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

Surface Engineering for Extremely Enhanced Charge Separation and Photocatalytic Hydrogen Evolution on g-C3 N4

Reinforcing the carrier separation is the key issue to maximize the photocatalytic hydrogen evolution (PHE) efficiency of graphitic carbon nitride (g-C₃ N₄ ). By a surface engineering of gradual doping of graphited carbon rings within g-C₃ N₄ , suitable energy band structures and built-in electric fields are established. Photoinduced electrons and holes are impelled into diverse directions, leading to a 21-fold improvement in the PHE rate.

Interfacial charge distributions in carbon-supported palladium catalysts

Controlling the charge transfer between a semiconducting catalyst carrier and the supported transition metal active phase represents an elite strategy for fine turning the electronic structure of the catalytic centers, hence their activity and selectivity. These phenomena have been theoretically and experimentally elucidated for oxide supports but remain poorly understood for carbons due to their complex nanoscale structure. Here, we combine advanced spectroscopy and microscopy on model Pd/C samples to decouple the electronic and surface chemistry effects on catalytic performance. Our investigations reveal trends between the charge distribution at the palladium-carbon interface and the metal's selectivity for hydrogenation of multifunctional chemicals. These electronic effects are strong enough to affect the performance of large (~5 nm) Pd particles. Our results also demonstrate how simple thermal treatments can be used to tune the interfacial charge distribution, hereby providing a strategy to rationally design carbon-supported catalysts.Control over charge transfer in carbon-supported metal nanoparticles is essential for designing new catalysts. Here, the authors show that thermal treatments effectively tune the interfacial charge distribution in carbon-supported palladium catalysts with consequential changes in hydrogenation performance.

Enhanced Cell Capture on Functionalized Graphene Oxide Nanosheets through Oxygen Clustering

With the global rise in incidence of cancer and infectious diseases, there is a need for the development of techniques to diagnose, treat, and monitor these conditions. The ability to efficiently capture and isolate cells and other biomolecules from peripheral whole blood for downstream analyses is a necessary requirement. Graphene oxide (GO) is an attractive template nanomaterial for such biosensing applications. Favorable properties include its two-dimensional architecture and wide range of functionalization chemistries, offering significant potential to tailor affinity toward aromatic functional groups expressed in biomolecules of interest. However, a limitation of current techniques is that as-synthesized GO nanosheets are used directly in sensing applications, and the benefits of their structural modification on the device performance have remained unexplored. Here, we report a microfluidic-free, sensitive, planar device on treated GO substrates to enable quick and efficient capture of Class-II MHC-positive cells from murine whole blood. We achieve this by using a mild thermal annealing treatment on the GO substrates, which drives a phase transformation through oxygen clustering. Using a combination of experimental observations and MD simulations, we demonstrate that this process leads to improved reactivity and density of functionalization of cell capture agents, resulting in an enhanced cell capture efficiency of 92 ± 7% at room temperature, almost double the efficiency afforded by devices made using as-synthesized GO (54 ± 3%). Our work highlights a scalable, cost-effective, general approach to improve the functionalization of GO, which creates diverse opportunities for various next-generation device applications.

Deciphering the Effect of Polymer-Assisted Doping on the Optoelectronic Properties of Block Copolymer-Anchored Graphene Oxide

Doping facilitates the tuning of band gap, providing an opportunity to tailor the optoelectronic properties of graphene in a simple way, and polymer-assisted doping is a new route to combine the optoelectronic properties of graphene with the properties of a polymer. In this endeavor, a linear diblock copolymer, polycaprolactone-block-poly(dimethyl aminoethyl methacrylate) (PCL₁₃-b-PDMAEMA₁₁₇) (GPCLD) is grafted from the graphene oxide (GO) surface via consecutive ring opening and atom transfer radical polymerization. GPCLD is characterized using proton nuclear magnetic resonance (¹H NMR), Fourier transform infrared spectroscopy, atomic force microscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and Raman spectroscopy. The phase transition behavior of the GPCLD solution with varying temperature and pH is monitored using fluorescence spectroscopy and dynamic light scattering. Temperature-dependent ¹H NMR spectra at pH 9.2 indicate the influence of temperature on the interaction between GPCLD and solvent (water) molecules causing the phase separation. Fluorescence spectra at pH 4 and 9.2 give the evidence of localized p- and n-type doping of graphene assisted by the pendent PDMAEMA chains. In the impedance spectra of GPCLD films, the Nyquist plots vary with pH; at pH 4, they exhibit a semicircle at higher frequencies and a spike at lower frequencies; at pH 7.0, the spike is replaced by an arc; and at pH 9.2, the semicircle at higher frequencies vanishes and only a spike is noticed, all of these suggesting different types of doping of graphene at different pH values. The dc-conductivity also varies with pH and temperature because of the different types of doping. The current (I)-voltage (V) property of GPCLD at different pH values is very unique: at pH 9.2, an interesting feature of negative differential resistance (NDR) is observed; at pH 7, the rectification property is observed; and at pH 4, again the NDR property is observed. The temperature-dependent I-V property at pH 7 and 9.2 clearly indicates a signature of doping, dedoping, and redoping because of the change in the interaction of GO with the grafted polymer arising from coiling and decoiling of polymer chains.

