IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 59, NO. 1, FEBRUARY 2012 215

Quantum Dot-Organic Polymer Composite Materialsfor Radiation Detection and Imaging

William G. Lawrence, Member, IEEE, Samta Thacker, Senerath Palamakumbura, Kent J. Riley, andVivek V. Nagarkar, Member, IEEE

Abstract—Colloidal semiconductor nanocrystals exhibit phys-ical properties that are characteristic of intermediate size scalesbetween molecular states and solid state materials, and are oftencalled quantum dots. Solid state semiconductor materials havebeen used extensively as scintillation detectors for ionizing radia-tion. We describe the use of semiconductor quantum dot-organicpolymer composites for use as scintillation detectors and reportthe use of quantum dot-polymer composite thin films for X-rayimaging.We have prepared quantum dot-polymer thin film samples using

both aqueous CdTe quantum dots in polyvinyl alcohol and non-aqueous CdSe quantum dots in polystyrene. Optical absorptionspectra and emission spectra are used to characterize the quantumdot morphology in the polymer film and to evaluate the impact ofthe host polymermatrix on the quantum dot dopant.We report thefluorescence lifetime changes of the quantum dots in the polymerhost and discuss the impact on high frame rate X-ray imaging. Theemission spectrum of the X-ray induced luminescence is found tohave the same spectral dependence as the laser induced fluores-cence. The details of the quantum dot-polymer composite scintil-lator fabrication and characterization will be discussed. The thinfilm substrates are coupled to a CCD camera and used to recordX-ray images.

Index Terms—Cadmium selenide, cadmium telluride, nanocom-posite, polymer, quantum dot, scintillator, X-ray imaging.

I. INTRODUCTION

T HE unique electronic properties of semiconductornanocrystal quantum dots have been used to develop

photonic devices that exploit the exciton production andexciton harvesting capabilities of quantum dot-polymer com-posite materials. The quantum dot-polymer composites havefound application in photovoltaic cells, electro-luminescentdevices [1]–[3], and scintillation detectors for ionizing radia-tion [4]–[6]. In quantum dot based photovoltaic systems, theincident light is absorbed by the quantum dot to produce a

Manuscript received January 03, 2011; revised September 09, 2011; ac-cepted November 28, 2011. Date of current version February 10, 2012. Thiswork was supported in part by the U.S. Department of Energy under GrantsDE-SC0000956 and DE-SC0000929.W. G. Lawrence was with Radiation Monitoring Devices, Watertown MA

02472 USA. He is now with the MIT Lincoln Laboratory, Lexington MA 02420USA (e-mail: william.lawrence@ll.mit.edu).S. Thacker, S. Palamakumbura, and V. V. Nagarkar are with Radiation Mon-

itoring Devices, Watertown MA 02472 USA (e-mail: sthacker@rmdinc.com;spalamakumbura@rmdinc.com; vnagarkar@rmdinc.com).K. J. Riley was with Radiation Monitoring Devices, Watertown MA 02472

USA. He is now with the Department of Radiation Oncology at MassachusettsGeneral Hospital, Boston MA 02114 USA (e-mail: kriley7@partners.org).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TNS.2011.2178861

localized electron—hole pair. When the quantum dot guestis embedded in a suitable polymer host material, the excitonpair can be separated and collected to produce current. Inelectroluminescent systems, the polymer host is used to trans-port charge to the quantum dots where the electron and holelocalize at the quantum dot and recombine to emit light. Asimilar mechanism is observed for the scintillation of quantumdot-polymer composite films upon exposure to ionizing radia-tion. The incident ionizing radiation produces free secondaryelectron and hole charge carriers. These free charge carrierscan trap at the quantum dot and recombine to produce emissionat optical frequencies, and we have used this to demonstrateX-ray imaging using quantum dot-polymer thin films.Colloidal semiconductor nanocrystals are described as

