Structural and electronic properties of oxidized graphene

Formation and catalytic activity of Pt supported on oxidized graphene for the CO oxidation reaction

Oxidized graphene as the reactive support can control the stability and reactivity of a single-atom Pt catalyst for CO oxidation.

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.

Direct observation of quantum-confined graphene-like states and novel hybrid states in graphene oxide by transient spectroscopy

Quantum-confined graphene-like electronic states are directly observed in graphene oxide and photothermally reduced graphene oxide via transient spectroscopy. An unexpected novel hybrid state arising from amorphous carbon-like peripheral structure with high sp(3) /sp(2) carbon ratio in close vicinity of confined graphene-like states is found commonly existent in various carbon nanomaterials, including graphene oxide, graphene quantum dots, and carbon dots.

Broadband photoresponse and rectification of novel graphene oxide/n-Si heterojunctions

We report a novel graphene oxide (GO) based p-n heterojunction on n-Si. The fabricated vertical GO/n-Si heterojunction diode shows a very low leakage current density of 0.25 µA/cm(2) and excellent rectification characteristics upto 1 MHz. The device on illumination shows a broadband (300-1100 nm) spectral response with a characteristic peak at ~700 nm, in agreement with the photoluminescence emission from GO. Very high photo-to-dark current ratio (>10(5)) is observed upon illumination of UV light. The transient photocurrent measurements indicate that the GO based heterojunction diodes can be useful for UV and broadband photodetectors, compatible with silicon device technology.

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.

Properties and applications of chemically functionalized graphene

The vast and yet largely unexplored family of graphene materials has great potential for future electronic devices with novel functionalities. The ability to engineer the electrical and optical properties in graphene by chemically functionalizing it with a molecule or adatom is widening considerably the potential applications targeted by graphene. Indeed, functionalized graphene has been found to be the best known transparent conductor or a wide gap semiconductor. At the same time, understanding the mechanisms driving the functionalization of graphene with hydrogen is proving to be of fundamental interest for energy storage devices. Here we discuss recent advances on the properties and applications of chemically functionalized graphene.

Chemical functionalization of silicene: spontaneous structural transition and exotic electronic properties

The use of newly discovered silicene for various optoelectronic applications depends largely on the possibility of controlling its electronic properties by chemical functionalization. To investigate this possibility, we systemically study the structural and electronic properties of chemically functionalized silicene by employing first-principles calculations combined with the cluster expansion approach. Interestingly, we find that chemically functionalized epitaxial silicene is generally accompanied by a spontaneous structural transition, which originates from the preference of sp(3) hybridization of silicon. To realized continuously tunable band gaps, chemical functionalization of freestanding silicene at ~900 K is proposed. Finally, we predict that metastable silicene can also be used as an important host material to produce novel functional materials via substitutional doping. For example, the discovered ordered Si(8)P(4) could be a strong candidate for thin-film solar cell absorbers beyond bulk Si.

Carbon nanomaterials for electronics, optoelectronics, photovoltaics, and sensing

In the last three decades, zero-dimensional, one-dimensional, and two-dimensional carbon nanomaterials (i.e., fullerenes, carbon nanotubes, and graphene, respectively) have attracted significant attention from the scientific community due to their unique electronic, optical, thermal, mechanical, and chemical properties. While early work showed that these properties could enable high performance in selected applications, issues surrounding structural inhomogeneity and imprecise assembly have impeded robust and reliable implementation of carbon nanomaterials in widespread technologies. However, with recent advances in synthesis, sorting, and assembly techniques, carbon nanomaterials are experiencing renewed interest as the basis of numerous scalable technologies. Here, we present an extensive review of carbon nanomaterials in electronic, optoelectronic, photovoltaic, and sensing devices with a particular focus on the latest examples based on the highest purity samples. Specific attention is devoted to each class of carbon nanomaterial, thereby allowing comparative analysis of the suitability of fullerenes, carbon nanotubes, and graphene for each application area. In this manner, this article will provide guidance to future application developers and also articulate the remaining research challenges confronting this field.

Graphene-based materials for hydrogen generation from light-driven water splitting

Hydrogen production from solar water splitting has been considered as an ultimate solution to the energy and environmental issues. Over the past few years, graphene has made great contribution to improving the light-driven hydrogen generation performance. This article provides a comprehensive overview of the recent research progress on graphene-based materials for hydrogen evolution from light-driven water splitting. It begins with a brief introduction of the current status and basic principles of hydrogen generation from solar water splitting, and tailoring properties of graphene for application in this area. Then, the roles of graphene in hydrogen generation reaction, including an electron acceptor and transporter, a cocatalyst, a photocatalyst, and a photosensitizer, are elaborated respectively. After that, the comparison between graphene and other carbon materials in solar water splitting is made. Last, this review is concluded with remarks on some challenges and perspectives in this emerging field.

