Novel Van der Waals Heterostructures and their Fabrication

Alyssa Simpson Author

07/29/2020 Added

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In this video I define heterostructures and their applications. I explain the methods of retrieval and assembly we employ during our research. I also give a summary of what the end goal looks like.

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[00:00:00.590]Hi, my name is Alyssa Simpson, a sophmore
[00:00:03.055]physics major doing research on Novel Van
[00:00:05.981]der Waals Heterostructures and their
[00:00:08.265]fabrication in the department of Physics &
[00:00:10.514]Astronomy. I would like to thank my
[00:00:12.594]collaborators Hanying Chen and Tianlin Li,
[00:00:15.427]and my advisor Professor Xia Hong for
[00:00:17.767]providing guidance and training. I would
[00:00:19.800]also like to thank the DOE, NSF and
[00:00:22.117]Nebraska UCARE for providing the funding
[00:00:24.132]for this project.
[00:00:27.255]A heterostructure is an artificial device
[00:00:30.104]made by stacking 2D materials, an example
[00:00:33.473]of which can be seen in the first image.
[00:00:35.740]Since the structure is completely
[00:00:37.825]artificial, we can stack as many different
[00:00:40.170]components as needed, these researchers
[00:00:42.873]used 5.
[00:00:44.541]The 2 most common materials that we have
[00:00:47.451]used in our research are graphene and hBN,
[00:00:51.093]hexagonal boron nitride, which both have
[00:00:53.728]honeycomb structures, as seen in the
[00:00:55.847]diagrams I plot using Autodesk Inventor, a
[00:00:58.962]CAD software. The qualities of both
[00:01:01.028]materials, graphene's two carbon atoms and
[00:01:04.727]hBN's differing boron and nitrogen atoms,
[00:01:07.159]cause a symmetry difference that gives
[00:01:09.476]them distinct transport properties. As
[00:01:11.458]graphene is a semi-metal with good
[00:01:14.192]conducting abilities, while hBN is a super
[00:01:16.623]insulative material.
[00:01:18.589]Using these materials, we would
[00:01:21.288]encapsulate the graphene between two
[00:01:23.220]layers of hBN to create a sandwich-like
[00:01:25.635]stack.
[00:01:27.651]Things that we use in our daily lives to
[00:01:30.240]carry around current make use of similar
[00:01:32.393]combinations. Cable wires feature an
[00:01:35.214]encapsulated conducting wire, in order to
[00:01:38.297]isolate it from the environment and
[00:01:40.496]protect the current flow from outside
[00:01:42.745]noise.
[00:01:43.761]In this stack, since the graphene would
[00:01:45.744]be encapsulated, the current flow will
[00:01:47.759]face no external influences, thus having
[00:01:51.558]no dissipation, making the transport non-
[00:01:54.740]Ohmic. In Ohmic transport, the movement
[00:01:57.422]of the electrons is highly perturbed and
[00:02:00.253]scattered, as such the current flow
[00:02:03.002]contains many vortexes instead of
[00:02:04.685]following a strict path. On the other
[00:02:08.350]hand, the electrons in this diagram do not
[00:02:11.199]deviate off the path, the single
[00:02:14.358]direction flow is evidence of ballistic
[00:02:16.447]transport. This phenomena can only be
[00:02:20.461]explained by quantum mechanics and not
[00:02:22.314]the classic mechanics we learnt in high
[00:02:24.659]school.
[00:02:25.712]During this presentation I will go through
[00:02:27.737]the processes by which we achieve this
[00:02:29.675]stack experimentally.
[00:02:32.150]The first step is to prepare the graphene
[00:02:34.472]and hBN samples. We use the traditional
[00:02:36.820]method of mechanical exfoliation. Using
[00:02:39.570]graphene for our demonstration we will
[00:02:42.152]carefully take a single graphite flake and
[00:02:44.551]place it on scotch tape, making sure to
[00:02:47.118]keep a silicon/silicon dioxide wafer that
[00:02:49.699]went through an acetone, IPA, and
[00:02:51.698]deionized water treatment, nearby. The
[00:02:54.763]next step is to cleave the graphite on
[00:02:57.221]the scotch tape by folding, which can be
[00:02:59.260]seen in this video where I fold the tape
[00:03:01.187]around four times. The amount of times
[00:03:03.342]the tape is folded is not concrete as the
[00:03:05.891]end goal is to evenly distribute the
[00:03:08.174]cleaved graphite over the tape. Lastly we
[00:03:11.406]cover the wafer with the tape and peel it
[00:03:13.771]off to obtain graphene.
[00:03:17.918]Here, we show some optical images of
[00:03:19.735]exfoliated graphene and hBN on their
