This research was supported as part of the Center for Energy Nanoscience, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science under Award Number DE-SC0001013. We also want to thank Dr. Max Zhang and Dr. Jianhua Yang of HP Labs for their help on TiO2 film sputtering and refractive indices measurement.

1. Prepare the Blank Polydimethylsiloxane (PDMS) Substrate for Nanoimprint Mold
<ol>
	<li>Silicon Wafer Treatment Process
	<ol>
		<li>Clean a 4 inch silicon wafer by rinsing with acetone, methanol and isopropanol.</li>
		<li>Blow it dry using the nitrogen gun.</li>
		<li>Clean it using piranha solution (3:1 mixture of sulfuric acid with 30% hydrogen peroxide) by soaking inside for 15 min.</li>
		<li>Rinse it with DI water. Blow dry using the nitrogen gun.</li>
		<li>Place the wafer in a glass desiccator. Add a drop (20 drops = 1 ml) of releasing agent (trichlorosilane) into the desiccator.</li>
		<li>Pump down the desiccator until the gauge reads -762 Torr and wait for 5 hr.</li>
		<li>Take the wafer out, which has been treated with releasing agent.</li>
	</ol>
	</li>
	<li>Preparation of PDMS Film (Used as Mold in Nanoimprint)
	<ol>
		<li>Weigh 10 g of silicone elastomer base and 1 g of curing agent.</li>
		<li>Add them in the same glass beaker.</li>
		<li>Stir and mix with a glass rod for 5 min.</li>
		<li>Put the mixture into a vacuum desiccator until the gauge reads -762 Torr to pump out all the trapped air bubbles.</li>
		<li>Spread them evenly onto the treated 4-inch silicon wafer.</li>
		<li>Bake the wafer with PDMS on top in the vacuum oven for 7 hr at 80 &#176;C to cure the PDMS film.</li>
	</ol>
	</li>
</ol>
2. Prepare the Nanoimprint Mold (Duplication from the Master Mold)
<ol>
	<li>Spin twelve drops (20 drops = 1 ml) of UV curable resist (15.2%) on a clean blank silicon wafer for 30 sec at 1,500 rpm.</li>
	<li>Carefully peel a piece of PDMS film off the treated silicon wafer.</li>
	<li>Put the PDMS film onto the UV curable resist and let it absorb the UV resist for 5 min then peel it off.</li>
	<li>Repeat 2.1-2.3 on the same PDMS film for two times. Absorb the UV resist for 3 min and 1 min respectively.</li>
	<li>Place the PDMS film (after three-time UV resist absorption) onto a silicon master mold.</li>
	<li>Put it into a chamber with nitrogen environment.</li>
	<li>Turn on UV lamp to cure the sample for 5 min.</li>
	<li>Peel off the PDMS film. The cured UV resist on the PDMS will keep the negative pattern of the master mold.</li>
	<li>Use RF O2 plasma to treat the PDMS mold. (RF power: 30 W, pressure: 260 mTorr, time: 1 min)</li>
	<li>Place the PDMS mold in a vacuum chamber with one drop (20 drops = 1 ml) of releasing agent for 2 hr.</li>
</ol>
3. Nanoimprint Pattern Transfer
<ol>
	<li>Spin eight drops (20 drops = 1 ml) of PMMA (996k, 3.1%) on the substrate to be imprinted for 50 sec at 3,500 rpm.</li>
	<li>Bake it on a hotplate for 5 min at 120 &#176;C.</li>
	<li>Wait for the sample to cool down.</li>
	<li>Spin eight drops (20 drops = 1 ml) of UV curable resist (3.9%) on the same substrate.</li>
	<li>Place the PDMS mold (prepared in step 2) onto the sample (with both UV resist and PMMA).</li>
	<li>Put it into a chamber with nitrogen environment.</li>
	<li>Turn on the UV lamp to cure for 5 min.</li>
