Symposium FI
Graphene and Other Emerging 2D-layered Nanomaterials: Synthesis, Properties and Potential Applications


FI-1:IL01  Nanometer-scale Characterization and Control of Two-dimensional Materials with Atomic Force Microscopy
M.R. Rosenberger, University of Notre Dame, Notre Dame, IN, USA

This talk will describe the use of atomic force microscopy (AFM) to characterize and/or control the electronic, optoelectronic, and infrared properties of two-dimensional materials (2DM) and 2DM heterostructures. I will discuss the nano-squeegee technique which uses an AFM tip to remove contaminants between layers in 2DM heterostructures. This technique largely eliminates unwanted extrinsic heterogeneity which leads to improved interlayer coupling and more accurate measurements of 2DM intrinsic properties. Next, I will describe the use of conductive AFM to detect and quantify point defects in 2DM, which enables direct correlation of defect density with optical properties. I will also present conductive AFM measurements of the local electronic properties of 2DM heterostructures. These measurements reveal that heterostructures of MoSe2/WSe2 and MoS2/WS2 at twist angles less than 1° undergo atomic-scale reconstruction to form domains of commensurate stacking. I will also describe an AFM-based measurement scheme which enables measurements of infrared absorption in 2DM at the nanometer-scale. Finally, I will present a general approach for imparting strain into 2DM with nanometer-scale precision. I will show that this technique can be used to create single photon emitters in WSe2.

FI-1:IL04  Quantum Hall Effect in Graphite Films
A. Mishchenko, Department of Physics and Astronomy, The University of Manchester, Manchester, UK

Graphite is considered a well-studied material which enjoyed more than 80 years of systematic experimental and theoretical research of electronic and transport properties. Recent developments in the field of graphene, 2D materials, and van der Waals heterostructures enabled us to revisit this elementary semimetal. Using van der Waals technology, we fabricated a series of high-quality samples of graphite films and characterised their properties using a density of states spectroscopy and transport measurements in high magnetic fields. To our surprise, we found that graphite films hundreds of graphene layers thick show robust quantum Hall effect (QHE). The unexpected QHE displays a striking dependence on layer number parity. Furthermore, in thinner graphite films we also observed fractional QHE, magnetic ordering, and other many-body physics. In this talk, I will overview these and other recent findings of unexpected physics in graphite films to catalyse further research of nontrivial physics and potential future applications in this elementary semimetal.

FI-1:IL05  Advances in Organic 2D Crystals
XINLIANG FENG, Center for Advancing Electronics Dresden & Faculty of Chemistry and Food Chemistry, Technische Universitaet Dresden, Germany

In contrast to the tremendous efforts dedicated to exploring graphene and inorganic 2D materials such as metal dichalcogenides, boron nitride, black phosphorus, metal oxides and nitrides, the study on organic 2D material systems including the bottom-up organic/polymer synthesis of graphene nanoribbons, 2D metal-organic frameworks, 2D polymers/supramolecular polymers as well as supramolecular approach to 2D organic nanostructures remains under development. In this lecture, we will present our recent efforts on the bottom-up synthetic approaches towards novel crystalline organic 2D materials with structural control at the atomic/molecular-level. 2D conjugated polymers and coordination polymers thus belong to such materials classes. The unique structures with possible tailoring of conjugated building block and conjugation length, adjustable pore size and thickness, as well as interesting electronic structure make them highly promising for a number of applications in electronics and spintronics. Other application potential of organic 2D materials will be also discussed.

FI-1:IL06  2D MXene Organic Dispersions and their Electronic Applications
CHONG MIN KOO, Materials Architecturing Research Center, Korea Institute of Science and Technology, Seoul, South Korea

MXenes, an emerging class of two-dimensional (2-D) materials composed of transition metals (M), and carbides, nitrides, or carbonitrides (X), have expanded their utility and importance in the fields of electromagnetic interference shielding, energy storage, energy harvesting, catalysis, gas sensing, electronics, and optics, due to their high electronic conductivity (~5000 S/cm), hydrophilicity, and solution processability. However, they suffer from poor stability against oxidative degradation and poor dispersion stability in organic environments. This presentation demonstrates simple and scalable ways to prepare Ti3C2Tx MXene dispersions in non-polar organic solvents. The MXene organic dispersions also exhibit strong oxidation resistance and stable long-term storage. Additionally, the stable MXene dispersions provide an opportunity to prepare printable flexible MXene films or electrodes for various flexible electronic applications including EMI shielding, flexible joule heater and LED display.

