II-VI Semiconductor Quantum Dots Research Articles

(Invited) Dynamics of Photoluminescence in Mn-Doped Pb-Halide Perovskite Nanocrystals

Nanocrystals of lead halide perovskites have been shown to have extraordinary photoluminescence properties, spanning the entire visible range achieved by the composition tuning via solid solutions at the halide site. Taking cues from Mn doping of traditional Group II-VI semiconductor quantum dots, Mn doping of the lead halide perovskites has been extensively explored by the community, establishing intense Mn-dopant induced emission that is significantly Stokes shifted from the excitonic emissions of the host. While all reports agree on the essential features of this emission, such as the peak position and the width of the Mn photoluminescence, there appears to be little agreement on the microscopic processes by which the host sensitizes Mn following its photoexcitation and the steps that control the deexcitation leading to the Mn emission. There are even reports of disagreement between different reports on the temperature dependence of the Mn emission and its origin. I shall discuss the present status of this field and establish, with the help of new experimental data as well as the already reported ones, what appears to be the microscopic processes controlling the excited dynamics in the Mn-doped lead halide perovskites.

Recent Progress in the Composites of Perovskite Nanocrystals and II-VI Quantum Dots: Their Synthesis, Applications, and Prospects

Abstract: The remarkable photoelectric characteristics of perovskite nanocrystals (NCs), including high fault tolerance, tunable photoluminescence (PL) emission, and high carrier mobility, contribute to making them especially attractive for photonic and optoelectronic applications. Unfortunately, the poor environmental thermal and light stability set obstacles to their industrial applications. Over the past 40 years, II-VI semiconductor quantum dots (QDs) have achieved many important photophysics findings and optoelectronic applications. Compared with perovskite NCs, II-VI semiconductor QDs still have a relatively weaker molar absorbance coefficient. Whereas, significant enhancement of both the stability and the optical performance of the composites of perovskite NCs and II-VI QDs are of interest for photovoltaic and optoelectronic devices. The composites of perovskite NCs and II-VI QDs come in two primary types: core/shell structures and heterojunction structures. To better understand the composites of perovskite NCs and II-VI QDs, the approaches of synthesis methods, their optoelectronic properties, carrier dynamics and potential applications in solar cells, light emitting diodes (LEDs) and photodetectors are summarized. Furthermore, the unmet problems and the potential applications are also presented.

Enhanced Spin Polarization from Biaxially Strained Colloidal Quantum Dots.

Electron and hole spin polarization is crucial for quantum dots to be used in spin lasers and quantum information processing. However, the degree of spin polarization in II-VI and III-V semiconductor quantum dots is low because of the degenerated valence band. Here, we increase the light and heavy hole degeneracy by introducing biaxial strain into CdSe-based quantum dots, enabling the degree of spin polarization to be increased from 20% to 50% under photoexcitation. The optical gain threshold measurement further reveals that the increase in polarization helps to reduce the gain threshold.

Resonant tunneling in a colloidal CdS semiconductor quantum-dot single-electron transistor based on heteroepitaxial-spherical Au/Pt nanogap electrodes.

Semiconductor quantum dots (QDs) have unique discrete energy levels determined by the particle size and material. Therefore, they have potential applications as novel optical and electronic devices. Among those, colloidal group II-VI semiconductor quantum dots stand out for their facile synthesis and band gaps aligned with the visible light spectrum. However, the electrical characterization studies of an individual quantum dot necessitate the size of nanogap electrodes being equal to the size of the quantum dot, which has conventionally been evaluated using techniques such as scanning tunneling microscopy (STM) and nanogaps fabricated by electromigration. The complexity of device fabrication has restricted research in this area. Here, we present a pioneering approach for the electrical characterization of single-QD: heteroepitaxial-spherical (HS) Au/Pt nanogap electrodes. We fabricated transistors through chemisorption, an anchoring colloidal CdS QD (3.8 nm) between the HS-Au/Pt nanogap electrodes (gap separation: 4.5 nm). The resulting device functions as a quantum-dot single-electron transistor (QD-SET), showing resonant tunneling-an inherent characteristic of the QD. A steep current increase was observed at a negative voltage, apart from the theoretical single-electron tunneling current by Coulomb blockade phenomena, which agreed with the theoretical resonant tunneling current through a discrete energy level of the QD. This underscores the promise of HS-Au/Pt nanogap electrodes in realizing single-QD devices, offering a pathway toward unlocking their full potential.

