Jaime Taha-Tijerina

Artificially stacked atomic layers: Toward new van der Waals solids

Strong in-plane bonding and weak van der Waals interplanar interactions characterize a large number of layered materials, as epitomized by graphite. The advent of graphene (G), individual layers from graphite, and atomic layers isolated from a few other van der Waals bonded layered compounds has enabled the ability to pick, place, and stack atomic layers of arbitrary compositions and build unique layered materials, which would be otherwise impossible to synthesize via other known techniques. Here we demonstrate this concept for solids consisting of randomly stacked layers of graphene and hexagonal boron nitride (h-BN). Dispersions of exfoliated h-BN layers and graphene have been prepared by liquid phase exfoliation methods and mixed, in various concentrations, to create artificially stacked h-BN/G solids. These van der Waals stacked hybrid solid materials show interesting electrical, mechanical, and optical properties distinctly different from their starting parent layers. From extensive first principle calculations we identify (i) a novel approach to control the dipole at the h-BN/G interface by properly sandwiching or sliding layers of h-BN and graphene, and (ii) a way to inject carriers in graphene upon UV excitations of the Frenkell-like excitons of the h-BN layer(s). Our combined approach could be used to create artificial materials, made predominantly from inter planar van der Waals stacking of robust bond saturated atomic layers of different solids with vastly different properties.

Binary and ternary atomic layers built from carbon, boron, and nitrogen.

Two-dimensional (2D) atomic layers derived from bulk layered materials are very interesting from both scientific and application viewpoints, as evidenced from the story of graphene. Atomic layers of several such materials such as hexagonal boron nitride (h-BN) and dichalcogenides are examples that complement graphene. The observed unconventional properties of graphene has triggered interest in doping the hexagonal honeycomb lattice of graphene with atoms such as boron (B) and nitrogen (N) to obtain new layered structures. Individual atomic layers containing B, C, and N of various compositions conform to several stable phases in the three-component phase diagram of B-C-N. Additionally, stacking layers built from C and BN allows for the engineering of new van-der-Waals stacked materials with novel properties. In this paper, the synthesis, characterization, and properties of atomically thin layers, containing B, C, and N, as well as vertically assembled graphene/h-BN stacks are reviewed. The electrical, mechanical, and optical properties of graphene, h-BN, and their hybrid structure are also discussed along with the applications of such materials.

Electrically Insulating Thermal Nano-Oils Using 2D Fillers

Different nanoscale fillers have been used to create composite fluids for applications such as thermal management. The ever increasing thermal loads in applications now require advanced operational fluids, for example, high thermal conductivity dielectric oils in transformers. These oils require excellent filler dispersion, high thermal conduction, but also electrical insulation. Such thermal oils that conform to this thermal/electrical requirement, and yet remain in highly suspended stable state, have not yet been synthesized. We report here the synthesis and characterization of stable high thermal conductivity Newtonian nanofluids using exfoliated layers of hexagonal boron nitride in oil without compromising its electrically insulating property. Two-dimensional nanosheets of hexagonal boron nitride are liquid exfoliated in isopropyl alcohol and redispersed in mineral oil, used as standard transformer oil, forming stable nanosuspensions with high shelf life. A high electrical resistivity, even higher than that of the base oil, is maintained for the nano-oil containing small weight fraction of the filler (0.01 wt %), whereas the thermal conductivity was enhanced. The low dissipation factor and high pour point for this nano-oil suggests several applications in thermal management.

Low-density three-dimensional foam using self-reinforced hybrid two-dimensional atomic layers

Low-density nanostructured foams are often limited in applications due to their low mechanical and thermal stabilities. Here we report an approach of building the structural units of three-dimensional (3D) foams using hybrid two-dimensional (2D) atomic layers made of stacked graphene oxide layers reinforced with conformal hexagonal boron nitride (h-BN) platelets. The ultra-low density (1/400 times density of graphite) 3D porous structures are scalably synthesized using solution processing method. A layered 3D foam structure forms due to presence of h-BN and significant improvements in the mechanical properties are observed for the hybrid foam structures, over a range of temperatures, compared with pristine graphene oxide or reduced graphene oxide foams. It is found that domains of h-BN layers on the graphene oxide framework help to reinforce the 2D structural units, providing the observed improvement in mechanical integrity of the 3D foam structure.

