Symposium F.MT03—Frontiers of Imaging and Spectroscopy in Electron Microscopy

Native defects in semiconductors are known to play an important role in determining their optoelectronic properties. Nickel oxide (NiO) is one of the few wide gap oxide semiconductors broadly applied as the hole transport layer in many optoelectronic devices. NiO exhibits p-type conductivity and its conductivity is believed to be controlled by Ni vacancy acceptors. Herein, we present a systematic study comparing the optical and electronic properties of p-type oxygen-rich NiO with Ni vacancies (NiO:V_Ni) and insulating Ni-rich NiO with O vacancies (NiO:V_O) and stoichiometric NiO. The optical properties were obtained by spectroscopic ellipsometry while valence band spectra were probed by high-resolution X-ray photoelectron spectroscopy. The experimental results are directly compared to first-principles DFT + U calculations. The absorption spectra for the NiO:V_Ni sample show significant defect-induced features below 3.0 eV compared to NiO, and NiO:V_O samples. The increase in sub-gap absorptions in the NiO:V_Ni can be attributed to gap states observed in the electronic density of states. Computational results confirm that gap states in NiO:Vo are predominantly Ni 3d states while those of NiO:V_Ni is composed of both the Ni 3d and O 2p states. This work reveals the relations between native vacancy defects and optical properties of NiO and provides insights that would aid in designing optoelectronic devices utilizing NiO with native defects.

Organohalide lead (hybrid) perovskites have emerged as competitive semiconducting materials for photovoltaic devices due to their high performance and low cost. To further the understanding and optimization of these materials, solution-based methods for interrogating and modifying perovskite thin films are needed. In this work, we report a hydrofluoroether (HFE) solvent-based electrolyte for electrochemical processing and characterization of organic–inorganic trihalide lead perovskite thin films. Organic perovskite films are soluble in most of the polar organic solvents, and thus until now, they were not considered suitable for electrochemical processing. We have enabled electrochemical characterization and demonstrated a processing toolset for these materials utilizing highly fluorinated electrolytes based on a HFE solvent. Our results show that chemically orthogonal electrolytes based on HFE solvents do not dissolve organic perovskite films and thus allow electrochemical characterization of the electronic structure, investigation of charge transport properties, and potential electrochemical doping of the films with in situ diagnostic capabilities

NiO_x is a transparent conductive oxide (TCO) that has attracted a lot of attention because of its electrical and optical properties. Most of the available TCOs are n-type semiconductors, while the development of efficient p-type TCOs remains a challenge. NiO_x is one of the promising p-type candidates, because of its superior chemical stability and optical transparency. Some potential optoelectronic applications of NiO_x are as a p-type channel in transparent thin film transistors (TFTs) and as a hole transport layer in perovskite solar cells. In this work thin NiO_x films were obtained by Pulsed Laser Deposition (PLD), d.c. sputtering, e-beam evaporation and sol-gel techniques.
All films were deposited on glass substrates with a size of 2×2 cm. The PLD films were deposited from a NiO target at room temperature. D.C sputtering and e-beam evaporation were used to deposit pure Ni films, which were then oxidized in air at various temperatures. The sol-gel NiO_x was obtained by depositing chemical precursors and subsequent thermal treatment. Various parameters were varied in the experiments and it was found that the structural, optical and electrical properties of the PLD films depend strongly on the oxygen pressure. The most important parameter for the other three types of oxides was the annealing temperature.
All samples were characterized by UV-Vis spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and atomic force microscopy. Results for the properties of NiO_x films deposited by the above techniques will be presented and a discussion for their possible application in TFTs and solar cells will be given.

Colloidal quantum dots (CQDs) are promising semiconducting materials to engineer photovoltaic and optoelectronic devices due to tunable size-dependent absorption and emission properties. Great efforts have been made to find suitable surface ligand to reduce spacing between the adjacent quantum dots to ease charge transport but there has been negligible attention given to purification strategies and systematic efforts to find the ideal ligand populations to optimize device performance. Charge transport could be hindered in the presence of excess free ligands forming interparticle barriers; on the contrary, an under-passivated QD can have high density of trap states and could also undermine the nanocrystal’s stability toward coalescence. Precipitation and redissolution (PR) methods are widely used purification techniques that must typically be repeated to get clean CQDs with appropriate populations of surface-bound molecules (ligands). PR often results in loss of native ligands bound to the surface which induces more surface traps and/or compromise the chemical stability of these nanocrystals.
Gel permeation chromatography (GPC) has been proven to be an effective technique to remove the byproducts and impurities from CQDs without damaging the QDs surface. In this study, we demonstrate the effectiveness of GPC in removing byproducts and unbound ligands from pre and post ligand exchanged PbS CQDs.
NMR and UV-Vis-NIR spectroscopy of oleate-capped PbS quantum dots shows that despite removal of unbound surfactants, PbS QDs remain stable in solution following GPC. These purified nanocrystals were used to made CQDs photodetectors and photovoltaic devices.
We show that films fabricated using GPC purified PbS QD solutions exhibit higher charge transport, and low contact resistance, as investigated through in-plane conductivity and photoresponse. The charge carrier mobility was extracted from the mobility lifetime product and recombination lifetime determined by the spatial profiles of the photocurrent obtained through scanning photocurrent microscopy (SPCM), and transient photocurrent measurements, respectively. Furthermore, our findings show that post synthetic purification and optimum ligand population can significantly improve physical characteristics including reduced surface roughness, suggesting that impurities and optimum ligand coverage play an important role in film morphology and device performance.

Dislocations in III-Nitrides constitute a long standing controversial topic. The lattice and thermal mismatch associated with the heteroepitaxy of GaN results in high dislocation densities in the order of 10⁸ to 10¹⁰ cm^-2. Although III-Nitride optoelectronic devices can operate even at these high dislocation densities, recent investigations indicate that c-type screw dislocations may have a detrimental effect on the properties and reliability of high-power electronic as well as of optoelectronic devices: For example, the observed high reverse (forward) leakage current in electronic (optolectronic) devices is attributed to screw type dislocations [1]. Recently, it has been proposed that these defects are associated with open core structures, thus forming nanopipes with diameters of a few nms [2]. However, imaging screw dislocations at atomic resolution by electron microscopy to validate this model is not trivial. Moreover, first principles calculations that can accurately describe the energetics and atomic and electronic structure of these defects, have been restricted to MBE growth and/or did not explicitly consider the highly strained core region of these defects [3].

In the present study we combine density functional theory (DFT) with large scale empirical potential calculations within an Implicit Boundaries Mutliscale Scheme calculations [4] to investigate the energetics, atomic structure and electronic properties of c-type screw dislocations in GaN. We explicitly consider both closed and open core screw dislocations as well as more than 100 different core reconstructions. Based on these calculations we construct a c-type screw dislocation phase diagram which describes the energetically most favorable core structures as function of the Ga, N and H chemical potentials. Our calculations indicate, in agreement with previous DFT studies, that growth under typical MBE growth conditions, i.e. metal rich growth, that these defects stabilize a Ga filled core. Moreover, under N rich conditions the core is stoichiometric, though a column of Ga-N dimers is removed form the core. However, the energy of the free m-plane surfaces under MBE conditions cannot be compensated by the core and strain energy and hence nanopipe formation is highly unfavorable. In contrast, under typical MOCVD or MOVPE growth conditions the Ga and N dangling bonds at the open core inner surfaces are passivated by NH_x molecules and H atoms, respectively. This reduces considerably the surface energy under these conditions enabling the formation of nanopipes with hexagonal cross section and diameters ranging for ≈1 to ≈2 nm. Our calculations show that these nanopipes obey electron counting rule and hence are semiconducting. Nevertheless, they have important implications on the efficient diffusion and/or incorporation of impurities and/or dopants at the side facets and at the neighborhood of the open core, which will be discussed in detail. We will also discuss in detail the energetics and structure of superscrew dislocations and their implications on the diameter of the nanopipes as well as the potential implications of the nanopipes on the properties and the efficiency of the electronic devices.

[1] K. Nomoto et al., IEEE Electron Device Lett. 37, 161 (2016).
[2] S. Usami et al., Appl. Phys. Lett. 112, 182106 (2018).
[3] J. E. Northrup, Appl. Phys. Lett. 78, 2288 (2001).
[4] L. Lymperakis et al., Phys. Rev. Lett. Phys. Rev. Lett. 93, 196401-1 (2004).

This project has received funding from the ECSEL Joint Undertaking (JU) project UltimateGaN under grant agreement No 826392. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and Austria, Belgium, Germany, Italy, Slovakia, Spain, Sweden, Norway, Switzerland.

Transparent electronics and optoelectronics have been garnering increasing attention for use in high performance devices such as next generation displays, photovoltaics, electrochromic devices and sensors. Metal oxides are considered to be necessary components in these applications due to their excellent optical transparency ranging from visible to near infrared regime as well as pseudo-metallic electrical conductivity[1, 2]. These metal oxides have been implemented in devices, depending on their electrical conductivity, as transparent electrodes by degenerately-doped oxides such as InSnO and doped ZnO; semiconducting active layers based on indium oxides, ZnO or SnO₂; and insulators (e.g., SiO₂, Al₂O₃, HfO₂) for dielectrics or encapsulations.

Thin film transistors (TFTs) are one of the key elements in displays as a pixel driving switch of which the required properties include high carrier mobility, low thermal budget processing, structural phase stability and reliable device performance in the presence of thermal and bias stress conditions. In recent years, amorphous oxide semiconductors based on indium oxides have been considerably investigated due to their promising carrier mobility, >5-25 cm²/Vs, compared to conventional amorphous Si (<1 cm²/Vs); low temperature processing; mechanical flexibility compared to the crystalline counterparts; and large area process-ability. Indium oxide-based binary and ternary cation material systems have shown tremendous promises for next generation displays since these materials meet the property and process needs that are described above. A binary cation material of indium zinc oxides (IZO) has been considered since the addition of Zn into In₂O₃ stabilizes the amorphous phase of In₂O₃ while the amorphous phase of undoped In₂O₃ is rapidly crystallized at very low homologous temperatures (T/T_m<0.19) at 150 °C [3, 4]. The reported carrier mobility of IZO is as high as 20-50 cm²/Vs for both Hall and TFT field effect mobilities. A ternary cation system of InGaZnO (IGZO) [5, 6] has been more popular than binary systems since the addition of Ga in IGZO suppresses the channel carrier density in TFT application, which is preferred for TFT devices to obtain low device off-state current. Other third cation species such as Hf, Si and Zr were implemented. However, the carrier mobility of those ternary cation systems is significantly lower, 3-10 cm²/Vs, which is 3-10 fold lower than that of In₂O₃ or IZO.

Therefore, securing strategies to develop a material system that maintains high carrier mobility (>20 cm²/Vs) and suppresses carrier generation for TFT channel application is of significant importance, which is expected to expedite the realization of next-generation transparent displays exhibiting reliable performance, fast switching speed and, consequently ultra high definition resolution.

Here, we report on a ternary cation material system of InAlZnO (IAZO) where the optimized carrier mobility is as high as 25-45 cm²/Vs. Further, the IAZO demonstrates an enhanced capability of stabilizing the amorphous phase during thermal stresses compared to InZnO. The effect of Al incorporation on the structural, electrical, optoelectronic and chemical properties of IAZO is systemically investigated and compared to those of IZO. The carrier transport behaviors (ionized impurity scattering; neutral scattering; charge screening) as well as the dominant mechanisms for carrier density and resistivity are discussed. The high mobility and greater phase stability may lead to high performance and sustainable device applications in next generation electronic and optoelectronic devices such as transparent displays.

References
[1]Lewis et al., MRS Bull., 25 (2000) 22-27.
[2] Coutts et al., MRS Bull., 25 (2000) 58-65.
[3] Paine et al., 85 (1999) 8445-8450.
[4] Lee et al., Journal of Materials Research, 27 (2012) 2299-2308.
[5] Nomura et al.,Nature, 432 (2004) 488-492.
[6] Hosono, Journal of Non-Crystalline Solids, 352 (2006) 851-858.

Indium nitride (InN) was reported to have a large optical bleaching effect ^[1], which provides potential application as high-speed and high-frequency optical switches in the optical communications. Due to its narrow direct bandgap (~0.7 eV), the bandgap engineering with GaN allows the ternary InGaN system to cover a range of bandgap energies from the infrared of InN to the ultraviolent of GaN. Such wide bandgap tuning provides spectral flexibility for InGaN as optical switches. In order to materialize the potential of InGaN for optical device applications, we investigated the optical bleaching behavior and ultrafast carrier dynamics in epitaxial InN and InGaN films, and proposed a phenomenological model to explain the carrier relaxation behavior.

InN and InGaN films with the thickness of 300 nm were deposited on c-sapphire substrates by molecular beam epitaxy. The optical bleaching effect and ultrafast carrier dynamics in InN-based epitaxial films was investigated by using femtosecond time-resolved pump-probe transmittance/reflectance measurements with a photon energy of 1.50 eV, which is based on open-aperture Z-scan technique.

Both epitaxial InN and InGaN films show large optical bleaching effect from Z-scan measurements, where a nearly 3 times increase in transmittance was observed at 800 nm without the dependence of the film composition. The time-resolved pump-probe transmittance results indicate that hot carriers are relaxed within a time scale of subpicosecond. The carrier lifetime was found to be independent of the film composition and the fabrication condition. The maximum value in the transient transmittance shows a nonlinear increase with increasing the pump-beam power, which may be associated to a change in the nonlinear refractive index. The detailed mechanism on the carrier relaxation behavior will be given in the conference.

This work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (C) (Grant No. 20K05368).

[1] V. Pacebutas, G. Aleksejenko, and A. Krotkus, Appl. Phys. Lett. 88, 191109 (2006).

The low open-circuit voltage V_OC of the earth-abundant kesterite Cu₂ZnSnS(Se)₄ (CZTSSe) solar absorbers is often associated with the large band tails, the origin of which has been tentatively assigned to the Cu-Zn disorder. Such an assignment has recently been questioned by some experiments. Here, by using the cluster expansion method and first-principles calculations, we investigate the role of cation disorder in the band tailing of CZTS. Our results show that the formation of local Cu-Cu-Cu-Sn tetrahedral motif as a result of the disorder is responsible for the band-gap narrowing, leading to a redshift of optical absorption onset by up to 0.2 eV. The Urbach energy, however, is hardly modified by the cation disorder, thereby excluding Cu-Zn disorder as a main source of the large band tails observed in stoichiometric CZTS. On the other hand, we find that extensive Cu-Zn disorder could stabilize the formation of point defects to a large extent. In particular, the defect concentration of Sn_Cu can be as high as 10¹² cm^-3 under typical Cu-poor growth conditions. The defect level is found at 0.2 eV above the VBM, suggesting that Sn_Cu could be a hole trap. Sn_Zn remains the predominant deep donor with defect levels near the midgap and hence is responsible for nonradiative recombination. We show that Sn antisites could be the main culprit of the low V_OC and the persistence of the two donors in the presence of cation disorder presents a major challenge towards a high-efficiency CZTS absorber.

