Direct observation of a long-lived single-atom catalyst chiseling atomic structures in graphene

Fabricating stable functional devices at the atomic scale is an ultimate goal of nanotechnology. In biological processes, such high-precision operations are accomplished by enzymes. A counterpart molecular catalyst that binds to a solid-state substrate would be highly desirable. Here, we report the direct observation of single Si adatoms catalyzing the dissociation of carbon atoms from graphene in an aberration-corrected high-resolution transmission electron microscope (HRTEM). The single Si atom provides a catalytic wedge for energetic electrons to chisel off the graphene lattice, atom by atom, while the Si atom itself is not consumed. The products of the chiseling process are atomic-scale features including graphene pores and clean edges. Our experimental observations and first-principles calculations demonstrated the dynamics, stability, and selectivity of such a single-atom chisel, which opens up the possibility of fabricating certain stable molecular devices by precise modification of materials at the atomic scale.

Graphene-MoS2 Hybrid Technology for Large-Scale Two-Dimensional Electronics

Two-dimensional (2D) materials have generated great interest in the last few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS2) and insulating Boron Nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency and favorable transport properties for realizing electronic, sensing and optical systems on arbitrary surfaces. In this work, we develop several etch stop layer technologies that allow the fabrication of complex 2D devices and present for the first time the large scale integration of graphene with molybdenum disulfide (MoS2) , both grown using the fully scalable CVD technique. Transistor devices and logic circuits with MoS2 channel and graphene as contacts and interconnects are constructed and show high performances. In addition, the graphene/MoS2 heterojunction contact has been systematically compared with MoS2-metal junctions experimentally and studied using density functional theory. The tunability of the graphene work function significantly improves the ohmic contact to MoS2. These high-performance large-scale devices and circuits based on 2D heterostructure pave the way for practical flexible transparent electronics in the future. The authors acknowledge financial support from the Office of Naval Research (ONR) Young Investigator Program, the ONR GATE MURI program, and the Army Research Laboratory. This research has made use of the MI.

Electric-Field Dependence of the Effective Dielectric Constant in Graphene

Not Available

Interplay of Defects, Magnetism, Ripples and Strain in Graphene

We present a comprehensive study based on first-principles calculations about the interplay of four important ingredients on the electronic structure of graphene: defects + magnetism + ripples + strain. So far they have not been taken into account simultaneously in a set of ab initio calculations. Furthermore, we focus on the strain dependence of the properties of carbon monovacancies, with special attention to magnetic spin moments. We demonstrated that such defects show a very rich structural and spin phase-diagram with many spin solutions as function of strain. At zero strain the vacancy shows a spin moment of 1.5 Bohrs that increases up to 2 Bohrs with stretching. Changes are more dramatic under compression: the vacancy becomes non-magnetic under a compression larger than 2%. This transition is linked to the structural modifications associated with the formation of ripples in the graphene layer. Our results suggest that such interplay could have important implications for the design of future spintronics devices based on graphene derivatives, as for example a spin-strain switch based on vacancies.

Strain-Tunable Spin Moment in Ni-Doped Graphene

Graphene, due to its exceptional properties, is a promising material for nanotechnology applications. In this context, the ability to tune the properties of graphene-based materials and devices with the incorporation of defects and impurities can be of extraordinary importance. Here we investigate the effect of uniaxial tensile strain on the electronic and magnetic properties of graphene doped with substitutional Ni impurities (Ni_sub). We have found that, although Ni_sub defects are non-magnetic in the relaxed layer, uniaxial strain induces a spin moment in the system. The spin moment increases with the applied strain up to values of 0.3-0.4 \mu_B per Ni_sub, until a critical strain of ~6.5% is reached. At this point, a sharp transition to a high-spin state (~1.9 \mu_B) is observed. This magnetoelastic effect could be utilized to design strain-tunable spin devices based on Ni-doped graphene.

