2, Indian Institute of Technology Delhi, New Delhi, NCR, India
Processes involving harvesting of near visible and near-infrared photons, followed by re-emission in the green region are relevant for fabrication of modified Si-based high efficiency photodetectors, as well as potentially Si-based solar cells. Requirements for such materials include a) low cost, b) highest possible upconversion PL efficiency to green, which is subsequently detected by a Si detector. The photophysics of doped NaYF4 materials has been extensively investigated with dopants such as Er3+, Tm3+, Ho3+, along with Yb3+ as a sensitizer for upconversion to visible regime at high incident optical power (~100 mW) for colloidal solutions. We report a moderate temperature (~ 320°C) synthesis of NaYF4:Yb(18%):Er(2%):Gd(15%) using thermal decomposition, resulting in highly crystalline nearly pure phase materials. High-resolution transmission electron microscopy (HRTEM) measurements reveal the formation of nearly spherical nanoparticles with an average diameter of 38 nm. Observed  lattice plane spacings (estimated to be 4.4±1.6 Å) in TEM data are in reasonable agreement with standard published X-ray diffraction (XRD) data (~5.17 Å) for comparable NaYF4-based materials. Powder XRD measurements of the unannealed material suggest the presence of a strongly hexagonal overall phase with lattice constant (~5.17 Å), again in agreement with TEM estimates. Deposited thin films of Gd-doped unannealed material exhibit a broadband (~ 400-1600 nm) near 2-fold enhancement in absorption over baseline well-studied Er3+ doped materials. Finally, photoluminescence (PL) upconversion response of the thin films with 785 nm laser excitation appears in green (539 nm) and red (665 nm). Thin films based on this Gd(III) doped material are thus potentially useable as optical antennas for higher efficiency devices based on Si photodetectors.