FI-1:L07  Mechanical Strain in Silicene Membranes
c. massetti2, C. Martella1, D.S. Dhungana1, C. Grazianetti1, A. Molle1, E. Bonera2, 1CNR-IMM Agrate Brianza unit, Agrate Brianza (MB), Italy; 2Università Milano-Bicocca, Milano, Italy

Since the isolation of graphene, the class of 2D materials has expanded at a fast pace. Monoelemental Xenes are an emerging class of graphene-like materials that show exceptional properties due to their 2D nature and, among them, silicene is one of the most promising aiming at the integration into devices [1].Here, we study the mechanical properties of epitaxial silicene and silicene/stanene heterostructures grown on Ag(111)/mica substrate by MBE [2]. We stress that two key technological solutions made possible the study: i)the use of cleavable Ag/mica stack as native substrates, ii)the encapsulation of the silicene layers into the Al2O3 and stanene layers to prevent silicene degradation. When disassembled from the native substrates [3], the silicene membranes can be manipulated and their mechanical response tested by micro-Raman spectroscopy under the introduction of an external strain. Under tensile strain, the bendable membranes showed a maximum Raman frequency shift rate up to 3.6 cm^-1/%strain and high stability up to one thousand bending cycles, thus envisioning their integration into flexible strain sensor devices.
[1] A.Molle et al.,Chem.Soc.Rev.47(2018)6370-6387 [2] D.S.Dhungana et al.,Adv.Funct.Mater.(2021)2102797 [3] C.Martella et al.,Adv.Funct.Mater.30(2020)2004546

FI-1:L08  Electronic Properties in MOCVD Mono- and Bilayer MoS2 on CVD Single-layer Graphene on Alpha-Al2O3 (0001) at Atomic Resolution
H. Wördenweber1, 2, S. Karthäuser1, A. Grundmann3, Z. Wang1, 2, H. Kalisch3, A. Vescan3, M. Heuken3, 4, R. Waser1, 2, S. Hoffmann-Eifert1, 1Peter Grünberg Institute 7&10, Forschungszentrum Jülich GmbH and JARA-FIT, Jülich, Germany; 2RWTH Aachen University, Aachen, Germany; 3Compound Semiconductor Technology, RWTH Aachen University, Aachen, Germany; 4AIXTRON SE, Herzogenrath, Germany

In the last decade graphene has emerged as a technologically viable bottom electrode for future 2D-device development. However, device design requires a comprehensive understanding of the correlation between structural and electrical properties from the micrometer scale down to atomic resolution. In this work we thoroughly performed a multiscale analysis on CVD single-layer graphene (SLG) on α-Al2O3 (0001) (sapphire) and on 2D-structures of the TMDC MoS2 deposited on top of SLG by means of MOCVD. In a first step we identified local variations in the SLG/sapphire interactions, i.e. differences in the bonding strength on either flat terraces or close to step-edges of sapphire leading to distinct electronic properties in the respective regions. While graphene is weakly electrostatically bound on sapphire terraces, it is nearly free-standing in the regions around step edges. Secondly, the electrical and structural properties of monolayer and bilayer semiconducting MoS2 on top of SLG were investigated. The influence of the different SLG regions on the electrical properties of MoS2 layer such as band gap and electronic states was examined. Regarding possible memristive device applications, local switching experiments of the MoS2 layers were performed using a STM and conductive-AFM.

FI-1:L09  New Way of Conceiving the Structure of Graphene
J. Niewiadomska-Kaplar, Scientific Publishing House Tab, Rome, Italy

The electrons belonging to energy sublevels s and p of the same energetic level do not coexist separately in blocks but are added together, reaching a maximum of 8 (2 + 6). In the alkanes, alkenes and alkynes the atoms do not change the way of distributing the orbitals around the nucleus but the atoms with 4 orbital s + p of which the vertices form the cube change the reciprocal spatial relations. The electrons s + p "occupy" the orbitals that are composed of 2 lobes present on opposite sides of the nucleus, which can be occupied by a single electron and a hole, by two electrons and by two holes. This new model of benzene is able to explain different characteristics of this compound and its derivatives, for example the differentiation of spin density of adjacent hydrogens in the ring, differentiation between the activating and deactivating substituents of electrophilic substitutions, degrees of aromaticity of the condensed rings and the annulenes. Graphene is the multiplication of the benzene fuel structure, therefore the determination of the exact structure of benzene is the first step in the knowledge of graphene.

