Phase-selective in-plane heteroepitaxial growth of H-phase CrSe2

Phase engineering of two-dimensional transition metal dichalcogenides (2D-TMDs) offers opportunities for exploring unique phase-specific properties and achieving new desired functionalities. Here, we report a phase-selective in-plane heteroepitaxial method to grow semiconducting H-phase CrSe2. The lattice-matched MoSe2 nanoribbons are utilized as the in-plane heteroepitaxial template to seed the growth of H-phase CrSe2 with the formation of MoSe2-CrSe2 heterostructures. Scanning tunneling microscopy and non-contact atomic force microscopy studies reveal the atomically sharp heterostructure interfaces and the characteristic defects of mirror twin boundaries emerging in the H-phase CrSe2 monolayers. The type-I straddling band alignments with band bending at the heterostructure interfaces are directly visualized with atomic precision. The mirror twin boundaries in the H-phase CrSe2 exhibit the Tomonaga-Luttinger liquid behavior in the confined one-dimensional electronic system. Our work provides a promising strategy for phase engineering of 2D TMDs, thereby promoting the property research and device applications of specific phases.


MBE growth and STM characterization of MoSe2 on the HOPG substrate
The growth temperature has been demonstrated to be one of the key factors in the MBE growth of MoSe2 nanoribbons by previous works 1,2 .When the substrate temperature is lower than 250 °C, the MBE-grown MoSe2 flakes have fractal shapes (as shown in Fig. S1a) due to the lower mobility of adatoms around the island edges.As the temperature is elevated, 2D MoSe2 islands with higher crystallinity can be obtained (Fig. S1b).Meanwhile, the density and length of MTBs in the MoSe2 monolayers can be controlled by tuning the growth temperature.More isolated and longer MTBs will be got at higher temperature (Fig. S1d).In the long MTBs, the STM instensity modulations can be observed with nonuniform amplitudes along the MTB.As shown in the Fig. S1d, the modulations are strongest near the edge of MTBs and decrease gradually from the edge to the middle postion of MTBs.The behavior can be decribed by the Friedel oscillations 4 .The MoSe2 nanoribbons with well-defined orientations form at the substrate temperature kept at about 550 °C (Fig. S1e).The monolayer and bilayer MoSe2 nanoribbons with straight edges obtained in our experiments are shown in Fig. S1e.Based on the previous study of MoSe2 nanoribbons 1,3 , the lefe edge of the MoSe2 nanoribbon in the close-up STM image (Fig. S1f) can be identified as the Se-edge and the right edge is the Mo-edge.
Apart from the growth temperature, the Se:Mo flux also has the equivalent effects on the growth favouring nanoribbon growth at lower Se concentrations.The evolution of MoSe2 island shapes is determined by the relative energies and growth rates of the different edge structures.The atomic growth mechanism of MoSe2 nanoribbon has been revealed by DFT calculations in the reported work 1 .The armchair edges were calculated to have the higher energy than the zigzag (Mo-and Se-terminated) edges.Therefore, the armchair edges grow much faster than the zigzag edges, which results in the ribbon structures.
The width control of MBE-grown MoSe2 nanoribbons through growth temperature has been demonstrated by previous works, in which the width will gradually decrease as the growth temperature is elevated 2 .The same tendency in the MBE growth of MoSe2 nanoribbons can also be observed in our experiments.The thickness can be controlled by tuning the growth duration.The density control of MoSe2 nanoribbons can be achieved by tuning the growth parameters of flux rate and growth duration.At the optimized growth parameters of growth temperature and Se:Mo flux, the 1D MoSe2 nanoribbons are able to be controlled at the monolayer level by tuning the flux rate and growth duration.Regarding to MoSe2 nanoribbons grown at 550 °C in our experiments, the statistical analysis of width and layer numbers was carried out (Fig. S1g and 1h).The widths of MoSe2 nanoribbons are mostly distributed from 10 to 25 nm.The statistical analysis on thickness distribution indicates the MoSe2 nanoribbons are mostly monolayer and bilayer.The band profile of the monolayer MoSe2 nanoribbon with a length of ~20 nm is directly visualized by the 2D plot of dI/dV spectra across the nanoribbon (Fig. S1j).The edge states can be observed at the edge termination due to the existence of dangling bongs.The upward band bending near the edge can be ascribed to the dangling electron (hole) states, which is consistent with the previous report 5 .

