Name: growth and electrostatic/chemical properties of metal/laalo3/srtio3 heterostructures
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This work received support from the ERC Consolidator Grant #615759 &#34;MINT&#34;, the region &#206;le-de-France DIM &#34;Oxymore&#34; (project &#34;NEIMO&#34;) and the ANR project &#34;NOMILOPS&#34;. H.N. was partly supported by the EPSRC-JSPS Core-to-Core Program, JSPS Grant-in-Aid for Scientific Research (B) (#15H03548). A.S. was supported by the Deutsche Forschungsgemeinschaft (HO 53461-1; postdoctoral fellowship to A.S.). D.C.V. thanks the French Ministry of Higher Education and Research and CNRS for financing of his PhD thesis. J.S. thanks the University Paris-Saclay (D&#39;Alembert program) and CNRS for financing his stay at CNRS/Thales.

Note: All 5 steps described in this protocol can be paused and restarted at any time, with the single condition that the sample is kept under high vacuum from step 3.4 through 5.
1. STO(001) Substrate Termination:
<ol>
	<li>Fill an ultrasonic cleaner (with a 40 kHz transducer) with water and heat it to 60 &#176;C. Fill a borosilicate glass beaker with acetone. Independent of the beaker size, be sure to fill it with at least 20% of its maximum volume, to ensure that the subst...

<ol>
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During substrate termination, one should be extremely careful with the submerging time in HF solution. We observed under- and over-etched surfaces by varying just 5 s with regard to the original recipe. Additionally, we observed a dependence between substrate step size and submerging time. For smaller step sizes (less than 100 nm) submerging 30 s might lead to over-etching, even though afterwards the annealing procedure might be sufficient to properly reconstruct the surface. Due to the risks of using HF based acids, we ...

Quasi 2D electron systems (q2DES) have been extensively used as a playground to study a multitude of low-dimensional and quantum phenomena. Starting from the seminal paper on the LaAlO3/SrTiO3 system (LAO/STO)1, a burst of different systems that host new interfacial electronic phases have been created. Combining different materials led to the discovery of q2DESs with additional properties, such as electric-field tunable spin polarization2, extremely high electron mobilities3 or ferroelectricity-coupled phenomena4. Although an immense body of work has been dedicated to unravel the creation and manipulation of these systems, several experiments and techniques have shown contradictory results, even in rather similar conditions. Additionally, the balance between electrostatic and chemical interactions was found to be essential to correctly understand the physics at play5,6,7.
In this article, we thoroughly describe the growth of different metal/LAO/STO heterostructures, using a combination of pulsed laser deposition (PLD) and in situ magnetron sputtering. Then, to understand the effect of different surface conditions in the buried q2DES at the LAO/STO interface, an electronic and chemical study is performed, using transport and electron spectroscopy experiments.
Since multiple methods have been previously used to grow crystalline LAO on STO, the choice of appropriate deposition techniques is a crucial step for the fabrication of high quality oxide heterostructures (in addition to possible cost and time constrains). In PLD, an intense and short laser pulse hits the target of the desired material, which is then ablated and gets deposited on the substrate as a thin film. One of the major advantages of this technique is the ability to reliably transfer the stoichiometry of the target to the film, a key element in order to achieve the desired phase formation. Furthermore, the capability of performing layer-by-layer growth (monitored in real time using reflection high-energy electron diffraction - RHEED) of a vast number of complex oxides, the possibility of having multiple targets inside the chamber at the same time (allowing the growth of different materials without breaking vacuum) and the simplicity of the setup make this technique one of the most effective and versatile.
Yet, other techniques such as molecular beam epitaxy (MBE) allow the growth of even higher quality epitaxial growth. Instead of having a target of a specific material, in MBE each specific element is sublimed towards the substrate, where they react with each other to form well defined atomic layers. Additionally, the absence of highly energetic species and more uniform energy distribution allows the fabrication of extremely sharp interfaces8. This technique is however much more complex than PLD when it comes to the growth of oxides, since it must be performed in ultra-high vacuum conditions (so that the long mean free path is not destroyed) and requires in general a larger investment, cost- and time-wise. Although the growth process used in the first LAO/STO publications was PLD, samples with similar characteristics have been grown by MBE9. It is also worth noting that LAO/STO heterostructures have been grown using sputtering10. Although atomically sharp interfaces were achieved at high temperatures (920 &#176;C) and high oxygen pressures (0.8 mbar), interfacial conductivity was not achieved.
For the growth of the metallic capping layers, we use magnetron sputtering, as it provides a good balance between quality and flexibility. Other chemical vapor deposition based techniques might however be used to achieve similar results.
Lastly, the combination of transport and spectroscopy techniques showed in this article exemplifies a systematic way of probing both electronic and chemical interactions, emphasizing the importance of crosschecking different approaches to fully understand the many features of these types of systems.

