Room-temperature Luminescence Research Articles

Low cost synthesis of silicon nanowires for photonic applications

Achieving light management in nanostructured materials is a recent challenge of great resonance among the scientific community, which is generally attained by expensive surface patterning requiring advanced technologies. In this paper, we report the realization of 2D random fractal arrays of silicon nanowires (NWs) synthesized with a low cost approach compatible with Si technology, without the use of any lithography or mask. Their innovative photonic properties are exploited in comparison to non-fractal nanowires. In particular, a remarkable room temperature luminescence is attained in Si NWs due to quantum confinement effect. The NWs fabrication was engineered in order to produce two-dimensional random fractal arrays of Si NWs whose structural properties were investigated and compared to other non-fractal Si NW systems. The impressive light trapping and strongly enhanced Raman signal of our fractal Si NW array is of significant interest for potential applications spanning from photonics, to photovoltaics and sensing.

Intramolecular electronic coupling for persistent room-temperature luminescence for smartphone based time-gated fingerprint detection

CzDPS emits persistent luminescence in both crystals and polymer matrixes, and can be used for time-gated luminescence imaging of fingerprints.

Europium luminescence from amorphous yttrium alumina films on fused silica substrates

In this work, we investigate photoluminescence of sol-gel derived films corresponding to yttrium alumina garnet Y3Al5O12 doped with trivalent europium. The films with three concentrations of europium 0,02; 0,025; 0,03 M with the thickness 210-220 nm were deposited on fused silica substrates by the corresponding sols’ spinning followed by drying and high temperature annealing at 1000 °C for 30 min. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis as well as ellipsometry, photoluminescence (PL) and photoluminescence excitation spectroscopy (PLE). All of the fabricated samples were amorphous and revealed room-temperature luminescence bands within the range of 550 – 750 nm corresponding to 5D0 → 7Fj (j = 0,..,4) transitions of trivalent europium ions with the most intensive bands at 610 – 630 nm and 680 – 710 nm. The intensity of europium luminescence increases with europium concentration. The photoluminescence excitation spectra measured at the emission wavelength 710 nm exhibit strong bands at 220 nm, which could be associated with absorption of exciting light within Eu3+ - O2-bands. Practical application of the obtained films transforming ultraviolet irradiation into visible is discussed.

Luminescent Silicon Nanocrystals Prepared by Laser-Assisted Synthesis in Liquid for Imaging and Photovoltaic Applications

Silicon nanocrystals (SiNCs) with photoluminescence in the visible spectral range were synthesized by laser irradiation of silicon wafers in solutions. Laser irradiation leads to the formation of well-dispersed Si–NCs with an average diameter of about 5 nm, which upon excitation with 300−450 nm light exhibited room temperature luminescence tunable in the range from 410 to 650 nm. The results showed that photoluminescence (PL) is affected by both sizes and surface states of the Si–NCs. The results obtained allow to conclude that the synthesized nanoparticles (NPs) can be suitable materials for applications in fluorescence imaging along with photovoltaics.

Room-temperature visible upconversion luminescence of Ni2+ sensitized by Yb3+ in transparent glass ceramics

Upconversion luminescence of transition metal Ni2+ ions is seldom at room temperature (RT) due to large non-radiative transition probability. Here, a green Ni2+ upconversion luminescence at RT is obtained by the near-infrared excitation of Yb3+ 2F7/2 → 2F5/2 at 980 nm in Ni2+/Yb3+ codoped transparent wide bandgap semiconductor γ-Ga2O3 glass ceramics, which can be assigned to the Ni2+ 1T2(1D) → 3A2(3F) transition. Lifetime measurement and upconversion power dependence data reveal energy transfer upconversion as the underlying upconversion mechanism for the Yb3+-Ni2+ systems incorporated into the γ-Ga2O3 nanocrystals. It is suggested that the low thermal quenching effect of the wide bandgap semiconductor γ-Ga2O3 and resonant sensitizing of the Yb3+ 2F5/2 state to the Ni2+ upconversion 3T2(3F) intermedia state are responsible for the achievement of room-temperature upconversion luminescence of Ni2+. The results demonstrate that wide bandgap semiconductor nanocrystal (γ-Ga2O3, TiO2, SnO2, et al.) glass ceramics may be a good candidate for hosting Ni2+ room-temperature upconversion luminescence.

