An upper bound for the magnetic force gradient in graphite

Cervenka et al. have recently reported ferromagnetism along graphite steps. We present Magnetic Force microscopy (MFM) data showing that the signal along the steps is independent of an external magnetic field. Moreover, by combining Kelvin Probe Force Microscopy (KPFM) and MFM, we are able to separate the electrostatic and magnetic interactions along the steps obtaining an upper bound for the magnetic force gradient of about16 microN/m, a figure six times lower than the lowest theoretical bound reported by Cervenka et al. Our experiments suggest absence of MFM signal in graphite at room temperature.Comment: 14 pages, including supplemetary informatio

Environmental effects in mechanical properties of few-layer black phosphorus

We report on the mechanical properties of few-layer black phosphorus (BP) nanosheets, in high vacuum and as a function of time of exposure to atmospheric conditions. BP flakes with thicknesses ranging from 4 to 30 nm suspended over circular holes are characterized by nanoindentations using an atomic force microscope tip. From measurements in high vacuum an elastic modulus of 46 ± 10 GPa and breaking strength of 2.4 ± 1 GPa are estimated. Both magnitudes are independent of the thickness of the flakes. Our results show that the exposure to air has substantial influence in the mechanical response of flakes thinner than 6 nm but small effects on thicker flake

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 micro-cavities are a simple but relevant example of nanoactuators. A drawback of this experimental set up 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 micro-cavities 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.Comment: pages 19, 4 figures + supplementary informatio

Fine defect engineering of graphene friction

Two-dimensional materials, in particular graphene, exhibit a low friction coefficient and good wear properties. However, the tribological properties of these materials strongly depend on faint differences at the atomic level, and the coexistence of different type of atomic defects in studied samples up to date led to experimental results difficult to reconcile. In our work, we quantified the influence of controlled induced atomic monovacancies on the frictional behaviour of graphene. Less than 0.1% of atomic vacancies induced a fivefold increase in the effective friction coefficient. We showed that friction force microscopy resolved monoatomic vacancies and provided the real-space distribution of their influence on the tribology of graphene. Two factors contributed to this increment in friction: one was related to enhanced reactivity of dangling bonds localized at the monovacancy (~1 nm2 ), that accounted for ~20% of the increase; and a more extended one (~25 nm2 ) arose from the long-range strain distribution around these defects, characteristic of graphene. These results unveil the subtle connection between friction, reactivity, and mechanical properties in two-dimensional materialsWe acknowledge financial support from Spanish MINECO (projects PID2019-106268GB-C31, PID2019-104272RB-C52, ENE2016-79282-C5-4, and MAT2017-83273-R); Comunidad de Madrid (S2018/NMT-4511, NMAT, 2D-CM); and Ramón Areces Foundation. RP, JG-H, and CG-N acknowledge support from the Spanish Ministry of Science and Innovation, through the “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M

Built-up AFM tips by metal nanoclusters engineering

The ability to probe tip-sample interactions by Atomic Force Microscopy (AFM) has recently boosted our understanding of the matter at the atomic scale, enabling the study of properties of surfaces and adsorbates which were previously inaccessible. Nevertheless, this sensitivity to forces presents some drawbacks, as the requirement of a sharp tip apex to prevent the loss of spatial resolution due to the existence of long-range interactions. In this work, we have overcome this long-standing challenge by investigating the controlled extraction of single metallic nanoclusters, selectively grown on graphene. Our results show that the successive extraction of cluster allows to grow nanotips, which minimize the long-range tip-sample interactions and greatly enhance the topographic resolution. We have demonstrated that the created nanotips are very stable, which enables exchanging the sample and using the same nanotip to explore different surfaces without loss of resolution. Since metallic clusters of very different materials and sizes can be grown and selectively extracted by AFM, ours work paves also the way to the specific functionalization of AFM-tips to sense a large variety of interactionsFinancial support from the Spanish Ministerio de Economía y Competitividad (MINECO) and Fondo Europeo de Desarrollo Regional (FEDER) under grants No. MAT2016-77852-C2-2-R and MAT2016-80907-P and by the Comunidad de Madrid NMAT2D-CM program under grant S2018/NMT-4511 is gratefully acknowledged. Financial support from the Spanish Ministerio de Ciencia e Innovacion under grant Nº PID2019-106268GB-C31 is also gratefully acknowledged. We thank Rubén Pérez and Oscar Custance for helpful discussions and Antonio J. Martínez-Galera for helpful discussions and technical assistanc

Mechanical Isolation of Highly Stable Antimonene under Ambient Conditions

Using mechanical exfoliation combined with a controlled double step transfer procedure we demonstrate that single layers of antimony can be readily produced. These flakes are not significantly contaminated upon exposure to ambient conditions and they do not react with water. DFT calculations confirm our experimental observations and predict a band gap of 1.2-1.3 eV (ambient conditions) for single layer antimonene, which is smaller than that calculated under vacuum conditions at 0 K. Our work confirms antimonene as a highly stable 2D material with promising relevant applications in optoelectronics.Comment: main paper: 5 pages, 4 figures supporting: 9 pages, 7 figures, Advanced Materials, 201

Mechanical and optical properties of ultralarge flakes of a metal-organic framework with molecular thickness

The isolation of 2D-materials is already a success for graphene, graphene oxide, boron nitride and a few clays or metal chalcogenides, however despite the fact that some of them show very interesting physical properties, they lack useful functionalities. Metal-Organic Frameworks (MOFs) are multifunctional materials showing a wide range of physical and chemical properties that can be structurally designed by suitable selection of their building-blocks. This strategy may allow the production of layers with a variety of useful electronic and molecular recognition functionalities. Herein we isolate 2D-MOF flakes with areas of hundreds of square microns and an excellent control of the molecular thickness (from single up to ca. 50 layers). The samples exhibit such good photoluminescence and mechanical properties as to allow free-standing characterization of few layers' flakesThe authors acknowledge financial support from MICINN (MAT2013-46753-C2-1-P and MAT2013-46753-C2-2-P and Consolider CSD2010-00024

Nanotribology and electrical properties of carbon nanotubes hybridized with covalent organic frameworks

Nanomanipulation of molecular materials such as carbon nanotubes (CNTs) or new covalent organic frameworks (COFs) is key not only for the study of their fundamental physicochemical properties, but also for building and probing nanodevices. Therefore, we have investigated the tribological properties of oxidized MWCNTs (ox-MWCNTs) and their hybridization with COF building blocks (ox-MWCNTs@COF) adsorbed on a mica surface. We used the AFM tip to apply torsional forces on individual nanotubes. Depending on the manipulation parameters, the lateral displacements of the AFM tip slide and/or bend nanotubes enabling the direct quantification of the nanotube-mica adhesion. We found striking changes in the behaviour of the lateral force needed to manipulate each carbon nanotube variant which indicates an increased adhesion of ox-MWCNTs@COF with respect to ox-MWCNTs (∼10x). In addition, the use of the AFM tip as a mobile electrode enabled the measurement of electrical transport through individual nanotubes that revealed a rectifying behaviour of the ox-MWCNTs@COF with high resistivity, which was in contrast with the near ohmic performance of ox-MWCNTsP. J.d.P. acknowledges support by grants from the Ministerio de Ciencia e Innovacion (FIS2017- 89549-R; “Maria de Maeztu” Program for Units of Excellence in R&D MDM2014-0377; and FIS2017-90701- REDT) and the Human Frontiers Science Program (HFSPO RGP0012/ 2018). R. M. ackowledges support by grant PID2019-110637RB-10