Model To Determine a Distinct Rate Constant for Carrier Multiplication from Experiments.

Carrier multiplication (CM) is the process in which multiple electron-hole pairs are created upon absorption of a single photon in a semiconductor. CM by an initially hot charge carrier occurs in competition with cooling by phonon emission, with the respective rates determining the CM efficiency. Up until now, CM rates have only been calculated theoretically. We show for the first time how to extract a distinct CM rate constant from experimental data of the relaxation time of hot charge carriers and the yield of CM. We illustrate this method for PbSe quantum dots. Additionally, we provide a simplified method using an estimated energy loss rate to estimate the CM rate constant just above the onset of CM, when detailed experimental data of the relaxation time is missing.

Efficient carrier multiplication in CsPbI3 perovskite nanocrystals.

The all-inorganic perovskite nanocrystals are currently in the research spotlight owing to their physical stability and superior optical properties-these features make them interesting for optoelectronic and photovoltaic applications. Here, we report on the observation of highly efficient carrier multiplication in colloidal CsPbI3 nanocrystals prepared by a hot-injection method. The carrier multiplication process counteracts thermalization of hot carriers and as such provides the potential to increase the conversion efficiency of solar cells. We demonstrate that carrier multiplication commences at the threshold excitation energy near the energy conservation limit of twice the band gap, and has step-like characteristics with an extremely high quantum yield of up to 98%. Using ultrahigh temporal resolution, we show that carrier multiplication induces a longer build-up of the free carrier concentration, thus providing important insights into the physical mechanism responsible for this phenomenon. The evidence is obtained using three independent experimental approaches, and is conclusive.

Asymmetric Optical Transitions Determine the Onset of Carrier Multiplication in Lead Chalcogenide Quantum Confined and Bulk Crystals.

Carrier multiplication is a process in which one absorbed photon excites two or more electrons. This is of great promise to increase the efficiency of photovoltaic devices. Until now, the factors that determine the onset energy of carrier multiplication have not been convincingly explained. We show experimentally that the onset of carrier multiplication in lead chalcogenide quantum confined and bulk crystals is due to asymmetric optical transitions. In such transitions most of the photon energy in excess of the band gap is given to either the hole or the electron. The results are confirmed and explained by theoretical tight-binding calculations of the competition between impact ionization and carrier cooling. These results are a large step forward in understanding carrier multiplication and allow for a screening of materials with an onset of carrier multiplication close to twice the band gap energy. Such materials are of great interest for development of highly efficient photovoltaic devices.

Broadband Cooling Spectra of Hot Electrons and Holes in PbSe Quantum Dots.

Understanding cooling of hot charge carriers in semiconductor quantum dots (QDs) is of fundamental interest and useful to enhance the performance of QDs in photovoltaics. We study electron and hole cooling dynamics in PbSe QDs up to high energies where carrier multiplication occurs. We characterize distinct cooling steps of hot electrons and holes and build up a broadband cooling spectrum for both charge carriers. Cooling of electrons is slower than of holes. At energies near the band gap we find cooling times between successive electronic energy levels in the order of 0.5 ps. We argue that here the large spacing between successive electronic energy levels requires cooling to occur by energy transfer to vibrational modes of ligand molecules or phonon modes associated with the QD surface. At high excess energy the energy loss rate of electrons is 1-5 eV/ps and exceeds 8 eV/ps for holes. Here charge carrier cooling can be understood in terms of emission of LO phonons with a higher density-of-states in the valence band than the conduction band. The complete mapping of the broadband cooling spectrum for both charge carriers in PbSe QDs is a big step toward understanding and controlling the cooling of hot charge carriers in colloidal QDs.

Photogeneration and Mobility of Charge Carriers in Atomically Thin Colloidal InSe Nanosheets Probed by Ultrafast Terahertz Spectroscopy.

The implementation of next generation ultrathin electronics by applying highly promising dimensionality-dependent physical properties of two-dimensional (2D) semiconductors is ever increasing. In this context, the van der Waals layered semiconductor InSe has proven its potential as photodetecting material with high charge carrier mobility. We have determined the photogeneration charge carrier quantum yield and mobility in atomically thin colloidal InSe nanosheets (inorganic layer thickness 0.8-1.7 nm, mono/double-layers, ≤ 5 nm including ligands) by ultrafast transient terahertz (THz) spectroscopy. A near unity quantum yield of free charge carriers is determined for low photoexcitation density. The charge carrier quantum yield decreases at higher excitation density due to recombination of electrons and holes, leading to the formation of neutral excitons. In the THz frequency domain, we probe a charge mobility as high as 20 ± 2 cm2/(V s). The THz mobility is similar to field-effect transistor mobilities extracted from unmodified exfoliated thin InSe devices. The current work provides the first results on charge carrier dynamics in ultrathin colloidal InSe nanosheets.