FI-2:IL02  Mechanical Properties of Graphene and 2D Material Reinforcements in Composites
I.A. Kinloch, Dept. of Materials and National Graphene Institute, University of Manchester, Manchester, UK

The intriguing combination of properties of graphene related materials (e.g. mechanical, thermal, morphological and electrical) make them ideal reinforcements in composites [1]. Herein, we use model experimental systems to establish the design rules for atomically thin reinforcements with regard to the role of their diameter, thickness, chemical structure and interface [2]. We then transfer these rules to produce bulk composites using a range of matrices (thermoset, thermoplastic, elastomer and inorganic). Particular promise is shown in hybrid composite systems where graphene is used in combination with conventional fillers. [1] I.A. Kinloch, J. Suhr, J. Lou, RJ.. Young, P.M. Ajayan, Science 362 6414, 547-553, (2018) [2] R.J. Young, M. Liu, I.A. Kinloch, S. Li, X. Zhao, C. Vallés, D.G. Papageorgiou Composites Science and Technology, 154, 110-116 (2018)

FI-2:IL05  Ferroelectric Domains and Networks of Piezoelectric Domains in Twistronic Bilayers of Transition Metal Dichalcogenides (TMD)
V. FALKO, National Graphene Institute, University of Manchester, Manchester, UK

Lattice reconstruction in small-angle-twisted bilayers of TMDs gives rise to the neworks of domains with the energetically preferential stacking and domain walls, which are similar to dislocations in bulk crystals. In bilayers with antiparallel orientation of the monolayers’ unit cells, these domains correspond to the 2H stacking of the crystals with a honeycomb domain wall network hosting spots of piezoelectric charges and the corresponding quantum dots for electrons and hole at its sites with MM’ and XX’ stacking. Bilayers with parallel orientation of the monolayers’ unit cells feature triangular-shape domains with a twin MX’ and XM’ stacking order, separated by partial screw dislocations network. Such domains feature weak ferroelectric polarisation at the interface of the two monolayer crystals, which gives rise to their tunability using the out-of-plane electric field which promotes domains with one energetically favourable polarisation (e.g., MX’). We find the threshold electric field at which parts of the partial domain wall dislocations merge into perfect (full) screw dislocation at the border of consecutive MX’ domains, and, then, demonstrate the universal scaling of the overall domain wall shapes.

FI-2:L09  2D Materials Beyond Graphene
Y. Gogotsi, C.E. SHUCK, A.J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, PA, USA

Since the discovery of graphene in 2004, two-dimensional (2D) materials have attracted attention owing to their desirable optical, electronic, and chemical properties. Following graphene, many classes of 2D materials were synthesized, with chemistries spanning nearly the entire periodic table, resulting in unique properties and applications for each family of 2D materials. MXenes, 2D transition metal carbides, nitrides, and carbonitrides were discovered in 2011 and have the general formula: Mn+1XnTx. To date, more than 30 stoichiometric MXenes have been discovered, including ordered double transition metal MXenes, in-plane ordered MXenes, and countless solid-solution MXenes allowing for tunable properties and capabilities. MXenes have high conductivity (>20,000 S/cm), are natively hydrophilic, can be produced scalably, have a redox-capable surface, and are easily processible. Due to these properties, MXenes have been widely studied for electrochemical energy storage, electromagnetic interference shielding, biomedicine, environmental remediation, and others. Special attention is placed in recent advancements in the MXene field, including the discovery of the M5X4Tx MXene and tunable solid-solution MXenes.

FI-3:IL01  The Xene Generations: Details, Methods and Perspectives of Epitaxial Single-element Two-dimensional Materials
A. Molle, CNR-IMM, unit of Agrate Brianza, Agrate Brianza, Italy

