Determination of Absolute Quantum Efficiency of X-ray Nano

Oct 4, 2014 - The absolute optical power at 611 nm emitting from Eu doped Gd2O3 nano phosphors upon X-ray excitation from a microfocus X-ray source ...
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Determination of Absolute Quantum Efficiency of X‑ray Nano Phosphors by Thin Film Photovoltaic Cells R. Andrew Davidson, Chad Sugiyama, and Ting Guo* Department of Chemistry, University of CaliforniaDavis, Davis, California 95616, United States ABSTRACT: The absolute optical power at 611 nm emitting from Eu doped Gd2O3 nano phosphors upon X-ray excitation from a microfocus X-ray source operated at 100 kV was measured with thin film photovoltaic cells (TFPCs), whose optical response was calibrated using an He−Ne laser at 632 nm. The same TFPCs were also used to determine the absorbed X-ray power by the nano phosphors. These measurements provided a convenient and inexpensive way to determine the absolute quantum efficiency of nano phosphors, normally a difficult task. The measured absolute X-ray-to-optical fluorescence efficiency of the nano phosphors annealed at 1100 °C was 3.2%. This is the first time such efficiency for Eu/Gd2O3 nano phosphors is determined, and the measured efficiency is a fraction of the theoretically predicted maximum efficiency of 10% reported in the literature.

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at high temperatures may be used to increase the quantum yield.10 Because it is difficult to measure the absolute quantum efficiency using the currently available technologies, most optimization in individual laboratories is done in a relative and qualitative manner. However, lack of cross-calibration of absolute quantum yield of X-rays to optical fluorescence conversion by nano phosphors made in different laboratories leads to insufficient optimization. For instance, a 10-fold improvement in fluorescence yield to a nano phosphor observed in one lab, though significant, does not mean the optimized nano phosphor is 10-fold superior than the same type of phosphor made in another lab. Nor does it mean these individual lab-optimized nano phosphors are fully optimized.11 An alternative is to compare the lab made phosphors against standard samples. However, this practice is rarely followed due to many difficulties. It is hence plausible that many of the currently available nano phosphors are still below the theoretically permitted performance limit. It is much more difficult to determine the absolute quantum yield of nano phosphors than the scintillation efficiency of crystalline scintillators.7 The latter is generally probed with devices such as photomultiplier tubes (PMTs) in the photon counting mode so the absolute efficiency can be readily obtained without the need to measure the absorbed power of ionizing radiation such as X-rays. In contrast, there is no uniform platform for measuring the absolute quantum efficiency of nano phosphors because it is difficult to determine absolute optical emission and X-ray absorption power using the same instrument. As a result, to date most characterization methods for phosphors are generally qualitative, detecting the

-ray nano phosphors are an important category of materials. They can be used in many applications ranging from X-ray imagers to X-ray detectors.1−4 Comparing with single crystal X-ray scintillators, nano phosphors have several advantages. First, nano phosphors can be more easily made into films, making it possible to fabricate flexible displays. Another advantage of nano phosphors over single crystal scintillators is that the former is inexpensive and more straightforward to synthesize. Yet another important aspect is that it is possible to scale up a film of phosphors more easily than the single crystal counterpart, making it possible to create large area X-ray detectors inexpensively. In addition, studying X-ray nano phosphors may help to understand complex carrier dynamics in scintillation materials and phosphors as well as to create new fluorophores to image tissues and cells.5 Other applications using nano phosphors are possible. For example, it is possible to generate electricity under certain conditions with highly penetrating ionizing radiation such as X-rays so that functions such as charging batteries buried in structures can be performed. X-ray nano phosphors are usually made synthetically, although the mechanisms and optimization are still being investigated.6−9 The X-ray to photon conversion mechanisms are complex, generally containing three main steps: (A) ionization of core electrons, (B) creation of electron−hole pairs through collisions between the ionized electrons and the phosphors materials, and (C) transport of these pairs to locations where they can undergo radiative recombination. For optimal performance, it is believed that electron−hole pairs created by X-ray irradiation should migrate to and be trapped at defects or interfaces and should not undergo nonradiative recombination in the body of the nano phosphors.7 Therefore, as-made nano phosphors must be optimized after initial synthesis to increase the quantum yield of X-ray to visible photon conversion. Several synthetic steps including annealing © XXXX American Chemical Society

