Research
Pioneering Research in Advanced Materials & Atomic Engineering
We are an electron microscopy group, focusing on radiation-solid interactions, optical and vibrational spectroscopy, and simulating materials in the environments in which they are designed to perform in.
research direction 1
Structural and Spectroscopic Analysis of Defects in 2D Materials
With funding from NASA and the Department of Energy, we aim to study the nature, and processing of defects in two-dimensional materials and their impact on electronic and optical materials. Here, we use charged projectiles and accelerate them at a target. The ion transfers its energy to the atom it interacts with by both elastic collisions with the target nuclei and inelastic collisions with the target electrons. A schematic of the ion-solid interaction is shown in Fig. 1a.
Our characterization of the films is done with a transmission electron microscope (TEM). Electrons are used as our analysis quanta because they propagate through space at a wavelength that is shorter than the atomic distance in a materials lattice. This allows us to visualize the atomic details of a material from a reconstruction of the electron wavefunction after the electron interacts and passes through a sample.
In Fig. 1b, we first shown a scanning TEM (STEM) pristine sample of MoS2, taken before an ion interacts with the film. Then, a defected sheet is shown after a high energy, 3 MeV Au2+ ion is accelerated at the target. The last image displays an implanted Au atom in an MoS2 lattice. The identity of the Au atom is verified using electron energy loss spectroscopy (EELS), a technique performed in the microscope to identify chemical fingerprints of a sample based on the amount of energy loss by the incident electron.
Figure 1: Structural and spectroscopic analysis of defects in MoS2.
EELS does have the limitation of not being sensitive to the Fermi-level. Therefore, if we want to extract spectroscopic information of excitations all the way up to the vacuum level, we must call on photoemission electron microscopy (PEEM). In Fig. 2, a simplified work function band diagram is shown for an ion beam modified MoS2 sample that is both free standing and on a substrate.
Interestingly, we see an increase in work function on free-standing films, and a decrease in work function on Au-supported films when defects are introduced in the system. This indicates that substrate recoil atoms play a major role in the manipulation of materials properties.
Figure 2: Simplified work function band diagram MoS2 that is irradiated with 10 keV He+ ions both free-standing (over vacuum) and supported by Au.
research direction 2
Atomic-scale Vibrational Spectroscopy of Crystalline and Amorphous Materials
We create a monochromatic electron beam in our STEM to greatly reduce the distribution of electrons from our source. This means we can excite low energy phenomena in a range of nanomaterials. This is even below the traditional regime used to excite optical phenomena (excitons and plasmons) but we now have access to the vibrational regime.
Accordingly, we can couple atomic-scale analysis in an aberration-corrected STEM with vibrational spectroscopy to unveil the direct correlation between atomic arrangements and atomic displacement modes.
First, we can use monochromated EELS to analyze the phonon modes in polymorphs of MoSe2, as shown in Fig. 3.
Next, we position a small probe at the micron-length scale to collect the vibrational modes at the interface between two different 2D materials: SnS and GeSe. While the phonon shift is close to the noise-limit of our experimental set-up, our measurements are still sensitive enough to pick up the variance in the displacement modes.
Figure 3: Corresponding aberration-corrected STEM and phonon-EELS of polymorphs in MoSe2.
Figure 4: SnS-GeSe heterostructure, where colors indicate where the probe is positioned to acquire each spectrum shown in the EELS plot.
Aside from polymorphs and heterointerfaces of 2D-materials, monochromated EELS inside an aberration-corrected STEM allows one to match one-to-one the aerial defect density with the vibrational modes in a material. This is a way to truly unveil the defect-bound phonon modes with altering defect inventories.
Figure 5: Aberration-corrected STEM images of pristine and irradiated WS2 with 2.6 MeV Fe2+ ions. The corresponding monochromated EELS plot of the micrograph regions are shown above.
Last in this section, we should consider a case where the phonon signal has a dipole component. This represents an ideal case to perform an off-axis EELS experiment. Hereby, we introduce the concept of off-axis EELS in Fig. 6. In Fig. 6, a 2D EELS signal is displayed to represent a bright field (BF) being displaced in q-space. The BF disc is electrically shifted outside of the EELS entrance aperture by the projector lenses such that a portion of the dark field disc now enters the spectrometer window to give information on the impact scattering in the off-axis regime.
