Name: fabricating van der waals heterostructures with precise rotational alignment
Uploaded: 2019-07-05T13:00:00
Description: Watch this Scientific Journal Video about Fabricating van der Waals Heterostructures with Precise Rotational Alignment at JoVE.com

The authors acknowledge funding from University of Ottawa and NSERC Discovery grant RGPIN-2016-06717 and NSERC SPG QC2DM.

1. Instrumentation for the transfer procedure
<ol>
	<li>In order to visualize the transfer process, utilize an optical microscope that can operate under bright-field illumination. Since the typical sizes of the 2D crystals are 1&#8211;500 &#181;m2, equip the microscope with 5x, 50x, and 100x long working distance objectives. The microscope must also be equipped with a camera that connects to a computer (Figure 1a).</li>
	<li>Use separate manipulators to individually control the ...

<ol>
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Tuning superconductivity in twisted bilayer graphene Available from: <a target="_blank" href="https://arxiv.org/abs/1808.07865">https://arxiv.org/abs/1808.07865</a> (2018)</li><li>Blake, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Making+graphene+visible.">Making graphene visible.</a> Applied Physics Letters. 91 (6), 063124 (2007).</li><li>Lin, Y. -. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-Layer+ReS2:+Two-Dimensional+Semiconductor+with+Tunable+In-Plane+Anisotropy.">Single-Layer ReS2: Two-Dimensional Semiconductor with Tunable In-Plane Anisotropy.</a> ACS Nano. 9 (11), 11249-11257 (2015).</li><li>Chenet, D. 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The home-built transfer setup presented here offers a method for building novel layered materials with both lateral and rotational control. Compared to other solutions described in the literature10,25, our system does not require complex infrastructure, yet it achieves the goal of controlled alignment of 2D crystals.
The most critical step in the procedure is that of aligning and placing the top crystal in contact with the bottom one. ...

Research in the burgeoning field of two-dimensional (2D) materials began after researchers developed a technique which enabled the isolation of graphene1,2,3 (an atomically flat sheet of carbon atoms) from graphite. Graphene is a member of a larger class of layered 2D materials, also referred to as van der Waals materials or crystals. They have strong covalent intralayer bonding and weak van der Waals interlayer coupling. Therefore, the technique for isolating graphene from graphite can also be applied to other 2D materials where one can break the weak interlayer bonds and isolate single layers. One key development in the field was the demonstration that just as the van der Waals bonds holding adjacent layers of two-dimensional materials together can be broken, they can also be put back together2,4. Therefore, crystals of 2D materials can be created by controllably stacking together layers of 2D materials with distinct properties. This spurred a great deal of interest, as materials previously inexistent in nature can be created with the goal of either uncovering formerly inaccessible physical phenomena4,5,6,7,8,9 or developing superior devices for technology applications. Therefore, having precise control over stacking 2D materials has become one of the main goals in the research field10,11,12.
In particular, the twist angle between adjacent layers in van der Waals heterostructures was shown to be an important parameter for controlling material properties13. For example, at some angles, the introduction of a relative twist between adjacent layers can effectively electronically decouple the two layers. This was studied both in graphene14,15 as well as in transition metal dichalcogenides16,17,18,19. More recently, it was surprisingly found that it can also alter the state of matter of these materials. The discovery that bilayer graphene oriented at a &#8220;magic angle&#8221; behaves as a Mott insulator at low temperatures and even a superconductor when the electron density is properly tuned has sparked great interest and a realization of the importance of the angular control when fabricating layered van der Waals heterostructures13,20,21.
Motivated by the scientific opportunities opened up by the idea of tuning the properties of novel van der Waals materials by adjusting the relative orientation between the layers, we present a home-built instrument along with the procedure to create such structures with angular control.

