Shell Nanoparticles by

Oct 14, 2009 - Tuning Photoluminescence of Ge/GeO2 Core/Shell Nanoparticles by Strain .... generated with TEM simulator Java Electron Microscopy...
2 downloads 0 Views 181KB Size
Download PDF
J. Phys. Chem. C 2009, 113, 19863–19866

19863

Tuning Photoluminescence of Ge/GeO2 Core/Shell Nanoparticles by Strain C. L. Yuan,*,† H. Cai,‡ P. S. Lee,‡ J. Guo,‡ and J. He§ Department of Physics, Jiangxi Normal UniVersity, Nanchang 330022, Jiangxi, People’s Republic of China, School of Materials Science and Engineering, Nanyang Technological UniVersity, Singapore 639798, and Department of Chemistry, Institute for Optical Sciences, and Centre for Quantum Information and Quantum Control, UniVersity of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada ReceiVed: August 4, 2009; ReVised Manuscript ReceiVed: September 22, 2009

The distribution of strain field plays an important role in deciding the physical properties of nanocrystals. The growth strain of Ge/GeO2 core/shell nanoparticles embedded in a regular array of Al2O3 nanoparticles and its resulting effect on the optical properties are investigated. Two-dimensional finite element calculations clearly demonstrate that Ge/GeO2 nanoparticles certainly experienced greater compressive strain in Al2O3 nanoparticles than in Al2O3 thin film, especially at the GeO2 shell area. This may lead to much more strainrelaxing defects produced at the GeO2 shell in Al2O3 nanoparticles. Meanwhile, the photogenerated excitons/ electron-hole pairs are localized by defects located at the GeO2 shell and are forced to recombine while being spatially confined in the Al2O3 nanoparticles. These effects might contribute to the observed intensity enhancement and blue shift of the photoluminescence peaks for the sample with Ge/GeO2 core/shell nanoparticles embedded in Al2O3 nanoparticles. The findings presented here provide physical insight and offer useful guidelines to controllably modify the optical properties of semiconductor nanoparticles through strain engineering. The impact of strain on materials is fundamentally important to a broad range of fields, from optoelectronics to biomechanics, because strain can yield nanostructures with novel properties,1,2 and nanomaterials respond to strain differently from their bulk counterparts.3-5 With the reducing sizes of materials, surfaces and interfaces become more important. For bulk materials, internal atoms dominate the properties of the materials, but for nanomaterials, surface and interface atoms may be dominant.6 In contrast to bulk crystals, a large fraction of the constituent atoms are located at the surface of isolated nanocrystals, which is favoring the formation of strain-induced defects at the interface of nanocrystals under strain field. The defects at the interface may strongly influence photo- and electroluminescence and carrier dynamics of nanocrystals.7-9 It has been reported that growth of nanocrystals embedded in a host matrix can lead to substantial strain.10-12 The strain may be relaxed through the growth process of nanocrystals and subsequently produce many more defects at the nanocrystal/matrix interface, and thus influence the physical properties of the nanocrystals. In the core/ shell nanoparticles embedded in the solid matrix, the interface states are particularly important because the substantial strain field can lead to extra strain-relaxing defects at the heterostructure interface, which can tune the optical properties of core/ shell nanoparticles significantly. However, there has been a lack of systematic study on the strain field of the core/shell nanoparticles induced by the nanoparticles formation in the host matrix, and its influence on the optical properties of the nanoparticles. Therefore, there is a strong motivation to pursue experimental and theoretical calculations approaches to investigate the growth strain of the core/shell nanoparticles. Further* To whom correspondence should be addressed. E-mail: clyuan@ jxnu.edu.cn. † Jiangxi Normal University. ‡ Nanyang Technological University. § University of Toronto.

