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Semiconductor quantum dots (QDs) are bright fluorophores that have significant utility for imaging and sensing applications. Core QDs are often employed in chemosensing via redox processes that modulates their fluorescence in the presence of an analyte. However, such particles lack robust surface passivation and generally contain a sizable portion of nonfluorescent QDs, which is detrimental to the detection limit. We investigated an approach to "turn on" non-fluorescent core QDs by lightly overcoating them with a thin shell of a higher bandgap semiconductor. The shell augments the population of sensing chromophores and increases the emission lifetime; however, it simultaneously mollifies redox processes that are responsible for analyte sensitivity to begin with. This balancing act was successfully applied to enhance the sensitivity of CdZnS/ZnS QDs towards 2,4,6-trinitrotoluene (TNT). Unexpectedly, it was found that CdZnS/ZnS QDs with very thick shells retained substantial sensitivity to TNT. This observation may be due to close coupling of the reduced substrate with the QD hole that is enabled by the near-degeneracy of holes in the core CdZnS and ZnS shell. The ability of core/shell QDs to retain substantial reducing power may have implications for other applications that can benefit from the enhanced stability of robust core/shell nanomaterials.

Semiconductor Quantum Dots (QDs) have great potential in applications for renewable energy generation due to their size-tunable redox potentials. QDs may also be doped to manipulate their electronic structure. Our group developed a method to dope each quantum dot with an exact number of guest ions by nucleating the QD around an organometallic seed cluster that contains guest ions. As a result, each QD has the same number of dopants, which eliminates problems due to inhomogeneity of the dot stoichiometry. These materials were studied using time-resolved X-ray absorption spectroscopy, which allows us to characterize the electronic and coordination structure in both the ground and excited states. It was found that, when dopants interact with charge carriers, they may alter their bonding to the underlying matrix. This phenomenon of charge carrier modulation of dopant bonding has a strong effect on the conductivity properties of doped semiconductors.

An epitaxial lift-off (ELO) technology has been demonstrated for monocrystalline CdTe/MgCdTe double-heterostructure (DH) with the assistant of photoresist. The increased photoluminescence (PL) intensity is observed from the lift-off thin films, and confirms that the CdTe/MgCdTe DHs maintain high optical quality after ELO. The proton radiation hardness of CdTe/MgCdTe DH samples is also studied. Compared with a reference sample without being irradiated, the DH samples bombarded with 63-MeV protons show increased PL intensity and decay time. Our findings indicate that the CdTe/MgCdTe DH thin-film solar cells are expected to be radiation-robust and suitable for space applications.

© 1963-2012 IEEE. High-energy (>40 MeV) protons are commonly used to characterize radiation tolerance of space electronics against damage caused by energy transfer to the nuclei and electrons of semiconductor materials while in orbit. While practically useful, these experiments are unrepresentative in terms of particle type and energy spectra, which results in disproportionate amounts of displacement damage and total ionizing dose. We compare these damages to those realized by bulk semiconductors used in optoelectronics in common low, medium, and high Earth orbits by calculating the duration in orbit required to achieve equivalent nuclear and electronic energy deposition. We conduct this analysis as a function of test proton energy, material, material thickness, and shielding thickness. The ratio of nuclear to electronic orbit duration, a value which would approach unity in an ideal radiation tolerance test, is found to exceed unity in the majority of cases but approaches unity as Al shielding increases. This study provides a connection between damage produced in terrestrial accelerator-based characterizations and orbit irradiation in terms of both damage modes which can cause optoelectronic components to fail: displacement damage and total ionizing dose.

Semiconductor-based mid-wave infrared photon detectors that functionalize space-based imaging systems are susceptible to both cumulative ionization and displacement damage, especially due to proton irradiation. Here, the dark current density and quantum efficiency of a mid-wave infrared detector utilizing a strain-balanced InGaAs/InAsSb superlattice active region are examined as a function of a 63 MeV proton radiation dose. Proton-irradiation is performed in an incremental stepwise dose up to a total ionizing dose of 100 krad(Si) or an equivalent proton fluence of 6.1 × 1011 protons/cm2. All characterization work is conducted with the detectors held at an operating temperature of 130 K throughout the experiment to limit thermal annealing effects. Prior to irradiation, the quantum efficiency of the top-side illuminated device without anti-reflection coating is 59.5%. The quantum efficiency is largely independent of temperature below 150 K, indicative of an electron minority carrier. As irradiation progressed the typical linear increase in inverse quantum efficiency with increasing proton fluence was observed, which led to a quantum efficiency damage factor of 1.12 × 1013 e cm2/ph. This value is shown to be an order of magnitude lower than typically observed in III-V nBn devices and is likely due to the higher mobility of minority electrons in the active region of this device. A full analysis of the characterization results suggests that displacement damage creates a significant population of donor states that modify the doping profile, in addition to Shockley–Read–Hall recombination centers that generally form as a result of proton irradiation.

Understanding the electronic structure of doped semiconductors is essential to realize advancements in electronics and in the rational design of nanoscale devices. Reported here are the results of time-resolved X-ray absorption studies on copper-doped cadmium sulfide nanoparticles that provide an explicit description of the electronic dynamics of the dopants. The interaction of a dopant ion and an excess charge carrier is unambiguously observed via monitoring the oxidation state. The experimental data combined with DFT calculations demonstrate that dopant bonding to the host matrix is modulated by its interaction with charge carriers. Furthermore, the transient photoluminescence and the kinetics of dopant oxidation reveal the presence of two types of surface-bound ions that create midgap states.

