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This paper describes the conductance of single-molecules and self-assembled monolayers comprising an oligophenyleneethynylene core, functionalized with acenes of increasing length that extend conjugation perpendicular to the path of tunneling electrons. In the Mechanically Controlled Break Junction (MCBJ) experiment, multiple conductance plateaus were identified. The high conductance plateau, which we attribute to the single molecule conformation, shows an increase of conductance as a function of acene length, in good agreement with theoretical predictions. The lower plateau is attributed to multiple molecules bridging the junctions with intermolecular interactions playing a role. In junctions comprising a self-assembled monolayer with eutectic Ga–In top-contacts (EGaIn), the pentacene derivative exhibits unusually low conductance, which we ascribe to the inability of these molecules to pack in a monolayer without introducing significant intermolecular contacts. This hypothesis is supported by the MCBJ data and theoretical calculations showing suppressed conductance through thePCfilms. These results highlight the role of intermolecular effects and junction geometries in the observed fluctuations of conductance values between single-molecule and ensemble junctions, and the importance of studying molecules in both platforms.

Biological electron transport is classically thought to occur over nanometre distances, yet recent studies suggest that electrical currents can run along centimetre-long cable bacteria. The phenomenon remains elusive, however, as currents have not been directly measured, nor have the conductive structures been identified. Here we demonstrate that cable bacteria conduct electrons over centimetre distances via highly conductive fibres embedded in the cell envelope. Direct electrode measurements reveal nanoampere currents in intact filaments up to 10.1 mm long (>2000 adjacent cells). A network of parallel periplasmic fibres displays a high conductivity (up to 79 S cm−1), explaining currents measured through intact filaments. Conductance rapidly declines upon exposure to air, but remains stable under vacuum, demonstrating that charge transfer is electronic rather than ionic. Our finding of a biological structure that efficiently guides electrical currents over long distances greatly expands the paradigm of biological charge transport and could enable new bio-electronic applications., Cable bacteria’ form long multicellular filaments that can transfer electrical currents over centimetre-long distances. Here, Meysman et al. show that the electrical currents run along highly conductive fibres embedded in the cell envelope, and charge transfer is electronic rather than ionic.

Despite theoretical predictions that graphene should be impermeable to all gases, practical experiments on sealed graphene nanodrums show small leak rates. Thus far, the exact mechanism for this permeation has remained unclear, because different potential leakage pathways have not been studied separately. Here, we demonstrate a sealing method that consists of depositing SiO2 across the edge of suspended multilayer graphene flakes using electron beam-induced deposition. By sealing, leakage along the graphene-SiO2 interface is blocked, which is observed to result in a reduction in permeation rate by a factor of 104. The experiments thus demonstrate that gas flow along the graphene-SiO2 interface tends to dominate the leak rate in unsealed graphene nanodrums. Moreover, the presented sealing method enables the study of intrinsic gas leakage through graphene membranes and can enable hermetic graphene membranes for pressure sensing applications.

Single-molecule junctions — devices in which a single molecule is electrically connected by two electrodes — enable the study of a broad range of quantum-transport phenomena even at room temperature. These quantum features are related to molecular orbital and spin degrees of freedom and are characterized by various energy scales that can be chemically and physically tuned: level spacings, charging energies, tunnel couplings, exchange energies, vibrational energies and Kondo correlation energies. The competition between these different energy scales leads to a rich variety of processes, which researchers are now starting to be able to control and tune experimentally. In this Technical Review, we present the status of the molecular electronics field from this quantum-transport perspective with a focus on recent experimental results obtained using break-junction devices, including scanning probe and mechanically controlled break junctions, as well as electromigrated gold and graphene break junctions.

Strain engineering is one of the most effective approaches to manipulate the physical state of materials, control their electronic properties, and enable crucial functionalities. Because of their rich phase diagrams arising from competing ground states, quantum materials are an ideal playground for on-demand material control, and can be used to develop emergent technologies, such as adaptive electronics or neuromorphic computing. It was recently suggested that complex oxides could bring unprecedented functionalities to the field of nanomechanics, but the possibility of precisely controlling the stress state of materials is so far lacking. Here we demonstrate the wide and reversible manipulation of the stress state of single-crystal WO3 by strain engineering controlled by catalytic hydrogenation. Progressive incorporation of hydrogen in freestanding ultra-thin structures determines large variations of their mechanical resonance frequencies and induces static deformation. Our results demonstrate hydrogen doping as a new paradigm to reversibly manipulate the mechanical properties of nanodevices based on materials control., 6 pages, 4 figures, 9 supplementary sections

Mechanical sources of nonlinear damping play a central role in modern physics, from solid-state physics to thermodynamics. The microscopic theory of mechanical dissipation suggests that nonlinear damping of a resonant mode can be strongly enhanced when it is coupled to a vibration mode that is close to twice its resonance frequency. To date, no experimental evidence of this enhancement has been realized. In this letter, we experimentally show that nanoresonators driven into parametric-direct internal resonance provide supporting evidence for the microscopic theory of nonlinear dissipation. By regulating the drive level, we tune the parametric resonance of a graphene nanodrum over a range of 40–70 MHz to reach successive two-to-one internal resonances, leading to a nearly two-fold increase of the nonlinear damping. Our study opens up a route towards utilizing modal interactions and parametric resonance to realize resonators with engineered nonlinear dissipation over wide frequency range., Nonlinear dissipation is frequently observed in nanomechanical resonators, but its microscopic origin remains unclear. Here, nonlinear damping is found to be enhanced in graphene nanodrums close to internal resonance conditions, providing insights on the mechanisms at the basis of this phenomenon.

