Sidney T. Malak

Assemblies of silver nanocubes for highly sensitive SERS chemical vapor detection

We suggest that silver nanocube (AgNC) aggregates within cylindrical pores (PAM–AgNC) can be employed as efficient nanostructures for highly efficient, robust, tunable, and reusable surface-enhanced Raman scattering (SERS) substrates for trace level organic vapor detection which is a challenging task in chemical detection. We demonstrate the ability to tune both the detection limit and the onset of signal saturation of the substrate by switching the adsorption behavior of AgNCs between highly aggregated and more disperse by varying the number of adsorption-mediating polyelectrolyte bilayers on the pore walls of the membrane. The different AgNC distributions show large differences in the trace vapor detection limit of the common Raman marker benzenethiol (BT) and a widely used explosive binder N-methyl-4-nitroaniline (MNA), demonstrating the importance of the large electromagnetic field enhancement associated with AgNC coupling. The SERS substrate with highly aggregated AgNCs within the cylindrical pores allows for consistent trace detection of mid ppb (∼500) for BT analyte, and a record limit of detection of low ppb (∼3) for MNA vapors with an estimated achievable limit of detection of approximately 600 ppt. The dispersed AgNC aggregates do not saturate at higher ppb concentrations, providing an avenue to distinguish between higher ppb concentrations and increase the effective range of the SERS substrate design. A comparison between the AgNC substrate and an electroless deposition substrate with silver quasi-nanospheres (PAM–AgNS) also demonstrates a higher SERS activity, and better detection limit, by the nanocube aggregates. This is supported by FDTD electromagnetic simulations that suggest that the higher integrated electromagnetic field intensity of the hot spots and the large specific interfacial areas impart greatly improved SERS for the AgNCs. Moreover, we demonstrated that the AgNC substrate can be reused multiple times without significant loss of SERS activity which opens up new avenues for in-field monitoring.

Crafting Core/Graded Shell-Shell Quantum Dots with Suppressed Re-absorption and Tunable Stokes Shift as High Optical Gain Materials.

The key to utilizing quantum dots (QDs) as lasing media is to effectively reduce non-radiative processes, such as Auger recombination and surface trapping. A robust strategy to craft a set of CdSe/Cd(1-x)Zn(x)Se(1-y)S(y)/ZnS core/graded shell-shell QDs with suppressed re-absorption, reduced Auger recombination rate, and tunable Stokes shift is presented. In sharp contrast to conventional CdSe/ZnS QDs, which have a large energy level mismatch between CdSe and ZnS and thus show strong re-absorption and a constrained Stokes shift, the as-synthesized CdSe/Cd(1-x)Zn(x)Se(1-y)S(y)/ZnS QDs exhibited the suppressed re-absorption of CdSe core and tunable Stokes shift as a direct consequence of the delocalization of the electron wavefunction over the entire QD. Such Stokes shift-engineered QDs with suppressed re-absorption may represent an important class of building blocks for use in lasers, light emitting diodes, solar concentrators, and parity-time symmetry materials and devices.

Silk Layering As Studied with Neutron Reflectivity

Neutron reflectivity (NR) measurements of ultrathin surface films (below 30 nm) composed of Bombyx mori silk fibroin protein in combination with atomic force microscopy and ellipsometry were used to reveal the internal structural organization in both dry and swollen states. Reconstituted aqueous silk solution deposited on a silicon substrate using the spin-assisted layer-by-layer (SA-LbL) technique resulted in a monolayer silk film composed of random nanofibrils with constant scattering length density (SLD). However, a vertically segregated ordering with two different regions has been observed in dry, thicker, seven-layer SA-LbL silk films. The vertical segregation of silk multilayer films indicates the presence of a different secondary structure of silk in direct contact with the silicon oxide surface (first 6 nm). The layered structure can be attributed to interfacial β-sheet crystallization and the formation of well-developed nanofibrillar nanoporous morphology for the initially deposited silk surface layers with the preservation of less dense, random coil secondary structure for the layers that follow. This segregated structure of solid silk films defines their complex nonuniform behavior in the D(2)O environment with thicker silk films undergoing delamination during swelling. For a silk monolayer with an initial thickness of 6 nm, we observed the increase in the effective thickness by 60% combined with surprising decrease in density. Considering the nanoporous morphology of the hydrophobic silk layer, we suggested that the apparent increase in its thickness in liquid environment is caused by the air nanobubble trapping phenomenon at the liquid-solid interface.

Enhancement of optical gain characteristics of quantum dot films by optimization of organic ligands

This work examines how the optimization of molecular dimensions and chemical functionality of the organic ligands of quantum dots (QDs) can be explored for dramatic enhancement of the optical properties of QD films, particularly, optical gain. We show that the replacement of traditional QD organic ligands with a much shorter ligand, butylamine, yields a dense QD-packing that results in a two-fold increase in optical gain. Overall, the highly packed QD films exhibit very large net gain values (∼500 cm−1) which greatly exceed typical Cd-based QD films with traditional ligands. In addition, thresholds for amplified-spontaneous emission (ASE) down to 50 μJ cm−2 were observed, which is exceptionally low for ns-pulse pumped QD systems. Our results confirm an additional route for obtaining high optical gain using QDs, and outline a strategy for modifying the optical gain characteristics by ligand exchange without needing to modify the QD selection. Consideration of the ligands along with QD compositional design could make it possible to fabricate photonic systems with exceptionally low lasing thresholds, and offers a route toward achieving high gain using steady state pumping, an extremely difficult feat to achieve in traditional QD systems.

Robust, Uniform, and Highly Emissive Quantum Dot–Polymer Films and Patterns Using Thiol–Ene Chemistry

This work demonstrates a facile and versatile method for generating low scattering cross-linked quantum dot (QD)-polymer composite films and patterned highly emissive structures with ultrahigh QD loading, minimal phase separation, and tunable mechanical properties. Uniform QD-polymer films are fabricated using thiol-ene chemistry, in which cross-linked polymer networks are rapidly produced in ambient conditions via fast UV polymerization in bulk to suppress QD aggregation. UV-controlled thiol-ene chemistry limits phase separation through producing highly QD loaded cross-linked composites with loadings above majority of those reported in the literature (<1%) and approaching 30%. As the QD loading is increased, the thiol and ene conversion decreases, resulting in nanocomposites with widely variable and tailorable mechanical properties as a function of UV irradiation time with an elastic modulus decreasing to 1 GPa being characteristic of reinforced elastomeric materials, in contrast to usually observed stiff and brittle materials under these loading conditions. Furthermore, we demonstrate that the thiol-ene chemistry is compatible with soft-imprint lithography, making it possible to pattern highly loaded QD films while preserving the optical properties essential for high gain and low optical loss devices. The versatility of thiol-ene chemistry to produce high-dense QD-polymer films potentially makes it an important technique for polymer-based elastomeric optical metamaterials, where efficient light propagation is critical, like peculiar waveguides, sensors, and optical gain films.

Structural Study of Star Polyelectrolytes and Their Porous Multilayer Assembly in Solution

Star polyelectrolytes with responsive properties to external stimuli, such as pH, temperature and ionic condition, were utilized to fabricate layer-by-layer (LbL) microcapsules . The microstructure of star polyelectrolytes was first studied in semi-dilute solution by in situ small-angle neutron scattering (SANS). These measurements show that with the addition of salts, arms of strong cationic star polyelectrolytes will contract and the spatial ordering of the stars would be interrupted. SANS measurements were also performed on the microcapsules in order to study their internal structure and responsive properties in solution. The results show that with the increase of shell thickness, microcapsules undergo a change of fractal dimension. Microcapsules with thinner shell have a surface fractal structure with rough interface, while those with thicker shell generally have a mass fractal structure of 3D random network. With the change of surrounding environment (pH, temperature, or ionic condition), the morphology and permeability of microcapsules are changed concurrently, for example, with the addition of multivalent salt, there is a surface- to mass-fractal transition, with the correlation length decreasing by around 50 %. This study provides insight into the mechanism of the responsiveness of novel star polyelectrolytes and their assembled multilayer structures.