Synthesis, Photophysics, and Switchable Luminescence Properties of a New Class of Ruthenium(II)-Terpyridine Complexes Containing Photoisomerizable Styrylbenzene Units.

We report here the synthesis and structural characterization of a new class of homoleptic terpyridine complexes of Ru(II) containing styrylbenzene moieties to improve room-temperature luminescence properties. Solid-state structure determination of 2 was done through single-crystal X-ray diffraction. Tuning of photophysical properties was done by incorporating both electron-donating and electron-withdrawing substituents in the ligand. The complexes exhibit strong emission having lifetimes in the range of 10.0–158.5 ns, dependent on the substituent and the solvent. Good correlations were also observed between Hammett σp parameters with the lifetimes of the complexes. Styrylbenzene moieties in the complexes induce trans–trans to trans–cis isomerization accompanied by huge alteration of their spectral profiles upon treating with UV light. Reversal of trans–cis to trans–trans forms was also achieved on interacting with visible light. Change from trans–trans to the corresponding trans–cis form leads to emission quenching, whereas trans–cis to the corresponding trans–trans form leads to restoration of emission. In essence, “on–off” and “off–on” photoswitching of luminescence was observed. Calculations involving density functional theory (DFT) and time-dependent-DFT methods were performed to understand the electronic structures as well as for appropriate assignment of the absorption and emission bands.

Room temperature luminescence of passivated InGaN quantum dots formed by quantum-sized-controlled photoelectrochemical etching

Room temperature luminescence of epitaxial InGaN quantum dots (QDs) formed by quantum sized-controlled photoelectrochemical (QSC-PEC) etching and passivation layer regrowth is demonstrated. QSC-PEC etching is performed on a 7.5 nm thick In0.20Ga0.80N layer emitting at ∼514–521 nm and with a laser diode emitting at 445 nm. Parameters such as etch bias (0.9 V and 1.5 V), laser average power (20 mW/cm2 and 100 mW/cm2), and laser operating conditions (pulsed and continuous wave) are explored. QSC-PEC etching of In0.20Ga0.80N requires a minimum bias (>0.9 V) and pulsed laser conditions in order to form QDs. After etching, the QDs do not exhibit photoluminescence due to defect recombination. Regrowth of passivation layers consisting of a 2 nm thick Al0.45Ga0.55N layer and a 11 nm thick GaN layer reduce the defect recombination, and room temperature photoluminescence is observed at room temperature at ∼435–445 nm with narrow full-width at half-maximum of ∼35 nm.

Synthesis and characterization of Ru(III) complexes with 2-((E)-((4-((4-bromophenyl)ethynyl)phenyl)imino)methyl-4-((E)-phenyldiazenyl)phenol and their use as a precursor for RuO2 nanoparticles

Some Ru(III) complexes of the type [Ru(L)(phen)X] (1a, 2a) and [Ru(L)(bipy)X] (1b, 2b) (where L = 2-((E)-((4-((4-bromophenyl)ethynyl)phenyl)imino)methyl-4-((E)-phenyldiazenyl)phenol, phen = 1,10-phenanthroline, bipy = 2,2′- bipyridine, X = N3 and NCS) have been prepared and characterized on the basis of elemental analysis, FTIR, 1H NMR and mass spectral studies. Room temperature luminescence is observed for all complexes not only due to π→π* intra-ligand transition but also due to metal to ligand charge transfer character and varies considerably with π–acidic character of the coligands. Thermal stability of phen and bipy complexes was studied by TG analysis. The representative complexes 1a and 2b were calcinated at 600 °C for 5 h and RuO2 obtained were characterized by FTIR, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) techniques. PXRD analysis showed that the obtained oxides are crystalline and corresponded to RuO2 phases. Crystal size, shape and morphology were determined by SEM and TEM studies.