Conduction tuning of graphene based on defect-induced localization

The conduction properties of graphene were tuned by tailoring the lattice by using an accelerated helium ion beam to embed low-density defects in the lattice. The density of the embedded defects was estimated to be 2-3 orders of magnitude lower than that of carbon atoms, and they functionalized a graphene sheet in a more stable manner than chemical surface modifications can do. Current modulation through back gate biasing was demonstrated at room temperature with a current on-off ratio of 2 orders of magnitude, and the activation energy of the thermally activated transport regime was evaluated. The exponential dependence of the current on the length of the functionalized region in graphene suggested that conduction tuning is possible through strong localization of carriers at sites induced by a sparsely distributed random potential modulation.

Regulating infrared photoresponses in reduced graphene oxide phototransistors by defect and atomic structure control

Defects play significant roles in properties of graphene and related device performances. Most studies of defects in graphene focus on their influences on electronic or luminescent optical properties, while controlling infrared optoelectronic performance of graphene by defect engineering remains a challenge. In the meantime, pristine graphene has very low infrared photoresponses of ~0.01 A/W due to fast photocarrier dynamics. Here we report regulating infrared photoresponses in reduced graphene oxide phototransistors by defect and atomic structure control for the first time. The infrared optoelectronic transport and photocurrent generation are significantly influenced and well controlled by oxygenous defects and structures in reduced graphene oxide. Moreover, remarkable infrared photoresponses are observed in photoconductor devices based on reduced graphene oxide with an external responsivity of ~0.7 A/W, at least over one order of magnitude higher than that from pristine graphene. External quantum efficiencies of infrared devices reach ultrahigh values of ~97%, which to our knowledge is one of the best efficiencies for infrared photoresponses from nonhybrid, pure graphene or graphene-based derivatives. The flexible infrared photoconductor devices demonstrate no photoresponse degradation even after 1000 bending tests. The results open up new routes to control optoelectronic behaviors of graphene for high-performance devices.

Mechanical properties of hydrogenated bilayer graphene

Using first principle methods, we study the mechanical properties of monolayer and bilayer graphene with 50% and 100% coverage of hydrogen. We employ the vdW-DF, vdW-DF-C09x, and vdW-DF2-C09x van der Waals functionals for the exchange correlation interactions that give significantly improved interlayer spacings and energies. We also use the PBE form for the generalized gradient corrected exchange correlation functional for comparison. We present a consistent theoretical framework for the in-plane layer modulus and the out-of-plane interlayer modulus and we calculate, for the first time, these properties for these systems. This gives a measure of the change of the strength properties when monolayer and bilayer graphene are hydrogenated. Moreover, comparing the relative performance of these functionals in describing hydrogenated bilayered graphenes, we also benchmark these functionals in how they calculate the properties of graphite.

Graphene-related nanomaterials: tuning properties by functionalization

In this review, we discuss the most recent progress on graphene-related nanomaterials, including doped graphene and derived graphene nanoribbons, graphene oxide, graphane, fluorographene, graphyne, graphdiyne, and porous graphene, from both experimental and theoretical perspectives, and emphasize tuning their stability, electronic and magnetic properties by chemical functionalization.

The impact of functionalization on the stability, work function, and photoluminescence of reduced graphene oxide

Reduced graphene oxide (rGO) is a promising material for a variety of thin-film optoelectronic applications. Two main barriers to its widespread use are the lack of (1) fabrication protocols leading to tailored functionalization of the graphene sheet with oxygen-containing chemical groups, and (2) understanding of the impact of such functional groups on the stability and on the optical and electronic properties of rGO. We carry out classical molecular dynamics and density functional theory calculations on a large set of realistic rGO structures to decompose the effects of different functional groups on the stability, work function, and photoluminescence. Our calculations indicate the metastable nature of carbonyl-rich rGO and its favorable transformation to hydroxyl-rich rGO at room temperature via carbonyl-to-hydroxyl conversion reactions near carbon vacancies and holes. We demonstrate a significant tunability in the work function of rGO up to 2.5 eV by altering the composition of oxygen-containing functional groups for a fixed oxygen concentration, and of the photoluminescence emission by modulating the fraction of epoxy and carbonyl groups. Taken together, our results guide the application of tailored rGO structures in devices for optoelectronics and renewable energy.