Received: August 30, 2014 Accepted: October 3, 2014

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optical fluorescence induced by X-rays using an optical dispersive device such as a spectrometer in conjunction with detectors such as PMTs, charge-coupled devices (CCD), or photodiodes. Most of the time, this qualitative characterization is adequate to evaluate the relative performance of a nano phosphor produced in one lab as it is optimized. Hence little effort has been invested in creating a more uniform platform of detection to more quantitatively measure the absolute quantum yield of the nano phosphors. As a result, the absolute efficiency are not measured and cannot be compared against others made in different laboratories, resulting in under-developed nano phosphors. If an inexpensive and convenient method can calibrate the absolute nano phosphors in individual laboratories, a task regarded extremely difficult in the past, then it is possible to synthesize and assess high quality nano phosphors. It is noted that other parameters such as absolute ef f iciency have been defined and used in quantifying nano phosphors, although they are not equivalent to the commonly defined absolute quantum efficiency.12 In this work, we wish to show an inexpensive and quantitative method to measure the absolute quantum efficiency of X-ray to visible photon conversion by nano phosphors. The new method is demonstrated on the optimization of a popular nano phosphors of Eu doped Gd2O3, which was annealed at high temperatures to improve the quantum efficiency. The nano phosphors are embedded in transparent polymer composite films, further simplifying the detection process. Both the visible and X-ray photons are detected with commercially available thin film photovoltaic cells (TFPCs). The results are compared with those obtained using a more conventional method employing all optical elements in a CCD-spectrometer combination that can measure only the relative efficiency of the nano phosphors without the determination of the absolute optical emission power or the X-ray absorption by the nano phosphors. The linearity and dynamic range of the new method are determined and it is demonstrated that this is a viable method to characterize the nano phosphors. The current method for the first time provides an inexpensive way to quantify X-ray nano phosphors. Europium(III) chloride hexahydrate (99.99%) was obtained from Sigma-Aldrich. Gadolinium(III) chloride hexahydrate (99.9%) was obtained from GFS Chemicals. Ammonium carbonate was obtained from Fischer Scientific. Poly(vinyl alcohol) (PVA), 86−89% hydrolyzed, was obtained from Alpha Aesar. These chemicals were used without further treatment. The europium doped gadolinium oxide (Eu3+/Gd2O3) was prepared by a coprecipitation method.13 A solution containing a 95:5 mol % ratio of Gd: Eu was prepared by dissolving 3.53 g of gadolinium(III) chloride hexahydrate and 0.183 g of europium(III) chloride hexahydrate in 20 mL Milli-Q water. After complete dissolution, 20 mL of 0.2 M ammonium carbonate was added with magnetic stirring to precipitate the oxide nanoparticles. The solution was then covered and stirred for 24 h. The precipitate was purified by centrifugation at 6k rpm for 15 min three times, each washed with Milli-Q water. The precipitate was dried on a hot plate, ground to a fine powder, and calcined in a tube furnace (Lindberg/Blue) at a given temperature in the range of 650−1100 °C for 2 h. Once calcined the powder was ground again to a fine powder. To create the nano phosphors pads (nanopads), a viscous solution of PVA was prepared by adding 2.4 g of PVA into 20 mL water under vigorous stirring. Once suspended, the solution was then heated to 80 °C to aid in dissolution. Once all PVA

was dissolved, stirring ceased, and the solution was allowed to cool. In a separate glass vial, a 0.78 wt % Eu3+/Gd2O3 in PVA solution was prepared by suspending the calcined Eu3+/Gd2O3 in the PVA solution. The solution was stirred until the suspension was homogeneous. The nanopads were formed by droppering a known mass of the suspension onto a glass slide of 0.780 × 0.945 in.2 (equal in size to a single section of the TFPCs (MP3-25, Powerfilm). Care was taken to prevent bubbles during formation of the nanopads, which were dried overnight in the dark. Once dried, a razor blade was used to peel the nanopads off the glass, after which the pads were pressed at 5k PSI to remove any wrinkles and create a film of uniform thickness. The known density of the nano phosphors in the suspension and deposited mass allowed us to determine the total amount of phosphors in the final nanopads. Absolute quantities of nano phosphors used here and corresponding pipetted masses of PVA solution in parentheses were: 0.488 mg (0.0625 g), 0.78 mg (0.10 g), 0.975 mg (0.125 g), 1.56 mg (0.20 g), 1.95 mg (0.25 g), 7.8 mg (1.0 g), and 11.7 mg (1.5 g). When the nanopads required more than 0.5 g pipetted mass, multiple layers were deposited with drying between depositions. Figure 1 shows the two instruments we developed in this work to investigate the performance of nano phosphors. The

Figure 1. X-ray instruments used to characterize X-ray nano phosphors. (A) shows the TFPC-based detection system that can determine the absolute quantum efficiency of the nano phosphors. (B) shows the layout of the spectrometer-based X-ray instrument in which a lens is used to collect the optical fluorescence upon X-ray excitation.

