Name: writing and low-temperature characterization of oxide nanostructures
Uploaded: 2014-07-18T15:00:00
Duration: 6 min 43 s
Description: Watch this Scientific Journal Video about Writing and Low-Temperature Characterization of Oxide Nanostructures at JoVE.com

The long-standing collaboration with Chang-Beom Eom at the University of Wisconsin-Madison, who provided the LAO/STO samples, is gratefully acknowledged. Video editing assistance from Christopher Solis is greatly appreciated. This work is supported by NSF (DMR-1104191, DMR-1124131), ARO (W911NF-08-1-0317), and AFOSR (FA9550-10-1-0524, FA9550-12-1-0268, FA9550-12-1-0057).

1. Obtain LAO/STO Heterostructures
<ol>
	<li>Obtain an oxide heterostructure consisting of 3.4 unit cells of LAO grown by pulsed laser deposition on TiO2-terminated STO substrates. Details of sample growth are described in Ref.&#160;22.</li>
</ol>
2. Photolithographic Processing of Samples
Create electrical contacts to the LAO/STO interface, with bonding pads for wiring canvases to a chip carrier. The individual processing steps are described in detail below.
<ol>
	<li>Spin photoresist
	<ol>
		<li>Spin photoresist on the samples at 600 rpm for 5&#160;sec, then at 4,000&#160;rpm for 30&#160;sec. The photoresist layer will be about 2&#160;&#181;m thick. Bake the samples at 95 &#176;C for 1 min.</li>
	</ol>
	</li>
	<li>Expose photoresist using a mask aligner with 320 nm light for 100 sec with a dose of 5&#160;mW/cm2.</li>
	<li>Develop the photoresist in photoresist developer for 1 min.</li>
	<li>Ion milling
	<ol>
		<li>Use an Ar+ ion mill to remove 15 nm of material (LAO and STO) in the areas not covered by photoresist. Place the samples at a 22.5&#176; angle to the direction perpendicular to the incoming Ar+ ion beam. If the Ar+ etching rate is not calibrated, perform a calibration run to ensure that the correct amount of material is removed. Determine the etching depth using AFM or equivalent profilmetry.</li>
	</ol>
	</li>
	<li>DC sputtering of Ti and Au
	<ol>
		<li>Deposit 4&#160;nm Ti, then 25&#160;nm Au onto the samples so that the Au makes electrical contact with the exposed STO layer. The sputtering pressure is in the range 2-6 x 10-7 Torr, and the sputtering takes place with the sample at RT. Pre-sputter Ti for 10 min with shutter closed at 100 W, then open shutter and sputter for 20 sec at 100 W. Upon completion, immediately pre-sputter Au for 1 min&#160;at 50&#160;W then sputter Au for 30 sec to the samples at 50&#160;W. Calibrate the time to produce the desired Ti and Au thicknesses.</li>
	</ol>
	</li>
	<li>Lift-off
	<ol>
		<li>Use Acetone/IPA ultrasonic wash to remove photoresist from the surface of the samples.</li>
	</ol>
	</li>
	<li>Second layer
	<ol>
		<li>A second lithographic process, excluding step 4 (i.e., excluding ion milling), is used to create gold wire connections to individual bonding pads. The two patterns must be well-aligned to ensure that they do not produce electrical shorts.</li>
	</ol>
	</li>
	<li>Plasma cleaning.
	<ol>
		<li>An IPC Barrel Etcher is used to remove the photoresist residue in the pattern trench. The instrument used at the 100 W and 1 Torr argon for 1 min</li>
	</ol>
	</li>
</ol>
3. Wire Bond a Sample to Prepare for Writing
<ol>
	<li>Mount the LAO/STO sample in a chip carrier (Figure 2A) with 28 available pins.</li>
	<li>Wire bond structure</li>
</ol>
NOTE: Use a wire bonder to make electrical connections between bonding pads on the sample and the chip carrier. Attach 1 mil (25 micrometer) gold wires between the electrical contacts and the chip carrier. Write nanostructures
4. Write Nanostructures
<ol>
	<li>Create an informal sketch of the conductive nanostructure (Figure 3A).</li>
	<li>Open the scalable vector graphics (SVG) editor (Figure 3B).
	<ol>
		<li>Use a template or define the window size to match that of the AFM image.</li>
		<li>Load the AFM image of the sample into the SVG editor.</li>
		<li>Create nanostructure elements overlaid on the AFM image.</li>
	</ol>
	</li>
	<li>Load the SVG file into the nanolithography program.</li>
	<li>Run the lithography software to create a conductive nanostructure.
	<ol>
		<li>Use Vtip=+10 V to create nanostructures, and Vtip=-10 V to erase nanostructures.</li>
		<li>Move the c-AFM tip at a speed ranging from 200 nm/sec to 2 &#181;m/sec.</li>
	</ol>
	</li>
</ol>
5. Cool Device and Take Measurements
<ol>
	<li>Turn off all white lights and use red filters/light sources.</li>
	<li>Extract the sample from the AFM system.</li>
	<li>Load the sample into the dilution refrigerator (A).</li>
	<li>Measure resistance vs. temperature (B) as the sample is cooled.</li>
	<li>Measure transport properties at low temperatures (C).</li>
</ol>