Spectral widths and Stokes shifts in InP-based quantum dots.

InP-based quantum dots (QDs) have Stokes shifts and photoluminescence (PL) line widths that are larger than in II-VI semiconductor QDs with comparable exciton energies. The mechanisms responsible for these spectral characteristics are investigated in this paper. Upon comparing different semiconductors, we find the Stokes shift decreases in the following order: InP > CdTe > CdSe. We also find that the Stokes shift decreases with core size and decreases upon deposition of a ZnSe shell. We suggest that the Stokes shift is largely due to different absorption and luminescent states in the angular momentum fine structure. The energy difference between the fine structure levels, and hence the Stokes shifts, are controlled by the electron-hole exchange interaction. Luminescence polarization results are reported and are consistent with this assignment. Spectral widths are controlled by the extent of homogeneous and inhomogeneous broadening. We report PL and PL excitation (PLE) spectra that facilitate assessing the roles of homogeneous and different inhomogeneous broadening mechanisms in the spectra of zinc-treated InP and InP/ZnSe/ZnS particles. There are two distinct types of inhomogeneous broadening: size inhomogeneity and core-shell interface inhomogeneity. The latter results in a distribution of core-shell band offsets and is caused by interfacial dipoles associated with In-Se or P-Zn bonding. Quantitative modeling of the spectra shows that the offset inhomogeneity is comparable to but somewhat smaller than the size inhomogeneity. The combination of these two types of inhomogeneity also explains several aspects of reversible hole trapping dynamics involving localized In3+/VZn2- impurity states in the ZnSe shells.

Electronic structure study of dual-doped II-VI semiconductor quantum dots towards single-source white light emission.

Single-source white light emitting colloidal semiconductor quantum dots (QDs) is one of the most exciting and promising high-quality solid-state light sources to meet the current global demand for sustainable resources. While most of the previous methods involve dual (green-red) emissive nanostructures coated on blue LEDs to achieve white light, this work describes a single-source white light emitter of robust and superior quality using dual-doping. A modified synthesis method for intense white light emitting Cu, Mn dual-doped ZnSe QDs is engineered such that the extent of doping and concentration of ligands can alter their electronic structures. This is then customized to obtain various types of white light emissions ranging from warm white to cool white. Further, the composition-driven change in the electronic structure of the host QDs is exploited to achieve emission tunability over the entire visible range.

Spin dynamics of positively charged excitons in Cr+ -doped quantum dots probed by resonant photoluminescence

We study the dynamics of the spin system that consists of a positively charged II-VI semiconductor quantum dot doped with a single ${\mathrm{Cr}}^{+}$ ion. The resonant photoluminescence (PL) of the positively charged exciton coupled with the ${\mathrm{Cr}}^{+}$ spin is used to analyze the main spin relaxation channels. The intensity of the resonant PL is reduced by an optical pumping of the spin of the resident hole-${\mathrm{Cr}}^{+}$ complex that can be seen as a nanomagnet. The spin memory can be partially erased by a nonresonant optical excitation. This leads to an increase of the resonant PL signal. The resonant PL is cocircularly polarized and corresponds to relaxation channels that conserve the ${\mathrm{Cr}}^{+}$ spin $|{S}_{z}|$. The observation in the resonant-PL excitation spectra of optical transitions with a change of the ${\mathrm{Cr}}^{+}$ spin permits to determine the magnetic anisotropy of the magnetic atom. Optical pumping, autocorrelation measurements, and the power dependence of the PL intensity distribution show that the effective temperature of the hole-${\mathrm{Cr}}^{+}$ spin system is affected by the optical excitation through the local generation of phonons.

