This work was supported by the Air Force Office of Scientific Research grant FA9550-16-1-0011 and University of Missouri startup funds. The authors would like to thank the University of Missouri Electron Microscopy Core facility for assistance with SEM imaging and use of patterning equipment and software.

1. Preparation of CNT Forest Sample for Milling
<ol>
	<li>CNT Synthesis
	<ol>
		<li>Deposit 10 nm of aluminum oxide (alumina) on a thermally oxidized silicon wafer using atomic layer deposition13 or other physical vapor deposition methods.</li>
		<li>Deposit 1 nm of iron on the alumina support layer by sputtering14 or other physical vapor deposition method.</li>
		<li>Synthesize CNTs using an established process, such as thermal chemical vapor deposition15.
		<ol>
			<li>Heat a 20 mm diameter tube furnace to 750 &#176;C in 400 standard cubic centimeters (sccm) of flowing helium and 100 sccm hydrogen. Introduce 100 sccm ethylene as a hydrocarbon feedstock gas for a growth rate of approximately 50 &#956;m/min.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>SEM Preparation
	<ol>
		<li>Apply carbon tape to a standard 1/2&#34; diameter SEM stub. If tilting the stage is required, overlap the region of the CNT forest sample to be milled over the edge of the stub. If software-controlled electron beam rastering will be used in the milling procedure, secure the CNT sample to an electron beam lithography mount in a similar manner.</li>
		<li>If milling the CNT cross section, secure the stub to a 45&#176; stub holder with a set screw.</li>
		<li>Vent the ESEM by selecting the &#34;Vent&#34; icon from the ESEM control software.</li>
		<li>Open the ESEM stage door, and secure the stub to the SEM stage with a set screw.</li>
		<li>Close the SEM chamber and select &#34;High Vacuum&#34; in the ESEM control software.</li>
		<li>While the ESEM chamber is pumping, select the electron beam parameters of 5 kV and spot size of 3.0 using the Beam Control tab within the control software.</li>
		<li>Select the secondary electron detector by selecting Detectors | ETD(SE) in the ESEM control software.</li>
		<li>Select the &#34;Beam On&#34; icon in the control software. The beam can be activated only once the chamber vacuum is less than 10-4 Torr. Use manual SEM focus control knobs to focus the sample.</li>
		<li>Tilt the sample to 45&#176; using manual tilt stage control knob or by inputting 45&#176; in the &#34;Tilt&#34; field in the &#34;Coordinates&#34; tab of the ESEM software. Focus on the highest sample. Link the focal distance to the working distance by selecting Stage | Link Z to FWD in the ESEM software menu. Input 7 mm into the &#34;Z&#34; field in the &#34;Coordinates&#34; tab within the control software.</li>
		<li>Adjust focus, stigmation, brightness and contrast using the manual control knobs to resolve a well-focused image.</li>
	</ol>
	</li>
	<li>Beam Adjustment in High Vacuum Mode
	<ol>
		<li>Locate a region for milling using navigation controls. Double click within the SEM image view or by manually turning the x and y control knobs of the SEM stage control to navigate.</li>
		<li>Navigate to an adjacent location approximately 100 &#956;m away from the milling region.</li>
		<li>Consult Figure 1 to estimate the material removal rate of the CNT forest as a function of pressure, acceleration voltage, dwell time per pixel, and beam current.</li>
		<li>Adjust the acceleration voltage to 30 kV and spot size to 5.0 using the ESEM control software. Adjust image focus, brightness, and contrast using the ESEM control knobs. For nanometer-scale milling of individual or few CNTs, select 5 kV and spot size of 3.0.</li>
		<li>Select a 1 mm aperture by manual aperture adjustment. Adjust focus, stigmation, brightness, and contrast to obtain a well-resolved image, as previously detailed.</li>
		<li>Decrease magnification to &#60;1,000X.</li>
	</ol>
	</li>
	<li>SEM Setup in Low Pressure Water Vapor
	<ol>
		<li>Select a pressure of 11 Pa in the control software dropdown box.</li>
		<li>Select &#34;Low Pressure&#34; mode in the &#34;Vacuum&#34; settings in the ESEM software to introduce water vapor.</li>
		<li>Select &#34;Beam On&#34; in the control software upon pressure stabilization. Select a dwell time of &#60;10 &#956;s and a resolution of 1,024 x 884 in the drop-down boxes of the control software.</li>
		<li>Adjust the image brightness, contrast, focus, and stigmation as previously detailed.</li>
