ABSTRACT Device applications of graphene such as ultrafast transistors and photodetectors benefit from the combination of both high-quality p- and n-doped components prepared in a

large-scale manner with spatial control and seamless connection. Here we develop a well-controlled chemical vapour deposition process for direct growth of mosaic graphene. Mosaic graphene is

produced in large-area monolayers with spatially modulated, stable and uniform doping, and shows considerably high room temperature carrier mobility of ~5,000 cm2 V−1 s−1 in intrinsic

portion and ~2,500 cm2 V−1 s−1 in nitrogen-doped portion. The unchanged crystalline registry during modulation doping indicates the single-crystalline nature of p–n junctions. Efficient hot

BY OTHERS DEMONSTRATION OF EPITAXIAL GROWTH OF STRAIN-RELAXED GAN FILMS ON GRAPHENE/SIC SUBSTRATES FOR LONG WAVELENGTH LIGHT-EMITTING DIODES Article Open access 03 June 2021 LARGE-AREA

SYNTHESIS AND TRANSFER OF MULTILAYER HEXAGONAL BORON NITRIDE FOR ENHANCED GRAPHENE DEVICE ARRAYS Article Open access 06 February 2023 DIMENSION- AND POSITION-CONTROLLED GROWTH OF GAN

MICROSTRUCTURE ARRAYS ON GRAPHENE FILMS FOR FLEXIBLE DEVICE APPLICATIONS Article Open access 01 September 2021 INTRODUCTION Graphene, a single layer of hexagonal carbon framework with

broadband photon absorption and extraordinary carrier mobility1, is attractive to high-performance electronic and optoelectronic devices, such as transistors2,3,4,5, optical modulators6 and

photodetectors7. Introduction of p–n junctions to graphene would allow for novel phenomena including Klein tunneling8, negative refractive index for Veselago lens9 and even photoelectric

conversion with a hot carrier-assisted photothermoelectric process10,11. However, existing methods for fabrication of graphene p–n junctions usually require external gate10 or unstable

adsorbed chemical dopants12, which are inconvenient for practical applications. In contrast, substitutional doping with heteroatoms via chemical vapour deposition (CVD) provides an effective

route for simple and stable tuning of doping levels in graphene. Production of graphene via CVD growth on transition metal substrates has been steadily maturing13,14. Recently, N-doped

graphene was successfully synthesized by mixing nitrogen compounds into forming gas during CVD growth15,16,17. Nevertheless, reported N-doped graphene typically has broad distribution of

thicknesses, high-density of grain boundaries, and variations over dopant concentration and distribution. Moreover, selective-area substitutional doping of graphene is even more challenging,

partly because traditional semiconductor selective doping techniques are less effective to this perfect atomically thin two-dimensional (2D) crystal formed by robust C–C bonds. It further

hinders the creation of single-crystalline graphene p–n junction, which is of great significance for potential optoelectronic applications. Modulation doping, a technique to integrate of

intricate n- and p-type segments in a tuned manner, is a powerful approach to achieve various nanostructure junctions with single-crystalline nature18. To improve the quality of N-doped

graphene, we established a controlled growth technique to achieve modulation doping during the synthesis in single process. We have produced a novel mosaic graphene, a continuous graphene

membrane with uniform thickness and regionally varied doping profile. RESULTS GROWTH OF MODULATION-DOPED MOSAIC GRAPHENE The schematic structure as well as its growth procedure of

modulation-doped mosaic graphene is shown in Fig. 1a. To initiate the growth, discrete intrinsic graphene grains with typical diameter of ~10 μm were first grown on an annealed

polycrystalline copper substrate at 1,000 °C13 (Supplementary Fig. S1). These discrete grains would serve as matrices for the following grafting stage. After a short purging period,

acetonitrile vapour was introduced as precursor gas for the laterally grafted growth of N-doped graphene15 (Supplementary Fig. S2). As the space between the intrinsic grains was previously

controlled, spontaneous nucleation of N-doped graphene grains is successfully suppressed (Supplementary Fig. S3). Coalescence of grains eventually yields a continuous monolayer mosaic

graphene film with uniform contrast under optical microscope (OM) (Fig. 1b). As shown in the scanning electron microscope (SEM) image (Fig. 1c), bright polygonal islands corresponding to

intrinsic graphene grains were clearly recognized, surrounded by dark intervals with substitutionally doped nitrogen atoms. The sample is predominantly of monolayer coverage due to the

suppression of graphene adlayers on copper13. In addition, once seeds for intrinsic grains were specifically predefined19, the consequent growth of mosaic grains can be as well templated,

thus leading to the creation of periodic mosaic graphene superlattice structure (Fig. 1d and Supplementary Fig. S4). More complex mosaic graphene structures were achieved through multiple

modulation cycles with period tuning. Figure 1e shows typical SEM images of the mosaic graphene with two and three modulation cycles, respectively. Spatially well-defined intrinsic and

