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Nanomechanical measurements of minimally twisted van der Waals materials remained elusive despite their fundamental importance for device realisation. Here, we use Ultrasonic Force Microscopy (UFM) to locally quantify the variation of out-of-plane Young's modulus in minimally twisted double bilayer graphene (TDBG). We reveal a softening of the Young's modulus by 7% and 17% along single and double domain walls, respectively. Our experimental results are confirmed by force-field relaxation models. This study highlights the strong tunability of nanomechanical properties in engineered twisted materials, and paves the way for future applications of designer 2D nanomechanical systems.

Preparation, analysis and lithium storage performance of a series of nitrogendoped carbon nanotube sponges (CNX) is presented in this work. The synthesis was performed using an aerosol-assisted chemical vapor deposition (AACVD) in a bi-sprayer system by using various carbon and nitrogen precursors made of mixtures of benzylamine with toluene, urea, pyridine and 1,2-dichlorbenzene, with ferrocene as catalyst. A series of physico-chemical analysis techniques are used to characterize the composition and the morphology of the obtained materials, and a correlation of these with the lithium storage performances is attempted. The samples reveal an interconnected core-shell CNX fiber morphology with a CNT-core surrounded by an amorphous carbon shell. Appealing lithium storage performances are attained, while also considering aspects of safety, low potential, and long-term cycling stability. The best performing sponges display a high specific capacity (223 mAh g−1) when cycled in a practically relevant voltage window (0.01–1V vs. Li), high first cycle (90%) and long-term cycling (99.3%) coulombic efficiencies and excellent capacity retention after 1500 cycles. This study further analyses the interplay between the morphology and the physico-chemistry of nitrogen-doped carbon nanotube materials for Lithium storage and provides guidelines for future developments.

We report on large spin-filtering effects in epitaxial graphene-based spin valves, strongly enhanced in our specific multilayer case. Our results were obtained by the effective association of chemical vapor deposited (CVD) multilayer graphene with a high quality epitaxial Ni(111) ferromagnetic spin source. We highlight that the Ni(111) spin source electrode crystallinity and metallic state are preserved and stabilized by multilayer graphene CVD growth. Complete nanometric spin valve junctions are fabricated using a local probe indentation process, and spin properties are extracted from the graphene-protected ferromagnetic electrode through the use of a reference Al2O3/Co spin analyzer. Strikingly, spin-transport measurements in these structures give rise to large negative tunnel magneto-resistance TMR = −160%, pointing to a particularly large spin polarization for the Ni(111)/Gr interface PNi/Gr, evaluated up to −98%. We then discuss an emerging physical picture of graphene−ferromagnet systems, sustained both by experimental data and ab initio calculations, intimately combining efficient spin filtering effects arising (i) from the bulk band structure of the graphene layers purifying the extracted spin direction, (ii) from the hybridization effects modulating the amplitude of spin polarized scattering states over the first few graphene layers at the interface, and (iii) from the epitaxial interfacial matching of the graphene layers with the spin-polarized Ni surface selecting well-defined spin polarized channels. Importantly, these main spin selection effects are shown to be either cooperating or competing, explaining why our transport results were not observed before. Overall, this study unveils a path to harness the full potential of low Resitance. Area (RA) graphene interfaces in efficient spin-based devices.

In the context of graphene-based composite applications, a complete understanding of charge conduction in multilayer reduced graphene oxides (rGO) is highly desirable. However, these rGO compounds are characterized by multiple and different sources of disorder depending on the chemical method used for their synthesis. Most importantly, the precise role of interlayer interaction in promoting or jeopardizing electronic flow remains unclear. Here, thanks to the development of a multiscale computational approach combining first-principles calculations with large-scale transport simulations, the transport scaling laws in multilayer rGO are unraveled, explaining why diffusion worsens with increasing film thickness. In contrast, contacted films are found to exhibit an opposite trend when the mean free path becomes shorter than the channel length, since conduction becomes predominantly driven by interlayer hopping. These predictions are favorably compared with experimental data and open a road toward the optimization of graphene-based composites with improved electrical conduction., The authors acknowledge support from the Flag-Era JTC 2017 project ‘ModElling Charge and Heat trANsport in 2D-materIals based Composites ─ MECHANIC’. M.N.Ç. and H.S. acknowledge support from TÜBİTAK (117F480). The numerical calculations reported in this paper were partially performed at TÜBİTAK ULAKBİM, High Performance and Grid Computing Center (TRUBA resources). A.A. and S.R. are supported by MECHANIC reference number: PCI2018-093120 funded by Ministerio de Ciencia, Innovacion y Universidades and the European Union Horizon 2020 research and innovation programme under Grant Agreement No. 881603 (Graphene Flagship). ICN2 is funded by the CERCA Programme/Generalitat de Catalunya, and is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). V.-H.N. and J.-C.C. acknowledge financial support from the Fédération Wallonie-Bruxelles through the ARC Grants (No. 16/21-077 and No. 21/26-116), from the European Union’s Horizon 2020 Research Project and Innovation Program ─ Graphene Flagship Core3 (No. 881603), from the Flag-ERA project (No. R.8010.19), and from the Belgium FNRS through the research projects (No. T.0051.18 and No. T.029.22F). Computational resources have been provided by the CISM supercomputing facilities of UCLouvain and the CECI consortium funded by F.R.S. -FNRS of Belgium (No. 2.5020.11). Computational resources have been provided by the CISM supercomputing facilities of UCLouvain and the CECI consortium funded by F.R.S. -FNRS of Belgium (No. 2.5020.11).

