G.A. Kumar

Stokes emission in GdF3:Nd3+ nanoparticles for bioimaging probes

There is increasing interest in rare earth (RE) doped nanoparticles (NPs) due to their sharp absorption and photoluminescence (PL) in the near infrared (NIR) spectral region. These NIR based nanoparticles (NPs) could allow biological imaging at substantial depths with enhanced contrast and high spatial resolution due to the absence of auto fluorescence in biological samples under infrared excitation. Here, we present the highly efficient infrared photoluminescence in GdF₃:Nd(3+) nanoparticles under 800 nm excitation within the hydrodynamic size limitations for bio-applications. The downconversion (Stokes emission) absolute quantum yields (QY) in powder, polymaleic anhydride-alt-1-octadicene (PMAO) coated powder and colloidal solutions have been investigated. QY measurements have revealed that downconversion (Stokes emission) QYs of approximately 5 ± 2 nm sized GdF₃:1% Nd(3+) colloidal NPs are 2000 times higher than those of efficient upconversion (UC) particles NaYF₄:20% Er/2% Yb of the same size. Furthermore, the utility of these NIR emitting nanoparticles for bioimaging probes has been demonstrated by confocal imaging and spectroscopic study.

Comparative studies of the spectroscopic properties of Nd 3+ : YAG nanocrystals, transparent ceramic and single crystal

Detailed comparative spectroscopic studies of Nd3+ doped YAG nanocrystals, transparent ceramic and single crystal have been performed. Although most of the radiative spectral properties of Nd3+ are almost in good agreement between the three hosts, the non-radiative losses are significantly high in nanocrystals, which are attributed due to the presence of large amount of hydroxyl groups on the nanocrystals surface which deteriorates the quality of the material for laser applications. In addition, wavelength dependent scattering loss for the Nd3+ doped YAG nanocrystals is found significantly high compared to those of Nd3+ doped single crystal and ceramic.

A bright yellow light from a Yb3+,Er3+-co-doped Y2SiO5 upconversion luminescence material

A bright yellow light from Yb3+ and Er3+ doped Y2SiO5 upconversion materials in particle and fiber form were prepared by a co-precipitation, and electrospinning method. The morphologies of the prepared samples were investigated through FT-IR, FE-SEM, XRD and Raman measurements. The upconversion properties of the samples are carefully studied based on the absorption, fluorescent and decay time measurements. Under 980 nm excitation, the prepared material shows bright yellow emission. By controlling the concentration of Yb3+ or Er3+ and the excitation laser power, the ratio of red to green emission intensity can be varied to adjust the color tuning properties of the phosphor. The upconversion mechanism, and transition probabilities were elucidated owing to Judd–Ofelt theory. The radiative quantum efficiencies for the red (4F9/2) and green (4S3/2) bands were estimated to be 87% and 55%, respectively. The color coordinates of the system were evaluated as a function of the dopant concentrations and plotted on a standard CIE index diagram. The change of band intensity ratio with dopant concentrations gives a promising potential of the current phosphor for lighting application.

Emission enhancement through Nd3+-Yb3+ energy transfer in multifunctional NaGdF4 nanocrystals

The growing need for biomedical contrast agents has led to the current development of multi-functional materials such as lanthanide-based nanoparticles (NPs). The optical and magnetic properties these nanoparticles (NPs) possess are important to enhance current biomedical imaging techniques. To increase the optical emissions of the nanoparticles, neodymium (Nd3+) and ytterbium (Yb3+) were introduced into a magnetic host of NaGdF4. The energy transfer between Nd3+ and the Yb3+ was then investigated at multiple concentrations to determine the optimal dopant levels. The NaGdF4:Nd3+,Yb3+ nanoparticles were synthesized through a modified solvothermal method, resulting in rectangular structures, with an average side length of 17.87 ± 4.38 nm. A double dopant concentration of 10% Nd3+ and 4% Yb3+ was found to be optimal, increasing the emission intensity by 71.5% when compared to the widely used Nd3+ single dopant. Decay measurements confirm energy transfer from Nd3+ to Yb3+, with a lifetime shortening from Nd3+ 1064 nm emission and a calculated lifetime of 12.72 ms with 98% efficiency. Despite NaGdF4:Nd3+,Yb3+ NPs showing a slight decrease in their magnetic response at the expense of optimizing optical emission, as it is directly dependent on the Gd3+ concentration, a strong paramagnetic behavior was still observed. These results corroborate that NaGdF4:Nd3+,Yb3+ NPs are viable candidates for multimodal imaging.

