Production and Analysis of Graphene – hBNHeterostructures of Certain Twist Angles

Item Type text; Electronic Thesis

Authors Ruden, Ximena Lu

Publisher The University of Arizona.

Rights Copyright © is held by the author. Digital access to this materialis made possible by the University Libraries, University of Arizona.Further transmission, reproduction or presentation (such aspublic display or performance) of protected items is prohibitedexcept with permission of the author.

Download date 09/03/2021 21:52:47

Link to Item http://hdl.handle.net/10150/625139

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Abstract This thesis gives instructions on how to create and verify good quality graphene and

hexagonal boron nitride (hBN) flakes, how to determine graphene’s edge conformation, and how to use that information to assemble stacks of graphene on hBN with a certain twist angle. It also gives information on how twist angle affects the Raman spectrum taken of a heterostructure and how to use that information to verify the twist angle. Data provided by literature is confirmed by experiment when possible.

Introduction

Graphene is a one atom thick layer of carbon atoms arranged in a hexagonal lattice. Many such layers on top of each other form the bulk graphite. The hexagonal lattice leads to two types of edge conformation: armchair and zigzag (Figure 1). Graphene is currently widely researched in many different forms, such as nanoribbons or as part of stacks.1 Its high carrier mobility (between 3000 and 200,000 cm2/Vs) and near-ballistic transport at room temperature make it appealing for nanoelectronics.2

Hexagonal Boron Nitride (hBN) is another two-dimensional material. It also forms a hexagonal lattice (Figure 1) but with a lattice constant 1.8% larger than graphene.3 hBN is popular because its bulk crystals have been shown to be an exceptional substrate for graphene, able to increase graphene’s electronic quality tenfold.1 While graphene is an excellent conductor, hBN is an insulator that can be used to create gate dielectrics and tunnel barriers.

Figure 1. The zigzag and armchair edge conformations of hexagonal lattices are shown. The lattice constant “a” is 1.8% larger for hBN than for graphene.1 (Left) Graphene lattice (Right) hBN lattice. Boron atoms are blue and nitrogen atoms are yellow.

Graphene and hBN along with other 2D materials can be assembled into van der Waals heterostructures, stacks held together by the van der Waals force.1 Heterostructures have properties that can be tuned by changing their components and their orientation. The possibilities of heterostructures have led to an increasing amount of research on them. This paper examines the simple stack of graphene on few-layer hBN crystals. In order to understand more complex heterostructures an understanding of simpler stacks are needed.

Stacks of graphene on hBN form Moiré patterns due to their mismatched lattices (Figure 2). The Moiré wavelength is largest when the lattices are perfectly aligned with a twist angle of zero, the Moiré pattern is then solely a result of the 1.8% difference in lattice parameter. As the

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twist angle increases, the Moiré wavelength decreases. The electronic energy due to the periodic potential of the Moiré superlattice is inversely proportional to the Moiré wavelength and affects the optical properties.3 As twist angle, Moiré wavelength, and energy due to the superlattice are all related to each other, it is sufficient to describe this phenomenon by only one parameter. Twist angle is used for this paper. Twist angle ranges from 0° to 30° due to rotational symmetry of the hexagonal lattices.

Figure 2. A Moiré lattice is formed by the twist of two hexagonal lattices. The Moiré wavelength is the distance between almost perfectly aligned hexagons.

Raman spectroscopy will be used to analyze the optical properties of graphene by itself as well as on hBN. Raman spectroscopy is performed by shining monochromatic light such as from a laser onto a sample and detecting the scattered light. The majority of scattered light is of the same frequency of the laser and is filtered out.4 However, a very small amount of the scattered light is shifted in frequency due to interactions with vibrational energy levels of the sample. The

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Raman spectrum is a plot of intensity versus frequency shift of the scattered light. Raman spectroscopy is useful as a fast, non-destructive characterization technique.

