Fatigue Response of MoS2 with Controlled Introduction of Atomic Vacancies

Fatigue-induced failure resulting from repetitive stress-strain cycles is a critical concern in the development of robust and durable nanoelectromechanical devices founded on 2D semiconductors. Defects, such as vacancies and grain boundaries, inherent in scalable materials can act as stress concentrators and accelerate fatigue fracture. Here, we investigate MoS2 with controlled atomic vacancies, to elucidate its mechanical reliability and fatigue response as a function of atomic defect density. High-quality MoS2 demonstrates an exceptional fatigue response, enduring 109 cycles at 80% of its breaking strength (13.5 GPa), surpassing the fatigue resistance of steel and approaching that of graphene. The introduction of atomic defect densities akin to those generated during scalable synthesis processes (∼1012 cm-2) reduces the fatigue strength to half the breaking strength. Our findings also point toward a sudden defect reconfiguration prior to global failure as the primary fatigue mechanism, offering valuable insights into structure-property relationships.

Confined Crack Propagation in MoS2 Monolayers by Creating Atomic Vacancies

In two-dimensional crystals, fractures propagate easily, thus restricting their mechanical reliability. This work demonstrates that controlled defect creation constitutes an effective approach to avoid catastrophic failure in MoS₂ monolayers. A systematic study of fracture mechanics in MoS₂ monolayers as a function of the density of atomic vacancies, created by ion irradiation, is reported. Pristine and irradiated materials were studied by atomic force microscopy, high-resolution scanning transmission electron microscopy, and Raman spectroscopy. By inducing ruptures through nanoindentations, we determine the strength and length of the propagated cracks within MoS₂ atom-thick membranes as a function of the density and type of the atomic vacancies. We find that a 0.15% atomic vacancy induces a decrease of 40% in strength with respect to that of pristine samples. In contrast, while tear holes in pristine 2D membranes span several microns, they are restricted to a few nanometers in the presence of atomic and nanometer-sized vacancies, thus increasing the material's fracture toughness.

Improved Graphene Blisters by Ultrahigh Pressure Sealing

Graphene is a very attractive material for nanomechanical devices and membrane applications. Graphene blisters based on silicon oxide microcavities are a simple but relevant example of nanoactuators. A drawback of this experimental setup is that gas leakage through the graphene-SiO₂ interface contributes significantly to the total leak rate. Here, we study the diffusion of air from pressurized graphene drumheads on SiO₂ microcavities and propose a straightforward method to improve the already strong adhesion between graphene and the underlying SiO₂ substrate, resulting in reduced leak rates. This is carried out by applying controlled and localized ultrahigh pressure (>10 GPa) with an atomic force microscopy diamond tip. With this procedure, we are able to significantly approach the graphene layer to the SiO₂ surface around the drumheads, thus enhancing the interaction between them, allowing us to better seal the graphene-SiO₂ interface, which is reflected in up to ∼ 4 times lower leakage rates. Our work opens an easy way to improve the performance of graphene as a gas membrane on a technological relevant substrate such as SiO₂.

AFM Manipulation of Gold Nanowires To Build Electrical Circuits

We introduce scanning-probe-assisted nanowire circuitry (SPANC) as a new method to fabricate electrodes for the characterization of electrical transport properties at the nanoscale. SPANC uses an atomic force microscope (AFM) to manipulate nanowires to create complex and highly conductive nanostructures (paths) that work as nanoelectrodes, allowing connectivity and electrical characterization of other nano-objects. The paths are formed by the spontaneous cold welding of gold nanowires upon mechanical contact, leading to an excellent contact resistance of ∼9 Ω/junction. SPANC is an easy to use and cost-effective technique that fabricates clean nanodevices. Hence, this new method can complement and/or be an alternative to other well-established methods to fabricate nanocircuits such as electron beam lithography (EBL). The circuits made by SPANC are easily reconfigurable, and their fabrication does not require the use of polymers and chemicals. In this work, we present a few examples that illustrate the capabilities of this method, allowing robust device fabrication and electrical characterization of several nano-objects with sizes down to ∼10 nm, well below the current smallest size able to be contacted in a device using the standard available technology (∼30 nm). Importantly, we also provide the first experimental determination of the sheet resistance of thin antimonene flakes.

