Publications

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Electro-optical organic materials hold great promise for the development of high-efficiency devices based on exciton formation and dissociation, such as organic photovoltaics (OPV) and organic light-emitting devices (OLEDs). However, the external quantum efficiency (EQE) of both OPV and OLEDs must be improved to make these technologies economical. Efficiency rolloff in OLEDs and inability to control morphology at key OPV interfaces both reduce EQE. Only by creating materials that allow manipulation and control of the intimate assembly and communication between various nanoscale excitonic components can we hope to first understand and then engineer the system to allow these materials to reach their potential. The aims of this proposal are to: 1) develop a paradigm-changing platform for probing excitonic processes composed of Crystalline Nanoporous Frameworks (CNFs) infiltrated with secondary materials (such as a complimentary semiconductor); 2) use them to probe fundamental aspects of excitonic processes; and 3) create prototype OPVs and OLEDs using infiltrated CNF as active device components. These functional platforms will allow detailed control of key interactions at the nanoscale, overcoming the disorder and limited synthetic control inherent in conventional organic materials. CNFs are revolutionary inorganic-organic hybrid materials boasting unmatched synthetic flexibility that allow tuning of chemical, geometric, electrical, and light absorption/generation properties. For example, bandgap engineering is feasible and polyaromatic linkers provide tunable photon antennae; rigid 1-5 nm pores provide an oriented, intimate host for triplet emitters (to improve light emission in OLEDs) or secondary semiconducting polymers (creating a charge-separation interface in OPV). These atomically engineered, ordered structures will enable critical fundamental questions to be answered concerning charge transport, nanoscale interfaces, and exciton behavior that are inaccessible in disordered systems. Implementing this concept also creates entirely new dimensions for device fabrication that could both improve performance, increase durability, and reduce costs with unprecedented control of over properties. This report summarizes the key results of this project and is divided into sections based on publications that resulted from the work. We begin in Section 2 with an investigation of light harvesting and energy transfer in a MOF infiltrated with donor and acceptor molecules of the type typically used in OPV devices (thiophenes and fullerenes, respectively). The results show that MOFs can provide multiple functions: as a light harvester, as a stabilizer and organizer or the infiltrated molecules, and as a facilitator of energy transfer. Section 3 describes computational design of MOF linker groups to accomplish light harvesting in the visible and facilitate charge separation and transport. The predictions were validated by UV-visible absorption spectroscopy, demonstrating that rational design of MOFs for light-harvesting purposes is feasible. Section 4 extends the infiltration concept discussed in Section to, which we now designate as "Molecule@MOF" to create an electrically conducting framework. The tailorability and high conductivity of this material are unprecedented, meriting publication in the journal Science and spawning several Technical Advances. Section 5 discusses processes we developed for depositing MOFs as thin films on substrates, a critical enabling technology for fabricating MOF-based electronic devices. Finally, in Section 6 we summarize results showing that a MOF thin film can be used as a sensitizer in a DSSC, demonstrating that MOFs can serve as active layers in excitonic devices. Overall, this project provides several crucial proofs-of- concept that the potential of MOFs for use in optoelectronic devices that we predicted several years ago [ 3 ] can be realized in practice.

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We report the first observation of coherent plasmon emission of THz radiation from arrays of semiconductor nanowires. The THz signal strength from InAs nanowires is comparable to a planar substrate, indicating the nanowires are highly efficient emitters. This is explained by the preferential orientation of plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection. Using a bulk Drude model, we identify the average donor density and mobility in the nanowires in a non-contact manner. Contact IV transconductance measurements provide order of magnitude agreement with values obtained from the THz spectra. © 2009 SPIE.

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We report on the fabrication and characterization of large-area 2-D square arrays of subwavelength holes in Ag and Al films. Fabrication is based on thermal nanoimprint lithography and metal evaporation, without the need for etching, and is compatible with low-cost, large-scale production. Reflectance spectra for these arrays display an intensity minimum whose amplitude, center wavelength, and line width depend on the geometry of the array and the reflectivity of the metal film. By placing various fluids in contact with the subwavelength aperture arrays, we observe that the center wavelength of the reflectance minimum varies linearly with the refractive index of the fluid with a sensitivity of over 500 nm per refractive index unit. The surface plasmon theory is used to predict sensitivities to refractive index change with accuracies better than 0.5%. © 2008 IEEE.

