Name: writing and low-temperature characterization of oxide nanostructures
Uploaded: 2014-07-18T15:00:00.000+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><tbody><tr><td>Equipment</td><td></td><td></td><td></td></tr><tr><td>Contact Aligner</td><td>Karl-Suss</td><td>MA6</td><td></td></tr><tr><td>Spinner</td><td>Solitec</td><td>5110C</td><td></td></tr><tr><td>Ion Mill</td><td>Commonwealth Scientific</td><td>8C</td><td></td></tr><tr><td>Sputtering System</td><td>Leybold-Heraeus</td><td>Z-650</td><td></td></tr><tr><td>Barrel Etcher</td><td>Branson/IPC</td><td>3000C</td><td></td></tr><tr><td>Wire Bonder</td><td>Westbond</td><td>7700E</td><td></td></tr><tr><td>AFM</td><td>Asylum Research</td><td>MFP-3D</td><td></td></tr><tr><td>Dilution Refrigerator</td><td>Quantum Design</td><td>P850</td><td></td></tr><tr><td>Ultrasonic Wash Machine</td><td>Fisher Scientific</td><td>15-335-6</td><td></td></tr><tr><td>Current Amplifier</td><td>Femto</td><td>DLPCA-200</td><td></td></tr><tr><td>Materials</td><td></td><td></td><td></td></tr><tr><td>LaAlO&lt;sub&gt;3&lt;/sub&gt;/SrTiO&lt;sub&gt;3&lt;/sub&gt;</td><td>Prof. Chang-Beom Eom</td><td></td><td>5 mm x 1 mm with ~3.4 unit cells of LAO (See Reference 18)</td></tr><tr><td>Photoresist</td><td>AZ Electronic Materials</td><td>P4210</td><td></td></tr><tr><td>Developer</td><td>AZ Electronic Materials</td><td>400K</td><td></td></tr><tr><td>Acetone</td><td>Fisher Scientific</td><td>A929SK-4</td><td></td></tr><tr><td>Isopropyl Alcohol</td><td>Fisher Scientific</td><td>A459-1</td><td></td></tr><tr><td>Deionized Water</td><td>Fisher Scientific</td><td>23-290-065</td><td></td></tr><tr><td>Gold Wire</td><td>DuPont</td><td>5771</td><td>1 mm diameter</td></tr><tr><td>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 ...

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9.9K Views.  University of Pittsburgh. 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.

Video: Writing and Low-Temperature Characterization of Oxide Nanostructures

Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures

The physical properties of a material are defined by its electronic structure. Electrons in solids are characterized by energy (&omega;) and momentum (k) and the probability to find them in a particular state with given &omega; and k is described by the spectral function A(k, &omega;). This function can be directly measured in an experiment based on the well-known photoelectric effect, for the explanation of which Albert Einstein received the Nobel Prize back in 1921. In the photoelectric effect the light shone on a surface ejects electrons from the material. According to Einstein, energy conservation allows one to determine the energy of an electron inside the sample, provided the energy of the light photon and kinetic energy of the outgoing photoelectron are known. Momentum conservation makes it also possible to estimate k relating it to the momentum of the photoelectron by measuring the angle at which the photoelectron left the surface. The modern version of this technique is called Angle-Resolved Photoemission Spectroscopy (ARPES) and exploits both conservation laws in order to determine the electronic structure, i.e. energy and momentum of electrons inside the solid. In order to resolve the details crucial for understanding the topical problems of condensed matter physics, three quantities need to be minimized: uncertainty* in photon energy, uncertainty in kinetic energy of photoelectrons and temperature of the sample.
In our approach we combine three recent achievements in the field of synchrotron radiation, surface science and cryogenics. We use synchrotron radiation with tunable photon energy contributing an uncertainty of the order of 1 meV, an electron energy analyzer which detects the kinetic energies with a precision of the order of 1 meV and a He3 cryostat which allows us to keep the temperature of the sample below 1 K. We discuss the exemplary results obtained on single crystals of Sr2RuO4 and some other materials. The electronic structure of this material can be determined with an unprecedented clarity.

The overall goal of this method is to determine the low-energy electronic structure of solids at ultra-low temperatures using Angle-Resolved Photoemission Spectroscopy with synchrotron radiation.

The physical properties of a material are defined by its  electronic structure. Electrons in solids are characterized by energy (&omega;)  and momentum ...

Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

Nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas allows the deposition of metal oxides with tunable morphology, structure, density and stoichiometry by a proper control of the plasma plume expansion dynamics. Such versatility can be exploited to produce nanostructured films from compact and dense to nanoporous characterized by a hierarchical assembly of nano-sized clusters. In particular we describe the detailed methodology to fabricate two types of Al-doped ZnO (AZO) films as transparent electrodes in photovoltaic devices: 1) at low O2 pressure, compact films with electrical conductivity and optical transparency close to the state of the art transparent conducting oxides (TCO) can be deposited at room temperature, to be compatible with thermally sensitive materials such as polymers used in organic photovoltaics (OPVs); 2) highly light scattering hierarchical structures resembling a forest of nano-trees are produced at higher pressures. Such structures show high Haze factor (&gt;80%) and may be exploited to enhance the light trapping capability. The method here described for AZO films can be applied to other metal oxides relevant for technological applications such as TiO2, Al2O3, WO3 and Ag4O4.

We describe the experimental method to deposit nanostructured oxide thin films by nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas. By using this method Al-doped ZnO (AZO) films, from compact to hierarchically structured as nano-tree forests, can be deposited.

Nanosecond Pulsed  Laser Deposition (PLD) in the presence of a background gas allows the deposition  of metal oxides with tunable morphology, structure, ...

Fabrication of Spatially Confined Complex Oxides

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from the inherent strong electron correlations that reside within them. These materials can also possess electronic phase separation in which regions of vastly different resistive and magnetic behavior can coexist within a single crystal alloy material. By reducing the scale of these materials to length scales at and below the inherent size of the electronic domains, novel behaviors can be exposed. Because of this and the fact that spin-charge-lattice-orbital order parameters each involve correlation lengths, spatially reducing these materials for transport measurements is a critical step in understanding the fundamental physics that drives complex behaviors. These materials also offer great potential to become the next generation of electronic devices 1-3. Thus, the fabrication of low dimensional nano- or micro-structures is extremely important to achieve new functionality. This involves multiple controllable processes from high quality thin film growth to accurate electronic property characterization. Here, we present fabrication protocols of high quality microstructures for complex oxide manganite devices. Detailed descriptions and required equipment of thin film growth, photo-lithography, and wire-bonding are presented.

We describe the use of pulsed laser deposition (PLD), photolithography and wire-bonding techniques to create micrometer scale complex oxides devices. The PLD is utilized to grow epitaxial thin films. Photolithography and wire-bonding techniques are introduced to create practical devices for measurement purposes.

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from ...

Atomically Defined Templates for Epitaxial Growth of Complex Oxide Thin Films

Atomically defined substrate surfaces are prerequisite for the epitaxial growth of complex oxide thin films. In this protocol, two approaches to obtain such surfaces are described. The first approach is the preparation of single terminated perovskite SrTiO3 (001) and DyScO3 (110) substrates. Wet etching was used to selectively remove one of the two possible surface terminations, while an annealing step was used to increase the smoothness of the surface. The resulting single terminated surfaces allow for the heteroepitaxial growth of perovskite oxide thin films with high crystalline quality and well-defined interfaces between substrate and film. In the second approach, seed layers for epitaxial film growth on arbitrary substrates were created by Langmuir-Blodgett (LB) deposition of nanosheets. As model system Ca2Nb3O10- nanosheets were used, prepared by delamination of their layered parent compound HCa2Nb3O10. A key advantage of creating seed layers with nanosheets is that relatively expensive and size-limited single crystalline substrates can be replaced by virtually any substrate material.

Various procedures are outlined to prepare atomically defined templates for epitaxial growth of complex oxide thin films. Chemical treatments of single crystalline SrTiO3 (001) and DyScO3 (110) substrates were performed to obtain atomically smooth, single terminated surfaces. Ca2 Nb3 O10- nanosheets were used to create atomically defined templates on arbitrary substrates.

Atomically defined substrate surfaces are prerequisite for the epitaxial growth of complex oxide thin films. In this protocol, two approaches to obtain ...

Chemistry

Preparation and Reactivity of Gasless Nanostructured Energetic Materials

High-Energy Ball Milling (HEBM) is a ball milling process where a powder mixture placed in the ball mill is subjected to high-energy collisions from the balls. Among other applications, it is a versatile technique that allows for effective preparation of gasless reactive nanostructured materials with high energy density per volume (Ni+Al, Ta+C, Ti+C). The structural transformations of reactive media, which take place during HEBM, define the reaction mechanism in the produced energetic&#160;composites. Varying the processing conditions permits fine tuning of the milling-induced microstructures of the fabricated composite particles. In turn, the reactivity, i.e., self-ignition temperature, ignition delay time, as well as reaction kinetics, of high energy density materials depends on its microstructure. Analysis of the milling-induced microstructures suggests that the formation of fresh oxygen-free intimate high surface area contacts between the reagents is responsible for the enhancement of their reactivity. This manifests itself in a reduction of ignition temperature and delay time, an increased rate of chemical reaction, and an overall decrease of the effective activation energy of the reaction. The protocol provides a detailed description for the preparation of reactive nanocomposites with tailored microstructure using short-term HEBM method. It also describes a high-speed thermal imaging technique to determine the ignition/combustion characteristics of the energetic materials. The protocol can be adapted to preparation and characterization of a variety of nanostructured energetic composites.

