Evidence of nanocrystalline semiconducting graphene monoxide during thermal reduction of graphene oxide in vacuum

Density functional theory and molecular dynamics simulation of water molecules confined between two-dimensional graphene oxide surfaces

In this work, the interaction potentials of water molecule with the two-dimensional graphene oxide (GO) surfaces containing epoxy groups have been determined using the M06-2X/6-31g (d,p) level of theory at different orientations and separations and fitted to the Born-Huggins-Meyer (BHM) potential. Good agreements were found between the computed and the well-known OPLS-AA and Dreiding potentials. We have also used some calculated potentials and the well-known models in the molecular dynamics (MD) simulations. Our results showed that some of the calculated force fields for both 2D GO structures almost represent similar results of average number of hydrogen bonds (), radial distribution functions (RDF), self-diffusion coefficient, and angle distribution function (ADF) with the OPLS-AA and Dreiding models which are due to their agreements of the interaction potentials. However, some models in both GO systems represent different results because of their shifted potentials to the larger distances. Our results also showed that the confined water molecules tend to orient toward the epoxy groups on the GO surfaces and the distributions at the angles of θ = 0o (or θ = 180o) is more than the other distributions. The water molecules confined between the bent GO surfaces showed less diffusion coefficients than the flat structure.

Exploration of isoelectronic substitution in graphene dioxide for photocatalytic and photovoltaic applications - an ab-initio study

Herein, we propose graphene dioxide (GDO) derivatives as promising materials for green hydrogen production by photocatalytic water splitting. The optoelectronic and photocatalytic properties of GDO, an insulator with a wide band gap, are tuned by designing new compositions through isovalent substitution of S/Se at the O site, Si and (B,N) at the C site. The newly predicted GDO derivatives were studied using hybrid functional calculations and our results show that several of these materials exhibit semiconducting behavior with a direct band gap value higher than 1.23 eV, hence appropriate for visible light-driven photocatalytic water splitting. The structural stability of these materials was analyzed by total energy and lattice dynamical calculations. The photo generated charge carriers possess lower effective mass and hence higher carrier mobility resulting in suppressed recombination rate and hence improving the water splitting efficiency. Apart from low excitonic binding energy, the electronic structure analysis shows that in several of these compounds the electrons and holes reside in two different atomic sites ensuring further reduction in recombination rate. The relatively higher absorption coefficient of GDO derivatives in the visible part of the solar spectrum indicates enhanced photoconversion efficiency suitable for solar cell applications also and it was further determined by photovoltaic performance parameter analysis. The band edge potential of GDO derivatives is well straddled by the water redox potential at different pHs, suggesting their potential for water splitting along with the possibility of CO2 reduction. Our findings indicate that the newly predicted compositions hold significant promise for photocatalytic as well as photovoltaic applications.

Radical anion functionalization of two-dimensional materials as a means of engineering simultaneously high electronic and ionic conductivity solids

A radical anion based functionalization of the basal plane of hexagonal boron nitride (h-BN) and other two-dimensional materials is proposed in the present study. The resulting materials can reversibly be oxidized without the detachment of the functional groups from the basal plane and can thus serve as surface-intercalation type cathode electroactive species and fast solid ion conductors at the same time. The functionalization of h-BN with [·OBX3]- radical anions (X=F, Cl) in the presence of Li, Na or Mg cations provides one example of such systems. This material can be realized in a proposed simple, two step synthesis. In the first step, a symmetric Lewis adduct of the corresponding Li, Na or Mg peroxides is formed with BX3. In the second step, the anion of the Lewis adduct is thermally split into two identical [·OBX3]- radical anions that covalently functionalize the B atoms of h-BN. In the maximum density surface packing functionalization, the product of the synthesis is A n [(BN)2OBX3] (A = Li, Na with n = 1 or A = Mg with n = 0.5). Its ionic conductivity is predicted to be in the order of 0.01-0.1 S cm-1 at room temperature, on the basis of Grotthus-like (or paddle-wheel) ion transport. In the highly oxidized states (0 ≤ n ≤ 1 for Li and Na and 0 ≤ n ≤ 0.5 for Mg), the electronic conductivity of this material is in the order of 1 S cm-1, similar to carbon black. In the fully reduced states (n = 2 for Li and Na and n = 1 for Mg), the material becomes an insulator, like h-BN. The tunability of the electronic properties of A n [(BN)2OBX3] via the cation concentration (n) allows for its application as multifunctional material in energy storage devices, simultaneously serving as cathode active species, solid electrolyte, electroconductive additive, separator, heat conductor and coating for metal anodes that enables dendrite-free plating. This multifunctionality reduces the number of phases needed in an all-solid-state battery or supercapacitor and thus reduces the interfacial impedance making energy storage devices more efficient. For example, Li[(BN)2OBF3] is predicted to have 5.6 V open circuit voltage versus Li metal anode, capacity of 191 mAh g- 1, specific energy of 1067 Wh kg- 1 and can store energy at a (materials only) cost of 24 USD kWh- 1.