Nano letters 21(18), 7617-7624 (2021). doi:10.1021/acs.nanolett.1c02237, Published by ACS Publ., Washington, DC

The dynamics of suspended two-dimensional (2D) materials has received increasing attention during the last decade, yielding new techniques to study and interpret the physics that governs the motion of atomically thin layers. This has led to insights into the role of thermodynamic and nonlinear effects as well as the mechanisms that govern dissipation and stiffness in these resonators. In this review, we present the current state-of-the-art in the experimental study of the dynamics of 2D membranes. The focus will be both on the experimental measurement techniques and on the interpretation of the physical phenomena exhibited by atomically thin membranes in the linear and nonlinear regimes. We will show that resonant 2D membranes have emerged both as sensitive probes of condensed matter physics in ultrathin layers, and as sensitive elements to monitor small external forces or other changes in the environment. New directions for utilizing suspended 2D membranes for material characterization, thermal transport, and gas interactions will be discussed and we conclude by outlining the challenges and opportunities in this upcoming field., Comment: 39 pages, 16 figures

Theoretical studies suggest that mastering the thermocurrent through single molecules can lead to thermoelectric energy harvesters with unprecedentedly high efficiencies.1,2,3,4,5,6This can be achieved by engineering molecule length,7optimizing the tunnel coupling strength of molecules via chemical anchor groups8or by creating localized states in the backbone with resulting quantum interference features.4Empirical verification of these predictions, however, faces considerable experimental challenges and is still awaited. Here we use a novel measurement protocol that simultaneously probes the conductance and thermocurrent flow as a function of bias voltage and gate voltage. We find that the resulting thermocurrent is strongly asymmetric with respect to the gate voltage, with evidence of molecular excited states in the thermocurrent Coulomb diamond maps. These features can be reproduced by a rate-equation model only if it accounts for both the vibrational coupling and the electronic degeneracies, thus giving direct insight into the interplay of electronic and vibrational degrees of freedom, and the role of spin entropy in single molecules. Overall these results show that thermocurrent measurements can be used as a spectroscopic tool to access molecule-specific quantum transport phenomena.

The charge density wave (CDW) state in van der Waals systems shows interesting scaling phenomena as the number of layers can significantly affect the CDW transition temperature, $T_{CDW}$. However, it is often difficult to use conventional methods to study the phase transition in these systems due to their small size and sensitivity to degradation. Degradation is an important parameter which has been shown to greatly influence the superconductivity in layered systems. Since the CDW state competes with the onset of superconductivity, it is expected that $T_{CDW}$ will also be affected by the degradation. Here, we probe the CDW phase transition by the mechanical resonances of suspended 2H-TaS2 and 2H-TaSe2 membranes and study the effect of disorder on the CDW state. Pristine flakes show the transition near the reported values of 75 K and 122 K respectively. We then study the effect of degradation on 2H-TaS2 which displays an enhancement of $T_{CDW}$ up to 129 K after degradation in ambient air. Finally, we study a sample with local degradation and observe that multiple phase transitions occur at 87 K, 103 K and 118 K with a hysteresis in temperature in the same membrane. The observed spatial variations in the Raman spectra suggest that variations in crystal structure cause domains with different transition temperatures which could result in the hysteresis. This work shows the potential of using nanomechanical resonance to characterize the CDW in suspended 2D materials and demonstrate that degradation can have a large effect on transition temperatures., The following article has been accepted by Applied Physics Letters. After it is published, it will be found at [https://doi.org/10.1063/5.0051112]

2D Materials 8(1), 015031 (2021). doi:10.1088/2053-1583/abbecd, Published by IOP Publ., Bristol

Paper has the potential to dramatically reduce the cost of electronic components. In fact, paper is 10 000 times cheaper than crystalline silicon, motivating the research to integrate electronic materials on paper substrates. Among the different electronic materials, van der Waals materials are attracting the interest of the scientific community working on paper-based electronics because of the combination of high electrical performance and mechanical flexibility. Up to now, different methods have been developed to pattern conducting, semiconducting and insulating van der Waals materials on paper but the integration of superconductors remains elusive. Here, the deposition of NbSe2, an illustrative van der Waals superconductor, on standard copy paper is demonstrated. The deposited NbSe2 films on paper display superconducting properties (e.g. observation of Meissner effect and resistance drop to zero-resistance state when cooled down below its critical temperature) similar to those of bulk NbSe2., Paper has the potential to dramatically reduce the cost of electronic components but the integration of electronic materials is challenging. Here the integration of NbSe2, a van der Waals superconductor, on paper is demonstrated.

Paper holds the promise to replace silicon substrates in applications like internet of things or disposable electronics that require ultra-low-cost electronic components and an environmentally friendly electronic waste management. In the last years, spurred by the abovementioned properties of paper as a substrate and the exceptional electronic, mechanical and optical properties of van der Waals (vdW) materials, many research groups have worked towards the integration of vdW materials-based devices on paper. Recently, a method to deposit a continuous film of densely packed interconnects of vdW materials on paper by simply rubbing the vdW crystals against the rough surface of paper has been presented. This method utilizes the weak interlayer vdW interactions and allows cleaving of the crystals into micro platelets through the abrasion against the paper. Here, we aim to illustrate the general character and the potential of this technique by fabricating films of 39 different vdW materials (including superconductors, semi-metals, semiconductors, and insulators) on standard copier paper. We have thoroughly characterized their optical properties showing their high optical quality: one can easily resolve the absorption band edge of semiconducting vdW materials and even the excitonic features present in some vdW materials with high exciton binding energy. We also measured the electrical resistivity for several vdW materials films on paper finding exceptionally low values, which are in some cases, orders of magnitude lower than those reported for analogous films produced by inkjet printing. We finally demonstrate the fabrication of field-effect devices with vdW materials on paper using the paper substrate as an ionic gate., Comment: 4 figures in main text, 21 figures in Supp. Info

Ultrafast optical control of magnetic anisotropy in a van der Waals antiferromagnet activates a sub-THz two-dimensional magnon., Van der Waals magnets provide an ideal playground to explore the fundamentals of low-dimensional magnetism and open opportunities for ultrathin spin-processing devices. The Mermin-Wagner theorem dictates that as in reduced dimensions isotropic spin interactions cannot retain long-range correlations, the long-range spin order is stabilized by magnetic anisotropy. Here, using ultrashort pulses of light, we control magnetic anisotropy in the two-dimensional van der Waals antiferromagnet NiPS3. Tuning the photon energy in resonance with an orbital transition between crystal field split levels of the nickel ions, we demonstrate the selective activation of a subterahertz magnon mode with markedly two-dimensional behavior. The pump polarization control of the magnon amplitude confirms that the activation is governed by the photoinduced magnetic anisotropy axis emerging in response to photoexcitation of ground state electrons to states with a lower orbital symmetry. Our results establish pumping of orbital resonances as a promising route for manipulating magnetic order in low-dimensional (anti)ferromagnets.