<table>
	<tbody>
		<tr>
			<td>5X objective lens</td>
			<td>Nikon Metrology</td>
			<td>MUE12050</td>
			<td>23.5 mm working distance and 0.15 numerical aperture</td>
		</tr>
		<tr>
			<td>50X objective lens</td>
			<td>Nikon Metrology</td>
			<td>MUE21500</td>
			<td>19 mm working distance and 0.4 numerical aperture</td>
		</tr>
		<tr>
			<td>100X objective lens</td>
			<td>Nikon Metrology</td>
			<td>MUE21900</td>
			<td>4.5 mm working distance and 0.8 numerical aperture</td>
		</tr>
		<tr>
			<td>Acetone</td>
			<td>Sigma-Aldrich</td>
			<td>270725</td>
			<td>Purity &#8805;99.90%</td>
		</tr>
		<tr>
			<td>Adhesive tape</td>
			<td>Ultron Systems, Inc.</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Anisole</td>
			<td>MicroChem</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Atomic force microscope</td>
			<td>Bruker</td>
			<td>Dimension Icon</td>
			<td>We typicall use the ScanAsyst mode</td>
		</tr>
		<tr>
			<td>Bottom stage rotation manipulator</td>
			<td>Zaber Technologies</td>
			<td>X-RSW60A-PTB2</td>
			<td>360&#176; travel with step size of 4.091 &#956;rad</td>
		</tr>
		<tr>
			<td>Bottom stage X manipulator</td>
			<td>Zaber Technologies</td>
			<td>X-LSM025A-PTB2</td>
			<td>25 mm travel with step size of 47.625 nm</td>
		</tr>
		<tr>
			<td>Bottom stage Y manipulator</td>
			<td>Zaber Technologies</td>
			<td>X-LSM025A-PTB2</td>
			<td>25 mm travel with step size of 47.625 nm</td>
		</tr>
		<tr>
			<td>Bottom stage Z manipulator</td>
			<td>Zaber Technologies</td>
			<td>X-VSR40A-KX14A</td>
			<td>40 mm travel with step size of 95.25 nm</td>
		</tr>
		<tr>
			<td>Isopropanol</td>
			<td>Sigma-Aldrich</td>
			<td>563935</td>
			<td>Purity 99.999%</td>
		</tr>
		<tr>
			<td>LabVIEW software</td>
			<td>National Instruments</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Macor</td>
			<td>McMaster-Carr</td>
			<td>8489K238</td>
			<td></td>
		</tr>
		<tr>
			<td>Microscope camera</td>
			<td>Zeiss</td>
			<td>426555-0000-000</td>
			<td>5 megapixel, 47 fps live frame rate, exposure time of 100 &#956;s - 2 s, color camera</td>
		</tr>
		<tr>
			<td>Molybdenum disulfide (MoS2)</td>
			<td>HQ Graphene</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Optical breadboard</td>
			<td>Thorlabs, Inc.</td>
			<td>MB4545/M</td>
			<td></td>
		</tr>
		<tr>
			<td>Optical microscope</td>
			<td>Nikon Metrology</td>
			<td>LV150N</td>
			<td></td>
		</tr>
		<tr>
			<td>Oxygen plasma etcher</td>
			<td>Plasma Etch, Inc.</td>
			<td>PE-50</td>
			<td></td>
		</tr>
		<tr>
			<td>PDMS stamp</td>
			<td>Gel-Pak</td>
			<td>PF-20-X4</td>
			<td></td>
		</tr>
		<tr>
			<td>PMMA 950 A6</td>
			<td>MichroChem Corp.</td>
			<td>M230006 0500L1GL</td>
			<td></td>
		</tr>
		<tr>
			<td>Polypropylene carbonate</td>
			<td>Sigma-Aldrich</td>
			<td>389021-100g</td>
			<td></td>
		</tr>
		<tr>
			<td>PVA Partall #10</td>
			<td>Composites Canada</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Rhenium disulfide (ReS2)</td>
			<td>HQ Graphene</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Si/SiO2 substrate</td>
			<td>Nova Electronics Materials</td>
			<td>HS39626-OX</td>
			<td></td>
		</tr>
		<tr>
			<td>Spin coater</td>
			<td>Laurell Technologies</td>
			<td>WS-650-23</td>
			<td></td>
		</tr>
		<tr>
			<td>Temperature controller</td>
			<td>Auber Instruments</td>
			<td>SYL-23X2-24</td>
			<td>Controls the temperature of the bottom stage via a J type thermocouple</td>
		</tr>
		<tr>
			<td>Top stage controller unit</td>
			<td>Mechonics</td>
			<td>CF.030.0003</td>
			<td></td>
		</tr>
		<tr>
			<td>Top stage X manipulator</td>
			<td>Mechonics</td>
			<td>MS.030.1800</td>
			<td>18 mm travel with step size of 11 nm</td>
		</tr>
		<tr>
			<td>Top stage Y manipulator</td>
			<td>Mechonics</td>
			<td>MS.030.1800</td>
			<td>18 mm travel with step size of 11 nm</td>
		</tr>
		<tr>
			<td>Top stage Z manipulator</td>
			<td>Mechonics</td>
			<td>MS.030.3000</td>
			<td>30 mm travel with step size of 11 nm</td>
		</tr>
		<tr>
			<td>Ultrasonic bath</td>
			<td>Elma Schmidbauer GmbH</td>
			<td>Elmasonic P 30 H</td>
			<td></td>
		</tr>
	</tbody>
</table>

