Multifunctional Two-Dimensional PtSe2‑Layer Kirigami Conductorswith 2000% Stretchability and Metallic-to-SemiconductingTunabilityEmmanuel Okogbue,†,‡,∇ Sang Sub Han,†,§,∇ Tae-Jun Ko,† Hee-Suk Chung,∥ Jinwoo Ma,§

Mashiyat Sumaiya Shawkat,†,‡ Jung Han Kim,† Jong Hun Kim,§ Eunji Ji,¶ Kyu Hwan Oh,§

Lei Zhai,†,⊥,# Gwan-Hyoung Lee,§ and Yeonwoong Jung*,†,‡,#

†NanoScience Technology Center, University of Central Florida, Orlando, Florida 32826, United States‡Department of Electrical and Computer Engineering, ⊥Department of Chemistry, and #Department of Materials Science andEngineering, University of Central Florida, Orlando, Florida 32816, United States§Department of Material Science and Engineering, Seoul National University, Seoul 08826, South Korea∥Analytical Research Division, Korea Basic Science Institute, Jeonju 54907, South Korea¶Department of Materials Science and Engineering, Yonsei University, Seoul 03722, South Korea

*S Supporting Information

ABSTRACT: Two-dimensional transition-metal dichalcogenide(2D TMD) layers are highly attractive for emerging stretchableand foldable electronics owing to their extremely small thicknesscoupled with extraordinary electrical and optical properties.Although intrinsically large strain limits are projected in them(i.e., several times greater than silicon), integrating 2D TMDs intheir pristine forms does not realize superior mechanical tolerancegreatly demanded in high-end stretchable and foldable devices ofunconventional form factors. In this article, we report a versatileand rational strategy to convert 2D TMDs of limited mechanicaltolerance to tailored 3D structures with extremely large mechanicalstretchability accompanying well-preserved electrical integrity andmodulated transport properties. We employed a concept of strainengineering inspired by an ancient paper-cutting art, known askirigami patterning, and developed 2D TMD-based kirigami electrical conductors. Specifically, we directly integrated 2Dplatinum diselenide (2D PtSe2) layers of controlled carrier transport characteristics on mechanically flexible polyimide (PI)substrates by taking advantage of their low synthesis temperature. The metallic 2D PtSe2/PI kirigami patterns of optimizeddimensions exhibit an extremely large stretchability of ∼2000% without compromising their intrinsic electrical conductance.They also present strain-tunable and reversible photoresponsiveness when interfaced with semiconducting carbon nanotubes(CNTs), benefiting from the formation of 2D PtSe2/CNT Schottky junctions. Moreover, kirigami field-effect transistors (FETs)employing semiconducting 2D PtSe2 layers exhibit tunable gate responses coupled with mechanical stretching upon electrolytegating. The exclusive role of the kirigami pattern parameters in the resulting mechanoelectrical responses was also verified by afinite-element modeling (FEM) simulation. These multifunctional 2D materials in unconventional yet tailored 3D forms arebelieved to offer vast opportunities for emerging electronics and optoelectronics.

KEYWORDS: 2D TMDs, 2D layers, PtSe2, Kirigami, stretchable electronics, strain engineering

Emerging stretchable and foldable electronics have soughtimproved performance and unconventional functionalities

to suit their increasing demands in various areas includingoptoelectronic, medical, and energy devices.1−6 Such devices ofexotic form factors require that active device components areinterconnected with conductive materials which can toleratesignificant mechanical deformation. Accordingly, the con-ductive interconnects should be able to retain their pristineelectrical and mechanical integrity upon repeatedly applyinghigh tensile and/or torsional stress.7−9 To ensure reliable

device operation in such a severe environment, it is aprerequisite to prepare for the materials to retain tailoredphysical structures to diminish electrical loss caused byexternally exerted stress. Conventionally employed electronicmaterials with pristine forms (e.g., thin-film metals orsemiconductors) exhibit a significant increase in electrical

Received: April 26, 2019Revised: June 11, 2019Published: June 17, 2019

Letter

pubs.acs.org/NanoLettCite This: Nano Lett. XXXX, XXX, XXX−XXX

© XXXX American Chemical Society A DOI: 10.1021/acs.nanolett.9b01726Nano Lett. XXXX, XXX, XXX−XXX