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The authors have nothing to disclose.

Successful creation of nanostructures depends on several critical steps. It is important that the LAO/STO samples are grown with a thickness that is known to be at the boundary between the insulating and conductive phase. (Details of sample growth fall outside the scope of this paper, but are crucial for overall success.) Second, it is important to have relative humidity within the range 25-45% for successful c-AFM writing. Values below 25% are unlikely to produce conductive nanostructures, while too high humidity will generally produce uncontrollably large features. Also, temperature control of the AFM is important if the c-AFM tip needs to achieve precise registry over long periods of time. Once the nanostructures are created, they must be placed in a vacuum environment if experiments lasting longer than a few hours are to be performed. For the experiments described here, the structure is created and within minutes transferred to a vacuum environment.
It is recommend before writing that a &#8220;writing test&#8221; be performed on all relevant electrodes. In such a test, two virtual electrodes are first created, and a single nanowire is written while simultaneously monitoring the conductance. A similar test of erasure can be performed by &#8220;cutting&#8221; the nanowire shortly afterwards. If the nanostructure is decaying rapidly, the issue is most likely due either to the interfacial contacts or the canvas itself. To distinguish between these two effects, a four-terminal measurement of the conductance should be performed, and the two-terminal conductance should be compared with the four-terminal conductance as a function of time. If the two-terminal conductance is decaying more rapidly than the four-terminal conductance, then the issue is related to the electrical contacts to the interface. If the four-terminal conductance is decaying at a comparable rate, then most likely the canvas is not suitable and should be replaced.
There are natural limitations of the current method for creating nanostructures. Specifically, the writing speed for the smallest devices is limited to a few hundred nanometers per second. Speeds far above that value lead to unpredictable results. Use of parallel writing techniques are possible27,28, but are not highly developed and have their own drawbacks. The size of nanostructures that can be created is naturally limited by the scan range of the AFM being used. A high-quality AFM with closed-loop feedback in the two scan directions is highly recommended. Tracking of point-like objects on the sample surface should be performed to monitor temporal drift of the sample.
Once creation of conductive nanostructures at oxide interfaces has been mastered, there are a wide range of experimental directions that can be explored. Using this technique, a wide variety of nanostructures and devices have already been demonstrated, including nanowires18, tunnel barriers29, rectifying junctions30, field-effect transistors18, single-electron transistors31, superconducting nanowires32, nanoscale optical detectors33, and nanoscale THz emitters and detectors34.