		<li>Navigate to the desired milling region. Rotate the image orientation by selecting Scan | Scan Rotation in the control software, if required. Select a suitable rotation angle that aligns with the native vertical and horizontal scan orientation of the SEM.</li>
		<li>For milling feature sizes on the order of 1 &#956;m, select a magnification of 40,000X. Select a magnification of 20,000X to mill features with dimensions up to 5 &#956;m.</li>
		<li>Pause the electron beam by selecting the &#39; &#34; &#39; icon . An image of the CNT forest will be displayed and may be used for selecting reduced area milling regions while the beam is paused.</li>
	</ol>
	</li>
</ol>
2. CNT Forest Milling
<ol>
	<li>Instructions for CNT forest milling using a rectangular selected area
	<ol>
		<li>Choose the &#39;Reduced Area&#39; tool in the control software, or select Scan-Reduced Area in the software menu. Extend a reduced area rectangle over the area to be milled.</li>
		<li>Adjust the image resolution to 2,048 x 1,768. Increase the dwell time to 2 ms. If 2 ms is not available, navigate to the Scan |Preferences and select the &#34;Scanning&#34; tab. Select an existing scan time and type &#34;2.0 ms&#34; into the &#34;Dwell Time&#34; field. Click &#34;OK&#34; to close the menu.</li>
		<li>Select the &#39; &#34; &#39; icon in the control software to activate the electron beam.</li>
		<li>Select the &#39; &#34; &#39; icon so that the beam rasters over the selected area one time. Select the icon immediately after step 2.1.3. The scan duration depends on the size of the selected area, resolution and dwell time and may be approximated by multiplying the number of pixels within the scan area and the dwell time per pixel.</li>
		<li>Decrease magnification to &#60;1,000X once the beam has completed rastering the selected area. Revert to the parameters used in step 1.3, including High Vacuum. Select &#34;Beam On&#34; to engage the beam.</li>
	</ol>
	</li>
	<li>Instructions for CNT forest milling along a horizontal line
	<ol>
		<li>Select the line scan feature by navigating to Scan | Line in the control software. The line width is determined by the size of the electron beam itself. Adjust the image resolution to 2,048 x 1,768 from the control software dropdown box. Increase the dwell time to 2 ms as detailed in step 2.1.2.</li>
		<li>Using the still image acquired before pausing the electron beam, place the line over the area to be milled.</li>
		<li>Select the videoscope icon or navigate to the Scan menu and select &#34;Videoscope.&#34; Using the videoscope tool provides feedback relative to when a line scan has fully completed.</li>
		<li>Select the &#39; &#34; &#39; icon to scan electron beam across the width of the line.</li>
		<li>Select the &#39; &#34; &#39; icon to blank the electron beam.</li>
	</ol>
	</li>
	<li>Instructions for CNT Forest milling using software-controlled electron beam rastering
	<ol>
		<li>Pattern Generation
		<ol>
			<li>Design a milling pattern of interest using a CAD software package such as AutoCAD.</li>
			<li>Using &#34;Nanometer Pattern Generation System&#34; (NPGS) software, import the CAD pattern file.</li>
			<li>Convert the shapes to solid features by selected &#34;Filled Polygons&#34; in the NPGS software.</li>
			<li>Save the drawing as a &#39;.dc2&#39; file in a designated project folder of NPGS.</li>
			<li>Using NPGS, navigate to the project folder containing the &#34;.dc2&#34; file. Right select the &#34;.dc2&#34; file and select &#34;Run File Editor&#34; to convert the drawing to NPGS code. Typical parameters used to pattern CNT forests at given conditions are as listed below: 
			Center-to-center distance = 5 nm 
			Line Spacing = 5 nm 
			Magnification = 10,000X 
			Desired Beam Current = 26 
			Line Dose = 100 nC/cm</li>
		</ol>
		</li>
		<li>Electron Beam Milling using NPGS Lithography Software</li>
		<li>Select the &#34;NPGS Mode&#34; in NPGS software button to give control of the SEM to NPGS.</li>
		<li>Highlight the pattern file and select &#34;Process Run File&#34; in NPGS to initiate milling.</li>
		<li>Select &#34;SEM Mode&#34; in the NPGS software when patterning is finished. Select &#34;High Vacuum&#34; in the ESEM control software.</li>
		<li>Select &#34;Beam On&#34; to inspect the milled region. Use conditions detailed in step 1.3.</li>
	</ol>
	</li>
</ol>
3. Sample Removal
<ol>
	<li>Vent the chamber by selecting &#34;Vent&#34; in the ESEM control software.</li>
	<li>Open the ESEM door. Remove the stub by loosening the set screw.</li>
	<li>Close the chamber door. Select &#34;High Vacuum&#34; in the control software.</li>
</ol>

