Origin of the optical phonon frequency shifts in ZnO quantum dots

Identifying the influence of the intrinsic defects in Gd-doped ZnO thin-films

“…The possible mechanisms that can be responsible for such a shift are (i) the biaxial strain, 48 (ii) phonon localization, 49,50 and (iii) laserinduced heating in materials and tensile strain during Raman measurements. 49,50 As E 2 (high) peak is expected to shift to the higher frequency value as the compressive biaxial stress increases (and c-parameter value decreases), we assume the unusual shift of E 2 (high) peak to lower frequencies as c-parameter decreases in our samples can be due the second assumption, which is phonon localization by defects, such as those related to oxygen deficiency, 51 zinc excess or surface impurities, are in line with the PL and XRD measurements, suggesting that defects related to oxygen deficiency such as V O accompanied Gd dopants. Furthermore, we excluded the explanation related to laser-induced heating in materials and tensile strain during Raman as all samples were measured at the same time using the same conditions, and the laser power on the samples was $0.07 mW (to avoid the heating effect).…”

Effect of NaOH concentration on optical properties of zinc oxide nanoparticles

“…These yellow and orange-red emissions are caused by oxygen interstitials and oxygen interstitials-zinc vacancies complexes respectively [28]. It may be inferred that high O − /O 2− ion concentration increases intrinsic strain in the lattice of the sample due to trapping of oxygen ions from the reaction solution into the interstitials which causes transitions from conduction bottom to the oxygen interstitial site emitting yellow emissions around 2.2 eV [22,28].…”

“…Moreover, it could have also caused zinc vacancies in the crystal thus resulting in the formation of zinc vacancies complexes causing transitions from vacant zinc sites to interstitial oxygen sites [28]. Most prominent emission bands observed from the emission spectra are in the blue and green region which may be due to lattice defects developed as a result of compressive strain produced at the intrinsic crystal lattice [21,28]. This leads to the formation of oxygen and zinc vacancies and interstitials simultaneously, which causes transition of energy from conduction band or zinc interstitials to the vacant zinc sites [21,22,28] resulting in blue emission, while the same originating from deep levels causes green emissions.…”

“…It may be inferred that high O − /O 2− ion concentration increases intrinsic strain in the lattice of the sample due to trapping of oxygen ions from the reaction solution into the interstitials which causes transitions from conduction bottom to the oxygen interstitial site emitting yellow emissions around 2.2 eV [22,28]. Moreover, it could have also caused zinc vacancies in the crystal thus resulting in the formation of zinc vacancies complexes causing transitions from vacant zinc sites to interstitial oxygen sites [28]. Most prominent emission bands observed from the emission spectra are in the blue and green region which may be due to lattice defects developed as a result of compressive strain produced at the intrinsic crystal lattice [21,28].…”

Atomistic origin and temperature dependence of Raman optical redshift in nanostructures: a broken bond rule

“…When the size is decreased, the rule of momentum conservation will be relaxed and the Raman active modes will not be limited to the center of the Brillouin zone. 13 However, Alim et al 21,22 who have measured resonant and nonresonant Raman scattering spectra for ZnO nanocrystals with an average diameter of 20 nm, proposed that the observed phonon redshift can be attributed to the local heating effects instead of phonon confinement effects. A Gaussian-type phonon confinement model 17 that has been used to fit the experimental data indicates that strong phonon damping is present, whereas calculations 26 using the correlation functions of the local dielectric constant ignore the role of phonon damping in the nanosolid.…”