High Photo Research Articles

Analysis of an unusual mixed-habit natural diamond by high spatial resolution techniques: Growth and fracture

Mixed-habit growth of natural diamond is relatively uncommon and the reasons why it occurs are not well-understood. The diamond studied here, unusually, alternated between cuboidal and octahedral growth over a very long period making it possible, using a variety of high resolution analytical techniques, to establish the properties of the different phases of growth. In particular, nickel, in the form of NE centres, was restricted the cuboidal regions which also contained very high levels of hydrogen. There was a high concentration of nitrogen in the sample, but photoluminescence microscopy revealed it was incorporated in the two different regions of growth in very different forms with strong N3 emission from the octahedral regions, but in the hydrogen-related 3107 cm−1 infra-red and NE centres in the cuboidal regions. These results suggest that the traditional means of assaying nitrogen levels by A- and B-centre absorption may require additions. The section of the sample studied was quite close to the nucleation of growth and study of this region by high spatial resolution photo- and cathodoluminescence microscopy revealed previously unreported aspects of this critical growth step. The experimental results also indicate that a major fracture occurred with new diamond grown between the mixed-habit and the later octahedral growth.

High Photo Sensing Performance With Electro-Optically Efficient Silicon Based ZnO/ZnMgO Heterojunction Structure

The bulk heterostructure having nanopillar growth on top surface of the proposed photodetector structure is a promising candidate of high detection for high resolution quantum optic and high speed optical communication. Here, we report Si-ZnO-ZnMgO having grown with aligned hexagonal nanopillars for high performance photo detector, due to having its high quantum efficiency. From the proposed structure (electro-optically modified) high responsivity of $2.857\times 10^{2}$ AW $^{-1}$ and detectivity of $1.33\times 10^{15}$ cm Hz $^{1/2}\,\,\text{W}^{-1}$ (3mm $\times$ 3mm) was achieved and also the small performance degradation with increase of temperature shows thermal stability.

Assay of Single-Cell Apoptosis by Ensemble and Single-Molecule Fluorescence Methods: Annexin-V/Polyethylene Glycol-Functionalized Quantum Dots as Probes

Apoptosis plays a critical role in many biological processes and the etiology of various diseases of the immune system. The study of apoptosis would allow both improving the diagnosis of certain diseases and serving as a target of drug screening. In this paper, we developed a sensitive assay of single-cell apoptosis using semiconductor quantum dots (QDs) as fluorescent-labeling probes. The principle of this assay is based on the detection of phosphatidylserine (PS) exposed on the plasma membrane during the drug-induced apoptosis. The QD-labeled annexin V (AV) was prepared to specifically target PS on the membrane of apoptotic cells, and PS was detected by fluorescent imaging, flow cytometer, and single-molecule fluorescence correlation spectroscopy (FCS). We developed the procedures for conjugation of QDs to AV and for purification of their conjugates by gel chromatography. The obtained conjugates were characterized by FCS, capillary electrophoresis, and zeta potential analyzer. We studied the nonspecific adsorption of cells to different surface-modified QDs and found that the nonspecific adsorption effects were significantly reduced by modification of QDs with polyethylene glycol in the detection of apoptosis. In this assay, the results obtained by flow cytometry were consistent with the commercial test kit. Furthermore, a home-built single-molecule FCS system was developed for in situ study the drug-induced apoptosis. We observed the significant change in the diffusion coefficients of QDs on cells during the progress of cell apoptosis. Compared with conventional methods, the fluorescent methods represented here possess high sensitivity because of the use of high photo stability and brightness QDs as labeling probes and provide the temp-spatial information on a single apoptotic cell.

High performance photodiode based on MoS2/pentacene heterojunction

Photodiodes based on monomolecular layer MoS2/pentacene heterojunction were fabricated and characterized. Similar to other organic photodetectors, the device performances are strongly dependent on the incident optical power. The unoptimized device with Al top electrode, exhibited a fairly high photo responsivity of 0.27 A/W, external quantum efficiency (EQE) of 51.3% and specific detectivity (D∗) of 4.81 × 1011 Jones for red light of 655 nm wavelength. These performances exceed not only the Schottky photodiode based on pentacene, and mono or few layer MoS2 based photodiodes, but also the most organic photodiodes reported recently. More importantly, with Au as the top electrode, the device performances could be further improved, the photoresponsivity, EQE and D∗ reached 0.31 A/W, 58.1%, and 1.55 × 1013 Jones, respectively at the same voltage biasing.

