Disentangling doping and strain effects at defects of grown MoS2 monolayers with nano-optical spectroscopy.

The role of defects in two-dimensional semiconductors and how they affect the intrinsic properties of these materials have been a widely researched topic over the past few decades. Optical characterization techniques such as photoluminescence and Raman spectroscopies are important tools to probe the physical properties of semiconductors and the impact of defects. However, confocal optical techniques present a spatial resolution limitation lying in a µm-scale, which can be overcome by the use of near-field optical measurements. Here, we use tip-enhanced photoluminescence and Raman spectroscopies to unveil the nanoscale optical properties of grown MoS2 monolayers, revealing that the impact of doping and strain can be disentangled by the combination of both techniques. A noticeable enhancement of the exciton peak intensity corresponding to trion emission quenching is observed at narrow regions down to a width of 47 nm at grain boundaries related to doping effects. Besides, localized strain fields inside the sample lead to non-uniformities in the intensity and energy position of photoluminescence peaks. Finally, two distinct MoS2 samples present different nano-optical responses at their edges associated with opposite strains. The edge of the first sample shows a photoluminescence intensity enhancement and energy blueshift corresponding to a frequency blueshift for E2g and 2LA Raman modes. In contrast, the other sample displays a photoluminescence energy redshift and frequency red shifts for E2g and 2LA Raman modes at their edges. Our work highlights the potential of combining tip-enhanced photoluminescence and Raman spectroscopies to probe localized strain fields and doping effects related to defects in two-dimensional materials.

Controlled Release and Cell Viability of Ketoconazole Incorporated in PEG 4000 Derivatives.

In recent years, polymeric materials have been gaining prominence in studies of controlled release systems to obtain improvements in drug administration. These systems present several advantages compared with conventional release systems, such as constant maintenance in the blood concentration of a given drug, greater bioavailability, reduction of adverse effects, and fewer dosages required, thus providing a higher patient compliance to treatment. Given the above, the present work aimed to synthesize polymeric matrices derived from polyethylene glycol (PEG) capable of promoting the controlled release of the drug ketoconazole in order to minimize its adverse effects. PEG 4000 is a widely used polymer due to its excellent properties such as hydrophilicity, biocompatibility, and non-toxic effects. In this work, PEG 4000 and derivatives were incorporated with ketoconazole. The morphology of polymeric films was observed by AFM and showed changes on the film organization after drug incorporation. In SEM, it was possible to notice spheres that formed in some incorporated polymers. The zeta potential of PEG 4000 and its derivatives was determined and suggested that the microparticle surfaces showed a low electrostatic charge. Regarding the controlled release, all the incorporated polymers obtained a controlled release profile at pH 7.3. The release kinetics of ketoconazole in the samples of PEG 4000 and its derivatives followed first order for PEG 4000 HYDR INCORP and Higuchi for the other samples. Cytotoxicity was determined and PEG 4000 and its derivatives were not cytotoxic.

Carbon nanotube-cellulose ink for rapid solvent identification.

In this work, a conductive ink based on microfibrillated cellulose (MFC) and multiwalled carbon nanotubes (MWCNTs) was used to produce transducers for rapid liquid identification. The transducers are simple resistive devices that can be easily fabricated by scalable printing techniques. We monitored the electrical response due to the interaction between a given liquid with the carbon nanotube-cellulose film over time. Using principal component analysis of the electrical response, we were able to extract robust data to differentiate between the liquids. We show that the proposed liquid sensor can classify different liquids, including organic solvents (acetone, chloroform, and different alcohols) and is also able to differentiate low concentrations of glycerin in water (10-100 ppm). We have also investigated the influence of two important properties of the liquids, namely dielectric constant and vapor pressure, on the transduction of the MFC-MWCNT sensors. These results were corroborated by independent heat flow measurements (thermogravimetric analysis). The proposed MFC-MWCNT sensor platform may help paving the way to rapid, inexpensive, and robust liquid analysis and identification.

Liquid phase exfoliation of talc: effect of the medium on flake size and shape.

