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Herein, we investigate the emission properties of Er - BiOCl in the range of ∼500 to 1700 nm under down - (488 and 973 nm) and NIR (telecom - wavelength at ~1500 nm) up - conversion excitation as well high energy X-ray excitation. The dependencies of red (~670 nm) and NIR (~800 nm) to green emission (~543 nm) ratio with Er concentration, excitation mode and delay after laser pulse as well as the up - conversion excitation spectra and decays are analyzed in terms of competitive ground state absorption/excited state absorption and energy transfer up - conversion mechanisms. The CIE chromaticity diagram show single excitation (~1500 nm), delay induced emission color change from yellowish green (delay of 0.001 ms) to reddish orange (delay of 1 ms). The X-ray induced emission of Er - BiOCl presents an atypical red to green emission ratio that exceeds that measured under optical down - conversion excitation by a factor of 13. The potential of Er - BiOCl for optical /X-ray imaging applications is discussed.
© 2015 Optical Society of America
1. Introduction
Over the past decade, BiOCl has drawn considerable attention for industrial applications, such as, photo-catalysts and magnetic materials, because of their unique electronic structure and catalytic performance [1–3]. BiOCl is a type of wide band gap semiconductor material with a band gap of 3.5 eV, with an open two-dimensional tetragonal crystal structure that is derived from the tridimensional fluorite (CaF2) structure [4,5]. Lanthanide doped host materials with a wide band gap are attractive for the optical applications because the lanthanide ions can emit within its optical window and do not suffer from quenching effects inherent to semiconductor hosts [6,7]. Bismuth metal component is considered one of the least toxic heavy metal, therefore is widely used in industry, biological and medical sciences. Bi has the largest atomic number, (Z = 83) among the non-radioactive metals and good X-ray attenuating properties which makes that Bi, either in nanoparticulate form or in various compounds, represents an excellent candidate as computed tomography contrast agent [8,9]. Despite the outstanding properties of BiOCl, such as low phonon energy (cut-off energy around 400 cm−1), high density, low toxicity, facile doping with lanthanide ions (due to size and charge compatibility of Bi3+ and Ln3+ ions) [10], little work has been dedicated to the lanthanide doped BiOCl materials as possible candidates for optical and X-ray imaging applications. Recently, a facile route to fabricate lanthanide - doped bismuth oxyhalide nanostructures, possessing both promising down-conversion luminescence and photocatalytic properties has been proposed [10]. Further, two recent reports highlighted the properties of BiOCl as an up - conversion luminescence host for single Er or Er, Yb couple dopants [11,12] by use of NIR excitation wavelength at 980 nm. To date, the NIR to Vis up - conversion emission in Er doped compounds is well understood with a special effort being devoted to NIR (telecom - wavelength around 1500 nm) to Vis and NIR UPC due to potential applications in photovoltaics, photonics, displays, imaging and sensors [13–16].
Herein, we report on the down and up-conversion emission properties of Er - BiOCl measured in the range of 500 to 1700 nm by use of optical excitation as well as X-ray excitation. Structural and morphological properties were investigated by use of X-ray diffraction (XRD), Raman, diffuse reflectance (DR-UV-Vis), transmission electron microscopy (TEM) and X-ray fluorescence spectroscopy. The analysis of the NIR (~1500 nm) excited up - conversion emission properties and mechanisms including the dependence of the red and NIR to green emission ratio on the delay after the laser pulse is discussed. It is suggested that this approach can be effective in differentiating among competitive ground state absorption/excited state absorption (GSA/ESA) and energy transfer up - conversion (ETU) mechanisms. It is also shown that Er - BiOCl exhibits an interesting optical response under high - energy excitation with X-rays that differs to that obtained under optical down - conversion excitation. The tunability of the color emission obtained under Vis - NIR down - conversion and NIR up - conversion excitation, the X-ray induced luminescence property of Er doped BiOCl along with BiOCl intrinsic features constitute attractive properties for use of this system in multimodal optical/X-ray imaging applications.
2. Experimental
2. 1. Synthesis
BiOCl, 1% and 5% Er doped BiOCl samples were prepared by solid state reaction. The starting materials used were NH4Cl (99,99%, from Aldrich), Bi2O3(99.999%, from Aldrich), and Er2O3 (99.99%, from Aldrich). The powders were mixed thoroughly for 30 minutes in an agate mortar and calcined at 500 °C for 3 h in air. NH4Cl was added in excess of 20% in order to compensate the volatilization losses.
