Organic-inorganic hybrid diodes are very promising for solution processing, low cost, high perfor... more Organic-inorganic hybrid diodes are very promising for solution processing, low cost, high performance optoelectronic devices. Here, we report a high quality p-n heterojunction diode composed of n-type inorganic Sb2S3 and p-type organic 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) with a rectification ratio of ∼10(2) at an applied bias of 1 V. On illumination with visible light (470 nm, 1.82 mW/cm(2)), the current value in our device becomes 8 × 10(2) times that of its dark value even at a zero bias condition. The estimated responsivity value at zero bias is 0.087 A/W which is so far the highest reported for any organic-inorganic hybrid photodiode, to the best of our knowledge. It also exhibits a fast photoresponse time of <25 ms (instrumental limit). More importantly, our device can also detect visible light with power density as low as 8 μW/cm(2) with a photocurrent density of 1.2 μA/cm(2) and a photocurrent to dark current ratio of more than 8. We also demonstrate that the values of responsivity, short circuit current, and open circuit voltage of the photodetector can be improved significantly using a thin layer of TiO2 hole-blocking layer. These findings suggest Sb2S3/spiro-OMeTAD heterojuncton as a promising candidate for efficient self-powered low visible light photodetector.
Surface trap states in copper indium gallium selenide semiconductor nanocrystals (NCs), which ser... more Surface trap states in copper indium gallium selenide semiconductor nanocrystals (NCs), which serve as undesirable channels for nonradiative carrier recombination, remain a great challenge impeding the development of solar and optoelectronics devices based on these NCs. In order to design efficient passivation techniques to minimize these trap states, a precise knowledge about the charge carrier dynamics on the NCs surface is essential. However, selective mapping of surface traps requires capabilities beyond the reach of conventional laser spectroscopy and static electron microscopy; it can only be accessed by using a one-of-a-kind, second-generation four-dimensional scanning ultrafast electron microscope (4D S-UEM) with subpicosecond temporal and nanometer spatial resolutions. Here, we precisely map the collective surface charge carrier dynamics of copper indium gallium selenide NCs as a function of the surface trap states before and after surface passivation in real space and time using S-UEM. The time-resolved snapshots clearly demonstrate that the density of the trap states is significantly reduced after zinc sulfide (ZnS) shelling. Furthermore, the removal of trap states and elongation of carrier lifetime are confirmed by the increased photocurrent of the self-biased photodetector fabricated using the shelled NCs.
Advanced materials (Deerfield Beach, Fla.), Jan 23, 2016
Heterostructured photoconductors based on hybrid perovskites and 2D transition-metal dichalcogeni... more Heterostructured photoconductors based on hybrid perovskites and 2D transition-metal dichalcogenides are fabricated and characterized. Due to the superior properties of CH3 NH3 PbI3 and WS2 , as well as the efficient interfacial charge transfer, such photoconductors show high performance with on/off ratio of ≈10(5) and responsivity of ≈17 A W(-1) . Furthermore, the response times of the heterostructured photoconductors are four orders of magnitude faster compared to the counterpart of a perovskite single layer.
Advanced materials (Deerfield Beach, Fla.), Jan 2, 2016
High-quality perovskite monocrystalline films are successfully grown through cavitation-triggered... more High-quality perovskite monocrystalline films are successfully grown through cavitation-triggered asymmetric crystallization. These films enable a simple cell structure, ITO/CH3 NH3 PbBr3 /Au, with near 100% internal quantum efficiency, promising power conversion efficiencies (PCEs) >5%, and superior stability for prototype cells. Furthermore, the monocrystalline devices using a hole-transporter-free structure yield PCEs ≈6.5%, the highest among other similar-structured CH3 NH3 PbBr3 solar cells to date.