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.

To illustrate the outcomes and effectiveness of our procedure we present a sequence of angle-controlled stacks of rhenium disulfide (ReS2) thin crystals. To emphasize that the described method can also be applied to atomically thin layers, we also exemplify the construction of two relatively twisted monolayers of molybdenum disulfide (MoS2).
To demonstrate the angular alignment capabilities of the transfer ...

Watch this Scientific Journal Video about Fabricating van der Waals Heterostructures with Precise Rotational Alignment at JoVE.com

Fabricating van der Waals Heterostructures with Precise Rotational Alignment

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 ...

This protocol allows for the creation of materials previously nonexistent in nature, with the goal of uncovering formerly-inaccessible physical phenomena or developing superior devices for technological applications. The main advantage of this technique is that the equipment can be operated remotely and in an environment control, and this can reduce the risk of manual error and also it can improve the sample cleanliness. Furthermore, the system is equipped with a rotational stage, and this allows the user to reach sub-degree angular alignment between the two flakes.As we are working with thin crystals and small surface areas, identification of an appropriate sized flake can be challenging to the untrained eye. Additionally, the crystal's small size can easily lead to misalignment during the transfer procedure. Therefore it is recommended to execute these steps meticulously.Many steps in the protocol have particular details that are difficult to explain but they're much simpler to follow when presented visually. In order to visualize the transfer process, utilize an optical microscope that can operate under bright-field illumination. Equip the microscope with 5x, 50x and 100x long-working-distance objectives.The microscope must also be equipped with a camera that connects to a computer. Equip two separate manipulators, shown here, to individually control the position of the crystals. Fabricate custom sample holders that can support a glass slide.This sample holder will be used to position the top crystal. For the bottom manipulators, place a flat heating element in a machined glass ceramic holder and affix it to the rotating stage. Then, connect the heating element to a power supply and a temperature controller.Submerge a one-by-one-centimeter square from a silicon silicon-oxide wafer in a beaker filled with acetone and then place the beaker into an ultrasonic cleaner. After 10 minutes, use a pair of tweezers to remove the wafer from the beaker and rinse it with isopropanol, then dry the wafer using a nitrogen gun. Next, carefully remove a portion of the crystal and place it on a piece of semiconductor-grade adhesive tape.Take a second piece of adhesive tape and firmly press it against the initial tape holding the crystal. Peel away the two pieces of tape, repeating several times, to produce many thin pieces of crystal. Press the adhesive tape with the thin 2D crystals onto a freshly-cleaned substrate, such that the crystal is in direct contact with the substrate and peel away the tape to leave exfoliated flakes on the substrate.To remove any residual adhesive, place the resulting sample into a beaker filled with acetone. After 10 minutes, remove the sample, rinse with isopropanol and dry with a nitrogen gun. Examine the exfoliated flakes using an optical microscope to estimate their thickness by assessing the flake's optical contrast with the substrate.Then, use AFM in tapping mode to better quantify the surface morphology and to measure the flake thickness. Prepare the top substrate for the transfer procedure by exfoliating the crystal on a glass slide with an attached polymethylmethacrylate film. After obtaining a clean substrate as previously shown, place the substrate on a spin coater and cover it with polyvinyl alcohol.Spin the substrate at 3, 000 RPM for one minute to produce a layer approximately 450 nanometers thick. Then, place the substrate directly onto a hot plate and bake it uncovered at 75 