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Microscopic and spectroscopic investigation of MoS2 nanotubes/P3HT nanocomposites
Abstract
Blends of conductive polymer poly(3-hexylthiophene) (P3HT)
and size controlled MoS2 nanotubes (NTs) were prepared
focusing on procedures which do not alter the electrical and
optical properties of nanotubes, i.e., without surfactants. After
disassembly of as-grown hedgehog-like self-assemblies of the
MoS2 NTs, relatively homogeneous dispersions of the NTs in
P3HT were prepared. Thin films of these blends were prepared
via spin coating.The NTs were foundefficiently wetted by the
P3HT and nearly completely immersed in the films.Thesurface
structure was composed of pure P3HT lamellae. Photoluminescence
spectra taken on P3HT/MoS2 NT thin films
revealed a quenching effect, which depends on the concentration
of NTs. Results of optical microscopy and spectroscopy
investigations, scanning tunnelling microscopy and scanning
electron microscopy studies are shown
Nanodielectrics based on metal oxide nanoparticles (NPs) exhibit significant improvement in dielectric performances. This study was carried out to examine the dielectric properties of Poly vinyl chloride (PVC) with the introduction of zinc oxide (ZnO) NPs synthesized by simple sol gel method. The microstructure and surface morphology of the synthesized ZnO NPs were characterized by X-ray diffraction (XRD) and field emission scanning microscopy (FE-SEM). The formation of the wurtzite-type ZnO was verified from XRD with average crystallite sizes around 35 nm. PVC/ZnO-NPs nanocomposites were prepared by the solution-cast technique with two different procedures of NPs addition. The dielectric properties of PVC/ZnO nanocomposites were studied by measuring the DC dielectric strength with constant 1 kV/s ramp and the dielectric response as function of frequency from 500 Hz to 1 MHz. A significant enhancement in breakdown strength has been observed with addition of ZnO NPs. The breakdown strength values reached up to 45 % (for PVC/0.14 vol% ZnO) more than obtained for the pristine PVC sample. The effect of dispersibility of NPs due to the used procedures of addition on the DC dielectric strength was studied and confirmed by FE-SEM. The dependency of the real part of permittivity (ε
′) on filler concentration was observed and confirmed the obtained results from DC dielectric strength.
The results of MoS 2 /carbon nanocomposite hydrothermal synthesis is presented. The synthesized material was studied by XRD, TEM, EDS, SAXS, and porosimetry. The multilayer nanospheres with the average size 40–70 nm were obtained. The investigations confirm the formation of particles with double‐hierarchical structure where 2H‐MoS 2 layers alternate with carbon layers. Atomic ratio of MoS 2 :C is about 1:1 (at.%) and does not depend on the heat treatment.
The proposed formation mechanism of MoS 2 /C double‐hierarchical structure.
Optical-absorption spectroscopy of inorganic fullerenelike MoS 2 and WS 2 IF-MoS 2 and IF-WS 2 is reported in the range 400–800 nm, at temperatures between 4–300 K, and compared to the corresponding bulk 2H material. A systematic study of the effect of IF size and number of atomic layers on the optical properties shows that the semiconductivity of the layered material is preserved in the IF structures. Nevertheless, all IF with number of layers (n)6 exhibit a decrease in the A and B exciton energies. This redshift becomes larger as additional inner layers are formed, until a saturation value is reached (n10). We assign this redshift to the deformations, curvature, and discommensuration between adjacent atomic layers the structure must accommo-date in order to form an IF structure. An increase in the exciton energies is observed in IF consisting of a few sulfide layers (n5). This blueshift is attributed to a quantum confinement in the z direction. Band-structure calculations show that an expansion of 7% along the c axis leads to a convergence of the levels K 1 and K 4 , which is displayed in the absorption spectra of IF with 1 or 2 layers. S0163-18299803111-7
Transport properties are studied in the MoS2 selective morphology system. Fluctuation-induced
tunneling dominates in pressed pellets at temperatures up to about 90 K. At higher temperatures,
variable range hopping (VRH) transport mechanism prevails, with diverse dimensionality and
temperature range. In Mo6S2I8 nanowires, 1D VRH is shown, in MoS2 plate-like crystals 2D VRH
and in MoS2 coaxial nanotubes and MoS2 size-controlled nanotubes 3D VRH is found. Decreasing
of dimensionality is accompanied by an increase of the conductivity in MoS2 nanotubes, compared
to the plate-like crystals. Transport properties are discussed in relation to the nature and density of
defects, giving rise to changes in the electronic structure and influencing the scattering mechanism
governing the charge transport.
