QD Structures Research Articles

Engineering Lewis-Acid Defects on ZnO Quantum Dots by Trace Transition-Metal Single Atoms for High Glycerol-to-Glycerol Carbonate Conversion.

Efficient conversion of biomass wastes into valuable chemicals has been regarded as a sustainable approach for green and circular economy. Herein, a highly efficient catalytic conversion of glycerol (Gly) into glycerol carbonate (GlyC) by carbonylation with the commercially available urea is presented using low-cost transition metal single atoms supported on zinc oxide quantum dots (M1-ZnO QDs) as a catalyst without using any solvent. A facile one-step wet chemical synthesis allows various types of metal single atoms to simultaneously dope and introduce Lewis-acid defects in the ZnO QD structure. It is found that doping with a trace amount of isolated metal atoms greatly boosts the catalytic activity with Gly conversion of 90.7%, GlyC selectivity of 100.0%, and GlyC yield of 90.6%. Congruential results from both Density Functional Theory (DFT)and in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (in situ DRIFTS) studies reveal that the superior catalytic performance can be attributed to the enriched Lewis acid sites that endow optimal adsorption, formation of the intermediate for coupling between urea and Gly, and desorption of GlyC. Moreover, the tiny size of ZnO QDs efficiently promotes the accessibility of these active sites to the reactants.

Modeling of light absorption in self-assembled truncated conical quantum dot structures

Quantum Dots have shown a significant potential as a top candidate for infrared photodetection at higher temperatures. In the presented work, a theoretical model for estimating the coefficient of optical absorption of self-assembled truncated conical quantum dot is developed. This model considers both bound-to-continuum and bound-to-bound absorption mechanisms that increase the accuracy of the absorption coefficient estimation. The developed model is based on estimating the bound states by diagonalizing the Hamiltonian matrix, where the density of states is computed using the Non-Equilibrium Greens function and the effective mass theory to obtain the unbound states. The kinetic equation of Green’s function is solved numerically by finite difference method. Besides, the effects of quantum dot size, height, aspect ratio, and density on the coefficient of the optical absorption are investigated. The results of the developed model are contrasted with those of other alternative QD structures where the truncated conical QD structure results in a higher absorption coefficient in infrared range than semispherical and conical QD structures.

Lifetime enhancement in QDLEDs via an electron-blocking hole transport layer

This study investigates the impact of an engineered hole transport layer (HTL) on the stability of electroluminescent quantum dot light-emitting devices (QDLEDs). The 9-Phenyl-3,6-bis(9-phenyl-9Hcarbazol-3-yl)-9H-carbazole (Tris-PCz) HTL, which possesses a shallower lowest unoccupied molecular orbital (LUMO) energy level compared to the widely used 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) HTL, is employed to confine electron overflow toward the HTL. Utilizing the Tris-PCz HTL results in a 20× improvement in the electroluminescence half-life (LT50) of QDLEDs compared with conventional QDLEDs using the CBP HTL. Electric and optoelectronic analyses reveal that the migration of excess electrons toward the HTL is impeded by the up-shifted LUMO level of Tris-PCz, contributing to prolonged operational device stability. Furthermore, the augmented electric field at the QD/Tris-PCz interface, due to accumulated electrons, expedites hole injection rates, leading to better charge injection balance and the confinement of the exciton recombination zone within the QD and thus the device stability enhancement. This study highlights the significant influence of the HTL on QDLED stability and represents one of the longest LT50 for a QDLED based on the conventional core/shell QD structure.

Soft ferromagnetic effect in FePc/CdS hybrid diluted magnetic organic/inorganic quantum dots

Manipulating magnetic interactions in diluted magnetic quantum dots (DMQDs) is a fundamental research field due to the potential for developing robust magnetic semiconducting materials that can operate effectively in various environmental conditions. Here we design a new class of DMQDs via hybridization of inorganic CdS QDs with organic iron phthalocyanine (FePc) molecules. By a combination of X-ray photoemission, Mӧssbauer spectroscopies, and vibrating sample magnetometry, we demonstrate that FePc molecules alter the original diamagnetic nature of CdS QDs. They turn them into room-temperature soft-ferromagnets, while preserving the CdS QD structure from further oxidation in a more efficient way than the standard Fe doping of CdS QDs. These hybrid materials open up new possibilities for the design of DMQDs of tunable and robust magnetism for optomagnetic and spintronic applications.