quantum dots because they exhibit unique physical propertiesthat are intermediate systems between molecular states andsolid state materials. The quantum dots have bright size-depen-dent emissions and unique electronic properties that have foundapplications as biological labels, light emitting devices, andphotovoltaic components. The diameter of the quantum dotstypically range from 3 to 6 nm, and the emission wavelengthof the quantum dot shifts from blue to red as the size of thenanocrystal increases [7]. Since the quantum dots are muchsmaller than the wavelength of the visible emission, isolatedquantum dots can be included in a polymer matrix withoutdegrading the optical clarity of the polymer [8].Solution phase synthesis methods are used to produce col-

loidal nanocrystals from II–VI, III–V, and IV–VI semiconductormaterials. The inorganic precursors are mixed with coordinatingsolvents such as trioctylphosphine oxide (TOPO), mercaptopro-pionic acid (MPA), or hexadecylamine (HDA) that couple tothe exposed surface of the nanocrystal and control the growthof the particle. The final quantum dot has a critical outer or-ganic passivation layer that prevents particle agglomeration inthe solvent, preserves the integrity of the particle and maintainshigh fluorescent quantum efficiency. Themixing of the quantumdots with the polymer matrix is controlled by the organic layeron the surface of the quantum dots. The integration of quantumdots with the polymer matrices is critical for the developmentof quantum dot-polymer photovoltaic electroluminescent de-vices and the quantum dot-polymer scintillators. Methods forintegrating the quantum dots with the polymer have been de-scribed in a recent review [9]. The integration of the quantumdots with the polymer can be accomplished by simple blendingof the quantum dots with the polymer [10], [11] or by blendingthe quantum dots with a polymer precursor followed by poly-merization [12], [13]. However, these methods require careful

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216 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 59, NO. 1, FEBRUARY 2012

attention to the compatibility of the quantum dot surface ligandswith bulk polymer since phase separation of the quantum dotstend to form agglomerates or larger clusters in the polymer.In this publication we present our work on the fabrication

of quantum dot-polymer composite materials for ionizing ra-diation detection and X-ray imaging. The samples are initiallycharacterized using optical emission spectra and time resolvedfluorescence measurements. The X-ray induced luminescencespectrum of solution phase quantum dot sample is presented.Solid state semiconductor materials have a long history as scin-tillation detectors for ionizing radiation. We describe the useof semiconductor quantum dot-organic polymer composites foruse as scintillation detectors and report the first use of quantumdot-polymer composite thin films for X-ray imaging.

II. FABRICATION OF QUANTUM DOT-POLYMER COMPOSITESAMPLES

Quantum dot samples were obtained from Phosphortech(Lithia Springs, GA). The quantum dots are prepared usingsolution phase synthesis methods to produce the semiconductornanocrystals with a narrow size distribution. The surfaces ofthe quantum dots are coated with an appropriate ligand to sta-bilize the quantum dots in solution and prevent agglomeration.Depending on the synthesis method, the final quantum dotsare produced in aqueous or non-aqueous solvent system. Thesolvent characteristics then impact the choice of compatiblepolymer matrix. We have prepared quantum dot-polymerthin film samples using both aqueous CdTe quantum dots inpolyvinyl alcohol (PVA) and non-aqueous CdSe quantum dotsin polystyrene. Thin films of CdTe quantum dots in polyvinylalcohol polymer are prepared by casting from an aqueoussolution that contains the water soluble quantum dots and thepolymer. A 65 micron thick sample is cast on a fiber opticwaveguide plate with a 25 mm diameter and 5 mm thickness.CdSe quantum dot doped polystyrene samples are prepared bymixing the styrene monomer with the quantum dot followedby thermal polymerization of the monomer using 2,2’-azobi-sisobutyronitrile (AIBN) initiator. The polymerization reactionproduces solid samples with 10 mm diameter and 5 mm thick-ness. The top and bottom surfaces of the solid disk samples arepolished for better light transmission. The phase separation ofthe quantum dots in the polymer can lead to agglomeration ofthe quantum dots and the formation of larger particles that arecomparable to or larger than the wavelength of visible light.This agglomeration creates scattering centers that reduce theoptical clarity of the samples.