Small (Weinheim an der Bergstrasse, Germany), Jan 28, 2015
Integrating nanomaterials with different dimensionalities and properties is a versatile approach ... more Integrating nanomaterials with different dimensionalities and properties is a versatile approach toward realizing new functionalities in advanced devices. Here, a novel diode-type heterostructure is reported consisting of 1D semiconducting ZnO nanorods and 2D metallic LaAlO3 -SrTiO3 interface. Tunable insulator-to-metal transitions, absent in the individual components, are observed as a result of the competing temperature-dependent conduction mechanisms. Detailed transport analysis reveals direct tunneling at low bias, Fowler-Nordheim tunneling at high forward bias, and Zener breakdown at high reverse bias. Our results highlight the rich electronic properties of such artificial diodes with hybrid dimensionalities, and the design principle may be generalized to other nanomaterials.
Here we report that mesoporous ternary oxide Zn2SnO4 can significantly promotes the crystallizati... more Here we report that mesoporous ternary oxide Zn2SnO4 can significantly promotes the crystallization of hybrid perovskite layers and serves as an efficient electron transporting material in perovskite solar cells. Such devices exhibit an energy conversion efficiency of 13.34%, which is even higher than that achieved with the commonly used TiO2 in the similar experimental conditions (9.1%). Simple one-step spin coating of CH3NH3PbI3-xClx on Zn2SnO4 is found to lead to rapidly crystalized bilayer perovskite structure without any solvent engineering. Furthermore, ultrafast transient absorption measurement reveals efficient charge transfer at the Zn2SnO4/perovskite interface. Most importantly, solar cells with Zn2SnO4 as the electron-transporting material exhibit negligible electrical hysteresis and exceptionally high stability without encapsulation for over one month. Besides underscoring Zn2SnO4 as a highly promising electron transporting material for perovskite solar cells, our result...
Integrating materials with distinct lattice symmetries and dimensions is an effective design stra... more Integrating materials with distinct lattice symmetries and dimensions is an effective design strategy toward realizing novel devices with unprecedented functionalities, but many challenges remain in synthesis and device design. Here, a heterojunction memory made of wurtzite ZnO nanorods grown on perovskite Nb-doped SrTiO3 (NSTO) is reported, the electronic properties of which can be drastically reconfigured by applying a voltage and light. Despite of the distinct lattice structures of ZnO and NSTO, a consistent nature of single crystallinity is achieved in the heterojunctions via the low-temperature solution-based hydrothermal growth. In addition to a high and persistent photoconductivity, the ZnO/NSTO heterojunction diode can be turned into a versatile light-switchable resistive switching memory with highly tunable ON and OFF states. The reversible modification of the effective interfacial energy barrier in the concurrent electronic and ionic processes most likely gives rise to the high susceptibility of the ZnO/NSTO heterojunction to external electric and optical stimuli. Furthermore, this facile synthesis route is promising to be generalized to other novel functional nanodevices integrating materials with diverse structures and properties.
A strong spin-spin interaction between oxygen vacancy and Mg in ZnMgO nanorods has been manifeste... more A strong spin-spin interaction between oxygen vacancy and Mg in ZnMgO nanorods has been manifested by electron paramagnetic resonance (EPR) studies from symmetrical hyperfine structure. Mg probably occupies Zn sites as a result of meta-stable coordination. 25 Mg have nuclear spin I = 5/2 that can give rise to spin resonance with the unpaired spin S = 1/2 of V-O(+) in ZnO having (2n + 1) = number of splitting as observed in the EPR spectra. On heating at 500 degrees C, the hyperfine structure disappears as Mg moves to the interstitial sites. The results are further supported by x-ray diffraction and photoluminescence results.
ZnO nanowires (NWs) have been decorated with Pd nanoparticles of sizes less than 10 nm (Pd-ZnO NW... more ZnO nanowires (NWs) have been decorated with Pd nanoparticles of sizes less than 10 nm (Pd-ZnO NWs) via a chemical solution route. The microstructural characterizations have been done using field emission scanning electron and high-resolution transmission electron microscopes. The effects of attaching Pd nanoparticles to the walls of ZnO NWs have been investigated by studying the ultraviolet (UV) photosensitivity and photoluminescence (PL) properties. The surface-modified NWs show a UV photosensitivity more than double and a response seven times faster compared to the bare NWs. The photocarrier relaxation under the steady UV illumination condition is quite different in Pd-ZnO NWs. The higher and faster photosensitivity has been explained on the basis of photocarrier transfer from the conduction band of ZnO to the Fermi level of Pd and subsequent electron trapping by the adsorbed O2 molecules on the NWs' surface, which have been presented through a proposed model. The PL spectrum of Pd-ZnO NWs shows that the intensities of the band-edge and defect-related emissions decrease and increase, respectively, due to Pd anchoring, the effect being pronounced as the density of Pd nanoparticles increases.
