Letter pubs.acs.org/NanoLett
Subangstrom Profile Imaging of Relaxed ZnO(101̅0) Surfaces Mo-Rigen He, Rong Yu, and Jing Zhu* Beijing National Center for Electron Microscopy, Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People's Republic of China S Supporting Information *
ABSTRACT: Relaxation is a most basic structural behavior of free surfaces, however, direct observation of surface relaxation remains challenging in atomic-scale. Herein, single-crystalline nanoislands formed in situ on ZnO nanowires and nanobelts are characterized using aberration-corrected transmission electron microscopy combined with ab initio calculations. For the first time, displacements of both Zn and O atoms in the fresh (101̅0) facets are quantified to accuracies of several picometers and the under-surface distributions of contractions and rotations of Zn−O bonds are directly measured, which unambiguously verify the theoretically predicted relaxation of ZnO (101̅0) free surfaces. Finally, the surface relaxation is directly correlated with the size effects of electromechanical properties (e.g., elastic modulus and spontaneous polarization) in ZnO nanowires. KEYWORDS: Surface structure, relaxation, ZnO, transmission electron microscopy, electromechanical properties
A
mong the most attractive materials for nanotechnology,1 ZnO is a promising candidate for a wide range of applications such as laser diodes,2 gas sensors,3 and catalysts4,5 with the atomic configurations and electronic structures of surfaces playing crucial roles in their performances. Moreover, most of basic physical properties in nanosized specimens are dominated by their free surfaces.6,7 For instance, the sizedependent enhancement of elastic modulus in [0001]-oriented ZnO nanowires (NWs),8,9 as well as their giant piezoelectricity,10 were recently attributed to the contraction of Zn−O bonds near (101̅0) side surfaces. Thus, probing into the surface structure of ZnO is of both scientific and practical significance. From an energetic viewpoint, the nonpolar (1010̅ ) (see Figure 1a) is one of the most favored surfaces in ZnO,11 the relaxation of which has been recognized in extensive atomistic simulations.11−13 For instance, the density functional theory (DFT) calculations by Meyer et al. predicted a contraction of ∼7% and inward rotation of ∼10° for the in-plane Zn−O bonds in the outmost surface layer (IP1, see Figure 1b for details).11 Compared with theoretical studies, however, experimental characterizations of surface relaxation lag far behind. On the basis of low-energy electron diffraction (LEED), Duke et al. first reported the inward relaxations of Zn1 and O1,14 which were later supported by Göpel et al. based on angle-resolved photoemission spectra,15 though no compelling evidence for in-plane relaxation was observed yet.14,15 On the contrary, the grazing incidence X-ray diffraction studies by Jedrecy et al. suggested nearly an inrelaxed surface.16 More importantly, only the relaxation in the outmost layer were measured by above techniques, but DFT calculations predicted © 2012 American Chemical Society
that surface relaxation decayed slowly into the bulk.11,13 Profile imaging in high-resolution transmission electron microscope (HRTEM) is potential for studying the atomic strutures under surfaces,17 however, conventional TEM imaging had suffered from delocalization effect and the limited resolution. Ding et al. reported the only HRTEM study of ZnO relaxed surfaces until now18 in which, however, the nearest-neighboring Zn and O atoms (with unrelaxed projective distance of 1.14 Å, see Figure 1a) were not resolved, the quantifying of atomic displacements thus suffered from severe errors. Thanks to the progress in aberration-corrected TEM,19 profile imaging is turning into a powerful method for resolving the relaxations and reconstructions in metallic and oxide surfaces.20−22 Most recently, the complex Co3O4(111) surfaces were directly measured at the subangstrom scale.23 Nonetheless, ZnO (101̅0), as one of the simplest and most typical surfaces in ionic crystals, has not yet been thoroughly characterized by TEM. Herein, relaxations of the fresh (101̅0) facets of ZnO nanoislands (NIs), which are formed in situ under e-beam (e-beam) irradiation, are studied using aberration-corrected TEM combined with DFT calculations. For the first time, the normal and in-plane displacements of both Zn and O atoms are quantified in picometer-level accuracies and the under-surface distributions of relaxation strains are measured. Furthermore, direct correlations between surface relaxation and the size-dependent electromechanical properties in ZnO NWs are demonstrated. Received: October 13, 2011 Revised: December 16, 2011 Published: January 3, 2012 704
dx.doi.org/10.1021/nl2036172 | Nano Lett. 2012, 12, 704−708
Nano Letters
Letter
Figure 1. Atomic configurations of ZnO (1010̅ ) surface (viewed in [121̅ 0] zone axis; only the first six layers, i.e., i = 1−6, are shown). Red, O; blue, Zn. (a) Unrelaxed structure and the interatomic projective distances. (b) Relaxed structure predicted by DFT calculations.11 Displacements from bulk positions (the light-colored atoms) are indicated by arrows, the lengths of which magnified by 5 times. IPi denotes the in-plane Zn−O bonds in the ith layer, and θi is the rotation of IPi.