Graphene quantum dots (QDs) are intensively studied as platforms for the next generation of quantum electronic devices. Fine tuning of the transport properties in monolayer graphene QDs, in particular with respect to the independent modulation of the tunnel barrier transparencies, remains challenging and is typically addressed using electrostatic gating. We investigate charge transport in back-gated graphene mechanical break junctions and reveal Coulomb blockade physics characteristic of a single, high-quality QD when a nanogap is opened in a graphene constriction. By mechanically controlling the distance across the newly formed graphene nanogap, we achieve reversible tunability of the tunnel coupling to the drain electrode by 5 orders of magnitude, while keeping the source-QD tunnel coupling constant. The break junction device can therefore become a powerful platform to study the physical parameters that are crucial to the development of future graphene-based devices, including energy converters and quantum calorimeters., S.C. acknowledges a Marie Skłodowska-Curie Individual Fellowship under grant BioGraphING (ID: 798851) and P.G. acknowledges a Marie Skłodowska-Curie Individual Fellowship under grant TherSpinMol (ID: 748642) from the European Union’s Horizon 2020 research and innovation programme. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, and the CREST (JPMJCR15F3), JST. This work was supported by the Graphene Flagship (a European Union’s Horizon 2020 research and innovation programme under grant agreement no. 649953), the Marie Curie ITN MOLESCO, an ERC advanced grant (Mols@Mols no. 240299), and a Spanish MCIU/AEI/FEDER project (PGC2018-094783).

In recent years, a wide range of single-molecule devices has been realized, enabled by technological advances combined with the versatility offered by synthetic chemistry. In particular, single-molecule diodes have attracted significant attention with an ongoing effort to increase the rectification ratio between the forward and reverse current. Various mechanisms have been investigated to improve rectification, either based on molecule-intrinsic properties or by engineering the coupling of the molecule to the electrodes. In this perspective, we first provide an overview of the current experimental approaches reported in literature to achieve rectification at the single-molecule level. We then proceed with our recent efforts in this direction, exploiting the internal structure of multi-site molecules, yielding the highest rectification ratio based on a molecule-intrinsic mechanism. We introduce the theoretical framework for multi-site molecules and infer general design guidelines from this. Based on these guidelines, a series of two-site molecules have been developed and integrated into devices. Using two- and three-terminal mechanically controllable break junction measurements, we show that depending on the on-site energies, which are tunable by chemical design, the devices either exhibit pronounced negative differential conductance, or behave as highly-efficient rectifiers. Finally, we propose a design of a single-molecule diode with a theoretical rectification ratio exceeding a million.

In transition metal dichalcogenides, defects have been found to play an important role, affecting doping, spin-valley relaxation dynamics, and assisting in proximity effects of spin-orbit coupling. Here, we study localized states in $\mathrm{WS}_2$ and how they affect tunneling through van der Waals heterostructures of h-BN/graphene/$\mathrm{WS}_2$/metal. The obtained conductance maps as a function of bias and gate voltage reveal single-electron transistor behavior (Coulomb blockade) with a rich set of transport features including excited states and negative differential resistance regimes. Applying a perpendicular magnetic field, we observe a shift in the energies of the quantum levels and information about the orbital magnetic moment of the localized states is extracted.

The appealing feature of molecular electronics is the possibility of exploiting functionality built within a single molecule. This functionality can be employed, for example, for sensing or switching purposes. Thus, ideally, the associated conductance changes should be sizable upon application of external stimuli. Here, we show that a molecular spring can be mechanically compressed or elongated to tune its conductance by up to an order of magnitude by controlling the quantum interference between electronic pathways. Oscillations in the conductance occur when the stress built up in the molecule is high enough to allow the anchoring groups to move along the surface in a stick-slip-like fashion. The mechanical control of quantum interference effects and the resulting large change in molecular conductance open the door for applications in, e.g., a minute mechanosensitive sensing device functional at room temperature., 23 pages, 6 figures

In the field of nanomechanics, parametric excitations are of interest since they can greatly enhance sensing capabilities and eliminate cross-talk. Above a certain threshold of the parametric pump, the mechanical resonator can be brought into parametric resonance. Here we demonstrate parametric resonance of suspended single-layer graphene membranes by an efficient opto-thermal drive that modulates the intrinsic spring constant. With a large amplitude of the optical drive, a record number of 14 mechanical modes can be brought into parametric resonance by modulating a single parameter: the pre-tension. A detailed analysis of the parametric resonance allows us to study nonlinear dynamics and the loss tangent of graphene resonators. It is found that nonlinear damping, of the van der Pol type, is essential to describe the high amplitude parametric resonance response in atomically thin membranes.