	<li>Peel the PDMS mold off the sample and the pattern on the PDMS mold gets transferred to the sample.</li>
</ol>
4. Cr Lift-off Process
<ol>
	<li>Reactive ion etching residual layer of UV resist and PMMA 
	Note: The SOP for ICP machine can be found at https://www.nanocenter.umd.edu/equipment/fablab/sops/etch-07/Oxford%20Chlorine%20Etcher%20SOP.pdf&#160;
	<ol>
		<li>Log in RIE ICP machine.</li>
		<li>Load a blank 4 inch silicon wafer. Run the clean recipe for 10 min.</li>
		<li>Take the blank silicon wafer out.</li>
		<li>Mount the sample on another clean silicon wafer and load it into the machine.</li>
		<li>Run the UV resist etching recipe for 2 min (the recipe can be found in Table 1).</li>
		<li>Take the sample out. Load a blank 4 inch silicon wafer. Re-run the clean recipe (can be found in Table 1) for 10 min.</li>
		<li>Mount the sample on a clean silicon wafer and load it into the machine.</li>
		<li>Run the PMMA etching recipe (can be found in Table 1) for 2 min. 
		Note: Now the residual resist has been etched and substrate is exposed.</li>
	</ol>
	</li>
	<li>Cr E-beam Evaporation
	<ol>
		<li>Log into e-beam evaporator.</li>
		<li>Load the Cr metal source and sample into the chamber.</li>
		<li>Set the thickness (20 nm) and deposition rate (0.03 nm/sec).</li>
		<li>Pump the chamber until required vacuum (10-7 Torr) is reached.</li>
		<li>Start the deposition process.</li>
		<li>Take the sample out after the deposition finishes.</li>
	</ol>
	</li>
	<li>Cr Lift-off Procedure
	<ol>
		<li>Immerse the sample in acetone with ultrasonic agitation for 5 min.</li>
		<li>Clean the sample by rinsing with acetone, methanol and isopropanol. 
		Note: The Cr evaporated on the resist will be lifted off and a Cr mask for substrate etching is formed.</li>
	</ol>
	</li>
</ol>
5. TiO2 Deposition
<ol>
	<li>Load sample.</li>
	<li>Set the parameters for the direct current magnetron sputtering machine
	<ol>
		<li>Use a chamber pressure of 1.5 mTorr, Ar flow of 100 sccm and a sputtering power of 130 W.</li>
		<li>Use a temperature of 27 &#176;C and a stage rotation speed of 20 rpm.</li>
	</ol>
	</li>
	<li>Start the sputter process and stop at desired thickness.</li>
	<li>Take the sample out and anneal the TiO2 film in oxygen environment at 300 &#176;C for 3 hr.</li>
</ol>
6. High Contrast Grating Etching
<ol>
	<li>Log in the inductively coupled plasma (ICP) reactive ion etching (RIE) machine.</li>
	<li>TiO2 etching
	<ol>
		<li>Load a blank 4-inch silicon wafer.</li>
		<li>Start and run the clean recipe (can be found in Table 1) for 10 min.</li>
		<li>Unload load the blank wafer and load the sample with Cr mask.</li>
		<li>Set etching time. Start TiO2 etching recipe. The etching process will automatically stop.</li>
		<li>Unload the sample.</li>
	</ol>
	</li>
	<li>SiO2 Etching
	<ol>
		<li>Repeat step 5.2 except use the SiO2 etching recipe.</li>
	</ol>
	</li>
</ol>
7. Reflectance Measurement
<ol>
	<li>Log in and turn on the measurement system.</li>
	<li>Place the reflectance standard mirror on the sample holder and align the optical path.</li>
	<li>Calibrate the system for the 100% reflectance.</li>
	<li>Take off the reflectance standard mirror and place the HCG.</li>
	<li>Measure the reflectance of the HCG.</li>
	<li>Save the data and log out of the measurement system.</li>
</ol>

