[Show abstract][Hide abstract] ABSTRACT: An appropriate way of realizing property nanoengineering in complex quaternary chalcogenide nanocrystals is presented for Cu2CdxSnSey(CCTSe) polypods. The pivotal role of the polarity in determining morphology, growth, and polytypic branching mechanism is demonstrated. Polarity is considered to be responsible for the formation of an initial seed that takes the form of a tetrahedron with four cation-polar facets. Size and shape confinement of the intermediate pentatetrahedral seed is also attributed to polarity as their external facets are anion-polar. The final polypod extensions also branch out as a result of cation-polarity-driven mechanism. Aberration-corrected scanning transmission electron microscopy is used to identify stannite cation ordering, while ab initio studies are used to show the influence of cation ordering/distortion, stoichiometry, and polytypic structural change on the electronic band structure.
[Show abstract][Hide abstract] ABSTRACT: 3D single crystalline well aligned GaN-InGaN rod arrays are fabricated by selective area growth (SAG) metal-organic vapor phase epitaxy (MOVPE) for visible light water splitting. Epitaxial InGaN layer grows successfully on 3D GaN rods to minimize defects within the GaN-InGaN heterojunctions. The indium concentration (In ~ 0.30 ± 0.04) is rather homogeneous in InGaN shells along the radial and longitudinal directions. The growing strategy allows to tune the band gap of the InGaN layer in order to match the visible absorption with the solar spectrum as well as to align the semiconductor bands close to the water redox potentials to achieve high efficiency. The relation between structure, surface and photoelectrochemical property of GaN-InGaN is explored by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), current-voltage and open circuit potential (OCP) measurements. The epitaxial GaN-InGaN interface, pseudomorphic InGaN thin films, homogeneous and suitable indium concentration and defined surface orientation are properties demanded for systematic study and efficient photoanodes based on III-Nitride heterojunctions.
[Show abstract][Hide abstract] ABSTRACT: Copper (II) oxide (CuO) is a metal oxide suitable for developing solid state gas sensors. Nevertheless, a detailed insight into the chemical-to-electrical transduction mechanisms between gas molecules and this metal oxide is still limited. Here, individual CuO nanowires were evaluated as ammonia (NH3) and hydrogen sulphide (H2S) sensors, validating the p-type character of this semiconductor. The working principle behind their performance was qualitatively modeled and it was concluded that adsorbed oxygen at the surface plays a key role necessary to explain the experimental data. Compared to their counterparts of SnO2 nanowires, an appreciable sensitivity enhancement to NH3 for concentrations below 100 ppm was demonstrated.
[Show abstract][Hide abstract] ABSTRACT: In this work, bottom-up devices based on individual monocrystalline SnO2 nanowires (NWs) were fabricated using FIB nanolithography on top of suspended microhotplates, with integrated heater and interdigitated microelectrodes. The electrical characterisation of such devices in the presence of different gases show that devices with improved gas sensing properties can be fabricated.
Microprocesses and Nanotechnology 2007. 20th International Microprocesses and Nanotechnology Conference|Microprocesses and Nanotechnology 2007. 20th International Microprocesses and Nanotechnology Conference. 12/2013;
[Show abstract][Hide abstract] ABSTRACT: Microelectromechanical systems (MEMS) with integrated heaters, originally designed for the readout of the resistance of metal oxide (MOX) layers, were configured to observe surface ionisation (SI) gas signals. Interdigital platinum (Pt) electrodes on top of the dielectric membranes acted as ion emitting layers while flat-plate counter electrodes, positioned at a short distance above them, were used for the current readout. In this work, we show that this device configuration leads to SI responses orders of magnitude higher than those previously reported for thin-film, flat-plate devices and fully comparable to the performance of parallel-nanowire devices (PNDs). The high ionisation efficiency of our MEMS devices, which are suitable for large-scale production and further integration steps, is attributed to the electric field enhancement that takes place at the sharp edges of the Pt electrodes.
