Da-Jun Shu

Self-Assembly of Nanometer-Scale Magnetic Dots with Narrow Size Distributions on an Insulating Substrate

The self-assembly of iron dots on the insulating surface of NaCl(001) is investigated experimentally and theoretically. Under proper growth conditions, nanometer-scale magnetic iron dots with remarkably narrow size distributions can be achieved in the absence of a wetting layer. Furthermore, both the vertical and lateral sizes of the dots can be tuned with the iron dosage without introducing apparent size broadening, even though the clustering is clearly in the strong coarsening regime. These observations are interpreted using a phenomenological mean-field theory, in which a coverage-dependent optimal dot size is selected by strain-mediated dot-dot interactions.

Confinement-induced ordering in dewetting and phase separation of polymer blend films.

Dewetting and phase separation of a thin polymer fi lm over a homogeneous surface has been extensively investigated in recent decades. Research was driven either by scientifi c curiosity for general principles of rupture mechanisms [ 1–3 ] or by technical anticipation in fi nding out the conditions for stable lithographic resist layers. [ 4 , 5 ] Depending on the interfacial interactions, there are two possible rupture mechanisms of a polymer fi lm: one occurs via nucleation of dry spots [ 6–8 ] and the other is by spontaneous amplifying the capillary waves caused by thermal fl uctuation. [ 9–11 ] To demonstrate the effect of intermolecular interactions in the process, the infl uence of pattern scale and frequency on morphological evolution of the dewetting pattern of the thin polymer fi lm can be tuned by topographically smooth, yet chemically inhomogeneous self-assembled monolayers. [ 12–15 ] In addition, physical modulations, such as periodic arrays of micro pillars, [ 16 ] stripes, [ 17 , 18 ]

Metallic stereostructured layer: An approach for broadband polarization state manipulation

In this letter, we report a full-metallic broadband wave plate assembled by standing metallic L-shaped stereostructures (LSSs). We show that with an array of LSSs, high polarization conversion ratio is achieved within a broad frequency band. Moreover, by rotating the orientation of the array of LSSs, the electric components of the reflection beam in two orthogonal directions and their phase difference can be independently tuned. In this way, all the polarization states on the Poincare sphere can be realized. As examples, the functionalities of a quarter wave plate and a half wave plate are experimentally demonstrated with both reflection spectra and focal-plane-array imaging. Our designing provides a unique approach in realizing the broadband wave plate to manipulate the polarization state of light.

Multi-tip nano-prisms: Controlled growth and emission enhancement properties

We report here the experimental observations that the tip topography of ZnO nano-prisms sensitively depends on the percentage of oxygen in the flux of the carrying gas in vapor growth. At a relatively high oxygen concentration, a number of thin filaments can be nucleated atop nano-prisms, forming a unique fish-spear–like multi-tip morphology. The length and density of the “spear tines” depend on the flux of the carrying gas. The field emission properties of the nanorod array with different tip morphology are investigated. The structures with longer and denser spear tines possess lower turn-on electric field and higher electric current density. The cathodoluminescence properties of the ZnO nano-prisms have also been studied. The luminescence related to defects in multi-tip nano-prisms possesses the strongest intensity, and the nanorod without any tine structure possesses the lowest defect luminescence intensity. The intrinsic luminescence of ZnO around 385 nm, however, has the opposite tendency. We suggest that our observation is inspiring in optimizing the emission properties of the nanowire devices.

Effects of external stress field on the charge stability of nitrogen vacancy centers in diamond

The interaction of the atom-like defects in semiconductors with external fields provides an avenue to quantum information processing and nanoscale sensors. Meanwhile, external fields may induce instability of the desired charge state of the defects. It is essential to understand how the charge state of a defect is affected by external fields that introduced in diverse applications. In this letter, we explore the stability of the negatively charged state (NV–) and the neutral state (NV0) of the nitrogen vacancy (NV) center in diamond under stress by first-principles calculations. We find that the relative stability of NV– to NV0 is always reduced by the stress if the NV center is free to relax its orientation. Once the NV center has formed and retains its orientation, however, the relative stability of NV– can be always enhanced by compressive stress along its trigonal symmetry axis. We believe that the results are not only significant for control of the charge stability of NV center but also enlightening ...

Nucleation‐Mediated Lateral Growth of Crystalline Islands on Foreign Substrate: an Origin of Long‐Range Ordering in Pattern Formation

Heteroepitaxial thin film growth may start by nucleating three‐dimensional (3D) or two‐dimensional (2D) islands on foreign substrate. Depending on the interfacial energies, heteroepitaxial growth can be categorized into three modes: Frank‐van der Merwe (FM), Stranski‐Krastanow (SK), and Volmer‐Weber (VW). In this lecture, we concentrate on the detail process of horizontal extension of a crystalline island on foreign substrate via successive nucleation at the concave corner of the crystal facet and the foreign substrate. It is demonstrated that due to the difference of surface tensions at the concave corner, once a nucleus appears at the corner, the crystallographic orientation of the nucleus is spontaneously twisted. By successive nucleation at the concave corner, the crystalline layer develops laterally, with its crystallographic orientation continuously rotated. Such a previously untouched effect could be the physical origin to form a few regular spatial patterns in the interfacial growth. A theory is d...