Mark W. Knight

Photodetection with Active Optical Antennas

An active optical antenna-diode combines the functions of light-harvesting and excited-electron injection. 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.

Aluminum for Plasmonics

Unlike silver and gold, aluminum has material properties that enable strong plasmon resonances spanning much of the visible region of the spectrum and into the ultraviolet. This extended response, combined with its natural abundance, low cost, and amenability to manufacturing processes, makes aluminum a highly promising material for commercial applications. Fabricating Al-based nanostructures whose optical properties correspond with theoretical predictions, however, can be a challenge. In this work, the Al plasmon resonance is observed to be remarkably sensitive to the presence of oxide within the metal. For Al nanodisks, we observe that the energy of the plasmon resonance is determined by, and serves as an optical reporter of, the percentage of oxide present within the Al. This understanding paves the way toward the use of aluminum as a low-cost plasmonic material with properties and potential applications similar to those of the coinage metals.

Photovoltaic materials: Present efficiencies and future challenges

Surveying the solar cell landscape The rate of development and deployment of large-scale photovoltaic systems over recent years has been unprecedented. Because the cost of photovoltaic systems is only partly determined by the cost of the solar cells, efficiency is a key driver to reduce the cost of solar energy. There are several materials systems being explored to achieve high efficiency at low cost. Polman et al. comprehensively and systematically review the leading candidate materials, present the limitations of each system, and analyze how these limitations can be overcome and overall cell performance improved. Science, this issue p. 10.1126/science.aad4424 BACKGROUND Photovoltaics, which directly convert solar energy into electricity, offer a practical and sustainable solution to the challenge of meeting the increasing global energy demand. According to the Shockley-Queisser (S-Q) detailed-balance model, the limiting photovoltaic energy conversion efficiency for a single-junction solar cell is 33.7%, for an optimum semiconductor band gap of 1.34 eV. Parallel to the development of wafer-based Si solar cells, for which the record efficiency has continually increased during recent decades, a large range of thin-film materials have been developed with the aim to approach the S-Q limit. These materials can potentially be deposited at low cost, in flexible geometries, and using relatively small material quantities. ADVANCES We review the electrical characteristics of record-efficiency cells made from 16 widely studied photovoltaic material geometries and illuminated under the standard AM1.5 solar spectrum, and compare these to the fundamental limits based on the S-Q model. Cells that show a short-circuit current (Jsc) lower than the S-Q limit suffer from incomplete light absorption or incomplete collection of generated carriers, whereas a reduced open-circuit voltage (Voc) or fill factor (FF) reflects unwanted bulk or interfacial carrier recombination, parasitic resistance, or other electrical nonidealities. The figure shows the experimental values for Jsc and the Voc × FF product relative to the S-Q limiting values for the different materials. This graph enables a direct identification of each material in terms of unoptimized light management and carrier collection (Jsc/JSQ < 1) or carrier management (Voc × FF/VSQ × FFSQ < 1). Monocrystalline Si cells (record efficiency 25.6%) have reached near-complete light trapping and carrier collection and are mostly limited by remaining carrier recombination losses. In contrast, thin-film single-crystalline GaAs cells (28.8%) show only minimal recombination losses but can be improved by better light management. Polycrystalline CdTe thin-film cells (21.5%) offer excellent light absorption but have relatively high recombination losses; perovskite cells (21.0%) and Cu(In,Ga)(Se,S)2 (CIGS) cells (21.7%) have poorer light management, although CIGS displays higher electrical quality. Aside from these five materials (Si, GaAs, CdTe, CIGS, perovskite) with efficiencies of >20%, a broad range of other thin-film materials have been developed with efficiencies of 10 to 12%: micro/nanocrystalline and amorphous Si, Cu(Zn,Sn)(Se,S)2 (CZTS), dye-sensitized TiO2, organic polymer materials, and quantum dot solids. So far, cell designs based on these materials all suffer from both light management and carrier management problems. Organic and quantum dot solar cells have shown substantial efficiency improvements in recent years. OUTLOOK The record-efficiency single-crystalline materials (Si, GaAs) have room for efficiency improvements by a few absolute percent. The future will tell whether the high-efficiency polycrystalline thin films (CdTe, CIGS, perovskite) can rival the efficiencies of Si and GaAs. Because the cost of photovoltaic systems is only partly determined by the cost of the solar cells, efficiency is a key driver to reduce the cost of solar energy, and therefore large-area photovoltaic systems require high-efficiency (>20%), low-cost solar cells. The lower-efficiency (flexible) materials can find applications in building-integrated PV systems, flexible electronics, flexible power generation systems, and many other (sometimes niche) markets. High-efficiency (>20%) materials find applications in large-area photovoltaic power generation for the utility grid as well as in small and medium-sized systems for the built environment. They will enable very large-scale penetration into our energy system, starting now and growing as the cost per kilowatt-hour is reduced further by a factor of 2 to 3. This can be achieved by nanophotonic cell designs, in which optically resonant and nonresonant structures are integrated with the solar cell architecture to enhance light coupling and trapping, in combination with continued materials engineering to further optimize cell voltage. Making big steps forward in these areas will require a coordinated international materials science and engineering effort. Limiting processes in photovoltaic materials. An efficient solar cell captures and traps all incident light (“light management”) and converts it to electrical carriers that are efficiently collected (“carrier management”). The plot shows the short-circuit current and product of open-circuit voltage and fill factor relative to the maximum achievable values, based on the Shockley-Queisser detailed-balance limit, for the most efficient solar cell made with each photovoltaic material. The data indicate whether a particular material requires better light management, carrier management, or both. Colors correspond to cells achieving <50% of their S-Q efficiency limit ηSQ (red), 50 to 75% (green), or >75% (blue). Recent developments in photovoltaic materials have led to continual improvements in their efficiency. We review the electrical characteristics of 16 widely studied geometries of photovoltaic materials with efficiencies of 10 to 29%. Comparison of these characteristics to the fundamental limits based on the Shockley-Queisser detailed-balance model provides a basis for identifying the key limiting factors, related to efficient light management and charge carrier collection, for these materials. Prospects for practical application and large-area fabrication are discussed for each material.

