Harold J. W. Zandvliet

The deterioration of transistor performance due to carrier scattering when the thickness of silicon is reduced below a few nanometers is one of the major concerns in silicon-based electronics. Atomically thin two-dimensional transition metal dichalcogenides (TMDCs) do not suffer from this problem and are thus appealing candidates for next generation transistors. To speed up the development of TMDCs for device applications, a fast, noninvasive, and reliable technique to measure the electrical properties on the micrometer scale is paramount. Here, we use multi-probe cantilevers with micrometer pitches to non-invasively probe the electronic properties of mono-, bi-, and trilayer molybdenum disulfide samples. We extract field-effect mobilities in the range from 2.7 to 16.3 cm2 V−1 s−1. Furthermore, atomic force microscopy images taken after the transport measurements reveal that landing damage from the multi-probe cantilever to the sample can be avoided, provided that the landing of the probe is gentle and the drain–source currents are limited.

In the realm of topological materials, bismuth is regarded as one of the prime candidates owing to its large spin–orbit coupling. Recent studies have revealed that the rectangular Bi(110) phase may host topological edge or higher-order topological hinge states. The topological nature of Bi(110) depends critically on the buckling of the (110) lattice. Only buckling below 0.1 Å may yield topologically non-trivial states. In this work, we examine the Bi(110) phase grown on Ge(111) using scanning tunneling microscopy and spectroscopy. The Bi(110) islands grow in four sets of threefold high-symmetry directions, which are rotated by ±13.7° and ±16.3° with respect to the high-symmetry directions of the (3×3)R30° Bi wetting layer covering the Ge(111) surface, revealing that the diagonals of the Bi(110) and Bi wetting layer unit cells align. Multilayer Bi(110) islands show a large buckling and an increased density of states at step edges near −0.35 eV. We find this state to emerge from the Bi wetting layer surface rather than the edge itself. The buckling of bilayer Bi(110) islands is lower and lies on the cusp of the trivial to topological transition. However, we find no evidence of electronic enhancement at its edges or corners.

The effect of moisture on paper is of great relevance for numerous applications ranging from printing to packaging. The uptake of water in the polymer network of the paper fibers via the liquid or vapor phase of water results in hydro- or hygro-expansion of the paper, respectively. Here, we present a detailed atomic force microscopy study of the out-of-plane hygro-expansion of paper by conducting a series of experiments for varying relative humidity. The high spatial resolution of the atomic force microscope enables measuring the out-of-plane hygro-expansion of paper fibers with subnanometer resolution. We find a linear relation between the relative humidity and the out-of-plane expansion of the paper fibers. A sorption isotherm measurement reveals a good fit with the Guggenheim-Anderson-de Boer model. The combination of the two measurements suggests that the vast increase in weight seen in the isotherm is due to water filling the pores between the fibers, where it does not result in additional out-of-plane expansion. In addition, by locking the atomic force microscopy tip at a predefined location on a paper fiber, we monitor the expansion and shrinkage of paper with a temporal resolution in the millisecond range. We find that the kinetics of the out-of-plane hygro-expansion is described using a double exponential function comprised of two exponents with different time scales, which we attribute to two separate adsorption sites.

Using cross-sectional atomic force microscopy, we investigated how different additives modify the adhesive properties of UV-cured polymer films used in coatings and printing primers.

Reversible, all-electric control of symmetry-protected zero-dimensional modes has been a long-standing goal. In buckled honeycomb lattices, a perpendicular field couples to the staggered sublattice potential providing the required handle. We combine scanning tunneling microscopy and tight-binding theory to switch zero-dimensional topological end states reversibly on and off in ultranarrow germanene nanoribbons by tuning the electric field in the tunnel junction. Increasing the field switches off the end modes of topological two-hexagon-wide ribbons, while the same field switches on zero-dimensional states in initially trivial three- and four-hexagon-wide ribbons. This atomic scale platform realizes a proof of principle for a zero-dimensional topological field effect device, opening a path for ultrasmall memory, controllable qubits, and neuromorphic architectures.

Palladium diselenide (PdSe ₂ ) is a two-dimensional (2D) transition metal dichalcogenide van der Waals material that exhibits a unique wrinkled pentagonal structure that results in strong anisotropic layer-dependent properties. Owing to these unique properties the material has a great potential for applications in electronic, optoelectronic, photonic and thermoelectric devices. Here, we study the electronic properties of monolayer (ML) PdSe ₂ in a Schottky junction by contacting the material with a tungsten scanning tunneling microscope (STM) tip. The STM-induced lifting of the 2D layer leads to structural warping of the top layer, which causes significant changes in the electronic properties of PdSe ₂ as well as the electrostatic potential across the junction. We compare the STM-determined work function of the warped PdSe ₂ ML with the work function of bulk PdSe ₂ as obtained by Kelvin probe force microscopy and electronic structure obtained by synchrotron-based angle-resolved photoemission spectroscopy, and x-ray photoelectron spectroscopy.

We show that twisted bilayers of silicene or germanene can be utilized for a novel transistor concept that relies on the quantum valley Hall effect. The application of an electric field normal to the twisted bilayer allows to tailor the size of the bandgap in AB- and BA-stacked domains of the twisted bilayer. In contrast to twisted bilayer graphene, the AB and BA bandgaps in twisted bilayer silicene and germanene are not inverted for small electric fields. However, above a critical electric field, the bandgaps invert, giving rise to a two-dimensional triangular network of topologically protected channels. The possibility to controllably switch these topologically protected states on and off using an electric field, combined with its inherent robustness against defects and impurities, establishes a foundation for a new type of transistor with an exceptional resilience.

Realizing a one-dimensional (1D) topological insulator and identifying the lower-dimensional limit of two-dimensional (2D) behavior are crucial steps toward developing high-density quantum state networks, advancing topological quantum computing, and exploring dimensionality effects in topological materials. Although 2D topological insulators have been experimentally realized, their lower dimensional limit and 1D counterparts remain elusive. Here, we fabricated and characterized arrays of zigzag-terminated germanene nanoribbons, a 2D topological insulator with a large topological bulk gap. The electronic properties of these nanoribbons strongly depend on their width, with topological edge states persisting down to a critical width (∼2 nm), defining the limit of 2D topology. Below this threshold, contrary to the tenfold way classification, we observe zero-dimensional (0D) states localized at the ends of the ultrathin nanoribbons. These end states, topologically protected by time-reversal and mirror symmetries, indicate the realization of a 1D topological insulator with strong spin-orbit coupling. Our findings establish germanene nanoribbons as a platform for investigating 1D topology and dimensionality effects in topological materials.