degrees Celsius for five minutes. Once cool, place the substrate back onto the spin coater and cover it with polymethylmethacrylate.Spin coat an 820-nanometer-thick layer of PMMA onto the substrate at 1, 500 RPM for one minute. After spin coating, place the substrate back onto the hot plate and bake it uncovered at 75 degrees Celsius. After five minutes, remove the substrate from the hot plate and place pieces of adhesive tape along its edges to create a tape frame.Then, mechanically exfoliate a 2D crystal onto the PMMA surface as shown in the previous section. Use a sharp pair of tweezers to separate the PMMA from the PVA, slowly peeling back the tape frame. The PMMA layer and exfoliated crystal, along with the tape frame, will detach from the PVA in the silicon silicon-oxide-wafer substrate.Next, invert the tape frame and place it on a machined support so that the crystal is facing downwards. Place the sample under an optical microscope and use a pair of sharp tweezers to place a small washer precisely on the PMMA film so that it surrounds the desired flake. Then, lower a glass slide and adhere it to the polymer by pressing it against the exposed tape.Place the prepared substrate on the bottom stage of the transfer setup. On this substrate, identify the position of the desired flake. This flake will become the bottom crystal.Then, place the top substrate into the top substrate holder of the transfer setup. Using a low magnification objective, bring the bottom substrate into focus and center the desired flake. Slowly lower the top substrate until it can be seen by the objective.Then, adjust its lateral position and the rotational alignment of the two flakes. When the flakes are close to being aligned as desired, switch to a 50x objective and continue to lower the top substrate, while adjusting the flake alignment. Then, once aligned, lower the top substrate until the top flake entirely contacts the bottom flake.The contact is noticeable due to a sudden change of color. When contact is made, heat the bottom substrate to 75 degrees Celsius for better adhesion of the PMMA to the bottom substrate. The PMMA will detach from the glass slide.Then, clean the bottom substrate to remove the PMMA film. To use the stamping method of flake transfer, first place the substrate on the bottom stage of the transfer setup. On this substrate, identify the position of the desired flake, which will be the bottom crystal.Next, place the top substrate into the holder of the transfer setup and heat the bottom substrate to 100 degree Celsius. Then, align and bring the top crystal into contact with the bottom flake. Once complete contact is made between the two flakes, slowly raise the top substrate.This results in the drop-off of the top flake from the stamp to the bottom substrate. Crystals of rhenium disulfide are ideal for demonstrating angular alignment, because it mechanically exfoliates as elongated bars with well-defined edges. Here, the top flake was placed at a 75 degree angle to the bottom crystal using the PDMS stamping method shown in this video.Atomic force microscopy was used to precisely measure the twist angle between the top and bottom flakes. Using the PMMA PVA procedure, the transfer setup was successfully used to create a structure consisting of two monolayer flakes of molybdenum disulfide. The individual monolayers are shown here exfoliated onto PMMA and silicon silicon-dioxide.The demonstrated procedure results in the structure shown here through an optical microscope. More details, such as the thickness and the relative position of the stacked monolayer can be confirmed using AFM. The most important parts of this procedure are the lateral and rotational alignment of the flakes.Other transfer methods exist and they follow a similar protocol, and different polymers and two-dimensional crystals can be used to fabricate particular structures. And of course, additional cleaning methods can be used to ensure pristine crystal surfaces and interfaces. When using chemicals such as PMMA, acetone and others, be sure to wear the proper personal protective equipment and to carry out the steps in a well-ventilated environment, such as a fume hood.