Crystal impurities, such as atomic vacancies, are known to modulate the charge transport characteristics of two-dimensional (2D) materials. Here, we apply a first-principles-enriched tight-binding modelling approach to assess the influence of sulfur vacancies on the electronic structure and quantum transport characteristics of MoS2 monolayers. To this end, an sp3d5 orthogonal tight-binding (oTB) model of the pristine and defective MoS2 monolayer is mapped with electronic structure calculations performed at the density functional theory level and subsequently used in the real-space Kubo−Greenwood (KG) scheme for charge transport simulations. The calculated charge carrier mobility is found to be sensitive to both the density and spatial arrangement of vacancies. Our oTB/KG simulations predict a drop of mobility by two orders of magnitude when the vacancy concentration is increased from 0.1 to 3%, in excellent agreement with experimental results. The simulation of realistic samples (including specific types of defects) pave a new route toward the accurate understanding and the possible prediction of 2D materials for nanoelectronic devices.

Strain engineering of graphene takes advantage of one of the most dramatic responses of Dirac electrons enabling their manipulation via strain-induced pseudo-magnetic fields. Numerous theoretically proposed devices, such as resonant cavities and valley filters, as well as novel phenomena, such as snake states, could potentially be enabled via this effect. These proposals, however, require strong, spatially oscillating magnetic fields while to date only the generation and effects of pseudo-gauge fields which vary at a length scale much larger than the magnetic length have been reported. Here we create a periodic pseudo-gauge field profile using periodic strain that varies at the length scale comparable to the magnetic length and study its effects on Dirac electrons. A periodic strain profile is achieved by pulling on graphene with extreme (>10%) strain and forming nanoscale ripples, akin to a plastic wrap pulled taut at its edges. Combining scanning tunneling microscopy and atomistic calculations, we find that spatially oscillating strain results in a new quantization different from the familiar Landau quantization observed in previous studies. We also find that graphene ripples are characterized by large variations in carbon-carbon bond length, directly impacting the electronic coupling between atoms, which within a single ripple can be as different as in two different materials. The result is a single graphene sheet that effectively acts as an electronic superlattice. Our results thus also establish a novel approach to synthesize an effective 2D lateral heterostructure - by periodic modulation of lattice strain., 18 pages, 5 figures and supplementary information

Spintronics is a quantum technology which aims at adding the spin quantum degree of freedom to conventional CMOS electronics. Since the discovery of the giant magneto-resistance in 1988, considered as the birth of this field, spintronics continues flooding the market with plethora of devices used in everyday life applications such as hard drive read heads or magnetic random-access memories, and so on. From a fundamental research perspective, the field is still blooming bringing post-CMOS perspectives technologically closer to the reality with, for instance, prototypes of all-spin-logic circuits and neuromorphic chips. To sustain this intense research activity, a quest for new platform materials is also taking place not only to enhance existing performances but also to generate novel functionalities. In this vein, carbon nanostructures such as molecules, graphene, and carbon nanotubes are among the most sought-after materials. In this review, spin transport experiments in carbon nanotubes and graphene are first detailed and then, the necessity to consider new hybrid interfaces are highlighted for a better control of the spin injection at the quantum device level.

When confined in circular cavities, graphene relativistic charge carriers occupy whispering gallery modes (WGMs) in analogy to classical acoustic and optical fields. The rich geometrical patterns of the WGMs decorating the local density of states offer promising perspectives to devise new disruptive quantum devices. However, exploiting these highly sensitive resonances requires the transduction of the WGMs to the outside world through source and drain electrodes, a yet unreported configuration. Here, we create a circular p-n island in a graphene device using a polarized scanning gate microscope tip and probe the resulting WGM signatures in in-plane electronic transport through the p-n island. Combining tight-binding simulations and the exact solution of the Dirac equation, we assign the measured device conductance features to WGMs and demonstrate mode selectivity by displacing the p-n island with respect to a constriction. This work therefore constitutes a proof of concept for graphene whisperitronic devices.

