Exciton characteristics in graphene epoxide

Graphene Oxide Chemistry Management via the Use of KMnO4/K2Cr2O7 Oxidizing Agents

In this paper, we propose a facile approach to the management of graphene oxide (GO) chemistry via its synthesis using KMnO4/K2Cr2O7 oxidizing agents at different ratios. Using Fourier Transformed Infrared Spectroscopy, X-ray Photoelectron Spectroscopy, and X-ray Absorption Spectroscopy, we show that the number of basal-plane and edge-located oxygenic groups can be controllably tuned by altering the KMnO4/K2Cr2O7 ratio. The linear two-fold reduction in the number of the hydroxyls and epoxides with the simultaneous three-fold rise in the content of carbonyls and carboxyls is indicated upon the transition from KMnO4 to K2Cr2O7 as a predominant oxidizing agent. The effect of the oxidation mixture's composition on the structure of the synthesized GOs is also comprehensively studied by means of X-ray diffraction, Raman spectroscopy, transmission electron microscopy, atomic-force microscopy, optical microscopy, and the laser diffraction method. The nanoscale corrugation of the GO platelets with the increase of the K2Cr2O7 content is signified, whereas the 10-100 μm lateral size, lamellar, and defect-free structure is demonstrated for all of the synthesized GOs regardless of the KMnO4/K2Cr2O7 ratio. The proposed method for the synthesis of GO with the desired chemistry opens up new horizons for the development of graphene-based materials with tunable functional properties.

Electric field quenching of graphene oxide photoluminescence

With the advent of graphene, there has been an interest in utilizing this material and its derivative, graphene oxide (GO) for novel applications in nanodevices such as bio and gas sensors, solid-state supercapacitors and solar cells. Although GO exhibits lower conductivity and structural stability, it possesses an energy band gap that enables fluorescence emission in the visible/near infrared leading to a plethora of optoelectronic applications. In order to allow fine-tuning of its optical properties in the device geometry, new physical techniques are required that, unlike existing chemical approaches, yield substantial alteration of GO structure. Such a desired new technique is one that is electronically controlled and leads to reversible changes in GO optoelectronic properties. In this work, we for the first time investigate the methods to controllably alter the optical response of GO with the electric field and provide theoretical modeling of the electric field-induced changes. Field-dependent GO emission is studied in bulk GO/polyvinylpyrrolidone films with up to 6% reversible decrease under 1.6 V µm^-1 electric fields. On an individual flake level, a more substantial over 50% quenching is achieved for select GO flakes in a polymeric matrix between interdigitated microelectrodes subject to two orders of magnitude higher fields. This effect is modeled on a single exciton level by utilizing Wentzel, Kremer, and Brillouin approximation for electron escape from the exciton potential well. In an aqueous suspension at low fields, GO flakes exhibit electrophoretic migration, indicating a degree of charge separation and a possibility of manipulating GO materials on a single-flake level to assemble electric field-controlled microelectronics. As a result of this work, we suggest the potential of varying the optical and electronic properties of GO via the electric field for the advancement and control over its optoelectronic device applications.

Oxygen migration and optical properties of coronene oxides and their persulfurated derivatives: insight into the electric field effect and the oxygen-site dependence

Oxygen migration and spectroscopic properties of coronene (C24) epoxides and persulfurated coronene (PSC) oxides have been investigated by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The rim-oxide is predicted to be more energetically favorable than the oxygen-centered configuration, and the application of an external electric field can accelerate the epoxy migration from the middle to the edge of the molecule. The predicted electronic absorptions and emissions of the C24 epoxides strongly depend on the location of oxygen. In particular, the stable edge-epoxide C24d3 has the largest radiative decay rate (kr) and the smallest non-radiative decay rate (knr), suggesting relatively strong fluorescence emission. On the contrary, absorptions and emissions of the PSC oxides are less changed, compared to those of the pristine PSC. On-the-fly trajectory surface hopping dynamics simulations reveal that the nonadiabatic S1 → S0 decay of the C24 epoxides is triggered by C-O bond stretching, and thus the radiative and nonradiative features depend on the C-O bond strength. The present results indicate that the oxygen diffusion on the basal plane of graphene oxides is easily tuned by the external electric field and their optoelectronic properties show a notable oxygen-site dependence.

Selective Reduction Mechanism of Graphene Oxide Driven by the Photon Mode versus the Thermal Mode

A two-dimensional nanocarbon, graphene, has attracted substantial interest due to its excellent properties. The reduction of graphene oxide (GO) has been investigated for the mass production of graphene used in practical applications. Different reduction processes produce different properties in graphene, affecting the performance of the final materials or devices. Therefore, an understanding of the mechanisms of GO reduction is important for controlling the properties of functional two-dimensional systems. Here, we determined the average structure of reduced GO prepared via heating and photoexcitation and clearly distinguished their reduction mechanisms using ultrafast time-resolved electron diffraction, time-resolved infrared vibrational spectroscopy, and time-dependent density functional theory calculations. The oxygen atoms of epoxy groups are selectively removed from the basal plane of GO by photoexcitation (photon mode), in stark contrast to the behavior observed for the thermal reduction of hydroxyl and epoxy groups (thermal mode). The difference originates from the selective excitation of epoxy bonds via an electronic transition due to their antibonding character. This work will enable the preparation of the optimum GO for the intended applications and expands the application scope of two-dimensional systems.

