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2008, 112, 4036-4041 Published on Web 02/28/2008
Fabrication of High Aspect Ratio Core-Shell CdS-Mn/ZnS Nanowires by a Two Step Solvothermal Process Soumitra Kar,*,† Swadeshmukul Santra,*,†,‡,§ and Helge Heinrich|,⊥,# NanoScience Technology Center, Department of Chemistry, Biomolecular Science Center, AdVanced Materials Processing and Analysis Center, Department of Physics, Department of Mechanical, Materials, and Aerospace Engineering, UniVersity of Central Florida, Orlando, Florida 32816 ReceiVed: January 11, 2008; In Final Form: February 14, 2008
Synthesis of thin (5-8 nm) CdS-Mn/ZnS core-shell nanowires (CSNWs) with high aspect ratio (up to ∼150) is reported for the first time. The modified solvothermal synthesis of CSNW involved two steps: the formation of Mn-doped CdS core followed by the ZnS shell. The CSNW growth process is engineered in such a way that the ZnS shell layer grew radially onto the prematurely grown CdS-Mn core, that is, prior to the formation of its well-faceted surface.
Introduction One-dimensional (1D) nanostructures have received tremendous attention in the field of electronics and optoelectronics since the discovery of carbon nanotubes.1 In particular, 1D semiconductor nanostructures are considered to be critical building blocks for nanoscale electronic and optoelectronic devices.2-8 To improve performance of these nanodevices, it is important that the efficiency of 1D semiconductor nanostructures be increased. Because surface defects are prominent due to large surface to volume ratio, efficiency of nanostructures and hence their performance in nanodevices9-13 could be improved by reducing surface defects. Thus, the focus now is on developing synthesis strategies for effective surface passivation of a 1D nanostructure that minimizes surface defects. To date, the best surface passivation approach is the creation of heterostructures such as core-shell nanostructures. In a typical surface passivation procedure, a shell structure of a wide band gap material is created over the core. In an effectively surface-passivated core-shell nanostructure, the core is completely covered by an epitaxial shell with minimal lattice mismatch between the core and the shell. Because of limited availability of appropriate shell layer material, to date only a few 1D core-shell nanostructures have been reported in the literature including Ge/Si,9 RuO2/ TiO2,14 CdSe/CdS,15 CdSe/ZnS,16 CdS/ZnS,17 and CdSe/ZnSe.18 Traditionally, 1D core-shell nanostructures have been synthesized using high-temperature vapor deposition methods such as chemical vapor deposition (CVD)9 and metallo-organic chemical vapor deposition (MOCVD).17 These methods are sophisticated and demand controlled stepwise supply of core * To whom correspondence should be addressed. Fax: 1 407 882 2819. Tel: 1 407 882 2848. E-mail: (S.K.)
[email protected]; (S.S.)
[email protected]. † NanoScience Technology Center. ‡ Department of Chemistry. § Biomolecular Science Center. | Advanced Materials Processing and Analysis Center. ⊥ Department of Physics. # Department of Mechanical, Materials, and Aerospace Engineering.
10.1021/jp800277x CCC: $40.75
and shell elements in the vapor state. It is therefore desirable to develop a relatively simple method of synthesizing 1D coreshell nanostructures. Moreover, it is desired that methods are developed to synthesize core-shell nanostructures with high aspect ratio, such as CSNWs. In this context, hot-phase chemical routes15,16,19 appear to be more attractive than traditional CVD methods. However, synthesis of 1D nanostructure with a large aspect ratio (i.e., length to diameter) using hot phase chemical route is challenging. So far, only short nanorods have been reported in the literature15,16,19 because of inherent limitations of the method. To the best of our knowledge, synthesis of dopant-based thin ( 6A1 Mn2+ ion transition.24,25 Also, as building blocks in nanoscale optoelectronic devices,26-28 these Mn2+ ions-doped CdS semiconductors are utilized as dilute magnetic semiconductors. To date, a few reports on the CdS-Mn/ZnS core-shell nanoparticles23,29,30 have been published, but to our knowledge there is none on the synthesis of CdS-Mn/ZnS CSNWs. Herein we report for the first time a simple two-step solvothermal route for the synthesis of thin single-crystal CdS-Mn/ZnS CSNWs with aspect ratio up to >150. Controlled growth of CSNWs is challenging as it demands favorable thermodynamic as well as kinetic conditions that © 2008 American Chemical Society
Letters
Figure 1. Schematic representation of the CdS-Mn/ZnS CSNW formation mechanism.
would allow crystalline growth in 1D. An attempt to incorporate dopants within the core of such CSNWs could be further challenging as the crystal growth process31 naturally excludes impurities (e.g., dopants). In this study, we intentionally created appropriate reaction conditions that allowed doping of Mn within CSNWs. In the past, solvothermal route has been successfully used to synthesize CdS nanorods with the help of a bidentate ligand, ethylenediamine (NH2CH2CH2NH2, en).32-34 This knowledge base is the foundation of the present study. First, we took advantage of the solvothermal route for the en-mediated 1D nanostructure template synthesis of CdS. Then, we initiated a controlled crystal growth process in the presence of shell layer source ions to obtain uniform CSNWs. The complete CSNW design scheme is depicted in Figure 1. Experimental Procedure The core-shell nanowire growth was carried out in a Teflonlined closed cylindrical stainless steel chamber. In a typical procedure, cadmium acetate and thiourea (1:3 molar ratios) were filled with appropriate amount of ethylenediamine (en), and 3 molar % of manganese acetate was used as the dopant source. The closed chamber was placed inside a preheated oven at 160 °C, and the reaction was continued for 6 h. In the next step, a mixed aqueous solution of zinc-acetate dihydrate (2.6 molar times that of Cd source) and thiourea (1:3 molar ratios) was added, and the reaction was continued for additional 4 h. Upon normal cooling, the product (yellow precipitate) was washed several times in water and ethanol and finally dried in vacuum for characterization. Results and Discussion In the traditional synthesis approach,32,33 cadmium acetate and thiourea (1:3 molar ratio) are mixed in en that also serve as a solvent. The manganese dopant content is kept at three molar percent of the Cd source to avoid concentration quenching.25 The Cd2+ metal ions interact with the lone pair of electrons of the en nitrogen atoms. A subsequent reaction of en-ligated Cd2+ ions with the S2- ions forms a 2D complex of CdS0.5en.34 This complex possesses an organic-inorganic lamellar structure with inorganic CdS layers separated by organic en spacers.34 As the reaction proceeds, the complex dissociates, collapsing the lamellar structure. As a result the 2D structure of the complex material disappeared leaving behind thin 1D CdS crystallites. These thin crystalline needlelike CdS nanostructures possessed highly energetic surfaces due to their undefined nature. Because of their high surface energies, they were prone to further growth following favorable crystal growth mechanism. Under normal solvothermal condition, the CdS-
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4037 0.5en and CdS phase remain in an equilibrium that favors enassisted recrystalization of the CdS templates. Over time, the CdS templates lead to the formation of well-faceted CdS nanorods. The mechanism of CdS nanorod formation could be due to en-assisted recrystalization of the CdS templates and/or Ostwald ripening. The elimination of the en from the CdS-0.5en complex could be enhanced by increasing the thermal energy or by introducing a foreign noncoordinating solvent such as water. It has been demonstrated that an increase of temperature leads to shortening of nanorod length and increase of their diameter.33 We hypothesize that addition of water at relatively low temperature (160 °C) could lead to the formation of well-crystallized thin (
