Thermal Contraction of Electrodeposited Bi/BiSb Superlattice Nanowires
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- Dou, X., Li, G., Huang, X. et al. Nanoscale Res Lett (2010) 5: 1118. doi:10.1007/s11671-010-9611-4
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The lattice parameter of Bi/BiSb superlattice nanowire (SLNW) has been measured using in situ high-temperature X-ray diffraction method. The single crystalline Bi/BiSb SLNW arrays with different bilayer thicknesses have been fabricated within the porous anodic alumina membranes (AAMs) by a charge-controlled pulse electrodeposition. Different temperature dependences of the lattice parameter and thermal expansion coefficient were found for the SLNWs. It was found that the thermal expansion coefficient of the SLNWs with a large bilayer thickness has weak temperature dependence, and the interface stress and defect are the main factors responsible for the thermal contraction of the SLNWs.
KeywordsSuperlattice nanowire Bismuth Electrodeposition Thermal contraction Anodic alumina membranes
The thermal expansion properties of low dimensional nanomaterials are important considerations for the application in thermoelectric fields. The positive thermal expansion is commonly observed in most bulk materials and can be understood in terms of Gruneisen parameter [1, 2, 3]. The negative thermal expansion, commonly originating from a structural related phase transition [4, 5, 6], has been observed among anisotropic systems, such as in AgI nanowires .
Bi and its alloy nanowires are potential candidates for low-temperature thermoelectric applications [8, 9] and have been extensively studied [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]. Theoretical calculation indicates that highly anisotropic bulk Bi has either positive or negative thermal expansion behavior depending strongly on crystallographic directions , and our previous experimental studies found that the lattice parameter and thermal expansion coefficient of single crystalline Bi nanowire depends strongly on its diameter and orientation [22, 25].
There are always certain kinds of defect and lattice stress in nanowires, especially for those fabricated by the electrochemical method [26, 27], and the surface defect and stress will increase substantially with decreasing nanowire diameter, which might be significant in determining the thermal behavior of nanowires. Thermal expansion results from the interplay between thermal stress and elasticity, leading to a rich variety of thermal behavior . Timmesfeld et al.  found that the point defects can lead to the changes in spring constants and anharmonicity around the defect, and these changes partly cancel and result in positive or negative thermal expansion coefficient. Xu et al. [30, 31] studied the thermal expansion behavior of the ordered silver nanowire arrays embedded in AAMs, and pointed out that the collective effects of the intrinsic expansion of silver nanowires together with surface pressure from the AAM and the vacancies incorporated into the silver lattice were responsible for the thermal expansion. Zhu et al.  pointed out that the first time XRD measurement is equivalent to an annealing process, which will partly eliminate the vacancies in Bi nanowires, and thus the lattice parameter will decrease at the second time measurement. Nevertheless, the elimination of defects is relatively more difficult than that of stresses and needs a particular annealing process. The existence of the interfaces in SLNWs will introduce excess defect and stress, which will have a strong influence on the thermal expansion behavior of SLNWs. In this paper, we report the thermal expansion behavior of Bi/BiSb SLNWs with different bilayer thicknesses by high-temperature XRD measurements.
We adopt the charge-controlled pulse electrodeposition to fabricate Bi/Bi0.5Sb0.5 SLNWs in a two-electrode plating cell in which the AAM sputtered with a layer of Au film (about 200 nm in thickness) serves as the cathode, and a graphite plate serves as the counter electrode [32, 33]. The AAM with the pore size of about 60 nm was prepared using the same procedures as in our previous reports [20, 21, 22, 23, 25, 32, 33, 34]. Four SLNW samples (SL1-SL4) with different average bilayer thicknesses were prepared (the lengths of segment Bi and BiSb nanowires are 15 and 50 nm for SL1, 7.5/25, 3/10 and 3/5 for SL2, SL3 and SL4, respectively).
Power X-ray diffraction (XRD, Philips PW 1700× with Cu Kα radiation), field emission scanning electron microscopy (FESEM; FEI Sirion-200), transmission electron microscopy (TEM and high-resolution TEM, JEOL-2010), selected area electron diffraction (SAED) were used to study the crystalline structure and morphology of the SLNW array. For SEM observations, the AAM was partly dissolved with 0.5 M NaOH solution and then carefully rinsed with deionized water several times. For TEM observations, the AAM was completely dissolved with 1 M NaOH solution and then rinsed with absolute ethanol and dispersed on a lacey carbon/copper TEM grid (SPI Supplies). In situ high-temperature XRD (Philips PW 1700) measurement was performed on Pt substrate in the temperature range from 298 to 508 K under high-vacuum atmosphere. Temperatures were kept constant at each point for 10 min before each measurement. The exact peak position of the SLNW arrays is obtained firstly strip Kα2 radiation, and then fits the selected peak by HighScore software.
Results and Discussion
Average segment length L1, L2 and room temperature lattice parameter
After 1st (Å)
From the aforementioned results, one can see that the thermal expansion coefficients of Bi nanowires are negative in the whole measuring temperature before and after annealing treatment, and that of BiSb alloy nanowires are either negative or positive depending on the measuring temperature, while that of the Bi/BiSb SLNWs are negative before annealing and become positive at high temperature after annealing. The thermal expansion coefficient of the SLNWs with a large bilayer thickness has weak temperature dependence, while that with a small bilayer thickness has strong temperature dependence and becomes weak upon annealing treatment, which clearly indicates that the stress and defect in the interface play a main role in determining the thermal contraction of the Bi/BiSb SLNWs.
In summary, the temperature dependence of the lattice parameter and thermal expansion coefficient of single crystalline Bi/BiSb SLNW arrays with different bilayer thicknesses were studied. There exists an obvious thermal contraction effect in the SLNWs. The thermal expansion coefficient of the SLNWs depends strongly on temperature and changes from negative to positive at elevated temperature. The thermal expansion coefficient of the SLNWs with a large bilayer thickness has weak temperature dependence, and the interface stress and defect plays a main role in the thermal contraction of the SLNWs. The SLNWs with a small bilayer thickness is considered to have a better thermoelectric performance, but we must consider the thermal expansion behavior in the practical applications of thermoelectric devices. The results will show a promise in studying the nature of the thermal expansion properties of nanowires.
We gratefully acknowledge the financial support from the National Natural Science Foundation of China (10704074) and the National Major Project of Fundamental Research for Nanomaterials and Nanostructures (No: 2005CB623603).
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