Recent research in two-dimensional (2D) materials has boosted a renovated interest in the p-n junction, one of the oldest electrical components which can be used in electronics and optoelectronics. 2D materials offer remarkable flexibility to design novel p-n junction device architectures, not possible with conventional bulk semiconductors. In this Review we thoroughly describe the different 2D p-n junction geometries studied so far, focusing on vertical (out-of-plane) and lateral (in-plane) 2D junctions and on mixed-dimensional junctions. We discuss the assembly methods developed to fabricate 2D p-n junctions making a distinction between top-down and bottom-up approaches. We also revise the literature studying the different applications of these atomically thin p-n junctions in electronic and optoelectronic devices. We discuss experiments on 2D p-n junctions used as current rectifiers, photodetectors, solar cells and light emitting devices. The important electronics and optoelectronics parameters of the discussed devices are listed in a table to facilitate their comparison. We conclude the Review with a critical discussion about the future outlook and challenges of this incipient research field., Review, 34 pages, 11 figures, 4 tables

Owing to their atomic-scale thickness, the resonances of two-dimensional (2D) material membranes show signatures of nonlinearities at forces of only a few picoNewtons. Although the linear dynamics of membranes is well understood, the exact relation between the nonlinear response and the resonator’s material properties has remained elusive. Here we show a method for determining the Young’s modulus of suspended 2D material membranes from their nonlinear dynamic response. To demonstrate the method, we perform measurements on graphene and MoS2 nanodrums electrostatically driven into the nonlinear regime at multiple driving forces. We show that a set of frequency response curves can be fitted using only the cubic spring constant as a fit parameter, which we then relate to the Young’s modulus of the material using membrane theory. The presented method is fast, contactless, and provides a platform for high-frequency characterization of the mechanical properties of 2D materials., The mechanical resonances of atomically thin membranes show nonlinear responses at driving forces in the picoNewton range. Here, the authors develop a contactless method to extract the Young’s modulus of 2D materials from the nonlinear dynamic response of these nanomechanical resonators.

Hexagonal boron nitride is a 2D material whose single-layer allotrope has not been intensively studied despite being the substrate for graphene electronics. Its transparency and stronger interlayer adhesion with respect to graphene makes it difficult to work with, and few applications have been proposed. We have developed a transfer technique for this extra-adhesive material that does not require its visual localization, and fabricated mechanical resonators made out of chemical vapor-deposited single-layer hexagonal boron nitride. The suspended material was initially contaminated with polymer residues from the transfer, and the devices showed an unexpected tensioning when cooling them to 3 K. After cleaning in harsh environments with air at 450 °C and ozone, the temperature dependence changed with f 0 Q products reaching 2 × 1010 Hz at room temperature. This work paves the way to the realization of highly sensitive mechanical systems based on hexagonal boron nitride, which could be used as an alternative material to SiN for optomechanics experiments at room temperature. An improved transfer method allows easy placement of highly transparent and strongly adhesive hexagonal boron nitride on target substrates. A team led by Santiago J. Cartamil-Bueno at Delft University of Technology developed a technique that enables the transfer of large-area, single-layer hexagonal boron nitride films grown by chemical vapor deposition onto a substrate of choice, whilst not requiring optical visualization. Following an additional cleaning step, the atomically thin membranes were transferred onto circular microcavities patterned on a silicon oxide substrate, resulting in the formation of suspended drums. Cleaning in harsh environments using a mixture of air and ozone is instrumental to a substantial improvement in the quality factor of the drums, indicating that undesired contamination causes damping of the mechanical motion. These results show promise for the development of sensitive hexagonal boron nitride resonators.

The unique properties and atomic thickness of two-dimensional (2D) materials enable smaller and better nanoelectromechanical sensors with novel functionalities. During the last decade, many studies have successfully shown the feasibility of using suspended membranes of 2D materials in pressure sensors, microphones, accelerometers, and mass and gas sensors. In this review, we explain the different sensing concepts and give an overview of the relevant material properties, fabrication routes, and device operation principles. Finally, we discuss sensor readout and integration methods and provide comparisons against the state of the art to show both the challenges and promises of 2D material-based nanoelectromechanical sensing., Comment: Review paper

We present experimental and theoretical studies of single-molecule conductance through nonplanar fullerocurcuminoid molecular dyads in ambient conditions using the mechanically controllable break junction technique. We show that molecular dyads with bare fullerenes form configurations with conductance features related to different transport channels within the molecules, as identified with filtering and clustering methods. The primary channel corresponds to charge transport through the methylthio-terminated backbone. Additional low-conductance channels involve one backbone side and the fullerene. In fullerenes with four additional equatorial diethyl malonate groups attached to them, the latter transport pathway is blocked. Density functional theory calculations corroborate the experimental observations. In combination with nonequilibrium green functions, the conductance values of the fullerocurcuminoid backbones are found to be similar to those of a planar curcuminoid molecule without a fullerene attached. In the nonplanar fullerocurcuminoid systems, the highest-conductance peak occurs partly through space, compensating for the charge delocalization loss present in the curcuminoid system., Financial support from the European Commission (COST Action MOLSPIN CA15128 and EU RISE (DAFNEOX) project SEP-210165479) is gratefully acknowledged. The work at the University of Chile was supported by Fondecyt Regular Project 1181080 (D.D.), Fondecyt Regular Project 1161775 (M.S.), Fondecyt Regular Project 1170524 (D.A.), and Fondequip EQM140055 and EQM180009 (D.D). Powered@NLHPC: this research was partially supported by the supercomputing infrastructure of the NLHPC (ECM-02). E.R. acknowledges MICIIN for grant PGC2018-093863-B-C21 and the Maria de Maeztu Excellence grant MDM-2017-0767, for the computer resources, technical expertise, and assistance provided by the Barcelona Supercomputing Centre and CSUC and to the Generalitat de Catalunya for an ICREA Academia award and grant 2017SGR1289. H.S.J.v.d.Z. acknowledges support from the Dutch Science foundation (NWO). N.A.-A. thanks MEC for grant MAT2016-77852-C2-1-R, to the Generalitat de Catalunya for the grant 2017SGR1277, and the Severo Ochoa Program for Centers of Excellence in R&D (SEV-2015-0496). This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement 724981). L.E. thanks the US National Science Foundation (NSF) for generous support of this work under the CHE-1801317 program. The Robert A. Welch Foundation is also gratefully acknowledged for an endowed chair to L.E. (grant AH-0033).

