Quantum Dot Structures Research Articles

Evaluation of the potential distribution in a multiple stacked Si quantum dots structure by hard X-ray photoelectron spectroscopy

Six-fold stacked Si quantum dots (Si-QDs) structures with ultrathin SiO2 interlayers were formed on ultrathin SiO2 layer/n-Si substrates by low-pressure chemical vapor deposition using a SiH4 gas and their vertical electric potential distributions were evaluated using hard X-ray photoelectron spectroscopy under a DC bias application to the semitransparent electrodes formed on the stacked Si-QDs structure. The Si1s photoelectron spectra due to Si–Si bonding units can be deconvoluted into seven components corresponding to the six Si-QDs layers and Si substrate. Obviously, the energy shift between the components for two adjacent dots layers becomes larger towards the upper side of the stacked dots structure. This result indicates that the electric field concentrates on the upper side and is consistent with the proposed model that can explain the ballistic electron emission characteristics from multiple stacked Si-QDs structures.

Role of Surface Morphology on Exciton Recombination in Single Quantum Dot-in-Rods Revealed by Optical and Atomic Structure Correlation.

The physical structure of colloidal quantum dot (QD) nanostructures strongly influences their optical and electronic behavior. A fundamental understanding of this interplay between structure and function is crucial to fully tailor the performance of QDs and their assemblies. Here, by directly correlating the atomic and chemical structure of single CdSe-CdS quantum dot-in-rods with time-resolved fluorescence measurements on the same structures, we identify morphological irregularities at their surfaces that moderate photoluminescence efficiencies. We find that two nonradiative exciton recombination mechanisms are triggered by these imperfections: charging and trap-assisted nonradiative processes. Furthermore, we show that the proximity of the surface defects to the CdSe core of the core-shell structures influences whether the charging or trap-assisted nonradiative channel dominates exciton recombination. Our results extend to other QD nanostructures and emphasize surface roughness as a crucial parameter when designing colloidal QDs with specific excitonic fates.

Effect of Cadmium Source on the Structure and Optical Properties of CdTe Quantum Dots

Effect of Cadmium Source on the Structure and Optical Properties of CdTe Quantum Dots

Structure, optical property, and reaction process of Si quantum dots embedded in an amorphous silicon carbide matrix

Silicon quantum dots (QDs) embedded in an amorphous silicon carbide matrix were prepared using silane, methane, and hydrogen as reactive gases at a low substrate temperature by radio-frequency plasma enhanced chemical vapor deposition followed by thermal annealing at 1000 °C in a N2 atmosphere in the quartz furnace. The structure, optical properties, and reaction process of the synthesized Si QDs embedded in an amorphous silicon carbide matrix under different discharge powers are investigated. With the increase in the discharge power from 40 to 100 W, the experimental result measured by X-ray diffraction, Raman spectroscopy, UV-Visible spectroscopy, and field emission scanning electron microscopy reveals that the average size and the crystal volume fraction of Si QDs decrease from 4.4 to 3.4 nm and from 61.1% to 46.3%, respectively, while the optical bandgap and the deposition rate increase from 1.75 to 1.96 eV and from 15.5 to 16.5 nm/min, respectively. Moreover, the real-time diagnosis of plasma by optical emission spectroscopy (OES) is used to determine the chemical species and excitation temperature under the glow discharge of silane, methane, and hydrogen gas mixtures. Based on the OES measurement, the chemical reactions happening in the plasma and growth mechanism for the synthesis of Si QDs embedded in an amorphous silicon carbide matrix have been proposed. This work plays a significant role in preparation of the Si QDs embedded in an amorphous silicon carbide matrix for third-generation photovoltaic solar cells.

