Effective work function modulation of graphene/carbon nanotube composite films as transparent cathodes for organic optoelectronics

A Wide-Range, Highly Stable Intelligent Flexible Pressure Sensor Based on Micro-Wrinkled SWCNT/rGO-PDMS with Efficient Thermal Shrinkage

Flexible pressure sensors have drawn growing attention in areas like human physiological signal monitoring and human-computer interaction. Nevertheless, it still remains a significant challenge to guarantee their long-term stability while attaining a wide detection range, a minute pressure testing limit, and high sensitivity. Inspired by the wrinkles on animal skins, this paper introduces a flexible pressure sensor with wrinkled microstructures. This sensor is composed of a composite of reduced graphene oxide (rGO), single-walled carbon nanotubes (SWCNTs), and polydimethylsiloxane (PDMS). After optimizing the proportion of the composite materials, the flexible pressure sensor was manufactured using highly efficient heat-shrinkable films. It has a sensitivity as high as 15.364 kPa-1. Owing to the wrinkled microstructures, the sensor can achieve an ultra-wide pressure detection range, with the maximum reaching 1150 kPa, and is capable of detecting water wave vibrations at the minimum level. Moreover, the wrinkled microstructures were locked by PDMS. The sensor acquired waterproof performance and its mechanical stability was enhanced. Even after 18,000 cycles of repeated loading and unloading, its performance remained unchanged. By combining with an artificial neural network, high-precision recognition of different sounds and postures when grasping different objects was realized, with the accuracies reaching 98.3333% and 99.1111%, respectively. Through the integration of flexible WIFI, real-time wireless transmission of sensing data was made possible. In general, the studied sensor can facilitate the application of flexible pressure sensors in fields such as drowning monitoring, remote traditional Chinese medicine, and intelligent voice.

Advancements in stretchable organic optoelectronic devices and flexible transparent conducting electrodes: Current progress and future prospects

The rapid evolution of flexible optoelectronic devices in consumer markets, such as solar cells, photonic skins, displays, lighting, supercapacitors, and smart windows, has spurred global innovation in the design and development of Stretchable Transparent Conducting Electrode (STCE) materials. These materials, which combine the flexibility of organic materials with the functionality of optoelectronic components, have drawn a lot of attention because of their potential uses in a variety of disciplines, such as medical equipment, wearable electronics, and soft robotics. Recent advancements in material science and device design have significantly improving performance, durability, and functionality of these stretchable organic optoelectronic devices. Furthermore, flexible conducting transparent electrodes play an essential role in a wide range of flexible and transparent electronics, including touch screens, displays, and solar cells. Traditional materials like indium tin oxide (ITO) electrodes, while effective, and constrained by their fragility and high cost. Recent innovations in alternative materials, such as metal mesh, nanowires, conducting polymers and graphene have ushered in a new era of affordable, flexible, and transparent conductive electrodes. Materials like graphene, metal nanowires, metallic grids, metal meshes, and dielectric-metal-dielectric electrodes are explored as potential substitutes for fragile ITO electrodes, thanks to their excellent combination of mechanical flexibility and electrical conductivity. This abstract delves into the opportunities and challenges in the development of flexible and transparent organic optoelectronic devices and flexible conducting transparent electrodes. In this review, we explain the technological advancements of transparent and stretchable electrodes, as well as their applications in organic optoelectronic devices such as organic and perovskite solar cells, OLED, heaters, and supercapacitors. We will specifically examine the basic characteristics, optoelectronic properties, and manufacturing procedures of transparent conducting electrodes. We also discuss the key criteria for evaluating proposals for new research lines in this burgeoning sector.

Effects of adatom species on the structure, stability, and work function of adatom-α-borophene nanocomposites

Work function-tunable borophene-based electrode materials are of significant importance because they promote efficient carrier extraction/injection, thereby enabling electronic devices to achieve maximum energy conversion efficiency. Accordingly, determining the work function of adatom-borophene nanocomposites within a series wherein the adatom is systematically changed will facilitate the design of such materials. In this study, we theoretically determined that the M-B bond length, binding energy, electron transfer between adatoms and BBP, and work function (ϕ) are linearly dependent on the ionization potential (IP) and electronegativity for thermodynamically and kinetically stable adatom-α-borophene (M/BBP) systems involving a series of alkali (earth) metal/BBP (M = Li-Cs; Be-Ba) and halogen/BBP (M = F-I), respectively. However, the binding energies of Li/BBP and Be/BBP deviate from these dependencies owing to their super small adatoms and the resulting significantly enhanced effective M-B bonding areas. By interpreting the electron transfer picture among the different parts of M/BBP, we confirmed that metallic M/BBP possesses ionic sp-p and dsp-p M-B bonds in alkali (earth) metal/BBP but covalent-featured ionic p-p interactions in halogen/BBP. In particular, the direct proportionality between IP and ϕ for alkali (earth) metal/BBP originates from the synergistic effect of charge rearrangement and the increased induced dipole moment; however, the inverse proportionality between electronegativity and ϕ for halogen/BBP arises from the adsorption induced charge redistribution. Our results provide guidance for experimental efforts toward the realization of work function-tunable borophene-based electrodes as well as insight into the bonding rules between various adatoms and α-borophene.

