This work is supported by the National Institute of Standards and Technology of the United States Department of Commerce.

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	<li>Doty, F. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Properties+of+CdZnTe+crystals+grown+by+a+high+pressure+Bridgman+method.">Properties of CdZnTe crystals grown by a high pressure Bridgman method.</a> Journal of Vacuum Science &amp; Technology B. 10 (4), 1418-1422 (1992).</li><li>Ikesue, A., Kinoshita, T., Kamata, K., Yoshida, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+and+optical+properties+of+high-performance+polycrystalline+Nd-YAG+Ceramics+for+Solid-State+Lasers.">Fabrication and optical properties of high-performance polycrystalline Nd-YAG Ceramics for Solid-State Lasers.</a> Journal of the American Ceramic Society. 78 (4), 1033-1040 (1995).</li><li>Elwell, D., Scheel, H. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Crystal Growth From High-Temperature Solutions. , (2011).</li><li>Doty, F. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Properties+of+CdZnTe+crystals+grown+by+a+high+pressure+Bridgman+method.">Properties of CdZnTe crystals grown by a high pressure Bridgman method.</a> Journal of Vacuum Science &amp; Technology B. 10 (4), 1418-1422 (1992).</li><li>Ikesue, A., Kinoshita, T., Kamata, K., Yoshida, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+and+optical+properties+of+high-performance+polycrystalline+Nd-YAG+Ceramics+for+Solid-State+Lasers.">Fabrication and optical properties of high-performance polycrystalline Nd-YAG Ceramics for Solid-State Lasers.</a> Journal of the American Ceramic Society. 78 (4), 1033-1040 (1995).</li><li>Elwell, D., Scheel, H. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Crystal Growth From High-Temperature Solutions. , (2011).</li><li>Therese, G. H. A., Kamath, P. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemical+Synthesis+of+Metal+Oxides+and+Hydroxides.">Electrochemical Synthesis of Metal Oxides and Hydroxides.</a> Chemistry of Materials. 12, 1195-1294 (2000).</li><li>Capper, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bulk+Crystal+Growth+-+Methods+and+Materials.">Bulk Crystal Growth - Methods and Materials.</a> Handbook of Electronic and Photonic Materials. , 231-254 (2007).</li><li>Seiler, D. G., Becker, W. M., Roth, L. 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S., 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=Two-dimensional+gas+of+massless+Dirac+fermions+in+graphene.">Two-dimensional gas of massless Dirac fermions in graphene.</a> Nature. 438, 197-200 (2005).</li><li>Wang, L., 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=One-Dimensional+Electrical+Contact+to+a+Two-Dimensional+Metal.">One-Dimensional Electrical Contact to a Two-Dimensional Metal.</a> Science. 342, 614-617 (2013).</li><li>Giessibl, F. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Advances+in+Atomic+Force+Microscopy.">Advances in Atomic Force Microscopy.</a> Reviews of Modern Physics. 75, 949-983 (2003).</li><li>Smith, K. C. A., Oatley, C. 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The authors declare no competing financial interests. Commercial materials, instruments and equipment are identified in this paper to specify the experimental procedure as completely as possible. In no case does such identification imply a recommendation or endorsement by the National Institute of Standards.

