We would like to acknowledge Dr. Felix Castellano (BGSU) and N. R. Neal for advice and valuable discussions. We gratefully acknowledge OBOR "Material Networks" program and Bowling Green State University for financial support. This work was partly supported by the NSF under Award CHE - 1112227.

1. Synthesis of ZnSe Core Nanocrystals8
<ol>
 <li>Place 7.0 g ODA and a magnetic stir bar into a three neck flask.</li>
 <li>In a separate flask, combine 0.063 g Se and 2.4 ml TOP and add a magnetic stir bar. The mixture of TOP and selenium should be degassed under vacuum for 30 min.</li>
 <li>Degas ODA for 90 min at 120 &deg;C, then put under Ar flow with a wide glass exhaust.</li>
 <li>Heat ODA to 300 &deg;C and, and inject Se mixture. Let temperature return to 300 &deg;C.</li>
 <li>Inject 1.0 ml of Et2Zn (10% by wt. in hexane) to the reaction flask and allow to react at 265 &deg;C for around 3 min, or until the exciton absorbance peak shifts to a desired wavelength (&lambda;=350-400 nm), upon which remove the flask from the heating mantle.</li>
 <li>Once the temperature of the reaction flask drops to ~60 &deg;C, add 12 ml methanol and split between two 15-ml centrifuge tubes, topping off with methanol. Centrifuge for 5 min and pour off the liquid phase. Redissolve the precipitated nanocrystals in toluene and repeat.</li>
</ol>
2. Growth of CdS Rods on ZnSe Cores9
<ol>
 <li>Combine 3.0 g TOPO, 0.280 g ODPA, 0.080 g HPA, and 0.090 g CdO and add a magnetic stir bar in a three neck flask.</li>
 <li>In a separate flask, combine 0.120 g S, and 4.0 ml TOP and add a magnetic stir bar.</li>
 <li>Degas CdO solution for 45 min at 150 &deg;C and TOP for 45 min at 120 &deg;C, then put under Ar flow with wide glass exhausts.</li>
 <li>Heat the CdO solution to 380 &deg;C until CdO is dissolved and solution is clear and colorless. Meanwhile, heat the S solution to 120 &deg;C until S is dissolved and the mixture is clear and colorless.</li>
 <li>Add all of the ZnSe from step 1 to the S solution.</li>
 <li>Add 2.0 ml of TOP to the Cd solution and let the temperature return to 380 &deg;C. Once 380 &deg;C is reached, immediately inject the S solution into the Cd solution.</li>
 <li>Allow the nanorods to grow for 6-9 min, including the temperature recovery time, and remove the flask from the heating mantle. The longer the solution is left on the heat, the longer the rods will be.</li>
 <li>Product may be a green gel, add chloroform to liquefy and split into two vials.</li>
 <li>Precipitate nanocrystals in ethanol, pour off the liquid phase, and redissolve the precipitated nanocrystals in chloroform.</li>
</ol>
3. Growth of Pt Tip on CdS Rods10
<ol>
 <li>Combine 0.2 ml OA, 0.2 ml oleylamine, 10 ml diphenyl ether, and 43 mg 1,2-hexadecanediol and add a magnetic stir bar in a flask.</li>
 <li>Degas the mixture for 1 hr at 80 &deg;C, then put under Ar flow and increase temp to 200 &deg;C.</li>
 <li>Add a mixture of CdS rods in chloroform and 20 mg Pt (II) acetylacetonate. After 5-7 min of the reaction mixture being at 190 &deg;C, the solution will turn black. Remove the flask from the heat.</li>
 <li>Precipitate nanocrystals in a 10:3 mixture of methanol to chloroform with a centrifuge and pour off the liquid phase. Redissolve the precipitated crystals in chloroform and repeat.</li>
</ol>
4. Ligand Exchange with MUA11
<ol>
 <li>In a vial, disperse nanorods in 10 ml chloroform.</li>