<ol>
	<li>Horne, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Solid+500+Sun+Compound+Concentrator+PV+Design.">A Solid 500 Sun Compound Concentrator PV Design.</a> Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on. , 694-697 (2006).</li><li>Guter, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Current-matched+triple-junction+solar+cell+reaching+41.1%+conversion+efficiency+under+concentrated+sunlight.">Current-matched triple-junction solar cell reaching 41.1% conversion efficiency under concentrated sunlight.</a> Applied Physics Letters. 94, 223504 (2009).</li><li>Shockley, W., Queisser, H. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Detailed+Balance+Limit+of+Efficiency+of+p-n+Junction+Solar+Cells.">Detailed Balance Limit of Efficiency of p-n Junction Solar Cells.</a> Journal of Applied Physics. 32, 510-519 (1961).</li><li>Green, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Potential+for+low+dimensional+structures+in+photovoltaics.">Potential for low dimensional structures in photovoltaics.</a> Materials Science and Engineering: B. 74, 118-124 (2000).</li><li>Karagodsky, V., Chang-Hasnain, C. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Physics+of+near-wavelength+high+contrast+gratings.">Physics of near-wavelength high contrast gratings.</a> Opt. Express. 20, 10888-10895 (2012).</li><li>Chou, S. Y., Krauss, P. R., Renstrom, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanoimprint+lithography.">Nanoimprint lithography.</a> Journal of Vacuum Science &amp; Technology B: Microelectronics and Nanometer Structures. 14, 4129-4133 (1996).</li><li>Namiki, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+FDTD+algorithm+based+on+alternating-direction+implicit+method.+Microwave+Theory+and+Techniques.">A new FDTD algorithm based on alternating-direction implicit method. Microwave Theory and Techniques.</a> IEEE Transactions on. 47, 2003-2007 (1999).</li><li>Moharam, M. G., Gaylord, T. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rigorous+coupled-wave+analysis+of+planar-grating+diffraction.">Rigorous coupled-wave analysis of planar-grating diffraction.</a> J. Opt. Soc. Am. 71, 811-818 (1981).</li><li>Yao, Y., Liu, H., Wu, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spectrum+splitting+using+multi-layer+dielectric+meta-surfaces+for+efficient+solar+energy+harvesting.">Spectrum splitting using multi-layer dielectric meta-surfaces for efficient solar energy harvesting.</a> Appl. Phys. A. 115, 713-719 (2014).</li><li>Yao, Y., Liu, H., Wu, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+of+high-contrast+gratings+for+a+parallel+spectrum+splitting+dispersive+element+in+a+concentrated+photovoltaic+system.">Fabrication of high-contrast gratings for a parallel spectrum splitting dispersive element in a concentrated photovoltaic system.</a> Journal of Vacuum Science &amp; Technology B. 32, 06FG04-06FG04-6 (2014).</li><li>Solak, H. H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sub-50+nm+period+patterns+with+EUV+interference+lithography.">Sub-50 nm period patterns with EUV interference lithography.</a> Microelectronic Engineering. 67, 56-62 (2003).</li><li>Li, Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hybrid+nanoimprint&#8722;+soft+lithography+with+sub-15+nm+resolution.">Hybrid nanoimprint&#8722; soft lithography with sub-15 nm resolution.</a> Nano letters. 9, 2306-2310 (2009).</li><li>Yu, Z., Chen, L., Wu, W., Ge, H., Chou, S. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+of+nanoscale+gratings+with+reduced+line+edge+roughness+using+nanoimprint+lithography.">Fabrication of nanoscale gratings with reduced line edge roughness using nanoimprint lithography.</a> Journal of Vacuum Science &amp; Technology B. 21, 2089-2092 (2003).</li></ol>

The authors have nothing to disclose.

First, the quality of the TiO2 film is very crucial for the HCG performance. The reflectance peak will be higher if the TiO2 film has less loss and surface roughness. The TiO2 film with a higher refractive index is also favorable because the optical mode confinement will be enhanced by a higher contrast in index, which can give rise to a flatter and broader reflectance band in HCG.
Second, the fabrication errors will have significant effects on the HCG and shou...

