Science - Photodetection with Active Optical Antennas
Nanoantennas are key optical components for light harvesting; photodiodes convert light into a current of electrons for photodetection. We show that these two distinct, independent functions can be combined into the same structure. Photons coupled into a metallic nanoantenna excite resonant plasmons, which decay into energetic, “hot” electrons injected over a potential barrier at the nanoantenna-semiconductor interface, resulting in a photocurrent. This dual-function structure is a highly compact, wavelength-resonant, and polarization-specific light detector, with a spectral response extending to energies well below the semiconductor band edge.
Basic scientific curiosity paid off in unexpected ways when Rice University researchers investigating the fundamental physics of nanomaterials discovered a new technology that could dramatically improve solar energy panels.
"We're merging the optics of nanoscale antennas with the electronics of semiconductors," said lead researcher Naomi Halas, Rice's Stanley C. Moore Professor in Electrical and Computer Engineering. "There's no practical way to directly detect infrared light with silicon, but we've shown that it is possible if you marry the semiconductor to a nanoantenna. We expect this technique will be used in new scientific instruments for infrared-light detection and for higher-efficiency solar cells."
More than a third of the solar energy on Earth arrives in the form of infrared light. But silicon -- the material that's used to convert sunlight into electricity in the vast majority of today's solar panels -- cannot capture infrared light's energy. Every semiconductor, including silicon, has a "bandgap" where light below a certain frequency passes directly through the material and is unable to generate an electrical current. By attaching a metal nanoantenna to the silicon, where the tiny antenna is specially tuned to interact with infrared light, the Rice team showed they could extend the frequency range for electricity generation into the infrared. When infrared light hits the antenna, it creates a "plasmon," a wave of energy that sloshes through the antenna's ocean of free electrons. The study of plasmons is one of Halas' specialties, and the new paper resulted from basic research into the physics of plasmons that began in her lab years ago.
It has been known that plasmons decay and give up their energy in two ways; they either emit a photon of light or they convert the light energy into heat. The heating process begins when the plasmon transfers its energy to a single electron -- a 'hot' electron. Rice graduate student Mark Knight, lead author on the paper, together with Rice theoretical physicist Peter Nordlander, his graduate student Heidar Sobhani, and Halas set out to design an experiment to directly detect the hot electrons resulting from plasmon decay.
Patterning a metallic nanoantenna directly onto a semiconductor to create a "Schottky barrier," Knight showed that the infrared light striking the antenna would result in a hot electron that could jump the barrier, which creates an electrical current. This works for infrared light at frequencies that would otherwise pass directly through the device.
"The nanoantenna-diodes we created to detect plasmon-generated hot electrons are already pretty good at harvesting infrared light and turning it directly into electricity," Knight said. "We are eager to see whether this expansion of light-harvesting to infrared frequencies will directly result in higher-efficiency solar cells."
Although the devices presented here enable us to investigate hot electron generation by plasmonic antennas, further optimization can significantly increase their quantum efficiency (at present, 0.01% of photons absorbed by each nanoantenna are converted into photocurrent). The role of the titanium layer appears to be quite critical: Numerical simulations show that the 1-nm layer is responsible for producing nominally 33% of the hot electrons, which would increase to more than 50% for a 5-nm thickness. Further experimental studies have indicated that reducing extraneous Ti oxidation during the fabrication process, improving ohmic contacts, and increasing the conductivity of the uppermost ITO layer can collectively increase device efficiency by more than an order of magnitude. In addition, a reverse bias of 1 V increases the photocurrent by a factor of 20. Together, these improvements would boost the quantum yield to nearly 2% over the spectral range of the device. A thin dopant layer could also boost efficiency, increase responsivity, and expand the spectral response of the devices by reducing the Schottky barrier height. Applying antireflection coatings or multipass geometries will also further increase the quantum yield.
The range of potential applications of this device concept is extremely diverse. As a silicon-based detector capable of detecting sub–band gap photons, this device could find widespread use in on-chip silicon photonics, ultimately eliminating the need to integrate additional semiconductor materials as detectors into chip designs, which would lower fabrication costs. The photodetection mechanism is compatible with existing, above–band gap photodetectors, which when combined could greatly extend the spectral range of silicon light-harvesting devices, such as silicon-based solar cells, into the infrared region of the spectrum. The broad infrared sensitivity of these devices could enable low-cost silicon infrared imaging detectors that may replace costly InGaAs detectors in this same spectral range. Antenna-diodes also offer functional aspects of photodetection not previously realized. By exploiting nanoantennas as a direct light-harvesting and carrier generation element, both polarization- and wavelength-selective detectors can be realized without additional optical components. We believe this mechanism of photodetection may give rise to additional unforeseen applications in photosensing, energy harvesting, imaging, and light detection technologies.
Science - Hot Electrons Cross Boundaries
When light hits the surface of gold or silver, it can excite collective oscillations of the conduction electrons called surface plasmons. The sensitivity of surface plasmons to changes in the surface region forms the basis of analytical tools such as surface plasmon resonance detection, which can be used in lab-on-a-chip applications to detect biomolecules. The excitation of surface plasmons also underlies surface-enhanced Raman spectroscopy. The surface plasmon of silver and gold surfaces that are rough at a nanoscale greatly increases local electric fields and boosts the signal from adsorbed molecules. The wavelength that excites surface plasmons can also be tuned by creating nanoparticles of different sizes, and on page 702 of this issue, Knight et al. exploit this effect to create a detector for near-infrared light. They fabricated a device consisting of rod-like nanoantennas that harvest light and convert a portion of the resulting plasmonic energy into an electric current without the need for an applied bias voltage.
Plasmons to electricity.
(A) Light excites surface plasmons (depicted as regions of positive and negative charge, top and bottom) that can decay into charge carriers, electrons e− and holes h+. Plasmons in shorter nanorods are excited at shorter wavelengths. The nanorods were grown on a titanium (Ti) buffer layer, 1 nanometer thick, on n-type silicon. (B) An energy diagram showing how excited electrons created by plasmon decay encounter a Schottky barrier at the metal-silicon interface, which share a common Fermi energy EF. Highly energetic electrons are either directly injected into the conduction band of silicon above its band edge, EC, or tunnel through the barrier. The barrier is less than the band gap energy (the difference between EC and valence band edge, EV). Holes and electrons produce a measurable photocurrent collected at the indium tin oxide and indium electrodes. "CREDIT: P. HUEY/SCIENCE"
Plasmonic systems can be designed to cover much of the solar spectrum, so this approach suggests a photosensitization strategy, much like the one exploited by Grätzel in dye-sensitized photovoltaics, but avoiding the problem of easily photodegradable organics. Although Knight et al. report very low quantum efficiencies, there is no physical reason why efficiencies cannot be much larger and lead to applications in energy conversion and photodetection.
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