A stable low-symmetry T-phase of MSi2Z4 (M = Mo, W; Z = P, As) nanosheets with promising electronic and photovoltaic properties: Insight from first-principles calculations

The synthesis of MoSi2N4 nanosheets boosts research on the layered MA2Z4 materials, which can be viewed as a MoS2-like MZ2 sheet sandwiched between two AZ surfaces. Most studies focus on the trigonal-prismatic (H-phase) MA2Z4 nanosheets, while the stable octahedral (T-phase) MA2Z4 ones are rarely explored. Here, based on first-principles calculations, we have identified a stable low-symmetry T-phase (Tl-phase) geometry for the MSi2Z4 (M = Mo, W; Z = P, As) materials. Such Tl-MSi2Z4 nanosheets are stabilized by the trimerization of metal atoms, which is distinct from the well-known T′-phase MoS2 system that favors the metal dimerization instead. The investigated Tl-MSi2Z4 nanosheets are all direct-gap semiconductors with moderate gap sizes around 1 eV and hence exhibit a strong absorption capacity to visible lights. High hole mobilities of 104–105 cm2/V s appear in the Tl-MSi2Z4 nanosheets that are dozens to hundreds of times larger than the electron mobilities. Moreover, superior photovoltaic performances are present in these Tl-MSi2Z4 materials, whose power-conversion efficiencies are estimated up to 26.1%–31.8% in a few micrometer thickness. In particular, the photovoltaic efficiency of the Tl-MoSi2P4 system can surpass the Shockley–Queisser limit and reaches 36.9%–39.5% under the concentration of 100–1000 suns illumination. Our study demonstrates that peculiar distorted T-phase geometries can exist in the layered MA2Z4 family, which exhibit promising electronic, transport, and photovoltaic behaviors for nanoelectronics, nano-devices, and green-energy applications.