The work was supported by Funda&#231;&#227;o de Amparo &#224; Pesquisa do Estado de S&#227;o Paulo (FAPESP), Centro de&#160;Desenvolvimento de Materiais Cer&#226;micos (CDMF- FAPESP&#160;N&#186; 2013/07296-2, 2017/11072-3, 2013/09963-6 and 2017/18916-2). Special thanks to Professor M&#225;ximo Siu Li for PL measurements.

1. Etching and cleaning the substrate
<ol>
	<li>Using a glass cutting system, form 2.5 x 2.5 cm substrates of fluoride thin oxide (FTO).</li>
	<li>Protect part of the substrate surface with a thermal tape leaving 0.5 cm of one side exposed.</li>
	<li>Deposit a small amount of zinc powder (enough to cover the area to be etched) on the top of the exposed FTO and drop concentrated hydrochloric acid (HCl) on the zinc powder slowly until all zinc powder is consumed by the reaction. Immediately after, rinse the substrate with deionized (DI) water. 
	CAUTION: Hydrogen gas in abundance is generated from zinc and HCl reaction.</li>
	<li>Remove the tape and wash with DI water and soap using a small brush. 
	NOTE: The brush helps to remove some residual glue from the tape.</li>
	<li>Leave the etched substrate in a soap solution (50% in water) and keep it for 15 min in an ultrasonicate bath. Then, sonicate for 15 min in DI water (2 times), followed by 10 more min in acetone and finally 10 min in isopropyl alcohol. Dry the substrate with nitrogen gas.</li>
</ol>
2. Deposition of niobium oxide films
<ol>
	<li>Fix the substrate through a shadow mask protecting 0.5 cm of both sides. 
	NOTE: On the side where the FTO was etched, it is important to certify that the FTO is covered in order to prevent short circuits when building the cell.</li>
	<li>Introduce the substrate into the sputtering chamber and seal the chamber.</li>
	<li>Start the mechanic pump. In the first 10 min, change the 3-way valve into roughing position to heat its oil and release water to improve the pumping. The primary pump works alone until the pressure is 6 x 10-2 Torr.</li>
	<li>Change the 3-way valve to backing position, and turn the turbo molecular pump on. Once the molecular pump is started, open the gate valve at the vacuum pump entry. Deposition starts when the pressure reaches 3 x 10-6 torr. 
	NOTE: Before starting the molecular pump, the primary vacuum should be better than 6 x 10-2 torr, however, not higher than 5 x 10-2 torr in order to prevent contaminating the chamber with pump oil.</li>
	<li>When the vacuum reaches 5 x 10-5 torr, open the water cooler system and turn on the substrate heating system. Set the temperature at 500 &#176;C. Increase the temperature slowly, 100 &#176;C every 5 min until it reaches the desired value.</li>
	<li>Set the gases parameters to be used in the deposition: argon of 40 sccm and oxygen of 3 to 10 sccm. 
	NOTE: The oxygen flow rate was varied in each deposition: 3, 3.5, 4 and 10 sccm. Oxygen reacts with niobium forming niobium oxide.</li>
	<li>Introduce argon onto the chamber, and set the pressure to 5 x 10-3 torr and the radio frequency (RF) to 120 W. Turn the RF on and tune using the impedance matching box. In case the plasma does not start, increase the pressure slowly until it reaches 2 x 10-2 Torr. In this pressure, the plasma should start. Set the pressure using a gate valve that can be opened or closed to change the pumping rate.</li>
	<li>Keep the plasma at 120 W for 10 min to clean the niobium target removing any oxide layer present in its surface. 