In certain high-power RF devices, such as isolators and circulators, a static magnetic field is present in parallel with the oscillating RF electric field. This parallel magnetic field imposes electron Larmor motion, which restricts the trajectories of electrons accelerated by the RF field. In open geometries, magnetic confinement reduces electron losses in the multipactor region and can reduce the breakdown threshold. In this paper, the effect of magnetic confinement on multipactor breakdown threshold was measured experimentally using a stripline fixture with an externally applied uniform magnetic field. Multipactor threshold power was reduced from the unmagnetized case by as much as 3 dB at magnetic fields of about 0.5 kG. Multipactor gap aspect ratio has a significant impact on the degree of threshold reduction.

Gallium is incorporated into the strain-balanced In(Ga)As/InAsSb superlattice system to achieve the same mid-wave infrared cutoff tunability as conventional Ga-free InAs/InAsSb type-II superlattices, but with an additional degree of design freedom to enable optimization of absorption and transport properties. Time-resolved photoluminescence measurements of InGaAs/InAsSb superlattice characterization- and doped device structures are reported from 77 to 300 K and compared to InAs/InAsSb. The low-injection photoluminescence decay yields the minority carrier lifetime, which is analyzed with a recombination rate model, enabling the determination of the temperature-dependent Shockley–Read–Hall, radiative, and Auger recombination lifetimes and extraction of defect energy levels and capture cross section defect concentration products. The Shockley–Read–Hall-limited lifetime of undoped InGaAs/InAsSb is marginally reduced from 2.3 to 1.4 μs due to the inclusion of Ga; however, given that Ga improves the vertical hole mobility by a factor of >10×, a diffusion-limited InGaAs/InAsSb superlattice nBn could expect a lower bound of 2.5× improvement in diffusion length with significant impact on photodetector quantum efficiency and radiation hardness. At temperatures below 120 K, the doped device structures are Shockley–Read–Hall limited at 0.5 μs, which shows promise for detector applications.

The structural and optical properties of two 210 nm thick InAsSbBi epilayers grown on (100) GaSb substrates by molecular beam epitaxy at 400 and 280 °C are investigated using X-ray diffraction, Rutherford backscattering spectroscopy, transmission electron microscopy, and photoluminescence spectroscopy. Both samples are free of observable defects. The higher temperature growth results in reduced Bi incorporation, good optical performance, smooth interfaces, and lateral composition modulation of the Bi mole fraction. The lower temperature growth results in near unity Bi incorporation, poor optical performance, interface roughness, and CuPtB -type atomic ordering on the {111}B planes.

Here, the design, fabrication, and characterization of an actively tunable long-wave infrared detector, made possible through direct integration of a graphene-enabled metasurface with a conventional type-II superlattice infrared detector, are reported. This structure allows for post-fabrication tuning of the detector spectral response through voltage-induced modification of the carrier density within graphene and, therefore, its plasmonic response. These changes modify the transmittance through the metasurface, which is fabricated monolithically atop the detector, allowing for spectral control of light reaching the detector. Importantly, this structure provides a fabrication-controlled alignment of the metasurface filter to the detector pixel and is entirely solid-state. Using single pixel devices, relative changes in the spectral response exceeding 8% have been realized. These proof-of-concept devices present a path toward solid-state hyperspectral imaging with independent pixel-to-pixel spectral control through a voltage-actuated dynamic response.

Lead selenide quantum dots (QDs) are low-bandgap IV-VI semiconducting nanomaterials that have been studied for a variety of applications. Their preparation using colloidal methods can produce small spherical to larger cubic nanocrystals, with an upper limit of ~17 nm reported to date. Here we describe methods for preparing cubic PbSe nanocrystals over a 20→40 nm size range using a twostep procedure. Specifically, ~10 nm PbSe QDs are generated using the rapid injection method, the products from which are overcoated with additional lead and selenium precursors. The use of two lead reagents were studied; lead oleate resulted in a maximum of 20 nm cubes, while more reactive lead hexyldecanoate resulted in much larger nanomaterials with bulk bandgaps. However, PbSe samples prepared with lead hexyldecanoate also contained agglomerates. Special care must be taken when characterizing larger strained nanomaterials with X-ray powder diffraction, for which the Scherrer equation is inadequate. A more rigorous approach using the Williamson-Hall method provides characterizations that are consistent with electron microscopy analysis.

Accurate p-type doping of the active region in III-V infrared detectors is essential for optimizing the detector design and overall performance. While most III-V detector absorbers are n-type (e.g., nBn), the minority carrier devices with p-type absorbers would be expected to have relatively higher quantum efficiencies due to the higher mobility of minority carrier electrons. However, there are added challenges to determining the hole carrier concentration in narrow bandgap InAsSb due to the potential for electron accumulation at the surface of the material and at its interface with the layer grown directly below it. Electron accumulation layers form high conductance electron channels that can dominate both resistivity and Hall-effect transport measurements. Therefore, to correctly determine the bulk hole concentration and mobility, temperature- and magnetic-field-dependent transport measurements in conjunction with Multi-Carrier Fit analysis were utilized on a series of p-doped InAs0.91Sb0.09 samples on GaSb substrates. The resulting hole concentrations and mobilities at 77 K (300 K) are 1.6 × 1018 cm−3 (2.3 × 1018 cm−3) and 125 cm2 V−1 s−1 (60 cm2 V−1 s−1), respectively, compared with the intended Be-doping of ∼2 × 1018 cm−3. A surface treatment experiment is conducted to associate one of the electron conducting populations to the surface. Variable temperature (15–390 K) measurements confirmed the different carrier species present in the sample and enabled the extraction of the bulk heavy hole, interface carriers and surface electron transport properties. For the bulk carrier, a thermal activation of intrinsic carriers is identified at high temperatures with a bandgap of EG ∼ 258 meV and the low temperature data suggests an activation energy of EA ∼ 22 meV for the Be dopant atoms. Finally, temperature analysis confirms a surface carrier electron with resulting mobilities and sheet concentrations at 30 K (300 K) of 4500 cm2 V−1 s−1 (4300 ± 100 cm2 V−1 s−1) and 5.6 × 1010 cm−2 (6 × 1010 ± 2 × 1010 cm−2), respectively.

