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A near-infrared emitting DNA-stabilized silver nanocluster (DNA-AgNC) with an unusually high fluorescence quantum yield is presented. The steady-state and time-resolved fluorescence properties of the DNA-AgNC were characterized, together with its ability to generate optically activated delayed fluorescence (OADF) and upconversion fluorescence (UCF).

International audience; Hydrogen bonding between nucleobases produces diverse DNA structural motifs, including canonical duplexes, guanine (G) quadruplexes, and cytosine (C) i-motifs. Incorporating metal-mediated base pairs into nucleic acid structures can introduce new functionalities and enhanced stabilities. Here we demonstrate, using mass spectrometry (MS), ion mobility spectrometry (IMS), and fluorescence resonance energy transfer (FRET), that parallel-stranded structures consisting of up to 20 G-AgI-G contiguous base pairs are formed when natural DNA sequences are mixed with silver cations in aqueous solution. FRET indicates that duplexes formed by poly(cytosine) strands with 20 contiguous C-AgI-C base pairs are also parallel. Silver-mediated G-duplexes form preferentially over G-quadruplexes, and the ability of Ag+ to convert G-quadruplexes into silver-paired duplexes may provide a new route to manipulating these biologically relevant structures. IMS indicates that G-duplexes are linear and more rigid than B-DNA. DFT calculations were used to propose structures compatible with the IMS experiments. Such inexpensive, defect-free, and soluble DNA-based nanowires open new directions in the design of novel metal-mediated DNA nanotechnology.

This work investigates the effects of dielectric environment and cluster shape on electronic excitations of fluorescent DNA-stabilized silver clusters, AgN-DNA. We first establish that the longitudinal plasmon wavelengths predicted by classical Mie-Gans (MG) theory agree with previous quantum calculations for excitation wavelengths of linear silver atom chains, even for clusters of just a few atoms. Application of MG theory to AgN-DNA with 400-850 nm cluster excitation wavelengths indicates that these clusters are characterized by a collective excitation process and suggests effective cluster thicknesses of ∼2 silver atoms and aspect ratios of 1.5 to 5. To investigate sensitivity to the surrounding medium, we measure the wavelength shifts produced by addition of glycerol. These are smaller than reported for much larger gold nanoparticles but easily detectable due to narrower line widths, suggesting that AgN-DNA may have potential for fluorescence-reported changes in dielectric environment at length scales of ∼1 nm.

The diverse optical and chemical properties of DNA-stabilized silver clusters (AgN-DNAs) have challenged the development of a common model for these sequence-tunable fluorophores. Although correlations between cluster geometry and fluorescence color have begun to shed light on how the optical properties of AgN-DNAs are selected, the exact mechanisms responsible for fluorescence remain unknown. To explore these mechanisms, we study four distinct purified AgN-DNAs in ethanol-water and methanol-water mixtures and find that the solvatochromic behavior of AgN-DNAs varies widely among different cluster species and differs markedly from prior results on impure material. Placing AgN-DNAs within the context of standard Lippert-Mataga solvatochromism models based on the Onsager reaction field, we show that such nonspecific solvent models are not universally applicable to AgN-DNAs. Instead, alcohol-induced solvatochromism of AgN-DNAs may be governed by changes in hydration of the DNA template, with spectral shifts resulting from cluster shape changes and/or dielectric changes in the local vicinity of the cluster.

Hydrogen bonding between nucleobases produces diverse DNA structural motifs, including canonical duplexes, guanine (G) quadruplexes, and cytosine (C) i-motifs. Incorporating metal-mediated base pairs into nucleic acid structures can introduce new functionalities and enhanced stabilities. Here we demonstrate, using mass spectrometry (MS), ion mobility spectrometry (IMS), and fluorescence resonance energy transfer (FRET), that parallel-stranded structures consisting of up to 20 G-Ag

We use high throughput near-infrared (NIR) screening technology to discover abundant new DNA-stabilized silver clusters, AgN-DNA, that fluoresce in the NIR. These include the longest wavelength AgN-DNA fluorophores identified to date, with peak emission beyond 950 nm that extends into the NIR II tissue transparency window, and the highest silver content.

