Strong Near-infrared Photoluminescence Research Articles

Type‐II CdSe/ZnO Core/Shell Nanorods: Nanoheterostructures with A Tunable Dual Emission in Visible and Near‐Infrared Spectral Ranges

AbstractA synthesis and characterization of luminescent nano‐heterostructures consisting of CdSe nanorod (NR) cores and a ZnO shell with up to three monolayers of ZnO is reported. The core/shell heterostructures show a tunable, dual photoluminescence (PL) in visible and Near Infrared (NIR) spectral ranges. Upon shelling the visible PL band attributed to the carrier recombination within the CdSe core shifts to lower energy by ≈0.05 to 0.15 eV relative to the bare CdSe NRs, due to a reduced quantum confinement. A NIR band, observed ≈0.4 – 0.5 eV below the PL energy of the CdSe core, is attributed to a type‐II carrier recombination across the CdSe/ZnO interface. The total PL quantum yield (PLQY) in the brightest heterostructures reaches ≈20%, increasing ≈100‐fold over the PLQY of the corresponding bare CdSe NRs. The average lifetimes of the visible PL in some heterostructures exceeds 100 ns, compared to ≈5 ns lifetime typical for bare CdSe NRs. The average PL lifetimes attributed to the type‐II charge separated states exceed one microsecond. Strong NIR PL, tunable in the 800–900 nm spectral range and the long‐lived charge separated state make the CdSe/ZnO core‐shell NRs appealing materials for exploitation in applications such as bioimaging, photocatalysis and optoelectronics.

Stable Near-Infrared Photoluminescence of Single-Walled Carbon Nanotubes Dispersed Using a Coconut-Based Natural Detergent.

We prepared single-walled carbon nanotube (SWNT) suspensions in phosphate buffer solutions containing 1% of a coconut-based natural detergent (COCO) or 1% of sodium dodecyl sulfate (SDS). The suspensions exhibited strong photoluminescence (PL) in the near-infrared region, suggesting that the SWNTs, such as those with (9, 4) and (7, 6) chiralities, were monodispersed. Upon diluting the suspensions with a detergent-free phosphate buffer solution, the PL intensity of the SDS-containing SWNT suspension was significantly lower than that of the COCO-containing SWNT suspension. The COCO-containing SWNT suspension was more stable than the SDS-containing SWNT suspension. The SWNT concentration of the suspensions prepared via bath-type sonication was lower than that of the suspensions prepared via probe-type sonication. However, near-infrared (NIR) PL intensity of the SWNT suspensions prepared via bath-type sonication was much higher than that of the SWNT suspensions prepared via probe-type sonication regardless of the detergent. This suggested that the fraction of monodispersed SWNTs of the suspensions prepared via bath-type sonication was larger than that of the suspensions prepared via probe-type sonication, although the SWNT concentration was low. Our results indicated that COCO favored the fabrication of SWNT suspensions with stable and strong NIR PL, which are useful for various biological applications.

Germanium Nanocrystal Properties from Photoluminescence

Morphological information has been obtained using the strong near-infrared photoluminescence emitted by germanium (Ge) nanocrystals (NCs) coherently imbedded in SiGe alloy layers, grown by molecular beam epitaxy on Si substrates. The emission spectra are analyzed for the effects of strain, carrier confinement, and disorder over a wide range of Ge concentrations in the surrounding SiGe medium. This analysis provided significant insight into the properties of the Ge nanocrystals, including their size and shape. We also discuss the mechanisms leading to the high quantum efficiency observed for emission from the Ge nanocrystals at low temperatures. We indicate how direct gap behavior might be achieved for Ge NCs lattice matched within dilute Ge1-ySny alloys, where tensile strain would be present in the NCs in all three directions.

CsSnI3: Semiconductor or Metal? High Electrical Conductivity and Strong Near-Infrared Photoluminescence from a Single Material. High Hole Mobility and Phase-Transitions

CsSnI(3) is an unusual perovskite that undergoes complex displacive and reconstructive phase transitions and exhibits near-infrared emission at room temperature. Experimental and theoretical studies of CsSnI(3) have been limited by the lack of detailed crystal structure characterization and chemical instability. Here we describe the synthesis of pure polymorphic crystals, the preparation of large crack-/bubble-free ingots, the refined single-crystal structures, and temperature-dependent charge transport and optical properties of CsSnI(3), coupled with ab initio first-principles density functional theory (DFT) calculations. In situ temperature-dependent single-crystal and synchrotron powder X-ray diffraction studies reveal the origin of polymorphous phase transitions of CsSnI(3). The black orthorhombic form of CsSnI(3) demonstrates one of the largest volumetric thermal expansion coefficients for inorganic solids. Electrical conductivity, Hall effect, and thermopower measurements on it show p-type metallic behavior with low carrier density, despite the optical band gap of 1.3 eV. Hall effect measurements of the black orthorhombic perovskite phase of CsSnI(3) indicate that it is a p-type direct band gap semiconductor with carrier concentration at room temperature of ∼ 10(17) cm(-3) and a hole mobility of ∼585 cm(2) V(-1) s(-1). The hole mobility is one of the highest observed among p-type semiconductors with comparable band gaps. Its powders exhibit a strong room-temperature near-IR emission spectrum at 950 nm. Remarkably, the values of the electrical conductivity and photoluminescence intensity increase with heat treatment. The DFT calculations show that the screened-exchange local density approximation-derived band gap agrees well with the experimentally measured band gap. Calculations of the formation energy of defects strongly suggest that the electrical and light emission properties possibly result from Sn defects in the crystal structure, which arise intrinsically. Thus, although stoichiometric CsSnI(3) is a semiconductor, the material is prone to intrinsic defects associated with Sn vacancies. This creates highly mobile holes which cause the materials to appear metallic.

