Flexible and transparent all-graphene circuits for quaternary digital modulations

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1 Received 27 Jun 22 Accepted 24 Jul 22 Pulished 2 Aug 22 DOI:.38/ncomms22 Flexile nd trnsprent ll-grphene circuits for quternry digitl modultions Seunghyun Lee, Kyunghoon Lee, Chng-Hu Liu, Girish S. Kulkrni & Zhohui Zhong In modern communiction systems, modultion is key function tht emeds the send signl (informtion) into crrier wve so tht it cn e successfully rodcsted through medium such s ir or cles. Here we report flexile ll-grphene modultor circuit with the cpility of encoding crrier signl with quternry digitl informtion. By exploiting the mipolrity nd the nonlinerity in grphene trnsistor, we demonstrte two types of quternry modultion schemes: quternry mplitude-shift keying nd qudrture phse-shift keying. Remrkly, oth modultion schemes cn e relized with just nd 2 ll-grphene trnsistors, respectively, representing drstic reduction in circuit complexity when compred with conventionl modultors. In ddition, the circuit is not only flexile ut lso highly trnsprent (~9% trnsmittnce) owing to its ll-grphene design with every component (chnnel, interconnects, lod resistor nd source/drin/gte electrodes) fricted from grphene films. Deprtment of Electricl Engineering nd Computer Science, University of Michign, Ann Aror, Michign 489, USA. Correspondence nd requests for mterils should e ddressed to Z.Z. (emil: zzhong@umich.edu). nture communictions 3:8 DOI:.38/ncomms Mcmilln Pulishers Limited. All rights reserved.

2 nture communictions DOI:.38/ncomms22 Physiclly complint electronics with the cpility to conform to non-plnr surfce is field of rpidly growing interest due to the numerous possiilities it offers. Applictions rnging from flexile solr cells, displys 2, e-ppers 3, werle electronics 4 nd iomedicl skin-like devices,6 open up new opportunities in the field of electronics. However, nerly ll flexile electronic devices require n externl power supply nd dt communiction modules, nd the lck of portility cn severely limit the functionlity of vrious pplictions. To drive the field forwrd, three key chllenges need to e ddressed: mens to generte or store power (for exmple, flexile tteries or power genertors), dt collecting scheme (for exmple, flexile sensors) nd system to trnsmit nd receive the collected dt (for exmple, flexile wireless communiction scheme). Recent dvnces in the field hve led to notle progress in the two res of flexile power 7,8 nd flexile sensors,6. However, designing nd mnufcturing flexile wireless communiction system is still chllenge owing to mteril constrints. Conventionl orgnic polymers 9, morphous silicon, or oxide-sed thin film trnsistors,2 show only modest performnce in this re owing to their limited crrier moilities. In this regrd, grphene is the idel mteril for flexile highspeed communiction systems owing to its unique electronic nd physicl properties, including high crrier moility 3, mipolrity 4,, trnsprency 6 nd mechnicl flexiility 7. For exmple, grphene trnsistors hve chieved unity gin cutoff frequencies of up to 3 GHz (refs 8,9). Grphene ws lso used s the chnnel nd the gte mteril for flexile trnsistors owing to its mechnicl flexiility 2,2. Despite grphene trnsistors low on/off rtio tht limits their usge in the digitl/logic pplictions, they re ttrctive in the nlogue/rdio frequency pplictions due to grphene s extremely thin structure tht llows shorter scling of chnnel length without the dverse short chnnel effects 22. Beyond individul trnsistor, pioneering works on grphene nlogue electronics led to the demonstrtion of grphene-sed frequency doulers 23 2, mixers 26,27 nd modultors 24,28,29 on rigid sustrtes. Grphene mixers were shown to effectively suppress odd-order intermodultions y exploiting the symmetric chrcter of grphene trnsistors 26. A high-performnce mixer fricted y integrtion of grphene trnsistors nd pssive components on single silicon cride wfer ws lso demonstrted 27. Tking it one step further, severl groups demonstrted inry digitl modultion schemes (inry phseshift keying (BPSK) nd inry frequency-shift keying (BFSK)) with grphene trnsistors on rigid sustrtes 24,28,29. Despite the remrkle progress, ll previous grphene nlogue circuits were demonstrted on rigid silicon sustrtes. A flexile digitl modultion scheme, which is the key uilding lock for flexile high-speed dt communiction, hs not een relized using grphene circuits. In ddition, previous grphene inry modultors cn only encode single it of dt per symol. To this end, we demonstrte, for the first time, flexile nd trnsprent ll-grphene circuits for quternry digitl modultions tht cn encode two