- Nano Express
- Open Access
Morphological variations in cadmium sulfide nanocrystals without phase transformation
© Dhage et al; licensee Springer. 2011
- Received: 22 December 2010
- Accepted: 14 June 2011
- Published: 14 June 2011
A very novel phenomenon of morphological variations of cadmium sulfide (CdS) nanorods under the transmission electron microscopy (TEM) beam was observed without structural phase transformation. Environmentally stable and highly crystalline CdS nanorods have been obtained via a chemical bath method. The energy of the TEM beam is believed to have a significant influence on CdS nanorods and may melt and transform them into smaller nanowires. Morphological variations without structural phase transformation are confirmed by recording selected area electron diffraction at various stages. The prepared CdS nanorods have been characterized by X-ray powder diffraction, TEM, UV-Vis spectroscopy, and photoluminescence spectroscopy. The importance of this phenomenon is vital for the potential application for CdS such as smart materials.
- Select Area Electron Diffraction Pattern
- Cadmium Sulfide
- Cadmium Selenide
- Structural Phase Transformation
- Zinc Blend
Intensive research has been conducted on one-dimensional semiconductors due to their fundamental significance for studying the dependence of various physical properties on dimensionality and size reduction, as well as the potential for applications in nanodevices [1, 2]. In recent years, controlling the morphology and size of nanomaterials has been a crucial issue in nanoscience research due to their fundamental shape- and size-dependent properties and significant applications. Cadmium sulfide (CdS) is one of the important direct band II-VI semiconductors. It has a band gap of 2.4 eV at room temperature, having vital optoelectronic applications for laser light-emitting diodes, and optical devices based on nonlinear properties [3, 4]. As an important II-VI semiconductor material, CdS nanocrystal has received considerable interest from researchers in control of its morphology and size.
The morphology of nanomaterials is a key factor that affects their properties. Nanostructures with novel morphologies have been considerably investigated. There are all kinds of highly faceted geometries such as rods, tetrapods, hexagons, cubes, and pyramids that have been obtained through sequential experiments within the cadmium selenide [5–8]. At the same time, theoretical discussion on the shape-property relation predicted that shape anisotropy induced optical polarization and single-particle electronic state differences. This would generate newer applications for the material and, in turn, stimulate chemists to pursue nanocrystals with novel shapes [9–11]. In recent years, the morphology effect of semiconductor nanocrystallites on their physical properties has aroused extensive attention [12, 13]. Since many fundamental properties of semiconductor materials have been expressed as a function of size and shape, controlling these aspects of semiconductor nanocrystallites would provide opportunities for tailoring properties of materials and offer possibilities for observing interesting and useful physical phenomena. Development of synthetic strategies for CdS nanocrystals of various shapes is still very significant to the field of materials science. The influence of various reaction parameters and solvents on the morphology of CdS nanostructures have been studied extensively by various researchers [14–17].
In this paper, we are reporting on a preparation of CdS nanorods and its novel morphological variation under the TEM beam. This report is the first of its kind to identify such morphological variations of CdS nanorods under a TEM beam. The morphological variations without phase transformations are supported by TEM images and corresponding selected area electron diffraction (SAED) patterns recorded at different stages. They are also supported by the characterization of CdS nanorods by X-ray powder diffraction (XRD), UV-Vis spectroscopy, and photoluminescence (PL) spectroscopy. The importance of this unique phenomenon in CdS nanorods is that it could potentially be applicable for smart materials.
All the chemicals utilized were of AR grade without any further purification (from Sigma-Aldrich). The synthetic method for CdS nanorods used in this work has been based on a previously reported chemical bath technique . The 0.16 M CdSO4 solution was first added to 7.5 M NH4OH solution under constant stirring. Following this, 0.6 M thiourea solution was slowly added to the mixture with rigorous stirring. The bath temperature and pH were maintained at about 65°C and 10, respectively. A precipitated yellow solid product was centrifuged and dried in the oven at 65°C for 4 h.
The crystal phase analysis of the synthesized nanorods was determined by XRD (Cu Kα radiation, X'pert, Philips) with a Bragg angle ranging from 20° to 80°. We then use a TEM (JEOL 100CX, JEOL) with a beam current of 80 μA at an accelerating voltage of 100 kV), to SAED patterns. These were obtained to examine the morphological variations and diffraction patterns at different stages. A TEM sample was then prepared by putting a minute amount of CdS nanorods powder on a carbon-coated copper grid, without dispersing powder in the solvent. The optical absorption of the CdS nanoparticles was then examined by a Perkin-Elmer lambda 20 UV/Visible spectrometer. Lastly, the photoluminescence spectrum was analyzed by a PTI fluorescence spectrometer.
The formation mechanism of CdS nanorods of cubic Zn-blend structure is due to the aqueous medium and the coordination of thiourea ligand as a molecular template mechanism, wherein temperature and pH are critical conditions. Similarly, Li et al  report the spherical morphology of CdS with cubic Zn-blend structure prepared in water and pyridine at 120°C. More research is being done towards the understanding of nanorod formation and its transformation into small nanowires after melting under a TEM beam.
The CdS nanorods of Zn-blend cubic crystal structure were prepared by a chemical bath method. We demonstrated the transformation of CdS nanorods to small nanowires under a TEM beam without a crystal phase transition. The morphological transformation of CdS nanorods into nanowires without phase transition is a novel and unique phenomenon observed in this specific material. This could be potentially applicable for smart materials, and various other applications can be explored.
We are thankful to the NSF IGERT Materials Creation Training Program (MCTP)-DGE-0654431 for the use of its analytical facilities.
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