Long-range Modulation Research Articles

Assessment of LoRaWAN Transmission Systems Under Temperature and Humidity, Gas, and Vibration Aging Effects Within IIoT Contexts

Within the big picture of the Internet of Things (IoT), a brand new paradigm has risen since few years ago in the context of industrial scenarios: the industrial IoT (IIoT), whose aim is to transplant all of the features, characteristics, and scopes of the IoT into industrial settings. The latter ones might be subject to a wide range of environmental conditions from the point of view of both temperature and relative humidity, along with harmful gases exposure and vibration due to, for instance, machinery. Therefore, whenever their monitoring is performed by means of devoted infrastructures, such extreme working conditions must be born in mind during the pertaining design phases. Mostly, industrial monitoring is put into effect by resorting to wireless sensor networks (WSNs) enabled by low-power wide area network (LPWAN) technologies. This set of facilities includes an ample heterogeneity of standards and techniques. However, the long-range (LoRa) modulation and the LoRa wide area network (LoRaWAN) protocol extensively proved to be reliable and robust alternatives. Thus, in this article, the variations of hardware performances of a LoRaWAN sensor node, in terms of transmission capabilities, due to the changes of temperature and relative humidity, vibration, and gaseous atmospheres, such as CO, NO, and NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , which are typical of industrial processes, are investigated. In so doing, an aging process due to the aforementioned phenomena was induced. To this end, a measurement campaign within controlled environments (i.e., a climatic chamber, a fume extraction plant, and an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ad hoc</i> vibration test bench) was sorted out, whose results show a physiological performances drop that neither undermines the network reliability nor damages the sensor node electronics.

Performance Improvement of LoRa Modulation With Signal Combining and Semi-Coherent Detection

In this letter, we investigate performance improvements of low-power long-range (LoRa) modulation when a gateway is equipped with multiple antennas. We derive the optimal decision rules for both coherent and non-coherent detections when combining signals received from multiple antennas. To provide insights on how signal combining can benefit LoRa systems, we present expressions of the symbol/bit error probabilities of both the coherent and non-coherent detections in AWGN and Rayleigh fading channels, respectively. Moreover, we also propose an iterative semi-coherent detection that does not require any overhead to estimate the channel-state-information (CSI) while its performance can approach that of the ideal coherent detection. Simulation and analytical results show very large power gains, or coverage extension, provided by the use of multiple antennas for all the detection schemes considered.

Machine Learning Prediction of Allosteric Drug Activity from Molecular Dynamics.

Allosteric drugs have been attracting increasing interest over the past few years. In this context, it is common practice to use high-throughput screening for the discovery of non-natural allosteric drugs. While the discovery stage is supported by a growing amount of biological information and increasing computing power, major challenges still remain in selecting allosteric ligands and predicting their effect on the target protein’s function. Indeed, allosteric compounds can act both as inhibitors and activators of biological responses. Computational approaches to the problem have focused on variations on the theme of molecular docking coupled to molecular dynamics with the aim of recovering information on the (long-range) modulation typical of allosteric proteins.

Moiré-induced electronic structure modifications in monolayer V2S3 on Au(111)

There is immense interest in how the local environment influences the electronic structure of materials at the single layer limit. We characterize moir\'e induced spatial variations in the electronic structure of in-situ grown monolayer V2S3 on Au(111) by means of low temperature scanning tunneling microscopy and spectroscopy. We observe a long-range modulation of the integrated local density of states (LDOS), and quantify this modulation with respect to the moir\'e superstructure for multiple orientations of the monolayer with respect to the substrate. Scanning tunneling spectroscopy reveals a prominent peak in the LDOS, which is shifted in energy at different points of the moir\'e superstructure. Comparing ab initio calculations with angle-resolved photoemission, we are able to attribute this peak to bands that exhibit a large out-of-plane d-orbital character. This suggests that the moir\'e driven variations in the measured density of states is driven by a periodic modulation of the monolayer-substrate hybridization.

