Carrier multiplication yields in CdSe and CdTe nanocrystals by transient photoluminescence

Engineering semiconductors to enhance carrier multiplication (CM) could lead to increased photovoltaic cell performance and a significant widening of the materials range suitable for future solar technologies. Semiconductor nanocrystals (NCs) have been proposed as a favourable structure for CM enhancement, and recent measurements by transient absorption have shown evidence for highly efficient CM in lead chalcogenide and CdSe NCs. We report here an assessment of CM yields in CdSe and CdTe NCs by a quantitative analysis of biexciton and exciton signatures in transient photoluminescence decays. Although the technique is particularly sensitive due to enhanced biexciton radiative rates relative to the exciton, kradBX > 2 kradX, we find no evidence for CM in CdSe and CdTe NCs up to photon energies E > 3 Eg, well above previously reported relative energy thresholds.Comment: 9 pages, 6 figure

Many-body processes in the photophysics of colloidal semiconductor nanocrystals

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Vita. Cataloged from student submitted PDF version of thesis.Includes bibliographical references (p. 135-143).In this work we have experimentally studied several aspects of two Coulomb processes that change the number of electrons and holes in colloidal semiconductor nanocrystals (NCs). Carrier Multiplication (CM) is the production of additional electron-hole pairs by collision of a highly excited carrier with valence electrons. Efficient CM would improve the performance of solar energy conversion devices, but it is weak in the bulk. Recent reports by several groups suggested highly efficient CM in semi-conductor NCs. We describe here our assessment of CM using transient photoluminescence in CdSe and lead chalcogenide NCs. Biexciton radiative and nonradiative rates were determined. In our study, no detectable CM was found in CdSe NCs photoexcited at a photon energy of up to 5.9 eV, and the CM yields observed for PbSe NCs at 3.1 eV were found consistent with bulk values. Reasons for the strong disagreement with prior measurements are discussed, and the low yields are theoretically accounted for. The second part of the thesis describes two studies of the "Auger" nonradiative recombination process whereby an electron-hole pair recombines while transferring its energy to a third particle. This mechanism is responsible for the short multiexciton lifetimes in NCs. In one study, we demonstrate a direct method for determining biexciton quantum yields in single nanocrystals by photon cross-correlation (antibunching) measurements. We find significant inhomogeneity in these values, indicating a previously obscured variation in Auger recombination rates.(cont.) Another set of experiments tests the conventional charging model of NC fluorescence intermittency ("blinking") which attributes off-state quenching to Auger decay, by studying single NCs with relatively long multiexciton Auger lifetimes. We find that off-state exciton quantum yields are significantly lower than the quantum yield of a biexciton and we demonstrate that multiexciton emission also shows strong intermittency. Both of these findings contradict the standard charging model. Alternatives are discussed.by Gautham Padmanabhan Nair.Ph.D

Carrier multiplication yields in PbS and PbSe nanocrystals measured by transient photoluminescence

We report here an assessment of carrier multiplication (CM) yields in PbSe and PbS nanocrystals (NCs) by a quantitative analysis of biexciton and exciton dynamics in transient photoluminescence decays. Interest in CM, the generation of more than one electron and hole in a semiconductor after absorption of one photon, has renewed in recent years because of reports suggesting greatly increased efficiencies in nanocrystalline materials compared to the bulk form, in which CM was otherwise too weak to be of consequence in photovoltaic energy conversion devices. In our PbSe and PbS NC samples, however, we estimate using transient photoluminescence that at most 0.25 additional e-h pairs are generated per photon even at energies hv > 5Eg, instead of the much higher values reported in the literature. We argue by comparing NC CM estimates and reported bulk values on an absolute energy basis, which we justify as appropriate on physical grounds, that the data reported thus far are inconclusive with respect to the importance of nanoscale-specific phenomena in the CM process.Comment: 10 pages, 7 figure

Conformations of 2-Carboxy-1,4-butanedioic Acid as a Function of Ionization State in Dimethyl Sulfoxide

