Welcome to the Quantum Dot Lab

The optical properties of quantum dots have created a lot of interest within the community due to their size tunable, fundamentally unique properties arising from the quantum confinement effect that are unforeseen in bulk. In this review we have highlighted some of the recent advances in the field including the synthetic challenges and advances to control not only the size but also their shape, internal structure and their coupling with the neighbouring nanocrystals leading to exciting new optical properties. We have further explored the various optical techniques that have been used to understand the electronic structure of not only ensemble and temporally averaged nanostructures but also with temporal resolution as well as single particle microscopy and hence understand the unique size dependent properties. We then explore a few of the unique properties or specialized class of nanocrystals using a combination of various optical properties.  The different classes of nanocrystals discussed in this review include materials such as transition metal doped nanocrystals as well as infrared emitting materials. The fundamentally unique processes such as blinking, that is observed in almost all single molecule nanocrystals, or ultrafast processes occurring in these nanocrystals that determine the properties of these materials have also been discussed. The review concludes with a brief discussion on the current and potential applications and a brief outlook for the future of this field.

Efficiency of the quantum dots based solar cells relies on charge transfer at the interface and hence on the relative alignment of the energy levels between materials. In spite of high demand to obtain size specific band offsets, very few studies exist and are studied using meticulous methods like photoelectron spectroscopy. However, semiconductor charging during measurements could result in indirect and possibly inaccurate measurements due to shift in valence and conduction band position. Here, in this report, we devise a novel method to study the band offsets by associating an atomic like state with the conduction band and hence obtaining an internal standard. This is achieved by doping copper in semiconductor nanocrystals, leading to the development of a characteristic intra-gap Cu-related emission feature assigned to the transition from the conduction band to the atomic like Cu d state. Using this transition we determine the relative band alignment of II-VI semiconductor nanocrystals as a function of size in the below 10 nm size regime. The results are in excellent agreement with the available photoelectron spectroscopy data as well as the theoretical data. We further use this technique to study the band edge variation as a function of temperature in CdSe nanocrystals. Additionally, surface electronic structure of CdSe nanocrystals have been studied using quantitative measurements of absolute quantum yield and PL decay studies of the Cu related emission and the band edge emission. The role of TOP and oleic acid as surface passivating ligand molecules has been studied for the first time.

Free standing ZnO nanocrystals simultaneously doped with Fe and Cu with varying Fe/Cu compositions have been synthesized using colloidal methods with a mean size of ~7.7 nm.  Interestingly, while Cu doped ZnO nanocrystal remains diamagnetic and Fe-doped samples show antiferromagnetic interactions between Fe sites without any magnetic ordering down to the lowest temperature investigated, samples doped simultaneously with Fe and Cu show a qualitative departure in exhibiting ferromagnetic interactions, with suggestions of ferromagnetic order at low temperature. XAS measurements establish the presence of Fe²⁺ and Fe³⁺ ions, with the concentration of the trivalent species increasing in presence of Cu doping, providing a direct evidence of Fe²⁺ + Cu²⁺  giving Fe³⁺ + Cu¹⁺ redox couple being correlated with the ferromagnetic property. Using DFT, the unexpected ferromagnetic nature of these systems is explained in terms of a double exchange between Fe atoms, mediated by the Cu atom, in agreement with experimental observations.

The incorporation of copper dopants into II-VI colloidal nanocrystals (NCs) leads to the introduction of intragap electronic states and the development of a new emission feature due to an optical transition, which couples the NC conduction band to the Cu-ion state. The mechanism underlying Cu related emission and specifically the factors that control the branching between the intrinsic and impurity-related emission channels remain unclear. Here, we address this problem by conducting spectroelectrochemical measurements on Cu-doped core/shell ZnSe/CdSe NCs. These measurements indicate that the distribution of photoluminescence (PL) intensity between

the intrinsic and the impurity bands as well as the overall PL efficiency can be controlled by varying the occupancy of surface defect sites. Specifically, by activating hole traps under negative electrochemical potential (the Fermi level is raised) we can enhance the Cu band at the expense of band-edge emission, which is consistent with the predominant Cu²⁺ character of the dopant ions. Further, we observe an overall PL “brightening” under negative potential and “dimming” under positive potential, which we attribute to changes in the occupancy of the electron trap sites (that is, the degree of their electronic passivation) that control nonradiative losses due to electron surface trapping.