N-doped portions are clearly recognized as homocentric growth rings with alternating contrast. The observation of sharp junction interface indicated the lateral extension growth and the high

thermal stability of N-doped portion. The width of either well-defined portion can be as narrow as several hundred nanometres. On the other hand, wafer-sized monolayer mosaic graphene was

successfully synthesized (Fig. 1g), which can be further scaled up for batch production of mosaic graphene, by using larger vessel and copper foil, or the roll-to-roll process14.

SPECTROSCOPIC CHARACTERIZATIONS OF MODULATION-DOPED MOSAIC GRAPHENE The concentration and uniformity of doping in either side of the lateral junctions, as well as the width of depletion

region, are important for the performance of p–n junction. To explore this, mosaic graphene grown on copper substrate was characterized by both photoemission electron microscope (PEEM) and

low-energy electron microscope (LEEM) under ultrahigh vacuum. As the photoemission threshold strongly depends on the position of Fermi level, doping modulation would lead to variations in

the productivity of photoelectrons20. As a result, the intrinsic graphene portion with higher work function exhibited relatively darker image contrast compared with N-doped portion in the

PEEM image (Fig. 2a). A clear boundary at the lateral junction of mosaic graphene was observed in the PEEM image. The profile line in Fig. 2a reveals the variation of work function across

graphene lateral junction. Despite the fluctuations in both portions, the profile exhibited an abrupt rise within 80 nm at the junction interface, indicating the sharpness of the lateral

junction. Similar with SEM, intrinsic portions in the same area of the mosaic graphene sample exhibited relatively lighter contrast in LEEM image taken at start voltage of 1.70 V (Fig. 2b,

inset). While the energy of incident electron is increasing from 0 eV, the reflected intensity _I_ gradually drops from _I_0. The threshold electron energy at which relative reflectivity

_I_/_I_0 decreases to 0.9 can be used to identify the work function of graphene21,22. We accordingly found the work function of intrinsic and N-doped graphene within the same copper grain

differed for ~0.2 eV at 330 K (Fig. 2b). To determine the quality and the spatial distribution of dopant in mosaic graphene, we conducted Raman mapping of mosaic graphene transferred on a

SiO2/Si substrate. As illustrated in Fig. 2c, the D band intensity map revealed a polygonal pattern with dark cores surrounded by bright loops. The uniformity of doping was again confirmed

through the distribution of D band intensity. Full spectra of both portions were collected and illustrated in Fig. 2d. The spectrum exhibited sharp G and 2D Raman bands with a ratio

_I_G/_I_2D<1 was recognized as intrinsic graphene23. The absence of the D band indicates that the high quality of intrinsic graphene was preserved despite of the following N-doped growth.

In contrast, the region exhibited strong D and D* bands, as well as broadening and shifting of both G band and 2D band in its spectrum (Supplementary Fig. S5) can be ascribed to N-doped

portion24,25. STRUCTURAL CHARACTERIZATIONS OF MODULATION-DOPED MOSAIC GRAPHENE The quality of graphene junction between intrinsic and N-doped portions is crucial for carrier conduction as

well as efficient photocurrent generation and collection. Especially, single-crystalline junction would provide much better performance with less carrier scattering than grain boundaries in

polycrystalline graphene19. To this end, we prepared discrete mosaic graphene grains, by halting the growth before full coverage in the stage of N-doped graphene, which facilitates

investigation of the interfacial structure after locating the lateral junction. From the SEM image (Fig. 3a), we confirmed that these discrete grains consist of intrinsic cores and ~2 μm

wide N-doped edges. The specific shape as well as the width of the edge could serve as marks for the locating of the lateral junction with atomic force microscopy (AFM) and transmission

electron microscopy (TEM). Consistent with the OM image, the interface identified by the dashed line showed ideal smoothness in AFM (Fig. 3b and Supplementary Fig. S6), with no observable

crack or overlap. To further confirm the single-crystalline essence of the junction, samples from the same batch were characterized by TEM and selected area electron diffraction (SAED).

