Seed Clusters

In addition to individual faculty projects, we are awarding Seed funds to small research clusters consisting of several faculty and at least two graduate students and/or postdoctoral research associates. Clusters will be evaluated principally on the basis of their potential as future IRGs and/or their ability to forge links to the industrial sector. The following potential clusters have been identified from submitted proposals and recommendations of a visiting panel consisting of some members of our External Academic Advisory Board.

(Click here to view Seed Projects)

Past Seed Clusters

Defect Control in Naturally-Assembled Photonic Crystals (D. Blank, CHEM; A. Stein, CHEM; D. Norris, CEMS) will explore photonic crystals – structures that are periodic on an optical length scale and can prohibit light from propagating in any direction. This property, commonly described as a photonic bandgap (PBG), can be exploited to manipulate photons, promising a variety of ultra-compact optical devices. A potentially simple and economical approach for creating 3-D photonic crystals relies on the natural assembly of monodisperse colloidal micron-scale spheres into a face-centered cubic (fcc) lattice. The resulting material, often regarded as a synthetic opal, can be filled with a high refractive index material. Subsequent removal of the template produces an ordered macroporous replica of the template, often described as an "inverse opal." Norris and Stein, experts in the synthesis and characterization of these materials, will address several challenges in this field. First, the existence of a PBG depends on the perfection of the structure. Random defects that commonly occur during conventional gravity-induced assembly will be minimized by combining vertical deposition with lithographic patterning of substrates, thereby obviating the formation of stacking faults during growth. In addition, higher quality inverse opals with improved uniformity and larger dimensions will be achieved by Stein through the use of functionalized colloidal templates that enhance the infiltration of the high index material and its properties (e.g., smoothness, density, optical nonlinearities). Second, most applications of PBG materials require introduction of intentional structural defects, which act as localized photonic states. Recently, Norris demonstrated a key initial step by creating random interstitial defects in a silicon inverted opal (see figure) through the addition of "dopant" spheres to the colloidal template. In order to obtain spatial control over doping, Blank and Norris will introduce defects into inverse opals with laser micromachining, including two-photon methods, and by focused ion beam techniques. Optical micro-spectroscopy methods developed by Norris will be used to analyze individual domains in the photonic crystals. The controlled-defect materials will be used to address important fundamental issues, such as the effects of disorder on photonic properties and the possibility of trapping light in photonic crystals. These phenomena are crucial to optical devices such as photonic crystal defect lasers, photonic switches, and memories. Third, in order to achieve wider PBGs, several approaches will be used to produce non-fcc structures, particularly diamond lattices. Stein will synthesize non-fcc crystals with derivatized spheres having suitable charge/radius ratios and/or through directed assembly with biological molecules, and Norris will employ optical trapping to guide the assembly into preordained architectures.