Industrial applications of nanomaterials require large-scale production methods, such as liquid phase exfoliation (LPE). Regarding this, it is imperative to characterize the obtained materials to tailor parameters such as exfoliation medium, duration, and mechanical energy source to the desired applications. This work presents results of statistical analyses of talc flakes obtained by LPE in four different media. Talc is a phyllosilicate that can be exfoliated into nanoflakes with great mechanical properties. Sodium cholate at two different concentrations (below and at the critical micelar concentration), butanone, and Triton-X100 were employed as exfoliation medium for talc. Using recent published statistical analysis methods based on atomic force microscopy images of thousands of flakes, the shape and size distribution of nanotalc obtained using the four different media are compared. This comparison highlights the strengths and weaknesses of the media tested and hopefully will facilitate the choice of the medium for applications that have specific requirements.

Aerosol-Printed MoS2 Ink as a High Sensitivity Humidity Sensor.

Molybdenum disulfide (MoS2) is attractive for use in next-generation nanoelectronic devices and exhibits great potential for humidity sensing applications. Herein, MoS2 ink was successfully prepared via a simple exfoliation method by sonication. The structural and surface morphology of a deposited ink film was analyzed by scanning electron microscopy (SEM), Raman spectroscopy, and atomic force microscopy (AFM). The aerosol-printed MoS2 ink sensor has high sensitivity, with a conductivity increase by 6 orders of magnitude upon relative humidity increase from 10 to 95% at room temperature. The sensor also has fast response/recovery times and excellent repeatability. Possible mechanisms for the water-induced conductivity increase are discussed. An analytical model that encompasses two ionic conduction regimes, with a percolation transition to an insulating state below a low humidity threshold, describes the sensor response successfully. In conclusion, our work provides a low-cost and straightforward strategy for fabricating a high-performance humidity sensor and fundamental insights into the sensing mechanism.

Nanomechanics of few-layer materials: do individual layers slide upon folding?

Folds naturally appear on nanometrically thin materials, also called "2D materials", after exfoliation, eventually creating folded edges across the resulting flakes. We investigate the adhesion and flexural properties of single-layered and multilayered 2D materials upon folding in the present work. This is accomplished by measuring and modeling mechanical properties of folded edges, which allows for the experimental determination of the bending stiffness (κ) of multilayered 2D materials as a function of the number of layers (n). In the case of talc, we obtain κ ∝ n 3 for n ≥ 5, indicating no interlayer sliding upon folding, at least in this thickness range. In contrast, tip-enhanced Raman spectroscopy measurements on edges in folded graphene flakes, 14 layers thick, show no significant strain. This indicates that layers in graphene flakes, up to 5 nm thick, can still slip to relieve stress, showing the richness of the effect in 2D systems. The obtained interlayer adhesion energy for graphene (0.25 N/m) and talc (0.62 N/m) is in good agreement with recent experimental results and theoretical predictions. The obtained value for the adhesion energy of graphene on a silicon substrate is also in agreement with previous results.

Gypsum: an environment-friendly, inexpensive and robust height calibration standard at nanometer-scale for atomic force microscopy.

Gypsum is an Earth-abundant mineral with enormous applications in agriculture and civil engineering. Here, we show it is also an excellent height calibration standard alternative for atomic force microscopy (AFM). Using plain water as etchant, gypsum flakes readily review 0.75 nm tall terraces which are easy to image (lateral dimensions from tens to hundreds of nanometers) and robust against time in ambient conditions. Therefore, the present work demonstrates a new height standard alternative which is easily-available for all AFM microscopists around the world.

Exfoliation and characterization of a two-dimensional serpentine-based material.