X-ray induced radiation damage in CLYC(Ce)

The radiation hardness of the 6Li loaded scintillator CLYC(Ce) to X-rays was investigated. Two crystals were studied; one crystal was irradiated with X-rays and one was kept as a control and only exposed to a moderated 241Am–Be source. The control crystal was used as a reference sample for photoluminescence excitation and emission measurements. The exposed crystal was given two doses of X-rays, the first was 2.3 Gy and the second was 118.2 Gy after a 22 week annealing period. The total dose was 120.5 Gy and was found to significantly reduce the light yield of the crystal. Pulse height spectra from the moderated 241Am–Be taken with the irradiated crystal showed a 54% decrease in pulse height and a broadening of the FHWM of the peak from 14.6 % to 44.2%. Analysis of the pulse shapes showed no change to the median decay time to 37% of the scintillator, with values of ∼350 ns for the irradiated and control crystal. Photoluminescence excitation and emission measurements revealed a reduction in the relative intensity of the main Ce3+ emission with respect to the self-trapped exciton luminescence. This indicated a modification to the Ce3+ luminescence center. There was little recovery at room temperature over the course of 22 weeks after the first irradiation. The irradiated crystal was also found to display significant room temperature luminescence after X-ray irradiation with it being measurable up to 120 min after the X-ray irradiation.

Sol-gel synthesis and nanostructured semiconductor analysis of undoped and Cd-doped ZnO thin films

The semiconductor thin films of undoped and cadmium (Cd) doped zinc oxide (ZnO) via different concentrations (Cd: 2, 5 and 10 wt.%) were prepared by sol-gel route technique using a dip-coating method onto glass and silicon substrates. The Zinc acetate dehydrate (ZnAc), the Cadmium acetate dehydrate (CdAc), the 2-Methoxyethanol and the Ethanolamine were used as a starting material, doping, solvent and stabilizer, respectively. All films were annealed for 120 min at 500 °C. The samples were characterized by different analysis techniques to understand their structural, morphological and optical properties and its components such as the thin films surface are a nanometric crystallite size with a ganglia-like structure as observed by Environmental Scanning Electron Microscopy. The films were characterized by X-ray diffraction, it was crystallized in a hexagonal crystal structure with a highly c-axis preferred (002) orientation, and the crystallites size decrease from 30 to 23 nm with the increasing of cadmium doping. The Rutherford Backscattering Spectrometry and the Auger Electron Spectroscopy spectra given an idea in the thickness of the film by simulation and relation to the etching time respectively, where did the two agree that the film of ZnO doped 10 wt.% Cd is the thickest. All films exhibit an optical transmittance above 75 to 85% nm and a sharp absorption onset about 375 nm corresponding to the fundamental absorption edge 3.08 to 3.21 eV. The room-temperature luminescence emission spectra of these films were excited under the same conditions with 250 nm Xenon lamp excitation, showed an ultraviolet and visible emissions.

Synthesis, Structural Characterization, and Luminescence Switching of Diarylethene-Conjugated Ru(II)-Terpyridine Complexes by trans-cis Photoisomerization: Experimental and DFT/TD-DFT Investigation.

We synthesized and thoroughly characterized a new family of diarylethene-conjugated mononuclear Ru(II)-terpyridine complexes and investigated in detail their photophysical, electrochemical, and spectroelectrochemical behaviors. Interestingly, the compounds show moderately strong room-temperature luminescence predominantly from their 3MLCT state with luminescence lifetime varying between 8.43 and 22.82 ns. Because of the presence of diarylethene unit, all the monometallic complexes underwent trans-to-cis photoisomerization upon interaction with UV light with substantial changes in their absorption and luminescence spectra. Reverting back from the cis to the trans form is also made possible upon treatment with visible light or by heat. Trans-to-cis isomerization leads to almost complete quenching of luminescence, while backward cis-to-trans isomerization gives rise to restoration of the original luminescence for all the complexes. Thus, "on-off" and "off-on" emission switching was made possible upon successive interaction of the complexes with UV and visible light. Computational investigation involving density functional theory (DFT) and time-dependent DFT methods was done for proper assignment of the experimental absorption and emission spectral bands in the complexes. Finally, experimentally observed trend on the absorption and emission spectral behaviors of the complexes upon photoisomerization was also compared with the calculated results.

