Prashant Nagpal

Ultrasmooth patterned metals for plasmonics and metamaterials.

Perfectly Flat? Plasmonic devices, which exploit the interactions of light with surface electrons, show great promise for applications in sensing, communications, and energy conversion. A key hindrance is the deposition of patterned metals used for plasmonics, because, as deposited, the terminal surfaces are rough and not amenable to patterning by directional dry-etching techniques. Nagpal et al. (p. 594) use patterned silicon substrates on which they add gold, silver, or copper and then apply an epoxy layer to the deposited metal. When pulled apart, the metal separates from the silicon, where the adhesion is poorer, leaving an ultra-smooth surface. The resulting surface plasmon propagation lengths approach the theoretical values for perfectly flat films. Films with enhanced surface-plasmon propagation may find use in sensing and communications devices. Surface plasmons are electromagnetic waves that can exist at metal interfaces because of coupling between light and free electrons. Restricted to travel along the interface, these waves can be channeled, concentrated, or otherwise manipulated by surface patterning. However, because surface roughness and other inhomogeneities have so far limited surface-plasmon propagation in real plasmonic devices, simple high-throughput methods are needed to fabricate high-quality patterned metals. We combined template stripping with precisely patterned silicon substrates to obtain ultrasmooth pure metal films with grooves, bumps, pyramids, ridges, and holes. Measured surface-plasmon–propagation lengths on the resulting surfaces approach theoretical values for perfectly flat films. With the use of our method, we demonstrated structures that exhibit Raman scattering enhancements above 107 for sensing applications and multilayer films for optical metamaterials.

Engineering metallic nanostructures for plasmonics and nanophotonics

Metallic nanostructures now play an important role in many applications. In particular, for the emerging fields of plasmonics and nanophotonics, the ability to engineer metals on nanometric scales allows the development of new devices and the study of exciting physics. This review focuses on top-down nanofabrication techniques for engineering metallic nanostructures, along with computational and experimental characterization techniques. A variety of current and emerging applications are also covered.

Template-stripped smooth Ag nanohole arrays with silica shells for surface plasmon resonance biosensing.

Inexpensive, reproducible, and high-throughput fabrication of nanometric apertures in metallic films can benefit many applications in plasmonics, sensing, spectroscopy, lithography, and imaging. Here we use template-stripping to pattern periodic nanohole arrays in optically thick, smooth Ag films with a silicon template made via nanoimprint lithography. Ag is a low-cost material with good optical properties, but it suffers from poor chemical stability and biocompatibility. However, a thin silica shell encapsulating our template-stripped Ag nanoholes facilitates biosensing applications by protecting the Ag from oxidation as well as providing a robust surface that can be readily modified with a variety of biomolecules using well-established silane chemistry. The thickness of the conformal silica shell can be precisely tuned by atomic layer deposition, and a 15 nm thick silica shell can effectively prevent fluorophore quenching. The Ag nanohole arrays with silica shells can also be bonded to polydimethylsiloxane (PDMS) microfluidic channels for fluorescence imaging, formation of supported lipid bilayers, and real-time, label-free SPR sensing. Additionally, the smooth surfaces of the template-stripped Ag films enhance refractive index sensitivity compared with as-deposited, rough Ag films. Because nearly centimeter-sized nanohole arrays can be produced inexpensively without using any additional lithography, etching, or lift-off, this method can facilitate widespread applications of metallic nanohole arrays for plasmonics and biosensing.

Three-dimensional plasmonic nanofocusing

We demonstrate three-dimensional plasmonic nanofocusing of light with patterned metallic pyramids obtained via template stripping. Gratings on the faces of these pyramids convert linearly polarized light into plasmons that propagate toward and converge at a approximately 10 nm apex. Experiments and computer simulations confirm that optical energy is focused into a nanoscale volume (5 x 10(-5) wavelength(3)). Because these structures are easily and reproducibly fabricated, our results could benefit many applications, including imaging, sensing, lithography, and nonlinear spectroscopy.

Plasmon-Enhanced Energy Transfer for Improved Upconversion of Infrared Radiation in Doped-Lanthanide Nanocrystals

Upconversion of infrared radiation into visible light has been investigated for applications in photovoltaics and biological imaging. However, low conversion efficiency due to small absorption cross-section for infrared light (Yb(3+)), and slow rate of energy transfer (to Er(3+) states) has prevented application of upconversion photoluminescence (UPL) for diffuse sunlight or imaging tissue samples. Here, we utilize resonant surface plasmon polaritons (SPP) waves to enhance UPL in doped-lanthanide nanocrystals. Our analysis indicates that SPP waves not only enhance the electromagnetic field, and hence weak Purcell effect, but also increase the rate of resonant energy transfer from Yb(3+) to Er(3+) ions by 6 fold. While we do observe strong metal mediated quenching (14-fold) of green fluorescence on flat metal surfaces, the nanostructured metal is resonant in the infrared and hence enhances the nanocrystal UPL. This strong Coulombic effect on energy transfer can have important implications for other fluorescent and excitonic systems too.

