Patterning via Optical Saturable Transitions - Fabrication and Characterization

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical ...

Reactive Vapor Deposition of Conjugated Polymer Films on Arbitrary Substrates

We demonstrate a method of conformally coating conjugated polymers on arbitrary substrates using a custom-designed, low-pressure reaction chamber. ...

The University of Wisconsin-Madison

University Of Massachusetts Amherst

A fluorescence turn-on mechanism to detect high explosives RDX and PETN.

A new polymerization technique that allows for the first-ever synthesis of poly(phenylenedicyanovinylene)s (PPCN2Vs) is described. PPCN2Vs, with their high electron affinities and structural versatility, seem ideally suited to address the need for new n-type polymers. Remarkably the polymers presented herein become more photoluminescent, in the thin film, under continuous irradiation.

Anionic oxidative polymerization: the synthesis of poly(phenylenedicyanovinylene) (PPCN2V).

In the past, the formation of microscale patterns in the far field by light has been diffractively limited in resolution to roughly half the wavelength of the radiation used. Here, we demonstrate lines with an average width of 36 nanometers (nm), about one-tenth the illuminating wavelength lambda1 = 325 nm, made by applying a film of thermally stable photochromic molecules above the photoresist. Simultaneous irradiation of a second wavelength, lambda2 = 633 nm, renders the film opaque to the writing beam except at nodal sites, which let through a spatially constrained segment of incident lambda1 light, allowing subdiffractional patterning. The same experiment also demonstrates a patterning of periodic lines whose widths are about one-tenth their period, which is far smaller than what has been thought to be lithographically possible.

Confining light to deep subwavelength dimensions to enable optical nanopatterning.

A series of 6,6-dicyanofulvene derivatives are synthesized starting from masked, dimeric, or monomeric cyclopentadienones. The reactivities of 6,6-dicyanofulvenes relative to their parent cyclopentadienones are discussed. 6,6-Dicyanofulvenes are capable of undergoing two consecutive, reversible, one-electron reductions and are presented as potential n-type small molecules.

Synthesis, reactivity, and electronic properties of 6,6-dicyanofulvenes.

The nitramine-containing explosive RDX and the nitroester-containing explosive PETN are shown to be susceptible to photofragmentation upon exposure to sunlight. Model compounds containing nitroester and nitramine moieties are also shown to fragment upon exposure to UV irradiation. The products of this photofragmentation are reactive, electrophilic NO(x) species, such as nitrous and nitric acid, nitric oxide, and nitrogen dioxide. N,N-Dimethylaniline is capable of being nitrated by the reactive, electrophilic NO(x) photofragmentation products of RDX and PETN. A series of 9,9-disubstituted 9,10-dihydroacridines (DHAs) are synthesized from either N-phenylanthranilic acid methyl ester or a diphenylamine derivative and are similarly shown to be rapidly nitrated by the photofragmentation products of RDX and PETN. A new (turn-on) emission signal at 550 nm is observed upon nitration of DHAs due to the generation of fluorescent donor-acceptor chromophores. Using fluorescence spectroscopy, the presence of ca. 1.2 ng of RDX and 320 pg of PETN can be detected by DHA indicators in the solid state upon exposure to sunlight. The nitration of aromatic amines by the photofragmentation products of RDX and PETN is presented as a unique, highly selective detection mechanism for nitroester- and nitramine-containing explosives and DHAs are presented as inexpensive and impermanent fluorogenic indicators for the selective, standoff/remote identification of RDX and PETN.

Detection of explosives via photolytic cleavage of nitroesters and nitramines.

Rylene dyes functionalized with varying numbers of phenyl trifluorovinylether (TFVE) moieties were subjected to a thermal emulsion polymerization to yield shape-persistent, water-soluble chromophore nanoparticles. Perylene and terrylene diimide derivatives containing either two or four phenyl TFVE functional groups were synthesized and subjected to thermal emulsion polymerization in tetraglyme. Dynamic light scattering measurements indicated that particles with sizes ranging from 70 - 100 nm were obtained in tetraglyme, depending on monomer concentration. The photophysical properties of individual monomers were preserved in the nanoemulsions and emission colors could be tuned between yellow, orange, red, and deep red. The nanoparticles were found to retain their shape upon dissolution into water and the resulting water suspensions displayed moderate to high fluorescence quantum yield.

Thermally-Polymerized Rylene Nanoparticles.

By saturating a photochromic transition with a nodal illumination (wavelength, λ), one isomeric form of a small molecule is spatially localized to a region smaller than the far-field diffraction limit. A selective oxidation step effectively locks this pattern allowing repeated patterning. Using this approach and a two-beam interferometer, we demonstrate isolated lines as narrow as λ/8 (78 nm) and spacing between features as narrow as λ/4 (153 nm). This is considerably smaller than the minimum far-field diffraction limit of λ/2. Most significantly, nanopatterning is achieved via single-photon reactions and at low light levels, which in turn allow for high throughput.

Breaking the far-field diffraction limit in optical nanopatterning via repeated photochemical and electrochemical transitions in photochromic molecules.

