Properties Of Semiconductor Quantum Dots Research Articles

Probing surface interactions in CdSe quantum dots with thiocyanate ligands.

Surface chemistry dictates the optoelectronic properties of semiconductor quantum dots (QDs). Tailoring these properties relies on the meticulous selection of surface ligands for efficient passivation. While long-chain organic ligands boast a well-understood passivation mechanism, the intricacies of short inorganic ionic ligands remain largely unexplored. This study sheds light on the surface-passivation mechanism of short inorganic ligands, particularly focusing on SCN- ions on CdSe QDs. Employing steady-state and time-resolved infrared spectroscopic techniques, we elucidated the surface-ligand interactions and coordination modes of SCN--capped CdSe QDs. Comparative analysis with studies on CdS QDs unveils intriguing insights into the coordination behavior and passivation efficacy of SCN- ions on Cd2+ rich QD surfaces. Our results reveal the requirement of both surface-bound (strong binding) and weakly-interacting interfacial SCN- ions for effective CdSe QD passivation. Beyond fostering a deeper understanding of surface-ligand interactions and highlighting the importance of a comprehensive exploration of ligand chemistries, this study holds implications for optimizing QD performance across diverse applications.

Assessing the Steric Impact of Surface Ligands on the Proteolytic Turnover of Quantum Dot-Peptide Conjugates.

Proteases are important biomarkers and targets for the diagnosis and treatment of disease. The advantageous properties of semiconductor quantum dots (QDs) have made these nanoparticles useful as probes for protease activity; however, the effects of QD surface chemistry on protease activity are not yet fully understood. Here, we present a systematic study of the impact of sterics on the proteolysis of QD-peptide conjugates. The study utilized eight proteases (chymotrypsin, trypsin, endoproteinase Lys C, papain, endoproteinase Arg C, thrombin, factor Xa, and plasmin) and 41 distinct surface chemistries. The latter included three molecular weights of each of three macromolecular ligands derived from dextran and polyethylene glycol, as well as anionic and zwitterionic small-molecule ligands, and an array of mixed coatings of macromolecular and small-molecule ligands. These surface chemistries spanned a diversity of thicknesses, densities, and packing organization, as characterized by gel electrophoresis, capillary electrophoresis, dynamic light scattering, and infrared spectroscopy. The macromolecular ligands decreased the adsorption of proteases on the QDs and decelerated proteolysis of the QD-peptide conjugates via steric hindrance. The properties of the QD surface chemistry, rather than the protease properties, were the main factor in determining the magnitude of deceleration. The broad scope of this study provides insights into the many ways in which QD surface chemistry affects protease activity, and will inform the development of optimized nanoparticle-peptide conjugates for sensing of protease activity and resistance to unwanted proteolysis.

State of the Art of Continuous and Atomistic Modeling of Electromechanical Properties of Semiconductor Quantum Dots.

The main intent of this paper is to present an exhaustive description of the most relevant mathematical models for the electromechanical properties of heterostructure quantum dots. Models are applied both to wurtzite and zincblende quantum dot due to the relevance they have shown for optoelectronic applications. In addition to a complete overview of the continuous and atomistic models for the electromechanical fields, analytical results will be presented for some relevant approximations, some of which are unpublished, such as models in cylindrical approximation or a cubic approximation for the transformation of a zincblende parametrization to a wurtzite one and vice versa. All analytical models will be supported by a wide range of numerical results, most of which are also compared with experimental measurements.

Impurity-related electronic properties in GaAs quantum disk under external excitations considering the confinement parabolicity effect

In this study, we investigated the ionization energy of the lowest bound-state donor-impurity in GaAs quantum disks under external electric and magnetic fields. We also examined its diamagnetic response, taking into account the parabolic confinement potential. Using the effective-mass approximation and Ritz's variational approach, we solved the Schrödinger equation. Our results demonstrate that both the electric and magnetic fields have a significant impact on the ionization energy and diamagnetic response. Additionally, the inclusion of the parabolic confinement potential changes the behavior of these properties. This study contributes to the understanding of the magneto-electrical properties of semiconductor quantum dots under external excitations and builds on our prior work on this topic.