two apparati shared a common microfocus X-ray source with a tungsten target (Thermo Kevex, PXS10-WB-10 mm). Figure 1A shows photovoltaics-based X-ray instrument that can measure both optical emission and absorbed X-ray power. The nanopad containing nano phosphors in polymer composite was sandwiched between two TFPCs. Upon X-ray irradiation, scintillation occurred to generate light and voltage produced in TFPCs was measured with a voltmeter. The whole device was housed in a homemade X-ray chamber.14 The same chamber was used to detect the optical fluorescence from X-ray irradiated nano phosphors. Figure 1B shows the instrument in which a lens was used to collect the fluorescence and the light was focused into a fiber optic connected to a spectrometer [SpectraPro 300i, Acton]. The signal was detected by a CCD. The overall absolute detection efficiency of this instrument was not calibrated. This instrument is shown in Figure 1B and no TFPCs were used. The measurements of X-ray induced optical fluorescence from nano phosphors were calibrated with a HeNe laser [05LHP991, Coherent] illuminating the same TFPC. Optical B

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absorptive neutral density filters [ANG-3, 10, and 11, CVI] were used to attenuate the HeNe light. Figure 2A shows a transmission electron microscope (TEM) image of the annealed rare earth nanoparticles. The nano-

Figure 3. (A) Voltage response from the TFPCs as a function of the power of He−Ne at 632 nm. (B) Voltage response from the photovoltaic as a function of the current for the X-ray source. Each sample contained 15.6 mg of nano phosphors. Also shown is the response of the TFPC alone without the nano phosphors (i.e., X-ray Only). (C) Net contribution from nano phosphors without “X-ray Only” contribution and (D) Corrected optical power from X-ray irradiation of nano phosphors (black) overlaid on the optical response from Figure 2D (gray). Dashed lines between 150 and 350 μA are drawn for visual guiding purpose for nano phosphors annealed at 950 and 1100 °C in D.

Figure 2. (A) TEM image of the annealed nano phosphors. (B) Nano phosphors embedded in polymer films coated on TFPCs with and without UV (254 nm) illumination. (C) Optical spectrum of the nano phosphors measured with the spectrometer. (D) Intensity of the peak at 611 nm as a function of calcination temperature.

then the voltage response will be linear only within a narrow dynamic range even if the fluorescence is linearly dependent on the X-ray power. Figure 3B shows the results of the voltage response to X-ray irradiation of several nanopads annealed at different temperatures. Each of the nanopads contained 15.6 mg nano phosphors. The results display the X-ray excited optical responses measured with the TFPCs as a function of X-ray power. On the basis of the measured voltage, the amount of light produced from nano phosphors irradiated with X-rays with greater than 100 μA at 100 kV placed the voltage response in the nonlinear regime shown in Figure 3A. It means that most of the measured voltages shown in Figure 3B are outside the linear region. At the lowest X-ray power (current equal to 50 μA) there is little saturation, and the voltage produced from the nano phosphors at this X-ray power is about twice that by the X-ray irradiated TFPC alone (X-ray Only). After removing the voltage contribution from the TFPCs themselves under X-ray irradiation, the results as shown in Figure 3C are more comparable to that shown in Figure 2D. The TFPCs and spectrometer measurements are similar, although there is a quantitative disagreement at high X-ray powers (voltage at 100 kV and current >100 μA), which is caused by the voltage saturation of the TFPCs at elevated X-ray power. This did not occur in optical spectrometer measurements. As a result of this saturation, phosphors annealed at 1100 °C is only 2.66 times as efficient as that at 650 °C after “X-ray Only” signal is subtracted. The results shown in Figure 3C are closer to those shown in Figure 2D if the saturation in Figure 3C is corrected using the

