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Optical Response of Monolayer CdTe/CdS Quantum Dots to X-rays and Gamma-rays

We investigate the influence of X-ray and gamma-ray irradiation on the photophysical properties of sub-monolayer CdTe/CdS quantum dots (QDs) immobilized in porous silica (PSiO2) scaffolds. The highly luminescent QD-PSiO2 thin films allow for straightforward monitoring of the optical properties of the QDs through continuous wave and time-resolved photoluminescence spectroscopy. The PSiO2 host matrix itself does not modify the QD properties. X-ray irradiation of the QD-PSiO2 films in air leads to an exponential decrease in QD emission intensity, an exponential blue-shift in peak emission energy, and substantially faster exciton decay rates with increasing exposure doses from 2.2 Mrad(SiO2) to 6.6 Mrad(SiO2). Gamma-ray irradiation of a QD-PSiO2 thin film at a total exposure dose of 700 krad(SiO2) in a nitrogen environment results in over 80% QD photodarkening but no concurrent blue-shift in peak emission energy due to a lack of photo-oxidative effects. Near-complete and partial reversal of irradiation-induced photodarkening was demonstrated on X-ray and gamma-ray irradiated samples, respectively, through the use of a surface re-passivating solution, suggesting that there are different contributing mechanisms responsible for photodarkening under different irradiation energies. This work contributes to improving the reliability and robustness of QD based heterogeneous devices that are exposed to high risk, high energy environments with the possibility of also developing QD-based large area, low-cost, re-useable, and flexible optical dosimeters.


Introduction
Over the past decade, there have been tremendous improvements in the synthesis of colloidal quantum dots (QDs), enabling the artificial engineering of wavelength specific, strong light emitters with controllable surface chemistries. Due to their unique optical properties, QDs have found many applications ranging from photovoltaics, 1-5 photodetectors, 6-7 fluorescence probes, [8][9][10] and LEDs [11][12][13][14][15] to radiation scintillators [16][17] , radiation oncology 18 , and X-ray imaging screens. 19 For some of these applications, knowledge of the influence of highly energetic photons on the optical properties of QDs immobilized on a substrate is essential for achieving reliable and robust QD-based device operation over extended periods of time in high risk and high radiation environments. Several studies have examined the radiation hardness of QD-based devices grown by metal organic chemical vapor deposition or molecular beam expitaxy. [20][21] Very few studies have investigated the effects of high energy radiation on colloidal QDs. One study reported rapid degradation of CdSe/ZnS colloidal QDs in hexanes under gamma-ray (γray) irradiation in air 22 and another study showed that 20 keV picosecond electron pulses incident on multilayer close-packed CdSe/ZnS colloidal QD films leads to charged exciton species and multiexciton states. 23 However, the influence of high energy photons on the optical properties of colloidal QDs immobilized on a substrate remains to be explored in detail.
In this work, we report on a detailed analysis of the optical properties of sub-monolayer CdTe/CdS colloidal QDs immobilized within porous silica (PSiO2) scaffolds under increasing X-ray and γ-ray exposure doses in air and nitrogen environments. Recent work demonstrated that colloidal QDs may be dispersed in a PSiO2 framework with little influence on their solution phase optical properties. 24 The exceptionally high surface area of PSiO2 (~200 m 2 cm -3 ) enables the attachment of a large quantity of QDs (~10 14 QDs for 1 -2% surface area coverage) spaced sufficiently far apart to suppress inter-QD exciton couplings, 24 which facilitates the formation of highly luminescent QD distributions that are surface-bound but still largely accessible for surface modification. In this context, using the QD-PSiO2 platform, we can study QD radiation sensitivity on a solid surface that mimics a potential device configuration while preventing inter-QD exciton couplings that may otherwise encourage other avenues for non-radiative exciton annihilation and interfere with conclusions that are drawn purely from radiation induced changes in exciton dynamics. [25][26][27] Here, a sub-monolayer of CdTe/CdS QDs (Supporting Information Figure S1) are electrostatically attached within a 10 μm thick PSiO2 framework (Supporting Information Figures S2-S5), although other types of QDs could be similarly studied using this approach. We show that cumulative 10 keV X-ray irradiation of the QD-PSiO2 samples in air from 2.2 Mrad(SiO2) to 6.6 Mrad(SiO2) leads to an exponential decrease in QD peak emission intensity and