Journal of Colloid and Interface Science, Volume: 353 Issue: 1 Pages: 30-38 DOI: 10.1016/j.jcis.2010.09.055, Jan 2011
ZnO nanoparticles in the form of quantum dots (QDs) have been dispersed in SiO2 matrix using StOb... more ZnO nanoparticles in the form of quantum dots (QDs) have been dispersed in SiO2 matrix using StOber method to form ZnO QDs-SiO2 nanocomposites. Addition of tetraethyl orthosilicate (TEOS) to an ethanolic solution of ZnO nanoparticles produces random dispersion. On the other hand, addition of ZnO nanoparticles to an already hydrolyzed ethanolic TEOS solution results in a chain-like ordered dispersion. The photoluminescence spectra of the as-grown nanocomposites show strong emission in the ultraviolet region. When annealed at higher temperature, depending on the sample type, these show strong red or white emission. Interestingly, when the excitation is removed, the orderly dispersed ZnO QDs-SiO2 composite shows a very bright blue fluorescence visible by naked eyes for few seconds indicating their promise for display applications. The emission property has been explained in the light of structure-property relationship. (C) 2010 Elsevier Inc. All rights reserved.
We have sputtered Zn onto quasi-one-dimensional ZnO nanowires (NWs) in order to investigate the e... more We have sputtered Zn onto quasi-one-dimensional ZnO nanowires (NWs) in order to investigate the effect of Zn diffusion on the photoluminescence and photoconduction properties of ZnO NWs. Elemental mapping clearly indicates higher Zn concentration in the NWs due to diffusion of Zn. The Zn-sputtered NWs show an enhanced ultraviolet emission with 7 nm red shift. Since the ionization energy of Zn-1 is 51 meV, the enhanced PL emission with a red shift is corelated to the coupling between free exciton and zinc interstitials (Zn-1) defects. The photocurrent transients show almost 20 times more photocurrent generation in Zn/ZnO NWs compared to the as-grown NWs. In contrast, the thin him shows no significant change in the photoluminescence and photoconductivity. Based on the photoconductivity and photoluminescence results, we predict that Zn diffusion in the NWs occurs easily compared to the films because of the smaller dimensions of the NWs.
New wormhole-like mesoporous TiO2 material has been synthesized through a convenient sol-gel meth... more New wormhole-like mesoporous TiO2 material has been synthesized through a convenient sol-gel method in the presence of a Schiff base secondary amine hexadecyl-2-pyrrole-methylamine (HPMA) containing chelating donor sites as template or structure directing agent (SDA). SDA molecules can be easily removed from the composite to generate mesoporosity and upon removal of the SDA molecule, this mesoporous TiO2 material showed very high surface area (480 +/- 10 m(2)/g) with an average pore diameter of 2.57 +/- 0.05 nm. When Rose Bengal dye is entrapped inside the nanopores of this material, it showed a drastic enhancement (ca. 40-folds) in the photoconductivity vis-a-vis mesoporous TiO2 alone under white light illumination. (C) 2010 Elsevier B.V. All rights reserved
ZnO nanowires (NWs) with a ZnS coating are synthesized in order to modify the surface without cha... more ZnO nanowires (NWs) with a ZnS coating are synthesized in order to modify the surface without changing the diameter of the NWs. They have the wurtzite ZnO at the core and a cubic ZnS at the outer layer. The NWs show a sharp ultraviolet and a broad visible emission of the photoluminescence spectra. Surface modification has led to a change in the position of the maxima of the visible emission in ZnO-ZnS NWs. The photocarrier relaxation under steady UV illumination occurs in ZnO NW arrays but is absent in ZnO-ZnS NW arrays. The dark current value for both type of NWs are similar, whereas the photocurrent value is much higher in the surface-modified NWs. Higher photocurrent value indicates a transport of the photogenerated carriers from the ZnS layer to ZnO during UV illumination. The carrier transport mechanism is proposed through a model.
