Munir Hasan Nayfeh

Reflection-based lensing is used to focus or collimate extreme ultraviolet (EUV)/x rays (x-ray optics) propagating in grazing incidence small-angle x-ray scattering (GISAXS) inside poly-capillary glass. In the process, the refractive index of silica is slightly less than that of vacuum (1.0) for EUV/x rays, leading to total external reflection with a critical angle slightly less than 90°. Here, we examine the feasibility of widening the total external reflection window by coating the capillary interior surface with a silicon nanofilm that assumes the shape of the polycapillary, which may enhance the collection and collimation power. We use slow dynamic self-assembly via circulation of the nanoparticles through the capillary. Electromagnetic scattering computations are conducted using the model of a flat glass substrate decorated with closely packed Si nanoparticles. Optical, luminescent, and SEM imaging techniques, as well as spectroscopy, show that the polycapillary coating is uniform and luminescent under UV irradiation. The computations show that, for visible wavelengths, a 3 nm silicon nanoparticle film reduces the grazing reflectivity at the visible/UV band-to-band transitions E1 and E2. For EUV/x ray, there is a significant reflectivity enhancement of ∼10% due to a larger index dip below that of vacuum, affording a smaller critical angle. Widening the external reflection regime promises superior collection and focusing power, improving the intensity and resolution. Being luminescent under UV makes it useful for imaging, therapy, and other clinical procedures, while also enabling the development of more compact and versatile instruments.

We use silicon-based nanograins as model nanodust in planetary nebulae and present photo-excitation and stability studies using synchrotron extreme ultraviolet radiation, while monitoring the induced cascade of visible/UV luminescence. We also conduct theoretical studies (atomistic simulations and classical Mie scattering), as well as stability studies of the grains under long-wavelength UV excitation using lasers or discharge lamps or under thermal treatments. We report that the luminescence of 1-nm grains remains stable for above ionization limit excitation [5–22 eV, 1012 (photons/cm2)/s]. Under excitation below the ionization limit, using lasers or discharge lamps (3.5 eV, 1014/cm2/s) or under thermal treatment, the luminescence exhibits partial stability at a steady ∼50%, with slow partial recovery. Time-dependent density functional theory shows the structural stability of neutral or ionized ultrasmall nanograins, while organic dye molecules are fully quenched with no recovery. Computations also show the enhancement of scattering of soft x rays over the geometrical cross section. We analyze the results in terms of quantum confinement induced effects, including inhibition of e–h and e–Coulomb scattering, enhancement of e–e correlation, and relativistic e–vibration coupling. These effects lead to multi-electron excitation, singlet–triplet intersystem conversion, and plasmon-type Mie “polarizmon scattering” by valence electrons. Such novel characteristics point to the survivability of ultrasmall grains in x-ray or UV environments, which may serve as a UV shield for large interstellar molecules, necessary to life.

Gold nanoparticles (AuNPs) have been targeted as novel contrast agent for computerized tomography (CT). However, AuNPs suffer from low‐contrast factor in the X‐ray regime. Functionalization of AuNPs with folic acid or sugar‐based molecules to induce selective uptake have displayed contrast enhancement with improved image brightness and CT signal intensity. However, it was not clear what the basic mechanism for the contrast enhancement was and whether it was related to the uptake enhancement or to a fundamental electromagnetic interaction effect. In this work, we conducted near‐field Mie as well as finite‐difference time‐domain (FDTD) field distribution of the scattering to discern the effect of a thin dielectric coating layer on the contrast functionality of AuNPs. Our results show that upon the incorporation of the dielectric shell (thin film or nanoparticle layer), the cross section of X‐ray scattering is enhanced, with silicon being more effective than silica coating, with multiresonance spectral response. The directionality and range and strength of the near field increase for silicon coating (high electron density or high k material in the visible). The effect may be understood in terms of several features. Even though the refractive indices of all materials in the X‐ray regime are ∼ 1.0, the wavelength dependence of their approach may exhibit sizeable differences The enhancement is understood in terms of high densities of polarization charge especially in silicon, which allows multipole resonances. The multiplicity of resonances leads to enhanced scattering and directionality (angular distribution) with reduced range. A silicon‐coating layer on AuNP may not only alleviate the contrast limitation, but it may afford synergistic integration of luminescence and scattering functionalities in the visible and X‐ray regimes.