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.

Ultrafast spectral migration of photoluminescence in graphene oxide

We use subpicosecond time-resolved photoluminescence measurements to study the nature of photoluminescence in graphene oxide and reduced graphene oxide. Our data indicate that, in contrast to prior suggestions, the photoluminescence spectra of graphene oxide and reduced graphene oxide are inhomogeneously broadened. We observe substantial energy redistribution and relaxation among the emitting states within the first few picoseconds, leading to a progressive red shift of the emission spectrum. Blue shifts that arise in time-integrated spectra upon photothermal reduction are easily understood within this dynamical context without invoking a modified distribution of dipole-coupled states. Rather, reduction increases the nonradiative electron-hole recombination rate and curtails the red-shifting process, which is consistent with an increase in quenching through the introduction of larger and/or more numerous sp(2) clusters. Polarization memory measurements show energetic signatures of electron-hole correlations, established on a subpicosecond time scale and developing little thereafter.

Molecular vibrational spectroscopy characterization of epoxy graphene oxide from density functional calculations

To further understand the structure of graphene oxide, several structures of graphene oxide were systematically investigated using density functional theory (DFT). Our models consisted of a hexagonal in-plane structure of graphene with epoxy groups, and different oxidation levels. We found that different arrangements of these units yielded a range of vibrational spectra. Raman positions of the D and G bands depend sensitively on the local atomic configurations. Both structure energy and spectra computations indicate that the oxidation functional groups are energetically favorable to aggregate together and to be close to one another on the opposite side of graphene surface.

The origin of fluorescence from graphene oxide

Time-resolved fluorescence measurements of graphene oxide in water show multiexponential decay kinetics ranging from 1 ps to 2 ns. Electron-hole recombination from the bottom of the conduction band and nearby localized states to wide-range valance band is suggested as origin of the fluorescence. Excitation wavelength dependence of the fluorescence was caused by relative intensity changes of few emission species. By introducing the molecular orbital concept, the dominant fluorescence was found to originate from the electronic transitions among/between the non-oxidized carbon regions and the boundary of oxidized carbon atom regions, where all three kinds of functionalized groups C-O, C = O and O = C-OH were participating. In the visible spectral range, the ultrafast fluorescence of graphene oxide was observed for the first time.

Mechanical properties of graphene oxides

The mechanical properties, including the Young's modulus and intrinsic strength, of graphene oxides are investigated by first-principles computations. Structural models of both ordered and amorphous graphene oxides are considered and compared. For the ordered graphene oxides, the Young's modulus is found to vary from 380 to 470 GPa as the coverage of oxygen groups changes, respectively. The corresponding variations in the Young's modulus of the amorphous graphene oxides with comparable coverage are smaller at 290-430 GPa. Similarly, the ordered graphene oxides also possess higher intrinsic strength compared with the amorphous ones. As coverage increases, both the Young's modulus and intrinsic strength decrease monotonically due to the breaking of the sp(2) carbon network and lowering of the energetic stability for the ordered and amorphous graphene oxides. In addition, the band gap of the graphene oxide becomes narrower under uniaxial tensile strain, providing an efficient way to tune the electronic properties of graphene oxide-based materials.

Room-temperature metastability of multilayer graphene oxide films

Graphene oxide potentially has multiple applications. The chemistry of graphene oxide and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is crucial to enable future applications of this material. Here, a combined experimental and density functional theory study shows that multilayer graphene oxide produced by oxidizing epitaxial graphene through the Hummers method is a metastable material whose structure and chemistry evolve at room temperature with a characteristic relaxation time of about one month. At the quasi-equilibrium, graphene oxide reaches a nearly stable reduced O/C ratio, and exhibits a structure deprived of epoxide groups and enriched in hydroxyl groups. Our calculations show that the structural and chemical changes are driven by the availability of hydrogen in the oxidized graphitic sheets, which favours the reduction of epoxide groups and the formation of water molecules.

