Interface‐tailored and nanoengineered polymeric materials for (opto)electronic devices

For plastic (opto)electronic devices such as light‐emitting diodes (LEDs), photovoltaic (PV) cells and field‐effect transistors (FETs), the processes of charge (hole/electron) injection, charge transport, charge recombination (exciton formation), charge separation (exciton diffusion and dissociation) and charge collection are critical to enhance their performance. Most of these processes are relevant to nanoscale and interfacial phenomena. In this review, we highlight the state‐of‐the‐art developments of interface‐tailored and nanoengineered polymeric materials to optimize the performance of (opto)electronic devices. These include (1) interfacial engineering of anode and cathode for polymer LEDs; (2) nanoengineered (C₆₀ and inorganic semiconductor nanoparticles) π‐conjugated polymeric materials for PV cells; and (3) polymer and monolayer dielectrics/interfaces for FETs and light‐emitting and nano‐FETs. Copyright © 2009 Society of Chemical Industry     

Metal nanocrystals as charge storage nodes for nonvolatile memory devices

The memory effects of the metal nanocrystals were found to be more pronounced than those of the semiconductor nanocrystals. Various metal nanocrystals as charge storage nodes are reviewed. The memory effects have strong relationship with the work function, and the work function can be modulated by changing the metal species. By tunneling dielectrics engineering, the optimum I_{G Write/Erase}/I_{G Retention} ratio can be obtained.     

α-Fe2O3/Pt hybrid nanorings and their enhanced photocatalytic activities

In this paper, α-Fe₂O₃ nanorings were synthesized and used as a support to further synthesize hybrid Pt/α-Fe₂O₃ nanorings. Transmission electron microscopy (TEM) analyses clearly suggest that Pt nanoparticles are deposited on the surface of α-Fe₂O₃ nanorings in a well-dispersed state. This metal–semiconductor hybrid system is expected to show high photocatalytic activity towards the degradation of environmental pollutants. Because of the Schottky contact between semiconductor and metal particles, the holes prefer to localize in energetically lower semiconductor, whereas the electrons can move through the heterointerface. This interfacial charge transfer and separation facilitate the redox reactions, results in a high photocatalytic activity. In our case, the as obtained α-Fe₂O₃/Pt hybrids exhibit enhanced photocatalytic performance with a degradation rate of 86.7% for methyl orange, which is much higher than that of pure α-Fe₂O₃ (33.2%).     

Effect of 2D MoS2 and Graphene interfaces with CoSb3 nanoparticles in enhancing thermoelectric properties of 2D MoS2-CoSb3 and Graphene-CoSb3 nanocomposites

The approach of incorporating a secondary phase in the bulk thermoelectric (TE) material has proved to be beneficial for enhancing the thermoelectric performance. We have investigated the effect of the presence of two-dimensional (2D) materials (MoS₂ or graphene) on the structural, electrical, and thermoelectric properties of CoSb₃ nanocomposite, in which CoSb₃ nanoparticles of sizes 20–50?nm are uniformly anchored on the surface of 2D-sheets of MoS₂ or graphene. The presence of 2D nanosheets enhances TE power factor and figure of merit (ZT). Inclusion of graphene in CoSb₃ causes large enhancement in power factor as a result of significantly high electrical conductivity and appreciable Seebeck coefficient. 2D graphene seems to work by providing extra carrier conduction channels along with a low interfacial potential barrier for charge transport. Homogeneously dispersed 2D-sheets of MoS₂ in CoSb₃ seem to cause interfacial modulation of charge carrier effective mass assisted by relatively larger interfacial barrier to result in significantly larger Seebeck coefficient and highly suppressed phonon conductivity, much more than graphene. The ZT value in both nanocomposites gets significantly enhanced in the entire studied temperature range of 300–700?K, the gain increasing with temperature over the CoSb₃. Whereas CoSb₃/graphene nanocomposites exhibit unusually high ZT at higher temperatures (550–700?K), the CoSb₃/2D-MoS₂ nanocomposites exhibit better performance (over graphene) in near room temperature range. The present study provides a possible strategy to enhance the conversion efficiency of various TE materials and has significant potential for waste heat recovery applications in various temperature ranges.     

