Nanophosphors-Based White Light Sources.

Maura Cesaria,Baldassare Di Bartolo

doi:10.3390/nano9071048

Abstract

Miniaturization requests and progress in nanofabrication are prompting worldwide interest in nanophosphors as white-emission mercury-free lighting sources. By comparison with their bulk counterparts, nanophosphors exhibit reduced concentration quenching effects and a great potential to enhance luminescence efficiency and tunability. In this paper, the physics of the nanophoshors is overviewed with a focus on the impact of spatial confinement and surface-to-volume ratio on the luminescence issue, as well as rare earth-activated multicolor emission for white light (WL) output. In this respect, the prominently practiced strategies to achieve WL emission are single nanophosphors directly yielding WL by means of co-doping and superposition of the individual red, green, and blue emissions from different nanophosphors. Recently, a new class of efficient broadband WL emitting nanophosphors has been proposed, i.e., nominally un-doped rare earth free oxide (yttrium oxide, Y2O3) nanopowders and Cr transition metal-doped garnet nanocrystals. In regard to this unconventional WL emission, the main points are: it is strictly a nanoscale phenomenon, the presence of an emitting center may favor WL emission without being necessary for observing it, and, its inherent origin is still unknown. A comparison between such an unconventional WL emission and the existing literature is presented to point out its novelty and superior lighting performances.

Highlights

Solid-state lighting based on light emitting diodes (LEDs) is a rapidly growing market that is progressively replacing the old lighting lamp technologies
We present and discuss the observed occurrence of unconventional white light (WL) emission, where “unconventional” refers to a new class of efficient broadband white-light-emitting nanophosphors consisting of nominally un-doped rare earth (RE)-free oxide nanopowders [17] and transition-metal (TM)-doped nanoscale hosts [18] excited by monochromatic infrared-excitation
Solid-state lighting devices exploiting LED pumping sources to irradiate phosphors materials dominate the actual market of white light emitting artificial sources for indoor and outdoor applications

Summary

Introduction

Solid-state lighting based on light emitting diodes (LEDs) is a rapidly growing market that is progressively replacing the old lighting lamp technologies (incandescent light bulbs, compact fluorescence lamps, and high-intensity discharge lamps). The paper is organized as follows: after a general introduction about the emission mechanisms and physics of phosphors and RE dopants, a section will be devoted to the spectroscopy of nanophosphors and, in addition, a section dealing with the experimental findings related to the observed unconventional WL emission will be presented In this respect, main points resulting from the experimental evidence are: first, such unconventional WL emission is a nanoscale phenomenon, second, its occurrence is Nanomaterials 2019, 9, 1048 strictly related to the host matrix because, doping can favor WL emission, doping is not Nanomaterials 2019, 9, x FOR PEER REVIEW needed, and third, its inherent origin is still unknown, despite an attempt of interpretation of the expesreimcoenndt,aitlsfioncdcuinrrgesnc[e19is].stBriyctlcyormelpataerdistoonthweihthostthmeaetxriixstbinecgausscee,naaltrhio,ucghadlloepninggincganqufaevsotrioWnsL to be addreemssiesdsioant,tdhoepfiungndisanmoet nnteaeldleedv,ealnwd itlhlibrde,pitosiinntheedreonut toaringdindisissctiullsusendk.nown, despite an attempt of interpretation of the experimental findings [19]. RRaaddiiaattiivvee eemmiissssiioonn ooff pphhoosspphhoorrss ccaann rreessuulltt ffrroomm ttwwoo mmaaiinn ccllaasssseess ooff pprroocceesssseess,, wwhhiicchh aarree,, sseennssiittiizzaattiioonn bbyy tthhee hhoosstt llaattttiiccee oorr ttrraannssiittiioonnss dduuee ttoo ddooppaannttss tteerrmmeedd ““aaccttiivvaattoorr”” ((AA)) iiff aaccttiinngg aass eemmiittttiinngg cceenntteerrss aanndd ““sseennssiittiizzeerr”” ((SS)) iiff tthheeiirr eenneerrggyy lleevveellss lleett ttrraannssffeerr eenneerrggyy ttoo tthhee eemmiittttiinngg aaccttiivvaattoorr iioonnss [[2244]]. Another investigated class of photo- and electro-luminescent materials is carbonaceous materials, including carbon nanotubes [61] and graphene [62,63,64,65]

Spectroscopy of Nanophosphors

Tuning of the Color Output

The Role of Surfaces

Nanoscale-Related Unconventional Production of White Light

Findings

Conclusions