Demonstrated Use of Quantum Dot LEDs to Eliminate the Need for External Laser and Aperture Enabled NSOM Probe
Nanotechnologies : Life Science Apps
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- Xiaojing (John) Zhang, Ph.D. , Biomedical Engineering
- Ashwini Gopal, Ph.D. , Electrical and Computer Engineering
- Kazunori Hoshino, Ph.D. , Biomedical Engineering
- Sunmin Kim , Biomedical Engineering
Quantum dot-based devices are advantageous in many ways. Solutions processing offers a much lower manufacturing cost than traditional evaporation/deposition methods. Quantum dots can be deposited on a wide range of substrates, and the films themselves are flexible. Traditional Light Emitting Diodes (LEDs) fabricated from bulk material are color-limited by the band gap, but quantum dot-based LEDs are tunable to any color because of quantum size effects.
The resolution of conventional (far-field) optical microscopes is limited by diffraction to the wavelength of light. The near-field scanning optical microscope (NSOM), a recent advancement, obtains high-resolution images (typically 80-200 nm) by using probe light source, whose size and probe-to-sample distance is shorter than the wavelength of light. NSOM has the ability to perform fluorescence and polarized imaging and ultraviolet, infrared, and Raman spectroscopy.
Currently, laser beams are tuned to achieve better resolution for smaller feature sizes by utilizing mirrors and decreasing aperture size. This leads to increased energy loss, since fewer of the photons can be directed through the aperture. Having an LED built into the probe tip itself would eliminate this problem and possibly enable even better resolution.
Commercially available NSOM probes consist of a fiber through which light from an external source is delivered to a tip with an 80-200 nm aperture. The probes are still made by hand, an inefficient manufacturing process, and the fibers are bulky, with a diameter of 80-100 micrometers. These drawbacks have prevented the NSOM from being used in further commercial applications such as nano-scale patterning and optical data storage devices.
Researchers at The University of Texas at Austin have developed a method for depositing uniform quantum dot films on a flat substrate. The films are CMOS compatible, which means they can undergo post-processing such as photolithography, etching, and patterning techniques to define contacts and the working area of the LEDs. The applications of this CMOS-compatible technology could include nanophotonic microsystems for sensing and imaging.
With modification, the deposition technique can be used in combination with a previous University technology (US patent 7,621,964) to fabricate an NSOM probe with a built-in LED light source on the tip. Fabricating NSOM probes in this manner is advantageous over existing methods, since the process could be automated using MEMS techniques, instead of typical manual assembly. Integrating the LED directly onto the tip would also eliminate the need to focus lasers through small apertures.
- Silicon-microfabrication may provide low-cost mass production of high-quality probes.
- Probe arrays will speed scanning, enabling scanning of larger areas.
- Enhanced resolution of 20-50nm means better acuity.
- Self-illuminating MEMS probe tip allows integration of self-content compact digital devices.
- Different emission wavelengths of light ranging from near UV to near-IR (sees in multiple colors)
- Can also be used in patterning and recording devices
- Ready to integrate with other silicon/MEMS functional elements
- Integrated nanoscale light source on tip of a scanning MEMS probe
- Lock-in amplification of signal
- Molecular-scale sensitivity
- Inexpensive, disposable, small-form factor probe array
- Can be used in existing AFM setup and other microscopy
- Provide a resolution beyond diffraction limit to several existing tools
Quantum dot-based light emitting devices have a wide range of applications in biology, imaging, sensing, security, and many other fields. Devices that emit in the infrared range could be used for medical imaging and diagnosis. Quantum dot-based LEDs could be used to develop a sensitive method for detecting cells, DNA, genes, and proteins. They could be used to develop security applications such as bio-warfare sensors. Such LEDs could also be integrated into probe tips for microscopy. This last application can offer vast improvement over current approaches, and it has been extensively investigated by the inventors.
Current and future applications impact a broad range of industrial markets including: biology and biomedicine; computing and memory; electronics and displays; optoelectronic devices such as LEDs, lighting, and lasers; optical components used in telecommunications; and security applications such as covert identification tagging or bio-warfare sensors.
This MEMS probe is compatible with modified NSOM imaging systems. It will open several new opportunities in biomedical and industrial applications of near-field microscopy. Potential imaging and characterization applications could include: semiconductor heterostructures; organic thin films and polymer blends; nanotubes, quantum dots, and other nanomaterials; cells, viruses, and DNA.
In addition, Dr. Zhang has received $500K in NSF funding to further this development.
- 1 U.S. patent issued: 8,193,010