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ARTICLE

Received 1 Jul 2014 | Accepted 2 Oct 2014 | Published 20 Nov 2014

Giant enhancement in vertical conductivity ofstacked CVD graphene sheets by self-assembledmolecular layersYanpeng Liu1,2,*, Li Yuan1,*, Ming Yang3, Yi Zheng1, Linjun Li1, Libo Gao1, Nisachol Nerngchamnong1,

Chang Tai Nai1,4, C.S. Suchand Sangeeth1, Yuan Ping Feng3, Christian A. Nijhuis1 & Kian Ping Loh1

Layer-by-layer-stacked chemical vapour deposition (CVD) graphene films find applications as

transparent and conductive electrodes in solar cells, organic light-emitting diodes and touch

panels. Common to lamellar-type systems with anisotropic electron delocalization, the plane-

to-plane (vertical) conductivity in such systems is several orders lower than its in-plane

conductivity. The poor electronic coupling between the planes is due to the presence

of transfer process organic residues and trapped air pocket in wrinkles. Here we show the

plane-to-plane tunnelling conductivity of stacked CVD graphene layers can be improved

significantly by inserting 1-pyrenebutyric acid N-hydroxysuccinimide ester between the

graphene layers. The six orders of magnitude increase in plane-to-plane conductivity is due to

hole doping, orbital hybridization, planarization and the exclusion of polymer residues.

Our results highlight the importance of interfacial modification for enhancing the perfor-

mance of LBL-stacked CVD graphene films, which should be applicable to other types of

stacked two-dimensional films.

DOI: 10.1038/ncomms6461

1 Department of Chemistry, Graphene Research Center, National University of Singapore, 3 Science Drive 3 117543 Singapore, Singapore. 2 NanoCore,National University of Singapore, 117576 Singapore, Singapore. 3 Department of Physics, National University of Singapore, 2 Science Drive 3, 117551 Singapore,Singapore. 4 NUS Graduate School for Integrative Sciences and Engineering, 28 Medical Drive #05-01, 117597 Singapore, Singapore. * These authorscontributed equally to this work. Correspondence and requests for materials should be addressed to C.A.N. (email: [email protected])or to K.P.L. (email: [email protected]).

NATURE COMMUNICATIONS | 5:5461 | DOI: 10.1038/ncomms6461 | www.nature.com/naturecommunications 1

& 2014 Macmillan Publishers Limited. All rights reserved.

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mailto:[email protected]

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Large-area graphene produced by chemical vapour deposition(CVD) is an attractive candidate for transparent andconducting electrodes due to its transparency, high electrical

conductivity and flexibility. CVD graphene has been deployed asdiffusion barriers in electrode modifiers, transparent conductorsin touch screen panels, organic light-emitting diodes and solarcells1–7. However, the conductivity of pristine graphene is limitedby the amount of carriers, and further modification such aschemical or electrostatic doping is needed to improve theconductivity. One way to improve the conductivity is to stacksingle-layer CVD graphene in a layer-by-layer (LBL) manner toform multilayers2,4,5,7. Although the overall conductivity (in-plane or out-of-plane) improves with layer thickness, weakelectronic coupling between the layers prevents a linear scaling ofthe conductivity with thickness of the stacked layers. Theseproblems will be amplified in large-area electronic devices whereseries resistance scales with lateral dimension. Most of theresearch efforts on graphene have thus far focused on improvingthe in-plane lateral conductivity, while scant attention has beenpaid to the plane-to-plane (vertical) conductivity, which isimportant in electrode applications. There is a need to probethe quality of interfaces in stacked CVD graphene films and tocorrelate that with the electronic properties. Pyrene and itsderivatives are widely used to functionalize graphene sheets dueto their p–p stacking ability, they can also act as hole-transportmaterials in photovoltaic devices due to their strong electrondelocalization8–10.