Spirally Wrapped Carbon Nanotube Microelectrodes for Fiber Optoelectronic Devices beyond Geometrical Limitations toward Smart Wearable E-Textile Applications

Fiber optoelectronics technology has recently attracted attention as enabling various form factors of wearable electronics, and the issue of how to control and optimize the configuration and physical properties of the electrode micropatterns in the microfiber devices has become important. Here, spirally wrapped carbon nanotube (CNT) microelectrodes with a controlled dimension are demonstrated for high-performance fiber optoelectronic devices. Inkjet-printed CNT microelectrodes with the desired dimension on an agarose hydrogel template are rolling-transferred onto a microfiber surface with an efficient electrical interface. A fiber organic field-effect transistor with spirally wrapped CNT microelectrodes verifies the feasibility of this strategy, where the transferred microelectrodes intimately contact the organic semiconductor active layer and the output current characteristics are simply controlled, resulting in characteristics that exceed the previous structural limitations. Furthermore, a fiber organic photodiode with spirally wrapped CNT microelectrodes, when used as a transparent electrode, exhibits a high I_light/I_dark ratio and good durability of bending. This fiber photodiode can be successfully incorporated into a textile photoplethysmography bandage for the real-time monitoring of human vital signals. This work offers a promising and efficient strategy to overcome the geometric factors limiting the performance of fiber-optic optoelectronic devices.

Single-step fabrication and work function engineering of Langmuir-Blodgett assembled few-layer graphene films with Li and Au salts

To implement large-area solution-processed graphene films in low-cost transparent conductor applications, it is necessary to have the control over the work function (WF) of the film. In this study we demonstrate a straightforward single-step chemical approach for modulating the work function of graphene films. In our approach, chemical doping of the film is introduced at the moment of its formation. The films are self-assembled from liquid-phase exfoliated few-layer graphene sheet dispersions by Langmuir-Blodgett technique at the water-air interfaces. To achieve a single-step chemical doping, metal standard solutions are introduced instead of water. Li standard solutions (LiCl, LiNO3, Li2CO3) were used as n-dopant, and gold standard solution, H(AuCl4), as p-dopant. Li based salts decrease the work function, while Au based salts increase the work function of the entire film. The maximal doping in both directions yields a significant range of around 0.7 eV for the work function modulation. In all cases when Li-based salts are introduced, electrical properties of the film deteriorate. Further, lithium nitrate (LiNO3) was selected as the best choice for n-type doping since it provides the largest work function modulation (by 400 meV), and the least influence on the electrical properties of the film.

Versatile and Tunable Electrical Properties of Doped Nonoxidized Graphene Using Alkali Metal Chlorides

With the rapid development of wearable and flexible electronics, graphene has been intensively studied for the transparent, hole transport electrode layer (HTL) of field-effect transistors, light-emitting diodes, and organic photovoltaic (OPV) cells. To modulate the sheet resistance and the work function of graphene as a HTL, the surface doping is versatile while retaining high transparency. In this work, we used a chemical doping method to control the charge carrier density, band gap, and work function of graphene with minimizing the damage of the carbon network, for which metal chlorides (NaCl, KCl, and AuCl₃) were used as chemical dopants. The high-quality graphene flakes were synthesized with large lateral sizes of more than 5 μm using ternary graphite intercalation compounds. Interestingly, the AuCl₃-doped graphene flake film with a film thickness of about 20 nm showed the lowest reported sheet resistance of ∼249 Ω/sq with ∼75% transmittance. Furthermore, it could control the work function from 4.32 to 5.1 eV. The interfacial dipole complexes of metal cations with a low work function and the reactive radicals such as -OH were discussed to explain this result. For the practical application, an OPV device using the AuCl₃-doped graphene flake film as the HTL was fabricated and it demonstrated enhanced power conversion efficiency while maintaining high optical transparency in visible light.