Voltammetric determination of electronic structure of quantum dots

Electronic structure plays an important role in determining the physiochemical properties of semiconductor quantum dots (QDs). Fabrication of high-performance QD devices relies on the reliable determination of electronic structure of QDs. Voltammetry enables the easily accessible detection on the energy levels of QDs. Herein, the fundamentals of voltammetric detection are first reviewed and discussed. Since the common ways used for tailoring electronic structure of QDs include tuning size, surface engineering, and varying composition, we next summarize the recent research on using voltammetry for probing the energy levels when studying these three effects.

Luminescence encoding of polymer microbeads with organic dyes and semiconductor quantum dots during polymerization

Luminescence-encoded microbeads are important tools for many applications in the life and material sciences that utilize luminescence detection as well as multiplexing and barcoding strategies. The preparation of such beads often involves the staining of premanufactured beads with molecular luminophores using simple swelling procedures or surface functionalization with layer-by-layer (LbL) techniques. Alternatively, these luminophores are sterically incorporated during the polymerization reaction yielding the polymer beads. The favorable optical properties of semiconductor quantum dots (QDs), which present broadly excitable, size-tunable, narrow emission bands and low photobleaching sensitivity, triggered the preparation of beads stained with QDs. However, the colloidal nature and the surface chemistry of these QDs, which largely controls their luminescence properties, introduce new challenges to bead encoding that have been barely systematically assessed. To establish a straightforward approach for the bead encoding with QDs with minimized loss in luminescence, we systematically assessed the incorporation of oleic acid/oleylamine-stabilized CdSe/CdS-core/shell-QDs into 0.5–2.5 µm-sized polystyrene (PS) microspheres by a simple dispersion polymerization synthesis that was first optimized with the organic dye Nile Red. Parameters addressed for the preparation of luminophore-encoded beads include the use of a polymer-compatible ligand such as benzyldimethyloctadecylammonium chloride (OBDAC) for the QDs, and crosslinking to prevent luminophore leakage. The physico-chemical and optical properties of the resulting beads were investigated with electron microscopy, dynamic light scattering, optical spectroscopy, and fluorescence microscopy. Particle size distribution, fluorescence quantum yield of the encapsulated QDs, and QD leaking stability were used as measures for bead quality. The derived optimized bead encoding procedure enables the reproducible preparation of bright PS microbeads encoded with organic dyes as well as with CdSe/CdS-QDs. Although these beads show a reduced photoluminescence quantum yield compared to the initially very strongly luminescent QDs, with values of about 35%, their photoluminescence quantum yield is nevertheless still moderate.

First-principles study of electronic and optical properties of small edge-functionalized penta-graphene quantum dots

Tailoring the optoelectronic properties of semiconductor quantum dots is essential for designing functionalized nanoscale devices. In this work, we use first-principles calculations to study the optoelectronic properties of small penta-graphene quantum dots (PGQDs) with various edge-functionalized groups, including hydrogen, halogen (fluorine, chlorine, and bromine), and hydroxyl functional groups. It is evident that these quantum dots, especially those passivated by hydrogen atoms, are thermally stable in vacuum. Moreover, the larger the quantum dots, the more negative the formation energy on stability could reach, thus forming thermodynamically more stable quantum dots. All investigated PGQDs exhibit semiconductor properties. Their bandgaps decrease with an increase in the size of the quantum dots, resulting from the hybridization of sp2 and sp3 carbon atoms and from the charge depletion or accumulation between the passivated atoms and the principal components upon interactions. Concurrently, this study aims to explain the optical absorption anisotropy induced by the edge-functionalized groups of PGQDs under multiple incident light polarizations. These results highlight the use of edge-functionalized groups to develop the next generation of optoelectronic devices.