	NOTE: While cleaning the target, the substrate shutter is kept closed to protect the substrate from any material deposition.</li>
	<li>Introduce oxygen into the chamber,&#160;after stabilization, set the radio frequency power to 240 W and&#160;open the substrate shutter. Deposition starts. Set the deposition time to have a final thickness of 100 nm based on previous studies that determined the deposition rate. For each deposition condition a different deposition rate is expected, so the deposition time does also differ.</li>
	<li>Once the deposition time is completed, close the shutter immediately, turn the RF off, the close the gases and decrease the substrate temperature to room temperature.</li>
	<li>As the substrate temperature reaches room temperature, introduce air to reestablish the ambient pressure and open the chamber.. 
	NOTE: Generally, the system takes 4 h to reach a temperature of 40 &#176;C.</li>
</ol>
3. Constructing the solar cells
<ol>
	<li>Preparing the solutions used to construct the devices
	<ol>
		<li>TiO2 paste solution: Mix 150 mg of TiO2 paste in 1 mL of DI water. Stir it for 1 day before use. 
		NOTE: Keep the suspension stirring even when you are not using it to be sure that the suspension is always homogeneous.</li>
		<li>Prepare the lead iodide solution (PbI2) by mixing 420 mg of PbI2 in 1 mL of anhydrous dimethylformamide. Use only anhydrous solvents.</li>
		<li>Prepare the methylammonium iodide (CH3NH3I) solution by adding 8 mg of CH3NH3I in 1 mL of isopropyl alcohol (IPA). 
		NOTE: The water content in IPA must be less than 0.0005%.</li>
	</ol>
	</li>
	<li>Deposit TiO2 mesoporous layer on top of the niobium oxide layer using a spin coater at 4,000 rpm for 30 s.</li>
	<li>Put the substrate on the oven following the steps: 270 &#176;C for 30 min; 370 &#176;C for 30 min and 500 &#176;C for 1 h. Wait until the oven reaches room temperature and remove the substrate. 
	NOTE: The heat treatment decomposes the organic part of the paste leaving a porous layer over the film.</li>
	<li>Deposit two layers of PbI2 on top of the TiO2 mesoporous using a spin coater at 6,000 rpm for 90 s and after each deposition put the substrate in a hot plate at 70 &#176;C for 10 min. 
	NOTE: The PbI2 deposition must be inside a glove box filled with pure nitrogen or argon and with controlled atmosphere (water and oxygen &#60; 0.1 ppm).</li>
	<li>Deposit the CH3NH3I solution. Drop 0.3 mL of CH3NH3I solution onto PbI2, wait 20 s and then spin at 4,000 rpm for 30 s. Put the substrate on a hot plate at 100 &#176;C for 10 min. 
	NOTE: The CH3NH3I deposition must be inside a glove box. The total amount of CH3NH3I solution must be dropped quickly in only one step.</li>
	<li>Deposit Spiro-OMeTAD solution on top of the perovskite layer by spin coating at 4,000 rpm for 30 s. Leave the substrate in an oxygen atmosphere overnight. 
	NOTE: The Spiro-OMeTAD deposition must be inside of a glove box. After the deposition, it is important to leave the substrate overnight in an oxygen atmosphere in order to oxidize the Spiro- OMeTAD increasing its conductivity.</li>
	<li>Evaporate 70 nm of gold contact using a shadow mask at a rate of 0.2 A/s until 5 nm is reached and then increase the rate to 1 A/s. 
	NOTE: It is important to use a slow rate at the beginning to prevent gold diffusion through the cell.</li>
</ol>

