Mariana C. Prado

Two-Dimensional Molecular Crystals of Phosphonic Acids on Graphene

The synthesis and characterization of two-dimensional (2D) molecular crystals composed of long and linear phosphonic acids atop graphene is reported. Using scanning probe microscopy in combination with first-principles calculations, we show that these true 2D crystals are oriented along the graphene armchair direction only, thereby enabling an easy determination of graphene flake orientation. We have also compared the doping level of graphene flakes via Raman spectroscopy. The presence of the molecular crystal atop graphene induces a well-defined shift in the Fermi level, corresponding to hole doping, which is in agreement with our ab initio calculations.

In vitro exposure to fullerene C60 influences redox state and lipid peroxidation in brain and gills from Cyprinus carpio (Cyprinidae)

Studies concerning the impact of nanomaterials, especially fullerene (C(60) ), in fresh water environments and their effects on the physiology of aquatic organisms are still scarce and conflicting. We aimed to assess in vitro effects of fullerene in brain and gill homogenates of carp Cyprinus carpio, evaluating redox parameters. A fullerene suspension was prepared by continued stirring under fluorescent light during two months. The suspension concentration was measured by total carbon content and ultraviolet-visible spectroscopy nephelometry. Characterization of C(60) aggregates was performed with an enhanced dark-field microscopy system and transmission electronic microscopy. Organ homogenates were exposed during 1, 2, and 4 h under fluorescent light. Redox parameters evaluated were reduced glutathione and oxidized glutathione, cysteine and cystine, total antioxidant capacity; activity of the antioxidant enzymes glutathione S-transferase and glutathione reductase (GR), and lipid peroxidation (TBARS assay). Fullerene induced a significant increase (p < 0.05) in lipid peroxidation after 2 h in both organs and reduced GR activity after 1 h (gills) and 4 h (brain) and antioxidant capacity after 4 h (brain). Levels of oxidized glutathione increased in the brain at 1 h and decreased at 2 h as well. Given these results, it can be concluded that C(60) can induce redox disruption via thiol/disulfide pathway, leading to oxidative damage (higher TBARS values) and loss of antioxidant competence.

Optimization of graphene dry etching conditions via combined microscopic and spectroscopic analysis

Single-layer graphene structures and devices are commonly defined using reactive ion etching and plasma etching with O2 or Ar as the gaseous etchants. Although optical microscopy and Raman spectroscopy are widely used to determine the appropriate duration of dry etching, additional characterization with atomic force microscopy (AFM) reveals that residual graphene and/or etching byproducts persist beyond the point where the aforementioned methods suggest complete graphene etching. Recognizing that incomplete etching may have deleterious effects on devices and/or downstream processing, AFM characterization is used here to determine optimal etching conditions that eliminate graphene dry etching residues.

Mixed self-assembled layers of phosphonic acids.

Self-assembled monolayers, bilayers, and other structures of mixed phosphonic acids were investigated using atomic force microscopy. Octadecylphosphonic acid (OPA, 18 carbon atoms in the alkyl chain) was mixed in solution, with different ratios, with octylphosphonic acid (OcPA, 8 carbon atoms in the alkyl chain) and deposited onto mica substrates. Self-assembled monolayers were formed, with no evidence of phase separation, from pure OPA up to 5:10 OPA/OcPA ratio (despite the large alkyl chain length difference). It was found that water plays a crucial role in the morphology of the self-assembled structures and their time evolution. The formation of bilayers instead of monolayers (OcPA-type behavior) is dominant for high water content (both in solution and/or the atmosphere). Mechanical and thermal resistance tests were performed on mixed and pure samples and revealed different properties that can be correlated to their composition.

Thermal Stability and Ordering Study of Long- and Short-Alkyl Chain Phosphonic Acid Multilayers

Long-range order evolution of self-assembled phosphonic acid multilayers as a function of temperature is studied here for two molecules with different alkyl chain length. By using synchrotron conventional diffraction, distinct order configurations are retrieved on phosphonic acid multilayers and their thermodynamic behavior monitored by energy-dispersive diffraction. This later technique allows us to observe the system behavior near order-disorder temperatures, as well as to determine the most stable configurations in the range from room temperature up to 120 °C. Planar order is also addressed by wide-angle X-ray scattering (WAXS) transmission experiments. Order parameter phase diagrams are built based on the experimental results, showing the dominant configuration at each temperature. The multilayer molecular long-range order retrieved from the experiments is corroborated by first principles calculations based on the Density Functional Theory. The bulk configurations depicted in this work are produced by molecule-molecule interactions and allow for future comparisons with the behavior of ordered molecules in few-monolayers configurations, commonly used in organic devices, where the presence of surfaces and interfaces strongly affects the molecule packing.

Crystal-oriented wrinkles with origami-type junctions in few-layer hexagonal boron nitride

AbstractUnderstanding layer interplay is the key to utilizing layered heterostructures formed by the stacking of different two-dimensional materials for device applications. Boron nitride has been demonstrated to be an ideal substrate on which to build graphene devices with improved mobilities. Here we present studies on the morphology and optical response of annealed few-layer hexagonal boron nitride flakes deposited on a silicon substrate that reveal the formation of linear wrinkles along well-defined crystallographic directions. The wrinkles formed a network of primarily threefold and occasionally fourfold origami-type junctions throughout the sample, and all threefold junctions and wrinkles formed along the armchair crystallographic direction. Furthermore, molecular dynamics simulations yielded, through spontaneous symmetry breaking, wrinkle junction morphologies that are consistent with both the experimental results and the proposed origami-folding model. Our findings indicate that this morphology may be a general feature of several two-dimensional materials under proper stress-strain conditions, resulting in direct consequences in device strain engineering.

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.

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. Microsc. Res. Tech. 76:1278–1283, 2013.