Sensors and Microsystems - Proceedings of the 10th Italian Conference, 2008
... system. 3 Results 3.1 Nana structured Titania Growth and Sensitization with Organic Molecules... more ... system. 3 Results 3.1 Nana structured Titania Growth and Sensitization with Organic Molecules Very interesting results in the synthesis of nanostructured titania have been obtained using cluster beams produced by the PMCS. ...
A problem of increasing interest in the last years, both for mass-spectrometry and for deposition... more A problem of increasing interest in the last years, both for mass-spectrometry and for deposition of organic semiconductor films, is to increase and control the flux of large organic molecules seeded in a carrier gas expanding towards a skimmer. This paper deals with experimental and numerical investigations on the dependence of kinetic energy of Pentacene molecules from the gas flow rate. Pentacene is a very important molecule in the field of organic semiconductors where it plays the role of a prototypical system for molecular electronics. The numerical results concerning the flow rate of helium through an optimized capillary as well as the kinetic energy of Pentacene molecules in the beam are in qualitative agreement with the results of measurements paving the way to the optimization of the device.
The flow in conical supersonic micronozzles and behind them by deposition of pentacene accelerate... more The flow in conical supersonic micronozzles and behind them by deposition of pentacene accelerated by helium was studied with the use of direct simulation Monte-Carlo method. The Knudsen number in critical cross-section of nozzles is 0.0043. A directedness of resulting accelerated pentacene flow was studied as a function of cone angle and geometrical Mach number of the nozzle. The intensity of heavy gas flow in the hypersonic region can be elevated by one order using supersonic nozzles.
An approach to the growth of films of π-conjugated organic materials, cluster assembled and nanoh... more An approach to the growth of films of π-conjugated organic materials, cluster assembled and nanohybrid materials combining supersonic free jets with a UHV deposition apparatus including surface characterization methods will be discussed. The unique control achievable with supersonic beams on initial kinetic energy, momentum and state of aggregation enables the growth of materials with controlled properties at different length scales. Results obtained with organic semiconductors and oligomers point out the crucial role of kinetic energy in growing organic crystalline films with well controlled morphologies and structures. By means of supersonic beams of clusters, nanocrystalline metal oxide films can be grown without annealing, so that grain size and morphology can be better controlled. In a co-deposition scheme these interesting features are combined in order to obtain a new class of hybrid functional materials with appealing properties for electronics, gas sensing and photovoltaic applications.
Supersonic beams of TiO2 clusters and metal phthalocyanines have been developed for the synthesis... more Supersonic beams of TiO2 clusters and metal phthalocyanines have been developed for the synthesis of hybrid for gas-sensing applications. This approach allows high degree of control on the properties of the synthesized materials and on the interface between clusters and organic molecules, so that new functional materials with novel and promising sensing properties are obtained. These materials can be synthesized
In this work, we show the influence of the liquid electrolyte adsorption by porous films made of ... more In this work, we show the influence of the liquid electrolyte adsorption by porous films made of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), PEDOT:PSS, on the operation of an Organic Electro-Chemical Transistor with an active channel based on these polymeric films. In particular, the effect of film hydration on device performance is evaluated by studying its electrical response as a function of the spatial position between the electrolyte and the channel electrodes. This is done by depositing a PEDOT:PSS film on a super-hydrophobic surface aimed at controlling the electrolyte confinement next to the electrodes. The device response shows that the confinement of ionic liquids near to the drain electrode results in a worsening of the current modulation. This result has been interpreted in the light of studies dealing with the transport of ions in semiconducting polymers, indicating that the electrolyte adsorption by the polymeric film implies the formation of liquid pathways inside its bulk. These pathways, in particular, affect the device response because they are able to assist the drift of ionic species in the electrolyte towards the drain electrode. The effect of electrolyte adsorption on the device operation is confirmed by means of moving-front measurements, and is related to the reproducibility of the device operation curves by measuring repeatedly its electrical response.
