Correction: Ando et al. Achieving Optical Ozone Sensing with Increased Response and Recovery Speed by Using Highly Dispersed CdSe/ZnS Quantum Dots in Porous Glass. Chemosensors 2024, 12, 254

CdSe/ZnS quantum dots (QDs) that were highly dispersed in porous glass showed a rapid decrease in the intensity of their photoluminescence (PL) in response to ozone at concentrations of 0–200 ppm in air (at room temperature and atmospheric pressure), followed by a similarly rapid recovery to full PL in air with no ozone. The response time of the PL quenching in the presence of ozone, and the recovery time to full PL in air after the ozone was removed, showed little dependence on the ozone concentration. Compared to conventional CdSe/ZnS QD films on planar glass substrates, the speed of ozone-induced decrease in the PL intensity of QDs increased, and the recovery speed of the PL intensity, once the ozone was removed from the air, was even more rapid compared to the recovery on planar glass. The 100% PL intensity recovery time in air was reduced to about 10% for CdSe/ZnS QDs that were dispersed in porous glass compared to CdSe/ZnS QD films on planar glass substrates. We hypothesize that this reflects the fact that ozone molecules that are adsorbed on the QD-layer-lined pore surfaces are quickly desorbed in ozone-free air, because the layer of CdSe/ZnS QDs is much thinner in the pores of porous glass than on a planar glass substrate. Thus, CdSe/ZnS QDs that were dispersed in porous glass showed a rapid response to ozone and a similarly rapid recovery in ozone-free air, which has not been seen in previous QD ozone gas sensors, indicating that they are promising as high-performance optical ozone sensor materials.

Semiconductor-type SnO2-based NO2 sensors operated at room temperature under UV-light irradiation

The photoluminescence response of semiconductor CdSe/ZnS quantum dots embedded in a borosilicate porous glass matrix to exposure to ammonia vapor is investigated. The formation of surface complexes on the quantum dots results in quenching of the photoluminescence and a shortening of the luminescence decay time. The process is reversible, desorption of ammonia molecules from the quantum dot surface causes the photoluminescence to recover. The sensitivity of the quantum dot luminescence intensity and decay time to the interaction time and the reversibility of the photoluminescence changes make the CdSe/ZnS quantum dots in porous glass system a candidate for use as an optical sensor of ammonia.

ABSTRACTSpatially quantized systems of III–V compounds have, in recent years, attracted considerable theoretical interest. However, the fabrication of quantum dots, a three-dimensionally quantum-confined microstructure, is particularly cumbersome and requires sophisticated lateral patterning techniques. A method, reported recently, which utilizes the microporosity of Vycor brand porous glass to produce quantum-confined microcrystals of II–VI and IV–VI semiconductors, is now extended to the fabrication of III–V quantum dots, by incorporating a microwave plasma assisted MOCVD technique. In this process, organometallic precursors impregnated in porous glass can be effectively cracked to deposit III–V microcrystals in glass. The results are discussed in light of the quantum size effect manifested by the optical absorption and photoluminescence data.

Assembly of Supra-Nanoclusters Within Crystalline and Amorphous 3-D Structures

ZnO nanostructure‐based photodetectors have a wide applications in many aspects, however, the response range of which are mainly restricted in the UV region dictated by its bandgap. Herein, UV–vis–NIR sensitive ZnO photodetectors consisting of ZnO nanowires (NW) array/PbS quantum dots (QDs) heterostructures are fabricated through modified electrospining method and an exchanging process. Besides wider response region compared to pure ZnO NWs based photodetectors, the heterostructures based photodetectors have faster response and recovery speed in UV range. Moreover, such photodetectors demonstrate good flexibility as well, which maintain almost constant performances under extreme (up to 180°) and repeat (up to 200 cycles) bending conditions in UV–vis–NIR range. Finally, this strategy is further verified on other kinds of 1D nanowires and 0D QDs, and similar enhancement on the performance of corresponding photodetecetors can be acquired, evidencing the universality of this strategy.

Improvement of isopropanol sensing performance by surface sensitization of CdSnO3 nanocubes with CdS quantum dots

In this work, we report measured effects on the fluorescent emission spectra of commercially produced core-shell (CdSe/ZnS) quantum dots (QDs). We report the effects on the fluorescent emission spectra of commercially produced CdSe/ZnS QDs of 2.5 nm, 3.3 nm and 6.3 nm size in toluene, following exposure to ~1 MeV gamma irradiation in the range 0.1-110 Gy. We show that damage depends on the size of the QDs and that increasing the concentration of QDs in the toluene decreases the effect. Recent work on the production of a prototype 2D imaging dosimeter, by absorbing a solution of green emitting QD in toluene into a sample of porous “Vycor” glass, has shown that QDs absorbed in the Vycor fluoresce under several hours of continual illumination and that the system continues to show fluorescence for several days after the initial preparation. Initial results of experiments to dynamically image the Vycor during electron irradiation are presented as is progress on the development of a second prototype device for 2D radiation dosimetry.

The exploration of nitrogen dioxide (NO2) gas sensor with high response, fast response and recovery speed, and low working temperature is still an urgent challenge. In this work, a novel SnO2 hierarchical hollow cube composed of ultrathin mesoporous nanosheets was successfully prepared via a simple sulfidation-oxidation method, and nitrogen-doped graphene quantum dots (N-GQDs) were evenly modified on the surface of SnO2. The prepared N-GQDs modified SnO2 (NG/Snx) shows an improved response, and the optimal sample (NG/Sn1.5) response (Rg/Ra = 417) toward 1 ppm NO2 is approximately 2.2 times that of pure SnO2 at 130 °C. Moreover, the NG/Sn1.5 sensing material exhibits fast response and recovery speed, good selectivity, repeatability, and long-term stability. More attractively, the NG/Sn1.5 sensor has outstanding detection ability (Rg/Ra = 25.3) for low concentration NO2 (100 ppb). The significantly improved NO2 sensing performance of NG/Sn1.5 sensing material is mainly attributed to the zero-dimensional/three-dimensional (0D/3D) heterostructure construction and N doping, which increases the space charge modulation depth and NO2 adsorption active sites of the composite material. In addition, the mesoporous hierarchical cubic structure is beneficial to the improvement of NO2 sensing performance due to its stable gas diffusion channels, large specific surface area, and abundant nanoscale grain boundaries. The results obtained herein may provide new ideas for the preparation of high performance NO2 sensors at low temperatures.

Room-temperature hydrogen gas sensor composed of palladium thin film deposited on NiCo2O4 nanoneedle forest

Thin films of semiconducting SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> were deposited by using RF sputtering technique under 30% oxygen and 70% argon in the reactive (Ar+O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) gas mixture using a metallic tin (Sn) target at 16 mTorr deposition pressure. The SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin film deposited at optimized sputtering conditions was found to be highly sensitive (sensing response ~1.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> ) to NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas (10 ppm) at comparatively low operating temperatures (~100°C), but with moderate response (4.1 minutes) and recovery speeds (33.4 minutes). Further the response and recovery times of the sensor structure were improved by loading Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and NiO nano clusters over SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> surface using E-beam evaporation and sputtering techniques respectively. Thickness of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nano clusters was varied from 10 to 18nm to get the best sensing characteristics. The quality of SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film and reaction kinetics of NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> with SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> surface at the Sn sites play an important role in enhancing the sensing response and response speed at low temperatures (<;60; 200°C).