Oxide heterostructures1-5 exhibit a remarkably wide variety of emergent physical phenomena which are both scientifically interesting and potentially useful for applications4. In particular, the interface between LaAlO3 (LAO) and SrTiO3 (STO)6 can exhibit insulating, conducting, superconducting7, ferroelectric-like8, and ferromagnetic9 behavior. In 2006, Thiel et al showed10 that there is a sharp insulator-to-metal transition as the thickness of the LAO layer is increased, with a critical thickness of 4 unit cells (4uc). It was subsequently shown that 3uc-LAO/STO structures exhibit a hysteretic transition that can be controlled locally with a conductive atomic-force microscope (c-AFM) probe11.
The properties of oxide interfaces such as LaAlO3/SrTiO3 depend on the absence or presence of conducting electrons at the interface. &#160;These electrons can be controlled using top gate electrodes 12,13, back gates 10, surface adsorbates14, ferroelectric layers15,16 and c-AFM lithography11. &#160;A unique feature of c-AFM lithography is that very small nanoscale features can be created.
Electrical top gating, combined with two-dimensional confinement, is often used to create quantum dots in III-V semiconductors 17. &#160;Alternatively, quasi-one-dimensional semiconducting nanowires can be electrically gated by proximity. &#160;The methods for producing these structures are time-consuming and generally irreversible. By contrast, the c-AFM lithography technique is reversible in the sense that a nanostructure can be created for one experiment, and then &#8220;erased&#8221; (similar to a whiteboard). Generally, c-AFM writing is performed with positive voltages applied to the AFM tip, while, erasing is performed using negative voltages. &#160;The time required to create a particular structure depends on the complexity of the device but is usually less than 30 min; most of that time is spent erasing the canvas. &#160;The typical spatial resolution is about 10 nanometers, but with proper tuning features as small as 2 nanometers can be created18.
A detailed description of the nanoscale fabrication procedure follows. The detail provided here should be sufficient to allow similar experiments to be performed by interested researchers. The method described here has many advantages over traditional lithographic approaches used to create electronic nanostructures in semiconductors. &#160;
The c-AFM lithography method described here is part of a much broader class of scanning-probe-based lithography efforts, including scanning anodic oxidation19, dip-pen nanolithography20, piezoelectric patterning21, and so on. &#160;The c-AFM technique described here, coupled with the use of novel oxide interfaces, can produce some of the highest-precision electronic structures with an unprecedented variety of physical properties. &#160;

<table border="0" cellpadding="0" cellspacing="0" width="889">
	<tbody>
		<tr height="20">
			<td height="20" style="height:20px;width:165px;">Name</td>
			<td style="width:169px;">Company</td>
			<td style="width:108px;">Catalog Number</td>
			<td style="width:447px;">Comments</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Equipment</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Contact Aligner</td>
			<td>Karl-Suss</td>
			<td>MA6</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Spinner</td>
			<td>Solitec</td>
			<td>5110C</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Ion Mill</td>
			<td>Commonwealth Scientific</td>
			<td>8C</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Sputtering System</td>
			<td>Leybold-Heraeus</td>
			<td>Z-650</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Barrel Etcher</td>
			<td>Branson/IPC</td>
			<td>3000C</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Wire Bonder</td>
			<td>Westbond</td>
			<td>7700E</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">AFM</td>
			<td>Asylum Research</td>
			<td>MFP-3D</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Dilution Refrigerator</td>
			<td>Quantum Design</td>
			<td>P850</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Ultrasonic Wash Machine</td>
			<td>Fisher Scientific</td>
			<td>15-335-6</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Current Amplifier</td>
			<td>Femto</td>
			<td>DLPCA-200</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;"></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Materials</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">LaAlO3/SrTiO3</td>
			<td>Prof. Chang-Beom Eom</td>
			<td>N/A</td>
			<td>5mm x 1mm with ~3.4 unit cells of LAO (See Reference 18)</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Photoresist</td>
			<td>AZ Electronic Materials</td>
			<td>P4210</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Developer</td>
			<td>AZ Electronic Materials</td>
			<td>400K</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Acetone</td>
			<td>Fisher Scientific</td>
			<td>A929SK-4</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Isopropyl Alcohol</td>
			<td>Fisher Scientific</td>
			<td>A459-1</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Deionized Water</td>
			<td>Fisher Scientific</td>
			<td>23-290-065</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Gold Wire</td>
			<td>DuPont</td>
			<td>5771</td>
			<td>1 mil diameter</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Chip Carrier</td>
			<td>NTK Technologies</td>
			<td>IRK28F1-5451D</td>
			<td></td>
		</tr>
	</tbody>
</table>

