Nanoscale Chemical Imaging by Scanning Tunneling Microscopy Assisted by Synchrotron Radiation

Nanoscale chemical imaging using scanning tunneling microscopy is demonstrated with a core-level excitation of the probed element by a synchrotron radiation light. Pronounced element-specific contrasts were observed in the spatial resolution of $\sim 10\mathrm{nm}$ on checkerboard-patterned Ni and Fe samples in differential photoinduced current images taken with the scanning tunneling microscopy tip under the synchrotron radiation irradiation whose photon energies are above and below the Ni (Fe) $L$ absorption edge. The local detection of the photoinduced secondary electrons through the surface barrier lowered by the proximate tip and/or via the tunneling process probably plays an important role in achieving the high-spatial resolution.

Figure 1

X-ray absorption spectra at (a) Fe $L$ edge and (b) Ni $L$ edge obtained by SRSTM with a glass-coated tungsten tip.

Figure 2

Images of an Fe and Ni checkerboard pattern observed by SRSTM with (b) constant current mode (topographic image, $V_s=-5\mathrm{V}$, $I_t=10\mathrm{pA}$), and with photocurrent at (c) $h\nu =706\mathrm{eV}$ and (d) 852 eV. Schematic illustration is presented in (a). Element-specific image of Fe is presented in (e) that is obtained by dividing a photocurrent image taken at the top of the Fe $L_3$ absorption edge ($h\nu =706\mathrm{eV}$) by that at the bottom of the edge (698 eV) [see Fig. 1a]. (f) Same as (e) but obtained with images around the Ni $L_3$ absorption edge ($h\nu =852$ and 843 eV).

Figure 3

Line profiles of photocurrent image (red +) in Fig. 2c and topographic image (blue ×) along the line indicated in Fig. 2b.

Figure 4

Schematic of the SRSTM measurement in the case that the tip-sample distance is long (a) and in the tunneling condition (b).