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Negatively charged colloidal poly(methyl methacrylate-co-butyl acrylate) (P(MMA-BA)) particles and positively charged dissolved poly(ethyleneimine) (PEI) were adsorbed onto a cement block using a layer-by-layer (LBL) assembly technique. The block was fashioned so as to have a cylindrical hole running from one face to another along the long axis of the rectangular block, and a fluid containing either of the two charged materials was pumped through the block. The result was a film tens of micrometers thick, and the pressure required to crack the cement block was measured after one end of the hole was sealed. Latex particles with a T(g) near the use temperature showed the maximum improvement in the cracking stress of the blocks. In a multilayer coating with identically sized particles, the cracking stress of the blocks increased to an improvement of 25% and then dropped off with increasing number of layers, even though the relationship between film thickness and the number of layers was linear. An improvement of about 30% in the cracking stress of the coated blocks was obtained when using multiple layers with different particle sizes. The effects of the number of layers and particle size on the cracking stress suggest that both the morphology and the thickness of the film play a role in performance. Tests done under confinement, e.g., with an external stress applied to the outside of the blocks, suggest that not only does a film-forming mechanism contribute to performance but that filling of microcracks in the rock may also play a role.
ACS Appl Mater Interfaces 2010 Apr
PMID:Strength improvement via coating of a cylindrical hole by layer-by-layer assembled polymer particles. 2042 42

A polymeric composite material composed of colloidal gold nanoparticles (<10 nm) and SU8 has been utilized for the fabrication of large-area, high-definition photonic crystal. We have successfully fabricated near-infrared photonic crystal slabs from composite materials using a combination of multiple beam interference lithography and reactive ion etching processes. Doping of colloidal gold nanoparticles into the SU8 photopolymer results in a better definition of structural features and hence in the enhancement of the optical properties of the fabricated photonic crystals. A 2D air hole array of triangular symmetry with a hole-to-hole pitch of approximately 500 nm has been successfully fabricated in a large circular area of 1 cm diameter. Resonant features observed in reflectance spectra of our slabs are found to depend on the exposure time, and can be tuned over a range of near-infrared frequencies.
ACS Appl Mater Interfaces 2010 Apr
PMID:Large-area, near-infrared (IR) photonic crystals with colloidal gold nanoparticles embedding. 2042 43

Reduced-symmetry plasmonic nanostructures can be designed to support a range of novel optical phenomena, such as nanoscale control of the far-field scattering profile and magnetic resonances at optical frequencies. A family of reduced-symmetry nanostructures--plasmonic semishells with specifically shaped and oriented perforations introduced into the metallic shell layer--can be tailored to control these effects. Unlike core-shell nanoparticles, perforated semishells can be fabricated using a combination of clean-room techniques. For a semishell with a single spherical perforation positioned on its symmetry axis, we examine how the resonant modes of the structure depend on hole size and shape. Placing the perforation off the symmetry axis allows a family of higher-order modes to be excited in the nanostructure, along with complex near-field charge distributions for the various resonant modes. This reduced-symmetry case provides a platform for optical studies, which agree quite well with theoretical analysis. Our study also examines two important variations of this structure: a semishell with multiple perforations in the shell layer, and a semishell with a wedge-like "slice" in the shell layer. A semishell with a wedge-like perforation can be thought of as a three-dimensional analogue of a split-ring resonator (SRR), an important nanoscale component in metamaterial design. Here we show that the dimensions of the wedge-like perforation, which control the effective optical frequency resistance, inductance, and capacitance of this structure, determine the frequency of the magnetic mode.
ACS Nano 2010 May 25
PMID:Perforated semishells: far-field directional control and optical frequency magnetic response. 2042 30

A simple template-free hydrothermal route was used for the synthesis of novel mesoporous CuO dandelion structures formed by self-organized CuO nanorods. A very high surface area approximately 325 m(2)/g and remarkably enhanced photoconductivity under white light irradiation of the CuO dandelions were observed compared to the nanocrystals. The extremely high photoconductivity is attributed to the presence of oxygen related hole-trap states at the large surface area of the dandelions. The fast response (tau = 24 s) of the photocurrent holds promise for the fast photo-sensing device applications.
ACS Appl Mater Interfaces 2010 May
PMID:Template-free synthesis of mesoporous CuO dandelion structures for optoelectronic applications. 2043 59

