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# Heterointerface effects in the electrointercalation of van der Waals heterostructures

## Abstract

Molecular-scale manipulation of electronic and ionic charge accumulation in materials is the backbone of electrochemical energy storage1,2,3,4. Layered van der Waals (vdW) crystals are a diverse family of materials into which mobile ions can electrochemically intercalate into the interlamellar gaps of the host atomic lattice5,6. The structural diversity of such materials enables the interfacial properties of composites to be optimized to improve ion intercalation for energy storage and electronic devices7,8,9,10,11,12. However, the ability of heterolayers to modify intercalation reactions, and their role at the atomic level, are yet to be elucidated. Here we demonstrate the electrointercalation of lithium at the level of individual atomic interfaces of dissimilar vdW layers. Electrochemical devices based on vdW heterostructures13 of stacked hexagonal boron nitride, graphene and molybdenum dichalcogenide (MoX2; X = S, Se) layers are constructed. We use transmission electron microscopy, in situ magnetoresistance and optical spectroscopy techniques, as well as low-temperature quantum magneto-oscillation measurements and ab initio calculations, to resolve the intermediate stages of lithium intercalation at heterointerfaces. The formation of vdW heterointerfaces between graphene and MoX2 results in a more than tenfold greater accumulation of charge in MoX2 when compared to MoX2/MoX2 homointerfaces, while enforcing a more negative intercalation potential than that of bulk MoX2 by at least 0.5 V. Beyond energy storage, our combined experimental and computational methodology for manipulating and characterizing the electrochemical behaviour of layered systems opens new pathways to control the charge density in two-dimensional electronic and optoelectronic devices.

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## Acknowledgements

We thank L. Jauregui, I. Fampiou and G. Kim for discussions, and S. Shirodkar for discussions and for sharing data from ref. 29. The major experimental work is supported by the Science and Technology Center for Integrated Quantum Materials, National Science Foundation (NSF) grant DMR-1231319. TEM analysis was supported by Global Research Laboratory Program (2015K1A1A2033332) through the National Research Foundation of Korea. D.K.B. acknowledges partial support from the international cooperation project under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER, B8-2463-05). P.K. acknowledges partial support from the Gordon and Betty Moore Foundation’s EPiQS Initiative through grant GBMF4543 and ARO MURI award W911NF14-0247. DFT calculations made use of the Odyssey cluster supported by the FAS Division of Science, Research Computing Group at Harvard University; and the Texas Advanced Computing Center at the University of Texas at Austin as part of the Extreme Science and Engineering Discovery Environment, which is supported by National Science Foundation grant ACI-1548562. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the Ministry of Education, Culture, Sports, Science and Technology, Japan and JSPS KAKENHI grant JP15K21722. Nanofabrication was performed at the Center for Nanoscale Systems at Harvard, supported in part by an NSF NNIN award ECS-00335765.

### Reviewer information

Nature thanks J. Cha and the other anonymous reviewer(s) for their contribution to the peer review of this work.

## Author information

Authors

### Contributions

D.K.B., M.R. and H.Y. performed the experiments and analysed the data. D.K.B., S.Y.F.Z. and P.K. conceived the experiment. D.T.L. and E.K. performed the theoretical computations. K.W. and T.T. provided bulk h-BN crystals. D.K.B., M.R. and P.K. wrote the manuscript. All authors contributed to the overall scientific interpretation and edited the manuscript.

### Corresponding author

Correspondence to Philip Kim.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Extended data figures and tables

### Extended Data Fig. 1 Additional electrochemical and Hall data for the structure-II graphene/MoSe2 stack.

a, Forward (solid lines) and reverse (dashed lines) sweeps of four-probe resistance (red), Hall carrier density (blue), and Hall mobility (purple) as a function of potential at the heterostructure (versus the counter electrode/electrolyte gate—that is, in a two-electrode electrochemical configuration) in a LiTFSI/PEO electrolyte at 325 K in the presence of a magnetic field, B = 0.5 T. Inset, optical micrograph of heterostructure stack working electrode. b, Identical experiment to that in a with the resistance (red) and Hall carrier density (blue) plotted as a function of the potential measured relative to a Pt pseudoreference electrode. c, Conventional cyclic-voltammetric electrochemical current response (green) overlaid with the resistance (red) over the course of the sweep, showing peaks that are difficult to assign directly to any specific reaction, probably incorporating side reactions at the Pt/electrolyte and Au/electrolyte interfaces. d, Hall resistance Rxy as a function of magnetic field at 325 K after intercalation (E = −4.5 V).

