation of Mahan excitons, which were also reported in other materials with large exciton binding energies 47 . As shown in Supplementary Fig.…”

“…3d ). This implies that exciton peak energy blue shifts as the hole density in CuI thin film increases, mainly due to the enhanced Coulomb screening induced reduction in exciton binding energy ( E b ), similar to that observed in anatase TiO 2 47 . The energy difference between the valence band edge E V and the Fermi level E F (i.e., E V - E F ) was calculated by using , where N is the hole density in the valence band, m h is the density of state hole effective mass ( ) which is found to be around 2.47 m e for CuI 16 , k B is the Boltzmann constant, T is the temperature, ℏ is the Plank constant, F 1/2 is the Fermi-Dirac integral which was calculated by method proposed by Bednarczyk et al 48 .…”

“…In addition, the major PB peak exhibits a blue-shift of ~8 meV with increasing |∆A| under a certain pump fluence (e.g., 0.64 mJ/cm 2 ). In case high carrier density filled in the band edges, long-range Coulomb screening may result in a blue-shift of the exciton peak (if excitons are present) due to the reduced exciton binding energy, while phase-space filling may also contribute to the blue-shift of exciton peak or the absorption edge 47 , which is actually revealed in Supplementary Fig. 4 .…”

Flexible Nano-TiO2 Sheets Exhibiting Excellent Photocatalytic and Photovoltaic Properties by Controlled Silane Functionalization─Exploring the New Prospects of Wastewater Treatment and Flexible DSSCs

“…In particular, the optoelectronic properties of both anatase and rutile polymorphs of TiO 2 have been intensely investigated for the prospects of using it in light-energy conversion applications. 3 − 5 TiO 2 is widely accepted as an n-type semiconductor with a wide indirect band gap. 6 Photoexcitation of anatase TiO 2 has been reported to generate a large number of strongly bound excitons that display an intermediate character between the Wannier–Mott and Frenkel excitons, that hold a two-dimensional wave-like nature, 7 and are confined in the 001 planes in the 3-dimensional lattice.…”

“…Of them, TiO 2 nanoparticles, which could serve both the abovementioned roles, have already revolutionized the fields of photocatalysis and photovoltaics; as evidenced by large numbers of research articles published each year on the use of TiO 2 nanoparticles in both areas. In particular, the optoelectronic properties of both anatase and rutile polymorphs of TiO 2 have been intensely investigated for the prospects of using it in light-energy conversion applications. − TiO 2 is widely accepted as an n-type semiconductor with a wide indirect band gap . Photoexcitation of anatase TiO 2 has been reported to generate a large number of strongly bound excitons that display an intermediate character between the Wannier–Mott and Frenkel excitons, that hold a two-dimensional wave-like nature, and are confined in the 001 planes in the 3-dimensional lattice .…”