Name: protocol for the evaluation of mri artifacts caused by metal implants to assess the suitability of implants and the vulnerability of pulse sequences
Uploaded: 2018-05-17T14:00:00
Description: Watch this Scientific Journal Video about Protocol for the Evaluation of MRI Artifacts Caused by Metal Implants to Assess the Suitability of Implants and the Vulnerability of Pulse Sequences at JoVE.c

The authors like to thank Stefanie Sauer, pharmacist at the Department of Pharmacy Heidelberg University Hospital, for her contributions to the MRI phantom. In addition, we would like to thank NORAS MRI products GmbH (H&#246;chberg, Germany) and especially Daniel Gareis for providing a prototype of the 16-channel multipurpose coil. Furthermore, we are grateful for the kind cooperation with SIEMENS Healthcare GmbH (Erlangen, Germany) and especially Mathias Nittka for their assistance in the sequence setup.

1. Phantom Preparation
<ol>
	<li>Determine the implant volume (e.g., by using the water displacement method). 
	NOTE: The volume of the CCT-T sample and the Z-T sample measured 0.65 mL and 0.73 mL, respectively.</li>
	<li>Fix the implant position in the middle of a non-ferromagnetic, plastic, waterproof box by using a thin thread. Use a box that is larger than the expected MRI artifacts. 
	NOTE: If no rough estimates of the artifact volumes of the implant and/or pulse sequence of interest are availab...

<ol>
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The number of patients with metallic implants and the number of MRI examinations is currently increasing1,2,3. In the past, MRI examinations were avoided after joint replacements. Today, however, MRI is not only requested for imaging such patients but should also allow for the evaluation of complications directly adjacent to joint arthroplasty. Thus, the MRI safety and MRI suitability of implants, as well as the robust pulse seq...

MRI has become an indispensable diagnostic tool. As a result, the number of MRI systems used in routine diagnostics is further increasing1. At the same time, the number of patients with implants is increasing as well2,3. In 2012, for instance, more than 1 million knee and joint replacements have been performed in the USA alone4. The prevalence of such implants was about 7 million in 2010, which corresponds to more than 10% of females in the age group 80-89 years5. As a result, the image quality and the diagnostic significance of MRI examinations are often impaired by artifacts due to metallic implants, resulting in a decreased diagnostic accuracy. Therefore, the MRI suitability of implants and the artifact vulnerability of pulse sequences are becoming increasingly important. Numerous approaches have been published to evaluate these characteristics. Due to strong discrepancies in the used evaluation methods, however, the respective results are hard to compare.
An evaluation of the MRI suitability of materials can be performed by calculating their magnetic susceptibility6. However, the vulnerability of different pulse sequences to artifacts cannot be compared with that approach for a given implant. Vice versa, the artifact volumes for a given pulse sequence can only be roughly estimated for different implants. In addition, the analysis is often performed with artificially shaped implants7,8. As the material volume and shape have an influence on the artifact size6, these features should be taken into account as well. As an alternative to magnetic susceptibility, the artifact size can be evaluated. Frequently, studies only rely on the qualitative evaluation of the artifact size9 or focus on the two-dimensional artifact size only covering one slice of the implant artifact10,11. Moreover, manual segmentation approaches are often used, which is not only time-consuming but also prone to intra- and inter-reader differences11. Finally, protocols often do not allow to test for non-fat-saturated and fat-saturated sequences at the same time12. This, however, would be desirable, since the applied fat suppression technique profoundly affects the artifact size.
Here, we present a protocol which allows for the reliable, semiautomatic, threshold-based, three-dimensional quantification of signal loss and pile-up artifacts of the entire implant, or all slices containing visible implant artifacts. Furthermore, it allows for testing T1- and T2-weighted images with or without fat-saturation. The protocol can be used to evaluate the MRI suitability of different implants or the vulnerability of different pulse sequences to metallic artifacts for a given implant.