In conventional LBL wet transfer process, the verticalconductivity of stacked CVD graphene films is significantly lowerthan that of graphite or dry-stacked mechanically exfoliatedgraphene films11. The reasons for low conductivity in LBL-stacked graphene films include the following: first, stackedgraphene films trap solvent and moisture, hence the measuredthickness of monolayer graphene using atomic force microscopy(AFM) is B0.7–1.0 nm compared with the 0.34 nm of exfoliatedgraphene4,12,13; second, wrinkles on the films arising from surfacetension during wet transfer trap air pockets and prevent goodelectronic coupling between the graphene layers3,4,14; and third,

polymer residues from the transfer process14–17. These factors notonly introduce scattering centres to in-plane carrier transport, butalso contribute to weak interlayer electronic coupling, as depictedin Fig. 1a.

Here we study whether the electronic coupling of stacked CVDgraphene layers can be improved by inserting self-assembledmolecules (SAM) between the layers, as measured via the EGaIn(eutectic gallium indium) technique. Through the insertion of aself-assembled monolayer (1-pyrenebutyric acid N-hydroxysucci-nimide ester, PBASE) between two LBL-stacked graphene films,we find that the interfacial resistance can be mitigated to a largeextent (current density: pristine one B10� 3 A cm� 2; modifiedone B103 A cm� 2). The large pendant N-hydroxysuccinimideester group of PBASE blocks a second layer of molecules fromstacking on top. This self-limiting nature of PBASE in forming amonolayer is important, as insertion of thick dielectric layerswould weaken the interlayer coupling. Other small moleculeswith similar pyrene backbone structures, but with pendant groupsthat can undergo intermolecular bonding, are prone to aggrega-tion (1-pyrenecarboxylic acid, 1-pyrenebutanol, like hydrogenbonding), which produces deleterious effects on the conductiv-ity18. Our study highlights the importance of selecting themolecules with the right chemical structure as an insertion layerto enhance the electrical performance of LBL-stacked CVDgraphene films.

ResultsPlane-to-plane conductivity measurement using EGaIn. Tostudy the plane-to-plane conductivity of stacked CVD graphenefilms, we used a non-invasive liquid metal alloy top electrode(EGaIn) to contact the upper graphene films softly19,20. Thismethod yields junctions in high yields (up to 100%) with goodstability because of the presence of a thin (B0.7 nm) highlyconductive layer of Ga2O3 that forms spontaneously in air21. Thisprotective layer of conductive oxide prevents the bulk metal fromdiffusing into the structures of interest and adds to themechanical stability of the junction22. This method also makes

IJ(-1 V)I (A cm–2)

WrinklesH20

Bottom CVD graphene

Top CVD graphene

H20N2

EGaIntip

Residue

Bottom CVD graphene

Top CVD graphene

N2

EGaIntip

e–

G/PBASE/GG/G

SA

M g

row

th ti

me

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)

J (A

cm

–2)

V (V)

–1.0 –0.5 0.0 0.5 1.010–8

10–6

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102

104

0 h

6 h

8 h10 h24 h

10–5 10–4 10–3 10–2 10–1 100 101 102 103 104 105

36 h

0 h

4 h

6 h

8 h

10 h

24 h

36 h

H 20

Figure 1 | Schematic illustrations of EGaIn measurements and the time evolution of current density. (a,b) Schematic drawings of the system of

recording the vertical tunnelling current density across two LBL-stacked CVD graphene films. Under ideal conditions, the gap between top and bottom

graphene films determines the tunnelling resistance and organic residues reduce interlayer coupling in model (a), which is the case for stacked CVD

graphene. (c) Experimental data of current density as a function of SAM growth time. The log-standard error bars are omitted for visual comparison

purpose. The full version could be found in Supplementary Fig. 3. (d) Histograms of the absolute tunnelling current density (J) distributions with various

PBASE SAM growth times at a bias of � 1 V. The Gaussian fitting curves are represented in blue.

ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6461

2 NATURE COMMUNICATIONS | 5:5461 | DOI: 10.1038/ncomms6461 | www.nature.com/naturecommunications



it possible to fabricate large numbers of highly repeatablejunctions for collecting statistics19–21. In contrast, conventionalmethods require harsh fabrication methods (for example, directmetal deposition of large electrode on graphene) and sometimescause damage (mainly penetration) to the graphene layersresulting in short or junctions dominated by artefacts22.