<ol>
	<li>Wasa, K., Kitabatake, M., Adachi, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Thin film materials technology : sputtering of compound materials. , (2004).</li><li>Kelly, P. J., Arnell, R. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Magnetron+sputtering:+A+review+of+recent+developments+and+applications.">Magnetron sputtering: A review of recent developments and applications.</a> Vacuum. 56, 159-172 (2000).</li><li>Chen, W. -. C., Peng, C. Y., Chang, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Heteroepitaxial+growth+of+TiN+film+on+MgO+(100)+by+reactive+magnetron+sputtering.">Heteroepitaxial growth of TiN film on MgO (100) by reactive magnetron sputtering.</a> Nanoscale Research Letters. 9, 551 (2014).</li><li>Guo, Q. X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Heteroepitaxial+growth+of+gallium+nitride+on+(+1+1+1+)+GaAs+substrates+by+radio+frequency+magnetron+sputtering.">Heteroepitaxial growth of gallium nitride on ( 1 1 1 ) GaAs substrates by radio frequency magnetron sputtering.</a> Journal of Crystal Growth. 239, 1079-1083 (2002).</li><li>Berg, S., Nyberg, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fundamental+understanding+and+modeling+of+reactive+sputtering+processes.">Fundamental understanding and modeling of reactive sputtering processes.</a> Thin Solid films. (476), 215-230 (2005).</li><li>Wong, M. S., Sproul, W. D., Chu, X., Barnett, S. 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L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hysteresis+dependence+on+CH3NH3PbI3+deposition+method+in+perovskite+solar+cells.">Hysteresis dependence on CH3NH3PbI3 deposition method in perovskite solar cells.</a> Proceedings of SPIE - International Society for Optics and Photonics. 9936, 9936 (2016).</li><li>Fernandes, S. L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nb2O5hole+blocking+layer+for+hysteresis-free+perovskite+solar+cells.">Nb2O5hole blocking layer for hysteresis-free perovskite solar cells.</a> Materials Letters. 181, 103-107 (2016).</li><li>Hamada, K., Murakami, N., Tsubota, T., Ohno, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Solution-processed+amorphous+niobium+oxide+as+a+novel+electron+collection+layer+for+inverted+polymer+solar+cells.">Solution-processed amorphous niobium oxide as a novel electron collection layer for inverted polymer solar cells.</a> Chemical Physics Letters. 586, 81-84 (2013).</li><li>Aegerter, M. a. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sol-gel+niobium+pentoxide:+A+promising+material+for+electrochromic+coatings,+batteries,+nanocrystalline+solar+cells+and+catalysis.">Sol-gel niobium pentoxide: A promising material for electrochromic coatings, batteries, nanocrystalline solar cells and catalysis.</a> Solar Energy Materials and Solar Cells. 68, 401-422 (2001).</li><li>Rani, R. A., Zoolfakar, A. S., O'Mullane, A. P., Austin, M. W., Kalantar-Zadeh, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thin+films+and+nanostructures+of+niobium+pentoxide:+fundamental+properties,+synthesis+methods+and+applications.">Thin films and nanostructures of niobium pentoxide: fundamental properties, synthesis methods and applications.</a> Journal Materials Chemistry A. 2, 15683-15703 (2014).</li><li>Foroughi-Abari, A., Cadien, K. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Growth,+structure+and+properties+of+sputtered+niobium+oxide+thin+films.">Growth, structure and properties of sputtered niobium oxide thin films.</a> Thin Solid Films. 519, 3068-3073 (2011).</li><li>Numata, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nb-doped+amorphous+titanium+oxide+compact+layer+for+formamidinium-based+high+efficiency+perovskite+solar+cells+by+low-temperature+fabrication.">Nb-doped amorphous titanium oxide compact layer for formamidinium-based high efficiency perovskite solar cells by low-temperature fabrication.</a> Journal Materials Chemistry A. 6, 9583-9591 (2018).</li><li>Gra&#231;a, M. P. F., Meireles, A., Nico, C., Valente, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nb2O5+nanosize+powders+prepared+by+sol-gel+-+Structure,+morphology+and+dielectric+properties.">Nb2O5 nanosize powders prepared by sol-gel - Structure, morphology and dielectric properties.</a> Journal of Alloys and Compounds. 553, 177-182 (2013).</li><li>Kogo, A., Numata, Y., Ikegami, M., Miyasaka, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nb+2+O+5+Blocking+Layer+for+High+Open-circuit+Voltage+Perovskite+Solar+Cells.">Nb 2 O 5 Blocking Layer for High Open-circuit Voltage Perovskite Solar Cells.</a> Chemistry Letters. 44, 829-830 (2015).</li><li>Ueno, S., Fujihara, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+an+Nb2O5+nanolayer+coating+on+ZnO+electrodes+in+dye-sensitized+solar+cells.">Effect of an Nb2O5 nanolayer coating on ZnO electrodes in dye-sensitized solar cells.</a> Electrochimica Acta. 56, 2906-2913 (2011).</li><li>Fernandes, S. L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Exploring+the+Properties+of+Niobium+Oxide+Films+for+Electron+Transport+Layers+in+Perovskite+Solar+Cells.">Exploring the Properties of Niobium Oxide Films for Electron Transport Layers in Perovskite Solar Cells.</a> Frontiers in Chemistry. 7, 1-9 (2019).</li><li>Shirani, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tribologically+enhanced+self-healing+of+niobium+oxide+surfaces.">Tribologically enhanced self-healing of niobium oxide surfaces.</a> Surface and Coatings Technology. 364, 273-278 (2014).</li><li>Yan, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nb2O5/TiO2+heterojunctions:+Synthesis+strategy+and+photocatalytic+activity.">Nb2O5/TiO2 heterojunctions: Synthesis strategy and photocatalytic activity.</a> Applied Catalysis B: Environmental. 152 (1), 280-288 (2014).</li></ol>

