Impacts of Molecular Orientation on the Hole Injection Barrier Reduction: CuPc/HAT-CN/Graphene

The molecular orientation affected by the interaction between a substrate and deposited molecules plays an important role in device performance. It is known that the molecular orientation influences not only the charge transport property but also its electronic structure. Therefore, the combined study of morphology and electronic structure is of high importance for device application. As a transparent electrode, graphene has many promising advantages. However, graphene itself does not have an adequate work function for either an anode or a cathode, and thus the insertion of a charge injection layer is necessary for it to be used as an electrode. In this study, the hole injection barrier (HIB) reduction was investigated at the interface of copper phthalocyanine (CuPc)/graphene with the insertion of a hexa­aza­triphenylene hexa­carbonitrile (HAT-CN) layer between them. The insertion of the HAT-CN layer roughens the originally flat graphene surface and it weakens the π-interaction between CuPc and of graphene. This induces face-on and edge-on mixed orientations of CuPc, while CuPc on bare graphene shows merely a face-on orientation. As a result, the HIB is reduced by the contribution of edge-on CuPc having lower ionization energy (0.37 eV) along with the high work function of the HAT-CN layer (0.26 eV)

Bifunctional Electrodeposited 3D NiCoSe₂/Nickle Foam Electrocatalysts for Its Applications in Enhanced Oxygen Evolution Reaction and for Hydrazine Oxidation

The development of stable and efficient oxygen evolutional electrocatalysts is fundamental to the production of hydrogen by water electrolysis. However, so far the majority of electrocatalysts require a substantial overpotential (η) (approximately >250 mV) to catalyze the bottleneck oxygen evolution reaction (OER). To overcome this large overpotential for OER, herein we report the growth of nickel–cobalt–selenide (NiCoSe₂) nanosheets over 3D nickel foam (NF) via a facile and scalable electrodeposition method. The resulting 3D NiCoSe₂/NF hybrid electrode requires an overpotential of merely 183 mV to reach the current density (<i>J</i>) of 10 mA cm^–2. To the best of our knowledge, this is the lowest η value reported so far for any earth-abundant material-based OER electrocatalyst to attain the same current density. Moreover, a significant reduction in Tafel slope (88 mV dec^–1) is observed between bare NF and NiCoSe₂/NF. Hence, as a result, the 3D hybrid NiCoSe₂/NF OER electrode outperforms the previously reported electrocatalysts including the expensive state-of-the-art OER electrocatalysts like RuO₂ and IrO₂. Such enhancement in the OER catalytic efficiency of NiCoSe₂ nanosheets over NF can be attributed to its enormous electrochemical active surface area (ECSA) (108 cm²), large roughness factor (270), highly conductive NF substrate, and the presence of multiple catalytically active OER species (NiOOH and CoOOH) on its surface. In addition, 3D hybrid NiCoSe₂/NF electrocatalyst was tested for hydrazine oxidation for its bifunctional utilization. Much lower onset potential values (−0.7 V vs SCE) and high current densities (>200 mA cm^–2) are observed for 3D hybrid NiCoSe₂/NF when benchmarked against bare NF (−0.4 V and <50 mA cm^–2). Furthermore, 3D hybrid NiCoSe₂/NF OER electrode shows excellent stability of 50 h for continuous OER in strongly alkaline solutions while maintaining its enormous ECSA, chemical composition, and structural morphology. The excellent bifunctional electrocatalytic activity, long-term stability, and facile preparation method enable NiCoSe₂/NF hybrid electrode to be a viable candidate for its widespread use in various water-splitting technologies

Bifunctional Electrodeposited 3D NiCoSe₂/Nickle Foam Electrocatalysts for Its Applications in Enhanced Oxygen Evolution Reaction and for Hydrazine Oxidation