more, the properties of nanoparticles may be influenced by assembling the particles into regular arrays.13,14 Meanwhile, in order to integrate the nanoparticle into practical applications, it is also necessary to control well the alignment of nanoparticles. However, it is still very difficult to fabricate monodispersed nanoparticles that exhibit a core/shell structure and form ordered arrays. In this paper, the growth strain of Ge/GeO2 core/shell nanoparticles embedded in a regular array of Al2O3 nanoparticles and its resulting effect on the optical properties are investigated. A regular array of Al2O3 nanoparticles embedded with Ge/ GeO2 core/shell nanoparticles on Si substrate were fabricated by the pulsed laser deposition (PLD) method, using an anodic aluminum oxide (AAO) nanopore template for selective deposition. AAO template with a typical pore size of 60 nm was selected and immersed into 5 wt % H3PO4 solution for 45 min to remove the barrier layer at the bottom of the pores. After that, the AAO membrane was bonded directly onto a Si (100) substrate via van der Waals force, where the Si substrates were first cleaned with SC1 (NH4OH:H2O2:H2O ) 1:1:5) and SC2 (HCl:H2O2:H2O ) 1:1:5) solutions, and then dipped in a 1% HF solution to remove the native oxide. A KrF pulsed laser was used to ablate the target in a high vacuum chamber. The wavelength of the excimer laser is 248 nm and the average energy density is about 1.5 J/cm2 with frequency of 10 Hz. The target consisted of a high-purity (99.99%) round Al2O3 target (diameter D ) 25 mm) and one small single crystal Ge square plate (about 2 mm in length). During the deposition process, the center of the Al2O3-Ge target assembly was set to spin slowly about its central axis and the laser beam vaporized the two component materials alternately. The thickness for the deposition is about 30 nm. After the deposition, the sample was subjected to a post deposition annealing at 400 °C for 120 s in air. Finally, the AAO template can be removed by N2 gas purging. The sample structure was examined using field emission scanning electron microscopy (FESEM) (JEOL JSM-

10.1021/jp907504q CCC: $40.75  2009 American Chemical Society Published on Web 10/14/2009

19864

J. Phys. Chem. C, Vol. 113, No. 46, 2009

Yuan et al. is along the (311) orientation. The GeO2 shell shows three grains oriented along (110), (002), and (200) for the top, middle, and bottom grains, respectively. The interspaces of the Ge nanocrystal and GeO2 nanocrystals show the Moire´ fringe patterns which are caused by interference between two sets of fine pattern grids of Ge and GeO2 nanocrystals. Considering a stress-free configuration, the D spacing of the Moire´ fringes is given by:16

Figure 1. (a, b) FESEM images taken from Al2O3 nanoparticles arrays grown by the PLD method.

Figure 2. (a) Typical low-magnification planar TEM image of a single Al2O3 nanoparticle with its corresponding electron diffraction pattern. (b) Enlarged planar HRTEM image of the adjacent regions between the Ge core and GeO2 shell. The insert is the planar HRTEM image of a single Ge/GeO2 core/shell nanoparticle embedded in the Al2O3 nanoparticle.

6340F) and a high-resolution transmission electron microscope (HRTEM) with a JEOL 2010 microscope. Photoluminescence (PL) spectra were measured by exciting the samples with the 325 nm line of a He-Cd laser. The PL signal was detected with a SPEX 1403 spectrometer (Horiba Jobin Yvon). Panels a and b of Figure 1 show the FESEM images taken from the Al2O3 nanoparticles after removing the AAO membrane. The right brighter areas of the FESEM images in Figure 1a indicate the residual AAO template remaining after the removal process. These FESEM images clearly show that an ordered Al2O3 nanoparticles array with an average diameter of 60 nm was formed by the combination of a self-organized alumina template with a PLD method. Figure 2a shows a typical low-magnification planar TEM image of single Al2O3 nanoparticle with its corresponding electron diffraction pattern. It shows the Al2O3 nanoparticle with a nearly spherical shape and size of about 60 nm. It can also be seen that there are two spherical shaped dark nanoparticles embedded in the Al2O3 nanoparticle. The diffraction pattern was matched against a simulated diffraction pattern for nanoparticles generated with TEM simulator Java Electron Microscopy Simulation (JEMS) software.15 With the experimental and the simulated diffraction patterns, it can be deduced that these nanoparticles consisted of Ge nanocrystals with cubic structure (space group FD3-MS) and GeO2 nanocrystals with tetragonal structure (space group P3121). Figure 2b is an enlarged HRTEM image showing the adjacent regions between the Ge core and GeO2 shell. The insert is the HRTEM image of a single Ge/ GeO2 core/shell nanoparticle embedded in an amorphous Al2O3 nanoparticle. This HRTEM examination clearly revealed that the nanoparticle is of core/shell structure. It shows that the core-shell nanoparticle has a single crystal core and a uniform shell. Obviously, the shell is composed of tiny nanocrystallites (polycrystalline) with different lattice orientations. The Ge core