Chemical Modification of Graphene Oxide by Nitrogenation: An X-ray Absorption and Emission Spectroscopy Study

Nitrogen-doped graphene oxides (GO:N_x) were synthesized by a partial reduction of graphene oxide (GO) using urea [CO(NH₂)₂]. Their electronic/bonding structures were investigated using X-ray absorption near-edge structure (XANES), valence-band photoemission spectroscopy (VB-PES), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS). During GO:N_x synthesis, different nitrogen-bonding species, such as pyrrolic/graphitic-nitrogen, were formed by replacing of oxygen-containing functional groups. At lower N-content (2.7 at%), pyrrolic-N, owing to surface and subsurface diffusion of C, N and NH is deduced from various X-ray spectroscopies. In contrast, at higher N-content (5.0 at%) graphitic nitrogen was formed in which each N-atom trigonally bonds to three distinct sp²-hybridized carbons with substitution of the N-atoms for C atoms in the graphite layer. Upon nitrogen substitution, the total density of state close to Fermi level is increased to raise the valence-band maximum, as revealed by VB-PES spectra, indicating an electron donation from nitrogen, molecular bonding C/N/O coordination or/and lattice structure reorganization in GO:N_x. The well-ordered chemical environments induced by nitrogen dopant are revealed by XANES and RIXS measurements.

Oxidized and Si-doped graphene: emerging adsorbents for removal of dioxane

The adsorption properties of oxidized graphene (GO) and Si-doped graphene (SiG) towards 1,4-dioxane were theoretically characterized.

A tunable volatile organic compound sensor by using PtOx/GQDs/TiO2 nanocomposite thin films at room temperature under visible-light activation

Pt/GQDs/TiO₂ nanocomposite thin film-based gas sensors show tunable VOC sensing behaviour at room temperature under visible-light activation.

Observing the Heterogeneous Electro-redox of Individual Single-Layer Graphene Sheets

Electro-redox-induced heterogeneous fluorescence of an individual single-layer graphene sheet was observed in real time by a total internal reflection fluorescence microscope. It was found that the fluorescence intensity of an individual sheet can be tuned reversibly by applying periodic voltages to control the redox degree of graphene sheets. Accordingly, the oxidation and reduction kinetics of an individual single-layer graphene sheet was studied at different voltages. The electro-redox-induced reversible variation of fluorescence intensity of individual sheets indicates a reversible band gap tuning strategy. Furthermore, correlation analysis of redox rate constants on individual graphene sheets revealed a redox-induced spatiotemporal heterogeneity or dynamics of graphene sheets. The observed controllable redox kinetics can rationally guide the precise band gap tuning of individual graphene sheets and then help their extensive applications in optoelectronics and devices for renewable energy.

Facile synthesis of diverse graphene nanomeshes based on simultaneous regulation of pore size and surface structure

Recently, graphene nanomesh (GNM) has attracted great attentions due to its unique porous structure, abundant active sites, finite band gap and possesses potential applications in the fields of electronics, gas sensor/storage, catalysis, etc. Therefore, diverse GNMs with different physical and chemical properties are required urgently to meet different applications. Herein we demonstrate a facile synthetic method based on the famous Fenton reaction to prepare GNM, by using economically fabricated graphene oxide (GO) as a starting material. By precisely controlling the reaction time, simultaneous regulation of pore size from 2.9 to 11.1 nm and surface structure can be realized. Ultimately, diverse GNMs with tunable band gap and work function can be obtained. Specially, the band gap decreases from 4.5-2.3 eV for GO, which is an insulator, to 3.9-1.24 eV for GNM-5 h, which approaches to a semiconductor. The dual nature of electrophilic addition and oxidizability of HO(•) is responsible for this controllable synthesis. This efficient, low-cost, inherently scalable synthetic method is suitable for provide diverse and optional GNMs, and may be generalized to a universal technique.

