Nanotwin Detection and Domain Polarity Determination via Optical Second Harmonic Generation Polarimetry

Abstract

We demonstrate that optical second harmonic generation (SHG) can be utilized to determine the exact nature of nanotwins in noncentrosymmetric crystals, which is challenging to resolve via conventional transmission electron or scanned probe microscopies. Using single-crystalline nanotwinned CdTe nanobelts and nanowires as a model system, we show that SHG polarimetry can distinguish between upright (Cd-Te bonds) and inverted (Cd-Cd or Te-Te bonds) twin boundaries in the system. Inverted twin boundaries are generally not reported in nanowires due to the lack of techniques and complexity associated with the study of the nature of such defects. Precise characterization of the nature of defects in nanocrystals is required for deeper understanding of their growth and physical properties to enable their application in future devices.

Inverting polar domains via electrical pulsing in metallic germanium telluride

“…1d,e). It must be noted that while diffraction-contrast TEM provides structural information on domains and domain walls illuminated in a particular zone, a suitable SHG polarimetry technique gives quantitative information on all the domain fractions1820.…”

“…Single-crystalline GeTe samples, though not standard, are the best option, although complications arise owing to the measured SHG signal being an interference of signals from sixteen domains. We designed polarimetry experiments18 on single-crystalline nanowires where the fundamental light (1,020 nm, spot size: 2 μm) was shone along the P α(+) direction (Fig. 1a, inset) and we varied its linear polarization direction from −180 o to 180 o with respect to the long-axis of the nanowire.…”

“…The ratios of volume fractions of domains following the same stacking sequence (either 1 or 2) were calculated as shown in Table 1, with the understanding that V d( i ) = V d(+, i )– V d(–, i ) , where ‘d' labels the ferroelastic domain α, β, γ or δ , and i =1 or 2 labels the type of stacking sequence (Supplementary Note 1). On 15 different nanowire devices of large diameters (>500 nm to avoid anisotropy in in-coupling and out-coupling of light)183335; we estimated the median value of the material constants d 31 /d 15 , and d 15 /d 33 to be 1.05 and −0.53, respectively (Supplementary Fig. 4).…”

“…Under each excitation polarization (0°, 60° and 120°), the SHG signals were imaged by a cooled charge-coupled device and detected by a spectrometer (Acton) with a 300 groove mm −1 500 nm blaze grating with a charge-coupled device detector (Princeton instruments) with a spectral resolution of 0.1 nm. The polarization properties of SHG signal was studied by placing a polarizer in front of the detector18.…”

“…Here, we developed a variant of the optical second harmonic generation (SHG) polarimetry181920, which is sensitive to material structure in crystals with no inversion symmetry212223242526 to quantify different domain fractions in single-crystalline GeTe. In conjunction with in situ transmission electron microscope (TEM) characterization of the GeTe devices27282930, we studied the interaction of these domains and domain boundaries with extended defects such as anti-phase boundaries (APBs) and associated partial dislocations created by heat shocks from current pulses.…”