Aachen Graphene Center Seminars


Title: Exotic 2D Lateral Heterostructures and Optoelectronic Devices

Abstract: Atomically thin layered materials such as graphene and transition metal dichalcogenides (TMDs) have opened a new and rich field with exotic physical properties and exciting potential applications in the “flatland” [1-8] There are enormous possibilities in combining diverse 2D materials for the unique design of ultra-smart and flexible optoelectronic devices, including transistors, light-emitting diodes, photovoltaics, photodetectors, and quantum emitters. Considerable efforts have been devoted to the van der Waals vertical hetero-integration of different 2D layered materials. On the other hand, lateral heterostructure can be fabricated only via direct growth. It can offer exciting opportunities for engineering the formation, confinement, and transport of electrons, holes, exciton, phonon, and polariton. We reported the direct fabrication of seamless, high-quality TMDs lateral heterostructures and superlattices in the chemical-vapor-deposition process, only changing the reactive gas environment in the presence of water vapor [1-5]. Our novel approach offers greater flexibility for the continuous growth of multi-junction TMDs lateral heterostructures/superlattices, controlled 1D interfaces, alloying, and layer numbers. The extent of the spatial modulation of individual TMD domains and their optical and electronic transition characteristics across the heterojunctions are studied in detail. Electrical transport measurements revealed diode behaviour across the 2D lateral junctions, promising for electroluminescence at room temperature [2-3]. Using photon energy-resolved photoconductivity mapping, long-term carrier accumulation in MoS2-WS2 lateral heterostructures was observed [5]. At the onset of photo-excitation, local carrier density was increased by two orders of magnitude and persisted for up to several days. Temperature-dependent photoluminescence from neutral exciton, trion, and defect-bound exciton provides a better understanding of the optical properties of these as-grown 2D lateral heterostructures. These studies will further supplement the quantitative evaluation of the optical properties of various 2D heterostructures to develop more complex and atomically thin superlattices and exotic 2D quantum devices.

Furthermore, the performance of most 2D heterostructure-based devices falls far below the predicted values owing to several intrinsic and extrinsic factors. These significant issues will also be discussed.


1. P. K. Sahoo et al., Nature, 553, 63–67 (2018)
2. P. K. Sahoo et al., ACS Nano 13, 12372 (2019)
3. F. Nugera et al. Small 2106600, 1 (2022)
4. Sousa et al. 2D Matererial 8, 035051 (2021)
5. S. Berweger et al. ACS Nano 14, 14080 (2020)
6. S. Chakraborty et al. iScience (2022)
7. C. Stevens et al., Nature Comm. 9, 3720 (2018)
8. M. Trushin et al. Phy. Rev. Lett. 125, 086803 (2020)

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!

04.07.2023 – Physik Hörsaal | 12:30
Prof. Yong P. Chen (Purdue University & Aarhus University)
Van der Waals Magnets based Heterostructures: platforms to engineer and probe novel magnetism

06.06.2023 – Physik Hörsaal | 12:30
Prof. Dr. Subho Dasgupta (Indian Institute of Science, Bangalore)
Printed 2D electronics with predominant intra flake transport

16.05.2023 – Physik Hörsaal | 12:30
Prof. Pawel Hawrylak (University of Ottawa)
Dirac Fermions in quantum dots in 2D materials

14.02.2023 – Physik Hörsaal | 12:00 noon
Prof. Ermin Malic (Philipps-Universität Marburg)
Exciton optics, dynamics, and transport in atomically thin materials

03.02.2023 – Physik Hörsaal | 4:00 p.m.
Prof. Jose A. Garrido (ICN2, ICREA, Spain)
Opportunities and Challenges of Graphene Neurotechnology in Neuroscience and Medical Applications


19.12.2022 – Physik Hörsaal *** Physics Colloquium Talk ***
Prof. Dmitri K. Efetov (Ludwig-Maximilians-Universität München)
Plethora of Many-Body Ground States in Magic Angle Twisted Bilayer Graphene

08.11.2022 – Physik Hörsaal
Prof. Silvia Viola Kusminskiy (RWTH Aachen University)
Light-matter interaction in antiferromagnets: probing and controlling antiferromagnetic magnons with optical photons

18.10.2022 – Physikzentrum, room 28A301 (Seminar Room II Institute of Physics) | 11:30
Prof. Afif Siddiki (Instambul University)
The local incompressibility of quantized Hall effects

14.10.2022 – Physik Hörsaal | 16:00
Prof. Yuhei Hayamizu (Tokyo Institute of Technology)
Self-assembled peptides for surface functionalization of 2D materials

11.10.2022 – Physik Hörsaal
Prof. Steven Koester (University of Minnesota & Theodore von Kármán Fellow at RWTH)
Enhancing Biosensor Response with Graphene “Lighting-Rods”

13.09.2022 – Physikzentrum, room 28A301 (Seminar Room II Institute of Physics) | 12:30
Dr. Jens Martin (Leibniz-Institut für Kristallzüchtung (IKZ) in Berlin)
“New perspectives on Layer Transfer and Artificial Crystalline Heterostructures”

