This work has been supported in part by the Spanish Ministry of Science and Innovation via Grant TEC2015-74329-JIN-(AEI/FEDER,EU), TEC2016-79898-C6-1-R (AEI/FEDER, EU), and TEC-2013-48147-C6-6R (AEI/FEDER, EU). SV acknowledges the support of the SoMoPro II Programme, co-&#64257;nanced by the European Union and the South Moravian Region, via Grant 4SGA8678. JC&#780; acknowledges the funding provided by MEYS, Project No. LQ1601 (CEITEC 2020). Part of this research has made use of the infrastructures of the SIX Research Centre, the core facilities of CEITEC under CEITEC-open access project via Grant LM2011020 funded by the Ministry of Education, Youth and Sports of the Czech Republic, and the Spanish ICTS Network MICRONANOFABS partially supported by MINECO.

Notes: For safety reasons, the reaction cell and the aerosol generator are placed inside a fume hood. Employ tweezers to handle the samples, wear gloves, a lab-coat, and goggles, and follow common laboratory safety practices.
1 . Preparation of Substrates and Set-up of Deposition Temperature
<ol>
	<li>Cut 10 mm x 10 mm silicon substrates using a diamond tip scribe (the substrate dimensions have been adapted to the size of our reaction cell). For this experiment, use a home-made stainless steel cylindrical reaction cell with an internal volume of ~7,000 mm3 (diameter: 30 mm, height: 10 mm) adapted to the dimensions of the silicon-based micromachined platforms employed for the fabrication of gas sensors.</li>
	<li>Clean the substrates in isopropanol, rinse with deionized water, and blow-dry the substrates with nitrogen to ensure good adherence of the films and uniform covering of the substrate.</li>
	<li>Place the substrate into the reaction cell. When using silicon-based micromachined platforms, instead of bare silicon substrates for the fabrication of gas sensors, place the micromachined platforms into the reaction cell and then align with a shadow mask to confine the growth of material to the area of interest.</li>
	<li>Close the reaction cell. Make sure that the lid of the reaction cell is properly sealed to avoid the leakage of reactive species.</li>
	<li>Switch-on the temperature control system, consisting of resistive heaters integrated with the reaction cell, a thermocouple to sense the temperature of the substrate and a proportional-integral-derivative (PID) controller.</li>
	<li>Set the temperature to 400 &#176;C and let it to stabilize (this process takes approximately 30 min, but it may change depending on the reaction cell dimensions and the characteristics of the temperature control system).</li>
</ol>
2 . Preparation of Solution for Aerosol Generation
<ol>
	<li>Add 50 mg of ZnCl2 to a 100 mL glass vial equipped with a vacuum trap (29/32 joint, 200 mm length, 5 mm hose barbs).</li>
	<li>Dissolve the ZnCl2 in 5 mL of ethanol and then cap the vial with the vacuum trap. Ensure the down-pipe end sits 60 mm above the bottom of the vial and without submerging in the solution. If needed, employ glass joint clips to secure the vial and the vacuum trap together during the CVD process.</li>
	<li>Clamp the vial to a universal support. Adjust the height to meet the bottom of the vial and the optimal focal point of the ultrasonic atomizer that operates at 1.6 MHz and delivers an average size of the aerosol droplets of &#8764;3 &#181;m.</li>
	<li>Connect the inlet and the exhaust of the vacuum trap to the nitrogen pipe and the reaction cell, respectively, as shown in the simplified scheme of the aerosol-assisted CVD system in Figure 1.</li>
	<li>Use a fresh solution of reactants for each deposition.</li>
</ol>
3 . CVD Process
<ol>
	<li>Before starting the CVD process, verify that the temperature in the reaction cell has reached the steady state.</li>
	<li>Adjust the nitrogen flow to 200 cm3/min and allow it to flow through the system (the flow rate has been tuned according to the dimensions of the reaction cell used in our experiments). The use of a mass-flow controller is recommended to ensure a constant flow during the deposition.</li>
	<li>Switch-on the aerosol generator and keep the aerosol constant during the process until the solution containing the zinc precursor is completely delivered to the reaction cell (this process takes approximately 120 min considering a solution volume of 5 mL and a flow rate of 200 cm3/min).</li>
	<li>As soon as the solution has been fully delivered to the reaction cell, switch-off the aerosol generator and the temperature system to cool down the reaction cell. Meanwhile keep the nitrogen flowing throughout the system.</li>
	<li>When the temperature has dropped to room temperature, close the nitrogen flow, open the reaction cell, and remove the samples. The substrate will show a greyish matte color on the surface, different from the shiny bare silicon wafer (the silicon-based micromachined platforms display a similar appearance after the CVD step). This matte color is associated with the presence of columnar ZnO structures in the form of rods like those observed by scanning electron microscopy (Figure 2).</li>
</ol>