Work Function Lowering of Graphite by Sequential Surface Modifications: Nitrogen and Hydrogen Plasma Treatment

Graphite-related materials play an important role in various kinds of devices and catalysts. Controlling the properties of such materials is of great significance to widen the potential applications and improve the performance of such applications as field emission devices and catalyst for fuel cells. In particular, the work function strongly affects the performance, and thus development of methods to tune the work function widely is urgently required. Here, we achieved wide-range control of the work function of graphite by nitrogen and hydrogen plasma treatments. The time of hydrogen plasma treatment and the amount of nitrogen atoms doped beforehand could control the work function of graphite from 2.9 to 5.0 eV. The formation of a surface dipole layer and the nitrogen-derived electron donation contributed to such lowering of the work function, which is advantageous for applications in various fields.

Dispersed Ag2O/Ag on CNT-Graphene Composite: An Implication for Magnificent Photoreduction and Energy Storage Applications

A simple hydrothermal route assisted by a triblock copolymer was used to synthesize Ag2O/Ag nanoparticles on a robotic support consists of functionalized MWCNTs and graphene composite (Ag2O/Ag/CNT-graphene). The composites together with the individual analog of Ag/CNT and Ag/graphene were characterized by means of XRD, TEM-SAED, N2 sorptiometry, Raman, FTIR, UV-Vis, and photoluminescence spectroscopy. These nanomaterials were then tested for the catalytic reduction of 4-nitrophenol (4-NP) to the technologically beneficial 4-aminophenol (4-AP). The Ag2O@Ag@CNT-graphene composite calcined at 400°C has shown fascinating reduction performances for 4-NP either in the dark (k = 0.014 s-1) or under visible light illumination (k = 0.039 s-1) in the presence of 5 mM NaBH4 compared to Ag/CNT (0.0112 s-1) and Ag/graphene (0.010 s-1) catalysts. This was chiefly because Ag2O@Ag@CNT-graphene comprises the highest pore volume (0.49 cm3/g) and involves three types of pores in the margin from 1.8 to 4.0 nm in front of only one modal type of pores for the rest of the catalysts and thus maximizes the adsorptive capacity of the reactants (4-NP and NaBH4). Moreover, the former composite exhibits the highest concentration of the Ag2O component as established by numerous techniques in addition to the cyclic voltammetry, proposing it's facile reaction with 4-NP along with the simultaneous transfer of surface hydrogen and electrons from NaBH4 ions to produce 4-AP. The promotion of the p-n junction evaluated using the Mott-schottky equation on Ag2O@Ag@CNT-graphene assisted by charges separation and surface plasmon resonance bands of Ag and Ag2O are found to be advantageous for 4-NP reduction. The latter composite delivers a specific capacitance of 355 F g-1 at 1.0 A g-1 exceeding those of Ag/CNT (230 F g-1) and Ag/graphene (185 F g-1). The EIS study establishes the high electronic conductivity of the metallic Ag and Ag2O moieties, low internal resistance of CNT-graphene as well as the marked ionic transfer facilitated by the composite porous nature.

Top Illuminated Hysteresis-Free Perovskite Solar Cells Incorporating Microcavity Structures on Metal Electrodes: A Combined Experimental and Theoretical Approach

Further technological development of perovskite solar cells (PSCs) will require improvements in power conversion efficiency and stability, while maintaining low material costs and simple fabrication. In this Research Article, we describe top-illuminated ITO-free, stable PSCs featuring microcavity structures, wherein metal layers on both sides on the active layers exerted light interference effects in the active layer, potentially increasing the light path length inside the active layer. The optical constants (refractive index and extinction coefficient) of each layer in the PSC devices were measured, while the optical field intensity distribution was simulated using the transfer matrix method. The photocurrent densities of perovskite layers of various thicknesses were also simulated; these results mimic our experimental values exceptionally well. To modify the cavity electrode surface, we deposited a few nanometers of ultrathin MoO₃ (2, 4, and 6 nm) in between the Ag and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) layers provide hydrophobicity to the Ag surface and elevate the work function of Ag to match that of the hole transport layer. We achieved a power conversion efficiency (PCE) of 13.54% without hysteresis in the device containing a 4 nm-thick layer of MoO₃. In addition, we fabricated these devices on various cavity electrodes (Al, Ag, Au, Cu); those prepared using Cu and Au anodes displayed improved device stability of up to 72 days. Furthermore, we prepared flexible PSCs having a PCE of 12.81% after incorporating the microcavity structures onto poly(ethylene terephthalate) as the substrate. These flexible solar cells displayed excellent stability against bending deformation, maintaining greater than 94% stability after 1000 bending cycles and greater than 85% after 2500 bending cycles performed with a bending radius of 5 mm.

