Jing
Lin
*a,
Benjamin
Dierre
b,
Yang
Huang
*a,
Yoshio
Bando
a,
Chengchun
Tang
ac,
Takashi
Sekiguchi
b and
Dmitri
Golberg
a
aInternational Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki, Japan. E-mail: Lin.Jing@nims.go.jp; Huang.Yang@aist.go.jp
bAdvanced Electronic Materials Center, National Institute for Materials Science, Namiki 1-1, Tsukuba, 305-0044, Ibaraki, Japan
cSchool of Material Science and Engineering, Hebei University of Technology, Tianjin, 300130, People's Republic of China
First published on 16th November 2010
Luminescence properties of individual BN-coated CaS:Eu nanowires have been studied by high-spatial-resolution cathodoluminescence (CL) spectroscopy. A broad red light-emitting band from an individual nanowire has been observed. Detailed local CL studies on the nanowires reveal spatial variations of luminescence from the structure surfaces toward their cores. Such variations are attributed to the different Eu2+ ions surroundings within the surface and core nanowire regions. The attractive luminescence properties are meaningful for fundamental studies of nanoscaled luminescent materials and may be of interest for novel optoelectronic applications.
Cathodoluminescence (CL) is the emission of light as a result of electron bombardment, which is different from the conventional photoluminescence (PL) spectroscopy.17–21 The excitation area during conventional PL is of the order of millimetres, while CL can independently excite a singlenanowire due to its high spatial resolution, up to a few nanometres. In addition, structural information can also be obtained by using secondary electron imaging. On the other hand, one can probe the luminescence behavior of a nanomaterial at different depths from the surface to the core through the use of electron beams with different energies. Therefore, CL is considered as a powerful tool to detect and identify local luminescence properties of individual nanostructures.
In the present work, we have explored the luminescence properties of individual BN-coated CaS:Eu nanowires by employing high-resolution CL spectroscopy. A broad visible red-emission band was observed. The detailed local luminescence studies also showed profound spatial variations of luminescence from the nanowire surfaces toward their cores. Such luminescent properties are envisaged to be valuable for novel optoelectronic nanodevices.
CL spectra were measured at room temperature by using an ultrahigh vacuum scanning electron microscope with a Gemini electron gun (Omicron, Germany) equipped with a CL system. The samples were put on a conductive carbon tape and then covered with a copper grid. The vacuum of the chamber was maintained at 10−10 mbar. This prevented the specimen contamination during observation. For CL measurement a beam current was kept at 1 µA and an accelerating voltage (Va) was varied from 1.5 to 7 kV.
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Fig. 1 (a) Typical SEM image of the BN-coated CaS:Eu nanowires. (b) HRTEM image and the corresponding Fast Fourier Transform (FFT) pattern (inset) of a nanowire. |
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Fig. 2 (a) SEM image of an individual BN-coated CaS:Eu nanowire. (b) The corresponding CL image. (c) CL spectrum obtained at 5 kV from the nanowire shown in (a). (d) CL intensity evolutions during the e-beam irradiation at 5 kV. (Inset) The corresponding SEM image. |
Then we have also investigated the local luminescent properties of a nanowire. The CL spectra (Va = 5 kV) were recorded within the same nanowire. Fig. 3b plots a number of CL spectra acquired at various locations along the growth axis of the nanowire, as marked in Fig. 3a. The CL spectra are fairly uniform. All peaks are located at the same positions, while the peak intensities are slightly different. Then we collected CL spectra taken at various locations across the nanowire (marked in Fig. 3c). As shown in Fig. 3d, obviously, the CL intensity from the wire core is much stronger than that from its edge. In addition, the peak positions taken in different locations across the wire show an interesting shift. From the edge to the core (from 1 to 3), the peaks have a slight blue shift, while from the core to the edge (from 4 to 7), the peaks have a slight red shift. This result is different from the measurements along the wire growth axis. This suggests that within an individual nanowire the luminescence from the surface and core regions is different. In addition, the spatially resolved CL measurements were also performed across different nanowires. Similar spatial variations of the luminescence were observed (Fig. S1 of the ESI†).
