Plasmon-enhanced multi-photon excited photoluminescence of Au, Ag, and Pt nanoclusters

In this work, we have studied the multi-photon excited photoluminescence from metal nanoclusters (NCs) of Au, Ag and Pt embedded in Al2O3matrix by ion implantation. The thermal annealing process allows to obtain a system composed of larger plasmonic metal nanoparticles (NPs) surrounded by photoluminescent ultra-small metal NCs. By exciting at 1064 nm, visible emission, ranging from 450 to 800 nm, was detected. The second and fourth-order nature of the multiphoton process was verified in a power-dependent study measured for each sample below the damage threshold. Experiments show that Au and Ag NCs exhibit a four-fold enhanced multiphoton excited photoluminescence with respect to that observed for Pt NCs, which can be explained as a result of a plasmon-mediated near-field process that is of less intensity for Pt NPs. These findings provide new opportunities to combine plasmonic nanoparticles and photoluminescent nanoclusters inside a robust inorganic matrix to improve their optical properties. Plasmon-enhanced multiphoton excited photoluminescence from metal nanoclusters may find potential application as ultrasmall fluorophores in multiphoton sensing, and in the development of solar cells with highly efficient energy conversion modules.

Photoluminescent properties of liposome-encapsulated amine-functionalized nanodiamonds

In the present work, amine-functionalized nanodiamonds (NDs) have been encapsulated in liposomes and studied in order to observe the modification of their photoluminescence properties. NDs were functionalized with aromatic amines such as 1-aminopyrene and 2-aminofluorene, and the aliphatic amine 1-octadecylamine. Morphology, structural and optical properties of NDs and amine-modified NDs were analyzed by transmission electron microscopy, atomic force microscopy, scanning electron microscopy, and photoluminescence. The amine-functionalized NDs were successfully encapsulated in lecithin liposomes prepared by the green and conventional methods. The obtained results show significant changes in photoluminescent properties of functionalized NDs, and were more potentialized after liposome encapsulation. Our findings could be applied in the development of new kinds of water-dispersible fluorescent hybrids, liposome-NDs, with the capability of drug encapsulation for use in diagnostics and therapy (theragnostic liposomes). All-optical sensors with possibilities for tailoring their response for other biomedical applications can be also contemplated.

Enhanced ultrafast optomagnetic effects in room-temperature ferromagnetic Pt nanoclusters embedded in silica by ion implantation

Ferromagnetic-like behavior at room temperature (300 K) was observed in Pt particles embedded in ion-implanted silica matrices. Results in samples integrated by ultra-small photoluminescent Pt clusters (<2 nm) were compared with samples containing exclusively larger plasmonic Pt nanoparticles (>3 nm). The ferromagnetic behavior coexists simultaneously with a diamagnetic response. Enhanced diamagnetic response of one order of magnitude was observed compared to typical diamagnetism in pure silica, and it is increased with the mean diameter of the Pt particles. Besides, a larger sensitivity to an external field was observed in the ferromagnetic response of the nanostructures with a characteristic saturation at 20 kOe. This ferromagnetic behavior was only observed in the samples with nucleated Pt particles. The magnitude of the saturation magnetization shows up to a fivefold increase in the samples with smaller particle size and larger particle density. Saturation magnetization was observed between 3-15 × 10^-4 emu g^-1, with remanent magnetization of 0.2-0.6 × 10^-4emu g^-1, measured at 300 K. Coercitive fields also decrease in samples with smaller size and particles density, with values of 114 and 300 Oe. At lower temperatures (5 K) the saturation magnetization increases, as it would be expected from a ferromagnetic state. Optomagnetic response was studied by inverse Faraday effects and induced photomagnetization with circular polarized picosecond pulsed light at 1064 nm wavelength. Results showed that samples with a stronger ferromagnetic response exhibit larger Faraday rotation up to 5.3 × 10³deg cm^-1 by light excitations with irradiances between 50 and 180 GW cm^-2. These findings have immediate applications in multifunctional solid-state magneto-optical devices such as optical isolators, high-data storage devices and ultrafast all-optical switching of magnetization.

Plasmon coupling interactions and inhibition of nonlinear absorption in a complex system with Ag and Pt nanoparticles in silica

The optical response exhibited by a complex hybrid system integrated by Pt ultrasmall fluorescent particles and plasmonic Ag nanoparticles is reported. The system was synthesized by coimplantation of Ag and Pt ions into a silica matrix followed by a proper thermal annealing. The energies and fluences were chosen in order to overlap the spatial regions of the Ag and Pt ion distributions below the silica surface. Optical absorption and emission spectroscopies show that the complex nanostructures exhibit an important plasmonic response, together with photoluminescence excited at 355 nm, which is enhanced when compared to the reference sample with only Pt particles. Off-resonance nonlinear transmission and Z-scan measurements were undertaken using ultrafast pulses. High-irradiance excitation at 1064 nm with picosecond pulses shows that the Pt or Ag nanoparticles exhibit a two-photon absorption effect, while the complex system shows the absence of any nonlinear absorption. Similar observations were made using femtosecond pulses at 800 nm wavelength. This inhibition of the two-photon absorption effect and enhancement in the emission of the complex hybrid samples by the synergic participation of Ag and Pt particles can be explained as a result of a plasmon coupling via the near-field interaction between plasmonic and emitting sources.

Superlinear Photoluminescence by Ultrafast Laser Pulses in Dielectric Matrices with Metal Nanoclusters

An intense photoluminescence emission was observed from noble metal nanoclusters (Pt, Ag or Au) embedded in sapphire plates, nucleated by MeV ion-implantation and assisted by an annealing process. In particular, the spectral photoluminescence characteristics, such as range and peak emission, were compared to the behavior observed from Pt nanoclusters embedded in a silica matrix and excited by UV irradiation. Correlation between emission energy, nanoclusters size and metal composition were analyzed by using the scaling energy relation EFermi/N1/3 from the spherical Jellium model. The metal nanocluster luminescent spectra were numerically simulated and correctly fitted using the bulk Fermi energy for each metal and a Gaussian nanoclusters size distribution for the samples. Our results suggest protoplasmonics photoluminescence from metal nanoclusters free of surface state or strain effects at the nanoclusters-matrix interface that can influence over their optical properties. These metal nanoclusters present very promising optical features such as bright visible photoluminescence and photostability under strong picosecond laser excitations. Besides superlinear photoluminescence from metal nanoclusters were also observed under UV high power excitation showing a quadratic dependence on the pump power fluence.