Interband and intraband optical transitions in InAs nanocrystal quantum dots: A pseudopotential approach

An atomistic pseudopotential method is used to investigate the electronic and optical properties of spherical InAs nanocrystals. Our calculated interband (valence-to-conduction) absorption spectra reproduce the features observed experimentally both qualitatively and quantitatively. The results relative to intraband (valence-to-valence and conduction-to-conduction) absorption successfully reproduce the recently measured photoinduced absorption spectra, which had so far been addressed only qualitatively. They exclude the hypothesis of a thermal activation process between dot-interior-delocalized hole states to explain the temperature dependence observed experimentally. Furthermore, based on the agreement of our data with the experimental valence intersublevel transitions and the almost complete overlap of the latter with scanning tunneling microscopic (STM) measurements, we question the simplistic attribution of the observed STM peaks obtained for negative bias

Exciton Dynamics in InSb Colloidal Quantum Dots

Extraordinarily fast biexciton decay times and unexpectedly large optical gaps are two striking features observed in InSb colloidal quantum dots that have remained so far unexplained. The former, should its origin be identified as an Auger recombination process, would have important implications regarding carrier multiplication efficiency, suggesting these nanostructures as potentially ideal active materials in photovoltaic devices. The latter could offer new insights into the factors that influence the electronic structure and consequently the optical properties of systems with reduced dimensionality and provide additional means to fine-tune them. Using the state-of-the-art atomistic semiempirical pseudopotential method we unveil the surprising origins of these features and show that a comprehensive explanation for these properties requires delving deep into the atomistic detail of these nanostructures and is, therefore, outside the reach of less sophisticated, albeit more popular, theoretical approaches

The effect of small elongations on the electronic and optical signatures in InAs nanocrystal quantum dots

We present a detailed theoretical investigation of the electronic structure and optical properties of InAs nanocrystals at the transition from spheres to rods. Using a semiempirical pseudopotential approach, we predict that, despite the qualitative similarity of both intra- and inter-band optical spectra, for NCs with R > 15 °A even slight elongations should result in shifts of the order of hundreds of meV in the spacings between STM peaks measured in the positive bias regime, in the position of the intra- band absorption peaks associated with transitions in the conduction band and in the separation between the first and the fifth peak in PLE experiments. Our results show that, based on the spectroscopic data, it should be possible to discriminate between spherical and elongated NCs with aspect ratios of length over diameter as small as 1.2. Indeed our results suggest that many nominally spherical experimental samples contained a large fraction of slightly elongated structures

Lifetime and polarization of the radiative decay of excitons, biexcitons and trions in CdSe nanocrystal quantum dots

Using the pseudopotential configuration-interaction method, we calculate the intrinsic lifetime and polarization of the radiative decay of single excitons (X), positive and negative trions (X+ and X−), and biexcitons (XX) in CdSe nanocrystal quantum dots. We investigate the effects of the inclusion of increasingly more complex many-body treatments, starting from the single-particle approach and culminating with the configuration-interaction scheme. Our configuration-interaction results for the size dependence of the single-exciton radiative lifetime at room temperature are in excellent agreement with recent experimental data. We also find the following. (i) Whereas the polarization of the bright exciton emission is always perpendicular to the hexagonal c axis, the polarization of the dark exciton switches from perpendicular to parallel to the hexagonal c axis in large dots, in agreement with experiment. (ii) The ratio of the radiative lifetimes of mono- and biexcitons (X):(XX) is ~1:1 in large dots (R=19.2 Å). This ratio increases with decreasing nanocrystal size, approaching 2 in small dots (R=10.3 Å). (iii) The calculated ratio (X+):(X−) between positive and negative trion lifetimes is close to 2 for all dot sizes considered

Ultrasound scan to detect acalculous cholecystopathy in immunocompromised hosts with unexplained fever.

We found a significant prevalence of acalculous cholecystopathy in a group of patients with hematologic malignancies and unexplained fever. Ultrasound scan (US) detected a case of acute cholecystitis, two of gallbladder overdistension and biliary sludge, and one of striated gallbladder wall thickening. US proved effective in early identification of abdominal infection site

Model-independent determination of the carrier multiplication time constant in CdSe nanocrystals

The experimental determination of the carrier multiplication (CM) time constant is complicated by the fact that this process occurs within the initial few hundreds of femtoseconds after excitation and, in transient-absorption experiments, cannot be separated from the buildup time of the 1p-state population. This work provides an accurate theoretical determination of the electron relaxation lifetime during the last stage of the p-state buildup, in CdSe nanocrystals, in the presence of a single photogenerated hole (no CM) and of a hole plus an additional electron–hole pair (following CM). From the invariance of the 1p buildup time observed experimentally for excitations above and below the CM threshold producing hot carriers with the same average per-exciton excess energy, and the calculated corresponding variations in the electron decay time in the two cases, an estimate is obtained for the carrier multiplication time constant. Unlike previous estimates reported in the literature so far, this result is model-independent, i.e., is obtained without making any assumption on the nature of the mechanism governing carrier multiplication. It is then compared with the time constant calculated, as a function of the excitation energy, assuming an impact-ionization-like process for carrier multiplication (DCM). The two results are in good agreement and show that carrier multiplication can occur on timescales of the order of tens of femtoseconds at energies close to the observed onset. These findings, which are compatible with the fastest lifetime estimated experimentally, confirm the suitability of the impact-ionization model to explain carrier multiplication in CdSe nanocrystals