Recent experiments have shown surprisingly large thermal time constants in suspended graphene ranging from 10 to 100 ns in drums with a diameter ranging from 2 to 7 microns. The large time constants and their scaling with diameter points towards a thermal resistance at the edge of the drum. However, an explanation of the microscopic origin of this resistance is lacking. Here, we show how phonon scattering at a kink in the graphene, e.g. formed by sidewall adhesion at the edge of the suspended membrane, can cause a large thermal time constant. This kink strongly limits the fraction of flexural phonons that cross the suspended graphene edge, which causes a thermal interface resistance at its boundary. Our model predicts thermal time constants that are of the same order of magnitude as experimental data, and shows a similar dependence on the circumference. Furthermore, the model predicts the relative in-plane and out-of-plane phonon contributions to graphene's thermal expansion force, in agreement with experiments. We thus show, that in contrast to conventional thermal (Kapitza) resistance which occurs between two different materials, in 2D materials another type of thermal interface resistance can be geometrically induced in a single material.

The high flexibility, impermeability and strength of graphene membranes are key properties that can enable the next generation of nanomechanical sensors. However, for capacitive pressure sensors, the sensitivity offered by a single suspended graphene membrane is too small to compete with commercial sensors. Here, we realize highly sensitive capacitive pressure sensors consisting of arrays of nearly ten thousand small, freestanding double-layer graphene membranes. We fabricate large arrays of small-diameter membranes using a procedure that maintains the superior material and mechanical properties of graphene, even after high-temperature annealing. These sensors are readout using a low-cost battery-powered circuit board, with a responsivity of up to $$47.8$$ aF Pa−1 mm−2, thereby outperforming the commercial sensors. Arrays of tiny graphene membranes exhibit excellent performance as pressure sensors, offering a competitive, low-cost alternative to commercially available systems. Graphene offers a durable and resilient material for sensors that generate a capacitive readout to pressure changes, but individual membranes are not sufficiently sensitive. Researchers led by Makars Siskins and Peter Steeneken of the Delft University of Technology in the Netherlands have devised a strategy for fabricating graphene sensor arrays that overcome this limitation. Their method yields millimeter-scale assemblies of 10,000 double-layered graphene membranes, which can in turn be connected to an inexpensive battery-powered circuit board. The authors demonstrate that this system can outperform a state-of-the-art commercial pressure sensor, and propose that further improvements to the design and fabrication of these arrays could improve their responsiveness by a full order of magnitude.

We fabricate paper-supported semiconducting devices by rubbing a layered molybdenum disulfide (MoS2) crystal onto a piece of paper, similarly to the action of drawing/writing with a pencil on paper. We show that the abrasion between the MoS2 crystal and the paper substrate efficiently exfoliates the crystals, breaking the weak van der Waals interlayer bonds and leading to the deposition of a film of interconnected MoS2 platelets. Employing this simple method, that can be easily extended to other 2D materials, we fabricate MoS2-on-paper broadband photodectectors with spectral sensitivity from the ultraviolet (UV) to the near-infrared (NIR). We also used these paper-based photodetectors to acquire pictures of objects by mounting the photodetectors in a homebuilt single-pixel camera setup., Comment: 6 main text figures + 4 Supp Info figures

$${\hbox {SrIrO}}_{3}$$ SrIrO 3 , the three-dimensional member of the Ruddlesden–Popper iridates, is a paramagnetic semimetal characterised by a the delicate interplay between spin–orbit coupling and Coulomb repulsion. In this work, we study the anisotropic magnetoresistance (AMR) of $${\hbox {SrIrO}}_{3}$$ SrIrO 3 thin films, which is closely linked to spin–orbit coupling and probes correlations between electronic transport, magnetic order and orbital states. We show that the low-temperature negative magnetoresistance is anisotropic with respect to the magnetic field orientation, and its angular dependence reveals the appearance of a fourfold symmetric component above a critical magnetic field. We show that this AMR component is of magnetocrystalline origin, and attribute the observed transition to a field-induced magnetic state in $${\hbox {SrIrO}}_{3}$$ SrIrO 3 .

On-chip temperature sensing on a micro- to nanometer scale is becoming more desirable as the complexity of nanodevices and size requirements increase and with it the challenges in thermal probing and management. This highlights the need for scalable and reliable temperature sensors which have the potential to be incorporated into current and future device structures. Here, we show that U-shaped graphene stripes consisting of one wide and one narrow leg form a single material thermocouple that can function as a self-powering temperature sensor. We find that the graphene thermocouples increase in sensitivity with a decrease in leg width, due to a change in the Seebeck coefficient, which is in agreement with our previous findings and report a maximum sensitivity of $\Delta S \approx$ 39 $\mathrm{\mu}$V/K., Comment: 7 pages, 4 figures

Franckeite is a naturally occurring layered mineral with a structure composed of alternating stacks of SnS2-like and PbS-like layers. Although this superlattice is composed of a sequence of isotropic two-dimensional layers, it exhibits a spontaneous rippling that makes the material structurally anisotropic. We demonstrate that this rippling comes hand in hand with an inhomogeneous in-plane strain profile and anisotropic electrical, vibrational and optical properties. We argue that this symmetry breakdown results from a spatial modulation of the van der Waals interaction between layers due to the SnS2-like and PbS-like lattices incommensurability., Comment: 6 main text figures, Supp. Info. available upon request

In two-dimensional materials research, oxidation is usually considered as a common source for the degradation of electronic and optoelectronic devices or even device failure. However, in some cases a controlled oxidation can open the possibility to widely tune the band structure of 2D materials. In particular, we demonstrate the controlled oxidation of titanium trisulfide (TiS3), a layered semicon-ductor that has attracted much attention recently thanks to its quasi-1D electronic and optoelectron-ic properties and its direct bandgap of 1.1 eV. Heating TiS3 in air above 300 °C gradually converts it into TiO2, a semiconductor with a wide bandgap of 3.2 eV with applications in photo-electrochemistry and catalysis. In this work, we investigate the controlled thermal oxidation of indi-vidual TiS3 nanoribbons and its influence on the optoelectronic properties of TiS3-based photodetec-tors. We observe a step-wise change in the cut-off wavelength from its pristine value ~1000 nm to 450 nm after subjecting the TiS3 devices to subsequent thermal treatment cycles. Ab-initio and many-body calculations confirm an increase in the bandgap of titanium oxysulfide (TiO2-xSx) when in-creasing the amount of oxygen and reducing the amount of sulfur.