We present an inexpensive, generalizable approach for modifying visible wavelength fluorescence microplate readers to detect emission in the near-infrared (NIR) I (650-950 nm) and NIR II (1000-1350 nm) tissue imaging windows. These wavelength ranges are promising for high sensitivity fluorescence-based cell assays and biological imaging, but the inaccessibility of NIR microplate readers is limiting development of the requisite, biocompatible fluorescent probes. Our modifications enable rapid screening of NIR candidate probes, using short pulses of UV light to provide excitation of diverse systems including dye molecules, semiconductor quantum dots, and metal clusters. To confirm the utility of our approach for rapid discovery of new NIR probes, we examine the silver cluster synthesis products formed on 375 candidate DNA strands that were originally designed to produce green-emitting, DNA-stabilized silver clusters. The fast, sensitive system developed here discovered DNA strands that unexpectedly stabilize NIR-emitting silver clusters.

DNA nucleobase sequence controls the size of DNA-stabilized silver clusters, leading to their well-known yet little understood sequence-tuned colors. The enormous space of possible DNA sequences for templating clusters has challenged the understanding of how sequence selects cluster properties and has limited the design of applications that employ these clusters. We investigate the genomic role of DNA sequence for fluorescent silver clusters using a data-driven approach. Employing rapid parallel silver cluster synthesis and fluorimetry, we determine the fluorescence spectra of silver cluster products stabilized by 1432 distinct DNA oligomers. By applying pattern recognition algorithms to this large experimental data set, we discover certain DNA base patterns, or "motifs," that correlate to silver clusters with similar fluorescence spectra. These motifs are employed in machine learning classifiers to predictively design DNA template sequences for specific fluorescence color bands. Our method improves selectivity of templates by 330% for silver clusters with peak emission wavelengths beyond 660 nm. The discovered base motifs also provide physical insights into how DNA sequence controls silver cluster size and color. This predictive design approach for color of DNA-stabilized silver clusters exhibits the potential of machine learning and data mining to increase the precision and efficiency of nanomaterials design, even for a soft-matter-inorganic hybrid system characterized by an extremely large parameter space.

In this paper we present a new near-IR emitting silver nanocluster (NIR-DNA-AgNC) with an unusually large Stokes shift between absorption and emission maximum (211 nm or 5600 cm-1). We studied the effect of viscosity and temperature on the steady state and time-resolved emission. The time-resolved results on NIR-DNA-AgNC show that the relaxation dynamics slow down significantly with increasing viscosity of the solvent. In high viscosity solution, the spectral relaxation stretches well into the nanosecond scale. As a result of this slow spectral relaxation in high viscosity solutions, a multi-exponential fluorescence decay time behavior is observed, in contrast to the more mono-exponential decay in low viscosity solution.

DNA-protected silver clusters (AgN-DNA) possess unique fluorescence properties that depend on the specific DNA template that stabilizes the cluster. They exhibit peak emission wavelengths that range across the visible and near-IR spectrum. This wide color palette, combined with low toxicity, high fluorescence quantum yields of some clusters, low synthesis costs, small cluster sizes and compatibility with DNA are enabling many applications that employ AgN-DNA. Here we review what is known about the underlying composition and structure of AgN-DNA, and how these relate to the optical properties of these fascinating, hybrid biomolecule-metal cluster nanomaterials. We place AgN-DNA in the general context of ligand-stabilized metal clusters and compare their properties to those of other noble metal clusters stabilized by small molecule ligands. The methods used to isolate pure AgN-DNA for analysis of composition and for studies of solution and single-emitter optical properties are discussed. We give a brief overview of structurally sensitive chiroptical studies, both theoretical and experimental, and review experiments on bringing silver clusters of distinct size and color into nanoscale DNA assemblies. Progress towards using DNA scaffolds to assemble multi-cluster arrays is also reviewed.