Formation of self-assembled nanoscale graphene/graphene oxide photomemristive heterojunctions using photocatalytic oxidation

Photocatalytic oxidation of graphene with ZnO nanoparticles was found to create self-assembled graphene oxide/graphene (G/GO) photosensitive heterostructures, which can be used as memristors. Oxygen groups released during photodecomposition of water molecules on the nanoparticles under ultraviolet light, oxidized graphene, locally forming the G/GO heterojunctions with ultra-high density. The G/GO nanostructures have non-linear current-voltage characteristics and switch the resistance in the dark and under white light, providing four resistive states at room temperature. Photocatalytic oxidation of graphene with ZnO nanoparticles is proposed as an effective method for creating two-dimensional memristors with a photoresistive switching for ultra-high capacity non-volatile memory.

Ab initio prediction and characterization of phosphorene-like SiS and SiSe as anode materials for sodium-ion batteries

In this work, a density functional theory (DFT) based first-principles study is carried out to investigate the potential of phosphorene-like SiS and SiSe monolayers as anode materials for sodium-ion (Na-ion) batteries. Results show that both SiS and SiSe have large adsorption energies towards single Na atom of -0.94 and -0.43eV, owing to the charge transfers from Na to SiS or SiSe. In addition, it is found that the highest Na concentration for both SiS and SiSe is x=1 with the chemical formulas of NaSiS and NaSiSe, corresponding to the high theoretical specific capacities for Na storages of 445.6 and 250.4mAhg^-1, respectively. Moreover, Na diffusions are very fast and show strong directional behaviors on SiS and SiSe monolayers, with the energy barriers of only 0.135 and 0.158eV, lower than those of conventional anode materials for Na-ion batteries such as Na₂Ti₃O₇ (0.19eV) and Na₃Sb (0.21eV). Finally, although SiS and SiSe show semiconducting behaviors, they transform to metallic states after adsorbing Na atoms, indicating enhanced electrical conductivity during battery cycling. Given these advantages, it is expected that both SiS and SiSe monolayers are promising anode materials for Na-ion batteries, and in principle, other Na-based batteries as well.

Quality evaluation of ultra-thin samples: Application to graphene

Many new materials emerging are strictly two dimensional (2D), often only one or two monolayers thick. They include transition metal dichalcogenides, such as MoS₂ , and graphene. Graphene in particular appears to have many potential applications. Typically the crystalline film without contamination is of interest. Therefore, a reliable method is needed to routinely evaluate the quality of the synthesized samples. Here, we present one such candidate method that utilizes standard electron diffraction and low/medium magnification imaging in a rudimentary transmission electron microscope. The electron irradiation dose is very low thus reducing electron irradiation damage of the investigated samples. As an example, the method was applied to the evaluation of as-grown graphene sample quality and a study on heating-induced change in graphene. It can be used to evaluate the volume and areal ratio of crystalline to noncrystalline component. The method is amiable to automated film quality evaluation.

