ZnS Barrier Research Articles

Quantum Shell in a Shell: Engineering Colloidal Nanocrystals for a High-Intensity Excitation Regime.

Many optoelectronic processes in colloidal semiconductor nanocrystals (NCs) suffer an efficiency decline under high-intensity excitation. This issue is caused by Auger recombination of multiple excitons, which converts the NC energy into excess heat, reducing the efficiency and life span of NC-based devices, including photodetectors, X-ray scintillators, lasers, and high-brightness light-emitting diodes (LEDs). Recently, semiconductor quantum shells (QSs) have emerged as a promising NC geometry for the suppression of Auger decay; however, their optoelectronic performance has been hindered by surface-related carrier losses. Here, we address this issue by introducing quantum shells with a CdS-CdSe-CdS-ZnS core-shell-shell-shell multilayer structure. The ZnS barrier inhibits the surface carrier decay, which increases the photoluminescence (PL) quantum yield (QY) to 90% while retaining a high biexciton emission QY of 79%. The improved QS morphology allows demonstrating one of the longest Auger lifetimes reported for colloidal NCs to date. The reduction of nonradiative losses in QSs also leads to suppressed blinking in single nanoparticles and low-threshold amplified spontaneous emission. We expect that ZnS-encapsulated quantum shells will benefit many applications exploiting high-power optical or electrical excitation regimes.

Unraveling the dual emission of single quantum-dot by single particle spectroscopy

Two-color emissive 0D–2D quantum-dot quantum-well (QD-QW) heteronanocrystals has created profound research activities. First multicolor emission in the visible region has been reported by Peng and co-workers in CdSe(core)–ZnS(barrier)-CdSe(shell) (core-barrier-shell) based heteronanostructures where the both CdSe phases (core and the shell) are emissive and tuneable as well. Owing to this enhanced and tuneable functionality, the QD-QW systems colloidal nanocrystals has fuelled their optical and imaging applications. Single particle spectroscopy has taken a giant step toward unravelling the features of individual particles and thus to provide direct information on their heterogeneity. To elucidate the dual emission characteristic of individual nanocrystals we performed energy mapped photoluminescence imaging. Surprisingly, the pseudo color PL intensity image shows that not all single particles are dual emissive in nature, few are either green emitting or red emitting. Photoluminescence spectrum of individual nanocrystals further confirms that individual nanocrystals can be dual emissive in nature. However, single color emissive dots are also present indicating the ensemble heterogeneity at single particle levels. The temporal evolution PL spectra of a single quantum shows spectral diffusion. The single dot experiments on the dual emissive QD-QW system unravels hidden photophysics which are otherwise not observed by ensemble spectroscopy.

Voltage-assisted SILAR deposition of CdSe quantum dots to construct a high performance of ZnS/CdSe/ZnS quantum dot-sensitized solar cells

The charge recombination on the interfaces of TiO2/quantum dots (QDs)/electrolyte is a key factor limiting the efficiency of quantum dot-sensitized solar cells (QDSSCs). Construction of double-layer barrier structure of ZnS/QDs/ZnS is a vital strategy to suppress the interfacial charge recombination. However, a large lattice mismatch (12%) at CdSe/ZnS interfaces causes CdSe to grow slowly on TiO2/ZnS mesoporous film, weakening the interaction between QDs and mesoporous film, which reducing the efficiency of CdSe QDSSCs with double ZnS barrier layers. Applying a voltage of 2 V in successive ionic layer adsorption reaction (VASILAR) to create an electric field, which assists Cd2+ and SeSO32− ions rapidly diffuse into the TiO2/ZnS mesoporous film to react forming CdSe QDs at room temperature. Optimizing the number of CdSe QDs deposition layers and combine with ZnS double-layer barrier structure, a best PCE of 4.34% for ZnS/CdSe/ZnS QDSSCs is achieved. This study gives a fast and simple approach to inhibit interfacial charge recombination to construct high performance CdSe QDSSCs.