Isolation of graphene is a milestone in condensed matter physics that paves the way to a new class of 2D synthetic single-element materials referred to as Xenes [1,2] This materials research frontier has currently come up into two generations of Xenes: the first one related to elements of the IV column of the periodic table (e.g., silicene, germanene, and stanine), and the second one to emerging elements of the adjacent columns (e.g. borophene, antimonene, tellurene, etc.). I will pay attention to the epitaxial methodologies, new Xene configurations and processes (e.g. Xene heterostructures [3]), and delamination schemes [4]) aiming at determining key points for nanotechnology applications, e.g. scalability, quality, and stability. Each aspects will be substantiated with atomically resolved and/or surface-sensitive data. Finally, the ongoing efforts to devise and realize Xene-based devices are summarized [5].
[1] A. Molle et al., Nature Materials 2017, 16, 163. [2] C. Grazianetti et al, Phys Status Sol RRL 2020, 14, 1900439. [3] C. Martella et al., Adv. Funct. Mater. 2020, 30, 2004546. [4] D. S. Dhungana et al., Adv. Funct. Mater. 2021, 31, 2102797. [5] A. Molle et al., Chem. Soc. Rev2018, 47,6370.

FI-3:IL02  Synthetic Route Towards Pure Phase of WS2 & MoS2 Inorganic Nanotubes and their Unusual Properties
A. Zak, S. Ghosh, C. Pallellappa, HIT-Holon Institute of Technology, Holon, Israel; T. Livneh, Nuclear Research Center, Negev, Israel; I. Kaplan-Ashiri, Weizmann Institute of Science, Israel; Y. Zhang, Max Planck Institute for Solid State Research, Stuttgart, Germany; Y. Iwasa, The University of Tokyo, Japan; V. Bruser, Leibnitz Institute of Plasma, Germany; A. Di Bartolomeo, University of Salerno, Italy

Inorganic nanotubes (INTs) of WS2 and MoS2 demonstrate unique properties due to their nanosize, closed-cage arrangement of the layers into chiral tubes and mechanical strength. An advance in extremely complicated high temperature synthesis of INT-MoS2 by vapor-gas-solid reaction of Mo oxides with H2/H2S gases will be presented. Bulk photovoltaic effect (BPVE) in INT-WS2 was recently discovered. The BPVE does not require p–n junctions of traditional PVE for generation of electric current, and occurs due to the intrinsic properties of INT-WS2: small band gap (1.4-2.1 eV), broken inversion symmetry and polar structure. The photocurrent in the nanotube-based device was orders of magnitude larger than in other BPVE materials. This progress is particularly important for environmentally benign energy harvesting because the efficiency of PVE has been almost reached the theoretical limit. It was shown that the resistivity of the individual WS2 nanotubes exponentially increases with strain withstanding over 12% elongation without rupture. The large variation of resistivity with stress enables the use of WS2 INTs as piezoresistive sensors. Single wall (SW) few nm diameter WS2 nanotubes were produced by high-power plasma irradiation of multiwall (MW) WS2 INTs. Low temperature cathodoluminescence (CL) of SWINTs demonstrates blue shift compared to MWINTs evidencing quantum confinement effect. By varying the diameter and number of layers in INTs the electronic structure of layered nanotubes can be engineered.

FI-3:IL05  2D Materials: Inorganic Nanotubes and Fullerene-like Nanoparticles, an Update
R. Tenne, Weizmann Institute of Science, Rehovot, Israel

After almost 100 years of research inorganic layered (2D) materials, like MoS2, are currently used as catalysts, lubricants, and perhaps most importantly in rechargeable Li- ion batteries. Much research is currently focused on monolayers (beyond graphene) of 2D materials and hybrids thereof in relation to their electronic and optoelectronics properties. After a short briefing on the history of 2D materials research,1,2 the concepts which led to the first synthesis of hollow-cage nanostructures, including nanotubes (INT) and fullerene-like (IF) nanoparticles from 2D compounds, will be presented. The progress with the high-temperature synthesis and characterization of new inorganic nanotubes (INT) and fullerene-like (IF) nanoparticles (NP) will be presented. In particular, the synthesis and structure of nanotubes from the ternary and more recently quaternary “misfit” layered compounds (MLC), like LnS-TaS2 (Ln= La, Ce, Gd, etc), CaCoO-CoO2 and numerous other MLC will be discussed. Major progress has been achieved in elucidating the structure of INT and IF using advanced microscopy techniques, like aberration corrected TEM and related techniques. Mechanical, electrical and optical measurements of individual WS2 nanotubes reveal their unique quasi-1D characteristics. This analyses demonstrate their altered behavior compared with the bulk phase, including quasi-1D superconductivity. Applications of the IF/INT as superior solid lubricants and reinforcing variety of polymers and light metal alloys was demonstrated. Some of this research resulted in commercial products (a few spin-off companies) which are exploited world-wide with rapidly expanding marketshare. Few recent studies indicate that this brand of nanoparticles is less toxic than most nanoparticles. With expanding product lines, manufacturing and sales, some of these nanomaterials are gradually becoming an industrial commodity.
1. L. Panchakarla, B. Visic and R. Tenne, “Perspective”, J. Am. Chem. Soc. 2017, 139, 12865-12878. 2. M. Serra, R. Arenal and R. Tenne, Nanoscale 11, 8073-8090 (2019).