© 2015 American Chemical Society.The remarkable precision that DNA scaffolds provide for arraying nanoscale optical elements enables optical phenomena that arise from interactions of metal nanoparticles, dye molecules, and quantum dots placed at nanoscale separations. However, control of ensemble optical properties has been limited by the difficulty of achieving uniform particle sizes and shapes. Ligand-stabilized metal clusters offer a route to atomically precise arrays that combine desirable attributes of both metals and molecules. Exploiting the unique advantages of the cluster regime requires techniques to realize controlled nanoscale placement of select cluster structures. Here we show that atomically monodisperse arrays of fluorescent, DNA-stabilized silver clusters can be realized on a prototypical scaffold, a DNA nanotube, with attachment sites separated by

DNA oligomers can form silver-mediated duplexes, stable in gas phase and solution, with potential for novel biomedical and technological applications. The nucleobase-metal bond primarily drives duplex formation, but hydrogen (H-) bonds may also be important for structure selection and stability. To elucidate the role of H-bonding, we conducted theoretical and experimental studies of a duplex formed by silver-mediated cytosine homopobase DNA strands, two bases long. This silver-mediated cytosine tetramer is small enough to permit accurate, realistic modeling by DFT-based quantum mechanics/molecular mechanics methods. In gas phase, our calculations found two energetically favorable configurations distinguished by H-bonding, one with a novel interplane H-bond, and the other with planar H-bonding of silver-bridged bases. Adding solvent favored silver-mediated tetramers with interplane H-bonding. Overall agreement of electronic circular dichroism spectra for the final calculated structure and experiment validates these findings. Our results can guide use of these stabilization mechanisms for devising novel metal-mediated DNA structures.

Discriminative base motifs within DNA templates for fluorescent silver clusters are identified using methods that combine large experimental data sets with machine learning tools for pattern recognition. Combining the discovery of certain multibase motifs important for determining fluorescence brightness with a generative algorithm, the probability of selecting DNA templates that stabilize fluorescent silver clusters is increased by a factor of >3.

Author(s): Markesevic, N; Oemrawsingh, SSR; Schultz, D; Gwinn, EG; Bouwmeester, D | Abstract: Polarization-resolved excitation and emission measurements on individual 10-15 atom silver clusters formed on DNA are presented. The emission is highly linearly polarized, typically around 90% for all emitters, whereas the polarization dependence of the excitation strongly varies from emitter to emitter. These observations support the hypothesis that the luminescence arises from collective electron oscillations along rod-shaped silver clusters, whereas the excitation process does not necessarily have a strong preferential polarization direction and thus may involve energy and/or charge transfer between the cluster and DNA. In addition to stabilizing the clusters, the DNA also appears to strongly influence the available paths for excitation. © 2014 WILEY-VCH Verlag GmbH a Co. KGaA, Weinheim.

DNA-stabilized silver clusters are remarkable for the selection of fluorescence color by the sequence of the stabilizing DNA oligomer. Yet despite a growing number of applications that exploit this property, no large-scale studies have probed origins of cluster color or whether certain colors occur more frequently than others. Here we employ a set of 684 randomly chosen 10-base oligomers to address these questions. Rather than a flat distribution, we find that specific color bands dominate. Cluster size data indicate that these "magic colors" originate from the existence of magic numbers for DNA-stabilized silver clusters, which differ from those of spheroidal gold clusters stabilized by small-molecule ligands. Elongated cluster structures, enforced by multiple base ligands along the DNA, can account for both magic number sizes and color variation around peak wavelength populations.