After acquisition of high quality bulk samples, characterized to ensure appropriate composition and structure, samples are patterned into the geometry depicted by exfoliating flakes of sample onto 1 cm &#215; 1 cm pieces of substrate. Substrates composed of heavily p-doped Si covered by approximately 300 nm of SiO2 are preferred as they increase the experimental parameter space by allowing the application of a back gate. Samples must be sufficiently thin &#8212; fewer than 10 nm&#160;&#8212; to produce a sufficient field effect to tune the chemical potential in the entirety of the conducting channel of the Hall bar device. Sample thickness is controlled by adequately exfoliating flakes from bulk material using standard wafer dicing tape and repeatedly pressing tape with flakes adhered to it to fresh tape until the flakes are of sufficient thickness for the purpose of the planned experiment. Samples transferred to substrate pieces are too small to see with the naked eye, so an optical microscope must be utilized to identify transferred pieces suitable to fabricate into a Hall bar. Sample flake thickness is accurately measured using atomic force microscopy (AFM), however researchers with sufficient experience may be able to identify samples of desired thickness based on the color of the flake.
A notable challenge to the technical procedure described in this manuscript arises when sample flakes do not adequately adhere to substrate pieces by van der Waals forces. In this case, during any number of steps of the fabrication procedure (especially during immersion in solvents) the sample flakes will be knocked or washed off of the substrate piece surface. This is addressed by the novel technique described in this manuscript whereby the sample flake is fixed to the substrate piece by coating the piece in sputtered SiO2. Once this is done, parts of the sample flake must be exposed to allow the direct adherence of metal contacts. The mask used for the deposition of these contacts may be used to suit this purpose by serving as a mask for the etching of the SiO2, as this would etch SiO2 exactly where the metal contacts will be deposited, permitting electrical contact to the sample flake while still keeping the flake secured with sputtered SiO2 over the majority of its area. Performing this step allows the transport measurement of novel bulk material that has been synthesized into crystals of insufficient size for conventional transport measurements of bulk materials, allowing for the transport study of many new materials as part of the pursuit of exploratory materials science research.
A profound technical advancement offered by the experimental techniques described in this paper come from the ability to stack multiple layered materials into heterostructures. This has numerous advantages. Hexagonal boron nitride (hBN) can be used to sandwich other 2D materials, such as graphene, to product the surface from defects arising from interaction with air, permitting more accurate, defect-free transport measurement of carrier states. Additionally, interesting emergent behaviors may be observable in heterostructures formed from stacks of different materials12. Following exfoliation, transfer to substrate piece, and identification of suitable sample flakes, a procedure may be followed to produce a heterostructure stack of multilayer materials involving sample transfer by the careful use of polymers polydimethylsiloxane (PDMA) and polypropylene carbonate (PPC). This method allows for such stacking without introducing globules of these polymers as contaminants between adjacent materials, as stacking occurs by pressing clean surfaces of the layered materials together. A completed heterostructure stack may be transferred to a new substrate piece for device fabrication.
Device fabrication is a rigorous process involving many steps. Once a suitable sample piece has been transferred, identified, and, if desired, stacked into a heterostructure comprised of several individual flakes, the steps of polymer mask application and patterning and several iterations of etching and metal deposition involved in the fabrication process can take several days to produce a single high quality sample. Due to the find-and-probe nature of this methodology, whereby a flake of desired size, thickness, and quality may be found anywhere on the substrate piece and Hall bar dimensions must be determined after the piece has been identified, lithography must be done by electron beam lithography. Electron beam lithography is an advanced lithography technique that allows the direct writing of structures down to nearly 5 nm dimensions via the use of a scanning of a focused beam of electrons. Specific device structures are prepared for each sample. Isotropic etching is done using the plasma generated in a reactive ion etching (RIE) system. For the etching of a hexagonal boron nitride/Graphene/hexagonal boron nitride stack, the gas used for this plasma etch is a mixture of O2 and CHF3. Deposited metal contacts consist of a thin layer of Cr, meant to serve as an adhesion layer, and a second layer of 750 nm of Au, chosen for its high electrical conductivity, deposited sequentially in high vacuum in an electron beam evaporation chamber. Fabrication of the device is complete after successful metal lift-off following metal deposition, after which point the device may be bonded to a package and loaded into a magnetotransport cryostat for experimental measurement.
Advancement of the fabrication and experimental techniques described in this manuscript will involve improvements in the procedure by which individual flakes may be stacked into heterostructures. Additionally, exfoliation of individual flakes and stacking of layered materials into heterostructures by the techniques described in this manuscript are limited to materials that are not effected by exposure to air. Additional consideration, such as undertaking much of the procedure in an inert environment, must be taken for materials that are destroyed by oxidation, such as the transition metal dichalcogenides and Bi-chalcogenide topological insulators. Magnetotransport systems will continue to see improvement as stronger magnets and lower temperature cryostats are engineered, leading to more powerful experimental measurement capability.