Quantum dots (QDs) of ZnO of 2-4 nm size have been encapsulated within a SiO2 matrix using aqueou... more Quantum dots (QDs) of ZnO of 2-4 nm size have been encapsulated within a SiO2 matrix using aqueous chemically grown ZnO nanoparticles in a precursor of tetraethyl orthosilicate The microstructure shows almost a uniform embedment of the QDs in the SiO2 matrix, resulting in a ZnO QDs-SiO2 composite structure The phorocurrent transients of the composite show an instant fall in the Current followed by an exponential decay under ultraviolet (UV) illumination, causing negative photoconductivity (NPC), in contrast to the positive photoconductivity in only ZnO nanoparticles The interface defect states due to the presence of the SiO2 network around ZnO act as charge trap centers for the photoexcited electrons and are responsible for the NPC The presence of interface-trapped charges under UV illumination has been further confirmed from capacitance-voltage measurements
The effect of surface capping with poly(vinyl,alcohol) (PVA) on the photocarrier relaxation of th... more The effect of surface capping with poly(vinyl,alcohol) (PVA) on the photocarrier relaxation of the aqueous chemically grown ZnO nanowires (NWs) has been investigated. The decay in the photocurrent during steady ultraviolet illumination due to the photocarrier relaxation has been reduced in the capped NWs, as evidenced From a decrease in the photocurrent only by 12% of Its maximum value under steady illumination For 15 min and a decrease in. the photocurrent by 49% of its maximum value during the same Interval of time in the as-grown NWs. The surface modification is confirmed from the FESEM, HRTEM, and FTIR results. The photoluminescence spectrum shows an enhanced ultraviolet emission and a reduced defect-related emission In the capped ZnO NWs compared to bare ZnO.
Organic-inorganic hybrid diodes are very promising for solution processing, low cost, high perfor... more Organic-inorganic hybrid diodes are very promising for solution processing, low cost, high performance optoelectronic devices. Here, we report a high quality p-n heterojunction diode composed of n-type inorganic Sb2S3 and p-type organic 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) with a rectification ratio of ∼10(2) at an applied bias of 1 V. On illumination with visible light (470 nm, 1.82 mW/cm(2)), the current value in our device becomes 8 × 10(2) times that of its dark value even at a zero bias condition. The estimated responsivity value at zero bias is 0.087 A/W which is so far the highest reported for any organic-inorganic hybrid photodiode, to the best of our knowledge. It also exhibits a fast photoresponse time of <25 ms (instrumental limit). More importantly, our device can also detect visible light with power density as low as 8 μW/cm(2) with a photocurrent density of 1.2 μA/cm(2) and a photocurrent to dark current ratio of more than 8. We also demonstrate that the values of responsivity, short circuit current, and open circuit voltage of the photodetector can be improved significantly using a thin layer of TiO2 hole-blocking layer. These findings suggest Sb2S3/spiro-OMeTAD heterojuncton as a promising candidate for efficient self-powered low visible light photodetector.
Surface trap states in copper indium gallium selenide semiconductor nanocrystals (NCs), which ser... more Surface trap states in copper indium gallium selenide semiconductor nanocrystals (NCs), which serve as undesirable channels for nonradiative carrier recombination, remain a great challenge impeding the development of solar and optoelectronics devices based on these NCs. In order to design efficient passivation techniques to minimize these trap states, a precise knowledge about the charge carrier dynamics on the NCs surface is essential. However, selective mapping of surface traps requires capabilities beyond the reach of conventional laser spectroscopy and static electron microscopy; it can only be accessed by using a one-of-a-kind, second-generation four-dimensional scanning ultrafast electron microscope (4D S-UEM) with subpicosecond temporal and nanometer spatial resolutions. Here, we precisely map the collective surface charge carrier dynamics of copper indium gallium selenide NCs as a function of the surface trap states before and after surface passivation in real space and time using S-UEM. The time-resolved snapshots clearly demonstrate that the density of the trap states is significantly reduced after zinc sulfide (ZnS) shelling. Furthermore, the removal of trap states and elongation of carrier lifetime are confirmed by the increased photocurrent of the self-biased photodetector fabricated using the shelled NCs.
