Tailoring the surface work function and/or surface functional group of oxide active layers in an optoelectronic device is an important means for performance improvement. One way to proceed is to adsorb molecules with varying dipole moment strength and sign. We report on the surface modification of mesoporous TiO2 using different self-assembled monolayers (SAMs) of acids. The energetics at the interface have been determined using a series of photoelectron spectroscopy techniques (UPS, IPES, XPS). We have shown that the observed changes in work function is correlated to the dipole moments of the respective acids, calculated by density functional theory. This interfacial engineering approach can be used to control the charge extraction from an optoelectronic device. A novel approach is proposed for boosting the performances of self-powered photodetectors. Visible-blind UVA photodetectors have been built by combining a mesoporous TiO2 layer with a Spiro-OMeTAD layer. SAMs interlayer induces a step in the vacuum energy, the formed dipole field dramatically affects the charge transfer and then the photocurrent/photoresponse of the device. The effect of para-substituted benzoic acids and β-alanine on the functioning of triple cation perovskite solar cells is then developed. We show then a best improvement for 4-chlorobenzoic acid SAMs which is due to the reduction of interfacial states, to the improvement of the quality of the perovskite material and to a better structural continuity.
Graphene/oxide composite structures can be employed for advanced applications owing to their synergistic properties. We prepared mesoporous TiO2 layers including (i) graphene oxide and (ii) reduced graphene oxide at various concentrations. The films deposited on glass were tested for photocatalytic degradation, in water and under UV light, of sulfathiazole, a model compound of an eco-persistent antibiotic. The results show the high photocatalytic activity of the TiO2 layers. Moreover, a significant improvement in the photodegradation rate is found for graphene oxide with an optimum concentration of 2 wt. %. It is assigned to a favorable energy level combination that favors the photogenerated charge separation and increases the lifetime of the electron/hole pairs.
Perovskite materials, despite displaying remarkable properties, suffer from the use of toxic lead and poor stability towards moisture and thermal stresses. These strongly hinder their future development. Recently, a new family of 3D perovskites, deficient in lead and iodide, the d-HPs, has been discovered. They involve large organic cations that circumvent the Goldschmidt tolerance factor. In the present study, we report that a new cation, the iodoethylammonium I-(CH2)2-NH3+ (IEA+), allows the synthesis of d-HP thin films based on the 3D MAPbI3 perovskite. This new compound has been tested for solar cell application. A final power conversion efficiency of 9.54 % was achieved.
Among alternative nanomaterials for energy related photonic applications, one-dimensional semiconductor nanowires are of a great interest due to their physical properties coming from electronic or quantum confinement. In particular, ZnO nanowires (or nanorods) has been widely investigated since ZnO has many unique properties such as wide direct band gap, large exciton binding energy and relatively high refractive index. Large optical gain also makes ZnO a well suited material for energy transfer in hybrid systems and especially optical energy transfer. There are however two issues remaining to be addressed, one is related to the control in size and dispersion in nanowires array and the other is related to the modeling of nanowires arrays. In this study, we report on a theoretical study on ZnO nanowires, in order to reach a better understanding of the mechanisms that govern the light propagation in nanowires arrays.
A phenomenological model has been developed and discussed. The model is able to describe the experimentally measured light transmission nanowires arrays. A slab of nanospheres and rough layers with thickness waviness were combined to simplify the nanowires structure description. This phenomenological description was proved to be feasible by fitting the experimental data. As a conclusion, light transmitted by randomly distributed nanowires can be explained by the combination of Mie theory and a rough Fresnel reflection at the interfaces.
Tb3+/Yb3+ co-doped CdF2 single crystals were successfully fabricated by the Bridgman technique from a vacuum furnace in fluoride atmosphere. The structural and luminescent properties were investigated by X-ray diffraction, optical absorption and luminescence techniques at room temperature. The emission spectra exhibit a weak blue and green emission under 350 nm excitation and a strong emission under 975 nm in the spectral range 450 – 510 nm and 510 – 570 nm which are assigned to 5D4 → 7F6 and 5D4 → 7F5 transitions of Tb3+ ions, respectively. Furthermore, the time resolved decay time spectra are also obtained using pulse laser. The obtained upconversion spectra under 940 nm diode laser excitation at different powers showed the two-photon absorption process responsible of blue, green and red emissions.