The unique optoelectronic properties of quantum dots (QDs) give them significant advantages over traditional organic dyes, not only as fluorescent labels for bioimaging, but also as emissive sensing probes. QD sensors that function via manipulation of fluorescent resonance energy transfer (FRET) are of special interest due to the multiple response mechanisms that may be utilized, which in turn imparts enhanced flexibility in their design. They may also function as ratiometric, or "color-changing" probes. In this review, we describe the fundamentals of FRET and provide examples of QD-FRET sensors as grouped by their response mechanisms such as link cleavage and structural rearrangement. An overview of early works, recent advances, and various models of QD-FRET sensors for the measurement of pH and oxygen, as well as the presence of metal ions and proteins such as enzymes, are also provided.

Remineralization of dentin during dental caries is of considerable clinical interest. Dentin matrix protein 1 (DMP1) is a non-collagenous calcium-binding protein that plays a critical role in biomineralization. In the present study, we tested if peptides derived from DMP1 can be used for dentin remineralization. Peptide pA (pA, MW = 1.726 kDa) and peptide pB (pB, MW = 2.185), containing common collagen-binding domains and unique calcium-binding domains, were synthesized by solid-phase chemistry. An extreme caries lesion scenario was created by collagenase digestion, and the biomineral-nucleating potential of these peptides was ascertained when coated on collagenase-treated dentin matrix and control, native human dentin matrix under physiological levels of calcium and phosphate. Scanning electron microscopy analysis suggests that peptide pB was an effective nucleator when compared with pA. However, a 1:4 ratio of pA to pB was determined to be ideal for dentin remineralization, based on hydroxyapatite (HA) morphology and calcium/phosphorus ratios. Interestingly, HA was nucleated on collagenase-challenged dentin with as little as 20 min of 1:4 peptide incubation. Electron diffraction confirmed the presence of large HA crystals that produced a diffraction pattern indicative of a rod-like crystal structure. These findings suggest that DMP1-derived peptides may be useful to modulate mineral deposition and subsequent formation of HA when exposed to physiological concentrations of calcium and phosphate.

With data storage moving further away from locally based storage and into an age of cloud storage, users are going to need their data on the go, and they're going to need it to be fast, and they're going to need it to be accurate. Eventual consistency is the theoretical guarantee that, provided no new updates to an entity are made, all reads of the entity return the last updated value. In this paper, a comparison is made on cloud-based studies to formulate an idea as to how to improve the synchronization of cloud storage systems by first benchmarking the eventual consistency and then utilizing a framework that dynamically learns the characteristics of a storage node so that eventually consistency is achieved quickly.

Skilled motor control is regulated by the convergence of somatic sensory and motor signals in brain and spinal motor circuits. Cervical deafferentation is known to diminish forelimb somatic sensory representations rapidly and to impair forelimb movements. Our focus was to determine what effect deafferentation has on the motor representations in motor cortex, knowledge of which could provide new insights into the locus of impairment following somatic sensory loss, such as after spinal cord injury or stroke. We hypothesized that somatic sensory information is important for cortical motor map topography. To investigate this we unilaterally transected the dorsal rootlets in adult rats from C4 to C8 and mapped the forelimb motor representations using intracortical microstimulation, immediately after rhizotomy and following a 2-week recovery period. Immediately after deafferentation we found that the size of the distal representation was reduced. However, despite this loss of input there were no changes in motor threshold. Two weeks after deafferentation, animals showed a further distal representation reduction, an expansion of the elbow representation, and a small elevation in distal movement threshold. These changes were specific to the forelimb map in the hemisphere contralateral to deafferentation; there were no changes in the hindlimb or intact-side forelimb representations. Degradation of the contralateral distal forelimb representation probably contributes to the motor control deficits after deafferentation. We propose that somatic sensory inputs are essential for the maintenance of the forelimb motor map in motor cortex and should be considered when rehabilitating patients with peripheral or spinal cord injuries or after stroke.

The primary photodynamics of 5-nm CdZnS core, CdZnS/ZnS core/shell, and CdZnS/ZnS·Pd nanoparticle adducts are characterized with broadband ultrafast transient absorption spectroscopy. Photogenerated excitons in the CdZnS and CdZnS/ZnS nanoparticles exhibit long-lived (>20 ns) lifetimes and further functionalizing of the type-I CdZnS/ZnS core/shells with Pd nanoparticles resulted in rapid exciton quenching (

The primary exciton dynamics of colloidal indirect bandgap AlP nanocrystals are characterized with ultrafast transient absorption spectroscopy. A 400-nm excitation results in a high yield formation of an emissive exciton with a ∼1-ns lifetime and a 50-nm bandwidth red-shifted emission. Multi-wavelength target analysis is used to decompose the measured signals into sequential and parallel models, which interpret the measured data as an emissive exciton with a 1.2-ns decay time and a dark exciton which is attributed to surface trapping. Reconstructed nonlinear absorption spectra resolve a broad optical gain persisting for >1 ns. The 15% emission yield demonstrates that colloidal AlP nanocrystals are useful as a potential broadband, high efficiency optoelectronics material.