J–V characteristics of dark current in truncated conical quantum dot infrared photodetectors (QDIPs)

Quantum Dot Infrared Photodetector (QDIP) is one of the promising candidates for infrared photodetection due to its controllable heterojunction bandgap and sensitivity to normal incident radiation. It is expected to be superior to infrared photodetectors of mature technologies such as Mercury Cadmium Telluride (HgCdTe) or a quantum well infrared photodetector. In the presented paper, we have developed a theoretical model for the dark current in truncated conical QDIP as the truncated conical shaped QD structure is more appropriate to describe the fabricated dots. The dark current model is based on the drift diffusion model solving the main governing Poisson’s and continuity equations. In this model, the carrier mobility is calculated by solving time-dependent Boltzmann transport equation in the photodetector material with embedded truncated conical QDs using finite difference technique. The results of the developed model have been compared with the dark current characteristics with published experimental results of Indium Arsenide/Gallium Arsenide (InAs/GaAs) truncated QDIP. The effects of QD volume, QD aspect ratio and QD density and the operating temperature on the dark current characteristics have also been investigated.

Review: Quantum Dot Light-Emitting Diodes.

Quantum dot light-emitting diodes (QD-LEDs) are one of the most promising self-emissive displays in terms of light-emitting efficiency, wavelength tunability, and cost. Future applications using QD-LEDs can cover a range from a wide color gamut and large panel displays to augmented/virtual reality displays, wearable/flexible displays, automotive displays, and transparent displays, which demand extreme performance in terms of contrast ratio, viewing angle, response time, and power consumption. The efficiency and lifetime have been improved by tailoring the QD structures and optimizing the charge balance in charge transport layers, resulting in theoretical efficiency for unit devices. Currently, longevity and inkjet-printing fabrication of QD-LEDs are being tested for future commercialization. In this Review, we summarize significant progress in the development of QD-LEDs and describe their potential compared to other displays. Furthermore, the critical elements to determine the performance of QD-LEDs, such as emitters, hole/electron transport layers, and device structures, are discussed comprehensively, and the degradation mechanisms of the devices and the issues of the inkjet-printing process were also investigated.

Enhanced current generation in quantum-dot intermediate band solar cells through optimizing the position of quantum dot layers

We present a numerical study using a self-consistent drift-diffusion approach to characterize the effect of light trapping on the quantum efficiency (QE) of a thin-film quantum dot intermediate band solar cell (QD-IBSC). In the concept of the IBSCs, the photons with energies lower than the bandgap of the host material can be absorbed in QDs due to the presence of the IB, leading to high photocurrent and conversion efficiency. However, in general, the quality of the QD structure is degraded due to the accumulated strain when the number of QD layers stacked vertically is increased, which is required to increase light absorption. In the present work, we adopted a Fabry-Pérot (FP) resonance structure to achieve high absorption with fewer QD layers. We demonstrate numerically, in a InAs/GaAs-based QDSC structure, that an increase in the QE by 8 times at maximum is achievable by tuning the position of the QD layers inserted.

Suppressing the Effect of the Wetting Layer through AlAs Capping in InAs/GaAs QD Structures for Solar Cells Applications.

Recently, thin AlAs capping layers (CLs) on InAs quantum dot solar cells (QDSCs) have been shown to yield better photovoltaic efficiency compared to traditional QDSCs. Although it has been proposed that this improvement is due to the suppression of the capture of photogenerated carriers through the wetting layer (WL) states by a de-wetting process, the mechanisms that operate during this process are not clear. In this work, a structural analysis of the WL characteristics in the AlAs/InAs QD system with different CL-thickness has been made by scanning transmission electron microscopy techniques. First, an exponential decline of the amount of InAs in the WL with the CL thickness increase has been found, far from a complete elimination of the WL. Instead, this reduction is linked to a higher shield effect against QD decomposition. Second, there is no compositional separation between the WL and CL, but rather single layer with a variable content of InAlGaAs. Both effects, the high intermixing and WL reduction cause a drastic change in electronic levels, with the CL making up of 1–2 monolayers being the most effective configuration to reduce the radiative-recombination and minimize the potential barriers for carrier transport.

Hydrothermal synthesis of water-soluble Mn- and Cu-doped CdSe quantum dots with multi-shell structures and their photoluminescence properties.

Optical properties of semiconductor quantum dots (QDs) can be tuned by doping with transition metal ions. In this study, water-soluble CdSe/ZnS:Mn/ZnS QDs with the core/shell/shell structure were synthesized through a hydrothermal method, in which the surface of the CdSe core was coated with a ZnS:Mn shell and ZnS capping shell. Herein, the CdSe core QDs were prepared first and then doped with Mn2+; therefore, the QD size and doping level could be controlled independently and interference from the self-purifying effect could be avoided. When CdSe cores with diameters less than 1.9 nm were used, Mn-related photoluminescence (PL) was observed as the main PL band, whereas the band-edge PL was mainly observed when larger CdSe cores were used. Furthermore, using ZnS:Cu as the doping shell layer, CdSe/ZnS:Cu/ZnS and ZnSe/ZnS:Cu/ZnS nanoparticles were successfully synthesized, and Cu-related PL was clearly observed. These results indicate that the core/shell/shell QD structure with doping in the shell layer is a versatile method for synthesizing doped QDs.

Modeling and characterization of carrier mobility for truncated conical quantum dot infrared photodetectors

In the present paper, a theoretical model for calculating the carrier mobility which is a result of the existence of a truncated conical quantum dots of n-type quantum dot infrared photodetectors (QDIPs) is developed. This model is built on solving Boltzmann’s transport equation that is a complex integro-differential equation describing the carrier transport. The time-domain finite-difference method is used in this numerical solution. The influences of dimensions and density of the QDs for this structure on the carrier mobility are studied. Eventually, the calculated mobility for truncated conical InAs/GaAs QDIP is contrasted to other conical, spherical, and hemispherical QD structures. The model put forward is a generic model that is applicable to various structures of truncated conical QDs devices.

Effect of doping on the opto-electronic properties of hollow spheroid ZnO quantum dots: A theoretical study

The hollow spheroid (HS) (ZnO)m QDs are computationally designed and optimized using DFT method. Again the optimized structures doped with F and La-dopant in each QD structures were also calculated by the same DFT method. The partial density of states (PDOS) for the F and La-doped (ZnO)m QDs were calculated and found that the La-doped bands are highly mixed with (ZnO)m QDs than the F-doped bands. The study of the excitation spectra shows that there is a blue shift in the undoped (ZnO)m QDs and clear red shifts in the F-doped and La-doped (ZnO)m QD structures. It is found that the F-doped (ZnO)m QDs have more energy gap than the La-doped (ZnO)m QDs in the calculation of HOMO-LUMO gap of the studied QDs. The electrostatic potential surface around the optimized F and La-doped (ZnO)m QDs were plotted and the result showed the potential on the surface with positive and negative surfaces.

Ag3PO4 NP@MoS2 nanosheet enhanced F, S-doped BN quantum dot electrochemiluminescence biosensor for K-ras tumor gene detection

In this research, a novel Ag3PO4 NPs@MoS2 nanosheet-based electrochemiluminescence (ECL) sensing system was developed to provide an effective method for tumor gene detection. At first, fluorine, sulfur-doped BN quantum dot (F, S-BN QD) were prepared as ECL emitter. Sulfur dopant can provide more reactive sites in the ECL reaction. Fluorine atoms in the QD structure further improved the stability of the crystal. Furthermore, Ag3PO4 NP@MoS2 nanosheets were fabricated via a hydrothermal route as ECL reaction catalyst. On the one hand, Ag3PO4 NP@MoS2 nanosheets promoted the generation of more oxidant of coreactant in the F, S-BN QD/H2O2 coreactant ECL pathway. On the other hand, the excellent conductivity of Ag3PO4 NP@MoS2 nanosheets facilitated the electron transfer and effectively reduce the damage of F, S-BN QD by excessive hot electrons. Finally, the proposed biosensor was designed to accurately quantify K-ras tumor gene from 10 fM to 100 pM with a limit of detection (LOD) of 0.2 fM. The sensing system was used to detect K-ras gene in human colorectal cancer tumor and tumor-adjacent tissues samples with satisfactory results. The amplified ECL sensing strategy with Ag3PO4 NPs@MoS2 nanosheet has significant potential value in the clinical detection.

Short-wave infrared (SWIR) photodetection by InAs/GaAs quantum dot heterostructures grown on Ge (100) substrate without Migration Enhanced Epitaxy layer

Integration of III-V based optoelectronic devices with Silicon (Si) technology is the prime choice of researchers for the futuristic Si-photonics. Profusion research is going on for direct growth of III-V heterostructures on the Germanium (Ge) substrate. Also, monolithic integration of III-V based device structures on Si using Ge layer and Migration Enhanced Epitaxy (MEE) technique have been endeavored. In this study, InAs based quantum dot infrared photodetectors (QDIPs) are grown on Ge substrate without MEE layers. The GaAs/AlAs superlattice buffer (SLB) layers are used to suppress the defects and dislocations. Two different QDIPs with GaAs capped and In 0.15 Ga 0.85 As/GaAs capped InAs QD are grown and characterized using photoluminescence (PL), high-resolution X-Ray Diffraction (HRXRD), and Raman spectroscopy (RS). Red-shifted and narrow PL spectrum of the InGaAs/GaAs capped QDIP with respect to the GaAs capped QDIP attributes to formation of larger dots and homogeneous size distribution. Low strain and high activation energy in the InGaAs capped QDs signify better strain relaxation via the InGaAs strain reducing layer (SRL) and better carrier confinement. Interband-assisted near-infrared (NIR) photoresponse is obtained from both devices till room temperature. However, the InGaAs/GaAs capped QDIP shows shortwave infrared (SWIR) response, which promulgates the formation of QDs with better crystallinity due to the presence of InGaAs SRL. The room temperature (300 K) dark current density of the GaAs and InGaAs capped QDIP is 1.2 × 10 −1 A/cm 2 and 6.6 × 10 −3 A/cm 2 respectively with −2 V bias voltage. This work demonstrates the pathway to uncomplicated growth of high quality QDIP structures on Ge substrate. • A novel technique is used to grow defect free InAs QD structures on Ge substrate. • GaAs/AlAs SLB layers are used to suppress the defects and dislocations. • InAs QDs with GaAs and InGaAs/GaAs capping are investigated and compared. • Both structures show low dark current at room temperature. • Infrared photoresponse till room temperature is obtained for both QDIPs.

A comprehensive analysis of strain profile in the heterogeneously coupled Stranski-Krastanov (SK) on Submonolayer (SML) quantum dot heterostructures

In this study, we demonstrate a detailed strain analysis of the heterogeneously coupled Stranski Krastanov (SK) on Submonolayer (SML) InAs quantum dot heterostructures with higher (0.1 ML/s) and lower (0.05 ML/s) growth rate of InAs. This hybrid structure utilizes advantages of both SK and SML QDs, and facilitates superior characteristics in the near-infrared energy harvesting. The spacer layer thickness between SK and SML QDs have been varied as 5, 7.5 and 10 nm to modulate the coupling between them and to get a better insight of the strain profile. Such strain modulation inside the heterostructure significantly affects the dot density and dot size distribution and thereby would ultimately impact the optoelectronic properties. It was observed that with increased spacer layer thickness, there was decrement in both in-plane and out-plane strain distribution within the entire heterostructure. The strain field obtained from the theoretical simulation of SK on SML QD structures supported the experimental results. In addition, the quantum dot infrared photodetector has been fabricated utilizing the optimized SK on SML QD heterostructure with the spacer layer thickness of 7.5 nm, which provided spectral response in the range of 2–6 μm and lower dark current as low as 1.05E-7 A/cm2 at 75 K at a bias of 1.5 V.

The 1200 nm‐Band InAs/GaAs Quantum Dot Intermixing by Dry Etching and Ion Implantation

In this article, the quantum dot intermixing (QDI) technique previously developed for 1550 nm‐band InAs/InAlGaAs QD is applied to 1200 nm‐band InAs/GaAs QD. Three methods of defect introduction for triggering the QDI are used such as inductively coupled plasma reactive ion etching (ICP‐RIE) (Ar+) and ion implantation (Ar+ and B+). As a result, about 80 nm photoluminescence (PL) peak wavelength shift is obtained for ICP‐RIE when annealing is performed at 575 °C, after etching down to 450 nm to the QD layer. On the contrary, about 110 nm PL peak wavelength shift is obtained for B+ ion implantation at an acceleration energy of 120 keV and a dose of 1.0 × 1014 cm−2 and subsequent annealing. Cross‐sectional image analyses by scanning transmission electron microscope (STEM) and energy‐dispersive X‐ray spectroscopy (EDX) clarified the modification of InAs QD structures by the QDI process.

Significant Enhancement in Quantum Dot Light-Emitting Device Stability via a Cascading Hole Transport Layer.

This work investigates the effect of the hole transport layer (HTL) on the stability of electroluminescent quantum dot light-emitting devices (QDLEDs). The electroluminescence half-life (LT50) of QDLEDs can be improved by 25× through the utilization of a cascading HTL (CHTL) structure with consecutive steps in the highest occupied molecular orbital energy level. Using this approach, a LT50 of 864,000 h (for an initial luminance of 100 cd m-2) is obtained for red QDLEDs using a conventional core/shell QD emitter. The CHTL primarily improves QDLED stability by shifting excessive hole accumulation away from the QD/HTL interface and toward the interlayer HTL/HTL interfaces. The wider electron-hole recombination zone in the CHTL for electrons that have leaked from the QD layer results in less HTL degradation at the QD/HTL interface. This work highlights the significant influence of the HTL on QDLED stability and represents the longest LT50 for a QDLED based on the conventional core/shell QD structure.

Modeling of carrier mobility for semispherical quantum dot infrared photodetectors (QDIPs)

Carrier mobility for quantum dot infrared photodetectors is considered as one of the critical parameters to determine many important device’s performance parameters such as the electrical conductivity, drift velocity, dark current and photocurrent. In this paper a complete theoretical model of the carrier mobility for semispherical quantum dot structures is developed. This model is based on the solution of Boltzmann transport equation all over the device. A parametric study of the effects of the QD density and the dimensions of the QD on the carrier mobility is investigated. Finally, the carrier mobility in semispherical QDIP is compared with other QD structures. The presented model is a generic model that can be applied for different semispherical QD structures.