The development of stable and efficient oxygen evolutional electrocatalysts is fundamental to the production of hydrogen by water electrolysis. However, so far the majority of electrocatalysts require a substantial overpotential (η) (approximately >250 mV) to catalyze the bottleneck oxygen evolution reaction (OER). To overcome this large overpotential for OER, herein we report the growth of nickel–cobalt–selenide (NiCoSe₂) nanosheets over 3D nickel foam (NF) via a facile and scalable electrodeposition method. The resulting 3D NiCoSe₂/NF hybrid electrode requires an overpotential of merely 183 mV to reach the current density (<i>J</i>) of 10 mA cm^–2. To the best of our knowledge, this is the lowest η value reported so far for any earth-abundant material-based OER electrocatalyst to attain the same current density. Moreover, a significant reduction in Tafel slope (88 mV dec^–1) is observed between bare NF and NiCoSe₂/NF. Hence, as a result, the 3D hybrid NiCoSe₂/NF OER electrode outperforms the previously reported electrocatalysts including the expensive state-of-the-art OER electrocatalysts like RuO₂ and IrO₂. Such enhancement in the OER catalytic efficiency of NiCoSe₂ nanosheets over NF can be attributed to its enormous electrochemical active surface area (ECSA) (108 cm²), large roughness factor (270), highly conductive NF substrate, and the presence of multiple catalytically active OER species (NiOOH and CoOOH) on its surface. In addition, 3D hybrid NiCoSe₂/NF electrocatalyst was tested for hydrazine oxidation for its bifunctional utilization. Much lower onset potential values (−0.7 V vs SCE) and high current densities (>200 mA cm^–2) are observed for 3D hybrid NiCoSe₂/NF when benchmarked against bare NF (−0.4 V and <50 mA cm^–2). Furthermore, 3D hybrid NiCoSe₂/NF OER electrode shows excellent stability of 50 h for continuous OER in strongly alkaline solutions while maintaining its enormous ECSA, chemical composition, and structural morphology. The excellent bifunctional electrocatalytic activity, long-term stability, and facile preparation method enable NiCoSe₂/NF hybrid electrode to be a viable candidate for its widespread use in various water-splitting technologies

Impact of Organic Molecule-Induced Charge Transfer on Operating Voltage Control of Both n‑MoS₂ and p‑MoTe₂ Transistors

Since transition metal dichalcogenide (TMD) semiconductors are found as two-dimensional van der Waals materials with a discrete energy bandgap, many TMD based field effect transistors (FETs) are reported as prototype devices. However, overall reports indicate that threshold voltage (Vth) of those FETs are located far away from 0 V whether the channel is p- or n-type. This definitely causes high switching voltage and unintended OFF-state leakage current. Here, a facile way to simultaneously modulate the Vth of both p- and n-channel FETs with TMDs is reported. The deposition of various organic small molecules on the channel results in charge transfer between the organic molecule and TMD channels. Especially, HAT-CN molecule is found to ideally work for both p- and n-channels, shifting their Vth toward 0 V concurrently. As a proof of concept, a complementary metal oxide semiconductor (CMOS) inverter with p-MoTe2 and n-MoS2 channels shows superior voltage gain and minimal power consumption after HAT-CN deposition, compared to its initial performance. When the same TMD FETs of the CMOS structure are integrated into an OLED pixel circuit for ambipolar switching, the circuit with HAT-CN film demonstrates complete ON/OFF switching of OLED pixel, which was not switched off without HAT-CN

Hole Injection Enhancements of a CoPc and CoPc:NPB Mixed Layer in Organic Light-Emitting Devices

The hole injection enhancement in organic light-emitting devices with the insertion of a cobalt phthalocyanine (CoPc) hole injection layer (HIL) between the indium tin oxide (ITO) anode and the <i>N</i>,<i>N</i>′-bis­(1-naphthyl)-<i>N</i>,<i>N</i>′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) hole transport layer (HTL) was demonstrated through current density–voltage–luminance measurements, in situ photoelectron spectroscopy experiments, and theoretical calculations. The CoPc HIL significantly reduces the hole injection barrier (HIB) and thus serves as an efficient HIL like the conventional copper phthalocyanine HIL. This commonality originates from their similar configurations of the highest occupied molecular orbital (HOMO), which consists of conducting macrocycle isoindole ligands, not related to the central metal. However, as the CoPc:NPB mixed HIL is inserted, the hole injection enhancements are inferior to that of a single CoPc HIL. This is due to the electron transfer from NPB to CoPc, which pulls the HOMO level of the mixed HIL down to the deeper position. The reduced hole injection with the mixed layer implies directly that the HIB between ITO and HIL dominates device performance as the so-called ladder effect of HILs

Interface Formation Between ZnO Nanorod Arrays and Polymers (PCBM and P3HT) for Organic Solar Cells

We investigated the interface formation between a ZnO nanorod array and active layers of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) and poly­[3-hexylthiophene] (P3HT) in organic solar cells (OSC). We measured the interfacial electronic structures with in situ photoemission spectroscopy combined with an electrospray deposition system. Different interfacial electronic structures were observed on the ZnO nanorod array, which were compared to those of a two-dimensional ZnO film. Comparing the interfacial orbital line-ups of the active layers on the nanorod array and the film, PCBM shows Fermi level pinning behavior, but P3HT does not. These induce nearly identical orbital line-ups at the interfaces of PCBM/film and PCBM/nanorod but different line-ups at the interfaces of P3HT/film and P3HT/nanorod. These differences are understood with the integer charge transfer model with the different thresholds of Fermi level pinning of PCBM and P3HT. These results give insight into the design not only of OSCs but also of any organic electronic devices with nanostructures: changes in electronic structure due to the nanostructure formation should be considered thoroughly