Tin(IV) oxide (SnO₂) is an n-type wide bandgap semiconductor, which is widely used as an electron transport layer (ETL) in organometal perovskite solar cells (PSCs) due to its promising properties such as high transmittance, high electron mobility, good photostability, good energy alignment and easy processing. In this work, SnO₂ nanostructures were synthesized by a solvothermal technique. The impacts of each growth parameter for the SnO₂ nanostructures were demonstrated. By investigating the relations among different growth conditions, optimized recipes were developed for SnO₂ nanostructured ETLs employing in PSCs. Furthermore, the interfaces between the FTO/ETL and ETL/perovskite are optimized in order to enhance the charge extraction efficiency of the devices. Detailed characterizations such as scanning electron microscopy, electrochemical impedance spectroscopy, atomic force microscopy spectroscopy, X-ray diffraction, I-V characteristics etc. have been performed on the SnO₂ ETL alone_, material interfaces and the SnO₂ nanostructures-based PSCs. The obtained results provide a valuable insight into the community for preparing desired PSCs with nanostructured ETL.

Since their introduction in the early 1990s, subwavelength grating guided-mode resonance (GMR) filters have attracted considerable attention due to their simple and compact structures and versatile spectral characteristics. These outstanding features allow miniaturization of filters to pixel scale and high-density of integration for visible and short-wave infrared (SWIR) image sensors where a mosaic of transmission filters of different resonances can produce selective spectral signals to individual pixels of focal plane arrays. However, the conventional GMR-based transmission filter design, essentially unchanged for more than two decades, remains as a limiting factor for practical implementation of such an array of filters on the same substrate. The conventional design based on a single-period grating necessarily involves a homogeneous layer (of subwavelength thickness) underneath a subwavelength grating of a dielectric material to achieve a narrow transmission peak and a wide stop-band. Maintaining the other spectral characteristics, the transmission peak can then be tuned to the desired wavelength by scaling all the dimensions including the grating and homogeneous layer heights. However, to integrate different filters into a compact mosaic, this conventional design requires either different substrates of corresponding thicknesses or a very complicated non-planar fabrication process to use a single wafer leading to low throughput. In this work, we demonstrate a dual-period design to eliminate this issue by fetching additional design flexibilities. First, the dual-period filters possess two fill-fractions and thus unlike their single-period counterparts allow more degrees of freedom to vary the lateral profile parameters (e.g., fill-fractions and periods) for producing desired resonances at different wavelengths as well as to tune a resonance while keeping the grating height constant. Second, to produce a narrow passband with a wide stopband, these filters, unlike the single-period counterparts, do not require a homogeneous layer, allowing a planar fabrication process for integration of different filters on the same substrate. Moreover, dual-period filters show less sensitivity to the fabrication deviations such as sidewall roughness and to those related to grating and/or homogeneous layer heights where the latter, especially, remains as another limiting problem for the conventional narrow passband single-period based filters. For example, a small variation in grating and/or homogeneous layer thickness (while other parameters remain constant) causes the resonance peaks to disappear or be suppressed significantly and thus the fabrication requires very high precision. In this work, we report dual-period GMR filters of dimensions smaller than on silicon-on-quartz (SOQ) wafers using conventional electron beam lithography (EBL) and reactive ion etching (RIE). The filters demonstrate efficiencies as high as 80% at the peak transmission wavelength, e.g. 1550 nm. The passbands are as small as 15 nm with over 350-nm-wide stopband. The spectra of the fabricated filters are in a good agreement with the results of simulations using the finite element method (FEM) in COMSOL Multiphysics® software. Tunability of narrow passband resonances has also been achieved by only varying the fill-factors and/or period while keeping the substrate thickness constant. An array of four filters (all based on dual-period) have been demonstrated on same substrate to generate well-separated passbands in the SWIR region (1.2-1.6 μm ) to work as a pixel. While smaller filter size would allow integration into compact chips, filters should be sufficiently spaced to eliminate the potential cross-talk. In order to produce an optimum pixel size to work in the concerned SWIR region, any potential cross-talk along with filter size effects are also analyzed theoretically and experimentally.

III-V compound solar cells have been considered as the leading photovoltaic devices due to their superior properties including the high carrier mobility, direct bandgap, and controllable bandgap against the solar spectrum. The extremely high solar-to-electric power conversion efficiency can be achieved with III-V solar cells by forming monolithic multi-junction structures through series processes of the epitaxial growth. Nevertheless, their prohibitively high materials and process (i.e. epitaxial growth) cost has limited the application to terrestrial photovoltaic systems. To reduce the cost per watts of III-V solar cells, several approaches have been suggested, such as transfer-printing-assisted assembly for re-using the expensive III-V substrate and solar light concentrator for boosting the power generation in a given cell area. A format of III-V solar cell arrays transfer-printed within the luminescent solar concentrator (LSC) has been particularly of interest due to the potential to mitigate the materials cost and simultaneously to augment the power generation. In spite of the successful demonstration with single-junction (e.g. GaAs) solar cells, the effect of power generation improvement by the LSC system cannot be simply achieved with multi-junction solar cells, because the balanced enhancement of photon absorption at each subcell is required for increasing overall photocurrent of series-connected multi-junction cells. In this regard, here we present a facile route to integrate the transfer-printed multi-junction III-V solar cells with the LSC medium for significantly raising their power generation. Monolithically grown indium gallium phosphide (InGaP)/gallium arsenide (GaAs) double-junction solar cells are completed implanted into a LSC printing medium by the transfer-printing-based fabrication assembly, where luminescent quantum dots (QDs) with the 610 nm emission wavelength are non-uniformly distributed in the LSC. Since the short-circuit current of InGaP/GaAs double-junction cells is limited by a level of photocurrent produced at the InGaP subcell, the LSC is designed for the emitted 610 nm photons of QDs to be preferentially guided into the InGaP subcell. At the optimal LSC configuration, the power generation of 26%-efficient InGaP/GaAs solar cells can be considerably improved by ~50% compared to the identical cells but without add of the LSC.

There has been a recent trend to use molecular dipole layers as a part of a novel
strategy to fabricate selective contacts. Conceptually, a thin film of oriented
molecular dipoles mimics a 2D capacitor, and the build-in voltage within the layer
affects the final contact equilibrium state. When the dipoles are oriented in a
certain direction one achieves larger accumulation of either holes or electrons by
modifying the workfunction of the metal. Amongst many of the dipols used amino
acids have particularly interesiting properties, for instance easy deposition by PVD
methods such as spincoating and thermal evaporation and strong molecular dipole
moment due to the presence within the molecule of a negatively charged
carboxylic group and a positively charged amino group.
We tested the effect on the contact resistance of a Silicon/dipole/Aluminium stack
using Transfer Length Method and testing different amino acids as the dipole
layers (e.g. Cysteine, Glycine, Histidine...).
Preliminary results show an enhanced contact resistivity of orders of magnitude
when dipoles are used on both p and n type silicon wafers, circumventing the
problematic Fermi level pinning in Silicon aluminium as deposited contacts.
Finally, experiments on Histidine using different passivating layers show a clear
difference in contact resistance with the change of the substrate dopant. Probably
indicating that macroscopic orientation of the dipole layer can be promoted when
using proper seed layers.

For many of their applications, 2D materials must be connected to the macroscale world, implying interfaces between the 2D atomic layer and a 3D bulk material. The structure and defects at the interface between a 2D material and a 3D thin film influence (opto-)electronic properties of nanoscale devices such as contact resistance, photo-response and high frequency performance [1]. If the interface is between a 2D material and a 3D nanostructure, the ability to control the nanostructure geometry and lateral positioning is important in enabling nano-optoelectronic applications such as plasmonic arrays or arrays of wires or lines in individual circuits interfaced with 2D materials. When 3D nanoislands are grown on conventional, bulk substrates, “directed self-assembly” is a useful approach where growth takes place spontaneously but is favored at particular nucleation sites. The control of nucleation sites involves control of surface topography, geometry and local interactions [2]. Here we focus on extending this approach to the case of 3D nanoislands on 2D surfaces. We create specific defect locations on 2D materials (graphene, MoS2, WSe2) that act as heterogeneous nucleation sites for 3D metals (Au, Ti), and we tune the 3D metal deposition conditions to produce specific island shapes by self-assembly.

Since directed self-assembly requires avoidance of extraneous nucleation, as well as reliable nucleation at the patterned sites, we first discuss the control of metal island nucleation on unpatterned 2D materials. In particular, the nucleation density is expected to depend on the adatom diffusion distance on the 2D material surface. On 2D materials in the form of suspended layers, we use post-growth imaging by transmission electron microscopy (TEM) to measure the shape and distance between metal islands deposited in an ultra-high vacuum (UHV) evaporation system. We find that nucleation density and island shape depend on factors such as the substrate temperature, topography, background pressure, time, and partial pressure of the deposited material. We achieve the largest island diffusion distance (>2 μm) by removing polymeric residues in the 2D material transfer process, suspending the 2D material samples on TEM grids, using medium (100-200 ^oC) temperature deposition, and minimising defect sites such as wrinkles. We next induce locally reduced adatom diffusion distances using He-Ion beam patterning to create periodic defects in the 2D material. The He-Ion dose rates are calibrated and resulting defects characterised using high resolution scanning transmission electron microscopy. We show that these defects act as heterogeneous sites and we discuss the extent to which they can be exploited to produce patterned, periodic Au nanoisland arrays. We develop automated image analysis techniques and perform statistical analysis of the nucleation site density and location. We believe that this templating and nucleation control opens routes towards self-assembled and tailored interface design. In particular, we discuss the possibilities of interface structures built from self-assembled 3D nanostructures for development of plasmonic nanoarrays with tunable and well-defined resonance frequencies.

[1] Wang, Y. et al. Van der Waals contacts between three-dimensional metals and two- dimensional semiconductors. Nature 568, 70–74 (2019).
[2] Gherasimova, M., Hull, R., Reuter, M. C. & Ross, F. M. Pattern-level assembly of Ge quantum dots on Si with focused ion beam templating. Appl. Phys. Lett. 93, 023106 (2008).

The nano-morphology of bulk heterojunction (BHJ) blends based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) blended with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) is systematically varied by changing the volume fraction of the solvent additive 1,8-diiodooctane (DIO) from 0 vol% to 20 vol% in the casting solution to prepare organic solar cells (OSCs). With increasing addition of DIO, the photoluminescence (PL) from the blend is reduced; however, a relative increase in PL from 750 nm onwards is observed for blends with 20 vol% DIO. As quenching of the blend PL is related to the donor/acceptor (D/A) interface, structural characterizations in real-space (microscopy) and k-space (scattering) are performed to unravel the morphology of blend systems to correlate nano-morphology with photophysical and charge transport processes. Blends prepared with 0 vol% DIO form large phase separated domains of PC₇₁BM, hundreds of nanometer in diameter. With the addition of 3 vol% DIO the size of PC₇₁BM domains is supressed resulting in a more mixed morphology due to the selective dissolution of DIO in PC₇₁BM. On the addition of up to 20 vol% of DIO, the film becomes rougher with a finer interconnected morphology due to polymer aggregation, which contrasts with previous reports. Electron-transport length scales measured by scanning photocurrent microscopy (SPM) are found to increase with the addition of up to 3 vol% DIO associated with the break-up of the large PC₇₁BM aggregates, while the hole-transport length is found to increase on adding DIO up to 20 vol% due to aggregation of polymer chains. Finite-difference time-domain (FDTD) simulations are performed to take into account for optical beam spread due to associated roughness of BHJ blend films which further strengthen the experimental findings to take into account for waveguiding/scattering components of excitation beam.The structural results are found to be in good agreement with the PL quenching and the effective transport lengths. Hence, this work represents a complete set of results systematically examining the effect of nanomorphology on structural and opto-electronic properties of BHJ blends, which can have a direct implication on better understanding of the emerging high efficiency OSC blend systems.

Diamond, by far, is the most thermally conductive material at room temperature and a great candidate for thermal management applications in microelectronic devices. Yet, its application has been discouragingly hindered due to its large thermal barrier at the interface when paired with other CMOS materials. Beryllium oxide (BeO) is a potential material candidate to bridge this thermal barrier between diamond and other wide bandgap semiconductors such as gallium nitride (GaN) and Silicon carbide (SiC) owing to its large bandgap, high thermal conductivity, and large spectral phonon overlap with diamond. In this study, using time-domain thermoreflectance we report the thermal conductivity and interfacial thermal resistance at BeO/diamond and Beo/Si interface across various temperatures. We measure the thermal conductivity of amorphous BeO for thicknesses below 30 nm to be ~10 +- 0.7 W/mK, which is significantly lower than its bulk crystalline value. Our results suggest that the interfacial thermal resistance at BeO/diamond interface is significantly lower than that of GaN/diamond and BeO/Si. This observation paves the way towards more thermally efficient integrated circuits.

Transparent electrodes are a necessary component of many optoelectronic devices. Networks of silver nanowires have emerged as a promising alternative to commonly-used metal oxides such as indium tin oxide (ITO). Their transparency and conductivity values are as high as ITO, while being mechanically flexible, less expensive and can be fabricated using high-throughput roll-to-roll methods. In this talk I will outline an additional advantage that has thus far received less attention: the high transparency of silver nanowire networks in the near-infrared region (NIR). This property is highly desirable for some applications such as window coatings and solar cells. I will show that for electrodes that are 96% transparent in the visible, ones made from ITO are only 35% transparent at a wavelength of 2500 nm, whereas those made from silver nanowires maintain a transparency as a high as 94%. I will present results that show that the nanowires in the network should be sparse and larger in diameter to maximize NIR transparency. Modelling of the nanowire networks will also be shown and help demonstrate the physical reasons this is the case.
At the end of my talk I will present my results on passivating silver nanowire electrodes for capacitive-type devices such as smart windows and displays. To fully realize the advantages of silver nanowire electrodes, candidate passivation materials must be optically transparent (including in the NIR range), inexpensive, mechanically flexible and simple to deposit using high-throughput methods. I will present our findings of the passivation effectiveness of polyurethane, PMMA, a commercial optical adhesive and cyclotene passivation layers on silver nanowire networks.

Charge transport layers play a fundamental role for efficient charge carrier injection and extraction in opto-electronic devices. Using interfacial layers, the open circuit voltage (V_OC) and fill factor (FF) of organic solar cells can improve significantly. Here we show experimentally that the extent of contact between a Molybdenum Oxide (MoO₃) hole extraction layer and the donor molecules in the photoactive layer of organic solar cells causes a proportional increase in non-radiative recombination losses. We quantify this effect by using several planar and bulk heterojunction solar cells device architectures, assigning it to surface recombination. We demonstrate that such losses can be suppressed by inserting a thin fullerenic anodic interlayer. This interfacial layer enhances the built-in potential and reduces the presence of minority charge carriers at the electrodes: a new perspective on the principle of selective charge extraction layers. The improvement to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. These findings are relevant to different branches of organic electronics, given the popularity of MoO₃ as an efficient hole extraction and injection layer, providing insights for future device design.