We describe the synthesis and characterization of a cross-linkable siloxane-derivatized tetraphenylbenzidine (DTMS-TPD), which was used for the fabrication of semiconducting highly ordered nanorod arrays on conductive indium tin oxide or Pt-coated substrates. The stepwise process allow fabricating of macroscopic areas of well-ordered free-standing nanorod arrays, which feature a high resistance against organic solvents, semiconducting properties and a good adhesion to the substrate. Thin films of the TPD derivate with good hole-conducting properties could be prepared by cross-linking and covalently attaching to hydroxylated substrates utilizing an initiator-free thermal curing at 160 degrees C. The nanorod arrays composed of cross-linked DTMS-TPD were fabricated by an anodic aluminum oxide (AAO) template approach. Furthermore, the nanorod arrays were investigated by a recently introduced method allowing to probe local conductivity on fragile structures. It revealed that more than 98% of the nanorods exhibit electrical conductance and consequently feature a good electrical contact to the substrate. The prepared nanorod arrays have the potential to find application in the fabrication of multilayered device architectures for building well-ordered bulk-heterojunction solar cells.
ACS Appl Mater Interfaces 2010 Jun
PMID:Template-based preparation of free-standing semiconducting polymeric nanorod arrays on conductive substrates. 2043 60

Polymers to be used in bulk heterojunction (BHJ) solar cells should maintain a low highest occupied molecular orbital (HOMO) energy level as well as a narrow band gap in order to maximize the open circuit voltage (V(oc)) and the short circuit current (J(sc)). To concurrently lower the HOMO energy level and the band gap, we propose to modify the donor-acceptor low band gap polymer strategy by constructing alternating copolymers incorporating a "weak donor" and a "strong acceptor". As a result, the "weak donor" should help maintain a low HOMO energy level while the "strong acceptor" should reduce the band gap via internal charge transfer (ICT). This concept was examined by constructing a library of polymers employing the naphtho[2,1-b:3,4-b']dithiophene (NDT) unit as the weak donor, and benzothiadiazole (BT) as the strong acceptor. PNDT-BT, designed under the "weak donor-strong acceptor" strategy, demonstrated both a low HOMO energy level of -5.35 eV and a narrow band gap of 1.59 eV. As expected, a noticeably high V(oc) of 0.83 V was obtained from the BHJ device of PNDT-BT blended with PCBM. However, the J(sc) ( approximately 3 mA/cm(2)) was significantly lower than the maximum expected current from such a low band gap material, which limited the observed efficiency to 1.27% (with a 70 nm thin film). Further improvements in the efficiency are expected from these materials if new strategies can be identified to (a) increase the molecular weight and (b) improve the hole mobility while still maintaining a low HOMO energy level and a narrow band gap.
ACS Appl Mater Interfaces 2010 May
PMID:A weak donor-strong acceptor strategy to design ideal polymers for organic solar cells. 2043 89

The utilization of graphene oxide (GO) thin films as the hole transport and electron blocking layer in organic photovoltaics (OPVs) is demonstrated. The incorporation of GO deposited from neutral solutions between the photoactive poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) layer and the transparent and conducting indium tin oxide (ITO) leads to a decrease in recombination of electrons and holes and leakage currents. This results in a dramatic increase in the OPV efficiencies to values that are comparable to devices fabricated with PEDOT:PSS as the hole transport layer. Our results indicate that GO could be a simple solution-processable alternative to PEDOT:PSS as the effective hole transport and electron blocking layer in OPV and light-emitting diode devices.
ACS Nano 2010 Jun 22
PMID:Solution-processable graphene oxide as an efficient hole transport layer in polymer solar cells. 2048 12

