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.

A novel interband-pump intersubband-probe technique is developed in order to study the dynamics of photogenerated carriers and excitons in semiconductor superlattices by time resolved photoinduced mid-infrared absorption. Using variable energies for both pump and probe, the measured photoinduced transients yielded the photoinduced absorption spectra at different excitation energies, the excitation spectra at different probe energies and both at varying delay times. We conclude from these measurements that a resonantly photogenerated excitonic population reaches thermal distribution within a few picoseconds of the excitation, much slower than photogenerated carries with excess energy. We find that it takes roughly 0.1 ns for the photogenerated population to equilibrate with the lattice temperature, and that this time depends on the excess energy by which the photogenerated population in created. After thermal equilibrium with the lattice is reached, the excitonic population decays on a sub-nanosecond time scale which is linearly dependent on the lattice temperature.

Time resolved photoinduced intersubband absorption: A novel and powerful way of studying the dynamics of quantum structure carriers

We describe a novel approach to experimentally quantify the effect of disorder in polymers using the dispersion of the resonant Raman scattering phonon frequencies with the excitation laser frequency. The method is applied to polyacetylene where two types of disorder are identified. For all-trans samples we found intrinsic disorder which results from variation in the coupling parameter. For cis rich trans samples the disorder is extrinsic and reflects breaking of the ground state degeneracy.

Disorder in Polyacetylene Probed By Resonant Raman Scattering

Carriers in small colloidal InP nanoparticles are in strong quantum confinement regime The low temperature photoluminescence spectrum of InP nanoparticles is composed of an excitonic luminescence at high energies and a non-excitonic defect emission band at lower energies HF etching of the nanoparticles reduces the defect emission and enhances the exciton process In this work we apply optically detected magnetic resonance spectroscopy (ODMR) both in continuous wave and time resolved mode (TR-ODMR) to study the defect luminescence in InP nanoparticles The results show that the defect luminescence originates from weakly coupled electron-hole pair, where the electron is trapped at the surface by phosphorous vacancy, Vp, and the hole is located at the valence band Additionally, the results suggest that the non-etched samples are dominated by Vp at the surface Those are mainly eliminated upon HF treatment, leaving behind small percent of Vp in the core of the nanoparticle We also find the electron-hole exchange interaction from circular polarized ODMR measurements The TR-ODMR measurement further clarifies the spin dynamics and characteristic of the magnetic sites Fitting these measurements to the simulated response of the PL intensity to the square wave modulated microwave power revealed that the spin relaxation time and radiativemore » lifetime of electron-hole pair in the nanoparticles are in the microseconds regime« less

Continuous and Time Resolved Optically Detected Magnetic Resonance Studies of InP Nanoparticles

We report on measurements of temporal and polarization correlations measurements of multi-color photons emitted from cw optically excited single semiconductor quantum dots. We show that there are tunable intensity and polarization correlations among photons emitted at the same, and at different wavelengths due to the recombination of electron-hole pairs from different collective many-carrier quantum states confined in the dots. The observed polarization correlations between the emitted photons are qualitatively explained in terms of long electronic spin memory.

Eitan Ehrenfreund

Papers

Time resolved photoinduced intersubband absorption: A novel and powerful way of studying the dynamics of quantum structure carriers

Disorder in Polyacetylene Probed By Resonant Raman Scattering

Continuous and Time Resolved Optically Detected Magnetic Resonance Studies of InP Nanoparticles

Quantum dots: a source of multicolor photons with tunable statistics and correlated polarizations

Time resolved intersubband opti al transitions in opti ally pumped semi ondu tor lasers