D)

dcoredshell |dshell - dcore |

(1)

where dcore and dshell are the corresponding d spacing of the overlapping planes for the Ge core and the GeO2 shell. The theoretical calculated value of the spacing of the Moire´ fringes is 0.43 nm, which is larger than our experimental observation (∼0.24 nm). This indicates the existence of strain fields and stress states existing in the Ge/GeO2 core/shell nanoparticles.17 Usually, the formation of nanoparticles in a matrix may be accompanied by the generation of tensile or compressive strain due to thermal expansion mismatch.18 The strain of the nanoparticle is mainly attributable to a volumetric difference.19 The model of the nanoparticle-matrix system is based upon the following assumptions: A spherical, isotropic, linear-elastic nanoparticle is considered to be embedded in an isotropic, linearelastic matrix. The nanoparticle surface is welded to the matrix. Assuming that the nanoparticle resides in a matrix cavity that is too small, this volumetric difference may arise from the matrix atoms not being able to move rapidly enough to accommodate the growing nanoparticle, which results in compressive strain for the nanoparticle. In our experiment, much higher strain can be tolerated in an Al2O3 nanoparticle than in an Al2O3 thin film. With radial compression from the Al2O3 matrix, the Ge/GeO2 nanoparticle is under isotropic, compressive strain. The matrix is under tensile strain in the tangential directions surrounding the Ge/GeO2 nanoparticle, and compressively strained in the radial direction. The strain in the matrix decays with increasing distance from the interface, but does not decay fully to zero. The small Al2O3 nanoparticle has a high surface area to volume ratio and highly curved surfaces, allowing the stress imposed on a surface to be distributed over a large fraction of the constituent atoms. In contrast, in Al2O3 thin films, the total number of atoms is larger, and the stress is imposed on a smaller fraction of the constituent atoms, favoring the formation of strain-relaxing defects in the matrix rather than homogeneous strain. Therefore, the Ge/GeO2 nanoparticles are expected to experience greater compressive stress in Al2O3 nanoparticles than in a thin film matrix. The strain distributions generated by the thermal expansion mismatch because of the growth of Ge/ GeO2 nanoparticle embedded in Al2O3 matrix are qualitatively simulated by two-dimensional (2D) finite element (FE) calculations, which are performed with a commercial software package ANSYS.20,21 In the simulation, the Young’s moduli are 103, 43.3, and 360 GPa for Ge, GeO2, and Al2O3, respectively, while the Poisson’s ratio is taken to be 0.26, 0.28, and 0.24 for Ge, GeO2, and Al2O3, respectively. It is assumed that a 1% thermal expansion is applied on the Ge/GeO2 nanoparticle. Figure 3 shows the distribution of strain intensity and the profile of strain intensity along the y direction (through the nanoparticle center) of the Ge/GeO2 nanoparticle embedded in the Al2O3 nanoparticle and a thin film. Obviously, the Ge/GeO2 nanoparticle experiences compressive strain in both the Al2O3 nanoparticle and a thin film. It should be noted that the compressive strain existing at the Ge/GeO2 nanoparticle in an Al2O3 nanoparticle is stronger

Ge/GeO2 Core/Shell Nanoparticles

Figure 3. Distribution of strain intensity and profile of strain intensity along the y direction (through the Ge/GeO2 nanoparticle center) of a Ge/GeO2 nanoparticle embedded in a Al2O3 nanoparticle and a thin film.

