Optical rectification and field enhancement in a plasmonic nanogap

Engineered nanomaterials for solar energy conversion

Understanding how to engineer nanomaterials for targeted solar-cell applications is the key to improving their efficiency and could lead to breakthroughs in their design. Proposed mechanisms for the conversion of solar energy to electricity are those exploiting the particle nature of light in conventional photovoltaic cells, and those using the collective electromagnetic nature, where light is captured by antennas and rectified. In both cases, engineered nanomaterials form the crucial components. Examples include arrays of semiconductor nanostructures as an intermediate band (so called intermediate band solar cells), semiconductor nanocrystals for multiple exciton generation, or, in antenna-rectifier cells, nanomaterials for effective optical frequency rectification. Here, we discuss the state of the art in p-n junction, intermediate band, multiple exciton generation, and antenna-rectifier solar cells. We provide a summary of how engineered nanomaterials have been used in these systems and a discussion of the open questions.

Fowler-Nordheim tunneling induced charge transfer plasmons between nearly touching nanoparticles

Reducing the gap between two metal nanoparticles down to atomic dimensions uncovers novel plasmon resonant modes. Of particular interest is a mode known as the charge transfer plasmon (CTP). This mode has been experimentally observed in touching nanoparticles, where charges can shuttle between the nanoparticles via a conductive path. However, the CTP mode for nearly touching nanoparticles has only been predicted theoretically to occur via direct tunneling when the gap is reduced to ~0.4 nm. Because of challenges in fabricating and characterizing gaps at these dimensions, experiments have been unable to provide evidence for this plasmon mode that is supported by tunneling. In this work, we consider an alternative tunneling process, that is, the well-known Fowler-Nordheim (FN) tunneling that occurs at high electric fields, and apply it for the first time in the theoretical investigation of plasmon resonances between nearly touching nanoparticles. This new approach relaxes the requirements on gap dimensions, and intuitively suggests that with a sufficiently high-intensity irradiation, the CTP can be excited via FN tunneling for a range of subnanometer gaps. The unique feature of FN tunneling induced CTP is the ability to turn on and off the charge transfer by varying the intensity of an external light source, and this could inspire the development of novel quantum devices.

A sub-1-volt nanoelectromechanical switching device

Nanoelectromechanical (NEM) switches have received widespread attention as promising candidates in the drive to surmount the physical limitations currently faced by complementary metal oxide semiconductor technology. The NEM switch has demonstrated superior characteristics including quasi-zero leakage behaviour, excellent density capability and operation in harsh environments. However, an unacceptably high operating voltage (4-20 V) has posed a major obstacle in the practical use of the NEM switch in low-power integrated circuits. To utilize the NEM switch widely as a core device component in ultralow power applications, the operation voltage needs to be reduced to 1 V or below. However, sub-1 V actuation has not yet been demonstrated because of fabrication difficulties and irreversible switching failure caused by surface adhesion. Here, we report the sub-1 V operation of a NEM switch through the introduction of a novel pipe clip device structure and an effective air gap fabrication technique. This achievement is primarily attributed to the incorporation of a 4-nm-thick air gap, which is the smallest reported so far for a NEM switch generated using a 'top-down' approach. Our structure and process can potentially be utilized in various nanogap-related applications, including NEM switch-based ultralow-power integrated circuits, NEM resonators, nanogap electrodes for scientific research and sensors.

Atomic-scale confinement of resonant optical fields

In the presence of matter, there is no fundamental limit preventing confinement of visible light even down to atomic scales. Achieving such confinement and the corresponding resonant intensity enhancement inevitably requires simultaneous control over atomic-scale details of material structures and over the optical modes that such structures support. By means of self-assembly we have obtained side-by-side aligned gold nanorod dimers with robust atomically defined gaps reaching below 0.5 nm. The existence of atomically confined light fields in these gaps is demonstrated by observing extreme Coulomb splitting of corresponding symmetric and antisymmetric dimer eigenmodes of more than 800 meV in white-light scattering experiments. Our results open new perspectives for atomically resolved spectroscopic imaging, deeply nonlinear optics, ultrasensing, cavity optomechanics, as well as for the realization of novel quantum-optical devices.