Utilizing embedded ultra-small Pt nanoparticles as charge trapping layer in flashristor memory cells

In this study, a methodology for producing highly controlled and uniformly dispersed metal nanoparticles were developed by atomic layer deposition (ALD) technique. All-ALD grown thin film flash memory (TFFM) cells and their applications were demonstrated with ultra-small platinum nanoparticles (Pt-NPs) as charge trapping layer and control tunnel oxide layer. The ultra-small Pt-NPs possessed sizes ranging from 2.3 to 2.6 nm and particle densities of about 2.5 × 1013 cm–b. The effect of Pt-NPs embedded on the storage layer for charging was investigated. The charging effect of ultra-small Pt-NPs the storage layer was observed using the electrical characteristics of TFFM. The Pt-NPs were observed by a high-resolution scanning electron microscopy (HR-SEM). The memory effect was manifested by hysteresis in the IDS-VDS and IDS-VGS curves. The charge storage capacity of the TFFM cells demonstrated that ALD-grown Pt-NPs in conjunction with ZnO layer can be considered as a promising candidate for memory devices. Moreover, ZnO TFFM showed a ION/IOFF ratio of up to 52 orders of magnitude and its threshold voltage (Vth) was approximately −4.1 V using Ids−a/b – Vgs curve. Fabricated TFFMs exhibited clear pinch-off and show n-type field effect transistor (FET) behavior. The role of atomic-scale controlled Pt-NPs for improvement of devices were also discussed and they indicated that ALD-grown Pt-NPs can be utilized in nanoscale electronic devices as alternative quantum dot structures.

Tunneling-induced ultraslow solitons in triangular quantum dot molecules

We theoretically investigate the propagation of a weak probe laser pulse in a triangular quantum dot molecules scheme based on the tunneling induced transparency. We find that the ultraslow optical solitons can be realized due to the destructive quantum interference induced by the interdot tunneling coupling which can be adjusted by the gate voltage appropriately. This work may provide practical applications such as electro-optic modulated devices and other information processes in semiconductor quantum dots structure.

Investigation of carrier confinement in direct bandgap GeSn/SiGeSn 2D and 0D heterostructures

Since the first demonstration of lasing in direct bandgap GeSn semiconductors, the research efforts for the realization of electrically pumped group IV lasers monolithically integrated on Si have significantly intensified. This led to epitaxial studies of GeSn/SiGeSn hetero- and nanostructures, where charge carrier confinement strongly improves the radiative emission properties. Based on recent experimental literature data, in this report we discuss the advantages of GeSn/SiGeSn multi quantum well and quantum dot structures, aiming to propose a roadmap for group IV epitaxy. Calculations based on 8-band k∙p and effective mass method have been performed to determine band discontinuities, the energy difference between Γ- and L-valley conduction band edges, and optical properties such as material gain and optical cross section. The effects of these parameters are systematically analyzed for an experimentally achievable range of Sn (10 to 20 at.%) and Si (1 to 10 at.%) contents, as well as strain values (−1 to 1%). We show that charge carriers can be efficiently confined in the active region of optical devices for experimentally acceptable Sn contents in both multi quantum well and quantum dot configurations.

Optimal electromechanical control of the excitonic fine structures of droplet epitaxial quantum dots

The intrinsic fine structure splittings (FSSs) of the exciton states of semiconductor quantum dots (QDs) are known to be the major obstacle for realizing the QD-based entangled photon pair emitters. In this study, we present a theoretical and computational investigation of the excitonic fine structures of droplet-epitaxial (DE) GaAs/AlGaAs QDs under the electro-mechanical control of micro-machined piezoelectricity actuators. From the group theory analysis with numerical confirmation based on the developed exciton theory, we reveal the general principle for the optimal design of micro-machined actuators whose application on to an elongated QD can certainly suppress its FSS. We show that the use of two independently tuning stresses is sufficient to achieve the FSS-elimination but is not always necessary as widely deemed. The use of a single tuning stress to eliminate the FSS of an elongated QD is possible as long as the crystal structure of the actuator material is in coincidence with that of the QD. As a feasible example, we show that a {\it single} symmetric bi-axial stress naturally generated from the $(001)$ PMN-PT actuator can be used as a single tuning knob to make the full FSS-elimination for elongated DE GaAs QDs.

Green Preparation of High Yield Fluorescent Graphene Quantum Dots from Coal-Tar-Pitch by Mild Oxidation.