Enabling DPP derivatives to show multistate emission and developing the multifunctional materials by rational branching effect

Conventional polycyclic organic molecules exhibit aggregation-caused quenching (ACQ) effect, and some organic molecules with twisted and propeller backbones could be made into aggregation-induced/enhanced emission (AIE/AEE). However, it was interesting but challenging to enable conjugated molecules derived from fused (hetero)cycles with high photo- and thermal-stability to show multistate emission, rather than only dual emission in solution and crystalline states. In the current work, we presented a rational strategy for realizing multistate emission of 1,4-diketo-pyrrolo [3,4-c]pyrrole (DPP) derivatives by employing imidazole as the additional acceptor bridge and constructing the branched DPP-centered molecule with four peripheral donors (triphenylamine). The resulting D‒A‒D compound (TIDPP) emitted yellow in solution with the fluorescence efficiency of ca. 70%, and its crystalline and amorphous states as well as aqueous suspensions also exhibited impressive fluorescence efficiencies of 40–50%, filling the gap between ACQ and AIE of DPP derivatives. Moreover, aggregation could greatly red-shift emission spectra and significantly enhance the two-photon absorption cross section and excitation fluorescence in the aqueous suspension. TIDPP solid was also found to exhibit the unique mechanochromic blue-shift luminescence and the commendable OFET performance, signifying the existence of aggregation-induced molecular planarization. This work demonstrated that the rational branching effect could manipulate the aggregation emission behaviors and develop the multifunctional materials.

Nano-crystalline CdS thick films: a highly sensitive photo-detector

This work reports a simple method for fabrication of highly sensitive photo-detector based on uniform nano-crystalline CdS thick films deposited on glass substrate by spray pyrolysis technique. The presented photo-detector platform contains CdS thick films (3, 6, 9, 12 and 15 μm) and Ag electrode structure. In order to detect incident light signals, changes in electrical signals were measured (conductance and current). For photo-detection, the current response between the ‘ON’ and ‘OFF ’ states of incident light was measured when exposed to different wavelengths ranging from 450 to 700 nm. The fabricated device showed significant detection of low-power (∼5 mW cm−2) light for optimized film thickness of 12 μm. Time response measurements at different wavelength show fast response and decay time (25 ms) and high photo sensitivity (1123) at low bias voltage, indicating good quality of the deposited 12 μm thick CdS film.

Black sesame-derived carbon dots for metal ion and amine vapour sensing

Carbon dots (CDs) are carbon nanoparticles with unique properties, including high photo stability and low toxicity. In this work, CDs were synthesized from black sesame via one-step hydrothermal method. CDs were characterized using UV-visible spectroscopy, fluorescence spectroscopy, Fourier-transform infrared spectroscopy, transmission electron spectroscopy, and x-ray photoelectron spectroscopy. The results showed that the size of CDs was around 7 nm, and they exhibited blue fluorescent emission with a quantum yield of 1%. The obtained CDs were applied in sensing applications for metal ion detection and ethylenediamine vapour using an electronic nose. This work thus demonstrated that carbon dots prepared from black sesame are intriguing sensing materials for various chemicals.

Bottom‐Up Synthesis of Carbon Quantum Dots With High Performance Photo‐ and Electroluminescence

AbstractAlthough carbon quantum dots (CQDs) are of great interest because of cost effectiveness and environmental compatibility with the facile tunability of their optical properties, poor photo‐ and electroluminescence (EL) of CQDs limits further implementation. Here, a novel bottom‐up synthetic route for fabricating highly crystalline CQDs suitable for high‐brightness blue light‐emitting diodes is demonstrated. The two‐step solution process is based on time‐controlled thermal carbonization of citric acid, followed by ligand exchange of the CQDs with oleylamine (OA) in solution. Carbonization allows for the nucleation and growth of crystalline CQDs, while OA treatment disperses the CQDs and stabilizes the solution, giving rise to CQDs with low structural defects and uniform sizes. The systematic study reveals the origin of the light emission of OA‐treated CQDs by photoluminescence (PL) analysis, which yields a high quantum efficiency of ≈30%. The photoluminescence‐optimized OA‐treated CQDs exhibit excellent blue EL performance with a low turn‐on voltage of ≈4 V and high brightness of 308 cd m−2; a negligible voltage‐dependent color shift when they are employed to an inverted light‐emitting diode.