We report on an experimental investigation of serpentine, an abundant phyllosilicate, as an alternative source of two-dimensional (2D) nanomaterials. We show, through scanning probe microscopy (SPM) measurements, that natural serpentine mineral can be mechanically exfoliated down to few-layer flakes, where monolayers can be easily resolved. The parent serpentine bulk material was initially characterized via conventional techniques like XRD, XPS, FTIR and Raman spectroscopies and the results show that it is predominantly constituted by the antigorite mineral. From ab initio calculations using density functional theory, we also determine the geometry and electronic structure of antigorite, the observed structural form of serpentine. Additionally, we further characterized electrical and mechanical properties of the obtained 2D material flakes using SPM and broadband synchrotron infrared nanospectroscopy. Wavelength tuning of the serpentine vibrational resonances, assigned to in- and out-of-plane molecular vibrations, are observed and compared with the FTIR characterization of the parent bulk material. They show that there is no degradation of serpentine's structural properties during its mechanical exfoliation down to nanometer-thin sheets. Therefore, our results introduce the serpentine mineral as an attractive low-cost candidate in 2D materials applications.

Humidity-Induced Photoluminescence Hysteresis in Variable Cs/Br Ratio Hybrid Perovskites.

Hybrid organic-inorganic perovskites containing Cs are a promising new material for light-absorbing and light-emitting optoelectronics. However, the impact of environmental conditions on their optical properties is not fully understood. Here, we elucidate and quantify the influence of distinct humidity levels on the charge carrier recombination in Cs xFA1- xPb(I yBr1- y)3 perovskites. Using in situ environmental photoluminescence (PL), we temporally and spectrally resolve light emission within a loop of critical relative humidity (rH) levels. Our measurements show that exposure up to 35% rH increases the PL emission for all Cs (10-17%) and Br (17-38%) concentrations investigated here. Spectrally, samples with larger Br concentrations exhibit PL redshift at higher humidity levels, revealing water-driven halide segregation. The compositions considered present hysteresis in their PL intensity upon returning to a low-moisture environment due to partially reversible hydration of the perovskites. Our findings demonstrate that the Cs/Br ratio strongly influences both the spectral stability and extent of light emission hysteresis. We expect our method to become standard when testing the stability of emerging perovskites, including lead-free options, and to be combined with other parameters known for affecting material degradation, e.g., oxygen and temperature.

Compression-Induced Modification of Boron Nitride Layers: A Conductive Two-Dimensional BN Compound.

The ability to create materials with improved properties upon transformation processes applied to conventional materials is the keystone of materials science. Here, hexagonal boron nitride (h-BN), a large-band-gap insulator, is transformed into a conductive two-dimensional (2D) material- bonitrol-that is stable at ambient conditions. The process, which requires compression of at least two h-BN layers and hydroxyl ions, is characterized via scanning probe microscopy experiments and ab initio calculations. This material and its creation mechanism represent an additional strategy for the transformation of known 2D materials into artificial advanced materials with exceptional properties.

Thionine Self-Assembled Structures on Graphene: Formation, Organization, and Doping.

The association of organic molecules with two-dimensional (2D) materials, creating hybrid systems with mutual influences, constitutes an important testbed for both basic science self-assembly studies and perspective applications. Following this concept, in this work, we show a rich phenomenology that is involved in the interaction of thionine with graphene, leading to a hybrid material formed by well-organized self-assembled structures atop graphene. This composite system is investigated by atomic force microscopy, electric transport measurements, Raman spectroscopy, and first principles calculations, which show (1) an interesting time evolution of thionine self-assembled structures atop graphene; (2) a highly oriented final molecular assembly (in accordance with the underlying graphene surface symmetry); and (3) a strong n-type doping effect introduced in graphene by thionine. The nature of the thionine-substrate interaction is further analyzed in experiments using mica as a polar substrate. The present results may help pave the way to achieve tailored 2D material hybrid devices via properly chosen molecular self-assembly processes.

Apparent Softening of Wet Graphene Membranes on a Microfluidic Platform.