<table border="0" cellpadding="0" cellspacing="0" width="638">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">Aqua B. Braun Ecotainer</td>
			<td style="width:288px;">B. Braun Melsungen AG, Melsungen, Germany</td>
			<td style="width:136px;"></td>
			<td style="width:64px;"></td>
		</tr>
		<tr height="33">
			<td height="33" style="height:33px;width:151px;">Semisynthetic fat: Witepsol W25</td>
			<td style="width:288px;">Caelo Caesar &#38; Loretz GmbH, Hilder, Germany</td>
			<td style="width:136px;">4051</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">Macrogol-8-stearate</td>
			<td style="width:288px;">Caelo Caesar &#38; Loretz GmbH, Hilder, Germany</td>
			<td style="width:136px;">3023</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">Plastic box: not specified</td>
			<td style="width:288px;"></td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">Implants: Nobel Replace</td>
			<td>Nobel Biocare, Z&#252;rich, Switzerland</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">Water bath Haake S5P</td>
			<td style="width:288px;">Thermo Scientific, Waltham, MA, USA</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="52">
			<td height="52" style="height:52px;width:151px;">Measuring cylinder Blaubrand Eterna, Class A, Boro 3.3</td>
			<td style="width:288px;">BRAND GmbH + Co Kg, Wertheim, Germany</td>
			<td style="width:136px;">32708</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">Coil: Variety</td>
			<td style="width:288px;">Noras MRI products GmbH, H&#246;chberg, Germany</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:151px;">MRI: Magnetom Trio</td>
			<td style="width:288px;">Siemens Healthcare GmbH, Erlangen, Germany</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="35">
			<td height="35" style="height:35px;width:151px;">Postprocesing software: Amira 6.4</td>
			<td style="width:288px;">Thermo Scientific, Waltham, MA, USA</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
	</tbody>
</table>

protocol for the evaluation of mri artifacts caused by metal implants to assess the suitability of implants and the vulnerability of pulse sequences

As the number of magnetic resonance imaging (MRI) scanners and patients with medical implants is constantly growing, radiologists increasingly encounter metallic implant-related artifacts in MRI, resulting in reduced image quality. Therefore, the MRI suitability of implants in terms of artifact volume, as well as the development of pulse sequences to reduce image artifacts, are becoming more and more important. Here, we present a comprehensive protocol which allows for a standardized evaluation of the artifact volume of implants on MRI. Furthermore, this protocol can be used to analyze the vulnerability of different pulse sequences to artifacts. The proposed protocol can be applied to T1- and T2-weighted images with or without fat-suppression and all passive implants. Furthermore, the procedure enables the separate and three-dimensional identification of signal loss and pile-up artifacts. As previous investigations differed greatly in evaluation methods, the comparability of their results was limited. Thus, standardized measurements of MRI artifact volumes are necessary to provide better comparability. This may improve the development of the MRI suitability of implants and better pulse sequences to finally improve patient care.

With the above-mentioned protocol, we evaluated the artifact volume of 2 different dental implants made of Titanium (T; see the Table of Materials) supporting different crowns [porcelain-fused-to-metal non-precious alloy (CCT-T) and monolithic zirconia (Z-T); Figure 1b and 1c]. The CCT-T sample represents a highly paramagnetic material composition predicting large artifacts (Cobalt 61%, Chrome 21%, and Tungsten 11%; CCT). The...

Watch this Scientific Journal Video about Protocol for the Evaluation of MRI Artifacts Caused by Metal Implants to Assess the Suitability of Implants and the Vulnerability of Pulse Sequences at JoVE.c

Protocol for the Evaluation of MRI Artifacts Caused by Metal Implants to Assess the Suitability of Implants and the Vulnerability of Pulse Sequences

We describe a standardized method to evaluate magnetic resonance imaging artifacts caused by implants to estimate the suitability of the implants for magnetic resonance imaging and/or the vulnerability of different pulse sequences to metallic artifacts simultaneously.

As the number of magnetic resonance imaging (MRI) scanners and patients with medical implants is constantly growing, radiologists increasingly encounter ...

