Marco Califano

Size-Dependent Valence and Conduction Band-Edge Energies of Semiconductor Nanocrystals

Through the use of photoelectron spectroscopy in air (PESA), we investigate the size-dependent valence and conduction band-edge energies of CdSe, CdTe, PbS, and PbSe semiconductor quantum dots (QDs). The results are compared to those of previous studies, based on differing experimental methods, and to theoretical calculations based on k·p theory and state-of-the-art atomistic semiempirical pseudopotential modeling. To accurately map out the energy level landscapes of QDs as a function of size, the QDs must be passivated by comparable surface chemistries. This is highlighted by studying the effect of surface chemistry on the valence band-edge energy in an ensemble of 4.7 nm CdSe QDs. An energy level shift as large as 0.35 eV is observed for this system through modification of surface chemistry alone. This shift is significantly larger than the size-dependent valence band-edge shift that is observed when comparable surface chemistries are used.

Optical properties of single semiconductor nanocrystals

We present an overview of the current progress in the understanding of the (steady state) optical properties of individual II-VI semiconductor nanocrystals. We begin with a presentation of the conceptual development of the theory required to model the electronic structure of these systems. This is followed by an overview of the current experimental results obtained from the spectroscopy of individual semiconductor nanocrystals, and in particular, we focus on the study of photoluminescence intermittency (blinking) and spectral diffusion. Where possible, we link the experimental observations to the predictions of current theories. We conclude that the surface of small semiconductor crystals plays an important role in determining their optical properties.

Auger-Assisted Electron Transfer from Photoexcited Semiconductor Quantum Dots

Although quantum confined nanomaterials, such as quantum dots (QDs) have emerged as a new class of light harvesting and charge separation materials for solar energy conversion, theoretical models for describing photoinduced charge transfer from these materials remain unclear. In this paper, we show that the rate of photoinduced electron transfer from QDs (CdS, CdSe, and CdTe) to molecular acceptors (anthraquinone, methylviologen, and methylene blue) increases at decreasing QD size (and increasing driving force), showing a lack of Marcus inverted regime behavior over an apparent driving force range of ∼0-1.3 V. We account for this unusual driving force dependence by proposing an Auger-assisted electron transfer model in which the transfer of the electron can be coupled to the excitation of the hole, circumventing the unfavorable Franck-Condon overlap in the Marcus inverted regime. This model is supported by computational studies of electron transfer and trapping processes in model QD-acceptor complexes.

Direct carrier multiplication due to inverse Auger scattering in CdSe quantum dots

Many optoelectronic devices could achieve much higher efficiencies if the excess energy of electrons excited well above the conduction band minimum could be used to promote other valence electrons across the gap rather than being lost to phonons. It would then be possible to obtain two electron–hole pairs from one. In bulk materials, this process is inherently inefficient due to the constraint of simultaneous energy and momentum conservation. We calculated the rate of these processes, and of selected competing ones, in CdSe colloidal dots, using our semi-empirical nonlocal pseudopotential approach. We find much higher carrier multiplication rates than in conventional bulk materials for electron excess energies just above the energy gap Eg. We also find that in a neutral dot, the only effective competing mechanism is Auger cooling, whose decay rates can be comparable to those calculated for the carrier multiplication process.

Hole surface trapping in CdSe nanocrystals: dynamics, rate fluctuations, and implications for blinking.

Carrier trapping is one of the main sources of performance degradation in nanocrystal-based devices. Yet the dynamics of this process is still unclear. We present a comprehensive investigation into the efficiency of hole transfer to a variety of trap sites located on the surface of the core or the shell or at the core/shell interface in CdSe nanocrystals with both organic and inorganic passivation, using the atomistic semiempirical pseudopotential approach. We separate the contribution of coupling strength and energetics in different systems and trap configurations, obtaining useful general guidelines for trapping rate engineering. We find that trapping can be extremely efficient in core-only systems, with trapping times orders of magnitude faster than radiative recombination. The presence of an inorganic shell can instead bring the trapping rates well below the typical radiative recombination rates observed in these systems.

Composition, volume, and aspect ratio dependence of the strain distribution, band lineups and electron effective masses in self-assembled pyramidal In1−xGaxAs/GaAs and SixGe1−x/Si quantum dots

We present a systematic investigation of the strain distribution of self-assembled pyramidal In1−xGaxAs/GaAs and SixGe1−x/Si quantum dots for the case of growth on a (001) substrate. The dependence of the biaxial and hydrostatic components of the strain on the quantum dot volume, aspect ratio, composition, and percentage of alloying x is studied using a method based on a Green’s function technique. The dependence of the carriers’ confining potentials and the electronic effective mass on the same parameters is then calculated in the framework of eight-band k⋅p theory. The results for which comparable published data are available are in good agreement with the theoretical values for strain profiles, confining potentials, and electronic effective mass.

