scanning time rather can cause the sample drift and carbon layer formation on the sample surface. For the further ptychographic measurements optimized dwell time values of 300-400 ms for pure magnetic contrast samples and 100-200 ms for charge scattering samples are used.
guess of illumination function, which no longer has a Gauss but a doughnut shape.
Dwell time optimization showed no considerable improvement of the image contrast of a highly scattering resolution target in the range from 100 to 700 ms. At the other hand, low scattering domain magnetic sample gained 20%of contrast with increase of dwell time from 100 to 300 ms and with no significant changes till 500 ms dwell time. It demonstrates that quality of the reconstructions can not be endlessly improved by increase of the dwell time values.
Chapter 5
Ptychographic chemical and magnetic contrast
5.1 Chemical contrast in LiFePO
4battery nanoparticles
TodayLiFePO4particles are prominent materials for energy conversion and storage, that is efficiently realized in electrochemical systems, where a phase-separating battery elec-trode consists of numerous nanoparticles packed in dense (up to1015cm−3) ensembles [108]. These electrodes can be chemically inhomogeneous on nanoscale that causes un-even distribution of current load, as a result directly affecting their life cycle. Therefore it is crucial to understand the nature of Li migration and its insertion on nanoscale level by investigation of the corresponding particle morphology and processes occurring during (de)lithiation. As an actual system for energy storage theLiFePO4battery nanoparticles have been investigated and spectroscopically imaged by soft X-ray ptychography.
The lithiation process, whenFePO4transforms toLiFePO4, results in a change of the Fe oxidation state fromF e3+ toF e2+. A high spatially integrated reference spectra of the lithiated and delithiated batteries obtained at different energies across the FeL2and L3absorption edges are shown in figure 5.1 a). As it is seen from the reference spectra these transformation causes a significant change, especially in the FeL3edge structure.
The corresponding energy scans taken at the MAXYMUS STXM in figure 5.1 b) are of reduced quality due the instabilities in the energy scan accompanied with difficulties of the I0 monitoring. Nevertheless the images taken at 708.2 eV and 710.3 eV allows a clear separation of these two phases in correspondence with the particle structure, size and morphology.
TheLiFePO4nanoplates were produced and studied in collaboration with Department
of Materials Science and Engineering of Stanford University (USA). The extensive inves-tigation of lithiation processes in these samples can be found in the following publications [109, 110, 111]. Till the recent time a significant limitation for studying these chemi-cal reactions in the single-crystal particles was the narrow range of tools which provided sufficient spatial resolution and chemical sensitivity for the visualization at sub microme-ter level. The first ptychography imaging of nano batmicrome-teries have been performed at ALS (Berkeley, USA) and presented in papers [50, 110, 151].
Figure 5.1: Absorption spectra ofLiFePO4nanoplates: a) absorption reference spectra of lithiated and delithiated areas of the battery. Adapted from [113]; b) the same spectra obtained at MAXY-MUS on theLiFePO4batteries overL3absorption edge with peaks at 708.2 eV and 710.3 eV.
The STXM image of a typical nano platelet is shown in the figure 5.2 d)-f), it has around 1-2µmwide and 2-3µmlong. The sample was scanned along Fe-edge for probing different oxidation states of Fe in charged and discharged phases. Ptychographic images, as well as STXM, show coexistence of two different oxidation phasesFe2+ andFe3+
at the same time within one nanoplate. Optical density (OD) images a), b) and c) show that the lithiated area is located inside of the nanoplate and surrounded by the delithiated regions. The value of complex refractive indexβof two phases differs for order of magni-tude. For the same reason comparing images at different absorption edges we see that OD is much higher for the delithiated area. It provides significant contrast of the interface ma-terials that makes this specimen a good test target for chemically sensitive high resolution ptychography.
The STXM images have obtained with FZP having∆r= 25nm with spatial resolu-tion of about 30 nm, while ptychographic reconstrucresolu-tions have the real space pixel size of 11 nm at Fe absorption edge and 8 cm distance from sample to detector. Chemical contrast of the ptychographic images is estimated by the step width between two chem-ical phases which show sharp border at the neighboring regions. The width of the edge
Figure 5.2: Comparison of STXM and ptychography imaging of chemical contrast inLiFePO4
nanoplates: a),b),c) OD images obtained by ptychography with a pixel size around 11.3 nm, d),e),f) OD STXM images obtained with 24 nm resolution focusing FZP with scanning step of 10 nm.
Images a),d) and b),e) showFe2+andFe3+oxidation states, respectively, e) and h) correspond to overlap maps of two phases, where red isFe2+and green isFe3+phase.
is determined by the distance from 10%to 90%of intensity as it is seen in figure 5.3 b).
The step width is 30.2 nm for ptychography and 59.7 nm for the STXM image that is about 2 times resolution improvement in the ptychographic image. The selected particles in the STXM and ptychography image sets are not the same and may have slightly differ-ent inclination relative to the beam direction. Therefore the measured profile values can not be used as an ultimate resolution evaluation giving just a general understanding of the resolution improvement.
The minor features, which would not be resolved at STXM images, are visible on the surface of the nanobatteries, e.g. cracks. It was shown in [50] that cracks occur along the longer side (c-axis) of the crystal due to lattice shrinkage along the shorter side (a-axis) during the transition ofLiFePO4toFePO4. The thin crack marked with blue arrow
Figure 5.3: Line profiles ofLiFePO4nanoplates: a) line profile crossing the border of two different chemical phases, that showed 30.2 nm for ptychography and 59.7 nm for STXM image; b) line profile over morphological border with 15.9 nm and 43.0 nm step widths for ptychography and STXM, respectively. The step width was determined as a distance from 10%to 90%of intensity.
of about 23-50 nm in thickness is visible on the right edge of the nanoplate and reveals unreacted region below.
Resolution evaluation was done by measuring width of the step of the line profile going over the morphological edge of the particle from the substrate. Ptychography and STXM showed 15.9 nm and 43.0 nm step width, respectively, that corresponds to more then 2.7 times resolution improvement (figure 5.3 b). Therefore the half pitch resolution of ptychographic images is around 8 nm.
Pre-edge images were taken initially in order to get pure chemical contrast of the lithiated and delithiated regions by subtracting them from on-edge images. However pre-edge images were significantly blurred with some artifacts pronounced on the pre-edges of the nanoplates. The observed artifacts are caused by the reduction of atomic scattering in the pre-edge region, where scattering factorf2has value close to zero andf1is negative.
At the same time the presence of high scattering from morphological features, i.e. edges, produces disconformity of the scattering signal causing the problems in image reconstruc-tion.