Regina Ragan

Measurement of the direct energy gap of coherently strained SnxGe1–x/Ge(001) heterostructures

The direct energy gap has been measured for coherently strained SnxGe1–x alloys on Ge(001) substrates with 0.035<x<0.115 and film thickness 50–200 nm. The energy gap determined from infrared transmittance data for coherently strained SnxGe1–x alloys indicates a large alloy contribution and a small strain contribution to the decrease in direct energy gap with increasing Sn composition. These results are consistent with a deformation potential model for changes in the valence and conduction band density of states with coherency strain for this alloy system.

Non‐lithographic SERS Substrates: Tailoring Surface Chemistry for Au Nanoparticle Cluster Assembly

Near-field plasmonic coupling and local field enhancement in metal nanoarchitectures, such as arrangements of nanoparticle clusters, have application in many technologies from medical diagnostics, solar cells, to sensors. Although nanoparticle-based cluster assemblies have exhibited signal enhancements in surface-enhanced Raman scattering (SERS) sensors, it is challenging to achieve high reproducibility in SERS response using low-cost fabrication methods. Here an innovative method is developed for fabricating self-organized clusters of metal nanoparticles on diblock copolymer thin films as SERS-active structures. Monodisperse, colloidal gold nanoparticles are attached via a crosslinking reaction on self-organized chemically functionalized poly(methyl methacrylate) domains on polystyrene-block-poly(methyl methacrylate) templates. Thereby nanoparticle clusters with sub-10-nanometer interparticle spacing are achieved. Varying the molar concentration of functional chemical groups and crosslinking agent during the assembly process is found to affect the agglomeration of Au nanoparticles into clusters. Samples with a high surface coverage of nanoparticle cluster assemblies yield relative enhancement factors on the order of 10⁹ while simultaneously producing uniform signal enhancements in point-to-point measurements across each sample. High enhancement factors are associated with the narrow gap between nanoparticles assembled in clusters in full-wave electromagnetic simulations. Reusability for small-molecule detection is also demonstrated. Thus it is shown that the combination of high signal enhancement and reproducibility is achievable using a completely non-lithographic fabrication process, thereby producing SERS substrates having high performance at low cost.

Direct energy gap group IV semiconductor alloys and quantum dot arrays in SnxGe1−x/Ge and SnxSi1−x/Si alloy systems

Abstract The narrow gap semiconductor alloys Sn x Ge 1− x and Sn x Si 1− x offer the possibility for engineering tunable direct energy gap Group IV semiconductor materials. For pseudomorphic Sn x Ge 1− x alloys grown on Ge (001) by molecular beam epitaxy, an indirect-to-direct bandgap transition with increasing Sn composition is observed, and the effects of misfit on the bandgap analyzed in terms of a deformation potential model. Key results are that pseudomorphic strain has only a very slight effect on the energy gap of Sn x Ge 1− x alloys grown on Ge (001) but for Sn x Ge 1− x alloys grown on Ge (111) no indirect-to-direct gap transition is expected. In the Sn x Si 1− x system, ultrathin pseudomorphic epitaxially-stabilized α-Sn x Si 1− x alloys are grown on Si (001) substrates by conventional molecular beam epitaxy. Coherently strained α-Sn quantum dots are formed within a defect-free Si (001) crystal by phase separation of the thin Sn x Si 1− x layers embedded in Si (001). Phase separation of the thin alloy film, and subsequent evolution occurs via growth and coarsening of regularly-shaped α-Sn quantum dots that appear as 4–6 nm diameter tetrakaidecahedra with facets oriented along elastically soft 〈100〉 directions. Attenuated total reflectance infrared absorption measurements indicate an absorption feature due to the α-Sn quantum dot array with onset at ∼0.3 eV and absorption strength of 8×10 3 cm −1 , which are consistent with direct interband transitions.

