Photocharging ZnO Nanocrystals: Picosecond Hole Capture, Electron Accumulation, and Auger Recombination

Abstract

Photochemical charging of colloidal ZnO nanocrystals has been studied using continuous-wave and time-resolved photoluminescence spectroscopies in conjunction with electron paramagnetic resonance spectroscopy. Experiments have been performed with and without addition of alcohols as hole quenchers, focusing on ethanol. Both aerobic and anaerobic conditions have been examined. We find that ethanol quenches valence-band holes within ∼15 ps of photoexcitation, but does not quench the trapped holes responsible for the characteristic visible photoluminescence of ZnO nanocrystals. Hole quenching yields "charged" nanocrystals containing excess conductionband electrons. The extra conduction-band electrons quench visible trap-centered luminescence via a highly effective electron/trap-state Auger-type cross-relaxation process. This Auger process is prominent even under aerobic photoexcitation conditions, particularly when samples are not stirred. Charging also reduces exciton nonradiative decay rates, resulting in increased UV luminescence. The dependence of charging on ethanol concentration and the reduced exciton nonradiative decay rates of charged ZnO nanocrystals are discussed. Finally, the results here provide a kinetic basis for understanding photochemical electron accumulation in colloidal ZnO nanocrystals.

Dispersible Plasmonic Doped Metal Oxide Nanocrystal Sensors that Optically Track Redox Reactions in Aqueous Media with Single‐Electron Sensitivity

“…To that end, our results are complementary to recent work by Gamelin and Mayer to advance the understanding of lightdriven redox chemistry of ZnO nanocrystals in organic media as it relates to their infrared plasmonic properties, [20][21][22][23][24][25][26] as well as work by Manna and Tassone, [27][28][29][30][31][32] Talapin and Feldmann, [ 33 ] and Jain and Alivisatos [ 34 ] investigating chemically induced NIR plasmon shifts in substrate-bound Cu 2− x E nanostructures, where E = S, Se, or Te. Our use here of plasmonic-doped metal oxides is expected to overcome the experimental challenges in using noble metal NCs to detect optically one or a few redox events per nanocrystal, owing to the signifi cantly higher carrier concentrations typifi ed by Au and Ag nanostructures.…”

Selenium Redox Reactivity on Colloidal CdSe Quantum Dot Surfaces

“…Localized surface redox chemistry has been shown to affect QD photoluminescence (PL) and blinking 9,10 and can sustain picosecond trap-assisted Auger recombination dynamics. 11 A greater understanding of this surface redox chemistry is necessary for advancing QD applications.…”

Quantum-size effects in visible defect photoluminescence of colloidal ZnO quantum dots: a theoretical analysis

“…Although there has been some controversy concerning the origin of the mechanism of the visible luminescence of ZnO, whether it is due to the radiative recombination of (i) a delocalized electron with a deeply trapped hole or (ii) a deeply trapped electron with a delocalized hole, 10,13,21 it is now well established that mechanism (i) is dominant 11,15,17 and in this work we adhere to this picture.…”

“…The two bands also appear in this case and their relative intensity is extremely dependent on whether the nanoparticles are in an atmosphere with or without the presence of oxygen. [8][9][10][11][12][13][14][15][16][17] Under aerobic conditions the visible band is prominent while the exciton band is a weak feature. However, under anaerobic conditions and upon irradiation with UV light, the visible emission band quenches as the exciton band increases in intensity.…”

“…However, under anaerobic conditions and upon irradiation with UV light, the visible emission band quenches as the exciton band increases in intensity. [8][9][10][11][12][13][14][15] Furthermore, an infrared absorption band develops due to the accumulation of electrons in the conduction band that leads to the formation of a local surface plasmon resonance in the nanoparticle. [16][17][18] Admission of oxygen provokes the opposite effect: the luminescent exciton emission disappears as the visible emission band recovers in intensity 12,13 and the infrared absorption is also quenched.…”