LORA Technology Basics and Applications

The Long Range (LoRa) technology was first developed by SemTech Company. LoRa is a wireless technology developed for long-range, low-power, low-bit rate and chirp spread spectrum (CSS) radio modulation technology, it also provides the ability to connect to sensors more than 15-30 miles away in rural areas.In this study, we present the LoRa system architecture with the functionality of each component and several typical application scenarios of LoRa network.LoRa is widely used into many applications, such as smart metering, factory monitoring and also it can be used to provide sensor information to communities to provide disaster alerts. LoRa networks allow for very long wireless links that can connect villages and towns. LoRa network is emerging as one of the most promising Low Power Wide Area networks (LPWAN).LPWANs represent a new trend in the evolution of the wireless communication designed to enable broad range of Internet of Things (IoT) applications.

Magnetocaloric behavior and magnetic ordering in MnPdGa

MnPdGa, a compound crystallizing in the ${\mathrm{Ni}}_{2}\mathrm{In}$ structure, is a material displaying magnetic ordering near room temperature and is a potential ambient-temperature magnetocaloric. Screening based on electronic structure calculations suggest that MnPdGa may exhibit a high magnetocaloric figure of merit due to its strong magnetostructural coupling. Here we report the preparation of MnPdGa and employ high--resolution synchrotron x-ray diffraction to confirm its hexagonal ${\mathrm{Ni}}_{2}\mathrm{In}$-type structure. The zero-field ground state is shown to be a conical spin-wave state, defined by a long-range modulation of the conventional conical antiferromagnet structure. Near the Curie temperature, the measurements carried out here coupled with electronic structure calculations suggest that a fully ferromagnetic state can form at elevated temperatures under an applied field. A peak magnetocaloric entropy change $\mathrm{\ensuremath{\Delta}}{S}_{M}=\ensuremath{-}3.54\phantom{\rule{0.28em}{0ex}}\mathrm{J}\phantom{\rule{0.28em}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ ($30.1\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$) is measured at ${T}_{C}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}315\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at an applied field $H=5\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. The exchange-driven, nontrivial magnetic structure found in MnPdGa is compared with the somewhat better-studied MnPtGa on the basis of electronic structure calculations.

O2.2. EVALUATION OF THE RELATIONSHIP BETWEEN GLUTAMATE AND BRAIN CONNECTIVITY IN ANTIPSYCHOTIC-NAïVE FIRST EPISODE PATIENTS – A COMBINED MAGNETIC RESONANCE SPECTROSCOPY AND RESTING STATE FUNCTIONAL CONNECTIVITY MRI STUDY

BackgroundSchizophrenia is thought to be a disorder of brain dysconnectivity. An imbalance between cortical excitation/inhibition is also implicated, but the link between these abnormalities remains unclear. The present study used resting state functional connectivity MRI (rs-fcMRI) and magnetic resonance spectroscopy (MRS) to investigate how measurements of glutamate + glutamine (Glx) in the anterior cingulate cortex (ACC) relate to rs-fcMRI in medication-naïve first episode psychosis (FEP) subjects compared to healthy controls (HC). Based on our previous findings, we hypothesized that in HC would show correlations between Glx and rs-fMRI in the salience and default mode network, but these relationships would be altered in FEP.MethodsData from 53 HC (age = 24.70 ±6.23, 34M/19F) and 60 FEP (age = 24.08 ±6.29, 38M/22F) were analyzed. To obtain MRS data, a voxel was placed in the ACC (PRESS, TR/TE = 2000/80ms). Metabolite concentrations were quantified with respect to internal water using the AMARES algorithm in jMRUI. rs-fMRI data were processed using a standard preprocessing pipeline in the CONN toolbox. BOLD signal from a priori brain regions of interest from posterior cingulate cortex (default mode network, DMN), anterior cingulate cortex (salience network, SN), and right posterior parietal cortex (central executive network, CEN) were extracted and correlated with the rest of the brain to measure functional connectivity (FC). Group analyses were performed on Glx, FC, and Glx-FC interactions while controlling for age, gender, and motion when applicable. FC and Glx-FC analyses were performed using small volume correction [(p < 0.01, threshold-free cluster enhancement corrected (TFCE)].ResultsNo significant between-group differences were found in Glx concentration in the ACC [F(1, 108) = 0.34, p = 0.56], but reduced FC was found on each network in FEP compared to HC (pTFCE corrected). Group Glx-FC interactions were found in the form of positive correlations between Glx and FC in DMN and SN in the HC group, but not in FEP; and negative correlations in CEN in HC, but not in FEP.DiscussionWhile we did not find significant group differences in ACC Glx measurements, ACC Glx modulated FC differentially in FEP and HC. Positive correlations between Glx and FC were found in the SN and DMN, suggesting long range modulation of the two networks in HC, but not in FEP. Additionally, negative correlations between Glx and FC were found in CEN in HC, but not in FEP. Overall, these results suggest that even in the absence of group differences in Glx concentration, the long-range modulation of these 3 networks by ACC Glx is altered in FEP.

