Synthesis and characterization of Type-Li CdS/Znse core/shell nanostructures

Nguyễn Xuân Ca và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 96(08): 45 - 48 45 SYNTHESIS AND CHARACTERIZATION OF TYPE-II CdS/ZnSe CORE/SHELL NANOSTRUCTURES Nguyen Xuan Ca1*, Nguyen Trung Kien1, Vu Thi Kim Lien2, Nguyen Xuan Nghia3 1 College of Science – TNU, 2 College of Education – TNU 3 Institute of Materials Science, Vietnam Academy of Science and Technology SUMMARY High-quality type-II CdS/ZnSe core/shell nanostructures (NSs) were synthesized by chemical method. Our synthesis involves fabrication CdS core particles that are subsequently overcoated with a layer of ZnSe in the noncoordinating solvent. An efficient spatial separation of electrons and holes between the core and the shell was observed by heterostructures. The emission wavelength of the CdS/ZnSe NSs can be changed from 568 to 589 nm for a fixed core radius. Because of a large offset of band edges at the core/shell interface, fabricated nanocrystals (NCs) exhibited a relatively low spectral overlap between emission and absorption profiles, with associated Stokes shifts of up to 70nm. Key word: spatial separations, nanostructures, semiconductor, reorganization energy. INTRODUCTION* Colloidal heterostructure semiconductor NCs were synthesized from multiple materials with different properties into a single nanoscale object, where the spatial localization of carriers can be precisely controlled by anipulating shapes and sizes of individual components [1,2]. Recently, considerable progress was made in the synthesis of type- II heterostructure NCs [3], that tends to spatially separate photoexcited electrons and holes in different parts of a composite NCs. These core/shell NCs are made of two semiconductor materials which both the conduction and valence bands of component lie lower in energy than the corresponding bands of the other component. As a result, one carrier is mostly confined to the core, while the other is mostly confined to the shell. The type-II NCs are expected to have many new properties that are fundamentally different from the type-I NCs because of the spatial separations of carriers. The type-II NCs can allow access to wavelengths that would otherwise not be available with a * Tel: 0985 338855, Email: nxuanca80@yahoo.com single material. Furthermore, the separation of charges in the lowest excited states of type-II NCs should make these materials more suitable in photovoltaic [4-6] or photoconduction applications [7,8]. One important application of type-II NCs is in lasing technologies [9], where they can be used for obtaining optical gain in the low- threshold single-exciton regime without complications associated with multiexciton nonradiative Auger recombination. In the present study, we report the synthesis of CdS/ZnSe type II NSs by the chemical method in a noncoordinating solvent The two- step synthesis involved growth and purification of the CdS core followed by deposition of a ZnSe shell. The QY of as- prepared nanocrystals was in the range of 7- 10%. Upon deposition of the ZnSe shell onto the CdS core, the fluorescence emission of NSs red-shifted from 568 to 589 nm, while main absorption edge remained near its original value measured in CdS core (465 nm). We noticed that for all fabricated combinations of the NSs sizes, the spectral overlap between absorption and emission profiles was relatively low, which is characteristic of spatially indirect carrier recombination. Nguyễn Xuân Ca và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 96(08): 45 - 48 46 Figure 1. The TEM images of