The Role of Charge Localization in Current-Driven Dynamics

We explore the role of charge localization in current-triggered, resonance-mediated, dynamical events in molecular junctions. To that end we use a simple model for a molecular rattle, a Li⁺C₉H⁻₉ zwitterion attached between two metal clusters. By varying the size of the metal clusters we systematically vary the degree of delocalization of the electronic orbitals underlying the resonant current, and thus can draw general conclusions regarding the effect of delocalization on dynamical processes induced by resonance inelastic current in molecular electronics. In the small cluster limit, we find interesting quantum dynamics in the nuclear subspace, corresponding to coherent tunneling of the wave packet through the barrier of an asymmetric double-well potential. These dynamics are rapidly damped with increasing charge delocalization in extended systems.     

Reforming of Oxygenates for H2 Production on 3d/Pt(111) Bimetallic Surfaces

We present results for H₂ production by reforming of oxygenates on Pt-based bimetallic surfaces using temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS) and density functional theory (DFT) calculations. Methanol, ethanol, ethylene glycol, and glycerol were employed as probe molecules. The formation of bimetallic surfaces with a 3d metal monolayer on Pt(111), designated 3d-Pt-Pt(111), led to increased H₂ production as compared to the parent metal surfaces. The combined experimental and DFT results suggest that the reforming activity tracks the energy of the surface d-band center of various monometallic and bimetallic surfaces.     

Metal Separations Using Emulsion Liquid Membranes

Abstract

Emulsion membrane systems consisting of an aqueous metal salt source phase, a toluene membrane containing the macrocyclic ligand dicyclohexano-18-crown-6 (DC18C6) (0.02 M) and the surfactant sorbitan monooleate (3% v/v), and an aqueous 0.05 M Li₄P₂O₇ receiving phase were studied with respect to the disappearance of metal from the source phase as a function of time. The salts Pb(NO₃)₂, Sr(NO₃)₂, TINO₃, and LiNO₃ were studied both singly and in mixtures of Pb(NO₃)₂ with each of the other salts. In all mixtures studied, Pb²⁺ was transported first, followed by the second cation (except Li⁺ which was not transported). An excess of a second salt with a common anion enhanced the transport of Pb²⁺. Modeling of these systems was discussed. Source phases containing basic (pH 11) K[Al(OH)₄] solutions were studied using the same membrane and a 0.15 M H₃PO₄ receiving phase. K⁺ and Al(III) (as aluminate anion) were both found to transport in this system, but no transport of Al(III) and little transport of K⁺ were detected when DC18C6 was absent.     

In situ synthesis of integrated dodecahedron NiO/NiCo2O4 coupled with N-doped porous hollow carbon capsule for high-performance supercapacitors

NiO and NiCo₂O₄ exhibit excellent synergistic effects and broad application prospects in electrochemical applications. However, the apparent interfacial instability between NiO and NiCo₂O₄ limits ion transport kinetics, charge/ion transfer, and electrochemical stability. In response, we developed and designed an integrated dodecahedron NiO/NiCo₂O₄ by a facile in-situ calcination method. Moreover, by utilizing the porous hollow structure of nitrogen-doped carbon capsules (N-Cc) as a conductive network, the N-Cc_x@NiO/NiCo₂O₄ heterostructures with stable interface structure, excellent electrolyte adsorption, and electron transfer pathways were carefully designed. The N-Cc_1.0@NiO/NiCo₂O₄ heterostructures are found to deliver an outstanding specific capacitance of 658.8 F g⁻¹, and a high energy density of 101.40 Wh kg⁻¹ at a power density of 775.03 W kg⁻¹, along with capacitance retention of more than 93.5% after 8000 cycles. Based on the DFT calculations and electrochemical experimental results, this work provides an effective in situ route for the construction of high-performance metal oxide heterostructure electrode materials for new energy storage devices.     