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resistance with accumulating strain owing to their structuralirreversibility. A variety of approaches have been explored toconvert intrinsically brittle electronic materials to stress-resilient forms by rationally engineering their physicalconfigurations and dimensions.10−12 Among them, formingthe materials into “kirigami” patterns inspired by the ancientpaper-cutting art13,14 offers distinguishable advantages forefficiently relieving external stress.15−20 This kirigami pattern-ing employs rows of designed cuts to a planar material, whichimproves its mechanical stretchability by converting appliedtensile stress to torsional stress at specific points between thecuts.20−26 In addition to structural “engineering” approaches,substantive efforts have been used to identify a new form ofelectronic materials which intrinsically possess suitablecrystallinity to enable superior mechanical tolerance. In thisendeavor, recently explored two-dimensional (2D) transition-metal dichalcogenides (TMDs) present highly unique andpromising aspects.27,28 For instance, they exhibit significantlylarger in-plane strain limits over covalently bonded three-dimensional crystals owing to their van der Waals molecularbonding, offering opportunities for futuristic stretchableelectronics.27,29 Despite the projected advantages, combiningthese two different compelling approaches (i.e., converting 2DTMDs into kirigami forms) has been rarely attempted, leavingtheir anticipated mechanoelectrical superiority largely unex-plored. While some proof-of-concept demonstrations havebeen predicted by theory,30,31 the experimental realization of“stand-alone” 2D TMD-based kirigami patterning has beenunavailable. This limitation is attributed to the technicaldifficulty associated with reliably handling extremely thin 2Dlayers in a free-standing form. As a result, limited success hasbeen demonstrated with the graphene kirigami of a very smalldimension (≲100 μm2) realized by highly sophisticatedlithographic fabrication processes.32 Directly integrating 2DTMDs into kirigami-patterned secondary substrates of highmechanical flexibility and electrical insulation (e.g., plastics)can be a viable alternative. Although it is desirable to grow 2DTMDs directly on flexible substrates for subsequent kirigamipatterning, the intrinsically high growth temperatures inherentto most 2D TMDs have made this approach technicallychallenging. For instance, the chemical vapor deposition(CVD) growth of conventional 2D TMDs such asmolybdenum (Mo) disulfides (MoS2) generally requires ahigh temperature of >500 °C which pristine plastics cannotwithstand.33−38 Alternately, the mechanical integration of

separately grown 2D MoS2 layers onto kirigami-patternedplastics has been pursued.39 This technique achieved poorelectrical retention under stretching (i.e., tensile strain <100%),significantly falling short of theoretically predicted advantages.Herein, we employed the kirigami strain engineering

concept in 2D TMDs by directly growing them on plasticsubstrates and investigated the resulting mechanoelectricalresponses. Among a variety of 2D TMDs, we employed 2Dplatinum diselenide (2D PtSe2) layers with an intrinsicallymuch lower growth temperature than for conventional 2DTMDs as well as layer-thickness-dependent semiconducting-to-metallic transition characteristics.40,41 We directly grew 2DPtSe2 layers of controlled morphology and transport character-istics on polyimide (PI) substrates at 400 °C via CVD reactionand subsequently created centimeter-scale kirigami patterns.We then explored their strain-tunable and/or invariantelectrical and optical properties via various characterization.The metallic 2D PtSe2/PI kirigami of optimized patterngeometries presented excellent electrical retention up to∼2000% tensile strain. Moreover, they exhibited strain-tunablephotocurrent generation and modulation upon incorporatingsemiconducting carbon nanotubes (CNTs) by the formationof CNT/2D PtSe2 Schottky junctions in kirigami forms. Wealso explored highly stretchable kirigami field-effect transistors(FETs) by employing semiconducting 2D PtSe2 layers andidentified their p-type responses to electrolyte gating. Theexclusive role of the kirigami pattern geometry in the resultingmechanoelectrical properties was also verified by finite elementmethod (FEM) simulation.

■ RESULTS AND DISCUSSION

Figure 1 is the schematic depiction of the process used tofabricate a 2D PtSe2/PI kirigami. The process begins withobtaining a kirigami-patterned electrically insulating PIsubstrate, which consists of symmetrical linear cuts achievedby laser beam cutting. The cutting parameters are optimized toensure minimal heat zones on the PI substrate and no damageto its pristine mechanical properties through the laserprocess.42 The substrate is subsequently cleaned in water bysonication to remove residual impurities resulting from thepattern fabrication. A Pt thin film of ∼6−8 nm thickness isdeposited onto the kirigami-patterned PI substrate via e-beamevaporation for subsequent thermal selenization.37 With thisthickness, 2D PtSe2 layers grow in a “vertical” orientation with

Figure 1. Schematic of the fabrication processes for kirigami-patterned, vertically aligned 2D PtSe2 layers on a flexible PI substrate.

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individual 2D layers standing upright, exposing their edges onthe surface as confirmed in the next section. The Pt-depositedkirigami substrate is then put into a CVD furnace to convert Ptto 2D PtSe2 layers at 400 °C. This temperature is high enoughto ensure the complete thermal selenization of Pt41,43,44 and islow enough not to deform the underlying PI substrate. Finally,gold (Au) contacts are deposited on the two ends of the 2DPtSe2/PI kirigami for electrical characterization under tensilestretching. During the stretching process, the kirigami patternleads to a reduction in its in-plane stiffness, causing the out-of-plane buckling of constituting struts allowing for high elasticityas depicted in the scheme (verified in a later section).45

Before evaluating the mechanoelectrical performance of 2DPtSe2/PI kirigami patterns, we first confirmed the successfulgrowth of 2D PtSe2 layers on PI substrates by characterizingtheir as-grown morphology. We employed a variety ofcharacterization techniques including X-ray photoelectronspectroscopy (XPS), transmission electron microscopy(TEM), and energy-dispersive X-ray spectroscopy (EDS).