The motivation of this study is to determine if small amounts of designer additives placed at the polymer-fullerene interface in bulk heterojunction (BHJ) solar cells can influence their performance. A series of AB-alternating side-chain-functionalized poly(thiophene) analogues, P1-6, are designed to selectively localize at the interface between regioregular poly(3-hexylthiophene) (rr-P3HT) and PC(n)BM (n = 61, 71). The side chains of every other repeat unit in P1-6 contain various terminal aromatic moieties. BHJ solar cells containing ternary mixtures of rr-P3HT, PC(n)BM, and varying weight ratios of additives P1-6 are fabricated and studied. At low loadings, the presence of P1-6 consistently increases the short circuit current and decreases the series resistance of the corresponding devices, leading to an increase in power conversion efficiency (PCE) compared to reference P3HT/PC(61)BM cells. Higher additive loadings (>5 wt %) lead to detrimental nanoscale phase separation within the active layer blend and produce solar cells with high series resistances and low overall PCEs. Small-perturbation transient open circuit voltage decay measurements reveal that, at 0.25 wt % incorporation, additives P1-6 increase charge carrier lifetimes in P3HT/PC(61)BM solar cells. Pentafluorophenoxy-containing polymer P6 is the most effective side-chain-functionalized additive and yields a 28% increase in PCE when incorporated into a 75 nm thick rr-P3HT/PC(61)BM BHJ at a 0.25 wt % loading. Moreover, devices with 220 nm thick BHJs containing 0.25 wt % P6 display PCE values of up to 5.3% (30% PCE increase over a control device lacking P6). We propose that additives P1-6 selectively localize at the interface between rr-P3HT and PC(n)BM phases and that aromatic moieties at side-chain termini introduce a dipole at the polymer-fullerene interface, which decreases the rate of bimolecular recombination and, therefore, improves charge collection across the active layer.

Improving the performance of P3HT-fullerene solar cells with side-chain-functionalized poly(thiophene) additives: a new paradigm for polymer design.

We investigate the bias-stress effect in field-effect transistors (FETs) consisting of 1,2-ethanedithiol-treated PbS quantum dot (QD) films as charge transport layers in a top-gated configuration. The FETs exhibit ambipolar operation with typical mobilities on the order of μ(e) = 8 × 10(-3) cm(2) V(-1) s(-1) in n-channel operation and μ(h) = 1 × 10(-3) cm(2) V(-1) s(-1) in p-channel operation. When the FET is turned on in n-channel or p-channel mode, the established drain-source current rapidly decreases from its initial magnitude in a stretched exponential decay, manifesting the bias-stress effect. The choice of dielectric is found to have little effect on the characteristics of this bias-stress effect, leading us to conclude that the associated charge-trapping process originates within the QD film itself. Measurements of bias-stress-induced time-dependent decays in the drain-source current (I(DS)) are well fit to stretched exponential functions, and the time constants of these decays in n-channel and p-channel operation are found to follow thermally activated (Arrhenius) behavior. Measurements as a function of QD size reveal that the stressing process in n-channel operation is faster for QDs of a smaller diameter while stress in p-channel operation is found to be relatively invariant to QD size. Our results are consistent with a mechanism in which field-induced nanoscale morphological changes within the QD film result in screening of the applied gate field. This phenomenon is entirely recoverable, which allows us to repeatedly observe bias stress and recovery characteristics on the same device. This work elucidates aspects of charge transport in chemically treated lead chalcogenide QD films and is of relevance to ongoing investigations toward employing these films in optoelectronic devices.

Bias-stress effect in 1,2-ethanedithiol-treated PbS quantum dot field-effect transistors.

P3HT/PC(61)BM bulk heterojunction solar cells containing varying amounts of different 6,6-dicyanofulvenes (DCFs) were fabricated and characterized. Photovoltaic cells containing ternary mixtures of P3HT, 0.5 equiv of PC(61)BM, and 0.5 equiv of 1,4-dimethyl-2,3-diphenyl-DCF (by weight) displayed average power conversion efficiencies of up to 4.5% under AM 1.5 irradiation, compared to 2.9% for reference P3HT-PC(61)BM solar cells. It was found that 1,4-dimethyl-2,3-diphenyl-6,6-dicyanofulvene could replace up to 50 wt % of PC(61)BM in 1:1 P3HT-PC(61)BM solar cells without sacrificing device performance.

Bulk heterojuction solar cells containing 6,6-dicyanofulvenes as n-type additives.

A thin-film transistor: An n-type polymer semiconductor, poly(2,3-bis(perfluorohexyl)thieno[3,4-b]pyrazine), was synthesized through a Pd-catalyzed polycondensation employing a perfluorinated multiphase solvent system. This is the first example of an n-type polymer semiconductor with exclusive solubility in fluorinated solvents. The fabrication of organic field effect transistors containing this new n-type polymer semiconductor is shown.

An air-stable low-bandgap n-type organic polymer semiconductor exhibiting selective solubility in perfluorinated solvents.

Absorbance modulation is an approach that enables the localization of light to deep sub-wavelength dimensions by the use of photochromic materials. In this article, we demonstrate the application of absorbance modulation on a transparent (quartz) substrate, which enables patterning of isolated lines of width 60 nm for an exposure wavelength of 325 nm. Furthermore, by moving the optical pattern relative to the sample, we demonstrate patterning of closely spaced lines, whose spacing is as small as 119 nm.

Trisha L. Andrew

Department of Chemistry

The University of Wisconsin-Madison

Patterning via Optical Saturable Transitions - Fabrication and Characterization

Reactive Vapor Deposition of Conjugated Polymer Films on Arbitrary Substrates

Trisha L. Andrew

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Chemistry

The University of Wisconsin-Madison

Patterning via Optical Saturable Transitions - Fabrication and Characterization

Reactive Vapor Deposition of Conjugated Polymer Films on Arbitrary Substrates

Department of Chemistry