Placental pathology and outcome after perinatal asphyxia and therapeutic hypothermia.

OBJECTIVE: To assess the relationship between placental pathology, pattern of brain injury and neurodevelopmental outcome in term infants with perinatal asphyxia receiving therapeutic hypothermia. STUDY DESIGN: Studies were performed in 76 infants. Death or survival with impairments at 18 to 24 months was used as a composite adverse outcome. Multivariable analysis was performed. RESULTS: Among the 75 infants analyzed, the predominant pattern of brain injury was: no injury (n=27), a white matter/watershed pattern (n=14), basal-ganglia-thalamic injury (n=13) or near-total brain injury (n=21). An adverse outcome was seen in 35 of the 76 infants. Elevated nucleated red blood cells were associated with white matter involvement. Small placental infarcts were more common among infants without brain injury. All other placental abnormalities were not related to both outcome measures. CONCLUSION: In our population of term infants receiving therapeutic hypothermia, no type of placental pathology was related to extensive brain injury or adverse neurodevelopmental outcome.

Radiative and Nonradiative Recombination in CuInS2 Nanocrystals and CuInS2-Based Core/Shell Nanocrystals.

Luminescent copper indium sulfide (CIS) nanocrystals are a potential solution to the toxicity issues associated with Cd- and Pb-based nanocrystals. However, the development of high-quality CIS nanocrystals has been complicated by insufficient knowledge of the electronic structure and of the factors that lead to luminescence quenching. Here we investigate the exciton decay pathways in CIS nanocrystals using time-resolved photoluminescence and transient absorption spectroscopy. Core-only CIS nanocrystals with low quantum yield are compared to core/shell nanocrystals (CIS/ZnS and CIS/CdS) with higher quantum yield. Our measurements support the model of photoluminescence by radiative recombination of a conduction band electron with a localized hole. Moreover, we find that photoluminescence quenching in low-quantum-yield nanocrystals involves initially uncoupled decay pathways for the electron and hole. The electron decay pathway determines whether the exciton recombines radiatively or nonradiatively. The development of high-quality CIS nanocrystals should therefore focus on the elimination of electron traps.

Organic Linker Defines the Excited-State Decay of Photocatalytic MIL-125(Ti)-Type Materials.

Recently, MIL-125(Ti) and NH2 -MIL-125(Ti), two titanium-based metal-organic frameworks, have attracted significant research attention in the field of photocatalysis for solar fuel generation. This work reveals that the differences between these structures are not only based on their light absorption range but also on the decay profile and topography of their excited states. In contrast to MIL-125(Ti), NH2 -MIL-125(Ti) shows markedly longer lifetimes of the charge-separated state, which improves photoconversion by the suppression of competing decay mechanisms. We used spectroelectrochemistry and ultrafast spectroscopy to demonstrate that upon photoexcitation in NH2 -MIL-125(Ti) the electron is located in the Ti-oxo clusters and the hole resides on the aminoterephthalate unit, specifically on the amino group. The results highlight the role of the amino group in NH2 -MIL-125(Ti), the electron donation of which extends the lifetime of the photoexcited state substantially.

Hole Cooling Is Much Faster than Electron Cooling in PbSe Quantum Dots.

In semiconductor quantum dots (QDs), charge carrier cooling is in direct competition with processes such as carrier multiplication or hot charge extraction that may improve the light conversion efficiency of photovoltaic devices. Understanding charge carrier cooling is therefore of great interest. We investigate high-energy optical transitions in PbSe QDs using hyperspectral transient absorption spectroscopy. We observe bleaching of optical transitions involving higher valence and conduction bands upon band edge excitation. The kinetics of rise of the bleach of these transitions after a pump laser pulse allow us to monitor, for the first time, cooling of hot electrons and hot holes separately. Our results show that holes cool significantly faster than electrons in PbSe QDs. This is in contrast to the common assumption that electrons and holes behave similarly in Pb chalcogenide QDs and has important implications for the utilization of hot charge carriers in photovoltaic devices.