Our modern society will not survive without moving a significant portion of energy consumption to renewable energy sources. To make this happen, we have to find a way to harvest renewable energy at a cost lower than petroleum-based energy sources in the near future. Solar energy is the most abundant renewable energy on earth. Despite that a lot of progresses have been made in solar energy harvesting, it is still very challenging to compete with petroleum-based energy sources. Improving the efficiency of solar cells is one of the most efficient ways to lower the system cost of solar energy harvesting.
Optical lenses and dish reflectors are usually used in most concentrated photovoltaic (CPV) systems1 to achieve a high concentration of solar power incidence on the small-area solar cells, so it is economically viable to exploit expensive tandem multi-junction solar cells2 in CPV systems, and to maintain a reasonable cost at the same time. However, for most non-concentrated photovoltaic systems, which usually require a large-area installment of solar cells, the high-cost tandem solar cells cannot be incorporated, although they usually have a broader solar spectrum response and a higher overall conversion efficiency than the single junction solar cells3.
Recently, with the help of the parallel spectrum splitting optics (i.e. dispersive element), the parallel spectrum splitting photovoltaic technology4 has made it possible that a similar or better spectrum coverage and conversion efficiency can be achieved without using the expensive tandem solar cells. The solar spectrum can be split into different bands and each band can be absorbed and converted to electricity by the specialized single-junction solar cells. In this way, the expensive tandem solar cells in CPV systems can be replaced by a parallel distribution of single-junction solar cells without any compromise on the performance.
The dispersive element that was designed in this report can be applied in a reflective CPV system (which is based on dish reflectors) to realize parallel spectrum splitting for the improved solar-electricity conversion efficiency and reduced cost. Multilayer high contrast gratings (HCG)5 is used as the dispersive element by designing each layer of HCG to work as an optical band reflector. The structures and parameters of the dispersive element are numerically optimized. Moreover, the fabrication of high contrast gratings for the dispersive element by using dielectric (TiO2) sputtering, nanoimprint lithography6 and reactive ion etching is studied and demonstrated.

<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>184 SILICONE ELASTOMER KIT</td>
			<td>SYLGARD</td>
			<td></td>
			<td>Polydimethylsiloxane (PDMS)</td>
		</tr>
		<tr>
			<td>4-inch silicon wafer</td>
			<td>Universitywafer</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>4-inch fused silica wafer</td>
			<td>Universitywafer</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Poly(methyl methacrylate)</td>
			<td>SIGMA-ALDRICH</td>
			<td align="right">182265</td>
			<td></td>
		</tr>
		<tr>
			<td>UV-curable resist</td>
			<td></td>
			<td></td>
			<td>Nor available on market</td>
		</tr>
		<tr>
			<td>PlasmaLab System 100</td>
			<td>Oxford Instruments</td>
			<td></td>
			<td>ICP IRE machine</td>
		</tr>
		<tr>
			<td>UV curing system for nanoimprint fabrication</td>
			<td></td>
			<td></td>
			<td>Not available on market</td>
		</tr>
		<tr>
			<td>Ocean Optics HR-4000&#160;</td>
			<td>Ocean Optics</td>
			<td>HR-4000</td>
			<td>Spectrometer with normal detector</td>
		</tr>
		<tr>
			<td>Lambda 950 UV / VIS</td>
			<td>PerkinElmer</td>
			<td></td>
			<td>spectrometer with hemisphere intergration detector</td>
		</tr>
		<tr>
			<td>JSM-7001F-LV</td>
			<td>JEOL</td>
			<td></td>
			<td>Field emission SEM</td>
		</tr>
		<tr>
			<td>DC magnetron sputtering machine</td>
			<td></td>
			<td></td>
			<td>Equipment is in HP labs, who helped us to sputter the TiO2</td>
		</tr>
		<tr>
			<td>Metal e-beam evaporator</td>
			<td>Temescal</td>
			<td>BJD-1800</td>
			<td></td>
		</tr>
	</tbody>
</table>

fabrication of high contrast gratings for the spectrum splitting dispersive element in a concentrated photovoltaic system