Multipactor breakdown is a resonant electron phenomena that is observed in spacecraft RF systems. While multipactor breakdown thresholds are well documented for parallel metal surfaces, susceptible breakdown geometries in passive RF components frequently deviate from this ideal parallel-plate geometry. A common feature of concern is an angled or chamfered edges placed on dielectrics inside RF components. Using a 3D electromagnetic tool coupled with an electron tracking tool, a simplified angled gap geometry is analyzed. The breakdown threshold increase from a parallel gap to an angle gap is calculated and discussed. The analysis results will be compared with laboratory measurements to demonstrate the applicability of this analysis technique.

Multipactor and ionization breakdown can cause critical damage to RF systems within spacecraft systems. In order to prevent these breakdown events and ensure proper satellite operation, fundamental physics and the RF system must both be well defined and understood. A standardized approach and methodology for breakdown prevention has been developed that encompasses the needed fundamental physics and systems engineering approach. In this presentation, a new approach is described which analyzes the RF satellite system to define worst case parameters for both multipator and ionization breakdown. New modeling techniques including 3D particle tracking and PIC codes are leveraged to improve accuracy and reduce overconservative margins. State-of-the-art diagnostics for multipactor and plasma detection are incorporated as standard techniques to ensure sensitive detection. Using improved modeling and test methods, this standard improves overall component design and the understanding of basic parameters of RF breakdown phenomena.

MgxCdi-xTe/Si tandem cells have the potential to reach a conversion efficiency greater than 40%. MgxCd1-xTe /MgyCd1-yTe (y>x) double heterostructures (DHs) grown by molecular beam epitaxy exhibit ∼1.7 eV bandgaps and very high absorption coefficients, as measured using photoluminescence (PL) and spectroscopic ellipsometry. Indium-doped n-type MgxCd1-xTe (x ∼ 13% Mg mole fraction) with a ∼1.7 eV bandgap shows strong PL, comparable to that from high-quality CdTe/MgCdTe double heterostructures. Devices consisting of an n-type MgxCd1-xTe DH absorber, a p-type hydrogenated amorphous silicon (a-Si:H) hole contact layer and an indium tin oxide (ITO) top electrode are demonstrated with promising performance.

Mg x Cd 1−x Te (1.7 eV)/Si (1.1 eV) (x∼0.13) tandem cells have the potential to reach conversion efficiencies greater than 30% with low cost. Mg 0.13 Cd 0.87 Te/Mg y Cd 1−y Te (y>0.13) double-heterostructures (DH) grown by molecular beam epitaxy feature 1.7 eV bandgaps and very high absorption coefficients measured by photoluminescence (PL) and ellipsometry. Both undoped and n-doped Mg 0.13 Cd 0.87 Te/Mg y Cd 1−y Te DHs exhibit comparable carrier lifetimes (50 ns) to that of high-quality doped CdTe/MgCdTe DHs, and show great promise to have even longer carrier lifetime through optimizing barrier height, barrier thickness, and doping concentration, leading to increased performance in solar cells.

Knowledge of the fate and transport of nanoparticles in the subsurface environment is limited, as techniques to monitor and visualize the transport and distribution of nanoparticles in porous media and measure their in situ concentrations are lacking. To address these issues, we have developed a light transmission and fluorescence method to visualize and measure in situ concentrations of quantum dot (QD) nanoparticles in variably saturated environments. Calibration cells filled with sand as porous medium and various known water saturation levels and QD concentrations were prepared. By measuring the intensity of the light transmitted through porous media exposed to fluorescent light and by measuring the hue of the light emitted by the QDs under UV light exposure, we obtained simultaneously in situ measurements of water saturation and QD nanoparticle concentrations with high spatial and temporal resolutions. Water saturation was directly proportional to the light intensity. A linear relationship was observed between hue-intensity ratio values and QD concentrations for constant water saturation levels. The advantages and limitations of the light transmission and fluorescence method as well as its implications for visualizing and measuring in situ concentrations of QDs nanoparticles in the subsurface environment are discussed.

Semiconductor nanocrystals (NCs or quantum dots) have significant potential for use in a variety of applications from renewable energy generation to biological imaging. Modern methods of colloidal synthesis can be used to create crystalline materials with tight size distributions; this assures high quantum yields and narrow emission profiles in the case of direct-bandgap semiconductors. The optical properties of NCs may also be tuned with size due to quantum confinement effects. Quantum confinement also creates problems when characterizing nanomaterials; specifically, the absorptivity of a sample is a function of the size and structure of the quantum dots. We demonstrate here a simple method for determining the molar absorptivity of aqueous CdSe/CdZnS NCs through Forster resonant energy transfer. Energy transfer from NC donors to dye acceptors was measured and modeled using standard Forster theory incorporating Poissonian statistics to calculate the acceptor/donor ratio leading to a direct determination o...

Developing simple synthetic methods to control the size and morphology of nanocrystals is an active area of research as these parameters control the material's electronic and optical properties. For a semiconductor with a symmetrical crystal structure such as lead selenide, anisotropic colloidal growth has been previously accomplished via the use of templates, seeds, or by block assembly of smaller, symmetrical subunits. Here, we present a simple method to create monodisperse lead selenide nanorods and multipods at low temperatures. The size distribution and the observed morphologies are consistent with a continuous, anisotropic growth of material. The syntheses of these anisotropic shapes are due to the nature of the nuclei that form upon injection of precursors into partially oxidized alkene solvents that may contain lactone and carbonate-functional derivatives.

This study addresses the issue of v. 15 in 1 Cor 11:2-16 from the perspective of a veiling custom. If veiling is in view (the position adopted here), then how does one confront the difficulty of reconciling the overall context with the exceptional statement in v. 15 that long hair for a woman is a glory. If, as the text argues, long hair is to be taken as a “glory,” by what logic could a woman understand that she should place a veil upon her head? This article provides a way out of the dilemma by showing how a veil can serve the double function of reflecting the hair’s beauty while at the same time preserving a symbol of female modesty.