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Fabricating Metamaterials Using the Fiber Drawing Method

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe their electromagnetic properties to the structure of their constituents, instead of the atoms that compose them. For example, sub-wavelength metal wires can be arranged to possess an effective electric permittivity that is either positive or negative at a given frequency, in contrast to the metals themselves 2. This unprecedented control over the behaviour of light can potentially lead to a number of novel devices, such as invisibility cloaks 3, negative refractive index materials 4, and lenses that resolve objects below the diffraction limit 5. However, metamaterials operating at optical, mid-infrared and terahertz frequencies are conventionally made using nano- and micro-fabrication techniques that are expensive and produce samples that are at most a few centimetres in size 6-7. Here we present a fabrication method to produce hundreds of meters of metal wire metamaterials in fiber form, which exhibit a terahertz plasmonic response 8. We combine the stack-and-draw technique used to produce microstructured polymer optical fiber 9 with the Taylor-wire process 10, using indium wires inside polymethylmethacrylate (PMMA) tubes. PMMA is chosen because it is an easy to handle, drawable dielectric with suitable optical properties in the terahertz region; indium because it has a melting temperature of 156.6 &deg;C which is appropriate for codrawing with PMMA. We include an indium wire of 1 mm diameter and 99.99% purity in a PMMA tube with 1 mm inner diameter (ID) and 12 mm outside diameter (OD) which is sealed at one end. The tube is evacuated and drawn down to an outer diameter of 1.2 mm. The resulting fiber is then cut into smaller pieces, and stacked into a larger PMMA tube. This stack is sealed at one end and fed into a furnace while being rapidly drawn, reducing the diameter of the structure by a factor of 10, and increasing the length by a factor of 100. Such fibers possess features on the micro- and nano- scale, are inherently flexible, mass-producible, and can be woven to exhibit electromagnetic properties that are not found in nature. They represent a promising platform for a number of novel devices from terahertz to optical frequencies, such as invisible fibers, woven negative refractive index cloths, and super-resolving lenses.

Metamaterials at terahertz frequencies offer unique opportunities, but are challenging to fabricate in bulk. We adapt the fabrication procedure for microstructured polymer optical fibers to inexpensively fabricate metamaterials potentially on an industrial scale. We produce polymethylmethacrylate fibers containing ~10 &mu;m diameter indium wires separated by ~100 &mu;m, which exhibit a terahertz plasmonic response.

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe ...

Characterization of Nanocrystal Size Distribution using Raman Spectroscopy with a Multi-particle Phonon Confinement Model

Analysis of the size distribution of nanocrystals is a critical requirement for the processing and optimization of their size-dependent properties. The common techniques used for the size analysis are transmission electron microscopy (TEM), X-ray diffraction (XRD) and photoluminescence spectroscopy (PL). These techniques, however, are not suitable for analyzing the nanocrystal size distribution in a fast, non-destructive and a reliable manner at the same time. Our aim in this work is to demonstrate that size distribution of semiconductor nanocrystals that are subject to size-dependent phonon confinement effects, can be quantitatively estimated in a non-destructive, fast and reliable manner using Raman spectroscopy. Moreover, mixed size distributions can be separately probed, and their respective volumetric ratios can be estimated using this technique. In order to analyze the size distribution, we have formulized an analytical expression of one-particle PCM and projected it onto a generic distribution function that will represent the size distribution of analyzed nanocrystal. As a model experiment, we have analyzed the size distribution of free-standing silicon nanocrystals (Si-NCs) with multi-modal size distributions. The estimated size distributions are in excellent agreement with TEM and PL results, revealing the reliability of our model.

We demonstrate how to determine the size distribution of semiconductor nanocrystals in a quantitative manner using Raman spectroscopy employing an analytically defined multi-particle phonon confinement model. Results obtained are in excellent agreement with the other size analysis techniques like transmission electron microscopy and photoluminescence spectroscopy.

