Atomic Layer Deposition on 2D Materials - American Chemical Society

Perspective pubs.acs.org/cm

Atomic Layer Deposition on 2D Materials† Hyun Gu Kim and Han-Bo-Ram Lee* Department of Materials Science and Engineering, Incheon National University, Incheon 22012, Korea ABSTRACT: 2D materials are layered crystalline materials and are the most attractive nanomaterials due to their potentials in next-generation electronics. Because most 2D materials are atomically thin, a suitable fabrication process without degradation of the original properties of the material is required to realize 2D-material-based devices. Atomic layer deposition (ALD) is an ideal technique for adding materials with atomic scaling precision to nanomaterials. Due to the surface-sensitive reactions of ALD, growth on 2D materials is strongly affected by the surface properties of the 2D materials. In this Perspective, ALD growth on 2D materials is reviewed and discussed with previously reported results to provide insights to readers who are investigating 2D materials and relevant topics.

1. INTRODUCTION Although most materials in the world have three-dimensional (3D) shapes, we intuitively know what a two-dimensional (2D) shape is. For instance, a sheet of paper looks like it has an ideal 2D shape because its thickness is very small, but other 2D shapes do not look as ideal. Similarly, in 2D materials, the thickness can be close to the atomic-layer scale, but the other two lateral dimensions are not, such as in a sheet of paper. Specifically, 2D materials are layered materials in which strong in-plane crystalline atomic layers are bonded to each other by weak out-of-plane van der Waals forces, so each atomic layer is easily split to single or several layers.1−4 The first 2D material was obtained from carbon. Few-layer graphene, which is a 2D material of carbon, was obtained from the mechanical exfoliation of graphite.5,6 Graphene shows many superior properties, such as high mobility, transparency, and mechanical strength, compared with conventional bulk materials.7,8 In addition, the 2D characteristic of graphene is a significant advantage for its integration with modern electronic devices, which have been developed from 2D thin films, compared with other nanomaterials, such as 1D nanowires and 1D nanotubes. Therefore, many researchers have focused attention on the great potential of graphene and have tried to extend the research on graphene to other 2D material systems. Other 2D materials have been found from several layered materials; for instance, 2D transition metal dichalcogenides (TMDs) were obtained from the mechanical exfoliation of bulk materials. Therefore, many 2D materials have been found and can be categorized into three groups based on their electrical properties: conductors, insulators, or semiconductors. For instance, graphene is a conducting 2D material, h-BN is an insulator, and some TMDs show semiconducting properties. In this Perspective, many 2D materials will be introduced, including graphene, h-BN, MoS2,

WS2, MoSe2, WSe2, WTe2, and phosphorene. Further lists of the various 2D materials that have been reported are well tabulated elsewhere.2,9−11 Researchers have tried to measure the superior properties of 2D materials to show the feasibility of 2D-material-based devices for next-generation electronics. For this, the device structure should be fabricated with atomically thin 2D materials; for instance, a field-effect transistor (FET) with MoS2 for measuring the channel properties of MoS2 could be fabricated with three electrodes (source, drain, and gate) and one dielectric layer, as shown in Figure 1. Modern microfabrication techniques are essential for realizing 2D-materialbased devices. However, current technology cannot realize a device of which components are composed of all 2D materials. Therefore, 2D materials are being used only for core components in specific devices with other components fabricated by modern microfabrication technology. For instance, many components of the graphene barristor were fabricated by thin-film deposition, photolithography, and etching, which are the main techniques of microfabrication for modern Si devices.12 If we want to make a transistor with MoS2, as shown in Figure 1, we need to deposit a dielectric layer on MoS2 using thin-film deposition methods and pattern it using etching methods. In particular, for thin-film deposition in microfabrication technology, the conventional deposition techniques include sputtering, evaporation, and chemical vapor deposition (CVD). These methods have been successfully adopted for Si device fabrication so far; however, they are not suitable for the fabrication of 2D-material-based devices. For instance, sputtering requires the use of harsh environments, such as plasma and molecular-level bombardments, and 2D materials can be easily Received: December 1, 2016 Revised: April 13, 2017 Published: April 25, 2017

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This Perspective is part of the Up-and-Coming series. © 2017 American Chemical Society

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DOI: 10.1021/acs.chemmater.6b05103 Chem. Mater. 2017, 29, 3809−3826

Perspective

Chemistry of Materials

Figure 1. Schematic drawing of a FET structure using MoS2.

Figure 2. Schematic drawings of ALD. A single ALD cycle consists of four steps: (a) exposure of a precursor, (b) purging with inert gas to remove unreacted precursor molecules, (c) exposure of a counter reactant, and (d) purging with inert gas to remove unreacted counter reactant molecules and byproducts.

damaged under these conditions due to their atomic-scale thickness. CVD and evaporation cannot fabricate high-quality films with precise tuning at the atomic scale on 2D materials because these techniques are developed for thin-film deposition in the range of tens to hundreds of nanometers.13−15 Atomic layer deposition (ALD) is a thin-film deposition method based on surface chemical reactions. Due to the unique mechanisms of surface self-saturated reactions, ALD has advantages, such as thin-film thickness control at the atomic scale and excellent conformality, over conventional thin-film deposition methods.16,17 These advantages of ALD are essential to realize 3D devices at the nanoscale, which cannot be achieved through conventional methods.18 Generally, one ALD cycle is composed of four steps, including precursor exposure, purging, counter-reactant exposure, and purging, as shown in Figure 2. Because of the sequential exposure of the precursor and counter reactant separated by the purging steps, chemical reactions only occur on the surface and strongly depend on the

surface properties of the original substrate. In other words, if there are two areas that have different surface properties on a single substrate, ALD will show different growth behaviors on them. Researchers sometimes utilize this surface-sensitive deposition behavior for selective deposition methodologies, which is routinely called area-selective ALD (AS-ALD).19 Sometimes, this surface-sensitive deposition mechanism of ALD results in a significant change in growth on different 2D materials. Because of these unique growth mechanisms and advantages, ALD has become the most important thin-film deposition technique for realizing 2D-material-based devices. Compared with CVD and sputtering, ALD is performed under much milder conditions that do not cause significant degradation of the 2D materials, and the thickness control of ALD at the atomic scale enables the formation of a highly tuned additional layer on 2D materials. In addition, the high quality of the film of ALD materials originating from the relatively slow growth rate 3810


3811

CVD graphene

Epitaxial graphene

Mechanically exfoliated graphene

HOPGa

Substrate

Al2O3

TiO2

Ta2O5

HfO2

ZnO Al2O3

HfO2

Al2O3

Pt

Al2O3

ALD material

H2O H2O H2O H2O O2 H2O H2O H2O H2O

TMA TMA TMA TMA TMAHr TDMAH TDEAHfs TDMAH TADMAt

H2O H2O H2O H2O H2O

TMA TMA TMA TMA TMA

H2O

H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O

TMA TMA TMA TDMAHh TDMAH TDMAH Hf(NMe2)4j HfCl4k DEZm TMA TEALo

TTIPu

H2O H2O

TMA TMA

PTCAe Al and Ti seed layers NO2 O3 Nonef HSQg NFC 1400-3CPi Temperature None Temperature 4MPl O3 Al and Ti seed layers PTCDAp Al and Ti seed layers HF+SC1q XeF2 HSQ NFC 1400-3CP PTCDA Al and Ti seed layers Al and Ti seed layers Al and Ti seed layers O2 plasma Al seed layer Cu and Ni−Au substrate Y2O3 seed layer None

