chemical vapor deposition growth of large-scale hexagonal ... · chemical vapor deposition growth...

Nano Res

1

Chemical vapor deposition growth of large-scale

hexagonal boron nitride with controllable orientation

Xiuju Song1, Junfeng Gao3, Yufeng Nie1, Teng Gao1, Jingyu Sun1, Donglin Ma1, Qiucheng Li1, Yubin

Chen1, Chuanhong Jin4, Alicja Bachmatiuk5, Mark H. Rümmeli6,7, Feng Ding3 ( ), Yanfeng Zhang1,2 (),

and Zhongfan Liu1 ( )

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0816-9

http://www.thenanoresearch.com on May 15, 2015

© Tsinghua University Press 2015

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DOI 10.1007/s12274-015-0816-9

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Chemical Vapor Deposition Growth of Large -scale

Hexagonal Boron Nitride with Controllable

Orientation.

Xiuju Song, Junfeng Gao, Yufeng Nie, Teng Gao,

Jingyu Sun, Donglin Ma, Qiucheng Li, Yubin Chen,

Chuanhong Jin, Alicja Bachmatiuk, Mark H. Rümmeli,

Feng Ding*, Yanfeng Zhang*, Zhongfan Liu*

Center for Nanochemistry (CNC), Peking University,

People’s Republic of China

Wafer-scale high-quality h-BN monolayer film is obtained with the

largest domain sizes up to 72 μm using a folded Cu enclosure

approach. The orientations of as-grown h-BN monolayers are strongly

correlated with the underneath Cu crystalline facets, with the Cu (111)

being the best substrate for growing high-quality single crystalline

h-BN monolayer.



Xiuju Song1, Junfeng Gao3, Yufeng Nie1, Teng Gao1, Jingyu Sun1, Donglin Ma1, Qiucheng Li1,

Yubin Chen1, Chuanhong Jin4, Alicja Bachmatiuk5, Mark H. Rümmeli6,7, Feng Ding3( ), Yanfeng

Zhang1,2( ), Zhongfan Liu1( )

3

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

hexagonal boron nitride,

Cu foil, domain size,

orientation, CVD

ABSTRACT

Chemical vapor deposition (CVD) synthesis of large-domain hexagonal boron

nitride (h-BN) with uniform thickness is of great challenge, mainly originating

from the extremely high nucleation density. We report herein the successful

growth of wafer-scale high-quality h-BN monolayer films with large

single-crystalline domain size up to ~72 μm in edge length using a folded Cu

enclosure approach. The highly-confined growth space and smooth Cu surface

inside the enclosure enable the effective suppression of precursor feeding rate

together with a drastic decrease of nucleation density. The orientations of

as-grown h-BN monolayer are found to be strongly correlated with

crystallographic orientations of Cu substrates, with Cu (111) being the best

substrate for growing aligned h-BN domains and even single-crystalline

monolayers, consistent with density functional theory calculations. The present

study offers a practical pathway for growing high-quality h-BN films by

deepening our fundamental understanding of its CVD growth process.

1 Introduction

Two-dimensional materials have received

increasing attention since the discovery of graphene

[1-3]. Specifically, hexagonal boron nitride (h-BN), a

structural analogue of graphene, possesses only a

1.8% lattice mismatch with graphene but has a large

band gap (~5.9 eV). The combinations of graphene

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2 Nano Res.

and h-BN, including both in-plane h-BN-graphene

hybrids and stacked graphene/h-BN (G/h-BN)

structures, have been demonstrated very intriguing

physical properties such as Hofstadter’s butterfly

[4-10]. In particular, G/h-BN vertical stacks exhibit

the excellent field effect transistor performance with

extremely high carrier mobility [11]. As the perfect

dielectric layer, h-BN has an atomically flat and

dangling bond-free surface, which ensures no charge

traps at the G/h-BN interface and results in an order

of magnitude increase of graphene’s carrier mobility

as compared to the typical SiO2/Si substrate [12, 13].

h-BN has also stimulated various applications in

deep ultraviolet light emitters [14], protective

coatings [15], and transparent electronics [16], due to

its excellent mechanical strength, chemical inertness

as well as nice optical transparency. These attractions

of h-BN have ignited numerous efforts on its

synthesis in the past few years, targeting uniform

thickness, large domain size and high crystallinity.

Indeed, the chemical vapor deposition (CVD)

growth of h-BN has been explored by using a variety

of transition metals as substrates, such as Ni films, Pt

foils, Fe films and Cu-Ni alloys [17-24]. Cu foil is the

most common substrate for h-BN growth due to its

low cost, commercially easy availability and

well-behaved catalytic performance for obtaining

high-quality h-BN films. Kim et al. pioneered the

monolayer h-BN growth on Cu foil via a low

pressure CVD (LPCVD) route and obtained h-BN

triangles with ~1 μm in edge length [25]. By

employing electropolished Cu foils, Teo et al.

obtained h-BN hexagons with a maximum edge

length of ~5 μm very recently, attributable to a

reduced nucleation density [26]. Nucleation sites can

also be suppressed simply by increasing the

pre-annealing time of Cu foil up to 6 h, which

resulted in the largest h-BN monolayer triangles with

an edge length of ~20 μm [27]. However, the

challenges still remain with respect to h-BN/Cu

synthesis including the uniformity control, thickness

control and domain size enlargement, which are

crucial for various applications of h-BN, especially in

high-performance G/h-BN devices.

