Aachen Graphene FLAGSHIP seminars
Tuesday, February 14, 2021, 12:00 to 13:00pm
Speaker: Ermin Malic, Philipps-University Marburg
Title: Exciton optics, dynamics, and transport in atomically thin materials
Abstract: Monolayer transition metal dichalcogenides (TMDs) and related van der Waals heterostructures exhibit a rich exciton physics including bright and a variety of dark states as well as spatially separated interlayer excitons. Solving 2D material Bloch equations for excitons, phonons and photons, we obtain a microscopic access to the interplay of optics, ultrafast dynamics and transport of excitons in these technologically promising materials.
In joint theory-experiment studies, we shed light on the importance of dark excitons on (i) low-temperature photoluminescence spectra of TMD monolayers and twisted homobilayers, (ii) temperature dependent exciton-exciton annihilation processes, (iii) ultrafast charge transfer dynamics in TMD heterostructures, (iv) exciton funneling in strained TMD monolayers as well as (v) anomalous exciton transport in TMD bilayers.
The gained microscopic insights into the spatiotemporal exciton dynamics are crucial for understanding and controlling many-particle phenomena governing exciton optics, dynamics and transport in technologically promising 2D materials and related heterostructures.
Tuesday, October 11, 2021, 12:00 to 13:00pm
Title: Enhancing Biosensor Response with Graphene “Lighting-Rods”
Abstract: Graphene field-effect transistor (GFET) sensors have emerged as a promising platform for
detection of clinical biomarkers, such as DNA, RNA, proteins, and antibodies [1]. As such,
graphene is interesting to create a new class of low-cost, point-of-need diagnostic systems. In
order to be competitive for these applications, GFET sensors must provide target specificity, have
high sensitivity, and achieve a result in a short period of time. While sensitivity and specificity can
readily be achieved using appropriate surface functionalization, rapid readout remains a
challenge.
In this talk, I will show how dielectrophoresis (DEP) can be used as an effective technique to
create graphene GFET sensors with ultra-fast response. DEP is a technique by which a neutral,
but polarizable particle can be attracted toward a location where a large electric field gradient
exists [2]. Due to its atomic-scale thickness, graphene can produce enormous electric field
gradients at its edges, effectively acting as a nanoscale “lightning rod” [3]. When an AC bias is
applied, this same effect can selectively attract polarizable neutral particles via the DEP effect.
Here, I will describe the theory of DEP trapping, and show how a variety of nanoparticles can be
trapped at extremely-low voltages at graphene edges [3]. I will also show recent results whereby
trapped particles at the graphene edges can be detected electrically, and also describe several
strategies to enhance the sensitivity of DEP GFET devices [4]. Finally, I will discuss future
challenges to realize fully-functional DEP-enhanced GFET biosensors.
As part of this presentation, I will also give an introduction to the University of Minnesota (UMN),
one of the largest universities in the United Sates by student population, the Minnesota Nano
Center, which is a primary driver of nanoscale fabrication innovation in the Upper Midwest of the
United States, and also other 2D-material research in the Koester NanoDevice Group at UMN.
[1] I. Prattis, E. Hui, P. Gubeljak, G. S. Kaminski Schierle, A. Lombardo, and L. G. Occhipinti, “Graphene for biosensing applications in point-of-care testing,” Trends in Biotechnology 39, 1065-1077 (2021).
[2] A. H. Pohl, Dielectrophoresis. Cambridge University, 1978.
[3] A. Barik, Y. Zhang, R. Grassi, B. P. Nadappuram, J. B. Edel, T. Low, S. J. Koester, and S.-H. Oh, “Graphene-edge dielectrophoretic tweezers for trapping of biomolecules,” Nat. Commun. 8, 1867 (2017).
[4] N. Izquierdo, R. Li, S.-H. Oh, and S. J. Koester, “Demanding more from graphene-based sensors: design and development of multi-functional GFET with dielectrophoresis enhanced sensing capabilities,” to be presented at the 2022 Fall MRS Meeting, Boston, MA, USA, Nov. 27 – Dec. 2, 2022.
