Pair Of Quantum Dots Research Articles

A quantum otto heat engine driven by three quantum dots

A quantum heat engine composed of three coupled quantum dots as a working substance is proposed. Since quantum dots naturally obey the Fermi Hubbard Hamiltonian, the strong coupling interaction regime allows the working substance to be evaluated under an effective Heisenberg Hamiltonian. Indeed, the influence of the strength coupling, between the three dots, on quantum machine efficiency and work in the presence of a homogeneous magnetic field is also examined. Furthermore, the influence of entanglement on the efficiency & work of the quantum dot Otto heat engine is well analyzed. As a tripartite working substance, we are interested in analyzing the local work and efficiency associated with each single and pair of quantum dots. The results show that the local efficiency associated with a pair of quantum dots achieves a maximum value, unlike the global efficiency. Indeed, the entanglement impact on Global/local work is studied.

Manipulation of electric and magnetic absorption coefficients using (In,Ga)As/GaAs chiral quantum dot medium

We demonstrate the manipulation of electric and magnetic absorption coefficients within the chiral quantum dots (QDs) system. The described setup involves a pair of double chiral quantum dots ((In,Ga)As/GaAs), operating as a four-level quantum system. The system is influenced by control, magnetic, and probe fields, along with the strength of inter-dot tunneling. The investigation demonstrates that the electric and magnetic absorption coefficients are notably affected by the strength of inter-dot tunneling and the intensity of the control field. Significantly, the study reveals a maximum observed electric coefficient of −3.4×1014 and a maximum magnetic coefficient of −3.4×10−12 within the proposed system. Furthermore, the characteristics of the incident light, including divergence angle, group index, and advance/delay time, are intricately influenced by the strength of inter-dot tunneling and the intensity of the control field. The maximum observed divergence angle measures 0.4 rad, with a group index of ±1.3×1020 and an advance/delay time of ±4.0×1010 in the proposed chiral QDs system. This investigation not only contributes to the advancement of quantum dot research but also presents promising prospects for various applications in optoelectronics and quantum information processing.

Enhanced upconversion luminescence of NaYF4:Er nanocrystals based on the synergy between whispering gallery mode excitation and förster resonant energy transfer

The upconversion luminescence at the visible band was excited by the high power density whispering gallery mode (WGM) on the surface of the microsphere. Under the WGM excitation, Er3+ of NaYF4:Er3+ directly absorbed three photons at 1525 nm, followed by the generation of upconversion luminescence. We used WS2 quantum dots (QDs) as donors and NaYF4:Er3+ upconversion nanoparticles (UCNPs) as acceptors to enhance upconversion luminescence by Förster resonant energy transfer (FRET). The physical mixture composed of monolayer WS2 and NaYF4:Er3+ nanoparticles was coated on SiO2 microsphere by immersion method. Excited by high power density WGM, three photons were absorbed by WS2-QDs based on the quantum size effect to generate excited electrons as donors and UCNPs as acceptors based on FRET, subsequently, the energy of the electrons were transferred to the luminescence center Er3+ to improve the upconversion luminescence efficiency. The mechanism model of upconversion luminescence enhancement based on FRET of WS2–NaYF4:Er3+ was proposed, and the enhancement effect was experimentally studied and demonstrated. To study the distance dependence of energy transfer between QD and UCNP pairs, we covered the surface of the silica microsphere with a controlled concentration of WS2 mixed with NaYF4:Er3+ nano-particles. When the mixed concentration of WS2-QDs was 0.15%, the upconversion luminescence reached the highest efficiency. The results showed that a twofold upconversion luminescence enhancement can be experimentally achieved. The proposed approach for enhancing the upconversion luminescence could benefit the improvement of imaging signal-to-noise ratio (SNR) using UCNPs for fluorescent labels in life sciences.

Effect of double quantum dot asymmetry on electron localization

In this paper, we study the localization of an electron in a binary quantum system formed by a pair of quantum dots (QDs). The traditional theoretical consideration of such systems is limited to the symmetrical case when QDs in such double quantum dot (DQD) are assumed identical in all respects. In this paper, we model the effects of breaking QD similarities in a DQD by studying two-dimensional (2D) DQDs as a double quantum well (DQW). This is done by solving the Schrödinger equation, with parameters chosen to describe an InAs/GaAs heterostructure. We calculate the energy spectrum of the electron confinement and the spectral distribution of localized/delocalized spatial states. Both symmetric and asymmetric QW shapes are considered and their effects are compared. The effects of symmetry breaking are explained within the framework of the two-level system theory. We delineate the QW weak and strong coupling cases in DQW. In particular, we show that the coherence in ideal DQW is unstable in the case of a weak QW coupling. Within the framework of the proposed approach, a charge qubit realized on a DQD is discussed and, as an example, a qubit based on an almost ideal DQD is proposed.

