ll

ArticleOrientation-engineered 2D electronics on vander Waals dielectrics
Weijun Wang, Yuxuan Zhang,
Wei Wang, ..., SenPo Yip, Weida
Hu, Johnny C. Ho
wdhu@mail.sitp.ac.cn (W.H.)
johnnyho@cityu.edu.hk (J.C.H.)

Highlights
Designed a hydromechanicalstrategy for aligned 2D material
synthesis
Quantitatively established theepitaxy relationship between 2D
materials and vdWs dielectrics
Demonstrated high-performancedevices with as-grown 2D
materials/vdWs dielectrics

Coordinated with Simcenter STAR CCM+ simulation, a hydromechanical strategy
is initiated to achieve the aligned epitaxy of 2D materials on vdWs mica dielectrics.
Combining density functional theory with Lagrange’s group theorem, a criterion
for how the oriented epitaxial growth of the 2D material on a 6-fold symmetric
vdWs dielectric is quantitatively established. Moreover, the electricalcharacteristics of ultrathin as-grown Bi2O2Se-based FETs supported on their
original epitaxial dielectric are evaluated at the device level.

Wang et al., Matter 7, 2236–2249
June 5, 2024 ª 2024 Elsevier Inc. All rights
reserved.
https://doi.org/10.1016/j.matt.2024.04.013

llArticleOrientation-engineered 2D electronics
on van der Waals dielectrics
Weijun Wang,1 Yuxuan Zhang,1 Wei Wang,1 Min Luo,2 You Meng,1 Bowen Li,1 Yan Yan,1 Di Yin,1Pengshan Xie,1 Dengji Li,1 Dong Chen,1 Quan Quan,1 SenPo Yip,3 Weida Hu,2,*
and Johnny C. Ho1,3,4,5,*

SUMMARY

PROGRESS AND POTENTIAL

van der Waals (vdWs) dielectrics are widely used in nanoelectronics
to preserve the intrinsic properties of two-dimensional (2D) semiconductors. However, achieving aligned growth of 2D semiconductors and their direct utilization on original vdWs epitaxial dielectrics
to avoid disorders poses significant challenges. Here, a hydromechanical strategy for aligned epitaxy of 2D materials on naturally
occurring vdWs mica dielectrics is developed. By combining density
functional theory with Lagrange’s group theorem, a quantitative criterion for 2D material epitaxy on 6-fold symmetric vdWs dielectrics
is established. Moreover, the as-grown ultrathin Bi2O2Se-channeledfield-effect transistor, with a hybrid dielectric layer, achieves a superior current on/off ratio (1.4 3 107) and high carrier mobility(22.4 cm2 V1 S1) by directly integrating as-grown 2D materials/
vdWs dielectrics. This work provides a powerful methodologicalplatform for aligned 2D material synthesis, alignment direction prediction, and intrinsic property investigation, laying the foundation
for advanced electronics on as-grown 2D materials/vdWs dielectrics.

Empowered by device
miniaturization approaching thephysical limits, 2D materials
emerge as virtuosos, significantly
influencing the trajectory ofelectronics in the post-Moore era.
Leveraged by thehydromechanical strategy, 2Dmaterials with preferential
orientations on van der Waals(vdWs) dielectrics can besynthesized, facilitating the direct
utilization of as-grown 2D
materials/vdWs dielectrics at thedevice level to minimize disorderinduced performancedegradation. Additionally, theestablishment of the quantitative
criterion for the epitaxy
relationship with vdWs dielectrics
can be aptly viewed as a measureof our understanding and canguide experimental decisions
effectively. This work introduces apowerful methodology platform
for synthesizing aligned 2D
materials, predicting alignment
directions, and preserving theirintrinsic properties, holding
substantial technologicalimplications for unlocking thetransformative potential of 2D
materials.