Molecular nanoarchitectures composed of porphyrins and carbon nanomaterials for light energy conversion

In this review, we report the recent advances in the construction of composite molecular nanoarchitectures of porphyrins and nanoscale carbon materials such carbon nanotubes (CNT), graphenes and polycyclic aromatic hydrocarbons (PAH) for photoinduced electron transfer and light energy conversion. First, we state novel single-wall carbon nanotubes (SWCNT)-driven aggregation of protonated porphyrins to produce supramolecular assemblies in the form of macroscopic bundles. Then, photoinduced electron transfer in self-assembled single-walled carbon nanotube (SWCNT)/zinc porphyrin (ZnP) hybrids utilizing (7,6)- and (6,5)-enriched SWCNTs having different band gaps is reported. Further, we discuss the structural and photoelectrochemical properties of porphyrin-based molecular assemblies of other carbon materials such as stacked-cup carbon nanotubes (SCCNT), carbon nanohorns (CNH) and graphenes. Finally, novel supramolecular patterning formation composed of triphenylene core-centered porphyrin hexamers for electronics is discussed.

Π-Bond maximization of graphene in hydrogen addition reactions

Thermodynamic stability of graphene hydrides increases in an approximately linear way with the numbers of π-bonds they contain. Thus, π-bond maximization is the primary driving force for hydrogen addition reactions of graphene. The previously reported thermal preference of sp(2)/sp(3)-phase separation of graphene hydrides is a straightforward effect of π-bond maximization. Although not well applicable to hydroxylation and epoxidation, the π-bond maximization principle also holds approximately for the fluorination reactions of graphene. The findings can be used to help locate the lowest-energy structures for graphene hydrides and to estimate the hydrogenation energy without first-principles calculations.

Titanium-decorated graphene oxide for carbon monoxide capture and separation

We propose titanium-decorated graphene oxide (Ti-GO) as an ideal sorbent for carbon monoxide (CO) capture and separation from gas mixtures. Based on first-principles calculations, Ti-GO exhibits a large binding energy of ~70 kJ mol(-1) for CO molecules, while the binding energies for other gases, such as N(2), CO(2), and CH(4), are significantly smaller. The gas adsorption properties of Ti-GO are independent of the local GO structures once Ti atoms are anchored by the oxygen-containing groups on the GO surface. The strong interaction between CO molecule and Ti is a result of dative bonding, i.e., hybridization between an empty d orbital of Ti and an occupied p orbital of CO. Adsorption isotherms from grand canonical Monte Carlo simulations clearly demonstrate the strong selectivity of Ti-GO for CO adsorption in a mixture with other gas.

Modulating optical properties of graphene oxide: role of prominent functional groups

To modulate the electronic and optical properties of graphene oxide via controlled deoxidation, a proper understanding of the role of the individual functional group in determining these properties is required. We, therefore, have performed ab initio density functional theory based calculations to study the electronic and optical properties of model structures of graphene oxide with different coverages and compositions. In particular, we considered various concentrations of major functional groups like epoxides, hydroxyls, and carbonyls, which mainly consititute the graphene oxide and the reduced graphene oxide. Our calculated electron energy loss spectra (EELS) demonstrate the π plasmon peak to be less sensitive, while π + σ plasmon is found to have a significant blue shift of about 1.0-3.0 eV, when the concentration of epoxy and hydroxyl functional groups in graphene oxide vary from 25% to 75%. However, the increase in carbonyl groups in the center of the graphene sheet creates holes, which lead to the red shift of the EELS. In the case of 37.5% of oxygen-to-carbon ratio, we find the π plasmon peak to be shifted by roughly 1.0 eV as compared to that of the pristine graphene. Our results agree well with the experimental findings which suggest a blue shift in the EELS of graphene oxide and an absorption feature due to a π electron transition of the carbonyl groups at a lower energy than that of epoxy and hydroxyl groups. We also show that the increase in the width of the hole created by the carbonyl groups significantly decreases the optical gap and opens the band gap, and thus, we argue that reduced graphene oxide with mostly carbonyl groups could be a useful material for developing tunable opto-electronic nanodevices.

Electrical conductivity, chemistry, and bonding alternations under graphene oxide to graphene transition as revealed by in situ TEM

A suspended graphene oxide device is fabricated and investigated using a transmission electron microscope (TEM) scanning tunneling microscope (STM) setup. A detailed study of step-by-step reduction of an individual graphene oxide sheet under current flow and Joule heating in tandem with conductivity measurements, atomic structure imaging, chemical composition, and bonding alternations tracing is performed. As monitored by electron energy loss spectroscopy, the oxygen content is tuned from that peculiar to a pristine graphene oxide (i.e., 23.8 at %) to oxygen-free pure graphene. Six orders of magnitude conductance rise is observed during this process with the final conductivity reaching 1.5 × 10(5) S/m. Quantification of plasma energy losses of the starting graphene oxide shows that ∼40% of the oxygen atoms are in the form of epoxy, and ∼60% oxygen atoms are in the form of hydroxyl. The total portion of sp(3) bonds in pristine graphene oxide is estimated to be ∼45%. The epoxy groups show a larger influence on the conductivity of graphene oxide than hydroxyl ones. Through analyzing consecutive plasma-loss energy spectra under gradual graphene oxide to graphene transformation, it is found that the oxygen atoms in epoxy groups decompose prior to those in hydroxyl groups.