O3 H2O H2O H2O O3 O2 O3 H2O

Counter reactant

TMA TMA TMA TMA TMA MeCpPtMe3d MeCpPtMe3 TMA

Precursor

None TMAb+NO2, O3 O3 DAc O3 None O3 NO2

Nucleation promotor

Table 1. ALD Processes on 2D Materials Growth type

Random69 Selective70

Random66,67 Random68 Random21

Random56 Improvement of dielectric constant66,67 Improvement of dielectric constant68

Improvement of dielectric constant60

Improvement of electron mobility62 Improvement of electron mobility63

Random62 Random63 Random64 Random65 Random57,59,60 Random56

Improvement of electron mobilityn,55

Improvement of electron mobility54

Improvement of electrical conductivity51 Improvement of electron mobility52

Improvement of mechanical property47 Improvement of Electron mobility48 and dielectric constant49

Improvement of dielectric constant60 Improvement of electron mobility and density61

Random56

Note

Improvement of electron mobility,44 Reduction of electron mobility45

Random57−60 Random61

Random47 Random48,49 Selective50 Random51 Random52 Random53 Random54 Random27 Random55 Random34 Random56

Random43 Random44−46

Random32 Random33 Random33−36 Random37 Random38 Selective39,40 Random41 Random42

Chemistry of Materials Perspective


Mechanically exfoliated MoS2

GO and RGO

Substrate

Table 1. continued

3812

TiO2

HfO2

TiO2 ZrO2 NiO Pd Al2O3

Pt

Pt

HfxAlyO2 ZnO

HfO2

ALD material

TDMAH No information

None None TTIP

TEMAH TDMAH TDMAH No information

O2 plasma Al seed layer UV-O3 UV-O3

None

TDMAH

TMA TMA TMA TMA No information

H2O H2O H2O H2O H2O H2O O2 O2

TEMAHw TDMAH, HfCl4 TDMAH TEMAH, TMA DEZ DEZ MeCpPtMe3 MeCpPtMe3, Pt (acac)2x MeCpPtMe3, Pt (acac)2 MeCpPtMe3 TiCl4y Zr(NMe2)4z NiCp2aa Pd(thd)2bb TMA

H2O H2O H2O No information H2O No information H2O

H2O H2O H2O H2O No information H2O

O2 H2O H2O O3 O2 H2O

O2

H2O H2O H2O H2O H2O H2O H2O H2O

Counter reactant

TMA TMA TMA TMA TMA TDEAHf TDMAH TDMAH

Precursor

None

UV-O3 Al seed layer O3 O2 plasma None

None None None O3 O3 None

O2 plasma

Temperature HMDSv Sequence control TMA+H2O H2O None None Hf and HfO2 seed layers None None Ar+ ion beam Temperature PTCA None None O2 plasma

Nucleation promotor Growth type

Improvement of dielectric constant,106 Improvement of sensitivity107 Improvement of dielectric constant108 Improvement of dielectric constant109

Improvement of sensitivity,112 Improvement of dielectric constant111,113 Reduction of contact resistance114

Random110 Random111−113 Random114

Improvement of electron mobility,103 Improvement of dielectric constant,104 Improvement of photoconductivity105

Improvement of dielectric constant101 Improvement of dielectric constant102

Improvement of electron mobility97 Improvement of dielectric constant98

Improvement of electron mobility,92 Improvement of dielectric constant93,94

Improvement of electrochemical capacity and electrical conductivity90

Random100 Random106,107 Random108 Random109

Random103−105

Random95−97 Random98 Random99 Random100,101 Random102

Note

Improvement of oxidation stability and conductivity,86 Improvement of electrical conductivity87 Improvement of electrochemical capacity and electrical conductivity88

Improvement of contact resistance28

Random28 Random86,87 Random88 Random89 Random90 Random91 Random92−94

Reduction of leakage current82 Improvement of dielectric constant83 Improvement of electron mobility84 Improvement of conductivity and sensitivity85 Improvement of electrical conductivity29 Improvement of contact resistance28


Reduction of leakage current74 Improvement of dielectric constant75


Selective80 Selective81 Random82 Random83 Random84 Random85 Selective29 Random28

Random71 Selective72 Random73 Random74 Random75 Random76 Random77,78 Random79



3813

TiOPc None

None

None

Al seed layer None

Al2O3 Al2O3

Al2O3

Al2O3

Al2O3 HfO2

ZrO2

Ti seed layer UV-O3 None

TMA TMA No information

TiOPccc None None

HfO2

TMA

Ti seed layer

Al2O3

TMA TDMAH

TMA

TMA

TMA TMA

TDMAH TDMAH No information

TTIP TMA TDMAH

None Perylene bisimide UV-O3

TiO2 Al2O3 HfO2

TMA TDMAH No information

None None None TDMAH

No information

None

Al seed layer

TMA TMA TMA

No information

Precursor

Al seed layer Perylene bisimide None

None

Nucleation promotor

HfO2

HfO2

Al2O3

ZrO2

ALD material

H2O H2O

H2O

H2O

H2O H2O No information H2O H2O No information H2O H2O

H2O

H2O H2O H2O

No information O2 plasma H2O No information H2O

No information H2O H2O H2O

Counter reactant

Improvement of dielectric constant132 Improvement of oxidation stability133 Improvement of dielectric constant93,94 Improvement of oxidation stability and device performances134−137 Improvement of oxidation stability,138 Improvement of dielectric constant139 Improvement of dielectric constant140

Random132 Random133 Random93,94 Random134−137 Random138,139 Random140


Random128

Improvement of dielectric constant128 Improvement of dielectric constant108 Improvement of dielectric constant115


Random128 Random108 Random115

Reduction of contact resistance127


Improvement of dielectric constant129 Improvement of dielectric constant130 Improvement of drive current131

Improvement of sensitivity107

Random107


Improvement of transport characteristic124 Improvement of dielectric constant125 Improvement of dielectric constant126


Random122,123

Improvement of dielectric constant116,117

Random116,117 Random23 Random118−121

Note

Improvement of dielectric constant,118 Improvement of photovoltaic performance,119 Improvement of electrical conductivity,120 Improvement of light absorption121 Improvement of electron mobility122,123


Growth type Random115

a Highly ordered pyrolitic graphite. bTrimethylaluminum. c1,10-Diaminodecane. dTrimethyl(methylcyclopentadienyl)-platinum(IV). e3,4,9,10-Perylene tetracarboxylic acid. fIdeal graphene ribbon for theoretical calculation. gHydrogen silsesquioxane. hTetrakis (dimethyl) amino hafnium. iOrganic polymer (JSR Micro, Inc.). jHafnium tetrakis(dimethylamide). kHafnium tetrachloride. l4-Mercaptophenol. mDiethyl zinc. nControl of carrier concentration according to the thickness of ZnO. oTriethylaluminum. pPerylene-3,4,9,10-tetracarboxylic dianhydride. qStandard Clean 1. r Tetramethylammonium hydroxide. sTetrakis (diethylamido) hafnium(IV). tPentakis (dimethylamino) tantalum(V). uTitanium(IV) isopropoxide. vHexamethyldisilazane. wTetrakis (ethylmethylamino) hafnium. xPlatinum(II)-acetylacetonate. yTitanium tetrachloride. zTetrakis (dimethylamido) zirconium(IV). aaNickelocene. bbPalladium 2,2,6,6-tetramethyl-3,5-heptanedione. ccTitanyl phthalocyanine.

