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Although 2D materials hold great potential for next-generation pressure sensors, recent studies revealed that gases permeate along the membrane-surface interface that is only weakly bound by van der Waals interactions, necessitating additional sealing procedures. In this work, we demonstrate the use of free-standing complex oxides as self-sealing membranes that allow the reference cavity of pressure sensors to be sealed by a simple anneal. To test the hermeticity, we study the gas permeation time constants in nano-mechanical resonators made from SrRuO3 and SrTiO3 membranes suspended over SiO2/Si cavities which show an improvement up to 4 orders of magnitude in the permeation time constant after annealing the devices for 15 minutes. Similar devices fabricated on Si3N4/Si do not show such improvements, suggesting that the adhesion increase over SiO2 is mediated by oxygen bonds that are formed at the SiO2/complex oxide interface during the self-sealing anneal. We confirm the enhancement of adhesion by picosecond ultrasonics measurements which show an increase in the interfacial stiffness by 70% after annealing. Since it is straigthforward to apply, the presented self-sealing method is thus a promising route toward realizing ultrathin hermetic pressure sensors., 7 pages, 5 figures

Suspended piezoelectric thin films are key elements enabling high-frequency filtering in telecommunication devices. To meet the requirements of next-generation electronics, it is essential to reduce device thickness for reaching higher resonance frequencies. Here, the high-quality mechanical and electrical properties of graphene electrodes are combined with the strong piezoelectric performance of the free-standing complex oxide, BaTiO

Porphyrin cyclophane 1, consisting of two rigidly fixed but still movable cofacial porphyrins and exposing acetate-masked thiols in opposed directions of the macrocycle, is designed, synthesized, and characterized. The functional cyclophane 1, as pioneer of mechanosensitive 3D materials, forms stable single-molecule junctions in a mechanically controlled break-junction setup. Its reliable integration in a single-molecule junction is a fundamental prerequisite to explore the potential of these structures as mechanically triggered functional units and devices.

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.

Intra- and intermolecular interactions are dominating chemical processes, and their concerted interplay enables complex nonequilibrium states like life. While the responsible basic forces are typically investigated spectroscopically, a conductance measurement to probe and control these interactions in a single molecule far out of equilibrium is reported here. Specifically, by separating macroscopic metal electrodes, two π-conjugated, bridge-connected porphyrin decks are peeled off on one side, but compressed on the other side due to the covalent mechanical fixation. We observe that the conductance response shows an exceptional exponential rise by two orders of magnitude in individual breaking events during the stretching. Theoretical studies atomistically explain the measured conductance behavior by a mechanically activated increase in through-bond transport and a simultaneous strengthening of through-space coupling. Our results not only reveal the various interacting intramolecular transport channels in a molecular set of levers, but also the molecules' potential to serve as molecular electro-mechanical sensors and switches.

Spin-crossover (SCO) active iron(II) complexes are an integral class of switchable and bistable molecular materials. Spin-state switching properties of the SCO complexes have been studied in the bulk and single-molecule levels to progress toward fabricating molecule-based switching and memory elements. Supramolecular SCO complexes featuring anchoring groups for metallic electrodes, for example, gold (Au), are ideal candidates to study spin-state switching at the single-molecule level. In this study, we report on the spin-state switching characteristics of supramolecular iron(II) complexes

We demonstrate charge detection with single-electron resolution at high readout frequency using a silicon field-effect transistor (FET) integrated with double resonant circuits. A FET, whose channel of 10-nm width enables a single electron to be detected at room temperature, is connected to resonant circuits composed of coupled inductors and capacitors, and these double resonant circuits provide two resonance frequencies. When the FET is driven by a carrier signal at the lower resonance frequency, a small signal applied to the FET's gate modulates the resonance condition, resulting in a reflected signal appearing near the higher resonance frequency. Such operation utilizing the double resonant circuits enables charge detection with a single-electron resolution of 3 × 10−3 e/Hz0.5 and a readout frequency of 200 MHz at room temperature. In addition, a variable capacitor used in the double resonant circuits allows charge-sensing characteristics to be controlled in situ.

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.

Unsupervised machine learning, and in particular data clustering, is a powerful approach for the analysis of datasets and identification of characteristic features occurring throughout a dataset. It is gaining popularity across scientific disciplines and is particularly useful for applications without a priori knowledge of the data structure. Here, we introduce an approach for unsupervised data classification of any dataset consisting of a series of univariate measurements. It is therefore ideally suited for a wide range of measurement types. We apply it to the field of nanoelectronics and spectroscopy to identify meaningful structures in data sets. We also provide guidelines for the estimation of the optimum number of clusters. In addition, we have performed an extensive benchmark of novel and existing machine learning approaches and observe significant performance differences. Careful selection of the feature space construction method and clustering algorithms for a specific measurement type can therefore greatly improve classification accuracies.

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.