CdS NCs and CdS/ZnSe NSs with difference size. The scale bars are 20 nm EXPERIMENTS Colloidal CdS/ZnSe NSs were prepared by the wet chemical method in octadecene (ODE) solvent. Trioctylphosphine and oleic acid were used as the ligands. The two-step synthesis involved the preparation of CdS core followed by the deposition of ZnSe shell. The ODE solutions of as-synthesized CdS core and CdS/ZnSe NSs were mixed with isopropanol, and then centrifuged. The obtained powder products were redissolved in toluene for the morphology analyses and room-temperature spectroscopic measurements. TEM images of CdS nanocrystals (NCs) and CdS/ZnSe NSs were obtained using a JEM 1010 microscope (Jeol). The optical absorption spectra were recorded with Jasco 670 spectrometer (Varian). The room- temperature PL spectra were measured using MicroSpec 2300i spectrometer with 325 nm excitation line of He-Cd laser. RESULTS AND DISCUSSION Figure 1 shows the TEM images of the initial CdS core (A), the type-II CdS/ZnSe core/shell NSs with a thin shell (B), a medium shell (C) and a thick shell (D) fabricated by overcoating CdS core with ZnSe shell at 250°C. While CdS particles are nearly spherical with a narrow dispersion size (core diameters are 5.5 nm), the final NSs have irregular shapes as previously observed for other types of core/shell structures [11,13]. However, the NSs size clearly increases following deposition of the shell. Basing on the TEM images, we estimate that the diameter of CdS/ZnSe NSs is from 7.7 to 12nm corresponding to the ZnSe shell 1.1 to 3.25nm thickness (from 2 to 6 monolayer (ML)). Despite the irregular shape of these NSs, we believe that the deposition of the ZnSe shell occurs over the entire surface of the CdS cores. Figure 2. The XRD spectra of CdS-core (A) and CdS/ZnSe (B-D) core/shell NSs. A B D C Nguyễn Xuân Ca và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 96(08): 45 - 48 47 Figure 3 shows the evolution of the optical absorption and emission spectra of the CdS/ZnSe NSs during the growth of the ZnSe shell on the CdS cores. Upon slow addition of Zn and Se precursors to the reaction flask, the CdS absorption peak began to broaden, developing a low-energy tail that extended into blue or yellow for large NSs. This indicates an onset of indirect type II absorption feature, which is characterized by electronic transitions across the core/shell interface with energies lower than absorption of CdS or ZnSe NCs. Previous studies of the type II NSs [11-13] also reported a similar broadening effect, which is proportional to the type II energy offset at the interface. A gradual increase in the type II charge separation is also seen in the evolution of the FL spectra during nanocrystal growth. The original CdS core exhibited a fluorescence peak at 465 nm. Upon the addition of Zn and Se precursors, this band quickly diminished, which is likely due to the removal of surface passivating ligands in CdS, while a new emission feature began to develop from 568 to 589 nm. We show that, the measured emission energies in the CdS/ZnSe NSs are smaller than those in either CdS or ZnSe NCs due to the spatially indirect character of electronic transitions across the core/shell interface. Throughout the entire growth process of all the three typical samples (B-D), the FWHM was controlled below 50 nm, indicating narrowsize distributions. By the change of core sizes and/or shell thicknesses, adjustable PL spanning most of the visible range was obtained. Figure 3. the evolution of the optical absorption and emission spectra of the NCs during the growth of the ZnSe shell on the CdS