<table border="0" cellpadding="0" cellspacing="0" style="width:1433px;" width="1433">
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	</colgroup>
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Pulsed Laser Deposition</td>
			<td style="width:200px;">SURFACE</td>
			<td style="width:305px;">PLD Workstation + UHV Cluster System</td>
			<td style="width:521px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">KrF Excimer Laser</td>
			<td style="width:200px;">Coherent</td>
			<td style="width:305px;">Compex Pro 201F</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Reflection High-Energy Electron Diffraction (electron gun)</td>
			<td style="width:200px;">R-Dec Co., Ltd.</td>
			<td style="width:305px;">RDA-003G</td>
			<td>Distributed in Europe by SURFACE.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Reflection High-Energy Electron Diffraction (CCD camera)</td>
			<td style="width:200px;">k-Space Associates, Inc.</td>
			<td style="width:305px;">kSA 400</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Variable Laser Beam Attenuator</td>
			<td style="width:200px;">Metrolux</td>
			<td style="width:305px;">ML 2100</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Excimer Laser Sensor</td>
			<td style="width:200px;">Coherent</td>
			<td style="width:305px;">J-50MUV-248</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:407px;">LaAlO3 target</td>
			<td style="width:200px;">CrysTec</td>
			<td style="width:305px;"></td>
			<td>Single-crystal target</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:407px;">SrTiO3 subtrates</td>
			<td style="width:200px;">CrysTec</td>
			<td style="width:305px;"></td>
			<td>Several different sizes. Possibility to order TiO2 terminated.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Buffered HF Acid</td>
			<td style="width:200px;">Technic</td>
			<td style="width:305px;">BOE 7:1</td>
			<td>buffered hydrofluoric acid = BOE 7:1 (HF : NH4F = 12.5 : 87.5%) in&#160;VLSI-quality.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Silver Paste</td>
			<td style="width:200px;">DuPont</td>
			<td style="width:305px;">4929N</td>
			<td>Conductive Silver Composite.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Ultrasonic Cleaner</td>
			<td style="width:200px;">Bransonic</td>
			<td style="width:305px;">12</td>
			<td>Ultrasonic Cleaning Bath</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Tube Furnace</td>
			<td style="width:200px;">AET Technologies</td>
			<td style="width:305px;">Heat Treatment Furnace</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Borosilicate Glass Beaker</td>
			<td style="width:200px;">VWR</td>
			<td>213-1128</td>
			<td>Iow form</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">PTFE Beaker</td>
			<td style="width:200px;">Dynalon</td>
			<td style="width:305px;">PTFE Beaker</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Substrate holder &#34;dipper&#34;</td>
			<td style="width:200px;">Eberl&#233;</td>
			<td style="width:305px;"></td>
			<td>Custom made dipper</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Magnetron Sputtering</td>
			<td style="width:200px;">PLASSYS</td>
			<td style="width:305px;">Sputtering system</td>
			<td>5 chambers for targets.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Metal targets</td>
			<td style="width:200px;">Neyco S.A.</td>
			<td style="width:305px;"></td>
			<td>Purity &#62; 99.9%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">X-Ray Photoelectron Spectroscopy System</td>
			<td style="width:200px;">Omicron</td>
			<td style="width:305px;">Custom XPS System</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">X-Ray Source</td>
			<td style="width:200px;">Omicron</td>
			<td style="width:305px;">DAR 400</td>
			<td>Twin Anode X-Ray Source.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Energy Analyser</td>
			<td style="width:200px;">Omicron</td>
			<td style="width:305px;">EA 125</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Atomic Force Microscopy</td>
			<td style="width:200px;">Bruker</td>
			<td style="width:305px;">Innova AFM</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Atomic Force Microscopy Probes</td>
			<td style="width:200px;">Olympus</td>
			<td style="width:305px;">OMCL-AC160TS-R3</td>
			<td>Micro Cantilevers</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Wire bonding</td>
			<td style="width:200px;">Kulicke &#38; Soffa</td>
			<td style="width:305px;">4523AD</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">PPMS</td>
			<td style="width:200px;">Quantum Design</td>
			<td style="width:305px;">PPMS Dynacool</td>
			<td>9T magnet.</td>
		</tr>
	</tbody>
</table>

growth and electrostatic/chemical properties of metal/laalo3/srtio3 heterostructures

