Materials Letters 63 (2009) 554–556
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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Self-assembled rings of EuBr2 nanostructures E.V. Mejía-Uriarte a,⁎, J.G. Bañuelos b, O. Kolokoltsev a, M. Navarrete c, F. Jaque d, E. Camarillo e, J. Hernández A e, H. Murrieta S e a
Laboratorio de Fotónica de Microondas, Centro de Ciencias Aplicadas y Desarrollo Tecnológico, AP 70-186 C.P. 04510, Universidad Nacional Autónoma de México, Mexico Laboratorio de Materiales y sensores, Centro de Ciencias Aplicadas y Desarrollo Tecnológico, AP 70-186 C.P. 04510, Universidad Nacional Autónoma de México, Mexico Instituto de Ingeniería, AP 70-472. C.P. 04510, Universidad Nacional Autónoma de México, Mexico d Departamento de Física de Materiales, Universidad Autónoma de Madrid, Cantoblanco, Madrid 28049, Spain e Instituto de Física, AP 20-364, C.P. 01000, Universidad Nacional Autónoma de México, Mexico b c
a r t i c l e
i n f o
Article history: Received 3 October 2008 Accepted 14 November 2008 Available online 25 November 2008 PACS: 61.72.Ss 68.37.Ps 78.55.Fv 78.67. Bf 81.16.Dn
a b s t r a c t The EuBr2 nanostructures self-assembled (SA) in ring form are presented in this letter. The atomic force microscopy (AFM) images show rings of ~190 to 2500 nm diameter formed on [100] surface. The sample optical response with rings displays an emission band peak (EBP) at 435 nm. The europium absorption bands reveal structural changes and shift toward infrared wavelength. The emission bandwidth of EuBr2 nanostructures in ring form is narrower than dispersed nanostructures of similar size. To our knowledge, is the first time that there is clear evidence of the arrangement in ring form in the KBr:Eu2+ crystal samples. © 2008 Elsevier B.V. All rights reserved.
Keywords: Atomic force microscopy Luminescence Nanostructures Self-assembly KBr:Eu2+
1. Introduction Alkali halides (AH) doped with divalent impurities have long been studied experimentally to better understand of solid state physics as well as their optical properties. Europium ion introduced substitutionally into the KBr crystal lattice shows a tendency towards formation of precipitates made up of Eu2+ and impurity-vacancy dipoles [1–3]. These precipitates are usually referred to as nanostructures (NS) [3]. They can be SA in different morphological structures, for example; in a ring. The SA rings have been observed on the surface of thin films, in particular, of arsenide. In that case, the rings are formed around nanoholes, which are created by etching on a GaAs cap layer covering InAs quantum dots [4,5]. In our case generally, the rings are formed inside the crystal on crystalline planes and NS are perfectly positioned forming a circumference. And these only are observed when the crystal is cleaved. Due to the europium high optical efficiency and its capacity to form new morphologies, as ring configuration, this crystal can be used for optoelectronic devices.
⁎ Corresponding author. Tel.: +52 55 5622 8602x1120; fax: +52 55 5622 8651. E-mail address:
[email protected] (E.V. Mejía-Uriarte). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.11.039
In this letter, we present the EuBr2 AFM images which are selfassembled in ring structure and their optical response. To our knowledge, this structure type formed by europium ions has not been previously observed by AFM. 2. Experimental Eu2+ doped (about 0.8% of europium) KBr crystals were grown in our laboratory by the Czochralski method, reported in previous works [1–3]. The samples were stored at room temperature (RT) and were not subjected to any heat treatment after they were grown eight years ago. It must be taken into account that after the crystal growth, it was subjected to a cooling process in the furnace which produced annealing by 20 h. During this process some NS were formed by precipitation effect. The europium concentration in the samples is ~200 ppm [6]. For each AFM images, a piece from freshly cleaved sample was prepared. The topographic images were obtained by contact mode with AFM AutoProbe CP (Park Scientific Instruments). The free software WSxM 4.0 Develop 12.21 (Nanotec SPM) was used for the analysis of AFM images [7]. To control the humidity, the samples were kept under a nitrogen vacuum chamber during the AFM experiments. All AFM images of KBr matrix and KBr doped with Eu2+ were taken on [100] surface. In this
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3. Experimental results
Fig. 1. Comparison of the optical response from KBr:Eu2+ single crystal samples with nanostructures; ( ) WOR and ( ) WR. (a) The absorption coefficient and (b) Emission spectrum (λexc = 355 nm) for both samples.
