Vlad Stolojan

Hybrid Carbon Nanotube Networks as Efficient Hole Extraction Layers for Organic Photovoltaics

Transparent, highly percolated networks of regioregular poly(3-hexylthiophene) (rr-P3HT)-wrapped semiconducting single-walled carbon nanotubes (s-SWNTs) are deposited, and the charge transfer processes of these nanohybrids are studied using spectroscopic and electrical measurements. The data disclose hole doping of s-SWNTs by the polymer, challenging the prevalent electron-doping hypothesis. Through controlled fabrication, high- to low-density nanohybrid networks are achieved, with low-density hybrid carbon nanotube networks tested as hole transport layers (HTLs) for bulk heterojunction (BHJ) organic photovoltaics (OPV). OPVs incorporating these rr-P3HT/s-SWNT networks as the HTL demonstrate the best large area (70 mm(2)) carbon nanotube incorporated organic solar cells to date with a power conversion efficiency of 7.6%. This signifies the strong capability of nanohybrids as an efficient hole extraction layer, and we believe that dense nanohybrid networks have the potential to replace expensive and material scarce inorganic transparent electrodes in large area electronics toward the realization of low-cost flexible electronics.

Thermal expansion coefficient of hydrogenated amorphous carbon

The coefficient of thermal expansion (CTE) of hydrogenated amorphous carbon (a-C:H) was investigated as a function of the concentration of sp2 hybridization. The CTE, determined using the thermally induced bending technique, depends on the concentration of sp2 bonded carbon, increasing to the value of graphite as the sp2 concentration approaches 100%. By using a combination of the thermally induced bending technique and nanohardness measurements, we extract separately the Young’s modulus and Poisson’s ratio of the a-C:H films as function of the sp2 concentration.

Structural and optoelectronic properties of C60 rods obtained via a rapid synthesis route

High purity single crystal C60 rods with uniform dimensions are synthesized by a rapid and facile approach which can be completed over a timescale of typically a few minutes. The morphology of the fullerene product has been characterized in detail by scanning electron microscopy, scanning transmission electron microscopy, and atomic force microscopy, demonstrating that the resulting materials are solid, hexagonal cross-sectioned rods with novel faceted tips. High resolution transmission electron microscopy investigations reveal that the rods are face-centered cubic packed single crystals. Vibrational and electronic spectroscopy studies provide compelling evidence that the rods are a van der Waals solid since the electronic structure of the component C60 molecules is largely preserved. The structures obtained are found to possess novel optoelectronic properties exhibiting low energy absorption not reported in related structures and materials to date. Furthermore significant room temperature photoluminescence is obtained from the C60 rods accompanied by a small blue shift of the spectra which is also observed for the first ‘allowed’ absorption transitions. Given their rapid synthesis, excellent purity, optical and charge transport properties these fullerene structures are expected to be a promising materials for nanoelectronic devices including thin film organic solar cells and photodetectors.

Ultrahigh Performance C60 Nanorod Large Area Flexible Photoconductor Devices via Ultralow Organic and Inorganic Photodoping

One dimensional single-crystal nanorods of C60 possess unique optoelectronic properties including high electron mobility, high photosensitivity and an excellent electron accepting nature. In addition, their rapid large scale synthesis at room temperature makes these organic semiconducting nanorods highly attractive for advanced optoelectronic device applications. Here, we report low-cost large-area flexible photoconductor devices fabricated using C60 nanorods. We demonstrate that the photosensitivity of the C60 nanorods can be enhanced ~400-fold via an ultralow photodoping mechanism. The photodoped devices offer broadband UV-vis-NIR spectral tuneability, exhibit a detectivitiy >109 Jones, an external quantum efficiency of ~100%, a linear dynamic range of 80 dB, a rise time 60 µs and the ability to measure ac signals up to ~250 kHz. These figures of merit combined are among the highest reported for one dimensional organic and inorganic large-area planar photoconductors and are competitive with commercially available inorganic photoconductors and photoconductive cells. With the additional processing benefits providing compatibility with large-area flexible platforms, these devices represent significant advances and make C60 nanorods a promising candidate for advanced photodetector technologies.

Confined crystals of the smallest phase-change material.

The demand for high-density memory in tandem with limitations imposed by the minimum feature size of current storage devices has created a need for new materials that can store information in smaller volumes than currently possible. Successfully employed in commercial optical data storage products, phase-change materials, that can reversibly and rapidly change from an amorphous phase to a crystalline phase when subject to heating or cooling have been identified for the development of the next generation electronic memories. There are limitations to the miniaturization of these devices due to current synthesis and theoretical considerations that place a lower limit of 2 nm on the minimum bit size, below which the material does not transform in the structural phase. We show here that by using carbon nanotubes of less than 2 nm diameter as templates phase-change nanowires confined to their smallest conceivable scale are obtained. Contrary to previous experimental evidence and theoretical expectations, the nanowires are found to crystallize at this scale and display amorphous-to-crystalline phase changes, fulfilling an important prerequisite of a memory element. We show evidence for the smallest phase-change material, extending thus the size limit to explore phase-change memory devices at extreme scales.

