Fig. 1. (a) Schematic 2D structure with z direction confinement. (b) The z-direction confinement spectrum of one-dimensional quantum well model. (c) Band structure and density of states for quasi-two-dimensional electron gas.

Fig. 2. (a) The QHE measured in a 6-nm-thick graphite flake at 0.25 K. (b) Schematic illustrations of electron trajectories under different conditions, (c) Energy gaps for the so-called 2.5D QHE as a function of thickness.^[26] Reproduced with permission from Ref. [26].

Fig. 3. Schematic illustrations of (a) free 2D electrons, (b) interlayer-interacted 2D electrons, (c) correlated 2D electrons, and (d) twisted 2D electronic systems.

Fig. 4. (a), (b) FeSe monolayer epitaxially grown on SrTiO₃(001) substrate, showing superconducting transition temperature above 100 K.^[114] (c) Schematic picture of transfer process of isolated freestanding SrTiO₃ layers.^[115] (d), (e) Characterizations of the SrTiO₃ few unit cells.^[115] (f)–(h) Superconducting behavior of monolayered BSCCO.^[9] Reproduced with permission from Refs. [9,114,115].

Fig. 5. Magnetic hysteresis loops measured in a 5-layered MnBi₂Te₄, with Hall resistance R_xy quantization indicated by dashed lines in (a) and color map of R_xx in (b).^[126] (c)–(e) Morphology of a Sb-doped MnBi₂Te₄device with 8 monolayers.^[127] (f) Phase diagram of Sb-doped MnBi₂Te₄ in the parameter space of gap energy, doping ratio, and chemical potential.^[127] Reproduced with permission from Refs. [126,127].

Fig. 6. (a) Schematic picture of a drag system with two separated 2D electrons. (b) Schematic picture of a GaAs/AlGaAs double quantum well sample.^[152] (c), (d) Demonstration of Coulomb drag in graphene at zero magnetic field.^[147] Width of the Hall bar is about 1.5 μm. (e), (f) Super fluid condensate realized in double bilayer graphene drag system.^[142] The excitonic condensate, indicated by the red arrow, was examined in the top drag layer in a counter flow configuration at ν_T = 1. (b)–(f) Reproduced with permission from Refs. [142,147,152].

Fig. 7. (a) Schematic picture of a TBLG. (b) Illustration of the electronic band structure of a flat band (blue) induced by the magic angle moiré superlattice. (c) Schematic picture of the moiré superlattice with their stacking order marked with colors. A full filling of the mini flat band corresponds to 4 electrons in the moiré unit cell originated from the 4-fold degeneracy. (d) Summarization of state-of-the-art experimentally observed correlated quantum states in TBLG with the rotation close to the magic angle.

Fig. 8. (a) Schematic picture of a double gate programmable MoS₂ transistor, which is capable to integrate photoswitching logic and memory in a single cell.^[205] (b) Ballistic avalanche transistor made of InSe/BP heterostructure.^[206] (c) Ferroelectric domains realized by local patterning and the resulted photo detector with arbitrarily defined shapes.^[207] (d) A FinFET using monolayered TMDC to replace the conventional Si fin channel.^[208] Reproduced with permission from Refs. [205–208].