a concurrent exponential blue-shift of the QD peak emission to higher energies due to accelerated photooxidative effects. Irradiating QD-PSiO2 samples with higher energy and more highly penetrating 662 keV γ-rays allows for enclosure of the samples in nitrogen purged vials to suppress the effects of radiationinduced accelerated photo-oxidation; however, the QDs experience severe photodarkening with significant loss of emission intensity for less than 1 Mrad(SiO2) total exposure dose. Importantly, it is shown that the effects of X-ray and γ-ray irradiation are largely reversible following a surface treatment procedure that involves the exposure of QDs to a thiol-containing solution. Consequently, this work provides a means of assessing the influence of high energy radiation and surface treatments on the photophysical properties of QDs for future QD-integrated device applications, and offers exciting avenues into developing quantifiable, low cost, flexible, large area, re-useable radiation dosimeters for space and other high-risk environments where low mass and robustness are key criteria for selecting dosimeters for extended space missions. [28][29] 2. Results and Discussions 2.1 10 keV X-ray Irradiations Figure 1 shows the exponential decrease in PL peak intensities for CdTe/CdS QDs attached to PSiO2 scaffolds under increasing 10-keV X-ray exposure (36.7 krad(SiO2)/min) in air. Since the photoelectric effect dominates for low energy (<100 keV) photon interactions with materials, the interaction of 10 keV X-rays with the PSiO2-QDs results in absorption of the primary photons through interactions with atoms and the generation of photoelectrons whose energy depends on the energy of the incident X-rays and the binding energies of electrons in the QDs and PSiO2 framework. [30][31] As a result, the primary source of radiation induced damage for X-ray irradiated colloidal CdTe QDs is most likely through the creation of secondary electron-hole pairs formed along the track of an ejected photoelectron.
As illustrated in the inset of Figure 1, the ejected primary photoelectron may interact with neighboring surface, core/shell interface or within core/shell structure itself and alter QD exciton dynamics through trap-mediated or Auger-assisted non-radiative carrier recombinations. Hence, the cumulative effects of increased carrier traps, photoionizations, and multiexciton creation are the most likely causes for the exponential decrease of peak QD intensities shown in Figure 1.
In order to investigate the role of photooxidation, X-ray photoelectron spectroscopy (XPS) measurements were carried out on an X-ray irradiated CdTe/CdS QD-PSiO2 sample. As shown in Figure 2, the analysis focused on the S2p 3/2 and Te3d 5/2 core levels associated with CdS and CdTe bonds, respectively. After Xray irradiation, there is an increase in the CdS binding energy from approximately 161 eV to 161.6 eV, which is consistent with oxidation, and a new peak appears at 575.9 eV, suggesting the formation of suboxides of Te. These results support the conclusion that the QDs are oxidized as a result of X-ray irradiation.
Previous studies have shown that the exposure of photocatalytically oxidized chalcogenide QDs to thiolcapping agents can help replace unstable disulfides that form during the photooxidation process with thiol ligands to maintain the stability of the QDs. 36 Accordingly, an X-ray irradiated CdTe/CdS QD-PSiO2 sample was exposed to a solution of glutathione containing free thiols to replace the disulfides, help repassivate surface dangling bonds of Cd ions, and possibly reduce photoionized Te atoms. 37 As shown in the XPS spectra in Figure 2, after the glutathione treatment, the peak at 575.9 eV is significantly Continuous wave photoluminescence (CWPL) measurements were then carried out to determine the effect of the glutathione treatment on the QD emission. As shown in Figure 3, following the surface treatment, there is an almost complete recovery of the net PL intensity, which had decreased by over an order of magnitude following a total X-ray exposure dose of 4.4 Mrad(SiO2) in air. A 0.035 eV blue-shift in QD emission wavelengths is also observed following exposure to the free-thiols in solution, which may be attributed to the etching of the oxide species of S and Te formed during the X-ray irradiation. QD emission energy can be related to the effective QD diameter as given by equation (1) where Eg(QD) is the bandgap of the QD, h is Planck's constant, a is the radius of the QD, meff is the effective mass of an electron, and Eg(bulk) is the bulk bandgap energy. 38 (1) Given that Eg(bulk) ~1.5 eV 39 and meff ~0.11me 40 for CdTe, we can estimate that the effective QD diameter decreases by Δa ~130 pm for the measured ΔEg(QD) ~0.035eV, given the initial QD diameter is ~3 nm.
