Spectrometer based on a compact disordered multi-mode interferometer.

We present a compact, CMOS compatible, photonic integrated circuit (PIC) based spectrometer that combines a dispersive array element of SiO2-filled scattering holes within a multimode interferometer (MMI) fabricated on the silicon-on-insulator (SOI) platform. The spectrometer has a bandwidth of 67 nm, a lower bandwidth limit of 1 nm, and a peak-to-peak resolution of 3 nm for wavelengths around 1310 nm.

Laser-Assisted Ultrafast Fabrication of Crystalline Ta-Doped TiO2 for High-Humidity-Processed Perovskite Solar Cells.

A titanium dioxide (TiO2) compact film is a widely used electron transport layer (ETL) for n-i-p planar perovskite solar cells (PSCs). However, TiO2 sufferers from poor electrical conductivity, leading to high energy loss at the perovskite/ETL/transparent conductive oxide interface. Doping the TiO2 film with alkali- and transition-metal elements is an effective way to improve its electrical conductivity. The conventional method to prepare these metal-doped TiO2 films commonly requires time-consuming furnace treatments at 450-600 °C for 30 min to 3 h. Herein, a rapid one-step laser treatment is developed to enable doping of tantalum (Ta) in TiO2 (Ta-TiO2) and to simultaneously induce the crystallization of TiO2 films from its amorphous precursor to an anatase phase. The PSCs based on the Ta-TiO2 films treated with the optimized fiber laser (1070 nm) processing parameters (21 s with a peak processing temperature of 800-850 °C) show enhanced photovoltaic performance in comparison to that of the device fabricated using furnace-treated films at 500 °C for 30 min. The ambient-processed planar PSCs fabricated under high relative humidity (RH) of 50-70% display power conversion efficiencies (PCEs) of 18.34% and 16.04% for devices based on Cs0.1FA0.9PbI3 and CH3NH3PbI3 absorbers, respectively. These results are due to the improved physical and chemical properties of the Ta-TiO2 films treated by the optimal laser process in comparison to those for the furnace process. The laser process is rapid, simple, and potentially scalable to produce metal-doped TiO2 films for efficient PSCs.

Manipulation of Molecular Vibrations on Condensing Er3+ State Densities for 1.5 µm Application.

Vibrational modes of chemical bonds in organic erbium (Er3+) materials play an important role in determining the efficiency of the 1.5 µm Er3+ emission. This work studies the energy coupling of the Er3+ intra-4f transitions and vibrational modes. The results demonstrate that the coupling introduces enormous nonradiative internal relaxation, which condenses the excited erbium population on to the 4I13/2 state. This suggests that vibrational modes can be advantageous for optimizing the branching ratio for the 1.5 µm transition in organic erbium materials. Through control of the quenching effect on to the 4I13/2 state and a reliable determination of intrinsic radiative rates, it is found that the pump power for population inversion can be reduced by an order of magnitude at high erbium concentrations compared to conventional inorganic erbium materials.

Non-diffracting beam generated from a photonic integrated circuit based axicon-like lens.

We demonstrate an on-chip silicon-on-insulator (SOI) device to generate a non-diffracting beam of ≈850 µm length from a diffractive axicon-like lens etched using a low resolution (200 nm feature size, 250 nm gap) deep-ultraviolet lithographic fabrication. The device consists of circular gratings with seven stages of 1x2 multimode interferometers. We present a technique to apodize the gratings azimuthally by breaking up the circles into arcs which successfully increased the penetration depth in the gratings from ≈5 µm to ≈60 µm. We characterize the device's performance by coupling 1300±50 nm swept source laser in to the chip from the axicon and measuring the out-coupled light from a grating coupler. Further, we also present the implementation of balanced homodyne detection method for the spectral characterization of the device and show that the position of the output lobe of the axicon does not change significantly with wavelength.

Multiple Roles of 1,4-Diazabicyclo[2.2.2]octane in the Solvothermal Synthesis of Iodobismuthates.