The pursuit of materials platforms for advanced electronics technology demands methods for high-throughput materials synthesis and subsequent characterization. Novel materials of interest in this pursuit may be produced in bulk by direct reaction synthesis1,2, electrochemical growth3,4, and other methods5 in a more rapid fashion than more involved single crystal thin film deposition techniques such as molecular beam epitaxy or chemical vapor deposition. The conventional method to measure the transport properties of bulk crystal samples is to cleave a rectangular prism-shaped fragment with dimensions of approximately 1 mm x 1 mm x 6 mm and attach wire leads to the sample in a Hall bar configuration6.
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. This can be because the crystals produced are too small to attach lead wires to, even under a powerful optical microscope, because the desired sample thickness is on the order of one to only a few monolayers, or because one aims to measure a stack of layered two-dimensional materials with near- or sub-nanometer thickness. The first category consists of, for example nanowires and certain preparations of molybdenum oxide bronzes7. The second category consists of single to very-few layers of two-dimensional materials such as graphene8, TMDs (MoS2, WTe2, etc.), and topological insulators (Bi2Se3, BixSb1-xTe3, etc.). The third category consists of heterostructures prepared by stacking individual layers of two-dimensional materials by manual assembly via layer transfer, most notably a trilayer stack of hBN-graphene-hBN9.
Exploratory research of novel electronic materials demands adequate methods for producing devices on difficult-to-measure samples. Often, the first batch of a new material synthesized by direct reaction or electrochemical growth yields very small single crystals with dimensions on the size order of microns. Such samples have historically proven enormously difficult to attach metal contacts to, necessitating improvement of sample growth parameters to achieve larger crystals for easier transport device fabrication, presenting an obstacle in the high-throughput research of novel materials. In order to enable rapid characterization of materials, 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, hBN, and TMDs, as well as multilayer heterostructures of such materials. Devices are adhered and wire-bonded to a package for insertion into a commercial superconducting magnet, dry helium close-cycle cryostat magnetotransport system. Transport measurements are taken at temperatures down to 0.300 K and magnetic fields up to 12 T.