This corresponds to a decrease in effective QD diameter by about one atomic layer and increased exciton confinement that shifts the QD emission to higher energies. The magnitude of the PL peak blue-shift is related to the total ionizing dose of X-rays irradiating the QD-PSiO2 samples. Figure 4 shows the corresponding increase in QD peak emission energy after the glutathione surface treatment procedure is performed on samples exposed to X-ray irradiation at total exposure doses of 2.2 Mrad(SiO2), 4.4 Mrad(SiO2), and 6.6 Mrad(SiO2). Control experiments show that the glutathione surface treatment procedure does not lead to increased PL intensity or a blue-shift in the PL spectrum when performed on a non-irradiated QD-PSiO2 sample (Supporting Information Figure S6). By correlating the degree of accelerated photo-oxidation to the X-ray total dose exposure, and re-passivating the surface, we could potentially enable a means of realizing quantitative, re-useable QD-based radiation dosimeters.  Time-resolved photoluminesce (TRPL) measurements were also carried out on the CdS/CdTe QD-PSiO2 samples before and after X-ray irradiation and the glutathione treatment, as shown in Figure 3b. Table 1 lists the average radiative lifetimes for peak QDs emission wavelengths before and after a total exposure dose of 4.4 Mrad(SiO2), and following the glutathione surface treatment. The decreased lifetime measured after X-ray irradiation is consistent with the CWPL measurements ( Figure 3) and can be attributed to a combination of increased carrier traps, photoionizations, and multiexciton creation that result from the Xray irradiation. The increase in carrier lifetime after the glutathione treatment is likely due to a reduction in surface defect states that results from the surface treatment etching away oxide species and partly reforming the CdS shell, as suggested by the XPS (Figure 2) and CWPL (Figure 3) data.

662 keV γ-ray Irradiations
Next, higher energy γ-ray irradiation (662 keV, Cesium-137 source) experiments were carried out on the QD-PSiO2 samples in air and nitrogen environments to determine how the different sources of radiation and ambient conditions affect the luminescence properties of the irradiated and glutathione-treated samples. The effects of γ-irradiation on the QD-PSiO2 samples in air were similar to those observed for X-ray irradiation (Supporting Information Figure S7 and Table S1). However, for γ-ray irradiation, a total exposure dose of only 700 krad/(SiO2) was sufficient to cause near complete photodarkening of the samples accompanied by a rapid decrease in lifetime. We note that due to the lower dose rate of the γ-irradiation (0.7 krad(SiO2)/min), the total exposure time of the samples to γ-rays as opposed to X-rays was significantly longer. In addition, after the glutathione surface treatment, γ-ray irradiated QD-PSiO2 samples experienced a significantly higher blue-shift in peak QD emission wavelength compared to X-ray irradiated samples after the same surface treatment procedure, and the surface treatment only partially reversed the γ-radiation induced photodarkening effects (Supporting Information Figure S7). We attribute the blue-shift in peak QD emission to γ-ray induced photo-oxidation and the subsequent etching of oxide species during the surface treatment. The lack of complete recovery of the QD emission intensity following the surface treatment is likely due to effects resulting from partial or incomplete passivation of the surface dangling bonds after the photocatalytic oxidation of the thiolated ligands, desorption of Cd 2+ ions or CdTe complexes from the core, and permanent lattice displacement damage effects. The minimum energy for lattice displacement damage in bulk CdTe crystals is approximately 250 keV and the maximum energies of secondary electrons generated by 662 keV γ-rays through Compton scattering is approximately 480 keV. 36,37 Secondary electrons possessing energies higher than 250 keV may therefore cause displacement of atoms in the QD core/shell structure, resulting in rearrangements in the core-shell QD lattice that may lead to significant loss of QD emission intensities through the creation of several nonradiative mid-gap defect states.