III. QUANTUM DOT SOLUTION PHASE EMISSION SPECTRA

The emission spectra of the solution phase and solid sam-ples are recorded using the Perkin Elmer LS 50 B fluorescencespectrometer. The instrument has both an excitation monochro-mator and an emission monochromator with the capacity toselect the excitation and emission wavelength. The emissionspectra of the samples were recorded upon excitation at 405 nmby scanning the emission monochromator. The minimum slitwidth was used on both the excitation and emission monochro-mators and provides 2 nm resolution. In some cases the emis-

Fig. 1. Absorption and emission spectra of solution phase CdSe quantum dots.The figure shows the absorption spectrum of the CdSe quantum dots togetherwith the emission from 405 nm excitation (lower trace) and X-ray excitation(upper trace; offset for clarity).

sion intensity was reduced using inconel neutral density fil-ters to avoid saturating the detector. The absorption spectra aremeasured over the 300 nm to 800 nm spectral range using aCary 1E (Varian Associates) double beam spectrometer. Dis-tilled water is used in the reference channel for the aqueoussample measurements. X-ray induced emission spectra are mea-sured using a copper target X-ray generator (8 keV Cu line)available at RMD. The X-ray source is operated at 40 kV with20 mA current and the X-ray output is directly incident on thesample. The resulting X-ray induced emission passes through a0.2 meter McPherson model 234/302 monochromator and thewavelength resolved emission is detected using a photomulti-plier tube (RCA C31034).The absorption and emission spectra of the solution phase

cadmium selenide (CdSe) quantum dots are shown in Fig. 1.The characteristic peaks of the first and second exciton statesof the quantum dot are observed in long wavelength region ofthe absorption spectrum followed by the broad continuum ab-sorption at shorter wavelengths. There is overlap of the opticalemission with the long wavelength tail of the absorption. Thisoverlap will result in the reabsorption of the emitted photonswithin the sample and any non-radiative relaxation processeswill reduce the light output of the sample. This small Stokesshift will be one of the limitations on the thickness of the sampleand the concentration of the quantum dots The bottom trace ofFig. 1 shows the emission spectrum of the CdSe quantum dotswith 405 nm optical excitation while the upper trace (offset forclarity) shows the emission spectrum of the sample upon contin-uous X-ray excitation. The same emission is observed for bothexcitation sources. This shows that the emission is characteristicof the electronic structure of the quantum dot and is independentof the excitation mechanism. The measured signal intensity ofthe X-ray induced emission was lower than the 405 nm inducedemission and results in lower signal to noise levels. The differ-ence in the observed widths of the 405 nm and X-ray emissionspectra is attributed to the lowermonochromator resolution usedto record the X-ray emission.

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Fig. 2. Images of the 10% CdTe640-PVA and 35% CdTe-PVA thin film sam-ples on 1 inch diameter fiber optic waveguide plates (a) Ambient light illumi-nation (b) UV lamp illumination (c) X-ray illumination (d) X-ray illuminationof 10% CdTe640-PVA sample with PVA film.

IV. QUANTUMDOT-POLYMER COMPOSITE CHARACTERIZATION

The quantum dot polymer composite thin films are preparedby mixing water soluble polyvinyl alcohol (PVA) polymer withaqueous quantum dots. The thin film composite materials aremade using quantum dots that have an emission maximumat 640 nm in aqueous solution and are designated QD640.We attempt to maximize the light output from the sampleby increasing the concentration of the quantum dots whilemaintaining uniform emission intensity across the sample. Twoquantum dot thin film samples with 10 percent concentrationby weight and 35 percent concentration by weight QD640in PVA are shown in Fig. 2. The samples consist of 65 mi-cron thick film (10% quantum dot loading) and 72 micronthick film (35% quantum dot loading) on 2 mm thick fiberoptic waveguide plate substrates. Images of both samples arerecorded under different illumination conditions. Images ofthe two samples under room light illumination are shown inFig. 2(a). This image shows the impact of the agglomeration ofquantum dots at high concentration which degrades the opticalclarity of the samples. The 35% QD640 sample contains darkspots corresponding to agglomerates of quantum dots. The10% QD640 sample has better uniformity across the surface.Fig. 2(b) shows the image of the same samples recorded underUV illumination. The visual impact of the poor uniformityacross the surface is enhanced by the UV induced emission.Fig. 2(c) shows the image of the thin film samples recordedwith X-ray induced emission. The 10% QD640 sample hasbetter uniformity across the film surface than the 35% sample.The agglomerate regions of the 35% have the brightest emis-sion intensity and show that uniform high density quantum dotsamples will have high scintillation efficiency. Also, the brightregions observed in the X-ray excited sample are differentfrom the UV excited image which indicates the difference inthe excitation mechanism and fluorescence efficiency of thesample regions. Fig. 2(d) compares the emission intensity ofthe 65 micron thick CdTe640-PVA film with a free standing 75micron thick PVA film.

