ZNO

Experimental evidence for a shallow donor hydrogen state in ZnO






Lower: Muon spin rotation signal from ZnO powder in 20mT, below the ionisation regime for the shallow donor state. Upper: Maximum entropy frequency transform of the signal. The extracted hyperfine parameters imply that the paramagnetic muon state is a shallow donor.

* ZnO is a wide-gap semiconductor with potential optoelectronic applications. It almost always exhibits strong n-type conductivity,whose source is controversial. It has been predicted recently, in numerical calculations (C Van de Walle, PRL 85(5), 1012, 2001), that hydrogen could form a shallow donor state in ZnO and so be a source of the n-type conductivity.
* This predicted hydrogen state has been observed for the first time experimentally by direct spectroscopic observation of its muonium counterpart.
* Hyperfine parameters extracted for the paramagnetic muonium state formed below 40K imply a shallow level with an ionisation energy of some 30meV.

Experimental confirmation of the predicted shallow donor hydrogen state in Zinc Oxide
SFJ Cox et al, Phys. Rev. Lett. 86(12), 2601, 2001

http://www.isis.rl.ac.uk/muons/science/index.htm



ZnO has a variety of promising properties such as a wide band gap (3.37 eV), a large electromechanical coupling factor, and a stable hexagonal phase. For these reasons, there has been considerable interest in recent years in the development of ZnO films for surface acoustic wave devices and ultraviolet optoelectronic devices. For ZnO films with the c-axis parallel to the surface plane of the piezoelectric substrates, the electro-mechanical coupling factor of the ZnO films can be enhanced by the excitation of its shear horizontal-type surface acoustic wave. …a critical problem is a high concentration of various lattice defects in the growth of ZnO films….. these defects in semi-conducting crystals can act a s traps and recombination centers. Thus, understanding of the characteristics of defects was a major emphasis of the present ZnO research.

High resolution electron microscopy of stacking faults in heteroepitaxial ZnO / LiTaO3, 2002, Oxford journals.org



Formation of ZnO nanocrystallites on cubic (3C) ZnS by electron beam irradiation has been investigated using a high-resolution analytical TEM. The specimen observed was a high-purity synthetic ZnS. In situ observations showed slight retreat of the synthetic ZnS surfaces and epitaxial formation of ZnO nanocrystallites, with orientation relationship of {lllJzns // (O^-OJzno and <110>ZnS//<10.0>ZnO. The ZnO nanocrystallites mostly adopt 2H structure, but 3C structure is also formed locally on ZnS, probably depending on the orientation and configuration of ZnS surfaces.

As is well known, zinc sulfide (ZnS) is utilized commonly as luminescent or semi-conductive material. During the investigation of microstructure of ZnS for
development of efficient luminescent materials, we observed the formation of numerous nanocrystallites on ZnS surfaces by electron beam irradiation in TEM.


...........To identify these nanocrystallites, TEM-EDS analysis was performed. EDS spectra from near the edge indicate that sulfur is decreased and a considerable amount of oxygen exists compared to the spearum from a thicker and fresh region of ZnS. This suggests that these nanocrystallites are ZnO.

......Two possibilities can be considered for the source of oxygen to form ZnO. Oxygen may be an impurity element in ZnS or it may come from residual gas in the TEM.
Maximum solubility of oxygen as ZnO to cubic ZnS was reported to be 0.7 wt% (0.84 mol.%). This solid solution decreases the unit cell parameter (a0) from 0.54093 nm
to 0.54065 nm.

Preliminary observations indicated that the ZnS basically adopts 3C (cubic) structure but stacking disorders (stacking faults, 1 = 3 twins and local 2H structure) are common on a specific (111) plane in ZnS, which is probably related
to the growth direction of the crystal. Grains which could be oriented with the electron beam parallel to the <110>directions, on which these stacking disorders are visible, were generally selected for the experiments. These defects are useful as markers to identify the same position among several images taken to record structural changes during beam irradiation.