Graphene’s Raman spectrum consists of a D peak, G peak, and 2D peak. The D peak appears around 1350 cm-1 due to defects in the graphene lattice and so for pristine graphene only appears near the edge.6 The G and 2D peaks appear around 1580 and 2690 cm-1 respectively due to in-plane vibrational modes. As the number of graphene layers increases, the 2D peak splits into an increasing number of modes that combine into a wider, shorter peak. Creating Graphene and hBN Flakes

Both graphene and hBN flakes were created by manual exfoliation using Scotch Matte Finish Magic Tape. The bulk material was pressed multiple times between the tape, spreading out the crystal. The flakes were then transferred to a silicon substrate coated in 285 nm of silicon dioxide. The silicon wafers were cut into chips with side length around half a centimeter by scratching with a glass cutter and snapping the wafer along the cut. The desired flakes were found by scanning the silicon chips with an optical microscope.

Manual exfoliation leads to high quality flakes of varying thicknesses and few impurities. From this method typical graphene flakes are around 10 μm in diameter or smaller while hBN flakes are slightly larger. Exfoliation of Graphene

Clean silicon chips were plasma cleaned with oxygen for around 10s. Then a flat piece of graphite around 1 cm in diameter was pressed a third of the way down a strip of tape around 30 cm in length then lifted off, leaving behind a residue. The strip of tape was brought together and apart until two regions of dark graphite form on the tape. This parent tape was stuck flat to the table with double-sided tape, another strip of tape laid on top in order to transfer the flakes to the daughter tape. A few daughter tapes were made from the parent tape. Silicon chips were placed on the graphite covered regions, pressing down hard. After heating at 130°C for 2.5 minutes and allowed to cool, the tape was peeled off fast, leaving behind graphene and graphite flakes. Exfoliation of hBN

Several small pieces of bulk hBN were placed on a strip of tape, which was brought together and apart to spread the hBN in two regions. Similarly to graphene exfoliation, a few daughter tapes were created from the parent tape, but pressing lightly. Silicon chips were placed lightly on the tape. Then the tape was peeled off very slowly, taking around 2 minutes per silicon chip.

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Identifying Good Graphene and hBN

First, graphene and hBN were identified by color with an optical microscope. Raman spectroscopy was used to verify monolayer graphene and atomic force microscopy (AFM) was used to determine the cleanliness of the flakes. Identification by Optical Microscope

Different thicknesses of graphite flakes look different colors due to thin film interference. Thick graphite flakes on silicon chips with a 285 nm thick coat of silicon dioxide look yellow. The color changes to blue then purple as the flakes get thinner, becoming a light purple when monolayer (Figure 3). With practice, monolayer graphene can be identified with great accuracy solely through optical observation.

Figure 3. Graphite of different thicknesses on a 285 nm silicon dioxide substrate, seen by an optical microscope. Monolayer graphene is a faint purple. The color darkens, becomes blue, then green, and finally yellow as the number of layers become thicker.

hBN flakes have a similar color spectrum on the silicon chips. hBN of around 20-50 nm thickness were used for the heterostructures and have a blue-green color (Figure 4). Flakes of at least 10 μm in diameter with little color variation or other imperfections were identified.

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Figure 4. This exemplifies the hBN flakes desired. It is around a uniform 30 nm thick, corresponding to a uniform blue-green color. Straight edges can be used to predict the lattice orientation. Raman Spectroscopy Verification of Monolayer Graphene

Raman spectroscopy can quickly verify if a graphene flake is monolayer, bilayer, or thicker. As Figure 5 shows, the 2D peak is larger than the G peak for monolayer while the peaks are around equal in size for bilayer. The G peak continues to grow relative to the 2D peak as the graphite becomes thicker. The D peak is not visible because these measurements were taken from the center of pristine graphene.

Figure 5. The Raman spectrum of monolayer graphene differs from bilayer graphene by the relative intensities of the G and 2D peaks. The 2D peak is larger than the G peak for monolayer graphene while the peaks are around equal intensity for bilayer graphene. The G peak continues to increase relative to the 2D peak as the number of layers grows.

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Atomic Force Microscopy (AFM) Determination of Surface Quality

AFM works by running a sharp cantilever tip over a sample. A laser bounces off the end of the cantilever into a detector, determining how it moves. Tapping mode was used, the cantilever bouncing up and down off of the sample. This mode gives information on the height of the sample as well as how the amplitude and phase of the tapping changes as the tip is moved over the sample. The amplitude of oscillation of the cantilever changes as it moves over objects of different height. The phase changes depending on the surface composition. Sticky tape shows up differently than graphene on the phase plot. The height plot was sufficient to distinguish any undesired features (Figure 6). AFM was used to verify the surface quality of the identified flakes. The good chips were narrowed down to those without cracks, ragged ends, or tape residue.