In all samples the nanostructures have a typical size of ~ 70–120 nm diameter. For this work, two types of samples were used: First, without rings (WOR), with nanostructures dispersed in the crystal; second, with self-assembled rings (WR) of ~ 190 to 2500 nm diameter. Fig.1 shows the optical response from KBr:Eu2+ crystal at RT. Fig.1(a) shows the optical absorption spectra for both WOR and WR samples. As can be seen by inspection of these spectra, the WR sample has a different structure than the WOR sample. The high-energy (HEB) and low-energy (LEB) absorption bands are due to the transition from lowest Stark component of the 4f7(8S7/2) ground state of the europium ions to the eg and t2g components of the 4f65d configuration, respectively. The separation between them is due to the 10Dq splitting of the 5d orbitals by the crystal field into two energy levels. The excitation with light in anyone of these two absorption bands exhibit only one emission band, as is shown in Fig. 1(b). The WOR and WR samples have NS of ~70–120 nm diameter, but they present changes in its optical response. The EBP from WOR samples is at 432 nm (0.17 eV, FWHM) and WR samples is at 435 nm (0.12 eV, FWHM), the bandwidth of WR samples is narrower than of the WOR samples. The shift of the EBP is due to a larger splitting of the 5d level. Fig. 2(a) shows an image of one freshly cleaved KBr single crystal sample, wide atomic flat terraces separated by steps of 0.33 nm were observed, which are approximately half of the lattice parameter of this crystal (0.6578) nm. Fig. 2(b) shows the surface image from the sample with EBP at 432 nm. These samples have only nanostructures of ~70–120 nm diameter and no ring or any other type of SA nanostructure. Fig. 2(c) shows the nanostructure profile with average size of ~75 nm diameter. Fig. 3 shows images from samples with SA rings structure of several diameters. Rings labeled with x, 1 and 2 are shown in Fig. 3(a). Fig. 3(b) shows profile from ring 1, these NS appears as if they were melted, this effect appear when they are very close together. In other cases, as in Fig. 3(c), ring 2, the assembly is made up of small nanostructures. The largest ring observed is shown in Fig. 3(d). As it may be appreciated, the NS appear to be fixed to form a circle, as if it were self-assembled in the same plane. Fig. 3(c) shows the ring profile. The large nanostructures in ring structure have ~ 100– 120 nm diameter, but the average size of the nanostructures that integrate the ring are of ~ 70 nm in diameter. Some samples have small well-defined rings; when these types of rings are present very few NS dispersed in the crystal are observed as in Fig. 4. Generally, all nanostructures are SA in ring form and apparently in the same plane, as the profile from ring 1 in Fig. 4(b). But, some SA rings are formed around a depression as is shown in Fig. 4(c), these holes were rarely observed during our experiments.
4. Conclusions Fig. 2. AFM images on [100] surface. (a) KBr matrix (1.5 × 1.5 µm2); (b) WOR KBr:Eu2+ sample (1.5 × 1.5 µm2); (c) profile of nanostructure (x), lateral distance (LD) vs height.
work, the NS diameter is considered as an average; the height is not well defined, because we do not know which portion of the nanostructure is under the crystalline planes [3].
We think that impurity quantity and thermal conditions during growth allow self-assembled nanostructures. The emission band from WR samples is shifted towards a longer wavelength, 435 nm, and its bandwidth is narrower than in the WOR samples with nanostructures of similar size, but dispersed in the KBr matrix. The bandwidth narrowness in SA rings might be related with a more
Fig. 3. AFM images and profiles from self-assembled ring nanostructures on [100] surface. (a) Sample with several self-assembled rings; (b) profile from ring 1, ~ 190 nm diameter; (c) profile from ring 2, ~ 585 nm diameter; (d) large ring, ~ 2.5 µm diameter.
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Fig. 4. 5.4 × 5.6 µm2, AFM image of EuBr2 self assembled rings. (a) sample with small rings of similar size; (b) profile from ring 1, ~ 460 nm diameter; (c) profile from hole 2, nanostructures forming a ring of ~ 400 nm diameter.
uniform crystal field at the europium ion site. On the other hand the shift of the EBP is the result of a larger splitting of the 4f65d1 Eg and 4f65d1 T2g levels as can be easily seen in Fig. 1, which in turn might be due to a larger interaction with the surroundings. The optical properties of these rings may have potential applications in optoelectronics and motivate further research into other types of selfassembled nanostructures. Acknowledgements This work is supported by CONACYT (J51441-F) and PAPIIT (IN 107408). The authors thank PFAMU-DGAPA program of UNAM (México),
Dra. América Vázquez for the facilities to obtain the emission spectra and Dr. Enrique Chicurel Uziel for revision of the English. References [1] López FJ, Murrieta S. H, Hernández A J, Rubio O. J. Phys Rev B 1980;22:6428–39. [2] Mejía-Uriarte EV, et al. J Phys: Condens Matter 2003;15:6889–98. [3] Mejía-Uriarte EV, et al. Influence of europium nanostructure size on the emission of KBr:Eu2+. 2008; September (submitted for publication). [4] Mano T, et al. Nano Lett 2005;5:425–8. [5] Songmuang R, Kiravittaya S, Schmidt OG. Applied Physics Lett 2003;82:2892–4. [6] Hernández JA, Cory WK, Rubio JO. J Chem Phys 1980;72:198–205. [7] Horcas J, Fernandez R, Gómez-Rodriguez JM, Colchero J, Gómez-Herrero J, Baro AM. Rev Sci Instrum 2007;78:13705–8.