Hybrid 2D nanomaterials as dual-mode contrast agents in cellular imaging

The design of multifunctional nanofluids is highly desirable for biomedical therapy/cellular imaging applications.[1–4] The emergence of hybrid nanomaterials with specific properties, such as magnetism and fluorescence, can lead to an understanding of biological processes at the biomolecular level.[1] Various hybrid systems have been analyzed in the recent past for several possible biomedical applications.[5–9] Carbon-based hybrid systems such as carbon nanotubes with various nanoparticles are being widely tested for their biological applications because of their ability to cross cell membranes and their interesting thermal and electrical properties.[10,11] Graphene oxide (GO) is a fairly new graphene-based system with a 2D carbon honeycomb lattice decorated with numerous functional groups attached to the backbone: these functional groups make it an excellent platform for further attachment of nanoparticles and synthesis of hybrid materials. Cell viability studies on GO have been recently attempted, showing biocompatibility. [12,13] Moreover, the intrinsic photoluminescence (PL) properties of GO can be utilized for cellular imaging.[13] The large surface area and non-covalent interactions with aromatic molecules make GO an excellent system for biomolecular applications and drug attachment.

Density Variant Carbon Nanotube Interconnected Solids

DOI: 10.1002/adma.201404995 from the individual dimensions of the CNTs, the interconnection also affects the density. Detailed microscopy and spectroscopy studies have been performed to explain the structure, properties, and other functional properties. In order to understand the structure, the top, bottom, and lateral sides of the 3D nanotube blocks were imaged using scanning electron microscopy (SEM). Figure 1 b,c shows the top and side view of the 3D-CNT solid blocks. SEM images show uniformly fl at top surfaces as well as the vertical alignment of the nanotubes. The top surface reveals the aligned CNTs are nicely spaced and form a microporous structure giving rise to the solid block. The side view shows aligned millimeter length CNTs exhibiting both buckling and bending due to their large height and network structure. In order to understand the in-depth morphology of the blocks, we sectioned the block at different planes and imaged using SEM as shown in Figure 2 . The 3D architectures entirely consist of entangled CNTs with different orientations producing spatially varying morphologies (Figure 2 a,e). The morphology for the intermolecular junctions reveals junctions of Y-type, X-type, multibranched, and ring-like confi gurations (Figure 2 b,c,d,f, respectively). Highly branched 3D-CNT network junctions are shown in Figure 2 d. Recent reports have shown that these kinds of multiterminal junctions are formed due to the presence of topological defects in the hexagonal carbon lattice within the junction region. [ 2,16,18 ] Solid blocks also contain solder-like junctions between CNT bundles (Figure 2 g,h). The nanoscale and solder-like junctions could be expected to improve mechanical and physical properties of the material. [ 2,19–21 ]

Antiwear and Extreme Pressure Properties of Nanofluids for Industrial Applications

This study investigates the effect of CuO, TiO2, Al2O3, and multiwalled nanotube (MWNT) nanoparticles at various treat rates on the tribological properties, namely, wear, coefficient of friction (COF), and pressure of seizure (poz), of metalworking fluids during lubricating processes in diverse industrial applications. Results are reported based on two methods: wear scar diameter (WSD) and COF by ASTM D5183 and poz by the Institute for Sustainable Technologies–National Research Institute (ITEePib) Polish method for testing lubricants under scuffing conditions. Results showed significant improvements with small filler concentrations of nanoparticles. CuO nanofluids showed a diminishment of 86% for WSD at 0.01 wt%, whereas TiO2 resulted in an increase in poz of up to ∼250% at 0.05 wt% compared to pure conventional fluid.