Paper based thermoresistive sensors are fabricated by rubbing WS2 powder against a piece of standard copier paper, like the way a pencil is used to write on paper. The abrasion between the layered material and the rough paper surface erodes the material, breaking the weak van der Waals interlayer bonds, yielding a film of interconnected platelets. The resistance of WS2 presents a strong temperature dependence, as expected for a semiconductor material in which charge transport is due to thermally activated carriers. This strong temperature dependence makes the paper supported WS2 devices extremely sensitive to small changes in temperature. This exquisite thermal sensitivity, and their fast response times to sudden temperature changes, is exploited thereby demonstrating the usability of a WS2-on-paper thermal sensor in a respiration monitoring device., 6 main text figures, 1 table, 6 supp. info. figures

We report on the fabrication and measurements of platinum-self-aligned nanogap devices containing cubed iron (core)/iron oxide (shell) nanoparticles (NPs) with two average different sizes (13 and 17 nm). The nanoparticles are deposited by means of a cluster gun technique. Their trapping across the nanogap is demonstrated by comparing the current vs voltage characteristics (I-Vs) before and after the deposition. At low temperature, the I-Vs can be well fitted to the Korotkov and Nazarov Coulomb blockade model, which captures the coexistence of single-electron tunneling and tunnel barrier suppression upon a bias voltage increase. The measurements thus show that Coulombblockaded devices can be made with a nanoparticle cluster source, which extends the existing possibilities to fabricate such devices to those in which it is very challenging to reduce the usual NP agglomeration given by a solution method., This study was supported by the EU Horizon 2020 research and innovation program under the Marie-Sklodowska-Curie Grant Agreement No. 645658 (DAFNEOX Project), by two FONDECYT REGULAR Grant Nos. 1181080 and 1161775, and by two FONDEQUIP Grant Nos. EQM140055 and EQM180009. We thank the Spanish Ministry of Science, Innovation and Universities (Project Nos. MAT2015-71664-R and RTI2018-099960-B-I00) and the Serbian Ministry of Education, Science and Technological Development (Project No. III45018) for their support. A.P. and Z.K. thank Senzor-INFIZ (Serbia) for the cooperation provided during their respective secondments.

We present measurements of weak localization on hexagonal boron nitride encapsulated bilayer ${\mathrm{MoS}}_{2}$. From the analysis we obtain information regarding the phase coherence and the spin diffusion of the electrons. We find that the encapsulation with boron nitride provides higher mobilities in the samples, and the phase coherence shows improvement, while the spin relaxation does not exhibit any significant enhancement compared to nonencapsulated ${\mathrm{MoS}}_{2}$. The spin relaxation time is in the order of a few picoseconds, indicating a fast intravalley spin-flip rate. Lastly, the spin-flip rate is found to be independent from electron density in the current range, which can be explained through counteracting spin-flip scattering processes based on electron-electron Coulomb scattering and extrinsic Bychkov-Rashba spin-orbit coupling.

Development of molecules that can switch between redox states with paired and unpaired electrons is important for molecular electronics and spintronics. In this work, a selection of redox-active indenofluorene-extended tetrathiafulvalenes (IF-TTFs) with thioacetate end groups was prepared from a readily obtainable dibromo-functionalized IF-TTF building block using palladium-catalyzed cross-coupling reactions, such as the Suzuki reaction. The end groups served as electrode anchoring groups for single-molecule conductance studies, and the molecules were subjected to mechanically controlled break-junction measurements with gold contacts and to low-bias charge transport measurements in gated three-terminal electromigration junctions. The neutral molecules showed clear conductance signatures, and somewhat surprisingly, we found that a meta-meta anchoring configuration gave a higher conductance than a para-meta configuration. We explain this behavior by "through-space" coupling between the gold electrode and the phenyl on which the anchoring group is attached. Upon charging the molecule in a gated junction, we found reproducibly a Kondo effect (zero-bias conductance) attributed to a net spin. Ready generation of radical cations was supported by cyclic voltammetry measurements, revealing stepwise formation of radical cation and dication species in solution. The first oxidation event was accompanied by association reactions as the appearance of the first oxidation peak was strongly concentration dependent.

Recent observations of destructive quantum interference in single-molecule junctions confirm the role played by quantum effects in the electronic conductance properties of molecular systems. We show here that the destructive interference can be turned ON or OFF within the same molecular system by mechanically controlling its conformation. Using a combination of ab-initio calculations and single-molecule conductance measurements, we demonstrate the existence of a quasi-periodic destructive quantum interference pattern along the breaking traces of {\pi}-{\pi} stacked molecular dimers. The detection of these interferences, which are due to opposite signs of the intermolecular electronic couplings, was only made possible by a combination of wavelet transform and higher-order statistical analysis of single-breaking traces. The results demonstrate that it is possible to control the molecular conductance over a few orders of magnitudes and with a sub-angstrom resolution by exploiting the subtle structure-property relationship of {\pi}-{\pi} stack dimers. These large conductance changes may be beneficial for the design of single-molecule electronic components that exploit the intrinsic quantum effects occurring at the molecular scale., Comment: 10 pages, 4 figures

We report on an approach to realize carbon-gold (C-Au) bonded molecular junctions without the need for an additive to deprotect the alkynyl carbon as endstanding anchor group. Using the mechanically controlled break junction (MCBJ) technique, we determine the most probable conductance value of a family of alkynyl terminated oligophenylenes (OPA(n)) connected to gold electrodes through such an akynyl moiety in ambient conditions. The molecules bind to the gold leads through an sp-hybridized carbon atom at each side. Comparing our results with other families of molecules that present organometallic C-Au bonds, we conclude that the conductance of molecules contacted via an sp-hybridized carbon atom is lower than the ones using sp(3) hybridization due to strong differences in the coupling of the conducting orbitals with the gold leads.

