Michael Altvater

The proximity-effect, whereby materials in contact appropriate each other’s electronic-properties, is widely used to induce correlated states, such as superconductivity or magnetism, at heterostructure interfaces. Thus far however, demonstrating the existence of proximity-induced charge-density-waves (PI-CDW) proved challenging. This is due to competing effects, such as screening or co-tunneling into the parent material, that obscured its presence. Here we report the observation of a PI-CDW in a graphene layer contacted by a 1T-TaS₂ substrate. Using scanning tunneling microscopy (STM) and spectroscopy (STS) together with theoretical-modeling, we show that the coexistence of a CDW with a Mott–gap in 1T-TaS₂ coupled with the Dirac-dispersion of electrons in graphene, makes it possible to unambiguously demonstrate the PI-CDW by ruling out alternative interpretations. Furthermore, we find that the PI-CDW is accompanied by a reduction of the Mott gap in 1T-TaS₂ and show that the mechanism underlying the PI-CDW is well-described by short-range exchange-interactions that are distinctly different from previously observed proximity effects.

Local laser‐induced oxidation is an extremely valuable technique to perform high‐throughput optimization across multidimensional parameter sets. In this work, a versatile method is presented for the synthesis of titanium dioxide (TiO₂) thin‐films with varying crystalline structures through the use of localized, visible, continuous‐wave laser‐processing. By controlling the laser intensity and the exposure time, the conversion of amorphous titanium disulfide (a‐TiS₂) precursor films into distinct phases of TiO₂ is achieved and a laser‐induced oxidation phase diagram is constructed with the resulting material phases, including anatase, rutile, and black TiO₂. By utilizing the dependence of phase formation on the rate and duration of laser energy input, mixtures of anatase and rutile phases are fabricated with controlled spatial arrangements. Photocatalytic properties of the synthesized films are evaluated using the degradation of nitrogen oxide (NO_x) gas under UV illumination and an organic dye under white‐light illumination, revealing that mixtures of anatase and rutile phases demonstrate superior photocatalytic activity. The laser‐induced oxidation method highlighted showcases a strategy for precisely tailored phase composition for directly tunable properties, paving the way for in‐depth studies into structure‐property relationships in photocatalysis and other applications of metal oxide films.

Charge density wave (CDW) phases native to the transition metal dichalcogenide 1T-TaS₂ are probed through a graphene layer covering its surface by using local scanning tunneling microscopy/spectroscopy and transport measurements. We show that over a wide range of temperatures, the graphene cover, while protecting the air-sensitive 1T-TaS₂ from oxidation, does not obstruct access to the electronic properties of the CDW. At room temperature, where the system is in the nearly commensurate CDW phase, we observe contiguous domains of the CDW phase separated by a network of topological defects. These topological defects, dubbed CDW vortices, form a lattice with a quasi-long range translational order, which resembles the Bragg glass phase in Abrikosov vortex lattices in type-II superconductors. While the quasi-long range order in the CDW itself is not uncommon, its existence in the CDW vortex lattice is observed here for the first time. Finally, when reducing the temperature to 78 K into the commensurate CDW phase, we find that the graphene cover layer acquires properties consistent with a proximity-induced CDW.

Devices made from two-dimensional (2D) materials such as graphene and transition metal dichalcogenides exhibit remarkable electronic properties of interest to many subdisciplines of nanoscience. Owing to their 2D nature, their quality is highly susceptible to contamination and degradation when exposed to the ambient environment. Protecting the 2D layers by encapsulation between hexagonal boron nitride (hBN) layers significantly improves their quality. Locating these samples within the encapsulant and assessing their integrity prior to further processing then becomes challenging. Here we show that conductive scanning probe techniques such as electrostatic force and Kelvin force microscopy make it possible to visualize the encapsulated layers, their charge environment and local defects on the sub-micrometer scale. Our techniques are employed without requiring electrical contact to the embedded layer, providing valuable feedback on the device’s local electronic quality prior to any device etching or electrode deposition. We show that these measurement modes, which are simple extensions of atomic force microscopy, are perfectly suited for imaging encapsulated conductors and their local charge environments.