For the recently emerging two-dimensional van der Waals ferroelectrics, their promising prospects in nanoelectronic applications are hindered by their low vertical polarizations. Despite recent experimental breakthroughs and theoretical high throughput screening of material database, the obtained vertical polarizations of two-dimensional ferroelectrics are still limited. Here we propose a strategy of constructing two-dimensional mixed-valence compounds. We show first-principles evidence that Sn₂S₃ monolayer can be synthesized via epitaxial growth of SnS on SnS₂ monolayer with degenerate mixed-valence bi-states. Such a system possesses a room-temperature robust vertical polarization higher than 10 pC/m, which can be switched via interchange of oxidation states between two layers crossing an energy-saving low barrier, either by applying an electric field or mechanical bending. If such Sn₂S₃ monolayer is utilized as a nanogenerator, an unprecedented alternating voltage of ~130 V can be generated by applying an oscillating driving force that repeatedly reverses its polarization. Moreover, it possesses multiple metastable phases with distinct electronic properties. The transformations between them are not only strain-tunable, but can also be controlled via external electric field or low frequency linearly polarized light. Due to the small energy difference between polar and nonpolar states with distinct thicknesses, akin to morphotoropic phases, an ultra-high piezoelectric coefficient (> 2700 pm/V) can be obtained in the phase transformation under pressure, which can be further greatly enhanced via critical doping.

对于最近出现的二维范德华铁电体，其在垂直电子方向的低极化阻碍了它们在纳米电子应用中的广阔前景。尽管最近在实验上取得了突破并且对材料数据库进行了理论上的高通量筛选，但是二维铁电体的垂直极化仍然受到限制。在这里，我们提出了一种构建二维混合价化合物的策略。我们显示出第一原理证据，即可以通过在SnS ₂上外延生长SnS来合成Sn ₂ S ₃单层具有简并混合价双态的单层。这样的系统具有高于10 pC / m的室温稳固垂直极化，可以通过施加电场或机械弯曲，通过跨越节能低势垒的两层之间的氧化态互换来进行切换。如果这样的Sn ₂ S ₃单层用作纳米发电机，通过施加反复反转其极化的振荡驱动力，可以产生约130 V的空前交流电压。此外，它具有多个具有不同电子特性的亚稳相。它们之间的变换不仅可以进行应变可调，而且还可以通过外部电场或低频线性偏振光进行控制。由于具有不同厚度的极性和非极性状态之间的能量差很小，类似于光致变相，因此在压力下的相变中可以获得超高压电系数（> 2700 pm / V），可以通过如下方法进一步提高压电系数关键掺杂。