Abstract

The mechanical and electronic properties of two-dimensional materials make them promising for use in flexible electronics1,2,3. Their atomic thickness and large-scale synthesis capability could enable the development of ‘smart skin’1,3,4,5, which could transform ordinary objects into an intelligent distributed sensor network6. However, although many important components of such a distributed electronic system have already been demonstrated (for example, transistors, sensors and memory devices based on two-dimensional materials1,2,4,7), an efficient, flexible and always-on energy-harvesting solution, which is indispensable for self-powered systems, is still missing. Electromagnetic radiation from Wi-Fi systems operating at 2.4 and 5.9 gigahertz8 is becoming increasingly ubiquitous and would be ideal to harvest for powering future distributed electronics. However, the high frequencies used for Wi-Fi communications have remained elusive to radiofrequency harvesters (that is, rectennas) made of flexible semiconductors owing to their limited transport properties9,10,11,12. Here we demonstrate an atomically thin and flexible rectenna based on a MoS2 semiconducting–metallic-phase heterojunction with a cutoff frequency of 10 gigahertz, which represents an improvement in speed of roughly one order of magnitude compared with current state-of-the-art flexible rectifiers9,10,11,12. This flexible MoS2-based rectifier operates up to the X-band8 (8 to 12 gigahertz) and covers most of the unlicensed industrial, scientific and medical radio band, including the Wi-Fi channels. By integrating the ultrafast MoS2 rectifier with a flexible Wi-Fi-band antenna, we fabricate a fully flexible and integrated rectenna that achieves wireless energy harvesting of electromagnetic radiation in the Wi-Fi band with zero external bias (battery-free). Moreover, our MoS2 rectifier acts as a flexible mixer, realizing frequency conversion beyond 10 gigahertz. This work provides a universal energy-harvesting building block that can be integrated with various flexible electronic systems.

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Acknowledgements

This work was financially supported by the MIT/Army Institute for Soldier Nanotechnologies, the Army Research Laboratory (grant W911NF-14-2-0102), the STC Center for Integrated Quantum Materials (National Science Foundation (NSF) grant number DMR-1231319), the Air Force Office of Scientific Research under the MURI-FATE program (grant number FA9550-15-1-0514), NSF grant DMR-1507806, the NSF Center for Energy Efficient Electronics Science (E3S; NSF grant number ECCS-0939514) and MIT International Science and Technology Initiatives (MISTI). Device fabrication was carried out at the MIT Microsystems Technology Laboratories. X-ray spectroscopy studies were done at the Cornell Center for Materials Research Shared Facilities. We acknowledge K. Gleason and A. Nourbakhsh for discussions.

Reviewer information

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

Author information

Affiliations

  1. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, USA

    • Xu Zhang
    • , Ujwal Radhakrishna
    • , Winston Chern
    • , Lin Zhou
    • , Yuxuan Lin
    • , Pin-Chun Shen
    • , Xiang Ji
    • , Ahmad Zubair
    • , Yuhao Zhang
    • , Jing Kong
    • , Mildred Dresselhaus
    •  & Tomás Palacios
  2. Information Processing and Telecommunications Center, Universidad Politécnica de Madrid, Madrid, Spain

    • Jesús Grajal
  3. Department of Signal Theory and Communications, University Carlos III of Madrid, Madrid, Spain

    • Jose Luis Vazquez-Roy
  4. Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

    • Xiaoxue Wang
  5. Department of Chemistry, Boston University, Boston, MA, USA

    • Xi Ling
  6. Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, CA, USA

    • Han Wang
  7. Army Research Laboratory, Adelphi, MD, USA

    • Madan Dubey
  8. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA, USA

    • Mildred Dresselhaus

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Contributions

X.Z. and T.P. conceived and designed the experiments. X.Z. fabricated the flexible devices. J.G., X.Z. and U.R. carried out the high-frequency measurements. J.G. and X.Z. did the circuit modelling and data analysis. X.W. and X.Z. performed the chemical phase change of the MoS2 samples. X.Z., X.W. and L.Z. conducted the spectroscopic study. W.C. and X.Z. carried out the CV measurement. J.L.V.R. and J.G. designed and fabricated the flexible antenna. All authors contributed to interpreting the data and writing the manuscript.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Tomás Palacios.

