Silver nanowires as plasmonic compensators of luminescence quenching in single up-converting nanocrystals deposited on graphene

Abstract

Single nanocrystal spectroscopy is employed to demonstrate metal-enhanced optical response of Er³⁺/Yb³⁺ doped up-conversion nanocrystals deposited on graphene upon coupling with silver nanowires. Direct interaction between nanocrystals and graphene results in quenching of up-conversion emission and shortening of luminescence decay times, due to the energy transfer to graphene. The amount of the energy absorbed by graphene can be enhanced by coupling Er³⁺/Yb³⁺ doped up-conversion nanocrystals with silver nanowires. Microscopy studies with high spatial resolution together with time-resolved analysis of nanocrystal luminescence show increase of the emission rates with fourfold enhancement of the intensity for nanocrystals placed in the vicinity of silver nanowires. This strong enhancement emerges despite simultaneous interaction with graphene. The hybrid nanostructure provides thus a way to combine optical activity of up-conversion nanocrystals and enhancement provided by metallic nanowires with excellent electrical and mechanical properties of graphene.

Figure 1

Extinction spectrum of a colloid of silver nanowires (blue line) together with the emission spectrum of a single nanocrystal (black line). The excitation of 980 nm laser diode was used, as indicated by the red line.

Figure 2

Illustratation of the examined sample (not to scale). Nanocrystals and silver nanowires deposited on graphene or glass are accesable.

Figure 3

Photoluminescence intensity maps of NaYF₄:Er³⁺/Yb³⁺ nanocrystals deposited on (a) glass and (b) graphene layer. Maps acquired for NCs coupled with silver nanowires placed on (c) glass and (d) graphene. Positions of nanowires on (e) glass and (f) graphene observed in a back scattering mode. Photoluminescence maps were collected at 550 nm under 980 nm laser excitation.

Figure 4

Histograms of photoluminescence intensity of (a,b) nanocrystals and (c,d) nanocrystals combined with silver nanowires, deposited on glass and graphene. Green (a,c) and red (b,d) emission of Er³⁺ is shown. Each histogram was obtained for approximately 150 individual NCs.

Figure 5

Luminescence transients of (a) green and (b) red emission of the NCs. Black and green lines/points correspond to NCs on glass and graphene, respectively, while blue and red lines/points correspond to NCs combined with NWs placed on glass and graphene, respectively.

Figure 6

Histograms of luminescence decay times, acquired for: (a,b) nanocrystals and (c,d) nanocrystals coupled with silver nanowires, deposited on glass and graphene. Pink bars denote residual population of NCs coupled to graphene and silver NWs. Each distribution contains about 150 experimental points. Decay times reflect intensity-weighted average lifetimes.