The charge transport properties of SCO [Fe(Htrz)2 (trz)](BF4 ) NPs covered with a silica shell placed in between single-layer graphene electrodes are reported. A reproducible thermal hysteresis loop in the conductance above room-temperature is evidenced. This bistability combined with the versatility of graphene represents a promising scenario for a variety of technological applications but also for future sophisticated fundamental studies.

Single-molecule break-junction measurements are intrinsically stochastic in nature, requiring the acquisition of large datasets of “breaking traces” to gain insight into the generic electronic properties of the molecule under study. For example, the most probable conductance value of the molecule is often extracted from the conductance histogram built from these traces. In this letter, we present an unsupervised and reference-free machine learning tool to improve the determination of the conductance of oligo(phenylene ethynylene)dithiol from mechanically controlled break-junction (MCBJ) measurements. Our method allows for the classification of individual breaking traces based on an image recognition technique. Moreover, applying this technique to multiple merged datasets makes it possible to identify common breaking behaviors present across different samples, and therefore to recognize global trends. In particular, we find that the variation in the extracted molecular conductance can be significantly reduced resulting in a more reliable estimation of molecular conductance values from MCBJ datasets. Finally, our approach can be more widely applied to different measurement types which can be converted to two-dimensional images.

Physical review research 2(1), 012058 (2020). doi:10.1103/PhysRevResearch.2.012058, Published by APS, College Park, MD

The energy dependent thermoelectric response of a single molecule contains valuable information about its transmission function and its excited states. However, measuring it requires devices that can efficiently heat up one side of the molecule while being able to tune its electrochemical potential over a wide energy range. Furthermore, to increase junction stability, devices need to operate at cryogenic temperatures. In this work, we report on a device architecture to study the thermoelectric properties and the conductance of single molecules simultaneously over a wide energy range. We employ a sample heater in direct contact with the metallic electrodes contacting the single molecule which allows us to apply temperature biases up to ΔT = 60 K with minimal heating of the molecular junction. This makes these devices compatible with base temperatures Tbath < 2 K and enables studies in the linear ( Δ T ≪ T molecule) and nonlinear ( Δ T ≫ T molecule) thermoelectric transport regimes.The energy dependent thermoelectric response of a single molecule contains valuable information about its transmission function and its excited states. However, measuring it requires devices that can efficiently heat up one side of the molecule while being able to tune its electrochemical potential over a wide energy range. Furthermore, to increase junction stability, devices need to operate at cryogenic temperatures. In this work, we report on a device architecture to study the thermoelectric properties and the conductance of single molecules simultaneously over a wide energy range. We employ a sample heater in direct contact with the metallic electrodes contacting the single molecule which allows us to apply temperature biases up to ΔT = 60 K with minimal heating of the molecular junction. This makes these devices compatible with base temperatures Tbath < 2 K and enables studies in the linear ( Δ T ≪ T molecule) and nonlinear ( Δ T ≫ T molecule) thermoelectric transport regimes.

Two-dimensional (2D) materials with strong in-plane anisotropy are of interest for enabling orientation-dependent, frequency-tunable, optomechanical devices. However, black phosphorus (bP), the 2D material with the largest anisotropy to date, is unstable as it degrades in air. In this work we show that As2S3 is an interesting alternative, with a similar anisotropy to bP, while at the same time having a much higher chemical stability. We probe the mechanical and optical anisotropy in As2S3 by three distinct angular-resolved experimental methods: Raman spectroscopy, atomic force microscopy (AFM), and resonance frequency analysis. Using a dedicated angle-resolved AFM force-deflection method, an in-plane anisotropy factor of EaEc=1.7 is found in the Young's modulus of As2S3 with Ea-axis = 79.1 ± 10.1 GPa and Ec-axis = 47.2 ± 7.9 GPa. The high mechanical anisotropy is also shown to cause up to 65% difference in the resonance frequency, depending on crystal orientation and aspect ratio of membranes.

Current wafer-scale fabrication methods for graphene-based electronics and sensors involve the transfer of single-layer graphene by a support polymer. This often leaves some polymer residue on the graphene, which can strongly impact its electronic, thermal, and mechanical resonance properties. To assess the cleanliness of graphene fabrication methods, it is thus of considerable interest to quantify the amount of contamination on top of the graphene. Here, we present a methodology for the direct measurement of the mass of the graphene sheet using quartz crystal microbalances (QCMs). By monitoring the QCM resonance frequency during removal of graphene in an oxygen plasma, the total mass of the graphene and contamination is determined with sub-graphene-monolayer accuracy. Since the etch-rate of the contamination is higher than that of graphene, quantitative measurements of the mass of contaminants below, on top, and between graphene layers are obtained. We find that polymer-based dry transfer methods can increase the mass of a graphene sheet by a factor of 10. The presented mass measurement method is conceptually straightforward to interpret and can be used for standardized testing of graphene transfer procedures in order to improve the quality of graphene devices in future applications.