its of informtion per symol. The circuits re oth flexile nd trnsprent with every prt of the circuit including the trnsistor chnnels, the interconnects etween trnsistors, the lod resistnce, nd the source/drin/gte electrodes fricted with grphene only. The monolithic structure llows unprecedented mechnicl flexiility nd ner-complete trnsprency tht is not possile with either silicon or metl. This structure is possile ecuse of grphene s unique property of eing zero-ndgp mteril retining the property of oth metl nd semiconductor. Importntly, the mipolrity of grphene trnsistors drsticlly reduces the circuit complexity when compred with silicon-sed modultors. No more thn couple of trnsistors re required for the two quternry modultion schemes demonstrted, wheres multitude of trnsistors re required for conventionl modultor circuits 3,3. Results Modultion mechnism nd trnsistor chrcteristics. The sic modultion techniques mp the informtion y vrying up to three different prmeters (mplitude, frequency nd phse) of the crrier wve to represent the dt. The most fundmentl inry digitl modultion techniques tht correspond to ech of these three prmeters re inry mplitude-shift keying (BASK), BFSK, BPSK. Furthermore, y comining two or more inry modultion schemes, it is possile to extend this technique into quternry digitl modultion schemes such s quternry mplitude-shift keying (4-ASK) nd qudrture phse-shift keying (QPSK) 32. Specificlly, QPSK explores ll four qudrnts of the constelltion, nd it is the key uilding unit for highly efficient modultion techniques tht re widely used in tody s telecommuniction stndrds such s Code division multiple ccess nd Long-term evolution. The ove-mentioned inry nd quternry digitl modultion schemes re plotted in polr constelltion with the rdil coordinte s the mplitude nd the ngulr coordinte s the phse (Fig. ). Importntly, ll of them cn e relized y using llgrphene circuits on the flexile nd trnsprent pltform. Figure (inset) shows the schemtic of the ll-grphene device fricted on endle plstic (polyethylene nphthlte) sustrte. The top, middle nd ottom grphene lyers form the top gte lyer, the chnnel/interconnect lyer, nd the ottom gte lyer, respectively. Two dielectric lyers re deposited using n tomic lyer deposition method to isolte the three grphene lyers. The finl device is highly trnsprent s shown in Fig. (~9% trnsmittnce; see Methods), nd fully endle s shown in Fig, c. The overll trnsmittnce is higher thn the expected vlue of 93% for tri-lyer grphene 6 ecuse the mjority of the re is covered with only one lyer of grphene. Under the opticl microscope, the grphene devices cn e identified y the contrst difference mong the top gte, chnnel nd ottom gte region of the ll-grphene trnsistor (Fig. c inset). The gte response curve ws mesured for ech ll-grphene trnsistors, nd the yield ws over 98%, with 64 out of 6 trnsistors eing fully functionl. Figure d (inset) shows typicl gte response curve from the fricted ll-grphene trnsistors. Slight shift in the chrge neutrlity point is oserved owing to environmentl doping. The crrier moility vlue cn e extrcted y fitting the experimentl vlue of source-to-drin conductnce over vrying gte voltges 33. The device presented in Fig. d (inset) hs holecrrier moility vlue of 3,342 ± 26 cm 2 Vs nd electron crrier moility of 2,83 ± cm 2 Vs (See Methods nd Supplementry Fig. S). Figure d lso presents histogrm for hole moility vlues extrcted from 3 different smples. The verge hole moility is,77 cm 2 Vs with stndrd devition of 983 cm 2 Vs. These moility vlues re severl orders of mgnitude higher thn those of lterntive mterils such s orgnic polymer 9 nd morphous mterils s expected. More importntly, the unique mipolr gte response of grphene trnsistors llows simple implementtion of previously mentioned inry modultion schemes s illustrted in Fig. e. The mplitude, frequency or phse of the output voltge will e determined y the operting gte is point of the grphene trnsistor. For exmple, mplitude modultion cn e chieved y utilizing the trnsconductnce chnge over the gte voltge difference. Frequency modultion is chieved y interchnging the is point from region dominted y electron (or hole) crriers to the chrge neutrlity point. Similrly, phse modultion is relized y chnging the is point from n electron (or hole) crrier-dominted region to the hole (or electron) crrier-dominted region. Binry nd quternry modultion with single trnsistor. We next demonstrted the three inry modultion schemes y using the ll-grphene circuit. Figure 2 shows the circuit digrm overlid on flse colour imge of the device. Green, grey nd red re nture communictions 3:8 DOI:.38/ncomms Mcmilln Pulishers Limited. All rights reserved.

