CrystEngComm PAPER Cite this: CrystEngComm, 2015, 17, 5734 Received 9th December 2014, Accepted 11th June 2015 DOI: 10.1039/c4ce02431a www.rsc.org/crystengcomm The effect of Se doping on the growth of Te nanorods†‡ Junghyeok Kwak, a Chang-Eun Kim, b Yuho Min, b Ji-Hwan Lee, b Aloysius Soon* b and Unyong Jeong * a In this study, we successfully conducted a series of computationally-assisted experiments, regarding the morphology control and chemical transformation of Te nanorods. The morphology of Te nanorods is con- trolled by introducing a minute amount of isovalent Se dopant. Density-functional theory calculations pre- dicted the Gibbs surface free energy change due to the adsorbent Se on the major facets of Te nanorods. Encouraged by the theoretical prediction, we conducted experiments on Te nanorod growth and did find significant variation of the morphology of Te nanorods due to Se injection. Furthermore, we demonstrated the chemical transformation of the shape-controlled Te nanorods to binary thermoelectric compounds such as PbTe and Bi 2 Te 3 without losing the tailored morphology. The transformed PbTe and Bi 2 Te 3 have nanoscale grain boundaries as seen from the cross-section HRTEM image. We emphasize that the robust production of morphology-controlled thermoelectric nanorods can be an optimal approach to develop an advanced thermoelectric composite material, by which the multiscale phonon scattering effect can be maximized. 1. Introduction Morphology control of nanocrystalline materials is of great interest, as the properties of the nanostructured materials are highly dependent on their dimensions; 1,2 thus, precise con- trol over these dimensions has been greatly sought after. When it comes to syntheses of nanocrystals, solution-based syntheses have notable advantages due to robust yield and uniform properties of the final product. There are several known mechanisms of growing nanocrystals by solution- based methods, including preferential growth of nanocrystal facets, 3,4 surfactant-assisted surface energy control, 5 oriented assembly of multiple nanocrystals, 6 sacrificial templating methods, 7 and chemical transformation of an existing nano- crystal into a completely different compound. 8 Although remarkable advances have been made as reviewed in other reports, 9,10,11 precise control over the geometric dimensions of these materials such as thickness or diameter is yet to be further investigated. Besides the geometric change of the nanocrystals due to dopants from a fundamental point of view, the shape- controlled Te nanorods can be used to produce an advanced thermoelectric composite. When shape-controlled nano- crystals are used to synthesize a bulk thermoelectric compos- ite, the size and shape of the nanocrystals have a critical impact on the packing factor, porosity, and grain boundary density of the final product. In this aspect, we further conducted chemical transformation of the shape-controlled Te nanorods into functional metal tellurides IJM x Te y ) such as PbTe and Bi 2 Te 3 . Metal tellurides IJM x Te y ) are considered promising mate- rials for developing advanced thermoelectric devices. 12 Spe- cifically, Bi 2 Te 3 , PbTe and their heterostructures have been widely investigated. 13,14 Generally, high electric conductivity, high Seebeck coefficient, and low thermal conductivity are required to improve thermoelectric efficiency. While the elec- tric conductivity and Seebeck coefficient are determined by the electronic structure, the thermal conductivity can be reduced by introducing microstructures such as grain bound- aries, line defects or point defects. According to Dresselhaus and Hicks, a remarkable reduction of thermal conductivity can be achieved when nanostructures induce robust phonon scattering phenomenon. 15–17 A straightforward approach to enhance phonon scattering is to increase the proportion of grain boundaries, mainly by using the powder-press method 5734 | CrystEngComm, 2015, 17, 5734–5743 This journal is © The Royal Society of Chemistry 2015 a Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Nam-gu, Pohang, Gyeongbuk 790-784, Republic of Korea. E-mail: ujeong@postech.ac.kr b Global E 3 Institute and Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-Ro, Seodaemun-Gu, Seoul 120-749, Republic of Korea. E-mail: aloysius.soon@yonsei.ac.kr † The authors declare no competing financial interest. ‡ Electronic supplementary information (ESI) available: Simulation codes and theoretical details for surface energy calculation; SEM and TEM images of the nanorods. See DOI: 10.1039/c4ce02431a Published on 11 June 2015. Downloaded by Yonsei University on 21/07/2015 15:43:18. View Article Online View Journal | View Issue