We have investigated strong optical nonlinearity of monolayer MoS2(1–x)Se2x across the exciton resonance, which is directly tunable by Se doping. The quality of monolayer alloys prepared by chemical vapor deposition is verified by atomic force microscopy, Raman spectroscopy, and photoluminescence analysis. The crystal symmetry of all of our alloys is essentially D 3h , as confirmed by polarization-dependent second-harmonic generation (SHG). The spectral structure of the exciton resonance is sampled by wavelength-dependent SHG (λ = 1000–1800 nm), where the SHG resonance red-shifts in accordance with the corresponding optical gap. Surprisingly, the effect of compositional variation turns out to be much more dramatic owing to the unexpected increase of B-exciton-induced SHG, which indeed dominates over the A-exciton resonance for x ≥ 0.3. The overall effect is therefore stronger and broader SHG resonance where the latter arises from different degrees of red-shift for the two exciton states. We report the corresponding absolute SHG dispersion of monolayer alloys, χ(2), as a function of Se doping. We believe that our finding is a critical step toward engineering highly efficient nonlinear optical van der Waals materials working in a broader performance range.

Precise control of the chemical valence or oxidation state of vanadium in vanadium oxide thin films is highly desirable for not only fundamental research, but also technological applications that utilize the subtle change in the physical properties originating from the metalinsulator transition (MIT) near room temperature. However, due to the multivalent nature of vanadium and the lack of a good understanding on growth control of the oxidation state, stabilization of phase pure vanadium oxides with a single oxidation state is extremely challenging. Here, we systematically varied the growth conditions to clearly map out the growth window for preparing phase pure epitaxial vanadium oxides by pulsed laser deposition for providing a guideline to grow high quality thin films with well-defined oxidation states ofA well pronounced MIT was only observed in VO 2 films grown in a very narrow range of oxygen partial pressure P(O 2 ). The films grown either in lower (< 10 mTorr) or higher P(O 2 ) (> 25 mTorr) result in V 2 O 3 and V 2 O 5 phases, respectively, thereby suppressing the MIT for both cases. We have also found that the resistivity ratio before and after the MIT of VO 2 thin films can be further enhanced by one order of magnitude when the films are further oxidized by post-annealing at a well-controlled oxidizing ambient. This result indicates that stabilizing vanadium into a single valence state has to compromise with insufficient oxidation of an as grown thin film and, thereby, a subsequent oxidation is required for an 3 improved MIT behavior. TextVanadium oxides are one of few binary oxides exhibiting intriguing strong correlation effects that are critically dependent upon the oxidation state of vanadium. . [6][7][8][9] In particular, the MIT near room temperature makes this phase most attractive. (3) V 2 O 5 (3d 0 , V +5 ) that is an insulator and has an orthorhombic layered structure (a = 11.54 Å, b = 3.57 Å, c = 4.38 Å). Its structure makes it attractive for electrode applications in, e.g., Li-ion batteries 10,11 and actuators. 12 As was found for perovskite oxides, [13][14][15] it is tempting to consider these vanadium oxides as oxygen sponges, by which one can obtain a strong contrast in the physical properties by reversibly transitioning between the phases. Moreover, the redox process Here, we report the growth control of the valence state in vanadium oxides by systematically controlling P(O 2 ) during pulsed laser deposition, followed subsequently by postannealing under highly oxidizing conditions. We found that the room temperature MIT could be 5 observed only in VO 2 thin films grown in a very narrow window of P(O 2 ). Moreover, interestingly, the MIT behavior could be further enhanced by post-annealing in high-pressure oxygen, implying that the optimal growth pressure to obtain phase pure films with high crystallinity was not enough to fully oxidize the films. This result ultimately stresses the importance of oxygen content in VO 2 that plays a critical role in the sharpness and resistivity r...

2Transition metal oxides have been extensively studied and utilized as efficient catalysts. However, the strongly correlated behavior which often results in intriguing emergent phenomena in these materials has been mostly overlooked in understanding the electrochemical activities. Here, we demonstrate a close correlation between the phase transitions and oxygen evolution reaction (OER) in a strongly correlated SrRuO3. By systematically introducing Ru-O vacancies into the singlecrystalline SrRuO3 epitaxial thin films, we induced phase transition in crystalline symmetry which resulted in corresponding modification in the electronic structure. The modified electronic structure significantly affect the electrochemical activities, so a 30% decrease in the overpotential for the OER activity was achieved. Our study suggests that a substantial enhancement in the OER activity can be realized even within single material systems, by rational design and engineering of their crystal and electronic structures. 3Transition metal oxides show promising chemical activities that can be applied in solid oxide fuel cells (SOFC), rechargeable batteries, catalytic converters, oxygen-separation membranes, and gas sensors. [1][2][3][4][5] Oxygen evolution reaction (OER, 4OH -→ O 2 + 2H 2 O + 4e -) is one of the most important steps in energy conversion and storage mechanisms, and is the efficiency-limiting process in electrolytic water splitting and metal-air batteries. 6,7 The ultimate goal of OER study is to develop low-cost, highly active, and stable catalysts. 8,9 Recently, perovskite oxides (ABO 3 ), such as Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ , Pr 0.5 Ba 0.5 CoO 3-δ , and LaCoO 3 , have attracted much attention owing to their intrinsically high OER activity. [10][11][12] More interestingly, properties such as surface oxygen binding energy, number of outer shell electrons in the transition metal ion, electron occupancy of the e g orbitals, and the proximity of the oxygen p-band to the Fermi level, have been proposed as descriptors for OER activity. 10,11,13,14 Such approaches, however, have been mainly tested by comparing systems containing different transition metal elements.Unintentionally, such variations in the identity of the elements therein involve commensurate changes in the atomic structures, valence states, electric resistivities, crystalline surfaces, and the overall and specific electronic structures of the materials. Therefore, approaches based on simplified electronic structure may not apply to distinctive material systems, and more carefully controlled study, for example, one using a single-material system, is necessary to precisely understand the effect of the catalyst's electronic structure on the OER. 15In order to probe the link between electronic structure and catalytic activity within a singlematerial system, we exploit the strongly correlated behavior in complex oxides. In particular, the strong coupling among the degrees of freedom of the d-electrons, i.e., charge, spin, orbital, and lattice, in transition...

Bonding geometry engineering of metal–oxygen octahedra is a facile way of tailoring various functional properties of transition metal oxides. Several approaches, including epitaxial strain, thickness, and stoichiometry control, have been proposed to efficiently tune the rotation and tilt of the octahedra, but these approaches are inevitably accompanied by unnecessary structural modifications such as changes in thin‐film lattice parameters. In this study, a method to selectively engineer the octahedral bonding geometries is proposed, while maintaining other parameters that might implicitly influence the functional properties. A concept of octahedral tilt propagation engineering is developed using atomically designed SrRuO 3 /SrTiO 3 (SRO/STO) superlattices. In particular, the propagation of RuO 6 octahedral tilt within the SRO layers having identical thicknesses is systematically controlled by varying the thickness of adjacent STO layers. This leads to a substantial modification in the electromagnetic properties of the SRO layer, significantly enhancing the magnetic moment of Ru. This approach provides a method to selectively manipulate the bonding geometry of strongly correlated oxides, thereby enabling a better understanding and greater controllability of their functional properties.