Graphene nanoplatelet doping of P3HT:PCBM photoactive layer of bulk heterojunction organic solar cells for enhancing performance

Hybrid organic photovoltaic (OPV) cells based on conjugated polymer photoactive materials are promising candidates for flexible, high-performance and low-cost energy sources owing to their inexpensive materials, cost-effective processing and ease of fabrication by simple solution processes. However, the modest PV performance obtained to date-in particular the low power conversion efficiency (PCE)-has impeded the large scale deployment of OPV cells. The low PCE in OPV solar cells is mainly attributed to the low carrier mobility, which is closely correlated to the transport diffusion length of the charge carriers within the photoactive layers. The 2D graphene material could be an excellent candidate for assisting charge transport improvement in the active layer of OPV cells, due to its huge carrier mobility, thermal and chemical stability, and its compatibility with the solution process. In this work, we report on the improvement of the optoelectronic properties and photovoltaic performance of graphene nanoplatelet (GNP)-doped P3HT:PCBM photoactive blended layers, integrated into a bulk heterojunction (BHJ) organic-photovoltaic-based device, using PEDOT:PSS on an ITO/glass substrate. First, the light absorption capacity was observed to increase with respect to the GNP content, while the photoluminescence showed clear quenching, indicating electron transfer between the graphene sheets and the polymeric matrix. Then, the incorporation of GNP into the BHJ active layer resulted in enhanced PV performance with respect to the reference cell, and the best PV performance was obtained with 3 wt.% of GNP loading, with an open-circuit voltage of 1.24 V, a short-circuit current density value of 6.18 mA cm^-2, a fill factor of 47.12%, and a power conversion efficiency of about 3.61%. We believe that the obtained results contribute to the development of organic photovoltaic devices and to the understanding of the impact of sp²-bonded carbon therein.

Oleylamine-functionalized graphene oxide as an electron block layer towards high-performance and photostable fullerene-free polymer solar cells

Oleylamine-functionalized graphene oxide (GO) has a shallower energy level of conduction band (E_CB) and a deeper energy level of the valence band (E_VB) as compared to common hole extraction layer (HEL) materials, which make the electron block layer (EBL). Photoluminescence, X-ray photoelectron spectroscopy (XPS), and current density-voltage (J-V) curves with a large reverse bias voltage range obtained under dark conditions are used to determine whether GO layers play important roles in blocking the electron transport to the MoO₃/Ag composite anode and prevent MoO₃ diffusion into a photoactive layer under light illumination. Moreover, GO inserted between a photoactive layer and an HEL enhances charge carrier transport and collection and avoids the monomolecular recombination between the photoactive layer and HEL. Photovoltaic parameters and photostability measurements of inverted and forward PSCs have shown that upon introduction of GO, the performance and photostability of PSCs are improved. On adding GO to PSCs, the power conversion efficiency (PCE) increases approximately 5% and 4% and reduces the decay ratio to approximately 50% and 65% of the initial value for the inverted and forward PSCs, respectively.

Optimization of self-catalyzed InAs Nanowires on flexible graphite for photovoltaic infrared photodetectors

The recent discovery of flexible graphene monolayers has triggered extensive research interest for the development of III-V/graphene functional hybrid heterostructures. In order to fully exploit their enormous potential in device applications, it is essential to optimize epitaxial growth for the precise control of nanowire geometry and density. Herein, we present a comprehensive growth study of InAs nanowires on graphitic substrates by molecular beam epitaxy. Vertically well-aligned and thin InAs nanowires with high yield were obtained in a narrow growth temperature window of 420-450 °C within a restricted domain of growth rate and V/III flux ratio. The graphitic substrates enable high nanowire growth rates, which is favourable for cost-effective device fabrication. A relatively low density of defects was observed. We have also demonstrated InAs-NWs/graphite heterojunction devices exhibiting rectifying behaviour. Room temperature photovoltaic response with a cut-off wavelength of 3.4 μm was demonstrated. This elucidates a promising route towards the monolithic integration of InAs nanowires with graphite for flexible and functional hybrid devices.