Traditional scanning transmission electron microscopy (STEM) detectors are large, monolithic regions that integrate a subset of the transmitted electron beam signal scattered from each electron probe position. These STEM imaging experiments record only 1-5 values per probe position, throwing away most of the diffracted signal information. With the introduction of extremely high speed direct electron detectors, we can now record a full image (2D data) of the diffracted electron probe scanned over the sample (2D grid of positions), producing a four-dimensional dataset we refer to as a 4D-STEM experiment. These diffraction images of the electron probe are extremely rich in atomic-scale information, such as the sample structure, orientation, composition, phonon spectra, defect structure and more. The spacing between adjacent STEM probes can be varied from below one Angstrom to hundreds of micrometers, allowing us to probe the functional length scale of materials and devices.

In this talk, I will discuss how 4D-STEM can be used to measure various properties important to materials science. I will show several examples of nanobeam electron diffraction used to measure sample structure, orientation and strain, for samples ranging from metallurgical alloys to conductive polymers. I will also show how 4D-STEM can enable phase contrast imaging methods. Finally, I will also show how the scattering matrix (S-matrix) formalism can be used both to numerically invert sample structure under conditions of multiple scattering of the electron beam, and also for very fast simulation of large 4D-STEM datasets. I will show an updated version of Prismatic, our open source simulation code that includes GUIs for easy application.

Recent progress in aberration correction and monochromation in the scanning transmission electron microscope (STEM) has enabled a wide range of new experiments that greatly enhance the depth of information becoming accessible in materials characterization. Direct detection of electron diffraction patterns and spectra became available just at the right time to push the limits of detectability and resolution of these electron optically advanced instruments even further and make experimental data fully quantifiable, to the point, where the match of simulations and experiment is ultimately only limited by counting statistics.

We will report on the recent development of advanced data reconstruction algorithms for recovering the dielectric function from relativistic EELS spectra [1], a versatile ptychography reconstruction algorithm that includes various regularization options [2], and other ways to process 4D-STEM data [3]. We will present results obtained by applying these algorithms to data having been recorded using a monochromated STEM (Nion HERMES microscope operating between 30 … 200 kV, EELS-resolution < 6 meV, spatial resolution < 0.07 nm) [4] equipped with a Dectris ELA hybrid pixel detector [5] that combines true single electron sensitivity, high speed (> 5000 fps for 514 x 1030 pixels at reduced bit depth, and 2250 full fps at 16 bit) and nearly perfect point spread function (1.3 pixels in integration mode) with the capacity for a high dynamic range (> 10^7). Being able to detect spectra and (zero-loss filtered) diffraction patterns without read-out noise greatly increases the level at which these reconstruction algorithms can extract the underlying dielectric function, local crystal structure or maps of the electrostatic potential.

Example applications that will be presented include the extraction of IR-dispersion information from high-resolution EELS of plasmonic nanostructures and a comparison of various data processing algorithms (ptychography, center-of-mass, etc.) applied to (zero-loss filtered) 4D-STEM data.

[1] A. Eljarrat and C.T. Koch, Ultramicroscopy 206 (2019) 112825
[2] M. Schloz, T.C. Pekin, Z. Chen, W. Van den Broek, D.A. Muller, C.T. Koch, arxiv :2005.01530
[3] B. Haas, M. Schloz, J. Müller, A. Mittelberger, O.L. Krivanek, L. Jones, W. Van den Broek, D. Cooper, J.-L. Rouvière, and C.T. Koch, EMC Abstract (2020)
[4] O.L. Krivanek, N. Dellby, J.A. Hachtel, J.-C. Idrobo, M.T. Hotz, B. Plotkin-Swing, N.J. Bacon, A.L. Bleloch, G.J. Corbin, M.V. Hoffman, C.E. Meyer, T.C. Lovejoy, Ultramicroscopy 203 (2019) 60-67
[5] T.C. Lovejoy, B. Plotkin-Swing, N. Dellby, C.E. Meyer, A. Mittelberger, A. Eljarrat, B. Haas, C. Koch, J. Müller and O.L. Krivanek, M&M Abstract (2020)

Convergent beam electron diffraction (CBED) provides a two-dimensional map of the intensity of scattered beams as a function of angle of incidence. Imprinted on the CBED pattern is a historical record of the multiple interactions of the electron wavefield as it is scattered within the specimen, making the pattern acutely sensitive to the specimen structure.

For more than half a century, this sensitivity has been exploited to determine crystal symmetries, refine crystal structures and measure bonding charge density distributions, to name just a few examples [1,2].

Today, with the advent of new, high speed, high dynamic range detectors, it is possible to read out the whole CBED pattern at each position of an electron beam as it is raster scanned across a specimen (often called “4D-STEM”) [3]. This delivers an unprecedented wealth of specimen information buried in terabytes of data! How can we decode this data to extract the specimen information we want? Various schemes, including ptychography and differential phase contrast, have been demonstrated successfully with fast detectors. These various approaches have enabled enhanced image resolution, mapping of electric and magnetic fields, strain, polarisation domains and octahedral tilts (for a review, see [3]).

In this paper, we will discuss some approaches that exploit the power and sensitivity of dynamical electron scattering that underpins 4D-STEM, to provide alternative imaging contrast mechanisms and interpretative schemes that can reveal targeted specimen information. We illustrate these with applications to metallic nanoparticles, beam-sensitive functional perovskites and semiconducting nanostructures.

[1] Zuo J.M. and Spence J.C.H. (1992) Electron Microdiffraction Springer. ISBN 978-1-4899-2353-0
[2] Tanaka M. (1997) Convergent-Beam Electron Diffraction. In: Dorset D.L., Hovmöller S., Zou X. (eds) Electron Crystallography. ISBN 978-94-015-8971-0
[3] Ophus C. (2019) Microscopy and Microanalysis 25, 563–582.

Confinement of magnetic structures geometrically as well as energetically, leads to novel and unexpected domain behavior. Lorentz transmission electron microscopy (LTEM) is ideally suited for quantitative analysis of magnetic domains and microstructure in functional nanoscale heterostructures. The current state of art LTEM enabled using aberration correctors allows for imaging down to sub-nanometer scale in field-free conditions. In this talk, the study of magnetic domain behavior in strongly interacting magnetic nanostructures namely artificial spin ices, curved 3D magnetic nanostructures, and functional materials that host skyrmions will be presented.
Artificial spin ice lattices consist of lithographically patterned arrays of interacting magnetic islands that exhibit magnetic frustration. We will present the results on emergence of magnetic excitations in such lattices which are topologically non-trivial as well as control of magnetic frustration and emergent ordering such as antiferromagnetic to ferromagnetic phase transition using lattice geometry. Furthermore, we will present a novel approach to understand such frustration behavior based on network topology that can be extended to use of graph neural networks to analyze and predict the behavior of spin ice lattices.
Topologically non-trivial spin textures such as skyrmions present unique opportunities to explore exciting fundamental phenomena, such as the topological Hall effect, as well as novel applications, such as skyrmion-based spintronics. The application of LTEM to visualize and understand the magnetic spin textures in multilayer [Pt/Co]_x thin films that can host skyrmions as well as controlled fabrication of skyrmions using Ga ion implantation will be discussed. In-situ magnetization reversal was used to understand the stability of the skyrmions.
Finally, we will also present a new approach based on machine learning for phase retrieval in LTEM that improves the achievable phase sensitivity and spatial resolution.

The layered oxide cathode materials are promising candidates for high energy density lithium-ion batteries (LIB). However, the Ni-rich layered compounds, especially the end member LiNiO₂ (LNO), suffer from stoichiometric challenges (Li/Ni ratios) and instability issues leading to structural degradation [1,2]. It is challenging to synthesize LNO without defects, which further results in severe capacity loss during electrochemical cycling. The phase transition structure/ mechanism is widely under debate, and obtaining a thorough insight is crucial for enhancing the cathode material performance. Directly visualizing the structure at the atomic scale can give us further insight, nonetheless detecting the light elements in energy materials is not trivial.

All the major studies over imaging lithium are carried out on free-standing single-crystalline nanoparticles [3]. However, the real cathode active materials (CAMs) are polycrystalline particles composed of densely packed primary particles/nanocrystals. This poses challenges in sample preparation to achieve the optimum thickness parameters to visualize lithium. In this report, we have investigated the phase transformed region using aberration-corrected scanning transmission electron microscopy (AC-STEM) combined with the pixelated detector (pnCCD). Selecting specific regions in the convergent beam electron diffraction (CBED) patterns and calculating the so-called virtual enhanced ABF (eABF) images is shown to enhance the contrast of the lighter elements even in the thicker sample of ~ 95 nm [4-6]. Moreover, to verify the dependency of this technique at various thicknesses, final gradational thinning of the focused ion beam (FIB) prepared lamellae is carried out by focused low energy Ar-ion NanoMill. Ultimately, we apply the HAADF (high angle annular dark field), ABF, and eABF imaging techniques simultaneously to image the entire layered-to-rock salt phase transition region.

Interestingly, we have been able to access the phase transformed region in between the layered and the rock-salt structure by collecting four-dimensional datasets (4D STEM). This has enabled us to atomically track Ni as well as the light elements, especially lithium and oxygen. The analysis of the layered-to-rock salt phase transition region suggests that there is no sharp interface, as previously assumed. Instead, the transformation appears to take place gradually [1]. The current findings warrant a re-examination of commonly assumed models on this important class of energy materials. Moreover, our results shine a light on the phase transformation mechanism at high spatial resolution and can influence the future design for stabilizing LNO against structural transitions.

References:

[1] S. Ahmed, M. Bianchini, A. Pokle et al. Visualization of Light Elements using 4D STEM: The Layered-to-Rock Salt Phase Transition in LiNiO2 Cathode Material. Advanced Energy Materials n/a, 2001026.
[2] M. Bianchini, M. Roca Ayats, P. Hartmann, T. Brezesinski, J. Janek, There and Back Again—The Journey of LiNiO2 as a Cathode Active Material, Angewandte Chemie International Edition. 58 (2019)10434–10458. https://doi.org/10.1002/anie.201812472.
[3] Y. Oshima, S. Lee, K. Takayanagi, Visualization of lithium ions by annular bright field imaging, Microscopy (Oxf). 66 (2017) 15–24. https://doi.org/10.1093/jmicro/dfw098.
[4] S.D. Findlay, Y. Kohno, L.A. Cardamone, Y. Ikuhara, N. Shibata, Enhanced light element imaging in atomic resolution scanning transmission electron microscopy, Ultramicroscopy. 136 (2014) 31–41. https://doi.org/10.1016/j.ultramic.2013.07.019.
[5] M. Ohtsuka, T. Yamazaki, Y. Kotaka, I. Hashimoto, K. Watanabe, Imaging of light and heavy atomic columns by spherical aberration corrected middle-angle bright-field STEM, Ultramicroscopy. 120 (2012) 48–55. https://doi.org/10.1016/j.ultramic.2012.06.006.
[6] Rose, H., Phase Contrast in Scanning Transmission Electron Microscopy, Optik 1974, 39, 416–436. (1974).

4D STEM has gained popularity in recent years and is now widely applied to the quantitative phase and orientation mapping[1], strain mapping[2], electromagnetic field measurements[3], and ptychographic reconstructions[4]. The realization of routine 4D STEM is closely related to the development of new detectors. Direct electron detectors, for example, have overcome the limitations faced by CCD and indirect CMOS sensors, providing much higher dynamic range and frame rate, while maintaining single-electron sensitivity. In some cases, e.g. the hybrid pixel array detectors, these detectors are limited in size. (Medipix3 has 256x256 pixels [5], the EMPAD has 128x128 pixels [6]). Therefore, it is of great value to be able to expand the effective size of a small detector.

In this presentation, we demonstrate a method to increase the effective size of a small detector by capturing and combining sub-regions of reciprocal space. We will highlight how to script the microscope to shift the diffraction pattern across the detector. Taking control of the diffraction shift also allows more flexibility in practical application. For example, this technique can be used to capture only certain segments of reciprocal space to save time and storage. It is possible to acquire along an annulus of the diffracted intensity to avoid the central beam and capture only high angle diffuse scattering. In this case, the diffraction pattern can be shifted in a ring pattern instead of a normal grid pattern. Furthermore, we will show how this technique expands a small detector’s application space. Electrons all the way out to the objective lens bore can be captured with a high sampling rate. Limitations and challenges of acquiring such data will also be discussed.

As for reconstruction, we will highlight our automated routine to accurately and precisely align the data post-acquisition. We will also examine the challenges associated with reconstructing the datasets, including rotating the pattern shift axes and lens hysteresis. Consistency of the alignment will also be discussed. We also explore the accuracy of the reconstruction as a function of pattern exposure time and sub-pattern overlap to help guide experimental parameters. Finally, we will also show that being able to capture the Bragg peak intensities at the same time as diffuse scattering provides opportunities for quantitative diffraction analysis.

References:

[1] Fundenberger et al., Ultramicroscopy 96 (2003), P127-137
[2] Muller et al., Appl. Phys. Lett.101(2012), P212110
[3] Ryll et al., Journal of Instrumentation 11(2016), P04006
[4] Lupini et al., Microsc. Microanal 21(2015), P1219-1220
[5] MW Tate et al., Microsc. Microanal 22 (2016), P237
[6] Mir et al., Ultramicroscopy 182(2017), P 44-53
[7] We acknowledge support for this work from the Air Force Office of Scientific Research (FA9550-17-1-0225. This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (award number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI).

We determine the correlation between the structural heterogeneity and unprecedented detailed parameters of medium range ordering (MRO) using 4-dimensional scanning transmission electron microscopy (4-D STEM). The details of MRO constitute nano-scaled structural heterogeneity, such as the type, size, and fraction of MRO structures, which are directly correlated with the important properties, including the ductility and glass forming ability of Zr-Cu-Co-Al metallic glasses. In this study, the quantitative analyses of the data are enabled by our 4-D STEM using the new-generation Electron Microscopy Pixel Array Detector (EMPAD). This new detector can not only support high dynamic range, which is necessary for the quantification, but also take nanodiffraction patterns continuously throughout a large sample area with oversampling. Thus, it allows us to obtain the statistically reliable information on the structural heterogeneity at the nanoscale. We confirm the fact that the important properties of Zr-Cu-Co-Al MGs and Ti-Zr-Ni-Be MGs are directly related to the determined MRO parameters. In the Zr-based MG system, the smaller and more diverse MRO types result in high ductility, and more structurally frustrated MRO type relates to the higher glass forming ability. In the case of Ti-based MGs, the addition of Be in Ti-Zr-Ni can break down icosahedral-MRO structure, which is our hypothesis based on the angular correlation analysis.

The spatial atomic structure of PEN is accessible for the first time with ptychographic reconstruction under low dosage illumination.

Introduction
Organic crystalline structures have been determined by diffraction methods that only generate average structural information from the characterization region. However, crystal structure and orientation in polymers, which depends on e.g. temperature, interfacial interactions and strain, will vary locally. To resolve the detailed microstructure, high resolution imaging is required.
In a case study of annealed thin films of polyethylene naphthalate (PEN) [1], crystalline domains were found to vary with the depth from a surface. We use PEN as a model system to demonstrate high resolution imaging of polymer crystals. High resolution imaging of polymers is limited by the low contrast generated by light elements and beam damage from the high-intensity illumination that is used to get sufficient resolution. Low dose imaging techniques applied here have proved very helpful to acquire data from the pristine structure before it is significantly altered by radiolysis.

