In Situ Laser Synthesis of Si Nanowires in the Dynamic TEM
Nanowires (NWs) are a crucial component in today's nanoscale devices and are vital to the further development of nanotechnology. To achieve reliable growth of NWs and uniform electronic properties, the specific mechanisms that control the structural development of NWs and the correlation between NWs and their nucleation and growth processes must be examined. Because imaging is not possible in Chemical Vapor Deposition (CVD) chambers, in situ transmission electron microscope (TEM) based growth methods provide a method for characterizing NW evolution. In this paper, we describe a new method of in situ synthesis and imaging of NWs using laser-assisted growth inside a TEM. The vapor-liquid solid (VLS) growth mechanism (Figure 1), first described by Wagner and co-workers, is a widely accepted description of how many 1D nanostructures are formed. VLS and similar models have been applied to such methods as hybrid pulsed-laser ablation/chemical vapor deposition (PLA/CVD) and laser assisted NW growth at elevated temperatures, in which a pulsed laser ablates a target with gas flowing through the reaction chamber held at elevated temperature. These methods have successfully produced semiconductor and conducting oxide NWs with size control determined by the catalyst droplet diameter. The VLS description of NW growth is a widely accepted model, but the exact role of vapor species and its interaction with the catalyst particle is not fully understood. For example, NW growth has been observed to occur far below predicted eutectic temperatures, which suggests that alternative mechanisms such as solid-liquid-solid growth (SLS) may play a role. Many NW fabrication techniques preclude direct in situ characterization of the growth process. Because of this need for direct observation during growth, TEM-based NW growth methods have become increasingly popular. Though progress has been made in the study of the VLS mechanism of NW growth using in situ TEM, conventional in situ growth experiments are limited by the standard video frame rate ({approx}30 Hz). Because of this limitation, relationships between reactant incorporation and transport, catalyst-wire interface development, and wire growth texture remain somewhat unclear. Moreover, laser-assisted phenomena (PLA or PLA/CVD) cannot be studied with conventional in situ TEM, which is typically performed with environmental TEM techniques using resistive heating. A novel form of in situ TEM for the study of NW growth by laser-assisted techniques is described in this paper. Here we present an in situ method for PLA synthesis of 1D nanostructures using the dynamic transmission electron microscope (DTEM)[18,19] that will permit the details of laser-assisted NW synthesis to be elucidated. The DTEM, schematically shown in Figure 2, consists of a TEM column (JEOL 2000FX) to which a pulsed Nd:YAG laser, labeled 'hydrodrive laser' in Fig. 2, is added. This laser is incident directly on the sample and can be operated at 1064, 532 or 355nm. The direct delivery of energy initiates a phase change or reaction that is investigated in situ during the experiment. Thus, the microscope can be used to fabricate NWs while simultaneously characterizing the reaction products created by each laser pulse. A second pulsed laser (15-ns-duration frequency-quintupled Nd:YLF) added to the column (labeled 'cathode laser' in figure 2) enables operation in both a pulsed photoemission and conventional thermionic continuous-wave (CW) electron beam imaging modes.[19] The pulsed mode's nanosecond-scale imaging capability greatly exceeds the {approx}30 Hz time resolution of conventional in situ TEM and can potentially provide a detailed understanding of NW growth. Time-resolved imaging experiments aimed at capturing the intermediate states at nanosecond time scales in PLA NW growth are underway and will be addressed in a future publication. In this paper, we present results obtained using in situ imaging in conventional CW imaging mode, but taking advantage of the DTEM's in situ laser drive capability. Specifically, we will present the first results using the sample ('hydro-drive') laser at 355nm to stimulate the growth of the nanostructures, therefore revealing the DTEM as a tool for in situ imaging of nanostructure synthesis (see the experimental section for details of the NW synthesis procedure).
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 950646
- Report Number(s):
- LLNL-JRNL-400555; TRN: US200910%%290
- Journal Information:
- Small, vol. 4, N/A, April 1, 2008, pp. 2187 - 2190, Vol. 4
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
42 ENGINEERING
CATALYSTS
CHEMICAL VAPOR DEPOSITION
DEPOSITION
ELECTRON BEAMS
ELECTRON MICROSCOPES
EUTECTICS
FABRICATION
HEATING
LASERS
MICROSCOPES
NANOSTRUCTURES
NUCLEATION
OXIDES
PHOTOEMISSION
SYNTHESIS
TARGETS
TEXTURE
THERMIONICS
TIME RESOLUTION