Frictional Behavior of Atomically Thin Sheets: Hexagonal-Shaped Graphene Islands Grown on Copper by Chemical Vapor Deposition

Single asperity friction experiments using atomic force microscopy (AFM) have been conducted on chemical vapor deposited (CVD) graphene grown on polycrystalline copper foils. Graphene substantially lowers the friction force experienced by the sliding asperity of a silicon AFM tip compared to the surrounding oxidized copper surface by a factor ranging from 1.5 to 7 over loads from the adhesive minimum up to 80 nN. No damage to the graphene was observed over this range, showing that friction force microscopy serves as a facile, high contrast probe for identifying the presence of graphene on Cu. Consistent with studies of epitaxially grown, thermally grown, and mechanically exfoliated graphene films, the friction force measured between the tip and these CVD-prepared films depends on the number of layers of graphene present on the surface and reduces friction in comparison to the substrate. Friction results on graphene indicate that the layer-dependent friction properties result from puckering of the graphene sheet around the sliding tip. Substantial hysteresis in the normal force dependence of friction is observed with repeated scanning without breaking contact with a graphene-covered region. Because of the hysteresis, friction measured on graphene changes with time and maximum applied force, unless the tip slides over the edge of the graphene island or contact with the surface is broken. These results also indicate that relatively weak binding forces exist between the copper foil and these CVD-grown graphene sheets

Load-Dependent Friction Hysteresis on Graphene

Nanoscale friction often exhibits hysteresis when load is increased (loading) and then decreased (unloading) and is manifested as larger friction measured during unloading compared to loading for a given load. In this work, the origins of load-dependent friction hysteresis were explored through atomic force microscopy (AFM) experiments of a silicon tip sliding on chemical vapor deposited graphene in air, and molecular dynamics simulations of a model AFM tip on graphene, mimicking both vacuum and humid air environmental conditions. It was found that only simulations with water at the tip–graphene contact reproduced the experimentally observed hysteresis. The mechanisms underlying this friction hysteresis were then investigated in the simulations by varying the graphene–water interaction strength. The size of the water–graphene interface exhibited hysteresis trends consistent with the friction, while measures of other previously proposed mechanisms, such as out-of-plane deformation of the graphene film and irreversible reorganization of the water molecules at the shearing interface, were less correlated to the friction hysteresis. The relationship between the size of the sliding interface and friction observed in the simulations was explained in terms of the varying contact angles in front of and behind the sliding tip, which were larger during loading than unloading

Water-Processable Self-Doped Hole-Injection Layer for Large-Area, Air-Processed, Slot-Die-Coated Flexible Organic Light-Emitting Diodes

Hole-injection layers (HILs) play a pivotal role in organic light-emitting diodes (OLEDs) by enabling the efficient injection of positive charge carriers (holes) into the active layer, thus facilitating light emission. This research paper focuses on enhancing the processability and performance of solution-processed HILs in OLEDs by utilizing a water-processable self-doped polymer, P2. The P2 film, deposited via slot-die coating, exhibits exceptional uniformity, high transmittance (85%) across the visible spectrum, and a smooth surface (with a root-mean-square roughness of 1.4 nm) comparable to state-of-the-art poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) films. The P2 HIL in a four-layer OLED structure, consisting of a PET/ITO/HIL/hole transport layer (HTL)/emissive layer (EML)/electron transport layer (ETL)/Ag, with poly­(9-vinylcarbazole) (PVK) as the HTL, Super Yellow (SY) as the EML, and poly­((9,9-bis­(3′-(N,N-dimethylamino)­propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) (PFN) as the ETL, demonstrates enhanced hole injection and transport properties. Flexible OLEDs incorporating P2 HILs, fabricated and tested under ambient conditions on a large-area (4 × 40 mm) indium–tin oxide (ITO)-coated polyethylene terephthalate (PET) substrate, demonstrate a maximum current efficiency of 1.24 cd/A, surpassing devices with PEDOT:PSS HILs by 82%. Moreover, a significant 50% reduction in turn-on voltage is observed compared with analogous devices using a PEDOT:PSS layer. This work contributes to the advancement of the OLED technology for various commercial optoelectronic applications

Water-Processable Self-Doped Hole-Injection Layer for Large-Area, Air-Processed, Slot-Die-Coated Flexible Organic Light-Emitting Diodes

Hole-injection layers (HILs) play a pivotal role in organic light-emitting diodes (OLEDs) by enabling the efficient injection of positive charge carriers (holes) into the active layer, thus facilitating light emission. This research paper focuses on enhancing the processability and performance of solution-processed HILs in OLEDs by utilizing a water-processable self-doped polymer, P2. The P2 film, deposited via slot-die coating, exhibits exceptional uniformity, high transmittance (85%) across the visible spectrum, and a smooth surface (with a root-mean-square roughness of 1.4 nm) comparable to state-of-the-art poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) films. The P2 HIL in a four-layer OLED structure, consisting of a PET/ITO/HIL/hole transport layer (HTL)/emissive layer (EML)/electron transport layer (ETL)/Ag, with poly­(9-vinylcarbazole) (PVK) as the HTL, Super Yellow (SY) as the EML, and poly­((9,9-bis­(3′-(N,N-dimethylamino)­propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) (PFN) as the ETL, demonstrates enhanced hole injection and transport properties. Flexible OLEDs incorporating P2 HILs, fabricated and tested under ambient conditions on a large-area (4 × 40 mm) indium–tin oxide (ITO)-coated polyethylene terephthalate (PET) substrate, demonstrate a maximum current efficiency of 1.24 cd/A, surpassing devices with PEDOT:PSS HILs by 82%. Moreover, a significant 50% reduction in turn-on voltage is observed compared with analogous devices using a PEDOT:PSS layer. This work contributes to the advancement of the OLED technology for various commercial optoelectronic applications