Two-dimensional shape memory graphene oxide

Driven by the increasing demand for micro-/nano-technologies, stimuli-responsive shape memory materials at nanoscale have recently attracted great research interests. However, by reducing the size of conventional shape memory materials down to approximately nanometre range, the shape memory effect diminishes. Here, using density functional theory calculations, we report the discovery of a shape memory effect in a two-dimensional atomically thin graphene oxide crystal with ordered epoxy groups, namely C₈O. A maximum recoverable strain of 14.5% is achieved as a result of reversible phase transition between two intrinsically stable phases. Our calculations conclude co-existence of the two stable phases in a coherent crystal lattice, giving rise to the possibility of constructing multiple temporary shapes in a single material, thus, enabling highly desirable programmability. With an atomic thickness, excellent shape memory mechanical properties and electric field stimulus, the discovery of a two-dimensional shape memory graphene oxide opens a path for the development of exceptional micro-/nano-electromechanical devices.

When reducing the size of shape memory materials to the nanoscale regime, the memory effect tends to diminish. Here, the authors report a theoretical proposal of a shape memory graphene oxide with ordered epoxy groups retaining excellent programmability and actuation capabilities.

Graphene Monoxide Bilayer As a High-Performance on/off Switching Media for Nanoelectronics

The geometries and electronic characteristics of the graphene monoxide (GMO) bilayer are predicted via density functional theory (DFT) calculations. All the possible sequences of the GMO bilayer show the typical interlayer bonding characteristics of two-dimensional bilayer systems with a weak van der Waals interaction. The band gap energies of the GMO bilayers are predicted to be adequate for electronic device application, indicating slightly smaller energy gaps (0.418-0.448 eV) compared to the energy gap of the monolayer (0.536 eV). Above all, in light of the band gap engineering, the band gap of the GMO bilayer responds to the external electric field sensitively. As a result, a semiconductor-metal transition occurs at a small critical electric field (EC = 0.22-0.30 V/Å). It is therefore confirmed that the GMO bilayer is a strong candidate for nanoelectronics.

Two-Dimensional SiS Layers with Promising Electronic and Optoelectronic Properties: Theoretical Prediction

Two-dimensional (2D) semiconductors can be very useful for novel electronic and optoelectronic applications because of their good material properties. However, all current 2D materials have shortcomings that limit their performance. As a result, new 2D materials are highly desirable. Using atomic transmutation and differential evolution global optimization methods, we identified two group IV-VI 2D materials, Pma2-SiS and silicene sulfide. Pma2-SiS is found to be both chemically, energetically, and thermally stable. Most importantly, Pma2-SiS has shown good electronic and optoelectronic properties, including direct bandgaps suitable for solar cells, good mobility for nanoelectronics, good flexibility of property tuning by layer control and applied strain, and good air stability as well. Therefore, Pma2-SiS is expected to be a promising 2D material in the field of 2D electronics and optoelectronics. The designing principles demonstrated in identifying these two tantalizing examples have great potential to accelerate the finding of new functional 2D materials.

Review of Graphene as a Solid State Diffusion Barrier

Conventional thin-film diffusion barriers consist of 3D bulk films with high chemical and thermal stability. The purpose of the barrier material is to prevent intermixing or penetration from the two materials that encase it. Adhesion to both top and bottom materials is critical to the success of the barrier. Here, the effectiveness of a single atomic layer of graphene as a solid-state diffusion barrier for common metal schemes used in microelectronics is reviewed, and specific examples are discussed. Initial studies of electrical contacts to graphene show a distinct separation in behavior between metallic groups that strongly or weakly bond to it. The two basic classes of metal reactions with graphene are either physisorbed metals, which bond weakly with graphene, or chemisorbed metals, which bond strongly to graphene. For graphene diffusion barrier testing on Si substrates, an effective barrier can be achieved through the formation of a carbide layer with metals that are chemisorbed. For physisorbed metals, the barrier failure mechanism is loss of adhesion at the metal–graphene interface. A graphene layer encased between two metal layers, in certain cases, can increase the binding energy of both films with graphene, however, certain combinations of metal films are detrimental to the bonding with graphene. While the prospects for graphene's future as a solid-state diffusion barrier are positive, there are open questions, and areas for future research are discussed. A better understanding of the mechanisms which influence graphene's ability to be an effective diffusion barrier in microelectronic applications is required, and additional experiments are needed on a broader range of metals, as well as common metal stack contact structures used in microelectronic applications. The role of defects in the graphene is also a key area, since they will probably influence the barrier properties.