Electron transfer/transport at metal-molecule interfaces probed by femtosecond time-resolved two-photon photoemission: Heptane and fullerene on Au(111)

Electron transfer and transport at metal-molecule interfaces has been a central problem in a number of disciplines. There is renewed interest in this problem from the rapidly growing field of molecule-based electronics, where the so-called “contact problem” is usually probed in transport measurements. Here, we probe interfacial electron transfer/transport using time-resolved two-photon photo-emission spectroscopy. Such an approach allows us to quantitatively measure the energetic alignment of occupied and unoccupied molecular orbitals to the metal Fermi level, the electronic coupling strength (or spectral density) between the molecular orbital and the metal substrate, and electronic-nuclear coupling leading to dynamic localization. These three types of information are in fact key ingredients in quantitative theories for electron transfer and transport at molecule-metal surfaces. Recent examples from our laboratory are used to illustrate the point. They include femtosecond dynamics of image potential resonances on heptane-covered Au(111) and exciton relaxation dynamics in C₆₀ epitaxial films on Au(111). On heptane/Au(111), the lifetime of the n = 1 image potential resonance increases from 52 fs at one monolayer (ML) coverage to 700 fs at 2 ML and 2.8 ps at 3 ML. On C₆₀/Au(111), the Frenkel exciton (involving the LUMO+1 level) couples over long distance to the metal substrate, with lifetime of 350 fs at 30 ML to 80 fs at 2 ML coverage. The film-thickness-dependent decay dynamics in both cases can be attributed to interfacial electron transfer. Fitting the distance-dependent electron transfer rates to a simple exponential function gives the characteristic distance parameter of ß = 0.6 ± 0.1 and 0.023 ± 0.005 Å⁻¹ for heptane- and C₆₀-covered Au(111), respectively. The difference in the ß parameters illustrates the essential role of the electronic structure of molecules in mediating interfacial electron transfer/transport.     

Improving photoelectric performance of MoS2 photoelectrodes by annealing

Semiconductor photoelectrochemistry (PEC) technology has attracted wide interest as it not only reveals the photoelectric properties of advanced materials, but also promotes energy conversion from the sunlight based on these materials. Herein, we investigated photoelectric properties of annealed and unannealed liquid phase exfoliated few-layer molybdenum disulfide (MoS₂) photoelectrodes by PEC measurement. The linear sweep voltammograms and photoelectric response I-t curves demonstrate enhanced performance of MoS₂ films after annealing. Nyquist impedance and Bode phase plots demonstrate a higher charge transfer property and a longer charge lifetime after annealing. The enhancement due to annealing could come from the increase of the crystalline and compact density of MoS₂ nano-sheets, which is proved by X-ray diffraction and Raman spectroscopy in line with the absorption spectroscopy. Our results pave the way for the improvement of two-dimensional material based photoanodes for high performance of PEC applications by a simple method.     

Materials with honeycomb structures for gate dielectrics in two-dimensional Field Effect Transistors – An ab initio study

The size of the Field Effect Transistor (FET) has been decreasing exponentially each year to keep up with the demands of the microelectronic industry. However, this scaling is now reaching its limits due to the performance of the materials currently used in the FET. Specifically, the leakage through silicon dioxide, the current gate dielectric, is beginning to inhibit the FET's performance. Recently, attention has been turning towards two-dimensional materials for electronic devices. However, insulating is especially challenging for two-dimensional materials due to the short diffusion length for charge carriers. We consider several materials that have been shown to have a stable two-dimensional honeycomb structure. In order to characterize these materials, we use Density Functional Theory (DFT) to calculate key properties that determine performance, including the dielectric function, band gap, and effective mass. We check the compatibility of these materials with Molybdenum Disulfide (MoS₂), a two-dimensional material that has emerged as a promising candidate for the semiconducting channel of a FET. We have identified several promising candidates, including hBN, which is the common choice as a dielectric material, and LiF and BeO, which are materials that have not been used before as gate dielectrics. We anticipate that this study will contribute to advancing the development of transistors that are made completely of two-dimensional materials.     