High-quality graphene films grown by CVD were transferredonto ultra-flat gold substrate (Cr/Au, 5/100 nm as bottomelectrode deposited at very low deposition rate) via conventionalPMMA(Poly(methyl methacrylate))-assisted wet transfer process.After removing PMMA layer, the samples were immersed in5 mM PBASE in dimethylformaldehyde (DMF) solution at 80 �Cto allow the PBASE molecules to form a SAM layer on graphenevia p–p interactions23. After drying, another CVD-growngraphene layer was directly stacked onto to form graphene/PBASE/graphene sandwich architecture (schematic drawing in

Fig. 1b and typical AFM images of G/PBASE/G in Fig. 2b).Ripples induced by surface tension, cracks and polymer particleresidues from wet transfer process are usually found on thesurface of as-growth graphene films, as shown in Fig. 2a. After theassembly of PBASE, most of these defects vanish, except for somepin holes arising from the incomplete coverage of PBASE. Due tostrong p–p stacking on graphene, the self-assembly of PBASE isable to supplant polymer residues (see Supplementary Fig. 1 fordark-field images illustrating the exclusion of polymer residues byPBASE monolayer growth) and also planarizes the defectivegraphene surface. The binding of a strong layer of PBASEincreases the bending rigidity and reduces the rippling (smoothsurface in Fig. 2c–f; similar phenomenon occurs on thickgraphene films in Supplementary Fig. 2 and SupplementaryNote 1). In addition, the hydrophilic side chain improves thewetting properties of graphene and avoids ripples formationduring wet transfer process of top graphene layer. All thesefactors contribute to strong interlayer coupling between the topand bottom CVD graphene layers.

Using the EGaIn technique, large amount of statistical currentdensity versus voltage (J(V)) data was readily collected (For moreinformation, please find Supplementary Note 2). Table 1 showsthe J(V) data for SAMs formed over time period ranging from 4to 36 h. For each device with specified SAM growth time, 360traces were recorded for three batches of samples to calculatelog7J719–21. The corresponding AFM images (in Fig. 2c–f) showthat these SAMs saturate at a thickness of 0.7±0.1 nm due to itsself-terminating ability. This is different from the self-assemblybehaviour of aromatic organic molecules where longer assemblytime leads to multilayer formation24. As can be seen in Fig. 1c, thelog-average current density (J) increases sharply to 103 A cm� 2

and saturates after 24 h of SAM growth (for J(V) curves, seeSupplementary Fig. 3). The tunnelling current density J (inA cm� 2) can be described by the simplified form of the Simmonsequation

J ¼ J0e�bd ð1Þ

where J0 (A cm� 2) is the current density for junction withd¼ 0 Å and b (Å� 1) is the tunnelling decay coefficient19–21,25.The relative low current density in the first 4 h of SAM growthprobably comes from the induction time needed for nucleationand organization of the SAM (Fig. 2c). Figure 1d presentsstatistical distribution histograms of the log-average J curves at abias of � 1 V, where the data is collected as a function of growthtime of the SAM. The good Gaussian-fitted histograms illustratethat stable and uniform junctions are formed over large areaof the sample. As shown in Fig. 1d, the current densityincreases from B10� 3 A cm� 2 of stacked G-G layers toB103 A cm� 2 of G-PBASE-G after the formation of a fullmonolayer of PBASE, which is a six order enhancement invertical conductivity. Statistics have been collected from 105

SiO2G/G

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SiO2 G/PBASE/G

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-----

0 nm

4 nm4 h

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0.00.20.40.6

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nm)

Distance (μm)

0.8 1.0 1.20.0

0.4

0.8

H (

nm)

Distance (μm)

Figure 2 | AFM imaging and representative height profiles.

(a,b) Topographies of two LBL-stacked CVD graphene before (G/G)

and after (G/PBASE/G) the insertion of PBASE. The corrugations on

G/PBASE/G films are obviously smaller than G/G films. The thickness of

this bilayer CVD graphene is B0.7–1.0 nm. Scale bar, 1mm. (c–f) Time

sequence of PBASE self-assembly on CVD graphene (PBASE/G). Scale bar,

500 nm. Topography line scans show that the height of PBASE SAM layer is

B0.7±0.1 nm.

