Atomistic origin and temperature dependence of Raman optical redshift in nanostructures: a broken bond rule

Abstract

Consistent insight is presented into the atomistic origin and temperature (T) dependence of the redshift of Raman optical modes in nanostructures from INTRODUCTIONThe vibration of under-coordinated atoms at a surface and in a nanosolid is of great importance because the behavior of surface phonons influences directly the electrical, optical, and dielectric properties in semiconductor materials and devices, such as electron-phonon coupling, photoabsorption, and photoemission, as well as phonon scattering in the transport dynamics of electrons, phonons, and photons in devices. 1 -5 With miniaturization of a solid down to nanometer scales, the optical Raman modes shift toward lower wavenumbers. 6 On the other hand, when the temperature is increased, the Raman peaks also shift down to low wavenumbers monotonically yet nonlinearly at low temperatures. 7 -9 It is expected that the magnitude of vibration for a surface atom is always greater than that in the bulk. 10,11 However, the underlying mechanisms behind the size-and temperatureinduced Raman redshift of the optical modes remain ambiguous.Ł Correspondence to: Chang Q. Sun, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore. E-mail: ecqsun@ntu.edu.sg Size-induced Raman redshiftThe size-induced redshift of Raman optical modes has usually been suggested to be activated by surface disorder, 12 surface stress, 13 -15 quantum confinement, 16 -19 local heating effects, 20 -22 or surface chemical passivation. The Raman shifts of TiO 2 particles are attributed to the effects of decreasing particle size on the force constants and the amplitudes of vibration of the nearest atomic neighbors. 23 However, the effect of stress can usually be ignored for hydrogenated silicon, 24,25 in which hydrogen atoms terminate the surface dangling bonds, which reduce the bond strains and hence the residual stress. The phonon quantum confinement argument 16 attributes the redshift of the asymmetric Raman line to the relaxation of the q-vector selection rule for the excitation of the Raman active phonons due to their localization. The relaxation of the momentum conservation rule arises from the finite crystalline size and the distribution of the diameters of the nanosolid in the films. When the size is decreased, the rule of momentum conservation will be relaxed and the Raman active modes will not be limited to the center of the Brillouin zone. 13 However, Alim et al. 21,22 who have measured resonant and nonresonant Raman scattering spectra for ZnO nanocrystals with an average diameter of 20 nm, proposed that the observed phonon redshift can be attributed to the local heating effects instead of phonon Atomistic origin and temperature dependence of Raman redshift in nanostructures 781 confinement effects. A Gaussian-type phonon confinement model 17 that has been used to fit the experimental data indicates that strong phonon damping is present, whereas calculations 26 using the correlation functions of the local dielectric constant...

Microstructure and optical properties of polycrystalline ZnO films sputtered under different oxygen flow rates

“…The other two peaks observed at 276 and 595 cm −1 are the results of the host lattice defects. In addition, at P O = 0 sccm, the peak near 408 cm −1 shifts to 418 cm −1 , and there appears such a weak peak at 380 cm −1 because the optical modes of the Zn-O bonds can be decided by the bond length, bond angle, bond strength, and atomic coordination numbers [29]. There are defects in the films fabricated at P O = 0 sccm so that the bond length, bond angle and bond strength of the Zn-O bonds can be affected.…”

Effects of the agglomeration state on the Raman properties of Co3O4 nanoparticles

“…The population of non‐equilibrium phonons decays into lower energy phonons as an effect of the anharmonic interaction or is scattered by the thermal phonons into modes with different frequencies. These phonon–phonon interactions are of special interest in nanostructured systems due to the lack of an adequate description in the literature . Previous studies have shown that the Raman properties of materials at high temperature are size dependent .…”

Anharmonicity Effects in Nanocrystals Studied by Raman Scattering Spectroscopy

“…Any frequency shift or the variation of the half--width Γ with temperature of the Raman modes are the measurements of the anharmonicity of the interactions. The phonon-phonon interactions are of special interest in nanostructured systems if we have in mind that in the literature there is still no adequate theory which describes the anharmonic effects in these systems [5,6].…”