Sergey Trishin

Coherent phonons describe the collective, ultrafast motion of atoms and play a central role in light-induced structural dynamics. Here, we use terahertz (THz) scanning tunneling microscopy to excite and detect coherent phonons in semiconducting 2H-MoTe ₂ and resolve how their excitation is influenced by atomic-scale defects. In a THz pump-probe scheme, we observe long-lived oscillatory signals that we assign to out-of-plane breathing and in-plane shear modes, which are both dipole forbidden in the bulk. The relative excitation strength of these modes varies near defects, indicating that tip-induced local band bending modulates the coupling to the THz field. This defect-tunable coupling offers previously unidentified opportunities to control selective excitation of vibrational modes at the nanoscale.

Controlling the surface diffusion of particles on 2D devices creates opportunities for advancing microscopic processes such as nanoassembly, thin-film growth, and catalysis. Here, we demonstrate the ability to control the diffusion of F₄TCNQ molecules at the surface of clean graphene field-effect transistors (FETs) via electrostatic gating. Tuning the back-gate voltage (V_G) of a graphene FET switches molecular adsorbates between negative and neutral charge states, leading to dramatic changes in their diffusion properties. Scanning tunneling microscopy measurements reveal that the diffusivity of neutral molecules decreases rapidly with a decreasing V_G and involves rotational diffusion processes. The molecular diffusivity of negatively charged molecules, on the other hand, remains nearly constant over a wide range of applied V_G values and is dominated by purely translational processes. First-principles density functional theory calculations confirm that the energy landscapes experienced by neutral vs charged molecules lead to diffusion behavior consistent with experiment. Gate-tunability of the diffusion barrier for F₄TCNQ molecules on graphene enables graphene FETs to act as diffusion switches.

Electrostatic potentials strongly affect molecular energy levels and charge states, providing the fascinating opportunity of molecular gating. Their influence on molecular vibrations remains less explored. Here, we investigate ethyl‐diaminodicyanoquinone molecules on a monolayer of MoS₂ on Au(111) using scanning tunneling and atomic force microscopy and spectroscopy. These molecules exhibit a large dipole moment in gas phase, which is found to (partially) persist on the MoS₂ monolayer. The self‐assembled structures consist of chains, where the dipoles of neighboring molecules are aligned antiparallel. Thanks to the decoupling efficiency of the molecular states from the metal by the MoS₂ interlayer, vibronic states of the molecules are resolved, which vary in intensity depending on the molecular surrounding. We suggest that the vibrations are strongly damped by electrostatic interactions with the environment.

Molecules consisting of a donor and an acceptor moiety can exhibit large intrinsic dipole moments. Upon deposition on a metal surface, the dipole may be effectively screened and the charge distribution altered due to hybridization with substrate electronic states. Here, we deposit Ethyl-Diaminodicyanoquinone molecules, which exhibit a large dipole moment in gas phase, on a Au(111) surface. Employing a combination of scanning tunneling microscopy and non-contact atomic force microscopy, we find that a significant dipole moment persists in the flat-lying molecules. Density-functional theory calculations reveal that the dipole moment is even increased on the metal substrate as compared to the gas phase. We also show that the local contact potential across the molecular islands is decreased by several tens of meV with respect to the bare metal. We explain this by the induced charge-density redistribution due to the adsorbed molecules, which confine the substrate's wavefunction at the interface. Our local measurements provide direct evidence of this so-called push-back or cushion effect at the scale of individual molecules.