Reversible Modulation of the Cr³⁺ Spin Dynamics in Colloidal SrTiO₃ Nanocrystals

Abstract

The spin relaxation dynamics of Cr3+ substitutional dopants in ∼10 nm colloidal SrTiO3 nanocrystals (NCs) has been studied by variable-temperature electron paramagnetic resonance (EPR) spectroscopy and continuous wave EPR power saturation measurements between 4.2 and 50 K. The presence of self-trapped electrons introduced by anaerobic ultraviolet irradiation (photodoping) is able to effectively accelerate the spin relaxation dynamics of localized spins of Cr3+ through a cross-relaxation process. The extra electrons introduced during photodoping trap on Ti-sites, creating Ti3+ defects with EPR transitions that are observed only at cryogenic temperatures because of fast spin–lattice relaxation. The change in the spin relaxation upon photodoping the Cr-doped SrTiO3 NCs is totally reversible upon opening the sample to air. This result provides a novel strategy to modulate spin dynamics in individual NCs using photons where long-lived spins on dopant ions couple to the fast-relaxing spins from metastable, transient defects.

On the formation of superoxide radicals on colloidal ATiO3 (A = Sr and Ba) nanocrystal surfaces

“…29 We also demonstrated in a more recent report that this g ¼ 2.003 signal can be increased upon exposing photochemically reduced (Ti 3+ -rich) Cr 3+ -doped SrTiO 3 NCs to air. 41 Others have also reported a TiO 2 -based NCs treated with H 2 O 2 that display similar absorption in the visible region originating from surface-adsorbed O 2 À defects. 43,44 Based on the similarity to previous reports and observations, 41,43 we attribute the reddish-brown coloration and EPR resonance at g ¼ 2.003 observed in Fig.…”

“…41 Others have also reported a TiO 2 -based NCs treated with H 2 O 2 that display similar absorption in the visible region originating from surface-adsorbed O 2 À defects. 43,44 Based on the similarity to previous reports and observations, 41,43 we attribute the reddish-brown coloration and EPR resonance at g ¼ 2.003 observed in Fig. 2 and 3, respectively, to the presence of O 2 À radicals on the surface of SrTiO 3 NCs prepared under aerobic hydrothermal conditions (method-I).…”

“…However, the electronic transition of the localized electron trapped on Ti 3+ sites to the conduction band minimum is broad and centered in the near-IR region. 40,41 To explore the origin of the brownish coloration of the SrTiO 3 NCs prepared by method-I, we carried out EPR spectroscopy measurements on all three types of SrTiO 3 NCs.…”

Reversible Control of the Mn Oxidation State in SrTiO3 Bulk Powders

“…The realization of a fast, low-energy method to modulate the oxidation state of transition-metal dopants in SrTiO 3 and related metal oxide semiconductors could impact various fields such as visible-light photocatalysis, sensing, and spin-based electronics. To this end, recent studies on the photodoping of colloidal Cr:SrTiO 3 nanocrystals show promise (Harrigan and Kittilstved, 2018).…”

Electron spin relaxation time of Ni(II) ion in hexapyrazole zinc(II) dinitrate at 300 K

“…The knowledge of electron spin relaxation rate is essential for distance measurement by EPR spectroscopy. So substantial works have been carried out to understand the electron spin relaxation rates of many biologically important metal ions like Fe(II), low spin Fe(III), Cu(II), and other metal ions.…”