04.07.2022 – ICT cube – Seminarraum 002 | 2:00 p.m.
Prof. Taiichi Otsuji (Research Institute of Electrical Communication, Tohoku University)
“Graphene Dirac plasmons for terahertz lasers, amplifiers, and detectors”

01.02.2022 – Online
Dr. Alwin Daus (Chair of Electronic Devices, RWTH Aachen University)
Flexible Electronics with Two-Dimensional and Layered Chalcogenide Compounds


09.11.2021 – Hybrid
Dr. Rebeca Ribeiro-Palau ( Université Paris-Saclay and CNRS, Centre de Nanosciences et de Nanotechnologies – C2N)
“Tunable valley currents in aligned bilayer graphene/BN”

05.10.2021 – Hybrid
Prof. Barış Şimşek, (Çankırı Karatekin University, Turkey)
“Optimum AgNPs-assisted Graphene Inks Coating on PVA-based Self-Healing Polymer Substrate with Nanostructures for Flexible Electronics”

04.05.2021 – Online
Nishta Arora, (Indian Institute of  Science in Bangalore & ELD, RWTH Aachen Universiity)
“Tuning Nonlinearities and Modal Coupling in Two Dimensional Electromechanical Resonators”

26.01.2021 – Online
Ellie Galanis & Jemma Allan (Paragraf)
“Developing and delivering graphene electronic devices at commercial quality and scale, starting with a graphene Hall effect sensor”


06.10.2020  –  Online 
Prof. Thomas Muelller (Vienna University of Technology)
“Ultrafast machine vision with 2D semiconductor photodiode arrays”

26.05.2020  –  Online 
Prof. Stephan Hofmann (Departement of Engineering, University of Cambridge, UK)
“On the Fundamental Mechanisms that underpin CVD Technology for Atomically Thin 2D Films”

Prof. Sergio O. Valenzuela
“Spin-orbit proximity phenomena and tunable spin-to-charge conversion in graphene”

Prof. Tomas Palacios (Massachusetts Institute of Technology)
“The Graphene Revolution: From Transistors to Synthetic Cells”

Prof. Barbaros Oezyilmaz (Centre for Advanced 2D materials, National University of Singapore)
“2D Amorphous Materials and other Research Efforts at the Centre for Advanced 2D Materials”

2 0 1 9

Dr. Lutz Waldecker (Dept. of Applied Physics, Stanford University, Stanford, USA)
“Dielectric screening and many-body interactions in 2D Semiconductors”

Dr. Mohammed Elsayed (RWTH Aachen)
“Thin-film Technology for Graphene-based Electronic Devices and Circuits”

Prof. Frank Schwierz (Technische Universität TU, Ilmenau, Germany)
” 2D Electronics – Opportunities and Challenges”

Prof. Tibor Grasser (Institute for Microelectronics, TU Wien, Austria)
“CaF2 Insulators for Ultrascaled 2D Field Effect Transistors”

Dr. Assaf Ya’akobovitz (Ben-Gurion University of the Negev, Israel)
“Electromechanical Behavior of Carbon Based Nano-Materials”

Dr. Sayanti Samaddar (2nd Institute of Physics, RWTH Aachen)
“Disorder induced Hydrodynamics in Graphene close to Charge Neutrality probed by Kelvin Probe Force Microscopy”

Dr. Angelika Knothe (Manchester University, UK)
“Electrostatically confined nanostructures in gaaped bilayer graphene”

Prof. Florian Libisch (TU Vienna, Austria)
“Nonlinear optics in two-dimensional materials”

Dr. Manohar Kumar (Laboratoire Pierre Aigrain Paris, France)
“Solidification of electrons in the flatland”

2 0 1 8

Dr. Daniel Schall (AMO GmbH Aachen)
“Graphene Photonics for Ultrafast Optical Communication”

Dr. Robin Dolleman (Faculty of Applied Sciences, TU Delft)
“Dynamics of interacting graphene membranes”

Dr. Inessa Bolshakova (Lviv Polytechnic Natianl University, Lviv, Ukraine)
“Development of radiation-resistant magnetic field sensors for fusion reactors”

15.06.2018, 13 bis 14 Uhr, Raum S3
Dr. Shu Nakaharai (NIMS & MANA)
“Polaritätskontrollierbare Transistoren auf 2D-Materialien”

Prof. Kirill Bolotin (FU Berlin)
“Bending, pulling, and cutting wrinkled two-dimensional materials”

Prof. Peter Bøggild (CNG-Center of Nanostructured Graphene, DTU Denmark)
“Graphene at the edge of perfection”

Dr. Gerard Verbiest (2nd Institute of Physics, RWTH Aachen)
“A graphene-based broadband displacement detector with pm resolution and its applications”

Dr. Zhenxing Wang (AMO GmbH)

“Graphene based diodes and integrated circuits for RF applications”

Stefan Wagner (Chair for Electronic devices, RWTH Aachen)
“Piezoresistive 2D material based nanoelectromechanical devices for strain sensing”

 2 0 1 7

Dr. Jens Sonntag (2nd Institute of Physics, RWTH Aachen)
“Impact of Many-Body Effects on Landau Levels in Graphene”