The complex interplay of dielectric substrate properties, semiconducting film growth, crystal str... more The complex interplay of dielectric substrate properties, semiconducting film growth, crystal structure, texture, and charge carrier transport is investigated for the case of tetracene films deposited on different dielectrics (polystyrene, parylene C, polymethylmethacrylate, hexamethyldisilazane-treated SiO 2 , and untreated SiO 2 ). The tetracene hole mobility, measured in the bottom-gate organic thin film transistor (OTFT) configuration, varies over more than one order of magnitude depending upon the dielectric layer used. Atomic force microscopy and synchrotron grazing incidence X-ray diffraction measurements, analyzed with the extended Rietveld method, were used to investigate the influence of film connectivity, crystalline phase, polymorphism, and texture on charge transport. The role of the surface polarity and the processing conditions of the gate dielectric layer are also discussed. Based on our results, we propose guidelines for the selection of a gate dielectric material favorable for charge transport in tetracene films. † Electronic supplementary information (ESI) available: AFM and uorescence microscopy images, XRD reconstructed pole gures, output and transfer characteristics of tetracene TFTs. See
Organic electrochemical transistors (OECTs) exploit electrolyte gating to achieve the transductio... more Organic electrochemical transistors (OECTs) exploit electrolyte gating to achieve the transduction of ionic currents. Therefore, they are ideally suitable to sense different chemo/bio species dissolved in the electrolyte. Current modulation in OECTs relies on doping or dedoping of the OECT channel by electrolyte ions. Nevertheless the role played by the specific physicochemical properties of an electrolyte on OECT operation is largely unknown. Here we investigate OECTs, making use of aqueous solutions of the micelle-forming cationic surfactant cetyltrimethylammonium bromide (CTAB) as the electrolyte. Micelle-forming salts are remarkable model systems to study the doping and dedoping mechanism of OECTs, because the aggregation of dissociated ions into micelles at the critical micelle concentration permits to modify the size and the type of the species that dope or dedope the OECT channel in situ. The current modulation of OECTs using a CTAB electrolyte shows a marked increase close to the critical micellar concentration. The measurement of the transistor's drain current as a function of CTAB concentration provides a simple, fast method to detect the formation of micelles from dissociated ions.
High sensitivity zinc oxide (ZnO) tetrapods (TPs) have been functionalized by nucleating cadmium ... more High sensitivity zinc oxide (ZnO) tetrapods (TPs) have been functionalized by nucleating cadmium sulphide (CdS) nanoparticles (NPs) directly on their surface with a spotted coverage thanks to an optimized synthesis in dimethylformamide (DMF). The obtained hybrid coupled material has been used to realize a gas sensing device with a highly porous nanostructured network, in which the proper alternation of ZnO-TPs and CdS-NPs gives rise to unconventional chemoresistive behaviours. Among the different tested gases and vapours, the sensor showed a unique fingerprint response-inversion between 300 °C and 400 °C only for nitrogen dioxide (NO2) and acetic acid (CH3COOH).
Selective detection of bioanalytes in physiological fluids, such as blood, sweat or saliva, by me... more Selective detection of bioanalytes in physiological fluids, such as blood, sweat or saliva, by means of lowcost and non-invasive devices, is of crucial importance to improve diagnosis and prevention in healthcare.
Organic Electrochemical transistors (OECTs) present unique features for their strategic combinati... more Organic Electrochemical transistors (OECTs) present unique features for their strategic combination with biomedical interfaces, simple and low voltage operation regime and sensing ability in aqueous environment, but they still lack selectivity, so that a significant effort in research is devoted to overcome this limitation. Here, we focus on the diffusion properties of molecular species in the electrolyte, which opportunely analyzed, modeled and compared to experimental data, serve as a simple and direct key factor in the recognition of species during OECT sensing. Specifically, we model the transient behavior of an OECT considering the effect of diffusion of the target species in the electrolyte. In doing so, we develop a general method that can be used to differentiate and distinguish different molecules from a complex mixture, on the basis of their diffusivity and thus mass. More importantly, the model can be realistically used to determine the physical characteristics of the transported species in a solution from a simple fitting procedure. On the basis of the obtained results, we discuss the contribution that our study could bring to OECT architecture to realize a new generation of devices with improved sensitivity, selectivity and reliability.
The capability of efficiently injecting charge carriers into organic films and finely tuning thei... more The capability of efficiently injecting charge carriers into organic films and finely tuning their morphology and structure is crucial to improve the performance of organic thin film transistors (OTFTs). In this work, we investigate OTFTs employing carbon nanotubes (CNTs) as the source-drain electrodes and, as the organic semiconductor, thin films of titanyl phthalocyanine (TiOPc) grown by supersonic molecular beam deposition (SuMBD). While CNT electrodes have shown an unprecedented ability to improve charge injection in OTFTs, SuMBD is an effective technique to tune film morphology and structure. Varying the substrate temperature during deposition, we were able to grow both amorphous (low substrate temperature) and polycrystalline (high substrate temperature) films of TiOPc. Regardless of the film morphology and structure, CNT electrodes led to superior charge injection and transport performance with respect to benchmark Au electrodes. Vacuum annealing of polycrystalline TiOPc film...