Ptychography
A thin polymer specimen can be treated as a weak phase object. When the electron probe scans through a thin specimen consisting of light elements, the amplitude is not expected to change significantly, while the phase contrast will offer more useful information.
Ptychography [2] is a dose efficient imaging technique with phase retrieval capacity by making use of a 4D STEM dataset. It has these advantages: i) Phase can be retrieved by Wigner-Distribution Deconvolution, and lens aberration can be corrected in post-acquisition computation. ii) Depth information can be obtained with optical sectioning, from the 4D dataset recorded in only one scan. This is very useful for beam sensitive materials. iii) Equipped with a fast-pixelated detector, the dataset recorded in low dose illumination can be adjusted to give sufficient signal for reconstruction.

High resolution image of PEN with discrepancy from XRD
Figure. 1 shows a ptychographic image of a grain in PEN. Many of the spatial frequencies present in the image, such as the (200) spacing, agree with previous models derived from X-ray diffraction studies. Noticeable in Figure. 1, however, there are some details between the (200) planes that are not predicted from the previous model. This information can only be uncovered by direct imaging.
This technique will enable the study of crystallography and defects of soft matter, hence assist us understand the property-structure relation of synthetic polymers at atomic level, and potentially the structure of natural macromolecules, e.g. protein, DNA, RNA.

Reference
[1] Shinotsuka, K. & Assender, H. In situ AFM study of near-surface crystallization in PET and PEN. J. Appl. Polym. Sci. 133, (2016).
[2] Yang, H. et al. Simultaneous atomic-resolution electron ptychography and Z-contrast imaging of light and heavy elements in complex nanostructures. Nat. Commun. 7, 12532 (2016).

Label-free spectroscopy and microscopy of live cells offers the capability to examine in-vivo conditions of cellular functions and structure providing valuable insight into biomolecular components of cells [1]. The physicochemical insulation of a samples' biologically relevant environment and instrumentation is critical for the proper understanding of underlying processes and reactions for label-free investigations. The recent development of two dimensional (2D) materials, i.e. graphene, makes it possible to isolate the sample environment from UHV conditions using these materials as molecularly impermeable, electron transparent membranes [2]. The latter opens the application of vacuum-based analytical techniques such as soft x-rays and electrons for in vivo cellular studies. Herein we report the design and fabrication of graphene encapsulated liquid cell platform designed to be suitable for in-situ studies of hydrated biological samples. The encapsulation with graphene allows the sample to be intimately confined and have its interstitial fluid environment preserved while becoming suitable to be studied with multiple analytical techniques [3]. This approach reduces the parasitic adsorption and scattering of the surrounding matrix while still providing nourishment to the cells for an enhanced lifetime [4]. The platform consists of an array of Si3N4 windows on which the samples are deposited allowing for combinatorial batch analysis. The platform has a small cross-section with photon and electrons making it compatible with a wide range of spectroscopies and microscopies. Implementation of sample immobilization with a biocompatible patternable hydrogel media allows for an extended lifetime of realistic conditions and potentially extended cellular lifetimes under measurement. Preliminary results into the viability of graphene encapsulation for biological samples were conducted with fluorescence optical, SEM, and FTIR microscopies.

References:
1. C. W. Freudiger et al, Science, 322 (2008) p. 1857-1861.
2. A. Kolmakov et al, Nature Nanotechnology 6 (2011) p. 651.
3. A. Yulaev et al, Advanced Materials Interfaces, 4, (2016) p.1600734
4. A. Matruglio et al, Journal of Instrumentation, 13, 05 (2018), p. C05016

Solution processing is a common first step in optoelectronic device fabrication, as it is a low-cost method with considerable flexibility. While it has been observed in several systems that solution chemistry affects both the morphology and properties of the deposited material, the manner in which these effects arise is rarely well understood; this is due in large part to the experimental difficulty of characterizing materials in solution. Here we demonstrate how cryo-electron microscopy (cryo-EM) can be used to overcome this challenge, sharing results from two technologically relevant systems: chalcogenide glasses and lead-halide perovskites. We show multiple cryo-EM methods, including single particle analysis, and discuss how an informed choice of method can reveal unique information about each material. Our results shine a light into the black box of solution processing, ultimately enabling better control over film morphology and optoelectronic performance.

Electronic and ionic transport in semiconducting polymers depends strongly on both molecular structure and mesoscale morphology. Transmission electron microscopy (TEM) studies of semiconducting polymers have proven to be challenging due to their inherent beam sensitivity and the interconnectedness of the physical processes occurring at various length scales. The complex, often hierarchical, internal structure of these molecular systems requires characterization tools that allow us to correlate structure from the nanoscale through the mesoscale all the way to their macroscopic properties. We leverage advances in high-resolution TEM (HRTEM) hardware, such as ultra-sensitive detectors and the use of cryogenic temperatures, and parallel computing data analytics tools to characterize the mesoscale organization of semiconducting polymers across areas similar in size to electronic devices (~ 10 μm²). The vast amount of data acquired from HRTEM imaging enables a statistical approach, which is necessary given the inherently disordered nature of semicrystalline materials.

We directly image the structure of a series of semicrystalline conjugated polymers using a fast read-out direct detector to record dose-fractionated exposure series (i.e., “movies”). The dose-fractionated exposure series are then analyzed using Fourier space image processing techniques and computer-based corrections for beam-induced specimen motion. The structure is spatially resolved through mapping of the alkyl (100) or backbone (001) spacing reflections, revealing the rich, peculiar nature of the nanoscale organization of semiconducting polymers. Through analysis of the local orientational correlations, we provide insights into the mesoscale organization of these materials. In addition, constructed orientation maps and director field lines with nanoscale resolution (~3 – 5 nm) allow us to visualize the alignment of molecular chains and understand electronic connectivity. The results show how beam-sensitive organic materials can be visualized and characterized at the nanoscale level using low-dose HRTEM to uncover their long and short-range organization, which is essential for understanding their macroscopic functional properties.

Organic semiconductors are of interest for a variety of applications such as biocompatible electronic devices, transistors, batteries, and solar cells. The morphology and self-assembly of these materials strongly influence charge and ion transport. While our fundamental understanding of structure-function relationships has improved greatly in recent years, significant questions remain about the nature of order at the nanoscale, the role of mesoscale organization, and its connection to the device transport properties. Here I discuss recent work examining the structure of a high-mobility semiconducting polymer previously thought to be amorphous. Thin-film synchrotron scattering, x-ray free-electron laser nano-diffraction, cryo-EM, and 4D-STEM are combined to demonstrate a rich, hierarchal organization with significant local fluctuations in molecular packing and signs of homoepitaxial order. These results combined with molecular modeling suggest a range of intra- and inter-molecular defects that break translational order but allow good long-range electronic coupling necessary for high mobility. This case highlights the need to develop and apply new high-resolution structural probes in heterogenous systems to self-consistently understand structure in disordered materials.

Sub-Ångström-resolution STEM has become a routine tool to study radiation-hard inorganic crystals, but only recently has it been achieved at cryogenic temperatures. This advancement has provided the capability to capture exotic electronic states, such as charge ordering [1-3]. However, precisely tracking the emergence of low-temperature states requires high-resolution and precision STEM across a broad range of temperatures, cryogenic and intermediate, which remains a serious challenge. Here, we demonstrate a new side-entry, dual-tilt cryogenic holder (HennyZ) that combines liquid nitrogen cooling with fast, local heating via a 6-pin MEMS device. This enables intermediate temperature studies at atomic resolution and the flexibility for more complex experiments, such as coupling with in situ biasing [4]. Sub-Å resolution is enabled by consistently low drift rates of 0.3-0.4 Å/s in the full temperature range from ~100-1000 K. Furthermore, the local heating in this continuously variable temperature (CVT) cryo-STEM approach allows the temperature to be cycled repeatedly and rapidly within a single experiment, affording novel dynamic studies. Understanding structural changes in materials at the atomic scale and at temperatures where they occur is critical for manipulating quantum states. We demonstrate CVT cryo-STEM as an effective local probe for revealing unexpected structural phenomena, such as metastable intermediate phases, in Nb₃Br₈—a 2D magnetic material [5,6].

References:
[1] I. El Baggari, et al., Proc. Natl. Acad. Sci. U.S.A. 115 (2018), p. 1445.
[2] B.H. Savitzky, et al., Ultramic. 191 (2018), p. 56.
[3] A.M. Minor, et al., MRS Bull. 44 (2019), p. 961.
[4] B.H. Goodge, et al., Microsc. Microanal. First View (2020).
[5] C. Pasco, et al., ACS Nano 13 (2019), p. 9457.
[6] This work is supported by PARADIM, an NSF-MIP (DMR-1539918), and NSF DMR
1429155 & DMR-1719875.

Oriented attachments are recognized as a common pathway for crystal growth in both natural and synthetic systems, leading interface free or twin related structures. The occurrence of 5-fold twinned nanoparticles leads to unique properties. However, nearly 200 years after their discovery, the formation mechanism is still ambiguous. Using in situ high-resolution transmission electron microscopy combined with molecular dynamics simulations, we demonstrate that 5-fold twinning occurs via repeated oriented attachment of ~3 nm Au nanoparticles. These oriented attachment events create high-energy grain boundaries, which accumulate strain during atomic rearrangements and consequently decompose via nucleation and growth of a special class of twins whose net strain is zero, inducing the 5-fold twin structures. The results provide a quantitative understanding of the 5-fold twinning process; this knowledge provides guidance for interpreting and controlling twin structures and morphologies of a wide range of materials.

Imaging chemical reactions in solution is essential for the understanding complex mechanisms involved in nanomaterial synthesis. Liquid phase electron microscopy (LP-EM) is an emerging technique capable of directly monitoring morphological changes of materials in solution with a unique combination of temporal and spatial resolution. LP-EM has been used to study the reaction mechanisms such as metal nanoparticle formation, polymer self-assembly, wet etching of semiconducting materials, growth of metal−organic frameworks, nucleation of calcium carbonate and Li electrodeposition. These studies provided an unprecedented insight into nanoscale reaction mechanisms that have significantly improved our understanding of nanomaterial synthesis. The primary challenge for the field of LP-EM is understanding the role that the electron beam plays in the observed mechanism. It is common to perform control experiments to establish a critical dose limit with which a process can be observed. A consequence of having to work below a critical dose limit is that LP-EM has mostly been used to monitor reactions over a period of seconds to minutes, despite the fact that many reactions take place over hours to days. Furthermore, even after a critical dose has been established, it is impossible to completely rule out the role of the electron beam using LP-EM experiments alone. Consequently, it is essential to study reaction mechanisms using multiple imaging modalities. Cryo-EM can also provide direct high-resolution information on reaction mechanisms in solution. The advantage of cryo-EM is that electron beam effects are well understood and, using appropriately low doses, are insignificant. The disadvantage of cryo-EM is that it is impossible to know the history or future of any individual particle being imaged. Consequently, the data on reaction mechanisms has to be presented by selecting images that the user determines to be representative of the process occurring in solution. For simple processes that evolve homogenously, it is possible to determine a mechanism with high confidence using cryo-EM alone. However, for complex processes that evolve heterogeneously, the confidence with which mechanisms can be determined by cryo-EM alone is low. Here, a simple process is defined as a process with a single step or single pathway and a complex process is defined as a process with multiple steps or multiple pathways. A homogenous evolution is characterized by a process where only one particle state is observed at each sampled time point, while a heterogeneous evolution is characterized by a process where multiple states are observed at each sampled time point. Consequently, cryo-EM is limited in its ability to resolve complex heterogeneous processes. Despite the obvious benefits of combining LP-EM and cryo-EM, most studies using these techniques have been only conducted with a single EM modality. Here we discuss the application of distributed electron microcopy for studying reaction mechanisms in solution. The term distributed is used because the data required to understand the mechanism is shared between multiple imaging modalities and sample preparation methods. The strength of this approach is that our proposed hypothesis is consistent with data taken from experiments with complementary benefits and limitations.

Electron microscopy has progressed extraordinarly to become a powerful tool for studying nanostructured catalysts at the atomic-scale. Advances in electron optics and detection have made atomic-resolution electron microscopy capable of resolving the three-dimensional surface structure of the complex catalytic nanomaterials. In parallel, the introduction of differentially pumped electron microscopes and micro-electro-mechanical-system devices has enabled in situ observations of catalysts during the exposure to gasses at pressures of up to atmospheric levels and temperatures of up to several hundred centigrade as well as operando studies by concurrent measurements of catalytic functionality. While these developments build a foundation for time-resolved observations of catalysts at the atomic-scale, such observations are challenged by electron-beam-induced alterations of the catalysts and mass-transport phenomena in the fluid phase. In this contribution, I will outline electron microscopy advances addressing the chemical relevance of atomic-scale visualizations. Moreover, I will showcase these “live” visualizations for exploring the dynamical and functional behavior of complex catalytic nanomaterials.

Transmission electron microscopy (TEM) is a powerful materials’ characterization technique for investigating their structure and properties at nanoscale dimensions. It has become an indispensable tool in the present era of nanoscience and nanotechnology. In this study, we employed the energy-filtered TEM (EFTEM) mode to investigate the effect of in-situ heating on the thin-layers of a stimuli-responsive (temperature) poly(N-isopropylacrylamide) (pNIPAm) which has been covalently attached to hollow titania (TiO₂) nanowires. The temperature-responsive behavior was characterized by observing the morphology and composition of samples as a result of heating. The pNIPAm has the swelling to shrinking transition temperature defined as lower critical solution temperature (LCST) of ~32 ^○C at standard conditions. We characterized the morphology and composition of the samples below and above the phase transition temperature of the pNIPAm by applying in-situ heating inside TEM. The EFTEM imaging and elemental mapping were applied at room temperature to determine the conformity of the pNIPAm layer and the elemental distributions of all elements present in samples. In the analysis, the elemental distribution of the element carbon (C) was attributed to the presence of the pNIPAm layer in the samples. The TiO₂ nanowires regions in the samples were identified by the overlapping of generated titanium (Ti) and oxygen (O) elemental maps. The EFTEM analysis was then repeated at the same sample regions by applying incremental in-situ heating, during which the temperature of samples was increased from ambient (23 ^○C) to high (100 ^○C) temperature with an interval of 10 ^○C. The acquired results revealed that the phase transition temperature of the pNIPAm came out to be 95 ^○C inside the TEM, which was higher than the expected value. Similarly, during the cooling of samples inside the TEM, the pNIPAm did not revert to the original morphology once the samples reached room temperature. However, the pNIPAm was found to recover the morphology by cooling the samples in standard atmospheric conditions. In conclusion, our findings show that the pNIPAm may exhibit a different swelling to shrinking behavior inside the TEM than under standard atmospheric conditions. The discrepancy is believed to be due to the rate of heat transfer between the surrounding environment and the samples and the relative humidity at ambient conditions, which evidently play a critical role for the pNIPAm transition from swelling to shrinking as well as from shrinking to swelling phase.