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The high surface to volume ratio of nanowires makes them attractive for exploiting exotic materials properties and nanoengineering new device structures. To realize these goals, a fundamental understanding of the morphology and growth of the nanowires must be attained in three dimensions, because a two-dimensional projection image of these complex three-dimensional nanomaterials is not sufficient to describe their properties. Scanning transmission electron tomography is used here to obtain three-dimensional tomograms of GaN/AIN core-shell nanowires. This technique reveals the overall morphology and triangular shape of the nanowires, as well as their relation to the catalyst particle, with a resolution of ∼1 nm in all three spatial dimensions. Defects that appear to be in the core of the nanowires in two-dimensional images are shown to be surface defects induced during growth, demonstrating the importance of this three-dimensional technique in analyzing nanomaterials. © 2008 American Chemical Society.

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We present the design, fabrication, and testing of a microelectromechanical systems (MEMS) light modulator based on pixels patterned with periodic nanohole arrays. Flexure-suspended silicon pixels are patterned with a two dimensional array of 150 nm diameter nanoholes using nanoimprint lithography. A top glass plate assembled above the pixel array is used to provide a counter electrode for electrostatic actuation. The nanohole pattern is designed so that normally-incident light is coupled into an in-plane grating resonance, resulting in an optical stop-band at a desired wavelength. When the pixel is switched into contact with the top plate, the pixel becomes highly reflective. A 3:1 contrast ratio at the resonant wavelength is demonstrated for gratings patterned on bulk Si substrates. The switching time is 0.08 ms and the switching voltage is less than 15V. © 2008 Optical Society of America.

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Optical filters based on resonant gratings have spectral characteristics that are lithographically defined. Nanoimprint lithography is a relatively new method for producing large area gratings with sub-micron features. Computational modeling using rigorous coupled-wave analysis allows gratings to be designed to yield sharp reflectance maxima and minima. Combining these gratings with microfluidic channels and micromechanical actuators produced using micro electromechanical systems (MEMS) technology forms the basis for producing tunable filters and other wavelength selective elements. These devices achieve tunable optical characteristics by varying the index of refraction on the surface of the grating. Coating the grating surface with water creates a 33% change in the resonant wavelength whereas bringing a grating into contact with a quartz surface shifts the resonant wavelength from 558 nm to 879 nm, a fractional change of 58%. The reflectivity at a single wavelength can be varied by approximately a factor of three. Future applications of these devices may include tunable filters or optical modulators. © 2008 Society of Photo-Optical Instrumentation Engineers.

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LDRD Project 102662 provided support to pursue experiments aimed at measuring the basic electrodynamic response and possible applications of carbon nanotubes and silicon nanowires at radiofrequency to microwave frequencies, approximately 0.01 to 50 GHz. Under this project, a method was developed to integrate these nanomaterials onto high-frequency compatible co-planar waveguides. The complex reflection and transmission coefficients of the nanomaterials was studied as a function of frequency. From these data, the high-frequency loss characteristics of the nanomaterials were deduced. These data are useful to predict frequency dependence and power dissipation characteristics in new rf/microwave devices incorporating new nanomaterials.

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Electrical contacts to semiconductors play a key role in electronics. For nanoscale electronic devices, particularly those employing novel low-dimensionality materials, contacts are expected to play an even more important role. Here we show that for quasi-one-dimensional structures such as nanotubes and nanowires, side contact with the metal only leads to weak band re-alignment, in contrast to bulk metal-semiconductor contacts. Schottky barriers are much reduced compared with the bulk limit, and should facilitate the formation of good contacts. However, the conventional strategy of heavily doping the semiconductor to obtain ohmic contacts breaks down as the nanowire diameter is reduced. The issue of Fermi level pinning is also discussed, and it is demonstrated that the unique density of states of quasi-one-dimensional structures make them less sensitive to this effect. Our results agree with recent experimental work, and should apply to a broad range of quasi-one-dimensional materials.

The film stress of Ni films deposited at near-ambient temperatures from sulfamate electrolytes was studied. The particulate filtering of the electrolyte, a routine industrial practice, becomes an important deposition parameter at lower bath temperatures. At 28 C, elevated tensile film stress develops at low current densities (<10 mA/cm{sup 2}) if the electrolyte is filtered. Filtering at higher current densities has a negligible effect on film stress. A similar though less pronounced trend is observed at 32 C. Sulfate-based Ni plating baths display similar film stress sensitivity to filtering, suggesting that this is a general effect for Ni electrodeposition. It is shown that filtering does not significantly change the current efficiency or the pH near the surface during deposition. The observed changes in film stress are thus attributed not to adsorbed hydrogen but instead to the effects of filtering on the formation and concentration of polyborate species due to the decreased solubility of boric acid at near-ambient temperatures.

