In 2020 it was demonstrated that a scanning tunneling microscope (STM) tip can electrostatically 'send' a single dibromoterfluorene molecule across a silver (111) surface over distances of order 100 nanometers, and even transfer it between two independent probes. The experiment, however, resolved only an upper bound on the molecule's transit time, left the interfacial charge that drives the electrostatic interaction uncharacterized, and could not control or explain the direction of repulsive sending. This problem asks for the quantitative completion of that demonstration: direct time-resolved measurement of transit-time distributions, instantaneous velocities, and kinetic energies of individually driven molecules; a microscopic determination of the interfacial charge (origin, magnitude, spatial distribution, and dependence on molecule, substrate, and tip); a systematic and predictive account of sending directionality; and extension of the method to other molecules, substrates, and to direct energy-dissipation measurements during single-molecule diffusion or collisions. No resolution of these cores is known in the literature to date, with partial progress only on controlled single-molecule collisions.

Background

On a crystal surface, a molecule usually moves by thermally activated hopping between neighboring lattice sites, a stochastic process that yields only statistical information about trajectories. A scanning tunneling microscope (STM) — an instrument that images and manipulates individual atoms and molecules by scanning an atomically sharp metal tip above the surface while measuring the tunneling current — can move adsorbates deliberately, but classic lateral manipulation keeps the tip close to the molecule and displaces it by discrete site-to-site hops or by continuous dragging; longer unguided flights were previously achieved only for 'hot' molecular fragments ejected by bond-breaking, whose trajectories cannot be steered. The situation changed with the experiment of Civita, Kolmer, Simpson, Li, Hecht, and Grill (Science 370, 957–960, 2020). They showed that single dibromoterfluorene (DBTF) molecules — a rigid, bromine-terminated aromatic 'wagon' consisting of three fused fluorene units — adsorbed on the atomically flat silver (111) surface at cryogenic temperature can be brought into an oriented state of very high translational mobility, in which the molecule travels along precisely defined tracks (the atomic rows of the surface) over distances up to about 150 nanometers with a precision of 0.1 angstrom. The force is electrostatic: the electric field of the biased tip repels or attracts the molecule over tens of nanometers without mechanical contact, which allowed the authors to 'send' a chosen molecule from one probe position and 'receive' it at another probe tens of nanometers away. Interpreting the observed attraction and repulsion requires the molecule–surface interface to carry a net positive charge (an 'interfacial charge' arising from charge rearrangement between adsorbate and metal), but the experiment inferred only its existence and sign, not its microscopic origin, magnitude, or spatial distribution. Crucially, the detection electronics were too slow to resolve the flight itself: only an upper bound of order a millisecond on the transit time could be deduced, so the molecule's instantaneous velocity, momentum, and kinetic energy — the quantities needed for a real-time picture of driven single-molecule transport and of energy dissipation during diffusion and collisions — remained unmeasured. The authors also found that the direction of repulsive displacement along an atomic row could not always be deduced (bias-polarity-swap imaging sometimes indicated opposite travel directions), and they identified the dependence of the effect on molecule design, substrate, and tip condition as open questions. In the terminology of the field, 'sending' denotes this remote, field-driven, contact-free long-range transport of one specific molecule, in contrast to dragging; 'bias-swap imaging' means repeating the same scan with reversed tip bias to separate electrostatic from other tip effects; kelvin probe force microscopy (KPFM), a scanning-probe variant that measures local contact-potential differences and can quantify nanoscale charge, is the natural complementary tool for charge characterization.

Problem Statement

Turn the demonstrated electrostatic 'sending' of single molecules on a metal surface from a kinematically uncharacterized effect into a quantitatively measured, mechanistically understood, and deterministically controllable transport process. Concretely, for the dibromoterfluorene/Ag(111) system in which the effect was established — and, for the extension component, beyond it — the question is: (a) direct time-resolved measurement of the flight of individually sent molecules, resolving the transit (not merely bounding it) so that transit-time distributions, instantaneous velocities, momenta, and kinetic energies of single molecules moving under tip-induced electrostatic forces are obtained; (b) determination of the microscopic interfacial charge that mediates the tip–molecule interaction — its physical origin, magnitude, and spatial distribution, and how it depends on the molecular species, the substrate, and the tip condition — at a level that quantitatively explains the observed attractive and repulsive forces; (c) a systematic and predictive account of the direction and reliability of repulsive sending along surface tracks, identifying the roles of tip apex geometry, bias waveform, and local surface defects, sufficient to send a chosen molecule in a chosen direction reproducibly rather than probabilistically; and (d) extension of the method toward other molecular species or substrates and to conditions permitting direct measurement of energy dissipation during single-molecule diffusion or collision events (the controlled-collision experiments of 2026 partially enter here, but without direct velocity/energy measurement of the transported molecules). The four components share the common objective of real-time quantitative dynamics of driven single-molecule transport, and the sending process is resolved only when it is simultaneously measured in real time, mechanistically explained, and directionally controllable.

References

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