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. Although electrostatic mechanisms have been proposed in the past to describe the existence of this critical thickness, the importance of chemical defects has been recently accentuated. Here, we describe the growth of metal/LAO/STO heterostructures in an ultra-high vacuum (UHV) cluster system combining pulsed laser deposition (to grow the LAO), magnetron sputtering (to grow the metal) and X-ray photoelectron spectroscopy (XPS). We study step by step the formation and evolution of the q2DES and the chemical interactions that occur between the metal and the LAO/STO. Additionally, magnetotransport experiments elucidate on the transport and electronic properties of the q2DES. This systematic work not only demonstrates a way to study the electrostatic and chemical interplay between the q2DES and its environment, but also unlocks the possibility to couple multifunctional capping layers with the rich physics observed in two-dimensional electron systems, allowing the fabrication of new types of devices.

The full experimental system used for growth and characterization is shown in Figure 2. Having different setups connected in UHV through a distribution chamber is highly recommended to ensure that the surface of the sample after each growth process is kept pristine. The PLD chamber (Figure 3), magnetron sputtering (Figure 7) and XPS chamber (Figure 8) are also described ...

Watch this Scientific Journal Video about Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures at JoVE.com

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures

We fabricate metal/LaAlO3/SrTiO3 heterostructures using a combination of pulsed laser deposition and in situ magnetron sputtering. Through magnetotransport and in situ X-ray photoelectron spectroscopy experiments, we investigate the interplay between electrostatic and chemical phenomena of the quasi two-dimensional electron gas formed in this system.

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide ...