FI-3:L06  Two-dimensional MXenes with 5 Atomic Layers of Transition Metals: Mo4VC4Tx and Beyond
C.E. Shuck, G. Deysher, K. Hantanasirisakul, K. Maleski, A. Sarycheva, B. Anasori, Y. Gogotsi, A.J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, PA, USA; N.C. Frey, A. Foucher, V.B. Shenoy, E.A. Stach, Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA, USA

MXenes have a general formula of Mn+1Xn, typically described as n=1-3, where M is a transition metal (Ti, Nb, V, Mo, etc.) and are interleaved with layers of C and/or N (shown as X). Here, we report on the synthesis and characterization of the first MXene Mo4VC4, with 5 atomic layers (n=4), as well as its precursor, Mo4VAlC4. TEM and XRD showed the structure of this phase is P63/mmc similar to other MAX phases. However, this structure has a twinned set of M-layers, unique in the MAX/MXene family. Compositionally studied via EDS and XPS, the MXene composition was determined to be Mo4.10V0.90C2.99. HRSTEM, Raman spectroscopy, and DFT indicate that the crystal structure contains a solid solution of Mo and V. DFT calculations also indicate that other n=4 transition metal MAX phases (M'4M"AlC4) may be possible, suggesting that more M5C4Tx MXenes can be synthesized. In addition, UV-vis-NIR spectroscopy, temperature-dependent resistivity measurements, and thermogravimetric analysis provide additional characterization on the optical, electronic, and thermal properties of this new Mo4VC4 MXene. This study provides a new subfamily of MXenes with five atomic layers of transition metals, allowing for wider range of compositions for more control over properties.

FI-3:IL07  Thickness and Lateral Engineering of 2D Materials
L. CAMILLI, Department of Physics, University of Rome “Tor Vergata”, Rome, Italy

The physical properties of two-dimensional (2D) materials depend strongly on the number of layers. Consequently, methods for controlling their thickness with atomic layer precision are highly desirable. Such methods can be divided in bottom-up or top-down methods. It depends on whether the number of layers of a given 2D material is controlled during the growth or its thickness is later reduced down to a chosen value. After reviewing some of the reported methods, we will present our own approach that is based on a scalable and controllable oxidation/etching process. Notably, the top layer(s) – or part thereof – is first oxidized in air; then the oxidized regions are selectively removed upon immersion in a proper etchant (water in our case). Here, we demonstrate that this thinning strategy works for several Ge-based 2D materials, however it can be extended to other crystals upon proper choice of the oxidation/etching reagent. This method supports 2D material-based device applications, e.g., in electronics or optoelectronics, where a precise control over the number of layers is needed. Finally, we also show that when used in combination with lithography, our method can be used to define precise local patterns in the 2D materials.

FI-3:L08  Sustainable and Scalable Liquid-phase Exfoliation of Graphene-like Materials with Nontoxic Polarclean Solvent
J. DE SANTIS1, V. Paolucci1, G. Di Iorio1, A. Politano2, C. Cantalini1, 1Department of Industrial and Information Engineering and Economics, University of L’Aquila, Italy; 2Department of Physical and Chemical Sciences, University of L’Aquila, Italy

Liquid phase exfoliation (LPE) is the most scalable technique to produce high quality 2D materials. The main open challenge is related to the quest of green solvents to replace conventional toxic ones. Here we demonstrate the suitability of Polarclean green solvent for LPE of layered materials for the case-study examples of Graphene, WS2 and MoS2. We performed a direct comparison, in the same processing conditions, with LPE by using NMP solvent. Inks obtained from LPE sonicated assisted, were characterized by AFM, SEM, HRTEM, STEM and Raman Spectroscopy. The yield of the flakes (with thickness <5 nm) obtained by using Polarclean is increased by ~350% with respect to the case of using NMP, maintaining comparable values of average lateral size, which even reaches ~10 µm for the case of graphene produced by exfoliation in Polarclean. Correspondingly, the density of defects is reduced with Polarclean-assisted exfoliation, as evidenced by the I(D)/I(G) ratio in Raman spectra of graphene, as low as 0.07±0.01. Our results indicate that Polarclean represents a green candidate for large-scale production of inks based on 2D materials, which allows to expand the use of 2D materials in different fields, for which conventional solvents have represented serious obstacles due to their toxicity.