Andreev and spin transport in carbon nanotube quantum dot hybrid devices

Semiconductor quantum dots are objects with dimensions of a few to a few tens of nanometers. Confinement on such a small scale, of the order of the electron de Broglie wavelength, confers upon the charge carriers inside quantum dots a discrete energy spectrum. These atomic-like energy levels give rise to several technologically interesting properties. For example, laser diodes using quantum dots as their active medium offer the possibility of very low threshold currents, due to the near impossibility of coupling to electromagnetic frequencies apart from the lasing mode. The combination of discrete excitations and the high degree of spatial separation from the surrounding environment provided by quantum dots also makes it possible to maintain the electrons inside them in coherent states for much longer than with many other material systems, making quantum dots very suitable candidates for quantum information technologies. In particular the study of individual quantum dots has received a great deal of attention in recent years, with a view toward developing devices for quantum computing and quantum key distribution. In this context, an individual quantum dot is the critical component of many envisaged devices. In this thesis, several of the basic properties of individual self-assembled quantum dots are investigated. Chapter 1 gives a brief introduction to the technology of self-assembled quantum dots, and outlines several of the milestones that have been achieved in the field of single quantum dot research. Time-correlated single photon counting, the central technique employed in this work, has been used to investigate the transient properties of photo-excited charges in quantum dots. This technique uses a sensitive photodetector, capable of registering single incident photons, to compare the arrival times of photons from the dots with a signal from a reference, allowing statistics that diagnose the transient luminescent properties to be built up. The reference may be a voltage pulse produced every time the pulsed excitation laser fires, which could allow the spontaneous emission time to be determined, or the reference may be from another detector exposed also to the photoluminescence signal from the sample, which might permit one to examine the degree to which events inside the single dot can occur simultaneously. Chapter 2 gives more detail into the experimental techniques employed during this work. The experimental work presented in this thesis is divided into three sections, (i) a study of the dependence of the spontaneous emission lifetime of a dot on its emission wavelength, (ii) an investigation into luminescence properties at high excitation powers, and (iii) a study of the transient interaction between electron spins in a quantum dot and the optically oriented spins of the nuclei of the dot. All these parts concern measurements of processes taking place within 10 ns after the arrival of a picosecond laser pulse. Chapter 3 describes measurements of the recombination lifetimes of several dots from a single sample, each with a different emission wavelength. Due to the self assembly process, each dot possesses dimensions, composition, and geometry that differ slightly from those of all the others. This accounts for the different emission wavelengths and makes it desirable to understand how variations in these properties affect the luminescence behaviour. It was found that the emission lifetime gets systematically longer, as the resonant wavelength gets larger, which is opposite to observations by others for dots that, like ours, are wider than the exciton Bohr radius. We find that our dots are so much larger than the Bohr radius that the emission wavelength is uncorrelated with the lateral dimension of the dots, but is correlated closely with the height. This is shown by examination of the diamagnetic coefficients of the dots as a function of emission wavelength. We argue that the oscillator strengths of these dots must be treated as similar to those of narrow quantum wells, such that the lifetime gets longer as the structure gets thicker, i. e. as the dots get higher. In Chapter 4, we examine photoluminescence transience at high excitation powers. Two findings in particular are described, (i) the exciton luminescence becomes delayed as the excitation power is increased, (ii) very strong background emission develops from the sample as the laser intensity is turned up. Three models are considered to account for these findings, (1) multiexciton emission, (2) dressed exciton emission, in which the background emission is actually from the exciton, but spectrally broadened by the presence of numerous charge carriers surrounding the dot, and (3) dressed exciton emission where the background emission is primarily from outside the dot, perhaps from a two-dimensionally confined continuum of states. In this latter case, the exciton emission from the dot is again broadened by the external carriers, becoming sharp only when sufficient of the external carriers have recombined. This leads to an apparent delay to the dot emission, brought about by the spectral filtering of the monochromator used in the experiments. This third mechanism is argued to be the most suitable explanation for the observations. Chapter 5 details the study of spin excitations in quantum dots. Spins were excited in photoluminescence experiments using circularly polarized laser light. Photoluminescence decays were recorded for the cases co- and counter-polarized with the pump laser, and these were combined to give the transience of the spin state of the photo-excited exciton. Normally in such experiments, one expects a monotonic decay of the photoluminescence polarization, but with several of the dots studied in this work, the polarization was found to first decay, before rising again after a few nanoseconds. This phenomenon is linked by several experiments to the transfer of spins from the optically orientated nuclear spin reservoir, mediated by the hyperfine interaction. It is confirmed that the nuclei are indeed aligned by the polarized excitation light by recording small Overhauser splittings of the spectra of individual quantum dot emission lines. These splittings are displacements of the emission energy (about 5 µeV) of the co- and counter-polarized emissions, relative to one another, when the circularly polarized excitation is used. The splitting changes direction, when the helicity of the excitation light is reversed. Next, the observed Overhauser effect is shown to exhibit power dependence, disappearing as the excitation power is reduced. Finally, the rising polarization transients, observed at relatively high power, are replaced by simple monotonic decays as the excitation is reduced to the level at which the Overhauser effect is known to be just vanishing. This last observation establishes clearly the role of the optically aligned nuclear spins in the observation of the rising polarization transients. A rate equation model is developed and used to demonstrate theoretically the feasibility to cause rising polarization transients by hyperfine interactions with optically oriented nuclear spins. The model shows that it should be possible to use measurements such as the reported transients to determine the degree of nuclear spin alignment in nuclear orientation experiments.

Stability promoted close-packed quantum dot-AlPO4 glass films with tunable and bimodal luminescence

To clarify the positive effect of the down-conversion process for ultraviolet (UV) and deep ultraviolet (DUV) light detection and visualization, we choose, synthesize, and characterize a spectrum of direct-bandgap CdSe-based colloidal quantum dot (QD) solvents and color-conversion layers (CCLs) across blue, green, yellow, orange, and red hues. Their optical absorption, emission, and response speeds under various UV and DUV wavelength of 280 nm, 372 nm, and 405 nm are evaluated. The blue QD CCL demonstrated the highest quantum yield up to 0.68. By integrating this blue QD CCL directly onto a silicon-based photodiode, the responded optical power to 280-nm DUV light is significantly enhanced by 27 times; this data decreases slightly to 23 times when using orange QDs, due to the comparatively lower quantum yield. For the optimal result in a communication system, the orange QDs help exhibit the highest response of 520 mV when stimulated with 372-nm UV light, compared with a substantial improvement over the original response of 120 mV. This enhancement makes the orange QDs significantly reduces the BER, especially at data rates below 70 Mb/s, due to the stronger response of the avalanche photodiode (APD) at 600 nm. Furthermore, to demonstrate the potential application of QDs for patterning and visualization, we have also produced CdSe-based QDs through inkjet printing, showcasing their printability, high stability in air, and pure color emission under DUV illumination. These results underscore the significant potential of CdSe-based QDs for full-color anti-counterfeiting solutions and their integration into flexible, printable wearables for a variety of visualization and DUV detection applications.