Quantum Dot Levels Research Articles

The modulation of Majorana bound states comb through quantum dots

Performing the scattering matrix approach, we calculate the current noise cross correlation and the differential conductance in the Majorana bound states (MBSs) comb. The teeth on the comb consist of topological super-conducting nanowire sandwiched between two quantum dots (QDs). We find that the nonlocal property of MBSs comb can be effectively modulated by the quantum dots. For only one “QD-MBSs-QD” building block, N = 1, the current noise cross correlation is negative at small bias and becomes positive at lager bias. A zero bias conductance peak of 2e2/h can be found, and it splits with ϵu increasing, where ϵu is the energy level of up-part QD. For N ≥ 2, the current noise cross correlation is zero at small bias and becomes positive at larger bias. Different from the case of N = 1, the zero bias conductance peak is 4e2/h due to the local Andreev reflection. Besides, we find that the differential conductance is robust against the influence of N.

Self-Assembly of Semiconductor Quantum Dots using Organic Templates.

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are regarded as brightly photoluminescent nanomaterials possessing outstanding photophysical properties, such as high photodurability and tunable absorption and emission wavelengths. Therefore, QDs have great potential for a wide range of applications, such as in photoluminescent materials, biosensors and photovoltaic devices. Since the development of synthetic methods for accessing high-quality QDs with uniform morphology and size, various types of QDs have been designed and synthesized, and their photophysical properties dispersed in solutions and at the single QD level have been reported in detail. In contrast to dispersed QDs, the photophysical properties of assembled QDs have not been revealed, although the structures of the self-assemblies are closely related to the device performance of the solid-state QDs. Therefore, creating and controlling the self-assembly of QDs into well-defined nanostructures is crucial but remains challenging. In this Minireview, we discuss the notable examples of assembled QDs such as dimers, trimers and extended QD assemblies achieved using organic templates. This Minireview should facilitate future advancements in materials science related to the assembled QDs.

Emerging Atomic Energy Levels in Zero-Dimensional Silicon Quantum Dots.

Driven by the emergence of colloidal semiconductor quantum dots (QDs) of tunable emission wavelengths, characteristic of exciton absorption peaks, outstanding photostability and solution processability in device fabrication have become a key tool in the development of nanomedicine and optoelectronics. Diamond cubic crystalline silicon (Si) QDs, with a diameter larger than 2 nm, terminated with hydrogen atoms are known to exhibit bulk-inherited spin and valley properties. Herein, we demonstrate a newly discovered size region of Si QDs, in which a fast radiative recombination on the order of hundreds of picoseconds is responsible for photoluminescence (PL). Despite retaining a crystallographic structure like the bulk, controlling their diameters in the 1.1-1.7 nm range realizes the strong PL with continuous spectral tunability in the 530-580 nm window, the narrow spectral line widths without emission tails, and the fast relaxation of photogenerated carriers. In contrast, QDs with diameters greater than 1.8 nm display the decay times on the microsecond order as well as the previous Si QDs. In addition to the five-orders-of-magnitude variation in the PL decay time, a systematic study on the temperature dependence of PL properties suggests that the energy structure of the smaller QDs does not retain an indirect band gap character. It is discussed that a 1.7 nm diameter is critical to undergo changes in energy structure from bulky to molecular configurations.

Antiresonance and its application in tri-quantum-dot systems

Antiresonance and its application in triple-quantum-dot systems are studied theoretically using nonequilibrium Green’s function method. The presence or absence of antiresonance point can be used to determine whether quantum dot impurity exists in the system. For a tri-quantum-dot ring, the conversion between 0 and 1 for the conductance can be realized by adjusting the energy levels of quantum dots, which makes the system applicable as a quantum switch. The coupling strength can be measured according to the energy level of the antiresonance point. Once the intradot Coulomb interactions are taken into account, three new Fano antiresonances emerge in the conductance spectrum. Under the action of the magnetic field, two antiresonance points disappear simultaneously. Our work sheds lights onto the design and implementation of new quantum functional devices for electronic measurements.