FI-4:IL02  Graphene for Waveguide-integrated Optoelectronics
C. Coletti, Center for Nanotechnology Innovation @ NEST, Istituto Italiano di Tecnologia, Pisa, Italy; Graphene Labs, Istituto Italiano di Tecnologia, Genova, Italy

To make graphene realistically appealing for a number of optoelectronic applications a feasible way to scale-up high-quality material with bottom-up synthetic approaches has to be found. In this talk, I will present the deterministic synthesis via chemical vapor deposition (CVD) of single-crystal graphene matrixes which can be straightforwardly integrated on existing photonic platforms [1]. The electrical properties of these graphene single-crystals are shown to be comparable to those of the gold standard, exfoliated graphene [2]. It will be presented how graphene matrixes can be integrated to fabricate high-performing photonic building blocks [3,4]. An alternative approach to obtain wafer-scale high-quality graphene on the c-plane of Al2O3(0001) substrates with a metal-free CVD approach will be discussed as a potential pathway for the front-end-of-line (FEOL) integration of this material in photonics [5].
1. Miseikis et al. 2D Materials 4, (2), pp. 021004 2017 2. Pezzini et al. 2D Materials 7 (4), 041003 2020 3. Giambra et al. ACS nano 15 (2), 3171-3187 2021 4. Miseikis et al. ACS Nano 2020, 14, 9, 11190–11204 2020 5. Mishra et al. Small, 15 (50), 1970273, 2019
Funding from European Union’s Horizon 2020 under grant agreement 881603-Graphene Core3 is acknowledged

FI-5:IL01  Modelling of WS2 Nanostructures: Optical Properties and Interaction with Hydrogen
J.A. Alonso, Dept. of Theoretical, Atomic and Optical Physics, University of Valladolid, Valladolid, Spain; A. Zak, A. Laikhtman, Physics Department, Faculty of Sciences, Holon Institute of Technlogy, Holon, Israel; J.I. Martinez, ESISNA Group, Institute of Materials Science of Madrid (CSIC), Madrid, Spain

The interaction of hydrogen with layered materials is relevant for catalysis, sensors and fuel cells. Density functional theory has been used to investigate the interaction of hydrogen with WS2 nanotubes and multilayers. H2 physisorbs on W atoms and atomic H chemisorbs on S atoms. The chemisorption energy depends on the tube diameter. Diffusion of H2 on the surface of WS2 finds low barriers, which helps explaining the experimental results on the dependence of hydrogen concentration with temperature. Intercalation of H2 between layers is endothermic. Intercalating H atoms is energetically favorable, but does not compensate the cost of dissociating the molecules. The presence of a full H2 monolayer on the surface facilitates the intercalation of H2 between layers underneath. Cathodoluminescence experiments confirmed by calculations give evidence of the effects of quantum confinement in single-wall WS2 nanotubes: as compared to multiwall nanotubes, a blue-shift is observed in the main peak of the emission spectrum. An opposite red-shift in the electronic gap is observed due to the curvature of the nanotrubes compared to planar layers. The capacity of manipulating the morphology and the number of layers to tailor the electronic structure opens the door to devices and applications.

FI-5:IL03  A High Throughput and Unbiased Machine Learning Approach for Classification of Graphene Dispersions
Md.J. Abedin, T. Barua, M. Shaibani, M. Majumder, Department of Mechanical & Aerospace Engineering, Monash University, Clayton, Australia

Significant research to define and standardize terminologies for describing stacks of atomic layers in bulk graphene materials has been undertaken. Most methods to measure the stacking characteristics are time consuming and are not suited for obtaining information by directly imaging dispersions. Conventional optical microscopy has difficulty in identifying the size and thickness of a few layers of graphene stacks due to their low photon absorption capacity. Utilizing a contrast based on anisotropic refractive index in 2D materials, it is shown that localized thickness-specific information can be captured in birefringence images of graphene dispersions. Coupling pixel-by-pixel information from brightfield and birefringence images and using unsupervised statistical learning algorithms, three unique data clusters representing flakes (unexfoliated), nanoplatelets (partially exfoliated), and 2D sheets (well-exfoliated) species in various laboratory-based and commercial dispersions of graphene and graphene oxide are identified. The high-throughput, multitasking capability of the approach to classify stacking at sub-nanometer to micrometer scale and measure the size, thickness, and concentration of exfoliated-species in generic dispersions of graphene/graphene oxide are demonstrated.