[00:03:22.450]respective silicon/silicon dioxide wafers.
[00:03:25.532]These photos were captured under a
[00:03:27.917]microscope with the magnification set at
[00:03:29.930]20. On the left side of the slide there
[00:03:32.329]are photos of graphene with multiple
[00:03:34.845]layers. The faintest areas represent
[00:03:39.293]monolayer graphene, and the slightly
[00:03:41.626]darker areas are either double layer or
[00:03:44.179]multi-layer. The opaque area in the
[00:03:46.940]corner would be bulk graphite that did not
[00:03:49.442]flake off. On the right side of the slide
[00:03:52.322]we have hBN, which is categorized by
[00:03:54.660]different colors that denote thickness.
[00:03:57.192]The top image shows non-uniform hBN, as it
[00:04:00.053]does not have a single color covering the
[00:04:02.600]surface, there are cracks and areas that
[00:04:04.980]are thicker than their counterparts.
[00:04:07.245]Underneath it is a uniform flake of hBN
[00:04:09.847]with no obvious color differences on its
[00:04:12.088]surface. For the later stages of our
[00:04:14.944]experiment, like the transfer, we will use
[00:04:17.390]that uniform hBN and the monolayered
[00:04:19.860]section of graphene.
[00:04:22.930]To prepare for the
[00:04:24.030]stacking of those materials, we will use
[00:04:26.480]the transfer technique. Here we show how
[00:04:29.458]to prepare the transfer stamp. We would
[00:04:31.788]cut two small squares out of gel film,
[00:04:34.503]which we then stack on a glass slide and
[00:04:36.463]cover with tape to create a smooth,
[00:04:38.481]transparent contact surface. For the stamp
[00:04:41.767]we use PPC, which stands for
[00:04:43.875]Polypropelene Carbonate, a polymer that is
[00:04:46.617]sticky at room temperature, allowing us to
[00:04:49.226]pick up samples, and turns to glass/
[00:04:51.332]undergoes vitrification at one hundred
[00:04:53.671]degrees celsius, which allows us to put
[00:04:56.036]down these samples. In order to remove
[00:05:00.018]the PPC from the wafer and attach it to
[00:05:02.343]the stamp, we need to place the tape on
[00:05:05.691]the wafer and carefully make sure that it
[00:05:08.183]is secure, using a swab if necessary, as
[00:05:12.161]can be seen in the video. Once done, the
[00:05:16.494]PPC will be pulled off and displayed as a
[00:05:23.432]film over the hole on the tape. Once the
[00:05:29.921]film has been retrieved, it will be placed
[00:05:36.229]over the stamp. In this slide, we
[00:05:39.437]show a schematic diagram of the transfer
[00:05:42.226]process. Since the PPC is sticky at room
[00:05:45.335]temperature, we can use the cold pickup
[00:05:48.897]method to start the stack with hBN. We can
[00:05:51.486]then use the hBN to pick up the
[00:05:53.485]monolayered graphene as there is a
[00:05:55.540]relatively strong van der Waals
[00:05:57.403]interaction between the two. To complete
[00:05:59.871]the stack, we would then drop down the hBN
[00:06:02.590]-GN combo on another flake of hBN by
[00:06:05.663]increasing the temperature. We end up with
[00:06:09.960]a sandwich-like structure, like the one on
[00:06:12.679]the left, with a top hBN, interbedded
[00:06:15.817]graphene, and bottom hBN. Another stack
[00:06:19.339]created using the same methodology is
[00:06:22.076]shown on the right side of the slide.
[00:06:24.626]Both optical images were taken with the
[00:06:27.031]magnification set at 50. This time we use
[00:06:30.908]PZT, a ferroelectric material, as the
[00:06:33.178]bottom gate instead of hBN. Later, we will
[00:06:37.300]conduct deeper research into the
[00:06:39.286]transport properties of these two devices.
[00:06:42.794]As a conclusion, we have exfoliated
[00:06:45.202]different 2D materials. We have assembled
[00:06:48.139]sandwich-like heterostructures by using
[00:06:50.823]the transfer technique. The encapsulated
[00:06:53.373]graphene is expected to have some quantum
[00:06:56.076]transport properties, such as the
[00:06:58.101]ballistic current flow. Here is the
[00:06:59.984]device that we fabricated based on the
[00:07:02.442]previous stack, and later we will perform
[00:07:05.045]some measurements to reveal the quantum
[00:07:07.916]transport phenomena. All in all,
[00:07:11.215]heterostructures provide a new way in
[00:07:13.867]realizing artificial transport devices.
[00:07:16.142]Thank you.

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Novel Van der Waals Heterostructures and their Fabrication

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