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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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><tbody><tr><td>ZnCl&lt;sub&gt;2&lt;/sub&gt; 99,999 % trace metal basis</td><td>Sigma-Aldrich&amp;nbsp;</td><td>229997</td><td>used as purchased from manufacturer</td></tr><tr><td>Ethanol &amp;ge;96%</td><td>Penta</td><td>71430</td><td>used as purchased from manufacturer</td></tr><tr><td>Reaction cell&amp;nbsp;</td><td>home-made</td><td></td><td>stainless steel cylindrical reaction cell (7000 mm&lt;sup&gt;3&lt;/sup&gt;, diameter: 30 mm, height: 10 mm) with integrated heaters to reach the temperature of deposition and provided with a PID controller</td></tr><tr><td>Ultrasonic liquid atomizer</td><td>Johnson Matthey</td><td></td><td>Operating frequency &amp;sim;1,6 MHz</td></tr><tr><td>Flowmeter</td><td></td><td></td><td>To have a better control of this step the use of a mass flow controller is recommended.&amp;nbsp;</td></tr><tr><td>Nitrogen</td><td>Linde Gas A.S.</td><td></td><td></td></tr><tr><td>Silicon wafers&amp;nbsp;&amp;nbsp;</td><td>MicroChemicals</td><td></td><td>&amp;lt;100&gt;, p-type, 525 &amp;micro;m thick, cut into pieces (10 mm &amp;times; 10 mm )</td></tr><tr><td>Glass vial - 100 ml</td><td></td><td></td><td>29/32 joint, 200 mm lenght</td></tr><tr><td>Vacuum trap</td><td></td><td></td><td>29/32 joint, 5 mm hose barbs&amp;nbsp;</td></tr><tr><td>Graduated cylinder - 10 ml</td><td></td><td></td><td></td></tr><tr><td>Universal support&amp;nbsp;</td><td></td><td></td><td></td></tr><tr><td>Balance</td><td></td><td></td><td></td></tr><tr><td>Scanning Electron Microscopy (SEM)</td><td>Tescan</td><td>Mira II LMU</td><td></td></tr><tr><td>X-ray diffraction (XRD)</td><td>Rigaku&amp;nbsp;</td><td>Smart Lab 3kW</td><td>Cu K&amp;alpha; radiation</td></tr><tr><td>X-ray Photoelectron spectroscopy (XPS)</td><td>Kratos</td><td>AXIS Supra</td><td>Monochromatic&amp;nbsp; K&amp;alpha; radiatio, 300 W emission power, magnetic lens, and charge compensation&amp;nbsp;</td></tr><tr><td>Transmission Electron Microscopy (TEM)</td><td>Jeol</td><td>JEM 2100F</td><td>operated at 200kV using Schottky cathode and equiped with EDX&amp;nbsp;</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>

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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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13.1K Views.  Instituto de Microelectrónica de Barcelona (IMB-CNM, CSIC). In kolomvorm zinkoxide structuren in de vorm van staven worden gesynthetiseerd via aerosol-bijgewoonde chemische damp afzetting zonder het gebruik van vooraf gedeponeerde katalysator-zaad deeltjes. Deze methode is schaalbaar en compatibel met verschillende ondergronden op basis van silicium, kwarts of polymeren.

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

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.

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 ...

Engineering

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, ...

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and reduction half reactions, and fast electron (hole) diffusion and charge separation. Nanowires present suitable architectures to meet these requirements. Axially segmented Ag|ZnO and radially segmented (coaxial) TiO2-Ag nanowires with a diameter of 200 nm and a length of 6-20 &#181;m were made by templated electrodeposition within the pores of polycarbonate track-etched (PCTE) or anodized aluminum oxide (AAO) membranes, respectively. In the photocatalytic experiments, the ZnO and TiO2 phases acted as photoanodes, and Ag as cathode. No external circuit is needed to connect both electrodes, which is a key advantage over conventional photo-electrochemical cells. For making segmented Ag|ZnO nanowires, the Ag salt electrolyte was replaced after formation of the Ag segment to form a ZnO segment attached to the Ag segment. For making coaxial TiO2-Ag nanowires, a TiO2 gel was first formed by the electrochemically induced sol-gel method. Drying and thermal annealing of the as-formed TiO2 gel resulted in the formation of crystalline TiO2 nanotubes. A subsequent Ag electrodeposition step inside the TiO2 nanotubes resulted in formation of coaxial TiO2-Ag nanowires. Due to the combination of an n-type semiconductor (ZnO or TiO2) and a metal (Ag) within the same nanowire, a Schottky barrier was created at the interface between the phases. To demonstrate the photocatalytic activity of these nanowires, the Ag|ZnO nanowires were used in a photocatalytic experiment in which H2 gas was detected upon UV illumination of the nanowires dispersed in a methanol/water mixture. After 17 min&#160;of illumination, approximately 0.2 vol% H2 gas was detected from a suspension of ~0.1 g of Ag|ZnO nanowires in a 50 ml 80 vol% aqueous methanol solution.