Figure 6. An AFM image of graphene. Colors represent how tall a feature is. The graphene is folded over itself in the center. There is some tape residue on the right side of the graphene and some much taller residue on the bottom of the image.

Identifying Edge Conformation of Graphene with Raman Spectroscopy

The D peak can be used to determine the edge conformation of graphene as its intensity depends on the edge conformation. According to literature, a pure zigzag edge should have no D peak while a pure armchair edge should have the largest intensity.5 No edge is purely zigzag however, so a primarily zigzag edge will have a small D peak. However, one has to be careful to take into account the other elements that also affect the peak’s intensity.

In graphene’s Raman spectrum, the intensity of the peaks change as one moves over an edge (Figure 7). The D peak appears due to defects in the graphene lattice and so only appears

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near the edge of pristine graphene created through manual exfoliation. The D peak’s intensity is largest directly on the edge of the graphene.

The intensity of the D peak also depends on the polarization angle of the laser. As Figure 8 shows, the intensity of the peak is largest when the laser is polarized parallel to the edge and smallest when perpendicular. Literature says that this is true no matter the conformation of the graphene edge.5 One should make sure to keep the polarization angle relative to the edges equivalent when comparing the D peak intensities of two edges. Examining an armchair edge with perpendicular polarization while examining a zigzag edge with parallel polarization may result in D peaks of similar size.

Figure 7. (Left) How the intensity of the D, 2D, and G peak changes as one moves across the edge of a piece of graphene. (Right) The path taken across the edge of the graphene.

Figure 8. The intensity of the Raman D peak, the laser shining on the edge of the graphene flake shown in Figure 7. The intensity changes as the polarization angle of the laser changes. It is maximum when the laser is polarized parallel to the graphene edge, in this case around 45°.

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Graphene flakes with corners a multiple of 30° have edge conformations of known relation.2 Corners with angles of 30°, 90°, and 150° have edges of different conformations while those with angles of 60° and 120° have edges with the same conformation (Figure 9).

Figure 9. The relative edge conformations around corners (armchair or zigzag) depends on the angle. (Left) Angles of 90° have edges with different conformations. (Middle) Angles of 60° and 120° have sides with the same conformation. (Right) Angles of 30° have sides of different conformations.

A graphene flake with two straight edges of different conformations is needed in order to determine the edge conformations. No edge is purely zigzag and so there will always be at least a small D peak near the edge. The straight edges will ensure the edge conformation is mostly homogeneous while the relative intensity of the D peaks across the edges can be used to determine the conformations. The edge with the larger intensity corresponds to the armchair edge and the other the zigzag edge. This information is used to create the following procedure to determine the edge conformation of graphene. Procedure: Finding Edge Conformation of Graphene

First, find a clean monolayer graphene flake with straight edges meeting at a corner of angle 30°, 90°, or 150°. Then, take Raman spectra across the corner varying the polarization angle of the laser with a half wave plate. Compare the intensity of the D peaks across the edges when the laser is polarized parallel to the edges. The edge with the larger peak intensity has an armchair conformation while the other has a zigzag conformation. Stacking Procedures

The apparatus shown in Figure 10 was used to stack graphene on hBN. At first polypropylene carbonate (PPC) film was used to assemble the stacks. However there were issues picking up graphene off the silicon oxide substrate, possibly due to the low humidity. A procedure using polycarbonate (PC) film provided by Dr. Schaibley was later used with success.

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Figure 10. This apparatus was used to create the heterostructures. The stage holds the silicon substrate down with vacuum suction. The stage can also be heated and cooled. The stamp holder allows for the fine movement of the stamp for alignment. Both the microscope and the stage can be moved in order to change the field of view. PPC Stacking Procedure

A few drops of PPC in chloroform (50 g/L) were used to spin coat a 1 cm3 SiO2 chip, spinning 1500 rpm for 1 minute. The chip was then heated at 90°C for two minutes and allowed to cool to room temperature. Then a piece of tape with a 1/16 inch hole punched through was placed on the chip and lifted up, pulling the film with it.