Analysis of the size distribution of nanocrystals is a critical requirement for the processing and optimization of their size-dependent properties. The ...

Simulation of Human-induced Vibrations Based on the Characterized In-field Pedestrian Behavior

For slender and lightweight structures, vibration serviceability is a matter of growing concern, often constituting the critical design requirement. With designs governed by the dynamic performance under human-induced loads, a strong demand exists for the verification and refinement of currently available load models. The present contribution uses a 3D inertial motion tracking technique for the characterization of the in-field pedestrian behavior. The technique is first tested in laboratory experiments with simultaneous registration of the corresponding ground reaction forces. The experiments include walking persons as well as rhythmical human activities such as jumping and bobbing. It is shown that the registered motion allows for the identification of the time variant pacing rate of the activity. Together with the weight of the person and the application of generalized force models available in literature, the identified time-variant pacing rate allows to characterize the human-induced loads. In addition, time synchronization among the wireless motion trackers allows identifying the synchronization rate among the participants. Subsequently, the technique is used on a real footbridge where both the motion of the persons and the induced structural vibrations are registered. It is shown how the characterized in-field pedestrian behavior can be applied to simulate the induced structural response. It is demonstrated that the in situ identified pacing rate and synchronization rate constitute an essential input for the simulation and verification of the human-induced loads. The main potential applications of the proposed methodology are the estimation of human-structure interaction phenomena and the development of suitable models for the correlation among pedestrians in real traffic conditions.

A protocol is presented for the characterization of the in-field pedestrian behavior and the simulation of the resulting structural response. Field-tests demonstrate that the in situ identified pacing rate and synchronization rate among the participants constitute an essential input for the simulation and verification of the human-induced loads.

For slender and lightweight structures, vibration serviceability is a matter of growing concern, often constituting the critical design requirement. With ...

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.

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.

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

A Fabrication and Measurement Method for a Flexible Ferroelectric Element Based on Van Der Waals Heteroepitaxy

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on high-density data storage and low-power consumption capabilities. However, the high-quality oxide based nonvolatile memory on flexible substrates is often constrained by the material characteristics and the inevitable high-temperature fabrication process. In this paper, a protocol is proposed to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica. The versatile deposition technique and measurement method enable the fabrication of flexible yet single-crystalline non-volatile memory elements necessary for the next generation of smart devices.

In this paper, we present a protocol to directly grow an epitaxial yet flexible lead zirconium titanate memory element on muscovite mica.

Flexible non-volatile memories have received much attention as they are applicable for portable smart electronic device in the future, relying on ...

Fabricating Reactive Surfaces with Brush-like and Crosslinked Films of Azlactone-Functionalized Block Co-Polymers

In this paper, fabrication methods that generate novel surfaces using the azlactone-based block co-polymer, poly (glycidyl methacrylate)-block-poly (vinyl dimethyl azlactone) (PGMA-b-PVDMA), are presented. Due to the high reactivity of azlactone groups towards amine, thiol, and hydroxyl groups, PGMA-b-PVDMA surfaces can be modified with secondary molecules to create chemically or biologically functionalized interfaces for a variety of applications. Previous reports of patterned PGMA-b-PVDMA interfaces have used traditional top-down patterning techniques that generate non-uniform films and poorly controlled background chemistries. Here, we describe customized patterning techniques that enable precise deposition of highly uniform PGMA-b-PVDMA films in backgrounds that are chemically inert or that have biomolecule-repellent properties. Importantly, these methods are designed to deposit PGMA-b-PVDMA films in a manner that completely preserves azlactone functionality through each processing step. Patterned films show well-controlled thicknesses that correspond to polymer brushes (~90 nm) or to highly crosslinked structures (~1-10 &#956;m). Brush patterns are generated using either the parylene lift-off or interface directed assembly methods described and are useful for precise modulation of overall chemical surface reactivity by adjusting either the PGMA-b-PVDMA pattern density or the length of the VDMA block. In contrast, the thick, crosslinked PGMA-b-PVDMA patterns are obtained using a customized micro-contact printing technique and offer the benefit of higher loading or capture of secondary material due to higher surface area to volume ratios. Detailed experimental steps, critical film characterizations, and trouble-shooting guides for each fabrication method are discussed.