We develop approaches to hold fluorescent silver clusters composed of only 10-20 atoms in nanoscale proximity, while retaining the individual structure of each cluster. This is accomplished using DNA clamp assemblies that incorporate a 10 atom silver cluster and a 15 or 16 atom silver cluster. Thermally modulated fluorescence resonance energy transfer (FRET) verifies assembly formation. Comparison to Förster theory, using measured spectral overlaps, indicates that the DNA clamps hold clusters within roughly 5 to 6 nm separations, in the range of the finest resolutions achievable on DNA scaffolds. The absence of spectral shifts in dual-cluster FRET pairs, relative to the individual clusters, shows that select few-atom silver clusters of different sizes are sufficiently stable to retain structural integrity within a single nanoscale DNA construct. The spectral stability of the cluster persists in a FRET pair with an organic dye molecule, in contrast to the blue-shifted emission of the dye.

We report on the first single emitter fluorescence spectra of DNA-stabilized silver clusters (Ag:DNAs) at ambient and cryogenic temperatures. While Ag:DNAs have received much attention recently due to their sequence-tunable emission wavelengths, the nature of the optical transitions (molecule-like or collective, cluster-like) is an open question. By removing the ensemble broadening present in previous spectroscopic studies, we probe the line widths Γ and brightness of individual Ag:DNA emitters. A roughly 5-fold increase in brightness from 295 to 1.7 K is accompanied by a factor of 2 decrease in Γ. The symmetric emission line shape, its insensitivity to embedding medium, and the independence of emission wavelength on excitation energy together indicate that the measured Γ represents the homogeneous line width, while the large, ∼100 meV values of Γ suggest rapid dephasing of a collective excited state of the silver cluster.

We report UV excitation through the DNA bases as a universal pathway to visible emission from DNA stabilized silver cluster fluorophores (AgDNAs). AgDNAs with peak emissions throughout the visible spectrum all exhibit two peaks in fluorescence excitation: a visible peak at a wavelength 50–150 nm shorter than the emission peak and a UV peak between 260 and 270 nm depending on the specifics of the DNA strand. UV excitation produces the same emission spectrum as visible excitation with 2–4 times greater intensity depending on the specific emitter. The excellent agreement between AgDNA excitation and absorption spectra, the similarity between AgDNA and bare DNA absorption spectra, and the ubiquity of UV excitation for all the AgDNAs studied strongly suggest that UV excitation proceeds via the DNA bases rather than by direct excitation of higher energy excited states of the Ag cluster. We explore the possibility that UV excitation can provide information about the type, number, and orientation of cluster-bound...

Few-atom Ag clusters self-assemble on single-stranded DNA and exhibit sequence-dependent fluorescence ranging from the blue to the NIR. DNA sequence presents a very large parameter space for creati...

Artificial structures built from synthetic DNA are emerging as platforms for device science that offer greatly enhanced resolution and simplicity of fabrication compared to traditional solid-state structures. These advantages arise from the nanometer-scale control over geometry provided by aqueous self-assembly of designed DNA sequences, which yields robust “scaffolds” in a palette of nanometer-scale geometries ranging from octahedra to two-dimensional lattices of all types. [1–10] Future applications that arise from such DNA scaffolds may include nanometer-scale optics, nanometer-scale electronics, and single-molecule detection proteomics. Realizing the promise that DNA-based materials hold for nanometer-scale device science will require the development of functional nanoelements for patterning of DNA scaffolds. In analogy to the layout of logic and memory elements on a semiconductor chip, nanoelements with properties that can be varied in a controlled manner across the underlying scaffold are needed. Such nanoelements should have lateral dimensions no larger than a few nanometers, in order to take advantage of the ultrafine, 6 nm resolution currently available in DNA scaffolds. Nanoelements should be hosted within DNA strands that can integrate into the scaffold structure, to enable precise positioning. Finally, nanoelement properties should depend on the base sequence of their host DNA strand, to provide site-specific behavior. Such a combination of properties would increase flexibility and function in comparison to current approaches for patterning DNA scaffolds with attachments, such as Au nanoparticles and proteins, which are relatively bulky and lack sensitivity to the sequence of nearby DNA bases. Here we show that silver nanoclusters bound to short, synthetic DNA strands provide optically functional nanoelements with the desired small size, sequence sensitivity, and suitability for integration into DNA scaffolds. Our work builds on the initial discovery of fluorescence from few-atom silver clusters attached to a 12-base, single-stranded DNA sequence. We use six 19-base DNA oligomers to show that visibly fluorescent silver clusters form only in single-stranded regions of the DNA hosts. This selectivity opens the possibility of precise positioning of optical nanoelements on DNA scaffolds through use of DNA “hairpin” sequences, which have already been used to create patterns of high complexity, with resolution below 10 nm, upon double-stranded DNA scaffolds. We find that the spectral characteristics of these silver-DNA nanoelements can be controlled by the base sequence and secondary structure of the DNA strands that host the silver atoms, a result that may ultimately contribute to achieving sequence-programmed optical addressing with nanometer-scale resolution. Scheme 1 represents the oligomers studied here. Colored circles indicate the bases: blue = cytosine (C), green = thymine (T), red = guanine (G), and yellow = adenine (A). C-Strand and G-Strand are complementary sequences that, alone in solution, are each predominantly single-stranded (ss), but form a purely double-stranded (ds) “Duplex” when thermally annealed together under our solution conditions. The hairpin oligomers C-loop, G-loop, A-loop and T-loop are partially self-complementary sequences with a common, 7-base C O M M U N IC A IO N