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve the solar conversion efficiency of a concentrated photovoltaic system. The proposed system will also lower the solar cell cost in the concentrated photovoltaic system by replacing the expensive tandem solar cells with the cost-effective single junction solar cells. The structures and the parameters of high contrast gratings for the dispersive elements were numerically optimized. The large-area fabrication of high contrast gratings was experimentally demonstrated using nanoimprint lithography and dry etching. The quality of grating material and the performance of the fabricated device were both experimentally characterized. By analyzing the measurement results, the possible side effects from the fabrication processes are discussed and several methods that have the potential to improve the fabrication processes are proposed, which can help to increase the optical efficiency of the fabricated devices.

Figure 1 shows the implementation of the dispersive element (multilayer high contrast grating (HCG)) in a concentrated photovoltaic system. The sun light is first reflected by the primary mirror and impinges on the reflective dispersive element, where the beam is reflected and split into different bands of different wavelengths. Each band will impinge on a certain location on the solar cell array for the best absorption and conversion to electricity. The key to this system is the design and implementatio...

Watch this Scientific Journal Video about Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System at JoVE.com

Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System

The fabrication of high contrast gratings as the parallel spectrum splitting dispersive element in a concentrated photovoltaic system is demonstrated. Fabrication processes including nanoimprint lithography, TiO2 sputtering and reactive ion etching are described. Reflectance measurement results are used to characterize the optical performance.

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve ...

fabrication-high-contrast-gratings-for-spectrum-splitting-dispersive

Research

JoVE Journal

Engineering

10.7K Aufrufe.  University of Sothern California. The fabrication of high contrast gratings as the parallel spectrum splitting dispersive element in a concentrated photovoltaic system is demonstrated. Fabrication processes including nanoimprint lithography, TiO2 sputtering and reactive ion etching are described. Reflectance measurement results are used to characterize the optical performance. 

Serie: Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System

Electron Channeling Contrast Imaging for Rapid III-V Heteroepitaxial Characterization

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging (ECCI) in a scanning electron microscope (SEM). ECCI allows for imaging of defects and crystallographic features under specific diffraction conditions, similar to that possible via plan-view transmission electron microscopy (PV-TEM). A particular advantage of the ECCI technique is that it requires little to no sample preparation, and indeed can use large area, as-produced samples, making it a considerably higher throughput characterization method than TEM. Similar to TEM, different diffraction conditions can be obtained with ECCI by tilting and rotating the sample in the SEM. This capability enables the selective imaging of specific defects, such as misfit dislocations at the GaP/Si interface, with high contrast levels, which are determined by the standard invisibility criteria. An example application of this technique is described wherein ECCI imaging is used to determine the critical thickness for dislocation nucleation for GaP-on-Si by imaging a range of samples with various GaP epilayer thicknesses. Examples of ECCI micrographs of additional defect types, including threading dislocations and a stacking fault, are provided as demonstration of its broad, TEM-like applicability. Ultimately, the combination of TEM-like capabilities &#8211; high spatial resolution and richness of microstructural data &#8211; with the convenience and speed of SEM, position ECCI as a powerful tool for the rapid characterization of crystalline materials.

The use of electron channeling contrast imaging in a scanning electron microscope to characterize defects in III-V/Si heteroexpitaxial thin films is described. This method yields similar results to plan-view transmission electron microscopy, but in significantly less time due to lack of required sample preparation.

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging ...

