Microscopy Conference 2023

Prof. Dr.

Session chairs

This session focuses on the development and application of advanced imaging, diffraction and spectroscopy to provide new insight into the structural, compositional and functional properties of low-dimensional and quantum materials. A specific focus will be placed on methods for imaging individual defects, enhancing the contrast of light atoms, preparing and cleaning ultra-thin samples, developing automated workflows for low dose imaging of beam sensitive materials and linking electron microscopy with complementary characterisation techniques. Other topics of interest include in situ experiments carried out at cryogenic or elevated temperature, as well as in the presence of external stimuli such as applied voltage, field, gases or light.

Analyzing and assembling layered materials, atom by atom and layer by layer, in 2D and 3D

Abstract text (incl. figure legends and references) Aberration-corrected electron microscopy is emerging as a versatile tool not only for analyzing, but also for manipulating materials down to the level of single atoms. I will discuss both of these aspects and present some of our recent developments in the context of layered, two-dimensional materials: On the analysis side, we developed means to identify the 3D atomic configuration of defects, grain boundaries or impurities, from only two exposures of the same structures [1,2]. The same approach, combined with the simple dependence of the intensity in annular dark field scanning transmission electron microscopy images on the atomic number provides (to some extent) chemical information about the sample, and hence allows an elemental identification in the case of light-element single-layer samples [3]. However, the intensity of individual atoms and atomic columns is affected by residual aberrations and the confidence of an identification is limited by the available signal to noise. As shown in Fig. 1, already in presence of small non-round aberrations, the histogram of intensities is significantly broadened if these aberrations are not taken into account. Here, we show that matching a simulation to an experimental image by iterative optimization provides a reliable analysis of atomic intensities even in presence of residual non-round aberrations [4]. We compare this method with other approaches demonstrating its high reliability at limited dose and with different aberrations. This is of particular relevance for analyzing moderately beam-sensitive materials, such as most 2D materials, where the limited sample stability makes it difficult to obtain spectroscopic information at atomic resolution. Towards an atomic-level manipulation, I will present a recent development where we combine spatially controlled modifications of 2D materials, using focused electron irradiation or electron beam induced etching, with the layer-by-layer assembly of van der Waals heterostructures [5] (Fig. 2). A new transfer and assembly process makes it possible to stack the layers under observation in an electron microscope, such that pre-patterned features can be aligned to each other. The aligned stacking of individually patterned 2D materials layers can be considered as a form of 3D printing, where each layer is only one or a few atoms thick, and features within each layer can be defined with a nm-scale resolution. Moreover, as each layer can be chosen from the large zoo of 2D materials, it should be possible to directly generate a wide variety of functional devices. Fig. 1. Intensity histograms from a simulated image in presence of small non-round aberrations (C1,0 = 2 nm, C1,2 = 1 nm, C2,1=50 nm, C2,3=50 nm). (a) Part of the simulated image, and (b) the simulated probe. (c) Intensity histogram using the optimization method, Gaussian fits, local maxima and the Voronoi cell integration (from left to right) [4]. Fig. 2. (a-c) Schematic of cutting a pattern into individual graphene layers (a,b) followed by aligned stacking (c). (d-f) Experimental realization, (d,e) SEM images of crosshair structures cut into graphene, (f) Dark-field TEM image showing the assembled structure, set for highlighting one of the two layers. Scale bars are (d,e) 500 nm and (f) 200 nm [5]. References [1] 2D Materials 5, 045029 (2018) [2] Appl. Phys. Lett. 114, 053102 (2019) [3] Nature Commun. 10, 4570, (2019) [4] Ultramicroscopy 227, 113292 (2021) [5] ACS Nano 16, 1836-46 (2022)