This paper addresses a number of major trends underlying the continuing effort to realize practical optoelectronic, electronic, and information-processing devices based on ensembles of quantum dots assembled in a variety of matrix materials. The great diversity of such structures makes it possible to fabricate numerous ensemble-based devices for applications underlying photoluminescent devices, light-emitting diodes, displays, photodetectors, photovoltaic devices, and solar cells. In addition, the application of colloidal quantum dots to allied technologies such as nanobiotechnology is considered for the case of monitoring conformational changes in biomolecules using luminescent quantum dots.

The Neolithic Revolution began 11,000 years ago in the Near East and preceded a westward migration into Europe of distinctive cultural groups and their agricultural economies, including domesticated animals and plants. Despite decades of research, no consensus has emerged about the extent of admixture between the indigenous and exotic populations or the degree to which the appearance of specific components of the “Neolithic cultural package” in Europe reflects truly independent development. Here, through the use of mitochondrial DNA from 323 modern and 221 ancient pig specimens sampled across western Eurasia, we demonstrate that domestic pigs of Near Eastern ancestry were definitely introduced into Europe during the Neolithic (potentially along two separate routes), reaching the Paris Basin by at least the early 4th millennium B.C. Local European wild boar were also domesticated by this time, possibly as a direct consequence of the introduction of Near Eastern domestic pigs. Once domesticated, European pigs rapidly replaced the introduced domestic pigs of Near Eastern origin throughout Europe. Domestic pigs formed a key component of the Neolithic Revolution, and this detailed genetic record of their origins reveals a complex set of interactions and processes during the spread of early farmers into Europe.

The beginning of farming in the Adriatic is a topic ripe for a new discussion and syn- thesis. Several lines of evidence suggest that immigration played a major role in the process. It invol- ved, however, both the actual movement of people and the active participation of the local popula- tion, and probably unfolded somewhat differently in different parts of the region. There is provocative evidence that the transition to farming occurred in a two-stage process. There was an initial stage of very rapid dispersal, perhaps by exploratory parties along the coast in the southern Adriatic. During the second stage, the eastern Adriatic littoral was probably colonized by farming communities, while the hinterland remained an agricultural frontier zone. IZVLE∞EK - Za≠etek poljedelstva na podro≠ju Jadrana je tema, ki omogo≠a diskusije in nove sinteze. Predstavljamo dokaze, da je glavno vlogo pri procesu igralo priseljevanje. V regiji se je proces odvi- jal razli≠no, vklju≠eval je tako selitve ljudi, kot tudi aktivno udele∫bo lokalnega prebivalstva. Doka- zujemo, da se je prehod h kmetovanju odvijal v dvostopenjskem procesu. V prvi stopnji je pri∏lo do hitre razpr∏itve, morda izvidnikov, vzdol∫ obale ju∫nega Jadrana. V drugi stopnji so skupine kmeto- valcev verjetno kolonizirale obalne predele vzhodnega Jadrana, medtem ko je zaledje ostalo mejno podro≠je kmetovanja.

