Showing papers on "Potential well published in 2021"

Why In2O3 Can Make 0.7 nm Atomic Layer Thin Transistors.

Abstract: In this work, we demonstrate enhancement-mode field-effect transistors by an atomic-layer-deposited (ALD) amorphous In2O3 channel with thickness down to 0.7 nm. Thickness is found to be critical on the materials and electron transport of In2O3. Controllable thickness of In2O3 at atomic scale enables the design of sufficient 2D carrier density in the In2O3 channel integrated with the conventional dielectric. The threshold voltage and channel carrier density are found to be considerably tuned by channel thickness. Such a phenomenon is understood by the trap neutral level (TNL) model, where the Fermi-level tends to align deeply inside the conduction band of In2O3 and can be modulated to the bandgap in atomic layer thin In2O3 due to the quantum confinement effect, which is confirmed by density function theory (DFT) calculation. The demonstration of enhancement-mode amorphous In2O3 transistors suggests In2O3 is a competitive channel material for back-end-of-line (BEOL) compatible transistors and monolithic 3D integration applications.

Amorphous Co3O4 quantum dots hybridizing with 3D hexagonal CdS single crystals to construct a 0D/3D p–n heterojunction for a highly efficient photocatalytic H2 evolution

Abstract: Herein, a novel amorphous monodisperse Co3O4 quantum dots/3D hexagonal CdS single crystals (0D/3D Co3O4 QDs/CdS) p-n heterojunction was constructed by a simple hydrothermal and electrostatic self-assembly method. The amorphous monodispersed Co3O4 QDs (≈4.5 nm) are uniformly and tightly attached to the surface of the hexagonal CdS single crystals. The sample, 0.5% CQDs/CdS exhibits outstanding hydrogen evolution activity of 17.5 mmol h-1 g-1 with a turnover number (TON) of 4214, up to 10.3 times higher than that of pure CdS. The enhanced photocatalytic activity can be attributed to the synergistic effect of the p-n heterostructure and the quantum confinement effect of Co3O4 QDs, which significantly promoted the separation efficiency of photo-generated electrons and holes. Additionally, the sulfur vacancy also can act as electron trappers to improve carrier separation and electron transfer. The photoelectrochemical and time-resolved fluorescence (TRPL) results further certify the effective spatial charge separation. This work gives an insight into the design of the 0D/3D Co3O4 QDs/CdS p-n heterostructure for a highly efficient photocatalysis.

High Color Purity and Efficient Green Light-Emitting Diode Using Perovskite Nanocrystals with the Size Overly Exceeding Bohr Exciton Diameter.

Abstract: Lead halide perovskite nanocrystals (PNCs) are emerging as promising light emitters to be actively explored for high color purity and efficient light-emitting diodes. However, the most reported lead halide perovskite nanocrystal light-emitting diodes (PNCLEDs) encountered issues of emission line width broadening and operation voltage elevating caused by the quantum confinement effect. Here, we report a new type of PNCLED using large-size CsPbBr3 PNCs overly exceeding the Bohr exciton diameter, achieving ultranarrow emission line width and rapid brightness rise around the turn-on voltage. We adopt calcium-tributylphosphine oxide hybrid ligand passivation to produce highly dispersed large-size colloidal CsPbBr3 PNCs with a weak size confinement effect and also high photoluminescence quantum yield (∼85%). Utilizing these large-size PNCs as emitters, we manifest that the detrimental effects caused by the quantum confinement effect can be avoided in the device, thereby realizing the highest color purity in green PNCLED, with a narrow full width at half-maximum of 16.4 nm and a high corrected maximum external quantum efficiency of 17.85%. Moreover, the operation half-life time of the large-size PNCLED is 5-fold of that based on smaller-size PNCs. Our work provides a new avenue for improving the performance of PNCLEDs based on unconventional large-size effects.

Impact of the stacking fault and surface defects states of colloidal CdSe nanocrystals on the removal of reactive black 5

Abstract: The photodegradation of the reactive black 5 (RB5) with CdSe-MSA nanocrystals (NCs) synthesized by using mercaptosuccinic acid (MSA) was studied. The quantum confinement effect was confirmed via the blue shift of absorbance band and higher energy band gap values. Photoluminescence (PL) emissions were riddled with low energy bands arising from the recombination of trapped charge carriers. The surface stress of the nanocrystals from the ligand capped surface and the atomic position relaxation at the stacking fault interface was linked to an observed hexagonal distortion. The comparison of SO and LO modes was used to analyze the quality of NCs, defect and disorder on the NCs surface. Contact time and RB5 degradation with NCs were investigated to understand the adsorption-photocatalysis synergy. Statistical physics parameters were calculated to explain the dye adsorption on tested nanocrystals. CdSe-MSA showed remarkable UV-light photocatalytic activity with a dye degradation of 89% in 120 min.