2.2 Characterization
Powder X-ray diffraction (XRD) patterns were recorded on a Schimadzu XRD-7000 diffractometer using Cu Kα radiation (λ = 1.5418 Å, 40 kV, 40 mA) at a scanning speed of 0.10 degrees min−1 in the 5 – 90 degrees 2Θ range. Diffuse reflectance optical (DR-UV-Vis) spectra were recorded at room temperature on a Analytik Jena Specord 250 spectrophotometer with an integrating sphere for reflectance measurements and MgO as the reflectance standard. DR-UV-Vis spectra of the photocatalysts were recorded in reflectance units and were transformed in Kubelka–Munk remission function F(R). Raman analysis was carried out with a Horiba Jobin Yvon - Labram HR UV-Visible-NIR Raman Microscope Spectrometer at 633, 514 and 488 nm. Microbeam X-ray fluorescence (micro-XRF) spectrometry was performed on a custom made instrument with an X-ray tube: Oxford Instruments, Apogee 5011, Mo target, focus spot ∼40 µm, max. high voltage - 50 kV, max current - 1 mA, Amptek X-123 complete X-Ray spectrometer with Si - PIN detector. The key element of the micro - XRF instrument is an X-ray policapillary minilens (IfG-Institute for Scientific Instruments) which provides a focal spot size on the sample of 15 - 20 micrometers. For transmission electron microscopy (TEM), the particles aggregates were dispersed in water and deposited on lacey carbon Cu greed. The TEM images were realized using a Jeol ARM 200F electron microscope working at 200kV. The TEM observations were performed at low magnification and low density of current to prevent the structure degradation of BiOCl particles.
2. 3. Optical excited luminescence
The photoluminescence measurements were carried out using a Fluoromax 4 spectrofluorometer (Horiba). Time resolved emission spectra were recorded at 300K using a wavelength tunable NT340 Series EKSPLA OPO (Optical Parametric Oscillator) operated at 10 Hz as excitation light source. The tunable wavelength laser has a narrow linewidth < 5 cm−1 with scanning step varying from 0.05 to 1 nm. As detection system, an intensified CCD (iCCD) camera (Andor Technology) coupled to a spectrograph (Shamrock 303i, Andor) was used. The time - resolved emission spectra were collected in the spectral range of 400 nm < λem < 900 nm by use of the box car technique. Photoluminescence was detected with a spectral resolution of 0.88 nm and the input slit of the spectrograph was set to 10 μm. The temperature of the iCCD was lowered to −20 °C to improve its signal to noise ratio. For the energy dependence measurements, the energy of the laser pulse was modified using neutral density filters and measured with a Gentec SOLO2 Laser Power & Energy Meter. The energy of the laser pulse at ∼1500 nm varied from 0.549 mJ to 3.11 mJ. For an extended range of the emission spectra (200 - 1200 nm) with the spectral resolution of 3.2 - 4.5 nm, the emission spectra were recorded by use of Avantes spectrometer (AvaSpec-HS1024x58/122TEC) by use of an integration time of 100 ms.
2.4 X-ray excited luminescence
To characterize the X-ray induced luminescence, the 1 and 5Er - BiOCl samples have been irradiated by use of X-ray tubes (Oxford Instruments, Apogee 5011, Mo and Wo targets, focus spot ∼40 µm, max. high voltage - 50 kV, max current - 1 mA). On both tubes the X-ray induced luminescence was collected in a coaxial transmission irradiation configuration with the sample holder placed at 40 mm of the X-ray focal spot. The emitted light was collected with a lens with focal distance of 8 mm on an Avantes spectrometer (AvaSpec-HS1024x58/122TEC). The following parameters were varied: X-ray tube kilovoltage and current and BiOCl powder specific weight. The X-ray debit dose was increased from few cGy/s up to 2 Gy/s.