Semiconductor nanocrystals (NCs) or quantum dots (QDs) show great promise for use in QD-LED (quantum dot lightemitting device) displays, owing to their unique optical properties and the continual development of new core and core–shell structures to meet specific color needs. This in combination with the recent development of more efficient and saturated QD-LEDs as well as new QD-LED fabrication techniques, suggests that QD-LEDs have the potential to become an alternative flat-panel display technology. The ideal red, green, and blue emission spectrum of an LED for a display application should have a narrow bandwidth and a wavelength such that its color coordinates on the Commission Internationale de l+Eclairage (CIE) chromaticity diagram lie outside the current National Television System Committee (NTSC) standard color triangle (see Figure 2). For a Gaussian emission spectrum with a full width at half maximum (FWHM) of 30 nm and a maximized perceived power, the optimal peak wavelength for display applications is l= 610– 620 nm for red, l= 525–530 nm for green, and l= 460– 470 nm for blue. For the red pixels, wavelengths longer than l= 620 nm become difficult for the human eye to perceive, while those shorter than l= 610 nm have coordinates that lie inside the standard NTSC color triangle. Optimization of wavelength for the blue pixels follows the same arguments as for the red pixel, but at the other extreme of the visible spectrum. For green pixels, l= 525–530 nm provides a color triangle with the largest area on the CIE chromaticity diagram (and therefore the largest number of colors accessible by a display). Wavelengths longer than l= 530 nm make some of the blue/green area of the triangle inaccessible. Wavelengths shorter than l= 525 nm compromise the yellow display emissions. To date, efficient red-emitting QD-LEDs with a peak emission wavelength optimized for display applications have been realized using (CdSe)ZnS core–shell NCs, while blue QD-LEDs with a peak wavelength of emission optimized for display applications have been realized with a (CdS)ZnS core–shell material. To date, although efficient green-emitting core–shell semiconductor NCs that emit at l= 525 nm have been synthesized, they have not been successfully incorporated into a QD-LED suitable for display applications. Previous work using (CdSe)ZnS core–shell NCs gave QD-LEDs that emit at wavelengths no shorter than l= 540–560 nm. 14] Using (CdSe)ZnS core–shell NCs to achieve l= 525 nm emission requires making small CdSe cores ( 2.5 nm in diameter). 16] Such small CdSe semiconductor NCs can be difficult to synthesize with narrow size distributions and high quantum efficiencies, and are also more difficult to process and overcoat with a higher-band-gap inorganic semiconductor, which is necessary for incorporation into solid-state structures. A core–shell composite, rather than an organically passivated NC, is desirable in a solid-state QD-LED device owing to the enhanced photoluminescence and electroluminescence (EL) quantum efficiencies of core– shell NCs and their greater tolerance to the processing conditions necessary for device fabrication. Larger NCs are also more desirable for use in QD-LEDs because the absorption cross section of NCs scales with size. Larger NCs with larger absorption cross sections lead to an increase in the efficiency of F?rster energy transfer from electroluminescing organic molecules to NCs in a working QD-LED, which in turn leads to more efficient devices. Herein, we report the synthesis of a CdxZn1 xSe alloy core on which we then grew a CdyZn1 yS shell to create a core– shell NC material with the ideal spectral characteristics for green emission in a QD-LED display and with a size large enough for fabricating a working QD-LED. Our CdxZn1 xSe core synthesis was based on work recently published, in which Cd and Se precursors were slowly introduced into a growth solution of ZnSe NCs. A three-step synthetic route was employed to prepare the (CdxZn1 xSe)CdyZn1 yS core–shell NCs. In the first step, ZnSe NCs were prepared by rapidly injecting 0.7 mmol of diethylzinc (Strem) and 1 mL of tri-noctylphosphine selenide (TOPSe; 1m) dispersed in 5 mL of tri-n-octylphosphine (TOP; 97% Strem) into a round-bottom flask containing 7 grams of degassed hexadecylamine (distilled from 90% Sigma–Aldrich) at 310 8C and by then growing the NCs at 270 8C for 90 min. The second step consisted of transferring 8 mL of the above ZnSe NC growth solution, at 160 8C, into a degassed solution of 16 grams of trin-octylphosphine oxide (TOPO; distilled from 90% Sigma– Aldrich) and 4 mmol of hexylphosphonic acid (HPA; Alfa Aesar), also at 160 8C. A solution of 1.1 mmol of dimethylcadmium (Strem) and 1.2 mL of TOPSe (1m) dispersed in 8 mL of TOP (97% Strem) was then introduced dropwise [*] Dr. J. S. Steckel, Dr. P. Snee, Dr. J. P. Zimmer, J. E. Halpert, Prof. M. G. Bawendi Massachusetts Institute of Technology Department of Chemistry Center for Materials Science and Engineering and The Institute for Soldier Nanotechnologies 77 Massachusetts Avenue, Room 6-221 Cambridge, MA 02139 (USA) Fax: (+1)617-253-7030 E-mail: mgb@mit.edu Dr. S. Coe-Sullivan, P. Anikeeva, L.-A. Kim, Prof. V. Bulovic Massachusetts Institute of Technology Laboratory of Organic Optics and Electronics Department of Electrical Engineering and Computer Science Cambridge, MA 02139 (USA) [] These authors contributed equally to this work.

We have utilized the high spatial resolution of near-field scanning optical microscopy (NSOM) to characterize nanoscopic electronic inhomogeneity in as-cast thin films of a light-harvesting dendrimer consisting of coumarin-343 (core) and coumarin-2 (peripheral) chromophores and in thermally annealed thin films of the semiconductive polymer poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). Using photoluminescence (PL) and third harmonic generation (THG) NSOM techniques, we have observed nanoscopic domains in these films that exhibit increased delocalization in the excited (using PL) and ground states (using THG). In addition, we have developed a procedure for examining NSOM images via calculation of radial distribution functions (RDFs). RDF analysis of the PL and THG NSOM images indicates that the domains exhibit correlated structure in annealed MEH-PPV films while the light-harvesting dendrimeric material does not. The existence or absence of such nanoscopic amorphous structure can be understood in terms of the molecular structure of each material.

The electronic structure of conjugated polymer films is of current interest due to the wide range of potential applications for such materials in optoelectronic devices. A central outstanding issue is the significance of interchain electronic species in films of these materials. In this paper, we investigate the nature of interchain species in films of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) both before and after thermal annealing. Our investigation employs a combination of third harmonic generation (THG) and near-field scanning optical microscopy to measure the wavelength and spatial dependence of the THG efficiency. These chemically selective imaging measurements reveal new, low-energy absorption features in nanometer-scale spatially distinct regions of annealed films that are only infrequently observed prior to annealing. This suggests that the polymer strands in annealed MEH-PPV films pack together closely enough that significant ground-state wave function overlap can occ...

Water confined to a repulsive ellipsoidal cavity with a variable aspect ratio is studied by computer simulation. The orientational order parameter of the OH bond with respect to the symmetry axis of the cavity, SOH, is examined as a function of the cavity dimensions for finite numbers of water molecules (N = 6 to 38 at constant density). We observe that the absolute value of SOH is increased by deforming a spherical cavity into either a prolate or an oblate ellipsoidal cavity. This trend is method independent, i.e., it is obtained using Monte Carlo simulations with TIP4P or SPC water and using molecular dynamics simulations. Generally ƒS OH ƒ decreases with increasing N and temperature for a fixed cavity aspect ratio. The calculated magnitudes of SOH and its temperature dependence are in order-of-magnitude agreement with experimental NMR determinations of SOH in hydrated fibers. Hence such spatial anisotropies should be considered in models of the residual orientation exhibited by probe molecules in fibrous synthetic and biological materials.

We introduce the use of quantum dot (QD) nanoparticles for near-surface velocimetry and provide preliminary data to demonstrate its feasibility. Evanescent wave illumination is used to image the motion of water-soluble (CdSe)ZnS QDs with a core size of 6 nm within a region of order 100 nm of a surface . Results are presented for the two in-plane components of the velocity field.