We have developed a new class of colloidal nanocrystals composed of Cu-doped ZnSe cores overcoated with CdSe shells. Via spectroscopic and magneto-optical studies, we conclusively demonstrate that Cu impurities represent paramagnetic 2+ species and serve as a source of permanent optically active holes. This implies that the Fermi level is located below the Cu2+/Cu1+ state, i.e., in the lower half of the forbidden gap, which is a signature of a p-doped material. It further suggests that the activation of optical emission due to the Cu level requires injection of only an electron without a need for a valence-band hole. This peculiar electron-only emission mechanism is confirmed by experiments in which the titration of the nanocrystals with holewithdrawing molecules leads to enhancement of Cu-related photoluminescence while simultaneously suppressing the intrinsic, band-edge exciton emission. In addition to containing permanent optically active holes, these newly developed materials show unprecedented emission tunability from near infrared (1.2 eV) to the blue (3.1 eV) and reduced losses from re-absorption due to a large Stokes shift (up to 0.7 eV). These properties make them very attractive for applications in light-emission and lasing technologies and especially for the realization of novel device concepts such as “zero-threshold” optical gain.

We report magneto-photoluminescence studies of strongly quantum-confined "0-D" diluted magnetic semiconductors (DMS), realized in Mn2+-doped ZnSe/CdSe core/shell colloidal nanocrystals. In marked contrast to their 3-D (bulk), 2-D (quantum well), 1-D (quantum wire), and 0-D (self-assembled quantum dot) DMS counterparts, the ubiquitous yellow emission band from internal d-d (4T1 6A1) transitions of the Mn2+ ions in these nanocrystals is not suppressed in applied magnetic fields and does become circularly polarized. This polarization tracks the Mn2+ magnetization, and is accompanied by a sizable energy splitting between right- and left-circular emission components that scales with the exciton-Mn sp-d coupling strength (which, in turn, is tunable with nanocrystal size). These data highlight the influence of strong quantum confinement on both the excitation and the emission mechanisms of magnetic ions in DMS nano-materials.

Spatial confinement of electronic excitations in semiconductor nanocrystals (NCs) results in a significant enhancement of nonradiative Auger recombination (AR), such that AR processes can easily dominate the decay of multiexcitons. AR is especially detrimental to lasing applications of NCs, as optical gain in these structures explicitly relies on emission from multiexciton states. In standard NCs, AR rates scale linearly with inverse NC volume. Here, we investigate multiexciton dynamics in hetero-NCs composed of CdSe cores and CdS shells of tunable thickness. We observe a dramatic decrease in the AR rates at the initial stage of shell growth, which cannot be explained by traditional volume scaling alone. Rather, fluorescence-line-narrowing studies indicate that the suppression of AR correlates with the formation of an alloy layer at the core−shell interface suggesting that this effect derives primarily from the “smoothing” of the confinement potential associated with interfacial alloying. These data highlight the importance of NC interfacial structure in the AR process and provide general guidelines for the development of new nanostructures with suppressed AR for future lasing applications.

A strong electron–hole exchange interaction (EI) in semiconductor nanocrystals (NCs) gives rise to a large (up to tens of meV) splitting between optically active ('bright') and optically passive ('dark') excitons. This dark–bright splitting has a significant effect on the optical properties of band-edge excitons and leads to a pronounced temperature and magnetic field dependence of radiative decay. Here we demonstrate a nanoengineering-based approach that provides control over EI while maintaining nearly constant emission energy. We show that the dark–bright splitting can be widely tuned by controlling the electron–hole spatial overlap in core–shell CdSe/CdS NCs with a variable shell width. In thick-shell samples, the EI energy reduces to <250 μeV, which yields a material that emits with a nearly constant rate over temperatures from 1.5 to 300 K and magnetic fields up to 7 T. The EI-manipulation strategies demonstrated here are general and can be applied to other nanostructures with variable electron–hole overlap.

We investigate the growth kinetics of CdS nanocrystals in the quantum confinement regime using time-resolved small-angle X-ray scattering. In contrast to earlier reports for similar systems, we establish that the growth kinetics in this case follows the Lifshitz−Slyozov−Wagner theory, for not only growth of the average diameter of the nanocrystals but also the time dependence of the size distribution and the temperature dependence of the rate constant. This is the first rigorous example of the coarsening process in the quantum confinement (<5 nm) regime. Ab initio studies for the reaction pathways provide a microscopic understanding of this finding.