An 8-nm-thick Sn-doped polycrystalline β-Ga2O3 MOSFET with a “normally off” operation

Abstract: Monoclinic gallium oxide (β-Ga2O3) has attracted the interest of the scientific community due to its application in power electronics. Power electronics that need to handle a high voltage often uses a “normally off” device with a positive threshold voltage due to its fail-safe operation and its simple system architecture. In this work, 8-nm-thick Sn-doped polycrystalline β-Ga2O3 thin films were investigated as a channel material for power electronics, and their properties were characterized. The optical bandgap of the 8-nm-thick Sn-doped β-Ga2O3 was determined to be 5.77 eV, which is larger than that of 100-nm-thick Sn-doped β-Ga2O3 due to the quantum confinement effect. The developed back-gated device demonstrated normally off behavior and exhibited a voltage handling capacity as high as 224 V (2.88 MV/cm). This ultrathin β-Ga2O3 layer could also be applied to fields other than power electronics, including displays, optical sensors, photocatalytic sensors, and solar cells.

Photoluminescence investigations of sulfur quantum dots synthesized by a bubbling-assisted strategy

Abstract: Sulfur quantum dots (S-dots) emerge as promising luminescent materials owing to their remarkable optical properties. However, the mechanisms of their formation and photoluminescence remain concealed. We reveal these mechanisms by the bubbling-assisted synthesis and spectroscopic study of S-dots formed from sulfur ions produced by the alkaline oxidation of bulk sulfur under the passivation of PEG. The emission colour of the S-dots depends on the size, explained by the quantum confinement effect. The dots' luminescent quantum efficiency is strongly affected by the surface sulfur species, which is optimized by the proper surface oxidation. The simple synthesis, excellent luminescence properties, and metal-free nature attract S-dots to optoelectronic and electroluminescence applications.

A Critical Review of Graphene Quantum Dots: Synthesis and Application in Biosensors

Abstract: Graphene quantum dots (GQDs) have aroused widespread attention because of their remarkable properties and potential applications. Herein, we discuss both the top-down and bottom-up strategies for t...

Core-Size-Dependent Trapping and Detrapping Dynamics in CdSe/CdS/ZnS Quantum Dots

Abstract: The fascinating optoelectronic properties of semiconductor quantum dots (QDs) originate from the quantum confinement effect; i.e., the band gap increases as the size decreases. Another significant ...

Stable deep blue emission with unity quantum yield in organic–inorganic halide perovskite 2D nanosheets doped with cerium and terbium at high concentrations

Abstract: Despite recent efforts, achieving stable deep blue emission with near unity quantum yield from lead-free perovskite has remained a great challenge. In this study, we have developed a novel strategy to achieve stable and deep blue emission with absolute unity photoluminescence (PL) quantum yield (QY) through Ce3+ and Tb3+ doping at high concentrations in 2D CH3NH3PbBr3 nanosheets (NSs) using a solvothermal method. We investigated the role of dopants in achieving the high QY and deep blue emission in a 2D perovskite using density functional theory (DFT) based calculations of its electronic structure. Our studies reveal that with Ce/Tb doping, the thickness of the NSs systematically goes down from 10 layers to bilayers (1.4 nm) at high doping levels and the bandgap of the 2D perovskite layer increases from 2.394 eV to 2.981 eV. The measured bandgap widening with doping is analyzed and explained on the basis of the quantum confinement effect and lattice contraction. Interestingly, by incorporating 70 mol% CeBr3 in the perovskite crystal, we achieved a deep blue emitting nanoplatelet with 100% QY, narrow linewidth (∼24 nm), and a color coordinate of (0.145, 0.054) closely matching with the standard color Rec. 2020 (0.131, 0.046) specification, making it one of the most efficient perovskite blue light emitters reported to date. We also demonstrate much improved storage stability of the Ce and Tb doped NS, fully consistent with the DFT calculations. The low temperature PL study reveals the coexistence of ordered and disordered orthorhombic phases. From DFT calculations, we show that the dopants stabilize the structure with lower formation energy and enrich the conduction band edge states without introducing deep trap states, which is responsible for the high PL QY. The calculation also reveals that Tb doping leads to a substantial increase in the bandgap, which is fully consistent with our experimental results. Finally, the Ce3+ doped CH3NH3PbBr3 blue-emitting nanoplatelet is used as a white light LED with CIE coordinates (0.334, 0.326). This work demonstrates a versatile approach to develop rare-earth doped deep blue-emitting 2D perovskites with exceptionally high PL QY and provides new insights into the structural stability and electronic structure of rare-earth doped 2D perovskites.