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L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Splendid+One-Dimensional+Nanostructures+of+Zinc+Oxide:+A+New+Nanomaterial+Family+for+Nanotechnology.">Splendid One-Dimensional Nanostructures of Zinc Oxide: A New Nanomaterial Family for Nanotechnology.</a> ACS Nano. 2 (10), 1987-1992 (2008).</li><li>Pal, J., Pal, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Faceted+metal+and+metal+oxide+nanoparticles:+design,+fabrication+and+catalysis.">Faceted metal and metal oxide nanoparticles: design, fabrication and catalysis.</a> Nanoscale. 7 (34), 14159-14190 (2015).</li><li>Sun, 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=The+Applications+of+Morphology+Controlled+ZnO+in+Catalysis.">The Applications of Morphology Controlled ZnO in Catalysis.</a> Catalysts. 6 (12), 188 (2016).</li><li>Vallejos, S., Umek, P., Blackman, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=AACVD+Control+parameters+for+selective+deposition+of+tungsten+oxide+nanostrucutres.">AACVD Control parameters for selective deposition of tungsten oxide nanostrucutres.</a> J Nanosci Nanotechnol. 11, 8214-8220 (2011).</li><li>Vallejos, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Aerosol+assisted+chemical+vapour+deposition+of+gas+sensitive+SnO2and+Au-functionalised+SnO2nanorods+via+a+non-catalysed+vapour+solid+(VS)+mechanism.">Aerosol assisted chemical vapour deposition of gas sensitive SnO2and Au-functionalised SnO2nanorods via a non-catalysed vapour solid (VS) mechanism.</a> Sci Rep. 6, 28464 (2016).</li><li>Hou, X., Choy, K. 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K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multifunctional+Au-ZnO+Plasmonic+Nanostructures+for+Enhanced+UV+Photodetector+and+Room+Temperature+NO+Sensing+Devices.">Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices.</a> Sci Rep. 4, 6481-6489 (2014).</li><li>Sutka, 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=A+straightforward+and+&amp;#34;green&amp;#34;+solvothermal+synthesis+of+Al+doped+zinc+oxide+plasmonic+nanocrystals+and+piezoresistive+elastomer+nanocomposite.">A straightforward and &amp;#34;green&amp;#34; solvothermal synthesis of Al doped zinc oxide plasmonic nanocrystals and piezoresistive elastomer nanocomposite.</a> RSC Advances. 5 (78), 63846-63852 (2015).</li><li>Vallejos, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chemoresistive+micromachined+gas+sensors+based+on+functionalized+metal+oxide+nanowires:+Performance+and+reliability.">Chemoresistive micromachined gas sensors based on functionalized metal oxide nanowires: Performance and reliability.</a> Sens Actuator B. 235, 525-534 (2016).</li><li>Bhachu, D. 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The authors have nothing to disclose

The aerosol-assisted CVD procedure detailed here leads to the formation of ZnO rods on silicon tiles of 10 mm x 10 mm. This procedure can be scaled-up to coat larger surfaces; however, notice that an increase in the reaction cell volume will require a readjustment of parameters, such as the carrier flow rate and the volume of solution. For larger reaction cells, it is also recommended to control the temperature gradients in the substrate, due to subtle gradients of less than 10 &#176;C possibly having a strong influence on the resulting morphology of the film, as demonstrated previously for the aerosol-assisted CVD of tungsten oxide8. To reproduce the results reported here, we recommend the use of an ultrasonic atomizer with similar operating frequency than that described in the protocol, as the average droplet size of the aerosol and in turn the resulting morphology of the film are influenced by this parameter7.
The selective deposition of other ZnO morphologies, rather than rods, can also be achieved by changing the precursor, deposition temperatures, or carrier solvents. For instance, the use of precursors such as diethyl zinc14 or zinc acetate15 has proved to lead to the formation of other morphological forms rather than hexagonal rods. We have also noticed that the use of different deposition temperatures during aerosol-assisted CVD produces changes in the morphology of the films, allowing for the formation of polycrystalline films at temperatures below 400 &#176;C, thicker hexagonal structures at temperatures exceeding 400 &#176;C, or degraded and less dense structures on the substrate when reaching 600 &#176;C. Similarly, the use of different solvents influences the morphology of the films, and for instance, we have proved recently that the use of methanol at the deposition temperature of 400 &#176;C encourages the formation of structures with flake-like morphology, whereas the use of acetone at the same temperature encourages the formation of upside-down cone-like structures9.
The role of the temperature and carrier solvents was also noticed previously on the aerosol-assisted CVD of other metal oxides structures (e.g., tungsten oxide5 and tin oxide6), and it was generally attributed to: chemical effects caused by reactive intermediates, which become active species for deposition or react homogeneously to form solid particles at the processing temperatures (this is more likely for solvents such as methanol and acetone, which can decompose at low temperatures e.g., &#60;500 &#176;C); and modulation of the rates of deposition (flux) and droplet evaporation (this is more likely dominant for solvents as ethanol, which do not form reactive radical species at the temperatures used in our experiments).
The protocol reported here is compatible with state-of-the-art microfabrication processes for silicon-based electronic devices and has the potential to be incorporated in processes involving high heat resistant flexible materials due to the relatively low temperatures for the aerosol-assisted CVD of structures. However, it is important to mention that the use of shadow masks for the selective growth of structures, such as in seeded methods based on the vapor-liquid-solid mechanism16, may have constraints in certain fabrication processes. On the other hand, the possibility to grow the structures via the non-catalyzed method presented here may have the advantage of less lithographic and metallization steps for in-chip integration of structures. Additionally, the relative low temperatures for the synthesis of ZnO rods may also allow for the use of this method with localized heating, a technique employed to confine the required thermal environment for both decomposition of the vapor-phase reactants and the growth kinetics of structures to a microscale area, reducing significantly the power consumption of high temperature (hot-wall) reactors17. The use of localized heating, for instance, has been shown feasible previously for the non-catalyzed aerosol-assisted CVD of tungsten oxide rods18. The growth of columnar ZnO structures with controlled morphology, that allow for their easy integration into different substrate and microfabrication processes, is of common interest in areas such as chemical sensing, photocatalysis, photonics and energy harvesting, amongst others.