Direct measurement of single-molecule electrical transparency by break junction experiments has become a major field of research over the two last decades. This review specifically and comprehensively highlights the use of porphyrins as molecular components and discusses their potential use for the construction of future devices. Throughout the review, the features provided by porphyrins, such as low level misalignments and very low attenuation factors, are shown with numerous examples, illustrating the potential and limitations of these molecular junctions, as well as differences emerging from applied integration/investigation techniques., Porphyrins have unique properties in electronic circuits. This review summarizes single molecule junction experiments and encourages the development of next generation molecular devices based on such building blocks.

Probing the universal low temperature magnetic field scaling of Kondo-correlated quantum dots via electrical conductance has proved to be experimentally challenging. Here, we show how to probe this in nonlinear thermocurrent spectroscopy applied to a molecular quantum dot in the Kondo regime. Our results demonstrate that the bias-dependent thermocurrent is a sensitive probe of universal Kondo physics, directly measures the splitting of the Kondo resonance in a magnetic field, and opens up possibilities for investigating nanosystems far from thermal and electrical equilibrium., Comment: Published version, including Supplementary Material main text:6 pages, 4 figures. SI: 55 pages, 22 figures

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

We report on charge transport across single short peptides using the Mechanically Controlled Break Junction (MCBJ) method. We record thousands of electron transport events across single-molecule junctions and with an unsupervised machine learning algorithm, we identify several classes of traces with multifarious conductance values that may correspond to different peptide conformations. Data analysis shows that very short peptides, which are more rigid, show conductance plateaus at low conductance values of about 10−3G0and below, withG0being the conductance quantum, whereas slightly longer, more flexible peptides also show plateaus at higher values. Fully stretched peptide chains exhibit conductance values that are of the same order as that of alkane chains of similar length. The measurements show that in the case of short peptides, different compositions and molecular lengths offer a wide range of junction conformations. Such information is crucial to understand mechanism(s) of charge transport in and across peptide-based biomolecules.

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.

Theoretical studies suggest that mastering the thermocurrent through single molecules can lead to thermoelectric energy harvesters with unprecedentedly high efficiencies.

We fabricate paper-supported semiconducting devices by rubbing a layered molybdenum disulfide (MoS

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.

By studying transport through seven structurally related porphyrin derivatives with a machine learning algorithm we could identify and distinguish three different electronic paths., Porphyrin derivatives are key components in natural machinery enabling us to store sunlight as chemical energy. In spite of their prominent role in cascades separating electrical charges and their potential as sensitizers in molecular devices, reports concerning their electronic transport characteristics are inconsistent. Here we report a systematic investigation of electronic transport paths through single porphyrin junctions. The transport through seven structurally related porphyrin derivatives was repeatedly measured in an automatized mechanically controlled break-junction set-up and the recorded data were analyzed by an unsupervised clustering algorithm. The correlation between the appearances of similar clusters in particular sub-sets of the porphyrins with a common structural motif allowed us to assign the corresponding current path. The small series of model porphyrins allowed us to identify and distinguish three different electronic paths covering more than four orders of magnitude in conductance.

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.

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 semiconductor that attracted much attention recently thanks to its quasi-1D electronic and optoelectronic 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 individual TiS3 nanoribbons and its influence on the optoelectronic properties of TiS3-based photodetectors. 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 of the bandgap of titanium oxysulfide (TiO2-xSx) when increasing the amount of oxygen and reducing the amount of sulfur.

Phase engineering of MoS2 transistors has recently been demonstrated and has led to record low contact resistances. The phase patterning of MoS2 flakes with laser radiation has also been realized via spectroscopic methods, which invites the potential of controlling the metallic and semiconducting phases of MoS2 transistors by simple light exposure. Nevertheless, the fabrication and demonstration of laser-patterned MoS2 devices starting from the metallic polymorph have not been demonstrated yet. Here, we study the effects of laser radiation on 1T/1 $\mathrm{T}^{\prime }$ -MoS2 transistors with the prospect of driving an in situ phase transition to the 2H-polymorph through light exposure. We find that although the Raman peaks of 2H-MoS2 become more prominent and the ones from the 1T/1 $\mathrm{T}^{\prime }$ phase fade after the laser exposure, the semiconducting properties of the laser-patterned devices are not fully restored, and the laser treatment ultimately leads to the degradation of the transport channel.

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

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

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

This is the post-peer reviewed version of the following article: Papadopoulos, Nikos et al. “Multi-terminal electronic transport in boron nitride encapsulated TiS3 nanosheets”. 2D Matererials, 2019, 7(1) 015009 doi:10.1088/2053-1583/ab4ef3 Which has been published in final form at: https://iopscience.iop.org/article/10.1088/2053-1583/ab4ef3, We have studied electrical transport as a function of carrier density, temperature and bias in multi-terminal devices consisting of hexagonal boron nitride (h-BN) encapsulated titanium trisulfide (TiS3) sheets. Through the encapsulation with h-BN, we observe metallic behavior and high electron mobilities. Below ∼60 K an increase in the resistance, and non-linear transport with plateau-like features in the differential resistance are present, in line with the expected charge density wave (CDW) formation. Importantly, the critical temperature and the threshold field of the CDW phase can be controlled through the back-gate, This work is in part financed by the Organization for Scientific Research (NWO) and the Ministry of Education, Culture, and Science (OCW). Growth of hexagonal boron nitride crystals was supported by the Elemental Strategy Initiative conducted by the MEXT, Japan and the CREST (JPMJCR15F3), JST. MIRE Group thanks the financial support from MINECO-FEDER through the project MA2015-65203-R

