Experimental observation of spatially resolved photo-luminescence intensity distribution in dual mode upconverting nanorod bundles

www.nature.com/scientificreports OPEN received: 14 September 2016 accepted: 06 January 2017 Published: 13 February 2017 Experimental observation of spatially resolved photoluminescence intensity distribution in dual mode upconverting nanorod bundles Pawan Kumar1,2, Satbir Singh1,2, V. N. Singh3, Nidhi Singh4, R. K. Gupta5 & Bipin Kumar Gupta1 A novel method for demonstration of photoluminescence intensity distribution in upconverting nanorod bundles using confocal microscopy is reported. Herein, a strategy for the synthesis of highly luminescent dual mode upconverting/downshift Y1.94O3:Ho3+0.02/Yb3+0.04 nanorod bundles by a facile hydrothermal route has been introduced. These luminescent nanorod bundles exhibit strong green emission at 549 nm upon excitations at 449 nm and 980 nm with quantum efficiencies of ~6.3% and ~1.1%, respectively. The TEM/HRTEM results confirm that these bundles are composed of several individual nanorods with diameter of ~100 nm and length in the range of 1–3 μm. Furthermore, two dimensional spatially resolved photoluminescence intensity distribution study has been carried out using confocal photoluminescence microscope throughout the nanorod bundles. This study provides a new direction for the potential use of such emerging dual mode nanorod bundles as photon sources for next generation flat panel optical display devices, bio-medical applications, luminescent security ink and enhanced energy harvesting in photovoltaic applications. Designing one-dimensional (1D) rare earth nanomaterials by template-free strategies is an ultimate challenge of cutting edge science1–6. In general, the chemical, physical and optical properties of inorganic nanostructures depend on their chemical composition, size and shape6–12. In recent times, rare earth-doped nanostructures have been recognized worldwide for their better chemical and optical properties originating from their unique electronic structures as well as wide range of applications in photovoltaic, bio-medical, anti-counterfeiting, solid state lighting, display technologies etc13–16. In comparison to organic dyes, metals, metal oxides, semiconductor quantum dots and core-shell structures; rare earth compounds present intense and sharp emission bands arising from f–f transitions and large Stokes shifts originating from their unique electronic configuration17–22. The 1D nanostructures of rare earth doped nanomaterials (like nanorods, nanowires, nanotubes etc.) have attracted enormous attention in recent years23–26. Tailoring of aspect ratio in rare earth based 1D nanostructure offers several advantages; like, quantum confinement, tunable electrical, magnetic and optical properties27. There are many reports on the synthesis of 1D nanomaterials; such as, III-V and II-VI semiconductors and oxide nanowires/ nanorods28–31. The widely used methods to prepare 1D structures are catalyst supported template as well as chemical vapour deposition. But, these methods have their own drawbacks; such as, complex procedure and impurities in the products. Therefore, the solution-phase methods for direct growth of 1D nanostructure without involving 1 Luminescent Materials and Devices Group, Materials Physics and Engineering Division, CSIR- National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi, 110012, India. 2Academy of Scientific and Innovative Research (AcSIR), CSIR-National Physical Laboratory Campus. Dr. K. S. Krishnan Road, New Delhi 110012, India. 3Advanced Materials and Devices Group, Physics of Energy Harvesting Division, CSIR - National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi, 110012, India. 4Metals, Alloys and Composites for Energy Applications Group, Physics of Energy Harvesting Division, CSIR - National Physical Laboratory, Dr K S Krishnan Road, New Delhi, 110012, India. 5 Department of Chemistry, Pittsburg State University, Pittsburg, KS, 66762, USA. Correspondence and requests for materials should be addressed to B.K.G. (email: ) Scientific Reports | 7:42515 | DOI: 10.1038/srep42515 1 www.nature.com/scientificreports/ catalysts or templates (such as hydrothermal method) are widely used for the synthesis of 1D nanomaterials with better purity, large scale production at economical cost and good homogeneity15. Now a days, bundle composed of rare earth based nanorods have gained much attention due to higher surface area, better quantum yield and optical properties1,13,15,32. These bundles of nanorods could be synthesized by using customized hydrothermal method without any external assistance32. Moreover, these luminescent bundles are highly desired for fabrication of flat panel optical display devices, which ignited us to explore the synthesis as well as spatially resolved photoluminescence (PL) intensity distribution on the surface of these nanorod bundles. Recently, the upconversion nanomaterials have received huge attention due to their various potential applications33,34. It is well established that the upconversion process involves an anti-Stokes shift in which absorption of multi-photons (two or more) of lower energy (infrared photons) results into emission of high energy photons. The upconverting phosphors are generally inorganic host lattice (chosen due to their low phonon energy) doped with emitters Er3+, Tm3+, Ho3+ and Yb3+ 34–36. Y2O3 is a one of the most explored host lattice due to its exceptional optical, thermal and mechanical properties37–39. In addition to this, the nucleation and formation of hexa-hydroxy rods and their conversion into oxide nanorods is quite easy in the binary system as compared to ternary system e.g. GdVO4, NaYF4, LaPO4 etc. Various synthesis methods have been used for the growth of upconverting nanophosphor with different morphologies; like, nanoparticles, nanotubes, nanoflakes, nanorods etc40–42. Moreover, it is interesting to note that the bundles composed of rare earth based nanorods with dual mode emission (both downshift/down conversion as well as upconversion) are meagrely reported in literature. These dual mode nanorod bundles open a new paradigm shift from nanorod to nanorod bundle structure for highly efficient next generation optical display applications. In order to establish a potential use of such luminescent nanorod bundles, it is extremely important to investigate the PL intensity distribution throughout the surface of nanorod bundles. Confocal PL mapping microscopy has gained recognition for the visualization of 2D spatial distribution of PL intensity in luminescent materails43,44. Furthermore, PL mapping provides a mapped image by integrating thousands of acquired PL spectra at every point and gives information about spectroscopic features at that particular point. Conceptually, image formation by PL mapping involves measuring a property from the entire field of view concurrently or by measuring a property of entire area sequentially from each points and combining it to recreate the image45. Hence, PL mapping is an important (...truncated)