M. Omari

Photothermovoltaic effect in carbon nanotubes: En route toward junctionless infrared photocells and light sensors

Optically induced voltage was studied in carbon nanotube films configured as two-terminal resistive elements and operating as junctionless photocells in the infrared. The photovoltage is found to appear only for asymmetric/off-contact illuminations and the effect is explained based on photogenerated heat flow model. The engineered cell prototypes were found to yield electrical powers of ∼30 pW while demonstrating improved conversion efficiency under high-flux illumination. The cell is also shown to act as uncooled infrared sensor with its dark-to-photocurrent ratio improving as temperature increases. The concept might enable nanotube’s use in applications ranging from heat recycling to self-powered infrared sensors.

Spectral investigation of carrier recombination processes in ZnO nanorods at high temperatures

The mechanism of near-band-edge (NBE) emission from crystalline ZnO (c‐ZnO) nanorods grown on c‐Si by a catalyst-assisted vapor-liquid-solid method has been investigated by performing temperature-, power-, and time-dependent photoluminescence (PL) measurements at a temperature (T) range of 143–503K. In contrast to previous reports, we find that the NBE PL is primarily associated with free exciton emission, whereas the contribution of band-to-band and free-to-bound radiative recombinations remains negligible up to the highest T studied. A spectral evolution of the NBE band with T was further analyzed within the framework of a three-parameter model, proposed recently, with the results presented and discussed. Finally, the ratio of excitonic-to-defect luminescence intensity has been observed to change nonmonotonically with T, which is explained based on the difference in the quenching mechanisms of exciton and defect PL.

Temperature-dependent studies of defect-assisted light emission and excitation processes in crystalline ZnO nanowire phosphors

A series of photoluminescence and photoluminescence-excitation spectroscopies have been performed to probe the processes regulating defect-assisted light emission from one-dimensional ZnO nanowire phosphors in a wide temperature range of 123–463 K. The observed nonmonotonic change of the integral defect-photoluminescence intensity as well as its peak position with temperature are explained based on the interplay of competing effects of thermal quenching and carrier redistribution over radiative channels. A temperature-induced broadening of the defect photoluminescence band is observed and attributed to the appearance of ∼2.1 eV band, the intensity of which is also found to quench quickly with the onset of higher temperature. The results of photoluminescence-excitation measurements show that band-to-band excitations remain a primary excitation channel of defects especially at low and moderate temperature range, whereas the role of direct, one-photon absorption channel is found to progress as temperature ap...

Crystallization and (Al,Ti)-oxide growth in annealed TiO2–Al2O3 multilayers

A family of TiO2–Al2O3 multilayers (Λ=2–72nmTiO2∕7nmAl2O3) is sputter deposited on fused silica substrates, sequentially annealed at 973 and 1273K, and analyzed by x-ray diffraction. The goal is to examine crystallization behavior upon annealing at temperatures at which thermodynamically stable mixed-cation phases should not form. The results show: (1) After the 973K anneal, films with Λ=18–72nmTiO2∕7nmAl2O3 weakly crystallize with a preferred (110) rutile orientation. In addition, enhancement of (200) rutile diffraction increases with increasing TiO2 layer thickness. (2) Significant crystallization occurs in films after the 1273K anneal. In films with Λ=36–72nmTiO2∕7nmAl2O3, a metastable pseudobrookite phase, Al0.95Ti2.05O5, crystallizes along with (110)r. However, only rutile TiO2 and α-Al2O3 crystallize in films with thinner TiO2 layers. An architecture-sensitive crystallization model is presented in which the first step common to all architectures is diffusive amorphization of TiO2 by Al2O3 at 973K to...

Crystallization and segregation in vitreous rutile films annealed at high temperature

Vitreous titania films with rutile short-range order were sputter deposited on unheated fused silica substrates, sequentially annealed at 973 and 1273 K, and examined by Raman microscopy, scanning electron microscopy, and x-ray diffraction. A segregated microstructure developed after the 1273 K anneal. This microstructure consists of supermicron-size craters dispersed in a matrix of submicron rutile crystals. Ti–O short-range order in the craters is characteristic of a mixture of two high pressure phases, m-TiO2 (monoclinic P21∕c space group) and α-TiO2 (tetragonal Pbcn space group). We calculated that a high average compressive stress parallel to the substrate must be accommodated in the films at 1273 K, caused by the difference in the thermal expansion coefficients of titania and fused silica. The formation of the segregated microstructure is modeled by considering two processes at work at 1273 K to lower a film’s internal energy: crystallization and nonuniform stress relief. The Gibbs–Thomson relation ...

