“Ultra-Atomic Resolution Electron Microscopy Project” Seminar Series“Revealing the role of individual molecules and strain in
nanostructured materials using advanced and in situ
electron microscopy”

Prof. Eva Olsson
Department of Physics, Chalmers University ofTechnology, Gothenburg, Sweden
The understanding of the structure-property relation of individual nanostructures is of critical importance for theadvancement of materials. The methods of advanced and in situ electron microscopy are well suited for characterizingthese relationships since the enable site-specific and direct correlation between atomic structure, electronic structure,mechanical behaviour, electrical transport and photoresponse. The results provide information for a basic understandingof the properties of these nanostructures and for tailoring device performances.
By in situ transmission electron microscopy (TEM), a direct correlation between mechanical and charge transport
properties was determined for InAs nanowires [1]. A uniaxial tensile stress was applied to individual nanowires andstrain mapping was performed by using 4D scanning TEM (4D STEM), while electrical measurements were carried out
simultaneously. A significant reduction of resistivity and enhanced piezoresistive response of the nanowires, compared
to bulk InAs, were observed with increasing strain. Individual GaAs nanowires were also studied using the same in situ
TEM approach [2]. Evidence for hole mobility modification by uniaxial strain was found. For bending deformation, thecurrent-voltage (I-V) characteristics of the nanowires change from linear to nonlinear [3].
For individual GaAs nanowires with build-in radial p-i-n junctions, being equivalent to individual nanoscale solar cells,
the photovoltaic properties, i.e., photocurrent and I-V characteristics, were investigated using an in situ scanningtunnelling microscope – scanning electron microscope (STM-SEM) setup [4,5]. A uniaxial tensile strain of 3% resulted
in an increase of photocurrent by more than a factor of 4 during near-infrared (NIR) illumination. This increase isattributed to a decrease of 0.2 eV in nanowire bandgap energy, thus reflecting the effect of tensile strain on lightabsorption. The quality of the electrical contacts is crucial when performing in situ biasing studies [5]. The contacts arealso of utmost importance in electronic devices and therefore currently attracts significant attention in the development
in devices based on two dimensional materials [6].
Molecular dopant species are used to improve and optimize the properties of organic semiconductors. A higher dopant
concentration may result in a higher electrical conductivity but the details of the spatial distribution of the dopant
molecules, clustering and cluster morphology are equally important [7]. The charge transfer between the dopants andthe surrounding polymer is promoted by a short distance between the dopant and the polymer. The dopant cluster
morphology determines if each dopant molecule is in direct contact with the polymer or if a portion of the dopants are
instead imbedded and parted from the polymer. Here, electron tomography was used to reveal the three dimensional(3D) distribution of individual dopants and dopant clusters.
This talk will show how advanced and in situ electron microscopy studies enable a quantitative understanding of theintriguing interplay between atomic structure, electronic structure, charge transport and photocurrent. It will also discuss
crucial aspects of the studies and illustrate the importance of control experiments to ensure that the in situ experiments
provide representative information of the correlation between atomic structure and properties.
References: [1] L. Zeng et al, Nano Lett. 18, 4949 (2018), [2] L. Zeng et al., Nano Lett. 21 3894 (2021), [3] L. Zeng etal., Phys Status Solidi., 13, 1900134 (2019). [4] J. Holmér et al., Nano Energy 53, 175 (2018). [5] J. Holmér et al., NanoLett. 21, 9038 (2021). [6] N. Shetty et al., ACS Appl. Nano Mat. 6, 6292 (2023). [7] G. Persson et al., Nanoscale 14,15404-15413 (2022).

Main meeting room at Institute of Engineering Innovation,Faculty of Engineering, Bldg. 9, 1F, 22 March 2024 (Fri.) 16:40～18:10