<table border="0" cellpadding="0" cellspacing="0" width="840">
	<tbody>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">Cryogenic Limited 12T CFMS</td>
			<td style="width:89px;">Cryogen Limited</td>
			<td style="width:135px;">CFM-12T-H3- IVTI-25</td>
			<td style="width:417px;">Magnetotransport system customized with modulated field magnet (step 4)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">7270 DSP Lock-in amplifier</td>
			<td style="width:89px;">Signal Recovery</td>
			<td style="width:135px;">7270</td>
			<td style="width:417px;">lock-in amplifier for source/drain and voltage measurements (step 4)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">GS200 DC Voltage/Current Source</td>
			<td style="width:89px;">Yokogawa</td>
			<td style="width:135px;">GS200</td>
			<td style="width:417px;">Voltage source for gate voltage application (step 4)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:200px;">2636B System Sourcemeter</td>
			<td style="width:89px;">Keithley</td>
			<td style="width:135px;">2636B</td>
			<td style="width:417px;">Sourcemeter for source/drain and voltage measurements</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">DWL 2000 Laser Pattern Generator</td>
			<td style="width:89px;">Heidelberg Instruments</td>
			<td style="width:135px;">DWL 2000</td>
			<td style="width:417px;">Generate chrome mask for lithography of substrate location/alignment feature pattern (step 1.3)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">Suss MicroTec MA6/BA6 Contact Aligner</td>
			<td style="width:89px;">Suss</td>
			<td style="width:135px;">MA6</td>
			<td style="width:417px;">Used for the lithography of substrate location/alignment feature pattern (step 1.4.12)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">JEOL Direct Write Electron Beam Lithography System</td>
			<td style="width:89px;">JEOL</td>
			<td style="width:135px;">JBX 6300-FS&#160;</td>
			<td style="width:417px;">Perform high-resolution lithography of devices</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">Discovery 550 Sputtering System</td>
			<td style="width:89px;">Denton Vacuum</td>
			<td style="width:135px;">Discovery 550</td>
			<td style="width:417px;">Perform SiO2 sputtering (step 2.5)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">Infinity 22 Electron Beam Evaporator</td>
			<td style="width:89px;">Denton Vacuum</td>
			<td style="width:135px;">Infinty 22</td>
			<td style="width:417px;">Perform Cr/Au deposition (steps 1.5 and 3.7)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">Unaxis 790 Reactive Ion Etcher</td>
			<td style="width:89px;">Unaxis</td>
			<td style="width:135px;">Unaxis 790</td>
			<td style="width:417px;">Etch sample into Hall bar structure (step 3.4)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:200px;">Name</td>
			<td style="width:89px;">Company</td>
			<td style="width:135px;">Catalog Number</td>
			<td style="width:417px;">Comments</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">PMMA 495 A4</td>
			<td style="width:89px;">MicroChem</td>
			<td style="width:135px;">PMMA 495 A4</td>
			<td style="width:417px;">Polymer coating/electron beam mask for lithography (step 3.5.1)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">PMMA 950 A4</td>
			<td style="width:89px;">MicroChem</td>
			<td style="width:135px;">PMMA 950 A4</td>
			<td style="width:417px;">Polymer coating/electron beam mask for sample dicing and lithography (steps 1.7.3, 3.3.1, and 3.5.2)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:200px;">S1813 positive photoresist</td>
			<td style="width:89px;">MicroChem</td>
			<td style="width:135px;">S1813 G2</td>
			<td style="width:417px;">Positive photoresist (step 1.4.8)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:200px;">LOR resist</td>
			<td style="width:89px;">MicroChem</td>
			<td style="width:135px;">LOR 3A</td>
			<td style="width:417px;">Lift off resist (step 1.4.3)</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">1:3 MIBK:IPA PMMA developer</td>
			<td style="width:89px;">MicroChem</td>
			<td style="width:135px;">1:3 MIBK:IPA</td>
			<td style="width:417px;">PMMA developer</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">MF-321 Developer</td>
			<td style="width:89px;">MicroChem</td>
			<td style="width:135px;">MF-321</td>
			<td style="width:417px;">Novolac positive photoresist-compatible developer solution (step 1.4.15)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">Diglycidiyl ether-terminated polydimethylsiloxane</td>
			<td style="width:89px;">Sigma Aldrich</td>
			<td style="width:135px;">SA 480282</td>
			<td style="width:417px;">For layered material stacking (step 2.6.1)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">Polypropylene carbonate</td>
			<td style="width:89px;">Sigma Aldrich</td>
			<td style="width:135px;">SA 389021</td>
			<td style="width:417px;">For layered material stacking (step 2.6.2)</td>
		</tr>
	</tbody>
</table>

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.

