Tuning Intermolecular Charge Transfer in Donor–Acceptor Two-Dimensional Crystals on Metal Surfaces

Abstract

Organic charge transfer (CT) compounds display a wide range of exotic electronic properties (charge-density wave stabilization, Peierls transitions, etc.) depending on the amount of charge transferred from the donor (D) to the acceptor (A) species. A complete exploration of the complex electronic phase diagrams for such compounds would thus require methods to systematically tune the amount of charge exchanged in the CT process. This has proven however challenging in the past: chemical functionalization of the constituent molecules can also affect the packing of the molecular units in the crystal, whereas changing D:A stoichiometry is often not possible in the bulk. Interestingly, it was recently found that multiple stoichiometries can actually be achieved by codeposition of different amounts of D and A molecules on metal surfaces. The question, however, of whether CT processes between D and A molecules can be tuned with the D:A ratio in such mixtures has not yet been studied, and it is no trivial matter, since competing CT processes between the metal surface and the organic adsorbates might hinder interadsorbate charge transfer. Here we demonstrate that the CT process from the organic donor tetrathiafulvalene (TTF) to the acceptor tetracyanoquino-p-dimethane (TCNQ) can be tuned with exquisite accuracy (∼0.1 e − ) by controlling the stoichiometry of D:A cocrystals deposited on Ag(111). This control opens new avenues to explore the complex phase diagrams of organic CT compounds and to tailor their electronic properties.

Ultrafast Dynamics of Electronic Resonances in Molecules Adsorbed on Metal Surfaces: A Wave Packet Propagation Approach

“…Indeed, the standard codes based on DFT are able to deal with enough atoms to describe properly many of the processes of interest such as molecular adsorption geometry, formation of molecule-metal bond, etc. [89][90][91][92][93][94][95][96][97][98][99][100][101][102] Typically, 4-5 atomic layers are enough for a proper description of these properties, provided large enough supercell in the lateral direction so that the effects owing to periodic representation of the system inherent to this codes can be neglected. However, if one is interested in the decay of molecule localized states into the metal, or in the photoemission, the computational domain has to be extended parallel to the surface as well as deep inside the metal and into the vacuum.…”

Electronic Characterization of a Charge-Transfer Complex Monolayer on Graphene

“…This anisotropy in the overlap results in pseudo-one-dimensional electronic dispersion, providing a suitable platform to investigate low-dimensional, as well as low-energy, physics. Despite the broad spectrum of intriguing physical phenomena that have been reported in bulk CTCs, their two-dimensional (2D) films have been much less studied. − In particular, the studies have been confined to metal substrates, which strongly interact with the molecular layer and mask the intrinsic electronic properties of the CTCs.…”

Electronic Characterization of a Charge-Transfer Complex Monolayer on Graphene

“…In bulk CTC crystals, donor and acceptor molecules typically stack in rows that maximize π − π electronic overlap along the rows only [7]. This anisotropy in the overlap results in pseudo one-dimensional electronic dispersion, providing a suitable platform to investigate low-dimensional, as well as low-energy, physics.Despite the broad spectrum of intriguing physical phenomena that have been reported in bulk CTCs, their two-dimensional (2D) films have been much less studied [8][9][10][11][12][13][14][15]. In particular, the studies have been confined to metal substrates, which strongly interact with the molecular layer and mask the intrinsic electronic properties of the CTCs.The CTC formed out of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) molecules is an archetypal example of a CTC.…”

“…Despite the broad spectrum of intriguing physical phenomena that have been reported in bulk CTCs, their two-dimensional (2D) films have been much less studied [8][9][10][11][12][13][14][15]. In particular, the studies have been confined to metal substrates, which strongly interact with the molecular layer and mask the intrinsic electronic properties of the CTCs.…”

“…The properties of 2D films of these CTCs on metallic substrates can be strongly influenced by the underlying substrate. For example, it is possible to form films with other than 1:1 stoichiometry [12][13][14]. In some cases, the effect of the substrate can be limited to doping of the film, e.g.…”