Visualization of the dynamic structure of the liquid, essential for studying the virus and cell activity, has remained elusive for decades due to lack of an effective tool for direct visualization beyond micrometer resolution. Here, we designed a simple liquid-cell for encapsulating the liquid state of sodium for transmission electron microscopic (TEM) observation. The real-time dynamic structure of the liquid-vapor interface was imaged and videoed by TEM on the sample of electron irradiated sodium chloride (NaCl) crystals, a well-studied sample with low melting temperature and quantum super-shells of clusters. The nanometer resolution images exhibit the fine structures of the capillary waves, composed of three zones of structures and features, i.e. flexible nanoscale fibers, nanoparticles/clusters, and a low-pressure area that sucks the nanoparticles from the liquid to the interface. Although the phenomenon was observed based on irradiated NaCl crystals, the similarity of the phenomenon to predictions suggest our first-time videos of the dynamic structure might be useful in validating long-debated theoretical models of the liquid-vapor interface. By implemented the technique to biology, we imaged virus and cell in wet for insights into the 3D structure of the intermediates of viral infection cell.

The oxygen evolution reaction (OER) plays an important role in energy conversion and storage applications, such as metal–air electrochemical cells and the production of hydrogen from water electrolysis. Metal oxide nanoparticles acting as high-quality electro-catalysts allows for anionic redox of oxygen at the particle surface. [1] However, this reaction may lead to considerable dissolution and degradation of the nanoparticle catalyst. Perovskite oxides have been previously demonstrated that, by enhancing the hybridization between the O 2p and transition metal 3d orbitals, the OER activity can be enhanced significantly. [2] KOH electrolyte with Fe impurities have been found to significantly increase the activity of Ni-based perovskite oxides electrocatalysts. [3] In this contribution, we evaluate the role of Fe impurities together with Sr-doping on LaCoO₃ based materials on the activity and stability for OER in alkaline electrolyte using combination of scanning transmission electron microscopy (STEM) imaging, electron energy loss spectrum (EELS) and energy dispersive x-ray spectroscopy (EDXS). Our analysis is conducted on the same particles on pristine and after cycling (Identical-Location TEM; ILTEM), which enables us to track the effect of electrochemical cycling on the particles. In addition to the La_1-xSr_xCoO₃ catalysts, we also studied MnWO particles decorated with Ir catalyst nanoparticles using the ILTEM approach. From the comparison of the same particles on pristine and after cycling, the dissolution of the Ir nanoparticle on the surface of MnWO particles was observed. The results will be combined with first principal calculation to show the effect of surface composition and structural evolution during activity of OER.

References:

[1] E Fabbri et al., Nature Materials 16 (2017), p. 925.
[2] JT Mefford et al., Nat Commun 7 (2016), p. 11053.
[3] L Trotochaud et al., Journal of the American Chemical Society 136 (2014), p. 6744.

Transmission electron microscopy (TEM) is a powerful tool for studying the properties of materials down to the atomic level. In many cases, the quantitative interpretation of images requires simulations based on atomistic structure models. Image simulation for TEM has typically relied on the independent atom model (IAM) that neglects bonding effects, which are measurable and important for understanding many systems. The rearrangement of charge due to bonding is directly detectable in HRTEM images and can be crucial for the interpretation of small differences in the atomic contrast [1]. Moreover, the recent rise in popularity of the four-dimensional (4D) scanning TEM has been partially due to promises of unprecedented sensitivity to electromagnetic fields [2]; however, such data call for rigorous theoretical interpretation through simulations.

Since all electrons and the nuclear cores contribute to the scattering potential, simulations that go beyond the IAM have relied on computationally highly demanding all-electron calculations. We have developed a method to generate ab initio electrostatic potentials when describing the core electrons by projector functions. Combined with an interface to quantitative image simulations, this implementation enables an easy and fast means to model electron scattering. By comparing simulated transmission electron microscopy images and diffraction patterns to experimental data, we can show an accuracy equivalent to earlier all-electron calculations at a much lower computational cost [3].

We have now implemented this method in a freely available open-source program based on Python, dubbed “abTEM” for ab initio Transmission Electron Microscopy. abTEM integrates directly with two popular open-source Python codes: the Atomic Simulation Environment (ASE) [4] for setting up atomistic models, and GPAW for calculating electrostatic potentials based on the real-space projector-augmented wave implementation of density functional theory [5]. A highly efficient first-principles description of van der Waals interactions can be included via the libvdwxc library [6].

abTEM is designed for accuracy and includes up to the second-order expansion of the multislice solution of the high-energy Schrödinger equation and uses the correct integration of the sub-slicing of the three-dimensional potential [7]. Due to integration with ASE, it is straightforward to model temperature in the frozen phonon approximation using realistic configurations from molecular dynamics simulations. abTEM works as a standard full-featured multislice simulation program, and we have additionally implemented features for speeding up simulations at some cost to accuracy.

We demonstrate the capabilities of our method by modeling twisted bilayer graphene, which has recently received considerable attention due to its extraordinary and highly tunable properties [8]. This system requires on the order of a thousand atoms to represent in simulations, which, to our knowledge, is the largest system ever to be used in a quantitative electron microscopy image simulation that includes an accurate representation of bonding effects. The differences are subtle due to the large contribution of the cores; hence a great deal of care needs to be taken when extracting charge rearrangements solely based on (S)TEM measurements.

More information about abTEM is available online [9].


References:

[1] S. Kurasch, Beilstein J. Nanotechnol. 2, 394 (2011)
[2] C. Ophus, Microsc. and Microanalysis 25, 563 (2017)
[3] T. Susi et al., Ultramicroscopy 197, 16 (2019)
[4] A.H. Larsen et al., J. Phys: Condens. Matter 29, 273002 (2017)
[5] J.J. Mortensen et al., Phys. Rev. B 71, 035109 (2005)
[6] A.H. Larsen et al., Model. Simul. Mater. Sci. Eng. 25, 065004 (2017)
[7] I. Lobato and D. van Dyck, Ultramicroscopy 156, 9 (2015)
[8] M. Yankowitz et al., Science 363, 1059 (2018)
[9] https://github.com/jacobma/abTEM

The work was supported by the European Research Council (ERC) Grant No. 756277-ATMEN.

Aberration-corrected scanning transmission electron microscopy (STEM) is a powerful technique to directly observe atomic-scale structures and chemistry inside materials and devices. In the state-of-the-art STEM, a probe size of less than 0.5Å in diameter has been experimentally realized. By combining with new elaborate STEM detectors, we can not only image single atoms, but can also image electric field distribution of atoms through differential phase contrast (DPC) imaging techniques. It then becomes tempting to observe magnetic fields at very high resolution, which should strongly correlate with the properties of magnetic materials and devices. However, high spatial resolution observation of magnetic materials is essentially very difficult, because high magnetic fields (~2T) are always exerted on samples inside the magnetic objective lens. In recent years, we have developed a new magnetic objective lens system that realizes a magnetic field free environment at the sample position. Using this new objective lens system combined with the state-of-the-art higher order aberration corrector, atom-resolved imaging in a magnetic field free environment has been achieved. The details of this magnetic-field-free STEM and the material application results will be presented in the talk.

Nanoscale properties rule the emission and absorption of light by nano-structures, for example the shape of metallic nanoparticles or the atomic design of semiconductor heterostructures. To overcome the diffraction limit of light (≈400 nm) one of the best techniques is electron microscopy. Indeed, the electron beam probes nano-object with sub-nanometer resolution. Both Cathodoluminescence (CL) and electron energy loss spectroscopy (EELS) study the optical properties of nanostructures. They probe, for example, the luminescence properties for CL [1] and the local density of state for EELS [2]. Time-resolved electron microscopy is emerging since a decade, expanding the possibilities of these two techniques. In this presentation, we will use a unique time-resolved transmission electron microscope based on a cold-FEG electron gun [3]. This technology allows, among other things, to reach a spatial resolution of 1 nanometer in pulsed mode. We will discuss the latest results of electron energy gain spectroscopy and the acquisition of the first cathodoluminescence decay traces in a transmission electron microscope.
Electron energy gain spectroscopy is the study of the coupling of a fast-electron with the near field created by the interaction of light with the nanostructure [4]. The group of Zewail in Caltech did the first experimental study in 2009 [5] followed by a few groups [6]. In this presentation, we will discuss electron gain spectroscopy using the first cold-FEG transmission electron gun [7]. This unique TEM allows the highest spatial resolution and highest brightness, leading to a spatial resolution of 1 nm and a strong interaction between the electron beam and the near field.
The development of time-resolved Cathodoluminescence (TR-CL) enabled the measurement of the lifetime of excited states in semiconductors with a sub-wavelength spatial resolution. It was used for example to measure the influence of stacking faults on the GaN exciton [12]. Recently, the first pump-probe cathodoluminescence experiment using diamond revealed the effect of electron excitation on the nitrogen vacancy color center [15]. These results demonstrate that TR-CL is essential to study the correlation between semiconductor optical and structural properties (composition, defects, strain…). While all these pioneering studies were done using a scanning electron microscope, the improvement of the spatial resolution and the combination with other electron-based spectroscopies offered by transmission electron microscopes will be a step forward for TR-CL. In this presentation, we will discuss the first time-resolved cathodoluminescence experiments within a transmission electron microscope.

[1] L. F. Zagonel et al. Nano Lett., vol. 11, pp. 568, 2011.
[2] A. Losquin et al., Nano-Letters, vol. 15, pp. 1229, 2015.
[3] F. Houdellier et al, Ultramicroscopy, vol. 186, pp. 128, 2018.
[4] F. J. G. de Abajo et al, Phys. Rev. Lett., vol. 100, 106804, 2008.
[5] B. Barwicketal, Nature, vol. 462, no. 7275, pp. 902, 2009.
[6] A. Feist et al, Nature, vol. 521, no. 7551, pp. 200, 2015.
[7] F. Houdellier, et al, Ultramicroscopy, vol. 186, pp. 128, 2018.
[8] B. G. Yacobi et al, Springer, 1990.
[9] L. F. Zagonel et al. Nano Lett., vol. 11, pp. 568, Feb. 2011.
[10] L. H. G. Tizei et al, Phys. Rev. Lett., vol. 110, p. 153604, 2013.
[11] S. Meuret et al., Phys. Rev. Lett., vol. 114, no. 19, 2015.
[12] P. Corfdir et al., J. Appl. Phys., vol. 105, 043102, 2009.
[13] R. J. Moerland et al, Opt. Express, vol. 24, 24760, 2016.
[14] X. Fu et al. ACS Nano, vol. 8, 3412, 2014.
[15] M. Solà-Garcia et al, ACS Photonics 7, 232 (2020).

Electrons, used as probes in electron microscopes, are of primal importance to measure material properties with unequalled combined spatial and energy resolution. For instance, inelastically scattered electrons can be collected and energetically separated in order to obtain spectra as a function of their energy loss or, with the same purpose, the radiated light can be collected in the far field and filtered in polarization and frequency. In particular, electron energy loss spectroscopy (EELS) can be considered the richest source of information as it carries the influence from the surface-sustained modes as well as from the bulk response of the system. Additionally, the synchronization of ft laser illumination with electron pulses at the sample opens the routes to study the ultrafast dynamics of nanostructured materials and their influence on optical near-fields. The evanescent components, produced by the excitations in the material, efficiently overcomes the unavoidable energy-momentum mismatch between free photons and free electrons yielding appreciable light-electron couplings. This strong interaction leads to multiple exchanges of quanta between the electron and the optical field, accompanied by a complex sub-fs dynamics. Based on this principle, photon-induced near-field electron microscopy (PINEM) is performed by analysing the resulting multiple gain and loss features in the electron spectra. PINEM experiments have so far relied on coherent light sources (i.e., lasers), for which the measured spectra are well reproduced by assuming sample bosonic excitations that are coherently populated with a large number of quanta. The probability of each electron spectral peaks associated with a net exchange of l quanta is then simply given by squared Bessel function. Additionally, this technique, performed with coherent light, has been combined with an electron free propagation of few millimeters to yield a temporal compression of the electron density of the order of attoseconds. Despite the numerous results obtained in this context, different aspects of the interaction between free electrons and confined light remain unexplored. For instance, the effects induced by the replacement of the coherent laser pulses with a quantum light source on electron spectra, are expected to show interesting physics significantly departing from the one previously studied.

In this work, we use quantum-optics methods in order to describe the quantum nature of the optical field and to understand how the resulting electron spectra strongly depend on the statistics of the sample excitations and their population, whose autocorrelation functions are directly retrieved from the ratios of electron gain intensities. In particular, in the limit of highly populated mode, we recover the usual results of classical PINEM for Fock and coherent states but we find a significant difference in the distribution of the gain and loss peaks of electron spectra in the case of thermal state. Furthermore, we study the role of quantum states of light, e.g. squeezed states, in shaping the longitudinal part of the electron wave function after a macroscopic propagation distance observing a faster compression of the electron density for light states with low phase uncertainty.

In conclusion, we consider this work of fundamental importance in the understanding of quantum many bodies excitations and their statistics as well as in the ability of tailoring the electron wave function via the use of light.

Over last decades, electron microscopy has become very powerful and versatile technique for nano- or atomic-scale imaging and spectroscopy [1]. Major advancements and outstanding capabilities have been made possible thanks better spatial and temporal control over the amplitude and phase of the wave function that characterizes the fast electrons used as sample probes. Control over the beam shape is commonly achieved by means of complex arrangements of magneto- and electrostatic electron lenses that enable sub-Ångstrom focusing and beam scanning, as well as correcting aberrations of electron optics. The phase of the electron wave function can be additionally modified by introducing static phase plates.

We envision an alternative to traditional electron-optics elements emerging by using optically-driven phase plates and enabling dynamical shaping of electron-beam wave functions both in space and time. This approach capitalizes recent experimental demonstrations of wave function control through optical near fields [2-4]. The electron-light interaction can in principle allow us to generate arbitrary electron beam shapes; as an application with high potential for improving the resolution of electron microscopes, we employ it here to correct for aberrations of standard electro- or magneto-static lenses used in current setups.

We demonstrate aberration correction via the interaction of the electron wave function with suitably profiled optical fields. We illustrate this concept by focusing on primary spherical aberration, which due to rotational invariance typically requires complex and expensive aberration correctors. Our work suggests that an optical corrector can help to significantly improve the focal spot and beam profile over the aberrated configuration, and that it can be in principle used to eliminate any undesired beam distortions in both standard and ultrafast (scanning) transmission electron microscopy. Optical aberration correctors could offer better versatility and compactness with respect to traditional corrector designs, and we foresee that they could open a new era of electron microscopy both in aberration correction and in the generation of on-demand electron beams.

[1] P. E. Batson, N. Dellby, and O. L. Krivanek, Nature 418, 617 (2002).
[2] B. Barwick, D. J. Flannigan, and A. H. Zewail, Nature 462, 902 (2009).
[3] A. Feist, K. E. Echternkamp, J. Schauss, S. V. Yalunin, S. Schäfer, and C. Ropers, Nature 521, 200 (2015).
[4] G. M. Vanacore, et al., Nat. Mater. 18, 573 (2019).