The control of spin current is pivotal for spintronic applications, especially for spin-orbit torque devices. Spin Hall effect (SHE) is a prevalent method to generate spin current. However, it is difficult to manipulate its spin polarization in nonmagnet. Recently, the discovery of spin current in ferromagnet offers opportunity to realize the manipulation. In the present work, the spin current in van der Waals ferromagnet Fe3GeTe2 (FGT) with varying magnetization is theoretically investigated. It has been observed that the spin current in FGT presents the nonlinear behavior with respect to magnetization. The in-plane and out-of-plane spin polarization emerges simultaneously, and the bilayer FGT can even exhibit arbitrary spin polarization thanks to the reduced symmetry. More intriguingly, the correlation between anomalous Hall effect (AHE) and spin anomalous Hall effect (SAHE) has been interpreted from the aspect of Berry curvature. This work illustrates that the interplay of symmetry and magnetism can effectively control the magnitude and spin polarization of the spin current, providing a practical method to realize exotic spin-orbit torques.

Close to a magical angle, twisted bilayer graphene (TBLG) systems exhibit isolated flat electronic bands and, accordingly, strong electron localization. TBLGs have hence been ideal platforms to explore superconductivity, correlated insulating states, magnetism, and quantized anomalous Hall states in reduced dimension. Below a threshold twist angle ($\sim$ $1.1^\circ$), the TBLG superlattice undergoes lattice reconstruction, leading to a periodic moir�� structure which presents a marked atomic corrugation. Using a tight-binding framework, this research demonstrates that superlattice reconstruction affects significantly the electronic structure of small-angle TBLGs. The first magic angle at $\sim$ $1.1^\circ$ is found to be a critical case presenting globally maximized electron localization, thus separating reconstructed TBLGs into two classes with clearly distinct electronic properties. While low-energy Dirac fermions are still preserved at large twist angles $> 1.1 ^\circ$, small-angle ($\lesssim 1.1^\circ$) TBLG systems present common features such as large spatial variation and strong electron localization observed in unfavorable AA stacking regions. However, for small twist angles below $1.1 ^\circ$, the relative contribution of the local AA regions is progressively reduced, thus precluding the emergence of further magic angles, in very good agreement with existing experimental evidence., 8 pages, 5 figures, 1 SI document, revised for publication

In this review we discuss the multifaceted problem of spin transport in hydrogenated graphene from a theoretical perspective. The current experimental findings suggest that hydrogenation can either increase or decrease spin lifetimes, which calls for clarification. We first discuss the spin-orbit coupling induced by local $\sigma-\pi$ re-hybridization and ${\bf sp}^{3}$ C-H defect formation together with the formation of a local magnetic moment. First-principles calculations of hydrogenated graphene unravel the strong interplay of spin-orbit and exchange couplings. The concept of magnetic scattering resonances, recently introduced \cite{Kochan2014} is revisited by describing the local magnetism through the self-consistent Hubbard model in the mean field approximation in the dilute limit, while spin relaxation lengths and transport times are computed using an efficient real space order N wavepacket propagation method. Typical spin lifetimes on the order of 1 nanosecond are obtained for 1 ppm of hydrogen impurities (corresponding to transport time about 50 ps), and the scaling of spin lifetimes with impurity density is described by the Elliott-Yafet mechanism. This reinforces the statement that magnetism is the origin of the substantial spin polarization loss in the ultraclean graphene limit., Comment: 10 pages, 8 figures, accepted for publication in 2D Materials as a Topical Review

arXiv:1405.3766v1, Unconventional magnetotransport fingerprints in the quantum Hall regime (with applied magnetic fields from one to several tens of Tesla) in chemically functionalized graphene are reported. Upon chemical adsorption of monoatomic oxygen (from $0.5\%$ to few percents), the electron-hole symmetry of Landau levels (LLs) is broken, while a double-peaked conductivity develops at low-energy, resulting from the formation of critical states conveyed by the random network of defects-induced impurity states. Scaling analysis hints towards the existence of an additional zero-energy quantized Hall conductance plateau, which is here not connected to degeneracy lifting of LLs by sublattice symmetry breaking. This singularly contrasts with usual interpretation, and unveils a new playground for tailoring the fundamental characteristics of the quantum Hall effect., J-CC and NL acknowledge financial support from the FRS-FNRS of Belgium. This research is directly connected to the ARC on ‘Graphene Nano-electromechanics’ (no. 11/16-037) sponsored by the Communauté Française de Belgique, and funded by the Spanish Ministry of Economy and Competitiveness for funding (MAT2012-33911). This research is partly funded by the European Union Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship.

Twisted bilayer graphene is created by slightly rotating the two crystal networks in bilayer graphene with respect to each other. For small twist angles, the material undergoes a self-organized lattice reconstruction, leading to the formation of a periodically repeated domain1-3. The resulting superlattice modulates the vibrational3,4 and electronic5,6 structures within the material, leading to changes in the behaviour of electron-phonon coupling7,8 and to the observation of strong correlations and superconductivity9. However, accessing these modulations and understanding the related effects are challenging, because the modulations are too small for experimental techniques to accurately resolve the relevant energy levels and too large for theoretical models to properly describe the localized effects. Here we report hyperspectral optical images, generated by a nano-Raman spectroscope10, of the crystal superlattice in reconstructed (low-angle) twisted bilayer graphene. Observations of the crystallographic structure with visible light are made possible by the nano-Raman technique, which reveals the localization of lattice dynamics, with the presence of strain solitons and topological points1 causing detectable spectral variations. The results are rationalized by an atomistic model that enables evaluation of the local density of the electronic and vibrational states of the superlattice. This evaluation highlights the relevance of solitons and topological points for the vibrational and electronic properties of the structures, particularly for small twist angles. Our results are an important step towards understanding phonon-related effects at atomic and nanometric scales, such as Jahn-Teller effects11 and electronic Cooper pairing12-14, and may help to improve device characterization15 in the context of the rapidly developing field of twistronics16.