Figure 2a shows an image of a bare PI substrate (left) and a Pt-deposited PI substrate after CVD selenization (right), revealingdistinguishable color contrast. We identified the chemicalcomposition of the Pt-deposited PI substrate which underwentCVD selenization. Figure 2b,c shows the XPS core-levelspectra of Pt 4f and Se 3d obtained from the Pt-deposited PIsubstrates after CVD selenization and their correspondingspin−orbit splitting values of 3.3 and 1.1 eV, respectively. Asshown in Figure 2b, the Pt 4f spectrum was featured to presenta high symmetry of the spectral profiles with a narrow full-width at half-maximum of ∼1 eV. Moreover, Pt 4f exhibits noobservable signal produced by elemental Pt (the expectedposition of Pt 4f7/2 is ∼71 eV), indicating that the entirespectrum can be assigned to Pt4+ corresponding to PtSe2. Se 3dspectra can be deconvoluted into two chemical states, aspresented in Figure 2c. The predominant spectrum representsPtSe2, whose spin-up state (j = 2 + 1/2) is located at 54.1 eV.On the other hand, the humplike small peak centered at ∼58.6eV is associated with the oxidation of Se to SeO2 or SeO3.

46

Figure 2. (a) Camera images showing a bare PI (left) and an as-grown 2D PtSe2 layers-on-PI substrate prepared by the CVD selenization of 8 nmPt (right). (b and c) XPS characterization of (b) Pt 4f and (c) Se 3d core levels obtained from 2D PtSe2 layers on PI substrates. (d) Low-magnification plane-view TEM image of 2D PtSe2 layers isolated from their PI growth substrates. (e) SAED indexing, (f) EDS spectrum profile,and (g) EDS mapping images obtained from the corresponding 2D PtSe2 layers. (h) HRTEM image of the corresponding sample showingvertically aligned 2D PtSe2 layers along with their schematic illustration (inset). (i) STEM image showing the edges of vertically aligned 2D PtSe2layers with an interlayer spacing of ∼0.54 nm, corresponding to the schematic illustration in the inset.

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The element ratio of Pt/Se calculated from the XPS spectrumis 0.55, which indicates the formation of stoichiometric 2DPtSe2 layers. TEM characterization was employed toinvestigate the microstructure of the 2D PtSe2 layers directlygrown on PI substrates. Figure 2d shows a low-magnificationTEM of isolated 2D PtSe2 layers revealing spatiallyhomogeneous imaging contrast. The corresponding selectedarea electron diffraction (SAED) pattern in Figure 2e presentsmultiple diffraction rings which are indexed to the crystallineplanes of (100), (001), (101), and (110) in hexagonal 2DPtSe2.

47,48 The EDS spectrum in Figure 2f obtained from thesame sample indicates that the stoichiometric atomic ratio ofPt/Se is 1:2, which is in agreement with the XPS results. Thecorresponding EDS mapping images in Figure 2g confirm thehighly homogeneous spatial distribution of Pt and Se,indicating the complete conversion of Pt and Se to PtSe2.Figure 2h shows a high-resolution TEM (HRTEM) image ofthe sample corresponding to Figure 2d−g. The image revealsthat the material consists of multiple domains of verticallyaligned 2D PtSe2 layers in various crystallographic orientationexposing their 2D edge sites on the surface. Figure 2i is thecorresponding scanning TEM (STEM) image revealingvertically aligned 2D PtSe2 layer edges with an interlayerspacing of ∼0.54 nm. This value corresponds to the (001)crystalline plane of 2D PtSe2 layers, as illustrated in theschematic of their atomic structures.49,50 The cross-sectionalTEM image presented in Supporting Information S1 directly

confirms the successful growth of vertically aligned 2D PtSe2layers on PI substrates identifying their thickness as ∼30 nm.Having confirmed the successful fabrication of 2D PtSe2/PI

kirigami patterns enabled by the low-temperature directgrowth of 2D PtSe2 layers, we investigated their mechanoelec-trical performances. Prior to that, we first studied the transportproperties of as-grown 2D PtSe2 layers on a SiO2/Si substratein a field-effect-transistor (FET) configuration to identify theirintrinsic carrier types. Our vertically aligned 2D PtSe2 layersexhibit metallic carrier transport with no detectable FET gateresponses (Supporting Information S2), consistent with thepreviously reported 2D PtSe2 layers of comparable thick-ness.43,51,52 Details of FET characterization are presented inthe next section. In designing the kirigami pattern, weconsidered the geometrical and dimensional influences of thepattern on the resulting electrical properties under mechanicaldeformation. We attempted to maximize the retention ofelectrical conductance under tensile stretch by applyingoptimized parameters to the kirigami pattern design. Theschematic in Figure 3a presents the kirigami pattern ofsymmetrical linear cuts where key characteristic parameters aredefined as follows: a is the cut length, b is the spacing betweentwo cuts perpendicular to the direction of tensile force, and c isthe vertical spacing between the linear cuts parallel to thedirection of tensile force. On the basis of the beam theorywhich models the mechanical deflection of a strut,53,54 therelationship between the critical tensile force (F) and thecharacteristic parameters is given as

Figure 3. (a) Variation of current in an optimized 2D PtSe2/PI kirigami up to 2000% strain. The inset illustrates the corresponding kirigami designwith a = 17.2 mm, b = 0.4 mm, and c = 0.375 mm. (b) Two-terminal I−V transport characterictics of the same kirigami compared at various strainlevels. (c) Electrical failure obtained from another kirigami beyond ∼2300% strain. (d) Cyclic test to show the variation in current with a largenumber of repeated stretchings measured at 3 V. (e) Camera images of the kirigami corresponding to (a) and (b) upon manual stretching up to2000% strain. (f) Schematic of an LED continuity circuit using a 2D PtSe2/PI kirigami as a stretchable conductor. (g) Demonstration of lighting upan LED by a 2D PtSe2/PI kirigami stretched to 0 and 1500%.

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FEct

a b( )

3

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where E is Young’s modulus and t is the thickness of the strutas deflection occurs along its length. The equation indicatesthat an increase in the cut length leads to a reduction in thecritical buckling length, thereby allowing for continuedelongation along the direction of applied force. Similarly,decreasing the vertical and horizontal spacing improves themaximum attainability of strain. Note that the strain is definedas ε = Δl/lo, where lo is the initial length of the kirigami beforestretching and Δl is the increase in the length upon stretching.The maximum strain attainable by the kirigami is proportionalthe cut length a and is inversely proportional to the lateral andvertical spacing of b and c, while the number of columns androws within the pattern has a negligible impact.45,53

Considering the above relationship between the maximumstrain and characteristic parameters, we applied the followingparameters to achieve maximum lateral stretchability in the 2DPtSe2/PI kirigami and investigated its resulting electricalproperties upon stretching: a = 17.2 mm, b = 0.4 mm, and c= 0.375 mm. Figure 3a shows the current response of a 2DPtSe2/PI kirigami pattern with these parameters under varyingstrain levels applied by manual stretching. The kirigami patternexhibits a negligible current drop of ≲5% even up to 2000%strain, confirming its excellent electrical retention. Figure 3bshows the two-terminal current−voltage (I−V) characteristicsof the corresponding sample revealing the ohmic nature ofmetallic 2D PtSe2 layers as manifested by the high linearity ofI−V. We note that this 2000% retention achieved with 2DPtSe2 layer-based kirigami is orders of magnitude higher thanthe stretchability of other experimentally demonstrated 2D

MoS2 layer-based kirigami patterns39 and outperforms most ofthe state-of-the-art kirigami-patterned electrical conduc-tors.19,22,45,53,54 We then investigated the failure characteristicsof kirigami patterns, for which we employed the in situmonitoring of current−strain variation until their electricalbreakdown. We first manually stretched another sample to1000% strain and loaded it into an automated tensile stretcherbefore applying controlled strain. Figure 3c shows that thesample exhibits an electrical failure at ∼2350% strainaccompanying a drastic current drop within ∼5% before thefailure point. The inset of the figure corresponds to the red-boxed regime detailing the failure characteristics. For furtherassessment of the strain-invariant electrical stability of kirigamipatterns, we performed cyclic tests by repeatedly stretchingand releasing them up to a fixed strain. Figure 3d presents thecurrent variation of another kirigami sample undergoing a largenumber of cyclic stretchingup to 1000 times at 1000%strain. The result confirms the excellent mechanical stability ofthe kirigami in terms of retaining its original electricalconductance throughout severe mechanical deformation. It isworth mentioning that previously developed nanomaterials-based kirigami patterns are mostly based on the solutionintegration of active materials,17 while our approach employsthe direct growth of 2D materials in kirigami form. Thisintegration advantage leads to the excellent structural integrityof 2D PtSe2 layers on PI substrates (i.e., seamless 2D PtSe2/PIinterfaces; TEM image in Supporting Information S1), whichis believed to be responsible for the observed superiority ofelectrical retention. In addition to the mechanical and physicalstability confirmed by the cyclic test and TEM characterization,respectively, we have investigated the environmental stabilityof 2D PtSe2/PI kirigami materials. Specifically, we have

Figure 4. (a) Camera image of CNT/2D PtSe2 layer-based kirigami. (b) Variation of current observed with CNT/2D PtSe2 layer-based kirigamiwith ON/OFF illumination cycles. (c) Energy band diagram illustrating photocurrent generation at the CNT/2D PtSe2 Schottky junction. (d)Plots of current vs strain obtained with the kirigami in (a) with/without illumination for up to 75% strain. (e) Plots of current vs strain obtainedwith the CNT/2D PtSe2 kirigami of optimized geometrical parameters stretched up to 1500% strain with/without illumination. (f) Demonstrationof the reversibility in photocurrent generation upon an application/release of tensile stretch.

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characterized the chemical states of 2D PtSe2/PI samples inthe as-grown state and after ∼10 months of air exposure atambient temperature and compared their characteristics. XPScharacterization (Supporting Information S3) identifies thatthe envelopes of both Pt 4f spectra are nearly identical, whichindicates that the 4+ valence state of Pt is well retained evenafter prolonged air exposure. The shapes of Se 3d spectrapertaining to PtSe2 are also very similar to a slight reduction inSeOx.Having proven the superior structural and electrical integrity

of 2D PtSe2/PI kirigami materials, we explored their potentialapplications which take advantage of strain-invariant metallictransports. We first utilized them as stretchable conductors foroperating small electronic devices for which we demonstratedthe powering up of a light emitting diode (LED), as illustratedin Figure 3f. The optimized 2D PtSe2/PI kirigami used forFigure 3a−e was connected to an LED light and was stretchedfrom 0 to 1500% by an automated tensile stretcher. Figure 3gdemonstrates that the light intensity (2 W/m2) of the LEDremains unchanged even up to 1500% strain (denoted by thered circles in both figures), indicating unimpaired chargetransports in 2D PtSe2 layers.In addition to the application for stretchable and foldable

electrical conductors, we further explored unconventionalopportunities for 2D PtSe2/PI kirigami by combining it withother functional materials. The metallic nature of verticallyaligned 2D PtSe2 layers confirmed by the FET characterization(Supporting Information S2) indicates the feasibility offorming Schottky junctions when they are interfaced withsemiconducting materials. In this approach, we consideredsemiconducting carbon nanotubes (CNTs) with intrinsicallyexcellent mechanical flexibility suitable for kirigami integration.We fabricated a lateral junction of CNT/2D PtSe2 layers on aPI kirigami (dimensional parameters: a = 13 mm, b = 1.63 mm,and c = 1.75 mm) by partially integrating CNTs using a drop-

casting method, as demonstrated in Figure 4a. We firstinvestigated the photoresponsiveness of this CNT/2D PtSe2kirigami upon illumination with white light without tensilestretch. Figure 4b confirms the generation of significantphotocurrent from the sample in Figure 4a in its pristinestate without stretching under repeating illumination cycles,measured at 1 V in 5 s intervals. We observe that 2D PtSe2layers without CNTs do not exhibit any photocurrent,indicating an exclusive role in the electronic junction ofCNT/2D PtSe2 layers upon its generation (SupportingInformation S4). The mechanism for photocurrent generationis attributed to the Schottky junction of CNT (semi-conducting)/2D PtSe2 layers (metallic), which is very similarto the case of the photocurrent generation in CNT−graphenejunctions.55 Figure 4c illustrates an approximate energy banddiagram to depict the formation of a CNT/2D PtSe2 Schottkyjunction where a band-gap energy of ∼0.6 eV is assigned forthe CNT used in this study as known from previous reports.34

From the diagram, it is apparent that illumination with a whitelight generates and separates photocarriers, leading to anefficient separation and collection of the majority of carriersacross the Schottky barrier. Qualitatively, similar photocurrentgeneration has been observed with CNT/graphene junctions,benefiting from the metallic transport of graphene layers assimilar to 2D PtSe2 layers.

55 After confirming the photocurrentgeneration in the CNT/2D PtSe2 kirigami in its pristine form(sample in Figure 4a), we then applied a tensile stretch to itand investigated the resulting photoresponsiveness. Figure 4dcompares the electrical responses of the CNT/2D PtSe2kirigami for up to the 75% strain level with and withoutillumination. Substantial photocurrent was observed underillumination measured at 0.5 V throughout the stretch,indicating the well-retained electrical and structural integrityof the CNT/2D PtSe2 Schottky junction. We also testedanother CNT/2D PtSe2 kirigami with optimized geometrical

Figure 5. (a) Schematics showing the defining geometrical parameters for the unoptimized (left) vs optimized (right) kirigami patterns. (b, c) FEMsimulations showing the strain distribution within the kirigami patterns of (b) unoptimized and (c) optimized parameters. (d) Graphicalcomparison of maximum in-plane principal strain vs stretch for the unoptimized (blue plot) vs optimized (red plot) kirigami patterns. The insetcompares the simulated vs experimentally demonstrated images of kirigami patterns while their failure point is approximated to be ∼0.1 times themaximum in-plane principal strain. (e−g) Plots of maximum in-plane principal strain vs stretch with varying geometrical parameters of (e) a, (f) b,and (g) c.

