Oleg A. Yeshchenko

Surface plasmon as a probe for melting of silver nanoparticles

The temperature dependence of the surface plasmon energy and bandwidth for silver nanoparticles in the size range 8-30 nm embedded in a silica matrix has been studied using diffuse reflection spectroscopy. The dependence shows a non-monotonic jump-like behaviour indicating a low-temperature size-dependent melting of silver nanoparticles. The melting point decreases with the decrease of the nanoparticle size. The hysteresis in the temperature dependence of the surface plasmon bandwidth has been observed, indicating the first-order phase transition.

Influence of interparticle interaction on melting of gold nanoparticles in Au/polytetrafluoroethylene nanocomposites

The temperature dependence of the surface plasmon resonance energy and width in 5 nm spherical gold nanoparticles embedded in a polymer polytetrafluoroethylene matrix has been studied using absorption spectroscopy. The jumplike features have been observed in these dependences indicating the melting of gold nanoparticles at temperatures considerably lower than the bulk melting point. The interaction between gold nanoparticles sufficiently affects the melting of nanoparticles. The increase in the filling factor of the particles leads to a decrease in the melting temperature of gold nanoparticles.

Optical study of ZnP2 nanoparticles in zeolite Na–X

We report that for the first time the nanoparticles of II-V semiconductor (ZnP2) were prepared and studied. ZnP2 nanoparticles were prepared by incorporation into zeolite Na-X matrix. Absorption, diffuse reflection (DR) and photoluminescence (PL) spectra of the ZnP2 nanoclusters incorporated into the supercages of zeolite Na-X were measured at the temperature 77 K. Five bands B1-B5 are observed in both the DR and PL spectra demonstrating the blue shift from the line of free exciton in bulk crystal. We attribute the B1-B5 bands to some stable nanoclusters with size less than the size of zeolite Na-X supercage. We observed Stokes shift of the PL bands from the respective absorption bands. The nonmonotonic character of its dependence on the cluster size can be explained as the result of competition of the Frank-Condon shift and the shift due to electronic relaxation.

Surface plasmon enhanced photoluminescence from fullerene C60 film on Au nanoparticles array: Resonant dependence on excitation frequency

The photoluminescence from the fullerene C60 thin films on array of the gold nanoparticles was studied at various energies of exciting laser photons. An enhancement of the photoluminescence of C60 film on array of Au nanoparticles was observed. The observed enhancement is due to strong local field of surface plasmon excited in the gold nanoparticles. Tuning of the exciting photon energy allowed to control the quantum yield of C60 film photoluminescence. When the exciting photon energy is close to the surface plasmon resonance, the quantum yield reaches its maximum. This fact confirms the resonant character of the surface plasmon enhancement of the studied photoluminescence of C60 film.

Electrodynamic coupling in regular arrays of gold nanocylinders

We report far-field experimental optical extinction spectra of linear chains of gold nanocylinders with interparticle separations close to the particles surface plasmon resonance (SPR) wavelength λSPR. The spectra reveal non-monotonic shift of the SPR peak position with increase in the distance d. The non-monotonic λSPR(d) dependence is rationalized considering electromagnetic coupling between the particles in the dipole approximation. Simple expressions for the modified SPR frequencies are obtained using the dipole scattered field and taking into account both the retardation and phase lag effects.

Temperature Dependence of Photoluminescence from Silver Nanoparticles

Strong temperature dependence of surface plasmon enhanced photoluminescence from silver nanoparticles embedded in a silica host matrix has been observed. The quantum yield of photoluminescence increases as the temperature decreases. Such an effect has been rationalized as being the result of an increase in the plasmonic enhancement factor as a consequence of the decrease in the plasmon damping constant. The decrease in the damping constant is due to a reduction in the electron–phonon scattering rate with the decrease in temperature. The temperature dependence of the photoluminescence quantum yield is stronger for small nanoparticles which reflects the strengthening of electron–phonon coupling in silver nanoparticles with a decrease of their size.

Multiserial Structure of Excitonic Energy Spectrum in Monoclinic ZnP2 Crystal

One-photon excitonic absorption spectra of the monoclinic ZnP 2 (β-ZnP 2 ) crystals are analysed. There exist four series characterized by the serial law E n = E g - Ry/n 2 + Δ/n 3 with the same value of energy gap Eg = 1.6026 meV. By variational calculations of the exciton energy dependence on the anisotropy parameter, we conclude an appearance of several hydrogen-like series caused by the splitting of excitonic states with different quantum numbers l and |m| due to the low symmetry of the crystal. Conclusions are made that two series with Δ = 0 can be attributed to the P-states of an exciton, and two others with Δ ¬= 0 to the S-states of an exciton. Calculations of e-h exchange interaction in an S-exciton are performed. Splitting of the series with Δ ¬= 0 is interpreted to have exchange origin (ortho-para-splitting). Its value is obtained to be 12.7 meV.

Surface Plasmon Modes of Sandwich-Like Metal–Dielectric Nanostructures

We established that metal–dielectric nanostructures (consisting of three stacked monolayers of silver nanoparticles incorporated into polymer cross-linked film) can demonstrate three different surface plasmon collective modes. The modes were detected when the extinction spectra of the nanostructures were studied as a function of the incident angle and polarization of the incident light. Two previously known surface plasmon collective modes, namely T and P, associated with particle dipoles parallel and perpendicular to plane of the layer were identified for the polymer films containing one, two, and three monolayers of the particles. The extinction bands of T and P modes exhibited different intensity and frequency dependences on the angle of incidence. More pronounced angular dependences for P mode band indicated the stronger coupling of dipoles for P mode than for T one. A new N mode was observed for the structures consisting of three nanoparticle layers. This mode originated from surface plasmon coupling between adjacent layers. The additional mode significantly increases amount of information that can be obtained from optical response of the nanostructures.

Surface plasmon enhanced photoluminescence from copper nanoparticles: Influence of temperature

Anomalous temperature dependence of surface plasmon enhanced photoluminescence from copper nanoparticles embedded in a silica host matrix has been observed. The quantum yield of photoluminescence increases as the temperature increases. The key role of such an effect is the interplay between the surface plasmon resonance and the interband transitions in the copper nanoparticles occurring at change of the temperature. Namely, the increase of temperature leads to the red shift of the resonance. The shift leads to increase of the spectral overlap of the resonance with photoluminescence band of copper as well as to the decrease of plasmon damping caused by interband transitions. Such mechanisms lead to the increase of surface plasmon enhancement factor and, consequently, to increase of the quantum yield of the photoluminescence.

Radiative transitions in an excitonic molecule

Abstract The ratio of intensities of inverse hydrogenlike series (IHS) luminescence bands which are due to two-electron transitions in an excitonic molecule (EM) have been calculated. The rates of two-photon and two-electron spontaneous transitions in the excitonic molecule have been estimated. Their ratio has been obtained as 0.13. The correlation between theoretical estimations and experimental results obtained from the luminescence spectra of β - ZnP2 crystals has been analyzed.