Efficient Low-Temperature Thermophotovoltaic Emitters from Metallic Photonic Crystals

We examine the use of metallic photonic crystals as thermophotovoltaic emitters. We coat silica woodpile structures, created using direct laser writing, with tungsten or molybdenum. Optical reflectivity and thermal emission measurements near 650 degrees C demonstrate that the resulting structures should provide efficient emitters at relatively low temperatures. When matched to InGaAsSb photocells, our structures should generate over ten times more power than solid emitters while having an optical-to-electrical conversion efficiency above 32%. At such low temperatures, these emitters have promise not only in solar energy but also in harnessing geothermal and industrial waste heat.

Single‐Crystalline Silver Films for Plasmonics

A simple route to flat, large-area, single-crystalline films for plasmonics is demonstrated by sputter deposition of silver onto mica substrates at elevated temperatures. The films exhibit improved dielectric properties and allow more precise patterning of high-quality nanostructures for plasmonic applications.

Spectral Dependence of Nanocrystal Photoionization Probability: The Role of Hot-Carrier Transfer

We conduct measurements of photocharging of PbSe and PbS nanocrystal quantum dots (NQDs) as a function of excitation energy (ℏω). We observe a rapid growth of the degree of photocharging with increasing ℏω, which indicates an important role of hot-carrier transfer in the photoionization process. The corresponding spectral dependence exhibits two thresholds that mark the onsets of weak and strong photocharging. Interestingly, both thresholds are linked to the NQD band gap energy (E(g)) and scale as ∼1.5E(g) and ∼3E(g), indicating that the onsets of photoionization are associated with specific nanocrystal states (tentatively, 1P and 2P, respectively) and are not significantly dependent on the energy of external acceptor sites. For all samples, the hot-electron transfer probability increases by nearly 2 orders of magnitude as photon energy increases from 1.5 to 3.5 eV, although at any given wavelength the photoionization probability shows significant sample-to-sample variations (∼10(-6) to 10(-3) for 1.5 eV and ∼10(-4) to 10(-1) for 3.5 eV). In addition to the effect of the NQD size, these variations are likely due to differences in the properties of the NQD surface and/or the number and identity of external acceptor trap sites. The charge-separated states produced by photoionization are characterized by extremely long lifetimes (20 to 85 s) that become longer with increasing NQD size.

Photocatalysis Deconstructed: Design of a New Selective Catalyst for Artificial Photosynthesis

A rapid increase in anthropogenic emission of greenhouse gases, mainly carbon dioxide, has been a growing cause for concern. While photocatalytic reduction of carbon dioxide (CO2) into solar fuels can provide a solution, lack of insight into energetic pathways governing photocatalysis has impeded study. Here, we utilize measurements of electronic density of states (DOS), using scanning tunneling microscopy/spectroscopy (STM/STS), to identify energy levels responsible for photocatalytic reduction of CO2-water in an artificial photosynthetic process. We introduce desired states in titanium dioxide (TiO2) nanoparticles, using metal dopants or semiconductor nanocrystals, and the designed catalysts were used for selective reduction of CO2 into hydrocarbons, alcohols, and aldehydes. Using a simple model, we provide insights into the photophysics governing this multielectron reduction and design a new composite photocatalyst based on overlapping energy states of TiO2 and copper indium sulfide (CIS) nanocrystals. These nanoparticles demonstrate the highest selectivity for ethane (>70%) and a higher efficiency of converting ultraviolet radiation into fuels (4.3%) using concentrated sunlight (>4 Sun illumination), compared with platinum-doped TiO2 nanoparticles (2.1%), and utilize hot electrons to tune the solar fuel from alkanes to aldehydes. These results can have important implications for the development of new inexpensive photocatalysts with tuned activity and selectivity.

Fabrication of carbon/refractory metal nanocomposites as thermally stable metallic photonic crystals

Metallic photonic crystals (MPhCs), three-dimensionally nanostructured metals, have been previously investigated as efficient emitters for thermophotovoltaic conversion of solar energy and waste heat into electricity. However, the thermal stability of these nanoscaled structures is limited at high temperatures. Here we present a fabrication scheme for preparing metal-coated carbon inverse opal photonic crystal structures that may be useful for thermal emission modification. Three-dimensionally ordered macroporous (3DOM) carbon films and monoliths, which can maintain their structure up to at least 2200 °C in argon, were used as thermally stable scaffolds for chemical vapor deposition (CVD) of the refractory metals tungsten, molybdenum, or tantalum, all with a thin hafnia interlayer. The tungsten-coated photonic crystals were found to be stable after heat treatment at 1000 °C for at least five hours, under high vacuum (10−6 torr). The thermal stability of these nanocomposite materials is mainly limited by the adhesion of the refractory metals on the 3DOM carbon scaffold. The hafnia interlayer serves as an adhesion promoter for this material, and structures without a hafnia coating show metal agglomeration after heat treatment at 1000 °C, as demonstrated with millimetre-sized 3DOM carbon monoliths that were only partially coated with hafnia. These results can have important implications not only for the development of efficient thermophotovoltaic emitters, but also for fabrication of other thermally stable, functional nanocomposite materials.