Name: flame experiments at the advanced light source: new insights into soot formation processes
Uploaded: 2014-05-26T13:00:00
Duration: 10 min 4 s
Description: Watch this Scientific Journal Video about Flame Experiments at the Advanced Light Source: New Insights into Soot Formation Processes at JoVE.com

Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the National Nuclear Security Administration under contract DE-AC04-94-AL85000. The work was also supported by the U.S. Department of Energy, Office of Basic Energy Sciences under the Single Investigator Small Group Research project (Grant No. DE-SC0002619) of Prof. Violi (University of Michigan, Ann Arbor). KRW is supported by the Department of Energy, Office of Science, Early Career Research Program under U.S. Department of Energy Contract No. DE-AC02-05CH11231. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. KKH acknowledges continuing support of part of this research by the DFG under contract KO 1363/18-3.

1. Gas-phase Experiments
<ol>
	<li>Igniting the Low-pressure Premixed Flame
	<ol>
		<li>Make sure that the cooling water is flowing through the burner and flame chamber wall and that the flame chamber is pumped down to ~0.1 mbar.</li>
		<li>Establish a 1 L/min flow of argon and a 1.5 L/min flow of oxygen through the burner surface and maintain a pressure of 80 mbar (60 Torr) in the flame chamber.</li>
		<li>Position the hot-wire igniter over the burner surface; set the hydrogen flow to 0.4 L/min, and quickly activate the igniter.</li>
		<li>After ignition, turn off the hot-wire igniter, and reposition it away from the burner.</li>
		<li>Establish the desired flows of argon, oxygen, hydrogen, and fuel. Adjust the pressure to match the conditions of the target flame [typically&#160;20-40 mbar (15-30 Torr)]. Note: Flame conditions for individual flames are provided in the original literature. For example, flows for the stoichiometric allene and propyne flames are listed in Hansen&#160;et al22.</li>
	</ol>
	</li>
	<li>Acquisition of Photoionization Efficiency (PIE) Curves &#8211; Energy Scan
	<ol>
		<li>When the pressure in the ionization chamber is &#8804;10-6 mbar, apply the voltages to the ion optics of the time-of-flight mass spectrometer and microchannel plate detector and open the beamline valves. Note: Earlier calibration experiments (not shown in the video) were used to find the voltage settings for optimal performance of the mass spectrometer.</li>
		<li>Start the Labview data acquisition program &#8220;General Interface.vi&#8221; (Figure 2) and move the burner to the desired position using the &#8220;Motor&#8221; tab in the software. Note: This Labview code was developed at the beamline and is available upon request.</li>
		<li>Use the &#8220;General&#8221; tab to define the scan parameters, i.e., the number of steps per eV of photon energy. Typically, a step size of 0.05 eV is used.</li>
		<li>Use the &#8220;ALS&#8221; tab to set the photon energy to the desired start value, and define the &#8220;ALS Energy&#8221; to be &#8220;active&#8221;.</li>
		<li>On the &#8220;Control&#8221; panel, activate &#8220;K6485&#8221; to read out the photocurrent measured by the photodiode.</li>
		<li>On the &#8220;P7886&#8221; tab, use the &#8220;Set Parameters&#8221; button to set the number of sweeps (normally between 219 and 221), the number of bins (normally 48k), and the bin width (500 psec).</li>
		<li>Enter a valid file path and name, and click &#8220;Start&#8221; to start the computer-controlled data acquisition process.</li>
	</ol>
	</li>
	<li>Acquisition of Mass Spectra &#8211; Burner Scan
	<ol>
		<li>Apply the voltages as for the energy scans to the ion optics of the time-of-flight mass spectrometer and microchannel plate detector.</li>
		<li>Open the beamline valves to allow the photon beam in the chamber.</li>
		<li>Open the Labview data acquisition program &#8220;General Interface.vi&#8221;.</li>
		<li>Using the &#8220;Motor&#8221; tab in the software, use the &#8220;Jogger&#8221; to move the burner surface as close as possible to the sampling cone, and define that position as the &#8220;Origin&#8221;. Define the motor to be &#8220;active&#8221;.</li>
		<li>Use to &#8220;General&#8221; tab to define the scan parameters, i.e., the number of steps per mm of burner movement. Typical values are 0-5 mm in 20 steps, 5-20 mm in 15 steps, and 20-30 mm in 5 steps.</li>
		<li>Use the &#8220;ALS&#8221; tab to set the photon energy to the desired value. Typical values are&#160;8-16.65 eV.</li>