FI-5:IL04  From the Atomic Structure to the Optoelectronic Properties Studies of 2D Materials via TEM
R. Arenal, Laboratorio Microscopias Avanzadas, U. Zaragoza, Zaragoza, Spain; Fundacion ARAID, Zaragoza, Spain; Instituto de Nanociencia y Materiales de Aragon (INMA), CSIC-U. Zaragoza, Zaragoza, Spain

The recent advances in transmission electron microscopes (TEM) bring access to electron probes of one angstrom within energy resolutions of <100 meV even working at low acceleration voltages. These performances offer new possibilities for probing the optical, dielectric and electronic properties of nanomaterials with unprecedented spatial information, as well as for studying the atomic configuration of nanostructures. In this contribution, I will present a selection of recent works involving all these matters. These works will concern the study of the atomic structure & configuration of 2D atomically thin nanostructures (in pristine and hybrid forms) as well as the optoelectronic properties studies carried out via EELS measurements. These works will illustrate the excellent capabilities offered by the use of a Cs probe-corrected (S)TEM, combined with the use of a monochromator, to study these properties within a very good spatial resolution. In summary, these studies elucidate critical questions concerning the local chemistry and the structure of these materials. This detailed knowledge is essential for better understanding their outstanding properties.
Research supported by MICINN (PID2019-104739GB) & EU-H2020 “Graphene Flagship” (881603) and “ESTEEM3” (823717).

FI-6:IL01  Synergizing High Capacitance with Fast Charging: Pseudocapacitance in 2D Materials
M.R. Lukatskaya, Department of the Mechanical and Process Engineering, ETH Zürich, Zürich, Switzerland

As electronic technology advances, the need in safe and long-lasting energy storage devices that occupy minimum volume arises. Short charging times of several seconds to minutes, with energy densities comparable to batteries, can be achieved in pseudocapacitors: a sub-class of supercapacitors, where capacitance is mediated by fast redox reactions and thus enables at least an order of magnitude more energy to be stored than in typical double layer capacitors. Transition metal oxides (e.g. RuO2, MnO2) and conducting polymers (e.g. polyaniline) serve as typical examples. However, these materials are often high in cost and/or suffer from low cycling stability. As a result, the search for new pseudocapacitive materials constitutes an important direction. In my talk, I will discuss how the key performance metrics of pseudocapacitors – capacitance and charging rates – can be pushed to the limits in the 2D materials that combine good electrical and ionic conductivities (ensuring fast charge transfer and hence charging rates) with high density of redox-active sites. In particular, I will discuss the electrochemistry of 2D transition metal carbides (MXenes) and 2D conductive metal-organic frameworks, with an emphasis on the mechanism of charge storage and electrode design.

FI-6:L02  Graphite Superlubricity Unabled by Triboinduced Nanocontacts
R. BUZIO, A. Gerbi, C. Bernini, CNR-SPIN, Genova, Italy; L. Repetto, Department of Physics, Università di Genova, Genova, Italy; A. Vanossi, International School for Advanced Studies (SISSA) and CNR-IOM, Trieste, Italy

A central playground for tribology is the investigation of ultralow friction regimes collectively named superlubricity [1]. This is usually observed at the nanoscale, but recent colloidal-probe Atomic Force Microscopy experiments involving graphite and 2D materials reported superlubricity at the microscale [2], enabled by formation of a transfer layer. It is claimed that superlubricity reflects crystalline incommensurability at the sliding interface, albeit direct evidences still lack. Here we explore the morphology and friction response of the triboinduced transfer layer, for a micrometric colloidal AFM probe sliding on graphite under ambient conditions [3]. We show that the transfer layer consists of nanosized multilayer graphene flakes, that behave as contact nanoasperities dissipating mechanical energy via atomic-scale stick-slip instabilities. Notably, we observe load-driven transitions to nealy dissipationless sliding, that agree with the single-asperity Prandtl-Tomlinson model. This indicates that the nanoscale superlubricity mechanisms may also underpin the ultralow friction states found in mesoscopic homointerfaces.
[1] M.Z. Baykara et al., Appl. Phys. Rev. 5, 041102 (2018) [2] J. Li et al., Advanced Science 1700616 (2017) [3] R. Buzio et al., Carbon 184, 875 (2021)

