Thickness-Dependent Nanoscale Elastic Stiffening of Chemical Vapor Deposited Atomically Thin 2H-MoS2 Films.

Understanding the nanoscale elastic-size-effects of atomically thin transition-metal dichalcogenides (TMDs) as a function of thickness underpins the avenue of flexible 2D electronics. In this work, we employed the atomic force acoustic microscopy (AFAM) technique to investigate the thickness-dependent elastic properties of CVD grown 2H-MoS2 films. The monolayer MoS2 exhibited a Young's modulus of 273 ± 27 GPa. Our systematic analysis from bulk to monolayer suggests that the 2H-MoS2 phase exhibits nanoscale elastic-stiffening behavior with decreasing number of layers (thickness). The Young's modulus increased by a factor of ∼2.7 for monolayer MoS2 when compared with the bulk. First-principle DFT calculations affirm the nanoscale elastic-stiffening behavior of MoS2 with decreasing number of layers. Our findings suggest that the observed elastic stiffening is due to the interlayer sliding, which may be facilitated by defects in MoS2 layers. The observed elastic stiffening may be of potential importance for understanding TMD based nanomechanical devices.

Recoverable and reusable visible-light photocatalytic performance of CVD grown atomically thin MoS2 films.

The decomposition of water pollutants including industrial dyes and chemicals via photocatalytic decontamination is one of the major investigated problems in recent years. Two-dimensional molybdenum disulfide (MoS2) layers have shown great promise as an efficient visible-light photocatalyst owing to its numerous active sites and large surface area. In this study, atomically thin MoS2 films of different thicknesses from monolayer to five-layer and ten layers were fabricated on sapphire substrates using chemical vapor deposition (CVD). We demonstrate that these MoS2 thin films can be used as a photocatalyst to degrade Methylene Blue (MB) dye and can be recovered completely with utmost structural and chemical stability. Under visible-light irradiation, the MB absorption peak completely disappears with ∼95.6% of degradation after 120 min. We also demonstrate the reusability of the MoS2 thin films without significantly losing the photocatalytic activity even after 5-cycles of degradation studies. The chemical and structural stability of the MoS2 films after 5-cycles of degradation studies were affirmed using various spectroscopic studies. Our findings suggest that the MB degradation efficiency increases from 19.01% to 98.46% with an increase in pH from 4 to 14. Our approach may facilitate a further design of other transition metal dichalcogenides-based recoverable photocatalysts for industrial applications.

Davydov Splitting, Double-Resonance Raman Scattering, and Disorder-Induced Second-Order Processes in Chemical Vapor Deposited MoS2 Thin Films.

Atomically thin MoS2 hosts rich and distinct vibrational spectral features, which are prominent under selective excitation energies near the excitonic transitions. In this work, we have investigated the resonant Raman scattering of the MoS2 layers of different thicknesses, from monolayer to five-layer samples, measured near resonance with the A excitonic transition. We show that the near-resonance excitation (1.96 eV) resulted in a Davydov splitting of the out-of-plane A-like phonon mode (A1g) around 406 cm-1 caused by the weak interlayer interaction. The number of Davydov splitting components (N) equals the number of layers (NL) of the MoS2, suggesting that it can be used as a thickness indicator. The origin of various Davydov components is understood based on a simple nearest-interlayer interaction. We extend our investigation to identify some acoustic phonon modes associated with characteristic second-order double-resonance Raman and disorder-induced bands.

Enhanced Fracture Resistance of Flexible ZnO:Al Thin Films in Situ Sputtered on Bent Polymer Substrates.

Improving the fracture resistance of inorganic thin films is one of the key challenges in flexible electronic devices. A nonconventional in situ sputtering method is introduced to induce residual compressive stress in ZnO:Al thin films during deposition on a bent polymer substrate. The films grown under a larger prebending strain resulted in a higher fracture resistance to applied strains by exhibiting a ∼ 70% improvement in crack-initiating critical strain compared with the reference sample grown without bending. This significant improvement is attributed to the induced residual stress, which helps to prevent the formation of cracks by counteracting the applied strain.

Prestress Driven Improvement in Fracture Behavior of in Situ Sputtered Zinc Oxide Thin Films on Stretched Polymer Substrates.

Flexible electronic devices need to survive bending or stretching operation without mechanical failure. If inorganic thin films are involved in the device structure, the evolution of cracks is a major challenge to overcome. Here, we report a novel way to substantially improve the fracture behavior of films that are based on intentional utilization of residual stress on the films by in situ sputtering on a stretched polymer substrate. The in situ sputtering combined with a stabilization stage yielded ZnO:Al thin films with a nearly 2-fold improvement in crack initiation strain, which indicates greater resistance to bending. The critical strain of the optimal ZnO:Al films was ∼1.83%, which is a significant improvement compared to the current tolerance value of ∼1%. This was accompanied by a ∼300% improvement in fracture energy. We attributed the improved fracture behavior to the presence of residual compressive stresses, which creates a barrier for crack formation by acting opposite to the applied bending strain.