Safe Synthesis of InP Quantum Dots for Biological Applications

Semiconductor nanocrystals (also known as quantum dots) have been extensively studied because of their unique optical properties which makes them promising candidates for bio imaging and bio sensing. II-VI semiconductor quantum dots are commonly used in biomedical applications because of their near unit quantum yields and narrow emission profiles. Despite these advantages, II-VI semiconductor nanocrystals contain toxic metals. As a result, III-V semiconductor nanocrystals such as InP are promising replacements for cadmium-based semiconductor nanocrystals. However, the synthesis of bright InP quantum dots with narrow size distribution is challenging mainly due to the use of overly reactive phosphorus precursors like tris(trimethylsilyl) phosphine (TMS)3P. This reagent is also highly flammable. We employed less pyrophoric sterically encumbered tris(triethylsilyl) phosphine and tris(tributylsilyl) phosphine for the synthesis and observed that quantum yield and the emission color saturation (fwhm) of InP nanocrystals are improved compared to those prepared with (TMS)3P, but not up to the levels realized with CdSe. HF treatment is often employed as a post-synthetic treatment to improve the optical properties of quantum dots. We demonstrate that ammonium bifluoride is a safer alternative to extremely hazardous HF. Ammonium bifluoride etched InP core dots were further passivated to create InP/ZnSeS core-shell quantum dots that were later water-solubilized and functionalized for biological applications. Figure 1

Doped or not doped? Importance of the local structure of Mn (II) in Mn doped perovskite nanocrystals

Transition metal doping of semiconductor nanocrystals (NCs) can generate new optical, magnetic, properties through dopant-host interaction. Although Mn2+ doping in semiconducting NCs has been studied for decades, Mn doped perovskite NCs have opened up new avenues for optoelectronic applications due to signature Mn d-d emission. However, Mn doping in bromide-based perovskite NCs have not shown this signature peak sowing doubts about the efficient doping in these systems. Here, we demonstrate that the chemical bonding and local environment of Mn obtained using electron paramagnetic resonance (EPR) and X-ray absorption fine structure (XAFS) is similar to that of chloride-based perovskites. However, the differences in optical properties between the chloride and bromide-based perovskites NCs arises due to fundamental difference in mechanism in perovskite NCs compared to the II-VI semiconductor quantum dots. This provides some insight into this problem from a fundamental perspective leading to more efficient synthesis techniques for applications.

Impact of transition metal ion doping on electron spin relaxation time in CdSe/ZnSe quantum dots

Theoretical calculations of spin relaxation time (SRT) of conduction electrons have been carried out considering the relaxation mediated by acoustic phonons using k.p perturbation theory and envelope function approximation in a transition metal doped II-VI semiconductor quantum dot under the strong confinement regime. In this calculation, we are considering the transitions in the Zeeman sublevels arising due to magnetic impurity doping and applied magnetic field in a Mn doped CdSe/ZnSe quantum dots. The occurrence of spin polarization switching at moderately low applied magnetic field is established in Cd1−xMnxSe/ZnSe quantum dots. The spin relaxation times have been found to be considerably longer with a higher dopant concentration in small magnetic fields (B < 2T) and at very low temperature (T < 50 K) regime. The results may help to demonstrate that, such small quantum dots can successfully be used as polarization switch in different spintronic nano-device.

DFT calculations of metal-organic I-III-VI semiconductor clusters: Benchmark of exchange-correlation functionals and localized basis sets

The primary objective of this work was to find the numerical methods suitable for DFT calculations of ternary I-III-VI semiconductor quantum dots emerging as new functional materials which demonstrate an increasing need to comprehend their complex physicochemical properties. The benchmarking analysis of 8 exchange-correlation functionals and 11 basis sets including all-electron and effective core potential ones was performed. Four metal-organic molecules, widely used as a single-precursor in the synthesis of A-In-X2 semiconductor quantum dots (A = Cu, Ag; X = S, Se) were considered as simple representatives of ternary semiconductor quantum dots. The geometrical parameters of the optimized structures were compared to the X-ray diffraction data. The hybrid PBE0 and B3PW91 functionals were found to be the best performing methods especially when connected with cc-PVDZ or Def2-SVP basis sets. The methods widely used in the previous calculations of II-VI semiconductor quantum dots, namely B3LYP functional and LANL2DZ or SBKJC basis sets, in this case resulted in lower agreement with the experimental data.

