



Chapter 1Graphene, Hexagonal Boron Nitride, and Heterostructure:  Properties

 and Applications
Chapter 1
Graphene, Hexagonal Boron Nitride, and
Heterostructure:  Properties and Applications
1.1Introduction to the 2D materials
 1.1.1Introduction to graphene

Graphene is a single layer of graphite sheet, constituting the basic unit of graphite, carbon nanotubes, fullerenes and other carbon materials (Figure 1げ1(a))［1げ2］. Before the experimental discovery of graphene, because of the effects of thermal expansion, theoretical and experimental circles believe that strict two dimensional(2D) crystals cannot be stable at finite temperatures. In 2004, Geim and Novoselov［3］, produced a single layer of carbon in atomsげlevel thickness by using micromechanical exfoliation (microexfoliation), studied electric field effect (Figure 1げ1(b)), and regarding the graphene, carried out a series of studies which broke the previous hypothesis［4げ7］. 


Figure 1げ1Graphene and carbon nanotube,and devices

(a) The basic unit of the other C material――graphene［1］;  (b) The relational graph between graphene and carbon nanotubes［7］;  (c) Graphene film and devices(Flow chart of graphene prepared by mechanical stripping method from A to E)［3］


Graphene is honeycombげdimensional crystals closely arranged by  sp2 hybridized carbon atoms, and its hexagonal geometry makes itюs structure very stable［8］. Each interlayer carbon atom bonds with the surrounding carbon atoms by sp2 hybridized, and contributes a nonげbonding electron to form a large ┪ bond, making electrons move freely between the layers.
Graphene is the thinnest and hardest nanoげmaterials［9］, its tensile strength is 125 GPa, and its elastic modulus is 1.1 TPa. The 2D ultimate plane strength is 42 N/m2. Carrier mobility is 2≠105 cm2/(V·s)［10げ11］, and it only affected by impurities and defects. Graphene’s thermal conductivity is up to 5.5 ≠103 W/(m·K)［12げ13］. Theoretically, graphene specific surface area is up to 2，630 m2/g. These unique physical properties make it widely apply to many areas of nanoelectronic devices, spin electronics, energy storage, and thermal conductivity materials.
Carbon nanotubes (deformation of graphene):  properties, compared with graphene.
Carbon nanotubes (CNTs) are oneげdimensional(1D) nanomaterials with a special structure［7,14］. It can be regarded as scale hollow tubular structure, which is made of a single or multiげlayer graphene sheet (Figure 1げ1(b)). According its number of layers, CNTs can be divided into SWNTs［15げ17］ and MWNTs［18］. According to the arrangement of carbon atoms in the cross section, the singleげwall,CNTs can be categorized into the  armchair and the zigzag［19げ20］. And again according to the electronic structures, SWNT can be categorized into the metallic and the semiげconductive,both being nonげintegral［21げ22］. 

The C = C covalent bond of carbon nanotubes makes their axial Youngюs modulus reaches 5TPa［20,23げ24］; 

 the lengthげdiameter ratio is as high as 104, the comparison area is greater than 1,500 m2/g［25げ26］,

 and the current carrying capacity is up to 109 A/cm2［27げ28］. 

 One of the optical properties of carbon nanotubes is wideげband absorption［29げ30］.

 As a good thermal conductivity, the axial thermal conductivity of carbon nanotubes is up to 6,600 W/(m·K), an excellent field emission characteristic, and its emission current mainly comes from the occupied states slightly lower than the Fermi level.
As representatives of 1D and 2D nanomaterials, while graphene is composed only by a single carbon atomic layer， that is the true sense of the 2D crystal structure. Compared with graphene, carbon nanotubes increase the total amount of carbon atoms, making themselves have lower energy of edge dangling bonds than graphene, which can stabilize the molecules in the air without the reaction with the air. From the performance standpoint, graphene has similar or more excellent characteristics than carbon nanotubes in conductivity, carrier mobility, thermal conductivity, freeげelectron moving space, strength and stiffness.
Graphene and carbonげnanotubes have different applications for many reasons, but ultimately can be attributed to the difference between 1D and 2D materials. For example, a single carbon nanotube can be regarded as a single crystal with high lengthげdiameter ratio;  however, the current synthesis and assembly technology cannot prepare the carbon nanotube crystals on macroscopic scale, which limits its applications. While, the advantage of graphene is its 2D crystal structure, and its strength, conductivity and thermal conductivity are the best among other 2D crystal materials, and it has broad application prospects because of the ability of a large area of continuous growth.
1.1.2Introduction to grapheneげlike 2D crystals―hexagonal boronnitride 
Hexagonal boron nitride (hげBN)［31］, is white block or powder. Its layered structure is similar to graphene lattice constant and characteristics, soげcalled “white graphene”［32げ34］.

