First-Principles Study of the Surface of Wurtzite ZnO and ZnS - Implications for Nanostructure Formation (original) (raw)
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Electronic and Structural Properties of the (101̅0) and (112̅0) ZnO Surfaces
The Journal of Physical Chemistry A, 2008
The structural and electronic properties of ZnO (101 j 0) and (112 j 0) surfaces were investigated by means of density functional theory applied to periodic calculations at B3LYP level. The stability and relaxation effects for both surfaces were analyzed. The electronic and energy band properties were discussed on the basis of band structure as well as density of states. There is a significant relaxation in the (101 j 0) as compared to the (112 j 0) terminated surfaces. The calculated direct gap is 3.09, 2.85, and 3.09 eV for bulk, (101 j 0), and (112 j 0) surfaces, respectively. The band structures for both surfaces are very similar. † Part of the special section for the "Symposium on Energetics and Dynamics of Molecules, Solids and Surfaces". * Corresponding authors.
Physical Review B, 2014
Using surface x-ray diffraction in combination with ab initio calculations we have studied the atomic structure of ultra-thin ZnO films deposited on Fe(110). In contrast to expectation that ZnO adopts the "graphitic" hexagonal Boron-nitride structure to the Wurtzite (WZ) structure is observed. Its formation is related to oxygen impurities in Fe(110) hollow sites inducing an anisotropic charge redistribution within the film which is characterized by a metallic surface. Our results provide a deeper understanding of depolarization mechanisms in ultrathin polar films at the atomic scale.
Comparative study of the (0001) and (0001¯) surfaces of ZnO
Applied Physics Letters, 2006
The authors compare the surface and optical properties of the Zn-polar (0001) and O-polar (0001¯) surfaces of bulk ZnO samples. For optical characterization, steady-state photoluminescence using a He–Cd laser was measured at 15 and 300K. At room temperature, the (0001¯) surface demonstrates nearly double the near-band-edge emission intensity seen for the (0001) surface. Using scanning Kelvin probe microscopy, the authors have measured surface contact potentials of 0.39±0.05 and 0.50±0.05V for the (0001) and (0001¯) surfaces, respectively. The resulting small difference in band bending for these two surfaces indicates that charge transfer between the surfaces is not a dominant stabilizing mechanism. Conductive atomic force microscopy studies show enhanced reverse-bias conduction in localized regions on the (0001¯) vs (0001) surface. The differences in surface conduction and band bending between the two polar surfaces can be attributed to their chemical interactions with hydrogen and ...
In this paper, all electron full-potential linearized augmented plane wave plus local orbitals method has been used to investigate the structural and electronic properties of polar (0001) and non-polar (10ī0) surfaces of ZnO in terms of the defect formation energy (DFE), charge density, and electronic band structure with the supercell-slab (SS) models. Our calculations support the size-dependent structural phase transformation of wurzite lattice to graphite-like structure which is a result of the termination of hexagonal ZnO at the (0001) basal plane, when the stacking of ZnO primitive cell along the hexagonal principle c-axis is less than 16 atomic layers of Zn and O atoms. This structural phase transformation has been studied in terms of Coulomb energy, nature of the bond, energy due to macroscopic electric field in the [0001] direction, and the surface to volume ratio for the smaller SS. We show that the size-dependent phase transformation is completely absent for surfaces with a non-basal plane termination, and the resulting structure is less stable. Similarly, elimination of this size-dependent graphite-like structural phase transformation also occurs on the creation of O-vacancy which is investigated in terms of Coulomb attraction at the surface. Furthermore, the DFE at the (10ī0)/(ī010) and (0001)/(000ī) surfaces is correlated with the slab-like structures elongation in the hexagonal a- and c-axis. Electronic structure of the neutral O-vacancy at the (0001)/(000ī) surfaces has been calculated and the effect of charge transfer between the two sides of the polar surfaces (0001)/(000ī) on the mixing of conduction band through the 4s orbitals of the surface Zn atoms is elaborated. An insulating band structure profile for the non-polar (10ī0)/(ī010) surfaces and for the smaller polar (0001)/(000ī) SS without O-vacancy is also discussed. The results in this paper will be useful for the tuning of the structural and electronic properties of the (0001) and (10ī0) ZnO nanosheets by varying their size.
