Steve Smout

Photoactivation of Silver-Exchanged Zeolite A†

Clusters of silver atoms and ions have attracted the interest of scientists because of their pronounced catalytic and emissive properties. To prevent aggregation of the clusters into larger particles, stabilization in gas matrices at cryogenic temperatures, or in scaffolds such as polyphosphates, DNA, peptides or polymers at ambient temperature, has been suggested. Alternatively, zeolites have been proposed to stabilize small ionic silver clusters. The molecular dimensions of the zeolite cages and channels prevent aggregation into larger nanoparticles (because of steric reasons) while the net negative charge of the zeolite lattice, the coordinating properties of the lattice oxygen atoms, and the presence of additional cations play a crucial role in stabilizing cationic clusters and unstable intermediates during reduction. Reduction of silver in zeolites is usually a bulk process that requires reductants, such as hydrogen gas or sodium borohydride, but also g irradiation and visible light can cause reduction. One of the most studied systems is silver-exchanged zeolite A (LTA topology), and several models have been proposed to explain the nature and location of the silver clusters formed in this zeolite upon reduction. Aside from their catalytic properties, oligonuclear silver clusters show particularly bright and stable luminescent properties. However, the existing fluorescence studies about Ag clusters in zeolites are limited to the excitation and emission spectra of bulk powder samples. Here, we report on bright fluorescent (spots in) individual silverexchanged zeolite 3A crystals obtained upon photoactivation using a focused UV irradiation on a fluorescence microscope. Photoactivation of silver has been demonstrated for nanoscale Ag2O particles (and interpreted as a photoreduction process). In our study, the emissive silver material is confined within a zeolite framework, which results in a better control of the type and location of the emissive species formed upon UV irradiation. Silver-cluster-loaded crystals are technologically very attractive—for example, as secondary light sources in fluorescence lamps—because of their high emission intensity, their excellent photostability upon UV irradiation, and their large Stokes shift. Moreover, the space-resolved selective activation of the emission intensity may have important applications in data storage. The emissive zeolite particles used herein were prepared by exchanging zeolite 3Awith (8 1)wt%Ag (fromAgNO3 solutions), followed by heating for one day at 450 8C (see the Supporting Information). The enhanced emission exhibited by the silver-exchanged zeolites after the thermal treatment has been ascribed to two possible causes: 1) to the formation of charge-transfer complexes between the partially (de)hydrated silver ions and the oxygen atoms in the zeolite lattice or 2) to the emissive properties of autoreduced oligoatomic silver clusters that may be formed during the high-temperature treatment. Although this report focuses on the photoactivation of thermally treated Ag zeolites, control experiments performed on not thermally activated Ag zeolites (dried at 110 8C) show an analogous photoactivation behavior (see the supporting Information). Figure 1a(1),b(1) shows typical confocal scanning images of a heat-treated Ag-containing zeolite crystal (roughly 3 ? 3 mm in size) taken under a confocal microscope using a picosecond-pulsed 375 nm excitation source (doubled Ti:Sapphire, Spectra Physics; see the Supporting Information). Figure 1a and 1b were taken at excitation intensities of 10 and 20 Wcm , respectively. Zeolite crystals that were not treated thermally exhibited an emission ten-times-weaker than that of the thermally treated samples. The confocal approach in combination with the pinhole in the emission path (see the Supporting Information) allows the collection of photoemission data from selected parts inside the crystals. Diffraction-limited bright spots can be generated at specific domains inside an individual crystal by simply focusing a low-power UV laser at the target position; for instance, in the crystal shown in Figure 1a, three individual spots were selectively activated by irradiation during [*] Dr. Y. Antoku, Dr. M. Sliwa, S. Smout, Prof. Dr. J. Hofkens, Dr. T. Vosch Department of Chemistry Katholieke Universiteit Leuven Celestijnenlaan 200F, 3001 Heverlee (Belgium) Fax: (+32)1632-7989 E-mail: tom.vosch@chem.kuleuven.be

Photophysics of 3,5-diphenoxy substituted BODIPY dyes in solution

We have prepared two fluorescent dyes derived from 8-(4-tolyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene with phenoxy and (o-bromo)phenoxy substituents at the 3,5-positions by a novel nucleophilic substitution reaction of the corresponding 3,5-dichloroBODIPY analogue. UV-vis absorption, steady-state and time-resolved fluorimetry have been used to investigate their solvent-dependent photophysical properties. The two BODIPY derivatives show narrow absorption and emission bands and display small Stokes shifts. The substituents at the 3,5-positions (phenoxy in 1 and o-bromophenoxy in 2) have a minor effect on the fluorescence quantum yields (0.16-0.40 for 1, 0.17-0.44 for 2) and lifetimes (1.09-2.51 ns for 1, 1.11-2.78 ns for 2). For both compounds, the fluorescence rate constant equals (1.5 +/- 0.1) x 10(8) s(-1).

Organic complementary oscillators with stage-delays below 1 μs

Recently, complex circuits of organic thin-film transistors have been shown. The use of complementary logic can significantly ease the design of large integrated circuits. However, the performance of complementary logic in organic thin-film technology has not been able to equivale that of unipolar logic, due to the difficulty to densely integrate and simultaneously optimize p-type and n-type transistors on a single substrate. Here, we develop an optimized complementary process for C60 n-type and pentacene p-type transistors, both having bottom-gate bottom-contact geometry. Using this complementary technology, we show ring-oscillators with a stage-delay below 1 μs at a supply-voltage of 20 V.