One-Pot Preparation of Mechanically Robust, Transparent, Highly Conductive, and Memristive Metal-Organic Ultrathin Film

The future of 2D flexible electronics relies on the preparation of conducting ultrathin films of materials with mechanical robustness and flexibility in a simple but controlled manner. In this respect, metal-organic compounds present advantages over inorganic laminar crystals owing to their structural, chemical, and functional diversity. While most metal-organic compounds are usually prepared in bulk, recent work has shown that some of them are processable down to low dimensional forms. Here we report the one-pot preparation, carried out at the water-air interface, of ultrathin (down to 4 nm) films of the metal-organic compound [Cu₂I₂(TAA)] _n (TAA= thioacetamide). The films are shown to be homogeneous over mm² areas, smooth, highly transparent, mechanically robust, and good electrical conductors with memristive behavior at low frequencies. This combination of properties, as well as the industrial availability of the two building blocks required for the preparation, demonstrates their wide range potential in future flexible and transparent electronics.

Confining crack propagation in defective graphene

Crack propagation in graphene is essential to understand mechanical failure in 2D materials. We report a systematic study of crack propagation in graphene as a function of defect content. Nanoindentations and subsequent images of graphene membranes with controlled induced defects show that while tears in pristine graphene span microns length, crack propagation is strongly reduced in the presence of defects. Accordingly, graphene oxide exhibits minor crack propagation. Our work suggests controlled defect creation as an approach to avoid catastrophic failure in graphene.

Conductance of partially disordered graphene: crossover from temperature-dependent to field-dependent variable-range hopping

We report an analysis of low-temperature measurements of the conductance of partially disordered reduced graphene oxide, finding that the data follow a simple crossover scenario. At room temperature, the conductance is dominated by two-dimensional (2D) electric field-assisted, thermally driven (Pollak-Riess) variable-range hopping (VRH) through highly disordered regions. However, at lower temperatures T, we find a smooth crossover to follow the exp(-E0/E)(1/3) field-driven (Shklovskii) 2D VRH conductance behaviour when the electric field E exceeds a specific crossover value EC(T)(2D) = (EaE(1/3)0 /3)(3/4) determined by the scale factors E0 and Ea for the high-field and intermediate-field regimes respectively. Our crossover scenario also accounts well for experimental data reported by other authors for three-dimensional disordered carbon networks, suggesting wide applicability.

Solvent-induced delamination of a multifunctional two dimensional coordination polymer

A coordination polymer is fully exfoliated by solvent-assisted interaction only. The soft-delamination process results from the structure of the starting material, which shows a layered structure with weak layer-to-layer interactions and cavities with the ability to locate several solvents in an unselective way. These results represent a significant step forward towards the production of structurally designed one-molecule thick 2D materials with tailored physico-chemical properties.

Intrinsic electrical conductivity of nanostructured metal-organic polymer chains

One-dimensional conductive polymers are attractive materials because of their potential in flexible and transparent electronics. Despite years of research, on the macro- and nano-scale, structural disorder represents the major hurdle in achieving high conductivities. Here we report measurements of highly ordered metal-organic nanoribbons, whose intrinsic (defect-free) conductivity is found to be 10(4) S m(-1), three orders of magnitude higher than that of our macroscopic crystals. This magnitude is preserved for distances as large as 300 nm. Above this length, the presence of structural defects (~ 0.5%) gives rise to an inter-fibre-mediated charge transport similar to that of macroscopic crystals. We provide the first direct experimental evidence of the gapless electronic structure predicted for these compounds. Our results postulate metal-organic molecular wires as good metallic interconnectors in nanodevices.

