Seminars

Atomically thin two-dimensional materials (2DM) have already demonstrated a wide range of both interesting and useful quantum properties (as well as unusual classical properties). Some of these are related to an ultimate surface-to-volume ratio in these materials. While it brings new physics and helps applications, such a dominating role of the surface, in case of non-uniformities, may drastically increase variability of materials behavior. This is where one needs a materials characterization technique with very fine spatial resolution, beyond diffraction limit of optical microscopy.

We use various near-field microscopy to study properties of novel materials, such as heteronanotubes, twisted graphene, TMDCs, MXenes and 2d atomic polar metals. In particular, scattering Scanning Near-field Optical Microscopy (sSNOM) will be discussed in detail. While, in principle, sSNOM is capable to beat the diffraction limit, multiple bottlenecks exist on this way. One of the biggest challenges is how to interpret the data and relate it to physics of materials.

A few methods of correlated optical (mid-IR) and hyperspectral imaging will be introduced using examples of twisted graphene and 2d polar metals.  sSNOM will be shown to map polaritonic wavefunctions. The new method will be introduced to directly extract polariton dispersion from the experimental data and the polaritonic confinement in plasmonic heterostructures. Using an example of a vertical 2DM heterostructure comprised of monolayer graphene and single layer flakes of MoS2, a multidimensional imaging will be introduced, the method capable to detect lattice mismatch and work function difference in the heterostructure. We correlate it to the near-field microscopy which allows mapping of distribution for doping and strain at sub-diffractional resolution.

The recent progress in the studies of 2D materials has brought forth many experimental and theoretical works on an interesting class of materials: the so-called transition metal phosphorus trichalcogenides with the structural formula MPX₃ (M: transition metal, X: chalcogen). Here, the diversity in the M/X combination opens the possibility to tune the electronic and magnetic properties of these materials over a wide range, resulting in many interesting physical phenomena and promoting their use in various application areas. In my talk, I will present several exciting examples of studies on the electronic structure of different MPX₃ materials using photoelectron spectroscopy methods (XPS, NEXAFS, ResPES, and ARPES). Additionally, I will discuss very recent spectroscopic results accompanied by large-scale systematic DFT calculations for the graphene/MPX₃ heterosystem, addressing some controversial aspects regarding the electronic and magnetic properties of such interfaces. This talk may be of interest to a wide audience of researchers working in different fields of solid-state physics/chemistry, materials science, and in the area of 2D materials.

The physics and transport properties of graphene devices are governed by their microscopic properties. Therefore, we combine in-situ transport and Atomic Force Microscopy (AFM) measurements at low temperatures and in a magnetic field to gain insight to the quantum hall edge states in a graphene Hall bar. In the presented study, carried out at NIST Gaithersburg, USA, we map the local chemical potential along the border of the hall bar and reveal the four-fold symmetry breaking of the electron energy levels in the zero energy Landau Level.

Disordered systems in the atomic limit offer several existing possibilities in both basic science and applications difficult to realize with 2D crystals. Examples range from higher order topological insulators to perfect Li ion membranes/solid state electrolytes. In my talk I will discuss two examples.

First, I will discuss monolayer amorphous carbon (or amorphous graphene) to date the only realization of an atomically thin, free standing amorphous material [1]. Despite significant advancements in using 2D materials for integrated circuits, one crucial building block, namely a 2D ultralow-k (ULK) dielectric is missing. The challenge lies in achieving a dielectric constant (k) less than three as traditional low-k dielectrics are inherently unstable at the 2D limit. Specifically, low-k materials are necessary to minimise parasitic capacitances as the distance between conductive elements shrinks below 10 nm. Moreover, advanced architectures like gate-all-around field effect transistors (GAA FET) require even lower dielectric constants (k<2) at sub-3nm thickness. Layer-by-layer grown amorphous carbon (ML-AC), as thin as 0.8 nm, is a mechanically robust 2D ULK dielectric with k of 1.35 and dielectric strength of 28-31 MVcm-1. Moreover, ML-AC overcomes the vulnerability of existing dielectrics to ion diffusion degradation with a record metal ion diffusion time to failure (TTF) of 1010 s for even a single layer. Therefore, otherwise necessary additional layers occupying up to 3 nm can be eliminated, which is especially significant as metal line widths approach 10 nm. Combined with its low-temperature, direct and conformal growth even on a dielectric, these critical features enable substantial improvements in silicon-based semiconductor electronics and ensure compatibility with future 2D electronics [2].