Observation of nanosilicon-based contributions to the interstellar nanodust is problematic because the indirect-bandgap of silicon makes its optical features wide, while carbon’s higher abundancy and ionization potential and the rising slope of extinction curves introduce heavy convolution. Recent macroscopic synthesis and charging of nanosilicon, the coming online of the Webb space telescope with unprecedented spectral resolution, and advances in modeling algorithms, light scattering, and fundamental atomistic computation may open opportunities for effective comparison between laboratory and space observation. Here, we study the transport of charged nanosilicon in electric/magnetic fields. We use high voltage across liquid colloids to charge and propel nanosilicon into external fields and imprint them on metal-coated substrates. We use absorption, luminescence, and light scattering in liquid, flight, and imprinted surfaces to study the field deflection of nanosilicon. We use the Mie/finite-difference time-domain theory to obtain scattering curves of nanosilicon and silica. Nanosilicon-based UV features near the 217.5-nm carbon bump are recorded and calculated using Time-Dependent Density Functional Theory (TDDFT) atomistic theory at 225, 280, and 153 nm resulting from bound–bound, and valence-continuum transitions, respectively. We also show that the constituents of silicates, oxygen and Mg and Fe metal ions, can attach to Si nanoparticles without interrupting luminescence, infrared, or UV signatures, respectively. Because charge defects allow nanosilicon transport over large distances via open B fields of solar holes as well as provide them with narrow “atomic-like” transitions, which are otherwise extended, sightlines with lower carbon and higher resolution afforded by Webb may allow the unmasking of Si-based features.

A thin film of silicon-based nanobubbles was recently suggested that could block a fraction of the sun’s radiation to alleviate the present climate crisis. But detailed information is limited to the composition, architecture, fabrication, and optical properties of the film. We examine here the optical response of Si nanobubbles in the range of 300–1000 nm to evaluate the feasibility using semi numerical solution of Maxwell’s equations, following the Mie and finite-difference time-domain procedures. We analyzed a variety of bubble sizes, thicknesses, and configurations. The calculations yield resonance scattering spectra, intensities, and field distributions. We also analyzed some many-body effects using doublets of bubbles. We show, due to high valence electron density, silicon exhibits strong polarization/plasmonic resonance scattering and absorption enhancements over the geometrical factor, which afford lighter but more efficient interception with a wide band neutral density filtering across the relevant solar light spectrum. We show that it is sufficient to use a sub monolayer raft with ∼0.75% coverage, consisting of thin (∼15 nm) but large silicon nanobubbles (∼550 nm diameter), to achieve 1.8% blockage of solar light with neutral density filtering, and ∼0.78 mg/m2 silicon, much less than the mass effective limit set earlier at 1.5 g/m2. We evaluated solid counterpart nanoparticles, which may be produced in blowing/inflation procedures of molten silicon, as well as aging by including silicon oxide capping. The studies confirm the feasibility of a space bubble filtering raft, with insignificant imbalance of the correlated color temperature (CCT) and color rendering index characteristics of sunlight.

The classical light interactions of nanosilicon, which is a dielectric material, are exceedingly weak for radius r ≪ λ (wavelength), scaling as r6. It exhibits geometrical anisotropy-based depolarization, which is the basis for the very weak response in isotropic structures (nanosphere). Recently, surface enhanced Raman scattering (SERS) in DNA decorated with ultrasmall Si nanoparticles has been demonstrated, affording an effective alternative to plasmon–metal particles. In this paper, we execute fundamental quantum atomistic computation of 1 nm hydrogenated Si particles, with different surface reconstruction and termination, including Si–H, H–Si–Si–H (dimer molecules), or oxygenated dimer bridges (H–Si–O–Si–H). We obtain the mechanical vibrational modes of the particles. Our results show that by changing the surface configuration one can control the symmetry and normal vibration modes, and enhance the polarizability, polarity, and light interactions (scattering, absorption, and depolarization/memory). The low frequency polarizability (Raman scattering) shifts spatially from the interior to the surface, while the infrared polarity remains on the surface, but its bandwidth narrows spectrally. The results support previous infrared absorption and light scattering and depolarization measurements, as well recent SERS, which enable Si nanoparticles to be an effective alternative to plasmonic metal particles. Molecular surface reconstruction in terms of Si dimers and bridges were suggested as the source of the novel nonlinear and anisotropic luminescence and photonic properties of Si nanoparticles. Such control affords potential for optimizing the design and operation of a wide range of opto-electronic advanced scattering and luminescence devices.

Silicon, a highly symmetric and homogeneous material, does not exhibit fast optical modulation. Recent classical electrodynamics simulations, however, demonstrated transient optical heterogeneity in silicon nanostructures, in which a high-density of excitonic electron–hole pair plasma and charge is created. The phenomenon, however, requires a specific particle size (∼100 nm diameter) and a high-density (10²³/cc) plasma. We examine here the quantum aspect of the heterogeneity in 1-nm Si nanoparticles. Due to the small number of atoms, 1 nm nanoparticles are amenable to the Hartree–Fock first principle atomistic quantum theory simulations procedure, while single ionization events are sufficient to provide high charge density (2 × 10²¹/cc). The simulations show that the charge distribution in singly charged 1-nm particles is nonlinear and heterogeneous, accompanied with structural distortion that produces an electric dipole moment. Electronically, the simulations show that the single charge induces stationary Coulomb states that riddle the bandgap of the neutral particle, with dipole-allowed transitions, effectively inducing partial conducting-like behavior. Optically, when the charge is produced by ionizing UV radiation, the ionized particle survives and exhibits both extended (wide-band) as well as atomic- or ion-like sharp emission, in agreement with infrared polarimetry and spectroscopy observations in the solar coronal holes, as well as under synchrotron irradiation. Not only do ionized Si nanoparticles (charged nanosilicon grains) afford fast optical modulations, but they may also prove pivotal for understanding features of interstellar medium, observed throughout the Milky Way and other galaxies, including spectroscopic and material composition, as well as neutral hydrogen abundancy.