particles after annealing shown in Figure 2A are shard like, transforming from spherical nanoparticles to aggregates after annealing at 650 °C. Pictures of the polymer composite with and without UV illuminating the nano phosphors are shown in Figure 2B. The composite is transparent at this nano phosphors concentration (∼4 mg loading), and the electrode patterns in the TFPCs are visible through the nanopad. Optical emission under X-ray excitation (in situ) is shown in Figure 2C, which is detected with the X-ray excitation−optical detection instrument shown in Figure 1B. Figure 2D shows the linearity of the emission measured with the spectrometer as a function of X-ray power (varying the current of the X-ray source at fixed voltage of 100 keV). 15.6 mg of the nano phosphors were used in each of the measurements. No saturation or other nonlinearity in optical fluorescence measurements is observed. On the basis of Figure 2D, the emission intensity of nano phosphors annealed at 1100 °C is 4.25 times those annealed at 650 °C, which was already much better than the as-made nanoparticles by at least an order of magnitude (not shown). Similar improvement results upon annealing have been reported in the literature.10 Figure 3A shows the calibrated optical responses from the TFPCs using the HeNe laser and neutral density optical filters. The sensitivity of the TFPCs allows the detection of 0.1 μW using only the central area (∼2 cm2) of a TFPC pad. The voltage response saturates quickly as the laser power is raised above 0.5 μW. In practical solar cell applications, more intense light generates more current, and therefore more power. The results shown in Figure 3A indicate that if voltage is used to measure the fluorescence from X-ray excited nano phosphors, C

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0.0064 V/mg K = = 0.0128 μW/mg Sop 0.5 V/μW

data displayed in Figure 3A. The results after correction are shown in Figure 3D (black lines), which agree better with Figure 2D (gray lines in Figure 3D). For instance, after the correction the efficiency for the nano phosphors annealed at 1100 °C is now 4.86 times that at 650 °C. Data between 150 and 350 μA of X-ray tube current in Figure 3D are not included due to lack of data points in the 3A. There are minor mismatches between the spectrometer measured data and those obtained with the TFPCs. Thin dashed lines (black) are drawn for visual guidance for nano phosphors annealed at 950 and 1100 °C. On the basis of Figure 3A, we can obtain the optical sensitivity of the TFPCs in the linear voltage region: Sop =

0.1 V 0.2 μW

The results shown so far still do not allow us to estimate the absolute fluorescence yield. In order to absolutely measure the conversion efficiency from X-rays to visible photons, another parameterthe absolute absorbed X-ray power by the nano phosphorsis needed. The absolute X-ray power absorbed by a unit mass of nano phosphors can be estimated based on (1) how much X-rays are absorbed by the nano phosphors and (2) the X-ray flux, which can be obtained from the factory calibrated flux, the literature values and calculations. The process is shown below. X-ray absorption by nanopads can be measured with TFPCs. We inserted different amounts of nano phosphors between two TFPCs shown in Figure 1A and measured the difference in voltage output shown in “X-ray Only” case. The voltage difference can be used to determine attenuation of X-rays. Thin Al foils were used to block the light emitted from the nano phosphors from reaching the TFPCs for “X-ray Only” measurements. The attenuation of X-rays by the nano phosphors was measured using ∼60 mg nano phosphors in polymer composites. The attenuation by the nanophosphors alone was 42%. The attenuation by the polymer itself was experimentally determined to be 16% with 500 mg of polymer. Both were measured with the Al foils in place, hence there was no contribution from the foil. These values are used to assist the calculations of the X-ray energy absorbed by the nanopads and the absolute conversion efficiency from X-rays to visible photons by the nano phosphors. X-ray flux can be determined with the help of the factory measured dose. Because the mechanism of X-ray generation is the same for both conventional X-ray tubes and microfocus sources, which is by electrons bombarding tungsten targets, the efficiency should be the similar for these two sources. The factory calibrated flux is 231 R/min from the microfocus source at 1 m away for 130 kV and 0.5 mA. This flux is 47.7% of the flux calculated with SpecCalc, a software used to calculate the flux and the X-ray spectrum.15 This correction factor is used for the source when calculating the X-ray spectrum and power at other power settings used in the experiments. Using these parameters listed here, the calculated X-ray power irradiating the sample is 0.208 mW through the sample area without the TFPC and 0.183 mW after passing through the front TFPC with the X-ray source operated at 100 kV and 50 μA. The absorbed X-ray power by 60 mg nano phosphors is calculated to be 46.6 μW or 0.776 μW/mg. It is important to point that this value cannot be directly obtained from the absorption measurement because the Si detector is biased toward absorbing more low energy X-rays, so the absorbed power must be calculated from the X-ray spectrum and measured absorption using the THPC. The unit mass X-ray (power) absorption is as follows:

(1)

which is 0.5 V/μW at 632 nm. This wavelength is close to 611 nm of the emission from the nano phosphors. Figure 4 shows the dependency of voltage as a function of the mass of the nano phosphors under identical X-ray

Figure 4. Voltage measurements of TFPC response to nano phosphors under X-ray excitation (50 μA and 100 kVp). The measurements (except for the highest mass) are best fitted to a straight line.