 <li>Add 0.1 g MUA to the nanorod solution and sonicate until all the MUA is dissolved.</li>
 <li>In a separate vial, dissolve 0.1 g KOH in 20 ml triple distilled water (TDW).</li>
 <li>Add approximately 5.0 ml of the TDW solution to the nanorod solution and shake vigorously.</li>
 <li>Centrifuge the mixture from step 4.4 at 6000 rpm for 1 min to separate the aqueous and organic phases.</li>
 <li>Collect the top (aqueous) phase and add methanol to achieve a solution that is 3:1 methanol to water.</li>
 <li>Centrifuge the mixture from step 4.6 at 6000 rpm for 2 min to precipitate nanocrystals. Pour off the liquid phase and redisperse the precipitated crystals by sonicating in TDW.</li>
 <li>If the organic phase in step 4.5 still retains some of the color of the original nanocrystal solution, repeat steps 4.4 through 4.7.</li>
</ol>
5. Synthesis of PbS Cores (Adapted from ref.12)
<ol>
 <li>Combine 0.49 g PbO, 18 ml ODE, and 1-16 ml OA (Depending on desired size, more OA yields larger particles) and add a magnetic stir bar in a three neck flask.</li>
 <li>In a separate flask, add 10 ml ODE and a magnetic stir bar.</li>
 <li>Degas both flasks for 1 hr at 120 &deg;C, then put under Ar flow.</li>
 <li>Heat the Pb solution to 135 &deg;C. Meanwhile, cool the flask containing only ODE to room temperature.</li>
 <li>Add 0.21 ml TMS to the room temperature ODE, then inject the mixture into the Pb solution at 135 &deg;C.</li>
 <li>Heat the mixture at 135 &deg;C for 1-5 min (depending on desired size, longer heating yields larger particles), and place into an ice bath to quench the reaction.</li>
 <li>Precipitate nanocrystals in distilled acetone, pour off the liquid phase, and redissolve precipitated crystals in toluene. Repeat two more times.</li>
</ol>
6. Growth of CdS Shell on PbS Cores13
<ol>
 <li>Combine 1.0 g CdO, 6 ml OA, and 15 ml ODE and add a magnetic stir bar in a three neck flask.</li>
 <li>In a separate flask add 20-40 mg of PbS cores dissolved in toluene and a magnetic stir bar.</li>
 <li>Heat the CdO solution to 280 &deg;C until the solution is clear and colorless, then cool to 100 &deg;C.</li>
 <li>Heat the PbS solution to 110 &deg;C for no more than 5 min to boil off excess, but not all, solvent, then inject the Cd solution.</li>
 <li>Slowly heat the reaction mixture to 120-160 &deg;C (depending on the desired shell thickness). For thinner shells (1-2 monolayers) quench the reaction immediately after the injection of the Cd solution.</li>
 <li>Once desired temperature is reached, quench the reaction by placing the flask in an ice bath.</li>
 <li>Precipitate crystals in ethanol, pour off the liquid phase, and redissolve the precipitated crystals in toluene. Repeat twice. On final cleaning cycle, redissolve and store the crystals in anhydrous octane.</li>
</ol>
7. Preparation of TiO2 on FTO/Glass Substrates
<ol>
 <li>Hand wash the FTO coated glass with detergent (Alconox) and rinse with deionized water.</li>
 <li>Sonicate the glass in methanol, acetone, then isoproponal, for 5 min each and dry with Ar flow.</li>
 <li>Place the glass in a bath of 75 mM TiCl4 in deionized water and heat (in air) for 30 min at 70 &deg;C.</li>
 <li>Rinse the glass with deionized water and dry with Ar, then heat (in air) at 450 &deg;C for 1 hr and let cool to room temperature.</li>
 <li>While the glass is cooling, dissolve TiO2 Dyesol paste in terpinol in a 3:1 ratio by weight.</li>