Ultra-thin (~5 nm thick) porous carbon membrane was used to support these discrete grains. In Fig. 3c, graphene grains were identified through the slight contrast from the support. Along the

white dashed arrow in Fig. 3c, extensive SAED patterns were captured with ~600 nm aperture and incident beam normal to the sample. Six typical SAED patterns among them were illustrated

(Fig. 3d), taken from the positions labelled in Fig. 3c, which together confirmed the single-crystalline nature of the discrete grain. The first and the last patterns were collected within

~2 μm from the edge correspond to the N-doped portion, whereas the other four lie in the intrinsic portion. It is worth noting that these patterns exhibit the same orientations of ~10° as

labelled in Fig. 3d. The thickness of the sample was further confirmed to be monolayer by analysing line profile of the diffraction pattern in Fig. 3d26. Histogram of pattern orientation

distributions from extensive SAED studies shown in Fig. 3e exhibited two pronouncing peaks separated by <1°, which could be reasonably ascribed to the wrinkle in the centre of the grain

(Fig. 3c). Moreover, lattice distance of the sample could also be obtained from SAED patterns. The histogram in Fig. 3f reveals an average lattice distance of ~2.43 Å. These structural

observations conclude that the crystal registry is retained during modulation doping growth of mosaic graphene, yielding single-crystalline lateral p–n junctions. TRANSPORT PROPERTIES OF

MODULATION-DOPED MOSAIC GRAPHENE Fundamental transport measurements were performed to evaluate the electronic properties of our modulation-doped graphene, in particular to verify the

single-crystalline lateral junction. Continuous mosaic graphene film transferred onto a silicon substrate with SiO2 as back gate was etched into narrow strips, and then embedded with

four-probe configuration (Fig. 4a, inset). Resistance of both portions and the junction are measured at room temperature. As shown in Fig. 4a, N-doped portion exhibited larger resistance

than intrinsic, arising from scattering defects. However, resistance across lateral junction is quite similar with that of intrinsic portion. The absence of scattering at the junction

evidently indicates the high quality of the lateral junction, resulting from the single-crystalline essence. Moreover, graphene p–n junction exhibited no rectification effect as expected

because of the absence of effective band gap, as indicated by the output properties shown in Supplementary Fig. S7. Transfer properties of mosaic graphene are further studied as shown in

Fig. 4b and Supplementary Fig. S8. Gate sweeping of each portion produced a single peak in resistivity. The distance between two charge neutrality points (Dirac points) corresponds to an

electron doping concentration of _n_d~2.70 × 1012 cm−2. This result resonates well with the work function difference measured by LEEM through the relation (ref. 27). Transfer characteristic

across the interface exhibits two separated peaks, hallmark of a graphene p–n junction. We further extracted carrier mobility near each Dirac point from these curves. Surprisingly, the room

temperature mobility of the N-doped portion can be as high as 2,500 (holes) and 1,800 (electrons) cm2 V−1 s−1, comparable to that of the intrinsic portion (4,000 cm2V−1 s−1 for holes and

2,500 cm2V−1 s−1 electrons). Extensive study on other 10 devices yielded a mobility of 1,000–2,500 cm2V−1 s−1 for N-doped and 2,500–5,000 cm2V−1 s−1 for intrinsic graphene, respectively.

Such mobility of N-doped graphene is over 1–2 orders of magnitude higher than that in previous reports16. In conventional CVD growth of N-doped graphene, the density of spontaneous

nucleation is remarkably high (Supplementary Fig. S9), yielding massive scattering grain boundaries. In contrast, grain boundaries in our N-doped mosaic graphene are limited by predefined

intrinsic grains, yielding high-carrier mobility with less grain boundary scattering28. PHOTOCURRENT GENERATION AT GRAPHENE P–N JUNCTION The remarkably high mobility of our mosaic graphene

with high-quality p–n junctions facilitates the generation of efficient photocurrent under illumination. As a demonstration, one grain of modulation-doped graphene was patterned and embedded

into a two-terminal device (Fig. 4c, inset). A focused 632.8nm laser spot (~1 μm, ~900 μW) was used to excite photocarriers. As illustrated in Fig. 4c, the p–n junction produced a

pronounced current shift when illuminated by laser, indicating its capability of photoelectric conversion. We further conducted photocurrent mapping of the device (Fig. 4c, inset). It was