We investigate theoretically and experimentally the singlet-triplet Kondo effect induced by a magnetic field in a molecular junction. Temperature dependent conductance, $G(T)$, is calculated by the numerical renormalization group, showing a strong imprint of the relevant low energy scales, such as the Kondo temperature, exchange and singlet-triplet splitting. We demonstrate the stability of the singlet-triplet Kondo effect against weak spin anisotropy, modeled by an anisotropic exchange. Moderate spin anisotropy manifests itself by lowering the Kondo plateaus, causing the $G(T)$ to deviate from a standard temperature dependence, expected for a spin-half Kondo effect. We propose this scenario as an explanation for anomalous $G(T)$, measured in an organic diradical molecule coupled to gold contacts. We uncover certain new aspects of the singlet-triplet Kondo effect, such as coexistence of spin-polarization on the molecule with Kondo screening and non-perturbative parametric dependence of an effective magnetic field induced by the leads., 15 pages, 9 figures; invited contribution to the "Virtual Special Issue" in honor of Abraham Nitzan in The Journal of Physical Chemistry C; v2: minor updates to the text and figures, added 2 paragraphs on numerical values of model parameters

One of the main challenges to upscale the fabrication of molecular devices is to achieve a mechanically stable device with reproducible and controllable electronic features that operates at room temperature1,2. This is crucial because structural and electronic fluctuations can lead to significant changes in the transport characteristics at the electrode-molecule interface3,4. In this study, we report on the realization of a mechanically and electronically robust graphene-based molecular junction. Robustness was achieved by separating the requirements for mechanical and electronic stability at the molecular level. Mechanical stability was obtained by anchoring molecules directly to the substrate, rather than to graphene electrodes, using a silanization reaction. Electronic stability was achieved by adjusting the π-π orbitals overlap of the conjugated head groups between neighbouring molecules. The molecular devices exhibited stable current-voltage (I-V) characteristics up to bias voltages of 2.0 V with reproducible transport features in the temperature range from 20 to 300 K.

Nano letters 19(2), 1282-1288 (2019). doi:10.1021/acs.nanolett.8b04862, Published by ACS Publ., Washington, DC

We use a scalable optical technique to characterize more than 21000 circular nanomechanical devices made out of suspended single- and double-layer graphene on cavities with different diameters ($D$) and depths ($g$). To maximize the contrast between suspended and broken membranes we used a model for selecting the optimal color filter. The method enables parallel and automatized image processing for yield statistics. We find the survival probability to be correlated to a structural mechanics scaling parameter given by $D^4/g^3$. Moreover, we extract a median adhesion energy of $\Gamma =$ 0.9 J/m$^2$ between the membrane and the native SiO$_2$ at the bottom of the cavities., Comment: 7 pages, 5 figures

Worlds first graphene-based Pirani pressure sensor is presented. Due to the decreased area and low thickness, the graphene-based Pirani pressure sensor allows for low power applications down to 0.9 mW. Using an innovative, transfer-free process, suspended graphene beams are realized. This allows for up to 100x miniaturization of the pressure sensor area, while enabling wafer-scale fabrication. The response of the miniaturized pressure sensor is similar to that of the much larger state-of-the-art Si-based Pirani pressure sensors, demonstrating the potential of graphene-based Pirani sensors.

We report on an experimental investigation of transport through single molecules, trapped between two gold nano-electrodes fabricated with the mechanically controlled break junction (MCBJ) technique. The four molecules studied share the same core structure, namely oligo(phenylene ethynylene) (OPE3), while having different aurophilic anchoring groups: thiol (SAc), methyl sulfide (SMe), pyridyl (Py) and amine (NH2). The focus of this paper is on the combined characterization of the electrical and mechanical properties determined by the anchoring groups. From conductance histograms we find that thiol anchored molecules provide the highest conductance; a single-level model fit to current–voltage characteristics suggests that SAc groups exhibit a higher electronic coupling to the electrodes, together with better level alignment than the other three groups. An analysis of the mechanical stability, recording the lifetime in a self-breaking method, shows that Py and SAc yield the most stable junctions while SMe form short-lived junctions. Density functional theory combined with non-equlibrium Green’s function calculations help in elucidating the experimental findings.

ConspectusThis Account provides an overview of our recent efforts to unravel charge transport characteristics of a metal-molecule-metal junction containing an individual π-conjugated molecule. The model system of our choice is an oligo(phenylene-ethynylene) consisting of three rings, in short OPE3, which represents a paradigmatic model system for molecular-scale electronics. Members of the OPE family are among the most studied in the field thanks to their simple and rigid structure, the possibility of chemically functionalizing them, and their clear transport characteristics. When investigating charge transport in molecular systems, two general directions can be distinguished: one in which assemblies composed of many molecules contacted in parallel are studied, while in the other a single molecule is investigated at a time. In the former approach, molecule-molecule interactions and ensemble-averaged quantities may play a role, thereby introducing broadening of spectral features and hindering the study of the behavior of individual molecules making it more difficult to deconvolute local and intrinsic molecular effects from collective ones. In contrast, single-molecule experiments directly probe individual molecular features and, when they are repeated many times, allow build up of a statistical representation of the changes introduced by, e.g., different junction configurations. Especially in recent years, experimental techniques have advanced such that now large sets of individual events can be measured and analyzed with statistical tools. To study individual single-molecule junctions, we use the break junction technique, in which two sharp movable electrodes are formed by breaking a thin metallic wire and used to contact a single or few molecules. By probing thousands of single-molecule junctions in different conditions, we show that their creation involves independent events justifying the statistical tools that are used. By combining room- and low-temperature data, we show that the dominant transport mechanism for electrons through the OPE3 molecule is off-resonant tunneling. The simplest model capturing transport details in this case is a single-level model characterized by three parameters: the level alignment of the frontier orbital with the Fermi energy of the leads and the electronic couplings to the leads. Variations in these parameters give a broad distribution (1 order of magnitude) in the observed conductance values, indicating that at the microscopic level both the hybridization with the metallic electrodes and the molecular electronic configuration can fluctuate. The low-temperature data show that these variations are due to abrupt changes in the configuration of the molecule in the junction leading to changes in either one of these parameters or both at the same time. The complementary information gained from different experiments is needed to build up a consistent and extended picture of the variability of molecular configurations, omnipresent in single-molecule studies. Knowledge of this variability can help one to better understand the behavior of molecules at the atomic level and at the metal-molecule interface in particular.