Direct Measurement of Hyperfine Shifts and Radio Frequency Manipulation of Nuclear Spins in Individual CdTe/ZnTe Quantum Dots.

We achieve direct detection of electron hyperfine shifts in individual CdTe/ZnTe quantum dots. For the previously inaccessible regime of strong magnetic fields B_{z}≳0.1 T, we demonstrate robust polarization of a few-hundred-particle nuclear spin bath, with an optical initialization time of ∼1 ms and polarization lifetime exceeding ∼1 s. Nuclear magnetic resonance spectroscopy of individual dots reveals strong electron-nuclear interactions characterized by Knight fields |B_{e}|≳50 mT, an order of magnitude stronger than in III-V semiconductor quantum dots. Our studies confirm II-VI semiconductor quantum dots as a promising platform for hybrid electron-nuclear spin qubit registers, combining the excellent optical properties comparable to III-V dots and the dilute nuclear spin environment similar to group-IV semiconductors.

The Middle Road Less Taken: Electronic-Structure-Inspired Design of Hybrid Photocatalytic Platforms for Solar Fuel Generation

The development of efficient solar energy conversion to augment other renewable energy approaches is one of the grand challenges of our time. Water splitting, or the disproportionation of H2O into energy-dense fuels, H2 and O2, is undoubtedly a promising strategy. Solar water splitting involves the concerted transfer of four electrons and four protons, which requires the synergistic operation of solar light harvesting, charge separation, mass and charge transport, and redox catalysis processes. It is unlikely that individual materials can mediate the entire sequence of charge and mass transport as well as energy conversion processes necessary for photocatalytic water splitting. An alternative approach, emulating the functioning of photosynthetic systems, involves the utilization of hybrid systems wherein different components perform the various functions required for solar water splitting. The design of such hybrid systems requires the multiple components to operate in lockstep with optimal thermodynamic driving forces and interfacial charge transfer kinetics. This Account describes a new class of nanoscale heterostructures comprising M xV2O5 nanowires, where M is a p-block cation with a ( n - 1) d10 ns2 np0 electronic configuration characterized by a stereoactive lone pair of electrons and x is its stoichiometry, interfaced with II-VI semiconductor quantum dots (QDs). Photocatalytic water splitting involves the transfer of excited-state holes from QDs to mid-gap states (derived from the stereoactive lone pairs of p-block cations) of nanowires, hole transport through nanowires, the reduction of protons at a QD-immobilized catalyst, and water oxidation at an anode. The M xV2O5/QD architectures provide a vast design space for evolutionary optimization of function with considerable tunability of composition and structure of the individual components as well as of the interfacial structure, thereby facilitating programmability of absorption spectra, energetic offsets, and charge-transfer reactivity. The available design space spans choice of the p-block cation M, its stoichiometry x, the composition and size of various QDs, and the nature of the nanowire/QD interface. This multivariate parameter space has been navigated by integrating first-principles modeling, diversified synthesis, spectroscopic measurements, and catalytic evaluation to facilitate the rational design of several generations of heterostructures and the systematic improvement of their photocatalytic performance. The electronic structures of the target heterostructures are predicted by DFT calculations in light of the revised lone pair model of stereoactive structural distortions and evaluated by hard X-ray photoelectron spectroscopy such as to systematically tune the interfacial band offsets. Central to this approach is the development of a topochemical "etch-a-sketch" intercalation approach that allows for facile installation of p-block cations in metastable polymorphs of V2O5. The interfacial charge transfer kinetics of M xV2O5/QD heterostructures is further evaluated by transient absorption spectroscopy to measure excited-state charge-transfer dynamics and is found to depend sensitively on interfacial structure and the thermodynamic driving forces in accordance with Marcus theory. The integration of theory and experiment has allowed for the design of viable photocatalytic architectures exemplified by the exceptional catalytic performance of β-Pb xV2O5/CdX (X= S, Se) architectures, which has subsequently been elaborated to other lone-pair M xV2O5 compounds, demonstrating the effective exploitation of the opportunities for programmability available in the design space.