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The authors declare that they have no competing financial interests.

The protocol details best practices for milling relatively large (micron-scale) features in CNT forests. In general, the material removal rate may be reduced by reducing the acceleration voltage, spot size, and aperture diameter. To trim a specific CNT within a forest, recommended conditions include 5 kV, a spot size of 3, and an aperture that is 50 &#956;m or less in diameter. Note that the milling technique using reduced area rectangles is detailed such that the electron beam rasters the enclosed region only one time. The reduced area may be scanned multiple times if additional cutting depth is desired; however, we illustrate a single scan for simplicity. We note that the extended electron beam dwell time, high current, and high acceleration voltage represent conditions that are frequently avoided for imaging of carbon-based materials; however, these aggressive parameters in a low-pressure water vapor ambient are critical for achieving large-scale milling. Further, we note that similar imaging conditions in the absence of low-pressure water vapor results in little CNT damage.
The ESEM-based milling method described in this work is a minimally disruptive machining method that preserves neighboring CNT forest structural morphology. The technique is amenable to removing nanoscale features such as segments of individual CNTs and also to removing regions spanning many microns. We demonstrate the technique using reduced area rectangles, lines, and arbitrary patterns using software-controlled electron beam rastering. While the technique is relatively clean compared to FIB-based milling, small amounts of carbon residue exists on milled surfaces. Current research is addressing avenues for reducing this residue. Additionally, the material removal rates shown in Figure 1 were obtained for a CNT forest with CNTs featuring an average outer and inner diameter of 10 and 7 nm, respectively. Material removal rates are expected to be a function of CNT density, CNT diameter, and CNT alignment. Figure 1 should be consulted as a guide, recognizing that the stated material removal rate is specific to this CNT forest morphology. While qualitative trends represented in the figure are expected to hold for all CNT forests, some experimentation may be required to find the optimal parameters for a different material system.
While the ESEM machining methodology is demonstrated using CNT forests, it is equally applicable for graphene and other carbon-based materials. The technique does not require delamination of the CNT forest for processing and does not introduce external heavy elements that may significantly alter the surrounding CNT forest morphology. The procedure may be used for inspecting CNT forest internal morphology, and perhaps for producing 3-D lattices structures for microscale prototyping which may be functionally coated (with alumina for enhanced stiffness17,18, for example).
The technique is currently being utilized to examine the internal structural morphology of CNT forests. Because structural morphology is intimately linked with functional properties16,19,20,21,22, characterization of CNT forest morphology in three dimensional space may provide additional insights into the governing structure-property relationships. With the ability to precisely mill into a forest and observe the internal nanotube interactions, CNT forest synthesis modeling and analytical models can be tuned and validated.
The emphasis of the ESEM milling technique to date has been directed towards rapid material removal with less focus on optimizing conditions for reducing the residual carbon residue. A future direction is to explore the mechanism of amorphous carbon deposition in the immediate vicinity of the cut surfaces when large volumes of materials are removed, as shown in Figure 5. With a broad parameter space available for exploration, including environmental gas composition, vapor pressure, acceleration voltage, probe current, and electron beam rastering conditions, enhanced surface cleanliness may be achieved.

Carbon nanotubes (CNTs) and graphene are carbon-based nanomaterials that have attracted significant attention because of their superior strength, durability, thermal, and electrical properties. Precision machining of carbon nanomaterials has become an emerging topic of research and offers the potential to engineer and manipulate these materials towards a variety of engineering applications. Machining CNTs and graphene requires nanoscale spatial precision to first locate a nanoscale area of interest and then to selectively remove only the material within the area of interest. As an example, consider the machining of vertically oriented CNT forests (also known as CNT arrays). The cross section of CNT forests may be precisely defined by lithographic patterning of catalyst films. The top surface of the vertically oriented forests, however, are frequently poorly ordered with non-uniform height. For surface-sensitive applications such as thermal interface materials, the irregular surface may impede optimal surface contact and reduce device performance. Precision trimming of the irregular surface to create a uniform flat surface could potentially offer better, more repeatable performance by maximizing the available contact area.
Precision machining techniques for nanomaterials frequently do not resemble conventional macroscale mechanical machining technologies such as drilling, milling, and polishing by means of hardened tooling. To date, techniques using energetic beams have been most successful at site-selective milling of carbon nanomaterials. These techniques include laser, electron beam, and focused ion beam (FIB) irradiation. Of these, laser machining techniques provide the most rapid material removal rate1,2; however, the spot size of laser systems is on the order of many microns and is too large to isolate nanometer-scale entities such as a single carbon nanotube segment within a densely populated forest. By contrast, electron and ion beam systems produce a beam that may be focused to a spot that is several nanometers or less in diameter.
FIB systems are specifically designed for nanoscale milling and deposition of materials. These systems utilize an energetic beam of gaseous metal ions (typically gallium) to sputter material from a selected area. FIB milling of CNTs is achievable, but often with unintended byproducts including gallium and carbon redeposition in surrounding regions of the forest3,4. When the technique is used for CNT forests, the redeposited material masks and/or alters the morphology of selected milling region, altering the native appearance and behavior of the CNT forest. The gallium may also implant within the CNT, providing electronic doping. Such consequences often make FIB-based milling prohibitive for CNT forests.
Transmission electron microscopes (TEMs) utilize a finely focused beam of electrons to probe the internal structure of materials. Acceleration voltages for TEM operation typically range from 80-300 kV. Because the knock-on energy of CNTs is 86.4 keV5, the electron energy produced by TEM is sufficient to directly remove atoms from the CNT lattice and induce highly localized milling. The technique mills CNTs with potentially sub-nanometer precision5,6,7; however, the process is very slow &#8211; often requiring minutes to mill a single CNT. Importantly, TEM-based milling approaches require CNTs to first be removed from a growth substrate and dispersed onto a TEM grid for processing. As a result, TEM-based methods are generally not compatible with CNT forest milling in which the CNTs must remain on a rigid substrate.
Milling of CNT forests by scanning electron microscopes (SEMs) has also received attention. In contrast to TEM-based techniques, SEM instruments are typically unable to accelerate electrons with sufficient energy to impart the knock-on energy required to directly remove carbon atoms. Rather, SEM-based techniques utilize an electron beam in the presence of a low-pressure gaseous oxidizer. The electron beam selectively damages the CNT lattice and may dissociate the gaseous ambient into more reactive species such as H2O2 and the hydroxyl radical. Water vapor and oxygen are the most commonly reported gases to achieve selective area etching. Because the SEM-based techniques rely on a multiple-step chemical process, numerous processing variables may influence the milling rate and precision of the process. It has been previously observed that increasing acceleration voltage and beam current directly increase the milling rate because of an increased energy flux, as expected11. The effect of chamber pressure is less obvious. A pressure that is too low suffers from a deficiency of the oxidizing agent, decreasing the milling rate. Further, an over-abundance of gaseous species scatters the electron beam and decreases the electron flux in the milling region, also decreasing the material removal rate.
To estimate the carbon removal rate, an approach similar to that used by Lassiter and Rack12 was employed, whereby electrons interact with precursor molecules near the surface to generate reactive species that etch the substrate surface. From this model, the etch rate is estimated as
<img alt="Equation" src="/files/ftp_upload/55149/55149eq1.jpg" />
where NA is the surface concentration of the etchant species, Z is the surface concentration of available reaction sites, x is a stoichiometry factor relating the volatile etching products generated relative to the reactants, A&#963; represents the probability of generating the desired etching species from an electron-water vapor collision, and &#915;e is the electron flux at the surface. The factors of x and A&#963; are assumed to be unity, while Z is assumed to be nearly constant and significantly larger than NA. Further details may be found in our previous work.11
In this article, a procedure is explored that uses low-pressure water vapor within a SEM to mill regions ranging from individual CNTs to large volume (tens of cubic micrometers) material removal. Here we demonstrate the technique used to mill CNT forests using an ESEM by the use of reduced area rectangles, horizontal line scans, and software-controlled rastering of the electron beam. Additional software and hardware are required for pattern generation, as outlined in the Materials List. Emphasis is placed on removing relatively large (100&#39;s of cubic microns) material volume from a CNT forest, so the following processing conditions are relatively aggressive.
When handling the sample and the sample stub, it is important to wear disposable nitrile gloves. This will prevent oils from being transferred to the stub or sample and consequently deteriorating the effectiveness of the pumps.