We report simultaneous amplified spontaneous emission from two different multiexcitonic transitions −1Se–1S3∕2 and 1Pe–1P3∕2− of colloidal CdSe nanocrystals (NCs) stabilized in high volume fraction in titania matrices. Two-state lasing from both multiexcitonic transitions is achieved in a surface-emitting distributed feedback CdSe NC laser. Variable stripe length measurements show that the gain from the 1Pe–1P3∕2 transition is approximately twice that of the −1Se–1S3∕2 transition for 4.2nm radius CdSe∕ZnS core-shell NCs.

The optical properties of bulk InAs0.936Bi0.064 grown by molecular beam epitaxy on a (100)-oriented GaSb substrate are measured using spectroscopic ellipsometry. The index of refraction and absorption coefficient are measured over photon energies ranging from 44 meV to 4.4 eV and are used to identify the room temperature bandgap energy of bulk InAs0.936Bi0.064 as 60.6 meV. The bandgap of InAsBi is expressed as a function of Bi mole fraction using the band anticrossing model and a characteristic coupling strength of 1.529 eV between the Bi impurity state and the InAs valence band. These results are programmed into a software tool that calculates the miniband structure of semiconductor superlattices and identifies optimal designs in terms of maximizing the electron-hole wavefunction overlap as a function of transition energy. These functionalities are demonstrated by mapping the design spaces of lattice-matched GaSb/InAs0.911Sb0.089 and GaSb/InAs0.932Bi0.068 and strain-balanced InAs/InAsSb, InAs/GaInSb, and...

Optical and structural properties of InAs/InAsSb type-II superlattices (T2SL) and their feasibility for mid- and longwavelength infrared (MWIR and LWIR) photodetector applications are investigated. The InAs/InAsSb T2SL structures with a broad bandgap range covering 4 μm to 12 μm are grown by molecular beam epitaxy and characterized by highresolution x-ray diffraction and photoluminescence (PL) spectroscopy. All of the samples have excellent structural properties and strong PL signal intensities of the same order of magnitude, indicating that non-radiative recombination is not dominant and the material system is promising for high performance MWIR and LWIR detectors and multiband FPAs.

Oleic acid capped lead sulfide (PbSe) cubes, spheres, rods and star-shaped nanocrystals with sizes ranging from 5 to 20 nm were synthesized, then subjected to a post-synthesis washing in a 1:1 ethanol/hexane solution. The relationship of third order nonlinear optical properties to nanocrystal surface chemistry as affected by this washing was analyzed using nanosecond 532 nm Z-scan. The results indicated a significant change in optical properties emerged only after the nanocrystals, regardless of shape, were washed in the ethanol/hexane mixture. The "as grown" nanocrystals displayed high emission efficiency with no nonlinear absorption, which converted to a highly-absorbing and strongly scattering media with quenching of optical emission following washing. Transmission electron microscopy showed that neither the size nor shape of the nanocrystals was changed by washing, indicating that all differences were related to changes in surface chemistry. Finally, the Z-scan of washed nanocrystals displayed a strong correlation between nonlinear scattering and nonlinear transmittance which was also not dependent upon shape. It is postulated that the washing allowed formation of deep trap states that permitted nonlinear optical absorption.

Crisis management research has largely ignored one of the most pressing challenges political leaders are confronted with in the wake of a large-scale extreme event: how to cope with what is commonly called the blame game. In this article, we provide a heuristic to help understand political leader responses to blame in the aftermath of crises, emphasizing the crucial role of their leadership style on the political management of Inquiries. After integrating theoretical and empirical findings on crisis management and political leadership styles, we illustrate our heuristic by applying it to the Bush administration's response to Hurrican Katrina in 2005. We conclude by offering suggestions for further research on the underdeveloped subject of the blame management challenges faced by political leaders in the wake of acute crisis episodes.

Infarcts from proximal middle cerebral artery (MCA) stroke can produce impairments in motor function, particularly finger movements in humans and digit flexion in rats. In rats, the extent of neural damage may be limited to basal ganglia structures or may also include portions of the frontal and parietal cortex in severe cases. Although the primary motor cortex (M1) is anatomically spared in proximal MCA occlusion, its functional integrity is suspect because even a small subcortical infarct can damage neural circuits linking M1 with basal ganglia, brainstem, and spinal cord. This motivated the present study to investigate the neurophysiological integrity of M1 after transient proximal MCA occlusion. Rats, preoperatively trained and non-preoperatively trained to reach for food, received extensive reach training/testing with the contralateral-to-lesion paw for several weeks after MCA occlusion. The forelimb movement representations were assayed from the ipsilateral-to-lesion M1 with intracortical microstimulation approximately 10 weeks after MCA occlusion. Digit flexion was impaired during food grasping in rats with relatively small subcortical infarcts and was completely abolished in rats that sustained at least moderate subcortical damage. Corresponding forelimb movement representations ranged from abnormally small to absent. The results suggest that ischemia in subcortical territories of the MCA does not spare the neurophysiological properties of M1 despite its apparent anatomical intactness, probably because of damage sustained to its descending fibers. Thus, M1 dysfunction contributes to the impairments that ensue from proximal MCA occlusion, even when the infarct is limited to subcortical regions.

The beginning of farming should be one of the most exciting issues in European prehistory. Instead, it runs repetitively in well-worn ruts. This chapter aims to open up a theoretical can of worms, working in similar directions to some recent essays in a non-dichotomized, nonessentializing archaeology of Europe in this period. These take two distinct but related directions. One is to re-evaluate standard interpretive tropes in classic cases such as the LBK, and to argue for much more complex processes at the forager–farmer encounter. A more radical approach is represented by theorists who question the idea that one can define essentialist identities based upon economies. The chapter first discusses the basic terms of argument critically, then poses several new models, and concludes by discussing the resolvability of the question. Beyond the Socratic aim of annoying all parties to the debate equally, it hopes to open a theoretical space in which Europe between 7000 and 4000 cal bc can be freed of encumbering conceptual baggage and viewed as a real ethnographic landscape.