 <li>Place 3 drops of the TiO2 mixture on the center of the FTO side of a dry glass slide that has been treated with TiCl4 and spin for 6 sec at 700 rpm and 1 min at 2,000 rpm.</li>
 <li>Anneal the slide in air at 450-500 &deg;C until the film turns brown, then clear.</li>
</ol>
8. Spin Coating PbS/CdS into a Film
<ol>
 <li>All spin coating steps are performed in an argon glovebox.</li>
 <li>Place 4-5 drops of PbS/CdS in octane (10 mg/ml) onto a still slide from step 7 and let spread until the center begins to dry, then spin for 5 sec at 600 rpm then 15 sec at 2,500 rpm.</li>
 <li>Place 10 drops of a 1:3 MPA:methanol solution on the slide, fully covering the surface, and spin for 5 sec at 600 rpm, then 15 sec at 2,500 rpm.</li>
 <li>Wash surface with methanol by placing 10 drops on the slide and spinning for 5 sec at 600 rpm, then 15 sec at 2,500 rpm.</li>
 <li>Wash surface with octane by same method as in step 8.4.</li>
 <li>Repeat steps 8.2-8.5 for each subsequent layer of the film.</li>
 <li>Anneal the film after every third layer at 150 &deg;C for 15 min. The final film should have an absorbance near 1.5 at the wavelength of the peak of the NC.</li>
</ol>
9. Dip Coating PbS/CdS Films
<ol>
 <li>Prepare a solution of 0.43 g cadmium acetate in 80 ml methanol in a beaker large enough to completely submerge the sample.</li>
 <li>Prepare a solution of sodium sulfide nonahydrate (Na2S&bull;9H2O) in 80 ml methanol in a beaker large enough to completely submerge the sample.</li>
 <li>Submerge the sample for 1 min in the Cadmium bath, and rinse with methanol. Then submerge for 1 min in the Sulfer bath, and rinse with methanol.</li>
 <li>Repeat step 9.3 until pores are filled (generally 4-8 times).</li>
 <li>Anneal sample at 150 &deg;C for 15 min.</li>
</ol>
10. Treating Films with CTAB14
<ol>
 <li>Place 0.25 ml of CTAB, dissolved in methanol, at a concentration of 10 mg/ml, on film from step 9 and let sit for 1 min, then spin at 2,500 rpm for 30 sec.</li>
 <li>Rinse slide with 10 drops of methanol and spin until dry.</li>
 <li>Repeat 10.1 and 10.2 once.</li>
</ol>
11. Representative Results
The evolution of the absorption and emission spectra corresponding to ZnSe/CdS/Pt NCs during each step of the synthesis is tracked in Figure 1. It can be seen that Figure 1b expresses absorbance peaks at ~350 nm and ~450, characteristic of ZnSe and CdS respectively, and, most notably, now displays the onset of a FL peak at ~550 nm. This FL feature is a result of emissive excitonic decay across the ZnSe/CdS interface. This type II interdomain FL is then quenched by the growth of the Pt tip (Figure 1c), due to the rapid injection of the delocalized electron into the metal moiety. This ultra-fast charge separation enables the utilization of the electron for the photocatalytic reduction of water. Hydrophilic MUA ligands are then added to facilitate the removal of the hole from the ZnSe domain, increasing stability by inhibiting oxidation of the semiconductor core, allowing for the sustained reduction of solar H2 (Figure 2). As a result of hole scavenging, the organic ligands become susceptible to photodegradation, but this can be simply mitigated by the addition of fresh ligands, as seen in Figure 2. Thus, the introduction of hydrophylic ligands not only render the NCs water soluble, but they also adjust the energetics of the system to protect the nanostructure at the cost of the inexpensive, easy to replace organic surfactants.