Colloidal quantum dot (CQD) photovoltaics combine low-cost solution processability with quantum size-effect tunability to match absorption with the solar spectrum. Rapid recent advances in CQD photovoltaics have led to impressive 3.6% AM1.5 solar power conversion efficiencies. Two distinct device architectures and operating mechanisms have been advanced. The first-the Schottky device-was optimized and explained in terms of a depletion region driving electron-hole pair separation on the semiconductor side of a junction between an opaque low-work-function metal and a p-type CQD film. The second-the excitonic device-employed a CQD layer atop a transparent conductive oxide (TCO) and was explained in terms of diffusive exciton transport via energy transfer followed by exciton separation at the type-II heterointerface between the CQD film and the TCO. Here we fabricate CQD photovoltaic devices on TCOs and show that our devices rely on the establishment of a depletion region for field-driven charge transport and separation, and that they also exploit the large bandgap of the TCO to improve rectification and block undesired hole extraction. The resultant depleted-heterojunction solar cells provide a 5.1% AM1.5 power conversion efficiency. The devices employ infrared-bandgap size-effect-tuned PbS CQDs, enabling broadband harvesting of the solar spectrum. We report the highest open-circuit voltages observed in solid-state CQD solar cells to date, as well as fill factors approaching 60%, through the combination of efficient hole blocking (heterojunction) and very small minority carrier density (depletion) in the large-bandgap moiety.
ACS Nano 2010 Jun 22
PMID:Depleted-heterojunction colloidal quantum dot solar cells. 2049 82

In an aim to harvest UV-near-visible (360-440 nm) photons as well as to increase the morphology in the bulk heterojunction solar cells, we report herein the strategic design, synthesis, and characterization of a novel excited-state intramolecular proton-transfer dye, 3-hydroxy-2-(5-(5-(5-(3-hydroxy-4-oxo-4H-chromen-2-yl)thiophen-2-yl)thiophen-2-yl)thiophen-2-yl)-4H-chromen-4-one (FT), which bears two key functional groups, namely 3-hydroxychromone chromophore and trithiophene backbone and is then exploited into the blends of regioregular poly(3-hexylthiophene) (RR-P3HT) and phenyl-C(61)-butyric acid methyl ester (PCBM). FT acts as an excellent UV-near visible absorber, which then undergoes excited-state intramolecular proton transfer, giving rise to an orange-red proton-transfer emission that was reabsorbed by P3HT via a Forster type of energy transfer. Introduction of FT to P3HT/PCBM blend films also improves the morphology of phase separated structure, in particular, enhances the interaction of P3HT chains and the hole mobility. In this work, under the optimized condition of P3HT: PCBM:FT of 15:9:2 in weight ratio, the best performance of the device B-FT2 revealed consistent enhancements in the efficiency (eta) 4.28% and short-circuit current (J(sc)) 12.53 mAcm(-2), which are higher than that (3.68% and 10.28 mAcm(-2)) of the best performance of the control device B (P3HT:PCBM 15:9 in weight ratio) by 16 and 22%, respectively.
ACS Appl Mater Interfaces 2010 Jun
PMID:Design and synthesis of trithiophene-bound excited-state intramolecular proton transfer dye: enhancement on the performance of bulk heterojunction solar cells. 2049 80

Coordination of phenyldithiocarbamate (PTC) ligands to solution-phase colloidal CdSe quantum dots (QDs) decreases the optical band gap, E(g), of the QDs by up to 220 meV. These values of DeltaE(g) are the largest shifts achieved by chemical modification of the surfaces of solution-phase CdSe QDs and are-by more than an order of magnitude in energy-the largest bathochromic shifts achieved for QDs in either the solution or solid phases. Measured values of DeltaE(g) upon coordination to PTC correspond to an apparent increase in the excitonic radius of 0.26 +/- 0.03 nm; this excitonic delocalization is independent of the size of the QD for radii, R = 1.1-1.9 nm. Density functional theory calculations indicate that the highest occupied molecular orbital of PTC is near resonant with that of the QD, and that the two have correct symmetry to exchange electron density (PTC is a pi-donor, and the photoexcited QD is a pi-acceptor). We therefore propose that the relaxation of exciton confinement occurs through delocalization of the photoexcited hole of the QD into the ligand shell.
ACS Nano 2010 Jun 22
PMID:Relaxation of exciton confinement in CdSe quantum dots by modification with a conjugated dithiocarbamate ligand. 2050 78


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