than that in the Al2O3 film. Especially, the GeO2 shell incurs the larger strain (matrix strain intensity is ∼ 1.7%) in Al2O3 nanoparticles. While in the Al2O3 film, the intensity of the strain exerted on the GeO2 shell decreases to ∼1.4%. Meanwhile, the Ge core also incurs a larger strain embedded in Al2O3 nanoparticles than in Al2O3 thin films. The strain experienced by the GeO2 shell area is quite large, even larger than the 1% thermal expansion applied in the FE calculations, which indicates the strain exerted on the GeO2 shell area in the real samples may be larger than the strain caused by the volume difference. The large strain will prefer to induce dislocations at the GeO2 shell area to relax the strain to reach the minimum energy requirement in the system. This may lead to many strainrelaxing defects being formed, and the larger the strain experienced, the more defects are generated.12 As shown in the HRTEM images, in the Ge/GeO2 core/shell nanoparticle, the Ge core is single crystal and the GeO2 shell is composed of tiny nanocrystallites (polycrystalline) with different lattice orientations. It prefers to induce defects at the boundaries of GeO2 nanocrystallites and the interface between the GeO2 shell and the Ge core to relax the strain. Since the GeO2 shell incurs larger strain in Al2O3 nanoparticles than in the Al2O3 film, there are also more defects produced around the Ge nanoparticle in Al2O3 nanoparticles even with the small difference in the strain applied to the Ge core between Ge/GeO2 in Al2O3 nanoparticles and that in the Al2O3 film. Therefore, such kinds of growth strain may have a significant influence on the physical properties of the Ge/GeO2 nanoparticle, like photoluminescence emission. Figure 4 shows the room temperature PL spectra of samples with Ge/GeO2 nanoparticles embedded in amorphous Al2O3 nanoparticles and the Al2O3 thin film at room temperature. Both samples were prepared at the same time with the same processes with the exception of the AAO template (as shown in the inserts of Figure 4). The average diameters of Ge/GeO2 nanoparticles in both the Al2O3 thin film and nanoparticles are similar (Ge core size and GeO2 shell thicknesses are about 11 and 3 nm). The area density of the Ge nanocrystals in Al2O3 nanoparticles is about 1 × 1010 cm-2, which is an order of magnitude lower than that in the Al2O3 thin film (∼1.3 × 1011 cm-2). For a meaningful comparison, both PL measurements were determined under the same conditions. Obviously, the PL intensity of the sample with Ge/GeO2 nanoparticles embedded in Al2O3 nanoparticles is about an order of magnitude higher than that of the

J. Phys. Chem. C, Vol. 113, No. 46, 2009 19865

Figure 4. Room temperature PL spectra of samples with Ge/GeO2 core/shell nanoparticles embedded in Al2O3 nanoparticles and a thin film.

sample with Ge/GeO2 nanoparticles embedded in the Al2O3 thin film, even with an order of magnitude lower Ge/GeO2 nanoparticles density. A comparison of the present results and those reported in literature indicates that the characteristic PL components observed arise from different types of defects. In the sample with Ge/GeO2 nanoparticles embedded in the Al2O3 film, two PL peaks are observed. The 3.0 eV peak is related to the Ge/O related defects22 and the origin of the PL peak at 2.2 eV is attributed to defect levels associated with the defects around Ge nanoparticles.23,24 Obviously, the intensity of the PL peak at 2.2 eV is higher than that of the PL peak at 3.0 eV, which may be due to a higher concentration of strain-relaxing defects at the GeO2 shell. In the sample with Ge/GeO2 nanoparticles embedded in Al2O3 nanoparticles, by contrast, two very interesting features are that the intensities of the PL spectra are enhanced significantly and the PL peaks undergo a blueshift with the observation of the PL peaks centered at 2.5 and 3.1 eV. The Ge/GeO2 nanoparticles experienced greater compressive strain embedded in Al2O3 nanoparticles than in the Al2O3 thin film. This may lead to much more strain-relaxing defects produced at the GeO2 shell. Meanwhile, instead of spreading over a large space in the Al2O3 thin film, the photogenerated excitons/electron-hole pairs will remain localized and be forced to recombine while being spatially confined in the Al2O3 nanoparticles. Therefore, in Al2O3 nanoparticles, many more photon-created exciton/electron-hole pairs are localized by defects located at the GeO2 shell. These effects might contribute to the observed intensity enhancement and blue shift of the PL peaks for the sample with Ge/GeO2 core/shell nanoparticles embedded in Al2O3 nanoparticles. In summary, the growth strain of Ge/GeO2 core/shell nanoparticles embedded in a regular array of Al2O3 nanoparticles and its resulting effect on the optical properties are investigated. A regular array of Al2O3 nanoparticles embedded with Ge/GeO2 core/shell nanoparticles on the Si substrate were fabricated by the PLD method with use of an AAO nanopore template for selective deposition. The high-resolution transmission electron microscope examination clearly revealed that the Ge/GeO2 core/ shell nanoparticles were composed of a single Ge nanocrystal core and tiny GeO2 nanocrystallites (polycrystalline) shell. 2D FE calculations clearly demonstrate that Ge/GeO2 nanoparticles certainly experienced greater compressive strain in Al2O3 nanoparticles than in the Al2O3 thin film, especially at the GeO2 shell area. This may lead to much more strain-relaxing defects

