Preparation of transparent and conductive cellulose nanocrystals/graphene nanoplatelets films

The manufacture of emerging products such as photovoltaic devices requires combinations of various novel materials to be leveraged into successful, scalable approach. In order to develop new electronic devices, it is necessary to find innovative solutions to the eco-sustainability problem of materials as substrates for circuits. We report on the demonstration of polymer solar cells fabricated on optically transparent and conductive graphene nanoplatelets (GNPs)–cellulose nanocrystals (CNC) film. The solar cells fabricated on the GNPs/CNC films display good rectification in the dark. Such GNPs–CNC functional films are expected to be attractive for eco-friendly electronics.     

Exciton polarizability in semiconductor nanocrystals

The response of charge to externally applied electric fields is an important basic property of any material system, as well as one critical for many applications. Here, we examine the behaviour and dynamics of charges fully confined on the nanometre length scale. This is accomplished using CdSe nanocrystals of controlled radius (1-2.5 nm) as prototype quantum systems. Individual electron-hole pairs are created at room temperature within these structures by photoexcitation and are probed by terahertz (THz) electromagnetic pulses. The electronic response is found to be instantaneous even for THz frequencies, in contrast to the behaviour reported in related measurements for larger nanocrystals and nanocrystal assemblies. The measured polarizability of an electron-hole pair (exciton) amounts to approximately 10(4) A(3) and scales approximately as the fourth power of the nanocrystal radius. This size dependence and the instantaneous response reflect the presence of well-separated electronic energy levels induced in the system by strong quantum-confinement effects.     

Cavity QED with semiconductor nanocrystals

We report on a strongly coupled cavity quantum electrodynamic (CQED) system consisting of a CdSe nanocrystal coupled to a single photon mode of a polymer microsphere. The strong exciton-photon coupling is manifested by the observation of a cavity mode splitting variant Planck's over 2piOmega(exp) between 30 und 45 microeV and photon lifetime measurements of the coupled exciton-photon state. The single photon mode is isolated by lifting the mode degeneracy in a slightly deformed microsphere cavity and addressing it by high-resolution imaging spectroscopy. This cavity mode is coupled to a localized exciton of an anisotropically shaped CdSe nanocrystal that emits highly polarized light in resonance to the cavity mode and that was placed in the maximum electromagnetic field close to the microsphere surface. The exciton confined in the CdSe nanorod exhibits an optical transition dipole moment much larger than that of atoms, the standard system for CQED experiments, and a low-temperature homogeneous line width much narrower than the high-Q cavity mode width. The observation of strong coupling in a colloidal semiconductor nanocrystal-cavity system opens the way to study fundamental quantum-optics phenomena and to implement quantum information processing concepts that work in the visible spectral range and are based on solid-state nanomaterials.     

Towards electrically conductive, self-healing materials.

A novel class of organometallic polymers comprising N-heterocyclic carbenes and transition metals was shown to have potential as an electrically conductive, self-healing material. These polymers were found to exhibit conductivities of the order of 10(-3) S cm-1 and showed structurally dynamic characteristics in the solid-state. Thin films of these materials were cast onto silicon wafers, then scored and imaged using a scanning electron microscopy (SEM). The scored films were subsequently healed via thermal treatment, which enabled the material to flow via a unique depolymerization process, as determined by SEM and surface profilometry. A method for incorporating these features into a device that exhibits electrically driven, self-healing functions is proposed.     

Transparent,electrically conductive,and flexible films made from multiwalled carbon nanotube/epoxy composites

Optically transparent, conductive, and mechanically flexible epoxy thin films are produced in the present study. Two types of multiwalled carbon nanotubes (MWCNTs) with different aspect ratios are dispersed in epoxy resin through an ultrasonication process. The MWCNT content is varied during the preparation of the thin films. The light transmittance and electrical conductivity of the thin films are characterized. Results show that composites containing MWCNTs with a lower aspect ratio exhibit enhanced electrical conductivity compared to those with a higher aspect ratio. A sheet resistance as low as 100 Ω/sq with nearly 60% optical transparency in 550 nm is achieved with the addition of MWCNTs in epoxy. In summary, transparent, conductive, and flexible MWCNT/epoxy thin films are successfully produced, and the properties of such films are governed by the aspect ratio and content of MWCNTs.     