Thermally stable, solvent resistant and flexible graphene oxide paper

The ability of graphene oxide (GO) aqueous suspensions to form robust GO paper is largely improved by basification of the suspension before processing.

Three-dimensional highly conductive graphene-silver nanowire hybrid foams for flexible and stretchable conductors

Graphene foams have showed huge application potentials owing to their unique 3D structure and superior properties. Thus, it is highly desired to develop a simple and effective pathway to fabricate high performance graphene-based foams. Here, we present a polymer template-assisted assembly strategy for fabricating a novel class of graphene/AgNW hybrid foams. The hybrid foams show 3D ordered microstructures, high thermal stability, and excellent electrical and mechanical properties, and demonstrate huge application potential in the fields of flexible and stretchable conductors. Importantly, the polymer-template assisted assembly technique is simple, scalable, and low-cost, providing a new synthesis protocol for various multifunctional graphene hybrid foam-based composites.

Density functional theory modeling of multilayer "epitaxial" graphene oxide

CONSPECTUS: Graphene oxide (GO) is a complex material of both fundamental and applied interest. Elucidating the structure of GO is crucial to achieve control over its properties and technological applications. GO is a nonstoichiometric and hygroscopic material with a lamellar structure, and its physical chemical properties depend critically on synthesis procedures and postsynthesis treatments. Numerous efforts are in place to both understand and exploit this versatile layered carbon material. This Account reports on recent density functional theory (DFT) studies of "epitaxial" graphene oxide (hereafter EGO), a type of GO obtained by oxidation of graphene films grown epitaxially on silicon carbide. Here, we rely on selected X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and X-ray diffraction (XRD) measurements of EGO, and we discuss in great detail how we utilized DFT-based techniques to project out from the experimental data basic atomistic information about the chemistry and structure of these films. This Account provides an example as to how DFT modeling can be used to elucidate complex materials such as GO from a limited set of experimental information. EGO exhibits a uniform layered structure, consisting of a stack of graphene planes hosting predominantly epoxide and hydroxyl groups, and water molecules intercalated between the oxidized carbon layers. Here, we first focus on XPS measurements of EGO, and we use DFT to generate realistic model structures, calculate core-level chemical shifts, and through the comparison with experiment, gain insight on the chemical composition and metastability characteristics of EGO. DFT calculations are then used to devise a simplistic but accurate simulation scheme to study thermodynamic and kinetic stability and to predict the intralayer structure of EGO films aged at room temperature. Our simulations show that aged EGO encompasses layers with nanosized oxidized domains presenting a high concentration of oxygen functionalities and local structural order, surrounded by regions of pristine graphene. Through the analysis of XRD and IR measurements, our DFT calculations finally show that in EGO, the oxidized domains of stacked layers overlap and locally confine about a monolayer of water molecules. The overall water content in EGO remains below 10%, and intralayer and interlayer spatial ditribution of oxygen species in EGO lead to a layered porous film with an interlayer spacing of about 10 Å. The basic insight gained from our DFT studies, from chemical composition to a nanoscale characterization of the film structure, will be used to fine-tune synthesis methods and EGO properties.

Toward optimal spatial and spectral quality in widefield infrared spectromicroscopy of IR labelled single cells