Figure 3 shows a typical Hall bar device patterned for the purpose of a low temperature magnetotransport experiment. The optical image in the upper figure shows a successfully-fabricated Graphene/hBN Hall bar; the lower image shows the device schematic with the Landauer-B&#252;ttiker edge states that arise from the Landau levels (LLs), a transport mechanism that can be used to calculate the values of the quantized Hall resistances, the experimental investigation of which will be discussed as a representative application of the experimental technique detailed in this paper. Often, fabrication of the Hall bar structure constitutes a tremendous challenge in the overall fabrication process. The steps involved in etching the sample into this shape may be skipped and leads may be attached directly to sample flakes as they are following transfer to the substrate piece. However, the imperfect geometry will not permit careful measurement of transport properties, so skipping the steps involved in etching the sample into a Hall bar structure should be constrained to initial measurements.
Experimental parameters include magnetic fields as high as 12 T, temperatures as low as 0.300 K, and gate voltages as high as 30 V. Alternating current may be supplied by the oscillator from a lock-in amplifier with associated lock-in AC voltage measurements, while direct current may be supplied by a sourcemeter with associated DC voltage measurements. Alternating current versus direct current and the magnitude of the current are parameters that must be carefully selected based on the properties, including resistance and breakdown characteristics, of the material being studied. The Hall resistance is defined as the potential difference between, or voltage measured across, leads 6 and 2 in Figure 3 divided by the applied current. Longitudinal resistance is defined as the potential difference between, or voltage measured across, leads 2 and 3 divided by the applied current. A Graphene Hall bar with the top surface protected by a hBN flake fully encapsulating the Graphene was measured at 1.7 K at magnetic fields ranging from -6 T to +6 T and back gate voltages ranging from -30 V to +30 V. Figure 4 shows how the Hall resistance changes within this parameter space. The behavior of the Hall resistance measured in the Graphene/hBN Hall bar, specifically the observable plateaus in the Hall resistance corresponding to Landau level filling, is a model example of the quantum Hall effect, a quantum mechanical phenomenon observable only by the application of such advanced magnetotransport technique as described in this paper.
Figure 5 shows a slice of the data presented in Figure 4 at B = 6 T, with the Hall resistance (Rxy) as a function of back gate voltage and the corresponding longitudinal resistance (Rxx) as a function of back gate voltage. The measurement clearly shows the that graphene exhibits a quantum Hall effect with quantized Hall resistance values of with integer-value Landau level number n, Planck&#39;s constant h, and electron charge e. The quantized Hall resistance plateaus coincide with vanishing longitudinal resistance.
<img alt="Figure 1" src="/files/ftp_upload/53506/53506fig1.jpg" /> 
Figure 1. Substrate pattern. (A) a region of the designed configuration for the lithographically-patterned positional/alignment marks for the sample substrate. (B) computer rendering of the patterned gold positional/alignment marks. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53506/53506fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53506/53506fig2.jpg" /> 
Figure 2. Graphene flake. An image of flake of monolayer graphene adjacent to a Cr/Au positional marker observed through an optical microscope. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53506/53506fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/53506/53506fig3.jpg" /> 
Figure 3. Hall bar device. Upper figure, optical image of Graphene/hBN Hall bar device with metal leads contacted to the terminals. Lower figure, device schematic with contact leads numbered corresponding to numerically labeled terminals in optical image of the device. Arrows demonstrate flow of current arising Landauer-B&#252;ttiker edge states. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53506/53506fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53506/53506fig4.jpg" /> 
Figure 4. 3D plot of Hall resistance as a function of magnetic field and global backgate. 3D plot of the Hall resistance of Graphene hall bar in the quantum Hall regime as function of magnetic field and global back gate at 1.7 K demonstrating quantum transport behavior. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53506/53506fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/53506/53506fig5.jpg" /> 
Figure 5. Hall and longitudinal magnetoresistance. Hall (Rxy) and longitudinal (Rxx) resistance as function of global back gate at fixed magnetic field &#9474;B&#9474;= 6 T demonstrating quantum Hall effect with quantized Hall resistance values coinciding with vanishing longitudinal resistance. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53506/53506fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Advanced Experimental Methods for Low-temperature Magnetotransport Measurement of Novel Materials at JoVE.com

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

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 ...

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10.5K Views.  National Institute of Standards and Technology. 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.