Calculations of Slater–Koster (SK) parameters appearing in the tight-binding method using sp³d⁵ basis sets for both the cationic and anionic species are presented for ZnS and CdS. We have adjusted these parameters to match the band structures obtained from the full potential linear augmented plane wave method. This operation has been carried out for a variety of structures namely zinc blende, wurtzite, rocksalt, CsCl and for a wide range of near-neighbor distances. The SK parameters have slightly different values for the same near-neighbor distance in different structures. Therefore, a least-squares fitting has been performed separately for each parameter as a function of only the near-neighbor distance to guarantee the transferability of these parameters to different structural environments. The fitted parameters are then used to calculate the electronic structure of small-sized clusters of ZnS and CdS in given geometries and the results are compared with ab initio results. A fairly good agreement found in the one-electron energy spectrum and total energy confirms transferability of the parameters to different length scales. A detailed account of the calculation procedure and calibration results is given in the present paper. These parameters can be used to study the electronic structure of large-sized clusters where first-principles methods are computationally demanding. It may be mentioned that the SK parameters do not satisfy the R^{− (l + l' + 1)} Harrison scaling law for larger values of the near-neighbor distance R.

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In this paper, we report on the growth and characterization of quantum dot-quantum well nanostructures with photoluminescence (PL) that is tunable over the visible range. The material exhibits a PL efficiency as high as similar to 60% and is prepared by reacting ZnS nanocrystals in turn with precursors for CdSe and ZnS in an attempt to form a simple "ZnS/CdSe/ZnS quantum-well structure". Through the use of synchrotron radiation-based photoelectron spectroscopy in conjunction with detailed overall compositional analysis and correlation with the size of the final composite nanostructure, the internal structure of the composite nanocrystals is shown to consist of a graded alloy core whose composition gradually changes from ZnS at the very center to CdSe at the onset of a CdSe layer. The outer shell is ZnS with a sharp interface, probably reflecting the relative thermodynamic stabilities of the parent binary phases. These contrasting aspects of the internal structure are discussed in terms of the various reactivities and are shown to be crucial for understanding the optical properties of such complex heterostructured nanornaterials.

The electronic structure of group II–VI semiconductors in the stable wurtzite form is analyzed using state-of-the-art ab initio approaches to extract a simple and chemically transparent tight-binding model. This model can be used to understand the variation in the bandgap with size, for nanoclusters of these compounds. Results complement similar information already available for same systems in the zinc blende structure. A comparison with all available experimental data on quantum size effects in group II–VI semiconductor nanoclusters establishes a remarkable agreement between theory and experiment in both structure types, thereby verifying the predictive ability of our approach. The significant dependence of the quantum size effect on the structure type suggests that the experimental bandgap change at a given size compared to the bulk bandgap, may be used to indicate the structural form of the nanoclusters, particularly in the small size limit, where broadening of diffraction features often make it difficult to unambiguously determine the structure.

Controlled doping is a critical step toward various unique nanostructures. This report shall demonstrate that doping chemistry of colloidal nanocrystals is much more complex than what has been proposed in the existing experimental and theoretical reports. Four individual processes, namely “surface adsorption”, “lattice incorporation”, “lattice diffusion”, and “lattice ejection”, will be identified, each of which possesses its own critical temperature. A given type of host nanocrystals can be switched from being impossible to dope to becoming successfully doped. The key is to program the reaction temperature to accommodate all elementary processes.

In this paper, we report the synthesis and self assembly of various sizes of ZnO nanocrystals. While the crystal structure and the quantum confinement of nanocrystals were mainly characterized using XRD and UV absorption spectra, the self assembly and long range ordering were studied using scanning tunneling microscopy after spin casting the nanocrystal film on the highly oriented pyrolytic graphite surface. We observe self assembly of these nanocrystals over large areas making them ideal candidates for various potential applications. Further, the electronic structure of the individual dots is obtained from the current-voltage characteristics of the dots using scanning tunneling spectroscopy and compared with the density of states obtained from the tight binding calculations. We observe an excellent agreement with the experimentally obtained local density of states and the theoretically calculated density of states.