cores. CONCLUSION The type-II CdS/ZnSe core/shell NSs were fabricated via colloidal synthesis by continuous deposition of Zn and Se organometallic precursors onto monodisperse CdS NCs. The samples were characterized by TEM, UV-vis absorption and FL spectroscopy, and X-ray diffractrometry. An efficient charge separation across the interface in the CdS/ZnSe NSs was witnessed by a strong PL originating from spatially indirect carrier recombination. The charge separation in CdS/ZnSe results in the core localization of excited electrons and shell localization of holes, which can be utilized in dye sensitized solar cells. Finally, the emission wavelength of fabricated CdS/ZnSe NSs is tunable in a wide spectral range, which is an important property for type-II NSs due to their potential use in hybrid LED, lasing applications and photovoltaics. REFERENCES [1]. K. Rajeshwar, C. R.Chenthamarakshan, N. R. D. Tacconi, Chem. Mater 13 (2001) 2765–2782. [2]. P. D. Cozzoli, T. Pellegrino, L. Manna, Chem.Soc. Rev 35 (2006) 1195–1208. [3]. N. Alexander, K. Maria, N. Nishshanka, K. Hewa, Z. Mikhail, J. Phys. Chem. C 112 (2008) 9301–9307 [4]. B. Jiwon, P. Juwon, J. H. Lee, W. Nayoun, N. Jutaek, L. Jongwoo, B. Y. Chang, H. J. Lee, C. Bonghwan, S. Junghan, J. B. Park, J. H. Choi, C. Kilwon, S. M. Park, J.Taiha, K. Sungjee, Chem. Mater 22 (2010) 233–240 [5]. J. Nanda, S. A. Ivanov, M. Achermann, I. Bezel, A. Piryatinski, V. I. Klimov, J. Phys. Chem. C 111 (2007) 15382-15390 [6]. S. Kim, B. Fisher, H. J. Eisler, M. J. Bawendi, Am. Chem. Soc 125 (2003) 11466-11467. [7]. O. Schops, N. L. Thomas, U. Woggon, M. V. J. Artemyev, Phys.Chem. B 110 (2006) 2074-2079 [8]. Y. K. Zaman, B. Romanova, S. Wang, D. Ripmeester, J. Small 1 (2005) 332-338 [9]. R. Xie, X. Zhong, T. Basche, Adv. Mater. 17 (2005) 2741-2746 [10]. C. T. Cheng, C. Y. Chen, C. W. Lai, W. H. Liu, S. C. Pu, P. T. Chou, Y. H. Chou, H. T. Chiu, J. Mater. Chem 15 (2005) 3409 - 3414 [11]. M. Danek, K. F. Jensen, C. B. Murray, M. G. Bawendi, Chem.Mater 8 (1996) 173-180 Nguyễn Xuân Ca và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 96(08): 45 - 48 48 [12]. S. A. Ivanov, J. Nanda, A. Piryatinski, M. Achermann, L. P. Balet, I. V. Bezel, P. O. Anikeeva, S. Tretiak, V. I. Klimov, J. Phys.Chem. B 108 (2004) 10625-10630. [13]. S. A. Ivanov, A. Piryatinski, J. Nanda, S. Tretiak, K. R. Zavadil, W. O. Wallace, D. Werder, V. I. Klimov, J. Am. Chem. Soc 129 (2007)11708 – 11719. TÓM TẮT CHẾ TẠO VÀ ĐẶC TRƯNG CỦA CẤU TRÚC NANO LÕI/VỎ LOẠI- II CdS/ZnSe Nguyễn Xuân Ca1*, Nguyễn Trung Kiên1, Vũ Thị Kim Liên2, Nguyễn Xuân Nghĩa3 1Trường Đại học Khoa học – ĐH Thái Nguyên, 2Trường Đại học Sư phạm – ĐH Thái Nguyên,3Viện Khoa học Công nghệ Việt Nam Cấu trúc nano dị chất lõi vỏ loại II CdS/ZnSe đã được chế tạo thành công bằng phương pháp hóa trong dung môi không liên kết. Công việc chế tạo của chúng tôi bao gồm việc chế tạo lõi CdS sau đó bọc lên lớp vỏ ZnSe. Hình dạng và kích thước của các nano tinh thể CdS/ZnSe đã được quan sát bằng kính hiển vi điện tử truyền qua. Phổ nhiễu xạ tia X đã cho thấy cấu trúc tinh thể lập phương giả kẽm của các nano tinh thể chế tạo được. Phổ quang huỳnh quang và hấp thụ của các nano tinh thể CdS/ZnSe cho thấy có sự dịch mạnh đỉnh phổ về phía bước sóng dài so với đỉnh phổ của lõi CdS. Một hiệu quả tách không gian của điện tử và lỗ trống giữa lõi và vỏ đã được quan sát với cấu trúc dị chất - một bằng chứng thực nghiệm quan trọng để chứng minh cho cấu trúc CdS/ZnSe chế tạo được là cấu trúc loại II. Từ khóa: tách không gian, cấu trúc nano, bán dẫn, năng lượng tái tổ hợp. * Tel: 0985 338855, Email: nxuanca80@yahoo.com