Coal tar pitch (CTP), a by-product of coking industry, has a unique molecule structure comprising an aromatic nucleus and several side chains bonding on this graphene-like nucleus, which is very similar to the structure of graphene quantum dots (GQDs). Based on this perception, we develop a facile approach to convert CTP to GQDs only by oxidation with hydrogen peroxide under mild conditions. One to three graphene layers, monodisperse GQDs with a narrow size distribution of 1.7 ± 0.4 nm, are obtained at high yield (more than 80 wt. %) from CTP. The as-produced GQDs are highly soluble and strongly fluorescent in aqueous solution. This simple strategy provides a feasible route towards the commercial synthesis of GQDs for its cheap material source, green reagent, mild condition, and high yield.

L-glutathione capped CdSeS/ZnS quantum dot sensor for the detection of environmentally hazardous metal ions

L-Glutathione functionalized CdSeS/ZnS quantum dots (QDs) were synthesized and tested as sensor for the detection of environmentally hazardous metal ions (Pb2+, Zn2+, Cr3+, Co2+ and Cu2+). L-GSH-CdSeS/ZnS QDs were synthesized by aqueous phase route and characterized by UV–visible and FTIR spectrometry. The structural properties were investigated by the scanning electron microscopy (SEM) image and energy-dispersive X-ray spectroscopy (EDX) analysis. The surface modification with L-GSH stabilizes CdSeS/ZnS QDs structure, decreases its toxicity and makes suitable for interaction with targeted subjects. Optical response of L-GSH-CdSeS/ZnS is observed to be dependent on pH of environment and lifetime of QDs. A strong “Turn-On” response is observed for Pb2+, Zn2+ and “Turn-Off” response is observed for Cr3+. Organic capping makes CdSeS/ZnS QDs suitable candidate to be used as sensing material for the development of selective and sensitive sensor to monitor the hazardous analytes in aqueous environment. This sensing system exhibits very low detection limit. The practical use of this system for metal ions determination in real samples is also demonstrated successfully.

Direct Measurement of Electronic Band Structure in Single Quantum Dots of Metal Chalcogenide Composites.

Metal chalcogenide quantum dots (QDs) are among the most promising materials as light harvesters in all-inorganic systems for applications in solar cells and production of solar fuels. The electronic band structure of composite QDs formed by lead and cadmium chalcogenides directly grafted on highly oriented pyrolytic graphite surfaces through successive ionic layer absorption and reaction is investigated. Atomic force microscopy and Kelvin probe force microscopy (KPFM) are applied to investigate PbS, CdS, and PbS/CdS QD systems. The variation of the surface potential of individual QDs is measured, investigating the evolution of the electronic band structure as a function of QD size and composition. A shift of the Fermi level toward more negative values occurs when QD size is increased. The shift is more pronounced in CdS than in PbS, while the composite PbS/CdS exhibits an intermediate behavior. The calculated shift is in good agreement with the experiments. These results highlight the ability of KPFM to directly measure the electronic band structure in individual QDs of metal chalcogenide composites. This feature regulates charge dynamics in composite systems, thereby affecting device performance. This work provides valuable insights for applications in several fields, in which charge injection plays a major role.

Fluorescent CdTe-QD-encoded nanocellulose microspheres by green spraying method

With water-soluble CdTe quantum Dots (QDs) as fluorescent labels and TEMPO-oxidized cellulose nanofibrils (CNF) as matrix, dual-color QD-encoded microspheres were prepared by mechanical spraying, freeze-molding and subsequent electrostatic cross-linking by calcium ion. The entire preparation process was accomplished in aqueous phase without any chemical treatment. The nanocellulose microspheres possessed porous structure and were able to be dispersed stably in water in the form of hydrogels. CdTe QDs were efficiently fixed in the network of intertwined nanofibrils via hydrogen bonding with CNF, making the microspheres to obtain high photoluminescence (PL) intensity and identifiable encoding signals consisting of seven kinds of microsphere arrays. The crystalline structure and chemical composition of both CdTe QDs and CNF were not affected during the preparation process. The comparision of PL behaviors of CdTe QDs and QD-encoded microspheres in different media indicate that CNF matrix provided good protection of QDs and improved the PL intensity and photostability. Dual-color CdTe-encoded nanocellulose microspheres were prepared via pure physical process in aqueous phase. The fluorescent microspheres possessed high photoluminescence intensity and identifiable seven kinds of encoding signals. The nanocellulose matrix provided good protection of CdTe QDs and improved the photoluminescence intensity and photostability.

