Showing papers on "Potential well published in 2020"

Band‐Edge Exciton Fine Structure and Exciton Recombination Dynamics in Single Crystals of Layered Hybrid Perovskites

Abstract: 2D perovskite materials have recently reattracted intense research interest for applications in photovoltaics and optoelectronics. As a consequence of the dielectric and quantum confinement effect, they show strongly bound and stable excitons at room temperature. Here, the band-edge exciton fine structure and in particular its exciton and biexciton dynamics in high quality crystals of (PEA)2PbI4 are investigated. A comparison of bulk and surface exciton lifetimes yields a room temperature surface recombination velocity of 2 × 103 cm s−1 and an intrinsic lifetime of 185 ns. Biexciton emission is evidenced at room temperature, with a binding energy of ≈45 meV and a lifetime of 80 ps. At low temperature, exciton state splitting is observed, which is caused by the electron–hole exchange interaction. Transient photoluminescence resolves the low-lying dark exciton state, with a bright/dark splitting energy estimated to be 10 meV. This work contributes to the understanding of the complex scenario of the elementary photoexcitations in 2D perovskites.

A facile one step hydrothermal synthesis of carbon quantum dots for label -free fluorescence sensing approach to detect picric acid in aqueous solution

Abstract: In this work, a facile one step approach has been applied for the synthesis of nitrogen and sulphur co-doped water soluble carbon quantum dots (NS-CQDs) through hydrothermal treatment of L-Lysine and thiourea. The obtained NS-CQDs have a high quantum yield (QY) of 53.19 % and emit strong blue fluorescence under UV light excitation of wavelength 365 nm. The morphology of NS-CQDs is spherical in shape and their sizes are distributed in the range 5–8 nm with average diameter 6.86 nm. Moreover the NS-CQDs show excitation dependent emission behavior due to quantum confinement effect. Additionally, NS-CQDs serve as a fluorescent probe for the selective and sensitive detection of picric acid (PA) in aqueous solution. A good linear response to PA in the concentration range 1–10 μM with a detection limit 0.24 μM has been obtained. The high selectivity of PA by NS-CQDs is suggested to arise from fluorescence resonance energy transfer (FRET) mechanism. Finally these NS-CQDs fluorescent probes have been examined in real water sample by measuring the concentration of PA in tap water.

Synergistic effect of quantum confinement and site-selective doping in polymeric carbon nitride towards overall water splitting

Abstract: Photocatalytic overall water splitting is a challenging topic in the research field of solar energy conversion. Herein, we proposed an effective strategy of adopting quantum confinement effect and site-selective doping to realize simultaneous hydrogen and oxygen production from pure water over polymeric carbon nitride (CN) photocatalysts. Selectively doping P atoms in the corner-carbon sites provided efficient charge transfer channels between different structural units. Meanwhile, the site-selective P doping could generate mid-gap states below the conduction band of CN, which extended the utilization of visible light up to 600 nm, and served as trapping centres to inhibit the charge recombination. In addition, the quantum confinement effect could ensure the sufficient driving force for the rate-determining water oxidation. As a consequence, the hydrogen production rate from overall water splitting reached 619.5 μmol·h−1 g−1 under simulated sunlight irradiation, with notable apparent quantum yields of 6.8% at 400 nm and 2.4% at 500 nm.

Dimensional reduction of the small-bandgap double perovskite Cs2AgTlBr6.