The authors have nothing to disclose.

The niobium oxide films prepared in this work was used as electron transport layer in perovskite solar cells. The most important characteristic required for an electron transport layer is to prevent recombination, blocking holes and transferring efficiently electrons.
In this respect the use of reactive sputtering technique is advantageous since it produces dense and compact films. Also, as already mentioned, compared to sol-gel, anodization, hydrothermal, and chemical vapor deposition synthes...

The sputtering technique is widely used to deposit high-quality films. Its main application is in the semiconductor industry, although it is also used in surface coating for improvement in mechanical properties, and reflective layers1. The main advantage of sputtering is the possibility to deposit different materials over different substrates; the good reproducibility and control over the deposition parameters. The sputtering technique allows deposition of homogeneous films, with good adhesion over large areas and at low-cost when compared with other deposition methods like chemical vapor deposition (CVD), molecular beam epitaxy (MBE) and atomic layer deposition (ALD)1,2. Commonly, semiconductor films deposited by sputtering are amorphous or polycrystalline, however, there are some reports on epitaxial growth by sputtering3,4. Nevertheless, the sputtering process is highly complex and the range of the parameter is wide5, so in order to achieve high-quality films, a good comprehension of the process and parameter optimization is necessary for each material.
There are several articles reporting on the deposition of niobium oxide films by sputtering, as well as niobium nitride6 and niobium carbide7. Among Nb-oxides, niobium pentoxide (Nb2O5) is a transparent, air-stable and water-insoluble material that exhibits extensive polymorphism. It is an n-type semiconductor with band gap values ranging from 3.1 to 5.3 eV, giving these oxides a wide range of applications8,9,10,11,12,13,14,15,16,17,18,19. Nb2O5 has attracted considerable attention as a promising material to be used in perovskite solar cells due to its comparable electron injection efficiency and better chemical stability compared to titanium dioxide (TiO2). In addition, the band gap of Nb2O5 could improve the open-circuit voltage (Voc) of the cells14.
In this work, Nb2O5 was deposited by reactive sputtering under different oxygen flow rates. At low oxygen flow rates, the conductivity of the films were increased without making use of doping, which introduces impurities on the system. These films were used as electron transport layer in perovskite solar cells improving the performance of these cells. It was found that decreasing the amount of oxygen induces the formation of oxygen vacancies, which increases the conductivity of the films leading to solar cells with better efficiency.

