Single-Crystalline Germanium Nanocrystals via a Two-Step Microwave-Assisted Colloidal Synthesis from GeI4.

Colloidal germanium (Ge) nanocrystals (NCs) are of great interest with possible applications for photovoltaics and near-IR detectors. In many examples of colloidal reactions, Ge(II) precursors are employed, and NCs of diameter ∼3-10 nm have been prepared. Herein, we employed a two-step microwave-assisted reduction of GeI4 in oleylamine (OAm) to prepare monodispersed Ge NCs with a size of 18.9 ± 1.84 nm. More importantly, the as-synthesized Ge NCs showed high crystallinity with single-crystal nature as indicated by powder X-ray diffraction, selected area electron diffraction, and high-resolution transmission electron microscopy. The Tauc plot derived from photothermal deflection spectroscopy measurement on Ge NCs thin films shows a decreased bandgap of the Ge NCs obtained from GeI4 compared with that of the Ge NCs from GeI2 with a similar particle size, indicating a higher crystallinity of the samples prepared with the two-step reaction from GeI4. The calculated Urbach energy indicates less disorder in the larger NCs. This disorder might correlate with the fraction of surface states associated with decreased particle size or with the increased molar ratio of ligands to germanium. Solutions involved in this two-step reaction were investigated with 1H NMR spectroscopy and high-resolution mass spectrometry (MS). One possible reaction pathway is proposed to unveil the details of the reaction involving GeI4 and OAm. Overall, this two-step synthesis produces high-quality Ge NCs and provides new insight on nanoparticle synthesis of covalently bonding semiconductors.

Structural Insights on Microwave-Synthesized Antimony-Doped Germanium Nanocrystals.

Doped and alloyed germanium nanocrystals (Ge NCs) are potential candidates for a variety of applications such as photovoltaics and near IR detectors. Recently, bismuth (Bi) as an n-type group 15 element was shown to be successfully and kinetically doped into Ge NCs through a microwave-assisted solution-based synthesis, although Bi is thermodynamically insoluble in bulk crystalline Ge. To expand the composition manipulation of Ge NCs, another more common n-type group 15 element for semiconductors, antimony (Sb), is investigated. Oleylamine (OAm)- and OAm/trioctylphosphine (TOP)-capped Sb-doped Ge NCs have been synthesized by the microwave-assisted solution reaction of GeI2 with SbI3. Passivating the Ge surface with a binary ligand system of OAm/TOP results in formation of consistently larger NCs compared to OAm alone. The TOP coordination on the Ge surface is confirmed by 31P NMR and SEM-EDS. The lattice parameter of Ge NCs increases with increasing Sb concentration (0.00-2.0 mol %), consistent with incorporation of Sb. An increase in the NC diameter with higher content of SbI3 in the reaction is shown by TEM. XPS and EDS confirm the presence of Sb before and after removal of surface ligands with hydrazine and recapping the Ge NC surface with dodecanethiol (DDT). EXAFS analysis suggests that Sb resides within the NCs on highly distorted sites next to a Ge vacancy as well as on the crystallite surface. High Urbach energies obtained from photothermal deflection spectroscopy (PDS) of the films prepared from pristine Ge NC and Sb-doped Ge NCs indicate high levels of disorder, in agreement with EXAFS data. Electrical measurements on TiO2-NC electron- and hole-only devices show an increase in hole conduction, suggesting that the Sb-vacancy defects are behaving as a p-type dopant in the Ge NCs, consistent with the vacancy model derived from the EXAFS results.

Quantum dot PbS(0.9)Se(0.1)/TiO2 heterojunction solar cells.

We report on photovoltaic cells based on ternary PbS(0.9)Se(0.1) quantum dots utilizing a heterojunction type device configuration. The best device shows an AM 1.5 power conversion efficiency of 4.25%. Furthermore, this ternary PbS(x)Se(1-x) quantum dot heterojunction device has a peak external quantum efficiency above 100% at 2.76 eV, approximately 2.7× the bandgap energy. The ternary quantum dots combine the higher short circuit currents of the binary PbSe system with the higher open circuit voltages of the binary PbS system.