The overall goal of this method is to evaluate the suitability of implants for magnetic resonance imaging, or estimate the vulnerability of pulse sequences to metal artifacts. This method can help to answer key questions in the radiology field, such as the evaluation of MRI suitability of implants and the estimation of vulnerability of pulse sequences from metal artifacts. The main advantage of this methodology is that it allows for a three-dimensional artifact evaluation in fat-suppressed and not fat-suppressed T1 and T2 weightings at the same time.The implications of this technique extend towards diagnosis of peri-implant diseases because it allows for optimization of MRI protocols with regard to reduction of peri-implant artifacts. Before constructing the phantom for this protocol, first determine the volume of the implant to be studied. Here, water displacement method is used, and the volume of the samples measured 0.65 milliliters and 0.73 milliliters respectively.Select a non-ferromagnetic plastic waterproof box that is larger than the expected MRI artifacts, then fix the implant position in the middle of the box using a thin thread. Next, carefully melt a mixture of semi-synthetic fat, water, and macrogol-8-stearate using a water bath at 50 degrees Celsius. Here, a 500 milliliter mixture was used for the embedding of each sample.Once the mixture becomes fluid, turn off the heat and then start stirring it slowly. While doing this, ensure that there is no separation of the fat and water phases in the mixture. Then, as soon as clotting begins, slowly pour the embedding mixture into the phantom box with the implant, being sure to pour gradually to avoid air inclusion.Finally, place the phantom into a refrigerator at four degrees Celsius overnight for desiccation, and decant any remaining residual fluid the next day. For the MRI examination, use a radiofrequency coil that allows for a homogeneous signal distribution within the imaging volume without severe and obvious signal drops. Be sure to position the phantom with the embedded implant within the radio frequency coil in the same orientation as it would be placed in vivo.Position the middle of the phantom in the iso center of the MRI. Next, set up the imaging sequences at the MRI console, making sure that the phantom box, including some air at the edges of the box, is within the imaging volume. Then acquire the images that will be evaluated for artifacts.Export the images from the MRI console without compression preferably using DICOM format. Then, import The images into post processing software that allows for placing regions of interest, or ROIs, evaluating signal intensities, threshold based segmentation, and a quantification of segmented volumes. To define the threshold for pile-up artifacts, and check for homogeneous signal distribution, first use the segmentation editor to place perpendicular lines adjacent to the outer border of the visible artifact on the slice with the maximum artifact size.Then, place a background ROI 10 millimeters in diameter outside each of the four intersection points. Next, in the project view, you use the material statistics tool to measure the mean signal intensity and standard deviation of all voxels within these four background ROI values and for each background ROI separately. ensure that the mean signal intensity of each background ROI is within 1.5 standard deviations of the mean signal of each of the other three counterparts to guarantee a homogeneous signal distribution.Then, calculate the threshold for pile-up artifacts by adding three standard deviations of the background ROI to the mean signal intensity of all voxels of the four ROI values. Next, perform a semi-automatic threshold based segmentation of pile-up artifacts. Use the masking tool to visualize the predefined signal intensity range and restrict the segmentation to it, selecting all voxels with signal intensities greater than the threshold adjacent to the signal loss artifact.To define the threshold for signal loss artifacts, first place four ROIs in air containing regions 10 millimeter in diameter at the corners of the phantom box, then measure the mean signal intensity and standard deviation of all voxels within them. Now, place an ROI in the core of the signal loss artifact defined by the largest connected area of low signal intensities. Then manually increase the size of this ROI to the largest possible size as defined on the screen here.Finally, measure the mean signal intensity and standard deviation of this core ROI. Also, calculate the signal intensity threshold for signal loss artifacts as seen here. Then perform a semi-automatic threshold based segmentation of these artifacts by selecting all voxels connected to the core ROI with signal intensities below this threshold.Use the masking tool of the segmentation editor to visualize the predefined signal intensity range and restrict the segmentation to it. If possible, use the fill function in the segmentation editor to include all voxels that are not yet selected. Subtract the physical implant volume from the calculated artifact volume to obtain the true artifact volume, and repeat this analysis at least three times.A time interval of at least two weeks should separate these multiple reads to exclude any learning bias. Typical positioning of the regions of interest for measuring the thresholds for pile-up artifacts and signal distribution in signal loss artifacts is seen here. The blue contour represents the semi-automatic segmentation for signal loss artifacts.The small red areas correspond to the result of pile-up artifacts. Dental implants supporting different single crowns used in the phantoms are seen here. These graphs show the mean three-dimensional artifact volume for each implant sample with standard deviations after subtracting the physical implant volume.Four MRI sequences were evaluated. Here, renderings of the artifact volumes for a cobalt chromium tungsten titanium implant and a zirconia titanium implant are shown. The blue areas represent signal loss artifacts and the red areas represent pile-up artifacts.Below we see the colored source images for all evaluated T-2 weighted sequences. While attempting this procedure, it's important to choose identical sequence parameters as in the Invivo scan, to allow for accurate transferability of the in vitro results. After watching this video you should have a good understanding of how to evaluate signal loss and pile-up artifacts in a standardized way.This allows you to assess the MRI suitability of implants, or vulnerability of pulse sequences to metal artifacts.

