Two-dimensional CP2 and LixCP2 ( x=1 and 2) monolayer with high-mobility transport anisotropy and extraordinary optical properties

Two-dimensional ${\mathrm{CP}}₂$ and ${\mathrm{Li}}_x{\mathrm{CP}}₂$ ($x=1$ and 2) monolayers with high-mobility transport anisotropy, and tunable electronic and optical properties were identified through density-functional theory calculations. Our results indicate that the carrier mobility of ${\mathrm{CP}}₂$ monolayer is highly anisotropic with high hole mobility of $5.192\ensuremath{\sim}6.826\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{2\phantom{\rule{4pt}{0ex}}}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{--1\phantom{\rule{4pt}{0ex}}}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^--1$ along the $x$ direction. The ${\mathrm{CP}}₂$ exhibits linear dichroism and extraordinary optical absorption in the ultraviolet- to visible-light spectrum range. More importantly, the electronic and optical properties of ${\mathrm{CP}}₂$ can be turned via lithiation. A semiconductor to semimetal transition is revealed by lithiated ${\mathrm{CP}}₂$ with the formation of ${\mathrm{LiCP}}₂$. The ${\mathrm{LiCP}}₂$ shows a high Fermi velocity of $0.69\ifmmode\times\else\texttimes\fi{}{10}⁶\phantom{\rule{0.16em}{0ex}}\mathrm{m}/\mathrm{s}$ and strong optical absorbance in midwave-infrared regions. Further increasing the Li concentration leads to a semimetal to direct band-gap semiconductor transition with the formation of ${\mathrm{Li}}₂{\mathrm{CP}}₂$. The ${\mathrm{Li}}₂{\mathrm{CP}}₂$ has a high intrinsic conductivity with an anisotropic character (a high electron mobility of $4.665\ensuremath{\sim}7.336\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{2\phantom{\rule{4pt}{0ex}}}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{--1\phantom{\rule{4pt}{0ex}}}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^--1$ along the $y$ direction) and intense optical absorbance in near-infrared regions. All these desirable properties make ${\mathrm{CP}}₂, {\mathrm{LiCP}}₂$, and ${\mathrm{Li}}₂{\mathrm{CP}}₂$ promising for a wide range of future applications in a variety of technologies, especially in electronics, and photovoltaic and optoelectronic devices.