<table><tbody><tr><td>100 mm diameter silicon wafer with 1 micron thermal oxide</td><td>University Wafer</td><td></td><td>Beginning substrate</td></tr><tr><td>Iron sputter target</td><td>Kurt J. Lesker</td><td>EJTFEXX351A2</td><td>Sputter target&amp;nbsp;</td></tr><tr><td>Savannah 200</td><td>Cambridge</td><td></td><td>For atomic layer deposition of alumina</td></tr><tr><td>Quanta 600F Environmental SEM</td><td>FEI</td><td></td><td>Environmental scanning electron microscope used to support a low-pressure water vapor ambient environment for CNT forest milling</td></tr><tr><td>xT Microscope Control software</td><td>FEI</td><td>4.1.7</td><td>Control software used on Quanta 600F ESEM</td></tr><tr><td>Nanometer Pattern Generation System - Software</td><td>JC Nabity Lithography Systems</td><td>Version 9</td><td>Software used for electron-beam lithography</td></tr><tr><td>Dedicated computer with PCI516 Lithography board</td><td></td><td></td><td>Equipment used for electron-beam lithography</td></tr><tr><td>DesignCAD software</td><td></td><td>V 21.2</td><td>Optional equipment used to generate patterns for electron-beam lithography</td></tr><tr><td>E-beam lithography mount</td><td>Ted Pella</td><td>16405</td><td>Electron beam lithography mount with a Faraday cup and gold nanoparticles on carbon tape</td></tr><tr><td>Picoammeter</td><td>Keithley</td><td>6485</td><td>Used with the Faraday cup to quantify beam current</td></tr><tr><td>12.7 mm diameter SEM stub</td><td>Ted Pella</td><td>16111</td><td>SEM stub</td></tr><tr><td>45 degree pin stub holder</td><td>Ted Pella</td><td>15329</td><td>Optional equipment used to mill the cross section of a CNT forest</td></tr></tbody></table>

precision milling of carbon nanotube forests using low pressure scanning electron microscopy

A nanoscale fabrication technique appropriate for milling carbon nanotube (CNT) forests is described. The technique utilizes an environmental scanning electron microscope (ESEM) operating with a low pressure water vapor ambient. In this technique, a portion of the electron beam interacts with the water vapor in the vicinity of the CNT sample, dissociating the water molecules into hydroxyl radicals and other species by radiolysis. The remainder of the electron beam interacts with the CNT forest sample, making it susceptible to oxidation from the chemical products of radiolysis. This technique may be used to trim a selected region of an individual CNT, or it may be used to remove hundreds of cubic microns of material by adjusting ESEM parameters. The machining resolution is similar to the imaging resolution of the ESEM itself. The technique produces only small quantities of carbon residue along the boundaries of the cutting zone, with minimal effect on the native structural morphology of the CNT forest.

