The effect of room temperature ultraviolet-ozone (UV-O 3) exposure of MoS 2 on the uniformity of subsequent atomic layer deposition of Al 2 O 3 is investigated. It is found that a UV-O 3 pre-treatment removes adsorbed carbon contamination from the MoS 2 surface and also functionalizes the MoS 2 surface through the formation of a weak sulfur-oxygen bond without any evidence of molybdenum-sulfur bond disruption. This is supported by first principles density functional theory calculations which show that oxygen bonded to a surface sulfur atom while the sulfur is simultaneously back-bonded to three molybdenum atoms is a thermodynamically favorable configuration. The adsorbed oxygen increases the reactivity of MoS 2 surface and provides nucleation sites for atomic layer deposition of Al 2 O 3. The enhanced nucleation is found to be dependent on the thin film deposition temperature. V

Layered transition metal dichalcogenides (TMDs) are being considered as a promising alternative channel material in ultra-thin and low power nanoelectronics, due to the significant tunability of their electronic properties via mechanisms such as mechanical strain, control of the material thickness, application of an external field, impurities, doping, alloying, or altering the substrate nature. Initially, monolayer TMDs as counterparts to graphene captured the attention of the scientific community owing to their semiconductor nature with sizable band gaps. However, certain physical and chemical properties of TMDs, such as their oxygen reactivity and stability in air need to be more completely understood in order to crystallize the promising superior performance of TMD-based electronic devices. Here, a comparative analysis of the stability of various TMDs (MX 2 : = M Mo, W; = X S, Se) in air is performed using density-functional theory (DFT) as well as x-ray photoelectron spectroscopy (XPS). We find that the surface chemistry of the basal plane of sulfides and selenides is relatively stable in air although for completely different reasons, which can be explained by investigating oxygen dissociative adsorption kinetics and thermodynamics. On the contrary, the edge of MX 2 nanoribbons shows strong driving forces towards O 2 dissociation and chemisorption. Our combined theoretical and experimental investigation reveals that the air stability of TMDs should not be placed in the same footing that other 2D materials, like graphene. Thus, this work highlights the importance of having controlled oxygen environment during crystal exfoliation/ growth and defect passivation in order to provide high quality uniform materials for TMD-based device fabrication.

In this work, phase stability of Ni-rich LiNi1–2x­Co x Mn x O2 (NCM) (x < 0.20) is investigated by means of a bond model based on the effective interaction of transition metal (TM) ions (represented as TM–TM bond), fitted to results obtained within the DFT+U framework. The developed bond model reveals the intrinsic relationship between phase stability and TM–TM bonds, which explains the different roles of TM ions in the phase stability of Ni-rich NCM. A sequence of TM–TM bond strengths (Mn4+Mn4+ > Ni2+Mn4+ > Ni3+Mn4+ > Co3+Mn4+ > Co2+Mn4+ > Ni2+Ni4+) is then predicted by bond model and subsequently used to understand the intricate layered LiTMO2 (TM = Ni, Co, Mn) phase triangle. Our results also show that Co and Mn ions segregate to form clusters within the Ni environment when x < 0.1 (Ni ≥ 80 at. %), and such segregation is responsible for the electrochemical degradation during cycling. The obtained results agree excellently with the validation experiment in the present work and also other experiments in the literature, and could help to clarify the existing controversies about the origin of the instability of Ni-rich NCM compounds. Finally, we show that, by tailoring the TM–TM interaction, i.e., the atomic uniformity of the as-synthesized cathode material, the electrochemical stability of the Ni-rich NCM can be substantially improved.

Transition metal dichalcogenides (TMDs) have been investigated extensively for potential application as device materials in recent years. TMDs are found to be stable in trigonal prismatic (H), octahedral (T), or distorted octahedral (Td) coordination of the transition metal. However, the detailed understanding of stabilities of TMDs in a particular phase is lacking. In this work, the detailed TMD phase stability using first-principles calculations based on density functional theory (DFT) has been investigated to clarify the mechanism of phase stabilities of TMDs, consistent with the experimental observation. Our results indicate that the phase stability of TMDs can be explained considering the relative strength of two competing mechanisms: ligand field stabilization of d-orbitals corresponding to transition metal coordination geometry, and charge density wave (CDW) instability accompanied by a periodic lattice distortion (PLD) causing the phase transition in particular TMDs.

The dependence of the “self cleaning” effect of the substrate oxides on substrate temperature during atomic layer deposition (ALD) of HfO2 on various chemically treated and native oxide InP (100) substrates is investigated using in situ X-ray photoelectron spectroscopy. The removal of In-oxide is found to be more efficient at higher ALD temperatures. The P oxidation states on native oxide and acid etched samples are seen to change, with the total P-oxide concentration remaining constant, after 10 cycles of ALD HfO2 at different temperatures. An (NH4)2 S treatment is seen to effectively remove native oxides and passivate the InP surfaces independent of substrate temperature studied (200 °C, 250 °C and 300 °C) before and after the ALD process. Density functional theory modeling provides insight into the mechanism of the changes in the P-oxide chemical states.