We describe a two-step synthesis of pure multiwall MoS2 nanotubes with a high degree of homogeneity in size. The Mo6S4I6 nanowires grown directly from elements under temperature gradient conditions in hedgehog-like assemblies were used as precursor
material. Transformation in argon-H2S/H2 mixture leads to the MoS2 nanotubes still grouped in hedgehog-like morphology. The described method enables a large-scale production of MoS2 nanotubes and their size control. X-ray diffraction, optical absorption and Raman spectroscopy, scanning electron microscopy
with wave dispersive analysis, and transmission electron microscopy were used to characterize the starting Mo6S4I6 nanowires and the MoS2 nanotubes. The unit cell parameters of the Mo6S4I6 phase are proposed. Blue shift in optical absorbance and metallic behavior of MoS2 nanotubes in two-probe measurement are explained by a high defect concentration.
Inorganic Nanotubes–MoS2–Conductivity–Defects–Mo6S4I6
We report on the first exfoliation of MoS2 coaxial nanotubes. The single-layer flakes, as the result of exfoliation, represent the transition metal dichalcogenides' analogue of graphene. They show a very low degree of restacking in comparison with exfoliation of MoS2 plate-like crystals. MoS2 monolayers were investigated by means of electron and atomic force microscopies, showing their structure, and ultraviolet-visible spectrometry, revealing quantum confinement as the consequence of the nanoscale size in the z-direction.
Demonstration of hybrid bulk heterojunction (BHJ) solar photovoltaic cell employing molybdenum disulfide (MoS2)/titanium dioxide (TiO2) nanocomposite (∼15 μm thick) and poly 3-hexylthiophene (P3HT) active layers is presented in this letter. The dominant Raman peak at 146 cm−1 confirmed TiO2, while two other peaks observed at 383 cm−1 and 407 cm−1 asserted MoS2 in the nanocomposite film. The demonstrated BHJ solar cell, having a stacked structure of indium tin oxide/TiO2/MoS2/P3HT/gold, exhibits a short circuit current density of 4.7 mA/cm2, open circuit voltage of 560 mV, and photoconversion efficiency of 1.3% under standard AM1.5 illumination condition. We observe that the quality of TiO2/MoS2/P3HT interfaces, as reflected in the dark saturation current in low- and medium-forward-bias region, plays a key role in impacting solar cell performance due to interfacial recombination effect.
Polymer solar cells based on regioregular poly(3-hexylthiophene) (P3HT) and ([6,6]-phenyl-C61-butyric acid methyl ester) (PCBM) were fabricated with two different architectures (normal and inverse). Normal cells using indium tin oxide (ITO) as anode and Al as cathode were fabricated on polyester foils and illuminated from substrate side. Inverse cells using Ti as cathode and ultrathin Au layer as anode were illuminated from the top side covered by a transparent Au contact. Both Au layer and PET/ITO show comparable transmission in the spectral range where P3HT absorbs. Inverse cells showed comparable device parameters to normal cell (open circuit voltage 550mV, short circuit current 6.25mA/cm2, fill factor 0.33 and white light power conversion efficiency 1.12%).
From band-structure calculations it is shown that MoSe2, MoS2, and WSe2 are indirect-gap semiconductors. The top of the valence band is at the Γ point and the bottom of the conduction band is along the line T of the hexagonal Brillouin zone, halfway between the points Γ and K. The A and B excitons correspond to the smallest direct gap at the K point. This assignment of the exciton peaks is shown to be consistent with the polarization dependence of their intensities, their effective masses, and the observed dependence of their splitting on the spin-orbit splittings of the constituent elements. The wave function at the top of the valence band is shown to be a metal-nonmetal antibonding state, which explains the observed high stability of these materials in photoelectrochemical cells against photocorrosion.