19866

J. Phys. Chem. C, Vol. 113, No. 46, 2009

at the GeO2 shell embedded in Al2O3 nanoparticles. Meanwhile, the photogenerated excitons/electron-hole pairs are localized by defects located in the GeO2 shell and are forced to recombine while being spatially confined in the Al2O3 nanoparticles. These effects might contribute to the observed intensity enhancement and blue shift of the PL peaks for the sample with Ge/GeO2 core/shell nanoparticles embedded in Al2O3 nanoparticles. Acknowledgment. This work in supported by the research fund of Jiangxi Normal University, Grant No. 1927. References and Notes (1) Robinson, R. D.; Sadtler, B.; Demchenko, D. O.; Erdonmez, C. K.; Wang, L. W.; Alivisatos, A. P. Science 2007, 317, 355. (2) Lee, J.; Kim, H.; Kahng, S. J.; Kim, G.; Son, Y. W.; Ihm, J.; Kato, H.; Wang, Z. W.; Okazaki, T.; Shinohara, H.; Kuk, Y. Nature 2002, 415, 1005. (3) Suhr, J.; Victor, P.; Ci, L.; Sreekala, S.; Zhang, X.; Nalamasu, O.; Ajayan, P. M. Nat. Nanotechnol. 2007, 2, 417. (4) Hall, A. R.; Falvo, M. R.; Superfine, R.; Washburn, S. Nat. Nanotechnol. 2007, 2, 413. (5) Roberts, M. M.; Klein, L. J.; Savage, D. E.; Slinker, K. A.; Friesen, M.; Celler, G.; Eriksson, M. A.; Lagally, M. G. Nat. Mater. 2006, 5, 388. (6) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (7) Fujii, M.; Inoue, Y.; Hayashi, S.; Yamamoto, K. Appl. Phys. Lett. 1996, 68, 3749. (8) Lombardo, S.; Coffa, S.; Bongiorno, C.; Spinella, C.; Castagna, E.; Sciuto, A.; Gerardy, C.; Ferrari, F.; Fazio, B.; Privitera, S. Mater. Sci. Eng., B 1996, 70, 295.

Yuan et al. (9) Franzo`, G.; Irrera, A.; Moreira, E. C.; Miritello, M.; Iacona, F.; Sanfolippo, D.; DiStefano, G.; Fallica, P. G.; Priolo, F. Appl. Phys. A: Mater. Sci. Process. 2002, 74, 1. (10) Choi, W. K.; Ng, V.; Ng, S. P.; Thio, H. H.; Shen, Z. X.; Li, W. S. J. Appl. Phys. 1999, 86, 1398. (11) Wellner, A.; Paillard, V.; Bonafos, C.; Coffin, H.; Claverie, A. J. Appl. Phys. 2003, 94, 5639. (12) Chew, H. G.; Zheng, F.; Choi, W. K.; Chim, W. K.; Foo, Y. L.; Fitzgerald, E. A. Nanotechnology 2007, 18, 065302. (13) Pileni, M. P. Appl. Surf. Sci. 2001, 171, 1. (14) Pozas, R.; Mihi, A.; Ocan˜a, M.; Mı´guez, H. AdV. Mater. 2006, 18, 1183. (15) Stadelmann, P. A. Ultramicroscopy 1987, 21, 131. (16) Hirsch, P. B.; Howie, A.; Nicholson, R. B.; Pashley, D. W.; Whelan, M. J. Electron Microscopy of Thin Crystals; Butterworths: London, UK, 1965; p 343 (17) Xie, H. M.; Shang, H. X.; Xue, Q. K.; Jia, J. F.; Dai, F. L. Opt. Lasers Eng. 2005, 43, 1071. (18) Hofmeister, H.; Dubiel, M.; Goj, H.; Thiel, S. J. Microsc. 1995, 177, 331. (19) Voronkov, V. V.; Falster, R. J. Appl. Phys. 2001, 89, 5965. (20) Benabbas, T.; Androussi, Y.; Lefebvre, A. J. Appl. Phys. 1999, 86, 1945. (21) Pei, Q. X.; Liu, C.; Wang, Y. Y. J. Appl. Phys. 2003, 93, 1487. (22) Torchynska, T. V.; Aguilar-Hernandez, J.; Herna´ndez, L. S.; Polupan, G.; Goldstein, Y.; Many, A.; Jedrzejewski, J.; Kolobov, A. Microelectron. Eng. 2003, 66, 83. (23) Giri, P. K.; Kesavamoorthy, R.; Panigrahi, B. K.; Nair, K. G. M. Solid State Commun. 2005, 133, 229. (24) Zacharias, M.; Fauchet, P. M. Appl. Phys. Lett. 1997, 71, 380.

JP907504Q