A plethora of plasmonics from the laboratory for nanophotonics at Rice University

The study of the surface plasmons of noble metals has emerged as one of the most rapidly growing and dynamic topics in nanoscience. Key advances in the synthesis of noble metal nanoparticles and nanostructures have resulted in a broad variety of structures whose geometries can be controlled systematically at the nanoscale. Arising from these efforts is a new level of insight and understanding regarding the fundamental properties of localized plasmons supported by these structures, and, in particular, the properties of interacting plasmon systems. This additional insight has led to the design of plasmonic systems that support coherent phenomena, such as Fano resonances. A broad range of applications are emerging for these structures: single- nanoparticle and nanogap chemical sensors, low-loss plasmon waveguides, and active plasmonic devices and detectors. Applications in biomedicine that exploit the strong photothermal response of nanoparticle plasmons have developed and advanced into clinical trials. The Laboratory for Nanophotonics at Rice has been home to many of these advances. Here, we showcase a variety of functional plasmonic materials and nanodevices emerging from our individual and collaborative efforts.

Electrically connected resonant optical antennas

Electrically connected resonant optical antennas hold promise for the realization of highly efficient nanoscale electro-plasmonic devices that rely on a combination of electric fields and local near-field intensity enhancement. Here we demonstrate the feasibility of such a concept by attaching leads to the arms of a two-wire antenna at positions of minimal near-field intensity with negligible influence on the antenna resonance. White-light scattering experiments in accordance with simulations show that the optical tunability of connected antennas is fully retained. Analysis of the electric properties demonstrates that in the antenna gaps direct current (DC) electric fields of 10(8) V/m can consistently be achieved and maintained over extended periods of time without noticeable damage.

How chain plasmons govern the optical response in strongly interacting self-assembled metallic clusters of nanoparticles

Self-assembled clusters of metallic nanoparticles separated by nanometric gaps generate strong plasmonic modes that support both intense and localized near fields. These find use in many ultrasensitive chemical and biological sensing applications through surface enhanced Raman scattering (SERS). The inability to control at the nanoscale the structure of the clusters on which the optical response crucially depends, has led to the development of general descriptions to model the various morphologies fabricated. Here, we use rigorous electrodynamic calculations to study clusters formed by a hundred nanospheres that are separated by ∼1 nm distance, set by the dimensions of the macrocyclic molecular linker employed experimentally. Three-dimensional (3D) cluster structures of moderate compactness are of special interest since they resemble self-assembled clusters grown under typical diffusion-limited aggregation conditions. We find very good agreement between the simulated and measured far-field extinction spectra, supporting the equivalence of the assumed and experimental morphologies. From these results we argue that the main features of the optical response of two- and three-dimensional clusters can be understood in terms of the excitation of simple units composed of different length resonant chains. Notably, we observe a qualitative difference between short- and long-chain modes in both spectral response and spatial distribution: dimer and short-chain modes are observed in the periphery of the cluster at higher energies, whereas inside the structure longer chain excitation occurs at lower energies. We study in detail different configurations of isolated one-dimensional chains as prototypical building blocks for large clusters, showing that the optical response of the chains is robust to disorder. This study provides an intuitive understanding of the behavior of very complex aggregates and may be generalized to other types of aggregates and systems formed by large numbers of strongly interacting particles.

Bridging quantum and classical plasmonics with a quantum-corrected model

Electromagnetic coupling between plasmonic resonances in metallic nanoparticles allows for engineering of the optical response and generation of strong localized near-fields. Classical electrodynamics fails to describe this coupling across sub-nanometer gaps, where quantum effects become important owing to non-local screening and the spill-out of electrons. However, full quantum simulations are not presently feasible for realistically sized systems. Here we present a novel approach, the quantum-corrected model (QCM), that incorporates quantum-mechanical effects within a classical electrodynamic framework. The QCM approach models the junction between adjacent nanoparticles by means of a local dielectric response that includes electron tunnelling and tunnelling resistivity at the gap and can be integrated within a classical electrodynamical description of large and complex structures. The QCM predicts optical properties in excellent agreement with fully quantum mechanical calculations for small interacting systems, opening a new venue for addressing quantum effects in realistic plasmonic systems.

Time-resolved imaging of near-fields in THz antennas and direct quantitative measurement of field enhancements

We investigate the interaction between terahertz waves and resonant antennas with sub-cycle temporal and λ/100 spatial resolution. Depositing antennas on a LiNbO₃ waveguide enables non-invasive electro-optic imaging, quantitative field characterization, and direct measurement of field enhancement (up to 40-fold). The spectral response is determined over a bandwidth spanning from DC across multiple resonances, and distinct behavior is observed in the near- and far-field. The scaling of enhancement and resonant frequency with gap size and antenna length agrees well with simulations.