Efficient small molecular organic light emitting diode with graphene cathode covered by a Sm layer with nano-hollows and n-doped by Bphen:Cs2CO3 in the hollows

Graphene is a favorable candidate for electrodes of organic light emitting diodes (OLEDs). Graphene has quite a high work function of ∼4.5 eV, and has been extensively studied when used as anodes of OLEDs. In order to use graphene as a cathode, the electron injection barrier between the graphene cathode and the electron transport layer has to be low enough. Using 4,7-diphenyl-1,10-phenanthroline (Bphen):Cs₂CO₃ to n-dope graphene is a very good method, but the electron injection barrier between the n-doped graphene and Bphen:Cs₂CO₃ is still too high to be ∼1.0 eV. In this work, in order to further reduce the electron injection barrier, a novel method is suggested. On the graphene cathode, a Sm layer with a lot of nano-hollows, and subsequently a layer of Bphen:Cs₂CO₃, are deposited. The Bphen:Cs₂CO₃ can n-dope graphene in the nano-hollows, and the Fermi level of the graphene rises. The nano Sm layer is very easily oxidized. Oxygen adsorbed on the surface of graphene may react with Sm to form an O^--Sm⁺ dipole layer. On the areas of the Sm oxide dipole layer without nano-hollows, the electron injection barrier can be further lowered by the dipole layer. Electrons tend to mainly inject through the lower electron barrier where the dipole layer exists. Based on this idea, an effective inverted small molecular OLED with the structure of graphene/1 nm Sm layer with a lot of nano-hollows/Bphen:Cs₂CO₃/Alq₃:C545T/NPB/MoO₃/Al is presented. The maximum current efficiency and maximum power efficiency of the OLED with a 1 nm Sm layer are about two and three times of those of the reference OLED without any Sm layer, respectively.

Cesium-doped graphene grown in situ with ultra-small TiO2 nanoparticles for high-performance lithium-ion batteries

A composite of ultra-small TiO₂ nanoparticles grown in situ on cesium-doped reduced graphene oxide with high electronic and Li⁺ conductivity.

Solution processed reduced graphene oxide electrodes for organic photovoltaics

Since the isolation of free standing graphene in 2004, graphene research has experienced a phenomenal growth. Due to its exceptional electronic, optical and mechanical properties, graphene is believed to be the next wonder material for optoelectronics. The enhanced electrical conductivity, combined with its high transparency in the visible and near-infrared regions of the spectrum, enabled graphene to be an ideal low cost indium-tin oxide (ITO) substitute. Solution-processed reduced graphene oxide combines the unique optoelectrical properties of graphene with large area deposition and flexible substrates rendering it compatible with roll-to-roll manufacturing methods. This paper provides an overview of recent research progress in the application and consequent physical-chemical properties of solution-processed reduced graphene oxide-based films as transparent conductive electrodes (TCEs) in organic photovoltaic (OPV) cells. Reduced graphene oxide (rGO) can be effectively utilized as the TCE in flexible OPVs, where the brittle and expensive ITO is incompatible. The prospects and future research trends in graphene-based TCEs are also discussed.

Work function-tunable transparent electrodes based on all graphene-based materials for organic–graphene photodetectors

A facile method was developed for the doping of graphene nanosheets grown via chemical vapor deposition (CVD) for all graphene-based organic hybrid devices.

Silver Nanowire Transparent Conductive Electrodes for High-Efficiency III-Nitride Light-Emitting Diodes

Silver nanowires (AgNWs) have been successfully demonstrated to function as next-generation transparent conductive electrodes (TCEs) in organic semiconductor devices owing to their figures of merit, including high optical transmittance, low sheet resistance, flexibility, and low-cost processing. In this article, high-quality, solution-processed AgNWs with an excellent optical transmittance of 96.5% at 450 nm and a low sheet resistance of 11.7 Ω/sq were demonstrated as TCEs in inorganic III-nitride LEDs. The transmission line model applied to the AgNW contact to p-GaN showed that near ohmic contact with a specific contact resistance of ~10(-3) Ωcm(2) was obtained. The contact resistance had a strong bias-voltage (or current-density) dependence: namely, field-enhanced ohmic contact. LEDs fabricated with AgNW electrodes exhibited a 56% reduction in series resistance, 56.5% brighter output power, a 67.5% reduction in efficiency droop, and a approximately 30% longer current spreading length compared to LEDs fabricated with reference TCEs. In addition to the cost reduction, the observed improvements in device performance suggest that the AgNWs are promising for application as next-generation TCEs, to realise brighter, larger-area, cost-competitive inorganic III-nitride light emitters.