This protocol describes the preparation of gasless nanostructured energetic materials (Ni+Al, Ta+C, Ti+C) using the short-term high-energy ball milling (HEBM) technique. It also describes a high-speed thermal imaging method to study the reactivity of mechanically fabricated nanocomposites. These protocols can be extended to other reactive nanostructured energetic materials.

High-Energy Ball Milling (HEBM) is a ball milling process where a powder mixture placed in the ball mill is subjected to high-energy collisions from the ...

Atomically Traceable Nanostructure Fabrication

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process being used in order to maintain exquisite control over both feature size and feature density. Here, we demonstrate a method for tracking atomically resolved and controlled structures from initial template definition through final nanostructure metrology, opening up a pathway for top-down atomic control over nanofabrication. Hydrogen depassivation lithography is the first step of the nanoscale fabrication process followed by selective atomic layer deposition of up to 2.8 nm of titania to make a nanoscale etch mask. Contrast with the background is shown, indicating different mechanisms for growth on the desired patterns and on the H passivated background. The patterns are then transferred into the bulk using reactive ion etching to form 20 nm tall nanostructures with linewidths down to ~6 nm. To illustrate the limitations of this process, arrays of holes and lines are fabricated. The various nanofabrication process steps are performed at disparate locations, so process integration is discussed. Related issues are discussed including using fiducial marks for finding nanostructures on a macroscopic sample and protecting the chemically reactive patterned Si(100)-H surface against degradation due to atmospheric exposure.

We report a protocol for combining the atomic metrology of the Scanning Tunneling Microscope for surface patterning with selective Atomic Layer Deposition and Reactive Ion Etching. Using a robust process involving numerous atmospheric exposures and transport, 3D nanostructures with atomic metrology are fabricated.

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process ...

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 ...

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 ...

Bulk and Thin Film Synthesis of Compositionally Variant Entropy-stabilized Oxides

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (Cu variant) entropy-stabilized oxides. Phase pure and chemically homogeneous (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (x = 0.11, 0.27) ceramic pellets are synthesized and used in the deposition of ultra-high quality, phase pure, single crystalline thin films of the target stoichiometry. A detailed methodology for the deposition of smooth, chemically homogeneous, entropy-stabilized oxide thin films by pulsed laser deposition on (001)-oriented MgO substrates is described. The phase and crystallinity of bulk and thin film materials are confirmed using X-ray diffraction. Composition and chemical homogeneity are confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The surface topography of thin films is measured with scanning probe microscopy. The synthesis of high quality, single crystalline, entropy-stabilized oxide thin films enables the study of interface, size, strain, and disorder effects on the properties in this new class of highly disordered oxide materials.

The synthesis of high quality bulk and thin film (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O entropy-stabilized oxides is presented.

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and ...

Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing

Electrical, optical, and magnetic properties of oxide materials can often be controlled by varying the oxygen content. Here we outline two approaches for varying the oxygen content and provide concrete examples for tuning the electrical properties of SrTiO3-based heterostructures. In the first approach, the oxygen content is controlled by varying the deposition parameters during a pulsed laser deposition. In the second approach, the oxygen content is tuned by subjecting the samples to annealing in oxygen at elevated temperatures after the film growth. The approaches can be used for a wide range of oxides and nonoxide materials where the properties are sensitive to a change in the oxidation state.
The approaches differ significantly from electrostatic gating, which is often used to change the electronic properties of confined electronic systems such as those observed in SrTiO3-based heterostructures. By controlling the oxygen vacancy concentration, we are able to control the carrier density over many orders of magnitude, even in nonconfined electronic systems. Moreover, properties can be controlled, which are not sensitive to the density of itinerant electrons.

Oxide materials show many exotic properties that can be controlled by tuning the oxygen content. Here, we demonstrate the tuning of oxygen content in oxides by varying the pulsed laser deposition parameters and by performing postannealing. As an example, electronic properties of SrTiO3-based heterostructures are tuned by growth modifications and annealing.

Electrical, optical, and magnetic properties of oxide materials can often be controlled by varying the oxygen content. Here we outline two approaches for ...