We report on spin transport in state-of-the-art epitaxial monolayer graphene based 2D-magnetic tunnel junctions (2D-MTJs). In our measurements, supported by ab-initio calculations, the strength of interaction between ferromagnetic electrodes and graphene monolayers is shown to fundamentally control the resulting spin signal. In particular, by switching the graphene/ferromagnet interaction, spin transport reveals magneto-resistance signal MR > 80% in junctions with low resistance × area products. Descriptions based only on a simple K-point filtering picture (i.e. MR increase with the number of layers) are not sufficient to predict the behavior of our devices. We emphasize that hybridization effects need to be taken into account to fully grasp the spin properties (such as spin dependent density of states) when 2D materials are used as ultimately thin interfaces. While this is only a first demonstration, we thus introduce the fruitful potential of spin manipulation by proximity effect at the hybridized 2D material / ferromagnet interface for 2D-MTJs., 2D materials are foreseen as an opportunity to tailor spintronics devices interfaces, a.k.a spinterfaces. Here, using state-of-the-art large-scale integration in spin-valves, authors demonstrate that hybridization of graphene with a metallic spin source results in strong spin filtering effects.

The temperature dependences of both heat capacity and thermal conductivity in nanolamellar Cr2AlC single crystals are measured using modulated photothermal radiometry and compared to first-principles calculations. The electronic contribution to the thermal conductivity of Cr2AlC single crystals is computed ab initio by determining the electronic transport coefficients using density functional theory and by solving the Bloch−Boltzmann transport equation with a temperaturedependent relaxation time. The lattice thermal conductivity is predicted by going beyond the quasi-harmonic approximation and considering renormalized second- and third-order force constant matrices, with anharmonic three-phonon scattering, isotopic scattering, and scattering by carbon vacancies. Isotopic scattering does not modify the lattice thermal conductivity. In contrast, even a small concentration of carbon vacancies induces a substantial decrease of the in-plane lattice thermal conductivity. The anisotropy measured in the thermal conductivity, with a ratio of ∼2 over the whole temperature range, is confirmed theoretically. This anisotropy seems to mainly arise from lattice contributions. A similar anisotropy is expected for other MAX phases with identical layered structures.

Herein we report on a subset of the elastic constants, ${c}_{ij}$, and hardness values of ${\mathrm{V}}_{2}\mathrm{AlC}$ and $\mathrm{C}{\mathrm{r}}_{2}\mathrm{AlC}$ single crystals by means of microindentation/nanoindentation techniques. Density functional theory (DFT) is also used to calculate the elastic constants. The ${c}_{33}$ and ${c}_{11}$ values determined using a Berkovich tip and those calculated by DFT are all found to fall in the relatively narrow range of 320--350 GPa. These results confirm once again that many of MAX phases are relatively elastically isotropic, especially when compared to many other known layered solids such as graphite and mica. Similarly, the hardness values, obtained using Vickers, Berkovich, and a 5 \textmu{}m spherical tip on the two orthogonal $\mathrm{C}{\mathrm{r}}_{2}\mathrm{AlC}$ surfaces, are quite comparable and average 9.0 \ifmmode\pm\else\textpm\fi{} 1 GPa. In all cases, the hardness values are at most 20% higher when the basal planes are loaded along [0001] than when they are loaded edge-on. The $\mathrm{C}{\mathrm{r}}_{2}\mathrm{AlC}$ surfaces record an average microyielding stress of 2.7 \ifmmode\pm\else\textpm\fi{} 0.3 GPa, while the less defective ${\mathrm{V}}_{2}\mathrm{AlC}$ crystals linearly sustain stresses of the order of 20 GPa, after which, in approximately 60% of the cases, pop-ins, some of which are substantial, are recorded. Postindentation scanning electron microscope micrographs clearly evidence the plastic anisotropy of these crystals. Large pileups near the indent edges and delamination cracks after loading along the [0001] and $[10\overline{1}0]$ directions, respectively, are consistent with deformation by ripplocations and ripplocation boundaries, as are the fully and spontaneously recoverable hysteresis stress-strain loops.