Extended data figures and tables

  1. Extended Data Fig. 1 Spectroscopic and transport study of MoS2 before and after phase change induced by n-butyllithium.

    a, Raman spectrum of MoS2 before and after the phase change induced by n-butyllithium. A new Raman peak around 286 cm−1 was observed after the phase change. b, X-ray photoelectron spectroscopy characterization of MoS2 samples (Mo 3d3/2 and Mo 3d5/2 peaks) before and after the 2H-to-1T/1T′ phase change induced by n-butyllithium. The Mo 3d3/2 and Mo 3d5/2 peaks were red-shifted after the phase change. c, IV transfer characteristics of a MoS2 field-effect transistor (FET) on a Si wafer capped with 300 nm SiO2, before and after phase change. The graph shows the drain current as a function of backgate bias for drain voltage Vds = 50 mV, channel length of 830 nm and channel width of 50 μm. The 300-nm-thick SiO2 serves as the backgate dielectric layer. Black line, pristine MoS2 FET; red line, MoS2 FET after phase change through n-butyllithium treatment; blue line, post-phase-change MoS2 FET after baking (180 °C for 3 min). The 1T-phase MoS2 is unstable in air at room temperature, and part of the 1T region is converted into the 1T′ phase. However, the 1T/1T′ mixture MoS2 retains metallic IV characteristics after baking.

  2. Extended Data Fig. 2 Investigation of Schottky contact in Pd/2H MoS2 and Ohmic contact in Au/1T MoS2/2H MoS2.

    a, IV characteristics of backgated MoS2 transistors on Si substrates (‘Si sub’) capped with 300 nm SiO2. Channel length, 4 μm. Pd is used as the source/drain contact metal. The nonlinear IdsVds characteristics are consistent with the behaviour of a Schottky contact. Vbg is the backgate bias. b, IV characteristics of backgated MoS2 transistors on Si substrates capped with 300 nm SiO2. Channel length, 4 μm. The source/drain region of MoS2 was phase-engineered into the metallic 1T/1T′ phase for use as contact. The linear IdsVds characteristics indicate an Ohmic contact. c, Transfer-length-method structure of 1T/1T′ MoS2. The contact resistance between Au and 1T/1T′ MoS2 is estimated to be about 168 Ω μm. d, Transfer-length-method structure of 2H MoS2 (channel width, 10 μm), in which the contact area is phase-engineered into the 1T/1T′ metallic phase before metal deposition. The contact resistance is estimated to be 56 kΩ μm.

  3. Extended Data Fig. 3 Comparison of measured S-parameter of the MoS2 diode and modelled S-parameter based on the equivalent circuit in Fig. 2a.

    a, Magnitude of S21. b, Phase of S21. c, Magnitude of S22. d, Phase of S22. Red circles, experimental data; blue line, modelled data.

  4. Extended Data Fig. 4 Resistive and capacitive components of MoS2 Schottky diode.

    a, Series resistance, Rs, obtained from S-parameter measurements at different external biases. b, Junction resistance, Rj, obtained from S-parameter measurements at different internal voltages. The internal voltage Vint is derived from Vint = Vext – I Rs. c, d, Quasi-static IV characteristics of the MoS2 Schottky diode. The modelling is based on I = Is{exp[e(VIRs)/(nkBT)]−1}, where Is = 700 nA, n = 2.9, Rs = 3,500 Ω and T = 300 K. The current density of a MoS2 phase-junction diode is shown in the logarithmic (c) and linear (d) scale. Blue line, modelling; red squares, measurements. e, CV characteristics of the MoS2 Schottky diode at f = 500 kHz. When the MoS2 diode is negatively biased or has a bias around zero, the overall capacitance is about 40–60 fF.