Floquet engineering of excitons in semiconductor quantum dots

Within the Floquet theory of periodically driven quantum systems, we demonstrate that a high-frequency electromagnetic field can be used as an effective tool to control excitonic properties of semiconductor quantum dots (QDs). It is shown, particularly, that the field both decreases the exciton binding energy and dynamically stabilizes the exciton, increasing its radiative lifetime. The developed theory can serve as a basis for the ultrafast method to tune spectral characteristics of the QD-based photon emitters by a high-frequency field.

Wafer-scale epitaxial modulation of quantum dot density

Precise control of the properties of semiconductor quantum dots (QDs) is vital for creating novel devices for quantum photonics and advanced opto-electronics. Suitable low QD-densities for single QD devices and experiments are challenging to control during epitaxy and are typically found only in limited regions of the wafer. Here, we demonstrate how conventional molecular beam epitaxy (MBE) can be used to modulate the density of optically active QDs in one- and two- dimensional patterns, while still retaining excellent quality. We find that material thickness gradients during layer-by-layer growth result in surface roughness modulations across the whole wafer. Growth on such templates strongly influences the QD nucleation probability. We obtain density modulations between 1 and 10 QDs/µm2 and periods ranging from several millimeters down to at least a few hundred microns. This method is universal and expected to be applicable to a wide variety of different semiconductor material systems. We apply the method to enable growth of ultra-low noise QDs across an entire 3-inch semiconductor wafer.

Core-Size-Dependent Trapping and Detrapping Dynamics in CdSe/CdS/ZnS Quantum Dots

The fascinating optoelectronic properties of semiconductor quantum dots (QDs) originate from the quantum confinement effect; i.e., the band gap increases as the size decreases. Another significant ...

Structural Dynamics and Electronic Properties of Semiconductor Quantum Dots: Computational Insights

Semiconductor quantum dots (QDs) exhibit exciting photophysical properties for a wide variety of applications in the field of energy conversion, lighting, and more recently quantum communication. The photophysics underpinning these applications at ambient conditions strongly depends on atomistic details of their dynamic surface. Neutral organic ligands used in surface passivation play a critical role in determining the structural and optoelectronic properties of these QDs. Small sizes and irregular atomic arrangements make these QD surface–ligand interfaces challenging to explore at the atomistic level. Here, we combine several computational simulation techniques to study thermally induced geometrical fluctuations in stable cadmium selenide (CdSe) QDs with and without ligand passivation. We find that structural fluctuations of surface atoms significantly depend on passivating molecules. The bulky and strongly binding ligands such as phosphine oxides induce a higher extent of interfacial dynamics. Though these stoichiometric QDs do not possess any permanent in-gap states, significant thermal distortion of QD–ligand interfaces can induce fluctuating defectlike states near band edges. Such vibronic dynamics in these QDs also modifies band-edge state positions on sub-picosecond timescale, impacting their functional properties. Our results further suggest that primary amine ligands are optimal choices for QD passivation. These insights may be helpful to make design principles for screening and optimizing passivating ligands best suited for various QD applications, such as for quantum communication technologies.

Vibronic Excitons and Conical Intersections in Semiconductor Quantum Dots.

Surface defects and organic surface-capping ligands affect the photoluminescence properties of semiconductor quantum dots (QDs) by altering the rates of competing nonradiative relaxation processes. In this study, broadband two-dimensional electronic spectroscopy reveals that absorption of light by QDs prepares vibronic excitons, excited states derived from quantum coherent mixing of the core electronic and ligand vibrational states. Rapidly damped coherent wavepacket motions of the ligands are observed during hot-carrier cooling, with vibronic coherence transferred to the photoluminescent state. These findings suggest a many-electron, molecular theory for the electronic structure of QDs, which is supported by calculations of the structures of conical intersections between the exciton potential surfaces of a small ammonia-passivated model CdSe nanoparticle.