Photoswitchable self-assembled monolayers (SAMs) in contact with a conductive or semiconductive layer can be used to remotely trigger changes in electrical current using light. In this study, we apply full-atomistic simulations to assess the changes in electronic structure and charge-transport properties of a graphene sheet in contact with an amorphous silica dielectric decorated by an azobenzene SAM. The simulations explicitly account for the structural and electrostatic disorder sourced by the dielectric, which turns out to be weakly affected by photoisomerization and spatially correlated over a length scale of 4–5 nm. Most interestingly, by combining large-scale (tight binding) density functional theory with Kubo–Greenwood quantum transport calculations, we predict that the trans-cis isomerization should induce a shift in surface electrostatic potential by a few tenths of a volt, accompanied by a variation in conductivity by a factor of about 3.

The recent discovery of quaternary MAX phases with chemical in-plane order allowed the addition of nontraditional MAX phase elements, such as rare-earth elements. In the present study, first-principles calculations are performed to investigate the electronic structure, elastic and hardness response, and bonding strengths of the novel $\mathrm{RE}\text{\ensuremath{-}}i\text{\ensuremath{-}}\mathrm{MAX}$ phases with the formula ${({\text{Mo}}_{2/3}{\text{RE}}_{1/3})}_{2}\text{AlC}$. The Voigt-Reuss-Hill bulk, shear, and Young's moduli are compared along the series of RE = Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu, and the global trend is found to depend on the unit cell volume. Nanoindentation experiments on Ho-based single crystals result in moduli that are within 10% of predicted values and a hardness of $\ensuremath{\sim}10$ GPa. The computation of the projected density of states, projected crystal orbital Hamilton population, and integrated projected crystal orbital Hamilton population, reveals that the bonding of Mo and RE atoms with the Al atoms are weaker than those with the C atoms, suggesting the exfoliation of $\mathrm{RE}\text{\ensuremath{-}}i\text{\ensuremath{-}}\mathrm{MAX}$ into two-dimensional $\mathrm{RE}\text{\ensuremath{-}}i$-MXenes to be feasible. Such a possibility to form two-dimensional crystals is further confirmed by the computation of the exfoliation energies, which demonstrates the process to be easier as the RE atomic mass decreases.

Beginning with an introduction to carbon-based nanomaterials, their electronic properties, and general concepts in quantum transport, this detailed primer describes the most effective theoretical and computational methods and tools for simulating the electronic structure and transport properties of graphene-based systems. Transport concepts are clearly presented through simple models, enabling comparison with analytical treatments, and multiscale quantum transport methodologies are introduced and developed in a straightforward way, demonstrating a range of methods for tackling the modelling of defects and impurities in more complex graphene-based materials. The authors also discuss the practical applications of this revolutionary nanomaterial, contemporary challenges in theory and simulation, and long-term perspectives. Containing numerous problems for solution, real-life examples of current research, and accompanied online by further exercises, solutions and computational codes, this is the perfect introductory resource for graduate students and researchers in nanoscience and nanotechnology, condensed matter physics, materials science and nanoelectronics.

The family of 2D materials has expanded quite rapidly, especially with the addition of transition metal carbides and nitrides called MXenes, in the last decade. Since their discovery in 2011, about 30 different MXenes have been synthesized, and the structure and properties of several dozens have been predicted by first-principles approaches. Given the outstanding advances in the MXene field, it is thus appropriate to review the most relevant properties of these MXenes and point out their potential applications. In this article, the structural, transport, magnetic, vibrational, mechanical, and electrochemical properties of MXenes are overviewed. The goal is to illustrate how the chemical versatility in the intrinsic composition and surface terminations combined with the potential addition of a fourth element enable to tune MXenes properties to meet the targeted applications.

A spin information is efficiently transmitted through a chemically modified multiwall carbon nanotube., Transporting quantum information such as the spin information over micrometric or even millimetric distances is a strong requirement for the next-generation electronic circuits such as low-voltage spin-logic devices. This crucial step of transportation remains delicate in nontopologically protected systems because of the volatile nature of spin states. Here, a beneficial combination of different phenomena is used to approach this sought-after milestone for the beyond–Complementary Metal Oxide Semiconductor (CMOS) technology roadmap. First, a strongly spin-polarized charge current is injected using highly spin-polarized hybridized states emerging at the complex ferromagnetic metal/molecule interfaces. Second, the spin information is brought toward the conducting inner shells of a multiwall carbon nanotube used as a confined nanoguide benefiting from both weak spin-orbit and hyperfine interactions. The spin information is finally electrically converted because of a strong magnetoresistive effect. The experimental results are also supported by calculations qualitatively revealing exceptional spin transport properties of this system.

We report on spin transport in WS

International audience; We report on spin transport in WS2-based 2D-magnetic tunnel junctions (2D-MTJs), unveiling a band structure spin filtering effect specific to the transition metal dichalcogenides (TMDCs) family. WS2 mono-, bi-, and trilayers are derived by a chemical vapor deposition process and further characterized by Raman spectroscopy, atomic force microscopy (AFM), and photoluminescence spec-troscopy. The WS2 layers are then integrated in complete Co/Al2O3 /WS2/Co MTJ hybrid spin-valve structures. We make use of a tunnel Co/Al2O3 spin analyzer to probe the extracted spin-polarized current from the WS2/Co interface and its evolution as a function of WS2 layer thicknesses. For monolayer WS2, our technological approach enables the extraction of the largest spin signal reported for a TMDC-based spin valve, corresponding to a spin polarization of PCo/WS2 = 12%. Interestingly, for bi-and trilayer WS2, the spin signal is reversed, which indicates a switch in the mechanism of interfacial spin extraction. With the support of ab initio calculations, we propose a model to address the experimentally measured inversion of the spin polarization based on the change in the WS2 band structure while going from monolayer (direct bandgap) to bilayer (indirect bandgap). These experiments illustrate the rich potential of the families of semiconducting 2D materials for the control of spin currents in 2D-MTJs.

