Understanding Hot-Electron Generation and Plasmon Relaxation in Metal Nanocrystals: Quantum and Classical Mechanisms

Generation of energetic (hot) electrons is an intrinsic property of any plasmonic nanostructure under illumination. Simultaneously, a striking advantage of metal nanocrystals over semiconductors lies in their very large absorption cross sections. Therefore, metal nanostructures with strong and tailored plasmonic resonances are very attractive for photocatalytic applications. However, the central questions regarding plasmonic hot electrons are how to quantify and extract the optically-excited energetic electrons in a nanocrystal. We develop a theory describing the generation rates and the energy-distributions of hot electrons in nanocrystals with various geometries. In our theory, hot electrons are generated owing to surfaces and hot spots. The formalism predicts that large optically-excited nanocrystals show the excitation of mostly low-energy Drude electrons, whereas plasmons in small nanocrystals involve mostly hot electrons. The energy distributions of electrons in an optically-excited nanocrystal show how the quantum many-body state in small particles evolves towards the classical state described by the Drude model when increasing nanocrystal size. We show that the rate of surface decay of plasmons in nanocrystals is directly related to the rate of generation of hot electrons. Based on a detailed many-body theory involving kinetic coefficients, we formulate a simple scheme describing the plasmon's dephasing. In most nanocrystals, the main decay mechanism of a plasmon is the Drude friction-like process and the secondary path comes from generation of hot electrons due to surfaces and electromagnetic hot spots. This latter path strongly depends on the size, shape and material of the nanocrystal, correspondingly affecting its efficiency of hot-electron production. The results in the paper can be used to guide the design of plasmonic nanomaterials for photochemistry and photodetectors.Comment: 90 pages, 21 figures, including Supplementary Informatio

Impurity-enhanced Aharonov-Bohm effect in neutral quantum-ring magnetoexcitons

We study the role of impurity scattering on the photoluminescence (PL) emission of polarized magnetoexcitons. We consider systems where both the electron and hole are confined on a ring structure (quantum rings) as well as on a type-II quantum dot. Despite their neutral character, excitons exhibit strong modulation of energy and oscillator strength in the presence of magnetic fields. Scattering impurities enhance the PL intensity on otherwise "dark" magnetic field windows and non-zero PL emission appears for a wide magnetic field range even at zero temperature. For higher temperatures, impurity-induced anticrossings on the excitonic spectrum lead to unexpected peaks and valleys on the PL intensity as function of magnetic field. Such behavior is absent on ideal systems and can account for prominent features in recent experimental results.Comment: 7 pages, 7 figures, RevTe

Aharanov-Bohm excitons at elevated temperatures in type-II ZnTe/ZnSe quantum dots

Optical emission from type-II ZnTe/ZnSe quantum dots demonstrates large and persistent oscillations in both the peak energy and intensity indicating the formation of coherently rotating states. Furthermore, the Aharanov-Bohm (AB) effect is shown to be remarkably robust and persists until 180K. This is at least one order of magnitude greater than the typical temperatures in lithographically defined rings. To our knowledge this is the highest temperature at which the AB effect has been observed in semiconductor structures

Plasmonic metamaterials and nanocomposites with the narrow transparency window effect in broad extinction spectra

Cataloged from PDF version of article.We propose and describe plasmonic nanomaterials with unique optical properties. These nanostructured materials strongly attenuate light across a broad wavelength interval ranged from 400 nm to S pm but exhibit a narrow transparency window centered at a given wavelength. The main elements used in our systems are nanorods and nanocrosses of variable sizes. The nanomaterial can be designed as a solution, nanocomposite film or metastructure. The principle of the formation of the transparency window in the broad extinction spectrum is based on the narrow lines of longitudinal plasmons of single nanorods and nanorod complexes. To realize the spectrum with a transmission window, we design a nanocomposite material as a mixture of nanorods of different sizes. Simultaneously, we exclude nanorods of certain lengths from the nanorod ensemble. The width of the plasmonic transparency window is determined by the intrinsic and radiative broadenings of the nanocrystal plasmons. Nanocrystals can be randomly dispersed in a solution or arranged in metastructures. We show that interactions between nanocrystals in a dense ensemble can destroy the window effect and, simultaneously, we design the metastructure geometries with weak destructive interactions. We also describe the effect of narrowing of the transparency window with increasing the concentration of nanocrystals. Two well-established technologies can be used to fabricate such nano- and metamaterials, the colloidal synthesis, and lithography. Nanocomposites proposed here can be used as optical materials and smart coatings for shielding of electromagnetic radiation in a wide spectral interval with a simultaneous possibility of communication using a narrow transparency window