MS 3: Low-dimensional and quantum materials

Plenary

Invited talk

Seeing into organic 2D materials: defects, dynamics, and resolution

Abstract text (incl. figure legends and references) Probing the structure of organic 2D materials (O2DMs), ultimately down to the atomic scale, has been a long-sought goal of materials science. AC-HRTEM is capable of direct imaging atomic structures with sub-Ångström resolution 1. However, electron irradiation damage leads to rapid disintegration of the molecular network during the imaging process, severely limiting achievable resolution regardless of TEMs' optical performance. Here we present various experimental efforts to extract high-resolution information from O2DMs in particular imine-based 2D polymer thin films 2,3. Two samples were synthesized using the surfactant-monolayer-assisted interfacial synthesis (SMAIS) 4 (Fig. 1). Using low-dose methods at room temperature, we examined 2D-PI-DhTPA 2, and directly observed the grain boundaries with a resolution of 2.3 Å at 300 kV, giving insight into the formation mechanism of the 2D crystals. Although HRTEM imaging of inorganic 2D materials can benefit substantially by restricting the electron energy below 80 keV 1, the rare TEM studies on organic 2D crystals are still usually carried out at 300 keV. We systematically evaluated the electron accelerating voltages for AC-HRTEM imaging of O2DMs 3. We reached an image resolution of 1.9 Å at 120 kV (Fig. 2a-b). The improvement in resolution has been achieved by maximizing the efficiency of electron usage, among the acceleration voltages of 300 kV, 200 kV, 120 kV, and 80 kV. Not only could the porphyrin pores be clearly resolved, but linkers with and without hydroxyl groups could also be distinguished. We occasionally observed abnormal contrast in the vicinity of the porphyrin cores (Fig. 2c-d), conflicting with their previously reported structures. DFT calculations revealed that the additional contrast could be attributed to molecular interstitial defects (Fig. 2e) – a defect type that had not been discovered in 2D polymers before. Recently, drawing from electron cryomicroscopy (cryoEM) in biology 5, we explored the potential of imaging our radiation-sensitive O2DMs at low temperature with direct electron detectors. This combination improved the information in a single micrograph such that reflections beyond a resolution of 1.3 Å are clearly resolved. We envisage that our results will bring new insights into the defect types and pore interfaces in O2DMS and promote a deeper understanding of structure-function correlations in this rising class of materials. Fig. 1. Reaction scheme and atomic models of 2D-PI-BPDA and 2D-PI-DhTPA. Fig. 2. a-d AC-HRTEM imaging of 2D-PI-BPDA at 120 kV, showing regions without and with TAPP interstitials. e, Plausible stacking modes of TAPP interstitials derived by DFTB calculations and corresponding simulated images. References Linck, M. et al. Phys. Rev. Lett. 117, 76101 (2016). Qi, H. et al. Sci. Adv. 6, eabb5976 (2020). Liang, B. et al. Nat. Commun. 13, 3948 (2022). Liu, K. et al. Nat. Chem. 11, 994–1000 (2019). Russo, C. J., &amp; Egerton, R. MRS Bulletin 44, 935–941 (2019). Acknowledgment We gratefully acknowledge the funding from DFG – 492191310; 426572620; 417590517 (SFB-1415), as well as the financial support from the European Union's Horizon2020 research and innovation program under Grant Agreement No. 881603 (GrapheneCore3). This work was also supported by an Astex SIPD Fellowship (HW) and Medical Research Council grant MC_UP_120117.

Prof.

Tunable eccentricity of Bloch skyrmions in Fe1.9Ni0.9Rh0.2P

Abstract text (incl. figure legends and references) 1. Introduction The experimental discovery of antiskyrmions in Mn1.4PtSn [1] has opened new pathways towards using microscopically small magnetic whirls as information carriers [2]. For this purpose, a thorough understanding of the detailed three-dimensional spin texture of these magnetic solitons, including their stabilization mechanisms is essential. Quite recently, the compound Fe1.9Ni0.9Pd0.2P was found to host antiskyrmions at room temperature, which can be transformed into skyrmions, e.g., by tuning the external magnetic field [3]. 2. Objectives In this study we investigate this elliptical skyrmion phase as a function of the external magnetic field and temperature in a similar material, Fe1.9Ni0.9Rh0.2P, where Pd was substituted by Rh, to gain further insight into the formation mechanisms of topological spin solitons. 3. Materials &amp; methods The occurrence and nature of skyrmions in a Fe1.9Ni0.9Rh0.2P lamella fabricated using a focused ion beam (FIB) were studied in a JEOL Jem F-200C by means of off-axis electron holography that allows for the quantitative determination of the projected in-plane magnetic induction Bip. The sample temperature was controlled with a Mel-Build LN2 Atmos Defend - In-situ double-tilt LN2 cooling holder. 4. Results In Fig. 1 the magnitudes of Bip in the lamella upon exposure to magnetic fields of 170 mT and 330 mT applied along [001] at room temperature are displayed. In zero field, the skyrmions initially possess an elliptical shape with the semi-major axes parallel to the ⟨110⟩ directions due to dipole–dipole interactions [5, 6]. Increasing the external out-of-plane magnetic field leads to (i) a reduction of the skyrmion size, (ii) an enhancement of Bip and (iii) a deformation of the skyrmions. The initially elliptical skyrmions are transformed into skyrmions with circular shapes until a critical field is reached, where the system undergoes a transition into the field-polarized phase. 5. Conclusion The current investigations provide a first insight how the balance between the symmetric and antisymmetric exchange and dipole-dipole interaction can tune the skyrmion structure in Fe1.9Ni0.9Rh0.2P. By combining our experimental results with magnetostatic simulations, we can shed light on the complex interplay of these interactions in the stabilization of topological spin textures in three dimensions. [1] A. K. Nayak et al., Nature 548 (2017), 561 – 566. [2] B. Göbel et al., Physics Reports 895 (2021), 1 – 28. [3] K. Karube et al., Nature Materials 20 (2021), 335 – 340. [4] K. Karube et al., Adv. Mater. 34 (2022), 2108770. [5] L. Peng et al., Nat. Nanotechnol. 15 (2020), 181 – 186. [6] J. Jena et al., Nature Communications 11 (2020), 1115. Fig. 1: Magnitude of Bip of Bloch skyrmions in Fe1.9Ni0.9Rh0.2P in external fields of 170 mT and 330 mT, respectively.