Hippocampal Dysconnectivity and Altered Glutamatergic Modulation of the Default Mode Network: A Combined Resting-State Connectivity and Magnetic Resonance Spectroscopy Study in Schizophrenia

BackgroundConverging lines of evidence point to hippocampal dysfunction in schizophrenia. It is thought that hippocampal dysfunction spreads across hippocampal subfields and to cortical regions by way of long-range efferent projections. Importantly, abnormalities in the excitation/inhibition balance could impair the long-range modulation of neural networks. The goal of this project was twofold. First, we sought to identify replicable patterns of hippocampal dysconnectivity in patients with a psychosis spectrum disorder. Second, we aimed to investigate a putative link between glutamatergic metabolism and hippocampal connectivity alterations. MethodsWe evaluated resting-state hippocampal functional connectivity alterations in two cohorts of patients with a psychosis spectrum disorder. The first cohort consisted of 55 medication-naïve patients with first-episode psychosis and 41 matched healthy control subjects, and the second cohort consisted of 42 unmedicated patients with schizophrenia and 41 matched control subjects. We also acquired measurements of glutamate + glutamine in the left hippocampus using magnetic resonance spectroscopy for 42 patients with first-episode psychosis and 37 healthy control subjects from our first cohort. ResultsWe observed a pattern of hippocampal functional hypoconnectivity to regions of the default mode network and hyperconnectivity to the lateral occipital cortex in both cohorts. We also show that in healthy control subjects, greater hippocampal glutamate + glutamine levels predicted greater hippocampal functional connectivity to the anterior default mode network. Furthermore, this relationship was reversed in medication-naïve subjects with first-episode psychosis. ConclusionsThese results suggest that an alteration in the relationship between glutamate and functional connectivity may disrupt the dynamic of major neural networks.

Long-range Regulation of Partially Folded Amyloidogenic Peptides

Neurodegeneration involves abnormal aggregation of intrinsically disordered amyloidogenic peptides (IDPs), usually mediated by hydrophobic protein-protein interactions. There is mounting evidence that formation of α-helical intermediates is an early event during self-assembly of amyloid-β42 (Aβ42) and α-synuclein (αS) IDPs in Alzheimer’s and Parkinson’s disease pathogenesis, respectively. However, the driving force behind on-pathway molecular assembly of partially folded helical monomers into helical oligomers assembly remains unknown. Here, we employ extensive molecular dynamics simulations to sample the helical conformational sub-spaces of monomeric peptides of both Aβ42 and αS. Our computed free energies, population shifts, and dynamic cross-correlation network analyses reveal a common feature of long-range intra-peptide modulation of partial helical folds of the amyloidogenic central hydrophobic domains via concerted coupling with their charged terminal tails (N-terminus of Aβ42 and C-terminus of αS). The absence of such inter-domain fluctuations in both fully helical and completely unfolded (disordered) states suggests that long-range coupling regulates the dynamicity of partially folded helices, in both Aβ42 and αS peptides. The inter-domain coupling suggests a form of intra-molecular allosteric regulation of the aggregation trigger in partially folded helical monomers. This approach could be applied to study the broad range of amyloidogenic peptides, which could provide a new path to curbing pathogenic aggregation of partially folded conformers into oligomers, by inhibition of sites far from the hydrophobic core.