writing and low-temperature characterization of oxide nanostructures

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions. Oxide interfaces exhibit a wide range of properties that can be controlled include conduction, piezoelectric behavior, ferromagnetism, superconductivity and nonlinear optical properties. Recently, methods for controlling these properties at extreme nanoscale dimensions have been discovered and developed. Here are described explicit step-by-step procedures for creating LaAlO3/SrTiO3 nanostructures using a reversible conductive atomic force microscopy technique. The processing steps for creating electrical contacts to the LaAlO3/SrTiO3 interface are first described. Conductive nanostructures are created by applying voltages to a conductive atomic force microscope tip and locally switching the LaAlO3/SrTiO3 interface to a conductive state. A versatile nanolithography toolkit has been developed expressly for the purpose of controlling the atomic force microscope (AFM) tip path and voltage. Then, these nanostructures are placed in a cryostat and transport measurements are performed. The procedures described here should be useful to others wishing to conduct research in oxide nanoelectronics.

The results shown here are representative of the transport behavior that can be exhibited by this class of nanostructures, and has been described elsewhere in detail23-26. In this example, a nanowire cavity has been constructed (Figure 4) from a 3.3 unit cell LAO/STO heterostructure. Conductive paths (shown in green) are typically 10 nm wide, as determined by nanowire &#8220;cutting&#8221; experiments11. The tip speed and voltage for each segment is independently configurable from the lithography front panel (Figure 4B), as is the tip writing speed. &#8220;Virtual electrodes&#8221; that interface with the interfacial contacts ensure that there is a highly conductive electrical connection to the nanostructures.
After the nanostructure is written, it is transferred to the dilution refrigerator. Exposure to light at or below 550 nm will produce unwanted photoconduction, so it is important to transfer the device in darkness or with the aid of a red &#8220;darkroom&#8221; light (Figure 5A). Electrical connections should be made at RT, and as with most semiconductor nanostructures, great care should be taken when changing electrical connections at cryogenic temperatures. If the devices is subjected to electrostatic discharge, it will most likely become insulating. Remarkably, the device functionality can be recovered by &#8220;cycling&#8221; the temperature to 300 K and cooling down again.
During cooldown, it is routine to monitor the two-terminal resistance, and even the four-terminal resistance, as a function of temperature. For these measurements an ac voltage (typically ~1&#160;mV) is applied at a low frequency (&#60;10&#160;Hz) to one of the electrodes, while the ac current is measured using a transimpedance amplifier. Lock-in demodulation and filtering is performed using a home-developed lock-in amplifier. The ac current is monitored as a function of temperature (Figure 5B).
Once the device is cooled to the base temperature of the dilution refrigerator (50 mK), four-terminal transport measurements are performed (Figure 5C). For these measurements, current is sourced through the main channel of the device, while voltage across the device is simultaneously measured. Instead of measuring with a lock-in amplifier, a full current-voltage (I-V) trace is measured. This method contains more information and the differential conduction can be calculated via numerical differentiation. For the particular device, the differential conduction is measured as a function of the side-gate voltage Vsg. This gate allows the chemical potential of the device to be changed. The transport through the device shows a strong non-monotonic dependence, indicating regions in which Coulomb blockade takes place for smaller values, and strong superconductivity for larger values of Vsg . Details about the physical interpretation for this class of device will be described elsewhere.
<img alt="Figure 1" src="/files/ftp_upload/51886/51886fig1highres.jpg" /> 
Figure 1. Photolithographic processing steps. Step 1: spin photoresist. Step 2: expose photoresist using mask aligner. Step 3: develop photoresist. Step 4: ion milling. Step 5: DC sputtering to deposit Ti and Au. Step 6: lift-off. Step 7: deposit the second layer. Step 8: plasma cleaning.
<img alt="Figure 2" src="/files/ftp_upload/51886/51886fig2highres.jpg" /> 
Figure 2. Images of lithographically patterned LAO/STO heterostructures.&#160;(A)&#160;Image showing 5mm x 5mm sample wire bonded to a chip carrier. (B) Optical image showing bonding pads and one of the canvases. (C)&#160;Close-up of a single canvas. <a href="https://www.jove.com/files/ftp_upload/51886/51886fig2highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/51886/51886fig3highres.jpg" /> 
Figure 3. (A)&#160;Informal design of LAO/STO nanostructure. (B)&#160;Precise layout of nanostructure using an open-source scalable vector graphics (SVG) editor.
<img alt="Figure 4" src="/files/ftp_upload/51886/51886fig4highres.jpg" /> 
Figure 4. (A)&#160;Lithography front panel for c-AFM patterning. (B)&#160;Screenshot from 3D simulator showing position and voltage of c-AFM tip. <a href="https://www.jove.com/files/ftp_upload/51886/51886fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/51886/51886fig5highres.jpg" /> 
Figure 5. (A)&#160;LAO/STO nanostructure being inserted into dilution refrigerator. (B)&#160;Monitoring of sample resistance as it is cooled from 300 K to 50 mK. (C)&#160;Monitoring of four-terminal differential conductance of device as a function of side gate voltage Vsg and voltage across the device (V4t). Intensity graph displayed in units of siemens (S), and voltages are displayed in units of volts (V).