Two Biexciton Types Coexisting in Coupled Quantum Dot Molecules.

Coupled colloidal quantum dot molecules (CQDMs) are an emerging class of nanomaterials, manifesting two coupled emission centers and thus introducing additional degrees of freedom for designing quantum-dot-based technologies. The properties of multiply excited states in these CQDMs are crucial to their performance as quantum light emitters, but they cannot be fully resolved by existing spectroscopic techniques. Here we study the characteristics of biexcitonic species, which represent a rich landscape of different configurations essentially categorized as either segregated or localized biexciton states. To this end, we introduce an extension of Heralded Spectroscopy to resolve the different biexciton species in the prototypical CdSe/CdS CQDM system. By comparing CQDMs with single quantum dots and with nonfused quantum dot pairs, we uncover the coexistence and interplay of two distinct biexciton species: A fast-decaying, strongly interacting biexciton species, analogous to biexcitons in single quantum dots, and a long-lived, weakly interacting species corresponding to two nearly independent excitons. The two biexciton types are consistent with numerical simulations, assigning the strongly interacting species to two excitons localized at one side of the quantum dot molecule and the weakly interacting species to excitons segregated to the two quantum dot molecule sides. This deeper understanding of multiply excited states in coupled quantum dot molecules can support the rational design of tunable single- or multiple-photon quantum emitters.

Nanoscale imaging of quantum dot dimers using time-resolved super-resolution microscopy combined with scanning electron microscopy

Time-resolved super-resolution microscopy was used in conjunction with scanning electron microscopy to image individual colloidal CdSe/CdS semiconductor quantum dots (QD) and QD dimers. The photoluminescence (PL) lifetimes, intensities, and structural parameters were acquired with nanometer scale spatial resolution and sub-nanosecond time resolution. The combination of these two techniques was more powerful than either alone, enabling us to resolve the PL properties of individual QDs within QD dimers as they blinked on and off, measure interparticle distances, and identify QDs that may be participating in energy transfer. The localization precision of our optical imaging technique was ∼3 nm, low enough that the emission from individual QDs within the dimers could be spatially resolved. While the majority of QDs within dimers acted as independent emitters, at least one pair of QDs in our study exhibited lifetime and intensity behaviors consistent with resonance energy transfer from a shorter lifetime and lower intensity donor QD to a longer lifetime and higher intensity acceptor QD. For this case, we demonstrate how the combined super-resolution optical imaging and scanning electron microscopy data can be used to characterize the energy transfer rate.

Enhanced Förster resonance energy transfer on layered metal-dielectric hyperbolic metamaterials: an excellent platform for low-threshold laser action.

Förster resonance energy transfer (FRET) is a well-known physical phenomenon, which has been widely used in a variety of fields, spanning from chemistry, and physics to optoelectronic devices. In this study, giant enhanced FRET for donor-acceptor CdSe/ZnS quantum dot (QD) pairs placed on top of Au/MoO3 multilayer hyperbolic metamaterials (HMMs) has been realized. An enhanced FRET transfer efficiency as high as 93% was achieved for the energy transfer from a blue-emitting QD to a red-emitting QD, greater than that of other QD-based FRET in previous studies. Experimental results show that the random laser action of the QD pairs is greatly increased on a hyperbolic metamaterial by the enhanced FRET effect. The lasing threshold with assistance of the FRET effect can be reduced by 33% for the mixed blue- and red-emitting as QDs compared to the pure red-emitting QDs. The underlying origins can be well understood based on the combination of several significant factors, including spectral overlap of donor emission and acceptor absorption, the formation of coherent closed loops due to multiple scatterings, an appropriate design of HMMs, and the enhanced FRET assisted by HMMs.

Spectroscopic investigation of energy transfer from composition dependent ternary QDs to hemicyanine dye

Systematic spectroscopic investigation of fluorescence (Förster) resonance energy transfer (FRET) is performed for a pair of ternary quantum dots (TQDs) CdSeS/ZnS with LDS698 dye. Composition of TQDs plays an important role in tuning emission wavelength keeping the size same. Coupling of-COOH functionalized QDs with dye results in decreasing fluorescence intensity and lifetimes of QDs analogous to quenching mechanism. The observed enhancement in acceptor intensity is essentially due to dipole–dipole interaction confirming energy transfer and its dependence on spectral overlap. FRET parameters like spectral overlap, Förster’s distance, intermolecular distance, etc., are calculated. The results strongly suggest that the chosen FRET pairs find potential application in bio-imaging and targeted drug delivery.