30.1 8b Thin-film microprocessor using a hybrid oxide-organic complementary technology with inkjet-printed P 2 ROM memory

We present an 8b general-purpose microprocessor realized in a hybrid oxide-organic complementary thin-film technology. The n-type transistors are based on a solution-processed n-type metal-oxide semiconductor, and the p-type transistors use an organic semiconductor. As compared to previous work utilizing unipolar logic gates [1], the higher mobility n-type semiconductor and the use of complementary logic allow for a >50x speed improvement. It also adds robustness to the design, which allowed for a more complex and complete standard cell library. The microprocessor consists of two parts, a processor core chip and an instruction generator. The instructions are stored in a Write-Once-Read-Many (WORM) memory formatted by a post-fabrication inkjet printing step, called Print-Programmable Read-Only Memory (P2ROM). The entire processing was performed at temperatures compatible with plastic foil substrates, i.e., at or below 250°C [2].

Low-temperature formation of source–drain contacts in self-aligned amorphous oxide thin-film transistors

We demonstrated self-aligned amorphous-Indium-Gallium-Zinc-Oxide (a-IGZO) thin-film transistors (TFTs) where the source–drain (S/D) regions were made conductive via chemical reduction of the a-IGZO via metallic calcium (Ca). Due to the higher chemical reactivity of Ca, the process can be operated at lower temperatures. The Ca process has the additional benefit of the reaction byproduct calcium oxide being removable through a water rinse step, thus simplifying the device integration. The Ca-reduced a-IGZO showed a sheet resistance (RSHEET) value of 0.7 kΩ/sq., with molybdenum as the S/D metal. The corresponding a-IGZO TFTs exhibited good electrical properties, such as a field-effect mobility (μFE) of 12.0 cm2/(V s), a subthreshold slope (SS−1) of 0.4 V/decade, and an on/off current ratio (ION/OFF) above 108.

Light, bias, and temperature effects on organic TFTs

In this work we present the instabilities observed in organic Thin-Film Transistors when subjected to stress test in different bias, temperature and illumination conditions. C-V measurements, indicate the presence of two distinct trapping phenomena. Appreciable charge trapping can be achieved using relatively high biases for long times (1000s). Illumination strongly enhances charge trapping only under positive gate biases, while it has no effect on the charge trapping/detrapping under negative gate bias. Charge detrapping is thermally activated coherently with trapping/detrapping at the SiO2/pentacene interface from hydrogenoid species. A first order model explaining the observed relaxation kinetics is also presented.

An organic integrated capacitive DC-DC up-converter

In this paper a fully integrated organic DC-DC up-converter is presented in a pentacene p-type only technology. This 3-stage Dickson converter reaches a voltage conversion factor of 3 for a purely capacitive load and 2.5 for a 10 µA load current. The maximal output voltage goes up to 75 V and the Dickson core efficiency is 48 %. The clock signal is generated on-chip with a 9-stage ring oscillator, built with zero-Vgs load inverters. A tunable input voltage provides a tuning range of 30 %. The presented converter is designed for the on-chip generation of voltages for biasing a capacitive load. This converter draws 560 µA from a 20 V supply voltage. The chip area measures 2.8×2.1 mm2. This converter fulfills a direct need for bias voltages beyond the supply voltage that is uncovered in recent work on organic circuits.

15.2 A flexible ISO14443-A compliant 7.5mW 128b metal-oxide NFC barcode tag with direct clock division circuit from 13.56MHz carrier

Flexible low-cost RFID/NFC tags have great potential to be embedded in everyday objects providing them a unique identifier or sensor readout facilitating the Internet-of-Everything, whereby a smartphone or tablet is the interface to the Internet-of-Things. The main challenge for flexible metal-oxide RFID tags is to fully comply with the ISO14443-A NFC standard to enable readout by standard NFC reader or handheld devices, due to the limited charge carrier mobility of the semiconductor and multiple sources of parameter variation caused by roughness, temperature and dimensional stability of the foils. Recent work by various groups [1–4] demonstrated only an incremental improvement in data rates from 50b/s to 396.5kb/s to be compatible with ISO14443 (105.9kb/s). In this work, we present a flexible metal-oxide NFC chip that is compliant with ISO14443-A, showcasing advancements on memory size, power consumption and a clock generation circuit.

16.5 A flexible thin-film pixel array with a charge-to-current gain of 59µA/pC and 0.33% nonlinearity and a cost effective readout circuit for large-area X-ray imaging

We report an active, medical-grade, high resolution, high dynamic range X-ray backplane based on a-IGZO thin-film technology with fast readout. This enables low dose, video rate X-ray imaging. Fast X-ray imaging will find its applications not only in medical, but also in non-destructive inspection. Curved imagers will yield sharper images when illuminated from a point source. This has been achieved thanks to superior a-IGZO technology and a pixel topology that improves noise performance and allows a cost-effective external readout.

Organic RFID Tags

Organic RFID tags are increasingly gaining credibility as a possible low-cost barcode replacement for product identification. This will only happen if organic RFID tags can operate in the frequency range defined by well-accepted EPC standards. This chapter evaluates the performance of existing organic RFID demonstrators and confirms that, based on lab scale demonstration of organic RFID tags, performance comparable to the EPC standards can be obtained. Moreover, the integration of sensors with the tags will enable added functionality and applications beyond pure identification.