High-rate low-temperature growth of vertically aligned carbon nanotubes

We report the low-temperature growth of vertically aligned carbon nanotubes (CNTs) at high growth rates by a photo-thermal chemical vapour deposition (PTCVD) technique using a Ti/Fe bilayer film as the catalyst. The bulk growth temperature of the substrate is as low as 370 °C and the growth rate is up to 1.3 µm min(-1), at least eight times faster than the values reported by traditional thermal CVD methods. Transmission electron microscopy observations reveal that as-grown CNTs are uniformly made of highly crystalline 5-6 graphene shells with an approximately 10 nm outer diameter and a 5-6 nm inner diameter. The low-temperature rapid growth of CNTs is strongly related to the unique top-down heating mode of PTCVD and the use of a Ti/Fe bimetallic solid solution catalyst. The present study will advance the development of CNTs as interconnects in nanoelectronics, through a CMOS-compatible low-temperature deposition method suitable for back-end-of-line processes.

Highly photoconductive amorphous carbon nitride films prepared by cyclic nitrogen radical sputtering

We report on the growth of amorphous carbon nitride films (a-CNx) showing the highest conductivity to date. The films were prepared using a layer-by-layer method (a-CNx:LL), by the cyclical nitrogen radical sputtering of a graphite radical, alternated with a brief hydrogen etch. The photosensitivity S of these films is 105, defined as the ratio of the photoconductivity σp to the dark conductivity σd and is the highest value reported thus far. We believe that the carriers generated by the monochromatic light (photon energy 6.2eV) in the a-CNx:LL films are primarily electrons, with the photoconductivity shown to increase with substrate deposition temperature.

A fast sonochemical approach for the synthesis of solution processable ZnO rods

A solution based sonochemical synthesis method for ZnO rods is presented with a resulting growth rate in excess of 15 times faster than previously reported. Such material is solution processable and could be exploited in the fabrication of transparent conductors and/or large area electronics via inkjet printing methods or solution based self-assembly techniques. To understand the crystal structure and defects chemistry, the as-synthesized wurtzite crystal structures were compared and contrasted with rods grown by the more traditional and well characterized hydrothermal growth method. Fluorescence spectra were recorded and the emission characteristics correlated with the structural and conductive properties of the ZnO rods. In particular, the sonochemical crystals appear to have a higher degree of order with fewer defects. This study represents a first step toward the tailoring of the electronic properties of ZnO rods. In particular, we will concentrate on the influence that native defects have on electrical conduction and on photoluminescence. Furthermore, we show how the intensity of the ultrasonic power exploited in this synthesis has a direct influence on the crystal quality as revealed by a comparative study. An optimum value between 30% and 35% of the maximum amplitude of a 20 kHz ultrasonic probe was found to give the best conditions for the growth of crystals with fewer defects density, while at ca. 25% of the maximum amplitude we observed the higher intensities for the fluorescence spectra both in the ultraviolet and in the visible range.

Solution processable multi-channel ZnO nanowire field-effect transistors with organic gate dielectric

The present work focuses on nanowire (NW) applications as semiconducting elements in solution processable field-effect transistors (FETs) targeting large-area low-cost electronics. We address one of the main challenges related to NW deposition and alignment by using dielectrophoresis (DEP) to select multiple ZnO nanowires with the correct length, and to attract, orientate and position them in predefined substrate locations. High-performance top-gate ZnO NW FETs are demonstrated on glass substrates with organic gate dielectric layers and surround source-drain contacts. Such devices are hybrids, in which inorganic multiple single-crystal ZnO NWs and organic gate dielectric are synergic in a single system. Current-voltage (I-V) measurements of a representative hybrid device demonstrate excellent device performance with high on/off ratio of ~10(7), steep subthreshold swing (s-s) of ~400 mV/dec and high electron mobility of ~35 cm(2) V(-1) s(-1) in N2 ambient. Stable device operation is demonstrated after 3 months of air exposure, where similar device parameters are extracted including on/off ratio of ~4 × 10(6), s-s ~500 mV/dec and field-effect mobility of ~28 cm(2) V(-1) s(-1). These results demonstrate that DEP can be used to assemble multiples of NWs from solvent formulations to enable low-temperature hybrid transistor fabrication for large-area inexpensive electronics.

Multi-Functional Carbon Fibre Composites using Carbon Nanotubes as an Alternative to Polymer Sizing

Carbon fibre reinforced polymers (CFRP) were introduced to the aerospace, automobile and civil engineering industries for their high strength and low weight. A key feature of CFRP is the polymer sizing - a coating applied to the surface of the carbon fibres to assist handling, improve the interfacial adhesion between fibre and polymer matrix and allow this matrix to wet-out the carbon fibres. In this paper, we introduce an alternative material to the polymer sizing, namely carbon nanotubes (CNTs) on the carbon fibres, which in addition imparts electrical and thermal functionality. High quality CNTs are grown at a high density as a result of a 35 nm aluminium interlayer which has previously been shown to minimise diffusion of the catalyst in the carbon fibre substrate. A CNT modified-CFRP show 300%, 450% and 230% improvements in the electrical conductivity on the ‘surface’, ‘through-thickness’ and ‘volume’ directions, respectively. Furthermore, through-thickness thermal conductivity calculations reveal a 107% increase. These improvements suggest the potential of a direct replacement for lightning strike solutions and to enhance the efficiency of current de-icing solutions employed in the aerospace industry.