Joseph S. Melinger

Quantum-dot/dopamine bioconjugates function as redox coupled assemblies for in vitro and intracellular pH sensing

The use of semiconductor quantum dots (QDs) for bioimaging and sensing has progressively matured over the past decade. QDs are highly sensitive to charge-transfer processes, which can alter their optical properties. Here, we demonstrate that QD-dopamine-peptide bioconjugates can function as charge-transfer coupled pH sensors. Dopamine is normally characterized by two intrinsic redox properties: a Nernstian dependence of formal potential on pH and oxidation of hydroquinone to quinone by O(2) at basic pH. We show that the latter quinone can function as an electron acceptor quenching QD photoluminescence in a manner that depends directly on pH. We characterize the pH-dependent QD quenching using both electrochemistry and spectroscopy. QD-dopamine conjugates were also used as pH sensors that measured changes in cytoplasmic pH as cells underwent drug-induced alkalosis. A detailed mechanism describing the QD quenching processes that is consistent with dopamines inherent redox chemistry is presented.

Comparison of error rates in combinational and sequential logic

A pulsed laser was used to demonstrate that, for transients much shorter than the clock period, error rates in sequential logic were independent of frequency, whereas error rates in combinational logic were linearly dependent on frequency. In addition, by measuring the error rate as a function of laser pulse energy for fixed clock frequency, the logarithmic dependence of the SEU vulnerable time period prior to the clock edge in combinational logic was established. A mixed mode circuit simulator program was used to successfully model the dynamic response of the logic circuit to pulses of laser light.

Critical evaluation of the pulsed laser method for single event effects testing and fundamental studies

In this paper we present an evaluation of the pulsed laser as a technique for single events effects (SEE) testing. We explore in detail the important optical effects, such as laser beam propagation, surface reflection, and linear and nonlinear absorption, which determine the nature of laser-generated charge tracks in semiconductor materials. While there are differences in the structure of laser- and ion-generated charge tracks, we show that in many cases the pulsed laser remains an invaluable tool for SEE testing. Indeed, for several SEE applications, we show that the pulsed laser method represents a more practical approach than conventional accelerator-based methods. >

Subbandgap laser-induced single event effects: carrier generation via two-photon absorption

Carrier generation based on subbandgap two-photon absorption is demonstrated and shown to be a viable alternative to the conventional single-photon excitation approach in laser-induced single event effects. The two-photon approach exhibits characteristics distinct from those of single-photon excitation, and may be advantageous for a range of single-event effect investigations. The charge track produced by two-photon absorption more closely resembles that of heavy-ion irradiation and, because the photon energy is subbandgap, backside injection through bulk silicon wafers is straightforward and three-dimensional mapping is possible.

Application of a pulsed laser for evaluation and optimization of SEU-hard designs [CMOS]

Pulsed laser single-event upset tests are used to pinpoint and characterize sensitive nodes of circuits and to provide feedback relevant to the development and optimization of radiation-hard designs. The results presented reveal the advantages of incorporating laser evaluation at an early stage into programs described for the development of radiation-hardened parts. A quantitative correlation is observed between the laser single-event upset and single-event latchup threshold measurements and those performed using accelerator-based heavy ion testing methods.

The contribution of nuclear reactions to heavy ion single event upset cross-section measurements in a high-density SEU hardened SRAM

Heavy ion irradiation was simulated using a Geant4 based Monte-Carlo transport code. Electronic and nuclear physics were used to generate statistical profiles of charge deposition in the sensitive volume of an SEU hardened SRAM. Simulation results show that materials external to the sensitive volume can affect the experimentally measured cross-section curve.

Adiabatic population transfer with frequency‐swept laser pulses

We present detailed experimental and theoretical results on population transfer with frequency‐swept picosecond laser pulses. Here, we demonstrate that intense frequency‐swept pulses, when applied in the adiabatic limit, lead to both more efficient and more selective excitation than do unmodulated laser pulses. The experimental work is performed on quasi‐two‐level systems (pentacene/p‐terphenyl crystal and Na vapor), quasi‐three‐level systems (Na vapor), and on more complex multilevel systems (I2 vapor). We discuss the different characteristics of adiabatic population transfer in both few‐level, and multilevel cases, and, in particular, present computer calculations to explore the effects of molecular rotations in multilevel adiabatic population transfer.