ABSTRACT Supersonic molecular beam deposition is a far-from-thermal-equilibrium kinetic activated... more ABSTRACT Supersonic molecular beam deposition is a far-from-thermal-equilibrium kinetic activated growth technique, which allows the fine control of the kinetic energy of molecular species. We present a study of the growth of very thin layers of copper phthalocyanine on nanostructured surfaces of titanium dioxide nanograins; the study combines time-of-flight secondary ion mass spectrometry, Raman, X-ray photoelectron spectroscopy, and multivariate statistical analysis. Different kinetic energies and layer thicknesses were considered to investigate bond formation and surface interaction between the organic molecules and the inorganic nanograins. This study allowed the clarification of the key role of the energetic properties of the supersonic beam in the surface activation, enabling bond formation, not available with other processes at equilibrium. In particular, high kinetic energy regimes for copper phthalocyanine molecules in high-dilution seeded beams allow the formation of stronger bonds at the interface, which is useful for producing innovative nanohybrid materials having specific improved structural and chemical characteristics. The comparison among different surface characterization measurements orchestrated by multivariate statistical analysis proved to be a valuable approach for surface interaction study and interpretation.
The polymorphism of titanyl phthalocyanine films, grown on atomically flat mica substrates, has b... more The polymorphism of titanyl phthalocyanine films, grown on atomically flat mica substrates, has been systematically studied by micro-Raman spectroscopy, correlating structure and optical properties. Different growth regimes, using hyperthermal seeded supersonic beams, have been explored as a function of the substrate temperature. Specific signatures in micro-Raman spectra, correlated to different phases, are identified and discussed. We demonstrate the unprecedented ability to grow crystalline films at low temperature, with improved structural order, and we show that different regimes lead to grain dimensions in a range from the nanometric to the micrometric scale. The local micro-Raman analysis, carried out on crystallites with regular shapes, allows discriminating different structural phases of the single crystalline grains. We provide evidence that different growth regimes are achieved and controlled, paving the way to phase selection, which is envisaged as a key feature to improve device performance.
Sensors and Microsystems - Proceedings of the 10th Italian Conference, 2008
... system. 3 Results 3.1 Nana structured Titania Growth and Sensitization with Organic Molecules... more ... system. 3 Results 3.1 Nana structured Titania Growth and Sensitization with Organic Molecules Very interesting results in the synthesis of nanostructured titania have been obtained using cluster beams produced by the PMCS. ...
A problem of increasing interest in the last years, both for mass-spectrometry and for deposition... more A problem of increasing interest in the last years, both for mass-spectrometry and for deposition of organic semiconductor films, is to increase and control the flux of large organic molecules seeded in a carrier gas expanding towards a skimmer. This paper deals with experimental and numerical investigations on the dependence of kinetic energy of Pentacene molecules from the gas flow rate. Pentacene is a very important molecule in the field of organic semiconductors where it plays the role of a prototypical system for molecular electronics. The numerical results concerning the flow rate of helium through an optimized capillary as well as the kinetic energy of Pentacene molecules in the beam are in qualitative agreement with the results of measurements paving the way to the optimization of the device.
The flow in conical supersonic micronozzles and behind them by deposition of pentacene accelerate... more The flow in conical supersonic micronozzles and behind them by deposition of pentacene accelerated by helium was studied with the use of direct simulation Monte-Carlo method. The Knudsen number in critical cross-section of nozzles is 0.0043. A directedness of resulting accelerated pentacene flow was studied as a function of cone angle and geometrical Mach number of the nozzle. The intensity of heavy gas flow in the hypersonic region can be elevated by one order using supersonic nozzles.
An approach to the growth of films of π-conjugated organic materials, cluster assembled and nanoh... more An approach to the growth of films of π-conjugated organic materials, cluster assembled and nanohybrid materials combining supersonic free jets with a UHV deposition apparatus including surface characterization methods will be discussed. The unique control achievable with supersonic beams on initial kinetic energy, momentum and state of aggregation enables the growth of materials with controlled properties at different length scales. Results obtained with organic semiconductors and oligomers point out the crucial role of kinetic energy in growing organic crystalline films with well controlled morphologies and structures. By means of supersonic beams of clusters, nanocrystalline metal oxide films can be grown without annealing, so that grain size and morphology can be better controlled. In a co-deposition scheme these interesting features are combined in order to obtain a new class of hybrid functional materials with appealing properties for electronics, gas sensing and photovoltaic applications.