Energy Dispersive X-ray Tomography for 3D Elemental Mapping of Individual Nanoparticles

Energy dispersive X-ray spectroscopy within the scanning transmission electron microscope (STEM) provides accurate elemental analysis with high spatial resolution, and is even capable of providing atomically resolved elemental maps. In this technique, a highly focused electron beam is incident upon a thin sample and the energy of emitted X-rays is measured in order to determine the atomic species of material within the beam path. This elementally sensitive spectroscopy technique can be extended to three dimensional tomographic imaging by acquiring multiple spectrum images with the sample tilted along an axis perpendicular to the electron beam direction.
Elemental distributions within single nanoparticles are often important for determining their optical, catalytic and magnetic properties. Techniques such as X-ray tomography and slice and view energy dispersive X-ray mapping in the scanning electron microscope provide elementally sensitive three dimensional imaging but are typically limited to spatial resolutions of &gt; 20 nm. Atom probe tomography provides near atomic resolution but preparing nanoparticle samples for atom probe analysis is often challenging. Thus, elementally sensitive techniques applied within the scanning transmission electron microscope are uniquely placed to study elemental distributions within nanoparticles of dimensions 10-100 nm.
Here, energy dispersive X-ray (EDX) spectroscopy within the STEM is applied to investigate the distribution of elements in single AgAu nanoparticles. The surface segregation of both Ag and Au, at different nanoparticle compositions, has been observed.

The use of energy dispersive X-ray tomography in the scanning transmission electron microscope to characterize elemental distributions within single nanoparticles in three dimensions is described.

Energy dispersive X-ray spectroscopy within the scanning transmission electron microscope (STEM) provides accurate elemental analysis with high spatial ...

Writing Bragg Gratings in Multicore Fibers

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong suppression at a single wavelength requires that all cores have matching transmission profiles. These gratings cannot be inscribed using the same method as for single-core fibers because the curved surface of the cladding acts as a lens, focusing the incoming UV laser beam and causing variations in exposure between cores. Therefore we use an additional optical element to ensure that the beam shape does not change while passing through the cross-section of the multicore fiber. This consists of a glass capillary tube which has been polished flat on one side, which is then placed over the section of the fiber to be inscribed. The laser beam enters the fiber through the flat surface of the capillary tube and hence maintains its original dimensions. This paper demonstrates the improvements in core-to-core uniformity for a 7-core fiber using this method. The technique can be generalized to larger multicore fibers.

We describe a technique for inscribing identical fiber Bragg gratings into each core of a multicore fiber. This is achieved by introducing an additional surface into the optical path to mitigate lensing by the curved surface of the fiber cladding.

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong ...

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials such as organo-lead halide perovskites is presented. The perovskite film thickness and absorption coefficient are initially characterized by profilometry and UV-VIS absorption spectroscopy. Calibration of both laser power and cavity sensitivity is described in detail. A protocol for performing Flash-photolysis Time Resolved Microwave Conductivity (TRMC) experiments, a non-contact method of determining the conductivity of a material, is presented. A process for identifying the real and imaginary components of the complex conductivity by performing TRMC as a function of microwave frequency is given. Charge carrier dynamics are determined under different excitation regimes (including both power and wavelength). Techniques for distinguishing between direct and trap-mediated decay processes are presented and discussed. Results are modelled and interpreted with reference to a general kinetic model of photoinduced charge carriers in a semiconductor. The techniques described are applicable to a wide range of optoelectronic materials, including organic and inorganic photovoltaic materials, nanoparticles, and conducting/semiconducting thin films.

A Time Resolved Microwave Conductivity technique for investigating direct and trap-mediated recombination dynamics and determining carrier mobilities of thin film semiconductors is presented here.

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials ...

