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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Aluminosilicate colloidal gels: from the early age to the precipitation of zeolites

Aluminosilicate hydrogels are often considered to be precursors for the crystallisation of zeolites carried out under hydrothermal conditions. The preparation of mechanically homogeneous aluminosilicate gels enables the study of these materials through bulk rheology and observation of the aging dynamics until the precipitation of crystalline zeolites. The first part of this study deals with the establishment of ternary state diagrams, in order to identify the range of chemical formulations that enable preparation of single-phase homogeneous gels. Then, by studying the viscoelastic moduli during the gelation reaction, and by yielding the gel under large deformation, we propose an empirical law considering the partial order of reaction on each chemical element, to predict the gelation time according to the chemical formulation. The scaling behavior of the elastic properties of this colloidal gel shows a transition from a strong link behavior to a weak link regime. Long term aging results in the shrinkage of the gel, accompanied by syneresis of interstitial liquid at the surface. Zeolites precipitate through crystallisation by a particle attachment mechanism, when thermodynamic equilibrium is reached. The stoichiometry of the precipitated zeolites is not only consistent with the concentration of the remaining species in the supernatant but, surprisingly, it is also very close to the partial order of the reaction of the chemical elements involved in the determination of the critical gel point. This indicates a strong correlation between the morphology of the soft amorphous gel network that is formed at an early age and those of the final solid precipitated crystals.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Metal ion dopant-induced famatinite to chalcostibite phase transformation of copper antimony sulphide colloidal nanostructures: effect on photophysical and pseudocapacitance properties

Here, the strategic doping of transition metal ions into copper antimony sulphide (CAS) semiconducting nanostructures can significantly influence their photophysical and pseudocapacitive properties, for which there are few reports and therefore is the focus of this study. Accordingly, highly crystalline metal ion (Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ and Zn 2+ ) doped off-stoichiometric copper rich/poor, antimony-rich/poor, and sulphur-poor CAS nanostructures (10–23 nm) were grown via colloidal (hot-injection) synthesis using metal diethyldithiocarbamate precursors. Importantly, metal ion doping significantly influences the structure and composition of the off-stoichiometric CAS nanostructures. Data from powder X-ray diffraction, Raman spectroscopy, high-resolution scanning/transmission electron microscopy, and energy dispersive X-ray spectroscopy confirm that the heavier metal ion dopants induce a novel phase transformation from famatinite (fCAS) to chalcostibite (cCAS) nanostructures. The influence of the metal ion dopants is also observed in the optical properties of as-synthesized nanostructures. This involves blue-shifted ultraviolet-visible absorption, reduced Urbach tailing, and tunable bandgaps between 2.17 and 2.38 eV. Also, the doped nanostructures display broad visible-near infrared photoluminescence via a triple radiative pathway with relatively short decay lifetimes between 0.2 and 6.1 ns. This is mediated by electronic transitions involving intrinsic (copper/antimony/sulphur interstitial) and extrinsic (metal ion interstitial) defect states. Additionally, electrodes prepared from Mn 2+ -doped fCAS nanostructures show enhanced pseudocapacitance via Na+ surface adsorption and intercalation relative to undoped fCAS electrodes, while Zn 2+ -doped cCAS electrodes exhibit pseudocapacitance via a combination of Na+ surface adsorption, intercalation, and redox reactions. These electrodes exhibit reduced charge transfer resistance, improved electronic conductivity and notably enhanced specific capacitance (∼222 F g −1 ), and charge transport, as measured in 1 M Na 2 SO 4 electrolyte via cyclic voltammetry and electrochemical impedance spectroscopy. To this end, the discovery of the metal ion-induced phase transformation presents a new avenue for optimizing the functional properties of fCAS and cCAS nanostructures, highlighting the critical role of metal ion-related defects in controlling the optical and electrochemical properties, towards potential solar absorption and energy storage applications.

