Solution Grown Se/Te Nanowires: Nucleation, Evolution, and The Role of Triganol Te seeds
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We have studied the nucleation and growth of Se–Te nanowires (NWs), with different morphologies, grown by a chemical solution process. Through systematic characterization of the Se–Te NW morphology as a function of the Te nanocrystallines (NCs) precursor, the relative ratio between Se and Te, and the growth time, a number of significant insights into Se–Te NW growth by chemical solution processes have been developed. Specifically, we have found that: (i) the growth of Se–Te NWs can be initiated from either long or short triganol Te nanorods, (ii) the frequency of proximal interactions between nanorod tips and the competition between Se and Te at the end of short Te nanorods results in V-shaped structures of Se–Te NWs, the ratio between Se and Te having great effect on the morphology of Se–Te NWs, (iii) by using long Te nanorods as seeds, Se–Te NWs with straight morphology were obtained. Many of these findings on Se–Te NW growth can be further generalized and provide very useful information for the rational synthesis of group VI based semiconductor NW compounds.
KeywordsSelenium Tellurium Nanowires Seeds
One-dimensional (1D) nanostructures such as nanowires (NWs), nanobelts, nanorods, and nanotubes, have been the focus of intensive research due to their novel electronic properties and potential applications in nanoscale devices [1, 2, 3, 4, 5, 6]. Among them, semiconductor NWs is investigated in more detail due to their important roles in fabricating nanoscale electronic or optoelectronic devices [7, 8, 9, 10, 11]. The growing interest in semiconductor NWs for electronic and photonic applications makes rational control over their morphology, structure, and key properties more and more important. It also requires thorough understanding of the growth mechanisms in specific material systems and techniques. Most group IV [12, 13], III–V , and II–VI  semiconductor compounds NWs had been fabricated via the vapor–liquid–solid (VLS) mechanism successfully. By this method, a liquid metal alloy initiates the growth of a solid whisker from vapor reactants. Compared to the VLS method, solution phase reactions have the advantage that seeds are not restricted to a two-dimensional (2D) growth plane, and copious quantities of well-defined nanostructures can be obtained easily compared to methods based on vapor-phase reactions. Chemical solution NW growth for group VI semiconductor material systems was envisioned to occur via the solution–solid–solution mechanism, in which trigonal Se or Te seeds initiate the growth of solid Se, Te, or Se/Te alloy NWs from solution reactants. Traditionally, Se [16, 17, 18] and Te  NWs have been synthesized by reduced selenious acid or orthotelluric acid at elevated temperatures, typically at 90–100 °C or by the reduction of metal–salt solutions with ascorbic acid at room temperature, or by a mild bio-molecule-assisted reduction method under hydrothermal conditions. As Se and Te have similar trigonal structures, it is possible that Se/Te alloy NWs of a single crystalline nature can be obtained by reducing selenious acid and orthotelluric acid at the same time in solution. More importantly, as Te tends to form in the trigonal phase more readily than Se, one may fabricate Se/Te heterojunctions by using Te nanorods as crystalline seeds. Although Xia et al.  had synthesized Se–Te alloy nanorods successfully by reducing selenious acid and orthotelluric acid with hydrazine at the temperature range of 90–100 °C, the lateral dimensions and morphology of the Se/Te NWs could not be controlled in this case due to lack of any surfactant and exact experimental control. On the other hand, the use of a trigonal Te NCs as a crystalline seed in Se/Te NW growth has received less attention. Qian Research Group [21, 22] had previously reported that by employing sodium dodecylbenzene sulfonate (SDBS) or other surfactants, Te nanorods with well controlled diameters and lengths could be reproducibly produced, which made the fabrication of Se/Te NWs by using Te NCs as crystalline seeds possible. Our research group  had further found that by using SDBS as the surfactant, the morphology and the lateral dimensions of Se/Te alloy NWs could be easily controlled. Following this, Se–Te alloy NWs with V-shaped structure has been prepared for the first time successfully by our research group with SDBS as surfactant .
In this article, we present new and simple methods for the fabrication of Se/Te NWs with different morphologies by using Te NCs seeds. We further investigated the nucleation and growth mechanism of Se–Te alloy with different morphology by controlling the experimental procedure. For the first time we have investigated the fabrication of Se/Te NWs with V-shape morphology, U-shape morphology, or straight morphology in the presence of SDBS surfactant by using different Te NCs seeds in detail. We prove here that such a method is a highly effective synthesis protocol to produce 1D nanostructures of Se/Te alloy NWs with different morphologies. Because of the mild reaction conditions and easily controlled synthesis, this method can be used in large-scale production of Te and Se/Te NW materials.