Bio-Conjugated CNT-Bridged 3D Porous Graphene Oxide Membrane for Highly Efficient Disinfection of Pathogenic Bacteria and Removal of Toxic Metals from Water

More than a billion people lack access to safe drinking water that is free from pathogenic bacteria and toxic metals. The World Health Organization estimates several million people, mostly children, die every year due to the lack of good quality water. Driven by this need, we report the development of PGLa antimicrobial peptide and glutathione conjugated carbon nanotube (CNT) bridged three-dimensional (3D) porous graphene oxide membrane, which can be used for highly efficient disinfection of Escherichia coli O157:H7 bacteria and removal of As(III), As(V), and Pb(II) from water. Reported results demonstrate that versatile membrane has the capability to capture and completely disinfect pathogenic pathogenic E. coli O157:H7 bacteria from water. Experimentally observed disinfection data indicate that the PGLa attached membrane can dramatically enhance the possibility of destroying pathogenic E. coli bacteria via synergistic mechanism. Reported results show that glutathione attached CNT-bridged 3D graphene oxide membrane can be used to remove As(III), As(V), and Pb(II) from water sample at 10 ppm level. Our data demonstrated that PGLa and glutathione attached membrane has the capability for high efficient removal of E. coli O157:H7 bacteria, As(III), As(V), and Pb(II) simultaneously from Mississippi River water.

2D Hybrid Nanostructured Dirac Materials for Broadband Transparent Electrodes

Broadband transparent electrodes based on 2D hybrid nanostructured Dirac materials between Bi2 Se3 and graphene are synthesized using a chemical vapor deposition (CVD) method. Bi2 Se3 nanoplates are preferentially grown along graphene grain boundaries as "smart" conductive patches to bridge the graphene boundary. These hybrid films increase by one- to threefold in conductivity while remaining highly transparent over broadband wavelength. They also display outstanding chemical stability and mechanical flexibility.

Hybrid Films of Graphene and Carbon Nanotubes for High Performance Chemical and Temperature Sensing Applications

A hybrid composite material of graphene and carbon nanotube (CNT) for high performance chemical and temperature sensors is reported. Integration of 1D and 2D carbon materials into hybrid carbon composites is achieved by coupling graphene and CNT through poly(ionic liquid) (PIL) mediated-hybridization. The resulting CNT/PIL/graphene hybrid materials are explored as active materials in chemical and temperature sensors. For chemical sensing application, the hybrid composite is integrated into a chemo-resistive sensor to detect a general class of volatile organic compounds. Compared with the graphene-only devices, the hybrid film device showed an improved performance with high sensitivity at ppm level, low detection limit, and fast signal response/recovery. To further demonstrate the potential of the hybrid films, a temperature sensor is fabricated. The CNT/PIL/graphene hybrid materials are highly responsive to small temperature gradient with fast response, high sensitivity, and stability, which may offer a new platform for the thermoelectric temperature sensors.

Three-Dimensional Conductive Nanocomposites Based on Multiwalled Carbon Nanotube Networks and PEDOT:PSS as a Flexible Transparent Electrode for Optoelectronics

We have synthesized conductive nanocomposites composed of multiwalled carbon nanotubes (MWCNTs) and Au nanoparticles (NPs). The Au NPs with an average size of approximately 4.3 nm are uniformly anchored on the MWCNT. After being exposed to microwave (MW) plasma irradiation, the anchored Au NPs melt and fuse, leading to larger aggregates (34 nm) that can connect the MWCNT forming a three-dimensional conducting network. The formation of a continuous MWCNT network can produce more a conductive pathway, leading to lower sheet resistance. When the Au-MWCNT is dispersed in the highly conductive polymer, poly(ethylene dioxythiophene):polystyrenesulfonate (

PEDOT

PSS), we can obtain solution-processable composite formulations for the preparation of a flexible transparent electrode. The resulting Au-MWCNT/PEDOT:PSS hybrid films possess a sheet resistance of 51 Ω/sq with a transmittance of 86.2% at 550 nm. We also fabricate flexible organic solar cells and electrochromic devices to demonstrate the potential use of the as-prepared composite electrodes. Compared with the indium tin oxide-based devices, both the solar cells and electrochromic devices with the composites incorporated as a transparent electrode deliver comparable performance.

Functionalized graphene and other two-dimensional materials for photovoltaic devices: device design and processing

2D materials have been successfully used in various types of solar cells as transparent electrodes, interfacial and active materials.