Chemical Makeup and Hydrophilic Behavior of Graphene Oxide Nanoribbons after Low-Temperature Fluorination

Here we investigated the fluorination of graphene oxide nanoribbons (GONRs) using H2 and F2 gases at low temperature, below 200 °C, with the purpose of elucidating their structure and predicting a fluorination mechanism. The importance of this study is the understanding of how fluorine functional groups are incorporated in complex structures, such as GONRs, as a function of temperature. The insight provided herein can potentially help engineer application-oriented materials for several research and industrial sectors. Direct (13)C pulse magic angle spinning (MAS) nuclear magnetic resonance (NMR) confirmed the presence of epoxy, hydroxyl, ester and ketone carbonyl, tertiary alkyl fluorides, as well as graphitic sp(2)-hybridized carbon. Moreover, (19)F-(13)C cross-polarization MAS NMR with (1)H and (19)F decoupling confirmed the presence of secondary alkyl fluoride (CF2) groups in the fluorinated graphene oxide nanoribbon (FGONR) structures fluorinated above 50 °C. First-principles density functional theory calculations gained insight into the atomic arrangement of the most dominant chemical groups. The fluorinated GONRs present atomic fluorine percentages in the range of 6-35. Interestingly, the FGONRs synthesized up to 100 °C, with 6-19% of atomic fluorine, exhibit colloidal similar stability in aqueous environments when compared to GONRs. This colloidal stability is important because it is not common for materials with up to 19% fluorine to have a high degree of hydrophilicity.

Heat Transfer Studies in Thermally Conducting and Electrically Insulating Nano-Oils in a Natural Circulation Loop

Mineral oil (MO), a dielectric insulating fluid, is commonly used as coolant and lubricant in various applications, such as in high voltage power transmission systems and machinery. The mode of heat transfer in most of these systems is natural convection. Prolonged operation at higher temperatures leads to the degradation of the dielectric coolant, which leads to diverse problems, such as shortage or breakdown of these devices and apparatuses. Increasing the heat transfer capability of the insulating fluid will minimize the energy consumption of the system, prolonging its useful life. It is proposed to improve the heat transfer performance of insulating fluid by the addition of hexagonal boron nitride (h-BN), which is synthesized and finally obtained in 2D-nanosheets through wet exfoliation technique, without affecting its electrical insulating property. h-BN was reported to have superb effect on thermal conductivity of MO (∼ 80% increase at 0.10wt.%) on addition at very low filler fraction [1], thermal stability of up to 800°C [2], and good electrical insulating properties due to its nature (electron band gap of approx. 4.5eV). The present work reports the application of nano-oil (MO + h-BN 2D-nanosheets) for enhanced heat dissipation. A rectangular thermosyphon loop was modeled as the thermal system in transformer. The aspect ratio of the loop and the positions of the heater and cooler were chosen according to the stability criterion so that the flow remains stable and unidirectional throughout the experiment. The effect on heat removal by varying the concentration of h-BN 2D-nanosheets (h-BNNS) in MO was measured and discussed.Copyright

2D Structures-Based Energy Management Nanofluids

Designing of compact electronic and electrical instruments needs the development of high efficient thermal and electrical management fluids. Recent advances in layered materials enable large scale synthesis of diverse two-dimensional (2D) structures. Some of these 2D materials are good choices as nanofillers in heat/electrical energy transfer fluids; mainly due to their high surface area available for energy conduction. Among various 2D nanostructures, hexagonal boron nitride (h-BN) or graphene (G) exhibit versatile properties such as outstanding thermal conductivity (TC), excellent mechanical stability, and remarkable chemical inertness. These 2D nanostructures have been used to create composite fluids for diverse thermal management applications, such as microelectronics, high voltage power transmission systems, automobiles, solar cells, biopharmaceuticals, medical therapy/diagnosis, and nuclear cooling, among others. The ever increasing thermal loads in applications now require advanced operational fluids, like high TC dielectric insulating fluids for electrical transformers. These fluids require superb filler dispersion, high thermal conduction, as well as electrical insulation. Such thermal oils that conform to this thermal/electrical requirement, and yet remain in highly suspended stable state, have not yet been synthesized. We discuss the synthesis and characterization of stable high TC and electrically conducting and non-conducting Newtonian nanofluids using liquid exfoliated layers of h-BN and G in dielectric mineral oil.Copyright