Abstract: Quantum confinement effects in lower-dimensional derivatives of the ABX3 (A = monocation, X = halide) single perovskites afford striking optical and electronic changes, enabling applications ranging from solar absorbers to phosphors and light-emitting diodes. Halide double perovskites form a larger materials family, known since the late 1800s, but lower-dimensional derivatives remain rare and prior work has revealed a minimal effect of quantum confinement on their optical properties. Here, we synthesize three new lower-dimensional derivatives of the 3D double perovskite Cs2AgTlBr6: 2D derivatives with mono- (1-Tl) and bi-layer thick (2-Tl) inorganic sheets and a quasi-1D derivative (1'-Tl). Single-crystal ellipsometry studies of these materials show the first clear demonstration that dimensional reduction can significantly alter the optical properties of 2D halide double perovskites. This large quantum confinement effect is attributed to the substantial electronic delocalization of the parent 3D Ag–Tl perovskite. Calculations track the evolution of the electronic bands with dimensional reduction and the accompanying structural distortions and show a direct-to-indirect bandgap transition as the 3D perovskite lattice is thinned to a monolayer in 1-Tl. This bandgap transition at the monolayer limit is also evident in the calculations for 1-In, an isostructural, isoelectronic analogue to 1-Tl in which In3+ replaces Tl3+, underscoring the orbital basis for the direct/indirect nature of the bandgap. Thus, in complement to the massive compositional diversity of halide double perovskites, dimensional reduction may be used as a systematic route for harnessing electronic confinement effects and obtaining new electronic structures.

A zero-dimensional hybrid lead perovskite with highly efficient blue-violet light emission

Abstract: Pursuits of high-performance blue light-emitting perovskites have attracted intensive attention due to an insufficient photo-luminescence quantum yield (PLQY) of 90%) in the green and red emission spectral regions of CsPbX3 (X = Br and I), respectively. Hence, it is very significant to improve the blue light emitting PLQYs to balance the development of three-primary-colour in high-definition displays. In this study, we have proposed a new structural design strategy of reducing the lattice dimension to enhance quantum confinement effect and further improve the blue-light emission efficiency. Herein, by rationally choosing a long chain-like organic cation to slice the [PbCl3]− skeleton, we have successfully constructed the first 0D perovskite of [BAPrEDA]PbCl6·(H2O)2, in which the isolated [PbCl6]4− units are confined by a closely assembled organic matrix. As expected, the bulk crystals of the 0D perovskite display broadband blue-violet light emission (392 nm) of radiative transition arising from triplet exciton states. Remarkably, the synergistic effects of enhanced quantum confinement and highly localized excitons from the 0D perovskite significantly boost the PLQY of blue light emission to 21.3%, which far exceeds than that of the typical 3D CsPbCl3. To the best of our knowledge, this study first realizes the lowest-dimensional structural transformation from 3D perovskite to 0D molecule but holding the intrinsic blue light emission, and it also represents a new record of highest-energy blue-violet light emission in single-crystalline 0D metal halides.

Size effects on structural and optical properties of tin oxide quantum dots with enhanced quantum confinement

Abstract: Size-controllable tin oxide (SnO2) quantum dots (QDs) in aqueous solution were prepared by using stannous chloride and thiourea as source materials and the hydrothermal treatment was employed for size control during the self-assembly of SnO2 QDs. The size effects on structural and optical properties of QDs were discussed. The prepared QDs were confirmed to be of rutile structure with (2 0 2), (2 1 1) and (3 3 0) facets observed. The crystal lattice constants changed their dimensions during grain growth and secondary grain boundaries appeared if grain size was over 7 nm. The composition and valence states were found independent with grain size. A negative correlation was observed between size and band gap, which exhibited much greater values than the calculation results from the theory of effective mass approximation and illustrated a strong quantum confinement in the present SnO2 QDs. The size effect of photoluminescence property illustrated a fluorescence inhibition, which could be ascribed to the additional energy levels provided by the lattice distortions at secondary grain boundaries. The depletion region in QDs was extended by these defects, which strengthened the quantum confinement effect.

Tellurene: An elemental 2D monolayer material beyond its bulk phases without van der Waals layered structures

Abstract: Due to the quantum confinement effect, atomically thin two-dimensional (2D) monolayer materials possess distinct characteristics from their corresponding bulk materials, which have received wide attention from science and industry. Among all the 2D materials, elemental 2D materials with the simplest components are most striking. As an emerging group-VIA elemental 2D monolayer material, tellurene exhibits many exciting fundamental properties, such as chemical and mechanical stabilities, bandgap and high carrier mobilities compared to phosphorene, graphene and MoS2, respectively. Besides, in further exploration, it was found that tellurene or tellurene-based device presents excellent thermoelectric properties, piezoelectric properties, quantum Hall effects, and superb optical properties especially nonlinear optics characteristics, etc. The properties of tellurene can be modulated by virtue of strain, defects, edges, and heterojunction effects. In view of so many unique properties, it has drawn significant interest since tellurene was predicted and fabricated successfully in 2017. In this paper, we review the 2D tellurene allotropes, experimental preparation, excellent properties, performance modulation and future development.

