Stefania Federici

Transport Physics and Device Modeling of Zinc Oxide Thin-Film Transistors Part I: Long-Channel Devices

Thin-film transistors (TFTs), which use zinc oxide (ZnO) as an active layer, were fabricated and investigated in detail. The transport properties of ZnO deposited by spray pyrolysis (SP) on a TFT structure are studied in a wide range of temperatures, electrical conditions (i.e., subthreshold, above-threshold linear, and saturation regions), and at different channel lengths. It is shown that ZnO deposited by SP is a nanocrystalline material; its field-effect mobility is temperature activated and increases with carrier concentration. On the basis of this analysis, we propose the multiple-trapping-and-release (MTR)-transport mechanism to describe the charge transport in ZnO. By means of numerical simulations, we prove that MTR is a suitable approach, and we calculate the density of states. We show that the tail states extend in a wide range of energy and that they strongly influence the transport properties. Finally, an analytical physical-based DC model is proposed and validated with experiments and numerical simulations. The model is able to reproduce the measurements on devices with different channel length in a wide range of bias voltages and temperatures by means of a restricted number of parameters, which are linked directly to the physical properties of the ZnO semiconductor. For the first time, the charge transport in the ZnO is investigated by means of the MTR, and a consistent analysis based on experiments, numerical simulations, and analytical modeling is provided.

Nanomechanical Recognition of N-Methylammonium Salts

Turning molecular recognition into an effective mechanical response is critical for many applications ranging from molecular motors and responsive materials to sensors. Herein, we demonstrate how the energy of the molecular recognition between a supramolecular host and small alkylammonium salts can be harnessed to perform a nanomechanical task in a univocal way. Nanomechanical Si microcantilevers (MCs) functionalized by a film of tetra-phosphonate cavitands were employed to screen as guests the compounds of the butylammonium chloride series 1-4, which comprises a range of low molecular weight (LMW) molecules (molecular mass < 150 Da) that differ from each other by one or a few N-methyl groups (molecular mass 15 Da). The cavitand surface recognition of each individual guest drove a specific MC bending (from a few to several tens of nanometer), disclosing a direct, label-free, and real-time mean to sort them. The complexation preferences of tetraphosphonate cavitands toward ammonium chloride guests 1-4 were independently assessed by isothermal titration calorimetry. Both direct and displacement binding experiments concurred to define the following binding order in the alkylammonium series: 2 > 3 ≈ 1 ≫ 4. This trend is consistent with the number of interactions established by each guest with the host. The complementary ITC experiments showed that the host-guest complexation affinity in solution is transferred to the MC bending. These findings were benchmarked by implementing cavitand-functionalized MCs to discriminate sarcosine from glycine in water.

Cavitand‐Grafted Silicon Microcantilevers as a Universal Probe for Illicit and Designer Drugs in Water

The direct, clean, and unbiased transduction of molecular recognition into a readable and reproducible response is the biggest challenge associated to the use of synthetic receptors in sensing. All possible solutions demand the mastering of molecular recognition at the solid-liquid interface as prerequisite. The socially relevant issue of screening amine-based illicit and designer drugs is addressed by nanomechanical recognition at the silicon-water interface. The methylamino moieties of different drugs are all first recognized by a single cavitand receptor through a synergistic set of weak interactions. The peculiar recognition ability of the cavitand is then transferred with high fidelity and robustness on silicon microcantilevers and harnessed to realize a nanomechanical device for label-free detection of these drugs in water.

Metal fractionation in soils and assessment of environmental contamination in Vallecamonica, Italy

Metal contamination was investigated in soils of the Vallecamonica, an area in the northern part of the Brescia province (Italy), where ferroalloy industries were active for a century until 2001. The extent in which emissions from ferroalloy plants affected metal concentration in soils is not known in this area. In this study, the geogenic and/or anthropogenic origin of metals in soils were estimated. A modified Community Bureau of Reference sequential chemical extraction method followed by inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses were employed to evaluate the potential bioavailability of Al, Cd, Mn, Fe, Cr, Zn, and Pb in soils. Principal component analysis (PCA) was used to assess the relationships among metal sources in soil samples from different locations. This approach allowed distinguishing of different loadings and mobility of metals in soils collected in different areas. Results showed high concentrations and readily extractability of Mn in the Vallecamonica soils, which may suggest potential bioavailability for organisms and may create an environmental risk and potential health risk of human exposure.

Quantifying the nanomachinery of the nanoparticle-biomolecule interface.