For PbS/CdS nanocrystal solids, Figure 3a and 3b shows a schematic of the fabrication process and the final device. Figure 3c, a TEM image of the core/shell nanocrystals, shows that the CdS infiltrates evenly around the PbS core. The nanocrystal solid is shown to be relatively free of pores in Figure 3d, an SEM image of the cross-section of a device. One result of the shell growth, that is observable, is a blueshift in both the absorbance and emission peaks. This shift is attributed to the PbS core shrinking as the Cd ions infiltrate further into the core, and can be seen in Figure 4. A large increase in the emission can also be seen in Figure 4, due to the enhanced quantum confinement provided by the CdS shell. The CdS layer not only increases the emission, it also protects the core, increasing the thermal stability of the solid, up to almost 200 &deg;C, almost 50 &deg;C higher than a PbS nanocrystal solid alone. Solar cells constructed using this nanocrystal solid architecture have not only shown better thermal stability, but have also been shown to have higher open circuit voltages (as high as 0.7 V) than the related organically linked films. These films have also shown a much higher tolerance to oxygen atmospheres, lasting for several days in normal atmospheric conditions with no degradation.
<img alt="Figure 1" src="/files/ftp_upload/4296/4296fig1.jpg" /> 
 Figure 1. Optical properties of fabricated heteronanocrystals. (a). The absorbance of ZnSe NCs showing an excitonic feature at &lambda; = 390 nm. Bandage emission was not observed for these samples. (b). Emission and absorbance of ZnSe/CdS nanorods grown from ZnSe core NCs. (c). Absorbance of ZnSe/CdS nanorods after Pt deposition.
<img alt="Figure 2" src="/files/ftp_upload/4296/4296fig2.jpg" /> 
 Figure 2. Evolution of hydrogen production on MUA capped ZnSe/CdS/Pt heteronanocrystals. The hydrogen production is resumed upon addition of new ligands to the ZnSe/CdS/Pt nanoparticles. The H2 production rate (the slope of the new experimental curve) after addition of fresh ligands (8-12 hr) is approximately the same as prior to the initial degradation (0-8 hr).
<img alt="Figure 3" src="/files/ftp_upload/4296/4296fig3.jpg" /> 
 Figure 3. Schematic representation of (a) the steps involved in fabrication of Nanocrystal solids and (b) the final photovoltaic device. Below are SEM images of (c) PbS/CdS core/shell nanocrystals and (d) a side view of the nanocrystal solid.
<img alt="Figure 4" src="/files/ftp_upload/4296/4296fig4.jpg" /> 
 Figure 4. Changes in the absorption (bottom) and emission (top of PbS NCs resulting from the deposition of approximately 1 monolayer of the CdS shell. The shrinking of the PbS core from the cation exchange is reflected as a &asymp;150 nm shift in both spectra. The PbS fluorescence intensity is enhanced due to the formation of type 1 heterostructure.
<img alt="Figure 5" src="/files/ftp_upload/4296/4296fig5.jpg" /> 
 Figure 5. Schematic representation of energy level alignment in (a) ZnSe/CdS/Pt core/rod/tip structures and (b) ZnTe/CdS/Pt core/rod/tip structures. Choosing appropriate materials is of the utmost importance in these devices, as holes from ZnTe seeded structures sit in an energy level that makes traveling to the ligand unfavorable.

