On-Chip Chemical Self-Assembly of Semiconducting Single-Walled Carbon Nanotubes (SWNTs): Toward Robust and Scale Invariant SWNTs Transistors.

In this paper, the fabrication of carbon nanotubes field effect transistors by chemical self-assembly of semiconducting single walled carbon nanotubes (s-SWNTs) on prepatterned substrates is demonstrated. Polyfluorenes derivatives have been demonstrated to be effective in selecting s-SWNTs from raw mixtures. In this work the authors functionalized the polymer with side chains containing thiols, to obtain chemical self-assembly of the selected s-SWNTs on substrates with prepatterned gold electrodes. The authors show that the full side functionalization of the conjugated polymer with thiol groups partially disrupts the s-SWNTs selection, with the presence of metallic tubes in the dispersion. However, the authors determine that the selectivity can be recovered either by tuning the number of thiol groups in the polymer, or by modulating the polymer/SWNTs proportions. As demonstrated by optical and electrical measurements, the polymer containing 2.5% of thiol groups gives the best s-SWNT purity. Field-effect transistors with various channel lengths, using networks of SWNTs and individual tubes, are fabricated by direct chemical self-assembly of the SWNTs/thiolated-polyfluorenes on substrates with lithographically defined electrodes. The network devices show superior performance (mobility up to 24 cm2 V-1 s-1 ), while SWNTs devices based on individual tubes show an unprecedented (100%) yield for working devices. Importantly, the SWNTs assembled by mean of the thiol groups are stably anchored to the substrate and are resistant to external perturbation as sonication in organic solvents.

Filling the Green Gap of a Megadalton Photosystem I Complex by Conjugation of Organic Dyes.

Photosynthesis is Nature's major process for converting solar into chemical energy. One of the key players in this process is the multiprotein complex photosystem I (PSI) that through absorption of incident photons enables electron transfer, which makes this protein attractive for applications in bioinspired photoactive hybrid materials. However, the efficiency of PSI is still limited by its poor absorption in the green part of the solar spectrum. Inspired by the existence of natural phycobilisome light-harvesting antennae, we have widened the absorption spectrum of PSI by covalent attachment of synthetic dyes to the protein backbone. Steady-state and time-resolved photoluminescence reveal that energy transfer occurs from these dyes to PSI. It is shown by oxygen-consumption measurements that subsequent charge generation is substantially enhanced under broad and narrow band excitation. Ultimately, surface photovoltage (SPV) experiments prove the enhanced activity of dye-modified PSI even in the solid state.

Organic-Inorganic Hybrid Solution-Processed H2-Evolving Photocathodes.

Here we report for the first time an H2-evolving photocathode fabricated by a solution-processed organic-inorganic hybrid composed of CdSe and P3HT. The CdSe:P3HT (10:1 (w/w)) hybrid bulk heterojunction treated with 1,2-ethanedithiol (EDT) showed efficient water reduction and hydrogen generation. A photocurrent of -1.24 mA/cm(2) at 0 V versus reversible hydrogen electrode (V(RHE)), EQE of 15%, and an unprecedented Voc of 0.85 V(RHE) under illumination of AM1.5G (100 mW/cm(2)) in mild electrolyte were observed. Time-resolved photoluminescence (TRPL), internal quantum efficiency (IQE), and transient photocurrent measurements were carried out to clarify the carrier dynamics of the hybrids. The exciton lifetime of CdSe was reduced by one order of magnitude in the hybrid blend, which is a sign of the fast charge separation upon illumination. By comparing the current magnitude of the solid-state devices and water-splitting devices made with identical active layers, we found that the interfaces of the water-splitting devices limit the device performance. The electron/hole transport properties investigated by comparing IQE spectra upon front- and back-side illumination evidenced balanced electron/hole transport. The Faradaic efficiency is 80-100% for the hybrid photocathodes with Pt catalysts and ∼70% for the one without Pt catalysts.

Charge collection enhancement by incorporation of gold-silica core-shell nanoparticles into P3HT:PCBM/ZnO nanorod array hybrid solar cells.

In this work, gold-silica core-shell (Au@silica) nanoparticles (NPs) with various silica-shell thicknesses are incorporated into P3HT:PCBM/ZnO nanorod (NR) hybrid solar cells. Enhancement in the short-circuit current density and the efficiency of the hybrid solar cells is attained with the appropriate addition of Au@silica NPs regardless of the silica-shell thickness. Compared to the P3HT:PCBM/ZnO NR hybrid solar cell, a 63% enhancement in the efficiency is achieved by the P3HT:PCBM/Au@silica NP/ZnO NR hybrid solar cell. The finite difference time domain simulations indicate that the strength of the Fano resonance, i.e., the electric field of the quasi-static asymmetric quadrupole, on the surface of Au@silica NPs in the P3HT:PCBM/ZnO NR hybrid significantly decreases with increasing thickness of the silica shell. Raman characterization reveals that the degree of P3HT order increases when Au@silica NPs are incorporated into the P3HT:PCBM/ZnO NR hybrid. The charge separation at the interface between P3HT and PCBM as well as the electron transport in the active layer are retarded by the electric field of the Fano resonance. Nevertheless, the prolongation of the electron lifetime and the reduction of the electron transit time in the P3HT:PCBM/ZnO NR hybrid solar cells, which result in an enhancement of electron collection, are achieved by the addition of Au@silica NPs. This may be attributed to the improvement in the degree of P3HT order and connectivity of PCBM when Au@silica NPs are incorporated into the P3HT:PCBM active layer.