3 nture communictions DOI:.38/ncomms22 ARTICLE BASK BPSK BFSK 3 4-ASK QPSK 8 Z Trnsmittnce (%) Grphene top gte Al 2 O 3 Plstic sustrte Chnnel & interconnects Grphene ottom gte Wvelength (nm) c Top gte Chnnel Bck gte d Device count I ds (na) V g e AM V out2 FM PM V out, 2, 3, 4, Hole moility (cm 2 Vs ) V in V in2 Figure Modultion mechnism nd device chrcteristics of flexile nd trnsprent ll-grphene trnsistors. () A constelltion digrm depicting five different digitl modultion techniques demonstrted in this work. The z xis, representing the frequency, is included to show the frequencymodulted signls. () A plot of the trnsmittnce s function of the wvelength nd n illustrtion of the ll-grphene trnsistor structure (inset). (c) A photogrph of grphene circuit on trnsprent nd endle plstic sustrte, nd microscopic imge of n ll-grphene trnsistor (inset). Scle- r, µm. (d) A histogrm of the hole moility extrcted from 3 trnsistors nd its Gussin fit (red line). The inset is plot of the mipolr current s function of gte voltge for typicl ll-grphene trnsistor. Voltge cross the drin nd the source is mv. (e) Illustrtions of mplitude, frequency nd phse modultion of sinusoidl wve chieved y operting single mipolr grphene trnsistor t different gte ises. the respective colours for the top, middle nd ottom grphene lyers. A grphene trnsistor ws used for the modultion, nd nother unised grphene trnsistor ws used s the lod resistor (R L ) for output (V out ). The middle grphene lyer (grey) serves s the trnsistor chnnel, the interconnect etween the trnsistor, the lod resistor nd the source/drin electrodes. To chieve digitl modultion, the crrier wve (V crrier ), the dt itstrem (V dt ) nd the DC gte is (V gs ) re dded together nd pplied to the top gte (green) of the modulting trnsistor. Both V gs nd V dt determine the operting is point of the trnsistor nd modulte the crrier signl ccordingly. The ottom gte (red) delivers dditionl flexiility to the mesurement, nd it cn lso e used to djust the chrge neutrlity point (V Dirc ) if there is environmentl doping. Figure 2 d shows plots of three sic inry modultion schemes demonstrted with the ll-grphene circuit. For BASK, the sum of V dt nd V crrier is superimposed on V gs so tht the different trnsconductnces on different is points will llow V crrier to chnge in mplitude t the output. Binry informtion of nd is successfully represented y the low nd high mplitude of crrier signl, respectively (Fig. 2). Similrly, we control V gs nd V dt to djust the is point for oth BFSK nd BPSK. The vlues nd re successfully differentited y the douling in frequencies (BFSK, Fig. 2c), or y the 8 phse chnge (BPSK, Fig. 2d). To the est of our knowledge, this is the first demonstrtion of BASK using grphene circuit, while previous