The mathematical model of quantum dots pair orientation under laser radiation field

One approach for the formation of structures with complex geometries at the nanoscale is the step-by-step assembly. In this case, it is necessary to be able to estimate the time required to establish orientational equilibrium for a preformed pair of particles. This process is statistical in nature and depends on the mechanism of interaction of the ensemble with the external field. The orientation of particles in an alternating field is associated with certain relaxation times, which depend on the viscosity and temperature of the medium, as well as on the geometric structure of the samples. This paper proposes an mathematical model of the process of establishing the distribution of nanoparticles pairs orientations taking into account the friction force, thermal motion, and the orienting laser field. A statistical orientation distribution was obtained for CdTe particles in the field of moderate laser radiation, and the average time for establishing orientational equilibrium was estimated.

High Extraction Efficiency Source of Photon Pairs Based on a Quantum Dot Embedded ina Broadband Micropillar Cavity.

The generation of photon pairs in quantum dots is in its nature deterministic. However, efficient extraction of photon pairs from the high index semiconductor material requires engineering of the photonic environment. We report on a micropillar device with 69.4(10)% efficiency that features broadband operation suitable for extraction of photon pairs. Opposing the approaches that rely solely on Purcell enhancement to realize the enhancement of the extraction efficiency, our solution exploits a suppression of the emission into the modes other than the cavity mode. Furthermore, the design of the device can be further optimized to allow for an extraction efficiency of 85%.

Controlled growth mechanism of ring-like In(Ga)As quantum Dot pairs on GaAs ring-ring-disk nanostructures templates

The ring-like In(Ga)As quantum dot pairs (QDPs) is prepared on GaAs ring-ring-disk (R-R-D) nanostructures template by droplet epitaxy (DE) method. The surface morphology of GaAs samples are characterized with Scanning Tunneling Microscope (STM), and studying local controlled growth mechanism of quantum dots (QDs) on GaAs R-R-D nanostructures. STM images show that In(Ga)As QDPs self-assembled a kind of complicated structure with one quantum dot (QD) pair in the center of nanostructures, other QDPs appearing both double rings and disk areas of original GaAs R-R-D nanostructures distributed like a exquisite ring. These In(Ga)As QDPs are uniformly arranged along the [10] direction of GaAs(001) surface. Combining Stranski-Krastanov (S-K) growth mode and the tensile effect of strain field in the [10] direction on sample surface, the nucleation position and distribution shape of ring-like In(Ga)As QDPs is locally controllable. It is found that high uniformly QDPs can be locally controlled by the original GaAs R-R-D nanostructures templates. These results can provide certain guiding significance for the controllable growth of QD by DE method.

A combination of a graphene quantum dots-cationic red dye donor-acceptor pair and cucurbit[7]uril as a supramolecular sensor for ultrasensitive detection of cancer biomarkers spermine and spermidine.

In a unique approach, the combination of a donor-acceptor pair of hydroxy graphene quantum dots (GQDs-OH) and a red-emissive donor-two-acceptor (D-2-A) type dye with pyridinium units (BPBP) and the well-known host cucurbit[7]uril (CB[7]) has been exploited as a supramolecular sensing assembly for the detection of cancer biomarkers spermine and spermidine in aqueous media at the sub-ppb level based on the affinity-driven exchange of guests from the CB[7] portal. In the binary conjugate, green fluorescent GQDs-OH transfers energy to trigger the emission of the dye BPBP and itself remains in the turn-off state. CB[7] withdraws the dye from the surface of GQDs-OH by strong host-guest interactions with its portal, making GQDs-OH fluoresce again to produce a ratiometric response. In the presence of spermine (SP) or spermidine (SPD), their strong affinity with CB[7] forces the ejection of the fluorophore to settle on the GQDs-OH surface, and the strong green emission of GQDs-OH turns off to device a supramolecular sensor for the detection of SP/SPD. The DFT studies revealed interesting excited-state charge-transfer conjugate formation between BPBP and GQDs leading to turn-on emission of the dye, and further supported the stronger binding modes of BPBP-CB[7], indicating the retrieval of the emission of GQDs. The assembly-disassembly based sensing mechanism was also established by Job's plot analysis, particle size analysis, zeta potential, time-resolved spectroscopy, ITC studies, microscopic studies, etc. The supramolecular sensing assembly is highly selective to SP and SPD, and showed nominal interference from other biogenic amines, amino acids, various metal ions, and anions. The limits of detection (LODs) were 0.1 ppb and 0.9 ppb for spermine and spermidine, respectively. The potential for the real-world application of this sensing assembly was demonstrated by spiking SP and SPD in human urine and blood serum with a high %recovery.