Advancements in widefield infrared spectromicroscopy have recently been demonstrated following the commissioning of IRENI (InfraRed ENvironmental Imaging), a Fourier Transform infrared (FTIR) chemical imaging beamline at the Synchrotron Radiation Center. The present study demonstrates the effects of magnification, spatial oversampling, spectral pre-processing and deconvolution, focusing on the intracellular detection and distribution of an exogenous metal tris-carbonyl derivative 1 in a single MDA-MB-231 breast cancer cell. We demonstrate here that spatial oversampling for synchrotron-based infrared imaging is critical to obtain accurate diffraction-limited images at all wavelengths simultaneously. Resolution criteria and results from raw and deconvoluted images for two Schwarzschild objectives (36×, NA 0.5 and 74×, NA 0.65) are compared to each other and to prior reports for raster-scanned, confocal microscopes. The resolution of the imaging data can be improved by deconvolving the instrumental broadening that is determined with the measured PSFs, which is implemented with GPU programming architecture for fast hyperspectral processing. High definition, rapidly acquired, FTIR chemical images of respective spectral signatures of the cell 1 and shows that 1 is localized next to the phosphate- and Amide-rich regions, in agreement with previous infrared and luminescence studies. The infrared image contrast, localization and definition are improved after applying proven spectral pre-processing (principal component analysis based noise reduction and RMie scattering correction algorithms) to individual pixel spectra in the hyperspectral cube.

Vibrational Excitations and Low Energy Electronic Structure of Epoxide-decorated Graphene

We report infrared studies of adsorbed atomic oxygen (epoxide functional groups) on graphene. Two different systems are used as a platform to explore these interactions, namely, epitaxial graphene/SiC(0001) functionalized with atomic oxygen (graphene epoxide, GE) and chemically reduced graphene oxide (RGO). In the case of the model GE system, IR reflectivity measurements show that epoxide groups distort the graphene π bands around the K-point, imparting a finite effective mass and contributing to a band gap. In the case of RGO, epoxide groups are found to be present following the reduction treatment by a combination of polarized IR reflectance and transmittance measurements. Similar to the GE system, a band gap in the RGO sample is observed as well.

Defect-induced semiconductor to metal transition in graphene monoxide

This study investigates the influence of point defects on the geometric and electronic structure of graphene monoxide (GMO) via density functional theory calculations.

Chemical structure and interlayer distance correlation of graphite oxide in the heating process as revealed by in situ Fourier transform infrared spectroscopy and wide-angle X-ray diffraction techniques

The thermal reduction behavior of graphite oxide (GO) film in an air atmosphere during a continuous heating process was monitored in situ using temperature-dependent infrared (IR) spectroscopy and synchrotron radiation wide-angle X-ray diffraction (WAXD) techniques. The results show that most of the water adsorbed by the GO sheets is removed by heating them to 130 °C. The dehydration process leads to a slight decrease of the interplane distance of the GO sheets. The IR data suggest that the thermal reduction occurs starting at 160 °C. The synchronous change of the of hydroxyl and carbonyl stretching mode (ν(-OH) and ν(C=C)) bands of GO between 160 and 210 °C suggest that the recovery of conjugated structure is mainly due to the reduction of -OH groups in this temperature region, in which the d spacing has not been affected. When the temperature reaches 210 °C, the rapid reduction of C=O groups together with the removal of the residual -OH and ether (C-O-C) groups leads to the sudden collapse of the GO sheets. Based on these findings, we present a schematic of the thermal stability of GO film in a continuous heating process, in which the thermal-induced chemical and crystallographic structural changes of the GO film have been correlated.

Silicene oxides: formation, structures and electronic properties

Understanding the oxidation of silicon has been critical to the success of all types of silicon materials, which are the cornerstones of modern silicon technologies. For the recent experimentally obtained two-dimensional silicene, oxidation should also be addressed to enable the development of silicene-based devices. Here we focus on silicene oxides (SOs) that result from the partial or full oxidation of silicene in the framework of density functional theory. It is found that the formation of SOs greatly depends on oxidation conditions, which concern the oxidizing agents of oxygen and hydroxyl. The honeycomb lattice of silicene may be preserved, distorted or destroyed after oxidation. The charge state of Si in partially oxidized silicene ranges from +1 to +3, while that in fully oxidized silicene is +4. Metals, semimetals, semiconductors and insulators can all be found among the SOs, which show a wide spectrum of electronic structures. Our work indicates that the oxidation of silicene should be exquisitely controlled to obtain specific SOs with desired electronic properties.

