The shell effect on the room temperature photoluminescence from ZnO/MgO core/shell nanowires: exciton–phonon coupling and strain

Abstract

The room temperature photoluminescence from ZnO/MgO core/shell nanowires (NWs) grown by a simple two-step vapor transport method was studied for various MgO shell widths (w). Two distinct effects induced by the MgO shell were clearly identified. The first one, related to the ZnO/MgO interface formation, is evidenced by strong enhancements of the zero-phonon and first phonon replica of the excitonic emission, which are accompanied by a total suppression of its second phonon replica. This effect can be explained by the reduction of the band bending within the ZnO NW core that follows the removal of atmospheric adsorbates and associated surface traps during the MgO growth process on one hand, and a reduced exciton-phonon coupling as a result of the mechanical stabilization of the outermost ZnO NW monolayers by the MgO shell on the other hand. The second effect is the gradual increase of the excitonic emission and decrease in the defect related emission by up to two and one orders of magnitude, respectively, when w is increased in the ∼3-17 nm range. Uniaxial strain build-up within the ZnO NW core with increasing w, as detected by x-ray diffraction measurements, and photocarrier tunneling escape from the ZnO core through the MgO shell enabled by defect-states are proposed as possible mechanisms involved in this effect. These findings are expected to be of key significance for the efficient design and fabrication of ZnO/MgO NW heterostructures and devices.

Effect of Material Structure on Photoluminescence of ZnO/MgO Core‐Shell Nanowires

“…showed a bimodal MgO‐thickness‐dependent PL enhancement with a minimum at 40 nm MgO thickness and 15‐fold enhancement maxima at MgO thicknesses of 20 nm and 60 nm. This bimodal enhancement which varies smoothly as a function of MgO thickness was not observed in the other studies,– and was attributed to the formation of optical cavity modes ,,. Thus the formation of optical cavity modes provides a mechanism for enhanced band‐edge PL beyond that due to ZnO surface passivation and band bending.…”

“…), formation of a quasi‐quantum‐well‐like structure (Shi et al . ), and lattice mismatch induced strain (Vega et al …”

Metastability effects on the photoluminescence of ZnO nano-micro structures grown at low temperature and influence of the precursors on their morphology and structure

“…Nano and microsized wide bandgap semiconductors exhibiting different morphologies, such as nanostructured films, nanowires, nanoparticles, nanobelts, microspheres and others, have important technological interest in many areas, such as electronic, optoelectronic and photovoltaic applications [1,2]. Among this type of materials, ZnO has been extensively studied due to its bandgap in the UV (3.37 eV at room temperature), large exciton binding energy (60 meV) [3,4] and an interesting combination of electronic and optical properties that make it suitable for electronic, photovoltaics, UV LEDs and UV photodetector applications [5][6][7][8]. Some of these properties can be tuned by varying the structure morphology (particle size, aspect ratio, surface area, shape, which depends on the growth methods and synthesis conditions [9,10].…”

“…In particular, the hydrothermal route allows ZnO synthesis at lower temperatures than those used in sol-gel (wide range of temperature) [15] and much lower than used in other techniques, such as chemical vapor deposition or vapor transport with carbothermal reduction (> 950 °C) [3,12,16]. These advantages and the fact that it is a rapid technique that can produce high quality product with high yields at low costs make the hydrothermal synthesis method a very promising candidate for future industrial applications.…”

ZnO nanowire sensitization with Ru polypyridyl complexes: charge-transfer probed by spectral and relaxation photocurrent measurements

“…Among the very interesting properties of ZnO are its direct bandgap of 3.37 eV, carrier mobility and electron diffusion coefficient higher than in TiO , low production and environmental costs [14], and high environmental stability [15]- [18]. In addition, ZnO can be easily grown in a wide variety of morphologies such as nanowires, nanotubes, nanosheets, nanoparticles, and nanoflowers [19]- [22], and can be combined with other materials in composites and core-shell systems [23], [24].…”