Ultrafast electron microscopy allows for the study of nanoscale material dynamics at picosecond time resolution. Electron pulses are typically generated using pulsed-laser induced photoemission from the electron cathode, and often multiple electrons are emitted within a single picosecond pulse. Here we study the dynamics of photon emission after excitation by dense collision cascades generated with controlled electron pulse densities. We use an ultrafast SEM cathodoluminescence (CL) microscope equipped with a pump-probe configuration, which enables a direct comparison between electron and light excitation.
Our pump-probe SEM-CL microscope is based on a 1-30 keV SEM equipped with a Schottky field-emission electron cathode. We use 250-femtosecond laser pulses at a wavelength of 258 nm to excite the W/ZrO electron cathode at a repetition rate from 200 kHz up to 25 MHz and create picosecond electron pulses by photoemission. By tuning the laser power we vary the average number of electrons per pulse from less than 1 up to 500. We record g⁽²⁾(τ) autocorrelation statistics of the CL emission at a wavelength of 450 nm from InGaN/GaN quantum wells using a Hanbury-Brown and Twiss interferometer with 70 ps temporal resolution.
Previous work using continuous and ns-pulsed electron beams has shown that strong bunching (g⁽²⁾(0) up to150) of CL photons can occur when exciting a sample with high-energy electrons, due to the fact that a single collision cascade can generate multiple CL photons [1,2]. Here, we extend this method to study bunching after excitation with ps electron pulses created using photoemission. We find that g⁽²⁾(0)-1 decreases inversely proportional to the number of electrons per pulse, similar to the trend with electron current found in previous work. We introduce an analytical and a statistical model to describe the bunching process and reproduce the measured dependence of g⁽²⁾(τ) on electron density in the ps pulses. Further analysis shows how the number of excitations/photons generated in each collision cascade can be derived from the data. The results suggest that despite the very high density of collision cascades, non-linear effects do not play a role in the plasmon generation, carrier diffusion and carrier recombination effects that follow electron impact on the InGaN/GaN quantum wells.
Next, we use the SEM-CL microscope in pump-probe configuration, by sending part of the femtosecond laser into the SEM chamber. We use this to further study the dynamics of carrier generation and recombination in electron and laser-excited semiconductors [3]. We probe the decay dynamics of bandgap emission in GaAs and CZTS (Cu₂ZnSnS₄) upon ps electron excitation (at 5-30 keV) and 250 fs laser excitation (λ=345 and 517 nm). We study the variations in carrier decay dynamics as a function of laser power and electron pulse density, to deduce the contribution of the different radiative and non-radiative decay mechanisms in each case. These results allow us to compare carrier densities in dense collision cascades and optically excited volumes in these semiconductors.
Finally, we combine CL and PL measurements to perform pump-probe spectroscopy, in which we use a delay stage to control the time between arrival of electron and laser pulses on the sample. We use the electron as a pump and the laser as a probe, and vice versa, aiming to probe non-linear effects in the evolution of dense electron cascades generated by picosecond electron pulses.

[1] S. Meuret, L. H. G. Tizei, T. Cazimajou, R. Bourrellier, H. C. Change, F. Treussart, and M. Kociak. Phys. Rev. Lett 114, 197404 (2015)
[2] S. Meuret, T. Coenen, H. Zeijlemaker, M. Latzel, S. Christiansen, S. Conesa-Boj, and A. Polman, Phys. Rev. B 96, 035308 (2017)
[3] M. Solà-Garcia, S. Meuret, T. Coenen, and A. Polman, ACS Photonics 7(1), 232-240 (2020)

The electrolyte 2.8 M lithium bis(fluorosulfonyl)imide (LiFSI) in dimethoxyethane (DME) has demonstrated good cycling behavior and stable Li morphology upon plating and stripping for high-capacity Li batteries, which is thought to be due to the formation of a stable solid-electrolyte interphase (SEI) layer for good ionic transport. Characterization of this SEI layer on lithium metal has been limited, due to the air-sensitivity and beam sensitivity of these materials, as well as the incompatibility of the electrolyte with the high vacuum environment necessary for nanoscale characterization with electron microscopy techniques. New cryogenic stages and transfer tools have enabled the characterization of frozen solid-liquid interfaces for lithium metal structures. Here, we used cryogenic focused ion beam (FIB) and ultrashort pulse laser ablation to create a cross-section of the lithium metal anode in contact with the electrolyte at various plating/stripping cycles to understand the morphological evolution of the lithium and SEI. Additionally, with cryo-FIB we utilized lift-out procedures under cryogenic temperatures for extraction of a lamella sample for investigation using scanning/transmission electron microscopy with energy dispersive x-ray spectroscopy and electron energy loss spectroscopy. The nanoscale chemical maps have allowed for a greater understanding of the SEI components and how they redistribute with reaction to the lithium metal over the course of many cycles. These results present a step towards visualization of the intact battery stack. We have determined a failure mechanism beyond short circuiting of the lithium metal anodes, due to reaction of the lithium with this LiFSI in DME chemistry.
This work was funded by Sandia National Laboratories’ Laboratory Directed Research and Development program. It was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

Transition metal oxide heterostructures host many novel functionalities and have stimulated large interest due to the possibilities of tailoring the functionalities at the atomic layer scale. It is the complex interactions at the interfaces of epitaxial oxide systems that contribute to intriguing physical effects induced by the local variation of electronic and ionic species. In this work, we focus on high-temperature superconductivity (HTSC)[1] of La₂CuO₄-based iso-structural and non–iso-structural systems. In particular, we fabricate (i) La_1.6A_0.4CuO₄–La₂CuO₄ (A = Ca, Sr, Ba) metallic–insulating (M–I) bilayers [2] and (ii) (La,Sr)₂CuO₄–SrMnO₃–LaMnO₃–La₂CuO₄ multilayers [3] using atomic-layer-by layer oxide molecular beam epitaxy (ALL-Oxide MBE) [4].
In order to correlate local structure and superconducting properties, we extensively probe the interfaces using aberration-corrected analytical scanning transmission electron microscopy (STEM) techniques including high-angle annular dark-field (HAADF) and annular bright-field (ABF) imaging, electron energy-loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy (EDXS). For the atomic-resolution analyses, a JEOL JEM-ARM200F STEM equipped with a cold field-emission electron source, a probe C_s-corrector (DCOR, CEOS GmbH), a Gatan GIF Quantum ERS spectrometer and a large solid-angle JEOL Centurio SDD-type EDXS detector was used. STEM imaging and EELS were performed at probe semi-convergence angles of 20 mrad and 28 mrad, respectively. The collection angles for HAADF and ABF images were 75-310 mrad and 11-23 mrad, respectively. O-O picker tool [5] software has been utilized for STEM image quantification.
The choice of the dopant substantially influences the superconducting mechanisms as a consequence of the dopant distribution near the nominal M–I bilayers interfaces. In the case of Sr- and Ca-doping, sharp interfaces induce striking interface effects, i.e. electronic redistribution. Differently, for Ba-doped bilayer, HTSC is attributed to “classical” homogeneous doping determined by cationic intermixing.[6] Moreover, we demonstrate that sharper Sr-doped La₂CuO₄ interfaces can be achieved by the heteroepitaxial contacts with manganite layers. The dopant distribution in La₂CuO₄ is affected by the elemental intermixing at the first atomic monolayer of the interfacial LaMnO₃ contact. Different superconducting behavior (interface vs filamentary) at the two different interface systems are correlated with the interface sharpness and will be discussed in detail.

References
[1] A. Gozar, G. Logvenov, L. F. Kourkoutis, A. T. Bollinger, L. A. Giannuzzi, D. A. Muller, I. Bozovic, Nature 2008, 455, 782.
[2] Y. E. Suyolcu, Y. Wang, F. Baiutti, A. Al-Temimy, G. Gregori, G. Cristiani, W. Sigle, J. Maier, P. A. van Aken, G. Logvenov, Sci. Rep. 2017, 7, 453.
[3] G. Kim, Y. Khaydukov, M. Bluschke, Y. E. Suyolcu, G. Christiani, K. Son, C. Dietl, T. Keller, E. Weschke, P. A. van Aken, G. Logvenov, B. Keimer, Phys. Rev. Mater. 2019, 3, 084420.
[4] Y. E. Suyolcu, G. Christiani, P. A. van Aken, G. Logvenov, J. Supercond. Nov. Magn. 2020, 33, 107.
[5] Y. Wang, U. Salzberger, W. Sigle, Y. Eren Suyolcu, P. A. van Aken, Ultramicroscopy 2016, 168, 46.
[6] Y. E. Suyolcu, Y. Wang, W. Sigle, F. Baiutti, G. Cristiani, G. Logvenov, J. Maier, P. A. van Aken, Adv. Mater. Interfaces 2017, 4, 1700737.

Domain walls (DWs) in ferroelectrics constitute the two-dimensional (2D) boundary region between domains characterized by different values of the order parameter, i.e. the ferroelectric polarization. One of the most striking properties of DWs is their ability to carry a bound charge, due to the screening by the free carriers of the material. In ferroelectric oxides, such charged DWs behave as 2D conductive pathways embedded in an insulating matrix. This special feature has raised a vast interest in conductive DWs for their potential use in future nanoelectronics. In particular, as the DWs in ferroelectric oxides are often movable under an external electric field, [1,2] they represent a very promising element for novel rewritable circuitry.
Deepening our knowledge about the charge screening processes is of fundamental relevance to achieve a complete understanding of their conduction properties. However, the lack of experimental techniques that are directly sensitive to the localized charges with sub-nanometer resolution makes the identification and the quantification of the screening charges a challenging problem.
In this study, we perform an atomic-scale study of the origin of the enhanced conductivity in charged 109° tail-to-tail (T-T) domain walls in the archetype multiferroic BiFeO₃ (BFO), by combining state-of-the-art scanning probe techniques – such as piezo-response force microscopy (PFM) and conductive atomic force microscopy (cAFM) – with differential phase-contrast (DPC) scanning transmission electron microscopy (STEM). [3]
Our study was conducted on the heterostructure BiFeO₃ (120 nm)/La_0.7Sr_0.3MnO₃ (5 nm)/SrRuO₃ (1 nm)/SrTiO₃ grown by pulsed laser deposition. The ferroelectric domain pattern observed by PFM displays the typical contrast of 71° DWs with head-to-tail DWs, which are oriented along the main directions of the cubic substrate. Interestingly, strongly charged T-T DWs are generated at the merging points of 71° stripes with different orientations. Thanks to their topologically charged state, these charged DWs display enhanced conductivity.
The analysis of the atomic displacements from high-resolution high-angle annular dark-field (HAADF) STEM demonstrates that the charged T-T domain wall presents a mixed Ising-Néel structure and is composed of a strained core with a lateral thickness of 6-8 unit cells. The charged-state of the DW was initially assessed by computing the density of bound charge (ρ_B) starting from the polarization maps. In particular, we image a distribution of positive charges mostly localized at the DW, with localized maxima separated a few nanometers from each other. The ρ_B reaches a maximum value of 1 oxygen vacancy (O_v) every 4 unit cells.
In order to directly image the topological charges at the DW, we employ the differential-DPC (dDPC) imaging technique. The differential form of DPC – thanks to its enhanced sensitivity to low Z species – has been recently been proven to be effective for imaging light elements, such as N, [4] Li, [5] and O. [6] By using dDPC, we successfully image the O columns in the BFO layer and thus map the O_v distribution along the DW core.
Our results elucidate the nature of the conductive DWs in BFO, providing new insights about their structure and charge screening phenomena. In particular, combining atomic displacement mapping and dDPC-STEM imaging we can image and quantify the local charges that are responsible for their high conductivity. This work paves the way for future studies about the structure-properties relationship in such charged DWs.
[1] T. Rojac et al., Adv. Funct. Mater. 25, 2099, 2015.
[2] J. C. Agar et al., Nat. Mater. 15, 549, 2016.
[3] M. Campanini et al., Nanoscale 12, 9186, 2020.
[4] E. Yücelen et al., Sci. Rep. 8, 2676, 2018.
[5] A. Carlsson et al., Microsc. Microanal. 24, 122, 2018.
[6] S. Findlay et al., Microsc. Microanal. 25, 1732, 2019.
[7] The authors acknowledge funding from the Swiss National Science Foundation (project Nr. 200021_175926).

The competition between polar distortions and the mobile charge carriers is a problem of longstanding interest to the development of a quantum theory of ferroelectricity. It is also a key issue for polar superconductors, which are a promising route to unconventional Cooper pairing.
SrTiO₃ is an incipient ferroelectric in unstrained, stoichiometric form, but easily becomes ferroelectric when subjected to small perturbations, including chemical substitution or applied stress. Ferroelectricity precedes superconductivity in doped, strained SrTiO₃, suggesting a complex relationship between two instabilities. Using an atomic-resolution scanning transmission electron microscope, we show atomic-scale evidence of the interplay between itinerant electrons and the correlated polar distortions above Curie temperature.
The undoped, strained SrTiO₃ films show correlated polar nanodomains above Curie temperature, suggesting an order-disorder type ferroelectric phase transition. We observe a systematic suppression of the correlated polar distortions, i.e. nanodomains, and ferroelectricity with introducing Sm dopants. The itinerant electrons screen polar distortions and disrupt nanodomains above Curie temperature. Increasing the dopant density, strain field of the dopants overlap and randomize the polar distortions. The results provide direct evidence that the long-range Coulomb interactions, already present in the paraelectric phase, are driving the ferroelectric transition and are becoming increasingly short-ranged with doping. Moreover, we show that the disorder caused by the dopant atoms themselves presents a second contribution to the destabilization of the ferroelectric state. This disorder completely destroys the nanodomains at high dopant concentrations, thereby. The results thus provide evidence for two distinct mechanisms that suppress the polar transition with doping and the presence of both competing and collaborative microscopic interactions in a ferroelectric superconductor. We also observe an inverse relationship between the critical temperature of superconductivity and the magnitude of polar distortions. Understanding the nature of ferroelectric transition and its interplay with itinerant electrons is of interest for testing the different proposed theoretical models for superconductivity in SrTiO₃.

Realization of precision, close-loop processes requires an integrated suite of integrated hardware [1], software [2] and modeling [3], and materials synthesis/optimization. With spatio-temporal control, deterministic synthesis can be achieved in quantum devices, patterned assemblies, and tailored additive manufacturing of materials. Real time processing multidimensional (hyperspectral) data signals benefit from machine learning approaches presenting an opportunity for implementation of a feedback loop for manufacturing of critical technologies, from quantum computers to 3D printed turbine blades. In order to probe specific phenomena, stimulus and detection must be linked; without real time “learning,” key events are left undetected due to temporal resolution limits or failed temporal triggering alignments. Novel detector designs enable manufacturing processes to be viewed “in operando” at high acquisition speeds (upwards of 4000 fps).The combination of machine learning with these detectors makes automation for atomic-scale manipulation a realistic goal. Here we present the application of machine learning techniques to hyperspectral data to demonstrate a framework for understanding rapidly evolving phenomena using convolutional neural networks trained on >10,000 spectra collected at 400 frames per second during an electron beam manipulation of complex oxide structures. We extend these studies to despite very noisy data collected at 400 fps. unsupervised learning via feature extraction instead of comparing to known labels, which enables the analysis of situations when otherwise inaccessible transformations or metastable states occur. These techniques provide the ability to precisely and predictively tune complex materials toward. To that end, we will highlight extension of these techniiques to surface termination manipulation in 2D materials.

