Role of Fluid Dynamics in Nanofabrication: Growth Mechanism and Optical Properties of Ligand Free CdS Nanotubes made using Alumina Nano-Reactor

Abstract

Cadmium sulfide is a well-known photoactive semiconducting material with many potential applications in optoelectronics. Most of these CdS nanostructures are grown using various physical and chemical routes. As such, it is well known that chemical synthesis process can tune the size and shape dependent physical properties of the material itself. Moreover, better knowledge on the role of hydrodynamics and kinetics of chemical reactions is very much needed to control the shape and also tuning the potential properties of nanostructures made using wet chemical nanofabrication process. Understanding such growth dynamics and subsequent modifications of physical properties of the concerned material is very important in optimizing the device quality for future applications. To probe this, we explore the role of hydrodynamics in nanofabrication process of ligand free cadmium sulfide [CdS] nanotubes self-assembled on porous alumina using a two-chamber nanoreactor based synthesis method. These CdS nanotubes show a significant photoluminescence [PL] red shift with increasing size as a result of the interplay between crystalline strain and defects incorporated during nucleation process.

Here, we present [chapter 3] a directionally asymmetric growth of CdS nanotubes on porous alumina as a result of nanoconfined reactions between two chemical precursors flowing horizontally in opposite directions. We show that the dissimilar flow rates of chemical precursors are responsible for initiating the one sided growth of CdS nanotubes on porous alumina. Structural evaluation using scanning electron microscopy images identified the nucleation sites, and also shows the elongation of CdS nanotubes. These incremental growths of CdS nanotubes may be due to plausible presence of chemo-hydrodynamic instability at the AAO-Na2S interface. However, the estimated values of dimensionless numbers are not sufficient enough to explain the occurrence of hydrodynamic instability at nanoscale. Therefore, using the electron microscope images, we investigate the unexplored roles of chemo-hydrodynamics at liquid-liquid reactive interface for such prolonged nucleation of CdS nanotubes.

To address the presence of hydrodynamic instability during nanotubular growth, we measured the length of CdS nanotubes [chapter 4] and observed an increasing runaway growth rate with growth duration using scanning electron microscopy, which cannot be explained by simple diffusion process of reactive ions. We argue this as a signature of instability at such reactive interface. We also identified the role of precursors in this particular CdS nanotubular growth and found that sodium sulfide (Na2S) is playing an important role in the elongation of nanotubes. Overall, many experiments are done to demonstrate the plausible chemo-hydrodynamic influence and presence of instability at nanoscale during this nanofabrication process.

To know more about the optical properties of these CdS nanotubes, we measured the PL. Interestingly, we see an enhanced PL [chapter 5] from CdS nanotubes as compared to CdS nanocrystallites made by bulk mixing of same precursors used during the synthesis of nanotubes. Even though the size of CdS nanotubes is much larger than its quantum confinement regime, we observed a sizeable shift in PL [chapter 6] with different diameter and length. In fact this can be verified from optical absorption studies, which reveals that near band edge absorption of CdS nanotubes is not affected by its size. As from experimental and theoretical results, we argue that the PL shift is mainly due to the influence of crystalline strain on cadmium vacancy minority defect at the nanotube surface as result of its unique growth process. We also show an increasing crystalline strain with size of these CdS nanotubes, which can ultimately red shift the PL peak. Calculations based on density functional theory also supported this explanation. Although sulfur interstitials having the lowest formation energy is the most abundant surface defect on these nanotubes but it is not participating in PL process. So, we predict that the nature of the defects is more important in determining the PL process as compared to defects with more concentration located on the surface.