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parameters and higher stretchability corresponding to Figure3a. Figure 4e compares the photoresponsiveness of thekirigami under a tensile stretch of up to 1500% strain with/without illumination. Significant photocurrent was generatedunder illumination, and it gradually decreases with increasingstretch levels, which is attributed to the decrease in the surfacearea of the kirigami exposed under illumination. Note that theillumination area from the optical source remains constantthroughout the tensile stretch, leading to the decrease in theactive surface area for absorbing incident photons upon thecontinued elongation of the kirigami. Moreover, we tested thereversibility of the strain-tunable photocurrent generation inthis kirigami under reversible mechanical deformation. Figure4f compares the electrical responses from the same kirigamistretched to 1500% and shrunk back to 0% as controlled by anautomated tensile stretcher under illumination of constantintensity (165 W/cm2). The results show nearly identicalvalues of photocurrent under stretching and shrinking at agiven strain level, confirming the excellent reversibility ofstrain-tunable electrical/optical properties.To clarify the mechanism behind the excellent strain-

invariant electrical performance of 2D PtSe2 layer-basedkirigami patterns, we employed FEM simulation and studiedthe geometrical effects of kirigami patterns on the strain

distribution introduced by a tensile stretch. We studied thepatterns of two different configurations (i.e., unoptimizedparameters of a = 6.5 mm, b = 1.63 mm, c = 1.75 mm andoptimized parameters of a = 17.2 mm, b = 0.4 mm, and c =0.375 mm as shown in the left and right schematics in Figure5a, respectively). FEM simulation images used to describe thespatial distribution of “global” strain on the unoptimized vsoptimized patterns under stretching are presented in Figure5b,c, respectively. The unoptimized pattern in Figure 5b showsthat the large vertical and horizontal spacings (b and c)between the struts limit their torsional rotation, causing a highstrain concentration at the cut edges even at a low strain levelof λ ≈ 1.6; λ is the stretch coefficient defined by λ = l/lo, wherelo and l denote the length before and after the stretch,respectively. On the other hand, upon stretching the kirigamiwith optimized parameters (Figure 5c), the applied tensileforce causes the pattern to readily rotate around the cut edges(orange and yellow boxes) of high strain localization. Thisrotation results in an efficient relaxation of accumulating strain,preserving the intrinsic electrical conduction with λ beingmuch larger than that observed with the unoptimized kirigami.Figure 5d compares the maximum in-plane principal strainvalues as a function of λ for the unoptimized vs optimizedpatterns. The maximum in-plane principal values are obtained

Figure 6. (a) HRTEM image of horizontally aligned thin 2D PtSe2 layers. (b) Ids−Vds transfer characteristics from a back-gated FET based onhorizontally aligned thin 2D PtSe2 layers configured on a SiO2/Si wafer. (c) Schematic illustration of an electrolyte gating of a 2D PtSe2/PI FET.(d) Ids−Vg and (e) Ids−Vds transfer characteristics from a 2D PtSe2/PI FET. The scale bar in the inset of (d) is 1 mm. (f) Ids−Vg transfercharacteristics from a 2D PtSe2/PI kirigami FET. (g) Camera images of the kirigami FETs corresponding to (f).

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from the edge cut areas of higher local strain presented inFigure 5b,c. Because kirigami patterns present complex straintensors owing to their multidirectional deformation, themaximum in-plane principal strain was chosen for performancecomparison. It is noted that significantly smaller maximum in-plane principal strain is realized in the optimized pattern at agiven λ, suggesting its high stretchability until the failure pointof >0.1. Moreover, the optical images of the experimentallyrealized kirigami patterns (insets; colored in gray) look nearlyidentical to the simulated ones, indicating the excellentcontrollability of our patterning process. It is noteworthythat the major motive for presenting the two cases of“unoptimized” vs “optimized” patterns is to demonstrate thecontrollability of lateral stretching in the kirigami patterns byrationally defining the dimensional parameters. The criterionfor achieving a large degree of stretchability is to increase thelength (a) of constituting trusts and simultaneously decreasingtheir widths (b and c). Accordingly, the failure point of thekirigami should appear at λ of a much larger value, whichcorresponds to larger stretchability. The unoptimized kirigamipattern electrically breaks down at ∼75% strain asexperimentally observed (Supporting Information S5), whichis significantly smaller than that achieved with the optimizedpattern (Figure 3e) as predicted above. To further demonstratethe role of kirigami pattern designs on controlling the resultantperformance, we have carried out additional FEM simulationsby varying the dimensional parameters of a, b, and c in eq 1.Figure 5e−g shows the variation of maximum in-planeprincipal strain with varying a, b, and c, respectively. Theanalysis confirms that the stretch levels corresponding to theapproximately defined failure point of 0.1 maximum in-planeprincipal strain drastically increase with larger a and smaller band c. This observation indicates that the kirigami pattern canbe significantly more stretched (e.g., up to ∼4000%) over whatwe have experimentally demonstrated by further adjusting thedimensional parameters. In this work, we have judiciously andintentionally selected the optimized parameters of a, b, and cand achieved the maximum stretchability of >2000% to avoidthe technical difficulty associated with manually handlingsamples with b and c parameters that are too small. In the FEMsimulation, we have considered that kirigami patterns exhibit avarying Poisson ratio during the course of their structuralchange. Details are presented in Supporting Information S6.Finally, we further explored the application aspects of 2D

