Interfacial and electronic properties of heterostructures of MXene and graphene

MXene-based heterostructures have received considerable interest owing to their unique properties. Herein, we examine various heterostructures of the prototypical MXene $\mathrm{T}{\mathrm{i}}_3{C}_2{T}_2$ ($T=\mathrm{O}$, OH, F; terminal groups) and graphene using density-functional theory. We find that the adhesion energy, charge transfer, and band structure of these heterostructures are sensitive not only to the surface functional group, but also to the stacking order. Due to its greatest difference in work function with graphene, $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\left(\mathrm{OH}\right)}_2$ has the strongest interaction with graphene, followed by ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{O}}_2$ and then ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{F}}_2$. Electron transfers from $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\left(\mathrm{OH}\right)}_2$ to graphene but from graphene to ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{O}}_2$ and ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{F}}_2$, which causes a shift in the Dirac point of the graphene bands in the heterostructures of monolayer graphene and monolayer MXene. In the heterostructures of bilayer graphene and monolayer MXene, the interface breaks the symmetry of the bilayer graphene; in the case of the AB-stacking bilayer, the electron transfer leads to an interfacial electric field that opens up a gap in the graphene bands at the $K$ point. This internal polarization strengthens both the interfacial adhesion and the cohesion between the two graphene layers. The MXene-graphene-MXene and graphene-MXene-graphene sandwich structures behave as two mirror-symmetric MXene-graphene interfaces. Our first-principles studies provide a comprehensive understanding for the interaction between a typical MXene and graphene.

Figure 1

Optimized geometries and band structures of the monolayer $\mathrm{T}{\mathrm{i}}_3{C}_2{T}_2\_\mathrm{graphene}$ heterostructure. (a), (b) ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{O}}_2\_\mathrm{graphene}$; (c), (d) $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\left(\mathrm{OH}\right)}_2\_\mathrm{graphene}$; (e), (f) ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{F}}_2\_\mathrm{graphene}$. The direction and number of electrons transferred between MXene and graphene are also given. The brown points in the band structures stand for the projected band structures of graphene and their sizes represent the magnitude of contribution. Fermi level is indicated by a red line in the band structure.

Figure 2

Electron-density difference at the interface of (a) ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{O}}_2\_\mathrm{graphene}$ ($\mathrm{MO}\_\mathrm{G}$), (b) $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\left(\mathrm{OH}\right)}_2\_\mathrm{graphene}$ ($\mathrm{MOH}\_\mathrm{G}$), and (c) ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{F}}_2\_\mathrm{graphene}$ ($\mathrm{MF}\_\mathrm{G}$). The excess (yellow) and depleted (cyan) electrons are shown: the isosurface value is $4\times {10}^{-4}\mathrm{e}/{\AA }^3$ for (a) and (b); $1.5\times {10}^{-4}\mathrm{e}/{\AA }^3$ for (c).

Figure 3

Optimized geometries and band structures of the heterostructures of $\mathrm{T}{\mathrm{i}}_3{C}_2{T}_2$ with AA-stacking bilayer graphene (${2\mathrm{G}}_{\mathrm{AA}}$). (a)–(c) ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{O}}_2\_{2\mathrm{G}}_{\mathrm{AA}}$; (d)–(f) $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\left(\mathrm{OH}\right)}_2\_{2\mathrm{G}}_{\mathrm{AA}}$; (e)–(i) ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{F}}_2\_{2\mathrm{G}}_{\mathrm{AA}}$. The direction and number of electron transfer between MXene and graphene bilayer are also given. The brown (blue) points in the band structures stand for the projected band structures of the lower (upper) graphene layer; their sizes represent the magnitude of contribution. The Fermi level is indicated by a red line in the band structure.

Figure 4

Optimized geometries and band structures of the heterostructures of $\mathrm{T}{\mathrm{i}}_3{C}_2{T}_2$ with AB-stacking bilayer graphene. (a)–(c) ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{O}}_2\_{2\mathrm{G}}_{\mathrm{AB}}$; (d)–(f) $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\left(\mathrm{OH}\right)}_2\_{2\mathrm{G}}_{\mathrm{AB}}$; (e)–(i) ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{F}}_2\_{2\mathrm{G}}_{\mathrm{AB}}$. The direction and number of electron transfer between MXene and graphene bilayer are also given. The brown (blue) points in the band structures stand for the projected band structures of the lower (upper) graphene layer; their sizes represent the magnitude of contribution. The Fermi level is indicated by a red line in the band structure.

Figure 5

Optimized geometries and band structures of the graphene-$\mathrm{T}{\mathrm{i}}_3{C}_2{T}_2$-graphene sandwich structures. (a)–(c) $\mathrm{G}\_{\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{O}}_2\_\mathrm{G}$; (d)–(f) $\mathrm{G}\_\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\left(\mathrm{OH}\right)}_2\_\mathrm{G}$; (e)–(i) $\mathrm{G}\_\mathrm{T}{\mathrm{i}}_3{C}_2{\mathrm{F}}_2\_\mathrm{G}$. Direction and number of electron transfer between MXene and graphene layers are also given. The brown (blue) points in the band structures stand for the projected band structures of the lower (upper) graphene layer; their sizes represent the magnitude of contribution. Fermi level is indicated by a red line in the band structure.

Figure 6

Electron-density difference at the interface of (a) $\mathrm{G}\_\mathrm{MO}\_\mathrm{G}$, (b) $\mathrm{G}\_\mathrm{MOH}\_\mathrm{G}$, (c) $\mathrm{G}\_\mathrm{MF}\_\mathrm{G}$ (d) $\mathrm{MO}\_\mathrm{G}\_\mathrm{MO}$, (e) $\mathrm{MOH}\_\mathrm{G}\_\mathrm{MOH}$, and (f) $\mathrm{MF}\_\mathrm{G}\_\mathrm{MF}$. The excess (yellow) and depleted (cyan) electrons are shown: the isosurface value is $4\times {10}^{-4}\mathrm{e}/{\AA }^3$ for (a) and (b); $1.5\times {10}^{-4}\mathrm{e}/{\AA }^3$ for (c).

Figure 7

Optimized geometries and band structures of the $\mathrm{T}{\mathrm{i}}_3{C}_2{T}_2\_\mathrm{graphene}\_\mathrm{T}{\mathrm{i}}_3{C}_2{T}_2$ sandwich structures. (a), (b) $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\mathrm{O}}_2\_\mathrm{G}\_\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\mathrm{O}}_2$; (c), (d) $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\left(\mathrm{OH}\right)}_2\_\mathrm{G}\_\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\left(\mathrm{OH}\right)}_2$; (e), (f) $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\mathrm{F}}_2\_\mathrm{G}\_\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\mathrm{F}}_2$. The direction and number of electron transfer between MXene and graphene layers are also given. The brown points in the band structures stand for the projected band structures of the graphene layer; their sizes represent the magnitude of contribution. Fermi level is indicated by a red line in the band structure.