MBE WSe2 Mechanically exfoliated WTe2 Mechanically exfoliated BN Exfoliated phosphorene

CVD WS2 Mechanically exfoliated MoSe2 Mechanically exfoliated WSe2

Mechanically exfoliated WS2

CVD MoS2

Substrate

Table 1. continued



Perspective


Figure 3. Schematic drawings of (a) an HOPG substrate on the macroscale and microscale and (b) selective growth of ALD on HOPG.

is another advantage for the fabrication of 2D-material-based devices.17 Therefore, many reports of device fabrication have employed ALD as a thin-film deposition method; in particular, metal oxides have been indispensable for dielectric layers in 2Dmaterial-based devices.20−22 At the beginning of research on graphene-based devices, some papers showed inconsistent results about ALD growth.23 Because many studies were focused on the investigation of the physical and electrical properties of graphene at the device scale, the inconsistencies were not highlighted but could be found in the experimental procedure and supporting materials.20,24−27 Several years later, the results in many papers about ALD growth have been analyzed by understanding the effects of the graphene synthetic process, graphene transfer process, ALD precursors, counter reactants, and substrates.28−30 Although there have been a couple of review articles about ALD on 2D materials, they only dealt with a limited number of metal oxides and focused on fabrication of a good insulator for graphene-based devices.30,31 In this Perspective, however, we extended summary and discussion to many papers dealing with 2D-materials-based devices using ALD as well as ALD growth on 2D materials. Moreover, various surface reactions of ALD that occur on various 2D materials are introduced and the correlations between the surface properties of 2D materials and ALD growth are discussed with experimental results and theoretical approaches. Subsequently, the methods for modifying the surface properties of 2D materials for ALD-reactive surfaces to form high-quality ALD films on 2D materials are discussed. In addition, the role of ALD for the fabrication of 2D-material-based devices and future applications is discussed. As described in the previous paragraph, ALD is a surfacesensitive deposition method, and the growth of ALD is a function of the precursor, counter reactant, surface properties, and deposition temperature. Therefore, information about ALD processes on 2D materials, including precursors, counter reactants, and synthetic methods of 2D materials, is

summarized in Table 1. This table should be useful for researchers who need to select an appropriate ALD material to fabricate their devices and want to know what kinds of pretreatments are required to form uniform ALD films.

2. ALD ON GRAPHITE In the early stages of research on ALD on graphene, graphite was employed as a substrate instead of graphene because graphite has almost identical surface properties to graphene but is much easier to handle. Highly ordered pyrolytic graphite (HOPG) is a suitable substrate for research purposes because it has an ideal, atomically flat surface morphology.141−143 In the context of chemical reactivity, the HOPG surface can be divided into two domains: the terrace region and the step-edge region.144−146 Microscopically, HOPG is a stacked bundle of a huge number of graphene layers, so the terrace region is identical to the basal plane of graphene, which is composed of sp2-bonded carbon atoms, as shown in Figure 3a. In contrast, the step edge is the end of the graphene sheet, so the sp2 bonding should be partially imperfect, resulting in high chemical reactivity, as depicted in Figure 3b. Because ALD is a surface-sensitive deposition method, the slight difference in chemical reactivity between the terrace and step-edge regions leads to interesting ALD growth behavior. ALD growth predominantly occurs along the step-edge lines but rarely on the terrace region, leading to the selective deposition of ALD. Metal oxides are indispensable materials for insulating and dielectric layers to realize graphene-based electronic devices, so several metal oxides, such as Al2O3, which have been evaluated on various substrates many times, were applied for ALD growth on HOPG. As addressed in the previous paragraph, Al2O3 was selectively deposited on a single HOPG substrate due to the existence of two chemical domains. TMA molecules predominantly adsorb on the step-edge regions, leading to subsequent reactions with the counter reactant. Therefore, the final morphology of Al2O3 has a laterally aligned nanowire shape, 3814


Perspective


Figure 4. (a) AFM images of an HOPG surface with graphene terraces and (b) a similar surface after 1 nm-thick ALD Al2O3 growth. Reprinted from ref 147 with the permission. Copyright 2008 AIP Publishing.

Figure 5. Schematic image of the physical and chemical properties of atomic steps on the surface of HOPG (left). Plot of the friction force versus the applied normal force for two different types of steps: external step edges and internal step edges (right). Reprinted with permission from ref40. Copyright 2015 American Chemical Society.

as shown in Figure 4.32,147 Additionally, HfO2 is an attractive material for high-k dielectrics in graphene-based devices because it has already been proven to be a dielectric layer in Si microelectronics. Similar to Al2O3, HfO2 ALD selectively formed along the step-edge region of the HOPG substrate.25 This selective growth behavior is also the same in metal ALD systems. The Bent research group reported that Pt nanowires (NWs) formed along the step edges of HOPG due to the difference in chemical reactivity between the two domains.39 The NW shape is more advantageous than the nanoparticle shape for catalysts in terms of durability, so the formation of Pt NWs shows another opportunity for catalyst applications. In the aforementioned report, it was found that there are two types of step edges using friction force microscopy (FFM), which is based on the architecture of atomic force microscopy (AFM) but measures the friction force between the tip and surface.40 One is an externally exposed step, and the other is an internal step covered by another graphene layer, as shown in Figure 5. Therefore, Pt ALD occurs on the external step edges only, not on the internal step edges, because the chemical reactivity of the internal step edges is screened by the top

graphene layer. These internal step edges are inherently formed during the fabrication process of HOPG.148−151 The purpose of the research on ALD of metal oxides is the fabrication of high-quality and continuous dielectric layers on 2D materials for further applications. However, the inherent chemical properties of graphite, which cause nonuniform selective growth, are a main obstacle to forming a uniform dielectric layer, and this is quite similar on the graphene surface. Therefore, many researchers have investigated the modification of the surface properties to tailor graphene reactivity toward ALD. At the beginning of this research, modification studies were performed on HOPG substrates. Before graphene was actively investigated, the surface functionalization of carbon nanotubes (CNTs) had been intensively studied, and it was well-known that NO2 molecules adsorb on the CNT surface and form reactive nucleation sites for ALD.152 Because the NO2 molecule freely desorbs from the CNT surface, the adsorbed NO2 molecules were exposed to TMA right after NO2 exposure to form noncovalent and stable NO2-TMA nucleation sites. Thus, pretreatment with NO2 gas was adopted to modify the HOPG and graphene surface.33,42 In contrast to the selective deposition of Al2O3 and HfO2, the NO2-treated HOPG surface 3815