The growing interest in designing functionalized, RNA-based nanoparticles (NPs) for applications such as cancer therapeutics requires simple, efficient assembly assays. Common methods for tracking RNA assemblies such as native polyacrylamide gels and atomic force microscopy are often time-intensive and, therefore, undesirable. Here we describe a technique for rapid analysis of RNA NP assembly stages using the formation of fluorescent silver nanoclusters (Ag NCs). This method exploits the single-stranded specificity and sequence dependence of Ag NC formation to produce unique optical readouts for each stage of RNA NP assembly, obtained readily after synthesis.

Metal ion interactions with DNA have far-reaching implications in biochemistry and DNA nanotechnology. Ag+ is uniquely interesting because it binds exclusively to the bases rather than the backbone of DNA, without the toxicity of Hg2+. In contrast to prior studies of Ag+ incorporation into double-stranded DNA, we remove the constraints of Watson-Crick pairing by focusing on homo-base DNA oligomers of the canonical bases. High resolution electro-spray ionization mass spectrometry reveals an unanticipated Ag+-mediated pairing of guanine homo-base strands, with higher stability than canonical guanine-cytosine pairing. By exploring unrestricted binding geometries, quantum chemical calculations find that Ag+ bridges between non-canonical sites on guanine bases. Circular dichroism spectroscopy shows that the Ag+-mediated structuring of guanine homobase strands persists to at least 90 °C under conditions for which canonical guanine-cytosine duplexes melt below 20 °C. These findings are promising for DNA nanotechnology and metal-ion based biomedical science.

Hybrid organic/inorganic devices may find applications as sensors and in futuristic molecular-electronic devices. Here, we demonstrate molecular control of vertical transport in semiconductor superlattices in strong magnetic fields by adsorption of organic molecules onto the sidewalls of a GaAs/AlGaAs device. The molecules have identical attachment groups functionalized by end groups with different electronegativities. For magnetic fields in quantized Hall states, we find that the adsorbate substantially modifies the network of edge states that carries the electrical current. The data indicate that molecules with appropriately chosen end groups can enhance or decrease the vertical conductivity of the edge state system.

We study extreme geometries in the field of 2DEG-coupled Josephson Junctions, at dilution refrigerator temperatures. The 2DEG is a continuous layer of InAs that contacts an overlying array of niobium stripes. The temperature dependence of other S–Semi–S systems has been studied in relatively small geometries. Our junctions are in a new regime with widths to 1mm and lengths of 1.5μm. We find temperature-dependences of the critical current, Ic, that are difficult to fit consistently with theories used for other InAs-based junctions.