Intersubband transitions in spherical quantum dot quantum well nanoparticle

In this paper, we present results about electronic spectra and optical properties of one-electron spherical quantum dot-quantum well (QDQW) structure. Investigated structure consists of CdSe core surrounded by ZnS shell and caped by infinitely high electron potential barrier which can be a good model for any high enough potential barriers. This specific QDQW structure is determined by CdSe and ZnS properties (effective masses and conductive band offset) and the core size and the shell thickness. We present calculation results for one-electron ground (1s) state and the first excited (1p) state transition. For this transition we have calculated oscillator strengths and linear and third-order nonlinear intersubband optical absorption coefficients for various core and shell size i.e. different CdSe core radii and ZnS barrier thickness. Change in core and shell dimensions induces change in one-electron wave function i.e. electron energy states and localization for both 1s and 1p states. As a result, intersubband 1s–1p transition of this system is greatly dependent on the core and shell size. For very small core radii, less than 0.7 nm, and core radii over 1.5 nm the most probable intersubband transition is 1s–1p, but in the region between 0.7 and 1.5 nm core radii, transitions from 1s to other p states dominates. Investigated properties depend mostly on the core radius. These results connect the dot structure and the optical properties of this particular structure. This behavior is than an illustration of similar systems.

A Photoetching‐After‐Growth Approach for the Synthesis of Nanocrystal Heterostructures Exhibiting Tunable Dual‐Band Emission

AbstractIntroduced herein is a new strategy for synthesizing sophisticated colloidal semiconductor nanostructures with targeted optical properties by combining a conventional colloidal growth (bottom‐up) process and a postgrowth photoetching (top‐down) process. To demonstrate the utility of such photoetching‐after‐growth approach, a complex quantum dot (QD)–quantum rod (QR) dual quantum‐confined domain nanocrystal heterostructure (NCH) is engineered which exhibits bright dual‐band emission precisely tunable in a broad visible region. (CdSe/CdS)/ZnS/CdSe QD/barrier/multi‐QR NCHs consisting of a single CdSe/CdS QD core and multiple CdSe QR branches with a wide‐bandgap ZnS barrier between them are prepared through a multistep colloidal growth. The as‐synthesized NCHs show a single emission band from QR; however, after photoetching, NCHs begin to display bright dual emission bands, a high‐energy emission band from QR in addition to the low‐energy emission band from QD. While the QD emission at 586 nm is determined by colloidal growth conditions, that of QR emission is precisely tunable in the 500–585 nm range by quantum‐size‐selective photoetching using a wavelength‐tunable laser. Notably, the QR emission bandwidth decreases to ≈20 nm, which is close to that of a single CdSe nanorod, emphasizing a significant quantum‐size‐focusing effect of the photoetching process.

Photoelectrochromic devices with cobalt redox electrolytes

In this work, the use of cobalt redox electrolytes in partly covered photoelectrochromic devices is investigated experimentally for the first time. The fabricated devices consist of a conductive glass photoanode coated with an electrochromic WO3 film of optical quality, including a mesoporous TiO2 layer (sensitized by the MK2 organic dye) that covers 20% of the device area. The liquid electrolyte is composed of 0.22 M Co(II)(bpy)3(PF6)2, 0.5 M LiClO4 and 0.5 M 4-tert-butylpyridine. A platinized conductive glass cathode completes the cell set-up. The fabricated devices are almost transparent in the bleached state with a Tlum value above 50%. They exhibit coloration speeds in the order of minutes, with a maximum contrast ratio of 2.9:1 after 21 min of illumination at 1000 W m−2 under open circuit conditions (OC), and high reversibility to fully bleached state in short circuit conditions. They provide a maximum measured power conversion efficiency of 0.28% due to limitations imposed by conflicting requirements of the photovoltaic and electrochromic elements, which is nonetheless sufficient to drive the coloration process. Since only the reduced specie Co2+ is present, initial illumination under OC for 3 min at 1000 W m−2 is necessary, prior to measurements, to oxidize Co2+ to Co3+ through the dye regeneration process in the electrolyte. The higher recombination losses of the Co2+/3+ redox shuttle compared to I−/I3−, which lead to a considerable reduction in coloration depth 20 days post fabrication due to loss of photoelectrons at the WO3/electrolyte interface, are suppressed by the use of a 35 nm thick ZnS barrier deposited on top of WO3. Remarkably, it results in a stabilized contrast ratio of 1.5:1, 23 days post fabrication. In addition, the color coordinates of the present devices resemble those of typical electrochromics: they exhibit blue coloration, as a result of the lack of the absorbing iodine in the electrolyte that produces a green tint.

Gel electrolytes for partly covered photoelectrochromic devices

This work comprises a systematic experimental investigation of the effect of gel electrolytes in the properties of partly covered photoelectrochromic devices.A series of gel electrolytes was fabricated with different concentrations of poly(methyl methacrylate), PMMA, (Mw 120.000), ranging from 0% w/w to 45% w/w, dissolved in acetonitrile with 0.5 M lithium iodide, 0.005 M iodine and 0.5 M 4-tert-butylpyridine. The ionic conductivity of the electrolytes was in the order of 10−5 S/cm, decreasing with increasing PMMA concentration.These electrolytes were incorporated in partly covered photoelectrochromic devices. It was found that with increasing PMMA concentration, the power conversion efficiency of the incorporated solar cells decreases (from 1.7% to 0.5%), mostly due to the reduction of the short-circuit current. Regarding coloration kinetics, the photocoloration efficiency, PhCE of the devices was nearly constant up to 35% w/w concentration of PMMA, decreasing with further increase in the PMMA concentration. Of the electrolytes tested, that with 35% w/w concentration of PMMA was favored as it leads to devices with satisfactory electrical and optical characteristics.The long-term stability of devices with the aforementioned electrolyte was tested. A gradual decrease of the coloration depth with storage time was observed, due to losses at the WO3/electrolyte interface. A 35 nm thin ZnS barrier on top of WO3 suppressed these losses, leading to a stabilized contrast ratio of 2.1:1, 137 days post fabrication. Devices exposed to insolation in open-circuit and in short-circuit conditions remained operational for more than 200 days, without the stability problems related to liquid electrolytes.

Assessment of the long-term performance of partly covered photoelectrochromic devices under insolation and in storage

This work comprises an experimental study of the long-term performance and stability of partly covered photoelectrochromic (PEC) devices under various operating conditions.The most durable PEC device was tested for nearly one year in open circuit storage in the dark exhibiting a 26% reduction in the power conversion efficiency of the photovoltaic (PV) unit. As regards optical performance, it was found to gradually converge to a contrast ratio of about 1.5:1, having passed through a transitional period of about 80 days, that caused a decrease in the coloration depth, without affecting reversibility. This gradual loss of coloration depth was attributed to the reaction of photoelectrons with triiodide ions at the electrochromic WO3 film-electrolyte interface, that takes place prior to the formation of the colored LixWO3 tungsten bronze, and is enhanced while the liquid electrolyte wets the WO3 film. This phenomenon was also observed in “redox” type electrochromic devices, verifying the proposed loss mechanism.PEC devices were also exposed to insolation in short (SC) and open (OC) circuit conditions. The PEC device tested in SC operation for more than four months, exhibited a contrast ratio comparable to that of the devices kept in storage, although an 80% reduction in the power conversion efficiency of the PV element has been observed. On the other hand, OC operation of 3 months did not affect the performance of the PV element, but, resulted in irreversible coloration of the device tested, due to trapping of Li ions within the WO3 matrix.Having identified the causes for the observed adverse phenomena, remedies can be sought, such as thin film barrier layers to prevent the loss of photoelectrons to the electrolyte, modified electrolytes to enhance the PV element stability, avoidance of prolonged OC operation to prevent irreversible coloration. Indeed, a 60 nm thick ZnS barrier deposited on top of the WO3 film was found to effectively suppress the loss current reactions, doubling the contrast ratio of the stabilized PEC devices to 2.8:1, at the expense of coloration speed.

Mn2+-ZnSe/ZnS@SiO₂ Nanoparticles for Turn-on Luminescence Thiol Detection.

Biological thiols are antioxidants essential for the prevention of disease. For example, low levels of the tripeptide glutathione are associated with heart disease, cancer, and dementia. Mn2+-doped wide bandgap semiconductor nanocrystals exhibit luminescence and magnetic properties that make them attractive for bimodal imaging. We found that these nanocrystals and silica-encapsulated nanoparticle derivatives exhibit enhanced luminescence in the presence of thiols in both organic solvent and aqueous solution. The key to using these nanocrystals as sensors is control over their surfaces. The addition of a ZnS barrier layer or shell produces more stable nanocrystals that are isolated from their surroundings, and luminescence enhancement is only observed with thinner, intermediate shells. Tunability is demonstrated with dodecanethiol and sensitivities decrease with thin, medium, and thick shells. Turn-on nanoprobe luminescence is also generated by several biological thiols, including glutathione, N-acetylcysteine, cysteine, and dithiothreitol. Nanoparticles prepared with different ZnS shell thicknesses demonstrated varying sensitivity to glutathione, which allows for the tuning of particle sensitivity without optimization. The small photoluminescence response to control amino acids and salts indicates selectivity for thiols. Preliminary magnetic measurements highlight the challenge of optimizing sensors for different imaging modalities. In this work, we assess the prospects of using these nanoparticles as luminescent turn-on thiol sensors and for MRI.

Strain-Induced Type II Band Alignment Control in CdSe Nanoplatelet/ZnS-Sensitized Solar Cells

Colloidal CdSe nanoplatelets (NPLs) deposited on TiO2 and overcoated by ZnS were explored as light absorbers in semiconductor-sensitized solar cells (SSSCs). Significant red shifts of both absorption and steady-state photoluminescence (PL) along with rapid PL quenching suggest a type II band alignment at the interface of the CdSe NPL and the ZnS barrier layer grown on the NPL layer, as confirmed by energy band measurements. The considerable red shift leads to enhanced spectral absorption coverage. Cell characterization shows an increased open-circuit voltage of 664 mV using a polysulfide electrolyte, which can be attributed to a photoinduced dipole effect created by the spatial charge separation across the nanoplatelet sensitizers. The observed short-circuit current density of 11.14 mA cm–2 approaches the maximal theoretical current density for this choice of absorber, yielding an internal quantum efficiency of close to 100%, a clear signature of excellent charge transport and collection yields. With thei...

Wavelength selective UV/visible metal-semiconductor-metal photodetectors

We report on growth, fabrication and characterization of metal-semiconductor-metal (MSM) photodetectors based on periodic heterostructures with ZnCdS quantum wells separated by ZnMgS and ZnS barrier layers. Heterostructures were grown on semi-insulating GaP substrates by MOVPE. Detecting properties of the MSM-heterophotodiodes have been investigated. We observe electrically tunable spectral response of these detectors. At low bias detectors provide narrowband response at the wavelength 350 nm with FWHM = 18 nm. Increasing bias shifts maximum detector sensitivity at the wavelength 450 nm while narrowband response at 350 nm remains. Thus, a two-color detection of light emission is provided.

Selective UV radiation detection on the basis of low-dimensional ZnCdS/ZnMgS/GaP and ZnCdS/ZnS/GaP heterostructures

The detecting properties of periodic heterostructures with ZnCdS quantum wells separated by ZnMgS or ZnS barrier layers are studied. Heterostructures are grown on semi-insulating GaP substrates by metal organic vapor-phase epitaxy (MOVPE). On their basis, metal–semiconductor–metal (MSM) diodes with interdigital Schottky contacts 3 μm, distances between them of 3 μm, and a total detector area of 100 × 100 μm are fabricated. The detectors have low dark currents (10–12 A); at low bias voltages, they provide a narrow- band response (full-width at half-maximum of FWHM = 18 nm at a wavelength of 350 nm) which is controlled by the ZnCdS quantum-well composition. As bias is increased to 70 V, the maximum detector sensitivity shifts by a wavelength of 450 nm, which is caused by penetration of the external-bias electric field into the semi-insulating GaP substrate. In this case, the narrow-band response of the detector at a wavelength of 350 nm is retained, i.e., the two-color detection of light is provided.

Electrically tunable spectral responsivity in metal–semiconductor–metal photodetectors based on low-dimensional ZnCdS/ZnMgS/GaP, ZnCdS/ZnS/GaP heterostructures

We report on growth, fabrication and characterization of metal–semiconductor–metal (MSM) photodetectors based on periodic heterostructures with ZnCdS quantum wells separated by ZnMgS and ZnS barrier layers. Heterostructures were grown on semi-insulating GaP substrates by MOVPE. MSM-diodes with width of Ni–Au interdigitated Schottky barrier contacts and gap between them of 3μm and total detector area of 100×100μm2 have been fabricated. Detecting properties of MSM-heterophotodiodes have been investigated. We observe electrically tunable spectral response of these detectors. At low bias detectors provide narrowband response (FWHM=18nm at the wavelength 350nm) determined by a composition of ZnCdS quantum well. Increasing bias up to 70V shifts maximum detector sensitivity at the wavelength 450nm while narrowband response at 350nm remains. Thus, a two-color detection of light emission is provided.

Controlling Charge Carrier Overlap in Type-II ZnSe/ZnS/CdS Core-Barrier-Shell Quantum Dots.

We describe the synthesis and spectroscopic characterization of colloidal ZnSe/ZnS/CdS nanocrystals, which exhibit a type-II electronic structure and wave function overlap that is strongly dependent on the thickness of the ZnS barrier. Barrier thickness is controlled by both the amount of deposited material and the reaction and annealing temperature of CdS shell growth. The results show that a single monolayer of ZnS mitigates the overlap significantly, while four and more monolayers effectively suppress band edge absorption and emission. Transient absorption spectra reveal a broad distribution of excitons with mixed S and P symmetry, which become allowed due to alloy formation and contribute to charge carrier relaxation across the barrier. We present a model of the core/shell interface based on cation diffusion, which allows one to estimate the extent of the diffusion layer from optical spectra.

Investigation of electronic and optical properties of (CdSe/ZnS/CdSe/ZnS) quantum dot–quantum well heteronanocrystal

In this article, we have investigated the photoluminescence intensity of quantum dot–quantum well heteronanocrystal with non-linear potential profile which has been analyzed by the finite element numerical methods and is compared with traditional potential profile of same heteronanocrystal. We have probed the effect of carrier localization in layers of heteronanocrystal on the photoluminescence intensity. Moreover, the effects of variation of radius layers such as CdSe core, shell, and ZnS barriers radius on the photoluminescence intensity are studied. Besides, for the first time, we demonstrated the shift of quantum dot–quantum well operation wavelength by introducing non-linear potential profile in the core of heteronanocrystal that can be drastically affected on biological application. Furthermore, in biological application, by tuning the emission wavelengths of quantum dot into the far-red and near-infrared ranges, the non-invasive in vivo imaging technique was developed. In this wavelength window, tissue absorption, scattering, and auto-fluorescence intensities have minimum quantities. In our article with new structure, the relation between size and operation wavelength don’t follow traditional relation.