Conjugated polymer-assisted dispersion of single-wall carbon nanotubes: the power of polymer wrapping.

The future application of single-walled carbon nanotubes (SWNTs) in electronic (nano)devices is closely coupled to the availability of pure, semiconducting SWNTs and preferably, their defined positioning on suited substrates. Commercial carbon nanotube raw mixtures contain metallic as well as semiconducting tubes of different diameter and chirality. Although many techniques such as density gradient ultracentrifugation, dielectrophoresis, and dispersion by surfactants or polar biopolymers have been developed, so-called conjugated polymer wrapping is one of the most promising and powerful purification and discrimination strategies. The procedure involves debundling and dispersion of SWNTs by wrapping semiflexible conjugated polymers, such as poly(9,9-dialkylfluorene)s (PFx) or regioregular poly(3-alkylthiophene)s (P3AT), around the SWNTs, and is accompanied by SWNT discrimination by diameter and chirality. Thereby, the π-conjugated backbone of the conjugated polymers interacts with the two-dimensional, graphene-like π-electron surface of the nanotubes and the solubilizing alkyl side chains of optimal length support debundling and dispersion in organic solvents. Careful structural design of the conjugated polymers allows for a selective and preferential dispersion of both small and large diameter SWNTs or SWNTs of specific chirality. As an example, with polyfluorenes as dispersing agents, it was shown that alkyl chain length of eight carbons are favored for the dispersion of SWNTs with diameters of 0.8-1.2 nm and longer alkyls with 12-15 carbons can efficiently interact with nanotubes of increased diameter up to 1.5 nm. Polar side chains at the PF backbone produce dispersions with increased SWNT concentration but, unfortunately, cause reduction in selectivity. The selectivity of the dispersion process can be monitored by a combination of absorption, photoluminescence, and photoluminescence excitation spectroscopy, allowing identification of nanotubes with specific coordinates [(n,m) indices]. The polymer wrapping strategy enables the generation of SWNT dispersions containing exclusively semiconducting nanotubes. Toward the applications in electronic devices, until now most applied approach is a direct processing of such SWNT dispersions into the active layer of network-type thin film field effect transistors. However, to achieve promising transistor performance (high mobility and on-off ratio) careful removal of the wrapping polymer chains seems crucial, for example, by washing or ultracentrifugation. More defined positioning of the SWNTs can be accomplished in directed self-assembly procedures. One possible strategy uses diblock copolymers containing a conjugated polymer block as dispersing moiety and a second block for directed self-assembly, for example, a DNA block for specific interaction with complementary DNA strands. Another strategy utilizes reactive side chains for controlled anchoring onto patterned surfaces (e.g., by interaction of thiol-terminated alkyl side chains with gold surfaces). A further promising application of purified SWNT dispersions is the field of organic (all-carbon) or hybrid solar cell devices.

Carbon nanotube network ambipolar field-effect transistors with 10(8) on/off ratio.

Polymer wrapping is a highly effective method of selecting semiconducting carbon nanotubes and dispersing them in solution. Semi-aligned semiconducting carbon nanotube networks are obtained by blade coating, an effective and scalable process. The field-effect transistor (FET) performance can be tuned by the choice of wrapping polymer, and the polymer concentration modifies the FET transport characteristics, leading to a record on/off ratio of 10(8) .

High performance photoelectrochemical hydrogen generation and solar cells with a double type II heterojunction.

We report on the fabrication of CdSe quantum dot (QD) sensitized electrodes by direct adsorption of colloidal QDs on mesoporous TiO2 followed by 3-mercaptopropionic acid (MPA) ligand exchange. High efficiency photoelectrochemical hydrogen generation is demonstrated by means of these electrodes. The deposition of ZnS on TiO2/CdSe further improves the external quantum efficiency from 63% to 85% at 440 nm under -0.5 V vs. SCE. Using the same photoelectrodes, solar cells with the internal quantum efficiency approaching 100% are fabricated. The ZnS deposition increases the photocurrent and chemical stability of the electrodes. Investigation of the carrier dynamics of the solar cells shows that ZnS enhances the exciton separation rate in CdSe nanocrystals, which we ascribe to the formation of a type II heterojunction between ZnS and CdSe QDs. This finding is confirmed by the dynamics of the CdSe photoluminescence, which in the presence of ZnS becomes noticeably faster.

Semiconducting single-walled carbon nanotubes on demand by polymer wrapping.

Efficient selection of semiconducting SWCNTs of large diameter range (0.8-1.6 nm) on demand is demonstrated. Different diameters of SWCNT are systematically selected by tuning the alkyl side-chain lengths of the wrapping polymers of similar backbone. The exceptional quality and high concentration of the SWCNTs is validated by the outstanding optical properties and the highly performing random network ambipolar field-effect transistors.

High performance ambipolar field-effect transistor of random network carbon nanotubes.

Ambipolar field-effect transistors of random network carbon nanotubes are fabricated from an enriched dispersion utilizing a conjugated polymer as the selective purifying medium. The devices exhibit high mobility values for both holes and electrons (3 cm(2) /V·s) with a high on/off ratio (10(6) ). The performance demonstrates the effectiveness of this process to purify semiconducting nanotubes and to remove the residual polymer.