Hybrid bismuth-containing halides are emerging as alternative candidates to lead-containing perovskites for light-harvesting applications, as Bi3+ is isoelectronic with Pb2+ and the presence of an active lone pair of electrons is expected to result in outstanding charge-carrier transport properties. Here, we report a family of one binary and three ternary iodobismuthates containing 1,4-diazabicyclo[2.2.2]octane (DABCO). These materials have been prepared solvothermally and their crystal structures, thermal stability, and optical properties determined. Reactions carried out in the presence of bismuth iodide and DABCO produced (C6H12N2)BiI3 (1), which consists of hybrid ribbons in which pairs of edge-sharing bismuth octahedra are linked by DABCO ligands. Short I···I contacts give rise to a three-dimensional network. Similar reactions in the presence of copper iodide produced (C8H17N2)2Bi2Cu2I10 (2) and [(C6H13N2)2BiCu2I7](C2H5OH) (3) in which either ethylated DABCO cations (EtDABCO)+ or monoprotonated DABCO cations (DABCOH)+ are coordinated to copper in discrete tetranuclear and trinuclear clusters, respectively. In the presence of potassium iodide, a unique three-dimensional framework, (C6H14N2)[(C6H12N2)KBiI6] (4), was formed, which contains one-dimensional hexagonal channels approximately 6 Å in diameter. The optical band gaps of these materials, which are semiconductors, range between 1.82 and 2.27 eV, with the lowest values found for the copper-containing discrete clusters. Preliminary results on the preparation of thin films are presented.

Synthesis of IR-emitting HgTe quantum dots using an ionic liquid-based tellurium precursor.

New scalable precursor chemistries for quantum dots are highly desirable and ionic liquids are viewed as an attractive alternative to existing solvents, as they are often considered green and recyclable. Here we report the synthesis of HgTe quantum dots with emission in the near-IR region using a phosphonium based ionic liquid, and without standard phosphine capping agents.

Preparation of solution processed photodetectors comprised of two-dimensional tin(ii) sulfide nanosheet thin films assembled via the Langmuir-Blodgett method.

We report the manufacture of fully solution processed photodetectors based on two-dimensional tin(ii) sulfide assembled via the Langmuir-Blodgett method. The method we propose can coat a variety of substrates including paper, Si/SiO2 and flexible polymer allowing for a potentially wide range of applications in future optoelectronic devices.

Loss of phenotypic inheritance associated with ydcI mutation leads to increased frequency of small, slow persisters in Escherichia coli.

Whenever a genetically homogenous population of bacterial cells is exposed to antibiotics, a tiny fraction of cells survives the treatment, the phenomenon known as bacterial persistence [G.L. Hobby et al., Exp. Biol. Med. 50, 281-285 (1942); J. Bigger, The Lancet 244, 497-500 (1944)]. Despite its biomedical relevance, the origin of the phenomenon is still unknown, and as a rare, phenotypically resistant subpopulation, persisters are notoriously hard to study and define. Using computerized tracking we show that persisters are small at birth and slowly replicating. We also determine that the high-persister mutant strain of Escherichia coli, HipQ, is associated with the phenotype of reduced phenotypic inheritance (RPI). We identify the gene responsible for RPI, ydcI, which encodes a transcription factor, and propose a mechanism whereby loss of phenotypic inheritance causes increased frequency of persisters. These results provide insight into the generation and maintenance of phenotypic variation and provide potential targets for the development of therapeutic strategies that tackle persistence in bacterial infections.

Confinement Effects and Charge Dynamics in Zn3N2 Colloidal Quantum Dots: Implications for QD-LED Displays.