Photoluminescence enhancement by deterministically site-controlled, vertically stacked SiGe quantum dots

The Si/SiGe heterosystem would be ideally suited for the realization of complementary metal-oxide-semiconductor (CMOS)-compatible integrated light sources, but the indirect band gap, exacerbated by a type-II band offset, makes it challenging to achieve efficient light emission. We address this problem by strain engineering in ordered arrays of vertically close-stacked SiGe quantum dot (QD) pairs. The strain induced by the respective lower QD creates a preferential nucleation site for the upper one and strains the upper QD as well as the Si cap above it. Electrons are confined in the strain pockets in the Si cap, which leads to an enhanced wave function overlap with the heavy holes near the upper QD’s apex. With a thickness of the Si spacer between the stacked QDs below 5 nm, we separated the functions of the two QDs: The role of the lower one is that of a pure stressor, whereas only the upper QD facilitates radiative recombination of QD-bound excitons. We report on the design and strain engineering of the QD pairs via strain-dependent Schrödinger-Poisson simulations, their implementation by molecular beam epitaxy, and a comprehensive study of their structural and optical properties in comparison with those of single-layer SiGe QD arrays. We find that the double QD arrangement shifts the thermal quenching of the photoluminescence signal at higher temperatures. Moreover, detrimental light emission from the QD-related wetting layers is suppressed in the double-QD configuration.

Dynamic generation of Greenberger-Horne-Zeilinger states with coupled charge qubits

In this paper, we present a proof-of-principle of the formation of pure maximally entangled states from the Greenberger-Horne-Zeilinger class, in the experimental context of charged quantum dots. Each qubit must be identified as a pair of quantum dots, sharing an excess electron, coupled by tunneling. The electron-electron interaction is accounted for and is responsible for the coupling between the qubits. The interplay between coherent tunneling events and many-body interaction gives rise to the formation of highly entangled states. We begin by treating the problem of encoding three-qubits in a system with three pairs of quantum dots, and the numerical analysis of the exact quantum dynamics to find the conditions for the generation of the GHZ states. An effective two-level model sheds light on the role of a high-order tunneling process behind the dynamics. The action of the main decoherence process, the charge dephasing, is quantified in the process. We then evaluate the physical requirements for the dynamical generation of GHZ states in a $N$ qubit scenario, and its challenges.

Theoretical study of excitons in GaAs quantum dot molecules obtained by nanoholes filling

In this work we report a theoretical study of excitons behavior in self-assembled strain-free GaAs quantum dot molecules (QDMs) obtained by nanoholes filling. More precisely, we study a single molecule formed by a vertically coupled GaAs/AlGaAs quantum dots pair (QDP). The vertical coupling governed by varying the spacer thickness layer reveals how the excitons behave in a QDM. A model based on resolving the three-dimensional effective-mass Schrödinger equation is adopted to reliably provide a complete overview of the fine structure of neutral and complexes excitons and to explain how a pair of quantum dots interact in between while varying the spacer layer thickness mediating the coupling in the barrier. Our calculations can be adopted to improve the growth conditions and boost the optical properties of strain-free GaAs QDMs for using them in quantum information technologies and quantum computing.

Morphological analysis of chromium in carbon quantum dots pairs Co-doped with zirconium and nitrogen and their applications in imaging of living cells

As a new nanomaterial in the biochemistry field, carbon quantum dots (CDs) have been widely applied by scientists. In this study, CDs co-doped with zirconium and nitrogen (Zr-N-CDs) were synthesized quickly with lemon, ethylenediamine, and zirconium chloride through a hydrothermal method. The yield of Zr-N-CDs reached as high as 82.7%. The Zr-N-CDs showed outstanding water solubility in aqueous solution. The formation of Zr-N-CDs was verified by characterization technologies, such as high-resolution transmission electron microscopy (HRTEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Moreover, the optical properties of Zr-N-CDs were investigated through fluorophotometer and ultraviolet spectroscopy. The synthesized Zr-N-CDs were applied to test hexavalent chromium (Cr (VI)), which showed a good linear relationship with the fluorescence quenching of Zr-N-CDs. The limit of detection was 0.52 µM. An analytical method for Cr morphology in natural water areas was developed in this experiment. The sensor showed good stability. The results demonstrate that the sensor detected 98.35%–100.9% Cr (VI) recovery rate in water samples. Based on the cytotoxicity of Zr-N-CDs to human cervical cancer cells (HeLa cells), the Zr-N-CDs had no evident cytotoxicity. The applications of Zr-N-CDs in bioimaging of cells were determined through laser scanning confocal microscopy.