We study the temperature dependence of vertical transport in GaAs/Al 0.1 Ga 0.9 As multilayers, in the regime of the integer quantum Hall effect. At low temperatures, vertical transport in quantum Hall states occurs on a two-dimensional chiral sheath of edge states near the sidewalls of the sample mesas. At higher temperatures, variable-range hopping through the bulk of the sample dominates. To extend the temperature range of sheath-dominated transport, we increase the device perimeters using fractal-shaped mesas defined by e-beam lithography. We report on the freeze-out of bulk transport and the nearly linear increase of the sheath conductivity with temperature.

Fluorescent, DNA-stabilized silver clusters are receiving much attention for sequence-selected colors and high quantum yields. However, limited knowledge of cluster structure is constraining further development of these "AgN-DNA" nanomaterials. We report the structurally sensitive, chiroptical activity of four pure AgN-DNA with wide ranging colors. Ubiquitous features in circular dichroism (CD) spectra include a positive dichroic peak overlying the lowest energy absorbance peak and highly anisotropic, negative dichroic peaks at energies well below DNA transitions. Quantum chemical calculations for bare chains of silver atoms with nonplanar curvature also exhibit these striking features, indicating electron flow along a chiral, filamentary metallic path as the origin for low-energy AgN-DNA transitions. Relative to the bare DNA, marked UV changes in CD spectra of AgN-DNA and silver cation-DNA solutions indicate that ionic silver content constrains nucleobase conformation. Changes in solvent composition alone can reorganize cluster structure, reconfiguring chiroptical properties and fluorescence.

We explore and demonstrate quantum confinement without walls in metal-clad InAs quantum wells. We observed intersubband absorption from confined states in InAs clad with Al, Sb, Nb, W, Pt, Ag, Au, Ti or In. We found that using this novel method, we can explore the physics and chemistry at the metal–semiconductor interface; reflection, autoionization, well-width fluctuation and interface reaction.

We study the conductance of the edge state sheath that forms in the integer quantum Hall effect from the coupled edge states of a GaAs/Al0.1Ga0.9As multilayer, as the magnetic field is tilted away from the direction perpendicular to the layers. As the tilt angle, and hence the total magnetic field, increases, the conductance perpendicular to the layers decreases. The effect is not strongly dependent on whether the sample surface is perpendicular or parallel to the in-plane component of the magnetic field. This result is inconsistent with theoretical predictions for a smooth surface and suggests that the edge state sheath may be rough.

We use mid-infrared (MIR) spectroscopy to investigate the contact region at the Nb/InAs superconductor/semiconductor interface. We observe a remnant of the intersubband (ISB) resonance in Nb-clad InAs quantum wells, both in samples where we have grown Nb directly on InAs in situ, and samples where we have etch exposed the InAs surface before depositing the Nb. The presence of this remnant ISB indicates partial confinement of the electrons to the InAs. The similar resonance widths in the MBE grown Nb/InAs sample and the samples with Nb on etched InAs suggest that coupling of electron states in the InAs to those in Nb, rather than etch-induced damage, produces most of the resonance broadening.

We study conductance fluctuations of the edge-state sheath that forms in the integer quantum Hall effect from the coupled edge states of a GaAs/Al x Ga 1 − x As multilayer. Comparison of the measured variance in the vertical conductance to recent theoretical predictions for mesoscopic fluctuations suggests dephasing lengths of 1–10 μm around the sheath perimeter at low temperatures. However, inconsistencies in the estimated inelastic lengths indicate that present understanding of dephasing on the sheath is incomplete.

We study tunneling through gated point contacts on GaAs/AlGaAs heterojunctions in the regime of the integer (IQHE) and fractional (FQHE) quantum Hall effects. The off-resonant conductance data at bulk filling factor ν= 1 3 has a steep temperature dependence that is roughly consistent with the predictions of Luttinger liquid theory for edge states. In the IQHE, we find only weak temperature dependence at ν=1 in the bulk. However, the line shape of tunneling resonances at ν=1 is not well fit by a Fermi-liquid form, perhaps indicating that complex edge structure affects resonant transmission.