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

Two-dimensional layered trichalcogenide materials have recently attracted the attention of the scientific community because of their robust mechanical and thermal properties and applications in opto- and nanoelectronics devices. We report the pressure dependence of out-of-plane Ag Raman modes in high quality few-layer titanium trisulfide (TiS3) nanoribbons grown using a direct solid-gas reaction method and infer their cross-plane thermal expansion coefficient. Both mechanical stability and thermal properties of the TiS3 nanoribbons are elucidated by using phonon-spectrum analyses. Raman spectroscopic studies at high pressure (up to 34 GPa) using a diamond anvil cell identify four prominent Ag Raman bands; a band at 557 cm-1 softens under compression, and others at 175, 300, and 370 cm-1 show normal hardening. Anomalies in phonon mode frequencies and excessive broadening in line width of the soft phonon about 13 GPa are attributed to the possible onset of a reversible structural transition. A complete structural phase transition at 43 GPa is inferred from the Ag soft mode frequency (557 cm-1) versus pressure extrapolation curve, consistent with recently reported theoretical predictions. Using the experimental mode Grüneisen parameters γi of Raman modes, we estimated the cross-plane thermal expansion coefficient Cv of the TiS3 nanoribbons at ambient phase to be 1.321 × 10-6 K-1. The observed results are expected to be useful in calibration and performance of next-generation nanoelectronics and optical devices under extreme stress conditions.

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

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 WO

Porphyrin derivatives are key components in natural machinery enabling us to store sunlight as chemical energy. In spite of their prominent role in cascades separating electrical charges and their potential as sensitizers in molecular devices, reports concerning their electronic transport characteristics are inconsistent. Here we report a systematic investigation of electronic transport paths through single porphyrin junctions. The transport through seven structurally related porphyrin derivatives was repeatedly measured in an automatized mechanically controlled break-junction set-up and the recorded data were analyzed by an unsupervised clustering algorithm. The correlation between the appearances of similar clusters in particular sub-sets of the porphyrins with a common structural motif allowed us to assign the corresponding current path. The small series of model porphyrins allowed us to identify and distinguish three different electronic paths covering more than four orders of magnitude in conductance.

A new concept to improve the reliability of functional single molecule junctions is presented using the E-field triggered switching of FeIIbis-terpyridine complexes in a mechanically controlled break junction experiment as model system. The complexes comprise a push-pull ligand sensing the applied E-field and the resulting distortion of the FeII ligand field is expected to trigger a spin-crossover event reflected in a sudden jump of the transport current. By molecular engineering, the active centre of the complex is separated from the gold electrodes in order to eliminate undesired side-effects. Two aspects are considered to isolate the central metal ion, namely the spacing by introducing additional alkynes, and the steric shielding achieved by bulky isopropyl groups. With this small series of model complexes, a pronounced correlation is observed between the occurrence of bistable junctions and the extent of separation of the central metal ion, affirming the hypothesized Enhanced Separation Concept (ESC). &nbsp

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.

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.

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.

TiS3 nanosheets have proven to be promising candidates for ultrathin optoelectronic devices due to their direct narrow band-gap and the strong light-matter interaction. In addition, the marked in-plane anisotropy of TiS3 is appealing for the fabrication of polarization sensitive optoelectronic devices. Herein, we study the optical contrast of TiS3 nanosheets of variable thickness on SiO2/Si substrates, from which we obtain the complex refractive index in the visible spectrum. We find that TiS3 exhibits very large birefringence, larger than that of well-known strong birefringent materials like TiO2 or calcite, and linear dichroism. These findings are in qualitative agreement with ab initio calculations that suggest an excitonic origin for the birefringence and linear dichroism of the material.

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 study single-molecule oligo(phenylene ethynylene)dithiol junctions by means of inelastic electron tunneling spectroscopy (IETS). The molecule is contacted with gold nano-electrodes formed with the mechanically controllable break junction technique. We record the IETS spectrum of the molecule from direct current measurements, both as a function of time and electrode separation. We find that for fixed electrode separation the molecule switches between various configurations, which are characterized by different IETS spectra. Similar variations in the IETS signal are observed during atomic rearrangements upon stretching of the molecular junction. Using quantum chemistry calculations, we identity some of the vibrational modes which constitute a chemical fingerprint of the molecule. In addition, changes can be attributed to rearrangements of the local molecular environment, in particular at the molecule–electrode interface. This study shows the importance of taking into account the interaction with the electrodes when describing inelastic contributions to transport through single-molecule junctions.