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.

The method of infrared photomodulation spectroscopy is used in order to study the nature of the (e1-e2)-type intersubband transitions in GaAs/AlGaAs multiple quantum wells. Both undoped and n-doped, narrow (d(GaAs) <100A) MQWs were studied. They were selectively photoexcited at various energies and intensities. The observed photoinduced, intersubband transitions involve free or bound excitons as well as bare electrons. The type of the transition is determined by its energy, polarization and the dependence of its intensity on the exciting (laser) intensity and on temperature.

Photo-Induced Intersubband Transitions in Quantum Wells

We apply intersubband electroabsorption spectroscopy to a current-carrying multiple quantum-well structure device, in order to simultaneously measure the steady state carrier density and the average time carriers spend in each quantum well. By comparing our measurements with a simple quantum mechanical tunneling model, we show directly that electrons sequentially tunnel through the periodic quantum structure. In addition, we use photoinduced absorption spectroscopy to measure the effect of the applied electric field on electron states within the minibands in the energy continuum. We show that already at relatively small fields, the optical transitions into the continuum minibands merge into a single spectral band. Using an eight-band model, we explain this observation in terms of field-induced miniband state localization.

Electroabsorption spectroscopy of intersubband transitions in multiple-quantum-well superlattices

We report on the photophysical properties of a soluble thienylene-vinylene π-conjugated polymer, namely imide poly-thienylene vinylene. Ultrafast pump probe spectroscopy reveals that a broad photoinduced absorption (PA) band is immediately photogenerated along with a narrower PA from the photoinduced singlet excitons. The broad PA is susceptible to magnetic field and thus is assigned to a correlated triple pair state. This feature shows a long lifetime persisting into the μs time domain, in contrast to the singlet exciton PA, which quickly decays on a 10-ps timescale. The steady-state PA spectrum is identical to the transient PA spectrum of the triplet pair state but shows a magnetic field response that is typical to isolated triplet excitons.

Long-lived-correlated triplet-pair state in an imide substituted poly-thienylene vinylene-based π-conjugated polymer

The three resonantly enhanced Raman lines of Poly-acetylene (PA) are studied under hydrostatic pressure up to 8kbar, with various laser excitation wavelengths. The results are interpreted in the framework of the Amplitude Modes theory (AM), with varying interchain coupling.

Resonant Raman Scattering of Trans Polyacetylene Under Pressure

Photoexcited eh pairs in poly(1,6 heptadiyne) are studied by photoinduced absorption and photoluminescence spectroscopy. Most of the pairs quickly recombine; some form metastable neutral states. Resonant Raman scattering spectra reveal coexistence of two polymer structures with two different gaps, each has a set of three strongly coupled phonons.

Eitan Ehrenfreund

Papers

Photo-Induced Intersubband Transitions in Quantum Wells

Electroabsorption spectroscopy of intersubband transitions in multiple-quantum-well superlattices

Long-lived-correlated triplet-pair state in an imide substituted poly-thienylene vinylene-based π-conjugated polymer

Resonant Raman Scattering of Trans Polyacetylene Under Pressure

Photoinduced Spectroscopy and Resonant Raman Study of Poly(1,6-Heptadiyne)