The ESEM technique was used to mill a CNT forest synthesized using thermal CVD15,16. Selected area removal of a few CNTs from within a forest is shown in Figure 211. For this demonstration, parameters include 5 kV, spot size of 3, 11 Pa, 170,000X magnification, 2 ms dwell time, and an aperture of 30 &#181;m.
To demonstrate a larger-scale area removal, the top surface of a CNT forest micropillar was selected for milling. SEM conditions are selected for rapid, large-area CNT forest removal. Namely, these conditions include a magnification of 20,000X, a pressure of 11 Pa, acceleration voltage of 30 kV, spot size of 5, dwell time of 2 ms, and a 1 mm aperture setting. A reduced area box is chosen such that the irregular top surface to be removed is enclosed within the selected area. SEM micrographs of the CNT forest pillar are shown in Figure 3 before and after the selective area milling process. The red line in the figure represents the lower bound of the reduced area box used for milling.
Non-rectangular geometries achieved using software-controlled electron beam rastering and a relatively short 20 &#956;m tall CNT forest. As shown in Figure 4, a 15 &#956;m diameter circle was machined into a CNT forest. For this demonstration, the CNT forest was milled parallel to the CNT growth direction (normal to the substrate). Milling parameters used for this demonstration include a magnification of 10,000X, a pressure of 11 Pa, acceleration voltage of 30 kV, spot size of 5, dwell time of 2 ms, and 1 mm aperture setting. Figure 4 shows that the process milled the CNTs fully to the underlying silicon substrate.
<img alt="Figure 1" src="/files/ftp_upload/55149/55149fig1.jpg" /> 
Figure 1: Material Removal Rate Variation. Material removal rate (MRR) variation. SEM micrographs demonstrate the MRR in the transverse direction (a) by varying operating pressure from 133, 66, 33, 66, and 11 Pa (top to bottom) and (b) in the axial cutting direction by varying dwell time from 3, 2, 1, and 0.5 ms/pixel (left to right). The MRR is plotted as a function of incremental changes in pressure, acceleration voltage, beam current, and dwell time in the (c) transverse and (d) axial cutting directions. The MRR as a function of electron dose varies nearly linearly in both the (e) transverse and (f) axial milling orientation. This figure is reproduced with permission from reference11. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55149/55149fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/55149/55149fig2.jpg" /> 
Figure 2: Milling of Individual CNTs. SEM micrograph showing individual CNTs from within a forest selected for local milling (a) before and (b) after milling. This figure is reproduced with permission from reference11. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55149/55149fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/55149/55149fig3.jpg" /> 
Figure 3: Milling of a CNT Forest. A 10 &#181;m wide CNT forest pillar (a) before and (b) after selective area milling using ESEM-based milling. Milling conditions include magnification of 20,000X, a pressure of 11 Pa, acceleration voltage of 30 kV, spot size of 5, dwell time of 2 ms per pixel, and 30 &#181;m aperture. The red line in the figure represents the lower bound of the selective area rectangle used in the milling process. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55149/55149fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/55149/55149fig4.jpg" /> 
Figure 4: Patterned Milling of CNT Forest. Software-controlled electron beam rastering is used to define and mill a 15 &#181;m diameter circle in a CNT forest. In this setup, the milling direction was parallel to the CNT growth direction from the top surface to the underlying substrate. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55149/55149fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/55149/55149fig5.jpg" /> 
Figure 5: Deposited Carbon after Milling. SEM micrographs showing the surface finish of ESEM milled CNT forests. (a) The top surface of a CNT forest shows the surface variation between the milled and as-synthesized regions. (b) Higher magnifications reveal that some amorphous carbon deposits are left behind during the cutting process. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55149/55149fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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Precision Milling of Carbon Nanotube Forests Using Low Pressure Scanning Electron Microscopy

Low pressure scanning electron microscopy in a water vapor ambient is used to machine nanoscale to microscale features in carbon nanotube forests.

A nanoscale fabrication technique appropriate for milling carbon nanotube (CNT) forests is described. The technique utilizes an environmental scanning ...

precision-milling-carbon-nanotube-forests-using-low-pressure-scanning

Nanyang Technological University

Research

JoVE Journal

Engineering

7.4K Views.  University of Missouri. Low pressure scanning electron microscopy in a water vapor ambient is used to machine nanoscale to microscale features in carbon nanotube forests.

Video: Precision Milling of Carbon Nanotube Forests Using Low Pressure Scanning Electron Microscopy

Fabrication, Densification, and Replica Molding of 3D Carbon Nanotube Microstructures

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip devices, and their applications. In particular, capabilities for cost-effective fabrication of polymer microstructures were transformed by the advent of soft lithography and other micromolding techniques 1, 2, and this led a revolution in applications of microfabrication to biomedical engineering and biology. Nevertheless, it remains challenging to fabricate microstructures with well-defined nanoscale surface textures, and to fabricate arbitrary 3D shapes at the micro-scale. Robustness of master molds and maintenance of shape integrity is especially important to achieve high fidelity replication of complex structures and preserving their nanoscale surface texture. The combination of hierarchical textures, and heterogeneous shapes, is a profound challenge to existing microfabrication methods that largely rely upon top-down etching using fixed mask templates. On the other hand, the bottom-up synthesis of nanostructures such as nanotubes and nanowires can offer new capabilities to microfabrication, in particular by taking advantage of the collective self-organization of nanostructures, and local control of their growth behavior with respect to microfabricated patterns. 
Our goal is to introduce vertically aligned carbon nanotubes (CNTs), which we refer to as CNT "forests", as a new microfabrication material. We present details of a suite of related methods recently developed by our group: fabrication of CNT forest microstructures by thermal CVD from lithographically patterned catalyst thin films; self-directed elastocapillary densification of CNT microstructures; and replica molding of polymer microstructures using CNT composite master molds. In particular, our work shows that self-directed capillary densification ("capillary forming"), which is performed by condensation of a solvent onto the substrate with CNT microstructures, significantly increases the packing density of CNTs. This process enables directed transformation of vertical CNT microstructures into straight, inclined, and twisted shapes, which have robust mechanical properties exceeding those of typical microfabrication polymers. This in turn enables formation of nanocomposite CNT master molds by capillary-driven infiltration of polymers. The replica structures exhibit the anisotropic nanoscale texture of the aligned CNTs, and can have walls with sub-micron thickness and aspect ratios exceeding 50:1. Integration of CNT microstructures in fabrication offers further opportunity to exploit the electrical and thermal properties of CNTs, and diverse capabilities for chemical and biochemical functionalization 3.