Graphene oxide/graphene vertical heterostructure electrodes for highly efficient and flexible organic light emitting diodes

The relatively high sheet resistance, low work function and poor compatibility with hole injection layers (HILs) seriously limit the applications of graphene as transparent conductive electrodes (TCEs) for organic light emitting diodes (OLEDs). Here, a graphene oxide/graphene (GO/G) vertical heterostructure is developed as TCEs for high-performance OLEDs, by directly oxidizing the top layer of three-layer graphene films with ozone treatment. Such GO/G heterostructure electrodes show greatly improved optical transmittance, a large work function, high stability, and good compatibility with HIL materials (MoO3 in this work). Moreover, the conductivity of the heterostructure is not sacrificed compared to the pristine three-layer graphene electrodes, but is significantly higher than that of pristine two-layer graphene films. In addition to high flexibility, OLEDs with different emission colors based on the GO/G heterostructure TCEs show much better performance than those based on indium tin oxide (ITO) anodes. Green OLEDs with GO/G heterostructure electrodes have the maximum current efficiency and power efficiency, as high as 82.0 cd A(-1) and 98.2 lm W(-1), respectively, which are 36.7% (14.8%) and 59.2% (15.0%) higher than those with pristine graphene (ITO) anodes. These findings open up the possibility of using graphene for next generation high-performance flexible and wearable optoelectronics with high stability.

Charge transfer and surface defect healing within ZnO nanoparticle decorated graphene hybrid materials

To harness the unique properties of graphene and ZnO nanoparticles (NPs) for novel applications, the development of graphene-ZnO nanoparticle hybrid materials has attracted great attention and is the subject of ongoing research. For this contribution, graphene-oxide-ZnO (GO-ZnO) and thiol-functionalized reduced graphene oxide-ZnO (TrGO-ZnO) nanohybrid materials were prepared by novel self-assembly processes. Based on electron paramagnetic resonance (EPR) and photoluminescence (PL) investigations on bare ZnO NPs, GO-ZnO and TrGO-ZnO hybrid materials, we found that several physical phenomena were occurring when ZnO NPs were hybridized with GO and TrGO. The electrons trapped in Zn vacancy defects (VZn(-)) within the core of ZnO NPs vanished by transfer to GO and TrGO in the hybrid materials, thus leading to the disappearance of the core signals in the EPR spectra of ZnO NPs. The thiol groups of TrGO and sulfur can effectively "heal" the oxygen vacancy (VO(+)) related surface defects of ZnO NPs while oxygen-containing functionalities have low healing ability at a synthesis temperature of 100 °C. Photoexcited electron transfer from the conduction band of ZnO NPs to graphene leads to photoluminescence (PL) quenching of near band gap emission (NBE) of both GO-ZnO and TrGO-ZnO. Simultaneously, electron transfer from graphene to defect states of ZnO NPs is the origin of enhanced green defect emission from GO-ZnO. This observation is consistent with the energy level diagram model of hybrid materials.

Mechanism for excitation-dependent photoluminescence from graphene quantum dots and other graphene oxide derivates: consensus, debates and challenges

Luminescent nanomaterials, with wide applications in biosensing, bioimaging, illumination and display techniques, have been consistently garnering enormous research attention. In particular, those with wavelength-controllable emissions could be highly beneficial. Carbon nanostructures, including graphene quantum dots (GQDs) and other graphene oxide derivates (GODs), with excitation-dependent photoluminescence (PL), which means their fluorescence color could be tuned simply by changing the excitation wavelength, have attracted lots of interest. However the intrinsic mechanism for the excitation-dependent PL is still obscure and fiercely debated presently. In this review, we attempt to summarize the latest efforts to explore the mechanism, including the quantum confinement effect, surface traps model, giant red-edge effect, edge states model and electronegativity of heteroatom model, as well as the newly developed synergistic model, to seek some clues to unravel the mechanism. Meanwhile the controversial difficulties for each model are further discussed. Besides this, the challenges and potential influences of the synthetic methodology and development of the materials are illustrated extensively to elicit more thought and constructive attempts toward their application.