Table 1 | Working device yields of G/PBASE/G devices withvarious PBASE SAM growth time.

Growthtime

Number ofjunctions

Number ofshorts

Number oftraces

Yield ofjunctions (%)

0 15 2 360 86.74 15 1 360 93.36 15 1 360 93.38 15 2 360 86.710 15 1 360 93.324 15 0 360 10036 15 0 360 100

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6461 ARTICLE




regions of our samples from three batches (2,520 traces in total),which shows consistently five to six order enhancement (statisticshistogram, see Supplementary Fig. 3). It is worth noting also thatthe in-plane conductivity of two stacked CVD graphene filmsis doubled after the introduction of PBASE layer (seeSupplementary Fig. 4 and Supplementary Note 3). Theenhancement along in-plane direction is much smaller thanthat out-of-plane direction, due to different transportmechanism11.

Characterizing PBASE-adsorbed CVD graphene. Ramanspectroscopy was employed to study the graphene film after itsinteraction with SAM26–28. A spatial colour map of the bottomgraphene using 532 nm excitation laser was carried out before andafter SAM growth over the same area (see Supplementary Fig. 5).In the low-frequency regions as shown in Fig. 3a, the dopingdependence of the G band is not as sensitive as thetwo-dimensional (2D) band, but several intrinsic signals due toPBASE molecules appear. The peak at 1,332.7 cm� 1 arises fromsp3 bonding. The peak at 1,611.4 cm� 1 is assignable to pyrenegroup resonance and the peak at 1,383.2 cm� 1 is due to theintroduction of disorder arising from orbital hybridization of themolecule with graphene plane. The mapping images of 2D peaks

are transformed to histograms for visual comparison. After SAMgrowth, the 2D peak of the graphene films is shifted to higherfrequency (B6 cm� 1; Fig. 3b) and has wider FWHM (full-widthat half-maximum; B6 cm� 1; Fig. 3c), which is a signature of holedoping by PBASE26,28. Ultraviolet photoelectron spectroscopywas used to record the work function change induced by theadsorption of PBASE. Consistent with hole doping, the workfunction is increased by B0.3 eV as shown in SupplementaryFig. 6a (refs 28,29). Back-gate field-effect transistor was alsofabricated to investigate the doping effect of PBASE molecules(see Supplementary Fig. 7 and Supplementary Note 4). Theresults show that the doping level of CVD graphene films byPBASE molecules could be up to B4� 1012 cm� 2 (the intrinsiccarrier density of pristine CVD bilayer graphene is in the order of1011 cm� 2). The enhanced carrier density will certainly improvethe overall conductivity (for in-plane conductivity, seeSupplementary Fig. 4), although the magnitude of increaseobserved for the vertical conductivity far exceeds contributiondue to increased carrier density alone.

The quality of the self-assembly monolayer is analysed byX-ray photoelectron spectroscopy. The wide scans show that nometallic element, arising from catalyst or etchant, is detected (seeSupplementary Fig. 8a). The presence of PBASE is confirmed bythe characteristic N 1s peak at 400.4 eV (Fig. 3d). The C 1s peak

PBASE growth

PBASE growth

Raman Pos of 2D (1 cm–1)

FWHM of 2D (1 cm–1)

Inte

nsity

(a.

u.)

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nsity

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1,576.4

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1,383.21,510

1,230.2

1,154.8

1,000

410 406 402 398 394 390 294 290 286 282

1,200

400.4 eV

N 1s C 1s sp2 [email protected]

C–[email protected]=C–O

@288.8

[email protected]

1,400 1,600 1,800

2,680

52 56 60 64

2,684 2,688

1,200 1,600 2,000 2,400 2,800

Pristine G

PBASE/G

Figure 3 | Raman and XPS studies of CVD graphene films adsorbed with a PBASE monolayer. (a) Representative Raman spectra before and after PBASE

SAM growth at the same spot near the G band. The inset figure of a shows the overall Raman spectrums comparison. (b,c) Histograms showing the

position and FWHM extracted from Raman colour mapping over the same area. (d,e) High-resolution N 1s and C 1s XPS spectrum of CVD graphene

adsorbed with PBASE, respectively. XPS, X-ray photoelectron spectroscopy.





can be deconvoluted into a weak C–N peak at 286.8 eV and themain C 1s peak at 284.5 eV, as shown in Fig. 3e. The asymmetricstretching modes of PBASE are detected by Fourier transforminfrared spectroscopy at 3,040, 2,935 and 2,868 cm� 1 for alkylgroup stretching and one sharp peak at 848 cm� 1 from C–Hwagging vibration (see Supplementary Fig. 6b. For morecharacterizations, see Supplementary Figs 12–16).