clearly observed that photocurrent was generated over the junction, as well as the two electrodes, with contrary directions. Moreover, the intensity of photocurrent generated at the junction

is approximately two times stronger than that at the graphene/electrodes junctions, indicating higher efficiency for potential photodetector applications. DISCUSSION The photocurrent of p–n

junction as function of carrier concentration was plotted together with the resistance in Fig. 4d. The resistance followed a typical curve with two neutral points, separating the whole

curve into three regimes labelled n-n+, p–n and p+-p. Meanwhile, the photocurrent curve exhibited a single peak with two polarity reversals, in contrast to those at source and drain

electrodes, both of which reversed only once as indicated in Supplementary Fig. S10. Moreover, the region whose photocurrent is determined by p–n junction could span over 2 μm along the

channel. The maximum current of ~125 nA pinned between the two neutral points in the p–n regime, when laser is right positioned at the junction. The polarity reversal of the photocurrent is

in contradiction with traditional photovoltaic process in which excited carriers were separated solely by the built-in electric filed, as the polarity of p–n junction remained unchanged

during tuning of the global gate voltage. To understand the reversal, photothermoelectric effect10 is considered as the primary scheme, where hot carriers excited by photon eventually result

in thermoelectricity. The photovoltage is determined by the temperature difference Δ_T_ of electrons inside and outside the excited zone. Considering the Seebeck coefficient of graphene

(~100 μV K−1) at room temperature29, Δ_T_ was estimated to be approaching 10 K in our case. The efficiency of photocurrent generation relies on the photon absorption and excitation rate of

carriers, as well as the separation of excitons. In our case, the unbiased photocurrent responsivity of ~0.1 mA W−1 is mainly impeded by the large channel resistance as well as energy

dissipation through the substrate. On the other hand, carrier multiplication predicted theoretically30 may further improve the carrier excitation rate in graphene. Enhancement of absorption

through plasmonic resonance31,32 or microcavity33 could also help to increase the absolute response of monolayer graphene. We further studied integration of multiple graphene photodetector

channels, as a model device for multiple signal computing. A photodetector array with seven individual p–n junction channels was fabricated monolithically from a single graphene hybrid

domain (Fig. 4e). A laser spot with ~5 μm in diameter shone over three p–n junction channels to produce photocarriers. Depending on the status of corresponding peripheral switches,

cooperative photodetection was achieved. Signals from individual channels as well as their additions were shown in Fig. 4f. It thus confirmed the reliability as well as stability of these

photodetectors based on single-crystalline graphene p–n junctions. In addition, the photocurrent strength generated by each individual channel is proportional to the power of illuminating

light, suggesting the possibility of imaging with spatial resolution. In summary, we have established a modulation doping technique for controlled growth of mosaic graphene with spatially

well-defined dopant and single-crystalline p–n junctions. The sample showed excellent transport properties in both intrinsic and N-doped portions. High-quality p–n junctions between two

portions can be used for efficient photocurrent generation under the photothermoelectric scheme. Arising from the improved doping, mosaic graphene would benefit not only graphene based

photocurrent generation but also fuel cells34, lithium batteries15 and even supercapacitors35. METHODS MOSAIC GRAPHENE GROWTH AND TRANSFER Large-area mosaic graphene was grown on annealed

copper foil loaded inside a homemade low-pressure CVD system, with methane and acetonitrile vapour for intrinsic and N-doped portions, respectively. The sample was then transferred to

silicon wafer covered with silicon oxide with the aid of poly(methyl methacrylate). Detailed growth procedure could be found in Supplementary Methods. CHARACTERIZATIONS OF MOSAIC GRAPHENE

Characterizations were done with OM (Olympus BX51), SEM (Hitachi S-4800, acceleration voltage 5–30 kV), AFM (DI Nanoscope IIIa), Raman spectrum (Horiba, LabRAM HR-800) and TEM (FEI Tecnai

T20, acceleration voltage 200 kV). PEEM and LEEM were carried out in an Elmitec LEEM/PEEM system with an aberration corrector under ultrahigh vacuum of ~1 × 10−10 Torr. The PEEM image was

acquired with a mercury lamb. DEVICE FABRICATION Mosaic graphene was first transferred onto a silicon substrate with silicon oxide as dielectric layer. SEM was used to identify specific

regions, while p–n junction from the same domain is preferred. Standard EBL (STRATA DB235, FEI) was carried out to define micro patterns. Designed graphene strips were shaped by plasma

etching. Afterward, trilayer metal electrodes (0.5 nm Cr/25 nm Pd/10 nm Au) were deposited by thermo/e-beam/thermo evaporation (UNIVEX 300, Leybold Vacuum) in one batch. The device was

lifted-off by acetone and washed with isopropanol. Finally, it was blow dried with nitrogen gas. TRANSPORT MEASUREMENT A homemade spherical chamber combined with turbo station (Pfeiffer,