Graphene/oxide composite structures are attracting increasing attention for many advanced applications. In the present work, mesoporous layers composed of TiO2 nanoparticles and graphene at various concentrations have been coated on conductive glass substrates. They have been tested for the photocatalytic degradation of 4-chlorophenol used as a model compound of an eco-persistent pollutant dilute in water. The formation of intermediate degradation products, namely, hydroquinone and benzoquinone, has been followed. The results show the high photocatalytic activity of the layers and a beneficial effect of graphene for an optimum concentration of 1.2 wt. %. The decrease in the activity observed at higher graphene content is assigned to the light absorption by this component. The key parameters for the enhancement of the photocatalytic performance are discussed.
The most important parameters of gas sensors are sensitivity and especially high selectivity to specific chemical species. To improve these parameters we developed sensor structures based on layered semiconducting oxides, namely CuO/Cu2O, CuO:Zn/Cu2O:Zn, NiO/ZnO. In this work, the ZnO/CuxO (where x = 1, 2) bi-layer heterostructure were grown via a simple synthesis from chemical solution (SCS) at relatively low temperatures (< 95 °C), representing a combination of layered n-type and p-type semiconducting oxides which are widely used as sensing material for gas sensors. The main advantages of the developed device structures are given by simplicity of the synthesis and technological cost-efficiency. Structural investigations showed high crystallinity of synthesized layers confirming the presence of zinc oxide nanostructures on the surface of the copper oxide film deposited on glass substrate. Structural changes in morphology of grown nanostructures induced by post-grown thermal annealing were observed by scanning electron microscopy (SEM) investigations, and were studied in detail. The influence of thermal annealing type on the optical properties was also investigated. As an example of practical applications, the ZnO/CuxO bi-layer heterojunctions and ZnO/CuO/Cu2O three-layered structures were integrated into sensor structures and were tested to different types of reducing gases at different operating temperatures (OPT), showing promising results for fabrication of selective gas sensors.
We have investigated the preparation of NiO layers by cathodic electrodeposition in various organic-based solvents, namely ethanol, dimethyl sulfoxide (DMSO), DMSO/2 vol.% H2O and DMSO/25 vol.% H2O mixtures. The layers were formed from the electrochemical reduction of nickel nitrate precursor. We show that, depending on the solvent used, various nickel compounds were deposited. In the case of ethanol, a transparent precursor layer was obtained that was transformed into NiO after an annealing treatment at 300°C. For DMSO and DMSO with 2 volume % of H2O, adherent, well-covering, mesoporous and rather thick NiO layers were obtained after an annealing treatment at 450°C. These layers, after growth, contained nickel oxide or hydroxide, metallic nickel and DMSO. The solvent acted as a blowing agent, being included in the deposit and giving rise to a mesoporous film after its elimination by thermal annealing. These porous layers of p-type oxide have been successfully sensitized by a push-pull dye (P1 dye) and showed photocurrent generation and an open circuit voltage (Voc) up to 167 mV in p-type dye-sensitized solar cells (p-DSSCs). For DMSO with 25 volume % of H2O, the deposited layers contained more metallic nickel and were dense even after annealing. They were unsuitable in p-DSSCs.
