Ian D. Hosein

Magnetically responsive and hollow colloids from nonspherical core-shell particles of peanut-like shape

Monodisperse peanut-shaped particles having various chemical compositions, architectures (core–shell and hollow) and properties (optical and magnetic) were prepared from hematite templates. The colloids enable shape programming alone, optical manipulation, or magnetic field approaches to assembly.

Rotator and crystalline films viaself-assembly of short-bond-length colloidal dimers

Nonspherical particles of pear-like and spherocylinder shape were organized into diverse two-dimensional (2D) structures, including the orientationally disordered rotator. Dry films with hexagonal, oblique, and centered rectangular symmetry were obtained by using convective assembly to condense and confine the system in a thin meniscus region. Monte Carlo simulations confirmed the transition from fluid to rotator simply as a function of system density and short-bond-length particle morphology.

Tuning manganese dopant spin interactions in single GaN nanowires at room temperature.

Control of electron spins in individual magnetically doped semiconductor nanostructures has considerable potential for quantum information processing and storage. The manipulations of dilute magnetic interactions have largely been restricted to low temperatures, limiting their potential technological applications. Among the systems predicted to be ferromagnetic above room temperature, Mn-doped GaN has attracted particular attention, due to its attractive optical and electrical properties. However, the experimental data have been inconsistent, and the origin of the magnetic interactions remains unclear. Furthermore, there has been no demonstration of tuning the dopant exchange interactions within a single nanostructure, which is necessary for the design of nanoscale spin-electronic (spintronic) devices. Here we directly show for the first time intrinsic magnetization of manganese dopants in individual gallium nitride nanowires (NWs) at room temperature. Using high-resolution circularly polarized X-ray microscopy imaging, we demonstrate the dependence of the manganese exchange interactions on the NW orientation with respect to the external magnetic field. The crystalline anisotropy allows for the control of dilute magnetization in a single NW and the application of bottom-up approaches, such as in situ nanowire growth control or targeted positioning of individual NWs, for the design of networks for quantum information technologies.

Electronic structure and magnetism of Mn dopants in GaN nanowires: Ensemble vs single nanowire measurements

We studied the electronic structure and magnetization of Mn dopants in GaN nanowires at the ensemble and single nanowire levels by near edge x-ray absorption fine structure spectroscopies. The results of single nanowire measurements indicate that Mn adopts tetrahedral coordination in GaN nanowires and has mixed oxidation state (Mn2+/Mn3+), with Mn2+ being in relative majority. Ensemble nanowire spectra suggest co-deposition of Mn secondary phases alongside nanowires. Single nanowire x-ray magnetic circular dichroism indicates intrinsic magnetic ordering of Mn dopants at 300 K. In contrast, as-grown nanowire samples show only residual magnetization, due to nanowire orientation dependence of magnetization.

Correlation between native defects and dopants in colloidal lanthanide-doped Ga2O3 nanocrystals: a path to enhance functionality and control optical properties

We report the synthesis and study of the photoluminescence properties of colloidal lanthanide(III)-doped Ga2O3 nanocrystals. The Ga2O3 nanocrystal host lattice acts as a sensitizer of the Eu3+ dopant red emission arising from intra-4f orbital transitions, and concurrently exhibits a strong blue photoluminescence originating from defect-based donor–acceptor pair (DAP) recombination. The Eu3+ sensitization, enabled by the energy transfer from the nanocrystal host lattice to the dopant centers, allows for the generation of dual blue-red emission. Increasing doping concentration leads to a decrease in the donor activation energy allowing for simultaneous control of the optical and electrical properties of these multifunctional nanocrystals. Analyses of the steady-state and time-resolved photoluminescence spectra suggest that Eu3+ ions occupy at least two different sites, which were tentatively designated as the six-coordinate internal and surface-related dopants. Uniquely, both DAP and Eu3+ emissions have long lifetimes (in milliseconds), although Eu3+ luminescence has a slower decay rate. These phenomena enable a temporal modulation of the dual emission and photoluminescence chromaticity on the millisecond timescale. The generality of these findings was demonstrated by preparing Tb3+-doped Ga2O3 nanocrystals, as a blue-green dual emitter. Owing to their optical transparency, electrical properties, emission color versatility, robustness, and fabricability, colloidal lanthanide(III)-doped Ga2O3 nanocrystals are a promising class of multifunctional materials and complex phosphors.

