Towards a holographic approach to spherical aberration correction in scanning transmission electron microscopy

Recent progress in phase modulation using nanofabricated electron holograms has demonstrated how the phase of an electron beam can be controlled. In this paper, we apply this concept to the correction of spherical aberration in a scanning transmission electron microscope and demonstrate an improvement in spatial resolution. Such a holographic approach to spherical aberration correction is advantageous for its simplicity and cost-effiectiveness

Quantitative measurement of mean inner potential and specimen thickness from high-Resolution off-axis electron holograms of ultrathin layered WSe2

The phase and amplitude of the electron wavefunction that has passed through ultra-thin flakes of WSe2 is measured from high-resolution off-axis electron holograms. Both the experimental measurements and corresponding computer simulations are used to show that, as a result of dynamical diffraction, the spatially averaged phase does not increase linearly with specimen thickness close to an [001] zone axis orientation even when the specimen has a thickness of only a few layers. It is then not possible to infer the local specimen thickness of the WSe2 from either the phase or the amplitude alone. Instead, we show that the combined analysis of phase and amplitude from experimental measurements and simulations allows an accurate determination of the local specimen thickness. The relationship between phase and projected potential is shown to be approximately linear for extremely thin specimens that are tilted by several degrees in certain directions from the [001] zone axis. A knowledge of the specimen thickness then allows the electrostatic potential to be determined from the measured phase. By using this combined approach, we determine a value for the mean inner potential of WSe2 of 18.9±0.8 V, which is 12% lower than the value calculated from neutral atom scattering factors

Quantitative measurement of mean inner potential and specimen thickness from high-resolution off-axis electron holograms of ultra-thin layered WSe2

Off-axis electron holography is a powerful tool to measure electrostatic and magnetic fields at the nanoscale inside a transmission electron microscope. The electron wave that has passed through a thin specimen can be recovered from an electron hologram and the phase can be related to the specimen thickness or the electrostatic potential in and around the specimen. However, dynamical diffraction may cause a deviation from the expected linear relationship between phase and specimen thickness, which emphasizes the need for comparisons with corresponding computer simulations.Here, we study few-layer-thick two-dimensional WSe2 flakes by off-axis electron holography. Voronoi tessellation is used to spatially average the phase and amplitude of the electron wavefunction within regions of unit-cell size (see Fig. 1). A determination of specimen thickness of the WSe2 is not possible from either the phase or the amplitude alone. Instead, we show that the combined analysis of phase and amplitude from experimental measurements and simulations allows an accurate determination of the local specimen thickness. Extremely thin specimens that are tilted slightly away from the [001] zone axis show an approximately linear relationship between phase and projected potential. If the specimen thickness is known, the electrostatic potential can be determined from the measured phase.We used this combined approach to determine a value for the mean inner potential of 18.9 ± 0.8 V for WSe2, which is approximately 10% lower than the value calculated from neutral atom scattering factors. In this way, a comparison of high-resolution electron holography data with simulations has been achieved on a quantitative level, enabling an assessment of the experimental conditions under which electrostatic potentials can be extracted directly from the phases of measured wavefunctions.The authors are grateful to L. Houben, M. Lentzen, A. Thust, J. Caron and C. B. Boothroyd for discussions, as well as A. Chaturvedi and C. Kloc from the School of Materials Science and Engineering, Nanyang Technological University, Singapore for providing the WSe2 crystals. We are also grateful to the European Research Council for an Advanced Grant and for funding by the German Science Foundation (DFG) grant MA 1280/40-

Controllable Atomic Scale Patterning of Freestanding Monolayer Graphene at Elevated Temperature

We show that by operating a scanning transmission electron microscope (STEM) with a 0.1 nm 300 kV electron beam, one can sculpt free-standing monolayer graphene with close-to-atomic precision at 600 °C. The same electron beam that is used for destructive sculpting can be used to image the sculpted monolayer graphene nondestructively. For imaging, a scanning dwell time is used that is about 1000 times shorter than for the sculpting. This approach allows for instantaneous switching between sculpting and imaging and thus fine-tuning the shape of the sculpted lattice. Furthermore, the sculpting process can be automated using a script. In this way, free-standing monolayer graphene can be controllably sculpted into patterns that are predefined in position, size, and orientation while maintaining defect-free crystallinity of the adjacent lattice. The sculpting and imaging processes can be fully computer-controlled to fabricate complex assemblies of ribbons or other shapes

Surface evolution of a Pt-Pd-Au electrocatalyst for stable oxygen reduction

Core- shell nanocatalysts have demonstrated potential as highly active low-Pt fuel cell cathodes for the oxygen reduction reaction (ORR); however, challenges remain in optimizing their surface and interfacial structures, which often exhibit undesirable structural degradation and poor durability. Here, we construct an unsupported nanoporous catalyst with a Pt- Pd shell of sub-nanometre thickness on Au, which demonstrates an initial ORR activity of 1.140Amg(Pt)(-1) at 0.9V. The activity increases to 1.471Amg(Pt)(-1) after 30,000 potential cycles and is stable over a further 70,000 cycles. Using aberration-corrected scanning transmission electron microscopy and atomically resolved elemental mapping, the origin of the activity change is revealed to be an atomic-scale evolution of the shell from an initial Pt-Pd alloy into a bilayer structure with a Pt-rich trimetallic surface, and finally into a uniform and stable Pt-Pd-Au alloy. This Pt-Pd-Au alloy possesses a suitable configuration for ORR, giving a relatively low free energy change for the final water formation from adsorbed OH intermediate during the reaction

Controllable Atomic Scale Patterning of Freestanding Monolayer Graphene at Elevated Temperature

We show that by operating a scanning transmission electron microscope (STEM) with a 0.1 nm 300 kV electron beam, one can sculpt free-standing monolayer graphene with close-to-atomic precision at 600 °C. The same electron beam that is used for destructive sculpting can be used to image the sculpted monolayer graphene nondestructively. For imaging, a scanning dwell time is used that is about 1000 times shorter than for the sculpting. This approach allows for instantaneous switching between sculpting and imaging and thus fine-tuning the shape of the sculpted lattice. Furthermore, the sculpting process can be automated using a script. In this way, free-standing monolayer graphene can be controllably sculpted into patterns that are predefined in position, size, and orientation while maintaining defect-free crystallinity of the adjacent lattice. The sculpting and imaging processes can be fully computer-controlled to fabricate complex assemblies of ribbons or other shapes