G. M. Lazzerini

Thermochemical nanopatterning of organic semiconductors

Patterning of semiconducting polymers on surfaces is important for various applications in nanoelectronics and nanophotonics. However, many of the approaches to nanolithography that are used to pattern inorganic materials are too harsh for organic semiconductors, so research has focused on optical patterning and various soft lithographies. Surprisingly little attention has been paid to thermal, thermomechanical and thermochemical patterning. Here, we demonstrate thermochemical nanopatterning of poly(p-phenylene vinylene), a widely used electroluminescent polymer, by a scanning probe. We produce patterned structures with dimensions below 28 nm, although the tip of the probe has a diameter of 5 microm, and achieve write speeds of 100 microm s(-1). Experiments show that a resolution of 28 nm is possible when the tip-sample contact region has dimensions of approximately 100 nm and, on the basis of finite-element modelling, we predict that the resolution could be improved by using a thinner resist layer and an optimized probe. Thermochemical lithography offers a versatile, reliable and general nanopatterning technique because a large number of optical materials, including many commercial crosslinker additives and photoresists, rely on chemical mechanisms that can also be thermally activated.

Large Work Function Shift of Gold Induced by a Novel Perfluorinated Azobenzene-Based Self-Assembled Monolayer

Tune it with light! Self-assembled monolayers on gold based on a chemisorbed novel azobenzene derivative with a perfluorinated terminal phenyl ring are prepared. The modified substrate shows a significant work function increase compared to the bare metal. The photo-conversion between trans and cis isomers chemisorbed on the surface shows great perspectives for being an accessible route to tune the gold properties by means of light.

Photoinduced work function changes by isomerization of a densely packed azobenzene-based SAM on Au: a joint experimental and theoretical study.

Responsive monolayers are key building blocks for future applications in organic and molecular electronics in particular because they hold potential for tuning the physico-chemical properties of interfaces, including their energetics. Here we study a photochromic SAM based on a conjugated azobenzene derivative and its influence on the gold work function (Φ(Au)) when chemisorbed on its surface. In particular we show that the Φ(Au) can be modulated with external stimuli by controlling the azobenzene trans/cis isomerization process. This phenomenon is characterized experimentally by four different techniques, kelvin probe, kelvin probe force microscopy, electroabsorption spectroscopy and ultraviolet photoelectron spectroscopy. The use of different techniques implies exposing the SAM to different measurement conditions and different preparation methods, which, remarkably, do not alter the observed work function change (Φ(trans)-Φ(cis)). Theoretical calculations provided a complementary insight crucial to attain a deeper knowledge on the origin of the work function photo-modulation.

Validation of the Compatibility Between a Porous Silicon-Based Gas Sensor Technology and Standard Microelectronic Process

The compatibility of a recently proposed porous silicon formation procedure for gas sensor integration with a commercial microelectronic process is analyzed. Porous silicon-based gas sensors have been produced on a test chip by means of a post-processing approach that enables silicon anodization in selected areas. The effects of the post-processing procedure on electronic circuits, integrated on the test chip as the sensors, have been investigated by electrical measurements. Critical electrical parameters of purposely-designed high-performance analog cells have been measured on several post-processed and not post-processed samples. Experimental outcomes demonstrate the actual compatibility of the post-processing procedure for porous silicon formation with commercial microelectronic processes.

Enhanced crystallinity and film retention of P3HT thin-films for efficient organic solar cells by use of preformed nanofibers in solution

We report the preparation of films of poly(3-hexylthiophene) nanofibers suitable for fabrication of efficient multilayer solar cells by successive deposition of donor and acceptor layers from the same solvent. The nanofibers are obtained by addition of di-tert-butyl peroxide (DTBP) to a solution of P3HT in chlorobenzene. Interestingly, by varying the concentration of DTBP we are able to control both crystallinity and film retention of the spin-cast films. We also investigate the influence of the DTBP-induced crystallization on charge transport by thin-film transistor measurements, and find a more than five-fold increase in the hole mobility of nanofiber films compared to pure P3HT. We attribute this effect to the synergistic effects of increased crystallinity of the fibers and the formation of micrometer-sized fiber networks. We further demonstrate how it is possible to make use of the high film retention to fabricate photovoltaic devices by subsequent deposition of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) from a chlorobenzene solution on top of the nanofiber film. The presence of a relatively large crystalline phase strongly affects the diffusion behavior of PCBM into the P3HT film, resulting in a morphology which is different from that of common bulk heterojunction solar cells and resembles a bilayer structure, as can be inferred from comparison of the external quantum efficiency spectra. However, a high power conversion efficiency of 2.3% suggests that there is still a significant intermixing of the two materials taking place.