Magnetic property characterization of magnetite (Fe3O4) nanorod cores for integrated solenoid rf inductors

The on-chip magnetic solenoid inductors with Fe3O4 magnetite nanorod (MN) cores are fabricated and characterized up to 40GHz. By vibrating-sample magnetometer measurements, the magnetic property of MN as a magnetic core for a solenoid inductor is investigated. In addition, high-frequency characterization with scattering parameter measurements is performed to estimate the high-frequency performance of the solenoid inductors with the MN cores.

Increasing light capture in silicon solar cells with encapsulants incorporating air prisms to reduce metallic contact losses.

Silicon solar cells are the most widely deployed modules owing to their low-cost manufacture, large market, and suitable efficiencies for residential and commercial use. Methods to increase their solar energy collection must be easily integrated into module fabrication. We perform a theoretical and experimental study on the light collection properties of an encapsulant that incorporates a periodic array of air prisms, which overlay the metallic front contacts of silicon solar cells. We show that the light collection efficiency induced by the encapsulant depends on both the shape of the prisms and angle of incidence of incoming light. We elucidate the changes in collection efficiency in terms of the ray paths and reflection mechanisms in the encapsulant. We fabricated the encapsulant from a commercial silicone and studied the change in the external quantum efficiency (EQE) on an encapsulated, standard silicon solar cell. We observe efficiency enhancements, as compared to a uniform encapsulant, over the visible to near infrared region for a range of incident angles. This work demonstrates exactly how a periodic air prism architecture increases light collection, and how it may be designed to maximize light collection over the widest range of incident angles.

Superhydrophobic Microporous Substrates via Photocuring: Coupling Optical Pattern Formation to Phase Separation for Process-Tunable Pore Architectures

We present a new approach to synthesize microporous surfaces through the combination of photopolymerization-induced phase separation and light pattern formation in photopolymer-solvent mixtures. The mixtures are irradiated with a wide-area light pattern consisting of high and low intensity regions. This light pattern undergoes self-focusing and filamentation, thereby preserving its spatial profile through the mixture. Over the course of irradiation, the mixture undergoes phase separation, with the polymer and solvent located in the bright and dark regions of the light profile, respectively, to produce a binary phase morphology with a congruent arrangement as the optical pattern. A congruently arranged microporous structure is attained upon solvent removal. The microporous surface structure can be varied by changing the irradiating light profile via photomask design. The porous architecture can be further tuned through the relative weight fractions of photopolymer and solvent in the mixture, resulting in porosities ranging from those with discrete and uniform pore sizes to hierarchical pore distributions. All surfaces become superhydrophobic (water contact angles >150°) when spray-coated with a thin layer of polytetrafluoroethylene nanoparticles. The water contact angles can be enhanced by changing the surface porosity via the processing conditions. This is a scalable and tunable approach to precisely control microporous surface structure in thin films to create functional surfaces and antiwetting coatings.

Coupling nonlinear optical waves to photoreactive and phase-separating soft matter: Current status and perspectives

Nonlinear optics and polymer systems are distinct fields that have been studied for decades. These two fields intersect with the observation of nonlinear wave propagation in photoreactive polymer systems. This has led to studies on the nonlinear dynamics of transmitted light in polymer media, particularly for optical self-trapping and optical modulation instability. The irreversibility of polymerization leads to permanent capture of nonlinear optical patterns in the polymer structure, which is a new synthetic route to complex structured soft materials. Over time more intricate polymer systems are employed, whereby nonlinear optical dynamics can couple to nonlinear chemical dynamics, opening opportunities for self-organization. This paper discusses the work to date on nonlinear optical pattern formation processes in polymers. A brief overview of nonlinear optical phenomenon is provided to set the stage for understanding their effects. We review the accomplishments of the field on studying nonlinear waveform propagation in photopolymerizable systems, then discuss our most recent progress in coupling nonlinear optical pattern formation to polymer blends and phase separation. To this end, perspectives on future directions and areas of sustained inquiry are provided. This review highlights the significant opportunity in exploiting nonlinear optical pattern formation in soft matter for the discovery of new light-directed and light-stimulated materials phenomenon, and in turn, soft matter provides a platform by which new nonlinear optical phenomenon may be discovered.

Multidirectional waveguide arrays in a planar architecture

We previously showed that large populations (<10, 000 cm-3) of self-induced cylindrical multimode waveguides spontaneously form when an incoherent white light field suffers modulation instability in a photopolymerizable medium. By deliberately modulating the optical field and employing multiple beams, we then generated a diverse range of waveguide lattices with 1-D, 2-D and 3-D geometries. Here, we describe the potential of this technique - optochemical organization – to provide an inexpensive, single-step, room temperature route to waveguide-inscribed planar architectures, which could serve as light-collecting, steering and focusing elements.