Collecting photoelectrons with a scanning tunneling microscope nanotip

Abstract

The collection of photoelectrons excited with a synchrotron via a nanotip placed near the surface of a sample is studied. Simulating the electron trajectory, we found that photoelectrons escaping from the surface are too weak to be the only source of electrons contributing to a photocurrent detected with a scanning tunneling microscope tip, as reported recently. The tunneling of low-energy electrons generated with synchrotron irradiation is suggested as an additional channel contributing to the photocurrent at a small separation between tip and sample. An image based on x-ray absorption is expected to attain a resolution comparable to a topographical image.

NanoXAS, a novel concept for high resolution microscopy

“…For a focus diameter of ∼ 35 nm the measured drain current at the Co L 3 absorption edge (778.4 eV) was about 3 pA. This results in a photocurrent density of about 4000 A/m 2 , so compared to a typical photocurrent density of a non-focused undulator source (U5 at the NSRRC [7]) of about 0.7 A/m 2 , we gain a factor of more than 1000.…”

“…Simulations performed by Chiu et al [7] discuss the resolution limits of insulated STM-tips irradiated by x-ray light. They define D prob as the diameter of a circular region on the sample surface within which emitted electrons contribute to 90 % of I tip .…”

X-ray nanotomography of SiO2-coated Pt90Ir10tips with sub-micron conducting apex

“…The prospects of the combination of scanning probes with synchrotron radiation has led to substantial efforts at synchrotron facilities worldwide. [6][7][8][9][10][11][12][13] Generally, the spatial resolution in STM depends on the sharpness of the tip. 14 However, in SXSTM, in addition to tunneling current, x-ray photoabsorption can yield extra electrons that are ejected from the sample and collected at the tip.…”

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

“…A specialized sharp tip [9,10] that is rastering over the sample surface during x-ray illumination detects x-ray enhanced currents. These currents can provide localized magnetic [11,12] and elemental contrast [8,13], because the specialized tip limits the detection of photo-ejected [6,14] and tunneling [3] electrons to a nanoscale region at the tip apex. Additionally, an electronic filter (topo filter) is used to separate the x-ray induced current signal and the conventional tunneling current signal into two separate channels simultaneously [15].…”

“…Synchrotron x-ray scanning tunneling microscopy (SX-STM) is an emerging technique that combines the chemical, electronic and magnetic sensitivity of synchrotron x-rays (SX) with the high spatial resolution of scanning tunneling microscopy (STM) [1][2][3][4][5][6][7]. Recently, elemental contrast with 2 nm lateral resolution and sensitivity at the single atomic layer limit has been presented [8].…”