26/11/2005 Manchester, United Kingdom
German periodical Advanced Materials publishes research by Los Alamos researchers involving Nanoco quantum dots… Figures are found in PDF Full Documentation
BY IAN H. CAMPBELL* AND BRIAN K. CRONE
Organic scintillators are widely used in radiation-detection applications due to their low cost, ease of fabrication, and fast response times. Their ionization energy is about 60 eV/photon and is nonlinear with particle energy for strongly ionizing radiation, such as alpha particles. Because of their large and nonlinear ionization energy, they are unsuitable for applications that require high energy resolution or detection of strongly ionizing particles. In contrast, inorganic semiconductors have ionization energies of about three times their energy gap (e.g., Si requires 3.6 eV per electron–hole pair), and they respond linearly to strongly ionizing particles. The ionization energy of a semiconductor quantum dot (qdot) is expected to be less than the bulk material and may approach the energy gap of the quantum dot.[2,3] Here, we propose a new class of radiation-detection materials, composites of inorganic semiconductor quantum dots and organic semiconductors, that possess the cost and processing advantages of organic scintillators and the ionization characteristics of inorganic semiconductors.
The qdot/organic semiconductor composite is designed so that ionizing radiation produces excitations predominantly in the inorganic semiconductor qdots, and these excitations are subsequently Förster-transferred to the organic material. Depending upon the application, the Förster-excited organic material(s) are chosen either to emit a Stokes-shifted photon or to dissociate the excitation and produce mobile charges. For scintillators, the composite material must be transparent to the emitted photon, and the large Stokes shift of the organic material is essential. Förster transfer can occur on subnanosecond timescales, so the fast response times of organic scintillators can be preserved. For charge-collection devices, the composite material must be trap free to allow efficient charge collection. Trap-free, conjugated organic materials are now available. Pure qdot solids are impractical radiation-detection materials because they are not transparent to their emission wavelength and have significant charge-carrier trapping.[ 6] However, qdot/organic semiconductor composites can be designed that have promising optical and electrical transport characteristics.[7,8] Gamma-ray, neutron, and charged-particle detection involve measuring the energy deposited by electrons or charged particles produced by the incident radiation. The deposited energy is measured by counting photons or mobile charges produced in the detecting material. The electron or charged-particle energy is transferred to the electrons of the detection material via Coulomb interactions that scale with electron density. Because inorganic qdots have a higher density than organic materials, most of the energy is deposited in the qdots for qdot volume fractions above about 0.15.
Here, we investigate a composite scintillator that uses a luminescent polymer as the organic semiconductor host.We first present optical properties of the composite that demonstrate Förster excitation transfer from the qdot to the polymer. We then demonstrate improved scintillation performance under electron-beam excitation using the cathodoluminescence (CL) attachment of a scanning electron microscope. These scintillation results are then discussed in the context of a simple model for the composite-material detector performance.
The composite detector materials were fabricated from two readily available constituents: CdSe–ZnSe core–shell qdots surface-passivated with hexadecylamine (HDA) and the soluble, conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)- p-phenylene vinylene] (MEH-PPV). The composite films were spin-cast from qdot/polymer solutions in chloroform.
Four qdot volume fractions were considered: 0, 0.07, 0.15, and 0.21. These volume fractions are of the inorganic core of the qdot; the HDA passivation is included in the organic volume fraction. For optical measurements, thin films were spin-cast on sapphire substrates and, for CL measurements, they were cast on gold-coated sapphire substrates.
Figure 1 shows the absorbance spectra of a pure and a 0.15 qdot-doped MEH-PPV film, and the photoluminescence (PL) spectra of a dilute qdot solution, and a pure and a 0.15 qdot-doped MEH-PPV film. The absorbance of MEHPPV peaks at about 500 nm and has a full width at half maximum (FWHM) of about 100 nm. The addition of qdots does not significantly alter the absorbance in this spectral region, because the absorption is weighted by volume fraction and not density, and because the qdot does not absorb strongly in this region. The qdot PL spectrum peaks at about 550 nm and has an FWHM of 30 nm. The emission spectrum of the qdots overlaps the absorption spectrum of MEH-PPV, so Förster energy transfer processes can occur. The PL spectra of pure and 0.15 qdot-doped MEH-PPV are also very similar. The peak is at about 580 nm, and the FWHM is 80–90 nm. The spectrum of the qdot-doped film has no significant contribution from qdot luminescence for excitation wavelengths down to 360 nm (the shortest available). At 360 nm, the optical absorbance is about half from the polymer and half from the qdots. This demonstrates that the Förster transfer process is occurring and that emission is from the polymer alone. The emission spectrum of the doped polymer is broader and has more intensity at longer wavelengths. These small changes are likely produced by disorder in the polymer chains induced by the qdots. Figure 2 shows the normalized CL signal from four samples with varying qdot fractions. The spectrally integrated CL was measured using 3 keV electrons at a current of 30 pA in all cases. The CL signal is normalized to that of pure MEH-PPV (qdot volume fraction = 0). The CL signal increases with qdot volume fraction for the first two samples but decreases to less than the pure MEH-PPV signal for the 0.21 qdot-doped sample.