The use of scanning transmission electron microscopes to manipulate substitutional defects in graphene has recently been demonstrated and modeled using ground state molecular dynamics, but the role of electronic excitations induced through inelastic electron scattering in promoting these transformations has so-far remained unexplored. In this work[1,2,3] we probe the effects of electronic excitations on the structural dynamics of localized-defects induced by an electron-beam (a point-charge perturbation), such as substitutional Si/P in graphene quantum-dots as well as the recently observed mobile Si-C₃ defects in graphene nanoflakes. Particularly, the ground and excited state potential energy barriers for the structural transformations are evaluated using time-dependent density functional theory and reveal that the barriers for such localized structural transformations can be effectively lowered upon excitation to the defect-centered excited state through an electronically nonadiabatic pathway. Optically bright excited states in which the barrier is decreased are identified in the low energy region of the electronic spectrum, suggesting that photoexcitation can modulate the reactivity of defects in graphene under electron beam irradiation. Implications of this discovery on directing localized nanoscale transformations using electron-beams will be further discussed, especially in quantum-materials.
[1] “Understanding Beam Induced Electronic Excitations in Materials”, David B. Lingerfelt, P. Ganesh, Jacek Jakowski, Bobby G Sumpter, J. Chem. Theory Comput.,16, 2, 1200–1214 (2020)
[2] “Electronically Nonadiabatic Structural Transformations Promoted by Electron Beams”, David B. Lingerfelt, P. Ganesh, Jacek Jakowski, Bobby G Sumpter, Advanced Functional Materials, 1901901, (2019)
[3] “Localized Electronic Excitations in Silicon-Doped Graphene and their Role in Defect Diffusion”, David B. Lingerfelt, Tao Yu, P. Ganesh, Jacek Jakowski and Bobby G. Sumpter (under review)

The enhanced energy resolution of recently developed monochromators now make it possible to use electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) to probe materials systems in new ways. This powerful spectroscopic capability combined with the small focused electron probe opens the door to explore photonic and phononic/vibrational excitations with high spatial resolution [1-3]. Here we describe recent advances leading to nanoscale and atomic resolution vibrational spectroscopy and novel approaches to probe guided photonic modes in dielectric particles. The fast electron in the electron microscope can excite vibrational modes in materials either through dipole interactions (similar to IR absorption spectroscopy) or impact scattering (similar to inelastic neutron scattering). The spatial localization of these two scattering mechanisms is very different. Impact scattering typically involves higher momentum transfers yielding a spatially localized spectral signal. In a crystal, high momentum transfer interactions are associated with exciting the short wavelength phonon modes at the Brillouin zone boundaries (BZB). Efficient excitation of these modes requires experimental conditions that favor Bragg scattering from the associated Miller planes. This can be easily accomplished in the conventional STEM EELS geometry by tilting the crystal into a suitable zone axis orientation and using a probe convergence angle that spans the Bragg angle associated with the BZB of interest. This convergence angle also permits a small probe to be formed which enables resolution of the associated Miller plane spacing. We have used this approach to demonstrate nanoscale and atomic resolution on a sample of Si, SiO₂ and BN [1]. EELS can also be employed to explore the characteristic photonic modes present in dielectric nanoparticles since the electron beam can provide a source of virtual photons to illuminate the object. We have explored the variation in photonic modes for high refractive index oxide particles such as TiO₂, Ta₂O₅ and CeO₂ [2]. The modes show up as peaks in the bandgap region of the spectrum. The energies of the modes are determined not only by the particle shape and size but also due to coupling between adjacent particles. Coupling between phonons and IR photons can give rise to phonon polaritons [3,4]. Polariton characteristics are also influenced by sample geometry, and so the polaritonic modes can be manipulated by changing the sample shape/size. Here we use STEM EELS to demonstrate variations in the polaritonic response on patterned SiO₂ thin films.

References:
[1] K. Venkatraman et al., Nature Physics 15 (2019) 1237-1241.
[2] Q. Liu et al., Phys. Rev. B 99 16 (2019) 165102.
[3] K. Venkatraman et al., Microscopy 67 (2018) i14 - 23.
[4] A. Konečná et al., Phys. Rev. B 98 20 (2018) 205409.
[5] The support from US National Science Foundation CHE-1508667 and U.S. DOE (DE-SC0004954), the use of (S)TEM at Eyring Materials Center at Arizona State University is gratefully acknowledged.

Advances in electron monochromation have led to a sea change in electron energy loss spectroscopy (EELS) over the last 6 years. Modern monochromators scanning transmission electron microscopes (STEM) have improved EELS energy resolution by an order of magnitude (from ~50 meV down to < 5 meV). This new level of energy resolution enables direct access to a host of new phenomena, such as phonons, vibrational modes, infrared optical excitations, and low-lying states in the electronic structure. Moreover, it also presents many new opportunities to probe difficult and complex systems by exploiting novel interactions between the electron beam and these ultra-low energy excitations. As a result, monochromated EELS is a highly versatile tool that can be applied to a wide range of different disciplines and materials systems and deliver exciting results.

In this talk, I will focus on three different materials systems where monochromated EELS presents a unique opportunity to provide new insights into important topics: biological, optical, and quantum materials. In terms of biological materials, I will demonstrate how we can the efficient long-range coupling of vibrational modes to the electron beam, to provide in depth spectroscopic analysis of ultra-beam sensitive biological materials. Furthermore, I will demonstrate how we can use the combine the natively high spatial and spectral resolution of monochromated EELS with various flavors of momentum-resolution to probe the dispersions of infrared quasiparticles directly at the nanoscale. Lastly, I will discuss how we can apply this technique to coupled networks of colloidal nanoparticles to examine emergent properties in quantum systems.

This work was performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. This research was conducted, in part, using instrumentation within ORNL’s Materials Characterization Core provided by UT-Battelle, LLC, under Contract No. DE-AC05- 00OR22725 with the DOE, and sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy.

Vibrational electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) has over the recent years emerged as a versatile tool for characterizing molecular to bulk or surface vibrational modes in a wide range of materials systems, at the nanoscale [1]. The possibility of going beyond the nanoscale, reaching atomic resolution, has been discussed extensively in the community. We recently demonstrated such a capability using a so-called dark-field (DF) experimental geometry, suppressing the relative spectral contribution of elastic and delocalized inelastic dipole scattering by phonons, favoring instead beam electrons having undergone localized impact phonon scattering [2]. This approach allows for acquiring phonon structure images using the acoustic and/ or optical phonon EEL peaks [2] and, is equally applicable to polar [2] and non-polar materials [3]. Moreover, these results verify experimentally that annular dark field (ADF) Z-contrast imaging is based on beam electrons being scattered inelastically by phonons, in agreement with the quantum excitation of phonons model [2]. As a next step, we applied this methodology to probe the vibrational response of a single tri-valent substitutional Si atom in a single layer of suspended graphene. Spatially resolved EEL spectra show a characteristic vibrational signature associated with the Si impurity, localized to the defect atom itself. This is rationalized by means of density functional theory (DFT) calculations, attributing the measured response to the Si atom inducing pseudo-localized phonon modes: i.e. defect modes hybridizing with the graphene bulk continuum, setting up resonant states [3]. Possible applications of the demonstrated atomic resolution and single atom sensitivity, as well as practical aspects of the experimental approach, will be discussed.

[1] O. L. Krivanek et al., Ultramicroscopy 203 (2019) p. 60–67.
[2] F. S. Hage et al., PRL 122 (2019) 016103.
[3] F. S. Hage et al., Science 367 (2020) p. 1124–1127.

Atomic vibrations and phonons are an excellent source of information on nanomaterials that we can access through a variety of methods including Raman scattering, infrared spectroscopy, and electron energy-loss spectroscopy (EELS). In the presence of a plasmon local field, vibrations are strongly modified and, in particular, their dipolar strengths are highly enhanced, thus rendering Raman scattering and infrared spectroscopy extremely sensitive techniques.
In this contribution [1], we experimentally demonstrate that the interaction between a relativistic electron and vibrational modes in nanostructures is fundamentally modified in the presence of plasmons. This could be observed due to the current developments in electron optics, making possible the generation of sub-nanometer wide sub-10 meV electron beams with 60 keV primary energy. We finely tune the energy of surface plasmons in metallic micrometer-long Ag nanowires in the vicinity of hexagonal boron nitride flakes, making it possible to monitor and disentangle both strong phonon–plasmon coupling and plasmon-driven phonon enhacement at the nanometer scale. Because of the near-field character of the electron beam–phonon interaction, optically inactive phonon modes are also observed. Besides increasing our understanding of phonon physics, our results hold great potential for investigating sensing mechanisms and chemistry in complex nanomaterials down to the molecular level.
The strong coupling limit is reached between surface plasmons and phonon modes as the largest coupling constant, g, reached was 52 meV and the full width at half maximum of the two coupled excitations were 23 and 19 meV. The signature of strong coupling is observed along the whole nanowires, including for nanowire regions far away from any h-BN (point to the coherence of the coupled modes along the nanowire). The spatial dependence at nanometer scale of these modes will be described.
More interestingly, the presence of plasmons modify the way fast electrons couple to bulk modes of h-BN, leading to enhancement of their signal and the observation of usually “dark modes”. As an example, the ZO-phonon mode of hBN, usually not observed for electron beams perpendicularly incident on an h-BN flake, can be observed when close to metallic nanowires. We will describe how this enhanced signature changes as a function of the distance of the electron probe to the nanowire surface, varying in the tens-of-nanometers scale. Experiments have been performed with the ChromaTEM microscope (TEMPOS project, ANR-10-EQPX-50), a modified Nion HERMES200 operated at 60 kV. The electron beam was set to have an energy spread in the 6-10 meV range and a nanometer size. Experiments are under way to achieve similar enhancement in molecular systems.
If time allows, recent nano-optics experiments using ChromaTEM will be discussed.

The excitations of quasiparticles govern the physical properties of low-dimensional materials. Furthermore, assessing the irregular stacking sequences and behaviours at imperfect sections (such as boundaries and edges) is undoubtedly important to understand the performance of nanodevices. However, such local information has usually been averaged in conventional inelastic scattering techniques using x-ray, neutron, and light sources because of their diffraction limit. We demonstrate the nanoscale optical and vibrational spectroscopy of 1D or 2D materials by using a monochromatic electron source mounted in a transmission electron microscope (TEM).
All experiments were performed using a TEM (JEOL TripleC#2 at 30–60 kV) equipped with a Schottky field emission gun, a double Wien filter monochromator, and delta correctors. The energy resolution was set to a value less than 50 meV and allowed to access the quasiparticle excitations (i.e. phonon, exciton, and plasmon) of low-dimensional materials by electron energy-loss spectroscopy (EELS). The EEL spectra were collected using a GATAN GIF quantum spectrometer designed for low-voltage operations.
The spatial and momentum resolutions in electron microscopes balance each other and can be finely tuned with magnetic/electrostatic lenses. For instance, an atomically thin probe can be formed with an integration of a large momentum space. This allows us to extract local information from single defects. We have successfully measured the optical gap transitions from a defect of an individual semiconducting carbon nanotube [1,2]. Recently, atomically localized vibrational spectroscopic techniques have also been developed [3,4].
In contrast, an electron probe consisting of a parallel beam has a higher momentum resolution and provides dispersions of quasiparticle excitations. The EEL spectra are collected from each momentum transfer (q) by a pinhole aperture or a q-slit in a diffraction plane. Using this method, we have obtained phonon dispersions of graphene and h-BN from a single layer. Interestingly, the inelastically scattered electrons by phonon excitations provide sufficient signals at a large q even in the second and third Brillouin zone. Such signal enhancement due to a charge modulation at a large q helps draw a phonon dispersion curve of graphene even from a single layer. In addition, the spatial resolution of this method (a few to a few tens of nanometres, depending on its momentum resolution) is more advantageous than other dispersion measurement techniques such as inelastic x-ray spectroscopy. By maintaining the momentum resolution better than ±0.2 Å^-1, each vibrational mode can be assigned at a region of a few tens of nanometres. Therefore, the propagation of each phonon mode at defects such as edges in graphene has been unambiguously visualized [5]. Such local spectroscopy with a large flexibility could help unravel the defect physics of quantum matters.

References
[1] R. Senga, T. Pichler and K. Suenaga Nano Letters 16 (2016), 3661.
[2] R. Senga et al., Nano Letters 18 (2018), 3920.
[3] F. Hage et al., Phys. Rev. Lett. 122 (2019), 016103
[4] K. Venkatraman et al., Nature Physics 15 (2019), 1237
[5] R. Senga et al., Nature 573 (2019), 247

Electron energy loss spectroscopy (EELS) can be used to identify polymers and similar organics, but is limited by the rate at which materials damage when probed with ~100 kV electrons. Any modern scanning transmission electron microscope (STEM) is routinely capable of producing a sub-nanometer probe, but the fluence rate (electrons/nm² s) needed to achieve enough signal at this level of resolution cannot be tolerated by most organics, even at low temperatures. Many polymers show signs of damage well below 10 electrons/Ų, a threshold frequently identified for studying organic material. Even with ongoing detector improvements, “damage-free” EELS fine structure mapping of polymers with nanometer-sized pixels does not seem conventionally attainable. However, the ways in which an electron beam modifies a given polymer—and the resulting changes that can be observed with EELS—are arguably even more unique than the initial EELS fine structure.

The type and extent of beam-induced chemistry for a given polymer depends on at least dose, dose-rate, sample temperature, and vacuum conditions. These dependencies provide opportunities to exploit beam damage as a means for polymer chemistry identification. Beam-induced chemistry significantly precedes substantial mass loss and/or sample distortion in many polymers, particularly at cryogenic temperatures. The simplest option is to probe the fluence dependence by recording a series of STEM-EELS maps with a fixed current and temperature. These 4D fluence-dependent EELS maps can be collected with total fluences normally considered too high for accurate polymer identification, while at lower fluence rates, enabling spatial higher resolution mapping. This will be demonstrated for model polymer blends for <20 nm pixels, using a spectrometer with a CCD-based EELS detector.

Many studies have unintentionally mapped damaged polymers, and/or specifically measured damage evolution with EELS, but resolving and using the beam-induced evolution of sample chemistry to improve mapping is novel. Ultimately, spatial resolution is still limited by beam-induced changes: e.g., the rate of mass loss and the diffusion of generated radicals and gases. While application to polymer EELS will be shown, the approach is quite generic to materials with a measurable chemical evolution preceding significant sample morphology change.

In this study, we perform first-principles calculations to investigate the effect of isotopic substitutions in the vibrational modes of hBN flakes. Using density functional theory calculations the ground state atomic configuration and the dynamical matrix of hBN flake are obtained. The eigenmodes and eigenfrequencies are calculated by diagonalization of the dynamical matrix. Using semiclassical formalism, the interaction between a focused electron beam and vibrations is described. We employ this approach to predict the effect of isotopes on the spatially-resolved electron energy-loss spectra (EELS) of hBN flakes.

