| 학회 | 한국재료학회 |
| 학술대회 | 2012년 가을 (11/07 ~ 11/09, 라카이샌드파인 리조트) |
| 권호 | 18권 2호 |
| 발표분야 | C. 에너지/환경 재료(Energy and Environmental Materials) |
| 제목 | Kinetics of individual nucleation in nanoscale Si-Au and Ge-Au systems |
| 초록 | The effect of size on phase stability and phase transformations is of both fundamental and applied interest. For example, during the nucleation and growth of self-assembled nanowires from nanoscale metal catalysts, the phase of the catalyst determines properties such as the growth rate and the structure of the nanowire. Based on our interest in Si and Ge nanowires catalyzed using Au, we have examined phase stability in the Si-Au and Ge-Au systems. Observations were made in situ in an ultra high vacuum transmission electron microscope that has gas delivery capabilities. We start with either size-selected aerosol particles or with evaporated and agglomerated Au films on electron transparent SiN membranes. The samples are heated by direct current, and Si or Ge is supplied by flowing the chemical vapor deposition precursor gases, disilane or digermane through a capillary into the polepiece region. We use a combination of bright and dark field imaging conditions to characterize the structures present and to quantify phase stability as a function of the conditions. For Si-Au, the transformation takes place as Si is added to the particle, and it can be seen that the reaction proceeds from the surface inwards, so that at intermediate times the system consists of a solid Au particle surrounded by liquid AuSi. [1] Modeling the kinetics of this transformation demonstrates that the Au-AuSi interface energy becomes important in modifying phase stability when the Au size becomes small. Analysis shows a substantial shift in the liquidus line, and a discontinuous change in the liquid composition at the transition. On further addition of Si, the AuSi eutectic liquid droplet becomes supersaturated with Si and eventually Si is precipitated. This nucleation event shows complex kinetics [2], as the nucleus rapidly jumps to a large volume and then grows more slowly at a rate dependent on the supply of Si. The jump size can be used to determine the supersaturation of Si in AuSi at the moment of nucleation. A comparison of kinetics at a range of temperatures, pressures and droplet sizes shows consistent behavior: supersaturation is not dependent on droplet volume, leading to predictable nucleation kinetics, which may be important in forming Si nanowires controllably. In the Ge-Au system, we find three distinct reaction pathways. The lowest temperature reaction is distinguished by the absence of any purely liquid state. From measurements of reaction rates and parameters such as supersaturation, we explain the sequence of pathways as arising from a kinetic competition between the imposed timescale for Ge addition and the inherent timescale for Ge nucleation. References [1] B. J. Kim, J. Tersoff, C.-Y. Wen, M. C. Reuter, E. A. Stach and F. M. Ross, Physical Review Letters 2009, 103, 155701. [2] B. J. Kim, J. Tersoff, S. Kodambaka, M. C. Reuter, E. A. Stach and F. M. Ross, Science 2008, 322, 1070. |
| 저자 | 김봉중 |
| 소속 | 광주과학기술원 신소재공학부 |
| 키워드 | Si-Au; Ge-Au; nucleation; phase transformation; kinetics; supersaturation |