HiCube 80 Eco) provided high-vacuum environment for the transport measurement of graphene p–n junction. Overnight pumping was usually required to achieve the intrinsic performance. Baking

was carefully avoided in case of any potential damage. A semiconductor analyser (Keithley, SCS-4200) was used to measure the four terminal electrical properties with Keithley 6517A providing

gate bias. PHOTOCURRENT MEASUREMENT Photocurrent from graphene p–n junction was measured on a modified Raman spectrometer (Renishaw-1000). Electrical cables were equipped to extract

photocurrent to the semiconductor analyser (Keithley, SCS-4200). A 632.8-nm Griot He–Ne laser was focused through an Olympus BH2 OM. Microstage on the Raman spectrometer is used for

alignment with accuracy better than 0.1 μm. ADDITIONAL INFORMATION HOW TO CITE THIS ARTICLE: Yan, K. _et al._ Modulation-doped growth of mosaic graphene with single-crystalline p–n junctions

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Scholar  Download references ACKNOWLEDGEMENTS We thank Alan Y. Liu, Desheng Kong, Gang Zhang, Dong Sun and Chuanhong Jin for helpful discussions and acknowledge financial support by the

National Natural Science Foundation of China (nos. 51121091, 51072004, 20973007, 21173004 and 21222303) and the National Basic Research Programme of China (nos. 2013CB932603, 2012CB933404,

2011CB921904 and 2011CB933003), the Programme for New Century Excellent Talents in universities and the Scientific Research Foundation for Returned Overseas Chinese Scholars, the State

Education Ministry (SRF for ROCS, SEM). AUTHOR INFORMATION Author notes * Kai Yan and Di Wu: These authors contributed equally to this work. AUTHORS AND AFFILIATIONS * Center for

Nanochemistry, Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular

Engineering, Peking University, Beijing, 100871, China Kai Yan, Di Wu, Hailin Peng & Zhongfan Liu * State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, Dalian, 116023, China Li Jin, Qiang Fu & Xinhe Bao Authors * Kai Yan View author publications You can also search for this author inPubMed Google Scholar * Di Wu

View author publications You can also search for this author inPubMed Google Scholar * Hailin Peng View author publications You can also search for this author inPubMed Google Scholar * Li

Jin View author publications You can also search for this author inPubMed Google Scholar * Qiang Fu View author publications You can also search for this author inPubMed Google Scholar *

Xinhe Bao View author publications You can also search for this author inPubMed Google Scholar * Zhongfan Liu View author publications You can also search for this author inPubMed Google

Scholar CONTRIBUTIONS K.Y., H.P. and Z.L. conceived and designed the experiments. K.Y., D.W. and H.P. performed the synthesis and the structural characterization. K.Y. and D.W. performed the

device fabrication, the transport and photocurrent measurements. L.J., Q.F. and X.B. performed LEEM and PEEM. K.Y., H.P. and Z.L. co-wrote the paper. H.P. and Z.L. supervised the project.

All authors contributed to the scientific planning and discussions. CORRESPONDING AUTHORS Correspondence to Hailin Peng or Zhongfan Liu. ETHICS DECLARATIONS COMPETING INTERESTS The authors

declare no competing financial interests. SUPPLEMENTARY INFORMATION SUPPLEMENTARY INFORMATION Supplementary Figures S1-S10, Supplementary Methods and Supplementary References (PDF 3756 kb)

RIGHTS AND PERMISSIONS This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. To view a copy of this license, visit

http://creativecommons.org/licenses/by-nc-sa/3.0/ Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Yan, K., Wu, D., Peng, H. _et al._ Modulation-doped growth of mosaic graphene

with single-crystalline p–n junctions for efficient photocurrent generation. _Nat Commun_ 3, 1280 (2012). https://doi.org/10.1038/ncomms2286 Download citation * Received: 17 September 2012 *

Accepted: 14 November 2012 * Published: 11 December 2012 * DOI: https://doi.org/10.1038/ncomms2286 SHARE THIS ARTICLE Anyone you share the following link with will be able to read this

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