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page 120 Ultrafast Cathodoluminescence for Improved Gamma-Ray Scintillators Jeffrey M. Pietryga 20100183ER Introduction Energy resolving gamma-ray detectors are crucial to national security, nonproliferation, and basic research. Principally, there is a need to identify radioactive materials (such as fissile material) and detect their movement. When gamma-rays interact with matter, much of the energy is converted into energetic electrons that cascade into showers of lower and lower energy electrons. In single-crystal scintillators, the electrons produce visible photons in a relatively well-understood manner. Energy resolution is simply a matter of counting the number of photons produced by a gamma-ray absorption event: the more photons produced, the higher the energy of the gamma ray that caused them. Semiconductor nanocrystals are of significant interest as scintillators because of their high gamma-stopping power, high emission quantum yields, solution processibility and low cost in comparison to large single crystals. While nanomaterials will convert incident gamma-rays into energetic electrons the same as other materials, the transduction of those electrons into photons is poorly understood, as it is made more complex in nanomaterials because of their unique optical properties, and the influences of interfaces throughout the very heterogeneous composite material. While ultrafast lasers and detectors have allowed us to achieve amazing insight into how optically excited nanocrystals relax to emit photons, similar experiments are not possible with a gamma-ray source, since its spontaneous emission cannot be synchronized to a detector. However, an ultrafast, short-pulse electron gun can act as a synchronizable surrogate for a gamma-ray source, allowing the type of time-resolved measurements needed to understand how electrons are converted into photons. In this work, we build just such an electron gun, and use it to perform transient cathodoluminescence experiments that will elucidate both favorable and undesirable relaxation pathways during electron-to-photon conversion in nanomaterials. Further, this insight will provide feedback to synthetic efforts aimed at producing high performance energy-resolving gamma-ray scintillators that are operable at room temperature. Benefit to National Security Missions Energy resolving gamma-ray detectors are vital to nuclear nonproliferation and security as these devices impair material diversion and movement/smuggling of fissile materials. Semiconductor nanocrystals are recognized candidates for application as highly fieldable, high-energy resolution, scalable scintillators. This project aims to determine the extent of their utility for this purpose and thereby aid in threat reduction. Low-cost detectors may also contribute to managing nuclear materials in the weapons complex. Progress As described in the technical description, this project consists of three main tasks: 1) establish a transient cathodoluminescence capability with adequate time resolution (on order 10 picoseconds) and reproducible operation at controllable pulse intensities (i.e. current) and voltages; 2) perform studies of known nanocrystal materials, such as cadmium selenide/zinc sulfide core/ shell nanocrystals, to produce a database of spectral signatures corresponding to known excitation states of semiconductor nanocrystals and inform the design of enhanced materials; and 3) development of new nanocrystals and/or composites with enhanced cathodoluminescence efficiency, applying continuous feedback from ongoing cathodoluminescence studies. A simplistic summation of our schedule expectations would roughly be that each task would take approximately one year of the three year project lifetime. Thus, at the end of year two, we should expect to have established the instrumental capability, performed a baseline survey of CdSe/ZnS nanocrystals of a range of sizes, and to have developed the mathematical analysis techniques required to turn the raw data into conclusions regarding the relaxation process in these nanocrystals that we will soon use to identify the next candidate material system for study. As will be described in more detail below, this is exactly the case. In the first quarter of FY11, we designed and constructed a new electron gun, featuring nested electron optics for full exclusion of stray external fields (Figure 1a). It Cheers: Axil On Wed, Feb 20, 2013 at 1:37 PM, Kevin O'Malley <[email protected]> wrote: > Axil: > > That article is exceedingly difficult to read. It's 2 pages embedded into > 969 pages of PDF and page 120 is blank. For the sake of others, so they do > not have to try 6 times to load the page, I have copied what I could, but > it does not contain page 120... because it is blank. > > > http://www.lanl.gov/science-innovation/_assets/FY11-Annual-Report.pdf > > > > > > was successfully mounted in the instrument chamber, was fired up, and was > shown to work extremely well, capable of producing a tightly focused beam > of electron pulses (~1 mm spot size, Figure 1b) controllable in intensity > from 5-150 picoamps, and at voltages from ~8 to 21 kilovolts. In the second > quarter of FY11, we focused on optimization of the luminescence collection > pathway, using a simple drop-cast film of cadmium selenide/zinc sulfide > core/shell nanocrystals to produce enough cathodoluminescence signal to > couple the instrument to a Hamamatsu streak camera for collection of time- > and spectrally-resolved emission. The optimization of the collection > pathway and synchronization were far from trivial, but as of March, the > instrument has been reliably producing beautiful streak camera traces of > nanocrystal emission produced by electrons. Further analysis has brought to > our attention a number of subtle yet extremely important details about the > operation of this streak camera, including how to avoid saturation effects > from the instrumental gain, and how to account for the effect of the > reverse sweep of the oscillating streak bias. > > ** > > While our appreciation of the complexities of the instrument was still > evolving, the third quarter of FY11 was devoted to collection