FI-6:L03  Intercalation of Alkaline-ions in 3D-aeromaterial Consisting of MoS2
P. HOLTZ,  S. Hansen, R. Adelung, Functional Nanomaterials, Department of Materials Science and Engineering, CAU Kiel, Kiel, Germany

The human brain works with an exceptionally low power consumption but high information output. Although the brain seems complex, there are some striking similarities to alkaline batteries as both function with a liquid electrolyte based electrochemical system and are able to store energy with different kinds of charge transport carriers. In order to mimic the spatial arrangement of a neuron inside the brain, a special 3D network is designed in this study using tetrapodal building blocks of sacrificial ZnO. For the realization of the battery-like electrodes, the 3D-network is infiltrated with a dispersion of both, the 2D transition metal dichalcogenide (TMDC) molybdenum disulfide (MoS2) and exfoliated graphene (EG). By etching ZnO away, one gets a light, mechanically stable and 3D shaped aero-TMDC as the anode material of a battery. The aero-material is incorporated into a coin cell as anode material with metallic Li as the cathode material. The electrolyte is varied with different conducting salts containing also Na+/K+-ions, which lead to a memristive switching effect by intercalating into the interlayer of the MoS2 structure. The systems are then characterized by cyclic voltammetry and galvanostatic tests and compared to further grasp the function of the different kind of ions.
Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) –Project-ID 434434223 –SFB 1461.

FI-6:L04  Nanotribology of AFM Probes Functionalized by Graphene
A. GERBI, R. BUZIO, C. Bernini, CNR-SPIN, Genova, Italy; L. Repetto, Department of Physics, Università di Genova, Genova, Italy; A. Vanossi, International School for Advanced Studies (SISSA) and CNR-IOM, Trieste, Italy

A central playground for tribology is the investigation of ultralow friction regimes collectively named superlubricity [1,2]. Superlubricity is observed at the nanoscale in Atomic Force Microscopy AFM experiments, due to crystalline incommensurability at the sliding interface. Strategies to promote structural lubricity across the length scales are currently underway, and include attempts to functionalize AFM tips by 2D materials [3-5]. This responds to the need to prepare probes of crystalline-defined structure to enhance manifestation and robustness of structural lubricity up to the microscale. Here we report on the nanotribology of AFM probes functionalized by graphene. Different strategies are considered, e.g. the formation of triboinduced transfer layers [6] or deposition of flakes from the liquid phase. An atomic-scale comparative study demonstrates overall effective achievement of superlubricity, albeit with a different friction phenomenology according to the functionalization method.
[1] M.Z. Baykara et al., Appl. Phys. Rev. 5, 041102 (2018); [2] O. Hod et al., Nature 563, 485 (2018); [3] M. Daly et al., ACS Nano 10, 1939 (2016); [4] Y. Liu et al., ACS Nano 12, 7638 (2018); [5] J. Li et al., Nanoscale 12, 5435 (2020); [6] R. Buzio, A. Gerbi et al. Carbon 184, 875 (2021).

FI-6:IL06  Computational and Experimental Approaches for Printed 2D-based Devices
G. FIORI, D. MARIAN, University of Pisa, Dipartimento Ingegneria Informazione, Pisa, Italy

The paradigm of distributed electronics as within the Internet of Things (IoT) is requiring a new generation of flexible, wearable, and conformable devices, which, in addition, have to comply with recyclability and environmentally friendliness requirements, much needed in our society. >From this point of view, paper represents the best available flexible substrates, while two-dimensional materials, with their outstanding electrical and mechanical properties could be the key enabling technology for the industrialization of flexible electronics. In this talk, we will provide an overview of the potential of this technology, showing already achieved results, both in terms of passive and active devices for flexible electronics, that can open the path towards the development of LSI integrated circuits on paper subtrates. Such goal can be achieved only through an approach based on synergic interactions between experimental and theoretical activity based on multi-scale simulations, which I will briefly introduced.


Cimtec 2022

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