Se/Te NWs were synthesized by a two-step solution process. First, fabrication of Te NCs by a chemical solution process similar to our previous report [23, 24]. Typically, 2 mmol of orthotelluric acid and 0.5 g SDBS were added to 100 mL pure water. The solution was then refluxed for 1 h until a clear solution was obtained. Then, the resulting solution was heated up to 95 °C at a rate of 10 °C/min in an argon atmosphere. After 30 min, 1.5 mL of hydrazine was quickly injected into the solution through a syringe and the solution turned black and cloudy immediately. The solution was kept at 95 °C for another 15 mins and then moved to an ice bath to quench the reaction to 0 °C. The resulting solution was refluxed at room temperature for different time periods in order to get NWs with different lengths. To obtain short Te nanorods and nanoparticles, the reflux time is about 1 or 2 days while it takes at least 6 days to obtain long Te nanorods. Second, to obtain the V-shaped or U-shaped Se/Te NWs, the resulting solution, which was refluxed for about 1 day, was heated to 95 °C, and then a 15 mL solution containing 1 mmol, 2 mmol, or 4 mmol selenious acid was added drop by drop through a funnel into the resulting solution containing trigonal Te nanorods and colloids. The corresponding feeding ratio between Se to Te is 1:2, 1:1, and 2:1, respectively. The solution was refluxed at 95 °C for another 3 h and cooled down to room temperature. On the other hand, to obtain Se/Te NW with a straight morphology, the resulting solution that had been refluxed for 4 days was heated to 95 °C, and then a 15 mL solution containing 2 mmol selenious acid was added drop by drop through a funnel into the resulting solution which contains trigonal Te nanorods and colloids. The solution was refluxed at 95 °C for another 3 h and cooled down to room temperature.
Results and Discussion
The growth of Se/Te NWs was performed by using Te nanorods as crystalline seeds through a chemical solution process. The detailed process and growth parameters for the NWs growth can be found in the experiment procedure. Specifically, we have found that Te tends to form rod-shaped structures more easily than Se and hence has the highest surface reactivity along its spiral chain direction. Therefore, the Se/Te NWs are expected to form a wire-like structure by using the Te nanorod as the crystalline seed. In order to study the effect of feeding ratio (molar ratio) between Se and Te sources on the final morphology of Se/Te NWs, we investigated the TEM images of the Se/Te final product prepared by using short Te nanorods prepared at the same condition as crystalline seeds with different Se to Te feeding ratios.
EDS analysis of Se and Te content at different part of V-shape and straight Se/Te NW
V-shape Se/Te NW
Straight Se/Te NW
The FFT pattern was recorded by focusing a convergent beam on the NW. Since this pattern remained unchanged along the length of the NW, we concluded that this NW was essentially single crystalline in nature. HRTEM images show well resolved lattice fringes (in the (001) planes) of the Se/Te lattice, with the interplane spacing of 5.8 Å and 5.4 Å, respectively, which are between the values of trigonal Se (c = 4.953 Å) and Te (c = 5.921 Å), indicating that the NWs grow along the (100) direction. We also checked other parts of V-shaped (inset of Fig. 4c), and got similar results. The pattern indicates that these Se/Te NWs are single crystalline in nature and have predominantly grown along the  direction, with the helical chains of Se/Te atoms parallel to the longitudinal axis.
In conclusion, we have studied the nucleation and growth evolution of Se/Te NWs prepared by chemical solution process. By varying key growth parameters such as using short Te nanorods or long Te nanorods as crystalline seeds, sequentially changing the Se to Te content, growth time, first time significant insights into the Se/Te NWs with different morphology growth have been developed. The trigonal Te nanorods were found to have the major role on the growth of single crystalline Se/Te NWs, while the present of SDBS surfactant is necessary to restrain the grow direction of Se/Te NWs. V-shape or U-shape Se/Te NWs are most likely formed by the competition of selenium and tellurium at the end of Te nanorods (seeds) and by the frequency of proximal interactions between nanorod tips. This conclusion is also supported by the analysis of Se and Te content in different part of Se/Te NW by EDAX. The input SDBS surfactant was shown to play a critical role in NW growth. These findings are very useful for understanding and rational synthesis of Se/Te NWs or other VI group compound semiconductor NWs.
This work is supported by NSFC project (No 50703012) and the MOST project (Nos 2009CB930604 and 2009CB623602).