The Metallic Photonic Crystals cluster (D. Norris, CEMS; D. Blank, CHEM; A. Stein, CHEM) proposes to utilize the expertise at UMN to fabricate these structures using self-assembly. We will use the tendency of monodisperse sub-micron spheres to spontaneously organize on a face-centered cubic (fcc) lattice. The resulting material, often referred to as a synthetic opal, will then act as a template into which the tungsten is infiltrated. Subsequent removal of the template leads to a tungsten photonic crystal, known as an inverse opal. Due to the simplicity of our approach, we should be able to obtain a large variety of samples and explore their optical properties in detail. For infiltration of the tungsten, we will pursue two strategies. (i) We will use chemical vapor deposition (CVD) to infiltrate tungsten into thin opaline films. Indeed, the first report of tungsten inverse opals, which very recently appeared, utilized this approach.3 Although UMN does not have the capability for tungsten CVD on campus, we have initiated a collaboration with researchers at IBM who specialize in tungsten deposition. (ii) We will use the reduction of sol-gel/salt precursors inside opaline particles. Previously, this strategy has generated nickel, cobalt, and iron inverse opals in our laboratories.4 Thus, it should be feasible to similarly obtain tungsten structures. Potential precursors include tungsten halides, oxyhalides or alkoxides, which can be converted to form tungsten oxide nanoparticles within the colloidal crystal template. These will then be reduced to tungsten by controlled hydrogen reduction or electrochemical reduction. Depending on the specific chemistry, the template can be removed either before or after the reduction process to produce the final structure. For photonic properties, it will be important to obtain a 3D skeleton with smooth surfaces. This can be achieved by maintaining small grain sizes, both during the synthesis of the oxide and conversion to the metal. We will employ doping methods which have been successful in grain size control of other oxides prepared in our labs, as well as modifications of other liquid phase methods that have been used to prepare tungsten or tungsten oxide nanoparticles. By following these two approaches, a variety of interesting materials (thin films, coatings, filaments, particles, etc.) should be possible. Afterwards, the structures will be structurally and optically characterized. Of particular interest, we plan to explore the thermal emission of different structures when they are resistively heated. For example, an interesting tungsten Òphotonic crystal filamentÓ could be formed by assembling an opal in a long groove etched in a flat substrate. After infiltration with tungsten and removal of the opal, the filament could then be released from the substrate and attached (under inert gas) to electrical contacts. Resistive heating should then lead to thermal spectrum that is modified by the structured nature of the filament. In addition to complete characterization of the linear optical response, including absorption, reflection, and emission, we will investigate the nonlinear optical properties of our materials. These experiments will involve a variety of state-of-the-art ultrafast nonlinear optical techniques that will evaluate the third-order optical response as a function of both frequency and response time. These experiments will cover from the IR to the UV with sub 50fs time resolution. Initial experiments will probe the nonlinear optical behavior of the band gap by measuring the modulated reflectance and transmission of the tungsten materials following an initial pump pulse that will induce a transient shift in the position of the band-gap. Time resolved fluorescence experiments will investigate the potential trapping of excited state that are created within the bandgap. Heterodyne detected four wave mixing experiments, FWM, set up in a reflective geometry will probe both the absorptive and refractive portion of the third-order optical response. In contrast to typical FWM experiments that are applied in transmission, the reflection FWM set up will limit the measured response to the vicinity of the surface and thereby allow probing of the inverted opal layer with minimal contributions from the substrate. Finally, to provide theoretical guidance, we propose to numerically simulate the optical behavior of photonic band gap structures to be developed under this effort, with a view towards acquiring further understanding of their properties. We propose to solve scattering problems for finite arrangements, which will deliver macroscopic observables (e.g., reflection and transmission coefficients) while accounting for size effects. Numerical codes will be based on fast solution algorithms for surface and volume integral formulations of the relevant scattering models.

Microstructural Mechanics of Bioartificial Tissues (V.H. Barocas, BME; T.R. Oegema, OS; R.T. Tranquillo, BME/CEMS) represents a core strength of the former Artificial Tissues IRG. The methodology for measuring the mechanical properties of these very compliant materials and assessing fibril alignment under stress, now possible with our Tissue Mechanics Laboratory (directed by J.L. Lewis, OS/MECH), is relatively undeveloped and poses several unique challenges that make this topic highly suitable for Seed funding. Furthermore, several companies have an interest in the Laboratory and have contributed financially and intellectually to its development. Artificial tissues based on cell-compacted biopolymer gels have the requisite biocompatibility, but it has proven difficult to attain the desired mechanical properties in a biopolymer-based artificial tissue, or tissue-equivalent (TE). Tranquillo and Oegema have demonstrated that the culture environment and choice of biopolymer can alter the microstructure of a TE. In particular, mechanical constraints on cell-mediated compaction of a fiber network in a gel can induce preferential alignment of tissue fibers and mechanical anisotropy in TEs. When fibrin rather than collagen is used as the biopolymer, the entrapped cells synthesize collagen at a higher rate and also synthesize elastin, an important component of native structural tissues, previously unreported in TEs. The MMBT cluster will combine two key tools to explore the role of the underlying biopolymer microstructure in macroscopic mechanical behavior. The Instron-Sacks biaxial testing system in the Tissue Mechanics Laboratory, coupled with our unique polarized light-based measurement of fibril alignment developed by Tranquillo, will allow simultaneous measurement of fibril alignment and mechanical behavior of a TE. This is complemented by our multiscale modeling technique developed by Barocas, which allows theoretical analysis of the relationship between microstructural changes and macroscopic behavior by incorporating microstructure directly into a finite element model instead of a traditional constitutive equation. The MMBT cluster will fabricate TEs with varying composition and fibril alignment, characterize fibril reorientation during mechanical testing, and use the multiscale model to analyze the fibril reorientation-mechanical property behavior. Once the microscale computer model has been refined, simulations will be performed to predict (i) the macroscopic stress-strain behavior of the TE (i.e., what the Instron measures), (ii) the rearrangement of the fibrils within the TE (i.e., what polarized light measures), and (iii) the tension or compression of individual representative fibrils within the TE. These studies will validate our model and provide a sound theoretical basis for the fabrication of TEs with mechanical properties matching those of native tissues.

