Surface modification of nanodiamond: Photoluminescence and Raman Studies ☆ J. Mona a , J.-S. Tu a , T.-Y. Kang a , Cheng-Yen Tsai a , E. Perevedentseva a, b , C.-L. Cheng a, ⁎ a Department of Physics, National Dong Hwa University, Hualien, 97410, Taiwan b P. N. Lebedev Physics Institute, Rus. Acad. Sci., Moscow, Russia abstract article info Article history: Received 20 June 2011 Received in revised form 18 December 2011 Accepted 21 December 2011 Available online 29 December 2011 Keywords: Nanodiamond Carboxylated nanodiamond Surface modification Photoluminescence Graphitization We report modifications in structural and surface properties of carboxylated nanodiamond (cND) due to thermal annealing from 150 °C to 900 °C and gas treatments (hydrogen/argon) at 650 °C. Modifications are manifested using FTIR, photoluminescence and Raman spectroscopic techniques. Significant enhancement of photoluminescence intensity and transformation of the shape of luminescence band is observed for gas treated cNDs and for thermally annealed cNDs at 850 °C. Similar enhancement is observed at various excita- tions as well as for different sized hydrogen treated cNDs. Surface transformation (graphitization) is affected by various treatments which presumably change the photoluminescence properties. Simultaneously, FTIR re- sults show enormous change in absorption frequency of carboxyl group (C=O). SEM results for gas treated samples show pyramidal structures of size ~ 26 nm. These results indicate modifying NDs surface improves the luminescence of ND, justifying their role in bio-labeling. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Nanodiamond (ND) has been extensively investigated during the last decade due to its pronounced optical properties such as strong Raman signal, photoluminescence, etc. [1-3]. At the same time NDs size and their aggregations or agglomerations are important parame- ters which might deteriorate their unique physical–chemical proper- ties [4]. Modifying functional groups present on ND's surface could prevail over these criticalities. Moreover, modified ND's surface can be conjugated with bio-molecules such as DNA, antigen, proteins etc. which make them convenient for various biomedical applications [5- 8]. As a next step, biomedical applications are directly linked to their photoluminescence (PL) and Raman spectroscopic properties. The PL properties of NDs are used for bio-labeling; to study the penetration of ND into cultured cells and are reliable for single particle tracking in living cells [9,10]. PL of ND is determined by a variety of luminescent centers associated with the crystal lattice defects and admixtures as well as by nanosize effect [11]. Some methods have been developed to increase the PL of NDs and obtained NDs are referred as fluorescent nanodiamond (FND). FNDs are more promising compared to conven- tional markers and labels, such as, organic dyes, quantum dots, fluores- cent proteins for bioimaging applications [9,10,12-14]. Additionally, high spectral and spatial resolution offered by Raman spectroscopy pro- vides complimentary approach to enable their use as nanobiolabels [15]. However, developing methods to obtain NDs with improved spec- troscopic properties for facilitating detection are still grave. It is important to develop different methods to obtain NDs with cer- tain specified surface groups. Until now widely known methods to modify ND's surfaces and to create carboxylated nanodiamond (cND) with carboxyl groups on the surface are through the use of strong acid mixtures such as sulfuric acid, nitric acid, phosphoric acid, etc. [8,16]. Analogously an efficient and homogeneous modification on NDs surface can be achieved with gas treatments, for example, hydrogenation [17], oxygen and ozone treatment [18], cold plasma treatment using CF 4 or SF 6 gas, etc. [19]. Additionally, temperature annealing on nanosized diamonds have also been used for surface cleaning purposes [20]. We assume that particular treatments could modify the NDs sur- face structure and thereby its spectroscopic properties. In the present report, we use both, temperature annealing as well as two different gas treatments (hydrogen/argon) and compare their effects to ad- dress the above issues. We analyze the surface chemical groups using FTIR spectroscopy and precisely explain C=O vibration fre- quency shift during thermal annealing from 300 °C to 1000 °C and gas treatments. In conjunction to this, observed enhancement in the PL intensity due to annealing and gas treatments is discussed. Results are confirmed using different sized NDs and with various excitation wavelengths for hydrogen treated cNDs. Correspondingly, structural changes are analyzed via Raman spectroscopy and SEM was used to observe the surface morphology. 2. Experimental Synthetic nanodiamond (ND) (100 nm and 300 nm in diameter) used for the investigation was purchased from Kay Industrial Diamond, Diamond & Related Materials 24 (2012) 134–138 ☆ Presented at NDNC 2011, the 5th International Conference on New Diamond and Nano Carbons, Suzhou, China. ⁎ Corresponding author at: Department of Physics, National Dong Hwa University, 1, Sec. 2 Da Hsueh Rd., Shoufeng, 97403 Hualien, Taiwan. Tel.: +886 3 8633696; fax: +886 3 8633690. E-mail address: clcheng@mail.ndhu.edu.tw (C.-L. Cheng). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.12.027 Contents lists available at SciVerse ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond