S. Aghion

The effect of selective interactions at the interface of polymer–oxide hybrid solar cells

The working mechanisms of excitonic solar cells are strongly dominated by interface processes, which influence the final device efficiency. However, it is still very challenging to clearly track the effects of inter-molecular processes at a mesoscopic level. We report on the realization of polymer-based hybrid solar cells made of prototypical materials, namely, poly(3-hexylthiophene) (P3HT) finely infiltrated in a TiO2 scaffold, with power conversion efficiency exceeding 1%. A step-change improvement in the device performance is enabled by engineering the hybrid interface by the insertion of an appropriate molecular interlayer. An unprecedented set of characterization techniques, including time-resolved optical spectroscopy, X-ray photoemission spectroscopy, positron annihilation spectroscopy and atomistic simulations, allows us to rationalize our findings. We show that a suitable chemical structure of the interlayer molecule induces selective intermolecular interactions, and thus a preferential surface energetic landscape and morphological order at the interface which consequently drives a strong improvement in charge generation and a decrease in recombination losses.

A moiré deflectometer for antimatter

The precise measurement of forces is one way to obtain deep insight into the fundamental interactions present in nature. In the context of neutral antimatter, the gravitational interaction is of high interest, potentially revealing new forces that violate the weak equivalence principle. Here we report on a successful extension of a tool from atom optics—the moiré deflectometer—for a measurement of the acceleration of slow antiprotons. The setup consists of two identical transmission gratings and a spatially resolving emulsion detector for antiproton annihilations. Absolute referencing of the observed antimatter pattern with a photon pattern experiencing no deflection allows the direct inference of forces present. The concept is also straightforwardly applicable to antihydrogen measurements as pursued by the AEgIS collaboration. The combination of these very different techniques from high energy and atomic physics opens a very promising route to the direct detection of the gravitational acceleration of neutral antimatter.

Positron annihilation spectroscopy: a new frontier for understanding nanoparticle-loaded polymer brushes

Nanoparticle-loaded polymer brushes are powerful tools for the development of innovative devices. However, their characterization is challenging and arrays of different techniques are typically required to gain sufficient insight. Here we demonstrate for the first time the suitability of positron annihilation spectroscopy (PAS) to investigate, with unprecedented detail and without making the least damage to samples, the physico-chemical changes experienced by pH-responsive polymer brushes after protonation and after loading of silver nanoparticles. One of the most important findings is the depth profiling of silver nanoparticles inside the brushes. These results open up a completely new way to understand the structure and behavior of such complex systems.

Spray-combustion synthesis: Efficient solution route to high-performance oxide transistors

Significance Although impressive progress in solution-processed metal-oxide (MO) electronics has been achieved, fundamental science challenges remain concerning whether solution-processed MO materials and particularly technologically relevant, indium-gallium-tin-oxide (IGZO), can achieve efficient and stable charge transport characteristics when processed at low temperatures for short times and how IGZO film density, porosity, carrier mobility, and charge trapping can be manipulated. Here, we report a coating technique, spray-combustion synthesis, and demonstrate IGZO semiconductor thickness, densification, nanoporosity, electron mobility, trap densities, and bias stress stability approaching the quality of sputtered films. Metal-oxide (MO) semiconductors have emerged as enabling materials for next generation thin-film electronics owing to their high carrier mobilities, even in the amorphous state, large-area uniformity, low cost, and optical transparency, which are applicable to flat-panel displays, flexible circuitry, and photovoltaic cells. Impressive progress in solution-processed MO electronics has been achieved using methodologies such as sol gel, deep-UV irradiation, preformed nanostructures, and combustion synthesis. Nevertheless, because of incomplete lattice condensation and film densification, high-quality solution-processed MO films having technologically relevant thicknesses achievable in a single step have yet to be shown. Here, we report a low-temperature, thickness-controlled coating process to create high-performance, solution-processed MO electronics: spray-combustion synthesis (SCS). We also report for the first time, to our knowledge, indium-gallium-zinc-oxide (IGZO) transistors having densification, nanoporosity, electron mobility, trap densities, bias stability, and film transport approaching those of sputtered films and compatible with conventional fabrication (FAB) operations.

AEgIS experiment commissioning at CERN

The AEgIS Experiment is an international collaboration based at CERN whose aim is to perform the first direct measurement of the gravitational acceleration g of antihydrogen in the gravitational field of the Earth. Cold antihydrogen will be produced with a pulsed charge exchange reaction in a cylindrical Penning trap where antiprotons will be cooled to 100mK. The cold antihydrogen will be produced in an excited Rydberg state and subsequently formed into a beam. The deflection of the antihydrogen beam will be measured by using Moire deflectometer gratings. After being approved in late 2008, AEgIS started taking data in a commissioning phase early 2012. This report presents an overview of the AEgIS experiment, describes its current status and shows the first measurements on antiproton catching and cooling in the 5 T Penning catching trap. We will also present details on the techniques needed for the 100mK antihydrogen production, such as pulsed positronium production and its excitation with lasers.