		<li>On the &#8220;Control&#8221; panel, activate &#8220;K6485&#8221; to read out the photocurrent measured by the photodiode.</li>
		<li>On the &#8220;P7886&#8221; tab, use the &#8220;Set Parameters&#8221; button to start a sub-VI (Figure 3) to set the number of sweeps, i.e., the number of mass spectra added on top of each other at every burner position (normally between 219 and 221), the number of bins (normally 48k), and the bin width (500 psec).</li>
		<li>Provide a valid file path and name, and click &#8220;Start&#8221; to begin the automated data acquisition process.</li>
	</ol>
	</li>
</ol>
<img alt="Figure 2" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig2highres.jpg" width="500" /> 
Figure 2.&#160;Graphical user interface of the data acquisition program. <a href="https://www.jove.com/files/ftp_upload/51369/51369fig2highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig3highres.jpg" width="500" /> 
Figure 3.&#160;Graphical user interface to input multichannel scaler parameters. <a href="https://www.jove.com/files/ftp_upload/51369/51369fig3highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
2. Aerosol Experiments
<ol>
	<li>Igniting the Near-atmospheric Pressure Opposed-flow Flame
	<ol>
		<li>Make sure that the cooling water is flowing through the burner, that the reactant outlets are approx. 10 mm apart, and that the flame chamber is pumped down to the minimum achievable pressure (~2 mbar).</li>
		<li>Fill the flame chamber with argon and bring the pressure up to ~860 mbar (650 Torr).</li>
		<li>Place the igniter coil approx. in the center of the two burner outlets.</li>
		<li>Set the gas flows as follows (Ar should already be flowing because it was used to fill chamber): Oxidizer stream: O2 0.3 L/min and Ar 1.6 L/min; Oxidizer stream coflow: Ar 2.5 L/min; Fuel stream: H2 0.3 L/min, Ar 2.5 L/min; Fuel stream coflow: Ar 2.5 L/min.</li>
		<li>Open the hydrogen and oxygen valves, and immediately switch on igniter coil.</li>
		<li>Once the flame is ignited, turn off the igniter coil, and retract it.</li>
		<li>Establish the desired flows of oxygen, argon, and fuel. Turn off the flow of hydrogen, and set pressure and reactant outlet separation to desired values for target flame. Note: The flow rates for the propane flame, which is shown in the video, are provided in Skeen&#160;et al4.</li>
	</ol>
	</li>
	<li>Acquisition of an Aerosol Mass Spectrum
	<ol>
		<li>Apply the appropriate voltages to the ion optics and detector of the AMS. Note: The voltage settings for an optimal performance were found in earlier calibration experiments, which are not shown in the video.</li>
		<li>Open the Labview data acquisition program &#8220;General Interface-Opposed-Flow.vi&#8221;. Note: This VI is a modification of the &#8220;General Interface.vi&#8221;, in which the motor control has been updated to match the new needs.</li>
		<li>Use the &#8220;Jogger&#8221; application on the &#8220;Motor&#8221; tab to translate the opposed-flow burner so that the quartz microprobe is at the position nearest to the fuel stream outlet (Figure 4). While at this position, reset the motor step position to zero. This procedure defines the &#8220;origin&#8221; position.</li>
		<li>Slowly open the quarter-turn ball valve allowing flow from the flame-sampling line into the aerodynamic lens (ADL) system. Confirm that the pressure at the outlet of the ADL is near 1 x 102 mbar.</li>
		<li>Use the &#8220;General&#8221; tab to define the scan parameters, i.e., the number of steps per mm of burner movement (burner scan) or photon energy (energy scan).</li>
		<li>Use the &#8220;ALS&#8221; tab to set the desired photon energy, and use the &#8220;Motor&#8221; tab to move the burner to the desired burner position.</li>
		<li>Use the &#8220;P7886&#8221; tab and the &#8220;Set Parameters&#8221; button therein to set the acquisition parameters.</li>
		<li>Define &#8220;Motor&#8221; (burner scan) or &#8220;ALS Energy&#8221; (energy scan) to be &#8220;active&#8221;.</li>
		<li>Insert a valid file path and name in the appropriate fields, and click &#8220;Start&#8221;. Aerosol mass spectra are now automatically taken.</li>
	</ol>
	</li>
</ol>
<img alt="Figure 4" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig4highres.jpg" width="500" /> 
Figure 4.&#160;Graphical user interface for burner movement for the opposed-flow flame assembly. <a href="https://www.jove.com/files/ftp_upload/51369/51369fig4highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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The authors have nothing to disclose.