 The hげBN is a lattice alternately arranged by B atoms and N atoms in the 2D plane by a hexagonal lattice formation laws, showing a honeycomb structure (Figure 1げ2). N atomic nucleus and B atom are combined with sp2 orbital to form a strong ┮ bond［35げ39］,

 the interlayer is combined by weak van der Waals forces, and slide easily between the layers with soft lubricating properties.


Figure 1げ2The flat structure diagram of hげBN. Blue on behalf of nitrogen atoms and pink on behalf of boron atoms, respectively［34］


In 1995, Nagashima et al. used an epitaxial growth on a variety of metal surfaces to obtain hげBN crystals［40］.

 The team in the University of Manchester adopting a microげmechanical peeling method in 2005, successfully prepared a 2D hげBN［41］.

 The band gap of hげBN is about 5.9 eV, the Mohs’ scale of hardness is about 2, the bulk modulus is about 36.5 GP,the heat conductivity layer up 600∥1,000 W/(m·K), the coefficient of thermal expansion layer is about -2.7≠10-6/→ (interlayer about 30≠10-6/→), the refractive index is about 1.8, and having a neutron absorption ability［42げ45］.

 Its antiげoxidation temperature is 900→, the thermos ability is up to 2,000→, an inert environment remaining stable at 2,700→. HげBN has good process abilities, such as thermal shock resistance to electrical vibration, high resistance to breakdown electric field strength, nonげtoxic and environmentally friendly, no wettability to various metals, chemical corrosion and other excellent physical and chemical properties［46げ53］.
1.1.3Introduction of graphene/hげBN,a 2D composite 
Graphene has very good electronic properties. However, it will be a challenging to make graphene into nanoげelectronic devices. Moreover, mechanical stability is required when using a scanning probe technology to detect micro graphene［54］.

 A substrate is introduced to solve this problem, while examining the substrate can open graphene band gap, thereby improving the switching performance of graphene electronics.
There have been already a lot studies of graphene and the substrate materials, such as Co［55］,

 Ni［56げ59］,

 Ru［60げ61］,

 Pt［62げ63］, as well as semiconductors SiO2［64げ66］,

 SiC［67げ69］,

 each of them can be used as a substrate graphene. However, experiments show that the graphene is on the top of these substrates is very uneven, and have a lot of wrinkles, restraining the properties of graphene. For example, SiO2 is the most common graphene substrate, but on its surface there are impurities, which cause the scattering of charge and a charge trap. Therefore, the growth and charge density distribution of graphene on the SiO2 substrate are very uneven (Figure 1げ3)［70］,

 which result in significant suppression on the carrier mobility of graphene. Recently, a lot of researches have proved that hげBN is an ideal substrate, which suits for graphene to maintain geometrical and electrical properties.
Monolayer graphene and hげBN have similar lattice structure, and lattice mismatch inげbetween is only about 1.5 %［72］.

 As a base, hげBN has smooth surface without charge trap, and also has a low dielectric constant, highげtemperature stability, high thermal conductivity and other properties. Again, hげBN with dielectric constant ┖ of 3∥4, breakdown electric field strength of about 0.7 V/nm, is great gate insulating layer for graphene. The surface optical phonon energy of hげBN is two orders of magnitude greater than that of SiO2, which indicates the using of hげBN as the substrate is likely to improve the performance of graphene device in conditions like higher temperature and higher electric field. In the single cell of hげBN,the difference of grid energy between nitrogen and boron atoms leads to a broad band gap of about 5.9 eV［73］,

 which is conducive to open the band gap of graphene. All above shows that hげBN is an ideal substrate material for graphene.
Dean et al. for the first time using hげBN as a supporting substrate of graphene, fabricated graphene transistor devices with high mobility［73］,

 on which clear graphene quantum Hall effect was observed. Ponomarenko et al. (Figure 1げ3)［71］

 used the technology of physical transfer to combine thin sheets of graphene and hげBN crystals produced hetero junction device with two single layers of graphene crystals. At the same time, researchers succeeded in achieving a variety of graphene heterojunctions and super lattice structures with more complex structures［74げ76］; 

 the United Kingdom and the United States researchers observed novel Hofstadter Butterfly phenomenon on graphene/hげBN heterojunction devices respectively［77げ78］.