Detection of low energy antiproton annihilations in a segmented silicon detector

The goal of the AEIS experiment at the Antiproton Decelerator (AD) at CERN, is to measure directly the Earths gravitational acceleration on antimatter by measuring the free fall of a pulsed, cold antihydrogen beam. The final position of the falling antihydrogen will be detected by a position sensitive detector. This detector will consist of an active silicon part, where the annihilations take place, followed by an emulsion part. Together, they allow to achieve 1% precision on the measurement of with about 600 reconstructed and time tagged annihilations. We present here the prospects for the development of the AEIS silicon position sentive detector and the results from the first beam tests on a monolithic silicon pixel sensor, along with a comparison to Monte Carlo simulations.

Particle tracking at cryogenic temperatures: the Fast Annihilation Cryogenic Tracking (FACT) detector for the AEgIS antimatter gravity experiment

The AEgIS experiment is an interdisciplinary collaboration between atomic, plasma and particle physicists, with the scientific goal of performing the first precision measurement of the Earths gravitational acceleration on antimatter. The principle of the experiment is as follows: cold antihydrogen atoms are synthesized in a Penning-Malmberg trap and are Stark accelerated towards a moire deflectometer, the classical counterpart of an atom interferometer, and annihilate on a position sensitive detector. Crucial to the success of the experiment is an antihydrogen detector that will be used to demonstrate the production of antihydrogen and also to measure the temperature of the anti-atoms and the creation of a beam. The operating requirements for the detector are very challenging: it must operate at close to 4 K inside a 1 T solenoid magnetic field and identify the annihilation of the antihydrogen atoms that are produced during the 1 μs period of antihydrogen production. Our solution—called the FACT detector—is based on a novel multi-layer scintillating fiber tracker with SiPM readout and off the shelf FPGA based readout system. This talk will present the design of the FACT detector and detail the operation of the detector in the context of the AEgIS experiment.

Measuring

experiments main goal is to measure the local gravitational acceleration of antihydrogen and thus perform a direct test of the weak equivalence principle with antimatter. In the first phase of the experiment the aim is to measure with 1% relative precision. This paper presents the antihydrogen production method and a description of some components of the experiment, which are necessary for the gravity measurement. Current status of the experimental apparatus is presented and recent commissioning results with antiprotons are outlined. In conclusion we discuss the short-term goals of the collaboration that will pave the way for the first gravity measurement in the near future.

\bar{g}

For the first time the AEgIS (Antihydrogen Experiment: Gravity, Interferometry, Spectroscopy) experiment will measure the Earths local gravitational acceleration g on antimatter through the evaluation of the vertical displacement of an antihydrogen horizontal beam. This will be a model independent test of the Weak Equivalence Principle at the base of the general relativity. The initial goal of a g measurement with a relative uncertainty of 1% will be achieved with less than 1000 detected antihydrogens, provided that their vertical position could be determined with a precision of a few micrometers. An emulsion based detector is very suitable for this purpose featuring an intrinsic sub-micrometric spatial resolution. Nevertheless, the AEgIS experiment requires unprecedented operational conditions for this type of detector, namely vacuum environment and very low temperature. An intense R&D activity is presently going on to optimize the detector for the AEgIS experimental requirements with rather encouraging results.

with

The principal aim of the AEgIS experiment at CERN is to measure the acceleration of antihydrogen due to Earths gravitational field. This would be a test of the Weak Equivalence Principle, which states that all bodies fall with the same acceleration independently of their mass and composition. The effect of Earths gravitational field on antimatter will be determined by measuring the deflection of the path of the antihydrogen from a straight line. The position of the antihydrogen will be found by detecting its annihilation on the surface of a silicon detector. The gravitational measurement in AEgIS will be performed with a gravity module, which includes the silicon detector, an emulsion detector and a scintillating fibre time-of-flight detector. As the experiment attempts to determine the gravitational acceleration with a precision of 1%, a position resolution better than 10 μm is required. Here we present the results of a study of antiproton annihilations in a 3D silicon pixel sensor and compare the results with a previous study using a monolithic active pixel sensor. This work is part of a larger study on different silicon sensor technologies needed for the development of a silicon position detector for the AEgIS experiment. The 3D detector together with its readout electronics have been originally designed for the ATLAS detector at the LHC. The direct annihilation of low energy antiprotons ( ~ 100 keV) takes place in the first few μm of the silicon sensor and we show that the charged products of the annihilation can be detected with the same sensor. The present study also aims to understand the signature of an antiproton annihilation event in segmented silicon detectors and compares it with a GEANT4 simulation model. These results will be used to determine the geometrical and process parameters to be adopted by the silicon annihilation detector to be installed in AEgIS.