Benzo-iridacyclopentadiene complexes: Mechanochromism and the effects of counter anions and halogen ligands

Five novel cationic cyclometalated Ir(III) complexes of type [(CˆC)IrX(PPh3)2(dmbpy)]A (X = halogen; dmbpy = 4,4′-dimethoxy-2,2′-bipyridine; A = counterion) were synthesised and characterised. Complexes 1–3 ([(CˆC)IrCl(PPh3)2(dmbpy)]A) have different anions (A = Cl−, CF3SO3−, PF6−) and complexes 4–5 ([(CˆC)IrX(PPh3)2(dmbpy)]PF6) have different halogen ligands (X = Br−, I−). Changing either the counter anion or halogen ligand altered the photophysical properties of the complexes. Interestingly, complex 1 (X = Cl, A = Cl−) did not exhibit mechanochromism, while the other four complexes all displayed mechanochromic properties. Crystal structures were determined for all compounds, and two types of single crystals are obtained for compounds 4 and 5. All complexes displayed bright solid-state phosphorescence, and reversible room-temperature mechanochromic luminescence was observed for complexes 2–5. The mechanism of the mechanochromic behaviour was investigated by combining powder and single-crystal X-ray diffraction analyses.

(Invited) Silicon Photonics Based on Ge/Sige Quantum Well (QW) Structures and Ge-Rich Materials for Near-IR and Mid-IR

Silicon photonics is a disruptive technology poised to revolutionize several areas of our society. It has often been considered as the natural step forward to overcome some of the inherent bottlenecks of current microelectronics technology. Importantly, photonic integrated circuits can be implemented using CMOS-like fabrication processes, hence benefiting from a mature technology that provides low-cost mass fabrication using advanced tools. A large pool of Si components has been demonstrated, standing for low-loss waveguides, filters, micro-ring resonators, Mach-Zehnder interferometers, array waveguide gratings or photonic crystals, among others [1]. Similarly, the incorporation of Ge in Si photonic platforms has yield the demonstration of several active optical devices such as high-speed optical modulators and photodetectors with unprecedented performance [2]. In addition, strong electro-absorption and electro-refraction has been demonstrated in Ge/SiGe quantum wells showing strong Quantum Confined Stark Effects (QCSE), allowing for efficient optoelectronic modulation and detection at telecom wavelengths [3]. Interestingly, tuning the QW design allows fine tuning of the absorption band edge of devices over a wideband range. Moreover, light emission from the direct bandgap transition HH1-CΓ1 has also been reported in SiGe/Ge quantum wells, opening the route towards the development of light emitting devices fully integrated in a silicon chip [4]. Noticeably, all the above described activities are meant to develop Si-based photonic platforms in the near-infrared wavelength range, where most applications are foreseen. Lately, however, there is an increasing interest in the exploitation of Ge-based photonic platforms at longer wavelengths up to the mid-infrared [5]. Having Ge a transparency window up to 15 µm, there are good prospects for the development of advanced photonic sensing platforms leveraging from the strong mid-IR molecular absorption bands of several chemical and biological substances. Besides, Ge-based platforms have also been proposed as ideal candidates to exploit the nonlinear properties at mid-IR wavelengths, owing to a higher χ(3) nonlinearity and the absence of two-photon absorption beyond λ ≈ 3.15 µm [6]. Important benefits are also expected in data communications, taking advantage from the two atmospheric transparency windows at λ ≈ 3-5 µm and λ ≈ 8-13 µm and the relaxed wavelength regulations put in place. Thus, in this presentation we will show our recent work on the development of Ge-based photonic devices for telecom and mid-IR applications. For that, a review of the advanced optoelectronic devices demonstrated so far in our group will be presented, including high-speed SiGe/Ge QWs electro-absorption modulators and detectors exploiting the QCSE. Then, further insight on the room temperature luminescence of QWs will be provided, followed by its integration in light emitting diodes in waveguide configuration for planar emission. In addition, a novel mid-IR photonic platform based on Ge-rich graded-index SiGe alloys will be presented. This platform offers an interesting set of advantages over other existing mid-IR approaches such as low-loss propagation and optimal modal area over an unprecedentedly broadband wavelength range [7]. As a consequence, the first demonstration of ultra-wideband mid-IR Mach-Zehnder interferometers operating from λ = 5.5 µm to 8.5 µm was performed using the graded-index SiGe platform [8]. Finally, the third-order nonlinear properties of this platform will be discussed, supported by experimental data on Ge-rich SiGe waveguides using a bidirectional top hat D-scan method [9]. Promising results were obtained, thus providing a promising scenario to develop broadband optical sources based on the supercontinuum generation.

