Davide Calestani

Zn vacancy induced green luminescence on non-polar surfaces in ZnO nanostructures

Although generally ascribed to the presence of defects, an ultimate assignment of the different contributions to the emission spectrum in terms of surface states and deep levels in ZnO nanostructures is still lacking. In this work we unambiguously give first evidence that zinc vacancies at the (1010) nonpolar surfaces are responsible for the green luminescence of ZnO nanostructures. The result is obtained by performing an exhaustive comparison between spatially resolved cathodoluminescence spectroscopy and imaging and ab initio simulations. Our findings are crucial to control undesired recombinations in nanostructured devices.

Aldehyde detection by ZnO tetrapod-based gas sensors

Zinc oxide (ZnO) tetrapods have been synthesized and prototypal gas sensors for volatile organic compounds (VOCs) have been realized with this sensing nanomaterial. Sensor characterization has been focused on aldehydes, such as acetaldehyde (or ethanal) and propionaldehyde (or propanal), whose detection at ppb levels is extremely important for monitoring environmental, domestic and food pollution, as well as for some sickness diagnosis and other medical applications. The response of gas sensor prototypes has been measured as a function of concentration, temperature, relative humidity and time. Good response values at ppb levels and some peculiar behaviours have been pointed out.

Low temperature thermal evaporation growth of aligned ZnO nanorods on ZnO film: a growth mechanism promoted by Zn nanoclusters on polar surfaces

ZnO aligned nanorods have been obtained on ZnO films by vapour phase at relatively low temperature (<500 °C). In this temperature range, which is for example interesting for glass-compatible processes for solar cell production, it is generally impossible to obtain a standard vapour–solid growth of these ordered nanostructures without the use of metal–organic precursors or metal catalysts. However, it is pointed out that, by means of a deposition of Zn nanoclusters on the polar surface of ZnO film grains, it is possible to obtain vertically aligned ZnO nanorods even at 480 °C. Zn nanocluster formation and rod growth can be achieved in a few minutes in a single process, without introducing any foreign element that might produce undesired material contamination and affect the physical properties of these nanostructures. The growth mechanism is discussed in detail, making comparisons with the results achieved on different substrates and under different growth conditions. Thanks to the comprehension of the growth mechanism, very uniform deposition of nanorods has been obtained on two square centimetres substrates in a small laboratory-scale reactor.

Unpredicted Nucleation of Extended Zinc Blende Phases in Wurtzite ZnO Nanotetrapod Arms

Tailoring the structural and electronic properties of 3D nanostructures via bottom-up techniques would pave the way for novel low-cost applications. One of such possibilities is offered by ZnO branched nanostructures like tetrapods, that have recently attracted attention for nanodevice applications from nanoelectronics to drug delivery. The conventional picture is that ZnO arms are thermodynamically stable only in the wurtzite phase. Here, we provide the first experimental evidence of unpredicted extended zinc blend phases (50-60 nm long) embedded in the arms of ZnO wurtzite tetrapods. In particular, decisive evidence is obtained from the one-to-one correlation between high lateral resolution cathodoluminescence spectroscopy, monochromatic contrast maps, and atomic resolution transmission electron microscopy images of ZnO single TPs. This observation is not specific to ZnO and can have a general validity for the understanding of the nucleation mechanisms in semiconducting 3D nanostructures for device applications.

A single cotton fiber organic electrochemical transistor for liquid electrolyte saline sensing

A single natural cotton fiber has been functionalized with poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) conductive polymer by a simple soaking process and used as a channel of an organic electrochemical transistor (OECT), directly interfaced with a liquid electrolyte in contact with an Ag wire gate. The device shows a stable and reproducible current modulation and has been demonstrated to be very effective for electrochemical sensing of NaCl concentration in water. The single wire cotton fiber OECT results to be a simple and low cost device, which is very attractive for wearable electronics in fitness and healthcare.

15% efficient Cu(In,Ga)Se2 solar cells obtained by low-temperature pulsed electron deposition

An approach to low-cost production of Cu(In,Ga)Se2 (CIGS) solar cells based on pulsed electron deposition (PED) has achieved a crucial milestone. Lab-scale solar cells with efficiencies exceeding 15% were obtained by depositing CIGS from a stoichiometric quaternary target at 270 °C and without any post-growth treatment. An effective control of the p-doping level in CIGS was achieved by starting the PED deposition with a layer of NaF tailored to generate the optimum Na diffusion. These results show that PED is a promising technology for the development of a competitive low-cost production process for CIGS solar cells.

