Size Confinement Effects on Electronic and Optical Properties of Silver Halide Nanocrystals as Probed by Cryo-EFTEM and EELS

Abstract

Cryo-energy-filtering transmission electron microscopy and electron energy-loss spectroscopy have been applied to study size confinement effects on electronic, dielectric, and optical properties of Ag(Br, I) nanocrystals. The dielectric permittivity, optical joint density of states, refractive index, extinction, and absorption for nanocrystals derived via Kramers-Kronig relations have been compared with experimental data for Ag(Br, I) tabular microcystals and ab initio linear muffin-tin orbital method in its atomic spheres approximation calculations for AgBr. Contrast tuning with the selected energy window between 0 and 100 eV enabled visualizing valence electron excitations in silver halides because of plasmons superimposed with interband transitions and Mott-Wannier excitons. The nonuniform contrast of nanocrystals revealed by cryoenergy-filtering transmission electron microscopy was referred to a size-confined coupling of surface and volume losses that could lead to oscillations of the image intensity with the nanocrystal size. A size-dependent enhancement of interband transitions * À 8 ; * À 6 ! * þ 6 at 4 eV correlated with the enhancement of exciton luminescence from nanocrystals because of contributions to the energy-level structure from carrier confinement and surface states.

Optical excitations in electron microscopy

“…This is the so-called Landau damping, which has been phenomenologically accounted for through a size-dependent local dielectric function, with the dephasing rate increased by ∼ a/v F (Batson, 1980;Hövel et al, 1993;Kreibig and Vollmer, 1995). Nonlocal effects in noble metal particles are noticeable up to a size ∼ 10−20 nm at optical frequencies, but it can be important in other materials for larger sizes ∼ 100 nm (e.g., silver halide nanocrystals, studied with EELS by Oleshko, 2008).…”

Growth Kinetics of Silver Bromide Nanoparticles in Aqueous Nonionic Surfactant Solutions

“…Although many studies are available on the preparation of silver halide nanoparticles, − there are limited studies on the kinetics of particle formation. , Studies on the kinetics of particle formation can provide important information about the nucleation and growth of the particles. Among the reported studies, different microemulsions have mostly been used as nanoreactors to control the particle size.…”

In situ Determination and Imaging of Physical Properties of Soft Organic Materials by Analytical Transmission Electron Microscopy

“…The method is based on analytical extraction of the material related signal from the valence bulk plasmon EFTEM image. The valence electron energy-loss spectroscopy technique, which records excitation of outer-shell electrons in the low energy-loss range (0–50 eV), is highly sensitive to the valence electron states that are primarily related to intrinsic properties of the material (Oleshko et al, 2002; Oleshko, 2012; 2008). For crystalline conductive solids (metals, ceramics, etc.)…”

“…Probing of low-loss electron excitation(<50 eV energy loss by electron energy-loss spectroscopy (EELS) within a ATEM) involves inter- and intraband transitions and collective excitation of bonding electrons which allows one to study a variety of characteristics and processes responsible for intrinsic physical and chemical properties of solids (Egerton, 2011). This includes (1) band structure, band gap, bonding, dielectric, and optical responses (Oleshko et al, 2000; Oleshko, 2002; Howe & Oleshko, 2004; Botton, 2007; Egerton, 2007; Nellist, 2007; Oleshko, 2008; Reimer & Kohl, 2008; Williams & Carter, 2009); (2) local electron excitation (surface or interface plasmons, exciton states) and size confinement effects due to contributions from carrier confinement and surface states (Egerton, 2007; Oleshko, 2008; Reimer & Kohl, 2008; Williams & Carter, 2009; García De Abajo, 2010); and (3) phase composition (Williams and Edington, 1976; Sigle et al, 2003). In this way, a low-loss EELS became the discipline which is concerned with characterization of solid materials on a variety of levels.…”