Journal of the Korean Physical Society, Vol. 60, No. 10, May 2012, pp. 1513∼1516 Study of the Thermal Conductivity of ZnO Nanowires/PMMA Composites Kh. T. Igamberdiev, Sh. U. Yuldashev, H. D. Cho and T. W. Kang ∗ Quantum-functional Semiconductor Research Center, Dongguk University, Seoul 100-715, Korea Sh. M. Rakhimova and T. Kh. Akhmedov Department of Thermophysics, Academy of Sciences, Tashkent 700135, Uzbekistan (Received 4 July 2011, in final form 20 January 2012) From thermal conductivity measurements on ZnO nanowires (NWs)/poly(methyl methacrylate) PMMA composites, the thermal conductivities of the ZnO nanowires were determined. The thermal conductivity of a ZnO NW decreases considerably with decreasing nanowire diameter, and for a ZnO nanowire with a diameter of 250 nm, the thermal conductivity at room temperature is approximately two times lower than that of bulk ZnO at the same temperature. The results of this study show that the thermal conductivity of a ZnO NW is mainly determined by increased phonon-surface boundary scattering. These results could be useful for the design of ZnO-nanowire-based devices. PACS numbers: 66.70.Df, 66.70.Lm, 63.22.Gh Keywords: ZnO nanowires/PMMA composite, Thermal conductivity DOI: 10.3938/jkps.60.1513 I. INTRODUCTION Semiconductor nanowires (NWs) and NW/organic polymer composites are receiving particular attention because of their promising applications for novel nano- sized electronics and optoelectronics devices such as piezoelectrical generators, light-emitting diodes, lasers and hybrid solar cells [1–3]. Management of the thermal properties of low-dimensional structures has some prob- lems because the nanocrystals have reduced thermal con- ductivity compared to the bulk counterpart due to the scattering of phonons at the surfaces [4]. The measure- ment of the thermal conductivity of a single nanowire is not an easy task, and some special suspended micro- devices need to be fabricated [5,6]. However, many fu- ture NW-based devices might be based on a large number of densely-packed NWs supported by a matrix material, so the physical properties of the combination of NWs and matrix would be important. It has been shown that the thermal conductivity of crystalline nanowires can be determined by measuring the thermal conductivity of the NW/poly(methyl methacrylate) composites [7,8]. PMMA is an important thermoplastic material with ex- cellent transparency. However, its lower thermal stabil- ity restrains it from application at higher temperatures. To improve its thermal properties, fillers such as silica, fullerenes and CdS nanoparticles have been introduced into PMMA [9–11]. ZnO is a wide-bandgap semiconduc- * E-mail: twkang@dongguk.edu tor, and it has a high transparency in the visible range of the optical spectrum. Therefore, the ZnO NW/PMMA composite is promising for use as an optical transparent material with a sufficiently high thermal conductivity. In this work, we report results for the thermal conductivity measurements of the ZnO NW/PMMA composites with ZnO nanowires of different diameters and alignments. II. EXPERIMENT For the study of the thermal conductivity properties, different samples of the ZnO NW/PMMA composites were prepared. The ZnO NW arrays were grown on Si (111) substrates by using a vapor-phase transport method at temperature of 800 – 1000 ◦ C without a cat- alyst. Zn (99.9999%) and O 2 (99.995%) were utilized as reactants, and N 2 (99.999%) was used as carrier gas. ZnO NW arrays with different diameters, lengths and densities of the nanowires were obtained by changing the growth conditions. The ZnO NW array was completely embedded in thick PMMA by using a spin-coating tech- nique. With an oxygen plasma, the PMMA layer was then etched down until the tops of the ZnO nanowires were exposed above the polymer’s surface. Also, other types of nonaligned ZnO NW/PMMA composites were prepared for comparison. For that, the ZnO NW arrays were scraped away from the Si substrates and the ZnO NW/PMMA composites were prepared by using an ap- proach similar to that described in Ref. 12. The ZnO NW were mixed with a methyl methacrylate (MMA) -1513-