Composite two-particle sources

Multi-particle sources constitute an interesting new paradigm following the recent development of on-demand single-electron sources. Versatile devices can be designed using several single-electron sources, possibly of different types, coupled to the same quantum circuit. However, if combined \textit{non-locally} to avoid cross-talk, the resulting architecture becomes very sensitive to electronic decoherence. To circumvent this problem, we here analyse two-particle sources that operate with several single-electron (or hole) emitters attached in series to the same electronic waveguide. We demonstrate how such a device can emit exactly two electrons without exciting unwanted electron-hole pairs if the driving is adiabatic. Going beyond the adiabatic regime, perfect two-electron emission can be achieved by driving two quantum dot levels across the Fermi level of the external reservoir. If a single-electron source is combined with a source of holes, the emitted particles can annihilate each other in a process which is governed by the overlap of their wave functions. Importantly, the degree of annihilation can be controlled by tuning the emission times, and the overlap can be determined by measuring the shot noise after a beam splitter. In contrast to a Hong-Ou-Mandel experiment, the wave functions overlap close to the emitters and not after propagating to the beam splitter, making the shot noise reduction less susceptible to electronic decoherence.

Berry phase in superconducting multiterminal quantum dots

We report on the study of the non-trivial Berry phase in superconducting multiterminal quantum dots biased at commensurate voltages. Starting with the time-periodic Bogoliubov-de Gennes equations, we obtain a tight binding model in the Floquet space, and we solve these equations in the semiclassical limit. We observe that the parameter space defined by the contact transparencies and quartet phase splits into two components with a non-trivial Berry phase. We use the Bohr-Sommerfeld quantization to calculate the Berry phase. We find that if the quantum dot level sits at zero energy, then the Berry phase takes the values $\varphi_B=0$ or $\varphi_B=\pi$. We demonstrate that this non-trivial Berry phase can be observed by tunneling spectroscopy in the Floquet spectra. Consequently, the Floquet-Wannier-Stark ladder spectra of superconducting multiterminal quantum dots are shifted by half-a-period if $\varphi_B=\pi$. Our numerical calculations based on Keldysh Green's functions show that this Berry phase spectral shift can be observed from the quantum dot tunneling density of states.

Antiresonance in electron transport through a four quantum dots system

A four quantum dots structure is designed. The influence of the interdot coupling strength, energy levels of quantum dots, magnetic flux and intradot Coulomb interactions on the conductance is discussed. An antiresonance point exactly emerges at the location of the energy level of side-coupled quantum dots (SCQD). The conversion between resonance band and antiresonance band can be achieved as parallel-coupled di-quantum dots with or without SCQD, which suggests a physical scheme of an effective quantum switch. The Breit–Wigner resonant and Fano antiresonant peaks may merge into one resonant band. By means of the mathematical formula, we analyze how the resonance band is formed. By varying intradot Coulomb interaction, the resonance band may evolve into a Fano antiresonance with an ultra-narrow width. This study provides theoretical basis for the design of quantum computing devices.

Cost-Effective and Semi-Transparent PbS Quantum Dot Solar Cells Using Copper Electrodes.

PbS quantum dots (QDs) have gained significant attention as promising solution-based materials for third generation of photovoltaic (PV) devices, thanks to their size-tunable band gap, air stability, and low-cost solution processing. Gold (Au), despite its high cost, is the standard electrode in the conventional PbS QD PV architecture because of its perfect alignment with valence levels of PbS QDs. However, to comply with manufacturing requirements for scalable device processing, alternative cost-effective electrodes are urgently required. Here, we employed an interface engineering approach and deposited poly(3-hexylthiophene-2,5-diyl) as a hole transport layer on 1,2-ethanedithiol-capped PbS QDs in order to adjust the valence band of QDs with the work function of inexpensive copper (Cu) electrodes. In fact, this is the first report of a Au-free PbS QD PV system employing the conventional device structure. Our Cu-based device shows a maximum power conversion efficiency (PCE) of 8.7% which is comparable with that of the Au-based device (10.2%). Interestingly, the P3HT-based device shows improved stability with relatively 10% PCE loss after 230 h under continuous illumination. Moreover, using an ultrathin Cu electrode, a semitransparent PbS QD PV is fabricated with a remarkably high average visible transparency of 26% and a PCE of 7.4%.

Electrical control of spins and giant g-factors in ring-like coupled quantum dots

Emerging theoretical concepts for quantum technologies have driven a continuous search for structures where a quantum state, such as spin, can be manipulated efficiently. Central to many concepts is the ability to control a system by electric and magnetic fields, relying on strong spin-orbit interaction and a large g-factor. Here, we present a mechanism for spin and orbital manipulation using small electric and magnetic fields. By hybridizing specific quantum dot states at two points inside InAs nanowires, nearly perfect quantum rings form. Large and highly anisotropic effective g-factors are observed, explained by a strong orbital contribution. Importantly, we find that the orbital contributions can be efficiently quenched by simply detuning the individual quantum dot levels with an electric field. In this way, we demonstrate not only control of the effective g-factor from 80 to almost 0 for the same charge state, but also electrostatic change of the ground state spin.