Graphene is regarded as the toughest two-dimensional material (highest in-plane elastic properties) and, as a consequence, it has been employed/proposed as an ultrathin membrane in a myriad of microfluidic devices. Yet, an experimental investigation of eventual variations on the apparent elastic properties of a suspended graphene membrane in contact with air or water is still missing. In this work, the mechanical response of suspended monolayer graphene membranes on a microfluidic platform is investigated via scanning probe microscopy experiments. A high elastic modulus is measured for the membrane when the platform is filled with air, as expected. However, a significant apparent softening of graphene is observed when water fills the microfluidic system. Through molecular dynamics simulations and a phenomenological model, we associate such softening to a water-induced uncrumpling process of the suspended graphene membrane. This result may bring substantial modifications on the design and operation of microfluidic devices which exploit pressure application on graphene membranes.

Electro-optical interfacial effects on a graphene/π-conjugated organic semiconductor hybrid system.

The influence of graphene and retinoic acid (RA) - a π-conjugated organic semiconductor - interface on their hybrid system is investigated. The physical properties of the interface are assessed via scanning probe microscopy, optical spectroscopy (photoluminescence and Raman) and ab initio calculations. The graphene/RA interaction induces the formation of a well-organized π-conjugated self-assembled monolayer (SAM) at the interface. Such structural organization leads to the high optical emission efficiency of the RA SAM, even at room temperature. Additionally, photo-assisted electrical force microscopy, photo-assisted scanning Kelvin probe microscopy and Raman spectroscopy indicate a RA-induced graphene doping and photo-charge generation. Finally, the optical excitation of the RA monolayer generates surface potential changes on the hybrid system. In summary, interface-induced organized structures atop 2D materials may have an important impact on both design and operation of π-conjugated nanomaterial-based hybrid systems.

Universal deformation pathways and flexural hardening of nanoscale 2D-material standing folds.

In the present work, we use atomic force microscopy nanomanipulation of 2D-material standing folds to investigate their mechanical deformation. Using graphene, h-BN and talc nanoscale wrinkles as testbeds, universal force-strain pathways are clearly uncovered and well-accounted for by an analytical model. Such universality further enables the investigation of each fold bending stiffness κ as a function of its characteristic height h 0. We observe a more than tenfold increase of κ as h 0 increases in the 10-100 nm range, with power-law behaviors of κ versus h 0 with exponents larger than unity for the three materials. This implies anomalous scaling of the mechanical responses of nano-objects made from these materials.

Raman evidence for pressure-induced formation of diamondene.

Despite the advanced stage of diamond thin-film technology, with applications ranging from superconductivity to biosensing, the realization of a stable and atomically thick two-dimensional diamond material, named here as diamondene, is still forthcoming. Adding to the outstanding properties of its bulk and thin-film counterparts, diamondene is predicted to be a ferromagnetic semiconductor with spin polarized bands. Here, we provide spectroscopic evidence for the formation of diamondene by performing Raman spectroscopy of double-layer graphene under high pressure. The results are explained in terms of a breakdown in the Kohn anomaly associated with the finite size of the remaining graphene sites surrounded by the diamondene matrix. Ab initio calculations and molecular dynamics simulations are employed to clarify the mechanism of diamondene formation, which requires two or more layers of graphene subjected to high pressures in the presence of specific chemical groups such as hydroxyl groups or hydrogens.The synthesis of two-dimensional diamond is the ultimate goal of diamond thin-film technology. Here, the authors perform Raman spectroscopy of bilayer graphene under pressure, and obtain spectroscopic evidence of formation of diamondene, an atomically thin form of diamond.

Nanometre-scale identification of grain boundaries in MoS2 through molecular decoration.

In this paper, we address the challenge of identifying grain boundaries on the molybdenum disulphide (MoS2) surface at the nanometre scale using a simple self-assembled monolayer (SAM) decoration method. Combined with atomic force microscopy, octadecylphosphonic acid monolayers readily reveal grain boundaries in MoS2 at ambient conditions, without the need of atomic resolution measurements under vacuum. Additional ab initio calculations allow us to obtain the preferred orientation of the SAM structure relative to the MoS2 beneath, and therefore, together with the experiments, the MoS2 crystalline orientations at the grain boundaries. The proposed method enables the visualization of grain boundaries with sub-micrometer resolution for nanodevice investigation and failure analysis.