Effect of two types of graphene nanoplatelets on the physico–mechanical properties of linear low–density polyethylene composites

The influence of two types of graphene nanoplatelets (GNPs) on the physico-mechanical properties of linear low-density polyethylene (LLDPE) was investigated. The addition of these two types of GNPs – designated as grades C and M – enhanced the thermal conductivity of the LLDPE, with a more pronounced improvement resulting from the M-GNPs compared to C-GNPs. Improvement in electrical conductivity and decomposition temperature was also noticed with the addition of GNPs. In contrast to the thermal conductivity, C-GNPs resulted in greater improvements in the electrical conductivity and thermal decomposition temperature. These differences can be attributed to differences in the surface area and dispersion of the two types of GNPs.

Electric Field Effects on Graphene Materials

Understanding the effect of electric fields on the physical and chemical properties of two-dimensional (2D) nanostructures is instrumental in the design of novel electronic and optoelectronic devices. Several of those properties are characterized in terms of the dielectric constant which play an important role on capacitance, conductivity, screening, dielectric losses and refractive index. Here we review our recent theoretical studies using density functional calculations including van der Waals interactions on two types of layered materials of similar two-dimensional molecular geometry but remarkably different electronic structures, that is, graphene and molybdenum disulphide (MoS2). We focus on such two-dimensional crystals because of they complementary physical and chemical properties, and the appealing interest to incorporate them in the next generation of electronic and optoelectronic devices. We predict that the effective dielectric constant (ε) of few-layer graphene and MoS2 is tunable by external electric fields (E ext). We show that at low fields (E ext < 0.01 V/Å) ε assumes a nearly constant value ∼4 for both materials, but increases at higher fields to values that depend on the layer thickness. The thicker the structure the stronger is the modulation of ε with the electric field. Increasing of the external field perpendicular to the layer surface above a critical value can drive the systems to an unstable state where the layers are weakly coupled and can be easily separated. The observed dependence of ε on the external field is due to charge polarization driven by the bias, which show several similar characteristics despite of the layer considered. All these results provide key information about control and understanding of the screening properties in two-dimensional crystals beyond graphene and MoS2

A boron nitride - graphene - C60 heterostructure

We have fabricated a new van-der-Waals heterostructure composed by BN/graphene/C60. We performed transport measurements on the preliminary BN/graphene device finding a sharp Dirac point at the neutrality point. After the deposition of a C60 thin film by thermal evaporation, we have observed a significant n-doping of the heterostructure. This suggests an unusual electron transfer from C60 into the BN/graphene structure. This BN/graphene/C60 heterostructure can be of interest in photovoltaic applications. It can be used to build devices like p-n junctions, where C60 can be easily deposited in defined regions of a graphene junction by the use of a shadow mask. Our results are contrasted with theoretical calculations.

Device Physics of Two-Dimensional Crystalline Materials

Thesis (Ph.D.)--University of Washington, 2016-07

Electrodeposited reduced-graphene oxide/cobalt oxide electrodes for charge storage applications

In the present work, electrochemically reduced-graphene oxide/cobalt oxide composites for charge storage electrodes were prepared by a one-step pulsed electrodeposition route on stainless steel current collectors and after that submitted to a thermal treatment at 200 degrees C. A detailed physico-chemical characterization was performed by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Raman spectroscopy. The electrochemical response of the composite electrodes was studied by cyclic voltammetry and charge-discharge curves and related to the morphological and phase composition changes induced by the thermal treatment. The results revealed that the composites were promising materials for charge storage electrodes for application in redox supercapacitors, attaining specific capacitances around 430 F g(-1) at 1 A g(-1) and presenting long-term cycling stability. (C) 2016 Elsevier B.V. All rights reserved.

STRAINING ENGINEERING OF GRAPHENE AND GRAPHENE-BASED NANOSTRUCTURES

Graphene has emerged as an extraordinary material with its capability to accommodate an array of remarkable electronic, mechanical and chemical properties. Extra-large surface-to-volume ratio renders graphene a highly flexible morphology, giving rise to intriguing observations such as ripples, wrinkles and folds as well as the potential to transform into other novel carbon nanostructures. Ultra-thin, mechanically tough, electrically conductive graphene films promise to enable a wealth of possible applications ranging from hydrogen storage scaffolds, electronic transistors, to bottom-up material designs. Enthusiasm for graphene-based applications aside, there are still significant challenges to their realization, largely due to the difficulty of precisely controlling the graphene properties. Controlling the graphene morphology over large areas is crucial in enabling future graphene-based applications and material design. This dissertation aims to shed lights on potential mechanisms to actively manipulate the graphene morphology and properties and therefore enable the material design principle that delivers desirable mechanical and electronic functionalities of graphene and its derivatives.