Synthesis of Quantum Dot-ZnS Nanosheet Inorganic Assembly with Low Thermal Fluorescent Quenching for LED Application.

In this report, to tackle the thermal fluorescent quenching issue of II-VI semiconductor quantum dots (QDs), which hinders their on-chip packaging application to light-emitting diodes (LEDs), a QD-ZnS nanosheet inorganic assembly monolith (QD-ZnS NIAM) is developed through chemisorption of QDs on the surface of two-dimensional (2D) ZnS nanosheets and subsequent assembly of the nanosheets into a compact inorganic monolith. The QD-ZnS NIAM could reduce the thermal fluorescent quenching of QDs effectively, possibly due to fewer thermally induced permanent trap states and decreased Förster resonance energy transfer (FRET) among QDs when compared with those in a reference QD composite thin film. We have demonstrated that the QD-ZnS NIAM enables QDs to be directly packaged on-chip in LEDs with over 90% of their initial luminance being retained at above 85 °C, showing advantage in LED application in comparison with conventional QD composite film.

Lead Halide Perovskite Nanocrystals in the Research Spotlight: Stability and Defect Tolerance.

This Perspective outlines basic structural and optical properties of lead halide perovskite colloidal nanocrystals, highlighting differences and similarities between them and conventional II–VI and III–V semiconductor quantum dots. A detailed insight into two important issues inherent to lead halide perovskite nanocrystals then follows, namely, the advantages of defect tolerance and the necessity to improve their stability in environmental conditions. The defect tolerance of lead halide perovskites offers an impetus to search for similar attributes in other related heavy metal-free compounds. We discuss the origins of the significantly blue-shifted emission from CsPbBr3 nanocrystals and the synthetic strategies toward fabrication of stable perovskite nanocrystal materials with emission in the red and infrared parts of the optical spectrum, which are related to fabrication of mixed cation compounds guided by Goldschmidt tolerance factor considerations. We conclude with the view on perspectives of use of the colloidal perovskite nanocrystals for applications in backlighting of liquid-crystal TV displays.

Stokes shifts of small ZnSe clusters from first-principles calculations

ABSTRACTThe large Stokes shifts of II-VI semiconductor quantum dots favour their application in bioimaging. Using a cluster model, we revealed that ZnSe clusters have large Stokes shifts at small cluster sizes by means of density functional theory calculations. Both the optical absorption and emission wavelengths of the clusters are size-dependent. While some trends were noted for the absorption spectra, the emission wavelength varies in a complicated way, leading to large fluctuations in their Stokes shifts with cluster size. These fluctuations, as well as their variations with solvent, were rationalised in terms of changes in their geometrical and electronic structures from size to size, and from ground- to excited-state.

Phase-Transfer Ligand Exchange of Lead Chalcogenide Quantum Dots for Direct Deposition of Thick, Highly Conductive Films.

The use of semiconductor nanocrystal quantum dots (QDs) in optoelectronic devices typically requires postsynthetic chemical surface treatments to enhance electronic coupling between QDs and allow for efficient charge transport in QD films. Despite their importance in solar cells and infrared (IR) light-emitting diodes and photodetectors, advances in these chemical treatments for lead chalcogenide (PbE; E = S, Se, Te) QDs have lagged behind those of, for instance, II-VI semiconductor QDs. Here, we introduce a method for fast and effective ligand exchange for PbE QDs in solution, resulting in QDs completely passivated by a wide range of small anionic ligands. Due to electrostatic stabilization, these QDs are readily dispersible in polar solvents, in which they form highly concentrated solutions that remain stable for months. QDs of all three Pb chalcogenides retain their photoluminescence, allowing for a detailed study of the effect of the surface ionic double layer on electronic passivation of QD surfaces, which we find can be explained using the hard/soft acid-base theory. Importantly, we prepare highly conductive films of PbS, PbSe, and PbTe QDs by directly casting from solution without further chemical treatment, as determined by field-effect transistor measurements. This method allows for precise control over the surface chemistry, and therefore the transport properties of deposited films. It also permits single-step deposition of films of unprecedented thickness via continuous processing techniques, as we demonstrate by preparing a dense, smooth, 5.3-μm-thick PbSe QD film via doctor-blading. As such, it offers important advantages over laborious layer-by-layer methods for solar cells and photodetectors, while opening the door to new possibilities in ionizing-radiation detectors.

