H.S. Tanvir Ahmed

Dispersions of Non-Covalently Functionalized Graphene with Minimal Stabilizer

We demonstrate that functionalized pyrene derivatives effectively stabilize single- and few-layer graphene flakes in aqueous dispersions. The graphene/stabilizer yield obtained by this method is exceptionally high relative to conventional nanomaterial stabilizers such as surfactants or polymers. The mechanism of stabilization by pyrene derivatives is investigated by studying the effects of various parameters on dispersed graphene concentration and stability; these parameters include stabilizer concentration, initial graphite concentration, solution pH, and type and number of functional groups and counterions. The effectiveness of the pyrene derivatives is pH-tunable, as measured by zeta potential, and is also a function of the number of functional groups, the electronegativity of the functional group, the counterion, the relative polarity between stabilizer and solvent, and the distance from the functional group to the basal plane. Even if the dispersion is destabilized by extreme pH or lyophilization, the graphene does not aggregate because the stabilizer remains adsorbed on the surface. These dispersions also show promise for applications in graphene/polymer nanocomposites (examined in this paper), organic solar cells, conductive films, and inkjet-printed electronic devices.

Measurement of Young's modulus variation with layer pair and interplanar spacing in gold–nickel nanolaminates using nanoindentation and the tapping mode

The features of grain size and interface separation strengthen the mechanical behavior of metallic nanolaminates. In addition, the presence of interlayer lattice strains can lead to a superlattice structure within the nanolaminate. The superlattice affects intrinsic properties of technological interest including electronic, magnetic, and elastic. The complex elastic and plastic behaviors of gold–nickel nanolaminate superlattice coatings as studied using nanoindentation are revisited with the tapping mode of a force microscope. Youngs modulus is determined with nanoindentation during the initial elastic unloading after plastic deformation at depths up to one-fifth the coating thickness. The tapping mode provides a measurement during the initial elastic deformation at depths of only a few nanometers. The tapping mode utilizes the shift in the resonant frequency of the probe-cantilever system as contact is made with the sample surface. Both of these nanoprobe test methods produce results for measurements co...

On the Tapping Mode Measurement for Young’s Modulus of Nanocrystalline Metal Coatings

Young’s modulus of nanocrystalline metal coatings is measured using the oscillating, that is, tapping, mode of a cantilever with a diamond tip. The resonant frequency of the cantilever changes when the diamond tip comes in contact with a sample surface. A Hertz-contact-based model is further developed using higher-order terms in a Taylor series expansion to determine a relationship between the reduced elastic modulus and the shift in the resonant frequency of the cantilever during elastic contact between the diamond tip and sample surface. The tapping mode technique can be used to accurately determine Young’s modulus that corresponds with the crystalline orientation of the sample surface as demonstrated for nanocrystalline nickel, vanadium, and tantalum coatings.