STM characterization of subsequently grown chromium selenide at different growth temperatures and deposition duration
In our experiment, Cr and Se atoms are codeposited onto the HOPG substrate after successful preparation of MoSe2 nanoribbons.Excessive selenium atoms were sublimed to maintain the Se-rich environment during the growth.The lowest growth temperature was set at 120 °C for thermally desorbing the excessive Se atoms from the substrate.At the grow temperature of 120 °C, only low-quality film of chromium selenide can be obtained (as shown in Fig. S2a).Thermal energy is essential for atoms diffusing and nucleating to form the better films.However, when the growth temperature is higher than 300 °C, most samples are the Cr2Se3, whose heights are 0.3 nm higher than that of MoSe2 (Fig. S2b).Thereofore, the proper substrate temperature was chosen at 180-250°C to grow 1H-CrSe2.When we deposite a small amount of Cr atoms, the MoSe2-CrSe2 lateral heterostructures accompanied with narrow-width CrSe2 nanoribbons can be obtained (Fig. S2c and d).When we increase the deposition duration, the isolated CrSe2 islands grown on the top of MoSe2 can be observed (as shown in Fig. S2e and f).Most samples are the 1T phase with non-layered structures.

Mechanism exploration of the in-planed template induced selective growth of H-phase CrSe2
When the spin polarization and Coulomb interaction were considered, the T-phase CrSe2 is calculated to have a larger lattice constant and a lower energy than the H-phase structure.However, as the lattice is restricted by the in-plane template of MoSe2 nanoribbons (3.3A), the H-phase CrSe2 is calculated to be the more stable configuration.To uncover the effect of in-plane epitaxial template, DFT calculations were carried out to simulate the binding energies of different numbers of H-phase and T-phase CrSex radicals combined at the Se-edge of the MoSe2 nanoribbon (Fig. S3a).The H-phase radicals possess the larger binding energies compared with the result of T-phase ones.The DFT calculations indicate that the formation of H-phase radicals at the edges of MoSe2 nanoribbons is energetically preferred.As the number of the CrSex radical increases and the radicals nucleate at the edges, the heterostructures with the 1H-1H or 1H-1T interface structures can be formed.Compared with the simulated interfacial structures between 1T-CrSe2 and 1H-MoSe2 monolayers, the H-phase CrSe2 structures connected to the Mo-edge and Se-edge of MoSe2 nanoribbon are demonstrated to be the more stable configuration than the T-phase structures (Fig. S3b).The continuity of 1H phase structures can avoid the occurrence of 1H-1T structures with higher interfacial energy.

Tomonaga−Luttinger liquid (TLL) behavior in the (quasi) onedimensional metallic MTBs
For 2D or 3D metallic systems, the electronic behavior can be described by Laudau Fermi liquid (FL) theory of non-interacting quasiparticles.When the electrons are confined in 1D systems, the quasiparticle excitation mechanism breaks down and electrons become a strongly correlated quantum liquid obeying the Tomonaga-Luttinger liquid (TLL) behavior 6,7 .As (quasi) one-dimensional metallic systems, TLL behavior has been revealed in the MTBs.As shown in Fig. S9b, the gap size increases with the length of MTBs getting shorter, which is the signature of TLL behavior.In the TLL theory, the energy gap of the finite system with length L can be described as Egap = [(πvc/2Kc) + (πvs/2Ks)](1/L), where vc and vs stand for the velocity of charge and spin excitation, respectively.Two Luttinger parameters Kc and Ks encode the interaction strength.Another signature of TLL behavior is the spin-charge separation which has the distinct dispersions of spin and charge excitations with velocities vs and vc.In the Fourier transformation of 2D plot of dI/dV spectra which can directly reveal the dispersion of confined states, two linear dispersion branches with different slopes corresponding to the spin and charge density excitations can be observed.