DFT rationalization of the room-temperature luminescence properties of Ru(bpy) 3 2+ and Ru(tpy) 2 2+ : 3MLCT–3MC minimum energy path from NEB calculations and emission spectra from VRES calculations

Extensive experimental data covering 40 years of research are available on Ru(bpy) 3 2+ and Ru(tpy) 2 2+ , which are the archetypes of inorganic photochemistry. The last decade has enabled computational chemists to tackle this topic through density functional theory and to shed some new light on our old friends. For the first time, this theoretical study maps the minimum energy path linking the 3MLCT (metal-to-ligand charge transfer) and the 3MC (metal-centred) states with the nudged elastic band method, also providing the calculation of the corresponding energy barrier. Remarkably, the obtained data are in very good agreement with the experimental activation energies reported from variable-temperature luminescence measurements. Calculation of vibrationally resolved electronic spectra is also in excellent agreement with the experimental emission maximum and bandshape of Ru(bpy) 3 2+ . Additionally, the 3MC–GS minimum energy crossing point was optimized for each complex. The combination of these data rationalizes the room-temperature luminescence of the bpy complex and non-luminescence of the tpy complex.

Mechano-responsive room temperature luminescence variations of boron conjugated pyrene in air.

Stimuli responsive tunable luminescence is a promising field of research. Even though mechanofluorescence is widely studied, mechanophosphorescence remains unexplored. Here we report the mechano-driven fluorescence and phosphorescence variations of a pyrene tetraboronic ester derivative. The fluorochromic and room temperature phosphorescence features are supported by theoretical studies and single crystal analysis. The mechanically ground fluorescence active but phosphorescence inactive pyrene tetraboronic ester exhibits room temperature phosphorescence in air with mechanical force. The efficient intermolecular electronic coupling in the dimer formed upon scratching enables a good communication between singlet and triplet states, hence resulting in room temperature mechanophosphorescence.

New Generation of Ultrasensitive Label-Free Optical Si Nanowire-Based Biosensors

We demonstrate the realization of the first label-free optical biosensor based on the room temperature luminescence of silicon nanowires (NWs) tested for the selective detection of C-reactive protein in human serum. High aspect ratio Si NW arrays used as sensing interface, are synthesized by a fast, low cost and Si industrially compatible approach. Si NW optical biosensors are fast and offer a broad concentration dynamic range that can be tuned according to different applications. Moreover, the platform is endowed with a high selectivity toward the target analyte and a sensitivity down to the fM limit of detection, opening the route toward noninvasive analysis in biofluids such as saliva.

Heterobimetallic acetylide bridged Cu(I)/Ru(II)-halide/pseudohalide hybrid complexes: Synthesis, structural characterization, luminescence and electrochemical studies

A new series of heterobimetallic complexes [Cu(PPh3)(NC5H4HCNC6H4CCC6H4CCRu (dppe)2Cl)X] (1a-5a) have been prepared by the reaction of trans-(NC5H4HCNC6H4CCC6H4CC Ru(dppe)2Cl) with copper salts in presence of triphenylphosphine (where X = Cl, Br, I, N3, NCS). Our synthetic attempts and successes are discussed in combination with spectroscopic and electronic characterization of the compounds. Comparison between halides and pseudohalides were studied by thermal and electrochemical analysis where, thermally robust complexes demonstrate quasireversible redox behaviour analogous to CuI/II/RuII/III couple. Room temperature luminescence with varying electron donating and quenching abilities of halides and pseudohalides in blue-green region were observed. Concentration and solvent dependant emission displays positive solvatochromism at ambient temperature.

