Nanoscale Colloidal Quantum Dots Providing Innovative Solutions for Evolving Imaging System Applications
Providing Innovative Solutions for Evolving Imaging System Applications
A focal plane array (FPA) is the modern day “film” of an imaging system. Photons emitted or reflected by a scene are collected by the camera optics and imaged onto the FPA. The FPA is composed of two components: the detector array and the readout integrated circuit (ROIC). The detector array contains thousands to millions of detector elements. Through a hybrid circuit manufacturing process, each of the detector elements in the detector array is connected electrically and mechanically to a companion unit cell circuit on the ROIC by an indium bump interconnection. The detector elements produce photocurrents that travel through the indium interconnects into ROIC unit cell circuitry, where the photocurrent is integrated and stored for subsequent readout via a multiplexer.
Detector characteristics usually limit operation to a certain band of photon wavelengths or spectral region. For example, to detect two widely separated bands (such as infrared [IR] and ultraviolet [UV]) two sets of FPAs are typically required, more than doubling the cost to the end-user. The FPA manufacturing process is also expensive, requiring extensive capital equipment not only to fabricate the small indium interconnects on each array, but also to carefully align and press the hybrid circuit layers together. Quantum dots offer a potential technology solution to mitigate both of these cost drivers.
Quantum dots are minute semiconductor crystals, typically a few nanometers (nm) in size. At these small dimensions, the physical extent of the quantum dot becomes smaller than the natural size of an electron-hole pair, and an effect called quantum confinement occurs. This can favorably change the optical properties that are governed by the size of the quantum dot. For example, by adjusting only the size of the quantum dot one can fine tune the photon absorption or emission spectra without requiring a complicated change of semiconducting material composition or stoichiometry. In addition, the photon absorption can be greater in quantum-confined nanocrystals, thus making for a more efficient detector.
Quantum dots are typically produced using one of two processes. They are either grown with molecular beam epitaxy onto a crystal substrate, or they are synthesized with solution-based chemistry, producing a nanocrystal colloidal suspension. The synthesis and application of colloidal quantum dots (CQDs) are discussed in the remainder of this article.
Figure1 shows the structure of a CQD; however, they need not be only spherical. Disks, rods and other structures have been produced by means of colloidal quantum dot processing. This example uses lead sulfide (PbS) CQDs that are made by injecting lead and sulfur organo-metallic precursors into a flask held within an inert atmosphere. Precise temperature and solvent combinations control the size of the CQDs. The CQDs are often encased in a semiconducting shell to protect and passivate the surface. Surface groups, such as ligands*, are attached to the shell and allow the CQDs to be suspended in solution. This solution-suspended state permits convenient deposition onto a variety of surfaces. Deposition techniques include drop casting, jet printing, stamping and spin casting to produce novel devices on a variety of substrates.
As shown in Figure 2, varying the CQD size modifies its physical properties, such as the bandgap, while using the same CQD material. In contrast, significantly changing the bandgap of a bulk semiconducting material typically requires a different chemical composition or stoichiometry. The left side of the figure shows that as the CQDs get smaller, the energy gap becomes larger, with discrete conduction and valence band levels. This means that photon absorption versus photon energy is no longer continuous, as in the bulk semiconductor case. Instead, the spectra are divided into a discrete set of levels that have enhanced absorption at each resonance. Similarly, photon emission depends on CQD size, which is demonstrated by using ultraviolet light to excite different size CQDs contained in a set of vials (right side of figure). The CQDs fluoresce with different colors and the smaller the CQD, the more blue-shifted is the emission. Figure 2 demonstrates this effect for CdSe dots ranging in size from 2 to 8 nm in diameter.
Quantum Dot Applications
Raytheon is developing quantum dot applications in collaboration with Prof. Moungi Bawendi’s Group at the Massachusetts Institute of Technology (MIT). The Institute of Soldier Nanotechnology (ISN), an Army University Affiliated Research Center (UARC), provides funding for this joint activity.1,2 These applications include not only short wavelength infrared (SWIR, nominally defined as the 1–2 micron spectral range) sensitive surfaces for use in FPAs, but also emitting surfaces for use in micro-displays, communications and dual-band detection devices.
Since CQDs can downshift the frequency of photons from the UV (nominally 200–400 nm) to the visible or SWIR region, a layer of CQDs on the detector transforms a SWIR FPA into a combination UV/SWIR FPA. So in addition to the usual SWIR imaging, the camera now has the ability to image in the UV for applications in UV optical communications (line-of-sight and non-line-of sight), in detection of covertly placed UV tags, in UV biometrics and in UV muzzle flash detection. In this way the technology increases the spectral range for broadband imaging applications.