INTRODUCTION
Batch fabrication of advanced two-dimensional (2D) field-effect transistors (FETs)
necessitates the synthesis of high-quality 2D films, which is commonly accomplishedby aligned coalescence of 2D material islands on specific substrates.1–3 However,
the as-grown 2D films via the mainstream chemical vapor deposition (CVD) method
are generally coalesced by enormous misoriented 2D material islands and may be
meticulously detached from the original substrates onto proposed dielectrics forfurther device fabrication.4,5 Such processes can separately cause grain-boundaryinduced disorders and surrounding disorders from chemical adsorbates or neighboring dielectrics, degrading the 2D material’s intrinsic properties,3,6 such as carrier
mobility and gate controllability. Compared to the 2D FETs based on conventionaloxide dielectrics (such as SiO2),7 the devices integrated with van der Waals (vdWs)
dielectrics exhibit improved mobility and better switching stability,8,9 attributable
to their atomically flat surfaces that are free of dangling-bond-induced disorders.
Thus, achieving aligned growth of 2D materials and directly using them on their original vdWs epitaxial dielectrics to avoid disorders is being pursued with purposefullyengineered off-cut angles toward future 2D nanoelectronics.10,11 Regrettably,
achieving non-destructive manipulation of as-grown 2D materials with vdWs dielectrics, e.g., h-BN,12 Sb2O3,13,14 etc., is currently challenging, hindering the direct
integration of as-grown 2D materials on their original vdWs dielectrics.

2236 Matter 7, 2236–2249, June 5, 2024 ª 2024 Elsevier Inc. All rights reserved.

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ArticleFortunately, the recent discovery of a naturally occurring vdWs dielectric, muscovite
mica, has shed light on exploring those above challenging 2D nanoelectronic issues.
Except for the dangling-bond-free surface that facilitates the aligned growth ofvarious 2D materials,15–18 the mica dielectric provides rich redundancy of being precisely manipulated to desired thicknesses, owing to the relatively weak interlayerinteraction.19 Also, it has a dielectric constant of 6.4–9.3,20 nearly twice that of theconventional SiO2 and h-BN, making it applicable for effective gate control andhigh-speed switching even down to a few atomic layers thick.3,21 Besides, mica
with a large band gap (5.1 eV),22 high dielectric strength (0.1–1 V/nm),23 and superior stability can make it an excellent dielectric layer, suitable for high electric fields
and high-temperature harsh environments.24,25 In this regard, relying on vdWs mica
dielectrics to develop advanced techniques for achieving aligned synthesis of 2D
materials and direct utilization of as-grown 2D materials/vdWs dielectrics are fundamentally feasible and of great significance.
Herein, a hydromechanical strategy for achieving the aligned epitaxy of 2D materials
on vdWs mica dielectrics, coordinated with Simcenter STAR CCM+ simulation, is firstdeveloped for the promising high-quality synthesis of large-scale 2D films via the
coalescence process. In order to gain further insights into the epitaxial relationshipbetween the 2D material and the 6-fold vdWs dielectric, we perform a systematictheoretical analysis by combining the density functional theory (DFT) calculation
with Lagrange’s group theorem. After that, an unreported criterion for how theepitaxial growth of the 2D material on a 6-fold symmetric vdWs dielectric is established. Moreover, a reproducible technique for high-precision manipulation of
as-grown 2D materials on their original vdWs mica dielectric is initiated, while theelectrical characteristics of ultrathin as-grown Bi2O2Se-based FETs supported on
its original vdWs epitaxial substrate are evaluated at the device level.

RESULTS AND DISCUSSION
Aligned fabrication of the 2D materials
Figure 1A depicts a conventional CVD setup with a horizontally positioned growth
substrate. As indicated by the carrier gas flow, prolonged precursor transport
reduces the control over precursor concentration and growth temperature,26–28 resulting in non-uniform deposition of various Bi–O–Se products (Figure 1B), including
in-plane Bi2O2Se, inclined Bi2O2Se, and Bi2Se3 nanoflakes, respectively. Disorderedepitaxial growth on mica is observed for each product (Figure S1), owing to the poor
controllability over the precursor transport process and growth temperature. In
contrast, as shown in Figure 1C, different compositions can be controllably synthesized by regulating the growth temperature and the precursor weight ratio in the hydromechanics-based CVD system (Note S1), where the growth substrate is inclinedly
placed in the CVD system (Figure S2). Figures 1D–1F show the optical microscope
images of as-grown 2D products on the titled mica with an angle of around 20 ,demonstrating significantly better thickness uniformity and flake orientation in themodified CVD system. Remarkably, the results show that the in-plane and inclined
Bi2O2Se crystals show three equivalent but different alignment angles on the mica
substrate, while only two different orientations are observed for synthesizing 2D
Bi2Se3 (detailed explanations will be provided later to investigate the intimate interplay between the 2D material and the mica substrate). A series of unique epitaxy
characteristics for different 2D products, including orientations and morphology
evolution, are observed (Figures S3–S9), showing the great potential of the hydromechanical synthesis strategy in the controllable synthesis of many other 2D
materials.