Microwave-assisted one-pot synthesis of SnC2O4/graphene composite anode material for lithium-ion batteries

Currently, SnC₂O₄ is considered as one of the most promising anode materials for high-energy lithium-ion batteries (LIBs) because its charge capacity is higher than that of metal oxides. Herein, a facile microwave-assisted solvothermal method was employed to obtain SnC₂O₄/GO composites within only 30?min, which is time-efficient. The amount of SnC₂O₄ was increased to 95.3?wt% to improve the capacity of the composite. Pure SnC₂O₄ with a high specific surface area of 19.6?m² g^?1 without any other tin compound was used for fabrication. The SnC₂O₄/GO composite exhibited excellent electrochemical performance, with reversible discharge/charge capacity of 657/659?mA?h?g^?1 after 100 cycles at 0.2?A?g^?1. Furthermore, at high current densities of 1.0 and 2.0?A?g^?1, the SnC₂O₄/GO composite anode exhibited high reversible discharge/charge capacities of 553/552 and 418/414?mA?h?g^?1, respectively, after 200 cycles at room temperature. These improvements were likely obtained because SnC₂O₄ was well composited with graphene, which not only offered rapid electron transfer but also released the tension produced by the volumetric effect during repeated lithiation/delithiation. Cyclic voltammetry (CV) was also performed to further study the electrochemical reactions of SnC₂O₄/GO. The facile microwave-assisted solvothermal method used herein is considered as a highly efficient method to fabricate metal oxalate/graphene composites for use as anode materials in LIBs.     

Structural and electronic properties of germanene/MoS2 monolayer and silicene/MoS2 monolayer superlattices

Superlattice provides a new approach to enrich the class of materials with novel properties. Here, we report the structural and electronic properties of superlattices made with alternate stacking of two-dimensional hexagonal germanene (or silicene) and a MoS₂ monolayer using the first principles approach. The results are compared with those of graphene/MoS₂ superlattice. The distortions of the geometry of germanene, silicene, and MoS₂ layers due to the formation of the superlattices are all relatively small, resulting from the relatively weak interactions between the stacking layers. Our results show that both the germanene/MoS₂ and silicene/MoS₂ superlattices are manifestly metallic, with the linear bands around the Dirac points of the pristine germanene and silicene seem to be preserved. However, small band gaps are opened up at the Dirac points for both the superlattices due to the symmetry breaking in the germanene and silicene layers caused by the introduction of the MoS₂ sheets. Moreover, charge transfer happened mainly within the germanene (or silicene) and the MoS₂ layers (intra-layer transfer), as well as some part of the intermediate regions between the germanene (or silicene) and the MoS₂ layers (inter-layer transfer), suggesting more than just the van der Waals interactions between the stacking sheets in the superlattices.     

Prediction of ultra-high ON/OFF ratio nanoelectromechanical switching from covalently-bound C60 chains

Applying a first-principles computational approach, we have systematically analyzed the effects of [2+2] cycloaddition oligomerization of fullerene C₆₀ chains on their junction electronic and charge transport properties. For hypothetical infinite C₆₀ chains, we first establish that the polymerization can in principle increase conductance by several orders of magnitude due to the strong orbital hybridizations and band formation. On the other hand, our simulations of the constant-height scanning tunneling microscope (STM) configuration shows that, in agreement with the recent experimental conclusion, the junction electronic structure and device characteristics are virtually unaffected by the C₆₀ chain oligomerization. We further predict that the switching characteristics including even the ON/OFF-state assignment will sensitively depend on the substrate metal species due to the Fermi-level pinning at the substrate-side contact and the subsequent energy level bending toward the STM tip-side contact. We finally demonstrate that a force-feedbacked nanoelectromechanical approach in which both of the C₆₀–electrode distances are kept at short distances before and after switching operations can achieve a metal-independent and significantly improved switching performance due to the Fermi-level pinning in both contacts and the large intrinsic conductance switching capacity of the C₆₀ chain oligomerization.     