Ordered arrays of rare-earth silicide nanowires on Si(0 0 1)

Rare earth silicides have been demonstrated to self-assemble during epitaxial growth as one-dimensional nanostructures with preferred orientation along Si <110> on Si[111] and Si[001]. The evolution of the one-dimensional structure during epitaxial growth has been attributed to an anisotropic lattice mismatch of the two orthogonal axis of the hexagonal unit cell with respect to Si. On highly oriented Si[001] substrates, nanowires align their long axis along Si <110>: thus, two orientations of nanowires were obtained having their long axes orthogonal to one another. We have now demonstrated that alignment of rare earth silicide nanowires can be achieved along a single direction by growth on vicinal Si[001] substrates. Self-assembled ErSi/sub 2/ and DySi/sub 2/ wires aligned along Si [110] have been grown at 600/spl deg/C with aspect ratios exceeding 100 and feature heights on the order of 1 atomic layer. The nanowires were characterized in situ with scanning tunneling microscopy. These rare earth silicide nanowires may have applications as non-lithographically fabricated small scale interconnects due to high electrical conductivity and low Schottky barrier to n-type Si.

Design of a versatile chemical assembly method for patterning colloidal nanoparticles

Poly(methyl methacrylate) (PMMA) domains in phase-separated polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer thin films were chemically modified for controlled placement of solution synthesized Au nanoparticles having a mean diameter of 24 nm. Colloidal Au nanoparticles functionalized with thioctic acid were immobilized on amine functionalized PMMA domains on the PS-b-PMMA template using 1-ethyl-3-[3-dimethylaminopropyl] carbodiimide hydrochloride linking chemistry and N-hydroxy sulfosuccinimide stabilizer. Atomic force microscopy and scanning electron microscopy images demonstrated immobilization of Au nanoparticles commensurate with PMMA domains. Nanoparticles form into clusters of single particles, dimers, and linear chains as directed by the PMMA domain size and shape. Capillary forces influence the spacing between Au nanoparticles on PMMA domains. Inter-particle spacings below 3 nm were achieved and these assemblies of closely spaced nanoparticle clusters are expected to exhibit strong localized electromagnetic fields. Thus, these processes and material systems provide an experimental platform for studying resonantly enhanced excitations of surface plasmons as a function of material and geometric structure as well as utilization in catalytic applications.

Void-mediated formation of Sn quantum dots in a Si matrix

Atomic scale analysis of Sn quantum dots (QDs) formed during the molecular beam-epitaxy (MBE) growth of SnxSi1-x (0.05 less than or equal to x less than or equal to 0.1) multilayers in a Si matrix revealed a void-mediated formation mechanism. Voids below the Si surface are induced by the lattice mismatch strain between SnxSi1-x layers and Si, taking on their equilibrium tetrakaidecahedron shape. The diffusion of Sn atoms into these voids leads to an initial rapid coarsening of quantum dots during annealing. Since this formation process is not restricted to Sn, a method to grow QDs may be developed by controlling the formation of voids and the diffusion of materials into these voids during MBE growth.

Shrink-induced sorting using integrated nanoscale magnetic traps

We present a plastic microfluidic device with integrated nanoscale magnetic traps (NSMTs) that separates magnetic from non-magnetic beads with high purity and throughput, and unprecedented enrichments. Numerical simulations indicate significantly higher localized magnetic field gradients than previously reported. We demonstrated >20 000-fold enrichment for 0.001% magnetic bead mixtures. Since we achieve high purity at all flow-rates tested, this is a robust, rapid, portable, and simple solution to sort target species from small volumes amenable for point-of-care applications. We used the NSMT in a 96 well format to extract DNA from small sample volumes for quantitative polymerase chain reaction (qPCR).

