Synthesis and Characterization of Superparamagnetic Iron Oxide Nanoparticles: A Series of Laboratory Experiments.

The following laboratory procedure provides students with hands-on experience in nanomaterial chemistry and characterization. This three-day protocol is easy to follow for undergraduates with basic chemistry or materials science backgrounds and is suitable for inclusion in upper-division courses in inorganic chemistry or materials science. Students use air-free chemistry procedures to synthesize and separate iron oxide magnetic nanoparticles and subsequently modify the nanoparticle surface by using a chemical stripping agent. The morphology and chemical composition of the nanoparticles are characterized using electron microscopy and dynamic light scattering measurements. Additionally, magnetic characterization of the particles is performed using an inexpensive open-source (3D-printed) magnetophotometer. Possible modifications to the synthesis procedure, including the incorporation of dopants to modify the magnetic response and alternative characterization techniques, are discussed. The three-day synthesis, purification, and characterization laboratory will prepare students with crucial skills for advanced technology industries such as semiconductor manufacturing, nanomedicine, and green chemistry.

Exploring the limits of sensitivity for strain gauges of graphene and hexagonal boron nitride decorated with metallic nanoislands.

The purpose of this work is to clarify the mechanism of piezoresistance in a class of ultra-sensitive strain gauges based on metallic films on 2D substrates ("2D/M" films). The metals used are gold or palladium deposited as ultrathin films (≤16 nm). These films transition from a regime of subcontiguous growth to a percolated morphology with increasing nominal thickness. The 2D substrates are either single-layer graphene or hexagonal boron nitride (hBN). By using either a conductor (graphene) or an insulator (hBN), it is possible to de-couple the relative contributions of the metal and the 2D substrate from the overall piezoresistance of the composite structure. Here, we use a combination of measurements including electron microscopy, automated image analysis, temperature-dependent conductivity, and measurements of gauge factor of the films as they are bent over a 1 µm step edge (0.0001% or 1 ppm). Our observations are enumerated as follows: (1) of the four permutations of metal and 2D substrate, all combinations except hBN/Au are able to resolve 1 ppm strain (considered extraordinary for strain gauges) at some threshold thickness of metal; (2) for non-contiguous (i.e., unpercolated) films of metal on hBN, changes in resistance for these small step strains cannot be detected; (3) for percolated films on hBN, changes in resistance upon strain can be resolved only for palladium and not for gold; (4) graphene does not exhibit detectable changes in resistance when subjected to step strains of either 1 or 10 ppm, but does so upon the deposition of any amount of gold or palladium, even for nominal thicknesses below the threshold for percolation. Our observations reveal unexpected complexity in the properties of these simple composite materials, and ways in which these materials might be combined to exhibit even greater sensitivity.

Graphene-Metal Composite Sensors with Near-Zero Temperature Coefficient of Resistance.

This article describes the design of piezoresistive thin-film sensors based on single-layer graphene decorated with metallic nanoislands. The defining characteristic of these composite thin films is that they can be engineered to exhibit a temperature coefficient of resistance (TCR) that is close to zero. A mechanical sensor with this property is stable against temperature fluctuations of the type encountered during operations in the real world, for example, in a wearable sensor. The metallic nanoislands are grown on graphene through thermal deposition of metals (gold or palladium) at a low nominal thickness. Metallic films exhibit an increase in resistance with temperature (positive TCR), whereas graphene exhibits a decrease in resistance with temperature (negative TCR). By varying the amount of deposition, the morphology of the nanoislands can be tuned such that the TCRs of a metal and graphene cancel out. The quantitative analysis of scanning electron microscope images reveals the importance of the surface coverage of the metal (as opposed to the total mass of the metal deposited). The stability of the sensor to temperature fluctuations that might be encountered in the outdoors is demonstrated by subjecting a wearable pulse sensor to simulated solar irradiation.

SERS-enhanced piezoplasmonic graphene composite for biological and structural strain mapping.

Thin-film optical strain sensors have the ability to map small deformations with spatial and temporal resolution and do not require electrical interrogation. This paper describes the use of graphene decorated with metallic nanoislands for sensing of tensile deformations of less than 0.04% with a resolution of less than 0.002%. The nanoisland-graphene composite films contain gaps between the nanoislands, which when functionalized with benzenethiolate behave as hot spots for surface-enhanced Raman scattering (SERS). Mechanical strain increases the sizes of the gaps; this increase attenuates the electric field, and thus attenuates the SERS signal. This compounded, SERS-enhanced "piezoplasmonic" effect can be quantified using a plasmonic gauge factor, and is among the most sensitive mechanical sensors of any type. Since the graphene-nanoisland films are both conductive and optically active, they permit simultaneous electrical stimulation of myoblast cells and optical detection of the strains produced by the cellular contractions.