Energy converting material for solar cell application

In this paper, we discuss the concept of an efficient infrared upconverting phosphor as an energy converting material that could potentially improve the efficiency of Si solar cells in bifacial configuration. Basic spectroscopic studies of Yb and Er-doped La2O2S phosphor was reported with particular attention to its upconversion properties under 1550 nm excitation. Different concentrations of phosphors were synthesized by solid state flux fusion method. The phosphor powders were well crystallized in a hexagonal shape with an average size 300-400 nm. The most efficient upconverting sample (1%Yb: 9% Er doped La2O2S) was also studied under the illumination with infrared (IR) broad band spectrum above 1000 nm. Our measurements show that even with an excitation power density of 0.159 W/cm2 using a tungsten halogen lamp the material shows efficient upconversion corroborating the fact that the present phosphors could be potential candidates for improving the efficiency of the present Si solar cells.

Depth-Resolved Multispectral Sub-Surface Imaging Using Multifunctional Upconversion Phosphors with Paramagnetic Properties.

Molecular imaging is very promising technique used for surgical guidance, which requires advancements related to properties of imaging agents and subsequent data retrieval methods from measured multispectral images. In this article, an upconversion material is introduced for subsurface near-infrared imaging and for the depth recovery of the material embedded below the biological tissue. The results confirm significant correlation between the analytical depth estimate of the material under the tissue and the measured ratio of emitted light from the material at two different wavelengths. Experiments with biological tissue samples demonstrate depth resolved imaging using the rare earth doped multifunctional phosphors. In vitro tests reveal no significant toxicity, whereas the magnetic measurements of the phosphors show that the particles are suitable as magnetic resonance imaging agents. The confocal imaging of fibroblast cells with these phosphors reveals their potential for in vivo imaging. The depth-resolved imaging technique with such phosphors has broad implications for real-time intraoperative surgical guidance.

Plasmon-enhanced upconversion in Yb3+/Er3+ doped inY2O3

In this paper, we study the computational modeling of the localized surface plasmonic and scattering field effect arising from of gold nanorods. We also report the synthesis and optical characterization of core-spacer-shell nanocomposites composed of gold nanorods coated with SiO2 and finally coated with Y2O3:Er3+/Yb3+ (Aunanorods@mSiO2@Y2O3:Er3+/Yb) through a layer-by-layer method. Preliminary upconversion analysis of singly (Aunanorods@mSiO2@Y2O3:Er3+/Yb) at 980 excitation indicates that the composition has to be optimized to understand the role of silica as a spacer and near field enhancer (gold nanorod) in the system.

Infrared excited Yb:Er: Y2O2S phosphors with intense emission for lighting applications

Yb and Er-doped Y2O2S phosphor was synthesized by solid state flux fusion method and their upconversion spectral properties were studied as a function of different Yb concentrations. The solid state flux fusion results in well crystallized hexagonal shaped phosphor particles of average size 3.8 μm. The detailed optical characterizations such as absorption, emission, and fluorescence decay were performed to explore the emission processes in the UV-VIS-NIR as well as to quantitatively estimate the fluorescence quantum yield. Upconversion spectral studies show that for all the compositions, green emissions are stronger, particularly; the green emission intensity is 1.7 times stronger than the red one with composition of 8 mol% Yb and 1 mol% Er. Mechanisms of upconversion by two photon and energy transfer processes are interpreted and explained. The color coordinates are measured and the color tunability was analyzed as a function of the 980 nm excitation power. Results show that the Y2O2S:Yb,Er phosphor offers power dependent color tuning properties where the emission color can be tuned from 490 to 550 nm by simply changing the 980 nm excitation power from 10 to 50 mW.

Highly efficient phosphors in cancer sensing and PDT

Highly efficient upconverting phosphors (NaYF4) doped with erbium ions are bio-conjugated and used for cancer imaging and photodynamic therapy. Once they are conjugated, the particles are injected into mice to demonstrate that cancer imaging with a near-infrared excitation source is possible. Finally, the particles are also conjugated with a photosensitive molecule with strong absorption near the upconversion emission peak (~ 550nm). The upconversion energy causes the photosensitive molecule to create highly reactive oxidative species, which puncture and kill the cell to which it is attached. These particles are then used in a mouse model, and the size of the tumors is modeled as a function of the dosage and duration of the photodynamic therapy.