Size-dependent structure of silver nanoparticles under high pressure

Abstract

Professor A Paul Alivisatos, ChairSilver noble metal nanoparticles, that have a diameter less than 10 nm, often possess multiply twinned grains allowing them to adopt shapes and atomic structures not observed in bulk materials. The properties exhibited by particles with multiply-twinned polycrystalline structures are often far different from those of single-crystalline particles and from the bulk. I will present experimental evidence that silver nanoparticles < 10 nm undergo a reversible structural transformation under hydrostatic pressures up to 10 GPa.Results for nanoparticles in the intermediate size range of 5 to 10 nm suggest a reversible linear pressure-dependent rhombohedral distortion which has not been previously observed in bulk silver. I propose a mechanism for this transition that considers the bond-length distribution in idealized multiply twinned icosahedral particles. At smaller sizes, results for 3.9 nm silver nanoparticles suggest a reversible linear orthorhombic distortion that continuously varies with hydrostatic pressure to 8GPa. This distortion is interpreted in the 2 context of idealized decahedral particles.In addition, given these size-dependent measurements of silver nanoparticle compression with pressure, we have constructed a pressure calibration curve. Encapsulating these silver nanoparticles in hollow metal oxide nanospheres then allows us to measure the pressure inside a nanoshell using x-ray diffraction. We demonstrate the measurement of pressure gradients across nanoshells and show that these nanoshells have maximum resolved shear strengths on the order of 500 MPa to 1GPa.Professor A Paul Alivisatos Dissertation Committee Chair i