Fig. 3. Emission spectra of CdTe640 quantum dots with 405 nm excitation.Lower Trace. CdTe640 in water solution. Middle Trace. 10% CdTe in PVA film.Top Trace. 35% CdTe640 in PVA.

The emission wavelength of the quantum dots shifts to lowerenergy when the quantum dot is embedded in the polymer ma-trix. Fig. 3 shows the emission spectrum of the QD640 quantumdots in solution together with the 10% PVA film and the 35%PVA film with excitation at 405 nm. The spectra show a red shiftof 31 nm for the 10% sample and 42 nm for the 35% sample. Theorigin of this red shift is attributed to a resonant energy transfereffect that is demonstrated by the wavelength dependence of thefluorescence lifetime described in the following section.

V. FLUORESCENCE LIFETIME

The fluorescence lifetimes of the quantum dots in solutionand in the polymer matrix are less than one microsecond.The fast and bright emission intensity of the quantum dotscan be used for high frame rate X-ray imaging systems. Thefluorescence lifetime also provides information about the ex-cited state dynamics of the quantum dots. The time resolvedfluorescence of the solid phase and liquid phase samples wererecorded at discrete wavelengths across the spectral emissionband. The time resolved measurements were carried out usingtime correlated single photon counting methods (TCSPC).TCSPC uses the photon arrival statistics based on the timeinterval between the excitation of the sample and the detec-tion of a single photon from the sample to measure the timedependent emission intensity. The excitation source is a col-limated pulsed picosecond diode laser (PicoQuant; laser headLDH-P-C-400M; driver PDL 800-B) with a maximum poweroutput of 20 mW. The laser pulse frequency was controlled byan external pulse generator (HP 2012B) and was typically 100to 500 kHz. Wavelength selection was accomplished using ashort path 1/8 meter monochromator with 100 micron opticalfibers at the entrance and exit ports. The emission from thesample was directly coupled into the fiber optic by placing thefiber optic proximal to the excitation spot in solid samples orexcitation line in liquid samples. The light from the opticalfiber at the exit of the monochromator is coupled by a lens to aCMOS Geiger mode avalanche photodiode also referred to asa Single Photon Avalanche Diode or SPAD [14]–[17]. The lens

218 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 59, NO. 1, FEBRUARY 2012

Fig. 4. Laser induced time dependent fluorescence emission decay ofCdTe-640 quantum dots. A): Time dependent emission at 740 nm for CdTe-640in solution and PVA polymer. B): Time dependent emission at 640 nm forCdTe-640 in solution and PVA polymer. Traces are offset for clarity.