Conductivity enhancement by PBASE for stacked CVDgraphene. To investigate the relationship between molecularstructure and the ability of the molecules to act as effectivemolecular linkers, six pyrene derivatives with different pendantgroups were self-assembled on CVD graphene. Several hundreddata were collected for each G/molecules/G sandwich device (forstatistics histogram, see Supplementary Fig. 9). It is obvious thatG/PBASE/G forms the most stable junction devices and has agood uniformity. To get rid of conductivity contribution from thesolvent, a control sample was soaked in pure DMF solvent underthe same condition and very marginal change is observed. Thelog-average J values at a bias of 1 V with log-s.d. error bar aresummarized in Fig. 4i for comparison purposes. It was found thatmolecules that can undergo intermolecular hydrogen bondingtend to aggregate and result in poor conductivity when used asmolecular linkers. The bond energy of p–p stacking is typicallyo10 kcal mol� 1, which is comparable to moderate and weakhydrogen bond energy. Hence, for pyrene derivatives which canundergo intermolecular hydrogen bonding, its packing order andstructure will be determined by competition between p–p stack-ing and hydrogen bonding. The topography of the SAM formedby two molecules, 1-pyrenecarboxylic acid and 1-pyrenebutanolon graphene, illustrates these competing effects. For 1-pyr-enebutanol, the SAM suffers from some degree of aggregation

(see Supplementary Fig. 10 and Supplementary Note 5). Thepresence of molecular clusters increases the tunnelling distanceand blocks the charge transfer, resulting in poor current density.Moreover, our density functional theory (DFT) calculation showsthat unlike PBASE, 1-pyrenebutanol does not undergo effectiveorbital hybridization with graphene (see below ‘DFT simulation’section). In the case of 1-pyrenecarboxylic acid, the thickness ofmolecular layer is B2 nm. The structure can be described by twohydrogen-bonded repeating units in the herringbone structure(see Supplementary Fig. 10e,f), originating from the strongintermolecular hydrogen bonding of the COOH groups18. In thiscase, the increase in interplanar distance d lowers the conductivitysignificantly (equation (1)) and offsets any positive effects oforbital hybridization of 1-pyrenecarboxylic acid with graphene.Although PBASE molecules have the largest side chain groupamong these molecules, its large pendant N-hydroxysuccinimideester group blocks a second layer of molecules from stacking ontop, hence it is self-limiting at a monolayer, thus ensuring that thetunnelling distance does not exceed the thickness of themonolayer. Stacked CVD graphene layers typically have aninterplanar distance that is much larger than the van der Waalsdistance due to inclusions of polymer residues and wrinkles,substitution of these by PBASE narrows the physical gap andresults in enhancement of the tunnelling current.

Influence of PBASE on mechanically exfoliated graphene stack.In the case of mechanically exfoliated graphene flakes, which arestacked by the dry transfer method, the coupling between thestacked graphene layers is better than that of CVD graphene dueto the absence of polymer residues and wrinkles30–32. In this case,the interplanar separation is close to the van der Waals distanceand the insertion of PBASE between the layers actually widens the

PBASE

DMF soakingBlank

1-Pyrenebutyric acidic acid

OH

O

1-Pyrenebutanolbutanol

OH

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0.5 1.0

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10–

10

10

10

10

10ylic acid

OHO

Pyrene-1-boronic acid

0 0.5 1.0

onic acid

B

HO OH

10–5

10–3

10–1

101

103

G/molecules/G

g a c b d e f

Figure 4 | Comparing the current density in G-SAM-G sandwich structure with different SAMs. (a–f) J(V) curves of G/PBASE/G, G/1-pyrenebutyric

acid/G, G/1-pyrenebutanol/G, G/1-pyreneacetic acid/G, G/1-pyrenecarboxylic acid/G and G/pyrene-1-boronic acid/G devices, respectively. (g) J(V)

curves of directly LBL-stacked G/G devices. (h) J(V) curves of LBL-stacked G/G devices with bottom graphene soaking in DMF solvent for 8 h.