Human stress monitoring through an organic cotton-fiber biosensor

Selective detection of bioanalytes in physiological fluids, such as blood, sweat or saliva, by means of low-cost and non-invasive devices, is of crucial importance to improve diagnosis and prevention in healthcare. To be really useful in everyday life a sensing system needs to be handy, non-invasive, easy to read and possibly wearable. Only a sensor that satisfies these requirements could be eligible for applications in healthcare and physiological condition monitoring. Herein an organic electrochemical transistor has been investigated as a simple, low-cost and e-textile biosensor, fully integrated on a single cotton yarn. The biosensor has been used for real-time detection of adrenaline, selectively compared to the saline content in human physiological fluids. The sensing mechanism is based on the oxidation of adrenaline at the Pt-gate electrode surface, with the formation of adrenaline-quinone and adrenochrome. Two independent organic electrochemical transistors, characterized by different gate-electrode materials, detect saline and adrenaline concentrations, respectively, in real human sweat. Measurements performed in real-time mode show the complete independence of adrenaline detection from NaCl and, hence, guarantee the simultaneous monitoring of both concentrations. The oxidation of adrenaline has been studied by means of absorption spectroscopy in air, with either silver or platinum working electrodes. Our results confirm that the oxidation reaction driven by the Pt-electrode leads to the formation of adrenochrome, while with the Ag-electrode the oxidation is similar to the spontaneous one occurring in air. The cotton-based biosensor shows the possibility of monitoring human performances (hydration and stress) in situ and using a non-invasive approach, opening new unexplored opportunities in healthcare, fitness and work safety.

Growth and Characterization of CZT Crystals by the Vertical Bridgman Method for X-Ray Detector Applications

CdZnTe crystals were grown by the vertical Bridgman method in closed quartz ampoules. The crystalline quality and the impurity content of these crystals were studied. Several X-ray detectors were cut out of these crystals. The resistivity, emission spectra, μτ product, and spectroscopic characteristics of these detectors were extensively measured and compared with the characteristics of detectors obtained from CdZnTe crystals grown by the boron oxide encapsulated vertical Bridgman technique. The detectors prepared from crystals grown without boron oxide show good μτ value, spectroscopic resolution, and higher reproducibility. The influence of growth method on impurity content and on detector response was discussed.

Extended functionality of ZnO nanotetrapods by solution-based coupling with CdS nanoparticles

Coupled nanostructures with “spotted” and “core–shell” morphologies have been obtained by a controlled heterogeneous nucleation of cadmium sulfide nanoparticles (CdS-NPs) onto the surface of zinc oxide nanotetrapods (ZnO-TPs). This result has been obtained by means of a modified chemical bath deposition reaction, without the need of any surfactant and surface passivating agents. The coupled nanostructure has been demonstrated to form an active p–n type-II heterojunction, clearly affecting the functional properties of ZnO nanostructures, as in gas-sensing or photocatalysis applications.

Microtexturing of the conductive PEDOT:PSS Polymer for superhydrophobic organic electrochemical transistors

Superhydrophobic surfaces are bioinspired, nanotechnology artifacts, which feature a reduced friction coefficient, whereby they can be used for a number of very practical applications including, on the medical side, the manipulation of biological solutions. In this work, we integrated superhydrophobic patterns with the conducting polymer PEDOT:PSS, one of the most used polymers in organic electronics because highly sensitive to ionized species in solution. In doing so, we combined geometry and materials science to obtain an advanced device where, on account of the superhydrophobicity of the system, the solutions of interest can be manipulated and, on account of the conductive PEDOT:PSS polymer, the charged molecules dispersed inside can be quantitatively measured. This original substrate preparation allowed to perform electrochemical measurements on ionized species in solution with decreasing concentration down to 10−7 molar. Moreover, it was demonstrated the ability of the device of realizing specific, combined time and space resolved analysis of the sample. Collectively, these results demonstrate how a tight, interweaving integration of different disciplines can provide realistic tools for the detection of pathologies. The scheme here introduced offers breakthrough capabilities that are expected to radically improve both the pace and the productivity of biomedical research, creating an access revolution.