Technical Report on the reduced graphene oxide biosynthesis protocol for biological applications

The interest of scientific community on carbon-based smart materials is growing and, especially focus on graphene oxide (GO) and reduced graphene oxide (rGO). An increasing number of bio-applications such as biological applications as bacterial inhibition, drug delivery and photothermal therapy aims the use of GO and rGO. For this reason, the methods used for the synthesis of graphene materials are more important because same of those procedures imply chemical reactions that involve hazardous and toxic reagents. In fact, the biocompatibility and toxicological activity of graphene-related materials is related to the methodologies employed for the synthesis that determine the carbon/oxygen (C/O) ratio of graphene oxide species. In this technical report, we focused on the synthesis of GO by means of that lead to a biocompatible GO form with a lower oxygen content. Thus, the synthesis of rAsGFP-rGO with the green fluorescent protein allowed us to obtain a biocompatible materials, without using hazardous and toxic reagents. This biocompatibility is the most important prerogative for the use of GO in biological activity assays as reported in several publications.

Seed layer technique for high quality epitaxial manganite films

We introduce an innovative approach to the simultaneous control of growth mode and magnetotransport properties of manganite thin films, based on an easy-to-implement film/substrate interface engineering. The deposition of a manganite seed layer and the optimization of the substrate temperature allows a persistent bi-dimensional epitaxy and robust ferromagnetic properties at the same time. Structural measurements confirm that in such interface-engineered films, the optimal properties are related to improved epitaxy. A new growth scenario is envisaged, compatible with a shift from heteroepitaxy towards pseudo-homoepitaxy. Relevant growth parameters such as formation energy, roughening temperature, strain profile and chemical states are derived.

MOLECULAR MASS MANIPULATION ENABLED BY GRAPHENE BASED NANOSTRUCTURES

The surge of interest in graphene, as epitomized by the Nobel Prize in Physics in 2010, is attributed to its extraordinary properties. Graphene is ultrathin, mechanically tough, and has amendable surface chemistry. These features make graphene and graphene based nanostructure an ideal candidate for the use of molecular mass manipulation. The controllable and programmable molecular mass manipulation is crucial in enabling future graphene based applications, however is challenging to achieve. This dissertation studies several aspects in molecular mass manipulation including mass transportation, patterning and storage. For molecular mass transportation, two methods based on carbon nanoscroll are demonstrated to be effective. They are torsional buckling instability assisted transportation and surface energy induced radial shrinkage. To achieve a more controllable transportation, a fundamental law of direction transport of molecular mass by straining basal graphene is studied. For molecular mass patterning, we reveal a barrier effect of line defects in graphene, which can enable molecular confining and patterning in a domain of desirable geometry. Such a strategy makes controllable patterning feasible for various types of molecules. For molecular mass storage, we propose a novel partially hydrogenated bilayer graphene structure which has large capacity for mass uptake. Also the mass release can be achieved by simply stretching the structure. Therefore the mass uptake and release is reversible. This kind of structure is crucial in enabling hydrogen fuel based technology. Lastly, spontaneous nanofluidic channel formation enabled by patterned hydrogenation is studied. This novel strategy enables programmable channel formation with pre-defined complex geometry.

Superlattice structures in twisted bilayers of folded graphene

The electronic properties of bilayer graphene strongly depend on relative orientation of the two atomic lattices. Whereas Bernal-stacked graphene is most commonly studied, a rotational mismatch between layers opens up a whole new field of rich physics, especially at small interlayer twist. Here we report on magnetotransport measurements on twisted graphene bilayers, prepared by folding of single layers. These reveal a strong dependence on the twist angle, which can be estimated by means of sample geometry. At small rotation, superlattices with a wavelength in the order of 10 nm arise and are observed by friction atomic force microscopy. Magnetotransport measurements in this small-angle regime show the formation of satellite Landau fans. These are attributed to additional Dirac singularities in the band structure and discussed with respect to the wide range of interlayer coupling models.