Hikaru Saito

Electron microscopy analysis of microstructure of postannealed aluminum nitride template

The microstructure of an AlN template after high-temperature annealing was investigated by transmission electron microscopy (TEM). The AlN template was prepared by depositing an AlN layer of about 200 nm thickness on a sapphire (0001) substrate by metal–organic vapor phase epitaxy. The AlN template was annealed under (N2 + CO) atmosphere at 1500–1650 °C. TEM characterization was conducted to investigate the microstructural evolution, revealing that the postannealed AlN has a two-layer structure, the upper and lower layers of which exhibit Al and N polarities, respectively. It has been confirmed that postannealing is an effective treatment for controlling the microstructure.

Three-dimensional visualization of dislocations in a ferromagnetic material by magnetic-field-free electron tomography

In conventional transmission electron microscopy, specimens to be observed are placed in between the objective lens pole piece and therefore exposed to a strong magnetic field about 2u202fT. For a ferromagnetic specimen, magnetization of the specimen causes isotropic and anisotropic defocusing, deflection of the electron beam as well as deformation of the specimen, which all become more severe when the specimen tilted. Therefore electron tomography on a ferromagnetic crystalline specimen is highly challenging because tilt-series data sets must be acquired without changing the excitation condition of a specific diffraction spot. In this study, a scanning transmission electron microscopy (STEM) tomography method without magnetizing a ferromagnetic specimen has been developed for three-dimensional (3D) visualization of dislocations in α-Fe, which is a typical ferromagnetic material. Magnetic-field-free environment down to 0.38u2009±u20090.07u202fmT at the specimen position is realized by demagnetizing the objective lens pole piece of a commercial STEM instrument. By using a spherical aberration corrector with the magnetic-field-free environment, an aberration corrected Low-Mag STEM mode with no objective lens field reaches a convergence semi angle ∼1u202fmrad and a spatial resolution ∼5u202fnm, and shows an adequate performance of imaging dislocations under a two-beam excitation condition for a low-index diffracted beam. The illumination condition for the aberration corrected Low-Mag STEM mode gives no overlap between the direct beam disk (spot) and neighboring diffraction disks. An electron channeling contrast imaging technique, in which an annular detector was located at a doughnut area between the direct beam and the neighboring diffracted beams, was effectively employed with the aberration corrected Low-Mag STEM mode to keep image intensity high enough even at large specimen-tilt angles. The resultant tomographic observation visualized 3D dislocation arrangements and active slip planes in a deformed α-Fe specimen.

Optically Coupled Plasmonic Nanopores Observed by Cathodoluminescence Scanning Transmission Electronmicroscopy

Control of the optical properties of nano-plasmonic structures is essential for next-generation optical circuits and high-throughput biosensing platforms. Realization of such nano-optical devices requires optical couplings of various nanostructured elements and field confinement at the nanoscale. In particular, symmetric coupling modes, also referred to as “dark modes”, have recently received considerable attention because these modes can confine light energy to small spaces. Although the coupling behavior of plasmonic nanoparticles has been relatively well-studied, couplings of inverse structures, i.e., holes and pores, remain partially unexplored. Even for the most fundamental coupling system of two dipolar holes, comparison of the symmetric and anti-symmetric coupling modes has not been performed. Here, we present a systematic study of the symmetric and anti-symmetric coupling of nanopore pairs using cathodoluminescence by scanning transmission electron microscopy (CL-STEM) and electromagnetic simulation. n n n nThe nanopore samples were fabricated by colloidal lithography and film transfer by wet etching of the sacrificial layer. To achieve very close separation of nanopore pairs and to obtain high spatial resolution in STEM, we chose ultra-thin, free-standing film structures. For the measurement of single and coupled pairs of nanopores, 135 nm nanopores in an AlN(8 nm)/Au(16 nm)/AlN(8 nm) trilayer membrane were used (Figure 1b). With this sandwich layer structure, it is possible to obtain very thin and stable metal layers, even at high temperatures. n n n nFor CL-STEM measurement, 80 kV acceleration was used to avoid possible damage due to relatively high beam current. (5nA) Depending on the electron beam position it is possible to distinguish the symmetric and anti-symmetric dipolar coupling modes.(Fig. 1) The observed symmetric coupling mode, approximated as a pair of facing dipoles, appeared at a lower energy than that of the anti-symmetric coupling mode, indicating that the facing dipoles attract each other. The anti-symmetric coupling mode splits into the inner- and outer-edge localized modes as the coupling distance decreases. These coupling behaviors cannot be fully explained as simple inverses of coupled disks. Electromagnetic simulation by finite difference time domain (FDTD) also showed consistent coupling behaviors. Models of FDTD simulation showed that the inner- and outer-edge anti-symmetric modes become fully localized with minimal influence of the opposite edges as the coupling distance decreases. Symmetric and anti-symmetric coupling modes are also observed in a short-range ordered pore array (Fig. 2), where one pore supports multiple local resonance modes, depending on the distance to the neighboring pores. n n nKeywords: n ncathodoluminescence; nnanopore; nplasmonics