Sensors and Actuators B Chemical 12/2013; 182:25–30. · 3.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The compositional versatility of I 2 –II–IV–VI 4 tetrahedrally-coordinated compounds allows for accommodating their functional properties to numerous technological applications. Among them, Cu2 ZnSnSe4 is an emerging photovol-taic material and Cu2CdSnSe4 displays excellent thermoelectric properties. The third compound of this family, Cu2HgSnSe4 , remains relatively unexplored. Herein, a synthetic route to produce Cu2HgSnSe4 nanoparticles with narrow size distribution and controlled composition is presented. Cu2HgSnSe4 nanoparticles were subsequently used as building blocks to produce bulk nanocrystalline materials, whose thermoelectric properties were analyzed. A very preliminary adjustment of the material composition yielded Seebeck coefficients up to 160 μV K−1 , electrical conductivities close to 10^4 Sm−1 and thermal conductivities down to 0.5 Wm−1K−1 .
[Show abstract][Hide abstract] ABSTRACT: Carbon nanofibres decorated with metal nanoparticles were deposited over kapton, a polyimide flexible substrate, onto which array interdigitated electrodes in one side and a common heater in the backside were printed using inkjet printing technique. The control over metal type (as-grown, Au, Pd) and the decoration percentage gave us the possibility to improve the sensors response, and also enhance the selectivity by taking advantage of the different interaction behaviors of tested target gas molecules with different hybrid materials that occur at room temperature.
[Show abstract][Hide abstract] ABSTRACT: We study the details of electronic transport related to the atomistic structure of silicon quantum dots embedded in a silicon dioxide matrix using ab initio calculations of the density of states. Several structural and composition features of quantum dots (QDs), such as diameter and amorphization level, are studied and correlated with transport under transfer Hamiltonian formalism. The current is strongly dependent on the QD density of states and on the conduction gap, both dependent on the dot diameter. In particular, as size increases, the available states inside the QD increase, while the QD band gap decreases due to relaxation of quantum confinement. Both effects contribute to increasing the current with the dot size. Besides, valence band offset between the band edges of the QD and the silica, and conduction band offset in a minor grade, increases with the QD diameter up to the theoretical value corresponding to planar heterostructures, thus decreasing the tunneling transmission probability and hence the total current. We discuss the influence of these parameters on electron and hole transport, evidencing a correlation between the electron (hole) barrier value and the electron (hole) current, and obtaining a general enhancement of the electron (hole) transport for larger (smaller) QD. Finally, we show that crystalline and amorphous structures exhibit enhanced probability of hole and electron current, respectively.
[Show abstract][Hide abstract] ABSTRACT: Titanates are suitable for many applications such as oxygen sensing and tunable HTS (high temperature superconducting) microwave filters. The potential advantages of the nanostructured forms have been however scarcely explored compared to other oxides. In this work, the structural and electrical properties of individual iron-doped strontium titanate nanotubes (Fe:SrTiO3) grown by electrophoretic deposition (EPD) were assessed for the first time, showing high stability and reproducibility. This result paves the way to further development of more complex titanate-based devices, as for instance nanostructured oxygen STFO sensors. From experimental data, it was concluded that the polycrystalline form of Fe:SrTiO3 nanotubes is the major limitation to attain high photoconductivity gains when exposed to UV-light.
Materials Chemistry and Physics 08/2013; 141(1):9-13. · 2.07 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Organic–inorganic hybrid gas sensors can offer outstanding performance in terms of selectivity and sensitivity towards single gas species. The enormous variety of organic functionalities enables novel flexibility of active sensor surfaces compared to commonly used pure inorganic materials, but goes along with an increase of system complexity that usually hinders a predictable sensor design. In this work, an ultra-selective NO2 sensor is realized based on self-assembled monolayer (SAM)-modified semiconductor nanowires (NWs). The crucial chemical and electronic parameters for an effective interaction between the sensor and different gas species are identified using density functional theory simulations. The theoretical findings are consistent with the experimentally observed extraordinary selectivity and sensitivity of the amine-terminated SnO2 NW towards NO2. The energetic position of the SAM–gas frontier orbitals with respect to the NW Fermi level is the key to ensure or impede an efficient charge transfer between the NW and the gas. As this condition strongly depends on the gas species and the sensor system, these insights into the charge transfer mechanisms can have a substantial impact on the development of highly selective hybrid gas sensors.