Cooling and Auger recombination of charges in PbSe nanorods: crossover from cubic to bimolecular decay.

The cooling and Auger recombination of electron-hole pairs in PbSe quantum dots (QDs) and a series of nanorods (NRs) with similar diameter and varying length was studied by ultrafast pump-probe laser spectroscopy. Hot exciton cooling rates are found to be independent of nanocrystal shape. The energy relaxation rate decreases during cooling of charges, due to reduction of the density of electronic states. Auger recombination occurs via cubic third-order kinetics of uncorrelated charges in the QDs and NRs with length up to 29 nm. On increasing the NR length to 52 nm, a crossover to bimolecular exciton decay is found. This suggests a spatial extent of the one-dimensional exciton of 30-50 nm, which is significantly smaller than the value of 92 nm for the three-dimensional exciton diameter in bulk PbSe. The Auger decay time increases with NR length, which is beneficial for applications in nanocrystal lasers as well as for generation of free charges in photovoltaics.

Third (fourth and fifth) nonocular tumors in survivors of retinoblastoma.

OBJECTIVE: This study aimed to investigate the incidence, timing, pattern, and distribution of, as well as survival as a result of, third, fourth, and fifth primary tumors in survivors of retinoblastoma. DESIGN: This study was a retrospective case series of patients diagnosed with retinoblastoma and a second malignant neoplasm. Records were examined for demographic, prior treatment, and second tumor information, as well as any evidence of the development of a third, fourth, or fifth nonocular tumor. When possible, telephone inquiries were conducted for follow-up. PARTICIPANTS: The study included 1506 patients followed in the Ophthalmic Oncology Center at New York-Presbyterian Hospital, New York Weill Cornell Medical Center, 211 of whom developed a second tumor and had sufficient treatment data to be useful for analysis. MAIN OUTCOME MEASURES: The development of third and additional nonocular tumors and survival from these tumors were the primary outcome measures. RESULTS: Of 211 second-tumor patients, 142 died before an additional malignancy developed (median survival time, 1.8 +/- 0.3 years) and in 28, third tumors developed (5-year incidence rate, 11%; 10-year incidence rate, 22%; median time to third tumor development, 5.8 +/- 8.3 years). The 5- and 10-year survival rates for this group were 41% and 30%, respectively (median survival time, 4.1 +/- 1.0 years). Of 28 patients in whom third tumors developed, 27 (96%) had received radiation therapy for their retinoblastoma. The most common sites for third tumors were soft tissues of the head (36% of all third tumors) and skin (36% of all third tumors). In six patients, a fourth tumor developed, and in two patients a fifth tumor developed. All fourth and fifth tumors were found in the soft tissues of the head, the skin, or the bones. CONCLUSIONS: Survivors of retinoblastoma in whom second malignant neoplasms develop are at a higher risk for the development of additional tumors than they were for the development of a second tumor. The locations and expected ages at which additional tumors develop are consistent with the patterns we have seen in second tumors.

A phase I/II study of subconjunctival carboplatin for intraocular retinoblastoma.

PURPOSE: To evaluate the efficacy and toxicity of subconjunctival carboplatin for intraocular retinoblastoma. DESIGN: A phase I/II clinical trial (noncomparative case series). PARTICIPANTS: Eleven children with bilateral retinoblastoma who had 13 eyes containing active tumor. METHODS: Subconjunctival carboplatin (1.4-2.0 ml of a 10-mg/ml solution) was administered. Ophthalmologic examinations with the patient under anesthesia were performed before each injection, and response and toxicity were assessed. MAIN OUTCOME MEASURES: Tumor response and toxicities. RESULTS: A median of three injections per eye was administered (range, 1-7 injections per eye) at a median interval of 21 days between injections (range, 14-35 days). The median dose was 20 mg/injection (range, 14-20 mg). Toxicities included transient periorbital edema in four eyes and optic atrophy in one eye that also received laser photocoagulation and cryotherapy. No nonocular toxicities were noted. Three of five eyes with vitreous disease that could be evaluated had major responses. Two of five eyes with retinal tumors had a response, but neither eye with subretinal disease responded. CONCLUSIONS: Subconjunctival carboplatin as used in this protocol is effective for intraocular retinoblastoma. It appears to be safe, but additional study and longer follow-up are required, particularly to determine the risk of optic atrophy.

