Sub-nanometer Lateral Resolution Research Articles

Study of multi-layered graphene by ultra-low energy SEM/STEM

Scanning electron microscopy with very slow electrons offers a novel tool enabling one to image the graphene samples at nanometer or even sub-nanometer lateral resolution in transmitted as well as reflected electrons and to count reliably the atomic layers in both imaging modes. The study was performed on graphene prepared by chemical vapor deposition on thin copper foil. Observation by slow electrons has also confirmed the underlayer mechanism of nucleation and growth bellow already existing graphene layers on copper. Moreover, electrons with impacted energy below 100eV can be used for “cleaning” of a material. It leads to elimination of the contamination process during the measurement, which enables to observe the predicted oscillations in reflection mode and to measure the transmissivity of various stacks of layers in transmission mode down to units of eV.

Optical Origin of Subnanometer Resolution in Tip-Enhanced Raman Mapping

Raman spectroscopy and imaging at the single-molecular level in both signal sensitivity and spatial resolution have been a long dream. A recent experimental study of single-molecule Raman mapping with a subnanometer resolution by using a tip-enhanced Raman scattering (TERS) technique is a great step closer to this great dream. However, how the subnanometer spatial resolution is possible with a Raman excitation light spot (“hot spot”) of diameter over 10 nm formed between the tip–substrate nanogap to excite and probe the molecule still remains mysterious. Here we present an optical theory of Raman scattering that accounts for the strong near-field self-interaction of molecule with the plasmonic nanogap due to multiple elastic scattering of light by the molecule. The result shows that the self-interaction effect strongly modulates the Raman excitation and radiation in both the signal intensity and spatial sensitivity, leading to a “super-hot spot” and subnanometer lateral resolution of Raman mapping. The optical theory can help to uncover the full picture of light-matter interaction of atoms and molecules with plasmonic nanostructures and explore unknown frontiers of physics and chemistry at nanoscale.

Sub-nanometer lateral resolution in photon emission from C 60 molecules on Au(110)

We have used a scanning tunneling microscope (STM) as a local electron source to excite photon emission from ordered monolayers of C 60 molecules on Au (110) surfaces. In maps of the STM stimulated photon emission the C 60 molecules have been spatially resolved. The photon emission is most intense when the tip is centered above a molecule. The diameter of the emission spot is approximately 4 Å. The results demonstrate the highest spatial resolution of light emission to date using a scanning probe technique.

Photon emission from adsorbed C60 molecules with sub-nanometer lateral resolution

We present the first experimental demonstration of spatially resolved photon emission of individual molecules on a surface. A scanning tunneling microscope (STM) was used as a local electron source to excite photon emission from hexagonal arrays of C60 molecules on Au(110) surfaces. Specifically, we show that in maps of photon emission intensities, C60 fullerenes appear as arrays of individual light emitters 4 A in diameter and separated by 10 A. Comparison with simultaneously recorded STM images reveals, that most intense emission is detected when the STM tip is centered above a molecule. The results demonstrate the highest spatial resolution of light emission to date using a scanning probe technique.

Photon emission from nanostructures in an STM

Using an STM we have created and investigated individual small particles with apparent lateral dimensions between 10 Å and 250 Å on metal single-crystal surfaces and metallic films. These particles are clearly observed in simultaneous measurements of STM topographs and emission of photons from the tunneling gap. The particles give rise to order-of-magnitude variations in photon intensity with subnanometer lateral resolution. We interpret the observed photon emission characteristics in terms of inelastic tunneling excitation of tip-induced, localized plasmon modes.

A comparative study of coldly- and warmly-condensed Ag films by scanning tunneling microscopy

Using scanning tunneling microscopy with a subnanometer lateral resolution we have studied silver films condensed at ∼ 90 K and at room temperature (RT). The coldly-condensed films are known to show the surface enhanced Raman effect, while the RT condensed films do not. We observe in real space that the coldly-condensed films exhibit nearly flat portions (average size 130 Å) separated by deep trenches of less than 30 Å width. In contrast, the RT condensed films do not exhibit trenches and their surface topography may be described in terms of a “rolling surface” with low-angle bounderies.