Nanoplasmonics: classical down to the nanometer scale

We push the fabrication limit of gold nanostructures to the exciting sub-nanometer regime, in which light-matter interactions have been anticipated to be strongly affected by the quantum nature of electrons in metals. Doing so allows us to (1) evaluate the validity of classical electrodynamics to describe plasmonic effects at this length scale and (2) witness the gradual (instead of sudden) evolution of plasmon modes when two gold nanoprisms are brought into contact. Using electron energy-loss spectroscopy and transmission electron microscope imaging, we investigated nanoprisms separated by gaps of only 0.5 nm and connected by conductive bridges as narrow as 3 nm. Good agreement of our experimental results with electromagnetic calculations and LC circuit models evidence the gradual evolution of the plasmonic resonances toward the quantum coupling regime. We demonstrate that down to the nanometer length scales investigated classical electrodynamics still holds, and a full quantum description of electrodynamics phenomena in such systems might be required only when smaller gaps of a few angstroms are considered. Our results show also the gradual onset of the charge-transfer plasmon mode and the evolution of the dipolar bright mode into a 3λ/2 mode as one literally bridges the gap between two gold nanoprisms.

Self-limited plasmonic welding of silver nanowire junctions

Nanoscience provides many strategies to construct high-performance materials and devices, including solar cells, thermoelectrics, sensors, transistors, and transparent electrodes. Bottom-up fabrication facilitates large-scale chemical synthesis without the need for patterning and etching processes that waste material and create surface defects. However, assembly and contacting procedures still require further development. Here, we demonstrate a light-induced plasmonic nanowelding technique to assemble metallic nanowires into large interconnected networks. The small gaps that form naturally at nanowire junctions enable effective light concentration and heating at the point where the wires need to be joined together. The extreme sensitivity of the heating efficiency on the junction geometry causes the welding process to self-limit when a physical connection between the wires is made. The localized nature of the heating prevents damage to low-thermal-budget substrates such as plastics and polymer solar cells. This work opens new avenues to control light, heat and mass transport at the nanoscale.

Nanoantennas for visible and infrared radiation

Nanoantennas for visible and infrared radiation can strongly enhance the interaction of light with nanoscale matter by their ability to efficiently link propagating and spatially localized optical fields. This ability unlocks an enormous potential for applications ranging from nanoscale optical microscopy and spectroscopy over solar energy conversion, integrated optical nanocircuitry, opto-electronics and density-of-states engineering to ultra-sensing as well as enhancement of optical nonlinearities. Here we review the current understanding of metallic optical antennas based on the background of both well-developed radiowave antenna engineering and plasmonics. In particular, we discuss the role of plasmonic resonances on the performance of nanoantennas and address the influence of geometrical parameters imposed by nanofabrication. Finally, we give a brief account of the current status of the field and the major established and emerging lines of investigation in this vivid area of research.

Dipole, quadrupole and octupole plasmon resonance modes in non-concentric nanocrescent/nanodisk structure: local field enhancement in the visible and near infrared regions

By deviating the nanodisk from the center in the silver nanocrescent/nanodisk structure, we find that the dipole, quadrupole and octupole modes can all induce very high local electric field enhancement (LFE, more than 750) for the coupling of nanocrescent and crescent gap modes, which makes the resonant wavelengths of the non-concentric nanostructures change from the visible to near infrared regions. In addition, the LFE factor of the quadrupole mode is more than 1000, which is suitable for single molecular detection by local surface enhanced spectroscopy.

Nanoscale Devices for Rectification of High Frequency Radiation from the Infrared through the Visible: A New Approach

We present a new and viable method for optical rectification. This approach has been demonstrated both theoretically and experimentally and is the basis fot the development of devices to rectify radiation through the visible. This technique for rectification is based not on conventional material or temperature asymmetry as used in MIM (metal/insulator/metal) or Schottky diodes, but on a purely sharp geometric property of the antenna. This sharp “tip” or edge with a collector anode constitutes a tunnel junction. In these devices the rectenna (consisting of the antenna and the tunnel junction) acts as the absorber of the incident radiation and the rectifier. Using current nanofabrication techniques and the selective atomic layer deposition (ALD) process, junctions of 1 nm can be fabricated, which allow for rectification of frequencies up to the blue portion of the spectrum. To assess the viability of our approach, we review the development of nanoantenna structures and tunnel junctions capable of operating in the visible region. In addition, we review the detailed process of rectification and present methodologies for analysis of diode data. Finally, we present operational designs for an optical rectenna and its fabrication and discuss outstanding problems and future work.