Next, I will discuss niobium diselenide (NbSe2) with dilute cobalt (Co) intercalation and show that such systems spontaneously display ferromagnetism below the superconducting transition temperature (T_C). The tunnelling magnetoresistance shows a bistable state, suggesting a ferromagnetic order in superconducting Co-NbSe2 [3]. We propose a RKKY exchange coupling mechanism based on spin-triplet superconducting order parameter to mediate such ferromagnetism. Non-local lateral spin valve measurements with Hanle spin precession signals up to micrometres below T_C suggest an intrinsic spin-triplet state in superconducting NbSe2 as key ingredient.

[1] Toh, C.-T. et al., Synthesis and properties of free-standing monolayer amorphous carbon. Nature (2020).

[2] Toh, C.-T. et al., 2D Ultralow-k Amorphous Carbon, under review.

[2] Qu, Tingyu et al., Ferromagnetic Superconductivity in Two-dimensional Niobium Diselenide (arXiv).

The unique physical properties of two-dimensional materials, combined with the ability to stack unlimited combinations of atomic layers with arbitrary crystal angle, has unlocked a new paradigm in designer quantum materials. For example, when two different monolayers are brought into contact to form a heterobilayer, the electronic interaction between the two layers results in a spatially periodic potential-energy landscape: the moiré superlattice. The moiré superlattice can create flat bands and quench the kinetic energy of electrons, giving rise to strongly correlated electron systems that underpin exotic forms of magnetism and superconductivity. I will first present how optically probing the behaviour of excitons – and their interactions with itinerant carriers – can be used to distinguish the properties of correlated states in semiconductor (transition metal dichalcogenide) based moiré systems. I will then describe how we can engineer the correlated states and investigate rich phase diagrams arising due to the interplay of charge, spin, lattice, and orbital degrees of freedom in multi-orbital moiré systems.

When two atomically-thin layers of a material are stacked one atop each other, with a relative twist angle between them, properties can emerge that bear little resemblance to the behavior of the individual layers. Though much can be predicted and designed about such structures, I will share two vignettes about how my students aimed for a particular behavior but found something quite different. The first led to the discovery of the first experimentally-known “orbital magnet”, a ferromagnet in which the tiny microscopic magnets that align with each other are not electron spins but tiny circulating current loops. The second surprise was observation of resistance that skyrocketed with the application of a magnetic field, along with other striking electronic properties — this one took years to figure out, but we’ve recently explained it.

Each of these two surprises turned out to be caused by a structural feature of the layered stack which had not previously been considered important. Finally, I’ll describe a refined approach to stacking and a newly-developed technique for mapping the structure of twisted layers, which together might help us get more repeatable control of structure and thus electronic properties in such twisted systems.

Atomic-Scale Processing (ASP) is a toolbox to deposit or etch films at the atomic scale. It started mostly with atomic layer deposition (ALD), but insights have advanced this to new concepts, including atomic layer etching (ALE) and area-selective deposition (ASD). Together with EUV, this toolbox has really enabled the last few chip nodes and is driving next generation nanoelectronics. But ASP is also gaining ground in solar, batteries, memory, … In our group in Eindhoven, we focus on mechanistic research of ASP: How the interaction of precursors, ions and photons with surfaces leads to film deposition and resulting film properties. This is often done using in-situ studies, while we mostly validate our films in devices through collaborations. My own research within the group is focused on ASP for future semiconductor devices, focusing on 2D TMDs, amorphous oxide semiconductors and fluorite ferroelectrics.

Aiming to complement my materials-based research, I am currently on a sabbatical at AMO in Aachen to learn more about the device processing side of semiconductor devices. In return, in this seminar I want to give a comprehensive introductory overview of what atomic-scale processing is. I will start “tutorial-like” at the basics of ALD, moving to the concept of ALE and ASD. I will cherry-pick a few of our recent in-situ studies and focus on the main practical insights we got from that. In the end, I will focus on my own research on ALD for 2D TMDs and amorphous oxide semiconductors.

As a last point to mention, I hope that by giving an overview of what we do in Eindhoven, this seminar might spark some ideas for further collaboration!