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No conflicts of interest declared.

This study demonstrates how composite architectures of inorganic nanocrystals can be employed to achieve a spatial separation of photoinduced charges. In particular, these composites allow fine-tuning of the distribution of charges across the two domains, which are then available to perform either photocatalytic or photovoltaic function. For instance, efficient photocatalysts can be made if donor and acceptor nanocrystal domains are built into a single nanoparticle. The energetics of such a system is shown in <st...

<table border="1">
 <tbody>
 <tr>
 <td>Name of the reagent</td>
 <td>Company</td>
 <td>Catalogue number</td>
 <td>Comments (optional)</td>
 </tr>
 <tr>
 <td>octadecylamine (ODA), 90%</td>
 <td>Fisher</td>
 <td>AC12932-0050 </td>
 <td></td>
 </tr>
 <tr>
 <td>selenium (Se), 200 mesh</td>
 <td>Acros</td>
 <td>AC19807-2500 </td>
 <td></td>
 </tr>
 <tr>
 <td>tri-n-octylphosphine (TOP), 97% </td>
 <td>Strem</td>
 <td>15-6655 </td>
 <td>Air Sensitive</td>
 </tr>
 <tr>
 <td>diethyl zinc (Et2Zn), 10% by wt.</td>
 <td>Aldrich</td>
 <td>22080 </td>
 <td>Air Sensitive, Light Sensitive</td>
 </tr>
 <tr>
 <td>methanol, 99.8%, anhydrous</td>
 <td>Aldrich</td>
 <td>179337</td>
 <td></td>
 </tr>
 <tr>
 <td>toluene, 99.8%, anhydrous</td>
 <td>Aldrich</td>
 <td>244511</td>
 <td></td>
 </tr>
 <tr>
 <td>tri-n-octylphosphine oxide (TOPO), 99% </td>
 <td>Aldrich</td>
 <td>223301 </td>
 <td></td>
 </tr>
 <tr>
 <td>n-octadecylphosphonic acid (ODPA), 98% </td>
 <td>PCI Synthesis</td>
 <td>104224</td>
 <td></td>
 </tr>
 <tr>
 <td>hexylphosphonic acid (HPA), 98%</td>
 <td>PCI Synthesis</td>
 <td>4721-24-8</td>
 <td></td>
 </tr>
 <tr>
 <td>cadmium oxide (CdO), 99.99%</td>
 <td>Aldrich</td>
 <td>202894</td>
 <td></td>
 </tr>
 <tr>
 <td>sulfur (S), 99.999%</td>
 <td>Acros</td>
 <td>AC19993-0500 </td>
 <td>Strong odor</td>
 </tr>
 <tr>
 <td>11-mercaptoundecanoic acid (MUA), 95%</td>
 <td>Aldrich</td>
 <td>450561</td>
 <td></td>
 </tr>
 <tr>
 <td>potassium hydroxide (KOH) </td>
 <td>Acros</td>
 <td>AC13406-0010 </td>
 <td></td>
 </tr>
 <tr>
 <td>chloroform</td>
 <td>VWR</td>
 <td>EM-CX1059-1 </td>
 <td></td>
 </tr>
 <tr>
 <td>lead oxide (PbO), 99.999%</td>
 <td>Aldrich</td>
 <td>32306-1KG </td>
 <td></td>
 </tr>
 <tr>
 <td>1-octadecene (ODE), 90%</td>
 <td>Aldrich</td>
 <td>O806-25ML</td>
 <td></td>
 </tr>
 <tr>
 <td>oleic acid (OA), 90%</td>
 <td>Aldrich</td>
 <td>O1008-1G</td>
 <td></td>
 </tr>
 <tr>
 <td>bis(trimethylsilyl) sulfide (TMS), synthetic grade</td>
 <td>Aldrich</td>
 <td>283134-25G</td>
 <td>Air sensitive, strong odor, highly reactive</td>
 </tr>
 <tr>
 <td>acetone</td>
 <td>EMD Chemicals</td>
 <td>AX0118-2</td>
 <td></td>
 </tr>
 <tr>
 <td>cadmium acetate</td>
 <td>Acros</td>
 <td>AC31713-5000 </td>
 <td></td>
 </tr>
 <tr>
 <td>sodium sulfide nonahydrate (Na2S&bull;9H2O), 98%</td>
 <td>Alfa Aesar</td>
 <td>CB1100945 </td>
 <td>Light sensitive</td>
 </tr>
 <tr>
 <td>hexadecyltrimethyl ammonium bromide (CTAB), 99%</td>
 <td>Sigma</td>
 <td>H6269-100G</td>
 <td></td>
 </tr>
 <tr>
 <td>oleylamine, 70%</td>
 <td>Aldrich</td>
 <td>O7805-5G</td>
 <td></td>
 </tr>
 <tr>
 <td>diphenyl ether</td>
 <td>Alpha Aesar</td>
 <td>101-84-8</td>
 <td></td>
 </tr>
 <tr>
 <td>1,2-hexadecanediol</td>
 <td>TCI</td>
 <td>6920-24-7</td>
 <td></td>
 </tr>
 <tr>
 <td>Pt (II) acetylacetonate, 97%</td>
 <td>Aldrich</td>
 <td>282782-5G</td>
 <td></td>
 </tr>
 <tr>
 <td>isopropanol, 99.8%, anhydrous</td>
 <td>Acros</td>
 <td>AC32696-0025 </td>
 <td></td>
 </tr>
 <tr>
 <td>titanium tetrachloride (TiCl4), 99.9%</td>
 <td>Aldrich</td>
 <td>697079-25G</td>
 <td>Extremely air sensitive</td>
 </tr>
 <tr>
 <td>titanium dioxide, DSL 90T</td>
 <td>DyeSol</td>
 <td>DSL 90T</td>
 <td></td>
 </tr>
 <tr>
 <td>terpineol</td>
 <td>MP Biomedical</td>
 <td>98-55-5</td>
 <td></td>
 </tr>
 <tr>
 <td>3-mercaptopropionic acid (MPA), 99%</td>
 <td>Alfa Aesar</td>
 <td>A10435</td>
 <td>Strong odor</td>
 </tr>
 <tr>
 <td>octane, anhydrous, 99%</td>
 <td>Aldrich</td>
 <td>412236</td>
 <td></td>
 </tr>
 </tbody>
</table>

