Hazard Screening Methods for Nanomaterials: A Comparative Study.

Hazard identification is the key step in risk assessment and management of manufactured nanomaterials (NM). However, the rapid commercialisation of nano-enabled products continues to out-pace the development of a prudent risk management mechanism that is widely accepted by the scientific community and enforced by regulators. However, a growing body of academic literature is developing promising quantitative methods. Two approaches have gained significant currency. Bayesian networks (BN) are a probabilistic, machine learning approach while the weight of evidence (WoE) statistical framework is based on expert elicitation. This comparative study investigates the efficacy of quantitative WoE and Bayesian methodologies in ranking the potential hazard of metal and metal-oxide NMs-TiO2, Ag, and ZnO. This research finds that hazard ranking is consistent for both risk assessment approaches. The BN and WoE models both utilize physico-chemical, toxicological, and study type data to infer the hazard potential. The BN exhibits more stability when the models are perturbed with new data. The BN has the significant advantage of self-learning with new data; however, this assumes all input data is equally valid. This research finds that a combination of WoE that would rank input data along with the BN is the optimal hazard assessment framework.

Odd-even effects in charge transport across n-alkanethiolate-based SAMs.

This paper compares rates of charge transport across self-assembled monolayers (SAMs) of n-alkanethiolates having odd and even numbers of carbon atoms (nodd and neven) using junctions with the structure M(TS)/SAM//Ga2O3/EGaIn (M = Au or Ag). Measurements of current density, J(V), across SAMs of n-alkanethiolates on Au(TS) and Ag(TS) demonstrated a statistically significant odd-even effect on Au(TS), but not on Ag(TS), that could be detected using this technique. Statistical analysis showed the values of tunneling current density across SAMs of n-alkanethiolates on Au(TS) with nodd and neven belonging to two separate sets, and while there is a significant difference between the values of injection current density, J0, for these two series (log|J0Au,even| = 4.0 ± 0.3 and log|J0Au,odd| = 4.5 ± 0.3), the values of tunneling decay constant, ß, for nodd and neven alkyl chains are indistinguishable (ßAu,even = 0.73 ± 0.02 Å(-1), and ßAu,odd= 0.74 ± 0.02 Å(-1)). A comparison of electrical characteristics across junctions of n-alkanethiolate SAMs on gold and silver electrodes yields indistinguishable values of ß and J0 and indicates that a change that substantially alters the tilt angle of the alkyl chain (and, therefore, the thickness of the SAM) has no influence on the injection current density across SAMs of n-alkanethiolates.

Introducing ionic and/or hydrogen bonds into the SAM//Ga2O3 top-interface of Ag(TS)/S(CH2)nT//Ga2O3/EGaIn junctions.

Junctions with the structure Ag(TS)/S(CH2)nT//Ga2O3/EGaIn (where S(CH2)nT is a self-assembled monolayer, SAM, of n-alkanethiolate bearing a terminal functional group T) make it possible to examine the response of rates of charge transport by tunneling to changes in the strength of the interaction between T and Ga2O3. Introducing a series of Lewis acidic/basic functional groups (T = -OH, -SH, -CO2H, -CONH2, and -PO3H) at the terminus of the SAM gave values for the tunneling current density, J(V) in A/cm(2), that were indistinguishable (i.e., differed by less than a factor of 3) from the values observed with n-alkanethiolates of equivalent length. The insensitivity of the rate of tunneling to changes in the terminal functional group implies that replacing weak van der Waals contact interactions with stronger hydrogen- or ionic bonds at the T//Ga2O3 interface does not change the shape (i.e., the height or width) of the tunneling barrier enough to affect rates of charge transport. A comparison of the injection current, J0, for T = -CO2H, and T = -CH2CH3--two groups having similar extended lengths (in Å, or in numbers of non-hydrogen atoms)--suggests that both groups make indistinguishable contributions to the height of the tunneling barrier.

Replacing Ag(TS)SCH(2)-R with Ag(TS)O(2)C-R in EGaIn-based tunneling junctions does not significantly change rates of charge transport.

This paper compares rates of charge transport by tunneling across junctions with the structures Ag(TS) X(CH2 )2n CH3 //Ga2 O3 /EGaIn (n=1-8 and X= SCH2  and O2 C); here Ag(TS) is template-stripped silver, and EGaIn is the eutectic alloy of gallium and indium. Its objective was to compare the tunneling decay coefficient (ß, Å(-1) ) and the injection current (J0 , A cm(-2) ) of the junctions comprising SAMs of n-alkanethiolates and n-alkanoates. Replacing Ag(TS) SCH2 -R with Ag(TS) O2 C-R (R=alkyl chains) had no significant influence on J0 (ca. 3×10(3)  A cm(-2) ) or ß (0.75-0.79 Å(-1) )-an indication that such changes (both structural and electronic) in the Ag(TS) XR interface do not influence the rate of charge transport. A comparison of junctions comprising oligo(phenylene)carboxylates and n-alkanoates showed, as expected, that ß for aliphatic (0.79 Å(-1) ) and aromatic (0.60 Å(-1) ) SAMs differed significantly.

Defining the value of injection current and effective electrical contact area for EGaIn-based molecular tunneling junctions.