IM 6: Phase-related techniques & 4D STEM

Abstract talk

Synthesis and characterization of metal sulfide nanoribbons on graphene by self-assembly of inorganic ions

Abstract text (incl. figure legends and references) Bottom-up synthesis of nanostructures on surfaces depends on the self-assembly of nanoscale building blocks. The diversity of accessible nanostructures, however, have been constrained by the limited choice of atomic and molecular building blocks that can be evaporated on a surface. Here, we bypass this limitation by using complex inorganic ions generated from electrospray ionization as building blocks to synthesize nanostructures on surfaces. We deposited HMonS3n+1&shy;– (n = 4-8) ions onto a freestanding single-layer graphene by electrospray ion-beam deposition (ESIBD), and imaged the resulting nanostructures by aberration-corrected scanning transmission electron microscopy (STEM). The molecules form anisotropic, single-layered, crystalline MoS2 nanoflakes (&lt; 20 nm2), which in turn self-assemble into MoS2 nanoribbons extending as far as 1 μm (Fig. 1). The widths of the nanoribbons were tuned by the precursors, as the HMonS3n+1&shy;– (n = 4-8) ions formed 4 ± 1 nm wide nanoribbons, the HMonS3n+1&shy;– (n = 5-8) ions 5 ± 2 nm wide nanoribbons, and the HMonS3n+1&shy;– (n = 6-8) ions formed 8 ± 3 nm wide ones. This first observation of such nanostructures evidences the potential of this approach to prepare previously inaccessible nanomaterials on surfaces. Figure 1 HMonS3n+1&shy;– ions were deposited onto single layer graphene by ESIBD, and self-assembled into 1 μm long monolayer MoS2 nanoribbons as observed by STEM.

Prof. Dr. rer. nat. habil.

Hexagonal hybrid bismuthene by molecular interface engineering

Abstract text (incl. figure legends and references) Since the rise of graphene as playground for scientists in different fields, the fascination for two-dimensional materials did not cool down but rather remains at a peak interest due to the exceptional properties those ultrathin, layered structures show. While physicists are intrigued by transport properties for TMD's and material scientists praise the mechanical stabilities of even single layer thick atomic layers, chemists mainly use 2D materials as perfectly defined substrate for functionalization approaches. In contrast to widely studied materials, we will present a chemical approach towards two-dimensional bismuth with unprecedented material quality and a surprisingly unexpected structure. In an one-pot colloidal reductive reaction, we synthesized single crystalline hexagonal platelets with diameters up to 2 µm and a thickness around 5 nm. Due to the chemical approach for the generation of these ultrathin bismuth material, we were able to introduce structurally modified bismuthene on the basal planes of the crystallites that are stabilized by a covalent functionalization. To reveal the nature and resulting properties of this sandwiched hybrid structure we made use of a battery of characterization techniques ranging from microscopy (HRTEM, HRSTEM, EELS, EDS, SAED, AFM) to spectroscopy (XPS, Raman) and finally simulations (DFT) and single-particle transport measurements. The combination of all these techniques allowed us to describe the stabilized bismuthene capping layers as fundamentally different from pristine rhombohedral bismuth. Firstly, while bismuth shows the typical zig-zag arrangement with a lattice constant of 4.5 Å, the bismuthene layers are flat with a lattice parameter of 7.5 Å. Monochromated STEM-EELS reveals LSPR modes that are highly unexpected for a semimetal like bismuth and the calculated band structure, based on the simulated structure, shows metallic behaviour. The metallic nature of individual crystals was measured by transport experiments at room temperature. In this contribution we show the wholistic process of an exceptional new material with a strong spin orbit coupling and postulated spin torque effects from the bottom-up synthesis to the characterization and conceivable future applications in devices. Fig. 1 A Colloidal synthesis of the sandwich material with equivalents for the reaction. B PXRD pattern of synthesized material with expected diffraction peaks indicated. C Typical unprocessed Raman spectrum. No indication of surface oxidation. Inset optical micrograph of crystallite. D BF-TEM data and E typical indexed diffraction pattern of hybrid material with superstructural reflections indicated. F Colorized large area HRSTEM data with FFT as inset. Superstructure indicated in FFT. G AFM with height profile. H Experimental series of integrated EELS intensity, showing localized surface plasmon modes and I simulation of plasmonic modes based on model hexagon. Fig. 2 A Integrated STEM-EDX intensity for individual crystallite with artistic representation of functionalized material. B Spatially integrated mean EDX spectrum with signals related to Bi. C Overview XPS before (red) and after (black) surface etching with Ar ions and D high resolution XPS of S/ Bi binding energies. A clear indication of S 2p before the surface etching can be noticed between 165–170 eV. E Structurally optimized bismuth/ bismuthene hybrid with simulated ED. Bottom right cross sectional HRTEM and respective image simulation.