In this work, we demonstrate the large

single-crystalline monolayer h-BN domain

synthesized on Cu foils by using an LPCVD

technique. The key to its success lies in effectively

suppressing the nucleation density during the CVD

growth process by using the inner surface of a folded

Cu foil enclosure as the substrate. Though Cu

enclosure has been used in graphene growth [28-30],

this is first time to be employed in h-BN growth.

Triangular-shaped single crystal h-BN flakes with a

domain size up to ~72 μm in edge length and a high

monolayer percentage up to 92% have been obtained

through kinetic control of the Cu-CVD process. More

importantly, we find that the orientations of the

as-grown h-BN flakes are strongly correlated to the

underlying Cu crystalline facets. In other words, the

symmetry of the Cu facet (representatively (111),

(110), or (100)) greatly affects the orientations of the

h-BN monolayers on it. The edge of an h-BN domain

tends to be aligned along a high symmetry direction

of the crystal facet, as evidenced by atomically

resolved scanning tunneling microscopy (STM)

images with a uniform large moiré (>10 nm)

formation, consistent with supporting density

functional theory (DFT) calculations. The Cu (111)

single crystal is found to be the ideal surface for

growing well-aligned h-BN domains. The present

study certainly provides a future direction for

growing high-quality h-BN films as well as

deepening the fundamental understanding of the

Cu-CVD process.

2 Results and Discussion

As schematically illustrated in Figure 1a, h-BN was

grown on polycrystalline Cu foils via a LPCVD

method by using ammonia borane (BH3-NH3) as the

precursor. Prior to the CVD growth, the Cu foil was

electropolished to reduce the surface roughness and

remove attached contaminations, followed by folding

it into an enclosure shape (Figure S1 in the Electronic

Supplementary Materials (ESM)). The BH3-NH3

precursor was put into a specially-designed

half-opened quartz cell and then loaded into the

CVD growth tube, where a heating belt was wrapped

around to aid the sublimation of BH3-NH3 at a

precursor evaporation temperature (Tp) range of

65 °C ~ 120 °C. The BH3-NH3 was sublimated and

decomposed into (BH2NH2)n, (BHNH)3 and hydrogen

(H2) [31], which were further pyrolyzed into B- and

N-containing intermediate species at the hot zone of

the reaction tube for h-BN growth. Before feeding

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

3 Nano Res.

with the precursors, the Cu enclosure was annealed

at 1000 °C for 2 h in a flow of 20 sccm H2 and 50 sccm

Ar. The schematic view in Figure 1b illustrates the

surface growth process of h-BN on Cu foils.

Figure 1 Large-area synthesis and characterizations of h-BN films. (a) Experimental setup of LPCVD. (b) Schematic diagram of the

formation process of h-BN flake on Cu foils. (c) SEM image of large-domain h-BN triangle showing the edge length of ~72 μm. (d)

Optical microscope image of transferred h-BN film on SiO2/Si. (e) AFM image of h-BN film transferred onto SiO2/Si. The white line

profile shows a typical thickness of monolayer h-BN on SiO2 (~0.7 nm); the red and blue rectangles indicate the roughness-measuring

area at SiO2 and h-BN, respectively. (f) Wafer-scale monolayer h-BN film. (g) XPS spectra of N 1s (left) and B 1s (right) of h-BN film

with binding energy peaks at 398.1 eV and 190.5 eV, respectively. (h) Raman spectrum of h-BN on SiO2/Si with a typical peak at 1369

cm-1. (i) UV-Vis absorption spectrum of h-BN film with a calculated band gap of 5.9 eV.

The scanning electron microscope (SEM)

micrograph in Figure 1c depicts the h-BN triangular

flakes grown for 120 min at 1000 °C with a Tp

temperature of 65 °C, which show a darker contrast

with regard to the bare Cu substrate. The middle

triangle presents a maximum edge length of ~72 μm,

larger than that reported on the Cu foils [27]. With an

increase of growth time, these individual flakes can

gradually merge into a continuous layer, enabling

full coverage of monolayer h-BN on Cu foils. The

optical microscope (OM) image in Figure 1d displays

an h-BN film after transferred onto 280 nm SiO2/Si

via a conventional wet etching technique [32]. A

uniform contrast can be seen for the h-BN covered

area, suggesting the formation of a uniform h-BN

layer. The atomic force microscopy (AFM) image of

the transferred sample reveals a thickness of ~ 0.7 nm,

as shown in Figure 1e. The surface roughness of

h-BN film is measured to be 0.145 nm, lower than

that of the SiO2/Si substrate (0.166 nm) [25, 33].

Further statistical analysis of AFM height

distribution (48 points in total, Figure S2 in the ESM)

manifests a thickness fluctuation between 0.5~0.9 nm,

well consistent with that for typical monolayer h-BN

[26]. These observations confirm the monolayer

nature of the CVD-grown h-BN film. Wafer-scale

h-BN film has been grown in such a way with over

92% coverage (Figure 1f and Figure S2 in the ESM).


