Anisotropic Rashba splitting in Pt-based Janus monolayers PtXY (X,Y = S, Se, or Te)

Recent studies have demonstrated the feasibility of synthesizing two-dimensional (2D) Janus materials which possess intrinsic structural asymmetry. Hence, we performed a systematic first-principles study of 2D Janus transition metal dichalcogenide (TMD) monolayers based on PtXY (X,Y = S, Se, or Te). Our calculated formation energies show that these monolayer Janus structures retain the 1T phase. Furthermore, phonon spectral calculations confirm that these Janus TMD monolayers are thermodynamically stable. We found that PtSSe, PtSTe, and PtSeTe exhibit an insulating phase with indirect band gaps of 2.108, 1.335, and 1.221 eV, respectively, from hybrid functional calculations. Due to the breaking of centrosymmetry in the crystal structure, the spin-orbit coupling (SOC)-induced anisotropic Rashba splitting is observed around the M point. The calculated Rashba strengths from M to Γ (α ^M-Γ _R) are 1.654, 1.103, and 0.435 eV Å^-1, while the calculated values from M to K (α ^M-K _R) are 1.333, 1.244, and 0.746 eV Å^-1, respectively, for PtSSe, PtSTe, and PtSeTe. Interestingly, the spin textures reveal that the spin-splitting is mainly attributed to the Rashba effect. However, a Dresselhaus-like contribution also plays a secondary role. Finally, we found that the band gaps and the strength of the Rashba effect can be further tuned through biaxial strain. Our findings indeed show that Pt-based Janus TMDs demonstrate the potential for spintronics applications.

Dimensionality-Mediated Semimetal-Semiconductor Transition in Ultrathin PtTe_{2} Films

Platinum ditelluride (PtTe_{2}), a type-II Dirac semimetal, remains semimetallic in ultrathin films down to just two triatomic layers (TLs) with a negative gap of -0.36  eV. Further reduction of the film thickness to a single TL induces a Lifshitz electronic transition to a semiconductor with a large positive gap of +0.79  eV. This transition is evidenced by experimental band structure mapping of films prepared by layer-resolved molecular beam epitaxy, and by comparing the data to first-principles calculations using a hybrid functional. The results demonstrate a novel electronic transition at the two-dimensional limit through film thickness control.

Engineering Surface Structure of Spinel Oxides via High-Valent Vanadium Doping for Remarkably Enhanced Electrocatalytic Oxygen Evolution Reaction

Spinel oxides (AB₂O₄) with unique crystal structures have been widely explored as promising alternative catalysts for efficient oxygen evolution reactions; however, developing novel methods to fabricate robust, cost-effective, and high-performance spinel oxide based electrocatalysts is still a great challenge. Here, utilizing a complementary experimental and theoretical approach, pentavalent vanadium doping in the spinel oxides (i.e., Co₃O₄ and NiFe₂O₄) has been thoroughly investigated to engineer their surface structures for the enhanced electrocatalytic oxygen evolution reaction. Specifically, when the optimal concentration of vanadium (ca. 7.7 at. %) is incorporated into Co₃O₄, the required overpotential to reach a certain j_GEOM and j_ECSA decreases dramatically for oxygen evolution reactions in alkaline media. Even after 30 h of chronopotentiometry, the required potential for V-doped Co₃O₄ just increases by 16.3 mV, being much lower than that of the undoped one. It is observed that the pentavalent vanadium doping introduces lattice distortions and defects on the surface, which in turn exposes more active sites for reactions. DFT calculations further reveal the rate-determining step changing from the step of *-O to *-OOH to the step of *-OH to *-O, while the corresponding energy barriers decrease from 1.73 to 1.57 eV accordingly after high-valent V doping. Moreover, the oxygen intermediate probing method using methanol as a probing reagent also demonstrates a stronger OH* adsorption on the surface after V doping. When vanadium doping is performed in the inverse spinel matrix of NiFe₂O₄, impressive performance enhancement in the oxygen evolution reaction is as well witnessed. All these results clearly illustrate that the V doping process can not only efficiently improve the electrochemical properties of spinel transition metal oxides but also provide new insights into the design of high-performance water oxidation electrocatalysts.

CO-induced Pd segregation and the effect of subsurface Pd on CO adsorption on CuPd surfaces

We report results of our study on the adsorption of CO on CuPd surfaces with bulk stoichiometric and nonstoichiometric layers using density functional theory (DFT). We found that the presence of Pd atoms in the subsurface layer promotes the adsorption of CO. We also observed CO-induced Pd segregation on the CuPd surface and we attribute this to the strong CO-Pd interaction. Lastly, we showed that the adsorption of CO promotes Pd-Pd interaction as compared to the pristine surface which promotes strong Cu-Pd interaction. These results indicate that CO adsorption on CuPd surfaces can be tuned by taking advantage of the CO-induced segregation and by considering the role of subsurface Pd atoms.