Band gap tailoring and photoluminescence performance of CdS quantum dots for white LED applications: influence of Ba2+ and Zn2+ ions

Abstract: Cadmium sulfide (CdS) is a suitable candidate in the II–VI wide band gap semiconductor family with robust applications in imaging, optical and optoelectronic fields. In the present investigations, various studies have shown that Ba2+ and Zn2+ ions are well incorporated in the CdS crystal structure without changing the original structure. The particle size was increased slightly as a function of Ba2+ concentration and the reason for this change was ascertained to the change in ionic radius of the Ba2+ and Cd2+ ions. UV–vis spectral studies showed that the absorption intensity was increased more due to the addition of Ba2+ ions and thereby the optical band gap has undergone a red-shift. This band gap narrowing occurrence was ascertained to the quantum confinement effect. The photoluminescence (PL) emission peaks were observed at 384 nm (UV region), 483 nm (blue), 523 nm (green), and 685 nm (red). The Ba2+ ion (2 wt. %) incorporation into Zn:CdS crystal structure showed an excellent optical property and offered various color emissions with high intensity. The primary studies (XRD, SEM) confirmed the crystalline nature and surface morphological information of the samples. The elemental and compositional analysis were made using EDX and FTIR results and they verified the purity of the synthesized nanomaterials. As the composition of Ba2+ and Zn2+ ion-incorporated CdS can be successfully prepared using facile co-precipitation method without using a capping agent, which possesses good absorption in UV region and offers an opportunity to tune the PL emission, these materials can be selected for the solar cells and white LED applications.

Metallic phase transition metal dichalcogenide quantum dots showing different optical charge excitation and decay pathways

Abstract: The charge excitation and decay pathways of two-dimensional heteroatomic quantum dots (QDs) are affected by the quantum confinement effect, bandgap structure and strong exciton binding energy. Recently, semiconducting transition metal dichalcogenides (TMDs) have been intensively studied; however, the charge dynamics of metallic phase QDs (mQDs) of TMDs remain relatively unknown. Herein, we investigate the photophysical properties of TMD-mQDs of two sizes, where the TMD-mQDs show different charge excitation and decay pathways that are mainly ascribed to the defect states and valence band splitting, resulting in a large Stokes shift and two excitation bands for maximum photoluminescence (PL). Interestingly, the dominant excitation band redshifts as the size increases, and the time-resolved PL peak redshifts at an excitation wavelength of 266 nm in the smaller QDs. Additionally, the lifetime is shortened in the larger QDs. From the structural and theoretical analysis, we discuss that the charge decay pathway in the smaller QDs is predominantly affected by edge oxidation, whereas the vacancies play an important role in the larger QDs. Metallic phase transition metal dichalcogenides quantum dots show different pathways of optical charge excitation and decay according to the size and sort of defects, resulting into the large Stoke shift, two bands for charge excitation, and TRPL peak shift. This result is mainly ascribed to the valance band splitting and the emerging defect states originated from atomic vacancy of basal plane and edge oxidation.

Highly efficient and blue-emitting CsPbBr3 quantum dots synthesized by two-step supersaturated recrystallization.

Abstract: Highly efficient and blue-emitting CsPbBr3 quantum dots were successfully synthesized by two-step supersaturated recrystallization under ambient condition. This method could control the particle size within 2.8 nm, thus resulting in strong quantum confinement effect of the products. The as-synthesized CsPbBr3 quantum dots presented outstanding optical properties with highest photo-luminescence quantum yield of 87.20% and longest PL lifetime of 12.24 ns. The blue light-emitting diode made from the CsPbBr3 quantum dots exhibited a CIE coordinate (0.14, 0.10), in good agreement with the standard blue CIE coordinate (0.14, 0.08) of National Television System Committee (NTSC).