Enhanced multiple exciton generation in quasi-one-dimensional semiconductors.

The creation of a single electron-hole pair (i.e., exciton) per incident photon is a fundamental limitation for current optoelectronic devices including photodetectors and photovoltaic cells. The prospect of multiple exciton generation per incident photon is of great interest to fundamental science and the improvement of solar cell technology. Multiple exciton generation is known to occur in semiconductor nanostructures with increased efficiency and reduced threshold energy compared to their bulk counterparts. Here we report a significant enhancement of multiple exciton generation in PbSe quasi-one-dimensional semiconductors (nanorods) over zero-dimensional nanostructures (nanocrystals), characterized by a 2-fold increase in efficiency and reduction of the threshold energy to (2.23 ± 0.03)E(g), which approaches the theoretical limit of 2E(g). Photovoltaic cells based on PbSe nanorods are capable of improved power conversion efficiencies, in particular when operated in conjunction with solar concentrators.

Self-Assembled Quantum Dot-Sensitized Multivalent DNA Photonic Wires

Combining the inherent scaffolding provided by DNA structure with spatial control over fluorophore positioning allows the creation of DNA-based photonic wires with the capacity to transfer excitation energy over distances greater than 150 Å. We demonstrate hybrid multifluorophore DNA-photonic wires that both self-assemble around semiconductor quantum dots (QDs) and exploit their unique photophysical properties. In this architecture, the QDs function as both central nanoscaffolds and ultraviolet energy harvesting donors that drive Förster resonance energy transfer (FRET) cascades through the DNA wires with emissions that approach the near-infrared. To assemble the wires, DNA fragments labeled with a series of increasingly red-shifted acceptor-dyes were hybridized in a predetermined linear arrangement to a complementary DNA template that was chemoselectively modified with a hexahistidine-appended peptide. The peptide portion facilitated metal-affinity coordination of multiple hybridized DNA-dye structures to a central QD completing the final nanocrystal-DNA photonic wire structure. We assembled several such hybrid structures where labeled-acceptor dyes were excited by the QDs and arranged to interact with each other via consecutive FRET processes. The inherently facile reconfiguration properties of this design allowed testing of alternate formats including the addition of an intercalating dye located in the template DNA or placement of multiple identical dye acceptors that engaged in homoFRET. Lastly, a photonic structure linking the central QD with multiple copies of DNA hybridized with 4-sequentially arranged acceptor dyes and demonstrating 4-consecutive energy transfer steps was examined. Step-by-step monitoring of energy transfer with both steady-state and time-resolved spectroscopy allowed efficiencies to be tracked through the structures and suggested that acceptor dye quantum yields are the predominant limiting factor. Integrating such DNA-based photonic structures with QDs can help create a new generation of biophotonic wire assemblies with widespread potential in nanotechnology.

Investigation of the Propagation Induced Pulse Broadening (PIPB) Effect on Single Event Transients in SOI and Bulk Inverter Chains

The propagation of single event transients (SET) is measured and modeled in SOI and bulk inverter chains. The propagation-induced pulse broadening (PIPB) effect is shown to determine the SET pulse width measured at the output of long chains of inverters after irradiation. Initially, narrow transients, less than 200 ps at the struck inverter, are progressively broadened into the nanosecond range. PIPB is induced by dynamic floating body effects (also called history effects) in SOI and bulk transistors, which depend on the bias state of the transistors before irradiation. Implications for SET hardness assurance, circuit modelling and hardening are discussed. Floating body and PIPB effects are usually not taken into account in circuit models, which can lead to large underestimation of SET sensitivity when using simulation techniques like fault injection in complex circuits.