Resonant coupling of bound excitons with LO phonons in ZnO: Excitonic polaron states and Fano interference

Multiphonon Process in Mn-Doped ZnO Nanowires

The multiphonon process plays an essential role in understanding electron-phonon coupling, which significantly influences the optical and transport properties of solids. Multiphonon processes have been observed in many materials, but how to distinguish them directly by their spectral characteristics remains controversial. Here, we report high-order Raman scattering up to 10 orders and hot luminescence involving 11 orders of phonons in Mn-doped ZnO nanowires by selecting the excitation energy. Our results show that the intensity distribution of high-order Raman scattering obeys an exponential decrease as the order number increases, while hot luminescence is fitted with a Poisson distribution with a resonance factor. Their linewidth and frequency can be well explained by two different transition models. Our work provides a paradigm for understanding the multiphonon-involved decay process of an excited state and may inspire studies of the statistical characteristics of excited state decay.

Extinction of the zero-phonon line and the first-order phonon sideband in excitonic luminescence of ZnO at room temperature: the self-absorption effect

It is firmly demonstrated in experiment that the self-absorption (SA) effect can lead to the extinction of the zero-phonon line and the first-order longitudinal optical phonon sideband of free excitonic luminescence of ZnO at room temperature. Moreover, effectiveness degree of SA effect is found to be dependent on both absorption coefficient and travelling distance of emitted photons, as well as even lattice temperature, which is uniquely reflected by the redshift amount in emission peak in ZnO. It is also unambiguously proved that the SA effect still strictly obeys the Beer-Lambert law of absorption. This work not only uncovers the long-term puzzle of significant redshift of emission peak of ZnO at higher temperatures, but also shows that the SA effect may have to be carefully taken into consideration in the study of spontaneous emission, laser and relevant optoelectronic processes in luminescent materials and optoelectronic devices.

Abnormal gas pressure sensitivity of the visible emission in ZnO quantum dots prepared by improved sol–gel method: the role of surface polarity

Polar surface induced band bending leads to the abnormal gas pressure sensitivity of visible emission in ZnO quantum dots.

Strong quantum confinement effect and reduced Fröhlich exciton-phonon coupling in ZnO quantum dots embedded inside a SiO2 matrix

ZnO quantum dots (QDs) embedded in an amorphous SiO2 matrix were examined in depth by using variable-temperature photoluminescence (PL) and optical reflectance spectroscopies. Compared with ZnO bulk crystals, ZnO quantum dots with an average size of 4 nm exhibit a strong quantum confinement effect, evidenced by a large blue shift in both PL and reflectance peaks of excitons. More interestingly, a remarkably reduced long-range Fröhlich interaction was revealed in ZnO QDs. These fascinating effects may make ZnO QDs a very appealing system in the fields of optoelectronics and others.

The effect of lithium intercalation on the electronic structure of the ternary compound semiconductors ZrSe(2-x)S(x)

The electronic properties of the lithium intercalated layered transition metal dichalcogenide semiconductors ZrS(x)Se(2-x) for x = 0-2 have been calculated by density functional theory (DFT) using the WIEN2k code. The calculations have been carried out by the PBE functional and the TB-MBJ potential as proposed by Tran and Blaha. The calculations have been performed with and without spin-orbit coupling and reveal that the intercalation of lithium causes the conduction bands of LiZrS(x)Se(2-x) to shift by about 2 eV towards lower binding energy. From this, a Fermi level crossing and metallic behavior in the three intercalated compounds result. Moreover, a number of trends can be observed. Due to the contributions of the dichalcogenide p-states in the valence band the inclusion of SO coupling in the calculations lifts the degeneracy at the points Γ and A of the Brillouin zone in the same way as in the parent compounds. With regard to crystal field effects for each compound the splitting is larger at the A point than at the Γ point and the absolute value of the splitting increases with the atomic number of the chalcogenide. In particular, the simple Fermi surface consisting solely of barrels centered along the LML line makes LiZrS(x)Se(2-x) a promising Fermi liquid reference compound.

Green Luminescence Band in ZnO: Fine Structures, Electron−Phonon Coupling, and Temperature Effect

The green emission band of ZnO has been investigated by both experimental and theoretical means. Two sets of equally separated fine structures with the same periodicity (close to the longitudinal optical (LO) phonon energy of ZnO) are well resolved in the low-temperature broad green emission spectra. As the temperature increases, the fine structures gradually fade out and the whole green emission band becomes smooth at room temperature. An attempt to quantitatively reproduce the variable-temperature green emission spectra using the underdamped multimode Brownian oscillator model taking into account the quantum dissipation effect of the phonon bath is done. Results show that the two electronic transitions strongly coupled to lattice vibrations of ZnO lead to the observed broad emission band with fine structures. Excellent agreement between theory and experiment for the entire temperature range enables us to determine the dimensionless Huang-Rhys factor characterizing the strength of electron-LO phonon coupling and the coupling coefficient of the LO and bath modes.