ZnO is a II - VI semiconductor with a wide direct band gap (3.37 eV), large exciton binding energy (60 meV), spontaneous polarization and piezoelectric constants that make it an attractive material for electronics, optoelectronics, energy generators, photocatalysis and chemical sensing. Most of the interesting functionalities of ZnO are related to its wurtzite crystal structure and its non-polar (e.g., {100}, {110}) and polar (e.g., {001}, {111}) surfaces associated to specific structured morphological forms (e.g., rods, pyramids, plates). The control of these morphological forms requires synthetic methods capable of producing well-defined crystals, with uniform size, shape, and surface structure1,2,3,4. In this context, new additive (bottom-up synthesis) manufacturing strategies, particularly based on vapor-phase routes are industrially attractive and potentially advantageous as they provide the ability to generate structured films in a continuous rather than batch mode with high purity and high throughputs. These routes have demonstrated the formation of ZnO structured films previously, but usually employing catalyst-seeds such as gold and/or high processing temperatures of 900 - 1,300 &#176;C2 {Wang, 2008 #491} (this might be inconvenient for certain fabrication processes due to the need of extra processing steps and/or temperature incompatibilities for in-chip integration).
Recently, we have used a vapor-phase method based on aerosol-assisted CVD of inorganic or metal-organic precursors to achieve the selective deposition of metal oxide structures (e.g., tungsten oxide5or tin oxide6), without the need of catalyst-seeds and at lower temperatures than those reported for traditional CVD. This method works at atmospheric pressure and can use less-volatile precursors compared to traditional CVD; solubility is the key precursor requirement, as the precursor solution is delivered to the reaction zone in an aerosol form7. In aerosol-assisted CVD, the nucleation and growth kinetics of structured materials and thin films are influenced by the synthesis temperature and concentration of reactive species, which in turn influence the morphological form of the film8. Recently, we have studied the morphology dependence of ZnO to various aerosol-assisted CVD conditions (including precursors, temperatures, carrier solvents, and precursor concentrations) and found routes for the formation of structured ZnO with rods-, flakes-, or upside-down-cone-like morphologies, among others9.
Herein, we present the protocol for the aerosol-assisted CVD of columnar ZnO structures in the form of rods composed in the majority by {100} surfaces. This protocol is compatible with various substrates including silicon, silicon-based micromachined platforms, quartz, or high heat resistant polyimide foils. In this report, we focus on the coating of bare silicon wafers and silicon-based micromachined platforms employed for the fabrication of gas sensors. The aerosol-assisted CVD of ZnO consists of three processing steps that include: the preparation of substrates and set-up of deposition temperature, the preparation of the solution for aerosol generation, and the CVD process. These steps are described in detail below and a schematic view showing the main elements of the system is displayed in Figure 1.