1Department of Materials Science andEngineering, City University of Hong Kong,
Kowloon 999077, Hong Kong SAR, China
2State Key Laboratory of Infrared Physics,

Shanghai Institute of Technical Physics, Chinese
Academy of Sciences, Shanghai 200083, China
3Institute for Materials Chemistry andEngineering, Kyushu University, Fukuoka
816-8580, Japan
4State Key Laboratory of Terahertz and Millimeter

Waves, City University of Hong Kong, Kowloon
999077, Hong Kong SAR, China
5Lead contact*Correspondence: wdhu@mail.sitp.ac.cn (W.H.),
johnnyho@cityu.edu.hk (J.C.H.)
https://doi.org/10.1016/j.matt.2024.04.013

Matter 7, 2236–2249, June 5, 2024 2237

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ArticleFigure 1. Hydromechanical strategy for aligned 2D material growth(A) Schematic illustration of a conventional CVD system exhibiting the spatial sectioning of the growth substrate for (I) in-plane Bi 2 O 2 Se, (II) inclined
Bi 2 O 2 Se, and (III) Bi 2 Se 3 , respectively.
(B) Schematic representation of the Bi–O–Se ternary phase diagram.
(C) Summary of the Bi–O–Se products via regulating the temperatures and weight ratios of Bi 2 Se 3 and Bi2 O 3 precursors.
(D–F) Optical microscope images of the well-orientated single-crystalline 2D islands along the high-symmetry direction. The crystallographic
orientations of 2D material flakes are positioned in red, blue, and purple colors, respectively. Compared to the conventional CVD synthesis system(Figure S1), the hydrodynamics system demonstrates excellent controllability by yielding a specific single product without misorientation, showcasing
its superiority.

2238 Matter 7, 2236–2249, June 5, 2024

ArticlellFigure 1. Continued(G) Simulated distribution of the carrier gas velocity (top) and the precursor concentration field (bottom) using Simcenter STAR CCM+ software for ahydromechanical CVD system.
(H) Simulated precursor concentration (4% on average) on the top surface of the substrate at equilibrium with a solution time of 18.13 s.