The thermodynamics of nanosystems is interesting, as they constitute the transition between the atomistic and solid states. This is empowered by the development of tools to manipulate individual atoms and perform atomistic simulations and fundamental thermos-science, such as microscopic time-symmetry and macroscopic time-asymmetry, the origin of time’s arrow, and photo-cryo-refrigeration. We examine here the photo-thermo and time dynamics in 1-nm silicon nanoparticles with tetrahedral-molecular core–shell structure prepared ex situ and suspended in solvents or re-constituted in films. We examined the temperature dependence of the quantum efficiency and time-dynamics of the Stokes luminescence and its energy dependence across the band. With temperature, we get flat lifetimes but with delay in the onset in agreement with a model calculation of above barrier emission. Our atomistic time-dependent density functional theory shows that Stokes heating takes place in the molecular-like shell where the lifetime is in the nanosecond regime, whereas anti-Stokes cooling takes place in the tetrahedral core where the lifetime is in the ms regime. Unlike doped glasses, we observed a 2-order of magnitude increase in the quantum efficiency of the Stokes luminescence at 10° K. The increase in the quantum efficiency at low temperature, the high quantum efficiency of stimulated anti-Stokes scattering and its anti-correlation with the luminescence, and the visible transparency/blindness due to quantum confinement are requirements for solid state photo-cooling, which may afford an all-silicon photo-cryo-refrigeration, with potential full integration into the CMOS silicon industry.

Metal nanostructures create near-field super hotspots under light irradiation with a range limited to a few nanometers. The intense field in the spot affords enhanced nonlinear optical processes, such as Raman spectroscopy. The intense field, however, can cause heavy distortion and thermal damage to the molecular specimen as well as heavy convolution with the metal electronic structure. In this work, we simulate concentric layered silicon–metal core–shell (and its inverse) nanostructures that may alleviate the disadvantages of the pure metal environment. Our results using Mie and finite-difference time-domain scattering studies show that, in addition to the super hotspot at the gold–silicon interface, there emerges a super hotspot at the silicon–vacuum interface, whose intensities anti-correlate and are tuned by tuning the silicon thickness. Moreover, the plasmonic resonance red shifts with the thickness of the silicon shell, reaching a terminal wavelength of ∼840 nm. These features are understood in terms of induced polarization charge at the silicon–metal and silicon–vacuum interfaces, which for high κ materials (13.32) can be significant. The metal–silicon system creates plasmon–polarizmon hotspots tunable in strength and wavelength content that can be designed to alleviate high field damage, useful for Raman scattering and photovoltaic applications. The integrated metal–silicon system also promises field enhancement of visible luminescence of silicon nanoparticles, useful for imaging and tracking applications.

Surface enhanced Raman scattering (SERS), in which sample molecules are placed in the proximity of conducting nanostructures, subjects the molecules to intense electron oscillation (plasmon) field. The intense field, however, may cause heavy distortion and thermal damage to the molecule as well as non-separable and heavy convolution with the metal electronic structure. We utilized 3-nm red luminescent Si nanoparticles decorating the DNA molecules (drawn electrostatically) to enhance Raman scattering in solution at 532 nm. We demonstrated that the nanoparticles enhance the spectral resolution and intensity of vibrations of DNA by two orders of magnitude and reveal vibrations that are otherwise weak or forbidden. Theoretically, we conducted calculations of Mie scattering and three-dimensional finite-difference time-domain scattering and obtained the wavelength dependence of the near-field distribution from single or dimer Si particles. The simulations show moderate intensity enhancement (25–40-fold) and exciton resonances. Moreover, it shows that the near field is highly confined, extending only to 3–5 Å from the Si particle (atomic scale) compared to several nanometers for metal nanoparticles. The observed SERS-type characteristics are understood in terms of polarization-based light scattering, which is possible by the use of Si of highly reduced size for which the polarizability and exciton processes are strong. However, multilayers contribute to metal SERS, and monolayers/single molecules dominate the Si case. Weaker but highly confined, ultra-short range polarization-based scattering provides an alternative to plasmon and Mie scattering, while providing practical, straightforward interpretation of vibration printing of bio-medical species without compromising the molecular structure.