Intraocular carboplatin concentrations following intravenous administration for human intraocular retinoblastoma.

Previous studies of the penetration of carboplatin into the vitreous have depended on unaffected animals or on animal models for other cancers. The objective of this study was to determine the intraocular levels of carboplatin following intravenous administration of carboplatin in the treatment of human intraocular retinoblastoma. Eight patients with bilateral intraocular retinoblastoma were treated in a consistent fashion with intravenous carboplatin. One additional patient was similarly treated, but enucleated one month later. Samples were taken from those nine eyes after enucleation one to two hours after the administration of 18.7 mg/kg (560 mg/m( 2) for patients more than 12 kg) of intravenous carboplatin, and carboplatin concentrations in the aqueous and vitreous were then measured by atomic absorption spectrometry. The mean concentration measured in the aqueous was 5.13 microg/ml and in the vitreous 4.05 microg/ml, and vitreal concentrations were an average of 80% of aqueous concentrations. In one patient, a vitreous concentration of carboplatin was detected after an interval of one month that was 10% of the levels found in the samples enucleated one hour post-administration. These concentrations are much higher than previous animal studies would predict, and are similar to levels measured in unaffected animals when the drug is given after the use of cryotherapy. The concentration also approaches levels previously shown to be toxic to the retina. This elevation in carboplatin concentration may be due to disruption of the blood-vitreous barrier by active tumor.

Second nonocular tumors in survivors of bilateral retinoblastoma: a possible age effect on radiation-related risk.

OBJECTIVE: This study aimed to investigate the relationship in bilateral retinoblastoma survivors between the incidence of second tumors and the age when external beam radiation (EBR) was used. DESIGN: A retrospective analysis of patients diagnosed with retinoblastoma was performed by examining records for background information and treatment information as well as reviewing documentation of patients with second nonocular tumors. Two telephone interviews were conducted for follow-up as well as inquiries directed to tumor registries and state databases. PARTICIPANTS: The original study included 1729 patients treated in New York and Boston; the current study includes only the 1506 patients treated in New York. Of those, 816 patients were diagnosed with bilateral retinoblastoma, had sufficient treatment data to be useful, and survived at least 1 year from diagnosis. MAIN OUTCOME MEASURES: The subjects were observed for evidence of the development of second nonocular tumors. RESULTS: There was a significant decrease in tumor-free survival among patients treated with EBR before the age of 12 months, but no significant difference between the group treated with EBR after the age of 12 months and the group not treated with EBR. For tumors in the field of radiation, patients treated with early EBR showed a significant decrease in tumor-free survival when compared to patients treated with late EBR, with no significant difference between late radiation and no radiation. There were no significant differences between groups for tumors out of the field of radiation. Significant differences attributable to the use of EBR were found only for tumors of the skull and face bones and for tumors of the soft tissue of the head. CONCLUSIONS: The long-term effect of radiation treatment on survivors of bilateral retinoblastoma is to increase the incidence and affect the distribution of second tumors. However, no increased risk is observed for tumors out of the field of radiation among patients who underwent radiation, and the risk for tumors in the field of radiation is heavily dependent on the age at which EBR is given and may be acceptably small to the patient after the age of 12 months.

Presenting signs of retinoblastoma.

A retrospective chart review was performed on the charts of 1265 patients with retinoblastoma who were on file at New York Hospital and whose conditions had been diagnosed between the years 1960 and 1990. The mean follow-up was 90 months, ranging from 0 to 409 months. Thirty-two distinct presenting signs of retinoblastoma were identified, the most common of which were leukocoria (56.2%), strabismus (23.6%), poor vision (7.7%), and family history (6.8%). No correlation was found between any of the presenting signs and laterality, sex, race, or survival. Leukocoria correlated to Reese-Ellsworth Group Va or Vb, whereas strabismus was invariably associated with either macular tumors or macular retinal detachments. Eighty-three (96.5%) patients presenting with a family history did so before the age of 24 months. The presenting sign leading to a diagnosis of retinoblastoma was correlated with degree of advancement and tumor location. Leukocoria and strabismus are the most common presenting signs, with leukocoria correlated to more advanced disease and strabismus always associated with macular involvement. Vitreous hemorrhage (1 case, 0.1%), microphthalmos (4 cases, 0.3%), and orbital cellulitis (3 cases, 0.2%) are extremely rare presenting findings in retinoblastoma.