SiC-based cermet with electrically conductive grain boundaries

The present paper deals with the characterization of structural and electrical properties of SiC-based cermets prepared by in situ reaction. The surface structure and electrical conductivity of the samples was investigated by the standard four-point probe method, SEM, TEM, AFM, and STM techniques. It was found that the electrical conductivity of the SiC-based ceramic–metal composites increases with increased fraction of metallic phases. Interestingly, samples containing app. 12 vol.% of non-percolated (isolated) metallic phases exhibit up to 2 orders of magnitude better electrical conductivity compared with the base-line liquid phase sintered SiC (LPS SiC). This effect results from doping of the SiC grains by diffusion of metallic components as well as from chemical modification of the grain boundary phases due to the reaction of sintering aids and metallic particles at high sintering temperatures. Absorbed current measurements using SEM, as well as AFM in spreading resistance and STM in tunneling mode were used for visualization of electrical pathways.     

Modifying the surface of electrically conductive porous alumina

Electrically conductive porous alumina has been paid great interest due to its excellent properties determined by its unique porous structure and electrical conductivity. In this paper, acid-treatment has been employed to activate the inner-connected graphite pathways along alumina grains, which is responsible for its high conductivity and erosion resistivity. By this method, various oxygen-functional groups can be grafted onto graphite defects, which could be used for a number of potential applications. Field-emission scanning electron microscope (FE-SEM) observation, X-ray diffraction (XRD) analysis and Raman spectroscopy evaluation were employed to characterize the structure and component changes after acid-treatment. Electrochemical measurements carried out in 0.5 M H₂SO₄ electrolyte indicated its possible application as electrode material.     

Pulsed electric current sintering of electrically conductive ceramics

The processing of yttria-stabilised zirconia (Y-ZrO₂)-based ceramic nanocomposites by means of pulsed electric current sintering (PECS) is described. A nanometer-sized electrically conductive secondary TiCN phase was added to the insulating zirconia matrix in order to make the composite electrically conductive. The paper focuses on the importance of processing conditions and highlights the benefits of the PECS method as compared to more traditional hot pressing. The mechanical and microstructural properties of the ZrO₂–TiCN composites have been determined, and the benefits of using an electrical current to densify these composites were explained in terms of the evolution of the electrical properties of the densifying powder compact.     

Nanopatterned carbon films with engineered morphology by direct carbonization of UV-stabilized block copolymer films

Nanopatterned thin carbon films were prepared by direct and expeditious carbonization of the block copolymer polystyrene- block-poly(2-vinylpyridine) (PS- b-P2VP) without the necessity of slow heating to the process temperature and of addition of further carbon precursors. Carbonaceous films having an ordered "dots-on-film" surface topology were obtained from reverse micelle monolayers. The regular nanoporous morphology of PS- b-P2VP films obtained by subjecting reverse micelle monolayers to swelling-induced surface reconstruction could likewise be transferred to carbon films thus characterized by ordered nanopit arrays. Stabilization of PS- b-P2VP by UV irradiation and the concurrent carbonization of both blocks were key to the conservation of the film topography. The approach reported here may enable the realization of a broad range of nanoscaled architectures for carbonaceous materials using a block copolymer ideally suited as a template because of the pronounced repulsion between its blocks and its capability to form highly ordered microdomain structures.     

Photothermal absorption spectroscopy of individual semiconductor nanocrystals

Photothermal heterodyne detection is used to record the first room-temperature absorption spectra of single CdSe/ZnS semiconductor nanocrystals. These spectra are recorded in the high cw excitation regime, and the observed bands are assigned to transitions involving biexciton and trion states. Comparison with the single nanocrystals photoluminescence spectra leads to the measurement of spectral Stokes shifts free from ensemble averaging.     

Magnetic properties of doped II-VI semiconductor nanocrystals

We investigate the magnetic properties of two typical II-VI semiconducting nanocrystals, namely, ZnS and ZnO, doped with various concentrations of a transition metal ion, Mn. A wide range of dopant concentrations was explored without changing the size of the nanocrystal, thereby allowing us to study the concentration dependence of various properties independent of any size variation of the host. However, only small doping concentrations could be realized in each case. All the studies were thus carried out with ZnS and ZnO nanocrystals with their respective average sizes fixed at about 1.6 and 4.7 nm. We show that the magnetic properties of such doped systems, remaining paramagnetic down to the lowest temperature (<3 K), can provide important and accurate information concerning the dopant level in such samples.     

Electrical and optical properties of semiconductor nanocrystals

Application of semiconductor nanocrystals in optoelectronic devices requires an understanding not only of their emission and absorption properties, but also of the processes of charge injection and transport in nanocrystalline films. Here, we present measurements of the electrical properties of nanocrystalline films and of blends of nanocrystals with conjugated polymers. We also describe the attachment of nanocrystals to semiconductor surfaces, and we investigate the emission of nanocrystalline films in microcavity structures and at high excitation intensities.     