Long-time stable colloidal Zn–Ag–In–S quantum dots with tunable midgap-involved emission

Abstract: Quaternary Zn–Ag–In–S (ZAIS) quantum dots (QDs) with efficient, tunable, and stable photoluminescence (PL) emission were prepared via a simple, effective, and low-cost reflux method. The structural analysis revealed the dominance of the quantum confinement effect. The calculated PL emission quantum yield was enhanced from 8.2% to 28.7% with experimental parameters indicating their marked influence on the PL emission properties of the final product. Particularly, it was found that by varying the precursors' feeding ratio, tunable emission from green to red was achieved. A set of direct and indirect pieces of evidence such as the broad-band emission spectrum (FWHM > 100 nm), large Stokes shift more than 120 nm, and predominantly a biexponentially long-lived decay profile with an average lifetime of about 366 ns were observed, showing the contribution of midgap localized energy levels in the recombination process. These data were obtained independently on the experimental condition used, which confirmed that this is mostly an intrinsic electronic property of quaternary In-based QDs. Finally, to ensure the stability of QDs in terms of colloidal and optical emission, their emission ability was evaluated after 26 months of storage. Colloidal QDs were still luminescent with strong yellowish-orange color with emission efficiency of ∼20.3% after 26 months. The combination of synthesis simplicity, compositional non-toxicity, PL emission superiority (strong, tunable, stable, and long lifetime emission), and colloidal stabilities confirms that the present ZAIS QDs are promising candidates for a wide range of applications in biomedicine, anticounterfeiting, and optoelectronics.

Phase transition and topological transistors based on monolayer Na3Bi nanoribbons.

Abstract: Recently, a topological-to-trivial insulator quantum-phase transition induced by an electric field has been experimentally reported in monolayer (ML) and bilayer (BL) Na3Bi. A narrow ML/BL Na3Bi nanoribbon is necessary to fabricate a high-performance topological transistor. By using the density functional theory method, we found that wider ML Na3Bi nanoribbons (>7 nm) are topological insulators, featured by insulating bulk states and dissipationless metallic edge states. However, a bandgap is opened for extremely narrow ML Na3Bi nanoribbons (<4 nm) due to the quantum confinement effect, and its size increases with the decrease in width. In the topological insulating ML Na3Bi nanoribbons, a bandgap is opened in the metallic edge states under an external displacement electric field, with strength (∼1.0 V A−1) much smaller than the reopened displacement electric field in ML Na3Bi (3 V A−1). An ultrashort ML Na3Bi zigzag nanoribbon topological transistor switched by the electrical field was calculated using first-principles quantum transport simulation. It shows an on/off current/conductance ratio of 4–71 and a large on-state current of 1090 μA μm−1. Therefore, a proof of the concept of topological transistors is presented.

Influence of different types of substrates on the physical properties of CdSe films

Abstract: Cadmium Selenide, CdSe, thin films deposited on different substrate types; FTO/glass,/glass, and ITO/glass substrates were produced by thermal evaporation method in the room temperature The influence of different types of substrates on the structural, optical and electrical properties of the films was studied by X-ray diffraction and absorption photo spectroscopy respectively X-ray diffractions revealed that all the CdSe films have a polycrystalline with cubic structure having preferred orientation (111) at 2θ ≈ 251° The optical energy band, Eg, values support the fact that the films have semiconductor behavior which can be attributed to the quantum confinement effect It was observed that the optical properties such as transmittance, reflectance, optical bandgap, and refractive index and some another parameters of CdSe films were strongly affected by types of substrates The electrical properties were measured at room temperature using two probe methods

Observation of quantum-confined exciton states in monolayer WS2 quantum dots by ultrafast spectroscopy

Abstract: Monolayer transition metal dichalcogenide quantum dots (TMDC QDs) could exhibit unique photophysical properties, because of both lateral quantum confinement effect and edge effect. However, there is little fundamental study on the quantum-confined exciton dynamics in monolayer TMDC QDs, to date. Here, by selective excitations of monolayer WS2 QDs in broadband transient absorption (TA) spectroscopy experiments, the excitation-wavelength-dependent ground state bleaching signals corresponding to the quantum-confined exciton states are directly observed. Compared to the time-resolved photophysical properties of WS2 nanosheets, the selected monolayer WS2 QDs only show one ground state bleaching peak with larger initial values for the linear polarization anisotropy of band-edge excitons, probably due to the expired spin–orbit coupling. This suggests a complete change of the band structure for monolayer WS2 QDs. In the femtosecond time-resolved circular polarization anisotropy experiments, a valley depolarization time of ∼100 fs is observed for WS2 nanosheets at room temperature, which is not observed for monolayer WS2 QDs. Our findings suggest a strong state-mixing of band-edge valley excitons responsible for the large linear polarization in monolayer WS2 QDs, which could be helpful for understanding the exciton relaxation mechanisms in colloidal monolayer TMDC QDs.