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Fig. 3 (a) SEM image of an individual nanowire. (b) CL spectra (Va = 5 kV) taken at various locations along the growth axis of the nanowire, as marked in the image (a). (d) CL spectra (Va = 5 kV) taken at various locations across the nanowire, as marked in (c), showing a blue shift from 1 to 3 and a red shit from 4 to 7. |
In order to further prove the spatial variation of the luminescence, we measured the CL spectra (Fig. 4a) at various accelerating voltage at a fixed position on an individual nanowire (as shown with a cross mark in the inset SEM image, Fig. 4a; the upper right inset is the Va dependence of the emission peak positions). It also displays a similar shift change. When Va increases from 1.5 kV to 7 kV, the peak positions have a slight blue shift. For the CL measurement, it is known that the lower Va allows exciting the subsurface region of a material, while the higher Va excites the deeper region.18 The reason is that the excitation volume of a lower electron-beam is smaller than the size of a nanowire, while for the higher electron-beam energy, the incident electrons can excite all the regions within a nanowire, including the core (Fig. 4b). Therefore, the increased Va leads to a blue shift of the peak position, which means that the luminescence from the core is blue-shifted compared with that from the surface. This result is consistent with the above-discussed location-sensitive measurements. For comparison, the CL measurement on bulk CaS:Eu without BN coating was also performed. However, with the same Va increase from 1.5 kV to 7 kV, the emission peak positions had no shift (Fig. 4c).
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Fig. 4 (a) CL spectra taken at various Va at the fixed position on an individual nanowire (seen in the SEM image, inset). The right inset is the Va dependence of the emission peak positions. (b) Schematic illustration of the CL measurements with different Va, indicating that the lower Va allows exciting the subsurface region of a nanowire and the higher Va excites the deeper region. (c) CL spectra taken from a bulk CaS:Eu without BN coating. The corresponding SEM image of the bulk crystal is shown on the inset. |
The spatial variation of the luminescence can be attributed to the different Eu2+ surroundings for the surface and core nanowire regions. Fig. 5a shows the structure model of cubic CaS and hexagonal BN within the coaxial heterostructures. During doping, Eu2+ ions prefer to substitute for Ca2+ ions in a CaS crystal lattice due to similarities in ionic radii. Compared with an uncoated bulk crystal, the conditions for the Eu2+ ions at the CaS nanowire surface region are different from the inner ions due to the influence of a BN tubular sheath. Eu2+ usually gives characteristic emission due to transitions between the crystal field components of the 4f65d1 excited state configuration and the 8S7/2(4f7) ground state, which is demonstrated in Fig. 5b. It should be noted that the luminescence of Eu2+ ions is strongly affected by the surrounding environments.22 The emission bands may shift to longer wavelength with the increment of crystal-field strength Δ. Compared with Eu2+ ions in the core, the crystal field strength around the Eu2+ ions within the interface region is stronger, which would enlarge the crystal-field splitting of the 5d level and result in the red-shifted emission.
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Fig. 5 (a) Structure model of cubic CaS and hexagonal BN coating on the nanowires. (b) Schematic diagram of the energies of 4f7 and 4f65d levels in Eu2+ influenced by a crystal field Δ. In most crystals, the lowest excited state of 4f level (6P7/2) is higher than the 4f65d level, so the emission band corresponds to transitions between ground state 8S7/2(4f7) and the crystal-field split 4f65d in Eu2+ ions. The emission will shift to longer wavelength with the strengthening of crystal field. |
Footnote |
† Electronic supplementary information (ESI) available: CL images of two different BN-coated CaS:Eu nanowires; CL spectra taken at various locations across the different nanowires. See DOI: 10.1039/c0nr00700e |
This journal is © The Royal Society of Chemistry 2011 |