<table><tbody><tr><td>2-propanol</td><td>Merck</td><td>67-63-0</td><td>solvent with maximum of 0.005% H&lt;sub&gt;2&lt;/sub&gt;O</td></tr><tr><td>4-tert-butylpyridine</td><td>Sigma Aldrich</td><td>3978-81-2</td><td>chemical with 96% purity</td></tr><tr><td>acetonitrile</td><td>Sigma Aldrich</td><td>75-05-8</td><td>anhydrous solvent , 99.8% purity</td></tr><tr><td>bis(trifluoromethane)sulfonimide lithium salt</td><td>Sigma Aldrich</td><td>90076-65-6</td><td>chemical with &amp;ge;99.95% purity</td></tr><tr><td>chlorobenzene</td><td>Sigma Aldrich</td><td>108-90-7</td><td>anhydrous solvent , 99.8% purity</td></tr><tr><td>ethanol</td><td>Sigma Aldrich</td><td>200-578-6</td><td>solvent</td></tr><tr><td>Fluorine doped tin oxide (SnO2:F) glass substrate</td><td>Solaronix</td><td>TCO22-7/LI</td><td>substrate to deposit films</td></tr><tr><td>Kaptom tape</td><td>Usinainfo</td><td>04227</td><td>thermal tape used to cover the substrates</td></tr><tr><td>Kurt J Lesker magnetron sputtering system</td><td>Kurt J Lesker</td><td>------</td><td>Sputtering equipment used to deposit compact films</td></tr><tr><td>Lead (II) iodide</td><td>Alfa Aesar</td><td>10101-63-0</td><td>PbI&lt;sub&gt;2&lt;/sub&gt; salt- 99.998% purity</td></tr><tr><td>methylammonium iodide</td><td>Dyesol</td><td>14965-49-2</td><td>CH&lt;sub&gt;3&lt;/sub&gt;NH&lt;sub&gt;3&lt;/sub&gt;I salt</td></tr><tr><td>N&lt;sup&gt;2&lt;/sup&gt;,N&lt;sup&gt;2&lt;/sup&gt;,N&lt;sup&gt;2&amp;prime;&lt;/sup&gt;,N&lt;sup&gt;2&amp;prime;&lt;/sup&gt;,N&lt;sup&gt;7&lt;/sup&gt;,N&lt;sup&gt;7&lt;/sup&gt;,N&lt;sup&gt;7&amp;prime;&lt;/sup&gt;,N&lt;sup&gt;7&amp;prime;&lt;/sup&gt;-octakis (4-methoxyphenyl)-9,9&amp;prime;-spirobi [9H-fluorene]-2,2&amp;prime;,7,7&amp;prime;-tetramine</td><td>Sigma Aldrich</td><td>207739-72-8</td><td>Spiro-OMeTAD salt, 99% purity</td></tr><tr><td>Niobium target of 3&amp;rdquo;</td><td>CBMM- Brazilian Metallurgy and Mining Company</td><td>------</td><td>niobium sputtering target used in the sputtering system</td></tr><tr><td>N-N dimethylformamide</td><td>Merck</td><td>68-12-2</td><td>solvent with maximum of 0.003% H&lt;sub&gt;2&lt;/sub&gt;O</td></tr><tr><td>TiO&lt;sub&gt;2&lt;/sub&gt; paste</td><td>Dyesol</td><td>DSL 30NR-D</td><td>titanium dioxide paste</td></tr><tr><td>tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide]</td><td>Dyesol</td><td>329768935</td><td>FK 209 Co(III) TFSL salt</td></tr></tbody></table>

niobium oxide films deposited by reactive sputtering: effect of oxygen flow rate

Reactive sputtering is a versatile technique used to form compact films with excellent homogeneity. In addition, it allows easy control over deposition parameters such as gas flow rate that results in changes on composition and thus in the film required properties. In this report, reactive sputtering is used to deposit niobium oxide films. A niobium target is used as metal source and different oxygen flow rates to deposit niobium oxide films. The oxygen flow rate was changed from 3 to 10 sccm. The films deposited under low oxygen flow rates show higher electrical conductivity and provide better perovskite solar cells when used as electron transport layer.

In the sputtering system, the deposition rate is strongly influenced by the oxygen flow rate. The deposition rate decreases when the oxygen flow is increased. Considering the present conditions of the target area used and plasma power, it is observed that from 3 to 4 sccm there is an expressive decrease on the deposition rate, however, when the oxygen is increased from 4 to 10 sccm it becomes less pronounced. In the regime of 3 sccm the deposition rate is 1.1 nm/s, decreasing abruptly to 0.1 nm/s for 10 sccm as seen in <...

Watch this Scientific Journal Video about Niobium Oxide Films Deposited by Reactive Sputtering: Effect of Oxygen Flow Rate at JoVE.com

Niobium Oxide Films Deposited by Reactive Sputtering: Effect of Oxygen Flow Rate

Here, we present a protocol for niobium oxide films deposition by reactive sputtering with different oxygen flow rates for use as an electron transport layer in perovskite solar cells.

Reactive sputtering is a versatile technique used to form compact films with excellent homogeneity. In addition, it allows easy control over deposition ...

niobium-oxide-films-deposited-reactive-sputtering-effect-oxygen-flow

Research

JoVE Journal

Chemistry

7.3K Views.  Federal University of São Carlos (UFSCAR). Here, we present a protocol for niobium oxide films deposition by reactive sputtering with different oxygen flow rates for use as an electron transport layer in perovskite solar cells.

Video: Niobium Oxide Films Deposited by Reactive Sputtering: Effect of Oxygen Flow Rate

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, ...