on the stabilization mechanisms of polar surfaces). Very importantly, all HRTEM images were recorded when the ebeam was spread to larger than TEM screen (∼15 cm in diameter), which suppressed the influence of irradiation by a factor of ∼10. Figure 3a shows another ZnO NI formed on the side surface of a ZnO nanobelt, and the following discussions on surface
Single-crystalline ZnO NWs and nanobelts were synthesized via thermal evaporation of Zn powder24 and were observed using a TEM (Titan 80−300, FEI) equipped with a spherical aberration (Cs) corrector for objective lens. Side surfaces of the original samples were clean and flat (Figure 2a), which were
Figure 2. ZnO NIs formed on the side surfaces of ZnO NW. (a) TEM image of an as-synthesized NW. (b) The same NW after an e-beam irradiation of ∼1 min. Dashed lines indicate the positions of original NW surfaces, which lie between the dedenda and addenda of NIs. The box C is magnified in (c). (c) HRTEM image of a single-crystalline NI (viewed in [12̅10] zone axis).
Figure 3. A ZnO NI formed on the side surface of ZnO nanobelt. (a) Experimental HRTEM image without any filtering applied. Measurements of the distributions of relaxations under (101̅0) surface are carried out in the pink box, which contains 20 layers of (1010̅ ) and 5 unit cells in [0001] direction. The relaxed distance of 1.03 Å between Zn1−O2 agrees quantitatively well with our DFT calculations. (b) Simulated HRTEM image and DFT-calculated structure model of the relaxed free surface. The different intensities between Zn and O columns agreed with the NCSI method.25,26
subsequently roughened under irradiation of the 300 kV ebeam (more experimental details are given in Supporting Information). After an irradiation of ∼1 min (the electron beam was converged to diameter of ∼5 cm on TEM screen during this process), the whole NW was covered by a layer of NIs with lateral sizes around 5 nm (Figure 2b), similar to the report by Ding et al.18 We noted that the dedenda of NIs were lower, while their addenda were higher, than the original NW surfaces, which indicated that the formation of NIs involved not only the e-beam sputtering of atoms, but also the selective regrowth of the low-energy surfaces. To support this, Figure 2c shows that single-crystalline ZnO NIs were formed epitaxially on the NW surface (also see Supporting Information for the evolution of an individual NI) and were enclosed with atomic-level clean (101̅0), (0001̅)-O, and (101̅3) instead of (0001)-Zn, facets. Since Zn and O atomic columns were resolved in the HRTEM images taken using the negative Cs imaging (NCSI) method,25,26 the polarity of NIs can be simply determined (see Supporting Information for some preliminary discussions
relaxation will be focused on the fresh (101̅0) facet of this triangle-shaped NI. Experimental HRTEM images were taken at a high tension of 300 kV, Cs was set at around −13 μm, and other residual aberrations were carefully corrected to 2-fold astigmatism
