Advanced optical spectroscopy of luminescent color centers in lithium fluoride thin films

Radiation Measurements 45 (2010) 359–361

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Advanced optical spectroscopy of luminescent color centers in lithium fluoride thin films M.A. Vincenti*, T. Marolo 1, R.M. Montereali ENEA, C.R. Frascati, Via E. Fermi 45, 00044 Frascati, Rome, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 July 2009 Received in revised form 12 October 2009 Accepted 30 November 2009

Lithium fluoride, LiF, is a well-known dosimeter material. In thin films the direct use of optical absorption spectra to individuate the stable formation of different kinds of point defects is often precluded by the presence of interference fringes due to the refractive index difference between film and substrate. Photoluminescence measurements are more sensitive. Recently a very effective investigation method is developing: it is often called combined excitation–emission spectroscopy (CEES) in the literature. In this work we present the basic characteristics of this technique and the first results of the investigation of polycrystalline LiF films grown by thermal evaporation on fused silica substrates and gamma irradiated at several doses up to 106 Gy in air. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Color center Lithium fluoride Photoluminescence Thin film

1. Introduction Lithium fluoride (LiF) is an interesting material for photonic applications (Montereali, 2002) because it is practically not hygroscopic and it can host laser active electronic defects, known as color centers (CC), characterized by a wide tunability and a good stability also at room temperature (RT). It is also a well-known dosimeter material in pure (McLaughlin et al., 1980) and doped (Lakshmanan et al., 1996) form. Gamma irradiation of LiF crystals and films induces the stable formation of primary and aggregate CC. The primary color center is the F one, which consists of an anionic vacancy occupied by an electron. Its absorption band is located at around 248 nm and up to now the photoluminescence originating from the F defect in LiF has not been detected unambiguously. The aggregate centers F2 and Fþ 3 (two electrons bound to two and three anion vacancies, respectively) possess almost overlapping absorption bands, around 450 nm, generally called M band (Nahum and Wiegand 1967); under optical pumping in this spectral region they emit broad photoluminescence bands peaked at around 678 nm and around 540 nm for F2 and Fþ 3 CC, respectively, even at RT. The thermoluminescence (TL) properties of LiF have been extensively investigated and recently it was possible to associate TL glow peaks to different kinds of CC in gamma irradiated LiF crystals (Baldacchini et al., 2008).

* Corresponding author. Tel.: þ39 6 9400 5668; fax: þ39 6 9400 5400. E-mail address: [email protected] (M.A. Vincenti). 1 ENEA Guest. 1350-4487/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2009.11.041

Recently new radiation detectors based on microcrystalline dispersion of LiF in a polymeric matrix have been introduced for gamma and electron high-dose dosimetry (Kovacs et al., 2000). In recent years the area of growth and characterization of LiF thin films have seen a considerable expansion. Polycrystalline LiF films grown by thermal evaporation were proposed and tested like novel X-ray imaging radiation detectors (Baldacchini et al., 2003) based on F2 and Fþ 3 photoluminescence as well as for nuclear sensors for neutrons (Cosset et al., 1997; Almaviva et al., 2008) and for gamma dosimetry (Montecchi et al., 2002). The great interest for new radiation detectors based on CC in this material prompted us to a careful investigation of the optical properties of gamma irradiated LiF films. In thin films the direct use of optical absorption spectra to individuate the presence of different kinds of point defects is often precluded by the presence of interference fringes due to the refractive index difference between film and substrate (Nichelatti et al., 2003). Moreover, a spectral identification of several light-emitting defects types is quite difficult because many optical transitions are possible and their different spectral features are often overlapping. Table 1 reports the typical values of the peak position (Ea, Ee) and of the full width at half-maximum (HWa, HWe) of the absorption and emission bands of F2 and Fþ 3 CC in LiF crystals at RT (Baldacchini et al., 1993). Their broad absorption bands are fully overlapped (M band) but photoluminescence ones, peaking in the red and green spectral regions, respectively (Table 1), are only partially superimposed because of a different Stokes shift. The photoluminescence measurements are more sensitive than the absorption ones.

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Table 1 RT spectroscopic parameters of F2 and Fþ 3 CC in LiF crystals at RT. Ea and HWa are peak positions and half-widths of the absorption bands; Ee and HWe are peak positions and half-widths of the emission bands. HWa (eV)

Ee (eV)

la (nm) F2

le (nm)

2.79 444 2.77 448

Fþ 3

HWe (eV)

0.16

1.83 678 2.29 541

0.29

0.36 0.31

4.5×107 4.0×107

PLE (arb. units)

Ea (eV)