The overall goal of this procedure is to fabricate and investigate metal lanthanum-aluminate-strontium type heterostructures using a combination of pulsed-laser deposition magnetron sputtering x-ray photoemission spectroscopy and magneto transport experiments. This method can help answer key questions about the interplay between electrostatic and chemical phenomenon of the quasi two-dimensional electron gas formed in these types of systems. The main advantages of this technique are it's simplicity and versatility, which together allow high-quality layer-by-layer growth of multi-power ultra-thin films.First, fill a 40 kilohertz ultra sonic cleaner with water and heat the bath to 60 degrees Celsius. Add to a borosilicate glass beaker sufficient acetone to fill the beaker to at least 20%of it's maximum volume. Place in the beaker an out-of-the-box mix-terminated single-side polished 001 oriented STO single crystal substrate.Sonicate the substrate in acetone for three minutes. Dry the substrate under a stream of nitrogen gas from a blow gun. Repeat the sonication in isopropanol and de-ionized water in sequence.Mount the clean substrate in a polyvinylidene fluoride dipper. Fill another borosilicate glass beaker with flowing de-ionized water. Then, fill about 20%of a similarly sized polytetrafluoroethylene beaker with hydrofluoric acid buffered with ammonium fluoride.Use the dipper to immerse the substrate in the buffered oxide etch for precisely 30 seconds. Immediately transfer the etched substrate to the beaker of flowing de-ionized water. Gently agitate the substrate in the flowing de-ionized water for two minutes.Dry the substrate under a stream of nitrogen gas and anneal the etched substrate in an oxygen environment at 1000 degrees Celsius. Next, gently mechanically polish a single-crystal one-inch LAO target with 180-grit aluminum oxide sandpaper lubricated with isopropanol. Once the surface appears homogenous dry the target under a flow of nitrogen gas.Mount the polished LAO target in a carousel that allows target rotation. Insert the carousel into the load-lock chamber, evacuate the chamber, and transfer the carousel to a pulsed-laser deposition chamber. Once the pulsed-laser deposition chamber pressure reaches 10 to the 9th millibars, supply oxygen to the chamber to achieve an oxygen partial pressure of 2x10 to the 4th millibars then place an excimer laser energy meter in the krypton fluoride laser beam path between the second converging lens and the quartz window.Set the laser to an arbitrary frequency and read the energy on the meter. Select the target rotation, remove the energy meter, and pre-ablate the target at three or four hertz for 20, 000 pulses in order to clean it's surface. Prior to sample preparation, use atomic force microscopy to confirm the termination, morphology, and cleanness of the terminated STO substrate surface.Use the conductive silver paste to fix the terminated STO substrate to a sample holder with the terminated surface face up, ensuring that the substrate is in the center of the holder. Heat the sample holder on a hot plate at 100 degrees Celsius for 10 minutes to evaporate solvent and solidify the paste for optimal thermal conduction. Then, allow the sample holder to cool to room temperature.Insert the sample holder into the load lock chamber and transfer the holder to the x-ray photoelectron spectroscopy chamber. Align the sample with the surface normal parallel to the electron analyzer axis. Move the x-ray gun as close as possible to the sample without allowing mechanical contact between the gun and the sample holder.After ensuring that the chamber is under ultra-high vacuum, turn on the x-ray gun. Acquire a survey spectrum between 12, 000 and 0 electron volts, with a selected step of 0.2 electron volts, a dwell time of 0.2 seconds, a pass energy between 30 and 60 electron volts, and the smallest spot size possible. Acquire a spectra of peaks of interest for later elemental analysis.After initial XPS analysis of the sample, transfer the sample holder to the PLD chamber and position the holder so the substrate faces the LAO target. Supply oxygen to the chamber to achieve an oxygen partial pressure of 2x10 to the 4th millibars. Ramp the sample holder temperature to 730 degrees Celsius at 25 degrees Celsius per minute, then set the reflection high-energy electron diffraction gun to a source voltage of 30 kilovolts, and a current of about 50 microamps.With the sample holder, place 63 millimeters away from the target, align the read electron beam with the substrate surface at a grazing angle between one and three degrees, so that the diffraction spots appear on the phosphor screen. Monitor the spot intensities in real-time via a CCD camera and image-analysis software. Once the reading stabilizes, start the laser.Observe the plume and monitor the read oscillations. Stop the laser at the peak of an oscillation upon achieving the desired thickness. Shut down the read gun, reduce the holder temperature to 500 degrees Celsius, and increase the oxygen partial pressure of the chamber to 0.1 millibars.Once the temperature and pressure are stable, increase the oxygen partial pressure to 300 millibars and anneal the sample at 500 degrees Celsius for 60 minutes. Analyze the LAO/STO sample with XPS. Transfer the sample holder with the LAO/STO substrate to the sputtering chamber under vacuum.Position the holder so that the substrate is about 7cm from the cobalt target. Supply pure argon gas to the sputtering chamber. With the target shutter closed, ramp the current to about 100 milliamps to ignite the plasma.Reduce the current to 80 milliamps and decrease the argon supply to 5.2 sccm, pre-sputter the cobalt target and then open the shutter and deposit cobalt for 25 seconds. Close the shutter to end deposition. For transport experiments, deposit a capping layer of three nanometers of aluminum to prevent oxidation of the underlying metal layer upon exposure to air.Otherwise, analyze the sample with XPS after cobalt deposition. Remove the sample holder from the system and transfer the sample to a transport measurement holder. On the sample stage of an ultrasonic wedge bonding machine, use aluminum or gold wires that are 0.025 millimeters in diameter to wire-bond the four corners of the sample to one channel of the transport measurement holder in hull geometry.Ensure that the wires stick well to the sample's surface. Then, wire bond the four corners to a second channel in vanderpow geometry. Check the contacts with a multimeter.Mount the holder in a transport setup and measure the magneto resistance and hull effect at various temperatures. Magneto transport measurements showed strikingly varied behavior depending on the metal layer deposited. LAO/STO samples capped with reactive metals showed s-shaped hull traces, indicating that an inter-facial quaside two-dimensional electron system had formed.Similar two-unit cell LAO/STO samples capped with noble metals, showed linear hull traces with resistance changes of only a few tens of milliohms over 4 teslas. Consistent with an insulating interface at which only the metal layer was detected. XPS of a tantalum-capped one unit cell LAO/STO sample showed tantalum oxide peaks and partial to total suppression of the metallic tantalum feature, suggesting that oxygen tended to diffuse toward the metal from the perovskite surface.Titanium atoms hosted some of the electrons released to the lattice by the formation of oxygen vacancies at the perovskite surface, changing the titanium valence state from 4 to 3, and forming a quaside 2D electron system. The titanium 3+to titanium 4+intensity ratio was about 20%Changing the electron take-off angle from 0 to 50 degrees had a minimal effect on the titanium 3+intensity, indicating that the quaside 2D electron system extended beyond the maximal XPS probing depth of five nanometers. Once mastered, the full procedure for one sample can be done in about one or two days.While attempting to perform PLD growth, pay special attention to the quality of both the substrates and the targets used. Perform extra analysis if the samples grown do not show the expected properties. Due to it's versatility, this procedure allows the highly-controlled creation and analysis of virtually any type of oxide heterostructures, as well as the possibility to explore a multitude of new interfacial and surface physical effects.

