Simultaneous optimization of charge-carrier mobility and optical gain in semiconducting polymer films

The combination of efficient light emission and high charge-carrier mobility has thus far proved elusive for polymer semiconductors, with high mobility typically achieved by cofacial pi-electron system to pi-electron system interactions that quench exciton luminescence. We report a new strategy, comprising the introduction of a limited number of more effective hopping sites between otherwise relatively isolated, and thus highly luminescent, polyfluorene chains. Our approach results in polymer films with large mobility (mu approximately 3-6 x 10(-2) cm2 V-1 s-1) and simultaneously excellent light-emission characteristics. These materials are expected to be of interest for light-emitting transistors, light-emitting diode sources for optical communications and may offer renewed hope for electrically pumped laser action. In the last context, optically pumped distributed feedback lasers comprising one-dimensional etched silica grating structures coated with polymer have state-of-the-art excitation thresholds (as low as 30 W cm(-2) (0.1 nJ per pulse or 0.3 microJ cm-2) for 10 Hz, 12 ns, 390 nm excitation) and slope efficiencies (up to 11%).     

Encapsulation of semiconducting polymers in vault protein cages

We demonstrate that a semiconducting polymer [poly(2-methoxy-5-propyloxy sulfonate phenylene vinylene), MPS-PPV] can be encapsulated inside recombinant, self-assembling protein nanocapsules called "vaults". Polymer incorporation into these nanosized protein cages, found naturally at approximately 10,000 copies per human cell, was confirmed by fluorescence spectroscopy and small-angle X-ray scattering. Although vault cellular functions and gating mechanisms remain unknown, their large internal volume and natural prevalence within the human body suggests they could be used as carriers for therapeutics and medical imaging reagents. This study provides the groundwork for the use of vaults in encapsulation and delivery applications.     

Controlling optical gain in semiconducting polymers with nanoscale chain positioning and alignment

We control the chain conformation of a semiconducting polymer by encapsulating it within the aligned nanopores of a silica host. The confinement leads to polarized, low-threshold amplified spontaneous emission from the polymer chains. The polymer enters the porous silica film from only one face and the filling of the pores is therefore graded. As a result, the profile of the index of refraction in the film is also graded, in the direction normal to the pores, so that the composite film forms a low-loss, graded-index waveguide. The aligned polymer chains plus naturally formed waveguide are ideally configured for optical gain, with a threshold for amplified spontaneous emission that is twenty times lower than in comparable unoriented polymer films. Moreover, the optimal conditions for ASE are met in only one spatial orientation and with one polarization. The results show that nanometre-scale control of semiconducting polymer chain orientation and position leads to novel and desirable optical properties.     

Enhanced mobility of confined polymers

Non-classical behaviour, brought about by a confinement that imposes spatial constraints on molecules, is opening avenues to novel applications. For example, carbon nanotubes, which show rapid and selective transport of small molecules across the nanotubes, have significant potential as biological or chemical separation materials for organic solvents or gaseous molecules. With polymers, when the dimensions of a confining volume are much less than the radius of gyration, a quantitative understanding of perturbations to chain dynamics due to geometric constraints remains a challenge and, with the development of nanofabrication processes, the dynamics of confined polymers have significant technological implications. Here, we describe a weak molecular-weight-dependent mobility of polymers confined within nanoscopic cylindrical pores having diameters smaller than the dimension of the chains in the bulk. On the basis of the chain configuration along the pore axis, the measured mobility of polymers in the confined geometry is much higher than the mobility of the unconfined chain. With the emergence of nanofabrication processes based on polymer flow, the unexpected enhancement in flow and reduction in intermolecular entanglements are of significant importance in the design and execution of processing strategies.     

Predictive study of charge transport in disordered semiconducting polymers

We present a theoretical study of charge transport in disordered semiconducting polymers that relates the charge mobility to the chemical structure and the physical morphology in a novel multiscale approach. Our studies, focusing on poly(9,9-dioctylfluorene) (PFO), show that the charge mobility is dominated by pathways with the highest interchain charge-transfer rates. We also find that disorder is not always detrimental to charge transport. We find good agreement with experimental time-of-flight mobility data in highly aligned PFO films.     