X-Ray magnetic circular dichroism (XMCD) experiments on diluted magnetic semiconductor nanocrystals (2–7 nm) are reported in order to study their local electronic structure and magnetic properties. ZnSe nanoparticles containing either single manganese ions (Mn²⁺) distributed in the lattice of the entire particle or a MnSe core in the center are prepared using high temperature approaches. The Mn²⁺ concentration is varied between less than one to several tens of manganese ions per nanocrystal. For all samples it is shown that the Mn²⁺ is exclusively present in the bulk of ZnSe nanoparticles with no evidence for oxidation to higher Mn-oxidation states. The magnetic ions are highly polarized inside the nanocrystals reaching about 80% of the theoretical value of a pure d⁵ state under identical conditions for the case of isolated manganese ions. Nanocrystals with a MnSe core ZnSe shell structure reach <50% of this value. Thus, their polarization is significantly more hindered, which is due to the significantly enhanced Mn–Mn interactions and a more distorted crystalline lattice. In contrast, no coupling between the manganese centers is observed in the nanoparticles doped samples with low concentrations of Mn²⁺, indicating that these ions are isolated in the bulk of the nanoparticles.

Formation of Mn2+-doped ZnSe quantum dots (Mn:ZnSe d-dots) with both branched and nearly spherical shapes has been studied. Structure analysis indicates that the Mn2+ dopants were localized in the core of a branched nanocrystal. The growth of branched d-dots, rather than spherical ones, was achieved by simply varying the concentration of two organic additives, fatty acids, and fatty amines. The photoluminescence properties of the branched nanocrystals were explored and compared with those of the nearly spherical particles.

Using extensive state-of-the-art experiments over a wide range of synthesis parameters, such as the temperature and concentrations of different reactants, we establish qualitatively different growth kinetics for ZnO nanocrystals compared to all growth kinetics of semiconductor nanocrystals, including ZnO, discussed so far in the literature. The growth rate is shown to be strongly dependent on the concentration of (OH)(-) in an intriguing nonmonotonic manner as well as on temperature and is almost invariably much slower than well-known and generally accepted growth mechanisms based on a diffusion-controlled Ostwald ripening process or that expected in the surface reaction controlled regime. We show that these qualitatively different results arise from the unexpected role played by a part of the reactants by inhibiting rather than facilitating the reaction; we explain this extraordinary result in terms of an effective passivating layer around the growing nanocrystals formed by a virtual capping shell of Na+ ions.

In this work, we report the study of growth of CdS as well as ZnS nanocrystals using in-situ small angle X-ray scattering (SAXS) technique, in presence of thio-glycerol as capping agent. We observe that the diameter of the nanocrystal is controlled between 1 and 3 nm by varying the temperature of the reaction. Further, the self-focusing of the size distribution can be observed and is more pronounced at higher temperatures reducing its relative width from 25% to 10%.

Zn edge and Mn edge EXAFS studies have been performed on bulk and nanocrystals (4.7 nm) of the dilute magnetic semiconductor Zn1-xMnxO, with x varying from 0.05 to 0.5. Preliminary analysis and linear combination fitting shows that Mn replaces Zn in the bulk sample, whereas in the nanocrystal samples, with different amounts of Mn, only a very small percentage of the Mn atoms replace Zn. The majority of the Mn atoms remain on the surface of the nanocrystallites in the form of Mn oxides.

We investigate the mechanism of growth of nanocrystals from solution using the case of ZnO. Spanning a wide range of values of the parameters, such as the temperature and the reactant concentration that control the growth, our results establish a qualitative departure from the widely accepted diffusion controlled coarsening (Ostwald ripening) process quantified in terms of the Lifshitz-Slyozov-Wagner theory. Further, we show that these experimental observations can be qualitatively and quantitatively understood within a growth mechanism that is intermediate between the two well-defined limits of diffusion control and kinetic control.

We report the first synthesis of Cu doped in the core region of ZnO nanocrystals and fluorescing in the blue region, establishing the novel possibility of using these as fluorescent probes.

We report the study of complex and unexpected dependencies of nanocrystal size as well as nanocrystalsize distribution on various reaction parameters in the synthesis of ZnO nanocrystals using poly(vinyl pyrollidone) (PVP) as a capping agent. This method establishes a qualitatively different growth mechanism to the anticipated Ostwald ripening behavior. The study of size-distribution kinetics and an understanding of the observed non-monotonic behaviors provides a route to rational synthesis. We used a simple, but accurate, approach to estimate the size-distribution function of nanocrystals from the UV-absorption spectrum. Our results demonstrate the accuracy and generality of this approach, and we also illustrate its application to various semiconducting nanocrystals, such as ZnO, ZnS, and CdSe, over a wide size range (1.8-5.3 nm).