Charge transfer between carbon nanotubes on surfaces.

The charge transfer between neighboring single-walled carbon nanotubes (SWNTs) on a silicon oxide surface was investigated as a function of both the SWNT nature (metallic or semiconducting) and the anode/cathode distance using scanning probe techniques. Two main mechanisms were observed: a direct electron tunneling described by the typical Fowler-Nordheim model, and indirect electron transfer (hopping) mediated by functional groups on the supporting surface. Both mechanisms depend on the SWNT nature and on the anode/cathode separation: direct electron tunneling dominates the charge transfer process for metallic SWNTs, especially for large distances, while both mechanisms compete with each other for semiconducting SWNTs, prevailing one over the other depending on the anode/cathode separation. These mechanisms may significantly influence the design and operation of SWNT-based electronic devices.

Chemical stabilization and improved thermal resilience of molecular arrangements: possible formation of a surface network of bonds by multiple pulse atomic layer deposition.

In this work, we make use of an atomic layer deposition (ALD) surface reaction based on trimethyl-aluminum (TMA) and water to modify O-H terminated self-assembled layers of octadecylphosphonic acid (OPA). The structural modifications were investigated by X-ray reflectivity, X-ray diffraction, and atomic force microscopy. We observed a significant improvement in the thermal stability of ALD-modified molecules, with the existence of a supramolecular packing structure up to 500 °C. Following the experimental observations, density functional theory (DFT) calculations indicate the possibility of formation of a covalent network with aluminum atoms connecting OPA molecules at terrace surfaces. Chemical stability is also achieved on top of such a composite surface, inhibiting further ALD oxide deposition. On the other hand, in the terrace edges, where the covalent array is discontinued, the chemical conditions allow for oxide growth. Analysis of the DFT results on band structure and density of states of modified OPA molecules suggests that besides the observed thermal resilience, the dielectric character of OPA layers is preserved. This new ALD-modified OPA composite is potentially suitable for applications such as dielectric layers in organic devices, where better thermal performance is required.

Characterizing inorganic crystals grown on organic self-assembled bilayers with scanning probe and electron microscopies.

Combined microscopy techniques are used to establish the usability of phosphonic acid layers as promoters of hydroxyapatite (HAp) growth. Using spread coating, octadecylphosphonic acid (OPA) self-assembled bilayers are delivered to the thin natural oxide layer of a titanium film surface with no prior treatment. These bilayers aggregate two major advantages of phosphonic moieties to titanium surfaces: nucleation of hydroxyapatite crystals from ionic solution and affinity for both titanium oxide surface and HAp crystals. The functionalized substrates and bare titanium (control) samples are immersed in an aqueous solution containing calcium and phosphorus ions. Over a 4-week immersion time, OPA-functionalized substrates present numerous large agglomerates of inorganic crystals, in contrast to control samples, with no significant amount of deposits. Initial sample characterization was performed with atomic force microscopy (AFM). Compositional and structural characterization of these agglomerates (using TEM, EDS, and electron diffraction), revealed that they are indeed HAp, the main component of the inorganic bone matrix.

Physiochemical properties of Caulobacter crescentus holdfast: a localized bacterial adhesive.

To colonize surfaces, the bacterium Caulobacter crescentus employs a polar polysaccharide, the holdfast, located at the end of a thin, long stalk protruding from the cell body. Unlike many other bacteria which adhere through an extended extracellular polymeric network, the holdfast footprint area is tens of thousands times smaller than that of the total bacterium cross-sectional surface, making for some very demanding adhesion requirements. At present, the mechanism of holdfast adhesion remains poorly understood. We explore it here along three lines of investigation: (a) the impact of environmental conditions on holdfast binding affinity, (b) adhesion kinetics by dynamic force spectroscopy, and (c) kinetic modeling of the attachment process to interpret the observed time-dependence of the adhesion force at short and long time scales. A picture emerged in which discrete molecular units called adhesins are responsible for initial holdfast adhesion, by acting in a cooperative manner.