Chloroform is a general solvent for poly(3-hexylthiophene) (P3HT) active layers in field-effect transistors. However, its low boiling point and rapid evaporation limit the time for crystallization during the spin-coating process, and field-effect mobilities achieved for P3HT films spin-coated from chloroform are typically on the order of 0.01 cm2/(V s). Here we investigate a range of solvents with higher boiling points. We find that 1,2,4-trichlorobenzene with good solubility and a high boiling point significantly improves the field-effect mobilities up to 0.12 cm2/(V s) with on:off ratios of 106. By controlling the microstructure through the choice of solvent while keeping the molecular weight fixed, we observe a clear correlation between the field-effect mobility and the degree of microcrystalline order as measured by X-ray diffraction, as well as the strength of polaronic relaxation of charge carriers in the accumulation layer as measured by optical spectroscopy of field-induced charge.
By extrapolating to 0 and 30 eV experimental data in the energy range from approximately 1-1.3 to 14 eV a Kramers-Kronig analysis of the reflectivity spectrum at room temperature from the basal plane (E perpendicular to c) of single crystals of 2H-MoS2, 2H-MoSe2 and 2H-MoTe2 has been performed. The optical data so obtained have been discussed and interpreted in terms of the proposed band model for these materials.
We have shown recently that the use of high-aspect-ratio inorganic nanorods in conjunction with conjugated polymers is a route to obtaining efficient solar cells processed from solution. Here, we demonstrate that the use of binary solvent mixtures in which one of the components is a ligand for the nanocrystals is effective in controlling the dispersion of nanocrystals in a polymer. By varying the concentration of the solvent mixture, phase separation between the nanocrystal and polymer could be tuned from micrometer scale to nanometer scale. In addition, we can achieve nanocrystal surfaces that are free of surfactant through the use of weak binding ligands that can be removed through heating. When combined, the control of film morphology together with surfactant removal result in nanorod–polymer blend photovoltaic cells with a high external quantum efficiency of 59 % under 0.1 mW cm–2 illumination at 450 nm.
Scanning tunneling microscopy images of thin films of polythiophene cast on highly oriented pyrolytic graphite gives structural insight on both local (chain-packing and conformational) and mesoscopic (polycrystal structure) scales. It is revealed that 2D polycrystals form monodomains (20 nm parallel and perpendicular to the polymer backbone) (see Figure), partially interconnected by folded chains.
This Full Paper focuses on the preparation of single-walled or multi-walled carbon nanotube solutions with regioregular poly(3-hexylthiophene) (P3HT) and a fullerene derivative 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C-61 (PCBM) using a high dissolution and concentration method to exactly control the ratio of carbon nanotubes (CNTs) to the P3HT/PCBM mixture and disperse the CNTs homogeneously throughout the matrix. The CNT/P3HT/PCBM composites are deposed using a spin-coating technique and characterized by absorption and fluorescence spectroscopy and by atomic force microscopy to underline the structure and the charge transfer between the CNTs and P3HT. The performance of photovoltaic devices obtained using these composites as a photoactive layer mainly show an increase of the short circuit current and a slight decrease of the open circuit voltage which generally leads to an improvement of the solar cell performances to an optimum CNT percentage. The best results are obtained with a P3HT/PCBM (1: 1) mixture with 0.1 wt % multi-walled carbon nanotubes with an open circuit voltage (V-oc) of 0.57 V, a current density at the short-circuit (I-sc) of 9.3 mA cm(-2) and a fill factor of 38.4 %, which leads to a power conversion efficiency of 2.0 % (irradiance of 100 mW cm(-2) spectroscopically distributed following AM1.5).