Surface fabrication methods for patterned deposition of nanometer thick brushes or micron thick, crosslinked films of an azlactone block co-polymer are reported. Critical experimental steps, representative results, and limitations of each method are discussed. These methods are useful for creating functional interfaces with tailored physical features and tunable surface reactivity.

In this paper, fabrication methods that generate novel surfaces using the azlactone-based block co-polymer, poly (glycidyl methacrylate)-block-poly (vinyl ...

Preparing an Isotopically Pure 229Th Ion Beam for Studies of 229mTh

A methodology is described to generate an isotopically pure 229Th ion beam in the 2+ and 3+ charge states. This ion beam enables one to investigate the low-lying isomeric first excited state of 229Th at an excitation energy of about 7.8(5) eV and a radiative lifetime of up to 104 seconds. The presented method allowed for a first direct identification of the decay of the thorium isomer, laying the foundations to study its decay properties as prerequisite for an optical control of this nuclear transition. High energy 229Th ions are produced in the &#945;&#160;decay of a radioactive 233U source. The ions are thermalized in a buffer-gas stopping cell, extracted and subsequently an ion beam is formed. This ion beam is mass purified by a quadrupole-mass separator to generate a pure ion beam. In order to detect the isomeric decay, the ions are collected on the surface of a micro-channel plate detector, where electrons, as emitted in the internal conversion decay of the isomeric state, are observed.

Preparing an Isotopically Pure 229Th Ion Beam for Studies of 229mTh

We present a protocol for the generation of an isotopically purified low-energy 229Th ion beam from a 233U source. This ion beam is used for the direct detection of the 229mTh ground-state decay via the internal-conversion decay channel. We also measure the internal conversion lifetime of 229mTh as well.

A methodology is described to generate an isotopically pure 229Th ion beam in the 2+ and 3+ charge states. This ion beam enables one to investigate the ...

Graphene-Assisted Quasi-van der Waals Epitaxy of AlN Film on Nano-Patterned Sapphire Substrate for Ultraviolet Light Emitting Diodes

This protocol demonstrates a method for graphene-assisted quick growth and coalescence of AlN on nano-pattened sapphire substrate (NPSS). Graphene layers are directly grown on NPSS using catalyst-free atmospheric-pressure chemical vapor deposition (APCVD). By applying nitrogen reactive ion etching (RIE) plasma treatment, defects are introduced into the graphene film to enhance chemical reactivity. During metal-organic chemical vapor deposition (MOCVD) growth of AlN, this N-plasma treated graphene buffer enables AlN quick growth, and coalescence on NPSS is confirmed by cross-sectional scanning electron microscopy (SEM). The high quality of AlN on graphene-NPSS is then evaluated by X-ray rocking curves (XRCs) with narrow (0002) and (10-12) full width at half-maximum (FWHM) as 267.2 arcsec and 503.4 arcsec, respectively. Compared to bare NPSS, AlN growth on graphene-NPSS shows significant reduction of residual stress from 0.87 GPa to 0.25 Gpa, based on Raman measurements. Followed by AlGaN multiple quantum wells (MQWS) growth on graphene-NPSS, AlGaN-based deep ultraviolet light-emitting-diodes (DUV LEDs) are fabricated. The fabricated DUV-LEDs also demonstrate obvious, enhanced luminescence performance. This work provides a new solution for the growth of high quality AlN and fabrication of high performance DUV-LEDs using a shorter process and less costs.

A protocol for graphene-assisted growth of high-quality AlN films on nano-patterned sapphire substrate is presented.

This protocol demonstrates a method for graphene-assisted quick growth and coalescence of AlN on nano-pattened sapphire substrate (NPSS). Graphene layers ...