The structural and optical properties of lattice-matched InAs0.911Sb0.089 bulk layers and strain-balanced InAs/InAs1−xSbx (x ∼ 0.1–0.4) superlattices grown on (100)-oriented GaSb substrates by molecular beam epitaxy are examined using X-ray diffraction, spectroscopic ellipsometry, and temperature dependent photoluminescence spectroscopy. The photoluminescence and ellipsometry measurements determine the ground state bandgap energy and the X-ray diffraction measurements determine the layer thickness and mole fraction of the structures studied. Detailed modeling of the X-ray diffraction data is employed to quantify unintentional incorporation of approximately 1% Sb into the InAs layers of the superlattices. A Kronig-Penney model of the superlattice miniband structure is used to analyze the valence band offset between InAs and InAsSb, and hence the InAsSb band edge positions at each mole fraction. The resulting composition dependence of the bandgap energy and band edge positions of InAsSb are described using the bandgap bowing model; the respective low and room temperature bowing parameters for bulk InAsSb are 938 and 750 meV for the bandgap, 558 and 383 meV for the conduction band, and −380 and −367 meV for the valence band.

We have developed a photostable nanocrystal-silica (NC-silica) laser that is robust under different chemical environments, making it suitable for integration with a microfluidic network. We demonstrate that the optical properties of this microscale laser can be a dynamic function of its local environment, thus providing a platform for potential applications such as nonlinear optical chemosensing on a miniaturized scale.

This communication describes a long (1 cm), laser-pumped, liquid core-liquid cladding (L2) waveguide laser. This device provides a simple, high intensity, tunable light source for microfludic applications. Using a core solution of 2 mM rhodamine 640 perchlorate, optically pumped by a frequency-doubled Nd:YAG laser, we found that the threshold for lasing was as low as 22 muJ (16-ns pulse length) and had a slope efficiency up to 20%. The output wavelength was tunable over a 20-nm range by changing the ratio of solvent components (dimethyl sulfoxide and methanol) in the liquid core.

Alloying bismuth with InAs provides a ternary material system near the 6.1 A lattice constant, which covers the technologically important mid- and long-wavelength infrared region. One challenge for this material system is that it is not straightforward to incorporate bismuth into the bulk InAs lattice, since bismuth has a tendency to surface-segregate and form droplets during growth. In this work, the conditions for InAsBi growth using molecular beam epitaxy are explored. A growth window is identified (temperatures ⪞ 270 °C, V/III flux ratios 0.98 ⪝ As/In ⪝ 1.02, and Bi/In ≅ 0.065) for droplet-free, high-quality crystalline material, where InAsBi layers with compositions of up to 5.8% bismuth (nearly lattice-matched to GaSb) are attained. The structural quality of InAsBi bulk and quantum well samples is evaluated using x-ray diffraction and transmission electron microscopy. The optical quality is assessed using photoluminescence, which is observed from quantum well structures up to room temperature and from thick, low Bi-content bulk layers at low temperatures. Bismuth is also used as a surfactant during the growth of InAs/InAsSb superlattices at 430 °C where it is observed that a small bismuth flux changes the surface reconstruction of InAs from (2×1) to (1×3), reduces the sticking coefficient of antimony, results in a slight increase in photoluminescence intensity, does not significantly incorporate, and does not alter the surface morphology.

The development of a reversible chemical sensor based on a CdSe/ZnS nanocrystal (NC) is described. Signal transduction is accomplished by fluorescence resonance energy transfer (FRET) between the NC and a fluorescent pH-sensitive squaraine dye attached to the surface of the NC. The efficiency of FRET, and consequently the relative intensity of NC and dye emissions, is modulated with the pH-dependent absorption cross section of the squaraine dye. The design of a NC sensor based on FRET results in a ratiometric sensor since the emission intensities of dye and NC may be referenced to the isosbestic point between NC and dye emissions. The ratiometric approach allows sensing to be performed, regardless of issues surrounding collection efficiency (scattering environment, light fluctuations, etc.) and dye:NC loadings.

The Large Area Picosecond Photodetector Collaboration is developing large-area fast photodetectors with time resolution/~10 ps and space resolution/~1 mm based on atomic layer deposition-coated glass Micro-Channel Plates (MCPs). We have assembled a facility at Argonne National Laboratory for characterizing the performance of a wide variety of microchannel plate configurations and anode structures in configurations approaching complete detector systems. The facility consists of a pulsed Ti:Sapphire laser with a pulse duration ≈100 fs, an optical system allowing the laser to be scanned in two dimensions, and a computer-controlled data-acquisition system capable of reading out 60 channels of anode signals with a sampling rate of over 10 GS/s. The laser can scan on the surface of a sealed large-area photodetector, or can be introduced into a large vacuum chamber for tests on bare 8 in.-square MCP plates or into a smaller chamber for tests on 33-mm circular substrates. We present the experimental setup, detector calibration, data acquisition, analysis tools, and typical results demonstrating the performance of the test facility.

We have developed a strategy for the ratiometric detection of toxic Hg(2+) ions using a semiconductor nanocrystal energy-transfer donor coupled to a mercury-sensitive "turn-on" dye acceptor. The results demonstrate a new paradigm of toxic metal sensing that resolves the difficulties with the use of semiconductor nanotechnology for this purpose.