Engineering

Fabrication of Spatially Confined Complex Oxides

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from the inherent strong electron correlations that reside within them. These materials can also possess electronic phase separation in which regions of vastly different resistive and magnetic behavior can coexist within a single crystal alloy material. By reducing the scale of these materials to length scales at and below the inherent size of the electronic domains, novel behaviors can be exposed. Because of this and the fact that spin-charge-lattice-orbital order parameters each involve correlation lengths, spatially reducing these materials for transport measurements is a critical step in understanding the fundamental physics that drives complex behaviors. These materials also offer great potential to become the next generation of electronic devices 1-3. Thus, the fabrication of low dimensional nano- or micro-structures is extremely important to achieve new functionality. This involves multiple controllable processes from high quality thin film growth to accurate electronic property characterization. Here, we present fabrication protocols of high quality microstructures for complex oxide manganite devices. Detailed descriptions and required equipment of thin film growth, photo-lithography, and wire-bonding are presented.

We describe the use of pulsed laser deposition (PLD), photolithography and wire-bonding techniques to create micrometer scale complex oxides devices. The PLD is utilized to grow epitaxial thin films. Photolithography and wire-bonding techniques are introduced to create practical devices for measurement purposes.

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from ...

Atomically Defined Templates for Epitaxial Growth of Complex Oxide Thin Films

Atomically defined substrate surfaces are prerequisite for the epitaxial growth of complex oxide thin films. In this protocol, two approaches to obtain such surfaces are described. The first approach is the preparation of single terminated perovskite SrTiO3 (001) and DyScO3 (110) substrates. Wet etching was used to selectively remove one of the two possible surface terminations, while an annealing step was used to increase the smoothness of the surface. The resulting single terminated surfaces allow for the heteroepitaxial growth of perovskite oxide thin films with high crystalline quality and well-defined interfaces between substrate and film. In the second approach, seed layers for epitaxial film growth on arbitrary substrates were created by Langmuir-Blodgett (LB) deposition of nanosheets. As model system Ca2Nb3O10- nanosheets were used, prepared by delamination of their layered parent compound HCa2Nb3O10. A key advantage of creating seed layers with nanosheets is that relatively expensive and size-limited single crystalline substrates can be replaced by virtually any substrate material.

Various procedures are outlined to prepare atomically defined templates for epitaxial growth of complex oxide thin films. Chemical treatments of single crystalline SrTiO3 (001) and DyScO3 (110) substrates were performed to obtain atomically smooth, single terminated surfaces. Ca2 Nb3 O10- nanosheets were used to create atomically defined templates on arbitrary substrates.

Atomically defined substrate surfaces are prerequisite for the epitaxial growth of complex oxide thin films. In this protocol, two approaches to obtain ...

Formation of Thick Dense Yttrium Iron Garnet Films Using Aerosol Deposition

Aerosol deposition (AD) is a thick-film deposition process that can produce layers up to several hundred micrometers thick with densities greater than 95% of the bulk. The primary advantage of AD is that the deposition takes place entirely at ambient temperature; thereby enabling film growth in material systems with disparate melting temperatures. This report describes in detail the processing steps for preparing the powder and for performing AD using the custom-built system. Representative characterization results are presented from scanning electron microscopy, profilometry, and ferromagnetic resonance for films grown in this system. As a representative overview of the capabilities of the system, focus is given to a sample produced following the described protocol and system setup. Results indicate that this system can successfully deposit 11 &#181;m thick yttrium iron garnet films that are &#160;&#62; 90% of the bulk density during a single 5 min deposition run. A discussion of methods to afford better control of the aerosol and particle selection for improved thickness and roughness variations in the film is provided.

This report describes the use of a custom-built system to perform aerosol deposition of thick films of yttrium iron garnet onto sapphire substrates at RT. The deposited films are characterized using scanning electron microscopy, profilometry, and ferromagnetic resonance to give a representative overview of the capabilities of the technique.

Aerosol deposition (AD) is a thick-film deposition process that can produce layers up to several hundred micrometers thick with densities greater than 95% ...

Chemical Precipitation Method for the Synthesis of Nb2O5 Modified Bulk Nickel Catalysts with High Specific Surface Area

We demonstrate a method for the synthesis of NixNb1-xO catalysts with sponge-like and fold-like nanostructures. By varying the Nb:Ni ratio, a series of NixNb1-xO nanoparticles with different atomic compositions (x = 0.03, 0.08, 0.15, and 0.20) have been prepared by chemical precipitation. These NixNb1-xO catalysts are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The study revealed the sponge-like and fold-like appearance of Ni0.97Nb0.03O and Ni0.92Nb0.08O on the NiO surface, and the larger surface area of these NixNb1-xO catalysts, compared with the bulk NiO. Maximum surface area of 173 m2/g can be obtained for Ni0.92Nb0.08O catalysts. In addition, the catalytic hydroconversion of lignin-derived compounds using the synthesized Ni0.92Nb0.08O catalysts have been investigated.