works hve only shown BPSK nd BFSK 24,28,29. By dding the BASK scheme, the three sic inry schemes hve een completed using flexile grphene circuits. We note tht the output voltge hs DC component for nd ecuse the trnsistor is operting t different is points on the gte response curve. The DC component cn e filtered out using highpss filter nd it hs een removed for clrity in this pper. Furthermore, y comining BASK nd BPSK, quternry modultion scheme 4-ASK ws demonstrted s shown in Fig. 2e. The inset of Fig. 2e illustrtes the four is points used in the 4-ASK scheme tht correspond to,, nd. Both the phse nd the mplitude informtion re used to distinguish the quternry signl tht is encoded in the crrier wve. Output of,, nd re represented y low mplitude, 27 phse; high mplitude, 27 phse; high mplitude, 9 phse; nd low mplitude, 9 phse in the crrier wve, respectively. A quternry digitl modultion scheme, 4-ASK, uses four points in the constelltion digrm (Fig. ) nd doules the dt trnsfer rte compred with inry scheme. Importntly, this is the first demonstrtion of quternry modultion with just one trnsistor (excluding the trnsistor tht is used s resistor), which is not possile in conventionl siliconsed modultors. Qudrture phse-shift keying with two grphene trnsistors. A more fundmentl quternry modultion scheme is QPSK, which explores ll four qudrnts of the constelltion. Figure 3 shows typicl QPSK trnsmitter structure used in modern digitl communiction. A inry dt strem is demultiplexed into the in-phse component (I) nd the qudrture-phse component (Q). I nd Q components re encoded onto two orthogonl sis functions, such s sine wve nd cosine wve, respectively, efore they re summed to generte QPSK-modulted signl. Here we used just two trnsistors with similr gte response in the ll-grphene circuit to demonstrte the QPSK modultion (Fig. 3). Actul microscope imges under lue filter re overlid on top of the circuit digrm. A sinusoidl wve from the function genertor ws connected to simple off-chip resistnce cpcitnce cpcitnce resistnce (RC CR) phse shift network to generte two orthogonl wve functions with 9 phse difference. The sinusoidl input is shifted y + 4 in the CR rnch nd y 4 in the RC rnch 34. Then, ech of these signls is summed internlly y the function genertors with two squre wves (I dt nd Q dt ) nd fed to the gtes of ech trnsistor ( detiled mesurement setup is shown in the Supplementry Fig. S2). The outputs (V I nd V Q ) re then summed to generte the finl QPSK-modulted signls. These signl components re plotted in Fig. 3c. The I crrier nd the Q crrier re the orthogonl crrier signls. The dt itstrem with,, nd is represented y the in-phse component I dt nd nture communictions 3:8 DOI:.38/ncomms Mcmilln Pulishers Limited. All rights reserved.

