Abstract
We perform scanning tunnelling microscopy (STM) in a regime where primary electrons are field-emitted from the tip and excite secondary electrons out of the target—the scanning field-emission microscopy regime (SFM). In the SFM mode, a secondary-electron contrast as high as 30% is observed when imaging a monoatomic step between a clean W(110)- and an Fe-covered W(110)-terrace. This is a figure of contrast comparable to STM. The apparent width of the monoatomic step attains the 1 nm mark, i.e. it is only marginally worse than the corresponding width observed in STM. The origin of the unexpected strong contrast in SFM is the material dependence of the secondary-electron yield and not the dependence of the transported current on the tip–target distance, typical of STM: accordingly, we expect that a technology combining STM and SFM will highlight complementary aspects of a surface while simultaneously making electrons, selected with nanometre spatial precision, available to a macroscopic environment for further processing.
Highlights
In scanning tunnelling microscopy (STM), the distance between the tip and the target is in the subnanometre range
We demonstrate an unexpectedly strong image contrast in secondary-electron imaging scanning field-emission microscopy (SFM)
A strong secondary-electron contrast is observed when going from a side of a surface consisting of Fe to a side of the same surface consisting of W, but no contrast is observed on a terrace consisting of the same atoms. These observations point to the element specificity of the strong secondary-electron contrast so that a technology combining STM and scanning field-emission microscopy regime (SFM) should provide a table-top instrument for elemental fingerprinting of materials at the nanoscale [16]
Summary
In scanning tunnelling microscopy (STM), the distance between the tip and the target is in the subnanometre range. The exponential dependence of the tunnelling probability on the tip-target distance produces the two distinct features that make STM almost unique: a subnanometre horizontal spatial resolution [1] and a strong image ‘contrast’. A strong secondary-electron contrast is observed when going from a side of a surface consisting of Fe to a side of the same surface consisting of W, but no contrast is observed on a terrace consisting of the same atoms These observations point to the element specificity of the strong secondary-electron contrast so that a technology combining STM and SFM should provide a table-top instrument for elemental fingerprinting of materials at the nanoscale [16]
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