Perspective


cannot produce graphene on a large scale.162,164 For large-scale synthesis, CVD and epitaxial methods using vacuum environments are advantageous.157,159,165,166 Single-layer graphene is formed on a catalyst substrate, such as Cu, through the thermal decomposition of CH4 gas in CVD, whereas wafer-scale graphene is epitaxially grown on a SiC substrate in UHV conditions. In many cases, a transfer process from the Cu substrate is required. The transfer process employs an adhesive transfer layer, such as poly(methyl methacrylate) (PMMA), so that removal of the transfer layer can be accomplished.157,159,165,166 In many cases, the residue of the transfer layer affects the surface properties of graphene. Single graphene layers are exfoliated from a graphite substrate by forming graphene oxide (GO), and the GO layers are reduced to graphene again in the following process. The reduced GO (RGO) is fabricated through several steps accompanying chemical reactions, so the surface of RGO contains various kinds of chemical species.160,167−169 As described above, reports about ALD growth on graphene have appeared in previous articles as a small part of the fabrication process to measure the physical and electrical properties of graphene. Therefore, the purpose of ALD is to fabricate a high-quality and continuous high-k dielectric layer without changing the graphene properties. The pristine graphene surface is chemically inert, similar to the surface of HOPG, so many surface functionalization and treatment approaches have been applied. In early research, mechanically exfoliated graphene was transferred onto an appropriate substrate, and then device components were fabricated in a variety of steps, including high-k deposition by ALD and electrode fabrication by e-beam lithography.170−173 Al2O3 has been the most widely applied material as a high-k dielectric on graphene devices. The nonuniform growth of Al2O3 on HOPG substrates had already been reported due to chemical inertness, so special treatment processes that modify the surface properties of graphene to be more active for ALD nucleation were developed. Williams et al. applied surface functionalization using NO2 and TMA, which had already been demonstrated on a CNT platform,152 for exfoliated graphene to investigate the quantum Hall effect.42 In another study, graphene was solely utilized as a sacrificial layer in a mechanical property measurement system of a 1 nm-thick Al2O3 layer.47 The Al2O3 layer was deposited on NO2-functionalized graphene, which was suspended on a hole pattern using ALD, and the graphene was etched out.47 Hydrogen silsesquioxane (HSQ) was used for a multipurpose layer. A 20 nm-thick HSQ layer was spin coated on graphene for an etching mask and e-beam resist, and the remaining HSQ layer on graphene was utilized as an ALD nucleation layer.51 The same scheme was applied to epitaxial graphene on a SiC substrate to fabricate a FET.64 In 2008, an interesting idea was suggested by the Dai group at Stanford University.43 They coated exfoliated graphene with 3,4,9,10-perylenetetracarboxylic acid (PTCA) molecules to form nucleation-promoting species. The perylene rings in the center of PTCA adsorbed on the graphene layer through π−π interactions, and the four carboxylic acids served as nucleation sites. A uniform and continuous Al2O3 layer was formed on the PTCA-functionalized graphene.43 This idea was extended to other functionalized molecules in the Hersam group at Northwestern University. They utilized 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), which has a similar molecular structure to PTCA, on epitaxial graphene grown on a SiC substrate under a UHV system.57 In the study, the

was reactive enough for ALD nucleation, resulting in the formation of a continuous ALD layer.33,42 NO2 and TMA formed a noncovalent layer on graphene and HOPG to promote ALD nucleation. A similar approach was developed using an alkane−amine molecule. The HOPG surface was coated with 1,10-diaminodecane (DA), and continuous Al2O3 was formed by ALD through noncovalent functionalization.37 Generally, the ALD process consists of the sequential exposure of two chemicals, the precursor and counter reactant. Therefore, the use of a very reactive counter reactant can be another approach for modifying the surface properties of graphene and graphite. O3 is a reactive oxidant for ALD processes and can be used as an alternative to O2 and H2O counter reactants in many cases. On the HOPG surface, the selective growth of Al2O3 was changed to random growth by solely changing the counter reactant from H2O to O3.32−35 From the XPS results, it was found that the inert surface of HOPG was changed to a reactive surface toward the nucleation of Al2O3 through the formation of oxygen-related species, such as epoxide, carbonyl, and carboxylic groups, at a specific temperature through the chemisorption of O3. At lower temperatures, however, O3 and precursor molecules physisorbed on the HOPG surface without forming chemical bonds with carbon.35 Theoretical calculations using density functional theory (DFT) revealed that the physisorbed O3 on the basal planes of HOPG was easily changed to chemisorption states, and epoxide groups that promoted the adsorption of TMA and the uniform nucleation of Al2O3 were formed through the chemical reaction of O3 and carbon.36 The O3 counter reactant has also been adopted for metal ALD processes. In many noble metal ALD processes, oxidants, such as O2 and O3, have been used for the counter reactant because noble metals are deposited through ligand combustion reactions by the oxidant.36,153 When O3 was applied as a counter reactant for Pt ALD on HOPG, selective deposition was not observed; instead, the random nucleation of Pt ALD occurred across the HOPG surface due to the formation of defects by O3.41 Pits that contained many circular step edges were formed through the surface etching of HOPG by O3, and they became nucleation sites for Pt ALD. The discrepancy of the effects of O3 on the HOPG surface in Al2O3 ALD and Pt ALD is probably due to the different deposition temperatures and O3 concentrations.

3. ALD GROWTH ON GRAPHENE Based on knowledge about ALD growth on HOPG, research has been extended to ALD growth on graphene. Although the surface of HOPG is ideal, being theoretically identical to the surface of graphene, there are many deviations in real cases, which can affect surface-sensitive ALD growth. For instance, a transfer process is required to collect a single graphene layer from the bulk, original substrate, and this process can change the surface properties of graphene.154−156 In addition, after the transfer process, graphene is placed on another substrate, so there may be substrate effects on the graphene properties.154−157 Such small or significant changes in graphene surface properties can change ALD growth behavior significantly. Various kinds of synthetic methods for graphene have been reported.5,158−163 For fabrication cost, process simplicity, and applicability, four methods are predominantly used. Mechanical exfoliation from graphite using an adhesive, such as Scotch tape, is a widely used method due to its simplicity; however, it 3816