Generic Process for Highly Stable Metallic Nanoparticle-Semiconductor Heterostructures via Click Chemistry for Electro/Photocatalytic Applications

Metallic nanoparticles (MNP) are utilized as electrocatalysts, cocatalysts, and photon absorbers in heterostructures that harvest solar energy. In such systems, the interface formed should be stable over a wide range of pH values and electrolytes. Many current nonthermal processing strategies rely on physical interactions to bind the MNP to the semiconductor. In this work, we demonstrate a generic chemical approach for fabricating highly stable electrochemically/photocatalytically active monolayers and tailored multilayered nanoparticle structures using azide/alkyne-modified Au, TiO2, and SiO2 nanoparticles on alkyne/azide-modified silicon, indium tin oxide, titania, stainless steel, and glass substrates via click chemistry. The stability, electrical, electrochemical, and photocatalytic properties of the interface are shown via electrochemical water splitting, methanol oxidation, and photocatalytic degradation of Rhodamine B (RhB) dye. The results suggest that the proposed approach can be extended for the large-scale fabrication of highly stable heterostructure materials for electrochemical and photoelectrocatalytic devices.

A Facile Approach for Assembling Lipid Bilayer Membranes on Template-Stripped Gold

Lipid vesicles are designed with functional chemical groups to promote vesicle fusion on template-stripped gold (TS Au) surfaces that does not spontaneously occur on unfunctionalized Au surfaces. Three types of vesicles were exposed to TS Au surfaces: (1) vesicles composed of only 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipids; (2) vesicles composed of lipid mixtures of 2.5 mol % of 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-poly(ethylene glycol)-2000-N-[3-(2-pyridyldithio)propionate] (DSPE-PEG-PDP) and 97.5 mol % of POPC; and (3) vesicles composed of 2.5 mol % of 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(poly(ethylene glycol))-2000] (DSPE-PEG) and 97.5 mol % POPC. Atomic force microscopy (AFM) topography and force spectroscopy measurements acquired in a fluid environment confirmed tethered lipid bilayer membrane (tLBM) formation only for vesicles composed of 2.5 mol % DSPE-PEG-PDP/97.5 mol % POPC, thus indicating that the sulfur-containing PDP group is necessary to achieve tLBM formation on TS Au via Au-thiolate bonds. Analysis of force-distance curves for 2.5 mol % DSPE-PEG-PDP/97.5 mol % POPC tLBMs on TS Au yielded a breakthrough distance of 4.8 ± 0.4 nm, which is about 1.7 nm thicker than that of POPC lipid bilayer membrane formed on mica. Thus, the PEG group serves as a spacer layer between the tLBM and the TS Au surface. Fluorescence microscopy results indicate that these tLBMs also have greater mechanical stability than solid-supported lipid bilayer membranes made from the same vesicles on mica. The described process for assembling stable tLBMs on Au surfaces is compatible with microdispensing used in array fabrication.

Comparison of electric field enhancements: Linear and triangular oligomers versus hexagonal arrays of plasmonic nanospheres

We investigate local electromagnetic field enhancements in oligomers of plasmonic nanospheres. We first evaluate via full-wave simulations the field between spheres in several oligomer systems: linear dimers, linear trimers, trimers 60°, trimers 90° and linear quadrumers. To gain a better understanding of the field enhancement values, we compare the results with local fields in a hexagonal close-packed (HCP) configuration with same structural dimensions. We then inter-relate the field enhancement values found via full-wave simulations to SERS enhancements of actual fabricated self-assembled oligomers. We find that linear oligomers provide the largest field enhancement values. Finally, we provide closed-form formulas for the prediction of the resonance frequency responsible for field enhancement in linear oligomers, namely dimers, trimers and quadrumers, modeling each nanosphere as a single electric dipole. These formulas provide with resonance values less than 7% shifted when compared to full-wave results even when the gap between spheres is only about one fifth of the radius, showing the powerfulness of dipolar approximations. The results shown in this paper demonstrate that ad hoc clusters of nanospheres can be designed and fabricated to obtain larger field enhancements than with the HCP structure and this may pave the way for the development of improved sensors for molecular spectroscopy.