Metal-insulator transition in organic ion intercalated ${\mathrm{VSe}}_2$ induced by dimensional crossover

The charge-density wave (CDW) transition has been extensively studied in transition metal dichalcogenides (TMDs), the underlying mechanism and the related metal-insulator transition are not as simple as Peierls instability in one dimension and still under hot debate. Here, through electrochemical intercalation of organic ions, we have observed a dimensional crossover induced metal-insulator transition in an organic ion intercalated TMDs: ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. In pristine ${\mathrm{VSe}}_2$, previous studies have revealed a three-dimensional CDW transition at $T_{\mathrm{CDW}}\sim 110\mathrm{K}$ with a metallic ground state. After intercalation of organic ions, the remarkable anisotropy of resistivity indicates a highly two-dimensional electronic state in ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$, which is consistent with our density functional theory (DFT) calculation. Interestingly, the dimensional crossover enhances the CDW transition with $T_{\mathrm{CDW}}$ of 165 K and leads to an insulating ground state in ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. Moreover, a commensurate superstructure with $3a\times 3a$ periodicity is also confirmed in this insulating CDW state. Although the DFT calculation suggests that the commensurate superstructure and the enhanced CDW temperature could be ascribed to the improved Fermi surface nesting, whether the metal-insulator transition is driven by a perfect Fermi surface nesting is still elusive at the present stage. The possible role of electronic correlation and electron-phonon coupling has also been discussed. Our work provides a different material platform to study the metal-insulator transition in TMDs.

Figure 1

The structure of pristine ${\mathrm{VSe}}_2$ and ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. (a) The side view for structure of pristine ${\mathrm{VSe}}_2$ and ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. (b) The top view for structure of pristine ${\mathrm{VSe}}_2$. (c) The optical images of pristine ${\mathrm{VSe}}_2$ and ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. (d) The x-ray diffraction patterns for pristine ${\mathrm{VSe}}_2$ and ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$, showing a series of $\left(00l\right)$ diffraction peaks.

Figure 2

(a) Temperature dependence of in-plane resistivity for pristine ${\mathrm{VSe}}_2$. The inset shows the enlarged view of resistivity curve in the vicinity of phase transition for pristine ${\mathrm{VSe}}_2$. (b) The temperature dependence of out-of-plane resistivity for pristine ${\mathrm{VSe}}_2$. The inset shows ${\rho }_{\mathrm{c}}/{\rho }_{\mathrm{ab}}$ versus T for pristine ${\mathrm{VSe}}_2$. (c) The temperature dependence of in-plane resistivity for ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. The inset shows the enlarged view of resistivity curve in the vicinity of phase transition for ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. (d) The temperature dependence of out-of-plane resistivity for ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. The inset shows ${\rho }_{\mathrm{c}}/{\rho }_{\mathrm{ab}}$ versus T for ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. The CDW transition temperature for ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$ is determined by the mean value of middle transition points in cooling and warming curves.

Figure 3

(a), (b) Temperature dependence of magnetic susceptibility of pristine ${\mathrm{VSe}}_2$ and ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$ measured at 5 T. The insets show zoom-in of magnetic susceptibility in the vicinity of phase transition. The CDW transition temperature for ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$ is determined by the mean value of middle transition points in cooling and warming curves. (c) The heat capacity of pristine ${\mathrm{VSe}}_2$. The upper inset shows the enlarged view of heat capacity curve for pristine ${\mathrm{VSe}}_2$ in temperature range of 90–120 K. The lower inset shows $C_{\mathrm{p}}/T$ as a function of $T^2$ at low temperature. (d) The heat capacity of ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. The inset shows $C_{\mathrm{p}}/T$ as a function of $T^2$ at low temperature. These results definitely indicate that the metal-insulating transition around 165 K for ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$ is a bulk first-order phase transition, while the CDW transition of pristine ${\mathrm{VSe}}_2$ is second-order.

Figure 4

The selected area electron diffraction patterns of ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$ along [001] direction at different temperatures. (a) Electron diffraction pattern at 300 K; (b) Electron diffraction pattern at 130 K. The red arrows denote the superstructure diffraction peaks in the CDW state; (c)–(f) Fermi surface topology of ${\mathrm{VSe}}_2$ with interlayer distance of 6.12, 8.62, 11.02, and 18.62 Å, respectively. (g)–(j) Fermi surface topology with raising Fermi surface by 0.00, 0.05, 0.10, and 0.15 eV at an interlayer distance of 18.62 Å, corresponding nesting vectors are $1/4a^{*}$, $0.26a^{*}$, $0.29a^{*}$, $1/3a^{*}$ respectively.

Figure 5

The discharge curve of ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$ during the electrochemical intercalation process.

Figure 6

The schematic structure of tetrabutyl ammonium ($\mathrm{TB}{\mathrm{A}}^{+}$).

Figure 7

The cross-section high-resolution TEM images of pristine ${\mathrm{VSe}}_2$ single crystal and ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. (a) The low magnification image of pristine ${\mathrm{VSe}}_2$ single crystal. (b) The high magnification image of pristine ${\mathrm{VSe}}_2$ single crystal. The interlayer spacing of 6.1 Å corresponds to the monolayer height of $1T\text{−}{\mathrm{VSe}}_2$. (c) The low magnification image of ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. (d) The high magnification image of ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. The interlayer spacing is 18.2 Å, which is consistent with the x-ray diffraction results.

Figure 8

(a) The in-plane SAED images of pristine ${\mathrm{VSe}}_2$. (b) The in-plane SAED images of ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$. The SAED results demonstrate that the in-plane lattice parameter of ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$ remains almost the same as pristine ${\mathrm{VSe}}_2$.

Figure 9

(a) Temperature dependence of magnetic susceptibility of pristine ${\mathrm{VSe}}_2$ single crystal measured at 5 T. The red solid line indicates the Curie-Weiss fitting of the susceptibility curves. (b) Temperature dependence of magnetic susceptibility of ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$ measured at 5 Tesla. The red solid line indicates the Curie-Weiss fitting of the susceptibility curves.

Figure 10

The magnetic field dependence of magnetic susceptibility $\left(M\text{−}H\right)$ with $H//ab$ plane for ${\left(\mathrm{TBA}\right)}_{0.3}{\mathrm{VSe}}_2$.

Figure 11

(a) Band structure and (b) Phonon-dispersion curve of the ${\mathrm{VSe}}_2$ with interlayer distance varying from 6.12 to 18.62 Å.