The described combination of flame-sampling and synchrotron-based VUV single-photon ionization with mass spectrometry provides the most detailed look into the chemical composition of laboratory-based model flames currently possible. The mass spectrometer provides universal detection of all sampled flame species simultaneously with high sensitivity (ppm range) over a wide dynamic range. Instrumental for the success of this technique is the use of synchrotron-generated VUV photons, whose energies can easily be tuned, to provide good selectivity between isomers and control of fragmentation. The latter factor is important when analyzing complex mixtures. The capabilities of the described experiment are unmatched by gas chromatography, which is commonly used for isomer-separation, and by conventional ionization techniques using energetic electrons. Limitations of the synchrotron-based technique arise from the fact that, especially for larger mass-to-charge ratios, many different isomers are conceivable, which then cannot be uniquely identified, and their contributions cannot be separated reliably1. The experimental results, in the form of isomer-resolved flame compositions, may yield improved kinetic models of combustion chemistry at an exceptionally detailed molecular level.
The described experiments are very complicated and a description of the troubleshooting procedures is beyond of what can be documented in the video and/or the protocol section of this manuscript. This fact is also true for the data analysis procedures. Modifications to the experimental set-up are normally done off-line in between the allocated &#8220;beamtime&#8221;. Because the emphasis of these experiments is on the quantitative determination of combustion intermediates, it is very critical to have stable and reproducible flames. Furthermore, it is necessary to wisely choose the photon energies and the other scan parameters to obtain an adequate set of experimental data that is sufficient for a reliable determination of the flame structure.
The flame experiments performed at the Advanced Light Source have successfully contributed to unravel the chemistry of benzene formation in hydrocarbon flames7.&#160;A prominent role of resonance-stabilized radicals as precursors has been established, for example, with the identification of the propargyl, allyl, and i-C4H5 radicals.
Because benzene formation is thought to be only the first step in the overall soot-formation process, additional efforts are under way at the Advanced Light Source to identify the chemical composition of flame-sampled soot particles. Compared to similar previous soot-sampling experiments28, this newly established aerosol-sampling experiment allows for recording near-threshold mass spectra, meaning that the photon energy can be tuned precisely to be only slightly above the components&#8217; ionization energies, thus avoiding fragmentation. Furthermore, fragmentations are also largely avoided by employing the process of flash-vaporization on the temperature-controlled copper block. However, the experiment is currently limited by not being able to provide quantitative data. Also, the recorded mass spectra are not particle specific, but averaged over many particles probably varying in composition and size. In addition, condensation can and does occur in the sampling probe, complicating the identification of species associated with particles in the flame. Furthermore, the species detected must be volatile enough to be vaporized at the temperature of the copper block (300-400 &#176;C) under vacuum. Nevertheless, the early qualitative data suggests that the compositions of soot precursor species are dependent on the chemical structure of the fuel and that soot-precursor-formation mechanisms are kinetically driven as opposed to thermodynamically. The aerosol mass spectrometry efforts are currently at the early stages, and the insights gained so far identify more research opportunities.
Future work on soot-formation processes is likely to focus on the chemistry beyond the first aromatic ring, i.e., the formation of indene, naphthalene, anthracene, etc, and their isomers. The ultimate goal is to understand the chemistry (and physics) of particle inception, and to develop a predictive model that can describe the entire soot-formation process (from fuel oxidation to particle coagulation).