Degradation and rejuvenation studies of AC electroluminescent ZnS:Cu,Cl phosphors.

We report detailed degradation and rejuvenation studies of AC electroluminescence (EL) of the phosphor ZnS:Cu,Cl, aiming to better understand the physical mechanisms that control EL emission. We find that the AC EL emission spectra vary considerably with the AC driving frequency but all spectra can be fit to a sum of four Gaussians. During degradation, although there is a large overall decrease in amplitude, the shape of the emission spectra measured at a given AC frequency does not change. Annealing the samples after they are significantly degraded can rejuvenate the phosphors with a maximum rejuvenation occurring (for fixed annealing times) near 180 °C. Further, these test cells can be degraded and rejuvenated multiple times. However studies at slightly higher annealing temperatures (240 °C) show significant thermal degradation and, perhaps more importantly, a change in the spectral shape; this likely indicates that two distinct mechanisms are then operative. In extended x-ray absorption fine structure (EXAFS) experiments we find that the CuS nanoprecipitates in the ZnS host (∼ 75% of the Cu is in the CuS precipitates) do not change significantly after the 240 °C anneal; these experiments also provide a more detailed comparison of the local structure about Cu in pure CuS, and in ZnS:Cu,Cl. In addition, the EXAFS experiments also place an upper limit on the fraction of possible interstitial Cu sites, proposed as a blue emission center, at less than 10%. The combined experiments place strong constraints on the mechanisms for degradation and rejuvenation and suggest that EL degradation is most likely caused by either Cu or Cl diffusion under high E-fields, while thermal diffusion at slightly elevated temperatures without E-fields present, re-randomizes the (isolated) dopant distributions. Higher T anneals appear to damage the sharp tips on the precipitates.

Electrosynthetically patterned conducting polymer films for investigation of neural signaling.

The ion-mediated conduction and versatility of device fabrication of conducting polymers provide a route to the study of neural signaling. Patterned junctions of conducting polypyrrole have been electropolymerized on commercially available microelectrode arrays, with typical dimensions 200 mum between electrodes, each electrode being 30 mum in diameter. Tetrabutylammonium perchlorate or sodium p-toluenesulfonate were used as electrolyte/counterion in the organic solvent. Individual polypyrrole junctions, when synthesized and connected in a three-electrode configuration, exhibit current-switching behavior analogous to neural weighting. Junctions copolymerized with thiophene exhibit current rectification and the nonlinear current-voltage behavior requisite for complex neural systems (i.e., the activation function).

Mechanism of thioflavin T binding to amyloid fibrils.

Thioflavin T is a benzothiazole dye that exhibits enhanced fluorescence upon binding to amyloid fibrils and is commonly used to diagnose amyloid fibrils, both ex vivo and in vitro. In aqueous solutions, thioflavin T was found to exist as micelles at concentrations commonly used to monitor fibrils by fluorescence assay ( approximately 10-20 microM). Specific conductivity changes were measured at varying concentration of thioflavin T and the critical micellar concentration was calculated to be 4.0+/-0.5 microM. Interestingly, changes in the fluorescence excitation and emission of thioflavin T were also dependent on the micelle formation. The thioflavin T micelles of 3 nm diameter were directly visualized using atomic force microscopy, and bound thioflavin T micelles were observed along the fibril length for representative fibrils. Increasing concentration of thioflavin T above the critical micellar concentration shows increased numbers of micelles bound along the length of the amyloid fibrils. Thioflavin T micelles were disrupted at low pH as observed by atomic force microscopy and fluorescence enhancement upon binding of thioflavin T to amyloid fibrils also reduced by several-fold upon decreasing the pH to below 3. This suggests that positive charge on the thioflavin T molecule has a role in its micelle formation that then bind the amyloid fibrils. Our data suggests that the micelles of thioflavin T bind amyloid fibrils leading to enhancement of fluorescence emission.