Temperature treatment of semiconducting polymers: An X-ray reflectivity study

The influence of temperature treatment on three types of conjugated polymer thin films, named poly(3-hexylthiophene) (P3HT), polyarylamine (PAA) and octylfluorene-bithiophene copolymer (F8T2) is studied. A detailed knowledge of the film morphology and crystalline structure is important since the performance of organic thin film transistors is extremely sensitive to small changes of morphology and structure. Samples are prepared via a spin-casting process on thermal oxidized silicon wafers. The influence of heat treatment in the range from ambient temperatures to 600 K is studied with specular X-ray reflectivity, X-ray diffraction and differential scanning calorimetry. The morphological parameters like layer thicknesses, electron densities and roughnesses of the interface and of the surface are calculated from the XRR measurements. The maximum change of layer thickness due to heat treatment is 15% of the initial layer thicknesses. The maximum variation of the mean electron densities are about 20% and the rms surface roughness vary from 2 Å up to 20 Å as a result of annealing. Interface roughness show variation of about 1 Å. Strong variations of the morphological parameters next to phase transition temperatures are observed as well.     

Recent development in metal halide perovskites synthesis to improve their charge-carrier mobility and photocatalytic efficiency

Over the past decade,all-inorganic metal halide perovskites(MHPs,CsPbX₃:X=Cl,Br,I) have been widely investigated as promising materials for optoelectronic devices such as solar cells and light-emitting diodes.MHPs are defecttolerant,which allows tuning of their bandgap without altering their photophysical properties.From a fundamental point of view,MHPs are excellent candidates for photocatalytic reactions due to their light-harvesting capability,high photogenerated charge-carrier mob...     

Covalent-bonded graphyne polymers with high hardness

Eight covalent-bonded graphyne polymers have been proposed using the newly developed USPEX and CALYPSO methods based on the first-principle calculations. These polymers are energetically more favorable than the corresponding graphyne under ambient pressure, and seven of them are more stable than fullerene C₆₀, indicating their existence possibilities. The mechanical and dynamic stabilities of these crystal structures have been confirmed by calculating their elastic constants and phonon dispersion curves, respectively. The newly developed variable-cell nudged elastic band (VC-NEB) simulations show that the graphyne → polymer transformation exhibits lower energy barrier than the graphite → diamond transformation. Two of the graphyne polymers have been found to be superhard, and the others are hard materials. These graphyne polymers possess tunable electronic properties from metallic to semiconductive.     

Two-dimensional semiconductors with possible high room temperature mobility

We have calculated the longitudinal acoustic phonon limited electron mobility of 14 twvo-dimensional semiconductors with composition of MX2, where M （= Mo, W, Sn, Hf, Zr and Pt） is the transition metal, and X is S, Se and Te. We treated the scattering matrix by the deformation potential approximation. We found that out of 14 compounds, MoTe2, HfSe2 and ZrSe2 are promising regarding to their possible high mobility and finite band gap. The phonon limited mobility can be above 2,500 cm^2·V^-1·s^-1 at room temperature.     

5, 10-linked naphthodithiophenes as the building block for semiconducting polymers

We present new semiconducting polymers incorporating naphtho[1, 2-b:5, 6-b′] dithiophene (NDT3) and naphtho[2, 1-b:6, 5-b′] dithiophene (NDT4), which are linked at the naphthalene positions, in the polymer backbone. It is interesting that the trend in the ordering structure and thus charge transport properties are quite different from what were observed in the isomeric polymers where the NDT3 and NDT4 cores are linked at the thiophene α-positions. In the thiophene-linked NDT system, the NDT3-based polymer (PNDT3BT) gave the better ordering in thin films and thus the high charge carrier mobility compared to the NDT4-based polymer (PNDT4BT). In the meantime, in the naphthalene-linked NDT system, the NDT4-based polymer (PNDT4iBT) provided the superior properties. Considering that PNDT4iBT has relatively low highest occupied molecular orbital (HOMO) energy level (−5.2 eV) and moderately high mobilities in the order of 10⁻² cm² V⁻¹ s⁻¹, the NDT4 core, when linked at the naphthalene positions, can be a good building unit for the development of high-performance semiconducting polymers for both organic field-effect transistors and photovoltaic devices.     