Establishing a consistent and predictive mechanism for molecular growth and soot formation processes is one of the greatest challenges in combustion chemistry research8,9. Combustion processes account for over half of the fine particle air pollution (PM2.5 &#8211; fine particles defined by an aerodynamic diameter of &#8804;2.5 &#956;m), and, to reduce the emission of these unwanted combustion byproducts, it is important to know their identities, concentrations, and formation pathways10. The nature of the combustion byproducts is influenced by the fuel and the conditions under which it is burned. Many studies have linked combustion emissions to acute environmental and health effects11-13. For example, combustion-generated particles have a strong influence on the air quality, atmospheric visibility, and the radiative balance of the Earth&#8217;s atmosphere. It is assumed that the chemical composition of the airborne combustion-generated particles determines their toxicity, which is commonly associated with polycyclic aromatic hydrocarbons (PAHs). The latter species are considered to be the molecular precursors of soot, and they are formed in incomplete combustion processes. Again, to identify these processes is still a challenging problem.
Generally speaking, the combustion reactions, which are at the origin of these emissions, follow complicated fuel decomposition and oxidation pathways, involving many different reactive species. They are connected within a network of hundreds or even thousands of reactions whose rates depend on temperature and pressure14,15.
Laminar, premixed, burner-stabilized flat flames, which can be established at pressures as low as 20-80 mbar (15-60 Torr), represent one of the standard combustion environments commonly used to unravel this complex chemical network and to investigate the pollutant potential of any given prototypical fuel16. In this configuration, the fuel and the oxidizer are already mixed when they reach the flame front; thus, the rate of combustion is dominated by chemical processes and not by mixing. By operating these flames at a sub-atmospheric pressure, the physical thickness of the reaction region is increased, allowing for improved spatial resolution of temperature and concentration gradients with laser-based or probe-sampling techniques1,17.
In order to precisely analyze the chemical composition of such flames, an analytical tool is required that provides universal detection of all species simultaneously, high sensitivity and dynamic range, good selectivity between isomers, and control of molecular fragmentation. A breakthrough in combustion-chemistry research was achieved with the use of flame-sampling mass spectrometry at synchrotron light sources where tunable vacuum-ultraviolet (VUV) radiation is used for near-threshold single-photon ionization5,6. In the flame experiments at the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory, which are shown in the accompanying video, gas samples are withdrawn from within the premixed flames by a quartz cone, expanded into higher vacuum, and ionized by VUV photons1,5. The experimental set-up is shown schematically in Figure 1. The key to the success of this experiment has been the ability to tune the energy of the ionizing photons in an appropriate range to minimize or even avoid photofragmentation and to allow isomer specificity1,3,5,18. As shown in the video, photoionization efficiency (PIE) curves can be recorded by tuning the photon energy19, which allow us to identify specific isomeric species in the complicated flame mixture. The PIE curves for individual species generally have distinct features, i.e., ionization thresholds, shapes, and intensities. The video also shows the experimental approach used to determine mole-fraction profiles of the individual components as a function of the distance to the burner surface.
These ALS-based combustion experiments have been focused on soot-formation processes in hydrocarbon flames and on the oxidation of oxygenated, next-generation, bio-derived fuels1,20. With regards to the soot-formation problem, the experiments revealed many new insights. In summary, it is now understood that the chemical structure of the fuel influences the identity (and the quantity) of the precursor molecules and that consequently many different pathways can contribute to the first step of the overall soot-formation process7,21.
Even deeper insights into the soot-formation chemistry were gained when identifying the chemical components of flame-generated soot nanoparticles with an ALS-based aerosol mass spectrometer. In this new experiment, which is explained in the second half of the video, non-premixed (diffusion) flames are used. The experimental setup is also shown in Figure 1. In this configuration, a flame is established at near-atmospheric pressure [933 mbar (700 Torr)] between two opposed laminar jets of fuel and oxidizer. Because the fuel and oxidizer streams remain separated outside the reaction zone, this configuration provides a good opportunity to examine molecular growth processes. Flame-generated particles are withdrawn from the flame using a quartz microprobe and subsequently focused with an aerodynamic lens system onto a heated copper target, where the particles flash vaporize and break apart into their individual constituents. These molecular building blocks are then ionized by the VUV photons from the ALS, and the corresponding ions are mass selected4. Not all the necessary work can be shown in the video, but the aerosol data suggest that the soot-formation mechanisms might be kinetically and not thermodynamically controlled. Furthermore, the data also indicate that the widely accepted H-abstraction-C2H2-addition (HACA) mechanism, in which small aromatic species grow to larger polycyclic aromatic hydrocarbons (PAHs) by a repetitive sequence of H-abstraction and C2H2-addition reactions, cannot explain all the observed particle constituents.
Combined with the video, the following protocol details the data acquisition procedures.
<img alt="Figure 1" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig1highres.jpg" width="500" /> 
Figure 1.&#160;Schematic diagram of the flame-sampling molecular beam and aerosol mass spectrometry experiments at the Advanced Light Source of the Lawrence Berkeley National Laboratory. With permissions from Refs. 2 and 4. <a href="https://www.jove.com/files/ftp_upload/51369/51369fig1highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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			<td height="42" style="height:42px;width:216px;">Name of Material/ Equipment</td>
			<td style="width:71px;">Company</td>
			<td style="width:119px;">Catalog Number</td>
			<td style="width:167px;">Comments/Description</td>
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			<td height="42" style="height:42px;width:216px;">flame-sampling mass spectrometer</td>
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			<td style="width:119px;"></td>
			<td>custom-built</td>
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			<td height="21" style="height:21px;width:216px;">aerosol mass spectrometer</td>
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			<td>custom-built</td>
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flame experiments at the advanced light source: new insights into soot formation processes

The following experimental protocols and the accompanying video are concerned with the flame experiments that are performed at the Chemical Dynamics Beamline of the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory1-4. This video demonstrates how the complex chemical structures of laboratory-based model flames are analyzed using flame-sampling mass spectrometry with tunable synchrotron-generated vacuum-ultraviolet (VUV) radiation. This experimental approach combines isomer-resolving capabilities with high sensitivity and a large dynamic range5,6. The first part of the video describes experiments involving burner-stabilized, reduced-pressure (20-80 mbar) laminar premixed flames. A small hydrocarbon fuel was used for the selected flame to demonstrate the general experimental approach. It is shown how species&#8217; profiles are acquired as a function of distance from the burner surface and how the tunability of the VUV photon energy is used advantageously to identify many combustion intermediates based on their ionization energies. For example, this technique has been used to study gas-phase aspects of the soot-formation processes, and the video shows how the resonance-stabilized radicals, such as C3H3, C3H5, and i-C4H5, are identified as important intermediates7. The work has been focused on soot formation processes, and, from the chemical point of view, this process is very intriguing because chemical structures containing millions of carbon atoms are assembled from a fuel molecule possessing only a few carbon atoms in just milliseconds. The second part of the video highlights a new experiment, in which an opposed-flow diffusion flame and synchrotron-based aerosol mass spectrometry are used to study the chemical composition of the combustion-generated soot particles4. The experimental results indicate that the widely accepted H-abstraction-C2H2-addition (HACA) mechanism is not the sole molecular growth process responsible for the formation of the observed large polycyclic aromatic hydrocarbons (PAHs).

A typical mass spectrum of flame-sampled gases from the low-pressure premixed burner is shown in Figure 5. The identities of the species contributing to the signal are revealed by the flame-sampled photoionization efficiency (PIE) curves for each mass-to-charge (m/z) ratio and their comparison to known isomer-specific ionization energies and PIE curves. Typical examples of flame-sampled PIE curves are shown in Figure 6 for m/z=39 (C3H3) and 41 (C3H5). The data are taken from a stoichiometric propyne flame22. The signal is unambiguously identified by their characteristic ionization thresholds to originate from the resonantly stabilized propargyl and allyl radicals. For many m/z values, multiple isomers are routinely identified by observing multiple thresholds. Many examples have been discussed extensively in the literature, for example, m/z=40 (allene and propyne), 44 (ethenol and acetaldehyde), 54 (1,3-butadiene, 1-butyne, and 2-butyne), or 78 (fulvene and benzene)23-27.
<img alt="Figure 5" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig5highres.jpg" width="500" /> 
Figure 5.&#160;Time-of-flight mass spectrum. Recorded with photons of 9.9 eV at 1.25 mm distance from the low-pressure premixed burner in a stoichiometric propyne-O2 flame. All peaks can be easily assigned to various combustion intermediates as outlined in the next step.
<img alt="Figure 6" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig6highres.jpg" width="500" /> 
Figure 6.&#160;Flame-sampled photoionization efficiency curves for m/z=39 and 41. The resonantly stabilized radicals propargyl and allyl can unambiguously be identified based on the observed ionization thresholds.
Once the isomeric composition is known, mass spectra are taken at various photon energies and from within different positions in the flame, as described above in the Protocol Section, to allow for the determination of isomer-specific mole fraction profiles of the individual species as a function of distance from the burner surface. Representative mole-fraction profiles of fulvene and benzene in a stoichiometric propyne flame from the low-pressure premixed burner are shown in Figure 7&#160;22.&#160;For each flame, typically a total of 40 to 50 individual mole-fraction profiles are determined for species ranging from m/z=1 (H atom) to m/z=78 (benzene and/or fulvene) or even higher, depending on the scientific goals. These mole fraction profiles are then used to assess the predictive capabilities of combustion-chemistry models and to validate them.
<img alt="Figure 7" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig7highres.jpg" width="500" /> 
Figure 7.&#160;Experimental mole fraction profiles.&#160;Profiles of fulvene and benzene in a stoichiometric propyne flame from the low-pressure premixed burner.
A typical aerosol mass spectrum is shown in Figure 8. It was taken from within a propane opposed-flow diffusion flame. Ion signal was observed for species with m/z ratios ranging from 150 to 600, with a peak around m/z=226. It is beyond the current experimental capabilities to identify all of the species observed in the mass spectrum or to unravel their possible formation pathways. Taking such mass spectra as a function of distance from the fuel outlet as described above (and shown in the video) provides spatially resolved profiles. A representative example is shown in the inlet of Figure 8 for the m/z=256 (C20H16) species. Similar profiles can be obtained for any of the other species as well, which consequently could be used as validation targets for any combustion-chemistry model.
<img alt="Figure 8" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig8highres.jpg" width="500" /> 
Figure 8.&#160;Flame-sampled aerosol mass spectrum from a propane-O2 opposed-flow diffusion flame. The inlet shows a representative spatially resolved profile for the C20H16 species at m/z=256.

Watch this Scientific Journal Video about Flame Experiments at the Advanced Light Source: New Insights into Soot Formation Processes at JoVE.com

Flame Experiments at the Advanced Light Source: New Insights into Soot Formation Processes

Gas sampling from laboratory-scale flames with online analysis of all species by mass spectrometry is a powerful method to investigate the complex mixture of chemical compounds occurring during combustion processes. Coupled with tunable soft ionization via synchrotron-generated vacuum-ultraviolet radiation, this technique provides isomer-resolved information and potentially fragment-free mass spectra.

The following experimental protocols and the accompanying video are concerned with the flame experiments that are performed at the Chemical Dynamics ...

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Research

JoVE Journal

Engineering

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.
The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.
To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 &mu;m thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm2, which can be increased up to 17-18 mA/cm2 (~25% higher) after application of a simple diffuse back reflector made of a white paint.
To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23&deg;C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF2, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF2, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm2, up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.
Introduction
Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) 1 and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) 2. For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector 3, silicon plasma texturing 4 or high refractive index nanoparticle reflector 5 have been suggested.
Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect 6. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment 6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested 8. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles 9. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance 10.
The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF2 or SiO2, from the rear reflector to avoid mechanical and chemical damage 7. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric 6. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.

Polycrystalline silicon thin-film solar cells on glass are fabricated by deposition of boron and phosphorous doped silicon layers followed by crystallisation, defect passivation and metallisation. Plasmonic light-trapping is introduced by forming Ag nanoparticles on the silicon cell surface capped with a diffused reflector resulting in ~45% photocurrent enhancement.

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To ...

Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures

The physical properties of a material are defined by its electronic structure. Electrons in solids are characterized by energy (&omega;) and momentum (k) and the probability to find them in a particular state with given &omega; and k is described by the spectral function A(k, &omega;). This function can be directly measured in an experiment based on the well-known photoelectric effect, for the explanation of which Albert Einstein received the Nobel Prize back in 1921. In the photoelectric effect the light shone on a surface ejects electrons from the material. According to Einstein, energy conservation allows one to determine the energy of an electron inside the sample, provided the energy of the light photon and kinetic energy of the outgoing photoelectron are known. Momentum conservation makes it also possible to estimate k relating it to the momentum of the photoelectron by measuring the angle at which the photoelectron left the surface. The modern version of this technique is called Angle-Resolved Photoemission Spectroscopy (ARPES) and exploits both conservation laws in order to determine the electronic structure, i.e. energy and momentum of electrons inside the solid. In order to resolve the details crucial for understanding the topical problems of condensed matter physics, three quantities need to be minimized: uncertainty* in photon energy, uncertainty in kinetic energy of photoelectrons and temperature of the sample.
In our approach we combine three recent achievements in the field of synchrotron radiation, surface science and cryogenics. We use synchrotron radiation with tunable photon energy contributing an uncertainty of the order of 1 meV, an electron energy analyzer which detects the kinetic energies with a precision of the order of 1 meV and a He3 cryostat which allows us to keep the temperature of the sample below 1 K. We discuss the exemplary results obtained on single crystals of Sr2RuO4 and some other materials. The electronic structure of this material can be determined with an unprecedented clarity.

The overall goal of this method is to determine the low-energy electronic structure of solids at ultra-low temperatures using Angle-Resolved Photoemission Spectroscopy with synchrotron radiation.

The physical properties of a material are defined by its  electronic structure. Electrons in solids are characterized by energy (&omega;)  and momentum ...

Integrating a Triplet-triplet Annihilation Up-conversion System to Enhance Dye-sensitized Solar Cell Response to Sub-bandgap Light

The poor response of dye-sensitized solar cells (DSCs) to red and infrared light is a significant impediment to the realization of higher photocurrents and hence higher efficiencies. Photon up-conversion by way of triplet-triplet annihilation (TTA-UC) is an attractive technique for using these otherwise wasted low energy photons to produce photocurrent, while not interfering with the photoanodic performance in a deleterious manner. Further to this, TTA-UC has a number of features, distinct from other reported photon up-conversion technologies, which renders it particularly suitable for coupling with DSC technology. In this work, a proven high performance TTA-UC system, comprising a palladium porphyrin sensitizer and rubrene emitter, is combined with a high performance DSC (utilizing the organic dye D149) in an integrated device. The device shows an enhanced response to sub-bandgap light over the absorption range of the TTA-UC sub-unit resulting in the highest figure of merit for up-conversion assisted DSC performance to date.

An integrated device, incorporating a dye-sensitized solar cell and triplet-triplet annihilation up-conversion unit was produced, affording enhanced light harvesting, from a wider section of the solar spectrum. Under modest irradiation levels a significantly enhanced response to low energy photons was demonstrated, yielding a record figure of merit for dye-sensitized solar cells.

The poor response of dye-sensitized solar cells (DSCs) to red and infrared light is a significant impediment to the realization of higher photocurrents ...

