Formation and stability of rocksalt ZnO nanocrystals in MgO

Abstract

Coimplantation of Zn and O ions into a single crystalline MgO and subsequent thermal annealing were applied in the synthesis of ZnO nanocrystals. Electron microscopy showed that rocksalt instead of wurtzite ZnO stabilizes for relatively large nanocrystals up to ϳ15 nm, resulting from its small lattice mismatch with MgO of ϳ1.7%. The vacancies initially created by implantation induce favorable nanocrystal growth kinetics and are effectively absorbed during the nucleation and growth processes. The optical band edge of the ZnO nanocrystals was detected at ϳ2.8 eV. 2-4 and can be tailored in band gap in the deep-ultraviolet range between ϳ4.8 eV for x ϳ 0.55 up to ϳ7.8 eV for MgO. ZnO is polymorphic and it is known to exist in two cubic structures. Stabilization of a rocksalt ZnO phase can be achieved as a result of external pressure. 5,6 Furthermore, ZnO also exists in a metastable sphalerite structure. The wurtzite and sphalerite arrangements are very much alike, with the wurtzite phase slightly more stable at about the same equilibrium density. 7Rocksalt ZnO is characterized by a significant ϳ0.8 eV /formula unit 7 smaller equilibrium cohesive energy at a ϳ25% higher density. Stabilization of a rocksalt phase in general may also occur for nanocrystals ͑NCs͒ typically in the range of 1 -10 nm as a result of the increasingly important influence of surfaces for colloidal quantum dots [8][9][10] or of interfaces for NCs embedded in the host matrices.11,12 The use of nanoparticles further leads to a size-dependent increase in optical band gap. 8,13 Synthesis of ZnO NCs embedded in MgO can therefore pave the way to extending the tunability of the rocksalt band gap by including the blueviolet to ultraviolet range between ϳ3 and ϳ5 eV.A promising all-solid-state synthesis route for compound NCs consists of sequential implantation of both types of ions and subsequent thermal annealing, 11,14 which is particularly interesting because the passivation of surface states can be achieved robustly by incorporation of the NCs in a wide band gap material such as MgO. Previous studies 15 approached the synthesis of embedded ZnO NCs using high dose Zn + ion implantation in MgO͑100͒ single crystals followed by thermal annealing under oxidizing conditions. However, this leads to the formation of metallic Zn NCs exclusively, 15 with interesting optical properties related with a pronounced Mie scattering resonance. Coimplantation of Zn and O ions, on the other hand, will directly lead to a situation of local supersaturation for both types of ions and is therefore a promising candidate for the synthesis of embedded ZnO NCs.MgO͑100͒ single crystals were implanted using 40 keV O + and 140 keV Zn + ions at a dose of 1 ϫ 10 17 ions/ cm 2 each. The stopping and range of ions in matter ͑SRIM͒ calculations 16 showed that maximum Zn and O concentrations of ϳ20 and 17 at % were reached at a depth of 62 and 66 nm, respectively, after implantation ͓see inset of Fig. 1͑a͔͒. The evolution of the implanted samples was monitored after each subsequent ...

Effects of Au layer thickness and number of bilayers on the properties of Au/ZnO multilayers

“…Similar growth was found on other substrates, sapphire (11)(12)(13)(14)(15)(16)(17)(18)(19)(20), silica glass and lime glass. 40 The displacement of the Bragg peak toward lower angle was already observed in sputtered ZnO films of similar thickness prepared under different conditions, as well as in Zn 0.95 Mn 0.05 O films.…”

“…RS-ZnO has been observed from W-ZnO under pressure, 20 by Zn and O co-implantation in MgO (Ref. 19) The lattice mismatch with Au is then only about 3%. The driving force for the structural transition could be the reduction of stress.…”

“…5(c), where the integrated intensity of the Au(111) Bragg peak plotted versus the total Au thickness, does not increase as expected. Finally, an attempt has been made to attribute the satellite peaks to ZnO under the wurtzite structure encountered for this oxide as bulk or deposited as thin film, 18 and under the rock salt (RS-ZnO) 19,20 or zinc-blende structures, 21 energetically less favored. In Table III, it can be seen that the (111) contributions of RS-ZnO together with the (101) of W-ZnO may eventually be encountered.…”

“…IR spectroscopy indicates a RS-ZnO atomic arrangement on top of this sample. RS-ZnO is an indirect semiconductor with a bandgap in the 2.45-2.8 eV range 19,20 with an absorption edge displaying a (1/k) 2 dependence. 19,20 The absorption versus k observed in the range 250-400 nm for ZnO[Au (1.8) /ZnO] 75 is close to a (1/k) 2 dependence.…”

Critical Review—Photocurrent Spectroscopy in Corrosion and Passivity Studies: A Critical Assessment of the Use of Band Gap Value to Estimate the Oxide Film Composition

“…Such a value is in quite good agreement with the experimental indirect optical bandgap values reported in the literature [85][86][87] for RS-ZnO under high pressure conditions (E g = 2.5 ± 0.15 eV) and with that (2.80 eV) reported for RS-ZnO nanocrystals embedded in MgO matrix. 89 By using the uncertainty range of χ Zn values around the value assumed for WZ-ZnO, we estimate a range of possible E g values for RS-ZnO equal to 2.74 ± 0.34 eV. We have to mention that indirect bandgap values ranging between 1.1 and 4.5 eV have been calculated by DFT for RS-ZnO.…”