Synthesis and topography of RuCl3]
Two structural phases of RuCl3 thin films were found in the growth process. First, β-RuCl3 chains and disordered α-RuCl3 fragments are self-assembled on the surface. Highly ordered α-RuCl3 monolayer appears and proliferates gradually in the annealing process. Figure 1(a)[(b)] shows the crystal structure of monolayer α-RuCl3 (β-RuCl3) where the edge (face)-sharing RuCl6-octahedra constitute the honeycomb lattice (zigzag chains) of α-form (β-form). In bulk, the β-phase is metastable and can be converted to α-phase at 450–600 oC 30 and Fig. 1(c) shows the RHEED patterns of the sample after preliminary growth, further deposition, and annealing from the top down. The preliminary growth of submonolayer RuCl3 gives the RHEED pattern [Top of Fig. 1(c)] of only β-phase and the corresponding STM image is given in Fig. 1 (d). Further deposition leads to the coexistence of α- and β-phase as shown in the RHEED pattern [middle of Fig. 1(c)]. Nanoscale STM topography [Fig. 1(e)] reveals compact domains of poorly ordered α-area and highly crystalized β-area. The image Fig. 1(f) gives the detailed structure of β-RuCl3 which shows neck-and-neck zigzag chains. The lattice constants of a = 5.5 Å, b = 6.1 Å are straightforward indications of β-RuCl331. Post annealing of fully covered monolayer film does not cause a significant change of lattice constant [bottom of Fig. 1(c)]. However, a structural phase transition is inferred by the STM topographic image [Fig. 1(h)], where a reduction of coverage [Fig. 1(g)] and a proliferation of α-RuCl3 monolayer with β-RuCl3 dressed in pieces can be seen. A blurred triangular lattice resolved at high tunneling resistance [Fig. 1(i)] stands for α-RuCl3 (See Fig. 3 for details) with a lattice constant of 6.19 Å, which can be well distinguished from the rectangular lattice of β-RuCl3. Comparing to 5.99 Å in the bulk31, the in-plane lattice constant of the α-RuCl3 layer is expanded by 3%, which fits the expectation for this heterostructure17.
Electronic properties and orbital textures of d-p bonds
Figure 2(a) shows the typical dI/dV experimental spectrum of α-RuCl3 monolayer (black solid line) and the calculated total DOS where majority and minority spins are represented by green and orange dashed lines respectively.The total DOS was given by the LSDA + SOC + U calculations, where the lattice constant a = b = 6.19 Å and a vacuum spacing of 15 Å were employed. The DFT calculations come out with a Mott phase and an energy splitting of t2g orbitals, which well explain the STS result. The Mott nature is supported by an ~ 0.6 eV full gap around Fermi level in the experimental curve. The ~ 1.0 eV peak-to-peak Mott gap is consistently given by both the experiment and calculation as marked in Fig. 2(a). In Fig. 2(b), the calculated partial density of states (PDOS) graph exhibits an ~ 2.0 eV splitting of t2g and eg orbitals due to the octahedral crystal field. The Cl-3p orbital occupies the valence band below \(\approx\)-2.0 eV, while the Ru-4d orbital resides between \(\approx\)-2.0 eV and \(\approx\)3.0 eV. Meanwhile, the orbital hybridization induces the mixture of p and d components in the whole energy scale. We projected the p orbitals onto the degeneracy-lifting basis of pπ and pσ orbitals and found that, specifically, the hybridization undergoes in the manner of t2g-pπ and eg-pσ bonding in Fig. 2(b). In the energy range of t2g orbitals, the PDOS of the pπ component is much larger than the pσ one due to the dominant pdπ bonding. In the energy range of eg orbitals, instead, the pσ component is much larger due to the pdσ bonding.
As expected, we observed distinct STM patterns in the energy ranges of different orbitals. Figure 3(a-f) are the typical constant-current STM images at indicated bias voltages. In the t2g and pπ dominating energy range (-3.0 to 1.5 eV), three protrusions are resolved in each surface unit cell forming a “Kagome-like” lattice. However, in the energy range of eg orbital, only one protrusion is observed appearing as a cluster of three, forming a triangular lattice of the trimers. Similar phenomena were observed in the STM images of CrI3 thin film33. Figure 3(g, h) show the simulated constant-current STM images, i.e. the isosurface of DOS integrated from Fermi level to the bias energy, based on Tersoff-Hamann model34.
Figure 3(g) is the typical simulated image at 0.9V while all the images of the t2g range show the same “Kagome-like” pattern. It is found that the images can be well explained by the two types of the Cl-3p states, specifically the pπ type for Fig. 3(g) and the pσ type for Fig. 3(h). In detail, the lobes of pπ are perpendicular to the corresponding Ru2Cl2 plane which is formed by two Ru ions and two bridging Cl ions [the case of experimental images Fig. 3(a-e) and the simulated image Fig. 3(g)], while the lobes of pσ lie in the Ru2Cl2 plane [the case of the experimental image Fig. 3(f) and the simulated image Fig. 3(h)]. The t2g (eg) orbital primarily hybridizes with the pπ (pσ) orbital based on the large orbital overlap and thus forms the pdπ (pdσ) bond. The STM images are dominated by the upper lobes of the topmost Cl- pπ and Cl- pσ states which reside on the opposite sides of the Cl ion and evolve into the vacuum. The distinct orientations of the Cl-p states of two types produce the shift (orange and green arrows) of image intensity with respect to the Cl atom sites (blue balls) as marked in Fig. 3(g, h). Thus, Cl- pπ and Cl- pσ orbitals directly link to the t2g and eg orbitals surrounding the Ru atoms, respectively. The same t2g- pπ textures across the fermi level clearly illustrates the monolayer system remains in Mott phase.