4 Nano Res.

Spectroscopic characterizations, including X-ray

photoemission spectroscopy (XPS), Raman

spectroscopy and UV-visible spectroscopy (UV-Vis),

were carried out to determine the elemental

composition and stoichiometry, lattice vibration

modes as well as band gap information of the

obtained monolayer h-BN. Two characteristic XPS

peaks located at 398.1 eV and 190.5 eV, which can be

assigned to N 1s and B 1s signals, respectively, were

observed. The N/B ratio is estimated to be 1.09,

indicative of the predominant B-N chemical bonding

in the h-BN lattice (Figure 1g and Figure S3 in the

ESM). Raman spectrum of the h-BN film on a SiO2/Si

substrate shows a characteristic peak at ~1369 cm-1,

originating from the boron-nitrogen bond stretching

of monolayer h-BN (Figure 1h) [34]. The full width at

half maximum of the peak is ~25 cm-1, likewise

suggesting the monolayer feature and high

crystallinity [34]. The optical band gap measured

from UV-Vis spectroscopy of the h-BN monolayer

transferred onto quartz substrate is 5.9 eV as shown

in Figure 1i, which is very close to the theoretical

value (6.0 eV) [35].

Figure 2 Effect of precursor evaporation temperature on the CVD growth of h-BN with Cu enclosure approach. (a-e) SEM images of

h-BN triangles grown at different Tp temperatures and growth time: (a) 120 °C, 2 min; (b) 100 °C, 5 min; (c) 70 °C, 90 min; (d) 65 °C,

120 min; (e) 55 °C, 180 min, respectively. (f) SEM image of a continuous monolayer h-BN obtained at 70 °C Tp temperature for 2 h. (g)

Change of nucleation density as a function of Tp. (h, i) SEM image of h-BN films obtained on the outer (h) and inner (i) surfaces of Cu

enclosure with Tp=70 °C for 30 min, respectively. (j, k, l) AFM images of the outer (j) and inner (k) surfaces of Cu enclosure after

annealing at 1000 °C for 1 h and their height profile.

It is a general trend that, the h-BN film grown on

Cu foils is of polycrystalline nature with small grains

and highly concentrated grain boundaries and

defects [27]. This is attributed to the extremely large

nucleation centers created in the CVD growth

process. Compared with the Cu-CVD-graphene

process, the nucleation density on Cu foils during

h-BN growth is generally much higher, which is most

possibly attributed to the high chemical affinity of

N-containing intermediate species to the Cu surface


5 Nano Res.

[36, 37]. In general, many factors determine the

nucleation density of h-BN on Cu foils, including

precursor evaporation temperature and rate,

substrate roughness, growth temperature, and

external impurities. Our systematic investigations

indicate that the feeding rate of precursors is a key

factor for nucleation density control. As a solid

precursor source was used, the amount of precursor

was fixed at 0.04 g and the feeding rate was adjusted

by varying the evaporation temperature of the

precursor cell. Figures 2a-e exhibit the SEM images of

h-BN monolayer triangles obtained on the inner

surface of Cu enclosure at different Tp temperatures

and hence feeding rates. At a 120 °C Tp temperature,

and a growth time of 2 min, h-BN triangles with an

average edge length of 2.8 μm were most available,

corresponding to a nucleation density of ca. 1.7×105

mm-2 (Figure 2a). When the Tp temperature was

decreased to 100 °C, h-BN triangular flakes with an

average edge length of 6.6 μm were obtained after 5

min growth (Figure 2b). If further reducing the Tp

temperature to 70 °C, the edge length of the h-BN

triangles increased to ~9.2 μm (Figure 2c) at a growth

time of 90 min. Further reducing the Tp temperature

to 55 °C, almost no h-BN was observed within a

growth time up to 3h (Figure 2e). The optimized Tp

temperature was 65 °C, with which large h-BN

triangles of ~50 μm in edge length were synthesized

for 120 min (Figure 2d). The nucleation density in

this case was estimated to be 2.7×103 mm-2, about two

orders of magnitude lower than the 120 °C

evaporation case. Simply increasing the growth time

only led to the formation of continuous monolayer

film with a full surface coverage and even

multilayers (Figure 2f and Figure S4 in the ESM). The

nucleation density on the inner surface of Cu

enclosure is plotted in Figure 2f against Tp

temperature, indicating that decreasing the Tp

temperature can effectively reduce the nucleation

density. On the other hand, when the growth time

was fixed at 2 h and the Tp temperature was changed

from 60 °C to 75 °C, submonolayer, monolayer and

multilayer h-BN could be obtained, respectively

(Figure S5 in the ESM).

The above experimental observations strongly

suggest that controlling the precursor feeding rate is

crucial for suppressing the nucleation of h-BN.

Indeed, the use of a Cu enclosure approach is the key

for drastically decreasing the feeding rate of

precursor species by a few orders of magnitude. To

achieve a deep understanding of the role of the Cu

enclosure, we examined both the outer and inner

surface of the Cu enclosures. As seen from the SEM

images in Figures 2h and i, very few h-BN triangles

exist on the inner surface of the Cu enclosure as

compared to that on the outer surface. As the growth

proceeds, h-BN flakes merged into a fully covered

monolayer on the outer surface (Figure S6a in the

ESM) while large h-BN triangles were obtained on

the inner surface (Figure S6b in the ESM). A uniform

h-BN monolayer can also be obtained on the inner

surface after prolonged growth (Figure S6c in the

ESM) while multilayer h-BN film was already

observed on the outer surface (Figure S6d in the

ESM). From these observations, it can be concluded

that the Cu enclosure approach can effectively reduce

the precursor concentration, nucleation density and

growth rate of h-BN on the inner surface.

