Scanning Tunneling Microscopy of the Organic Conductor [(η-C₅Me₅)₂Ru(η⁶,η⁶-[2₂](1, 4)-cyclophane)][TCNQ]₄

Scanning tunneling microscopy (STM) studies of the (001) and (010) faces, of the molecular semiconductor [(η-C₅Me₅)₂Ru(η⁶,η⁶-[2₂](1, 4)-cyclophane)]²⁺[TCNQ]₄^2- (1, TCNQ = tetracyanoquinodimethane) are reported. The lattice constants determined from the density of states corrugation of the STM images are a = 13.8 ± 0.3 Å, b = 15.9 ± 0.2 Å, c = 16.6 ± 2 Å, β = 88° ± 2°, γ = 82° ± 2°, in good agreement with the X-ray crystal structure. STM images of both faces reveal local density of states (LDOS) associated with stacking of TCNQ molecules along the [100] direction in two crystallographically unique stacks. The tunneling current contrast conforms to the tetrameric periodicity of the TCNQ stacks observed in the crystal structure. Columnar regions of negligible tunneling current on the ac face are attributed to stacks of (η-C₅Me₅)₂Ru(η⁶,η⁶-[2₂](1, 4)cyclophane)²⁺ dications. Each TCNQ column exhibits a tunneling current corrugation repeating at intervals of a that is attributed to tunneling into the conduction band of antiferromagnetic 2κ_F charge density wave (CDW) structure. The CDWs also exhibit corrugation, and antiphase modulation, at a/2 with respect to adjacent stacks. This is consistent with appreciable interstack Coulomb interactions and contributions from the magnetic 4κ_F structure, which based on the tight-binding approximation is equivalent to the canonical description (TCNQ)₂^-(TCNQ)₂^-. The STM data are in agreement with magnetic susceptibility and EPR studies, which indicate significant contribution of the 4κ_F state to the electronic structure of 1. The STM therefore provides characterization of the local electronic structure that is manifested in the bulk electronic properties of 1.