Due to the deeper penetration depth of 662 keV γ-rays into materials, it is also possible to conduct γ-ray irradiation studies on QD-PSiO2 samples enclosed in a nitrogen purged glass vial with a septum. This approach enables suppression of the effects of atmospheric humidity and oxygen on QD surface states that is not possible in the case of X-rays, which attenuate rapidly in glass such that any sample enclosed in a vial would be shielded from the radiation. CWPL measurements for a QD-PSiO2 sample prior to and following a 700 krad/SiO2 γ-irradiation in a nitrogen environment are shown in Figure 5a. An 80% decrease in net QD emission results from the γ-irradiation. Following exposure to the thiolated surface treatment solution, the net QD emission increases to 72% of the pre-irradiated value but is not completely reversible. Unlike the X-ray γ-irradiated samples in air, QD-PSiO2 samples irradiated by γ-rays in nitrogen experience no observable blue-shift in the measured QD emission spectra following the glutathione surface treatment, confirming the absence of photo-oxidative effects on the thiolated ligands.
Consequently, we can conclude that the damage to surface states is lower in the absence of accelerated photo-oxidative effects on the thiolated ligands. The recovery in CWPL observed following exposure to the free-thiol solution ( Figure 5)  TRPL measurements for a QD-PSiO2 sample prior to and following a 700 krad/SiO2 γ-irradiation in a nitrogen environment are shown in Figure 5b. After prolonged γ-irradiation, the exciton lifetime may be modeled with a bi-exponential fit given by I(t) = A1 exp(-t/τ1) + A2 exp(-t/τ2), where τ1 and τ2 represent the time constants, and A1 and A2 represent the amplitudes of the components, respectively ( Table 2). 42 The faster decay time constant τ1 is usually attributed to exciton recombinations and the much slower decay time constant τ2 may be attributed to emission of dark excitons or other trap states. [43][44] Following γirradiation in nitrogen, the faster PL decay component (A1%) is seen to decrease accompanied by faster carrier recombination times, possibly due to increased defects. The longer PL component (A2%) is seen to significantly increase accompanied by a lengthening of the PL decay time following irradiation. The reason for this is not entirely clear but could be due to a contribution from carrier trapping for extended periods of time. After the glutathione surface treatment, there is an overall decrease in both PL decay times.

Conclusions
The effects of X-ray and γ-ray irradiation on the photophysical properties of colloidal CdTe/CdS QDs immobilized within PSiO2 3D scaffolds have been characterized. The QD emission is significantly reduced following irradiation. XPS measurements confirm photo-oxidation plays a role when irradiation is performed in air. Photoionization, carrier traps, and multiexcition generation also likely play a role in the radiation-induced photodarkening and reduced exciton lifetimes that were measured following X-ray and γ-ray irradiation of the QD-PSiO2 samples. More detailed investigations of QD exciton dynamics that can resolve sub-nanosecond lifetimes would be able to shed further light on competing mechanisms of photo-oxidation, long lived carrier traps, and mid-gap defect states. Due to their higher energy, γ-rays also likely cause lattice displacements in the QDs that lead to a permanent reduction in the QD emission.
CdTe/CdS QDs demonstrate near complete recovery of QD peak intensity and lifetime after X-ray irradiation when a thiol-rich surface treatment procedure is performed; partial luminescence recovery was observed for γ-irradiated samples. We believe the surface treatment is instrumental in not only reforming a CdS shell but also reducing photoionized QDs that have entered into a dark state. Quantification of the total exposure dose through monitoring changes in QD peak emission intensity and energy may enable applications in passive radiation dosimetry in high risk, high radiation environments. Lightweight and flexible QD-based thin film substrates could be realized for a variety of applications with sub-monolayer QD distributions in PSiO2 3D scaffolds that reduce carrier-trapping and charge transfer between QDs and maintain access to QDs surfaces for re-passivating solutions.

Experimental Section
Sample Preparation: Nanostructured porous silicon films were fabricated by electrochemical etching of boron doped p+ silicon wafers (<100>, 0.01Ω-cm, Silicon Quest) in a two-electrode configuration. A platinum wire counter-electrode and silicon wafer with an exposed area of 1.