High performance CNT point emitter with graphene interfacial layer

Carbon nanotubes (CNTs) have great potential in the development of high-power electron beam sources. However, for such a high-performance electronic device, the electric and thermal contact problem between the metal and CNTs must be improved. Here, we report graphene as an interfacial layer between the metal and CNTs to improve the interfacial contact. The interfacial graphene layer results in a dramatic decrease of the electrical contact resistance by an order of 2 and an increase of the interfacial thermal conductivity by 16%. Such a high improvement in the electrical and thermal interface leads to superior field emission performance with a very low turn-on field of 1.49 V μm(-1) at 10 μA cm(-2) and a threshold field of 2.00 V μm(-1) at 10 mA cm(-2), as well as the maximum current of 16 mA (current density of 2300 A cm(-2)).

Low vacuum annealing of cellulose acetate on nickel towards transparent conductive CNT-graphene hybrid films

We report a versatile method based on low vacuum annealing of cellulose acetate on nickel (Ni) surface for rapid fabrication of graphene and carbon nanotube (CNT)-graphene hybrid films with tunable properties. Uniform films mainly composed of tri-layer graphene can be achieved via a surface precipitation of dissociated carbon at 800 °C for 30 seconds under vacuum conditions of ∼0.6 Pa. The surface precipitation process is further found to be efficient for joining the precipitated graphene with pre-coated CNTs on the Ni surface, consequently, generating the hybrid films. As expected, the hybrid films exhibit substantial opto-electrical and field electron emission properties superior to their individual counterparts. The finding suggests a promising route to hybridize the graphene with diverse nanomaterials for constructing novel hybrid materials with improved performances.

Gadolinium-based nanoparticles for highly efficient T1-weighted magnetic resonance imaging

We developed Pyrene-Gadolinium (Py-Gd) nanoparticles as pH-sensitive magnetic resonance imaging (MRI) contrast agents capable of showing a high-Mr signal in cancer-specific environments, such as acidic conditions. Py-Gd nanoparticles were prepared by coating Py-Gd, which is a complex of gadolinium with pyrenyl molecules, with pyrenyl polyethyleneglycol PEG using a nano-emulsion method. These particles show better longitudinal relaxation time (T1) MR signals in acidic conditions than they do in neutral conditions. Furthermore, the particles exhibit biocompatibility and MR contrast effects in both in vitro and in vivo studies. From these results, we confirm that Py-Gd nanoparticles have the potential to be applied for accurate cancer diagnosis and therapy.

A highly sensitive NADH sensor based on a mycelium-like nanocomposite using graphene oxide and multi-walled carbon nanotubes to co-immobilize poly(luminol) and poly(neutral red) hybrid films

Hybridization of poly(luminol) (PLM) and poly(neutral red) (PNR) has been successfully performed and further enhanced by a conductive and steric hybrid nanotemplate using graphene oxide (GO) and multi-walled carbon nanotubes (MWCNTs). The morphology of the PLM-PNR-MWCNT-GO mycelium-like nanocomposite is studied by SEM and AFM and it is found to be electroactive, pH-dependent, and stable in the electrochemical system. It shows electrocatalytic activity towards NADH with a high current response and low overpotential. Using amperometry, it has been shown to have a high sensitivity of 288.9 μA mM(-1) cm(-2) to NADH (Eapp. = +0.1 V). Linearity is estimated in a concentration range of 1.33 × 10(-8) to 1.95 × 10(-4) M with a detection limit of 1.33 × 10(-8) M (S/N = 3). Particularly, it also shows another linear range of 2.08 × 10(-4) to 5.81 × 10(-4) M with a sensitivity of 151.3 μA mM(-1) cm(-2). The hybridization and activity of PLM and PNR can be effectively enhanced by MWCNTs and GO, resulting in an active hybrid nanocomposite for determination of NADH.

Immobilization of individual nanotubes in graphitic layers for electrical characterization

A simple route is followed to produce an abundance of individual carbon nanotubes (CNTs) immobilized in graphitic layers to counter the challenge of locating individual CNTs and restrict the lateral displacement of CNTs due to the high electrostatic force exerted by a scanning tunnelling microscope tip for electrical characterization. Graphitic layers are selected for the embedding matrix as graphite and the nanotubes have a similar work function and hence would not perturb the electrical configuration of the nanotube. Solvent mediated exfoliation of graphite layers to insert the nanotubes was preferred over oxidative expansion, as oxidation could perturb the electrical configuration of graphite. During the exfoliation of graphite the optimized amount of nanotubes was introduced into the medium such that an individual nanotube could be immobilized in few-layer graphene followed by precipitation and centrifugation. The dose and the time of sonication were optimized to ensure that damage to the walls of the nanotubes is minimized, although the ultrasonication causes scissoring of the nanotube length. This procedure for immobilizing nanotubes in graphitic layers would be equally applicable for functionalized CNTs as well. The capability of embedding individual nanotubes into a similar work function material in an organic solvent, which could then be transferred onto a substrate by simple drop casting or spin coating methods, has an added advantage in sample preparation for the STM characterization of CNTs.