<table border="0" cellpadding="0" cellspacing="0" width="824">
	<tbody>
		<tr height="25">
			<td height="25" style="height:25px;width:272px;">ZnCl2 99,999 % trace metal basis</td>
			<td style="width:124px;">Sigma-Aldrich&#160;</td>
			<td style="width:121px;">229997</td>
			<td style="width:307px;">used as purchased from manufacturer</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Ethanol &#8805;96%</td>
			<td style="width:124px;">Penta</td>
			<td style="width:121px;">71430</td>
			<td style="width:307px;">used as purchased from manufacturer</td>
		</tr>
		<tr height="108">
			<td height="108" style="height:108px;width:272px;">Reaction cell&#160;</td>
			<td style="width:124px;">home-made</td>
			<td style="width:121px;"></td>
			<td style="width:307px;">stainless steel cylindrical reaction cell (7000 mm3, diameter: 30 mm, height: 10 mm) with integrated heaters to reach the temperature of deposition and provided with a PID controller</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Ultrasonic liquid atomizer</td>
			<td style="width:124px;">Johnson Matthey</td>
			<td style="width:121px;"></td>
			<td style="width:307px;">Operating frequency &#8764;1,6 MHz</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:272px;">Flowmeter</td>
			<td style="width:124px;"></td>
			<td style="width:121px;"></td>
			<td style="width:307px;">To have a better control of this step the use of a mass flow controller is recommended.&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Nitrogen</td>
			<td style="width:124px;">Linde Gas A.S.</td>
			<td style="width:121px;"></td>
			<td style="width:307px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:272px;">Silicon wafers&#160;&#160;</td>
			<td style="width:124px;">MicroChemicals</td>
			<td style="width:121px;"></td>
			<td style="width:307px;">&#60;100&#62;, p-type, 525 &#181;m thick, cut into pieces (10 mm &#215; 10 mm )</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Glass vial - 100 ml</td>
			<td style="width:124px;"></td>
			<td style="width:121px;"></td>
			<td style="width:307px;">29/32 joint, 200 mm lenght</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Vacuum trap</td>
			<td style="width:124px;"></td>
			<td style="width:121px;"></td>
			<td style="width:307px;">29/32 joint, 5 mm hose barbs&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Graduated cylinder - 10 ml</td>
			<td style="width:124px;"></td>
			<td style="width:121px;"></td>
			<td style="width:307px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Universal support&#160;</td>
			<td style="width:124px;"></td>
			<td style="width:121px;"></td>
			<td style="width:307px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Balance</td>
			<td style="width:124px;"></td>
			<td style="width:121px;"></td>
			<td style="width:307px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">Scanning Electron Microscopy (SEM)</td>
			<td style="width:124px;">Tescan</td>
			<td style="width:121px;">Mira II LMU</td>
			<td style="width:307px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:272px;">X-ray diffraction (XRD)</td>
			<td style="width:124px;">Rigaku&#160;</td>
			<td style="width:121px;">Smart Lab 3kW</td>
			<td style="width:307px;">Cu K&#945; radiation</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:272px;">X-ray Photoelectron spectroscopy (XPS)</td>
			<td style="width:124px;">Kratos</td>
			<td style="width:121px;">AXIS Supra</td>
			<td style="width:307px;">Monochromatic&#160; K&#945; radiatio, 300 W emission power, magnetic lens, and charge compensation&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:272px;">Transmission Electron Microscopy (TEM)</td>
			<td style="width:124px;">Jeol</td>
			<td style="width:121px;">JEM 2100F</td>
			<td style="width:307px;">operated at 200kV using Schottky cathode and equiped with EDX&#160;</td>
		</tr>
	</tbody>
</table>

aerosol-assisted chemical vapor deposition of metal oxide structures: zinc oxide rods

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, their high cost production and/or incompatibility with microfabrication technologies, due to the use of pre-deposited catalyst-seeds and/or high processing temperatures exceeding 900 &#176;C, represent a drawback for a widespread use of these methods. Here, however, we report the synthesis of ZnO rods via a non-catalyzed vapor-solid mechanism enabled by using an aerosol-assisted chemical vapor deposition (CVD) method at 400 &#176;C with zinc chloride (ZnCl2) as the precursor and ethanol as the carrier solvent. This method provides both single-step formation of ZnO rods and the possibility of their direct integration with various substrate types, including silicon, silicon-based micromachined platforms, quartz, or high heat resistant polymers. This potentially facilitates the use of this method at a large-scale, due to its compatibility with state-of-the-art microfabrication processes for device manufacture. This report also describes the properties of these structures (e.g., morphology, crystalline phase, optical band gap, chemical composition, electrical resistance) and validates its gas sensing functionality towards carbon monoxide.