Observing Multiexciton Correlations in Colloidal Semiconductor Quantum Dots via Multiple-Quantum Two-Dimensional Fluorescence Spectroscopy.

Correlations between excitons, that is, electron-hole pairs, have a great impact on the optoelectronic properties of semiconductor quantum dots and thus are relevant for applications such as lasers and photovoltaics. Upon multiphoton excitation, these correlations lead to the formation of multiexciton states. It is challenging to observe these states spectroscopically, especially higher multiexciton states, because of their short lifetimes and nonradiative decay. Moreover, solvent contributions in experiments with coherent signal detection may complicate the analysis. Here we employ multiple-quantum two-dimensional (2D) fluorescence spectroscopy on colloidal CdSe1-xSx/ZnS alloyed core/shell quantum dots. We selectively map the electronic structure of multiexcitons and their correlations by using two- and three-quantum 2D spectroscopy, conducted in a simultaneous measurement. Our experiments reveal the characteristics of biexcitons and triexcitons such as transition dipole moments, binding energies, and correlated transition energy fluctuations. We determine the binding energies of the first six biexciton states by simulating the two-quantum 2D spectrum. By analyzing the line shape of the three-quantum 2D spectrum, we find strong correlations between biexciton and triexciton states. Our method contributes to a more comprehensive understanding of multiexcitonic species in quantum dots and other semiconductor nanostructures.

Method for analyzing the electrophysical properties of semiconductor quantum dots

Some important properties of the InSb and CdSe quantum dots, such as size and energy spectrum, were studied by method of normalized differential tunneling current-voltage characteristics. The results of size evaluation are qualitatively and quantitatively consistent with the results obtained by TEM and analysis of spectral dependence of absorption coefficient and luminescence with an error less than 15%. During the study it was also shown, that method of normalized differential tunneling current-voltage characteristics also allows us to analyze the energy spectrum of semiconductor quantum dots (position of the first three energy levels).

Effect of p-doping on the intensity noise of epitaxial quantum dot lasers on silicon.

This work experimentally investigates the impact of p-doping on the relative intensity noise (RIN) properties and subsequently on the modulation properties of semiconductor quantum dot (QD) lasers epitaxially grown on silicon. Owing to the low threading dislocation density and the p-modulation doped GaAs barrier layer in the active region, the RIN level is found very stable with temperature with a minimum value of -150dB/Hz. The dynamical features extracted from the RIN spectra show that p-doping between zero and 20 holes/dot strongly modifies the modulation properties and gain nonlinearities through increased internal losses in the active region and thereby hinders the maximum achievable bandwidth. Overall, this Letter is important for designing future high-speed and low-noise QD devices integrated in future photonic integrated circuits.

A highly sensitive fluorescent immunosensor for sensitive detection of nuclear matrix protein 22 as biomarker for early stage diagnosis of bladder cancer.

A novel strategy is reported for highly sensitive, rapid, and selective detection of nuclear matrix protein NMP22 using two-color quantum dots based on fluorescence resonance energy transfer (FRET). Quantum dots (QDs) are highly advantageous for biological imaging and analysis, particularly when combined with (FRET) properties of semiconductor quantum dot (QDs) are ideal for biological analysis to improve sensitivity and accuracy. In this FRET system narrowly dispersed green emitting quantum dot CdTe core is used as a donor and labelled by monoclonal (mAb) antibody, while orange emitting quantum dot CdTe/CdS core shell is used as an accepter and labelled by polyclonal (pAb) antibody. The quantum dots are labelled by antibodies using EDC/NHS as crosslinking agent. Bovine serum albumin (BSA) solution was added to block nonspecific binding sites. The fluorescence intensity of QDs accepter decreased linearly with the increasing concentrations of NMP22 from 2–22 pg mL−1 due to FRET system and fluoroimmunoassay reaction. This method has good regression coefficient (R2 = 0.998) and detection limit was 0.05 pg mL−1. The proposed FRET-based immunosensor provides a quick, simple and sensitive immunoassay tool for protein detection, and can be considered as a promising approach for clinical applications. The proposed FRET-based immunosensor provides a quick, simple and sensitive immunoassay tool for protein detection, and can be considered as a promising approach for clinical applications.