There is an increasing demand for sensors to monitor environmental levels of ultraviolet (UV) radiation and pollutant gases. In this work, an individual nanowire of Pd modified ZnO nanowire (ZnO:Pd NW) was integrated in a nanosensor device for efficient and fast detection of UV light and CH4 gas at room temperature. Crystalline ZnO:Pd nanowire/nanorod arrays were synthesized onto fluorine doped tin oxide (FTO) substrates by electrochemical deposition (ECD) at relative low-temperatures (90 °C) with different concentrations of PdCl2 in electrolyte solution and investigated by SEM and EDX. Nanodevices were fabricated using dual beam focused electron/ion beam (FIB/SEM) system and showed improved UV radiation response compared to pristine ZnO NW, reported previously by our group. The UV response was increased by one order in magnitude (≈ 11) for ZnO:Pd NW. Gas sensing measurements demonstrated a higher gas response and rapidity to methane (CH4 gas, 100 ppm) at room temperature, showing promising results for multifunctional applications. Also, due to miniature size and ultra-low power consumption of these sensors, it is possible to integrate them into portable devices easily, such as smartphones, digital clock, flame detection, missile lunching and other smart devices.
Transition metal doped-oxide semiconductor nanostructures are important to achieve enhanced and new properties for advanced applications. We describe the low temperature preparation of ZnO:Ag nanowire/nanorod (NW/NR) arrays by electrodeposition at 90 °C. The NWs have been characterized by SEM, EDX, transmittance and photoluminescence (PL) measurements. The integration of Ag in the crystal is shown. Single nanowire/nanorod of ZnO:Ag was integrated in a nanosensor structure leading to new and enhanced properties. The ultraviolet (UV) response of the nanosensor was investigated at room temperature. Experimental results indicate that ZnO:Ag (0.75 μM) nanosensor possesses faster response/recovery time and better response to UV light than those reported in literature. The sensor structure has been also shown to give a fast response for the hydrogen detection with improved performances compared to pristine ZnO NWs. ZnO:Ag nanowire/nanorod arrays electrochemically grown on p-type GaN single crystal layer is also shown to act as light emitter in LED structures. The emission wavelength is red-shifted compared to pristine ZnO NW array. At low Ag concentration a single UV-blue emission is found whereas at higher concentration of dopant the emission is broadened and extends up to the red wavelength range. Our study indicates that high quality ZnO:Ag NW/NR prepared at low temperature by electrodeposition can serve as building nanomaterials for new sensors and light emitting diodes (LEDs) structures with low-power consumption.
We present the cathodic electrochemical preparation of NiO porous films on transparent conducting oxide substrates from nickel nitrate precursor. Two approaches are developed. The first one consists in depositing a precursor by pulsed deposition using an aqueous medium. The precursor layer is shown to be a mixture of nickel oxide and hydroxide. It is subsequently transformed into NiO by thermal annealing. We have also developed the direct electrodeposition of NiO porous layers using dimethyl sulfoxide (DMSO) organic medium and higher deposition temperature. The films have been sensitized by the P1 dye and tested as the hole transport layers for p-type dye-sensitized solar cells.
Scintillation materials have been used widely in either military or civil areas, but most of them emit lights in the spectral range of ultraviolet or visible. There are few candidates with an emission in the spectral range of 650 to 1200nm. Here, we report a Bi2+ doped phosphor of Sr2P2O7:Bi2+, which once exposed to X-ray can emit deep red peaking at ~700nm due to the typical 2P3/2 to 2P1/2 transition of Bi2+. Deep red radioluminescence manifests the potential application of the phosphor as implantable scintillator for instance or other sensor which can obtain real time dose information and reduce serious radiation accidents in the case of radiation therapy.
ZnO sphere made of aggregated nanoparticles with a mean diameter of 19 nm have been prepared by the forced thermohydrolysis in polyol medium technique which is a versatile synthesis method for the preparation of metal oxide particles with controlled properties. The sphere were polydisperse and sub-micrometric in size. Porous layers have been prepared using these building blocks. They showed a large specific surface area and were highly light scattering in the visible wavelength region. We have investigated the performances of D149-dyesensitized solar cells (DSSCs) based on these layers. The annealing temperature as well as the layer thickness has been optimized. Finally the best cells reached an overall conversion efficiency of about 4.7% for layers with a thickness ranging between 27 and 35 μm and annealed at 400°C.
Nanowire (NW) based light emitting diodes (LEDs) have drawn great research interest due to many advantages
compared to thin film based devices. Marked improved performances are expected from nanostructured active layers
for light emission. Semiconducting oxide nanowires can act as direct waveguides and favor emitted light extraction
without use of lens and reflectors in LEDs. Moreover, the use of ZnO wires avoids the presence of grain boundaries
and then the emission efficiency is boosted by the absence of non-radiative recombinations at the joint defects.
In this context, europium (Eu):Chelate/ZnO:Mg-nanowires/p-GaN light-emitting-diode (LED) structures have been
fabricated showing near-UV/violet electroluminescence and red emission from trivalent europium. Fabricated LED
structures exhibit UV-blue light at about 380 nm coming from the n-(ZnO:Mg)/p-GaN and a sharp red emission at
∼611 nm related to the intra-4f transition of Eu ions. It is found that in the case of the ZnO:Mg, the emission
wavelength is slightly shifted to smaller wavelength to be well adapted to the trivalent europium excitation band.
Radiative energy transfer is achieved through strong overlap between the emission wavelength from n-(ZnO:Mg)/p-
GaN heterojunction and chelate ligand intensive absorption band. Indeed the Eu:chelate/(ZnO:Mg)-nanowires/p-GaN
structure appears well adapted to UV/blue and red dual emission. Our results shows that the design of LEDs based on
the chelate ligands are important issue to enhance the performance of electroluminescence devices based on ZnO
nanowire arrays/p-GaN heterojunction and rare-earth metal complexes.
Electrochemical deposition (ECD) is a versatile technique for the preparation of ZnO nanowires (NWs) and
nanorods (NRs) with high structural and optical quality. The bandgap of the ZnO NWs can be engineered by
doping. Depending on the doping cation and concentration, the bandgap is increased or decreased in a controlled
manner. The NW arrays have been grown on various substrates. The epitaxial growth on single-crystal conducting
substrates has been demonstrated. By using p-type GaN layers, heterostructures have been fabricated with a high
rectifying electrical behavior. They have been integrated in low-voltage LEDs emitting in the UV or in the visible
region depending on the NW composition. For visible-blind UV-photodetector application, ZnO NW ensembles,
electrochemically grown on F:SnO2, have been contacted on their top with a transparent graphene sheet. The
photodetector had a responsivity larger than 104 A/W at 1V in the near-UV range. ECD ZnO NWs have also been
isolated and electrically connected on their both ends by Al contacts. The obtained nanodevice, made of an
individual NW, was shown to be a H2 gas sensor with a high selectivity and sensitivity. Moreover, it was shown that
Cd-doping of ZnO NWs significantly improved the performance of the sensor.
The bandgap control of doped-ZnO nanowires is important for tunable light emitting diodes (LEDs). Ultraviolet (UV), blue and violet LED structures based on Ag-doped ZnO /p-GaN and Cd-alloyed ZnO (Zn1-xCdxO) nanorods/p-GaN heterojunction have been fabricated by epitaxial electrodeposition at low temperatures and thermal annealing. UV electroluminescence (EL) peak around 397 nm observed from pure nanowires-ZnO/p-GaN at room temperature was shifted to 406 nm or 423 nm by using heterojunction between Ag-doped ZnO (ZnO:Ag) and Zn1-xCdxO-nanorods grown on p-GaN substrate, respectively. The electroluminescence emission threshold voltage was low at about 5.0 V and EL intensity increased with rise in the applied voltage bias. Presented experimental results demonstrate the tunable emission from silver and cadmium-doping in ZnO-based nanoLEDs.
We have investigated a large variety of ZnO structures prepared by sol-gel, electrodeposition and occlusion electrolysis for dye-sensitized solar cell (DSSC) application. The micro-/nanostructures include (nano)porous films, nanowire arrays and hierarchical structures. Several dyes (D149, D205, N719, Z907) have been tested. We have found that the best system is the nanoporous ZnO film prepared by electrodeposition using EY as a structure directing agent and sensitized by the D149 indoline dye in the presence of octanoic acid. Electrochemical impedance spectroscopy (EIS) investigation of the cells’ functioning shows a high charge collection efficiency of the ZnO-based photoelectrodes (>92%). Combined with IPCE measurements, we conclude to a high charge injection efficiency (>95%). However, the cell optimizing must now focus on the light harvesting efficiency that must be increased by broadening the sunlight wavelength range collected at the ZnO-based photoelectrode.