Supersonic beams of TiO2 clusters and metal phthalocyanines have been developed for the synthesis... more Supersonic beams of TiO2 clusters and metal phthalocyanines have been developed for the synthesis of hybrid for gas-sensing applications. This approach allows high degree of control on the properties of the synthesized materials and on the interface between clusters and organic molecules, so that new functional materials with novel and promising sensing properties are obtained. These materials can be synthesized
In this work, we show the influence of the liquid electrolyte adsorption by porous films made of ... more In this work, we show the influence of the liquid electrolyte adsorption by porous films made of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), PEDOT:PSS, on the operation of an Organic Electro-Chemical Transistor with an active channel based on these polymeric films. In particular, the effect of film hydration on device performance is evaluated by studying its electrical response as a function of the spatial position between the electrolyte and the channel electrodes. This is done by depositing a PEDOT:PSS film on a super-hydrophobic surface aimed at controlling the electrolyte confinement next to the electrodes. The device response shows that the confinement of ionic liquids near to the drain electrode results in a worsening of the current modulation. This result has been interpreted in the light of studies dealing with the transport of ions in semiconducting polymers, indicating that the electrolyte adsorption by the polymeric film implies the formation of liquid pathways inside its bulk. These pathways, in particular, affect the device response because they are able to assist the drift of ionic species in the electrolyte towards the drain electrode. The effect of electrolyte adsorption on the device operation is confirmed by means of moving-front measurements, and is related to the reproducibility of the device operation curves by measuring repeatedly its electrical response.
The complex interplay of dielectric substrate properties, semiconducting film growth, crystal str... more The complex interplay of dielectric substrate properties, semiconducting film growth, crystal structure, texture, and charge carrier transport is investigated for the case of tetracene films deposited on different dielectrics (polystyrene, parylene C, polymethylmethacrylate, hexamethyldisilazane-treated SiO 2 , and untreated SiO 2 ). The tetracene hole mobility, measured in the bottom-gate organic thin film transistor (OTFT) configuration, varies over more than one order of magnitude depending upon the dielectric layer used. Atomic force microscopy and synchrotron grazing incidence X-ray diffraction measurements, analyzed with the extended Rietveld method, were used to investigate the influence of film connectivity, crystalline phase, polymorphism, and texture on charge transport. The role of the surface polarity and the processing conditions of the gate dielectric layer are also discussed. Based on our results, we propose guidelines for the selection of a gate dielectric material favorable for charge transport in tetracene films. † Electronic supplementary information (ESI) available: AFM and uorescence microscopy images, XRD reconstructed pole gures, output and transfer characteristics of tetracene TFTs. See
Organic electrochemical transistors (OECTs) exploit electrolyte gating to achieve the transductio... more Organic electrochemical transistors (OECTs) exploit electrolyte gating to achieve the transduction of ionic currents. Therefore, they are ideally suitable to sense different chemo/bio species dissolved in the electrolyte. Current modulation in OECTs relies on doping or dedoping of the OECT channel by electrolyte ions. Nevertheless the role played by the specific physicochemical properties of an electrolyte on OECT operation is largely unknown. Here we investigate OECTs, making use of aqueous solutions of the micelle-forming cationic surfactant cetyltrimethylammonium bromide (CTAB) as the electrolyte. Micelle-forming salts are remarkable model systems to study the doping and dedoping mechanism of OECTs, because the aggregation of dissociated ions into micelles at the critical micelle concentration permits to modify the size and the type of the species that dope or dedope the OECT channel in situ. The current modulation of OECTs using a CTAB electrolyte shows a marked increase close to the critical micellar concentration. The measurement of the transistor's drain current as a function of CTAB concentration provides a simple, fast method to detect the formation of micelles from dissociated ions.
High sensitivity zinc oxide (ZnO) tetrapods (TPs) have been functionalized by nucleating cadmium ... more High sensitivity zinc oxide (ZnO) tetrapods (TPs) have been functionalized by nucleating cadmium sulphide (CdS) nanoparticles (NPs) directly on their surface with a spotted coverage thanks to an optimized synthesis in dimethylformamide (DMF). The obtained hybrid coupled material has been used to realize a gas sensing device with a highly porous nanostructured network, in which the proper alternation of ZnO-TPs and CdS-NPs gives rise to unconventional chemoresistive behaviours. Among the different tested gases and vapours, the sensor showed a unique fingerprint response-inversion between 300 °C and 400 °C only for nitrogen dioxide (NO2) and acetic acid (CH3COOH).