growth-electrostaticchemical-properties-metallaalo3srtio3

Research

JoVE Journal

Engineering

Concurrent Quantitative Conductivity and Mechanical Properties Measurements of Organic Photovoltaic Materials using AFM

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. Thus, understanding of spatial variations in composition as well as electrical properties of OPV materials is of paramount importance for moving PV technology forward.1,2 In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution. Currently, materials properties measurements performed using commercially available AFM-based techniques (PeakForce, conductive AFM) generally provide only qualitative information. The values for resistance as well as Young's modulus measured using our method on the prototypical ITO/PEDOT:PSS/P3HT:PC61BM system correspond well with literature data. The P3HT:PC61BM blend separates onto PC61BM-rich and P3HT-rich domains. Mechanical properties of PC61BM-rich and P3HT-rich domains are different, which allows for domain attribution on the surface of the film. Importantly, combining mechanical and electrical data allows for correlation of the domain structure on the surface of the film with electrical properties variation measured through the thickness of the film.

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution.

Organic photovoltaic  (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale  inhomogeneity of OPV materials affects performance ...

Dry Oxidation and Vacuum Annealing Treatments for Tuning the Wetting Properties of Carbon Nanotube Arrays

In this article, we describe a simple method to reversibly tune the wetting properties of vertically aligned carbon nanotube (CNT) arrays. Here, CNT arrays are defined as densely packed multi-walled carbon nanotubes oriented perpendicular to the growth substrate as a result of a growth process by the standard thermal chemical vapor deposition (CVD) technique.1,2 These CNT arrays are then exposed to vacuum annealing treatment to make them more hydrophobic or to dry oxidation treatment to render them more hydrophilic. The hydrophobic CNT arrays can be turned hydrophilic by exposing them to dry oxidation treatment, while the hydrophilic CNT arrays can be turned hydrophobic by exposing them to vacuum annealing treatment. Using a combination of both treatments, CNT arrays can be repeatedly switched between hydrophilic and hydrophobic.2 Therefore, such combination show a very high potential in many industrial and consumer applications, including drug delivery system and high power density supercapacitors.3-5
The key to vary the wettability of CNT arrays is to control the surface concentration of oxygen adsorbates. Basically oxygen adsorbates can be introduced by exposing the CNT arrays to any oxidation treatment. Here we use dry oxidation treatments, such as oxygen plasma and UV/ozone, to functionalize the surface of CNT with oxygenated functional groups. These oxygenated functional groups allow hydrogen bond between the surface of CNT and water molecules to form, rendering the CNT hydrophilic. To turn them hydrophobic, adsorbed oxygen must be removed from the surface of CNT. Here we employ vacuum annealing treatment to induce oxygen desorption process. CNT arrays with extremely low surface concentration of oxygen adsorbates exhibit a superhydrophobic behavior.

This article describes a simple method to fabricate vertically aligned carbon nanotube arrays by CVD and to subsequently tune their wetting properties by exposing them to vacuum annealing or dry oxidation treatment.

In this article, we describe a simple method to reversibly  tune the wetting properties of vertically aligned carbon nanotube (CNT) arrays.  Here, CNT ...