Solution-Processed Copper/Reduced-Graphene-Oxide Core/Shell Nanowire Transparent Conductors

Copper nanowire (Cu NW) based transparent conductors are promising candidates to replace ITO (indium-tin-oxide) owing to the high electrical conductivity and low-cost of copper. However, the relatively low performance and poor stability of Cu NWs under ambient conditions limit the practical application of these devices. Here, we report a solution-based approach to wrap graphene oxide (GO) nanosheets on the surface of ultrathin copper nanowires. By mild thermal annealing, GO can be reduced and high quality Cu r-GO core-shell NWs can be obtained. High performance transparent conducting films were fabricated with these ultrathin core-shell nanowires and excellent optical and electric performance was achieved. The core-shell NW structure enables the production of highly stable conducting films (over 200 days stored in air), which have comparable performance to ITO and silver NW thin films (sheet resistance ∼28 Ω/sq, haze ∼2% at transmittance of ∼90%).

Friction and conductance imaging of sp(2)- and sp(3)-hybridized subdomains on single-layer graphene oxide

We investigated the subdomain structures of single-layer graphene oxide (GO) by characterizing local friction and conductance using conductive atomic force microscopy. Friction and conductance mapping showed that a single-layer GO flake has subdomains several tens to a few hundreds of nanometers in lateral size. The GO subdomains exhibited low friction (high conductance) in the sp(2)-rich phase and high friction (low conductance) in the sp(3)-rich phase. Current-voltage spectroscopy revealed that the local current flow in single-layer GO depends on the quantity of hydroxyl and carboxyl groups, and epoxy bridges within the 2-dimensional carbon layer. The presence of subdomains with different sp(2)/sp(3) carbon ratios on a GO flake was also confirmed by chemical mapping using scanning transmission X-ray microscopy. These results suggest that spatial mapping of the friction and conductance can be used to rapidly identify the composition of heterogeneous single-layer GO at nanometer scale, which is essential for understanding charge transport in nanoelectronic devices.

Moving beyond flexible to stretchable conductive electrodes using metal nanowires and graphenes

Stretchable and/or flexible electrodes and their associated electronic devices have attracted great interest because of their possible applications in high-end technologies such as lightweight, large area, wearable, and biointegrated devices. In particular, metal nanowires and graphene derivatives are chosen for electrodes because they show low resistance and high mechanical stability. Here, we review stretchable and flexible soft electrodes by discussing in depth the intrinsic properties of metal NWs and graphenes that are driven by their dimensionality. We investigate these properties with respect to electronics, optics, and mechanics from a chemistry perspective and discuss currently unsolved issues, such as how to maintain high conductivity and simultaneous high mechanical stability. Possible applications of stretchable and/or flexible electrodes using these nanodimensional materials are summarized at the end of this review.

Oscillatory behaviour of the surface reduction process of multilayer graphene oxide at room temperature

We report the observation of oscillatory redox reactions on the surface of multilayer graphene oxide (GO) films at room temperature.

Modulation of the exfoliated graphene work function through cycloaddition of nitrile imines

1,3-Dipolar cycloaddition between nitrile imines and graphene is studied. The work function of functionalized-graphene depends on the nature of functionalization.

Enhanced power factor in flexible reduced graphene oxide/nanowires hybrid films for thermoelectrics

Highly-flexible RGO/Te NW hybrid films with superior thermoelectric performance (optimal PF value = 80 μW (m K²)⁻¹) were fabricated and the transport characteristics of the charge carrier was investigated.

The role of reduction extent of graphene oxide in the photocatalytic performance of Ag/AgX (X = Cl, Br)/rGO composites and the pseudo-second-order kinetics reaction nature of the Ag/AgBr system

The role of reduction extent of GO in a graphene-based photocatalyst has been studied using Ag/AgX (X = Cl, Br)/rGO composites as a model. The pseudo-second-order kinetics reaction nature of the Ag/AgBr system was also observed.