Watch this Scientific Journal Video about Writing and Low-Temperature Characterization of Oxide Nanostructures at JoVE.com

Writing and Low-Temperature Characterization of Oxide Nanostructures

Oxide nanostructures provide new opportunities for science and technology. The interfacial conductivity between LaAlO3 and SrTiO3 can be controlled with near-atomic precision using a conductive atomic force microscopy technique. The protocol for creating and measuring conductive nanostructures at LaAlO3/SrTiO3 interfaces is demonstrated.

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions.  Oxide ...

writing-and-low-temperature-characterization-of-oxide-nanostructures

Research

JoVE Journal

Engineering

A Method to Fabricate Disconnected Silver Nanostructures in 3D

The standard nanofabrication toolkit includes techniques primarily aimed at creating 2D patterns in dielectric media. Creating metal patterns on a submicron scale requires a combination of nanofabrication tools and several material processing steps. For example, steps to create planar metal structures using ultraviolet photolithography and electron-beam lithography can include sample exposure, sample development, metal deposition, and metal liftoff. To create 3D metal structures, the sequence is repeated multiple times. The complexity and difficulty of stacking and aligning multiple layers limits practical implementations of 3D metal structuring using standard nanofabrication tools. Femtosecond-laser direct-writing has emerged as a pre-eminent technique for 3D nanofabrication.1,2 Femtosecond lasers are frequently used to create 3D patterns in polymers and glasses.3-7 However, 3D metal direct-writing remains a challenge. Here, we describe a method to fabricate silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm. The method enables the fabrication of patterns not feasible using other techniques, such as 3D arrays of disconnected silver voxels.8 Disconnected 3D metal patterns are useful for metamaterials where unit cells are not in contact with each other,9 such as coupled metal dot10,11or coupled metal rod12,13 resonators. Potential applications include negative index metamaterials, invisibility cloaks, and perfect lenses.
In femtosecond-laser direct-writing, the laser wavelength is chosen such that photons are not linearly absorbed in the target medium. When the laser pulse duration is compressed to the femtosecond time scale and the radiation is tightly focused inside the target, the extremely high intensity induces nonlinear absorption. Multiple photons are absorbed simultaneously to cause electronic transitions that lead to material modification within the focused region. Using this approach, one can form structures in the bulk of a material rather than on its surface.
Most work on 3D direct metal writing has focused on creating self-supported metal structures.14-16 The method described here yields sub-micrometer silver structures that do not need to be self-supported because they are embedded inside a matrix. A doped polymer matrix is prepared using a mixture of silver nitrate (AgNO3), polyvinylpyrrolidone (PVP) and water (H2O). Samples are then patterned by irradiation with an 11-MHz femtosecond laser producing 50-fs pulses. During irradiation, photoreduction of silver ions is induced through nonlinear absorption, creating an aggregate of silver nanoparticles in the focal region. Using this approach we create silver patterns embedded in a doped PVP matrix. Adding 3D translation of the sample extends the patterning to three dimensions.

