Turning ZnO into an Efficient Energy Upconversion Material by Defect Engineering

Abstract

Photon upconversion materials are attractive for a wide range of applications from medicine, biology to photonics. Among them, ZnO is of particular interest owing to its outstanding combination of materials and physical properties. Though energy upconversion has been demonstrated in ZnO, the exact physical mechanism is still unknown that prevents a control of the processes. Here, we show that defects formed in bulk and nanostructured ZnO synthesized using standard growth techniques play a key role in promoting efficient energy upconversion via two-step two-photon absorption (TS-TPA). From photoluminescence excitation of the anti-Stokes emissions, the threshold energy of the TS-TPA process is determined as being 2.10 -2.14 eV in all studied ZnO materials irrespective of the employed growth techniques. Our photo-electron paramagnetic resonance studies show that this threshold closely matches the ionization energy of the zinc vacancy -a common grown-in intrinsic defect in ZnO, thereby identifying the zinc vacancy as being the dominant defect responsible for the observed efficient energy upconversion. The upconversion is found to persist even at a low excitation density, making it attractive for photonic and photovoltaic applications.1

Zinc-Vacancy–Donor Complex: A Crucial Compensating Acceptor in ZnO

“…Here, the key point defects to be considered include zinc vacancies (V Zn ), zinc interstitials (Zn i ), oxygen vacancies (V O ), and oxygen interstitials (O i ). Among them, V Zn is probably the most relevant defect, since it has the lowest formation energy among native point defects in n-type ZnO [1] and is commonly found in bulk and nanostructured materials [4][5][6][7][8]. V Zn is also suggested to be the origin of the observed n-type doping limit in ZnO [9,10] by forming complexes with donors leading to their compensation [11][12][13].…”

Dual-wavelength excited photoluminescence spectroscopy of deep-level hole traps in Ga(In)NP

“…There are in principle several possible physical mechanisms that could lead to this PL enhancement. The first possibility is that P2 with a below-bandgap photon energy could generate free electrons and holes via defect-mediated upconversion or two-photon nonlinear processes, 35,36 leading to enhanced PL emissions. This possibility is not plausible here because photo-excitation with only the NIR light P2 did not result in any sizable PL emissions.…”

Efficient Auger Charge-Transfer Processes in ZnO

“…The magneto-PL measurements are performed in the Faraday configuration (with an applied magnetic field B parallel to the c axis of the crystal and also parallel to the wave vector k of the detected light) and in the Voigt configuration (B⊥c and kkc). Additionally, the angles between B and the c axis are varied from 0 to 90°w ithin the (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) plane at the fixed magnetic field of 8 T. EPR measurements are carried out within the temperature range of 4.2-77 K at a microwave frequency of 9.4 GHz. For photo-EPR studies, several diode lasers and a wavelength-tunable pulsed Ti:sapphire laser with a repetition rate of 76 MHz are used to provide illumination at specific wavelengths.…”

“…ZnO has a wide and direct band gap of 3.3 eV and a large exciton binding energy. It is also nontoxic, sustainable, and cheap and can be readily synthesized with high quality as bulk crystals and thin films and in various nanostructured morphologies [12][13][14][15][16][17]. All these attributes make ZnO a very promising material for a wide range of applications including light-emitting diodes and lasers [18,19], gas sensors [20], solar cells [21], and photodetectors [22] and as transparent conductive oxide in displays [23,24].…”