4 nture communictions DOI:.38/ncomms22 e Dt Crrier 3 3 V dt V crrier. Off-chip Top gte Bottom gte V gs Chnnel G V DD... D S 4ASK R L V out 4ASK 2. Dt Crrier c Dt Crrier d Dt Crrier BASK BFSK BPSK Figure 2 Binry nd quternry digitl modultions using single ll-grphene trnsistor. () A circuit digrm with flse-colour imge of grphene trnsistors connected in common-source configurtion. The V dt signl is the digitl dt tht is encoded onto the crrier signl V crrier. The V dt signl is squre wve for ll three inry digitl modultion schemes nd four level step-like wve for the 4-ASK modultion scheme. () Time domin plot of the BASK. V DD of V is the supply voltge. (c) Time domin plot of the BFSK t V DD of V. (d) Time domin plot of the inry phse-shift keying t V DD of V. (e) A time domin plot of the 4-ASK modultion of crrier signl. The inset is n illustrtion descriing the four operting gte is points used in 4-ASK. V DD is V. the qudrture-phse component Q dt s shown in the plot. Modulting I crrier with I dt results in phse chnges in I chnnel nd the sme pplies to Q crrier, Q dt nd Q chnnel. Dt it nd in I dt corresponds to phse of 8 nd in I chnnel. Similrly, Dt it nd of Q dt corresponds to phse of 9 nd 27 in Q chnnel. The sum of I chnnel nd Q chnnel is the finl output signl (I + Q) tht hs distinct phse shifts of 22, 3, 3 nd 4, ech corresponding to inry dt of,, nd. To vlidte the result, the instntneous phse informtion ws extrcted from the finl output signl (I + Q) nd plotted s demodulted phse (Fig. 3c, ottom pnel). This mthemticl form of demodultion ws chieved y extrcting the phse informtion from the Hilert trnsform of the output signl (I + Q). The plot of the demodulted phse indictes cler distinction of phse shift etween different QPSK signls. The crrier to noise rtio (C/N), which is the rtio of signl power to the white-noise power, ws found to e 2. db from the frequency spectrum using conventionl signl nlysis progrm (see Methods). The corresponding it error rte (BER) ssuming dditive white Gussin noise chnnel is much lower thn the performnce threshold BER of 4, ove which the rdio link is considered to e in outge 3. This confirms the roustness nd ccurcy of the grphene-sed QPSK modultor. From the output signl (I + Q) nd the input crrier signls, the gin of the QPSK modultor is clculted to e round.6, one of lrgest ever mesured with grphene modultors. The gins of our ll-grphene inry nd quternry modultors re comprle or lrger thn ll the previous modultor works s shown in the Supplementry Tle S. Although these gin vlues re less thn, in modern trnsmitter structure, the mplifiction of the signl is primrily ccomplished y n udio mplifier or power mplifier, nd gin is not n essentil component of the modultor. All-grphene modultor circuits under mechnicl strin. Lst, we exmine the performnce of ll-grphene circuit under mechnicl strin (Fig. 4). Frequency modultion (tht is, frequency douling, Supplementry Fig. S3) ws first evluted t different ending rdii. To quntify the comprison, fst Fourier trnsform ws pplied to the douled output voltge, yielding peks t ω nd 2ω corresponding to the originl frequency nd the douled frequency (Supplementry Fig. S4). The rtios of these two peks, which indicte the spectrl purity of the frequency douling, re plotted s function of the ending rdii in Fig. 4. Very little chnge is oserved under different ending rdii, indicting the roustness in circuit performnce under mechnicl strin. In ddition, vrious inry nd quternry digitl modultion schemes re lso successfully demonstrted t mximum strin level of 2.7% (ending rdius of. mm) under the test set-up (Fig. 4). The results further confirm tht the trnsprent ll-grphene modultors re fully functionl under highly strined conditions. Discussion The operting principle nd technique of flexile nd trnsprent llgrphene modultors descried here cn e pplied to widely used network technologies in tody s multimedi nd communiction nture communictions 3:8 DOI:.38/ncomms Mcmilln Pulishers Limited. All rights reserved.

