Intercalation induced quasi-freestanding layer in TiSe$_2$

TL;DR

The paper investigates how potassium intercalation into TiSe2 affects its electronic structure, using μ-ARPES and core-level spectroscopy to track changes upon room-temperature deposition. It reports a conduction-band splitting into two branches, with the energy separation reaching approximately 130 meV at 40 K. Photon-energy dependent ARPES reveals one branch with two-dimensional character and the other with three-dimensional bulk dispersion, indicating the formation of a quasi-freestanding TiSe2 layer with K impurities occupying the top vdW gap. DFT calculations comparing monolayer and bulk TiSe2 support the experimental observations and show a lattice expansion along the c-axis leading to a 2D-3D crossover; PLD is suppressed upon intercalation. The study demonstrates dimensional control of 1T-TiSe2 via intercalation, offering a route to explore low-temperature phenomena like superconductivity.

Abstract

Angle-resolved photoemission spectroscopy is employed to study the electronic structure of bulk TiSe2 before and after doping with potassium impurities. A splitting in the conduction band into two branches is observed after room-temperature deposition. The splitting energy increases to approximately 130 meV when the sample is cooled to 40 K. One branch exhibits a non-dispersive two-dimensional feature, while other one shows the characteristics of three dimensional bulk band dispersion. Core level spectroscopy suggests that the K impurities predominantly occupy the intercalated sites within the van derWaals gap. The results indicate the formation of a quasi-freestandingTiSe2 layer. Additionally, doping completely suppresses the periodic lattice distortion in the surface region. These findings are further supported by density functional theory calculations, which compare the band structure of monolayer and bulk TiSe2 with experimental data. Thus, the dimensional and intrinsic electronic properties of 1T-TiSe2 can be controlled through the intercalation procedure used in this work.

Figures (5)

• Figure 1: (a-b) Se 3d and K 3p core levels as a function of K-deposition level, respectively. (c-d) ARPES spectra along the $L$-$A$-$L$ direction of the Brillouin zone taken from pristine and K-doped samples, respectively. (e) 2D and 3D representation of the hexagonal Brillouin zone of TiSe$_2$. (g-j) ARPES spectra as a function of increasing K-doping level. All spectra are taken at room temperature with 119 eV linear horizontal polarized lights.
• Figure 2: (a-b) Temperature dependence of Se 3d and K 3p core levels, respectively. Se 3d peaks are calibrated to the same height and same scaling is applied to K 3p peaks to visualize the changes. (c-d) Temperature dependent ARPES pectra along the $L$-$A$ (119 eV photon energy, k$_z$ = $A$) and $\overline{M}$-$\overline{\Gamma}$ (99 eV photon energy, k$_z$ = 0.17$\Gamma$$A$) directions, respectively. Orange arrows in d mark UB and LB. (e-f) Low temperature ARPES data of pristine (left panel) and S3 (right panel). Pristine sample is cleaved at 40 K sample temperature. FVBs due to the PLD are marked for the pristine sample.
• Figure 3: (a) Photon energy dependent ARPES maps taken from S3 along the $\overline{M}$ - $\overline{\Gamma}$. (b) k$_z$ versus k$_\parallel$ dispersion at the Fermi level. Dashed yellow and red lines represent the out of plane dispersion of LB and UB, respectively. (c-d) Fermi surfaces at k$_z$ = $\Gamma$ and $L$-points. Black hexagons in c and d represent the 2D Brillouin zone. Sample temperature was kept at 40 K during the measurements.
• Figure 4: (a-b) Experimental valence band spectra along the $M$ - $\Gamma$ - $M$ and $L$ - $A$ - $L$, directions, respectively. (c-d) Experimental conduction bands along the $M$ - $\Gamma$ and $L$ - $A$ directions, respectively. Calculated band structures of monolayer (red lines) and bulk (green lines) TiSe$_2$ are superimposed onto the measured band structures. Color contrast in a and b is enhanced to increase the visibility of valence bands. Experimental data are collected from S3 at 40 K sample temperature and spin orbit coupling is included in the band structure calculations.
• Figure 5: (a-b) Ball and stick representations of pristine TiSe$_2$ and K$_{0.25}$TiSe$_2$ supercell, respectively. Upper and lower panels display side and top views. (c-d) Electronic structures of pristine TiSe$_2$ and K$_{0.25}$TiSe$_2$ supercell along the $A$-$\Gamma$-$M$-$L$ direction. Supercell band structure is unfolded into the Brillouin Zone of TiSe$_2$ unit cell. In these band structure calculations, spin orbit coupling and U-term are not included.