3. Results and discussion
3.1 Summary of structural properties
Powder X-ray diffraction (XRD), Raman, diffuse reflectance in the UV to Vis range (DR-UV-Vis), X-ray fluorescence (XRF) spectroscopy and transmission electron microscopy (TEM) were used to characterize the crystalline phase, absorption properties, chemical composition, size and morphology of BiOCl particles. The XRD patterns of the examined samples are shown in Fig. 1(a).The XRD pattern of BiOCl, 1Er - BiOCl and 5Er - BiOCl corresponds to tetragonal bismuth oxide chloride with space group of P4/nmm (PDF Card No. 00-006-0249). In addition, the intense and sharp diffraction peaks suggest the well - crystallized form of the samples. As shown in Fig. 1(b), all samples show similar Raman spectra, which consist of two bands at approximately 150, 200 cm−1 assigned to A1g internal Bi - Cl stretching and Eg internal Bi - Cl stretching mode, as well as a barely detectable band at about 400 cm−1 assigned to Eg and B1g modes produced by the motion of the oxygen atoms. Figure 1(c) illustrates the diffuse reflectance (DR-UV-Vis) spectra in the range of 200 - 1100 nm. The white colored samples have a broad intense absorption below 500 nm, and indicate the absorption behavior of the wide - band - gap semiconducting oxide. The optical absorption edge was shifted more to the red with Er addition. The f-f absorptions of Er metal can be detected around 490, 525, 655, 805 and 980 nm. BiOCl particles are well crystallized and some of them are welded in aggregates, according to TEM images, Fig. 1(e) and 1(f). The crystallite sizes range between 100 and 500 nm and have a dominant lamellar morphology.
Fig. 1 (a) XRD patterns, (b) Raman, (c) DR-UV-Vis and (d) micro - XRF spectra 1/5Er-BiOCl; (e) and (f) TEM of BiOCl.
3. 2 Down – conversion emission properties
The excitation spectrum of 5Er - BiOCl measured around red emission at 670 nm (corresponding to 4F9/2 - 4I15/2 transition) shows the Er f-f absorptions at ~380, 411, 450, 488 and 520 nm, corresponding to 4I15/2 - 4G11/2, 2H9/2, 4F3/2, 4F7/2 and 2H11/2, respectively, Fig. 2(a).In essence, the excitation spectrum evidences that, there is no absorption at energies higher than that corresponding to 360 nm that fuel the red as well as the green emission (the latter emission has an identical excitation spectrum with that monitored around 670 nm) and also that BiOCl host is not involved in Er sensitization. The wide-range (500 to 1700 nm) down- conversion emission spectra of 5Er - BiOCl measured upon excitation at λex = 488 and 973 nm are represented in Fig. 2(b).
Fig. 2 (a) DC excitation spectrum of 5Er - BiOCl measured around 670 nm; (b) DC emission spectra of 5Er - BiOCl measured upon excitation at 488 and 973 nm.
These spectra contains all relevant emission transitions of Er, corresponding to the (thermalized) 2H11/2/4S3/2 - 4I15/2, 4F9/2 - 4I15/2, 4S3/2 - 4I13/2, 4I9/2 - 4I15/2, 4I11/2 - 4I15/2 and 4I13/2 - 4I15/2 transitions in the spectral region of 515 - 532, 532 - 570, 640 - 685, 780 - 840, 845 - 860, 950 - 1050 and 1450 - 1650 nm, respectively.
Fig. 3 Er energy level scheme with the observed luminescence transitions and the proposed UPC mechanisms (see text).
3. 3 Up – conversion emission properties under NIR excitation at ~1500 nm
Despite the potential interest for numerous applications, such as photovoltaics, displays, sensors and photonics, the NIR (~1500 nm) excited UPC emission of Er is far less studied than NIR (~980 nm) excited UPC [13,17–26].
Here, the UPC spectra excited around 1500 nm are measured using both spectral and temporal resolution (time - gated/time - resolved up - conversion) provided by a time - gated image intensified CCD [27]. The time - resolved UPC emission spectra of 1Er (Fig. 4(a)) and 5Er - BiOCl (Fig. 4(b)) measured upon pulsed excitation into the maximum of the 4I13/2 level absorption at 1498 nm show the emission bands around 540, 670, 800 and 980 nm assigned to transitions from the 2H11/2/4S3/2, 4F9/2, 4I9/2 to the 4I15/2 ground state, respectively. By using an additional detector (integration time of 100 ms), it is also observed that the 4I11/2 related emission around 980 nm dominates the overall spectrum according to the following percentage distribution: 8% (green emission), 15% (red emission), 23% (NIR emission around 800 nm) and 54% (NIR emission around 980 nm) (Fig. 4(c)). In contrast, the DC emission spectrum shows a completely different distribution of the transition intensities, with the green emission representing 78% from the overall intensity along with the much weaker red emission (10%) and NIR emission around 800 nm (12%).
