Miho Tagawa

Stabilization of DNA nanostructures by photo-cross-linking

Developing methods for stabilizing DNA nanostructures is a major challenge for next-generation nanofabrication, because stable DNA nanostructures are expected to work as building materials in the bottom-up assembly of functional biomolecules and nano-electronic components. Here we show the availability of cross-linking-type photoreaction with 3-cyanovinylcarbazole nucleosides (CNVKs) on DNA nanostructures. DNA double-crossover AB-staggered (DXAB) tiles including cross-linking molecules, CNVKs, self-assembled into two-dimensional (2D) periodic DNA arrays and were covalently connected by photo-cross-linking. The self-assembled DNA arrays before and after photo-cross-linking have been visualized by high-resolution, tapping mode atomic force microscopy (AFM) in buffer. The improvement of the heat tolerance of photo-cross-linked DNA arrays was confirmed by heating and visualizing the DNA nanostructures. The heat-resistant DNA arrays may expand the potential of DNA as a functional material in biotechnology and nanotechnology.

Evolution of threading screw dislocation conversion during solution growth of 4H-SiC

Evolution of threading screw dislocation (TSD) conversion during the solution growth of 4H-SiC on a vicinal crystal of 4H-SiC(0001) was investigated by synchrotron X-ray topography. Selecting appropriate X-ray wavelength and g vector, we can change the penetration of X-ray, and the dislocation behaviors with the different depth were successfully observed. Evidently TSDs parallel to the c-axis having c-component Burgers vector were changed into defects on the (0001) basal planes with the same Burgers vector as the TSDs, propagating to the [112¯0] step-flow direction by advancing macrosteps during the solution growth. The TSD conversions stochastically took place during the growth. The conversion rate was almost uniform and finally almost all TSDs were converted to the basal plane defects. The conversion rate was low at the very early stage of the growth, which implies that the macrosteps were not formed at the initial stage of the solution growth.

Low-dislocation-density 4H-SiC crystal growth utilizing dislocation conversion during solution method

We report a marked reduction in the dislocation density of a 4H-SiC crystal using a high-efficiency dislocation conversion phenomenon. During the solution growth, threading dislocations were efficiently converted to basal plane defects by the step flow of macrosteps. Utilizing this dislocation conversion phenomenon, we achieved the marked reduction of threading dislocation density. Consequently, the threading screw dislocation density was only 30 cm−2, which was two orders of magnitude lower than that of the seed crystal. The 4H-SiC polytype of the seed crystal was replicated in the grown crystal, which was attributed to the spiral growth owing to a few remaining threading screw dislocations upstream of the step flow.

Heat-resistant DNA tile arrays constructed by template-directed photoligation through 5-carboxyvinyl-2′-deoxyuridine

Template-directed DNA photoligation has been applied to a method to construct heat-resistant two-dimensional (2D) DNA arrays that can work as scaffolds in bottom-up assembly of functional biomolecules and nano-electronic components. DNA double-crossover AB-staggered (DXAB) tiles were covalently connected by enzyme-free template-directed photoligation, which enables a specific ligation reaction in an extremely tight space and under buffer conditions where no enzymes work efficiently. DNA nanostructures created by self-assembly of the DXAB tiles before and after photoligation have been visualized by high-resolution, tapping mode atomic force microscopy in buffer. The improvement of the heat tolerance of 2D DNA arrays was confirmed by heating and visualizing the DNA nanostructures. The heat-resistant DNA arrays may expand the potential of DNA as functional materials in biotechnology and nanotechnology.

Surface Morphology and Threading Dislocation Conversion Behavior during Solution Growth of 4H-SiC Using Al-Si Solvent

Surface morphology and threading dislocation conversion behavior during solution growth of 4H-SiC using pure Si and Al-Si solvents was investigated. The growth surfaces on the C face were smoother than the Si face. By the addition of Al to the solvent, the growth surface became smooth on the C face and rough on the Si face. Threading screw dislocation conversion took place only in the grown crystals on the Si face and threading edge dislocation conversion occurs both on the Si face and the C face using the pure Si solvent. On the other hand, in the grown crystal on the C face using the Al-Si solvent, the threading dislocation conversion was hardly observed. These results indicate that the threading dislocation conversion behavior is influenced by the surface morphology.