In situ confined growth of ultrasmall perovskite quantum dots in metal–organic frameworks and their quantum confinement effect

Abstract: The metal halide perovskite quantum dots (APbX3 PeQDs; A = Cs or CH3NH3; X = Cl, Br or I) have emerged as a new type of promising optoelectronic material for light-emitting and photovoltaic applications because of their excellent optical properties. However, the precise control over the size and photoluminescence (PL) emission of APbX3 PeQDs remains a great challenge, which has been one of the main obstacles to the applications of PeQDs. Herein, we report a unique strategy for in situ confined growth of MAPbBr3 (MA = CH3NH3) PeQDs by using porous metal–organic framework (MOF) UiO-66 as a matrix. By introducing Pb(Ac)2 and MABr precursors into the pores of UiO-66 via a stepwise approach, ultrasmall MAPbBr3 PeQDs were in situ grown in the matrix with the size tuned from 6.4 to 3.3 nm by changing the concentration of the Pb(Ac)2 precursor. Accordingly, the PL emission wavelength of the resulting MAPbBr3 PeQDs was blue-shifted from 521 to 486 nm with the size reduction, owing to the strong quantum confinement effect of the PeQDs. Due to the surface passivation effect endowed by the UiO-66 matrix, the ultrasmall MAPbBr3 PeQDs also displayed a high PL quantum yield (PLQY) of 43.3% and a long PL lifetime of 100.3 ns. This study proposes a new strategy to prepare ultrasmall PeQDs and effectively control their sizes and PL emissions, which may open up new avenues for the development of high-performance luminescent PeQDs for diverse applications.

Optical Quantum Confinement in Ultrasmall ZnO and the Effect of Size on Their Photocatalytic Activity

Abstract: Zinc oxide is a well-known metal oxide semiconductor with a wide direct band gap that offers a promising alternative to titanium oxide in photocatalytic applications. ZnO is studied here as quantum ...

Engineering the energy bandgap of lead cobalt sulfide quantum dots for visible light optoelectronics

Abstract: The energy bandgap of ternary alloyed lead cobalt sulfide quantum dots has been engineered for visible light optoelectronic applications. Ternary Pb0.8Co0.2S QDs were synthesized in situ onto TiO2 electrodes using a sub-sequential chemical bath deposition method up to 7 times. The surface morphology of the prepared alloyed Pb0.8Co0.2S QDs photoanodes was characterized using a transmission electron microscope. The X-ray diffraction technique was used to study the structural properties of the prepared alloyed photoanodes. The optical properties were characterized using a UV–visible–NIR spectrophotometer in the visible region range. The absorption of the prepared photoanodes increases as the no. of deposition times rises up to 7. Besides, the energy bandgap of the alloyed photoanodes is red-shifted from 3.15 eV (393 nm) to 1.68 eV (738 nm). These bandgap red shifts are mainly attributed to the quantum confinement effect. Based on optical properties measurements, the prepared ternary alloyed QDs could be utilized effectively in visible light optoelectronic applications.