Novel Single Walled Carbon Nanotube Based Magnetic-Fluorescent Nanohybrids as Dual-Modal MRI/Optical Imaging Probes

Gadolinium chelate and Quantum Dots (QDs) decorated single walled carbon nanotubes (SWNTs) nanohybrids (SWNT-Gd-QDs) were designed for dual-modal cellular imaging via magnetic resonance imaging (MRI) and optical imaging. The nanohybrids were prepared via a simple and novel layer-by-layer (LBL) assembly in combination with covalent connection strategy. The SWNT-Gd-QDs nanohybrids showed a strong near-infrared photoluminescence (PL) emission at room temperature and enhanced T1-weighted and T2-weighted MRI signals in water. Thus, the nanohybrids combine magnetic and fluorescent properties and can serve as dual-modal MRI/optical imaging contrast agent.

Yb3+ → Er3+ energy transfer in Al2O3 and temperature characteristic of near-infrared photoluminescence

For the Er3+–Yb3+ codoped Al2O3 powders, the strong near-infrared photoluminescence (PL) centered at 1.535 μm derived from the energy transfer (ET) from Yb3+ to Er3+ was detected by a 978 nm laser diode excitation. Compared with that of Er3+ doped Al2O3 powders, the PL intensity enhanced about 9 times, the full width at half maximum (FWHM) extended from 82 to 90 nm, and the lifetime increased from 3.22 to 4.17 ms for Er3+–Yb3+ codoped Al2O3 powders at room temperature. The ET coefficient of 2.18 × 10−18 cm3 s−1 from Yb3+ to Er3+ was obtained based on the rate equations. The decrease of PL intensity with increasing temperature in the range of 298–733 K was observed, due to thermally enhanced nonradiative relaxation 4I13/2 → 4I15/2 dominated over thermally enhanced phonon-assisted ET in the Er3+–Yb3+ codoped Al2O3.

Strong near-infrared photoluminescence in erbium/ytterbium codoped porous silicon

Erbium ions and ytterbium ions (1021∕cm3) were electrochemically codoped into porous silicon. When compared with the Er-doped porous silicon we found two salient features in its photoluminescence. One is that the 1.54μm emission of Er3+ ions was 50 times stronger under 980nm laser excitation at room temperature. This is due to the high efficiency of the Yb3+→Er3+ energy transfer. The other is that the photoluminescence intensity increases rapidly with temperature, while that in Er-doped porous silicon decreases monotonically with temperature. Our results show that the codoped Er∕Yb porous silicon has a promising potential for the application in optical communications.

Heteroepitaxial growth of wurtzite InN films on Si(111) exhibiting strong near-infrared photoluminescence at room temperature

High-quality InN epitaxial films have been grown by nitrogen-plasma-assisted molecular-beam epitaxy on Si(111) substrates using a double-buffer technique. Growth of a (0001)-oriented single crystalline wurtzite–InN layer was confirmed by reflection high-energy electron diffraction, x-ray diffraction, and Raman scattering. At room temperature, these films exhibited strong near-infrared (0.6–0.9 eV) photoluminescence (PL). In addition to the optical absorption measurement of absorption edge and direct band nature, the PL signal was found to depend linearly on the excitation laser intensity over a wide intensity range. These results indicate that the observed PL is due to the emission of direct band-to-band recombination rather than the band-to-defect (or impurity) deep emission.

Colloidal PbS Nanocrystals with Size‐Tunable Near‐Infrared Emission: Observation of Post‐Synthesis Self‐Narrowing of the Particle Size Distribution

The synthesis of nanocrystalline PbS quantum dots using organometallic precursors is reported. The samples exhibit tunable, strong near‐infrared photoluminescence. A remarkable spontaneous self‐directed narrowing of the particle size distribution is shown to occur after the particles are synthesized and dispersed in organic solvent. The Figure shows the tuning of band‐edge exciton absorption features through the near‐infrared spectral region according to particle size.

Strong near-infrared photoluminescence and absorption from Si/Si1−xGex type-II multiple quantum wells on bulk crystal SiGe substrates

We report a study of photoluminescence (PL) and optical absorption in high-quality tensile-strained Si type-II multiple quantum wells (QWs) grown on bulk crystal SiGe substrates by using low-temperature ultrahigh-vacuum chemical vapor deposition. Detailed PL experiments, as a function of excitation density together with applied uniaxial stress, were performed to study the band alignment and to help elucidate the origin of the observed PL peaks. Spatially direct and indirect transitions of the Si QWs and an intense broad subgap PL were observed. With increasing excitation power, the PL lines of Si QWs shift to lower energies, whereas the broad PL shifts to higher energy. The applied [110] compressive stress results in a redshift of the substrate PL lines, as expected, but a blueshift of both the Si QW PL and the broad PL lines. The optical absorption data also show strong absorption in the near-infrared region where the broad PL was observed. Near the absorption edge (Eg), the energy dependence obeys a square law rather than a square root law, suggesting that Eg is associated with a quasi-direct transition. The Eg value deduced according to the square law agrees well with the broad PL peak energy provided that temperature-dependent Eg, excitonic binding energy, and exciton localization energy, as well as quantum confinement are considered.

Polygermyne—A Prototype System for Layered Germanium Polymers

Crystalline polygermyne—a new compound consisting of H-terminated germanium double layers (see Figure) stacked to form a three-dimensional crystal—is reported to have been produced by the topochemical transformation of epitaxial thin CaGe2 films at –30 °C. Polygermyne is found to exhibit strong near-infrared photoluminescence with an intensity comparable to that of siloxene, its silicon homologue.