Preparation and characterization of WSe2 nano-films by magnetron sputtering and vacuum selenization

Graphene-structure WSe2 nano-films (NFMs) were prepared by magnetron sputtering and vacuum selenization. The as synthesized WSe2 NFMs were characterized using various techniques such as SEM, XRD and EDS. Our work provides a simple and effective method to prepare WSe2 NFMs. We have observed the high photo responsivity of WSe2 NFMs up to 10 A W−1, which is 40% higher than the previously reported data. Our photoelectrochemical tests demonstrate that WSe2 NFMs have excellent photoelectric properties, indicating that NFMs have a wide application potentialsin solar cells and optoelectronic devices.

Growth and characterization of porous sp2-BN films with hollow spheres under hydrogen etching effect via borazane thermal CVD

Porous sp2-boron nitride (sp2-BN) films were directly grown on c-plane Al2O3 substrates by low pressure thermal chemical vapor deposition (CVD) using borazane (NH3-BH3) as a single precursor and without any catalyst. It was found that the BN films grown under 20 mbar are porous and consist of nano-scaled hollow BN spheres of various sizes, with outer radius (ro) ranging from 7.20 nm to 13.32 nm. The B-N bonding was confirmed to be sp2-bonded by the Raman scattering whereas, the crystal structure was determined by TEM and XRD to be turbostratic BN (t-BN) having a lattice constant ‘c’ larger than the hexagonal BN (h-BN) bulk crystal. Homogenous nucleation played a vital role in the porous sp2-BN formation that is contrary to normal CVD growth under such low pressure i.e. 2 mbar for denser BN films. Additionally, because of the H2 chemical etching effect, the H2/N2 ratio shows a strong influence on the film growth. All of the as-grown films have shown good ultraviolet (UV) absorption edge near 210 nm, hence providing a feasible route towards the synthesis of porous sp2-BN with higher surface-volume ratio for high performance photo sensors and smart energy storage devices.

Dynamics of Charge Carriers in Silicon Nanowire Photoconductors Revealed by Photo Hall Effect Measurements.

Photoconductors have extraordinarily high gain in quantum efficiency, but the origin of the gain has remained in dispute for decades. In this work, we employ photo Hall effect to reveal the gain mechanisms by probing the dynamics of photogenerated charge carriers in silicon nanowire photoconductors. The results reveal that a large number of photogenerated minority electrons are localized in the surface depletion region and surface trap states. The same number of excess hole counterparts is left in the nanowire conduction channel, resulting in the fact that excess holes outnumber the excess electrons in the nanowire conduction channel by orders of magnitude. The accumulation of the excess holes broadens the conduction channel by narrowing down the depletion region, which leads to the experimentally observed high photo gain.

GaN-UV photodetector integrated with asymmetric metal semiconductor metal structure for enhanced responsivity

Fabrication of very thin GaN ultraviolet photodetectors on Si (111) substrate integrated with asymmetric (Pt–Ag, Pt–Cr) metal–semiconductor–metal (MSM) structure have been illustrated. Designed GaN photodetection device displays significant enhancement in responsivity for asymmetric (Pt–Ag) MSM structure (280 mA/W) in comparison to symmetric (Pt–Pt) MSM structure (126 mA/W) at 10 V bias. The fabricated asymmetric and symmetric devices also exhibit fast response time in the range of 30–59 ms. The enhancement in responsivity using asymmetric MSM structure ascribed to large difference in work function which lead to change in Schottky barrier height of the metal semiconductor junction. Additionally, power dependent photoresponse analysis of GaN asymmetric (Pt–Ag) ultraviolet photodetector was showing a responsivity of 116 mA/W at low optical power of 1 mW. Such GaN asymmetric MSM ultraviolet photodetectors having high responsivity can extensively be used for low power, high speed ultraviolet photo detection applications.

The different paths and potential risks of photo(-electro)-catalytic degradation for rhodamine B in water by graphene/TiO2 membrane.

