Platinum-based transition metal dichalcogenides have been gaining renewed interest because of the development of a new method to synthesize thin film structures. Here, using first-principles calculation, we explore the electronic properties of PtX 2 (X = S, Se, and Te) with respect to film thickness. For bulk and layered structures (1 to 10 layers), octahedral 1T is the most stable. Surprisingly, we also find that the 3R structure has comparable stability relative to the 1T, implying possible synthesis of 3R. For a bulk 1T structure, PtS 2 is semiconducting with an indirect band gap of 0.25 eV, while PtSe 2 and PtTe 2 are both semi-metallic. Still, all their corresponding monolayers exhibit an indirect semiconducting phase with band gaps of 1.68, 1.18, and 0.40 eV for PtS 2 , PtSe 2 , and PtTe 2 , respectively. For the band properties, we observe that all these materials manifest decreasing/closing of indirect band gap with increasing thickness, a consequence of quantum confinement and interlayer interaction. Moreover, we discover that controlling the thickness and applying strain can manipulate van Hove singularity resulting to high density of states at the maximum valence band. Our results exhibit the sensitivity and tunability of electronic properties of PtX 2 , paving a new path for future potential applications.

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Spinel oxides (AB 2 O 4 ) 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., Co 3 O 4 and NiFe 2 O 4 ) 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 Co 3 O 4 , 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 Co 3 O 4 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 NiFe 2 O 4 , 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.

Among the families of transition metal dichalcogenides (TMDs), Pd-based TMDs have been one of the less explored materials. In this study, we investigate the electronic properties of PdX 2 (X = S, Se, or Te) bulk and thin films. The analysis of structural stability shows that the bulk and thin film (1 to 5 layers) structures of PdS 2 exhibit pyrite, while PdTe 2 exhibits 1T. Furthermore, PdSe 2 exhibits pyrite in bulk and thin films down to the bilayer. Most surprisingly, PdSe 2 monolayer transits to 1T phase. For the electronic properties of the stable bulk configurations, pyrite PdS 2 and PdSe 2 , and 1T PdTe 2 , demonstrate semi-metallic features. For monolayer, on the other hand, the stable pyrite PdS 2 and 1T PdSe 2 monolayers are insulating with band gaps of 1.399 eV and 0.778 eV, respectively, while 1T PdTe 2 monolayer remains to be semi-metallic. The band structures of all the materials demonstrate a decreasing or closing of indirect band gap with increasing thickness. Moreover, the stable monolayer band structures of PdS 2 and PdSe 2 exhibit flat bands and diverging density of states near the Fermi level, indicating the presence of van Hove singularity. Our results show the sensitivity and tunability of the electronic properties of PdX 2 for various potential applications.

Ultrathin Janus two-dimensional (2D) materials are attracting intense interest currently. Substitutional doping of 2D transition metal dichalcogenides (TMDs) is of importance for tuning and possible enhancement of their electronic, physical and chemical properties toward industrial applications. Using systematic first-principles computations, we propose a class of Janus 2D materials based on the monolayers MX 2 (M = V, Nb, Ta, Tc, or Re; X = S, Se, or Te) with halogen (F, Cl, Br, or I) or pnictogen (N, P, As, Sb, or Bi) substitution. Nontrivial phases are obtained on pnictogen substitution of group VB (V, Nb, or Ta), whereas for group VIIB (Tc or Re), the nontrivial phases are obtained for halogen substitution. Orbital analysis shows that the nontrivial phase is driven by the splitting of M-d yz and M-d xz orbitals. Our study demonstrates that the Janus 2D materials have the tunability and suitability for synthesis under various conditions.

The electronic and magnetic properties of transition metal dichalcogenides are known to be extremely sensitive to their structure. In this paper we study the effect of structure on the electronic and magnetic properties of mono-and bilayer VSe 2 films grown using molecular beam epitaxy. VSe 2 has recently attracted much attention due to reports of emergent ferromagnetism in the two-dimensional (2D) limit. To understand this compound, high-quality 1T and distorted 1T films were grown at temperatures of 200°C and 450°C, respectively, and studied using 4 K scanning tunneling microscopy and spectroscopy. The measured density of states and the charge density wave (CDW) patterns were compared to band structure and phonon dispersion calculations. Films in the 1T phase reveal different CDW patterns in the first layer compared to the second. Interestingly, we find the second layer of the 1T film shows a CDW pattern with 4a × 4a periodicity which is the 2D version of the bulk CDW observed in this compound. Our phonon dispersion calculations confirm the presence of a soft phonon at the correct wave vector that leads to this CDW. In contrast, the first layer of distorted 1T phase films shows a strong stripe feature with varying periodicities, while the second layer displays no observable CDW pattern. Finally, we find that the monolayer 1T VSe 2 film is weakly ferromagnetic, with ∼3.5 μ B per unit similar to previous reports.

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.

Probing the effects of thin-film thickness on transition metal dichalcogenides offer novel insights into their electronic properties and tunability, which leads to a new avenue of research and applications. A comprehensive first-principles study on thickness-dependent structural stabilities and electronic properties of ZrX2 (X = S, Se, or Te) thin films from 1 layer (L) to 6L and bulk was performed. The calculated formation energies show that ZrX2 adopts the 1T phase as the most stable structure. Furthermore, 1T-ZrS2 and ZrSe2 thin films and bulk are indirect semiconductors and their band gaps decrease as the number of layers is increased up to 6L, while 1T-ZrTe2 thin films and bulk are semimetallic. Interestingly, we demonstrate that the surface band structure of bulk and monolayer ZrTe2 under generalized gradient approximation + U and HSE06 methods is in excellent agreement with the angle-resolved photoelectron spectroscopy measurement. Finally, we discover the existence of van Hove singularities in strained 2L and unstrained 3L 1T-ZrS2 thin films, implying the existence of superconductivity in these thin films. These results showcase the tunable electronic properties of ZrX2 thin films because of thickness dependence and strain.