A study is presented of the nanomechanical phenomena experienced by nanoparticle-conjugated biomolecules. A thermodynamic framework is developed to describe the binding of thrombin-binding aptamer (TBA) to thrombin when the TBA is conjugated to nanorods. Binding results in nanorod aggregation (viz. directed self-assembly), which is detectable by absorption spectroscopy. The analysis introduces the energy of aggregation, separating it into TBA-thrombin recognition and surface-work contributions. Consequently, it is demonstrated that self-assembly is driven by the interplay of surface work and thrombin-TBA recognition. It is shown that the work at the surface is about -10 kJ mol(-1) and results from the accumulation of in-plane molecular forces of pN magnitude and with a lifetime of <1 s, which arises from TBA nanoscale rearrangements fuelled by thrombin-directed nanorod aggregation. The obtained surface work can map aggregation regimes as a function of different nanoparticle surface conditions. Also, the thermodynamic treatment can be used to obtain quantitative information on surface effects impacting biomolecules on nanoparticle surfaces.

Protein thin film machines

We report the first example of microcantilever beams that are reversibly driven by protein thin film machines fueled by cycling the salt concentration of the surrounding solution. We also show that upon the same salinity stimulus the drive can be completely reversed in its direction by introducing a surface coating ligand. Experimental results are throughout discussed within a general yet simple thermodynamic model.

On the difference of equilibrium constants of DNA hybridization in bulk solution and at the solid-solution interface.

The origin of the difference between the equilibrium (affinity) constants of ligand–receptor binding in bulk solution and at a solid‐solution interface is discussed in terms of Gibbsian interfacial thermodynamics. It results that the difference is determined by the surface work that the ligand–receptor interaction spends to accommodate surface binding, and in turn that the value of the surface equilibrium constant (strongly) depends on the surface that confines the event. This framework consistently describes a wide set of experimental observations of DNA surface hybridization, correctly predicting that within the surface work window for DNA hybridization, that ranges from −90 to 75 kJmol−1, the ratio between surface and bulk equilibrium constants ranges from 10−16 to 1013, spanning 29 orders of magnitude. Copyright

On the thermodynamics of biomolecule surface transformations.

Biological surface science is receiving great and renewed attention owing the rising interest in applications of nanoscience and nanotechnology to biological systems, with horizons that range from nanomedicine and biomimetic photosynthesis to the unexpected effects of nanomaterials on health and environment. Biomolecule surface transformations are among the fundamental aspects of the field that remain elusive so far and urgently need to be understood to further the field. Our recent findings indicate that surface thermodynamics can give a substantial contribution toward this challenging goal. In the first part of the article, we show that biomolecule surface transformations can be framed by a general and simple thermodynamic model. Then, we explore its effectiveness by addressing some typical cases, including ligand-receptor surface binding, protein thin film machines, nanomechanical aspects of the biomolecule-nanoparticle interface and nanomechanical biosensors.

Nanoliter contact angle probes tumor angiogenic ligand-receptor protein interactions

Any molecular recognition reaction supported by a solid phase drives a specific change of the solid-solution interfacial tension. Sessile contact angle (CA) experiments can be readily used to track this thermodynamic parameter, prompting this well-known technique to be reinvented as an alternative, easy-access and label-free way to probe and study molecular recognition events. Here we deploy this technique, renamed for this application CONAMORE (CONtact Angle MOlecular REcognition), to study the interaction of the tumor-derived pro-angiogenic vascular endothelial growth factor-A (VEGF-A) with the extracellular domain of its receptor VEGFR2. We show that CONAMORE recognizes the high affinity binding of VEGF-A at nanomolar concentrations to surface-immobilized VEGFR2 regardless of the presence of a ten-fold excess of a non-specific interacting protein, and that it further proofs its specificity and reliability on competitive binding experiments involving neutralizing anti-VEGF-A antibodies. Finally, CONAMORE shows the outstanding capability to detect the specific interaction between VEGFR2 and low molecular weight ligands, such as Cyclo-VEGI, a VEGFR2 antagonist cyclo-peptide, that weighs about 2 kDa.

Nanomechanics of surface DNA switches probed by captive contact angle.

Self-assembled monolayers of Thrombin Binding Aptamers (TBA) were prepared on gold surfaces with typical surface densities of close-packed ssDNA (4×10(12) and 8×10(12)molecules/cm(2)). CONtact Angle MOlecular REcognition (CONAMORE) in captive bubble geometry was then assessed to scan the surface work triggered by TBAs when switching from the elongated to the G-quadruplex conformation upon binding with Na(+) or K(+) cations. We found Na(+) and K(+) could induce comparable linear to G-quadruplex strokes, and resulted in values for surface work of ~-70 pN nm/molecule (~18 kBT). The strokes change the in-plane van der Waals and weak electrostatic interactions and accumulate to result in macroscopic surface work. Micro- to macroscopic translation strongly depends on the nature of the cation and TBA surface density. In particular, the K(+) stimulus triggers a macroscopic surface work of -2.2±0.2 and -5.3±0.2 mN/m for low and high packed monolayers, respectively, while Na(+) triggers up to -6.7±1.0 mN/m in the highly packed monolayer, but creates negligible work for the low packed monolayer. These results show that CONAMORE can contribute important information for the development of devices based on DNA switches, and ultimately help address some of the open challenges for DNA-based nanomachinery.