harvesting solar energy by means of charge-separating nanocrystals and their solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

Watch this Scientific Journal Video about Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids at JoVE.com

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

harvesting-solar-energy-means-charge-separating-nanocrystals-their

Research

JoVE Journal

Engineering

14.0K Views.  Bowling Green State University. A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Video: Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Characterization of Surface Modifications by White Light Interferometry: Applications in Ion Sputtering, Laser Ablation, and Tribology Experiments

In materials science and engineering it is often necessary to obtain quantitative measurements of surface topography with micrometer lateral resolution. From the measured surface, 3D topographic maps can be subsequently analyzed using a variety of software packages to extract the information that is needed.
In this article we describe how white light interferometry, and optical profilometry (OP) in general, combined with generic surface analysis software, can be used for materials science and engineering tasks. In this article, a number of applications of white light interferometry for investigation of surface modifications in mass spectrometry, and wear phenomena in tribology and lubrication are demonstrated. We characterize the products of the interaction of semiconductors and metals with energetic ions (sputtering), and laser irradiation (ablation), as well as ex situ measurements of wear of tribological test specimens.
Specifically, we will discuss:
<ol class="lroman">
	<li>Aspects of traditional ion sputtering-based mass spectrometry such as sputtering rates/yields measurements on Si and Cu and subsequent time-to-depth conversion.</li>
	<li>Results of quantitative characterization of the interaction of femtosecond laser irradiation with a semiconductor surface. These results are important for applications such as ablation mass spectrometry, where the quantities of evaporated material can be studied and controlled via pulse duration and energy per pulse. Thus, by determining the crater geometry one can define depth and lateral resolution versus experimental setup conditions.</li>
	<li>Measurements of surface roughness parameters in two dimensions, and quantitative measurements of the surface wear that occur as a result of friction and wear tests.</li>
</ol>
Some inherent drawbacks, possible artifacts, and uncertainty assessments of the white light interferometry approach will be discussed and explained.

White light microscope interferometry is an optical, noncontact and quick method for measuring the topography of surfaces. It is shown how the method can be applied toward mechanical wear analysis, where wear scars on tribological test samples are analyzed; and in materials science to determine ion beam sputtering or laser ablation volumes and depths.

