1 Ultrasonic Delamination Based Adhesion Testing for High - Throughput Assembly of van der Waals Heterostructures Tara Pe ña Jewel Holt Arfan Sewaket and Stephen M. Wu

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Ultrasonic Delamination Based Adhesion Testing for High-

Throughput Assembly of van der Waals Heterostructures

Tara Peña,∗,† Jewel Holt,† Arfan Sewaket,† and Stephen M. Wu†,¶

†Department of Electrical & Computer Engineering, University of Rochester, Rochester,

NY, USA.

¶Department of Physics & Astronomy, University of Rochester, Rochester, NY, USA.

E-mail: tpena@ur.rochester.edu, stephen.wu@rochester.edu

Abstract

Two-dimensional (2D) materials assembled into van der Waals (vdW) heterostructures

contain unlimited combinations of mechanical, optical, and electrical properties that can be

harnessed for potential device applications. Critically, these structures require control over

interfacial adhesion in order for them to be constructed and to have enough integrity to survive

industrial fabrication processes upon their integration. Here, we promptly determine the

adhesion quality of various exfoliated 2D materials on conventional SiO2/Si substrates using

ultrasonic delamination threshold testing. This test allows us to quickly infer relative substrate

adhesion based on the percent area of 2D flakes that survive a fixed time in an ultrasonic bath,

allowing for control over process parameters that yield high or poor adhesion. We leverage this

control of adhesion to optimize the vdW heterostructure assembly process, where we show

that samples with high or low substrate adhesion relative to each other can be used selectively

to construct high-throughput vdW stacks. Instead of tuning the adhesion of polymer stamps to

2D materials with constant 2D-substrate adhesion, we tune the 2D-substrate adhesion with

constant stamp adhesion to 2D materials. The polymer stamps may be reused without any

polymer melting steps, thus avoiding high temperatures (<120°C) and allowing for high-

throughput production. We show that this procedure can be used to create high-quality 2D

twisted bilayer graphene on SiO2/Si, characterized with atomic force microscopy and Raman

spectroscopic mapping, as well as low-angle twisted bilayer WSe2 on h-BN/SiO2/Si, where we

show direct real-space visualization of moiré reconstruction with tilt angle-dependent scanning

electron microscopy.

Introduction

Two-dimensional (2D) materials - such as graphene, hexagonal boron nitride (h-BN), and

transition metal dichalcogenides (TMDs) - exhibit a vast range of intriguing (opto)electronic properties.

Because of the exceptionally weak out-of-plane van der Waals (vdW) interactions, these 2D materials

can be mechanically isolated then layered into vdW heterostructures. This vdW heterostructure

assembly procedure typically involves dry-transfer processes that rely on temperature tunable

adhesion between 2D materials and polymer stamps. The success of the vdW heterostructure

assembly process depends critically on the control over the relative adhesion of 2D materials to

stamps, substrates, and other 2D materials, leaving adhesion as the most important process parameter

to control in 2D vdW stacking. High-throughput, simple, and quick tests of relative adhesion are

therefore highly important for the optimization of any vdW heterostructure assembly process and may

open doors to new techniques that may supplant the current status quo methods.

Interfacial adhesion has been experimentally investigated through a number of experiments such

as interactive scanning probe tips1, pressurized blister tests2, and buckle delamination tests3. All of

these works paved the way for quantitatively obtaining interfacial adhesion energies between 2D

materials and various 3D-bonded substrates. However, because this parameter entirely depends on

the interaction between two interfaces, it varies easily with the cleanliness of the substrate or

environmental conditions4. Additionally, since these are specialized tests, they do not allow for a

simple high-throughput mechanism to determine if certain process parameters are contributing to

these variations in adhesive properties.

In this work, we present ultrasonic delamination threshold testing as a method to infer the

interfacial adhesion quality between 2D materials and their respective substrates. Ultrasonic cleaning

is a standard cleaning technique, where ultrasonic sounds waves are generated to agitate fluids and

subsequently any submerged samples. This aggressive cleaning technique has been used previously to

corroborate quantified adhesion measurements of polymer/substrate interfaces5. While this method

does not yield a direct quantitative measure of 2D material/substrate interfacial adhesion energies, it

can be used to judge relative adhesion between two different samples in a simple, quick, and reliable

manner. Using this method of adhesion testing, we can judge adhesion quality between exfoliated

graphite flakes and a conventional SiO2/Si substrate under different process conditions, and then use

this knowledge to intentionally fabricate graphene flakes with high and poor adhesion to the SiO2

surface. This allows for direct control over an adhesion variable that otherwise would not be accounted

for in the vdW heterostructure assembly process.