In this chapter we first present an efficient real-space transport methodology, which is rooted in first-principles simulations and combination with tight-binding models, for computing electronic conductance in disordered forms of graphene-based nanoribbons. We then illustrate our approach by studying the effects of chemical modifications, structural defects, and magnetic fields on the resulting quantum transport in micrometer-long defected graphene nanoribbons. Physical phenomena such as the impurity resonance, transport mobility gaps and transistor functionalities are analyzed in the light of our numeric. When possible, comparison with experimental works is also discussed.

In this work, density functional theory is used to predict the voltage of newly designed PTMA-based radical molecules. Organic radical molecules have recently been tested as promising materials for secondary batteries due to their processability and structural diversity. In order to tune the redox potentials, we investigate the substitution of functional groups on the actual TEMPO organic radical. Our first-principles simulations predict voltages ranging between 3.4 and 6.2 V allowing for a fine-tuning of the corresponding voltage for specific needs.

Graphene coatings reduce surface adhesion owing to a low surface energy. In the present work, a single CVD-grown graphene layer on Cu is shown to modify the elastic contact behavior by eliminating adhesion. Nanoindentation load-displacement curves exhibit higher load bearing capacity for Cu/graphene in the elastic regime compared to bare Cu and a closer agreement with Hertz law. Molecular dynamics simulations confirm the quasi-absence of adhesion between graphene and indentor tip. These results open new opportunities regarding tribological issues related to coatings or MEMS applications.

Chains of carbon atoms in the sp 1 hybridization are the one-dimensional analog of graphene. The first experimental studies of electrical transport in atomic carbon chains have shown a much lower conductivity than the quantum conductance limit. Here we explain, experimentally and by ab-initio transport modeling, the limited conductivity by studying the influence of carbon contacts in different hybridization states on the electrical properties of carbon chains. In-situ measurements in an electron microscope allow the synthesis and electrical characterization of carbon chains. Current-voltage curves of carbon chains, spanning between carbon contacts with sp 2 -or sp 3 -hybridized contact atoms, are measured and calculated. Contact atoms in the sp 2 -hybridization allow up to two orders of magnitude higher current than through sp 3 contacts. Another important factor is the electron distribution in the chain which is determined by an even or odd number of atoms. On the other hand, it is shown that the overall length of the chain and strain have only minor influence on the conductivity. A current carrying capacity of up to 6.5 μA at an applied voltage of 1.5 V is measured.

For centuries, cutting and folding the papers with special patterns have been used to build beautiful, flexible and complex three-dimensional structures. Inspired by the old idea of kirigami (paper cutting), and the outstanding properties of graphene, recently graphene kirigami structures were fabricated to enhance the stretchability of graphene. However, the possibility of further tuning the electronic and thermal transport along the 2D kirigami structures have remained original to investigate. We therefore performed extensive atomistic simulations to explore the electronic, heat and load transfer along various graphene kirigami structures. The mechanical response and thermal transport were explored using classical molecular dynamics simulations. We then used a real-space Kubo-Greenwood formalism to investigate the charge transport characteristics in graphene kirigami. Our results reveal that graphene kirigami structures present highly anisotropic thermal and electrical transport. Interestingly, we show the possibility of tuning the thermal conductivity of graphene by four orders of magnitude. Moreover, we discuss the engineering of kirigami patterns to further enhance their stretchability by more than 10 times as compared with pristine graphene. Our study not only provides a general understanding concerning the engineering of electronic, thermal and mechanical response of graphene but more importantly can be useful to guide future studies with respect to the synthesis of other 2D material kirigami structures, to reach highly flexible and stretchable nanostructures with finely tunable electronic and thermal properties., Comment: 29 pages, 9 figures, 1 supplementary figure

The recent introduction of two-dimensional materials into magnetic tunnel junctions (2D MTJs) offers very promising properties for spintronics, such as atomically defined interfaces, spin filtering, perpendicular anisotropy, and modulation of spin-orbit torque. Nevertheless, the difficulty of integrating exfoliated 2D materials into spintronic devices has limited exploration. Here the authors find a fabrication process leading to superior performance in MTJs based on transition-metal dichalcogenides, and further suggest a path to alleviate basic issues of technology and physics for 2D MTJs.