(i) The average J characteristics of G/molecules/G device with a bias of 1.0 V for all these devices. All SAMs were grown for 8 h.





spacing, and will be expected to reduce the tunnelling current.To verify if this is true, the I–V curves of devices made frommechanically exfoliated graphene flakes that are stacked by drytransfer method are recorded with four-probe stations and listedin Fig. 5c, where we compare the tunnelling current density ofG/G and G/PBASE/G. In contrast to the conductivityenhancement for stacked CVD graphene system reported earlier(Fig. 5a,b), there is now a slight reduction in the currentfrom B8.0� 102 (linear I–V curve, at 70.5 V7 bias) to8.2� 101 A cm� 2 after the introduction of PBASE SAM layerbetween two graphene flakes. From equation (1), the value of bwill vary under different junction systems. In graphene–insulatortunnelling system, the tunnelling current density has anexponential dependence on the thickness of the tunnellingbarrier d. Compared with graphene–boron nitride–graphenesystem, with tunnel distance of DdE6 Å, there should be a threeorder of magnitude drop in current density from graphene–graphene device33. However, as shown in Fig. 5c,d, the currentdensity only decreases by one order of magnitude, which revealsthat the presence of SAM molecules as an interlayer has a positiveeffect in enhancing the vertical conductivity to counteract thecurrent decay due to the increase in tunnelling distance, althoughthe effect of the latter, which has an exponential dependence,dominates the current density eventually. We can inferthat compared with mechanically exfoliated, dry-transferredgraphene, the much larger increase in tunnelling currentdensity in the case of stacked CVD graphene after insertingPBASE is due to the supplanting of polymer residues as well assuppression of wrinkles.

DFT calculations of structure and electronic properties. Tounderstand why subtle changes in the functional groups of thepyrene-based SAM affects the vertical conductivity when it isinserted between two graphene layers, DFT calculations wereperformed to investigate effect of PBASE molecules on electronicproperties of two stacked CVD graphene. First, the out-of-planeseparation (d) between two graphene sheets illustrated in Fig. 6a

(left) was adjusted to investigate the separation-dependent tun-nelling barrier and out-of-plane band gap, where the tunnellingbarrier (DV) is defined as the potential energy above the asso-ciated Fermi energy29,34–36. As expected, both the tunnellingbarrier and out-of-plane band gap of two graphene sheets arealmost constant with d varying from 14 to 8 Å and then decreasedramatically with further reduction of out-of-plane separation d.The interfacial configuration of PBASE molecular layers shown inFig. 6c (left) is most energetically favourable, in which thecalculated separation between top and bottom graphene is about13.4 Å, in good agreement with above experimental result. Theequilibrium distance between molecules and bottom graphenelayer (db

eq) is 2.3 Å (Fig. 6c) and is similar to typical metal–graphene chemisorption separation distance (Pd, deq¼ 2.3 Å).From the partial density of states (see Supplementary Fig. 11a),the p orbital of C atom at bottom graphene layer and s orbital ofnearest H atom in PBASE molecule are slightly hybridized29.With PBASE as the molecular linker, the tunnelling barrierdecreases by 0.98 eV (from 4.22 to 3.24 eV). It is noted that theenergy gap for pure two graphene layers at this separationdistance is about 4.06 eV at M-point. After the introduction ofSAM in between graphene layers, the out-of-plane band gapappears at k-point and is reduced to 0.71 eV. Therefore, thecombined effects of orbital hybridization, reduction in band gap(by 3.35 eV) and tunnelling barrier contribute to the significantenhancement of vertical conductivity after SAM insertion. Inaddition, from the out-of-plane band structure in Fig. 6d, theshifting of the Fermi level towards lowest unoccupied molecularorbital also indicates that holes are donated by the absorbedPBASE molecules to graphene, resulting in p-doping. However,for the G/1-pyrenebutanol/G fully relaxed structure, db