The stamp used to assemble the heterostructure (Figure 11) consists of a clean square of polydimethylsiloxane (PDMS) on a glass microscope slide. The PDMS is of dimensions around 5mm x 5mm x 2mm, larger than the hole punched in the tape so that the circle of uncovered PPC lies completely over the PDMS. The stamp is heated at 90°C for 50 seconds to flatten the film. Then, the stamp was used to pick up an hBN flake. With the stacking stage set at 40°C, the PPC film was brought into contact with the SiO2 chip near the desired hBN flake and left to spread and cover it. A test touchdown may first be done on a clean region of the chip to note where the PPC film first comes in contact with the chip. The temperature may be raised a few degrees to

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facilitate the spread, but is returned to 40°C before lifting the PPC film. A rainbow interference pattern (Figure 12) shows the speed and direction the film is likely to spread. When the hBN flake is completely covered with the PPC film, the stamp is slowly raised, lifting the PPC film off of the SiO2 substrate and hopefully taking the hBN with it. The hBN flake should disappear then reappear a faint yellow color if it is lifted by the PPC. The stamp may be heated at 90°C for 50 seconds to flatten the film.

Next, the graphene is picked up on the hBN. The location of the desired graphene and surrounding features are outlined. Then the stamp is moved so that the hBN flake is directly over the graphene. Small adjustments may need to be made as the hBN is brought closer to the graphene. The PPC film should touch down near where it did the first time and slowly spread to cover the graphene with the hBN flake. The temperature may be raised to facilitate the spread, but is returned to 40°C before lifting the PPC film.

To transfer the stack to a clean SiO2 chip, the tape with the PPC film was removed from the stamp and placed sticky-side down so the hole in the tape is over the chip. Then, it is heated to 120°C for 10 minutes. After a few minutes, the tape around the hole is pressed to ensure the PPC film is touching the substrate. After the 10 minutes, the tape is lifted up when still hot, leaving behind a circle of PPC film with the stack. The PPC film is removed by annealing the chip in a tube furnace under vacuum at 350°C for 15 minutes.

Figure 11. These are the design of the stamps used when assembling heterostructures. (Top) The PC film is stretched over the PDMS, the hole in the tape larger than the PDMS. (Bottom) The PPC film is wholly on the PDMS. The tape is stretched over the PDMS.

Figure 12. PPC film spreading over a silicon wafer. The film is touching the chip were it is yellow. An hBN flake is covered. The red and green interference pattern is large and intense, indicating that the touch down spot will grow quickly. The touch down spot will grow in the direction of the arrows.

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PC Stacking Procedure

The PC film was created by putting a few drops of PC in chloroform (188 g/L) on a clean glass slide. Another slide was placed on top then flicked off, flattening the PC. The thicker solution makes spin-coating not as effective in making a smooth film. Tape with a ¼ inch hole is placed over the film to pick it up. The stamp is similar to the PPC stamp except the PC film completely covers the PDMS (Figure 11). Double-sided tape around the PDMS is also used to hold the film in place.

The procedure for touching down is similar to the PPC procedure except the temperature is raised more to spread the touchdown point as PC film does not spread as easily as PPC. First, the graphene is picked up. After the test touchdown, the PC film is brought to touch down near the graphene with the stage at 55°C. Then the temperature is raised ~2°C/min to around 95°C or until the graphene is completely covered. Then the temperature is lowered back to 55°C at ~5°C/min. Then after drawing the location of the graphene and nearby features, slowly lift the stamp with the graphene.

Next align the graphene over the hBN flake, slowly bringing them closer together until the PC film has touched down near the flake. Then raise the temperature around 10°C/min to 210°C. The graphene should touch down on the hBN. Wait 10 minutes then pull up the stamp while it is hot, leaving behind the PC film.