We study spontaneous and stimulated interactions between high-energy electrons and photons in nanoscale plasmonic near fields using Electron Energy Loss Spectroscopy (EELS), Cathodoluminescence (CL) spectroscopy and Photon-Induced Near-field Electron Microscopy (PINEM) in a Scanning (Transmission) Electron Microscope (S(T)EM). In recent years, EELS, CL and PINEM have enabled tremendous advances in probing (resonant) optical materials excitations with extreme spatial, temporal and energy resolution [1,2]. In EELS and CL, the time-varying evanescent electric field of the swift electrons serves as an ultra-broadband excitation source that can couple to a wide range of optical modes in a material [3]. The energy transfer during this interaction is a spontaneous process, resulting in a characteristic electron energy loss (EELS) and the subsequent emission of light (CL). Conversely, in PINEM, the electrons interact with an intense laser-driven near-field, giving rise to both electron energy-gain and -loss transitions by stimulated absorption and emission of photons at an energy determined by the laser frequency [4,5].

We demonstrate spatially-resolved EELS, CL and PINEM measurements of highly-localized dipolar optical fields at the sharp tip apexes of a chemically-synthesized Au nanostar [6]. We highlight similarities and differences between the measured EELS, CL and PINEM distributions, and discuss the role of the underlying excitation mechanisms driven either by the electron itself (EELS and CL) or the external laser (PINEM). Supported by electromagnetic boundary element method calculations for a simplified model geometry, we show that in the limit of a single isolated tip mode, spatial variations in the spontaneous and stimulated electron-photon interactions are purely determined by the modal electric near field profile, irrespective of the excitation source.

The interaction strength between free electrons and localized optical fields scales with the Fourier amplitude of the electric near field component along the electron trajectory at a single spatial frequency that is determined by the electron velocity [3,7,8]. As a consequence, the measured EELS, CL and PINEM distributions depend on the initial electron energy, with the maximum interaction strength shifting towards smaller electron energies for increasing field confinement. In general, the full three-dimensional (3D) reconstruction of an electric near-field distribution is therefore only possible by extensive tomographic measurements at multiple electron energies. Yet, using PINEM we show that for an axially-symmetric tip, we can determine the laser-induced surface charge density profile along the tip contour and thus obtain the desired 3D near-field distribution from a single measurement at a fixed electron energy and incident angle.

References:
[1] A. Losquin and T. T. A. Lummen, Front. Phys. 12, 127301 (2017).
[2] A. Polman, M. Kociak, and F. J. Garcia de Abajo, Nature Mater. 18, 1558-1171 (2019).
[3] F. J. Garcia de Abajo, Rev. Mod. Phys. 82, 209-275 (2010).
[4] B. Barwick, D. J. Flannigan, and A. H. Zewail, Nature 462, 902-906 (2009).
[5] A. Feist, K. E. Echternkamp, J. Schauss, S. V. Yalunin, S. Schaefer, and C. Ropers, Nature 521, 200-203 (2015).
[6] N. Pazos-Perez, L. Guerrini, and R. A. Alvarez-Puebla, ACS Omega 3, 17173-17179 (2016).
[7] A. Losquin and M. Kociak, ACS Photonics 2, 1619-1627 (2015).
[8] F. J. Garcia de Abajo and M. Kociak, New J. Phys. 10, 073035 (2008).

Confinement of magnetic structures geometrically as well as energetically, leads to novel and unexpected domain behavior. Lorentz transmission electron microscopy (LTEM) is ideally suited for quantitative analysis of magnetic domains and microstructure in functional nanoscale heterostructures. The current state of art LTEM enabled using aberration correctors allows for imaging down to sub-nanometer scale in field-free conditions. In this talk, the study of magnetic domain behavior in strongly interacting magnetic nanostructures namely artificial spin ices, curved 3D magnetic nanostructures, and functional materials that host skyrmions will be presented.
Artificial spin ice lattices consist of lithographically patterned arrays of interacting magnetic islands that exhibit magnetic frustration. We will present the results on emergence of magnetic excitations in such lattices which are topologically non-trivial as well as control of magnetic frustration and emergent ordering such as antiferromagnetic to ferromagnetic phase transition using lattice geometry. Furthermore, we will present a novel approach to understand such frustration behavior based on network topology that can be extended to use of graph neural networks to analyze and predict the behavior of spin ice lattices.
Topologically non-trivial spin textures such as skyrmions present unique opportunities to explore exciting fundamental phenomena, such as the topological Hall effect, as well as novel applications, such as skyrmion-based spintronics. The application of LTEM to visualize and understand the magnetic spin textures in multilayer [Pt/Co]_x thin films that can host skyrmions as well as controlled fabrication of skyrmions using Ga ion implantation will be discussed. In-situ magnetization reversal was used to understand the stability of the skyrmions.
Finally, we will also present a new approach based on machine learning for phase retrieval in LTEM that improves the achievable phase sensitivity and spatial resolution.

Advances in electron monochromation have led to a sea change in electron energy loss spectroscopy (EELS) over the last 6 years. Modern monochromators scanning transmission electron microscopes (STEM) have improved EELS energy resolution by an order of magnitude (from ~50 meV down to < 5 meV). This new level of energy resolution enables direct access to a host of new phenomena, such as phonons, vibrational modes, infrared optical excitations, and low-lying states in the electronic structure. Moreover, it also presents many new opportunities to probe difficult and complex systems by exploiting novel interactions between the electron beam and these ultra-low energy excitations. As a result, monochromated EELS is a highly versatile tool that can be applied to a wide range of different disciplines and materials systems and deliver exciting results.

In this talk, I will focus on three different materials systems where monochromated EELS presents a unique opportunity to provide new insights into important topics: biological, optical, and quantum materials. In terms of biological materials, I will demonstrate how we can the efficient long-range coupling of vibrational modes to the electron beam, to provide in depth spectroscopic analysis of ultra-beam sensitive biological materials. Furthermore, I will demonstrate how we can use the combine the natively high spatial and spectral resolution of monochromated EELS with various flavors of momentum-resolution to probe the dispersions of infrared quasiparticles directly at the nanoscale. Lastly, I will discuss how we can apply this technique to coupled networks of colloidal nanoparticles to examine emergent properties in quantum systems.

This work was performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. This research was conducted, in part, using instrumentation within ORNL’s Materials Characterization Core provided by UT-Battelle, LLC, under Contract No. DE-AC05- 00OR22725 with the DOE, and sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy.

The enhanced energy resolution of recently developed monochromators now make it possible to use electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) to probe materials systems in new ways. This powerful spectroscopic capability combined with the small focused electron probe opens the door to explore photonic and phononic/vibrational excitations with high spatial resolution [1-3]. Here we describe recent advances leading to nanoscale and atomic resolution vibrational spectroscopy and novel approaches to probe guided photonic modes in dielectric particles. The fast electron in the electron microscope can excite vibrational modes in materials either through dipole interactions (similar to IR absorption spectroscopy) or impact scattering (similar to inelastic neutron scattering). The spatial localization of these two scattering mechanisms is very different. Impact scattering typically involves higher momentum transfers yielding a spatially localized spectral signal. In a crystal, high momentum transfer interactions are associated with exciting the short wavelength phonon modes at the Brillouin zone boundaries (BZB). Efficient excitation of these modes requires experimental conditions that favor Bragg scattering from the associated Miller planes. This can be easily accomplished in the conventional STEM EELS geometry by tilting the crystal into a suitable zone axis orientation and using a probe convergence angle that spans the Bragg angle associated with the BZB of interest. This convergence angle also permits a small probe to be formed which enables resolution of the associated Miller plane spacing. We have used this approach to demonstrate nanoscale and atomic resolution on a sample of Si, SiO₂ and BN [1]. EELS can also be employed to explore the characteristic photonic modes present in dielectric nanoparticles since the electron beam can provide a source of virtual photons to illuminate the object. We have explored the variation in photonic modes for high refractive index oxide particles such as TiO₂, Ta₂O₅ and CeO₂ [2]. The modes show up as peaks in the bandgap region of the spectrum. The energies of the modes are determined not only by the particle shape and size but also due to coupling between adjacent particles. Coupling between phonons and IR photons can give rise to phonon polaritons [3,4]. Polariton characteristics are also influenced by sample geometry, and so the polaritonic modes can be manipulated by changing the sample shape/size. Here we use STEM EELS to demonstrate variations in the polaritonic response on patterned SiO₂ thin films.

References:
[1] K. Venkatraman et al., Nature Physics 15 (2019) 1237-1241.
[2] Q. Liu et al., Phys. Rev. B 99 16 (2019) 165102.
[3] K. Venkatraman et al., Microscopy 67 (2018) i14 - 23.
[4] A. Konečná et al., Phys. Rev. B 98 20 (2018) 205409.
[5] The support from US National Science Foundation CHE-1508667 and U.S. DOE (DE-SC0004954), the use of (S)TEM at Eyring Materials Center at Arizona State University is gratefully acknowledged.

Traditional scanning transmission electron microscopy (STEM) detectors are large, monolithic regions that integrate a subset of the transmitted electron beam signal scattered from each electron probe position. These STEM imaging experiments record only 1-5 values per probe position, throwing away most of the diffracted signal information. With the introduction of extremely high speed direct electron detectors, we can now record a full image (2D data) of the diffracted electron probe scanned over the sample (2D grid of positions), producing a four-dimensional dataset we refer to as a 4D-STEM experiment. These diffraction images of the electron probe are extremely rich in atomic-scale information, such as the sample structure, orientation, composition, phonon spectra, defect structure and more. The spacing between adjacent STEM probes can be varied from below one Angstrom to hundreds of micrometers, allowing us to probe the functional length scale of materials and devices.

In this talk, I will discuss how 4D-STEM can be used to measure various properties important to materials science. I will show several examples of nanobeam electron diffraction used to measure sample structure, orientation and strain, for samples ranging from metallurgical alloys to conductive polymers. I will also show how 4D-STEM can enable phase contrast imaging methods. Finally, I will also show how the scattering matrix (S-matrix) formalism can be used both to numerically invert sample structure under conditions of multiple scattering of the electron beam, and also for very fast simulation of large 4D-STEM datasets. I will show an updated version of Prismatic, our open source simulation code that includes GUIs for easy application.

Imaging chemical reactions in solution is essential for the understanding complex mechanisms involved in nanomaterial synthesis. Liquid phase electron microscopy (LP-EM) is an emerging technique capable of directly monitoring morphological changes of materials in solution with a unique combination of temporal and spatial resolution. LP-EM has been used to study the reaction mechanisms such as metal nanoparticle formation, polymer self-assembly, wet etching of semiconducting materials, growth of metal−organic frameworks, nucleation of calcium carbonate and Li electrodeposition. These studies provided an unprecedented insight into nanoscale reaction mechanisms that have significantly improved our understanding of nanomaterial synthesis. The primary challenge for the field of LP-EM is understanding the role that the electron beam plays in the observed mechanism. It is common to perform control experiments to establish a critical dose limit with which a process can be observed. A consequence of having to work below a critical dose limit is that LP-EM has mostly been used to monitor reactions over a period of seconds to minutes, despite the fact that many reactions take place over hours to days. Furthermore, even after a critical dose has been established, it is impossible to completely rule out the role of the electron beam using LP-EM experiments alone. Consequently, it is essential to study reaction mechanisms using multiple imaging modalities. Cryo-EM can also provide direct high-resolution information on reaction mechanisms in solution. The advantage of cryo-EM is that electron beam effects are well understood and, using appropriately low doses, are insignificant. The disadvantage of cryo-EM is that it is impossible to know the history or future of any individual particle being imaged. Consequently, the data on reaction mechanisms has to be presented by selecting images that the user determines to be representative of the process occurring in solution. For simple processes that evolve homogenously, it is possible to determine a mechanism with high confidence using cryo-EM alone. However, for complex processes that evolve heterogeneously, the confidence with which mechanisms can be determined by cryo-EM alone is low. Here, a simple process is defined as a process with a single step or single pathway and a complex process is defined as a process with multiple steps or multiple pathways. A homogenous evolution is characterized by a process where only one particle state is observed at each sampled time point, while a heterogeneous evolution is characterized by a process where multiple states are observed at each sampled time point. Consequently, cryo-EM is limited in its ability to resolve complex heterogeneous processes. Despite the obvious benefits of combining LP-EM and cryo-EM, most studies using these techniques have been only conducted with a single EM modality. Here we discuss the application of distributed electron microcopy for studying reaction mechanisms in solution. The term distributed is used because the data required to understand the mechanism is shared between multiple imaging modalities and sample preparation methods. The strength of this approach is that our proposed hypothesis is consistent with data taken from experiments with complementary benefits and limitations.

Oriented attachments are recognized as a common pathway for crystal growth in both natural and synthetic systems, leading interface free or twin related structures. The occurrence of 5-fold twinned nanoparticles leads to unique properties. However, nearly 200 years after their discovery, the formation mechanism is still ambiguous. Using in situ high-resolution transmission electron microscopy combined with molecular dynamics simulations, we demonstrate that 5-fold twinning occurs via repeated oriented attachment of ~3 nm Au nanoparticles. These oriented attachment events create high-energy grain boundaries, which accumulate strain during atomic rearrangements and consequently decompose via nucleation and growth of a special class of twins whose net strain is zero, inducing the 5-fold twin structures. The results provide a quantitative understanding of the 5-fold twinning process; this knowledge provides guidance for interpreting and controlling twin structures and morphologies of a wide range of materials.

The electrolyte 2.8 M lithium bis(fluorosulfonyl)imide (LiFSI) in dimethoxyethane (DME) has demonstrated good cycling behavior and stable Li morphology upon plating and stripping for high-capacity Li batteries, which is thought to be due to the formation of a stable solid-electrolyte interphase (SEI) layer for good ionic transport. Characterization of this SEI layer on lithium metal has been limited, due to the air-sensitivity and beam sensitivity of these materials, as well as the incompatibility of the electrolyte with the high vacuum environment necessary for nanoscale characterization with electron microscopy techniques. New cryogenic stages and transfer tools have enabled the characterization of frozen solid-liquid interfaces for lithium metal structures. Here, we used cryogenic focused ion beam (FIB) and ultrashort pulse laser ablation to create a cross-section of the lithium metal anode in contact with the electrolyte at various plating/stripping cycles to understand the morphological evolution of the lithium and SEI. Additionally, with cryo-FIB we utilized lift-out procedures under cryogenic temperatures for extraction of a lamella sample for investigation using scanning/transmission electron microscopy with energy dispersive x-ray spectroscopy and electron energy loss spectroscopy. The nanoscale chemical maps have allowed for a greater understanding of the SEI components and how they redistribute with reaction to the lithium metal over the course of many cycles. These results present a step towards visualization of the intact battery stack. We have determined a failure mechanism beyond short circuiting of the lithium metal anodes, due to reaction of the lithium with this LiFSI in DME chemistry.
This work was funded by Sandia National Laboratories’ Laboratory Directed Research and Development program. It was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.