of a > database of cathodoluminescence traces from a size-diverse series of > cadmium selenide/ zinc sulfide (CdSe/ZnS) core/shell nanocrystals. In > parallel, we collected time-resolved photoluminescence on the same exact > samples, which provide a vital starting point for analysis of features > found in the more complex cathodoluminescence traces. A comparison of the > two is shown in Figure 2. At this point, we can confidently say that the > quality of the data between the two experiments is completely equivalent, > which is a significant achievement in its own right. As of now, we have > full data sets for four different sizes of CdSe/ZnS nanocrystals, emitting > from 530 to 665 nm. > > > > *Figure 2. (a-c) Time-resolved photoluminescence for a sample of CdSe/ZnS > nanocrystals emitting at 630 nm. In (a), only single-exciton dynamics are > observed. As power is increased (b), a faster biexciton peak emerges at the > same energy. Finally, at very high powers (c), a very fast triexciton peak > is seen at shorter wavelengths. (d) A cathodoluminescence trace of the > same sample shows strong contributions from the multiexciton states. When > these contributions are deconvolved, hints of charged “trion” emission > emerge.* > > Through careful analysis over the final quarter of FY11, we have both > fully characterized the time resolution of our instrument, and developed > the methodology for identifying and deconvolving the contributions of > several types of excited states to the cathodoluminescence decay traces. A > side-by-side comparison of traces recorded at several beam currents > revealed that decay lifetimes associated with a given size of nanocrystal > showed no dependence on the beam current, as expected (Figure 3a). However, > the apparent rise-time of the signal depended strongly on the current, but > not at all on the size of the nanocrystals. Effective pulse widths were > extracted from the rise times measured for a range of currents and sizes. > When plotted together, the dependence of the pulse widths on current > becomes clear (Figure 3b). Extrapolating to 0 current, we see that our > nominal response time for cathodoluminescence is 9 ps, which compares > favorably to the measurements using the streak camera with laser excitation > (~6 ps). We > > > > believe that the additional rise stems from a combination of Coulombic > pulse spread and possible bulk charging effects within the film itself > during measurements. Ongoing experiments are aimed at minimizing the > importance of these effects. > > *Figure 3. (a) Although rise time slows with increasing current, the > decay lifetimes of a single nanocrystal sample do not vary, and are > superimposable at later times. (b) An analysis of the effect of current on > lifetime for several nanocrystal sizes reveals a consistent trend that > implies an instrument response time of only 9 ps, which is further > broadened by pulse effects that can be processed out after the measurement. > * > > At the present time, we are using the rise-time measurements and > laser-based photoluminescence experiments to deconvolve the chief features > in our cathodluminescence traces. Preliminarily, we report contributions > from a range of excited states, including single exciton, biexciton and > tri- and higher multi-exciton states (these last states being > indistinguishable by our method). In addition, we have evidence of a > substantial signal from charged nanocrystal states, i.e., excited states > with an imbalance in electrons and holes resulting in a net charge on the > nanocrystal, the simplest being a “trion” (two electrons and one hole). > Currently, we are refining a model for inferring the fraction of charged > nanocrystals in the whole population. > > *Future Work* > > The exact fraction of nanocrystals that are charged by the initial > high-energy excitation is the subject of current analysis. However, it is > fairly clear that it is significant. Since multiple states with a charge > imbalance decay by radiative recombination (slow) and non-radiative Auger > recombination (fast) to eventually produce non-emissive ground-state > charged nanocrystals, this result yields a possible reason why > nanocrystal-based scintillators have underperformed. This suggests two new > immediate goals. The first is to show that the electron excitation method > we employ does not preferentially result in more charged nanocrystals than > would be produced by a gamma-ray of similar energy. This will be the > subject of near term studies based on varying the sample form factor, by > which we hope to modify the ability of potentially excess charges to be > captured by or flow out of the sample. The second is to study nanocrystals > or composites that should exhibit modified charging probability (that is, > materials that are easier or harder to ionize). Intentional manipulation of > the fraction of charged > > > > nanocrystals would be the first step towards true optimization of the > gamma-to-photon transduction process. > > *Conclusion* > > Now that we have optimized and benchmarked the performance of our > cathodoluminescence instrument, we have been able to collect the first > substantial set of decay traces on a size series of CdSe/ZnS NCs, which is > the material system most studied for gamma scintillation. In conjunction > with photoluminescence studies, we have been able to identify the > contributions of a number of familiar excited states to the decay dynamics. > Most surprising so far is the not-insignificant presence of charged > nanocrystals. While more complete and quantitative analysis is under way, > this observation already suggests the next logical steps for our studies, > which will focus on the determining the source and possibly manipulating > the fraction of charged states. If this turns out to be the dominant factor > in the performance of semiconductor nanocrystals as gamma scintillators, > our study will be the first ever to suggest an active pathway toward > realizing the latent potential of these materials. This could very well > reinvigorate worldwide efforts in developing cheap, rugged replacements for > single-crystal scintillators based on this fascinating class of materials. >