The aerosol-assisted CVD of ZnCl2 dissolved in ethanol leads to the formation of greyish uniform and adherent films on bare silicon wafers (relatively easily abraded mechanically). Characterization of the films using scanning electron microscopy (SEM) above 8,000X magnification displays quasi-aligned hexagonal shaped ZnO rods with lengths of &#8764;1,600 and diameters of &#8764;380 nm (Figure 2). Large errors in the set-point temperature or the presence of temperature gradients along the substrate during the CVD may cause the deposition of other ZnO morphologies (Figure 3) or films with non-uniform structures. In addition, uneven or non-adherent coatings may be related in part to poor temperature control, incorrect adjustment of the flow, and/or the use of a different carrier solvent than that specified in this protocol.
X-ray diffraction (XRD) analysis of the rods shows diffraction patterns associated with a hexagonal ZnO phase (P63mc space group, a = 3.2490 &#197;, b = 3.2490 &#197;, and c = 5.2050 &#197;; ICCD Card No. 5-0664). These patterns display a high intensity diffraction peak at 34.34&#176; 2&#952;, corresponding to the (002) plane of the hexagonal ZnO phase, along with other seven low intensity diffraction peaks at 31.75, 36.25, 47.54, 56.55, 62.87, 67.92, and 72,61&#176; 2&#952;, corresponding to the (100) (101) (102) (110) (103) (201) and (004) planes of the hexagonal ZnO phase, respectively. Characterization of the rods by high-resolution transmission electron microscopy (TEM) shows marked planar spacing (0.26 nm) consistent with the internal lattice of the (002) plane (d = 0.26025 nm) of the hexagonal ZnO phase identified by XRD. Energy-dispersive X-ray (EDX) spectroscopy shows the presence of Zn with relatively low chlorine contamination (found for Cl:Zn 0.05 at.%).
The estimation of the optical bandgap of the rods by means of diffuse reflectance measurements of films indicates an optical bandgap of 3.2 eV, consistent with the literature values for ZnO10. The analysis of the films using X-ray photoelectron spectroscopy (XPS) is characterized by Zn 2p1/2 and Zn 2p3/2 core level peaks spectra at 1,045 and 1,022 eV, respectively, consistent with those observed previously for ZnO11,12.
The use of this protocol on silicon-based micromachined platforms intended for gas sensing lead to the direct integration of columnar ZnO rods confined on the sensing-active area (400 x 400 &#181;m2), which is defined by a shadow mask. The electrical resistance of the films is in the order of k&#937; (&#8764; 100 k&#937;) measured at room temperature by using the interdigitated electrodes integrated into the silicon-based micromachined platforms. Figure 4 displays the picture of an array of four micromachined gas sensors based on aerosol-assisted CVD rods. The characteristics and fabrication process for the micromachined platforms have been described previously13. These microsystems are sensitive to relative low concentrations of carbon monoxide, with the maximum responses recorded (using a continuous gas flow test chamber13) when the sensors were operated at 360 &#176;C using the resistive microheaters integrated in the system (Figure 5).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56127/56127fig1.jpg" /> 
Figure 1: Schematic View of the Aerosol-assisted CVD System.
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56127/56127fig2.jpg" /> 
Figure 2: Top (A) and Cross-sectional (B) SEM Images of the ZnO Rods Deposited via Aerosol-Assisted CVD. <a href="https://cloudflare.jove.com/files/ftp_upload/56127/56127fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/56127/56127fig3.jpg" /> 
Figure 3: Cross-sectional SEM Images of ZnO Deposited via Aerosol-assisted CVD at 300 (A), 400 (B), 500 (C), and 600 &#176;C (D). <a href="https://cloudflare.jove.com/files/ftp_upload/56127/56127fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/56127/56127fig4.jpg" /> 
Figure 4: Silicon-based Micromachined Platform with 4 Microsensors Mounted on a TO8-package (A), and Detailed View of a Microsensor (B) and the ZnO Rods Deposited on the Edge of an Electrode (C). <a href="https://cloudflare.jove.com/files/ftp_upload/56127/56127fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/56127/56127fig5.jpg" /> 
Figure 5: Electrical Resistance Changes of the ZnO Rods Towards Various Concentrations (25, 20, 10 and 5 ppm) of Carbon Monoxide. <a href="https://cloudflare.jove.com/files/ftp_upload/56127/56127fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Aerosol-assisted Chemical Vapor Deposition of Metal Oxide Structures: Zinc Oxide Rods at JoVE.com

Aerosol-assisted Chemical Vapor Deposition of Metal Oxide Structures: Zinc Oxide Rods

Columnar zinc oxide structures in the form of rods are synthesized via aerosol-assisted chemical vapor deposition without the use of pre-deposited catalyst-seed particles. This method is scalable and compatible with various substrates based on either silicon, quartz or polymers.

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, ...

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13.0K Views.  Instituto de Microelectrónica de Barcelona (IMB-CNM, CSIC). Columnar zinc oxide structures in the form of rods are synthesized via aerosol-assisted chemical vapor deposition without the use of pre-deposited catalyst-seed particles. This method is scalable and compatible with various substrates based on either silicon, quartz or polymers.

Video: Aerosol-assisted Chemical Vapor Deposition of Metal Oxide Structures: Zinc Oxide Rods

Preparation of Hydrophobic Metal-Organic Frameworks via Plasma Enhanced Chemical Vapor Deposition of Perfluoroalkanes for the Removal of Ammonia

Plasma enhanced chemical vapor deposition (PECVD) of perfluoroalkanes has long been studied for tuning the wetting properties of surfaces. For high surface area microporous materials, such as metal-organic frameworks (MOFs), unique challenges present themselves for PECVD treatments. Herein the protocol for development of a MOF that was previously unstable to humid conditions is presented. The protocol describes the synthesis of Cu-BTC (also known as HKUST-1), the treatment of Cu-BTC with PECVD of perfluoroalkanes, the aging of materials under humid conditions, and the subsequent ammonia microbreakthrough experiments on milligram quantities of microporous materials. Cu-BTC has an extremely high surface area (~1,800 m2/g) when compared to most materials or surfaces that have been previously treated by PECVD methods. Parameters such as chamber pressure and treatment time are extremely important to ensure the perfluoroalkane plasma penetrates to and reacts with the inner MOF surfaces. Furthermore, the protocol for ammonia microbreakthrough experiments set forth here can be utilized for a variety of test gases and microporous materials.