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Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra shallow and sharp doping profiles at the nanometer scale. In MLCD process the dopant source is a monolayer containing dopant atoms.
In this article a detailed procedure for surface doping of silicon substrate as well as silicon nanowires is demonstrated. Phosphorus dopant source was formed using tetraethyl methylenediphosphonate monolayer on a silicon substrate. This monolayer containing substrate was brought to contact with a pristine intrinsic silicon target substrate and annealed while in contact. Sheet resistance of the target substrate was measured using 4 point probe. Intrinsic silicon nanowires were synthesized by chemical vapor deposition (CVD) process using a vapor-liquid-solid (VLS) mechanism; gold nanoparticles were used as catalyst for nanowire growth. The nanowires were suspended in ethanol by mild sonication. This suspension was used to dropcast the nanowires on silicon substrate with a silicon nitride dielectric top layer. These nanowires were doped with phosphorus in similar manner as used for the intrinsic silicon wafer. Standard photolithography process was used to fabricate metal electrodes for the formation of nanowire based field effect transistor (NW-FET). The electrical properties of a representative nanowire device were measured by a semiconductor device analyzer and a probe station.

A detailed procedure for surface doping of Silicon interfaces is provided. The ultra-shallow surface doping is demonstrated by using phosphorus containing monolayers and rapid annealing process. The method can be used for doping of macroscopic area surfaces as well as nanostructures.

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra ...

Fast Imaging Technique to Study Drop Impact Dynamics of Non-Newtonian Fluids

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Drop impact of non-Newtonian fluids is a complex process since different physical parameters influence the dynamics over a very short time (less than one tenth of a millisecond). A fast imaging technique is introduced in order to characterize the impact behaviors of different non-Newtonian fluids.

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We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A single gold nanoparticle held by an optical tweezer is used as a sensor to quantify the rhythmic motion of a Nauplius larva (Artemia salina) in a water sample. This is achieved by monitoring the time dependent displacement of the trapped nanoparticle as a consequence of the Nauplius activity. A Fourier analysis of the nanoparticle&#39;s position then yields a frequency spectrum that is characteristic to the motion of the observed species. This experiment demonstrates the capability of this method to measure and characterize the activity of small aquatic larvae without the requirement to observe them directly and to gain information about the position of the larvae with respect to the trapped particle. Overall, this approach could give an insight on the vitality of certain species found in an aquatic ecosystem and could expand the range of conventional methods for analyzing water samples.

Analyzing the Movement of the Nauplius &#039;Artemia salina&#039; by Optical Tracking of Plasmonic Nanoparticles

We use optical tracking of plasmonic nanoparticles to probe and characterize the frequency movements of aquatic organisms.

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A ...

Integrating a Triplet-triplet Annihilation Up-conversion System to Enhance Dye-sensitized Solar Cell Response to Sub-bandgap Light

The poor response of dye-sensitized solar cells (DSCs) to red and infrared light is a significant impediment to the realization of higher photocurrents and hence higher efficiencies. Photon up-conversion by way of triplet-triplet annihilation (TTA-UC) is an attractive technique for using these otherwise wasted low energy photons to produce photocurrent, while not interfering with the photoanodic performance in a deleterious manner. Further to this, TTA-UC has a number of features, distinct from other reported photon up-conversion technologies, which renders it particularly suitable for coupling with DSC technology. In this work, a proven high performance TTA-UC system, comprising a palladium porphyrin sensitizer and rubrene emitter, is combined with a high performance DSC (utilizing the organic dye D149) in an integrated device. The device shows an enhanced response to sub-bandgap light over the absorption range of the TTA-UC sub-unit resulting in the highest figure of merit for up-conversion assisted DSC performance to date.

An integrated device, incorporating a dye-sensitized solar cell and triplet-triplet annihilation up-conversion unit was produced, affording enhanced light harvesting, from a wider section of the solar spectrum. Under modest irradiation levels a significantly enhanced response to low energy photons was demonstrated, yielding a record figure of merit for dye-sensitized solar cells.

The poor response of dye-sensitized solar cells (DSCs) to red and infrared light is a significant impediment to the realization of higher photocurrents ...