The graphene (GR)/TiO2 membrane was prepared by the sol-gel method and coated on the indium tin oxide (ITO) conductive glass, which showed high and stable photo(-electro)-catalytic activities to rhodamine B (Rh-B) in water. Characterization results showed that the GR was dispersed and wrapped in the needle-like TiO2. With GR/TiO2 membrane and simulated sunlight irradiation, the removal efficiency of Rh-B (10mgl-1 and pH at 5.4) arrived at 87.6% within 300min. However, the higher removal efficiency for Rh-B reached to 97.8% by the photo-electro-catalytic degradation with the applied voltage 4v for 30min. The ·OH that generated in the photo-catalytic degradation process were responsible for Rh-B decomposition. The ·O2- played the significant role in the photo-electro-catalytic degradation of Rh-B. Furthermore, the decarboxylation was also occurred in the photo-electro-catalytic degradation for the Rh-B in water except for the deethylation and hydroxylation in the photo-catalytic degradation. In addition, the toxicities of the intermediates were calculated using the ECOSAR program and the EPIWIN software. The results indicated that the toxicities of intermediates from photo-electro-catalytic degradation for the Rh-B were higher than photo-catalytic degradation, due to the generation of decarboxylate.

Broadband surface plasmon resonance enhanced self-powered graphene/GaAs photodetector with ultrahigh detectivity

A type of broadband (325−980 nm) self-powered photodetector based on graphene/GaAs van der Waals heterojunction is reported. By simply spinning a layer of Ag nano-particles (Ag NPs) onto graphene/GaAs heterostructure, the responsivity and detectivity of our devices for the whole spectrum range are enhanced significantly. The maximum photocurrent responsivity of 210 mA W−1 (increased by 38%) and detectivity of 2.98 × 1013 Jones (increased by 202%) are achieved at 405 nm, which is about two or three orders of magnitude larger than other graphene based self-powered photodetectors. The mechanism of the improvement originates from the overlap the depletion region of graphene/GaAs heterostructure, the surface plasmon enhanced light field region below the surface and the light absorption region of the GaAs layer. The external quantum efficiency (EQE) measurement, transient photoluminescence (PL) test and the comparative theoretical simulation show that the surface plasmon enhancement should only be applicable for graphene/direct band gap semiconductor heterostructure, where the semiconductor should have a high optical absorption coefficient. The obtained high performance broadband photodetector with excellent detectivity is a promising candidate for many important optoelectronic applications, such as ultrasensitive image sensor in charge coupled displayer field, which requires colour sensors not only with high photo responsivity but also in a wide spectral range.

10-Mesityl-1,8-diphenylanthracene Dimer: Synthesis, Structure, and Properties.

Macrocyclic 10-mesityl-1,8-diphenylanthracene dimer 4 was synthesized by using the electron-transfer oxidation of Lipshutz cuprate derived from 1,8-bis(4-bromophenyl)-10-mesityl-anthracene 7 in moderate yield. This dimer 4 is a considerably fluorescent molecule (ΦF 0.40) with high thermal, photo, and air stability. The X-ray analysis of 4 revealed a unique structure with a small inner cavity which can incorporate a small molecule or atom. 1H NMR spectra in solution and emission spectra of 4 in the solid state showed that copper(I) ion was incorporated to form a 1:1 complex 4·CuOTf, whereas silver(I) ion only weakly interacted with 4 under similar conditions.

Silicon photoresistive sensors with improved performance

This paper describes the properties of photoresistive sensors built on a thin silicon substrate. These fully restriction of hazardous substances compliant devices show high photo sensitivity, comparable to the best performing CdS/CdSe photocells, within a wide range of illumination intensities extending down to low illuminance levels of less than 5 lx of a black-body radiation at 2850 K color temperature. A comparative analysis of the photoconductive gain confirmed better efficiency of the devices with a shorter photoconductive channel length. The photoconductive gain was shown to increase with the illuminance level, exceeding unity at certain illuminance levels specific for the photoconductive channel length and applied voltage bias. Such behavior was suggested to occur due to the retardation of the minority carrier collection by the potential barrier formed by a high-low junction within the photoresistor's contact regions, which allows more than one majority carrier collection by the electrode per each collected non-equilibrium minority carrier. The physical mechanisms affecting the photocurrent and resistance dependence on the applied voltage bias and illumination intensity are discussed.

Amorphous to crystalline phase transformation and band gap refinement in ZnSe thin films

Zinc selenide (ZnSe) thin films are deposited by physical vapor deposition on indium tin oxide (ITO) substrates with different thicknesses. The crystalline phase refinement is studied by carrying out post-deposition annealing in air at various temperatures. The composition analyses carried out by the Rutherford backscattering spectroscopy have shown the ratios of Zn:Se to be 1:1. As-deposited samples have shown predominant crystalline phase with a small inclusion of the amorphous phase. The contribution of amorphous phase diminishes after the post-deposition annealing of the samples in air. However, the peaks of tin oxide and ITO begin to appear in the X-ray diffraction patterns after post-deposition annealing, showing that substrate material has started reacting with ZnSe films. The crystallinity of the ZnSe samples increases with annealing temperature, which results in the increase of conductivity and optical bandgap of the samples. It is proposed that as-prepared un-annealed samples grow with a large population of trapping centers filled with electrons near the top edge of the valance band. These electrons are elevated to the conduction band when exposed to the light, resulting in a very high photo current in as-prepared samples in comparison with post-annealed samples. The population of the localized trapping centers near the top edge of the valence band of ZnSe diminishes significantly with the post-annealing that refines the band gap; optimize their electrical and optical properties.