It is essential to mention that although enormous research efforts have been
devoted to Bi2O2Se synthesis, this is the first report on the preferentially aligned
growth of in-plane Bi2O2Se nanoflakes on a substrate. A series of Simcenter STAR
CCM+ simulations are performed for the conventional and hydrodynamic systems,
keeping all other growth parameters constant, such as growth temperature, pressure, etc., to semiquantitatively compare their controllability of precursor transport(Figures S10–S12). The simulations highlight the significance of flow rate and precursor concentration across the growth substrate in a CVD reactor, providing strong
evidence for robustly achieving the aligned growth of 2D materials in the hydrodynamics system. Figures 1G and 1H show the simulated result of carrier gas velocity
and precursor concentration with the titled angle of 20 and a solution time of 17.17s, respectively. The result indicates that both gas flow velocity and precursor concentration near the mica substrate are effectively regulated since the titled mica substrate (length 3 width = 2 3 1.8 cm) can serve as a ‘‘valve’’ owing to the limited
tube size (1 inch). More importantly, a lower precursor concentration dissipation is
obtained for the hydrodynamic CVD system, compared to the conventional CVD
system, with respect to the point source, demonstrating better controllability ofthe vapor precursor by simply regulating the valve angle. The above simulation results support our experimental observations and highlight the importance of deliberate process management to achieve a controllable synthesis of 2D materials in the
CVD method by manipulating process parameters and reactor geometry.
Characteristics of the aligned 2D materials
The morphology and crystal structure of the 2D Bi–O–Se products are then investigated by a combination of experimental techniques. The atomic force microscopy(AFM) images of the 2D Bi–O–Se products are shown in Figures 2A–2C, revealinga clean and atomically flat surface topography with root-mean-square surface roughness of 5.08, 5.12, and 5.25 Å, respectively, within the area of 2 3 2 mm. Adopting
the hydrodynamic strategy, the 2D Bi–O–Se products exhibit significantly improved
controllability, with characteristic average thicknesses of 7.2 nm for in-plane
Bi2O2Se, 20.9 nm for tilted Bi2O2Se, and 7.1 nm for Bi2Se3 (Figures 2D–2F). Figure 2G exhibits that X-ray diffraction (XRD) measurements of in-plane Bi2O2Se,29 inclined Bi2O2Se,30 and Bi2Se331 are well aligned with the precious works. Notably, thetilted Bi2O2Se shows an additional (013) peak in the XRD measurements corresponding to the (013) crystal planes from the cross-sectional scanning transmission
electron microscope images via focused ion beam sectioning.30 The unmarked
diffraction peaks originated from the pristine mica substrate match well with thestandard XRD card PDF no. 16-0344 (Figure S13). X-ray photoelectron spectroscopy(XPS) reveals the chemical bonding states of Bi and Se for the in-plane/flat Bi2O2Se
and Bi2Se3 (Figure S14), and all the resolved peaks are consistent with the composition of previous works.32,33 The inclined Bi2O2Se flakes are converted into flat ones
for XPS measurements through proper ultrasonication since the XPS instrument can
only analyze 5–10 nm of the top surface,34 making the signal intensity of inclined
samples too weak for identification. Figures 2H and 2I display the related Raman
spectra and optical analysis of each 2D product, which are well-aligned with theircharacteristic vibration peaks.30,35–37 Compared to in-plane or flat Bi2O2Se flakes,
a relatively large redshift of the A1g peak for the inclined Bi2O2Se is observed, which
could be attributed to the inharmonicity of lattice vibrations.38 (Note S2) The above

Matter 7, 2236–2249, June 5, 2024 2239

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ArticleFigure 2. Characterization of the aligned 2D materials(A–C) AFM measurements of 2D Bi–O–Se flakes with feature thicknesses of 7.8 (in-plane Bi 2 O 2 Se), 23.2 (inclined Bi2 O 2 Se), and 7.6 nm (Bi 2 Se 3 ),respectively.
(D–F) Average thickness distribution of the well-aligned Bi–O–Se flakes on mica substrate: in-plane Bi 2 O2 Se (7.2 nm), inclined Bi2 O 2 Se (20.9 nm), andBi 2 Se 3 flakes (7.1 nm), respectively.
(G) XRD patterns of the pristine mica substrate and as-grown 2D Bi–O–Se flakes on mica substrates.
(H) Raman spectra of different 2D Bi–O–Se products on mica substrates. Meanwhile, flatwise Bi2 O 2 Se flakes refer to the inclined Bi2 O 2 Se flakes after 2 sof ultrasound.
(I) Optical measurements of the related 2D flakes. (I) Raman mapping of the as-grown Bi 2 O2 Se A1g peak. (II and III) Optical microscope image andcorresponding Raman mapping of Bi2 O 2 Se A1g peak for the inclined and flat Bi 2 O 2 Se, respectively. (IV) Raman mapping of the as-grown Bi2 Se 3 A1g peak.