Advanced materials (Deerfield Beach, Fla.), Jan 23, 2016
Heterostructured photoconductors based on hybrid perovskites and 2D transition-metal dichalcogeni... more Heterostructured photoconductors based on hybrid perovskites and 2D transition-metal dichalcogenides are fabricated and characterized. Due to the superior properties of CH3 NH3 PbI3 and WS2 , as well as the efficient interfacial charge transfer, such photoconductors show high performance with on/off ratio of ≈10(5) and responsivity of ≈17 A W(-1) . Furthermore, the response times of the heterostructured photoconductors are four orders of magnitude faster compared to the counterpart of a perovskite single layer.
Advanced materials (Deerfield Beach, Fla.), Jan 2, 2016
High-quality perovskite monocrystalline films are successfully grown through cavitation-triggered... more High-quality perovskite monocrystalline films are successfully grown through cavitation-triggered asymmetric crystallization. These films enable a simple cell structure, ITO/CH3 NH3 PbBr3 /Au, with near 100% internal quantum efficiency, promising power conversion efficiencies (PCEs) >5%, and superior stability for prototype cells. Furthermore, the monocrystalline devices using a hole-transporter-free structure yield PCEs ≈6.5%, the highest among other similar-structured CH3 NH3 PbBr3 solar cells to date.
Small (Weinheim an der Bergstrasse, Germany), Jan 28, 2015
Integrating nanomaterials with different dimensionalities and properties is a versatile approach ... more Integrating nanomaterials with different dimensionalities and properties is a versatile approach toward realizing new functionalities in advanced devices. Here, a novel diode-type heterostructure is reported consisting of 1D semiconducting ZnO nanorods and 2D metallic LaAlO3 -SrTiO3 interface. Tunable insulator-to-metal transitions, absent in the individual components, are observed as a result of the competing temperature-dependent conduction mechanisms. Detailed transport analysis reveals direct tunneling at low bias, Fowler-Nordheim tunneling at high forward bias, and Zener breakdown at high reverse bias. Our results highlight the rich electronic properties of such artificial diodes with hybrid dimensionalities, and the design principle may be generalized to other nanomaterials.
Here we report that mesoporous ternary oxide Zn2SnO4 can significantly promotes the crystallizati... more Here we report that mesoporous ternary oxide Zn2SnO4 can significantly promotes the crystallization of hybrid perovskite layers and serves as an efficient electron transporting material in perovskite solar cells. Such devices exhibit an energy conversion efficiency of 13.34%, which is even higher than that achieved with the commonly used TiO2 in the similar experimental conditions (9.1%). Simple one-step spin coating of CH3NH3PbI3-xClx on Zn2SnO4 is found to lead to rapidly crystalized bilayer perovskite structure without any solvent engineering. Furthermore, ultrafast transient absorption measurement reveals efficient charge transfer at the Zn2SnO4/perovskite interface. Most importantly, solar cells with Zn2SnO4 as the electron-transporting material exhibit negligible electrical hysteresis and exceptionally high stability without encapsulation for over one month. Besides underscoring Zn2SnO4 as a highly promising electron transporting material for perovskite solar cells, our result...