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Procedures are outlined to prepare segmented and coaxial nanowires via templated electrodeposition in nanopores. As examples, segmented nanowires consisting of Ag and ZnO segments, and coaxial nanowires consisting of a TiO2 shell and a Ag core were made. The nanowires were used in photocatalytic hydrogen formation experiments.

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and ...

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 ...

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O substrates outside vacuum at low temperature. The performance of photovoltaic devices featuring atmospherically fabricated ZnO/Cu2O heterojunction was dependent on the conditions of AP-SALD film deposition, namely, the substrate temperature and deposition time, as well as on the Cu2O substrate exposure to oxidizing agents prior to and during the ZnO deposition. Superficial Cu2O to CuO oxidation was identified as a limiting factor to heterojunction quality due to recombination at the ZnO/Cu2O interface. Optimization of AP-SALD conditions as well as keeping Cu2O away from air and moisture in order to minimize Cu2O surface oxidation led to improved device performance. A three-fold increase in the open-circuit voltage (up to 0.65 V) and a two-fold increase in the short-circuit current density produced solar cells with a record 2.2% power conversion efficiency (PCE). This PCE is the highest reported for a Zn1-xMgxO/Cu2O heterojunction formed outside vacuum, which highlights atmospheric pressure spatial ALD as a promising technique for inexpensive and scalable fabrication of Cu2O-based photovoltaics.

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO

Here we present a protocol for synthesizing Zn1-xMgxO/Cu2O heterojunctions in open-air at low temperature via atmospheric pressure spatial atomic layer deposition (AP-SALD) of Zn1-xMgxO on cuprous oxide. Such high quality conformal metal oxides can be grown on a variety of substrates including plastics by this cheap and scalable method.

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O ...

Synthesis and Characterization of High c-axis ZnO Thin Film by Plasma Enhanced Chemical Vapor Deposition System and its UV Photodetector Application

In this study, zinc oxide (ZnO) thin films with high c-axis (0002) preferential orientation have been successfully and effectively synthesized onto silicon (Si) substrates via different synthesized temperatures by using plasma enhanced chemical vapor deposition (PECVD) system. The effects of different synthesized temperatures on the crystal structure, surface morphologies and optical properties have been investigated. The X-ray diffraction (XRD) patterns indicated that the intensity of (0002) diffraction peak became stronger with increasing synthesized temperature until 400 oC. The diffraction intensity of (0002) peak gradually became weaker accompanying with appearance of (10-10) diffraction peak as the synthesized temperature up to excess of 400 oC. The RT photoluminescence (PL) spectra exhibited a strong near-band-edge (NBE) emission observed at around 375 nm and a negligible deep-level (DL) emission located at around 575 nm under high c-axis ZnO thin films. Field emission scanning electron microscopy (FE-SEM) images revealed the homogeneous surface and with small grain size distribution. The ZnO thin films have also been synthesized onto glass substrates under the same parameters for measuring the transmittance.
For the purpose of ultraviolet (UV) photodetector application, the interdigitated platinum (Pt) thin film (thickness ~100 nm) fabricated via conventional optical lithography process and radio frequency (RF) magnetron sputtering. In order to reach Ohmic contact, the device was annealed in argon circumstances at 450 oC by rapid thermal annealing (RTA) system for 10 min. After the systematic measurements, the current-voltage (I-V) curve of photo and dark current and time-dependent photocurrent response results exhibited a good responsivity and reliability, indicating that the high c-axis ZnO thin film is a suitable sensing layer for UV photodetector application.

We offered a method to directly synthesize high c-axis (0002) ZnO thin film by plasma enhanced chemical vapor deposition. The as-synthesized ZnO thin film combined with Pt interdigitated electrode was used as sensing layer for ultraviolet photodetector, showing a high performance through a combination of its good responsivity and reliability.