Herein the procedures for plasma enhanced chemical vapor deposition of perfluoroalkanes on microporous materials such as metal-organic frameworks to enhance their stability and hydrophobicity are described. Furthermore, breakthrough testing of milligram quantities of samples is described in detail.

Plasma enhanced chemical vapor deposition (PECVD) of perfluoroalkanes has long been studied for tuning the wetting properties of surfaces. For high ...

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 ...

Chemical Vapor Deposition of an Organic Magnet, Vanadium Tetracyanoethylene

Recent progress in the field of organic materials has yielded devices such as organic light emitting diodes (OLEDs) which have advantages not found in traditional materials, including low cost and mechanical flexibility. In a similar vein, it would be advantageous to expand the use of organics into high frequency electronics and spin-based electronics. This work presents a synthetic process for the growth of thin films of the room temperature organic ferrimagnet, vanadium tetracyanoethylene (V[TCNE]x, x~2) by low temperature chemical vapor deposition (CVD). The thin film is grown at &#60;60 &#176;C, and can accommodate a wide variety of substrates including, but not limited to, silicon, glass, Teflon and flexible substrates. The conformal deposition is conducive to pre-patterned and three-dimensional structures as well. Additionally this technique can yield films with thicknesses ranging from 30 nm to several microns. Recent progress in optimization of film growth creates a film whose qualities, such as higher Curie temperature (600 K), improved magnetic homogeneity, and narrow ferromagnetic resonance line-width (1.5 G) show promise for a variety of applications in spintronics and microwave electronics.

We present the synthesis of the organic-based ferrimagnet vanadium tetracyanoethylene (V[TCNE]x, x~2) via low temperature chemical vapor deposition (CVD). This optimized recipe yields an increase in Curie temperature from 400 K to over 600 K and a dramatic improvement in magnetic resonance properties.

Recent progress in the field of organic materials has yielded devices such as organic light emitting diodes (OLEDs) which have advantages not found in ...

Photochemical Oxidative Growth of Iridium Oxide Nanoparticles on CdSe@CdS Nanorods

We demonstrate a procedure for the photochemical oxidative growth of iridium oxide catalysts on the surface of seeded cadmium selenide-cadmium sulfide (CdSe@CdS) nanorod photocatalysts. Seeded rods are grown using a colloidal hot-injection method and then moved to an aqueous medium by ligand exchange. CdSe@CdS nanorods, an iridium precursor and other salts are mixed and illuminated. The deposition process is initiated by absorption of photons by the semiconductor particle, which results with formation of charge carriers that are used to promote redox reactions. To insure photochemical oxidative growth we used an electron scavenger. The photogenerated holes oxidize the iridium precursor, apparently in a mediated oxidative pathway. This results in the growth of high quality crystalline iridium oxide particles, ranging from 0.5 nm to about 3 nm, along the surface of the rod. Iridium oxide grown on CdSe@CdS heterostructures was studied by a variety of characterization methods, in order to evaluate its characteristics and quality. We explored means for control over particle size, crystallinity, deposition location on the CdS rod, and composition. Illumination time and excitation wavelength were found to be key parameters for such control. The influence of different growth conditions and the characterization of these heterostructures are described alongside a detailed description of their synthesis. Of significance is the fact that the addition of iridium oxide afforded the rods astounding photochemical stability under prolonged illumination in pure water (alleviating the requirement for hole scavengers).

A protocol for the photochemical oxidative growth of small crystalline iridium oxide nanoparticles on the surface of CdSe@CdS seeded rod nanoparticles is presented.

We demonstrate a procedure for the photochemical oxidative growth of iridium oxide catalysts on the surface of seeded cadmium selenide-cadmium sulfide ...

Highly Stable, Functional Hairy Nanoparticles and Biopolymers from Wood Fibers: Towards Sustainable Nanotechnology

Nanoparticles, as one of the key materials in nanotechnology and nanomedicine, have gained significant importance during the past decade. While metal-based nanoparticles are associated with synthetic and environmental hassles, cellulose introduces a green, sustainable alternative for nanoparticle synthesis. Here, we present the chemical synthesis and separation procedures to produce new classes of hairy nanoparticles (bearing both amorphous and crystalline regions) and biopolymers based on wood fibers. Through periodate oxidation of soft wood pulp, the glucose ring of cellulose is opened at the C2-C3 bond to form 2,3-dialdehyde groups. Further heating of the partially oxidized fibers (e.g., T = 80 &#176;C) results in three products, namely fibrous oxidized cellulose, sterically stabilized nanocrystalline cellulose (SNCC), and dissolved dialdehyde modified cellulose (DAMC), which are well separated by intermittent centrifugation and co-solvent addition. The partially oxidized fibers (without heating) were used as a highly reactive intermediate to react with chlorite for converting almost all aldehyde to carboxyl groups. Co-solvent precipitation and centrifugation resulted in electrosterically stabilized nanocrystalline cellulose (ENCC) and dicarboxylated cellulose (DCC). The aldehyde content of SNCC and consequently surface charge of ENCC (carboxyl content) were precisely controlled by controlling the periodate oxidation reaction time, resulting in highly stable nanoparticles bearing more than 7 mmol functional groups per gram of nanoparticles (e.g., as compared to conventional NCC bearing &#60;&#60; 1 mmol functional group/g). Atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) attested to the rod-like morphology. Conductometric titration, Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), dynamic light scattering (DLS), electrokinetic-sonic-amplitude (ESA) and acoustic attenuation spectroscopy shed light on the superior properties of these nanomaterials.