 Since then, researchers have used a variety of methods to fabricate graphene/hげBN heterostructure functional devices with their 


Figure 1げ3Comparing topography and charge density for graphene/BN vs. graphene/SiO2［71］


original purpose of improving the quality and the speed in the preparation of graphene instead of studying the performance of graphene/hげBN heterostructures.
1.2Graphene
1.2.1Structure of graphene

Ideal graphene is a single layer of 2D atomic crystal with orthohexagonal lattice structure. The length of C―C bond is around 0.142 nm, and the thickness of the layer is 0.35 nm. In each state, single carbon atom forms strong ┮ bond with three nearest neighbours respectively by sp2 orbital hybridization, causing occupied and unoccupied states keep away from each other. Each unit cell of graphene has two types of subげlattices, type A and B (Figure 1げ4(a)), and there are chiral characteristics in the spin of between electrons of A and B［79］.


Figure 1げ4Structure and Brillouin zone of graphene

(a) The relationship between distributions of A and B atoms in the unit cell of graphene;  (b) Corresponding Brillouin zone. The Dirac cones are located at the K and M points［79］


1.2.2Preparation of graphene
Preparation of graphene can be classified into physical and chemical methods, including mechanical peeling method［1］ and epitaxial growth method［80］,

  method of chemical cleavage［81］,

 chemical vapor deposition (CVD)［11］, lowげthermal expansion method［82］,

 nanotubes cutting method［83］,

 metal catalysis and so on［84げ85］,

 as shown in Figure 1げ5.



Figure 1げ5Several kinds of preparing methods of graphene

Spectral analysis of graphene prepared by CVD method is shown from a to e in the left group figures. Sample image of graphene by epitaxial growth is shown from a to e in the middle of group figures. Epitaxial growth of graphene samples and their characteristics is shown from A to F in the right group figures［11,84げ85］


Yu et al. summarized the progress of the direct growth of graphene on the substrate of semiconductor and insulator in recent years［86］.
 Table 1げ1 shows the comparison of physical properties between several materials and hげBN as substrate.


Table 1げ1Comparison of physical properties between several materials and hげBN



Al2O3
SiO2
Graphite
Graphite

diamond
hげBN
Lattice constant
Faceげcentered： 1.27
Orthogonal
Faceげcentered
a=1.383
b=1.741
c=0.504
1.936
a=0.246 nm

c=0.667 nm
0.3567
a=0.2504 nm

c=0.6661nm
Thermal conductivity/
(W/(m·K))
40
1.4
25∥470
22
25.1
Dielectric constant
6.8
3.9
8.7
5.7
4

1.2.3Physical properties of graphene
1. Mechanical and thermal characteristics of graphene

2D honeycombげshaped crystal structure endows graphene excellent inげplane mechanical properties. Lee et al. found that the Youngюs modulus of graphene can reach(130÷10) GPa under the assumption of the thickness of the graphene layer 0.35 nm［9］,

 as shown in Figure 1げ6 and the 2D ultimate plane strength is 42 N/m2. GŦmezげNavarro et al.［87］

 and Poot et al.［88］ obtained intensity values of different graphene layers by various methods.

 As shown in above studies, graphene as novel nanoげmaterials has excellent mechanical properties.


 Figure 1げ6Fracture test results［9］

(a) The test line corresponding to different film diameters and tip radiis;  (b) Histogram and Gauss distribution of two kinds of fracture forces［9］


Because of high modulus of elasticity and long mean free path of electrons, the thermal conductivity of graphene can reach up to 3,000∥6,000 W/(m·K). Hong et al.［11］ showed that graphene’s thermal conductivity is 5,300 W/(m·K) and would decrease while temperature increasing［12,89］.

 Seol et al. have studied graphene’s thermal conductivity on the substrate of SiO2［90］,

 and the results showed that the value of thermal conductivity was still as high as 600 W/(m·K) even the phenomenon of phonon scattering happens due to the transfer of heat across the interface between the two materials. In addition, defects and the unordered arrangement of edges would reduce the thermal conductivity of graphene (Figure 1げ7)［91］.


Figure 1げ7Different widths of graphene nanosheetsюs thermal conductivity
changes at different temperatures

(a) The thermal conductivity change images of different materials at 300 K;  (b) The anisotropy test images of different widths of nanoribbons at different temperatures［91］


2. Optical properties of graphene 
Singleげlayer graphene is colorless, so we always observe graphene with substrate. The light absorption intensity of graphene is irrelevant to the frequency of light, but appears a linear correspondence relationship with the amount of graphene layers (Figure 1げ8)［92げ93］.

 For the singleげlayer graphene, the absorption and transmittance of visible light are 2.3 % and 97.7 %, respectively［94げ95］.

 Graphene has characteristics of wideげband absorption and zeroげenergy 


Figure 1げ8Looking through oneげatomげthick crystals［93］