Using this control over substrate adhesion, we show that we can optimize the throughput of

vdW heterostructures and side-step many of the undesirable constraints set by the standardized

procedure. Here, we choose to vary adhesion by exfoliating 2D flakes on plasma-treated SiO2/Si

substrates that vary with ambient exposure. By varying between high and low adhesion at the 2D

material/substrate interface, we can control which 2D flakes can be delaminated by the polymer stamp

during vdW heterostructure construction. Moreover, we find when a 2D flake on the polymer contacts

a highly-adhered flake on SiO2/Si, the 2D flake on the polymer will prefer to remain on the highly-

adhered flake because of the high adhesion to the substrate and the additional vdW interactions

between the two layers, therefore constructing a vdW heterostructure directly on SiO2/Si. This process

represents the reverse process of the standard dry-transfer technique, where the flakes remain on the

polymer stamp after contact and transfer to a SiO2/Si substrate requires a polymer melt. This new

method allows for vdW heterostructures to be constructed directly onto the substrate and the polymer

stamps may be reused. Melting the polymer is disadvantageous since this may need high-temperatures

(~200°C) and will inevitably lead to residues. To combat these polymer residues, the entire sample

must go through rigorous solvent baths and/or a h-BN encapsulation is required to protect the

interface of interest underneath. Directly constructing the vdW heterostructure onto a target substrate

significantly enhances the throughput of this process, furthermore the structure quality is retained

without the need of extensive post-processing steps to clean polymer residues.

Using this new high-throughput vdW heterostructure assembly process, we can quickly produce

several TBG samples varying in twist angle, then probe sample quality with atomic force microscopy

(AFM) and Raman spectroscopy. Finally, we show that this procedure can be extended to other 2D

materials by constructing twisted bilayer WSe2 on h-BN/SiO2/Si (with high adhesion engineered

between h-BN and SiO2 interface) and show direct real-space imaging of moiré reconstruction through

tilt-angle dependent scanning electron microscopy. These results show us that our adhesion testing

techniques and new vdW heterostructure assembly process can reliably generate high-quality samples,

by reproducing various important effects currently at the forefront of exploration in 2D

heterostructures constructed with vdW assembly6.

Methods

All SiO2/Si substrates are placed in an acetone then isopropanol ultrasonic baths for 15 minutes

each, then dried with N2. Subsequently, the substrates are oxygen plasma cleaned at 100 W and 250

mtorr for 3 minutes. The reactive ion etching chamber is also oxygen plasma cleaned for 30 minutes

prior to putting any substrates inside, this ensures no contaminates can be redeposited onto the SiO2

surface. The graphene flakes are produced with the scotch tape method onto pre-treated 300 nm

SiO2/Si substrates, then the substrates are heated at 100°C for 90 s with the graphite tape still in

contact. The tape is slowly removed after the heating procedure is completed and the substrate has

cooled. For monolayer WSe2, we use Gel-Pak polydimethylsiloxane (PDMS) exfoliation to produce large

area monolayers, then these monolayers are transferred onto 90 nm SiO2/Si substrates. Both graphene

and WSe2 monolayers are identified with optical microscopy, which we thoroughly confirm prior to

vdW heterostructure assembly with AFM and Raman spectroscopy11.