eq and dteq

(dteq, the most energetically favourable distance between

molecules and the top graphene) increase to 2.5 and 3.6 Å,respectively, as shown in Fig. 6e, which increase the tunnellingbarrier height DV to 3.41 eV (3.24 eV for PBASE molecules).Although its out-of-plane band gap is also at k-point, thereduction in energy gap (3.09 eV, from B4.06 to 0.97 eV) issmaller than that of PBASE molecules. Beside this, the orbital

–0.5 0.0 0.510–4

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Figure 5 | Average J(V) curves’ comparison of devices prepared by different methods. (a,b) J(V) curves of G/G and G/PBASE/G devices via wet

transfer of CVD graphene. (c,d) J(V) curves of G/G and G/PBASE/G devices via dry transfer of micromechanically cleaved graphene monolayer. The error

bars refer to the log-s.d. of the log-average J values.





hybridization of 1-pyrenenbutanol with graphene is muchweaker (see Supplementary Fig. 11b). These results illustratethat PBASE molecules are unique in bridging two stackedCVD graphene sheets and improving its out-of-planeconductivity.

DiscussionOur studies reveal that the poor plane-to-plane charge transportof LBL-stacked CVD graphene films, which is related to organicinclusions in the interlayers and wrinkles inherited from the wettransfer process, can be alleviated by using self-assembled PBASEas a molecular linker between the stacked graphene sheets. WithPBASE inserted between two stacked CVD graphene, the plane-to-plane conductivity is enhanced by six orders of magnitude(current density increases from B10� 3 to B103 A cm� 2). Theimprovement is due to a combination of factors, which includedoping, planarization effect of PBASE on wrinkles created duringthe growth or transfer process, as well as the supplanting ofpolymeric residues by PBASE, which undergoes strong p–pstacking with graphene. The unique molecular structure ofPBASE with a pyrene base and a pendant side group also favoursorbital hybridization and molecular bridging between thegraphene layers. The conductivity enhancement by molecularlinkers applies mainly to wet-transferred film. On dry-transferred,mechanically exfoliated graphene film, the presence of PBASE

contributes to a one order reduction in vertical conductivity.Nevertheless, the use of self-assembled molecular layers asmolecular plaster to improve the electronic coupling betweentwo stacked layers remains highly relevant to large area, CVD-grown 2D films, since these are generally transferred by wettransfer methods, and are thus affected by wrinkles and organicinclusions when stacked in LBL manner.

MethodsJunction of CVD graphene via wet transfer. Using PMMA as a backing layersimilar to the transfer methods reported in the previous literatures, the as-grownfilms were transferred onto Au/Cr (100 nm/5 nm) substrates3,5. The graphene filmswere soaked in 5 mM PBASE in DMF solution for various hours at 80 �C to allowthe PBASE molecules to adsorb by p–p interactions23. After drying, anothergraphene film (smaller in surface area) was directly stacked onto the centre of firstbottom graphene films to avoid the direct contact of upper graphene with theunderlying gold substrates.

Junction of mechanically cleaved graphene via dry transfer. The bottommonolayer graphene was directly scotch taped onto silicon wafer (pre-cleaned byoxygen plasma), as a result, this graphene layer is almost wrinkle free and have nopolymer residues. After PBASE SAM formation, another fresh exfoliated graphenewas transferred on top to form the junctions via dry transfer stage30–32. Thesedevices were fabricated in Class 1000 cleanroom.