Femtosecond-laser direct-writing is frequently used to create three-dimensional (3D) patterns in polymers and glasses. However, patterning metals in 3D remains a challenge. We describe a method for fabricating silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm.

The standard nanofabrication toolkit includes  techniques primarily aimed at creating 2D patterns in dielectric media. Creating  metal patterns on a ...

Probing and Mapping Electrode Surfaces in Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen 1-7. The performance of SOFCs and the rates of many chemical and energy transformation processes in energy storage and conversion devices in general are limited primarily by charge and mass transfer along electrode surfaces and across interfaces. Unfortunately, the mechanistic understanding of these processes is still lacking, due largely to the difficulty of characterizing these processes under in situ conditions. This knowledge gap is a chief obstacle to SOFC commercialization. The development of tools for probing and mapping surface chemistries relevant to electrode reactions is vital to unraveling the mechanisms of surface processes and to achieving rational design of new electrode materials for more efficient energy storage and conversion2. Among the relatively few in situ surface analysis methods, Raman spectroscopy can be performed even with high temperatures and harsh atmospheres, making it ideal for characterizing chemical processes relevant to SOFC anode performance and degradation8-12. It can also be used alongside electrochemical measurements, potentially allowing direct correlation of electrochemistry to surface chemistry in an operating cell. Proper in situ Raman mapping measurements would be useful for pin-pointing important anode reaction mechanisms because of its sensitivity to the relevant species, including anode performance degradation through carbon deposition8, 10, 13, 14 ("coking") and sulfur poisoning11, 15 and the manner in which surface modifications stave off this degradation16. The current work demonstrates significant progress towards this capability. In addition, the family of scanning probe microscopy (SPM) techniques provides a special approach to interrogate the electrode surface with nanoscale resolution. Besides the surface topography that is routinely collected by AFM and STM, other properties such as local electronic states, ion diffusion coefficient and surface potential can also be investigated17-22. In this work, electrochemical measurements, Raman spectroscopy, and SPM were used in conjunction with a novel test electrode platform that consists of a Ni mesh electrode embedded in an yttria-stabilized zirconia (YSZ) electrolyte. Cell performance testing and impedance spectroscopy under fuel containing H2S was characterized, and Raman mapping was used to further elucidate the nature of sulfur poisoning. In situ Raman monitoring was used to investigate coking behavior. Finally, atomic force microscopy (AFM) and electrostatic force microscopy (EFM) were used to further visualize carbon deposition on the nanoscale. From this research, we desire to produce a more complete picture of the SOFC anode.

We present a unique platform for characterizing electrode surfaces in solid oxide fuel cells (SOFCs) that allows simultaneous performance of multiple characterization techniques (e.g. in situ Raman spectroscopy and scanning probe microscopy alongside electrochemical measurements). Complementary information from these analyses may help to advance toward a more profound understanding of electrode reaction and degradation mechanisms, providing insights into rational design of better materials for SOFCs.

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen ...

