Yabin Yan

Ferroelectric critical size and vortex domain structures of PbTiO3 nanodots: A density functional theory study

The ferroelectric critical size and microscopic domain structure of PbTiO3 nanodots with unit cells of N × N × N (N = 1–3) have been investigated by ab initio (first-principles) density functional theory calculations. Nanodots with PbO and TiO surface terminations are investigated, and the ground state of TiO-terminated nanodots is found to be paraelectric regardless of the size. However, for PbO-terminated nanodots, the ferroelectric state is energetically favorable even in the smallest nanodot, indicating the absence of an intrinsic critical size for ferroelectricity in the nanodot structure. Moreover, the distributions of polarizations in nanodots with different sizes are analyzed. The vortex polarizations rotating around both the central [001] axis and diagonal [11¯1] directions of nanodots can stably exist. The vortex polarization arises from the opposite rotation between the cations and anions around the [001] and the [11¯1] directions of nanodots, respectively. On the other hand, the toroidal momen...

Investigation into the Breakdown of Continuum Fracture Mechanics at the Nanoscale: Synthesis of Recent Results on Silicon

The present contribution reviews some recent results on the experimental characterisation of the nanoscale fracture toughness of silicon by using pre-cracked specimens and alternatively the theory of critical distances (TCD). Later, the results are discussed to provide the ultimate dimensional limit of the continuum fracture mechanics at the nanoscale in the light of sophisticated discrete atomic simulations at the onset of brittle fracture. The results show that the fracture toughness of Si is independent of the scale, crystal orientation and the singular stress field length. This confirms the atomistic nature of the brittle fracture. Moreover, the continuum fracture mechanics fails below a singular stress field approaching 2 nm.

A Nano-Cantilever Method for Crack Initiation at the Free Edge of the Cu/Si Interface in Nanoscale Components

Straight and Bent nano-cantilever specimens are respectively proposed to investigate the single-mode and mixed-mode crack initiation at the Cu/Si interface edge in nanoscale components. With a minute loading apparatus, all nanoscale samples are in situ loaded and observed. Numerical analysis is employed to acquire the critical interfacial stress distributions during crack initiation. The stress concentration regions near the edge of Cu/Si interface in all specimens are within the scale of 100 nm, and the critical normal and shear stresses have a circular relation in nanoscale components, which represents the fracture criterion of the interface in nanoscale components.

Interfacial Delamination of PZT Thin Films

The interface strength of Pb(Zr,Ti)O3(PZT) thin films on a silicon substrate is studied experimentally and numerically in this work. First, a sandwiched cantilever specimen is proposed to perform the delamination tests. The experimental results show that the multilayered Cr/PZT/PLT/Pt/Ti thin films deposited on single-crystal silicon substrates are delaminated along the interface between Cr and PZT layers in a brittle manner. Second, based on cohesive zone models (CZMs), numerical simulations are carried out to extract the fracture toughness of the interface Cr/PZT. Three types, exponential, bilinear and trapezoidal, of CZMs are adopted. Characteristic CZM parameters are obtained through comparisons with experimental results. The simulation results indicate that (i) cohesive strength and work of separation are the dominating parameters in the CZMs; (ii) the bilinear CZM is more suitable in describing this brittle interfacial delamination; and (iii) compared with typical film/coating materials of several millimetre thicknesses, the fracture energy of this weak interface Cr/PZT is quite low. Our study demonstrates a methodology of how to measure and determine the interface bonding strength parameter of PZT thin films, which will be useful for assessing the structural integrity and reliability of PZT devices.