Package-free flexible organic solar cells with graphene top electrodes

Package-free flexible organic solar cells are fabricated with multilayer graphene as top transparent electrodes, which show the highest power conversion efficiency of about 3.2% and excellent flexibility and bending stability. The devices also show good air stability, indicating that multilayer graphene is a promising environmental barrier that can protect the organic solar cells from air contamination.

Aligned carbon nanotubes for high-efficiency Schottky solar cells

The development of low-cost and high-efficiency silicon Schottky solar cells has drawn considerable interest in recent years. A facial approach for the fabrication of carbon nanotube-silicon (CNT-Si) Schottky solar cells by using aligned double-walled CNTs drawn from a CNT array is demonstrated. The aligned CNTs help to form high CNT-Si junction density and provide efficient charge-transport paths. The power conversion efficiency (PCE) reaches 10.5%, which is higher than that of solar cells fabricated using pristine and random CNT networks. Furthermore, the cell fabrication is scalable, and the solar cells fabricated in one batch show very small PCE fluctuations.

Graphene Films for Flexible Organic and Energy Storage Devices

Graphene and its derivatives have been the subject of extensive research in fundamental science and have viable applications in current and future technology. The exceptionally high electronic and thermal conductivity, optical transparency, and high specific surface area, combined with excellent mechanical flexibility and environmental stability leave graphene poised to be a material of the future. This perspective introduces the importance of graphene electrodes, discusses the synthesis of graphene and transfer onto desired substrates and the role of graphene in electrodes for a broad range of flexible devices such as photovoltaic, electronic, and electrochemical energy storage.

Al-TiO₂ composite-modified single-layer graphene as an efficient transparent cathode for organic solar cells

While there are challenges in tuning the properties of graphene (surface wettability, work function alignment, and carrier transport) for realizing an efficient graphene cathode in organic solar cells (OSCs), we propose and demonstrate using an Al-TiO₂ composite to modify single-layer graphene as an efficient cathode for OSCs. To unveil the contributions of the composite in addressing the aforementioned challenges, the evaporated aluminum nanoclusters in the composite benefit the graphene cathode by simultaneously achieving two roles of improving its surface wettability for subsequent TiO₂ deposition and reducing its work function to offer better energy alignment. To address challenges related to charge transport, solution-processed TiO₂ with excellent electron transport can offer charge extraction enhancement to the graphene cathode, which is essential to efficient devices. However, it is a well-known issue for methods such as spin-coating to produce uniform films on the initially hydrophobic graphene, even with improved wettability. The undesirable morphology of TiO₂ by such methods considerably inhibits its effectiveness in enhancing charge extraction. We propose a self-assembly method to deposit the solution-processed TiO₂ on the Al-covered graphene for forming the Al-TiO₂ composite. Compared with spin-coating, the self-assembly method is found to achieve more uniform coating on the graphene surface, with highly controllable thickness. Consequently, the graphene cathode modified with the Al-TiO₂ composite in inverted OSCs gives rise to enhanced power conversion efficiency of 2.58%, which is 2-fold of the previously best reported efficiency (1.27%) for graphene cathode OSCs, reaching ∼75% performance of control devices using indium tin oxide.

Structure, stability, and electronic interactions of polyoxometalates on functionalized graphene sheets

Polyoxometalates (H(3)PMo(12)O(40), H(3)PW(12)O(40), H(4)PMo(11)VO(40)) supported on oxygen- and alkyl-functionalized graphene sheets were investigated. Discrete molecular species were directly observed by electron microscopy at loadings below 20 wt.%. The interaction between the polyoxometalates and the graphene surface was found to significantly impact their vibrational spectra and a linear correlation between the frequency of the M-O(c)-M vibration and the dispersion was evidenced by FTIR. While bulk-like electronic properties were observed for small aggregates (2-5 nm), UV-vis spectroscopy and cyclic voltammetry revealed changes in the electronic structure of isolated molecular species as a result of their interaction with graphene. Because of the ability to disperse alkyl-functionalized graphene in a variety of polar and nonpolar solvents, the materials synthesized in this work provide an opportunity to disperse polyoxometalates in media in which they would not dissolve if unsupported.