Large Quantum Dots Research Articles

Determination of the particle size distribution of cube-shaped colloidal perovskite quantum dots from photoluminescence spectra: A combined theoretical-experimental approach.

A theoretical-experimental approach is proposed to convert the photoluminescence spectra of colloidal perovskite quantum dot ensembles into accurate estimates for their intrinsic particle size distribution functions. Two main problems were addressed and properly correlated: the size dependence of the first excitonic transition in a single cube-shaped quantum dot and the inhomogeneous broadening of the fluorescence line shape due to the size nonuniformity of the chemically prepared quantum dot suspension in addition to the single-dot homogeneous broadening. By applying the reported methodology to CsPbBr3 quantum dot samples belonging to the strong and intermediate confinement regimes, the calculated size distributions exhibited close agreement with those obtained from transmission electron microscopy, with precise estimates for the average particle size and standard deviation. Specifically for strongly confined ultrasmall CsPbBr3 quantum dots, the presented spectroscopic model for size distribution computation is based on a new analytical expression for the size-dependent bandgap, which was developed within the framework of the finite-depth square-well effective mass approximation accounting for band nonparabolicity effects. Such a quantum mechanical approach correctly predicts the expected transition to the intermediate confinement regime in sufficiently large quantum dots, which are traditionally described by the well-known bandgap equation in the infinite potential barrier limit with a spatially correlated electron-hole wavefunction and nonparabolic carrier effective masses. The proposed calculation scheme originates from general theoretical considerations so that it can be readily adapted to semiconductor quantum dots of many other systems, from all inorganic metal halides to hybrid perovskite materials, regardless of the adopted chemical synthesis route.

QArray: A GPU-accelerated constant capacitance model simulator for large quantum dot arrays

Semiconductor quantum dot arrays are a leading architecture for the development of quantum technologies. Over the years, the constant capacitance model has served as a fundamental framework for simulating, understanding, and navigating the charge stability diagrams of small quantum dot arrays. However, while the size of the arrays keeps growing, solving the constant capacitance model becomes computationally prohibitive. This paper presents an open-source software package able to compute a 100 × 100100×100 pixels charge stability diagram of a 16-dot array in less than a second. Smaller arrays can be simulated in milliseconds - faster than they could be measured experimentally, enabling the creation of diverse datasets for training machine learning models and the creation of digital twins that can interface with quantum dot devices in real-time. Our software package implements its core functionalities in the systems programming language Rust and the high-performance numerical computing library JAX. The Rust implementation benefits from advanced optimisations and parallelisation, enabling the users to take full advantage of multi-core processors. The JAX implementation allows for GPU acceleration.

Nanoshell-driven carrier engineering of large quantum dots enables ultra-stable and efficient LEDs

Nanoshell-driven carrier engineering of large quantum dots enables ultra-stable and efficient LEDs

(Invited) Development of Ag- and Sn-Based Colloidal Quantum Dots for Near Infrared Photodetection

Low-cost photodetectors with sensitivity in the second near-infrared window (NIR-II, 1000–1700 nm) are highly demanded. Recently, we made the first demonstration of an Ag2Se colloidal quantum dots (QDs) photodiode with sensitivity up to 1200 nm. By employing secondary phosphine to elevate the precursor reactivity, the Ag2Se QDs with a distinct excitonic absorption peak was achieved. These nanocrystals were deposited from solution into a mesoporous TiO2 scaffold to increase the light absorption and charge separation and reduce the exciton diffusion length. By incorporating a suitable hole-transporting layer between the active layer and Ag anode, the resulting devices showed a responsivity of 4.17 mA/W at 1200 nm.1 Also, Ag2Te QDs are excellent for advancing the detection wavelength. Their synthesis with desired particle sizes, narrow size distribution and high photoluminescence quantum yield (PL QY) is challenging. We systematically investigate critical parameters affecting the synthesis in an organic phase. It shows that high Ag/Te feed ratio leads to smaller size and higher PL QY; under 4:1 Ag/Te feed molar ratio, addition of secondary phosphine leads to narrower size distribution and excellent colloidal stability; under 6:1 Ag/Te feed molar ratio, excess 1-dodecanethiol as a strong ligand slows the nucleation and results in fewer nuclei, leading to a broad size distribution and poor optical properties; additional n-trioctyl phosphine as a weak ligand provides better colloidal stability; and another weak ligand n-tributylphosphine improves Ag2Te QD colloidal stability, focuses size distribution, and enhances PL QY. After optimization relatively large Ag2Te QDs with distinct excitonic absorption peaks (~1050 – 1450 nm) and PL emission peak 1.3 – 1.7 µm (QY up to 6.2%) were obtained. NIR-II photodetection has been demonstrated with a responsivity of ~1.5 mA/W at 1400 nm.2 More recently, we have been developing Sn-based chalcogenide QDs and obtained preliminary photodetection results. These results demonstrate that Ag and Sn chalcogenide QDs offer a low-toxicity route for low-cost fabrication of NIR-II photodetection. Reference: [1] Graddage N, Ouyang J, Lu J, Chu TY, Zhang Y, Li Z, Wu X, Malenfant PRL, Tao Y. Near-Infrared-II Photodetectors Based on Silver Selenide Quantum Dots on Mesoporous TiO2 Scaffolds. ACS Applied Nano Materials 2020, 3: 12209–12217.[2] Ouyang J, Graddage N, Lu J, Zhong Y, Chu TY, Zhang Y, Wu X, Kodra O, Li Z, Tao Y, Ding J. Ag2Te Colloidal Quantum Dots for Near-Infrared-II Photodetectors. ACS Applied Nano Materials 2021, 4: 13587–13601. Keywords: Environmentally Friendly; Near Infrared; Quantum Dots; Photodetection.

InAs/GaAs SK quantum dots stacking: Impact of spacer layer on optical properties

In this paper, we report the optical properties of vertically stacked multilayers of self-organized InAs/GaAs quantum dots QDs with different GaAs spacer layer thicknesses of 10 and 15 nm, using photoluminescence PL measurement. The 10 K PL spectrum exhibits double-emission peaks ambiguously identified in PL spectra where the excitation power dependence reveals that these emission peaks are attributed to fundamental ground state GS of large quantum dots in the low energy side, and first excited state 1ES transitions of large quantum dots in coincidence with fundamental ground state GS of smaller ones, in the high energy side. Both the excitation power dependence and the temperature dependence measurement results exhibited competition between the latter's optical transitions in the QDs, which becomes more prevalent at higher excitation powers. The main PL peak is quenched above 190 K, giving rise to “a 1D miniband”, ascribed to the electronic coupling between QDs along the stacking direction and tuning the emission wavelength around 1.3 μm.Our results emphasize the Key role of the vertically stacked InAs/GaAs QDs structures with thin GaAs spacer layers in optimizing design for long-wavelength devices.

Effective Charge Collection of Electron Transport Layers for High-Performance Quantum Dot Infrared Solar Cells.

Infrared (IR) solar cells, capable of converting low-energy IR photons to electron-hole pairs, are promising optoelectronic devices by broadening the utilization range of the solar spectrum to the short-wavelength IR region. The emerging PbS colloidal quantum dot (QD) IR solar cells attract much attention due to their tunable band gaps in the IR region, potential multiple exciton generation, and facile solution processing. In PbS QD solar cells, ZnO is commonly utilized as an electron transport layer (ETL) to establish a depleted heterostructure with a QD photoactive layer. However, band gap shrinkage of large PbS QDs makes it necessary to tailor the behaviors of the ZnO ETL for efficient carrier extraction in the devices. Herein, the characteristics of ZnO ETL are efficiently and flexibly tailored to match the QD layer by handily adjusting the postannealing process of ZnO ETL. With a suitable temperature, the well-matched energy level alignment and suppressed trap states are simultaneously achieved in the ZnO ETL, effectively reducing the nonradiative recombination and accelerating the electron injection from the QD layer to ETL. As a consequence, a high-performance PbS QD photovoltaic device with power conversion efficiencies (PCEs) of 10.09% and 1.37% is obtained under AM 1.5 and 1100 nm filtered solar illumination, demonstrating a simple and effective approach for achieving high-performance IR photoelectric devices.

Size Control and Photoluminescence Characteristics of InSb Quantum Dots on GaSb Substrates Grown by Molecular Beam Epitaxy

InSb self‐assembled quantum dots (QDs) have the possibility to increase the luminescence intensity of light‐emitting diodes operating in the midinfrared region. As a first step toward the midinfrared luminescence, the single‐layer InSb QDs are grown on GaSb (100) substrates by molecular beam epitaxy and the effects of V/III ratio and InSb coverage on size and density of InSb QDs and photoluminescence (PL) characteristics are investigated. As the V/III ratio increases, the distribution of QD height broadens and goes from monomodal to multimodal. The increase in the number of large QDs and their multimodal distribution are thought to be due to the ripening process. The PL intensity is maximum at a V/III ratio of 2 with PL wavelength of 1.76 μm. As the InSb coverage increases at a V/III ratio of 2, QD size and density increase while maintaining a monomodal distribution in QD size. However, the PL peak intensity decreases with increasing InSb coverage due to the increase of nonradiative QDs. The highest PL peak intensity at an InSb coverage of 2.0 monolayer and a V/III ratio of 2 with PL wavelength of 1.69 μm are obtained. The present single‐layer InSb QD shows the emission in the near‐infrared region.

Size-tunable and monodisperse lead sulfide quantum dots for broadband photodetectors

Lead sulfide quantum dots (PbS QDs) are used in broadband photodetectors due to their excellent size tunability, photosensitivity, and solution processability. However, due to the risk of Ostwald ripening, synthesizing high-quality PbS QDs with absorption peaks over 2000 nm with high monodispersity is a challenge. In this study, by controlling the molar ratio of Pb to S and the number of injections of S precursor, we successfully prepared large PbS QDs with an absorption peak at 2122 nm, corresponding to an average diameter of 11.42 nm, with a super-mono-dispersity of 5.50%. Broadband photodetectors ranging from visible light to short-wave infrared are prepared using the large PbS QDs, corresponding to a response rate of 5.98 mA/W at 1940 nm.

Modelling and experimental characterization of double layer InP/AlGaInP quantum dot laser

Spectrum of an InP/AlGaInP self- assembled double-layer quantum dot (QD) laser fabricated by metal–organic vapor-phase epitaxy is theoretically and experimentally investigated. A bimodal QD size distribution (small and large QD groups) was detected which is formed during the fabrication. A model is proposed based on rate equations accounting for the superposition of two inhomogeneous QD groups. The total output power and the power spectral density (PSD) of the fabricated QD laser are experimentally characterized at room temperature. The output spectrum is segmented into the sum of two Gaussians curves (super Gaussian) belonging to the small and large QD groups. The peak PSD and the spectral width of each group are extracted and their dependency on the injected current density is analysed. The peak of the large QDs is found to be dominant at small current while the peak of the small QDs dominated at high current alongside a reduction in its spectral width leading to lasing based on them. This behaviour is attributed to the saturation of the large QDs energy levels due to its relatively long radiative lifetime. The experimental analysis is in a good agreement with the theoretical results.

Formation of Colloidal In(As,P) Quantum Dots Active in the Short-Wave Infrared, Promoting Growth through Temperature Ramps.

Colloidal InAs quantum dots (QDs) are widely studied as a printable optoelectronic material for short-wave infrared (SWIR) that is not restricted by regulations on hazardous substances. Such applications, however, require synthetic procedures that yield QDs with adjustable sizes at the end of the reaction. Here, we show that such one-size-one-batch protocols can be realized through temperature profiles that involve a rapid transition from a lower injection temperature to a higher reaction temperature. By expediting the transition to the reaction temperature and reducing the overall synthesis concentration, we can tune QD sizes from 4.5 to 10 nm, the latter corresponding to a band gap transition at 1600 nm. We argue that the temperature ramps provide a more distinct separation between nucleation at low temperature and growth at high temperature such that larger QDs are obtained by minimizing the nucleation time. The synthetic procedures introduced here will strongly promote the development of a SWIR optoelectronic technology based on InAs QDs, while the use of temperature profiles to steer a colloidal synthesis can find applications well beyond the specific case of InAs QDs.

Minimizing heat generation in quantum dot light-emitting diodes by increasing quasi-Fermi-level splitting.

Minimizing heat accumulation is essential to prolonging the operational lifetime of quantum dot light-emitting diodes (QD-LEDs). Reducing heat generation at the source is the ideal solution, which requires high brightness and quantum efficiency at low driving voltages. Here we propose to enhance the brightness of QD-LEDs at low driving voltages by using a monolayer of large QDs to reduce the packing number in the emitting layer. This strategy allows us to achieve a higher charge population per QD for a given number of charges without charge leakage, enabling enhanced quasi-Fermi-level splitting and brightness at low driving voltage. Due to the minimized heat generation, these LEDs show a high power conversion efficiency of 23% and a T95 operation lifetime (the time for the luminance to decrease to 95% of the initial value) of more than 48,000 h at 1,000 cd m-2.