Perspective


been proven to be a reactive counter reactant, was also applied to modify the surface reactivity toward ALD reactions. The Kim group performed intensive studies on the effects of O3 on graphene surface properties and the nucleation of Al2O3 through experimental and theoretical methods.48,49 They revealed that O3 exposure at room temperature leads to the physisorption of O3 and the formation of a reactive nucleation site without the generation of defects on exfoliated graphene. In addition, a more reactive counter reactant, O2 plasma, was used for the formation of an Al2O3 layer on graphene by PE-ALD.66 During the PE-ALD process, O2 plasma was remotely generated to minimize damage to the graphene layer and a uniform and continuous Al2O3 layer was formed on CVD graphene. The electrical properties of PE-ALD Al2O3 showed improvements compared with other thermal ALD Al2O3. A cleaning process of epitaxial graphene was utilized for surface functionalization.66 HF cleaning followed by the SC1 step (NH4OH + H2O2) generated active surface species on epitaxial graphene, leading to the formation of a uniform Al2O3 layer. Interestingly, some papers have shown the uniform growth of dielectric layers by ALD without any functionalization or treatment processes. For instance, Liu et al. fabricated a graphene transistor using exfoliated graphene with a 20 nmthick ALD HfO2 layer without treatment or a seed layer to improve ALD nucleation.53 In addition, Metic et al. successfully fabricated a graphene FET with an ALD HfO2 dielectric layer without any nucleation promoter on CVD graphene.76 This discrepancy could be due to many reasons, including both controllable and uncontrollable conditions, such as the cleanliness of the graphene, transfer process, synthetic process, kinds of precursors, and conditions of the ALD chamber. In fact, compared with HOPG, exfoliated graphene, and epitaxial graphene, CVD graphene is synthesized by a more rapid and multistep process, resulting in the formation of more defects on CVD graphene, such as grain boundaries, cracks, and transfer layer residues. In 2015, Oh and co-workers performed a comparative study to reveal the effects of the synthetic process and ALD precursor on the growth of HfO2 ALD on graphene.81 The nucleation rate of HfO2 ALD was much higher on CVD graphene than on exfoliated graphene due to the existence of oxygen-related species, which could have formed during the synthetic process. Because CVD graphene is synthesized on a Cu substrate through a thermal decomposition process, many defect sites caused by Cu, such as wrinkles and folds, could be formed together with inherent defects, such as grain boundaries, cracks, and internal step edges.174−176 In addition, two Hf precursors, HfCl4 and TDMAH, showed different growth behaviors on the same CVD graphene. DFT calculations revealed that HfCl4 is more actively adsorbed on CVD graphene through physisorption than TDMAH, resulting in a greater nucleation rate.81 In addition, Karasulu and coworkers performed a similar comparative study on the effect of Pt precursors on Pt ALD and the oxidation of graphene on graphene using DFT calculations.28 MeCpPtMe3 and Pt(acac)2 precursors were modeled, and the details of the reaction pathways were calculated. MeCpPtMe3 was more reactive on an oxidized graphene surface than Pt(acac)2. They showed that the nonselective physisorption of the precursor facilitates the random nucleation of the ALD material on the chemically inert graphene surface. The Samsung research team maximized this nonselectivity to form a uniform dielectric layer.71 They calculated the physisorption conditions of Al and Hf precursors by DFT and fabricated uniform Al2O3 and HfO2 layers on CVD

close-packed structure of PTCDA was revealed by scanning tunneling microscopy (STM), as shown in Figure 6, and Al2O3

Figure 6. (a) STM image of an epitaxial graphene surface prepared by UHV graphitization of SiC (0001) (imaging conditions: sample voltage VS = −2.0 V, tunneling current It = 0.05 nA). (b) Atomically resolved graphene bilayer (VS = −0.3 V, It = 0.1 nA). (c) Large-area STM image of a PTCDA monolayer, which conformally coats several step edges in a uniform ordered monolayer (VS = −2.0 V, It = 0.05 nA). (d) Molecular structure of PTCDA. (e) High-resolution STM image of the area marked by the square in (c), showing a herringbone arrangement of the PTCDA molecules and continuous ordering over a substrate step edge (VS = −2.0 V, It = 0.07 nA). Reprinted with permission from ref 57. Copyright 2011 American Chemical Society.

and HfO2 were successfully deposited by ALD on the PTCDA seeding layer. In the following papers, the growth characteristics of the PTCDA seeding layer on epitaxial graphene were investigated using Raman spectroscopy58 and scanning microwave microscopy (SSM)59 and applied to the fabrication of platforms for Al2O3 and HfO2 dielectric durability measurements.60 A similar seed layer concept was applied to ZnO ALD.55 4-Mercaptophenol (4MP) has an aromatic ring, which can interact with graphene through π−π interactions and two functional groups (−OH and −SH), allowing ZnO to nucleate easily on the 4MP-functionalized graphene surface. Another approach for a functionalized layer on graphene is the formation of a metallic seed layer. Kim and co-workers deposited a very thin, approximately 1 to 2 nm, Al metallic layer on exfoliated graphene by evaporation, and then the oxidized Al layer (by air exposure) served as a nucleation seed layer for subsequent Al2O3 ALD.44 Robinson and co-workers evaluated the formation of several dielectric materials on epitaxial graphene, including Al2O3, TiO2, HfO2, and Ta2O5, with metallic seed layers and correlated the kind of dielectric layer to the electron mobility in the graphene layer.56 The TiO2 layer showed the highest improvement of electron mobility; however, the exact reason behind this was not fully understood. In many following papers, researchers adopted this idea for the fabrication of graphene FETs.45,46,61 O3, which had already 3817


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RGO.89−91,177 In ZrO2, transition of the growth mode from island growth to layer growth was observed.89

graphene through a physisorption mechanism by decreasing the deposition temperature. Furthermore, they controlled the composition of HfxAlyO to improve the electrical properties as a dielectric layer, such as the dielectric constant.83 A similar approach was proposed to improve Al2O3 ALD nucleation on CVD graphene. Before Al2O3 ALD, graphene was exposed to TMA molecules followed by H2O exposure without a purging step.74 Due to the lack of a purging step, a nucleationpromoting layer was formed through the direct reaction of physisorbed TMA and subsequent H2O. In another report, the continuous growth of Al2O3 was investigated on CVD graphene by solely controlling the precursor pulsing sequence without pretreatment.73 Dlubak and co-workers suggested an interesting approach to improve the nucleation of ALD on graphene. They evaluated the nucleation of Al2O3 ALD on graphene placed on various substrates, including SiO2, Cu, and Ni−Au alloy, and observed a drastic improvement of the nucleation on the graphene/ metallic substrate. It was found that the metallic substrate affected the adsorption of the ALD precursor through the very thin graphene layer and promoted the nucleation of Al2O3 through the formation of polar sites.21 Meanwhile, the selective growth of ALD on defect sites of graphene was utilized positively to improve the graphene properties.29 Lee and coworkers suggested that the selective deposition of ALD metal improves the electrical conductivity of graphene without undesired degradation of the properties. They selectively deposited Pt by ALD on grain boundaries and showed an increase in conductivity. Similar to exfoliated graphene, several kinds of seed layers were applied to CVD graphene as nucleation promoters. A continuous ALD ZnO2 film was formed on CVD graphene using a PTCA functionalization layer.84 The hydroxyl surface species that formed on the PTCA layer reacted with DEZ precursor molecules, leading to the nucleation of ZnO. An ultrathin HfO2 layer was deposited by e-beam evaporation as a seed layer followed by HfO2 ALD.79 The researchers investigated the effect of seed-layer thickness on the film continuity of ALD HfO2 and concluded that a minimum seedlayer thickness was required to form a continuous ALD HfO2 layer. As mentioned earlier, RGO has various kinds of surface species that have functionalities compared with graphene synthesized by different methods because it is exposed to strong chemicals during the multistep synthetic process. Therefore, most reports about ALD growth on RGO and GO have consistently shown the random nucleation of ALD deposits without seed layers or pretreatment processes. In addition, RGO and GO have flake-like forms due to chemical exfoliation, so they have much longer edge lines, which are chemically active sites, than epitaxial graphene, exfoliated graphene, and CVD graphene. Nucleated Pt randomly forms on RGO and GO sheets without selective deposition due to the existence of oxygen species.86,87 Due to the inherent growth behavior of metals, the growth of Pt is not a layer-by-layer process but an island growth mode, resulting in the formation of Pt nanoparticles (NPs) instead of a Pt film. This Pt NPs/ RGO (GO) system has shown superior properties in catalytic applications due to the large surface area of the NPs and the uniform formation of NPs on 3D carbon supports given by the advantages of ALD. Other catalyst materials, including Pd, NiO, and ZrO2, were also applied to a composite catalyst system with

