Abstract
Growing nanowires inside a transmission electron microscope (TEM) and observing the process in situ has contributed immensely to understanding nanowire growth mechanisms. Majority of such studies were on elemental semiconductors – either Si or Ge – both of which are indirect bandgap semiconductors. Several compound semiconductors on the other hand have a direct bandgap making them more efficient in several applications involving light absorption or emission. During compound nanowire growth using a metal catalyst, the difference in miscibility of the nanowire species inside the metal catalyst are different, making its growth dynamics different from elemental nanowires. Thus, studies specifically focusing on compound nanowires are necessary for understanding its growth dynamics. This chapter reviews the recent progresses in the understanding of compound semiconductor nanowire growth obtained using in situ TEM. The concentrations of the nanowire species in the catalyst was studied in situ. This concentration difference has been shown to enable independent control of layer nucleation and layer growth in nanowires. In situ TEM has also enabled better understanding of the formation of metastable crystal structures in nanowires.
Highlights
IntroductionThe high surface-to-volume ratio and the high aspect ratio of the nanowire geometry paves the way to a plethora of interesting advantages
Materials generally grown in the nanowire morphology can be broadly classified as elemental and compound [9]
transmission electron microscope (TEM) resolution is currently restricted by technical limitations, not by the physically attainable limit; over the years TEM resolution has been constantly improving and this evolution is visible if we look at reports of nanowire growth with in situ TEM as well
Summary
The high surface-to-volume ratio and the high aspect ratio of the nanowire geometry paves the way to a plethora of interesting advantages. Some of the common techniques for growing nanowires with gas phase precursors by the aforementioned mechanisms include chemical vapor deposition (CVD), metalorganic CVD (MOCVD) and molecular beam epitaxy (MBE). These systems were initially designed for growing thin films and later adapted for growing nanowires. The supplied vapor phase precursor species dissolves in the liquid catalyst and at appropriately high supersaturation crystallizes atomic layers of the solid nanowire. Some MOCVD systems are equipped with in situ light reflectance monitors which can be used to estimate the increase in sample height and surface roughening These methods are used conventionally for tracking growth of thin films from large areas of the sample. An example of a phenomenon which was discovered only due to in situ observation of individual nanowires is truncation — where the nanowire-catalyst interface has a dynamic non-flat surface near the triple-phase-line [44–48]. (Triple-phase-line refers to the periphery of the interface between the nanowire and the catalyst droplet where the vapor, liquid and solid phases meet.) Another interesting in situ observation was that the nucleation of wurtzite layer happens at, or at least very close to, a corner of the triple-phase-line (observation of the precise location being elusive due to the ‘limited’ temporal resolution compared to the expected extremely rapid growth of the nucleus to beyond the critical size) [49]
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