Dependence of the Photoluminescence of Hydrophilic CuInS2 Colloidal Quantum Dots on Cu-to-In Molar Ratios

Copper indium sulfide (CIS) quantum dots (QDs) are attractive as potential optoelectronic devices, since they have large absorption coefficients across a broad spectral range, size- and composition-tunable photoluminescence from the visible to the near-infrared, and low toxicity. However, CIS QDs should be hydrophilic in certain applications. In this present work, hydrophilic CIS and CIS/ZnS colloidal QDs were synthesized by a one-pot environmentally-friendly route employing N,N-dimethylformamide (DMF) as the solvent and in situ generated H2S as the sulphur precursor. The effect of stoichiometry was investigated by varying the feeding molar ratios of Cu/In, which did not affect the crystalline structure of the CIS colloidal QDs, while the optical property was influenced significantly. With the decrease of the feeding molar ratios of Cu/In, the band gap of the resulting CIS colloidal QDs had blue-shifts and relative photoluminescence quantum yield of the corresponding CIS/ZnS colloidal QDs increased monotonically.

Effect of amine type on the structure and luminescent properties of CdSe quantum dots

A series of CdSe QDs are synthesized using four kinds of ligands including dodecylamine, tetradecylamine, 1-hexadecanamine and octadecylamine by organic metal preparation method. The XRD patterns revealed that all the CdSe QDs with different amine ligands belong to cubic phase. The monodisperse CdSe QDs are obtained and their average size increases with the Carbon chain length of amine ligands. The absorption peaks of CdSe QDs capping with different amines shift from 526 to 557 nm. The results indicated that the luminescent properties and size of CdSe QDs can be modulated by amine ligand in the process of synthesis.

Performance of quantum-dot-based tunnel-injection lasers: A theoretical analysis

Tunnel-injection lasers promise advantages in the modulation bandwidth and temperature stability in comparison with conventional laser designs. In this paper, we present results of a microscopic theory for laser properties of tunnel-injection devices and a comparison with a conventional quantum-dot laser structure. In general, the modulation bandwidth of semiconductor lasers is affected by the steady-state occupations of electrons and holes via the presence of spectral hole burning. For tunnel-injection lasers with InGaAs quantum dot emitting at an telecom wavelength of 155 μm, we demonstrate that the absence of spectral hole burning favors this concept over conventional quantum-dot based lasers.

In0.5Ga0.5As Bilayer Quantum Dot Heterostructure for mid-infrared photodetection

Strain-coupled Bilayer quantum dot (QD) heterostructures have taken over single layer QD structures due to their longer wavelength emission, homogeneity in dot size distribution and optimum dot density. The optoelectronic characteristics of In0.5Ga0.5As Bilayer QDIP along with photoresponse in the mid-infrared regime have been shown in this article. The heterostructure is grown with a 6.5 nm GaAs spacer between the seed and top QD layers, following a new growth strategy. The monolayer coverage for top QD layer is less compared to the seed layer. However, a similar QD size has been observed for both layers due to the existing strain propagation from seed to top QD layer through the GaAs spacer. Moreover, the presence of strain reducing In0.15Ga0.85As capping layer on the top QD has an active participation in preserving the QD size, inhibiting In-Ga intermixing. The room temperature photoluminescence (PL) emission peak from the device is obtained near communication wavelength (1.31 μm). The lower full width half maximum (FWHM) of 21 meV and higher activation energy of 326 meV for the QDIP attribute homogeneity in dot size distribution and higher barrier potential respectively. The proposed Bilayer QDIP has a broad spectral response in the mid-infrared regime (6–8 μm) with very low dark current density (4E−7 A/cm2) and high responsivity (33 mA/W) at −2V and 75 K. The measured efficiency for the QDIP is 0.18%.

