Archis Marathe

Functionalization of graphene sheets through fullerene attachment

Fullerene-functionalized graphene shows a hierarchical structure, and is promising for many potential applications. In this paper, a new method is presented to synthesize such a novel nanostructure. Graphite flakes were intercalated to graphite oxide, and then functionalized with zero-dimensional fullerene nanocrystals. The resultant compounds were characterized by FT-IR spectroscopy, UV–Vis spectroscopy, atomic force microscopy, and transmission electron microscopy. The characterization results confirmed that fullerene crystals were successfully attached onto the single-layer graphene sheets and significantly facilitated the exfoliation of graphite to monolayer graphene. The fullerene grafted on the graphene surface serves as a space impediment to prevent the re-stacking of exfoliated graphene sheets. This attempt provides an effective route for large-scale exfoliation and functionalization of monolayer graphene, and is expected to significantly facilitate the application of graphene in the electronic devices, energy storage, and functional materials.

Tunable photoluminescence and energy transfer of YBO3:Tb3+, Eu3+ for white light emitting diodes

Rare-earth (Eu 3+, Tb 3+) ion co-doped YBO 3 phosphors with a morphology of uniform flower-like assembly are fabricated by a facile hydrothermal method without the use of surfactant, organic ligands, or further sintering treatment. Photoluminescence (PL) studies have demonstrated that under the excitation of 365 nm ultraviolet (UV) light, YBO 3:Tb 3+, Eu 3+ emits a white light, including three emissions: a blue band attributed to self-trapped excitons (STEs), a green band due to the Tb 3+ transition of 5D 4– 7F j ( j = 6, 5, 4, and 3), and a red band due to the Eu 3+ transition of 5D 0– 7F j ( j = 0, 1, 2, 3, and 4). Energy transfers from host YBO 3 to Tb 3+, and Eu 3+ and Tb 3+ to Eu 3+, as well as tunable emission by varying the relative doping ratios are demonstrated. The combination of blue emission from STEs with green and red emissions from activators provides a novel and efficient technique to make white light emitting diodes (WLEDs).

Shear-induced phase transition of nanocrystalline hexagonal boron nitride to wurtzitic structure at room temperature and lower pressure

Disordered structures of boron nitride (BN), graphite, boron carbide (BC), and boron carbon nitride (BCN) systems are considered important precursor materials for synthesis of superhard phases in these systems. However, phase transformation of such materials can be achieved only at extreme pressure–temperature conditions, which is irrelevant to industrial applications. Here, the phase transition from disordered nanocrystalline hexagonal (h)BN to superhard wurtzitic (w)BN was found at room temperature under a pressure of 6.7 GPa after applying large plastic shear in a rotational diamond anvil cell (RDAC) monitored by in situ synchrotron X-ray diffraction (XRD) measurements. However, under hydrostatic compression to 52.8 GPa, the same hBN sample did not transform to wBN but probably underwent a reversible transformation to a high-pressure disordered phase with closed-packed buckled layers. The current phase-transition pressure is the lowest among all reported direct-phase transitions from hBN to wBN at room temperature. Usually, large plastic straining leads to disordering and amorphization; here, in contrast, highly disordered hBN transformed to crystalline wBN. The mechanisms of strain-induced phase transformation and the reasons for such a low transformation pressure are discussed. Our results demonstrate a potential of low pressure–room temperature synthesis of superhard materials under plastic shear from disordered or amorphous precursors. They also open a pathway of phase transformation of nanocrystalline materials and materials with disordered and amorphous structures under extensive shear.

Synthesis and photoluminescence properties of hierarchical architectures of YBO3:Eu3+

We present a general hydrothermal method without any organic solvent or surfactant for the synthesis of YBO3:Eu3+ nano- and micro-structures. The samples were of excellent quality with uniform, well dispersed, self-assembled and self-purified YBO3:Eu3+. Single crystal nanoflakes gathered together to evolve into a hierarchical architecture with eight different types of three dimensional morphologies. Strong photoluminescence spectra were obtained at 592, 611 and 627 nm at excitations wavelengths 254 and 363 nm. The emission spectra are associated with the transitions from the excited 5D0 level to the 7FJ (J = 1, 2, 3, 4) levels of Eu3+ activators. YBO3:Eu3+ prepared using ethanol solvent exhibited the highest chromaticity values yet reported.

Superior white light emission and color tunability of tri-doped YBO3:Tb3+, Eu3+ and Dy3+ for white light emitting diodes

A tri-doped YBO3:Tb3+, Eu3+ and Dy3+ phosphor, which is capable of producing white light by combining blue, green, yellow, orange and red emissions when excited at 365 nm ultraviolet (UV) light, was developed using a general hydrothermal method. The samples showed strong photoluminescence spectra at 485, 541, 578, 591, 611, and 627 nm under the excitation wavelength of 365 nm (ultraviolet light) indicating the presence of blue, green, yellow, orange and red light due to the transitions 4F9/2 → 6H15/2 (Dy3+), 5D4 → 7F5 (Tb3+), 4F9/2 → 6H13/2 (Dy3+), 5D0 → 7F1 (Eu3+) and 5D0 → 7F2 (Eu3+), respectively. Tri-doping is seemed to be more successful in the creation of white light. The effect of the variations in doping percentages on color tunability was also evident. Evidence of efficient energy transfers from the host excitations to activators Tb3+ and Eu3+, and from Dy3+ to Tb3+ to Eu3+ existed. Although there is a weak energy transfer from host to Dy3+ occurred, strong photoluminescence excitation bands existed in Dy3+.