Fig. 4 Time - resolved UPC emission spectra of 1Er - BiOCl (a) and 5Er - BiOCl (b) measured at delays of 0.001 and 0.65 ms after the laser pulse. For comparison, the DC emission spectra measured at identical delay after the laser pulse are also included. The UPC and DC emission spectra in (a), (b) and (c) were normalized to 543 nm peak intensity. In (a) and (b) the RGR and NIRGR refer to red to green and NIR to green emission ratio, respectively. These are estimated as the ratio of integrated intensities in the spectral ranges of 600 - 700 nm and 500 - 600 nm and 780 - 860 nm to 500 - 600 nm, respectively (See Media 1).
It is noteworthy to remark (Fig. 4(a) and 4(b)), following comparison of the UPC and DC spectra which are normalized to the peak intensity of green emission and measured at identical delays after the laser pulse, that: i) with the increase of the delay after the laser pulse the red to green emission ratio, RGR and the intensity ratio of 4I9/2 emission to green emission, NIRGR, are enhanced; ii) the RGR is strongly enhanced under UPC excitation compared to DC excitation, and iii) the above effects are concentration dependent. Thus, at a delay of 0.65 ms the value RGR of 1Er - BiOCl or 5Er - BiOCl under UPC excitation is 37 or 19 times greater than the value measured under DC excitation (Fig. 4(a) and 4(b)). Also the NIR emission around 800 nm that is practically negligible under DC excitation (that is NIRGR is close to 0, irrespective of delay and Er concentration) is remarkably enhanced under UPC excitation with a NIRGR of 2.5 (1Er - BiOCl) or 5.2 (5Er - BiOCl) (measured at a delay of 0.65ms). All these results indicate that the red and NIR emission around 670 and 800 nm, respectively, are fuelled by mechanisms which are different to those manifesting in DC excitation, most likely by energy transfer up - conversion (ETU). To illustrate better the different evolutions of Er emission under DC and UPC excitation with delay after the laser pulse, a movie (Media 1) is provided in the supplement data. The time - resolved emission spectra of 5Er - BiOCl are measured using the time scale: δt (µs) = 0.5 + 0.5*(i). (1), (where i = number of recorded spectra and δt is the delay after the laser pulse) under excitation at 488 (blue line spectra) and 1498 nm (red contour spectra). Commercial Matlab software was used to illustrate the temporal evolution of the first 80 frames (corresponding to a maximum δt of 1.5 ms)
To confirm further the contribution of ETU to UPC emission, the UPC excitation spectra and decays were also measured (Fig. 5(a) and 5(b)). It is well - known that in the case of ETU, both ions act as sensitizers and therefore the excitation spectrum should be very similar to the absorption spectrum. In the case of ground - state absorption followed by excited state absorption, GSA/ESA, additional lines in the excitation spectrum, corresponding to the transitions of the second absorption step from the intermediate state into the upper state, should be detected [24,25]. Figure 5(a) shows the UPC excitation spectra of 1/5Er - BiOCl by monitoring the green (543 nm), red (675 nm) and NIR (802 nm) emissions. For 5Er - BiOCl, the UPC spectra are similar and most likely correspond to inter Stark transitions from 4I15/2 to 4I13/2 states, indicating the presence of ETU. For 1Er - BiOCl, the excitation spectra show some weak additional lines (especially for emissions at 543 and 802 nm) that suggest the prevalence of GSA/ESA over ETU mechanisms.
Fig. 5 (a) UPC excitation spectra of 1Er - BiOCl and 5Er - BiOCl monitoring the green (543 nm), red (675 nm) and NIR (803 nm) emission measured at 1µs delay after the laser pulse. All spectra were normalized at 1498 nm peak intensity; (b) Emission decays of 5Er - BiOCl under DC (λex = 488) and UPC (λex = 1498 nm) excitations; (c) Dependence of UPC emission intensity of 5Er - BiOCl on the pulse energy; (d) CIE chromaticity diagram showing the temporal evolution of the (x,y) color coordinates.