Selective detection of bioanalytes in physiological fluids, such as blood, sweat or saliva, by me... more Selective detection of bioanalytes in physiological fluids, such as blood, sweat or saliva, by means of lowcost and non-invasive devices, is of crucial importance to improve diagnosis and prevention in healthcare.
Organic Electrochemical transistors (OECTs) present unique features for their strategic combinati... more Organic Electrochemical transistors (OECTs) present unique features for their strategic combination with biomedical interfaces, simple and low voltage operation regime and sensing ability in aqueous environment, but they still lack selectivity, so that a significant effort in research is devoted to overcome this limitation. Here, we focus on the diffusion properties of molecular species in the electrolyte, which opportunely analyzed, modeled and compared to experimental data, serve as a simple and direct key factor in the recognition of species during OECT sensing. Specifically, we model the transient behavior of an OECT considering the effect of diffusion of the target species in the electrolyte. In doing so, we develop a general method that can be used to differentiate and distinguish different molecules from a complex mixture, on the basis of their diffusivity and thus mass. More importantly, the model can be realistically used to determine the physical characteristics of the transported species in a solution from a simple fitting procedure. On the basis of the obtained results, we discuss the contribution that our study could bring to OECT architecture to realize a new generation of devices with improved sensitivity, selectivity and reliability.
The capability of efficiently injecting charge carriers into organic films and finely tuning thei... more The capability of efficiently injecting charge carriers into organic films and finely tuning their morphology and structure is crucial to improve the performance of organic thin film transistors (OTFTs). In this work, we investigate OTFTs employing carbon nanotubes (CNTs) as the source-drain electrodes and, as the organic semiconductor, thin films of titanyl phthalocyanine (TiOPc) grown by supersonic molecular beam deposition (SuMBD). While CNT electrodes have shown an unprecedented ability to improve charge injection in OTFTs, SuMBD is an effective technique to tune film morphology and structure. Varying the substrate temperature during deposition, we were able to grow both amorphous (low substrate temperature) and polycrystalline (high substrate temperature) films of TiOPc. Regardless of the film morphology and structure, CNT electrodes led to superior charge injection and transport performance with respect to benchmark Au electrodes. Vacuum annealing of polycrystalline TiOPc film...
ABSTRACT Supersonic molecular beam deposition is a far-from-thermal-equilibrium kinetic activated... more ABSTRACT Supersonic molecular beam deposition is a far-from-thermal-equilibrium kinetic activated growth technique, which allows the fine control of the kinetic energy of molecular species. We present a study of the growth of very thin layers of copper phthalocyanine on nanostructured surfaces of titanium dioxide nanograins; the study combines time-of-flight secondary ion mass spectrometry, Raman, X-ray photoelectron spectroscopy, and multivariate statistical analysis. Different kinetic energies and layer thicknesses were considered to investigate bond formation and surface interaction between the organic molecules and the inorganic nanograins. This study allowed the clarification of the key role of the energetic properties of the supersonic beam in the surface activation, enabling bond formation, not available with other processes at equilibrium. In particular, high kinetic energy regimes for copper phthalocyanine molecules in high-dilution seeded beams allow the formation of stronger bonds at the interface, which is useful for producing innovative nanohybrid materials having specific improved structural and chemical characteristics. The comparison among different surface characterization measurements orchestrated by multivariate statistical analysis proved to be a valuable approach for surface interaction study and interpretation.
The polymorphism of titanyl phthalocyanine films, grown on atomically flat mica substrates, has b... more The polymorphism of titanyl phthalocyanine films, grown on atomically flat mica substrates, has been systematically studied by micro-Raman spectroscopy, correlating structure and optical properties. Different growth regimes, using hyperthermal seeded supersonic beams, have been explored as a function of the substrate temperature. Specific signatures in micro-Raman spectra, correlated to different phases, are identified and discussed. We demonstrate the unprecedented ability to grow crystalline films at low temperature, with improved structural order, and we show that different regimes lead to grain dimensions in a range from the nanometric to the micrometric scale. The local micro-Raman analysis, carried out on crystallites with regular shapes, allows discriminating different structural phases of the single crystalline grains. We provide evidence that different growth regimes are achieved and controlled, paving the way to phase selection, which is envisaged as a key feature to improve device performance.
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