25 ENERGY STORAGE↗
DOE OSTI2025

Colloidal synthesis of ultrathin KFeS 2 and RbFeS 2 magnetic nanowires with non-van der Waals 1D structures

Design of one-dimensional (1D) nanomaterials based on non-van der Waals (non-vdW) 1D chain structures is emerging as a new materials frontier, owing to their strong intrinsic anisotropy and broad compositional diversity. However, achieving ultrathin 1D morphology in such systems remains a significant challenge. In this work, we report the colloidal synthesis of ultrathin KFeS 2 and RbFeS 2 nanowires—representing the first fabrication of ultrathin 1D nanomaterials driven by non-vdW 1D crystal structures. The nanowires exhibit diameters of ∼5 nm and lengths of microns, with anisotropic growth directed by covalent [FeS 2 ] − chains. Magnetic characterization reveals significantly reduced antiferromagnetic transition temperatures and suppressed interchain ferromagnetic interactions, demonstrating pronounced size and morphology effects. Control experiments on structurally related materials indicate that direct nucleation of the 1D phase is essential for achieving the nanowire morphology. These findings establish a new synthetic pathway to an understudied family of non-vdW 1D nanomaterials, enabling exploration of their emergent quantum and magnetic properties.

Sun, Zhaohong [University of Southern California, ↗
DOE OSTI2014

A differential dielectric spectroscopy setup to measure the electric dipole moment and net charge of colloidal quantum dots

A sensitive dielectric spectroscopy setup is built to measure the response of nanoparticles dispersed in a liquid to an alternating electric field over a frequency range from 10{sup −2} to 10{sup 7} Hz. The measured complex permittivity spectrum records both the rotational dynamics due to a permanent electric dipole moment and the translational dynamics due to net charges. The setup consists of a half-transparent capacitor connected in a bridge circuit, which is balanced on pure solvent only, using a software-controlled compensating voltage. In this way, the measured signal is dominated by the contributions of the nanoparticles rather than by the solvent. We demonstrate the performance of the setup with measurements on a dispersion of colloidal CdSe quantum dots in the apolar liquid decalin.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Wavelength-tunable colloidal quantum dot laser on ultra-thin flexible glass

A mechanically flexible and wavelength-tunable laser with an ultra-thin glass membrane as substrate is demonstrated. The optically pumped hybrid device has a distributed feedback cavity that combines a colloidal quantum dot gain film with a grating-patterned polymeric underlayer, all on a 30-μm thick glass sheet. The total thickness of the structure is only 75 μm. The hybrid laser has an average threshold fluence of 450 ± 80 μJ/cm{sup 2} (for 5-ns excitation pulses) at an emitting wavelength of 607 nm. Mechanically bending the thin-glass substrate enables continuous tuning of the laser emission wavelength over an 18-nm range, from 600 nm to 618 nm. The correlation between the wavelength tunability and the mechanical properties of the thin laser structure is verified theoretically and experimentally.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Charge transport in colloidal ZnO nanocrystal solids: The significance of surface states

We investigate charge transport behaviour in colloidal ZnO nanocrystal solids with different surface states. Our results show that the logarithm of the conductivity scales with −T{sup −1/4}, suggestive of Mott variable-range hopping. Analysis of the density of states at the Femi level suggests that the charge hopping occurs through surface or defect states, rather than by direct hopping between quantum-confined conduction band states of the nanocrystals.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Real time drift measurement for colloidal probe atomic force microscope: a visual sensing approach

Drift has long been an issue in atomic force microscope (AFM) systems and limits their ability to make long time period measurements. In this study, a new method is proposed to directly measure and compensate for the drift between AFM cantilevers and sample surfaces in AFM systems. This was achieved by simultaneously measuring z positions for beads at the end of an AFM colloidal probe and on sample surface through an off-focus image processing based visual sensing method. The working principle and system configuration are presented. Experiments were conducted to validate the real time drift measurement and compensation. The implication of the proposed method for regular AFM measurements is discussed. We believe that this technique provides a practical and efficient approach for AFM experiments requiring long time period measurement.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Photoinduced band filling in strongly confined colloidal PbS quantum dots

Increase in continuous wave laser excitation (6 W/cm{sup 2} to 120 W/cm{sup 2}) of colloidal PbS quantum dots in the strongly quantized regime (diameters 2.0 nm and 4.7 nm) deposited on semi-insulating GaAs and glass causes a clear blue shift (0.019 eV and 0.080 eV) of the emission spectra. Proof of the applicability of a dynamic three-dimensional band filling model is the significance of the presented results and demonstrates the effective electronic coupling in quantum dot arrays similar to superlattices. The work also reveals the influence of quantum dot sizes on photo-doping effects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Solution-processed high-performance colloidal quantum dot tandem photodetectors on flexible substrates

