Size dependence of carrier dynamics and carrier multiplication in PbS quantum dots

The time dynamics of the photoexcited carriers and carrier-multiplication efficiencies in PbS quantum dots (QDs) are investigated. In particular, we report on the carrier dynamics, including carrier multiplication, as a function of QD size and compare them to the bulk value. We show that the intraband 1P - \u3e 1S decay becomes faster for smaller QDs, in agreement with the absence of a phonon bottleneck. Furthermore, as the size of the QDs decreases, the energy threshold for carrier multiplication shifts from the bulk value to higher energies. However, the energy threshold shift is smaller than the band-gap shift and, therefore, for the smallest QDs, the threshold approaches 2.35 E(g), which is close to the theoretical energy conservation limit of twice the band gap. We also show that the carrier-multiplication energy efficiency increases with decreasing QD size. By comparing to theoretical models, our results suggest that impact ionization is sufficient to explain carrier multiplication in QDs

Affordable Medical X-ray Imaging for the Developing World: a Global Vision

Abstract-Imaging technologies, already used routinely for diagnostics, are becoming an essential part of the work flow included in screening, treatment progress' monitoring and outcomes' evaluation. However, various imaging modalities considered standard practice for the medical institutions in the developed world remain today rather inaccessible for the populations at large in the developing countries. One of the limiting factors is the high cost of medical imaging equipment. To develop and produce low cost, medical imaging systems of good quality requires disruptive innovations and novel approaches to engineering. The sophistication and capabilities of ubiquitous consumer products available today provide a real opportunity to capitalize on their low cost components to drive down significantly the cost of medical imaging equipment. Following this approach, XLV Diagnostics, a Canadian start-up, is developing a completely new technology, X-ray Light Valve (XLV), for producing very affordable large area, flat panel, x-ray detectors based on which economical medical imaging systems can be built. The first XLV product will be a digital mammography machine for breast cancer screening that will be sold at a fraction of the prices asked for presently available machines. The paper will present the conceptual approach and innovative engineering to create from consumer type components a good quality digital imaging system and will show several of its parametric characteristics

A tunable colloidal quantum dot photo field-effect transistor

We fabricate and investigate field-effect transistors in which a light-absorbing photogate modulates the flow of current along the channel. The photogate consists of colloidal quantum dots that efficiently transfer photoelectrons to the channel across a charge-separating (type-II) heterointerface, producing a primary and sustained secondary flow that is terminated via electron back-recombination across the interface. We explore colloidal quantum dot sizes corresponding to bandgaps ranging from 730 to 1475 nm and also investigate various stoichiometries of aluminum-doped ZnO (AZO) channel materials. We investigate the role of trap state energies in both the colloidal quantum dot energy film and the AZO channel. Most photodetectors function either as photodiodes or as photoconductors: photodiodes offer fast response and low dark current, while photoconductors provide built-in gain associated with the use of long-lived sensitizing centers. The concept of a phototransistor, such as a photo-field-effecttransistor (photoFET), 1 offers an attractive possibility: gain united with a lowered dark current compared to the photoconductor, achieved if the thickness of the current-carrying channel can be chosen independently from the thickness of the light-absorbing layer. Here, we report a versatile photoFET that benefits from a broad spectrum of detection, spanning the visible and portions of the short-wavelength infrared spectral region. We present two of the four ingredients necessary to the realization of a highly sensitive photodetector based on the photo-FET concept: we successfully transfer photoelectrons from the sensitizing material to the electron-accepting channel (EAC), and we obtain photocurrents through the recirculation of secondary injected photocarriers flowing along the channel. Further work will be required to realize a sensitizing layer having sufficiently high electron mobility that a fully light-absorbing thickness of this medium can efficiently inject all carriers into the channel; and to deposit an ultrathin channel compatible with low dark current. Our device uses colloidal quantum dots (CQDs) as the sensitizing material to form the photo gate. CQDs offer tunability of the bandgap, and thus the spectral range of light sensing in a photodetector, through the size of the nanoparticles. Since CQDs are solution processable, they are compatible with low-cost, large-area processes. We utilize a sub-monolayer film of PbS CQD of a variety of sizes as the sensitizing material. Aluminum-doped zinc oxide (AZO) serves as the electron-accepting channel. We employ no conductivity-enhancing treatments to the CQD films-instead of employing crosslinking treatments using short dithiols, 2 we leave the quantum dots capped with oleic acid. This, combined with our use of a submonolayer of CQDs, ensures that our study focuses on electron injection into the channel, followed by conduction within the channel, since the photogate material has negligible conductance compared to the channel. As schematically drawn i

A tunable colloidal quantum dot photo field-effect transistor

We fabricate and investigate field-effect transistors in which a light-absorbing photogate modulates the flow of current along the channel. The photogate consists of colloidalquantum dots that efficiently transfer photoelectrons to the channel across a charge-separating (type-II) heterointerface, producing a primary and sustained secondary flow that is terminated via electron back-recombination across the interface. We explore colloidalquantum dot sizes corresponding to bandgaps ranging from 730 to 1475 nm and also investigate various stoichiometries of aluminum-doped ZnO (AZO) channel materials. We investigate the role of trap state energies in both the colloidalquantum dot energy film and the AZO channel