Integrating materials with distinct lattice symmetries and dimensions is an effective design stra... more Integrating materials with distinct lattice symmetries and dimensions is an effective design strategy toward realizing novel devices with unprecedented functionalities, but many challenges remain in synthesis and device design. Here, a heterojunction memory made of wurtzite ZnO nanorods grown on perovskite Nb-doped SrTiO3 (NSTO) is reported, the electronic properties of which can be drastically reconfigured by applying a voltage and light. Despite of the distinct lattice structures of ZnO and NSTO, a consistent nature of single crystallinity is achieved in the heterojunctions via the low-temperature solution-based hydrothermal growth. In addition to a high and persistent photoconductivity, the ZnO/NSTO heterojunction diode can be turned into a versatile light-switchable resistive switching memory with highly tunable ON and OFF states. The reversible modification of the effective interfacial energy barrier in the concurrent electronic and ionic processes most likely gives rise to the high susceptibility of the ZnO/NSTO heterojunction to external electric and optical stimuli. Furthermore, this facile synthesis route is promising to be generalized to other novel functional nanodevices integrating materials with diverse structures and properties.
A strong spin-spin interaction between oxygen vacancy and Mg in ZnMgO nanorods has been manifeste... more A strong spin-spin interaction between oxygen vacancy and Mg in ZnMgO nanorods has been manifested by electron paramagnetic resonance (EPR) studies from symmetrical hyperfine structure. Mg probably occupies Zn sites as a result of meta-stable coordination. 25 Mg have nuclear spin I = 5/2 that can give rise to spin resonance with the unpaired spin S = 1/2 of V-O(+) in ZnO having (2n + 1) = number of splitting as observed in the EPR spectra. On heating at 500 degrees C, the hyperfine structure disappears as Mg moves to the interstitial sites. The results are further supported by x-ray diffraction and photoluminescence results.
ZnO nanowires (NWs) have been decorated with Pd nanoparticles of sizes less than 10 nm (Pd-ZnO NW... more ZnO nanowires (NWs) have been decorated with Pd nanoparticles of sizes less than 10 nm (Pd-ZnO NWs) via a chemical solution route. The microstructural characterizations have been done using field emission scanning electron and high-resolution transmission electron microscopes. The effects of attaching Pd nanoparticles to the walls of ZnO NWs have been investigated by studying the ultraviolet (UV) photosensitivity and photoluminescence (PL) properties. The surface-modified NWs show a UV photosensitivity more than double and a response seven times faster compared to the bare NWs. The photocarrier relaxation under the steady UV illumination condition is quite different in Pd-ZnO NWs. The higher and faster photosensitivity has been explained on the basis of photocarrier transfer from the conduction band of ZnO to the Fermi level of Pd and subsequent electron trapping by the adsorbed O2 molecules on the NWs' surface, which have been presented through a proposed model. The PL spectrum of Pd-ZnO NWs shows that the intensities of the band-edge and defect-related emissions decrease and increase, respectively, due to Pd anchoring, the effect being pronounced as the density of Pd nanoparticles increases.
Journal of Colloid and Interface Science, Volume: 353 Issue: 1 Pages: 30-38 DOI: 10.1016/j.jcis.2010.09.055, Jan 2011
ZnO nanoparticles in the form of quantum dots (QDs) have been dispersed in SiO2 matrix using StOb... more ZnO nanoparticles in the form of quantum dots (QDs) have been dispersed in SiO2 matrix using StOber method to form ZnO QDs-SiO2 nanocomposites. Addition of tetraethyl orthosilicate (TEOS) to an ethanolic solution of ZnO nanoparticles produces random dispersion. On the other hand, addition of ZnO nanoparticles to an already hydrolyzed ethanolic TEOS solution results in a chain-like ordered dispersion. The photoluminescence spectra of the as-grown nanocomposites show strong emission in the ultraviolet region. When annealed at higher temperature, depending on the sample type, these show strong red or white emission. Interestingly, when the excitation is removed, the orderly dispersed ZnO QDs-SiO2 composite shows a very bright blue fluorescence visible by naked eyes for few seconds indicating their promise for display applications. The emission property has been explained in the light of structure-property relationship. (C) 2010 Elsevier Inc. All rights reserved.
