Highly fluorescing biological labels with excitation in near-infrared window II have attracted the interest of scientific community as they are capable of increasing both penetration depth and imaging quality. However, studies on the utilization of quantum dots (QDs) in biological imaging appear to be rather limited to the near-infrared window I (NIR-I: 650−950 nm). We herein report on the observation of efficient photoluminescence (PL) in Mn 2+ -doped ZnS QDs excited by two-photon absorption (2PA) in near-infrared window II (NIR-II: 1000−1350 nm). Multiphoton-absorptioninduced PL measurements indicate that these biocompatible QDs exhibit a two-photon action cross-section of 265 GM at 1180 nm, the highest value reported to date among conventional fluorescent probes on excitation in NIR-II. This value is 1−2 orders of magnitude higher than that for organic dye molecules excited by NIR-I photons and 3−4 times greater than that of fluorescent proteins excited in the NIR-II. The underlying NIR-II excitation mechanism for the Mn 2+ emission at 586 nm on account of the 4 T 1 − 6 A 1 transition is attributed to the transitions from the valence subband of ZnS QDs (or ground states of Mn 2+ ions) to the excited states of Mn 2+ ions by direct two-photon absorption. Transient PL measurements reveal single exponential decay with a PL lifetime of 0.35 ± 0.03 ms irrespective of excitation wavelength, which are 4−5 orders longer than that of conventional fluorescent probes. With the excitation in NIR-II window and the unique combination of photophysical properties such as a greater two-photon action cross-section, a longer PL lifetime, and larger anti-Stokes shift (450 nm or more), Mn 2+ -doped ZnS QDs appear to be a promising candidate for deep tissue imaging applications.
Steeping interest on graphene research in basic sciences and applications emphasizes the need for an economical means of synthesizing it. We report a method for the synthesis of graphene on commercially available stainless steel foils using direct thermal chemical vapor deposition. Our method of synthesis and the use of relatively cheap precursors such as ethanol (CH 3 CH 2 OH) as a source of carbon and SS 304 as the substrate, proved to be economically viable. Presence of singleand few-layer graphene was confirmed using confocal Raman microscopy/spectroscopy. X-ray photoelectron spectroscopic measurements were further used to establish the influence of various elemental species present in stainless steel on graphene growth. Role of cooling rate on surface migration of certain chemical species (oxides of Fe, Cr and Mn) that promote or hinder the growth of graphene is probed. Such analysis of the chemical species present on the surface can be promising for graphene based catalytic research.
The prospect of engineering the optical bandgap in semiconductor nanostructures all the way from ultraviolet to visible is highly significant in various applications such as photocatalysis, sensing, opto-electronics, and biomedical applications. Since many of the semiconductors have their optical bandgaps in the UV region, various techniques are used for the tuning of their bandgaps to the visible region. Doping and co-doping with metals and nonmetals have been found highly effective in bandgap narrowing as doping creates a continuum of mid-bandgap states which effectively reduces their bandgap. Other than these techniques, the modulation of intrinsic vacancies is an effective way to control the optical bandgap. Among all semiconductors, Titanium dioxide is a well-studied material for UV photocatalytic applications. TiO2 has oxygen and Titanium vacancies as intrinsic defects which influence the bandgap based on its phase of existence. The creation of oxygen vacancies generates unpaired electrons associated with Ti3+ species resulting in the creation of donor levels within the bandgap. Trivacancies give p-type nature to TiO2 due to excess holes and generate acceptor levels in the bandgap. The existence of a continuum of such intrinsic defect states within the bandgap appears to narrow the bandgap and enhance the visible light absorption in TiO2 though the effect is an apparent narrowing. Doping and co-doping of TiO2 with metals Au, Ag, Fe, Co, Ni, Pt, Pd, etc., and non-metals B, C, N, Br, Cl, etc, doping with Ti3+ ions, hydrogenation etc have been found to narrow down the optical bandgap of TiO2. In this review, we focus on such intrinsic vacancy modulated optical bandgap narrowing in TiO2. This review covers significantly recent advancements in bandgap engineering of TiO2.
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