Modeling of porous silicon junction field effect transistor gas sensors: Insight into NO2 interaction

In this paper a lumped parameter electrical model for porous silicon junction field effect transistor (PSJFET) gas sensors is presented and experimentally validated. The PSJFET is an integrated p-channel JFET device modified with a porous silicon layer, the latter acting as sensing element. The model is described by using an analytical closed-form expression, which quantitatively links the sensor current to the analyte concentration in the environment, and validated by using experimental data of PSJFETs exposed to both synthetic air and NO2 with concentration of 300 ppb. Best fitting of experimental data with the proposed model allows one to get quantitative information on the effect of NO2 adsorption/desorption at the PS surface on electrical PS parameters, such as its conductance and surface charge density.

Low-temperature treatment of semiconducting interlayers for high-efficiency light-emitting diodes based on a green-emitting polyfluorene derivative

We investigate the scope for low-temperature treatment of exciton/electron blocking interlayers in light-emitting diodes based on poly(9,9′-dioctylfiuorene-alt-benzothiadiazole) (F8BT). We focus on poly(9,9′-dioctylfluorene-alt-N-(4-butylphenyl)-diphenylamine) (TFB) interlayers processed at temperatures up to 50 °C, i.e., far below the glass transition temperature of TFB (∼156 °C). Continuous-wave and time-resolved photoluminescence studies confirm the formation of both excitons and exciplex species, as a result of the F8BT/TFB intermixing. Interestingly, however, we can still increase the electroluminescence external quantum efficiency from 0.05% to 0.5% and 1% for progressively thicker TFB films. We propose that a degree of intermixing may become acceptable as a trade-off to achieve low-temperature processability.

Addressing Reliability and Degradation of Chemitransistor Sensors by Electrical Tuning of the Sensitivity

Here we show that electrical tuning of the sensitivity of chemitransistor sensors, namely field-effect-transistors (FETs) exploiting nano/mesostructured sensing materials, can be used to effectively address two chief problems of state-of-the-art gas sensors, specifically fabrication reliability and degradation by aging. Both experimental evidences and theoretical calculations are provided to support such a result, using as a case-of-study junction field-effect-transistors (JFETs) exploiting mesostructured porous silicon (PS) as sensing material (PSJFETs) for the detection of nitrogen dioxide (NO2) at hundreds ppb. Proof of concept is given by fully compensating the effect of fabrication errors on the sensitivity of two PSJFETs integrated on the same chip, which, though identical in principle, feature sensitivities to NO2 differing from about 30% before compensation. Although here-demonstrated for the specific case of PSJFETs, the concept of sensor reliability/aging problem compensation by sensitivity electrical-tuning can be applied to other chemitransistor sensors that exploit sensing materials different than PS.

Increased efficiency of light-emitting diodes incorporating anodes functionalized with fluorinated azobenzene monolayers and a green-emitting polyfluorene derivative

We investigate the functionalization of gold anodes with azobenzene-based self-assembled monolayers (AZO-SAM) and the influence of such functionalization on the external quantum efficiency (EQE) of polyfluorene-based light-emitting diodes (LEDs). Photoluminescence and electroluminescence measurements show that the AZO-SAMs do not modify the shape of the emission spectrum of the active layer. Instead, AZO-SAMs enhance the EQE of LEDs by an order of magnitude (from 0.018% to 0.18%) and decrease the turn-on voltage from 7.9 V to 6.2 V by reducing the injection barrier at the anode, thus promoting a better balance between hole and electron populations in the active layer.

A porous silicon JFET gas sensor: Experimental and modeling

In this work, electrical measurements and modeling of an integrated porous silicon (PS) JFET (PSJFET) gas sensor are reported. The experimental IDS-VDS curves were measured for different VGS voltages, both in synthetic air and in presence of 300 ppb of NO2. The modeling was carried out by taking both the PS layer and the FET structure into account. By best fitting the experimental data with the given model, a quantitative evaluation of the effect of NO2 on both the PS layer and the FET structure was performed. Firstly, an indirect estimation of the PS resistance values in air (RPS_AIR~31 kOmega) and in NO2 (RPS_NO2~3.2 kOmega) was achieved, in agreement with data reported in literature and acquired by direct electrical measurements on PS. Moreover, it was also estimated that exposure to 300 ppb of NO2 resulted in a reduction of the FET channel section of ~0.2 mum, with respect to air.