- Email: [email protected]
Fibers rival lens-based measuring systems
RESEARCH NEWS
Solution-cast photodetectors outclass semiconductors ELECTRONIC MATERIALS
University of Toronto researchers have fabricated a solution-processed infrared photodetector with sensitivity superior to the best epitaxially grown device [Konstantatos et al., Nature (2006) 442, 180]. Their findings indicate the potential of solution-cast electronics and optoelectronics as a viable alternative to conventional technologies. Solution-grown electronics and optoelectronics offer clear-cut advantages over epitaxially grown crystalline semiconductor devices. The ‘wet’ route to manufacture is less expensive, easier to apply over large device areas, and typically poses fewer problems with materials compatibility. However, solution-cast devices need to match the performance of traditional devices if they are to be anything other than niche products. Photodetection is a particularly promising area for non-Si technology. Near infrared (NIR) and
short-wavelength infrared (SWIR) photodetection (beyond 1 µm) is relevant to applications such as night surveillance, security cameras, and in vivo biological imaging. But the photosensitive properties of Si deteriorate beyond 800 nm and end abruptly at 1.1 µm. Gerasimos Konstantatos and colleagues fabricated their novel photodetector devices on glass slides patterned with 100 nm high Au electrodes at 5 µm intervals. They spin-coated the planar electrode arrays with PbS quantum dots in chloroform solution. Evaporation of the solvent leaves a smooth semiconductor film on the glass consisting of a layer of colloidal nanocrystals 800 nm thick. The most sensitive infrared photodetectors exhibited a D* value, the normalized detectivity, of 1.8 x 1013 jones (1 jones = 1 cm Hz1/2 W-1) at 1.3 µm and room temperature. This compares favorably with high-performance, epitaxially
grown InGaAs photovoltaic devices that have a peak D* at room temperature of ~1012 jones. Little additional work would be needed to scaleup the approach for mass manufacture, says Ted Sargent. The team can already produce enough material to coat 1 m2 in a single synthesis. “For photodetectors and imaging applications, this is a massive quantity,” he says. However, realizing the devices’ potential still poses a challenge. “Photoconductive gain is a mixed blessing,” says Sargent. “On one hand, it has allowed us to show superior sensitivity measured through the normalized detectivity, D*. On the other, it is accompanied by a generally greater current density flow. Circuits in imaging chips can be designed to ‘tame’ photoconductive gain, but they rely on creativity on the part of the pixel-by-pixel read-out scheme designer.” Paula Gould
Fibers rival lens-based measuring systems OPTICAL MATERIALS Optical-field measurements generally require a number of carefully-arranged lenses, filters, beam splitters, and detector arrays. Measurements are consequently limited by the equipment’s size, durability, weight, and field-of-view. Researchers from Massachusetts Institute of Technology (MIT) have now revealed a radically different – and less constrained – way to acquire optical information based on polymeric light-sensitive fibers [Abouraddy et al., Nat. Mater. (2006) 313, 186]. The MIT team abandoned linear lens-based optical systems in favor of web-like photodetecting arrays with a closed sphere geometry. The spherical arrays comprise polysulfone-clad fibers with a Sn-doped chalcogenide glass photoconductive core. Sandwiched between the glass core and polymer cladding are four metal (Sn-5% Ag) electrodes running the length of the fibers. The resulting 1 mm thick fibers are lightweight, mechanically tough, flexible, and can be manufactured in arbitrarily long lengths. Any light reaching the fiber surface – from whatever angle – is detected as a change to the current flowing in an external circuit. Combining several fibers into an array permits the angle of incidence of the light to be determined from the distribution of detected signals. “When you’re looking at something with your eyes, there’s a particular direction you’re looking in,”
explains Ayman F. Abouraddy. “The field of view is defined around that direction. Depending on the lens, you may be able to capture a certain field of view around that direction, but that’s it. Until now, almost every optical system was limited by an optical axis or direction.” The MIT team has also experimented with planar fiber webs. They found that a single 32 x 32 array could measure the intensity of an incident optical field and, hence, generate rough images of lit objects placed nearby. Putting two such arrays in parallel enables the amplitude and phase information of incident patterns to be determined. The detail of images generated could be improved by reducing the fiber diameter, and packing more photodetecting strands into arrays, say the researchers. This is unlikely to pose any practical difficulties. For example, a 40 mm diameter, 30 cm long perform can be drawn into a 48 km long fiber with a 100 µm diameter. Thinner fibers could also be used to make giant-sized webs. The weight of such a 15 x 15 m2 construct with a total photosensitive area of 4.5 m2 would be just 700 g. This is negligible when compared with the weight of traditional optical instruments, note the researchers. Fibers sensitive to sound, heat, chemical contaminants, or other wavelengths in the electromagnetic spectrum could also be produced if
The spherical photodetecting array. Inset: scanning electron micrograph of the fiber crosssection, showing the glass core, metal electrodes, and polymer cladding. (Courtesy Yoel Fink, MIT.) the composition of the glass core was altered, the researchers suggest. Webs constructed of such fibres will yield ‘images’ in these parameter spaces, and can be said to see, hear, sense, and smell.