Possible quantum fluctuations in the vicinity of the quantum critical point of (Sr,Ca)3Ir4Sn13 revealed by high-energy x-ray diffraction

We explore the evolution of the structural phase transition of $\rm{(Sr, Ca)_3Ir_4Sn_{13}}$, a model system to study the interplay between structural quantum criticality and superconductivity, by means of high-energy x-ray diffraction measurements at high pressures and low temperatures. Our results confirm a rapid suppression of the superlattice transition temperature $T^*$ against pressure, which extrapolates to zero at a critical pressure of $\approx 1.79(4)$ GPa. The temperature evolution of the superlattice Bragg peak in $\rm{Ca_3Ir_4Sn_{13}}$ reveals a drastic decrease of the intensity and an increase of the linewidth when $T \rightarrow 0$ K and $p \rightarrow p_c$. Such anomaly is likely associated to the emergence of quantum fluctuations that disrupt the formation of long-range superlattice modulation. The revisited temperature-pressure phase diagram of $\rm{(Sr, Ca)_3Ir_4Sn_{13}}$ thus highlights the intertwined nature of the distinct order parameters present in this system and demonstrates some similarities between this family and the unconventional superconductors.

Coexistence of Van Hove singularities and pseudomagnetic fields in modulated graphene bilayer

The stacking and bending of graphene are trivial but extremely powerful agents of control over graphene’s manifold physics. By changing the twist angle, one can drive the system over a plethora of exotic states via strong electron correlation, thanks to the moiré superlattice potentials, while the periodic or triaxial strains induce discretization of the band structure into Landau levels without the need for an external magnetic field. We fabricated a hybrid system comprising both the stacking and bending tuning knobs. We have grown the graphene monolayers by chemical vapor deposition, using 12C and 13C precursors, which enabled us to individually address the layers through Raman spectroscopy mapping. We achieved the long-range spatial modulation by sculpturing the top layer (13C) over uniform magnetic nanoparticles (NPs) deposited on the bottom layer (12C). An atomic force microscopy study revealed that the top layer tends to relax into pyramidal corrugations with C3 axial symmetry at the position of the NPs, which have been widely reported as a source of large pseudomagnetic fields (PMFs) in graphene monolayers. The modulated graphene bilayer (MGBL) also contains a few micrometer large domains, with the twist angle ∼10°, which were identified via extreme enhancement of the Raman intensity of the G-mode due to formation of van Hove singularities (VHSs). We thereby conclude that the twist-induced VHSs coexist with the PMFs generated in the strained pyramidal objects without mutual disturbance. The graphene bilayer modulated with magnetic NPs is a non-trivial hybrid system that accommodates features of twist-induced VHSs and PMFs in environs of giant classical spins.