Nanocrystalline anatase TiO2/reduced graphene oxide composite films as photoanodes for photoelectrochemical water splitting studies: the role of reduced graphene oxide

Schematic of a photoelectrochemical cell using TiO₂/RGO film as the photoanode.

Oxygenated amorphous carbon for resistive memory applications

Carbon-based electronics is a promising alternative to traditional silicon-based electronics as it could enable faster, smaller and cheaper transistors, interconnects and memory devices. However, the development of carbon-based memory devices has been hampered either by the complex fabrication methods of crystalline carbon allotropes or by poor performance. Here we present an oxygenated amorphous carbon (a-COx) produced by physical vapour deposition that has several properties in common with graphite oxide. Moreover, its simple fabrication method ensures excellent reproducibility and tuning of its properties. Memory devices based on a-COx exhibit outstanding non-volatile resistive memory performance, such as switching times on the order of 10 ns and cycling endurance in excess of 10(4) times. A detailed investigation of the pristine, SET and RESET states indicates a switching mechanism based on the electrochemical redox reaction of carbon. These results suggest that a-COx could play a key role in non-volatile memory technology and carbon-based electronics.

The interactions between TiO2 and graphene with surface inhomogeneity determined using density functional theory

TiO2/graphene composites have shown promise as photocatalysts, leading to improved electronic properties. We have modeled using density functional theory TiO2/graphene interfaces formed between graphene with various defects/functional groups (C vacancy, epoxide, and hydroxyl) and TiO2 clusters of various sizes. We considered clusters from 3 to 45 atoms, the latter a nanoparticle of ∼1 nm in size. Our results show that binding to pristine graphene is dominated by van der Waals forces, and that C vacancies or epoxide groups lead to much stronger binding between the graphene and TiO2. Such sites may serve to anchor TiO2 to graphene. Graphene surfaces with hydroxyls however lead to OH transfer to TiO2 and weak interactions between the graphene and the hydroxylated TiO2 cluster. Charge transfer may occur between TiO2 and graphene in various directions (graphene to TiO2 or TiO2 to graphene), depending on the state of the graphene surface, based on overlap of the density of states. Our work indicates that graphene surface defects or functional groups may have a significant effect on the stability, structure, and photoactivity of these materials.

Sensitive photo-thermal response of graphene oxide for mid-infrared detection

This study characterizes the effects of incident infrared (IR) radiation on the electrical conductivity of graphene oxide (GO) and examines its potential for mid-IR detection. Analysis of the mildly reduced GO (m-GO) transport mechanism near room temperature reveals variable range hopping (VRH) for the conduction of electrons. This VRH behavior causes the m-GO resistance to exhibit a strong temperature dependence, with a large negative temperature coefficient of resistance of approximately -2 to -4% K(-1). In addition to this hopping transport, the presence of various oxygen-related functional groups within GO enhances the absorption of IR radiation significantly. These two GO material properties are synergically coupled and provoke a remarkable photothermal effect within this material; specifically, a large resistance drop is exhibited by m-GO in response to the increase in temperature caused by the IR absorption. The m-GO bolometer effect identified in this study is different from that exhibited in vanadium oxides, which require added gold-black films that function as IR absorbers owing to their limited IR absorption capability.

Atomistic understandings of reduced graphene oxide as an ultrathin-film nanoporous membrane for separations

The intrinsic defects in reduced graphene oxide (rGO) formed during reduction processes can act as nanopores, making rGO a promising ultrathin-film membrane candidate for separations. To assess the potential of rGO for such applications, molecular dynamics techniques are employed to understand the defect formation in rGO and their separation performance in water desalination and natural gas purification. We establish the relationship between rGO synthesis parameters and defect sizes, resulting in a potential means to control the size of nanopores in rGO. Furthermore, our results show that rGO membranes obtained under properly chosen synthesis conditions can achieve effective separations and provide significantly higher permeate fluxes than currently available membranes.

Ultrathin-film nanoporous membranes promise low-cost and high-performance separation for applications such as water desalination and the purification of natural gas. Here, the authors adopt a molecular dynamics approach to assess the potential of reduced grapheme oxide as such a material.