5 nture communictions DOI:.38/ncomms22 ARTICLE Binry its Ι dt Demultiplexer Q dt V I +4 NRZ encoder Crrier NRZ encoder V out RC CR Network sin c t RC CR network cos c t V Q i (t) q (t) QPSK (t) V DD c Ι crrier Q crrier I dt Q dt I chnnel (mv) Q chnnel (mv) I+Q (mv) I dt V crrier Q dt Demodulted phse ( ) Figure 3 QPSK demonstrted with two ll-grphene trnsistors. () A conceptul digrm of conventionl QPSK trnsmitter structure. NRZ encoder is non-return-to-zero encoder where is represented y positive voltge stte nd is represented y negtive voltge stte. RC CR network is the resistnce-cpcitnce cpcitnce-resistnce phse shift network tht genertes two orthogonl wve functions with 9 phse difference. () An llgrphene circuit digrm of the QPSK system using two trnsistors. The ctul microscopic imge of the ll-grphene circuit under lue filter is shown. The trnsistor dimension is µm µm. (c) Time domin plots of the input nd output signls demonstrting QPSK modultion scheme. V DD of 7 V ws the supply voltge. devices y either introducing pulse shping with signl delys or coupling forementioned modultion schemes 32. Severl recent works lso demonstrted voltge gin in grphene trnsistors showing possiility of grphene-sed mplifiers nd rdiofrequency front-ends in communiction system The comintion of n efficient modultion method with relile rdiofrequency front-end will e the key fctor in determining the prcticlity of moile nd flexile pprtus. In conjunction with conventionl thin film technology nd high-resolution lithogrphy, ll-grphene modultor circuits will hve pivotl role in relizing high speed, mechniclly complint, nd trnsprent electronic system in the ner future. Methods Smple friction. Grphene films used in this work re synthesized using chemicl vpour-deposition method on copper foil After the chemicl vpour-deposition synthesis, one side of the copper smple with grphene ws coted with 9PMMA A2 (Microchem) resist nd cured t 8 C for min. The other side of the smple ws exposed to O 2 plsm for 3 s to remove the grphene on tht side. The smple ws then left in Ammonium persulfte (Sigm Aldrich, G) solution (.2 g ml ) for t lest 6 h to completely dissolve wy the copper lyer. Then the grphene ws trnsferred to -µm thick polyethylene nphthlte sustrte. The PMMA coting is removed with cetone nd the sustrte is rinsed with deionized wter severl times. The grphene lyer ws then ptterned with conventionl stepper tool (GCA AS2 AutoStepper) using SPR22 3. (Microchem) resist. The process temperture ws kept under the glss trnsition temperture of the plstic sustrte (2 C) t ll times. After grphene ws ptterned, 2 nm of Al 2 O 3 ws deposited s uffer lyer using e-em evportion. Then, 6 nm of Al 2 O 3 ws deposited s the dielectric using tomic lyer deposition t 8 C. Another grphene lyer ws trnsferred on top of the Al 2 O 3 lyer to form the chnnel lyer nd then it ws ptterned with lithogrphy gin. E-em evportion nd tomic lyer deposition of Al 2 O 3, with the sme thickness s the ottom dielectric, ws repeted on top of the chnnel lyer. Finl grphene lyer ws trnsferred gin on top of the dielectric nd ptterned with lithogrphy to e used s the top gte. Trnsmittnce mesurement. The trnsmittnce mesurement set-up consists of monochromtor (Acton SP23 triple grting monochromtor/spectrogrph, Princeton Instruments) coupled with 2-W tungsten hlogen lmp (Hmtsu), collimtor nd photodetector. An iris ws used to prevent the photodetector from soring the scttered light from the sustrte. Opticl power mesurements were crried out using 928-C power meter (Newport) coupled to n ultrvioletenhnced 98UV Si photodetector (Newport). A lnk polyethylene nphthlte sustrte ws used s reference for sutrction. DC chrcteriztion. The contct resistnce nd the moility cn e extrcted y fitting the experimentl vlue of resistnce cross the source nd drin of the grphene trnsistors with the following eqution 33, V L Rtotl = ds = Rcontct + Ids Vg VDirc qmw n 2 ( ) + ( Cox ) 2 q where the vriles re defined s drin/source voltge V ds, drin/source current I ds, contct resistnce R contct, gte cpcitnce C ox, residul crrier concentrtion n, the gte voltge V g, the chrge neutrlity point V Dirc, drin/source width W nd length L. Device in Supplementry FS indicted hole moility of 3,342 ± 26 cm 2 Vs nd electron moility of 2,83 ± cm 2 Vs with residul concentrtion of n = (2.47 ±.), cm 2 nd R contct = 6.4 ±. kω. For ll the cses, the residul concentrtion mtched well with the reported vlues 2 cm 2 (ref. 33). Notly, the high contct resistnce is resulted from series resistnce of long grphene strips tht hve een used s the interconnects etween the drin/source electrodes nd the contcts. Although the lrge series resistnce currently limits the frequency performnce of the devices, this prolem cn e resolved y prtil doping of grphene interconnects in the future. Severl works hve shown it is possile to lower the grphene sheet resistnce significntly y room temperture doping 4,42. Extrction of crrier to noise rtio (C/N). Signl to noise rtio is defined s, 2 VSignl log 2 [ db] VNoise () (2) nture communictions 3:8 DOI:.38/ncomms Mcmilln Pulishers Limited. All rights reserved.