(Invited) Silicon Coupled Quantum Dots for Spin-Based Quantum Bits

Recently quantum computation devices have been intensively studied in various systems. Spins in silicon quantum dots (Si QDs) are one of the promising candidates for implementing quantum bits (qubits). Since only 4.7% of Si atoms are isotopes with nuclear spins, long coherence times of electron spins reflecting small hyperfine interactions were demonstrated [1-3]. Moreover, using isotopically-engineered Si [4], qubits with extended coherence time and gate operation with high fidelity were demonstrated [5-7]. Si qubit research has accelerated following the demonstration of these high fidelity single qubit operations and two-qubit logic gates [8]. The major remaining issue is large scale integration of qubits. The advantage of Si qubits is that matured CMOS technologies can be applied for integration of qubits. At Tokyo Tech, we have studied physically-defined coupled QDs in MOS structures prepared on Si-on-insulator (SOI) substrates [9-20], which are promising for high density integration. The physically-defined devices do not require gates to create confinement potentials for the QDs. These devices thus represent a technological simplification owing to the reduced number of gates. Common fabrication processes normally used to produce Si MOS was applied to make the physically-defined Si QDs. We have developed various device structures of physically-defined Si QDs. By measuring double QD devices, we succeeded in charge sensing of few-electron regime [11, 15, 16] and observation of Pauli-spin blockade [12]. In triple QDs arranged in an equilateral triangle, we demonstrated for the first time in the world a silicon-based two-dimensional arrayed QD structure [20]. We also fabricated and characterized Si double QDs in p-channel MOS and few-hole regime and Pauli-spin blockade were observed [14, 18, 19]. Furthermore, we formed Si single QDs on ultrathin (~6 nm) SOI and obtained comparatively large charging energy (~ 20 meV). We performed three-dimensional calculations of capacitance matrix and transport properties through the QD and found a good quantitative agreement with experiment [17]. These achievements are the important steps for realizing quantum computation devices. We have also studied gate-defined QDs in Si/SiGe heterostructures to demonstrate high-fidelity gate operation through joint research with the University of Tokyo [3,6,7]. The fusion of Si devices and established GaAs qubit technology was advanced. Manipulation of spin states in Si QDs and single-shot readout using a measurement system constructed through development of GaAs qubits, demonstrating that the coherence time is about 100 times longer than that of GaAs systems [3]. The fidelity of single-qubit gate operation has been demonstrated to be 99.6% for native Si QDs [3] and 99.93% for isotopically-engineered Si QDs [6]. Both fidelity values are above the quantum error correction threshold of 99%. This work was financially supported by Q-LEAP of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and CREST (JPMJCR1675) of Japan Science and Technology Agency (JST) in Japan. B.M. Maune, et al., Nature 481, 344 (2012).E. Kawakami, et al., Nature Nanotech. 9, 666 (2014).K. Takeda, TK, et al., Sci. Adv., 2, 8, e1600694 (2016)K.M. Itoh and H. Watanabe, Mater. Res. Soc. Commun. 4, 143 (2014).M. Veldhorst, et al., Nature Nanotech. 9, 981 (2014).J. Yoneda, TK, et al., Nature Nanotech., 13, 102 (2018)K. Takeda, TK, et al., npj Quantum Information 4, 54 (2018)M. Veldhorst, et al., Nature 526, 410 (2015).A. Sulthoni, TK, et al., J. Appl. Phys., 110, 054511 (2011).A. Rossi, TK, et al., Appl. Phys. Lett. 98, 133506 (2011). K. Horibe, TK, et al., J. Appl. Phys. 111, 093715 (2012).G. Yamahata, TK, et al., Phys. Rev. B 86, 115322 (2012).T. Ferrus, TK, et al., New J. Phys. 16, 1, 013016 (2014).K. Yamada, TK, et al., Appl. Phys. Lett. 105, 113110 (2014).K. Horibe, TK, et al., Appl. Phys. Lett. 106, 083111 (2015).K. Horibe, TK, et al., Appl. Phys. Lett. 106, 053119 (2015).S. Ihara, TK, et al., Appl. Phys. Lett. 107, 013102 (2015)Y. Yamaoka, TK, et al., Jpn. J. Appl. Phys. 56, 04CK07 (2017)S. Mizoguchi, TK, et al., Jpn. J. Appl. Phys. 57, 04FK03 (2018)R. Mizokuchi, TK, et al., Appl. Phys. Lett. 114, 073104 (2019)

Facile in situ Synthesis of Ag-Doped CdSe Supra-Quantum Dots and their Characterization.

A supra-quantum dot (SQD) is a three-dimensionally assembled QD structure composed of several hundreds to thousands of QDs connected through oriented attachments. Owing to their three-dimensional interconnected structures and relatively large volumes, impurity atoms are thermodynamically more stable in SQDs than in conventional QDs. Herein, we report the facile in-situ synthesis of colloidal Ag-doped CdSe SQDs. Ag dopants were efficiently incorporated into CdSe SQDs through the three-dimensional interconnection of Ag-doped primary CdSe QDs, as confirmed by elemental analysis combined with chemical etching. Photoelectron spectroscopic studies revealed that the Ag-doped CdSe SQDs exhibit n-type doping behavior, since the valence electrons from the interstitial Ag atoms are directly donated to the lattice.

InTlSb quantum dot structures

Abstract In this work, total (linear and nonlinear) optical absorption from InTl1−xSbx QD structures was studied. It is shown that these Tl-based QD structures have high absorption at long wavelengths. Nonlinear absorption at high power levels was preferred at some slow-light applications. The structures studied assess the possibility of infrared applications of Tl-based structures.