MS 1: Energy-related materials and catalysts

Ordered electronic states in 2D quantum materials: imaging effects of hydrostatic pressure at the atomic scale

Abstract text (incl. figure legends and references) Introduction Charge density waves (CDW) are an ordered electronic state with periodic modulation of electron density and periodic lattice distortion (PLD) modulation of the atomic positions.1 CDW is represented by an order parameter Ψ =Δeiφ characterized by a phase φ, giving position of the CDW with relation to the lattice, and amplitude,Δ, giving the value of the energy gap.1 In some quantum materials, CDW are in competition / coexistence with the superconducting phase.2-3 The nature of this relationship is still not well understood. Application of pressure is one experimental approach used to probe the nature of this relationship. 3 However, this also leads to the necessity to understand effects of pressure on the nature and the structure of the CDW at the atomic scale. Objectives Here, we have used atomic-scale transmission electron microscopy imaging to determine the influence of applied pressure on the structural properties of 1T-TaS2 and the CDW at the atomic-scale.4 Materials &amp; Methods 1T-TaS2 samples were exposed to hydrostatic pressures of up to 2.5 GPa in a diamond anvil cell (DAC) (Fig.1 (a)). Pressurized samples were then prepared for TEM investigations through mechanical exfoliation. In high-resolution TEM (HRTEM) imaging CDW wave vectors and their respective phases are accessible and their evolution as a function of applied pressure can be mapped at the atomic scale.4 Results A HRTEM image from a pressurized sample characterized by a deformation defect is shown in Fig. 1(b). Strong coupling of lattice and electronic degrees of freedom means that changes in lattice structure and ensuing deformation defects due to the applied pressure have an effect on the structure of the CDW electronic state. The modulation of CDW around the structural defect is shown in Figs 1(d)-(e). The magnitude of the CDW wave-length |λ1| and the respective phase φ1 are shown as intensity maps in Figs. 1(d) and (e) respectively. The high and low intensity in the magnitude images respectively show the elongated or shortened CDW wavelength. The magnitude of the CDW wavelength and its phase are strongly modulated in the vicinity of the deformation defects. This strong modulation is characterized by (1) CDW strain which leads to local changes in the modulation wavelenght (2) Topological phase defects in the CDW order parameter in form of CDW dislocations (marked by dotted circles in Figs. 1(c)-(d)). Conclusion In conclusion we have mapped the atomic-scale response of the CDW order parameter in 1T-TaS2 exposed to 2.5 GPa hydrostatic pressure. The CDW order parameter responds to the lattice defects arising from the applied pressure by showing an elastic-like strain response and topological phase defects in the electronic order. References [1].J. Wilson, F. D. Salvo, and S. Mahajan, Adv. Phys. 24, 117 (1975). [2] E. Fradkin et al., Rev. Mod. Phys. 87, 457 (2015) [3] A. F. Kusmartseva et al., Phys. Rev. Lett. 103, 236401(2009) [4] M. K. Kinyanjui et al.Phys. Rev. B 104, 125106(2021) FIG. 1 (a) Diamond anvil cell set-up used for pressure experiments. (b) A HRTEM image showing a deformation defect in a 1T-TaS2 sample pressurized up to 2.5 GPa (b) FT of the HRTEM image with the Bragg spots from the atomic lattice (solid circles) and super lattice spots from the CDW ( triangles). Temperature intensity maps representing (d) the magnitude (e) phase map of CDW wavelength. The dotted circles show the positions of CDW dislocations

MS 3
Low-dimensional and quantum materials

Appointment

Session chairs

Description

Programme

MS 3Low-dimensional and quantum materials

Appointment

Session chairs

Description

Programme

MS 3
Low-dimensional and quantum materials