Tuning the optical band gap of monolayer WSe2 in ferroelectric field-effect transistors

Ferroelectric field-effect transistors (FeFETs) hold great promises for application in modern semiconducting industry as memory or logic devices. Intensive studies have been done on improving the performance of FeFET, which mostly rely on the electrical polarization properties of ferroelectric gate. However, the influence of the piezoelectric effect of ferroelectric gate on the semiconducting channel has not been explored to a large extent. In this work, we use gate voltage-dependent photoluminescence (PL) spectroscopy to study the piezoelectric effect of P(VDF-TrFE) gate on monolayer WSe₂ channel in FeFETs. The PL peak of monolayer WSe₂ undergoes a blue shift or red shift when gate voltage is applied through the ferroelectric layer, while no peak shift is observed when gate voltage is applied through the SiO₂ layer. Raman spectroscopy measurements confirm the presence of in-plane strain in monolayer WSe₂. Qualitative theoretical analysis and discussion suggest that the piezoelectricity of monolayer WSe₂ and the P(VDF-TrFE) deformation caused by inverse piezoelectric effect are the main factors that cause strain in monolayer WSe₂. These results demonstrate that the band structure of two-dimensional channel, especially monolayer two-dimensional materials, can be modulated in FeFETs, which should not be neglected under large gate voltage in FeFETs.     

A spin-valve-system in ferromagnetic semiconductor Sr2FeMoO6/SrMoO4

In this study, we designed a novel magnetic semiconductor Sr₂FeMoO₆/SrMoO₄ with a sharp metal-insulator transition based on the spin-glass freezing and antiferromagnetic transition behavior, which has been confirmed by various magnetic and transport measurements. To investigate the charge transport mechanisms of the sample, the electrical resistivities have been measured over a wide range of temperatures from 400?K down to liquid helium temperatures under different magnetic fields. The typical semiconductor behavior of the sample is observed. The relatively high conduction behavior is blocked below 80?K, and thereafter into the insulator state. The Vogel-Fulcher and the Mott variable-range-hopping (VRH) law are also used to study its conduction features and the relationship between magnetic and electrical properties. The spin-glass like component around the Sr₂FeMoO₆ crystal is initially observed by TEM and this provides a robust evidence able to verify the results from magnetic measurements. The novel features of the metal-insulator transition shielding external magnetic effects in highly disordered Sr₂FeMoO₆ double perovskite is dominantly controlled by a new spin-valve-system present in the spin-glass layers. Our insights into the role of the B-site disorder, spin-glass layer around the soft ferromagnetic grains, grain boundaries, and non-magnetic second phase effects demonstrate how the static and dynamic properties of magnetic materials might be tuned by designing the composition with a high B-site cation disordered matrix ferromagnetic material.     

2D/2D SnO2 nanosheets/Ti3C2Tx MXene nanocomposites for detection of triethylamine at low temperature

Highly active two-dimensional (2D) nanocomposites have been widely concerned in the field of gas sensors because of their unique advantages and synergistic effects. 2D/2D SnO₂ nanosheets/Ti₃C₂T_x MXene nanocomposites were synthesized by using layered Ti₃C₂T_x MXene and uniform SnO₂ nanosheets by hydrothermal method. Characterization results show that the SnO₂ nanosheets are well dispersed and vertically anchored on the layered Ti₃C₂T_x MXene surface, forming heterogeneous interfaces. Based on the gas-adsorption capabilities and synergistic effects of electronic properties, SnO₂ nanosheets/Ti₃C₂T_x MXene nanocomposites show high triethylamine (TEA) gas-sensing performance at low temperature (140 °C). The sensor responses of the nanocomposites and pure SnO₂ nanosheets to 50 ppm of TEA are 33.9 and 3.4, respectively. An enhancement mechanism for SnO₂ nanosheets/Ti₃C₂T_x MXene nanocomposites is proposed for highly sensitive and selective detection of TEA at low temperature. The combination strategy of two-dimensional metal oxide semiconductor and multilayer MXene provides a new way for the development of cryogenic gas sensors in the future.     