Waterborne polyurethane/carbon quantum dot nanocomposite as a surface coating material exhibiting outstanding luminescent performance

A luminescent waterborne polyurethane (WPU)/carbon quantum dot (CQDs) nanocomposite has been prepared by incorporation of low dose levels (0.05, 0.15, and 0.25 wt%) of carbon quantum dots via an in stiu polymerization, using isophorone diisocyanate (IPDI), 1,4-butanediol (BDO) as the hard segment, polypropylene glycol(PPG) (Mn = 1000) as soft segment, dimethylolpropionic acid (DMPA) as hydrophilic monomer, and CQDs as a modified agent. We characterized the CQDs-WPU nanocomposite and investigated its properties by Fourier transform infrared spectrometer (FT-IR), fluorescence spectra, Thermogravimetric analyzer (TGA), scanning electron microscope (SEM), etc. The results indicated that CQDs was successfully introduced into the main chains of polyurethane. The incorporation of CQDs endowed CQDs-WPU nanocomposites the excellent mechanical properties and thermostability. Interestingly, the fabricated nanocomposite film exhibited good transparency and photoluminescence properties, the photoluminescence intensity of CQDs-WPU film increased with the increase of CQDs dosage, and an excitation wavelength independent photoluminescence behavior of CQDs-WPU was witnessed.

Interfacial charge transfer between CsPbBr3 quantum dots and ITO nanoparticles revealed by single-dot photoluminescence spectroscopy

The interfacial charge transfer between single CsPbBr3 perovskite quantum dots (QDs) and indium tin oxide (ITO) is investigated by single-dot photoluminescence spectroscopy. It is found that when the Fermi level of single perovskite QDs aligns with that of ITO nanoparticles, the QD surface cannot be charged by the ITO through interfacial electron transfer. Therefore, the QD/ITO system with Fermi level alignments can exclude exciton nonradiative recombination processes involving the additional surface electrons, such as the exciton Auger recombination and the valence band hole transfer processes. Hence the photovoltaic devices based on perovskite QD/ITO system with the Fermi level alignments have the improved photoelectric conversion efficiency.

Se-Assisted Performance Enhancement of Cu2ZnSn(S,Se)4 Quantum-Dot Sensitized Solar Cells via a Simple Yet Versatile Synthesis.

The earth-abundant Cu2ZnSnS4 (CZTS) quantum dots (QDs) have emerged as one potential substitute to toxic cadmium or rare indium QDs, but their application in quantum dot-sensitized solar cells (QDSSCs) is still limited by the improper particle size and the rigorous synthesis and ligand exchange conditions. Herein, we developed a one-pot hot injection method by using Tri-n-octylphosphine oxide (TOPO) as the solvent and oleylamine as the capping agent to synthesize Cu2ZnSn(S,Se)4 (CZTSSe) QDs with adjustable size and narrow size distribution. The key feature of this approach is that we can take advantage of the high-temperature nucleation, low-temperature growth, and strong reducibility of NaHB4 to prepare small-sized CZTSSe QDs without using 1-dodecanethiol (DDT) and to extend the light harvesting range through Se incorporation. After Se incorporation, it turns out that the conduction band (CB) level of CZTSSe QDs decreases, implying that the injection driving force of the electron to the CB of TiO2 films becomes weaker and a larger recombination would be induced at the TiO2/QDs/electrolyte interface. Benefiting from the broadened optoelectronic response range, the induced higher Jsc (16.80 vs 14.13 mA/cm2) finally leads to the increase of the conversion efficiency of CZTSSe QDSSC from 3.17% to 3.54% without further modification. Despite the fact that the efficiency is still far behind those of literature reported values through use of other chalcogenide sensitizers, this DDT-free approach solves the main hindrance for the application of CZTSSe QDs in QDSSCs and holds a more convenient way for ligand exchange, light absorption improvement, and particle size control.