Visible-light Induced Reduction of Graphene Oxide Using Plasmonic Nanoparticle

Present work demonstrates the simple, chemical free, fast, and energy efficient method to produce reduced graphene oxide (r-GO) solution at RT using visible light irradiation with plasmonic nanoparticles. The plasmonic nanoparticle is used to improve the reduction efficiency of GO. It only takes 30 min&#160;at RT by illuminating the solutions with Xe-lamp, the r-GO solutions can be obtained by completely removing gold nanoparticles through simple centrifugation step. The spherical gold nanoparticles (AuNPs) as compared to the other nanostructures is the most suitable plasmonic nanostructure for r-GO preparation. The reduced graphene oxide prepared using visible light and AuNPs was equally qualitative as chemically reduced graphene oxide, which was supported by various analytical techniques such as UV-Vis spectroscopy, Raman spectroscopy, powder XRD and XPS. The reduced graphene oxide prepared with visible light shows excellent quenching properties over the fluorescent molecules modified on ssDNA and excellent fluorescence recovery for target DNA detection. The r-GO prepared by recycled AuNPs is found to be of same quality with that of chemically reduced r-GO. The use of visible light with plasmonic nanoparticle demonstrates the good alternative method for r-GO synthesis.

A simple protocol for the preparation of reduced graphene oxide using visible light and plasmonic nanoparticle is described.

Present work demonstrates the simple, chemical free, fast, and energy efficient method to produce reduced graphene oxide (r-GO) solution at RT using ...

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe2) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal&#8211;semiconductor contact of the FIB-fabricated MoSe2 devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe2-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 &#937;&#8722;1 cm&#8722;1, with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe2 multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe2 are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS2 and WS2.

We describe the approaches for the device fabrication and electrical characterization of molybdenum diselenide (MoSe2) layer semiconductor nanostructures with different thicknesses. In addition, the fabrication of ohmic contacts for MoSe2-layer nanocrystals by the focused-ion beam deposition method using platinum (Pt) as a contact metal is described.

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor ...

Fabrication of Low Temperature Carbon Nanotube Vertical Interconnects Compatible with Semiconductor Technology

We demonstrate a method for the low temperature growth (350 &#176;C) of vertically-aligned carbon nanotubes (CNT) bundles on electrically conductive thin-films. Due to the low growth temperature, the process allows integration with modern low-&#954; dielectrics and some flexible substrates. The process is compatible with standard semiconductor fabrication, and a method for the fabrication of electrical 4-point probe test structures for vertical interconnect test structures is presented. Using scanning electron microscopy the morphology of the CNT bundles is investigated, which demonstrates vertical alignment of the CNT and can be used to tune the CNT growth time. With Raman spectroscopy the crystallinity of the CNT is investigated. It was found that the CNT have many defects, due to the low growth temperature. The electrical current-voltage measurements of the test vertical interconnects displays a linear response, indicating good ohmic contact was achieved between the CNT bundle and the top and bottom metal electrodes. The obtained resistivities of the CNT bundle are among the average values in the literature, while a record-low CNT growth temperature was used.

A method for the growth of low temperature vertically-aligned carbon nanotubes, and the subsequent fabrication of vertical interconnect electrical test structures using semiconductor fabrication is presented.

We demonstrate a method for the low temperature growth (350 &#176;C) of vertically-aligned carbon nanotubes (CNT) bundles on electrically conductive ...

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

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

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

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

Writing Bragg Gratings in Multicore Fibers

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong suppression at a single wavelength requires that all cores have matching transmission profiles. These gratings cannot be inscribed using the same method as for single-core fibers because the curved surface of the cladding acts as a lens, focusing the incoming UV laser beam and causing variations in exposure between cores. Therefore we use an additional optical element to ensure that the beam shape does not change while passing through the cross-section of the multicore fiber. This consists of a glass capillary tube which has been polished flat on one side, which is then placed over the section of the fiber to be inscribed. The laser beam enters the fiber through the flat surface of the capillary tube and hence maintains its original dimensions. This paper demonstrates the improvements in core-to-core uniformity for a 7-core fiber using this method. The technique can be generalized to larger multicore fibers.