PtSe2/PI kirigami patterns by taking advantage of the tunableelectrical properties of 2D PtSe2 layers of precisely controlledmorphology. Two-dimensional PtSe2 layers exhibit a semi-conducting-to-metallic transition as their layer morphology andthickness vary, as modulated by CVD growth condi-tions49−52,56 (i.e., vertically aligned thick 2D PtSe2 layersexhibit metallic transport while horizontally aligned thin layersare semiconducting56). While the above demonstrations(Figures 3 and 4) focus on utilizing the metallic transport of2D PtSe2 layers, we further extend the kirigami approach toexploit their semiconducting properties in unconventionalelectronic applications, including kirigami FETs. To this end,we grew semiconducting 2D PtSe2 layers in the horizontalorientation by selenizing Pt of small thickness (typically <1nm) under CVD conditions identical to those mentionedabove.56 Figure 6a shows a HRTEM image of horizontallyaligned 2D PtSe2 layers directly grown on PI revealing multiplecrystalline grains and Moire fringes. Corresponding cross-sectional TEM and EDS characterization further confirm their

stoichiometric growth maintaining the horizontal-layer ori-entation (Supporting Information S7). Prior to exploring thekirigami FET applications, we first carried out controlexperiments to verify the intrinsic carrier type of thehorizontally aligned 2D PtSe2 layers by characterizing FETgate responses. Figure 6b presents the transfer characteristicsof drain-source current vs voltage (Ids−Vds) with varying gatevoltage (Vg) from a back-gated FET configured on a SiO2/Sisubstrate (inset). The plots reveal p-type semiconducting FETgate responses (i.e., increasing Ids with decreasing Vg at a givenVds). Having confirmed the “intrinsic” semiconducting natureof horizontally aligned 2D PtSe2 layers, we then attempted todevelop FETs incorporating them on PI substrates andeventually their kirigami forms. For the reliable operation ofFETs even under mechanical stretching, we employedelectrolyte gates (e.g., potassium chloride (KCl)) as a mediumto gate the devices as shown in Figure 6c. Horizontally aligned2D PtSe2 layers on PI substrates were in top contact with metalelectrodes for source-drain patterns which were electricallyisolated by poly(dimethylsiloxane) (PDMS). Then, a dropletof KCl (10 mM) was applied on top of the 2D PtSe2 layers,and it was electrically biased for gating. Figure 6d shows theIds−Vg transfer characteristics of an electrolyte-gated 2D PtSe2/PI FET (device image in the inset), and Figure 6e presents itscorresponding Ids−Vg transfer characteristics with varying Vg.These characterization results confirm the reliable gateresponse of 2D PtSe2/PI FETs, offering a basis for extendingthem in kirigami form. The identical electrolyte gatingapproach was applied to 2D PtSe2/PI kirigami FETs, andFigure 6f exhibits Ids−Vg transfer characteristics from a kirigamisample of varying stretch levels. The plots clearly reveal that p-type semiconducting transports are well retained with slightlydecreasing Ids during the increasing mechanical stretch. Figure6g shows the images of the kirigami FETs corresponding toFigure 6f where PDMS is used as a chamber to contain KCl ofconstant concentration (10 mM). It is worth mentioning thatour finding of the strain-invariant Ids−Vg transfer characteristicsqualitatively agrees with the observation with the graphenekirigami FETs32 of smaller dimensions gated under similarelectrolyte conditions.

■ CONCLUSIONS

We have developed a multifunctional 2D PtSe2/PI kirigamiwhich exhibits strain-invariable electrical conductance andstrain-tunable photoresponsiveness. Metallic 2D PtSe2 kirigamipatterns of optimized dimensions and geometries presentedexcellent electrical retention up to ∼2000% tensile stretch,significantly outperforming previously explored other nanoma-terial-based stretchable electrical conductors. Moreover, theypresented substantive photocurrent modulation which wasreversibly retained throughout significant mechanical deforma-tion. Kirigami FETs were realized by utilizing semiconducting2D PtSe2 layers and exhibited electrolyte-driven tunable gateresponses under mechanical stretching. This unconventionaltype of 2D material configured in 3D form offers excitingopportunities for futuristic stretchable and foldable electronics.

■ METHODS

Preparation of a Kirigami-Patterned PI Substrate.Kirigami patterning of the PI substrate with systematic linearcuts was carried out using a CO2 laser cutter (UniversalILS12.150D, 150 W capacity) with a 10.6 μm infrared laser.

