Nozomi Shirato

Elemental fingerprinting of materials with sensitivity at the atomic limit.

By using synchrotron X-rays as a probe and a nanofabricated smart tip of a tunneling microscope as a detector, we have achieved chemical fingerprinting of individual nickel clusters on a Cu(111) surface at 2 nm lateral resolution, and at the ultimate single-atomic height sensitivity. Moreover, by varying the photon energy, we have succeeded to locally measure photoionization cross sections of just a single Ni nanocluster, which opens new exciting opportunities for chemical imaging of nanoscale materials.

Controlled modulation of hard and soft X-ray induced tunneling currents utilizing coaxial metal-insulator-metal probe tips

The effect of a local external electric field on the barrier potential of a tunneling gap is studied utilizing an emerging technique, synchrotron x-ray scanning tunneling microscopy. Here, we demonstrate that the shape of the potential barrier in the tunneling gap can be altered by a localized external electric field, generated by voltages placed on the metallic outer shield of a nanofabricated coaxial metal-insulator-metal tip, resulting in a controlled linear modulation of the tunneling current. Experiments at hard and soft x-ray synchrotron beamlines reveal that both the chemical contrast and magnetic contrast signals measured by the tip can be drastically enhanced, resulting in improved local detection of chemistry and magnetization at the surface.

Detecting element specific electrons from a single cobalt nanocluster with synchrotron x-ray scanning tunneling microscopy

We use a nanofabricated scanning tunneling microscope tip as a detector to investigate local X-ray induced tunneling and electron emission from a single cobalt nanocluster on a Au(111) surface. The tip-detector is positioned a few angstroms above the nanocluster, and ramping the incident X-ray energy across the Co photoabsorption K-edge enables the detection of element specific electrons. Atomic-scale spatial dependent changes in the X-ray absorption cross section are directly measured by taking the X-ray induced current as a function of X-ray energy. From the measured sample and tip currents, element specific X-ray induced current components can be separated and thereby the corresponding yields for the X-ray induced processes of the single cobalt nanocluster can be determined. The detection of element specific synchrotron X-ray induced electrons of a single nanocluster opens an avenue for materials characterization on a one particle at-a-time basis.

Local X-ray magnetic circular dichroism study of Fe/Cu(111) using a tunneling smart tip

A tunneling smart tip of a synchrotron X-ray scanning tunneling microscope provides simultaneously localized topographic, elemental and magnetic information.

X-ray magnetic circular dichroism and near-edge X-ray absorption fine structure of buried interfacial magnetism measured by using a scanning tunneling microscope tip

Magnetism at buried interfaces plays a crucial role in many emerging phenomena, but detection of interfacial magnetism in close proximity to a surface with elemental and chemical sensitivity is a challenging task. Here, we use low temperature synchrotron x-ray scanning tunneling microscopy to investigate x-ray magnetic circular dichroism and the near edge x-ray absorption fine structure of La0.67Sr0.33MnO3-LaNiO3 superlattices. In stark contrast to the weak magnetic signal of Mn when the La0.67Sr0.33MnO3 layers are located on top, a robust x-ray magnetic circular dichroism signal is detected when they are buried underneath the LaNiO3 layers. The near edge x-ray absorption fine structure reveals the valence states of manganese, while the oxygen K-edge x-ray absorption spectra show an increase in hole formation, indicating a cogent charge transfer at the LaNiO3/La0.67Sr0.33MnO3 interface. This work demonstrates that scanning tunneling microscopy can be extended to the synchrotron X-ray study of buried interfaces by controlling the tip-sample separation in the nanometer regime.Magnetism at buried interfaces plays a crucial role in many emerging phenomena, but detection of interfacial magnetism in close proximity to a surface with elemental and chemical sensitivity is a challenging task. Here, we use low temperature synchrotron x-ray scanning tunneling microscopy to investigate x-ray magnetic circular dichroism and the near edge x-ray absorption fine structure of La0.67Sr0.33MnO3-LaNiO3 superlattices. In stark contrast to the weak magnetic signal of Mn when the La0.67Sr0.33MnO3 layers are located on top, a robust x-ray magnetic circular dichroism signal is detected when they are buried underneath the LaNiO3 layers. The near edge x-ray absorption fine structure reveals the valence states of manganese, while the oxygen K-edge x-ray absorption spectra show an increase in hole formation, indicating a cogent charge transfer at the LaNiO3/La0.67Sr0.33MnO3 interface. This work demonstrates that scanning tunneling microscopy can be extended to the synchrotron X-ray study of buried inter...

Ultra-high vacuum compatible optical chopper system for synchrotron x-ray scanning tunneling microscopy

High-speed beam choppers are a crucial part of time-resolved x-ray studies as well as a necessary component to enable elemental contrast in synchrotron x-ray scanning tunneling microscopy (SX-STM). However, many chopper systems are not capable of operation in vacuum, which restricts their application to x-ray studies with high photon energies, where air absorption does not present a significant problem. To overcome this limitation, we present a fully ultra-high vacuum (UHV) compatible chopper system capable of operating at variable chopping frequencies up to 4 kHz. The lightweight aluminum chopper disk is coated with Ti and Au films to provide the required beam attenuation for soft and hard x-rays with photon energies up to about 12 keV. The chopper is used for lock-in detection of x-ray enhanced signals in SX-STM.