Direct and reliable patterning of plasmonic nanostructures with sub-10-nm gaps

Nanoscale gaps in metal films enable strong field enhancements in plasmonic structures. However, the reliable fabrication of ultrasmall gaps (<10 nm) for real applications is still challenging. In this work, we report a method to directly and reliably fabricate sub-10-nm gaps in plasmonic structures without restrictions on pattern design. This method is based on a lift-off process using high-resolution electron-beam lithography with a negative-tone hydrogen silsesquioxane (HSQ) resist, where the resulting nanogap size is determined by the width of the patterned HSQ structure, which could be written at less than 10 nm. With this method, we fabricated densely packed gold nanostructures of varying geometries separated by ultrasmall gaps. By controlling structure sizes during lithography with nanometer precision, the plasmon resonances of the resulting patterns could be accurately tuned. Optical and surface-enhanced Raman scattering (SERS) measurements on the patterned structures show that this technique has promising applications in the fabrication of passively tunable plasmonic nanostructures with ultrasmall gaps.

Highly effective SERS substrates based on an atomic-layer-deposition-tailored nanorod array scaffold

When two metallic surfaces supporting plasmonic excitation are brought into close proximity of each other, a nanogap (of width on the subwavelength scale) will form, which boosts greatly the local optical field. Based on this idea, we fabricated two types of three-dimensional plasmonic substrates with such nanogaps, taking advantage of both atomic layer deposition (ALD) and the capillary effect. Owing to the counteraction of the gap-reducing and capillary, nanogaps with different widths and profiles have been formed on the scaffold of aligned ZnO nanorods and shown to induce large field enhancement with enhancement factor up to 2.64 × 10(6) for surface-enhanced Raman scattering (SERS).

Plasmonic polymer tandem solar cell

We demonstrated plasmonic effects in an inverted tandem polymer solar cell configuration by blending Au nanoparticles (NPs) into the interconnecting layer (ICL) that connects two subcells. Experimental results showed this plasmonic enhanced ICL improves both the top and bottom subcells' efficiency simultaneously by enhancing optical absorption. The presence of Au NPs did not cause electrical characteristics to degrade within the tandem cell. As a result, a 20% improvement of power conversion efficiency has been attained by the light concentration of Au NPs via plasmonic near-field enhancement. The simulated near-field distribution and experimental Raman scattering investigation support our results of plasmonic induced enhancement in solar cell performance. Our finding shows a great potential of incorporating the plasmonic effect with conventional device structure in achieving highly efficient polymer solar cells.

Accurate determination of plasmonic fields in molecular junctions by current rectification at optical frequencies

Current rectification, i.e., induction of dc current by oscillating electromagnetic fields, is demonstrated in molecular junctions at an optical frequency. The magnitude of rectification is used to accurately determine the effective oscillating potentials in the junctions induced by the irradiating laser. Since the gap size of the junctions used in this study is precisely determined by the length of the embedded molecules, the oscillating potential can be used to calculate the plasmonic enhancement of the electromagnetic field in the junctions. With a set of junctions based on alkyl thiolated molecules with identical HOMO-LUMO gap and different lengths, an exponential dependence of the plasmonic field enhancement on gap size is observed.

Plasmon resonance in individual nanogap electrodes studied using graphene nanoconstrictions as photodetectors

We achieve direct electrical readout of the wavelength and polarization dependence of the plasmon resonance in individual gold nanogap antennas by positioning a graphene nanoconstriction within the gap as a localized photodetector. The polarization sensitivities can be as large as 99%, while the plasmon-induced photocurrent enhancement is 2-100. The plasmon peak frequency, polarization sensitivity, and photocurrent enhancement all vary between devices, indicating the degree to which the plasmon resonance is sensitive to nanometer-scale irregularities.

Single molecule electronic devices

Single molecule electronic devices in which individual molecules are utilized as active electronic components constitute a promising approach for the ultimate miniaturization and integration of electronic devices in nanotechnology through the bottom-up strategy. Thus, the ability to understand, control, and exploit charge transport at the level of single molecules has become a long-standing desire of scientists and engineers from different disciplines for various potential device applications. Indeed, a study on charge transport through single molecules attached to metallic electrodes is a very challenging task, but rapid advances have been made in recent years. This review article focuses on experimental aspects of electronic devices made with single molecules, with a primary focus on the characterization and manipulation of charge transport in this regime.