Two-dimensional metal-organic coordination networks of Mn-7,7,8,8-tetracyanoquinodimethane assembled on Cu(100): Structural, electronic, and magnetic properties

A compound two-dimensional (2D) monolayer mixing Mn atoms and 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules was synthesized by supramolecular assembly on a Cu(100) surface under ultrahigh vacuum conditions. Its structural, electronic, and magnetic properties were analyzed by scanning tunneling microscopy experiment and theory, low-energy electron diffraction, x-ray photoemission spectroscopy, and density-functional theory calculations. The 2D compound has a long-range ordered square planar network structure consisting of fourfold coordinated Mn centers with a Mn:TCNQ ratio of 1:2. The electronic-state analysis revealed a complex charge-transfer scenario indicating strong bonding of TCNQ with both the Mn adsorbates and the Cu surface atoms. The calculations reveal that the Mn centers carry a magnetic moment close to $5{\mu }_{\text{B}}$ with very weak coupling between adjacent Mn centers.

Figure 1

(Color online) (a) STM topograph of the Mn-TCNQ assembly on a Cu (100) surface. (b) High-resolution STM topograph of the $\text{Mn}{\left(\text{TCNQ}\right)}_2$ network. (c) LEED pattern of the $\text{Mn}{\left(\text{TCNQ}\right)}_2$ network. Inset: chemical structure of the molecule TCNQ. [(d) and (e)] Top and side views of the DFT-optimized structure of the $\text{Mn}{\left(\text{TCNQ}\right)}_2$ network on the Cu(100) surface color code: Cu red, C gray, N blue, H white, and Mn purple. STM images recorded at (a) 0.4 nA and $-0.96\text{V}$ and (b) 0.3 nA and $-0.72\text{V}$. LEED image (c) recorded at 16 eV.

Figure 2

(Color online) XPS $\text{N}1s$ signal of (a) TCNQ powder, (b) TCNQ monolayer grown on Cu(100), and (c) $\text{Mn}{\left(\text{TCNQ}\right)}_2$ networks grown on Cu(100).

Figure 3

(Color online) (a) Simulated constant-current STM image at 0.5 nA and 0.5 V (giving an average tip-to-molecule height of $5\text{Å}$), using DFT results for the $\text{Mn}{\left(\text{TCNQ}\right)}_2$ network structure on Cu(100). (b) Same as (a) but with a Hubbard $U$ correction, $U_{eff}=4.2\text{eV}$. [(c) and (d)] Spin PDOS for Mn and TCNQ adsorbed on a Cu(100) surface, calculated with VASP without Hubbard $U$ (c), and using $U_{eff}=4.2\text{eV}$ (d). The PDOS of Mn is indicated in blue. The PDOS of the TCNQ molecules is indicated as red solid lines, with positive (negative) values for the majority-spin (minority-spin) PDOS. Square unit cells indicated in (a) and (b) with side length 1.28 nm.

Figure 4

(Color online) (a) The PDOS of the top copper layer, of the TCNQ molecule, and of the $\text{Mn}{d}_{xz}$ and $d_{x^2-y^2}$ orbitals. (b) The PDOS of the $\text{Mn}{d}_{xz}$ $\left(d_{yz}\right)$ and $d_{z^2}$ orbitals and of the four N atoms bonded to Cu. Note that the PDOS of the $d_{xz}$ and $d_{yz}$ orbitals are equal.