Center

5.0×107 106 Gy λexc = 670 nm

RT

3.5×107 3.0×107 2.5×107 2.0×107 1.5×107

Performing a conventional spectroscopic characterization requires a lot of time and intuition, but recently a very effective investigation method is developing; it is often called combined excitation– emission spectroscopy (CEES) in the literature (Gill et al., 1994; Dierolf et al., 2000). In this work we present the basic characteristics of this technique and the first results obtained in the investigation of polycrystalline LiF films grown by thermal evaporation (Montereali, 2002) on fused silica substrates and gamma irradiated at several doses up to 106 Gy in air (Montecchi et al., 2002). 2. Experimental Polycrystalline LiF films, 3 mm thick, were deposited by thermal evaporation on radiation hard fused silica substrates, kept at 250  C during the growth, in the Solid State Laser and Spectroscopy Laboratory of ENEA, C.R. Frascati (Rome). Their structural, morphological and optical properties are strongly dependent on the nature of the substrate and on the main deposition parameters, i.e. substrate temperature, thickness and evaporation rate. They were exposed to gamma irradiation from a 60Co source at the Calliope plant of ENEA, C.R. Casaccia (Rome) at several doses ranging from 103 Gy to 106 Gy in air, with the same dose rate of 2.4 kGy/s. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra at RT were measured with a Jobin Yvon Fluorolog-3 spectrofluorimeter adopting a front-face detecting geometry. The excitation wavelength, lexc, was provided by a xenon lamp (450 W) filtered by a monochromator. The acquired spectra are automatically corrected for the instrumental response as well as for the realtime variation of the lamp pumping intensity. In CEES approach a large number of excitation (or emission) spectra are recorded at a sequence of emission (or excitation) wavelengths. In particular, PLE spectra at RT were collected shifting the emission wavelength by 5 nm after every acquisition in the

λexc = 540 nm

1.0×107 5.0×106 0.0 380

400

420

440

460

480

500

520

Wavelength (nm) Fig 2. RT photoluminescence excitation spectra of the LiF film gamma irradiated at 106 Gy at the emission wavelengths of 540 nm and 670 nm.

spectral interval extending from 430 nm to 800 nm, where the F2 and Fþ 3 emission bands are located. The measurement conditions were setted in order to collect all data set of PLE spectra for the LiF film gamma irradiated at the highest dose (106 Gy).

3. Results and discussion Fig. 1 shows the PL spectra of the three LiF films gamma irradiated at different doses, from 104 Gy to 106 Gy, at RT under optical excitation at 458 nm. At the lowest dose (103 Gy) the instrumental sensitivity was not sufficient to clearly distinguish the PL signal in standard measurements conditions. Each emission spectrum consists of two broad visible emission bands ascribed to F2 and Fþ 3 CC, peaked at 674 nm and at 540 nm, respectively, according with the values reported in Table 1. Fig. 2 shows the PLE spectra of a LiF film gamma irradiated at the highest dose of 106 Gy collected at two different emission wavelength, lemi, 540 nm (green emission, Fþ 3 ) and 670 nm (red emission, F2). According with the absorption spectral features in Table 1, the Fþ 3 PLE band is broader than the F2 one and its peak position is located at a highest wavelength. In CEES approach, all the collected PLE spectra are shown in a 3D graph, reported in Fig. 3, where the resulting data set of excitation

λexc = 458 nm

PL (arb. units)

4×106

RT

3×106 106 Gy

2×106 1.6×105 Gy

1×106 104 Gy

0

500

550

600

650

700

750

Wavelength (nm) Fig 1. RT photoluminescence spectra of the gamma irradiated LiF films at three different doses under optical pumping at lexc ¼ 458 nm.

Fig 3. 3D graph of photoluminescence excitation measurements for LiF film gamma irradiated at dose 106 Gy, collected at RT.

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intensities as a function of excitation and emission wavelengths is visualized. This kind of representation gives a global view of all the emission and excitation peaks and allows to distinguish every emission band and correlates it to the appropriate excitation wavelengths. Fig. 3 shows only two peaks, due to F2 and Fþ 3 CC, at excitation wavelengths around 450 nm, according with the selected spectra reported in Fig. 2. The intensity scale allows to unambiguously identify them in the twodimensional projection in the lemi, lexc plane. This is not a trivial result because it is possible to exclude the presence of a third yellow emission band often reported in the literature (Shchepina et al., 1984; Gu et al., 1988), whose emission peak is located between the F2 and Fþ 3 ones. It should be pointed out that gamma colored LiF films are under careful spectroscopic investigation for the first time. This approach is very suitable for investigations of colored thin films, as it is possible to neglect the effects of optical absorption (Skuja, 2000) and to use directly the PLE spectra as acquired. On the contrary, this correction is significant in colored LiF crystals (Marolo et al., 2005). 4. Conclusions An advanced spectroscopic characterization has been performed for the first time on gamma colored LiF films by using CEES technique. The same information could have been obtained from conventional systematic PLE spectra but CEES technique makes easier and effective to isolate the different spectral contributions buried in the broad absorption and emission bands peaking in the visible range. This approach is very suitable for the investigation of low-absorbing, efficient light-emitting thin films, like gamma colored LiF films, in order to individuate correlations between CC stable formation and their growth conditions. Acknowledgements The authors thank S. Agnello, S. Baccaro, F. Bonfigli, M. Montecchi and E. Nichelatti for useful discussions. References Almaviva, S., Marinelli, M., Milani, E., Prestopino, G., Tucciarone, A., VeronaRinati, C., Angelone, M., Lattanzi, D., Pillon, M., Montereali, R.M., Vincenti, M.A.,

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