| Summary: | 碩士 === 東海大學 === 應用物理學系 === 104 === The purpose of this thesis is to synthesizing large grains few-layer hexagonal boron nitride on copper foil without particles and carbon contamination by low pressure vapor phase transport approach.
In this study, a home-build horizontal low pressure chemical vapor deposition reactor was used to grow few layer h-BN sheets on polycrystalline copper foils of 10 micrometer thickness. Before synthesizing, a series of surface pretreatment included CH3COOH acid etching, 10000C and 4 hour annealing under argon and hydrogen ambience. Solid ammonia borane powder that place in a stainless steel tube was used as precursors. Borazine (B3N3H6) produced from thermal decomposition of ammonia borane was carried by argon and hydrogen gases while aminoborane was filtered out by PTFE filter. At the end of the growth, the furnace was quickly cooled down to room temperature by moving the oven. The synthesized products were then transferred from copper foil to silicon oxide substrate for further characterizations and applications.
The few layer hexagonal boron nitride sheets were successfully synthesized on copper foil. The scanning electron microscopy images of h-BN grains demonstrate triangular and wrinkle morphology with size of a few micrometer. It is speculated that the large difference of thermal expansion coefficient between hexagonal boron nitride sheets and copper foil. The Raman scattering spectra of synthesized sheets on oxide substrate show a peak located at about 1363 cm-1 and confirmed hexagonal boron nitride phase. It is also noted that the use of PTFE filters in the system actually efficiently filtered out the aminoborane particles with micrometer size. However, we still observed some nanometer clusters located outside the triangle h-BN grains. It is believed that these nanometer clusters is not similar with aminoborane particles but could possibly be formed from the aggregation of extra boron atoms precipitated from the edge of h-BN grains. For verifying the stability of synthesized h-BN sheets, a continue 1000oC and 10 minutes heating after successful synthesis of h-BN sheets was conducted. It is observed that a continuous heating would reduce the h-BN grains size and destroy the triangular shape. This phenomena could possibly be resulted from the evaporation of cooper atoms beneath the h-BN sheets.
Further investigations through changing the process time from 30 to 150 minutes indicated that the nucleation density increased with increasing the process time. For verifying the stability of synthesized h-BN sheets, a continue 1000oC and 10 minutes heating after successful synthesis of h-BN sheets was conducted. It is observed that a continuous heating would reduce the h-BN grains size and destroy the triangular shape. This phenomena could possibly be resulted from the evaporation of cooper atoms beneath the h-BN sheets. It would be possible also happened during processing and contribute to be new nucleation sites. The observed grain size of h-BN sheets with increasing processing time did not continuous increase due to the dense nucleation sites.
Our results indicated that it is necessary not only to decrease the nucleation sites of cooper foils but also to optimize the processing parameters to decrease the evaporation rate of cooper atoms for enlarging the grain size.
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