Failure Analysis of Batteries Using Synchrotron-based Hard X-ray Microtomography

Imaging morphological changes that occur during the lifetime of rechargeable batteries is necessary to understand how these devices fail. Since the advent of lithium-ion batteries, researchers have known that the lithium metal anode has the highest theoretical energy density of any anode material. However, rechargeable batteries containing a lithium metal anode are not widely used in consumer products because the growth of lithium dendrites from the anode upon charging of the battery causes premature cell failure by short circuit. Lithium dendrites can also form in commercial lithium-ion batteries with graphite anodes if they are improperly charged. We demonstrate that lithium dendrite growth can be studied using synchrotron-based hard X-ray microtomography. This non-destructive imaging technique allows researchers to study the growth of lithium dendrites, in addition to other morphological changes inside batteries, and subsequently develop methods to extend battery life.

Synchrotron-based hard X-ray microtomography is used to image the electrochemical growth of dendrites from a lithium metal electrode through a solid polymer electrolyte membrane.

Imaging morphological changes that occur during the lifetime of rechargeable batteries is necessary to understand how these devices fail. Since the advent ...

Measurement of X-ray Beam Coherence along Multiple Directions Using 2-D Checkerboard Phase Grating

A procedure for a technique to measure the transverse coherence of synchrotron radiation X-ray sources using a single phase grating interferometer is reported. The measurements were demonstrated at the 1-BM bending magnet beamline of the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). By using a 2-D checkerboard &#960;/2 phase-shift grating, transverse coherence lengths were obtained along the vertical and horizontal directions as well as along the 45&#176; and 135&#176; directions to the horizontal direction. Following the technical details specified in this paper, interferograms were measured at different positions downstream of the phase grating along the beam propagation direction. Visibility values of each interferogram were extracted from analyzing harmonic peaks in its Fourier Transformed image. Consequently, the coherence length along each direction can be extracted from the evolution of visibility as a function of the grating-to-detector distance. The simultaneous measurement of coherence lengths in four directions helped identify the elliptical shape of the coherence area of the Gaussian-shaped X-ray source. The reported technique for multiple-direction coherence characterization is important for selecting the appropriate sample size and orientation as well as for correcting the partial coherence effects in coherence scattering experiments. This technique can also be applied for assessing coherence preserving capabilities of X-ray optics.

The measurement protocol and data analysis procedure are given for obtaining transverse coherence of a synchrotron radiation X-ray source along four directions simultaneously using a single 2-D checkerboard phase grating. This simple technique can be applied for complete transverse coherence characterization of X-ray sources and X-ray optics.

A procedure for a technique to measure the transverse coherence of synchrotron radiation X-ray sources using a single phase grating interferometer is ...

Using Synchrotron Radiation Microtomography to Investigate Multi-scale Three-dimensional Microelectronic Packages

Synchrotron radiation micro-tomography (SR&#181;T) is a non-destructive three-dimensional (3D) imaging technique that offers high flux for fast data acquisition times with high spatial resolution. In the electronics industry there is serious interest in performing failure analysis on 3D microelectronic packages, many which contain multiple levels of high-density interconnections. Often in tomography there is a trade-off between image resolution and the volume of a sample that can be imaged. This inverse relationship limits the usefulness of conventional computed tomography (CT) systems since a microelectronic package is often large in cross sectional area 100-3,600 mm2, but has important features on the micron scale. The micro-tomography beamline at the Advanced Light Source (ALS), in Berkeley, CA USA, has a setup which is adaptable and can be tailored to a sample&#39;s properties, i.e., density, thickness, etc., with a maximum allowable cross-section of 36 x 36 mm. This setup also has the option of being either monochromatic in the energy range ~7-43 keV or operating with maximum flux in white light mode using a polychromatic beam. Presented here are details of the experimental steps taken to image an entire 16 x 16 mm system within a package, in order to obtain 3D images of the system with a spatial resolution of 8.7 &#181;m all within a scan time of less than 3 min. Also shown are results from packages scanned in different orientations and a sectioned package for higher resolution imaging. In contrast a conventional CT system would take hours to record data with potentially poorer resolution. Indeed, the ratio of field-of-view to throughput time is much higher when using the synchrotron radiation tomography setup. The description below of the experimental setup can be implemented and adapted for use with many other multi-materials.

For this study synchrotron radiation micro-tomography, a non-destructive three-dimensional imaging technique, is employed to investigate an entire microelectronic package with a cross-sectional area of 16 x 16 mm. Due to the synchrotron&#39;s high flux and brightness the sample was imaged in just 3 min&#160;with an 8.7 &#181;m spatial resolution.

Synchrotron radiation micro-tomography (SR&#181;T) is a non-destructive three-dimensional (3D) imaging technique that offers high flux for fast data ...

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices have used devices prepared by spin coating, a process that is not amenable to large-scale fabrication. This mismatch in device fabrication makes it difficult to translate quantitative results obtained in the laboratory to the commercial level, making optimization difficult. Using a mini-slot die coater, this mismatch can be resolved by translating the commercial process to the laboratory and characterizing the structure formation in the active layer of the device in real time and in situ as films are coated onto a substrate. The evolution of the morphology was characterized under different conditions, allowing us to propose a mechanism by which the structures form and grow. This mini-slot die coater offers a simple, convenient, material efficient route by which the morphology in the active layer can be optimized under industrially relevant conditions. The goal of this protocol is to show experimental details of how a solar cell device is fabricated using a mini-slot die coater and technical details of running in situ structure characterization using the mini-slot die coater.

