Plasma-enabled growth of single-crystalline SiC/AlSiC Core–Shell nanowires on porous alumina templates

We report the catalyst-free synthesis of the arrays of core–shell, ultrathin, size-uniform SiC/AlSiC nanowires on the top of a periodic anodic aluminum oxide template. The nanowires were grown using an environmentally friendly, silane-free process by exposing the silicon supported porous alumina template to CH4 + H2 plasmas. High-resolution scanning and transmission electron microscopy studies revealed that the nanowires have a single-crystalline core with a diameter of about 10 nm and a thin (1–2 nm) amorphous AlSiC shell. Because of their remarkable length, high aspect ratio, and very high surface area-to-volume ratio, these unique structures are promising for nanoelectronic and nanophotonic applications that require efficient electron emission, light scattering, etc. A mechanism for nanowire growth is proposed based upon the reduction of the alumina template to nanosized metallic aluminum droplets forming between nanopores. The subsequent incorporation of silicon and carbon atoms from the plasma leads to nucleation and growth from the top of the alumina template.

Minimizing the Gibbs–Thomson effect in the low-temperature plasma synthesis of thin Si nanowires

An advanced combination of numerical models, including plasma sheath, ion- and radical-induced species creation and plasma heating effects on the surface and within a Au catalyst nanoparticle, is used to describe the catalyzed growth of Si nanowires in the sheath of a low-temperature and low-pressure plasma. These models have been used to explain the higher nanowire growth rates, low-energy barriers, much thinner Si nanowire nucleation and the less effective Gibbs–Thomson effect in reactive plasma processes, compared with those of neutral gas thermal processes. The effects of variation in the plasma sheath parameters and substrate potential on Si nanowire nucleation and growth have also been investigated. It is shown that increasing the plasma-related effects leads to decreases in the nucleation energy barrier and the critical nanoparticle radius, with the Gibbs–Thomson effect diminished, even at low temperatures. The results obtained are consistent with available experimental results and open a path toward the energy- and matter-efficient nucleation and growth of a broad range of one-dimensional quantum structures.

From nucleation to nanowires : a single-step process in reactive plasmas

This feature article introduces a deterministic approach for the rapid, single-step, direct synthesis of metal oxide nanowires. This approach is based on the exposure of thin metal samples to reactive oxygen plasmas and does not require any intervening processing or external substrate heating. The critical roles of the reactive oxygen plasmas, surface processes, and plasma-surface interactions that enable this growth are critically examined by using a deterministic viewpoint. The essentials of the experimental procedures and reactor design are presented and related to the key process requirements. The nucleation and growth kinetics is discussed for typical solid-liquid-solid and vapor-solid-solid mechanisms related to the synthesis of the oxide nanowires of metals with low (Ga, Cd) and high (Fe) melting points, respectively. Numerical simulations are focused on the possibility to predict the nanowire nucleation points through the interaction of the plasma radicals and ions with the nanoscale morphological features on the surface, as well as to control the localized 'hot spots' that in turn determine the nanowire size and shape. This generic approach can be applied to virtually any oxide nanoscale system and further confirms the applicability of the plasma nanoscience approaches for deterministic nanoscale synthesis and processing.

Plasma-enabled growth of ultralong straight, helical, and branched silica photonic nanowires

This article reports on the lowerature inductively coupled plasma-enabled synthesis of ultralong (up to several millimeters in length) SiO2 nanowires, which were otherwise impossible to synthesize without the presence of a plasma. Depending on the process conditions, the nanowires feature straight, helical, or branched morphologies. The nanowires are amorphous, with a near-stoichiometric elemental composition ([O] / [Si] =2.09) and are very uniform throughout their length. The role of the ionized gas environment is discussed and the growth mechanism is proposed. These nanowires are particularly promising for nanophotonic applications where long-distance and channelled light transmission and polarization control are required.

Beneficial defects : exploiting the intrinsic polishing-induced wafer roughness for the catalyst-free growth of Ge in-plane nanowires

We outline a metal-free fabrication route of in-plane Ge nanowires on Ge(001) substrates. By positively exploiting the polishing-induced defects of standard-quality commercial Ge(001) wafers, micrometer-length wires are grown by physical vapor deposition in ultra-high-vacuum environment. The shape of the wires can be tailored by the epitaxial strain induced by subsequent Si deposition, determining a progressive transformation of the wires in SiGe faceted quantum dots. This shape transition is described by finite element simulations of continuous elasticity and gives hints on the equilibrium shape of nanocrystals in the presence of tensile epitaxial strain.