We describe a technique for inscribing identical fiber Bragg gratings into each core of a multicore fiber. This is achieved by introducing an additional surface into the optical path to mitigate lensing by the curved surface of the fiber cladding.

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong ...

Iridium Oxide-reduced Graphene Oxide Nanohybrid Thin Film Modified Screen-printed Electrodes as Disposable Electrochemical Paper Microfluidic pH Sensors

A facile, controllable, inexpensive and green electrochemical synthesis of IrO2-graphene nanohybrid thin films is developed to fabricate an easy-to-use integrated paper microfluidic electrochemical pH sensor for resource-limited settings. Taking advantages from both pH meters and strips, the pH sensing platform is composed of hydrophobic barrier-patterned paper micropad (&#181;PAD) using polydimethylsiloxane (PDMS), screen-printed electrode (SPE) modified with IrO2-graphene films and molded acrylonitrile butadiene styrene (ABS) plastic holder. Repetitive cathodic potential cycling was employed for graphene oxide (GO) reduction which can completely remove electrochemically unstable oxygenated groups and generate a 2D defect-free homogeneous graphene thin film with excellent stability and electronic properties. A uniform and smooth IrO2 film in nanoscale grain size is anodically electrodeposited onto the graphene film, without any observable cracks. The resulting IrO2-RGO electrode showed slightly super-Nernstian responses from pH 2-12 in Britton-Robinson (B-R) buffers with good linearity, small hysteresis, low response time and reproducibility in different buffers, as well as low sensitivities to different interfering ionic species and dissolved oxygen. A simple portable digital pH meter is fabricated, whose signal is measured with a multimeter, using high input-impedance operational amplifier and consumer batteries. The pH values measured with the portable electrochemical paper-microfluidic pH sensors were consistent with those measured using a commercial laboratory pH meter with a glass electrode.

The study demonstrates the growth of iridium oxide-reduced graphene oxide (IrO2-RGO) nanohybrid thin films on irregular and rough screen-printed carbon substrate through a green electrochemical synthesis, and their implementation as a pH sensor with a patterned paper-fluidic platform.

A facile, controllable, inexpensive and green electrochemical synthesis of IrO2-graphene nanohybrid thin films is developed to fabricate an easy-to-use ...

Advanced Experimental Methods for Low-temperature Magnetotransport Measurement of Novel Materials

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film synthesis) for the purpose of exploratory materials research. Certain materials pose a challenge wherein the traditional bulk Hall bar device fabrication method is insufficient to produce a measureable device for sample transport measurement, principally because the single crystal size is too small to attach wire leads to the sample in a Hall bar configuration. This can be, for example, because the first batch of a new material synthesized yields very small single crystals or because flakes of samples of one to very few monolayers are desired. In order to enable rapid characterization of materials that may be carried out in parallel with improvements to their growth methodology, a method of device fabrication for very small samples has been devised to permit the characterization of novel materials as soon as a preliminary batch has been produced. A slight variation of this methodology is applicable to producing devices using exfoliated samples of two-dimensional materials such as graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenides (TMDs), as well as multilayer heterostructures of such materials. Here we present detailed protocols for the experimental device fabrication of fragments and flakes of novel materials with micron-sized dimensions onto substrate and subsequent measurement in a commercial superconducting magnet, dry helium close-cycle cryostat magnetotransport system at temperatures down to 0.300 K and magnetic fields up to 12 T.

We describe the methodology of mechanical exfoliation and deposition of flakes of novel materials with micron-sized dimensions onto substrate, fabrication of experimental device structures for transport experimentation, and the magnetotransport measurement in a dry helium close-cycle cryostat at temperatures down to 0.300 K and magnetic fields up to 12 T.

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film ...

Fabrication and Characterization of Superconducting Resonators

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.

Superconducting microwave resonators are of interest for detection of light, quantum computing applications and materials characterization. This work presents a detailed procedure for fabrication and characterization of superconducting microwave resonator scattering parameters.

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors ...