ARTICLE

Bidirectional mid-infrared communicationsbetween two identical macroscopic graphene fibresBo Fang1,5, Srikrishna Chanakya Bodepudi2,5, Feng Tian2,3, Xinyu Liu2, Dan Chang1, Sichao Du 2, Jianhang Lv2,

Jie Zhong4, Haiming Zhu 4, Huan Hu 3, Yang Xu 2,3✉, Zhen Xu 1✉, Weiwei Gao1 & Chao Gao 1✉

Among light-based free-space communication platforms, mid-infrared (MIR) light pertains to

important applications in biomedical engineering, environmental monitoring, and remote

sensing systems. Integrating MIR generation and reception in a network using two identical

devices is vital for the miniaturization and simplification of MIR communications. However,

conventional MIR emitters and receivers are not bidirectional due to intrinsic limitations of

low performance and often require cryogenic cooling. Here, we demonstrate that macro-

scopic graphene fibres (GFs) assembled from weakly-coupled graphene layers allow room-

temperature MIR detection and emission with megahertz modulation frequencies due to the

persistence of photo-thermoelectric effect in millimeter-length and the ability to rapidly

modulate gray-body radiation. Based on the dual-functionality of GFs, we set up a system that

conducts bidirectional data transmission by switching modes between two identical GFs. The

room-temperature operation of our systems and the potential to produce GFs on industrial

textile-scale offer opportunities for simplified and wearable optical communications.

https://doi.org/10.1038/s41467-020-20033-2 OPEN

1MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, 38 ZhedaRoad, 310027 Hangzhou, People’s Republic of China. 2 College of Micro-Nano Electronics, ZJU-Hangzhou Global Scientific and Technological InnovationCentre, State Key Laboratory of Silicon Materials and Modern Optical Instruments, Zhejiang University, 38 Zheda Road, 310027 Hangzhou, People’s Republicof China. 3 Zhejiang University/University of Illinois at Urbana-Champaign Joint Institute (ZJU-UIUC), Zhejiang University, 314400 Haining, Zhejiang,People’s Republic of China. 4 Department of Chemistry, Zhejiang University, 310027 Hangzhou, Zhejiang, People’s Republic of China. 5These authorscontributed equally: Bo Fang, Srikrishna Chanakya Bodepudi. ✉email: yangxu-isee@zju.edu.cn; xuzhen199@163.com; chaogao@zju.edu.cn

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Optical communications based on mid-infrared (MIR)light are vital in molecular “fingerprint” imaging,healthcare, and in a wide variety of applications including

security, environmental monitoring, metrology, and atmosphericscience1–6. Creating bidirectional MIR communication systemsusing two identical devices facilitates the design of simplifiedMIR-based optoelectronic systems. Such conceptual advanceshave been witnessed in visible and near-infrared regions, but notyet in the MIR range7,8. This is because the majority of traditionalMIR detectors and emitters, e.g., narrow-band gapsemiconductors9,10, are predominantly designed to be eitheremitters or detectors, and, therefore, are not compatible for dual-functionality with satisfactory performance.

Single and few-layer graphene (FLG) are a suitable choice forboth MIR emission and detection. FLG can emit MIR throughrecombination of electrically injected electron-holes, or gray-bodyirradiation11,12. Besides, it can effectively detect MIR via directband transition and due to the presence of longer hot-carrierlifetime13,14. However, despite their promises in terms of band-width and flexibility, their performance is still intrinsically limitedby weak absorption and short light-matter interaction lengths inthe MIR region. Microscale thick stacks of multilayer graphenecan be considered as a viable alternative as it shows improvedMIR absorption and emission due to increased interaction length.Yet, critical challenges, including a high recombination rate andshort carrier transport lengths, remained. Recent reports suggestthat the weak interlayer coupling can potentially minimize theout-of-plane scattering and exhibit intrinsic mobility like gra-phene, and effectively confine photoexcited carriers to individuallayers15,16. Such uncorrelated multilayer graphene can retain hotelectron relaxation times similar to single-layer graphene andsustain photo-thermoelectric effect for long spatial scales16. Sofar, there is no practical demonstration of room-temperature MIRcommunication in freestanding, flexible bulk graphene systems.

In this work, we use a scalable twist-draw protocol to assemblegraphene nanosheets into macroscopic graphene fibers (GFs)with weakly coupled graphene layers. As a receiver, GF exhibitsroom-temperature MIR detection at 0.25MHz as a result ofphoto-thermoelectric effect. As an emitter, GF is an efficient gray-body radiator to irradiate bias-dependent MIR spectrum at 10MHz. Based on these merits, we developed the bidirectional MIRcommunication link using two identical GFs, where two GFsoptically couple two separate electrical circuits and exchangeinformation mediated by MIR light.

ResultsFabrication and characterization of GFs. Meters-long GFs(Fig. 1a) with ~50 μm diameter are prepared from continuouslyassembled wet-spun graphene oxide (GO) films using home-made twist-draw equipment17 (Supplementary Fig. 1), on thesame scale as producing yarns or threads from cotton in thetextile industry18. The twisting procedure imparts long-rangetwists (Fig. 1b), a spring-like structure (Fig. 1c) on loosely stackedgraphene sheets (Fig. 1e, f) in GFs, which bring a large tensileelongation beyond 15% in the axial direction19 (SupplementaryFig. 1h and Movie 1), and further, GFs are woven into a naturalwearable fabric (Fig. 1d). The quality of graphene sheets in GFsis verified with the high-resolution transmission electron micro-scope (HR-TEM) in Fig. 1g, the negligible characteristic Ramandefect peak (D band, Fig. 2d and Supplementary Fig. 2), and thesharp reflection peak at 26.89° in X-ray diffraction (XRD) spectra(Fig. 1h). The transition from insulating to metallic behavior ofconductivity with power-law dependence (Fig. 1i) suggests thatGF is a clean multilayer system with minimal defect density ordoping20. The mechanical endurance through 100 cyclical tensile

tests (Fig. 1j) further demonstrates the potential of GF in durableoptoelectronic devices.

GFs in MIR detection mode. To investigate our hypothesis onGF for MIR detection, we study a 1 mm long, 50 μm diameter GFstructure suspended between two terminals at room-temperatureand irradiated with laser from 2 to 10 μm wavelength range. Thecontact resistance (RC) of the GF measured from the transferlength method (TLM) is ~1.27Ω (Supplementary Fig. 3), and isconsistent with the 4-point measurement. This value is almost anorder of magnitude lower than the resistance of 1 mm long GF(12–30Ω), and thus insignificant in the emission and detectionresponses of the GF. The generation of hot spot at the center ofthe GF and the gradual drop from the center to the electrodesalong the GF while emitting MIR further supports that themaximum resistance contribution is from the GF (SupplementaryFig. 4). As shown in Fig. 2a, we record a photoresponse with apulse width of ~0.9 μs (the rise and fall time are ~130 ns and ~4μs, respectively) by illuminating GF with a 200-fs pulsed 4 μmMIR laser at 100 kHz repetition rate. This demonstrates that GF iscapable of detecting MIR light at 0.25MHz. Relatively higher butthe opposite polarity of the photocurrent, Iph, at GF-metalinterfaces and weaker response away from interfaces (Fig. 2b) at arelatively weak constant bias of 3 mV suggests that the MIRresponse of GF is mainly dominated by photo-thermoelectric(PTE) effect21. Work function differences22 can also induce sucha built-in electric field but usually limited to nanoscale lengthfrom the interface in the bias range used in our device.

We propose that the MIR response in GFs is dominated by thephoto-thermoelectric effect in the weakly coupled multilayerstructure. Here, photoexcited hot electrons generated within thelaser spot diffuse to GF-metal junctions by the electrontemperature gradient14,21,23,24. We argue that unlike what is seenin nanosized devices, photo-thermoelectric effect in the GF alongits length is largely influenced by interlayer hot electron transferand scattering rather than Seebeck coefficient difference at GF-metal junction. The symmetric Raman 2D band in Fig. 2drepresents the weak interlayer coupling in GFs25, complyingrandomly stacked graphene layers as displayed in Fig. 1f and theinset in Fig. 2d. Weakly coupled graphene layers can retain themobility of single-layer graphene and increase the in-plane chargerelaxation lengths15, and sustaining MIR response in long GFs(Fig. 2e, f). In such uncorrelated layers, hot electron relaxationlength typically extends to a few micro-meters16,24,26. Thepersistence of photoresponse when the laser spot is focused atthe center of the GF (L= 0.4 mm) indicates that the PTE effectcan be extended into much longer spatial scales through multiplegraphene flakes. Also, it has been reported that the hot electronscan tunnel through weakly coupled graphene layers prior tothermalization27 that can further increase the hot electronrelaxation length.

The observed photocurrent response of GF is much broader(~900 ns) than the 200-fs incident laser pulse width. Thisbroadening is due to the combination of resistance-capacitance(RC) time constant of electrodes, external circuit and the totalcooling time of the system. The rise time of ~120 ns independentto the relative distance of the laser spot position on GF from theelectrodes (Supplementary Fig. 5) supports that the photocurrentmaximum is delayed due to RC of external circuit. A detaileddiscussion on time constant contribution of external circuit ispresented in the Supplementary information. Further, trappingand de-trapping of photocarriers can increase the time of flight ofcarriers in GF, which can also reflect in the broadening of thephotocurrent response. The extended trailing edge of ~4 μs of thephotocurrent response is based on the total cooling time or the

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time taken for the system to reach thermal equilibrium with thesurroundings. The self-suspended nature of GF and weakinterlayer coupling can significantly reduce the heat dissipationto the environment and thus the lattice cooling mainly happensvia thermal conductivity. Extremely slow cooling in the order of100 μs have been recorded in carbon-based systems such ascarbon nanotubes (CNTs) and graphite that are limited by theweak thermal dissipation channels and low heat capacitance28,29.Thus, the 4 μs trailing edge is due to above discussed factors and,therefore, we believe that the observed temporal photoresponse isa lower limit of the GF. Although, the intrinsic response of thePTE effect in GF can be much quicker (in few picoseconds), thecombination of above discussed effects significantly broadens theresponse pulse to ~900 ns with a long trailing edge of ~4 μs in ourmeasurement set-up.

GF exhibits the responsivity of up to 0.67 AW−1 in thewavelength range 2–10 μm (Fig. 2c and Supplementary Fig. 7a–h),superior to other room-temperature MIR detectors such as

quantum dots30, black arsenic phosphorus31, hybrid graphene/Si32 or Ge photodetectors33, as well as photodetectors based onCNTs34. We propose that the comparatively higher responsivityin our device is attributed to the relaxation time and the densityof hot electrons that reach GF-metal interface. At lowerwavelength region, the dominant hot electron cooling mechanism

is optical phonon emission. Therefore, hot electrons with energiescloser to ~0.2 eV relax mostly by emitting optical phonons andthus only a limited number of hot electrons diffuse to the GF-metal interface, resulting in lower responsivity. Whereas, for thehot electron energies much lower than the optical phonon energy,the dominant cooling mechanism is via the emission of acousticphonons, which is less efficient cooling process in graphene inmid-infrared range (2–10 μm). This enables a higher density ofhot electrons to reach GF-metal interface with the increase inwavelength, and thus a gradual rise in responsivity followed byweak saturation in 6–10 μm region. We also estimated the specificdetectivity (D*) and noise equivalent power (NEP) of GF in theMIR region (2–10 μm) as displayed in Supplementary Fig. 8.Minor differences in the fluence of the laser pulse at differentwavelengths are depicted in error bars in the D* and NEP values.The wavelength dependency of D* is similar to the responsivityand consistent with the dominant cooling channels such asacoustic phonon emission at low photon energies. Moreover,measurements at 403 K demonstrate that the GF holds theresponsivity of 0.03 AW−1 at comparatively higher temperatures(Supplementary Fig. 9), which offers advantage over conventionalinfrared detectors limited to cryogenic operation35.

On the other hand, the MIR photoresponse is absent in highlyoriented pyrolytic graphite (HOPG, the inset in Fig. 2g, i), a

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Fig. 1 Structural characteristics of GFs. a Photograph of meters-long GF collected on a graphite roller. b, c SEM images of the GF surface at differentmagnifications. d Weaving continuous GFs into a cotton cloth and wearing the composite fabric on a finger. e, f SEM images of GF cross-section showingrandomly stacked graphene sheets. g TEM image of a crystallized graphene sheet in GF. h XRD spectrum of GF. The full width at half maximum (W1/2) of~0.272° indicates restoration of ~29.7 nm thick graphene crystals in GF as described by Scherrer equation (LC= 0.9λ/(W1/2 cosθ)). i Four-point electricalconductivity of GF as a function of inverse temperature. j Mechanical durability test of GF up to 100 cycles.

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Fig. 2 GF operation in MIR detection mode. a Schematic of 1 mm long GF suspended between gold electrodes illuminated by 4 μm wavelengthfemtosecond laser (fluence= 4.9W cm−2) at constant voltage bias of 3 mV. b Photoresponse of GF when the laser spot moved from one electrode to theother. The position of the laser spot along the GF length is labeled next to the response signal. Photoresponse curves are shifted for clarity. c A comparisonof responsivities of state-of-the-art MIR photodetectors with GF. Raman spectra, the corresponding MIR response, and the simplified schematics ofpresumed photo-carrier transport mechanisms of GF (d–f), highly oriented pyrolytic graphite (HOPG, g–i) and reduced graphene oxide fiber (RGOF, j–l),respectively. The solid circles in e, h, and k represent electrons, and the hollow circles represent the hole. The orange lines in k represent sp3-hybridizedtopologic defects.

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strongly coupled multilayer graphene system with parabolicenergy dispersion, where hot electrons quickly transfer energy tolattice due to strong electron-phonon coupling. The stronglycoupled layers display splitting in the 2D Raman band (Fig. 2g),induced by orbital hybridization that can substantially increasecarrier scattering and further hinders carrier transport36 (Fig. 2h).Besides, the quality of the graphene lattice also contributes to thefast MIR detection response. In the case of oxygen-containeddefective multilayer graphene systems, such as reduced grapheneoxide fibers (RGOF, Fig. 2j), charge-carriers are usually trappedby sp3-hybridized topologic defects37 (Fig. 2k), and yield no MIRresponse (Fig. 2l).

GFs in MIR emission mode. We investigated the MIR emissionin GFs (Fig. 3a) based on gray-body radiation, where the gener-ated thermal energy of electrically driven Joule heating is con-verted to MIR radiance with a broad bias-dependentspectrum38,39. In Fig. 3b, we plot the theoretical emission curvesof a gray-body at different temperatures according to the Planck’s

function: Iλ λ;Tð Þ ¼ 2εhc2ð Þ= λ5 exp hcλkBT

� �� 1

�� �� �, where ε is

the emissivity, h is the Planck constant, c is the speed of light, andKB is the Boltzmann constant. The emission spectra blue-shift tothe visible light region at high temperatures and the MIR windowis estimated to be below ~700 K since λ is mostly longer than 2μm in this range. In the case of GF emitters, the emission spectraare easily controlled in the MIR window by directly adjusting theinput electric field, F. Based on the emission spectra of GF inFig. 3c, the estimated temperature is <662 K when F is within3.53 V cm−1. The definite match of the measured emissionspectra with theoretical fit indicates that the MIR emission of GFis indeed the gray-body radiation.

Apparently, GFs are effective gray-body radiators as the requiredF to operate GF emitter is at least five orders of magnitude lowerthan that of few-layer graphene (FLG) MIR emitters13. Such animprovement is related to the high σ (3 × 104 Sm−1, Fig. 1i), whichenables GFs to consume low electrical energy and high ε, a factorthat determines the thermal emission ability of a radiator. Thestrong light absorption of the multilayer graphene in GFs leads to a

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Fig. 3 GF operation in MIR emission mode. a The schematic of our device. b The theoretical MIR window of gray-body irradiance. c The IR images andemission spectra (hollow dots) of GF at 332, 578, and 662 K, which match well with the gray-body radiation simulation (solid lines). The applied electricfields are 1.17 V cm−1, 2.35 V cm−1, and 3.53 V cm−1, respectively. The emissivity (d) and emission efficiency (e) of the GF emitter at different operationtemperatures. f, A GF continuously emits MIR light over 40 h under a constant electric field of 3.53 V cm−1. The schematic (g) and the real-time detection(h) of the intensity modulation of the GF emission at 10MHz by using a commercial InGaAs detector. The Vpeak of input bias is 6 V, and the peak F is 3.53V cm−1.

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ε above 0.6 (Fig. 3d), much higher than that of single-layergraphene40 (SLG, ~0.02) and metals (<0.3). Such high ε permits GFto release high thermal energy under a low F. Thus, GF emitterexhibits a high thermal emission efficiency (η, see the Fig. 3e) of~0.052% at 400K, where η is termed as14: η ¼ εδT4=AP*

in. Here, δis Stefan’s constant, A is the surface area of GF, and Pin* is the inputelectric energy per density. This value is three orders of magnitudehigher than that of FLG emitters (~10−6)13. The decompositiontemperature of GF approaches 800K in the air41, enabling stableMIR emission of GF above room-temperature. For example, whenF is 3.53V cm−1, GF displays stable emission at ~662 K for >40 hwithout the loss of emissivity (Fig. 3f).

We recorded the MIR emission frequency of GF using acommercial InGaAs photodetector (Fig. 3g). As shown in Fig. 3h,we observed a synchronized MIR emission while modulating F upto 10MHz. The emission speed in GF might be related to theweak interlayer coupling and the suspended structure, whichminimizes vertical heat dissipation to substrate and confines mostof the Joule heating14,40.

MIR signal exchange between two identical GFs and demon-strations. Dual-functional GFs as MIR detectors and emitters,show the potential in the simplification of MIR-based optoelec-tronic systems. We set up a bidirectional MIR communicationsystem (Fig. 4a and Supplementary Fig. 5) to exchange infor-mation between two identical GFs using MIR as a medium. Thedigital data are modulated to the on/off status of one GF, whichacts as the transmitter, and the modulated MIR light is thenreceived by the other GF and demodulated. The circuit designand working principle are explained in the Methods section. Ourbidirectional communication system is switched between down-stream and upstream modes by adjusting the delay and the biasvoltage. The working distance is generally 0.1–3 cm, which issuperior to the results in other related work42,43. Although thesignal attenuation is inevitable (Supplementary Fig. 11), thedetected photocurrent observed at the distance of 3 cm is suffi-ciently strong to make the bidirectional MIR communicationwork smoothly. As shown in Fig. 4b, the system exhibits anincreasing current transfer ratio, CTR, the ratio of output current(Iout, the photocurrent at the GF detector) to the input current(Iin, the input current at GF emitter), with the decrease inworking distance, and reaching an CTR of 0.37% at the distanceof 0.3 cm. Provided that the incident MIR light from GF emitter isfocused in a narrow active area of the GF receiver combined witha proper medium of transmission such as coupling with anoptical fiber, high communication frequency is likely realized atmuch longer distances in this system.

The input electric field of GF emitter, F, well controls the Iout atthe GF receivers (Fig. 4c), and an F of 3.2 V cm−1 generates anIout of ~0.4 mA. Weak atmospheric absorption of light in the MIRrange enables the system to achieve high transmission and alsofree from the interference of visible light blockers. As recorded inFig. 4d, Iout is almost unchanged while shielding with 500 nm or800 nm filters. The ability to switch the same GF from emission todetection eliminates the use of two sets of transmitters andreceivers, resolving the problems of conventional communicationsystems, such as light fidelity (Li-Fi)44 and nonlinear conversiontechnique45. The bidirectional data transmission can modulate at1–125 Hz (Supplementary Fig. 12). Since GF is capable offunctioning at high frequencies both as a MIR emitter anddetector, the communication frequency is likely limited by the RCtime constants of electrodes and encoding/decoding circuitry.Thus, this communication frequency can be further improved bychoosing low noise circuits that can function at high speed. Asshown in Fig. 4h, MIR communication between two identical GFs

reaches up to 100 kHz after removing the signal processingmodules (see the supplementary Fig. 13). Both MIR downstreamand upstream transmissions are highly stable (Fig. 4e, f), asdemonstrated by the output of a dialog box (SupplementaryMovie 2). Signal transmission in traditional communicationsystems are hampered by atmospheric absorption and variationin ambient pressure. In this system, the electrical signals ofcommunication quickly stabilize at 0.45 times of initial valuewhen the air pressure increases from 1 × 10−5 to 25 mbar,suggesting the ability of the GF-based MIR communicationsystem to function at surroundings under different pressures (seethe Supplementary Fig. 14).

