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these parameters complicates the efforts in enhancing the thermoelectric performance. [1,3] To date, high ZT values have been obtained through enhancing electrical transport properties, reducing lattice thermal conductivities, and exploring the thermoelectric materials with intrinsically low thermal conductivity. [7][8][9][10][11][12][13][14][15][16][17][18][19] Lead chalcogenides are kinds of superior thermoelectrics because of their special physical and chemical properties. [20] In particular, PbTe and PbSe possess complex band structures (such as small energy offsets between light L and heavy Σ hole valence bands), which lead to larger band effective mass, higher Seebeck coefficients, and favor better electrical performance than that with single valence band. [9,[20][21][22] Besides, the intrinsic lattice thermal conductivities of lead chalcogenides are quite low due to the phonon anharmonicity. [20] Even that PbS possesses simple band structures and poor electrical and thermal transport properties, the features of low-cost, earth-abundant, higher melting point (1391 K), and larger energy band gap (0.41 eV) make PbS also an attractive thermoelectric candidate. [23][24][25] In the past few years, several approaches have been employed to enhance the thermoelectric performance of lead chalcogenides, such as enhancing electrical properties through introducing resonant states [8,26,27] and manipulating band structures, [28] suppressing thermal conductivities through introducing nanostructures, [25,[29][30][31][32] and all-scale hierarchical structures. [10,33] Among these methods, elements alloying for the solid solution with atomic-scale substitutions have been the most generally employed. Indeed, promising thermoelectric performance was achieved through simple elements alloying in the Pb-Sn-Te-Se solid solution system. [34] Similarly, extraordinary thermoelectric performance has been achieved in PbTe-PbSe, [28,35] PbTe-PbS, [36][37][38] PbTe-MTe (M = Mg, Sr), [10,33,39] PbSe-PbS, [40,41] PbSe-CdSe, and PbS-CaS solid solutions. [22,25] The above achievements in lead chalcogenides motivate us to fully investigate the thermoelectric transport properties of p-type PbTe-PbSe-PbS alloys and deeply understand this system, in which the carrier concentration was fixed with 2 mol% Na doping. Specifically, an ultrahigh power factor ≈25 µW cm −1 K −2 and a low lattice thermal conductivity ≈0.5 Wm −1 K −1 at 873 K, and thus a high ZT ≈ 1.9 are obtained in Solid solution alloying is one of the quite powerful approaches to enhance thermoelectric performance because it can simultaneously optimize electrical and thermal transport properties. Herein, a comprehensive investigation on p-type PbTe-PbSe-PbS alloys is reported, in which the carrier concentration is fixed with 2 mol% Na doping. High thermoelectric performance is achieved via synergistically tuning carrier concentration, manipulating electronic band structure, introducing nanostructures, and separating phases. Thus, a high ZT value ≈1.9 is obtained in (PbTe) 1−x (PbSe) x...

Two-dimensional checkerboard lattice, the simplest line-graph lattice, has been intensively studied as a toy model, while material design and synthesis remain elusive. Here, we report theoretical prediction and experimental realization of the checkerboard lattice in monolayer Cu2N. Experimentally, monolayer Cu2N can be realized in the well-known N/Cu(100) and N/Cu(111) systems that were previously mistakenly believed to be insulators. Combined angle-resolved photoemission spectroscopy measurements, first-principles calculations, and tight-binding analysis show that both systems host checkerboard-derived hole pockets near the Fermi level. In addition, monolayer Cu2N has outstanding stability in air and organic solvents, which is crucial for further device applications.

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