Effects of Substrate Bias Voltage on Structure of Diamond-Like Carbon Films on AISI 316L Stainless Steel: A Molecular Dynamics Simulation Study.

With the recent significant advances in micro- and nanoscale fabrication techniques, deposition of diamond-like carbon films on stainless steel substrates has been experimentally achieved. However, the underlying mechanism for the formation of film microstructures has remained elusive. In this study, the growth processes of diamond-like carbon films on AISI 316L substrate are studied via the molecular dynamics method. Effects of substrate bias voltage on the structure properties and sp3 hybridization ratio are investigated. A diamond-like carbon film with a compact structure and smooth surface is obtained at 120 V bias voltage. Looser structures with high surface roughness are observed in films deposited under bias voltages of 0 V or 300 V. In addition, sp3 fraction increases with increasing substrate bias voltage from 0 V to 120 V, while an opposite trend is obtained when the bias voltage is further increased from 120 V to 300 V. The highest magnitude of sp3 fraction was about 48.5% at 120 V bias voltage. The dependence of sp3 fraction in carbon films on the substrate bias voltage achieves a high consistency within the experiment results. The mechanism for the dependence of diamond-like carbon structures on the substrate bias voltage is discussed as well.

Intrinsic and extrinsic effects on the electrotoroidic switching in a ferroelectric notched nanodot by a homogeneous electric field.

The control of topological defects in ferroelectrics, in particular by a homogeneous electric field, has emerged as an active research direction. A polarization vortex, which is a fundamental topological defect formed in ferroelectric nanodots, has recently been demonstrated to be switchable by a homogeneous electric field through the control of the built-in electrical distribution using low-symmetry nanodots. Such electrotoroidic switching is investigated for nearly ideal systems, e.g., free-standing nanodots. However, the electrotoroidic switching may be impacted by several factors, for instance, the nanoscale effect of flexoelectricity (intrinsic effect), epitaxial strain and the frequency of the applied field (extrinsic effects). In the present study, the switching of the polarization vortex in a notched nanodot under a homogeneous electric field is investigated. The emphasis is put on a comparison between intrinsic and extrinsic effects on the vortex switching. The results show that the vortex switching takes place through alternate vortex-to-polar and polar-to-vortex transformations due to the appearance of the notch. Although the flexoelectricity breaks the symmetry of the polarization field in the notched nanodot during the polarization transformation and gives rise to an unusual behavior of the vortex core, which departs from the symmetry axis of the notched nanodot, this intrinsic effect plays a relatively insignificant role in the switching behavior of the polarization vortex. In comparison to the intrinsic effect, interestingly, the extrinsic effects strongly influence the vortex switching behavior. Specifically, the frequency of the applied electric field can alter both the shape of the toroidal hysteresis loop and the domain transformation process of the vortex switching. In addition, under substrate constraints, the magnitude of the coercive electric fields at which the vortex-to-polar and polar-to-vortex transformations occur linearly decreases with the increase of strain. The present study provides instructive information on the efficient control of a polarization vortex, which is dominated by extrinsic factors rather than intrinsic ones.

Formation of polarization needle-like domain and its unusual switching in compositionally graded ferroelectric thin films: an improved phase field model.

Compositionally graded ferroelectrics (cgFEs), which possess a spatial variation in material composition, exhibit anomalous or even unprecedented properties. Despite several breakthroughs having been achieved in experiments, there surprisingly is a lack of an effective simulation approach for cgFEs, thereby greatly hindering a deep understanding about underlying mechanisms hidden behind the observed phenomena. In this study, an improved phase field model is proposed for a cgFE made of PbZr(1-x)Ti x O3 based on the Ginzburg-Landau theory. The improved approach enables us to capture several key phenomena occurring in the cgFE PbZr0.8Ti0.2O3 ⇔ PbZr0.2Ti0.8O3 thin film that are observed from experiments, such as the formation of needle-like domains with curved domain walls, ferroelastic switching under an electric field, and the voltage offset of the hysteresis loop. Results obtained from the improved approach indicate that the high elastic energy near the needle-tip of an a-domain gives rise to a deflection of the domain wall from the regular a/c domain wall, while the high concentration of the depolarization field shrinks a part of the a-domain near the needle-tip and terminates the a-domain within the cgFE thin film. These facilitate the stabilization of needle-like domains with curved domain walls in the cgFE thin film. Furthermore, the flexoelectricity is proven to play an important role in the voltage offset of the hysteresis loop. On the other hand, the ferroelectric switching process in the cgFE thin film exhibits a vastly different response in comparison to that in homogeneous ferroelectric thin films, including local switching initiation and formation and annihilation of vortex-antivortex pairs during the switching. The present study, therefore, provides an incisive approach for investigations on cgFEs, which may bring new understanding and unique insights into these complex materials, as well as novel potential applications.