Generation of Non‐Classical Light Using Semiconductor Quantum Dots

AbstractSources of non‐classical light are of paramount importance for future applications in quantum science and technology such as quantum communication, quantum computation and simulation, quantum sensing, and quantum metrology. This Review is focused on the fundamentals and recent progress in the generation of single photons, entangled photon pairs, and photonic cluster states using semiconductor quantum dots. Specific fundamentals which are discussed are a detailed quantum description of light, properties of semiconductor quantum dots, and light–matter interactions. This includes a framework for the dynamic modeling of non‐classical light generation and two‐photon interference. Recent progress is discussed in the generation of non‐classical light for off‐chip applications as well as implementations for scalable on‐chip integration.

Quantum Dot Based Solar Cells: Role of Nanoarchitectures, Perovskite Quantum Dots, and Charge-Transporting Layers.

Quantum dot solar cells (QDSCs) are attractive technology for commercialization, owing to various advantages, such as cost effectiveness, and require relatively simple device fabrication processes. The properties of semiconductor quantum dots (QDs), such as band gap energy, optical absorption, and carrier transport, can be effectively tuned by modulating their size and shape. Two types of architectures of QDSCs have been developed: 1) photoelectric cells (PECs) fabricated from QDs sensitized on nanostructured TiO2 , and 2) photovoltaic cells fabricated from a Schottky junction and heterojunction. Different types of semiconductor QDs, such as a secondary, ternary, quaternary, and perovskite semiconductors, are used for the advancement of QDSCs. The major challenge in QDSCs is the presence of defects in QDs, which lead to recombination reactions and thereby limit the overall performance of the device. To tackle this problem, several strategies, such as the implementation of a passivation layer over the QD layer and the preparation of core-shell structures, have been developed. This review covers aspects of QDSCs that are essential to understand for further improvement in this field and their commercialization.

Influence of capping ligands on the assembly of quantum dots and their properties

ABSTRACTIn order to understand the effect of capping ligand on optical and electrical properties of semiconductor quantum dots (QDs), we have added liquid crystal (8CB) to the CdSe/CdS/ZnS QDs monolayer formed via the Langmuir Blodgett technique And studied emission spectra and conducting properties of resulting QDs. The assembly of QDs monolayer modified by liquid crystal (8CB) can be tuned by varying the temperature. The mechanism of the influence of capping ligands on the gap energy, dipole moment and charge distribution within (CdSe)13 cluster was studied via quantum chemical calculations, i.e. we have used density functional theory to systematically investigate the equilibrium configuration of QDs passivated by oleic acid and liquid crystal ligands.

(Invited) Photoionization in Doped and Undoped Semiconductor Quantum Dots

Optoelectronic properties of semiconductor quantum dots strongly depend on the propensity of their band carriers to transition to states extrinsic to the nanocrystal. Photoionization events typically reduce the photoluminescence quantum yield and induce emission intermittency and therefore they are often suppressed in light-emitting applications by the use of passivating barriers in the form of protective ligands or core/shell heterostructuring. In other uses of quantum dots, e.g. as photocatalysts, photocathodes, photodetectors, or solar cells, fast and efficient extraction of carriers to extrinsic states is of key importance. Here, we investigate the photoemission of carriers induced by carrier-carrier or carrier-dopant interactions in intrinsic and intentionally doped nanocrystals. Our studies reveal the ultrafast time scale, spectral response, and interaction mechanism of hot-carrier photoemission in these systems. Application-relevant performance parameters such as quantum efficiency and temporal response will be discussed.