Zinc nitride (Zn3N2) colloidal quantum dots are composed of nontoxic, low-cost, and earth-abundant elements. The effects of quantum confinement on the optical properties and charge dynamics of these dots are studied using steady-state optical characterization and ultrafast fluence-dependent transient absorption. The absorption and emission energies are observed to be size-tunable, with the optical band gap increasing from 1.5 to 3.2 eV as the dot diameter decreased from 8.9 to 2.7 nm. Size-dependent absorption cross sections (σ = 1.22 ± 0.02 × 10-15 to 2.04 ± 0.03 × 10-15 cm2), single exciton lifetimes (0.36 ± 0.02 to 0.65 ± 0.03 ns), as well as Auger recombination lifetimes of biexcitons (3.2 ± 0.4 to 5.0 ± 0.1 ps) and trions (20.8 ± 1.8 to 46.3 ± 1.3 ps) are also measured. The degeneracy of the conduction band minimum (g = 2) is determined from the analysis of the transient absorption spectra at different excitation fluences. The performance of Zn3N2 colloidal quantum dots thus broadly matches that of established visible light emitting quantum dots based on toxic or rare elements, making them a viable alternative for QD-LED displays.

Solution Processable 1D Fullerene C60 Crystals for Visible Spectrum Photodetectors.

Visible spectrum photodetector devices fabricated using molecular crystals of carbon C60 are reported. The devices operate efficiently, extending over and beyond the full visible light spectrum (300-710 nm) with a bias voltage tunable responsivity of 4 mA-0.5 mA W-1 . Across this range of wavelengths, the noise equivalent power of these devices remains below 102 nW Hz-1/2 , providing a detectivity of 107 Jones. The noise current in these devices is found to have a strong dependence on both bias voltage and frequency, varying by 4 orders of magnitude from 1 nA Hz-1/2 to 0.1 pA Hz-1/2 . The devices also display a near-linear dependence of photocurrent on light intensity over 4 orders of magnitude, providing a dynamic range approaching 80 dB. The 3 dB bandwidth of the devices is found to be above 102 Hz, while the 18 dB bandwidth exceeds 1 kHz. The transient photocurrents of the devices have a rise time of ≈50 µs and a long fall time of ≈4 ms. The spectral photocurrent of the devices is found to quench gradually with a reduction in temperature from ≈300 K and is fully quenched at temperatures below T ≈ 100 K. Upon reheating, the device performance is fully recovered.

The Role of Substituent Effects in Tuning Metallophilic Interactions and Emission Energy of Bis-4-(2-pyridyl)-1,2,3-triazolatoplatinum(II) Complexes.

The photoluminescence spectra of a series of 5-substituted pyridyl-1,2,3-triazolato Pt(II) homoleptic complexes show weak emission tunability (ranging from λ=397-408 nm) in dilute (10(-6) M) ethanolic solutions at the monomer level and strong tunability in concentrated solutions (10(-4) M) and thin films (ranging from λ=487-625 nm) from dimeric excited states (excimers). The results of density functional calculations (PBE0) attribute this "turn-on" sensitivity and intensity in the excimer to strong Pt-Pt metallophilic interactions and a change in the excited-state character from singlet metal-to-ligand charge transfer ((1)MLCT) to singlet metal-metal-to-ligand charge transfer ((1)MMLCT) emissions in agreement with lifetime measurements.

n-type chalcogenides by ion implantation.

Carrier-type reversal to enable the formation of semiconductor p-n junctions is a prerequisite for many electronic applications. Chalcogenide glasses are p-type semiconductors and their applications have been limited by the extraordinary difficulty in obtaining n-type conductivity. The ability to form chalcogenide glass p-n junctions could improve the performance of phase-change memory and thermoelectric devices and allow the direct electronic control of nonlinear optical devices. Previously, carrier-type reversal has been restricted to the GeCh (Ch=S, Se, Te) family of glasses, with very high Bi or Pb 'doping' concentrations (~5-11 at.%), incorporated during high-temperature glass melting. Here we report the first n-type doping of chalcogenide glasses by ion implantation of Bi into GeTe and GaLaSO amorphous films, demonstrating rectification and photocurrent in a Bi-implanted GaLaSO device. The electrical doping effect of Bi is observed at a 100 times lower concentration than for Bi melt-doped GeCh glasses.