We have made electronic transport studies of the integer quantum Hall effect in a three-dimensional, layered semiconductor structure. When in-plane transport is quantized, the dependence of the vertical conductance Gzz on device size shows that current flows via extended surface states, which form from the coupled edge states in each layer. As the temperature rises, bulk transport begins to dominate Gzz, giving roughly activated behavior at high T. Data on in-plane transport demonstrate a very small energy scale for the rise in longitudinal resistance, RL.

Recent work on peridic SNS microstructures, in which a superconductor grating contacts an underlying InAs quantum well, has demonstrated that supercurrents can flow across the strips of quasi-two-dimensional electron gas between the superconductor grating lines. Here we show that the loss of DC resistance is accompanied by the emergence of striking features in the samples' excitation spectra, for energies below ∼ 2†. Although such series weak-link structures may display a Josephson plasmon resonance, the changes in the samples' far-infrared transmission in an applied H field appear to be inconsistent with the behavior expected for this mode. The transmission spectra also differ from the form expected for an energy gap in the quantum well strips between adjacent superconductor lines. We speculate that the observed absorption features instead arise from transitions between quasi-particle states produced by Andreev reflections at the superconductor/InAs interfaces.

We have applied high-frequency radiation to a one-dimensional array of superconducting-normal-superconducting junctions, comprised of Nb2DInAsNb, and observed Shapiro steps in the I–V curve which are dominated by a step at V = hv/4e, half the voltage of the usual AC Josephson effect. This result is discussed in view of a coupling between the Nb stripes that differs from the usual jc sin Φ form. The zero-bias resistance of the sample is finite and increases exponentially with temperature. The Shapiro steps, however, persist up to the Nb transition temperature. These results imply that the finite resistance of the sample originates from excitations in the superconducting state.

We present experimental studies of energy loss from quasi-two-dimensional electron gases that are driven to a nonequilibrium steady state by intense, ultrahigh-frequency fields. For the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs heterostructures studied, longitudinal-optical (LO) phonon emission dominates energy loss across the previously unexplored frequency range from 0.2 to 3.5 THz, at moderate electron temperatures. We find an unexpected resonance in the absorbed power that is consistent with an increase near 0.5 THz in either the net LO phonon emission rate, or in the high-frequency mobility.

We report that adsorption of monolayers of organic molecules onto the surface of ferromagnetic semiconductor heterostructures produces large, robust changes in their magnetic properties. The heterostructures have half a monolayer of MnAs embedded in GaAs, 50 A beneath the surface. The molecules investigated are alkylphosphonic acids that bind to GaAs via a phosphonate group. The organization of the organic monolayer determines the reduction in the Curie temperature, with ordered monolayers producing nearly complete suppression of ferromagnetism. We attribute this striking chemical modulation of magnetic properties to electronic changes brought about by the binding of the molecules to the semiconductor surface.

Fluorescent DNA-stabilized silver nanoclusters contain both cationic and neutral silver atoms. The absorbance spectra of compositionally pure solutions follow the trend expected for rod-shaped silver clusters, consistent with the polarized emission measured from individual nanoclusters. The data suggest a rod-like assembly of silver atoms, with silver cations mediating attachment to the bases.

Large third-order, free-carrier nonlinear susceptibilities, X(3) (to approximately 0.2 esu), and subcubic dependence of the third-harmonic power on the incident intensity, have been observed between 19 cm-1 and 23 cm-1 for InAs/AlSb quantum wells with electron sheet densities between 2.5*1012 cm-2. and 8*1012 cm-2. We find that the transmission of the fundamental, and the samples' DC conductivity, decrease with increasing incident intensity, indicating a large rise in the scattering rate. Using the intensity-dependent transmission to account for absorption in the sample is not sufficient to recover a cubic power law for the third-harmonic intensity. In addition, given the increased scattering rate indicated by the conductivity data, the bulk free-carrier X(3) due to non-parabolicity should decrease dramatically with increasing fundamental intensity, contrary to our results. Thus, non-parabolicity alone cannot account for the observed third-harmonic response.