Fast and non-invasive conductivity determination by the dielectric response of reduced graphene oxide: an electrostatic force microscopy study

The high dispersion found in the literature for the conductivity of Reduced Graphene Oxide (RGO) layers makes it highly desirable to develop fast and non-invasive methods for their characterization. Here we show that Electrostatic Force Microscopy (EFM) is an in situ, fast, and contactless technique to evaluate the conductivity of chemically derived graphene layers. The dielectric response of RGO flakes is observed to depend on their conductivity in the range of 0-3 S m(-1). Interestingly, we also find that for electrostatic purposes, a graphene layer is equivalent to an extremely thin dielectric layer with an effective permittivity (ε(eff)) that depends on the conductivity of the layers and spans from 5 for the insulating layers, to 2000 for the more conductive ones. We discuss how these high values of ε(eff) are a consequence of the incomplete screening of electric fields through graphene layers.

2D materials: to graphene and beyond

This review is an attempt to illustrate the different alternatives in the field of 2D materials. Graphene seems to be just the tip of the iceberg and we show how the discovery of alternative 2D materials is starting to show the rest of this iceberg. The review comprises the current state-of-the-art of the vast literature in concepts and methods already known for isolation and characterization of graphene, and rationalizes the quite disperse literature in other 2D materials such as metal oxides, hydroxides and chalcogenides, and metal-organic frameworks.

Atomic structure of reduced graphene oxide

Using high resolution transmission electron microscopy, we identify the specific atomic scale features in chemically derived graphene monolayers that originate from the oxidation-reduction treatment of graphene. The layers are found to comprise defect-free graphene areas with sizes of a few nanometers interspersed with defect areas dominated by clustered pentagons and heptagons. Interestingly, all carbon atoms in these defective areas are bonded to three neighbors maintaining a planar sp(2)-configuration, which makes them undetectable by spectroscopic techniques. Furthermore, we observe that they introduce significant in-plane distortions and strain in the surrounding lattice.

Electrical conduction mechanism in chemically derived graphene monolayers

We have performed a detailed study of the intrinsic electrical conduction process in individual monolayers of chemically reduced graphene oxide down to a temperature of 2 K. The observed conductance can be consistently interpreted in the framework of two-dimensional variable-range hopping in parallel with electric-field-driven tunneling. The latter mechanism is found to dominate the electrical transport at very low temperatures and high electric fields. Our results are consistent with a model of highly conducting graphene regions interspersed with disordered regions, across which charge carrier hopping and tunneling are promoted by strong local electric fields.

Elastic properties of chemically derived single graphene sheets

The elastic modulus of freely suspended graphene monolayers, obtained via chemical reduction of graphene oxide, was determined through tip-induced deformation experiments. Despite their defect content, the single sheets exhibit an extraordinary stiffness ( E = 0.25 TPa) approaching that of pristine graphene, as well as a high flexibility which enables them to bend easily in their elastic regime. Built-in tensions are found to be significantly lower compared to mechanically exfoliated graphene. The high resilience of the sheets is demonstrated by their unaltered electrical conductivity after multiple deformations. The electrical conductivity of the sheets scales inversely with the elastic modulus, pointing toward a 2-fold role of the oxygen bridges, that is, to impart a bond reinforcement while at the same time impeding the charge transport.

Electronic transport properties of individual chemically reduced graphene oxide sheets

Individual graphene oxide sheets subjected to chemical reduction were electrically characterized as a function of temperature and external electric fields. The fully reduced monolayers exhibited conductivities ranging between 0.05 and 2 S/cm and field effect mobilities of 2-200 cm2/Vs at room temperature. Temperature-dependent electrical measurements and Raman spectroscopic investigations suggest that charge transport occurs via variable range hopping between intact graphene islands with sizes on the order of several nanometers. Furthermore, the comparative study of multilayered sheets revealed that the conductivity of the undermost layer is reduced by a factor of more than 2 as a consequence of the interaction with the Si/SiO2 substrate.