Casting Protocols for the Production of Open Cell Aluminum Foams by the Replication Technique and the Effect on Porosity

Metal foams are interesting materials from both a fundamental understanding and practical applications point of view. Uses have been proposed, and in many cases validated experimentally, for light weight or impact energy absorbing structures, as high surface area heat exchangers or electrodes, as implants to the body, and many more. Although great progress has been made in understanding their structure-properties relationships, the large number of different processing techniques, each producing material with different characteristics and structure, means that understanding of the individual effects of all aspects of structure is not complete. The replication process, where molten metal is infiltrated between grains of a removable preform material, allows a markedly high degree of control and has been used to good effect to elucidate some of these relationships. Nevertheless, the process has many steps that are dependent on individual &#8220;know-how&#8221;, and this paper aims to provide a detailed description of all stages of one embodiment of this processing method, using materials and equipment that would be relatively easy to set up in a research environment. The goal of this protocol and its variants is to produce metal foams in an effective and simple way, giving the possibility to tailor the outcome of the samples by modifying certain steps within the process. By following this, open cell aluminum foams with pore sizes of 1&#8211;2.36 mm diameter and 61% to 77% porosity can be obtained.

Replication is one of the processing techniques used for the production of porous metal sponges. In this paper one implementation of the method for the production of open celled porous aluminum is shown in detail.

Metal foams are interesting materials from both a fundamental understanding and practical applications point of view. Uses have been proposed, and in many ...

Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis

Nuclear reaction analysis (NRA) via the resonant 1H(15N,&#945;&#947;)12C reaction is a highly effective method of depth profiling that quantitatively and non-destructively reveals the hydrogen density distribution at surfaces, at interfaces, and in the volume of solid materials with high depth resolution. The technique applies a 15N ion beam of 6.385 MeV provided by an electrostatic accelerator and specifically detects the 1H isotope in depths up to about 2 &#956;m from the target surface. Surface H coverages are measured with a sensitivity in the order of ~1013 cm-2 (~1% of a typical atomic monolayer density) and H volume concentrations with a detection limit of ~1018 cm-3 (~100 at. ppm). The near-surface depth resolution is 2-5 nm for surface-normal 15N ion incidence onto the target and can be enhanced to values below 1 nm for very flat targets by adopting a surface-grazing incidence geometry. The method is versatile and readily applied to any high vacuum compatible homogeneous material with a smooth surface (no pores). Electrically conductive targets usually tolerate the ion beam irradiation with negligible degradation. Hydrogen quantitation and correct depth analysis require knowledge of the elementary composition (besides hydrogen) and mass density of the target material. Especially in combination with ultra-high vacuum methods for in-situ target preparation and characterization, 1H(15N,&#945;&#947;)12C NRA is ideally suited for hydrogen analysis at atomically controlled surfaces and nanostructured interfaces. We exemplarily demonstrate here the application of 15N NRA at the MALT Tandem accelerator facility of the University of Tokyo to (1) quantitatively measure the surface coverage and the bulk concentration of hydrogen in the near-surface region of a H2 exposed Pd(110) single crystal, and (2) to determine the depth location and layer density of hydrogen near the interfaces of thin SiO2 films on Si(100).

We illustrate the application of 1H(15N,&#945;&#947;)12C resonant nuclear reaction analysis (NRA) to quantitatively evaluate the density of hydrogen atoms on the surface, in the volume, and at an interfacial layer of solid materials. The near-surface hydrogen depth profiling of a Pd(110) single crystal and of SiO2/Si(100) stacks is described.

Nuclear reaction analysis (NRA) via the resonant 1H(15N,&#945;&#947;)12C reaction is a highly effective method of depth profiling that quantitatively and ...