.......The image corresponding to the diffraction pattern showed clear Moire fringes on the ZnS crystal, indicating these nanocrystallites are formed not only at the edges, but on the whole surface of ZnS grains. From these observations, we conclude these nanocrystallites are ZnO with 2H structure and they are formed epitaxially on ZnS.


Formation of ZnO nanocrystaMites on ZnS surfaces by electron beam irradiation, Journal of Electron Microscopy 47(2): 135-141 (1998)

Oxides: Manganites

Manganites with perovskite structure show some peculiar properties, such as colossal magnetoresistance (CMR) effect and metal-to-insulator MI transition accompanying the charge and orbital ordering CO/OO. Extensive studies on CMR manganites re-ported that the spatial inhomogeneous state of magnetic, electric and/or lattice sys-tems is important for understanding the peculiar properties of manganites. Recently, it was revealed by using both the resistivity measurement and the transmission electron microscopy TEM that La5/8-x Pr x Ca 3/8 MnO3 shows unexpected coexistence of large domain structures of the CO/OO state and the ferromagnetic FM metallic one at low temperature below 60 K, which is the MI transition temperature on cooling. simi-lar results were also reported using scanning tunnelling microscopy applied to La0.7 Ca0.3 MnO3. this coexistence of two competing ground states, the CO/OO insulating state and the FM metallic one,is recognized as a common phenomenon in CMR man-ganites. Note that this coexistence of two different phases is characterized as phase separation. Thus, it is important to examine the spatial distribution of the CO/OO state and the FM metallic state in the phase separated region found in CMR manganites. …..note that CO/OO state is characterized as the alternating arrangement of Mn 3+ and Mn 4+ ions along the [100] direction.

www.oxfordjournals.org

Rao , 1998, Colossal Magnetoresistance, Charge ordering, and Related Physical Prop-erties of Manganese Oxides; World Scientific, Singapore
Kaplan, 1999, Physics of Manganites; Plenum publishers, NY


...The two titanium chain compounds NaTiSi2O6 and TiOCl exhibit spin gap
formation at unusually high temperatures due to unconventional dimerization
mechanisms. A model allowing the comparison of X-ray diffraction data, dimer-
ization, and the magnitude of the spin gap is proposed. This is tested against
both magnetic susceptibility and μSR data for both compounds. For NaTiSi2O6
both experimental techniques are in reasonable agreement, whereas in TiOCl the
results are conclusively different. The origin of this disparity in TiOCl is explored.


.....The question of whether exceptionally small defect concentrations are capable
of destroying long range orbital order or seed the local orbital ordering is not
clear, and the influence this would have on the magnetic properties has not been
investigated theoretically. Some general arguments have been made by Petit et
al. [Peti06], noting that while Ni2+ ions occupying Li sites are not Jahn-Teller
active, a distribution of them within the NiO2 layer could affect the orbital or-
dering. Their effect on the magnetic interactions might well be more significant,
however, since they would change the relative strength of the inter- and intra-layer
exchange pathways, a point discussed by Lewis et al.

Baker P J, 2007, Tuning the dimensionality and interactions in transition metal oxides: a μSR study, Thesis of Doctor of Philosophy, Balliol College




Zinc oxide nanostructures: growth, properties and applications

ZnO is a wide band-gap (3.37 eV) compound semiconductor that is suitable for short wavelength optoelectronic applications. The high exciton binding energy (60meV) in ZnO crystal can ensure efficient excitonic emission at room temperature and room
temperature ultraviolet (UV) luminescence has been reported in disordered nanoparticles and thin films. ZnO is transparent to visible light and can be made highly conductive by doping. ZnO is a versatile functional material that has a diverse group of growth morphologies, such as nanocombs, nanorings, nanohelixes/nanosprings,nanobelts, nanowires and nanocages.

JOURNAL OF PHYSICS: CONDENSED MATTER (2004)
http://www.iop.org/EJ/article/0953-8984/16/25/R01/cm4_25_R01.pdf
?request-id=1c1534ca-d4b3-4021-843f-7c3a689a6dfe

http://www.iop.org/EJ/abstract/0953-8984/16/25/R01/