We present methods for fabrication of patterned microstructures of vertically aligned carbon nanotubes (CNTs), and their use as master molds for production of polymer microstructures with organized nanoscale surface texture. The CNT forests are densified by condensation of solvent onto the substrate, which significantly increases their packing density and enables self-directed formation of 3D shapes.

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip ...

Simultaneous Synthesis of Single-walled Carbon Nanotubes and Graphene in a Magnetically-enhanced Arc Plasma

Carbon nanostructures such as single-walled carbon nanotubes (SWCNT) and graphene attract a deluge of interest of scholars nowadays due to their very promising application for molecular sensors, field effect transistor and super thin and flexible electronic devices1-4. Anodic arc discharge supported by the erosion of the anode material is one of the most practical and efficient methods, which can provide specific non-equilibrium processes and a high influx of carbon material to the developing structures at relatively higher temperature, and consequently the as-synthesized products have few structural defects and better crystallinity.
 To further improve the controllability and flexibility of the synthesis of carbon nanostructures in arc discharge, magnetic fields can be applied during the synthesis process according to the strong magnetic responses of arc plasmas. It was demonstrated that the magnetically-enhanced arc discharge can increase the average length of SWCNT 5, narrow the diameter distribution of metallic catalyst particles and carbon nanotubes 6, and change the ratio of metallic and semiconducting carbon nanotubes 7, as well as lead to graphene synthesis 8. 
 Furthermore, it is worthwhile to remark that when we introduce a non-uniform magnetic field with the component normal to the current in arc, the Lorentz force along the J&times;B direction can generate the plasmas jet and make effective delivery of carbon ion particles and heat flux to samples. As a result, large-scale graphene flakes and high-purity single-walled carbon nanotubes were simultaneously generated by such new magnetically-enhanced anodic arc method. Arc imaging, scanning electron microscope (SEM), transmission electron microscope (TEM) and Raman spectroscopy were employed to analyze the characterization of carbon nanostructures. These findings indicate a wide spectrum of opportunities to manipulate with the properties of nanostructures produced in plasmas by means of controlling the arc conditions.

Anodic arc discharge is one of the most practical and efficient methods to synthesize various carbon nanostructures. To increase the arc controllability and flexibility, a non-uniform magnetic field was introduced to process the one-step synthesis of large-scale graphene flakes and high-purity single-walled carbon nanotubes.

Carbon nanostructures such as single-walled carbon nanotubes (SWCNT) and graphene attract a deluge of interest of scholars nowadays due to their very ...

Bioengineering

Atomic Force Microscopy of Red-Light Photoreceptors Using PeakForce Quantitative Nanomechanical Property Mapping

Atomic force microscopy (AFM) uses a pyramidal tip attached to a cantilever to probe the force response of a surface. The deflections of the tip can be measured to ~10 pN by a laser and sectored detector, which can be converted to image topography. Amplitude modulation or &#8220;tapping mode&#8221; AFM involves the probe making intermittent contact with the surface while oscillating at its resonant frequency to produce an image. Used in conjunction with a fluid cell, tapping-mode AFM enables the imaging of biological macromolecules such as proteins in physiologically relevant conditions. Tapping-mode AFM requires manual tuning of the probe and frequent adjustments of a multitude of scanning parameters which can be challenging for inexperienced users. To obtain high-quality images, these adjustments are the most time consuming.
PeakForce Quantitative Nanomechanical Property Mapping (PF-QNM) produces an image by measuring a force response curve for every point of contact with the sample. With ScanAsyst software, PF-QNM can be automated. This software adjusts the set-point, drive frequency, scan rate, gains, and other important scanning parameters automatically for a given sample. Not only does this process protect both fragile probes and samples, it significantly reduces the time required to obtain high resolution images. PF-QNM is compatible for AFM imaging in fluid; therefore, it has extensive application for imaging biologically relevant materials.
The method presented in this paper describes the application of PF-QNM to obtain images of a bacterial red-light photoreceptor, RpBphP3 (P3), from photosynthetic R. palustris in its light-adapted state. Using this method, individual protein dimers of P3 and aggregates of dimers have been observed on a mica surface in the presence of an imaging buffer. With appropriate adjustments to surface and/or solution concentration, this method may be generally applied to other biologically relevant macromolecules and soft materials.

A method for investigating the structure of a protein photoreceptor using atomic force microscopy (AFM) is described in this paper. PeakForce Quantitative Nanomechanical Property Mapping (PF-QNM) reveals intact protein dimers on a mica surface.

Atomic force microscopy (AFM) uses a pyramidal tip attached to a cantilever to probe the force response of a surface. The deflections of the tip can be ...

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 ...