Aharonov-Bohm (AB) interferences in the quantum Hall regime can be achieved, provided that electrons are able to transmit between two edge channels in nanostructures. Pioneering approaches include quantum point contacts in 2DEG systems, bipolar graphene p-n junctions, and magnetic field heterostructures. In this work, defect scattering is proposed as an alternative mechanism to achieve AB interferences in polycrystalline graphene. Indeed, due to such scattering, the extended defects across the sample can act as tunneling paths connecting quantum Hall edge channels. Consequently, strong AB oscillations in the conductance are predicted in polycrystalline graphene systems with two parallel grain boundaries. In addition, this general approach is demonstrated to be applicable to nano-systems containing two graphene barriers with functional impurities and perspectively, can also be extended to similar systems of 2D materials beyond graphene., 8 pages, 5 figures, Supplementary Information, submitted

With the advent of high mobility encapsulated graphene devices, new electronic components ruled by Dirac fermions optics have been envisioned and realized. The main building blocks of electron-optics devices are gate-defined p-n junctions, which guide, transmit and refract graphene charge carriers, just like prisms and lenses in optics. The reflection and transmission are governed by the p-n junction smoothness, a parameter difficult to tune in conventional devices. Here we create p-n junctions in graphene, using the polarized tip of a scanning gate microscope, yielding Fabry-P\'erot interference fringes in the device resistance. We control the p-n junctions smoothness using the tip-to-graphene distance, and show increased interference contrast using smoother potential barriers. Extensive tight-binding simulation reveal that smooth potential barriers induce a pronounced quasi-confinement of Dirac fermions below the tip, yielding enhanced interference contrast. On the opposite, sharp barriers are excellent Dirac fermions transmitters and lead to poorly contrasted interferences. Our work emphasizes the importance of junction smoothness for relativistic electron optics devices engineering.

Ab initio calculations have been performed on the liquid gallium−hydrogen system at 100 °C. Gallium was found to interact with both free hydrogen atoms and H2, transferring charge in the process. First-principles computer modeling showed that the dissociation of H2 is a consequence of charge transfer in liquid gallium−hydrogen systems, even at low temperature. Once negatively charged, hydrogen atoms interact with each other and are stabilized by positively charged gallium atoms inducing local atomic arrangements analogous to parts of the digallane molecule.

Recent outbreaks in bottom-up chemical techniques have demonstrated the synthesis of atomically perfect zigzag graphene nanoribbons (ZGNRs) and of their corresponding analogs with phenyl-edge functionalization [P. Ruffieux et al., Nature 531, 489 (2016)]. Since spin-polarized currents can be generated at the edges of the ZGNRs, the control of phenyl decoration at the zigzag edges could accurately tune the electronic properties of these carbon-based nanoribbons. In the present paper, using first-principles calculations and a Landauer-B\"uttiker approach, the electronic and magnetic properties as well as the spin-resolved transmissions are investigated in various phenyl-edge-modified ZGNRs. The understanding of the spin-dependent transport properties in relation to the atomic structure opens the way to design devices such as spin valves for future spintronics applications.

Graphene charge carriers behave as relativistic massless fermions, thereby exhibiting a variety of counter-intuitive behaviors. In particular, at p-n junctions, they behave as photons encountering a negative index media, therefore experiencing a peculiar refraction known as Veselago lensing. However, the way Dirac fermions flow through a Veselago lens remains largely unexplored experimentally. Here, a novel approach to create a movable and tunable circular p-n junction in graphene is proposed, using the polarized tip of a scanning gate microscope. Scanning the tip in the vicinity of a graphene constriction while recording the device conductance yields images related to the electron flow through a circular Veselago lens, revealing a high current density in the lens core, as well as two low current density zones along transport axis. Tight-binding simulations reveal the crucial role of the p-n junction smoothness on these phenomena. The present research adds new dimensions in the control and understanding of Dirac fermions optical elements, a prerequisite to engineer relativistic electron optics devices., Accepted for publication in Physical Review B, Rapid Communications

Two-dimensional (2D) conjugated polymers exhibit electronic structures analogous to that of graphene with the peculiarity of π-π* bands which are fully symmetric and isolated. In the present letter, the suitability of these materials for electronic applications is analyzed and discussed. In particular, realistic 2D conjugated polymer networks with a structural disorder such as monomer vacancies are investigated. Indeed, during bottom-up synthesis, these irregularities are unavoidable and their impact on the electronic properties is investigated using both ab initio and tight-binding techniques. The tight-binding model is combined with a real space Kubo-Greenwood approach for the prediction of transport characteristics for monomer vacancy concentrations ranging from 0.5% to 2%. As expected, long mean free paths and high mobilities are predicted for low defect densities. At low temperatures and for high defect densities, strong localization phenomena originating from quantum interferences of multiple scattering paths are observed in the close vicinity of the Dirac energy region while the absence of localization effects is predicted away from this region suggesting a sharp mobility transition. These predictions show that 2D conjugated polymer networks are good candidates to pave the way for the ultimate scaling and performances of future molecular nanoelectronic devices.