We report a high-performance colloidal quantum dot (CQD)-based near-infrared tandem photodetector fabricated on flexible substrates via solution-processed method. The tandem photodetector on poly(ethylene terephthalate) substrates exhibited low dark current and high detectivities over ∼8.8 × 10{sup 11} Jones at near infrared range at −0.5 V bias and over ∼10{sup 13} Jones near 0 bias. The critical bend radii of ∼8 mm and ∼3 mm have been demonstrated for tensile and compressive bending, respectively. The performance of photodetectors remains stable under mechanical stress, making PbSe CQD material a promise candidate for flexible infrared sensing applications.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Probing the structural dependency of photoinduced properties of colloidal quantum dots using metal-oxide photo-active substrates

We used photoactive substrates consisting of about 1 nm coating of a metal oxide on glass substrates to investigate the impact of the structures of colloidal quantum dots on their photophysical and photochemical properties. We showed during irradiation these substrates can interact uniquely with such quantum dots, inducing distinct forms of photo-induced processes when they have different cores, shells, or ligands. In particular, our results showed that for certain types of core-shell quantum dot structures an ultrathin layer of a metal oxide can reduce suppression of quantum efficiency of the quantum dots happening when they undergo extensive photo-oxidation. This suggests the possibility of shrinking the sizes of quantum dots without significant enhancement of their non-radiative decay rates. We show that such quantum dots are not influenced significantly by Coulomb blockade or photoionization, while those without a shell can undergo a large amount of photo-induced fluorescence enhancement via such blockade when they are in touch with the metal oxide.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Hybrid systems of AlInP microdisks and colloidal CdSe nanocrystals showing whispering-gallery modes at room temperature

We report on the realization of hybrid systems composed of passive optical microdisk resonators prepared from epitaxial layer systems and nanocrystal quantum emitters synthesized by colloidal chemistry. The AlInP disk material allows for the operation in the visible range, as probed by CdSe-based nanocrystals. Photoluminescence spectra at room temperature reveal sets of whispering-gallery modes consistent with finite-difference time-domain simulations. In the experiments, a special sample geometry renders it possible to detect resonant optical modes perpendicular to the disk plane.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Optofluidic taming of a colloidal dimer with a silicon nanocavity

We report here the optical trapping of a heterogeneous colloidal dimer above a photonic crystal nanocavity used as an on-chip optical tweezer. The trapped dimer consists of a cluster of two dielectric microbeads of different sizes linked by van der Waals forces. The smallest bead, 1 μm in diameter, is observed to be preferentially trapped by the nanotweezer, leaving the second bead untrapped. The rotational nature of the trapped dimer Brownian motion is first evidenced. Then, in the presence of a fluid flow, control of its orientation and rotation is achieved. The whole system is found to show high rotational degrees of freedom, thereby acting as an effective flow-sensitive microscopic optical ball joint.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Air-stable short-wave infrared PbS colloidal quantum dot photoconductors passivated with Al{sub 2}O{sub 3} atomic layer deposition

A PbS colloidal quantum dot photoconductor with Al{sub 2}O{sub 3} atomic layer deposition (ALD) passivation for air-stable operation is presented. Two different types of inorganic ligands for the quantum dots, S{sup 2−} and OH{sup −}, are investigated. PbS/S{sup 2−} photoconductors with a cut-off wavelength up to 2.4 μm are obtained, and a responsivity up to 50 A/W at 1550 nm is reported. The corresponding specific detectivity is ∼3.4 × 10{sup 8} Jones at 230 K. The 3-dB bandwidth of the PbS/S{sup 2−} and PbS/OH{sup −} photodetectors is 40 Hz and 11 Hz, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Fluorescence resonance energy transfer measured by spatial photon migration in CdSe-ZnS quantum dots colloidal systems as a function of concentration

The study of the spatial photon migration as a function of the concentration brings into attention the problem of the energy transfer in quantum dot embedded systems. By measuring the photon propagation and its spatial dependence, it is possible to understand the whole dynamics in a quantum dot system, and also improve their concentration dependence to maximize energy propagation due to radiative and non-radiative processes. In this work, a confocal microscope was adapted to scan the spatial distribution of photoluminescence from CdSe-ZnS core-shell quantum dots in colloidal solutions. The energy migration between the quantum dots was monitored by the direct measurement of the photon diffusion length, according to the diffusion theory. We observed that the photon migration length decreases by increasing the quantum dot concentration, this kind of behavior has been regarded as a signature of Förster resonance energy transfer in the system.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Cholesterics of colloidal helices: Predicting the macroscopic pitch from the particle shape and thermodynamic state