Chemical Precipitation Method for the Synthesis of Nb2O5 Modified Bulk Nickel Catalysts with High Specific Surface Area

A protocol for the synthesis of sponge-like and fold-like Ni1-xNbxO nanoparticles by chemical precipitation is presented.

We demonstrate a method for the synthesis of NixNb1-xO catalysts with sponge-like and fold-like nanostructures. By varying the Nb:Ni ratio, a series of ...

Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 &#176;C. To grow VO2 on a wide variety of substrates, with wafer-scale uniformity and angstrom level control of thickness, the method of atomic-layer deposition was chosen. This ALD process enables high-quality, low-temperature (&#8804;150 &#176;C) growth of ultrathin films (100-1000 &#197;) of VO2. For this demonstration, the VO2 films were grown on sapphire substrates. This low temperature growth technique produces mostly amorphous VO2 films. A subsequent anneal in an ultra-high vacuum chamber with a pressure of 7x10-4 Pa of ultra-high purity (99.999%) oxygen produced oriented, polycrystalline VO2 films. The crystallinity, phase, and strain of the VO2 were determined by Raman spectroscopy and X-ray diffraction, while the stoichiometry and impurity levels were determined by X-ray photoelectron spectroscopy, and finally the morphology was determined by atomic force microscopy. These data demonstrate the high-quality of the films grown by this technique. A model was created to fit to the data for VO2 in its metallic and insulating phases in the near infrared spectral region. The permittivity and refractive index of the ALD VO2 agreed well with the other fabrication methods in its insulating phase, but showed a difference in its metallic state. Finally, the analysis of the films&#39; optical properties enabled the creation of a wavelength- and temperature-dependent model of the complex optical refractive index for developing VO2 as a tunable refractive index material.

Thin films (100-1000 &#197;) of vanadium dioxide (VO2) were created by atomic-layer deposition (ALD) on sapphire substrates. Following this, the optical properties were characterized through the metal-insulator transition of VO2. From the measured optical properties, a model was created to describe the tunable refractive index of VO2.

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 &#176;C. To grow VO2 on a wide variety of substrates, with ...

Bulk and Thin Film Synthesis of Compositionally Variant Entropy-stabilized Oxides

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (Cu variant) entropy-stabilized oxides. Phase pure and chemically homogeneous (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (x = 0.11, 0.27) ceramic pellets are synthesized and used in the deposition of ultra-high quality, phase pure, single crystalline thin films of the target stoichiometry. A detailed methodology for the deposition of smooth, chemically homogeneous, entropy-stabilized oxide thin films by pulsed laser deposition on (001)-oriented MgO substrates is described. The phase and crystallinity of bulk and thin film materials are confirmed using X-ray diffraction. Composition and chemical homogeneity are confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The surface topography of thin films is measured with scanning probe microscopy. The synthesis of high quality, single crystalline, entropy-stabilized oxide thin films enables the study of interface, size, strain, and disorder effects on the properties in this new class of highly disordered oxide materials.

The synthesis of high quality bulk and thin film (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O entropy-stabilized oxides is presented.

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and ...

U2O5 Film Preparation via UO2 Deposition by Direct Current Sputtering and Successive Oxidation and Reduction with Atomic Oxygen and Atomic Hydrogen