coupling of the light to the detector provides room to insertwavelength selection filters to block stray light including straylaser light. The photon counting detector is a single elementof a 16-element linear array of 30 micron diameter detectorson a 300 micron pitch. The detector was designed at RadiationMonitoring Devices and has been described and characterizedin a previous publication [18].Fig. 4 shows the wavelength resolved fluorescence lifetimes

measured for the CdTe640 quantum dots in solution and in thepolymer matrix with 405 nm excitation. The fluorescence life-time was measured at 20 nm intervals across the emission band.The figure shows the 640 nm and 740 nmmeasurements for boththe solution phase sample and the polymer matrix sample whereat the short wavelength, high energy edge and long wavelengthlow energy edge of the emission envelope. The figure illustratesthedramaticdifferencesinthephotoluminescencedecaybetweenthe solution phase quantum dots and the polymer matrix boundquantum dots. The fluorescence lifetimes of the solution phasequantum dots are well characterized by a single exponential life-time. The fluorescence lifetimes of the CdTe640 quantum dots inthe PVA matrix show very fast decay at higher energy and a risetime followed by a longer decay time at lower energy emission.Thiswavelengthdependent change in thefluorescencedecay ratehas been observed before in dense quantum dot films [17]–[21]and in quantum dot-polymer composites [23], [24]. The changein thefluorescent lifetimes is the result of Forster resonant energytransfer (FRET) between the quantum dots.The quantum dot sample is composed of a narrow size dis-

tribution of nanocrystals. However, this distribution still con-tains smaller quantum dots with higher energy states and largerquantum dots with lower energy states. When the quantum dotsare in solution, the individual nanoparticles are isolated by a sol-vent shell and do not interact with each other. The fluorescence

lifetime observed for the quantum dots in solution is charac-teristic of the isolated nanocrystals across the emission spec-trum as shown in Fig. 4. In contrast, the quantum dots in thepolyvinyl alcohol matrix demonstrate strikingly different fluo-rescence decay rates across the emission spectrum. The emis-sion from the smaller quantum dots is observed on the blue edgeof the emission band. The blue emitting quantum dots have afast decay rate which is a result of the radiative decay rate to-gether with the fast resonant energy transfer to larger, lower en-ergy quantum dots. The emission from the large quantum dots isobserved on the red edge of the emission band. The time depen-dence of the red emission shows the rise time in the emissionintensity due to the transfer from the small quantum dots fol-lowed by the relatively long radiative decay that approaches thesolution phase lifetime.The red shift in the emission of the quantum dot-polymer

composite relative to the solution phase emission is the resultof depletion of blue emission from the electronically excitedsmaller quantum dots and the subsequent increase in red emis-sion from electronically excited population of larger quantumdots. The energy transfer rate of the small, blue quantum dotscompetes with the radiative rate and reduces the emission inten-sity on the high energy side of the emission profile. The FRETmechanism shifts the excited state population to the low energy,red side of the emission. Crooker et al. [19] measured the timegated emission spectra of a solid phase quantum dot sample.These measurements showed that the early time emission spec-trum, which occurs before significant radiative transfer has oc-curred, are similar to the solution phase emission spectra. Thedelayed time emission spectrum is red shifted relative to the so-lution phase emission spectrum since there has been excitationtransfer from the smaller quantum dots to the larger, red emit-ting, quantum dots.

VI. EXCITON HARVESTING AND ENERGY TRANSFER WITHPPO

The details of the mechanism for the excitation of thequantum dots depend strongly on the incident ionizing radi-ation. However, in general, the incident radiation undergoesinelastic collisions with atoms in the bulk material that ejectcore electrons to produce hole states and electrons with highexcess kinetic energy. These primary charge carriers produceadditional secondary charge carriers by impact ionization of thebulk material. The subsequent cascade produces thermalizedelectrons and holes. These electrons and holes can localize onan individual quantum dot to form an exciton with subsequentradiative relaxation of the exciton and the emission of light. Aspart of this program we have investigated ways to increase theemission intensity of the quantum dot-polymer composite filmsby increasing the trapping efficiency of the electrons and holesgenerated by the incident radiation.PPO (2,5-diphenyl oxazole)is commonly used as an organic

scintillator in which the free charge carriers produced by in-cident ionizing radiation generates electronically excited PPOmolecules. The PPO can radiatively relax to produce light at340 nm. PPO is often combined with 1–4,bis-2-(5-phenyloxa-zolyly)-benzene (POPOP) to shift the emission wavelength to400 nm where the electronically excited POPOP is produced by