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

Novel 3D/VR Interactive Environment for MD Simulations, Visualization and Analysis

The increasing development of computing (hardware and software) in the last decades has impacted scientific research in many fields including materials science, biology, chemistry and physics among many others. A new computational system for the accurate and fast simulation and 3D/VR visualization of nanostructures is presented here, using the open-source molecular dynamics (MD) computer program LAMMPS. This alternative computational method uses modern graphics processors, NVIDIA CUDA technology and specialized scientific codes to overcome processing speed barriers common to traditional computing methods. In conjunction with a virtual reality system used to model materials, this enhancement allows the addition of accelerated MD simulation capability. The motivation is to provide a novel research environment which simultaneously allows visualization, simulation, modeling and analysis. The research goal is to investigate the structure and properties of inorganic nanostructures (e.g., silica glass nanosprings) under different conditions using this innovative computational system. The work presented outlines a description of the 3D/VR Visualization System and basic components, an overview of important considerations such as the physical environment, details on the setup and use of the novel system, a general procedure for the accelerated MD enhancement, technical information, and relevant remarks. The impact of this work is the creation of a unique computational system combining nanoscale materials simulation, visualization and interactivity in a virtual environment, which is both a research and teaching instrument at UC Merced.

A new computational system featuring GPU-accelerated molecular dynamics simulation and 3D/VR visualization, analysis and manipulation of nanostructures has been implemented, representing a novel approach to advance materials research and promote innovative investigation and alternative methods to learn about material structures with dimensions invisible to the human eye.

The increasing development of computing (hardware and software) in the last decades has impacted scientific research in many fields including materials ...

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.

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 ...

Detection and Recovery of Palladium, Gold and Cobalt Metals from the Urban Mine Using Novel Sensors/Adsorbents Designated with Nanoscale Wagon-wheel-shaped Pores

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in industrialized countries. Here, the development of optical mesosensors/adsorbents (MSAs) for efficient recognition and selective recovery of Pd(II), Au(III), and Co(II) from urban mine was achieved. A simple, general method for preparing MSAs based on using high-order mesoporous monolithic scaffolds was described. Hierarchical cubic Ia3d wagon-wheel-shaped MSAs were fabricated by anchoring chelating agents (colorants) into three-dimensional pores and micrometric particle surfaces of the mesoporous monolithic scaffolds. Findings show, for the first time, evidence of controlled optical recognition of Pd(II), Au(III), and Co(II) ions and a highly selective system for recovery of Pd(II) ions (up to ~95%) in ores and industrial wastes. Furthermore, the controlled assessment processes described herein involve evaluation of intrinsic properties (e.g., visual signal change, long-term stability, adsorption efficiency, extraordinary sensitivity, selectivity, and reusability); thus, expensive, sophisticated instruments are not required. Results show evidence that MSAs will attract worldwide attention as a promising technological means of recovering and recycling palladium, gold and cobalt metals.

Because of the importance and extensive use of palladium, gold and cobalt metals in high-technology equipment, their recovery and recycling constitute an important industrial challenge. The metal recovery system described herein is a simple, low-cost means for the effective detection, removal, and recovery of these metals from the urban mine.

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in ...

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

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

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

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

Experimental Methods for Investigation of Shape Memory Based Elastocaloric Cooling Processes and Model Validation

Shape Memory Alloys (SMA) using elastocaloric cooling processes have the potential to be an environmentally friendly alternative to the conventional vapor compression based cooling process. Nickel-Titanium (Ni-Ti) based alloy systems, especially, show large elastocaloric effects. Furthermore, exhibit large latent heats which is a necessary material property for the development of an efficient solid-state based cooling process. A scientific test rig has been designed to investigate these processes and the elastocaloric effects in SMAs. The realized test rig enables independent control of an SMA&#39;s mechanical loading and unloading cycles, as well as conductive heat transfer between SMA cooling elements and a heat source/sink. The test rig is equipped with a comprehensive monitoring system capable of synchronized measurements of mechanical and thermal parameters. In addition to determining the process-dependent mechanical work, the system also enables measurement of thermal caloric aspects of the elastocaloric cooling effect through use of a high-performance infrared camera. This combination is of particular interest, because it allows illustrations of localization and rate effects &#8212; both important for efficient heat transfer from the medium to be cooled.
The work presented describes an experimental method to identify elastocaloric material properties in different materials and sample geometries. Furthermore, the test rig is used to investigate different cooling process variations. The introduced analysis methods enable a differentiated consideration of material, process and related boundary condition influences on the process efficiency. The comparison of the experimental data with the simulation results (of a thermomechanically coupled finite element model) allows for better understanding of the underlying physics of the elastocaloric effect. In addition, the experimental results, as well as the findings based on the simulation results, are used to improve the material properties.

