Transition-metal dichalcogenides (TMDs) have emerged in recent years as a special group of two-dimensional materials and have attracted tremendous attention. Among these TMD materials, molybdenum disulfide (MoS) has shown promising applications in electronics, photonics, energy, and electrochemistry. In particular, the defects in MoS play an essential role in altering the electronic, magnetic, optical, and catalytic properties of MoS, presenting a useful way to engineer the performance of MoS. The mechanisms by which lattice defects affect the MoS properties are unsettled. In this work, we reveal systematically how lattice defects and substrate interface affect MoS electronic structure. We fabricated single-layer MoS by chemical vapor deposition and then transferred onto Au, single-layer graphene, hexagonal boron nitride, and CeO as substrates and created defects in MoS by ion irradiation. We assessed how these defects and substrates affect the electronic structure of MoS by performing X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopies, and scanning tunneling microscopy/spectroscopy measurements. Molecular dynamics and first-principles based simulations allowed us to conclude the predominant lattice defects upon ion irradiation and associate those with the experimentally obtained electronic structure. We found that the substrates can tune the electronic energy levels in MoS due to charge transfer at the interface. Furthermore, the reduction state of CeO as an oxide substrate affects the interface charge transfer with MoS. The irradiated MoS had a faster hydrogen evolution kinetics compared to the as-prepared MoS, demonstrating the concept of defect controlled reactivity in this phase. Our findings provide effective probes for energy band and defects in MoS and show the importance of defect engineering in tuning the functionalities of MoS and other TMDs in electronics, optoelectronics, and electrochemistry.
Vapor-phase atomic layer deposition (ALD) of nickel sulfide (NiS x ) is comprehensively reported for the first time. The deposition process employs bis(N,N′-di-tertbutylacetamidinato)nickel(II) and H 2 S as the reactants and is able to produce fairly smooth, pure, godlevskite-structured NiS x thin films following an ideal layer-by-layer ALD growth fashion for a relatively wide process temperature range from 90−200 °C. Excellent conformal coating is demonstrated for this ALD process, as the deposited NiS x films are able to uniformly and conformally cover deep narrow trenches with aspect ratio as high as 10:1, which highlights the general and broad applicability of this ALD process for fabricating complex 3D-structured nanodevices. Further, we demonstrate the applications of this ALD NiS x for oxygen-evolution reaction (OER) electrocatalysis. The ALD NiS x is found to convert to nickel (oxy)hydrate after electrochemical aging, and the aged product shows a remarkable electrocatalytic activity and long-term stability, which is among the best electrocatalytic performance reported for nonprecious OER catalysts. Considering that ALD can be easily scaled up and integrated with 3D nanostructures, we believe that this ALD NiS x process will be highly promising for a variety of applications in future energy devices.
Electrochemical experiments were conducted on {100}, {110}, and {111} silicon wafers to characterize the kinetics of the initial lithiation of crystalline Si electrodes. Under constant current conditions, we observed constant cell potentials for all orientations, indicating the existence of a phase boundary that separates crystalline silicon from the amorphous lithiated phase. For a given potential, the velocity of this boundary was found to be faster for {110} silicon than for the other two orientations. We show that our measurements of varying phase boundary velocities can accurately account for anisotropic morphologies and fracture developed in crystalline silicon nanopillars. We also present a kinetic model by considering the redox reaction at the electrolyte/lithiated silicon interface, diffusion of lithium through the lithiated phase, and the chemical reaction at the lithiated silicon/crystalline silicon interface. From this model, we quantify the rates of the reactions at the interfaces and estimate a lower bound on the diffusivity through the lithiated silicon phase.
Atomic layer deposition (ALD) of cobalt sulfide (Co9S8) is reported. The deposition process uses bis(N,N'-diisopropylacetamidinato)cobalt(II) and H2S as the reactants and is able to produce high-quality Co9S8 films with an ideal layer-by-layer ALD growth behavior. The Co9S8 films can also be conformally deposited into deep narrow trenches with aspect ratio of 10:1, which demonstrates the high promise of this ALD process for conformally coating Co9S8 on high-aspect-ratio 3D nanostructures. As Co9S8 is a highly promising electrochemical active material for energy devices, we further explore its electrochemical performance by depositing Co9S8 on porous nickel foams for supercapacitor electrodes. Benefited from the merits of ALD for making high-quality uniform thin films, the ALD-prepared electrodes exhibit remarkable electrochemical performance, with high specific capacitance, great rate performance, and long-term cyclibility, which highlights the broad and promising applications of this ALD process for energy-related electrochemical devices, as well as for fabricating complex 3D nanodevices in general.
Transition metal dichalcogenides (TMDs) have attracted much attention due to their promising optical, electronic, magnetic, and catalytic properties. Engineering the defects in TMDs represents an effective way to achieve novel functionalities and superior performance of TMDs devices. However, it remains a significant challenge to create defects in TMDs in a controllable manner or to correlate the nature of defects with their functionalities. In this work, taking single-layer MoS 2 as a model system, defects with controlled densities are generated by 500 keV Au irradiation with different ion fluences, and the generated defects are mostly S vacancies. We further show that the defects introduced by ion irradiation can significantly affect the properties of the single-layer MoS 2 , leading to considerable changes in its photoluminescence characteristics and electrocatalytic behavior. As the defect density increases, the characteristic photoluminescence peak of MoS 2 first blueshifts and then redshifts, which is likely due to the electron transfer from MoS 2 to the adsorbed O 2 at the defect sites. The generation of the defects can also strongly improve the hydrogen evolution reaction activity of MoS 2 , attributed to the modified adsorption of atomic hydrogen at the defects.
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