Preparation and characterisation of high surface area semiconducting oxides

The conventional method of preparing doped semiconducting oxides involves solid state sintering at elevated temperatures and the resultant products have a very low surface area (< 1 m² g^–1). This severely limits the activity of semiconducting oxide catalysts, such as lithiated NiO. A cryochemical method based on the freeze drying of mixed salt solutions, followed by low temperature calcining has yielded lithiated NiO of over 60 m² g^–1. DTA, conductivity measurements and chemical analyses confirmed that under these conditions, very uniform mixing between Li₂O and NiO is achieved and that Li₂O can diffuse into the NiO lattice at about 400° C, as compared to 950 to 1000° C for products prepared by conventional means.     

Direct spectroscopic evidence of energy transfer from photo-excited semiconducting polymers to single-walled carbon nanotubes

Single-walled carbon nanotubes have been efficiently dispersed in an organic solvent using the semiconducting polymers MEHPPV and PFO. It has been found that energy is transferred to the carbon nanotubes when the polymer is photo-excited across its minimum energy gap. This is shown using photoluminescence excitation mapping in the range of both the polymer's and the nanotube's optical absorbance. Possible mechanisms for the energy transfer are discussed.     

Direct discrimination between semiconducting and metallic single-walled carbon nanotubes with high spatial resolution by SEM

Single-walled carbon nanotube (SWCNT) films with a high density exhibit broad functionality and great potential in nanodevices,as SWCNTs can be either metallic or semiconducting in behavior.The films greatly benefit from characterization technologies that can efficiently identify and group SWCNTs based on metallic or semiconducting natures with high spatial resolution.Here,we developed a facile imaging technique using scanning electron microscopy (SEM) to discriminate between semiconducting and metallic SWCNTs based on black and white colors.The average width of the single-SWCNT image was reduced to ～9 nm,～1/5 of previous imaging results.These achievements were attributed to reduced surface charging on the SiO2/Si substrate under enhanced accelerating voltages.With this identification technique,a CNT transistor with an on/off ratio of ＞105 was fabricated by identifying and etching out the white metallic SWCNTs.This improved SEM imaging technique can be widely applied in evaluating the selective growth and sorting of SWCNTs.     

AlGaN/GaN high electron mobility transistors with implanted ohmic contacts

Selective area silicon implantation for source/drain regions was integrated into the fabrication of molecular beam epitaxy-grown AlGaN/GaN HEMTs. Dopant activation was achieved by rapid thermal annealing at 1100 °C in flowing N₂ ambient for 120 s with an AlN encapsulation. Linear transmission line measurements showed that the resistance of the overlay Ti/Al/Ni/Au ohmic contacts was reduced by 61% compared to the control sample. After the Schottky Ni/Au gate formation, the typical DC characteristics displayed a higher current drive, smaller knee voltage and better gate control properties for HEMTs with implanted source and drain regions.     

Chirality-enriched semiconducting carbon nanotubes synthesized on high surface area MgO-supported catalyst

Single-walled carbon nanotubes (SWCNTs) with a narrow diameter distribution were synthesized by radio frequency-Catalytic Chemical Vapor Deposition (RF-CCVD) through the pyrolysis of CH4. Fe-Co bimetallic catalytic nanoclusters were supported on high-surface area MgO nanopowders and used in the nanotube synthesis process. Nanolog absorption fluorescence analysis was used to characterize the chiralities of the as-produced SWCNTs over this nanostructural catalyst. In the final SWCNT sample, the (7,5) semiconducting carbon nanotube species were found to be dominant, with a low chirality variation.