  5. Extended Data Fig. 5 Demonstration of the MoS2-based RF energy-harvesting circuit.

    a, Circuit diagram. The decoupling capacitors at the input can block the d.c. current while permitting flow of RF signals. The d.c. and RF signal paths are indicated by blue and red dashed lines, respectively. The output signal was measured by an oscilloscope with an impedance of 1 MΩ, which was in shunt with a load resistance. The capacitor in this circuit blocks the d.c. current and protects the signal generator, and it is not necessary for the demonstration using the antenna. ‘L.P.F.’ indicates the low-pass filter and Pdel is the power delivered to the MoS2 diode. b, Power efficiency of MoS2-based rectifiers (red stars) compared with state-of-the-art rigid technologies at 2.4 GHz (Si Schottky diodes and GaAs Schottky diodes; black symbols)35,36,37,38,39,40. To ensure a fair comparison, all the data points here show the RF–d.c. conversion efficiency for rectifiers without the antenna effect. In this proof-of-concept demonstration, the flexible MoS2 rectifiers exhibit competitive power efficiency. We note that the power efficiency was obtained in an academic laboratory before careful optimization. Through optimization of the material engineering, the phase-change process and the matching circuit, the power efficiency of the presented MoS2-based rectifiers can be further improved.

  6. Extended Data Fig. 6 Fully flexible MoS2 rectenna harvesting electromagnetic radiation energy in the Wi-Fi band (5.9 GHz).

    a, Fabrication of integrated MoS2 rectenna. The phase-engineered MoS2 rectifier arrays are fabricated on Kapton substrates. After high-frequency characterization by S-parameter measurements, we integrated the MoS2 rectifier with a flexible 5.9-GHz Wi-Fi antenna on the same piece of Kapton film. b, Demonstration of energy harvesting of electromagnetic radiation in the 5.9-GHz Wi-Fi band using the as-fabricated flexible MoS2 rectenna. The input power available to the MoS2 rectenna was about 3 dBm (about 2 mW). The measurement was carried out in a parallel configuration (as shown in Fig. 3d). The transmitter Wi-Fi-band antenna was powered by a signal generator and approached the receiver antenna of the MoS2 rectenna. The rectified output voltage Vout was about 250 mV, which was measured with an oscilloscope in shunt with a 10-kΩ load resistance. c, Simulated directivity pattern of the flexible dipole antenna including the feeding line effect. Theta and phi are the polar and azimuthal angles for spherical coordinates, respectively. The principal yz, xz and xy planes of the antenna are indicated by red, green and blue circles, respectively. The total antenna gain is expected to be only −0.38 dB below the directivity (D0 = 2.64 dB) owing to the low Ohmic loss and the good impedance matching with respect to a reference impedance of 50 Ω. d, Return loss of the flexible antenna. The simulation (blue) and measurement (pink) data match well at the operating frequency.

  7. Extended Data Fig. 7 High-frequency MoS2 mixers.

    a, Conversion loss of the MoS2 mixer at different RF powers delivered at the input. The conversion loss is defined as the power difference between the input RF signal (1.4 GHz) and the output intermediate frequencies (IF; downconverted at 0.4 GHz and upconverted at 2.4 GHz). b, Input 1-dB compression point of the MoS2 mixer. The 1-dB compression point is a measure of an RF mixer’s linearity and is defined as the input RF power for which the conversion loss is increased by 1 dB from the ideal. For the upconversion IF of 2.4 GHz, the 1-dB compression point is about −32.3 dBm. For the downconverted IF of 0.4 GHz, the 1-dB compression point is about −36.4 dBm. The high-frequency performance of the flexible MoS2 mixer can be further optimized by improving the impedance matching, which was not optimized in this proof-of-concept demonstration.

  8. Extended Data Table 1 Comparison of state-of-the-art high-frequency rectifiers

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