This subsequently allows access to highly local electronic, optical, and vibrational information (i.e., energies less than a few eV). The plot in Fig. 6 yields a direct comparison of off axis vs. on axis spectrum for a thin film of hexagonal boron nitride. On axis yields more signal, but the signal is delocalized and contains dipole scattering. Off-axis has the benefit of having a highly localized signal and contains only impact scattering. Accordingly, we can separate out phonons that are dominated by impact scattering events in a region of interest. See the transverse optical (TO) phonon mode, for example. When the on-axis contribution is highlighted, the signal is dominated by excitations that are delocalized over a range of unit cells. Off-axis EELS suppresses the long-range contributions and highlights only the localized excitations occurring at the limit of individual atoms. The scattering collected off-axis resembles that of Rutherford scattering.
Figure 6: Off-axis EELS of hBN. 2D-EELS representation of hBN with BF disc displaced outside of EELS aperture and plot of on- and off-axis EELS comparison.
research direction 3
Data-driven in-situ TEM
In situ transmission electron microscopy (TEM) allows one to observe the structure of a material at unprecedented spatial resolution, while probing non-equilibrium phenomena with an external stimulus. However, with the dynamic frame rate of cameras now collecting over 1000 frames per second, an individual experiment can easily create 100,000 frames to process. While it simply isn’t feasible for a human to extrapolate metrics from this amount of information, data-driven approaches emerge to mitigate this issue. This research directorate sheds light on how data-driven approaches can probe defects in time-series micrographs for nuclear materials applications, as well as laser-matter interactions, space applications, and non-aqueous corrosion.
Figure 7: Overview of environmental extremes we can couple inside a TEM. (a) A 532 and 1064-nm laser are coupled in the TEM to simulate a gaussian beam laser ablation in a range of materials. (b) Example of an in situ TEM holder (thermal anneal). Holders allow one to subject an external stimulus to a material while observing lattice dynamics. (c) Ion irradiation source couples in-line with the TEM (in EDS port) to simulate generation IV nuclear reactor environments.
The in situ videos show both a self-ion irradiation of Cu (used to simulated displacement cascades) and an isochronal thermal anneal of He bubbles in a Pd-Ni alloy to simulate materials aging. The Cu sample is on the left, while the PdNi alloy is on the right.
Figure 8: in situ ion irradiation of Cu (a) and thermal anneal of PdNi in (b).
There are many problems with the databases collected from these experiments in that they are massive, susceptible to bias from human analysis, take a huge amount of time to extract analytics from the experiment, and are difficult to reproduce the statistics from one scientist to another. Our research group works on customized workflows to process images and extract trends in pseudo real-time. Fig. 9 displays a case of supervised learning. Detailed information found here: APL Mach. Learn. 2, 016117 (2024).
Figure 9: Image analysis workflow generated for decoupling the evolution of defects in time-series micrographs.
Figure 10: Learning latent-space manifolds of He bubbles in PdNi alloys. (a) In situ isochronal thermal anneal of PdNi alloys. (b) Process for semi-supervised learning for micrographs in (a). (c) Contamination matrix, (d) shear deformation matrix, and (e) artificial noise generation proposed to build up synthetic database for training.
One problem with supervised learning is that it can be difficult to scale your solution to different datasets (i.e., training on dislocation identification and migrating to precipitates). This is where autoencoders may be suitable!
Here, we show some results from using a variational autoencoder with rotational and translational invariances to create and apply artificial constraints to the dataset. This serves as a proposed method to make interpretation of a dataset reproducible from a customized workflow. A great explanation can be found here.
research direction 4
Low-temperature Optical Spectroscopy
Veering away from our traditional electron spectroscopy focus, optical spectroscopy gives us more signal, more energy resolution, and is (usually) less destructive on the sample. It is a vital tool in our characterization wheelhouse to determine transition frequencies of atoms in functional materials.
Going to liquid He temperatures (~4K) allows for significantly improved resolution and sensitivity in the measurements. Additionally, a quartz zero-order half-waveplate from Newport is used either pre- or post-sample to rotate plane-polarized light at low temperatures. We do this to resolve the symmetry of quasiparticles and interpret anisotropic properties in 2D materials. In Fig. 11a, a schematic of the experiment shows where a waveplate (green) is with respect to a polarizer (blue). In Fig. 11b and 11c, polar plots showing 4-fold and 2-fold symmetry of the same displacement mode in SnS. Panel b is linearly-polarized light, while panel c is co-polarized light, rotated before the sample.
Figure 11: Schematic showing polarization-resolved spectroscopy experiments
the latest
Recent Publications & Key Findings
Explore our latest breakthroughs and contributions to materials science. Our founder, Dr. Kory Burns’ is regularly published and has presented at leading conferences.