Challenges in Rheological Characterization of Highly Concentrated Suspensions &#8212; A Case Study for Screen-printing Silver Pastes

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the manifold requirements during processing and application of such complex fluids. In this investigation, measuring protocols for a conclusive assessment of different process relevant rheological parameters have been evaluated. This includes the determination of yield stress, viscosity, wall slip velocity, structural recovery after large deformation and elongation at break as well as tensile force during filament stretching.
The importance of concomitant video recordings during parallel-plate rotational rheometry for a significant determination of rheological quantities is demonstrated. The deformation profile and flow field at the sample edge can be determined using appropriate markers. Thus, measurement parameter settings and plate roughness values can be identified for which yield stress and viscosity measurements are possible. Slip velocity can be measured directly and measuring conditions at which plug flow, shear banding or sample spillover occur can be identified clearly. 
Video recordings further confirm that the change in shear moduli observed during three stage oscillatory shear tests with small deformation amplitude in stage I and III but large oscillation amplitude in stage II can be directly attributed to structural break down and recovery. For the pastes investigated here, the degree of irreversible, shear-induced structural change increases with increasing deformation amplitude in stage II until a saturation is reached at deformations corresponding to the crossover of G' and G'', but the irreversible damage is independent of the duration of large amplitude shear. 
A capillary breakup elongational rheometer and a tensile tester have been used to characterize deformation and breakup behavior of highly filled pastes in uniaxial elongation. Significant differences were observed in all experiments described above for two commercial screen-printing silver pastes used for front side metallization of Si-solar cells.

Challenges in Rheological Characterization of Highly Concentrated Suspensions &amp;#8212; A Case Study for Screen-printing Silver Pastes

A protocol for a robust and application relevant rheological characterization of highly concentrated suspensions is presented. Silver pastes used for screen-printing application in solar cell production are employed as model systems.

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the ...

High Temperature Fabrication of Nanostructured Yttria-Stabilized-Zirconia (YSZ) Scaffolds by In Situ Carbon Templating Xerogels

We demonstrate a method for the high temperature fabrication of porous, nanostructured yttria-stabilized-zirconia (YSZ, 8 mol% yttria - 92 mol% zirconia) scaffolds with tunable specific surface areas up to 80 m2&#183;g-1. An aqueous solution of a zirconium salt, yttrium salt, and glucose is mixed with propylene oxide (PO) to form a gel. The gel is dried under ambient conditions to form a xerogel. The xerogel is pressed into pellets and then sintered in an argon atmosphere. During sintering, a YSZ ceramic phase forms and the organic components decompose, leaving behind amorphous carbon. The carbon formed in situ serves as a hard template, preserving a high surface area YSZ nanomorphology at sintering temperature. The carbon is subsequently removed by oxidation in air at low temperature, resulting in a porous, nanostructured YSZ scaffold. The concentration of the carbon template and the final scaffold surface area can be systematically tuned by varying the glucose concentration in the gel synthesis. The carbon template concentration was quantified using thermogravimetric analysis (TGA), the surface area and pore size distribution was determined by physical adsorption measurements, and the morphology was characterized using scanning electron microscopy (SEM). Phase purity and crystallite size was determined using X-ray diffraction (XRD). This fabrication approach provides a novel, flexible platform for realizing unprecedented scaffold surface areas and nanomorphologies for ceramic-based electrochemical energy conversion applications, e.g. solid oxide fuel cell (SOFC) electrodes.

High Temperature Fabrication of Nanostructured Yttria-Stabilized-Zirconia YSZ Scaffolds by In Situ Carbon Templating Xerogels

A protocol for fabricating porous, nanostructured yttria-stabilized-zirconia (YSZ) scaffolds at temperatures between 1,000 &#176;C and 1,400 &#176;C is presented.

We demonstrate a method for the high temperature fabrication of porous, nanostructured yttria-stabilized-zirconia (YSZ, 8 mol% yttria - 92 mol% zirconia) ...