On-demand tuning of charge accumulation and carrier mobility in quantum dot solids for electron transport and energy storage devices

Abstract: Assemblies of colloidal quantum dots (CQDs) are attractive for a broad range of applications because of the ability to exploit the quantum confinement effect and the large surface-to-volume ratio due to their small dimensions. Each application requires different types of assemblies based on which properties are intended to be utilized. Greater control of assembly formation and optimization of the related carrier transport characteristics are vital to advance the utilization of these materials. Here, we demonstrate on-demand control of the assembly morphology and electrical properties of highly crosslinked CQD solids through the augmentation of various assembly methods. Employment of electric-double-layer (EDL) gating on these assembly structures (i.e., an amorphous assembly, a hierarchical porous assembly, and a compact superlattice assembly) reveals their intrinsic carrier transport and accumulation characteristics. Demonstrations of high electron mobility with a high current modulation ratio reaching 105 in compact QD films and of a record-high areal capacitance of 400 μF/cm2 in an electric-double-layer supercapacitor with very thin (<100 nm) QD hierarchical porous assemblies signify the versatility of CQDs as building blocks for various modern electronic devices. A method for creating assemblies of nanoparticles with a desired geometry has been developed by researchers in Japan and Indonesia. Colloidal quantum dots are nanoscale semiconductors. Their small size tightly confines the motion of the electrons within, giving the dots extraordinary electrical and optical properties. Each potential application requires the quantum dots to be assembled with a specific structure and composition. Satria Bisri from the RIKEN Center for Emergent Matter Science and the Tokyo Institute of Technology and co-workers have developed a method for the on-demand control of the structure, and therefore the electrical properties, of colloidal quantum dot assemblies. The team optimized existing quantum dot deposition techniques to create quantum dot assemblies with a record high capacitance per area in a film thinner than a hundred nanometers. Precise control of colloidal-semiconductor-quantum-dots (CQD) assembly morphologies and the related carrier transport characteristics are vital to advance their utilisations. Each application requires different assembly types to exploit either the quantum confinement effect or the large surface-to-volume ratio. On-demand control of CQD-solids‘ morphology are demonstrated using variety of assembly methods. Employment of the electric-double-layer gating on varieties of CQD solids reveals their intrinsic carrier transport and accumulation characteristics. Compact superlattice structure shows high conductivity, and the hierarchical porous assembly exhibits high carrier accumulations. These flexibilities in assembly controls and characteristic tunings signify CQD versatilities as building blocks for different modern electronics.

Ag2ZnSnS4 Nanocrystals Expand the Availability of RoHS Compliant Colloidal Quantum Dots

Abstract: The demonstration of the quantum confinement effect in colloidal quantum dots (QDs) has been extensively studied and exploited mainly in Pb and Cd chalcogenide systems. There has been an urgent nee...

Visible-light driven photocatalytic hydrogen generation by water-soluble all-inorganic core–shell silicon quantum dots

Abstract: The photocatalytic hydrogen (H2) generation by boron (B) and phosphorus (P) codoped silicon quantum dots (Si QDs) with diameters in the quantum confinement regime is investigated. The codoped Si QDs have an amorphous shell made from B, Si and P. The shell induces negative potential on the surface and makes codoped Si QDs dispersible in water. The hydrophilic shell offers enhanced stability and efficiency in photocatalytic H2 generation and provides the opportunity to study the size dependence of the H2 generation rate. A drastic increase of H2 generation rate with decreasing QD size is observed. Analyses based on the Marcus theory reveal that the upper shift of the lowest unoccupied molecular orbital level of Si QDs by the quantum confinement effect is responsible for the enhanced photocatalytic activity.

CuInS2 Quantum Dots with Thick ZnSexS1–x Shells for a Luminescent Solar Concentrator

Abstract: The photoluminescence quantum yield (PLQY) of copper indium sulfide (CIS) quantum dots (QDs) improves significantly after a shelling procedure as the shell materials, like zinc sulfide, mitigate surface defects and reduce nonradiative recombinations. However, it is widely observed that the PLQY reduces when QDs are cast as a solid film from their solution, and PL peak emission also red-shifts, which suggests a relaxation of the quantum confinement effect. This could be due to thin zinc sulfide shells. Unlike cadmium-based QDs, reports on a thick zinc chalcogenide shell on CIS QDs are limited. Efforts to grow larger shells have been stymied by zinc diffusion toward the core, causing cation exchange and alloy formation. Thick zinc chalcogenide shell growth typically requires higher temperatures, a regime which would irreversibly degrade the PL of a CIS QD. Here, we develop a technique to grow thicker shells on CIS QDs to improve the quantum confinement effect between the QDs. With these thick shell CIS QDs, we demonstrate better emission as well as thermal and chemical stability. Finally, we demonstrate their application in a luminescent solar concentrator and showed that it has better light harvesting characteristics than its thin shell counterpart.