Ultrahigh performance C60 nanorod large area flexible photoconductor devices via ultralow organic and inorganic photodoping.

One dimensional single-crystal nanorods of C60 possess unique optoelectronic properties including high electron mobility, high photosensitivity and an excellent electron accepting nature. In addition, their rapid large scale synthesis at room temperature makes these organic semiconducting nanorods highly attractive for advanced optoelectronic device applications. Here, we report low-cost large-area flexible photoconductor devices fabricated using C60 nanorods. We demonstrate that the photosensitivity of the C60 nanorods can be enhanced ~400-fold via an ultralow photodoping mechanism. The photodoped devices offer broadband UV-vis-NIR spectral tuneability, exhibit a detectivitiy>10(9) Jones, an external quantum efficiency of ~100%, a linear dynamic range of 80 dB, a rise time 60 µs and the ability to measure ac signals up to ~250 kHz. These figures of merit combined are among the highest reported for one dimensional organic and inorganic large-area planar photoconductors and are competitive with commercially available inorganic photoconductors and photoconductive cells. With the additional processing benefits providing compatibility with large-area flexible platforms, these devices represent significant advances and make C60 nanorods a promising candidate for advanced photodetector technologies.

Phase separation in garnet solid solutions and its effect on optical properties.

Phase behavior is studied in erbium-doped Y3 Al5 O12 (YAG) garnets synthesized by solid-state reactions. High resolution synchrotron XRD and SEM-EDX studies reveal phase separation at an erbium content between 8 and 50 at%, depending upon the processing conditions. Similar results are found in closely-related garnet systems. The phase separation has a striking effect on the optical properties of YAG:Er(3+) .

On the analogy between photoluminescence and carrier-type reversal in Bi- and Pb-doped glasses.

Reaction order in Bi-doped oxide glasses depends on the optical basicity of the glass host. Red and NIR photoluminescence (PL) bands result from Bi(2+) and Bin clusters, respectively. Very similar centers are present in Bi- and Pb-doped oxide and chalcogenide glasses. Bi-implanted and Bi melt-doped chalcogenide glasses display new PL bands, indicating that new Bi centers are formed. Bi-related PL bands have been observed in glasses with very similar compositions to those in which carrier-type reversal has been observed, indicating that these phenomena are related to the same Bi centers, which we suggest are interstitial Bi(2+) and Bi clusters.

Facile synthesis of titania nanowires via a hot filament method and conductometric measurement of their response to hydrogen sulfide gas.

Titania nanostructures are of increasing interest for a variety of applications, including photovoltaics, water splitting, and chemical sensing. Because of the photocatalytical properties of TiO2, chemical processes that occur at its surface can be exploited for highly efficient nanodevices. A facile and fast synthesis route has been explored that is free of catalysts or templates. An environmental scanning electron microscopy (ESEM) system was employed to grow titania nanowires (NWs) in a water vapor atmosphere (∼1 mbar) and to monitor the growth in situ. In addition, the growth process was also demonstrated using a simple vacuum chamber. In both processes, a titanium filament was heated via the Joule effect and NWs were found to grow on its surface, as a result of thermal oxidation processes. A variety of nanostructures were observed across the filament, with morphologies changing with the wire temperature from the center to the end points. The longest NWs were obtained for temperatures between ∼730 °C and 810 °C. Typically, they have an approximate thickness of ∼300 nm and lengths of up to a few micrometers. Cross sections prepared by focused-ion-beam milling revealed the presence of a porous layer beneath the NW clusters. This indicates that the growth of NWs is driven by oxidation-induced stresses in the subsurface region of the Ti filament and by enhanced diffusion along grain boundaries. To demonstrate the potential of titania NWs grown via the hot filament method, single NW devices were fabricated and used for conductometric sensing of hydrogen sulfide (H2S) gas. The NW electric resistance was found to decrease in the presence of H2S. Its variation can be explained in terms of the surface depletion model.

A microfluidic system for long-term time-lapse microscopy studies of mycobacteria.