Near-ultraviolet optical absorption behavior of TiO2–Al2O3 multilayer films

The fundamental optical absorption edge of sputter-deposited titania-alumina (TiO2–Al2O3) multilayers on fused SiO2 substrates is studied by near ultraviolet-visible spectrophotometry. We examine a family of films with bilayer architecture Λ=9–72nm TiO2∕7nm Al2O3 (TiO2 volume fraction from 0.56 to 0.91). Neither the TiO2 or Al2O3 layers have long-range crystallographic order. The absorption coefficient α in the region of optical density from 2 to 7 is determined as a function of energy E. The first significant finding is that the onset of absorption for these multilayers is determined by their TiO2 component, independent of TiO2 layer thickness. The nondirect optical gap Eo is determined by extrapolating linear α1∕2 vs E curves to α=0. Eo=2.95 (±0.02)eV for all multilayers, identical to Eo for a single layer TiO2 film with rutile short-range atomic order. The second significant finding is that the strength of α in the multilayers is diminished compared to a Vegard’s rule analysis based on the weighted sum...

Zirconia-alumina nanolaminate for perforated pitting corrosion protection of stainless steel

The subject of this article is control of perforated pit growth in 316L stainless steel by application of an artificial overlayer film. The protection afforded by 250 nm thick films with two ZrO2–Al2O3 nanolaminate architectures, single-layer ZrO2, and single-layer Al2O3 under cyclic polarization in saline electrolyte is reported here. The post-exposure morphology is characterized by scanning electron microscopy with in situ electron energy dispersive spectroscopy. The films’ diverse behavior is analyzed in terms of a model for perforated pit growth that requires partial pit occlusion until an autocatalytic geometry is established. The results show that the key property a film must have to arrest the development of autocatalytic geometry is the capability to fracture locally so that the electrolyte from the pit freely mixes with the bulk electrolyte and the pit interior becomes passivated. We show how a nanolaminate with 5.0 nm tetragonal ZrO2–5.0 nm amorphous Al2O3 bilayers sustains local fracture (blow holes) without widespread cracking and is protective against perforated pit growth.The subject of this article is control of perforated pit growth in 316L stainless steel by application of an artificial overlayer film. The protection afforded by 250 nm thick films with two ZrO2–Al2O3 nanolaminate architectures, single-layer ZrO2, and single-layer Al2O3 under cyclic polarization in saline electrolyte is reported here. The post-exposure morphology is characterized by scanning electron microscopy with in situ electron energy dispersive spectroscopy. The films’ diverse behavior is analyzed in terms of a model for perforated pit growth that requires partial pit occlusion until an autocatalytic geometry is established. The results show that the key property a film must have to arrest the development of autocatalytic geometry is the capability to fracture locally so that the electrolyte from the pit freely mixes with the bulk electrolyte and the pit interior becomes passivated. We show how a nanolaminate with 5.0 nm tetragonal ZrO2–5.0 nm amorphous Al2O3 bilayers sustains local fracture (blow ...

Transition Metal-Doped ZnO Nanowires: En Route Towards Multi-colour Light Sensing and Emission Applications

Opto-electronics is one of the largest and fastest evolving market segments, with revenues expected to surpass

INVESTIGATION OF TIME-RATED DEFECT FORMATION, INFRARED ABSORPTION AND TRANSPORT CHARACTERISTICS OF SINGLE-WALLED CARBON NANOTUBES WET-PROCESSED IN PHOSPHORIC ACID

1.2 trillion by 2017 (Optoelectronics Industry, 2007). Continuous improvements in the size, detection limit, reliability and spectral sensitivity of existing solidstate light sensors and image detectors, which remain a key component of almost every opto-electronic system and circuit, drive the field to new heights every year. Compared with other components, light sources and detectors recently have shown the most significant revenue gains, whereas highly-miniaturized and low-power solid-state photodetectors operating in extended, UV-visible spectral ranges will continue to have an important role, as they are expected to be ubiquitously used in many electronic systems ranging from highcapacity information storage to biochemical sensing, chemical and biological analysis, and astronomy.

Comment on: “Photocurrent Amplification at Carbon Nanotube‐Metal Contacts”

Time-dependent wet-processing of HiPCo nanotubes in ~ 0.5 M phosphoric acid and its effect on the structural, transport, infrared light absorption and photoconduction characteristics have been studied. Nanotubes were treated for nominal time intervals of 1, 2 and 3 h. The treatment is found to be a two-step process that initially results in the removal/partial replacement of most pre-existing C-O, O–H and CHx groups with phosphorous oxy and carbonyl groups. According to T-dependent current–voltage measurements, the differential conductance, G of nanotube network varies with temperature as ~ Ta, with a exhibiting a slight increase as a result of the treatment, attributed to a slight increase in disorder and not doping effects. The nanotubes processed for three hours also show an order of magnitude improvement in photoconduction response time compared to that of untreated tubes, with growth/decay characteristic time constants approaching a sub-second range.