Fabrication of highly efficient pure blue-emitting electroluminescentdevices using ZnSe/ZnSe x S 1-x /ZnS QDs

Abstract: Since electroluminescent (EL) quantum dots (QDs) are considered a key component of the next-generation display, and large-scale production of environment-friendly QDs is required for their wide use in commercial displays. Therefore, several studies on non-cadmium QDs, such as indium phosphide (InP) QDs in the III-V category, graphene QDs, and copper indium sulfide (CuInS2) or silver indium sulfide (AgInS2) QDs in the I-III-VI2 category, have been conducted owing to their non-toxicity and good optical properties. Subsequently, significant results have been reported for green and red colors. However, for synthesis of blue QDs, pure blue emission in the range of 440-460 nm has been achieved with few materials. Among them, zinc selenide (ZnSe) is a promising candidate for synthesizing blue QDs. However, owing to the wide band gap (2.7 eV) of ZnSe, highly effective QDs were attained in the violet region (420-440 nm). Here, for the first time, we have synthesized ZnSe/ZnSexS1-x/ZnS QDs emitting at a wavelength of 444 nm with high photoluminescence quantum yield (PLQY) of 77.2%. Also, full width at half maximum (FWHM) of 23.3 nm ensured its excellent color purity. Use of a gradient intermediate shell of ZnSeS in the original ZnSe/ZnS QDs was the key factor behind this achievement. The intermediate gradient shell of ZnSeS around the core delocalizes the electrons, weakening the quantum confinement effect (QCE), hence rendering the emission color of the QDs tunable from violet to blue by manipulating the ratio of selenium (Se) and sulfur (S) in the composites. A blue emission peak centered at 452 nm was observed for the quantum dot light-emitting diodes (QD-LEDs) fabricated using the above-mentioned QDs, and an external quantum efficiency (EQE) of 5.32%, current efficiency of 1.51 cd/A, and power efficiency of 0.74 lm/W were reported. Furthermore, our fabricated device exhibited a maximum brightness of 3,754 cd/m2 and a half operational time (LT50) with 100 cd/m2 initial luminance of 1.27 h, which are the highest values of these parameters to be reported till date for a blue QD-LED fabricated using ZnSe core based QDs in pure blue region.

Cathodoluminescence in single and multiwall WS2 nanotubes: Evidence for quantum confinement and strain effect

Abstract: For nanoparticles with sub-10 nm diameter, the electronic bandgap becomes size dependent due to quantum confinement; this, in turn, affects their electro-optical properties. Thereby, MoS2 and WS2 monolayers acquire luminescent capability, due to the confinement-induced indirect-to-direct bandgap transition. Rolling up of individual layers results in single wall inorganic nanotubes (SWINTs). Up to the present study, their luminescence properties were expected to be auspicious but were limited to theoretical investigations only, due to the scarcity of SWINTs and the difficulties in handling them. By optimizing the conditions in the plasma reactor, relatively high yields of WS2 SWINTs 3–7 nm in diameter were obtained in this work, compared to previous reports. A correlative approach, transmission electron microscopy coupled with a scanning electron microscope, was adapted to overcome handling obstacles and for testing individual nanotubes by low-temperature cathodoluminescence. Clear cathodoluminescence spectra were obtained from WS2-SWINTs and compared with those of WS2 multiwall nanotubes and the corresponding bulk material. Uniquely, the optical properties of INTs acquired from cathodoluminescence were governed by the opposite impact from quantum size effect and strain in the bent triple S-W-S layers. The experimental findings were confirmed by the Density Functional and Time-Dependent Density Functional theoretical modeling of monolayer and bilayer nanotubes of different chiralities and diameters. This study provides experimental evidence of the quantum confinement effect in WS2 SWINTs akin to WS2 monolayer. The ability to tune the electronic structure with morphology or number of layers may be exploited toward photoelectrochemical water splitting with WS2 catalysts, devising field effect transistors, photodetectors, and so on.