In materials science and engineering it is often necessary to obtain quantitative measurements of surface topography with micrometer lateral resolution. ...

Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

Nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas allows the deposition of metal oxides with tunable morphology, structure, density and stoichiometry by a proper control of the plasma plume expansion dynamics. Such versatility can be exploited to produce nanostructured films from compact and dense to nanoporous characterized by a hierarchical assembly of nano-sized clusters. In particular we describe the detailed methodology to fabricate two types of Al-doped ZnO (AZO) films as transparent electrodes in photovoltaic devices: 1) at low O2 pressure, compact films with electrical conductivity and optical transparency close to the state of the art transparent conducting oxides (TCO) can be deposited at room temperature, to be compatible with thermally sensitive materials such as polymers used in organic photovoltaics (OPVs); 2) highly light scattering hierarchical structures resembling a forest of nano-trees are produced at higher pressures. Such structures show high Haze factor (&gt;80%) and may be exploited to enhance the light trapping capability. The method here described for AZO films can be applied to other metal oxides relevant for technological applications such as TiO2, Al2O3, WO3 and Ag4O4.

We describe the experimental method to deposit nanostructured oxide thin films by nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas. By using this method Al-doped ZnO (AZO) films, from compact to hierarchically structured as nano-tree forests, can be deposited.

Nanosecond Pulsed  Laser Deposition (PLD) in the presence of a background gas allows the deposition  of metal oxides with tunable morphology, structure, ...

Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm2. Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Tin sulfide (SnS) is a candidate material for Earth-abundant, non-toxic solar cells. Here, we demonstrate the fabrication procedure of the SnS solar cells employing atomic layer deposition, which yields 4.36% certified power conversion efficiency, and thermal evaporation which yields 3.88%.

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Quantitative Hardness Measurement by Instrumented AFM-indentation

In this work, a combination of amplitude-modulated non-contact atomic force microscopy and atomic force spectroscopy is applied for instrumented hardness measurements on an Au(111) surface with atomistic resolution of single plasticity events. A careful experimental procedure is described that includes the force sensor selection, its calibration, the calibration of the cantilever deflection detection system, and the minimization of instrumental drift for accurate and reproducible force-distance measurements. Also, a method for the data analysis is presented that allows the extraction of force-penetration curves from recorded force-distance curves. A typical curve displays a clear elastic deformation regime up to the first plasticity event, or pop-in, with a length in the range of one to two Burger's vectors. Later plasticity events exhibit the same magnitude. The work of plasticity is further extracted from the measurements. Finally, the hardness is determined in combination with the indentation curve using non-contact atomic force microscopy images of the remaining indents.

This experimental protocol describes how atomic force microscopy can be used to measure hardness at the true nanometer scale and to detect single atomistic plasticity events.

In this work, a combination of amplitude-modulated non-contact atomic force microscopy and atomic force spectroscopy is applied for instrumented hardness ...

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical ...

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) ...

Well-aligned Vertically Oriented ZnO Nanorod Arrays and their Application in Inverted Small Molecule Solar Cells

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule (SMPV1) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), by utilizing ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The inverted SMPV1:PC71BM solar cells with ZnO NRs that grew on both a sputtered and sol-gel processed AZO seed layer are fabricated. Compared with the AZO thin film prepared by the sol-gel method, the sputtered AZO thin film exhibits better crystallization and lower surface roughness, according to X-ray diffraction (XRD) and atomic force microscope (AFM) measurements. The orientation of the ZnO NRs grown on a sputtered AZO seed layer shows better vertical alignment, which is beneficial for the deposition of the subsequent active layer, forming better surface morphologies. Generally, the surface morphology of the active layer mainly dominates the fill factor (FF) of the devices. Consequently, the well-aligned ZnO NRs can be used to improve the carrier collection of the active layer and to increase the FF of the solar cells. Moreover, as an anti-reflection structure, it can also be utilized to enhance the light harvesting of the absorption layer, with the power conversion efficiency (PCE) of solar cells reaching 6.01%, higher than the sol-gel based solar cells with an efficiency of 4.74%.