Sputter Growth and Characterization of Metamagnetic B2-ordered FeRh Epilayers

Chemically ordered alloys are useful in a variety of magnetic nanotechnologies. They are most conveniently prepared at an industrial scale using sputtering techniques. Here we describe a method for preparing epitaxial thin films of B2-ordered FeRh by sputter deposition onto single crystal MgO substrates. Deposition at a slow rate onto a heated substrate allows time for the adatoms to both settle into a lattice with a well-defined epitaxial relationship with the substrate and also to find their proper places in the Fe and Rh sublattices of the B2 structure. The structure is conveniently characterized with X-ray reflectometry and diffraction and can be visualised directly using transmission electron micrograph cross-sections. B2-ordered FeRh exhibits an unusual metamagnetic phase transition: the ground state is antiferromagnetic but the alloy transforms into a ferromagnet on heating with a typical transition temperature of about 380 K. This is accompanied by a 1% volume expansion of the unit cell: isotropic in bulk, but laterally clamped in an epilayer. The presence of the antiferromagnetic ground state and the associated first order phase transition is very sensitive to the correct equiatomic stoichiometry and proper B2 ordering, and so is a convenient means to demonstrate the quality of the layers that can be deposited with this approach. We also give some examples of the various techniques by which the change in phase can be detected.

A method to prepare epitaxial layers of ordered alloys by sputtering is described. The B2-ordered FeRh compound is used as an example, as it displays a metamagnetic transition that depends sensitively on the degree of chemical order and the exact composition of the alloy.

Chemically ordered alloys are useful in a variety of magnetic nanotechnologies. They are most conveniently prepared at an industrial scale using ...

Using Microwave and Macroscopic Samples of Dielectric Solids to Study the Photonic Properties of Disordered Photonic Bandgap Materials

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). In this article we will describe the methods for constructing and characterizing macroscopic disordered photonic structures using microwaves. The microwave regime offers the most convenient experimental sample size to build and test PBG media. Easily manipulated dielectric lattice components extend flexibility in building various 2D structures on top of pre-printed plastic templates. Once built, the structures could be quickly modified with point and line defects to make freeform waveguides and filters. Testing is done using a widely available Vector Network Analyzer and pairs of microwave horn antennas. Due to the scale invariance property of electromagnetic fields, the results we obtained in the microwave region can be directly applied to infrared and optical regions. Our approach is simple but delivers exciting new insight into the nature of light and disordered matter interaction.

Our representative results include the first experimental demonstration of the existence of a complete and isotropic PBG in a two-dimensional (2D) hyperuniform disordered dielectric structure. Additionally we demonstrate experimentally the ability of this novel photonic structure to guide electromagnetic waves (EM) through freeform waveguides of arbitrary shape.

Disordered structures offer new mechanisms for forming photonic bandgaps and unprecedented freedom in functional-defect designs. To circumvent the computational challenges of disordered systems, we construct modular macroscopic samples of the new class of PBG materials and use microwaves to characterize their scale-invariant photonic properties, in an easy and inexpensive manner.

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). ...

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. ...

Electroactive Polymer Nanoparticles Exhibiting Photothermal Properties

A method for the synthesis of electroactive polymers is demonstrated, starting with the synthesis of extended conjugation monomers using a three-step process that finishes with Negishi coupling. Negishi coupling is a cross-coupling process in which a chemical precursor is first lithiated, followed by transmetallation with ZnCl2. The resultant organozinc compound can be coupled to a dibrominated aromatic precursor to give the conjugated monomer. Polymer films can be prepared via electropolymerization of the monomer and characterized using cyclic voltammetry and ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy. Nanoparticles (NPs) are prepared via emulsion polymerization of the monomer using a two-surfactant system to yield an aqueous dispersion of the polymer NPs. The NPs are characterized using dynamic light scattering, electron microscopy, and UV-Vis-NIR-spectroscopy. Cytocompatibility of NPs is investigated using the cell viability assay. Finally, the NP suspensions are irradiated with a NIR laser to determine their effectiveness as potential materials for photothermal therapy (PTT).

A protocol is presented for the synthesis and preparation of nanoparticles consisting of electroactive polymers.

A method for the synthesis of electroactive polymers is demonstrated, starting with the synthesis of extended conjugation monomers using a three-step ...