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non-radiative energy transfer from PPO to POPOP. When thequantum dot sample is mixed with PPO, the quantum dots canbe excited by both the exciton trapping of the electron hole pair,and by non-radiative energy transfer from PPO to the quantumdot.We demonstrate this mechanism by measuring the X-ray

induced luminescence from polystyrene samples that containquantum dots, PPO, or quantum dots with PPO. We usedCdSe/ZnS core—shell quantum dots with 607 nm emis-sion(QD607)and prepared a set of four samples that include asample with 0.2% PPO and 0.2% quantum dot in polystyrene,a sample with 0.2% PPO in polystyrene, a sample with 0.2%quantum dots, and a blank polystyrene sample. Solid disk sam-ples are cast with a 1 cm diameter and the ends of the sampleare polished to produce a 3 mm thick disk. These samples haverelatively low (0.2%) concentration of the PPO and quantumdots. Samples with high concentrations of quantum dots werenot optically clear and had reduced light transmission due toscattering within the sample. The scattering is attributed tothe formation of quantum dot agglomerates with sizes thatare comparable to the wavelength of visible light and fromscattering centers. Samples prepared with high concentrationsof PPO were soft and flexible, but with poor optical quality. ThePPO presumably inhibited the polymerization of the styrenemonomer and can also act as plasticizer to reduce the samplehardness. These soft samples could not be polished to get ad-equate optical quality. These factors limited the concentrationof quantum dots and PPO in the sample. Since the scintillationintensity of the quantum dots and the energy transfer efficiencyfrom the PPO to the quantum dots both depend on concentra-tion, the measured signal intensities were low. However thesemeasurements can be used to demonstrate scintillation andenergy transfer in these materials. Fig. 5 shows a photographof the samples recorded under ambient lighting conditions,together with the image of all of the samples under X-rayexcitation. The emission from the samples that contain onlyPPO or only quantum dot are not significantly brighter thanthe blank polystyrene. In contrast, the sample with the PPOand quantum dot is brighter than the sample of the quantumdot alone or the PPO alone. The scintillation intensities in thebright region of the samples are 5 digital units (D.U.) for PST,4 D.U. for PST/PPO, 7 D.U. for PST/QDot, and 23 D.U. forPST/PPO/QDot. The intensity of the PST/PPo/QDot sampleis higher than the sum of the PST/PPO and PST/QDot anddemonstrates a cooperative effect of the PPO with the quantumdots that produces enhanced emission intensity. This effectcan be attributed to efficient capture of the energy depositedinto the sample by the PPO with subsequent energy transferto the quantum dots. This process is expected to be stronglydependent of the mean separation of the PPO and the quantumdot and should be more efficient at higher concentrations. How-ever, as described previously, the quantum dot-polymer andPPO-polymer interactions limited the loading of the sample.

VII. ALPHA INDUCED EMISSION

We have measured the alpha particle induced emission fromthe set of PPO/quantum dot/polymer samples in order to demon-strate the role of PPO on the efficiency of the exciton harvesting

Fig. 5. Images of polystyrene (PST), polystyrene/PPO, Polystyrene/QuantumDot (PST/Qdot), and Polystyrene, PPO, and quantum dot samples. Top: Imagerecorded under ambient light illumination. Bottom: Relative X-ray induced flu-orescence intensity.

and subsequent quantum dot emission. We measured the alphaparticle induced emission from the four samples described inthe previous section (0.2% PPO, 0.2% QD607 quantum dot,0.2% PPO/0.2% QD607 quantum dots, and a blank polysty-rene sample). The relative light output of each of the sampleswas measured upon exposure to alpha particles. Since the objec-tive of these measurements is to demonstrate the energy transferfrom the PPO to the quantum dot we have used an optical filterto block the 340 nm emission of the PPO. For each measure-ment, one of the polystyrene samples shown in Fig. 6 is placedon a 540 nm long wave pass optical filter sitting directly abovea XP2203 PMT (Photonis). The americium source that pro-duces 5.486 MeV alpha particles is placed on the flat top ofthe disk. The sample, source, and PMT window are sealed ina light tight housing. A multichannel analyzer is used to recordthe light output from each sample. The low quantum dot con-centration limits the efficiency of the energy transfer from thePPO to quantum dot. Therefore, if no optical filter is used thePPO doped polystyrene sample has the highest light output. A540 nm long wave pass filter is used to block the direct emis-sion of the PPO and emphasize the emission from the quantum