Size tunable and controllable synthesis of PbS quantum dots for broadband photoelectric response

PbS quantum dots (QDs) with size-dependent photoelectric characteristics and wide spectral absorption show a great potential material for preparing near-infrared photodetectors, attracting numerous studies. This paper prepared size-tunable PbS QDs with a mean size varying from 3 nm to 9 nm via a modified hot injection method. And the effects of different factors on the size of PbS QDs in the synthesis were systematically summarized. Moreover, size-dependent optoelectronic properties of PbS QDs were studied. With the increase in size, the dark current, photocurrent, and responsivity of devices show an increase, while the 3 dB bandwidth decreases. In addition, the larger the size, the wider the detection spectrum coverage, the detectivity of the corresponding absorption peak in PbS QDs is better than another size PbS QDs, but the maximum detectivity of the large size PbS QDs is lower than that of the small size PbS QDs in the near-infrared. Therefore, selecting different PbS QDs to prepare photodetector is of great significance for practical applications with different detection requirements. Significantly, these size-tunable PbS QDs photodetectors show fast response and excellent responsivity, and detectivity.

Multiterminal open quantum dot circuit operating in the fractional quantum Hall regime

We study theoretically a large quantum dot in the fractional quantum Hall regime that is strongly coupled to $\mathcal{M}$ leads via single-mode quantum point contacts. In the case $\mathcal{M}=2$, when the system is mapped onto the two-channel charge Kondo problem, we predict a universal expression for the conductance in the vicinity of a strong-coupling fixed point. The power of the leading temperature correction to the maximal conductance is determined by the fractional filling factor $\ensuremath{\nu}=1/m$. For $\mathcal{M}>2$, we examine the case in which $\mathcal{M}\ensuremath{-}1$ quantum point contacts are fully open, reproducing a single-channel circuit coupled to a dissipative Ohmic environment. The system is treated as a Luttinger liquid with an impurity, whose effective interaction parameter is defined as $K=\ensuremath{\nu}(\mathcal{M}\ensuremath{-}1)/\mathcal{M}$. Conductance scaling in the weak and strong tunnel regimes is used to discuss the low-temperature transport behavior of multichannel single- and double-charge Kondo devices.

Exciton radiative lifetime in CdSe quantum dots

Colloidal CdSe quantum dots (QDs) are promising materials for solar cells because of their simple preparation process and compatibility with flexible substrates. The QD radiative recombination lifetime has attracted enormous attention as it affects the probability of photogenerated charges leaving the QDs and being collected at the battery electrodes. However, the scaling law for the exciton radiative lifetime in CdSe QDs is still a puzzle. This article presents a novel explanation that reconciles this controversy. Our calculations agree with the experimental measurements of all three divergent trends in a broadened energy window. Further, we proved that the exciton radiative lifetime is a consequence of the thermal average of decays for all thermally accessible exciton states. Each of the contradictory size-dependent patterns reflects this trend in a specific size range. As the optical band gap increases, the radiative lifetime decreases in larger QDs, increases in smaller QDs, and is weakly dependent on size in the intermediate energy region. This study addresses the inconsistencies in the scaling law of the exciton lifetime and gives a unified interpretation over a widened framework. Moreover, it provides valuable guidance for carrier separation in the thin film solar cell of CdSe QDs.

Optical Gain of Vertically Coupled Cd0.6Zn0.4Te/ZnTe Quantum Dots.

The optical modal gain of Cd0.6Zn0.4Te/ZnTe double quantum dots was measured using a variable stripe length method, where large and small quantum dots are separated with a ZnTe layer. With a large (~18 nm) separation layer thickness of ZnTe, two gain spectra were observed, which correspond to the confined exciton levels of the large and small quantum dots, respectively. With a small (~6 nm) separation layer thickness of ZnTe, a merged single gain spectrum was observed. This can be attributed to a coupled state between large and small quantum dots. Because the density of large quantum dots (4 × 1010 cm-2) is twice the density of small quantum dots (2 × 1010 cm-2), the density of the coupled quantum dots is determined by that of small quantum dots. As a result, we found that the peak gain (123.9 ± 9.2 cm-1) with the 6 nm separation layer is comparable to that (125.2 ± 29.2 cm-1) of the small quantum dots with the 18 nm separation layer.