Combining Microfluidics and Microrheology to Determine Rheological Properties of Soft Matter during Repeated Phase Transitions

The microstructure of soft matter directly impacts macroscopic rheological properties and can be changed by factors including colloidal rearrangement during previous phase changes and applied shear. To determine the extent of these changes, we have developed a microfluidic device that enables repeated phase transitions induced by exchange of the surrounding fluid and microrheological characterization while limiting shear on the sample. This technique is &#181;2rheology, the combination of microfluidics and microrheology. The microfluidic device is a two-layer design with symmetric inlet streams entering a sample chamber that traps the gel sample in place during fluid exchange. Suction can be applied far away from the sample chamber to pull fluids into the sample chamber. Material rheological properties are characterized using multiple particle tracking microrheology (MPT). In MPT, fluorescent probe particles are embedded into the material and the Brownian motion of the probes is recorded using video microscopy. The movement of the particles is tracked and the mean-squared displacement (MSD) is calculated. The MSD is related to macroscopic rheological properties, using the Generalized Stokes-Einstein Relation. The phase of the material is identified by comparison to the critical relaxation exponent, determined using time-cure superposition. Measurements of a fibrous colloidal gel illustrate the utility of the technique. This gel has a delicate structure that can be irreversibly changed when shear is applied. &#181;2rheology data shows that the material repeatedly equilibrates to the same rheological properties after each phase transition, indicating that phase transitions do not play a role in microstructural changes. To determine the role of shear, samples can be sheared prior to injection into our microfluidic device. &#181;2rheology is a widely applicable technique for the characterization of soft matter enabling the determination of rheological properties of delicate microstructures in a single sample during phase transitions in response to repeated changes in the surrounding environmental conditions.

We demonstrate the fabrication and use of a microfluidic device that enables multiple particle tracking microrheology measurements to study the rheological effects of repeated phase transitions on soft matter.

The microstructure of soft matter directly impacts macroscopic rheological properties and can be changed by factors including colloidal rearrangement ...

Uncoupling Coriolis Force and Rotating Buoyancy Effects on Full-Field Heat Transfer Properties of a Rotating Channel

An experimental method for exploring the heat transfer characteristics of an axially rotating channel is proposed. The governing flow parameters that characterize the transport phenomena in a rotating channel are identified via the parametric analysis of the momentum and energy equations referring to a rotating frame of reference. Based on these dimensionless flow equations, an experimental strategy that links the design of the test module, the experimental program and the data analysis is formulated with the attempt to reveal the isolated Coriolis-force and buoyancy effects on heat transfer performances. The effects of Coriolis force and rotating buoyancy are illustrated using the selective results measured from rotating channels with various geometries. While the Coriolis-force and rotating-buoyancy impacts share several common features among the various rotating channels, the unique heat transfer signatures are found in association with the flow direction, the channel shape and the arrangement of heat transfer enhancement devices. Regardless of the flow configurations of the rotating channels, the presented experimental method enables the development of physically consistent heat transfer correlations that permit the evaluation of isolated and interdependent Coriolis-force and rotating-buoyancy effects on the heat transfer properties of rotating channels.

Here, we present an experimental method for decoupling the interdependent Coriolis-force and rotating-buoyancy effects on full-field heat transfer distributions of a rotating channel.

An experimental method for exploring the heat transfer characteristics of an axially rotating channel is proposed. The governing flow parameters that ...

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

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

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

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

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

Fabricating van der Waals Heterostructures with Precise Rotational Alignment

In this work we describe a technique for creating new crystals (van der Waals heterostructures) by stacking distinct ultrathin layered 2D materials. We demonstrate not only lateral control but, importantly, also control over the angular alignment of adjacent layers. The core of the technique is represented by a home-built transfer setup which allows the user to control the position of the individual crystals involved in the transfer. This is achieved with sub-micrometer (translational) and sub-degree (angular) precision. Prior to stacking them together, the isolated crystals are individually manipulated by custom-designed moving stages that are controlled by a programmed software interface. Moreover, since the entire transfer setup is computer controlled, the user can remotely create precise heterostructures without coming into direct contact with the transfer setup, labeling this technique as &#8220;hands-free&#8221;. In addition to presenting the transfer set-up, we also describe two techniques for preparing the crystals that are subsequently stacked.

In this work we describe a technique that is used to create new crystals (van der Waals heterostructures) by stacking ultrathin layered 2D materials with precise control over position and relative orientation.

In this work we describe a technique for creating new crystals (van der Waals heterostructures) by stacking distinct ultrathin layered 2D materials. We ...