In view of its epitaxial seeding capability, c-plane single crystalline sapphire represents one of the most enticing, industry-compatible templates to realize manufacturable deposition of single crystalline two-dimensional transition metal dichalcogenides (MX 2 ) for functional, ultrascaled, nanoelectronic devices beyond silicon. Despite sapphire being atomically flat, the surface topography, structure, and chemical termination vary between sapphire terraces during the fabrication process. To date, it remains poorly understood how these sapphire surface anomalies affect the local epitaxial registry and the intrinsic electrical properties of the deposited MX 2 monolayer. Therefore, molybdenum disulfide (MoS 2 ) is deposited by metal−organic chemical vapor deposition (MOCVD) in an industry-standard epitaxial reactor on two types of c-plane sapphire with distinctly different terrace and step dimensions. Complementary scanning probe microscopy techniques reveal an inhomogeneous conductivity profile in the first epitaxial MoS 2 monolayer on both sapphire templates. MoS 2 regions with poor conductivity correspond to sapphire terraces with uncontrolled topography and surface structure. By intentionally applying a substantial off-axis cut angle (1°in this work), the sapphire terrace width and step heightand thus also surface structurebecome more uniform across the substrate and MoS 2 conducts the current more homogeneously. Moreover, these effects propagate into the extrinsic MoS 2 device performance: the field-effect transistor variability reduces both within and across wafers at higher median electron mobility. Carefully controlling the sapphire surface topography and structure proves an essential prerequisite to systematically study and control the MX 2 growth behavior and capture the influence on its structural and electrical properties.

As interest in layered van der Waals (vdW) materials keeps increasing, fundamental knowledge about their synthesis is gaining more and more value. The defect-free heteroepitaxial integration of vdW materials on large-area substrates is currently thoroughly being researched since it might encompass a successful transition of these materials to industrial applications.To date, Transition Metal Dichalcogenides (TMDs) are considered as one of the most promising vdW materials within the field of nanoelectronics. Nevertheless, the electrical characterization of heteroepitaxially grown TMDs still shows inferior performance as compared to exfoliated TMD flakes. This is mainly attributed to the high density of defects resulting from their challenging vdW heteroepitaxial synthesis. In this work, we have investigated in depth the vdW homoepitaxial synthesis of the WSe2 TMD compound. We have demonstrated that even for homoepitaxy, the simplest type of crystal growth, challenges such as the formation of 60 o twins need to be addressed. We evidenced the presence of 60 o twins during vdW homoepitaxy which is assigned to stacking faults. The formation of these stacking faults is associated with their very similar binding energy as revealed by Density Functional Theory (DFT) calculations. Therefore, stacking faults are identified in this work to be the fundamental limitation of lowly-defective TMD vdW epitaxy. Furthermore, a generalized model is developed that determines the lower limit of the defect density based on the degree of control on the bilayer stacking phase and the nucleation density of the TMD compound. This model therefore assesses and quantifies for the first time the ultimate defect density level that can be achieved with vdW epitaxially grown 2D materials.

Layered materials held together by weak van der Waals (vdW) interactions are a promising class of materials in the field of nanotechnology. Besides the potential for single layers, stacking of various vdW layers becomes even more promising since unique properties can hence be precisely engineered. The synthesis of stacked vdW layers, however, remains to date, hardly understood. Therefore, in this work, the vdW epitaxy of transition metal dichalcogenides (TMDs) on single-crystalline TMD templates is investigated in depth. It is demonstrated that the role of lattice mismatch is insignificant. More importantly is the role of surface energy, calculated using density functional theory, which plays an essential role in the activation energy for adatom diffusion, hence nucleation density. This in turn correlates with defect density since the stacking sequence in vdW epitaxy is generally poorly controlled. Moreover, the vapor pressure of the transition metal is also found to correlate with adatom diffusion. Consequently, the proposed study enables important and new insight in the vdW epitaxy of multilayer 2D homo-/heterostructures.

Superhydrophobic surfaces are highly promising for self-cleaning, anti-fouling and anti-corrosion applications. However, accurate assessment of the lifetime and sustainability of super-hydrophobic materials is hindered by the lack of large area characterization of superhydrophobic breakdown. In this work, attenuated total reflectance−Fourier transform infrared spectroscopy (ATR-FTIR) is explored for a dynamic study of wetting transitions on immersed superhydrophobic arrays of silicon nanopillars. Spontaneous breakdown of the superhydrophobic state is triggered by in-situ modulation of the liquid surface tension. The high surface sensitivity of ATR-FTIR allows for accurate detection of local liquid infiltration. Experimentally determined wetting transition criteria show significant deviations from predictions by classical wetting models. Breakdown kinetics is found to slow down dramatically when the liquid surface tension approaches the transition criterion, which clearly underlines the importance of more accurate wetting analysis on large-area surfaces. Precise actuation of the superhydrophobic breakdown process is demonstrated for the first time through careful modulation of the liquid surface tension around the transition criterion. The developed ATR-FTIR method can be a promising technique to study wetting transitions and associated dynamics on various types of superhydrophobic surfaces.

Area-selective deposition (ASD) is a "bottom-up" substrate-selective material deposition process, considered as a promising alternative to current "top-down" pattering techniques. The most studied and successful ASD strategies envisage a combination of atomic layer deposition (ALD) and a passivation layer, which prevents material deposition on the non-growth areas. As ASD targets increasingly smaller dimensions, metrology challenges are prominent along with preserving confined film growth. For patterned substrates with nanometric critical dimensions, only a few characterization techniques can be employed to assess the ASD performance. However, these techniques provide no or little insight into the passivation layer. This is a crucial limitation as the blocking film plays a key role in the ASD process. In this work, pulsed force mode atomic force microscopy (AFM) is used to characterize and monitor the quality of the passivation films by measuring the surface energy fluctuations occurring on the patterned substrate undergoing ASD. As the evolution of the relative adhesion force distribution of the sample under ALD conditions is recorded, the octadecanethiol (ODT) coverage on non-growth areas is accurately estimated. The heavily temperature-dependent self-assembled monolayer degradation revealed by the nanomechanical characterization is supported by X-ray photoelectron spectroscopy. As Hf 3 N 4 ALD is performed, the top-down scanning electron microscopy investigation is employed to show the strong relationship between ASD quality upon ALD and pulsed force AFM-derived ODT coverage.