Building a general theoretical framework to describe the microscopic origin of macroscopic chirality in (colloidal) liquid crystals is a long-standing challenge. Here, we combine classical density functional theory with Monte Carlo calculations of virial-type coefficients to obtain the equilibrium cholesteric pitch as a function of thermodynamic state and microscopic details. Applying the theory to hard helices, we observe both right- and left-handed cholesteric phases that depend on a subtle combination of particle geometry and system density. In particular, we find that entropy alone can even lead to a (double) inversion in the cholesteric sense of twist upon changing the packing fraction. We show how the competition between single-particle properties (shape) and thermodynamics (local alignment) dictates the macroscopic chiral behavior. Moreover, by expanding our free-energy functional, we are able to assess, quantitatively, Straley’s theory of weak chirality, which is used in several earlier studies. Furthermore, by extending our theory to different lyotropic and thermotropic liquid-crystal models, we analyze the effect of an additional soft interaction on the chiral behavior of the helices. Finally, we provide some guidelines for the description of more complex chiral phases, like twist-bend nematics. Our results provide new insights into the role of entropy in the microscopic origin of this state of matter.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Nanomechanical and topographical imaging of living cells by atomic force microscopy with colloidal probes

Atomic Force Microscopy (AFM) has a great potential as a tool to characterize mechanical and morphological properties of living cells; these properties have been shown to correlate with cells’ fate and patho-physiological state in view of the development of novel early-diagnostic strategies. Although several reports have described experimental and technical approaches for the characterization of cellular elasticity by means of AFM, a robust and commonly accepted methodology is still lacking. Here, we show that micrometric spherical probes (also known as colloidal probes) are well suited for performing a combined topographic and mechanical analysis of living cells, with spatial resolution suitable for a complete and accurate mapping of cell morphological and elastic properties, and superior reliability and accuracy in the mechanical measurements with respect to conventional and widely used sharp AFM tips. We address a number of issues concerning the nanomechanical analysis, including the applicability of contact mechanical models and the impact of a constrained contact geometry on the measured Young’s modulus (the finite-thickness effect). We have tested our protocol by imaging living PC12 and MDA-MB-231 cells, in order to demonstrate the importance of the correction of the finite-thickness effect and the change in Young’s modulus induced by the action of a cytoskeleton-targeting drug.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Two-color beam improvement of the colloidal particle lens array assisted surface nanostructuring

We consider laser nanostructuring of the material surface by means of a colloidal particle lens array. Here, the monolayer of dielectric micro- or nanospheres placed on the surface acts as an array of near-field lenses that focus the laser radiation into the multitude of distinct spots, allowing the formation of many structures in a single stage. We show that conversion of a small part of the energy of the femtosecond beam into the second harmonic (SH) is an efficient way to increase the surface density of obtained nanostructures. By combining the fundamental frequency and the SH, one benefits both from the power of the former and from the focusing ability of the latter. This combination provides an efficient nanostructuring with sphere diameter close to the wavelength of the second harmonic. The possibility to create arrays of nanostructures with surface density above 5×10{sup 8} cm{sup −2} with femtosecond Ti:sapphire laser operating at 800 nm was demonstrated by employing 0.45 μm spheres.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Shear-induced breaking of cages in colloidal glasses: Scattering experiments and mode coupling theory

We employ x-ray scattering on sheared colloidal suspensions and mode coupling theory to study structure factor distortions of glass-forming systems under shear. We find a transition from quadrupolar elastic distortion at small strains to quadrupolar and hexadecupolar modes in the stationary state. The latter are interpreted as signatures of plastic rearrangements in homogeneous, thermalized systems. From their transient evolution with strain, we identify characteristic strain and length-scale values where these plastic rearrangements dominate. This characteristic strain coincides with the maximum of the shear stress versus strain curve, indicating the proliferation of plastic flow. The hexadecupolar modes dominate at the wavevector of the principal peak of the equilibrium structure factor that is related to the cage-effect in mode coupling theory. We hence identify the structural signature of plastic flow of glasses.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