The Plasma Surface Passivation of Luminescent Silicon Nanocrystals cluster (U. Kortshagen, ME; S. Girshick, ME; J. Roberts, CHEM) proposes to produce silicon nitride and a-SiCx:H passivation layers on silicon nanoparticles, and to study their photoluminescent properties and surface chemistries. The proposed plasma process would use two plasma reactors in series. In the first plasma process, silicon quantum dots will be generated. Currently, we produce silicon nanocrystals in this plasma process in a mixture of in Ar-He-silane plasma highly diluted in hydrogen, and it is likely that the particles are hydrogen-capped. According to the literature, these are optimal conditions for depositing high-quality silicon nitride or amorphous silicon carbide layers. These particles will be extracted from the initial plasma reactor and immediately reintroduced into a second plasma flow reactor. By adding additional nitrogen and silane into the gas mixtures, we will be able to produced silicon nitride layers, and by adding silane and methane we will be able to form silicon carbide layers. We have previously shown that by varying the power density in the plasma, the gas temperature can be varied between slightly above room temperature and >1200 K. By varying the gas composition and gas temperature in the plasma, we hope to be able to deposit very high quality passivation layers. We will characterize the silicon particles, the quality of the passivation layer, and the resistance of the passivation layer to oxidation in a number of ways. Particle crystallinity will be assessed with high resolution TEM in collaboration with Barry Carter’s group, or with InnovaLight, which has access to the facilities at the University of Texas in Austin. The surface elemental composition of the particles will be studies by X-ray Photoelectron Spectroscopy (XPS). The uptake of ambient gases, including water and oxygen, by gas-borne particles will be studied in Roberts’ laboratory using tandem differential mobility analysis (T-DMA). In T-DMA, the kinetics and reaction mechanisms of reactions involving nanometer-sized aerosol particles are studied. The only requirement is that the reaction result in a change in particle size that is measurable by a differential mobility analyzer (DMA). The Roberts lab is equipped with DMAs capable of measuring diameter changes as small as 1% of the initial particle diameter. For small enough particles (Dp ² 10 nm), this translates into monolayer sensitivity.

The Silicon Nanoparticles cluster (S. Campbell, ECE; W. Gerberich, CEMS; J. Kakalios, PHYS; U. Kortshagen, MECH) will explore single crystal silicon nanoparticles and their unique electronic and optical properties. Making use of techniques developed recently in the UMN Particle Technology Laboratory (www.me.umn.edu/divisions/environmental/ptl), Kortshagen will create, size-select, extract, and process 5 - 80 nm diameter silicon nanoparticles in a high density plasma reactor, and will synthesize thin films of nanostructured hydrogenated silicon (ns-Si:H) consisting of silicon nanoparticles embedded in an amorphous hydrogenated silicon matrix (alpha-Si:H). The mid-gap defect density of ns-Si:H is nearly ten times lower than in RF-deposited alpha-Si:H, and the transport of photo-excited charge carriers is 100 times larger than that observed in alpha-Si:H. Furthermore, ns-Si:H based photovoltaic devices exhibit no appreciable decrease in efficiency after illumination. These features, coupled with the ability of plasma processing to make large-area films, make ns-Si:H an ideal candidate for photovoltaics. Kakalios will use transport and thermopower measurements to assess local and long-range order, which will be correlated with the immediate and long-term photoresponse. Kortshagen will rely on these measurements to optimize the particle size distribution. The cluster also will explore the fabrication of integrated transistor-like devices based on single crystal silicon nanoparticles alternately deposited with amorphous insulator films. The use of nanoparticles also allows the integration, in both 2-D and 3-D, of otherwise incompatible single crystal materials such as Si and GaN. Kortshagen will deposit the nanoparticles through plasma processing. Campbell will fabricate contacts to the nanoparticles using proximal probes and e-beam lithography, and will measure the temperature-dependent current-voltage, capacitance-voltage, and capacitance-time behavior of metal-semiconductor-metal structures (see figure). Kortshagen and Campbell will investigate the dependence of electrical behavior on particle formation conditions, the presence of surface state-passivating oxides, and low-resistance ohmic and Schottky contacts formed by reactions between the silicon nanoparticles and the metallic film.