Structural characterization of two solvates of a luminescent copper(II) bis-(pyridine)-substituted benzimidazole complex.

Copper(II) complexes of benzimidazole are known to exhibit biological activity that makes them of inter-est for chemotherapeutic and other pharmaceutical uses. The complex bis-(acetato-κO){5,6-dimethyl-2-(pyridin-2-yl)-1-[(pyridin-2-yl)meth-yl]-1H-benzimidazole-κ2N2,N3}copper(II), has been prepared. The absorption spectrum has features attributed to intra-ligand and ligand-field transitions and the complex exhibits ligand-centered room-temperature luminescence in solution. The aceto-nitrile monosolvate, [Cu(C2H3O2)2(C20H18N4)]·C2H3N (1), and the ethanol hemisolvate, [Cu(C2H3O2)2(C20H18N4)]·0.5C2H6O (2), have been structurally characterized. Compound 2 has two copper(II) complexes in the asymmetric unit. In both 1 and 2, distorted square-planar N2O2 coordination geometries are observed and the Cu-N(Im) bond distance is slightly shorter than the Cu-N(py) bond distance. Inter-molecular π-π inter-actions are found in 1 and 2. A weak C-H⋯π inter-action is observed in 1.

Heterobimetallic Cu(I)/Ru(II) Acetylide Bridged Hybrid Complexes Containing Coordination and Organometallic Sites

AbstractThe heterobimetallic complexes [Cu(phen)(NC5H4HC=NC6H4C≡CC6H4C≡CRu(dppe)2Cl)]X (1 a‐4 a) and [Cu(bipy)(NC5H4H C=NC6H4C≡CC6H4C≡CRu(dppe)2Cl)]X (1 b‐4 b) (where X= I, ClO4, BF4, PF6; phen= 1, 10 phenanthroline, bipy= 2, 2′ bipyridine) have been prepared by the reaction of (NC5H4HC=NC6H4C≡CC6H4C≡CRu(dppe)2Cl) with copper salts in presence of phen or bipy as a coligand. Our synthetic attempts and successes are discussed in combination with spectroscopic and electronic characterization of the compounds. Comparison between phen and bipy complexes were studied by thermal and electrochemical analysis where, thermally robust complexes demonstrate quasireversible redox behaviour analogous to CuI/II/RuII/III couple. SEM of representative complexes 2 a and 2 b clarify the average size and morphology of the complexes. Room temperature luminescence influenced by varying coordinating ability of counter anions along with π‐acidic character in green‐yellow region was observed. Solvent dependant emission displays positive solvatochromism at ambient temperature. The second harmonic generation (SHG) efficiency of the complexes was measured by Kurtz‐powder technique indicating that all complexes display the second harmonic generation (SHG) property.

Luminescence emission from Al0.3Ga0.7N/GaN multi quantum disc core/shell nanowire: Numerical approach

In this work, a numerical approach to investigate the room temperature luminescence emission from core/shell nanowire is presented where GaN quantum discs (QDiscs), periodically distributed in AlxGa1−xN nanowire, is considered as core and AlxGa1−xN as shell. Thin disc shaped (Ring shaped) n-doped region has been placed at the GaN/ AlxGa1−xN (AlxGa1−xN /air) interface in AlxGa1−xN region in axial (radial) directions. To obtain energy levels and related wavefunctions, self-consistent procedure has been employed to solve Schrodinger-Poisson equations with considering the spontaneous and piezoelectric polarization. Then luminescence spectrum is studied in details to recognize the parameters influent in luminescence. The results show that the amount of doping, size of QDiscs and theirs numbers have remarkable effects on the band to band luminescence emission. Our numerical calculations gives some insights into the luminescence emission of core/shell nanowire and exhibits a useful tool to analyze findings in experiments.