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Characterisation and properties

Knowing the composition, structure and properties is the basis to understand and further develop the produced nanostructure. We use a number of techniques to characterise the nanostructures, e.g. TEM, XEDS, AFM, PL, CL and synchrotron beamlines.

Project areas:

3-D reconstruction from TEM and STEM.

By using several different techniques, such as HREM imaging, Dark-field STEM at high and low angles and electron induced x-ray emission, we try to acquire more information from images of nanostructures. The classical way is to use either absorption or, the opposite,  high angle scattering to get a monotonically proportional signal to the projected density, and reconstruct volumes with the same density by surface rendering. However, including information like chemical composition and crystallographic orientation, we may extract more information which is not merely equivalent to the 3-D shape of the structure.3D tomography

Image: A full 3-D tomographic reconstruction of a Sn-doped GaAs nanowire including the chemical composition. The seed particle has two components; Au/Sn alloy and Au/Ga alloys. The surfaces of the wire is decorated with pure metallic Ga nanoparticles

 

In-situ characterisation during growth of nanowires

In a new approach, demanding serious investment in instrument and time, we attempt to follow the growth of a nanowire from the nucleation stage at a single metal nanoparticle, to a full grown nanowire. This is performed with atomic resolution imaging, in which each layer, or bilayer of atoms as it proves to be, can be studied in real time.  The growth mechanism of the wire is depending on the pressure of the gaseous precursors, the proportion between them, and the temperature in a complex way. With the new Environmental Transmission Electron Microscope (ETEM) we can control these parameters, and also measure the composition of the seed particle all through the growth process. With the ETEM, we can emulate the growth parameters used in large scale growth machines, but also investigate new combinations which have not been tried before, or even is impossible to achieve with today’s growth systems. We can also perform more growth runs per day than possible on a large system, and at the same time use microscopic amounts of the expensive and sometimes harmful substances, thus minimising our ecological footprint.

Surface characterisation

Direct access to electronic and geometric structure of solid surfaces down to the Ångstrom length scale is available by STM. Its importance in the study of surface electronic and geometric structure of low dimensional object cannot be underestimated, and have shown its usefulness as a tool to study electronic and geometric structure in numerous cases. We have demonstrated that atomic resolved structural and electrical measurements by STM is possible onside, inside and topside of III-V nanowires - and put this to good use. We developed wire atomic hydrogen cleaning procedures necessary for atomic resolved STM, tested them on full nanowire devices (with all interconnects in place) and observed that device function was maintained after cleaning. We use our combined STM and AFM setup to measure atomic scale surface chemistry and surface electronic/geometric structure directly on functioning low dimensional components. At MAXIV , new possibilities arise for in-situ characterisation of surfaces.

Optical and electronic properties of nanostructured materials

Light absorption, emission and light-matter interactions in general are phenomena originated from structure of the electronic states and excited state dynamics in materials. These properties can be modulated by shape and size effects when the materials possess structures at the nanoscale. We use time-resolved absorption, luminescence and multi-dimensional coherent spectroscopies to probe the excited state dynamics in semiconductor nanowires, nanoparticles, organic semiconductors and other electronic materials. Moreover, combination with optical or electron microscopy allows to study these properties with high spatial resolution (for example within one nanowire) and probe variations of the properties from one individual nano system to another. Monitoring the fate of the excited states is the basis for rationalization of fundamental properties of functional nanomaterials and the origin of parasitic losses in devices.

Key publications

Electron tomography reveals the droplet covered surface structure of nanowires grown by aerotaxy. Axel R Persson, Wondwosen Metaferia, Sudhakar Sivakumar, Lars Samuelson, Martin H Magnusson, Reine Wallenberg. Small 14 (33), 1801285. DOI: 10.1002/smll.201801285
See article electron tomography at publisher's site

“Supertrap” at Work: Extremely Efficient Nonradiative Recombination Channels in MAPbI3 Perovskites Revealed by Luminescence Super-Resolution Imaging and Spectroscopy. A. Merdasa, Y. Tian, R. Camacho, A. Dobrovolsky, E. Debroye, E. L. Unger, J. Hofkens, V. Sundström, and I. G. Scheblykin: ACS Nano, 2017, 11 (6), pp 5391–5404. DOI: 10.1021/acsnano.6b07407
See article supertrap at work at publisher's site

Key faculty

Recent theses

Filip Lenrick, Focused ion beam preparation and transmission electron microscopy of materials for energy applications. PhD Thesis, Lund University 2016.
See Filip Lenrick's thesis at the Research Portal

Axel R. Persson, Nanoscale electron tomography and compositional analysis of Aerotaxy nanowires. Licentiate Thesis, 2018.
See Axel Persson's thesis at the Research Portal

Aboma Merdasa, Super-resolution Luminescence Micro-Spectroscopy: A nano-scale view of solar cell material photophysics. PhD thesis, Lund University 2017.
See Aboma Merdasa'a thesis at the Research Portal

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Videos

Recording of growth of gold-seeded indium arsenide nanowires in the Lund ETEM. At the beginning of the sequence we drop the temperature from 600ºC to 420ºC, so that growth initiates, and at the end of the sequence, we raise the temperature to 600ºC again, so that the nanowires start etching away again.