irradiation conditions (100 kV and 50 μA). The polymer matrix is transparent to the red scintillation light. Under this configuration, 50% of the fluorescence light is collected by the top TFPC, as shown in Figure 1. The responses shown in Figure 4 can be absolutely calibrated using the response from the HeNe excitation results shown in Figure 3A. For example, the amount of light at 611 nm created from 8 mg of the nano phosphors by X-ray excitation is equivalent to that of 0.05 μW of HeNe light at 632 nm. We need these results to estimate the absolute efficiency of the nano phosphors, as shown below. Similar to those shown in Figure 3A, the voltage saturates as a function of the mass of nano phosphors. The saturation is due to the natural voltage response from the TFPCs. Figure 4 indicates the mass region in which the nano phosphors generate linear voltage response. As the amount of the nano phosphors increases to tens of mg, X-ray to optical power conversion calculation becomes less accurate due to the saturation of the voltage response. From Figure 4, the voltage produced per unit mass of nano phosphors from X-ray excitation K is as follows: K = slope = 0.0064 V/mg

(3)

PX−ray = 0.776 μW/mg

(4)

This gives rise to 1.65% conversion efficiency from X-rays to photons at 611 nm. η=

(2)

Therefore, the overall optical sensitivity from the X-ray nano phosphors per unit mass using 100 kV and 50 μA on the X-ray source is as follows:

K PX−raySop

=

0.0128 = 0.0165 or 1.65% 0.776

(5)

This efficiency was obtained with a one side detector collecting 50% of the emitted light from the nanopad, so the D

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efficiency from X-ray photons absorbed by the nano phosphors to visible photons emitted by the annealed Eu/Gd2O3 nano phosphors is 3.3%, a fraction of the maximum efficiency estimated based on literature values.

actual total efficiency is 3.3% to consider all the light generated from the phosphors material. This is the first time such efficiency is obtained experimentally for this nano phosphor. In the literature the maximum efficiency has been theoretically estimated and with several approximations, the highest efficiency of a similar nano phosphor of Eu/La2O3 is estimated to be 10%.6 The TFPC approach presents a novel and simple means to directly determine the absolute X-ray nano phosphor efficiency by measuring both the emitted light power and absorbed X-ray power. This is partially enabled by the optical power response in the selected spectral range. The simplicity of TFPCs and low cost of these cells makes this approach accessible to any laboratories. Hence it is now possible to combine the measurement of X-ray power using calibrated TFPCs and the optical power measurement of the nano phosphors using the same TFPCs to determine the absolute quantum yield of the nano phosphors. No dispersive elements or sensitive optical detectors are needed for the determination of the absolute quantum yield. The novel method developed here can be used to determine the absolute quantum efficiency of X-ray nano phosphors through cross-calibration between X-ray induced optical fluorescence, laser irradiation of the TFPCs, and measurement of absorbed X-ray power. This more quantitative method of directly determining the absolute efficiency of X-ray nano phosphors may help guide the development of better nano phosphors in the future. Once calibrated with light and X-rays, the TFPCs can be used in different laboratories to determine the absolute quantum efficiency without the use of a spectrometer and X-ray detector. We only used the data in the linear voltage response region to determine the quantum efficiency of the conversion of X-ray to visible photon η for the nano phosphor. The results shown here suggest that if the voltage is used for characterization, then the dynamic range is narrow, accommodating only a few mg of nano phosphors over the 1 in2 area of the polymer film before saturating the voltage output of the TFPCs. It is possible to use power instead of voltage, which may significantly increase the dynamic range and can measure much higher densities of nano phosphors. We did not observe the degradation of the nano phosphors in the time span of our measurement. Because the illuminating area is only a fraction of a whole piece of the commercially available TFPC, the voltage measured here may be different from those using different TFPCs whose size is smaller than the illuminating area because the unilluminated area may have a much different internal resistance. Each separate TFPCs will need to be calibrated, but the methodology developed here should be the same. On the basis of the study shown here, it is possible to use Xray nano phosphors to create X-ray voltaic cells (XVs). To match the sunlight by the nano phosphors, a kW level rotating anode X-ray source would be needed to excite improved nano phosphors of nearly 50% quantum efficiency so that the volume of XV can be kept small. Another advantage of using nano phosphors and photovoltaics to harvest X-ray energy rather than directly using crystalline scintillators is to avoid direct electron−hole pair recombination in bulk semiconductors. We have for the first time employed thin film photovoltaic cells to measure the absolute quantum efficiency of X-rays induced optical fluorescence from nanoparticle X-ray phosphors. The determination of the absolute efficiency was assisted by the HeNe laser light and estimated X-ray flux. The highest

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partially supported by the National Science Foundation (CHE1307527). REFERENCES

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