Aluminum Plasmonic Nanoantennas

The use of aluminum for plasmonic nanostructures opens up new possibilities, such as access to short-wavelength regions of the spectrum, complementary metal-oxide-semiconductor (CMOS) compatibility, and the possibility of low-cost, sustainable, mass-producible plasmonic materials. Here we examine the properties of individual Al nanorod antennas with cathodoluminescence (CL). This approach allows us to image the local density of optical states (LDOS) of Al nanorod antennas with a spatial resolution less than 20 nm and to identify the radiative modes of these nanostructures across the visible and into the UV spectral range. The results, which agree well with finite difference time domain (FDTD) simulations, lay the groundwork for precise Al plasmonic nanostructure design for a variety of applications.

Substrates Matter: Influence of an Adjacent Dielectric on an Individual Plasmonic Nanoparticle

Studying the plasmonic properties of metallic nanoparticles at the individual nanostructure level is critical to our understanding of nanoscale metallic systems. Here we show how the presence of a nearby dielectric substrate modifies the energies of the plasmon modes of a metallic nanoparticle. The adjacent dielectric lifts the degeneracy of the dipole plasmon modes oriented parallel and perpendicular to the substrate, introducing a significant energy splitting that depends strongly on the permittivity of the substrate. This energy splitting can easily be misinterpreted as an anomalously broadened plasmon line shape for excitation of an individual nanoparticle with unpolarized light.

