In the coming decade, the ability to sense and detect the state of biological systems and living organisms optically, electrically and magnetically will be radically transformed by developments in materials physics and chemistry. The emerging ability to control the patterns of matter on the nanometer length scale can be expected to lead to entirely new types of biological sensors. These new systems will be capable of sensing at the single-molecule level in living cells, and capable of parallel integration for detection of multiple signals, enabling a diversity of simultaneous experiments, as well as better crosschecks and controls.

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The use of nanocrystals in biological detection

Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron–hole pairs through enhanced impact ionization processes. Three QD solar cell configurations are described: (1) photoelectrodes comprising QD arrays, (2) QD-sensitized nanocrystalline TiO 2 , and (3) QDs dispersed in a blend of electron- and hole-conducting polymers. These high-efficiency configurations require slow hot carrier cooling times, and we discuss initial results on slowed hot electron cooling in InP QDs.

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Quantum dot solar cells

Quantum communication relies on the availability of light pulses with strong quantum correlations among photons. An example of such an optical source is a single-photon pulse with a vanishing probability for detecting two or more photons. Using pulsed laser excitation of a single quantum dot, a single-photon turnstile device that generates a train of single-photon pulses was demonstrated. For a spectrally isolated quantum dot, nearly 100% of the excitation pulses lead to emission of a single photon, yielding an ideal single-photon source.

https://science.sciencemag.org/content/sci/290/5500/2282.full.pdf

A Quantum Dot Single-Photon Turnstile Device

Raman spectroscopy is a standard characterization technique for any carbon system. Here we review the Raman spectra of amorphous, nanostructured, diamond-like carbon and nanodiamond. We show how to use resonant Raman spectroscopy to determine structure and composition of carbon films with and without nitrogen. The measured spectra change with varying excitation energy. By visible and ultraviolet excitation measurements, the G peak dispersion can be derived and correlated with key parameters, such as density, sp(3) content, elastic constants and chemical composition. We then discuss the assignment of the peaks at 1150 and 1480 cm(-1) often observed in nanodiamond. We review the resonant Raman, isotope substitution and annealing experiments, which lead to the assignment of these peaks to trans-polyacetylene.

Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond

As a growing applied science, nanotechnology has considerable global socioeconomic value, and the benefits afforded by nanoscale materials and processes are expected to have significant impacts on almost all industries and all areas of society. A diverse array of engineered nanoscale products and processes have emerged [e.g., carbon nanotubes, fullerene derivatives, and quantum dots (QDs)], with widespread applications in fields such as medicine, plastics, energy, electronics, and aerospace. With the nanotechnology economy estimated to be valued at $1 trillion by 2012, the prevalence of these materials in society will be increasing, as will the likelihood of exposures. Importantly, the vastness and novelty of the nanotechnology frontier leave many areas unexplored, or underexplored, such as the potential adverse human health effects resulting from exposure to novel nanomaterials. It is within this context that the need for understanding the potentially harmful side effects of these materials becomes clear. The reviewed literature suggests several key points: Not all QDs are alike; engineered QDs cannot be considered a uniform group of substances. QD absorption, distribution, metabolism, excretion, and toxicity depend on multiple factors derived from both inherent physicochemical properties and environmental conditions; QD size, charge, concentration, outer coating bioactivity (capping material and functional groups), and oxidative, photolytic, and mechanical stability have each been implicated as determining factors in QD toxicity. Although they offer potentially invaluable societal benefits such as drug targeting and in vivo biomedical imaging, QDs may also pose risks to human health and the environment under certain conditions.

A toxicologic review of quantum dots: toxicity depends on physicochemical and environmental factors.

Summary form only given. We use visible-infrared dual beam time-resolved spectroscopy to study the dynamics of carriers in optically pumped laser devices containing quantum heterostructures within their active layers. By tuning the probe energy through the intersubband absorption resonance, we directly measure the electron population in various momentum states within the first conduction subband of the lasing device. We use, for the first time, time-resolved intersubband absorption to directly probe the dynamics of the excited electronic plasma. We study GaAs/AlGaAs semiconductor quantum well (QW) lasers at various excitation densities, below and above their lasing threshold. The excitation, by an above bandgap, spectrally tunable, picosecond optical laser pump pulse, is followed by a mid-IR picosecond probe pulse, spectrally tuned to the QW intersubband optical transition resonance. By changing the delay time between the pump and the probe pulses, we investigate the temporal evolution of the photoexcited electrons before, during and after the lasing action.

Time resolved intersubband optical transitions in optically pumped semi conductor lasers

We used photoinduced absorption (PIA) spectroscopy of optical transitions between valence subbands of a nominally undoped SiGe/Si quantum wells to study the recombination processes of photogenerated carriers in these heterostructures. We measured the PIA as a function of the ambient temperature, photo-excitation density and modulation frequency. Our measurements show that the recombination kinetics is governed by both an extrinsic mono-molecular and intrinsic bi-molecular terms. We account for this density dependence by a simple rate model which takes into account thermal activation of the photogenerated holes out of the SiGe wells. The model describes very well the measured temperature dependence of the PIA and it provides quite an accurate determination of the intrinsic recombination rate of electrons and holes within the well regions in these heterostructures. The rate that we obtain is faster than the measured recombination rates in bulk Si and Ge. This recombination rate enhancement is possibly due to carriers confinement within the SiGe well regions.

Recombination of carriers in SiGe/Si heterostructures measured by photomodulated intersubband absorption

Magnetic Field Effects in Organic Semiconductors; Low and High Fields, Steady State and Time Resolved

Recent advances in spin response of organic semiconductors include long polaron spin coherence time measured
by optically detected magnetic resonance (ODMR); substantive room-temperature magneto-electroluminescence
and magneto-conductance obtained in organic light emitting diodes (OLED); and spin-polarized carrier injection
from ferromagnetic electrodes in organic spin valves (OSV). Although the hyperfine interaction (HFI) has been
foreseen to play an important role in organic spin response, clear experimental evidence has been lacking. Using
the chemical versatility advantage of the organics, we studied and compared spin responses in films, OLED and
OSV devices based on π-conjugated polymers made of protonated, H-, and deuterated, D-hydrogen having a
weaker HFI strength. We demonstrate that the HFI plays a crucial role in all three spin responses. OLEDs and
films based on the D-polymers show substantial narrower magneto-electroluminescence, magneto-conductivity
and ODMR responses; whereas due to the longer spin diffusion, OSV devices based on D-polymers show
substantially larger magnetoresistance that reaches ~330% at small bias voltage and low temperatures.

Eitan Ehrenfreund

Papers

Time resolved intersubband optical transitions in optically pumped semi conductor lasers

Recombination of carriers in SiGe/Si heterostructures measured by photomodulated intersubband absorption

Magnetic Field Effects in Organic Semiconductors; Low and High Fields, Steady State and Time Resolved

Hyperfine interaction in spin response of organic devices

Polaron Pair Model of Magnetic Field Effects in Devices Based on Organic Semiconductors and Hybrid Perovskites