Electronic measurement via EGaIn Tip. In a home-built set-up similar to thatreported by Whitesides and Chiechi et al.19,20, eutectic metal alloy EGaIn(75.5 wt.% gallium and 24.5 wt.% indium, from Sigma-Aldrich) was used to record

H

ONd

(Å)

Eletrostatic potential (eV)

HO

Eletrostatic potential (eV)

5

10

15

20

–40–30–20–10010

ΔV=3.41 eV

5

10

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20

–40–30–20–10010

ΔV=3.24 eV

d teq

d beq

d teq

d beq

d(Å)

–1

0

1

2

HOMO

H K

H K

LUMO

Ene

rgy

(eV

)

0.71

–1

0

1

2

Ene

rgy

(eV

)

–15 –10 –5 0 5

2

4

6

8

10

12

14Top CVD graphene

Bottom CVD graphene

d

ΔV (eV)

d(Å)

ΔV

Eg (eV)

HOMO

LUMO

0.97

Eletrostatic potential

G/1-Pyrenebutanol/G

G/PBASE/G

Ef

Ef

3.4 3.6 3.8 4.0

2

4

6

8

10

12

14

d(Å)

Figure 6 | DFT-simulated structures and their electronic properties. (a) Typical electrostatic potential plot and tunnelling barrier height DV as a function

of separation distance between two graphene layers (d). (b) d-dependent band gap. (c) Left, optimized G/PBASE/G structure, where dteq ¼ 2:9 A and

dbeq ¼ 2:3 A; right, its corresponding electrostatic potential. (d) Band structure of G/PBASE/G at k-point. (e) Left, optimized G/1-pyrenebutanol/G device.

The dteq and db

eq are about 3.6 and 2.5 Å; right, its corresponding electrostatic potential. (f) Band structure of G/1-pyrenebutanol/G at k-point.





the J(V) characteristics at room temperature. The cone-shaped Ga2O3/EGaIn topelectrodes were fabricated and lifted by a 10-ml glass syringe (Hamilton, 1701RNR)with a metal needle (Hamilton, conical shape 26s). To isolate from vibrations thatare dominant sources of the failure of the junctions, a series of vibration dampingsystems were employed, one of which critical components of the EGaIn-setup is themicromanipulator (Leica) with only 1.0 nm per hour drifting. For the contact withbottom electrode, a micro-positioner (Crest innovation, S-725PLM) was used tohold a gold probe tip to form direct electrical contact with bottom gold substrate.All the I–V measurements were recorded using a Keithley 6485 picoammeter.

DFT simulation. All calculations were carried out using DFT-based Vienna abinitio simulation package (VASP) with Perdew–Burke–Ernzerhof approximationfor the exchange-correlation functional, and frozen-core all-electron projector-augmented wave method37–39. The cut-off energy for the expansion of plane-wavebasis was set to 400 eV. The graphene/molecular/graphene junctions were modelledby sandwiching the molecules in between two 7� 7� 1 graphene supercells, andthe related Brillouin zone was sampled by gamma-centred 3� 3� 1 k-pointmeshes. To minimize the interaction between neighbour graphene surfaces, a 15-Åvacuum was applied normal to graphene surface. All structures were optimizeduntil the force on each atom is smaller than 0.02 eV Å� 1, in which van der Waalseffects were included using DFT-D2 method of Grimme40.

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AcknowledgementsK.P.L. thanks MOE Tier II grant ‘Interface Engineering of Graphene Hybrids for EnergyConversion’. Grant Number: R-143-000-488-112. The Singapore National ResearchFoundation (NRF Award No. NRF-RF 2010-03 to C.A.N.) is kindly acknowledged forsupporting this research. We thank J. WU and S.F. WANG for helpful discussions onFET device fabrication.

Author contributionsK.P.L. and C.A.N. supervised the project. Y.L. and L.Y. designed and performed theexperiments. Y.P.L. prepared graphene samples and conducted the SAM formation.L.Y. performed the J(V) measurements. M.Y performed VASP DFT calculations.Y.Z., C.T.N. and N.N. helped to record and analyse the AFM, Raman, XPS, FTIR andUPS spectra. L.G. and L.L. helped to prepare graphene films and electrode fabrication.C.S.S.S. recorded the I–V curves of exfoliated graphene samples. All authors contributedto writing the manuscript.

Additional informationSupplementary Information accompanies this paper at http://www.nature.com/naturecommunications

Competing financial interests: The authors declare no competing financial interests.

Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/

How to cite this article: Liu, Y. et al. Giant enhancement in vertical conductivity ofstacked CVD graphene sheets by self-assembled molecular layers. Nat. Commun. 5:5461doi: 10.1038/ncomms6461 (2014).






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