Impact of Organic Molecule-Induced Charge Transfer on Operating Voltage Control of Both n‑MoS₂ and p‑MoTe₂ Transistors

Since transition metal dichalcogenide (TMD) semiconductors are found as two-dimensional van der Waals materials with a discrete energy bandgap, many TMD based field effect transistors (FETs) are reported as prototype devices. However, overall reports indicate that threshold voltage (Vth) of those FETs are located far away from 0 V whether the channel is p- or n-type. This definitely causes high switching voltage and unintended OFF-state leakage current. Here, a facile way to simultaneously modulate the Vth of both p- and n-channel FETs with TMDs is reported. The deposition of various organic small molecules on the channel results in charge transfer between the organic molecule and TMD channels. Especially, HAT-CN molecule is found to ideally work for both p- and n-channels, shifting their Vth toward 0 V concurrently. As a proof of concept, a complementary metal oxide semiconductor (CMOS) inverter with p-MoTe2 and n-MoS2 channels shows superior voltage gain and minimal power consumption after HAT-CN deposition, compared to its initial performance. When the same TMD FETs of the CMOS structure are integrated into an OLED pixel circuit for ambipolar switching, the circuit with HAT-CN film demonstrates complete ON/OFF switching of OLED pixel, which was not switched off without HAT-CN

Chrysanthemum-Like CoP Nanostructures on Vertical Graphene Nanohills as Versatile Electrocatalysts for Water Splitting

CoP is a promising catalyst material to replace noble metals in water electrolysis. To further explore the potential of CoP in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), we utilize vertical graphene nanohills (VGNHs) that are known to enhance catalytic performances through superaerophobicity. Unique CoP chrysanthemum-like structures are formed on VGNHs through a facile, one-step electrodeposition reaction. Because of the highly conductive VGNH support and the modified CoP nanostructures, the optimized CoP/VGNHs hybrid catalyst exhibits excellent electrocatalytic activities toward HER in 0.5 M H2SO4, such as a low overpotential at 10 mA cm–2 (η10) of 51 mV, a small Tafel slope of 36 mV dec–1, and a long-term stability. Specifically, the overpotential at 100 mA cm–2 (η100) is merely 125 mV, an outstanding performance for a noble metal-free catalyst. Furthermore, the HER performance in 1.0 M KOH (η10 of 93 mV) and the OER performance in the same alkaline medium (η10 of 300 mV) are highly competitive, making CoP/VGNHs also an excellent bifunctional electrocatalyst yielding a current density of 10 mA cm–2 at a low voltage of 1.63 V. This novel nanostructure offers an efficient strategy for the development of nonprecious metal catalysts for water electrolysis

Bistable Organic Memory Device with Gold Nanoparticles Embedded in a Conducting Poly(<i>N</i>-vinylcarbazole) Colloids Hybrid

We report on the nonvolatile memory characteristics of a bistable organic memory (BOM) device with Au nanoparticles (NPs) embedded in a conducting poly(N-vinylcarbazole) (PVK) colloids hybrid layer deposited on flexible poly(ethyleneterephthalate) (PET) substrates. Transmission electron microscopy (TEM) images show the Au nanoparticles distributed isotropically around the surface of a PVK colloid. The average induced charge on Au nanoparticles, estimated using the C−V hysteresis curve, was large, as much as 5 holes/NP at a sweeping voltage of ±3 V. The maximum ON/OFF ratio of the current bistability in the BOM devices was as large as 1 × 105. The cycling endurance tests of the ON/OFF switching exhibited a high endurance of above 1.5 × 105 cycles, and a high ON/OFF ratio of ∼105 could be achieved consistently even after quite a long retention time of more than 1 × 106 s. To clarify the memory mechanism of the hole-mediated bistable organic memory device, the interactions between Au nanoparticles and poly(N-vinylcarbazole) colloids was studied by estimating the density of states and projected density of state calculations using density functional theory. Au atom interactions with a PVK unit decreased the band gap by 2.96 eV with the new induced gap states at 5.11 eV (HOMO, E0) and LUMO 4.30 eV and relaxed the HOMO level by 0.5 eV (E1). E1 at ∼6.2 eV is very close to the pristine HOMO, and thus the trapped hole in E1 could move to the HOMO of pristine PVK. From the experimental data and theoretical calculation, it was revealed that a low-conductivity state resulted from a hole trapping at Eo and E1 states and subsequent hole transportation through Fowler−Nordheim tunneling from E1 state to Au NPs and/or interface trap states leads to a high conductivity state