Improvement of monodispersity of PbS quantum dots by filtration with organic solvent-resistant polyamide hollow fiber membranes

We report that membrane filtration can replace centrifugation as a highly efficient size classification process of colloidal quantum dots (QDs) after chemical synthesis. The production of colloidal QDs requires the separation of the targeted QDs dispersed in organic solvents from other by-products. The separation process has been conventionally performed by centrifugation. We investigated replacing the centrifugation with filtration using organic solvent-resistant polyamide hollow fiber membranes (HFMs). By choosing the pore size of HFMs, QDs of arbitrary size were classified. It was also demonstrated that not only large QDs but also small QDs could be separated simultaneously by using HFM with a dense layer of polyamide inside. Consequently, highly monodisperse QDs were easily obtained in a single filtration operation using HFMs.

Strain Relaxation of InAs Quantum Dots on Misoriented InAlAs(111) Metamorphic Substrates

We investigate in detail the role of strain relaxation and capping overgrowth in the self-assembly of InAs quantum dots by droplet epitaxy. InAs quantum dots were realized on an InAlAs metamorphic buffer layer grown on a GaAs(111)A misoriented substrate. The comparison between the quantum electronic calculations of the optical transitions and the emission properties of the quantum dots highlights the presence of a strong quenching of the emission from larger quantum dots. Detailed analysis of the surface morphology during the capping procedure show the presence of a critical size over which the quantum dots are plastically relaxed.

Microscopic Origin of the Effective Spin-Spin Interaction in a Semiconductor Quantum Dot Ensemble

We present a microscopic model for a singly charged quantum dot (QD) ensemble to reveal the origin of the long-range effective interaction between the electron spins in the QDs. Wilson's numerical renormalization group (NRG) is used to calculate the magnitude and the spatial dependency of the effective spin-spin interaction mediated by the growth-induced wetting layer. Surprisingly, we found an antiferromagnetic Heisenberg coupling for very short inter-QD distances that is caused by the significant particle-hole asymmetry of the wetting layer band at very low filling. Using the NRG results obtained from realistic parameters as input for a semiclassical simulation for a large QD ensemble, we demonstrate that the experimentally reported phase shifts in the coherent spin dynamics between single- and two-color laser pumping can be reproduced by our model, solving a long-standing open problem of the microscopic origin of the inter-QD electron spin-spin interaction.

A shuttling-based two-qubit logic gate for linking distant silicon quantum processors

Control of entanglement between qubits at distant quantum processors using a two-qubit gate is an essential function of a scalable, modular implementation of quantum computation. Among the many qubit platforms, spin qubits in silicon quantum dots are promising for large-scale integration along with their nanofabrication capability. However, linking distant silicon quantum processors is challenging as two-qubit gates in spin qubits typically utilize short-range exchange coupling, which is only effective between nearest-neighbor quantum dots. Here we demonstrate a two-qubit gate between spin qubits via coherent spin shuttling, a key technology for linking distant silicon quantum processors. Coherent shuttling of a spin qubit enables efficient switching of the exchange coupling with an on/off ratio exceeding 1000, while preserving the spin coherence by 99.6% for the single shuttling between neighboring dots. With this shuttling-mode exchange control, we demonstrate a two-qubit controlled-phase gate with a fidelity of 93%, assessed via randomized benchmarking. Combination of our technique and a phase coherent shuttling of a qubit across a large quantum dot array will provide feasible path toward a quantum link between distant silicon quantum processors, a key requirement for large-scale quantum computation.

Synthesis and Optical Properties of Ag–Ga–S Quantum Dots

Ag–Ga–S quantum dots (QDs) are obtained by colloidal synthesis in aqueous solutions with glutathione ligands at mild conditions. The QDs can be assigned to metastable orthorhombic, rhombohedral, or rocksalt‐type phases, assuming a significant internal pressure within crystallites, while the conventional tetragonal AgGaS2 phase is less likely formed. Similarly to these metastable phases, the QDs reveal a relatively narrow indirect bandgap contrary to the wide and direct bandgap of tetragonal AgGaS2. Size‐selected fractions of the colloidal solutions are separated by repeated centrifuging with addition of 2‐propanol. The QD size is evaluated from atomic force microscopy, while the crystallite size determined using the Scherrer formula is underestimated. X‐ray photoemission spectroscopy reveals that the QD composition changes from nearly stoichiometric for the fraction with the largest QDs toward Ag deficient and S rich for smaller QDs. The synthesized QDs are characterized by the absorption edge blueshift with decreasing QD size. A blueshift of the photoluminescence (PL) peak with decreasing QD size is observed as well as a nonmonotonous size dependence of the PL intensity. Raman spectra of the Ag–Ga–S QDs are discussed compared to those of bulk crystals and a strong contribution of surface phonons is revealed.