Sonochemical nanoplungers : crystalline gold nanowires by cavitational extrusion through nanoporous alumina

Rapid, simple, catalyst-free, room-temperature sonochemical fabrication of long (up to 30 mm), ultra-thin (about 20 nm), crystalline gold nanowires on nanoporous anodic alumina membranes is reported. It is demonstrated that the nanowires nucleate and grow inside the nanosized pores and then form a dense network on the bottom side of the membrane. A growth mechanism is proposed based on the formation of through channels in the Al2O3 membrane by sonochemical etching, followed by nanowire nucleation in the channels and their further extrusion out of the pores by acoustic cavitation. This process can be used for the fabrication of metal nanowires with highly controllable diameter and density, suitable for numerous applications such as nanoelectronic, nanofluidic, and optoelectronic components and devices.

Optical properties of silver nanowires grown electrochemically in nanoporous alumina

We report fabrication and optical properties of electrochemically deposited silver nanowires into nanoporous alumina template. A finite element method is used to study plasmonic coupling of dipole emitters with the silver nanowires.

Hybrid polymer/metal nanoparticles with surface enhanced raman tracking for potential use in nanomedicine

This project was a preliminary step towards the development of novel methods for early stage cancer diagnosis and treatment. Diagnostic imaging agents with high Raman signal enhancement were developed based on tailored assemblies of gold nanoparticles, which demonstrated potential for non-invasive detection from deep under the skin surface. Specifically designed polymers were employed to assemble gold nanoparticles into controlled morphologies including dimers, nanochains, nanoplates, globular and core-satellite nanostructures. Our findings suggest that the Raman enhancement is strongly dependent on assembly morphology and can be tuned to adapt to the requirements of the diagnostic agent.

High mobility top-gate and dual-gate polymer thin-film transistors based on diketopyrrolopyrrole-naphthalene copolymer

In this letter, the performance characteristics of top-gate and dual-gate thin-film transistors (TFTs) with active semiconductor layers consisting of diketopyrrolopyrrole-naphthalene copolymer are described. Optimized top-gate TFTs possess mobilities of up to 1 cm 2 /V s with low contact resistance and reduced hysteresis in air. Dual-gate devices possess higher drive currents as well as improved subthreshold and above threshold characteristics compared to single-gate devices. We also describe the reasons that dual-gate devices result in improved performance. The good stability of this polymer combined with their promising electrical properties make this material a very promising semiconductor for printable electronics.

Charge carrier velocity distributions in high mobility polymer field-effect transistors

In this letter, the velocity distributions of charge carriers in high-mobility polymer thin-film transistors (TFTs) with a diketopyrrolopyrrole- naphthalene copolymer (PDPP-TNT) semiconductor active layer are reported. The velocity distributions are found to be strongly dependent on measurement temperatures as well as annealing conditions. Considerable inhomogeneity is evident at low measurement temperatures and for low annealing temperatures. Such transient transport measurements can provide additional information about charge carrier transport in TFTs which are unavailable using steady-state transport measurements.

Charge-carrier velocity distributions in high-mobility polymer dual-gate thin-film transistors

We report charge-carrier velocity distributions in high-mobility polymer thin-film transistors (PTFTs) employing a dual-gate configuration. Our time-domain measurements of dual-gate PTFTs indicate higher effective mobility as well as fewer low-velocity carriers than in single-gate operation. Such nonquasi-static (NQS) measurements support and clarify the previously reported results of improved device performance in dual-gate devices by various groups. We believe that this letter demonstrates the utility of NQS measurements in studying charge-carrier transport in dual-gate thin-film transistors.

Charge transport study of high mobility polymer thin-film transistors based on thiophene substituted diketopyrrolopyrrole copolymers

In this paper, we report on the device physics and charge transport characteristics of high-mobility dual-gated polymer thin-film transistors with active semiconductor layers consisting of thiophene flanked DPP with thienylene-vinylene-thienylene (PDPP-TVT) alternating copolymers. Room temperature mobilities in these devices are high and can exceed 2 cm2 V-1 s-1. Steady-state and non-quasi-static measurements have been performed to extract key transport parameters and velocity distributions of charge carriers in this copolymer. Charge transport in this polymer semiconductor can be explained using a Multiple-Trap-and-Release or Monroe-type model. We also compare the activation energy vs. field-effect mobility in a few important polymer semiconductors to gain a better understanding of transport of DPP systems and make appropriate comparisons.

