Electronic and vibrational properties of ${\mathrm{V}}_2\mathrm{C}$-based MXenes: From experiments to first-principles modeling

In the present work, the electronic and vibrational properties of both pristine ${\mathrm{V}}_2\mathrm{C}$ and fully terminated ${\mathrm{V}}_2\mathrm{C}{T}_2$ (where $T=\mathrm{F},$ O, OH) two-dimensional monolayers are investigated using density functional theory. First, the atomic structures of ${\mathrm{V}}_2\mathrm{C}$-based MXene phases are optimized, and their respective dynamical stabilities are discussed. Second, electronic band structures are computed indicating that ${\mathrm{V}}_2\mathrm{C}$ is metallic as well as all the corresponding functionalized systems. Third, the vibrational properties (phonon frequencies and spectra) of ${\mathrm{V}}_2\mathrm{C}$-based MXenes are computed with the density functional perturbation theory and reported. Both Raman-active ($E_g$, $A_{1g}$) and infrared-active ($E_u$, $A_{2u}$) vibrational modes are predicted ab initio with the aim to correlate the experimental Raman peaks with the calculated vibrational modes and to assign them to specific atomic motions. The effect of the terminal groups on the vibrational properties is emphasized, along with the effect on the presence and position of the corresponding Raman peaks. Our results provide insights for the identification and characterization of ${\mathrm{V}}_2\mathrm{C}$-based samples using Raman spectroscopy.

Figure 1

Atomic structure of (a) pristine ${\mathrm{V}}_2\mathrm{C}$ and four models for terminated ${\mathrm{V}}_2{\mathrm{CT}}_2$: (b) MD1, (c) MD2, (d) MD3, and (e) MD4, top and side views. Vanadium and carbon atoms are in red (medium gray) and brown (dark gray), respectively, while the functional group ($T=\mathrm{F}$, O, OH) is in blue (light gray). In (a), A and B indicate the two types of hollow sites; $a$ and $d$ are the lattice constant and the layer thickness, respectively.

Figure 2

Electronic band structures of (a) ${\mathrm{V}}_2\mathrm{C}$, (b) ${\mathrm{V}}_2{\mathrm{CF}}_2$, (c) ${\mathrm{V}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2$, and (d) ${\mathrm{V}}_2\mathrm{CF}\left(\mathrm{OH}\right)$ in their high-symmetry configuration. The Fermi level is fixed as the reference of zero energy.

Figure 3

Local and projected densities of states of (a) ${\mathrm{V}}_2\mathrm{C}$, (b) ${\mathrm{V}}_2{\mathrm{CF}}_2$, and (c) ${\mathrm{V}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2$ in their high-symmetry configuration. The Fermi level is positioned at zero energy.

Figure 4

Schematic representations of the Raman-active modes of pristine ${\mathrm{V}}_2\mathrm{C}$ MXene: (a) and (b) in-plane vibration of V atoms at 224.4 ${\mathrm{cm}}^{-1}$ corresponding to the $E_g$ mode and (c) out-of-plane vibration of V atoms at 358.9 ${\mathrm{cm}}^{-1}$ corresponding to the $A_{1g}$ mode. Vanadium atoms are in red (light gray) and carbon atoms are in brown (dark gray).

Figure 5

Phonon band structures and densities of states of (a) ${\mathrm{V}}_2\mathrm{C}$, (b) ${\mathrm{V}}_2{\mathrm{CF}}_2$, (c) ${\mathrm{V}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2$, and (d) ${\mathrm{V}}_2\mathrm{CF}\left(\mathrm{OH}\right)$ in their high-symmetry nonmagnetic configuration. For the three systems, the phonon density of states for each atom is also plotted, allowing for better visualization of the contributing atoms in each normal mode.

Figure 6

(a) Scanning electron microscopy image and (b) Raman spectrum of the 3D MAX ${\mathrm{V}}_2\mathrm{AlC}$ and the 2D MXene produced by HF etching of a ${\mathrm{V}}_2\mathrm{AlC}$ sample. Note the sample consists of the stacking of numerous 2D layers, with curved paperlike morphology.

Figure 7

Raman spectrum of the exfoliated ${\mathrm{V}}_2\mathrm{C}$-based sample collected at room temperature. The calculated Raman-active frequencies and the total phonon densities of states of the ${\mathrm{V}}_2\mathrm{C}$, ${\mathrm{V}}_2{\mathrm{CF}}_2$, ${\mathrm{V}}_2\mathrm{C}{\left(\mathrm{OH}\right)}_2$, and ${\mathrm{V}}_2\mathrm{CF}\left(\mathrm{OH}\right)$ monosheets are also included under the experimental spectrum for comparison. The matching between the predicted normal-mode frequencies and the experimental spectrum confirms the presence of heterogeneous terminal groups randomly distributed at the ${\mathrm{V}}_2\mathrm{C}$ surface.