### Extended Data Fig. 2 Additional electrochemical and Hall data of the structure-II graphene/MoS2 stack.

a, b, Resistance (red) and Hall carrier density (blue) as a function of potential in a two-electrode (potential versus counter; a) and three-electrode (potential versus Pt pseudoreference; b) electrochemical configuration in a LiTFSI/PEO electrolyte at 325 K in the presence of a magnetic field, B, of 0.5 T. Inset, optical micrograph of heterostructure stack ‘working electrode’. c, Conventional cyclic-voltammetric electrochemical current response (grey) overlaid with the resistance (red) over the course of the sweep. d, Temperature dependence of resistance (red) and Hall mobility (purple) between 200 K and 1.8 K. e, Hall resistance, Rxy, as a function of magnetic field after cooling to 200 K immediately after the termination of a sweep to −4.8 V. f, Hall resistance Rxy as a function of magnetic field at 1.8 K.

### Extended Data Fig. 3 Additional Raman and photoluminescence spectroscopy data.

a, Raman spectra of an h-BN/graphene/MoS2 structure-II device (identical device to that in Fig. 2b) over the course of electrochemical intercalation, showing the disappearance of spectral features of graphene and MoS2 after full intercalation at −5.0 V, consistent with Pauli blocking in addition to the 2H→1T′ phase transition of MoS2. Deintercalation restores graphene peaks, and annealing at 300 °C for 1 h restores the 2H-MoS2 peaks. Each spectrum is offset for clarity. bg, Schematic diagram (b), photoluminescence spectra (ce), photoluminescence map (f) and Raman map over the 350–450 cm−1 range (g) of an h-BN-encapsulated multi-structure device (identical device to that in Fig. 2d–g) that consists of a graphene monolayer straddling a monolayer MoS2 crystal at one end and a bilayer MoS2 crystal at the other. Data were acquired on the pristine stack before intercalation (c), after deintercalation followed by removal of electrolyte (d) and after subsequent annealing at 300 °C for 1 h (eg). The sharp peak at almost 2 eV is the graphene two-dimensional (Raman scattering) peak. Photoluminescence spatial maps in the pristine state and after deintercalation are presented in Fig. 2e, f and the map of the spatial intensity of the J2 Raman peak of the T′ phase (around 226 cm−1) after annealing is shown in Fig. 2g.

### Extended Data Fig. 4 Electrochemical and Hall data of structure III graphene/MoS2 stack.

a, Resistance (red) and Hall carrier density (blue) as a function of potential in a two-electrode (potential versus counter; a) and three-electrode (potential versus Pt pseudoreference; b) electrochemical configuration in a LiTFSI/PEO electrolyte at 325 K in the presence of a magnetic field B of 0.5 T. Inset, optical micrograph of heterostructure stack ‘working electrode’. c, Conventional cyclic-voltammetric electrochemical current response (grey) overlaid with the resistance (red) over the course of the sweep. d, Temperature dependence of resistance (red) and Hall mobility (purple) between 200 K and 1.8 K. e, Hall resistance Rxy as a function of magnetic field at 1.8 K. This device shows a carrier density of 1.4 × 1014 cm−2. Maximum carrier density observed for structure-III devices is 1.9 × 1014 cm−2.

### Extended Data Fig. 5 Dependence of carrier densities of intercalated structure I on backgate voltage.

a, b, Hall resistance (a) and magnetoresistance, (b; individually offset for clarity), as a function of magnetic field strength, B, in the case of a structure-I device with varying backgate voltage, Vg. c, Dependence of change in Hall (filled circles) and SdH (open circles) carrier densities on Vg. Solid lines represent fits that assume a Si backgate capacitance of 1.2 × 10−8 F cm−2.