The synthesis of hybrid bulk-heterojunction solar cells from zinc oxide (ZnO) nanocrystalline (nc) particles and a conjugated polymer, poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene ] (MDMO-PPV), was investigated. The cells exhibited an incident photon to current conversion efficiency up to 40%. The photoinduced charge separation of the MDMO-PPV:nc-ZnO mixture was analyzed using photoinduced absorption (PIA) spectroscopy. The kinetics of charge generation of the mixture was investigated with pump-probe spectroscopy by monitoring the intensity of low-density radical cation absorption band at 2200 nm (0.56 eV). Transmission electron microscopy (TEM) was used to analyze the morphology of the MDMO-PPV:ncZnO blend.
Three different industrially available multiwalled carbon nanotube (MWNT) materials were directly incorporated into polycarbonate by melt mixing using a small-scale compounder. Despite of similar aspect ratios the electrical percolation behaviour was different. TEM investigations reveal significant differences in the nanotube dispersion which can be attributed to different dispersability of the raw MWNT materials. It is shown that the investigation of the sedimentation behaviour of aqueous MWNT dispersions is a simple method to estimate the nanotube dispersability.The relationships between melt processing conditions and MWNT dispersion and distribution were studied on polycarbonate samples containing 0.875 wt% MWNT prepared by masterbatch dilution. During melt mixing only high shear forces can provide suitable MWNT dispersion because firstly the MWNT disentanglement is facilitated and secondly secondary agglomeration is prevented. At low shear agglomeration of formerly well dispersed MWNT could be observed. During hot pressing the network or MWNT arrangement and the resulting electrical conductivity can be manipulated by the processing conditions like melt temperature and pressing speed. A certain nanotube agglomeration can enhance the development of an electrical percolated network as shown by dielectric spectroscopy.
High-quality CdSe quantum dots (Qdots) were produced using the wet chemical synthetic method. These size-controlled CdSe Qdots were characterized by UV–vis absorption spectroscopy, photoluminescence (PL) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The XRD patterns and TEM images showed that the CdSe Qdots had a wurtzite crystalline structure. Bandgap-tuning CdSe Qdots were used to fabricate solar cells. We report the extensive experimental results of the CdSe Qdots–conjugated polymer composites used for the fabrication of the hybrid solar cells, by blending them between CdSe Qdots–poly (3-hexylthiophene) (P3HT) and CdSe Qdots/[poly(1-methoxy-4-(2-ethylhexyloxy-2,5-phenylenevinylene))] (MEH-PPV) polymer.
Photovoltaic devices based on regioregular poly(3-hexylthiophene) (P3HT) and ([6,6]-phenyl-C61-butyric acid methyl ester) (PCBM) were fabricated and characterized using 5×5 cm ITO polyester foils with an active cell area of 0.5×0.5 cm2. The HOMO/LUMO of P3HT and PCBM were estimated from cyclic voltammetry data. The complete quenching of photoluminescence of P3HT after mixing with PCBM indicates an effective charge transfer from P3HT to PCBM. The absorption spectrum of a blend (1:3 wt%) of both components shows that there is no ground state doping. Following device parameters without any special postproduction treatment were determined: VOC=600 mV, ISC=6.61 mA/cm2, FF=0.39 and ηAM1.5 (PIN:100 mW/cm2)=1.54%.
Bulk heterojunction photovoltaic devices based on blends of a conjugated polymer poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) as electron donor and crystalline ZnO nanoparticles (nc-ZnO) as electron acceptor have been studied. Composite nc-ZnO:MDMO-PPV films were cast from a common solvent mixture. Time-resolved pump-probe spectroscopy revealed that a photoinduced electron transfer from MDMO-PPV to nc-ZnO occurs in these blends on a sub-picosecond time scale and produces a long-lived (milliseconds) charge-separated state. The photovoltaic effect in devices, made by sandwiching the active nc-ZnO:MDMO-PPV layer between charge-selective electrodes, has been studied as a function of the ZnO concentration and the thickness of the layer. We also investigated changing the degree and type of mixing of the two components through the use of a surfactant for ZnO and by altering the size and shape of the nc-ZnO particles. Optimized devices have an estimated AM1.5 performance of 1.6% with incident photon to current conversion efficiencies up to 50%. Photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy have been used to gain insight in the morphology of these blends.
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