Morphological Features and Band Bending at Nonpolar Surfaces of ZnO
The Journal of Physical Chemistry C, 2015
We employ hybrid density functional calculations to analyze the structure and stability of the (101̅ 0) and (112̅ 0) ZnO surfaces, confirming the relative stability of the two surfaces. We then examine morphological features, including steps, dimer vacancies, and grooves, at the main nonpolar ZnO surface using density functional methods. Calculations explain why steps are common on the (101̅ 0) surface even at room temperature, as seen in experiment. The surface structure established has been used to obtain the definitive ionization potential and electron affinity of ZnO in good agreement with experiment. The band bending across the surface is analyzed by the decomposition of the density of states for each atomic layer. The upward surface band bending at the (101̅ 0) surface affects mostly the valence band by 0.32 eV, which results in the surface band gap closing by 0.31 eV; at the (112̅ 0) surface, the valence band remains flat and the conduction band bends up by 0.18 eV opening the surface band gap by 0.12 eV. Figure 1. ZnO wurtzite structure. The lattice vectors, a and c, and the internal parameter u are shown. The ions in darker colors represent the primitive unit cell, which is shown on the right-hand side. Red is reserved for O and gray for Zn. Article pubs.acs.org/JPCC
Materials
ZnO nanostructures were grown on a Si(111) substrate using a vapor–liquid–solid (VLS) growth procedure (pristine ZnO) and annealed via a rapid thermal-annealing process in an argon atmosphere at 1100 °C (Ar-ZnO). The synthesized ZnO nanostructures were investigated through structural, electronic structural, morphological, optical, and magnetic characterizations. X-ray diffraction and selective area electron diffraction (SAED) measurements revealed that both samples exhibited the hexagonal wurtzite phase of nanocrystalline ZnO. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy carried out at the O K-edge inferred the presence of the intrinsic-defect states. Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy images displayed the formation of ZnO nanostructures. The photoluminescence (PL) spectra demonstrated an emission band in the UV region along with an additional defect band in the visible region. PL spectral analysis confirmed t...
Electronic Band Structures and Native Point Defects of Ultrafine ZnO Nanocrystals
ACS Applied Materials & Interfaces, 2015
Ultrafine ZnO nanocrystals with a thickness down to 0.25 nm are grown by a metalorganic chemical vapor deposition method. Electronic band structures and native point defects of ZnO nanocrystals are studied by a combination of scanning tunneling microscopy/spectroscopy and first-principles density functional theory calculations. Below a critical thickness of about 1 nm ZnO adopts a graphitic-like structure and exhibits a wide band gap similar to its wurtzite counterpart. The hexagonal wurtzite structure, with a well-developed band gap evident from scanning tunneling spectroscopy, is established for a thickness starting from about 1.4 nm. With further increase of the thickness to 2 nm, V O −V Zn defect pairs are easily produced in ZnO nanocrystals due to the self-compensation effect in highly doped semiconductors.
Unravelling the origin of the giant Zn deficiency in wurtzite type ZnO nanoparticles
Scientific Reports, 2015
Owing to its high technological importance for optoelectronics, zinc oxide received much attention. In particular, the role of defects on its physical properties has been extensively studied as well as their thermodynamical stability. In particular, a large concentration of Zn vacancies in ZnO bulk materials is so far considered highly unstable. Here we report that the thermal decomposition of zinc peroxide produces wurtzite-type ZnO nanoparticles with an extraordinary large amount of zinc vacancies (>15%). These Zn vacancies segregate at the surface of the nanoparticles, as confirmed by ab initio calculations, to form a pseudo core-shell structure made of a dense ZnO sphere coated by a Zn free oxo-hydroxide mono layer. In others terms, oxygen terminated surfaces are privileged over zinc-terminated surfaces for passivation reasons what accounts for the Zn off-stoichiometry observed in ultra-fine powdered samples. Such Zn-deficient Zn1-xO nanoparticles exhibit an unprecedented pho...