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The substrate was placed on the equipment bed, and the lasercutting process was performed in the presence of constant aircooling and gas suction to prevent the release of any toxicfumes and the burning of the material. The optimal cuttingconditions for minimizing the heat zone and burning effectfrom the laser have been identified as follows: laser power of60%, laser speed of 100%, and pulse per inch of 600.Two-Dimensional PtSe2 Layer Growth. Two-dimen-

sional PtSe2 layers were grown on the kirigami-patterned PIsubstrate in a home-built CVD system based on a horizontalquartz tube furnace (Lindberg/Blue M Mini-Mite). Anelectron beam evaporator (Temescal) was used to deposit Ptseeds of controlled thickness at a constant evaporation rate of0.15 Å/s. The Pt-deposited patterned PI substrate was placedin the middle zone of the CVD furnace with an alumina boatcontaining selenium (Se) preloaded at the furnace’s upstreamside. The quartz tube was pumped down to a pressure of ∼1mTorr and was subsequently purged with argon (Ar) toremove residual gases. The furnace was heated to a growthtemperature of 400 °C while Se was evaporated at ∼200 °C fora dwell time of 50 min under a continuous flow of Ar gas(∼100 standard cubic centimeters per minute (SCCM)).Upon completion of the CVD reaction, the furnace wasallowed to cool to room temperature naturally.Electrical and Optoelectrical Characterization. For

electrical characterization, two-terminal devices were fabricatedby depositing gold contacts on the ends of the kirigami 2DPtSe2/PI pattern. I−V transfer characteristics were recorded byusing a home-built probe station connected to a semi-conductor parameter analyzer (HP 4156 A). For opto-electrical characterization, the electrical response of the devicewas recorded under illumination from a white LED ofcontrolled intensity. Strain-dependent electrical and opto-electrical properties were obtained by operating the devicesmounted on a home-built tensile stretcher.XPS and TEM/STEM Characterization. XPS character-

ization was performed using a Thermo VG Scientific K-αsystem equipped with an Al Kα-ray source (1486.3 eV) underultrahigh vacuum. The energy resolution of the instrument isbetter than 0.5 eV, and a 100 W X-ray spot of 400 μm2 wasused for surface scans with a pass energy of 50 eV. All XPSpeaks are calibrated using the binding energy of C 1s, 284.5 eV,and a Shirley-type background and a Voigt function were usedfor peak fitting. TEM/STEM characterization was performedusing JEOL ARM200F FEG-TEM/STEM with a Cs (sphericalaberration) corrector. Plane-view TEM samples were preparedby delaminating the 2D PtSe2 layers from PI substrates insidethe water and by transferring them to carbon TEM grids.Cross-sectional TEM samples were prepared by focused ionbeam (FIB) milling and lift-out techniques using a Quanta 2DFEG, FEI. As-grown 2D PtSe2 layers/PI substrates were coatedwith a carbon film of ∼100 nm thickness and weresubsequently cross-sectioned using a FIB-Ga ion beam of 30keV. All TEM/STEM operations were performed at anaccelerating voltage of 200 kV.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theweb. The Supporting Information is available free of charge onthe ACS Publications website at DOI: 10.1021/acs.nano-lett.9b01726.

Cross-sectional TEM image, EDS profile, FET character-ization, XPS characterization, strain−current measure-ment, and photocurrent measurement (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: Yeonwoong.Jung@ucf.edu.ORCIDLei Zhai: 0000-0002-3886-2154Gwan-Hyoung Lee: 0000-0002-3028-867XYeonwoong Jung: 0000-0001-6042-5551Author Contributions∇These authors contributed equally to this work.Author ContributionsY.J. conceived the project and directed it. E.O. and S.S.H.developed the 2D PtSe2/PI kirigami patterns and evaluatedtheir electrical and optoelectrical performances under theguidance of Y.J. E.O. developed the CNT/2D PtSe2 kirigamiunder the guidance of Y.J. and L.Z. S.S.H. and H.-S.C.performed the TEM characterization. T.-J.K. participated inthe development of kirigami patterns and the growth of 2DPtSe2 layers. J.M. performed the FEM simulation under theguidance of K.H.O. J.H.K. participated in the development ofkirigami patterns. M.S.S. and S.S.H. fabricated and charac-terized the FET devices under the guidance of Y.J. J.H.K. (JongHun Kim) and E.J. performed the XPS characterization underthe guidance of G.-H.L. E.O., S.S.H., and Y.J. wrote themanuscript with input from all authors.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSY.J. acknowledges financial support from the University ofCentral Florida (grant no. 20080742 and VPR AECR award).The work at the University of Central Florida was alsosupported by the Korea Institute of Energy TechnologyEvaluation and Planning (KETEP) and the Ministry of Trade,Industry & Energy (MOTIE) of the Republic of Korea (no.20173010013340, Y.J. and S.S.H.). The research at SeoulNational University was supported by a National ResearchFoundation of Korea (NRF) grant funded by the Koreangovernment (grant nos. 2018M3D1A1058794 and2017R1A5A1014862; SRC program: vdWMRC center).Also, the work at Seoul National University was supportedby the Korea Institute of Energy Technology Evaluation andPlanning (KETEP) and the Ministry of Trade, Industry &Energy (MOTIE) of the Republic of Korea (no.20173010013340). K.H.O. was supported by the project titled“Development of cleanup technology for spilled oil andfloating HNS using nanostructured structures”, funded bythe Korea Coast Guard of the Korean government.

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