AFM was used to obtain the morphology

information of Cu enclosure on the inner and outer

surfaces. As shown in Figures 2j and k, the inner

surface was much smoother than the outer surface.

The average roughness of the outer surface was

estimated to be ~3.92 nm (Figures 2j and l) while that

of the inner surface was only ~1.08 nm (Figures 2k

and l). The considerably high outer surface

roughness was attributed to the thermal evaporation

of Cu atoms at high temperature [29]. For the inner

surface of Cu enclosure, the evaporative loss of Cu

atoms was strongly suppressed by re-deposition

effect in the limited space. Obviously, such a

remarkable morphology difference on the outer and

inner surfaces resulted in the difference in nucleation

density and hence growth results. In addition, a

numerous number of BN nanoparticles were

observed on the outer surface of Cu enclosure

(Figures S7a in the ESM). In contrast, the inner

surface of Cu enclosure was very clean, featured with

large h-BN triangles under the same experimental

conditions (Figures S7b in the ESM).

In brief, the Cu enclosure approach has the

following three advantages for achieving

high-quality h-BN monolayer growth: (a) drastically

reducing the feeding rate of precursors and hence the

nucleation density; (b) effectively suppressing the Cu

loss and providing a smooth surface; (c) preventing


6 Nano Res.

the Cu surface from external contaminations.

Figure 3 TEM characterizations of the atomic lattice, thickness and domain orientation of h-BN films. (a) Bright-field TEM image of an

h-BN flake. (b) Overlaid SAED patterns randomly taken on the h-BN triangle shown in (a). (c) Atomic-resolution TEM image of the

h-BN film. (d) High-resolution TEM image of a folded edge reconfirming the monolayer nature of the h-BN film. (e) EELS spectrum of

the h-BN film. (f) False color dark-field TEM image and corresponding SAED pattern of an h-BN triangle (g). (h) False color dark-field

TEM image of two triangles merged with the same orientation and the corresponding SAED pattern (i). (j) False color dark-field TEM

image of mirror-twins with the corresponding SAED pattern in (k). (l) Dark-field TEM image of merged h-BN flakes with a relative

rotation of ~21°, as proved by the SAED pattern (m).

Transmission electron microscopy (TEM)

combined with selected area electron diffraction

(SAED) and electron energy loss spectroscopy (EELS)

were employed to probe the layer thickness,

crystallinity, and elemental stoichiometry of the

obtained h-BN flakes. Transmission electron

microscopy (TEM) combined with selected area

electron diffraction (SAED) and electron energy loss

spectroscopy (EELS) were employed to probe the

layer thickness, crystallinity, and elemental

stoichiometry of the obtained h-BN flakes. Figure 3a

displays a bright-field TEM image of an h-BN

triangle (with edge length of ~40 μm) transferred

onto TEM grid. SAED patterns recorded at five

random positions (marked by red dots in Figure 3a)

on this h-BN triangle were overlaid with an image

processing tool into one frame shown in Figure 3b.

Apparently, the only one set of six-fold symmetric

diffraction pattern with sharp spots justifies the large

area uniform single crystalline nature [27]. Figure 3c


7 Nano Res.

shows the clear atomic-resolution TEM image of

h-BN with the lattice constant of ~0.25 nm, the same

as that of bulk h-BN [37]. Moreover, the

high-resolution TEM image on the film edge (Figure

3d) with a line shape contrast also verifies the single

layer feature of the as-grown h-BN film. To

investigate the chemical composition of the sample,

an EELS spectrum was recorded (Figure 3e) which

show the representative peaks of boron and nitrogen

K-shell ionization edges with the characteristic π*

and σ* energy loss peaks at boron and nitrogen,

indicating the sp2 hybridization nature of the h-BN

flake [38].

Figure 4 Orientation dependence of h-BN triangles on Cu crystalline facet. (a) EBSD mapping of polycrystalline Cu foil. (b)

Corresponding SEM image of the as-grown h-BN on polycrystalline Cu foil. (c) X-ray diffraction pattern of Cu foil after growth,

consisting of three facets: Cu (111), Cu (100), Cu (110). (d-f) Representative SEM images of h-BN grown on Cu (111), Cu (100) and Cu

(110), respectively. (g-i) Statistical distributions of the edge angles of individual triangular h-BN domains grown on Cu (111), Cu (100)

and Cu (110), respectively.