Electronic structure methods are combined into a multiscale framework to investigate the electronic transport properties of recently synthesized pristine and nitrogen-doped graphene nanowiggles and their heterojunctions deposited on a substrate. Real-space Kubo-Greenwood transport calculations reveal that charge carrier mobilities reach values up to 1000 cm2 V−1 s−1 as long as the amount of substrate impurities is sufficiently low. Owing to their type-II band alignment, atomically precise heterostructures between pristine and N-doped graphene nanowiggles are predicted to be excellent candidates for charge carrier separation devices with potential in photoelectric and photocatalytic water splitting applications.

Electronic structure methods are combined into a multiscale framework to investigate the electronic transport properties of recently synthesized pristine and nitrogen-doped graphene nanowiggles and their heterojunctions deposited on a substrate. Real-space Kubo-Greenwood transport calculations reveal that charge carrier mobilities reach values up to 1000 cm²/Vs as long as the amount of substrate impurities is sufficiently low. Owing to their type-II band alignment, atomically precise heterostructures between pristine and N-doped graphene nanowiggles are predicted to be excellent candidates for charge carrier separation devices with potential in photoelectric and photocatalytic water splitting applications.

Quantum transport in nanodevices is usually probed thanks to measurements of the electrical resistance or conductance, which lack the spatial resolution necessary to probe local-scale electron behavior. Here, we will discuss how to get real-space local information on peculiar quantum transport phenomena inside graphene constrictions. The results were obtained using low temperature scanning gate microscopy (SGM), which consists in mapping the electrical conductance of a device as an electrically-biased sharp metallic tip scans in its vicinity [1]. If the tip-induced perturbation is relatively small, SGM is an imaging technique giving access to the local density of states inside mesoscopic devices [2]. Here, we will focus on the regime of large tip-induced perturbation. This regime is particularly interesting in the case of graphene: depending on tip voltage and tip-sample distance, one can tune the shape, size, and smoothness of the moving scattering region associated with the tip, and eventually create a moving circular pn-junction within the device. We use this moving pn-junction to probe peculiar aspects of relativistic charge carrier dynamics within graphene devices: Klein tunnelling and current focusing and defocusing [3], Fabry-Pérot interferometry [4] and signatures of whispering gallery modes. This work unlocks new possibilities in the field of electron optics in graphene. [1] M.A. Eriksson et al., Appl. Phys. Lett. 69, 671 (1996); M. Topinka et al., Science 289, 671 (2000). [2] F. Martins et al., Phys. Rev. Lett. 99, 136807 (2007) ; M. Pala et al., Phys. Rev. B 77, 125310 (2008). [3] B. Brun et al., Phys. Rev. B 100, 041401 (2019). [4] B. Brun et al., arXiv:1911.12031.

As in many countries, the rise of nanosciences in Belgium has been triggered in the eighties in the one hand, by the development of scanning tunneling and atomic force microscopes offering an unprecedented possibility to visualize and manipulate the atoms, and in the other hand, by the synthesis of nano-objects in particular carbon nanostructures such as fullerene and nanotubes. Concomitantly, the increasing calculating power and the emergence of computing facilities together with the development of DFT-based ab initio softwares have brought to nanosciences field powerful simulation tools to analyse and predict properties of nano-objects. Starting with 0D and 1D nanostructures, the floor is now occupied by the 2D materials with graphene being the bow of this 2D ship. In this review article, some specific examples of 2D systems has been chosen to illustrate how not only density functional theory (DFT) but also tight-binding (TB) techniques can be daily used to investigate theoretically the electronic, phononic, magnetic, and transport properties of these atomically thin layered materials.

Increasing the storage capacity of lithium electrodes, without altering their cyclability, is one of the key challenges for modern ion-based batteries. For graphite-based anodes, the well-known capacity limit is $\ensuremath{\sim}370$ mAh/g, which corresponds to a lithium composition of ${\mathrm{Li}}_{1}{\mathrm{C}}_{6}$. Lithium intercalation is accompanied by a volume expansion of $\ensuremath{\sim}10%$. In the present work, accurate first-principles methods are used to investigate the performance of different bulk ${sp}^{2}$ carbon allotropes as anodes in lithium-ion batteries. Compared to graphite, which is an alternated stack of graphene layers (Bernal stacking) exhibiting a perfect hexagonal tiling, the layers of the other stacked systems considered are constructed from various polygonal carbon rings, such as squares, pentagons, hexagons, heptagons, octagons, and dodecagons. These ${sp}^{2}$ allotropes, which appear locally in defective graphene and grain boundaries, can exhibit a substantial increase in specific capacity with respect to graphite (up to a factor of two, i.e., ${\mathrm{Li}}_{2}{\mathrm{C}}_{6}$) with only a relatively small volume expansion (at most 25%). The mechanisms for this predicted increase in the number of lithium atoms that can be hosted in these still hypothetical carbon crystals are analyzed in detail, yielding global strategies for improving lithium capacity in ${sp}^{2}$ carbon-based batteries. In addition, these results offer an insight on the local mechanism of Li incorporation in randomly defective graphite.