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Two-dimensional (2D) transition metal dichalcogenides (TMD) are of specific interest for a variety of applications when they present a semiconducting character. The band gap between the valence and the conducting band can be tuned and is dependent on the phase of the TMD flake, the number of layers, and how the flakes are chemically modified to impart new surface functionalities. We investigate the surface functionalization of molybdenum disulfide (MoS2) flakes with electron-acceptor 4-nitrothiophenol (NTP) and measure how the band gap of the flake is altered by the presence of this small organic molecule. The functionalized MoS2surface with NTP is investigated with confocal and tip-enhanced Raman spectroscopy as well as with photoluminescence microscopy under 532 nm excitation. Photoconversion of NTP into dimercaptoazobenzene (DMAB) and aminothiophenol (ATP) is reported even under weak irradiation conditions, highlighting the potential of MoS2for its photocatalytic properties. The intensity and spectral changes of the photoluminescence of modified MoS2are indicative of the charge transfer character between MoS2and NTP.

We report on tungsten disulfide (WS2) flakes grown by chemical vapor deposition (CVD), which exhibit a flower-like surface structure above the primary few-layer flake with a triangular shape. The fine structure is only revealed in the mechanical, chemical, and electronic properties of the flake but not in the topography. The origin of this structure is the peculiar one-step growth during the CVD process that permits to control the sulfur concentration at any time. A high concentration of S at the onset of the deposition process leads to a rapid growth of the flake, resulting in tungsten vacancies. Reducing the sulfur concentration toward the end of the growth slows down the reaction and leads to sulfur vacancies. These microscale domains were studied by confocal- and tip-enhanced Raman spectroscopy revealing their chemical composition with high spatial resolution. A strong quenching of the photoluminescence in the tungsten-vacancy domains is observed. Atomic force microscope measurements, performed in intermittent contact mode, force modulation mode (including lateral force mode), and PeakForce quantitative nanomechanics mode, show that the mechanical properties of these domains differ. Within the tungsten-vacancy domains, the adhesion force is reduced, while the friction force increased. Kelvin probe force microscopy measurements show that the electronic properties of the flakes are modulated by these domains. The combined nanomechanical and nanospectroscopy measurements provide detailed insights on the inhomogeneous surface properties of the single WS2flake, further highlighting how its multidomain properties can be finely tuned using CVD.

We report on the formation of stimuli-responsive structured hydrogel thin films whose pattern geometry can be adjusted on demand and tuned reversibly by varying solvent quality or by changing temperature. The hydrogel films, ∼100 nm in thickness, were prepared by depositing layers of random copolymers comprising N-isopropylacrylamide and ultraviolet (UV)-active methacryloyloxybenzophenone units onto solid substrates. A two-beam interference pattern technique was used to cross-link the selected areas of the film; any unreacted material was extracted using ethanol after UV light-assisted cross-linking. In this way, we produced nanoholes, perfectly ordered structures with a narrow size distribution, negligible tortuosity, adjustable periodicity, and a high density. The diameter of the circular holes ranged from a few micrometers down to several tens of nanometers; the hole periodicity could be adjusted readily by changing the optical period of the UV interference pattern. The holes were reversibly closed and opened by swelling/deswelling the polymer networks in the presence of ethanol and water, respectively, at various temperatures. The reversible regulation of the hole diameter can be repeated many times within a few seconds. The hydrogel sheet with circular holes periodically arranged may also be transferred onto different substrates and be employed as tunable templates for the deposition of desired substances.

We report on molybdenum disulfide (MoS2) flakes grown by chemical vapor deposition (CVD), which exhibit an increased photoluminescence (PL) intensity at the edge of the flake under moderate irradiation. The photo-induced chemisorption of O2and H2O to sulfur vacancies in the edge regions is the origin of the modulated PL intensity. The MoS2flakes were analyzed utilizing correlated multispectral wide-field microscopy with stochastic optical reconstruction microscopy (STORM). The advantage of the wide-field approach compared to conventional point scanning microscopy is a faster acquisition over a larger field of view without exposing the sample to high irradiance conditions. This enables in situ and real-time measurements of the PL intensity fluctuations and the identification of defects within the flake. The STORM postprocessing reveals fine structures within the flake and enhances the contrast, which cannot be revealed by point scanning microscopy. In particular, by increasing the intensity over the nondestructive threshold, the stochastic optical reconstruction enables the study of the kinetics of defect domain generation with reduced PL and Raman intensity caused by lattice defects or photo-oxidation of MoS2to MoO3. Additional measurements of the work function by Kelvin force microscopy, the topography, and the friction force by AFM reveal that the defect domains start to form at the edge of the flake, with a larger interlayer height and work function but reduced friction force compared to MoS2. The stochastic optical reconstruction microscopy postprocessing is a powerful technique to understand the layer-by-layer photo-oxidation of MoS2flakes and gives new insights in the design of, for example, MoS2/MoO3heterostructures.