Dark-field TEM (DF-TEM) was then employed to

examine the orientation of h-BN triangular flakes and

their aggregates. Figure 3f displays a false color

DF-TEM image of an equilateral h-BN triangle with

all perfect interior angles of 60°, indicative of its

single crystalline nature (Figure 3g). During CVD

growth, such kinds of h-BN triangles gradually

expand their sizes and finally merge with each other,

forming an entire film. Apparently, the orientations

of these triangles will determine the merging

boundaries and the finally-formed polycrystalline

films. One of the typical merging behavior is shown

in Figure 3h, in which two perfectly aligned flakes

coalesce together. The corresponding overlaid SAED


8 Nano Res.

pattern exhibits only one set of hexagonal spots

(Figure 3i), suggesting their identical crystalline

orientation. Another frequently-observed case for

merging is the coalescence of two triangles with 180°

rotation forming a mirror-twinned structure (Figure

3j) [39]. Their SAED patterns surprisingly show only

one set of hexagonal spots as seen in Figure 3k,

indicating the precise alignment of two triangles with

an edge angle of 60°. The above experimental

observations suggest that the h-BN triangles with the

same orientation or with a relative rotation of 180°

are able to coalesce into a well-aligned film, which

contribute to the high crystalline quality with

reduced grain boundaries. Moreover, misoriented

polygonal h-BN flakes can also be occasionally

detected. The DF-TEM image in Figure 3l clearly

displays the merging of two single-crystalline

domains with different orientations (marked in

different colors). The relative rotation can be

determined from the corresponding SAED pattern,

which are ~21° in Figure 3m. Apparently, grain

boundaries are created between these misaligned

domains, leading to the polycrystalline nature of the

h-BN film. It is hence a natural conclusion that

controlling the orientations of individual triangles is

the prerequisite for achieving a single-crystalline

h-BN monolayer film.

Figure 5 DFT calculations of the binding energies for h-BN flakes on different Cu facets. (a, b) Calculated binding energies between

h-BN flakes and Cu substrates as a function of angle to the close-packing directions of Cu (111) and Cu (100), respectively. (c)-(h)

Schematic orientations corresponding to two maximum binding energies of h-BN flakes on Cu (111) (c, d) and four binding energies

peaks on Cu (100) (e-h), respectively. The insert images show the three equivalent close-packing directions of Cu (111) (i) and two

equivalent close-packing directions of Cu (100) (j).

On a highly polycrystalline surface, the

orientation of adlayer is usually affected by the

crystalline facet [40]. The dependence of h-BN

growth behavior on different crystalline facets of Cu

foil was systematically examined by employing

electron backscatter diffraction mapping (EBSD) and


9 Nano Res.

SEM. Figure 4a shows the EBSD map of a typical area

on Cu foils (corresponding SEM image shown in

Figure 4b), which displays the coexistence of Cu (111),

Cu (100) and Cu (110) facets, consistent with the XRD

result shown in Figure 4c. The growth behaviors of

h-BN films on these crystalline facets are exhibited in

Figures 4d-f, respectively. Obviously, the nucleation

density and domain sizes of h-BN islands do not

show remarkable facet dependent behavior. However,

the orientation distributions of h-BN triangles are

distinctly different (Figures 4g-i). The h-BN triangles

are well aligned on Cu (111), where the difference in

relative orientation can be expressed as multiples of

60o (Figure 4d). The two dominant orientations of

h-BN on Cu (111) are 0° and 60°, as clearly seen from

the orientation distribution plot (Figure 4g). This

implies that the h-BN flakes may be aligned along the

lattice of the Cu (111) surface. In contrast, on Cu (100),

there are four typical orientations, i.e. 0°, 30°, 60°, 90°

(Figures 4e and h) with the alignment deviations of

±30°. Similarly, the h-BN flakes on Cu (110) facet have

six dominant orientations with the deviations of ±10°

(Figures 4f and i). These facet dependent orientations

of h-BN on Cu (100) and Cu (111) facets can be

attributed to the alignment of h-BN on the 4-fold and

6-fold symmetries of facets. However, the reason for

the six orientations on Cu (110) is not clear, possibly

caused by the reconstruction of Cu (110) facet under

the experimental conditions.

To achieve a deeper understanding of how the

symmetry of the Cu facet affects the orientation of

the grown h-BN, the interaction between the h-BN

flakes and Cu facets was studied by DFT calculations

(See Supporting information for details of the

computation). It is known that the inert h-BN wall

interacts weakly with the substrate surface (e.g., the

calculated height of h-BN monolayer to Cu (111)

surface is about 3.17 Å and the van der Waals

interaction between them is only 0.104 eV per atom

according to our calculations) and thus its orientation

should be determined by the edge-catalyst

interaction during the early stage of its growth,

similar as that for graphene CVD growth [41]. That

means the binding of an h-BN domain on metallic

substrate mainly occurs at the edge, and thus, the

average binding energy is a function of its perimeter,

1 2

3 3A A B B C CE E L E L E L

L

(1)

where E(θ) is the average bind energy (in eV/nm) of

an triangular h-BN flake on the substrate; EA(θ),EB

(θ+2π/3), EC(θ+2π/3) are the binding energies of its

three edges as shown in Figure 5c. L = LA + LB + LC is

the perimeter of the h-BN flake and LA, LB and LC are

the lengths of the three edges of the triangle. θ is the

angle between the A edge and one high symmetric

direction of the Cu surface. Due to the symmetry of

the h-BN, and corresponding direction of edge B and

C to the direction is θ+π/3 and θ+2π/3, respectively.

As shown in Figures 5i and j, θ is the angle between

edge A and the [-110] direction of Cu (111) or [00-1]

direction of Cu (100).