Functionalization of Single-walled Carbon Nanotubes with Thermo-reversible Block Copolymers and Characterization by Small-angle Neutron Scattering

We demonstrate a protocol for single-walled carbon nanotube functionalization using thermo-sensitive PEO-PPO-PEO triblock copolymers in an aqueous solution. In a carbon nanotube/PEO105-PPO70-PEO105 (poloxamer 407) aqueous solution, the amphiphilic poloxamer 407 adsorbs onto the carbon nanotube surfaces and self-assembles into continuous layers, driven by intermolecular interactions between constituent molecules. The addition of 5-methylsalicylic acid changes the self-assembled structure from spherical-micellar to a cylindrical morphology. The fabricated poloxamer 407/carbon nanotube hybrid particles exhibit thermo-responsive structural features so that the density and thickness of poloxamer 407 layers are also reversibly controllable by varying temperature. The detailed structural properties of the poloxamer 407/carbon nanotube particles in suspension can be characterized by small-angle neutron scattering experiments and model fit analyses. The distinct curve shapes of the scattering intensities depending on temperature control or addition of aromatic additives are well described by a modified core-shell cylinder model consisting of a carbon nanotube core cylinder, a hydrophobic shell, and a hydrated polymer layer. This method can provide a simple but efficient way for the fabrication and in-situ characterization of carbon nanotube-based nano particles with a structure-tunable encapsulation.

A method for the functionalization of carbon nanotubes with structure-tunable polymeric encapsulation layers and structural characterization using small-angle neutron scattering is presented.

We demonstrate a protocol for single-walled carbon nanotube functionalization using thermo-sensitive PEO-PPO-PEO triblock copolymers in an aqueous ...

Chemistry

Fabrication of Carbon Nanotube High-Frequency Nanoelectronic Biosensor for Sensing in High Ionic Strength Solutions

The unique electronic properties and high surface-to-volume ratios of single-walled carbon nanotubes (SWNT) and semiconductor nanowires (NW) 1-4 make them good candidates for high sensitivity biosensors. When a charged molecule binds to such a sensor surface, it alters the carrier density5 in the sensor, resulting in changes in its DC conductance. However, in an ionic solution a charged surface also attracts counter-ions from the solution, forming an electrical double layer (EDL). This EDL effectively screens off the charge, and in physiologically relevant conditions ~100 millimolar (mM), the characteristic charge screening length (Debye length) is less than a nanometer (nm). Thus, in high ionic strength solutions, charge based (DC) detection is fundamentally impeded6-8.
We overcome charge screening effects by detecting molecular dipoles rather than charges at high frequency, by operating carbon nanotube field effect transistors as high frequency mixers9-11. At high frequencies, the AC drive force can no longer overcome the solution drag and the ions in solution do not have sufficient time to form the EDL. Further, frequency mixing technique allows us to operate at frequencies high enough to overcome ionic screening, and yet detect the sensing signals at lower frequencies11-12. Also, the high transconductance of SWNT transistors provides an internal gain for the sensing signal, which obviates the need for external signal amplifier.
Here, we describe the protocol to (a) fabricate SWNT transistors, (b) functionalize biomolecules to the nanotube13, (c) design and stamp a poly-dimethylsiloxane (PDMS) micro-fluidic chamber14 onto the device, and (d) carry out high frequency sensing in different ionic strength solutions11.

We describe the device fabrication and measurement protocol for carbon nanotube based high frequency biosensors. The high frequency sensing technique mitigates the fundamental ionic (Debye) screening effect and allows nanotube biosensor to be operated in high ionic strength solutions where conventional electronic biosensors fail. Our technology provides a unique platform for point-of-care (POC) electronic biosensors operating in physiologically relevant conditions.

The unique electronic properties and high surface-to-volume ratios of single-walled carbon nanotubes (SWNT) and semiconductor nanowires (NW) 1-4 make them ...

Localization and Relative Quantification of Carbon Nanotubes in Cells with Multispectral Imaging Flow Cytometry

Carbon-based nanomaterials, like carbon nanotubes (CNTs), belong to this type of nanoparticles which are very difficult to discriminate from carbon-rich cell structures and de facto there is still no quantitative method to assess their distribution at cell and tissue levels. What we propose here is an innovative method allowing the detection and quantification of CNTs in cells using a multispectral imaging flow cytometer (ImageStream, Amnis). This newly developed device integrates both a high-throughput of cells and high resolution imaging, providing thus images for each cell directly in flow and therefore statistically relevant image analysis. Each cell image is acquired on bright-field (BF), dark-field (DF), and fluorescent channels, giving access respectively to the level and the distribution of light absorption, light scattered and fluorescence for each cell. The analysis consists then in a pixel-by-pixel comparison of each image, of the 7,000-10,000 cells acquired for each condition of the experiment. Localization and quantification of CNTs is made possible thanks to some particular intrinsic properties of CNTs: strong light absorbance and scattering; indeed CNTs appear as strongly absorbed dark spots on BF and bright spots on DF with a precise colocalization.
This methodology could have a considerable impact on studies about interactions between nanomaterials and cells given that this protocol is applicable for a large range of nanomaterials, insofar as they are capable of absorbing (and/or scattering) strongly enough the light.