4. ALD GROWTH ON TMDS One of the reasons why the research on ALD growth on 2D materials has focused mainly on graphene systems is that graphene was the first discovered 2D material. For 2D-materialbased electronic devices, growth of dielectric layers on other semiconducting 2D materials is more necessary because graphene has metallic or semimetallic properties. In other words, a dielectric layer should be formed by ALD on a semiconducting 2D material, instead of a metallic graphene layer, to fabricate 2D-material-based field effect transistors, as shown in Figure 1. Thus, because several semiconducting 2D materials have been reported, many fundamental and applied studies about ALD on 2D materials have shifted from graphene to semiconducting 2D materials. TMDs are the most widely studied group of materials as semiconducting 2D materials. There are several materials that have 2D layered structures: for instance, MoS2, WS2, MoSe2, WSe2, and WTe2.6,178−181 Similar to graphene, TMDs are obtained from bulk materials by mechanical exfoliation. MoS2 is one of the most widely researched semiconducting 2D materials. Based on the knowledge of ALD growth on graphene, the Ye group investigated Al2O3 ALD on exfoliated MoS2 by experimental and theoretical routes.93 The uniform growth of Al2O3 ALD was dependent on the growth temperature, which was in the experimental results, and it was revealed that the TMA precursor physically (instead of chemically) adsorbed on MoS2 using DFT calculations, leading to nonselective growth on MoS2 at low temperatures. Unlike Al2O3 using TMA and H2O, HfO2 ALD showed nonuniform growth on MoS2 flakes. In fact, researchers have reported the formation of a uniform HfO2 layer by ALD in part of the MoS2 transistor fabrication.103,104 However, the Wallace group proved that HfO2 was not continuous on the surface of MoS2 flakes by experimental results and DFT calculations and claimed that the uniform growth in previous reports was probably due to the existence of adhesive layer residues that promoted ALD nucleation.105 At higher temperature, a UV-O3 pretreatment was applied to Al2O3 and HfO2 ALD on exfoliated MoS2.95−97,99,108 UV-O3 produced S−O bonds, facilitating the adsorption of TMA and resulting in the formation of continuous Al2O3. In addition, the UV-O3 treatment was applied to other TMDs. Similarly, the pretreatment and seeding layer, which were successfully applied for as promoters for ALD nucleation on graphene, were adopted for a MoS2 surface. Multilayered MoS2 flakes obtained from bulk MoS2 were exposed to reactive O2 plasma for surface functionalization prior to Al2O3 and HfO2 ALD.100 Under O2 plasma exposure, the top layers of MoS2 were oxidized to MoO3, and the oxide layer facilitated the nucleation of Al2O3 and HfO2. However, this plasma treatment could not be applied for a single MoS2 layer due to oxidization of the top layer. To demonstrate the channel layer properties of MoS2 in a charge-trap memory device, Al2O3 and HfO2 layers were deposited by ALD on MoS2.98,106,182 For this, an Al seed layer was evaporated on the MoS2 surface before the ALD process. Due to the limitations of the mechanical exfoliation method in large-scale synthesis, researchers have strived to develop other synthetic methods that enable the large-area production of TMDs. CVD, which was already proven as a large-area synthetic method of graphene, was adopted for the syntheses of 3818


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Figure 7. STM images of bare WSe2/HOPG and a monolayer of TiOPc on WSe2/HOPG. STM images are recorded at 100 K. (a) Bare MBEdeposited WSe2 layer on HOPG (VS = 2.0 V, IT = 30 pA). (b) Expanded STM image of WSe2 with atomic resolution. Hexagonal atomic array of the topmost Se layer in WSe2 (VS = −2.0 V, IT = 130 pA). The inset shows the Fourier transform of the atomic array of Se. (c) TiOPc ML deposited onto a WSe2 ML on HOPG. The inset shows the molecular structure of TiOPc (VS = 2.0 V, IT = 20 pA). (d) Submolecular resolution of MBE TiOPc ML/WSe2 ML/HOPG. The central O is observed as a bright spot (VS = +2.0 V, IT = 40 pA). The inset shows the Fourier transform of the TiOPc ML. (e) Three-dimensional STM image of the TiOPc ML with molecular resolution. (f) STM image of TiOPc molecules and a schematic of the TiOPc ML, including the WSe2 ML and HOPG. The two TiOPc molecules on the left are overlapped with the STM image. (g) (dI/dV)/(I/V) spectrum of bare WSe2ML/HOPG and TiOPc ML/WSe2 ML/HOPG. Reprinted with permission from ref 132. Copyright 2016 American Chemical Society.

charges. Similarly, the formation of the ALD Al2O3 dielectric layer affected the electrical properties of MoS2 in a FET. The mobility of MoS2 was much improved solely by formation of Al2O3, and the Al2O3 layer probably depressed the impurity scattering and phonon scattering.97 Meanwhile, the continuous growth of ALD Al2O3 without a seed layer or pretreatment, which is inconsistent with previous reports,23 could be explained by physisorption of the precursor at low temperature, surface residues from CVD reactions, and effects of the bottom substrate through the single TMD layer. In fact, most research showing continuous growth of ALD Al2O3 employed low deposition temperatures below 200 °C, at which the TMA precursor physisorbs on MoS2, leading to random nucleation of Al2O3.92,93,118,119,121 Similarly, there were reports on continuous growth of other ALD metal oxides on various TMDs, including MoS2, WS2, WTe2, and WSe2, via physisorption of the precursors at low temperature.125,127,133 Selenide 2D materials of TMDs were pretreated by UV−O3 or coated with a seed layer for better ALD nucleation. Azcatl and co-workers intensively investigated the surface property changes of MoSe2 and WSe2 under UV-O3 pretreatment using an in situ XPS technique.108 The top oxidized layer was reduced again by the UV-O3 exposure during the subsequent HfO2 ALD process, and MoOx was more effectively reduced than WOx due to the higher stability of WOx. A very thin Ti metallic layer was deposited on exfoliated WSe2 followed by air exposure to oxidize the Ti seed layer.128 ALD Al2O3 and HfO2 layers were continuously formed on this TiOx seed layer to fabricate a WSe2 transistor. Instead of a metallic Ti layer, an organic Ti seed layer was utilized for the functionalization of