The binding energies of several h-BN edges with

different orientation angles (θ = 0.0°, 8.1°, 18.4°, 26.6°,

31.0°, 36.9°, 39.8°, 45.0° respectively on the Cu (100)

surface and θ = 0.0°, 6.6°, 19.1°, 23.4°, 26.3°, 30.0°,

respectively on the Cu (111) surface) were calculated

with the DFT method and shown in the Figures S8

and S9 of ESM. The binding energies of the h-BN

edge with other angles are obtained by linear

interpolation method and those with angles beyond

this range (0°-45° for Cu (100) surface and 0°-30° for

Cu (111) surface) can be obtained by simply

considering the symmetry of the system. Then the

binding energy of an h-BN triangular flake on Cu

surface is calculated by Eq. (1). As aforementioned,

the equilateral triangular h-BN flake has an

periodicity of 120°, E(θ)=E(θ+i×120°). Thus, as shown

in Figures 5a and b, the binding energies of h-BN

flakes on Cu (111) and Cu (100) versus the

orientation of the flake are plotted in the range of

0°-120°. It is distinct that there are two high binding

energy peaks for an h-BN triangle on the Cu (111) (0°

and 60°) and four peaks on the Cu (100) ((0°, 30°, 60°

and 60°), in excellent agreement with the

experimental observed orientations of the h-BN

flakes on them. The orientations of h-BN flakes

corresponding to these peaks are schematically

shown on the right panel of Figure 5. It is apparent

that all these h-BN flakes possess at least one edge

parallel to the close-packing direction of Cu surface

(labeled with white lines). This can be understood

that the close-packing direction has much dense Cu

atoms, which passivate the edge of h-BN more

effectively. Therefore, the highly populated

orientation angles of h-BN flakes on Cu (111) and Cu

(100) surfaces can be interpreted as the energetically


10 Nano Res.

preferred orientation of the h-BN flakes. This

indicates that the growth of h-BN on Cu foil follows

the edge-epitaxial growth mode [42], which may be

benefited from the low pressure environment on the

clean inner cavity of Cu enclosure.

Figure 6 Alignment of h-BN flakes with underlying Cu facet and large-area h-BN monolayers on Cu (111) single crystal. (a, b) Typcial

STM images of h-BN moiré pattern with a large period (~10.8 nm) showing the alignment of h-BN with Cu (111) facet (a: 300 nm×300

nm; VT=-0.123 V, IT=2.665 nA; b: 45 nm×45 nm; VT=-0.019 V, IT=2.582 nA). The inserted height profile along the indicated line

demonstrates the period of moiré pattern is up to 10.8 nm. (c) Schematic illustration of the non-rotated h-BN unit cell on Cu (111). (d)

Optical microscope image of h-BN transferred onto Cu grid. (e-i) SAED patterns taken randomly on (d) over 600×600 μm2.

To confirm the alignment of h-BN with underlying

Cu facets, STM was employed to investigate the

moiré and the atomic-scale feature of h-BN grown on

Cu foils. Considering the small lattice mismatch

between Cu (111) (2.556 Å ) and h-BN (2.500 Å ), a

specific moiré structure can evolve due to the lattice


11 Nano Res.

mismatch between h-BN and Cu (111), and different

moiré structures can also appear by changing their

relative orientation from oriented to disorientated

[43]. Figures 6a and b display the representative STM

images with h-BN moiré pattern (~10.8 nm in period),

indicating a less than 1° rotation of h-BN with Cu

(111). Figure 6c shows the schematic illustration of a

non-rotated h-BN unit cell on Cu (111). The

atomic-resolution STM image in the Figure S10a of

ESM demonstrates its high crystal quality. It is noted

that, most of the h-BN flakes are nearly aligned with

the underlying Cu (111) facet, forming a ~10.8 nm

moiré pattern, consistent with DFT calculation results.

As clearly seen from Figure S10b of ESM, when

grown on Cu (111) single crystal, nearly all the h-BN

triangles are aligned along the same or reversed

orientation. We transferred a fully covered h-BN

monolayer grown on Cu (111) onto Cu grids (Figure

6d) and obtained the SAED patterns at random

positions over 600×600 μm2. As shown in Figures 6e-i,

the SAED patterns exhibits less than 1.06° rotation of

the h-BN lattice throughout the entire film, indicative

of the excellent alignment of h-BN film along Cu (111)

crystalline facet.

3 Conclusions

In summary, high-quality h-BN monolayer films

with the large single-crystalline domain size up to

~72 μm in edge length have been achieved on Cu

foils using a LPCVD technique. The folded Cu

enclosure approach has been proved to perfectly

suppress the nucleation centers during CVD growth

process, which leads to the remarkable improvement

of single-crystalline domain size and preferential

monolayer growth. It is revealed that the orientations

of as-grown h-BN monolayers are strongly correlated

with crystallographic orientations of Cu substrates,

with Cu (111) being the best substrate for growing

high-quality single-crystalline h-BN monolayer films.

DFT calculations well explain these crystalline facet

effects. The present work provides a future direction

for growing high-quality h-BN monolayer films and

opens a practical pathway for high-performance

G/h-BN electronics.

