Intramolecular bonds resolved on a semiconductor surface

Abstract

Noncontact atomic force microscopy (NC-AFM) is now routinely capable of obtaining submolecular resolution, readily resolving the carbon backbone structure of planar organic molecules adsorbed on metal substrates. Here we show that the same resolution may also be obtained for molecules adsorbed on a reactive semiconducting substrate. Surprisingly, this resolution is routinely obtained without the need for deliberate tip functionalization. Intriguingly, we observe two chemically distinct apex types capable of submolecular imaging. We characterize our tip apices by "inverse imaging" of the silicon adatoms of the Si(111) −7×7 surface and support our findings with detailed density functional theory (DFT) calculations. We also show that intramolecular resolution on individual molecules may be readily obtained at 78 K, rather than solely at 5 K as previously demonstrated. Our results suggest a wide range of tips may be capable of producing intramolecular contrast for molecules adsorbed on semiconductor surfaces, leading to a much broader applicability for submolecular imaging protocols.

Measuring the reactivity of a silicon-terminated probe

“…3 does not show clear bondlike resolution at close approach such that the orientation of the molecule can be determined. This must be reconciled with other results showing that silicon-terminated probes can resolve the structure of planar organic molecules [2,4]. Most likely the contrast at close approach is the result of multiatom interactions between the tip apex and the C 60 molecule.…”

“…results with support from ab initio density functional theory (DFT) calculations and compare our data to simulations investigating the interaction of metal and silicon clusters with carbon nanotubes [12,13] and recent intramolecular imaging results on semiconductor substrates [2,4].…”

“…Very recently it has been shown that similar contrast can also be obtained on semiconductor surfaces at liquid nitrogen temperatures [2,3] and even at room temperature [4]. In experiments conducted on metallic surfaces it is widely assumed that intramolecular resolution can only be obtained via functionalization of the scanning probe tip with an unreactive atom or molecule, since otherwise the high reactivity of the metallic tip causes manipulation of the molecule before the tip is able to enter into the Pauli repulsion regime where intramolecular contrast is obtained.…”

“…In experiments conducted on metallic surfaces it is widely assumed that intramolecular resolution can only be obtained via functionalization of the scanning probe tip with an unreactive atom or molecule, since otherwise the high reactivity of the metallic tip causes manipulation of the molecule before the tip is able to enter into the Pauli repulsion regime where intramolecular contrast is obtained. However, in recent combined experimental and theoretical studies, it has been suggested that semiconductor tips may counterintuitively be able to provide similar resolution [2][3][4], despite silicon traditionally being thought of as a highly reactive tip material [5][6][7].…”

Unique Determination of “Subatomic” Contrast by Imaging Covalent Backbonding

“…3a of Welker et al 31 ). We also observe a marked increase in the occurrence of this contrast mode after deposition of molecules with passivated end groups, 33 and, consequently, it seems likely that the contrast arises from the passivation of the tip by an unreactive atom or molecule and the resulting suppression of the chemical interaction between the tip and surface.…”