Capacitor coupling induces synchronization between neural circuits

The signal propagation and information encoding in neurons are much dependent on the activation of synapses function. Electric synapse often plays short-range modulation so that neurons can give fast responses to external stimulus while chemical synapse keeps active under long-range modulation on neural activities. Each neuron can be considered as an intelligent signal processor and appropriate setting in parameters in the electric devices can build reliable neural circuits to detect effective signal exchange and propagation. In this paper, a two-variable FitzHugh–Nagumo electrical oscillator driven by a sinusoidal voltage source is used to investigate the synchronization when the coupling capacitor is activated, which both of the plates of the coupling capacitor can trigger time-varying electric field in the capacitor, and energy flow is propagated to modulate the outputs of neural circuits. The circuit equations are obtained, and scale transformation is applied to get a dimensionless dynamical system under field coupling, and the error function for output voltage and phases is calculated to judge whether complete synchronization and phase synchronization can be realized by selecting different capacitances for the coupling capacitor. It is found that synchronization realization between neurons can be controlled by adjusting the capacitance of coupling capacitor. For two identical neurons driven by the same stimulus, complete synchronization is reached, while phase synchronization is stabilized when neurons are driven by different stimuli as the capacitor coupling is switched on. Furthermore, the realization of complete synchronization is verified on the analog circuits. It provides another new scheme for synchronization realization between neurons and a new mechanism for signal encoding in neurons via field coupling is explained.

Conformational Complexity and Dynamics in a Muscarinic Receptor Revealed by NMR Spectroscopy.

The M2 muscarinic acetylcholine receptor (M2R) is a prototypical GPCR that plays important roles in regulating heart rate and CNS functions. Crystal structures provide snapshots of the M2R in inactive and active states, but the allosteric link between the ligand binding pocket and cytoplasmic surface remains poorly understood. Here we used solution NMR to examine the structure and dynamics of the M2R labeled with 13CH3-ε-methionine upon binding to various orthosteric and allosteric ligands having a range of efficacy for both G protein activation and arrestin recruitment. We observed ligand-specific changes in the NMR spectra of 13CH3-ε-methionine probes in the M2R extracellular domain, transmembrane core, and cytoplasmic surface, allowing us to correlate ligand structure with changes in receptor structure and dynamics. We show that the M2R has a complex energy landscape in which ligands with different efficacy profiles stabilize distinct receptor conformations.

Probing greenhouse gases in turbulent atmosphere by long-range open-path wavelength modulation spectroscopy

Open-path measurements of greenhouse gases yield valuable data for atmospheric research. We present such a method based on wavelength modulation spectroscopy to measure greenhouse gases. A movable platform was developed capable of detecting atmospheric variations of the concentrations of methane and carbon dioxide. The system was calibrated in the laboratory and then moved to the field for continuous automated measurements over paths of up to 2.6 km. The intensity modulation caused by turbulence was also measured. The continuous monitoring of methane was carried out for more than 30 h. Similar measurements of carbon dioxide were performed for about 10 h. The results show that the detection limits of methane and carbon dioxide are ∼2 ppb and ∼20 ppm with integration times of 60 s and 20 s, respectively. We conclude by discussing the achieved parameters in comparison to results obtained with other techniques, which shows that the developed sensor is a promising tool for monitoring of greenhouse gases.

Simultaneous Acquisition of Multicolor Information From Neural Circuits in Resin-Embedded Samples.

Resin embedding has been widely used for precise imaging of fluorescently labeled biological samples with optical and electron microscopy. The low preservation rate of fluorescence, especially for red fluorescent proteins, has limited the application of resin embedding in multifluorescent protein-labeled samples. Here, we optimized the embedding method to retain the intensity of multiple fluorescent proteins during resin embedding. By reducing the polymerization temperature from 50 to 35°C and adding a fluorescent protein protection reagent during the embedding process, we successfully increased the fluorescence preservation rate by nearly twofold for red fluorescent proteins, including tdTomato, mCherry, and DsRed. Meanwhile, the background fluorescence decreased significantly in the optimized embedding method. This method is suitable not only for red fluorescent protein-labeled samples but also for blue (BFP) and green fluorescent protein (GFP)-labeled samples. We embedded brains labeled with BFP, DsRed, and GFP via AAV and rabies virus and acquired the distribution of input neurons to different cortical areas. With GFP/tdTomato double-labeled samples in resin, we obtained the cholinergic projectome of the pedunculopontine tegmental nucleus (PPTg) and the distribution of cholinergic neurons at single-neuron resolution in the whole brain simultaneously. Input cholinergic terminals from the PPTg were found to innervate the cholinergic soma and fiber in the neocortex, basal forebrain and brainstem, indicating that local cholinergic neurons received long-range cholinergic modulation from the midbrain. Our optimized method is useful for embedding multicolor fluorescent protein-labeled samples to acquire multidimensional structural information on neural circuits at single-neuron resolution in the whole brain.