Sponge-Templated Macroporous Graphene Network for Piezoelectric ZnO Nanogenerator

We report a simple approach to fabricate zinc oxide (ZnO) nanowire based electricity generators on three-dimensional (3D) graphene networks by utilizing a commercial polyurethane (PU) sponge as a structural template. Here, a 3D network of graphene oxide is deposited from solution on the template and then is chemically reduced. Following steps of ZnO nanowire growth, polydimethylsiloxane (PDMS) backfilling and electrode lamination completes the fabrication processes. When compared to conventional generators with 2D planar geometry, the sponge template provides a 3D structure that has a potential to increase power density per unit area. The modified one-pot ZnO synthesis method allows the whole process to be inexpensive and environmentally benign. The nanogenerator yields an open circuit voltage of ∼0.5 V and short circuit current density of ∼2 μA/cm(2), while the output was found to be consistent after ∼3000 cycles. Finite element analysis of stress distribution showed that external stress is concentrated to deform ZnO nanowires by orders of magnitude compared to surrounding PU and PDMS, in agreement with our experiment. It is shown that the backfilled PDMS plays a crucial role for the stress concentration, which leads to an efficient electricity generation.

Phosphorene oxide: stability and electronic properties of a novel two-dimensional material

Phosphorene, the monolayer form of (black) phosphorus, was recently exfoliated from its bulk counterpart. Phosphorene oxide, by analogy to graphene oxide, is expected to have novel chemical and electronic properties, and may provide an alternative route to the synthesis of phosphorene. In this research, the physical and chemical properties of phosphorene oxide including its formation by oxygen adsorption on the bare phosphorene was investigated. Analysis of the phonon dispersion curves finds stoichiometric and non-stoichiometric oxide configurations to be stable at ambient conditions, thus suggesting that the oxygen adsorption may not degrade the phosphorene. The nature of the band gap of the oxides depends on the degree of functionalization of phosphorene; an indirect gap is predicted for the non-stoichiometric configurations, whereas a direct gap is predicted for the stoichiometric oxide. Application of mechanical strain or an external electric field leads to tunability of the band gap of the phosphorene oxide. In contrast to the case of the bare phosphorene, dependence of the diode-like asymmetric current-voltage response on the degree of stoichiometry is predicted for the phosphorene oxide.

Functional holey graphene oxide: a new electrochemically transformed substrate material for dopamine sensing

Increasing active sites through generating holes within the basal plane of graphene sheets is an effective strategy to enhance catalytic performance in various applications such as sensors, electrocatalysis, and electronics.

Oxidation of CO on a carbon-based material composed of nickel hydroxide and hydroxyl graphene oxide, (Ni4(OH)3–hGO) – a first-principles calculation

The mechanism of CO oxidation on a composite of nickel hydroxide and hydroxyl graphene oxide, Ni₄(OH)₃–hGO.

Fabrication and characteristics of GaN-based light-emitting diodes with a reduced graphene oxide current-spreading layer

A reduced graphene oxide (GO) layer was produced on undoped and n-type GaN, and its effect on the current- and heat-spreading properties of GaN-based light-emitting diodes (LEDs) was studied. The reduced GO inserted between metal electrode and GaN semiconductor acted as a conducting layer and enhanced lateral current flow in the device. Especially, introduction of the reduced GO layer on the n-type GaN improved the electrical performance of the device, relative to that of conventional LEDs, due to a decrease in the series resistance of the device. The enhanced current-spreading was further of benefit, giving the device a higher light output power and a lower junction temperature at high injection currents. These results therefore indicate that reduced GO can be a suitable current and heat-spreading layer for GaN-based LEDs.

Engineering the work function of buckled boron α-sheet by lithium adsorption: a first-principles investigation

First-principles density functional theory calculations were performed to study the effect of Li adsorption on the structural and electronic properties, particularly the work function, of boron α-sheet. The calculated binding energies indicated that boron α-sheet could be well stabilized by the adsorption of Li atoms. Furthermore, the work functions of Li-adsorbed boron α-sheets were observed to decrease drastically with increasing Li coverage. The work functions are lower than that of Mg and even, for some of them, lower than that of Ca, indicating a considerable potential application of Li-adsorbed boron α-sheets as field-emission and electrode materials. Based on the calculated geometric and electronic structures, we discuss in details some possible aspects affecting the work function. The Li coverage dependence of the work functions of Li-adsorbed boron α-sheets was further confirmed by electrostatic potential analyses. The relationship between the work function variation and the Fermi and vacuum energy level shifts was also discussed, and we observed that the variation of the work function is primarily associated with the shift of the Fermi energy level. It is the surface dipole formed by the interaction between adatoms and substrate that should be responsible for the observed variation of the work function, whereas the increasing negative charge and rumpling for boron α-sheet only play minor roles. Additionally, the effect of Li adatoms on the work function of boron α-sheet was confirmed to be much stronger than that of graphene or a graphene double layer.