The GSA/ESA and ETU contributions to UPC were further differentiated by comparing the emission decays measured under DC and UPC excitations (Fig. 5(b)). UPC by GSA/ESA process occurs within pulse width, therefore the decays measured under DC and UPC conditions should be similar. Generally, UPC by ETU leads to a decay curve for the up - conversion emission that is longer than that measured in DC mode, with a rise time and decay time that are determined by both the intermediate and the upper excited state lifetimes [29]. For 5Er - BiOCl the UPC emission decays of green and red emission show a marked lengthening compared to the DC emission decays that confirm the occurrence of ETU mechanism. Nevertheless, the UPC emission decays do not start from zero, irrespective of the monitored emission, evidencing thus the occurrence of a competing GSA/ESA mechanism [22,25]. A small rise time of ca. 0.03 ms is however measured for red UPC emission. The slope of the curve of log(Intensity) versus log(Energy) approaches two for NIR emission and three for the green and red emissions consistent with a two - and three - photon UPC processes, respectively (Fig. 5(c)). Note that UPC excitation spectra are narrower when monitoring the green and red emissions compared to NIR emission since sharper lines are expected for a higher order process [28]. In line with the time - gated UPC emission spectra illustrated in Fig. 4(a) and 4(b), the CIE chromaticity diagram (Fig. 5(d)) show single excitation, delay induced emission color change from yellowish green (delay of 0.001 ms) to reddish orange (delay of 1 ms).
Several processes can lead to the population of higher excited states of Er from the 4I13/2 state that is first excited upon absorption of a 1498 nm photon (see also Fig. 3). An Er ion can be promoted to the 4I9/2 excited state, from where emission around 800 nm occurs, via the 4I13/2 → 4I9/2 ESA but also the (4I13/2, 4I13/2) → (4I15/2, 4I9/2) ETU1 processes. The (4I13/2, 4I13/2) → (4I15/2, 4I9/2) ETU process followed by the 4I9/2 → 4I11/2 multiphonon relaxation is responsible for the relative strong UPC emission of 4I11/2 level illustrated in Fig. 4(c). Nevertheless, the important contribution of 4I9/2 related emission around 800 nm (Fig. 4(c)) suggest that this multiphonon relaxation is not very efficient. UPC process leading to 4I11/2 emission is confirmed to occur via a two photon process according to the dependency of UPC emission intensity on laser pulse energy (Fig. 5(c)). Further, the 4F9/2 level is populated via the (4I11/2, 4I9/2) → (4F9/2, 4I15/2) and (4I11/2, 4I13/2) → (4F9/2, 4I15/2) ETU processes giving rise to red UPC emission bands centered at 670 nm (ETU I and ETU II on Fig. 3). The temporal evolution of the red emission relative to green emission (Fig. 4(b)) and the longer decays compared to those of green emission under UPC excitation mode (Fig. 5(b)) suggest that the red - emitting 4F9/2 level is fed by additional ETU bypassing the green-emitting 4S3/2 level or cross- relaxation (CR), (Fig. 3). Finally, the 2H11/2 / 4S3/2 states can be reached by two sequential ESA processes via 4I13/2 and 4I9/2 levels. However, the prolonged decay of 4S3/2 level measured under UPC at 1500 nm compared to that measured under DC excitation at 488 nm (Fig. 5(b)) suggest the presence of ETU. In this case, the green emission can be induced by (4I11/2, 4I11/2) → (4F7/2, 4I15/2) ETU mechanism followed by nonradiative relaxation of the 4F7/2 to the 2H11/2 / 4S3/2 - green emitting level [13].
3. 4. Emission properties under X-ray excitation
Under X-ray excitation (by use of a Mo target X-ray tube with characteristic spectrum relevant for the mammography diagnostics), Er - BiOCl exhibits luminescence or scintillation which is readily assignable to Er dopant. Figure 6 presents the comparison between the emission spectra under optical DC excitation at λex = 488 nm and X- ray excitation of the most emissive sample (5Er - BiOCl). The most obvious difference between the two spectra concerns the RGR that is much greater under X-ray excitation than optical DC excitation. To extend the basis of the comparison, also included in Fig. 6 are the UPC emission spectra of 5Er - BiOCl under excitation at 802 and 976 nm. We only briefly remark that Er - BiOCl displays a relative strong up - conversion emission under 976 and 802 nm excitation wavelengths, corresponding to 4I15/2 - 4I11/2 and 4I15/2 - 4I9/2 absorption transitions, respectively (a detailed description will be submitted elsewhere). It is readily observed that the highest red to green emission ratio RGR is measured under X-ray excitation (2.8), followed by UPC excitation at 1498 nm (1.8) UPC excitation at 802 nm (1.2), UPC excitation at 976 nm (0.5) and for DC excitation at 488 nm (0.2).