Experimental methods for investigation of solid state cooling processes and characterization of elastocaloric material properties of Shape Memory Alloys (SMA) are presented. A custom-built test rig has been designed for controlling and comprehensive monitoring of elastocaloric cooling processes. Furthermore, it provides a validation platform for thermomechanically coupled modeling approaches.

Shape Memory Alloys (SMA) using elastocaloric cooling processes have the potential to be an environmentally friendly alternative to the conventional vapor ...

Measurement of Outgassing Rates of Steels

Steels are commonly used materials in the fabrication of vacuum systems because of their good mechanical, corrosion, and vacuum properties. A variety of steels meet the criterion of low outgassing required for high or ultrahigh vacuum applications. However, a given material can present different outgassing rates depending on its manufacturing process or the various pretreatment processes involved during the fabrication. Thus, the measurement of outgassing rates is highly desirable for a specific vacuum application. For this reason, the rate-of-pressure rise (RoR) method is often used to measure the outgassing of hydrogen after bakeout. In this article, a detailed description of the design and execution of the experimental protocol involved in the RoR method is provided. The RoR method uses a spinning rotor gauge to minimize errors that stem from outgassing or the pumping action of a vacuum gauge. The outgassing rates of two ordinary steels (stainless steel and mild steel) were measured. The measurements were made before and after the heat pretreatment of the steels. The heat pretreatment of steels was performed to reduce the outgassing. Extremely low rates of outgassing (on the order of 10&#8722;11 Pa m3 sec&#8722;1 m&#8722;2) can be routinely measured using relatively small samples.

A protocol for the measurement of outgassing rates of hydrogen from ordinary steel vacuum chambers using the rate-of-pressure rise method is presented.

Steels are commonly used materials in the fabrication of vacuum systems because of their good mechanical, corrosion, and vacuum properties. A variety of ...

Experimental Procedure for Warm Spinning of Cast Aluminum Components

High performance, cast aluminum automotive wheels are increasingly being incrementally formed via flow forming/metal spinning at elevated temperatures to improve material properties. With a wide array of processing parameters which can affect both the shape attained and resulting material properties, this type of processing is notoriously difficult to commission. A simplified, light-duty version of the process has been designed and implemented for full-size automotive wheels. The apparatus is intended to assist in understanding the deformation mechanisms and the material response to this type of processing. An experimental protocol has been developed to prepare for, and subsequently perform forming trials and is described for as-cast A356 wheel blanks. The thermal profile attained, along with instrumentation details are provided. Similitude with full-scale forming operations which impart significantly more deformation at faster rates is discussed.

An experimental protocol for instrumented warm rotary forming of cast aluminum alloys employing a bespoke industrially scaled apparatus is presented. Experimental considerations including thermal and mechanical effects are discussed, as well as similitude with full-scale processing of automotive wheels.

High performance, cast aluminum automotive wheels are increasingly being incrementally formed via flow forming/metal spinning at elevated temperatures to ...

Standardized Method for Measuring Collection Efficiency from Wipe-sampling of Trace Explosives

One of the limiting steps to detecting traces of explosives at screening venues is effective collection of the sample. Wipe-sampling is the most common procedure for collecting traces of explosives, and standardized measurements of collection efficiency are needed to evaluate and optimize sampling protocols. The approach described here is designed to provide this measurement infrastructure, and controls most of the factors known to be relevant to wipe-sampling. Three critical factors (the applied force, travel distance, and travel speed) are controlled using an automated device. Test surfaces are chosen based on similarity to the screening environment, and the wipes can be made from any material considered for use in wipe-sampling. Particle samples of the explosive 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) are applied in a fixed location on the surface using a dry-transfer technique. The particle samples, recently developed to simulate residues made after handling explosives, are produced by inkjet printing of RDX solutions onto polytetrafluoroethylene (PTFE) substrates. Collection efficiency is measured by extracting collected explosive from the wipe, and then related to critical sampling factors and the selection of wipe material and test surface. These measurements are meant to guide the development of sampling protocols at screening venues, where speed and throughput are primary considerations.