Cytotoxicity studies of II-VI semiconductor quantum dots on various cancer cell lines

Enhanced understanding of diseases at the molecular level has made a paradigm shift towards identifying new biological indicators especially in nanomaterials. It is important to make Quantum Dots (QDs) more than just passive bio-probes/labels for biological imaging and cellular studies as they offers “smart” multifunctional nano-platforms. For any biomedical, optoelectronic device application, evaluation of cytotoxicity coupled with cellular uptake and internalization of QDs are imperative. This paper describes the cytotoxic studies of hydrophilic and hydrophobic QDs, capped with polyvinyl pyrrolidone (PVP) and oleic acid in human breast adenocarcinoma MCF-7, Human Embryonic Kidney (HEK-293) and Ehrlich Ascites Carcinoma (EAC) cancer cells that indicated a concentration and time dependent response in a 48 hr assay. The enhanced fluorescence emitted from the cytoplasm confirmed that the QDs were efficiently internalized by the cells. 35% cytotoxicity was observed by core-shell ZnSe/CdSe QDs in HEK-293 cells, while the hydrophobic CdSe exhibited less cytotoxicity in both MCF-7 and EAC cell lines in 48 hrs. Increased LDH leakage and decreased MTT reduction was observed in a time dependent manner. The decrease rate of LDH was found in PVP-CdSe relative to the value obtained from untreated/control cells post 24 hr. The oleic acid coating renders the core-shell CdSe QDs to be more hydrophobic thus making them less toxic due to possibly weak interaction with the cells, and low ionization of cadmium. Based on our experimental observation the sequence of cytotoxicity of tested QDs was hydrophilic greater than hydrophobic in all three cell lines.

Addition of Fluorescence Lifetime Spectroscopy to the Tool Kit Used to Study the Formation and Degradation of Luminescent Quantum Dots in Solution.

The increasing commercialization of consumer electronic products that make use of II-VI semiconductor quantum dots (QDs) has raised significant concerns about their impact on natural systems and human health once they are released into the environment at the end of the product's lifetime. In this paper, we demonstrate the addition of fluorescence lifetime spectroscopy to the existing tool kit of spectroscopic techniques to quantitatively monitor changes in QD properties as they form and degrade in solution. Our study reveals that because of its rich information content, fluorescence lifetime spectroscopy has a limited utility as a stand-alone technique in the study of QD formation and degradation. However, combining fluorescence lifetime spectroscopy with the commonly used emission quantum yield and peak width measurements along with other analytical methods, including ultraviolet-visible spectroscopy, transmission electron microscopy, and inductively coupled plasma mass spectrometry measurements, significantly enhances the existing analytical tool kit and provides the capability to monitor in real time, the formation and degradation of luminescent QDs in organic and aqueous solutions under environmentally relevant conditions.

Spin dynamics of an individual Cr atom in a semiconductor quantum dot under optical excitation

We studied the spin dynamics of a Cr atom incorporated in a II-VI semiconductor quantum dot using photon correlation techniques. We used recently developed singly Cr-doped CdTe/ZnTe quantum dots to access the spin of an individual magnetic atom. Auto-correlation of the photons emitted by the quantum dot under continuous wave optical excitation reveals fluctuations of the localized spin with a timescale in the 10 ns range. Cross-correlation gives quantitative transfer time between Cr spin states. A calculation of the time dependence of the spin levels population in Cr-doped quantum dots shows that the observed spin dynamics is dominated by the exciton-Cr interaction. These measurements also provide a lower bound in the 20 ns range for the intrinsic Cr spin relaxation time.