Analysis of rates of tunneling across self-assembled monolayers (SAMs) of n-alkanethiolates SCn (with n = number of carbon atoms) incorporated in junctions having structure Ag(TS)-SAM//Ga2O3/EGaIn leads to a value for the injection tunnel current density J0 (i.e., the current flowing through an ideal junction with n = 0) of 10(3.6±0.3) A·cm(-2) (V = +0.5 V). This estimation of J0 does not involve an extrapolation in length, because it was possible to measure current densities across SAMs over the range of lengths n = 1-18. This value of J0 is estimated under the assumption that values of the geometrical contact area equal the values of the effective electrical contact area. Detailed experimental analysis, however, indicates that the roughness of the Ga2O3 layer, and that of the Ag(TS)-SAM, determine values of the effective electrical contact area that are ~10(-4) the corresponding values of the geometrical contact area. Conversion of the values of geometrical contact area into the corresponding values of effective electrical contact area results in J0(+0.5 V) = 10(7.6±0.8) A·cm(-2), which is compatible with values reported for junctions using top-electrodes of evaporated Au, and graphene, and also comparable with values of J0 estimated from tunneling through single molecules. For these EGaIn-based junctions, the value of the tunneling decay factor ß (ß = 0.75 ± 0.02 Å(-1); ß = 0.92 ± 0.02 nC(-1)) falls within the consensus range across different types of junctions (ß = 0.73-0.89 Å(-1); ß = 0.9-1.1 nC(-1)). A comparison of the characteristics of conical Ga2O3/EGaIn tips with the characteristics of other top-electrodes suggests that the EGaIn-based electrodes provide a particularly attractive technology for physical-organic studies of charge transport across SAMs.

Replacing -CH2CH2- with -CONH- does not significantly change rates of charge transport through Ag(TS)-SAM//Ga2O3/EGaIn junctions.

This paper describes physical-organic studies of charge transport by tunneling through self-assembled monolayers (SAMs), based on systematic variations of the structure of the molecules constituting the SAM. Replacing a -CH(2)CH(2)- group with a -CONH- group changes the dipole moment and polarizability of a portion of the molecule and has, in principle, the potential to change the rate of charge transport through the SAM. In practice, this substitution produces no significant change in the rate of charge transport across junctions of the structure Ag(TS)-S(CH(2))(m)X(CH(2))(n)H//Ga(2)O(3)/EGaIn (TS = template stripped, X = -CH(2)CH(2)- or -CONH-, and EGaIn = eutectic alloy of gallium and indium). Incorporation of the amide group does, however, increase the yields of working (non-shorting) junctions (when compared to n-alkanethiolates of the same length). These results suggest that synthetic schemes that combine a thiol group on one end of a molecule with a group, R, to be tested, on the other (e.g., HS~CONH~R) using an amide-based coupling provide practical routes to molecules useful in studies of molecular electronics.

Active and non-active large-area metal-molecules-metal junctions.

The study of charge transport processes through organic molecules by using molecular junctions has generated great attention in the last few years, partially triggered by the possibility of developing molecular electronic devices to be implemented somehow into current silicon-based technology. As experimental tools, a large variety of conceptually and geometrically different metal-molecule(s)-metal junctions has been proposed. While the intrinsic conductivity of a molecule is still elusive, parameters crucial for molecular electronics have been extracted by using a variety of junctions. Significantly, the results extracted from molecular junctions and those obtained by the kinetic approach in supramolecular D-B-A systems are complementary. For the sake of a practical discussion, a distinction is made between "active junctions" and "non-active junctions". Active junctions are those aimed at switching the electrical response by an external stimulus acting "in situ" to modify the electronic structure of the molecular system. Non-active junctions are those aimed at studying different conduction regimes by incorporating molecules of different electronic structures. Depending on their geometry, the junctions can incorporate different numbers of molecules. Large area molecular junctions present two main advantages: (1) a simpler assembly, by requiring less sophisticated fabrication and (2) a higher versatility, relative to single molecule junctions, towards potential applications in organic electronics. The present chapter focuses on the fabrication of a variety of large-area molecular junctions and summarizes and compares the experimental results.

Additive nanoscale embedding of functional nanoparticles on silicon surface.

We present a novel additive process, which allows the spatially controlled integration of nanoparticles (NPs) inside silicon surfaces. The NPs are placed between a conductive stamp and a silicon surface; by applying a bias voltage a SiO(2) layer grows underneath the stamp protrusions, thus embedding the particles. We report the successful nanoembedding of CoFe(2)O(4) nanoparticles patterned in lines, grids and logic structures.

Bridging the gap between nanoparticles and single crystal surfaces.

Using density functional theory calculations and the extended ab initio atomistic thermodynamics approach, we studied the adsorption of oxygen on the different surface faces, which are involved in the faceting of Ir(210). Constructing the (p,T)-surface phase diagrams of the corresponding surfaces in contact with an oxygen atmosphere, we find that at high temperatures the planar surfaces are stable, while lowering the temperature stabilizes those nano-facets found experimentally. Afterwards, we constructed the (a,T,deltaphi)-phase diagram for Ir(210) in contact with an aqueous electrolyte and found that the same nano-facets should be stable under electrochemical conditions. Motivated by this prediction from theory, experiments were performed using cyclic voltammetry and in-situ scanning tunneling microscopy. The presence of nanofacets for Ir(210) gives rise to a characteristic current-peak in the hydrogen adsorption region for sulfuric acid solution. Furthermore, first results on the electrocatalytic behavior of nano-faceted Ir(210) are presented.