DiscussionThe past decades have witnessed a significant development inbidirectional optical signal transmission, which made a greatcontribution to the monolithic integration and miniaturization ofoptoelectronic devices. Recently, Bao et al. proposed the bidir-ectional communication system between two identical perovskite-based devices42. However, most of this work focused on thevisible and near-infrared region due to the material and perfor-mance limitations. Our system extends it to the MIR region usingdual-functional and high-performance GFs. Furthermore, theMIR emission and detection properties of our GFs reflect thegreat potential of weakly coupled multilayer graphene systems inMIR sensors.

The GF-based bidirectional MIR communication systemexhibit advances in both technology and materials. In principle,any materials that can detect MIR light are capable of bifunc-tionality as observed in GF, since they can emit MIR light bygray-body radiation. Since there are no practical demonstration ofMIR optocoupling devices to reasonably evaluate the perfor-mance our GF system, we deduce a quality factor (I*, Supple-mentary Fig. 15), which represents Iout per input electric field, andexpresses as:

I* ¼ SηεσR*; ð1Þwhere S is the cross-sectional area with a determined radical sizeand R* is the MIR responsivity. Judging from this relationship, weconclude that the suitable MIR sensors are required to possess alarge cross-section area, high-energy conversion efficiency, highemissivity, high electrical conductivity, and high MIR respon-sivity. We summarize the I* of state-of-the-art high-performanceMIR sources and sensors, including nanocarbons such as CNT,SLG, FLG, etc. and carbon macro-materials, such as GF and CNTfilms. As shown in Fig. 4g, the I* of GF is 7.35 × 10−6 m S AW−1,which is at least three orders of magnitude higher than that ofother carbon-based MIR sensors. Among them, SLG, FLG, andCNT are mainly limited by small S, while CNT film is furtherrestricted by the low R*.

Compared to conventional semiconductor MIR emitters andreceivers46, GFs are highly stretchable and soft. Owing to thehydrophobic nature and mechanical durability, GF withstands thestrains and stresses of the textile manufacturing process andrough washing conditions (Supplementary Fig. 14a–c andMovie 3). MIR photoresponse of GF-based fabric after multiplecycles of washing remains virtually unchanged, presenting aviable path towards the wearable daily use47. The compatibility ofGFs in wearable textile with its intrinsic merits of flexibility,room-temperature functionality, and compatibility for massproduction lends well to future research towards wearablebidirectional optical communications.

MethodsGF fabrication. Ultra-large GO liquid crystalline dispersions in DMF were com-mercially purchased from GaoxiTech (www.gaoxitech.com). GO suspensions (at a

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concentration of 10 mgmL−1) were perfused into a pumping apparatus with a2-mm-wide microfluidic channel, and then being extruded into ethyl acetatebath at a rate of 20 meters per minute. Solvents exchange and interlayer forcesinduce the coagulation of GO dispersions into gel belts. To guarantee the rapiddrying of GO belts, we used organic solvents (DMF & EA) as the dispersive andcoagulative agents. GO belts exhibit an elongation beyond 20%, and such goodflexibility smooths the following twisting processing on our homemade

machine. Flexible GO belts were processed into continuous graphene oxidefibers (GOFs) on a twist-draw apparatus, where a normal torque was applied totwist GO belts into a thread-like structure. After undergoing air drying, thetwisted fibers were thermally treated in a vacuum oven for 2 h at 60 °C toremove residual solvents. GOFs were insulating, which were reduced into highlyelectrically conductive optoelectronics by chemical and thermal treatment at3100 K.

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0.30

0.35

0.400.3 cm1 cm2 cm3 cm

CT

R (

%)

Iin (mA)

–0.5

0.0

0.5

1.0

1.5

2.0

2.5

0 40 80 120 160 200

–0.164

–0.160

–0.156

–0.152

–0.148

Vol

tage

(V

)V

olta

ge (

V)

Write data

Time (μs)

Read data

h

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Characterizations. SEM images of GFs, GO belts and GO sheets were collected ona Hitachi S4800 field-emission SEM system. HR-TEM observations of the graphenesheet in GF were conducted on a JEM-2010 HR-TEM. As-prepared GFs werebroken into individual graphene sheets after undergoing strong ultrasonic treat-ment in water. A drop of a dispersion containing individual graphene sheets wasadded on a copper mesh, and then were taken to HR-TEM observations afterdrying. Electrical conductivities were measured using the four-probe method on aKeithley 2400 multi-function source-meter. Mechanical tensile behaviors of GFwere inspected on an HS-3002C at a loading rate of 10% per minute. To observethe changes in GF electrical conductivities with the increase of elongation, wecombined source-meter with HS-3002C. XRD data were obtained with an X’PertPro (PANalytical) diffractometer employing monochromatic Cu Kα1 radiation (λ= 1.5406 Å) at 40 kV by adding a bundle of GFs in the field. Raman spectra werecollected on a Labram HRUV spectrometer operating at 532 nm while varying thelaser spot to different positions on GF.