Grain size and texture of Ni electrodeposited from sulfamate baths depend greatly on current density. Increasing grain size is observed with increasing current density and the deposit texture changes from (110) at current densities lower than 5 mA cm{sup -2} to (100) for higher current densities. Co-deposition of Mn modifies the deposit structure by favoring the growth of the (110) texture and decreasing the average grain size even as the current density increases. While the average Mn film content increases with increasing current density, local Mn concentrations are a more complex function of deposition parameters, as indicated by atom probe tomography measurements. In both direct-current plated and pulse plated films, large variations on a nanometer scale in local Mn concentration are observed.

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DNA-wrapped carbon nanotubes (DNA-CNT) have generated attention due the ability to disperse cleanly into solution, and by the possibility of sorting nanotubes according to size and conductivity. In order to learn more about the effects of DNA on the electrical transport characteristics of single wall carbon nanotubes, we fabricate and test a series of devices consisting of DNA-wrapped CNTs placed across gold, palladium, and palladium oxide electrodes. In addition, we look at how DNA functionalized CNTs react to presence of hydrogen, which has previously been shown to affect the conductivity of CNTs when in contact with palladium.

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We present experimental observation and theoretical analysis of looping carbon nanotubes connecting two electrodes. The measured conductance of the nanotubes is not strongly affected by the presence of these conformational defects, a result that is confirmed by quantum transport calculations. Our work indicates that solution-based fabrication methods for carbon nanotube devices can have high conformational defect tolerance, except for defects with 5-10 nanometer bending radius. © 2005 American Institute of Physics.

We present nanometer-scale resolution, ballistic electron emission microscopy (BEEM) studies of Au/octanedithiol/n-GaAs (001) diodes. The presence of the molecule dramatically increases the BEEM threshold voltage and displays an unusual transport signature as compared to reference Au/GaAs diodes. Furthermore, BEEM images indicate laterally inhomogeneous interfacial structure. We present calculations that address the role of the molecular layer at the interface. Our results indicate that spatially resolved measurements add new insight to studies using conventional spatial-averaging techniques.

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We report the surface acoustic wave (SAW) correlator devices fabricated using nanoimprint lithography. Using step-and-flash imprint lithography (S-FIL), we produced SAW correlator devices on 100 mm diameter z-cut LiNbO 3 devices and an aluminum metal etch process. On the same chip layout, we fabricated SAW filters and compared both the filters and correlators to similar devices fabricated using electron-beam lithography (EBL). Both S-FIL- and EBL-patterned correlators and SAW filters were analyzed using a bit-error rate tester to generate the signal and a parametric signal analyzer to evaluate the output. The NIL niters had an average center frequency of 2.38 GHz with a standard deviation of 10 MHz. The measured insertion loss averaged -31 dB. In comparison, SAW filters fabricated using EBL exhibited a center frequency of 2.39 GHz and a standard deviation of 100 kHz. Based on our preliminary results, we believe that S-FIL is an efficient and entirely viable fabrication method to produce quality SAW filters and correlators. © 2004 American Vacuum Society.

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Twinning is ubiquitous in electroplated metals. Here, we identify and discuss unique aspects of twinning found in electrodeposited Ni-Mn alloys. Previous reports concluded that the twin boundaries effectively refine the grain size, which enhances mechanical strength. Quantitative measurements from transmission electron microscopy (TEM) images show that the relative boundary length in the as-plated microstructure primarily comprises twin interfaces. Detailed TEM characterization reveals a range of length scales associated with twinning beginning with colonies ({approx}1000 nm) down to the width of individual twins, which is typically <50 nm. We also consider the connection between the crystallographic texture of the electrodeposit and the orientation of the twin planes with respect to the plating direction. The Ni-Mn alloy deposits in this work possess a 110-fiber texture. While twinning can occur on {l_brace}111{r_brace} planes either perpendicular or oblique to the plating direction in {l_brace}110{r_brace}-oriented grains, plan-view TEM images show that twins form primarily on those planes parallel to the plating direction. Therefore, grains enclosed by twins and multiply twinned particles are produced. Another important consequence of a high twin density is the formation of large numbers of twin-related junctions. We measure an area density of twin junctions that is comparable to the density of dislocations in a heavily cold-worked metal.