A. Orsini

Towards high-performance, low-cost quartz sensors with high-density, well-separated, vertically aligned ZnO nanowires by low-temperature, seed-less, single-step, double-sided growth

Resonant sensors with nanostructured surfaces have long been considered as an emergent platform for high-sensitivity transduction because of the potentially very large sensing areas. Nevertheless, until now only complex, time-consuming, expensive and sub-optimal fabrication procedures have been described; in fact, especially with reference to in-liquid applications, very few devices have been reported. Here, we first demonstrate that, by immersing standard, ultra-low-cost quartz resonators with un-polished silver electrodes in a conventional zinc nitrate/HMTA equimolar nutrient solution, the gentle contamination from the metallic package allows direct growth on the electrodes of arrays of high-density (up to 10 μm⁻²) and well-separated (no fusion at the roots) ZnO nanowires without any seed layer or thermal annealing. The combination of high-density and good separation is ideal for increasing the sensing area; moreover, this uniquely simple, single-step process is suitable for conventional, ultra-low-cost and high-frequency quartzes, and results in devices that are already packaged and ready to use. As an additional advantage, the process parameters can be effectively optimized by measuring the quartz admittance before and after growth. As a preliminary test, we show that the sensitivity to the liquid properties of high-frequency (i.e. high sensitivity) quartzes can be further increased by nearly one order of magnitude and thus show the highest ever reported frequency shifts of an admittance resonance in response to immersion in both ethanol and water.

Interplay between crystallographic orientation and electric transport properties in La2/3Sr1/3MnO3 films

The effect of crystallographic interface orientation on the electric transport properties of fully strained La2/3Sr1/3MnO3 films grown onto LaAlO3 substrates has been investigated. It is found that, relative to the (001) orientation, the (110) orientation strongly enhances the transport properties for film thickness in the range between 3 and 12 nm. Such an effect was ascribed to reduced [relative to the (001) substrates] tetragonal distortion induced by epitaxy onto (110)-oriented substrates. The reduced tetragonal distortion quenches the occupational imbalance between the Mn eg orbitals thus, ultimately, reinforcing the ferromagnetic double exchange transport mechanism.

Real-time monitoring of the solution growth of ZnO nanorods arrays by quartz microbalances and in-situ temperature sensors

Wet-chemistry methods have crucial advantages for the synthesis of nanostructures, including simple, low-cost, large-area, and low-temperature deposition on almost arbitrary substrates. Nevertheless, the rational design of improved wet-chemistry procedures is extremely difficult because, in practice, only post-synthesis characterization is possible. In fact, the only methods for on-line monitoring the growth of nanostructures in liquids are complex, expensive and introduce intricate artifacts. Here we demonstrate that electro-mechanically resonating substrates and in-situ temperature sensors easily enable an accurate real-time investigation of reaction kinetics and, in combination with conventional SEM imaging, greatly facilitate the rational design of optimized synthesis procedures; in particular, such a simple approach provides useful insight for the development of processes where one or more key parameters are dynamically adjusted. As a proof-of-concept, first, we accurately characterize a process for fabricating arrays of ZnO nanorods; afterwards, we design a dynamic-temperature process that, in comparison with the corresponding constant-temperature procedure, is almost-ideally energy efficient and results in ZnO nanorods with improved characteristics in terms of length, aspect ratio, and total deposited nanorods mass. This is a major step towards the rational design of dynamic procedures for the solution growth of nanostructures.

Solution-Grown Zn/Al layered double hydroxide nanoplatelets onto Al thin films: fine control of position and lateral thickness

We have grown nanostructured films of Zn/Al Layered Double Hydroxide (LDH) on different substrates by combining the deposition of an aluminum micropatterned thin layer with a successive one-step room-temperature wet-chemistry process. The resulting LDH film is made of lamellar-like nanoplatelets mainly oriented perpendicular to the substrate. Since the aluminum layer acts as both reactant and seed for the synthesis of the LDH, the growth can be easily confined with submicrometric-level resolution (about ±0.5 µm) by prepatterning the aluminum layer with conventional photolithographic techniques. Moreover, we demonstrate real-time monitoring of the LDH growth process by simply measuring the resistance of the residual aluminum film. If the aluminum layer is thinner than 250 nm, the morphology of LDH nanoplatelets is less regular and their final thickness linearly depends on the initial amount of aluminum. This peculiarity allows accurately controlling the LDH nanoplatelet thickness (with uncertainty of about ±10%) by varying the thickness of the predeposited aluminum film. Since the proposed growth procedure is fully compatible with MEMS/CMOS technology, our results may be useful for the fabrication of micro-/nanodevices.

Disentangling strain effects in manganite heterostructures

Understanding the physical behavior of an interface between two different oxides is made difficult because of the many competing physical mechanisms such as epitaxial strain, broken symmetry, elemental interdiffusion, and polarity discontinuity, which may be at play simultaneously. We propose an approach, based on heterostructures engineering, to single out the effect of the epitaxial strain from the other physical-chemical effects at the interface between the substrate and the La07Sr0.3MnO3 film. It was found that the degradation of magnetotransport properties, reported for relatively thick films, is a consequence of epitaxial strain alone and is not affected by interface chemistry.

3D Reconstruction of Quasi-1D Single-Crystal Nanostructures.

The accurate determination of the 3D geometries of single-crystal quasi-1D nanostructures is described, including sidelengths, perimeters, areas, lengths, and azimuth and elevation angles. This is a major step toward the synthesis of quasi-1D nanostructures with superior uniformity, and tightly controlled geometrical or geometry-dependent properties.

CMOS Compatible, Low Power, High-Sensitivity Zn/Al Layered Double Hydroxides Humidity Micro-Sensor

After depositing on glass substrates aluminum thin film micro-patterned tracks, we have used a wet chemistry process in order to transform the aluminum metal into Zn/Al layered double hydroxides (LDHs). This technique consists in placing the substrates into a zinc nitrate solution with a basic agent at temperatures lower than 90 °C. Such a hydrothermal method is very simple, low cost, CMOS/MEMS compatible, suitable for large area substrates and results in the growth of Zn/Al LDHs thin films with nanoplatelets shape. We have then tested the nanostructured Zn/Al LDH micro-sized tracks as a humidity sensor and found a high sensitivity at room temperature. Our approach seems ideal for the fabrication of CMOS-compatible, low power, on-chip integrated humidity micro-sensors.

Quartz Crystal Microbalances for On-line Monitoring of Nanostructures Growth

Quartz Crystal Microbalances (QCM) are widely used for monitoring materials growth in vacuum physical deposition techniques; as a more complex application, QCMs are also used for on-line monitoring of thin films deposition in liquid thanks to electrochemical reactions onto quartz electrode (EQCM). Our goal here is to demonstrate that QCMs can also be used for on-line monitoring of hydrothermal nanostructures growth as well as for obtaining information on the synthesis reaction kinetic. In practice, first, we model the QCM in solution by means of a lumped element circuit; afterwards, rather than only the resonant frequency, we measure the entire QCM admittance spectrum around the resonant frequency, thus obtaining more information. As an example, we show that this approach allows to clearly distinguish the growth of different types of ZnO nanostructures, namely nanorods and nanoplatelets, on the QCM surface and the reaction dynamics.