This manuscript describes how to design and fabricate efficient inverted SMPV1:PC71BM solar cells with ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The well-aligned vertically oriented ZnO NRs exhibit high crystalline properties. The power conversion efficiency of solar cells can reach 6.01%.

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule ...

Indoor Experimental Assessment of the Efficiency and Irradiance Spot of the Achromatic Doublet on Glass (ADG) Fresnel Lens for Concentrating Photovoltaics

We present a method to characterize achromatic Fresnel lenses for photovoltaic applications. The achromatic doublet on glass (ADG) Fresnel lens is composed of two materials, a plastic and an elastomer, whose dispersion characteristics (refractive index variation with wavelength) are different. We first designed the lens geometry and then used ray-tracing simulation, based on the Monte Carlo method, to analyze its performance from the point of view of both optical efficiency and the maximum attainable concentration. Afterwards, ADG Fresnel lens prototypes were manufactured using a simple and reliable method. It consists of a prior injection of plastic parts and a consecutive lamination, together with the elastomer and a glass substrate to fabricate the parquet of ADG Fresnel lenses. The accuracy of the manufactured lens profile is examined using an optical microscope while its optical performance is evaluated using a solar simulator for concentrator photovoltaic systems. The simulator is composed of a xenon flash lamp whose emitted light is reflected by a parabolic mirror. The collimated light has a spectral distribution and an angular aperture similar to the real Sun. We were able to assess the optical performance of the ADG Fresnel lenses by taking photographs of the irradiance spot cast by the lens using a charge-coupled device (CCD) camera and measuring the photocurrent generated by several types of multi junction (MJ) solar cells, which have been previously characterized at a solar simulator for concentrator solar cells. These measurements have demonstrated the achromatic behavior of ADG Fresnel lenses and, as a consequence, the suitability of the modelling and manufacturing methods.

Indoor Experimental Assessment of the Efficiency and Irradiance Spot of the Achromatic Doublet on Glass ADG Fresnel Lens for Concentrating Photovoltaics

The achromatic doublet on glass (ADG) Fresnel lens makes use of two materials with differing dispersion to reduce chromatic aberration and increase attainable concentration. In this paper, a protocol for the complete characterization of the ADG Fresnel lens is presented.

We present a method to characterize achromatic Fresnel lenses for photovoltaic applications. The achromatic doublet on glass (ADG) Fresnel lens is ...

Developing High Performance GaP/Si Heterojunction Solar Cells

To improve the efficiency of Si-based solar cells beyond their Shockley-Queisser limit, the optimal path is to integrate them with III-V-based solar cells. In this work, we present high performance GaP/Si heterojunction solar cells with a high Si minority-carrier lifetime and high crystal quality of epitaxial GaP layers. It is shown that by applying phosphorus (P)-diffusion layers into the Si substrate and a SiNx layer, the Si minority-carrier lifetime can be well-maintained during the GaP growth in the molecular beam epitaxy (MBE). By controlling the growth conditions, the high crystal quality of GaP was grown on the P-rich Si surface. The film quality is characterized by atomic force microscopy and high-resolution x-ray diffraction. In addition, MoOx was implemented as a hole-selective contact that led to a significant increase in the short-circuit current density. The achieved high device performance of the GaP/Si heterojunction solar cells establishes a path for further enhancement of the performance of Si-based photovoltaic devices.

Here, we present a protocol to develop high-performance GaP/Si heterojunction solar cells with a high Si minority-carrier lifetime.

To improve the efficiency of Si-based solar cells beyond their Shockley-Queisser limit, the optimal path is to integrate them with III-V-based solar ...