Paula Gould
SEPTEMBER 2006 | VOLUME 9 | NUMBER 9
13
Solution-cast photodetectors outclass semiconductors ELECTRONIC MATERIALS
University of Toronto researchers have fabricated a solution-processed infrared photodetector with sensitivity superior to the best epitaxially grown device [Konstantatos et al., Nature (2006) 442, 180]. Their findings indicate the potential of solution-cast electronics and optoelectronics as a viable alternative to conventional technologies. Solution-grown electronics and optoelectronics offer clear-cut advantages over epitaxially grown crystalline semiconductor devices. The ‘wet’ route to manufacture is less expensive, easier to apply over large device areas, and typically poses fewer problems with materials compatibility. However, solution-cast devices need to match the performance of traditional devices if they are to be anything other than niche products. Photodetection is a particularly promising area for non-Si technology. Near infrared (NIR) and
short-wavelength infrared (SWIR) photodetection (beyond 1 µm) is relevant to applications such as night surveillance, security cameras, and in vivo biological imaging. But the photosensitive properties of Si deteriorate beyond 800 nm and end abruptly at 1.1 µm. Gerasimos Konstantatos and colleagues fabricated their novel photodetector devices on glass slides patterned with 100 nm high Au electrodes at 5 µm intervals. They spin-coated the planar electrode arrays with PbS quantum dots in chloroform solution. Evaporation of the solvent leaves a smooth semiconductor film on the glass consisting of a layer of colloidal nanocrystals 800 nm thick. The most sensitive infrared photodetectors exhibited a D* value, the normalized detectivity, of 1.8 x 1013 jones (1 jones = 1 cm Hz1/2 W-1) at 1.3 µm and room temperature. This compares favorably with high-performance, epitaxially
grown InGaAs photovoltaic devices that have a peak D* at room temperature of ~1012 jones. Little additional work would be needed to scaleup the approach for mass manufacture, says Ted Sargent. The team can already produce enough material to coat 1 m2 in a single synthesis. “For photodetectors and imaging applications, this is a massive quantity,” he says. However, realizing the devices’ potential still poses a challenge. “Photoconductive gain is a mixed blessing,” says Sargent. “On one hand, it has allowed us to show superior sensitivity measured through the normalized detectivity, D*. On the other, it is accompanied by a generally greater current density flow. Circuits in imaging chips can be designed to ‘tame’ photoconductive gain, but they rely on creativity on the part of the pixel-by-pixel read-out scheme designer.” Paula Gould
Fibers rival lens-based measuring systems OPTICAL MATERIALS Optical-field measurements generally require a number of carefully-arranged lenses, filters, beam splitters, and detector arrays. Measurements are consequently limited by the equipment’s size, durability, weight, and field-of-view. Researchers from Massachusetts Institute of Technology (MIT) have now revealed a radically different – and less constrained – way to acquire optical information based on polymeric light-sensitive fibers [Abouraddy et al., Nat. Mater. (2006) 313, 186]. The MIT team abandoned linear lens-based optical systems in favor of web-like photodetecting arrays with a closed sphere geometry. The spherical arrays comprise polysulfone-clad fibers with a Sn-doped chalcogenide glass photoconductive core. Sandwiched between the glass core and polymer cladding are four metal (Sn-5% Ag) electrodes running the length of the fibers. The resulting 1 mm thick fibers are lightweight, mechanically tough, flexible, and can be manufactured in arbitrarily long lengths. Any light reaching the fiber surface – from whatever angle – is detected as a change to the current flowing in an external circuit. Combining several fibers into an array permits the angle of incidence of the light to be determined from the distribution of detected signals. “When you’re looking at something with your eyes, there’s a particular direction you’re looking in,”
explains Ayman F. Abouraddy. “The field of view is defined around that direction. Depending on the lens, you may be able to capture a certain field of view around that direction, but that’s it. Until now, almost every optical system was limited by an optical axis or direction.” The MIT team has also experimented with planar fiber webs. They found that a single 32 x 32 array could measure the intensity of an incident optical field and, hence, generate rough images of lit objects placed nearby. Putting two such arrays in parallel enables the amplitude and phase information of incident patterns to be determined. The detail of images generated could be improved by reducing the fiber diameter, and packing more photodetecting strands into arrays, say the researchers. This is unlikely to pose any practical difficulties. For example, a 40 mm diameter, 30 cm long perform can be drawn into a 48 km long fiber with a 100 µm diameter. Thinner fibers could also be used to make giant-sized webs. The weight of such a 15 x 15 m2 construct with a total photosensitive area of 4.5 m2 would be just 700 g. This is negligible when compared with the weight of traditional optical instruments, note the researchers. Fibers sensitive to sound, heat, chemical contaminants, or other wavelengths in the electromagnetic spectrum could also be produced if
The spherical photodetecting array. Inset: scanning electron micrograph of the fiber crosssection, showing the glass core, metal electrodes, and polymer cladding. (Courtesy Yoel Fink, MIT.) the composition of the glass core was altered, the researchers suggest. Webs constructed of such fibres will yield ‘images’ in these parameter spaces, and can be said to see, hear, sense, and smell.
Paula Gould
SEPTEMBER 2006 | VOLUME 9 | NUMBER 9
13