Observation of Size Confinement Effects of Excitons in AgX Nanocrystals by Cryo-Energy-Filtering TEM/EELS
- PDF / 400,322 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 26 Downloads / 166 Views
N3.9.1
Observation of Size Confinement Effects of Excitons in AgX Nanocrystals by Cryo-EnergyFiltering TEM/EELS Vladimir P. Oleshko University of Virginia, Department of Materials Science & Engineering, Charlottesville, VA 22904-4745, USA ABSTRACT The non-uniform size-dependent contrast of AgBr0.95I0.05 nanocrystals (NCs) ranging from 22 to 80 nm in equivalent diameter (dc) observed by cryo-energy-filtering TEM is referred to predominant excitations at the surfaces and near the edges. When the fields due to surface losses reach throughout the structure, they couple and the probability for their generation becomes periodic in the NC size. Since electronic sum rules must be satisfied, the surface excitations reduce the strength of the bulk excitations. Coupling of surface and volume losses may cause oscillations of the image intensities with the NC size. The appearance of such oscillations demonstrates a size confinement of excitations of valence electrons due to contributions to the energy-level structure from carrier confinement and surface states. The imaginary part of relative dielectric permittivity derived from electron energy-loss spectra shows an enhanced intensity of the band at 4 eV for NCs with dc = 50±4 nm as compared to those of 109±7 nm in size, while the bands at 7 eV and at 10 eV appear to be suppressed. An increase of the intensity of excitonassisted direct interband transition at 4 eV (Γ8−, Γ6− → Γ6) correlates with the size-dependent enhancement of free exciton luminescence from AgBr NCs, when their size is less than 100 nm. INTRODUCTION Considerable complexity in the valence-band structure caused by hybridization of halogen pstates and Ag 4d-states and occurrence of a low energy indirect gap between the valence and conduction bands in AgX (X = Cl, Br, I) lead to unique size restriction effects on the electronic properties of nanocrystalline silver halides, when the particle size decreases below 100 nm [1-4]. The confinement occurs when the spatial extent of a material (boundary conditions) begins to affect the eigenenergies of the electron wave function, causing the electronic properties to differ from those of bulk material. Considering the relaxation properties of excitons in AgBr, size effects may be expected for crystals much larger than the Bohr radius of the indirect exciton ~3 nm. This is because the restricted geometry leads to the occurrence of surface recombination that becomes a dominant relaxation channel in crystals with sizes less than 500 nm [5]. Quantum size effects of excitons in AgX nanocrystals (NCs) have been demonstrated by the appearance of the indirect exciton luminescence forbidden by selection rules in large crystals and by an increase of the quantum yield by several orders of magnitude likely affected by surface states when the particle size is decreased from 80 nm to 7.9 nm [6,7]. A high energy shift of luminescence band maxima of free excitons in the case of Ag(Br,I) NCs with sizes of 40 nm and less was also observed [7,8]. In order to determine the critical size for t
Data Loading...