Spectral broadening of optical transitions at tunneling resonances in InAs/GaAs coupled quantum dot pairs

We report on linewidth analysis of optical transitions in InAs/GaAs coupled quantum dots as a function of bias voltage, temperature, and tunnel coupling strength. A significant line broadening up to 100 $\mu$eV is observed at hole tunneling resonances where the coherent tunnel coupling between spatially direct and indirect exciton states is maximized, corresponding to a phonon-assisted transition rate of 150 ns${}^{-1}$ at 20 K. With increasing temperature, the linewidth shows broadening characteristic of single-phonon transitions. The linewidth as a function of tunnel coupling strength tracks the theoretical prediction of linewidth broadening due to phonon-assisted transitions, and is maximized with an energy splitting between the two exciton branches of 0.8$-$0.9 meV. This report highlights the linewidth broadening mechanisms and fundamental aspects of the interaction between these systems and the local environment.

One SYK single electron transistor

We study the behaviour of a single electron transistor (SET) represented by a dissipative tunnel junction between a pair of quantum dots described by two (possibly, different) Sachdev–Ye–Kitaev (SYK) models. A combined influence of the soft collective charge and energy modes on charge transport is discussed, alongside the competing effects of the Coulomb blockade and emergent Kondo resonances which might all conspire to result in a non-monotonic behaviour of the system’s conductance.

Photoluminescence characterization of wetting layer and carrier dynamics for coupled InGaAs/GaAs surface quantum dot pair structures.

The optical properties are investigated by spectroscopic characterizations for bilayer InGaAs/GaAs quantum dot (QD) structures consisting of a layer of surface quantum dots (SQDs) separated from a layer of buried quantum dots (BQDs) by different GaAs spacers with thicknesses of 7 nm, 10.5 nm and 70 nm. The coupling from the BQDs to SQDs leads to carrier transfer for the two samples with thin spacers, 7 nm and 10.5 nm, in which QD pairs are obtained while not for the 70 nm spacer sample. The carrier tunneling time is measured to be 0.145 ns and 0.275 ns from BQDs to SQD through the 7 nm and 10.5 nm spacers, respectively. A weak emission band can be observed at the wavelength of ∼ 960 nm, while the excitation intensity dependent PL and PLE spectra show that this is from the wetting layer (WL) of the SQDs. This WL is very important for carrier dynamics in bilayer structures of BQDs and SQDs, including for carrier generation, capture, relaxation, tunneling, and recombination. These results provide useful information for understanding the optical properties of InGaAs SQDs and for using such hybrid structures as building blocks for surface sensing devices.

(Invited) Photoelectrochemical Properties of All-Inorganic Core/Shell Silicon Quantum Dots

Silicon (Si) quantum dots (QDs) are an environmentally friendly nano-functional material potentially suitable for biophotonics applications. Recently, we have developed a new type of all-inorganic Si QDs that can be dispersed in water almost perfectly without organic ligands and exhibit size-dependent photoluminescence (PL) in the near-infrared range in water [1-2]. The unique property of the QDs arises from the unique structure achieved by simultaneous doping of boron (B) and phosphorus (P). The figure shows a typical transmission electron microscope (TEM) image of a B and P codoped Si QD [3]. The lattice fringe corresponds to {111} planes of crystalline Si. On the surface of the crystalline core, an amorphous shell is formed. The shell is composed of Si, B and P, the concentration of B and P being in the range from a few % to 10 at.%. The shell induces negative potential on the surface and prevents agglomeration of Si QDs in polar solvents by the electrostatic repulsion. To the ligand-free surface of codoped Si QDs, molecules in a solution can access easily, and thus a variety of charge-transfer-induced phenomena such as a photocatalytic effect and chemical doping are expected [4-5]. In a codoped Si QD, B and P are also doped in the crystalline core. Formation of donor and acceptor states in the band gap strongly modifies the luminescence property. Combination of the quantum size effect and the doping effect leads to the control of the luminescence energy in the range much wider than that of conventional Si QDs [6, 7]. By comparing the luminescence energy with those obtained by theoretical calculations, we estimate the number of active B and P pairs in a Si QD to be 2-10 depending on the doping concentration and the size. In the presentation, we will discuss the luminescence and photoelectrochemical properties, including photocatalytic hydrogen evolution, of the B and P codoped Si QDs.