Solution-processed dual-gate polymer field-effect transistors for display applications

We describe the advantages of dual-gate thin-film transistors (TFTs) for display applications. We show that in TFTs with active semiconductor layers composed of diketopyrrolopyrrole-naphthalene copolymer, the on-current is increased, the off-current is reduced, and the sub-threshold swing is improved compared to single-gate devices. Charge transport measurements in steady-state and under non-quasi-static conditions reveal the reasons for this improved performance. We show that in dual-gate devices, a much smaller fraction of charge carriers move in slow trap states. We also compare the activation energies for charge transport in the top-gate and bottom-gate configurations.

Poly(2,5-bis(2-octyldodecyl)-3,6-di(furan-2-yl)-2,5-dihydro-pyrrolo[3,4-c] pyrrole-1,4-dione-co-thieno[3,2-b]thiophene): A high performance polymer semiconductor for both organic thin film transistors and organic photovoltaics

A new diketopyrrolopyrrole (DPP)-containing donor-acceptor polymer, poly(2,5-bis(2-octyldodecyl)-3,6-di(furan-2-yl)-2,5-dihydro-pyrrolo[3,4-c] pyrrole-1,4-dione-co-thieno[3,2-b]thiophene) (PDBF-co-TT), is synthesized and studied as a semiconductor in organic thin film transistors (OTFTs) and organic photovoltaics (OPVs). High hole mobility of up to 0.53 cm 2 V -1 s -1 in bottom-gate, top-contact OTFT devices is achieved owing to the ordered polymer chain packing and favoured chain orientation, strong intermolecular interactions, as well as uniform film morphology of PDBF-co-TT. The optimum band gap of 1.39 eV and high hole mobility make this polymer a promising donor semiconductor for the solar cell application. When paired with a fullerene acceptor, PC 71BM, the resulting OPV devices show a high power conversion efficiency of up to 4.38% under simulated standard AM1.5 solar illumination.

One-step macroscopic alignment of conjugated polymer systems by epitaxial crystallization during spin-coating

The one-step preparation of highly anisotropic polymer semiconductor thin films directly from solution is demonstrated. The conjugated polymer poly(3-hexylthiophene) (P3HT) as well as P3HT:fullerene bulk-heterojunction blends can be spin-coated from a mixture of the crystallizable solvent 1,3,5-trichlorobenzene (TCB) and a second carrier solvent such as chlorobenzene. Solidification is initiated by growth of macroscopic TCB spherulites followed by epitaxial crystallization of P3HT on TCB crystals. Subsequent sublimation of TCB leaves behind a replica of the original TCB spherulites. Thus, highly ordered thin films are obtained, which feature square-centimeter-sized domains that are composed of one spherulite-like structure each. A combination of optical microscopy and polarized photoluminescence spectroscopy reveals radial alignment of the polymer backbone in case of P3HT, whereas P3HT:fullerene blends display a tangential orientation with respect to the center of spherulite-like structures. Moreover, grazing-incidence wide-angle X-ray scattering reveals an increased relative degree of crystallinity and predominantly flat-on conformation of P3HT crystallites in the blend. The use of other processing methods such as dip-coating is also feasible and offers uniaxial orientation of the macromolecule. Finally, the applicability of this method to a variety of other semi-crystalline conjugated polymer systems is established. Those include other poly(3-alkylthiophene)s, two polyfluorenes, the low band-gap polymer PCPDTBT, a diketopyrrolopyrrole (DPP) small molecule as well as a number of polymer:fullerene and polymer:polymer blends. Macroscopic spherulite-like structures of the conjugated polymer poly(3-hexylthiophene) (P3HT) grow directly during spin-coating. This is achieved by processing P3HT or P3HT:fullerene bulk heterojunction blends from a mixture of the crystallizable solvent 1,3,5-trichlorobenzene and a second carrier solvent such as chlorobenzene. Epitaxial growth of the polymer on solidified solvent crystals gives rise to circular-symmetric, spherulite-like structures that feature a high degree of anisotropy.