### Extended Data Fig. 6 Additional data on multi-structure-device 1.

a, Optical micrograph (false colour) of a device consisting of several h-BN-encapsulated graphene/MoS2 heterostructure types (depicted in the associated illustration) arrayed along a single graphene monolayer (identical device to that in Fig. 4b). b, c, Zonal resistances as a function of potential in a two-electrode (potential versus counter; b) and three-electrode (potential versus Pt pseudoreference; c) electrochemical configuration. Intercalation (indicated by the arrows) initiates at potentials approximately 0.6 V more positive at zones B (structure III) and D (structure II) than at zone A (structure I). d, Conventional cyclic-voltammetric electrochemical current response (grey) of the entire device overlaid with the resistances of the various device regions over the course of the sweep. Cyclic voltammetry cannot distinguish between the intercalation of graphene/MoS2 and graphene/h-BN regions in this device. e, Hall resistance Rxy as a function of magnetic field at 1.8 K for the different regions of the device after electrochemical polarization up to −5.0 V, displaying the resulting Hall carrier densities obtained. f–h, Magnetoresistance data at 1.8 K for zones A (f), C (g) and D (h), showing associated SdH carrier densities nSdH extracted from the periodicities of oscillations in B−1. i, Temperature dependence of resistance for the various device regions between 200 K and 1.8 K during warming.

### Extended Data Fig. 7 Additional data on multi-structure-device 2.

a, Optical micrograph (false colour) of a device consisting of multiple h-BN-encapsulated graphene/MoS2 heterostructure types (depicted in the associated illustration) arrayed along a single graphene monolayer. b, c, Zonal resistances as a function of potential in a two-electrode (potential versus counter; b) and three-electrode (potential versus Pt pseudoreference; c) electrochemical configuration. Intercalation (indicated by the arrows) initiates at potentials approximately 0.7 V more positive at zones B (structure II) and C (structure V) than at zone A (structure I). d, Conventional cyclic-voltammetric electrochemical current response (grey) of the entire device overlaid with the resistances of the various device regions over the course of the sweep. Cyclic voltammetry cannot distinguish between the intercalation of graphene/MoS2 and graphene/h-BN regions in this device. e, Hall resistance Rxy as a function of magnetic field at 1.8 K for the different regions of the device after electrochemical polarization up to −5.5 V, displaying the resulting Hall carrier densities obtained. f–h, Magnetoresistance data at 1.8 K for regions A (f), B (g), and C (h) that reveal associated SdH carrier densities, nSdH from the periodicities of oscillations.

### Extended Data Fig. 8 Electrochemical gating of non-encapsulated few-layer (4–5 layers) MoX2.

a, b, Four-terminal resistance, Rxx, of a few-layer MoSe2 crystal on a linear (a) and a logarithmic (b) scale, during electrochemical gating in an electrolyte comprising LiTFSI dissolved in diethylmethyl(2-methoxyethyl)ammonium TFSI (DEME-TFSI). Intercalation takes place between −2.5 V and −3 V (red arrow) and the device loses electrical contact (demonstrated by the disruption in the phase of the lock-in amplifier (inset)) beyond –3 V. c, Four-terminal resistance, Rxx, of a few-layer MoS2 device during electrochemical gating in a LiTFSI/PEO electrolyte. As in a, the resistance of this device begins to increase at around −3.5 V and is completely insulating beyond −4.25 V, which is indicative of conversion to lithium polysulfide.

### Extended Data Fig. 9 Onset potentials and charge capacities of various heterostructures.

a, Intercalation onset potentials (versus Pt pseudoreference electrode) for different vdW heterostructure types as well as few-layer MoX2. Error bars represent standard deviations (from left to right, n = 3, 5, 4, 2, 1, 3) of measurements from multiple devices or distinct contact pairs. b, Carrier densities attained after intercalation of various h-BN/graphene/MoX2 heterostructures. Circles, squares and triangles represent densities reached after intercalation at up to −5, −5.5, and −6 V, respectively. Filled symbols designate densities determined from Hall data (revealing approximate MoX2 carrier densities, except in the case of structure I), whereas hollow symbols represent densities extracted from SdH oscillations (revealing graphene carrier densities). c, Average capacity values from devices in b, expressed in units of C g−1 (gravimetric capacity) and (C cm−3) volumetric capacity.