We describe a method to produce U2O5 films in situ using the Labstation, a modular machine developed at JRC Karlsruhe. The Labstation, an essential part of the Properties of Actinides under Extreme Conditions laboratory (PAMEC), allows the preparation of films and studies of sample surfaces using surface analytical techniques such as X-ray and ultra-violet photoemission spectroscopy (XPS and UPS, respectively). All studies are made in situ, and the films, transferred under ultra-high vacuum from their preparation to an analyses chamber, are never in contact with the atmosphere. Initially, a film of UO2 is prepared by direct current (DC) sputter deposition on a gold (Au) foil then oxidized by atomic oxygen to produce a UO3 film. This latter is then reduced with atomic hydrogen to U2O5. Analyses are performed after each step involving oxidation and reduction, using high-resolution photoelectron spectroscopy to examine the oxidation state of uranium. Indeed, the oxidation and reduction times and corresponding temperature of the substrate during this process have severe effects on the resulting oxidation state of the uranium. Stopping the reduction of UO3 to U2O5 with single U(V) is quite challenging; first, uranium-oxygen systems exist in numerous intermediate phases. Second, differentiation of the oxidation states of uranium is mainly based on satellite peaks, whose intensity peaks are weak. Also, experimenters should be aware that X-ray spectroscopy (XPS) is a technique with an atomic sensitivity of 1% to 5%. Thus, it is important to obtain a complete picture of the uranium oxidation state with the entire spectra obtained on U4f, O1s, and the valence band (VB). Programs used in the Labstation include a linear transfer program developed by an outside company (see Table of Materials) as well as data acquisition and sputter source programs, both developed in-house.

U2O5 Film Preparation via UO2 Deposition by Direct Current Sputtering and Successive Oxidation and Reduction with Atomic Oxygen and Atomic Hydrogen

This protocol presents the preparation of U2O5 thin films obtained in situ under ultra-high vacuum. The process involves oxidation and reduction of UO2 films with atomic oxygen and atomic hydrogen, respectively.

We describe a method to produce U2O5 films in situ using the Labstation, a modular machine developed at JRC Karlsruhe. The Labstation, an essential part ...

Plasma-Assisted Molecular Beam Epitaxy Growth of Mg3N2 and Zn3N2 Thin Films

This article describes a procedure for growing Mg3N2 and Zn3N2 films by plasma-assisted molecular beam epitaxy (MBE). The films are grown on 100 oriented MgO substrates with N2 gas as the nitrogen source. The method for preparing the substrates and the MBE growth process are described. The orientation and crystalline order of the substrate and film surface are monitored by the reflection high energy electron diffraction (RHEED) before and during growth. The specular reflectivity of the sample surface is measured during growth with an Ar-ion laser with a wavelength of 488 nm. By fitting the time dependence of the reflectivity to a mathematical model, the refractive index, optical extinction coefficient, and growth rate of the film are determined. The metal fluxes are measured independently as a function of the effusion cell temperatures using a quartz crystal monitor. Typical growth rates are 0.028 nm/s at growth temperatures of 150 &#176;C and 330 &#176;C for Mg3N2 and Zn3N2 films, respectively.

Plasma-Assisted Molecular Beam Epitaxy Growth of Mg3N2 and Zn3N2 Thin Films

This article describes the growth of epitaxial films of Mg3N2 and Zn3N2 on MgO substrates by plasma-assisted molecular beam epitaxy with N2 gas as the nitrogen source and optical growth monitoring.

This article describes a procedure for growing Mg3N2 and Zn3N2 films by plasma-assisted molecular beam epitaxy (MBE). The films are grown on 100 oriented ...

Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing

Electrical, optical, and magnetic properties of oxide materials can often be controlled by varying the oxygen content. Here we outline two approaches for varying the oxygen content and provide concrete examples for tuning the electrical properties of SrTiO3-based heterostructures. In the first approach, the oxygen content is controlled by varying the deposition parameters during a pulsed laser deposition. In the second approach, the oxygen content is tuned by subjecting the samples to annealing in oxygen at elevated temperatures after the film growth. The approaches can be used for a wide range of oxides and nonoxide materials where the properties are sensitive to a change in the oxidation state.
The approaches differ significantly from electrostatic gating, which is often used to change the electronic properties of confined electronic systems such as those observed in SrTiO3-based heterostructures. By controlling the oxygen vacancy concentration, we are able to control the carrier density over many orders of magnitude, even in nonconfined electronic systems. Moreover, properties can be controlled, which are not sensitive to the density of itinerant electrons.

Oxide materials show many exotic properties that can be controlled by tuning the oxygen content. Here, we demonstrate the tuning of oxygen content in oxides by varying the pulsed laser deposition parameters and by performing postannealing. As an example, electronic properties of SrTiO3-based heterostructures are tuned by growth modifications and annealing.

Electrical, optical, and magnetic properties of oxide materials can often be controlled by varying the oxygen content. Here we outline two approaches for ...

Niobium Oxide Films Deposited by Reactive Sputtering: Effect of Oxygen Flow Rate

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