Experimental method

h-BN growth: The growth of h-BN film was

performed on copper foils (25 μm in thickness; Alfa

Aesar; purity 99.8%) by using a low pressure

chemical vapor deposition technique. Prior to the

growth, copper foil was electrochemically polished

for 30 min to remove the surface impurities as well as

reduce the surface roughness. Ammonia borane

(BH3-NH3) precursor was placed inside a 1 inch

nested quartz tube. The furnace was ramped up to

1000 °C in 40 min, followed by sample annealing for

2 h in Ar (50 sccm) and H2 (50 sccm). BN precursor

was introduced by sublimation with the aid of a

heating belt (heating temperature range: 55~120 °C),

which could be delivered onto the Cu substrate by

Ar/H2 carrier gas for h-BN synthesis. After growth,

the furnace was cooled down to room temperature.

Transfer: As-grown h-BN was transferred onto

SiO2/Si substrates, quartz plates, and Cu grids after

growth with the aid of Poly (methyl methacrylate)

(PMMA). Briefly, PMMA was spin-coated onto the

sample and cured for 5 min on hot plate (180 °C),

followed by etching the Cu foil in iron chloride (FeCl3)

solution. The PMMA-supported film was then

repeatedly rinsed and washed by DI-water for

several times, after which the film was transferred

onto desired substrates, where the removal of PMMA

was achieved by using hot acetone vapor.

Characterization: The as-grown h-BN samples were

characterized by using scanning electron microscopy

(SEM, Hitachi S-4800, 2 kV), Electron backscatter

diffraction mapping (EBSD was collected using a

Hitachi S-4500 analytical SEM with Oxford

Technology EBSD System. During EBSD collection,

the probe current is 5 nA, the accelerating voltage is

20 kV, and the angle of incidence is 70 degrees.), STM

(Omicron UHV-VT-SPM-MBE System), and X-ray

photoelectron spectroscopy (XPS, Kratos Axis Ultra).

The transferred samples were examined by using

optical microscope (Olympus DP71), Raman

spectroscopy (Horiba HR-800, 457.8 nm laser

excitation), atomic force microscopy (AFM, Veeco

Nanoscope III, tapping mode), transmission electron

microscopy (TEM, FEI TecnaiF20; FEI Tecnai T20,

acceleration voltage of 200 kV), and UV-Vis

absorption spectroscopy (Perkin Elmer Lambda 950).

The atomic-resolution transmission electron

microscopy (TEM) investigations were taken on a

third-order aberration corrected (objective lens) FEI

Titan 300-80 operating with an acceleration voltage of

80 kV.


12 Nano Res.

Acknowledgements

The work was supported by the Natural Science

Foundation of China (Grants 51432002, 50121091,

51290272, 51222201), the Ministry of Science and

Technology of China (Grants 2013CB932603,

2012CB933404, 2011CB933003, 2011CB921903,

2012CB921404), and the Ministry of Education (Grant

20120001130010).

Electronic Supplementary Material: Further details

of XPS data, AFM, SEM, DFT calculations and STM

images regarding the h-BN sample is available in the

online version of this article at

http://dx.doi.org/10.1007/s12274-***-****-*

(automatically inserted by the publisher).

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Nano Res.

Electronic Supplementary Material



Xiuju Song1, Junfeng Gao3, Yufeng Nie1, Teng Gao1, Jingyu Sun1, Donglin Ma1, Qiucheng Li1,

Yubin Chen1, Chuanhong Jin4, Alicja Bachmatiuk5, Mark H. Rümmeli6,7, Feng Ding3( ), Yanfeng

Zhang1,2( ), Zhongfan Liu1( )

3

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

Figure S1 Photograph of a Cu enclosure.


Nano Res.

Figure S2 AFM height measurement of h-BN transferred onto SiO2/Si. AFM images of an h-BN triangle (a) and

near-fully covered h-BN film (c). (b) and (d) are the height histograms of green rectangular region in image (a)

and (c), respectively. (e) Distribution of height measurement of h-BN are measured at 48 regions, showing that

the thickness of 92% h-BN film is between 0.5-0.9 nm, which is consistent with the thickness of monolayer h-BN

films [1].

Figure S3 A survey XPS spectrum of as-grown h-BN on Cu foils. The existence of B 1s and N 1s peaks

demonstrates the presence of h-BN [2].


Nano Res.

Figure S4 Time dependence of h-BN growth on Cu foils. SEM images of h-BN grown on Cu foils at different Tp

temperatures and growth time: (a) 120 °C, 2 min; (b) 120 °C, 3 min; (c) 70 °C, 30 min; (d) 70 °C, 90 min; (e) 70 °C,

120 min.

Figure S5 Precursor evaporation temperature dependence of h-BN growth on Cu foils. SEM images of h-BN

grown on Cu foils for 2 h at different Tp temperatures: (a) 75 °C; (b) 70 °C; (c) 65 °C; (d) 60 °C.


Nano Res.

Figure S6 h-BN growth on the outer and inner surface of Cu enclosure. SEM images of h-BN grown on the

outer (a) and inner surface (b) of Cu enclosure with Tp=70 °C for 2 h. SEM images of h-BN grown on the inner (c)

and outer surface (d) of Cu enclosure with the Tp =100 °C for 3 min.

