Two-dimensional (2D) materials, consisting of atomically thin crystal layers bound by the van der Waals force, have attracted much interest because of their potential in diverse technologies, including electronics, optoelectronics and catalysis1,2,3,4,5,6,7,8,9,10. In particular, solution-processable 2D semiconductor (such as MoS2) nanosheets are attractive building blocks for large-area thin-film electronics. In contrast to conventional zero- and one-dimensional nanostructures (quantum dots and nanowires, respectively), which are typically plagued by surface dangling bonds and associated trapping states, 2D nanosheets have dangling-bond-free surfaces. Thin films created by stacking multiple nanosheets have atomically clean van der Waals interfaces and thus promise excellent charge transport11,12,13,14,15. However, preparing high-quality solution-processable 2D semiconductor nanosheets remains a challenge. For example, MoS2 nanosheets and thin films produced using lithium intercalation and exfoliation are plagued by the presence of the metallic 1T phase and poor electrical performance (mobilities of about 0.3 square centimetres per volt per second and on/off ratios of less than 10)2,12, and materials produced by liquid exfoliation exhibit an intrinsically broad thickness distribution, which leads to poor film quality and unsatisfactory thin-film electrical performance (mobilities of about 0.4 square centimetres per volt per second and on/off ratios of about 100)14,16,17. Here we report a general approach to preparing highly uniform, solution-processable, phase-pure semiconducting nanosheets, which involves the electrochemical intercalation of quaternary ammonium molecules (such as tetraheptylammonium bromide) into 2D crystals, followed by a mild sonication and exfoliation process. By precisely controlling the intercalation chemistry, we obtained phase-pure, semiconducting 2H-MoS2 nanosheets with a narrow thickness distribution. These nanosheets were then further processed into high-performance thin-film transistors, with room-temperature mobilities of about 10 square centimetres per volt per second and on/off ratios of 106 that greatly exceed those obtained for previous solution-processed MoS2 thin-film transistors. The scalable fabrication of large-area arrays of thin-film transistors enabled the construction of functional logic gates and computational circuits, including an inverter, NAND, NOR, AND and XOR gates, and a logic half-adder. We also applied our approach to other 2D materials, including WSe2, Bi2Se3, NbSe2, In2Se3, Sb2Te3 and black phosphorus, demonstrating its potential for generating versatile solution-processable 2D materials.
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The data that support the findings of this study are available from the corresponding authors on reasonable request.
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Xiangfeng Duan acknowledges support from the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering through award DE-SC0018828. (material synthesis and structural characterizations). Y.H. acknowledges financial support from National Science Foundation grant EFRI-1433541 (thin film device fabrication and characterizations). We acknowledge the Electron Imaging Center at UCLA for TEM technical support and the Nanoelectronics Research Facility at UCLA for device fabrication technical support.
Nature thanks K. Xiao and the other anonymous reviewer(s) for their contribution to the peer review of this work.
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
a, Schematic of the electrochemical intercalation of MoS2 with THAB. b, Schematic of the intercalation and exfoliation of the MoS2 crystal to produce 2D-nanosheet ink, which can be processed further into continuous large-area thin films with broad-area plane-to-plane contacts between stacked nanosheets. c, Photographs of a thin piece of MoS2 crystal before (left) and after (right) THAB intercalation. The MoS2 crystal expands by a factor of about 20 along the (001) direction. Scale bars, 5 mm. d, Schematic of the interlayer spacing expansion from the original 6.1 Å to 22.9 Å, caused by the intercalation of THAB molecules.
a, XRD patterns of the THAB-intercalated (inter-MoS2) and pure MoS2. The MoS2 interlayer spacing expands from the original 6.1 Å to 22.9 Å after the electrochemical intercalation of THAB. The peaks indicated with asterisks are for the intercalated MoS2; the rest are from MoS2 itself. For XRD, the MoS2 crystal was allowed to be intercalated for a short period (about 1 min) to capture the intermediate state where the intercalation occurs but the expansion is not sufficient to cause severe shape deformation. b, XRD patterns for MoS2 crystals intercalated with various ammonium molecules. C2, C3, C4, and C10 represent tetraethylammonium bromide ((C2H5)4NBr; TEAB), tetrapropylammonium bromide ((C3H7)4NBr; TPAB), tetrabutylammonium bromide ((C4H9)4NBr; TBAB) and tetradecylammonium bromide ((C10H21)4NBr; TDAB), respectively. With the increasing molecule size (from C2 to C3, C4 and C10), the interlayer distance expansion is more substantial, ranging from 10.9 Å to 14.3 Å, 18.5 Å and 24.2 Å.