In this study, zinc oxide (ZnO) thin films with high c-axis (0002) preferential orientation have been successfully and effectively synthesized onto ...

Seedless Growth of Bismuth Nanowire Array via Vacuum Thermal Evaporation

Here a seedless and template-free technique is demonstrated to scalably grow bismuth nanowires, through thermal evaporation in high vacuum at RT. Conventionally reserved for the fabrication of metal thin films, thermal evaporation deposits bismuth into an array of vertical single crystalline nanowires over a flat thin film of vanadium held at RT, which is freshly deposited by magnetron sputtering or thermal evaporation. By controlling the temperature of the growth substrate the length and width of the nanowires can be tuned over a wide range. Responsible for this novel technique is a previously unknown nanowire growth mechanism that roots in the mild porosity of the vanadium thin film. Infiltrated into the vanadium pores, the bismuth domains (~ 1 nm) carry excessive surface energy that suppresses their melting point and continuously expels them out of the vanadium matrix to form nanowires. This discovery demonstrates the feasibility of scalable vapor phase synthesis of high purity nanomaterials without using any catalysts.

A protocol for seedless and high yield growth of bismuth nanowire arrays through vacuum thermal evaporation technique is presented.

Here a seedless and template-free technique is demonstrated to scalably grow bismuth nanowires, through thermal evaporation in high vacuum at RT. ...

Preparation of ZnO Nanorod/Graphene/ZnO Nanorod Epitaxial Double Heterostructure for Piezoelectrical Nanogenerator by Using Preheating Hydrothermal

Well-aligned ZnO nanostructures have been intensively studied over the last decade for remarkable physical properties and enormous applications. Here, we describe a one-step fabrication technique to synthesis freestanding ZnO nanorod/graphene/ZnO nanorod double heterostructure. The preparation of the double heterostructure is performed by using thermal chemical vapor deposition (CVD) and preheating hydrothermal technique. In addition, the morphological properties were characterized by using the scanning electron microscopy (SEM). The utility of freestanding double heterostructure is demonstrated by fabricating the piezoelectric nanogenerator. The electrical output is improved up to 200% compared to that of a single heterostructure owing to the coupling effect of the piezoelectricity between the arrays of ZnO nanorods on the top and bottom of graphene. This unique double heterostructure have a tremendous potential for applications of electrical and optoelectrical devices where the high number density and specific surface area of nanorod are needed, such as pressure sensor, immuno-biosensor and dye-sensitized solar cells.

One-step fabrication method for obtaining freestanding epitaxial double heterostructure is presented. This approach could achieve ZnO coverage with a higher number density than that of the epitaxial single heterostructure, leading to a piezoelectric nanogenerator with an increased output electrical performance.

Well-aligned ZnO nanostructures have been intensively studied over the last decade for remarkable physical properties and enormous applications. Here, we ...

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 ...

Zinc-Sponge Battery Electrodes that Suppress Dendrites

We report two methods to create zinc-sponge electrodes that suppress dendrite formation and shape change for rechargeable zinc batteries. Both methods are characterized by creating a paste made of zinc particles, organic porogen, and viscosity-enhancing agent that is heated under an inert gas and then air. During heating under the inert gas, the zinc particles anneal together, and the porogen decomposes; under air, the zinc fuses and residual organic burns out, yielding an open-cell metal foam or sponge. We tune the mechanical and electrochemical properties of the zinc sponges by varying zinc-to-porogen mass ratio, heating time under inert gas and air, and size and shape of the zinc and porogen particles. An advantage of the reported methods is their ability to finely tune zinc-sponge architecture. The selected size and shape of the zinc and porogen particles influence the morphology of the pore structure. A limitation is that resulting sponges have disordered pore structures that result in low mechanical strength at low volume fractions of zinc (&#60;30%). Applications for these zinc-sponge electrodes include batteries for grid-storage, personal electronics, electric vehicles, and electric aviation. Users can expect zinc-sponge electrodes to cycle up to 40% depth of discharge at technologically relevant rates and areal capacities without the formation of separator-piercing dendrites.

The goal of the reported protocols is to create rechargeable zinc-sponge electrodes that suppress dendrites and shape change in zinc batteries, such as nickel&#8211;zinc or zinc&#8211;air.

We report two methods to create zinc-sponge electrodes that suppress dendrite formation and shape change for rechargeable zinc batteries. Both methods are ...

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

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