Long-Range Modulations of Electric Fields in Proteins

Electrostatic interactions are essential for controlling the protein structure and function. Whereas so far experimental and theoretical efforts focused on the effect of local electrostatics, this work aims at elucidating the long-range modulation of electric fields in proteins upon binding to charged surfaces. The study is based on cytochrome c (Cytc) variants carrying nitrile reporters for the vibrational Stark effect that are incorporated into the protein via genetic engineering and chemical modification. The Cytc variants were thoroughly characterized with respect to possible structural perturbations due to labeling. For the proteins in solution, the relative hydrogen bond occupancy and the calculated electric fields, both obtained from molecular dynamics (MD) simulations, and the experimental nitrile stretching frequencies were used to develop a relationship for separating hydrogen-bonding and non-hydrogen-bonding electric field effects. This relationship provides an excellent description for the stable Cytc variants in solution. For the proteins bound to Au electrodes coated with charged self-assembled monolayers (SAMs), the underlying MD simulations can only account for the electric field changes Δ Eads due to the formation of the electrostatic SAM-Cytc complexes but not for the additional contribution, Δ Eint, representing the consequences of the potential drops over the electrode/SAM/protein interfaces. Both Δ Eads and Δ Eint, determined at distances between 20 and 30 Å with respect to the SAM surface, are comparable in magnitude to the non-hydrogen-bonding electric field in the unbound protein. This long-range modulation of the internal electric field may be of functional relevance for proteins in complexes with partner proteins (Δ Eads) and attached to membranes (Δ Eads + Δ Eint).

Correlated insulator behaviour at half-filling in magic-angle graphene superlattices.

A van der Waals heterostructure is a type of metamaterial that consists of vertically stacked two-dimensional building blocks held together by the van der Waals forces between the layers. This design means that the properties of van der Waals heterostructures can be engineered precisely, even more so than those of two-dimensional materials. One such property is the 'twist' angle between different layers in the heterostructure. This angle has a crucial role in the electronic properties of van der Waals heterostructures, but does not have a direct analogue in other types of heterostructure, such as semiconductors grown using molecular beam epitaxy. For small twist angles, the moiré pattern that is produced by the lattice misorientation between the two-dimensional layers creates long-range modulation of the stacking order. So far, studies of the effects of the twist angle in van der Waals heterostructures have concentrated mostly on heterostructures consisting of monolayer graphene on top of hexagonal boron nitride, which exhibit relatively weak interlayer interaction owing to the large bandgap in hexagonal boron nitride. Here we study a heterostructure consisting of bilayer graphene, in which the two graphene layers are twisted relative to each other by a certain angle. We show experimentally that, as predicted theoretically, when this angle is close to the 'magic' angle the electronic band structure near zero Fermi energy becomes flat, owing to strong interlayer coupling. These flat bands exhibit insulating states at half-filling, which are not expected in the absence of correlations between electrons. We show that these correlated states at half-filling are consistent with Mott-like insulator states, which can arise from electrons being localized in the superlattice that is induced by the moiré pattern. These properties of magic-angle-twisted bilayer graphene heterostructures suggest that these materials could be used to study other exotic many-body quantum phases in two dimensions in the absence of a magnetic field. The accessibility of the flat bands through electrical tunability and the bandwidth tunability through the twist angle could pave the way towards more exotic correlated systems, such as unconventional superconductors and quantum spin liquids.

Unusual Long-Range Ordering Incommensurate Structural Modulations in an Organic Molecular Ferroelectric.