Graphene oxide as a promising hole injection layer for MoS₂-based electronic devices

The excellent physical and semiconducting properties of transition metal dichalcogenide (TMDC) monolayers make them promising materials for many applications. The TMDC monolayer MoS2 has gained significant attention as a channel material for next-generation transistors. However, while n-type single-layer MoS2 devices can be made with relative ease, fabrication of p-type transistors remains a challenge as the Fermi-level of elemental metals used as contacts are pinned close to the conduction band leading to large p-type Schottky barrier heights (SBH). Here, we propose the utilization of graphene oxide (GO) as an efficient hole injection layer for single-layer MoS2-based electronic and optoelectronic devices. Using first-principles computations, we demonstrate that GO forms a p-type contact with monolayer MoS2, and that the p-type SBH can be made smaller by increasing the oxygen concentration and the fraction of epoxy functional groups in GO. Our analysis shows that this is possible due to the high work function of GO and the relatively weak Fermi-level pinning at the MoS2/GO interfaces compared to traditional MoS2/metal systems (common metals are Ag, Al, Au, Ir, Pd, Pt). The combination of easy-to-fabricate and inexpensive GO with MoS2 could be promising for the development of hybrid all-2D p-type electronic and optoelectronic devices on flexible substrates.

Volta potential phase plate for in-focus phase contrast transmission electron microscopy

We describe a phase plate for transmission electron microscopy taking advantage of a hitherto-unknown phenomenon, namely a beam-induced Volta potential on the surface of a continuous thin film. The Volta potential is negative, indicating that it is not caused by beam-induced electrostatic charging. The film must be heated to ∼ 200 °C to prevent contamination and enable the Volta potential effect. The phase shift is created "on the fly" by the central diffraction beam eliminating the need for precise phase plate alignment. Images acquired with the Volta phase plate (VPP) show higher contrast and unlike Zernike phase plate images no fringing artifacts. Following installation into the microscope, the VPP has an initial settling time of about a week after which the phase shift behavior becomes stable. The VPP has a long service life and has been used for more than 6 mo without noticeable degradation in performance. The mechanism underlying the VPP is the same as the one responsible for the degradation over time of the performance of thin-film Zernike phase plates, but in the VPP it is used in a constructive way. The exact physics and/or chemistry behind the process causing the Volta potential are not fully understood, but experimental evidence suggests that radiation-induced surface modification combined with a chemical equilibrium between the surface and residual gases in the vacuum play an important role.

Improving the efficiency of organic photovoltaics by tuning the work function of graphene oxide hole transporting layers

A facile, fast, non-destructive and roll-to-roll compatible photochemical method for simultaneous partial reduction and doping of graphene oxide (GO) films through ultraviolet laser irradiation in the presence of a Cl2 precursor gas is demonstrated. The photochemical chlorinated GO-Cl films were fully characterized by XPS and Raman measurements, in which grafting of chloride to the edges and the basal plane of GO was confirmed. By tuning the laser exposure time, it is possible to control the doping and reduction levels and therefore to tailor the work function (WF) of the GO-Cl layers from 4.9 eV to a maximum value of 5.23 eV. These WF values match with the HOMO level of most polymer donors employed in OPV devices. Furthermore, high efficiency poly(2,7-carbazole) derivative (PCDTBT):fullerene derivative (PC71BM) based OPVs with GO-Cl as the hole transporting layer (HTL) were demonstrated with a power conversion efficiency (PCE) of 6.56% which is 17.35% and 19.48% higher than that of the pristine GO and PEDOT:PSS based OPV devices, respectively. The performance enhancement was attributed to more efficient hole transportation due to the energy level matching between the GO-Cl and the polymer donor.

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.

