第124回先端計測オープンセミナー

2018年5月10日 (木) 15:00-16:00

国立研究開発法人 物質・材料研究機構
千現地区 研究本館8階 中セミナー室
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Dr. Ovidiu Cretu (ICYS Researcher)

In-situ experiments on low-dimensional materials inside the transmission electron microscope

 In this talk I will show some results which have been obtained by performing in-situ experiments on 2D monolayered nanosheets and 1D individual nanotubes and nanowires inside a transmission electron microscope (TEM).

The first part of the talk discusses the idea of electron irradiation damage in a very beam-sensitive material - hexagonal boron nitride. I will show that the interplay between electron irradiation and sample temperature allows us to obtain a wide range of new structures. This includes well-known defects with triangular symmetry, but also completely new configurations, such as hexagonal-shaped and boron-terminated vacancies or - in more extreme cases - boron nitride atomic chains. Due to the nature of the experiments, these structures can be immediately characterized. The data is discussed in relation to various damage mechanisms, showing that the transformations are determined by an interplay between electron irradiation and thermally-enhanced reconstructions. In support of these findings, I will show that electron beam damage in this material can be completely avoided by overlapping the sample with a monolayer of graphene.

The second part of the talk shows data acquired on carbon nanotubes and CdS nanowires inside the TEM, using a combination of commercial and custom-made instrumentation. This approach is centered on sample holders which allow for an optical fiber and/or a metallic probe to be positioned close to the region of interest of the sample, via a piezoelectric mechanism, with nanometer precision. Firstly, I will show the possibility of mapping the luminescence centers in CdS nanowires through cathodoluminescence. The results reveal the distribution of centers with nanometer-precision, and then associate them to various intrinsic defects in CdS. Subsequent experiments improve on the previous method and describe optoelectronic measurements. CdS nanowires are probed before, during and after mechanical bending, revealing details such as their optical ON/OFF ratios and wavelength-dependent photocurrent generation. Lastly, I will introduce the idea of carbon nanotube telescoping and highlight the stability of these structures, which remain electrical conductive while extended and recover their initial properties once retracted, over the course of several cycles.