The incommensurate (IC) behaviors of ferroelectrics have been widely investigated in inorganic oxides as an exciting branch for aperiodic materials, whereas it still remains a great challenge to achieve such intriguing effects in organic systems. Here, we present that successive ordering of dynamic dipoles in an organic molecular ferroelectric, N-isopropylbenzylaminium trichloroacetate (1), enables unusual incommensurately modulated structures between its paraelectric phase and ferroelectric phase. In particular, 1 exhibits three distinct IC states coupling with a long-range ordering modulation. That is, the incommensurately modulated lattice is ∼7 times as large as its periodic prototype, and the IC structure is well solved using a (3 + 1)D superspace group with the modulated wavevector q = (0, 0, 0.1589). To the best of our knowledge, 1 is the first organic ferroelectric showing such a long-range ordering IC structural modulation. In addition, structural analyses reveal that slowing down dynamic motions of anionic moieties accounts for its modulation behaviors, which also results in dramatic reorientation of dipolar moments and concrete ferroelectric polarization of 1 (∼0.65 μC/cm2). The combination of unique IC structural modulations and ferroelectricity makes 1 a potential candidate for the assembly of an artificially modulated lattice, which will allow for a deep understanding of the underlying chemistry and physics of aperiodic materials.

Moiré superlattice-level stick-slip instability originated from geometrically corrugated graphene on a strongly interacting substrate

Two dimensional (2D) materials often exhibit novel properties due to various coupling effects with their supporting substrates. Here, using friction force microscopy (FFM), we report an unusual moiré superlattice-level stick-slip instability on monolayer graphene epitaxially grown on Ru(0 0 0 1) substrate. Instead of smooth friction modulation, a significant long-range stick-slip sawtooth modulation emerges with a period coinciding with the moiré superlattice structure, which is robust against high external loads and leads to an additional channel of energy dissipation. In contrast, the long-range stick-slip instability reduces to smooth friction modulation on graphene/Ir(1 1 1) substrate. The moiré superlattice-level slip instability could be attributed to the large sliding energy barrier, which arises from the morphological corrugation of graphene on Ru(0 0 0 1) surface as indicated by density functional theory (DFT) calculations. The locally steep humps acting as obstacles opposing the tip sliding, originates from the strong interfacial electronic interaction between graphene and Ru(0 0 0 1). This study opens an avenue for modulating friction by tuning the interfacial atomic interaction between 2D materials and their substrates.

Quantitative calculation of atomic-scale frictional behavior of two-dimensional material based on sliding potential energy surface

The excellent tribological characteristics of two-dimensional (2D) materials have received great attention, however, how to effectively predict their frictions is still lacking. Here, we propose to obtain the sliding potential energy surface by density functional theory calculations, instead of simplified potential energy function. Thus it is able to solve the frictional behaviors of 2D materials with irregular complex potential energy surfaces. Firstly, we reveal the mechanism of dual-scale stick-slip behavior between a tip and a graphene/Ru(0001) heterostructure. With a dual-wavelength potential energy surface, we observe a similar frictional behavior to those captured in atomic force microscopy experiments, in which a significant long-range stick-slip sawtooth modulation emerges with a period coinciding with the Moir superlattice structure. Secondly, we discuss the interlayer frictions of 2D materials, including graphene/graphene, fluorinated graphene/fluorinated graphene, MoS2/MoS2, graphene/MoS2 and fluorinated graphene/MoS2. With sliding potential energy surface obtained by density functional theory calculations, the interlayer friction is estimated according to the Prandtl-Tomlinson model calculation method. Compared with the friction between homostructures, the friction between heterostructures is lowered by orders of magnitude, which could be attributed to its ultralow sliding potential barrier. The stick-slip instability could be observed in homostructure, while heterostructure exihibits smooth friction loops. The 2D sliding path between the layers is recorded in the sliding process, showing its dependence on both the potential energy barrier and the spring constant. The sliding path shift increases with the increase of potential energy barrier and the decrease of spring constant in the y direction. This method is also applicable to tribological systems with dominated interfacial van der Waals interaction.