We analyze the electronic structure of group III-V semiconductors obtained within full potential linearized augmented plane wave (FP-LAPW) method and arrive at a realistic and minimal tight-binding model, parametrized to provide an accurate description of both valence and conduction bands. It is shown that the cation sp(3)- anion sp(3)d(5) basis along with the next nearest neighbor model for hopping interactions is sufficient to describe the electronic structure of these systems over a wide energy range, obviating the use of any fictitious s(*) orbital, employed previously. Similar analyses were also performed for the II-VI semiconductors, using the more accurate FP-LAPW method compared to previous approaches, in order to enhance reliability of the parameter values. Using these parameters, we calculate the electronic structure of III-V and II-VI nanocrystals in real space with sizes ranging up to about 7 nm in diameter, establishing a quantitatively accurate description of the bandgap variation with sizes for the various nanocrystals by comparing with available experimental results from the literature.

We present a scanning tunnelling microscopy and spectroscopy study of polyaniline nanostructure. Our results show the possibility of achieving a high degree of self-organization in polyaniline nanorods in the undoped as well as the doped states. The spectroscopy results clearly establish a direct correlation between the extent of ordering and the density of states at the Fermi energy of the doped samples. Small angle x-ray scattering experiment helps to understand the formation of such self-organized structures.

We investigate the magnetic properties of two typical II-VI semiconducting nanocrystals, namely, ZnS and ZnO, doped with various concentrations of a transition metal ion, Mn. A wide range of dopant concentrations was explored without changing the size of the nanocrystal, thereby allowing us to study the concentration dependence of various properties independent of any size variation of the host. However, only small doping concentrations could be realized in each case. All the studies were thus carried out with ZnS and ZnO nanocrystals with their respective average sizes fixed at about 1.6 and 4.7 nm. We show that the magnetic properties of such doped systems, remaining paramagnetic down to the lowest temperature (<3 K), can provide important and accurate information concerning the dopant level in such samples.

We report a quantitatively accurate description of the electronic structure of semiconductor nanocrystals using the sp(3)d(5) orbital basis with the nearest neighbor and the next nearest neighbor interactions. The use of this model for II-VI and III-V semiconductors is reviewed in article. The excellent agreement of the theoretical predictions with the experimental results establishes the feasibility of using this model for semiconductor nanocrystals.

In the present work, we report the synthesis of high quality ZnO nanocrystals with sharp absorption edges in four different sizes, namely 3.0, 3.5, 4.7 and 5.4 nm, characterized by X-ray and electron diffraction, as well as transmission electron microscopy. The bandgaps of these samples, in conjunction with further data from the published literature, exhibit a systematic dependence on the nanocrystal size. In absence of any prior reliable theoretical results in the literature to understand this dependence quantitatively, we have analyzed for the first time, the electronic structure of bulk ZnO obtained from the full potential linearized augmented plane wave method using fatbands, density of states and partial density of states. The crystal orbital Hamiltonian population is obtained from linearized Muffin-Tin orbital band structure calculations to understand the range of hopping interactions relevant for an accurate description of the electronic structure. Using these analyses, a realistic tight binding model is proposed. Based on this model, we calculate the variation of the bandgap with the size of ZnO nanocrystals. These theoretical results agree well with all available data over the entire range of sizes, establishing the effectiveness of this approach.

We report the synthesis and characterization of several sizes of Mn-doped ZnO nanocrystals, both in the free-standing and the capped particle forms. The sizes of these nanocrystals could be controlled by capping them with polyvinyl pyrollidone under different synthesis conditions and were estimated by X-ray diffraction and transmission electron microscopy. The absorption properties of PVP-capped Mn-doped ZnO exhibit an interesting variation of the band gap with the concentration of Mn. Fluorescence emission, electron paramagnetic resonance, and X-ray absorption spectroscopy provide evidence for the presence of Mn in the interior as well as on the surface of the nanocrystals.

We obtain an effective parametrization of the bulk electronic structure of InP within the tight-binding scheme. Using these parameters, we calculate the electronic structure of InP clusters with the size ranging up to 7.5 nm. The calculated variations in the electronic structure as a function of the cluster size is found to be in excellent agreement with experimental results over the entire range of sizes, establishing the effectiveness and transferability of the obtained parameter strengths.