Water-Processable Self-Doped Hole-Injection Layer for Large-Area, Air-Processed, Slot-Die-Coated Flexible Organic Light-Emitting Diodes

Hole-injection layers (HILs) play a pivotal role in organic light-emitting diodes (OLEDs) by enabling the efficient injection of positive charge carriers (holes) into the active layer, thus facilitating light emission. This research paper focuses on enhancing the processability and performance of solution-processed HILs in OLEDs by utilizing a water-processable self-doped polymer, P2. The P2 film, deposited via slot-die coating, exhibits exceptional uniformity, high transmittance (85%) across the visible spectrum, and a smooth surface (with a root-mean-square roughness of 1.4 nm) comparable to state-of-the-art poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) films. The P2 HIL in a four-layer OLED structure, consisting of a PET/ITO/HIL/hole transport layer (HTL)/emissive layer (EML)/electron transport layer (ETL)/Ag, with poly­(9-vinylcarbazole) (PVK) as the HTL, Super Yellow (SY) as the EML, and poly­((9,9-bis­(3′-(N,N-dimethylamino)­propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) (PFN) as the ETL, demonstrates enhanced hole injection and transport properties. Flexible OLEDs incorporating P2 HILs, fabricated and tested under ambient conditions on a large-area (4 × 40 mm) indium–tin oxide (ITO)-coated polyethylene terephthalate (PET) substrate, demonstrate a maximum current efficiency of 1.24 cd/A, surpassing devices with PEDOT:PSS HILs by 82%. Moreover, a significant 50% reduction in turn-on voltage is observed compared with analogous devices using a PEDOT:PSS layer. This work contributes to the advancement of the OLED technology for various commercial optoelectronic applications

Water-Processable Self-Doped Hole-Injection Layer for Large-Area, Air-Processed, Slot-Die-Coated Flexible Organic Light-Emitting Diodes

Hole-injection layers (HILs) play a pivotal role in organic light-emitting diodes (OLEDs) by enabling the efficient injection of positive charge carriers (holes) into the active layer, thus facilitating light emission. This research paper focuses on enhancing the processability and performance of solution-processed HILs in OLEDs by utilizing a water-processable self-doped polymer, P2. The P2 film, deposited via slot-die coating, exhibits exceptional uniformity, high transmittance (85%) across the visible spectrum, and a smooth surface (with a root-mean-square roughness of 1.4 nm) comparable to state-of-the-art poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) films. The P2 HIL in a four-layer OLED structure, consisting of a PET/ITO/HIL/hole transport layer (HTL)/emissive layer (EML)/electron transport layer (ETL)/Ag, with poly­(9-vinylcarbazole) (PVK) as the HTL, Super Yellow (SY) as the EML, and poly­((9,9-bis­(3′-(N,N-dimethylamino)­propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) (PFN) as the ETL, demonstrates enhanced hole injection and transport properties. Flexible OLEDs incorporating P2 HILs, fabricated and tested under ambient conditions on a large-area (4 × 40 mm) indium–tin oxide (ITO)-coated polyethylene terephthalate (PET) substrate, demonstrate a maximum current efficiency of 1.24 cd/A, surpassing devices with PEDOT:PSS HILs by 82%. Moreover, a significant 50% reduction in turn-on voltage is observed compared with analogous devices using a PEDOT:PSS layer. This work contributes to the advancement of the OLED technology for various commercial optoelectronic applications

Electrode Materials for Enhancing the Performance and Cycling Stability of Zinc Iodide Flow Batteries at High Current Densities

Aqueous redox flow battery systems that use a zinc negative electrode have a relatively high energy density. However, high current densities can lead to zinc dendrite growth and electrode polarization, which limit the battery’s high power density and cyclability. In this study, a perforated copper foil with a high electrical conductivity was used on the negative side, combined with an electrocatalyst on the positive electrode in a zinc iodide flow battery. A significant improvement in the energy efficiency (ca. 10% vs using graphite felt on both sides) and cycling stability at a high current density of 40 mA cm–2 was observed. A long cycling stability with a high areal capacity of 222 mA h cm–2 is obtained in this study, which is the highest reported areal capacity for zinc–iodide aqueous flow batteries operating at high current density, in comparison to previous studies. Additionally, the use of a perforated copper foil anode in combination with a novel flow mode was discovered to achieve consistent cycling at exceedingly high current densities of >100 mA cm–2. In situ and ex situ characterization techniques, including in situ atomic force microscopy coupled with in situ optical microscopy and X-ray diffraction, are applied to clarify the relationship between zinc deposition morphology on the perforated copper foil and battery performance in two different flow field conditions. With a portion of the flow going through the perforations, a significantly more uniform and compact zinc deposition was observed compared to the case where all of the flow passed over the surface of the electrode. Results from modeling and simulation support the conclusion that the flow of a fraction of electrolyte through the electrode enhances mass transport, enabling a more compact deposit