2240 Matter 7, 2236–2249, June 5, 2024

Articlellanalyses demonstrate that our 2D products synthesized by hydrodynamic strategy
via the CVD method are of high quality with excellent uniformity, stoichiometry, crystallinity, etc.
Insight into the epitaxy relationship with vdWs dielectrics
Experimental observations indicate that the alignment of a 2D material on the mica substrate is synergistically influenced by the symmetries of the material itself and the underlying substrate. Thus, a comprehensive theory on 2D materials epitaxy, capable of predicting the alignment of various 2D products on mica substrates, is highly desirable as
an experimental design guideline for large-scale 2D film synthesis. However, no general
theory exists to explain the epitaxial relationship between different 2D materials on mica.
Thanks to the weak bonding between the potassium (K) ions and aluminosilicate layers,
vdWs mica dielectrics possess a perfect cleavage parallel to {001} planes,39 which is intrinsically favorable for the lateral migration of precursor atoms and epitaxial growth of 2D
materials.40 The {001} cleavage planes, consisting of the K ions, with an ideal surface,possess a 6-fold rotational symmetry (C6) (Figure S15), and the alignment of 2D materials
with self-passivated edges on mica substrate is dominated by the weak vdWs forces.41,42
In such scenery, the interaction between 2D materials and mica {001} planes is required toreveal the alignment of 2D materials on mica by utilizing the periodic boundary condition
models. Previous studies have shown that 2D materials (edges of graphene, h-BN, andtransition-metal dichalcogenides [TMDCs]) tend to align their high-symmetry directions
with those of their substrates, such as the <110> directions of Cu (111),43 <11–20> of hBN44, and (0001) planes of Al2O3.45 This rule is consistent with our experimental observations on the epitaxial growth of in-plane Bi2O2Se, inclined Bi2O2Se, and Bi2Se3 on mica
substrates (Figures 1D–1F), as well as being in perfect agreement with many experimental
observations on 2D materials’ growth on the vdWs mica dielectrics known up to now(Table S1).
To verify the assumption that a 2D material tends to align its high-symmetry direction withthe high-symmetry direction of the mica surface, comprehensive DFT calculations are
conducted based on our well-aligned 2D materials synthesis. Figures 3A–3C exhibit
the schematic models of our 2D materials clusters aligning along the high-symmetry directions of the mica dielectric for the related DFT calculations. In the calculations, various
angles between different 2D material clusters regarding vdWs mica dielectrics aremodeled to ensure the accuracy of the calculations (Figures S16–S18). Detailed methods
for calculations are provided in the experimental procedures. Figure 3D shows the binding energy of different 2D material clusters on a mica substrate as a function of the alignment angle between the self-passivated edges of 2D materials and the high-symmetry<110> direction of the mica {001} surface. These results show that the system’s overall energy maintains the lowest value when the self-passivated edges of the 2D materials follow
the high-symmetry direction of the neighboring vdWs substrate. Although we calculatevery limited epitaxial systems of 2D materials on mica, the above results allow us to summarize the rule for the alignment of an arbitrary 2D material on mica, i.e., a high-symmetry
direction of the 2D island prefers to align along a high-symmetry direction of the mica
substrate.
With the principle that determines the alignment of an arbitrary 2D material on amica substrate established, we will further discuss how the symmetries of both thesubstrate and the 2D material interact in the epitaxial growth. Considering a 2D material with a G2D symmetry group placed on a 6-fold mica substrate (GMica), the symmetry group of the system, G2D&Mica, must be a subgroup of either G2D or GMica
because the alignment of the 2D material and the substrate remains unaffected byany symmetry operation of G2D&Mica. As derived in Note S3, the number ofMatter 7, 2236–2249, June 5, 2024 2241

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ArticleFigure 3. Study on the epitaxy relationship with vdWs dielectrics(A–C) Schematic model of 2D materials clusters aligning along the high-symmetry direction of themica dielectric.
(D) The calculated binding energies of the 2D materials cluster on the mica surface as a function of
the alignment angle (q) between a high-symmetry direction of 2D materials and a high-symmetry
mica surface.
(E) The number of equivalent but distinct alignments of our experimental 2D products on the mica
substrate. The symmetry groups of the mica substrate, 2D materials, and the system of the 2D islandon mica are provided. The number of symmetry operations of the mica substrate and the wholesystem are N Mica and N 2D&Mica , respectively. Consistent with our experimental observations, these2D products, including in-plane Bi 2 O2 Se, inclined Bi2 O 2 Se, and Bi2 Se 3 , exhibit 3, 3, and 2
equivalent but different orientations, respectively.