Spatial Optimization of Modulation Doping in P-I-P QDIPs: Towards Achieving Higher Operating Temperature

We report short wavelength infrared (SWIR) photodetectors using multi-stacked modulation doped Indium-Arsenide (InAs) self-assembled quantum dots (QDs). The detriments of direct doping on the energy levels of InAs QDs have been eliminated by choosing modulation doping. We have considered three P-I-P Indium Arsenide/Gallium Arsenide (InAs/GaAs) quantum dot infrared photodetector (QDIP) heterostructures in which a thin (3 nm) P-doped GaAs layer is introduced at a distance of 7, 12 and 17 nm away from each InAs dot layer respectively. The structures have been analyzed for their morphological and optical characteristics and an optimum separation between the active layer and modulation doped layer has been achieved. The fabricated single pixel detectors exhibit a broad spectral response from 1 to 3.5 μm in the SWIR region for all the samples. The infrared photocurrent survives up to a maximum temperature of 200K for the optimized device with a peak responsivity of 1.439 A/W at wavelength λ = 1.77 μm and temperature T = 200 K.

Effect of Si Doping in Self-Assembled InAs Quantum Dots on Infrared Photodetector Properties

We investigate the characteristics of self-assembled quantum dot infrared photodetectors(QDIPs) based on doping level. Two kinds of QDIP samples are prepared using molecular beam epitaxy : n+-i(QD)-n+ QDIP with undoped quantum dot(QD) active region and n+-n−(QD)-n+ QDIP containing Si direct doped QDs. InAs QDIPs were grown on semi-insulating GaAs (100) wafers by molecular-beam epitaxy. Both top and bottom contact GaAs layer are Si doped at 2×1018/cm3. The QD layers are grown by two-monolayer of InAs deposition and capped by InGaAs layer. For the n+-n−(QD)-n+ structure, Si dopant is directly doped in InAs QD at 2×1017/cm3. Undoped and doped QDIPs show a photoresponse peak at about 8.3 μm, ranging from 6~10 μm at 10 K. The intensity of the doped QDIP photoresponse is higher than that of the undoped QDIP on same temperature. Undoped QDIP yields a photoresponse of up to 50 K, whereas doped QDIP has a response of up to 30 K only. This result suggests that the doping level of QDs should be appropriately determined by compromising between photoresponsivity and operating temperature.

(Invited) Beyond Si and SiO x : SiN x and SiC x Anode Materials for Lithium-Ion Batteries

The research of silicon (Si) anode with commercial feasibility for high energy lithium ion batteries continues to accelerate because of its high specific capacity of 3592 mAh g-1. Since the massive volume change upon repeated charging-dischrage process turned out as the main origin of insufficient cycling stability, the various material designs has recently been proposed. Nanostructure engineering has been spotlighted because the stress is highly mitigated during lithiation when its size decreases to the nano level. However, although theoretical simulation results have concluded that the size of silicon with quantum level (1~5 nm) shows extremely low stress evolution during (de)lithiation process, previously reported Si-based anodes have been too much focusing on void space or morphological properties with the size of over 10 nm. Alternatively, phase control of active-inacitve matrix has received considerable attention because the nano-engineering advantages (low stress intensification) are strenthened and the nano-property weakenesses (low tap density, high surface area and poor elecrical properties) are compensated simultaneously. For example, micron-sized SiO x consisting of nano-Si (active matrix) with the size of the under 5 nm and SiO1-2 (inactive matrix) has been highlighted owing to its high cycling performance and tap density. Nevertheless, its utilization as a commercial anodes are still hindered by its inferior battery performance including low coulombic efficiency of < 85% caused by intrinsic characteristics of inactive matrix (irreversible formation of Li2O and Li4SiO4). As a result, there is strong motivation to develop the technology realizing the silicon size up to the quantum dot level (1~5 nm) in addition to the phase control of fascinating inacitve matrix. In this aspect, the SiNx and SiCx anodes will be proposed as alternative high capacity anodes with commercial feasibility for high energy lithium ion batteries. In addition to its fundamental features and variety of strategies according to the synthetic method, the critical factors affecting the electrochemical performances of SiN x and SiC x anodes will be covered. Furthermore, these anodes will be compared with the state of the art benchmarking samples including SiOx and Si-carbon composites under the industrial standard conditions.