We have sputtered Zn onto quasi-one-dimensional ZnO nanowires (NWs) in order to investigate the e... more We have sputtered Zn onto quasi-one-dimensional ZnO nanowires (NWs) in order to investigate the effect of Zn diffusion on the photoluminescence and photoconduction properties of ZnO NWs. Elemental mapping clearly indicates higher Zn concentration in the NWs due to diffusion of Zn. The Zn-sputtered NWs show an enhanced ultraviolet emission with 7 nm red shift. Since the ionization energy of Zn-1 is 51 meV, the enhanced PL emission with a red shift is corelated to the coupling between free exciton and zinc interstitials (Zn-1) defects. The photocurrent transients show almost 20 times more photocurrent generation in Zn/ZnO NWs compared to the as-grown NWs. In contrast, the thin him shows no significant change in the photoluminescence and photoconductivity. Based on the photoconductivity and photoluminescence results, we predict that Zn diffusion in the NWs occurs easily compared to the films because of the smaller dimensions of the NWs.
New wormhole-like mesoporous TiO2 material has been synthesized through a convenient sol-gel meth... more New wormhole-like mesoporous TiO2 material has been synthesized through a convenient sol-gel method in the presence of a Schiff base secondary amine hexadecyl-2-pyrrole-methylamine (HPMA) containing chelating donor sites as template or structure directing agent (SDA). SDA molecules can be easily removed from the composite to generate mesoporosity and upon removal of the SDA molecule, this mesoporous TiO2 material showed very high surface area (480 +/- 10 m(2)/g) with an average pore diameter of 2.57 +/- 0.05 nm. When Rose Bengal dye is entrapped inside the nanopores of this material, it showed a drastic enhancement (ca. 40-folds) in the photoconductivity vis-a-vis mesoporous TiO2 alone under white light illumination. (C) 2010 Elsevier B.V. All rights reserved
ZnO nanowires (NWs) with a ZnS coating are synthesized in order to modify the surface without cha... more ZnO nanowires (NWs) with a ZnS coating are synthesized in order to modify the surface without changing the diameter of the NWs. They have the wurtzite ZnO at the core and a cubic ZnS at the outer layer. The NWs show a sharp ultraviolet and a broad visible emission of the photoluminescence spectra. Surface modification has led to a change in the position of the maxima of the visible emission in ZnO-ZnS NWs. The photocarrier relaxation under steady UV illumination occurs in ZnO NW arrays but is absent in ZnO-ZnS NW arrays. The dark current value for both type of NWs are similar, whereas the photocurrent value is much higher in the surface-modified NWs. Higher photocurrent value indicates a transport of the photogenerated carriers from the ZnS layer to ZnO during UV illumination. The carrier transport mechanism is proposed through a model.
Quantum dots (QDs) of ZnO of 2-4 nm size have been encapsulated within a SiO2 matrix using aqueou... more Quantum dots (QDs) of ZnO of 2-4 nm size have been encapsulated within a SiO2 matrix using aqueous chemically grown ZnO nanoparticles in a precursor of tetraethyl orthosilicate The microstructure shows almost a uniform embedment of the QDs in the SiO2 matrix, resulting in a ZnO QDs-SiO2 composite structure The phorocurrent transients of the composite show an instant fall in the Current followed by an exponential decay under ultraviolet (UV) illumination, causing negative photoconductivity (NPC), in contrast to the positive photoconductivity in only ZnO nanoparticles The interface defect states due to the presence of the SiO2 network around ZnO act as charge trap centers for the photoexcited electrons and are responsible for the NPC The presence of interface-trapped charges under UV illumination has been further confirmed from capacitance-voltage measurements
The effect of surface capping with poly(vinyl,alcohol) (PVA) on the photocarrier relaxation of th... more The effect of surface capping with poly(vinyl,alcohol) (PVA) on the photocarrier relaxation of the aqueous chemically grown ZnO nanowires (NWs) has been investigated. The decay in the photocurrent during steady ultraviolet illumination due to the photocarrier relaxation has been reduced in the capped NWs, as evidenced From a decrease in the photocurrent only by 12% of Its maximum value under steady illumination For 15 min and a decrease in. the photocurrent by 49% of its maximum value during the same Interval of time in the as-grown NWs. The surface modification is confirmed from the FESEM, HRTEM, and FTIR results. The photoluminescence spectrum shows an enhanced ultraviolet emission and a reduced defect-related emission In the capped ZnO NWs compared to bare ZnO.
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