Enhanced reduction of graphene oxide by means of charging and electric fields applied to hydroxyl groups

We present a first-principles study of the effects of charging and perpendicular electric fields on hydroxyl groups, both of which mediate the reduction of graphene oxide through the formation of H2O and H2O2. Starting with an investigation of the interaction between the hydroxyl groups and graphene, we determine the equilibrium binding geometry, binding energy, and the diffusion path with a minimum energy barrier and show that those equilibrium properties are strongly affected by external agents. While co-adsorbed H and O form bound OH, co-adsorbed H and OH in close proximity form H2O with almost no energy barrier. When negatively charged or subjected to a perpendicular electric field, the energy barrier between two OH co-adsorbed in close proximity is weakened or totally suppressed, forming an oxygen atom strongly bound at the bridge site, together with a water molecule. The water molecule by itself is very weakly bound to graphene and is prone to desorb from the surface, leading to the reduction of graphene oxide. It is therefore demonstrated that the reduction of graphene oxide is promoted to a large extent by negative charging or an applied perpendicular electric field, through the formation of weakly bound water molecules from hydroxyl groups.

Overcoming the phase inhomogeneity in chemically functionalized graphene: the case of graphene oxides

The inhomogeneous phase, which usually exists in graphene oxides (GOs), is a long-standing problem that has severely restricted the use of GOs in various applications. By using first-principles based cluster expansion, we find that the existence of phase separation in conventional GOs is due to the extremely strong attractive interactions of oxygen atoms at different graphene sides. Our Monte Carlo simulations show that this kind of phase separation is not avoidable under the current experimental growth temperature. In this Letter, the idea of oxidizing graphene on a single side is proposed to eliminate the strong double-side oxygen attractions, and our calculations show that well-ordered GOs could be obtained at low oxygen concentrations. These ordered GOs behave as quasi-one-dimensional narrow-gap semiconductors with quite small electron effective masses, which can be useful in high-speed electronics. Our concept could be widely applied to overcome the phase inhomogeneity in various chemically functionalized two-dimensional systems.

Semiconducting graphene: converting graphene from semimetal to semiconductor

Interest in graphene has grown extensively in the last decade or so, because of its extraordinary physical properties, chemical tunability, and potential for various applications. However, graphene is intrinsically a semimetal with a zero bandgap, which considerably impedes its use in many applications where a suitable bandgap is required. The transformation of graphene into a semiconductor has attracted significant attention, because the presence of a sizable bandgap in graphene can vastly promote its already-fascinating potential in an even wider range of applications. Here we review major advances in the pursuit of semiconducting graphene materials. We first briefly discuss the electronic properties of graphene and some theoretical background for manipulating the band structure of graphene. We then summarize many experimental approaches proposed in recent years for producing semiconducting graphene. Despite the relatively short history of research in semiconducting graphene, the progress has been remarkable and many significant developments are highly anticipated.

Fourier transform infrared spectrochemical imaging: review of design and applications with a focal plane array and multiple beam synchrotron radiation source

The beamline design, microscope specifications, and initial results from the new mid-infrared beamline (IRENI) are reviewed. Synchrotron-based spectrochemical imaging, as recently implemented at the Synchrotron Radiation Center in Stoughton, Wisconsin, demonstrates the new capability to achieve diffraction limited chemical imaging across the entire mid-infrared region, simultaneously, with high signal-to-noise ratio. IRENI extracts a large swath of radiation (320 hor. × 25 vert. mrads(2)) to homogeneously illuminate a commercial infrared (IR) microscope equipped with an IR focal plane array (FPA) detector. Wide-field images are collected, in contrast to single-pixel imaging from the confocal geometry with raster scanning, commonly used at most synchrotron beamlines. IRENI rapidly generates high quality, high spatial resolution data. The relevant advantages (spatial oversampling, speed, sensitivity, and signal-to-noise ratio) are discussed in detail and demonstrated with examples from a variety of disciplines, including formalin-fixed and flash-frozen tissue samples, live cells, fixed cells, paint cross-sections, polymer fibers, and novel nanomaterials. The impact of Mie scattering corrections on this high quality data is shown, and first results with a grazing angle objective are presented, along with future enhancements and plans for implementation of similar, small-scale instruments.