a–c, XPS spectra for the as-deposited (black), TFSI-treated (red), and TFSI-treated and annealed (blue) MoS2 film, including the Mo 3d scan (a), S 2p scan (b) and N 1s scan (c). All three MoS2 films have the same 2H crystal structure. The absence of any N signal after TFSI treatment and thermal annealing indicates the complete removal of ammonium molecules and PVP. a.u., arbitrary units. d, SEM image of a single-nanosheet device fabricated on a 300-nm-thick SiO2/Si substrate. Electrodes are false-coloured. Scale bar, 1 μm. e, Isd–Vg transfer characteristics of the device with Vsd = 1 V. f, EDS spectrum of the intercalated THAB–exfoliated MoS2 superlattice. It shows the co-existence of Mo and S from MoS2 and that of N and Br from THAB. The atomic ratio N/Br is approximately 2% of Mo/S. During the reaction, Br2 was produced at the anode, as indicated by the emergence of the dark yellow Br2 solution. However, after the electrochemical potential was withdrawn, the highly active Br2 quickly back-reacted with the THA+–MoS2− layers to form the final MoS2–THAB superlattice structure. Consequently, N and Br elements were both detected in the THAB–exfoliated MoS2 structure.
a, Photograph of the photolithographically patterned MoS2 thin film on a standard 100-mm-diameter SiO2/Si wafer. b, c, SEM image of the transistor fabricated with photolithography (b) and AFM analysis (c). Scale bars, 10 μm (b) and 2 μm (c). d, e, Cross-sectional TEM images of plane-to-plane contacts between MoS2 nanosheets; d and e show two different regions of the thin film. The red dashed boxes indicate the regions where two nanosheets exhibit a contact that is nearly indistinguishable from a van der Waals interface between atomic layers of MoS2. Scale bars, 5 nm.
Vg ranges from −10 V (black) to 40 V (dark blue) in steps of 10 V.
a, b, Isd on linear (a) and logarithmic (b) scales. Vsd = 1 V.
Extended Data Fig. 7 Isd–Vg transfer characteristics of MoS2 films annealed at various temperatures.
a, Annealed at 200 °C. b, Annealed at 400 °C. Both devices were fabricated on a 90-nm-thick SiO2/Si substrate, with Vsd = 1 V. The threshold voltage shifts to more positive values when the film is annealed at a higher temperature, suggesting a reduced carrier concentration owing to the removal of impurity doping at the higher temperature.
Extended Data Fig. 8 Constructing logic gates and computational circuits using MoS2 thin-film transistors with a local back gate.
a, Schematic of the MoS2 transistor with a local back gate (Ti/Au, 10 nm/40 nm) buried under a 30-nm-thick Al2O3 dielectric layer. b, Optical image of an individual transistor. The gate electrode was fabricated after etching the underlying Al2O3 layer. Scale bar, 20 μm. c, Isd–Vg transfer characteristics of the transistor constructed on the buried gate with a 30-nm-thick Al2O3 dielectric layer andVsd = 1 V. The device was annealed at 400 °C for 1 h. d–h, Schematics of the logic inverter (d) and the NAND (e), NOR (f), AND (g) and XOR (h) gates. GND, ground.
a–l, AFM (a–f) and TEM (g–l) images of exfoliated nanosheets of WSe2 (a, g), Bi2Se3 (b, h), NbSe2 (c, i), In2Se3 (d, j), Sb2Te3 (e, k) and black phosphorus (BP; f, l). Insets are photographs of the corresponding ink solutions. Scale bars, 1 μm (a, c, f), 500 nm (b, d, e, g, i, l) and 200 nm (h, j, k).
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Lin, Z., Liu, Y., Halim, U. et al. Solution-processable 2D semiconductors for high-performance large-area electronics. Nature 562, 254–258 (2018). https://doi.org/10.1038/s41586-018-0574-4
- MoS2 Nanosheets
- MoS2 Thin Films
- Black Phosphorus
- Tetraheptylammonium Bromide
- Thin Film Transistors
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