Preparation of Liquid-exfoliated Transition Metal Dichalcogenide Nanosheets with Controlled Size and Thickness: A State of the Art Protocol

We summarize recent advances in the production of liquid-exfoliated transition metal dichalcogenide (TMD) nanosheets with controlled size and thickness. Layered crystals of molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are exfoliated in aqueous surfactant solution by sonication. This yields highly polydisperse mixtures containing nanosheets with broad size and thickness distributions. However, for most purposes, specific sizes (in terms of both lateral dimension and thickness) are required. For example, large and thin nanosheets are desired for (opto) electronic applications, while laterally small nanosheets are interesting for catalytic applications. Therefore, post-exfoliation size selection is an important step that we address here. We provide a detailed protocol on the efficient size selection in large quantities by liquid cascade centrifugation and the size and thickness quantification by statistical microscopic analysis (atomic force microscopy and transmission electron microscopy). The comparison of MoS2 and WS2 shows that both materials are size-selected in a similar way by the same procedure. Importantly, the dispersions of size-selected nanosheets show systematic changes in their optical extinction spectra with size due to edge and confinement effects. We show how these optical changes are related quantitatively to the nanosheets dimensions and describe how mean nanosheets length and layer number can be extracted reliably from the extinction spectra. The exfoliation and size selection protocol can be applied to a broad range of layered crystals as we have previously demonstrated for graphene, gallium sulphide (GaS) and black phosphorus.

A protocol for the liquid exfoliation of layered materials to nanosheets, their size selection and size measurement by microscopic and spectroscopic techniques is presented.

We summarize recent advances in the production of liquid-exfoliated transition metal dichalcogenide (TMD) nanosheets with controlled size and thickness. ...

Preparation of Large-area Vertical 2D Crystal Hetero-structures Through the Sulfurization of Transition Metal Films for Device Fabrication

We have demonstrated that through the sulfurization of transition metal films such as molybdenum (Mo) and tungsten (W), large-area and uniform transition metal dichalcogenides (TMDs) MoS2 and WS2 can be prepared on sapphire substrates. By controlling the metal film thicknesses, good layer number controllability, down to a single layer of TMDs, can be obtained using this growth technique. Based on the results obtained from the Mo film sulfurized under the sulfur deficient condition, there are two mechanisms of (a) planar MoS2 growth and (b) Mo oxide segregation observed during the sulfurization procedure. When the background sulfur is sufficient, planar TMD growth is the dominant growth mechanism, which will result in a uniform MoS2 film after the sulfurization procedure. If the background sulfur is deficient, Mo oxide segregation will be the dominant growth mechanism at the initial stage of the sulfurization procedure. In this case, the sample with Mo oxide clusters covered with few-layer MoS2 will be obtained. After sequential Mo deposition/sulfurization and W deposition/sulfurization procedures, vertical WS2/MoS2 hetero-structures are established using this growth technique. Raman peaks corresponding to WS2 and MoS2, respectively, and the identical layer number of the hetero-structure with the summation of individual 2D materials have confirmed the successful establishment of the vertical 2D crystal hetero-structure. After transferring the WS2/MoS2 film onto a SiO2/Si substrate with pre-patterned source/drain electrodes, a bottom-gate transistor is fabricated. Compared with the transistor with only MoS2 channels, the higher drain currents of the device with the WS2/MoS2 hetero-structure have exhibited that with the introduction of 2D crystal hetero-structures, superior device performance can be obtained. The results have revealed the potential of this growth technique for the practical application of 2D crystals.

Through the sulfurization of pre-deposited transition metals, large-area and vertical 2D crystal hetero-structures can be fabricated. The film transferring and device fabrication procedures are also demonstrated in this report.

We have demonstrated that through the sulfurization of transition metal films such as molybdenum (Mo) and tungsten (W), large-area and uniform transition ...

Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating

A method of carrier number control by electrolyte gating is demonstrated. We have obtained WS2 thin flakes with atomically flat surface via scotch tape method or individual WS2 nanotubes by dispersing the suspension of WS2 nanotubes. The selected samples have been fabricated into devices by the use of the electron beam lithography and electrolyte is put on the devices. We have characterized the electronic properties of the devices under applying the gate voltage. In the small gate voltage region, ions in the electrolyte are accumulated on the surface of the samples which leads to the large electric potential drop and resultant electrostatic carrier doping at the interface. Ambipolar transfer curve has been observed in this electrostatic doping region. When the gate voltage is further increased, we met another drastic increase of source-drain current which implies that ions are intercalated into layers of WS2 and electrochemical carrier doping is realized. In such electrochemical doping region, superconductivity has been observed. The focused technique provides a powerful strategy for achieving the electric-filed-induced quantum phase transition.

Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating

Here, we present a protocol to control the carrier number in solids by using the electrolyte.

A method of carrier number control by electrolyte gating is demonstrated. We have obtained WS2 thin flakes with atomically flat surface via scotch tape ...