Magnetic and Optical Studies of NiFe2O4 Micro- and Nanoparticles

Abstract: The nickel ferrite micro- and nanoparticles have been prepared via solid-state and sol-gel auto-combustion technique and its single-phase formation, cubic spinel crystal structure (space group Fd-3m), and surface morphology were confirmed by XRD, FESEM, and TEM techniques, respectively. The optical band gap was estimated from reflectance spectra using the Kubelka-Munk equation and the direct optical band gap enhances in nanoparticles (1.905 eV) than microparticles (1.75 eV) due to the quantum confinement effect. The small value of remanence magnetization (Mr) and coercive field (Hc) of nanoparticles was observed from narrow hysteresis M-H loop which recommends its superparamagnetic states near room temperature (296 K). The absence of blocking temperature for microparticles from 0 to 300 K confirms the non-existence of a single-domain region and superparamagnetic states.

Two-dimensional CaFCl: ultra-wide bandgap, strong interlayer quantum confinement, and n-type doping.

Abstract: Two-dimensional (2D) ultra-wide bandgap (UWBG) semiconductors have attracted tremendous attention because of their unique electronic properties and promising applications. Using first-principles calculations, monolayer (bilayer) CaFCl has a cleavage energy of 0.93 J m−2 (0.72 J m−2), suggesting that the exfoliation of monolayer and few-layer materials from the bulk phase could be feasible. The CaFCl monolayer is an UWBG semiconductor with a direct bandgap of 6.62 eV. In addition to the dynamic and thermodynamic stability, it can remain thermally stable at 2200 K, suitable for operation in high-temperature environments. The bandgap of monolayer CaFCl can be tuned by external strain and layer thickness. The decrease of the layer thickness leads to not only a bandgap increase but also an indirect-to-direct bandgap transition, suggesting a strong interlayer quantum confinement effect. Under biaxial strain, the direct bandgap can also be turned into an indirect one. The adsorption of a tetrathiafulvalene (TTF) molecule introduces deep donor states in the gap of CaFCl. Under an external electric field with direction from CaFCl to TTF, the TTF-derived donor states move closer to the conduction band edge of CaFCl and then the adsorption complex becomes effectively n-doped. Furthermore, monolayer CaFCl exhibits pronounced optical absorption in the ultraviolet range of the solar spectrum. These results render CaFCl an attractive 2D material for applications in flexible nanoelectronic and optoelectronic devices.

Evolutional carrier mobility and power factor of two-dimensional tin telluride due to quantum size effects

Abstract: There has been emerging research of novel two-dimensional (2D) layered materials recently, due to their striking geometric, electronic and thermoelectric properties caused by the quantum confinement effect. However, the current reported thermoelectric performance of thin films is usually size-sensitive and hence may not be superior to that of their bulk counterparts. It is thus important to determine the size effect for low-dimensional thermoelectric materials, based on the theoretical tools of quantum mechanics. To achieve this goal, we studied 2D SnTe single crystals with a varied layer thickness as the characteristic length of materials, to eliminate the other coupled effects of interface engineering or universal defects in polycrystalline thin films. In this work, we demonstrate that the strategy of the quantum confinement effect is highly sensitive to the layer thickness of 2D SnTe materials, and a critical size of 3 layers exists, above which an abrupt degradation of the mobility and thermoelectric parameters occurs. The thermoelectric performance is optimal in monolayer SnTe and then gradually decays, until 6 layers, which gets close to the bulk feature. Correspondingly, the power factor (PF) and the ZT values exhibit evident layer-tunability as a combined effect of layer-dependent relaxation time, effective mass, electrical conductivity and Seebeck coefficient. Our study provides a profound physical understanding of the low-dimensionality strategy for high-performance thermoelectric materials. The intrinsic thermoelectric properties of ultra-thin 2D materials can be favourable as compared to those of the bulk single crystals.