The stabilization mechanism and size effect of nonpolar-to-polar crystallography facet tailored ZnO nano/micro rods via a top-down strategy.

A simple and efficient top-down strategy, the chemical vapor etching method, is reported for synthesizing corrugated ZnO nano/micro rods (NRs). The stabilization mechanism of this unique nanostructure has been determined through a combination of aberration-corrected field emission scanning electron microscopy, high-resolution transmission electron microscopy, and first-principles calculations. The experimental data are in good agreement with the theoretical calculations, and a remarkable nonpolar-to-polar surface faceting transition is demonstrated. The corrugated-shaped structure results from the remarkable stability of the defect-induced reconstructions (O vacancy, Zn-Zn dimer), which makes the high-index polar {303[combining macron]1} and {101[combining macron]1[combining macron]} planes lower in energy compared to the nonpolar {101[combining macron]0} plane. Based on the results of first-principles surface calculations, a general formula is established to provide an accurate description of the unusual size effect of the length of the corrugated unit vs. the NR diameter, and it also offers direct explanations for certain experimental observations. The present study deepens our atomic-level understanding of the detailed structure and stability of polar surface decorated corrugated ZnO NRs, and points to a viable path towards designing polar-stable wurtzite structures.

Tailoring the surface of ZnO nanorods into corrugated nanorods via a selective chemical etch method.

Using the chemical vapour deposition method, we successfully converted smooth ZnO nanorods (NRs) into corrugated NRs by simply increasing the reaction time. The surface morphology and crystallographic structure of the corrugated NRs were investigated. The corrugated NRs were decorated by alternant [Formula: see text] and [Formula: see text] planes at the exposed side surfaces while the conventional [Formula: see text] planes disappeared. No twinning boundaries were found in the periodically corrugated structures, indicating that they were type II corrugated NRs. Further investigation told us that they were selectively etched. We introduced a hydrothermal method to synthesize the smooth ZnO NRs and then etched them in a tube furnace at 950 °C with a flow of carbon monoxide. By separating the growth stage and the selective etching stage, we explicitly demonstrated a successfully selective etching effect on ZnO NRs with a carbon monoxide reducing atmosphere for the first time. An etching mechanism based on the selective reaction between carbon monoxide and the different exposed surfaces was proposed. Our results will improve the understanding of the growth mechanism on coarse or corrugated NRs and provide a new strategy for the application of surface controlled nanostructured materials.

Grain-boundary-enhanced carrier collection in CdTe solar cells.

When CdTe solar cells are doped with Cl, the grain boundaries no longer act as recombination centers but actively contribute to carrier collection efficiency. The physical origin of this remarkable effect has been determined through a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles theory. Cl substitutes for a large proportion of the Te atoms within a few unit cells of the grain boundaries. Density functional calculations reveal the mechanism, and further indicate the grain boundaries are inverted to n type, establishing local p-n junctions which assist electron-hole pair separation. The mechanism is electrostatic, and hence independent of the geometry of the boundary, thereby explaining the universally high collection efficiency of Cl-doped CdTe solar cells.

Carrier separation at dislocation pairs in CdTe.

Through the use of aberration corrected scanning transmission electron microscopy, the atomic configuration of CdTe intragrain Shockley partial dislocation pairs has been determined: Single Cd and Te columns are present at opposite ends of both intrinsic and extrinsic stacking faults. These columns have threefold and fivefold coordination, indicating the presence of dangling bonds. Counterintuitively, density-functional theory calculations show that these dislocation cores do not act as recombination centers; instead, they lead to local band bending that separates electrons and holes and reduces undesirable carrier recombination.

Hybrid-functional calculations of electronic structure and phase stability of MO (M = Zn, Cd, Be, Mg, Ca, Sr, Ba) and related ternary alloy M x Zn1-x O.

Using the hybrid exchange-correlation functional within the density-functional theory, we have systematically investigated the structural and electronic properties of MO (M = Be, Mg, Ca, Sr, Ba, Zn, Cd) in binary rock salt (B1), zinc-blende (B3) and wurtzite (B4) phases, including the structural parameters, bulk moduli, band gaps and deformation potentials. Our results agree well with the experimental data and other theoretical results, and give a better understanding of the relationship between the geometric and electronic structure. After calculating the band alignment, we find that in both the B1 and B3 structures, the valence band maximum (VBM) has an obvious decrease from BeO to MgO to CaO, then it goes up from SrO to BaO to ZnO to CdO. Moreover, the properties of the ternary alloys M x Zn1-x O were studied through the application of the special quasi-random structure method. The critical value of the ZnO composition for the transition from the B3 structure to the B1 structure gradually increases from (Ca, Zn)O to (Mg, Zn)O to (Sr, Zn)O to (Ba, Zn)O to (Cd, Zn)O, indicating that (Ca, Zn)O can exist in the B3 structure with the lowest ZnO composition. These results provide a good guideline for the accessible phase space in these alloy systems.

Hole-Induced Spontaneous Mutual Annihilation of Dislocation Pairs.

Dislocations are always observed during crystal growth, and it is usually desirable to reduce the dislocation density in high-quality crystals. Here, the annihilation process of the 30° Shockley partial dislocation pairs in CdTe is studied by first-principles calculations. We found that the dislocations can glide relatively easily due to the weak local bonding. Our systematic study of the slipping mechanism of the dislocations suggests that the energy barrier for the annihilation process is low. Band structure calculations reveal that the band bending caused by the charge transfer between the two dislocation cores depends on the core-core distance. A simple linear model is proposed to describe the mechanism of formation of the dislocation pair. More importantly, we demonstrate that hole injection can affect the core structure, increase the mobility, and eventually trigger a spontaneous mutual annihilation, which could be employed as a possible facile way to reduce the dislocation density.