Phenotypic heterogeneity in bacterial populations is thought to contribute to a number of important phenomena including sporulation and persistence. The latter has clinical implications in many diseases such as tuberculosis, where persistence of Mycobacterium tuberculosis within the human host is believed to be the root cause of latent tuberculosis and the ability of a minority population of cells to survive antibiotic exposure, despite being genetically identical to the bulk population that are killed. However, phenotypic variations caused by non-genetic mechanisms are difficult to study because of the transient nature of the persistent state and thereby the requirement to observe individual cells in real-time. Recently, microfluidics, combined with time-lapse microscopy, has become a powerful tool for studying population heterogeneity in bacteria. However, growth and replication of mycobacterial cells provide particular problems for the development of microfluidic systems due to their tendency to grow in three dimensions. We here describe a novel microfluidic device for the observation of growth and antibiotic killing in individual mycobacterial cells. We constructed a microfluidic device suitable for studying single cell behavior in mycobacteria. The growth of single cells of Mycobacterium smegmatis expressing green fluorescent protein was monitored using a confocal laser scanning microscope. Within the device M. smegmatis cells were tightly confined within a hydrogel matrix thus promoting planar growth. Cell growth and killing was observed in the device with dead cells highlighted by uptake of propidium iodide. Conclusions/Significance. We demonstrate that our device allows real-time analysis and long-term culture of single cells of mycobacteria, and is able to support the study of cell death during the application of antibiotics. The device will allow observation of individual cells' cell genealogy to be determined and direct observation of rare states, such as persistence.

Efficient white light emission by upconversion in Yb(3+)-, Er(3+)- and Tm(3+)-doped Y2BaZnO5.

We report efficient white upconversion luminescence in Yb(3+)-, Er(3+)- and Tm(3+)-doped monophasic and biphasic Y(2)BaZnO(5) phosphors under 977 nm near-infrared excitation and at low excitation power densities (down to ∼25 mW mm(-2)).

Optimization of hybrid organic-inorganic interdigitated photovoltaic device structure using a 2D diffusion model.

To improve the efficiency of organic photovoltaic devices the inclusion of semiconducting nanoparticles such as PbS has been used to enhance near-infrared absorption. Additionally the use of interdigitated heterojunctions has been explored as a means of improving charge extraction. In this paper we provide a two-dimensional model taking into account these approaches with the aim of predicting an optimized device geometry to maximize the efficiency. The steady-state exciton population has been calculated in each of the active regions taking into account the full optical response based on using a finite difference approach to obtain approximate numerical solutions to the 2D exciton diffusion equation. On the basis of this we calculate the contribution of each active material to the device short circuit current and power conversion efficiency. We show that optimized structures can lead to power conversions efficiencies of ∼50% compared to a maximum of ∼17% for planar heterojunction devices. To achieve this the interdigitated region thickness should be ∼800 nm with PbS and C(60) widths of ∼60 and 20 nm, respectively. Even modest nanopatterning using much thinner active regions provides improvements in efficiency and may be approached using a variety of methods including nanoimprinting lithography, nanotemplating, or the incorporation of presynthesized nanorod structures.

Charge transfer in hybrid organic-inorganic PbS nanocrystal systems.

Charge transfer interactions between PbS nanocrystals (NCs) and tetrathiafulvalene (TTF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) are studied using optical spectroscopy. Selective quenching of PbS NC photoluminescence (PL) by TTF is observed and related to the relative alignment of the highest occupied molecular orbital (HOMO) of TTF and the PbS NC 1s(h) energy level. TCNQ is also found to quench PbS NC PL irrespective of the NC bandgap. A ground-state charge transfer mechanism between PbS and TCNQ is proposed to account for the observed quenching indirectly supported by observed changes in the absorption spectra of PbS-TTF and PbS-TCNQ solutions. Additionally, a second emission band in the visible spectral region ( approximately 675 nm) is found upon excitation of PbS-TCNQ solutions. These results are of interest for the future design of charge-transfer systems for use in hybrid organic-inorganic systems.