We examine the relationship between the transport and magnetic properties of digital ferromagnetic heterostructure superlattices in which 0.5 monolayer MnAs planes alternate with undoped GaAs spacer layers. The data show that as the thickness t of the GaAs spacers increases, charge transport and the Curie temperature both approach their independent-layer limits at comparable values of t. An increase in the per-layer conductivity with decreasing t accompanies a rise in TC. This behavior is consistent with an enhancement of interlayer ferromagnetic interactions by charge coupling across the spacers.

We investigate a Josephson field-effect transistor in which the electron gas in an InAs quantum well serves as the weak link between superconducting Nb electrodes. We modulate the density Ns of electrons and the critical current Ic in the weak link by applying a voltage to an insulated gate. Measurements of the dependence of Ic on Ns are in good agreement with a model in which each occupied subband of the InAs quantum well makes an independent contribution to Ic. Inclusion of the nonparabolic band structure in InAs is crucial to this agreement.

We examine the effects of threading magnetic flux through the chiral metal that forms near the surface of 3D semiconductor multilayers in the regime of the quantum Hall effect. In low-temperature studies of GaAs/AlGaAs multilayers, we find that the response of the sheath of edge states to fields B plane along the planes of the multilayer depends on whether B plane is perpendicular or parallel to this chiral metal. The ν =1 and 2 edge-state sheaths respond differently to fields B plane that are perpendicular to the sheath.

Novel emitters consisting of silver nanoclusters that are bound to single-stranded DNA show great promise as accurately positioned single photon sources that can interact coherently. Here, we present first results of a spectroscopic study.

We report studies of the surface plasmon modes of a layer of electron gas that is embedded in a wider, effective positive background, provided by a parabolic Al x Ga 1-x As well. Although conditions in the interior of the electron gas are nearly identical to those in a neutral hellium slab, the more abrupt electronic surfaces in the parabolic well produce a qualitatively different surface plasmon spectrum

InAs quantum wells (QWs) have been used as weak links in many recent studies of novel superconductor-normal metal–superconductor junctions. The degree of coupling between the superconducting electrodes depends sensitively on both the superconductor/InAs interface and the QW material in the weak link, factors that are difficult to separate in dc transport studies. Here we used midinfrared spectroscopy to investigate the superconductor/semiconductor contact region. The remnant intersubband absorption we observe in Nb-clad InAs shows that the superconductor/InAs interface produced some confinement of electrons in the InAs. This confinement is, however, consistent with phase coherent transport in the InAs. We find no evidence for charge transfer from the superconductor to the InAs on cooling below the critical temperature of Nb.

We report low-temperature magnetotransport measurements on remotely-doped wide parabolic GaAs/Al/inf>Ga−Ga1−xAs well structures which contain thick (> 1000 A) layers of high mobility (> 2×105 cm2/V-sec) electron gas. Similar magnitudes and temperature dependences for the Hall mobilities are found in three samples with widely different well widths, sheet densities, and 3D design densities, leading to information on the possible mobility-limiting scattering mechanisms. Comparisons of the subband energy splittings obtained from Shubnikov-de Haas oscillation measurements with those obtained from self-consistent calculations are used to comment on the uniformity of the electron density profile and deviations from parabolicity in the well potential for these samples.

Demonstration de la production d'une couche mince d'un gaz d'electrons de mobilite elevee par comparaison des oscillations de Shubnikov de Haas avec des calculs auto-coherents des niveaux d'energie des etats fondamentaux

We have measured energy relaxation and longitudinal-optical (LO) phonon emission times in modulation-doped InAs quantum wells driven by high dc fields, and by intense ac fields at frequencies from 0.49 to 0.66 terahertz. We find that for electron temperatures between 50 and 200 K, LO phonon emission dominates energy relaxation. We determine a large net LO phonon emission time, indicating a strong LO phonon bottleneck both in high dc and in intense terahertz fields.