In this study, the main purpose was to monitor the cellular uptake and eventual exocytosis of carbon nanotubes (CNTs). From this perspective, we proposed here a unique method using multispectral imaging flow cytometry allowing a quantification and localization of CNTs in a statistically relevant number of cells.

Carbon-based nanomaterials, like carbon nanotubes (CNTs), belong to this type of nanoparticles which are very difficult to discriminate from carbon-rich ...

Nanoscale Characterization of Liquid-Solid Interfaces by Coupling Cryo-Focused Ion Beam Milling with Scanning Electron Microscopy and Spectroscopy

Physical and chemical processes at solid-liquid interfaces play a crucial role in many natural and technological phenomena, including catalysis, solar energy and fuel generation, and electrochemical energy storage. Nanoscale characterization of such interfaces has recently been achieved using cryogenic electron microscopy, thereby providing a new path to advancing our fundamental understanding of interface processes.
This contribution provides a practical guide to mapping the structure and chemistry of solid-liquid interfaces in materials and devices using an integrated cryogenic electron microscopy approach. In this approach, we pair cryogenic sample preparation which allows stabilization of solid-liquid interfaces with cryogenic focused ion beam (cryo-FIB) milling to create cross-sections through these complex buried structures. Cryogenic scanning electron microscopy (cryo-SEM) techniques performed in a dual-beam FIB/SEM enable direct imaging as well as chemical mapping at the nanoscale. We discuss practical challenges, strategies to overcome them, as well as protocols for obtaining optimal results. While we focus in our discussion on interfaces in energy storage devices, the methods outlined are broadly applicable to a range of fields where solid-liquid interface play a key role.

Cryogenic Focused Ion Beam (FIB) and Scanning Electron Microscopy (SEM) techniques can provide key insights into the chemistry and morphology of intact solid-liquid interfaces. Methods for preparing high quality Energy Dispersive X-ray (EDX) spectroscopic maps of such interfaces are detailed, with a focus on energy storage devices.

Physical and chemical processes at solid-liquid interfaces play a crucial role in many natural and technological phenomena, including catalysis, solar ...

Microscopic Visualization of Porous Nanographenes Synthesized through a Combination of Solution and On-Surface Chemistry

On-surface synthesis has recently been regarded as a promising approach for the generation of new molecular structures. It has been particularly successful in the synthesis of graphene nanoribbons, nanographenes and intrinsically reactive and instable, yet attractive species. It is based on the combination of solution chemistry aimed at preparation of appropriate molecular precursors for further ultra-high vacuum surface assisted transformations. This approach also owes its success to an incredible development of characterization techniques, such as scanning tunneling/atomic force microscopy and related methods, which allow detailed, local characterization at atomic scale. While the surface-assisted synthesis can provide molecular nanostructures with outstanding precision, down to single atoms, it suffers from basing on metallic surfaces and often limited yield. Therefore, the extension of the approach away from metals and the struggle to increase productivity seem to be significant challenges toward wider applications. Herein, we demonstrate the on-surface synthesis approach for generation of non-planar nanographenes, which are synthesized through a combination of solution chemistry and sequential surface-assisted processes, together with the detailed characterization by scanning probe microscopy methods.

A combination of solution and surface assisted synthesis opens new directions in the atomically precise synthesis of nanostructures. Scanning tunneling microscopy (STM) supplemented by non-contact atomic force microscopy (nc-AFM) enables detailed characterization of newly designed and generated carbon-based nano-objects.

On-surface synthesis has recently been regarded as a promising approach for the generation of new molecular structures. It has been particularly ...

Atomic Force Microscopy Cantilever-Based Nanoindentation: Mechanical Property Measurements at the Nanoscale in Air and Fluid

An atomic force microscope (AFM) fundamentally measures the interaction between a nanoscale AFM probe tip and the sample surface. If the force applied by the probe tip and its contact area with the sample can be quantified, it is possible to determine the nanoscale mechanical properties (e.g., elastic or Young&#39;s modulus) of the surface being probed. A detailed procedure for performing quantitative AFM cantilever-based nanoindentation experiments is provided here, with representative examples of how the technique can be applied to determine the elastic moduli of a wide variety of sample types, ranging from kPa to GPa. These include live mesenchymal stem cells (MSCs) and nuclei in physiological buffer, resin-embedded dehydrated loblolly pine cross-sections, and Bakken shales of varying composition.
Additionally, AFM cantilever-based nanoindentation is used to probe the rupture strength (i.e., breakthrough force) of phospholipid bilayers. Important practical considerations such as method choice and development, probe selection and calibration, region of interest identification, sample heterogeneity, feature size and aspect ratio, tip wear, surface roughness, and data analysis and measurement statistics are discussed to aid proper implementation of the technique. Finally, co-localization of AFM-derived nanomechanical maps with electron microscopy techniques that provide additional information regarding elemental composition is demonstrated.

Quantifying the contact area and force applied by an atomic force microscope (AFM) probe tip to a sample surface enables nanoscale mechanical property determination. Best practices to implement AFM cantilever-based nanoindentation in air or fluid on soft and hard samples to measure elastic modulus or other nanomechanical properties are discussed.

An atomic force microscope (AFM) fundamentally measures the interaction between a nanoscale AFM probe tip and the sample surface. If the force applied by ...

Precision Milling of Carbon Nanotube Forests Using Low Pressure Scanning Electron Microscopy

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