TMDs. Unlike CVD graphene, which must be transferred to other substrates from the original Cu substrate after CVD, TMDs synthesized by CVD do not require catalytic substrates, such as Cu, but form directly on specific substrates. Therefore, the negative effects caused by defects and contaminants could be minimized by the transfer-free process. In early stages of research, a single MoS2 layer was synthesized via the sulfurization of MoO3 on an SiO2 substrate in a CVD chamber.116 To evaluate the basic properties of MoS2 as a channel material of a transistor, Al2O3ALD was used with an Al seed-layer method.116,117 In addition, the concept of using a seed-layer molecule was applied to TMDs. Wirtz and coworkers deposited perylene bisimide, which has a similar molecular structure to the previously suggested seed molecules (PTCA and PTCDA), on CVD MoS2 and WS2 to form a uniform Al2O3 layer.23 In the report, they also observed selective deposition of Al2O3 ALD on monolayer and doublelayer MoS2 and WS2. ALD Al2O3 only formed on the double layer; however, the reason was not clearly understood.23 The Kim group tried to determine the effect of a top Al2O3 layer on single-layer MoS2.118 They synthesized single-layer MoS2 by CVD with MoCl5 and (CH3)2S and deposited a continuous Al2O3 layer by ALD without a seed layer on CVD MoS2. They revealed that a long exposure of H2O oxidant on MoS2 deteriorated the electrical properties of MoS2 devices due to the formation of Mo−O bonds at the interface. A solar cell structure consisting of a single layer of n-type MoS2 and a ptype Si substrate was improved by passivation of MoS2 by Al2O3 ALD.119 The thin ALD Al2O3 passivation layer reduced the interface charge trap and increased the fixed positive 3819


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6. SUMMARY AND OUTLOOK Because 2D materials have no dangling bonds on their surface, they are considered chemically inert. Thus, theoretically, ALD growth, which requires chemical species on the surface for precursor adsorption, does not occur on 2D materials. In reality, 2D materials have inherent defects, such as grain boundaries, cracks, wrinkles, and edge sites, allowing ALD materials to nucleate on these defects and leading to selective deposition. For the roles of ALD layers in 2D-material-based devices, continuous and uniform layers are desired; thus, various methods to modify the surface properties of 2D materials allowing for reactivity toward ALD nucleation have been suggested. Seed layers using metallic layers and organic molecule have been widely used for promoting ALD nucleation. Pretreatment using gas and plasma has been employed as another route to make the surface of 2D materials reactive. Controlling the deposition parameters enables uniform growth of ALD materials on 2D surfaces without a seed layer or pretreatment. The deposition temperature has been controlled to induce the physisorption of precursor, resulting in random nucleation without a dependency on the surface chemical reactivity. As described in the introduction, the early research on ALD growth on 2D materials was driven by the fabrication of 2Dmaterial-based devices. In the subsequent reports, fundamental studies about ALD on 2D materials were conducted. Many discrepancies and unclear behaviors of ALD on 2D materials are now being explained by experimental and theoretical approaches. However, more in-depth studies on ALD on 2D materials are required. For instance, ALD materials are now limited to a few oxides and metals. Other material systems, such as sulfides and nitrides, which could serve as passivation layers and semiconducting layers, for ALD on 2D materials should be investigated, and the effects of precursors and counter reactants are also important to understand the flexibilities of the fabrication processes. The effects of the substrate underneath 2D materials are also important. Because 2D materials are one atomic layer thick, the substrate could affect the reaction taking place on 2D materials. ALD can open further opportunities for 2D materials. In many cases, an ALD layer with a continuous morphology is desired; however, discontinuous shapes, such as nanoparticles and nanowires, could be advantageous for specific applications. The control of the surface properties of 2D materials changes the growth behavior of ALD, enabling nanostructuring. In addition, selective deposition on different 2D materials by ASALD could be a big advantage for simplifying fabrication processes. The direct synthesis of 2D materials by ALD is one of the attractive applications. From the meaning of the name, ALD is the most suitable synthetic method for 2D materials. The Kim group at Yonsei University has published several papers about the direct syntheses of 2D materials by ALD.190−193 In early reports, they transformed WO3 films deposited by ALD to layered WS2 through sulfurization due to the difficulties of the direct deposition of TMDs. In the following study, they successfully developed a direct synthetic method of TMDs by controlling the deposition temperature191 and extended it to synthesize alloys of TMDs by ALD for optoelectronic device applications.193 Loh and co-workers reported a direct synthesis of crystalline MoS2 layers by ALD with a MoCl4 precursor and H2S reactant.194 In addition, polycrystalline TMD films have been reported many times by

WSe2, as shown in Figure 7. Titanyl (TiO) phthalocyanine (C32H18N8) (TiOPc), which is an organic semiconductor, was coated on MBE-grown WSe2 and exfoliated WSe2 for functionalization.129,132 STM revealed that a monolayer of TiOPc molecules were well aligned with 4-fold symmetry and became adsorption sites for the TMA precursor of Al2O3 ALD.

5. ALD GROWTH ON OTHER 2D MATERIALS Beyond graphene and TMDs, many researchers have explored other layered materials that could be utilized as 2D materials. Phosphorene is an allotrope of phosphorus, similar to graphene being an allotrope of graphite, and individual phosphorene layers are weakly bonded by van der Waals forces, as shown in Figure 8. Unlike semimetallic graphene,

Figure 8. Schematic drawing of phosphorene layers.

phosphorene has a direct bandgap of 0.3−1.5 eV; furthermore, it has very high electron mobility, up to 1000 cm2/(V s).183−187 Thus, phosphorene has received much attention from various research groups because it was mechanically exfoliated from black phosphorus in 2014.183,186−188 However, the phosphorene and black phosphorus flakes are easily oxidized by moisture and oxygen in air, and morphology and property degradations occur, which can be major disadvantages for device components in many future applications. ALD is a suitable deposition method to form a passivation layer of phosphorene against air exposure. Therefore, ALD metal oxides are utilized for two purposes: a dielectric layer for a phosphorene device and a passivation layer on phosphorene. Because Al2O3 is one of the reliable ALD materials and has been applied as passivation layers in other applications several times, it was selected as a phosphorene passivation material. In a back-gate transistor using phosphorene, ALD Al2O3 was deposited on phosphorene as a top passivation layer because the dielectric layer was formed underneath phosphorene.135 In contrast, ALD Al2O3 is deposited on phosphorene before the formation of the gate so that it could play two roles: protection and insulation. Similar to other 2D materials, metallic thin layers were used as seed layers. Prior to ALD, a thin Al layer was formed by evaporation for two purposes: a first protection layer from oxidation by H2O during the Al2O3 ALD process and a seed layer for Al2O3 ALD.138,139 In addition to a protection layer, Al2O3 ALD passivation positively affects the electrical properties of phosphorene, such as decreasing the trap charge density and increasing the device reliability.134 In another report, the change in the surface properties of black phosphorus was analyzed by in situ XPS analysis.136 The interface between ALD Al2O3 and black phosphor deteriorated more on black phosphor with the native oxide layer than freshly exfoliated phosphorus, likely due to the effects of the native oxide on ALD nucleation. To avoid oxidation and damage of phosphorene during Al2O3 ALD, the initial process was performed at two temperature regimes.137,189 Initial ALD cycles were carried out at room temperature to minimize the oxidation of phosphorene by the H2O counter reactant, and then the temperature was elevated to 150 °C for further film growth. 3820


