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Controllable van der Waals gaps by water adsorption

Nature Nanotechnology volume 19, pages 448–454 (2024)Cite this article

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Abstract

Van der Waals (vdW) gaps with ångström-scale heights can confine molecules or ions to an ultimately small scale, providing an alternative way to tune material properties and explore microscopic phenomena. Modulation of the height of vdW gaps between two-dimensional (2D) materials is challenging due to the vdW interaction. Here we report a general approach to control the vdW gap by preadsorption of water molecules on the material surface. By controlling the saturation vapour pressure of water vapour, we can precisely control the adsorption level of water molecules and vary the height of the vdW gaps of MoS2 homojunctions from 5.5 Å to 53.6 Å. This technique can be further applied to other homo- and heterojunctions, constructing controlled vdW gaps in 2D artificial superlattices and in 2D/3D and 3D/3D heterojunctions. Engineering the vdW gap has great practical potential to modulate the device performance, as evidenced by the vdW-gap-dependent diode characteristics of the MoS2/gap/MoS2 junction. Our work introduces a general strategy of molecular preadsorption that can extend to various precursors, creating more tunability and variability in vdW material systems.

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Fig. 1: Illustration and structural characterizations of conventional vdW integration and vdW gap engineering.
Fig. 2: Dynamic evolutions of water molecule adsorption and structural characterizations of controllable vdW gaps.
Fig. 3: Extendable material systems, realizable superlattice concepts and extendable adsorption precursors of vdW gap engineering.
Fig. 4: Electrical properties of MoS2/gap/MoS2 junction and DFT calculations.

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Data availability

All the data generated and analysed in this study are included in the Article and its Supplementary Information. The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (grant number XDB0580000), the National Key Research and Development Program of the Ministry of Science and Technology (grant numbers 2021YFA1200700, 2022YFB4400100), China National Funds for Distinguished Young Scientists Grant 61925403, China National Funds for Outstanding Young Scientists Grant 62122024, the China National Postdoctoral Program for Innovative Talents (grant number BX20230392), the Creative Research Groups Program of the National Natural Science Foundation of China (grant number 62321003), the National Natural Science Foundation of China (grant numbers 91964203, 62134001, 12174094, 62274060, 62311540157), the Natural Science Foundation of Hunan Province (grant numbers 2021RC5004, 2021JJ20028), the Guangdong Basic and Applied Basic Research Foundation–Regional Joint Fund (2020B1515120040), the Shenzhen Science and Technology Research Funding (JCYJ20200109115408041), the Key Research and Development Plan of Hunan Province (grant number 2022WK2001) and the Natural Science Foundation of Changsha (grant number kq2004002).

Author information

Author notes

  1. These authors contributed equally: Chang Liu, Xuming Zou, Yawei Lv.

Authors and Affiliations

  1. Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education & Hunan Provincial Key Laboratory of Low-Dimensional Structural Physics and Devices, School of Physics and Electronics, Hunan University, Changsha, China

    Chang Liu, Xuming Zou, Yawei Lv, Chao Ma, Kenli Li & Yuan Liu

  2. State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China

    Chang Liu

  3. State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Semiconductors (College of Integrated Circuits), Hunan University, Changsha, China

    Xingqiang Liu & Lei Liao

  4. Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China

    Yang Chai

  5. School of Physics and Electronic Engineering, Harbin Normal University, Harbin, China

    Lei Liao

  6. Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University, Wuhan, China

    Jun He

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Contributions

X.Z., L.L. and C.L. designed the research. C.L. prepared and characterized a series of vdW gap structures. C.L. and X.Z. performed device fabrication, electrical measurements and analysis. X.L., Y. Liu, C.M., K.L., Y.C. and J.H. participated in the experiment. Y. Lv conducted the DFT calculations and wrote the related discussions. C.L., X.Z, Y. Lv, L.L. and J.H. co-wrote the manuscript with inputs from all of the authors. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Xuming Zou, Lei Liao or Jun He.

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Nature Nanotechnology thanks Simon Fleischmann and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Schematics depicting the generation and control of SVP.

a, Schematic diagram of the SVP controlling apparatus. In the N2-filled glove box, a PTFE bottle containing 4/5 UP-water with a hole in the top is heated by water bath. Since the experiments are performed in the glove box, the water bath is simply replaced by a beaker filled with water and a hot plate. b, Side view of the SVP generation apparatus. The horizontal line of the water bath is as close as possible to the top of the PTFE bottle to ensure that the gas and liquid inside the PTFE bottle are at the same temperature. When the system reaches stability at a specific temperature, the sample on the substrate (PDMS substrate) is fixed to the hole in the PTFE cap. At the same time, the PTFE bottle is sealed and the material underneath is in a saturated vapor state. Therefore, the required SVP can be precisely controlled by controlling the temperature of water bath. c, The correspondence between SVP and temperature. The SVP at different temperature can be calculated using the Antoni equation: lgP = A – (B / (t + C), here, P represents the SVP in kPa, t represents the temperature of the liquid in °C, and A, B and C are constants. For UP-water, the values of A, B, and C are 7.07, 1657.46 and 227.02, respectively.

Extended Data Fig. 2 Dynamic evolution of water molecule adsorption at 80 °C and 100 °C.

a,b, AFM images of MoS2 surface after at 80 °C for 10 mins (a) and 100 °C for 10 mins (b). Below: height profiles corresponding to the water islands on the AFM images, here the heights of water islands increase with temperature, indicating the feasibility of SVP to control the adsorption level.

Extended Data Fig. 3 The calculated vdW energy between water molecule and MoS2 by DFT.

a, Water molecule on defect-free MoS2 surface. b, Water molecule on defective MoS2 surface (missing an S atom).