Treating Surfaces with a Cold Atmospheric Pressure Plasma using the COST-Jet

In recent years, non-thermal atmospheric pressure plasmas have been used extensively for surface treatments, in particular, due to their potential in biological applications. However, the scientific results often suffer from reproducibility problems due to unreliable plasma conditions as well as complex treatment procedures. To address this issue and provide a stable and reproducible plasma source, the COST-Jet reference source was developed.
In this work, we propose a detailed protocol to perform reliable and reproducible surface treatments using the COST reference microplasma jet (COST-Jet). Common issues and pitfalls are discussed, as well as the peculiarities of the COST-Jet compared to other devices and its advantageous remote character. A detailed description of both solid and liquid surface treatment is provided. The described methods are versatile and can be adapted for other types of atmospheric pressure plasma devices.

This protocol is presented to characterize the setup, handling, and application of the COST-Jet for the treatment of diverse surfaces such as solids and liquids.

In recent years, non-thermal atmospheric pressure plasmas have been used extensively for surface treatments, in particular, due to their potential in ...

Etalon Assembly and Fiber-Etalon Alignment

To begin, place the 3D-printed cell on the table with the etalon pit facing upward. Insert an O-ring into the etalon pit and press it slightly into the designated groove. Place the beam splitter with the reflective surface facing upward onto the O-ring in the etalon pit.Using a tweezer, carefully place the two spacers onto the beam splitter to generate a clear aperture for the gas and excitation laser, which enters the air cavity via the through-hole running from one side of the cell to the other. Align the mirror on top of the spacers with the reflective side facing down so that the beam splitter, spacers, and mirror are aligned concentrically. Take the 3D-printed etalon cap and put the O-rings into the designated grooves.Align the cap to the rectangular groove of the cell, lift the cell, and apply pressure on the cap to fix the spacers in place while simultaneously inserting four M4 screws through the designated holes from the back side. Mount the screws with four M4 nuts on the front side and tighten them until the pressure from the cap is enough to hold the spacers in place and the O-rings are sufficiently compressed. For fiber-etalon alignment, mount the pigtailed ferrule and the GRIN lens system with the ferrule clamp and ensure that the translation stage in the Z direction is moved to its maximum height.Align the 3D-printed cell underneath this system, fixing its position at a height slightly below the GRIN lens, pointing directly to the center of the opening. Apply one or two drops of adhesive on the front end of the GRIN lens with a pipette. Lower down the translation stage in the Z direction until contact with the anti-reflection coated surface of the beam splitter is insured.Continue to lower the GRIN lens until sufficient pressure is applied and the springs are under enough tension. Turn on the modulated laser and the oscilloscope. Ensure the oscilloscope has the highest possible resolution when starting the alignment process.Then, set the time resolution so that the two to three periods of the modulation are visible. To start the alignment process, ensure that the GRIN lens points normally on the beam splitter surface. Step by step, deflect the first goniometric stage slightly and then move the other goniometric stage around the zero position.If no change is observed on the oscilloscope, deflect the first goniometric stage slightly more and repeat this iterative process until the triangular modulation becomes visible on the oscilloscope. Once a strong back-reflection is observed, adjust the oscilloscope's resolution and ensure the peak of the etalon's reflectance function sits centrally on the triangular modulation slopes. Tune the etalon's peak by changing the temperature of the laser until the peak is centered on the slope.With slight movements of the goniometric stages, try to maximize the peak strength while simultaneously maximizing the peak-to-peak ratio of the triangular modulation. When the alignment is finished, mount the UV lamp close to the GRIN lens mounted at a 45-degree angle. Next, cure the adhesive applied on the front end of the GRIN lens.After 5 to 10 minutes, turn off the UV lamp and apply more adhesive around the GRIN lens without touching it. Expose the adhesive to UV light for another five to 10 minutes. Repeat this step until the opening of the cell is completely filled with a homogenous layer of adhesive and perform the final cure for at least one hour.The representative image shows a good and worse alignment. The better the alignment, the higher the peak-to-peak ratio of the triangular modulation and the more the reflectance peak approaches zero.