Multiwalled carbon nanotubes/ZnO nanowires composite structure with enhanced ultraviolet emission and faster ultraviolet response M. Dutta, D. Basak * Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India article info Article history: Received 22 July 2009 In final form 8 September 2009 Available online 10 September 2009 abstract We report on the fabrication of multiwalled carbon nanotubes (MWCNTs)/ZnO nanowires (NWs) com- posite structures obtained by coating the vertically grown ZnO NWs with a solution of MWCNTs, dis- persed in 2-propanol. By increasing the number of coatings, a network of MWCNTs layer is formed. The composite structure shows enhanced ultraviolet emission and faster ultraviolet photoresponse time constants for multiple MWCNTs coatings (n = 5 and 2) compared to the bare ZnO NWs. The enhanced emission and faster photoresponse are correlated with the MWCNTs mediated surface plasmon reso- nance effect and faster electron transfer between the ZnO and the MWCNTs, respectively. Ó 2009 Elsevier B.V. All rights reserved. Because of its wide band gap (E g  3.4 eV), large exciton binding energy (60 meV), low cost, and ease of manufacturing, ZnO is emerging as a potential candidate in the optoelectronic applica- tions in the ultraviolet (UV) spectral range. During the past decade, the demonstration of a large variety of functional ZnO nanowires (NWs) devices such as optically pumped lasers [1], photodetectors [2], optical switches [3], field effect transistors [4], and chemical and biological sensors [5] have aroused further growing interest in this material. Recently, ZnO-based composite structures have re- ceived much attention because these provide the scope to tune the material properties depending on the nature of the counter part of the composite structure – such as by hybridizing two semiconduc- tors [6] and semiconductor–metal [7] systems, it is possible to enhance the charge separation and improve the efficiency of the interfacial charge transfer process. The co-operative electronic interactions between these two materials are the main reason be- hind the interesting electrical and optical properties [8]. One tech- nologically attractive class of ZnO-based composite structure is obtained by hybridization of ZnO with carbon nanotubes (CNTs) owing to the fascinating structural, electronic, mechanical, optical and thermal properties of the CNTs. Although there are reports on ZnO/CNT composite heterostructures [9–15], a very few attempts have been reported on their optical properties [8,14,15]. Neverthe- less, especially the study on the UV photoresponse properties of the composite structure is not reported which generates a curiosity to investigate the photoresponse properties of CNT/ZnO composite structure. CNT/ZnO composite structure is expected to show faster response to the UV light which is usually slow in only ZnO based thin films and nanostructures owing to the defects. Therefore, here we report on the fabrication of the MWCNTs/ZnO NWs composite structure using a simple technique. This shows enhanced UV emis- sion and fast photoresponse characteristics due to the surface plas- mon (SP) resonance and enhanced charge separation and fast electron transfer, respectively. The MWCNTs/ZnO NWs composite structure was made by fol- lowing two steps. First the ZnO NWs arrays are deposited on a glass substrate. The detail of the deposition technique was described elsewhere [16]. An uniform MWCNT suspension was made by dis- persing 0.4 mg of MWCNT (purchased from Cheap tube, USA) with an external diameter 20–40 nm and length 10–30 lm in 30 ml of 2-propanol with the help of sodium dodecyl benzene sulfonate (SDDBS) and sonicating the solution for 45 min, which yielded a stable dark suspension. This solution was centrifuged at 8000 rpm for 15 min and the clear supernatant liquid containing unbound SDDBS was discarded. This process was repeated few times and the final centrifugate was again suspended in 2-propa- nol following a procedure followed by Kamat et al. [17]. Secondly, a dropcast method was employed to cast a film of the suspended MWCNTs solution on the ZnO NWs. The MWCNTs’ network was formed by increasing the number (n) of coatings. A field-emission scanning electron microscope (FESEM) (JEOL JSM-6700F) and a high-resolution transmission electron microscope (HRTEM) (JEOL JSM-2010 attached with energy dispersive X-ray analysis (EDAX) facility, INCA, Oxford) were used for microstructural characteriza- tions. Room-temperature photoluminescence (PL) measurement was done using a high resolution spectrometer (Horiba Jobin Yvon, Model: iHR 320) together with a photomultiplier tube with a He- Cd laser (Kimmon Koha Co. Ltd.; Model: KR1801C) as the 325 nm excitation source. The photocurrent spectra between two gold 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.09.024 * Corresponding author. Fax: +91 33 24732805. E-mail address: sspdb@iacs.res.in (D. Basak). Chemical Physics Letters 480 (2009) 253–257 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett