R-3 Repository :: Browsing by Author "Halas, Naomi"

Browsing by Author "Halas, Naomi"

Now showing 1 - 14 of 14

A Comparison of Plasmon-induced and Photoexcited Hot Carriers in Metallic Nanostructures
(2015-12-02) Zhao, Hangqi; Halas, Naomi; Nordlander, Peter; Link, Stephan
The incompressible oscillations of electrons in metallic nanostructures, known as surface plasmons, have provided a promising route to increasing light-matter coupling and boosting the efficiency of solar energy conversion in photovoltaic devices. When plasmons decay, energetic electron-hole pairs are created through a non-radiative channel. These hot electrons have found applications in photodetection and photocatalysis but remain poorly understood in terms of mechanisms. In this work1, we made a comprehensive comparison between plasmon-induced hot carrier generation and direct excitations of hot carriers by photon absorption. Using a gold nanowire based hot carrier device, which either forms a Schottky barrier or an Ohmic barrier between nanostructures and a wide-bandgap semiconductor substrate, we are able to distinguish between these two mechanisms of hot carrier generation. We show that plasmon-induced hot electrons have higher energies than directly excited carriers, and can be characterized by the integration of electrical field enhancement within the nanostructures, while photoexcited carriers are correlated with material absorption. Our work paves the way for increasing the energy conversion efficiency by decreasing the Schottky barrier and collecting both the plasmonic and interband photocurrent, which may find wide applications in future photovoltaic devices.
A first principles approach to describing novel plasmonic phenomena
(2015-04-09) Kulkarni, Vikram; Nordlander, Peter J.; Hafner, Jason; Halas, Naomi
Plasmonic phenomena are described using first principles approaches such as time-dependent density functional theory (TDDFT) and molecular dynamics. These techniques are used to study hot electron generation via plasmon decay, charge transfer plasmons, plasmons in doped semiconductor nanocrystals, and heat dissipation around nanostructures. The theory presented is fully developed for spherical nanoparticles yet the physics is qualitatively the same for nanostructures of arbitrary complexity. The quantum nature of the electron gas is present in all investigations. Effects of size quantization, electronic lifetimes, resonant tunneling, exchange and correlation, Friedel oscillations and Kapitza resistance are all incorporated. Non-radiative plasmon decay into electron-hole pairs is shown to be dependent on the size of the nanoparticle and the lifetime of the electronic levels. Small nanoparticles and systems with long lifetimes are more efficient at generating high energy carriers. Charge transfer plasmons are demonstrated in dimer systems with quantized conducting junctions. An energy level of the junction must be resonant with the Fermi energy of the nanoparticles to facilitate the charge transfer. Thus the optical properties of the dimer are dictated by the electronic structure of the junction. The plasmon energies of semiconductor nanocrystals are tuned via doping. Plasmons consisting of a few hundred charge carriers are observed in the mid and far infrared regions of the electromagnetic spectrum. Many body effects are shown to correct the calculated plasmon energies when compared to classical theory. Heated nanoparticles are shown to distort the density of the surrounding solvent when illuminated at high intensities on resonance. The distortion of water around a nanoparticle leads to a significant interfacial resistance and a nonlinearity of the steady state temperature of the nanoparticle. However the nanoparticle does not produce a well-defined bubble in its immediate surroundings. This work is important for applications such as plasmon-enhanced catalysis, photocurrent generation, molecular electronics, steam generation and nanoscale heating.
Development of an Active Display Using Plasmonic Nanorods and Liquid Crystals
(2015-08-26) Olson, Jana; Link, Stephan; Halas, Naomi; Nordlander, Peter
The current generation of display technology has already provided the public with displays that exhibit high color fidelity, faster than the eye can see frame rates, and screens spanning an order of magnitude in size, from hand-held to wall-sized. What has yet to be achieved is for display technology to become versatile, fitting into the human environment in a way that is unobtrusive, ergonomic, and promotes an improved quality of life. This thesis points to nanotechnology, particularly plasmonic nanomaterials, as a way to bring display technologies to that next level. The work presented in this thesis is the development of a nanomaterial that is vividly colored, electronically controllable, and highly versatile in its possible applications. First, an understanding of the electronic switching of nematic liquid crystals is gained via combination of randomly oriented gold nanorods with homogeneously aligned nematic liquid crystal. The nanorods are used to probe the birefringence of the liquid crystal as an in-plane bias is used to switch the alignment of the liquid crystal from a uniform parallel alignment to a 90 degree twisted alignment. This mechanism is confirmed with theoretical modeling using Jones Calculus. Second, the polarization sensitivity of the nanorods is exploited by creating a hexagonal array of co-oriented nanorods to form a plasmonic pixel using electron beam lithography. Aluminum was chosen as the plasmonic material because its plasmon resonance can span the whole visible region, and because of its compatibility with the semiconductor manufacturing industry. The outstanding vivid color of these pixels is dependent on the physical characteristics of the individual nanorods, and also on the inter-rod spacing. Dipole coupling within the array of ~300 nm separated nanorods is used to restrict plasmon scattering at both long- and short-wavelengths. A simple 3-step color control mechanism was developed that others can use to produce aluminum plasmonic pixels with colors on-par with standard red, green, and blue displays. Finally, mm-scale colored pixels are demonstrated to be switchable with a liquid crystal display, and therefore immediately compatible with current liquid crystal display technology.
Electron Energy Loss Spectroscopy and Optical Properties of Plasmonic Nanostructure
(2015-04-15) Cao, Yang; Nordlander, Peter J.; Geurts, Frank; Halas, Naomi
Plasmon is considered to be the incompressible self-oscillation of conducting electrons in small nanoparticles. A classical spring model could be used to describe plasmon’s behavior. Many different techniques have been applied to understand nanostructure’s plasmonic properties. Electron energy loss spectroscopy (EELS) is one of these tools, which is helpful for us to understand the interaction between fast moving electrons and nanomaterials. It could achieve very high spatial and energy resolution. Here, we develop a new finite-difference time-domain method to calculate EELS spectra and maps, which is based on a commercial software package “Lumerical”. The calculated results for different cases are compared with the well-known boundary element method (BEM) and show an excellent agreement. Our finite-difference time-domain (FDTD) method to calculate EELS spectra has further been proven really helpful by high-density plasmonic dimers’ experimental results. There are basically two different numerical techniques. One is based on finite difference method (FEM) and another is according to finite-difference time-domain method (FDTD). Both of them are very important to perform optical calculations in nanophotonics and plasmonics area. In general, they will try to solve Maxwell equations with many different boundary conditions numerically. Optical properties of nanomaterials are also very tremendous for us to understand plasmonics behavior in the external electromagnetic fields. We systematically performed FEM simulations for different dimensions’ split ring structure and identified each plasmon mode via induced charge plot. Later we also studied hollow Au Nanoshells: hollow Au-Ag Nanoshell and hollow Au-Co Nanoshell. The former showed the surprising in vivo instability in the near infrared region while the later has potential application in hot electron generation.
Engineered Plasmonic Nanostructures for Infrared Spectroscopy, Refractive Index Sensing and Nonlinear Optics
(2020-03-19) Dong, Liangliang; Halas, Naomi
Surface plasmon – the electromagnetic interaction in metal nanoparticles and nanostructures – has been the topic of intense research activities for many years. Early researchers studied the dependence of plasmon resonance frequency on the size, shape and dielectric environment of the nanoscale system, mainly for sensing applications. The intense and localized field, generated by two adjacent metallic nanostructures when appropriately illuminated, has been utilized for enhancing the sensitivity of vibrational spectroscopy. The near-field enhancement is also responsible for benefiting device properties, such as improving nonlinear frequency conversion efficiency. In this thesis, I will present plasmonic structures with interesting optical properties and discuss their applications in infrared spectroscopy, refractive index sensing and subwavelength nonlinear optics.
High Chromaticity Aluminum Plasmonic Pixels for Active Liquid Crystal Displays
(American Chemical Society, 2016) Olson, Jana; Manjavacas, Alejandro; Basu, Tiyash; Huang, Da; Schlather, Andrea E.; Zheng, Bob; Halas, Naomi; Nordlander, Peter; Link, Stephan; Laboratory for Nanophotonics
Chromatic devices such as flat panel displays could, in principle, be substantially improved by incorporating aluminum plasmonic nanostructures instead of conventional chromophores that are susceptible to photobleaching. In nanostructure form, aluminum is capable of producing colors that span the visible region of the spectrum while contributing exceptional robustness, low cost, and streamlined manufacturability compatible with semiconductor manufacturing technology. However, individual aluminum nanostructures alone lack the vivid chromaticity of currently available chromophores because of the strong damping of the aluminum plasmon resonance in the visible region of the spectrum. In recent work, we showed that pixels formed by periodic arrays of Al nanostructures yield far more vivid coloration than the individual nanostructures. This progress was achieved by exploiting far-field diffractive coupling, which significantly suppresses the scattering response on the long-wavelength side of plasmonic pixel resonances. In the present work, we show that by utilizing another collective coupling effect, Fano interference, it is possible to substantially narrow theﾠshort-wavelengthﾠside of the pixel spectral response. Together, these two complementary effects provide unprecedented control of plasmonic pixel spectral line shape, resulting in aluminum pixels with far more vivid, monochromatic coloration across the entire RGB color gamut than previously attainable. We further demonstrate that pixels designed in this manner can be used directly as switchable elements in liquid crystal displays and determine the minimum and optimal numbers of nanorods required in an array to achieve good color quality and intensity.
Hot Carrier Generation in Nanostructures for Efficient Photocatalysis and Photodetection
(2017-09-28) Zhao, Hangqi; Halas, Naomi; Nordlander, Peter
Surface plasmons are incompressible oscillations of conduction band electrons in metallic nanostructures and have provided a promising route for light-harvesting and light-driven catalysis. Energetic electron-hole pairs, known as hot carriers, are created when plasmons decay through a non-radiative channel and hold extraordinary potential for boosting the efficiency of both photocurrent generation in photovoltaic devices and plasmon-enhanced photocatalysis. In this thesis, the fundamentals and mechanisms of plasmon-induced hot carrier generation were firstly introduced. Then we demonstrated how hot carrier generation could facilitate chemical reactions with the antenna-reactor concept. In this picture, we showed that by directly combining plasmonic and catalytic nanoparticles, the plasmonic nanoantenna could couple strongly with light and induce a forced plasmon in the catalytic reactor, enabling significantly enhanced generation of hot carriers within the catalyst nanoparticles and dramatically increased chemical reaction rates consequently. This could overcome the weak light coupling of traditional transition metal catalysts and provide independent control of chemical and light-harvesting properties of the catalysts by modular design. This approach is investigated and demonstrated by various heterometallic antenna-reactor complexes, including Pd islands decorated Al nanocrystals, Al-Pd heterodimers and Al-Cu2O nanoshell structures. In the second part of the thesis, a novel device for Mid-infrared photodetection was introduced based on efficient collections of hot holes. Apart from its high responsivity rivalling commercially available IR detectors, this photodetector could work on room temperature, which is significantly advantageous over conventional IR detector that requires cryogenic cooling. The devices consists of a plasmonic Al grating that operates both as an electric contact and optical filter, and a p-doped silicon substrates acting as a MIR absorber through free carrier absorption, generally regarded as detrimental in IR detection. The photodetector achieves its high performance through a modulation of the carrier mobility in silicon. Direct electrical read-outs of the absorption spectra of two molecules were performed using this detector, demonstrating its great potential for on-chip molecular vibrational spectroscopy.
Implementation of Hot Electrons in Hybrid Antenna-Graphene Structures
(2013-09-16) Wang, Yumin; Nordlander, Peter J.; Halas, Naomi; Link, Stephan
Graphene, a one-atom-thick sheet of hexagonally packed carbon atoms, is a novel material with high electron mobility due to its unique linear and gapless electronic band structure. Its broadband absorption and unusual doping properties, along with superb mechanical flexibility make graphene of promising application in optoeletronic devices such as solar cell, ultrafast photodetectors, and terahertz modulators. How- ever, the current performance of graphene-based devices is quite unacceptable owning to serious limitations by its inherently small absorption cross section and low quan- tum efficiency. Fortunately, nanoscale optical antennas, consisting of closely spaced, coupled metallic nanoparticles, have fascinating optical response since the collective oscillation of electrons in them, namely surface plasmons, can concentrate light into a subwavelength regime close to the antennas and enhance the corresponding field considerably. Given that optical antenna have been applied in various areas such as subwavelength optics, surface enhanced spectroscopies, and sensing, they are also able to assist graphene to harvest visible and near-infrared light with high efficiency. Moreover, the efficient production of hot electrons due to the decay of the surface plasmons can be further implemented to modulate the properties of graphene. Here we choose plasmonic oligomers to serve as optical antenna since they pos- sess tunable Fano resonances, consisting of a transparency window where scattering is strongly suppressed but absorption is greatly enhanced. By placing them in di- rect contact with graphene sheet, we find the internal quantum efficiency of hybrid antenna-graphene devices achieves up to 20%. Meanwhile, doping effect due to hot electron is also observed in this device, which can be used to optically tune the elec- tronic properties of graphene.
Molecular Plasmonics
(2016-12-16) Cui, Yao; Nordlander, Peter; Halas, Naomi
Graphene supports surface plasmons that have been observed to be both electrically and geometrically tunable in the midto far-infrared spectral regions. In particular, it has been demonstrated that graphene plasmons can be tuned across a wide spectral range spanning from the mid-infrared to the terahertz. The identification of a general class of plasmonic excitations in systems containing only a few dozen atoms permits us to extend this versatility into the visible and ultraviolet. As appealing as this extension might be for active nanoscale manipulation of visible light, its realization constitutes a formidable technical challenge. We experimentally demonstrate the existence of molecular plasmon resonances in the visible for ionized polycyclic aromatic hydrocarbons (PAHs), which we reversibly switch by adding, then removing, a single electron from the molecule. The charged PAHs display intense absorption in the visible regime with electrical and geometrical tunability analogous to the plasmonic resonances of much larger nanographene systems. Finally, we also use the switchable molecular plasmon in PAHs to demonstrate a proof-of-concept low-voltage electrochromic device.
Molecular Plasmonics: Graphene Plasmons in the Picoscale Limit
(2015-08-20) Lauchner, Adam; Halas, Naomi; Nordlander, Peter; Link, Stephan
Doped graphene supports surface plasmons in the mid- to far-infrared that are both electrically and spatially tunable. Graphene has been shown to enable greater spatial confinement of the plasmon and fewer losses than typical noble metals. Reduced-dimensional graphene structures, including nanoribbons, nanodisks, and other allotropes including carbon nanotubes exhibit higher frequency plasmons throughout the mid- and near-infrared regimes due to additional electronic confinement of the electrons to smaller length scales. Recent theoretical predictions have suggested that further spatial confinement to dimensions of only a few nanometers (containing only a few hundred atoms) would result in a near-infrared plasmon resonance remarkably sensitive to the addition of single charge carriers. At the extreme limit of quantum confinement, picoscale graphene structures known as Polycyclic Aromatic Hydrocarbons (PAHs) containing only a few dozen atoms should possess a plasmon resonance fully switched on by the addition or removal of a single electron. This thesis reports the experimental realization of plasmon resonances in PAHs with the addition of a single electron to the neutral molecule. Charged PAHs are observed to support intense absorption in the visible regime with geometrical tunability analogous to plasmonic resonances of much larger nanoscale systems. To facilitate charge transfer to and from PAH molecules, a three-electrode electrochemical cell with optical access was designed, where current is passed through a nonaqueous electrolyte solution that contains a known concentration of PAH molecules. In contrast to larger graphene nanostructures, the PAH absorption spectra possess a rich and complex fine structure that we attribute to the coupling between the molecular plasmon and the vibrational modes of the molecules. The natural abundance, low cost, and extremely large variety of PAH molecules available could make extremely large-area active color-switching applications, such as walls, windows or other architectural elements, even vehicles, a practical technology.
Plasmonic Nanostructures: Optical Nanocircuits, Tunable Charge Transfer Plasmons, and Properties of Fano Resonant Nanoclusters
(2015-09-22) Wen, Fangfang; Halas, Naomi; Nordlander, Peter; Link, Stephan
Metallic Nanoparticles have attracted increasing interest due to their abilities to confine and manipulate light at the nanoscale via the excitation of surface plasmons, the collective oscillation of conduction band electrons. Surface plasmons can focus electromagnetic field into a subwavelength dimension and sense the change of the local dielectric environment, promising properties for surface enhanced spectroscopy and sensing applications. New interesting properties emerge, such as the Fano resonance, when clusters of nanoparticles are brought into close proximities. The reduced light scattering within the Fano resonance corresponds to the intense local fields around and within the clusters, a promising feature for the development of ultrasensitive chemical sensors. Cluster of nanoparticles also support a new plasmon resonance known as the charge transfer plasmon (CTP) when their junctions are made conductive. This thesis will focus on exploring new properties of complex plasmonic nanoclusters and applying them in applications of optical nanocircuits, frequency modulation, and surface enhanced Raman scattering. First, this thesis demonstrates the realization of 3D optical nanocircuits using plasmonic dimer antenna composed of two Au nanodisks separated by a gap. Individual antennas are loaded with media of specific geometries and dielectric properties, acting as optical nanocircuits that tune the resonance of the nanoantennas at visible wavelengths. Series and parallel combinations of nanocircuit elements (nanocapacitors, nanoinductors and nanoresistors) can be realized by appropriately loading specific arrangements of dielectric, semiconducting and metallic nanoparticles in the antenna gap. Second, this thesis investigates the CTP in nanowire-bridged dimer nanoantennas. The CTP arises at lower energies and depends sensitively on the junction conductance, offering a new route for achieving tunable plasmon resonances by modifying junction geometries or materials. Third, this thesis examines the complex near field properties of the Fano resonant plasmonic nanoclusters using the surface enhanced Raman scattering (SRES) both from molecules distributed randomly on the structure and from carbon nanoparticles deposited at specific locations within the structure. It is found that the largest SERS enhancement is achieved when the Fano resonance overlaps with the laser excitation wavelength and the specific stokes mode of the analyte. Finally, the plasmonic properties of the Fano nanoclusters are shown to be substantially modified by the addition of carbon nanoparticles. The placement of several carbon nanoparticles in junctions between multiple adjacent Au particles introduces a collective magnetic plasmon mode into the existing Fano dip, giving rise to an additional subradiant mode in the metallodielectric nanocluster.
Plasmonic Properties of Aluminum Nanostructures
(2015-02-13) Liu, Lifei; Nordlander, Peter J.; Halas, Naomi; Link, Stephan
The plasmonic properties of metallic nanoscale systems have been widely investigated. However the plasmon resonances of the most common plasmonic materials, like gold and silver, are challenging to be tuned into the ultra-violet (UV) region due to their inherent limitations. Recently, aluminum has attracted increasing attention because its plasmon resonances can be extended from the whole visible spectrum into UV region. Also aluminum is a low-cost material of the compatibility to manufacturing process including complementary metal-oxide-semiconductor (CMOS), which allows aluminum to serve as an alternative plasmonic material for commercial applications. In this thesis, measuring the scattering spectra of aluminum nanostructures has been performed, which confirms the tunability of aluminum plasmon resonances. We also directly image the local density of optical states (LDOS) of aluminum nanostructures. Furthermore, the dependence of aluminum plasmon resonances on oxide fractions within the system has been investigated. Finally we demonstrate the feasibility of a promising application of aluminum nanorods for plasmonic-based full-color display.
