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Nanotwin Detection and Domain Polarity Determination via optical second harmonic generation polarimetry Mingliang Ren, Rahul Agarwal, Pavan Nukala, Wenjing Liu, and Ritesh Agarwal Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b01537 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on June 29, 2016
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Nano Letters
Nanotwin Detection and Domain Polarity Determination via optical second harmonic generation polarimetry Ming-Liang Ren†, Rahul Agarwal†, Pavan Nukala, Wenjing Liu, Ritesh Agarwal* Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA * E-mail:
[email protected] †
These authors have equal contribution to this work
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.
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Key words: second harmonic generation, twin boundary, nanowire, transmission electron microscope, crystal structure, nonlinear optics
Defects in crystalline materials such as grain boundaries, twins and dislocations are of significant interest as they influence their mechanical, electronic and optical properties. For example, defects such as dislocation pairs can induce local band bending that efficiently separates electrons and holes impacting performance in CdTe photovoltaics1. The creation of dislocations in a phase change memory device can induce crystalline-to-amorphous phase transition with large ON/OFF resistance ratio, and have been recently utilized to engineer ultralow power memory devices2-4. Moreover, planar defects such as twin boundaries and stacking faults (SFs) are commonly found in II-VI and III-V semiconducting nanostructures of zincblende (ZB) and wurtzite (WZ) crystals grown by the vapor-liquid-solid (VLS) mechanism5-10. Twin boundaries can be formed in the upright (or ortho) configuration with cation-anion bonding and also in the inverted (or para) configuration with cation-cation or anion-anion bonding11, 12, and have been observed to enhance cathodoluminescence (CL)13 and increase the efficiency of charge separation for solar hydrogen generation14. Not only in crystalline semiconductors, nanotwins in metals have been observed to induce high electrical conductivity, high strength and duplicity (tensile elongation)15, 16. Other materials, like diamond, are also known to achieve unprecedented hardness and thermal stability after introducing nanotwins17. Therefore, in order to understand the effect of nanotwins on material properties, detection and analysis of the nature of nanotwins is of critical importance.
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Nano Letters
While established techniques such as diffraction and phase contrast transmission electron microscopy (TEM), conductive atomic force microscopy (c-AFM) and piezo-force microscopy (PFM) are routinely used to characterize such extended defects such as dislocation pairs and twin boundaries, each of them suffer from certain limitations. Conventional diffraction and phase contrast TEM techniques are limited in distinguishing between intensities of diffraction spots with opposite k-vectors owing to Friedel’s law (a property of Fourier transform of a real function, F(k)=F(−k)* and then |F(k)|2=|F(−k)|2)18,
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. This translates to a fundamental limitation of
diffraction contrast TEM in not being able to detect the nature of the stacking twin boundaries. The convergent beam electron diffraction (CBED)20 and scanning tunneling microscope (STM)21 are fundamentally limited by the sample thickness and are very challenging experiments. Although c-AFM, using the electrical current to measure the surface profile of conducting samples has been performed on polycrystalline CdTe thin film to distinguish twin boundaries from grain boundaries22, no further studies have been reported to ascertain the nature of twin boundaries. PFM, typically probing ferroelectric domains, is not applicable to semiconductor nanotwins due to high electrical conductivity of typical semiconductors22, 23. Optical second harmonic generation (SHG) polarimetry, sensitive to crystal structure and electronic transitions24, is another promising way to probe ferroelectric domains25 and visualize boundaries of two antiparallel domains26. Polar nature of ferroelectric twin boundaries in CaTiO3 can be resolved directly via SHG because of distinct SHG polarimetric signals associated with twin boundaries with specific polarizations27. Since the domain size is very large (>100 µm) and each domain exhibits very weak SHG signal in this case, an individual twin boundary that only has one single polarity can be found easily via SHG and then characterized in detail. For an intermediate-sized domain (>10 µm), the nature of the boundaries can be determined by 3 ACS Paragon Plus Environment
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detecting each domain via 2D or 3D SHG microscopy if it is non-centrosymmetric, e.g. polycrystalline monolayer MoS228. At the nanoscale (