Superbound Excitons in 2D Phosphorene Oxides

The optical excitations in layered phosphorene oxides are studied via ab initio calculation together with GW approximation for the self-energy and solving the Bethe-Salpeter equation (BSE) for the excitations. It is found that the electronic structure of phosphorene oxides closely depends on the oxygen concentration. For the high oxygen coverage structure P₄O₁₀, it shows a strong localized molecular-like electronic structure with exciton binding ( E_b) energy up to 3.0 eV, which is several times larger than the ordinary E_b value in various low-dimensional materials. This study may provide an alternative way to design functional layered materials with large exciton binding energies by controlling the oxidation level in phosphorene oxides.

Thin Nacre-Biomimetic Coating with Super-Anticorrosion Performance

The rigorous organic and inorganic laminated structure of nacre has been developed by millions of years of biological evolution against various external impacts, including mechanical loadings and chemical attacks. Nacre-biomimetic materials have been recognized as an effective strategy to achieve high strength and toughness simultaneously. However, the understanding of nacre-like structure from the perspective of corrosion protection is still very limited. This work investigates the anticorrosion performance of nacre-biomimetic GO/epoxy (NBGE) coatings with alternating layers. Potentiodynamic polarization measurements indicated that the corrosion rate of steel protected by the NBGE coating with 5 layers of GO and 6 layers of epoxy (5NBGE) and a total thickness of 17 μm was 20 times slower than that of steel under the pure epoxy coating twice as thick in 3.5 wt % NaCl solution. Electrochemical impedance spectroscopy measurements revealed the importance and functions of the GO layers in NBGE coatings. The 5NBGE coating exhibited better performance than carbon-based nanoparticle/epoxy mixed coatings. The superior anticorrosion performance of the NB5G6E coating was supported by photographic observations, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and chloride diffusion measurements. The strong cross-linking layer-by-layer structure of NBGE coatings was proved by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction analyses. The anticorrosion mechanism of the NBGE coatings was interpreted by the mitigation of chemical reactions occurring at the steel-coating interface due to the restricted intrusion of O₂, H₂O, and Cl^- through the reduced pores and defects by the intercalated GO layers in the coatings.

Room-temperature excitonic emission with a phonon replica from graphene nanosheets deposited on Ni-nanocrystallites/Si-nanoporous pillar array

Graphene nanosheets (GNSs) were grown on a Si nanoporous pillar array (Si-NPA) via chemical vapour deposition, using a thin layer of pre-deposited Ni nanocrystallites as catalyst. GNSs were determined to be of high quality and good dispersivity, with a typical diameter size of 15 × 8 nm. Light absorption measurements showed that GNSs had an absorption band edge at 3.3 eV. They also showed sharp and regular excitonic emitting peaks in the ultraviolet and visible region (2.06-3.6 eV). Moreover, phonon replicas with long-term stability appeared with the excitonic peaks at room temperature. Temperature-dependent photoluminescence from the GNSs revealed that the excitonic emission derived from free and bound excitonic recombination. A physical model based on band energy theory was constructed to analyse the carrier transport of GNSs. The Ni nanocrystallites on Si-NPA, which acted as a metal-enhanced fluorescence substrate, were supposed to accelerate the excitonic recombination of GNSs and enhanced the measured emission intensity. Results of this study would be valuable in determining the luminescence mechanism of GNSs and could be applied in real-world optoelectronic devices.

Selective engineering of oxygen-containing functional groups using the alkyl ligand oleylamine for revealing the luminescence mechanism of graphene oxide quantum dots

Oxygen-containing functional groups such as epoxy, hydroxyl, carboxylic, and carboxyl groups have a great influence on the luminescence properties of graphene oxide quantum dots (GOQDs). Understanding their roles is essential for the design and optimization of GOQD performance. Herein, we investigate the effect of epoxide functional groups in GOQDs on the luminescence mechanism through passivation of the epoxide functional groups using the alkyl ligand oleylamine. Luminescence in the as-synthesized GOQDs has two separate origins: intrinsic states derived from localized sp² carbon subdomains and extrinsic states formed by oxygen-functional groups. When the oleylamine ligand is conjugated on the GOQDs, intrinsic PL emission from the localized sp² carbon subdomains decreases. This is discussed in detail, based on optical characterization and first-principles density functional theory calculations, which reveal that the role of the epoxide functional groups is to form localized sp² carbon subdomains emitting intrinsic PL. To the best of our knowledge, this is the first investigation of the role of epoxide functional groups on the luminescence mechanism in GOQDs.