[Show abstract][Hide abstract] ABSTRACT: The controlled filling of the pores of highly ordered mesoporous antiferromagnetic Co3O4 replicas with ferrimagnetic FexCo3-xO4 nanolayers is presented as a proof-of-concept toward the integration of nanosized units in highly ordered, heterostructured 3D architectures. Antiferromagnetic (AFM) Co3O4 mesostructures are obtained as negative replicas of KIT-6 silica templates, which are subsequently coated with ferrimagnetic (FiM) FexCo3-xO4 nanolayers. The tuneable magnetic properties, with a large exchange bias and coercivity, arising from the FiM/AFM interface coupling, confirm the microstructure of this novel two-phase core-shell mesoporous material. The present work demonstrates that ordered functional mesoporous 3D-materials can be successfully infiltrated with other compounds exhibiting additional functionalities yielding highly tuneable, versatile, non-siliceous based nanocomposites.
[Show abstract][Hide abstract] ABSTRACT: We present a theoretical model describing the photo-electrical response of a
system composed of quantum dots embedded in a dielectric matrix. The model is
based on the non-coherent rate equations and the Transfer Hamiltonian
formalism. The self charge was included. Within this methodology, the response
of the system only depends on fundamental material parameters and its geometry.
Transport through several quantum dot configurations was simulated obtaining
current-voltage curves in dark and illuminating conditions for three different
scenarios: single one quantum dot and two quantum dots in parallel and serial
configurations. Despite the simplicity of the model, it has been used to
reproduce successfully experimental results.
[Show abstract][Hide abstract] ABSTRACT: Herein, we present the fabrication and characterization of a flexible gas sensor based on carbon nanofibers. The sensing device is composed of interdigitated silver electrodes deposited by inkjet printing on Kapton substrates, subsequently coated with carbon nanofibers as sensing element. Gas sensing response to CO, NH3 and humidity has been characterized in detail. Thermal, mechanical and electromagnetic radiation effects have also been studied and discussed from the point of view of the cross-sensitivity. The obtained results open the door for a new generation of flexible sensors with multifunctional sensing features, which are producible with scalable techniques based on low cost nanomaterials.
[Show abstract][Hide abstract] ABSTRACT: Tin oxide (SnO2) represents a major fraction of research for developing solid-state gas sensors. Nevertheless, a detailed insight into the chemical-to-electrical transduction mechanisms between ammonia (NH3) molecules and this metal oxide is still limited. Here, the adsorption of NH3 on SnO2 was examined by density functional theory (DFT) calculations and confronted to experimental data obtained with individual nanowire devices. It was concluded that under real working conditions nonlattice oxygens (O5c) adsorbed on SnO2 exhibit a more basic character than lattice bridging oxygens (O2c), and consequently, they play a key role in the dehydrogenation of NH3 on SnO2, with N2 and H2O as the main resulting products. The sensing process of ammonia on tin oxide nanowires not only involves physical mechanisms but also has a concomitant chemical nature that requires two molecules of NH3 for the reaction to take place. Our theoretical modeling reveals why ammonia sensing is competitive to the adsorption of water molecules. As a result, interfering effects in monitoring traces of NH3 intrinsically occur in humid conditions.
The Journal of Physical Chemistry C. 02/2013; 117(7):3520–3526.
[Show abstract][Hide abstract] ABSTRACT: Herein we report a solar diode sensor (SDS) based on new designed CdS@n-ZnO/p-Si nanoelements which unify gas sensing (CdS@n-ZnO) and solar energy harvesting (n-ZnO/p-Si diode) functionalities in a singular material unit and device. A novel The SDS sensing mechanism (change of open circuit voltage, ΔVoc), in comparison to the well-known conductometric sensors (change of resistance, ΔR), is systematically studied and explained in terms of gas–material surface interactions and the subsequent changes in the doping level (ND) of n-ZnO, which is manifested in the variation of Voc in CdS@n-ZnO/p-Si. The fabricated SDS was capable of quantitatively detecting oxidising and reducing gases with reproducible response at room temperature and without the need of any other energy sources except solar illumination to deliver a self-sustained gas sensor.