Onion-like (CdSe)ZnS/CdSe/ZnS quantum-dot-quantum-well heteronanocrystals for investigation of multi-color emission

We investigate multi-color spontaneous emission from quantum-dot- quantum-well heteronanocrystals made of onion-like (CdSe)ZnS/CdSe/ZnS (core)shell/shell/shell structures, with our theoretical results explaining experimental measurements for the first time. In such multi-layered heteronanocrystals, we discover that the carrier localization is tuned from type-1-like to type-2-like localization by controlling CdSe and ZnS shell thicknesses, and that 3-monolayer ZnS barriers are not necessarily sufficient for carrier localization, unlike in conventional (CdSe)ZnS (core)shell structures. We demonstrate that exciton localization in distinct layers of (CdSe)ZnS/CdSe/ZnS heteronanocrystals with high transition probability (for n=1 states in CdSe core and n=2 states in CdSe shell) is key to their multi-color emission.

Light Harvesting and Carrier Transport in Core/Barrier/Shell Semiconductor Nanocrystals

Excitation transfer pathways in colloidal core/barrier/shell nanomaterials were investigated in the CdSe/ZnS/CdSe system. Absorption of light in the outer CdSe shell results in emission from the band edge of the CdSe core. The CdSe quantum shell acts as a light harvester that indirectly increases the brightness of the CdSe quantum-dot core. Spectroscopic evidence suggesting that the CdSe core and shell are coupled by tunneling of excitons through the ZnS barrier is provided. Competition kinetic analysis shows that charge transport competes effectively with hole capture by pyridine at the outer CdSe shell and exciton relaxation within the outer CdSe shell.

Coupling between ferromagnetic electrodes through ZnS barrier

Magnetization measurements are performed on CoFe2/ZnS/CoFe2/NiFe structures to investigate the interactions between ferromagnetic electrodes through the ZnS barrier. Negative shifts observed in magnetization minor loops indicate a ferromagnetic interaction. The influence of the hard-layer deposition temperature on this shift and on the hard-layer coercive field is considered. The amplitude of the shift decreases as the thickness of the ZnS layer increases. The decrease in this shift at low temperature confirms the presence of an indirect exchange coupling between the magnetic electrodes mediated by spin-polarized quantum tunneling through the ZnS layer.

Ferromagnetic exchange interaction between hard and soft ferromagnetic layers through ZnS semiconductor

Abstract We experimentally evidenced the presence of an indirect exchange coupling between hard and soft ferromagnetic electrodes through a ZnS barrier in magnetic tunnel junctions. For a 2 nm thick ZnS barrier, a negative shift of about –25 Oe is observed in asymmetric magnetization minor loop. This is attributed to a ferromagnetic interaction between the CoFe/Fe soft bilayer and the thick CoFe layer. The amplitude of the observed shift decreased as the thickness of the ZnS barrier increased, which agrees with theoretical models that the exchange interaction is mediated by spin polarized tunneling.

Indirect exchange coupling between two ferromagnetic electrodes through ZnS barrier in magnetic tunnel junctions

Room- and low-temperature magnetization measurements have been performed on magnetic tunnel junctions before patterning to evidence the presence of an indirect ferromagnetic exchange interaction between the hard and the soft ferromagnetic electrodes through the ZnS barrier. The magnetic junctions are composed of an artificial ferrimagnet CoFe/Ru/CoFe sandwich as a hard layer separated by a ZnS barrier from a soft CoFe/Fe soft layer as follows: Fe6 nmCu3 nm(CoFe)1.8 nmRu0.8 nm(CoFe)3 nmZnSx nmCoFe1 nmFe4 nmCu1 nmRu3 nm. At room temperature, a large shift of about −25 Oe is observed in a magnetization minor loop that indicates the presence of a ferromagnetic interaction. The decrease of the amplitude of this shift at low temperature provides a signature of an indirect exchange coupling mediated by spin-polarized quantum tunneling.