Dislocation Conversion During SiC Solution Growth for High-Quality Crystals

Solution growth of SiC has attracted significant attention due to its potential for the production of high-quality SiC wafers. We have recently investigated the dislocation propagation behavior during SiC solution growth with the aim of reducing the dislocation density. Threading dislocations were found to be converted to defects on the basal planes during solution growth. Utilizing this dislocation conversion phenomenon, we have proposed a dislocation reduction process during solution growth and achieved high-quality 4H-SiC crystal growth. Here we confirm the potential of SiC solution growth for the production of high-quality SiC wafers.

Effect of magnesium ion concentration on two-dimensional structure of DNA-functionalized nanoparticles on supported lipid bilayer

The effect of divalent cations on lipid-bilayer-assisted DNA-functionalized nanoparticle (DNA-NP) assembly has been studied. We previously reported the lateral diffusion of DNA-NPs on planar lipid bilayer patches, owing to the mobility of lipid molecules in a supported lipid bilayer (SLB), and the resultant two-dimensional (2D) assembly of DNA-NPs. We here report the structural change of the assembled 2D DNA-NP lattices by magnesium ion concentration control on a successfully formed uniform SLB. In the magnesium-free buffer solution, DNA-NPs on SLB loosely assembled into quasi-hexagonal ordered lattices. In buffer solution containing 1 mM magnesium acetate, the interparticle distance of DNA-NPs decreased and the lattice structure became disordered. In buffer solution containing 5 mM magnesium acetate, the structure of DNA-NP arrays changed markedly and square lattices appeared. It is suggested that magnesium ions affected DNA molecules, which linked nanoparticles, and enabled the control of the structure of DNA-NP 2D arrays.

Control of Interface Shape by Non-Axisymmetric Solution Convection in Top-Seeded Solution Growth of SiC Crystal

We achieved the convex growth interface shape in top-seeded solution growth of SiC applying non-axisymmetric solution convection induced by non-axisymmetric temperature distribution. The detailed solution flow, temperature distribution and carbon concentration distribution were calculated by 3-dimensional numerical analysis. In the present case, the solution flow below the crystal was unidirectional and the supersaturation was increased along the solution flow direction. By the rotation of the crystal in the unidirectional flow and the temperature distribution, we successfully obtained the crystal with the convex growth interface shape.

SiC solution growth on Si face with extremely low density of threading screw dislocations for suppression of polytype transformation

In order to achieve a high-quality SiC crystal in solution growth, one of the most difficult issues is to grow a thick layer on Si face avoiding polytype transformation. In this case, two-dimensional nucleation, which leads to the polytype transformation, is frequently induced because a density of threading screw dislocations acting as a source of spiral step decreases and wide terraces form by step bunching as growth proceeds. Therefore, it is very difficult to stabilize the polytype of crystals grown with extremely low density of threading screw dislocations. In this study, we tried to overcome these problems by using specially designed seed crystal and optimizing growth temperature and temperature distribution. We successfully grew thick low-threading-dislocation density SiC crystal without polytype transformation under the condition of high growth temperature and homogeneous temperature distribution.

Polytype control by activity ratio of silicon to carbon during SiC solution growth using multicomponent solvents

We report on the relationship between grown polytypes and the activity ratio of silicon to carbon during SiC solution growth using multicomponent solvents. From the thermodynamic calculation as well as crystallization experiments, we revealed that a high activity ratio (aSi/aC) in the solution leads to the growth of low-hexagonality polytypes, and low aSi/aC results in the growth of high-hexagonality polytypes. 4H-SiC is stable when aSi/aC is relatively low (~101 > aSi/aC), 3C-SiC is stable when aSi/aC is relatively high (~104 < aSi/aC), and 6H-SiC is stable in the intermediate aSi/aC range (~102 < aSi/aC < ~103). From these results, the Cr–Si solvent at high temperatures is expected to be suitable for 4H-SiC growth, and Sc–Si and Fe–Si solvents at relatively low temperatures are expected to be suitable for 3C-SiC growth.