The conformational equilibria of 2-carboxy-1,4-butanedioic acid and its mono-, di-, and trianions were estimated by NMR couplings in dimethyl sulfoxide (DMSO). Intramolecular hydrogen bonding was inferred for the mono- and dianions, but not for the triacid. For the di- and trianions, the ^3/_(HH) couplings were consistent with the negative carboxylate groups being much closer together than might be expected from electrostatic repulsion considerations. The successive triacid pK_a values were estimated as 7.0, 13.4, and ∼20(?) on the Bordwell scale

Tumor endothelial marker 1-specific DNA vaccination targets tumor vasculature

Tumor endothelial marker 1 (TEM1; also known as endosialin or CD248) is a protein found on tumor vasculature and in tumor stroma. Here, we tested whether TEM1 has potential as a therapeutic target for cancer immunotherapy by immunizing immunocompetent mice with Tem1 cDNA fused to the minimal domain of the C fragment of tetanus toxoid (referred to herein as Tem1-TT vaccine). Tem1-TT vaccination elicited CD8+ and/or CD4+ T cell responses against immunodominant TEM1 protein sequences. Prophylactic immunization of animals with Tem1-TT prevented or delayed tumor formation in several murine tumor models. Therapeutic vaccination of tumor-bearing mice reduced tumor vascularity, increased infiltration of CD3+ T cells into the tumor, and controlled progression of established tumors. Tem1-TT vaccination also elicited CD8+ cytotoxic T cell responses against murine tumor-specific antigens. Effective Tem1-TT vaccination did not affect angiogenesis-dependent physiological processes, including wound healing and reproduction. Based on these data and the widespread expression of TEM1 on the vasculature of different tumor types, we conclude that targeting TEM1 has therapeutic potential in cancer immunotherapy

Biexciton Quantum Yield of Single Semiconductor Nanocrystals from Photon Statistics

Biexciton properties strongly affect the usability of a light emitter in quantum photon sources and lasers but are difficult to measure for single fluorophores at room temperature due to luminescence intermittency and bleaching at the high excitation fluences usually required. Here, we observe the biexciton (BX) to exciton (X) to ground photoluminescence cascade of single colloidal semiconductor nanocrystals (NCs) under weak excitation in a g(2) photon correlation measurement and show that the normalized amplitude of the cascade feature is equal to the ratio of the BX to X fluorescence quantum yields. This imposes a limit on the attainable depth of photon antibunching and provides a robust means to study single emitter biexciton physics. In NC samples, we show that the BX quantum yield is considerably inhomogeneous, consistent with the defect sensitivity expected of the Auger nonradiative recombination mechanism. The method can be extended to study X,BX spectral and polarization correlations.United States. Dept. of Energy (grant no. DE-FG02-07ER46454)Massachusetts Institute of Technology. George R. Harrison Spectroscopy Laboratory (grant no. NIH P41 RR02594)United States. Dept. of Energy. Office of Basic Energy Sciences (award no. DE-SC0001088

Machine Learning in Combustion: Optimization of Fuel Combustion of Rockets

Application of Machine Learning (ML) Techniques in research has seen a drastic increase in past few years and the main aspect of it in the rocket propulsion is to understand the combustion process and finding various methos to optimize it. This paper provides an overview of the relationship between machine learning and combustion with a specific focus on optimizing the rocket fuel combustion. The introduction presents an overview of Combustion and various ways which the ML is associated with it. Subsequently, the paper also discusses about the various ML algorithms which extends its discussions with the supervised, unsupervised and semi-supervised learning techniques and some of its types. An overview of different types of rocket engine is presented for the understanding of the characteristics, advantages and disadvantages of the commonly used rocket engines such as solid, liquid and hybrid propellant ones. Focusing on rocket fuel combustion, the discussion extends to various methods of optimization of the combustion process.  Finally, the paper presents comprehensive results and discussions derived from the studies conducted on rocket fuel combustion optimization