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Article Cite This: ACS Omega 2018, 3, 3129−3135
Strain Mapping and Raman Spectroscopy of Bent GaP and GaAs Nanowires Hyung Soon Im,† Kidong Park,† Jundong Kim,† Doyeon Kim,† Jinha Lee,† Jung Ah Lee,† Jeunghee Park,*,† and Jae-Pyoung Ahn*,‡ †
Department of Chemistry, Korea University, Sejong 30019, Korea Korea Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, Korea
‡
S Supporting Information *
ABSTRACT: Strain engineering of nanowires (NWs) has been recognized as a powerful strategy for tuning the optical and electronic properties of nanoscale semiconductors. Therefore, the characterization of the strains with nanometer-scale spatial resolution is of great importance for various promising applications. In the present work, we synthesized single-crystalline zinc blende phase GaP and GaAs NWs using the chemical vapor transport method and visualized their bending strains (up to 3%) with high precision using the nanobeam electron diffraction technique. The strain mapping at all crystallographic axes revealed that (i) maximum strain exists along the growth direction ([111]) with the tensile and compressive strains at the outer and inner parts, respectively; (ii) the opposite strains appeared along the perpendicular direction ([2̅11]); and (iii) the tensile strain was larger than the coexisting compressive strain at all axes. The Raman spectrum collected for individual bent NWs showed the peak broadening and red shift of the transverse optical modes that were well-correlated with the strain maps. These results are consistent with the larger mechanical modulus of GaP than that of GaAs. Our work provides new insight into the bending strain of III−V semiconductors, which is of paramount importance in the performance of flexible or bendable electronics.
1. INTRODUCTION One-dimensional semiconductor nanostructures [typically nanowires (NWs)] have attracted much attention over the past two decades because they are promising bottom-up building blocks of future nanodevices.1 Recently, strain engineering of NWs has been recognized as a powerful strategy for tuning the optical and electronic properties of nanoscale semiconductors, and thus it is receiving an increasing amount of attention.3 The prominent properties of tensile- and compressive-strained NWs are smaller and larger band gaps, respectively, as compared to those of corresponding unstrained NWs.2−8 For bent (or buckled) NWs, a red-shifted emission has been observed, which has been explained by the piezotronic effect, in which under a piezoelectric field, the number of photons emitted from the outer surface of the NW is much larger than that from the inner surface.9−13 The outstanding piezoelectric properties of ZnO and CdS NWs have facilitated the development of various piezotronic and piezo-phototronic devices such as nanogenerators, field-effect transistors, and light-emitting diodes, which rely on the piezoelectric potential created along the strained NWs.3,14−16 Moreover, the enhanced carrier mobility or photocurrent of strained NWs has attracted widespread research interest owing to their great potential in a rich variety of applications.17−24 To expedite the fabrication of advanced flexible electronic devices, it is essential to examine the strains with nanometerscale spatial resolution so that the modification of the © 2018 American Chemical Society
properties can be better understood. Such nanoscale examination can be carried out using transmission electron microscopy (TEM) techniques such as aberration-corrected high-resolution TEM (HRTEM),25 electron diffraction (e.g., convergent beam and nanobeam),26−30 geometrical phase analysis of high-angle annular dark-field scanning TEM,31 and dark-field electron holography.32 In particular, a precession-assisted nanobeam electron diffraction (NBED) technique had been developed to visualize the strain of materials with high precision.28−30 This technique makes use of electron beam precession together with spot diffraction pattern recognition, resulting in reliable strain maps with a spatial resolution down to 2 nm and excellent precision (0.02%) on a field emission gun TEM.33,34 Furthermore, the NBED technique has a great advantage to provide the strain distribution at a specific crystallographic axis. Nevertheless, strain mapping using NBED for strained NWs has not been reported yet. The strain distribution at individual crystallographic axis would provide valuable information to understand their novel properties. In the present work, GaP and GaAs NWs were synthesized using the chemical vapor transport method. The NWs exhibited a single-crystalline zinc blende (ZB) phase with a uniform growth direction ([111]). The NWs were bent by the Received: January 11, 2018 Accepted: February 21, 2018 Published: March 15, 2018 3129
DOI: 10.1021/acsomega.8b00063 ACS Omega 2018, 3, 3129−3135
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lattice fringe distortion across the NWs to be evaluated with respect to an unstrained area within the projected lattice, which is taken as a reference. The two-dimensional mapping of the lattice expansion (positive value) or contraction (negative value) relative to a reference (d0), that is, (d − d0)/d0 × 100%, was carried out for each NBED pattern spot. The reference was taken at the center of the NW (marked by the dots in the maps), where presumably no lattice deformation occurred. The precision of the strain was
