Characteristics of charge density waves on the surfaces of quasi-one-dimensional charge-transfer complex layered organic crystals

Abstract

We performed scanning tunneling microscope (STM) studies to observe the surface electronic structure of the following semiconducting charge-transfer complexes: dipropylamine-tetracyanoquinodimethane [DPA(TCNQ)${}_{2}$], triethylammonium [TEA (TCNQ)${}_{2}$], and $N$-ethylmorpholinium [HEM(TCNQ)${}_{2}$], which all possess the molecularly flat $\mathit{bc}$ surface terminated predominately with the quasi-one-dimensional (1D) TCNQ${}^{\ensuremath{-}0.5}$ anion chains or the corresponding DPA${}^{+}$, TEA${}^{+}$, and HEM${}^{+}$ cations contributing a lesser fraction. On the $\mathit{bc}$ surfaces terminated with TCNQ${}^{\ensuremath{-}0.5}$ anions for all crystals, the $4{k}_{F}$[DPA(TCNQ)${}_{2}$] or $2{k}_{F}$ [TEA(TCNQ)${}_{2}$ and HEM(TCNQ)${}_{2}$] charge density wave (CDW) driven by the spin-Peierls or Peierls instability for the quasi-1D TCNQ${}^{\ensuremath{-}0.5}$ chains, respectively, which occur already within the bulk, were probed by use of STM on the crystal surfaces. In that the surface lattice constants obtained from CDW are in good agreement with the bulk ones obtained by x-ray diffraction, such CDWs are commensurate. This modulation of the electric structure is so distinct that the geometric corrugation of individual TCNQ anions was hardly traced during our STM measurements. On the $\mathit{bc}$ surfaces terminated with DPA${}^{+}$, TEA${}^{+}$, and HEM${}^{+}$ cations, the STM images with submolecular resolution represent the geometric corrugation of individual molecular cations, and the surface lattice constants obtained deviate greatly from that of the x-ray crystal structures, indicating self-reassembling of the surface cations.