Chi-Hang Lam

Competing roughening mechanisms in strained heteroepitaxy: a fast kinetic Monte Carlo study.

We study the morphological evolution of strained heteroepitaxial films using kinetic Monte Carlo simulations in two dimensions. A novel Greens function approach, analogous to boundary integral methods, is used to calculate elastic energies efficiently. We observe island formation at low lattice misfit and high temperature that is consistent with the Asaro-Tiller-Grinfeld instability theory. At high misfit and low temperature, islands or pits form according to the nucleation theory of Tersoff and LeGoues.

Direct TEM observations of growth mechanisms of two-dimensional MoS2 flakes.

A microscopic understanding of the growth mechanism of two-dimensional materials is of particular importance for controllable synthesis of functional nanostructures. Because of the lack of direct and insightful observations, how to control the orientation and the size of two-dimensional material grains is still under debate. Here we discern distinct formation stages for MoS2 flakes from the thermolysis of ammonium thiomolybdates using in situ transmission electron microscopy. In the initial stage (400 °C), vertically aligned MoS2 structures grow in a layer-by-layer mode. With the increasing temperature of up to 780 °C, the orientation of MoS2 structures becomes horizontal. When the growth temperature reaches 850 °C, the crystalline size of MoS2 increases by merging adjacent flakes. Our study shows direct observations of MoS2 growth as the temperature evolves, and sheds light on the controllable orientation and grain size of two-dimensional materials.

Mechanisms of imprint effect on ferroelectric thin films

We have developed a single/double layer model to explain horizontal shifting of measured D-E hysteresis loops (imprint) for ferroelectric thin films. Such phenomenon can be explained by considering three mechanisms or their multiple effects: (1) stress induced by film/electrode lattice mismatch or clamping, (2) domain pinning induced by, e.g., oxygen vacancies, or (3) degradation of ferroelectric properties in film/electrode surface layers. First, it is found that hysteresis loops under the influence of stress exhibit large horizontal shifts with magnitudes comparable to those observed in experiments. Second, a pseudo-non-switching layer with a large coercive field is assumed to be present at the film/electrode interface in an otherwise homogeneous ferroelectric thin film, and in this case our simulation also shows a large imprint effect. Third, it is also found that time-dependent space-charge-limited conduction is likely to be one origin for the occurrence of imprint.

Time-dependent space-charge-limited conduction as a possible origin of the polarization offsets observed in compositionally graded ferroelectric films

We investigated the effects of free space charges on hysteresis-loop measurement of compositionally graded ferroelectrics and found that they are quite likely to be responsible for the “polarization offsets” observed in experiments. Taking into account conduction by those free space charges, or time-dependent space-charge-limited conduction, our computer simulation of compositionally graded lead zirconate titanate, which is placed in the Sawyer–Tower circuit and driven by an alternating applied voltage, produced shifting of measured hysteresis loops where the shift magnitudes are comparable with published experimental data. It also produced the key features as observed in experiments: The “offsets:” (a) have a monotonous increase with electric-field amplitude, (b) change in direction when the composition gradient is inverted, and (c) develop like the typical charging-up of a capacitor. All these results suggest that time-dependent space-charge-limited conduction is a possible origin of the polarization of...

Island, pit, and groove formation in strained heteroepitaxy.

We study the morphological evolution of strained heteroepitaxial films using a kinetic Monte Carlo method in three dimensions. The elastic part of the problem uses a Greens function method. Isolated islands are observed under deposition conditions for deposition rates slow compared with intrinsic surface roughening rates. They are hemispherical and truncated conical for high and low temperature cases, respectively. Annealing of films at high temperature leads to the formation of closely packed islands as in instability theory. At low temperature, pits form via a multistep layer-by-layer nucleation mechanism in contrast to the conventional single-step nucleation process. They subsequently develop into grooves, which are energetically more favorable.

Probing the quantum behavior of a nanomechanical resonator coupled to a double quantum dot

We propose a current correlation spectrum approach to probe the quantum behaviors of a nanome-chanical resonator (NAMR). The NAMR is coupled to a double quantum dot (DQD), which acts as a quantum transducer and is further coupled to a quantum-point contact (QPC). By measuring the current correlation spectrum of the QPC, shifts in the DQD energy levels, which depend on the phonon occupation in the NAMR, are determined. Quantum behaviors of the NAMR could, thus, be observed. In particular, the cooling of the NAMR into the quantum regime could be examined. In addition, the effects of the coupling strength between the DQD and the NAMR on these energy shifts are studied. We also investigate the impacts on the current correlation spectrum of the QPC due to the backaction from the charge detector on the DQD.

Ultrahigh refractive index sensing performance of plasmonic quadrupole resonances in gold nanoparticles

The refractive index sensing properties of plasmonic resonances in gold nanoparticles (nanorods and nanobipyramids) are investigated through numerical simulations. We find that the quadruple resonance in both nanoparticles shows much higher sensing figure of merit (FOM) than its dipolar counterpart, which is attributed mainly to the reduction in resonance linewidth. More importantly, our results predict that at the same sensing wavelength, the sensing FOM of the quadrupole mode can be significantly boosted from 3.9 for gold nanorods to 7.4 for gold nanobipyramids due to the geometry-dependent resonance linewidth, revealing a useful strategy for optimizing the sensing performance of metal nanoparticles.

Method to Measure the Viscosity of Nanometer Liquid Films from the Surface Fluctuations

We describe a method to measure the viscosity of polystyrene liquid films with thicknesses ∼5 and ∼80 nm spin-cast on oxide-coated silicon. In this method, temporal evolution of the film surface is monitored and modeled according to the dynamics of the surface capillary waves. Viscosities obtained from the ∼80 nm films display an excellent agreement with those of the bulk polymer, but those from the ∼5 nm films are up to 106 times reduced. By modeling the data to the Vogel–Fulcher–Tammann relation, we find that the observations are consistent with the thickness dependence of the glass transition temperature previously reported of these films.

Pipe network model for scaling of dynamic interfaces in porous media

We present a numerical study on the dynamics of imbibition fronts in porous media using a pipe network model. This model quantitatively reproduces the anomalous scaling behavior found in imbibition experiments [Phys. Rev. E 52, 5166 (1995)]. Using simple scaling arguments, we derive a new identity among the scaling exponents in agreement with the experimental results.

Cross-Kerr effect on an optomechanical system

We study the cross-Kerr (CK) effect on an optomechanical system driven by two-tone fields. We show that in the presence of the CK effect, a bistable behavior of the mean photon number in the cavity becomes more robust against the fluctuations of the frequency detuning between the cavity mode and the control field. The bistability can also be turned into a tristability within the experimentally accessible range of the system parameters. Also, we find that the symmetric profile of the optomechanically induced transparency is broken and the zero-absorption point is shifted in the presence of the CK effect. This shift can be used to measure the strength of the CK effect and the asymmetric absorption profiles can be employed to engineer a high quality factor of the cavity.