Electronic Characterization of a Charge-Transfer Complex Monolayer on Graphene

Abstract

Organic charge-transfer complexes (CTCs) formed by strong electron acceptor and strong electron donor molecules are known to exhibit exotic effects such as superconductivity and charge density waves. We present a low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) study of a two-dimensional (2D) monolayer CTC of tetrathiafulvalene (TTF) and fluorinated tetracyanoquinodimethane (F₄TCNQ), self-assembled on the surface of oxygen-intercalated epitaxial graphene on Ir(111) (G/O/Ir(111)). We confirm the formation of the charge-transfer complex by dI/dV spectroscopy and direct imaging of the singly occupied molecular orbitals. High-resolution spectroscopy reveals a gap at zero bias, suggesting the formation of a correlated ground state at low temperatures. These results point to the possibility to realize and study correlated ground states in charge-transfer complex monolayers on weakly interacting surfaces.

Keywords: F4TCNQ; TTF; charge density wave (CDW); charge-transfer complex; epitaxial graphene; scanning tunneling microscopy (STM).

Figure 1

Assembly and structure of the CTC on oxygen-intercalated graphene. (a) STM topography image of oxygen-intercalated graphene on Ir(111). The additional superstructure apart from the moiré is due to reconstruction of subsurface oxygen. The scale bar is 3 nm. Imaging parameters: 1.2 nA and 10 mV. (b) Few large islands of CTC on the G/O/Ir(111) surface showing various domains and the domain boundaries. The scale bar is 30 nm. Imaging parameters: 0.4 pA and 0.75 V. (c) Zoomed-in STM image of the CTC showing the arrangement of TTF and F₄TCNQ molecules. Each molecule forms a row next to the row of the other molecule. A molecular structure along with a unit cell is overlaid to elucidate the molecular arrangement within the unit cell. The scale bar is 2 nm. Imaging parameters: ∼5 pA and 0.1 V. (d) DFT-simulated STM image of the CTC close to the Fermi energy resembling the recorded topography closely. Molecular structure and unit cells are overlaid for clarity.

Figure 2

Charge transfer across the molecules. (a) Long-range dI/dV spectra on F₄TCNQ molecules in a single-component chain on the G/O/Ir(111) surface (red line) and on the F₄TCNQ sites in the CTC (black line). (b) Long-range dI/dV spectra on TTF molecules in a single-component assembly on G/O/Ir(111) (blue line) and on the TTF sites in the CTC (black line). (c) Bias-dependent STM images of the CTC at the sample biases indicated in the figure. The size of each image is 4.7 × 3.2 nm².

Figure 3

Short-range dI/dV spectroscopy on the CTC. (a) Short-range dI/dV spectra on the TTF and F₄TCNQ sites in the CTC showing a dip at zero bias. (b) Magnetic field dependent dI/dV spectra on a TTF site in the CTC showing that the shape and size of the zero-bias dip do not change with magnetic field up to 10 T. (c) Temperature-dependent dI/dV spectra on a TTF site in the CTC showing that the dip is washed away with increasing temperature and the asymmetric background is also decreased at higher temperatures. (d) Temperature dependence of the zero-bias conductance (ZBC, normalized at the dI/dV at a bias of 20 mV) showing saturation at 15–20 K.

Figure 4

Deconvoluting the low-bias features of the dI/dV spectra. (a) Short-range dI/dV spectrum on TTF molecules. The curve has been fitted with the sum of two Fano functions: Fano-1 (broken black line) represents the central dip and Fano-2 (red line) represents the step. The final fit is indicated by a blue line. (b) Temperature-dependent evolution of HWHM extracted from the two Fano functions (Fano-1: left, Fano-2: right) from the fits. (c) Short-range dI/dV spectrum on CTC islands, recorded on an F₄TCNQ molecule showing steps at energies ∼2 (shown by arrow 1), ∼31 (arrow 2), ∼35 (arrow 3), and ∼52 meV (arrow 4). (d) Temperature-dependent evolution of the steps at ∼2 meV (step-1: left) and at ∼52 meV (step-4: right).

Figure 5

(a) STM topography image of a CTC at imaging parameters 5 pA and −500 mV. The scale bar is 3 nm. (b) Contrast-optimized version of the topography in panel (a) showing periodic topography modulations (white lines are a guide to the eyes). (c, d) Two-dimensional fast-Fourier transform (2D-FFT) of panel (a) showing features corresponding to the CTC rectangular lattice (marked by vectors b₁ and b₂), spots due to the underlying graphene moiré (white hexagon), and charge density wave modulations by vectors u₁, u₂, and u₃. CDW wavelengths corresponding to u₁ and u₂ are approximately 3.25 × l₁ and 3.25 × l₂, while that corresponding to u₃ is ∼5 nm. The scale bar is 1 nm^–1.