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(7) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (8) Geim, a K. Graphene: Status and Prospects. Science 2009, 324, 1530−1534. (9) Mas-Ballesté, R.; Gómez-Navarro, C.; Gómez-Herrero, J.; Zamora, F. 2D Materials: To Graphene and Beyond. Nanoscale 2011, 3, 20−30. (10) Han, W. Perspectives for Spintronics in 2D Materials. APL Mater. 2016, 4, 032401. (11) Lim, H.; Yoon, S. I.; Kim, G.; Jang, A. R.; Shin, H. S. Stacking of Two-Dimensional Materials in Lateral and Vertical Directions. Chem. Mater. 2014, 26, 4891−4903. (12) Yang, H.; Heo, J.; Park, S.; Song, H. J.; Seo, D. H.; Byun, K.-E.; Kim, P.; Yoo, I.; Chung, H.-J.; Kim, K. Graphene Barristor, a Triode Device with a Gate-Controlled Schottky Barrier. Science 2012, 336, 1140−1143. (13) Mattevi, C.; Kim, H.; Chhowalla, M. A Review of Chemical Vapour Deposition of Graphene on Copper. J. Mater. Chem. 2011, 21, 3324−3334. (14) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312−1314. (15) Bonaccorso, F.; Lombardo, A.; Hasan, T.; Sun, Z.; Colombo, L.; Ferrari, A. C. Production and Processing of Graphene and 2d Crystals. Mater. Today 2012, 15, 564−589. (16) Kim, H.; Lee, H.-B.-R.; Maeng, W. Applications of Atomic Layer Deposition to Nanofabrication and Emerging Nanodevices. Thin Solid Films 2009, 517, 2563−2580. (17) George, S. M. Atomic Layer Deposition: An Overview. Chem. Rev. 2010, 110, 111−131. (18) Dasgupta, N. P.; Lee, H.-B.-R.; Bent, S. F.; Weiss, P. S. Recent Advances in Atomic Layer Deposition. Chem. Mater. 2016, 28, 1943− 1947. (19) Atomic Layer Deposition of Nanostructured Materials; Pinna, N., Knez, M., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2011. (20) Garces, N. Y.; Wheeler, V. D.; Gaskill, D. K. Graphene Functionalization and Seeding for Dielectric Deposition and Device Integration. J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom. 2012, 30, 030801. (21) Dlubak, B.; Kidambi, P. R.; Weatherup, R. S.; Hofmann, S.; Robertson, J. Substrate-Assisted Nucleation of Ultra-Thin Dielectric Layers on Graphene by Atomic Layer Deposition. Appl. Phys. Lett. 2012, 100, 173113. (22) Choi, K.; Lee, Y. T.; Im, S. Two-Dimensional van Der Waals Nanosheet Devices for Future Electronics and Photonics. Nano Today 2016, 11, 626−643. (23) Wirtz, C.; Hallam, T.; Cullen, C. P.; Berner, N. C.; O’Brien, M.; Marcia, M.; Hirsch, A.; Duesberg, G. S. Atomic Layer Deposition on 2D Transition Metal Chalcogenides: Layer Dependent Reactivity and Seeding with Organic Ad-Layers. Chem. Commun. 2015, 51, 16553− 16556. (24) Meric, I.; Han, M. Y.; Young, A. F.; Ozyilmaz, B.; Kim, P.; Shepard, K. L. Current Saturation in Zero-Bandgap, Top-Gated Graphene Field-Effect Transistors. Nat. Nanotechnol. 2008, 3, 654− 659. (25) Keefer, D. Atomic Layer Deposition of Thin Film Hafnium Oxide as Top Gate Oxide in Graphene Field Effect Transistors. 2008 NNIN REU Res. Accompl. 2008, 120−121. (26) Liao, L.; Bai, J.; Qu, Y.; Lin, Y.; Li, Y.; Huang, Y.; Duan, X. HighK Oxide Nanoribbons as Gate Dielectrics for High Mobility TopGated Graphene Transistors. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 6711−6715. (27) Alles, H.; Aarik, J.; Aidla, A.; Fay, A.; Kozlova, J.; Niilisk, A.; Pärs, M.; Rähn, M.; Wiesner, M.; Hakonen, P.; Sammelselg, V. Atomic Layer Deposition of HfO2 on Graphene from HfCl4 and H2O. Cent. Eur. J. Phys. 2011, 9, 319−324.

ALD for lubricant applications; however, they did not have layered 2D structures.195−197 These sulfide ALD processes that have already been reported could have potential for the direct syntheses of 2D materials by ALD with careful selections of precursors and process parameters. However, there are many challenges to producing a single-layer 2D material by ALD. For instance, layer-by-layer growth has to be precisely controlled without island growth on a specific surface, and suitable precursors for 2D materials should be synthesized.

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AUTHOR INFORMATION

Corresponding Author

*H.-B.-R. Lee. Email: [email protected]. ORCID

Han-Bo-Ram Lee: 0000-0002-0097-6738 Notes

The authors declare no competing financial interest. Biographies Hyun Gu Kim is a graduate student in the Department of Materials Science and Engineering at Incheon National University in Korea. He received a B.S. degree in Polymer Science and Engineering at Korea National University of Transportation. In Spring 2014, he joined Prof. Han-Bo-Ram Lee’s group at Incheon National University as a Ph.D. student, and he is studying Pt ALD on reduced graphene oxide and carbon powder and their applications in transparent heaters and functional materials. Dr. Han-Bo-Ram Lee is an associate professor in the Department of Materials Science and Engineering at Incheon National University in Korea. He received a B.S. degree in Materials Science and Engineering at Sungkyunkwan University and a Ph.D. in Materials Science and Engineering at POSTECH in 2009, followed by a postdoctoral scholar position at Stanford University. Han-Bo-Ram Lee’s current research interests and topics are focused on understanding and controlling surface chemistry and reactions and applying this knowledge to various applications of which properties could be improved by the functionalization of surfaces on the nanoscopic to macroscopic ranges. ALD has been one of Prof. Lee’s major research fields in the last 10 years for surface engineering and functionalization. More information about Prof. Lee’s research can be found here (http://nanomaterial.kr).

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1D1A1B03935611).

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REFERENCES

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Atomic Layer Deposition on 2D Materials - American Chemical Society

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