Extended Data Fig. 4 Single-layer and multilayer MoS2 stacked gap structures and related analysis.

a-c, Optical micrographs of top single-layer MoS2 (a), bottom multilayer MoS2 (b) and vdW gap structure (c) assembled at Teva = 100 °C. d, AFM image of the surface of single-layer gap structure, where a large number of water droplets are confined and visible due to the high flexibility of the single-layer MoS2. e,f Schematic of the single-layer (e) and multilayer (f) MoS2 gap structure. g, The surface AFM images of different thickness stacking, including the 3-layer, 6-layer and 12-layer MoS2, showing that multilayer stacking is able to form flat surfaces due to the hardness of the multilayer material. h, Low magnification STEM image of the top 12-layer MoS2 gap structure, showing that multilayer stacking is capable of creating large-sized and atomically flat gaps.

Extended Data Fig. 5 AFM and STEM studies to demonstrate thermal stability of gap structures.

a, Edge information of gap structures (assembled at Teva = 100 °C) traced by AFM at different annealing temperatures. Below 90 °C, the water gap can be maintained, while at 250 °C, the edge of the gap completely collapses. b, STEM image of a gap structure with collapsed edges due to high temperature annealing. c, Schematic illustration of the edge collapse process of a gap structure. d, Schematic demonstrating the high temperature stability of an h-BN pre-packaged gap structure. e, Height variations in the h-BN pre-packaged gap structure traced with AFM, and here the pre-packaged gap structure is able to remain structurally stable even at 250 °C annealing. The yellow shading in (a,e) defines the AFM scan region (in the corresponding inset).

Extended Data Fig. 6 The vdW gap structure of diverse building blocks and dimensions prepared by vdW gap engineering.

We show that vdW gap engineering is able to be applied to different materials and different dimensions, including 2D/2D homojunction (such as Graphene/Graphene, PdSe2/PdSe2, and MoTe2/MoTe2), 2D/2D heterojunctions including p–n junctions and semimetal-semiconductor junctions (such as WS2/MoS2, PtS2/MoS2, ReS2/MoS2, WSe2/MoS2 and PdSe2/MoS2), even beyond 2D system including 2D/3D and 3D/3D metal-semiconductor junctions (such as Au/MoS2 and Au/P+Si). The M-S junctions are defined as metal/semiconductor junctions. All the scale bars, 5 nm.

Extended Data Fig. 7 Low-magnification STEM images to demonstrate the homogeneity of the vdW gaps.

Here, all obtained vdW gap structures can be highly consistent over the micrometer length range.

Extended Data Fig. 8 Elemental analysis of gaps in cross-sections prepared by FIB.

a, FIB process for preparing cross-sectional slices. The slices are ultimately thinned to less than 100 nm for subsequent STEM observations. b,c, EDS elemental maps of the cross-sections of a MoS2/gap/MoS2 structure (b) and a PtS2/gap/MoS2 structure (c), where a significant amount of Si elements are found in the gaps in both structures. d, STEM image and elemental analysis of a Graphene/gap/Graphene structure, where the re-deposited amorphous-Si in the gap can be directly seen due to the close Z-contrast (related to atomic-number) of C and Si (in HAADF-STEM). e, STEM image and elemental analysis of a P+Si/gap/Au structure, here, the re-deposited Si in the gap can also be seen. f, EDS energy spectra corresponding to the four gaps above.

Extended Data Fig. 9 Schematics related to droplet gap structure and transport, as well as polarization analysis of defect sites.

a, Schematic explaining the water droplet gap, where the gap is alternately filled with N2 and water droplets. b, The 2 × 2 MoS2 supercell model and the in-x-y-plane potential on its top where an S vacancy (S-vac.) exists. c, Variation of polarization potential of water molecules on MoS2 surface without and with S-vacancy defects calculated by DFT. d, Polarization-potential-difference in the vertical direction of the MoS2/H2O model without and with S-vac. defects.

Extended Data Fig. 10 MoS2/gap/MoS2 diodes for pressure sensing.

a-d, Sound response characteristics of MoS2/gap/MoS2 diodes with different gap height at the voltage of −1 V, including ~1.3 nm gap (a), ~3.9 nm gap (b), ~4.8 nm gap (c) and ~5.3 nm gap (d). Inset of (a): schematic of MoS2/gap/MoS2 structure for pressure sensing test, and a computer-controlled speaker served as an acoustic source instead of the z-motor for obtaining smaller and higher frequency pressures. For a continuous steady electronic-drum-sound pulse (~91 dB), device with maximum gap height (~5.3 nm) shows the most significant and stable response. e, Current response of ~5.3 nm gap device for specific 10 kHz sound waves (~94 dB), highlighting the fast response of our devices (<25 μs, limited by our measurement set-up). f, Pressure sensitivity to the acoustic wave with different frequencies (300–10,000 Hz). g, Pressure response in ultralow pressure range (<1 Pa). Here, the device is consistently sensitive to different pressures with an average sensitivity Save of ~3.9 × 103 kPa−1. h, Repeated test on musical sound waves, demonstrating the stability of the device under rapidly changing pressure. i, Discrimination of human speech sounds, for example, "Standing on the shoulders of giants" and "The important thing is not to stop questioning".

Supplementary information

Supplementary Information

Supplementary Discussions 1–5, Notes 1–6 and Figs. 1–16.

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Liu, C., Zou, X., Lv, Y. et al. Controllable van der Waals gaps by water adsorption. Nat. Nanotechnol. 19, 448–454 (2024). https://doi.org/10.1038/s41565-023-01579-w

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