Fig. 6 Comparison between the emission spectra of 5Er - BiOCl under DC, UPC and X-ray excitation irradiation. All spectra were normalized to 543 nm peak intensity.
While in the optical excitation mode, the laser excitation is performed directly into the f-f absorption of Er at 488 nm, in the X-ray excitation mode, it is BiOCl host that is excited, with the subsequent formation of a great number of e - h pairs that migrate within its structure. Migration stage is strongly affected by defects, surfaces and interfaces that can introduce energy levels into the forbidden gap of the material [30]. The scintillation (luminescence) occurs when these e - h pairs recombine at the Er sites with the net scintillation intensity being determined by the competition between radiative recombination at the Er sites versus nonradiative recombination at quenching centers and trapping of the carriers [31]. Due to relatively short X-ray attenuation length in BiOCl, the emitted light photons has longer path in thick powder layers and the wavelength dependent absorption/scattering process might induce a slight variation of the red to green emission intensity ratio. To account for this effect, we measured the dependence of the total emission intensity on the powder layer thickness and estimated the optimum thickness around 23.5 mg /cm2 (Fig. 7(a)). A slight decrease of the RGR value with the decrease of the powder specific weight was observed under X-ray excitation; however, the decrease of 2.8 to 2.2 cannot account for the large difference between the RGR measured with X-ray excitation and optical DC excitation (2.8 to 0.2). Further experiments are needed to decipher the mechanism of the red emission enhancement under X-ray excitation. It was shown in literature that the red to green emission ratio can depend on many parameters, such as incident laser power, temperature, concentration, particle size and shape, phase purity and the presence of other impurities [32–34]. To the best of our knowledge, this is a first observation of strong dependence of red to green emission ratio on the excitation mode that is optical down /up - conversion and X-ray excitation.
Fig. 7 (a) Dependence of the total X-ray induced luminescence (XEOL) intensity of 5Er - BiOCl on the powder specific weight. (b) X-ray induced luminescence (XEOL) spectra of 5Er - BiOCl collected at different X-ray intensities (linearity test, Upper Inset). Also included is the dependence of the XEOL intensity on the tube kilovoltage (2nd order polynomial dependence, Lower Inset).
The XEOL intensity linearly correlates with the X-ray tube current for Mo and has a 2nd order polynomial dependence on the tube kilovoltage (Fig. 7(b)). Similar evolutions were measured by use of Wo X-ray target tube which has a markedly different X-ray energy spectrum (not shown). As the X-ray emission intensity (X-ray photon flux) is linearly dependent on the tube current and quadratically dependent on the tube kilovoltage, we can draw the conclusion that the XEOL yield is linearly correlated with the X-ray photon flux and the X-ray energy spectra influence on XEOL yield is negligible. It is interesting to note that the RGR remained constant, at ∼2.8, irrespective the target source, tube kilovoltage and current.
Due to combined down - / up - conversion luminescence and X-ray induced luminescence properties, Er doped high - Z BiOCl may represent a promising, single composition based platform for synergistic optical/X-ray imaging. The spectroscopically demonstrated linear response of scintillation intensity with X-ray energy and flux at medically relevant energy and exposure ranges (from few cGy/s up to 2 Gy/s), underscores the potential utility of these Er - BiOCl nanomaterials for dosimetry [35,36]. For the X-ray energy range suitable for the mammography diagnostics as used here, the X-ray mass attenuation coefficient of BiOCl is up to two order of magnitude greater than the value for the breast tissue [37] and therefore, it is expected that biocompatible and biologically size relevant BiOCl (< 50 nm) [38] represents a potential candidate for cancer radiosensitizer and X-ray contrast agent [39,40].
4. Conclusions
We investigate the optical properties of Er doped BiOCl measured within extended Vis to IR spectral range under optical down - and up - conversion as well as X-ray excitation. The Er related tunable luminescence in the 500 to 1700 nm combined with BiOCl host properties, such as reportedly low toxicity, high X-ray absorption and low phonon cutoff energy, render Er doped high - Z BiOCl as promising material for optical and X-ray imaging applications.
Acknowledgments
This work was supported in part by ANCS LAPLAS project PN 09.39. The authors also thank Dr. Cristina Gheorghe for help with NIR down - conversion.
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