Optimized sampling protocols and the development of new wipe materials can be facilitated by standardized measurements of collection efficiency from wipe-sampling. Our approach for sampling trace explosives uses an automated device to control speed, force, and distance during wipe-sampling followed by extraction of collected explosives.

One of the limiting steps to detecting traces of explosives at screening venues is effective collection of the sample. Wipe-sampling is the most common ...

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and forging. Adoption of this technique is proceeding despite disagreement concerning the underlying mechanism responsible for EAD. The experimental procedure described herein enables a more explicit study compared to previous EAD research by removing thermal effects, which are responsible for disagreement in interpreting previous EAD results. Furthermore, as the procedure described here enables EAD observation in situ and in real time in a transmission electron microscope (TEM), it is superior to existing post-mortem methods that observe EAD effects post-test. Test samples consist of a single crystal copper (SCC) foil having a free-standing tensile test section of nanoscale thickness, fabricated using a combination of laser and ion beam milling. The SCC is mounted to an etched silicon base that provides mechanical support and electrical isolation while serving as a heat sink. Using this geometry, even at high current density (~3,500 A/mm2), the test section experiences a negligible temperature increase (&#60;0.02 &#176;C), thus eliminating Joule heating effects. Monitoring material deformation and identifying the corresponding changes to microstructures, e.g. dislocations, are accomplished by acquiring and analyzing a series of TEM images. Our sample preparation and in situ experiment procedures are robust and versatile as they can be readily utilized to test materials with different microstructures, e.g., single and polycrystalline copper.

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Isolating electrical and thermal effects on electrically assisted deformation (EAD) is very difficult using macroscopic samples. Metallic sample micro- and nanostructures together with a custom test procedure have been developed to evaluate the impact of applied current on the formation without joule heating and evolution of dislocations on these samples.

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and ...

Micro/Nano-scale Strain Distribution Measurement from Sampling Moir&#233; Fringes

This work describes the measurement procedure and principles of a sampling moir&#233; technique for full-field micro/nano-scale deformation measurements. The developed technique can be performed in two ways: using the reconstructed multiplication moir&#233; method or the spatial phase-shifting sampling moir&#233; method. When the specimen grid pitch is around 2 pixels, 2-pixel sampling moir&#233; fringes are generated to reconstruct a multiplication moir&#233; pattern for a deformation measurement. Both the displacement and strain sensitivities are twice as high as in the traditional scanning moir&#233; method in the same wide field of view. When the specimen grid pitch is around or greater than 3 pixels, multi-pixel sampling moir&#233; fringes are generated, and a spatial phase-shifting technique is combined for a full-field deformation measurement. The strain measurement accuracy is significantly improved, and automatic batch measurement is easily achievable. Both methods can measure the two-dimensional (2D) strain distributions from a single-shot grid image without rotating the specimen or scanning lines, as in traditional moir&#233; techniques. As examples, the 2D displacement and strain distributions, including the shear strains of two carbon fiber-reinforced plastic specimens, were measured in three-point bending tests. The proposed technique is expected to play an important role in the non-destructive quantitative evaluations of mechanical properties, crack occurrences, and residual stresses of a variety of materials.

A sampling moir&#233; technique featuring 2-pixel and multi-pixel sampling methods for high-accuracy strain distribution measurements at the micro/nano-scale is presented here.

This work describes the measurement procedure and principles of a sampling moir&#233; technique for full-field micro/nano-scale deformation measurements. ...