Figure S7 Contaminations on the outer surface of Cu foils. SEM images of h-BN grown on the outer (a) and

inner surface (b) of Cu enclosure with Tp=65 °C for 2h. There are a numerous number of BN nanoparticles were

observed on the outer surface of Cu enclosure.


Nano Res.

DFT calculations

First-principle calculations were performed by using the DFT and plane wave pseudopotential technique, as

implemented in the Vienna Ab-initio Simulation Package (VASP) [3, 4]. Generalized gradient approximation

(GGA) with the Perdew–Burke–Ernzerhof (PBE) functional [5] was used to describe the exchange-correlation

interaction.

Projector-augmented wave (PAW) method [6] was used to describe the core electrons. The plane-wave basis

kinetic energy cutoff of 400 eV and convergence criterion criteria of 10-4 eV were used in all the calculations. A

conjugate-gradient algorithm was used to relax the ions until the force was less than 0.02 eV/Å . The partial

occupancies for each wavefunction were determined by method of Methfessel-Paxton with an order of 1 and

the width of the smearing is 0.2 eV. In all calculations, the Brillouin zone was sampled with dense reciprocal

meshes (the separation is less than 0.2 Å -1).

Four-row wide zigzag BN ribbon with hydrogen-terminated B edges was put on the Cu surfaces. To evaluate

the interaction of h-BN edges with different orientations on Cu surface, six supercell with one co-periodic

dimension of h-BN zigzag periods and Cu direction were built for BN edges on Cu (111) surface. That is 1

zigzag periodic BN edge versus 1R0° direction (in related to the lattice vectors of [-110] and [01-1]) of Cu

subsurface (1ZZ@1R0°), 8 Zigzag periodic BN edge versus 57 R6.6° (8ZZ@ 57 R6.6°), and similar (8ZZ@

3 7 R19.1°), (9ZZ@ 2 19 R23.4°), (8ZZ@ 61 R26.3°) and (7ZZ@ 4 3 R30.0°) with same abbreviated formation.

Similar to on Cu (111), BN edges with eight orientations were sampled on Cu (100) surface, that is (1ZZ@1R0°),

(7ZZ@5 2 R8.1°), (10ZZ@3 10 R18.4°), (7ZZ@3 5 R26.6°), (6ZZ@ 34 R31.0°), ([email protected]°), (8ZZ@ 61

R39.8°), (3ZZ@ 2 2 R45.0°) versus lattice vectors of [01-1] and [011] of Cu (100) surface.

All the Cu surface is modeled with three layers metal slab with bottom layer fixed. In all calculations,

periodic boundary conditions (PBC) are applied along all the three directions. The vacuum space larger than 10

Å is adopted between the neighboring images to eliminate their interactions.

There are three close-packed directions of the family [-110] for Cu (111) surface and two close-packed

directions of the family [01-1] for Cu (100) surface. The binding energy between h-BN edge with an angel to

one close-packed direction of Cu surface was defined as:

E()= (Etot – Efree – Esub)/L

where Etot is the total energy of BN edges on Cu surfaces, Efree is energy of free BN ribbons with

hydrogen-terminated B edges in vacuum, spin-polarized DFT was used for free edge calculation. Esub is the

energy of Cu surface for each model, and L is the length of h-BN edge.


Nano Res.

Figure S8 Binding energies of h-BN edges with different angles on Cu (111). Binding energy as a function of

angle of h-BN edge to the close-packed direction on Cu (111) surface and six models of BN edges with different

orientations on Cu (111) surface. Considered on top layer of Cu (111) surface, E () = E (60° - ).

Figure S9 Binding energies of h-BN edges with different angles on Cu (100). Binding energy as a function of

angle of BN edge to the close-packing direction on Cu (100) surface and eight models of h-BN edges with

different orientations on Cu (100) surface. Considered on top layer of Cu (100) surface, E () = E (90° - ).


Nano Res.

Figure S10 h-BN growth on Cu (111) single crystal. (a) Atomic-resolution STM image of h-BN lattice (VT=-0.002

V, IT=9.498 nA). (b) SEM image of h-BN grown on Cu (111) single crystal with Tp=75 °C for 20 min.

Reference

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M.; Palacios, T.; Kong, J. Synthesis of monolayer hexagonal boron nitride on Cu foil using chemical vapor

deposition. Nano Lett. 2012, 12, 161-1616.

[2] Tay, R. Y.; Griep, M. H.; Mallick, G.; Tsang, S. H.; Singh, R. S.; Tumlin, T.; Teo, E. H.; Karna, S. P. Growth of

large single-crystalline two-dimensional boron nitride hexagons on electropolished copper. Nano Lett. 2014, 14,

839-846.

[3] Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors

using a plane-wave basis set. Comp. Mater. Sci. 1996, 6, 15-50.

Kresse, G.; Furthmüller, J. Efficient iterative schemes for \textit{ab initio} total-energy calculations using a

plane-wave basis set. Phys. Rev. B 1996, 54, 11169-11186.

[4] Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett.

1996, 77, 3865-3868.

[5] Blöchl, P. E., Projector augmented-wave method. Phys. Rev. B, 1994, 50, 17953-17979.

Address correspondence to [email protected]; [email protected]; [email protected]

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