equivalent but distinct directions of a 2D material on mica {001} surfaces, defined as
NE, can be computed using the following formula, derived from Lagrange’s theoremof the group theory46,47:
NE =jGMica j;
jG2D&Mica jwhere |GSub| and |G2D&Mica| are the numbers of distinct symmetry operations of GSuband G2D&Sub, respectively. According to the abovementioned principle of arbitrary
2D materials on mica, a high-symmetry edge of a 2D material preferably aligned
along a high-symmetry direction of the mica substrate, the symmetry group of thewhole system, G2D&Mica, must be the largest subgroup of both GMica and G2D. Given
all possible combinations of the symmetries of the 2D material and the mica substrate, the number of equivalent but distinct alignments of various 2D materials on
mica {001} surfaces are summarized (Table 1). Consequently, according to our2242 Matter 7, 2236–2249, June 5, 2024

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ArticleTable 1. Summary of the number of equivalents
G2D

C6V

C4V

C3V

C2V

GMica

C6V

C6V

C6V

C6V

G2D&Mica

C6V

C2V

C3V

C2V

|GMica|

12

12

12

12

|G2D&Mica|

12

4

6

4

NE

1

3

2

3

The number of equivalent but different orientations of a 2D material on a mica substrate is based on theinterplay between their symmetries.

experimental observation and theoretical analysis (Figure 3E), we believe that the NE
for most 2D materials synthesized on mica can be reasonably predicted, providing adesirable experimental design guideline before the general synthesis of 2D materials on the vdWs mica dielectric.
High-precision manipulation of the vdWs dielectric
After demonstrating the superiority of the oriented growth of 2D materials, devicelevel implementations based on as-grown 2D materials/vdWs dielectrics to reveal
the ‘‘true’’ properties of as-grown 2D materials are further explored, utilizing vdWs
mica/SiO2 as the hybrid dielectric layers. Figure 4A illustrates the device fabrication
process of the FETs, in which a high-fidelity transfer method is developed by utilizing
the thermal releasing tape to assist in the fabrication of hybrid mica/SiO2 dielectric
layers. Figure 4B depicts the schematic of the backgated FET, and its optical and
AFM images demonstrate the thicknesses of the Bi2O2Se layer and mica dielectric
layer with atomic-scale flatness as 3.4 and 100 nm, respectively (Figure S19). It is
worth mentioning that the thickness of the as-grown Bi2O2Se/vdWs mica dielectric
heterostructures is controlled by the exfoliation time with blue tape before thermallyreleasing onto SiO2. As shown in Figure 4C, the corresponding transfer curves of thedevice on a logarithmic scale reveal a record-high current on/off ratio of 1.4 3 107 at
a relatively small source/drain bias of VDS = 20 mV. Figure 4D exhibits the linear
output curves of the device at VGS sweeping from 15 to 15 V, demonstrating theohmic contact between the Bi2O2Se semiconductor channel and Au electrodes.
Compared to many state-of-the-art 2D-material-channeled FETs integrated with various
dielectrics (Figure 4E), our Bi2O2Se-based FETs with hybrid mica/SiO2 gate dielectrics
show high mobility of 22.3 G 1.2 cm2 V1 S1 for a channel thickness down to 3.4 nmof about 6 layers (Figures S20 and S21), which is superior or comparable to the mostwidely investigated non-vdWs dielectrics, e.g., Al2O3, SiO2, HfO2, etc. (Table S2). Thishigh carrier mobility of the ultrathin Bi2O2Se flake not only demonstrates the high qualityof our CVD-grown ultrathin 2D Bi2O2Se materials but also shows the low interface trap
density and coulomb impurity concentration as compared to the other device techniques(Figures S22–S24). Figure 4F gives the high current on/off ratio (1.4 3 107) of our Bi2O2Se
FETs, which is significantly better or comparable to many promisingly 2D materials,
including TMDCs, black phosphorus (BP), b-Ga2O3, etc., demonstrating the excellent
coupling of as-grown Bi2O2Se flakes on the transferable hybrid dielectrics. By leveraging
the as-grown Bi2O2Se on its original vdWs epitaxial dielectric, the FETs exhibit improved
performance attributes and mitigate the detrimental effects that can arise from additional
device fabrication steps, showing their superiority in reducing the performance degradation of the CVD-grown 2D semiconductors (Figure S25). Considering the high transparency, excellent flexibility, and suitable insulating properties of mica, this research lays thefoundation for potential device-level studies on the transferable dielectric of mica, e.g.,
flexible transparent electronics.