Quantum Dot Imaging Technology
CQD enhanced imagers can take advantage of both direct detection photocurrent generation and the frequency downshifting properties of CQDs.
Figure 3 illustrates how Raytheon employs the photocurrent properties of CQDs. The upper illustration depicts conventional technology that uses a hybridization technique. The lower illustration (cross section and top views) shows the CQDs deposited on the top over-glass layer of a CMOS (complementary metal oxide semiconductor) ROIC that has been post-processed with a gold grid structure. The CQDs are photo-active in the annular sections of the grid and are surrounded by the gold electrodes. The CQDs absorb photons and produce photocurrents, which are injected via the gold electrodes into the underlying CMOS circuitry for further processing.
The CQD detecting film is applied by spin coating onto a packaged ROIC. The ROIC itself forms the detector substrate, so no hybridization is required. The conventional hybrid technology shown above requires several layers of processing with many more interfaces and interconnections, which increase the fabrication cost.
Figure 4 shows how CQD technology can significantly reduce complexity, cycle time, size, weight, power and cost by employing its downshifting properties. CQDs are deposited onto the detector surface of a conventional hybrid focal plane array. The CQDs once again absorb photons; but instead of producing a photocurrent, they emit longer-wavelength (red-shifted) photons that are subsequently detected by the FPA. This extends the detection band of the imager without the need for a second FPA. The middle section of Figure 4 shows the conventional technology, and the lower section depicts the reduction in complexity through the use of CQDs.
Figure 5 illustrates an imaging setup for evaluating the performance of directly deposited CQDs onto ROICs. Separate quad structures with circular depositions of CQDs on three of the four quadrants of the ROIC allow testing of different CQD materials. Also shown is the image formed by an infrared light beam focused by the camera onto the CQD focal plane.
The image shown in Figure 5 demonstrates that an IR focal plane can be produced by simply drop casting CQD material onto an appropriately modified silicon ROIC. Visible focal planes can be made in a similar fashion. It is also possible to extend the spectral response of the resultant FPA device beyond the visible range into the SWIR spectral band consistent with the spectral characteristics of the CQDs. While this nascent CQD FPA technology does not currently match the performance of state-of-the-art SWIR FPAs, it does significantly reduce complexity and, therefore, cost. This has the potential to address more cost-sensitive applications in which the higher level of performance is not required. For example, one application may be the formation of non-planar conformal focal plane arrays as opposed to the current rigid, rectangular structures. Another example is to replace a few expensive systems that monitor a field of regard, with numerous, less expensive networked systems that can monitor and expand the field of regard with system redundancy.
We have also demonstrated the downshifting capability by depositing CQDs on a quartz plate that is displaced off-focal plane. The CQDs absorb in the UV and visible bands and emit at ~1,300 nm in the SWIR region. Using a re-imaging technique that focuses the CQD emissions from the downshifting plate onto an InGaAs (indium gallium arsenide) FPA, it was possible to extend the spectral response of this FPA into the UV region.
Raytheon is working with the MIT Bawendi Group on several CQD applications, including biomedical imaging research. By functionalizing** the CQD surface groups, they can be designed to stick to stem cells, tumors and other biological structures.3 This allows for direct imaging and monitoring of medical and biological processes using Raytheon SWIR cameras set to detect certain tissue transparency bands. Recently this was demonstrated at MIT by imaging a mouse liver. The liver collects the functionalized CQDs that were injected into the blood stream. These CQDs were irradiated, and the SWIR downshifted emissions were imaged with a SWIR camera using one of the SWIR transparency bands of the epidermal tissue.
CQDs can be used to modify the capabilities of conventional FPA imaging devices. As the technology matures, CQDs will not only enable more cost-effective defense products, but they will also help drive new products and innovations in both adjacent sciences and commercial marketplaces.
* Ligands – Ions or molecule that bind to transition metal ions to form complex ions.
** Functionalizing – Adding new features by altering the surface chemistry.
1Geyer, Scott, Ph.D. Dissertation: Science and Applications of Infrared Semiconductor Nanocrystals, MIT Chemistry Dept- Bawendi Group, 2010.
2Efficient Luminescent Down-shifting Detectors Based on Colloidal Quantum Dots for Dual-Band Detection Applications, May 2011, ACS Nano (American Chemical Society).
3W. Liu, et al. Compact Biocompatible Quantum Dots Functionalized for Cellular Imaging, JACS, Jan. 5, 2008.