Results & Discussion

For the delamination testing, we fabricate numerous exfoliated graphite flakes on plasma-

treated SiO2/Si substrates that vary in adhesion. Oxygen plasma treatment is commonly used to clean

SiO2/Si substrates before exfoliation to increase monolayer flakes from a few μm2 to well over 100 μm2

in area7. Oxygen plasma treatment on the SiO2 surface prior to exfoliation removes hydrocarbon

contaminants and environmental adsorbates that pre-exist on the surface, additionally the plasma

increases the surface reactivity of the SiO2 (Fig. 1a,b)8. This means that the highest adhesion can be

achieved by directly exfoliating the 2D materials onto the plasma-treated surface immediately after

taking the substrate out of the chamber, ensuring the cleanest interface between the 2D material and

substrate and high surface reactivity of the substrate’s surface (Fig. 1c). The longer the SiO2 is re-

exposed to ambient conditions, the more environmental adsorbates are redeposited onto the surface

and limits interactions with the exfoliated 2D flakes (Fig. 1d)4,9. The adhesion energies between

monolayer graphene and SiO2 varies tremendously in the literature between 100 and 450 mJ/m2 in the

literature2,10, which is possibly a manifestation of these discrepancies with substrate cleaning and

ambient exposure. We choose to control adhesion by varying the time the plasma-treated substrates

are in ambient prior to exfoliation.

To observe this variation in graphite adhesion to plasma-treated substrates, we conduct

ultrasonic delamination threshold testing. Here, we take the several graphite/SiO2 samples that vary in

the amount of time the plasma-treated SiO2 was left in ambient prior to exfoliation between 1 minute

and 60 minutes. We image several regions with graphite flakes over the samples (Fig. 1f,h), then place

them into an IPA ultrasonic bath for 5 minutes, and finally repeat imaging over those exact regions (Fig.

1g,i). From this procedure, we find almost 90% of the graphite flakes survive the ultrasonic bath when

the exfoliation is immediate (< 1 minute), which falls off quickly to only 15% after a 60 minute ambient

exposure time (Fig. 1e). These results suggest that adhesion is changing rapidly with the substrates’

exposure to ambient, more importantly we can infer that exfoliating under 1 minute and after 60

minutes of ambient exposure will guarantee either graphite flakes with high or low adhesion

respectively. We can clearly observe how fixed graphite flakes are preserved from the ultrasonic bath

test (Fig. 2a,b), while the free graphite flakes almost entirely are no longer on the substrate after (Fig.

2c,d).

Using a free and readily available image analysis software (ImageJ), we can quantitatively

extract the area of graphite flakes that survive the ultrasonic bath testing presented in Fig. 1e. The

optical micrographs of these areas were converted into binary images using ImageJ, where the

substrate is highlighted in white and the graphite flakes in black (Fig. 2). To construct the binary

images, we utilize two methods (Fig. 2a,b and Fig. 2c,d). Fig. 2a,b presents a method which outlines the

edges of the graphite flakes, through an edge detection algorithm (a plugin provided by ImageJ). The

algorithm calculates intensity gradients throughout the original image (Fig. 2e), where the gradient

values are presented in Fig. 2a. Once the graphite edges are identified from the calculated gradient

values, they are “filled in” to construct the final binary image in Fig. 2b. Fig. 2c,d presents a second

method, where image thresholding is employed instead. Most graphite flakes within this thickness

range have RGB values below the substrate value11, therefore the background (substrate) of the

original image can be brought to saturation (RGB = 255) by increasing the overall RGB value of the

image (Fig. 2c). If any of the graphite flakes have RGB values above that of the substrate mean, the

overall image’s RGB value can be decreased such that the substrate is all black (RGB = 0), then the

image can be inverted. Once the background is saturated, image thresholding will similarly yield a

binary image where the graphite flakes are highlighted in black (Fig. 2d). Finally, these two images can

be overlaid to extract the final binary image (Fig. 2f), two methods are used to affirm most of the

exfoliated graphite features in the optical micrograph are accounted for. We also note, because

monolayer graphene’s contrast is extremely close to that of the substrate (|RGB1L - RGBsubstrate| < 5),

these methods are optimized to specifically capture these monolayer graphene features. Using this

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1UltrasonicDelaminationBasedAdhesionTestingforHigh-ThroughputAssemblyofvanderWaalsHeterostructuresTaraPeña,∗,†JewelHolt,†ArfanSewaket,†andStephenM.Wu†,¶†DepartmentofElectrical&ComputerEngineering,UniversityofRochester,Rochester,NY,USA.¶DepartmentofPhysics&Astronomy,UniversityofRochester,Rochester,NY...

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1 Ultrasonic Delamination Based Adhesion Testing for High - Throughput Assembly of van der Waals Heterostructures Tara Pe ña Jewel Holt Arfan Sewaket and Stephen M. Wu

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