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ArticleFigure 4. Device fabrication and performance evaluations(A) Device fabrication process of the Bi 2 O2 Se FETs using mica/SiO2 as hybrid dielectric layers via a high-fidelity transfer method assisted by the thermal
releasing tape (TRT).
(B) Schematic illustration of the back-gated FETs where the Bi 2 O2 Se semiconductor and mica/SiO 2 hybrid layer are used as the transistor channel andgate dielectric, respectively.
(C) Transfer curves of the mica-based Bi 2 O2 Se FETs as V DS increases from 0.02–0.18 V, revealing typical n-type characteristics with a current on/off ratioof 1.4 3 10 7 .
(D) Output characteristics of the Bi2 O 2 Se FET as V GS increases from 15 to 15 V.
(E) Mobility of our Bi2 O 2 Se-based FETs with the hybrid mica/SiO 2 gate dielectric compared to other state-of-the-art 2D FETs integrated with various
dielectrics. 48–65(F) On/off ratio of the device at a relatively small bias of V DS = 0.02 V as gate voltages sweep from 15 to 15 V, and the performance comparison with thestate-of-the-art 2D FETs based on distinct categories of 2D materials, e.g., TMDCs, BP, b-Ga 2 O 3 , etc. 30,48,57,60–70

Conclusions
In summary, we investigate the aligned epitaxy of 2D materials and develop amethod for the high-fidelity manipulation of as-grown 2D materials on the vdWs
mica dielectric at the device level. The aligned epitaxy of distinct CVD-grown 2D
materials is realized by adopting the hydromechanical strategy. Simcenter STAR

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CCM+ simulation is employed to understand the enhanced precursor controllability
during the CVD synthesis. In addition, the insight into the epitaxy relationship between our 2D products and vdWs mica dielectrics is uncovered by DFT calculations,demonstrating the energy-favorable directions of 2D clusters on mica dielectrics.
Specifically, combined with Lagrange’s theorem of the group theory, the numberof equivalent but different directions of a 2D material on mica is first established,potentially applicable to many other 2D epitaxy systems. More importantly, detailed
electrical device characterization is performed for the high-fidelity manipulation of
the as-grown Bi2O2Se-based device with hybrid dielectrics, exhibiting a record
high current on/off ratio of 1.4 3 107 and excellent carrier mobility of 22.4 cm2V1 S1. Overall, this work provides a powerful methodology platform for synthesizing the aligned 2D materials, predicting the alignment direction, evaluating theirintrinsic properties, and laying the foundation for probing device functionalities,e.g., flexible transparent electronics.

EXPERIMENTAL PROCEDURES
Resource availability
Lead contact
Further information and requests for resources and reagents should be directed to
and will be fulfilled by the lead contact, Johnny C. Ho (johnnyho@cityu.edu.hk).
Materials availability
This study did not generate new unique reagents.
Data and code availability
Any additional information required to reanalyze the data reported in this paper isavailable from the lead contact upon request.

MATERIALS SYNTHESIS AND CHARACTERIZATION
Bi2O2Se flakes were synthesized by the CVD method using Bi2Se3 (Aldrich, 99.99%)
and Bi2O3 (Thermo Scientific, 98%) powders as the precursors.71,72 The quartz boatof the Bi2O3 side was put in the center of the heating zone, and the mica substrate
was placed at a proper downstream position accordingly. The furnace was heated to635 C–650 C with a ramping time of 10 min and a holding time of 10–25 min under124 torr. Ahead of the heating process, the furnace tube was flushed with high-purity argon gas to thoroughly exhaust the atmospheric moisture and oxygen. After
high-temperature deposition, the furnace was naturally cooled to room temperature. The topological morphologies of the materials were characterized by opticaland atomic force microscopes (Dimension Icon, Bruker). Raman spectra werecollected by a confocal microscope spectrometer (Alpha 300R, WITec).
Simcenter STAR CCM+ simulation
As the flow velocity in the tubular furnace remains relatively flat, a laminar flow model
coupled with dilute material transfer modules was selected to simplify the numericalcomputation for mass transport in the CVD systems. The detailed models for the
related Simcenter STAR CCM+ simulation are shown in Figure S26. Except for thebasic geometries of the two systems, other key parameters, e.g., pressure, carriergas, etc., were consistent with the experimental condition provided in the materials
synthesis and characterization section. It is worth noting that the precursor with avolume fraction of precursor gases (1%) was viewed as a continuous point source
diffusion model (which is denoted as a normalized unit concentration). A balanced
concentration distribution of the precursor vapor on growth substrates was obtainedusing the computational fluid dynamics solver.