Nanowires (NWs)-based light emitting diodes (LEDs) have drawn large interest due to many advantages compared to thin film based devices. We have successfully prepared epitaxial n-ZnO(NW)/p-GaN heterojunctions using low temperature soft electrochemical techniques. The structures have been used in LED devices and exhibited highly interesting performances. Moreover, the bandgap of ZnO has been tuned by Cu or Cd doping at controlled atomic concentration. A result was the controlled shift of the LED emission in the visible spectral wavelength region. Using DFT computing calculations, we have also shown that the bandgap narrowing has two different origins for Zn1-xCdxO (ZnO:Cd) and ZnO:Cu. In the first case, it is due to the crystal lattice expansion, whereas in the second case Cu-3d donor and Cu-3d combined to O-2p acceptor bands appear in the bandgap which broadnesses increase with the dopant concentration. This leads to the bandgap reduction.
Nanowires (NWs)-based light emitting diodes (LEDs) have drawn large interest due to many advantages compared to
thin film based devices. Markedly improved performances are expected from nanostructured active layers for light
emission. Nanowires can act as direct waveguides and favor light extraction without the use of lenses and reflectors.
Moreover, the use of wires avoids the presence of grain boundaries and then the emission efficiency should be boosted
by the absence of non-radiative recombinations at the joint defects. Electrochemical deposition technique was used for
the preparation of ZnO-NWs based light emitters. Nanowires of high structural and optical quality have been epitaxially
grown on p-GaN single crystalline films substrates. We have shown that the emission is directional with a wavelength
that was tuned and red-shifted toward the visible region by doping with Cu in ZnO NWs.
The bandgap engineering of ZnO nanowires by doping is of great importance for tunable light emitting diode (LED)
applications. We present a combined experimental and computational study of ZnO doping with Cd or Cu atoms in the
nanomaterial. Zn1-xTMxO (TM=Cu, Cd) nanowires have been epitaxially grown on magnesium-doped p-GaN by
electrochemical deposition. The Zn1-xTMxO/p-GaN heterojunction was integrated in a LED structure. Nanowires act as
the light emitters and waveguides. At room temperature, TM-doped ZnO based LEDs exhibit low-threshold emission
voltage and electroluminescence emission shifted from ultraviolet to violet-blue spectral region compared to pure ZnO
LEDs. The emission wavelength can be tuned by changing the transition metal (TM) content in the ZnO nanomaterial
and the shift is discussed, including insights from DFT computational investigations.
The electrochemical deposition technique was used for the preparation of Cu-doped ZnO-nanowire-based emitters. Nanowires of high structural and optical quality were epitaxially grown on p-GaN single crystalline film substrates. We found that the emission is directional with a wavelength that is tuned and redshifted toward the visible region by doping with Cu in nanowires. Furthermore, Cu-doped ZnO-nanowires show an enhancement of the transition probability under magnetic field.
Electrodeposition is a versatile method for the synthesis of ZnO thin films. Various nanostructures can be prepared in a
controlled manner. The formation of ZnO nanowire arrays is observed in a deposition bath depleted in zinc salt
precursor. Compared to dense ZnO films, the ZnO nanowire arrays present a high UV near band edge PL emission due
to a better structural quality with lower structural defects. Recent works assign the formation of ZnO nanowires to the
increase in the interfacial pH during the growth. At high pH, zinc ions are complexed by hydroxide ions and are
negatively charged. This leads to the quenching of the lateral growth and the slowdown of the vertical growth rate.
Mesoporous ZnO films can be prepared by electrodeposition by using a structure directing agent. Results obtained with
eosin Y dye are interesting : after optimizing of the dye concentration, a maximum porosity of more than 62% and a
maximum of specific surface area of 75 m2.cm-3 is obtained. These films are promising for photocatalysis and solar cell applications.
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