Tuesday, September 13, 2021, 12:30 to 13:30pm
Title: New perspectives on Layer Transfer and Artificial Crystalline Heterostructures
Abstract: Layer transfer introduces a paradigm shift in the way crystalline heterostructures could be
synthesized. In contrast to established growth methods of epitaxial heterostructures which
must oblige to matching conditions at the interface with respect to surface energies, lattice
parameter and crystal symmetry, layer transfer allows in principle for the combination of any
crystalline materials.
In particular, the potential to create novel artificial crystalline heterostructures has been
demonstrated with 2-dimensional van der Waals (2D vdW) materials. New functional
interfaces can be realized by novel combinations of different 2D vdW materials. More
astonishing, emerging physical properties can be produced by reassembling layers of the
same material in different ways for example by introducing a twist angle between the layers.
In this way so called Moiré materials can be created which for 2D vdW materials allow for
unique ways of band structure engineeringc, famously the formation of flat bands at magic
angle twisted bilayer graphene. It also has been realized that stacking of 2D materials creates
periodic strain patterns. Yet, because the interlayer interaction in 2D vdW materials is
intrinsically weak and intralayer bonds are always stronger, it is not possible to create defect
structures within the layer that is to produce artificial twist boundaries characterized by
screw dislocation networks. This is different with covalently or ionic bonded materials such as
oxides. For example, in oxidic perovskites the bonding character is almost ionic, making it
possible to produce nm thin films in (100)-orientation virtually without surface
reconstruction. Yet interaction to other polar surfaces remains strong and as demonstrated
in the past via wafer bonding of single crystals dislocation networks at the interface occur
after thermal annealing.
The potential of layer transfer for future crystal assembly has been recognized by Leibniz
Institut für Kristallzüchtung IKZ. After providing a brief general overview of the IKZ and the
research activities of the section "Semiconductor Nanostructures", I will discuss preliminary
results on growth, release and transfer of thin epitaxial perovskite films. As outlook, we
propose that artificial twist boundaries in nm thin perovskite films can serve as innovative
nanoengineering platform for epitaxial oxides. Advanced control of local strain gradients
offers manifold ways to influence internal degrees of freedom at the nanoscale beyond the
possibilities of classical strain engineering in heteroepitaxy and opening up new avenues to
investigate flexoelectric effects. In addition, I will present recent advances of in situ growth
of 2D vdW multilayers and heterostructures.
Tuesday, May 4, 2021, 12 to 1pm
Title: Tuning Nonlinearities and Modal Coupling in Two Dimensional Electromechanical Resonators
Abstract: The advent of two-dimensional materials such as graphene and transition metal dichalcogenides have generated immense interests in the area of nanoelectromechanical systems (NEMS). Their extraordinary properties such as low mass, atomically thin dimensions, high mechanical strength, and unique electrical and optical properties make them an excellent candidate for NEMS. These resonators are extremely sensitive to external stimuli. This is particularly important for research, from both application and fundamental point of view. Nonlinearities in these devices play a vital role in the dynamics as dimensions are reduced to atomic scale. A clear understanding of nonlinearities and the ability to control and manipulate them to enhance the performance are pivotal for applications of these devices.
In this talk, I will discuss the ability to tune and effectively nullify Duffing nonlinearity in molybdenum disulfide (MoS2) resonators. I will discuss the effect of in-built strain in the resonator on the cancellation voltage of Duffing nonlinearity. The simulations and observed results can serve as a simple guide to design and control nonlinearities and to effectively improve the linear dynamic range in these devices for next generation two dimensional resonant sensors.
Further, the multiple vibrational modes of a two-dimensional resonator are inherently coupled through the strain in the membrane and can be manipulated through parametric techniques. Parametric manipulation enables us to control the coupling among mechanical modes of resonator. I will discuss our experimental observation of strong coupling between mechanical modes of MoS2 resonator. Parametric pumping is utilized for cooling and heating of mechanical modes. This work holds promise for phonon manipulation studies in 2D membranes.