Phonon replicas of type-II GaSb/GaAs quantum dot structure grown by liquid phase epitaxy

Type-II GaSb/GaAs quantum dot (QD) structures have attracted a great deal of attention owing to their unique optical and electrical properties associated with type-II band alignment. In this study, GaSb QDs were grown on GaAs substrates using liquid phase epitaxy (LPE) technique. The morphology of the obtained QDs was studied by scanning electron microscope and atom force microscope. The optical properties of the QDs were investigated by photoluminescence (PL) spectra. Strong first- and second-order longitudinal optical-phonon replicas were observed on the low-energy side of the wetting layer peak in the PL spectra.

InPBi Quantum Dots for Super-Luminescence Diodes.

InPBi thin film has shown ultra-broad room temperature photoluminescence, which is promising for applications in super-luminescent diodes (SLDs) but met problems with low light emission efficiency. In this paper, InPBi quantum dot (QD) is proposed to serve as the active material for future InPBi SLDs. The quantum confinement for carriers and reduced spatial size of QD structure can improve light emission efficiently. We employ finite element method to simulate strain distribution inside QDs and use the result as input for calculating electronic properties. We systematically investigate different transitions involving carriers on the band edges and the deep levels as a function of Bi composition and InPBi QD geometry embedded in InAlAs lattice matched to InP. A flat QD shape with a moderate Bi content of a few percent over 3.2% would provide the optimal performance of SLDs with a bright and wide spectrum at a short center wavelength, promising for future optical coherence tomography applications.

Phase transition and charge transport through a triple dot device beyond the Kondo regime**Project supported by the National Natural Science Foundation of China (Grant No. 11504102), the Scientific Research Items Foundation of Hubei Educational Committee (Grant Nos. Q20161803 and D20171803), the Doctoral Scientific Research Foundation of Hubei University of Automotive Technology (Grant No. BK201407).

Semiconductor quantum dot structure provides a promising basis for quantum information processing, within which to reveal the quantum phase and charge transport is one of the most important issues. In this paper, by means of the numerical renormalization group technique, we study the quantum phase transition and the charge transport for a parallel triple dot device in the strongly correlated limit, focusing on the effect of inter-dot hopping t beyond the Kondo regime. We find the quantum behaviors depend closely on the initial electron number on the dots, and the present model may map to single, double, and side-coupled impurity models in different parameter spaces. An orbital spin-1/2 Kondo effect between the conduction leads and the bonding orbital, and several magnetic-frustration phases are demonstrated when t is adjusted to different regimes. To understand these phenomena, a canonical transformation of the energy levels is given, and important physical quantities with respect to increasing t and necessary theoretical discussions are shown.

Interfacially Bound Exciton State in a Hybrid Structure of Monolayer WS2 and InGaN Quantum Dots.

van der Waals heterostructures that are usually formed using atomically thin transition-metal dichalcogenides (TMDCs) with a direct band gap in the near-infrared to the visible range are promising candidates for low-dimension optoelectronic applications. The interlayer interaction or coupling between two-dimensional (2D) layer and the substrate or between adjacent 2D layers plays an important role in modifying the properties of the individual 2D material or device performances through Coulomb interaction or forming interlayer excitons. Here, we report the realization of quasi-zero-dimensional (0D) photon emission of WS2 in a coupled hybrid structure of monolayer WS2 and InGaN quantum dots (QDs). An interfacially bound exciton, i.e., the coupling between the excitons in WS2 and the electrons in QDs, has been identified. The emission of this interfacially bound exciton inherits the 0D confinement of QDs as well as the spin-valley physics of excitons in monolayer WS2. The effective coupling between 2D materials and conventional semiconductors observed in this work provides an effective way to realize the 0D emission of 2D materials and opens the potential of compact on-chip integration of valleytronics and conventional electronics and optoelectronics.