Characterisation of Photonic Nanostructures in an Electron Microscope

July 1, 2019

Shima Kadkhodazadeh of DTU Nanolab provides insight into Photonic Nanostructures following research conducted at the Technical University of Denmark

Humanity has always been fascinated by light and its interaction with matter, manifested by the ageless desirability of precious stones. Today, nanostructures exploiting optical phenomena are prevalent in fields ranging from telecommunications to solar cells, catalysis, biological sensing and medical diagnostics. A key requirement for future progress in these technologies is obtaining better understanding of the structure – functionality relationship of their constituent components on the nanometre scale. This calls for characterisation methods that are capable of probing structure and properties with nanometre spatial resolution and beyond. A powerful tool for nanoscale characterisation is transmission electron microscopy (TEM). Today’s aberration-corrected TEMs can routinely image structures with atomic resolution. In aberration-corrected scanning TEM (STEM) electrons can be focussed into a probe below 1 Å in diameter, allowing both imaging and chemical analysis to be carried out with atomic resolution. Other characteristics such as strain, polarity and electric field strengths can also be measured in today’s TEMs, all of which influence the optical performance of structures.

Our studies of photonic nanostructures have included III/V semiconductor quantum dots. Low dimensional semiconductor structures are of interest in a number of fields, due to their characteristics that differ from those of the bulk, and which can be manipulated by controlling their structure and chemistry. Quantum dots have dimensions so small that quantum mechanical effects result in the quantisation of their electronic density of states in 3 dimensions. The exact configuration of the electronic states and subsequently, the optical properties of the quantum dots are largely determined by their structure and chemistry. Thus, a significant step in optimising their performance and synthesis is understanding their subtleties on the nanometre or even atomic scale. Our studies have included atomic resolution imaging to determine the morphology and chemical distribution¹, 3-dimensional reconstruction of morphology² and strain measurement³ (Figure 1). The results have contributed to tailoring the synthesis process for fabricating structures with desired optical properties, and formulating more accurate theoretical models to predict the properties of such structures.

Figure 1: (a) Atomic resolution STEM image of an InAs/InP quantum dot in cross-sectional view. The position of different atomic species can be directly detected in the image and the dimensions and profile of the quantum dot can be accurately measured1. (b) Electron tomography analysis of InAs/InGaAsP quantum dots before and after embedding in InGaAsP matrix, revealing the morphological changes undergone as the result2. (c) Electron holography measurement of strain in InAs/InP quantum dots3.

A very unique capability in TEM is the possibility of probing optical properties with sub-nanometre resolution. While light-based methods are the obvious choice for optical characterisation, they are limited to spatial resolutions on the order of tens of nanometres at best. Traditionally, the energy resolution attainable in TEM was insufficient to access the energy ranges relevant for optical excitations. However, continuous advances in instrumentation have bridged this hurdle and improved the energy resolution of state-of-the-art TEMs to values comparable to optical techniques. Another focus of our TEM studies of photonic nanostructures has been metallic nanostructures for plasmonic applications. Plasmonics is an emerging field in which metallic structures are used to guide light or to enhance interactions between light and materials placed in their vicinity. This is due to the ability of these nanostructures to support surface plasmon resonances in the optical range of the electromagnetic spectrum. Our studies have included probing the evolution of the surface plasmon resonances in gold and silver nanoparticles as a function of particle size⁴ and separation distance between particles^5,6, including sub-nanometre separations (Figure 2). The results have provided novel insight into the characteristics of plasmonic nanostructures, otherwise unreachable by optical methods, as well as testing and validating new theoretical descriptions concerned with the plasmonic properties of low dimensional metallic structures⁷.

Recently, we have expanded the research to include metal alloy nanoparticles showing superior performance for plasmonic ultrafast hydrogen detection^8,9.

Figure 2: (a) Electron energy-loss spectra probing the surface plasmon resonances in silver dimer nanoparticles at separation distances below 1 nm, revealing the evolution of surface plasmons resonances in a non-classical regime. (b) Electron energy-loss measurement of the surface plasmon resonance wavelength for gold and silver dimers as a function of separation distance. Establishing an accurate scaling of the surface plasmon resonance with distance is highly important for nanometrology applications.

These examples highlight how improvements in the spatial and energy resolutions of modern TEMs contribute to bridging the gap in our knowledge of the structure – functionality relationship in photonic nanostructures. A step forward in this quest entails in-situ TEM probing of nanostructures under working conditions. In-situ methods in TEM are rapidly expanding, and currently include heating/cooling, electrical biasing, light injection and detection, and gas/liquid environment. Future efforts in this field will focus on combining these tools with TEM measurements of morphology, crystal structure, chemistry, strain, electric field, etc., to “shed better light” on the structure – functionality link in photonic nanostructures.

References

¹ S. Kadkhodazadeh, E.S. Semenova, K. Yvind, and R.E. Dunin-Borkowski, J. Cryst. Growth 329, 57 (2011).

² S. Kadkhodazadeh, E.S. Semenova, M. Schubert, M. Thuvander, K.M. Stiller, K. Yvind, and R.E. Dunin-Borkowski, J. Phys. Conf. Ser. 326, 12046 (2011).

³ D. Cooper, J.-L. Rouviere, A. Béché, S. Kadkhodazadeh, E.S. Semenova, K. Yvind, and R. Dunin-Borkowski, Appl. Phys. Lett. 99, 261911 (2011).

⁴ S. Raza, S. Kadkhodazadeh, T. Christensen, M. Di Vece, M. Wubs, N.A. Mortensen, and N. Stenger, Nat. Commun. 6, (2015).

⁵ S. Kadkhodazadeh, J.R. de Lasson, M. Beleggia, H. Kneipp, J.B. Wagner, and K. Kneipp, J. Phys. Chem. C 118, 5478 (2014).

⁶ S. Kadkhodazadeh, J.B. Wagner, H. Kneipp, and K. Kneipp, Appl. Phys. Lett. 103, 83103 (2013).

⁷ S. Kadkhodazadeh, T. Christensen, M. Beleggia, N. A. Mortensen, J. B. Wagner, ACS Photonics 4, 251 (2017)

⁸ S. Kadkhodazadeh, F. A. A. Nugroho, C. Langhammer, M. Beleggia, J. B. Wagner, ACS Photonics 6, 779 (2019)

⁹ F. A. A. Nugroho, I. Darmadi, L. Cusinato, A. Susarrey-Arce, H. Schreuders, L. J. Bannenberg, A. Bastos da Silva Fanta, S. Kadkhodazadeh, J. B. Wagner, T. J. Antosiewicz, A. Hellman, V. P. Zhdanov, B. Dam, C. Langhammer, Nature Materials 18, 489 (2019)