Voltage and length-dependent phase diagram of the electronic transport in carbon nanotubes

In this work, we report experimental data on the evolution of the resistance with applied voltage in nonsuspended single-walled carbon nanotubes (SWNTs) of lengths ranging from 100 nm up to 6 microm. At low bias, the differential resistance as a function of length is well described by a linear fitting. At high biases, this magnitude first saturates and then decreases for nanotubes longer than 1 microm. We also present Monte Carlo numerical simulations for the one-dimensional Boltzmann's equation, describing how the electrons propagate along the tube and how they interact with acoustic and optical phonons. Our theoretical results show a remarkable agreement with the experimental differential resistance, allowing us to give a detailed description of the electron distribution function and the chemical potential along the nanotube. Finally, we present experimental results on the transition from Anderson localization at low bias to high diffusive regime at high bias in defected SWNTs. This result is combined with those of defect-free SWNTs to present a general landscape of the electronic transport in carbon nanotubes.

Conductance oscillations in squashed carbon nanotubes

We report measurements on the radial electromechanical properties of single walled carbon nanotubes. By measuring the conductance of the nanotube, we show that a gap is opened while squashing the nanotubes and that during the deformation stages we observe at least two open-close cycles of the gap. We employ a novel experimental setup where an atomic force microscope tip is used both as an electrode and to induce radial deformations. In contrast with prior experiments reported, this technique allows direct probing of the local electronic structure of carbon nanotubes as they are radially deformed.

Tuning the conductance of single-walled carbon nanotubes by ion irradiation in the Anderson localization regime

Carbon nanotubes are a good realization of one-dimensional crystals where basic science and potential nanodevice applications merge. Defects are known to modify the electrical resistance of carbon nanotubes; they can be present in as-grown carbon nanotubes, but controlling their density externally opens a path towards the tuning of the electronic characteristics of the nanotube. In this work, consecutive Ar+ irradiation doses are applied to single-walled nanotubes (SWNTs) producing a uniform density of defects. After each dose, the room-temperature resistance versus SWNT length (R(L)) along the nanotube is measured. Our data show an exponential dependence of R(L) indicating that the system is within the strong Anderson localization regime. Theoretical simulations demonstrate that mainly di-vacancies contribute to the resistance increase induced by irradiation, and that just a 0.03% of di-vacancies produces an increase of three orders of magnitude in the resistance of a SWNT of 400 nm length.

Ordered self-assembled monolayers of Peptide nucleic acids with DNA recognition capability

We report on the formation of ordered self-assembled monolayers (SAMs) of single-stranded peptide nucleic acids (ssPNA). In spite of their remarkable length (7 nm) thiolated PNAs assemble standing up on gold surfaces similarly to the SAMs of short alkanethiols. SAMs of ssPNA recognize complementary nucleic acids, acting as specific biosensors that discriminate even a point mutation in target ssDNA. These results are obtained by surface characterization techniques that avoid labeling of the target molecule: x-ray photoemission, x-ray absorption and atomic force microscopy.

Contactless experiments on individual DNA molecules show no evidence for molecular wire behavior

A fundamental requirement for a molecule to be considered a molecular wire (MW) is the ability to transport electrical charge with a reasonably low resistance. We have carried out two experiments that measure first, the charge transfer from an electrode to the molecule, and second, the dielectric response of the MW. The latter experiment requires no contacts to either end of the molecule. From our experiments we conclude that adsorbed individual DNA molecules have a resistivity similar to mica, glass, and silicon oxide substrates. Therefore adsorbed DNA is not a conductor, and it should not be considered as a viable candidate for MW applications. Parallel studies on other nanowires, including single-walled carbon nanotubes, showed conductivity as expected.

Nonlinear resistance versus length in single-walled carbon nanotubes

In this work fundamental properties of the electrical transport of single-walled carbon nanotubes as a function of their length are investigated. For this purpose, we have developed a new technique that allows us to characterize electronic transport properties of single-walled carbon nanotubes by probing them at different spots. This technique uses scanning force microscopy to make mechanical and electrical nanocontacts at any selected spot of a given image. We have applied this technique to molecules with high intrinsic resistance. The results show a nonlinear resistance vs distance behavior as the nanotube is probed along its length. This is an indication of elastic electronic transport in one-dimensional systems.