6 nture communictions DOI:.38/ncomms22 Pek 2 / Pek (dbv) Rdius of curvture (mm) This figure chrcterizes the rtio of the fundmentl signl to the noise spectrum. The noise spectrum includes ll non-fundmentl spectrl components such s spurs nd the noise floor in the Nyquist frequency rnge (smpling frequency/2) without the DC component, the fundmentl itself nd the hrmonics. Six hrmonics were considered in our clcultion. Crrier-to-noise rtio (C/N) (tht is, signl-to noise rtio of -modulted signl) of 2. db ws extrcted from the fst Fourier trnsform plot using conventionl progrm, SBench 6. (Spectrum GmH). The BER from this C/N vlue is significntly etter thn the performnce threshold of QPSK system s indicted in the ref. 3. References. Lipomi, D. J., Tee, B. C. K., Vosgueritchin, M. & Bo, Z. Stretchle orgnic solr cells. Adv. Mter. 23, (2). 2. Mch, P., Rodriguez, S. J., Nortrup, R., Wiltzius, P. & Rogers, J. A. Monolithiclly integrted, flexile disply of polymer-dispersed liquid crystl driven y ruer-stmped orgnic thin-film trnsistors. Appl. Phys. Lett. 78, (2). 3. Rogers, J. A. et l. Pper-like electronic displys: lrge-re ruer-stmped plstic sheets of electronics nd microencpsulted electrophoretic inks. Proc. Ntl Acd. Sci. USA 98, (2). 4. Ctrysse, M. et l. Towrds the integrtion of textile sensors in wireless monitoring suit. Sens. Actutors A 4, 32 3 (24).. Kim, D.- H. et l. Epiderml electronics. Science 333, (2) Figure 4 Flexile nd trnsprent ll-grphene digitl modultor circuits under mechnicl strin. () The plot of the signl mplitude rtio of the originl nd the douled frequency s function of the curvture rdius for grphene frequency douler. The inset is photogrph of the mesurement set-up. () Time domin plots of BASK (lck), inry phse-shift keying (red), BFSK (lue) nd 4-ASK (green) schemes chieved with mechniclly strined ll-grphene circuits t. mm rdius of curvture (2.7% strin) Lipomi, D. J. et l. 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7 nture communictions DOI:.38/ncomms22 ARTICLE Acknowledgements Acknowledgement is mde to the Ntionl Science Foundtion Sclle Nnomnufcturing Progrm (DMR-287). Prt of the work ws conducted in the Lurie Nnofriction Fcility t University of Michign, memer of the Ntionl Nnotechnology Infrstructure Network funded y the Ntionl Science Foundtion. Author contriutions S.L. nd Z.Z. conceived the experiments. S.L. fricted the devices, developed the electricl mesurement set-up nd performed the mesurements. K.L. provided support for friction, C.L. provided support for trnsmittnce mesurement, nd G.S.K. contriuted to the electricl mesurement set-up. S.L. wrote the pper nd Z.Z. supervised the work. All uthors discussed the results nd commented on the mnuscript. Additionl informtion Supplementry Informtion ccompnies this pper t nturecommunictions Competing finncil interests: The uthors declre no competing finncil interests. Reprints nd permission informtion is ville online t reprintsndpermissions/ How to cite this rticle: Lee, S. et l. Flexile nd trnsprent ll-grphene circuits for quternry digitl modultions. Nt. Commun. 3:8 doi:.38/ncomms22 (22). nture communictions 3:8 DOI:.38/ncomms Mcmilln Pulishers Limited. All rights reserved.