BTO/GaN heterostructure based on Schottky junction for high-temperature selective ultra-violet photo detection

Ferroelectric/semiconductor heterostructures have attracted great interest for future optoelectronic and electronic devices due to ferroelectric polarization induced interfacial charge coupling at the junction. Here, we report on the fabrication of GaN/BaTiO3 (GaN/BTO) heterostructure based Schottky junction on c-sapphire using pulsed laser deposition and its performance in UV detection. The Schottky-type BTO/GaN heterojunction for high temperature selective ultraviolet photo detection is demonstrated ranging from 313 to 423 K. The temperature-dependent parameters such as responsivity (Rλ), sensitivity (S), specific detectivity (D*), and barrier height (φb) and ideality factor (η) of the device have been quantified. Calculated parameters are systematically compared with the effect of dark and UV light. The responsivity increases with respect to temperature up to 413 K and then decrease. The highest UV (λ = 360 nm) responsivity is found to be 45.54 and 45.21 A W−1, at temperature 393 and 413 K respectively under +5 V bias. The Schottky barrier height of BTO/GaN heterojunction increased with increasing temperature in presence of dark as well as UV light while ideality factor decrease in both the cases. Overall, the efficiency of BTO/GaN device is optimized at 413 K. We have observed high sensitivity of BTO/GaN device as compared to GaN device with respect to applied bias.

AC conductivity studies of PMMA/TiO2 composites

Polymeric materials have special interest because they are useful for the development of advanced high energy electrochemical devices and photo electro-chemical cells. The importance of polymers is mainly because polymers are still regarded as a cheap alternative material. Recently polymer matrix-ceramic composites receive increased attention due to their electrical and electronic properties. PMMA is highly transparent to visible light, mechanically stable and excellent in insulating properties which made this polymer an candidate for many optical and electrical applications. TiO2 is a UV-resistant material. Used in making chemical fiber, plastics, printing ink, coating, etc. In this work, PMMA/TiO2 composites were prepared by using sol gel method. The composite samples were characterized by XRD, FTIR, DSC and SEM. The incorporation of TiO2 improved the structural, morphological, and conductivity properties of the composite. The dielectric and a.c. conductivity of the samples were found to increase with increase in the concentration of TiO2 and temperature. The synthesized polymer composites were suitable for different technological applications.

Fabrication of Organic Photo Detectors Using Inkjet Technology and Its Comparison to Conventional Deposition Processes

The current upscaling of large area electronics is pushing the technological and cost limits of conventional processing and materials. Introduction of new materials, sophisticated device architectures and advanced deposition techniques has made organic photo detector (OPD) attractive for many applications. Spin coated Titanium Oxide (TiOx) has been successfully established as an electron transport layer (ETL) in many optoelectronic devices, including OPDs. In this paper, on one hand OPDs are fabricated with TiOx as ETL deposited by sputtering and electron beam techniques, where the dark current density is reduced by two orders of magnitude up to 10−7 A/cm2 while retaining high photo responsivity. On the other hand, inkjet printing is demonstrated as an alternative additive and digital manufacturing route for the vacuum-free deposition of TiOx and active layer composition of poly(3-hexylthiophene) and [6,6]-phenyl-C60-butyric acid methyl ester (P3HT:PC60BM). OPDs fabricated with inkjet printed TiOx layers showed comparable performance as the state of the art devices fabricated by spin coated TiOx. In addition, OPDs were manufactured using inkjet technology for both the active and ETL layers. Our research proves that there is a significant influence of the deposition technique to the corresponding device performance. The results obtained in terms of figure of merit for OPDs (dark current density of $2.7\times 10^{-5}$ A/cm2, signal to noise ratio of 320, external quantum efficiency of 45% and fabrication yield of 30%–60%) from the developed inkjet printing process shows that further investigation on the ink formulation and optimizations can result into better performing OPDs.