The PC film is washed off by soaking the chip 15 minutes in chloroform, then 10 minutes in acetone and 5 minutes in isopropanol. Making Stacks of Certain Twist Angle

In order to make stacks of certain twist angle one needs to align straight edges of the hBN and graphene flakes. To make stacks with twist angles around 0° or 30°, the edges should be aligned at an angle varying by a multiple of 30°. Figure 9 illustrates how rotating certain angles will result in either the same or opposite edge conformation. No quick way to determine the edge conformation of hBN with Raman spectroscopy was found. This uncertainty in conformation results in two possible twist angles for every alignment. Raman spectroscopy can be used to verify if the twist angle is small or large, as described in the next section.

Raman Spectroscopy of Graphene on hBN

As explained in the introduction, graphene on hBN forms a Moiré pattern that affects graphene’s Raman spectrum. The phenomenon can be divided into two regimes: lattices with small twist angle <2° and misaligned lattices with twist angle >2°.

For misaligned stacks with twist angle larger than 2° have 2D peaks narrower than graphene directly on the silicon wafer.3 The 2D peak is also upshifted by around 9 cm-1 while the G peak is unchanged.

For small twist angle of less than 2°, the 2D peak broadens as the lattices change from misaligned to completely aligned.3 The G peak also broadens as the lattices become more aligned.

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These features of the Raman spectrum can be used to identify if the graphene hBN stacks are close or far from alignment. For example, Figure 13 shows a stack, one edge of the graphene rotated around 60° from an edge of the hBN. This could lead to a stack of twist angle around 0° or 30° depending if the edges are the same or different conformation. One can determine which it is by Raman spectroscopy. Its Raman spectrum (Figure 14) has a 2D peak of similar width and location to graphene directly on a silicon wafer. The G peak is broader and less intense than for graphene on silicon. These factors support that there is a small twist angle less than 2° between the graphene and hBN.

Figure 13. Graphene on hBN. (Left) Image of stack by optical microscope. The graphene is circled. The top edge of the graphene flake is rotated around 60° from an hBN edge, and so the lattices may match up with a small twist angle. (Middle) Height plot of stack by AFM. (Right) Zoomed-in on the graphene flake.

Figure 14. (Top) Raman spectrum of the stack shown in Figure 13. There is an additional peak due to the hBN. (Bottom) Raman spectrum of monolayer graphene on a silicon chip, to compare.

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Conclusion

Graphene has an auspicious future as an electronic component. Someday nanoelectronics may be made of heterostructures composed of graphene and hBN. In order to make those future devices, one must first understand the basics of how graphene’s properties are affected by an hBN substrate.

The techniques covered in this thesis are not only useful for assembling stacks of graphene on hBN. Similar stacking techniques can be used to assemble a wide variety of heterostructures. Also, the technique to identify the edge conformation of graphene can be useful when studying graphene nanoribbons, their thinness causing their properties to be significantly affected by edge conformation.2

[1] Geim, A. K., and I. V. Grigorieva. "Van der Waals heterostructures." Nature 499, no. 7459 (2013): 419-25. doi:10.1038/nature12385. [2] You, Yumeng, Zhenhua Ni, Ting Yu, and Zexiang Shen. "Edge chirality determination of graphene by Raman spectroscopy." Applied Physics Letters 93, no. 16 (2008): 163112. doi:10.1063/1.3005599. [3] Eckmann, Axel, Jaesung Park, Huafeng Yang, Daniel Elias, Alexander S. Mayorov, Geliang Yu, Rashid Jalil, Kostya S. Novoselov, Roman V. Gorbachev, Michele Lazzeri, Andre K. Geim, and Cinzia Casiraghi. "Raman Fingerprint of Aligned Graphene/h-BN Superlattices." Nano Letters 13, no. 11 (2013): 5242-246. doi:10.1021/nl402679b. [4] "What is Raman Spectroscopy?" InPhotonics. http://www.inphotonics.com/raman.htm. [5] Casiraghi, C., A. Hartschuh, H. Qian, S. Piscanec, C. Georgi, A. Fasoli, K. S. Novoselov, D. M. Basko, and A. C. Ferrari. "Raman Spectroscopy of Graphene Edges." Nano Letters 9, no. 4 (2009): 1433-441. doi:10.1021/nl8032697. [6] Childres, Isaac. "Raman Spectroscopy of Graphene an Related Materials ." https://www.physics.purdue.edu/quantum/files/Raman_Spectroscopy_of_Graphene_NOVA_Childres.pdf.