Thursday, January 16, 2020, 2 to 3pm
Tomás Palacios (MIT) on the topic: "From Transistors to Synthetic Cells"
The lecture will take place in the "Physikhörsaal" (28D001) in the physics centre of RWTH Aachen at Campus Melaten.
Abstract:
Two-dimensional materials enjoy a vast array of unique properties, from extreme thinness and mechanical
flexibility to amazing quantum physics. These properties will have a tremendous impact in future
electronics by enabling large area, high speed, ubiquitous sensing and processing. This talk will review
some of the recent progress on the use of graphene and other two-dimensional materials in these
applications. In particular, it will discuss state-of-the-art MoS and WSe transistors for ultra-low power 2 2
CMOS circuits [1-2], graphene-based chemical [3] and infrared sensors [4], large area devices for energy
harvesting [5], and a new generation of micro-systems that probe the limits of electronics.
[1] NanoLetters, 16 (2016) 7798-7806.; [2] NanoLetters, 15 (2015) 4928-4934; [3] Applied Materials
and Interfaces, 10 (2018) 16169-16176. [4] Heterogeneous Integration of 2D Materials and Devices on
a Si Platform, Chapter in Beyond CMOS Technologies for Next Generation Computer Design (2019),
Springer. [5] Nature (2019) https://doi.org/10.1038/s41586-019-0892-1
Tuesday, January 7, 2020, 12 to 1 pm
Prof. Barbaros Ozyilmaz from the National University of Singapore (NSU) gives a talk about "2D Amorphous Materials and other Research Efforts at the Centre for Advanced 2D Materials"
Tuesday, April 10, 2018, 12 to 1pm
Peter Bøggild of DTU Denmark gives a talk about "Graphene at the edge of perfection"
Friday, June 15, 2018, 1 to 2pm
Dr. Shu Nakaharai (NIMS & MANA) on the topic: "Polarity-Controllable Transistors on 2D Materials"
The lecture will take place in room S3 in building Otto-Blumenthal-Str. 2 (above the workshop) at Campus Melaten.
Abstract:
Two-dimensional (2D) materials such as transition metal dichalcogenides (TMDCs) have been expected for future channel materials due to their atomically-thin and smooth surfaces, which can lead to reducing the short channel effects in the aggressively scaled CMOS technology. On the other hand, there remain some challenging issues of carrier doping, control of carrier type and transistor polarity. In this talk, at first, we will discuss how we can overcome the issue of carrier type control in TMDC semiconductors. Here, it will be reviewed that MoTe2, in contrast to MoS2, exhibits only weak Fermi level pinning at the Schottky junctions, and it can behave as both n- and p-type transistors depending on the work function of the contact metals. Finally, it will be discussed that MoTe2 can be expected to be the ideal ambipolar channel in a unique concept of polarity-controllable transistors in which the transistor polarity (n/p) can be changed by electrostatic gating.
Wednesday, June 5, 2018, 12 to 1pm
Dr. Shu Nakaharai (NIMS & MANA) on the topic: "Polarity-Controllable Transistors on 2D Materials"
Abstract:
Two-dimensional (2D) materials such as transition metal dichalcogenides (TMDCs) have been expected for future channel materials due to their atomically-thin and smooth surfaces, which can lead to reducing the short channel effects in the aggressively scaled CMOS technology. On the other hand, there remain some challenging issues of carrier doping, control of carrier type and transistor polarity. In this talk, at first, we will discuss how we can overcome the issue of carrier type control in TMDC semiconductors. Here, it will be reviewed that MoTe2, in contrast to MoS2, exhibits only weak Fermi level pinning at the Schottky junctions, and it can behave as both n- and p-type transistors depending on the work function of the contact metals. Finally, it will be discussed that MoTe2 can be expected to be the ideal ambipolar channel in a unique concept of polarity-controllable transistors in which the transistor polarity (n/p) can be changed by electrostatic gating.