The CL signal from the 0.15 qdot-doped sample is more than twice as large as that from pure MEH-PPV. The film thickness was approximately 1 lm in all cases. The maximum electron-penetration depth (polymer only) at 3 keV is less than 0.5 lm, and thus the electron beam was stopped within the film in all cases. The luminescence yield is not corrected for the change in electron backscattering due to the changing composition of the composite. This is at most a 10% correction and increases the normalized yield of the qdot-doped samples because more electrons are backscattered from the high-atomic-number qdots. The luminescence yield is also not corrected for the increase in optical index of refraction of the sample with increasing qdot concentration. The index at the emission wavelength increases from about 1.7 to 2.0 for the 0.21 qdot-fraction sample. This decreases the amount of light escaping the material by about 5%.
The ideal ionization energy of the composite is determined from the density-weighted contribution from each component, i.e.,
1/I = q1V1/I1 + q2V2/I
q1V1 + q2V2
(1) where I is the ionization energy, q is the density, V is the volume fraction, and the subscripts refer to the components of the composite. The ideal ionization energy and normalized
CL intensity is shown in Figure 3. The CL is the inverse of the ionization energy normalized to a qdot volume fraction of zero. The calculation used ionization energies and densities, respectively, of 3 eV/photon and 5.8 g cm–3 for the qdots and 40 eV/photon and 1.1 g cm–3 for the polymer. These estimates show that the performance of an organic scintillator can be improved by about an order of magnitude and that most of the improvement is obtained for qdot volume fractions less than 0.4.
Figure 3 implies that we could expect an improvement of about a factor of eight in the normalized CL measurements shown in Figure 2. The measured CL intensities are lower than expected and decrease sharply for the 0.21 qdot-doped sample. The decreased intensity is most likely due to phase separation of the qdots and polymer, which was observed in the SEM images of the 0.21 qdot-doped sample. In order for the composite to perform ideally, each qdot should be surrounded by polymer. If they are not, then Förster transfer can occur between qdots, making non-radiative recombination processes more significant. The PL quantum efficiency of the qdots used here was about 25 %, so non-radiative processes become more important with Förster transfer between qdots.
Although not implied by these results, these composites also have the potential to be used for fast neutron energy resolution. The nanoscale mixture of organic and inorganic components should allow the energy from proton recoils off the organic matrix to be efficiently and linearly converted to charge by the inorganic semiconductor qdots. Quantum dots in inorganic sol–gel matrices have recently been investigated for neutron detection. We have presented initial electron-excited scintillation results of quantum-dot/organic semiconductor composites that are promising for radiation-detection applications. The incorporation of inorganic semiconductor quantum dots has the potential to improve the light output of organic scintillators by more than one order of magnitude and make them suitable for high-resolution spectroscopy applications. These composite materials have advantages for gamma-ray, neutron, and charged-particle detection. They have processing characteristics similar to organic molecules and polymers and thus are promising for low-cost, large-area/volume applications.
The CdSe/ZnSe quantum dots were purchased from Nanoco Technologies, and MEH-PPV (ADS100RE) was purchased from American Dye Source. Both materials were obtained as powders and used as received. The CdSe/ZnSe quantum dots were approximately 2.6 nm in diameter. MEH-PPV had a molecular weight of 140 000 (polystyrene standards) and a polydispersity of 3.91. The materials were loaded into a glovebox (Vacuum Atmospheres) containing an argon atmosphere with typically < 1 ppm of oxygen and water. The materials were dissolved in dry, deoxygenated chloroform. The solutions were placed in sealed vials and heated and stirred at about 50 °C for at least 12 h. For optical measurements, the composite solutions were spin-cast onto sapphire disks. The absorption spectra were recorded using a Hewlett–Packard 8452A diode-array spectrophotometer. The photoluminescence spectra were recorded using two SPEX 270M spectrometers. One spectrometer was used with a 100 W lamp to provide variable wavelength excitation, and the other spectrometer was scanned to record the photoluminescence as a function of wavelength. For the scanning electron microscopy (SEM) measurements, the composite solutions were spin-cast onto sapphire disks that had been previously coated with 5 nm of titanium followed by 200 nm of gold to provide a conductive path for the electron-beam current. After spin-casting in the argon-atmosphere glovebox, the samples were loaded into a vacuum-tight container and transported to the scanning electron microscope. The samples were exposed to air for a few minutes while being loaded into the microscope sample chamber. The cathodoluminescence measurements were performed using a JEOL 5800LV SEM instrument with an Oxford Instruments cathodoluminescence attachment. The cathodoluminescence was collected using a parabolic mirror and detected with a photomultiplier tube.
Received: July 13, 2005
Final version: September 14, 2005
Published online: November 21, 2005
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