Stability and Properties of Two-Dimensional Graphene Hydroxide

Systematic study of interaction between graphene and hydroxyls is carried out by first-principles calculations. Although single hydroxyl adsorbed on graphene presents magnetic properties, hydroxyls prefer to adsorb on graphene in pairs without magnetic properties. The formation energy of hydroxyl pairs with graphene is coverage-dependent, and the most stable structure is half-covered by hydroxyl pairs along zigzag chains with alternative sp2 and sp3 hybridization between carbon atoms. The bandgap of this structure is 0.97 eV in GW approximation, close to the bandgap of Si, and this structure is stable at room temperature. It is possible to build graphene-based electronic circuits from graphene hydroxide without the need for cutting or etching.

Formation and electronic properties of hydrogenated few layer graphene

Motivated by the controversial experimental conclusions on the affinity of few layer graphenes (FLGs) towards hydrogen plasma, we systematically investigate the hydrogenation of FLGs within the framework of density functional theory. The approaching hydrogen atoms from both sides of an FLG induce a structural transition from a layered structure into a hydrogen passivated thin diamond film (HP-TDF). The very low transition barrier of FLG hydrogenation indicates the feasibility of FLG hydrogenation through the proposed mechanism. The increasing formation energy with the thickness of FLGs implies that hydrogenation of single layer graphene is easier than that of FLG, which is in agreement with most experimental observations. Moreover, the electronic properties of HP-TDFs and the hydrogenated bilayer graphene ribbons are also studied.

sp-Electron magnetic clusters with a large spin in graphene

Motivated by recent experimental data (Sepioni, M.; et al. Phys. Rev. Lett. 2010, 105, 207-205), we have studied the possibility of forming magnetic clusters with spin S > (1)/(2) on graphene by adsorption of hydrogen atoms or hydroxyl groups. Migration of hydrogen atoms and hydroxyl groups on the surface of graphene during the delamination of HOPG led to the formation of seven atom or seven OH-group clusters with S = (5)/(2) that were of a special interest. The coincidence of symmetry of the clusters with the graphene lattice strengthens the stability of the cluster. For (OH)(7) clusters that were situated greater than 3 nm from one another, the reconstruction barrier to a nonmagnetic configuration was approximately 0.4 eV, whereas for H(7) clusters, there was no barrier and the high-spin state was unstable. Stability of the high-spin clusters increased if they were formed on top of ripples. Exchange interactions between the clusters were studied and we have shown that the ferromagnetic state is improbable. The role of the chemical composition of the solvent used for the delamination of graphite is discussed.

Reversible electrical reduction and oxidation of graphene oxide

We demonstrate that graphene oxide can be reversibly reduced and oxidized using electrical stimulus. Controlled reduction and oxidation in two-terminal devices containing multilayer graphene oxide films are shown to result in switching between partially reduced graphene oxide and graphene, a process which modifies the electronic and optical properties. High-resolution tunneling current and electrostatic force imaging reveal that graphene oxide islands are formed on multilayer graphene, turning graphene into a self-assembled heterostructure random nanomesh. Charge storage and resistive switching behavior is observed in two-terminal devices made of multilayer graphene oxide films, correlated with electrochromic effects. Tip-induced reduction and oxidation are also demonstrated. Results are discussed in terms of thermodynamics of oxidation and reduction reactions.

Gate-voltage control of oxygen diffusion on graphene

We analyze the diffusion of oxygen atoms on graphene and its dependence on the carrier density controlled by a gate voltage. We use density functional theory to determine the equilibrium adsorption sites, the transition state, and the attempt frequency for different carrier densities. The ease of diffusion is strongly dependent on carrier density. For neutral graphene, we calculate a barrier of 0.73 eV; however, upon electron doping the barrier decreases almost linearly to reach values as low as 0.15 eV for densities of -7.6×10(13)  cm(-2). This implies an increase of more than 9 orders of magnitude in the diffusion coefficient at room temperature. This dramatic change is due to a combined effect of bonding reduction in the equilibrium state and bonding increase at the transition state and can be used to control the patterning of oxidized regions by an adequate variation of the gate voltage.