Light-Induced Release of DNA from Gold Nanoparticles: Nanoshells and Nanorods

Plasmon-resonant nanoparticle complexes show highly promising potential for light-triggered, remote-controlled delivery of oligonucleotides on demand, for research and therapeutic purposes. Here we investigate the light-triggered release of DNA from two types of nanoparticle substrates: Au nanoshells and Au nanorods. Both light-triggered and thermally induced release are distinctly observable from nanoshell-based complexes, with light-triggered release occurring at an ambient solution temperature well below the DNA melting temperature. Surprisingly, no analogous measurable release was observable from nanorod-based complexes below the DNA melting temperature. These results suggest that a nonthermal mechanism may play a role in plasmon resonant, light-triggered DNA release.

Narrowband photodetection in the near-infrared with a plasmon-induced hot electron device

In gratings, incident light can couple strongly to plasmons propagating through periodically spaced slits in a metal film, resulting in a strong, resonant absorption whose frequency is determined by the nanostructure periodicity. When a grating is patterned on a silicon substrate, the absorption response can be combined with plasmon-induced hot electron photocurrent generation. This yields a photodetector with a strongly resonant, narrowband photocurrent response in the infrared, limited at low frequencies by the Schottky barrier, not the bandgap of silicon. Here we report a grating-based hot electron device with significantly larger photocurrent responsivity than previously reported antenna-based geometries. The grating geometry also enables more than three times narrower spectral response than observed for nanoantenna-based devices. This approach opens up the possibility of plasmonic sensors with direct electrical readout, such as an on-chip surface plasmon resonance detector driven at a single wavelength.

Au Nanomatryoshkas as Efficient Near-Infrared Photothermal Transducers for Cancer Treatment: Benchmarking against Nanoshells

Au nanoparticles with plasmon resonances in the near-infrared (NIR) region of the spectrum efficiently convert light into heat, a property useful for the photothermal ablation of cancerous tumors subsequent to nanoparticle uptake at the tumor site. A critical aspect of this process is nanoparticle size, which influences both tumor uptake and photothermal efficiency. Here, we report a direct comparative study of ∼90 nm diameter Au nanomatryoshkas (Au/SiO2/Au) and ∼150 nm diameter Au nanoshells for photothermal therapeutic efficacy in highly aggressive triple negative breast cancer (TNBC) tumors in mice. Au nanomatryoshkas are strong light absorbers with 77% absorption efficiency, while the nanoshells are weaker absorbers with only 15% absorption efficiency. After an intravenous injection of Au nanomatryoshkas followed by a single NIR laser dose of 2 W/cm2 for 5 min, 83% of the TNBC tumor-bearing mice appeared healthy and tumor free >60 days later, while only 33% of mice treated with nanoshells survived the same period. The smaller size and larger absorption cross section of Au nanomatryoshkas combine to make this nanoparticle more effective than Au nanoshells for photothermal cancer therapy.

Designing and Deconstructing the Fano Lineshape in Plasmonic Nanoclusters

By varying the relative dimensions of the central and peripheral disks of a plasmonic nanocluster, the depth of its Fano resonance can be systematically modified; spectral windows where the scattering cross section of the nanocluster is negligible can be obtained. In contrast, electron-beam excitation of the plasmon modes at specific locations within the nanocluster yields cathodoluminescence spectra with no Fano resonance. By examining the selection rules for plasmon excitation in the context of a coupled oscillator picture, we provide an intuitive explanation of this behavior based on the plasmon modes observed for optical and electron-beam excitation in this family of nanostructures.

Embedding Plasmonic Nanostructure Diodes Enhances Hot Electron Emission

When plasmonic nanostructures serve as the metallic counterpart of a metal-semiconductor Schottky interface, hot electrons due to plasmon decay are emitted across the Schottky barrier, generating measurable photocurrents in the semiconductor. When the plasmonic nanostructure is atop the semiconductor, only a small percentage of hot electrons are excited with a wavevector permitting transport across the Schottky barrier. Here we show that embedding plasmonic structures into the semiconductor substantially increases hot electron emission. Responsivities increase by 25× over planar diodes for embedding depths as small as 5 nm. The vertical Schottky barriers created by this geometry make the plasmon-induced hot electron process the dominant contributor to photocurrent in plasmonic nanostructure-diode-based devices.