[Show abstract][Hide abstract] ABSTRACT: Herein, we present a flexible gas sensor based on carbon nanofibers (CNFs). The sensing device was fabricated by means of low-cost techniques: inkjet printing and spray coating. Characterization on the use of ultraviolet (UV) light in order to modulate the sensor signal was performed. Sensor response and characteristics times (response time, τresp; and recovery time, τrec) were fully characterized. UV illumination was also chosen to anneal the sensing active layer in the fabrication process and to fasten desorption of the gas adsorbed, achieving a stable sensor signal. Furthermore, better conditions of UV radiation to optimize τresp and τrec were found.
Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS & EUROSENSORS XXVII), 2013 Transducers & Eurosensors XXVII: The 17th International Conference on; 01/2013
[Show abstract][Hide abstract] ABSTRACT: We present a combined study of the morphology and electronic transport of silicon nanowires (SiNWs). On the one hand, we used a model for the Gibbs free energy of the nanowires in order to assess the importance of the contribution of volume, surface, edge, and planar defects in the different diameter ranges and polymorphs found experimentally, namely, cubic diamond (“cubic”) and hexagonal diamond (“hexagonal”). Surface contribution to the Gibbs free energy explains the appearance of the hexagonal phase in intermediate diameters. Structural relaxations via planar defects explain the abundance of the cubic phase in thinner NWs. On the other hand, in order to approximate the transport in the range of thick SiNWs, we studied the electron transport properties of bulk silicon in the main crystallographic growth directions of SiNWs, namely, 001d, 110d, and 111d for the cubic phase and 001h for the hexagonal phase, with a density functional theory-non-equilibrium Green functions formalism (DFT-NEGFF) code. We found the 001d direction the one with the greatest conductivity, related to the fact that the absolute minimum of the bulk silicon band diagram is located in this direction, it has the lowest effective mass among all the studied directions, and the transport is also favorable due to the shape of the conduction band minimum orbital (i.e., LUMO, low unoccupied molecular orbital).
The Journal of Physical Chemistry C. 10/2012; 116(41):22078–22085.
[Show abstract][Hide abstract] ABSTRACT: TiO2 nanorod arrays grown on conductive substrates were converted using chemical strategies into CdS@TiO2 and CdS@anatase@rutile TiO2 heterostructures to fabricate visible-light harvesting assemblies. Compared to pure TiO2 nanorods, CdS@TiO2 heterostructures evidently extended the absorption edge and exhibited enhanced photoelectrochemical (PEC) response in the visible region. Further enhancement of PEC performance was achieved by introducing an intermediate anatase TiO2 layer in the CdS@rutile TiO2 heterostructures. An excitonic cascade of band alignment (CdS, anatase-TiO2 and rutile-TiO2) was constituted by arranging different semiconductors in order to align the edges of their conducting band, which improved charge separation and suppressed the recombination processes by facilitating the transfer of forward electrons and limiting the reverse processes due to spatial separation of the electron and hole in different material regions.
Journal of Materials Chemistry 09/2012; 22(38):20472-20476. · 5.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Radial p–n nanowire heterojunction devices represent a favorable geometry to maximize the interfacial area and charge carrier separation due to the built-in field established across the junction. This report presents the functional characterization of a heterojunction device based on a single coaxial p-Si/n-ZnO nanowire that was integrated in a circuit by FIB nanolithography to study the electrical properties. Specifically, their photovoltaic and gas sensing performances were preliminary assessed. The gas sensing response of the p–n heterojunction could be usefully modulated by controlling the bias currents through the device, showing a complementary functionality of these nanoarchitectured devices.
Materials Chemistry and Physics. 08/2012; 135(s 2–3):618–622.