MIR detection measurements. To examine the MIR response, photoconductivityexperiments were conducted with a GF suspended between two supporting blocksthat also served as electrical contacts. Silver paste was used to glue GF on gold padsfor better electrical contact. The distance between the two ends of electrical contactwas 1 mm. For the convenience of comparison, all the measurement systems wereplaced in natural surroundings: room-temperature, relative air humidity of 40%,and illuminance of 200 Lux. The two-probe MIR response of GF was collected atvarious bias values in both dark and light conditions. Moreover, the light sourceswere produced by a femtosecond pulse MIR laser (Standard 20W model, Δt= 200fs). This laser system can emit MIR light with the wavelength from 3 to 10 μm,being equipped with integrated pulse picker, power supply, and air-water or water-water chiller. To eliminate any possible interference from lower wavelength light,we used a near-infrared (NIR) absorptive filter to purify MIR lasers in everymeasurement. The time-resolved response of GF was recorded by illuminating ourdevice with a pulsed laser at low bias. By shifting the laser spots along the axialdirection of GF, we collected the responsive signals at different positions. Toquantitatively evaluate the MIR responsive performance of GFs, we further cal-culated their spectral responsivity (R) from the measured photocurrent (Ip). Ip isobtained by subtracting the value of current under MIR irradiance from the valueof dark current in I–V curves (Supplementary Fig. 5). Then R can be calculated as

R ¼ IpP*A

;

where P* is the power density of lasers, and A is the illuminated area of GF.

MIR emission measurements. GF emitter was driven by a low bias under ambientconditions with a two-terminal suspended structure. The emission spectra werecollected at Wuhan Institute of Product Quality Supervision and Inspection, using aJobin-Yvon iHR550 grating spectrometer, together with a cooled HgCdTe detector,with a 1–12 μm response. At such a long wavelength, MIR light could be easilyabsorbed by shelters, such as glass and polymer-based materials. To rule out thedisturbance of MIR absorbers and achieve an accurate observation of the emissionspectra, we directly collected the MIR spectra at the same surroundings like that ofMIR detection measurements. In the case of gray-body irradiance, the wavelengthand power of emission were entirely controlled by the input electric field (F). Whenthe input F was lower than 3.53 V cm−1, GF functioned to emit a bias-dependentMIR light ranges between 2 and 10 μm. A higher F induces a blue-shift of emissionspectra to shorter wavelengths closer to NIR and visible light region. The highertemperature produced at higher electric fields can damage our GF emitter. TheMIR emission power can be calculated by the Stefan-Boltzmann law, J= εσT4,where J is the radiated power, ε is the emissivity, σ is Stefan’s constant (5.67 × 10−8

Wm−2 K−4), and T is the temperature of GF. To precisely determine the realtemperature (T) of GF at the center position, we used a double color infraredthermometer, which was able to eliminate the effects of emissivity. When Fincreases from 1.17 to 3.53 V cm−1, the values of corresponding T also increasesfrom 372 to 578 K, and the value of MIR emission power was calculated to be at therange from 760 to 4113Wm−2, several orders of magnitude higher than that ofpreviously reported MIR emitters.