Detecting the Water-soluble Chloride Distribution of Cement Paste in a High-precision Way

To improve the accuracy of the chloride distribution along the depth of cement paste under cyclic wet-dry conditions, a new method is proposed to obtain a high-precision chloride profile. Firstly, paste specimens are molded, cured, and exposed to cyclic wet-dry conditions. Then, powder samples at different specimen depths are grinded when the exposure age is reached. Finally, the water-soluble chloride content is detected using a silver nitrate titration method, and chloride profiles are plotted. The key to improving the accuracy of the chloride distribution along the depth is to exclude the error in the powderization, which is the most critical step for testing the distribution of chloride. Based on the above concept, the grinding method in this protocol can be used to grind powder samples automatically layer by layer from the surface inward, and it should be noted that a very thin grinding thickness (less than 0.5 mm) with a minimum error less than 0.04 mm can be obtained. The chloride profile obtained by this method better reflects the chloride distribution in specimens, which helps researchers to capture the distribution features that are often overlooked. Furthermore, this method can be applied to studies in the field of cement-based materials, which require high chloride distribution accuracy.

A protocol for obtaining a water-soluble chloride profile by using a high precision milling method is presented.

To improve the accuracy of the chloride distribution along the depth of cement paste under cyclic wet-dry conditions, a new method is proposed to obtain a ...

Method for Recording Broadband High Resolution Emission Spectra of Laboratory Lightning Arcs

Lightning is one of the most common and destructive forces in nature and has long been studied using spectroscopic techniques, first with traditional camera film methods and then digital camera technology, from which several important characteristics have been derived. However, such work has always been limited due to the inherently random and non-repeatable nature of natural lightning events in the field. Recent developments in lightning test facilities now allow the reproducible generation of lightning arcs within controlled laboratory environments, providing a test bed for the development of new sensors and diagnostic techniques to understand lightning mechanisms better. One such technique is a spectroscopic system using digital camera technology capable of identifying the chemical elements with which the lightning arc interacts, with these data then being used to derive further characteristics. In this paper, the spectroscopic system is used to obtain the emission spectrum from a 100 kA peak, 100 &#181;s duration lightning arc generated across a pair of hemispherical tungsten electrodes separated by a small air gap. To maintain a spectral resolution of less than 1 nm, several individual spectra were recorded across discrete wavelength ranges, averaged, stitched, and corrected to produce a final composite spectrum in the 450 nm (blue light) to 890 nm (near infrared light) range. Characteristic peaks within the data were then compared to an established publicly available database to identify the chemical element interactions. This method is readily applicable to a variety of other light emitting events, such as fast electrical discharges, partial discharges, and sparking in electrical equipment, apparatus, and systems.

Emission spectroscopy techniques have traditionally been used to analyze inherently random lightning arcs occurring in nature. In this paper, a method developed to obtain the emission spectroscopy from reproducible lightning arcs generated within a laboratory environment is described.

Lightning is one of the most common and destructive forces in nature and has long been studied using spectroscopic techniques, first with traditional ...

Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of nanowires. DEP is used for nanowire analysis, characterization, and for solution-based fabrication of semiconducting devices. The method works by applying an alternating electric field between metallic electrodes. The nanowire formulation is then dropped onto the electrodes which are on an inclined surface to create a flow of the formulation using gravity. The nanowires then align along the gradient of the electric field and in the direction of the liquid flow. The frequency of the field can be adjusted to select nanowires with superior conductivity and lower trap density. 
In this work, flow-assisted DEP is used to create nanowire field effect transistors. Flow-assisted DEP has several advantages: it allows selection of nanowire electrical properties; control of nanowire length; placement of nanowires in specific areas; control of orientation of nanowires; and control of nanowire density in the device. 
The technique can be expanded to many other applications such as gas sensors and microwave switches. The technique is efficient, quick, reproducible, and it uses a minimal amount of dilute solution making it ideal for the testing of novel nanomaterials. Wafer scale assembly of nanowire devices can also be achieved using this technique, allowing large numbers of samples for testing and large-area electronic applications.

In this paper, flow assisted dielectrophoresis is demonstrated for the self-assembly of nanowire devices. The fabrication of a silicon nanowire field effect transistor is shown as an example.

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of ...