Synthesis schemes to prepare highly stable wood fiber-based hairy nanoparticles and functional cellulose-based biopolymers have been detailed.

Nanoparticles, as one of the key materials in nanotechnology and nanomedicine, have gained significant importance during the past decade. While ...

In Situ Detection and Single Cell Quantification of Metal Oxide Nanoparticles Using Nuclear Microprobe Analysis

Micro-analytical techniques based on chemical element imaging enable the localization and quantification of chemical composition at the cellular level. They offer new possibilities for the characterization of living systems and are particularly appropriate for detecting, localizing and quantifying the presence of metal oxide nanoparticles both in biological specimens and the environment. Indeed, these techniques all meet relevant requirements in terms of (i) sensitivity (from 1 up to 10 &#181;g.g-1 of dry mass), (ii) micrometer range spatial resolution, and (iii) multi-element detection. Given these characteristics, microbeam chemical element imaging can powerfully complement routine imaging techniques such as optical and fluorescence microscopy. This protocol describes how to perform a nuclear microprobe analysis on cultured cells (U2OS) exposed to titanium dioxide nanoparticles. Cells must grow on and be exposed directly in a specially designed sample holder used on the optical microscope and in the nuclear microprobe analysis stages. Plunge-freeze cryogenic fixation of the samples preserves both the cellular organization and the chemical element distribution. Simultaneous nuclear microprobe analysis (scanning transmission ion microscopy, Rutherford backscattering spectrometry and particle induced X-ray emission) performed on the sample provides information about the cellular density, the local distribution of the chemical elements, as well as the cellular content of nanoparticles. There is a growing need for such analytical tools within biology, especially in the emerging context of Nanotoxicology and Nanomedicine for which our comprehension of the interactions between nanoparticles and biological samples must be deepened. In particular, as nuclear microprobe analysis does not require nanoparticles to be labelled, nanoparticle abundances are quantifiable down to the individual cell level in a cell population, independently of their surface state.

In Situ Detection and Single Cell Quantification of Metal Oxide Nanoparticles Using Nuclear Microprobe Analysis

We describe a procedure for the detection of chemical elements present in situ in human cells as well as their in vitro quantification. The method is well-suited to any cell type and is particularly useful for quantitative chemical analyses in single cells following in vitro metal oxide nanoparticles exposure.

Micro-analytical techniques based on chemical element imaging enable the localization and quantification of chemical composition at the cellular level. ...

Reactive Vapor Deposition of Conjugated Polymer Films on Arbitrary Substrates

We demonstrate a method of conformally coating conjugated polymers on arbitrary substrates using a custom-designed, low-pressure reaction chamber. Conductive polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4-propylenedioxythiophene) (PProDOT), and a semiconducting polymer, poly(thieno[3,2-b]thiophene) (PTT), were deposited on unconventional highly-disordered and textured substrates with high surface areas, such as paper, towels and fabrics. This reported deposition chamber is an improvement of previous vapor reactors because our system can accommodate both volatile and nonvolatile monomers, such as 3,4-propylenedioxythiophene and thieno[3,2-b]thiophene. Utilization of both solid and liquid oxidants are also demonstrated. One limitation of this method is that it lacks sophisticated in situ thickness monitors. Polymer coatings made by the commonly used solution-based coating methods, such as spin-coating and surface grafting, are often not uniform or susceptible to mechanical degradation. This reported vapor phase deposition method overcomes those drawbacks and is a strong alternative to common solution-based coating methods. Notably, polymer films coated by the reported method are uniform and conformal on rough surfaces, even at a micrometer scale. This feature allows for future application of vapor deposited polymers in electronics devices on flexible and highly textured substrates.

This paper presents a protocol for reactive vapor deposition of poly(3,4-ethylenedioxythiophene), poly(3,4-propylenedioxythiophene), and poly(thieno[3,2-b]thiophene) films on glass slides and rough substrates, such as textiles and paper.

We demonstrate a method of conformally coating conjugated polymers on arbitrary substrates using a custom-designed, low-pressure reaction chamber. ...