Annular oligomeric amyloid intermediates observed by in situ atomic force microscopy.

Amyloidoses and related protein deposition diseases involve the transformation of normally soluble proteins into insoluble deposits, usually fibrillar in nature. Although it was originally assumed that the fibrils were the toxic species, this assumption has recently been called into question. Accumulating evidence in several systems suggests that oligomeric intermediates on the aggregation pathway may be toxic. In the present study we used in situ atomic force microscopy to monitor aggregation in aqueous solution in real time. The sample used was an amyloidogenic immunoglobulin light chain, involved in AL or light chain amyloidosis. The nature of the observed oligomeric intermediates was dependent on the conditions of incubation, especially pH and ionic strength. Several different aggregation intermediates with a variety of morphologies, including annular or torus-shaped species, were observed. The data indicate that protein aggregation can be very complex, involving a variety of different oligomeric intermediates whose population will be determined by the kinetic and thermodynamic competition between them.

A model for amyloid fibril formation in immunoglobulin light chains based on comparison of amyloidogenic and benign proteins and specific antibody binding.

In an attempt to understand the mechanism of amyloid fibril formation in light chain amyloidosis, the properties of amyloidogenic (SMA) and benign (LEN) immunoglobulin light chain variable domains (VL) were compared. The conformations of LEN and SMA were measured using secondary and tertiary structural probes over the pH range from 2 and 8. At all pH values, LEN was more stable than SMA. The CD spectra of LEN at pH 2 were comparable to those of SMA at pH 7.5, indicating that the low pH conformation of LEN closely resembles that of SMA at physiological pH. At low pH, a relatively unfolded intermediate conformation is populated for SMA and rapidly leads to amyloid fibrils. The lack of such an intermediate with LEN, is attributed to sequence differences and accounts for the lack of LEN fibrils in the absence of agitation. A kappa IV-specific monoclonal antibody that recognizes the N-terminal of SMA caused unraveling of the fibrils to the protofilaments and was observed to bind to one end of SMA protofilaments by atomic force microscopy. The antibody result indicates that each protofilament is asymmetric with different ends. A model for the formation of fibrils by SMA is proposed.

A general model for amyloid fibril assembly based on morphological studies using atomic force microscopy.

Based on atomic force microscopy analysis of the morphology of fibrillar species formed during fibrillation of alpha-synuclein, insulin, and the B1 domain of protein G, a previously described model for the assembly of amyloid fibrils of immunoglobulin light-chain variable domains is proposed as a general model for the assembly of protein fibrils. For all of the proteins studied, we observed two or three fibrillar species that vary in diameter. The smallest, protofilaments, have a uniform height, whereas the larger species, protofibrils and fibrils, have morphologies that are indicative of multiple protofilaments intertwining. In all cases, protofilaments intertwine to form protofibrils, and protofibrils intertwine to form fibrils. We propose that the hierarchical assembly model describes a general mechanism of assembly for all amyloid fibrils.

Surface-catalyzed amyloid fibril formation.

Light chain (or AL) amyloidosis is characterized by the pathological deposition of insoluble fibrils of immunoglobulin light chain fragments in various tissues, walls of blood vessels, and basement membranes. In the present investigation, the in vitro assembly of a recombinant amyloidogenic light chain variable domain, SMA, on various surfaces was monitored using atomic force microscopy. SMA formed fibrils on native mica at pH 5.0, conditions under which predominantly amorphous aggregates form in solution. Fibril formation was accelerated significantly on surfaces compared with solution; for example, fibrils grew on surfaces at significantly faster rates and at much lower concentrations than in solution. No fibrils were observed on hydrophobic or positively charged surfaces or at pH >7.0. Two novel types of fibril growth were observed on the surface: bidirectional linear assembly of oligomeric units, and linear growth from preformed amorphous cores. In addition to catalyzing the rate of fibrillation, the mechanism of fibril formation on the surfaces was significantly different from in solution, but it may be more physiologically relevant because in vivo the deposits are associated with surfaces.