Tuning of the Gap in a Laughlin-Bychkov-Rashba Incompressible Liquid

We report on our investigation of the influence of Bychkov-Rashba spin-orbit interaction (SOI) on the incompressible Laughlin state. We find that experimentally obtainable values of the spin-orbit coupling strength can induce as much as a 25% increase in the quasiparticle-quasihole gap Eg at low magnetic fields in InAs, thereby increasing the stability of the liquid state. The SOI-modulated enhancement of Eg is also significant for filling factors 1/5 and 1/7, where the FQH state is usually weak. This raises the intriguing possibility of tuning, via the SO coupling strength, the liquid to solid transition to much lower densities.Comment: 4 pages, 3 figure

Neck dissection versus “watchful-waiting” in early squamous cell carcinoma of the tongue our experience on 127 cases

Background Early oral squamous cell carcinoma (EOSCC) represents about 90% of the oral cancers especially in older males. The etiology is multifactorial, strongly related to tobacco and alcohol abuse, but also infective agents, Human papillomaviruses (HPV16-18), genetic factors and pre-neoplastic lesions seem to be implicated. There is no consensus in the literature for the treatment of early squamous cell carcinoma of the tongue (stages I–II); both an elective neck dissection policy and a watchful-waiting policy have their proponents in the different centers. Methods The records of 127 patients with EOSCC of the tongue treated in our Department between 2007 and 2011, with cN0 neck staging, who underwent resection of the primary tumor with or without elective neck dissection, were reviewed. Results We divided the patients into two groups, in Group 1 the 66 patients who received an elective neck dissection 30 days later from the primary surgery have been included, and in Group 2 the 61 patients undergoing “watchful waiting” observation for the development of nodal metastases have been collected. Statistical calculations were performed using Chi-square and t student test. Conclusions A significant difference was found between the two groups as concerns tumor stage and pathologic tumor classification (p < 0.001). No significant differences were present between the two groups as concerns mean follow up (P = 0.2), relapse rate (p = 0.3) and relapse-free survival time (p = 0.2). In T1 stage tumors with depth of infiltration ≤4 mm, or low grade (G1-G2), the “watchful waiting” strategy for cervical metastases is appropriate, given the low regional recurrence rate (15%) and overall survival of 100%. In case of T2 lesions with depth of infiltration ≥4 mm or high grade (G3) we prefer to perform the elective neck dissection, with 13% of local recurrence and 100% of survival at 6 years

Interwell relaxation times in p-Si/SiGe asymmetric quantum well structures: the role of interface roughness

We report the direct determination of nonradiative lifetimes in Si∕SiGe asymmetric quantum well structures designed to access spatially indirect (diagonal) interwell transitions between heavy-hole ground states, at photon energies below the optical phonon energy. We show both experimentally and theoretically, using a six-band k∙p model and a time-domain rate equation scheme, that, for the interface quality currently achievable experimentally (with an average step height ⩾1 Å), interface roughness will dominate all other scattering processes up to about 200 K. By comparing our results obtained for two different structures we deduce that in this regime both barrier and well widths play an important role in the determination of the carrier lifetime. Comparison with recently published experimental and theoretical data obtained for mid-infrared GaAs∕AlxGa1−xAs multiple quantum well systems leads us to the conclusion that the dominant role of interface roughness scattering at low temperature is a general feature of a wide range of semiconductor heterostructures not limited to IV-IV material

Origins of Photoluminescence Decay Kinetics in CdTe Colloidal Quantum Dots

Recent experimental studies have identified at least two nonradiative components in the fluorescence decay of solutions of CdTe colloidal quantum dots (CQDs). The lifetimes reported by different groups, however, differed by orders of magnitude, raising the question of whether different types of traps were at play in the different samples and experimental conditions and even whether different types of charge carriers were involved in the different trapping processes. Considering that the use of these nanomaterials in biology, optoelectronics, photonics, and photovoltaics is becoming widespread, such a gap in our understanding of carrier dynamics in these systems needs addressing. This is what we do here. Using the state-of-the-art atomistic semiempirical pseudopotential method, we calculate trapping times and nonradiative population decay curves for different CQD sizes considering up to 268 surface traps. We show that the seemingly discrepant experimental results are consistent with the trapping of the hole at unsaturated Te bonds on the dot surface in the presence of different dielectric environments. In particular, the observed increase in the trapping times following air exposure is attributed to the formation of an oxide shell on the dot surface, which increases the dielectric constant of the dot environment. Two types of traps are identified, depending on whether the unsaturated bond is single (type I) or part of a pair of dangling bonds on the same Te atom (type II). The energy landscape relative to transitions to these traps is found to be markedly different in the two cases. As a consequence, the trapping times associated with the different types of traps exhibit a strikingly contrasting sensitivity to variations in the dot environment. Based on these characteristics, we predict the presence of a sub-nanosecond component in all photoluminescence decay curves of CdTe CQDs in the size range considered here if both trap types are present. The absence of such a component is attributed to the suppression of type I traps