Syntheses, Crystallization, and Spectroscopic Characterization of 3,5-Lutidine N-Oxide Dehydrate

The synthesis of 3,5-lutidine N-oxide dehydrate, 1, was achieved in the synthesis route of 2-amino-pyridine-3,5-dicarboxylic acid. Ochiai first used the methodology for non-substituted pyridines in 1957 in a 12 h process, but no X-ray suitable crystals were obtained. The substituted ring used in the methodology presented here clearly influenced the addition of water molecules into the asymmetric unit, which confers a different nucleophilic strength in 1. The X-ray suitable crystal compound 1 was possible due to the stabilization of the negative charge in the oxygen by the presence of two water molecules where the hydrogen atoms donate positive charge into the ring; such water molecules serve well to construct a supramolecular interaction. The hydrated molecules may be possible for the alkaline system that is reached by adjusting the pH to 10. Importantly, the double methyl substituted ring and a reaction time of 5 h, makes it a more versatile method and with wider chemical applications for future ring insertions.

Syntheses, Crystallization, and Spectroscopic Characterization of 3,5-Lutidine N-Oxide Dehydrate

Herein, we report the synthesis and crystallization of 3,5-lutidine N-oxide dehydrate by a simple protocol that differs from the classical synthesis of pyridine N-oxide. This protocol utilizes different starting material and involves less reaction time to yield a new solvated supramolecular structure, which crystallizes under slow evaporation.

The synthesis of 3,5-lutidine N-oxide dehydrate, 1, was achieved in the synthesis route of 2-amino-pyridine-3,5-dicarboxylic acid. Ochiai first used the ...

Solvothermal Synthesis of MIL-96 and UiO-66-NH2 on Atomic Layer Deposited Metal Oxide Coatings on Fiber Mats

Metal-organic frameworks (MOFs), which contain reactive metal clusters and organic ligands allowing for large porosities and surface areas, have proven effective in gas adsorption, separations, and catalysis. MOFs are most commonly synthesized as bulk powder, requiring additional processes to adhere them to functional devices and fabrics that risk decreasing the powder porosity and adsorption capacity. Here, we demonstrate a method of first coating fabrics with metal oxide films using atomic layer deposition (ALD). This process creates conformal films of controllable thickness on each fiber, while providing a more reactive surface for MOF nucleation. By submerging the ALD coated fabric in solution during solvothermal MOF synthesis, the MOFs create a conformal, well-adhered coating on the fibers, resulting in a MOF-functionalized fabric, without additional adhesion materials that may block MOF pores and functional sites. Here we demonstrate two solvothermal synthesis methods. First, we form a MIL-96(Al) layer on polypropylene fibers using synthetic conditions that convert the metal oxide to MOF. Using initial inorganic films of varying thicknesses, diffusion of the organic linker into the inorganic allows us to control the extent of MOF loading on the fabric. Second, we perform a solvothermal synthesis of UiO-66-NH2 in which the MOF nucleates on the conformal metal oxide coating on polyamide-6 (PA-6) fibers, thereby producing a uniform and conformal thin film of MOF on the fabric. The resulting materials can be directly incorporated into filter devices or protective clothing and eliminate the maladroit qualities of loose powder.

Solvothermal Synthesis of MIL-96 and UiO-66-NH2 on Atomic Layer Deposited Metal Oxide Coatings on Fiber Mats

Metal-organic frameworks are effective in gas storage and heterogeneous catalysis, but typical synthesis methods result in loose powders that are difficult to incorporate into smart materials. We demonstrate a method of first coating fabrics with ALD metal oxides, resulting in conformal films of MOF on the fabrics during solvothermal synthesis.

Metal-organic frameworks (MOFs), which contain reactive metal clusters and organic ligands allowing for large porosities and surface areas, have proven ...

Preparation of Polyoxometalate-based Photo-responsive Membranes for the Photo-activation of Manganese Oxide Catalysts

This paper presents a method to prepare charge-transfer chromophores using polyoxotungstate (PW12O403-), transition metal ions (Ce3+ or Co2+), and organic polymers, with the aim of photo-activating oxygen-evolving manganese oxide catalysts, which are important components in artificial photosynthesis. The cross-linking technique was applied to obtain a self-standing membrane with a high PW12O403- content. Incorporation and structure retention of PW12O403- within the polymer matrix were confirmed by FT-IR and micro-Raman spectroscopy, and optical characteristics were investigated by UV-Vis spectroscopy, which revealed successful construction of the metal-to-metal charge transfer (MMCT) unit. After deposition of MnOx oxygen evolving catalysts, photocurrent measurements under visible light irradiation verified the sequential charge transfer, Mn &#8594; MMCT unit &#8594; electrode, and&#160;the photocurrent intensity was consistent with the redox potential of the donor metal (Ce or Co). This method provides a new strategy for preparing integrated systems involving catalysts and photon-absorption parts for use with photo-functional materials.

Here, we present a protocol to prepare charge transfer chromophores based on a polyoxometalate/polymer composite membrane.

This paper presents a method to prepare charge-transfer chromophores using polyoxotungstate (PW12O403-), transition metal ions (Ce3+ or Co2+), and organic ...