Superlattice nanowire heat engines with direction-dependent power output and heat current

Heat engines (HEs) made of low dimensional structures offer promising applications in energy harvesting due to their reduced phonon thermal conductance. Many efforts have been devoted to the design of HEs made of quantum-dot (QD) superlattice nanowire (SLNW), but only SLNWs with uniform energy levels in QDs were considered. Here we propose a HE made of SLNW with staircase-like QD energy levels. It is demonstrated that the nonlinear Seebeck effect can lead to significant electron transports for such a nanowire with staircase-like energy levels. The asymmetrical alignment of energy levels of quantum dots embedded in nanowires can be controlled to allow resonant electron transport under forward temperature bias, while they are in off-resonant regime under backward bias. Under such a mechanism,the power output and efficiency of such a SLNW are better than SLNWs with uniform QD energy levels. The SLNW HE has direction-dependent power output and heat current. In addition, the HE has the functionality of a heat diode with impressive negative differential thermal conductance under open circuit condition.

Discrete quantum levels and Zeeman splitting in ultra-thin gold-nanowire quantum dots

We fabricate ultrathin gold nanowires (AuNWs) by means of a wet-chemical synthesis involving a reduction reaction. Our low-temperature transport measurements reveal the presence of the Coulomb-blockade effect and the formation of discrete quantum levels in an individual AuNW. We also observe the Zeeman splitting of the quantum levels in AuNW quantum dots under the application of magnetic fields via single-electron transport measurements using excitation spectroscopy. Our experimental results indicate that spin–orbit coupling strongly suppresses the estimated g-factor.

Overcoming the Electroluminescence Efficiency Limitations in Quantum‐Dot Light‐Emitting Diodes

AbstractDevelopment of quantum dots (QDs) based light‐emitting diodes (QLEDs) is driven by attractive properties of these fluorophores such as precise Gaussian distribution, tunable emission, and facile solution processability. The performance of QLED devices is limited by intrinsic factors such as luminance quenching in quantum dots due to imbalanced carrier injection predominantly caused by a large hole injection barrier as well as by extrinsic processes such as nonradiative recombination at active layer interfaces. The Auger recombination problem is overcome by charge siphoning at the interfaces between QDs and charge‐transporting material. A simplest trilayer (p–i–n) LED structure is fabricated using an all‐solution processing method: a carefully engineered p‐type polymeric hole transport layer with a gradient work function is incorporated. The gradient work function creates the cascading energy levels from the moderate Fermi level anode to the deep‐lying valence band level of QDs. As a result, the QLEDs exhibit significantly improved external quantum efficiencies and luminous efficiencies of 15.9% and 31.8 cd A−1, 17.4% and 59.3 cd A−1, and 12.8% and 14.4 cd A−1 for red, green, and blue light‐emitting devices, respectively. It is expected that the concept demonstrated here will facilitate the design and development of efficient solution‐processible QLEDs for full‐color displays.

Discrete energy states induced broadband emission from self-assembly InGaN quantum dots

Light-emission contributed from discrete energy levels of self-assembly InGaN quantum dots is explored to fabricate broadband light-emitting diodes (LEDs). A series of InGaN quantum dots samples are self-assembly grown on sapphire substrate by metal-organic vapor phase epitaxy. Emission from the discrete energy states of the InGaN quantum dots is detected by micro-zone photoluminescence under cryogenic temperature. The peaks assignments are also verified by theoretical simulation. A green LED incorporating InGaN multi-quantum dots as active medium is fabricated, demonstrating a broader emission than the referred multi-quantum wells LED. It suggests quantum dots be of great potential to improve the color-rendering index of white LEDs.

Plasmon-assisted two-photon Rabi oscillations in a semiconductor quantum dot – metal nanoparticle heterodimer

Tho-photon Rabi oscillations hold potential for quantum computing and quantum information processing, because during a Rabi cycle a pair of entangled photons may be created. We theoretically investigate the onset of this phenomenon in a heterodimer comprising a semiconductor quantum dot strongly coupled to a metal nanoparticle. Two-photon Rabi oscillations in this system occur due to a coherent two-photon process involving the ground-to-biexciton transition in the quantum dot. The presence of a metal nanoparticle nearby the quantum dot results in a self-action of the quantum dot via the metal nanoparticle, because the polatization state of the latter depends on the quantum state of the former. The interparticle interaction gives rise to two principal effects: (i) - enhancement of the external field amplitude and (ii) - renormalization of the quantum dot's resonance frequencies and relaxation rates of the off-diagonal density matrix elements, both depending on the populations of the quantum dot's levels. Here, we focus on the first effect, which results in interesting new features, in particular, in an increased number of Rabi cycles per pulse as compared to an isolated quantum dot and subsequent growth of the number of entangled photon pairs per pulse. We also discuss the destructive role of radiative decay of the excitonic states on two-photon Rabi oscillations for both an isolated quantum dot and a heterodimer.