Bidirectional GF-based MIR communication system. In this system, an liquidcrystal display (LCD) is used as a module to input and display digital signals. Thetransmit control unit is mainly composed of three parts, a digital-analog converteron a microcontroller unit (MCU), an operational amplifier, and a GF emitter. Thereceive control units were mainly composed of four parts, a GF receiver, a sub-tractor module (OP07), an amplifier module (INA114) and an analog-digitalconverter on MCU. To realize the bidirectional transmission of data, both thetransmitting and receiving control units were connected to both the GF receiverand the emitter. To demonstrate bidirectional communication, a series of binarycodes representing the message, ZJU, do you copy?, were transmitted through theMCU in transmit control unit, which automatically recognized the input signals astransmitted signals, and switched to the transmit mode. Further, the digital-analogconverter translated the input digitals into voltage signals based on ASCII code.Then, the operational amplifier adjusted the voltage to an accurate value, ensuringthe input electric field on GF emitter lies within the range of 1.13 to 3.35 V cm−1.Synchronous MIR light was produced from GF emitter by altering input bias. Thetransmitted MIR emission was received by the GF at the receiver end while varyingdistances in the range 0.1–3 cm away from GF emitter. Owing to a reliable MIRdetecting ability, synchronous voltage signals were generated in the GF receiverwith the irradiance of GF emitter. We used subtractor and amplifier modules toprocess weaker received signals. The subtractor module increased the spacingbetween light and dark voltages, and the amplifier module enhanced the overallvoltage signals. Clear and smooth voltage signals were extracted from the amplifiermodule. Through the analog-digital converter on MCU, voltage signals weretranslated into digital format, which was displayed on the LCD. Thus, the down-stream transmission is finished. The upstream transmission at the reverse directionis conducted by a completely symmetric process. The transmit and the receivercontrol units can spontaneously switch GF from emission to detection modes byusing a relay.

Data availabilityThe data that support the findings of this study are available from the correspondingauthor upon reasonable request. Correspondence and requests for materials should beaddressed to Y.X., Z.X., and C.G.

Received: 30 April 2020; Accepted: 10 November 2020;

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Fig. 4 Bidirectional MIR communication system using two identical GFs. a Block diagram of the prototype of the bidirectional MIR communicationsystem. Receiving and transmitting sides of the system can write/read MIR data into the system by encoding/decoding transmitting messages into binarycodes. b The dependence of current transfer ratio (CTR) of GF on the increasing input current, Iin at various working distance (0.3–3 cm). The dependenceof output current, Iout, on the increasing of F applied to the GF emitter (c), and on the selection of filters (d) the GF–GF communication system.e Downstream and upstream transmissions. Details of signal processing are present in the Methods section. In IR images, “E” refers to the GF emitter and“R” refers to the GF receiver. f The electric signals of E are stably digitized in the system. Electrically driven GF transmits a signal of binary code “1”, and GFin the dormant state represents a signal of “0.” g The communication currents per input electrical field, I*, of state-of-the-art MIR sensors in bidirectionalMIR communications. h MIR communication between two GFs reaches 100 KHz without the interference of signal processing modules.

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AcknowledgementsWe thank Dr. Tawfique Hasan from University of Cambridge, Prof. F. Koppens, Prof. X.Duan and Prof. X. Wang for discussions. This work is supported by the National KeyR&D Program of China (Nos. 2016YFA0200200), National Natural Science Foundationof China (Nos. 51533008, 51703194, 51603183, 51873191, 61874094, 61674127,61474099 and 61950410606), Hundred Talents Program of Zhejiang University(188020*194231710/113), Fujian Provincial Science and Technology Major Projects (No.2018HZ0001-2), the Fundamental Research Funds for the Central Universities (Nos.2016XZZX001-05, 2017XZZX008-06, 2017QNA4036, and 2017XZZX009-2), and ZJ-NSF (LZ17F040001).

Author contributionsC. G., Y. X., and Z. X. designed the project and experiments. B.F. fabricated the graphenefibers and carried out the main characterizations. F.T., B.F., J.L., and X.L. constructed theMIR communication systems. X.L., S.D., and J.Z. helped to conduct MIR detection tests.D.C. fabricated the fabrics. H.Z., H.H., and W.G. provided suggestions to the char-acterizations. B.F., S.C.B. and Y. Xu wrote the manuscript draft. All authors participatedin discussions, read and revised the manuscript.

Competing interestsThe authors declare no competing interests.

Additional informationSupplementary information is available for this paper at https://doi.org/10.1038/s41467-020-20033-2.

Correspondence and requests for materials should be addressed to Y.X., Z.X. or C.G.

Peer review information Nature Communications thanks the anonymous reviewer(s) fortheir contribution to the peer review of this work. Peer reviewer reports are available.

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