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Here, we present a protocol to fabricate organic thin film solar cells using a mini-slot die coater and related in-line structure characterizations using synchrotron scattering techniques.

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices ...

Studying Dynamic Processes of Nano-sized Objects in Liquid using Scanning Transmission Electron Microscopy

Samples fully embedded in liquid can be studied at a nanoscale spatial resolution with Scanning Transmission Electron Microscopy (STEM) using a microfluidic chamber assembled in the specimen holder for Transmission Electron Microscopy (TEM) and STEM. The microfluidic system consists of two silicon microchips supporting thin Silicon Nitride (SiN) membrane windows. This article describes the basic steps of sample loading and data acquisition. Most important of all is to ensure that the liquid compartment is correctly assembled, thus providing a thin liquid layer and a vacuum seal. This protocol also includes a number of tests necessary to perform during sample loading in order to ensure correct assembly. Once the sample is loaded in the electron microscope, the liquid thickness needs to be measured. Incorrect assembly may result in a too-thick liquid, while a too-thin liquid may indicate the absence of liquid, such as when a bubble is formed. Finally, the protocol explains how images are taken and how dynamic processes can be studied. A sample containing AuNPs is imaged both in pure water and in saline.

This protocol describes the operation of a liquid flow specimen holder for scanning transmission electron microscopy of AuNPs in water, as used for the observation of nanoscale dynamic processes.

Samples fully embedded in liquid can be studied at a nanoscale spatial resolution with Scanning Transmission Electron Microscopy (STEM) using a ...

Experimental Implementation of a New Composite Fabrication Method: Exposing Bare Fibers on the Composite Surface by the Soft Layer Method

The bipolar plate is a key component in proton exchange membrane fuel cells (PEMFCs) and vanadium redox flow batteries (VRFBs). It is a multi-functional component that should have high electrical conductivity, high mechanical properties, and high productivity.
In this regard, a carbon fiber/epoxy resin composite can be an ideal material to replace the conventional graphite bipolar plate, which often leads to the catastrophic failure of the entire system because of its inherent brittleness. Though the carbon/epoxy composite has high mechanical properties and is easy to manufacture, the electrical conductivity in the through-thickness direction is poor because of the resin-rich layer that forms on its surface. Therefore, an expanded graphite coating was adopted to solve the electrical conductivity issue. However, the expanded graphite coating not only increases the manufacturing costs but also has poor mechanical properties.
In this study, a method to expose fibers on the composite surface is demonstrated. There are currently many methods that can expose fibers by surface treatment after the fabrication of the composite. This new method, however, does not require surface treatment because the fibers are exposed during the manufacture of the composite. By exposing bare carbon fibers on the surface, the electrical conductivity and mechanical strength of the composite are increased drastically.

A protocol to expose bare fibers on the composite surface by eliminating resin rich area is presented. The fibers are exposed during fabrication of the composites, not by the post surface treatment. The exposed carbon composites exhibit high electrical conductivity in the through-thickness direction and high mechanical property.

The bipolar plate is a key component in proton exchange membrane fuel cells (PEMFCs) and vanadium redox flow batteries (VRFBs). It is a multi-functional ...

Synchrotron X-ray Microdiffraction and Fluorescence Imaging of Mineral and Rock Samples

In this report, we describe a detailed procedure for acquiring and processing&#160;x-ray microfluorescence (&#956;XRF), and Laue and powder microdiffraction two-dimensional (2D) maps at beamline 12.3.2 of the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. Measurements can be performed on any sample that is less than 10 cm x 10 cm x 5 cm, with a flat exposed surface. The experimental geometry is calibrated using standard materials (elemental standards for XRF, and crystalline samples such as Si, quartz, or Al2O3 for diffraction). Samples are aligned to the focal point of the x-ray microbeam, and raster scans are performed, where each pixel of a map corresponds to one measurement, e.g., one XRF spectrum or one diffraction pattern. The data are then processed using the in-house developed software XMAS, which outputs text files, where each row corresponds to a pixel position. Representative data from moissanite and an olive snail shell are presented to demonstrate data quality, collection, and analysis strategies.

We describe a beamline setup meant to carry out rapid two-dimensional x-ray fluorescence and x-ray microdiffraction mapping of single crystal or powder samples using either Laue (polychromatic radiation) or powder (monochromatic radiation) diffraction. The resulting maps give information about strain, orientation, phase distribution, and plastic deformation.