### Extended Data Fig. 10 Transmission electron microscopy data of incompletely intercalated structure-II devices.

a, Resistance, Rxx, as a function of applied potential, E, of an h-BN/MoS2/graphene vdW heterostructure fabricated onto a 50 nm holey amorphous silicon nitride membrane. The electrochemical reaction is suspended as the increase in Rxx is commencing by immediately sweeping the potential back to 0 V. b, $${g}_{{{\rm{MoS}}}_{{\rm{2}}}}=11\bar{2}0$$ dark-field TEM image of the device after removal of the electrolyte. c, SAED patterns acquired from the regions designated 1, 2, and 3 in b. SAED data reveal a pristine MoS2 structure in region 1, but splitting of the Bragg spots (insets) at the edges of the heterostructure (regions 2 and 3) indicative of the formation of two or more domains. d, Resistance, Rxx, as a function of applied potential, E, of an h-BN/MoS2/graphene/h-BN vdW heterostructure. The electrochemical reaction is suspended as Rxx approaches a maximum by immediately sweeping the potential back to 0 V. e, $${g}_{{{\rm{MoS}}}_{{\rm{2}}}}=11\bar{2}0$$ dark-field TEM image of the device after removal of the electrolyte. f, SAED patterns of the regions designated 1, 2, 3, and 4 in e. SAED data reveal a pristine MoS2 structure in region 1, but strong splitting of the Bragg spots (insets) towards the edge of the heterostructure (region 3) indicative of the formation of several domains. In region 4, the diffuse scattering from the underlying amorphous silicon nitride membrane obscures any diffraction features from the MoS2, which in that region must be considerably disordered with any domain sizes 300 nm (the aperture size).

### Extended Data Fig. 11 (Scanning) transmission electron microscopy data of the fully intercalated structure-II device.

a, Resistance, Rxx, as a function of applied potential, E, of an h-BN/MoS2/graphene/h-BN vdW heterostructure fabricated onto a 50 nm holey silicon nitride membrane. The potential is reversed to 0 V after Rxx returns to a minimum (full intercalation) at around −5 V. b, Bright-field TEM image of the device after removal of the electrolyte. c, SAED patterns of the regions designated 1 and 2 in b in both the [0001] zone-axis (beam perpendicular to the plane of the heterostructure; middle panel) and off-zone-axis (sample tilted) conditions (left and right panels). The off-zone axis condition permits the minimization of double-diffraction phenomena associated primarily with the top and bottom h-BN flakes. SAED data at the suspended (no amorphous silicon nitride) window reveal two rings associated with the MoS2 layer, indicating considerable disorder in the xy plane with a domain size 300 nm (the aperture size). SAED data acquired over the membrane (region 2) cannot resolve these MoS2 diffraction features owing to the diffuse scattering from the amorphous silicon nitride membrane in that region. d, Aberration-corrected bright-field STEM image of the heterostructure (left, raw data; right, filtered data), which is dominated by the h-BN in the structure. The bright periodic patches arise from the moiré pattern of the two h-BN crystals. e, Aberration-corrected HAADF STEM image of the device showing the nanostructure of the MoS2 layer after one cycle. Filtered inverse fast Fourier transform (FFT) data resolve xy rotational disorder in the MoS2 atomic chains (top right, white dashed lines, revealing the approximate domain sizes as 5–10 nm (bottom).

### Extended Data Fig. 12 DFT-computed electronic structures of graphene/MoS2 heterobilayers over the course of Li intercalation.

Relaxed geometries (left), band structures (middle), and density-of-states plots (right) for graphene/MoS2 structures as Li atoms are incrementally added (top to bottom) and the phase of MoS2 is changed from H to T′. The reason for the large carrier density in MoS2 compared with that in graphene upon intercalation is evident from the relative density of states associated with MoS2 compared to that of graphene.

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Bediako, D.K., Rezaee, M., Yoo, H. et al. Heterointerface effects in the electrointercalation of van der Waals heterostructures. Nature 558, 425–429 (2018). https://doi.org/10.1038/s41586-018-0205-0

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