Graphene Oxide: A One- versus Two-Component Material

The structure of graphene oxide (GO) is a matter of discussion. While established GO models are based on functional groups attached to the carbon framework, another frequently used model claims that GO consists of two components, a slightly oxidized graphene core and highly oxidized molecular species, oxidative debris (OD), adsorbed on it. Those adsorbents are claimed to be the origin for optical properties of GO. Here, we examine this model by preparing GO with a low degree of functionalization, combining it with OD and studying the optical properties of both components and their combination in an artificial two-component system. The analyses of absorption and emission spectra as well as lifetime measurements reveal that properties of the combined system are distinctly different from those of GO. That confirms structural models of GO as a separate oxygenated hexagonal carbon framework with optical properties governed by its internal structure rather than the presence of OD. Understanding the structure of GO allows further reliable interpretation of its optical and electronic properties and enables controlled processing of GO.

Green Preparation of Epoxy/Graphene Oxide Nanocomposites Using a Glycidylamine Epoxy Resin as the Surface Modifier and Phase Transfer Agent of Graphene Oxide

In studies of epoxy/graphene oxide (GO) nanocomposites, organic solvents are commonly used to disperse GO, and vigorous mechanical processes and complicated modification of GO are usually required, increasing the cost and hindering the development and application of epoxy nanocomposites. Here, we report a green, facile, and efficient method of preparing epoxy/GO nanocomposites. When triglycidyl para-aminophenol (TGPAP), a commercially available glycidyl amine epoxy resin with one tertiary amine group per molecule, is used as both the surface modifier and phase transfer agent of GO, GO can be directly and rapidly transferred from water to diglycidyl ether of bisphenol A and other types of epoxy resins by manual stirring under ambient conditions, whereas GO cannot be transferred to these epoxy resins in the absence of TGPAP. The interaction between TGPAP and GO and the effect of the TGPAP content on the dispersion of GO in the epoxy matrix were investigated systematically. Superior dispersion and exfoliation of GO nanosheets and remarkably improved mechanical properties, including tensile and flexural properties, toughness, storage modulus, and microhardness, of the epoxy/GO nanocomposites with a suitable amount of TGPAP were demonstrated. This method is organic-solvent-free and technically feasible for large-scale preparation of high-performance nanocomposites; it opens up new opportunities for exploiting the unique properties of graphene or even other nanofillers for a wide range of applications.

Electronic and optical properties of surface hydrogenated armchair graphene nanoribbons: a theoretical study

The hydrogen coverage on armchair graphene nanoribbons affects the spatial distribution of the wavefunction locally, revealing a confinement phenomenon, and influences the electronic and optical properties as well.

Electronic and optical properties of surface-functionalized armchair graphene nanoribbons

The functional groups on armchair graphene nanoribbons affect the spatial distribution of the wavefunction and influence the electronic and optical properties as well.

Optical properties of armchair graphene nanoribbons with Stone–Wales defects and hydrogenation on the defects

Armchair graphene nanoribbons display interesting optical properties with the existence of Stone–Wales defects and hydrogenation on the defects.

Extremely High Two-Photon Absorbing Graphene Oxide for Imaging of Tumor Cells in the Second Biological Window

Cancer, a life-threatening disease, has become a global pandemic. Targeted tumor imaging using near-infrared (NIR) light is the key to improve the penetration depth and it is highly promising for clinical tumor diagnostics. Driven by this need, in this Letter we have reported aptamer conjugated graphene oxide-based two-photon imaging of breast tumor cells selectively. Reported data indicate that there is an extremely high two-photon absorption from aptamer conjugated graphene oxide (σ2PA = 46890 GM). Experimental data show that two-photon luminescence signal remains almost unchanged even after 2 h of illuminations. Reported results show that S6 RNA aptamers conjugated graphene oxide-based two-photon fluorescence can be used for selective two-photon imaging of SK-BR-3 breast tumor cell in second biological transparency windows using 1100 nm wavelength. Experimental data demonstrate that it is highly capable of distinguishing targeted breast cancer SK-BR-3 cells from other nontargeted MDA-MB-231 breast cancer cells.

Excitonic Photoluminescence from Nanodisc States in Graphene Oxides

The origin of near-infrared (NIR) luminescence from graphene oxide (GO) is investigated by photoluminescence (PL) excitation spectroscopy, time-resolved PL spectroscopy, and density functional theory based many body perturbation theories. The energy of experimentally observed NIR PL peak depends on the excitation energy, and the peak broadens with increasing excitation energy. It is found that the PL decay curves in time-resolved spectroscopy show build-up behavior at lower emission energies due to energy transfer between smaller to larger graphene nanodisc (GND) states embedded in GO. We demonstrate that the NIR PL originates from ensemble emission of GND states with a few nanometers in size. The theoretical calculations reveal the electronic and excitonic properties of individual GND states with various sizes, which accounts for the inhomogeneously broadened NIR PL. We further demonstrate that the electronic properties are highly sensitive to the protonation and deprotonation processes of GND states using both the experimental and theoretical approaches.