Metal–organic frameworks-derived TiO2 for photocatalytic degradation of tetracycline hydrochloride

This work will change the common understanding that C doping of MIL-125(Ti)-derived TiO₂ is a key factor in improving its photocatalytic performance, and it can also help to understand the internal relationship between the structure and performance of photocatalytic materials deeply. It provides a simple synthesis method for the wider application of TiO₂ in the field of photocatalysis. Compared with previous studies, this article uses the titanium-based metal-organic framework MIL-125(Ti) to prepare the semiconductor photocatalyst M-TiO₂ by calcination in the air at a lower temperature and shorter time. After analyzing the M-TiO₂ prepared in the experiment, the results can be received that there is no obvious agglomeration and the morphology is almost unchanged, as the frame structure does not collapse at the same time. As a result, the advantages of the large specific surface area and porousness of metal–organic frameworks (MOF) as precursor derivatives are preserved. As for the changes in the micro-morphology, pore structure, and specific surface area of M-TiO₂ compared with the precursor, they are investigated seriatim. The results show that, compared with commercial TiO₂-P25, the performance of M-TiO₂ photocatalytic degradation of tetracycline hydrochloride is 5.7 times that of the precursor metal-organic framework MIL-125(Ti) and 2.2 times that of P25, and has good cycle stability.     

ZnCo2O4/CNT composite for efficient supercapacitor electrodes

Due to their combination of enhanced electrical conductivity and high-performance electron and ion transport channels, binary metal oxides with well-morphological optimized electrode materials have been attracted the greatest research attention for high-performance supercapacitor applications. An easy co-precipitation method is used to synthesize ZnCo₂O₄ nanoparticles using NaOH and Urea as precipitation agents. To facilitate electrical conductivity, suitable carbon material such as carbon nanotube (CNT) has been added to make a composite material. The three-electrode system was preferred for estimating specific capacitance of prepared material and optimally efficient ZnCo₂O₄/CNT electrode delivered a moderate 888 F/g capacitance at 1 A/g in 3 M KOH and after 5000 charge discharge cycles 94.72% of cycling stability retained at 5 A/g. This paper presents a little price and simple procedure for preparation of ZnCo₂O₄/CNT electrode that promotes creative sprit for energy storage applications.     

Coloring ionic trapping states in WO3 and Nb2O5 electrochromic materials

Complex electro-optical analysis is a very useful approach to separate different kinetic processes that occur during ionic insertion reactions in electrochromic oxide materials. In this paper, we use this type of combined technique to investigate ionic and optical changes in different oxide host systems, i.e., in two oxide hosts, specifically WO₃ and Nb₂O₅. A comparison of their electro-optical responses revealed the presence of an ionic trapping contribution to the kinetics of the coloring sites, which was named here as coloring ionic trapping state. As expected, this coloring trapping process is slower in Nb₂O₅ since the reduction potential of Nb₂O₅ is more negative (more energy is needed for a higher degree of coloration). A phenomenological solid-state model that encompasses homogeneous charge transfer and valence trapping was proposed to explain the coloring ionic trapping process. Basically the model is able to explain how ionic dynamics at low frequency region, i.e., the slower kinetic step, controls the coloring kinetics, i.e., how it is capable to regulate the coloring rates.Optical transient analyses demonstrated the possibility of the presence of more than one coloring ionic trap, indicating the complexity of the processes involved in coloration phenomenon in metal oxide host systems.