Vertical junction photodetectors based on reduced graphene oxide/silicon Schottky diodes

Reduced graphene oxide (RGO) has been employed as an electrode for a series of vertically structured photodetectors. Compared with mechanically exfoliated or chemical vapor deposited graphene, RGO possesses more oxygen containing groups and defects, which are proved to be favorable to enhance the performance of photodetectors. As a matter of fact, RGO with different reduction levels can be readily obtained by varying the annealing temperature. The synthesis procedures for the RGO material are suitable for large scale production and its performance can be effectively improved by functionalization or element doping. For RGO-based devices, the Schottky junction properties and photoelectric conversion have been investigated, primarily by analyzing their current-voltage characteristics. Subsequently, the ON/OFF ratio, responsivity and detectivity of the photodetectors were closely examined, proving that the RGO material could be effectively utilized as the electrode material; also, their relationship with the RGO reduction levels has also been explored. By analyzing the response/recovery speed of the RGO-based photodetectors, we have studied the effects of oxygen-containing functional groups and crystalline defects on the photoelectric conversion.

Work Function Engineering of Graphene

Graphene is a two dimensional one atom thick allotrope of carbon that displays unusual crystal structure, electronic characteristics, charge transport behavior, optical clarity, physical & mechanical properties, thermal conductivity and much more that is yet to be discovered. Consequently, it has generated unprecedented excitement in the scientific community; and is of great interest to wide ranging industries including semiconductor, optoelectronics and printed electronics. Graphene is considered to be a next-generation conducting material with a remarkable band-gap structure, and has the potential to replace traditional electrode materials in optoelectronic devices. It has also been identified as one of the most promising materials for post-silicon electronics. For many such applications, modulation of the electrical and optical properties, together with tuning the band gap and the resulting work function of zero band gap graphene are critical in achieving the desired properties and outcome. In understanding the importance, a number of strategies including various functionalization, doping and hybridization have recently been identified and explored to successfully alter the work function of graphene. In this review we primarily highlight the different ways of surface modification, which have been used to specifically modify the band gap of graphene and its work function. This article focuses on the most recent perspectives, current trends and gives some indication of future challenges and possibilities.

Origin of the chemical and kinetic stability of graphene oxide

At moderate temperatures (≤ 70°C), thermal reduction of graphene oxide is inefficient and after its synthesis the material enters in a metastable state. Here, first-principles and statistical calculations are used to investigate both the low-temperature processes leading to decomposition of graphene oxide and the role of ageing on the structure and stability of this material. Our study shows that the key factor underlying the stability of graphene oxide is the tendency of the oxygen functionalities to agglomerate and form highly oxidized domains surrounded by areas of pristine graphene. Within the agglomerates of functional groups, the primary decomposition reactions are hindered by both geometrical and energetic factors. The number of reacting sites is reduced by the occurrence of local order in the oxidized domains, and due to the close packing of the oxygen functionalities, the decomposition reactions become - on average - endothermic by more than 0.6 eV.

Work function modulation and thermal stability of reduced graphene oxide gate electrodes in MOS devices

Work function (WF) tuning of the contact electrodes is a key requirement in several device technologies, including organic photovoltaics (OPVs), organic light-emitting diodes (OLEDs), and complementary metal oxide semiconductor (CMOS) transistors. Here, we demonstrate that the WF of the gate electrode in an MOS structure can be modulated from 4.35 eV (n-type metal) to 5.28 eV (p-type metal) by sandwiching different thicknesses of reduced graphene oxide (rGO) layers between top contact metals and gate dielectric SiO2. The WF of the gate electrode shows strong dependence on the rGO thickness and is seen to be nearly independent of the contact metals used. The observed WF modulation is attributed to the different amounts of oxygen concentrations in different thicknesses of rGO layers. Importantly, this oxygen concentration can also be varied by the reduction extent of the graphene oxide as experimentally demonstrated. The results are verified by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses. The obtained WF values are thermally stable up to 800 °C. At further high temperatures, diffusion of metal through the rGO sheets is the main cause for WF instability, as confirmed by cross-sectional high-resolution transmission electron microscopy analysis. These findings are not limited to MOS devices, and the WF modulation technique has the potential for applications in other technologies such as OLEDs and OPVs involving graphene as conducting electrodes.

Heteroatom-doped graphene materials: syntheses, properties and applications

Heteroatom doping endows graphene with new or improved properties and greatly enhances its potential for various applications.

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.