220 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 59, NO. 1, FEBRUARY 2012

Fig. 6. Energy spectra of Am alpha particles recorded using the quantum dot-PPO-polymer composite samples shown in Fig. 6. Sample composition as noted.The trace intensities are normalized by peak height and offset by 0.4 units each.

dots with an emissionmaximum at 607 nm. The results recordedusing the long wave pass filter are shown in Fig. 6.The polystyrene sample that contains both the PPO and the

QD607 quantum dot produces the brightest scintillation emis-sion intensity with the incident alpha particles. The poly-styrene with PPO and polystyrene with QD607 both producescintillation emission above the blank polystyrene sample. Ex-citation of the PST/PPO and PST/QDot samples by the inci-dent alpha particles shows less emission than the excitation ofthe PST/PPO/QDot sample. In the PST/PPO/QDot sample theemission is observed from the quantum dot which can excitedby two different pathways and act at as either a primary scintil-lator or a wavelength shifter. Direct excitation of the quantumdot by the incident alpha particle results in scintillation. Excita-tion of the PPO by the incident alpha particle followed by energytransfer to the quantum dot results in wavelength shifting. Thepolymer composite with the combination of PPO and quantumdots produces the brightest emission in the emission band withwavelengths longer than 540 nm.

VIII. X-RAY IMAGING

The quantum dot-polymer composite thin films are used todemonstrate X-ray imaging using semiconductor nanocrystalscintillators. Thin films of the 10% CdTe640 quantum dots inpolyvinyl alcohol were cast onto fiber optic waveguide plates

Fig. 7. X-ray imaging using the thin film CdTe-640 polymer composite scintil-lator. The image shows the line pair phantom and four X-ray images of differentquarters of the line pair phantom.

to produce a 65-micron thick film. The quantum dot-polymerfilm and substrate are coupled to the detector array usinga permanently bonded 3 to 1 fiber optic taper. The imagesare recorded using a thermoelectrically cooled PhotometricsCE200A CCD camera that has a 512 512 detector array with19 micron pixels. The phantoms to be imaged were placeddirectly above the quantum dot loaded polymer film in the pathof the X-ray source. The excitation source for the images isa Gendex X-ray source operating at 15 mA with 60 kVp. Wehave used the quantum dot – polymer composite thin films todemonstrate the first example of X-ray imaging using semi-conductor nanocrystal scintillators. The 10% CdTe640 filmhad the best uniformity with high quantum dot concentration.This sample was used to record images of a line pair phantomshown in Fig. 7. The active area of the scintillating quantumdot film was 25 mm in diameter, which is smaller than the linepair phantom. Multiple images of the phantom were recordedwith the thin film sample at different regions of the phantom.The four X-ray images of the line pair phantom are shown inFig. 7. The lines per mm for each section are shown outside theimages. We observe a 5% contrast modulation at 5 line pairsper mm.

IX. CONCLUSIONS

We have demonstrated the fabrication of quantum dot-or-ganic polymer thin film composites that retain optical efficiencyof the quantum dots and high optical clarity of the polymer.These films show X-ray induced luminescence at the charac-teristic quantum dot emission wavelengths that can be used forscintillation detection. The intensity of the scintillation lightoutput scales with the quantum dot concentration. The time re-solved fluorescence measurements show the fast radiative re-laxation of the quantum dots and indicate that these scintillationmaterials can be used for high frame rate imaging. The thin scin-tillation films are used to make the first X-ray images recordedusing colloidal quantum dot scintillators. PPO can be used to

LAWRENCE et al.: QUANTUM DOT-ORGANIC POLYMER COMPOSITE MATERIALS 221

increase the emission intensity of the quantum dots upon exci-tation with ionizing radiation.

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