Sub-100 nm gold nanomatryoshkas improve photo-thermal therapy efficacy in large and highly aggressive triple negative breast tumors
(Elsevier, 2014) Ayala-Orozco, Ciceron; Urban, Cordula; Bishnoi, Sandra; Urban, Alexander; Charron, Heather; Mitchell, Tamika; Shea, Martin; Nanda, Sarmistha; Schiff, Rachel; Halas, Naomi; Joshi, Amit
There is an unmet need for efficient near-infrared photothermal transducers for the treatment of highly aggressive cancers and large tumors where the penetration of light can be substantially reduced, and the intra-tumoral nanoparticle transport is restricted due to the presence of hypoxic or necrotic regions. We report the performance advantages obtained by sub 100 nm gold nanomatryushkas, comprising concentric gold–silica–gold layers compared to conventional ~ 150 nm silica core gold nanoshells for photothermal therapy of triple negative breast cancer. We demonstrate that a 33% reduction in silica–core–gold-shell nanoparticle size, while retaining near-infrared plasmon resonance, and keeping the nanoparticle surface charge constant, results in a four to five fold tumor accumulation of nanoparticles following equal dose of injected gold for both sizes. The survival time of mice bearing large (> 1000 mm3) and highly aggressive triple negative breast tumors is doubled for the nanomatryushka treatment group under identical photo-thermal therapy conditions. The higher absorption cross-section of a nanomatryoshka results in a higher efficiency of photonic to thermal energy conversion and coupled with 4–5 × accumulation within large tumors results in superior therapy efficacy.
Theranostic gold nanoshells and nanomatryoshkas for cancer therapy
(2015-01-20) Ayala-Orozco, Ciceron; Halas, Naomi; Marti, Angel; Nordlander, Peter; Joshi, Amit
This dissertation describes the synthesis of multifunctional gold nanoparticles designed for therapy and diagnosis of cancer (theranostics), and the evaluation of their therapeutic efficacy and bioimaging of tumors in mice. The design of these metallic nanoparticles is aimed to incorporate imaging agents (MRI contrasts and fluorophores) in compact structures with dimensions below 100 nm while keeping their NIR-light-absorbing properties and optimum surface chemistry to enhance accumulation in tumor. The therapeutic response of these metallic nanoparticles is derived from the photoexcitation of their plasmon resonance, the collective oscillation of the conduction band electrons, which was advantageously utilized to enhance the quantum yield of fluorophores resonant in the NIR where the penetration of light is maximal in biological tissue and minimally destructive. Gold nanoshells as absorbers of NIR light can convert the absorbed light into heat consequently causing hyperthermia in the surrounding medium which leads to tumor cell death. To extent the application of previously developed theranostic nanoshells to the highly lethal pancreatic cancer, chapter 2 describes a magneto-fluorescent theranostic nanocomplex targeted to neutrophil gelatinase associated lipocalin (NGAL) receptor in pancreatic cancer. Gold nanoshells (SiO2-Au core-shell nanoshell) resonant at 810 nm were encapsulated in silica epilayers doped with iron oxide and the NIR dye ICG, resulting in a theranostic gold nanoshells, which provided contrast for both T2 weighted MRI and NIR fluorescence optical imaging. The large size of this complex (200 nm) potentially can hinder the accumulation in tumor. Seeking to reduce the size of the theranostic nanoparticles, chapter 3 presents the sub-100 nm Au nanomatryoshkas (Au/SiO2/Au). 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 tumor-bearing mice appeared healthy and tumor free >60 days later, while only 40% 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. Chapter 4 presents the therapeutic efficacy in mice bearing large (>1000 mm3) and highly aggressive triple negative breast tumors. To equip the Au nanomatryoshkas with imaging contrast agents, fluorophores were encapsulated in the internal SiO2 layer of the Au/SiO2/Au matryoshkas as described in chapter 5. We observed strong fluorescence enhancements of the NIR dyes Cy7 and IR800. This behavior can be understood by taking into account the near field enhancement induced by the Fano resonance of the nanomatryoshka, which is responsible for enhanced absorption of the fluorophores incorporated into the nanocomplex. The combination of compact size and enhanced light emission with internal encapsulation of the fluorophores for increased biocompatibility suggests outstanding potential for this type of nanoparticle complex in biomedical applications as it is investigated and presented in chapter 6.

Browsing by Author "Halas, Naomi"

Results Per Page

Sort Options