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ArticleDFT calculations of the binding energies
The DFT calculations were performed with the Vienna Ab-initio Simulation Packagecode with the full-potential projected augmented wave formalism.73,74 The generalized gradient approximation of the Perdew-Burke-Ernzerhof functional was used todescribe the exchange and correlation functions.75 The structure was relaxed until
the atomic force was less than 0.01 eV Å1, and the self-consistent calculation was
terminated when the energy converged to 104 eV. A 2 3 2 unit cell of mica and a
2 3 2 unit cell of Bi2O2Se or Bi2Se3 were constructed to match each other. To eliminate the perturbation from neighboring slabs, a vacuum layer of 20 Å was used. The
vdWs interaction between matched layers was considered by the DFT-D3 method of
Grimme with a zero-damping function.76 The cutoff energy for the plane-wave
expansion was set to 450 eV, and a k-point mesh of 3 3 3 3 1 was employed to sample the Brillouin zone.
To separately calculate the interaction between 2D products (in-plane Bi2O2Se, inclined Bi2O2Se, and Bi2Se3 nanoflakes), different pristine clusters with their typicalgeometry shapes were stacked onto a mica (001) surface under periodic boundary
conditions and with different alignment angles (5 per step). During the geometry
optimization, the top atomic layer of the K metal slab was fixed, as the edges of
the clusters are generally self-passivated. The binding energy between the calculated 2D materials and the mica substrate is defined as
Eb = Etotal  E2D  Emica ;where Etotal, E2D, and Emica are the energies of the whole system, the 2D materials
layer, and the mica substrate, respectively.
Device fabrication and evaluation
For the back-gated FETs, the as-grown Bi2O2Se flakes were detached from mica andtransferred to the desired dielectric substrate by a poly(methyl methacrylate)(PMMA)-assisted wet-transfer method (Figure S13). The Au electrodes were
patterned by electron-beam lithography and thermally evaporated with a thicknessof 40 nm. The electronic performance was characterized by a semiconductor
analyzer (Agilent 4155C) and probe station.

SUPPLEMENTAL INFORMATION
Supplemental information can be found online at https://doi.org/10.1016/j.matt.
2024.04.013.

ACKNOWLEDGMENTS
This research was financially supported by a fellowship award from the Research
Grants Council of the Hong Kong Special Administrative Region, China (CityU
RFS2021-1S04), and the Shenzhen Municipality Science and Technology Innovation
Commission (grant no. SGDX2020110309300402 [‘‘Modulation and Detection of
Terahertz Waves based on Semi-Metallic Two-Dimensional Materials,’’ CityU]).

AUTHOR CONTRIBUTIONS
Weijun Wang, Y.Z., and Wei Wang contributed equally to this work. J.C.H. and Weijun Wang conceived the project. Y.Z. and Wei Wang performed the DFT calculations
and Simcenter STAR CCM+ simulation. Weijun Wang, Y.Z., and Wei Wang conducted AFM, Raman, and XPS characterization and analysis. Weijun Wang, M.L., andY.M. fabricated the devices and performed the electrical tests and data processing.
B.L., Y.Y., D.Y., P.X., D.L., D.C., Q.Q., and S.Y. helped to prepare the samples and2246 Matter 7, 2236–2249, June 5, 2024

ll

Articleperform the basic material characterizations. W.H., J.C.H., and Weijun Wang wrote
the paper. All authors reviewed and commented on the manuscript.

DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: January 3, 2024
Revised: February 23, 2024
Accepted: April 7, 2024
Published: May 7, 2024

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