Universal Trapping Mechanism in Semiconductor Nanocrystals

Size tunability of the optical properties and inexpensive synthesis make semiconductor nanocrystals one of the most promising and versatile building blocks for many modern applications such as lasers, single-electron transistors, solar cells, and biological labels. The performance of these nanocrystal-based devices is however compromised by efficient trapping of the charge carriers. This process exhibits different features depending on the nanocrystal material, surface termination, size, and trap location, leading to the assumption that different mechanisms are at play in each situation. Here we revolutionize this fragmented picture and provide a unified interpretation of trapping dynamics in semiconductor nanocrystals by identifying the origins of this so far elusive detrimental process. Our findings pave the way for a general suppression strategy, applicable to any system, which can lead to a simultaneous efficiency enhancement in all nanocrystal-based technologies.

Approximate methods for the solution of quantum wires and dots: Connection rules between pyramidal, cuboidal, and cubic dots

Energy eigenvalues of the electronic ground state are calculated for rectangular and triangular GaAs/Ga0.6Al0.4As quantum wires as well as for cuboidal and pyramidal quantum dots of the same material. The wire (dot) geometries are approximated by a superposition of perpendicular independent finite one-dimensional potential wells. A perturbation is added to the system to improve the approximation. Excellent agreement with more complex treatments is obtained. The method is applied to investigate the ground state energy dependence on volume and aspect ratio for finite barrier cubic, cuboidal, and pyramidal quantum dots. It is shown that the energy eigenvalues of cubes are equal to those of cuboids of the same volume and aspect ratio similar to one. In addition, a relationship has been found between the volumes of pyramidal quantum dots (often the result of self-assembling in strain layered epitaxy) and cuboidal dots with the same ground state energy and aspect ratios close to one.

Quantum box energies as a route to the ground state levels of self-assembled InAs pyramidal dots

A theoretical investigation of the ground state electronic structure of InAs/GaAs quantum confined structures is presented. Energy levels of cuboids and pyramidal shaped dots are calculated using a single-band, constant-confining-potential model that in former applications has proved to reproduce well both the predictions of very sophisticated treatments and several features of many experimental photoluminescence spectra. A connection rule between their ground state energies is found which allows the calculation of the energy levels of pyramidal dots using those of cuboids of suitably chosen dimensions, whose solution requires considerably less computational effort. The purpose of this work is to provide experimentalists with a versatile and simple method to analyze their spectra. As an example, this rule is then applied to successfully reproduce the position of the ground state transition peaks of some experimental photoluminescence spectra of self-assembled pyramidal dots. Furthermore the rule is used t...

Photoinduced surface trapping and the observed carrier multiplication yields in static CdSe nanocrystal samples.

Photocharging has been suggested recently as the explanation for the spread of carrier multiplication yields reported by different groups. If this hypothesis can be plausible in the case of PbSe, it is inconsistent with the reported experimental data relative to CdSe nanocrystals and cannot therefore explain the large discrepancies found in that material system between static and stirred samples. An alternative explanation, photoinduced surface trapping, is suggested here, based on the results of atomistic semiempirical pseudopotential calculations of the Auger recombination rates in a number of excitonic configurations including a variety of surface traps, which show that the photoinduced surface trapping of the hole, which leaves the core negatively charged (but the nanocrystal neutral overall), can lead to recombination rates that are indistinguishable from those of a conventional biexciton with four core-delocalized carriers and therefore result in exaggerated CM yields in static samples. In contrast, the recombination rate of a charged exciton is found to be at least a factor of 2.3 smaller than that of the biexciton and therefore easily distinguishable from it experimentally. Although increased trapping at surface states was dismissed as unlikely for PbSe nanocrystals, in the case of CdSe, this hypothesis is further supported by much experimental evidence including recent spectroscopic measurements on CdSe nanostructures, single-nanocrystal photoionization studies on CdSe core/shell nanocrystals, and state-resolved transient absorption studies of biexcitonic states, all showing increased probability of surface trapping for highly excited states. These results suggest that multicarrier processes could be mediated by different mechanisms in CdSe and PbSe nanocrystals.