Blue‐Light Emissive Type II ZnO@5‐Amino‐2‐Naphthalene Sulfonic Acid Core–Shell Quantum Dots

Abstract

Blue‐light emissive type II ZnO@5‐amino‐2‐naphthalene sulfonic acid (ANSA) core–shell quantum dots (QDs) is first fabricated. Green luminescence (GL) of 2.25 eV of ZnO QDs disappears at the threshold excitation wavelengths (λex) > 400 nm due to the electronic transition from conduction band (CB) or shallow donor centers to VZn2− at ≈0.9 eV above valence band (VB). Blue emissions from ZnO@ANSA QDs are well elucidated by the charge transfer from CB of ZnO QDs to lowest unoccupied molecular orbital (LUMO) of ANSA and from shallow donor Zn interstitial (Zni) or extended Zni states to CO, CN, CS (π*)‐states, and then the electronic transitions to nonbonding CO, CN, CS (π) states of ANSA. Spectral‐dependent electron paramagnetic resonance (SDEPR) peak g = 1.96 of ZnO QDs emerges only with illumination of light shorter than 400 nm in wavelength and completely disappears in ZnO@ANSA QDs. Electrons trapped in CB or Zni's are unveiled to participate in GL and the paramagnetic defect to derive electron paramagnetic resonance (EPR) peak g = 1.96 of ZnO. The estimated spin–spin relaxation time (T2) of ZnO QDs was about 32.5 ns, which is two orders of magnitude longer than known values for bulk or thin‐film ZnO.

Keywords

• blue-emissive quantum dots
• oxygen vacancy (VO+)
• spectral-dependent electron paramagnetic resonance
• zinc vacancy (VZn−)
• ZnO@5-amino-2-naphthalene sulfonic acid core–shell quantum dots