Colloidal synthesis of ultrathin two-dimensional semiconductor nanocrystals

2D semiconductor quantum wells have been recognized as potential candidates for various quantum devices. In quantum wells, electrons and holes are spatially confined within a finite thickness and freely move in 2D space. Much effort has focused on shape control of colloidal semiconductor nanocrystals(NCs), and synthesis of 2D colloidal NCs has been achieved very recently. Here, recent advances in colloidal synthesis of uniform and ultrathin 2D CdSeNCs are highlighted. Structural and optical property characterization of these quantum-sized 2D CdSe NCs is discussed. Additionally, 2D CdSe NCs doped with Mn 2+ ions for dilute magnetic semiconductors (DMS) are presented.These 2D CdSe-based NCs can be used as model systems for studying quantum-well structures.     

Trapped-dopant model of doping in semiconductor nanocrystals

We propose a framework for describing the impurity doping of semiconductor colloidal nanocrystals. The model is applicable when diffusion of impurities through the nanocrystal is sufficiently small that it can be neglected. In this regime, the incorporation of impurities requires that they stably adsorb on the nanocrystal surface before being overgrown. This adsorption may be preempted by surfactants in the growth solution. We analyze numerically this competition for the case of Mn doping of CdSe nanocrystals. Our model is consistent with recent experiments and offers a route to the rational optimization of doped colloidal nanocrystals.     

On the characterization of an electrically conductive polyaniline complex

This paper reports on the characterization techniques performed to evaluate the suitability of three different samples of a polyaniline (PANI) complex (PANIPOL) for the production of composites exhibiting a phase separated morphology with continuous elongated structures of PANI embedded in the bulk of an insulating polymer matrix by following an in-situ deformation process. The characterization techniques included rheometry, differential scanning calorimetry and thermogravimetry. In addition, X-ray diffraction, gravimetry, infrared spectroscopy, conductivity measurements, and optical and scanning electron microscopy were used to fully characterize the samples. The thermal limitations and stability of the samples were determined. At the same time, their flow properties in the molten state under different levels of shear were also analysed. The experimental results assisted in the identification of the samples' components and revealed that the PANI particles were 7 m in diameter or smaller.     

In-situ production of electrically conductive fibres in polyaniline-SBS blends

Blending of electro-conductive monofilaments with more traditional insulating materials is a promising approach for the production of composites for applications in static dissipative packaging and in industrial textiles. Accordingly, we report on a favourable method for manufacturing these kinds of material which involves generating the fibres in-situ, that is, during the actual forming process. Electrically conductive polyaniline (PANI) was thermally blended with polystyrene-polybutadiene-polystyrene (SBS) at different weight compositions. The resultant blends were capillary extruded in order to induce a drawing process in the dispersed phase (PANI) of the blend and hence, the in-situ formation of PANI fibres within the above mentioned polymeric matrix. Microscopic analysis on the extrudates revealed that PANI was deformed during the process to produce elongated structures, i.e. ellipsoids or even short fibres, in the blends. Electrical measurements were performed and it was found that blending SBS with no more than 20 weight percent of PANI could produce an electrically conductive composite with a good level of conductivity. The relationship between the volume conductivity and content of PANI in the PANI-SBS blends, was found to be characteristic of a percolation system, with a threshold as low as 5 weight percent of PANI.     

Electrical and mechanical properties of electrically conductive polyethersulfone composites

Electrically conductive polyethersulphone (pes) composites containing carbon fibres, nickel fibres, stainless steel fibres or aluminium flakes at various volume fractions up to 40% were fabricated and tested. For electromagnetic interference (emi) shielding effectiveness > 50 dB, the minimum filler volume fraction was 40% for carbon fibres of length 200 or 400 μm, 20% for nickel or stainless steel fibres, and 30% for aluminium flakes. The tensile strength first increased and then decreased with increasing filler content, such that the highest tensile strength occurred at 30 volume% (vol%) for carbon fibres (of length 200 or 400 μm) as the filler and at 10 vol% for nickel or stainless steel fibres. However, for carbon fibres (of length 100 μm) and aluminium flakes, the tensile strength increases up to at least 40 vol%. The best overall performance was provided by aluminium flakes at 40 vol%; the resistivity was 7 × 10⁻⁵ Ω cm, the emi shielding effectiveness was > 50 dB and tensile strength was 67 MPa. The resistivity of the aluminium flake composites was not affected by heating in air at 140°C for up to at least 144 h.